Rigid Polyurethane Casting Resins
Rigid casting resins are two-part polyurethanes formulated to behave like the thermoplastic the part will eventually be molded in: an ABS-like grade cures to roughly 2 GPa flexural modulus and 40–55 MPa tensile strength, a PC-like grade adds impact and about 10–20 °C of heat resistance, a PP-like grade drops to about 1 GPa so it flexes and hinges, and a glass-filled nylon-like grade stiffens to several GPa. Mixed, degassed and poured into a silicone mold, they demold in 30–120 minutes and are the standard way to get 10–300 molded-looking housings, brackets and covers without cutting a steel tool.
Popular rigid casting resin grades
Compare in the explorer| Grade | Yield, MPa | Tensile, MPa | Density, g/cm³ | Machinability | Best for |
|---|---|---|---|---|---|
| ABS-Like | 45 | 52 | 1.12 | Good | The default cast housing, cover or bezel |
| PC-Like | 52 | 60 | 1.16 | Good | Drop-tested parts and a little more heat |
| PP-Like | 30 | 36 | 1.15 | Good | Living hinges and flexible clips |
| GF Nylon-Like | 75 | 60 | 1.4 | Good | Stiffest, most heat-resistant castings |
| General Purpose | 45 | 40 | 1.2 | Fair | Plain rigid castings for fit and form checks |
| ABS-Like FR | 45 | 50 | 1.17 | Very good | UL 94 V-0 samples for flammability review |
| PU-HighTemp | — | 65 | 1.16 | Very good | Post-cured parts for warm environments |
Choose rigid casting resins for
Pilot and bridge production of enclosures, bezels, covers and brackets in 10–300 pieces before the injection tool exists; trade-show and photography samples that must match the production colour and texture; functional prototypes with real snap-fits, screw bosses, living hinges and cast-in inserts; and flame-rated or glass-filled variants of the same part for a design review.
Look elsewhere when
Only one to five parts are needed and the geometry still changes daily (SLA Tough Resin or a machined ABS or PC block is faster), the part carries sustained load or runs above about 80–100 °C for years (machine POM-C, PA6 or PC, or mold it), tolerances tighter than ±0.15 mm are needed from a flexible silicone mold, it must be optically clear (Rigid Clear), it must flex like rubber (Rubber-Like), or volumes pass a few hundred and a soft aluminum injection tool becomes cheaper per part.
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Rigid casting resin by application
- Housings that look molded
- ABS-Like by default, General Purpose when the part is only a fit check; pigment goes in the mix, texture and gloss come from the master, and paint or soft-touch coating follows.
- Impact and warmth
- PC-Like for parts that are dropped, snapped or sit near electronics; PU-HighTemp when a post-cure for service near 120 °C is worth the extra cycle.
- Hinges and snap-fits
- PP-Like bends and snaps repeatedly like molded polypropylene — lids, cable clips, one-piece boxes; keep the hinge thin, uniform and radiused at the root.
- Stiff frames and flame-rated parts
- GF Nylon-Like for brackets, chassis and gear housings; ABS-Like FR or PU-FR-V0 when the sample must carry a V-0 rating.
Working with rigid casting resin
- Casting
- Two-part resin is metered by weight, pigmented, mixed and vacuum-degassed, then poured under vacuum into a silicone mold held at 60–70 °C; demold time runs from 30 minutes for fast ABS-like grades to a few hours for filled and high-temperature grades, and a post-cure brings the mechanical properties up. One mold gives 20–50 shots; several molds run in parallel for pilot quantities.
- Tolerance and design
- Expect ±0.15 mm on features under 50 mm and ±0.3 % of length beyond that, set by 0.15–0.3 % resin shrinkage and the flex of the silicone. Walls of 1.5–3 mm are ideal, 0.75 mm locally; undercuts demold freely, but put the parting line where a witness is acceptable, and cast threaded inserts and bushings in place rather than cutting threads in resin.
- Machining
- Castings drill, tap and mill cleanly with sharp tools and light cuts; glass-filled grades wear tooling faster and chip at edges. Critical bores and datum faces are machined after casting to reach ±0.05 mm — leave 0.3–0.5 mm of stock on those features in the master.
- Finishing
- Parts inherit the master surface — gloss, matte or textured — and are colour-matched in the resin to a Pantone or RAL reference. They take paint, soft-touch coating, EMI shielding paint and pad or silk-screen printing after a light abrade; no plating, anodizing or welding. Unpainted urethane yellows and chalks outdoors within months, so anything that sees sunlight gets a UV-stable topcoat.
Other polyurethane series
Rigid Polyurethane Casting Resins — frequently asked
What is rigid casting resin?
Rigid casting resins are two-part polyurethanes formulated to behave like the thermoplastic the part will eventually be molded in: an ABS-like grade cures to roughly 2 GPa flexural modulus and 40–55 MPa tensile strength, a PC-like grade adds impact and about 10–20 °C of heat resistance, a PP-like grade drops to about 1 GPa so it flexes and hinges, and a glass-filled nylon-like grade stiffens to several GPa. Mixed, degassed and poured into a silicone mold, they demold in 30–120 minutes and are the standard way to get 10–300 molded-looking housings, brackets and covers without cutting a steel tool.
Which grade of rigid casting resin should I use?
ABS-Like (or General Purpose, its plainer cousin) for any housing, cover or bezel that only has to look and feel molded. PC-Like when the part will be dropped, snapped hard or sits somewhere warm — the tougher, slightly higher-temperature choice at a small cost in stiffness. PP-Like for living hinges, flexible clips and one-piece boxes that must bend thousands of times. GF Nylon-Like for brackets, frames and gear housings that would otherwise be machined nylon: the stiffest and most heat-resistant of the group, with a duller surface and chippier edges. ABS-Like FR or PU-FR-V0 when the sample must pass a UL 94 V-0 review; PU-HighTemp when a post-cured part must serve near 120 °C.
Which grade of rigid casting resin is the strongest?
GF Nylon-Like has the highest yield strength in the series at 75 MPa; PU-PMMA-like follows at 58 MPa. Values are for each grade's default condition.
Which grade of rigid casting resin is easiest to machine?
PU-Foam-Rigid (machinability excellent, 88/100), then RPU (85/100).
ABS-like or PC-like for a housing?
ABS-Like for anything that only has to look right and survive handling: it is stiffer, cheaper, easier to cast bubble-free and the grade most masters are dialed in for. PC-Like when the prototype will be drop-tested, snapped repeatedly or sits near a warm component — it keeps its toughness in thinner walls and gains roughly 10–20 °C of heat deflection. Neither is the resin the production tool will run, so treat both as form, fit and function stand-ins.
How strong is a glass-filled nylon-like casting?
Stiffer than any unfilled cast urethane — a flexural modulus of several GPa against about 2 GPa for ABS-Like — with the highest heat resistance in the rigid group after post-cure. It is not the equal of molded PA66-GF30: strength is around half, the fibres are short and randomly oriented, and edges chip more easily. Use it to prove a bracket or frame in the right stiffness class, then machine the production part from nylon or mold it if the load case is real.