Multi-cavity injection mold economics look obvious on a spreadsheet — until fill imbalance creates dimensional spread and cosmetic rejects across cavities. Cavitation pays when the process window is wide enough to hold all cavities in correlation without constant operator tuning. Procurement sees lower piece price; quality sees four ways for a program to fail instead of one.
The buyer pain is predictable: award a 4-cavity tool to hit a target FOB, then spend the first production quarter fighting cavity 3 flash, cavity 1 short fill, and SPC charts that never stabilize. Root cause is often cavitation chosen before DFM proved the part could fill balanced — not “bad operator.”
When multi-cavity makes sense
- Stable design and frozen material grade
- Annual volume typically > ~50k–100k (part size and cycle dependent)
- Balanced fill achievable with gate and runner layout
- Press tonnage and tie-bar clearance fit projected tool size
- Quality system tracks cavity-to-cavity correlation in FAI and SPC
- Amortization math clears added tooling vs single-cavity piece price
- Cosmetic requirements allow consistent gate vestige across cavities
When to stay single-cavity longer
- Ramp uncertainty — design or volume still moving quarter to quarter
- Large part footprint limits practical cavitation on available press
- Extreme cosmetic control on small visible features — isolate variables first
- Debugging complex warp or shrink — multi-cavity masks root cause
- Low volume NPI where tooling cash is better spent on steel quality than cavity count
- First-time material or supplier — prove process in 1-cavity before multiplying risk
Fill balance: the technical gate most RFQs skip
Multi-cavity tools share one injection unit feeding multiple cavities through a runner system. If path length, diameter, or heat history differs, cavities fill at different rates. Faster cavities pack more; slower cavities short or sink.
Mold designers use runner balance, gate size tuning, and sometimes sequential valve gates to manage fill. Ask explicitly: “How will fill balance be achieved, and how will we verify it at T0?”
For tight tolerances, require dimensional study on all cavities at FAI — not cavity 1 only. Regulated and automotive-adjacent programs often mandate this; industrial OEMs should adopt the same habit before field issues teach it.
Simple amortization example (illustrative)
Suppose single-cavity piece price is $0.42 and cycle is 42 seconds. A 4-cavity tool adds $35,000 tooling but targets $0.14 per part equivalent at similar cycle per mold open (simplified — actual math includes yield and scrap).
At 80,000 parts/year, annual molding cost difference can exceed tooling premium within 18–24 months. Below 20,000 parts/year, the same math often favors single-cavity or 1+1 family layout. Run your own numbers — do not trust supplier cavitation upsell without TCO.
Family molds: economics and traps
Multiple part numbers in one tool lowers tooling cost per SKU but ties production scheduling — strong for product families launched together, weak when one SKU EOLs early and blocks plate capacity.
Family mold traps include:
- One SKU changes CAD — entire plate may stop until rework
- Volume imbalance — high runner one cavity idle while others backlog
- Quality hold on one part stops shipment of all parts in the tool
- Blank-off of unused cavities adds cost and process risk
Define family mold exit strategy before cut: blank-off plan, plate swap, or retirement path when one SKU dies.
What the quote must state
| Item | Why it matters |
|---|---|
| Cavity count and layout drawing | Validates press fit and balance assumptions |
| Runner balance approach | Explains fill risk on tight tolerances |
| Expected cycle at stated cavitation | TCO vs single-cavity alternative |
| Spare cavity component policy | Downtime if one cavity damages |
| FAI scope per cavity | Regulated programs often require all cavities |
| Hot runner drop map | Gate-to-cavity traceability |
| Press requirement (tonnage, tie-bar) | Avoid “tool does not fit” at receiving molder |
Integrate with DFM review before cavitation is locked — adding cavities later often means new plates, not a simple edit.
Production quality: cavity correlation and SPC
Multi-cavity production should tag parts by cavity during FAI and early SPC — not just lot number. If cavity 4 drifts, you need traceability to quarantine affected inventory.
OEM quality teams should define:
- Which CTQs are tracked per cavity vs pooled
- When a single bad cavity triggers plate stop
- Spare insert availability and lead time
Link to PPAP / FAI quality guide for contract language.
Staged cavitation strategy
Some programs start 1-cavity for launch, add cavity plates at volume ramp. This spreads tooling cash and reduces early correlation risk. Not every mold geometry allows plate swaps — ask during mold design if staged cavitation is feasible.
How Deuchi evaluates cavitation
We model piece price at 1-cavity vs target cavitation with realistic cycle and scrap assumptions. If correlation risk outweighs savings at your volume, we recommend staged cavitation rather than overspecifying day one.
Cavitation decisions connect to runner type and tooling cost breakdown — all three should be quoted consistently, not optimized in isolation.
Press selection and cavitation
Cavitation is limited by press tonnage, shot size, tie-bar clearance, and daylight opening. A 4-cavity tool that fits steel economics but not your largest press forces suboptimal machines or manual insert loading — erasing cycle savings.
Require mold height, width, and weight in the quote. Receiving molder should confirm fit before steel cut — not at shipment.
Quality hold scenarios
When one cavity fails CTQ, define whether you stop all cavities or run good cavities with quarantine rules. Multi-cavity without a policy defaults to chaos — shipping mixed-quality cartons or stopping entire SKU while cavity 2 insert ships from overseas.
Document hold rules in quality agreement before production release, especially for enclosure programs with field safety implications.
OEE impact of cavitation decisions
Multi-cavity tools reduce cycle-per-part when all cavities run good parts — but OEE drops if one cavity is blocked off or if correlation issues force sorting. Model not only ideal cycle but expected yield per cavity based on historical fill balance risk on similar geometry.
Automation and robotics favor consistent gate trim and predictable part ejection — multi-cavity with robot takeout needs defined cavity ID marking if downstream vision checks part variants.
Tooling investment review gate
Before approving cavitation upsell, require written comparison: 1-cavity piece price and cycle vs N-cavity at same quality assumptions. Procurement should reject cavitation increases that only reduce piece price on paper while adding correlation risk without FAI plan per cavity.
Executive summary for leadership: cavitation is a quality-capacity trade, not only a unit cost lever.
FAQ
Can we add cavities later?
Sometimes via new cavity plates — often cheaper to design target cavitation up front if volume forecast is confident. Retrofit depends on mold base size and press capacity reserved at design.
Does multi-cavity hurt tight tolerances?
It can — require FAI on all cavities and ongoing SPC on CTQs; one outlier cavity is a common production headache.
What is a family mold risk?
If one part EOLs, you may run inefficiently or pay for cavity blank-off — plan SKU lifecycle before family tooling.
Is 2-cavity always safer than 4?
Often yes for first production steel — lower balance risk, easier debug, still improves output vs 1-cavity. Step up when process proves stable.
Next step: Send volume forecast and part size for cavitation recommendation.