Injection Molding vs. CNC Machining: How to Choose the Right Process

Every custom parts program eventually hits the same fork in the road: mold the part, or machine it. The right answer depends on five variables — and getting it wrong early is expensive.

The economics

CNC machining wins below roughly 100–500 units. There is no tooling cost, and per-part price declines only slightly with volume. Injection molding carries tooling cost (typically $3K–$40K for a production mold) but drives per-part cost down dramatically above 1,000 units — often by 5–20× versus machining.

| Factor | Injection Molding | CNC Machining | |---|---|---| | Tooling cost | High (mold required) | None | | Per-part cost @ 10K pcs | Very low | Moderate | | Lead time (first parts) | 2–6 weeks (with tooling) | 3–10 days | | Volume sweet spot | 1K – 1M+ | 1 – 5K |

Tolerances and materials

Precision CNC holds ±0.005 mm on critical features and machines nearly any engineering material, including titanium and 17-4PH. Injection molding typically holds ±0.05 mm (±0.02 mm with careful process control) and is limited to thermoplastics — but delivers parts with consistent surface texture and zero tool marks on visible faces.

Design constraints

• Undercuts: trivial for 4/5-axis CNC; expensive or impossible in molding without sliders or lifters
• Wall thickness: molding wants uniform 1.5–3 mm walls; machining accepts whatever the stock allows
• Cosmetic surfaces: molding produces Class-A faces straight from the tool; machined parts need finishing to match

A hybrid path worth considering

For many programs, the cheapest risk reduction is CNC prototypes first, then production tooling. You validate fit and function on machined parts (in the production material, not a printed substitute), capture DFM corrections, and only then commit to steel. Budget two extra weeks; save months of mold rework.

Rule of thumb: prototyping quantities, irregular geometry or metal parts → CNC. 5,000+ plastic parts with stable designs → molding. Everything else is a conversation with your supplier's engineer.

Need help deciding? Submit your drawing and our engineers will recommend the most cost-effective route with numbers, not opinions.