Number and Layout of Mold Cavities
The number of mold cavities defaults to the value of the cost model variable defaultNumCavities (1 in starting point VPEs). The user can override the default with the setup option Number of Cavities (see User Inputs for Plastic Molding), and direct aPriori to do one of the following:
• Use an explicitly specified cavity count (powers of 2 between 1 and 128 inclusive).
• Calculate the number of cavities in the Most Cost-efficient Cavity Layout.
• Calculate the minimum number of cavities that Meets the Production Requirements of the current scenario.
In all cases, aPriori selects a cavity layout (a rectangular array of cavities) from the lookup table layoutNumCav.
It selects a layout that is as close as square as possible, using the given number of cavities; that is, it finds a row that satisfies the following (“PFR” stands for “plastic forming region”):
• The product of the values in the columns Cavities Lengthwise and Cavities Widthwise is the given number of cavities.
• The values of Cavities Lengthwise and Cavities Widthwise minimize the difference between the mold base length and width; that is, minimize the difference between, roughly, the following quantities:
o Cavities Lengthwise * PFR Length + Runner Region Allowance
o Cavities Widthwise * PFR Width + Runner Region Allowance
(The mold base dimensions are also augmented by some additional allowances related to temperature control.)
See also Dimensions of the Plastic Forming Region and Runner System Layout.
Most Cost-efficient Cavity Layout
aPriori chooses the most cost-efficient number of cavities as follows:
• For each candidate cavity count (powers of 2 between 1 and 128 inclusive):
o Select the layout that is closest to square, given the Dimensions of the Plastic Forming Region.
o Determine the fully burdened part cost using the selected layout.
• Choose the number of cavities in the layout that results in the lowest cost.
Meets the Production Requirements
Calculation of the number of cavities that can accommodate production requirements depends on the following:
• Required annual volume
• Values of the following VPE variables (starting point values shown in parentheses)
o Production Days Per Year (253)
o Shifts Per Day (2)
o Hours Per Shift (8)
o Plant Efficiency (0.95)
• Injection/cool/ejection time (estimated as 1.2 times the cool time—see Times for Plastic Molding)
In the lookup table layoutNumCav aPriori selects the first row whose value for Number of Cavities is greater than or equal to the required number of cavities. In starting point VPEs, this is the smallest power of 2 (up to 128) that can meet the production requirements. (The maximum number of cavities is 128; if the required number of cavities is greater, the number of cavities is set to 128.)