Last updated: 2026-09-02 · 5 min read

Injection Molding vs. CNC Machining — When Does Each Process Make Economic Sense?

Injection molding produces parts at low per-unit cost after a high tooling investment; CNC machining produces parts at high per-unit cost with no tooling — and the breakeven volume (typically 100–500 parts depending on geometry) determines which process costs less over the program’s life. Below breakeven, CNC is cheaper; above breakeven, injection molding’s amortized tooling cost drops below the cumulative CNC cost per part.

Cycle time — The total time for one injection molding cycle: injection + packing + cooling + mold open/close + ejection. Typical range: 15–60 seconds. Cycle time determines production rate and cost per part — reducing cycle by 5 seconds saves 10–15% annual production cost. See also related guide.

Key Facts

  • CNC machining has zero tooling cost but high per-part cost ($5–50+ per part depending on complexity); injection molding has high tooling cost ($15,000–$150,000+) but low per-part cost ($0.05–$2.00).
  • The breakeven for a medium-complexity part is typically 100–500 units — below that, CNC wins; above that, injection molding wins. The exact number depends on part geometry, material and tolerance.
  • Tolerance: CNC holds ±0.025mm on metals and ±0.05mm on plastics; injection molding holds ±0.1mm — CNC is 2–4× more precise, which matters for mating surfaces and optical features.

    • Cycle time — The total time for one injection molding cycle: injection + packing + cooling + mold open/close + ejection. Typical range: 15–60 seconds. Cycle time determines production rate and cost per part — reducing cycle by 5 seconds saves 10–15% annual production cost. The related guide covers adjacent topics.

    Cost Comparison by Volume

    Volume CNC Total Cost Injection Total Cost Winner
    50 parts $1,500–$2,500 $20,000–$25,000 CNC
    500 parts $15,000–$25,000 $20,500–$26,000 Breakeven zone
    5,000 parts $150,000–$250,000 $25,000–$35,000 Injection
    50,000 parts $1.5M–$2.5M $50,000–$120,000 Injection

    When CNC Is the Right Choice

  • Prototype and pre-production: 1–100 parts for fit-check, functional testing and market validation before committing to tooling.
  • Tight tolerance features: mating surfaces, bearing bores and optical features that require ±0.025mm — injection molding cannot hold these without secondary machining.
  • Low-volume production: specialized equipment, medical devices and aerospace components where annual volume is under 500 and the tooling investment cannot be justified.
  • Design iteration: each design change requires no tooling modification — the CNC program is updated and the next part is cut from the revised file.

    • What This Means for Your Quotation

    The DFM review should compare both processes with total cost at the expected volume — not just tooling price or per-part price in isolation. When the volume is below breakeven, CNC is the lower total cost; when it is above, injection molding wins. For programs that start with CNC prototypes and transition to injection production, the quotation should show both paths and the volume at which the transition makes economic sense.

    Frequently Asked Questions

    Can CNC parts be used as production parts, not just prototypes?

    Yes — CNC-machined parts are used in production when volumes are low (under 500/year) or when the tolerance exceeds what molding can hold. Aerospace, medical and semiconductor components are often CNC-machined in production volumes because the precision requirement outweighs the higher per-part cost.
    The key advantage is repeatability without tooling: each part is cut from the same program, so dimensions are consistent across the production run without the mold wear that affects injection-molded parts over time.

    What about 3D printing — does that change the breakeven?

    3D printing (FDM, SLA, SLS) is competitive with CNC for prototypes at very low volumes (1–20 parts) and for complex geometries that CNC cannot . But 3D-printed parts have lower mechanical strength, poorer surface finish and higher per-part cost than CNC at volumes above 50. The breakeven between 3D printing and injection molding is similar to CNC — around 100–300 parts.

    Can you combine CNC and injection molding in one program?

    Yes — the standard approach is CNC prototypes for fit and function testing, then injection molding for production. Some programs use CNC for critical features (tight-tolerance bores, mating surfaces) and injection molding for the rest of the part, with secondary CNC machining after molding. The quotation should show the combined cost if both processes are needed.

    Key Takeaways

    1. CNC machining wins at low volume (under 100–500 parts) and tight tolerance; injection molding wins at higher volume where the amortized tooling cost drops below CNC’s per-part cost.
    2. The breakeven volume depends on part geometry, material and tolerance — the DFM should calculate it for the specific part, not use a generic rule of thumb.
    3. For programs that transition from prototype to production, the quotation should show both paths and the volume at which the switch makes economic sense.

    Send aCatMould the part drawing and annual volume, and the DFM review compares CNC and injection molding with total cost at your volume — with the breakeven point calculated for your specific geometry, so the process decision is based on total program cost, not per-part price alone.

    Sources
    Plastics Technology — injection molding vs. CNC machining economics
    SPE — process selection and cost comparison
    MoldMaking Technology — prototype to production process selection

    Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet