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Factors involved
The number of cavities depends on the available production time, product quantity
required, machine shot size and plasticizing capacities, shape and size of the moldings,
and mold costs.
Formulas
Following are simple formulas for determining the number of cavities. Use the minimum value
derived from the following formulas.
Product quantity
If the dimensional tolerance of the part is not very critical and a large number of moldings are
required, multi-cavity molds are preferred. The number of cavities is dependent on the time
available to supply a specific lot of parts ™, the number of parts in the lot (L), the cycle time to
produce a single set of parts (tc), the reject factor (K), expressed as K = 1/(1-reject rate). The
relation is:
Number of cavities = L x K x tc / tm
Shot capacity
The injection machine shot capacity is also a factor in determining the number of cavities. Take 80
percent of the machine capacity as the shot weight (S) and divide by the part weight (W) to get the
number of cavities. The relation is:
Number of cavities = S / W
Plasticizing capacity
The injection machine plasticizing capacity is also a factor. Divide the plasticizing capacity (P) of
the machine by the estimated number of shots per minute (X) and part weight (W). The relation is:
Number of cavities = P / ( X x W )
Determining Number of Cavities