RIM Molding, Reaction Injection Molding

Reaction Injection Molding Design Guide

RIM Molding, Reaction Injection Molding - This technique was developed in Germany in the 1960's for molding urethanes. It is used currently for rigid urethane foam, flexible microcellular foam, and rigid microcellular foam. The process embodies high-pressure-impingement mixing of the liquid components before they are injected into the mold. RIM has advantages over the standard low pressure mechanical-mixing systems in that larger parts are possible, mold cycles are shorter, there is no need for solvent-cleaning cycles,surface finishes are improved, and rapid injection into the mold is possible.

The RIM process involves a high-pressure delivery of two or more liquid urethane components to a very small mixing chamber where they are continuously mixed and injected into a closed mold at rates approaching 650 lb/min. The liquid components are heated to maintain low viscosities. The heart of the system is the mixing chamber, where the liquid components must be thoroughly mixed without imparting turbulence. Continuous delivery of the components to the mold is accomplished by high-volume, high-pressure recirculating pumps from liquid-storage tanks. Automatic controls are used to maintain precise flow and temperature of the resins.

Since mold pressures in RIM processes are usually below 100 psi (0.69 MPa), mold-clamp-pressure requirements can accordingly be low. Most RIM equipment is in the 25-100 ton range. Depending upon production quantity and quality requirements, molds for RIM may be made from aluminum-filled epoxy for low production, or from cast or machined aluminum or chrome-plated steel for longer runs. Molds must be heated to 120-160°F (49-71°C). Applications include automobile bumpers, radio and TV cabinets, furniture and business-machine housings.

Typically, RIM molding cannot economically compete with other processes for medium volumes in the range of 500 to 50,000 parts because of high tooling and operating costs.