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## Determining the number of cavities

Posted by admin on July 31st, 2012
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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 )

Author: ZHILIAN MOULD