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Plastics Processing: Extrusion

Extrusion accounts for the majority of plastic resin processing in the U.S. by weight. It consists of forcing molten plastic resin through a die, or orifice, under constant pressure - a procedure similar to squeezing toothpaste from a tube. The process results in a continuous length and uniform cross-section. The shape of the opening determines its "profile". Since it is a continuous process, large volumes of resin may be produced in a comparatively short time.

The most important part of this process is the extruder. In the most common type of extruder, the screw variety, dry resin is fed into a hopper on the machine and enters the flutes of a long, motor-driven screw. The rotating action of the screw serves the dual purpose of promoting the melting of the resin through pressure and friction, as well as transporting the melt to the die ahead.

There are four different classes of extrusion:

Straight Extrusion - extruded profiles emerge from the die in a straight line. The shapes are then either air- or water-cooled, cut to length, and are then basically ready for use.
Common Applications: plastic pipe, window trim, housing siding, tubing

Extruded Sheet - also produced by passing a continuous resin melt through a die. In this case, however, the die will be of a thin and slotted configuration.
Common Applications: packaging material, protective window films

Blown film - a continuous extrusion where the molten resin is literally blown into a tubular shape through a vertically mounted circular die.
Common Applications: trash bags, dry cleaning bags, grocery bags, other bags

Coextrusion - two or more resins of different colors or dissimilar properties may be combined in a common product with this process.
Common Applications: drinking cups with white interior and color exterior