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Should I use urethane casting or injection molding?
Urethane casting is cheaper for about 10–200 parts: a master and silicone mold cost about $500–$2,500, and first parts ship in 1–2 weeks. Injection molding wins above a few hundred parts, because each silicone mold makes only 20–25 parts while an aluminum injection mold makes 10,000–100,000. Molding also gives the real production resin and ±0.005 in (±0.13 mm) tolerances.
Which is cheaper at your quantity?
The same housing, each process in a suitable material (PC/ABS and Cast urethane).
| Quantity | Injection molding | Urethane casting |
|---|---|---|
| 10 | $1,047 | $136 |
| 50 | $209 | $81.50 |
| 100 | $105 | $71.40 |
| 500 | $21.50 | $68.60 |
| 1,000 | $11.10 | $68.20 |
| 5,000 | $4.24 | $68.00 |
| 10,000 | $2.50 | $67.90 |
| 50,000 | $1.19 | $67.90 |
| 100,000 | $1.02 | $67.90 |
Where each pays off
Plastic injection molding
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- Tooling
- 3–14 weeks
- First article
- 1–2 weeks after tool completion
- Production
- 2–4 weeks per release
±0.005 in (±0.13 mm); ±0.002 in (±0.05 mm) on small critical features
Urethane casting
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- Tooling
- 3–7 days for master and silicone mold
- First article
- 1–2 weeks
- Production
- 2–3 weeks for up to about 200 parts
About ±0.010 in (±0.25 mm) on features under a few inches; about 0.15% shrink
Which should you pick?
Pick Injection molding when
- Volume is above about 200–500 parts
- Certification or testing must use the production resin
- Tolerances must be tighter than ±0.010 in (±0.25 mm)
- Heat exceeds urethane's typical 140–200 °F (60–95 °C) heat deflection
Pick Urethane casting when
- You need 10–200 parts for pilots, beta units or trade shows
- Parts need a molded look and color without tooling spend
- The design will change before launch
- Soft-touch or overmolded prototypes run from 30 Shore A to 85 Shore D
Key differences
| Injection molding | Urethane casting | |
|---|---|---|
| Tool | Machined aluminum or steel, $3,000–$40,000+ | Silicone from a master, $500–$2,500 |
| Tool life | Aluminum 10,000–100,000 shots; hardened steel 1,000,000+ | About 20–25 parts per mold |
| Material | Production thermoplastics | Two-part thermoset polyurethane simulating ABS, PC, PP or rubber |
| Tolerance | ±0.005 in (±0.13 mm) | ±0.010 in (±0.25 mm), about 0.15% shrink |
| Draft and undercuts | 1–2° draft; each slide adds $1,000–$5,000 | No draft required; moderate undercuts are fine |
| Lead time | 3–14 weeks for tooling | 3–7 days for the mold; first parts in 1–2 weeks |
Questions
Is urethane casting the same as injection molding?
No. Urethane casting pours two-part thermoset resin into a silicone mold at low pressure. Injection molding forces molten thermoplastic into a machined metal mold at high pressure. Cast parts look molded but are a different material.
How close are cast urethane properties to molded plastic?
Close for many rigid grades. ABS-like urethanes match ABS on tensile strength and hardness but differ in impact, heat and chemical resistance. Use cast parts for fit, form and most function checks, and run certification tests on molded parts.
What about an aluminum prototype injection mold?
It fills the gap above about 200 parts. A simple aluminum tool typically costs $3,000–$15,000, takes 3–6 weeks, and molds the production resin, which makes it the better bridge when testing requires the real material.
Try both on your part.
The builder prices the same geometry in every suitable process and material, with DFM for each.