Dyna-Purge Troubleshooting Tips

Listed below is a collection of the most common purging problems and how additional alterations to our “Suggested Purging Procedure” can help you. Please click on your purging problem to see the solution.

Streaking Degradation/Black Specks Displacing the purge Transitioning from one temperature to anotherHeat sensitive resinsClumping or “balling-up” in the feed throat“Windmilling” or screw slippage
Purge overspray

Streaking

Streaking may be caused by a multitude of reasons. For example, if color streaking occurs in the same location on the part, then the color entrapment is likely to be in the nozzle or die. If the streaks appear randomly, then the entrapment is likely to be in the barrel.

If you are experiencing any of the following problems and the suggested purging procedure isn’t enough, click on the problem to find additional recommendations.

Streaking due to using a highly pigmented color.

Options:

a) Raise the barrel temperature approximately 50° – 100ºF (28° – 56ºC) above the normal operating temperature, so that the pigmented resin will be flushed out more effectively by the purging compound.

b) Soak Dyna-Purge at either the standard temperature or higher temperatures. In this case, begin purging until only Dyna-Purge is exiting the machine, then allow it to soak for 5 to 10 minutes before continuing to purge.

Streaking due to contamination at the check ring.

Run several short, high velocity shots. This will help to dislodge and remove contamination around the check ring.

Streaking due to contamination in the nozzle or die.

Raise the temperature of the nozzle or die 50° – 100ºF (28° – 56ºC) to reduce the viscosity of the resident resin. This makes it easier for the high viscosity purge to push out the low viscosity resin.

Streaking due to contamination in the hot runners.

Options:

a) Raise the temperature of the hot runners 50° – 100ºF (28° – 56ºC) to reduce the viscosity of the resident resin. This makes it easier for the high viscosity purge to push out the low viscosity resin.

b) If you have valve gated hot runners and are experiencing contamination in one or more gates, close the other gates and purge through the troublesome gates separately to increase pressure and turbulence.

c) Use short high velocity shots to dislodge and remove contamination.

Streaking due to contamination in your thermal or “hot tip” gates.

Thermal gate or “hot tip” systems form a small solidified cold slug or “bubble” of resin at the gate tip that acts as an insulator. This accumulation of resin makes it very difficult to purge, since you are often merely removing layers and not displacing the entire slug of resin. In many instances, the problem can be solved by installing “heat resistant nozzle tip insulators” or inserts. Tip insulators are available from your hot runner manufacturer. However, not all hot runners work well with tip insulators and the following step-by-step procedure breaks down what to do in those situations.

  1. Increase the temperature at the tips by 50°-100°F (28°-56°C). This will reduce the viscosity of the resident resin, making it easier for the high viscosity purge to push out the lower viscosity resin.
  2. Increase the back pressure between 50% and 100% to improve the purging experience.
  3. Purge with Dyna-Purge following one of the recommended Suggested Purging Procedures (SPP).
  4. Upon start-up of the next production resin inspect the parts to make sure that they are clean and free of contamination. If necessary, proceed to Step 5.
  5. Next, take off the cavity plate and remove the slug of resin from the tip with a clean, soft cloth. Inspect the cavity plate and remove any additional resin from the plate.
  6. Reinstall the cavity plate and continue with the next production resin.
Streaking due to contamination in the accumulator.

Options:

a) Fill the accumulator with Dyna-Purge. Purge a small amount of Dyna-Purge through the die, then soak for 30 minutes. If the accumulator has internal agitation, turn on periodically while soaking. When there are more than two heads, raise the temperature of the outboard heads 50° – 100ºF (28° – 56ºC) higher than the rest of the system. This will compensate for the longer flow path. Empty the accumulator, adjust temperature settings and, using your next resin, flush out the Dyna-Purge.

b) Run the die as close to closed as possible to create more pressure.

Streaking due to contamination in the screw and barrel.

Options:

a) Short bursts of agitation at higher screw speed loosens the carbon or pigment. Subsequently slowing the screw speed down allows the purge to expand and push the loosened contamination out.

b) Raise the barrel temperature approximately 50° – 100ºF (28° – 56ºC) above the normal operating temperature, so that the pigmented resin will be flushed out more effectively by the purging compound.

c) Soak Dyna-Purge at either the standard temperature or higher temperatures. In this case, begin purging until only Dyna-Purge is exiting the machine, then allow to soak for 5 to 10 minutes before continuing to purge.

d) Increase back pressure to improve results by adding force and agitation to the purging process. Be sure to use enough material to fill the barrel, so Dyna-Purge can scrub and push through contamination, including carbon build up. Increasing the back pressure up to 100% may improve the purging experience.

e) Shutting off the machine between a resin or color change may allow for additional contamination to be removed from the barrel and screw. This shut down/start-up procedure allows for Dyna-Purge to bond with any residual contamination on the barrel and screw during cooling. As temperatures are brought up, Dyna-Purge will remove the contamination from the surface and machine. For additional details on this procedure click here and follow the directions for your application.

Streaking due to contamination in the gear pump.

Options:

a) Operate at low delta P (low difference in pressure) to improve purging.

b) To get the optimal cleaning effect in the extruder (and some in the viscoseal of the pump), vary the inlet pressure.

Streaking due to contamination in the vent hole.

Options:

a) Purge with the vent capped to help clean the vent hole. In this case, care should be taken not to allow pressure to build up beyond specifications.

b) Cleaning a vent hole can also be accomplished by introducing small amounts of Dyna-Purge through the vent hole. During the normal purging process, place a small quantity of Dyna-Purge through the vent hole.

Degradation or Black Specks

Black specks/carbon buildup are often caused by one or more of the following:

  • “Dead spots” or negative flow areas in the barrel, nozzle or die.
  • Hot temperature spots in the machine.
  • Additives, fillers and color concentrates from the carrier system that are left behind when they are separated from the resin as it is processed.
  • Degradation of polymers by excess heat, shear action, or residence time in the barrel.
  • Unstable resins being left in the machine for an extended period of time.
  • Lack of a regular preventive maintenance program.

The carbon buildup will eventually break off into pieces and be moved forward by the screw, displaying itself as black specks in the finished product. We recommend regular purging with Dyna–Purge after every color change (to remove separated pigment and additives that adhere to the metal of the screw and barrel), resin change (to remove old resin residue that may have a different viscosity from the next resin and therefore, clings to the screw and degrades), and for shutdown or start-up.

If you are experiencing any of the following problems and the suggested purging procedure isn’t enough, click on the problem to find additional recommendations.

Degradation at the check ring.

Run several short, high velocity shots. This will help to dislodge and remove contamination around the check ring.

Degradation in the nozzle or die.

Raise the temperature of the nozzle or die 50° – 100ºF (28° – 56ºC) to reduce the viscosity of the resident resin. This makes it easier for the high viscosity purge to push out the low viscosity resin.

Degradation in the hot runners.

Options:

a) Raise the temperature of the hot runners 50° – 100ºF (28° – 56ºC) to reduce the viscosity of the resident resin. This makes it easier for the high viscosity purge to push out the low viscosity resin.

b) If you have valve gated hot runners and are experiencing contamination in one or more gates, close the other gates and purge through the troublesome gates separately to increase pressure and turbulence.

c) Use short high velocity shots to dislodge and remove contamination.

Degradation in your thermal or “hot tip” gates.

Thermal gate or “hot tip” systems form a small solidified cold slug or “bubble” of resin at the gate tip that acts as an insulator. This accumulation of resin makes it very difficult to purge, since you are often merely removing layers and not displacing the entire slug of resin. In many instances, the problem can be solved by installing “heat resistant nozzle tip insulators” or inserts. Tip insulators are available from your hot runner manufacturer. However, not all hot runners work well with tip insulators and the following step-by-step procedure breaks down what to do in those situations.

  1. Increase the temperature at the tips by 50° – 100°F (28° – 56°C). This will reduce the viscosity of the resident resin, making it easier for the high viscosity purge to push out the lower viscosity resin.
  2. Increase the back pressure between 50% and 100% to improve the purging experience. As with any purging application, extra caution should be taken not to exceed any of the equipment manufacturer’s operating requirements.
  3. Purge with Dyna-Purge following one of the recommended Suggested Purging Procedures (SPP).
  4. Upon start-up of the next production resin inspect the parts to make sure that they are clean and free of contamination. If necessary, proceed to Step 5.
  5. Next, take off the cavity plate and remove the slug of resin from the tip with a clean, soft cloth. Inspect the cavity plate and remove any additional resin from the plate.
  6. Reinstall the cavity plate and continue with next production resin.
Degradation in the accumulator.

Options:

a) Fill the accumulator with Dyna-Purge. Purge a small amount of Dyna-Purge through the die, then soak for 30 minutes. If the accumulator has internal agitation, turn on periodically while soaking. When there are more than two heads, raise the temperature of the outboard heads 50° – 100ºF (28° – 56ºC) higher than the rest of the system. This will compensate for the longer flow path. Empty the accumulator, adjust temperature settings and, using your next resin, flush out the Dyna-Purge.

b) Run the die as close to closed as possible to create more pressure.

Degradation in the screw and barrel.

Options:

a) Short bursts of agitation at higher screw speed loosens the carbon or pigment. Subsequently slowing the screw speed down allows the purge to expand and push the loosened contamination out.

b) Increase back pressure to improve results by adding force and agitation to the purging process. Be sure to use enough material to fill the barrel, so Dyna-Purge can scrub and push through contamination, including carbon build up. Increasing the back pressure up to 100% may improve the purging experience.

c) Increase the screw speed to help Dyna-Purge flow downstream faster. Start purging at a low speed. When Dyna-Purge starts coming out, increase the screw speed to the maximum safe level. This increases the level of agitation and allows for the Dyna-Purge to plasticize sooner. The combination of producing more thermal heat and agitation may enhance the purging process.

d) Shutting off the machine between a resin or color change may allow for additional contamination to be removed from the barrel and screw. This shut down/start-up procedure allows for Dyna-Purge to bond with any residual contamination on the barrel and screw during cooling. As temperatures are brought up, Dyna-Purge will remove the contamination from the surface and machine. For additional details on this procedure click here and follow the directions for your application.

Degradation in the gear pump.

Options:

a) Operate at low delta P (low difference in pressure) to improve purging.

b) To get the optimal cleaning effect in the extruder (and some in the viscoseal of the pump), vary the inlet pressure.

Degradation in the vent hole.

Options:

a) Purge with the vent capped to help clean the vent hole. In this case, care should be taken not to allow pressure to build up beyond specifications.

b) Cleaning a vent hole can also be accomplished by introducing small amounts of Dyna-Purge through the vent hole. During the normal purging process, place a small quantity of Dyna-Purge through the vent hole.

Degradation occurring on start-up.

Make sure to keep the barrel full to eliminate oxidation. Oxidation is a primary source of degradation. During shutdown, filling the barrel with Dyna-Purge, a thermally stable compound, will minimize this risk.

Displacing the purge

While Dyna–Purge is often “stiffer” than the resident resin, it is also formulated to be “post purged.” However, if the next production resin has a high melt flow, the material might not be able to adequately displace Dyna–Purge, since the higher melt resin will flow over Dyna–Purge rather than displace it. One solution might be using a different Dyna–Purge product, with a carrier system that flows more easily. If that doesn’t work, we suggest “bridging” the melt flow difference by blending the Dyna–Purge, at a 50/50 ratio, with a higher melt resin. The higher melt resin becomes the carrier system for Dyna–Purge and makes it easier to displace the Dyna–Purge and transition to the next production resin.

Transitioning from one resin to another with a different temperature profile

When transitioning from a high temperature profile to a low temperature profile, or vice versa, it is important to use the appropriate purging compounds. For instance, transitioning from PEEK to a TPO, it is important to remove all of the PEEK with a high temperature purging compound such as E2 before transitioning to a lower temperature purging compound such as K before introducing the TPO. This will ensure that all of the residual PEEK and E2 is removed and will make it easier for the TPO to displace the remaining purge.

Conversely, transitioning from a TPO to PEEK would result in degradation of that material and black specks if the TPO and K are not completely displaced.

Heat sensitive resins

Some resins, such as PVC, are heat sensitive. Carbonization due to heat oxidation can lead to degradation within just a few minutes. Therefore, we suggest that you not starve the screw. Dyna-Purge should be introduced at the end of the resin run by adding the Dyna–Purge on top of the resident resin in the hopper. Continue to purge until the purge is “clean and free of contamination.” (see Suggested Purging Procedure- Quick Color change)

Clumping or “balling-up” in the feed throat

Clumping or “balling-up” in the feed throat can be avoided by keeping the feed throat cool. The temperature of the feed throat coolant should be maintained at 80° – 120°F (27° – 49°C). It is best to control the feed throat temperature as close to 100°F (38°C) as possible. In humid weather, the temperature should be just warmer than the dew point to avoid condensation. Also, in some cases, keeping the screw turning at a low RPM will help to prevent bridging.

“Windmilling” or screw slippage

This phenomenon is due to poor solids conveying. Solids conveying occurs when the plastic material “sticks” to the barrel and “slips” on the screw. This is necessary for the material to be moved forward.

Normally you can solve this problem by adjusting your barrel temperature profile. Try switching from the commonly used “flat” profile to a reverse temperature profile, where the barrel temperatures are higher in the rear and lower at the discharge end of the barrel. For example, if your current profile, starting in the feed zone, is 450F, 450F, 450F, and 450F, adjust that to 500F, 480F, 460F, and 440F.

Some molders believe that higher settings in the rear zones will increase their melt temperature. Not so – the plastic is still in pellet form in the feed section of the screw; increasing the heat there will improve the coefficient of friction at the barrel wall between the pellets and barrel, and, therefore, improve the solids conveying and eliminate “windmilling.”

– Timothy Womer, TWWomer and Associates, LLC
Mr. Womer has been a recognized authority in plastics technology and machinery for over 35 years and is a member of the Plastics Hall of Fame.

Purge overspray

To minimize purge overspray, hang a scrap part or a piece of cardboard* on the injector pins of the movable platen and close the gap between the movable and stationary platens. After purging, discard the scrap part or cardboard.

*Dyna-Purge offers Lava-Shield®, a safe, customizable and reusable device to capture molten plastic. For more information on Lava-Shield, click here.

“We have developed the following tips that in some instances may enhance your purging results.”

Darci Amos

Sales Consultant, Dyna-Purge