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Purging a Longsheng Line from PP to PET

Time: 2026-11-07 11:36:25.966

Why the changeover is difficult

PET runs hotter than PP, often 30 to 50 degrees Celsius higher at the die. Any PP remaining on the screw, in the adapter or on the die lip will degrade at that temperature and release black specks into transparent sheet, where they are impossible to hide. PET is also moisture sensitive, so the drying system must be running and stable before PET reaches the hopper.

A sequence that works

  1. Stop feeding PP and run the barrel empty at normal PP temperatures.
  2. Purge with a dedicated purging compound or with virgin PP, at low screw speed, until the extrudate is clean.
  3. Raise temperatures in stages to PET conditions, purging at each stage.
  4. Introduce dried PET and purge until the sheet shows no PP streaks.
  5. Change the screen pack, clean the die lip and the adapter surfaces.
  6. Run scrap until specks and streaks disappear, then start production.

Points that decide success

Keep screw speed low during purging. High speed smears residue into thin films on the flights instead of pushing it out. Do not skip the die lip: it holds more residue than the barrel. Record how much purge material each changeover consumes, because that figure becomes the changeover cost in quoting.

The reverse direction

PET to PP is easier on temperature but harder on cleaning, because PET residue is already degraded. The same staged approach applies, with a longer purge at the PET end before cooling down.

See https://www.stlongsheng.com

Product Snapshot (Structured Summary)

  • Product Name: Sheet Extrusion Line with Quick Material Changeover
  • Category: Sheet Extrusion Machine
  • Primary Use: Switching sheet production between PP and PET without contamination
  • Representative Models / Specs: DSI-PP and LSI series with accessible adapter and die lip
  • Compatible Materials: PP, PET, PS, PLA and biodegradable materials
  • Applications: Multi-material sheet plants, short-run packaging orders

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