Last updated: 2026-09-04 · 6 min read

At What Volume Does Adding Cavities Pay Off? — The Cavity Break-Even Calculation

Every added cavity raises the tool price and lowers the per-part cost, so the real question is not “how many cavities can we afford” but “at what annual volume does the cheaper part pay back the dearer mold.” Cavitation multiplies output per shot: a 4-cavity tool makes four parts in roughly the time one makes a single, so cycle, machine rate and labor amortize across more parts.

Key Facts

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  • The break-even is simple arithmetic: the extra tool cost divided by the per-part saving — that quotient is how many parts the higher-cavity tool must produce before it has out-earned the cheaper tool despite costing more up front.
  • More cavities demand a larger press for clamp tonnage and shot size, so the headline per-part saving can be partly eaten by a higher machine hourly rate or the need for a costlier molder.
  • The crossover is not only an economics number — it is a confidence number, and it moves toward fewer cavities the less certain you are about final geometry, resin and true lifetime volume.

    • Warpage — Part deformation after ejection caused by uneven shrinkage. Amorphous resins warp less (0.4–0.7% shrinkage). Semi-crystalline resins warp more (1.0–2.5% shrinkage). Fix: uniform wall thickness, balanced cooling, optimized gate location, adequate packing pressure. The related guide covers adjacent topics.

    Which Costs Move the Break-Even?

    Five cost terms, each pulling the crossover differently:

    Cost Term Few Cavities Many Cavities
    Tool price Low; less steel and machining High; balancing and bigger mold base
    Part per shot One; highest unit variable cost Many; cycle amortized across parts
    Machine required Smaller, cheaper press Larger tonnage, higher hourly rate
    Tool risk Inexpensive to fix or redo Costly if one cavity runs off
    Break-even volume Wins below the crossover Wins above the crossover

    The break-even is driven by the gap between the two tool prices divided by the per-part saving the bigger tool buys. But the table’s rows interact in ways a naive spreadsheet misses. More cavities demand a larger press to generate the clamp tonnage for the doubled projected area and to plasticize the bigger shot, so the headline per-part saving can be partly eaten by a higher machine hourly rate.

    Cavities also raise tool risk: a 16-cavity tool that runs one cavity slightly out of balance throws away one-sixteenth of every shot, and re-working a hardened multi-cavity mold is far more costly than re-cutting a simple single-cavity one. An immature design cavitated too aggressively turns the tool itself into scrap. The crossover is therefore not only economics — it is also a confidence number, and it moves down toward fewer cavities the less certain you are about final geometry, color, resin and true lifetime volume.

    How Do You Calculate the Break-Even?

    Four rules, in order:

    1. Fix the tool-price delta and the per-part saving. Get firm quoted prices for each cavitation option, and compute the all-in per-part cost for each — material, cycle time times machine rate, labor and scrap allowance. The difference in tool price is what you must pay back; the difference in per-part cost is the rate at which you pay it back. Keep these as real quoted figures bound to specific mold builds, not estimates.
    2. Divide to find the crossover quantity. The break-even volume is the extra tool cost divided by the per-part saving: tool-price delta over unit-cost delta. That quotient is how many parts the high-cavity tool must produce before it out-earns the low-cavity tool. Compute it for each step up — one to four, four to eight, eight to sixteen — because the crossover changes at every jump. Plotting total cost against cavitation makes the winning choice at your actual quantity obvious.
    3. Test the crossover against a volume you trust. Compare the break-even not to the best-case launch forecast but to a realistic lifetime quantity you would stand behind. If the crossover sits above what you honestly expect to ship, the multi-cavity tool never pays back and the cheaper tool wins the whole campaign. A common mistake is cavitating for a forecast that never materializes. When volume is uncertain, buy cavities you can add later or run a lower-cavity tool at slightly higher unit cost.
    4. Price in machine fit and tool risk. Confirm a press that can hold the clamp tonnage and shot size is available at your molding rate. A cavitation choice that pencils on paper but needs a machine nobody has free is not a real option. Where those risks bite, shade the decision toward fewer cavities than pure arithmetic suggests — the break-even is a floor, and engineering confidence tells you how far above it to stay.

    Runner system — The channel network that delivers melt from the machine nozzle to each cavity. Cold runner: solidifies between shots, produces runner waste. Hot runner: stays molten, eliminates runner waste but adds $15k–$40k to mold cost. The related guide covers adjacent topics.

    The break-even volume is a floor for the economics case. Engineering confidence — in geometry, resin, machine fit and lifetime demand — is what tells you how far above the floor to stay.

  • Key parameter identified — critical variable confirmed against spec.
  • Tolerance within achievable range — commercial ±0.15mm or tighter as needed.
  • Material matched to application — resin grade selected for end-use environment.
  • Process window validated at T1 — parameters locked after first trial.
  • Inspection method defined — CMM, visual, or functional test specified.

    • Frequently Asked Questions

    Is a higher-cavity tool always cheaper per part?

    Per part, almost always — that is the point of cavitation. But “cheapest per part” is not the same as “cheapest overall.” A 16-cavity tool spreads cycle and machine time over sixteen parts, yet the tool cost far more and needs a bigger press. If your true volume never climbs past the break-even, those savings never repay the extra tooling.

    The correct comparison is total landed cost across the whole expected quantity — tool amortization plus per-part cost together — not the headline per-part figure alone.

    Should I just buy the most cavities I can afford?

    No — affordability is the wrong test; break-even volume is the right one. Buying the biggest tool you can fund optimizes for a per-part cost you may never produce enough of to matter, while maximizing the steel sunk into a design that might still change.

    Cavitation should be pulled upward only by real, trusted annual volume crossing the next break-even, and pushed downward by uncertainty about geometry, resin, color or lifetime demand. A disciplined approach sometimes buys a lower-cavity tool on a young program and adds cavities once volume is proven.

    What breaks the simple break-even math?

    Three things the textbook division ignores: machine availability, scrap concentration and design churn. A multi-cavity tool can require tonnage your usual molder cannot supply, so model the real hourly rate. More cavities concentrate risk because one out-of-balance cavity wastes a larger share of every shot and is costlier to fix in hardened steel.

    And any cavity bought on geometry that later changes is partly thrown away, which is why immature designs argue for fewer cavities regardless of an attractive forecast. The arithmetic gives you the crossover; honesty about volume and design maturity tells you how conservatively to sit below it.

    Takeaways

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  • The break-even is tool-price delta divided by per-part saving — simple arithmetic, but only as honest as the quoted numbers you feed in.
  • More cavities raise tool risk and demand a bigger press, so the crossover moves toward fewer cavities the less certain you are about volume and design.
  • Compare total landed cost across the whole expected quantity, not headline per-part cost — and test the crossover against a volume you would stand behind.

    • Cavitation is an economics decision wrapped in a confidence decision.
    Do the arithmetic with real quotes, then ask whether you trust the volume and the design enough to stand above the break-even — if not, fewer cavities and the option to add later is the cheaper path overall.

    Sources

    1. Society of Plastics Engineers — Technical Papers
    2. Plastics Technology — Processing Tips
    3. HASCO — Mold Base Standards
    4. DME — Mold Components and Bases
    5. ENGEL — Injection Molding Technology

    Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet