"This product of mine—how many cavities should I run?"
It's the question injection molders hear most often. And it's also the question most likely to get you a biased answer. The mold shop's recommendation isn't necessarily wrong—but it's often not optimized for your best interest. Maybe it's the cavity count they're most comfortable with, or the one that fits their existing injection molding machines, or simply "what everyone else in the industry does."
Get the cavity count right, and you strike the perfect balance between tooling cost and production cost. Get it wrong, and you either waste money on the mold or lose money on every single part you produce.
This article gives you three key decision factors. After reading, you'll be able to make a rough calculation yourself.
The starting point for cavity selection is always how many parts you need to make. No compromise here.
Under 10,000 units/year: Go with 1 cavity
For low-volume runs, prototyping, or niche products, a single-cavity mold is the way to go. It's cheaper, faster to build, and easier to modify. If you build a 4-cavity mold, the tooling cost alone is 2–3 times higher—and you'll never use the full capacity. The per-part tooling amortization ends up being worse, not better.
10,000–100,000 units/year: 2–4 cavities
This is the sweet spot. The mold costs more than single-cavity (a 4-cavity mold typically runs 2.5–3× the price), but productivity quadruples, and per-part molding costs drop by over 60%.
Over 100,000 units/year: 4–8 cavities or more
At high volumes, more cavities mean lower per-part cost. Whether you should go to 16, 32, or even 64 cavities depends entirely on how large the volume is. For consumer goods like food containers or bottle caps, with annual runs in the millions, 64-cavity molds are common.
Quick Reference Table:
| Annual Volume | Recommended Cavities | Rationale |
|---|---|---|
| Under 10,000 | 1 cavity | Lowest tooling cost, maximum flexibility |
| 10,000–100,000 | 2–4 cavities | Best cost-performance trade-off |
| Over 100,000 | 4–8 cavities or more | Higher volume justifies higher cavity count |
This is the factor people overlook most often. You can't just keep adding cavities—mold size is the hard ceiling.
The larger your part, the more mold space each cavity consumes. And the overall mold size is ultimately constrained by the tie-bar spacing and platen size of your injection molding machine. Big parts like automotive bumpers or large appliance housings? You're looking at 1 or maybe 2 cavities—there's simply no room for more.
Small parts are a different story. Buttons, connectors, bottle caps, toy building blocks—these have small projected areas, and you can pack dozens or even hundreds of cavities into a single mold. Toy bricks running 1×32 or 1×64? Not unusual at all.
A quick rule of thumb: If your part weighs under 30 grams and its largest dimension is under 80 mm, you can consider multi-cavity tooling. If it weighs over 150 grams, start conservative—2 to 4 cavities is a safer bet.
A higher-cavity mold costs more. But can the extra tooling cost be recovered through production savings? That's what matters.
Here's the thing: mold cost doesn't scale linearly with cavity count. A 4-cavity mold typically costs 2.5–3× a single-cavity mold. An 8-cavity mold might run 4–5×, and a 16-cavity mold can be 8× or more. Meanwhile, per-part molding cost drops as cavities increase—4 cavities cuts per-part cost by over 60% compared to single-cavity, but 16 cavities might only save you a few more cents compared to 8.
That's diminishing returns in action. So it's not about "more cavities = always better." It's about running the full math with your production volume and tooling cost.
One more thing to watch: more cavities = more risk. With a 16-cavity mold, if just one cavity has an issue, the whole mold stops for maintenance. If the failure rate per cavity is 1%, the chance of at least one cavity failing in a 16-cavity mold jumps to about 15%. Most people don't factor this "risk cost" into their decision—but it's real.
If you're still unsure, follow this process:
Start with annual volume — this is non-negotiable
Then check part size — can you physically fit the cavities, and will it fit your press?
Run the total cost math — how much more is the mold? how much do you save in production? how long until you break even?
A good mold supplier will help you run these numbers. But the person who knows your business best—and what "good enough" really means—is you.
