Shot Size Weight and Melt Volume Calculator

For injection setup technicians sizing a shot before the first mold trial, this tool calculates the mass of the cold part and runner, and the melt volume the screw must actually dose at melt density. This accounts for the fact that shot weight is derived from cold part and runner volume at solid density, while the melt volume to dose takes into account the melt density and cushion volume.

How to Use This Calculator

1. Enter the cold part volume in cm³ for one shot, multiplying a single cavity's volume by the cavity count first.
2. Enter the runner volume in cm³ for that same shot.
3. Enter the cushion volume in cm³ you intend to hold ahead of the screw.
4. Enter the polymer's solid density and melt density in g/cm³, keeping melt density lower than solid density.
5. Read the shot weight in grams and the melt volume to dose in cm³, which update as you type.

FAQ

Q: Why is shot weight calculated from the cold part volume and solid density instead of the melt?

A: Mass does not change on melting — the cold part and runner volume times solid density gives the true mass, so the shot weight is fixed once the part is molded.

Q: Why does the melt volume to dose differ from the shot weight?

A: The screw doses a volume, and the same mass occupies more volume at melt density, so the tool divides the shot weight by melt density; adding the cushion volume gives the total the screw must dose.

Q: What happens if I enter a melt density higher than the solid density?

A: The tool flags the pair as invalid, because melt density must be lower than solid density — using cold volume times melt density would understate the mass.

Q: Is the shot weight the same as the machine's rated shot capacity?

A: No — the shot weight here is the mass of your part plus runner, while the melt volume to dose includes the cushion you hold ahead of the screw.