Reaction Injection Molding is commonly referred to as RIM molding. It is a very dynamic and cost-effective molding technology that can be used when producing parts in lower annual volumes (200-20,000 per year). RIM molding is popular for a number of reasons but mainly due to the superior physical properties available to the part designer and the host of processing and polyurethane (PUR) material formulation possibilities.
RIM Molding Explained
RIM molding is very similar to traditional injection molding in terms of the types of parts, tolerances and aesthetics that can be achieved but it does have numerous differences that traditional injection can’t offer. Unlike traditional injection molding where you heat up a thermoplastic pellet and inject the tool under high pressure to fill the cavity, RIM is a liquid process capable of achieving a broad range of physical properties by injecting two liquid raw materials into a heated mold with very little pressure into a mold cavity. The two liquid raw materials polyol and isocynate (polyurethane -PUR) are mixed at a mix head and injected into the tool. Once the material meets the heated tool surface, an exothermic reaction begins to occur and allows the material to expand and fill the cavity.
Depending on the formulation and mix ratio’s of the two raw material components of the polyurethane, the finished parts can be foamed, a solid, dual density foam and can vary from being flexible to very rigid. In addition, parts can be very large in size, lightweight and also be able to vary wall sections within the same part without sink or distortion. This is due in part to the foaming process which enables less built in stress in the part to occur especially in large parts or parts with variable wall sections (up to 2”).