Learn Up: What is Rapid Injection Molding?
Injection Molding

What is Rapid Injection Molding?

July 13, 2021

Learn Up: Rapid Injection Molding

The Rapid Injection Molding (RIM) process incorporates a rapid tooling method to deliver production-grade injection molded parts quickly. Not only is the turnaround time extremely fast, with parts in hand possible in less than two weeks, it’s also at lower cost than with standard tooling. For prototype quantities or low-volume production runs, you can get parts made with production resins and move right into your function and form-fit tests.

This technology is ideal for buyers who need between a few parts to perhaps 10,000 or more, and who want their parts in a couple weeks and with minimal investment.

A molding recap

We’ve discussed plastic injection molding previously. In our ”What is Plastic Injection Molding?” blog, readers learned the basics of this deeply technical process. They discovered that there’s a veritable universe of polymers out there, each with its own specific properties and uses. They picked up some high-level explanations of molding terms like overmolding, side actions, and pulls. Lastly, readers learned of the need for sound part design, and were introduced to the term “Design for Manufacturability,” or DFM. Without a good handle on this important concept, they were told, the road to cost-effective, high-quality plastic injection molded parts will be a rocky one.

Semi-crystalline thermoplastic polymers are strong, engineering grade materials with excellent chemical resistance.

What we didn’t talk about are the various types of plastic injection molding services available, how they differ from one another, and why a careful analysis of the options can spell the difference between an overwhelming success and an expensive lesson. Here are some of the alternatives to Rapid Injection Molding and their trade-offs:

  • Some 3D printing service bureaus promise prototype molds practically overnight. Because these tools are themselves made of polymer, however, their lifespan is generally limited to less than 100 parts (often way less). In addition, the parts will be restricted to “easy” molding plastics such as polypropylene or TPU, and geometries with relatively simple shapes, with no options for undercuts and other features that require side actions.
  • So called “e-machine shops” advertise much the same thing, but typically use longer lasting aluminum for their mold components, and cut them on CNC milling machines. The production quantities possible with these tools might hit 1000 pieces or so, but again, part geometries and material selection are somewhat limited. And as with 3D-printed tools (at least those made of polymer), there’s little possibility for the cooling channels used on production tools, making for long cycle times.
  • Production molding houses trumpet their ability to make cooled, multi-cavity, highly efficient molds able to spit out even complex parts in record time. Yet this capability comes at a cost, both in the lead-time needed to design and build the mold and a commensurately higher tooling investment up front. The result, however, is the lowest cost per piece possible.
Industrial metal mold/blank milling. Metalworking. Lathe, milling and drilling industry. CNC technology.

A quick Google search for plastic injection molded parts illustrates an important fact: buyers of such products have plenty of options. The challenge, then, is finding a reputable shop able to produce the required number of parts within the budget, timeframe, and quality requirements said buyers can work with. And there’s the rub.

Because there are so many ways to crack the plastic injection molding nut, anyone who’s not an expert on the topic might find supplier selection challenging. Worse, those who want fully functional prototype and low to mid-volume parts quickly might make an expensive mistake. As you’ll see, RIM is often the best solution for a variety of manufacturing needs.


Of course, some of you might argue “why not just 3D-print the parts, especially if we only need a handful?” It’s a good question, but one that’s easy to answer: 3D printed parts do not have the same mechanical properties as those that have been injection-molded, and are therefore unsuitable for functional testing. With RIM, you’ll get fully production-grade parts to facilitate all your testing and troubleshooting efforts.


Not so muddy waters

But what does the word “rapid” really mean, and what differentiates RIM practitioners from the manufacturers offering injection molded parts “right now,” or those in the production business who likely won’t even quote on a job unless it involves parts by the gazillions? As you’ll see, there’s much more to rapid than being fast.

At Prismier, RIM begins with another acronym, MUD. It’s short for Master Unit Die, and it is to the molding industry what modular furniture is to IKEA. As the name implies, a MUD is a quick-change die system that utilizes standardized mold inserts made of P20 tool steel, a tough but readily machinable low carbon alloy.


Because the customer only pays for the inserts and not the entire mold, the up-front investment is much lower. And as already mentioned, this is a quick-change system, so setup times are significantly shorter than with traditional mold tooling, further reducing part costs. Slides and lifters are possible, as is overmolding. The only restriction is size—Prismier’s proprietary MUD system is limited to parts measuring 12″ x 12″ x 4″ high.

Hard, Soft, or Rapid?

Aside from MUD, the use of P20 alloy is one of the key differentiators between aluminum or 3D printed plastic molds. That’s because steel is far more wear and deformation-resistant than the alternatives, drastically increasing tool life. P20 also supports the injection molding of abrasive polymers, and those with higher melt temperatures. These include super strong glass-filled or fiber-filled Nylons and engineering-grade materials like Delrin. Higher molding temperatures also help to reduce or eliminate problems such as knit lines and “halo” marks, common sights in parts made from quick-turn prototype molds.

3D render image representing an plastic moulding machine / Plastic injection moulding

Because we also have sinker EDM capabilities, we’re able to generate square corners and other part features not possible with a milling cutter. This opens numerous doors on design flexibility. That said, RIM molds are not hardened like their production counterparts, so the machining process is both faster and less expensive. The result is lead-times of between 3 to 12 days, with a far lower price per piece than competing methods can offer.

Finish, already

Texturing is another consideration. It can be difficult to achieve industry standard “Mold-Tech” textures and SPI (Society of Plastics Industry) finishes on aluminum molds and especially 3D printed ones. They might be okay for prototype purposes, but if the part quantities increase and a production tool is required, the chances of the old and new parts matching is slim. Similar arguments can be made about the limitations on part geometries as discussed earlier—these might be just fine during the project’s early stages, but later on? Correcting design shortcomings after the fact is often an expensive undertaking.

Next to packaging, plastic injection molded parts consume the lion’s share of the world’s polymers.

Checking the boxes

Of course, Prismier also offers production molding. In fact, it’s a large portion of what we do here. The difference is, we’re happy to quote part quantities well below the “gazillions” threshold. We’re also willing to work with customers to find the most timely, cost-effective solution possible, whether that means RIM or a more customized, high-volume tooling approach. The takeaway is clear. We encourage anyone with a plastic injection molding project—whatever the quantity—to kick the tires and check all the boxes. Just do yourself a favor and be sure that Prismier is one of them.

If you'd like to know more, pick up the phone and call us at (630) 592-4515 or email us at sales@prismier.com. Or if you're ready for a quote, email quotes@prismier.com. We'll be happy to discuss your options.