3D Printing vs Injection Moulding

These two processes are not really competitors. They answer different questions. Moulding asks "how cheap can the ten-thousandth part be?" Printing asks "how quickly and cheaply can I have this one?"

The shape of each cost curve

Injection moulding melts plastic and rams it into a steel or aluminium tool. The tool is the whole story. Cutting one is a precision machining job that takes weeks and costs somewhere from low thousands for a simple aluminium tool to tens of thousands for a multi-cavity hardened steel one. Once it exists, each part takes seconds and costs pennies.

So the curve is: enormous fixed cost, near-zero marginal cost.

FDM printing has no tool. The fixed cost is essentially the time to check and prepare the file. Each part then takes hours of machine time and a few pounds of plastic, every single time. Printing the hundredth part costs about what the first one did, minus a modest quantity discount for setup being amortised and the bed being packed efficiently.

So the curve is: near-zero fixed cost, meaningful marginal cost.

Two straight lines with very different slopes cross at exactly one point.

Where the crossover sits

For a typical small functional part — say something that fits in your hand, in PETG — the honest answer is usually somewhere in the hundreds to low thousands of units.

That range is wide because the crossover moves with:

• Part size. Print cost scales with volume of plastic and hours on the machine. A big part shifts the crossover down, sometimes to the low hundreds, because each print is expensive. Moulding a big part also needs a bigger tool, which pushes back the other way.
• Tool complexity. A part with undercuts, side actions or threads needs a more complicated tool. Every complication adds thousands and moves the crossover up — potentially well past a thousand units.
• How settled the design is. This is the one people underestimate. A tool change is a machining operation on hardened steel. If you are not certain the design is final, the true cost of moulding includes the tool you will scrap.
• Time. Weeks of lead time for a tool versus days for prints. If you need parts this month, the crossover is irrelevant.

The rough decision

• 1 to 50 parts — print. Not close.
• 50 to 500 — print, almost always, unless the part is tiny and the design is frozen. This is where small-batch printing lives, and small batch costs covers how quantity moves the number.
• 500 to a few thousand — genuinely depends. Get a tooling quote and a print quote and compare. Sometimes the answer is "print the first 500 while the tool is cut".
• Above a few thousand — mould. We will say so.

The bridge nobody mentions

Printing and moulding are not either/or in time. The normal sequence is: print prototypes until the design stops changing, print the first production batch to prove demand and find the problems, then commit to a tool once you know the part is right. Every design change caught by a printed part is a change you did not pay for in steel.

That is what prototype printing is for, and it is the cheapest insurance in manufacturing.

Where printing is the wrong process

• High volume, settled design, simple geometry. Mould it. Our unit cost will never beat a tool at scale, and pretending otherwise would waste your money.
• You need moulding's surface finish or material properties. Moulded parts are fully dense and isotropic; printed parts have layer lines and a weaker cross-layer direction. See how strong are 3D printed parts.
• You need a specific engineering polymer or an approved food-contact grade. The moulding world has a far wider material shelf than filament does.
• Very thin-walled, very high-count consumables. Lids, caps, closures. Not our fight.

Working it out for your part

Send the geometry and the quantity. We will price the print run properly — material, time, size and quantity all move it, as set out in how pricing works — and if the number says "go and get a tool quote", that is what we will tell you.

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