When left in the barrel at operating temp, the resin will begin to break down and leave carbon deposits on the screw and barrel. These will then come out as black specks in the material. This can also happen if plastic is left in the barrel with the heats off. As the plastic is heated back up to operating temperature, the resin can break down, again causing black specks. You can check with the supplier for a hold temperature, but all that I have seen recommend leaving the temp's at 300 to 350 degrees F when press is not running.
We use clear or natural polypro for long term shutdowns and also for screw removal. One must starve feed the polypro into screw throat to avoid bridging or creating a slug from the higher temperatures of polycarb.
You will be better of leaving the polycarb in the barrel and the temp set at 350 degrees F. I have used it in numerous medical jobs, and we would leave it for up to a week at a time. If the job you are using it for is highly visual, either clear or white, you can get black spec's. Purging will get rid of most of the polycarb, but you will always leave trace amounts behind. If you want to remove as much as possible, purging compounds such as Asaclean do a pretty good job, but are rather expensive. A cheaper option is to purge with cast acrylic. This is made from crushed acrylic sheets. This works well, but make sure you feed it slowly or it will bridge up in the feed throat worse than any other material I have ever seen.
