Silicone Mold Casting

Silicone mold casting makes short-run polyurethane parts by pouring resin into flexible silicone molds, capturing fine detail with low tooling cost but moderate tolerances.

Overview

Silicone mold casting is a urethane casting method where a master pattern is used to create a flexible silicone tool, then liquid polyurethane resins are mixed and poured (or injected) into the mold to form parts. The silicone tool reproduces fine features, textures, and thin walls better than many hard tools, and tooling lead time is typically days.

Use it for prototype-to-bridge volumes when you need production-like plastics (rigid, rubber-like, clear) without committing to injection tooling. Expect limited mold life (often ~10–30 shots depending on geometry and resin), part-to-part variation from mold wear and resin shrink, and more post-processing for gates, vents, and parting lines. Tight fits usually require secondary machining or design clearance, and large flat parts can warp.

Common Materials

  • Polyurethane (rigid)
  • Polyurethane (elastomer)
  • ABS-like urethane
  • PP-like urethane
  • PC-like urethane
  • Clear urethane

Tolerances

±0.010" (±0.005" achievable on small, well-controlled features)

Applications

  • Consumer product housings (prototype/bridge)
  • Overmold-like grips and seals (urethane elastomer)
  • Transparent light pipes and lenses (clear urethane)
  • Automotive interior trim prototypes
  • Connector shells and protective caps
  • Medical device enclosures (prototype builds)

When to Choose Silicone Mold Casting

Choose silicone mold casting for 5–50+ parts when you need plastic parts that look and behave close to production materials with fast, low-cost tooling. It works best for moderate-size components with complex geometry, cosmetic surfaces, and multiple material durometers. Plan for secondary ops if you need precision fits, threaded features, or very flat large panels.

vs Vacuum Casting

Pick silicone mold casting when you don’t need vacuum degassing for cosmetic perfection or bubble-sensitive clear parts. It’s often lower setup complexity and cost for simple geometries and opaque resins. If your part has deep pockets, thin clear walls, or strict cosmetic requirements, vacuum casting usually delivers more consistent surfaces.

vs Injection Molding

Pick silicone mold casting when volumes are too low to justify steel/aluminum tooling or when the design is still changing. It gives fast tooling turns and material “look-alike” options with acceptable variation. If you need hundreds to thousands of parts with tight repeatability and engineered thermoplastics, injection molding is the better fit.

vs CNC Machining (Plastics)

Pick silicone mold casting when geometry is hard to machine, when you need multiple identical parts quickly, or when a molded-like surface/texture matters. Casting avoids long cycle times and complex fixturing for organic shapes and undercut-like features (within mold-parting constraints). If you need tight tolerances, crisp edges, and predictable flatness, machining wins.

vs SLA 3D Printing

Pick silicone mold casting when you need production-like material behavior (impact, flexibility, Shore hardness) and better heat resistance than typical photopolymers. It’s also a good path when you need several identical parts from one master pattern. If you only need one or two parts fast and can accept photopolymer properties, SLA is usually quicker.

Design Considerations

  • Add 1–3° draft on vertical faces and avoid reverse drafts unless you accept mold splitting and higher scrap risk
  • Define cosmetic surfaces and place parting lines, gates, and vents on non-cosmetic faces
  • Use consistent wall thickness where possible and avoid large flat thin panels that can warp as resin shrinks
  • Design clearance into mating features; avoid relying on cast dimensions for press fits without secondary machining
  • Specify threaded inserts for durable threads; avoid casting fine internal threads directly
  • Call out surface texture/gloss targets and color matching requirements up front to prevent rework and quote churn