In reaction injection molding (RIM), a two part liquid polyurethane is mixed under low pressure and then forced into a mold cavity. Polyurethane is a thermoset and it chemically crosslinks during the mixing and injection cycle. The part hardens by chemical reaction in the mold and does not require heating or cooling.

The two part component resin must be controlled accurately for temperature, pressure and viscosity. The complete polymerization of the polyurethane takes place in the mold. A complete purging of the mixing head after each shot is required or hardened cross linked material clogs the mixing chamber. Molded part surfaces are smooth; no pinholes or surface blemishes mar their surface. The density, weight, and elasticity of each part can be controlled by changing the types of components. Hard rubber-like parts are often produced in large structural machine bases. The rubber like density of the polyurethane absorbs sound and dampens machine vibrations. Relatively protracted cycle times ( 2-3 minutes) and the narrow range of materials (mostly polyurethanes) available for use with this process have restricted the growth of RIM.

RIM products are similar to large structural foamed injection molded products. However, RIM parts are produced at significantly less cost and shorter production times. RIM does not require high temperatures (85 to 100 degree F) to crosslink the material. Likewise, the relatively low pressure of the injection cycle (less than 100 PSI) is 500 to 4,000 times less than found in injection molding. These lower pressures and temperatures make it possible to construct molds rapidly out of low strength inexpensive materials. For example, many RIM molds are constructed out of aluminum sheet stock. RIM molds king-size box springs and mattresses have been fabricated from glass reinforced polyester.

Products using RIM methods are generally thick walled large structural components. They can be molded in high-density, high-modulus, flexible elastomers and lower density structural foamed parts. The first RIM polyurethane products, the 1978 Corvette bumpers, were molded by General Motors. Since its introduction the lightweight, soft, and flexible bumper have become a standard on many automobiles. Thermoset polyurethane, nylon, polyethylene, epoxy and polyester have all been successfully molded with RIM methods. Nylon commercial applications include: computer housings, television cabinets, radio cabinets, and the fenders of some Oldsmobiles.