Reaction injection molding: The other RIM

With the incredible success of the BlackBerry handheld device, it’s hard to open the business pages of a Canadian newspaper without reading about our homegrown wireless device company Research In Motion. They generally abbreviate it as RIM. Whenever I see or hear the acronym, however, I don’t think about Research in Motion, but about what I regard as the real RIM: reaction injection molding.

RIM (which henceforth means the molding operation, not the BlackBerry guys) is one of those things that hides some incredibly complex chemistry behind a deceptively simple process. A common part that’s well suited to the RIM process is polyurethane auto fascia and bumpers.

Up front, it’s a little more complex than conventional injection molding; the raw material is a two-part system. The chemistry is pretty involved, but for our purposes, the two components can be thought of as an isocyanate and a form of alcohol…think epoxy glue. To use epoxy, you mix the “resin” with a “hardener” and, after a certain amount of time, it sets. Polyurethanes use a catalyst to speed the setting process, and as a result the mixing happens in a “mixing head”, after which the shot is immediately introduced into the mold. I say “introduced” because unlike thermoset injection molding, you don’t need crushing pressure to fill the mold. And as typical PU formulations set up, a little gas is evolved, which increases the polymer volume and aids flow.

Disadvantages? RIM polymers involve substances that can be difficult, costly, and dangerous to handle, especially isocyanate precursors. The equipment is specialized and expensive for small parts and conventional shapes, and the process has unique maintenance and QA needs. But there’s a big positive, too: With RIM, you’re essentially making the polymer inside the mold, which means the stuff you’re injecting can be engineered to flow very easily by thermoplastic standards. That means lighter, cheaper molds, and the ability to mold very large parts with quick cycle times.

Why not just mold TPE with gas assist on conventional equipment? You can, and in many applications that’s a preferred process, but the ability to shoot a viscous liquid into a big mold without a major mold engineering effort is a big advantage of RIM. If you’re working with PU, a blowing agent like pentane or even a controlled amount of water lets you create rigid foams, allowing light, strong parts without the network of webbed reinforcement needed behind rigid thermplastics.

There’s a lot more to RIM than this simple “rant” can explain, but think of it this way: if conventional thermoplastic injection molding is like stuffing sausages, RIM is like making waffles. Both taste great with my breakfast.

(By the way, last issue I promised to expand on my mold cooling theme in this issue; watch for it next issue, instead.)