A 32-cavity mold produces parts faster, but if it takes eight years to recoup the additional investment, that speed is a liability, not an asset. Cavitation decisions balance tooling investment against per-part production cost, with the correct answer depending entirely on volume expectations, demand certainty, and time horizon. Understanding the economics of cavitation prevents both over-investment in tooling that never pays back and under-investment that creates capacity constraints.
How Cavitation Affects Production
Adding cavities multiplies parts per cycle. A single-cavity mold running 30-second cycles produces 120 parts per hour. A four-cavity mold at the same cycle time produces 480 parts per hour. At first glance, quadrupling cavities quadruples output.
Reality is more nuanced. Multi-cavity molds typically run longer cycles than single-cavity equivalents for several reasons.
Cooling time increases because the total heat load on the mold increases. Four cavities release four times the thermal energy per cycle. Unless cooling capacity scales proportionally (requiring larger cooling channels, more circuits, higher flow rates), the mold runs hotter and parts need longer to solidify.
Fill balance challenges arise because material must reach all cavities simultaneously through identical flow paths. Imbalanced filling causes some cavities to fill before others, creating quality variation and potentially flash in early-filling cavities. Achieving balance may require larger runners, which add to material usage and cooling time.
Process window narrows with more cavities. A single-cavity mold can be optimized for that one cavity. Eight cavities must all produce acceptable parts from the same process settings, and the acceptable range is the intersection of eight individual windows rather than one.
Complexity increases in all aspects: mold construction, maintenance, quality monitoring, troubleshooting. More cavities mean more potential failure points.
The practical result: doubling cavities typically delivers 1.6 to 1.9 times the output, not 2.0 times. This still provides substantial per-part cost reduction, but the advantage diminishes as cavitation increases.
Single-Cavity Economics
Single-cavity molds offer advantages that extend beyond lower initial cost.
Lower tooling investment suits applications where demand is uncertain. A single-cavity mold producing a new product can validate the market before committing to higher-cavitation production tooling. If the product fails, the tooling loss is minimized.
Faster development results from simpler mold construction. Single-cavity molds have fewer components, simpler runner systems, and less complex cooling. Build time is shorter, often by weeks.
Easier troubleshooting narrows problems to one cavity’s behavior. Multi-cavity troubleshooting must determine which cavity causes problems, then identify whether the issue is mold-specific or process-related.
Quality advantages come from not needing to balance multiple cavities. The process optimizes for one geometry without compromise.
When single-cavity makes sense:
Annual volumes under 50,000 parts, where cavitation payback takes many years.
Uncertain demand where volume forecasts have low confidence.
Developmental products where design may change before production stabilizes.
Complex geometries where multi-cavity molds face diminishing balance returns.
Bridge production where limited tooling investment buys time for demand assessment.
Multi-Cavity Economics
Multi-cavity molds reduce per-part cost at volumes where tooling amortization becomes small relative to production cost savings.
Cavitation planning typically follows geometric progressions: 2, 4, 8, 16, 32 cavities. This pattern matches standard mold and runner configurations. Odd numbers are possible but may create balancing challenges.
Cost per part reduction at each cavitation step follows a pattern:
| Cavities | Mold Cost Multiple | Parts/Cycle | Effective Cost/Part Multiple |
|---|---|---|---|
| 1 | 1.0x | 1 | 1.0x |
| 2 | 1.4-1.6x | 2 | 0.70-0.80x |
| 4 | 1.8-2.2x | 4 | 0.45-0.55x |
| 8 | 2.5-3.5x | 8 | 0.31-0.44x |
| 16 | 4.0-6.0x | 16 | 0.25-0.38x |
These multiples are approximate; actual costs depend on part geometry, runner system, and mold construction requirements.
Diminishing returns set in at higher cavitation. Going from 1 to 2 cavities might cut per-part cost by 25 percent. Going from 16 to 32 cavities might cut per-part cost by only 10 percent. Each doubling requires proportionally more mold investment while delivering smaller incremental savings.
Machine requirements increase with cavitation. A 16-cavity mold may require a larger tonnage machine than a 4-cavity version of the same part, increasing machine hour rates. Shot size requirements increase proportionally with cavities plus runner volume.
Family Molds
Family molds produce multiple different parts in a single mold, typically components that assemble together. A container and lid, or a set of related components, can be molded simultaneously.
Advantages:
Reduced total tooling cost compared to separate molds for each part.
Simultaneous production ensures component availability matches.
Reduced setups when both parts are always needed together.
Challenges:
Fill balance is extremely difficult when parts have different geometries. A large part and small part in the same mold create significant imbalance.
When one cavity has problems, production of all parts stops. If the lid cavity develops a crack, body production also halts even though that cavity is fine.
Process optimization for different geometries requires compromise. Thick and thin parts in one mold cannot both have optimal cooling parameters.
Volume matching assumes parts are always needed in fixed ratios. If container demand exceeds lid demand, family molds create excess lids.
Family molds make sense when parts are always used together in fixed ratios, geometries are similar enough for reasonable balance, and volume doesn’t justify separate optimized molds.
The Calculation Framework
A quantitative approach compares cavitation options objectively.
Variables needed:
Annual volume forecast (parts/year)
Single-cavity mold cost (baseline)
Multi-cavity mold cost at each option (2, 4, 8…)
Machine hour rate for appropriate machine sizes
Cycle time at each cavitation level
Expected production life (years)
Payback period calculation:
Additional investment = Multi-cavity cost minus Single-cavity cost
Annual savings = (Single-cavity cost per part minus Multi-cavity cost per part) × Annual volume
Payback years = Additional investment ÷ Annual savings
Example:
Single-cavity mold: $50,000, cycle time 30 seconds, 120 parts/hour at $75/hour machine rate = $0.625/part.
Four-cavity mold: $95,000, cycle time 36 seconds (10 percent longer), 400 parts/hour at $90/hour machine rate = $0.225/part.
At 200,000 annual volume:
Additional investment: $45,000
Annual savings: ($0.625 – $0.225) × 200,000 = $80,000
Payback: 45,000 ÷ 80,000 = 0.56 years (about 7 months)
At 50,000 annual volume:
Annual savings: $0.40 × 50,000 = $20,000
Payback: 45,000 ÷ 20,000 = 2.25 years
Sensitivity analysis addresses volume uncertainty. If forecasted volume is 200,000 but actual could be anywhere from 100,000 to 300,000, calculate payback at each scenario:
At 100,000: Payback = 1.1 years
At 200,000: Payback = 0.56 years
At 300,000: Payback = 0.37 years
If payback is acceptable even at lower scenarios, higher cavitation is defensible.
Beyond Pure Economics
Some factors override pure payback calculations. Financial analysis provides a starting point, but operational realities often determine final decisions.
Capacity constraints force cavitation decisions. If machines are fully scheduled and demand exceeds capacity, higher-cavitation molds that produce more parts per machine hour may be necessary regardless of payback. The alternative is declining orders or adding machines.
Lead time requirements may require capacity that only multi-cavity molds provide. A customer requiring 1,000 parts daily needs production capability to match, whatever the economics.
Quality considerations sometimes favor lower cavitation. Critical medical or optical parts may achieve tighter tolerances with single-cavity molds optimized for one geometry. The quality value may exceed cost efficiency value.
Flexibility value argues for lower cavitation when product changes are expected. A single-cavity mold of a product that will evolve costs less to obsolete than a 16-cavity mold made obsolete by the same change.
Risk distribution suggests starting lower. Beginning with a 4-cavity mold allows adding another 4-cavity mold if demand materializes, distributing investment over time and providing redundancy. Starting with an 8-cavity mold concentrates risk. This staged approach also provides operational flexibility: two 4-cavity molds can run different colors simultaneously, accommodate maintenance without full production stoppage, and adapt to demand fluctuations more gracefully than a single high-cavitation tool.
Prototype to production progression follows a natural path. Many successful programs begin with single-cavity prototype tooling, move to 2 or 4 cavity production tooling after design validation, then invest in higher cavitation only after demand proves stable. Each stage provides learning that improves subsequent decisions. Skipping directly to high cavitation assumes knowledge about demand, design stability, and process requirements that may not exist.
Supplier relationships influence cavitation strategy. A molder with excess capacity may prefer lower-cavitation molds that keep machines running longer. A capacity-constrained molder may push for higher cavitation to free machine time. Understanding supplier incentives helps evaluate their cavitation recommendations.
Cavitation decisions depend on confidence in volume forecasts. Conservative approaches preserve flexibility; aggressive approaches optimize cost if volumes materialize. The right answer balances financial analysis against strategic considerations, choosing cavitation that makes sense across the realistic range of demand scenarios.
Sources
- Rees, Herbert. “Mold Engineering.” Hanser, 2002.
- Beaumont, John. “Runner and Gating Design Handbook.” Hanser, 2004.
- Kazmer, David O. “Injection Mold Design Engineering.” Hanser, 2007.
- Plastics Technology. “Mold Cavitation Economics.” https://www.ptonline.com/
- Society of Plastics Engineers. “Tooling Investment Analysis.”