ABSTRACT
In extrusion a material is forced through a die that shapes the profile. Continuous flow of material results in a long shape of constant cross section. Unlike the extrudate from a toothpaste tube or a meat grinder, plastic extrudates generally approach truly continuous formation. They may attain dimensional stability in various ways. Like the usual meat grinder, the extruder (Figure 14.1) is essentially a screw conveyer carrying cold plastic pellets forward and compacting them in the compression section with heat from external heaters and the friction of viscous flow. The pressure is highest right before the plastic enters the die that shapes the extrudate. The screen pack and breaker plate between the screw and the die filter out dirt and unfused polymer lumps. When thermoplastics are extruded, it is necessary to cool the extrudate below Tm or Tg in order to gain dimensional stability. In some cases this can be done by simply running the product through a tank of water or, even more simply, by air cooling. When rubber is extruded, dimensional stability comes about by cross-linking (vulcanization). Special attachments for continuous vulcanization are described later. Actually, rubber extrusion for wire coating was the first application of the screw extruder in the 1880s. Standard sizes of single-screw extruders (specified by the inside diameter of the barrel) are 1.5, 2, 2.5, 3, 3.5, 4.5, 6, and 8 in. Smaller sizes are available for laboratory evaluations.
