An extrusion company asked us for a consultation to purge rigid PVC from single and twin screw extruders. The current method used a powder purge that could run through the die. The consideration was to seek a possible improvement in cleaning and performance to the recent purge, which could also be left inside the machine to seal and prevent PVC degradation, which can easily occur during machine downtime and start-up.
This application for purging rigid PVC carried the common concerns for the material: avoiding temperature increases to prevent degradation of the PVC and ensuring that the PVC itself can displace the purge to return to production without delays due to residue. The user intelligently set about running tests first on the 3.5 ”single screw extruder before running the same test on the twin screw extruder. This decision allowed them to observe the behavior of the new purge in the simpler of the two extruders as a flow and displacement test with the purge and PVC.
If satisfactory, they’d try it on the twin screw extruder, which is larger in barrel capacity and more complex in design. I recommended a mechanical purge because mechanical purging compounds do not require temperature increases to function properly, and being a grade approved for rigid PVC, it would be easily displaced by the resin, leaving little to no residue issues.
The result was better than expected, achieving cleaning with approximately 20 lbs. of Asaclean when the previous method took 40 lbs. On top of the 50% savings in purge quantity, they obtained a 20% reduction in the cost of the purge consumed per application, plus additional savings due to the decrease in both start-up time and scrap compared to the previous method.