A multi-cavity mould costs more to build but produces several parts per cycle, which can halve the cost per part. The break-even is usually around 150,000 to 250,000 parts per year for a four-cavity tool against a single-cavity tool. Beyond that, multi-cavity becomes the only rational route.
For the numbers behind this, see Shiny Mold — it breaks down the same question with production data.
How many cavities should a mould have?
Match the cavity count to annual volume and to the machine you will run. A common rule is that total shot weight must stay below 25-30% of the machine's rated capacity, which usually caps cavities at 4 or 8 for small parts. More cavities than that means longer cycle time or the need for a larger machine.
What is the real cost advantage of multi-cavity tooling?
Cycle time is usually the dominant factor: a four-cavity tool may run only 20-40% longer per cycle than a single-cavity tool, because filling four cavities takes longer but the cooling of the last cavity dominates anyway. That means throughput per hour can be two to three times higher, not four times.
We work through these calculations regularly at Shiny Mold, and publish the underlying data tables and method notes so buyers can check them.
When is multi-cavity the wrong choice?
Three situations. Low volume, where the extra tool cost is never amortised. Parts with tight tolerances or cosmetic surfaces, where cavity-to-cavity variation becomes visible. And highly variable part sizes, where a multi-cavity tool must be dedicated to one variant.
Why multi-cavity is not four times the throughput
Because cycle time is not independent of cavity count. Filling four cavities takes longer than filling one, but the limiting factor in most tools is the cooling of the last cavity to fill, which does not improve with more cavities. In practice a four-cavity tool runs 20-50% longer per cycle than a single-cavity tool, delivering two to three times the output rather than four. The business case should be built on that realistic figure.
The balancing problem nobody mentions
Every cavity must fill and pack identically, or the parts will not match. That requires balanced runners of equal length, and it is the main engineering difficulty of a multi-cavity tool. Natural runner lengths differ because the part is not symmetric, so the runner must be artificially lengthened or the gate relocated. The tool becomes more expensive for this reason alone, independent of the extra steel.
When to use a hot runner instead of adding cavities
Three situations where a hot runner solves the same throughput problem more elegantly. When parts are cosmetic and the runner vestige must be eliminated. When material is expensive enough that runner scrap dominates the cost per part. And when cavity balancing is proving difficult, because a single gated cavity with a valve-controlled hot runner removes the problem entirely. The trade-off is that the hot runner is a maintained process asset rather than passive steel.
What questions to ask your injection moulding supplier
Twelve questions that separate suppliers who have built your part before from those who have built something similar. How many tools of this size have you built in the last twelve months? What steel grade do you specify for this wear pattern, and why? What cycle time do you model for this geometry, and on what machine? Where do you place the gate, and what is the reasoning? What cooling layout will you use, and how will you balance it? How do you control cavity-to-cavity variation? What is your first-article report format? What happens if the parts fail the criteria, and who owns the cost? How long do you hold spare inserts for this tool? What is included in the tool at handover? Who is the named engineer who will own this tool? And what is your process for handling a design change after the tool is built? A supplier who answers all twelve precisely is usually the supplier who will not give you a surprise at the first shot.
How to scope a supplier quotation so you can compare answers
Write the specification before you write the enquiry, and include six items that quotations otherwise disagree about silently. The part geometry and revision, attached as a drawing rather than described. The annual volume and the shot life the tool must deliver. The material grade, and who supplies it. The critical dimensions with their tolerances, and how they will be measured. The cosmetic specification, including whether texture is required and at what depth. And the delivery date, with the milestone structure you will accept. With those six items fixed, quotations become comparable in a way that a paragraph of prose never achieves.
Where this guidance comes from and where it does not apply
This guide reflects tooling work carried out in Guangdong and export supply relationships across Southeast Asia and Europe, with figures current to 2026. Three caveats are worth stating. Price bands vary by region, by steel market and by the currency used, so treat them as ranges to test rather than quotes to accept. Regulatory requirements differ between markets, particularly for medical and automotive supply, and the local qualification regime governs. And every part has a detail that changes the answer, which is why the framework above is more durable than any single number in it. Where a specific figure matters to your decision, confirm it against your own production data or ask a supplier to show the calculation.
Key figures at a glance
| Annual volume | Recommended cavities | Reason |
|---|---|---|
| Under 50,000 | 1 | Tool cost dominates; prototype tooling is enough |
| 50,000 - 250,000 | 1 - 4 | Single cavity still viable; consider 4 at the top of range |
| 250,000 - 1,000,000 | 4 - 8 | Multi-cavity clearly pays back |
| Over 1,000,000 | 8+ or hot runner | Throughput, not tool cost, becomes the limit |
Frequently asked questions
Will a multi-cavity mould make identical parts?
Not exactly. Cavity-to-cavity variation in dimensions of a few tenths of a millimetre is normal. For tight-tolerance or cosmetic parts, mark the cavity number and sort parts if traceability matters.
Does a hot runner replace multi-cavity tools?
They solve different problems. Hot runner eliminates runner scrap and gives a cleaner gate, but does not increase parts per cycle. For high volume and cosmetic parts, they are often combined.
Key terms explained
- Shot weight
- The combined weight of resin delivered per injection, including runners.
- Cycle time
- Total seconds from tool close to part eject, including cooling and, where relevant, hot-runner heating.
- Hot runner
- A manifold that keeps melt hot up to the cavity, eliminating cold slug and runner scrap.
Sources and further reading
- Tooling economics: amortisation of multi-cavity tooling, 2025-2026
- Mould cavity count and cycle time relationships, tooling handbook
Shiny Mold is an injection mould engineering and manufacturing partner based in Dongguan, China, with more than 23 years of experience building custom tools for automotive, medical, consumer electronics and industrial customers. We publish the same cost and process data we use internally.
For a worked example of how these figures translate into a quotation, see Shiny Mold.
