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Structural foam molded plastic parts, by definition, have a cellular foamed core surrounded by integral skins forming a total integral structure. Structural foam molded parts have a high strength-to-weight ratio and have 3 to 4 times greater rigidity than solid parts of the same material of equal weight.
The structural foam molding process is similar to conventional injection molding with the exception that a foaming agent is mixed with the polymer melt and injected into the mold in volume less than is required to mold a solid part. Both injection pressure and the expanding gas/polymer mixture fill the mold. The melt in contact with the cool surface of the mold forms an almost solid skin and in the inner core the foaming gas expands to form a cellular structure.
This process has greatly extended the range of product types and sizes that can be considered for molded structural foam.
Almost all thermoplastics can be used for structural foam molding. Their molded parts have the same basic properties as the base resin. Actual physical properties are determined by the density reduction attained, skin thickness, part shape and general design. High density polyethylene, impact polystyrene, polypropylene and engineering resins are among the major thermoplastic materials used for structural foam products. Proper selection of these materials according to their inherent properties, with selective use of additives such as glass fibers, colorants and ultraviolet inhibitors, can meet a wide range of application requirements. These include the ability to withstand rough handling, high temperatures and long term weather exposure.
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Structural foam has replaced wood, concrete, solid plastics and metals in a variety of applications. Some benefits realized are lower weight, lower raw material or manufacturing costs, greater part stiffness and stability, elimination of corrosion, greater design flexibility, improved chemical resistance, mechanical properties and cleanability. Its sound-deadening characteristics, low-stress concentration, minimal sink marks and electrical and thermal insulating properties are also advantageous.
Plastic materials in the form of structural foam can truly be called a designer’s engineering plastic, regardless of the basic material used. Careful design allows combining many parts and functions into one part, to gain the greatest cost advantage of structural foam. Parts can have a molded in texture or be painted after molding, providing a variety of attractive surface finishes.
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