Vacuum Casting with Silicone Molds: When 10–500 Copies Beat Injection Molding

Vacuum casting copies a master pattern into silicone molds to produce 10–500 urethane parts at a fraction of injection tooling cost — with first articles in 5–10 days and per-part prices that beat molded parts below roughly 100–200 units. It is the fastest economical route from a validated CAD model to a batch of production-look, production-feel plastic parts.

How the process works

We start from a master — CNC-machined or SLA-printed and hand-finished to the surface quality you want, because every casting inherits it. The master is suspended in liquid silicone and cured, then cut open along a parting line to create the mold. Two-part polyurethane resin is mixed under vacuum (eliminating bubbles), poured into the cavity and cured in an oven. Each silicone mold yields roughly 20–50 copies before detail degrades; a 200-piece order simply uses several molds, still without any steel.

Material properties: honest numbers

• Rigid resins: ABS-like (Shore 80D), PC-like translucent, glass-filled grades
• Flexible resins: TPU-like Shore 40–90A for gaskets, grips, overmolds
• Heat resistance: typically 60–80 °C continuous; specialty resins reach ~120 °C
• Color: pigment-tinted in the resin or painted; clear resins available for lenses

Cast urethane parts are excellent for marketing samples, user trials, exhibition units and low-volume sales — but they are not chemically identical to injection-molded ABS or PC. For certification, drop tests and fatigue validation, treat casting as a visual-and-assembly proxy, and validate the final material via bridge injection molding.

Cost logic vs injection molding

A simple aluminum injection mold might cost 10–20× a silicone mold. Spread over 100 parts, casting wins decisively; over 1,000 parts, aluminum tooling amortizes below cast unit cost. The practical crossover sits between 100 and 500 parts depending on part size and finish. SkillsProto quotes casting, aluminum tooling and steel bridge tooling side by side so you can see the real break-even for your geometry.

Design guidelines that matter

• Draft: 1–2° keeps parts releasing cleanly from soft silicone
• Wall thickness: 1.5–4 mm typical; sink shows above ~5 mm
• Inserts and threads: metal inserts cast in place; heat-set inserts after casting
• Undercuts: silicone flexes, so mild undercuts are free — a casting superpower
• Sharp internal corners: add 0.5 mm radii for resin flow

When to move on from casting

Switch to rapid aluminum tooling when volume passes roughly 200–500 units, when the resin must be a true engineering polymer (real PA12-GF, real PC), or when certification testing demands molded material. Send us your STEP file and target quantity — our 24-hour DFM quote will show casting vs molding cost curves for your exact part.