ABSTRACT
Reaction injection molding, or RIM for short, began as a low volume conversion process utilizing thermoset polymers. Recently, it has gained popularity in the automotive markets. Figure 5.1 shows the equipment used in this process involving two reactive chemicals: an isocyanate and a polyol. The two liquids are mixed at pressures between 1500 psi and 3000 psi (1021 MPa) in the mixing chamber and injected into the mold from the lowest point of the mold in order to minimize trapped air.1 The part is then allowed to cure before it is removed from the mold. Once the part has been de-molded, it may be necessary to clean the mold before the next cycle. The cured part is then ready for post-molding operations, which include trimming the flash, filling voids, adding inserts, priming, and painting. The filling process of reaction injection molding occurs at very low pressures, around 50-150 psi (0.34-1 MPa), and low flow rates to prevent air entrapment.2 This process takes place at room temperature and, compared to injection molding, does not require complex tooling to produce parts. Another unique feature of reaction injection molding is that the cycle times are measured in minutes rather than seconds like regular injection molding cycle times.
