Structural foam shapes can be valuable as integral components. Some product components are best suited to be made out of metal, other solid plastic, wood, ceramic, glass, or composite materials that can be complex combinations in a matrix of the other components. Structural foam is really in a different category of component.
Structural foam parts are made by having a negative cavity in the in the shape of the final part, typically molded in a solid substance, usually aluminum. A molten plastic, leavened with an inert gas such as nitrogen is injected typically at a low pressure into the cavity mold, which may have been preheated. The foam expands into the cavity, and the surface bubbles are broken and compressed forming a dense skin against the surface of the cavity. The rest of the bubbles stay intact, less dense, and disperse their density more or less evenly. This volume of closed cells of foam forms a structural matrix that has great relative strength. So the final part will have a dense, handle able and washable skin, and the body of the part will have great strength per weight.
Most consumers have been exposed to the polystyrene version of these products that have been used largely in the shipping and packaging industries, as these structural foam shapes are readily utile for surrounding oddly shaped consumer goods, and providing a compressible structure around them that will fit inside a rectilinear container, like a box. However, what most consumers may not know is that houses are now being made with the assistance of this same type of foam. It is now formed into sheets and interlocking blocks that can be integrated with steel rebar, and cast in the center of concrete walls and foundations. This process adds tremendous thermal insulation capabilities to what is otherwise a heat and moisture sink.
The design, making, and usage of molds for these foams are now handled by well established business experts. The molds are typically aluminum, are created with water flowing heat radiators/absorbers, and have a very long industrial life span.