Polyurethane foam products are made in a closed aluminum mold by means of Reaction Injection Molding (RIM). Two liquid substances, usually polyols and isocyanates, are injected into a mold under pressure. Gasification occurs, whereby the material starts to foam, expands and forms to the mold. This gives the product a porous, lightweight core with a closed surface. Both hard products and flexible parts can be made with this technique.