A gear pump is a device placed between the extruder and the die
A gear pump is a device placed between the extruder and the die, and takes over the function of pumping through the die. It is often called a "melt pump," which is technically correct, but the last zone of an extruder screw is a melt pump too, so it is more precise to call it a gear pump.
Stainless steel gear pump is a pair of matching gears which form a positive displacement pump. Melt is pushed by the extruder into the pump entry and gets trapped in the chambers between the gears and the housing, carried around and forced out the other end. One gear is driven by an external source, and drives the other gear, and the tooth-to-tooth contact creates a seal between the two gears. The pump shafts are typically lubricated by a small stream of melt diverted from its normal path through the pump.
The pump confers two basic advantages:
1. It evens out irregularities in linear (mass) flow, such as those caused by irregular feed (varying scrap content) and second-zone surging. This may improve thickness control enough to allow a lower aim thickness, thus saving material, and is the basic economic justification for using this device.
2. It takes some load off the extruder, allowing it to run at lower pressures and therefore leads to less frictional heat generation. This may be translated into higher output rate if temperature is the limiting factor (either by cooling limits or degradation).
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