I was just asked the question…
Kevin
We are making some products using a PC+PET blend. We are having issues with material feeding / screw slip and screw stall. Do you have any idea what the exact issue may be?
My Response
Since you are processing a blend of two materials, the melting characteristics of each will be slightly different.
The goal during screw recovery is to maximize the material’s adhesion (tact) to the barrel in the feed zone to ensure the best conveyance to the transition zone.
The rear zone temperature is often lower than the final melt temperature in such blends. This is because only one component generally needs softening to ensure proper tact between the pellet and barrel while in the feed zone. Too much melting can often result in one component actually behaving like a lubricant which can inhibit conveyance.
The best way to ensure the optimal recovery is to perform a tact temperature study:
My Thoughts
It may be beneficial to pull the screw and ensure there is no significant wear in the feed zone of the screw, this region of the screw may need polishing or surface treatment to ensure proper conveyance.
Also, make sure you are using either a recovery delay or suck-back before screw recovery to ensure there is no excessive pressure on the screw and material when recovery begins.
-Andy
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