This question came in as a follow-up to a recent post.
Recently we were processing Acetal material with black masterbatch. During the whole run the screw recovery time kept going up excessively before coming back to normal values after a few minutes. This was repeated throughout the run and it happens on this particular job all the time. we are not allowed to change either screw speed or back pressure excessively. Purging the barrel through with purging material helps for a while but it goes back doing the same thing.could it be the masterbatch causing screw slip. Your thoughts on this please.
Essentially, the colorant (masterbatch) has a different melting profile than the base resin. In many cases, the additives melt faster, causing slippage on the barrel during recovery. Optimal recovery occurs when the melted material has the highest tact with the barrel causing the best and most consistent conveyance down the screw. To locate this optimal temperature, you must conduct a rear zone temperature study.
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