How small can I make the runner?

Only as small as your resin supplier allows, because a runner that freezes early stops packing the part. For Delrin, DuPont sets the minimum at the part wall thickness plus 1 mm, and never below 1.5 mm.

How do I enter a trapezoidal or half-round runner?

Enter the diameter of a round runner with the same cross-section area: 2 × √(area ÷ π). A half-round runner of radius r matches a round runner 1.41 r in diameter.

Does a hot sprue bushing take the sprue out of the shot?

Yes. Its melt stays molten and goes into the next shot instead of being ejected, so enter sprue length 0.

Why does the share matter if every runner is reground?

Because feeding it back to the same press makes that share of the feed regrind. If the print or resin datasheet caps regrind lower, the surplus has to go to another job or be sold.

Injection Molding calculator

Runner Weight Calculator

Estimate the cold runner and sprue weight per shot from the runner layout, before there is a runner to weigh. You need runner and sprue dimensions from the mold drawing, the resin density and the part weight per shot.

What this calculator does

Cold runner and sprue weight per shot from the runner layout, and its share of every shot.

Formula used

Runner volume (mm³) = π ÷ 4 × runner diameter² × total runner length
Sprue volume (mm³) = π ÷ 4 × mean sprue diameter² × sprue length
Runner and sprue weight (g) = (runner + sprue volume) ÷ 1,000 × resin density
Shot weight = part weight per shot + runner and sprue weight
Share of shot (%) = runner and sprue weight ÷ shot weight × 100

Inputs explained

Runner Diameter: Full-round runner diameter from the mold drawing.
Total Runner Length: All branches from sprue to gates, plus cold slug wells.
Sprue Length: Nozzle seat to parting line; 0 for a hot sprue.
Mean Sprue Diameter: Average of the orifice and parting-line diameters.
Resin Density: Solid density of your exact grade from its datasheet.
Part Weight per Shot: All parts from one shot, runner removed: part weight × cavities.

How to use the result

Best suited to sizing the runner at mold design review, quoting resin before the mold is built, weighing a cold sprue against a hot sprue.
Stepped runners: run each diameter separately, sprue length 0 after the first, and add. For a hot runner feeding cold sub-runners, enter only the sub-runners with sprue length 0.

Current U.S. benchmarks

The producer price index for plastic resins and materials stands at 280.569 (BLS, Aug 2026), up 6.6% from a year earlier. Quotes priced off last quarter's material cost miss this move.
The U.S. has 9,635 plastics product manufacturing establishments employing about 677,302 workers (Census County Business Patterns, 2023).

Common questions

How small can I make the runner? Only as small as your resin supplier allows, because a runner that freezes early stops packing the part. For Delrin, DuPont sets the minimum at the part wall thickness plus 1 mm, and never below 1.5 mm.
How do I enter a trapezoidal or half-round runner? Enter the diameter of a round runner with the same cross-section area: 2 × √(area ÷ π). A half-round runner of radius r matches a round runner 1.41 r in diameter.
Does a hot sprue bushing take the sprue out of the shot? Yes. Its melt stays molten and goes into the next shot instead of being ejected, so enter sprue length 0.
Why does the share matter if every runner is reground? Because feeding it back to the same press makes that share of the feed regrind. If the print or resin datasheet caps regrind lower, the surplus has to go to another job or be sold.

Related guides

Managing Cold Runner Scrap Like a Cost Center

Last reviewed 2026-10-01.