Guide · Molding · 7 min read

Low-volume injection molding

Molded parts in the hundreds to low thousands, without paying for a production steel mold. The tool options, what drives the cost, when it's worth it, and a live calculator for your part.

The short answer

Low-volume molding usually means a simpler, shorter-lived tool: aluminum or soft steel, one cavity, often built into a standard mold base. For a small, simple part it commonly costs a few thousand dollars, and it's good for thousands to tens of thousands of parts.

It starts to make sense once you need more parts than casting or printing can make cheaply. Against vacuum casting that's often a few hundred parts; against desktop 3D printing, whose parts look and perform differently, it can be a thousand or more. The calculator below shows both for a part of your size.

Try it with your part's size

Set the size and drag the quantity. Watch where the aluminum mold line drops below printing and casting, and how much tooling each option needs.

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The tool options

Aluminum tools. Quicker and cheaper to machine than hardened steel, so you get parts sooner. They wear faster, especially with glass-filled or abrasive plastics, and fine cosmetic finishes are harder to hold.
Inserts in a shared mold base. Some molders keep standard frames and machine only the inserts that shape your part. You pay for the inserts, not a whole mold.
Family molds. Several different parts in one tool, such as both halves of an enclosure. Cheaper tooling overall, but the parts come out in fixed ratios and can be harder to fill evenly.
Hand-loaded inserts. Instead of a sliding action for an undercut, an operator places and removes a loose insert each shot. The tool is cheaper; each part takes longer.
3D-printed mold inserts. High-temperature resin inserts can mold tens to a few hundred parts in the real material. Useful for testing fit and material before cutting metal.
Benchtop molding machines. For very small parts, a benchtop machine and simple aluminum molds can bring short runs in house. Worth it mainly for repeat small batches.

What drives the cost at low volume

The tool, spread over few parts. At 500 parts, a $4,000 tool adds $8 to every part. At 5,000 it adds 80 cents. That's why quantity matters more than anything else.
Machining time. Most of a mold's cost is machining. Aluminum cuts several times faster than steel, so for small, simple parts an aluminum tool can cost half or less. The catch is that it wears out sooner.
Setup per run. Each production run has a setup charge. Four runs of 500 cost noticeably more than one run of 2,000.
Wall thickness. A part spends most of its cycle cooling, and cooling time grows quickly with wall thickness. Thin, even walls make every part cheaper.
Minimums. Minimum order quantities and minimum material purchases matter more when the run is small. Ask about both.

Ways to make it cheaper

Make the part smaller, or split a big part into identical smaller ones.
Thin and even out the walls, and core out thick sections.
Design out undercuts, or accept a hand-loaded insert.
Use a common material and a standard texture.
Only tolerance the dimensions that matter.
Order fewer, larger runs.

Pilotrun's Part lab ranks these levers for your own part, with what each one saves per unit.

When it isn't worth it yet

If you need under a few hundred parts, or the design will still change, a mold is usually the most expensive way to get them. Industrial printing and vacuum casting need little or no tooling, and changing a print costs nothing, while changing a mold can cost as much as a new insert.

Questions to ask a low-volume molder

What tool material, and how many shots is it guaranteed for?
Who owns the tool, and can it be moved to another molder?
What happens, and who pays, if it wears out early?
How many first samples (T1) are included, and what do changes cost?
What's the setup charge per run, the minimum order, and the minimum material buy?

What a mold costs, and why quotes for the same part can be thousands apart.