What Is Urethane Casting?
Urethane casting (also called polyurethane casting or vacuum casting) produces plastic parts using silicone molds and liquid polyurethane resin. A master pattern — usually 3D printed or CNC machined — is used to create a silicone rubber mold. Two-part urethane resin is then mixed and poured (or vacuum-drawn) into the mold, where it cures into a finished part.
It’s the go-to process for bridge production: the gap between prototyping and injection molding. Need 10 to 500 parts that look and perform like production plastic, but can’t justify a $30,000 mold? Urethane casting.
How the Process Works
- Master pattern. A high-quality prototype is made by 3D printing (usually SLA or SLS) or CNC machining. Surface finish on the master transfers directly to every cast part, so it’s worth finishing it well.
- Mold making. The master is suspended in a box and liquid silicone rubber is poured around it. After curing (12–24 hours), the silicone is cut open and the master is removed, leaving a flexible negative cavity.
- Casting. Two-part polyurethane resin is mixed (by weight or volume, depending on the system), degassed under vacuum, and poured into the silicone mold. Some setups use vacuum to pull resin into the cavity and eliminate air bubbles.
- Curing. The resin cures at room temperature or in an oven (typically 60–80°C for faster cycles). Demold time ranges from 30 minutes to several hours depending on the resin system.
Common Materials
Urethane resins can be formulated to simulate almost any production plastic:
- ABS-like — Rigid, impact-resistant, paintable. The most common choice for functional prototypes and enclosures.
- PP-like — Flexible, living-hinge capable, chemical-resistant.
- PC-like (clear) — Optically clear, can be polished for lens-quality transparency.
- Glass-filled — Stiff, dimensionally stable, higher temperature resistance.
- Elastomeric — Shore A 20 to Shore D 80. Rubber-like parts for gaskets, seals, grips, overmolds.
- Flame-retardant — UL94 V-0 rated formulations available for electrical enclosures.
What It Costs
| Cost Element | Typical Range |
|---|---|
| Master pattern (SLA print) | $50 – $500 |
| Silicone mold | $200 – $2,000 |
| Per-part cost (small-medium) | $15 – $150 |
| Mold life | 15 – 25 pulls (typical) |
Total project cost for 50 parts might run $2,000–$5,000. That same geometry in injection molding would cost $15,000–$40,000 just for the mold before a single part is made.
Design Considerations
- Surface finish = master finish. Every detail on the master — layer lines, machining marks, texture — transfers to the mold and every cast part. Invest time in finishing the master.
- Undercuts are (mostly) free. Silicone molds are flexible, so moderate undercuts that would require side actions in injection molding simply flex out during demolding.
- Wall thickness. Less critical than injection molding since there’s no packing pressure, but very thin walls (<1 mm) may not fill reliably. Air bubbles are the enemy — vacuum degassing helps.
- Inserts. Metal inserts, threaded bushings, and overmolded components can be placed in the mold before casting.
- Color and finish. Pigments can be mixed into the resin. Parts can be painted, plated, or pad-printed just like injection molded parts.
- Mold life. Silicone molds degrade with each pull. Aggressive geometry and high-temperature cures reduce mold life. Plan for mold replacement on longer runs.
When to Use Urethane Casting
- 10 to 500 parts (sweet spot is 20–200)
- Bridge production while injection mold tooling is being built
- Functional prototypes that need production-like material properties
- Parts with complex geometry or undercuts that would make injection tooling expensive
- Quick turnaround (first parts in 1–2 weeks vs. 8–12 weeks for injection tooling)
- Market testing before committing to hard tooling
When to Consider Alternatives
- Over 500 parts: Injection molding cost per part drops below casting
- One-off or 1–5 parts: 3D printing is simpler and cheaper
- Hollow parts: Blow molding or rotational molding
- Thermoset requirements: Compression molding
Bottom Line
Urethane casting fills the gap that no other process covers well: real parts, real materials, real fast, without real tooling cost. It won’t replace injection molding at volume, and it’s overkill for one-offs. But for bridge production, market testing, and short runs of functional parts, it’s the most flexible and forgiving process available. If you have a 3D printer and a vacuum chamber, you can do it in your shop.