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CNC Machining vs Injection Molding — Which to Choose

RCRay Chan·2026-08-18·21 min read
Table of Contents

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 — CNC for the prototype and bridge run while the production mold is being cut, then molding for the run itself.

This guide compares them on the numbers that decide the choice: setup, per-part cost, tolerance, geometry, materials and lead time. Every figure below is a published competitor capability (FirstMold, Protolabs, Xometry, HLH Rapid), an industry-standard datasheet range, or a verified value from our own plant in Dongguan, marked [OUR PLANT]. Nothing is invented — where a number has no public source, we say so.

The 2026 context makes the comparison sharper than it used to be. Prototype-to-production cycles are shorter, low-volume molding is cheaper than it was a decade ago, and buyers increasingly want one supplier that runs both processes so the bridge parts and the production parts come from the same DFM logic. That is the setup we describe below.

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 a C11000 copper insert with PA6 GF30 molding at 250,000+ units/year).
  • One supplier: CNC bridge parts and molded production parts from the same DFM review — the cheapest path to launch.

Table of Contents

  1. It is a volume question first
  2. The Snapshot
  3. How CNC machining works
  4. How injection molding works
  5. Cost and volume break-even
  6. Geometry: where each wins
  7. Tolerances, materials and machinability
  8. Machine selection and shop capability
  9. Secondary operations and finishing
  10. Metrology and quality control
  11. Cost structure: where the money goes
  12. Lead time and ramp path
  13. Where the choice goes wrong
  14. Decision pass
  15. Frequently Asked Questions
  16. Sources

How CNC machining works

CNC removes material from a solid block with rotating cutting tools, programmed from the CAD model. There is no tool to build and no shrink to predict — the part is cut to the print, and the same machine can make a different part tomorrow.

  • 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 CMM 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.
  • Materials: metals (Al 6061/7075, stainless 304/316, Ti-6Al-4V, brass, C11000 copper) and engineering plastics (PEEK, ABS, PC, POM).

The machine-hour economics are simple: a bracket that takes 20 minutes on a machining center costs roughly the same at part 1 and part 1,000. That is CNC’s weakness at volume and its strength everywhere else — there is no amortization cliff, so small batches are priced honestly per part.

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. Once the mold exists, the machine produces identical parts every 15–60 seconds, and multi-cavity tools multiply that output.

  • 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 [OUR PLANT]).
  • 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.
  • Shrink: amorphous resins shrink 0.4–0.8%, semi-crystalline 1.0–2.5%; the cavity is cut oversized to compensate.

The process fundamentals of the two are summarized below:

Factor CNC machining Injection molding
Setup CAM programming, no tooling Production tool first: 6–12 weeks
First part 1–10 days 3–5 weeks rapid / 8–12 weeks production
Per-part cost Roughly flat (machine time) Dollars → cents as volume grows
Tolerance ±0.01–0.05 mm (metal & plastic) ±0.05 mm critical / ±0.1–0.2 mm general
Surface Ra 0.8–3.2 µm milled Molded finish, textured or glossy
Materials Metals + plastics Thermoplastics & elastomers
Shrink control None needed Cut into tool (0.4–2.5%)

Cost and volume break-even

The decision is mostly arithmetic on the tool amortization: molding’s per-part cost is (tool cost ÷ volume) + resin + cycle, so the tool cost shrinks toward zero per part as volume climbs. CNC’s per-part cost has no such denominator — it is machine hours × rate, flat at every volume.

  • 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.
  • Published tool-cost anchors: HLH Rapid quotes simple molds at $3,000–6,000 and complex steel/multi-cavity tools from $7,000 up, with total projects running $10,000–100,000 (https://www.hlhrapid.com/capabilities/injection-molding/). FirstMold publishes low-volume molding from 50 pieces at $1.2/part, about 40% cheaper than SLS 3D printing on the same geometry (https://firstmold.com/pp-injection-molding/) — the point where even small batches stop needing CNC.
  • 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.
Volume band Likely winner Why
< 100 parts CNC No tooling to amortize; molding’s tool can’t pay back
100–500 CNC or rapid tooling Compare CNC rate vs tool ÷ volume; published small-batch molding from 50 pcs
500–5,000 Case-by-case Mold amortization starts beating machine hours
5,000–100K Molding Per-part cost drops to cents; multi-cavity tools multiply output
100K+ Molding, high-cavitation 4/8/16/32/64 cavities; our button program at 5M units/year

The crossover point is not fixed — it moves with part size, tolerance and cavitation. A large housing that takes two hours to machine breaks even at a few hundred parts; a tiny bracket that machines in 5 minutes may not justify a tool until several thousand.

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. FirstMold’s design handbook, for example, recommends ribs at ≤50% of wall thickness with height ≤3× wall and a root fillet ≥0.25T (https://firstmold.com/pp-injection-molding/).
  • CNC limits: slow on very high volume; internal threads and hidden features need extra setups.
Feature CNC machining Injection molding
Undercuts Free (tool access permitting) Needs slides/actions, added cost
Sharp internal corners Yes No — fillets required
Thin walls Machinable, slow at depth 1–4 mm uniform wall; down to 0.5 mm in thin enclosures
Snap-fits, living hinges Machined hinge is fragile Molded in one shot, millions of cycles
Ribs Any depth Rib ≤ 50% of wall, height ≤ 3× wall
Draft Not required 0.5–2° needed
Metal parts Yes No (except metal inserts)
Cosmetic surface at scale Per-part finishing Consistent molded texture

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.
  • Protolabs’ published numbers put the machining tolerance into the tool at ±0.003 in (0.076 mm) and the finished resin part at ≥±0.002 in/in (0.051 mm/mm); ABS shrinks 0.003 in/in (0.076 mm/mm) while PP shrinks 0.018 in/in (0.457 mm/mm) — so a tool built for ABS switched to PP yields parts roughly 0.015 in/in smaller (https://www.protolabs.com/resources/blog/injection-molding-tolerances/). This is the single biggest reason a machined prototype and a molded production part can disagree dimensionally.
  • FirstMold publishes ±0.127 mm (±0.005 in) as its standard molded-part tolerance for automotive programs (https://firstmold.com/industries/automotive/), ±0.0254 mm (±0.001 in) for medical (https://firstmold.com/industries/medical/), and CNC ±0.002 in / molded ±0.005 in for consumer electronics (https://firstmold.com/industries/consumer-electronic/).
  • 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 at 250,000+ units/year.

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). Titanium is the extreme case for waste: FirstMold cites roughly 35% of titanium material ending up as chips on CNC programs (https://firstmold.com/industries/aerospace/).
  • 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.

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

Machine selection and shop capability

The process choice is only half the decision — the machine matters too, and the right shop runs both.

On the CNC side:

  • 3-axis vertical machining centers handle the majority of prismatic parts: brackets, plates, housings, fixtures. Fast to program, fast to set up, cheapest machine hour.
  • 5-axis machines tilt the spindle to reach compound angles, deep pockets and undercut-adjacent features in one setup — fewer setups mean better accuracy (±0.005 mm with CMM verification) and less handling.
  • Lathes / turn-mill centers cover round parts: shafts, bushings, threaded collars. Many bracket programs actually pair a turned feature with a milled feature.
  • Wire EDM cuts hardened steel and complex profiles — at our plant it is what cuts the mold inserts for the molding side of the same program ([OUR PLANT]: 9+4 wire EDM machines in-house).

On the molding side:

  • Clamp tonnage is selected from projected area: Kemal MFG publishes a rule of 1.5–2.5 tons per square inch of projected part area (https://www.kemalmfg.com/pvc-injection-molding/). Under-sizing gives flash; over-sizing wastes energy and slows cycles.
  • Cavitation is a business decision: a 4-cavity tool costs more than a single cavity but quadruples output per cycle. High-cavitation (8/16/32/64) is how per-part cost collapses at 100K+ units.
  • Our plant runs 18+3 Sodick injection molding machines [OUR PLANT] — the “+3” reserved for overmolding, insert molding and LSR work that needs separate process cells.

The one-roof advantage: when CNC and molding share a shop, the same DFM team reviews the part, the CNC bridge parts are made while the mold is cut, and the first molded shots are checked against the machined reference parts that were already approved.

Secondary operations and finishing

Neither process ships a finished product straight off the machine. Secondary work is where cost overruns hide, and it differs by process.

After CNC:

  • Deburring and edge breaking — every machined edge needs a pass; sharp corners are a liability in assembly and handling.
  • Bead blasting — matte, uniform texture that hides tool marks; a cheap upgrade on aluminum.
  • Anodizing (aluminum) — FirstMold prices anodizing at the $$ tier of its surface-finishing menu (https://firstmold.com/surface-finishing/); Type II for color/corrosion, Type III for wear.
  • Powder coating ($$$ tier) for durable color; electroplating ($$$) for decorative chrome/nickel — plating-grade substrate required.
  • Laser engraving ($$) for logos, part numbers, 2D codes.
  • Threads and inserts — internal threads need tapping or thread milling; press-fit inserts add a step but enable metal threads in plastic parts.

After molding:

  • Gate and runner trimming — automatic (tunnel/submarine gates) or manual; gate vestige placement is a DFM decision.
  • Pad printing ($$) and silk screen ($$) for graphics; laser engraving ($$) for permanent marks.
  • Spray painting — FirstMold lists screen printing, laser engraving and spray painting as in-house capabilities (https://firstmold.com/surface-finishing/); surface-finish cost coefficients from its materials page rank painting ★★ and electroplating ★★★★ (electroplating-grade ABS only), with texture etching at ★ (https://firstmold.com/materials/injection-molding-materials/).

The planning rule is the same for both: list every secondary operation in the RFQ. An unlisted tapped hole or a cosmetic surface requirement is where budgets die.

Metrology and quality control

Tolerance claims are only as good as the measurement behind them.

  • First article inspection (FAI) — CMM measurement of critical features against the print before production runs; for molding this happens at T1 samples.
  • In-process checks — height gauges, calipers and bore gauges on the shop floor; CMM patrol checks at defined intervals.
  • Our plant holds ±0.02 mm on controlled dimensions [OUR PLANT] and runs CMM, height gauges, moisture analyzers, pressure gauges and color controllers — the same QA equipment list FirstMold publishes for its keycap programs (https://firstmold.com/pbt-injection-molding/).
  • Process capability — for automotive programs, capability indices of Cpk ≥ 1.33 are the standard acceptance bar (industry practice under PPAP); a process that can’t hit it needs tool or process changes, not more inspection.
  • Material traceability — metals ship with mill certificates; plastics with resin lot certificates. Molding under IATF 16949 / ISO 13485 / ISO 9001 [OUR PLANT] extends the same traceability to regulated parts.

Protolabs’ own tolerance guidance makes the deeper point: tolerance is controlled through DFM, material selection, tool design and process control, not through inspection alone (https://www.xometry.com/resources/injection-molding/injection-molding-tolerances/). The machined prototype and the molded part are both measured against the same print, so any drift between them shows up at T1 — before production spend.

Cost structure: where the money goes

Understanding where the money goes in each process keeps the comparison honest.

  • Molding cost structure (FirstMold’s published breakdown): raw material 40–60%, processing 20–35%, mold 15–25%, post-processing 5–20% (https://firstmold.com/materials/injection-molding-materials/). Material dominates — which is why thin-wall design and high-MFI resins matter: FirstMold notes high-MFI materials can cut production time by roughly 30%, while reinforced compounds accelerate mold wear about 3×.
  • CNC cost structure: dominated by machine time (setup + cycle + programming share) and material. Machine-hour rates compound with part complexity; a 5-axis part costs more per hour than a 3-axis part even when the material is identical.
  • Material waste: CNC is subtractive — the billet is bigger than the part. Titanium is the worst case at roughly 35% chip waste (FirstMold, https://firstmold.com/industries/aerospace/); aluminum and brass are recycled but still carry a buy-to-fly penalty. Molding wastes only the runner system, and hot-runner tools cut even that.
  • Tooling is molding’s compensating line item: $3,000–6,000 simple, $7,000+ complex, $10,000–100,000 total projects (HLH Rapid, https://www.hlhrapid.com/capabilities/injection-molding/).
  • Hidden costs both processes share: secondary operations, finishing, inspection documentation and logistics. One supplier covering both eliminates the double handling — parts don’t ship from the machine shop to the molding shop and back.

The sourcing rule of thumb: tooling drives the decision to mold; per-part price drives the volume decision. If the tool amortization doesn’t pencil out against CNC machine hours at your annual volume, molding is wrong — no matter how good the quote looks.

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.
  • Xometry’s published workflow targets T1 samples in 5 business days fastest, 3 weeks typical, with mold grades from Class 105 (prototype) to Class 101 (high-volume production) (https://www.xometry.com/capabilities/injection-molding-service/).
  • 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: two DFM reviews, two sets of fixtures, two inspection reports, and finger-pointing when a fit issue spans the interface. 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.
  • Tolerance over-spec → paying for ±0.005 mm CMM-controlled features on a surface that only needs ±0.2 mm; tolerance should be as loose as function allows.
  • Ignoring shrink in the handoff → a machined prototype approved at nominal dimensions and a molded part that shrinks 1–2% are different parts unless the cavity compensates.
  • No metrology plan → parts measured differently at the machine shop and the molding shop disagree in the report, and nobody knows which is right.

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 — and we run both processes under one roof, so the answer doesn’t depend on which shop you called first.

Frequently Asked Questions

1. At what volume should I switch from CNC to injection molding? Around 200–2,000 parts, depending on part size, tolerance and cavitation. Under ~500 parts CNC almost always wins; above ~5,000 molding dominates. The crossover is where (tool cost ÷ volume) drops below CNC machine-hour cost.

2. What tolerance can CNC machining hold? ±0.01–0.05 mm standard on metals and plastics; ±0.005 mm on precision 5-axis work with CMM inspection. Injection molding holds ±0.05 mm on critical features and ±0.1–0.2 mm general.

3. Is CNC machining more accurate than injection molding? On the machined feature itself, yes — CNC cuts to print with no shrink. Molded parts inherit the cavity accuracy plus resin shrinkage, which is why a tool’s steel is cut to ±0.02–0.076 mm but the finished part is quoted looser (Protolabs: tool machining ±0.003 in / 0.076 mm, finished part ≥±0.002 in/in).

4. Can CNC machines cut metal that molding can’t? Yes. CNC spans aluminum (Al 6061/7075), stainless 304/316, Ti-6Al-4V, brass and C11000 copper; injection molding is limited to thermoplastics and elastomers (plus metal inserts via insert molding).

5. How much does a mold cost compared to CNC setup? CNC setup is near-zero — just CAM programming. Molds run $3,000–6,000 simple and $7,000+ for complex multi-cavity tools (HLH Rapid), with total programs $10,000–100,000. That tool cost is what break-even math amortizes.

6. How fast can I get parts from each process? CNC first parts in 1–10 days; rapid-tooling molded parts in 3–5 weeks; production-tooling parts in 8–12 weeks from DFM release. Xometry’s published T1 target is 5 business days fastest, 3 weeks typical.

7. What draft angle does a molded part need? Typically 0.5–2° per side (Protolabs recommends 1–2°), depending on texture and depth. CNC parts need no draft at all — a key geometric advantage for prototypes.

8. Why do molded parts differ from my CNC prototype? Shrinkage. Amorphous resins shrink 0.4–0.8%, semi-crystalline 1.0–2.5%; the cavity compensates, but a tool built for ABS switched to PP yields parts roughly 0.015 in/in smaller (Protolabs). The DFM review must carry the material and shrink into the tool design.

9. What surface finish can each process achieve? CNC milling gives Ra 0.8–3.2 µm, then bead blasting or anodizing. Molding gives the tool’s surface — polished, textured or glossy — consistently across millions of parts, with gate/weld-line placement decided in DFM.

10. Can I use CNC for bridge production while the mold is being made? Yes — that is the standard ramp path: CNC bridge parts in days to prove form/fit and keep the launch moving, rapid tool in 3–5 weeks, production tool in 8–12 weeks. One supplier keeps the bridge and production parts dimensionally consistent.

11. Does CNC machining cost more than molding per part? At volume, yes — typically 5–50× more at 100K+ units. Below a few hundred parts, CNC is cheaper because there is no tool to amortize. The crossover is the break-even volume.

12. When should I combine CNC and molding in one program? When the product mixes metal and plastic — a CNC aluminum bracket with a molded cover, or an insert-molded busbar like our HV busbar (PA6 GF30 over C11000 copper at 250,000+ units/year). Hybrid programs are cheaper with one supplier running both processes.

Sources

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Written by

Ray Chan

Manufacturing Engineer · Custom Manufacturing Specialist. Ray helps global importers and integrators source factory-direct plastic parts and tooling.

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