Silicone Mold Casting

Silicone mold casting uses soft silicone tooling to cast polyurethane parts with production-like surfaces, ideal for complex, low-volume plastic components.

Overview

Silicone mold casting uses a master pattern and poured silicone rubber to create a flexible mold, then casts polyurethane or similar resins into that mold. The process captures fine detail from the master, delivers smooth, injection-molding-like surfaces, and supports complex geometries with undercuts using flexible tooling or inserts.

This method shines for low-volume runs where you need production-like plastic parts without paying for hard tooling. Typical batch sizes range from a handful of parts up to a few dozen per mold, depending on geometry and resin. Expect moderate tolerances and some variation from shrink and mold wear, so it’s best for enclosures, covers, cosmetic parts, and functional prototypes rather than tight-tolerance mechanisms. Tradeoffs: limited material selection vs. true injection-grade resins, reduced dimensional stability over time, and higher per-part cost as quantities grow. It’s a strong choice for bridge production and design validation before committing to injection molds.

Common Materials

  • Rigid polyurethane (ABS-like)
  • Flexible polyurethane (rubber-like)
  • Clear polyurethane (PC-like)
  • High-temperature polyurethane
  • Color-matched polyurethane

Tolerances

±0.005" to ±0.015"

Applications

  • Electronics housings and bezels
  • Handheld device enclosures and grips
  • Automotive interior trim prototypes
  • Overmolded handles and soft-touch features
  • Medical device covers and shells
  • Low-volume cosmetic panels and covers

When to Choose Silicone Mold Casting

Use silicone mold casting for low-volume plastic parts where you need production-like appearance and feel from soft tooling. It fits complex geometries, good surface finish, and moderate tolerances for prototypes, pilot runs, and bridge production. Best in quantities from a few parts up to a few hundred before hard tooling becomes more economical.

vs Vacuum Casting

Choose silicone mold casting when you can accept gravity or simple pressure casting and want maximum flexibility on mold design and quick iteration on tooling. Vacuum casting is preferable when you need better void control, more consistent properties, and finer detail in thin or intricate sections, especially for clear parts.

vs Injection Molding

Choose silicone mold casting when you need production-like plastic parts but can’t justify injection mold tooling cost or lead time. It’s ideal for design validation, market samples, and low-volume runs before freezing the design for steel tooling.

vs CNC machining

Choose silicone mold casting when geometry is complex, has undercuts, or demands molded-style textures that are difficult or expensive to machine. It often wins on cost and lead time for multiple copies of an organic or highly contoured part, especially in plastic-like materials.

vs 3D printing

Choose silicone mold casting when you want smoother surfaces, better cosmetics, and material behavior closer to production plastics than typical printed parts. It works well when you need several identical parts; print one high-quality master, then replicate it economically via silicone tooling.

Design Considerations

  • Keep wall thickness as uniform as possible; large transitions or thick sections increase sink, warpage, and cure time
  • Add draft (1–3° where possible) and generous fillets to improve mold filling, part release, and silicone mold life
  • Avoid ultra-thin walls below ~1.0–1.5 mm; thin sections trap bubbles and may not fill reliably
  • Account for resin shrinkage (typically 0.15–0.3%) in critical dimensions and communicate any key datums clearly on the drawing
  • Minimize deep undercuts or specify where you can accept split molds or inserts, as complex parting schemes drive cost and lead time
  • Specify color, transparency, and surface finish (e.g., gloss, matte, texture) up front so the shop can choose suitable resins and finishing steps