Most extrusion screws have hard-surfacing materials welded on the flights to provide longer wear life. When extreme wear occurs in the absence of abrasive fillers, a burr on the flights provides a clue as to what is happening. A burr on the trailing side indicates a very high side force is causing the flights to gall or even weld to the barrel material, literally pulling the two surfaces apart. This is usually due to ‘wedging,’ which is caused by the screw momentarily plugging with solid polymer at a radial location. If pressure from the plugged channel is not balanced by a similar force on the other side, the screw is pushed with tremendous force against the barrel.
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