Melt Pump for Extruder

The melt pump for extruder simply takes away whatever the extruder is pumping. As soon as the output of the extruder is sufficient to fill the teeth, however, the situation changes dramatically. Now it is just as if the extruder is pumping into a brick wall. The melt pump can accept only so much plastic and no more. Attempting to pump more plastic skyrockets the pressure upwards untilit blows the safety blow-out plug, at which time the pressure drops to zero, and the ball game is over Our problem, however, is that if we are going to use the melt pump to meter out the plastic, operation has to be in the steep vertical part of this pressure characteristic, because the melt pump cadt do any metering unless the teeth are fllled, and that only occurs on the steep vertical portion of the curve. But no human operator can maintain that exact balance for very long. Manual control is simply not feasible!

An examination of the plastic path in the extruder/melt pump combination turns up some other interesting facts: Most of the change in output shown in Fig. 6.5 is not proportional to the level of the pressure in the input to the melt pump. It is proportional to the difj'erence in pressure across th.e pump! In the melt pump of Fig. 6.4, for example, there is a 68 6 Control of the Plastic difference of 1200 psi trying to drive the plastic through the leakage paths around the sides of the gears, around their periphery, and through the bearings.

We can never eliminate these leakage paths. But as long as the pressure drop across them remains constant, the amount of plastic passing through them will remain constant too, and so will the net throughput of the melt pump/extruder combination.

Since a microprocessor controller is generally used to control the temperature anyhow, it poses no problem to control not just the inlet pressure to the pump but also rather the difj'erential pressure across that pump (Fig. 6.8). Now the curve looks like the one in Fig. 6.7, where operation is always at the same point in the pressure throughput curve. Controlling the differential pressure across the pump in this manner improves the throughput stability of the extruder/melt pump combination to about 0.i% or better!

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