The structural foam molding process requires an inert gas (such as nitrogen) to be mixed with the chosen plastic material inside an extruder barrel. This mixture is then injected into a mold using less pressure than what is common in injection molding
Once injected, the gas expands, filling the mold cavity. Alternatively, a chemical reaction within the blend itself may lead to the desired foaming action. When the foaming plastic touches the cold mold wall it solidifies, producing a thin layer of skin. This skin is supported by a cellular structure that is not dissimilar to wood products. Unlike injection molding, high pressures are not needed. The foaming action caused by the gas is sufficient to create the part.