CNC Machining vs Injection Molding: When to Switch from Machined to Molded

CNC machining is the right choice below roughly 100-200 plastic parts, for every metal part, and whenever you need production tolerances this week; injection molding wins above that volume because the tooling cost amortizes into a unit price machining cannot reach. The break-even typically sits between 100 and 500 pieces depending on part size and complexity — and the fastest way to find yours is to quote both routes side by side.

The cost crossover, with real numbers

Injection molding front-loads cost: an aluminum rapid tool runs a fraction of steel and still delivers 1,000-5,000 shots, while a hardened production mold costs several times more and lasts 100,000+. Machining has almost no setup cost but every part consumes machine time. Below roughly 100 pieces, machine time is cheaper than a tool; above roughly 500, the tool disappears into the unit price. Between those numbers, part geometry decides — a small simple bracket crosses over earlier than a large ribbed housing.

• 1-50 parts: CNC machining (or vacuum casting for copies of a master)
• 50-200 parts: quote both — crossover zone
• 200-1,000 parts: rapid aluminum tooling almost always wins
• 1,000+ parts: injection molding, no contest
• Metal parts at any volume: machining, casting or fabrication — molding does not apply

Tolerance and surface finish

Machining holds ±0.02-0.05 mm as standard and ±0.005 mm on precision features, because the cutter follows a programmed path. Molding holds roughly ±0.05-0.1 mm on a well-designed part, but that figure moves with wall thickness, gate position and resin shrink — and shrink varies by polymer. Machined parts also avoid weld lines, sink marks and gate vestige. If a bore carries a bearing or a face carries a gasket, machining is the safer specification.

Same polymer, different behavior

A machined POM or nylon part is cut from extruded or cast stock; a molded part is the same polymer melted and cooled under pressure. The molded part carries orientation, residual stress and shrink anisotropy the machined one does not, and glass-filled grades abrade cutting tools but flow well in a mold. For regulated fatigue or drop testing, validate in the route you will actually produce with — not the cheaper proxy.

Lead time: days versus weeks

• CNC first article: 1-3 working days from STEP file
• Rapid aluminum tool + T1 samples: 7-10 days tooling, samples about two weeks from order
• Hardened steel production tool: 15-25 days depending on cavities and cooling
• Design changes after tooling: expensive in molding, nearly free in machining

Design rules molding imposes that machining does not

Molding demands uniform wall thickness (typically 1.5-3 mm), draft of 1-2 degrees on vertical faces, radii instead of sharp internal corners, and ribs no thicker than about 60% of the wall to hide sink. Bosses need proper base radii; undercuts need slides or lifters. Machining ignores all of it — which is why early prototypes are machined even when production will be molded.

How to decide in 60 seconds

Ask three questions: is the part metal? (machine it) Is the quantity under 200? (machine it, or vacuum-cast if you need more than a handful of identical copies) Is the design still changing? (machine it until it freezes). If quantity is high and geometry is frozen, mold it. Send us the STEP file with target quantity and our 24-hour DFM quote returns machining, vacuum casting and injection molding cost curves for your exact part.