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The term “cellular plastics” encompasses a range of materials with widely varying properties and fields of application. Virtually any polymer, thermoplastic or thermoset, can be made into a cellular or foamed form with the resulting products having densities ranging from 60 pounds per cubic foot (pcf) all the way down to 0.1 pcf.
Basically, cellular plastics can have either of two structural configurations: (a) the closed-cell type, in which each individual cell, more or less spherical in shape, is completely enclosed by a wall of plastic, or (b) the open-cell type, in which the individual cells are intercommunicating. The foams can be rigid, semirigid, or flexible. In general, the properties of the respective plastics are present in the foamed products, except, of course, those that are changed by conversion to the cellular structure.
In this chapter, the more common foamed plastics now in use are discussed. Readers are cautioned, however, that there may be little in common between many of these materials, other than some degree of cellular structure. Cellular plastics can be produced in the form of slabs, blocks, boards, sheets, molded shapes, and sprayed coatings. Some can also be “foamed-in-place” in an open cavity. The type of process used (casting, extrusion, injection molding) will affect the properties of the end-product.
As originally conceived, most foamed plastics were completely cellular in structure; today, it is possible to arrange the cells so that a product may have an essentially solid skin surface and a cellular core.
Air is whipped into a suspension or solution of the plastic, which is then hardened by heat or catalytic action or both.
A gas is dissolved in the mix and expands when pressure is reduced.
A component of the mix is volatilized by heat.
Water produced in an exothermic chemical reaction is volatilized within the mass by the heat of reaction.
Carbon dioxide gas is produced within the mass by chemical reaction.
A gas, such as nitrogen, is liberated within the mass by thermal decomposition of a chemical blowing agent.
Tiny beads of resin or even glass (e.g., micro-balloons) are incorporated in a plastic mix.
Finally, readers should be cautioned about the terminology used to identify cellular plastics. Unfortunately, it is loose and generally relates to the starting material (e.g., polyethylene) from which the foam is made. However, an expression such as “polyethylene foam” can have many meanings. This term can refer to special low-density polyethylene foams and special high-density polyethylene foams that are quite different in character. It could include cross-linked polyethylene foams, which differ even more, or it can refer to polyethylene foam films made by extruding low-density polyethylene with a nitrogen blowing agent. Or, to complicate matters still further, it also could encompass low-density polyethylene structural foams or high-density polyethylene structural foams that bear no resemblance or relationship to any of the other foams that have been mentioned thus far. In this chapter, an attempt will be made to maintain these distinctions and to point up the differences between the various cellular plastics whenever possible.
KeywordsInjection Molding Polystyrene Foam Cellular Plastic Blow Agent Conventional Injection Molding
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