These two are very important to mix the plastic and remove temperature variations for Plastic pumps, but they do not produce a constant displacement device. It can be seen that it takes a 68% change in the RPM to keep the throughput constant in the face of a 4000 psiincrease in the screw output pressure.
Note that these data were collected within a short period of time with the same lot of plastic, with highly sophisticated controls for all parameters of the extruder, and with constant operating conditions in the extruder. Performance in an actual operating environment, with random lots of plastic, varying temperatures of the plastic feed, and the usual differences of opinion on operating conditions among operators, would be much worse.
There are those who may argue that constancy of output is not important in these days of closed-loop controls over the various facets of the processod plastic pumps. This argument falls short for two reasons:
1. First, no corrective control can ever completely compensate for basic parameters that are not stable. Stabilizing those basic parameters - and throughput stability is certainly at the top of the list - is an important first step in taming any process.
2. Furthermore, some of the most potent closed-loop control schemes start with the assumption of constant throughput as a given. There simply is no substitute!