Engineering tools

Shot size formula in injection moulding — calculator and worked example

The shot size formula: shot weight = (part weight × number of cavities) + runner weight. To choose a machine, apply the working rule that your shot weight should sit between roughly 25% and 65% of the machine’s rated shot capacity — so divide your shot weight by 0.65 for the smallest sensible machine capacity, and by 0.25 for the largest. The calculator below does both steps.

Worked example

A 42 g part in a 4-cavity mould with an 18 g runner: shot weight = 42 × 4 + 18 = 186 g. Dividing by 0.65 and 0.25 gives a machine window of 286 g to 744 g rated capacity. A machine rated around 400 g would run this shot at 47% of capacity — comfortably in the middle of the window.

Why 25–65%? The reasoning behind the rule

Below about 25% of capacity, material sits too long in the barrel — residence time rises, and heat-sensitive polymers degrade before they reach the cavity. Above about 65%, the screw runs out of headroom: shot-to-shot consistency suffers, and short shots appear the moment anything drifts. The window is a rule of thumb, not a law — heat-stable materials tolerate lower utilisation, and some machines plasticise fast enough to run higher — but a supplier who cannot discuss where your part sits in this window is guessing about your machine allocation.

What the formula does not tell you

Shot weight sizes the injection unit; it says nothing about the clamp. Clamping force is calculated separately from the part’s projected area and the material’s cavity pressure — a large flat part with a small shot weight can still demand a big machine for its clamp. Check both before committing tooling to a machine size.