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What is Extrusion...
Extrusion is a method of making a thermoplastic polymer into a useful shape, such as a pipe, profile, sheet or film. In essence, a plastics extruder melts or plasticizes a solid polymer and pumps it through a die to form the desired shape. In many cases, the extruder also mixes the polymer with other ingredients, such as color, reinforcing fibers, mineral fillers, or processing aids, to name just a few. The extruder’s job is to adequately disperse and distribute all ingredients in the mixture, and to provide a melt with consistent temperature and pressure.
In general, an extrusion system has several components:
- • feed hoppers and mechanisms for dosing the desired percentage of the polymer and other ingredients
- • an extruder to melt and mix the polymer and other ingredients
- • a melt filtration system for removing contaminants
- • an adapter to connect the extruder to the die
- • a die for forming the molten polymer into the desired shape
- • a cooling mechanism (air ring, water bath, etc.) to freeze the molten polymer into the final shape
- • a puller to pull the polymer out of the die and through the cooling mechanism.
Different types and sizes of extruders are used in this system, depending on the job requirements. The size of the extruder is described by the screw or barrel diameter (commonly ranging from 0.5 inch to 10 inch, or 15 to 250 mm) and by the barrel length-to-diameter ratio (such as 24 L/D).
Types of extruders
In an extruder, the extruder motor turns the screw(s) inside the barrel, and the solid material is carried down the extruder by the screw flights. Although the barrel has heating or cooling devices to help control temperature, the solid polymer is melted primarily by the frictional heat created between the solid bed and the extruder barrel. In some cases, the polymer is melted and mixed first in a roll mill, batch mixer, continuous mixer, or another extruder, then fed to a short, melt-fed extruder, which acts as a melt pump to feed a uniform melt to the die. Two types of mixing - distributive and dispersive - occur in the extruder. Dispersive mixing, which breaks up agglomerated particles into the smallest particle size, requires high energy and shear stress. Distributive mixing, which uniformly distributes ingredients throughout the mixture, requires less energy. Different types of extruders and screw designs have different mixing ability.
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