CNC Machining vs Injection Molding: 9 Proven Selection Rules

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  CNC machining vs injection molding at a glance
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  Which process fits your part
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  How each process makes a part, and why that sets the cost floor
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  Tooling cost and the real break-even point for CNC machining vs injection molding
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  Tolerances and surface finish: where each process holds up
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  Material options and part geometry decide more than price
• 7
  Bridge production and lead time when you are close to the line

CNC machining starts with a CAD model and a billet. A CAM programmer picks toolpaths, workholding, and cutters, then posts G-code for a 3-axis or 5-axis mill. The finished shape is whatever the tool can reach without colliding with the part or the fixture.

Setup is the first real cost. A simple 3-axis job may need two or three orientations, each with its own fixture and zero point. A 5-axis machine can often reach five faces in one setup, which removes handling time and tightens feature-to-feature accuracy. Simple workholding runs under an hour. A custom tombstone or soft jaws can eat a full shift.

Injection molding removes no material. A toolmaker cuts a cavity and core from steel, then a press injects molten polymer at 200–300 °C and packs it under pressure. The part must release from the tool, so draft, wall thickness, and gate location are fixed before the first shot.

Rule 1: if the geometry cannot be drafted and ejected, molding is off the table regardless of volume. Rule 2: if you need more than one design revision during the program, keep it in CNC. Rule 3: if annual volume is under about 1,000 parts, tooling amortization rarely pays back.

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  Machining cutsNo draft needed, undercuts are fine, tool reach is the only limit.
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  Molding formsDraft of 1–2° minimum, uniform walls, gate and ejector marks are permanent.
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  Cost driverMachining pays for setup and cycle time. Molding pays for tool steel up front.

A single-cavity mold for a small bracket commonly lands between $5,000 and $15,000. A multi-cavity tool with lifters, slides, and hot runners can pass $80,000. That money sits on the balance sheet before a single sellable part exists.

The break-even quantity is where the mold cost plus the molded unit price equals the machined unit price times quantity. With a $12,000 tool, a $0.60 molded part, and a $14 machined part, you cross over near 900 pieces. Change the machined price to $6 and the crossover moves past 2,000.

This is why the decision is not about which process is cheaper. It is about where your part sits relative to a number that geometry, tolerance, material, and annual volume all push in different directions. Run the arithmetic with your own quotes before you approve tooling.

Rule 4: build a payback model with real unit prices, not catalog averages. Rule 5: treat any design that is still changing as a CNC part, even if the volume looks like a molding job.

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  Low tool cost$5,000 to $15,000 for a simple single-cavity tool.
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  High tool costAbove $40,000 with slides, lifters, and hot runners.
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  CrossoverUsually between 500 and 2,000 parts for a small enclosure.

CNC machining holds ±0.005 mm on critical features when the setup is rigid and the tool is fresh. That matters for bearing bores, seal grooves, mating faces, and any feature that sets an assembly stack-up. Molding typically holds ±0.05 mm on stable dimensions. Tightening a molded tolerance to ±0.01 mm means tool adjustments, tighter process windows, and higher scrap.

Molded tolerances also drift. Warp, shrink, and gate location move dimensions between shots and between cavities. If your drawing calls out a flatness of 0.05 mm across a 200 mm panel, molding may need a fixture to hold the part during measurement.

Surface finish follows the same split. As-machined finishes land at Ra 1.6–3.2 μm. Fine passes reach Ra 0.2–0.8 μm. A mold can produce a mirror polish from the start, but every texture is locked into the tool steel. Changing it later means pulling the tool.

Rule 6: if a feature controls assembly fit, machine it. Rule 7: if the surface is cosmetic and fixed, molding gives you repeatability at volume.

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  Machined finishRa 0.8–1.6 μm typical, Ra 0.2–0.8 μm with fine passes.
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  Molded finishSPI grades from glossy to matte, set by the tool.
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  RiskMolded dimensions move with shrink and warp between cavities.

Machining covers the full material list: 6061 and 7075 aluminum, 303 and 17-4PH stainless, 4140 steel, titanium, Inconel, and engineering plastics from POM to PEEK. If the part needs to be metal, structural, or heat resistant, molding is usually not an option.

Molding works with thermoplastics that flow and freeze: ABS, PC, PMMA, PP, HDPE, and glass-filled grades. Glass fill raises stiffness but wears the tool and changes shrink. Long fibers give strength in one direction and warp in another.

Geometry is where molding wins cleanly. A thin-walled enclosure with snap fits, ribs, and bosses is expensive to machine because every feature costs cycle time. Molded, the whole part forms in one shot. Deep pockets, sharp internal corners, and undercuts push the other way: they need EDM or slides in a mold, and a 5-axis machine handles them without extra tooling.

Rule 8: match the process to the material and the feature list first. Cost is the last filter, not the first.

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  Metal partsMachining, unless you switch to die casting.
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  Thin-wall plasticMolding wins above a few thousand parts.
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  UndercutsFree in machining, slides and cost in molding.

Many programs sit in the middle: 2,000 to 8,000 parts a year, a design that is 90% frozen, and a launch date that cannot slip. Machining covers the gap. Parts ship in 3–5 days, and the same program can run from one prototype to a 10,000-part order without a tooling commitment.

That bridge buys time to validate the design in the field, collect real usage data, and only then cut steel. If the design changes after tooling, you pay for welding, recutting, or a new tool. A CNC program change costs a CAM edit and a new setup.

The trade-off is unit price. Bridge parts carry a machined price that never drops. If the annual volume is genuinely above 5,000 parts and the design is stable, that premium becomes hard to defend after the crossover point.

Rule 9: use machining as bridge production when the design is not frozen, then move to molding once the drawing stops changing. Do not order tooling to hit a price target on a part that still has open questions.

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  Bridge window3–5 day turnaround holds a launch date while tooling is cut.
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  No MOQOne prototype to 10,000+ parts on the same process.
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  Watch the priceMachined unit cost stays flat at every quantity.