Extrusion: Why Is Material Backing Up Into the Feed Throat?

Q. I am running a 6-in. extruder, which is starve-fed through a gravimetric feeder. We are running a raw-grade PP. I have days where the material will not move through the feed section and backs up into the feed throat. I have tried reverse heat profiles, flat profiles, and ramp-up profiles, but none of them seem to make this problem go away. I have recently built a new feedscrew with more feed zones but that did not solve the problem. Any ideas? —Ontario processor

A. Based on the information you supplied, it appears there is nothing wrong with the screw, since it works at least part of the time. The problem is likely the starve-feeding. If a length of steel (the screw) is heated to 450 F at one end, it will eventually approach 450 F at the other end unless something is removing the heat. In a flood-fed process, the incoming polymer removes the heat from the screw. In a starve-fed situation, the polymer is not in intimate contact with the screw until the screw fills, generally in the compression section. As a result the tumbling pellets do not absorb much heat in the feed area and the screw gets hot. As soon as the screw is stopped, the polymer melts on the screw surface in the feed section. Once melted onto the screw, it takes considerable force to get the plastic off. Other pellets stick to the melted polymer and disrupt the feed. Feeding depends on the frictional forces of the plastic against the barrel exceeding the frictional forces of the plastic against the screw. Have your screw bored for cooling in just the feed section and introduce a small amount of cooling water. That should solve your problem.

Jim Frankland
Frankland Plastics Consulting, LLC
(724) 651-9196 • jim.frankland@comcast.net