The screw in an extruder is basically a pump. There are other types of pumps: the piston pump, the peristaltic pump, and the gear pump are three common types.
At first glance, the screw type pump would seem to be an odd choice for use in an extruder. Because in an extruder constancy of output is very important, and the screw type pump is the only one of the four types that is not a positive displacement pump - that is, a pump which always pumps a specified volume of product for every revolution. The other three pump types- the gear pump, the piston pump, and the peristaltic pump - are all positive displacement types.
The screw type pump is chosen for the plastic extruder over the other pump types because it is an ine[jicient pump! That is not to say that it is a bad pump - just that it generates a lot of heat. But think about that for a minute. How else would you melt a gaylord of plastic pellets? If you just put your pellets in a pot with a fire under it, there isdt a polymer in the world that would not be hopelessly degraded on the outside long before the plastic in the middle was even warm. The only practical way to melt a lot of plastic is to take a little bit ofit at a time and subject it to frictional heat generated by a screw type pump.
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