A number of factors should be considered when deciding how many cavities can be filled from a runner system.
The number of cavities which can be filled from any given runner system depends on the following:
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Machine size (available clamp force)
-
Available shot volume
-
Available production time
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Required product quantity
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Shape and size of the moldings
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Mold costs
The number of cavities filled from a particular runner system can affect the ease with which the program arrives at a runner balancing solution. The possible combinations of pressure drops in the runner system itself and in filling the cavity volumes can rapidly increase the complexity of the balancing problem.
Following are simple formulas for determining the number of cavities. Use the minimum value derived from these formulae.
Production schedule
If the dimensional tolerance of the part is not very critical and a large number of moldings are required, multi-cavity molds are preferred.
Number of cavities = 
where:
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= number of parts in the lot -
= the reject factor, expressed as = 1/(1-reject rate) -
= cycle time to produce a single set of parts -
= available time to supply a lot of parts
Shot capacity
Number of cavities = 
where,
-
= 80% of machine capacity -
= part weight
Plasticizing capacity
Number of cavities = 
where,
-
= machine plasticizing capacity -
= estimated number of shots per minute - = part weight