This is arrant nonsense, spread primarily by one large extruder manufacturer who does not make melt pumps. Melt pumps designed specifically for PVC have been available for at least 15-18 years. At least one extruder manufacturer estimates that its melt pumps are used more on PVC than any other polymer.
Notes also that, more recently, melt pumps designed particularly for fluoropolymers have become available and are in routine daily use.
“You can get the same stability of output using a barrier screw.”
This is absolutely untrue – a flat lie and a deliberate one. Anyone who tells you this either does not understand how a screw works, or he is assuming that you do not know and is trying to take advantage of that fact. It is a close question which is worse!
A barrier screw does three things:
1. It prevents unmelt plastics from passing through to the screens.
2. It provides the same shear type mixing as a spiral Maddock mixer.
3. It makes enormous profits for the screw vendors, since it costs little more than a standard screw with a spiral Maddock and sells for two to three times more.
The throughput stability of a barrier screw is no more and no less than other screws. It does not even come close to the 0.1% throughput stability of a melt pump.