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Plastics Processing: Foam Processing

Plastic foam is used in a wide variety of foam products on the market today - everything from carpet padding to aircraft arm rests to drinking cups.

The most familiar type of foam is termed expanded polystyrene (EPS). EPS beads contain a blowing agent which, when heated, causes the beads to expand from 2 to 50 times their original size. The amount of expansion is controlled by time and temperature. Processors heat the beads mostly using continuous steam. They are agitated in a large steam chamber. As they expand, they flow out the top - much like popcorn out of a popper - from 3 to 12 hours before final processing.

Shape Molding
For small, finished parts, the beads are fed into closed molds for shape molding, where steam is injected to expand them to fill the mold cavity. The mold is then cooled and the completed product is removed.

Block Molding
For larger applications processors use block molding, where a large mold cavity is filled with beads, closed, and subjected to an influx of steam which causes the block to fuse and set. After curing, the block is cut, usually by hot wires, into sheet, slab, or custom shapes.

Flexible Foam Sheet
Flexible foam sheet is produced by extrusion. A variety of resins may be used, but polystyrene (PS) is by far the most popular.