Process selection
Vacuum Casting vs Injection Molding
Vacuum casting pours polyurethane resin into a silicone mold cast from a 3D printed master, degassed under vacuum. Injection molding fills a machined metal tool with real thermoplastic at high pressure. The core trade: vacuum casting delivers 10 to 25 parts per mold in days for hundreds of dollars; injection molding delivers thousands of parts for dollars each after a five-figure tool.
| Vacuum Casting | Injection Molding | |
|---|---|---|
| Tooling cost | $500 to $2,500 silicone mold | $8,000 to $25,000 aluminum, $25,000 to $150,000 steel |
| Mold life | 15 to 25 shots per mold | 100,000 aluminum to 1M+ steel |
| Lead time to first part | 5 to 10 days | 4 to 12 weeks |
| Cycle time | 30 to 60 min pour and demold, oven cure | 15 to 60 s |
| Per-part cost | $30 to $150 | $0.50 to $5 at volume |
| Materials | PU resins simulating ABS, PP, PC, rubber | Actual production thermoplastics |
| Tolerance | ±0.15 mm plus 0.15 to 0.25% shrink | ±0.05 to 0.1 mm |
Choose Vacuum Casting when
- 10 to 100 parts for pilot builds, marketing units, and field trials
- Parts in hand within 5 to 10 days of final CAD
- Design still moving: a $1,500 mold is disposable, a steel tool is not
Choose Injection Molding when
- Volumes past a few hundred parts, where $1 to $5 each beats $50
- Real production resin behavior: creep, UV, chemical, flame ratings
- Tolerances under ±0.1 mm and certified, repeatable process control
The verdict
Vacuum casting is a bridge, not a destination. Use it for 10 to 100 units while the design settles or the injection tool is being cut. Once demand clears a few hundred parts, or the application needs true production resin properties, the numbers and the material data sheet both point to injection molding.
Cost comparison
A silicone mold costs $500 to $2,500 including the SLA master and dies after 15 to 25 shots, so budget one mold per 20 parts. Each PU casting runs $30 to $150 in resin and labor with a 30 to 60 minute demold cycle. A soft aluminum injection tool starts near $8,000 to $25,000 with parts at $1 to $5. The crossover sits between roughly 50 and 300 parts: below it vacuum casting wins on tooling; above it injection's per-part cost dominates everything.
Common questions
How close are cast polyurethane parts to production plastic?
Close in look and feel, not in data-sheet performance. ABS-like and PP-like PU grades match stiffness and surface finish well, but long-term creep, UV stability, chemical resistance, and flame ratings differ from the real thermoplastic. Never run certification or life testing on cast PU and assume the molded part behaves the same.
How many parts can one silicone mold produce?
Plan on 15 to 25 shots before edge detail degrades and parting lines get ragged; aggressive geometry or dark pigmented resins wear molds faster. For a 60-part pilot run, quote three molds from the same master. Past roughly 100 to 300 parts, a soft aluminum injection tool usually costs less per part.