CNC Lathe Machining vs Injection Molding

How the two processes make the part

CNC Machining is subtractive: a metal or plastic blank is cut to the CAD model, so the first article is available in days with no part-specific tooling and design changes are cheap. Injection moulding is formative in plastic: a steel mould is filled with molten thermoplastic under pressure. The mould is the entire economic equation — expensive to make, but negligible marginal cost once paid for — and it only applies to plastics, never to metal.

Side-by-side comparison

Factor	CNC machining	Injection moulding
Tooling	CAD/CAM + cutters; first article in days, no part-specific tool	Steel mould $5k–$100k+; design, machining and sampling take weeks
Lead time (first article)	1–3 days from CAD	4–10 weeks including mould
Economic batch	1-off to hundreds	1,000–100,000+ to amortise the mould
Materials	Metals and plastics cut from solid stock	Thermoplastics (ABS, PC, PA, PEEK, POM) and some thermosets
Tolerance	±0.003–0.05 mm	±0.1–0.3 mm (mould shrinkage varies)
Wall thickness	No absolute minimum	Min ~0.5–1 mm to fill the cavity
Geometry	Solid 3D within cutter access	Hollow, ribbed, snap-fit shells come naturally
Material structure	From solid stock — isotropic	Flow lines, weld lines, sink marks possible
Design change cost	CAD edit, near zero	New or modified mould — expensive
Post-processing	Deburr, anodise, plate	Gate removal, deflash, sometimes vapour smooth
Typical applications	Metal parts, precision, prototypes	High-volume plastic enclosures, housings

Where CNC Machining wins, where injection moulding wins

CNC Machining wins whenever the part is metal, must hold ±0.003–0.05 mm, needs internal or asymmetric 3D detail, or is needed in any quantity below the mould amortisation point (often 1,000–10,000 parts). Injection moulding wins for very high volumes of a plastic component where wall thickness, ribs and snap-fits are wanted and the tooling can be absorbed by the run. For metal requirements the comparison ends immediately — moulding simply does not apply.

Material structure and properties

CNC Machining from solid stock yields isotropic properties and predictable performance traceable to the mill certificate. Moulded plastic parts show flow lines, potential weld lines at merge points and sink marks on thick sections; these are managed by gate placement and wall-thickness control rather than eliminated. For a metal requirement, CNC Machining is the only of the two that can deliver it.

Process-selection checklist

• Part must be metal? → CNC Machining (injection moulding is plastic only).
• Need <1,000 parts or a quick prototype? → CNC Machining.
• Plastic part, >10,000 off, with thin walls/ribs? → injection moulding.
• Need ±0.01 mm or better? → CNC Machining.
• Moulded part must locate precisely? → machine critical faces by CNC Machining.

Post-processing

CNC Machining parts are deburred and finished (anodise, plate, paint, passivate). Moulded parts need gate and flash removal, may be vapour-smoothed, and often have the precision datums machined if the application demands it; cosmetic moulded surfaces still depend on mould polish rather than a separate finish.

Frequently asked questions

Cost crossover in practice

When you are choosing, the question is rarely which process is "better" and almost always about batch size. As a working rule for a part of moderate complexity: CNC Machining costs a few tens of dollars in machine time per piece with no upfront tooling, so 100 parts might total a few thousand dollars. Injection moulding needs tooling in the low five figures, so the same 100 parts would be far more expensive once the tool is paid for — but at 10,000 parts the per-piece tooling share falls to cents and the formative process wins decisively. The practical crossover for most components lands between 1,000 and 5,000 pieces; above it, amortised tooling dominates, below it, CNC Machining is cheaper and far more flexible. Always quote both before you commit, because material yield, secondary operations and scrap rate shift the exact break-even for your part.

Design-for-manufacturing tips

• Keep functional tolerances at ±0.02–0.05 mm where cnc lathe machining is used; reserve ±0.003 mm (milling) / ±0.002 mm (turning) for critical datums only — every tighter band adds cost.
• Design for standard stock sizes and minimise deep, narrow cavities that need long-reach or 5-axis tooling.
• If volume will rise, design the part so it can later move to injection molding without changing the interface — locate on the same datums.
• Specify surface finish only where it matters; machined finish is cheap, plating and anodising are the cost drivers.

What this shop actually runs

Capability evidence should come as specifics, not slogans. Behind this guide is a 6,800 m2 plant in Dongguan, China running 100+ CNC machines with 100+ staff. The machine list by category:

• 5-axis machining centres (10+) - Mazak VARIAXIS I-600 (510/910/510 mm travel, 12,000 rpm, 22 kW, 71.6 N·m, 30-tool BT40) and Mazak INTEGREX I-250H S (B-axis 0.0001°, 24 kW, 124 N·m, 38-tool HSK-A63). Intended for compound-angle parts, single-setup multi-face prismatic work, and aerospace fittings where datum transfer loss is unacceptable.
• 3-axis / 4-axis machining centres (50+) - Brother SPEEDIO S500Z2N / S700Z2N (10,000 rpm, 14/21-tool ATC, 0.7 s tool-to-tool, Z-axis 2.2 G) and Mazak VCE-430A L / VCE-570C L (10,000 rpm, 118 N·m, 24-tool ATC). Intended for prismatic parts, plates, brackets, housings.
• Swiss-type turning centres (10+) - Citizen Cincom A20-3F7 (Φ20 mm, 10,000 rpm), TSUGAMI B0206-III / B0205-III (Φ20 mm, 0.1 µm), STAR SR-32JII / SR-32J Type N (Φ32 mm, 8,000 rpm). Intended for long, slender rotational parts at ±0.002 mm.
• CNC turning centres (20+) - HEADMAN T70M/1250 (720 mm swing, 117 mm bore, 3,000 rpm, BMT75 12-station turret). Intended for shafts, bushings, flanges up to ±0.002 mm.
• Wire EDM (5+) - Sodick AQ535L (550/350/300 mm travel, ±20° taper, 800 kg workpiece). Intended for hardened tool steel, internal splines, sharp internal corners that milling cannot reach.
• Inspection: Hexagon CMM + 20+ gauges - Hexagon CMM for coordinate measurement, plus vision system, profilometer, hardness tester, coating thickness gauge, XRF for material verification. The 10:1 rule (gauge 10x more accurate than the band) is applied to every feature.
• Machine highlights - VARIAXIS I-600: one-setup multi-face 5-axis + MAZA-CHECK auto-calibration; INTEGREX I-250H S: DONE-IN-ONE mill-turn, B-axis 0.0001° + Ai thermal shield; SPEEDIO S500Z2N / S700Z2N: Z-axis 2.2 G + 0.7 s ATC; Swiss-type: guide-bush switchable + 0.1 µm resolution.

Tolerance: ±0.003 mm CNC milling / ±0.002 mm CNC turning / ±0.002 mm grinding and wire EDM. Capability: Ppk >= 1.67 initial on key characteristics, Cpk >= 1.33 ongoing.

Certificates: ISO 9001:2015 (116024/A/0001/UK/En), IATF 16949:2016 (131941/A/0001/SM/En, with design clause 8.3 excluded, stated openly) and ISO 14001:2015 (116024/B/0001/UK/En), issued to Dongguan Zhuohang Ltd, registration 91441900MA4UWT6X39, founded 2005.

Where we ship: Austria, Belgium, the Czech Republic, Denmark, France, Germany, Greece, Hungary, Ireland, Italy, the Netherlands, Spain, Sweden and the United Kingdom in Europe; Brazil, Canada, Mexico and the United States in the Americas; China, Israel, Japan, Singapore and Sri Lanka in Asia; Australia and New Zealand in Oceania; and South Africa in Africa — plus other countries and regions worldwide, shipped from our base in Dongguan, China.

Contact: Felipe Iglesias, [email protected], +86 13126536955.

Standards referenced on this page

• ASTM B209/B209M-21 — Standard Specification for Aluminum-Alloy Sheet and Plate
• ASTM A582/A582M-12 — Standard Specification for Free-Machining Stainless Steel Bars
• ISO 9001 — Quality management systems — Requirements