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Throughput of a melt pump with simple input pressure control

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By far the best way to get a stable throughput from an extruder is to use a melt pump on the output. In concept the melt pump is very simple. It uses a pair of closely meshed gears, placed in the main plastic stream. The teeth are filled at the input,leveled off by the housing, and then emptied at the meshing point. The device can be thought of as "measuring cups on a wheel". The teeth are loaded at the input and emptied at the output.

The important thing to note is that the gear pump is a positive displacement device. There is no channel open between input and output, as in a screw type pump. Every time the gears advance one tooth, precisely two teethful of plastic are delivered from input to output, and insofar as the primary pumping mechanism is concerned, the amount of plastic delivered will be independent of back pressure. Overall, the throughput is not really completely independent of the pressure, of course. Extrusion grade gear pumps are made to extremely close tolerances. Both the gears and the housing are precision ground to size rather than being simply machined. Never- theless, there will still be clearances required for the gears to turn, and where there are clearances, there will be leakage. The gear pump in Fig. 6.4, for example, has 1200 psi tending to push the plastic back from output to input - along the sides of the gears, around the periphery of the gears, and through the bearings.

The "efficiency" curve, or throughput plot, of the gear pump used with simple input pressure control will typically look like Fig. 6.5. It has the same shape as the curve for the extruder shown earlier, but the throughput variations with pressure are reduced by a factor of about i5 : i. As we shall see in the following, this reduction in throughput variation can be reduced even further through proper control.

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