Green-lighting a 300-unit beta run before a Kickstarter fulfillment window can put an NPI manager in a costly bind. Cut a steel injection mold for 300 housings and you’ve spent $9,000 on a tool capable of 100,000 shots—most of that capacity will never be used. Run 300 urethane castings through rebuilt silicone molds instead, and the per-part price quietly climbs past $35, while injection at the same volume would have been $11. The real question isn’t which process is superior; it’s where the crossover point sits. At XC Machining, we quote both processes from the same CAD model dozens of times a month, and the line is more predictable than most startup teams expect.
Vacuum casting—also called urethane casting or silicone tooling—and injection molding solve the same problem at different volumes. They should not be treated as competitors but as sequential stages: vacuum casting bridges early production, while injection is the long-term destination. This guide walks through soft-tooling and hard-tooling price ranges, lead times, tolerance windows, and the geometry factors that can shift the break-even point by hundreds of units in either direction.
In vacuum casting, a silicone RTV mold is formed around a 3D-printed or CNC-machined master, then polyurethane resin is poured in under vacuum and cures in one to four hours. A single silicone mold typically yields 15–25 production-quality parts before surface detail begins to drift, and a complex mold can be rebuilt two to three times from the same master before the master itself needs a refresh. Vacuum casting quotes typically range from $18 to $90 per part depending on size, finish, and resin grade—and that price stays flat regardless of order quantity. That flatness is exactly what makes the process attractive at low volumes, but it becomes the problem once volume climbs into the 200–600 unit range.
Injection molding flips the economics by amortizing fixed tooling cost across the production run. A simple aluminum prototype mold—often called a rapid tool or soft tool—costs $3,000–$8,000 and lasts for 5,000–20,000 shots. A hardened P20 or H13 steel production mold costs $8,000–$45,000 or more and holds tolerances across 250,000+ shots. Injection also delivers consistent ±0.05 to ±0.1 mm tolerances, supports specified surface finishes from SPI-A1 mirror to textured MT-11020, and accepts production-grade resins with verified material certificates—critical for any FDA, automotive, or UL submission.
Using real quotes from Q1 and Q2 2026 on a typical 120 x 80 x 40 mm electronics enclosure in ABS or ABS-like resin, the crossover for a mid-complexity part lands between 200 and 300 units. For very simple geometries like a flat lid or snap cap, the crossover shifts down to 120–180 units because the injection mold is cheaper. For complex parts with overmolds, multi-shot color, or insert molding, vacuum casting can hold its cost advantage up to 400–500 units because the equivalent injection tool gets expensive quickly.
Engineers can move the crossover by another 100–300 units by designing with the right levers in mind. A 30-minute DFM review against factors like uniform wall thickness, draft angles, rib-to-wall ratios, and sink risk often reveals whether a bridge-production strategy is viable. The sharpest hardware startups don’t commit to one process from day one. They sequence: vacuum casting from beta through first commercial launch, then injection from the second production run onward, while keeping the master CAD and tooling strategy aligned so the transition never forces a redesign. Teams that follow this path typically spend $15,000–$28,000 less on tooling across the first 18 months than those that jump straight to injection at unit 50, and they reach market 4–7 weeks sooner because vacuum casting first parts ship in days, not weeks.
For typical hand-sized parts in single-color resin without complex undercuts, vacuum casting and aluminum-tool injection molding cross over between 200 and 300 units at 2026 pricing. Below that, vacuum casting avoids the $4,000–$8,000 tooling sunk cost; above that, injection’s per-part price drops fast enough that even a single-cavity aluminum mold pays back within 350–500 parts. Complex geometries can push the crossover up to 400–600 units.
Regulatory requirements can override cost math. Vacuum casting parts generally are not accepted for FDA, UL, or production automotive submissions because polyurethane resins are simulant materials, not certified production resins. Final qualification usually requires injection-molded parts in certified materials like medical-grade ABS, UL-listed PC, or automotive PP. Also keep in mind silicone mold life: a 200-unit run typically needs 8–12 mold builds, and that cost is already built into the per-part quote. For runs of 1–30 units, 3D printing is often cheaper because there is no mold cost at all—MJF nylon or SLA resin parts run $8–$45 each with no setup fee and 3–7 day lead times. Vacuum casting starts winning above 30–40 units.
At XC Machining, vacuum casting, injection molding, and 3D printing live under one roof, so we can quote all three from the same CAD model and show the crossover for your exact part at your exact volume. Prototypes ship in 3–7 business days; bridge production runs of 100–500 parts ship in 10–15 business days; injection molding from a frozen design ships in 30–45 days. Upload your CAD files for a same-day comparison at https://www.xcmachining.com/contact/.
