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CNC Machining vs Injection Molding — Which to Choose

Process

It is a volume question first

Below a few hundred parts, CNC machining usually wins — no tooling, fast setup, any geometry. Above that, injection molding overtakes because the tooling cost is spread across the run and per-part cost drops sharply. The two are not rivals; they are adjacent rungs on the same volume ladder, and many programs use both.

This guide compares them on the numbers that decide the choice: setup, per-part cost, tolerance, geometry and lead time.

The Snapshot

  • CNC setup: near-zero tooling; first part in hours–days after CAM programming; molding needs a 6–12 week tool first.
  • Per-part cost: CNC holds roughly flat per part; molding drops from dollars per part at 100 units to cents per part at 100K+ as the tool amortizes.
  • Break-even: molding overtakes CNC somewhere around 200–2,000 parts, depending on part size, cavitation and tolerance.
  • Tolerance: CNC reaches ±0.01–0.05 mm easily on metals and plastics; molding holds ±0.05 mm critical / ±0.1–0.2 mm general.
  • Geometry: CNC makes undercuts, deep pockets and internal features freely; molding needs draft, uniform wall and no trapped steel.
  • Materials: CNC cuts metals (Al, steel, Ti, Cu) and plastics; molding is plastics/elastomers only (our HV busbar paired C11000 copper insert with PA6 GF30 molding).

How CNC machining works

CNC removes material from a solid block with rotating cutting tools, programmed from the CAD model.

  • 3-axis mills the top and sides; 5-axis tilts the part to reach complex angles in one setup, holding ±0.01 mm on critical features.
  • Setup: CAM programming hours–1 day; first chip in 1–3 days; no tooling spend.
  • Tolerance: ±0.01–0.05 mm standard; ±0.005 mm achievable on precision 5-axis with inspection.
  • Surface: milled finish Ra 0.8–3.2 µm; no weld lines, no gate marks, no shrink.
  • Cost curve: per-part cost stays high because every part pays for machine time — roughly flat across the run.

How injection molding works

Injection molding melts plastic and forces it into a steel cavity; the tool is the capital cost, the cycle is cheap.

  • Tooling: a production mold runs 6–12 weeks (rapid tooling 3–5 weeks) and costs the bulk of the program up front.
  • Cycle: 15–60 s per shot; multi-cavity tools multiply output (our small-button program ran high-cavitation tooling at 5M units/year, ±0.03 mm, 12 weeks).
  • Tolerance: ±0.05 mm critical / ±0.1–0.2 mm general (mold steel ±0.02 mm).
  • Surface: can be textured, glossy or molded-in; gate/weld-line positions need DFM.
  • Cost curve: high fixed tool cost, then cents per part — cost collapses with volume.

Cost and volume break-even

The decision is mostly arithmetic on the tool amortization.

  • CNC: part cost ≈ machine hours × rate. At 100 parts of a small bracket, CNC is cheaper because molding’s 6–12 week tool isn’t amortized.
  • Molding: part cost ≈ (tool cost ÷ volume) + resin + cycle. Past the break-even (~200–2,000 parts), molding wins decisively.
  • Rule of thumb: under ~500 parts, almost always CNC; 500–5,000, compare carefully; above 5,000, molding dominates.
  • High-cavitation molding (4/8/16/32/64 cavities) drives per-part cost lower still — our 5M units/year button program is the extreme end.

Geometry: where each wins

  • CNC wins on undercuts, sharp internal corners, deep ribs, thin walls, and parts too large or too few to tool. It cuts metals molding can’t touch.
  • Molding wins on high-volume organic shells, snap-fits, living hinges and parts needing consistent surface finish across millions of units.
  • Molding limits: needs draft (0.5–2°), uniform wall (1–4 mm), no trapped steel, controlled shrink — all DFM items.
  • CNC limits: slow on very high volume; internal threads and hidden features need extra setups.

Tolerances, materials and machinability

  • CNC: ±0.01–0.05 mm on both metals and plastics; isotropic, no shrink.
  • Molding: ±0.05 mm critical features; shrink (amorphous 0.4–0.8%, semi-crystalline 1.0–2.5%) must be cut into the tool.
  • Materials: CNC spans Al 6061/7075, stainless 304/316, Ti-6Al-4V, brass, C11000 copper, PEEK, ABS, PC; molding is thermoplastics/elastomers (PA66, PPS, PP, PC/ABS, POM, LSR, TPE).
  • Mixed programs: a metal CNC bracket with a molded plastic cover is common — our HV busbar molded PA6 GF30 around a C11000 copper insert.

CNC reaches metals molding cannot touch:

  • Al 6061 (easy, yield ~70–150 MPa), Al 7075 (high strength ~500 MPa), SS 304/316 (corrosion, ~200 MPa), Ti-6Al-4V (aerospace, ~880 MPa, low thermal conductivity → slow feeds), brass and C11000 copper (our HV busbar insert, 250,000+ units/year with PA6 GF30 molding).

  • Machining tolerance: ±0.01–0.05 mm standard, ±0.005 mm on precision 5-axis with CMM inspection.

  • Surface finish: milled Ra 0.8–3.2 µm; can be bead-blasted or anodized post-machining.

  • Material cert: metals ship with mill cert; plastics (PEEK, ABS, PC) with resin lot cert — molding matches this for regulated parts.

  • <100 parts: CNC every time — molding’s 6–12 week tool can’t amortize.

  • 100–500 parts: CNC bridge or rapid tooling at 3–5 weeks.

  • 500–5,000 parts: compare tool cost ÷ volume against CNC machine rate.

  • >5,000 parts: molding dominates; high-cavitation (8/16/32/64 cavities) cuts per-part cost further.

Quick reference for the materials each process handles:

Material Process Service / note
ABS Both Melt ~200–240 °C, mold ±0.05 mm
PC/ABS Both 110–125 °C continuous, ±0.05 mm
PA66 (GF30) Molding 120–140 °C, shrink 1.0–2.0%
PPS Molding 200–220 °C, ±0.02 mm critical
PEEK Both 250 °C continuous, ±0.05 mm
PP Molding 100–120 °C, shrink 1.0–2.5%
POM Molding 90–100 °C, shrink 1.8–2.5%
LSR Molding -40 to 200 °C, shrink 2–3%
Al 6061 CNC ±0.01–0.05 mm, anodizable
Ti-6Al-4V CNC ±0.005 mm precision, ~880 MPa

Lead time and ramp path

  • CNC: 1–10 days to first part; ideal for bridge production and validation.
  • Molding rapid tooling: 3–5 weeks to first parts for fit/program proof.
  • Molding production tooling: 8–12 weeks from DFM release to shipment.
  • Combine: use CNC for bridge parts while the production mold is cut, then switch to molding at volume — one supplier keeps both in sync.

Keeping CNC and molding under one roof means the bridge parts and the production parts come from the same DFM logic.

  1. DFM review — geometry, wall, draft, tolerance and material reviewed before any spend.
  2. CNC bridge — machined prototypes in days to prove form/fit while tooling is planned.
  3. Rapid tooling — soft-steel mold; first molded parts in 3–5 weeks.
  4. Production tooling — hardened multi-cavity; shipment at 8–12 weeks.

Running CNC at shop A and molding at shop B doubles the validation cost. One supplier from prototype to shipment is the cheaper path.

Where the choice goes wrong

  • Molded too early → tool cost never amortizes below break-even volume.
  • CNC’d at volume → per-part cost stays 5–50× too high at 100K+ units.
  • Wrong material assumption → specced a metal part as molded plastic, or vice versa.
  • No bridge plan → production tooling delay stalls the whole launch.
  • Two suppliers → DFM re-learned, cost doubled, interface gaps.

Decision pass

Bring the annual volume, the material (metal vs plastic), and the tolerance. We return a CNC-vs-molding call with break-even volume, tool amortization and a bridge plan that keeps the launch on schedule.

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