Injection molding · Machine sizing
Shot Size Calculator
Calculate injection molding shot weight including cavities, runner, and cushion. This practical tool is an engineering estimate—validate decisions against machine, material, mold, and process data.
Decision chain
From estimate to the next check
- Complete shotPart mass × cavities plus runner or sprue
- CushionA stated process allowance, not scrap
- Volume basisMelt density when the next check is volumetric
- Machine checkDocumented screw, stroke, and injection-unit rating
Formula and method
Shot mass includes all cavity parts, runner/sprue, and the selected cushion.
Inputs and units
Enter measured or specified values in the displayed units. Keep units consistent; results are rounded for practical planning. This first release uses the displayed engineering units rather than a unit-switcher.
Worked example
(45 g × 4 cavities + 12 g runner) × 1.10 cushion allowance = 211.2 g.
Build a complete shot record
Use a weighed finished part or validated CAD mass, the actual cavity count, and a weighed cold runner or sprue. Record cushion as a process assumption rather than treating it as scrap.
Compare like with like at the injection unit
Take the machine rating, screw, and material basis from the machine data sheet. A quoted PS shot-weight rating is not a material-neutral barrel capacity; use shot volume when a volumetric comparison is needed.
Sanity check before trials
The calculated shot must be greater than total molded part mass because it includes runner mass and cushion. If it is not, a mass or cavity input has been omitted.
Interpretation, assumptions, and limitations
FAQ
Does a hot runner make runner mass zero?
It may remove cold-runner scrap, but the injection unit still needs a documented shot and cushion. Confirm the actual system and process with the mold documentation.
Next: validate the measured process
Continue with Process Validation & Stability Tools to compare fill behavior, cavity weights, pressure demand, gate seal and short-term repeatability.