Mold amortization and cavity economics: the arithmetic behind tooling decisions

More cavities means a more expensive tool and a cheaper part — up to a point. How to spread mold cost honestly, and how to compare two cavitation options with numbers instead of instinct.

Updated September 1, 2026

A mold is a large payment made once, in exchange for a lower cost on every part afterwards. Everything difficult about costing it comes from one question: **how many parts do you divide it by?**

That is not a fact you look up. It is an assumption about the future, and the honest way to handle it is to write it down rather than bury it.

Choosing the denominator

Divide by	Effect	When it is right
This order only	Highest part cost; you can never lose money on the tool	Tool is specific to one customer or one order
Expected program volume	Lower part cost; you are betting on future orders	There is a committed program with a stated volume
Tool's mechanical life	Lowest part cost; the most optimistic	Rarely appropriate — the tool usually outlives the product

Whichever you choose, record the number on the sheet. Two cost sheets that spread tooling over different volumes are not comparable, and nothing else on the sheet reveals the difference. This is the same discipline the machining guide asks for around lot size, applied to a much larger number.

If the customer paid for the tool

Then it is not in your part cost at all, and including it is double charging. But settle ownership explicitly: if the customer owns the tool and can move it to another molder, you cannot amortize it across future work either — you have neither the cost nor the security.

Comparing cavitation options

More cavities costs more tool and less machine time per part. Whether that trade is worth taking depends entirely on volume, and it is straightforward to settle with numbers:

Cost the part with the low-cavitation tool: tool cost spread over volume, plus cycle divided by its parts per cycle.
Cost it again with the high-cavitation tool: a bigger tool number spread over the same volume, a cycle that is somewhat longer, divided by more parts.
Compare. The crossover volume is the answer, and it is usually not where instinct puts it.

A larger tool usually needs a larger press, and a larger press has a higher hourly rate. Leaving that out is what makes high cavitation look better than it is — check which press each option actually runs on before comparing.

What the arithmetic cannot tell you

Cycle time for a tool that does not exist yet is an estimate, and the whole comparison rests on it. Get it from your toolmaker, record that you did, and note it as an estimate on the sheet.

This guide covers how to structure the comparison and what to record. It does not tell you what cycle a given part will run at or how many cavities your part can support — those depend on the part, the resin and the press, and the people who will build the tool know them better than any general document.

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