Shot Size Calculator
Calculate injection molding shot weight and volume from part weight, cavity count and runner allowance, plus hourly throughput from the cycle time.
Introduction
Shot size determines whether a job fits a given injection molding machine: too large and the barrel cannot deliver the shot, too small and the resin degrades from excessive residence time. The Shot Size Calculator adds part weight and runner weight to get total shot weight, converts that to melt volume using the melt density of your resin, and projects hourly material throughput from your cycle time. With those three numbers you can match a job to a press, plan material consumption per shift, and sanity-check whether the quoted cycle time is physically consistent with the dryer and feed system capacity.
How it Works
Three chained equations run on every calculation. Shot weight (g) = part weight + runner weight. Shot volume (cm³) = shot weight ÷ melt density (g/cm³) — melt density is lower than solid density, which is why volume checks against barrel capacity must use the melt figure. Output (kg/h) = shot weight ÷ cycle time (s) × 3.6, where the 3.6 factor converts grams-per-second into kilograms-per-hour (3600 s/h ÷ 1000 g/kg). A 50 g shot on a 20 s cycle therefore consumes 9 kg of resin per hour per cavity set.
Usage Scenarios
- Verifying that a 4-cavity job with a 48 g total shot fits within the recommended 20–80% utilization window of a press with a 120 g barrel capacity.
- Planning resin purchasing: converting a 30 s cycle and 65 g shot into kg/h, then into pallets per week for a 24/5 production schedule.
- Sizing a dryer — hourly throughput from the calculator tells you the minimum drying capacity needed to keep hygroscopic resins like PA6 within moisture spec.
- Quoting a new job: combining shot weight with material price per kg to get raw material cost per cycle before adding press time.
- Checking residence time risk: a tiny shot in an oversized barrel means long melt residence and potential degradation for heat-sensitive resins like POM.
FAQ
Why use melt density instead of solid density?
The barrel meters molten polymer. Melt density is typically 10–25% lower than solid density, so using the solid figure understates shot volume and can overload the barrel stroke.
Does runner weight matter on a hot-runner tool?
On a full hot-runner system there is no solidified runner, so enter zero. For cold runners or hybrid tools, weigh the actual runner and include it — it often rivals the part weight.
Where does the 3.6 factor come from?
Output in kg/h equals grams per second times 3600 seconds per hour divided by 1000 grams per kilogram, which simplifies to shot weight ÷ cycle time × 3.6.
What barrel utilization should I target?
A common guideline is 20–80% of barrel capacity per shot. Below that range residence time grows and resin may degrade; above it the screw may not recover plasticized melt in time.