At what volume does injection moulding beat CNC machining on cost?

For plastic parts the crossover typically lands between 100 and 1,000 parts. A machined plastic part at €15-60 each crosses a €5,000-8,000 mould at roughly 150-600 parts, depending on size and cycle time. Above the crossover the moulded part usually costs 5-20× less.

How much material does CNC machining waste compared with moulding?

Machining a plastic part from solid stock commonly removes 50-90% of the billet as swarf, and engineering plastics like POM, PA or PEEK cost €10-60/kg. Moulding wastes only the runner — typically 5-30% of shot weight in a cold-runner tool and nothing in a hot runner.

Which process holds tighter tolerances?

CNC machining holds ±0.02-0.05 mm routinely and tighter with care; injection moulding holds ±0.05-0.1 mm on critical features. Machining wins on absolute precision, but moulding's tolerance is adequate for most assemblies and stays stable across millions of identical parts.

When must a part stay machined rather than moulded?

Three cases: metals that cannot be replaced by plastic, volumes permanently below the break-even, and geometries that violate moulding physics — fully enclosed cavities, zero draft on deep walls, or sharp internal corners everywhere. Tight-tolerance prototypes in final material also stay machined.

Is a machined plastic part stronger than a moulded one?

Marginally, in unfilled resins: machined stock has no weld lines, no gate stress and no fibre-orientation effects, so a machined POM or PA test bar can read slightly higher. In glass-filled grades the difference is small, and moulded parts gain design features — ribs, bosses, snap-fits — that add real strength.

Can I machine prototypes and then mould the series?

Yes, and it is the standard path for tight-tolerance parts: machine 10-100 prototypes in the final resin to validate fit and function, then cut the mould. Watch for features that machine easily but cannot eject from a mould — undercuts, zero-draft walls — and fix them at DFM stage, not in steel.

Injection Moulding vs CNC Machining: Break-Even, Tolerances & Cost Curves

Machining cuts a part out of solid stock one at a time; moulding forms it in a cavity that repeats forever. For plastic parts the economics cross over between roughly 100 and 1,000 parts — below that, machining avoids tooling entirely; above it, the mould pays for itself quickly and then produces parts for a fraction of the machined price. This guide puts numbers on the crossover and the engineering differences around it.

How do the two processes compare?

Factor	CNC machining (plastics)	Injection moulding
Tooling cost	€0-500 (fixtures, CAM programming)	€4,000-60,000+ depending on tool class
Per-part cost	€15-60+ typical, flat with volume	€0.50-3.00 at volume, falling with cavitation
Break-even zone	Wins below ~100-1,000 parts	Wins above the crossover, often 5-20× cheaper
Lead time to first parts	1-2 weeks	4-12 weeks for tooling, then T1 samples
Tolerances	±0.02-0.05 mm routine	±0.05-0.1 mm on critical features
Material waste	50-90% of the billet becomes swarf	Runner only: 5-30% cold runner, ~0% hot runner
Material range	Stock shapes: POM, PA, PC, PEEK, PTFE, PEI	Thousands of grades including filled and FR compounds
Design freedom	Internal corners, zero draft, deep pockets all possible	Bounded by draft, uniform wall, ejection
Part-to-part consistency	Machine-dependent, operator-dependent	Steel-defined, identical across the series

Where is the cost break-even?

The calculation mirrors any tooling decision: break-even = tool price ÷ (machined price − moulded price). A POM insulator machined at €28 each, mouldable at €1.40 from a €6,500 single-cavity tool, crosses at about 245 parts. A larger PC enclosure machined at €55 crosses the same tool at around 120 parts. Because machined price barely moves with volume — every part costs a full cycle of spindle time — the curves diverge hard once the tool exists.

The crossover moves with part size, material and machinability. Small parts in cheap POM cross late (the machined part is cheap too); large parts in PEEK at €60/kg cross very early, because the machined part carries both the machine hour and 5-10 kg of swarf. When the comparison is close, cost per part modelling on your actual geometry settles it in one quotation.

What about material waste and material grades?

Machining plastic is subtractive: a part that weighs 80 g may be cut from a 700 g billet, and engineering stock costs €10-60/kg. The swarf has essentially no recovery value. Moulding is near-net-shape: a cold runner adds 5-30% scrap that is often reground back into production, and a hot runner adds none. At 10,000 parts a year, the waste difference alone can exceed the tool price.

Grade availability cuts the other way in one respect: stock shapes come in a shorter list of resins, and some moulding compounds (long-glass grades, some TPEs) are not available as machinable plate. But for POM, PA6, PC, PTFE, PEI and PEEK, machining gives you the exact resin in prototype quantities with no minimum order.

How do design freedom and DFM constraints differ?

The machine does not care about draft. Zero-draft walls, sharp internal corners, deep narrow pockets, threads cut to full depth, sealed cavities machined from two halves and bonded — all routine on a mill, all impossible or expensive in a mould. Moulding demands draft angles (typically 0.5-2°), uniform wall thickness, radiused corners and a parting line the part can eject from.

In exchange, the mould gives you features machining cannot reach economically: ribs at 60% of wall, bosses, snap-fits, living hinges, moulded-in threads and inserts, textures and logos — all formed in the same 20-60 second cycle. A part designed for moulding is usually a different, better part than the same function designed for machining; the DFM rules guide covers the constraints before they cost you a steel change.

When must the part stay on the machine?

Three cases are settled. First, metals: if the part must be aluminium, brass or steel for strength, temperature or conductivity, moulding is not in the running (though the metal-to-plastic question is worth asking once). Second, permanently low volumes: 50 parts a year for a decade never repays a mould. Third, moulding-impossible geometry that cannot be redesigned — rare, but it exists in manifolds and sealed fluidics.

Nordmould's free DFM review is the fast way to settle your own part: send the STEP file you have been machining, and the review returns mouldability feedback, draft and wall-thickness corrections, and a transparent quotation with tooling and per-part price side by side. T1 sampling then verifies tolerances on parts from the real tool before you commit the series.

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Send Nordmould the drawing you currently machine — the free DFM review returns your break-even volume, a transparent quotation and a T1 sampling plan.