The decision in one paragraph
For plastic parts in the 50–500 piece range, vacuum casting usually wins on total cost once volume passes ~40 units, and almost always wins on finish quality. CNC wins when the design is still iterating (tooling commitment is premature), when tolerances exceed vacuum casting's ±0.15 mm envelope, or when lead time under 10 days is critical.
Cost math — the numbers that matter
Take a typical enclosure part: 150 × 80 × 25 mm, 2 mm wall, moderate complexity, requires a clean cosmetic finish. Here's the math we walk through on customer quote calls:
| Volume | CNC (total) | Vacuum cast (total) | Better choice |
|---|---|---|---|
| 10 pcs | $850 | $1,800 | CNC |
| 25 pcs | $1,900 | $2,100 | CNC (barely) |
| 50 pcs | $3,500 | $3,000 | Vacuum cast |
| 100 pcs | $6,500 | $4,200 | Vacuum cast |
| 250 pcs | $15,000 | $7,500 | Vacuum cast |
| 500 pcs | $28,000 | $12,000 | Vacuum cast |
Representative numbers for a 150×80×25 mm enclosure. Actual quotes vary by geometry and material.
The break-even point lands around 30–40 pieces for a typical enclosure part. Below that, CNC's zero tooling cost wins. Above that, vacuum casting's per-unit economics dominate. For very large parts (> 500 mm) or very simple parts (flat plates), the break-even shifts — larger parts push it higher because mold cost scales with volume; simpler parts push it lower because CNC time is minimal.
Lead time — where the calculus shifts
The cost math favors vacuum casting above 40 pieces, but lead time tells a different story. CNC can deliver first parts in 5–7 days. Vacuum casting requires 7–10 days for pattern creation and silicone tooling, then 2–5 days for the actual pour cycles, plus finishing. First finished vacuum-cast parts typically land 15–20 days after order placement.
If you need 100 parts in two weeks, CNC wins despite costing more. If you have a month, vacuum casting wins on cost and finish quality. This is why our standard recommendation is: lock your design first, then pick the process. Don't commit to vacuum casting tooling while design is still iterating — the tooling is specific to a CAD revision.
Finish quality — vacuum casting's quiet advantage
CNC-machined plastic parts require post-processing to remove toolmarks from cosmetic surfaces. Bead blasting, vapor polishing (for PC and ABS), or hand sanding are all options, each adding $3–15 per part in labor. For parts where cosmetic finish matters, this labor narrows CNC's apparent cost advantage.
Vacuum-cast parts come out of the silicone mold with near-injection-molding surface quality. Paint, texture, and cosmetic detail (Mold-Tech textures can be embossed into the master pattern) are reproduced faithfully in every poured part. For consumer hardware products, the finish quality difference is significant.
Material options
CNC machining handles virtually any engineering plastic — ABS, PC, POM, PEEK, PTFE, PMMA, PEI. Material comes from stock plates and rods, so you get the exact spec listed on the material datasheet.
Vacuum casting uses urethane resins that simulate engineering plastics. ABS-like, PC-like, PP-like, rubber-like (shore 40A–90A), clear-optical, high-temp (150 °C), and flame-retardant (UL94 V-0) resins cover most common needs. Mechanical properties are 80–95% of the injection-molded equivalent. For parts needing certified specific material (FDA food-contact, medical implant-grade, aerospace flame-retardant), CNC from certified stock is the safer choice. See our vacuum casting page for resin options.
When CNC is the right call despite the cost
Four scenarios where we recommend CNC even above the break-even volume:
- Design still iterating. Vacuum casting tooling is CAD-revision-specific. If you're on revision 6 and expecting a revision 7, don't pour silicone. Stay on CNC until design is locked.
- Tolerance tighter than ±0.15 mm. Vacuum casting shrinkage is predictable but not zero. For mating interfaces requiring ±0.05 mm, machine it.
- Lead time < 10 days. Silicone tooling adds 7–10 days minimum. If the deadline is tight, CNC.
- Certified material requirement. Medical implant-grade, FDA food-contact, aerospace FAR 25.853 — these need CNC from certified stock.
The recommended workflow
For most startup and low-volume hardware projects, we recommend a staged approach:
- Phase 1 (design iteration, 1–10 pcs per rev): CNC. Fast, cheap per unit at this volume, no tooling commitment.
- Phase 2 (pre-production, 50–300 pcs): Vacuum casting. Lower per-unit cost, production-quality finish, preserves capital for mold investment later.
- Phase 3 (production, 1,000+ pcs): Injection molding. Pays off tooling around unit 1,500–2,000, then stays cheap forever.
Most of our hardware startup customers bridge phases 1 and 2 in-house with us, then move to an injection molder once volumes justify hard tooling.