Purging Procedures

For Blow Molding

The ASACLEAN purging compound grade for your blow molding process depends on your thermoplastic resin and its processing temperature. Extrusion blow molding processes are classified as either “extrusion blow” or “accumulator blow.”

It is necessary to contact a Purging Process Expert at ASACLEAN for proper purging compound grade selection for use in your specific blow molding application.

Extrusion Blow Purging Procedures

  1. Raise the temperatures of the die about 20°C – 30°C (35°F – 55°F).
  2. Extrude the remaining resin from the system.
  3. Clean the hopper and feed throat.
  4. For unfilled grades of ASACLEAN purging compound, screens and dies may be left in place for purging. Consult your technical service representative for appropriate mesh size and die clearance.
  5. If the machine is heavily contaminated and our glass-filled grades (EX, PX2, NF) are required, remove the screen pack to prevent plugging. Remove the die as well, if necessary (recommended for highly polished surfaces).
  6. Feed the required amount of ASACLEAN purging compound into the hopper.
  7. Start purging at low screw speed first; increase the screw speed to the maximum safe level after material begins coming out from the head.
  8. Periodically stop the screw and allow the ASACLEAN purging compound to sit for a few minutes. Resume screw rotation at maximum safe screw speed.
  9. When the purge is free of contamination, displace the remaining ASACLEAN purging compound with the next processing resin, using the same procedure above.
  10. Change the machine parameters back to normal processing conditions.
  11. Replace the screen pack with a clean one.

Accumulator Blow Purging Procedures

  1. Raise the temperatures of the die about 20°C – 30°C (35°F – 55°F).
  2. Extrude the remaining resin from the system.
  3. Clean the hopper and feed throat.
  4. For unfilled grades of ASACLEAN purging compound, screens and dies may be left in place for purging. Consult your technical service representative for appropriate mesh size and die clearance.
  5. If the machine is heavily contaminated and our glass-filled grades (EX, PX2, NF) are required, remove the screen pack to prevent plugging. Remove the die as well, if necessary (recommended for highly polished surfaces).
  6. Feed the required amount of ASACLEAN purging compound into the hopper.
  7. Start purging at low screw speed first; increase the screw speed to the maximum safe level after material begins coming out from the head. If possible, reduce the accumulator volume until the purge material from the screw appears free of contamination.
  8. Periodically stop the screw and allow the ASACLEAN purging compound to sit for a few minutes. Resume screw rotation at maximum safe screw speed.
  9. If machine allows, increase back pressure to at least 80 – 90 percent.
  10. Fill the accumulator head, and increase the accumulator discharge speed to maximum safe level.
  11. Purge 25 percent of the ASACLEAN purging compound from the accumulator then refill back to 100 percent.
  12. Purge 50 percent of the ASACLEAN purging compound from the accumulator then refill back to 100 percent.
  13. Purge 75 percent of the ASACLEAN purging compound from the accumulator then refill back to 100 percent.
  14. Purge 100 percent of the ASACLEAN purging compound.
  15. When the purge is free of contamination, displace the remaining ASACLEAN purging compound with the next processing resin, using the same procedures above.
  16. Change the machine parameters back to normal processing conditions.
  17. Replace the screen pack with a clean one.

NCH Grade Extrusion Blow Molding

For Resins 175°C – 360°C (350°F – 680°F)

  1. Empty the system of the production resin, and make sure the feed areas are clean.
  2. If possible, and safe, raise heats in all zones after the screw and barrel to at least 230⁰C (450⁰F). Best results will be obtained at 260⁰C to 315⁰C (500⁰F to 600⁰F). Temperatures of outboard heads (in multiple head setups) should be 30⁰C (50⁰F) hotter than inboard heads. Do not exceed 360°C (680°F).

If purging a heat sensitive resin, do not exceed the safe processing temperature of the material. This purge procedure may need to be repeated at elevated temperatures to remove all contamination.

  1. Manually clean and cap all barrel vents, and increase temperatures in the decompression zones by 15⁰C (25⁰F), and make sure the feed areas are clean.
  2. Flush the machine by running 1-4 system volumes of clean, natural blow molding grade HDPE. It is best to use an HDPE that is stiffer than the production resin.
  3. Empty the system of the flush resin, and add Asaclean NCH at normal screw speed until the purge extrudes uniformly from the head(s). If using accumulators, increase the setpoints by 10% and cycle on automatic 2 to 4 times to ensure complete filling.
  4. Turn the screw at minimum controllable rpm for a 30 minute heat soak. Asaclean NCH is heat activated, so expansion will occur during the heat soak. For reciprocating screws, stop the screw during the soak phase, and take a shot every 5 minutes to ensure the head stays full.
  5. Empty the screw and barrel.
  6. If the last of the purge carries contamination, repeat steps 5-7.
  7. Wait 5 minutes with an empty barrel for the Asaclean NCH residue to break down.
  8. Change temperatures to the profile required for the next material.
  9. When safe, displace the Asaclean NCH with the next resin.
  10. Begin production under normal processing conditions.

Injection Blow Molding Purging Procedures

For injection blow molding, please click here to view and follow the purging procedures for injection molding.

For further insight into our recommended purging procedures, visit the Purging Process Resource Center. This page features training videos, guidelines, technical documentation and more to answer your purging compound questions.

Ready to learn even more about ASACLEAN purging compounds and services? Click here to schedule your free, 10-minute consultation with a Purging Process Expert at ASACLEAN.

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