Choosing the Number of Mold Cavities

Cavity count is one of the first decisions on a new mold and one of the biggest drivers of its price. More cavities cost more to build but make more parts per cycle. The right number comes out of simple arithmetic.

Start with press hours

Multiply annual volume by cycle time and divide by the number of cavities. Take 500,000 parts a year on a 30 second cycle:

Then compare the extra mold cost with the press time it saves, at your press rate. Cycle time also changes with cavity count: more cavities can mean a bigger press and a slightly longer cycle.

What more cavities cost

• A bigger, more expensive mold, and often a bigger press.
• Balancing. Every cavity has to fill the same way or parts vary. Naturally balanced runners and process tuning get close, but cavity-to-cavity variation still has to be measured.
• Measurement. Each cavity needs its own dimensional check at approval, and parts should be traceable by cavity number.
• Risk. One damaged cavity can stop the whole tool unless it can be blocked off.

When one cavity makes sense

Low volumes, large parts, tight tolerances and designs that may still change all favor a single cavity. It's also the usual choice for a first production tool: prove the part, then build a multi-cavity tool when volumes justify it.

Family molds

A family mold makes different parts in the same shot, like the two halves of a housing. It saves a mold, with some catches:

• All the parts have to be the same material and color.
• Different sized parts fill differently, which makes the process harder to balance.
• You make every part in equal numbers, even if you need more of one, unless cavities can be shut off.

Family molds work best for matched sets at moderate volumes. At high volumes, separate molds usually run better.

Tell us your annual volume and we'll suggest a cavity count with the numbers behind it. See tooling.

Related reading

• Hot runner vs cold runner molds
• Injection mold cost factors
• Aluminum vs steel injection molds