Thermoplastics are plastics that soften when heated but harden again when they are cooled, and this allows them to be shaped easily. Softening and rehardening does not significantly alter the properties of a thermoplastic. As it is heated, a thermoplastic first becomes flexible and elastic, like rubber, and then completely plastic, like a viscous liquid.
Extrusion, carried out in an extruder, is a common method of shaping thermoplastics. An extruder consists of a heated, pressure-resistant barrel in which there is a helical screw, as in a domestic meat grinder. The screw conveys granules of the plastic through a heated die at a temperature of about 392° F (200° C) and a pressure of from 1450 to 4350 psi (100 to 300 bar).
A wide range of differently shaped products can be made by this method, depending on the shape of the die aperture. If it is circular, rods or filaments are produced; if it is annular, pipes and tubes are formeeand if it is a slit, plastic sheeting or film will be produced. The continuous products emerging from the extruder die are cooled by air, water, cooled rollers or by contact with cooled metal surfaces which give the exact shape to the section. The finished product is then rolled up or cut into suitable lengths. Large amounts of PVC are used in the production of cladding for window frames.