Anyone who processes polycarbonate knows how heat-sensitive the material can be. PC operates in a fairly high temperature range around 550 to 600 F. It is a hygroscopic material that needs to be dried in the low to mid 200s. Polycarbonate can begin to degrade and lose clarity and optics at around 284F. Polycarbonate needs to keep moving through the production process without any type of extended residence times. In other words, PC degrades very easily and turns to carbon. PC is also a very dense material.
Now, here is the time bomb trap of using PC to purge your machine. Because it’s dense, clear, and available (as the processor may have some scrap on hand) to use for purging, it would seem like a good a good idea. It seems to push out previous resin or colorant effectively and then appear clear. This is not the case.
After purging with polycarbonate you leave behind residual polycarbonate inside the machine. The dense PC will coat the screw and barrel. If the processor goes down in temp to run some other resin, then the PC will crystallize on the screw and barrel and continue down the path of degradation. Once you first purge with PC, it's only a matter of time before the contamination becomes a huge issue. The screw and barrel will first have an amorphous carbon bleeding out a black contamination and carbon deposits will form in the microscopic cavities on the barrel walls. This will flake of as black specs.
Purging with polycarbonate scrap is a major example of short-sighted thinking of processors. Sure, it seems easy and much cheaper in the short-term. But it becomes very expensive in lost production and extended downtime when contamination inevitably occurs from residual polycarbonate left in the machine.
This time bomb situation can be easily avoided and actually costs much less with the appropriate thermally-stable purge product. With the appropriate grade, the purge will not only help with the material or color changeovers, but will also conduct a preventative maintenance on the machine to avoid carbon contamination in the first place.
Ready to reduce your production downtime to protect your profits? Learn more about how purging compounds and process efficiency work in tandem.
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