Extrusion basics: Just the right amount of cool

16/09/2015

Much of plastics processing is concerned with temperature. We know about the four evils of overheating:

Degradation—heat x time destroys chains, resulting in weakness, discoloration and contamination.
Cooling problems, which may limit the production rate.
Sizing problems, such as sagging sheet or sticky profiles.
Additive effects, like premature foaming, loss of volatile additives and color shifts.
But the feed, extrudate and product also can be too cool. A cold feed may not extrude as fast, and if feed temperature varies, such as day-night, it may lead to cyclical production rates. Preheating the feed usually is done only for drying purposes, but it can be helpful even when no drying is needed by stabilizing feed temperatures, and it may allow faster rates by getting in more heat sooner.

Cooler temperature of the emerging extrudate will help against some or all of the above overheating evils. It also usually reduces gloss, which could be good, bad or irrelevant. It may raise head pressure, however, which requires more power to push through, and returns some of the heat taken out.
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