Cavity count is the largest single lever on both tooling cost and part cost, and it is decided by arithmetic rather than by preference: annual volume divided by the hours available at the expected cycle time gives the parts per cycle needed. Choosing it too high wastes capital; too low leaves you short of capacity with no cheap fix.
Doing the arithmetic
Take the annual volume, divide by the production hours you are willing to buy, and divide again by the cycles per hour the part's cooling time allows. That gives the cavities needed. Then check the press: total projected area sets clamp tonnage and total shot weight sets the barrel, and either can force a bigger machine whose hourly rate changes the answer.
Balance is what makes it work
Every cavity must fill at the same time with the same pressure, or the parts differ from cavity to cavity. Naturally balanced runner layouts, where the melt travels the same distance to each gate, are the reliable way to achieve this. Artificially balanced layouts, where runner diameters are varied instead, work at one process setting and drift at another.
Family tools and their trap
A family tool puts different parts in one tool, which is attractive when a product needs a set of components. The trap is that different parts have different volumes and fill at different rates, so balancing is difficult and one cavity is always over or under packed. They also force you to make every part in the same ratio, whatever demand actually looks like.
When fewer cavities is right
When volume is uncertain, because a single cavity tool is cheap and a second tool can follow. When the part is large, because clamp tonnage rises with projected area. When the resin is abrasive and tool maintenance will be frequent. And when time to market matters more than unit cost, since a single cavity tool samples sooner.
Questions people ask about multi cavity injection molding
How many cavities do I need?
Annual volume divided by the production hours you will buy, divided by cycles per hour at the part's cooling time. Then check that the press can hold the clamp and deliver the shot.
What is a balanced runner?
A layout in which melt travels the same distance to every gate, so all cavities fill together at the same pressure. It is the reliable way to get identical parts from every cavity.
Are family tools a good idea?
Sometimes, and they carry a real trap: parts of different sizes fill at different rates and cannot be balanced well, and you must make them in a fixed ratio.
Is a multi cavity tool always cheaper per part?
Only if the volume uses it. Unused cavities are idle capital, and a larger press with a higher hourly rate can cancel the saving.