Injection moulding cost: tooling, cycle time, resin and the break-even

Injection moulding cost is mostly the tool, divided by how many parts you make, plus a small per-part cost for resin and machine time. On a few hundred parts the mould can be most of the price; on tens of thousands it shrinks to cents and the cycle time takes over. To estimate a price, cost the tool, the cycle, the resin and the setup separately, then spread the fixed parts over the batch.

What is injection moulding, and why is the price shaped like this?

Injection moulding (injection molding in the US) forces molten plastic into a steel or aluminium mould, holds it under pressure while it cools, then opens the mould and ejects the part. The machine does the same thing every few seconds for as long as you keep feeding it resin.

That is the whole economic story. The mould is designed and machined once, at real cost. After that, each part costs very little. A price for a moulded part is therefore two prices glued together: a large fixed amount for the tooling, and a small variable amount per part. Every decision about whether to mould, and how, comes down to how those two lines trade off.

The technology page for injection moulding covers the process itself. This article is about the arithmetic.

What drives the cost of the mould?

The tool is usually the largest single number on the quote, and it is often shown as a separate NRE (non-recurring engineering) line. What makes it bigger:

As a rough guide, a simple single-cavity aluminium tool for a small part might cost in the low thousands, while a multi-cavity hardened steel production tool can run to tens of thousands or more. Those are wide ranges on purpose: the only way to know a tool’s price is to have a toolmaker quote the actual part.

Tool life matters as much as tool price. An aluminium tool may be good for thousands to tens of thousands of shots; a hardened steel tool can run hundreds of thousands to a million or more. If you need 200,000 parts, the cheap tool is not cheap: you will buy it several times.

What sets the cycle time?

The cycle time is the time from one mould close to the next. It has four phases: injection, holding, cooling and ejection. Cooling is usually the longest, and it is governed by the thickest wall in the part. Cooling time grows roughly with the square of wall thickness, so a part with a 4 mm section can take far longer to cool than the same part designed at 2 mm.

This is why moulding design guides insist on uniform walls, coring out thick sections and using ribs instead of solid mass. It is not only about sink marks; it is about money per shot.

Per-part machine cost is then simple:

• Machine cost per shot = press hourly rate × cycle time in hours.
• Machine cost per part = machine cost per shot ÷ number of cavities.

A bigger press has a higher hourly rate, so part size feeds the price twice: once through the tool, once through the machine.

How do you cost the resin?

Resin is priced by weight. For each shot, take the part weight plus its share of the runner (the channels that feed the cavities), then multiply by the resin price per kilogram. A cold runner is often reground and reused for less demanding parts, but some customers and some materials forbid regrind, so check before you count it as free.

Engineering resins with glass fill, flame retardants or medical grades cost several times more than commodity polypropylene or ABS. For a small part, resin is often a minor line next to the tool. For a large, heavy part in an engineering resin it can be the biggest variable cost.

What about setup and the batch?

Every production run needs the mould hung in the press, heated, purged and dialled in, and the first parts inspected. That is a setup cost charged per run, not per part. It is small next to the tool, but on short runs it still shows up, and it is why moulders often have a minimum run size.

Worked example: amortising a tool across four batch sizes

Two things stand out. First, the unit cost falls by a factor of about 30 between 1,000 and 100,000 parts, and almost all of that fall is the tool share. Second, below a few thousand parts the tool is nearly the whole price, which is exactly where other processes start to compete. The comparison with 3D printing and the comparison with vacuum casting work through those break-evens.

Should you amortise the tool into the part price?

Tool amortisation means folding the tool into the unit price instead of invoicing it separately. Both approaches are common:

• Tool invoiced up front. The customer pays for the mould, usually owns it, and then pays a low unit price. Clear, and the moulder carries no risk if the second order never comes.
• Tool amortised over an agreed quantity. The unit price is higher until that quantity is reached. Easier for the customer’s cash flow, but the moulder carries the risk, so the agreement needs a clause covering what happens if the volume never materialises.

Whichever you choose, write down the quantity the tool is amortised over. It is the number everyone forgets, and it is the one that decides whether the job made money. The same logic of fixed cost per run versus cost per part runs through quantity pricing and batch discounts for every process.

How do you bring the cost down?

• Design for the tool. Add draft to every face that slides out of the mould, remove undercuts that need side actions, and keep walls uniform. The minimum wall thickness guide has typical ranges for moulded parts.
• Match the tool to the volume. Do not buy a hardened multi-cavity tool for a 2,000-part pilot, and do not run 500,000 parts from aluminium.
• Add cavities when the run is long. Doubling the cavities roughly halves machine time per part. It pays back only when the extra tool cost is smaller than the machine time it saves over the tool’s life.
• Prototype first. A printed or cast part catches design errors before they are cut into steel, where every change means welding, re-machining and re-sampling.

How shops quote moulding today

Moulding quotes are still built mostly by hand, because the tool needs a toolmaker’s eye on the actual part. The variable side, resin per shot, cycle time and setup, is formula work and benefits from the same structured approach as any other process: rates, quantities and a visible price breakdown.

Quotometry gives a budgetary injection moulding estimate from the uploaded file: the tool, from projected area, depth, cavities, side actions and the steel class the buyer picks, and the part, from shot weight and a cycle dominated by cooling. Every moulding line goes to the shop’s review before it is confirmed, because draft is not measured and flow is not simulated; the instant figure tells a buyer early whether moulding fits their volume (see injection moulding).

Questions people ask

• Injection moulding
• Tooling
• Tool amortisation
• Cycle time
• Mould cavity
• NRE
• Draft angle