Injection Molding vs CNC Machining: When Each Wins
Volume, geometry, lead time and material all play into the molded-vs-machined decision. Here's how to think about the crossover point.
The crossover question
For a typical engineering plastic part, the breakeven volume between machining and molding lands somewhere between 250 and 2,000 pieces, depending on geometry complexity and tooling cost.
## What pushes you toward CNC machining
- Annual volumes under 500 pieces - Geometry that traps a tool (deep undercuts, multiple sliding actions) - Material that doesn't mold well (PEEK, Ultem in some grades, PTFE) - Tight tolerances on critical features that benefit from post-mold machining anyway - Lead time critical: machining can ship in days, tooling takes 6–10 weeks
## What pushes you toward injection molding
- Annual volumes above ~1,500 pieces - Repeating features (snaps, ribs, bosses) that machine slowly - Cosmetic surfaces (Class A finish, textures, in-mold decoration) - Insert molding for fasteners or terminals - Per-part cost dominates over tooling amortization
## A worked example
Consider a 4" enclosure half:
- CNC route: $42 per piece, ships in 8 business days, no tooling. - Mold route: $9,500 tool + $2.85 per piece, ships first article in 7 weeks.
Crossover: (9500) / (42 - 2.85) ≈ 243 pieces. Above ~250 lifetime pieces, the mold pays for itself.
## When in doubt, build a bridge
A common pattern: machine the first 100–500 pieces while the production tool is being built. You hit market on time, validate the design, and switch to molded parts when the tool lands.