After years of working with manufacturers who asked the same question, we have developed a good sense of what works and what doesn't in Reaction Injection Molding. Their collective experience provides valuable insights into this versatile but sometimes misunderstood process.
At its core, RIM uses polyurethane chemistry—a family of materials rather than just one type of plastic. Manufacturers have successfully used:
Rigid systems when they need something sturdy and heat-resistant providing the highest surface quality and exact replication of the mold surfaces.
Elastomeric systems when they need something that can take a beating while also enhancing comfort and ergonomic needs
Poly-DCPD when they need exceptional chemical resistance and thermal performance
Poly-DCPD is much improved over simple DCPD's of the past. Poly-DCPD provides high strength and impact resistance with exceptional chemical and thermal performance.
What fascinates materials engineers about these systems is how they can be fine-tuned.
One medical device manufacturer needed a housing that could withstand hospital-grade disinfectants without deteriorating or cracking. The team formulated a specific polyurethane blend that provided reliable parts, and it has been in production for over 25 years.
Painted surfaces tend to improve chemical resistance and moisture absorption properties.
One thing that still surprises RIM newcomers is how large these parts can be. Experienced molders have seen clients' jaws drop when shown they could mold 6-foot equipment housings as single pieces.
The secret lies in those liquid components. They're about as viscous as maple syrup, flowing effortlessly through the mold at pressures you could almost create by hand. This means designers aren't fighting against the material but working with it.
The larger the part (size and weight), the more competitive the RIM process becomes compared to other molding process options.
Wall thickness is another area where RIM shines compared to other processes. Successful designs have included:
Delicate 1/8" sections that transition smoothly to structural 1" sections
Paper-thin cosmetic features alongside chunky mounting bosses
Gradual thickness changes that would cause nightmare sink marks in injection molding
One automotive manufacturer approached us after struggling to make a dashboard component with traditional processes. The varying thickness kept causing warping and sink marks. With RIM, they molded it perfectly on the first production run.
Industrial designers familiar with the design freedoms inherent in RIM molding continue to rely on the process to meet customer requirements.
Over the years, engineers have pushed the limits of what can be RIM molded. Features that typically work well include:
Variable wall thickness
Stiffening ribs without the normal sink-marks
Deep, textured surfaces that create striking visual effects
Encapsulated electronics—like a weather station housing where the circuit board was surrounded by polyurethane, protecting it from years of harsh conditions
Encapsulations with low internal pressures and temperatures
That said, not everything works. Experienced designers have learned (sometimes the hard way) to be careful with:
Draft angles—typically recommending at least 1/2° for shallow parts and more for deeper sections
Inner corners—sharp 90° angles can cause stress concentration points
Very thin sections—anything under 1/16" can be tricky to fill consistently
One aspect of RIM that doesn't get enough attention is how beautifully these RIM parts take paint. Seasoned professionals have seen painted RIM components next to their metal counterparts, and even experienced engineers couldn't tell the difference.
The slightly porous surface of polyurethane creates an excellent mechanical bond with paint. Combined with the right primer, finishes can rival automotive quality.
RIM isn't for every project. After working with hundreds of clients, manufacturers have found it's typically most economical when:
Production needs fall between 100-5,000 parts annually
Designs are complex with varying wall thicknesses
Size requirements are significant
Tool budgets are limited (RIM tools typically cost 40-60% less than comparable injection molds)
RIM molds are similar in design and accuracy to injection molds, but they are often half the cost due to the low molding pressures and temperatures.
Honest molders are upfront about RIM's limitations. It's probably not right for projects requiring:
Optically clear parts
Annual volumes exceeding 10,000 units
Extremely tight tolerances (beyond ±0.005")
Ultra-thin walls critical to the design
Cosmetically correct without paint is not realistic
When helping clients decide whether RIM is right for their project, experienced consultants typically examine:
How big and complex is the part?
What are the annual volume needs?
Does the design require varying wall thicknesses?
Is encapsulation needed?
What surface quality is required?
What's the tooling budget?
How quickly is market entry needed?
When engineers understand these parameters, they make better decisions—and often discover design possibilities they hadn't considered before.
RIM isn't magic, but sometimes, it feels close. It has transformed projects that seemed impossible with conventional manufacturing methods into successful products.
The key is understanding its capabilities and limitations and designing with those parameters in mind.