Processes used in plastics manufacturing

[German version]

Process	Application
Extrusion	Extrusion is performed in extruders. This process is used to produce tubes, profiles, films etc. from thermoplastics. Extrusion proceeds as follows: The starting material (mainly polyethylene, polypropylene, polyvinyl chloride and polystyrene), which is usually in powder or pellet form, is placed in a feed hopper. The material is then heated and homogeneously mixed in the extruder to form a melt and a conveying screw expels the melt through a shaping die at the end of the extruder. Dies of differing shapes produce the various products.
Injection molding	Injection molding is used not only for thermoplastics and thermosets but also for elastomers. As in extrusion, the starting material, which is usually in pellet form, is placed in a hopper, heated, plasticized and expelled through a nozzle by a screw conveyor. On the far side of the nozzle, there is a mold cavity which serves to shape the plastic molding. Injection molding is frequently used for mass-producing relatively small parts (e.g. screw caps for beverage bottles).
Calendering	Calendering or rolling is used for film production. The calender consists of several highly polished and very rapidly rotating rolls. The rolls may be chilled or heated as required. Calendering is used in particular when film thickness specifications are particularly tight. Films with embossed patterns or the like may, for example, be obtained by using special calenders.
Blow molding	Blow molding is used to produce hollow articles. One particular kind of blow molding is extrusion blow molding, in which an extruded parison or preform is inserted into a two-part mold, where it is inflated with compressed air and so pressed against the cold mold wall, where it cools. The mold halves are then separated and the finished hollow article may be removed. In packaging applications, blow molding is primarily used for producing plastic bottles.