How do you calculate runner weight?

Treat the runner and sprue as cylinders: π / 4 x diameter² x length gives the volume in mm³, divided by 1,000 for cc, times the resin density. A 6 mm runner 160 mm long and a 60 mm sprue at 5.25 mm mean diameter in POM at 1.42 g/cc weigh 8.27 g, 8.6% of a shot with 88 g of parts, the Runner Weight calculator's own example.

What is the share of shot for?

Reground into the same job, the runner becomes the regrind share of the feed. If the share is above the regrind limit on the part print, not all of the runner can go back into that job; the rest has to go to another product or be scrapped.

What runner length should I enter?

Every branch from the sprue to the gates, plus cold slug wells, from the mould drawing. For a sprue-gated part enter 0; for a hot runner feeding cold sub-runners enter only the sub-runners with a sprue length of 0.

How do I handle trapezoidal or stepped runners?

Enter a trapezoidal runner at the diameter of a round runner with the same area. A stepped runner needs each diameter separately, or one equivalent diameter that gives the same volume.

Does it include the gates?

No. Gate volume is small next to the runner. Weigh a real runner once the mould runs and compare it with the estimate.

Does the workbook use macros?

No. It is a plain .xlsx with ordinary formulas, tables and native charts. The Verify sheet re-checks the formulas against fixed test moulds and shows Self-check: PASS.

Production and Throughput

Runner Weight Excel Template

Runner and sprue weight = π ÷ 4 × diameter² × length ÷ 1,000 × resin density, and its share of the shot = runner and sprue ÷ (parts + runner and sprue). Give every cold-runner mould a row and see its runner weight, its share of the shot against the print's regrind limit, and the resin a run puts into runners.

Free, and a plain .xlsx with no macros to enable. A CSV with live formulas and a printed how-to guide come with it.

Overview

This runner weight template is a working Excel workbook for moulding process and tooling engineers, estimators and materials planners. Each cold-runner mould is one row with its resin, cavities and part weight, the runner diameter and total length of every branch, the sprue length and mean diameter, the regrind limit on the part print and the shots in a run. The workbook works out the runner and sprue weight from the drawing, its share of every shot, whether that share can all go back into the job as regrind, and the resin a run puts into runners. It ships with ten moulds in five resins at a fictional moulder, so every tile and chart is filled in when you open it.

In the example a 4-cavity POM gear housing with a 6 mm runner 160 mm long and a 60 mm sprue moulds 8.27 g of runner and sprue with 88 g of parts: 8.6% of the shot, inside its 15% regrind limit. A 32-cavity clip mould feeds 0.9 g parts through 640 mm of 4 mm runner: 30% of every shot is runner against a 20% limit, 753 kg of POM a run, so it is the candidate for a hot runner. A medical housing allows no regrind at all, so its runner cannot go back into the job however small it is.

Runner weight grows with the square of the diameter, so the runner diameter is the lever. The download is a plain .xlsx with no macros. A CSV with live formulas is also included for other tools; the dashboard, charts, dropdowns and self-check are in the .xlsx.

What this template includes

Setup sheet with your plant and currency, and the resins with their density and price per kg
Moulds table, one row per cold-runner mould: part, resin, cavities, part weight, runner diameter and length, sprue length and mean diameter, regrind limit and shots per run, with a Check column
Per mould: parts per shot, runner, sprue and runner-and-sprue weight, shot weight, share of shot, a regrind status in words, runner resin per run and its value
Dashboard with four tiles (moulds, runner resin per run, its value, moulds over their regrind limit), share of shot against regrind limit, runner and sprue weight per shot, and the mould table
Method sheet with every formula, and a Verify sheet that re-checks the formulas against an independent reference and the Runner Weight calculator (Self-check: PASS)
CSV with live formulas for the Moulds table; the dashboard, charts, dropdowns and self-check are in the .xlsx

Suggested use case

Use it to estimate runner weight from a new mould's drawing before there is a runner to weigh, to check runner share against each part's regrind limit, or to put a number on the resin your runners take each run when deciding on a hot runner.

How to use it

On Setup, type your plant and currency, and list your resins with their density and price per kg.
On Moulds, give each mould a row: resin, cavities, part weight, runner and sprue dimensions, the print's regrind limit and shots per run.
Fix anything the Check column names.
Open the Dashboard: runner share of shot against regrind limit, and the resin each run puts into runners.

Frequently Asked Questions

How do you calculate runner weight?: Treat the runner and sprue as cylinders: π / 4 x diameter² x length gives the volume in mm³, divided by 1,000 for cc, times the resin density. A 6 mm runner 160 mm long and a 60 mm sprue at 5.25 mm mean diameter in POM at 1.42 g/cc weigh 8.27 g, 8.6% of a shot with 88 g of parts, the Runner Weight calculator's own example.
What is the share of shot for?: Reground into the same job, the runner becomes the regrind share of the feed. If the share is above the regrind limit on the part print, not all of the runner can go back into that job; the rest has to go to another product or be scrapped.
What runner length should I enter?: Every branch from the sprue to the gates, plus cold slug wells, from the mould drawing. For a sprue-gated part enter 0; for a hot runner feeding cold sub-runners enter only the sub-runners with a sprue length of 0.
How do I handle trapezoidal or stepped runners?: Enter a trapezoidal runner at the diameter of a round runner with the same area. A stepped runner needs each diameter separately, or one equivalent diameter that gives the same volume.
Does it include the gates?: No. Gate volume is small next to the runner. Weigh a real runner once the mould runs and compare it with the estimate.
Does the workbook use macros?: No. It is a plain .xlsx with ordinary formulas, tables and native charts. The Verify sheet re-checks the formulas against fixed test moulds and shows Self-check: PASS.