Injection molding has high upfront tooling cost ($5k-100k+) but low per-part cost ($0.10-2 per unit). CNC has zero tooling cost but higher per-part cost ($5-50 per unit). Somewhere in between is a crossover quantity. Below it, CNC is cheaper. Above it, molding is cheaper. Getting this right saves or wastes tens of thousands of dollars.
CNC machining cost:
Injection molding cost:
The economics: CNC has low fixed cost + high variable cost. Molding has high fixed cost + low variable cost. Crossover is where total costs meet.
Example: a 50-gram plastic housing. Assume:
Crossover point:
CNC total = 22 × N
Molding total = 15,000 + 0.65 × N
Setting equal: 22N = 15,000 + 0.65N → 21.35N = 15,000 → N ≈ 702 units.
Below 702 units, CNC wins. Above, molding wins. Given that a 50g housing typically has a production annual volume of 5,000-50,000, molding is clearly right for production.
For 100-unit prototype runs of the same part: CNC at $22 × 100 = $2,200, molding at $15,000 + $65 = $15,065. CNC wins by 7× at prototype quantities.
| Tooling type | Cost range | Typical lifetime |
|---|---|---|
| Prototype mold (aluminum, 1 cavity) | $3,000-8,000 | 100-1,000 shots |
| Production mold (P20 steel, 1 cavity) | $10,000-25,000 | 500,000 shots |
| Production mold (H13 steel, 1 cavity) | $20,000-50,000 | 1,000,000+ shots |
| Multi-cavity mold (4-8 cavity) | $30,000-100,000 | 1,000,000+ shots |
| Family mold (multiple different parts) | $40,000-150,000 | 500,000+ shots |
| Complex mold (undercuts, lifters, sliders) | +$5,000-30,000 to above | varies |
Key insight: low-quantity prototype tooling ($3-8k) dramatically lowers the CNC-to-molding crossover. For 100-500 units, prototype molding becomes competitive with CNC.
Using the $22 per-part CNC example with different tooling investments:
| Tool cost | Per-part molding cost | Crossover quantity |
|---|---|---|
| $5,000 | $0.75 | ~235 units |
| $10,000 | $0.65 | ~470 units |
| $25,000 | $0.55 | ~1,170 units |
| $50,000 | $0.50 | ~2,330 units |
| $100,000 | $0.45 | ~4,650 units |
Conclusion: if annual or lifetime volume is below ~250 units, CNC is almost always better. Above 500 units, consider prototype tooling. Above 1,000 units, almost always go with injection molding.
CNC-machined and injection-molded parts have different mechanical properties even in the same plastic. Molded parts have:
Injection molding requires:
CNC has no draft requirement, can handle complex geometry. For parts not designed for molding, CNC accepts the geometry; molding requires rework.
CNC prototype: 3-10 days. Molding prototype: 3-6 weeks for tool + first shots. For time-sensitive programs, CNC bridges to molding while tool is being cut.
Mid-production, you can re-quote CNC in a different material with no tooling changes. Molding changes require re-validated molds and sometimes separate mold for each material.
For programs in transition from prototype to production, "bridge tooling" fills the gap:
Done in sequence, this reduces total spend on low-volume launches while providing capacity for success. For 1,000-unit annual runs with uncertain future, aluminum mold + CNC prototype is a cost-effective approach.
Email [email protected] with your part and expected annual volume. We offer CNC plastic machining, prototype aluminum tooling, and production steel tooling — we'll quote the combination that minimizes your total cost.
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