Unlike thermoplastic injection molding, Reaction Injection Molding is a process capable of achieving a broad range of properties.  While both processes use molds to form parts, the RIM process uses polyurethanes to produce molded parts. 

The polyurethanes are two liquid components (Polyol and Isocyanate) developed in two-part formulations to create RIM systems.  The system will determine whether the part molded will be a foam or a solid and whether the part will be flexible or extremely rigid.  The specific gravity will range from 0.2 to 1.6.

The RIM Process involves a chemical reaction between the two liquid components.  Initially, the liquids are held in separate, temperature-controlled feed tanks.  The chemicals are fed through supply lines to metering units that precisely meter both chemicals, at high pressure, to a mixhead device.

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The liquids are injected – or shot – into the mold at pressures between 1,500 and 3,000 psi, and are mixed by high-velocity impingement.  The shot and cycle times vary depending on the part size and the polyurethane system used.

Inside the mold the liquid undergoes an exothermic chemical reaction, which forms the polyurethane polymer in the mold.  When processing foams, significant forces created inside the mold must be resisted to ensure the integrity of the part.  The clamping pressure required can be up to many tons depending on the size, expansion rates, and the desired density of the part, along with other material factors.

After the shot, the chemicals are continuously circulated at low pressure by the pumping system and through the mixhead.  After reaching the mixhead the chemicals are recirculated back through the feed tanks and then through the same path again back out to the mixhead.  This low-pressure recirculation is used to maintain temperature, nucleation, and will help keep added ingredients such as fillers or pigments evenly dispersed.