Reaction injection molding (RIM) is similar to injection molding except thermosetting polymers are used, which requires a curing reaction to occur within the mold. The most common RIM processable material is polyurethane.

The two parts of the polymer are mixed together. The mixture is then injected into the mold under high pressure using an mixer. The mixture is allowed to sit in the mold long enough for it to expand and cure. If reinforcing agents are added to the mixture then the process is known as reinforced reaction injection molding (RRIM). Common reinforcing agents include glass fibers and mica. A subset of RIM is structural reaction injection molding (SRIM), which uses fiber meshes for the reinforcing agent. The fiber mesh is first arranged in the mold and then the polymer mixture is injection molded over it.

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 bi-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.

The disadvantages are slow cycle times, compared to injection molding, and expensive raw materials.

 Making a RIM mold is a important procedure to assure the production of RIM. It need professional engineers and powerful craftsman working together.