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How do I work out how many cavities my mould needs?
Divide the time you need (annual demand × cycle time) by the time the press is available (productive hours × 3,600 × efficiency). The result is the minimum number of cavities, which you round up to a practical layout such as 1, 2, 4 or 8. For example, 1,200,000 parts a year on a 30-second cycle, with 5,000 press hours at 80 percent efficiency, needs 2.5 cavities, so a 4-cavity mould.
Cavity count is one of the biggest drivers of mould cost and part price. Too few cavities and the press cannot keep up with demand; too many and the mould costs more, needs a bigger machine and is harder to keep consistent.
The formula
Several moulding references publish the same calculation:
Minimum cavities = (annual demand × cycle time in seconds) ÷ (productive press hours per year × 3,600 × OEE)
- Annual demand: parts needed in a 12-month period, including spares and expected scrap.
- Cycle time: seconds per shot, from injection to the mould closing again. Early on this is an estimate; cooling usually dominates it.
- Productive press hours: Axiom Molds' guide uses about 5,000 hours a year for two shifts, five days a week, and about 7,000 hours for continuous running.
- OEE (overall equipment effectiveness): the share of planned time that produces good parts, allowing for downtime, changeovers and rejects. Published examples use 0.80 to 0.85.
Worked examples
| Input | Example A (MoldingHub) | Example B (Axiom Molds) | Example C (a smaller product) |
|---|---|---|---|
| Annual demand | 1,200,000 | 3,500,000 | 60,000 |
| Cycle time | 30 s | 12 s | 40 s |
| Press hours | 5,000 | 5,500 | 2,000 (part-time on a shared press) |
| OEE | 0.80 | 0.85 | 0.80 |
| Minimum cavities | 2.5 | 2.5 | 0.42 |
| Practical choice | 4 | 4 or 8 | 1 |
Example C is our own illustration using the same formula: 60,000 × 40 = 2,400,000 seconds needed, against 2,000 × 3,600 × 0.8 = 5,760,000 seconds available, so one cavity has plenty of spare capacity.
Why round to 2, 4 or 8
MoldingHub notes that results are rounded up to standard counts (1, 2, 4, 8, 16, 32 and so on) because symmetric layouts are easier to balance, so every cavity fills at the same time and pressure. Three or five cavities are possible but harder to fill evenly.
Checks that can cap the number
- Clamp force. Total projected area of all cavities plus the runner sets the press size. More cavities may push the mould onto a larger, dearer machine.
- Shot size. Axiom Molds suggests the total shot (parts plus runner) should fall between about 20 and 80 percent of the barrel's capacity.
- Tolerances. Each cavity is cut separately and fills slightly differently. Axiom's guide recommends smaller counts, around 4 to 8 cavities, for parts with very tight tolerances.
- Design maturity. Every design change has to be made in every cavity, so an unsettled design argues for fewer.
- Total cost. Compare mould cost plus part cost over the expected life for two or three counts, rather than choosing the count with the lowest part price.
The formula gives a starting point. The moulder's real cycle time, press availability and the toolmaker's quote decide the final count.
Questions buyers ask
What cycle time should I assume before the mould exists?
Ask the moulder for an estimate based on wall thickness and resin, since cooling time rises with the square of the wall. A thick-walled part can need far longer than a thin one of the same size.
Should I add cavities for future growth?
Sometimes it is cheaper to build a second mould later than to pay for unused cavities now, especially if demand is uncertain. Some moulds are designed so cavities can be blocked off and run fewer at a time.
Does more cavities always mean a cheaper part?
No. Press rate, mould cost and scrap all change with cavity count. Past a certain point, the extra tooling cost outweighs the saving per part.
How is cavity-to-cavity variation checked?
At sampling, parts from every cavity are measured, and each cavity is usually marked with a number on the part so problems can be traced.
Sources
- Axiom Molds, How to determine cavity count
- MoldingHub, Annual demand in injection molding
- BASF, Estimating cooling times in injection molding (PDF)