The number that decides everything
Mould cost is close to proportional to the number of cavities. That is the whole commercial argument for multi-cavity tooling — spread the mould cost over more parts and the cost per part falls. It is also the whole trap. Past a certain point, adding a cavity does not reduce unit cost; it starts degrading the part, or simply will not fill, and you have paid for the privilege.
The uncomfortable truth
Most multi-cavity moulds in the market are cavity-counted by the quote, not calculated. A quote for "8 cavities" tells you nothing about whether 8 was the right number or whether the moulder simply had that tool available. Ask for the calculation. A supplier who cannot show it is guessing, and you are paying for the guess.
Four limits decide how many cavities you can actually run
Cavity count is not a free choice. It is bounded by whichever of these four constraints binds first, and on a real part it is usually fill time or pressure drop — not clamp force, whatever the quote implies.
1. Fill time — the usual real limit
All cavities must fill completely within the mould's cooling-critical window, before the gate freezes. The calculation that decides it:
Run a worked example. A 250 g part, 4 cavities, 30 g/s effective fill rate on a 300-ton machine:
- Total shot = 250 g × 4 = 1,000 g ≈ 1,050 cm³ for a 1.05 g/cm³ resin
- Fill time = 1,050 ÷ 30 = 35 seconds
- Cooling time for this part is roughly 22 s, so available fill time ≈ 7–11 s
- Result: it does not fill. Short shots on the last cavity, every shot.
The machine is not too small in tonnage terms — it has plenty of clamp force to spare. It is the plasticising capacity that is short. Fill time is checked against cycle time, not against machine tonnage, and this is the mistake that gets a multi-cavity tool quoted and then rejected after the first trial.
The most common multi-cavity error
A 4-cavity tool quoted on a 300-ton machine that needs 40 seconds to fill will never work, and no amount of injection pressure fixes it. The screw cannot deliver plastic fast enough. If you are told otherwise, ask for the fill-time calculation.
2. Clamp force — the limit everyone quotes, rarely the binding one
Total projected area must stay below roughly 70–80% of rated clamping force, to leave holding force margin against pressure spikes. Note what this calculation actually does: it grows with projected area per cavity, so doubling cavities doubles the clamp requirement. Doubling cavities is not free on the machine — it requires a proportionally larger machine or a lower clamp safety factor.
3. Pressure drop — the limit that shapes the geometry
Each additional cavity adds a runner. The gate has to feed all of them, and splitting the flow front between cavities requires a balanced runner. Past a certain cavity count the runner itself becomes significant volume — you are injecting a large slug that becomes reclaim, and the pressure to drive it is higher than the pressure the part actually needs.
| Part weight class | Typical cavity range | What binds first | Runner style |
|---|---|---|---|
| Under 20 g | 4–16 | Cycle time — too few cavities makes the cycle uneconomic | Hot runner, multi-drop |
| 20–100 g | 2–8 | Fill time | Valve-gated hot runner |
| 100–500 g | 1–4 | Fill time and pressure drop | Hot runner, or balanced cold runner with sprue |
| Above 500 g | 1–2 | Clamp force and machine capacity | Direct gate, no runner |
Above roughly 500 g per cavity, cavity count stops being a cost question at all — the machine cannot hold the projected area, and the answer is 1 or 2.
4. Cooling — the constraint that sets cycle time, and therefore economics
Cavities are not thermally independent in a simple way. A tightly packed multi-cavity tool has less steel between cavities, so heat transfers between them and the effective cooling area per cavity shrinks. The practical effect: eight cavities in the same mould do not cool twice as fast as four. The cooling benefit of adding cavities flattens well before the cost benefit does.
This is why a hot runner, despite its higher tool cost, often wins on total cost for high-volume multi-cavity work: no runners to cool, no sprue to remove, and uniform cavity pressure from cycle to cycle.
How to calculate the cavity count yourself
Work in this order. Each step can reduce the number you were hoping for — that is the point.
Max cavities = (Fill rate × Available fill time) ÷ Shot volume
Max cavities = (Rated clamp × 0.75) ÷ Projected area per cavity
Cost per part = (Mould cost ÷ expected lifetime shots) + (Cycle seconds ÷ 3600 × hourly rate)
Step 4 is where cavity count actually gets decided, and it has nothing to do with physics. If the hourly machine rate is high, extra cavities only pay off if cycle time genuinely falls. If cycle time does not fall much because cooling dominates, you have bought a more expensive mould for a marginal gain — and that is a bad trade.
Runner balancing, briefly but properly
Unbalanced runners give you cavities that fill at different times and cool to different densities. The symptom is a part whose dimensions vary across the batch depending on which cavity produced it. Three approaches cover most cases:
- Natural balancing — arrange cavities in a geometric pattern around the gate so flow path length is equal by symmetry. Cheapest, works for simple round or square parts.
- Balanced cold runner — full-balance layout plus a runner sized so pressure drop to the furthest cavity matches the nearest. Standard for 4–8 cavities.
- Valve gating — each cavity gated and switched in sequence or simultaneously. Costs more and needs a control system, but gives cavity-to-cavity consistency nothing else matches, and lets you shut off one cavity while still filling the others.
If your parts vary in dimension and the mould is multi-cavity
Test before blaming the resin. Mark the parts by cavity — a tiny drill mark, or cavity position in the stack — and measure each group separately. If cavity A is consistently larger than cavity D, it is runner balance, and it is fixable. If the whole batch moves together, the cause is process, not tooling.
Three mistakes that cost real money
1. Quoting cavities from the machine's capacity, not the fill rate
The machine can hold the clamp force and still not fill. Fill time is the number to check first, and it is checked against cycle time. Most expensive multi-cavity mistakes are made here.
2. Ignoring the sprue and runner as product
A cold-runner tool makes a sprue that is regrind, and often runners. On a small part those runners can weigh more than the part itself. That material is not free, it is not scrap-free, and its colour has to match. Quote the cycle as producing parts plus reclaim, not just parts.
3. Adding cavities for unit cost without a volume commitment
A 16-cavity tool costs several times a 4-cavity tool. If your volume does not actually consume the extra cavities over the mould's life, you have paid more per part instead of less. Ask the supplier to show the break-even volume, and check it against your real forecast — not your optimistic one.
What to send us for a quotation
If you want a cavity count you can defend rather than accept, send this and expect a calculation back:
- Part drawing or a photograph with dimensions, plus material and grade
- Part weight and projected area — we can estimate from a sample if you send one
- Annual volume, and the realistic shot-weight tolerance you need
- Hot runner acceptable, or cold runner preferred? Any budget ceiling?
- Machine tonnage already fixed at your end, if so
We return the cavity count with the fill-time and clamp calculations shown, plus the tool cost at one cavity less and one cavity more — so you can see where the curve flattens.
Common questions
How many cavities should an injection mould have?
There is no standard number — it is bounded by whichever of four limits binds first: clamp force, fill time, runner pressure drop, and cooling. In practice small parts under 20 g commonly run 4 to 16 cavities, 20 to 100 g parts 2 to 8, 100 to 500 g parts 1 to 4, and above 500 g per cavity the question stops being economic and the answer becomes 1 or 2.
Does more cavities always mean lower cost per part?
No. A tightly packed multi-cavity tool has less steel between cavities, so heat transfers between them and the effective cooling area per cavity shrinks — eight cavities do not cool twice as fast as four. Fill time grows linearly with cavity count at the same time. If cycle time does not fall proportionally, per-part cost can actually rise.
Why does my 4-cavity mould short shot on the last cavity?
Almost always fill time rather than clamp force. If the screw cannot deliver the total shot volume within the available window — roughly 30 to 50% of cooling time — the last cavity receives insufficient melt. More injection pressure will not fix it, because pressure is not the constraint; delivery rate is. The fixes are a larger screw diameter, higher plasticising capacity, fewer cavities, or valve gating.
Is a hot runner worth the extra cost for multi-cavity work?
Frequently yes, for high volume. It removes the sprue and runners entirely, so there is nothing to cool and nothing to trim from the part, and cavity pressure stays uniform cycle to cycle. It costs more in the tool and needs temperature control. The trade favours it when cycle time is long enough for runner cooling to matter and volume is high enough to amortise the tool.
How do I know if a mould quote is properly calculated?
Ask for the cavity calculation, not just the number. A defensible quote shows fill time against cycle time, projected area against rated clamp force, and the resulting maximum cavity count as the lower of those two, plus the runner style. A cavity number with no working behind it was probably taken from a similar tool rather than derived from your part.