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Injection Molding Shot Weight Calculator

Estimate shot weight, PS-equivalent shot size, and injection machine capacity usage for plastics molding jobs.

Cited category: Manufacturing & Materials. Formula used: shot weight = (part volume x cavities + runner/sprue volume) x material density.

Inputs

Use per-part volume or per-part weight, then add cavity count, runner volume, resin density, and machine PS-equivalent capacity.

Part input basis
Use the finished molded part volume before multiplying by cavities.
Enter the total cold runner, sprue, and gate volume for one complete shot.
Preset values are nominal. Use your resin supplier data for production decisions.
The barrel usage check compares PS-equivalent shot size to this capacity. Target range: 20-80%.

Results

PS-equivalent values use a polystyrene density of 1.05 g/cm3.

Enter valid values to calculate the shot size.
Shot weight Not calculated
PS-equivalent shot Not calculated
Machine capacity used Not calculated
Status Not calculated

Shot build-up

Part input basis
Not calculated
Part volume
Not calculated
Part weight
Not calculated
Cavities
Not calculated
Runner/sprue weight
Not calculated
Total shot volume
Not calculated

Machine check

Material density
Not calculated
Machine capacity
Not calculated
Material capacity
Not calculated
Target range
20-80%
Warning
Not calculated

Self-tests

Runs golden checks against the pure shot-weight calculation functions.

Self-tests have not been run.

About the Injection Molding Shot Weight Calculator

Injection molders, tooling engineers, estimators, and product developers use an injection molding shot weight calculator to estimate the material mass required for parts, cavities, runners, sprues, and cushion. Enter volume, density, cavity count, and runner volume to compare total shot with machine capacity. It supports early machine selection and material planning before mold trials.

How it works

  1. Enter part volume or part weight and the number of cavities.
  2. Add runner, sprue, cold slug, and expected cushion volume where applicable.
  3. Select or enter the material density used for the estimate.
  4. Calculate total shot weight and compare it with machine shot capacity.
  5. Confirm the final setup with the molder and material supplier.

Frequently asked questions

How is injection molding shot weight calculated?

Shot weight is the combined weight of all cavities plus runner, sprue, gates, and cushion allowance. If you start with volume, multiply by material density to estimate mass.

Should I use solid density or melt density?

For early estimating, published solid density is often used because part weight is measured after cooling. Process engineering may require melt density or machine-specific data for a more precise shot-size check.

Why should shot size be matched to the molding machine?

A shot that is too large may exceed machine capacity, while a shot that is too small can cause residence-time, control, or repeatability problems. Use the machine maker's recommended operating range.

Do runners count if the tool uses a hot runner?

Hot runner systems usually reduce or eliminate cold runner scrap, but manifold and gate design still affect processing. Include only the material that is injected and recovered or discarded for the actual tool design.

Can this estimate replace a mold flow analysis?

No. Shot weight is only one sizing input. Mold flow, clamp tonnage, fill pressure, cooling, shrinkage, venting, and gate design require separate engineering checks.

References