Can Poly-DCPD Be Molded in 'Soft' Composite Molds? A Materials Engineer's Perspective
When engineers first encounter polydicyclopentadiene (Poly-DCPD) molding, one of the most frequent...
4 min read
The hardest decision in a RIM project is often not whether to use the process, but which material to mold. RIM is not a single material. It is a family of polyurethane and polyolefin thermoset systems that range from soft, flexible elastomers to rigid structural foams stiff enough to stand in for sheet metal. Pick the wrong end of that range and you either over-build a part that should flex or under-build one that needs to carry load.
Quick answer: Choose a RIM material by working backward from the part’s load, stiffness, weight, environment, impact exposure, temperature range, finish requirements, and production volume. Elastomers fit flexible or energy-absorbing parts, solid RIM systems fit dense structural housings, structural foam fits large rigid parts where stiffness-to-weight matters, and Poly-DCPD fits rigid, impact-resistant parts exposed to demanding environments.
This is a designer's framework for narrowing the choice early, before tooling commitments lock the decision in.
What material families does RIM cover?
At the soft end sit elastomers, with hardness starting around Shore A 30. These handle energy absorption, sealing, bumpers, and flexible covers. Moving up in rigidity, solid RIM systems give you a dense, tough part with good surface finish for housings and structural components. Structural foam sits at the rigid end, using a cellular core with a solid skin to deliver high stiffness-to-weight in thick walls. Flexible foam covers cushioning and soft-touch applications. Across the polyurethane family, rigidity is the main sorting axis. A rigid system carries a higher flexural modulus and hardness along with good thermal, electrical, and chemical resistance, while an elastomeric system trades stiffness for flexibility and impact tolerance.
Poly-DCPD, a polyolefin thermoset system, occupies a useful corner of the map for parts that need toughness and chemical resistance together. Exothermic's Poly-DCPD resin data shows a flexural modulus in the range of roughly 287,000 to 383,000 psi (ASTM D790), notched Izod impact of about 2.8 to 9.8 ft-lb/in (ASTM D256), and a Shore D hardness around 80 to 83 (ASTM D2240), with polymer density between 0.98 and 1.12 g/ml (ASTM D792). Those numbers describe a rigid, impact-resistant material that holds up in demanding environments.
What questions actually drive the choice?
Start with load. Does the part carry structural load, or does it cover and protect? A structural housing points toward solid RIM or structural foam. A flexible boot or bumper points toward an elastomer.
Then weigh stiffness against weight. Structural foam earns its place when you need rigidity in a large, thick-walled part without the mass of a solid section. For solid materials, wall thickness usually centers around 1/8 inch, though parts as thick as 1/2 inch have been molded. Structural foam parts commonly run from 1/4 inch to 1-1/2 inches, according to established RIM design references.
Environment comes next. Chemical exposure, UV, and temperature swings can be engineered into the formulation through material selection. Where rigidity needs a boost, glass or mineral fillers can be added to solid systems, a reinforced approach that raises stiffness without changing the basic process. Finish and cosmetics matter for any part a customer sees. Solid RIM takes paint and texture well and supports automotive-grade finishing, which is why it shows up on visible panels and enclosures.
How do impact and temperature factor in?
Impact resistance is where material choice gets specific. Elastomers absorb energy by deforming. Rigid systems resist it through strength and stiffness. If a part will see drops, vibration, or field abuse, the notched Izod figure is worth checking against the application, and Poly-DCPD's impact range makes it a candidate for rugged housings. Temperature behavior also varies across the family. RIM's curing reaction is itself exothermic and can drive the core of a thick section past 400 degrees F during molding, which is one reason wall thickness and cross-section design belong in the material conversation rather than apart from it.
What is the simplest way to narrow the list?
Work from the application back to the material, not the other way around. Define the load the part must carry, the environment it lives in, the finish it needs, and the volume you plan to build. Those four inputs usually point to a single family before you ever discuss formulation details. From there, a short engineering review can confirm the specific system and wall strategy.
Frequently asked questions
Can one RIM part combine soft and rigid sections?
Not within a single shot of one material, but RIM's range lets you match a material to the dominant requirement, and consolidation or over-molding strategies can address parts that need two behaviors.
Is structural foam strong enough to replace metal?
For many large enclosures, yes. Structural foam delivers high stiffness in thick walls at a fraction of the weight, which is why it competes with sheet metal and castings in low-to-medium volume parts.
When does Poly-DCPD make sense over polyurethane?
When you need a rigid, impact-tolerant part with strong chemical resistance. Its property profile suits rugged housings and components exposed to harsh conditions.
Get a material recommendation for your part
Material selection is easier with a real part in front of it. Send Exothermic your performance requirements and the team will recommend a material family and wall strategy as part of an engineering consultation.
