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Notes on injection moulding: part design, tooling and cost

Front page / Do I need a single-cavity, multi-cavity or family mould?

Do I need a single-cavity, multi-cavity or family mould?

A single-cavity mould makes one part per shot and is the usual choice for prototypes, large parts and uncertain demand. A multi-cavity mould makes several identical parts per shot and pays off when volumes are high and the design is settled. A family mould makes different parts of the same assembly in one shot, which can cut tooling cost, but only when the parts are similar in size, made in the same material and needed in the same quantities.

The cavity is the hollow in the mould that forms the part. How many cavities a mould has, and whether they are all the same, is one of the biggest choices behind a mould quote.

The three layouts compared

LayoutParts per shotTooling costBest forWatch out for
Single cavityOneLowestPrototypes, large or complex parts, changing designsHigher price per part at volume
Multi-cavityTwo, four, eight or more of the same partHigher: bigger mould, more steel and machiningStable, high-volume partsEvery cavity must fill and cool the same
FamilyOne each of several different partsUsually less than separate moulds for each partMatching parts of an assembly at low to medium volumeUneven fill, unequal demand, sorting

What the published guidance says

  • Protolabs describes the two reasons to choose multiple cavities as meeting demand and lowering the piece-part price, but warns that more cavities do not automatically mean lower total cost; the crossover depends on tool price, part price and quantity.
  • Protolabs offers 2, 4 or 8 cavities per mould on its multi-cavity service, depending on part size and complexity, and says multi-cavity tools need a tab or runner gate rather than direct gating.
  • For family tools, Protolabs limits how different the parts can be: the allowed difference in part volume ranges from about 20 percent for small parts to about 10 percent for large ones, and cavities must fill within 2,000 psi (13.79 MPa) of each other.
  • Protolabs' UK family-tooling guide says grouping suitable parts in one family mould can save up to 40 percent of costs, provided they fit a standard mould size, use the same material and are needed in the same volumes.
  • Dienamics, an Australian toolmaker, points out that a running family tool drops all its parts into one container, so they must be sorted by hand or by automation, which adds labour.

Why family moulds are harder than they look

Plastic takes the easiest path. If one cavity in a family mould is larger or thicker than its neighbour, it fills differently, so one part can be over-packed while another is short. That is why the published guidance insists on similar sizes and geometries. Demand is the other trap: Protolabs gives the example of a lid that wears out faster than its base, so a 1:1 family mould would build up spare bases. Options include a separate multi-cavity mould for the faster-moving part, or a layout with two lids and one base.

How to decide

  1. Start with annual volume and how confident you are in it. Low or uncertain demand favours one cavity.
  2. Freeze the design first. Every design change has to be repeated in every cavity.
  3. Price at least two layouts at your expected quantities and compare total cost, not just the mould price.
  4. For a family mould, check that the parts share a resin and colour, are similar in size, and are used in the same ratio.
  5. Avoid hand-loaded inserts in multi-cavity tools. Protolabs notes that a pick-out that is simple in one cavity becomes slow across eight.
  6. Ask how cavity balance will be checked at first samples, for example by measuring parts from every cavity.

Limits and savings here are published by specific suppliers and vary by toolmaker. Use them to frame questions, not as a quote.

Questions buyers ask

Can I start with one cavity and add more later?

Often, yes. Many products start in a single-cavity mould and move to a multi-cavity production tool once demand is proven. Some suppliers credit part of the first mould against the second.

Are parts from different cavities identical?

They should be close, but each cavity is cut separately, so small differences are normal. Parts are often marked with a cavity number so any problem can be traced.

Can a family mould run only one of its parts?

Some moulds are built with runner shut-offs so a cavity can be blocked, which Dienamics describes as a way to run different parts or colours. It adds complexity and is worth asking about at the quote stage.

Does more cavities mean a bigger press?

Usually. Clamp force scales with the total projected area of all cavities plus the runner, so a multi-cavity mould needs a larger machine than a single-cavity one for the same part.

Sources