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Injection Molding Shot Weight and Shot Size Calculator

Shot size ties the part, the runner and the machine together. Pick a press whose barrel is too big and heat-sensitive resin degrades from long residence time. Pick one too small and you cannot fill. This calculator checks the match.

Your numbers

Part volume

cm3

From CAD mass properties.

Cavities

Cold runner and sprue volume per shot

cm3

Use 0 for a hot runner.

Resin melt-state density factor (solid density)

g/cm3

Solid density from the datasheet, for example 1.05 for ABS, 0.91 for PP, 1.20 for PC.

Machine shot capacity (general purpose PS)

The rated shot size on the machine data sheet is normally stated in grams of polystyrene.

Cycle time

Your results

Total shot weight

Grams per shot, including runner.

Shot as percent of barrel

46.7%

Aim for roughly 20 to 80 percent.

Part weight

18.9

Grams per part.

Runner share of shot

10%

Barrel capacity for this resin

180

PS capacity scaled by density ratio to 1.05 g/cm3.

Estimated melt residence time

0.7143

Minutes. Compare with the resin's heat-history limit.

Residence time here is a simple barrel-volume estimate. Real values depend on screw design and the amount of melt held in the screw tip and nozzle. Check the resin supplier data for limits.

Save results:

Press selection helper from ERP data

Optional: discuss automating the mold-to-press checks you do by hand.

Optional. The tool and every export work without this. No spam.

How to use it

Enter the volume of one part from CAD, the number of cavities, and the cold runner and sprue volume (zero for hot runner). Add the solid density of the resin, the rated shot capacity of the machine in grams of polystyrene, and the cycle time. The results show part weight, total shot weight, runner share, the machine capacity converted to your resin, utilization and residence time.

Use the datasheet solid density, not melt density.
Machine data sheets rate capacity in grams of PS.
Include sprue puller and cold slug in runner volume.

Formulas and conversion

Shot weight is the part volume times cavity count plus runner volume, multiplied by density. Machine ratings are normally published for general purpose polystyrene at a density near 1.05 g/cm3, so capacity for another resin scales by the density ratio: a PP at 0.91 holds about 87 percent of the PS rating, and a PC at 1.20 holds about 114 percent. Utilization is shot weight divided by that scaled capacity. Residence time is barrel capacity divided by shot weight, times cycle time, expressed in minutes.

Reading the result

A shot between about 20 and 80 percent of barrel capacity is the usual working band. Below 20 percent the melt sits too long, and heat-sensitive materials such as PVC, POM, PC and some flame-retardant grades can degrade. Above 80 percent there is little room for process adjustment and you risk short shots when conditions drift. Residence time should stay well under the supplier's limit at processing temperature, often a few minutes for sensitive grades. If the runner share is over 20 to 30 percent, evaluate a hot runner.

Common mistakes

Mixing up grams of PS with grams of the real resin gives wrong utilization. Forgetting the runner underestimates shot weight on cold runner tools. Ignoring the cushion also matters, since the screw should not bottom out. Finally, glass-filled materials have higher density than the base polymer, so use the compound datasheet value rather than the unfilled grade.

Frequently Asked Questions

How do I calculate shot weight in injection molding?

Add the volume of all parts and the runner for one shot, then multiply by the resin's solid density. Volume in cm3 times density in g/cm3 gives grams per shot.

What percent of the barrel should a shot use?

A common guideline is 20 to 80 percent of rated capacity, with 30 to 70 percent preferred for sensitive resins. Lower uses lead to long residence time, higher leave no adjustment margin.

Why is machine shot capacity given in polystyrene?

Manufacturers rate shot size in grams of general purpose polystyrene as a common basis. You scale the rating by the density ratio to find the capacity for your actual resin.

What is residence time and why does it matter?

It is how long resin spends in the heated barrel before injection. Long residence breaks down heat-sensitive resins, causing discoloration, splay, brittleness or black specks.

If you keep press and resin data in an ERP or a spreadsheet, we can help build a press-selection helper that applies this check automatically.

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