The parallel screws are so close together that providing enough thrust bearing is a problem. Obviously, one screw shaft must extend beyond the other to accommodate the proper thrust bearing. In the case of tapered screws, this problem is solved due to the larger diameter of the feed end, which provides a wide spacing between the screws.
This allows the use of standard thrust bearings and a simple, robust distribution gear system. Counter-rotating conical extruders have large capacity feed sections to accommodate lower bulk density compounds. This provides flexibility for handling a wide variety of compounds.
Over time, the output of the parallel screw gradually decreases due to wear. Whereas in the case of tapered screws, the output can be maintained by periodically advancing the screw. When the polymer is plasticized, an outward force is applied to the screw.
