Single Screw Extruder

The simplest extruder is a ram extruder, as shown in Figure 3.1. Pressure is applied to a piston, forcing the extrudate out of the die. Heat applied to the barrel melts the material and lowers its viscosity. With the correct heat and ram movement pressure, the polymer is pushed out of the die in the desired shape. There are many problems associated with a ram extruder. First, it is a discontinuous process. Second, plastic is a great insulator; consequently, it takes a relatively long time to heat the billet uniformly from the barrel surface to the center. Using a high barrel temperature to increase the melting rate tends to degrade the resin at the barrel wall before the billet is molten. Lowering heater band temperatures to minimize degradation requires longer heat soak times to soften the billet properly. Third, shear heating from the ram movement is minimal.

The key components in a single screw extruder are shown in Figure 3.2(a). Figure 3.2(b) shows a cutaway of a 2.5-in. American Kuhne extruder.