Barrel heaters have very little impact on melting, an aspect of polymer processing that is misunderstood by quite a few. I recently discussed processing with someone who has been in extrusion for almost 30 years, and he asked me if he would need greater wattage-capacity heaters for more output. Even he did not understand that most melting occurs through shear heating and is the result of the drive power being converted to heat via the screw. The main pu rposes of the heaters are to melt the polymer that remains in the barrel at cold startup, to assist in forming the initial melt, and to "trim" the barrel temperatures for specific purposes such as improving feed rate.
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