What Is a Vacuum Casting?
Vacuum casting is a low-volume process that makes plastic parts by pouring liquid resin, usually polyurethane, into a silicone mold under vacuum so it fills fine detail without air bubbles. Also called urethane or polyurethane casting, it produces small batches of near-production-quality parts without the cost of hard tooling.
In short: one master pattern, one silicone mold, and a few dozen cast parts that look and feel close to injection-molded ones.
How Does Vacuum Casting Work?
- Master pattern. A precise model of the part is made, usually by SLA 3D printing or CNC machining. Its surface finish is copied into every casting.
- Silicone mold. The master, with its gates and risers attached, is suspended in a mold box and liquid silicone is poured around it. Once cured, the mold is cut open along a parting line and the master is removed, leaving a cavity.
- Resin mixing. The casting resin and any color pigment are mixed and degassed under vacuum.
- Casting. The closed mold sits in a vacuum chamber while resin is poured in. The vacuum draws out air, so the resin reaches thin walls and fine texture.
- Curing and demolding. The mold goes into an oven until the resin hardens, then the part is removed and the gates are trimmed off.
At NICE Rapid the silicone cures for 8–16 hours, parts cure at 60°C (140°F), and each mold yields 20–25 parts before it needs replacing.
Is Urethane Casting the Same as Vacuum Casting?
Yes, in practice. "Vacuum casting" describes the method; "urethane casting" and "polyurethane casting" describe the material. Polyurethane (PU) resins are used because their grades can mimic many production plastics, though epoxy and polyester resins can also be vacuum cast.
NICE Rapid's 30+ PU resins include grades that imitate ABS, polypropylene, polycarbonate, nylon and rubber (Shore A 40–80), plus clear, high-temperature (HDT above 120°C / 248°F) and glass-filled grades. They behave like those plastics but are not the same material, so final validation of material-dependent properties is best done on molded parts.
Vacuum Casting vs 3D Printing vs Injection Molding
The figures below are NICE Rapid's published capabilities.
| SLA 3D printing | Vacuum casting | Injection molding | |
|---|---|---|---|
| Tooling | None | Silicone mold from a master | Aluminum or steel mold |
| Typical quantity | 1–10 parts | 20–25 parts per mold | 50–1,000 (aluminum); 10,000+ per year (steel) |
| Tolerance | ±0.1 mm (±0.004 in) | ±0.3 mm (±0.012 in) | Set by the mold; cavities cut to ±0.05 mm |
| Materials | Photopolymer resins | PU resins simulating ABS, PP, PC, rubber | The production thermoplastic |
| Lead time | 1–3 days | 7–14 business days | 1–2 weeks (aluminum tool); 4–6 weeks (production steel tool) |
When Should You Use Vacuum Casting?
Vacuum casting fits the gap between the first printed prototype and the first mold:
- Appearance models and marketing samples that need a painted, textured or clear finish.
- Functional and fit testing of snap fits, overmolded soft grips and assemblies, in quantities too large to print economically.
- Bridge and pilot builds while production tooling is being made.
Design the part as if it were going to be injection molded, with consistent walls and sensible draft. The cast parts then represent the production part, and the same CAD file can move to tooling. Silicone molds flex, so casting tolerates small undercuts that a steel mold would need a side action for, but the production design must still solve them.
Vacuum casting jobs start from one piece, with no minimum order.
Frequently Asked Questions
Is urethane casting the same as vacuum casting?
Yes. Urethane casting and polyurethane casting are other names for vacuum casting. The names refer to the polyurethane resin, while vacuum casting refers to the vacuum used to fill the silicone mold without bubbles.
What materials are used in polyurethane casting?
Polyurethane resins are the usual choice. Grades are formulated to imitate ABS, polypropylene, polycarbonate, nylon and rubber, with clear, heat-resistant and glass-filled versions. Epoxy and polyester resins can also be cast.
How many parts can a vacuum casting mold make?
A silicone mold typically yields about 20–25 parts before it needs replacing, so larger quantities need additional molds. That limit is why vacuum casting suits small series rather than production volumes.
Related Services at NICE Rapid
- Vacuum Casting — clear, overmolded and textured castings in 30+ PU resins, typically ±0.3 mm, delivered in 7–14 business days.
- 3D Printing — in-house SLA for high-accuracy master patterns and one-off prototypes.
- Request a Quote — send a STEP file, quantity, material and finish; the team responds within 24 hours.
Sources
- Oleksy M. et al., "Rapid Prototyping Technologies: 3D Printing Applied in Medicine," Pharmaceutics (2023), section 2.5 Vacuum Casting — silicone mold poured around a master pattern and split in two; resin poured under vacuum to remove gas bubbles and reproduce the model; oven curing; gates removed after casting; polyester, polyurethane or epoxy resins; parts should be designed to injection-molding principles; used to duplicate parts in small series — pmc.ncbi.nlm.nih.gov
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