At What Volume Does RIM Make More Sense Than Injection Molding?

If you are running between 100 and 5,000 parts a year, reaction injection molding almost always costs less over the life of the program than injection molding. Above that, the math flips.

Where does the volume crossover fall?

Between 100 and 5,000 parts a year, RIM wins on total program cost against injection molding. In that window the tooling savings are large enough to matter and the volume stays small enough that the higher per-part cost never catches up.

The arithmetic is simple enough to do on the back of a drawing. Take the tooling delta between the two processes and divide it by the per-part cost difference. That gives you the break-even quantity. Then compare it against how many parts you expect to build over the life of the program, not in year one.

We run this calculation with engineers most weeks. The programs where the answer surprises people are usually the ones where somebody quoted a five-year volume and the product got redesigned in year two.

Why does a RIM tool cost so much less?

RIM fills a mold at 50 psi. Injection molding fills at pressures more than an order of magnitude higher, per the Covestro RIM Part and Mold Design Guide. That is why an injection tool has to be hardened steel sitting in a press that can hold it shut.

We can build a RIM tool out of softer material. The relative mold cost comparison in that same guide indexes machined steel at 100%. Machined aluminum lands near 80%, cast aluminum and Kirksite near 60%, sprayed zinc near 40%, and cast epoxy near 30% for prototype work. Softer material machines faster. Faster machining is less tool time, and tool time is most of what you are paying for.

The second saving is press. Many of our molds are free-standing and hand-clamped, which works for parts up to 1,000 lbs. No press tonnage sized to the projected area of the part means no capital cost buried in the quote.

Why is the RIM per-part cost higher?

A RIM part comes out of the mold needing more hands than an injection molded part. Trimming, drilling, bonding, sanding, and painting all happen after demold, and that post-molding labor lands in the piece price.

Cycle time is competitive. An average mold fills in a second or less and the part demolds in 30 to 60 seconds. What follows the demold is where the cost sits.

That is the honest tradeoff. You pay less to get started and slightly more per part forever, so the process rewards programs that never reach the volume where a cheap piece price would have paid back an expensive tool.

How do you run the crossover math on your own part?

Two numbers decide it: what the two tools cost, and what the two piece prices are. Everything else is detail. For a wider look at how RIM stacks up against thermoforming, cast urethane, structural foam, and sheet metal, see our RIM process comparison chart.

What if your annual volume is under 100 parts?

Below about 100 pieces a year, look at machining first. Our own process selection guide puts RIM as an economical alternative to machining at quantities as low as 50 pieces, so the boundary is soft, and it moves depending on part size and geometry.

Large parts move it the most. A machined enclosure panel measuring several feet across gets expensive fast, and at that size RIM can beat machining well below 50 pieces. We covered that case in more detail in why RIM beats sheet metal and injection molding for large enclosures.

What if your annual volume is over 5,000 parts?

Above 5,000 parts a year, injection molding is probably your process, and we will tell you so on the phone rather than after you have paid for a RIM tool. If you are weighing the two at the low end of that range, low volume injection molding: when it works and when RIM wins walks through the tradeoffs.

Three exceptions are worth checking before you commit. Parts too large for available press capacity stay in RIM territory regardless of volume. So do parts with wall sections that swing from an eighth of an inch to over an inch, and parts that need components encapsulated during the shot.

There is a fourth case. Sometimes a single RIM shot replaces an assembly of several injection molded pieces plus fasteners and labor. The comparison is no longer part against part, so run the volume math on the whole assembly.

Quick answers for sourcing teams

Send us a sketch and an annual quantity and we will run the crossover math against your part before either of us spends money on tooling. Exothermic has been building RIM tools and molding parts in New Jersey since 1971.