Reaction injection molding (RIM) is the injection of liquid polyurethane systems into a mold. The 2 liquid components (isocyanate and resin) react to form the part. Reaction injection molding is similar to injection molding except thermosetting polymers are used, which requires a curing reaction to occur within the mold.
Reaction injection molding can produce strong, flexible, lightweight parts which can easily be painted. It also has the advantage of quick cycle times compared to typical vacuum cast materials. The two-component mixture injected into the mold has a much lower viscosity than molten thermoplastic polymers, therefore large, light-weight, and thin-walled items can be successfully RIM processed. This thinner mixture also requires less clamping forces, which leads to smaller equipment and ultimately lower capital expenditures. Advantage of RIM processed foam is that a high-density skin is formed with a low-density core and that the interior of the part can be molded to take advantage of design for manufacture.