Injection molding is one of the most important methods of producing shaped articles in large numbers. As in an extruder, the plastic is softened by being passed along a heated barrel by means of a screw. However, the screw has the additional function of a ram, which injects the softened plastic rapidly into a cooled steel mold at temperatures of from about 355 to 570° F (180 to 300° C) and pressures of 14,500 psi (1000 bar) or more. After cooling, the mold opens and the article is removed.
Blow molding is a process used to manufacture hollow articles with small openings, such as bottles, cans, drums, tanks and toys. A piece of softened plastic (frequently polyethylene) tubing is extruded and introduced into a two-piece blow mold. As the mold closes, it presses one end of the tubular section together to seal it. Compressed air is blown into the other end of the tube until it fits against the cooled mold and solidifies.
Thermoforming is a process for making shaped articles, such as bowls or beakers, from plastic sheeting or film. The sheeting or film is heated with infrared radiation to a temperature of between 250 and 355° F (120-180° C) until it is soft. It is thenucked against a single-section mold by applying a racuum. After cooling (solidifying), the molded Lrticle is separated from the remainder of the sheet. In rotational molding, plastics powder is placed in L heated, hollow metal mold which is rotated slowly Lbout two axes that are perpendicular to each other. the powder melts and is distributed evenly over the vhole inner surface of the mold. After cooling, the mold is opened and the molded hollow article emoved.
Thermoplastics are often modified with fillers, such as glass fiber and talc, to improve trength and resistance to heat and warpage. Addiives are used for colors and to give resistance to staic electricity, slip, flame, and light.