What is the fundamental difference between CNC machining and injection molding?

CNC machining is a subtractive process — a cutting tool removes material from a solid workpiece according to a computer-controlled toolpath. Injection molding is a net-shape process — molten plastic is injected into a precisely machined mold cavity and cooled into the part's final shape. CNC removes material to reveal the part; injection molding forms the part directly from the mold's geometry.

Why does CNC machining achieve tighter tolerances than injection molding?

CNC machining's precision is limited primarily by the machine's own mechanical precision (ballscrew accuracy, spindle runout), since the cutting tool directly removes material along a precisely programmed path. Injection molding's precision is affected by additional factors like material shrinkage as plastic cools and flow behavior within the mold, which introduce variability CNC machining doesn't have to the same degree.

How do I decide between CNC machining and injection molding for a plastic part?

The decision comes down almost entirely to production volume. CNC machining has low upfront cost but higher cost per part, making it economical for prototypes and low-volume production (roughly tens to a few hundred parts). Injection molding has substantial upfront tooling cost but very low cost per part, becoming economical once volume is high enough (often several thousand parts and up) that the mold cost is offset by the per-part savings.

Why do injection-molded parts need draft angles while CNC-machined parts generally don't?

Injection molding requires a slight taper (draft angle) on vertical walls so the solidified part can be ejected cleanly from the mold — without it, the part would physically stick. CNC machining removes material directly rather than forming a part inside an enclosed cavity, so there's no equivalent ejection constraint requiring draft angles in the same way.

Engineering·9 min read·July 29, 2026

🏭 CNC Machining and Injection Molding: How Parts Actually Get Manufactured

How CNC machining removes material to create precision parts, how injection molding shapes molten plastic at high volume, the real trade-offs between the two processes in cost, tolerance, and volume, and how design decisions in CAD directly affect manufacturability with each method.

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CNC Machining: Subtractive Manufacturing at Precision Scale

CNC (computer numerical control) machining is a subtractive manufacturing process — a cutting tool removes material from a solid workpiece (metal, plastic, or other material) according to a computer-controlled toolpath, progressively cutting away everything that isn't part of the final shape until the finished part remains. This is fundamentally different from additive manufacturing (3D printing, covered elsewhere on this site), which builds a part up layer by layer rather than removing material from a solid block, and the subtractive-versus-additive distinction drives real, practical differences in what each process is actually good at.

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