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USEFUL MOULDING CALCULATIONS

Method of Calculating shot capacity and locking force
Example : Rectangular box 100 x 60 x 20 mm. height, wall thickness
Example : 2 mm. moulded in polypropelene.
Weight of Moulding : (100 x 20 x 2) x 2 + (60 x 20 x 2) x 2 + 100 x 60 x 2 (all 5 sides)
= 8000 + 4800 + 12000
3
= 24800 mm OR 24.8 cm 3
3
density of PP = 0.91 gm/cm , so
24.8 x 0.91
= 22.5 gms. Say 30 gms. with sprue.
Normally shot capacity of injection moulding machine is defined in polystyrene and
70% of that value can be taken for polypropylene

Shot Capacity in PP = 30/ 0.7 = 43 gms.
2
Locking force projected area of mould = 100 x 60 = 6000 mm OR 60 cm 2
Assuming injection pressure required is 800 kg. / cm 2
Force that tends to open a mould = 400 x 60
= 24000 kg OR 24 tonnes
Therefore to mould this component machine should have minimum shot capacity of
43 gm. and a locking force of 24 tonnes.
Other Formulas :
1) Required Tonnage = (Projected Area of the part X Number of Cavities +
Projected area of the Runner) X Tons / sq. inch required for the resin Projected Area
is the 2-Dimensional area of the part as seen from the direction of the flow of the
plastic from the sprue.
PE, PP, ABS, Acetal, Acrylics - 3 to 3.5 tons/sq. in (for approx 0.060 wall thickness)
Nylons, PC, Polyesters = 3.5 to 4.5 tons/sq. in (for approx 0.060 wall thickness)
Thicker the wall, more is the required tonnage.
2) Residence Time of the plastic in the barrel =
[ Shot Size of the Machine [

(Part Weight X Number of Cavities + Weight of the runner)
X 1.06 / Density of the plastic.

3) Percentage of shot used =

[ (Part Weight X Number of Cavities + Weight of the runner) [
X 1.06 / Density of the plastic
Shot Size of the Machine

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