Injection Molding vs CNC Machining for Plastic Parts: Where the Math Turns

You need 500 ABS covers, 90 × 60 × 18 mm, with four bosses and a snap feature. The molding quote comes back at $9,400 for the tool plus $0.44 per part. The machining quote is $11.80 per part with a $180 programming charge. At 500 pieces, molding lands at $19.24 each and machining at $12.16 — so machining wins. At 2,000 pieces the numbers flip hard, and molding comes in at $5.14 against $11.70.

The crossover sits somewhere around 800 pieces for this part. That number is not a universal constant. It moves with part size, wall thickness, tool complexity, cosmetic requirements and how likely the design is to change. But the shape of the curve is always the same, and once you can estimate the crossover for your own part you stop guessing.

Two curves, one decision

Molding front-loads cost. You pay for a tool before you get a single part, and the tool price does not change whether you order 500 parts or 500,000. After that, the per-part price is small: material, machine time of a few seconds, and a share of the scrap rate.

Machining spreads cost evenly. There is a small programming and fixture charge, and then every part carries its own machine time. There is no volume discount beyond a few percent, because the thousandth part takes exactly as long as the first.

Plot those two on the same axes and they cross once. To the left of the crossing point, machining is cheaper. To the right, molding is. The whole exercise is finding where your part's crossing point sits, and then deciding how much confidence you have in the volume forecast.

What the tool actually costs

Tooling price is driven by four things, in roughly this order:

Cavity count. A single-cavity tool is the baseline. Two cavities roughly add 60% to the tool cost; eight cavities can quadruple it. Cavity count is chosen from annual volume and the press size available, not from the size of your first order.

Tool steel versus aluminum. An aluminum tool for a simple part might cost $2,500 to $8,000 and last 500 to 5,000 shots. A hardened steel tool runs $12,000 to $60,000 and lasts 100,000 to a million or more. For 5,000 parts a year over three years, aluminum is often the rational choice even though it wears faster.

Part complexity. Every undercut needs a side action or a lifter. Every tight cosmetic requirement needs more polishing. Every deep rib needs a thinner, more fragile core insert. A tool with two side actions can cost twice what the same envelope costs without them.

Tolerance and finish. A textured cosmetic surface adds etching and polishing cost. A part with a ±0.05 mm tolerance on a dimension across the parting line will need tool trials and possibly a tool change, because molded tolerance is a function of shrinkage, which varies with wall thickness and flow direction.

Running the numbers for your part

The table assumes a $9,400 single-cavity tool, a $0.44 molded part, $11.80 machined part and $180 programming charge. Those are realistic figures for a hand-sized ABS cover at a Chinese contract manufacturer.

Why machining keeps winning past the crossover

Volume is not the only variable, and buyers who treat it as the only variable get burned.

Design churn. A tool change costs $800 to $4,000 and takes one to three weeks. A CNC program change costs nothing and takes an afternoon. If your product is still being validated, or if you expect a revision after field trials, machining absorbs that risk at almost no cost. Many teams machine the first 200 units, run pilots, then cut steel.

Bridge volume. If the tool takes five weeks and your launch is in three, machining is the only path. It is common to machine the launch batch and mold the production batch, accepting a higher unit cost for the first few hundred.

Large parts. Above roughly 400 mm in the longest dimension, tooling cost and press availability change the economics. A 600 mm machined HDPE panel may be cheaper than a tool that needs a 1,000 tonne press.

Tight tolerances. Molding holds ±0.1 mm on small dimensions comfortably and ±0.3 mm on large ones, and those numbers move with shrinkage variation. Machining holds ±0.05 mm without drama. If a bearing bore or a sealing groove is in the part, it may need a machining operation after molding regardless.

Low-volume, high-mix. If you sell twelve variants of a housing at 80 units each, twelve tools is a different proposition from one machining program with a parameter change.

Where the design rules differ

Molding requires draft, typically 1° to 2° on vertical walls. It requires uniform wall thickness, because thick sections sink and thin sections short-shot. It cannot produce a true undercut without a side action. It handles ribs efficiently — a 0.6 mm rib is normal in ABS — and it puts a gate mark somewhere on the part, which matters if the surface is cosmetic.

Machining removes all of those constraints but adds its own. Internal corners need a cutter radius, so a molded pocket with a 0.5 mm corner becomes a 1.5 mm radius unless you accept a slower small-tool operation. Deep pockets are limited by tool reach. And material cost per part is much higher, because you buy a solid block and ship a fraction of it.

A part designed for molding usually machines fine as a prototype, provided you accept the draft and the rib thickness. A part designed for machining often molds badly, because it has sharp internal corners, non-uniform walls and features that would need side actions.

Material behaviour is not identical

A machined ABS part and a molded ABS part are not the same material in the same condition. Machining cuts through the skin and leaves tool marks that can act as stress concentrators. Molded parts have flow orientation, weld lines and a skin that is denser than the core, which changes impact strength and dimensional stability.

For glass-filled grades, machining exposes filler on the cut surface, which affects chemical resistance and appearance. For PC and acrylic, machining can leave internal stress that shows up as crazing after solvent exposure.

If the part must pass a drop test or a chemical compatibility test, validate the machined prototype knowing it may perform differently from the molded production part. That is a reason to mold early if those tests are gating.

Frequently asked questions

What is the typical break-even volume?

For a hand-sized plastic part with a simple single-cavity tool, the crossover is usually between 500 and 1,500 pieces. Complex parts with side actions or cosmetic requirements push it to 2,000 to 5,000. Very simple parts in aluminum tools can break even under 400.

Can I machine a part designed for injection molding?

Yes, usually with minor changes. Keep the draft, keep the wall thickness, and accept that sharp molded corners become radiused machined corners. The prototype will look close but not identical to the production part.

How long does each route take?

Machining typically delivers in 5 to 10 working days from approved drawings. A new mold takes 3 to 6 weeks for a simple tool and 8 to 12 weeks for a complex one, plus trial shots and any correction loops.

Is low-volume molding worth it with an aluminum tool?

Often yes, if the geometry is stable and the volume is above 500. Aluminum tools are cheaper and faster to make, but they wear and they transfer heat differently, so cycle times are longer and part quality drifts after a few thousand shots.