How RIM works
At the heart of the polyurethane RIM process is a chemical reaction between the two liquid components, which are held in separate, temperature-controlled feed tanks equipped with agitators. From these tanks, the isocyanate and polyol feed through supply lines to metering units that precisely meter both components, at high pressure, to a mixhead device.
When injection of the liquids into the mold begins, the valves in the mixhead open. The liquid reactants enter a chamber in the mixhead at pressures between 100 and 200 bar, and they are intensively mixed by high-velocity impingement. From the mix chamber, the liquid then flows into the mold at approximately atmospheric pressure. Inside the mold, the liquid undergoes an exothermic chemical reaction, which forms the polyurethane polymer in the mold.
Shot and cycle times vary, depending on the part size and the polyurethane system used. An average mold for an elastomeric part may be filled in one second or less and be ready for demolding in 30-60 seconds. Special extended gel-time polyurethane RIM systems allow the processor enough time to fill very large molds using equipment originally designed for smaller molds.