Extruded Products and Solidification

Extrusion is a workhorse method in polymer processing and used for several purposes. First, extruders are used at the end of primary synthesis lines to convert the newly formed polymer to pellets. Pellets are produced by chopping a narrow diameter extrudate. Second, extrusion is used to produce a product or “profile.” Shapes such as sheets, rods, and tubes are made by extrusion. Third, extrusion is used to prepare and pressurize a melt for other forming operations. For example, an extruder with a special die is used to make thin sheets by the blown film process, extruders create parisons for blow molding, and extruders with reciprocating screws are used in injection molding. Here, we focus on the shaping aspect of profile extrusion.

Examples of Special Dies. Tubular shapes require a special die with a mandrel for the interior hole in the tube. The polymer melt must flow around the mandrel, which is held in place by a support known as a spider, as shown in Figure 3.63. The polymer “knits” together after going around the “legs” of the spider. Tubes are also extruded with dies that are oriented vertically. In this way, the tube shape can be better stabilized through solidification.