Fillers & Additives

Many different fillers and additives such as Talc, Calcium Carbonate, Sodium Sulfate, Glass Fibers, and colorants can be added for many reasons. Additives and fillers are introduced early in the polymer blending process. These could be added to reduce the cost of the material, increase the strength of the material, change the color, adjust the gloss, or even increase the density of the material.

Plasticizers

Due to the rigidity of PVC and CPVC, plasticizers are added during blending to help the polymer flow when processed. Large amounts of plasticizers are added when flexible PVC polymers are being blended. Plasticizers are usually comprised of smaller molecules which reduce the intermolecular entanglement or molecular attraction.

Rigid PVC and CPVC have a high viscosity because they use a minimal percentage of plasticizers to help flow, yet allow the final product to retain its stiffness. Rigid PVC and CPVC are commonly used for high strength applications such as plumbing pipes, pipe fittings, house siding, window frames, and control panels on ‘white goods’ such washing machines and dish washers.

Flexible PVC uses a much higher percentage of plasticizer to reduce the stiffness of the intended products. A further benefit of using a plasticizer is that it reduces the viscosity of the PVC. Common uses for flexible PVC are tubing, synthetic leather, shower curtains, films, and gaskets.

Heat Stabilizers

PVC and CPVC degrade very easily. Heat stabilizers are added during polymer blending to help reduce degradation and improve thermal stability when processing. The most common heat stabilizers are metal-based and often include multiple elements such as tin, barium, and calcium. These additives can withstand heat much better than PVC or CPVC alone, which improves thermal stability during processing.

Proactive Process Adjustments

For performance PVC or CPVC applications, good process documentation allows the processor to make predictive adjustments based on an expected change in the time, temperature, and/or shear in an upcoming run. For example, if an upcoming production run is going to contain regrind, the barrel temperatures can be reduced to help mitigate the additional heat the regrind adds to the process. If a production run is scheduled to be run in a machine with a smaller barrel, the barrel temperatures or pressures might need to be increased to put more temperature or shear into the material – this can help compensate for the shorter time the PVC or CPVC will spend in the smaller barrel.

When using different equipment or material from run to run, a modified process may be necessary to get the same product performance. As a processor, you can control aspects of time, temperature, and shear on the PVC or CPVC, but many times it is the role of the processor to compensate for changes they cannot control such as a different machine or lot of material. The more a processor can predict these changes, the less scrap will be produced and the higher the production efficiency your process will have.

A proactive approach is important for performance PVC and CPVC applications. This is because most performance testing requires a specified time to pass such as 12 or 24 hours before the product can be properly tested. If a processor can gain some skill and experience in making effective proactive process adjustments, then it is often possible to run production while the product waits to be tested because there is a high-level of confidence that the product being produced will meet all quality parameters including performance testing.

Reactive Process Adjustments

The first use of this data is to make process adjustments on setting up, followed by adjustments after the product is found to be defective. If the PVC or CPVC shows degradation, then there needs to be a reduction in time, temperature, and/or shear such as a reduction in screw speed, barrel temperature, or a change to a machine with a smaller barrel. Conversely, if there is not enough gelation, then the time, temperature and/or shear applied to the material will need to be increased to get more gelation.

When using the same or similar processing equipment, the same process should be used to maintain the same balance of time, temperature, and shear. The part is likely to perform the same in testing, but good documentation will help you determine and compensate for differences during startup.

Keep in mind that many flexible PVC or CPVC applications can be considered performance PVC or performance CPVC applications if they must meet specific performance requirements such as chemical resistance or chemical characterization testing which require the PVC to be at near peak gelation or fusion to pass.

Advantages & Disadvantages

There are several advantages and disadvantages associated with using the Chemical Immersion Test to evaluate the gelation or fusion of PVC and CPVC materials.

Advantages:

• Many chemicals are available for use in this test
• Testing is easy to perform
• Cost is low

Disadvantages:

• The test takes a significant amount of time to complete
• The chemicals used can be very dangerous
• Proper PPE and ventilation are required
• Evaluation requires training and experience
• Evaluation of results are very subjective
• Test results are not quantitative

Pendulum Impact Strength Test

The most common form of impact strength test is the pendulum test. In this test, a weighted pendulum is swung through the test sample to break it. The pendulum starts on one side of the testing apparatus from a fixed point and the highest position of the pendulum’s upswing is measured. This measurement corresponds to the amount of energy lost as the pendulum passed through the sample. The less the pendulum swings afterward, the more energy has been lost during the breaking of the sample. In theory, properly gelated product will have a higher energy loss when being tested than low gelation product does.

Different tests may use different weights and methods for holding the sample. For example, ASTM D2560-10 titled ‘Standard Test Methods for Determining the Izod Pendulum Impact Resistance of Plastics’ uses a notched sample which is struck while clamped in a vertical orientation.

The general methodology of different pendulum impact testers is common:

1. The test sample is prepared by cutting it to size and often is notched to reduce the effects of the outer polymer skin from skewing the test results
2. The sample is secured in the testing apparatus
3. The swinging weight is lifted and released from a fixed point
4. The energy loss is measured

Falling Weight Impact Strength Test

The falling weight impact test is a simple way to test the impact resistance of a product. These tests use a weight with a piercing element which is dropped from a specific height. The test is designed to determine whether the product resists puncture in a pass or fail evaluation. The energy used in the test is created by the dropping of the weight in the test. In most cases, the product has a specific weight and height impact that it is designed to resist. When the test is designed properly, a well-produced and gelated PVC or CPVC product will pass this test by resisting puncture, while poorly manufactured product will fail the test. Different tests use different weights and points for puncturing the sample. In many applications, the final product is fixtured under the falling weight. This tests how a product will perform when impacted by something realistic such as a hammer.

Different tests may use different weights and methods for holding the sample. For example, ASTM D5420-21 titled ‘Standard Test Method for Impact Resistance of Flat, Rigid Plastic Specimen by Means of a Striker Impacted by a Falling Weight (Gardner Impact)’ offers different product geometries, weights, and heights for standardized testing.

The general methodology of falling weight impact testers is common:

1. The test sample is prepared by cutting it to size (where applicable)
2. The cut sample or finished device is secured in the testing apparatus
3. The falling weight is lifted and released from a fixed height
4. The sample either passes or fails the test based on whether the product is punctured

Advantages & Disadvantages

There are several advantages and disadvantages associated with using Impact Strength testing to evaluate the gelation or fusion of PVC and CPVC materials.

Advantages:

• Many testing systems are available
• The tests are relatively easy to perform
• Results are easy to evaluate
• Pendulum impact testing is quantitative
• Falling weight impact testing is easy to perform final product

Disadvantages:

• Apparatus used can be dangerous
• Poor results are not always caused by poor gelation or fusion
• Falling weight impact testing is not quantitative
• Extensive testing of good and bad product required to determine pass or fail criteria

Advantages & Disadvantages

There are several advantages and disadvantages associated with using Mechanical Strength testing to evaluate the gelation or fusion of PVC and CPVC materials.

Advantages:

• Many testing systems are available
• The tests are relatively easy to perform
• Results are easy to evaluate
• Most strength testing is quantitative
• Strength testing can often be used on the final product

Disadvantages:

• Apparatus used can be very dangerous
• Poor results are not always caused by poor gelation or fusion
• Extensive testing of good and bad product required to determine pass or fail criteria

Advantages & Disadvantages

There are several advantages and disadvantages associated with using Thermogravimetric Analysis (TGA) to test the gelation or fusion of PVC and CPVC.

Advantages:

• Many low-cost TGA systems are available
• The tests are relatively easy to perform
• The test is evaluating the polymer itself

Disadvantages:

• High-accuracy TGA systems are expensive
• Low-cost TGA systems have poor ventilation
• Evaluation requires training and experience
• Comparative testing of good and bad product is required to determine pass or fail curves

Advantages & Disadvantages

There are several advantages and disadvantages associated with using Differential Scanning Calorimetry (DSC) to evaluate the gelation or fusion of PVC and CPVC.

Advantages:

• The test is evaluating the polymer itself
• Percentage gelation is analyzed and provided by the DSC system

Disadvantages:

• DSC systems are expensive
• Sample preparation requires training and experience

Quality Assurance for PVC & CPVC

With respect to PVC and CPVC, the process-specific documentation portion of the process sheet should be duplicated each and every time. On every production run, the product should have the same cooling time, short shot weight, packing pressure, etc., as these parameters do not have a significant effect on gelation or fusion of performance PVC/CPVC. In this case, quality assurance involves making sure these remain the same every time the process runs, regardless of the shift personnel.

With PVC and CPVC, the run-specific documentation portion of the process sheet should be duplicated whenever the source materials are similar. When the source material is different, these may be adjusted to achieve the proper time, temperature, and shear balance complication to reach the amount of gelation or fusion required to meet end-use requirements

For performance PVC/CPVC, you may need to reduce the material temperature or back pressure to compensate for a material lot that degrades easily or was dried twice. Conversely, you may have a part that is visually acceptable but is failing performance testing. A rise in material temperature or back pressure could be helpful in increasing the amount of gelation or fusion in your final performance PVC/CPVC product.

The more documented history you have from your material suppliers, the more likely you will be able to compensate for these changes. When possible, work with your PVC/CPVC supplier to obtain more consistent source material over time.

Process Inputs vs. Process Outputs

Process inputs are the machine-dependent parameters entered into the processing equipment. These settings include parameters such as: injection speeds, barrel temperatures, and transfer position.

Process outputs are machine-independent, process-specific parameters, which result from the process. These outputs include: temperatures, pressures, weights, times, as well as any additional information important to the product or process. Additional information which are also considered process outputs include: part measurements, material dewpoint, quality information, clamp tonnage, photographs, and observations.

Open Loop vs. Closed Loop Process Controls

Open loop process control systems guide the process through a series of predetermined steps. After the process is complete, the cycle starts over again. As the process is in operation, the operator is left to examine the process outputs for inconsistencies. The operator is then responsible for correcting the process for inconsistencies. The more inconsistent a process is, the harder it becomes for the operator to correct. The accuracy of an open loop process is solely dependent on the knowledge and experience of the operator.

Closed loop process control systems are designed to automatically correct variations in the process. Electrical measuring devices, such as pressure gauges, are used to measure the process outputs. A feedback loop is used to transfer process outputs to the closed-loop process control system. A microprocessor is then used to evaluate the process outputs for variations. The microprocessor compares the actual process outputs to the desired process outputs. If a variation is detected, the microprocessor determines how much of a particular action is required to correct the process. The signal is then sent back to the equipment, and the process is adjusted. The quick response of closed-loop process control systems makes them effective in controlling and reducing variations within a process.

General Information

• Date & Time
• Material Type, Grade, and Lot #
• Material Supplier
  • Blending Plant & Line
  • Pellitizer Plant & Line
  • Regrind, Additives, & Colorants
  • Source
  • % Used
• Mold, Machine, Auxiliary, & Downstream Equipment
  • ID, type, & condition

Molding Machinery Setpoints

• Injection *
  • Times: Delay and Maximum
  • Max. Pressure
  • Speed
  • Positions: Shot Size and Transfer
• Packing *
  • Time
  • Pressure
  • Speed
• Cooling
  • Time
  • Temperature: A-side, B-side, and other setpoint(s)
  • Cooling Line Layout Diagram
• Recovery
  • Times: Delay & Maximum
  • Back Pressure
  • Speeds: Decomp. Before, RPM, and Decomp. After
  • Temperatures: Feedthroat, Barrel, and Nozzle
  • Positions: Decomp. Before, Shot Size, and Decomp. After
• Mold Open
  • Times: Delay & Maximum
  • Speeds: Breakaway, Fast, and Final
  • Positions: Breakaway, Fast, and Fi
  • Forces: Breakaway, Fast, and Final
• Ejection Forward
  • Times: Delay and Maximum
  • Speeds: Slow and Fast
  • Positions: Slow and Fast
• Ejection Retract
  • Times: Delay and Maximum
  • Speeds: Fast and Slow
  • Positions: Fast and Slow
• Mold Close
  • Times: Delay, Maximum, Protect, and Tonnage
  • Speeds: Fast, Slow, and Protect
  • Positions: Fast, Slow, and Protect
  • Forces: Fast, Slow, Protect, and Tonnage
• Upper & Lower Process Limits
  • Alarms for All Critical Parameters
• All Auxiliary & Downstream Equipment
  • All Setoints and Configuration
  • Pictures (when possible)

NOTES:

* If profiling is used, be sure to include multiple values — and a photograph of the profile curve, if possible.

Process Process Sheet: Process Outputs

Each Mold or Product should have its own set of process outputs, typically referred to as a Process Sheet. Process sheets are a record of the actual process that made a good product. This process-specific documentation is very helpful to technicians when starting up and troubleshooting the process. There should be an approved standard process sheet used as a reference during each production run, as well as run-specific process sheets filled out during each first-piece approval.

The process outputs shown here should be consistent from one production run to another, regardless of the machine and auxiliary equipment used. If a future machine or piece of auxiliary equipment is incapable of performing as documented, it should not be used to manufacture that product, as it will not be able to duplicate the documented standard.

The following are examples of common process outputs and information that can be obtained:

• Injection *
  • Time
  • Pressures: Peak and Transfer
  • Short Shot Weight (and photo)
• Packing *
  • Time
  • Pressure
  • Final Part Weight
• Cooling *
  • Time
  • Temperature
  • Pressure
  • Flow Rate
  • Cooling Line Layout

NOTES:

The entire shot should be weighed at transfer and after packing, but the individual cavity weights can also be documented for additional information for multi-cavity molds.

*If profiling is used, be sure to include multiple values — and a photograph of the profile curve, when possible.

*Be sure to include values both to and from the process for each controller — and note the number of cooling line loops, if applicable

Time, Temperature & Shear Considerations

• Primary Time Considerations
  • Cycle Time
  • Estimated Barrel Residence Time
  • Regrind Percentage
• Secondary Time Considerations
  • Dryer Residence Time
  • Material Moisture Content
  • Smear Tip Utilized (YES/NO)
• Primary Tempertaure Considerations
  • Material Temperature Measurement
  • Screw Cooling Utilized (YES/NO)
  • Screw Coolant Temperatures: To and From Process
  • Barrel Cooling Fans Utilized (YES/NO)
  • Barrel Zone Temperatures (actual)
• Secondary Temperature Considerations
  • Dryer Temperature
  • Hot Runner Temperatures: Actual Temps
• Primary Shear Considerations
  • Back Pressure
  • Recovery Time
  • Injection Time
  • PVC-Specific Screw Utilized (YES/NO)
• Gelation or Fusion Test Results
  • Testing Results (PASS/FAIL)
  • Testing Measurements: Actual Data

Injection Speed

Since the higher viscosity PVC or CPVC encounters high shear rates when it enters the gate, a low injection speed is commonly used in the gate area. This low injection speed causes a large increase in material viscosity, making it very difficult to fill the whole mold. To decrease the viscosity of the PVC or CPVC during injection, a higher injection speed is typically used while filling most of the mold cavity.

Keep in mind, high injection speeds also increase shear on the polymer, but this shear is very quick. Shear during injection does not have a significant impact on the mixing of performance PVC or CPVC polymers with respect to improving gelation. High injection speeds can cause over-shearing and degradation of the PVC or CPVC. Low injection speeds will increase the polymer viscosity, which can prevent mold filling or packing.

Injection Speed Profiling

Although most polymers mold well with one injection speed, PVC & CPVC often process better with multiple injection speeds. The easiest way to determine the preferred injection speeds is to conduct an injection speed study with packing turned off. An Injection Speed Study involves creating short shots with 5 to 10 different injection speeds. The parts produced are labeled with the Injection Speed and evaluated to determine which speeds look the best in the runner, gate, middle, and end, as well as any concern areas. These typically indicate what speeds are best in what areas.

For example, suppose an injection speed study determined the gate area looks best at 0.2 in/s (5mm/s), but the rest of the part looks good at 0.6in/s (15mm/s). In this case, the best starting point when developing the injection profile would be using 0.2in/s & 0.6in/s (5 & 15mm/s). If the short shot looks good in all areas at one speed, such as 0.4in/s (10mm/s), then you should use just one injection speed to mold the part.

When establishing the profile, you will need to conduct short shots at each stage to determine when each area is filled so you can begin the next injection speed. Ensure the transfer position for the first speed selected passes far enough beyond the area of concern before transferring to the next speed. You will need to document the time, pressure, and short shot weight at each transition to be able to properly repeat your profile during startups and troubleshooting.

The advanced method for Injection Profiling is to conduct a Rheology Curve at 10 different injection speeds while collecting and labeling the short shots for an Injection Speed Study. Although PVC and CPVC have very high viscosities, the polymers still show shear thinning behavior as the injection speed increases. When evaluating the viscosity curve, if no significant shear thinning is seen, then the gate is likely too large to cause adequate shear thinning during injection – such a large gate may make it very difficult or impossible to fill and pack the part properly. If the Viscosity Curve shows immediate shear thinning and the part shows evidence of excessive shear in the gate, then the gate may be causing too much shear because it is too small. The injection speed or speeds would be selected in the same manner as the speed study, but the advanced method allows the gates to be evaluated as appropriate or inappropriate for the application.

Maximum Injection Pressure

When injecting, is it very important to ensure you have adequate injection pressure available to avoid a pressure-limited process. A pressure-limited process is one where the maximum injection pressure is reached. This situation prevents the desired injection speed from being maintained – this causes the screw to slow down during injection. Since the screw speed during injection controls the viscosity of the polymer, injection speed control is very important for PVC and CPVC. A pressure-limited process causes a slowdown of the screw, resulting in an increase in polymer viscosity. This causes a significant pressure loss, which may prevent the part from being injected or packed completely.

During process development, a high injection pressure should be used to allow for changes in speed to not cause the process to become pressure-limited. This allows the process developer to conduct the necessary studies and create the correct injection profile.

Once the process is being prepared for production, the max injection pressure should be set to 20% above the peak injection pressure. Too much pressure during production can cause excessive pressure to build up if a flow obstruction, such as a blocked cavity, occurs. This high pressure can often cause significant mold damage. Too little injection pressure available during production can cause the process to become pressure-limited, which will cause variation and defects, such as shorts, sinks, and voids.

Maximum Injection Time

When injecting, is it very important to ensure you have adequate injection time available to ensure the process injects until it reaches the transfer position.

During process development, a high injection time should be used to allow for changes in speed to not cause the process to stop injecting prematurely. This allows the process developer to conduct the necessary studies and create the correct injection profile.

Once the process is being prepared for production, the max injection time should be set to 20% above the injection time. Too much time available during injection can cause pressure to be applied for excessive periods of time if a flow obstruction, such as a blocked cavity, occurs. This situation can often cause mold damage. Too little injection time available during production can cause the process to transfer prematurely, which can cause variation and defects such as shorts, sinks, and voids.

Transfer Position

The transfer from injection to packing should take place when all mold cavities have a short shot. When developing the transfer position, the part should be considered 100% injected when the first mold cavity is full (or the flow is stopped by a gas trap). At this point, all the mold cavities should be weighed, and this should be documented as the part weight at 100% full. Multiply the full injection weight by 90 and 95% to calculate the desired part weight range at transfer. The transfer position should be adjusted to get an injection weight between this 90 and 95% full range. This transfer position ensures all cavities remain short shot at the time of transfer during normal variation.

If all the mold cavities are not short at transfer, then they will start to pack during injection, causing variation in the process. Since PVC and CPVC tend to be injected very slowly, the process can be easily affected by small shifts in viscosity. It is always recommended to have all the mold cavities short to ensure normal variation does not significantly affect the process and cause packing to occur during injection.

Packing Pressure

The packing pressure must be high enough to fill & pack the part without flashing or over-packing the mold. Although your molding machine may be capable of utilizing multiple 2nd stage packing pressures, you should only apply a single pressure setting until the end of packing. Using one packing speed makes the process more repeatable with less variation.

To determine the appropriate packing pressure, you should determine the minimum and maximum packing pressure, which provides an acceptable packed-out part. With PVC & CPVC, it is best to start the lab with times currently in use with similar processes. If you are unsure as to what packing time to start with, it may be best to perform the 2nd stage packing pressure study at 2 or 3 different packing times.

This acceptable packing pressure range (minimum to maximum acceptable packing pressure) is your process window. Select a final packing pressure in the middle of this range. If the process window is large, the process should be easy to maintain over time. If the window is small, the process will be very difficult to maintain, thus some parameters, such as temperature, injection speed, or packing time, might need to be changed to obtain a larger process window.

Packing Speed

Most molding machines allow you to select a packing speed. This packing speed should be set to the same speed as your injection speed. If you are using multiple injection speeds, the packing speed should be set to the last speed in your profile. Using the same speed when packing allows the screw to slow down naturally. If a different speed is used, the screw may speed up or slow down at transfer, which may cause defects such as shorts or flash.

Packing Time

For most non-PVC or CPVC applications, the gate will seal or freeze off during 2nd Stage Packing. When the gate seals, it prevents material from flowing in and out of the mold cavity when packing pressure is turned off.

Many PVC and CPVC materials have very high viscosities that require very high injection pressures using very large gates. Many of these large gates require such a long time to achieve gate seal that it is unprofitable to wait for gate seal to occur when molding PVC and CPVC parts.

For PVC parts, longer packing times will tend to provide a better-formed and packed part, but lower packing times will save cycle time and reduce part weight. Packing time studies on PVC parts involve molding parts at different packing times and evaluating them for part quality, as well as weighing them for material consumption. If the part weight does not increase and the part achieves gate seal, then select a time after gate seal as your packing time. If the part does not show a gate seal, find the packing time range that provides a good product and run the process near the middle of this time.

Keep in mind, as you increase packing time, you will need to reduce the cooling time to maintain the same balance of time, temperature, and shear during the packing time study. For more about the time, temperature, and shear balance, please review the relevant chapter in this guide.

Clamp Tonnage

PVC and CPVC have high viscosities and typically require a large gap in the parting line or mold damage for flash to occur. Too low a clamp tonnage will allow flash to occur. Excessive clamp tonnage causes excessive wear to mold components, as well as block vents causing burning or dieseling.

A clamp tonnage study is the best way to determine the minimum amount of tonnage necessary to prevent flash. To conduct this study, mold a good part and document the clamp tonnage. Then gradually reduce and document the clamp tonnage until the bad parts are produced. Lastly, add 10% tonnage to the minimum tonnage that makes an acceptable product. This is your recommended clamp tonnage.

Multi-Stage Packing for Pressure-Sensitive Molds

In a small percentage of molds, the packing pressure required to complete filling can cause defects in the molded product if one packing pressure is used. If a mold cannot be processed with a standard 2-Stage Scientific Molding Process, then a multi-stage packing approach can be used.

The initial packing stage is determined by conducting a DOE (Design of Experiments) with multiple packing times & pressures to create a packing window where the mold cavity is filled, but the over-packing defect is not present. Once this window is created, select a packing time and pressure in the center of this window. This initial packing time and pressure should consistently fill the mold cavities, while not over-packing the part.

Initial Packing Window for Multi-Stage Packing

The final packing pressure and packing time are determined in the same manner as mentioned in the preceding sections.

Screw Delay or Decompression Before Recovery

Immediately after packing, there is a large amount of pressure present in front of the screw. This back pressure must be relieved or the screw will be subjected to the force of packing at the same time it is subjected to the torque of packing during recovery. This combined force & torque puts a large strain on the screw, which can cause the it to break or bend during recovery. If this pressure is not relieved, the PVC or CPVC will also be subjected to a brief moment of extremely high shear when the screw begins to turn. This short period of high shear will contribute to the degradation of your PVC or CPVC.

Screw delay before recovery is the most common way to relieve the pressure in front of the screw before recovery. This option uses a recovery delay, such as 0.2 seconds, to allow the pressure in front of the screw to be relieved by pushing the screw back before recovery starts.

Screw decompression pulls the screw back a short distance to reduce the packing pressure before recovery. This method is not recommended for PVC and CPVC as decompression before recovery might create a negative pressure. If the gate is not sealed at the end of packing, screw decompression may pull the polymer out of the mold cavity and into the runner system, causing defects such as sinks or voids near the gate.

Screw RPM

With most materials, it is preferred to complete screw recovery in 80% of your cooling time when possible. Since PVC and CPVC are very shear sensitive, most molders reduce shear with a lower screw RPM. If screw recovery exceeds the cooling time, the screw will extend the cycle time until the shot is recovered. This should be avoided, as longer cycle times can contribute to the degradation of your PVC or CPVC. It is important to ensure your screw recovery consumes no more than 95% of your cooling time for PVC/CPVC.

The primary purpose of Screw RPM is to turn the screw fast or slow enough to achieve your intended recovery time. If the PVC or CPVC process needs more heat, the barrel temperatures should be raised. If more shear is needed in the plastic, back pressure should be increased.

If necessary, you will need to extend your cooling time to ensure you have a consistent cycle time which is not affected by your screw recovery. To calculate the maximum screw recovery time, multiply your cooling time by 0.95.

Barrel Temperatures: Rear Half

For PVC or CPVC, the rear half of the barrel should use a lower temperature in the rear zone. The zone temperatures should increase incrementally to the front half temperatures. In general, a low rear zone temperature improves the conveyance of PVC or CPVC powders, pellets, or regrind in the feed zone of the screw. Increasing temperatures in the middle zones help the polymer melt begin melting as the screw channel depths decrease. You should avoid high temperatures in the rear zone, as PVC and CPVC prefer to be heated gradually during recovery.

A rear half temperature study is used to determine the rear zone temperature, which provides the best material conveyance and recovery. Before starting this test, start with a moderate rear-half temperature and stabilize the process. Once the process is stabilized, record the rear zone temperature and screw recovery time. Incrementally reduce the rear-half temperatures, stabilize the process, and document the recovery time. The optimal rear-half temperature profile is the one that provides the best recovery time while still making acceptable parts.

Barrel Temperatures: Front Half

Since the polymer heat history is important to both gelation and degradation of performance in PVC or CPVC materials, the zones in the front half of the barrel should all be set to the same temperature. This straight profile in the front provides the most consistent control of the material temperature during recovery. These front-half temperature zones should be raised or lowered together to ensure accurate control of the polymer temperature.

Barrel Temperatures: Barrel Temperatures: Nozzle &ano; End Cap

The nozzle and end cap do not influence material melting. Therefore, these zones should be increased or lowered to compensate for cycling issues. For example, these temperatures may need to be lowered to correct for heat issues, such as drooling or degradation. These temperatures may need to be increased to compensate for cycling issues such as a stuck sprue or blocked nozzle.

Back Pressure

Back pressure forces the screw to pump harder to force plastic to accumulate at the front of the screw. Increasing back pressure puts work and energy into the plastic, which increases the shear applied to the polymer. This increase in energy applied to the PVC also increases the temperature of the PVC or CPVC material. Back pressure should only be increased when more material mixing is needed or when more shear is needed to reach peak gelation in performance PVC/CPVC applications.

Adding back pressure slows the pumping of material to the front of the screw, thus, screw RPM will also need to be increased to maintain your desired recovery time. Always add back pressure incrementally, allowing the process to stabilize at each step to help avoid a dangerous chain reaction of PVC or CPVC degradation.

Screw Decompression After Recovery: Check Ring Machines

Injection molding machines equipped with a check ring require decompression after recovery to relieve pressure on the check ring after recovery. This pressure relief ensures the check ring will seat properly when the screw begins to move forward during injection. Without screw decompression, check rings will provide an inconsistent injection from shot to shot. Check rings are not recommended for processing PVC and CPVC, as they have additional surfaces where polymer flow can easily stagnate and cause degradation. Smear tips are the preferred screw tip for processing PVC and CPVC because they have a streamlined design that does not allow the polymer flow to stagnate.

Screw Decompression After Recovery: Smear Tip Machines

Injection molding machines equipped specifically for PVC or CPVC will have a smear tip at the end of the screw. The smear tip design allows for smooth material flow around the tip of the screw without any material stagnation. The smear tip does not need decompression after recovery to ensure a consistent injection from shot to shot. Some molders may use a small amount of decompression after recovery to help reduce material drooling and stringing when the mold opens for part removal.

Coolant Temperature

The heated PVC or CPVC polymer is cooled using water as the coolant. The higher the coolant temperature, the slower the cooling rate since it takes longer for the heat to be removed from the polymer. For this reason, a higher coolant temperature causes smaller part dimensions while a lower coolant temperature cooling time will result in larger dimensions.

Temperature vs. Part Dimensions

Cooling Time

For Injection Molding, cooling time is the time the mold remains closed, and the coolant removes heat from the polymer. The longer the cooling time, the quicker the heat is being removed from the polymer. This results in a faster cooling rate. Shorter Cooling Times remove less heat from the polymer, resulting in a slower Cooling Rate. For these reasons, a longer cooling time causes larger part dimensions, while a shorter cooling time will result in smaller dimensions.

Cooling Time vs. Part Dimensions

Basic Cooling Time & Coolant Temperature Study

For PVC and CPVC applications, the most common approach is to first determine the lowest mold temperature which produces good parts. Next, the lowest cooling time is determined and 10% cooling time is added for safety. If these final parts meet specifications, it is a considered a good process.

Advanced Cooling Time & Coolant Temperature Study

If the final parts from the basic study do not meet specifications, a basic Design of Experiments (DOE) should be conducted. In this scenario, parts are produced at mutiple coolant times and temperatures.

The low values in the study should represent the minimum coolant temperature and cooling time that produced an acceptable part.

A 2-factor DOE is most common when the part is already close to the desired final dimensions. A 3-factor DOE is more suitable when the part is not close to the desired dimensions. If, at any point in the study, degradation occurs, stop the study and thoroughly purge the barrel.

Basic 2-Factor Coolant Temperature & Cooling Time DOE

Basic 3-Factor Coolant Temperature & Cooling Time DOE

Mold Breakaway

As the mold initially opens, it is known as ‘breakaway.’ It is important to determine a safe speed at which the mold halves can separate safely without damaging the part. In most cases, the slower the mold opens, the better the cavity separates from the part, but this is not always the case. For this reason, start with a slow speed, and then increase it until you determine a breakaway speed that is faster and yields acceptable parts. This breakaway speed should end when the part separates from the mold cavity, the leader pins are completely out of their bushings, and any slide-actuating pins are no longer engaged.

Mold Open Fast

After the part and leader pins are cleared, a significantly higher mold opening speed should then be used until the final mold opening distance is reached. This speed should be one that the machine can handle smoothly without causing the machine to shift during mold opening. In some cases, you may need to use a slower third mold open speed to help the mold stop at a precise location for robotic part removal.

Mold Open Distance

The final mold opening distance should be what is necessary to safely remove the part from the mold during part ejection. Every additional inch or centimeter of mold opening can consume a large amount of energy and time over the course of time. For example, if a mold opens one inch (2.5 centimeters) more than required, it will travel an additional three miles (5 kilometers) beyond what is necessary after one hundred thousand cycles. All this wasted travel consumes energy and increases wear on the platens, tie bars, and foot plates.

Eject on the Fly

Whenever possible, initiate the part ejection as the mold opens. This is often referred to as ‘eject on the fly.’ This reduces the overall time required to remove the part from the mold. The eject-on-the-fly process should not start until the mold has opened the same distance as the ejection travel. This ensures there is no way the ejector pins can touch the opposite side of the mold if a mistake is made.

Ejection Cycles

Many injection molders cycle the ejection system multiple times. The intent is to ensure all the parts are off the mold before the mold closes, but in practice, this wastes time, energy, and reduces the life of your ejection components. For example, if the ejection cycle takes 3 seconds, using 2 extra ejection cycles will consume over 160 hours after 100,000 cycles. You should cycle the ejection system only once. If multiple ejection cycles are required to remove the part from the mold, then a change to the ejection system should be done to improve this situation, as this can save hundreds of hours of production over the life of the mold.

Ejection Profiling

Since fast ejection speeds can damage the part, many injection molders will use multiple ejection speeds. The initial ejection speed should be slow enough to prevent part damage. This initial speed should continue until the part is separated from the core. The second, faster ejection speed should be used to push the part away from the core to aid in part removal. Using multiple ejection speeds can help save many seconds per cycle.

Ejection Retract

To save time, the ejection retract speed should be as fast as the machine can perform smoothly. The ejection should retract immediately without any delay to help separate the ejector pins of the plate from the part. Delays in ejection retraction should only be used if a robot or machine operator is being used to remove the part from the ejector system.

Initial Mold Close Speed

As the mold closes, the initial speed should be moderately fast. This mold closing should stop just before any mold components, such as leader pins or slides, interact.

Mold Close Slowdown

Prior to any mold component contact, the mold should slow down to a lower speed. This helps ensure components, such as leader pins, engage properly.

Mold Protect

Immediately before the mold is closed, the closing speed should go into ‘mold protect mode.’ In this mode, the clamp is closed very slowly under low force. This setting should be tested by placing a business card or a piece of cardboard between the mold halves. If mold protection prevents the mold from clamping on an obstruction, the mold protection is properly set. If the mold protection allows the mold to close up on an obstruction, it is improperly set and must be adjusted.

Initial Troubleshooting

• Clean & Inspect the Vents
• Verify Process:
  • Injection Time
  • Part Weight at Transfer (no runner or sprue)
  • Peak and Transfer Pressure
  • Packing Pressure & Speed
  • Final Part Weight
  • Recovery Time
  • Clamp Force
  • Coolant Temperature & Flow
  • Material Temperature & Back Pressure
  • Barrel Residence Time
• Check:
  • Cushion is Approximately 10% of Shot Size
  • Process is Not Pressure Limited
  • Cooling Line Layout
  • Check Ring Performance (if being used)
• Look for flow obstructions or faulty equipment in nozzle, gate, or hot runner

Possible Causes: Injection

Inadequate or Blocked Venting

Inadequate venting will make it very difficult to fill the mold consistently. Molds should be vented anywhere possible. They must be vented to the atmosphere, and cross-cavity venting should be avoided. If regular cleaning is needed, there should be a procedure for scheduled vent cleaning before they become blocked.

Flow Obstructions or Restrictions

Check for obstructions in the nozzle, gate, or hot runner, as well as a faulty heater band on the barrel, nozzle, or hot runner system.

Low Injection Time

This indicates the material is entering the mold faster than the mold can vent the air out. Reducing injection speed will help increase the injection time, as well as help the gas better vent from the mold.

High Injection Time

This indicates material is entering the mold, causing an increase in material viscosity, which may prevent the material from filling the mold cavity. Increasing injection speed will help you decrease injection time, as well as the reduce the pressure loss in the mold cavity. Verify the maximum injection pressure is high enough for the machine to maintain the desired injection speed.

Low Part Weight at Transfer

This indicates too little material is entering the mold during injection, leaving the mold cavity underfilled at the start of packing. If other parameters are similar and only the weight is different, raising the shot size or decreasing the transfer position should increase the part weight at transfer.

High Part Weight at Transfer

This indicates too much material is entering the mold during injection. This condition may cause a cavity pressure spike during injection, but can also cause the screw to bounce back at the time of transfer, resulting in a high cavity pressure loss. If other parameters are similar and only the weight is different, lowering shot size or increasing transfer position should increase part weight at transfer.

High Peak or Transfer Pressure

This indicates the material viscosity has increased or there is an obstruction. First, check for flow obstructions, verify the short shot weight, and then check the material temperature before investigating other causes.

Pressure Limited Process

A pressure-limited process cannot maintain the desired injection speed. If a change in lot #, colorant, or regrind is causing a viscosity increase, you may need to raise the shot size or decrease the transfer position to compensate for the higher viscosity material. Flow obstructions and faulty equipment should always be checked when the maximum pressure is being reached. In some cases, an increase in the maximum injection pressure is needed to accommodate a higher viscosity material, but the remainder of the process should be verified to ensure there are no other causes for a high injection pressure before this change is made.

Possible Causes: Packing

Low Packing Pressure

This indicates there is not enough pressure to complete mold filling during packing. The packing pressure should be set to match the process-specific documentation, and the packing speed should be set to the same value as your last injection speed before transferring from injection to packing.

Low Packing Speed

A low packing speed will artificially slow down the screw at transfer, which can raise material viscosity and prevent it from reaching the end of fill. To avoid complications, the packing speed should be set to the same value as your last injection speed before transferring from injection to packing.

Low Packing Time

A significantly low packing time may not provide enough time to complete mold filling. A packing time study can be done to ensure the packing time is enough to achieve adequate mold filling and packing.

Low Final Part Weight

This indicates insufficient material is entering the mold cavity during Injection and packing. This is indicative of a change in the process, but you need to find other differences between the process and the documented standard before action can be taken.

High Clamp Force

Excessive clamp force will prevent the air from venting as quickly as the material is entering the mold. You may see or smell burning at the short-shot locations if this is the cause. The clamping force should be set to match the process-specific documentation. You should clean the vents at this time, as high clamp tonnage restricts airflow, which causes vents to clog faster.

No Cushion:

A process without a cushion does not have enough material to fill the part and compensate for shrinkage during packing. The cushion should be approximately 10% of the shot size. Both the shot size and transfer position must be increased to raise the cushion.

High Cushion

Melted PVC & CPVC are highly compressible plastics, thus, a process with an excessive cushion, such as 25% of the shot size, can lose significant injection & packing pressure. The cushion should always be approximately 10% of the Shot Size. Both the shot size and transfer position must be decreased to lower the cushion.

Possible Causes: Cooling

Low Coolant Temperature or High Coolant Flow

PVC and CPVC tend to be very temperature sensitive, and a cold mold can cause excessive pressure loss during injection and packing. This pressure loss often creates defects at the end of fill, such as short shots. Returning the coolant temperature to match the process-specific documentation should reduce the pressure loss in the mold. Using a coolant temperature controller with a lower-rated coolant pump or reducing the coolant flow with a flow control valve may return the water flow to match that of the documented standard.

Poor Cooling Line Layout

An incorrect cooling line layout will affect the material flow pattern. Verify that the cooling line layout matches the process-specific documentation whenever cooling is in question.

Possible Causes: Recovery

Low Material Temperature or Low Back Pressure

PVC and CPVC tend to be very temperature sensitive, and a cold material might cause excessive pressure loss during injection & packing. Low material temperature often causes with defects, such as short near the end of fill. Returning the material temperature and back pressure to match the process-specific documentation should reduce the pressure loss in the mold. It may be necessary to check the thermocouples & heater bands on the barrel, as well as the hot runner system if one is being used.

Low Recovery Time

A high screw RPM can cause inconsistent recovery and excessive shear during recovery. The screw RPM should be adjusted so the recovery time consumes most of the cooling time.

Low Barrel Residence Time

A short barrel residence time may prevent the PVC & CPVC from fully melting during recovery. This usually is due to a significantly smaller barrel being used than was used on the documented standard. In some cases, barrel temperature can be increased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

Possible Causes: Material

Material Contamination

Contaminants can cause a flow restriction if they get caught in the nozzle and hot runner system. These could be any non-melting material, such as wood, metal, or cloth, as well as degradation and any polymer that does not melt at the current processing temperature. Contamination can come from many locations, including storage, hoppers, hoses, filters, screens, surge bins, access doors, seals, and blenders. All potential sources of contamination must be checked to determine where the contamination is coming from. Once the source is determined, everything that came in contact with the material afterward must be thoroughly cleaned and inspected before returning to use.

Possible Causes: Machinery

Check Ring Performance (Intermittent Defect)

If a check ring is being used, perform a check ring performance study by weighing 10 consecutive short shots (weight all cavities + runner) and calculating variability. Over 2% variability means the check ring is suspect, and should be changed immediately if over 3% Variability. Purging the Barrel or increasing Decompression after Recovery may improve check ring performance.

% Variability = (100%) * (Max - Min) / (Average)

Possible Causes: Mold & Design

High Cavity Imbalance (Cavity-to-Cavity Variation)

Significant differences in cavity filling can cause some cavities to fill and start packing during injection, while other cavities are significantly short at transfer. Make sure all the mold cavities are short at the time of transfer to minimize any cavity-to-cavity variation in the final molded part. There are steps that can improve cavity imbalance, such as using a different injection speed, adjusting hot runner gate drops, cleaning the mold vents, increasing the number of vents, or adjusting gate width when necessary.

Part or Mold Design

With the high viscosity of PVC & CPVC, it is very difficult to fill thin sections, especially at the end of the fill. It may be necessary to increase the thickness of the part in areas that are difficult to fill with PVC & CPVC if this is a recurring issue.

Initial Troubleshooting

• Inspect for Damage, Wear, Rust, or Corrosion
• Verify Process:
  • Injection Time
  • Part Weight at Transfer (no runner or sprue)
  • Peak and Transfer Pressure
  • Packing Pressure & Speed
  • Final Part Weight
  • Clamp Force
  • Recovery Time
  • Coolant Temperature & Flow
  • Material Temperature & Back Pressure
  • Barrel Residence Time
  • Regrind Percentage & Quality
• Ensure the Mold:
  • Covers at least 2/3 of the distance between the tie bars
  • Does not extend beyond the platens
• Check Material Drying:
  • Material Moisture
  • Dryer Residence Time
  • Dryer Temperature
• Clean & Inspect the Vents
• Check:
  • Cooling Line Layout
  • Check Ring Performance (if used)

Possible Causes: Injection

Inadequate or Blocked Venting (Thin Flash)

If the flash is very thin, it may result from inadequate venting. In this situation, the clamp is forced open by the escaping gas, allowing some of the material to exit the mold cavity with the gas. The clamp then closes down on the flash, making it very thin and wispy. This must be remedied immediately, as clamping on flash will cause significant parting line damage over time. Molds should be vented anywhere possible. They must be vented to the atmosphere, and cross-cavity venting should be avoided. If regular cleaning is needed, there should be a procedure for scheduled vent cleaning before they become blocked.

Low Injection Time

This indicates the material is entering the mold faster than expected, resulting in a lower material viscosity that can flash more easily. Reduced viscosity can also cause more material to enter the mold during injection. This should be evident in a higher part weight at transfer. Reducing the injection speed will help you increase the injection time.

Low Peak or Transfer Pressure

This indicates the material viscosity has decreased and there will be less pressure loss during injection. If the mold or material temperatures are higher than the documented standard, these can be decreased to increase the viscosity of PVC & CPVC. If a change in lot #, colorant, or regrind is causing the viscosity to decrease, you may need to decrease the shot size or increase the transfer position to compensate for the lower viscosity material.

High Part Weight at Transfer

This indicates too much material is entering the mold during injection, which can cause flash. If other parameters are similar and only the weight is different, lowering shot size or increasing transfer position should decrease part weight at transfer.

Possible Causes: Packing

High Packing Pressure

Too much pressure after mold filling can force the mold to open and cause flash. The packing pressure should be set to match the process-specific documentation.

High Packing Speed

A high packing speed can reduce the material velocity if the packing pressure is significantly higher than the injection speed at transfer. To avoid complications, the packing speed should be set to the same value as your last injection speed before transferring to packing.

High Final Part Weight

This indicates excessive material is entering the mold cavity during Injection and packing. This is indicative of a change in the process, however, you need to find other changes in the process before action can be taken.

Low Clamp Force

Insufficient clamp force will allow plastic to flow between the parting lines. The packing pressure should be verified to match the process-specific documentation, or it will overcome the clamp tonnage.

High Clamp Force (Thin Flash)

Excessive clamp force will prevent the air from venting as fast as the material is entering the mold. If the flash is very thin, it can be the result of inadequate venting. In this situation, the clamp is forced open by the escaping gas, and some of the material exits the mold cavity with the gas. The clamp then closes down on the flash, making it very thin and wispy. This must be remedied immediately, as clamping on flash will cause significant parting line damage over time. This is not a common cause of flash, but reducing clamp tonnage may fix thin flash if too much parting line damage has not already occurred.

Possible Causes: Cooling

High Coolant Temperature or Low Coolant Flow

PVC & CPVC tend to be temperature sensitive, and a hot mold reduces the pressure loss during injection & packing. Returning the coolant temperature to match the process-specific documentation should return the pressure loss in the mold to normal. Check the cooling system for obstructions, as this will reduce coolant flow. Using a coolant temperature controller with a higher-rated coolant pump may raise the coolant flow to match the process-specific documentation.

Poor Cooling Line Layout

An incorrect cooling line layout will affect the material flow pattern. Verify the Cooling Line Layout matches the Process-Specific Documentation whenever cooling is in question.

Possible Causes: Recovery

High Material Temperature or High Back Pressure

PVC & CPVC tend to be very temperature sensitive, and a hot material will flow more easily due to a lower viscosity. Returning the material temperature and back pressure to match the process-specific documentation should return the material viscosity to normal. If regrind is being used, reduce the regrind percentage to increase the viscosity. You should always purge the barrel when degradation is suspected with PVC & CPVC. It may be necessary to check the cooling fans, screw cooling, thermocouples, & heater bands.

Low Recovery Time

A high Screw RPM can cause inconsistent recovery and excessive shear during recovery. The screw RPM should be adjusted so the recovery time consumes most of the cooling time.

High Barrel Residence Time

A long barrel residence time can cause the PVC & CPVC to remain in the barrel too long and degrade.

Possible Causes: Materials

High Regrind Percentage

PVC & CPVC tend to be very temperature sensitive and reground PVC & CPVC will cause an increase in material temperature. If too much regrind is being used, the barrel cooling fans may be inadequate to remove this increased heat. Reducing the regrind percentage should lower the material temperature and increase the material viscosity.

Poor Quality Regrind

Regrind with any degraded PVC & CPVC will cause a dangerous chain reaction of degradation, which can cause many issues, including flash. Any regrind containing degradation or burning should be discarded before it causes more defects or a serious safety concern.

Material Degradation

Degradation of PVC & CPVC causes extra volatiles and gases to occur in the polymer, which can lower the material viscosity and cause flash. You should also notice a distinct burnt PVC & CPVC smell if this is the case. Removing the bad material from the barrel helps prevent the degraded polymer from contaminating all other materials it contacts. If a Hot Runner System is being used, purge it out as well.

Poor Material Drying

Although PVC & CPVC do not always need drying, they are hygroscopic polymers. The absorbed moisture will come out of the plastic during processing, which can cause visual defects, as well as lower the polymer viscosity. Although grades vary, most PVC & CPVC resins should be dried for 1-2 hours at 65 to 80°C (150 to 175°F) with dry air (approx. -40°C or -40°F Dewpoint) to a Moisture Content below 0.3%.

Possible Causes: Machinery

Check Ring Performance (Intermittent Defect)

If a check ring is being used, perform a check ring performance study by weighing 10 consecutive short shots (weight all cavities + runner) and calculating variability. Over 2% variability means the check ring is suspect, and should be changed immediately if over 3% Variability. Purging the Barrel or increasing Decompression after Recovery may improve check ring performance.

% Variability = (100%) * (Max - Min) / (Average)

Excessive Platen Deflection

High deflection in the center of the stationary platen will cause the center of the mold to lose effective clamp force, which can cause flash in the center area of the mold. Platen deflection can be measured by the deflection of the stationary platen during mold clamping, injection, and packing. This deflection is measured using an indicator holder mounted to the machine base with the indicator touching as close to the center of the platen as possible. This measurement can be compared to the measured deflection during routine machine maintenance. To compensate for this, you can add shims around the locating ring or add bolster plates to the mold.

Improper Clamp Selection

If the mold does not cover at least 2/3 of the distance between the tie bars or if the mold extends beyond the platens, it will not receive adequate clamp tonnage distribution.

Poor Clamp Condition

The clamp tonnage will not be properly distributed if the machine is not level, the platens are out of alignment, the platens are concave, or a tie bar is stretched. These can be checked with equipment, such as levels, a tie bar strain gauge, a laser alignment tool, or inside micrometers. Damage or misalignment must be repaired immediately to prevent excessive equipment and mold damage.

Possible Causes: Mold & Design

Excessive Mold Deflection

High deflection in the center of the mold will cause the center of the mold to lose effective clamp force, this can cause flash in the center area of the mold. This can be measured by the movement of the injection unit during injection and packing. This is measured with an indicator holder mounted to the machine base and the indicator touching the injection unit. This measurement can be compared to the measured deflection during mold qualifications. To compensate for excessive mold deflection, you can add shims around the locating ring, add bolster plates to the mold, or add support pillars behind the support plate.

Inspect for Damage, Wear, Rust, or Corrosion

All mating surfaces, including parting lines, ejector pins & holes, wear plates, and all other mating surfaces must be checked for damage, wear, rust, or corrosion. Any of these issues can cause flash, and complications in one area of the tool may prevent another area from properly sealing.

High Cavity Imbalance (Cavity to Cavity Variation)

Significant differences in cavity filling can cause some cavities to fill and start packing during injection, while other cavities are significantly short at transfer. Make sure all the mold cavities are short at the time of transfer to minimize any cavity-to-cavity variation in the final molded part. There are steps that can improve cavity imbalance, such as using a different injection speed, adjusting hot runner gate drops, cleaning the mold vents, increasing the number of vents, or adjusting gate width when necessary.

Part or Mold Design

Make sure the mold has adequate support in the center or edges of the mold. To ensure the clamp force is concentrated on the core and cavity blocks, they should stand out of the mold base (also known as proud). Locating features, such as parting line locks, should be used on the mold to help prevent the two halves from shifting during clamping and molding. Most shut-offs and slide locks should have preloads to ensure adequate interference during lockup. The shut-offs can be tested by applying machining fluids, such as Dykem, to one side and then clamping up the mold. The machining fluid will transfer to the other side in the areas where the two halves are shut off, areas where this does not happen may require repair.

Initial Troubleshooting

• Clean & Inspect the Vents
• Verify Process:
  • Injection Time
  • Part Weight at Transfer (no runner or sprue)
  • Peak and Transfer Pressure
  • Packing Pressure, Time, and Speed
  • Final Part Weight
  • Clamp Force
  • Recovery Time
  • Coolant Temperature & Flow
  • Cooling Time
  • Material Temperature & Back Pressure
  • Barrel Residence Time
• Check:
  • Cushion is approximately 10% of Shot Size
  • Process is Not Pressure Limited
  • Cooling Line Layout
  • Check Ring Performance (if used)
• Look for obstructions or faulty equipment in the nozzle, gate, or hot runner

Possible Causes: Injection

Inadequate or Blocked Venting

Poor venting will make it very difficult to fill the mold consistently. Molds should be vented anywhere possible. They must be vented to the atmosphere, and cross-cavity venting should be avoided whenever possible. If regular cleaning is needed, there should be a procedure for scheduled vent cleaning before they become blocked.

Flow Obstructions or Restrictions

Check for obstructions in the nozzle, gate, or hot runner, as well as a faulty heater band on the barrel, nozzle, or hot runner system.

Low Injection Time

This indicates the material is entering the mold faster than the mold can vent the air out, resulting in a gas trap. Reducing the injection speed will help increase the injection time.

High Injection Time

This indicates material is entering the mold slowly, causing an increase in material viscosity. Increasing injection speed will help decrease injection time. Check the maximum injection pressure in case there is not enough injection pressure available to maintain the injection speed.

Low Part Weight at Transfer

This indicates too little material is entering the mold during injection, leaving the mold cavity underfilled at the start of packing. If other parameters are similar and only the weight is different, raising the shot size or decreasing the transfer position should increase the part weight at transfer.

High Part Weight at Transfer

This indicates too much material is entering the mold during injection. This condition may cause a cavity pressure spike during injection, but can also cause the screw to bounce back at the time of transfer, resulting in a high cavity pressure loss. If other parameters are similar and only the weight is different, lowering shot size or increasing transfer position should increase part weight at transfer.

High Peak or Transfer Pressure

This indicates the material viscosity has increased or there is a flow obstruction. First, check for flow obstructions, verify the short shot weight, and then check the material temperature.

Pressure Limited Process

A pressure-limited process cannot maintain the desired injection speed. If a change in lot #, colorant, or regrind is causing a viscosity increase, you may need to raise the shot size or decrease the transfer position to compensate for the higher viscosity material. Flow obstructions and faulty equipment should always be checked when the maximum pressure is being reached. In some cases, an increase in the maximum injection pressure is needed to accommodate a higher viscosity material, but the remainder of the process should be verified to ensure there are no other causes for high-injection pressure before this change is made.

Possible Causes: Packing

Low Packing Pressure

This indicates there is not enough pressure to complete mold filling during packing. The packing pressure should be set to match the process-specific documentation, and the packing speed should be set to the same value as your last injection speed before transferring from injection to packing.

Low Packing Time

With PVC & CPVC, it is difficult to get a full gate seal at the end of packing. Insufficient packing time can cause voids near the gate since material will leave the part into the runner or sprue. A packing time study should be done to ensure the packing time is enough to achieve a good gate appearance.

Low Packing Speed

A low packing speed will artificially slow down the screw at transfer, which can raise material viscosity and prevent packing at the end of fill. To avoid complications, the packing speed should be set to the same value as your last injection speed before transferring from injection to packing.

Low Final Part Weight

This indicates insufficient material is entering the mold cavity during Injection and packing. This is indicative of a change in the process, but you need to find other changes in the process before action can be taken.

High Clamp Force

Excessive clamp force will prevent the air from venting as fast as the material is entering the mold. The pressure used to pack out the trapped gas takes away from the pressure needed to pack out the part. The clamp force should be set to match the process-specific documentation, and you will likely need to clean the vents at this time.

No Cushion

A process without a cushion does not have enough material to fill the part and compensate for shrinkage during packing. The cushion should be approximately 10% of the shot size. Both the shot size and transfer position must be increased to raise the cushion.

High Cushion

Melted PVC & CPVC are highly compressible plastics, thus, a process with an excessive cushion, such as 25% of the shot size, can lose significant injection & packing pressure. The cushion should always be approximately 10% of the Shot Size. Both the shot size and transfer position must be decreased to lower the cushion.

Possible Causes: Cooling

Low Coolant Temperature or High Coolant Flow

PVC & CPVC tend to be very temperature sensitive, and a cold mold can cause excessive pressure loss during injection & packing. This pressure loss often creates defects, such as sinks at the end of the fill. Returning the coolant temperature to match the process-specific documentation should reduce the pressure loss in the mold. Using a coolant temperature controller with a lower-rated coolant pump or reducing the coolant flow with a flow control valve may return the water flow to match that of the documented standard.

High Coolant Temperature or Low Coolant Flow

PVC & CPVC tend to be very temperature sensitive, and a hot mold increases material shrinkage due to a slower cooling rate. Returning the coolant temperature to match the process-specific documentation should return the pressure loss in the mold to normal. Check the cooling system for obstructions, as this will reduce coolant flow. Using a coolant temperature controller with a higher-rated coolant pump may raise the coolant flow to match the process-specific documentation.

Poor Cooling Line Layout

An incorrect cooling line layout will affect the material flow and material shrinkage. Verify the cooling line layout matches the process-specific documentation whenever cooling is in question.

Low Cooling Time

A low cooling time will cause the part to eject at a hotter temperature, which slows the cooling rate. This reduced cooling rate will cause an increase in material shrinkage. The cooling time should be set to match the process-specific documentation

Possible Causes: Recovery

Low Material Temperature or Low Back Pressure

PVC & CPVC tend to be very temperature sensitive, and a cold material might cause excessive pressure loss during injection & packing. This pressure loss can cause sinks near the end of the fill. Returning the material temperature and back pressure to match the process-specific documentation should reduce the pressure loss in the mold. It may be necessary to check the thermocouples & heater bands on the barrel & hot runner system.

High Material Temperature or Back Pressure

Higher material temperatures cause an increase in material shrinkage, which can result in sinks. Returning the material temperature and back pressure to match the process-specific documentation should return the pressure loss in the mold to normal. If regrind is being used, reduce the regrind percentage. You should always purge the barrel if degradation is suspected with PVC & CPVC. It may be necessary to check the cooling fans, screw cooling, thermocouples, & heater bands.

Low Recovery Time

A high screw RPM can cause inconsistent recovery and excessive shear and heating during recovery. This often increases shrinkage due to a material temperature increase. The screw RPM should be adjusted so the recovery time consumes most of the cooling time.

Low Barrel Residence Time

A short barrel residence time may prevent the PVC & CPVC from fully melting during recovery. This low-temperature material causes excessive pressure loss during injection & packing, resulting in sinks near the end of the fill. This can be due to a significantly smaller barrel being used than what was used on the documented standard. In some cases, barrel temperature can be increased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

High Barrel Residence Time

A long barrel residence time can cause the heat-sensitive PVC & CPVC to heat up or degrade. This heated material will shrink more than expected, often resulting in sinks. In some cases, barrel temperature can be decreased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

Possible Causes: Material

High Regrind Percentage

PVC & CPVC tend to be very temperature sensitive, and reground PVC & CPVC will cause an increase in material temperature and shrinkage. If too much regrind is being used, the barrel cooling fans may be inadequate to remove this increased heat. Reducing the regrind percentage should lower the material temperature and decrease material shrinkage.

Poor Quality Regrind

Regrind with any degraded PVC & CPVC will cause a dangerous chain reaction of degradation, which can cause many issues, including excessive shrinkage. Any regrind containing degradation or burning should be discarded before it causes more defects or a serious safety concern.

Material Degradation

Degradation of PVC & CPVC cause the polymer to heat up and increase shrinkage. You should also notice a distinct burnt PVC & CPVC smell if this is the case. Removing the bad material from the barrel helps prevent the degraded polymer from contaminating all other materials it contacts. If a Hot Runner System is being used, purge it out as well.

Material Contamination

Contaminates can cause a flow restriction if they get caught in the nozzle and hot runner system. These could be any non-melting material, such as wood, metal, or cloth, as well as degradation & any polymer that does not melt at the current processing temperature. Contamination can come from many locations, including storage, hoppers, hoses, filters, screens, surge bins, access doors, seals, and blenders. All potential sources of contamination must be checked to determine where the contamination is coming from. Once the source is determined, everything that came in contact with the material afterward must be thoroughly cleaned and inspected before returning to use.

Possible Causes: Machine

Check Ring Performance (Intermittent Defect)

If a check ring is being used, perform a check ring performance study by weighing 10 consecutive short shots (weight all cavities + runner) and calculating variability. Over 2% variability means the check ring is suspect, and should be changed immediately if over 3% Variability. Purging the Barrel or increasing Decompression after Recovery may improve check ring performance.

Possible Causes: Mold & Design

High Cavity Imbalance (Cavity to Cavity Variation)

Significant differences in cavity filling can cause some cavities to fill and start packing during injection, while other cavities are significantly short at transfer. Make sure all the mold cavities are short at the time of transfer to minimize any cavity-to-cavity variation in the final molded part. There are steps that can improve cavity imbalance, such as using a different injection speed, adjusting hot runner gate drops, cleaning the mold vents, increasing the number of vents, or adjusting gate width when necessary.

Part or Mold Design

Reducing the thickness of the part or the size of the part feature opposite the sink will reduce the shrinkage in the area of the sink. Gating into thick sections will improve the pressure distribution in the part. Increased cooling in the areas with Sinks will decrease Shrinkage and improve the condition. More uniform thicknesses throughout the part may also help with sinks.

Initial Troubleshooting

• Clean & Inspect the Vents
• Verify Process:
  • Injection Time
  • Part Weight at Transfer (no runner or sprue)
  • Peak and Transfer Pressure
  • Packing Pressure, Time, and Speed
  • Final Part Weight
  • Clamp Force
  • Recovery Time
  • Coolant Temperature, Pressure, and Flow
  • Cooling Time
  • Material Temperature & Back Pressure
• Check:
  • Cushion is approximately 10% of Shot Size
  • Process is Not Pressure Limited
  • Cooling Line Layout
  • Check Ring Performance (if used)
• Look for obstructions or faulty equipment in the nozzle, gate, or hot runner

Possible Causes: Injection

Inadequate or Blocked Venting

Poor venting will make it very difficult to fill the mold consistently. Molds should be vented wherever possible and must be vented to the atmosphere. Cross-cavity venting should be avoided whenever possible. If cleaning is needed, there should be a procedure for scheduled vent cleaning before they become blocked.

Flow Obstructions or Restrictions

Check for obstructions in the nozzle, gate, or hot runner, as well as a faulty heater band on the nozzle or hot runner system.

Low Injection Time

This indicates the material is entering the mold faster than the mold can vent the air out. You may see or smell burning at the end of the flow if this is the cause. Reducing Injection Speed will help you increase Injection Time.

High Injection Time

This indicates material is entering the mold slowly, causing an increase in material viscosity. Increasing injection speed will help decrease injection time. Check the maximum injection pressure in case there is not enough injection pressure available to maintain the injection speed.

Low Part Weight at Transfer

This indicates too little material is entering the mold during injection. If other parameters are similar and only the weight is different, raising the shot size or decreasing the transfer position should increase the part weight at transfer.

High Part Weight at Transfer

This indicates too much material is entering the mold during injection. This condition may cause a cavity pressure spike during injection, but can also cause the screw to bounce back during transfer, resulting in a cavity pressure loss at transfer. If other parameters are similar and only the weight is different, lowering shot size or increasing transfer position should increase part weight at transfer.

High Peak or Transfer Pressure

This indicates the material viscosity has increased or there is an obstruction. Check for flow obstructions. If the mold or material temperatures are lower than the documented standard, these can be increased to allow the PVC & CPVC to flow more easily into the mold.

Pressure Limited Process

A pressure-limited process cannot maintain the desired injection speed. If a change in lot #, colorant, or regrind is causing a viscosity increase, you may need to raise the shot size or decrease the transfer position to compensate for the higher viscosity material. Flow obstructions and faulty equipment should always be checked when the maximum pressure is being reached. In some cases, an increase in the maximum injection pressure is needed to accommodate a higher viscosity material, but the remainder of the process should be verified to ensure there are no other causes for high pressure before this change is made.

Possible Causes: Packing

Low Packing Pressure

This indicates there is not enough pressure to fill the mold and compensate for shrinkage during packing. The Packing Pressure should be set to match the process-specific documentation, and the packing speed should be set to the same value as your last Injection Speed before Transfer from Injection to Packing.

Low Packing Time

With PVC & CPVC, it is difficult to get a full gate seal at the end of packing. Insufficient packing time can cause voids near the gate since the material will leave the part in the runner or sprue. A packing time study should be done to ensure the packing time is enough to achieve a good gate quality.

Low Packing Speed

A low packing speed will artificially slow down the screw at transfer, which can raise viscosity and increase pressure loss. To avoid complications, the packing speed should be set to the same value as your last injection speed before transferring from injection to packing.

Low Final Part Weight

This indicates insufficient material is entering the mold cavity during Injection and packing. This is indicative of a change in the process, but you need to find other changes in the process before action can be taken.

High Clamp Force

Excessive clamp force will prevent the air from venting as fast as the material is entering the mold. You will sometimes see or smell burning at the end of fill locations if this is the cause. The clamp force should be set to match the process-specific documentation, you will likely need to clean the vents at this time.

No Cushion

A process without a cushion does not have enough material to fill the part and compensate for shrinkage during packing. The cushion should always be approximately 10% of the Shot Size. Both the shot size and transfer position must be increased to raise the cushion.

High Cushion

Melted PVC & CPVC are highly compressible plastics, thus, a process with an excessive cushion, such as 25% of the shot size, can lose significant injection & packing pressure. The cushion should always be approximately 10% of the Shot Size. Both the shot size and transfer position must be decreased to lower the cushion.

Possible Causes: Cooling

Low Coolant Temperature or High Coolant Flow

PVC & CPVC tend to be very temperature sensitive, and a cold mold can cause excessive pressure loss during injection & packing. This pressure loss often creates defects, such as voids, at the end of the fill. A cold mold can also cause the material to freeze at the mold surface, but later shrink away from itself internally, causing a void. Returning the coolant temperature to match the process-specific documentation should reduce the pressure loss in the mold. Using a coolant temperature controller with a lower-rated coolant pump or reducing the coolant flow with a flow control valve may return the water flow to match that of the documented standard.

Poor Cooling Line Layout

An incorrect cooling line layout will affect the material flow and material shrinkage. Verify the Cooling Line Layout matches the Process-Specific Documentation whenever cooling is in question.

High Cooling Time

A longer cooling time can cause the part surface to freeze in place. If shrinkage is prevented on the outer layers of the part, the center of the part may compensate with excessive shrinkage and voids. The cooling time should be set to match the process-specific documentation.

Possible Causes: Recovery

Low Material Temperature or Back Pressure

PVC & CPVC tend to be very temperature sensitive, and a cold material might cause excessive pressure loss during injection & packing. This pressure loss can cause voids near the end of fill. Returning the material temperature and back pressure to match the process-specific documentation should reduce the pressure loss in the mold. It may be necessary to check the thermocouples & heater bands on the barrel & hot runner system.

High Material Temperature or Back Pressure

Higher material temperatures cause an increase in material shrinkage, which can result in voids. Returning the material temperature and back pressure to match the process-specific documentation should return the pressure loss in the mold to normal. If regrind is being used, reduce the regrind percentage. You should always purge the barrel if degradation is suspected with PVC & CPVC. It may be necessary to check the cooling fans, screw cooling, thermocouples, & heater bands.

Low Recovery Time

A high screw RPM can cause inconsistent recovery and excessive shear and heating during recovery. This often increases shrinkage due to a material temperature increase. The screw RPM should be adjusted so the recovery time consumes most of the cooling time.

Low Barrel Residence Time

A short barrel residence time may prevent the PVC & CPVC from fully melting during recovery. This low-temperature material causes excessive pressure loss during injection & packing resulting in sinks near the end of fill. This can be due to a significantly smaller barrel being used than was used on the documented standard. In some cases, barrel temperature can be increased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

High Barrel Residence Time

A long barrel residence time can cause the heat sensitive PVC & CPVC to heat up or degrade. This heated material will shrink more than expected often resulting in sinks. In some cases, barrel temperature can be decreased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

Possible Causes: Materials

High Regrind Percentage

PVC & CPVC tend to be very temperature sensitive, and reground PVC & CPVC will cause an increase in material temperature and shrinkage. If too much regrind is being used, the barrel cooling fans may be inadequate to remove this increased heat. Reducing the regrind percentage should lower the material temperature and decrease material shrinkage.

Poor Quality Regrind

Regrind with any degraded PVC & CPVC will cause a dangerous chain reaction of degradation, which can cause many issues, including excessive shrinkage. Any regrind containing degradation or burning should be discarded before it causes more defects or a serious safety concern.

Material Degradation

Degradation of PVC & CPVC causes the polymer to heat up and increase shrinkage. You should also notice a distinct burnt PVC & CPVC smell if this is the case. Removing the bad material from the barrel helps prevent the degraded polymer from contaminating all other materials it contacts. If a Hot Runner System is being used, purge it out as well.

Material Contamination

Contaminates can cause a flow restriction if they get caught in the nozzle and hot runner system. These could be any non-melting materials, such as wood, metal, or cloth, as well as degradation & any polymer that does not melt at the current processing temperature. Contamination can come from many locations, including storage, hoppers, hoses, filters, screens, surge bins, access doors, seals, and blenders. All potential sources of contamination must be checked to determine where the contamination is coming from. Once the source is determined, everything that came in contact with the material afterward must be thoroughly cleaned and inspected before returning to use.

Possible Causes: Machinery

Check Ring Performance (Intermittent Defect)

If a check ring is being used, perform a check ring performance study by weighing 10 consecutive short shots (weight all cavities + runner) and calculating variability. Over 2% variability means the check ring is suspect, and should be changed immediately if over 3% Variability. Purging the Barrel or increasing Decompression after Recovery may improve check ring performance.

% Variability = (100%) * (Max - Min) / (Average)

Possible Causes: Mold & Design

High Cavity Imbalance (Cavity to Cavity Variation)

Significant differences in cavity filling can cause some cavities to fill and start packing during injection, while other cavities are significantly short at transfer. Make sure all the mold cavities are short at the time of transfer to minimize any cavity-to-cavity variation in the final molded part. There are steps that can improve cavity imbalance, such as using a different injection speed, adjusting hot runner gate drops, cleaning the mold vents, increasing the number of vents, or adjusting gate width when necessary.

Part or Mold Design

Reducing the thickness of part or specific part features will help prevent voids. Gating into thick sections will also improve the pressure distribution in the part. More uniform thickness throughout the part may also help with voids.

Initial Troubleshooting

• Purge the Barrel
• Clean & Inspect the Vents
• Check for Mold Surface for Volatile Buildup or Damage
• Verify Process:
  • Injection Time & Profile
  • Part Weight at Transfer (no runner or sprue)
  • Peak and Transfer Pressure
  • Packing Pressure, Time, and Speed
  • Final Part Weight
  • Clamp Force
  • Recovery Time
  • Coolant Temperature, Pressure, and Flow
  • Material Temperature & Back Pressure
  • Barrel Residence Time
• Check Material Drying:
  • Material Moisture
  • Dryer Residence Time
  • Dryer Temperature
• Check:
  • Cushion is approximately 10% of Shot Size
  • Process is Not Pressure Limited
  • Cooling Line Layout
  • Check Ring Performance (if used)
• Look for obstructions or faulty equipment in nozzle, gate, or
  hot runner

Possible Causes: Injection

Inadequate or Blocked Venting

Poor venting can cause gas entrapment on the mold surface. Molds should be vented anywhere possible. They must be vented to the atmosphere, and cross-cavity venting should be avoided whenever possible. If routine vent cleaning is needed, there should be a procedure for scheduled vent cleaning before they become blocked.

Flow Obstructions or Restrictions

Check for obstructions in the nozzle, gate, or hot runner, as well as a faulty heater band on the nozzle or hot runner system. These can increase shear and affect the material flow pattern within the mold.

Low Injection Time

This indicates the material is entering the mold faster than the mold can vent the air out, causing gas entrapment. You may see or smell burning at the end of flow if this is the cause. Reducing Injection Speed will help you increase Injection Time.

High Injection Time

This indicates material is entering the mold slowly, causing an increase in material viscosity. A higher viscosity PVC & CPVC may flow differently and may not be able to properly contact and duplicate the mold surface. Increasing Injection Speed will help you decrease Injection Time. You should also check your Maximum Injection Pressure in case there is not enough Injection Pressure available to maintain the Injection Speed.

Injection Profile

PVC & CPVC typically require an injection profile to make a good part. For example, the gate area often needs a lower injection speed to improve appearance. A significant change in the injection profile will have a significant effect on the appearance of the part and gate. The Injection Profile should match the Process-Specific Documentation.

Low Part Weight at Transfer

This indicates too little material is entering the mold during injection which may cause poor surface finish due to inadequate packing. If other parameters are similar and only the weight is different, raising the shot size or decreasing the transfer position should increase the part weight at transfer.

High Part Weight at Transfer

This indicates too much material is entering the mold during injection and often contributes to gas entrapment. This condition may cause a cavity pressure spike during injection, but can also cause the screw to bounce back during transfer, resulting in a cavity pressure loss at transfer. If other parameters are similar and only the weight is different, lowering shot size or increasing transfer position should increase part weight at transfer.

Low Peak or Transfer Pressure

This indicates the material viscosity has decreased and there will be less pressure loss during injection. If the mold or material temperatures are higher than the documented standard, these can be decreased to reduce the flow of PVC & CPVC. If a change in lot #, colorant, or regrind is causing a decrease in viscosity, you may need to decrease the shot size or increase the transfer position to compensate for the lower viscosity material.

High Peak or Transfer Pressure

This indicates the material viscosity has increased or there is an obstruction. Check for flow obstructions. If the mold or material temperatures are lower than the documented standard, these can be increased to allow the PVC & CPVC to flow more easily into the mold.

Pressure Limited Process

A pressure-limited process cannot maintain the desired injection speed. If a change in lot #, colorant, or regrind is causing a viscosity increase, you may need to raise the shot size or decrease the transfer position to compensate for the higher viscosity material. Flow obstructions and faulty equipment should always be checked when the maximum pressure is being reached. In some cases, an increase in the maximum injection pressure is needed to accommodate a higher viscosity material, but the remainder of the process should be verified to ensure there are no other causes for high pressure before this change is made.

Possible Causes: Packing

Low Packing Pressure

This indicates there is not enough pressure to fill the mold and compensate for shrinkage during packing. An under-packed part may not properly replicate the mold surface. The packing pressure should be set to match the process-specific documentation, and the packing speed should be set to the same value as your last Injection Speed before Transfer from Injection to Packing.

High Packing Pressure

This indicates there is too much pressure after mold filling, which might cause the plastic to get too much mold surface detail. This is usually a concern only when there are volatiles, rust, corrosion, or damage on the mold surface. If there are concerns with the mold surface, it should be corrected before the problems worsen. The packing pressure should be set to match the process-specific documentation.

Low Packing Time

With PVC & CPVC, it is difficult to get a full gate seal at the end of packing. Insufficient packing time will cause defects near the gate because the material will leave the part in the runner or sprue. A packing time study should be done to ensure the packing time is adequate to achieve a good gate appearance.

Low Packing Speed

A low packing speed will artificially slow down the screw at transfer, which can raise viscosity and cause under packing. An under-packed part may not properly replicate the mold surface. To avoid complications, the packing speed should be set to the same value as your last injection speed before transferring from injection to packing.

Low Final Part Weight

This indicates insufficient material is entering the mold cavity during Injection and packing. This is indicative of a change in the process, but you need to find other changes in the process before action can be taken.

High Final Part Weight

This indicates excessive material enters the mold cavity during Injection and packing. This is indicative of a change in the process, but you need to find other changes in the process before action can be taken.

High Clamp Force

Excessive clamp force will prevent the air from venting as fast as the material is entering the mold. This condition causes poor surface finish due to gas entrapment. The clamp tonnage should be set to match the process-specific documentation, and you will likely need to clean the vents at this time.

No Cushion

A process without a cushion does not have enough material to fill the part and compensate for shrinkage during packing. The cushion should always be approximately 10% of the Shot Size. Both the shot size and transfer position must be increased to raise the cushion.

High Cushion

Melted PVC & CPVC are highly compressible plastics, thus, a process with an excessive cushion, such as 25% of the shot size, can lose significant injection & packing pressure. The cushion should always be approximately 10% of the shot size. Both the shot size and transfer position must be decreased to lower the cushion.

Possible Causes: Cooling

Low Coolant Temperature or High Coolant Flow

PVC & CPVC tend to be very temperature sensitive, and a cold mold can cause excessive pressure loss during injection & packing. This often causes defects at the end of the fill. Cold molds can also prevent the material from properly replicating the mold surface. Returning the coolant temperature to match the process-specific documentation should reduce the pressure loss in the mold. Using a coolant temperature controller with a lower rated coolant pump or reducing the coolant flow with a flow control valve may return the water flow to match that of the documented standard.

High Coolant Temperature or Low Coolant Flow

PVC & CPVC tend to be very temperature sensitive, and a warm mold will change how the plastic reaches and duplicates the mold surface. Returning the coolant temperature to match the process-specific documentation should return the pressure loss in the mold to normal. Check the cooling system for obstructions, as this will reduce coolant flow. Using a coolant temperature controller with a higher rated coolant pump may raise the coolant flow to match the process-specific documentation.

Cooling Line Layout

An incorrect cooling line layout can affect the material flow and shrinkage. Verify the Cooling Line Layout matches the Process-Specific Documentation.

Low Cooling Time

Small changes in cooling time tend not to significantly affect the part surface appearance, but large changes impact the amount of post-mold shrinkage. A significant drop in cooling time may cause enough post-mold shrinkage to change the gloss and surface texture of the molded part.

High Cooling Time

Small changes in cooling time tend not to significantly affect the part surface appearance, but large impact the amount of post-mold shrinkage. A significant rise in cooling time may cause the material to show too much mold surface detail. This is usually a concern only when there is volatile, rust, corrosion, or damage on the mold surface, but can also cause a change in the gloss of the part surface. If there is are concerns with the mold surface, it should be corrected before the problem worsens.

Possible Causes: Recovery

Low Material Temperature or Back Pressure

PVC & CPVC tend to be very temperature sensitive, and a cold material might cause excessive pressure loss during injection & Packing, as well as affect the gloss of the molded part. Cold molds can also prevent the material from properly replicating the mold surface. Returning the Material Temperature and Back Pressure to match the Process-Specific Documentation should reduce the pressure loss in the mold. It may be necessary to check the thermocouples & heater bands on the barrel & hot runner system.

High Material Temperature or Back Pressure

PVC & CPVC tend to be very temperature sensitive, and a hot material might cause low-pressure loss during injection & packing, as well as affect the gloss of the molded part. A significant rise in material temperature may cause the material to give off too many volatiles, which can get between the part and the mold surface. These volatiles can change the gloss of the final part, as well as cause rust, corrosion, or damage to the mold surface. Returning the material temperature and back pressure to match the process-specific documentation should return the pressure loss in the mold to normal. If regrind is being used, reduce the regrind percentage. You should always purge the barrel when degradation is suspected with PVC & CPVC. It may be necessary to check the cooling fans, screw cooling, thermocouples, & heater bands.

Low Recovery Time

A high Screw RPM can cause inconsistent recovery and excessive shear during recovery. The screw RPM should be adjusted so the Recovery Time consumes most of the Cooling Time.

Low Barrel Residence Time

A short barrel residence time may prevent the PVC & CPVC from fully melting during recovery. This low-temperature material causes excessive pressure loss during injection & packing, as well as defects near the end of fill. This can be due to a significantly smaller barrel being used in comparison to what was used on the documented standard. In some cases, barrel temperature can be increased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

High Barrel Residence Time

A long barrel residence time can cause the heat-sensitive PVC & CPVC to heat up or degrade. This heated material will shrink more than expected, which will affect the surface gloss and replication of mold detail. A significant rise in residence time may cause the material to give off too many volatiles, which can get between the part and the mold surface. These volatiles can change the gloss of the final part, as well as cause rust, corrosion, or damage to the mold surface. In some cases, barrel temperature can be decreased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

Possible Causes: Materials

High Regrind Percentage

PVC & CPVC tend to be very temperature sensitive, and reground PVC & CPVC will cause an increase in material temperature and shrinkage. If too much regrind is being used, the barrel cooling fans may be inadequate in removing this increased heat, causing the release of volatiles, which can get between the part and the mold surface. Reducing the regrind percentage should lower the material temperature and decrease the creation of volatiles.

Poor Quality Regrind

Regrind with any degraded PVC & CPVC will cause a dangerous chain reaction of degradation, which can cause many issues, including excessive release of volatiles. Any regrind containing degradation or burning should be discarded before it causes more defects or a serious safety concern.

Material Degradation

Degradation of PVC & CPVC causes extra volatiles and gasses to occur in the polymer which will give off volatiles which will reduce your part surface finish. You should also notice a distinct burnt PVC & CPVC smell if this is the case. Removing the bad material from the barrel helps prevent the degraded polymer from contaminating all other materials it contacts. If a Hot Runner System is being used, purge it out as well.

Material Contamination

Any foreign liquid or polymer that melts at processing temperatures may turn to gas and become volatiles during injection. Contamination can come from many locations, including storage, hoppers, hoses, filters, screens, surge bins, access doors, seals, and blenders. All potential sources of contamination must be checked to determine where the contamination is coming from. Once the source is determined, everything that came in contact with the material afterward must be thoroughly cleaned and inspected before returning to use.

Poor Material Drying

Although PVC & CPVC do not always need drying, they are hygroscopic polymers. The absorbed moisture will come out of the plastic during processing, which can cause visual defects, as well as lower the polymer viscosity. Although grades vary, most PVC & CPVC resins should be dried for 1-2 hours at 65 to 80°C (150 to 175°F) with dry air (approx. -40°C or -40°F Dewpoint) to a moisture content below 0.3%.

Possible Causes: Machinery

Check Ring Performance (Intermittent Defect)

If a check ring is being used, perform a check ring performance study by weighing 10 consecutive short shots (weight all cavities + runner) and calculating variability. Over 2% variability means the check ring is suspect, and should be changed immediately if over 3% Variability. Purging the Barrel or increasing Decompression after Recovery may improve check ring performance.

% Variability = (100%) * (Max - Min) / (Average)

Machine Settings

It is very easy to damage the part surface during part removal. In most cases, higher mold breakaway and ejection speeds will increase the stresses applied to the molded part during part removal. In any automation & robotics application, there is the expectation that the movements are synchronized. In some cases, a slow mold open speed is needed at the end of the mold opening to ensure the clamp stops in the correct position for the robotics to properly remove the part. In other cases, the ejection timing & speed must be properly set to move at the same speed as the robot.

Robot Settings & Condition

Inspection should start with the components that contact the part, ensuring there is no damage, such as worn suction cups, dented grippers, dull cutters, parts stuck in sorting equipment, damaged vacuum lines, or bent end of arm tooling. Your company should have spare parts for all automation components, such as grippers, holders, cutters, suction cups, springs, hoses, rails, bolts, nuts, etc. so any suspect component can be replaced immediately. In any automation & robotics application, there is the expectation that the movements are synchronized. The robot must move with the machine ejection system when grabbing the part, as well as move with the automation when setting down the part. To avoid damage to the part surface, there should be smooth movements whenever a transfer is taking place from the mold to the robot and the robot to the automation.

Possible Causes: Mold & Design

Buildup of Volatiles on Mold Surface

Extensive trapped gas on the texture or polish will build up on the mold surface, which may occur in areas with bad venting, or show up as bands when overall venting is poor. Solvents or surface clearing solutions like a dry ice-blaster will help clean up the mold surface.

Mold Surface Damage

Extensive trapped gas on the texture or polish will corrode the surface over time. This will likely need repair to significantly improve the part's appearance. If this occurs, a strategy for fixing the mold venting should be developed.

High Cavity Imbalance (Cavity to Cavity Variation)

Significant differences in cavity filling can cause some cavities to fill and start packing during injection, while other cavities are significantly short at transfer. Make sure all the mold cavities are short at the time of transfer to minimize any cavity-to-cavity variation in the final molded part. There are steps that can improve cavity imbalance, such as using a different injection speed, adjusting hot runner gate drops, cleaning the mold vents, increasing the number of vents, or adjusting gate width when necessary.

Part or Mold Design

Good part design strategies, such as gating into thick sections, avoiding sharp transitions, maintaining the same thickness when possible, and providing adequate venting are important to the overall surface finish.

Initial Troubleshooting

• Purge the Barrel
• Clean & Inspect the Vents
• Verify Process:
  • Injection Time & Profile
  • Part Weight at Transfer (no runner or sprue)
  • Packing Pressure & Speed
  • Clamp Force
  • Recovery Time
  • Material Temperature & Back Pressure
  • Barrel Residence Time
• Check Material Drying:
  • Material Moisture
  • Dryer Residence Time
  • Dryer Temperature
• Check mold surface for volatile buildup or damage

Possible Causes: Injection

Inadequate or Blocked Venting

Poor venting can cause gas entrapment during injection & packing. Molds should be vented anywhere possible, and must be vented to the atmosphere, and cross-cavity venting should be avoided whenever possible. If routine vent cleaning is needed, there should be a procedure for scheduled vent cleaning before they become blocked.

Low Injection Time

This indicates the material is entering the mold faster than the mold can vent the air out. You may see or smell burning at the end of the flow. If this is the cause, reducing injection speed will help you increase Injection Time.

Injection Profile

PVC & CPVC typically require an injection profile to make a good part. A significant increase in speed within the injection profile can have a significant effect on mold venting. The injection profile should match the process-specific documentation.

High Part Weight at Transfer

This indicates more material is entering the mold during injection than the mold can effectively vent. This condition may cause a cavity pressure spike during Injection, but can also cause the screw to bounce back during transfer resulting in a cavity pressure loss at transfer. If other parameters are similar and only the weight is different, lowering the shot size or increasing the transfer position should increase the part weight at transfer. If other parameters are significantly different than the standard, then follow the associated suggestions below.

Possible Causes: Packing

High Packing Pressure

This indicates there is too much pressure after mold filling, which might finish mold filling faster than the mold can vent. The packing pressure should be set to match the process-specific documentation, and the packing speed should be set to the same value as your last injection speed before transferring from injection to packing.

High Packing Speed

A high packing speed can cause a rise in material velocity if the packing pressure is significantly higher than the injection speed at transfer. This can cause burning by compressing the gas remaining in the front of the screw at transfer. To avoid complications, the packing speed should be set to the same value as your last injection speed before transferring from injection to packing.

High Clamp Force

Excessive clamp force will prevent the air from venting as fast as the material is entering the mold. You will sometimes see or smell burning at the end of fill locations if this is the cause. The clamp force should be set to match the process-specific documentation, and you will likely need to clean the vents at this time.

Possible Causes: Recovery

High Material Temperature or Back Pressure

PVC & CPVC tend to be very temperature sensitive, and a hot material might cause low-pressure loss during injection & packing, as well as more shrinkage. Returning the material temperature and back pressure to match the process-specific documentation should return the pressure loss in the mold to normal. If regrind is being used, reduce the regrind percentage. You should always purge the barrel when degradation is suspected with PVC & CPVC. It may be necessary to check the cooling fans, screw cooling, thermocouples, & heater bands.

Low Recovery Time

A high Screw RPM can cause inconsistent recovery and excessive shear during recovery, making it more susceptible to burning. The screw RPM should be adjusted so the recovery time consumes most of the cooling time.

High Barrel Residence Time

A long barrel residence time can cause the heat sensitive PVC & CPVC to heat up or degrade. A significant rise in residence time may cause the material to give off too many volatiles which can increase the likelihood of burning. In some cases, barrel temperature can be decreased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

Possible Causes: Materials

High Regrind Percentage

PVC & CPVC tend to be very temperature sensitive and reground PVC & CPVC will cause an increase in material temperature and shrinkage. If too much regrind is being used, the barrel cooling fans may be inadequate in removing this increased heat, leading to the release of volatiles, which can increase the likelihood of burning. Reducing the regrind percentage should lower the material temperature and decrease the creation of volatiles.

Poor Quality Regrind

Regrind with any degraded PVC & CPVC will cause a dangerous chain reaction of degradation, which can cause many issues, including excessive release of volatiles. Any regrind containing degradation or burning should be discarded before it causes more defects or a serious safety concern.

Material Degradation

Degradation of PVC & CPVC causes extra volatiles and gasses to occur in the polymer, which will increase likelihood of burning. You should also notice a distinct burnt PVC & CPVC smell if this is the case. Removing the bad material from the barrel helps prevent degraded polymer from contaminating all other material it contacts. If a Hot Runner System is being used, purge it out as well.

Poor Material Drying

Although PVC & CPVC do not always need drying, they are hygroscopic polymers. The absorbed moisture will come out of the plastic during processing as gas, which can increase the likelihood of burning. Although grades vary, most PVC & CPVC resins should be dried for 1-2 hours at 65 to 80°C (150 to 175°F) with dry air (approx. -40°C or -40°F Dewpoint) to a moisture content below 0.3%.

Possible Causes: Mold & Design

Buildup of Volatiles

Extensive trapped gas on the texture or polish will build up on the mold surface, causing the matte surface finish associated with burning. Solvents or surface clearing solutions like a dry ice-blaster will help clean up the mold surface.

Mold Surface Damage

Extensive trapped gas on the texture or polish will corrode the surface over time, oftentimes giving the rough surface a finish of burning. This will likely need repair to significantly improve the part's appearance. If this occurs, a strategy for fixing the mold venting should be developed.

Design Concerns

If the end of flow is at a location that cannot be vented, it may be necessary to adjust the design, part thickness, or add flow leaders to move the end of fill to another location that is easy to vent. Adjusting the gate location can also improve this situation.

Initial Troubleshooting

• Clean & Inspect the Vents
• Verify Process:
  • Injection Time & Profile
  • Part Weight at Transfer (no runner or sprue)
  • Peak and Transfer Pressure
  • Packing Pressure, Time, and Speed
  • Final Part Weight
  • Clamp Force
  • Recovery Time
  • Coolant Temperature, Pressure, and Flow
  • Material Temperature & Back Pressure
  • Barrel Residence Time
• Check Material Drying:
  • Material Moisture
  • Dryer Residence Time
  • Dryer Temperature
• Check:
  • Cushion is approximately 10% of Shot Size
  • Process is Not Pressure Limited
  • Cooling Line Layout
  • Check Ring Performance (if used)
• Look for obstructions or faulty equipment in nozzle, gate, or
  hot runner

Possible Causes: Injection

Inadequate or Blocked Venting

Poor venting can cause gas entrapment during injection & packing. Molds should be vented anywhere possible. They must be vented to the atmosphere, and cross-cavity venting should be avoided whenever possible. If routine vent cleaning is needed, there should be a procedure for scheduled vent cleaning before they become blocked.

Flow Obstructions or Restrictions

Check for obstructions in the nozzle, gate, or hot runner, as well as a faulty heater band on the nozzle or hot runner system. These can increase shear and affect the material flow pattern within the mold, reducing the pressure present at the weld or meld line location.

Low Injection Time

This indicates the material is entering the mold faster than the mold can vent the air out, causing gas entrapment. This trapped gas will reduce the strength and appearance of a weld or meld line. You may see or smell burning at the end of the flow if this is the cause. Reducing injection speed will help you increase injection time.

High Injection Time

This indicates material is entering the mold slowly, causing an increase in material viscosity. A higher viscosity PVC & CPVC may encounter too much pressure loss during injection, which can reduce the pressure applied to the weld or meld line location. Increasing the injection speed will help you decrease the injection time. You should also check your maximum injection pressure in the event that there is not enough injection pressure available to maintain the injection speed.

Injection Profile

PVC & CPVC typically require an injection profile to make a good part. For example, the gate area often needs a lower injection speed to improve appearance. A significant change in the injection profile will have a significant effect on the appearance of the part and gate. The injection profile should match the process-specific documentation.

Low Part Weight at Transfer

This indicates too little material is entering the mold during injection, which may cause poor surface finish due to inadequate packing. If other parameters are similar and only the weight is different, raising the shot size or decreasing the transfer position should increase the part weight at transfer.

High Part Weight at Transfer

This indicates too much material is entering the mold during injection and often contributes to gas entrapment. This condition may cause a cavity pressure spike during injection, but can also cause the screw to bounce back during transfer, resulting in a cavity pressure loss at transfer. If other parameters are similar and only the weight is different, lowering shot size or increasing transfer position should increase part weight at transfer.

High Peak or Transfer Pressure

This indicates the material viscosity has increased or there is an obstruction. Check for flow obstructions. If the mold or material temperatures are lower than the documented standard, these can be increased to allow the PVC & CPVC to flow more easily into the mold.

Pressure Limited Process

A pressure-limited process cannot maintain the desired injection speed. If a change in lot number, colorant, or regrind is causing a viscosity increase, you may need to raise the shot size or decrease the transfer position to compensate for the higher viscosity material. Flow obstructions and faulty equipment should always be checked when the maximum pressure is being reached. In some cases, an increase in the maximum injection pressure is needed to accommodate a higher viscosity material, but the remainder of the process should be verified to ensure there are no other causes for high pressure before this change is made.

Possible Causes: Packing

Low Packing Pressure

This indicates there is not enough pressure to fill the mold and pack out the weld or meld lines during packing. The packing pressure should be set to match the process-specific documentation, and the packing speed should be set to the same value as your last injection speed before transfer to packing.

Low Packing Speed

A low packing speed will artificially slow down the screw, raise viscosity, and may cause under-packed weld or meld lines. To avoid complications, the packing speed should be set to the same value as your last injection speed before transferring from injection to packing.

Low Packing Time

With PVC & CPVC, it is difficult to get a full gate seal at the end of packing. Decreased packing time may reduce the amount of molecular interaction that takes place across the weld or meld line. A packing time study should be done to ensure the packing time is adequate to achieve a good weld or meld line development.

Low Final Part Weight

This indicates insufficient material is entering the mold cavity during Injection and packing. This is indicative of a change in the process, but you need to find other changes in the process before action can be taken.

High Clamp Force

Excessive clamp force will prevent the air from venting as fast as the material enters the mold, which can trap gas and volatiles at the weld or meld line location. You will sometimes see or smell burning at the end of fill locations if this is the cause. The Clamp Force should be set to match the Process-Specific Documentation and you will likely need to clean the vents at this time.

No Cushion

A process without a cushion does not have enough material to fill the part and compensate for shrinkage during packing. The cushion should always be approximately 10% of the Shot Size. Both the shot size and transfer position must be increased to raise the cushion.

High Cushion

Melted PVC & CPVC are highly compressible plastics. Therefore, a process with an excessive cushion, such as 25% of the shot size, can lose significant injection & packing pressure. The cushion should always be approximately 10% of the Shot Size. Both the shot size and transfer position must be decreased to lower the cushion.

Possible Causes: Cooling

Low Coolant Temperature or High Coolant Flow

PVC & CPVC tend to be very temperature sensitive, and a cold mold can cause excessive pressure loss during injection & packing. This can reduce the pressure reaching the weld or meld line location. Returning the coolant temperature to match the process-specific documentation should reduce the pressure loss in the mold. Using a coolant temperature controller with a lower-rated coolant pump or reducing the coolant flow with a flow control valve may return the water flow to match that of the documented standard.

Poor Cooling Line Layout

An incorrect cooling line layout will affect the material flow and material shrinkage. Verify the Cooling Line Layout matches the Process-Specific Documentation whenever cooling is in question.

Possible Causes: Recovery

Low Material Temperature or Back Pressure

PVC & CPVC tend to be very temperature sensitive, and a cold material might cause excessive pressure loss during injection & packing. This will reduce the pressure available at the weld & meld line location. Returning the material temperature and back pressure to match the process-specific documentation should reduce the pressure loss in the mold. It may be necessary to check the thermocouples & heater bands on the barrel & hot runner system.

High Material Temperature or Back Pressure

PVC & CPVC tend to be very temperature sensitive, and a rise in material temperature may cause the material to give off too many volatiles. These volatiles can get trapped between the two flow fronts at the weld or meld line location. Returning the material temperature and back pressure to match the process-specific documentation should return the pressure loss in the mold to normal. If regrind is being used, reduce the regrind percentage. You should always purge the barrel when degradation is suspected with PVC & CPVC. It may be necessary to check the cooling fans, screw cooling, thermocouples, & heater bands.

Low Recovery Time

A high Screw RPM can cause inconsistent recovery and excessive shear during recovery. The screw RPM should be adjusted so the recovery time consumes most of the cooling time.

Low Barrel Residence Time

A short barrel residence time may prevent the PVC & CPVC from fully melting during recovery. This low-temperature material causes excessive pressure loss during injection and may not have enough pressure available to properly pack the weld or meld line. This can be due to a significantly smaller barrel being used than that of the documented standard. In some cases, barrel temperature can be increased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

High Barrel Residence Time

A long barrel residence time can cause the heat-sensitive PVC & CPVC to heat up or degrade. A significant rise in residence time may cause the material to give off too many volatiles, which can get between flow fronts forming the weld or meld line. In some cases, barrel temperature can be decreased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

Possible Causes: Materials

High Regrind Percentage

PVC & CPVC tend to be very temperature sensitive and reground PVC & CPVC will cause an increase in material temperature and generation of volatiles. If too much regrind is being used, the barrel cooling fans may be inadequate in removing this increased heat, causing the release of volatiles, which can get between the part and the mold surface. Reducing the regrind percentage should lower the material temperature and decrease the creation of volatiles.

Poor Quality Regrind

Regrind with any degraded PVC & CPVC will cause a dangerous chain reaction of degradation, which can cause many issues, including excessive release of volatiles. These volatiles can negatively impact the strength and appearance of your weld or meld line. Any regrind containing degradation or burning should be discarded before it causes more defects or a serious safety concern.

Material Degradation

Degradation of PVC & CPVC causes extra volatiles and gases to occur in the polymer. These volatiles can negatively impact the strength and appearance of your weld or meld line. You should also notice a distinct burnt PVC & CPVC smell if this is the case. Removing the bad material from the barrel helps prevent degraded polymer from contaminating all other material it contacts. If a Hot Runner System is being used, purge it out as well.

Material Contamination

Any foreign liquid or polymer that melts at processing temperatures may turn to gas and become volatiles during injection. Contamination can come from many locations, including storage, hoppers, hoses, filters, screens, surge bins, access doors, seals, and blenders. All potential sources of contamination must be checked to determine where the contamination is coming from. Once the source is determined, everything that came in contact with the material afterwards must be thoroughly cleaned and inspected before returning to use.

Poor Material Drying

Although PVC & CPVC do not always need drying, they are hygroscopic polymers. The absorbed moisture will come out of the plastic during processing, which negatively impacts the strength and appearance of your weld or meld line. Although grades vary, most PVC & CPVC resins should be dried for 1-2 hours at 65 to 80°C (150 to 175°F) with dry air (approx. -40°C or -40°F Dewpoint) to a moisture content below 0.3%.

Possible Causes: Machinery

Check Ring Performance (Intermittent Defect)

If a check ring is being used, perform a check ring performance study by weighing 10 consecutive short shots (weight all cavities + runner) and calculating variability. Over 2% variability means the check ring is suspect, and should be changed immediately if over 3% Variability. Purging the Barrel or increasing Decompression after Recovery may improve check ring performance.

% Variability = (100%) * (Max - Min) / (Average)

Possible Causes: Mold & Design

Buildup of Volatiles on Mold Surface

Extensive Trapped Gas on the texture or polish will build up on the mold surface at the weld or meld line location. This may not affect the strength, but can have a significant effect on the appearance of the weld of the meld line. Solvents or surface clearing solutions, like a dry ice-blaster, will help clean up the mold surface.

Mold Surface Damage

Extensive trapped gas on the texture or polish will corrode the surface over time. This will likely need repair to significantly improve the appearance of your weld or meld line. If this occurs, a strategy for fixing the mold venting should be developed.

High Cavity Imbalance (Cavity to Cavity Variation)

Significant differences in cavity filling can cause some cavities to fill and start packing during injection, while other cavities are significantly short at transfer. Make sure all the mold cavities are short at the time of transfer to minimize any cavity-to-cavity variation in the final molded part. There are steps that can improve cavity imbalance, such as using a different injection speed, adjusting hot runner gate drops, cleaning the mold vents, increasing the number of vents, or adjusting gate width when necessary.

Design Concerns

If the end of flow is at a location that cannot be vented, adjust the design or part thickness, or add flow leaders to move the end of fill to another location that is easy to vent. Adjusting the gate location can also improve this situation.

Initial Troubleshooting

• Verify Process:
  • Injection Time & Profile
  • Recovery Time
  • Coolant Temperature, Pressure, and Flow
  • Material Temperature & Back Pressure
  • Barrel Residence Time
• Check for Mold Surface for Volatile Buildup or Damage
• Check Cooling Line Layout
• Check for Gate Damage

Possible Causes: Injection

Low Injection Time

This indicates the material is entering the mold, creating more shear at the gate area. Reducing injection speed will help you increase injection time.

High Injection Time

This indicates the material is entering the mold, creating less shear at the gate area. In most cases, this is better for reducing jetting, but a significantly slow injection speed can cause the material viscosity to become so high that it cannot create a smooth laminar flow as it enters the mold cavity. Increasing the injection speed will help you decrease injection time.

Injection Profile

PVC & CPVC typically require an injection profile to make a good part. A significant decrease in the injection speed at the gate area will reduce the shear in the gate area. The Injection Profile should match the Process-Specific Documentation.

Possible Causes: Cooling

Low Coolant Temperature or High Coolant Flow

PVC & CPVC tend to be very temperature sensitive, and a cold mold can cause excessive shear in the gate area. Returning the coolant temperature to match the process-specific documentation should reduce the pressure loss in the mold. Using a coolant temperature controller with a lower-rated coolant pump or reducing the coolant flow with a flow control valve may return the water flow to match that of the documented standard.

Poor Cooling Line Layout

An incorrect cooling line layout will affect the material flow. Verify the Cooling Line Layout matches the Process-Specific Documentation whenever cooling is in question.

Possible Causes: Recovery

Low Material Temperature or Back Pressure

PVC & CPVC tend to be very temperature sensitive, and a cold material will encounter additional shear at the gate. Returning the material temperature and back pressure to match the process-specific documentation should reduce the pressure loss in the mold. It may be necessary to check the thermocouples & heater bands on the barrel & hot runner system.

High Material Temperature or Back Pressure

PVC & CPVC can receive too much heat or shear during recovery, which will make it more shear-sensitive during injection. Returning the material temperature and back pressure to match the process-specific documentation should return the material to normal. If regrind is being used, reduce the regrind percentage. You should always purge the barrel when degradation is suspected with PVC & CPVC. It may be necessary to check the cooling fans, screw cooling, thermocouples, & heater bands.

Low Recovery Time

A high screw RPM can cause inconsistent recovery and excessive shear during recovery, making it more susceptible to jetting. The screw RPM should be adjusted so the recovery time consumes most of the cooling time.

Low Barrel Residence Time A short barrel residence time may prevent the PVC & CPVC from fully melting during recovery. This can be due to a significantly smaller barrel being used than what was used on the documented standard. In some cases, barrel temperature can be increased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

High Barrel Residence Time

A long barrel residence time can cause the heat-sensitive PVC & CPVC to heat up or degrade. A significant rise in residence time may cause the material to be more shear-sensitive during injection. In some cases, barrel temperature can be decreased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

Possible Causes: Materials

High Regrind Percentage

PVC & CPVC tend to be very temperature sensitive, and reground PVC & CPVC will cause an increase in material temperature and degradation of material. Degraded material can be easily over-sheared during injection. If too much regrind is being used, the barrel cooling fans may be inadequate in removing this increased heat causing the release of volatiles which can get between the part and the mold surface. Reducing the regrind percentage should lower the material temperature and decrease the creation of volatiles.

Poor Quality Regrind

Regrind with any degraded PVC & CPVC will cause a dangerous chain reaction of degradation, which can cause many issues, including low-strength material. Any regrind containing degradation or burning should be discarded before it causes more defects or a serious safety concern.

Material Degradation

Degradation of PVC & CPVC reduces the polymer strength. Degraded material can be easily over-sheared during injection. You should also notice a distinct burnt PVC & CPVC smell if this is the case. Removing the bad material from the barrel helps prevent the degraded polymer from contaminating all other materials it contacts. If a hot runner system is being used, purge it out as well.

Poor Material Drying

Although PVC & CPVC do not always need drying, they are hygroscopic polymers. The absorbed moisture will come out of the plastic during processing, which may affect polymer flow. Excessive material drying will degrade the PVC & CPVC, which will make it more shear sensitive during injection. Although grades vary, most PVC & CPVC resins should be dried for 1-2 hours at 65 to 80°C (150 to 175°F) with dry air (approx. -40°C or -40°F Dewpoint) to a moisture content below 0.3%.

Possible Causes: Mold & Design

Buildup of Volatiles on mold Surface

Extensive trapped gas on the texture or polish will build up on the mold surface at the jetting location. This buildup can cause the appearance of jetting even after the defect has been corrected. Solvents or surface clearing solutions like a dry ice-blaster will help clean up the mold surface.

Mold Surface Damage

Extensive trapped gas on the texture or polish will corrode the surface over time, causing the appearance of jetting even after the defect has been corrected. This will likely need repair to significantly improve the part's appearance. Small burs or dents at the gate area can prevent the material from adhering to the mold wall and guide the polymer to shoot into the mold cavity.

Mold or Part Design

Adjusting the gate design to promote laminar flow will reduce the likelihood of jetting. Gating into a wall may also stop jetting and force a laminar flow to develop.

Initial Troubleshooting

• Clean & Inspect the Vents
• Verify Process:
  • Injection Time & Profile
  • Clamp Force
  • Coolant Temperature, Pressure, and Flow
  • Material Temperature & Back Pressure
  • Barrel Residence Time
• Check Cooling Line Layout

Possible Causes: Injection

Inadequate or Blocked Venting

Poor venting can cause gas entrapment that may cause racetracking. Molds should be vented wherever possible. They must be vented to the atmosphere, and cross-cavity venting should be avoided whenever possible. If routine vent cleaning is needed, there should be a procedure for scheduled vent cleaning before they become blocked.

Flow Obstructions or Restrictions

Check for obstructions in the nozzle, gate, or hot runner, as well as a faulty heater band on the nozzle or hot runner system. These can increase shear and can affect the material flow pattern within the mold.

Low Injection Time

This indicates the material is entering the mold faster than the mold can vent the air out. Gas entrapment can negatively affect the material flow pattern within the mold. Reducing Injection Speed will help you increase Injection Time.

High Injection Time

This indicates material is entering the mold slowly, causing an increase in material viscosity. A higher viscosity PVC & CPVC may only flow in the thickest sections where flow is the easiest. Increasing Injection Speed will help decrease Injection Time. Check your maximum injection pressure in case there is not enough injection pressure available to maintain the injection speed.

Injection Profile

PVC & CPVC typically require an injection profile to make a good part. For example, the gate area often needs a lower injection speed to improve appearance. Increasing the injection speed in the areas of the part where racetracking occurs may improve the condition. The Injection Profile should match the Process-Specific Documentation.

Possible Causes: Packing

High Clamp Force

Excessive clamp force will prevent the air from venting as fast as the material is entering the mold, which may help cause racetracking. The clamp force should be set to match the process-specific documentation. You will likely need to clean the vents at this time.

Possible Causes: Recovery

Low Material Temperature or Back Pressure

PVC & CPVC tend to be very temperature sensitive, and a cold material will have a higher viscosity. This higher viscosity material may only flow through the thickest areas of the mold. Returning the material temperature and back pressure to match the process-specific documentation should reduce the pressure loss in the mold. It may be necessary to check the thermocouples & heater bands on the barrel & hot runner system.

Low Barrel Residence Time

A short barrel residence time may prevent the PVC & CPVC from fully melting during recovery. This higher viscosity material may only flow through the thickest areas of the mold. This can be due to a significantly smaller barrel being used than was used on the documented standard. In some cases, barrel temperature can be increased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

Possible Causes: Mold & Design

Buildup of Volatiles on Mold Surface

Extensive trapped gas on the texture or polish will build up on the mold surface at the racetracking location. This buildup can cause the appearance of racetracking even after the defect has been corrected. Solvents or surface clearing solutions, like a dry ice-blaster, will help clean up the mold surface.

Mold Surface Damage

Extensive Trapped Gas on the texture or polish will corrode the surface over time, effecting the appearance of racetracking even after the defect has been corrected. This will likely need repair to significantly improve the part appearance.

Design Concerns

Increasing part thickness when flow stagnates can help improve even material flow through the mold. Adjusting the gate design to promote a laminar flow will reduce the likelihood of Racetracking. Gating into a thicker section can often help a laminar flow develop.

Initial Troubleshooting

• Clean & Inspect the Vents
• Verify Process:
  • Injection Time & Profile
  • Packing Time
  • Clamp Force
  • Coolant Temperature, Pressure, and Flow
  • Material Temperature & Back Pressure
  • Barrel Residence Time
• Check Material Drying:
  • Material Moisture
  • Dryer Residence Time
  • Dryer Temperature
• Check Cooling Line Layout
• Check for Mold Surface for Volatile Buildup or Damage
• Look for obstructions or faulty equipment in nozzle, gate, or
  hot runner

Possible Causes: Injection

Inadequate or Blocked Venting

Poor venting can cause gas entrapment during injection. Molds should be vented wherever possible. They must be vented to the atmosphere, and cross-cavity venting should be avoided whenever possible. If routine vent cleaning is needed, there should be a procedure for scheduled vent cleaning before they become blocked.

Flow Obstructions or Restrictions

Check for obstructions in the nozzle, gate, or hot runner, as well as a faulty heater band on the nozzle or hot runner system. These can increase shear and affect the material flow pattern through the gate.

Low Injection Time

This faster injection rate can cause excessive shear in the gate area, which can discolor the polymer, causing gate blush. Reducing Injection Speed will help increase Injection Time.

High Injection Time

This indicates material is entering the mold slowly, causing an increase in material viscosity. A higher viscosity PVC & CPVC may encounter excessive shear in the gate area, which usually appears as discoloration. Increasing Injection Speed will help you decrease Injection Time. You should also check your Maximum Injection Pressure in case there is not enough Injection Pressure available to maintain the Injection Speed.

Injection Profile

PVC & CPVC typically require an injection profile to make a good part. A significant decrease in the injection speed at the gate area will typically improve the appearance of the gate area. The injection profile should match the process-specific documentation.

Possible Causes: Packing

High Packing Time

With PVC & CPVC, gates tend to be large, and thus gate seal may not take place. Long packing times can often cause excessive packing near the gate area in hot runner molds. Packing times that extend beyond gate seal can cause material to leak past the gate, causing stress and appearance issues at the gate.

High Clamp Force

Excessive clamp force will prevent the air from venting as fast as the material is entering the mold, even at the gate area. The clamp force should be set to match the process-specific documentation. You will likely need to clean the vents at this time.

Possible Causes: Cooling

Low Coolant Temperature or High Coolant Flow

PVC & CPVC tend to be very temperature sensitive, and a cold mold can cause excessive shear in the gate area. Returning the coolant temperature to match the process-specific documentation should reduce the pressure loss in the mold. Using a coolant temperature controller with a lower rated coolant pump or reducing the coolant flow with a flow control valve may return the water flow to match that of the documented standard.

Poor Cooling Line Layout

An incorrect cooling line layout will affect the material flow. Verify the Cooling Line Layout matches the Process-Specific Documentation whenever cooling is in question.

Possible Causes: Recovery

Low Material Temperature or Back Pressure

PVC & CPVC tend to be very temperature sensitive and a cold material might cause increased shear at the gate area. Returning the material temperature and back pressure to match the process-specific documentation should reduce the pressure loss in the mold. It may be necessary to check the thermocouples & heater bands on the barrel & hot runner system.

High Material Temperature or Back Pressure

PVC & CPVC tend to be very temperature sensitive, and a rise in material temperature may cause the material to give off too many volatiles. These volatiles often become deposited at the gate area as the material enters the mold cavity. Returning the material temperature and back pressure to match the process-specific documentation should improve the gate appearance if this is the cause. If regrind is being used, reduce the regrind percentage. You should always purge the barrel when degradation is suspected with PVC & CPVC. It may be necessary to check the cooling fans, screw cooling, thermocouples, & heater bands.

Low Recovery Time

A high Screw RPM can cause inconsistent recovery and excessive shear during recovery. The screw RPM should be adjusted so the Recovery Time consumes most of the Cooling Time.

Low Barrel Residence Time

A short barrel residence time may prevent the PVC & CPVC from fully melting during recovery. This low temperature material can cause excessive shear in the gate area. This can be due to a significantly smaller barrel being used than was used on the documented standard. In some cases, barrel temperature can be increased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

High Barrel Residence Time

A long barrel residence time can cause the heat-sensitive PVC & CPVC to heat up or degrade. A significant rise in residence time may cause the material to give off too many volatiles, which can become deposited in the gate area during injection. In some cases, barrel temperature can be decreased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

Possible Causes: Materials

High Regrind Percentage

PVC & CPVC tend to be very temperature sensitive, and reground PVC & CPVC will cause an increase in material temperature and generation of volatiles. These volatiles often become deposited at the gate area as the material enters the mold cavity. Reducing the regrind percentage should lower the material temperature and decrease the creation of volatiles.

Poor Quality Regrind

Regrind with any degraded PVC & CPVC will cause a dangerous chain reaction of degradation, potentially causing many issues, including excessive release of volatiles. These volatiles can negatively impact the strength and appearance of the gate area. Any regrind containing degradation or burning should be discarded before it causes more defects or serious safety concerns.

Material Degradation

Degradation of PVC & CPVC causes extra volatiles and gases to appear in the polymer. These volatiles can negatively impact the strength and appearance of the gate. You should notice a distinct burnt PVC & CPVC smell if this is the case. Removing the bad material from the barrel helps prevent degraded polymer from contaminating all other material it contacts. If a Hot Runner System is being used, purge it out as well.

Material Contamination

Any foreign liquid or polymer that melts at processing temperatures may turn to gas and become volatile during injection. Contamination can come from many locations, including storage, hoppers, hoses, filters, screens, surge bins, access doors, seals, and blenders. All potential sources of contamination must be checked to determine where the contamination is coming from. Once the source is determined, everything that came in contact with the material afterwards must be thoroughly cleaned and inspected before returning to use.

Poor Material Drying

Although PVC & CPVC do not always need drying, they are hygroscopic polymers. The absorbed moisture will come out of the plastic in the gate area. This is typically referred to as splay, but if it is minor, it can sometimes be mistaken for gate blush. Although grades vary, most PVC & CPVC resins should be dried for 1-2 hours at 65 to 80°C (150 to 175°F) with dry air (approx. -40°C or -40°F Dewpoint) to a moisture content below 0.3%.

Possible Causes: Mold & Design

Buildup of Volatiles on a Mold Surface

Extensive trapped gas on the texture or polish will build up on the mold surface at the gate location. This buildup can cause the appearance of gate blush even after the defect has been corrected. Solvents or surface clearing solutions, like a dry ice-blaster, will help clean up the mold surface.

Mold Surface Damage

Extensive Trapped Gas on the texture or polish will corrode the surface over time, causing the appearance of gate blush even after the defect has been corrected. This will likely need to be repaired to significantly improve the part's appearance.

Design Concerns

Adjusting the gate design to promote a laminar flow will reduce the likelihood of this defect. Gating into a thick section may also help laminar flow develop.

Initial Troubleshooting

• Purge The Barrel
• Clean & Inspect the Vents
• Verify Process:
  • Injection Time & Profile
  • Part Weight at Transfer (no runner or sprue)
  • Peak and Transfer Pressure
  • Packing Pressure, Time, and Speed
  • Final Part Weight
  • Clamp Force
  • Recovery Time
  • Coolant Temperature, Pressure, and Flow
  • Material Temperature & Back Pressure
  • Barrel Residence Time
• Check Material Drying:
  • Material Moisture
  • Dryer Residence Time
  • Dryer Temperature
• Check Cooling Line Layout
• Check the Mold Surface for Volatile Buildup or Damage
• Look for obstructions or faulty equipment in nozzle, gate, or
  hot runner

Possible Causes: Injection

Inadequate or Blocked Venting

With poor venting, gas can get caught inside the part and interrupt the polymer flow, causing the polymer to separate into layers. Molds should be vented wherever possible. They must be vented to the atmosphere, and cross-cavity venting should be avoided whenever possible. If routine vent cleaning is needed, there should be a procedure for scheduled vent cleaning before they become blocked.

Flow Obstructions or Restrictions

Check for obstructions in the nozzle, gate, or hot runner, as well as a faulty heater band on the nozzle or hot runner system. These can increase shear and cause the polymer to weaken and separate into layers.

Low Injection Time

This indicates a faster injection rate, which can cause excessive shear in the gate area, possibly reducing the strength of the polymer and causing the polymer to weaken and separate into layers. Reducing Injection Speed will help increase Injection Time.

High Injection Time

This indicates material is entering the mold slowly, causing an increase in material viscosity. A higher viscosity PVC & CPVC may encounter excessive shear and stress in the gate area if the flow is too slow. Increasing Injection Speed will help you decrease Injection Time. You should also check your maximum injection pressure in case there is not enough injection pressure available to maintain the injection speed.

Injection Profile

PVC & CPVC typically require an injection profile to make a sufficient part. A significant decrease in the injection speed at the gate area and thin sections typically improve the flow through these restrictive areas. The injection profile should match the process-specific documentation.

High Part Weight at Transfer

This indicates more material is entering the mold during injection than expected. This condition may cause a cavity pressure spike during injection, but it can also cause the screw to bounce back as the polymer flows out of the mold cavity and back into the runner system. This rapid flow reversal can cause layers between the cooled polymer and the retreating flow. If other parameters are similar and only the weight is different, lowering the shot size or increasing the transfer position should increase the part weight at transfer and help maintain forward screw travel during transfer. High Peak or Transfer Pressure

This indicates the material viscosity has increased or there is an obstruction, both of which can contribute to delamination. Check for flow obstructions. If the mold or material temperatures are lower than the documented standard, these can be increased to allow the PVC & CPVC to flow more easily into the mold.

Pressure Limited Process

A pressure-limited process cannot maintain the desired injection speed. If a change in lot number, colorant, or regrind is causing a viscosity increase, you may need to raise the shot size or decrease the transfer position to compensate for the higher viscosity material. Flow obstructions and faulty equipment should always be checked when the maximum pressure is reached. In some cases, an increase in the maximum injection pressure is needed to accommodate a higher viscosity material, but the remainder of the process should be verified to ensure there are no other causes for high pressure before this change is made.

Possible Causes: Packing

Low Packing Pressure

This indicates there is not enough pressure to fill the mold and compensate for shrinkage during packing. An under-packed part will often have reduced strength properties, which can make it easier for the part to separate into layers during use. The packing pressure should be set to match the process-specific documentation, and the packing speed should be set to the same value as your last Injection Speed before transferring from Injection to Packing.

High Packing Pressure

This indicates there is too much pressure after mold filling, which might cause excessive stress in the part. The packing pressure should be set to match the process-specific documentation, and the packing speed should be set to the same value as your last injection speed before transfer.

High Packing Time

With PVC & CPVC, gates tend to be large, and thus gate seal may not take place. Long packing times can often cause excessive stress near the gate area. Packing times that extend beyond the gate seal can also cause material to leak past the gate, potentially causing a small outer layer of material to form in the gate area.

Low Final Part Weight

This indicates insufficient material is entering the mold cavity during Injection and packing. This is indicative of a change in the process, but you need to find other changes in the process before action can be taken.

High Final Part Weight

This indicates excessive material enters the mold cavity during Injection and packing. This is indicative of a change in the process, but you need to find other changes in the process before action can be taken.

High Clamp Force

Excessive clamp force will prevent the air from venting from the mold. Poor venting gas can get caught inside the part and interrupt the polymer flow, causing the polymer to separate into layers. The Clamp Force should be set to match the Process-Specific Documentation. You will likely need to clean the vents at this time.

No Cushion

A process without a cushion will often result in an under-packed part with reduced strength. The cushion should always be approximately 10% of the Shot Size. Both the shot size and transfer position must be increased to raise the cushion.

High Cushion

Melted PVC & CPVC are highly compressible plastics. Thus, a process with an excessive cushion of over 25% of the shot size can lose a lot of injection & packing pressure. This can result in an under-packed part with reduced strength. The cushion should always be approximately 10% of the Shot Size. Both the shot size and transfer position must be decreased to decrease the cushion.

Possible Causes: Cooling

Low Coolant Temperature or High Coolant Flow

PVC & CPVC tend to be very temperature sensitive, and cold mold might cause excessive shear & stress during injection & packing. Returning the coolant temperature to match the process-specific documentation should reduce the pressure loss in the mold. Using a coolant temperature controller with a lower-rated coolant pump or reducing the coolant flow with a flow control valve may return the water flow to match that of the documented standard.

Poor Cooling Line Layout

An incorrect cooling line layout will affect the material flow. Verify the cooling line layout matches the process-specific documentation whenever cooling is in question.

Possible Causes: Recovery

Low Material Temperature or Back Pressure

PVC & CPVC tend to be very temperature sensitive, and a cold material might cause excessive shear & stress during injection & packing. Returning the material temperature and back pressure to match the process-specific documentation should reduce the pressure loss in the mold. It may be necessary to check the thermocouples & heater bands on the barrel & hot runner system.

High Material Temperature or Back Pressure

PVC & CPVC tend to be very temperature sensitive, and a rise in material temperature may cause the material to degrade and weaken. Overheated PVC & CPVC will also give off volatiles, which can become trapped within the part, causing it to separate into layers. Returning the material temperature and back pressure to match the Process-Specific Documentation should return the pressure loss in the mold to normal. If regrind is being used, reduce the regrind percentage. You should always purge the barrel when degradation is suspected with PVC & CPVC. It may be necessary to check the cooling fans, screw cooling, thermocouples, & heater bands.

Low Recovery Time

A high Screw RPM can cause inconsistent recovery and excessive shear during recovery. The screw RPM should be adjusted so the Recovery Time consumes most of the Cooling Time.

Low Barrel Residence Time

A short barrel residence time may prevent the PVC & CPVC from fully melting during recovery. This low temperature material can cause excessive shear & stress during injection & packing. This can be due to a significantly smaller barrel being used than was used on the documented standard. In some cases, barrel temperature can be increased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

High Barrel Residence Time

A long barrel residence time can cause the heat-sensitive PVC & CPVC to degrade or weaken. Overheated PVC & CPVC will also give off volatiles, which can become trapped within the part, causing it to separate into layers. In some cases, barrel temperature can be decreased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

Possible Causes: Materials

High Regrind Percentage

Too much PVC & CPVC regrind will cause an increase in material temperature and generation of volatiles. These volatiles can become trapped within the part, causing it to separate into layers. Reducing the regrind percentage should lower the material temperature and decrease the creation of volatiles.

Poor Quality Regrind

Regrind with any degraded PVC & CPVC will cause a dangerous chain reaction of degradation, possibly causing numerous issues, including an excessive release of volatiles. These volatiles can become trapped within the part, causing it to separate into layers. Any regrind containing degradation or burning should be discarded before it causes more defects or serious safety concerns.

Material Degradation

Degradation of PVC & CPVC causes extra volatiles and gases to appear in the polymer, which will give off volatiles. These volatiles can become trapped within the part, causing it to separate into layers. You should notice a distinct burnt PVC & CPVC smell if this is the case. Removing the bad material from the barrel helps prevent degraded polymer from contaminating all other material it comes in contact with. If a Hot Runner System is being used, purge it out as well.

Material Contamination

Any foreign liquid or polymer that melts at processing temperatures may turn to gas and become volatiles. Contamination can come from many locations, including storage, hopper, hoses, filters, screens, surge bins, access doors, seals, and blenders. All potential sources of contamination must be checked to determine where the contamination is coming from. Once the source is determined, everything that came in contact with the material afterwards must be thoroughly cleaned and inspected before returning to use.

Poor Material Drying

Although PVC & CPVC do not always need to be dried, they are hygroscopic polymers. The absorbed moisture can become trapped within the part, causing it to separate into layers. Although grades vary, most PVC & CPVC resins should be dried for 1-2 hours at 65 to 80°C (150 to 175°F) with dry air (approx. -40°C or -40°F Dewpoint) to a moisture content below 0.3%.

Possible Causes: Mold & Design

Part or Mold Design

Good part design strategies, such as gating into thick sections, avoiding sharp transitions, maintaining the same thickness when possible, and providing adequate venting, are important to overall part quality.

Initial Troubleshooting

• Clean & Inspect the Vents
• Check for Mold Surface for Volatile Buildup or Damage
• Verify Process:
  • Injection Time & profile
  • Part Weight at Transfer (no runner or sprue)
  • Peak and Transfer Pressure
  • Packing Pressure & Speed
  • Clamp Force
  • Coolant Temperature, Pressure, and Flow
  • Material Temperature & Back Pressure
  • Barrel Residence Time
• Check process is not Pressure Limited

Possible Causes: Injection

Inadequate or Blocked Venting

Poor venting can cause gas entrapment on the mold surface, which can become trapped behind the flow front, often leaving volatiles on the part surface. This trapped gas can also cause the polymer at the mold surface to skip forward, leaving a surface blemish across the flow front. Molds should be vented wherever possible. They must be vented to the atmosphere, and cross-cavity venting should be avoided whenever possible. If routine vent cleaning is needed, there should be a procedure for scheduled vent cleaning before they become blocked.

Flow Obstructions or Restrictions

Check for obstructions in the nozzle, gate, or hot runner, as well as a faulty heater band on the nozzle or hot runner system. These can increase shear and affect the material flow pattern within the mold.

Low Injection Time

This indicates the material is entering the mold faster than the mold can vent the air out, causing gas entrapment. This trapped gas can also cause the polymer at the mold surface to skip forward, leaving a surface blemish across the flow front. You may see or smell burning at the end of the flow if this is the cause. Reducing Injection Speed will help you increase Injection Time.

High Injection Time

This indicates material is entering the mold slowly, causing an increase in material viscosity. A higher viscosity PVC & CPVC may flow differently, causing sections of the flow to stop for some time before flowing again. Any abrupt change in the flow can result in a flow line on the final part. Increasing Injection Speed will help you decrease Injection Time. You should also check your Maximum Injection Pressure in case there is not enough Injection Pressure available to maintain the Injection Speed.

Injection Profile

PVC & CPVC typically require an injection profile to make a good part. An abrupt change in the injection profile will often cause flow lines to occur. The Injection Profile should match the process-specific documentation.

Low Part Weight at Transfer

This indicates too little material is entering the mold during injection, which may cause a flow line when the process transfers to packing. If other parameters are similar and only the weight is different, raising the shot size or decreasing the transfer position should increase part weight at transfer.

High Part Weight at Transfer

This indicates too much material is entering the mold during injection and often contributes to flow lines through gas entrapment. This condition may cause a cavity pressure spike during injection, but can also cause the screw to bounce back during transfer resulting in a cavity pressure loss at transfer. A screw bounce back causes the flow front to stop, which can result in a flow line. If other parameters are similar and only the weight is different, lowering the shot size or increasing the transfer position should increase the part weight at transfer.

High Peak or Transfer Pressure

This indicates the material viscosity has increased or there is an obstruction that can change the polymer flow during injection. Check for flow obstructions. If the mold or material temperatures are lower than the documented standard, these can be increased to allow the PVC & CPVC to flow more easily into the mold.

Pressure Limited Process

A pressure-limited process cannot maintain the desired injection speed. If a change in lot number, colorant, or regrind is causing a viscosity increase, you may need to raise the shot size or decrease the transfer position to compensate for the higher viscosity material. Flow obstructions and faulty equipment should always be checked when the maximum pressure is being reached. In some cases, an increase in the maximum injection pressure is needed to accommodate a higher viscosity material, but the remainder of the process should be verified to ensure there are no other causes for high pressure before this change is made.

Possible Causes: Packing

Low Packing Pressure

This indicates there is not enough pressure to fill the mold and compensate for shrinkage during packing. A low packing pressure can cause the flow front to stagnate at transfer, resulting in a flow line. The packing pressure should be set to match the process-specific documentation, and the packing speed should be set to the same value as your last injection speed before transferring from injection to packing.

Low Packing Speed

A low packing speed will artificially slow down the screw at transfer which can create a flow line. To avoid complications, the packing speed should be set to the same value as your last injection speed before transferring from injection to packing.

Low Final Part Weight

This indicates insufficient material is entering the mold cavity during Injection and packing. This is indicative of a change in the process, but you need to find other changes in the process before action can be taken.

High Clamp Force

Excessive clamp force will prevent the air from venting from the mold. Trapped gas can become trapped behind the flow front, often leaving volatiles on the part surface. This trapped gas can also cause the polymer at the mold surface to skip forward, leaving a surface blemish across the flow front. The Clamp Force should be set to match the Process-Specific Documentation. You will likely need to clean the vents at this time.

Possible Causes: Cooling

Low Coolant Temperature or High Coolant Flow

PVC & CPVC tend to be very temperature sensitive, and a cold mold might cause excessive pressure loss during injection & packing. Excessive pressures can cause the front to slide along the mold surface, which can leave a surface blemish along the flow front. Returning the coolant temperature to match the process-specific documentation should reduce the pressure loss in the mold. Using a coolant temperature controller with a lower-rated coolant pump or reducing the coolant flow with a flow control valve may return the water flow to match that of the documented standard.

Poor Cooling Line Layout

An incorrect cooling line layout will affect the material flow and material shrinkage. Verify the cooling line layout matches the process-specific documentation whenever cooling is in question.

Possible Causes: Recovery

Low Material Temperature or Back Pressure

PVC & CPVC tend to be very temperature sensitive, and a cold material might not properly adhere to the mold surface during filling. Poor adherence can cause the flow front to slide along the mold surface, which can leave a surface blemish along the flow front. Returning the material temperature and back pressure to match the process-specific documentation should reduce the pressure loss in the mold. It may be necessary to check the thermocouples & heater bands on the barrel & hot runner system.

High Material Temperature or Back Pressure

PVC & CPVC tend to be very temperature sensitive, and a rise in material temperature may cause the material to degrade, giving off volatiles. Volatiles can become trapped behind the flow front, often leaving blemishes on the part surface. Volatiles can also cause the polymer at the mold surface to skip forward, leaving a surface blemish across the flow front. Returning the material temperature and back pressure to match the process-specific documentation should return the pressure loss in the mold to normal. If the regrind is being used, reduce the regrind percentage. You should always purge the barrel when degradation is suspected with PVC & CPVC. It may be necessary to check the cooling fans, screw cooling, thermocouples, & heater bands.

Low Recovery Time

A high screw RPM can cause inconsistent recovery and excessive shear during recovery. The screw RPM should be adjusted so the recovery time consumes most of the cooling time.

Low Barrel Residence Time

A short barrel residence time may prevent the PVC & CPVC from fully melting during recovery. PVC & CPVC tend to be very temperature sensitive, and a cold material might not properly adhere to the mold surface during filling. In some cases, barrel temperature can be increased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

High Barrel Residence Time

A long barrel residence time can cause the heat-sensitive PVC & CPVC to degrade and give off volatiles. Volatiles can also cause the polymer at the mold surface to skip forward, leaving a surface blemish across the flow front. In some cases, barrel temperature can be decreased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

Possible Causes: Materials

High Regrind Percentage

PVC & CPVC tend to be very temperature sensitive, and reground PVC & CPVC will cause an increase in material temperature and generation of volatiles. Volatiles can become trapped behind the flow front, often leaving blemishes on the surface part. Volatiles can also cause the polymer at the mold surface to skip forward, leaving a surface blemish across the flow front. Reducing the regrind percentage should lower the material temperature and decrease the creation of volatiles.

Poor Quality Regrind

Regrind containing degraded PVC & CPVC will cause a dangerous chain reaction of degradation, which can cause many issues, including excessive release of volatiles. Any regrind containing degradation or burning should be discarded before it causes more defects or serious safety concerns.

Material Degradation

The degradation of PVC & CPVC causes extra volatiles and gases to occur in the polymer, which will give off volatiles. Volatiles can also cause the polymer at the mold surface to skip forward, leaving a surface blemish across the flow front. You should notice a distinct burnt PVC & CPVC smell if this is the case. Removing the bad material from the barrel helps prevent degraded polymer from contaminating all other material it contacts. If a Hot Runner System is being used, purge it out as well.

Poor Material Drying

Although PVC & CPVC do not always need drying, they are hygroscopic polymers. The moisture can become trapped behind the flow front, often leaving blemishes on the part surface. Moisture can also cause the polymer at the mold surface to skip forward, leaving a surface blemish across the flow front. Although grades vary, most PVC & CPVC resins should be dried for 1-2 hours at 65 to 80°C (150 to 175°F) with dry air (approx. -40°C or -40°F Dewpoint) to a moisture content below 0.3%.

Possible Causes: Mold & Design

Buildup of Volatiles on Mold Surface

Extensive Trapped Gas on the texture or polish will build up on the mold surface. This may occur in areas with bad venting or show up as a flow line when overall venting is poor. Solvents or surface clearing solutions, like a dry ice-blaster, will help clean up the mold surface.

Mold Surface Damage

Extensive Trapped Gas on the texture or polish will corrode the surface over time. This will likely need repair to significantly improve the part's appearance. If this occurs, a strategy for fixing the mold venting should be developed.

High Cavity Imbalance (Cavity to Cavity Variation)

Significant differences in cavity filling can cause some cavities to fill and start packing during injection, while other cavities are significantly short at transfer. Make sure all the mold cavities are short at the time of transfer to minimize any cavity-to-cavity variation in the final molded part. There are steps that can improve cavity imbalance, such as using a different injection speed, adjusting hot runner gate drops, cleaning mold vents, increasing the number of vents, or adjusting the gate width if necessary.

Part or Mold Design

Good part design strategies, such as gating into thick sections, avoiding sharp transitions, maintaining a constant thickness when possible, and providing adequate venting, are important to the overall surface finish.

Initial Troubleshooting

• Verify Process:
  • Part Weight at Transfer (no runner or sprue)
  • Packing Time
  • Final Part Weight
  • Recovery Time
  • Coolant Temperature, Pressure, and Flow
  • Material Temperature & Back Pressure
  • Barrel Residence Time
• Check:
  • Cushion is approximately 10% of Shot Size
  • Process is not Pressure Limited
  • Cooling Line Layout

Possible Causes: Injection

High Part Weight at Transfer

This indicates more material is entering the mold during injection than expected. This condition may cause a cavity pressure spike during injection, but can also cause the screw to bounce back. This rapid flow reversal can cause significant pressure loss and shrinkage in the gate area. If other parameters are similar and only the weight is different, lowering shot size or increasing transfer position should increase part weight at transfer, as well as help maintain forward screw travel at transfer.

Possible Causes: Packing

Low Packing Time

With PVC & CPVC, it is difficult to get a full gate seal at the end of packing. Insufficient packing time will cause material to leave the gate area into the runner or sprue. A packing time study should be done to ensure the packing time is adequate to achieve a good gate appearance.

Low Final Part Weight

This indicates that insufficient material is entering the mold cavity during Injection and packing. This is indicative of a change in the process, but changes must be identified in the process before action can be taken.

No Cushion

A process without a cushion will allow material to flow back through the gate prematurely. The cushion should always be approximately 10% of the shot size. Both the shot size and transfer position must be increased to raise the cushion.

Possible Causes: Cooling

High Coolant Temperature or Low Coolant Flow

Higher mold temperatures will increase the gate seal time. An inadequate gate seal will cause material to leave the gate area into the runner or sprue. Returning the coolant temperature to match the process-specific documentation should return the pressure loss in the mold to normal. Check the cooling system for obstructions, as this will reduce coolant flow. Using a coolant temperature controller with a higher-rated coolant pump may raise the coolant flow to match the process-specific documentation.

Poor Cooling Line Layout

An incorrect cooling line layout will affect the material flow and material shrinkage. Verify the cooling line layout matches the process-specific documentation whenever cooling is in question.

Possible Causes: Recovery

High Material Temperature or Back Pressure

Higher material temperatures increase the gate seal time, causing material to leave the gate area into the runner or sprue. Returning the material temperature and back pressure to match the Process-Specific Documentation should return the pressure loss in the mold to normal. If regrind is being used, reduce the regrind percentage. You should always purge the barrel when degradation is suspected with PVC & CPVC. It may be necessary to check the cooling fans, screw cooling, thermocouples, & heater bands.

High Barrel Residence Time

A long barrel residence time can cause the heat-sensitive PVC & CPVC material to increase temperature. Higher material temperatures increase the gate seal time, causing material to leave the gate area into the runner or sprue. In some cases, barrel temperature can be decreased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

Possible Causes: Materials

High Regrind Percentage

PVC & CPVC tend to be very temperature sensitive, and reground PVC & CPVC will cause an increase in material temperature. Higher material temperatures increase the gate seal time, causing material to leave the gate area into the runner or sprue. Reducing the regrind percentage should lower the material temperature and decrease the creation of volatiles.

Poor Quality Regrind

Regrind with any degraded PVC & CPVC will cause a dangerous chain reaction of degradation, potentially causing issues such as an excessive temperature increase. Higher material temperatures increase the gate seal time, causing material to leave the gate area and enter the runner or sprue. Any regrind containing degradation or burning should be discarded before it causes more defects or serious safety concerns.

Material Degradation

The degradation of PVC & CPVC will cause a dangerous chain reaction of degradation, which can cause potentially causing many issues such as an excessive temperature increase. You should notice a distinct burnt PVC & CPVC smell if this is the case. Removing the bad material from the barrel helps prevent degraded polymer from contaminating all other material it contacts. If a Hot Runner System is being used, purge it out as well.

Possible Causes: Mold & Design

Mold Design

Reducing the size of the gate can improve the gate seal. Keep in mind, PVC & CPVC are shear sensitive and can become easily over-sheared if the gate thickness is too small.

Initial Troubleshooting

• Clean & Inspect the Vents
• Verify Process:
  • Injection Time
  • Part Weight at Transfer (no runner or sprue)
  • Peak and Transfer Pressure
  • Packing Pressure, Speed, and Time
  • Final Part Weight
  • Clamp Force
  • Coolant Temperature, Pressure, and Flow
  • Material Temperature & Back Pressure
  • Barrel Residence Time
• Verify Hot Runner Gate Tip Temperatures
• Check Cooling Line Layout

Possible Causes: Injection

Venting Concerns

Poor venting will make it very difficult to fill the mold consistently, causing pressure loss during injection. Molds should be vented wherever possible. They must be vented to the atmosphere, and cross-cavity venting should be avoided whenever possible. If cleaning is needed, there should be a procedure for scheduled vent cleaning before they become blocked.

High Injection Time

This indicates material is entering the mold slowly, causing an increase in material viscosity and pressure loss. Increasing Injection Speed will help you decrease Injection Time. You should also check your Maximum Injection Pressure in case there is not enough Injection Pressure available to maintain the Injection Speed.

High Peak or Transfer Pressure

This indicates the material viscosity has increased or there is an obstruction that can change the polymer flow during injection. An increase in the polymer’s viscosity can cause excessive pressure loss during injection. If the mold or material temperatures are lower than the documented standard, these can be increased to allow the PVC & CPVC to flow more easily into the mold.

Possible Causes: Packing

High Packing Pressure

This indicates there is too much pressure after mold filling, which can cause overpacking. The Packing Pressure should be set to match the Process-Specific Documentation.

Low Packing Speed

A low packing speed will artificially slow down the screw at transfer, causing a pressure loss at transfer. To avoid complications, the packing speed should be set to the same value as your last injection speed before transferring from injection to packing.

High Packing Time

With PVC and CPVC, gates tend to be large, and thus gate seal may not occur. Long Packing Times can often cause excessive packing near the gate area. Packing Times that extend beyond the gate seal can cause material to leak past the gate, causing stress and appearance issues at the gate.

High Final Part Weight

This indicates excessive material is entering the mold cavity during Injection and packing. This is indicative of a change in the process, but you need to find other changes in the process before action can be taken.

High Clamp Force

Excessive clamp force will prevent the air from venting, causing excessive pressure loss during injection. The clamp force should be set to match the process-specific documentation. You will likely need to clean the vents at this time.

Possible Causes: Cooling

Low Coolant Temperature or High Coolant Flow

PVC & CPVC tend to be very temperature sensitive, and a cold mold might cause excessive pressure loss during injection & packing. Returning the coolant temperature to match the process-specific documentation should reduce the pressure loss in the mold. Using a coolant temperature controller with a lower rated coolant pump or reducing the coolant flow with a flow control valve may return the water flow to match that of the documented standard.

Poor Cooling Line Layout

An incorrect cooling line layout will affect the material flow and shrinkage. Verify the Cooling Line Layout matches the Process-Specific Documentation whenever cooling is in question.

Possible Causes: Recovery

Low Material Temperature or Back Pressure

PVC & CPVC tend to be very temperature sensitive, and a cold material will increase pressure loss during injection. Returning the material temperature and back pressure to match the process-specific documentation should reduce the pressure loss in the mold. It may be necessary to check the thermocouples & heater bands on the barrel & hot runner system.

Low Barrel Residence Time

A short barrel residence time may prevent the PVC & CPVC from fully melting, causing increased pressure losses during injection. In some cases, barrel temperature can be increased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

Possible Causes: Machinery

Excessive Stationary Platen Deflection

High deflection in the center of the stationary platen will cause the center of the mold to lose effective clamp force. This deflection during injection and packing can cause a sprue-gated part to be thicker in the center. This is measured with an indicator holder mounted to the machine base, with the indicator touching as close to the center of the platen as possible. This measurement can be compared to the measured deflection during routine machine maintenance. To compensate for this, you can add shims around the locating ring or add bolster plates to the mold.

Possible Causes: Mold & Design

Excessive Mold Deflection

High deflection in the center of the mold will cause a loss of effective clamp force in that area, which can cause a sprue-gated part to be thicker in the center. This can be measured by the movement of the injection unit during injection and packing. This is measured with an indicator holder mounted to the machine base, with the indicator touching the injection unit. This measurement can be compared to the measured deflection during mold qualifications. To compensate for excessive mold deflection, you can add shims around the locating ring, add bolster plates to the mold, or add support pillars behind the support plate.

Part or Mold Design

Good part design strategies, such as gating into thick sections, avoiding sharp transitions, maintaining the same thickness when possible, and providing adequate venting, are important to even part shrinkage.

Initial Troubleshooting

• Clean & Inspect the Vents
• Verify Process:
  • Injection Time
  • Part Weight at Transfer (no runner or sprue)
  • Peak and Transfer Pressure
  • Packing Pressure, Time, and Speed
  • Final Part Weight
  • Clamp Force
  • Coolant Temperature, Pressure, and Flow
  • Material Temperature & Back Pressure
  • Barrel Residence Time
• Check:
  • Cushion is approximately 10% of Shot Size
  • Process is not Pressure Limited
  • Cooling Line Layout
  • Check Ring Performance (if used)
• Look for obstructions or faulty equipment in nozzle, gate, or
  hot runner

Possible Causes: Injection

Venting Concerns

Poor venting will make it very difficult to fill the mold consistently, causing pressure loss during injection. Molds should be vented wherever possible. They must be vented to atmosphere, and cross-cavity venting should be avoided whenever possible. If cleaning is needed, there should be a procedure for scheduled vent cleaning before they become blocked.

Flow Obstructions or Restrictions

Check for obstructions in the nozzle, gate, or hot runner, as well as a faulty heater band on the nozzle or hot runner system. These can cause pressure losses during injection and packing.

High Injection Time

This indicates material is entering the mold slowly, causing an increase in material viscosity and pressure loss. Increasing injection speed will help you decrease injection time. You should also check your maximum injection pressure in case there is not enough injection pressure available to maintain the injection speed.

Low Part Weight at Transfer

This indicates too little material is entering the mold during injection. If other parameters are similar and only the weight is different, raising the shot size or decreasing the transfer position should increase the part weight at transfer.

High Peak or Transfer Pressure

This indicates the material viscosity has increased or there is an obstruction that can change the polymer flow during injection. An increase in the viscosity polymer can cause excessive pressure loss during injection. If the mold or material temperatures are lower than the documented standard, these can be increased to allow the PVC & CPVC to flow more easily into the mold.

Pressure Limited Process

A pressure-limited process cannot maintain the desired injection speed. If a change in lot number, colorant, or regrind is causing a viscosity increase, you may need to raise the shot size or decrease the transfer position to compensate for the higher viscosity material. Flow obstructions and faulty equipment should always be checked when the maximum pressure is being reached. In some cases, an increase in the maximum injection pressure is needed to accommodate a higher viscosity material, but the remainder of the process should be verified to ensure there are no other causes for high pressure before this change is made.

Possible Causes: Packing

Low Packing Pressure

This indicates there is not enough pressure to fill the mold and compensate for shrinkage during packing. The packing pressure should be set to match the process-specific documentation, and the packing speed should be set to the same value as your last injection speed before transferring from injection to packing.

Low Packing Speed

A low packing speed will artificially slow down the screw at transfer, causing a pressure loss at transfer. To avoid complications, the packing speed should be set to the same value as your last injection speed before transferring from injection to packing.

Low Final Part Weight

This indicates insufficient material is entering the mold cavity during injection and packing. This is indicative of a change in the process, but you need to find other changes in the process before action can be taken.

High Clamp Force

Excessive clamp force will prevent the air from venting, causing excessive pressure loss during injection. The clamp force should be set to match the process-specific documentation. You will likely need to clean the vents at this time.

No Cushion

A process without a cushion does not have enough material to fill the part and compensate for shrinkage during packing. The cushion should always be approximately 10% of the shot size. Both the shot size and transfer position must be increased to raise the cushion.

High Cushion

Melted PVC & CPVC are highly compressible plastics. Thus, a process with an excessive cushion such as 25% of the shot size can lose significant injection & packing pressure. The cushion should always be approximately 10% of the shot size. Both the shot size and transfer position must be decreased to lower the cushion.

Possible Causes: Cooling

Low Coolant Temperature or High Coolant Flow

PVC & CPVC tend to be very temperature sensitive, and a cold mold might cause excessive pressure loss during injection & packing. Returning the coolant temperature to match the process-specific documentation should reduce the pressure loss in the mold. Using a coolant temperature controller with a lower rated coolant pump or reducing the coolant flow with a flow control valve may return the water flow to match that of the documented standard.

Poor Cooling Line Layout

An incorrect cooling line layout will affect the material flow and material shrinkage. Verify the Cooling Line Layout matches the Process-Specific Documentation whenever cooling is in question.

Possible Causes: Recovery

Low Material Temperature or Back Pressure

PVC & CPVC tend to be very temperature sensitive, and a cold material might cause excessive pressure loss during injection & packing. Low material temperature often causes defects near the end of fill. Returning the material temperature and back pressure to match the process-specific documentation should reduce the pressure loss in the mold. It may be necessary to check the thermocouples & heater bands on the barrel as well as the hot runner system if being used.

Low Recovery Time

A high screw RPM can cause inconsistent recovery and excessive shear during recovery. The screw RPM should be adjusted so that the recovery time consumes most of the cooling time.

Low Barrel Residence Time

A short barrel residence time may prevent the PVC & CPVC from fully melting during recovery. This cold material might cause excessive pressure loss during injection & packing. This condition is usually due to a significantly smaller barrel being used than was used on the documented standard. In some cases, barrel temperature can be increased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

Possible Causes: Material

Material Contamination

Contaminants can cause a flow restriction if they get caught in the nozzle and hot runner system, resulting in a pressure loss during injection. These could be any non-melting material, such as wood, metal, or cloth, as well as degradation & any polymer which does not melt at the current processing temperature. Contamination can come from many locations, including storage, hopper, hoses, filters, screens, surge bins, access doors, seals, and blenders. All potential sources of contamination must be checked to determine where the contamination is coming from. Once the source is determined, everything that came in contact with the material afterwards must be thoroughly cleaned and inspected before returning to use.

Possible Causes: Machinery

Check Ring Performance (Intermittent Defect)

If a check ring is being used, perform a check ring performance study by weighing 10 consecutive short shots (weight all cavities + runner) and calculating variability. Over 2% variability means the check ring is suspect, and should be changed immediately if over 3% Variability. Purging the barrel or increasing Decompression after Recovery may improve check ring performance.

% Variability = (100%) * (Max - Min) / (Average)

Possible Causes: Mold & Design

Part or Mold Design

With the high viscosity of PVC & CPVC, it is very difficult to fill thin sections, especially at the end of fill. It may be necessary to increase the thickness of the part in areas that are difficult to fill with PVC & CPVC if this is a recurring issue. Good part design strategies, such as gating into thick sections, avoiding sharp transitions, maintaining the same thickness when possible, and providing adequate venting, are important to improving mold filling.

Initial Troubleshooting

• Purge Barrel
• Verify Process:
  • Material Temperature
  • Back Pressure
  • Barrel Residence Time
  • Recovery Time
  • Coolant Temperature and Flow
  • Cooling Time
  • Injection Time & Profile
  • Part Weight at Transfer (no runner or sprue)
  • Peak Pressure & Transfer Pressure
  • Packing Pressure & Packing Speed
  • Clamp Force
• Check Material Drying:
  • Material Moisture
  • Dryer Residence Time
  • Dryer Temperature
  • Screw & Barrel Cooling Equipment
  • Volatile buildup or damage on mold surface
  • Clean & Inspect the Vents
• Check Look for obstructions or faulty equipment at barrel, nozzle, gate, or hot runner

Possible Causes: Recovery

High Material Temperature or Back Pressure

PVC & CPVC tend to be very temperature sensitive, and excessive heat or shear can quickly cause degradation. Returning the material temperature and back pressure to match the process-specific documentation should return the pressure loss in the mold to normal. If regrind is being used, reduce the regrind percentage. You should always purge the barrel when degradation is suspected with PVC & CPVC. It may be necessary to check the cooling fans, screw cooling, thermocouples, & heater bands.

Low Recovery Time

A high Screw RPM can cause inconsistent recovery and excessive shear during recovery. The screw RPM should be adjusted so the Recovery Time consumes most of the Cooling Time.

High Barrel Residence Time

A long barrel residence time can cause the heat sensitive PVC & CPVC to heat up or degrade. In some cases, barrel temperature can be decreased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

Possible Causes: Cooling

High Cooling Time

A significantly higher cooling time will increase the cycle time as well as the barrel residence time. The cooling time should match the process-specific documentation.

Possible Causes: Materials

High Regrind Percentage

PVC & CPVC tend to be very temperature sensitive, and reground PVC & CPVC will cause an increase in material temperature. If too much regrind is being used, the barrel cooling fans may be inadequate to remove this increased heat, causing degradation. Reducing the regrind percentage should lower the material temperature and decrease the creation of volatiles.

Poor Quality Regrind

Regrind with any degraded PVC & CPVC will cause a dangerous chain reaction of degradation, creating a very dangerous situation. Any regrind containing degradation or burning should be discarded before it causes more defects or a serious safety concern.

Material Degradation

Degradation of PVC & CPVC causes extra volatiles and gases to occur in the polymer. Removing the bad material from the barrel helps prevent degraded polymer from contaminating all other material it contacts. If a Hot Runner System is being used, purge it out as well.

Poor Material Drying

Although PVC & CPVC do not always need drying, they are hygroscopic polymers. The absorbed moisture will come out of the plastic during processing as gas, which can increase the likelihood of burning. Although grades vary, most PVC & CPVC resins should be dried for 1-2 hours at 65 to 80°C (150 to 175°F) with dry air (approx. -40°C or -40°F Dewpoint) to a moisture content below 0.3%.

Possible Causes: Injection

Inadequate or Blocked Venting

Poor venting can cause gas entrapment during injection, which can burn the polymer at the flow front. Molds should be vented wherever possible; they must be vented to the atmosphere, and cross-cavity venting should be avoided whenever possible. If routine vent cleaning is needed, there should be a procedure for scheduled vent cleaning before they become blocked.

Flow Obstructions or Restrictions

Excessive shear due to obstructions can cause degradation during injection. Check for obstructions in the nozzle, gate, or hot runner as well as a faulty heater band on the barrel, nozzle, or hot runner system.

Low Injection Time

The material may be entering the mold fast enough to over-shear the PVC & CPVC. If the polymer is experiences too much shear, the entire part may change color. Reducing the injection speed will help increase the injection time.

Possible Causes: Packing

High Packing Pressure

This indicates there is too much pressure after mold filling, which might cause the mold to fill faster than the mold can vent. This can cause degradation or burning at the end of fill. The packing pressure should be set to match the process-specific documentation, and the packing speed should be set to the same value as your last injection speed before transferring from injection to packing.

High Packing Speed

A high packing speed can cause a rise in material velocity if the packing pressure is significantly higher than the injection speed at transfer. This can cause burning by compressing the gas remaining in the front of the screw during transfer. To avoid complications, the packing speed should be set to the same value as your last injection speed before transferring from injection to packing.

High Clamp Force

Excessive clamp force will prevent the air from venting as fast as the material is entering the mold. This can cause degradation or burning at the end of fill. The clamp force should be set to match the process-specific documentation. You will likely need to clean the vents at this time.

Possible Causes: Mold & Design

Buildup of Volatiles

Extensive trapped can block the vents, causing burning due to gas entrapment during injection & packing. Solvents or surface-clearing solutions like a dry ice-blaster will help clean up the mold surface.

Mold Surface Damage

Extensive trapped gas on the texture or polish will corrode the surface over time, sometimes giving a rough surface finish or burning. This will likely need repair to significantly improve the part's appearance. If this occurs, a strategy for fixing the mold venting should be developed.

Design Concerns

If the end of flow is at a location that cannot be vented, adjust the design, part thickness, or add flow leaders to move the end of fill to another location that is easy to vent. Adjusting the gate location can also improve this situation. Increasing the number or the size of the gate can help prevent over shear during injection with PVC & CPVC.

Initial Troubleshooting

• Inspect for and correct any existing defects such as:
  • Degradation
  • Short Shots
  • Burning
  • Delamination
  • Flow Lines
  • Racetracking
  • Poor Weld or Meld Lines
  • Brittleness/Cracking/Crazing
  • Gels
  • Contamination
  • Bubbles
  • Splay
• Purge Barrel
• Verify Process:
  • Material Temperature & Back Pressure
  • Recovery Time
  • Barrel Residence Time
  • Coolant Temperature & Flow
  • Cooling Time
  • Injection Time & Profile
  • Part Weight at Transfer (no runner or sprue)
  • Peak and Transfer Pressure
  • Packing Pressure, Time, and Speed
  • Final Part Weight
• Check Material Drying:
  • Material Moisture
  • Dryer Residence Time
  • Dryer Temperature
• Check:
  • Cushion is approximately 10% of Shot Size
  • Process is not Pressure Limited
  • Cooling Line Layout
  • Check Ring Performance (if used)
  • Screw & Barrel Heating and Cooling Equipment
• Look for obstructions or faulty equipment at barrel, nozzle, gate, or hot runner

Possible Causes: Recovery

Low Material Temperature or Back Pressure

A cold material might cause excessive pressure loss during injection & packing, which can result in overpacking near the gate area. This can also reduce the amount of gelation or fusion the PVC & CPVC experiences during the molding process. Returning the material temperature and back pressure to match the process-specific documentation should reduce the pressure loss in the mold. It may be necessary to check the thermocouples & heater bands on the barrel & hot runner system. Raising the material temperature or back pressure may also be needed to add more temperature or shear into the polymer to improve gelation or fusion.

High Material Temperature or Back Pressure

A high temperature or back pressure can cause the heat sensitive PVC & CPVC to degrade, which will reduce the polymer strength. Returning the material temperature and back pressure to match the process-specific documentation should help these situations. You may need to further reduce the barrel temperature or back pressure if material degradation is suspected. You should always purge the barrel when degradation is suspected with PVC & CPVC. It may be necessary to check the cooling fans, screw cooling, thermocouples, & heater bands.

Low Recovery Time

A high Screw RPM can cause inconsistent recovery and excessive shear during recovery. The screw RPM should be adjusted so that the recovery time consumes most of the cooling time.

Low Barrel Residence Time

A short barrel residence time may prevent the PVC & CPVC from fully melting during recovery. This low-temperature material causes excessive pressure loss during injection and may not have enough pressure available to properly pack the part. A low barrel residence time can also reduce the amount of gelation or fusion the PVC & CPVC experiences during the molding process. This situation can be due to a significantly smaller barrel being used than was used on the documented standard. In some cases, barrel temperature or back pressure can be increased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

High Barrel Residence Time

A long barrel residence time can cause the heat sensitive PVC & CPVC to heat up or degrade, which will reduce the polymer strength. In some cases, barrel temperature can be decreased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

Possible Causes: Cooling

Low Coolant Temperature or High Coolant Flow

PVC & CPVC tend to be very temperature sensitive, and a cold mold can cause excessive pressure loss during injection & packing. Cold molds can also cause molded-in stresses, which can weaken the molded part. Returning the coolant temperature to match the process-specific documentation should reduce the pressure loss in the mold. Using a coolant temperature controller with a lower-rated coolant pump or reducing the coolant flow with a flow control valve may return the water flow to match that of the documented standard.

High Coolant Temperature or Low Coolant Flow

Since PVC & CPVC tend to be molded without a gate seal, a warmer mold will change how much the gate seals. This warmer gate will affect how much material leaves the gate at the end of packing. If the molded PVC & CPVC has some degradation, a high mold temperature can slow the cooling rate and allow material degradation to continue after injection. Returning the coolant temperature to match the process-specific documentation should return the pressure loss in the mold to normal. Check the cooling system for obstructions, as this will reduce coolant flow. Using a coolant temperature controller with a higher rated coolant pump may raise the coolant flow to match the process-specific documentation.

Cooling Line Layout

An incorrect cooling line layout can affect the material flow and shrinkage. Verify the Cooling Line Layout matches the Process-Specific Documentation.

Low Cooling Time

Lower cooling times provide a lower cooling rate to the PVC & CPVC part since the water-cooled mold removes heat more efficiently than the ambient air. If the molded PVC & CPVC has some degradation, a low cooling time can allow existing material degradation to continue after part ejection. If degradation is occurring in this manner, you may not see signs of degradation on the surface of the part, but you can see degraded polymer when the part is cut open. The cooling time should match the documented standard.

High Cooling Time

A high cooling time will increase the cooling rate of the polymer, which may cause molded stresses that can weaken the molded part. The cooling time should match the documented standard.

Possible Causes: Materials

High Regrind Percentage

PVC & CPVC tend to be very temperature sensitive, and reground PVC & CPVC will cause an increase in material temperature and shrinkage. If too much regrind is being used, the barrel cooling fans may be inadequate to remove this increased heat, resulting in a weakened part due to material degradation. Reducing the regrind percentage should lower the material temperature and decrease the creation of volatiles.

Poor Quality Regrind

Regrind with any degraded PVC & CPVC will cause a dangerous chain reaction of degradation, which can weaken the part. Any regrind containing degradation or burning should be discarded before it causes more defects or a serious safety concern.

Material Degradation

Degradation of PVC & CPVC significantly reduces the strength and performance of PVC & CPVC. Removing the bad material from the barrel helps prevent degraded polymer from contaminating all other material it contacts. If a Hot Runner System is being used, purge it out as well.

Material Contamination (intermittent defect)

Any foreign materials, such as dust, will reduce the mechanical and resistance properties of your polymer. Contamination is typically a random condition that would cause some parts to fail while others may perform very well. Contamination can come from many locations, including storage, hopper, hoses, filters, screens, surge bins, access doors, seals, and blenders. All potential sources of contamination must be checked to determine where the contamination is coming from. Once the source is determined, everything that came in contact with the material afterwards must be thoroughly cleaned and inspected before returning to use.

Poor Material Drying

Although PVC & CPVC do not always need drying, they are hygroscopic polymers. The absorbed moisture will come out of the plastic during processing, which can affect the viscosity and physical properties of the molded part. Although grades vary, most PVC & CPVC resins should be dried for 1-2 hours at 65 to 80°C (150 to 175°F) with dry air (approx. -40°C or -40°F Dewpoint) to a moisture content below 0.3%.

Possible Causes: Injection

Inadequate or Blocked Venting

Poor venting can cause gas entrapment during injection. This gas can interfere with mold filling as well as weld & meld line strength. It requires a lot of pressure to force trapped gas out of the mold, which affects the amount of pressure available to pack the molded part. Molds should be vented wherever possible; they must be vented to the atmosphere, and cross-cavity venting should be avoided whenever possible. If routine vent cleaning is needed, there should be a procedure for scheduled vent cleaning before they become blocked.

Flow Obstructions or Restrictions

Check for obstructions in the nozzle, gate, or hot runner, as well as a faulty heater band on the barrel, nozzle, or hot runner system.

Low Injection Time

This indicates the material is entering the mold faster than the mold can vent the air out, causing gas entrapment. PVC & CPVC are shear-sensitive materials that can be degraded if injected too fast. Reducing injection speed will help you increase injection time to match the documented standard.

High Injection Time

This indicates material is entering the mold slowly, causing an increase in material viscosity. A higher viscosity PVC & CPVC experiences an increased pressure loss during injection and packing, which may weaken the molded part. Increasing injection speed will help you decrease injection time. You should also check your maximum injection pressure in case there is not enough injection pressure available to maintain the injection speed.

Injection Profile

PVC & CPVC typically require an injection profile to make a good part. For example, the gate area often needs a lower injection speed to reduce shear. A significant change in the injection profile will have a significant effect on the appearance of the part and gate. The injection profile should match the process-specific documentation.

Low Part Weight at Transfer

This indicates too little material is entering the mold during injection, which may reduce strength due to inadequate packing. If other parameters are similar and only the weight is different, raising shot size or decreasing transfer position should increase part weight at transfer.

High Part Weight at Transfer

This indicates too much material is entering the mold during injection and often contributes to gas entrapment. This condition may cause a cavity pressure spike during injection, but can also cause the screw to bounce back during transfer, resulting in a cavity pressure loss at transfer. If other parameters are similar and only the weight is different, lowering shot size or increasing transfer position should decrease part weight at transfer.

Low Peak or Transfer Pressure

This indicates the material viscosity has decreased, and there will be less pressure loss during injection. If the mold or material temperatures are higher than the documented standard, these can be decreased to reduce the flow of PVC & CPVC. If a change in lot number, colorant, or regrind is causing the viscosity decrease, you may need to decrease the shot size or increase the transfer position to compensate for the lower viscosity material.

High Peak or Transfer Pressure

This indicates the material viscosity has increased or there is an obstruction. Check for flow obstructions. If the mold or material temperatures are lower than the documented standard, these can be increased to allow the PVC & CPVC to flow more easily into the mold.

Pressure-Limited Process

A pressure-limited process cannot maintain the desired injection speed. If a change in lot number, colorant, or regrind is causing a viscosity increase, you may need to raise the shot size or decrease the transfer position to compensate for the higher viscosity material. Flow obstructions and faulty equipment should always be checked when the maximum pressure is reached. In some cases, an increase in the maximum injection pressure is needed to accommodate a higher viscosity material, but the remainder of the process should be verified to ensure there are no other causes for a high injection pressure before this change is made.

Possible Causes: Packing

Low Packing Pressure

This indicates there is not enough pressure to fill the mold and compensate for shrinkage during packing. An under-packed part will typically have reduced strength. The packing pressure should be set to match the process-specific documentation, and the packing speed should be set to the same value as your last injection speed before transfer to packing.

High Packing Pressure

This indicates there is too much pressure after mold filling, which might over-pack the part. An over-packed part typically has molded-in stresses, which can weaken the final product. The packing pressure should be set to match the process-specific documentation, and the packing speed should be set to the same value as your last injection speed before transferring from injection to packing.

Low Packing Time

With PVC & CPVC, it is difficult to get a full gate seal at the end of packing. Insufficient packing time will cause material to leave the part in the runner or sprue. This tends to weaken the part in the gate area. A packing time study should be done to ensure the packing time is adequate to achieve a good gate appearance and performance.

High Packing Time

With PVC & CPVC, it is difficult to get a full gate seal at the end of packing. Excessive packing time may cause overpacking and stress in the gate area. A packing time study should be done to ensure the packing time is adequate to achieve a good gate appearance and performance.

Low Packing Speed

A low packing speed will artificially slow down the screw at transfer, which can cause under packing. An under-packed part is typically weaker than expected. To avoid complications, the packing speed should be set to the same value as your last injection speed before transferring to packing.

Low Final Part Weight

This indicates insufficient material is entering the mold cavity during Injection and packing. This is indicative of a change in the process, but you need to find other changes in the process before action can be taken.

High Final Part Weight

This indicates excessive material is entering the mold cavity during Injection and packing. This is indicative of a change in the process, but you need to find other changes in the process before action can be taken.

High Clamp Force

Excessive clamp force will prevent the air from venting as fast as the material is entering the mold. This gas can interfere with mold filling as well as weld & meld line strength. It requires a lot of pressure to force trapped gas out of the mold, which reduces the amount of pressure available to pack the molded part. The clamp tonnage should be set to match the process-specific documentation. You will likely need to clean the vents at this time.

No Cushion

A process without a cushion does not have enough material to fill the part and compensate for shrinkage during packing. This results in an under-packed part, which is typically weaker than expected. The cushion should always be approximately 10% of the shot size. Both the shot size and transfer position must be increased to raise the cushion.

High Cushion

Melted PVC & CPVC are highly compressible plastics. Thus, a process with an excessive cushion, such as 25% of the shot size, can lose significant injection & packing pressure. The cushion should always be approximately 10% of the Shot Size. Both the shot size and transfer position must be decreased to lower the cushion.

Possible Causes: Machine

Check Ring Performance (Intermittent Defect)

If a check ring is being used, perform a check ring performance study by weighing 10 consecutive short shots (weight all cavities + runner) and calculating variability. Over 2% variability means the check ring is suspect, and should be changed immediately if over 3% Variability. Purging the Barrel or increasing Decompression after Recovery may improve check ring performance.

% Variability = (100%) * (Max - Min) / (Average)

Possible Causes: Mold & Design

High Cavity Imbalance (Cavity to Cavity Variation)

Significant differences in cavity filling can cause some cavities to fill and start packing during injection, while other cavities are significantly short at transfer. Make sure all the mold cavities are short at the time of transfer to minimize any cavity-to-cavity variation in the final molded part. There are steps that can improve cavity imbalance, such as using a different injection speed, adjusting hot runner gate drops, cleaning the mold vents, increasing the number of vents, or adjusting gate width when necessary.

Design Concerns

Good part design strategies, such as gating into thick sections, avoiding sharp transitions, maintaining the same thickness when possible, and providing adequate venting, are important to improving mold filling. Increasing wall thickness will also increase the Mechanical or Resistance properties of PVC & CPVC. Since it is difficult to achieve full theoretical strength with PVC & CPVC, a higher safety factor should be used than is common with most polymers. For example, a designer may typically design a Polypropylene part 15% thicker as a safety margin, but a PVC & CPVC part should be 25% or 30% thicker than needed to get a similar safety margin.

Initial Troubleshooting

• Purge the Barrel
• Verify Process:
  • Material Temperature & Back Pressure
  • Barrel Residence Time
  • Coolant Temperature, Pressure, & Flow
  • Recovery Time
  • Cooling Time
  • Injection Time & Profile
  • Part Weight at Transfer (no runner or sprue)
  • Peak and Transfer Pressure
  • Packing Pressure, Time, & Speed
  • Final Part Weight
• Check Material Drying:
  • Material Moisture
  • Dryer Residence Time
  • Dryer Temperature
• Review Part Removal
• Check:
  • Screw & Barrel Cooling Equipment
  • Cushion is approximately 10% of Shot Size
  • Process is not Pressure Limited
  • Cooling Line Layout
  • Check Ring Performance (if used)
• Look for obstructions or faulty equipment at the barrel, nozzle, gate, or hot runner

Possible Causes: Recovery

Low Material Temperature or Back Pressure

A cold material might cause excessive pressure loss during injection & packing, which can result in overpacking near the gate area. This can also reduce the amount of gelation or fusion the PVC & CPVC experiences during the molding process. Returning the material temperature and back pressure to match the process-specific documentation should reduce the pressure loss in the mold. It may be necessary to check the thermocouples & heater bands on the barrel & hot runner system. Raising the material temperature or back pressure may also be needed to add more temperature or shear into the polymer to improve gelation or fusion.

High Material Temperature or Back Pressure

A high temperature or back pressure can cause the heat-sensitive PVC & CPVC to degrade, which will reduce the polymer strength. Returning the material temperature and back pressure to match the process-specific documentation should help these situations. You may need to further reduce the barrel temperature or back pressure if material degradation is suspected. You should always purge the barrel when degradation is suspected with PVC & CPVC. It may be necessary to check the cooling fans, screw cooling, thermocouples, & heater bands.

Low Recovery Time

A high Screw RPM can cause inconsistent recovery and excessive shear during recovery. The screw RPM should be adjusted so that the recovery time consumes most of the cooling time.

Low Barrel Residence Time

A short barrel residence time may prevent the PVC & CPVC from fully melting during recovery. This low-temperature material causes excessive pressure loss during injection and may not have enough pressure available to properly pack the part. A low barrel residence time can also reduce the amount of gelation or fusion the PVC & CPVC experiences during the molding process. This situation can be due to a significantly smaller barrel being used than was used on the documented standard. In some cases, barrel temperature or back pressure can be increased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

High Barrel Residence Time

A long barrel residence time can cause the heat-sensitive PVC & CPVC to heat up or degrade, which will reduce the polymer strength. In some cases, barrel temperature can be decreased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

Possible Causes: Cooling

Low Coolant Temperature or High Coolant Flow

PVC & CPVC tend to be very temperature sensitive, and a cold mold can cause excessive pressure loss during injection & packing. Cold molds can also cause molded-in stresses, which can weaken the molded part. Returning the coolant temperature to match the process-specific documentation should reduce the pressure loss in the mold. Using a coolant temperature controller with a lower-rated coolant pump or reducing the coolant flow with a flow control valve may return the water flow to match that of the documented standard.

High Coolant Temperature or Low Coolant Flow

Since PVC & CPVC tend to be molded without a gate seal, a warmer mold will change how much the gate seals. This warmer gate will affect how much material leaves the gate at the end of packing. If the molded PVC & CPVC has some degradation, a high mold temperature can slow the cooling rate and allow material degradation to continue after injection. Returning the coolant temperature to match the process-specific documentation should return the pressure loss in the mold to normal. Check the cooling system for obstructions, as this will reduce coolant flow. Using a coolant temperature controller with a higher-rated coolant pump may raise the coolant flow to match the process-specific documentation.

Cooling Line Layout

An incorrect cooling line layout can affect the material flow and shrinkage. Verify the Cooling Line Layout matches the Process-Specific Documentation.

Low Cooling Time

Lower cooling times provide a lower cooling rate to the PVC & CPVC part since the water-cooled mold removes heat more efficiently than the ambient air. If the molded PVC & CPVC has some degradation, a low cooling time can allow existing material degradation to continue after part ejection. If degradation is occurring in this manner, you may not see signs of degradation on the surface of the part, but you can see degraded polymer when the part is cut open. The cooling time should match the documented standard.

High Cooling Time

A high cooling time will increase the cooling rate of the polymer, which may cause molded-in stresses that can weaken the molded part. The cooling time should match the documented standard.

Possible Causes: Materials

High Regrind Percentage

PVC & CPVC tend to be very temperature sensitive, and reground PVC & CPVC will cause an increase in material temperature and shrinkage. If too much regrind is being used, the barrel cooling fans may be inadequate in removing this increased heat, resulting in a weakened part due to material degradation. Reducing the regrind percentage should lower the material temperature and decrease the creation of volatiles.

Poor Quality Regrind

Regrind with any degraded PVC & CPVC will cause a dangerous chain reaction of degradation, which can weaken the part. Any regrind containing degradation or burning should be discarded before it causes more defects or a serious safety concern.

Material Degradation

Degradation of PVC & CPVC significantly reduces the strength and performance of PVC & CPVC. Removing the bad material from the barrel helps prevent degraded polymer from contaminating all other material it contacts. If a Hot Runner System is being used, purge it out as well.

Material Contamination (intermittent defect)

Any foreign materials, such as dust, will reduce the mechanical and resistance properties of your polymer. Contamination is typically a random condition that would cause some parts to fail while others may perform very well. Contamination can come from many locations, including storage, hopper, hoses, filters, screens, surge bins, access doors, seals, and blenders. All potential sources of contamination must be checked to determine where the contamination is coming from. Once the source is determined, everything that came in contact with the material afterwards must be thoroughly cleaned and inspected before returning to use.

Poor Material Drying

Although PVC & CPVC do not always need drying, they are hygroscopic polymers. The absorbed moisture will come out of the plastic during processing, which can affect the viscosity and physical properties of the molded part. Although grades vary, most PVC & CPVC resins should be dried for 1-2 hours at 65 to 80°C (150 to 175°F) with dry air (approx. -40°C or -40°F Dewpoint) to a moisture content below 0.3%.

Possible Causes: Injection

Inadequate or Blocked Venting

Poor venting can cause gas entrapment during injection. This gas can interfere with mold filling as well as weld & meld line strength. It requires a lot of pressure to force trapped gas out of the mold, which affects the amount of pressure available to pack the molded part. Molds should be vented wherever possible; they must be vented to the atmosphere, and cross-cavity venting should be avoided whenever possible. If routine vent cleaning is needed, there should be a procedure for scheduled vent cleaning before they become blocked.

Flow Obstructions or Restrictions

Check for obstructions in the nozzle, gate, or hot runner as well as a faulty heater band on the barrel, nozzle, or hot runner system.

Low Injection Time

This indicates the material is entering the mold faster than the mold can vent the air out, causing gas entrapment. PVC & CPVC are shear-sensitive materials that can be degraded if injected too fast. Reducing injection speed will help you increase injection time to match the documented standard.

High Injection Time

This indicates material is entering the mold slowly, causing an increase in material viscosity. A higher viscosity PVC & CPVC experiences an increased pressure loss during injection and packing, which may weaken the molded part. Increasing injection speed will help you decrease injection time. You should also check your maximum injection pressure in case there is not enough injection pressure available to maintain the injection speed.

Injection Profile

PVC & CPVC typically require an injection profile to make a good part. For example, the gate area often needs a lower injection speed to reduce shear. A significant change in the injection profile will have a significant effect on the appearance of the part and gate. The injection profile should match the process-specific documentation.

Low Part Weight at Transfer

This indicates too little material is entering the mold during injection, which may reduce strength due to inadequate packing. If other parameters are similar and only the weight is different, raising shot size or decreasing transfer position should increase part weight at transfer.

High Part Weight at Transfer

This indicates too much material is entering the mold during injection and often contributes to gas entrapment. This condition may cause a cavity pressure spike during injection, but can also cause the screw to bounce back during transfer, resulting in a cavity pressure loss at transfer. If other parameters are similar and only the weight is different, lowering shot size or increasing transfer position should decrease part weight at transfer.

Low Peak or Transfer Pressure

This indicates the material viscosity has decreased, and there will be less pressure loss during injection. If the mold or material temperatures are higher than the documented standard, these can be decreased to reduce the flow of PVC & CPVC. If a change in lot number, colorant, or regrind is causing the viscosity decrease, you may need to decrease the shot size or increase the transfer position to compensate for the lower viscosity material.

High Peak or Transfer Pressure

This indicates the material viscosity has increased or there is an obstruction. Check for flow obstructions. If the mold or material temperatures are lower than the documented standard, these can be increased to allow the PVC & CPVC to flow more easily into the mold.

Pressure-Limited Process

A pressure-limited process cannot maintain the desired injection speed. If a change in lot number, colorant, or regrind is causing a viscosity increase, you may need to raise the shot size or decrease the transfer position to compensate for the higher viscosity material. Flow obstructions and faulty equipment should always be checked when the maximum pressure is reached. In some cases, an increase in the maximum injection pressure is needed to accommodate a higher viscosity material, but the remainder of the process should be verified to ensure there are no other causes for a high injection pressure before this change is made.

Possible Causes: Packing

Low Packing Pressure

This indicates there is not enough pressure to fill the mold and compensate for shrinkage during packing. An under packed part will typically have reduced strength. The packing pressure should be set to match the process-specific documentation and the packing speed should be set to the same value as your last injection speed before transfer to packing.

High Packing Pressure

This indicates there is too much pressure after mold filling which might over-pack the part. An over-packed part typically has molded-in stresses, which can weaken the final product. The packing pressure should be set to match the process-specific documentation, and the packing speed should be set to the same value as your last injection speed before transfer from injection to packing.

Low Packing Time

With PVC & CPVC, it is difficult to get a full gate seal at the end of packing. Insufficient packing time will cause material to leave the part in the runner or sprue. This tends to weaken the part in the gate area. A packing time study should be done to ensure the packing time is adequate to achieve a good gate appearance and performance.

High Packing Time

With PVC & CPVC, it is difficult to get a full gate seal at the end of packing. Excessive packing time may cause overpacking and stress in the gate area. A packing time study should be done to ensure the packing time is adequate to achieve a good gate appearance and performance.

Low Packing Speed

A low packing speed will artificially slow down the screw at transfer, which can cause under packing. An under-packed part is typically weaker than expected. To avoid complications, the packing speed should be set to the same value as your last injection speed before transferring to packing.

Low Final Part Weight

This indicates insufficient material is entering the mold cavity during Injection and packing. This is indicative of a change in the process, but you need to find other changes in the process before action can be taken.

High Final Part Weight

This indicates excessive material is entering the mold cavity during Injection and packing. This is indicative of a change in the process, but you need to find other changes in the process before action can be taken.

High Clamp Force

Excessive clamp force will prevent the air from venting as fast as the material is entering the mold. This gas can interfere with mold filling as well as weld & meld line strength. It requires a lot of pressure to force trapped gas out of the mold, which reduces the amount of pressure available to pack the molded part. The clamp tonnage should be set to match the process-specific documentation. You will likely need to clean the vents at this time.

No Cushion

A process without a cushion does not have enough material to fill the part and compensate for shrinkage during packing. This results in an under-packed part, which is typically weaker than expected. The cushion should always be approximately 10% of the shot size. Both the shot size and transfer position must be increased to raise the cushion.

High Cushion

Melted PVC & CPVC are highly compressible plastics; thus, a process with an excessive cushion, such as 25% of the shot size, can lose significant injection & packing pressure. The cushion should always be approximately 10% of the Shot Size. Both the shot size and transfer position must be decreased to lower the cushion.

Possible Causes: Machine

Check Ring Performance (Intermittent Defect)

If a check ring is being used, perform a check ring performance study by weighing 10 consecutive short shots (weight all cavities + runner) and calculating variability. Over 2% variability means the check ring is suspect, and should be changed immediately if over 3% Variability. Purging the Barrel or increasing Decompression after Recovery may improve check ring performance.

% Variability = (100%) * (Max - Min) / (Average)

Machine Settings

It is very easy to damage the part during part removal. In most cases, higher mold breakaway and ejection speeds will increase the stresses applied to the molded part during part removal.

Possible Causes: Mold & Design

High Cavity Imbalance (Cavity to Cavity Variation)

Significant differences in cavity filling can cause some cavities to fill and start packing during injection, while other cavities are significantly short at transfer. Make sure all the mold cavities are short at the time of transfer to minimize any cavity-to-cavity variation in the final molded part. There are steps that can improve cavity imbalance, such as using a different injection speed, adjusting hot runner gate drops, cleaning the mold vents, increasing the number of vents, or adjusting gate width when necessary.

Design Concerns

Good part design strategies, such as gating into thick sections, avoiding sharp transitions, maintaining the same thickness when possible, and providing adequate venting, are important to improving mold filling. Increasing wall thickness will also increase the Mechanical or Resistance properties of PVC & CPVC. Since it is difficult to achieve full theoretical strength with PVC & CPVC, a higher safety factor should be used than is common with most polymers. For example, a designer may typically design a Polypropylene part 15% thicker as a safety margin, but a PVC & CPVC part should be 25% or 30% thicker than needed to get a similar safety margin. Decreasing undercuts, increasing draft angles, increasing ejection components, and draw polishing cores and cavities will help the part separate from the mold during the part removal process.

Initial Troubleshooting

• Verify Process:
  • Material Temperature & Back Pressure
  • Barrel Residence Time
  • Recovery Time
  • Cooling Time
• Check Screw & Barrel Heating and Cooling Equipment

Possible Causes: Recovery

Low Material Temperature or Back Pressure

Low temperature PVC & CPVC may not adequately melt during recovery. Returning the Material Temperature and Back Pressure to match the Process-Specific Documentation should reduce gels in the molded part. It may be necessary to check the thermocouples & heater bands on the barrel & hot runner system.

Low Recovery Time

A high Screw RPM can cause inconsistent recovery and may force unmelted pellets or regrind to the front of the screw. The Screw RPM should be adjusted so the Recovery Time consumes most of the Cooling Time.

Low Barrel Residence Time

A short barrel residence time may prevent the PVC & CPVC from fully melting during recovery. In some cases, barrel temperature or back pressure can be increased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

Possible Causes: Materials

High Regrind Percentage

Regrind tends to be comprised of pieces larger than the base resin. These large regrind pieces can take longer than pellets or powder to melt. Reducing the screen size in the grinder or incorporating a pulverizer will reduce the size of the regrind pellets that must be melted during recovery. Reducing the regrind percentage will also reduce the number of large regrind pieces that must be melted.

Material Contamination (intermittent defect)

Any foreign materials that do not melt may appear as gels in the final part. Contamination is typically a random condition that would cause some parts to fail while others may perform very well. Contamination can come from many locations, including storage, hopper, hoses, filters, screens, surge bins, access doors, seals, and blenders. All potential sources of contamination must be checked to determine where the contamination is coming from. Once the source is determined, everything that comes in contact with the material afterwards must be thoroughly cleaned and inspected before returning to use.

Initial Troubleshooting

• Verify Process:
  • Material Temperature & Back Pressure
  • Barrel Residence Time
  • Cooling Time
• Rear Zone Temperature Study
• Check Screw & Barrel Heating and Cooling Equipment
• Check Material Drying:
  • Material Moisture
  • Dryer Residence Time
  • Dryer Temperature
• Pre-Heat Material with Hopper Dryer

Possible Causes: Recovery

Low Material Temperature

Low temperature PVC & CPVC has a very high viscosity and may be difficult to pump to the front of the screw. Returning the material temperature and back pressure to match the process-specific documentation improves the recovery time. It may be necessary to check the thermocouples & heater bands on the barrel & hot runner system.

High Back Pressure

Higher back pressures require the screw to pump harder to accumulate plastic to the front of the screw. Decreasing the back pressure should help reduce screw recovery time.

High Recovery Time

Initially, the screw RPM should be increased to attempt a reduction in the recovery time. Amorphous Polymers such as PVC & CPVC convey best in the feed zone when the rear zone barrel temperature is relatively low, thus a rear zone temperature study can help select the right temperature. A rear zone temperature study consists of stabilizing the molding process with a relatively high rear zone temperature and documenting the recovery time. Then, the rear zone temperature is lowered, the process is stabilized, and the recovery time is recorded. This process is repeated until you reach the lower end of the manufacturer’s recommended temperature range. The best rear zone temperature is the one that provides the lowest recovery time without causing degradation.

Possible Causes: Materials

High Regrind Percentage

Regrind tends to be comprised of pieces larger than the base resin. These large regrind pieces can take longer than pellets or powder to melt. Reducing the screen size in the grinder or incorporating a pulverizer will reduce the size of the regrind pellets that must be melted during recovery. Reducing the regrind percentage will also reduce the number of large regrind pieces that must be melted.

Possible Causes: Machine

Machine Settings

If the machine is set with sprue break, low nozzle contact force, incorrect nozzle diameter, or incorrect nozzle alignment, the material may drool instead of building a shot. Look for drooling buildup or leaking around the nozzle. If this is occurring, determine what the cause is and correct it immediately.

Initial Troubleshooting

• Verify Process:
  • Material Temperature & Back Pressure
  • Barrel Residence Time
  • Recovery Time
  • Cooling Time
  • Injection Time & Profile
  • Packing Time
• Rear Zone Temperature Study
• Check Screw & Barrel Heating and Cooling Equipment
• Pre-Heat Material with Hopper Dryer

Possible Causes: Recovery

Low Material Temperature

Low temperature PVC & CPVC have a very high viscosity and may be difficult to pump to the front of the screw. Returning the material temperature and back pressure to match the process-specific documentation improves the recovery time. It may be necessary to check the thermocouples & heater bands on the barrel & hot runner system.

High Back Pressure

Higher back pressures require the screw to pump harder to accumulate plastic to the front of the screw. Decreasing the back pressure should help reduce screw recovery time.

High Recovery Time

Initially, the screw RPM should be increased to attempt a reduction in the recovery time. Amorphous Polymers such as PVC & CPVC convey best in the feed zone when the rear zone barrel temperature is relatively low, thus a rear zone temperature study can help select the right temperature. A rear zone temperature study consists of stabilizing the molding process with a relatively high rear zone temperature and documenting the recovery time. Then the rear zone temperature is lowered, the process is stabilized, and the recovery time is recorded. This process is repeated until you reach the lower end of the manufacturer’s recommended temperature range. The best rear zone temperature is the one that provides the lowest recovery time without causing degradation.

Possible Causes: Cooling

High Cooling Time

A high cooling time will delay mold opening and cause an increase in the overall cycle time. The cooling time should match the documented standard.

Possible Causes: Materials

High Regrind Percentage

Regrind tends to be comprised of pieces larger than the base resin. These large regrind pieces can take longer than pellets or powder to melt. Reducing the screen size in the grinder or incorporating a pulverizer will reduce the size of the regrind pellets that must be melted during recovery. Reducing the regrind percentage will also reduce the number of large regrind pieces that must be melted.

Possible Causes: Injection

High Injection Time

This indicates material is entering the mold slowly, resulting in an overall longer cycle time. Increasing Injection Speed will help you decrease Injection Time. You should also check your maximum injection pressure in case there is not enough injection pressure available to maintain the injection speed.

Injection Profile

PVC & CPVC typically require an injection profile to make a good part. A significant change in the injection profile will result in a significant increase in the injection time and cycle time. The injection profile should match the process-specific documentation.

Pressure-Limited Process

A pressure-limited process cannot maintain the desired injection speed. If a change in lot number, colorant, or regrind is causing a viscosity increase, you may need to raise the shot size or decrease the transfer position to compensate for the higher viscosity material. Flow obstructions and faulty equipment should always be checked when the maximum pressure is reached. In some cases, an increase in the maximum injection pressure is needed to accommodate a higher viscosity material, but the remainder of the process should be verified to ensure there are no other causes for a high injection pressure before this change is made.

Possible Causes: Packing

High Packing Time

Excessive packing time will increase the overall cycle time. A packing time study should be done to ensure the packing time is adequate to achieve a good gate appearance.

Possible Causes: Machine

Machine Settings

Verify the clamp and part removal functions are correctly set and working properly. An incorrectly set mold protect may increase the part removal time. Increasing the mold open, mold close, ejection forward, & ejection retract speeds will decrease the overall cycle time. If this is a hydraulic machine, verify that the hydraulic oil is not overheating, as this will usually slow down all machine functions, including the clamp and ejectors. Verify the quality of the oil in the hydraulic molding machine has been tested by a lab, as low-quality hydraulic fluid will slow the speed of the molding machine components. Verify adequate clamp and ejection force are available to maintain their set speeds.

Robot Settings & Condition

If there is significant wear or poor lubrication, the robot may not be capable of reaching the desired clamp and ejector speeds, which will increase the cycle time. Inadequate force settings will cause the robot to slow down, resulting in a longer part removal time.

Poor Clamp Condition

If there is significant wear or poor lubrication, the machine may not be capable of reaching the desired clamp and ejector speeds, resulting in an increased part removal time as well as increased wear on the machine components.

Initial Troubleshooting

• Purge Barrel
• Verify Process:
  • Material Temperature & Back Pressure
  • Barrel Residence Time
  • Recovery Time
  • Cooling Time
  • Injection Time & Profile
  • Part Weight at Transfer (no runner or sprue)
  • Peak and Transfer Pressure
• Verify Setup Sheet
  • Decompression After Recovery
  • Nozzle or Hot Runner Gate Drop Temperature
• Check Screw & Barrel Heating and Cooling Equipment

Possible Causes: Recovery

High Material Temperature or Back Pressure

A high temperature or back pressure can cause the heat-sensitive PVC & CPVC to degrade, which will reduce the polymer strength and contribute to strings. Returning the material temperature and back pressure to match the process-specific documentation should help these situations. You should always purge the barrel when degradation is suspected with PVC & CPVC. It may be necessary to check the cooling fans, screw cooling, thermocouples, & heater bands.

Low Recovery Time

A high Screw RPM can cause inconsistent recovery and excessive shear during recovery. The screw RPM should be adjusted so that the recovery time consumes most of the cooling time.

High Barrel Residence Time

A long barrel residence time can cause the heat-sensitive PVC & CPVC to heat up or degrade, which will reduce the polymer strength. In some cases, barrel temperature can be decreased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

Possible Causes: Materials

High Regrind Percentage

PVC & CPVC tend to be very temperature sensitive, and reground PVC & CPVC will cause an increase in material temperature and a decrease in material viscosity. If too much regrind is being used, the barrel cooling fans may be inadequate to remove this increased heat, resulting in a weakened part due to material degradation. Reducing the regrind percentage should lower the material temperature and decrease the creation of volatiles.

Poor Quality Regrind

Regrind with any degraded PVC & CPVC will cause a dangerous chain reaction of degradation, which can weaken the polymer. Any regrind containing degradation or burning should be discarded before it causes more defects or a serious safety concern.

Material Degradation

Degradation of PVC & CPVC significantly reduces the strength and performance of PVC & CPVC. Removing the bad material from the barrel helps prevent degraded polymer from contaminating all other material it contacts. If a Hot Runner System is being used, purge it out as well.

Poor Material Drying

Although PVC & CPVC do not always need drying, they are hygroscopic polymers. The absorbed moisture will come out of the plastic during processing, which can reduce the viscosity and physical properties of the polymer. Although grades vary, most PVC & CPVC resins should be dried for 1-2 hours at 65 to 80°C (150 to 175°F) with dry air (approx. -40°C or -40°F Dewpoint) to a moisture content below 0.3%.

Possible Causes: Injection

Flow Obstructions or Restrictions

Check for obstructions in the nozzle, gate, or hot runner as well as a faulty heater band on the barrel, nozzle, or hot runner system.

Low Injection Time

This indicates the material is entering the mold faster with increased shear at the gate. PVC & CPVC are shear-sensitive materials that can be degraded and weakened if injected too fast. Reducing injection speed will help you increase injection time to match the documented standard.

Injection Profile

PVC & CPVC typically require an injection profile to make a good part. For example, the gate area often needs a lower injection speed to reduce shear. A significant change in the injection profile will have a significant effect on the shear applied to the PVC & CPVC. The Injection Profile should match the Process-Specific Documentation.

High Part Weight at Transfer

This indicates too much material is entering the mold during injection. This condition may cause a cavity pressure spike during injection, but can also cause the screw to bounce back during transfer, resulting in material backflow through the gate. This material backflow creates a lot of stress at the gate, which can weaken the polymer. If other parameters are similar and only the weight is different, lowering shot size or increasing transfer position should decrease part weight at transfer.

Low Peak or Transfer Pressure

This indicates the material viscosity has decreased, and there will be less pressure loss during injection. If the mold or material temperatures are higher than the documented standard, these can be decreased to reduce the flow of PVC & CPVC. If a change in lot number, colorant, or regrind is causing the viscosity decrease, you may need to decrease the shot size or decrease the material temperature to compensate for the lower viscosity material.

High Peak or Transfer Pressure

This indicates the material viscosity has increased or there is an obstruction. Check for flow obstructions that might be causing shear and weakening the polymer.

Possible Causes: Packing

Low Packing Pressure

Too low a packing pressure might cause the screw to bounce back during transfer, resulting in material backflow through the gate. This material backflow creates a lot of stress at the gate, which can weaken the polymer. The packing pressure should be set to match the process-specific documentation, and the packing speed should be set to the same value as your last Injection Speed before Transfer from Injection to Packing.

Low Packing Time

With PVC & CPVC, it is difficult to get a full gate seal at the end of packing. Insufficient packing time will cause material to leave the part and backflow into the runner or sprue. This tends to weaken the polymer in the gate area and may cause strings. A packing time study should be done to ensure the packing time is adequate to achieve a good gate appearance.

High Packing Time

With PVC & CPVC, it is difficult to get a full gate seal at the end of packing. Excessive packing time may cause overpacking and stress in the gate area. A packing time study should be done to ensure the packing time is adequate to achieve a good gate appearance.

Low Final Part Weight

This indicates insufficient material is entering the mold cavity during Injection and packing. This is indicative of a change in the process, but you need to find other changes in the process before action can be taken.

High Final Part Weight

This indicates excessive material is entering the mold cavity during injection and packing. This is indicative of a change in the process, but you need to find other changes in the process before action can be taken.

No Cushion

A process without a cushion does not have enough material to properly pack the part, often causing the screw to remain stationary. This stationary screw is no longer pushing plastic; as a result, the material begins to backflow through the gate. This tends to weaken the polymer in the gate area and may cause strings. The cushion should always be approximately 10% of the shot size. Both the shot size and transfer position must be increased to raise the cushion.

Possible Causes: Machine

Machine Settings

Sprue break can often be used to separate the nozzle from the sprue bushing after recovery. Sprue break can sometimes prevent stringing, but sometimes contributes to drooling. Although PVC & CPVC often run in machines without check rings, decompression after recovery can be used to help relieve this pressure in front of the screw before mold opening. This reduction in pressure helps keep the polymer inside the nozzle or hot runner when the mold opens. When processing PVC & CPVC with a cold runner system, the molding machine should use a full taper nozzle tip. This tip has a large diameter at the nozzle, which tapers down all the way to the orifice diameter. The full taper nozzle tip helps reduce shear while promoting a clean break from the sprue when the mold opens. The nozzle tip temperature can be reduced in some cases to improve stringing.

Possible Causes: Mold & Design

High Cavity Imbalance (Cavity to Cavity Variation)

Significant differences in cavity filling can cause some cavities to fill and start packing during injection, while other cavities are significantly short at transfer. Make sure all the mold cavities are short at the time of transfer to minimize any cavity-to-cavity variation in the final molded part. There are steps that can improve cavity imbalance, such as using a different injection speed, adjusting hot runner gate drops, cleaning the mold vents, increasing the number of vents, or adjusting gate width when necessary.

Part or Mold Design

Hot Runner Systems can be dangerous with PVC & CPVC if the polymer is left in it for any extended period of time. To reduce the risk of material flow stagnation, externally heated hot runner drops should be used with high flow gate tips. Removable cavity plates are beneficial to allow for safe & quick removal of the PVC & CPVC plastic at the gate tip once the hot runner system is properly purged. The gate tip temperature can be reduced in some cases to improve stringing.

Initial Troubleshooting

• Purge Barrel
• Verify Process:
  • Material Temperature & Back Pressure
  • Barrel Residence Time
  • Recovery Time
  • Cooling Time
• Verify Setup Sheet
  • Decompression After Recovery
  • Nozzle or Hot Runner Gate Drop Temperature
• Check Screw & Barrel Heating and Cooling Equipment

Possible Causes: Recovery

High Material Temperature or Back Pressure

A high temperature or back pressure can cause the heat-sensitive PVC & CPVC to degrade, which will reduce the polymer strength and contribute to drooling. Returning the material temperature and back pressure to match the Process-Specific Documentation should help these situations. You may need to further reduce the barrel temperature or back pressure if material degradation is suspected. You should always purge the barrel when degradation is suspected with PVC & CPVC. It may be necessary to check the cooling fans, screw cooling, thermocouples, & heater bands.

Low Recovery Time

A high Screw RPM can cause inconsistent recovery and excessive shear during recovery. The screw RPM should be adjusted so that the recovery time consumes most of the cooling time.

High Barrel Residence Time

A long barrel residence time can cause the heat-sensitive PVC & CPVC to heat up or degrade, which will reduce the polymer strength and build up pressure in the barrel. In some cases, barrel temperature can be decreased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

Possible Causes: Materials

High Regrind Percentage

PVC & CPVC tend to be very temperature sensitive, and reground PVC & CPVC will cause an increase in material temperature and a decrease in material viscosity. If too much regrind is being used, the barrel cooling fans may be inadequate to remove this increased heat, resulting in pressure buildup in the barrel due to material degradation. Reducing the regrind percentage should lower the material temperature and decrease the creation of volatiles.

Poor Quality Regrind

Regrind with any degraded PVC & CPVC will cause a dangerous chain reaction of degradation which can weaken the polymer. Any regrind containing degradation or burning should be discarded before it causes more defects or a serious safety concern.

Material Degradation

Degradation of PVC & CPVC significantly reduces the strength and performance of PVC & CPVC. Removing the bad material from the barrel helps prevent degraded polymer from contaminating all other material it contacts. If a Hot Runner System is being used, purge it out as well.

Poor Material Drying

Although PVC & CPVC do not always need drying, they are hygroscopic polymers. The absorbed moisture will turn to steam in the barrel, reducing the viscosity as well as building up pressure. Although grades vary, most PVC & CPVC resins should be dried for 1-2 hours at 65 to 80°C (150 to 175°F) with dry air (approx. -40°C or -40°F Dewpoint) to a moisture content below 0.3%.

Possible Causes: Machine

Machine Settings

Although PVC & CPVC often run in machines without check rings, decompression after recovery can be used to help relieve this pressure in front of the screw before mold opening. This reduction in pressure helps keep the polymer inside the nozzle or hot runner when the mold opens. When processing PVC & CPVC with a cold runner system, the molding machine should use a full taper nozzle tip. This tip has a large diameter at the nozzle, which tapers down all the way to the orifice diameter. The full taper nozzle tip helps reduce shear while promoting a clean break from the sprue when the mold opens. The nozzle tip temperature can be reduced in some cases to improve drooling.

Possible Causes: Mold & Design

Part or Mold Design

Hot Runner Systems can be dangerous with PVC & CPVC if the polymer is left in it for any extended period of time. To reduce the risk of material flow stagnation, externally heated hot runner drops should be used with high flow gate tips. Removable cavity plates are beneficial to allow for safe & quick removal of the PVC & CPVC plastic at the gate tip once the hot runner system is properly purged. The gate tip temperature can be reduced in some cases to improve drooling.

Initial Troubleshooting

• Purge Barrel
• Verify Process:
  • Material Temperature & Back Pressure
  • Barrel Residence Time
  • Recovery Time
  • Coolant Temperature, Pressure, & Flow
  • Cooling Time
• Verify Setup Sheet
  • Decompression After Recovery
  • Nozzle or Hot Runner Gate Drop Temperature
• Check Screw & Barrel Heating and Cooling Equipment

Possible Causes: Recovery

Low Material Temperature or Back Pressure

PVC & CPVC tend to be very temperature sensitive, and a cold material will have a very high viscosity. Returning the material temperature and back pressure to match the process-specific documentation should help the polymer flow. It may be necessary to check the thermocouples & heater bands on the barrel & hot runner system. Raising the material temperature or back pressure may also be needed.

Low Recovery Time

A high Screw RPM can cause inconsistent recovery and excessive shear during recovery. The screw RPM should be adjusted so that the recovery time consumes most of the cooling time.

Low Barrel Residence Time

A short barrel residence time may prevent the PVC & CPVC from fully melting during recovery. This low-temperature material will have a very high viscosity. This situation can be due to a significantly smaller barrel being used than was used on the documented standard. In some cases, barrel temperature or back pressure can be increased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

Possible Causes: Cooling

Low Coolant Temperature or High Coolant Flow

PVC & CPVC tend to be very temperature sensitive, and a cold mold might draw heat away from the nozzle or hot runner. If too much heat is drawn away, it can cause a freeze-off. Returning the coolant temperature to match the process-specific documentation should reduce the pressure loss in the mold. Using a coolant temperature controller with a lower-rated coolant pump or reducing the coolant flow with a flow control valve may return the water flow to match that of the documented standard.

Cooling Line Layout

An incorrect cooling line layout can affect the material flow and cooling. Verify the Cooling Line Layout matches the Process-Specific Documentation.

High Cooling Time

A high cooling time will increase the amount of heat that is removed from the nozzle or hot runner system. The cooling time should match the documented standard.

Possible Causes: Materials

High Regrind Percentage

PVC & CPVC tend to be very temperature sensitive, and excessive reground PVC & CPVC can cause crosslinking and freeze off. If too much regrind is being used, the barrel cooling fans may be inadequate to remove this increased heat, resulting in a weakened part due to material degradation. Reducing the regrind percentage should lower the material temperature and decrease the creation of volatiles.

Poor Quality Regrind

Regrind with any degraded PVC & CPVC will cause a dangerous chain reaction of degradation, which can cause crosslinking and freeze off. Any regrind containing degradation or burning should be discarded before it causes more defects or a serious safety concern.

Material Degradation

Excessive degradation of PVC & CPVC can cause crosslinking and freeze off. Removing the bad material from the barrel helps prevent degraded polymer from contaminating all other material it contacts. If a Hot Runner System is being used, purge it out as well.

Material Contamination

Contaminants can cause a flow restriction if they get caught in the nozzle and hot runner system. These could be any non-melting material, such as wood, metal, or cloth, as well as degradation & any polymer that does not melt at the current processing temperature. Contamination can come from many locations, including storage, hopper, hoses, filters, screens, surge bins, access doors, seals, and blenders. All potential sources of contamination must be checked to determine where the contamination is coming from. Once the source is determined, everything that came in contact with the material afterwards must be thoroughly cleaned and inspected before returning to use.

Possible Causes: Injection

Flow Obstructions or Restrictions

Check for obstructions in the nozzle, gate, or hot runner as well as a faulty heater band on the barrel, nozzle, or hot runner system.

Possible Causes: Machine

Machine Settings

Sprue break can often be used to separate the nozzle from the sprue bushing after recovery. Sprue break can prevent excessive heat loss from the nozzle to the bushing, but sometimes contributes to drooling. When processing PVC & CPVC with a cold runner system, the molding machine should use a full taper nozzle tip. This tip has a large diameter at the nozzle, which tapers down all the way to the orifice diameter. The full taper nozzle tip helps reduce shear while promoting a clean break from the sprue when the mold opens. The nozzle tip temperature can be increased in some cases to prevent freeze-off.

Possible Causes: Mold & Design

Part or Mold Design

Hot Runner Systems can be dangerous with PVC & CPVC if the polymer is left in it for any extended period of time. To reduce the risk of material flow stagnation, externally heated hot runner drops should be used with high flow gate tips. Removable cavity plates are beneficial to allow for safe & quick removal of the PVC & CPVC plastic at the gate tip once the hot runner system is properly purged. The gate tip temperature can be increased in some cases to prevent freeze-off.

Initial Troubleshooting

• Verify Process:
  • Material Temperature & Back Pressure
  • Barrel Residence Time
  • Recovery Time
  • Coolant Temperature, Pressure, and Flow
  • Cooling Time
  • Injection Time & Profile
  • Part Weight at Transfer (no runner or sprue)
  • Packing Pressure, Time, & Speed
  • Final Part Weight
• Verify Setup Sheet
  • Mold Breakaway Speed
  • Decompression After Recovery
  • Nozzle or Hot Runner Gate Drop Temperature
• Check
  • Cavity parting line damage
  • Volatile Buildup or damage on mold surface
  • Screw & Barrel Heating and Cooling Equipment

Possible Causes: Recovery

Low Material Temperature or Back Pressure

A lower material temperature increases the cooling rate, resulting in lower part shrinkage. This larger part is more difficult to remove from the cavity during mold opening. Returning the material temperature and back pressure to match the process-specific documentation should reduce part shrinkage. It may be necessary to check the thermocouples & heater bands on the barrel & hot runner system.

Low Recovery Time

A high Screw RPM can cause inconsistent recovery and excessive shear during recovery. The screw RPM should be adjusted so that the recovery time consumes most of the cooling time.

Low Barrel Residence Time

A short barrel residence time may prevent the PVC & CPVC from fully melting during recovery. A lower material temperature increases the cooling rate, resulting in lower part shrinkage. This larger part is more difficult to remove from the cavity during mold opening. This situation can be due to a significantly smaller barrel being used than was used on the documented standard. In some cases, barrel temperature or back pressure can be increased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

Possible Causes: Cooling

Low Coolant Temperature or High Coolant Flow

A lower mold temperature increases the cooling rate, resulting in lower part shrinkage. This larger part is more difficult to remove from the cavity during mold opening. Returning the coolant temperature to match the process-specific documentation should reduce the pressure loss in the mold. Using a coolant temperature controller with a lower-rated coolant pump or reducing the coolant flow with a flow control valve may return the water flow to match that of the documented standard.

Cooling Line Layout

An incorrect cooling line layout can affect the material flow and cooling. Verify the Cooling Line Layout matches the Process-Specific Documentation.

High Cooling Time

A high cooling time increases the cooling rate, resulting in lower part shrinkage. This larger part is more difficult to remove from the cavity during mold opening. The cooling time should match the documented standard.

Possible Causes: Materials

Poor Material Drying

Although PVC & CPVC do not always need drying, they are hygroscopic polymers. The absorbed moisture will turn to steam in the barrel, reducing the viscosity and increasing the likelihood of flash. Although grades vary, most PVC & CPVC resins should be dried for 1-2 hours at 65 to 80°C (150 to 175°F) with dry air (approx. -40°C or -40°F Dewpoint) to a moisture content below 0.3%.

Possible Causes: Injection

Low Injection Time

This indicates the material is entering the mold faster, which will reduce the pressure loss during injection and packing. Decreased pressure loss may result in better packing pressure distribution and larger dimensions. Reducing injection speed will help you increase injection time to match the documented standard.

Injection Profile

PVC & CPVC typically require an injection profile to make a good part. For example, the gate area often needs a lower injection speed to reduce shear. A significant change in the injection profile will have a significant effect on the filling and packing of the part. The Injection Profile should match the process-specific documentation.

High Part Weight at Transfer

This indicates too much material is entering the mold during injection, which may cause over-packing of the part. If other parameters are similar and only the weight is different, lowering shot size or increasing transfer position should decrease part weight at transfer.

Possible Causes: Packing

High Packing Pressure

This indicates there is too much pressure after mold filling, which might over-pack the part. An over-packed part typically has molded-in stresses, which can weaken the final part. The Packing Pressure should be set to match the Process-Specific Documentation, and the Packing Speed should be set to the same value as your last Injection Speed before Transfer from Injection to Packing.

High Packing Time

With PVC & CPVC, it is difficult to get a full gate seal at the end of packing. Excessive packing time may cause overpacking and stresses in the gate area. A packing time study should be done to ensure the packing time is adequate to achieve a good gate appearance.

High Final Part Weight

This indicates excessive material is entering the mold cavity during Injection and packing. This is indicative of a change in the process, but you need to find other changes in the process before action can be taken.

Low Clamp Force

Insufficient clamp force will allow plastic to flow between the parting lines or mold components. Depending on where the flash occurs, it can sometimes cause the part to stick to the wrong side of the mold during mold opening. The clamp tonnage should be set to match the documented standard.

Possible Causes: Machine

Machine Settings

It is very easy to damage the part during part removal. In most cases, slower mold breakaway and ejection speeds will help the part remain on the core side of the mold during mold open. Sprue break can often be used to separate the sprue from the nozzle in cases where the sprue remains stuck during mold opening.

Poor Clamp Condition

If flash occurs, the part may stick to the mold during part removal. The clamp tonnage will not be properly distributed if the machine is not level, the platens are out of alignment, the platens are concave, or a tie bar is stretched. These can be checked with equipment such as levels, a tie bar strain gauge, a laser alignment tool, or inside micrometers. Damage or misalignment must be repaired immediately to prevent excessive equipment and mold damage.

Possible Causes: Mold & Design

Mold Surface Damage

PVC & CPVC are both highly corrosive polymers and may damage the cavity surface over time. This corrosion may not look like much visually, but the mold surface becomes very rough, which can help it hold onto the part during mold opening. This situation will likely need repair to significantly improve the surface finish.

High Cavity Imbalance (Cavity to Cavity Variation)

Significant differences in cavity filling can cause some cavities to fill and start packing during injection, while other cavities are significantly short at transfer. Make sure all the mold cavities are short at the time of transfer to minimize any cavity-to-cavity variation in the final molded part. There are steps that can improve cavity imbalance, such as using a different injection speed, adjusting hot runner gate drops, cleaning the mold vents, increasing the number of vents, or adjusting gate width when necessary.

Design Concerns

Good part design strategies, such as gating into thick sections, avoiding sharp transitions, maintaining the same thickness when possible, and providing adequate venting, are important to improving mold filling will help maintain more consistent shrinkage across the whole part. Increasing wall thickness will also increase part shrinkage in areas that are sticking. Decreasing undercuts, increasing draft angles, and draw polishing the cavities will help the part separate from the cavity during mold opening. It is not uncommon for minor undercuts to be added to the core side of the mold to help pull the part away from the cavity during mold opening.

Initial Troubleshooting

• Purge Barrel
• Verify Process:
  • Material Temperature & Back Pressure
  • Recovery Time
  • Coolant Temperature, Pressure, & Flow
  • Cooling Time
  • Injection Time & Profile
  • Part Weight at Transfer (no runner or sprue)
  • Packing Pressure, Time, & Speed
  • Final Part Weight
• Verify Setup Sheet
  • Initial Ejection Speed
• Check cavity parting line for damage
• Check mold Surface for volatile buildup or damage
• Check:
  • Cushion is approximately 10% of Shot Size
  • Process is not Pressure Limited
  • Cooling Line Layout
  • Check Ring Performance (if used)
  • Screw & Barrel Heating and Cooling Equipment
  • Cavity parting line for damage
  • Mold surface for volatile buildup or damage
• Look for obstructions or faulty equipment in the nozzle, gate, or hot runner

Possible Causes: Recovery

High Material Temperature or Back Pressure

A high temperature or back pressure can cause the warmer PVC & CPVC to stick to the core due to increased shrinkage. Returning the material temperature and back pressure to match the Process-Specific Documentation should help these situations. You may need to further reduce the barrel temperature or back pressure if material degradation is suspected. You should always purge the barrel when degradation is suspected with PVC & CPVC. It may be necessary to check the cooling fans, screw cooling, thermocouples, & heater bands.

Low Recovery Time

A high Screw RPM can cause inconsistent recovery and excessive shear during recovery. The screw RPM should be adjusted so that the recovery time consumes most of the cooling time.

High Barrel Residence Time

A long barrel residence time can cause the PVC & CPVC temperature to increase. This may cause it to stick to the core due to increased shrinkage. In some cases, barrel temperature can be decreased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

Possible Causes: Cooling

High Coolant Temperature or Low Coolant Flow

Since PVC & CPVC tend to be molded without a gate seal, a warmer mold will change how much the gate seals. This warmer gate will affect how much material leaves the gate at the end of packing. If the molded PVC & CPVC has some degradation, a high mold temperature can slow the cooling rate and allow material degradation to continue. Returning the coolant temperature to match the process-specific documentation should return the pressure loss in the mold to normal. Check the cooling system for obstructions, as this will reduce coolant flow. Using a coolant temperature controller with a higher-rated coolant pump may raise the coolant flow to match the process-specific documentation.

Cooling Line Layout

An incorrect cooling line layout can affect the material flow and shrinkage. Verify the Cooling Line Layout matches the Process-Specific Documentation.

Low Cooling Time

Lower cooling times result in a warmer part with a lower strength when being ejected. Decreased material strength can often cause ejector marks in the part. The cooling time should match the documented standard.

High Cooling Time

A high cooling time will result in a cooler and stronger plastic. A stronger plastic part can sometimes be more difficult to eject, especially if the core has any undercuts that are difficult to remove. The cooling time should match the documented standard.

Possible Causes: Materials

High Regrind Percentage

PVC & CPVC tend to be very temperature sensitive, and reground PVC & CPVC will cause an increase in material temperature and shrinkage. Decreased material strength will also result, which can contribute to ejector marks in the part. If too much regrind is being used, the barrel cooling fans may be inadequate to remove this increased heat, resulting in a weakened part due to material degradation. Reducing the regrind percentage should lower the material temperature and decrease the creation of volatiles.

Poor Quality Regrind

Regrind with any degraded PVC & CPVC will cause a dangerous chain reaction of degradation, which can weaken the part. Decreased material strength can often cause ejector marks in the part. Any regrind containing degradation or burning should be discarded before it causes more defects or a serious safety concern.

Material Degradation

Degradation of PVC & CPVC significantly reduces the strength and performance of PVC & CPVC. Removing the bad material from the barrel helps prevent degraded polymer from contaminating all other material it contacts. If a Hot Runner System is being used, purge it out as well.

Poor Material Drying

Although PVC & CPVC do not always need drying, they are hygroscopic polymers. The absorbed moisture will turn to steam in the barrel, which will reduce the material viscosity as well as the part strength. Although grades vary, most PVC & CPVC resins should be dried for 1-2 hours at 65 to 80°C (150 to 175°F) with dry air (approx. -40°C or -40°F Dewpoint) to a moisture content below 0.3%.

Possible Causes: Injection

Flow Obstructions or Restrictions

Check for obstructions in the nozzle, gate, or hot runner as well as a faulty heater band on the barrel, nozzle, or hot runner system.

High Injection Time

This indicates material is entering the mold slowly, causing an increase in material viscosity. A higher viscosity PVC & CPVC experiences increased pressure loss during injection and packing, which may weaken the molded part. Increasing injection speed will help you decrease injection time. You should also check your Maximum Injection Pressure in case there is not enough Injection Pressure available to maintain the Injection Speed.

Injection Profile

PVC & CPVC typically require an injection profile to make a good part. For example, the gate area often needs a lower injection speed to reduce shear. A significant change in the injection profile will have a significant effect on the strength of the molded part. The Injection Profile should match the Process-Specific Documentation.

Low Part Weight at Transfer

This indicates too little material is entering the mold during injection, which may reduce strength due to inadequate packing. If other parameters are similar and only the weight is different, raising shot size or decreasing transfer position should increase part weight at transfer.

High Part Weight at Transfer

This indicates too much material is entering the mold during injection and often contributes to gas entrapment. This condition may cause a cavity pressure spike during injection, which can overpack the part and make it stick. If other parameters are similar and only the weight is different, lowering shot size or increasing transfer position should decrease part weight at transfer.

Low Peak or Transfer Pressure

This indicates the material viscosity has decreased, and there will be less pressure loss during injection, which may result in overpacking. If the mold or material temperatures are higher than the documented standard, these can be decreased to reduce the flow of PVC & CPVC. If a change in lot #, colorant, or regrind is causing the viscosity decrease, you may need to decrease the shot size or increase the transfer position to compensate for the lower viscosity material.

High Peak or Transfer Pressure

This indicates the material viscosity has increased or there is an obstruction. Check for flow obstructions. If the mold or material temperatures are lower than the documented standard, these can be increased to allow the PVC & CPVC to flow more easily into the mold.

Possible Causes: Packing

Low Packing Pressure

This indicates there is not enough pressure to fill the mold and compensate for shrinkage during packing. An under-packed part will typically have reduced strength, which can be damaged during ejection. The packing pressure should be set to match the process-specific documentation, and the packing speed should be set to the same value as your last Injection Speed before Transfer from Injection to Packing.

High Packing Pressure

This indicates there is too much pressure after mold filling, which might over-pack the part. An over-packed part may be difficult to eject from the core. The Packing Pressure should be set to match the Process-Specific Documentation, and the Packing Speed should be set to the same value as your last Injection Speed before Transfer from Injection to Packing.

Low Packing Time

With PVC & CPVC, it is difficult to get a full gate seal at the end of packing. Insufficient packing time will cause material to leave the part in the runner or sprue. This tends to weaken the part, especially near the gate area. A packing time study should be done to ensure the packing time is adequate to achieve a good gate appearance.

High Packing Time

With PVC & CPVC, it is difficult to get a full gate seal at the end of packing. Excessive packing time may cause overpacking and weakness in the gate area. An over-packed part may be difficult to eject from the core. A packing time study should be done to ensure the packing time is adequate to achieve a good gate appearance.

Low Final Part Weight

This indicates insufficient material is entering the mold cavity during Injection and packing. This is indicative of a change in the process, but you need to find other changes in the process before action can be taken.

High Final Part Weight

This indicates excessive material is entering the mold cavity during Injection and packing. This is indicative of a change in the process, but you need to find other changes in the process before action can be taken.

No Cushion

A process without a cushion does not have enough material to fill the part and compensate for shrinkage during packing. This results in an under-packed part, which is typically weaker than expected. The cushion should always be approximately 10% of the Shot Size. Both the shot size and transfer position must be increased to raise the cushion.

High Cushion

Melted PVC & CPVC are highly compressible plastics; thus, a process with an excessive cushion, such as 25% of the shot size, can lose significant injection & packing pressure. The cushion should always be approximately 10% of the Shot Size. Both the shot size and transfer position must be decreased to lower the cushion.

Possible Causes: Machine

Check Ring Performance (Intermittent Defect)

If a check ring is being used, perform a check ring performance study by weighing 10 consecutive short shots (weight all cavities + runner) and calculating variability. Over 2% variability means the check ring is suspect, should be changed immediately if over 3% Variability. Purging the Barrel or increasing Decompression after Recovery may improve check ring performance.

% Variability = (100%) * (Max - Min) / (Average)

Machine Settings

It is very easy to damage the part during part removal. In most cases, a slower initial ejection speed will decrease the stress on the part during ejection.

Poor Clamp Condition

If flash occurs, the part may stick to the mold during part removal. The clamp tonnage will not be properly distributed if the machine is not level, the platens are out of alignment, the platens are concave, or a tie bar is stretched. These can be checked with equipment such as levels, a tie bar strain gauge, a laser alignment tool, or inside micrometers. Damage or misalignment must be repaired immediately to prevent excessive equipment and mold damage.

Possible Causes: Mold & Design

Mold Surface Damage

PVC & CPVC are both highly corrosive polymers and may damage the core surface over time. This corrosion may not look like much visually, but the mold surface becomes very rough, which can help it hold onto the part during part ejection. This situation will likely need repair to significantly improve the surface finish.

High Cavity Imbalance (Cavity to Cavity Variation)

Significant differences in cavity filling can cause some cavities to fill and start packing during injection, while other cavities are significantly short at transfer. Make sure all the mold cavities are short at the time of transfer to minimize any cavity-to-cavity variation in the final molded part. There are steps that can improve cavity imbalance, such as using a different injection speed, adjusting hot runner gate drops, cleaning the mold vents, increasing the number of vents, or adjusting gate width when necessary.

Design Concerns

Good part design strategies, such as gating into thick sections, avoiding sharp transitions, maintaining the same thickness when possible, and providing adequate venting, are important to improving mold filling. Increasing wall thickness will also increase the mechanical or resistance properties of PVC & CPVC. Since it is difficult to achieve full theoretical strength with PVC & CPVC, a higher safety factor should be used than is common with most polymers. For example, a designer may typically design a Polypropylene part 15% thicker as a safety margin, but a PVC & CPVC part should be 25% or 30% thicker than needed to get a similar safety margin. Decreasing undercuts, increasing draft angles, increasing ejection components, and draw polishing cores and cavities will help the part separate from the mold during the part removal process.

Initial Troubleshooting

• Inspect and repair any faulty robot/automation components
• Verify automation/robotics synchronization and alignment
• Inspect & measure part and correct any other part defects
• Verify Process:
  • Material Temperature & Back Pressure
  • Coolant Temperature, Pressure, and Flow
  • Cooling Time
  • Part Weight at Transfer (no runner or sprue)
  • Packing Pressure
  • Final Part Weight
• Check:
  • Screw & Barrel Cooling Equipment
  • Cushion is approximately 10% of Shot Size
  • Process is not Pressure Limited
  • Cooling Line Layout
  • Check Ring Performance (if used)
• Look for obstructions or faulty equipment in the nozzle, gate, or hot runner

Possible Causes: Recovery

Low Material Temperature or Back Pressure

Cold PVC & CPVC will likely result in a final part with larger dimensions. Returning the material temperature and back pressure to match the process-specific documentation should reduce the pressure loss in the mold. It may be necessary to check the thermocouples & heater bands on the barrel & hot runner system. Raising the material temperature or back pressure may also be needed to add more temperature or shear into the polymer to improve gelation or fusion.

High Material Temperature or Back Pressure

A high temperature or back pressure can cause the heat-sensitive PVC & CPVC to degrade, which will reduce the polymer strength. The warmer material will cause the part to be ejected warmer with less strength than expected. This high-temperature material will also shrink more than expected. Returning the material temperature and back pressure to match the Process-Specific Documentation should help these situations. You may need to further reduce the barrel temperature or back pressure if material degradation is suspected. You should always purge the barrel when degradation is suspected with PVC & CPVC. It may be necessary to check the cooling fans, screw cooling, thermocouples, & heater bands.

Low Recovery Time

A high Screw RPM can cause inconsistent recovery and excessive shear during recovery. The screw RPM should be adjusted so that the recovery time consumes most of the cooling time.

Low Barrel Residence Time

A short barrel residence time may prevent the PVC & CPVC from fully melting during recovery. A cold material will likely result in a final part with large dimensions. A low barrel residence time can also reduce the amount of gelation or fusion the PVC & CPVC experiences during the molding process, making the part weaker. This situation can be due to a significantly smaller barrel being used than was used on the documented standard. In some cases, barrel temperature or back pressure can be increased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

High Barrel Residence Time

A long barrel residence time can cause the heat-sensitive PVC & CPVC to heat up or degrade, which will reduce the polymer strength. This high-temperature material will also shrink more than expected. In some cases, barrel temperature can be decreased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

Possible Causes: Cooling

Low Coolant Temperature or High Coolant Flow

Cold molds will likely result in a final part with larger dimensions. Cold molds can also cause molded-in stresses, which can weaken the molded part. Returning the coolant temperature to match the process-specific documentation should reduce the pressure loss in the mold. Using a coolant temperature controller with a lower-rated coolant pump or reducing the coolant flow with a flow control valve may return the water flow to match that of the documented standard.

High Coolant Temperature or Low Coolant Flow

This high temperature mold will cause the part to shrink more than expected. The warmer mold will also cause the part to be ejected warmer with less strength than expected. Returning the coolant temperature to match the process-specific documentation should return the pressure loss in the mold to normal. Check the cooling system for obstructions, as this will reduce coolant flow. Using a coolant temperature controller with a higher-rated coolant pump may raise the coolant flow to match the process-specific documentation.

Cooling Line Layout

An incorrect cooling line layout can affect the material flow and shrinkage. Verify the Cooling Line Layout matches the Process-Specific Documentation.

Low Cooling Time

Lower cooling times provide a lower cooling rate to the PVC & CPVC part, resulting in a smaller final part. The lower cooling time will cause the part to be ejected warmer with less strength than expected. If the molded PVC & CPVC has some degradation, a low cooling time can allow existing material degradation to continue. If degradation is occurring in this manner, you may not see signs of degradation on the surface of the part, but you can see degraded polymer when the part is cut open. The cooling time should match the documented standard.

High Cooling Time

A high cooling time will increase the cooling rate of the polymer, which may result in molded-in stresses that can weaken the molded part. This longer cooling time can also increase the dimensions of the molded part. The cooling time should match the documented standard.

Possible Causes: Materials

High Regrind Percentage

PVC & CPVC tend to be very temperature sensitive, and reground PVC & CPVC will cause an increase in material temperature and shrinkage. Decreased material strength will also result, which can contribute to robot & automation marks on the part. If too much regrind is being used, the barrel cooling fans may be inadequate to remove this increased heat, resulting in a weakened part due to material degradation. Reducing the regrind percentage should lower the material temperature and decrease the creation of volatiles.

Poor Quality Regrind

Regrind with any degraded PVC & CPVC will cause a dangerous chain reaction of degradation, which can weaken the part. Any regrind containing degradation or burning should be discarded before it causes more defects or a serious safety concern.

Material Degradation

Degradation of PVC & CPVC significantly reduces the strength and performance of PVC & CPVC. Removing the bad material from the barrel helps prevent degraded polymer from contaminating all other material it contacts. If a Hot Runner System is being used, purge it out as well.

Poor Material Drying

Although PVC & CPVC do not always need drying, they are hygroscopic polymers. The absorbed moisture will come out of the plastic during processing, which can affect the viscosity and physical properties of the molded part. Although grades vary, most PVC & CPVC resins should be dried for 1-2 hours at 65 to 80°C (150 to 175°F) with dry air (approx. -40°C or -40°F Dewpoint) to a moisture content below 0.3%.

Possible Causes: Injection

Inadequate or Blocked Venting

Poor venting can cause gas entrapment during injection. This gas can cause short shots and sinks, which can interfere with part handling. Molds should be vented wherever possible; they must be vented to the atmosphere, and cross-cavity venting should be avoided whenever possible. If routine vent cleaning is needed, there should be a procedure for scheduled vent cleaning before they become blocked.

Flow Obstructions or Restrictions

Check for obstructions in the nozzle, gate, or hot runner as well as a faulty heater band on the barrel, nozzle, or hot runner system.

Injection Profile

PVC & CPVC typically require an injection profile to make a good part. For example, the gate area often needs a lower injection speed to reduce shear. A significant change in the injection profile will have a significant effect on the appearance of the part and gate. The Injection Profile should match the Process-Specific Documentation.

Low Part Weight at Transfer

This indicates too little material is entering the mold during injection, which may reduce part dimensions due to inadequate packing. If other parameters are similar and only the weight is different, raising shot size or decreasing transfer position should increase part weight at transfer.

High Part Weight at Transfer

This indicates too much material is entering the mold during injection and often results in overpacking. If other parameters are similar and only the weight is different, lowering shot size or increasing transfer position should decrease part weight at transfer.

Pressure-Limited Process

A pressure-limited process cannot maintain the desired injection speed. If a change in lot #, colorant, or regrind is causing a viscosity increase, you may need to raise the shot size or decrease the transfer position to compensate for the higher viscosity material. Flow obstructions and faulty equipment should always be checked when the maximum pressure is reached. In some cases, an increase in the maximum injection pressure is needed to accommodate a higher viscosity material, but the remainder of the process should be verified to ensure there are no other causes for a high injection pressure before this change is made.

Possible Causes: Packing

Low Packing Pressure

This indicates there is not enough pressure to fill the mold and compensate for shrinkage during packing. An under-packed part will typically have smaller dimensions and reduced strength. The packing pressure should be set to match the process-specific documentation, and the packing speed should be set to the same value as your last Injection Speed before Transfer from Injection to Packing.

High Packing Pressure

This indicates there is too much pressure after mold filling, which might over-pack the part. An over-packed part typically has larger dimensions as well as molded-in stresses, which can weaken the final part. The packing pressure should be set to match the process-specific documentation, and the packing speed should be set to the same value as your last injection speed before transfer from injection to packing.

Low Packing Time

With PVC & CPVC, it is difficult to get a full gate seal at the end of packing. Insufficient packing time will cause material will leave the part in the runner or sprue. This tends to make the dimensions smaller and weaken the part in the gate area. A packing time study should be done to ensure the packing time is adequate to achieve a good gate appearance.

High Packing Time

With PVC & CPVC, it is difficult to get a full gate seal at the end of packing. Excessive packing time will cause an overpacked part. This tends to make the dimensions large. A packing time study should be done to ensure the packing time is adequate to achieve the correct dimensions.

Low Final Part Weight

This indicates insufficient material is entering the mold cavity during Injection and packing. This is indicative of a change in the process, but you need to find other changes in the process before action can be taken.

High Final Part Weight

This indicates excessive material is entering the mold cavity during Injection and packing. This is indicative of a change in the process, but you need to find other changes in the process before action can be taken.

Low Clamp Force

Insufficient clamp force will allow plastic to flow between the parting lines. This flash can interfere with part handling. The packing pressure should be verified to match the process-specific documentation, or it will overcome the clamp tonnage.

High Clamp Force

Excessive clamp force will prevent the air from venting as fast as the material is entering the mold. This gas can cause short shots, which can interfere with part handling. The Clamp Tonnage should be set to match the Process-Specific Documentation. You will likely need to clean the vents at this time.

No Cushion

A process without a cushion does not have enough material to fill the part and compensate for shrinkage during packing. This results in an under-packed part with smaller dimensions than expected. The cushion should always be approximately 10% of the Shot Size. Both the shot size and transfer position must be increased to raise the cushion.

High Cushion

Melted PVC & CPVC are highly compressible plastics; thus, a process with an excessive cushion, such as 25% of the shot size, can lose significant injection & packing pressure. The cushion should always be approximately 10% of the Shot Size. Both the shot size and transfer position must be decreased to lower the cushion.

Possible Causes: Machine

Check Ring Performance (Intermittent Defect)

If a check ring is being used, perform a check ring performance study by weighing 10 consecutive short shots (weight all cavities + runner) and calculating variability. Over 2% variability means the check ring is suspect, and should be changed immediately if over 3% Variability. Purging the Barrel or increasing Decompression after Recovery may improve check ring performance.

% Variability = (100%) * (Max - Min) / (Average)

Machine Settings

In any automation & robotics application, there is the expectation that the movements are synchronized. In some cases, a slow mold open speed is needed at the end of mold open to ensure the clamp stops in the correct position for the robotics to properly remove the part. In other cases, the ejection timing & speed must be properly set to move at the same speed as the robot.

Robot Settings & Condition

Inspection should start with the components that contact the part, ensuring there is no damage such as worn suction cups, dented grippers, dull cutters, parts stuck in sorting equipment, damaged vacuum lines, or bent end-of-arm tooling. Your company should have spare parts of all automation components, such as grippers, holders, cutters, suction cups, springs, hoses, rails, bolts, nuts, etc., so any suspect component can be replaced immediately. In any automation & robotics application, there is the expectation that the movements are synchronized. The robot must move with the machine ejection system when grabbing the part as well as move with the automation when setting down the part. To avoid damage to the part surface, there should be smooth movements whenever a transfer is taking place from the mold to the robot and the robot to the automation.

Poor Clamp Condition

Poorly greased or malfunctioning clamp and ejection components will affect the synchronization of movements between the machine and the robotics. Since hydraulic clamps require a lot of hydraulic fluid, poorly maintained hydraulics will make it impossible to have consistent clamp movements from shot to shot.

Possible Causes: Mold & Design

High Cavity Imbalance (Cavity to Cavity Variation)

Significant differences in cavity filling can cause some cavities to fill and start packing during injection, while other cavities are significantly short at transfer. Make sure all the mold cavities are short at the time of transfer to minimize any cavity-to-cavity variation in the final molded part. There are steps that can improve cavity imbalance, such as using a different injection speed, adjusting hot runner gate drops, cleaning the mold vents, increasing the number of vents, or adjusting gate width when necessary.

Design Concerns

Increasing the number of grippers or suction cups or the surface area that the robot grabs the part will reduce the overall pressure or vacuum needed to control the part during part removal and secondary operations. Contacting the part in non-cosmetic areas of the part may also help reduce the impact of automation and robotics on the part. Good part design strategies, such as gating into thick sections, avoiding sharp transitions, maintaining the same thickness when possible, and providing adequate venting, are important to improving mold filling. Increasing wall thickness will also increase the Mechanical or Resistance properties of PVC & CPVC. Since it is difficult to achieve full theoretical strength with PVC & CPVC, a higher safety factor should be used than is common with most polymers. For example, a designer may typically design a Polypropylene part 15% thicker as a safety margin, but a PVC & CPVC part should be 25% or 30% thicker than needed to get a similar safety margin.

Initial Troubleshooting

• Verify Process:
  • Material Temperature
  • Back Pressure
  • Recovery Time
  • Coolant Temperature, Pressure, and Flow
  • Cooling Time
  • Injection Time & Profile
  • Part Weight at Transfer (no runner or sprue)
  • Packing Pressure, Time, and Speed
  • Final Part Weight

Possible Causes: Recovery

Low Material Temperature or Back Pressure

A lower material temperature increases the cooling rate, resulting in lower part shrinkage. Returning the material temperature and back pressure to match the process-specific documentation should reduce part shrinkage. It may be necessary to check the thermocouples & heater bands on the barrel & hot runner system.

Low Recovery Time

A high Screw RPM can cause inconsistent recovery and excessive shear during recovery. The screw RPM should be adjusted so that the recovery time consumes most of the cooling time.

Low Barrel Residence Time

A short barrel residence time may prevent the PVC & CPVC from fully melting during recovery. A lower material temperature increases the cooling rate, resulting in lower part shrinkage. This situation can be due to a significantly smaller barrel being used than was used on the documented standard. In some cases, barrel temperature or back pressure can be increased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

Possible Causes: Cooling

Low Coolant Temperature or High Coolant Flow

A lower mold temperature increases the cooling rate, resulting in less part shrinkage. Returning the coolant temperature to match the process-specific documentation should reduce the pressure loss in the mold. Using a coolant temperature controller with a lower-rated coolant pump or reducing the coolant flow with a flow control valve may return the water flow to match that of the documented standard.

Cooling Line Layout

An incorrect cooling line layout can affect the material flow and cooling. Verify the Cooling Line Layout matches the Process-Specific Documentation.

High Cooling Time

A high cooling time increases the cooling rate, resulting in lower part shrinkage. The cooling time should match the documented standard.

Possible Causes: Materials

High Regrind Percentage

If too much PVC & CPVC regrind is being used, the barrel cooling fans may be inadequate to remove this increased heat, resulting in material degradation. Degraded PVC tends to give off gases, which can cause the dimensions to increase if they get trapped in the part. Reducing the regrind percentage should lower the material temperature and decrease the creation of volatiles.

Poor Quality Regrind

Regrind with any degraded PVC & CPVC will cause a dangerous chain reaction of degradation. Degraded PVC tends to give off gases, which can cause the dimensions to increase if they get trapped in the part. Any regrind containing degradation or burning should be discarded before it causes more defects or a serious safety concern.

Material Degradation

Degradation of PVC & CPVC significantly reduces the strength and performance of PVC & CPVC. Degraded PVC tends to give off gases, which can cause the dimensions to increase if they get trapped in the part. Removing the bad material from the barrel helps prevent degraded polymer from contaminating all other material it contacts. If a Hot Runner System is being used, purge it out as well.

Possible Causes: Injection

Low Injection Time

This indicates the material is entering the mold faster, which will reduce the pressure loss during injection and packing. Decreased pressure loss may result in better packing pressure distribution and larger dimensions. Reducing injection speed will help you increase injection time to match the documented standard.

Injection Profile

PVC & CPVC typically require an injection profile to make a good part. For example, the gate area often needs a lower injection speed to reduce shear. A significant change in the injection profile will have a significant effect on the filling and packing of the part. The Injection Profile should match the process-specific documentation.

High Part Weight at Transfer

This indicates too much material is entering the mold during injection, which may cause over-packing of the part. If other parameters are similar and only the weight is different, lowering shot size or increasing transfer position should decrease part weight at transfer.

Possible Causes: Packing

High Packing Pressure

This indicates there is too much pressure after mold filling, which might over-pack the part. An over-packed part will have larger dimensions than expected. The packing pressure should be set to match the process-specific documentation, and the packing speed should be set to the same value as your last injection speed before transfer from injection to packing.

High Packing Time

With PVC & CPVC, it is difficult to get a full gate seal at the end of packing. Excessive packing time may cause overpacking and stress in the gate area. A packing time study should be done to ensure the packing time is adequate to achieve consistent part dimensions.

High Final Part Weight

This indicates excessive material is entering the mold cavity during Injection and packing. This is indicative of a change in the process, but you need to find other changes in the process before action can be taken.

Low Clamp Force

Insufficient clamp force will allow plastic to flow between the parting lines or mold components. Flash on the edges of the part can affect part measurements. If the mold is being forced open during injection or packing, then the part thickness will likely increase. Depending on where the flash occurs, it can sometimes cause the part to stick to the wrong side of the mold during mold opening. The packing pressure should be verified to match the process-specific documentation, or it will overcome the clamp tonnage.

Possible Causes: Mold & Design

High Cavity Imbalance (Cavity to Cavity Variation)

Significant differences in cavity filling can cause some cavities to fill and start packing during injection, while other cavities are significantly short at transfer. Make sure all the mold cavities are short at the time of transfer to minimize any cavity-to-cavity variation in the final molded part. There are steps that can improve cavity imbalance, such as using a different injection speed, adjusting hot runner gate drops, cleaning the mold vents, increasing the number of vents, or adjusting gate width when necessary.

Design Concerns

Decreasing the mold dimensions will decrease the dimensions of the final molded part. Good part design strategies, such as gating into thick sections, avoiding sharp transitions, maintaining the same thickness when possible, and providing adequate venting are important to improving mold filling and will help maintain more consistent shrinkage across the whole part. Increasing wall thickness will also increase part shrinkage in areas that are sticking. Decreasing undercuts, increasing draft angles, and draw polishing the cavities will help the part separate from the cavity during mold opening. It is not uncommon for minor undercuts to be added to the core side of the mold to help pull the part away from the cavity during mold opening.

Initial Troubleshooting

• Purge Barrel
• Verify Process:
  • Material Temperature & Back Pressure
  • Recovery Time
  • Coolant Temperature, Pressure, & Flow
  • Cooling Time
  • Injection Time & Profile
  • Part Weight at Transfer (no runner or sprue)
  • Packing Pressure, Time, & Speed
  • Final Part Weight
• Check:
  • Cushion is approximately 10% of Shot Size
  • Process is not Pressure Limited
  • Cooling Line Layout
  • Check Ring Performance (if used)
  • Screw & Barrel Cooling Equipment
• Look for obstructions or faulty equipment in the nozzle, gate, or hot runner

Possible Causes: Recovery

High Material Temperature or Back Pressure

A high temperature or back pressure can slow the cooling rate, resulting in increased shrinkage. Returning the material temperature and back pressure to match the process-specific documentation should help these situations. You may need to further reduce the barrel temperature or back pressure if material degradation is suspected. You should always purge the barrel when degradation is suspected with PVC & CPVC. It may be necessary to check the cooling fans, screw cooling, thermocouples, & heater bands.

Low Recovery Time

A high screw RPM can cause inconsistent recovery and excessive shear during recovery. The screw RPM should be adjusted so that the recovery time consumes most of the cooling time.

High Barrel Residence Time

A long barrel residence time can cause the PVC & CPVC temperature to increase. This may cause increased part shrinkage. In some cases, barrel temperature can be decreased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

Possible Causes: Cooling

High Coolant Temperature or Low Coolant Flow

PVC & CPVC tend to be very temperature sensitive, and a hot mold increases material shrinkage due to a slower cooling rate. Returning the coolant temperature to match the process-specific documentation should return the pressure loss in the mold to normal. Check the cooling system for obstructions, as this will reduce coolant flow. Using a coolant temperature controller with a higher-rated coolant pump may raise the coolant flow to match the process-specific documentation.

Cooling Line Layout

An incorrect cooling line layout can affect the material flow and shrinkage. Verify the cooling line layout matches the process-specific documentation.

Low Cooling Time

Lower cooling times cause a slower cooling rate because the part cools faster inside the mold than it does outside the mold. This slower cooling rate will cause more shrinkage in the final part. The cooling time should match the documented standard.

Possible Causes: Materials

High Regrind Percentage

PVC & CPVC tend to be very temperature sensitive, and reground PVC & CPVC will cause an increase in material temperature and shrinkage. If too much regrind is being used, the barrel cooling fans may be inadequate to remove this increased heat. Reducing the regrind percentage should lower the material temperature and decrease the creation of volatiles.

Poor Quality Regrind

Regrind with any degraded PVC & CPVC will cause a dangerous chain reaction of degradation. This hotter material will typically shrink more than expected. Any regrind containing degradation or burning should be discarded before it causes more defects or a serious safety concern.

Material Degradation

Degradation of PVC & CPVC significantly reduces the strength and performance of PVC & CPVC. Removing the bad material from the barrel helps prevent degraded polymer from contaminating all other material it contacts. If a Hot Runner System is being used, purge it as well.

Possible Causes: Injection

Inadequate or Blocked Venting

Poor venting can cause gas entrapment during injection. This gas can cause short shots, which may also result in small part dimensions when measured. Molds should be vented wherever possible; they must be vented to the atmosphere, and cross-cavity venting should be avoided whenever possible. If routine vent cleaning is needed, there should be a procedure for scheduled vent cleaning before they become blocked.

Flow Obstructions or Restrictions

Check for obstructions in the nozzle, gate, or hot runner as well as a faulty heater band on the barrel, nozzle, or hot runner system.

High Injection Time

This indicates material is entering the mold slowly, causing an increase in material viscosity. A higher viscosity PVC & CPVC experiences increased pressure loss during injection and packing, which may result in a smaller under-packed part. Increasing Injection Speed will help you decrease Injection Time. You should also check your Maximum Injection Pressure in case there is not enough Injection Pressure available to maintain the Injection Speed.

Injection Profile

PVC & CPVC typically require an injection profile to make a good part. For example, the gate area often needs a lower injection speed to reduce shear. A significant change in the injection profile will have a significant effect on the strength of the molded part. The injection profile should match the process-specific documentation.

Low Part Weight at Transfer

This indicates too little material is entering the mold during injection, which may reduce part dimensions due to inadequate packing. If other parameters are similar and only the weight is different, raising shot size or decreasing transfer position should increase part weight at transfer.

Pressure-Limited Process

A pressure-limited process cannot maintain the desired injection speed. If a change in lot number, colorant, or regrind is causing a viscosity increase, you may need to raise the shot size or decrease the transfer position to compensate for the higher viscosity material. Flow obstructions and faulty equipment should always be checked when the maximum pressure is reached. In some cases, an increase in the maximum injection pressure is needed to accommodate a higher viscosity material, but the remainder of the process should be verified to ensure there are no other causes for a high injection pressure before this change is made.

Possible Causes: Packing

Low Packing Pressure

This indicates there is not enough pressure to fill the mold and compensate for shrinkage during packing. An under-packed part will typically have reduced smaller dimensions. The packing pressure should be set to match the process-specific documentation, and the packing speed should be set to the same value as your last Injection Speed before Transfer from Injection to Packing.

Low Packing Time

With PVC & CPVC, it is difficult to get a full gate seal at the end of packing. Insufficient packing time will cause material will leave the part in the runner or sprue. This tends to reduce part dimensions. A packing time study should be done to ensure the packing time is adequate to achieve a good gate appearance.

Low Final Part Weight

This indicates insufficient material is entering the mold cavity during Injection and packing. This is indicative of a change in the process, but you need to find other changes in the process before action can be taken.

High Clamp Force

Excessive clamp force will prevent the air from venting as fast as the material is entering the mold. This gas can cause short shots, which can be measured as small dimensions. The Clamp Tonnage should be set to match the Process-Specific Documentation. You will likely need to clean the vents at this time.

No Cushion

A process without a cushion does not have enough material to fill the part and compensate for shrinkage during packing. This results in an under-packed part with smaller dimensions than expected. The cushion should always be approximately 10% of the Shot Size. Both the shot size and transfer position must be increased to raise the cushion.

High Cushion

Melted PVC & CPVC are highly compressible plastics; thus, a process with an excessive cushion, such as 25% of the shot size, can lose significant injection & packing pressure. The cushion should always be approximately 10% of the Shot Size. Both the shot size and transfer position must be decreased to lower the cushion.

Possible Causes: Machine

Check Ring Performance (Intermittent Defect)

If a check ring is being used, perform a check ring performance study by weighing 10 consecutive short shots (weight all cavities + runner) and calculating variability. Over 2% variability means the check ring is suspect, and should be changed immediately if over 3% Variability. Purging the Barrel or increasing Decompression after Recovery may improve check ring performance.

% Variability = (100%) * (Max - Min) / (Average)

Possible Causes: Mold & Design

High Cavity Imbalance (Cavity to Cavity Variation)

Significant differences in cavity filling can cause some cavities to fill and start packing during injection, while other cavities are significantly short at transfer. Make sure all the mold cavities are short at the time of transfer to minimize any cavity-to-cavity variation in the final molded part. There are steps that can improve cavity imbalance, such as using a different injection speed, adjusting hot runner gate drops, cleaning the mold vents, increasing the number of vents, or adjusting gate width when necessary.

Design Concerns

Increasing the mold dimensions will increase the dimensions of the final molded part. Good part design strategies, such as gating into thick sections, avoiding sharp transitions, maintaining the same thickness when possible, and providing adequate venting are important to improving mold filling and will help maintain more consistent shrinkage across the whole part. Increasing wall thickness will also increase part shrinkage in areas that are sticking. Decreasing undercuts, increasing draft angles, and draw polishing the cavities will help the part separate from the cavity during mold opening. It is not uncommon for minor undercuts to be added to the core side of the mold to help pull the part away from the cavity during mold opening.

Initial Troubleshooting

• Check Part Dimensions and Troubleshoot:
  • Large Part Dimensions
  • Small Part Dimensions
  • Parts Smaller at Gate
  • Parts Larger at Gate
  • Parts Smaller at End of Fill
• Purge Barrel
• Verify Process:
  • Material Temperature & Back Pressure
  • Barrel Residence Time
  • Recovery Time
  • Coolant Temperature, Pressure, & Flow
  • Cooling Time
  • Injection Time & Profile
  • Part Weight at Transfer (no runner or sprue)
  • Packing Pressure, Time, & Speed
  • Final Part Weight
• Verify Setup Sheet
  • Initial Ejection Speed
  • Mold Breakaway
  • Cooling Line Layout
• Verify Automation & Robotics
  • Alignment
  • Synchronization
  • Proper Functionality
• Check:
  • Core & Cavity for damage
  • Cushion is approximately 10% of Shot Size
  • Process is not Pressure Limited
  • Check Ring Performance (if used)
  • Screw & Barrel Heating and Cooling Equipment
• Look for obstructions or faulty equipment in the nozzle, gate, or hot runner

Possible Causes: Recovery

Low Material Temperature or Back Pressure

A lower material temperature increases the cooling rate, resulting in lower part shrinkage than expected. Returning the material temperature and back pressure to match the process-specific documentation should reduce part shrinkage. It may be necessary to check the thermocouples & heater bands on the barrel & hot runner system.

High Material Temperature or Back Pressure

A high temperature or back pressure can slow the cooling rate, resulting in more part shrinkage than expected. Returning the material temperature and back pressure to match the Process-Specific Documentation should help these situations. You may need to further reduce the barrel temperature or back pressure if material degradation is suspected. You should always purge the barrel when degradation is suspected with PVC & CPVC. It may be necessary to check the cooling fans, screw cooling, thermocouples, & heater bands.

Low Recovery Time

A high Screw RPM can cause inconsistent recovery and excessive shear during recovery. The screw RPM should be adjusted so that the recovery time consumes most of the cooling time.

Low Barrel Residence Time

A short barrel residence time may prevent the PVC & CPVC from fully melting during recovery. A lower material temperature increases the cooling rate, resulting in lower part shrinkage. This situation can be due to a significantly smaller barrel being used than was used on the documented standard. In some cases, barrel temperature or back pressure can be increased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

High Barrel Residence Time

A long barrel residence time can cause the PVC & CPVC temperature to increase. This may cause increased part shrinkage. In some cases, barrel temperature can be decreased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

Possible Causes: Cooling

Low Coolant Temperature or High Coolant Flow

PVC & CPVC tend to be very temperature sensitive, and a cold mold can cause excessive pressure loss during injection & packing. This pressure loss often creates inconsistent shrinkage as well as excessive stresses within the part. Returning the coolant temperature to match the process-specific documentation should reduce the pressure loss in the mold. Using a coolant temperature controller with a lower-rated coolant pump or reducing the coolant flow with a flow control valve may return the water flow to match that of the documented standard.

High Coolant Temperature or Low Coolant Flow

PVC & CPVC tend to be very temperature sensitive, and a hot mold increases material shrinkage due to a slower cooling rate. Returning the coolant temperature to match the process-specific documentation should return the pressure loss in the mold to normal. Check the cooling system for obstructions, as this will reduce coolant flow. Using a coolant temperature controller with a higher-rated coolant pump may raise the coolant flow to match the process-specific documentation.

Cooling Line Layout

An incorrect cooling line layout can affect the material flow and cooling. Verify the Cooling Line Layout matches the Process-Specific Documentation.

Low Cooling Time

Lower cooling times cause a slower cooling rate because the part cools faster inside the mold than it does outside the mold. This slower cooling rate will cause more shrinkage in the final part. The cooling time should match the documented standard.

High Cooling Time

A high cooling time increases the cooling rate, resulting in lower part shrinkage. The cooling time should match the documented standard.

Possible Causes: Materials

High Regrind Percentage

PVC & CPVC tend to be very temperature sensitive, and reground PVC & CPVC will cause an increase in material temperature and shrinkage. Adversely, degraded PVC tends to give off gases, which can cause the dimensions to increase if they get trapped in the part. If too much regrind is being used, the barrel cooling fans may be inadequate to remove this increased heat, resulting in a weakened part due to material degradation. Reducing the regrind percentage should lower the material temperature and decrease the creation of volatiles.

Poor Quality Regrind

Regrind with any degraded PVC & CPVC will cause a dangerous chain reaction of degradation. This hotter material will typically shrink more than expected. Adversely, degraded PVC tends to give off gases, which can cause the dimensions to increase if they get trapped in the part. Any regrind containing degradation or burning should be discarded before it causes more defects or a serious safety concern.

Material Degradation

Degradation of PVC & CPVC significantly reduces the strength and performance of PVC & CPVC. Removing the bad material from the barrel helps prevent degraded polymer from contaminating all other material it contacts. If a Hot Runner System is being used, purge it out as well.

Poor Material Drying

Although PVC & CPVC do not always need drying, they are hygroscopic polymers. The absorbed moisture will come out of the plastic during processing, which can affect the viscosity and physical properties of the molded part. Although grades vary, most PVC & CPVC resins should be dried for 1-2 hours at 65 to 80°C (150 to 175°F) with dry air (approx. -40°C or -40°F Dewpoint) to a moisture content below 0.3%.

Possible Causes: Injection

Inadequate or Blocked Venting

Poor venting can cause gas entrapment during injection. This gas can cause short shots, which may also result in small part dimensions when measured. Molds should be vented wherever possible; they must be vented to the atmosphere, and cross-cavity venting should be avoided whenever possible. If routine vent cleaning is needed, there should be a procedure for scheduled vent cleaning before they become blocked.

Flow Obstructions or Restrictions

Check for obstructions in the nozzle, gate, or hot runner as well as a faulty heater band on the barrel, nozzle, or hot runner system.

Low Injection Time

This indicates the material is entering the mold faster, which will reduce the pressure loss during injection and packing. Decreased pressure loss may result in better packing pressure distribution and larger part dimensions. Reducing injection speed will help you increase injection time to match the documented standard.

High Injection Time

This indicates material is entering the mold slowly, causing an increase in material viscosity. A higher viscosity PVC & CPVC experiences increased pressure loss during injection and packing, which may result in a smaller under-packed part. Increasing Injection Speed will help you decrease injection time. You should also check your Maximum Injection Pressure in case there is not enough Injection Pressure available to maintain the Injection Speed.

Injection Profile

PVC & CPVC typically require an injection profile to make a good part. For example, the gate area often needs a lower injection speed to reduce shear. A significant change in the injection profile will have a significant effect on the strength of the molded part. The Injection Profile should match the Process-Specific Documentation.

Low Part Weight at Transfer

This indicates too little material is entering the mold during injection, which may reduce part dimensions due to inadequate packing. If other parameters are similar and only the weight is different, raising shot size or decreasing transfer position should increase part weight at transfer.

High Part Weight at Transfer

This indicates too much material is entering the mold during injection, which may cause over-packing of the part. If other parameters are similar and only the weight is different, lowering shot size or increasing transfer position should decrease part weight at transfer.

Low Peak or Transfer Pressure

This indicates the material viscosity has decreased, and there may be overpacking, but it might also indicate underfilling during injection. If the mold or material temperatures are higher than the documented standard, these can be decreased to reduce the flow of PVC & CPVC. If a change in lot #, colorant, or regrind is causing the viscosity decrease, you may need to decrease the shot size or increase the transfer position to compensate for the lower viscosity material.

High Peak or Transfer Pressure

This indicates the material viscosity has increased or there is an obstruction. Check for flow obstructions. If the mold or material temperatures are lower than the documented standard, these can be increased to allow the PVC & CPVC to flow more easily into the mold.

Pressure-Limited Process

A pressure-limited process cannot maintain the desired injection speed. If a change in lot #, colorant, or regrind is causing a viscosity increase, you may need to raise the shot size or decrease the transfer position to compensate for the higher viscosity material. Flow obstructions and faulty equipment should always be checked when the maximum pressure is reached. In some cases, an increase in the maximum injection pressure is needed to accommodate a higher viscosity material, but the remainder of the process should be verified to ensure there are no other causes for a high injection pressure before this change is made.

Possible Causes: Packing

Low Packing Pressure

This indicates there is not enough pressure to fill the mold and compensate for shrinkage during packing. An under-packed part will typically have reduced strength, which can get damaged or warp during ejection. The packing pressure should be set to match the process-specific documentation, and the packing speed should be set to the same value as your last Injection Speed before Transfer from Injection to Packing.

High Packing Pressure

This indicates there is too much pressure after mold filling, which might over-pack the part. An over-packed part will have larger dimensions than expected. The packing pressure should be set to match the process-specific documentation, and the packing speed should be set to the same value as your last injection speed before transfer from injection to packing.

Low Packing Time

With PVC & CPVC, it is difficult to get a full gate seal at the end of packing. Insufficient packing time will cause material will leave the part in the runner or sprue. This tends to reduce part dimensions. A packing time study should be done to ensure the packing time is adequate to achieve a good gate appearance.

High Packing Time

With PVC & CPVC, it is difficult to get a full gate seal at the end of packing. Excessive packing time may cause overpacking and stress in the gate area. A packing time study should be done to ensure the packing time is adequate to achieve consistent part dimensions.

Low Final Part Weight

This indicates insufficient material is entering the mold cavity during Injection and packing. This is indicative of a change in the process, but you need to find other changes in the process before action can be taken.

High Final Part Weight

This indicates excessive material is entering the mold cavity during Injection and packing. This is indicative of a change in the process, but you need to find other changes in the process before action can be taken.

High Clamp Force

Excessive clamp force will prevent the air from venting as fast as the material is entering the mold. This gas can cause short shots, which can be measured in small dimensions. The clamp tonnage should be set to match the process-specific documentation. You will likely need to clean the vents at this time.

No Cushion

A process without a cushion does not have enough material to fill the part and compensate for shrinkage during packing. This results in an under-packed part with smaller dimensions than expected. The cushion should always be approximately 10% of the shot size. Both the shot size and transfer position must be increased to raise the cushion.

High Cushion

Melted PVC & CPVC are highly compressible plastics; thus, a process with an excessive cushion, such as 25% of the shot size, can lose significant injection & packing pressure. The cushion should always be approximately 10% of the shot size. Both the shot size and transfer position must be decreased to lower the cushion.

Possible Causes: Machine

Check Ring Performance (Intermittent Defect)

If a check ring is being used, perform a check ring performance study by weighing 10 consecutive short shots (weight all cavities + runner) and calculating variability. Over 2% variability means the check ring is suspect, and should be changed immediately if over 3% Variability. Purging the Barrel or increasing Decompression after Recovery may improve check ring performance.

% Variability = (100%) * (Max - Min) / (Average)

Machine Settings

If the part is sticking to the cavity during mold open, slower mold breakaway and ejection speeds will help the part remain on the core side of the mold during mold open. Sprue break can often be used to separate the sprue from the nozzle in cases where the sprue remains stuck during mold opening. If the part is being damaged during part ejection, a slower initial ejection speed will decrease the stress on the part. In any automation & robotics application, there is the expectation that the movements are synchronized. In some cases, a slow mold open speed is needed at the end of mold open to ensure the clamp stops in the correct position for the robotics to properly remove the part. In other cases, the ejection timing & speed must be properly set to move at the same speed as the robot.

Robot Settings & Condition

Poor part handling can easily stress & warp a molded part. Inspection should start with the components that contact the part, ensuring there is no damage, such as worn suction cups, dented grippers, dull cutters, parts stuck in sorting equipment, damaged vacuum lines, or bent end-of-arm tooling. Your company should have spare parts of all automation components, such as grippers, holders, cutters, suction cups, springs, hoses, rails, bolts, nuts, etc., so any suspect component can be replaced immediately. In any automation & robotics application, there is the expectation that the movements are synchronized. The robot must move with the machine ejection system when grabbing the part, as well as move with the automation when setting down the part. Whenever possible, there should be smooth movements when a transfer is taking place from the mold to the robot and the robot to the automation. If the parts are being dropped, packed, or placed into a box, make sure it is adequately cooled beforehand, or retained heat will cause the parts to shrink more than expected after molding. In some cases, it is necessary to fixture the part after molding to ensure it maintains the desired dimensions.

Excessive Stationary Platen Deflection

High deflection in the center of the stationary platen will cause the center of the mold to lose effective clamp force. This deflection during injection and packing can cause a sprue-gated part to be thicker in the center. This is measured with an indicator holder mounted to the machine base and the indicator touching as close to the center of the platen as possible. This measurement can be compared to the measured deflection during routine machine maintenance. To compensate for this, you can add shims around the locating ring or add bolster plates to the mold.

Possible Causes: Mold & Design

High Cavity Imbalance (Cavity to Cavity Variation)

Significant differences in cavity filling can cause some cavities to fill and start packing during injection, while other cavities are significantly short at transfer. Make sure all the mold cavities are short at the time of transfer to minimize any cavity-to-cavity variation in the final molded part. There are steps that can improve cavity imbalance, such as using a different injection speed, adjusting hot runner gate drops, cleaning the mold vents, increasing the number of vents, or adjusting gate width when necessary.

Design Concerns

Increasing the number of grippers or suction cups or the surface area that the robot grabs the part will reduce the overall pressure or vacuum needed to control the part during part removal and secondary operations. Contacting the part in non-cosmetic areas of the part may also help reduce the impact of automation and robotics on the part. Decreasing undercuts, increasing draft angles, increasing ejection components, and draw polishing cores and cavities will help the part separate from the mold during the part removal process. Good part design strategies, such as gating into thick sections, avoiding sharp transitions, maintaining the same thickness when possible, and providing adequate venting are important to improving mold filling. Increasing wall thickness will also increase the Mechanical or Resistance properties of PVC & CPVC. Since it is difficult to achieve full theoretical strength with PVC & CPVC, a higher safety factor should be used than is common with most polymers. For example, a designer may typically design a Polypropylene part 15% thicker as a safety margin, but a PVC & CPVC part should be 25% or 30% thicker than needed to get a similar safety margin.

Initial Troubleshooting

• Purge Barrel with Purging Compound
• Clean and Inspect Material Handling Equipment, Including:
  • Hopper
  • Filters
  • Screens
  • Bins
  • Dryer
  • Blender
  • Access Doors
  • Hoses
• Inspect Material and Replace if Contaminated
• Verify Process:
  • Material Temperature & Back Pressure
  • Barrel Residence Time
  • Recovery Time
  • Cycle Time
• Check Material Drying:
  • Material Moisture
  • Dryer Residence Time
  • Dryer Temperature
• Check Screw & Barrel Cooling Equipment

Possible Causes: Materials

High Regrind Percentage

PVC and CPVC tend to be very temperature sensitive, and reground PVC or CPVC can cause degradation, which may appear as contamination. If too much regrind is being used, the barrel cooling fans may be inadequate to remove this increased heat, causing degradation, which will contaminate the polymer. Reducing the regrind percentage should lower the material temperature and decrease the creation of volatiles.

Poor Quality Regrind

Regrind with any degraded PVC or CPVC will cause a dangerous chain reaction of degradation, creating a very dangerous situation. Contaminated regrind will cause contamination in the final part. Any regrind containing degradation or burning should be discarded before it causes more defects or a serious safety concern. Any contaminated regrind should be discarded.

Material Degradation

Degraded PVC & CPVC give off a very distinct burnt PVC or CPVC smell. Removing the bad material from the barrel helps prevent degraded polymer from contaminating all other material it contacts. If a Hot Runner System is being used, purge it out as well.

Material Contamination

Contamination can come from many locations, including storage, hopper, hoses, filters, screens, surge bins, access doors, seals, and blenders. All potential sources of contamination must be checked to determine where the contamination is coming from. Once the source is determined, everything that came in contact with the material afterwards must be thoroughly cleaned and inspected before returning to use.

Poor Material Drying

Although PVC and CPVC do not always need drying, they are hygroscopic polymers. Excessive drying will degrade the polymer, which can contaminate the final product. The dryer should also be thoroughly checked for any sources of contamination, including hoses, seals, and filters. Although grades vary, most PVC & CPVC resins should be dried for 1-2 hours at 65 to 80°C (150 to 175°F) with dry air (approx. -40°C or -40°F Dewpoint) to a moisture content below 0.3%.

Possible Causes: Recovery

High Material Temperature or Back Pressure

PVC and CPVC tend to be very temperature sensitive, and excessive heat or shear can quickly cause degradation. Small amounts of degradation can often appear as contamination. Returning the material temperature and back pressure to match the process-specific documentation should return the pressure loss in the mold to normal. If regrind is being used, reduce the regrind percentage. You should always purge the barrel when degradation is suspected with PVC and CPVC. It may be necessary to check the cooling fans, screw cooling, thermocouples, & heater bands.

Low Recovery Time

A high Screw RPM can cause inconsistent recovery and excessive shear during recovery. The screw RPM should be adjusted so the Recovery Time consumes most of the Cooling Time.

High Barrel Residence Time

A long barrel residence time can cause the heat-sensitive PVC or CPVC to heat up or degrade. Small amounts of degradation can often appear as contamination. In some cases, barrel temperature can be decreased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

Possible Causes: Machinery

Machine Settings

Always verify there is a delay or decompression prior to screw recovery to reduce the pressure in front of the screw. If screw recovery begins immediately after packing, the material will experience a brief moment of very high shear. This high shear can create small amounts of degraded PVC or CPVC in the barrel.

Initial Troubleshooting

• Purge Barrel with Purging Compound
• Check Material Drying:
  • Material Moisture
  • Dryer Residence Time
  • Dryer Temperature
• Inspect Material and Replace if Contaminated
• Clean and Inspect Material Handling Equipment, Including:
  • Hopper
  • Filters
  • Screens
  • Bins
  • Dryer
  • Blender
  • Access Doors
  • Hoses
• Clean & Inspect the Vents
• Verify Process:
  • Material Temperature & Back Pressure
  • Barrel Residence Time
  • Recovery Time
  • Cycle Time
  • Clamp Force
• Check Screw & Barrel Cooling Equipment

Possible Causes: Materials

High Regrind Percentage

PVC and CPVC tend to be very temperature sensitive, and reground PVC or CPVC will cause volatiles in the material to burn or vaporize. These volatiles can often appear as splay in the final molded part. If too much regrind is being used, the barrel cooling fans may be inadequate to remove this increased heat, causing degradation. Reducing the regrind percentage should lower the material temperature and decrease the creation of volatiles.

Poor Quality Regrind

Regrind with any degraded PVC or CPVC will cause a dangerous chain reaction of degradation, creating a very dangerous situation of degradation. During degradation, volatiles escaping the polymer melt during injection will often create a splay condition. Any regrind containing degradation or burning should be discarded before it causes more defects or a serious safety concern. Any contaminated regrind should be discarded.

Material Degradation

Degraded PVC & CPVC give off a very distinct burnt PVC or CPVC smell. Removing the bad material from the barrel helps prevent degraded polymer from contaminating all other material it contacts. If a Hot Runner System is being used, purge it out as well.

Material Contamination

Liquids and non-compatible polymers will often cause a splay condition when injected. Contamination can come from many locations, including storage, hopper, hoses, filters, screens, surge bins, access doors, seals, and blenders. All potential sources of contamination must be checked to determine where the contamination is coming from. Once the source is determined, everything that came in contact with the material afterwards must be thoroughly cleaned and inspected before returning to use.

Poor Material Drying

Although PVC and CPVC do not always need drying, they are hygroscopic polymers. The absorbed moisture will come out of the plastic during processing as gas, which can increase the likelihood of splay. Excessive drying will create volatiles, which can cause splay on the final part. The dryer should also be thoroughly checked for any sources of contamination, including hoses, seals, and filters. Although grades vary, most PVC & CPVC resins should be dried for 1-2 hours at 65 to 80°C (150 to 175°F) with dry air (approx. -40°C or -40°F Dewpoint) to a moisture content below 0.3%.

Possible Causes: Recovery

Low Material Temperature or Back Pressure

PVC and CPVC tend to be very shear-sensitive, and a cold material might cause excessive shear during injection, which can release volatiles and give the appearance of splay. Returning the Material Temperature and Back Pressure to match the Process-Specific Documentation should reduce the generation of volatiles. It may be necessary to check the thermocouples & heater bands on the barrel & hot runner system.

High Material Temperature or Back Pressure

PVC and CPVC tend to be very temperature sensitive, and a hot material might cause degradation. During degradation, volatiles escaping the polymer melt during injection will often create a splay condition. Returning the material temperature and back pressure to match the process-specific documentation should return the pressure loss in the mold to normal. If regrind is being used, reduce the regrind percentage. You should always purge the barrel when degradation is suspected with PVC and CPVC. It may be necessary to check the cooling fans, screw cooling, thermocouples, & heater bands.

Low Recovery Time

A high Screw RPM can cause inconsistent recovery and excessive shear during recovery. The screw RPM should be adjusted so the Recovery Time consumes most of the Cooling Time.

Low Barrel Residence Time

A short barrel residence time may prevent the PVC or CPVC from fully melting during recovery. This cold material might cause excessive shear during injection, which can release volatiles and give the appearance of splay. This can be due to a significantly smaller barrel being used than was used on the documented standard. In some cases, barrel temperature can be increased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

High Barrel Residence Time

A long barrel residence time can cause the heat-sensitive PVC or CPVC to heat up or degrade. During degradation, volatiles escaping the polymer melt during injection will often create a splay condition. In some cases, barrel temperature can be decreased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

Possible Causes: Cooling

Low Coolant Temperature or High Coolant Flow

Water will condense on the mold surface if the temperature is below the dew point of the surrounding air. When the hot polymer touches the condensation, it will vaporize during injection, causing splay on the part surface. Returning the coolant temperature to match the process-specific documentation should reduce the pressure loss in the mold. Using a coolant temperature controller with a lower-rated coolant pump or reducing the coolant flow with a flow control valve may return the water flow to match that of the documented standard. If the plant is not climate-controlled, the mold temperature will have to be increased above the dew point of the surrounding air, which will require a new process to be developed.

Possible Causes: Injection

Inadequate or Blocked Venting

Poor venting can reduce the venting of volatiles, resulting in splay during injection. Molds should be vented wherever possible; they must be vented to the atmosphere, and cross-cavity venting should be avoided whenever possible. If routine vent cleaning is needed, there should be a procedure for scheduled vent cleaning before they become blocked.

Possible Causes: Packing

High Clamp Force

Excessive clamp force can reduce the venting of volatiles, resulting in splay during injection. The Clamp Force should be set to match the process-specific documentation. You will likely need to clean the vents at this time.

Possible Causes: Machine

Machine Settings

Excessive screw decompression after recovery may draw some air into the barrel. Sprue break can expose the heater polymer at the nozzle tip and may start burning and creating volatiles when exposed during sprue break. If a small amount of drooling is present at the nozzle tip during sprue break, it may enter the sprue during carriage-forward and become injected with the polymer into the mold. This cooled material may cause a splay-like appearance as it is over-sheared during injection.

Possible Causes: Mold & Design

Buildup of Volatiles on Mold Surface

Extensive splay on the texture or polish will build up on the mold surface and eventually cause the appearance of splay. Solvents or surface-clearing solutions like a dry ice-blaster will help clean up the mold surface.

Mold Surface Damage

Extensive splay on the texture or polish will corrode the surface over time. This will likely need repair to significantly improve the part's appearance.

Initial Troubleshooting

• Purge Barrel with Purging Compound
• Check Material Drying:
  • Material Moisture
  • Dryer Residence Time
  • Dryer Temperature
• Inspect Material and Replace if Contaminated
• Clean and Inspect Material Handling Equipment, Including:
  • Hopper
  • Filters
  • Screens
  • Bins
  • Dryer
  • Blender
  • Access Doors
  • Hoses
• Clean & Inspect the Vents
• Verify Process:
  • Material Temperature & Back Pressure
  • Barrel Residence Time
  • Recovery Time
  • Cycle Time
• Check Screw & Barrel Cooling Equipment

Possible Causes: Materials

High Regrind Percentage

PVC and CPVC tend to be very temperature sensitive, and reground PVC or CPVC will cause volatiles in the material to vaporize. These volatiles can get trapped beneath the part surface, causing bubbles or blisters on the final molded part. If too much regrind is being used, the barrel cooling fans may be inadequate to remove this increased heat, causing degradation. Reducing the regrind percentage should lower the material temperature and decrease the creation of volatiles.

Poor Quality Regrind

Regrind with any degraded PVC or CPVC will cause a chain reaction of degradation, creating a very dangerous situation. During degradation, volatiles get trapped beneath the part’s surface, causing bubbles or blisters on the final molded part. Any regrind containing degradation or burning should be discarded before it causes more defects or a serious safety concern. Any contaminated regrind should be discarded.

Material Degradation

Degraded PVC & CPVC give off a very distinct burnt PVC or CPVC smell. Removing the bad material from the barrel helps prevent degraded polymer from contaminating all other material it contacts. If a Hot Runner System is being used, purge it out as well.

Material Contamination

Liquids and non-compatible polymers can cause a bubble or blister condition if they vaporize and become trapped within the part. Contamination can come from many locations, including storage, hopper, hoses, filters, screens, surge bins, access doors, seals, and blenders. All potential sources of contamination must be checked to determine where the contamination is coming from. Once the source is determined, everything that came in contact with the material afterwards must be thoroughly cleaned and inspected before returning to use.

Poor Material Drying

Although PVC and CPVC do not always need drying, they are hygroscopic polymers. The absorbed moisture will come out of the plastic during processing as gas, which can increase the likelihood of bubbles or blisters if trapped within the part. Excessive drying will create volatiles, which can cause bubbles or blisters in the final part. The dryer should also be thoroughly checked for any sources of contamination, including hoses, seals, and filters. Although grades vary, most PVC & CPVC resins should be dried for 1-2 hours at 65 to 80°C (150 to 175°F) with dry air (approx. -40°C or -40°F Dewpoint) to a moisture content below 0.3%.

Possible Causes: Recovery

Low Material Temperature or Back Pressure

PVC and CPVC tend to be very shear-sensitive, and a cold material might cause excessive shear during injections, which can release volatiles that can get trapped beneath the part surface. Returning the Material Temperature and Back Pressure to match the Process-Specific Documentation should reduce the generation of volatiles. It may be necessary to check the thermocouples & heater bands on the barrel & hot runner system.

High Material Temperature or Back Pressure

PVC and CPVC tend to be very temperature sensitive, and a hot material might cause degradation. During degradation, volatiles trapped in the polymer melt will often create a bubble or blister condition. Returning the material temperature and back pressure to match the process-specific documentation should return the pressure loss in the mold to normal. If regrind is being used, reduce the regrind percentage. You should always purge the barrel when degradation is suspected with PVC and CPVC. It may be necessary to check the cooling fans, screw cooling, thermocouples, & heater bands.

Low Recovery Time

A high Screw RPM can cause inconsistent recovery and excessive shear during recovery. The screw RPM should be adjusted so the Recovery Time consumes most of the Cooling Time.

Low Barrel Residence Time

A short barrel residence time may prevent the PVC or CPVC from fully melting during recovery. This cold material might cause excessive shear during injection, which can generate volatiles that can become trapped beneath the part surface. This can be due to a significantly smaller barrel being used than was used on the documented standard. In some cases, barrel temperature can be increased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

High Barrel Residence Time

A long barrel residence time can cause the heat-sensitive PVC or CPVC to heat up or degrade. During degradation, volatiles in the polymer melt can often create a bubble or blister condition. In some cases, barrel temperature can be decreased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

Possible Causes: Cooling

Low Coolant Temperature or High Coolant Flow

Water will condense on the mold surface if the temperature is below the dew point of the surrounding air. The mold surface moisture may get trapped in the part, resulting in bubbles or blisters. Returning the coolant temperature to match the process-specific documentation should reduce the pressure loss in the mold. Using a coolant temperature controller with a lower-rated coolant pump or reducing the coolant flow with a flow control valve may return the water flow to match that of the documented standard. If the plant is not climate-controlled, the mold temperature will have to be increased above the dew point of the surrounding air, which will require a new process to be developed.

Possible Causes: Injection

Inadequate or Blocked Venting

Poor venting can reduce the venting of volatiles, which can get trapped in the polymer, causing bubbles or blisters. Molds should be vented wherever possible; they must be vented to the atmosphere, and cross-cavity venting should be avoided whenever possible. If routine vent cleaning is needed, there should be a procedure for scheduled vent cleaning before they become blocked.

High Part Weight at Transfer

This indicates more material is entering the mold during injection than expected. This condition may cause a cavity pressure spike during Injection, but can also cause the screw to bounce back as the polymer flows out of the mold cavity and back into the runner system. This rapid flow reversal can cause a layering of polymer between the polymer cooling and freezing and the retreating flow. Although this condition is technically delamination, the appearance can often look like bubbles or blisters. If other parameters are similar and only the weight is different, lowering shot size or increasing transfer position should increase part weight at transfer, as well as help maintain forward screw travel at transfer.

Possible Causes: Packing

High Clamp Force

Excessive clamp force can reduce the venting of trapped volatiles, resulting in bubbles or blisters. The clamp force should be set to match the process-specific documentation. You will likely need to clean the vents at this time.

Possible Causes: Machine

Machine Settings

Excessive screw decompression after recovery may draw some air into the barrel. Sprue break can expose the heater polymer at the nozzle tip and may start burning and creating volatiles when exposed during sprue break.

Initial Troubleshooting

• Purge Barrel with Purging Compound
• Clean and Inspect Material Handling Equipment, Including:
  • Hopper
  • Filters
  • Screens
  • Bins
  • Dryer
  • Blender
  • Access Doors
  • Hoses
• Clean & Inspect the Vents
• Verify Process:
  • Material Temperature
  • Back Pressure
  • Barrel Residence Time
  • Recovery Time
  • Cycle Time
  • Coolant Temperature & Flow
  • Injection Time
  • Packing Pressure
• Check Material Drying:
  • Material Moisture
  • Dryer Residence Time
  • Dryer Temperature
• Check Screw & Barrel Cooling Equipment

Possible Causes: Materials

High Regrind Percentage

PVC and CPVC tend to be very temperature sensitive, and reground PVC or CPVC can cause degradation, which will change the material color. If too much regrind is being used, the barrel cooling fans may be inadequate to remove this increased heat, causing degradation. Reducing the regrind percentage should lower the material temperature and decrease the creation of volatiles.

Poor Quality Regrind

Regrind with any degraded PVC or CPVC will cause a dangerous chain reaction of degradation, creating a very dangerous situation. Contaminated regrind will cause a color change in the final part. Any regrind containing degradation or burning should be discarded before it causes more defects or a serious safety concern. Any contaminated regrind should be discarded.

Material Degradation

Degraded PVC & CPVC give off a very distinct burnt PVC or CPVC smell. Removing the bad material from the barrel helps prevent degraded polymer from contaminating all other material it contacts. If a Hot Runner System is being used, purge it out as well.

Material Contamination

Many forms of contamination can change the color of the polymer. This can come from many locations, including storage, hopper, hoses, filters, screens, surge bins, access doors, seals, and blenders. All potential sources of contamination must be checked to determine where the contamination is coming from. Once the source is determined, everything that came in contact with the material afterwards must be thoroughly cleaned and inspected before returning to use.

Poor Material Drying

Although PVC and CPVC do not always need drying, they are hygroscopic polymers. The absorbed moisture will come out of the plastic, which can affect the mixing of the colorant as well as the surface gloss. Excessive drying will degrade the polymer, which will change the color. The dryer should also be thoroughly checked for any sources of contamination, including hoses, seals, and filters. Although grades vary, most PVC & CPVC resins should be dried for 1-2 hours at 65 to 80°C (150 to 175°F) with dry air (approx. -40°C or -40°F Dewpoint) to a moisture content below 0.3%.

Poor Colorant Mixing

The polymer and colorant must be thoroughly mixed before entering the feedthroat. The colorant must be added at the correct weight ratio and then mixed using the correct material mixers or blenders for the application. If the blending is done using a volumetric blender, then the bulk density differences between the base material and the colorant must be tested and considered when calculating the volumetric blending ratios.

Possible Causes: Recovery

Low Material Temperature or Back Pressure

Pellet or powder-based colorants are designed to melt early so they begin to disperse and mix as soon as possible. If the polymer takes too long to heat up, the colorant will not have enough time to disperse. Returning the material temperature and back pressure to match the process-specific documentation should improve mixing during recovery. It may be necessary to check the thermocouples & heater bands on the barrel & hot runner system.

High Material Temperature or Back Pressure

PVC and CPVC tend to be very temperature sensitive, and an excessive heat or shear can quickly cause degradation and change the material color. Returning the material temperature and back pressure to match the process-specific documentation should return the color to normal. If regrind is being used, reduce the regrind percentage. You should always purge the barrel when degradation is suspected with PVC and CPVC. It may be necessary to check the cooling fans, screw cooling, thermocouples, & heater bands.

Low Recovery Time

A high screw RPM can cause inconsistent recovery and excessive shear during recovery. The screw RPM should be adjusted so that the recovery time consumes most of the cooling time.

Low Barrel Residence Time

A low barrel residence time may not provide enough time for the pellet or powder-based colorant to melt and properly disperse for even mixing. In some cases, barrel temperature can be increased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

High Barrel Residence Time

A long barrel residence time can cause the heat-sensitive PVC or CPVC to heat up or degrade. Any degradation can change the color of PVC or CPVC. In some cases, barrel temperature can be decreased to compensate for this, but the material temperature should always be verified before any temperature changes are made.

Possible Causes: Cooling

Low Coolant Temperature or High Coolant Flow

PVC and CPVC tend to be very temperature sensitive, and a cold mold can prevent the material from properly replicating the mold surface. This can change the gloss of the final part, making the part appear to have a different color to the eye. Returning the coolant temperature to match the process-specific documentation should reduce the pressure loss in the mold. Using a coolant temperature controller with a lower-rated coolant pump or reducing the coolant flow with a flow control valve may return the water flow to match that of the documented standard.

High Coolant Temperature or Low Coolant Flow

PVC and CPVC tend to be very temperature sensitive, and a warm mold will change how the plastic reaches and duplicates the mold surface. This can change the gloss of the final part, making the part appear to have a different color to the eye. Returning the coolant temperature to match the process-specific documentation should return the pressure loss in the mold to normal. Check the cooling system for obstructions, as this will reduce coolant flow. Using a coolant temperature controller with a higher-rated coolant pump may raise the coolant flow to match the process-specific documentation.

Possible Causes: Injection

Inadequate or Blocked Venting

Poor venting can cause gas entrapment on the mold surface. This can change the gloss of the final part, making the part appear to have a different color to the eye. Molds should be vented wherever possible; they must be vented to the atmosphere, and cross-cavity venting should be avoided whenever possible. If routine vent cleaning is needed, there should be a procedure for scheduled vent cleaning before they become blocked.

Low Injection Time

This indicates that the material is entering the mold too quickly, which reduces its viscosity and may cause gas entrapment. Both of these conditions can change the gloss of the final part. Reducing injection speed will help you increase injection time.

High Injection Time

This indicates material is entering the mold slowly, causing an increase in material viscosity. A higher viscosity PVC or CPVC may flow differently and may not be able to properly contact and duplicate the mold surface, resulting in a change in surface gloss. Increasing Injection Speed will help you decrease Injection Time. You should also check your Maximum Injection Pressure in case there is not enough Injection Pressure available to maintain the Injection Speed.

Possible Causes: Packing

Low Packing Pressure

This indicates there is not enough pressure to fill the mold and compensate for shrinkage during packing. An under-packed part may not properly replicate the mold surface. The packing pressure should be set to match the process-specific documentation, and the packing speed should be set to the same value as your last Injection Speed before Transfer from Injection to Packing.

High Packing Pressure

This indicates there is too much pressure after mold filling, possibly causing the plastic to get too much mold surface detail. This is usually a concern only when there are volatiles, rust, corrosion, or damage on the mold surface. If there are concerns with the mold surface, it should be corrected before the problems worsen. The packing pressure should be set to match the process-specific documentation.

High Clamp Force

Excessive clamp force will prevent the air from venting as fast as the material is entering the mold. This condition can change the part's appearance due to gas entrapment. The clamp tonnage should be set to match the process-specific documentation. You will likely need to clean the vents at this time.

Possible Causes: Mold & Design

Buildup of Volatiles on Mold Surface

Extensive trapped gas on the texture or polish will build up on the mold surface, changing the color appearance. This may occur in areas with bad venting or show up as bands when overall venting is poor. Solvents or surface-clearing solutions like a dry ice-blaster will help clean up the mold surface.

Mold Surface Damage

Extensive trapped gas on the texture or polish will corrode the surface over time, changing the color appearance. This will likely need repair to significantly improve the part's appearance. If this occurs, a strategy for fixing the mold venting should be developed.

Celsius to Fahrenheit (°C to °F)

Millimeters to Inches (mm to in)

Milliliters to Fluid Ounces (ml to fl oz)

Grams To Ounces (g to oz)

bar to psi