How to decide cavities number for plastic injeciton mould?

Multi-cavity molds produced parts in a much higher efficiency way, it's employed frequently on the occasion when the demands volume is high, the number of the cavities and mold construction depend on both economic and technical factors, quantity of parts to be molded at one cycle, required cycle time, and unit price are related with mold making cost. See below figure, mold cost and plastic parts cost need to be considered to achieved the best result of the project.

There are a few points need to be pay attention when we decide the numbers of cavity:

Delivery time 
The more numbers of cavities, the longer manufacturing time. When the required mold delivery time is short, reducing the mold cavity number with the customer's approval can be used to shorten the mold manufacturing time.

Quality and tolerance
As the numbers of cavity increase, the tolerance of the molded parts reduce if the same manufacturing process employed, it would also much more difficult to ensure tolerance during molding process, it's also not as easy as single cavity molds while modification required.

Layout and melt flow
To have quality parts, Multi-cavity mold need to have an balanced runner layout, the melt flow much fill all the cavities equivalent under same injection pressure, especially for the raw materials with bad melt flow performance, such as PC, it's easy to have some cavities are fulfilled or even overfilled with flash created while others are not.

Injection machine capacity
The injection molding machine is part of the cost of injection molded parts, so usually the mold is required to be manufactured at a predetermined injection molding machine, the size of the mold have to be considered during design process, the designed mold not allowed to be too large or too small, the injection molding machine also have certain amount of maximum injection volume, the total volume of all cavities plus runners should not be exceed 80% of it.