Multi-cavity molds

The number of cavities and mold construction depend both on economical and technical factors. Important is the number of parts to be molded, the required time, and price in relation to mould manufacturing costs. The figure below shows the relation between the total part costs and the number of cavities. Total part costs in relation to number of cavities.

The gating system and gate location can limit the design freedom for multi-cavity molds. Dimensional accuracy and quality requirements should be accounted for. The runner layout of multiple-cavity molds should be designed for simultaneous and even cavity filling. The maximum number of cavities in a mold depends on the total cavity volume including runners in relation to the maximum barrel capacity and clamping force of the injection molding machine. Number of cavities A given molding machine has a maximum barrel capacity of 254 cm3, a plasticizing capacity of 25 g/s, 45 mm screw and a clamping force of 1300 kN. A PC part of 30 cm3, (shot weight 36 g) and a projected area of 20 cm2 including runners requires about 0.5/tons/ cm2  (5 kN/cm2 ) clamping force. The maximum number of cavities based on the clamping force would be 12. It is advisable to use only 80% of the barrel capacity, thus the number of cavities in this example is limited to 6. When very short cycle times are expected the total number of cavities may be further reduced.  A 6-cavity mold in this example requires a shot weight of 216 g. The cooling time must be at least 8.7 seconds.