![]() ![]() In order for the screw to rotate it has to stretch the rubber bands, and that requires a certain amount of power from the drive motor. It’s like the polymer were made up of many tangled rubber bands all attached to the barrel and screw. The rotating screw then stretches or “shears” the polymer in that space. The polymer melt sticks rigidly to both the barrel wall and the screw surface (see the highlighted strip in the illustration). The barrel is heated to help initiate the first melting, but then provides primarily a thermal jacket over that stretching process. They tried drastically reduced barrel temperatures-but not only were they unable to bring the melt temperature down, they overloaded the drive by extracting too much heat from the system.īasically, the melting and melt-temperature rise in an extruder occur largely by “stretching” any melted polymer contained in the space between the barrel bore and the screw (see illustration). They had changed from a fractional-melt-index (0.35 MI) HDPE to a 0.06 MI bimodal HDPE, and were trying to hold the temperature down to accommodate their downstream cooling capacity. I was recently asked by an experienced pipe processor why the melt temperature was so high on a new polymer his company was evaluating. Plastics extrusion has been around for almost 100 years, but despite its extensive history, polymer shear rate/viscosity curves are still not widely used or understood. ![]()
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