Injection molding vs CNC machining

What injection molding is

Molten plastic is injected under pressure into a steel or aluminum tool cut with the negative of the part. The tool closes, the shot fills and cools, the tool opens, the part ejects. Once the tool exists, the cycle repeats in seconds to minutes with almost no variation between one part and the next, which is what makes molding a production process rather than a one-off process.

• The tool is the cost, not the part. A production tool runs 4 to 8 weeks to design, cut, and sample here, by tool class and complexity, and that cost has to be paid once regardless of whether the run is 500 parts or 500,000.
• Per-part cost drops hard with volume. Once the tool is paid off, a molded part is mostly material and a few seconds of machine time, which is why molding wins decisively at production volumes and loses badly on a single prototype.
• Tolerance is tool-limited, then repeatable. We hold ±0.05 mm on critical, CMM-verified features and ±0.1 mm typical, with uncalled dimensions to DIN 16742, and every part off that tool holds the same number, not just the first one.
• Material choice is resin, not alloy. ABS, PC, PC-ABS, PP, PA6, PA66, glass-filled PA, POM, PBT, PMMA, TPE, and TPU run in-house, with further grades available on request, all across a 25 to 3,000 ton press range with an 8 kg shot capacity in-house and to 15 kg through partner presses.

What CNC machining is

A rotating cutter removes material from a solid block of metal or plastic, either moving the tool against a fixed part (milling) or spinning the part against a fixed tool (turning). Nothing has to be built before the first part exists, which is the opposite trade to molding: no tooling investment, but every part costs close to what the first one cost.

• No tool, so no wait before part one. Typical lead time here is 3 to 15 working days by setup count and finish, most of it machine time and programming, not tool build.
• Tolerance can beat molding, at a price. ±0.02 mm is achievable on 5-axis work with in-process probing, ±0.05 mm is standard on 3-axis, tighter than what an as-molded part typically holds, but every tightened tolerance adds inspection time and cost per part, with no tooling amortization to spread it across.
• Material choice is real engineering alloy. Aluminum 6061-T6 and 7075-T6, stainless 304, 316L and 17-4PH, steels 1018 and 4140, brass C360, titanium Ti-6Al-4V, plus engineering plastics such as POM, PEEK, and PTFE, materials no molding resin can substitute for where a metal property is the requirement, not a preference.
• Cost scales with setups and time on the machine, not volume. A part cut in four setups on a 3-axis mill costs roughly four times the machine time of the same part in one setup on 5-axis, and that relationship holds whether it is the first part or the thousandth.

Injection molding against CNC machining

Which one, for which part

• Under a few hundred parts, or a design still changing, machining almost always wins. Paying for a production tool before a design is frozen is money spent on a shape that may not survive the next revision.
• Real metal properties, not just a metal look, is CNC machining specifically: strength-to-weight in 7075, corrosion resistance in 316L, or wear resistance in hardened 4140. There is no resin substitute for these.
• Thousands of identical parts in a thermoplastic or elastomer is molding's case, and the per-part cost gap over machining widens every time the tool runs again.
• A tolerance tighter than ±0.05 mm on a feature that also needs to be produced by the thousand usually means machining the specific feature after molding, not choosing one process for the whole part; ask at quoting stage rather than assuming either process alone covers it.

Where the crossover actually sits

The tool is the entire reason molding costs more up front and less per part later, so the crossover volume is wherever the tool cost, divided across the run, drops below the gap between a machined part's price and a molded part's price. That point moves with part size and complexity, which is exactly why we ask for annual volume and part geometry before quoting either route rather than defaulting to one. As a rule of thumb across the parts we see, the crossover for small to mid-size plastic parts sits somewhere in the low hundreds to low thousands of units; for anything smaller than that, machining's zero tooling cost usually wins outright, and for anything larger, the tool pays for itself and keeps paying.

Our status

Trumould runs both processes in-house rather than treating one as the default and the other as an outsourced afterthought: 25 to 3,000 ton presses for molding, and 3-, 4- and 5-axis milling plus turning and mill-turn for machining, on the same quoting desk. Send the part and the expected annual volume, and the quote states which process is recommended and why, not just a price for whichever one you asked about first.