Structural foam molding is a modification of the injection molding process previously discussed. Many large structural parts require thicker walls than standard injection molding can produce. Structural foam molding allows for the injection molding of thick wall parts through the addition of a foam agent. Any injection moldable thermoplastics can be foam molded. To produce the thicker wall part, a small amount of nitrogen or chemical blow agent is introduced into the plastic material. Under pressure inside the injection screw, the foaming agents do not expand. When the melted plastic enters the mold cavity, foaming occurs. As the foaming plastics fill the mold cavity, the wall of the part solidifies against the cold mold wall. A thin layer of plastic solidifies without foaming along the mold wall. This thin layer forms a skin structure over the foamed inter core. The thin solid wall is supported by the interior cellular foamed structure. A part produced this way results in a heavy cellular structure similar to wood products.

The amount of foaming agent used controls the size of the cellular structures and the density of the par. An increase or decrease in the size and number of foam cells is determined by the amount of foaming agents. Structural foaming reduces the amount of material required to fill the mold and eliminates sink marks. It also adds greater strength and stiffness to parts due to the increased thickness and integrity of the cell structure. The dimensionally stable foam part requires less ribs, fillets, and bosses than injection molded parts.

But, the foaming of plastics causes a swirling blemish on the surface of the plastic part. If the outside appearance is important the structural foam part must be filled, sanded, and painted. The foaming process also traps gas glowing agent inside the part cellular structure. Degassing (slow evaporation from the part) the blowing agent for three to four months after molding, can discolor and damage the surface finish. Post curing to accelerate the degassing is sometimes required. The insulating qualities of the foamed structure require a longer time to cool which results in longer production times than its injection molded counter part.

Common structural parts take advantage of the large size, thick walls (less than .500 inch) and dimensionally stable properties of a foamed-product. Typical products are large trash containers, freeway sand safety containers, and in-ground housing for water systems. Large instrument case housing has been successfully molded in thin walls of more than .250 inch using high impact and dimensionally stable polycarbonates. However, the post molding costs of filling and finishing the swirl surface and the corrosive problems of the blow agents has restricted the application of formed molding.