Injection molds offer a tradeoff between the up-front cost of a mold and the per-piece cost of production. If you're making five parts, injection molding isn't the right choice, because you'll pay the cost of the mold and only be able to amortize it over five parts - much more expensive than, say, 3D printing. If you're still iterating the design, it's too early to injection mold it.

But what if you need 10,000 parts? 100,000 parts? Quickly, injection molding becomes vastly lower cost, because the incremental cost of each part is very low and you're spreading the up-front cost of the mold over many more parts.

So what does a mold cost? The answer is, as always, "it depends." But there are some rough guidelines to follow.

Part Size

The bigger the part, the bigger the mold. Molds have to be machined out of high-strength steel, and if your part is large, that means there's going to be a lot of machining - which drives up both the raw material cost (steel) and the cost of transforming it (machining).

What does "big" mean? It includes all three dimensions of the part size: width, length, and height. Wider, longer, or taller: more expense.

A "small" part is typically 3-4" in width and length, and similar in height, or smaller. A "medium" sized part may increase to 6x6x6". And beyond that, the part may be considered "large." Large can get pretty big: it is possible to make objects as large as a pallet (40x48"), but these require very large machines.

A single-cavity (see below) mold for a "small" part is typically $10-20,000. A medium-sized part will typically be $15-40k. And a large part may run $30k and up.

This all assumes an uncomplicated mold design that does not require moving parts. Good part design helps ensure the mold is uncomplicated. If the mold must have moving parts to allow the part to eject, costs begin increasing.

Cavitation

Cavitation throws a wrench in our math. If you're producing a large number of parts, you can make a tradeoff: a higher-output mold that costs more to build, but will reduce your part cost. How? By producing more than one part in every cycle.

A two-cavitation mold will produce two parts per cycle. This doubles your output. It reduces your part cost because there is a fixed cost to running an injection molding machine, and doubling the output allows the manufacturer to spread that cost over a greater number of parts per hour. (But it does not change the cost of material per part.)

Adding cavitation adds mold expense, because the mold is bigger and it also becomes more complex. A multi-cavity tool requires a distribution system to allow molten plastic to flow to each of the cavities so they can fill at the same time.

Multi-cavity molds usually cost at least $30-40k, and from there can range up to $100-200k depending on the size of each part and the number of cavities.

Doing the math

So should you go multi-cavity or not? If you're still fairly early in the scaling process, it often makes sense to not build a multi-cavity tool. But if you already have steady demand for your product, the math can become easier to justify a bigger mold.

Suppose you have a part with the following characteristics: