How Many Cavities Should a Mold Have? — The Cavity-Count Decision, Its Effect on Piece Cost, Tool Price and Risk
Last updated: 2026-09-04 · Reading time: about 6 min
Cavity count trades tool price against piece cost. Each cavity adds matched steel, electrodes, gating and cooling — an 8-cavity tool can cost several times a 1-cavity, but parts per shot multiply. Two constraints cap the count: projected area × clamping pressure must fit press tonnage; shot weight must stay within the machine’s plasticizing limit. Model cost per shot at 1/2/4/8/16 to find where added tooling stops paying back over your volume.
See also related article.
Key facts
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Parting line — The line where the two mold halves (A-side and B-side) meet. Parting line location affects: gate placement, ejector pin marks, witness lines, and cosmetic appearance. A well-designed parting line minimizes visible witness marks. See also related guide.
What drives the economics of cavity count?
Five factors move together as cavities increase. Understanding the direction of each one is the basis for the cost model.
| Factor | As Cavities Increase | Consequence |
|---|---|---|
| Tool price | Rises faster than linearly — every cavity needs matched steel, electrodes, gate and cooling | A 16-cavity tool can cost many times a single; volume must repay the gap |
| Parts per shot | Multiply exactly with cavity count | Fixed cycle, setup and labor split across more parts, cutting piece cost |
| Press tonnage | Grows with total projected area (parts + runner) | At some count you jump to a larger press; tonnage often caps cavities first |
| Shot weight | Rises with material per shot plus runner volume | Must stay inside machine shot size and plasticizing rate |
| Inter-cavity variation | More cavities mean more chances one runs slightly off | Balanced fill and matched steel get harder; cavity-to-cavity drift grows |
Cavity count. The number of identical part-forming cavities in a single mold. Each produces one part per shot. Higher counts lower piece cost by spreading cycle overhead, but raise tool price, tonnage and the difficulty of keeping cavities matched. The optimum is the count whose added tool cost is repaid within realistic demand.
The trap is optimizing piece cost alone. A 16-cavity tool gives the lowest theoretical piece price but demands a big press, larger budget and disciplined balancing. If volume is modest or the design is maturing, that overhead is wasted. A single cavity inflates piece cost because cycle overhead falls on one part. The correct count is the highest whose added tool price is repaid inside your demand horizon, with margin for balancing.
How do you pick the right cavity count?
Four steps. The method moves from volume anchor through physical ceilings to a cost model, then locks interchangeability.
- Anchor on realistic lifetime volume. Savings from more cavities only materialize if you run enough shots to amortize the bigger tool. Take committed volume plus upside, spread over the tool’s life, and use the middle case. If demand is uncertain, favor a count comfortable at the low case and add cavities later through duplicate inserts.
- Compute tonnage and shot-weight ceilings. Estimate projected area per cavity (part plus runner share), multiply by clamping pressure, add margin, and confirm an available press. Then check total shot weight against machine shot-size and plasticizing rate. Whichever limit hits first is the real cap — usually press tonnage, not tool cost.
- Model cost per shot across 1/2/4/8/16. For each count, add tool amortized over lifetime volume, cycle time per shot, material per good part including runner scrap, and labor divided across parts. The best count is the lowest total cost over your volume, not simply the lowest per-shot rate.
- Protect interchangeability. Require matched electrodes, hardened interchangeable inserts, and a runner gated to fill every cavity evenly. Designate a reference cavity and document spread across cavities on the trial report. Parts from cavity 1 and 16 must assemble identically — correct tooling before acceptance, not sorting in production.
“Cavity count is arithmetic, not preference. Model cost per shot at 1/2/4/8/16, find where added tooling stops paying back over your volume, and hold one cavity as the quality master the others match.”
— Tooling Engineer, aCatMould
Frequently asked questions
Is doubling cavities just doubling the tool price?
No — it costs more than double for the cavity work, but the base plate, clamping and mounting hardware are shared.
The increase is steeper than linear for the steel-and-electrode portion yet less than buying two complete tools.
Matched cavities are expensive because of precision, not duplication: every added cavity needs its own electrode, gate and cooling, and the runner must be balanced. That is why a 16-cavity tool is a substantially larger commitment than a 2-cavity.
Can I start low-cavity and add cavities later?
Often yes, and a sensible hedge when demand is uncertain — if the tool is designed for it from the start. Build the mold with a plate layout and runner that accept additional matched inserts, or plan a duplicate insert set to the same electrodes.
Adding cavities changes fill balance, shot weight and press requirement, so the step-up must be re-trialed, not just bolted in. Deciding the target count early lets you design the base tool to scale — far cheaper than re-cutting a tool never meant to grow.
Does a higher cavity count slow the cycle?
Usually somewhat, but total output still rises. More cavities mean more material to inject and often a larger projected area, lengthening fill and cooling. The key is that cycle grows far slower than part count: going from 4 to 8 might raise cycle by a fifth but doubles parts per shot, so cost per part still drops.
The real risks are balance and quality — a longer runner that must fill evenly and a heavier shot that taxes plasticizing. The payback model should include expected scrap and maintenance, not just the optimistic shot rate.
Key takeaways
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Whichever hits first is the real cap.
Cavity count trades tool price against piece cost: each cavity adds steel, electrodes, gating and cooling, while spreading cycle overhead across more parts. Two physical ceilings — press tonnage and shot weight — cap the count.
Model cost per shot at 1/2/4/8/16, find where added tooling stops paying back over your volume, and hold one cavity as the quality master.
Sources
- MoldMaking Technology — Tooling economics and cavity-count decisions. www.mmtech.com
- Plastics Technology — Molding process data and multi-cavity practice. www.ptonline.com
- SPE — Society of Plastics Engineers, injection molding fundamentals. www.spe.org
- HASCO — Standard mold components, multi-cavity tool hardware. www.hasco.com
- Plastics Industry Association — Industry resources on plastics processing. www.plasticsindustry.org
Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet