Polyurethane Casting Resins
Polyurethane casting resins are how you get 10 to 300 injection-molding-quality parts without cutting a steel tool. A master (usually 3D printed or CNC machined) is cast in silicone; two-part urethane is mixed, degassed and poured into the silicone mold under vacuum; twenty minutes to a few hours later the part comes out with the surface, colour and feel of a production molding. Grades are formulated to mimic the thermoplastics they will eventually be molded in — ABS-like, PC-like, PP-like, glass-filled nylon-like — plus optically clear and rubber-like Shore A resins for lenses and overmolds. They are prototype materials: excellent for form, fit and function checks, marketing samples and pilot runs, less good for years of service in the sun or under sustained load.
Casting resins by type
Rigid engineering-like resinsABS-like, PC-like, PP-like, GF nylon-like, flame-retardant1724 – 75Housings, brackets, bezels, covers, structural prototypes — ABS-like is the general-purpose grade; PC-like for impact and a little more heat; PP-like for living hinges and flex; glass-filled nylon-like for the stiffest, most heat-resistant castings; FR grades for UL 94 V-0 samples.
Rigid clear resinsWater-clear, UV-stabilized urethane1—Lenses, light pipes, display windows, fluidic manifolds and see-through housings — polished silicone molds give optically clear parts that can be tinted; polish or coat for the best transmission.
Rubber-like resins20–90 Shore A elastomeric urethane, clear or pigmented47 – 8Grips, buttons, keypads, seals, gaskets, bumpers, wearable straps and two-shot overmolds onto rigid castings — durometer chosen per part; tear strength and recovery are close to molded TPU.
Choose a urethane casting resin for
Pilot runs and bridge production of housings, covers, bezels and enclosures before the steel mold is ready; marketing and trade-show samples that must look and feel molded; functional prototypes that need a real snap-fit, living hinge or overmolded grip; clear lenses, light guides and fluid-handling parts (rigid clear); soft-touch buttons, gaskets, bumpers and wearables (rubber-like, 20–90 Shore A); and any part where 10–300 pieces in production-like colour, texture and finish are worth more than the isotropy of a machined part.
Look elsewhere when
Volumes exceed a few hundred (a silicone mold lasts 20–50 shots; injection molding becomes cheaper), the part must carry sustained load or run above 80–100 °C (cast urethanes creep and soften; machine from engineering plastic or mold it), tolerances tighter than ±0.15 mm or ±0.3 % are required (silicone molds flex and the resin shrinks), the part will live outdoors for years (unpigmented urethanes yellow and chalk in UV), or one-off geometry is needed fast — a single printed part is quicker and cheaper than making a mold for one cast.
Loading the library…
Polyurethane by application
- Housings and covers that look molded
- ABS-like is the default; PC-like when the part will be dropped or sees a little heat; pigment in the mix, texture from the master, paint or soft-touch coating after.
- Stiff, warm or flame-rated
- Glass-filled nylon-like for brackets and frames that would otherwise be machined nylon; FR grades for samples that must pass a flammability review.
- Living hinges and flexible snap-fits
- Bends and snaps repeatedly like molded polypropylene — lids, clips, cable management, one-piece boxes.
- Clear windows and lenses
- Polished masters, polished molds, vacuum degassing and a UV-stabilized clear grade — then a clear coat if the surface must stay optical.
- Soft-touch, seals and overmolds
- Specify the durometer; cast as a separate part or as a second shot over a rigid casting for grips, gaskets and bumpers.
AI suggestions — verify with an engineer before ordering. Estimates are indicative; the quote page prices your exact geometry.
Manufacturing notes
- Master and silicone mold
- The master is SLA-printed or CNC-machined and hand-finished to the surface you want on every part — every scratch or texture on the master appears on all castings. Silicone is poured around it under vacuum and cured; one mold yields 20–50 parts depending on the resin and geometry. Complex parts use multi-piece molds and cores.
- Casting
- Two-part urethane is metered, pigmented, mixed and degassed, then poured under vacuum so the mold fills bubble-free; the mold is cured at 60–70 °C and demolded. Inserts, threaded bosses and overmolded second shots are cast in place. Cycle time is 1–4 hours per shot per mold; a set of molds runs in parallel for pilot quantities.
- Tolerance and design
- Expect ±0.15 mm on small features and ±0.3 % of length overall; wall thickness 1.5–3 mm is ideal, down to 0.75 mm locally. Design for undercuts freely — silicone flexes — but keep parting lines where a witness line is acceptable. Threads are best as inserts; shrinkage is 0.15–0.3 %.
- Finishing
- Parts come out with the master's finish and colour-matched in the resin (Pantone / RAL). Post-processing options: painting and soft-touch coating, texture, silk-screen and pad printing, vapour polishing of clear parts, EMI shielding paint, insert installation, assembly. No plating or anodizing.
Most-quoted polyurethane grades
| Grade | Type | Yield, MPa | Tensile, MPa | Density, g/cm³ | Machinability | Weldability | Best for |
|---|---|---|---|---|---|---|---|
| ABS-Like | Rigid engineering-like resins | 45 | 52 | 1.12 | Good | Not recommended | General-purpose cast housings |
| PC-Like | Rigid engineering-like resins | 52 | 60 | 1.16 | Good | Not recommended | Impact and a little more heat |
| PP-Like | Rigid engineering-like resins | 30 | 36 | 1.15 | Good | Not recommended | Living hinges, flexible clips |
| GF Nylon-Like | Rigid engineering-like resins | 75 | 60 | 1.4 | Good | — | Stiffest, most heat-resistant castings |
| Rigid Clear | Rigid clear resins | 38 | 40 | 1.06 | Good | Not recommended | Lenses, light guides, windows |
| Rubber-Like | Rubber-like resins | 8 | 16 | 1.15 | Poor | Poor | Grips, seals, soft overmolds |
| General Purpose | Rigid engineering-like resins | 45 | 40 | 1.2 | Fair | Poor | Everyday rigid cast PU |
| ABS-Like FR | Rigid engineering-like resins | 45 | 50 | 1.17 | Very good | Not recommended | Flame-retardant rigid PU (UL94 V-0 capable) |
| Clear Rubber-Like | Rubber-like resins | 7 | 5 | 1.13 | Poor | Not recommended | Transparent flexible PU |
Finishes for polyurethane
All 111 finishes3 of our 111 surface finishes take polyurethane: 3 recommended outright. Each finish opens to its datasheet — thickness, hardness, salt-spray hours, spec codes and the polyurethane-specific notes — and the configurator prices it on the grade you pick.
| Finish | Group | On polyurethane | Thickness, µm | Hardness, HV | Salt spray, h | Cost |
|---|---|---|---|---|---|---|
| Degating | Mechanical finishing & post-processing | Yes | 0 | — | — | $ |
| Texture Finishing | Mechanical finishing & post-processing | Yes | 0 | — | — | $$$ |
| Trimming | Mechanical finishing & post-processing | Yes | 0 | — | — | $ |
Polyurethane — frequently asked
When does vacuum casting beat 3D printing?
From about 10 pieces up, when the parts must look and feel production-molded: smooth textured surfaces with no layer lines, colour in the material, true clear parts, real rubber-like overmolds, and mechanical properties (impact, snap-fit fatigue) closer to molded ABS or PC than any printed resin. For one to five parts, or when the geometry is still changing daily, printing is faster and cheaper.
How close is an ABS-like casting to real ABS?
Close enough for form, fit and most function checks: similar stiffness, impact and surface finish, slightly lower heat deflection (typically 70–90 °C versus 95–100 °C for molded ABS), and a shorter life in UV without paint. Snap-fits, screw bosses and living hinges work. It is not a substitute for a molded part in long-term or high-temperature service, and it is not the same resin the production tool will run.
How many parts can one silicone mold make?
Typically 20–50 shots for rigid resins and 15–25 for aggressive clear or rubber-like grades, depending on geometry (thin cores and sharp features wear the silicone). Above that the mold tears or loses detail and a new one is cast from the same master. For 100–300 parts we run several molds in parallel; beyond that a soft aluminum injection tool is usually the cheaper route.
What tolerances can cast urethane hold?
±0.15 mm on features under 50 mm and about ±0.3 % of the nominal on larger dimensions — set by resin shrinkage and the flexibility of the silicone mold rather than by the master, which can be machined far more precisely. Critical fits are best designed with a machining or post-fit step, or achieved with inserts cast in place.
Can cast urethane parts be painted or coated?
Yes — urethane takes paint, soft-touch coatings, EMI shielding and pad printing readily after a light abrade and wipe; clear parts can be polished, tinted or hard-coated. Colour is usually mixed into the resin instead so scratches do not show a different colour underneath. Plating, anodizing and welding are not options.