A multi-cavity mold means several identical cavities machined into one tool, so one machine cycle produces several parts instead of one. It is the single biggest lever on unit part cost, and the most common source of expensive mistakes — because cavity count is a production decision made long before anyone has run the part.
Key takeaways
- Cavitation is arithmetic. A four-cavity tool running a 30-second cycle produces 9,600 shots a day against 2,400 from a single cavity.
- The cavities must be balanced. Identical geometry in every cavity is what makes multi-cavity tools predictable; imbalance trades a small speed loss for a very large scrap rate.
- More cavities is not always cheaper. Cycle time rises with cavity count once the barrel runs out of shot capacity, and the tool cost rises faster than either.
- 48 cavities is the practical ceiling for this class of tool. Past that, the answer is a different architecture, not more steel.
- Decide cavitation against real annual demand, not against the number that sounds impressive in a quote comparison.
What a multi-cavity tool actually is
A multi-cavity tool is not several small tools assembled together. It is one steel base with the same cavity geometry repeated N times, drilled around a common cooling and gating system. The repetition is what makes it economical: the base, the cooling circuits, the hot runner manifold, the ejector system and the machine setup are all paid for once and amortised across every part the tool makes.
That is the entire argument for multi-cavity tooling. A one-cavity tool on a 200,000-piece annual program spends 80% of its cycle doing nothing, and the cost of that idle time lands in the part price.
Cavitation is arithmetic, not preference
Run the numbers before anything else. A single cavity on a 30-second cycle gives 2,400 shots per day; a four-cavity tool on the same cycle gives 9,600; a sixteen-cavity tool gives 38,400. At $6,000 of annual demand a one-cavity tool is the correct answer, and no amount of enthusiasm changes it.
The error is symmetrical and expensive in both directions. Under-cavitating means paying for machine time you do not use. Over-cavitating means writing a five-figure tool cheque to serve a demand that never arrives. Both mistakes are common because cavity count is usually chosen in a vacuum, before anyone has confirmed the volume.
| Annual demand | Reasonable cavitation | Typical tool range |
|---|---|---|
| Up to 10,000 | 1 | Aluminum or pre-hardened prototype |
| 10,000 – 60,000 | 2 – 4 | Production steel, cold or hot runner |
| 60,000 – 300,000 | 4 – 16 | Production steel, hot runner typical |
| Above 300,000 | 16 – 48 | Hot runner with valve gating |
Our cycle time reduction guide covers the other half of this equation, because cavitation and cycle time are the same conversation from two directions.
Balanced versus unbalanced cavities
This is the distinction that separates a tool that works from a tool that produces a bin of good parts and a pile of rejects. In a balanced tool every cavity is identical — same geometry, same cooling, same gating, same pressure drop. Every cavity fills at the same rate and packs to the same density.
In an unbalanced tool the cavities are deliberately different, usually to save steel or to fit an awkward core. The first cavity fills fast, the last fills slowly, and the parts come out of the same cycle with different weights and different shrinkages.
Unbalanced tools are legitimate and sometimes the only option for large or complex parts. They require a sorting step downstream, because the parts are not interchangeable even though they came from the same tool in the same cycle. For a buyer who has to sort, the multi-cavity tool may cost more to own than a one-cavity tool with no sorting.
What actually limits how many cavities you can fit
Cavity count is bounded by four things, in this order:
- Part envelope. Cavities have to be spaced far enough apart for steel between them, and that spacing is non-negotiable.
- Mold base size. Standard base sizes step up in discrete sizes. The next base up often costs more than the cavities it would add would earn back.
- Cooling. Every cavity needs its own cooling circuit, and the drill paths from one circuit to the next must not intersect. This is the usual reason a 32-cavity layout turns into a 16-cavity one.
- Shot capacity. Beyond a certain point the machine cannot charge the barrel fast enough to fill all cavities before the cooling time starts, and cycle time rises instead of falling.
Roughly 60–70% of a mold bill is machining hours on the cavity and core, so adding the eighth cavity does not add eight times the work — but it does add steel, base size, cooling and assembly, and those compound. Our reference for this class of tool is up to 48 cavities, maximum tool size 1,600 × 1,000 × 500 mm, maximum weight 15 t.
Cold runner or hot runner
At two or four cavities, a cold runner with an open sprue bus is usually the cheaper answer. At eight and above, hot runner systems start to win, and by sixteen cavities they are normally the only practical choice.
The reason is scrap. A cold runner means runners and sprues are cut off and discarded on every cycle. At sixteen cavities the waste is not a rounding error, and the sprue itself becomes a variable in the cycle that you cannot shortcut. A hot runner keeps the melt hot all the way to the cavity, so there is nothing to cut off and the whole charge becomes part.
On every tool we build, hot runners come from HRS, Yudo, Husky, Incoe or Mold-Masters. Ask for those names in writing. “Standard hot runner” without a brand is not a specification, and the difference between a cheap manifold and a good one is often the difference between a stable process and a process that needs a babysitter.
Premium components — hot runner systems, hardened ejector pins, guided ejection — add precision and longevity but increase upfront cost by $2,000–$8,000. In practice the base and the hot runner are the two line items most often swapped quietly to hit a target price.
Cavity layout and part orientation
Layout is decided by geometry, not by preference. Two constraints dominate: the cooling circuits must reach every cavity face, and the part must eject without distortion.
Deep pockets pull heat away from the core side, so the cooling on that side has to be more aggressive. Thin ribs and long flow paths favour a different orientation than a deep box. This is where a tool that was laid out on a screen without a thermal model goes wrong, and it is the argument for running a real simulation before the steel is cut.
Run the simulation before you commit the layout
A multi-cavity tool multiplies every thermal problem in the design across all of them. A cooling weakness in a one-cavity tool costs you one part; the same weakness in a sixteen-cavity tool costs you sixteen.
At RCH Plastic your STEP or IGES file is modelled in UG, run through Moldflow, and returned as a written DFM report within 3 working days, at no cost. The report tells you fill and pack behaviour, weld line positions, and whether the cooling layout is doing the work the design assumes it is doing. Reading it before the tool is machined costs nothing; reading it after costs a set of inserts.
Our mold flow simulation guide explains what to look for in that report.
Why two quotes for the same multi-cavity tool differ so much
When the same drawing comes back as $9,000 from one shop and $26,000 from another, the difference is almost never a pricing decision. It is an assumption decision. One shop has quoted a cold-runner four-cavity tool in pre-hardened steel. The other has quoted a hot-runner sixteen-cavity valve-gated tool in production steel. Both are correct answers to different questions.
Before comparing, normalise five items: cavity count, steel grade, runner type, expected mold life, and what quality documentation is included. Our five pricing factors guide works through each one. Until all five match, the price comparison is meaningless.
Hot runner systems from HRS, Yudo, Husky, Incoe or Mold-Masters are the single largest source of spread, and they are the first place to look when two quotes diverge.
Mold base brands, and why the name matters
Mold bases from brands like HASCO, DME or LKM add predictable cost starting around $800–$2,500. The premium buys something specific: standardized hole patterns, plates that are through-hardened and stress-relieved rather than only surface-hardened, and a published tolerance on the guide pillars that the cavity alignment depends on.
On every tool we build the base is LKM, DME or HASCO — or machined in-house to the same standard. A cheaper base with softer plates will deflect slightly under clamping load, and a multi-cavity tool amplifies that deflection across every cavity. Cavity alignment is not an assembly tolerance on a multi-cavity tool; it is the thing that determines whether all sixteen cavities produce interchangeable parts.
Steel selection for a multi-cavity tool
Cavity count changes the steel question, because the tool spends its life in service and any variation between cavities is hard to see until parts come out different. Our reference is 1.2344 (H13) and 1.2343 (H11) — hot-work steels, hardened to 48–52 HRC, for high-volume production tools. Tool life on those is up to 1,000,000 shots, depending on steel grade and maintenance.
For a lower-volume multi-cavity tool, pre-hardened P20 or NAK80 is a defensible choice and costs less up front. Our aluminum vs steel comparison sets out that trade-off properly. The decision rule is the same as for single-cavity tools: the volume decides the material, not the cavity count.
Ejection is where multi-cavity tools reveal themselves
Sixteen parts have to release cleanly, at the same moment, every cycle. Ejection force that works on one cavity can distort parts on the last cavity, because the pack pressure has already dropped across the fill.
Guided ejection, hardened ejector pins and a balanced layout are not luxury options at high cavitation. They are what keeps the parts dimensionally consistent enough to be interchangeable. If a supplier has not mentioned ejection strategy, ask directly. Our reference tools use guided ejection and hardened pins at higher cavitation.
Tool life in this context is worth understanding as a number you can measure rather than a number you were promised. Every claim we make is backed by a document issued against your mold number — the DFM report, the CMM dimensional report on trial samples, the full process parameter sheet, trial photos and video.
Tooling and part cost, separated
Mold bases and premium components are one-time capital costs. Part price is a recurring unit cost driven by material, cycle time, cavitation and volume. They are frequently confused, and confusing them is how buyers end up paying for a tool that does not earn out.
A project that looks expensive at the tooling stage is often the cheapest over three years, and the reverse is also true. Ask for a quote broken into tooling, unit price at your volumes, material grade and lead time — a single lump-sum number hides the levers you can negotiate. Our injection molding cost guide works through the full picture, and the cost breakdown guide itemises where every dollar lands.
Reference figures for a multi-cavity tool
For comparison, these are the figures we quote against. They are reference ranges, not a quotation, and the actual number depends on your geometry.
| Parameter | RCH Plastic reference (2026) |
|---|---|
| Cavitation | Up to 48 |
| Maximum mold size | 1,600 × 1,000 × 500 mm |
| Maximum mold weight | 15 t |
| Lead time to T1 sample | 35 days (simple) / 42 days (medium) / 50 days (complex) |
| Fastest achievable | 30 days, when the design is frozen and components are in stock |
| Tool life | Up to 1,000,000 shots, depending on steel grade and maintenance |
| Machining tolerance (mold) | ±0.005 mm on critical dimensions |
| Molded part tolerance | ±0.01 mm on critical dimensions, subject to resin and geometry |
| Mold base | LKM / DME / HASCO, or machined in-house to the same standard |
| Hot runner brands | HRS, Yudo, Husky, Incoe, Mold-Masters |
Our machining tolerance is ±0.005 mm on critical mold dimensions. The molded part tolerance that tool can then hold is ±0.01 mm on critical dimensions, subject to the resin and the part geometry. Ask for both numbers separately — a supplier who quotes a single figure for both has not thought the question through.
Molds built per month
We build 20–30 mold sets per month. That number is worth reading for a reason other than capacity: it is the difference between a shop that machines one cavity and inspects it, and a shop running a multi-cavity tool through programming, drilling, assembly and trial as a single scheduled job. At high cavitation, a tool that is not inspected between operations is a tool with a scrap problem you will pay for in parts.
Related guides in this series
Multi-cavity design sits inside a larger set of decisions. These five usually come next:
- Injection molding design guide — the design choices that determine what a tool can be.
- Mold flow simulation guide — how fill, pack and cooling are actually verified.
- How to reduce injection molding cycle time — the other half of the cavitation calculation.
- Five factors that affect injection mold cost — normalise quotes before comparing.
- How much does your first injection mold cost — if the cavitation decision is still open, start there.
Get a multi-cavity tool quoted
Send the 3D file, the resin, your annual quantity and your target date. We will tell you what cavitation is right at that volume, whether cold or hot runner fits, what the tool will cost, and how fast it can run — and if a one-cavity tool is genuinely the better answer, we will say so. Request a multi-cavity quote.