Work out injection moulding shot size instantly with clear inputs, formula shown and shareable results.
Shot weight is the weight of all cavities plus the runner system that feeds them. Machine selection then aims for 40-80 percent of rated shot capacity: below that the material sits too long in the barrel and degrades, above it the screw has no cushion and shot-to-shot consistency suffers.
Shot weight
Shot = part weight x cavities x (1 + runner allowance)
Utilisation
Utilisation = shot weight / machine shot capacity
Shot = part weight x cavities x (1 + runner allowance). Shot weight is the weight of all cavities plus the runner system that feeds them.
The screw needs residual cushion to pack the part and compensate for shrinkage. Without it, dimensions drift and sink marks appear.
This calculator takes 4 inputs: Part weight, Number of cavities, Runner and sprue allowance, Machine shot capacity. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.