Annual volume and break-even point
Cavity count is often decided by looking only at annual volume, but that variable alone does not define the best option. A single-cavity mold costs less to build and carries less technical risk, while a multi-cavity mold lowers unit cost in exchange for a higher initial investment and more balancing complexity.
The break-even point between the two depends on the part price, the machine hourly cost and the expected mold life. A return-on-investment analysis, not volume alone, is what really determines how many cavities make sense.
Fill balance between cavities
A multi-cavity mold only delivers consistent parts if every cavity receives the same volume of material, at the same pressure and at the same time. A cold runner that is not geometrically balanced favors the cavities closest to the sprue.
From four cavities upward, it is worth evaluating a geometrically balanced runner or a hot runner with individual valve gate control. Without balance, the far cavities show sink marks, flow lines or short shots while the near ones are already flashing.
Part geometry and complexity
Parts with thin walls, deep ribs, metal inserts or critical cosmetic areas are more sensitive to cavity-to-cavity variation. In these cases, fewer well-controlled cavities usually produce less scrap than a high-cavitation mold with tight tolerances.
Geometry also affects the ejection system: the more cavities, the more ejectors, springs and slides must be synchronized every cycle, which adds mechanical failure points to maintain.
Cycle time and machine capacity
More cavities do not always mean more productivity. If the injection machine lacks the clamping force, shot volume or plasticizing capacity, the cycle gets longer and offsets the gain in parts per shot.
Check the clamping force required by the projected area of all cavities together, not just one, before setting the final number.
Build cost and future maintenance
Each additional cavity increases machining and polishing cost and the number of moving components to maintain. An eight-cavity mold does not cost twice as much as a four-cavity one; it usually costs more, because the cooling, ejection and runner systems grow as well.
Before deciding, project the maintenance cost over five years: wear parts, downtime for service and spare part availability, not just the initial build cost.
A mold flow analysis before machining lets you check the balance between cavities and adjust the runner system without rework costs.
Checklist before defining the cavity count
☐ Confirm projected annual volume
☐ Calculate break-even per cavity
☐ Define expected mold life
☐ Validate target part price
☐ Identify critical finish areas
☐ Review thin walls and ribs
☐ Define key dimensional tolerances
☐ Locate candidate gate positions
☐ Check available clamping force
☐ Confirm plasticizing capacity
☐ Evaluate cold vs. hot runner
☐ Simulate fill balance
☐ Project wear parts
☐ Define spare parts plan
☐ Estimate service downtime
☐ Document acceptance criteria
Common questions.
Is it always cheaper to produce in a mold with more cavities?
Not necessarily. Unit cost goes down, but build cost, maintenance and the risk of defects from unbalanced filling go up. The decision should rest on a return analysis, not on part price alone.
When does a hot runner make sense?
When the cavity count is high, when the material is sensitive to degradation along long cold runners, or when eliminating sprue and runner waste is needed to cut material cost.
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