18.3.2 Structural-Foam Molding
The sirucrurul-foarn molding process is used to make plastic products with a solid outer skin and a cellular inner structure Typical products made arc furniture components, TV cabi¬nets. business-machine housings, and storage-battery cases. Although there are several foam molding processes, they are all basically similar to injection molding and to extrusion. Both thermoplastics and theriuoscts can be used for foam molding, bu: thermosets are in A liquid processing form similar to the condition of polymers in reaction-injection nxilding.
In injection foam molding, thermoplastics are mixed with a blowing agent (usually an inert gas such as nitrogen), which expands the material. The core of the part is cellular, and the skin is rigid. The thickness of the skin can be as much as 2 mm (0.03 in.), and part densities arc as low as 40% of the density of the solid plastic. Thus parts have a desirably high stiffness-to-weight ratio, even when weighing as much as 55 kg (120 lb).