Injection Molding worked example

Runner Weight with runner system volume of 21 cc: a worked example

What does the result look like when runner system volume reaches 21 cc? The full calculation is worked below with real intermediate numbers. Use this when calculating total shot weight for cold runner molds, estimating regrind volume, or comparing cold runner waste to hot runner investment.

The inputs for this scenario

  • Runner system volume: 21 cc (raised for this scenario; the documented default is 8.5)
  • Resin density: 1.04 g/cc (unchanged)
  • Geometry/sprue factor: 1.1 x (unchanged)

Working through the calculation

  • Applying the documented formula (Runner weight = Runner volume x Resin density x Geometry factor) to the inputs above produces each figure below.
  • At this operating point the engine returns 24.02 g for result, the number this scenario is built around.
  • At this operating point the engine returns 24.02 value for basic runner weight (volume x density).
  • At this operating point the engine returns 1 x for multiplier.
  • At this operating point the engine returns 21.84 value for factor a x b.

How this compares with the baseline

  • Against the tool's baseline example, where runner system volume sits at 8.5 cc and the headline result is 9.72 g, this scenario comes in 147% above the baseline at 24.02 g.
  • A figure at this level is achievable when runner system volume is genuinely sustained, not just peaked for a shift. It uses a single geometry factor to approximate sprue taper and fittings; complex or unbalanced runner layouts may need a direct CAD mass measurement instead.

Results at a glance

  • Result: 24.02 g (headline result)
  • Basic runner weight (volume x density): 24.02 value
  • Multiplier: 1 x
  • Factor A x B: 21.84 value

Run it with your numbers

  • Every input above is editable in the live Runner Weight calculator, which recalculates instantly and can be shared with the inputs intact.

Last reviewed 2026-05-12.