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Views: 0 Author: Site Editor Publish Time: 2025-09-10 Origin: Site
The extrusion process is basically designed to continuously convert a soft material into a particular form. The heart of this processing/fabrication machine is a screw conveyer. It carries the cold plastic material (in granular or powdered form) forward by the action of the screw and squeezes it, and, with heat from external heaters and the friction of viscous flow, changes it to a molten stream(refer to Figure 1). As it does this, it develops pressure on the material, which is highest right before the molten plastic enters the die. The screen pack, consisting of a number of fine or coarse mesh gauzes supported on a breaker plate and placed between the screw and the die, filter out dirt and unfused polymer lumps. The pressure on the molten plastic forces it through an adapter and into the die, which dictates the shape of the final extrudate.
Figure 1. Features of a single-screw extruder.
A die with a round opening produces pipe; a square die opening produces a square profile, etc. Other continuous shapes, such as the film, sheet, rods, tubing, and filaments, can be produced with appropriate dies. Extruders are also used to apply insulation and jacketing to wire and cable and to coat substrates such as paper, cloth, and foil.
When thermoplastic polymers are extruded, it is necessary to cool the extrudate below Tm (melting temperature) or Tg (glass transition temperature) to impart dimensional stability. This cooling can often be done simply by running the product through a tank of water, by spraying cold water, or, even more simply, by air cooling. When rubber is extruded, dimensional stability results from cross-linking (vulcanization).
Extruders have several other applications in polymer processing: in the blow-molding process they are used to make hollow objects such as bottles; in the blown-film process they are used for making wide films; they are also used for compounding plastics (i.e., adding various ingredients to a resin mix) and for converting plastics into the pellet shape commonly used in processing.
