Multi-Cavity vs Single-Cavity Molds: Economics at Different Volumes

A multi-cavity mold pays off when the annual volume is high enough, and certain enough, that the lower machine cost per part repays the extra tooling cost within the period the buyer will accept. Below that volume, or when demand and design are still uncertain, a single-cavity or low-cavitation mold is the cheaper and safer choice. The break-even volume is calculated from quoted tool prices, measured or estimated cycle times, and machine rates for the press each mold needs, not from a rule-of-thumb volume threshold.

What Changes When Cavities Are Added

Parts per cycle rise with the cavity count, but the other inputs to cost move too:

• Cycle time. Each added cavity adds melt, and the heat it brings has to leave through the mold’s cooling circuits. If cooling capacity does not grow with the cavity count, the cycle gets longer.
• Fill balance. Every cavity must fill and pack under the same process settings. When the runner does not deliver melt evenly, early-filling cavities can flash or overpack while late ones run short, and cavity-to-cavity dimensional variation appears.
• Process window. The acceptable settings for an eight-cavity mold are the overlap of eight individual windows, which is narrower than the window for any single cavity.
• Machine size. Projected area and shot volume grow with cavities, which can move the mold onto a larger press with a higher hour rate. The tonnage side of that is covered in matching clamp force to part requirements.
• Availability. When one cavity is damaged, a four-cavity mold may be able to run with that cavity blocked off at up to three-quarters of its output, while a single-cavity mold stops. Whether it can depends on the runner or hot-runner design, fill balance, ejection, and re-qualifying the process with the cavity blocked, which the moldmaker and the customer’s approval rules decide.

Output therefore rises by less than the cavity multiple whenever the cycle lengthens. For example, a single-cavity mold at 30 seconds makes 120 parts per hour. A four-cavity mold at 36 seconds makes 3,600 ÷ 36 × 4 = 400 parts per hour, not 480. The four-cavity cycle in that example is 20% longer (36 ÷ 30 − 1), and output is 3.33 times the single-cavity rate rather than 4 times.

The Basic Payback Calculation

For example, assume these quoted and estimated inputs (illustrative figures, not market prices):

1. Extra investment: $95,000 − $50,000 = $45,000.

1. Machine cost saving per part: $0.625 − $0.225 = $0.40.
2. Annual saving: $0.40 × annual volume.
3. Payback: $45,000 ÷ annual saving.

Turned around, the volume that pays back the extra $45,000 in exactly one year is 45,000 ÷ 0.40 = 112,500 parts. That figure, computed from the program’s own quotes, is the useful threshold; a general rule such as “under 50,000 parts, stay single-cavity” ignores the inputs that actually decide it.

Adding the Costs the Basic Model Leaves Out

Machine time is only one cost that changes with cavitation. A fuller per-part comparison adds:

• Yield. Scrap from cavity imbalance or a narrower window raises the cost of every good part. Divide machine and material cost by the expected yield.
• Material and runner. With a cold runner, runner weight per part changes with layout; if regrind is not allowed, the runner is waste.
• Tool maintenance. More cavities mean more inserts, gates, and ejector sets to maintain.
• Inspection. Cavity-to-cavity variation means sampling per cavity, which adds measurement time.
• Setup frequency. A higher-output mold runs shorter campaigns for the same demand, so setup cost is spread over fewer hours.

For example, keep the figures above, add $0.30 of material per part, and assume a 99% yield for the single-cavity mold and 97% for the four-cavity mold:

1. Single-cavity cost per good part: ($0.625 + $0.30) ÷ 0.99 = about $0.934.
2. Four-cavity cost per good part: ($0.225 + $0.30) ÷ 0.97 = about $0.541.
3. Saving per good part: about $0.393.
4. One-year payback volume: 45,000 ÷ 0.393 = about 114,500 parts.

The two-point yield difference barely moves the answer in this case, because the machine-time saving dominates. It would move it much more on a part with expensive material, which is exactly the case where the fuller model earns its effort. The cost-per-part build-up is worked through in detail in calculating per-part cost in injection molding, and the drivers of the tool quotes themselves in injection mold tooling costs.

Testing the Forecast, Not Just the Arithmetic

Payback is only as good as the volume forecast. Run the calculation at the low, expected, and high volumes the business can defend, as in the payback table above. If the extra investment repays within the acceptable period even at the low case, the higher cavitation is defensible. If it repays only at the high case, the decision is a bet on the forecast.

The cycle time used for each option should also be tested. If the four-cavity cycle in the example were 40 seconds instead of 36, output would fall to 360 parts per hour and the machine cost per part would rise to $0.25, cutting the saving from $0.40 to $0.375 per part. The cycle time guide explains why the thickest section, not the cavity count, sets the cooling floor.

Family Molds

A family mold makes different parts in one tool, such as a container and its lid. It reduces the number of tools, and the parts come off the press in the ratio they are assembled. It also ties the parts together: when one cavity needs repair, both stop, and when demand for one part rises faster than the other, the mold makes surplus of the slower part. Parts of different size and wall thickness are hard to fill and cool evenly from one set of process settings. A family mold fits best when the parts are always used together in a fixed ratio and are similar in size and wall thickness.

Decisions the Payback Number Does Not Settle

• Capacity. If presses are fully booked and demand exceeds capacity, more parts per machine hour may be needed whatever the payback.
• Design stability. A product still changing can make a high-cavitation mold obsolete; the cost of that risk rises with the tool price.
• Staging. Two four-cavity molds instead of one eight-cavity mold spread the investment, allow two colors or two presses at once, and keep half the output during maintenance, at a higher combined tool cost.
• Supplier incentives. A molder with idle presses gains from lower cavitation that keeps machines running; a capacity-constrained molder gains from higher cavitation. Knowing which applies helps a buyer read a cavitation recommendation.

A cavitation choice holds up when it still repays its extra tooling within the accepted period at the low-case volume, using cost per good part built from real quotes. If it pays back only at the high end of the forecast, it is a bet on demand rather than a tooling decision, and capacity, design stability, and staging should carry more weight.