Vacuum Casting

Vacuum casting produces high-detail polyurethane parts by pouring degassed resin into silicone molds under vacuum, suited for prototype-to-bridge volumes and cosmetic finishes.

Overview

Vacuum casting (urethane casting) makes parts by pouring mixed polyurethane resin into a silicone mold inside a vacuum chamber. Vacuum pulls out trapped air, so thin walls, fine textures, and cosmetic surfaces replicate well from a master pattern. Molds are typically silicone and built from a 3D-printed or CNC-machined master.

Choose vacuum casting for realistic prototypes and bridge production when you need 10–50+ parts that look and feel like molded plastic: good surface finish, optional color matching, and a wide range of urethane “material feels” (rigid, elastomeric, clear). Lead times are often days once a master exists.

Tradeoffs: tolerances are moderate and can drift with silicone mold wear and resin shrink; sharp edges and long, thin sections can warp. Heat resistance and long-term mechanical properties depend on the urethane system and usually trail true thermoplastics. Expect limited mold life and higher per-part cost than injection molding at scale.

Common Materials

  • Rigid polyurethane
  • Elastomeric polyurethane (Shore A)
  • Clear polyurethane
  • Flame-retardant polyurethane
  • Glass-filled polyurethane

Tolerances

±0.010 in (±0.25 mm) typical; tighter on small features

Applications

  • Consumer product appearance models
  • Overmold-like soft-touch grips and seals
  • Clear light pipes and lenses (prototype)
  • Low-volume housings and covers
  • Medical device enclosures (prototype/bridge)
  • Automotive interior trim prototypes

When to Choose Vacuum Casting

Vacuum casting fits when you need low-to-medium quantities of plastic-like parts with strong cosmetic requirements and fast turnaround. It works best for parts that can be demolded from silicone without complex actions and where ±0.010 in-level tolerances are acceptable. Typical volumes are 1–50 parts per mold set, with additional molds for higher quantities.

vs Silicone Mold Casting

Choose vacuum casting when air entrapment would create bubbles, voids, or surface pitting—especially in thin walls, clear parts, or textured surfaces. Vacuum improves fill consistency and cosmetic quality, reducing rework and scrap on appearance-critical parts.

vs Injection molding

Choose vacuum casting when volumes are too low to justify steel/aluminum tooling or when you need parts in days, not weeks. It’s a good bridge while designs are still changing, before committing to production tools.

vs CNC machining (plastics)

Choose vacuum casting when you need multiple copies with consistent appearance, molded-like surfaces, or elastomeric/clear materials that are slow or expensive to machine. It also helps when geometry includes deep textures, undercuts suitable for silicone, or thin walls that chatter in machining.

vs SLA 3D printing

Choose vacuum casting when you need 10+ parts with better isotropy and more “real plastic” feel than typical photopolymers, or when you want color matching and production-like surface finish. It’s also useful when SLA support marks and post-processing time become the bottleneck.

Design Considerations

  • Add draft (1–3°) on vertical faces to reduce tearing and improve mold life
  • Avoid razor-sharp edges; use radii/chamfers to improve fill and reduce edge chipping in silicone
  • Keep wall thickness uniform where possible to reduce shrink, sink-like distortion, and warpage
  • Call out cosmetic surfaces and texture requirements; specify gloss level and acceptable parting lines
  • Design for simple demolding; minimize severe undercuts unless you can accept split molds or flexible de-mold features
  • Provide clear tolerance priorities; only tighten critical features and plan secondary machining for holes/locating datums