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.

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:

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.