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A lesser-known subset of injection molding is reaction injection molding. Here we’ll explain what it is, show photos of reaction injection molded products, and provide some helpful info for designers.

What’s the difference between injection molding and reaction injection molding? And what does the “reaction” part refer to?

In standard injection molding, thermoplastic is injected into a mold as a liquid. As the part cools, it solidifies.

In reaction injection molding or RIM, a thermosetting resin is used, rather than a thermoplastic. Two different resins that will react when combined and heated are squirted into the mold, which is then heated up in order for the part to cure. Thus the “reaction” refers to the thermosetting, as when you mix up a two-part epoxy.

What specific material is the thermosetting resin?

The thermosetting resin commonly used in RIM is polyurethane. (Specifically, aromatic polyurethane.*)

What are the applications for reaction injection molding?

RIM is commonly used for large, sturdy but lightweight parts like automotive bumpers and spoilers, boat dashboards, ATM machines, business machines, medical machines, et cetera.

Why would I use reaction injection molding rather than standard injection molding? What are the benefits and drawbacks?

RIM arguably offers more design freedom. Parts that are tricky to create with standard injection-molding, like large parts with wall thicknesses that vary from thick to thin, are well-suited to RIM. And because the material is polyurethane, it’s possible to mold pieces that have a softer foam core and a harder outer shell, yielding a rigid part that is lightweight. By tweaking the polyurethane blend and curing process, you can dial in the desired weight, strength, density and hardness of your part.

One drawback is that aromatic polyurethane doesn’t have good UV resistance and must be painted for protection.

As for cost benefits and drawbacks, your company’s resident nerd would have to do the math, weighing the following factors. On the one hand, RIM has lower tooling costs; aluminum, rather than steel, molds can be used, and aluminum molds are cheaper to machine. On the other hand, the cycle times for RIM are slower than with standard injection molding. If you’ve gone with a cheaper aluminum mold, it won’t last for as many cycles as a steel one, and if you opt for a steel one, there go your cost savings. Also, the materials cost of the polyurethane needed for RIM is generally higher than the polypropylene and polyethylene commonly used in standard injection molding.

Resources

– RIM Manufacturing put together this video explaining and demonstrating the process:

– ThomasNet explains the RIM process in depth here.

– For nitty-gritty details on designing parts for RIM—wall thickness, draft angle, rib placement, undercuts, etc.—the polymer manufacturer formerly known as Bayer Material Science, now called Covestro, has produced this free RIM Part and Mold Design Guide.

–from the RIM Part and Mold Design Guide

– *Broadly speaking there are two types of polyurethane: Aromatic and aliphatic. You can read about the technical differences between the two here.