EXTRUSION

More plastic resin has processed through the extrusion process than through any other plastics manufacturing technique. This fact, however, is misleading. Very big amounts of plastics are processed by extrusion into pipes, films, sheets, coatings and so on. Extrusion is also used for producing pellets of thermoplastic materials. Therefore, the majority of plastic material produced in the world today has had at least one extrusion through an extruder. Then those materials will be used in other plastic processing methods, like injection moulding, blow moulding and other melt processing methods, including the extrusion.
Extrusion is a typical method to produce continuous products (pipes, sheets, films, coatings) of different kinds of materials. The first extruders were developed for food and building materials industry. Nowadays very many materials can be extruded into products. In the area of technical materials the polymeric materials are the most common ones.
During extrusion, a polymer melt is pumped through a shaping die into the final profile form. The first extruders were ram-type extruders and the first extruders were built in 1797 to extrude seamless lead pipes. The first ram-type extruders for rubber industry were designed 1845. In 1846 a patent for cable coating was filed for trans-gutta-percha and cis-hevea rubber and the first insulated wire was laid across the Hudson River for the Morse Telegraph Company 1849. The first screw extruder was patented 1879 for wire coating. The extrusion of thermoplastic polymers started in 1935.
Ram and screw extruders are both used to pump high viscous polymer melts through a die to form a continuous profile.
Schematic of pumping principles
The extrusion process
The extruder is a melt pump. In the first stage the polymeric material is melted and then the melted material is forced via the die into the final form.
Today nearly all extruders are so called screw extruders, earlier there were different kinds of development stages. The most important step is the melting and homogenization of the polymeric materials. It can also be carried out with calenders. Multiscrew extruders have been developed for special purposes, especially for different kinds of mixing applications. One of the newest developments is the cone extruder, which will have many applications in pipe extrusion and other applications.
The extrusion process is best understood, when it is divided into key steps:

1. Pretreatment of extruded material. This includes drying of materials, feeding of additives, preheating.
2. Material is fed into the extruder through the throat, an opening that links the material hopper into the extruder barrel
3. Forced feeders, if the feeding of materials is difficult or it is important that the material feed is very constant
4. Inside the cylinder the raw material is conveyed from the feeding zone to the die. In this case it is very important that the friction between the cylinder is higher than the friction between the screw.
5. The screw can be divided into three parts and a conventional screw is presented in fig. 11. The zones are: feeding zone, compression zone and homogenisation zone.
6. The die. The melted material is pumped through the die into the final form.
7. Calibration of the extrudate in the final dimensions and form
8. Postprocessing of extrudates

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