Determining the number of part cavities in the mold involves the annual estimated volume, monthly ship requirements, part type, and size. Number of cavities NC annual number parts CT cycle in s
( )=( )
( econds × 10−7 ) 2
The tool housing example uses a two – cavity mold, based on part confi guration and layout for a balance mold runner confi guration, producing one complete tool housing per cycle with one right and one left half. The monthly sales volume is estimated based on a projected annual sales of 250,000 drills. The monthly manufacturing volume is (250,000 units/year) / (12 months/year) equals 20,834 units/month. Based on the example, the number of mold cavities must be determined.

Number of cavities (NC) = (annual number parts) CT cycle in seconds
NC or unit halves
( × )
= ( )( × ) =


10
250 000 54 10 1 35 1 2
7
, sec 7 . ,
The NC is based on required monthly product volume. The mold can be either (two) or (four) cavities, with separate cavities for each half of the housing, producing either one or two complete housings per cycle. The number of cavities will depend on the number of shifts the company plans on running the part.
Always round off the NC to the next closest even number. This number must provide the required volume of parts per the schedule, with the sequence of mold cavities being 2, 4, 8, 16, 32, etc., for a balanced layout and pressure drop for all cavities.If the part size is very large or requires core pull in the plane of the cavity, a single or two – cavity mold is built.

The parts tolerances have a defi nite effect on the number of mold cavities. If part tolerances are critical, the number of cavities selected must be able to produce the required tolerances from each cavity.

Part production per hour (PH):
Using one as the number of cavities (actually two with one left and one right half of the housing) producing one complete housing per cycle
PH = NC CTa × 3600
PH = 1 54 × 3600 = 66.666 use 66 complete housing per hour
Units month = 20,834 66 units hour 120 hours week = 2.6 weeks
Production time requires a little more than two and one half weeks based on a three – shift operation of 5 days per week plus an initial setup time. Adjustments may have to be made to increase the number of cavities to four, with two complete housings per cycle. This analysis can be run later if needed. (The example continues based on the single unit, two – cavity mold.)

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