Vacuum Casting vs Injection Molding: Which One for Your Quantity?

For 10–100 parts, vacuum (urethane) casting is typically 60–80% cheaper than injection molding and delivers finished parts in 5–10 days — injection molding only becomes the economical choice above roughly 500 units, once aluminum tooling (7–10 days) or production steel tooling (4–6 weeks) is amortized across the run.

CMM probe checking a machined component against its drawing — Comparing processes only means something once both parts are measured the same way.

Cost ladder: casting vs molding by quantity

The two processes price completely differently. Casting carries a small silicone-tool cost that you pay once and a per-part resin price that barely falls; molding carries a real tooling investment that only pays back as the run grows. That is why the cheapest process flips as quantity increases:

Quantity	Vacuum casting	Injection molding	Usually cheaper
10 parts	No meaningful tool cost; parts in 5–10 days	Tool cost dominates; weeks of payback per part	Casting, by a wide margin
50 parts	Per-part cost falls; still no heavy tooling	Aluminum tool (7–10 days) begins to make sense	Casting
100 parts	Still the sweet spot for cast urethane	Tooling amortization still heavy	Casting, usually
500+ parts	Multiple silicone molds needed (20–30 casts each)	Tool cost now spread thin; per-part price drops sharply	Molding

Switching checklist: when to move to injection molding

Casting is not a lesser molding — it is the right tool for a different job. Move to molding when any of these is true:

Annual demand passes roughly 500 parts, or lifetime demand exceeds ~5,000 pieces of the same design.
The part needs certified production material data — cast PU resins mimic ABS, PC and PA but are not the certified engineering resin.
A stable aluminum bridge tool (7–10 days) or production steel tool (4–6 weeks) fits the program timeline.
Wall thickness, rib depth or undercuts need the consistency of a rigid tool across thousands of shots.

Stay with casting when the design is still moving, the quantity is 10–100, or you need painted, printed or soft-touch finishes on parts that look and feel molded — at a fraction of the tooling exposure.

Material fidelity: how close is cast urethane to the real resin?

PU resins are formulated to mimic ABS, PC and PA grades in feel, Shore hardness and flexural behaviour; glass-filled and transparent variants are available, and overmolded soft-touch grips are routine.
Dimensional accuracy is about ±0.2 mm on cast parts — plenty for housings, covers and fit-checks, not enough for precision mating features (those should be machined or molded).
One silicone mold yields roughly 20–30 casts before detail degrades, which is exactly why casting suits short runs and molding suits volume.
Injection molding runs the final engineering resin with drawing-level tolerances (typically ±0.1 mm general), so it is the only route when material certification or tight tolerances are contractual.

A practical middle path many teams miss: cast 50–100 bridge parts now while the aluminum tool is cut in parallel — you get sales samples and test units in 5–10 days without waiting for the mold, then switch to molded supply mid-programme. Send your STEP file and the 24-hour DFM review will price both routes side by side.

Frequently Asked Questions

Is vacuum casting cheaper than injection molding?

Below roughly 100 units, yes — silicone tooling costs a fraction of an aluminum or steel mold, so casting wins on total cost. Above about 500 units, the mold is amortized and molding is usually cheaper per part.

How many parts can one silicone mold produce?

Typically 20–30 casts per mold before surface detail degrades, which is exactly why casting suits short runs and injection molding suits volume.

When should I switch from vacuum casting to injection molding?

When annual volume passes ~500 parts, lifetime demand exceeds ~5,000 parts, or you need certified production materials. An aluminum bridge tool covers the gap in 7–10 days.

How close are vacuum-cast parts to molded parts?

Visually and mechanically very close — PU resins mimic ABS, PC and PA grades and dimensions hold about ±0.2 mm. But they are prototype or bridge parts, not certified production material.