Extrusion basics: Just the right amount of coolExtrusion basics: Just the right amount of cool
Much of plastics processing is concerned with temperature. We know about the four evils of overheating:
September 13, 2015
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.
It's not easy to cool a melt in an extruder, because barrel cooling thickens the melt at the wall (higher viscosity), which draws more power from the motor and returns some of the heat that has been taken out. Water is more effective than air, but both have this higher-power disadvantage. Cooling after the screw doesn't have this problem, but it may lead to temperature variations from point to point on the product, unless a static mixer is in the line after, or within, the cooler to homogenize the melt. That may be why the dies themselves are seldom cooled, but this is an area worth more attention, as we often adjust die temperatures and even add local insulation to affect the product.
Faster cooling of the product itself may be good for production rates—but don't run faster unless you can sell more, as increased speed may result in a loss in precision and require thicker product (hence more money) to avoid product failure. Very fast cooling also may freeze in stresses, which may slowly relax and cause problems in printing registry, warpage and permanent bends in coiled products. One case where fast cooling is needed is clear PET—slow cooling will produce cloudy product because of the larger crystallite size. And in cooling profiles immersed in a water bath, remember that the product is moving, so there is no need to move the water: Just strip the hot layer off the surface with a sponge, soft plastic or even a rag, and there will be more circulation around the profile and, thus, more effective cooling.
Some final words: Make sure your instruments are reliable, especially the melt temperature reading, and make sure they are read by people who understand them and can react to problems they may reveal.
And remember that no plastics are toxic. None of them. The bad public image comes from fear of chemistry, arising from fear of science, which challenges the belief in the impossible. Mystery is OK, but magic is not. Sorry, Disney.
Allan Griff is a veteran extrusion engineer, starting out in tech service for a major resin supplier, and working on his own now for many years, as a consultant, expert witness in law cases, and especially as an educator via webinars and seminars, both public and in-house. He wrote the first practical extrusion book back in the 1960s as well as the Plastics Extrusion Operating Manual, updated almost every year, and available in Spanish and French as well as English. Find out more on his website, www.griffex.com, or email him at [email protected].
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