Runner Scrap

Managing Cold Runner Scrap Like a Cost Center

Cold runners consume resin every shot and most shops never weigh them. A playbook for runner weight standards, ratio benchmarks, and hot runner conversion math.

On small parts the runner is often the biggest material line nobody manages. Take an 8 g part in a 4 cavity cold runner tool with a 14 g runner system: that is 3.5 g of runner per part, a 44 percent material penalty on every shot. At 2 million parts a year and $2.40 per kilogram, the runner alone consumes 7,000 kg and $16,800 of resin. If it cannot be reground back into the process, that is pure cost. Even at full regrind, you paid to melt, inject, cool, and granulate it. The runner is a cost center, and it deserves a standard, a trend chart, and an owner.

Estimate runner weight before the tool exists so the quote carries it. Take the volume of each segment times resin density, with a geometry factor for the sprue and sub-runner transitions. A full-round 6 mm runner has a cross section of 0.283 cm2, so 300 mm of total runner length holds 8.5 cm3, which in ABS at 1.04 g/cm3 is 8.8 g before the sprue. Add a typical sprue at 3 to 6 g and you are at 12 to 15 g. The Runner Weight calculator handles the segment math and the geometry factor; your job is honest lengths and diameters off the layout.

Benchmark runner-to-part weight ratio by part size. Under 20 percent is solid for mid-size parts, under 10 percent is excellent, and anything over 50 percent should trigger a design review. Small parts under 5 g routinely run 80 to 150 percent ratios in cold runner tools, which is exactly where hot runners earn their keep. Track the ratio per tool, not per plant: a plant average hides the three tools generating half the regrind. In most shops I have audited, 20 percent of tools produce 60 to 70 percent of runner scrap by weight.

Run hot runner conversion math once a year on the worst ratio tools. A 4 drop valve gate system runs $25,000 to $45,000 installed. A tool making 500,000 shots a year with a 12 g runner generates 6,000 kg of runner; if half of that value is lost to regrind handling, downgrading, and scrap, you are burning $7,000 to $14,000 a year at $2.40 per kilogram. Add 1 to 2 seconds of cycle saved because the fat sprue no longer controls cooling, worth another $3,000 to $8,000 in press time, and payback lands at 18 to 36 months. Under 3 years, convert.

Regrind is not free resin. Granulating, conveying, and blending cost $0.10 to $0.25 per kilogram in labor and energy, and most specs cap regrind at 10 to 25 percent because each heat history knocks 5 to 10 percent off impact properties in ABS and nylon. Balance the loop: a tool with a 40 percent runner ratio generates more regrind than a 25 percent blend cap can consume, so the surplus gets sold as scrap at 20 to 40 percent of virgin price. That gap between virgin cost and scrap recovery is the real runner cost, and it belongs on the tool's cost sheet.

The design levers are diameter and layout. Runner volume scales with diameter squared, so taking a runner from 8 mm to 6 mm cuts its volume 44 percent. Most cold runners are oversized because nobody wants a fill problem at trials; size to the calculated pressure drop instead, and leave steel-safe room to open up if needed. Shorten total runner length with a tighter cavity layout, use trapezoidal sections only where the parting line demands them since they carry 15 to 25 percent more volume than full-round for the same flow, and add cold slug wells so you are not upsizing the runner to fix a slug defect.

The failure modes repeat everywhere. Runners get sized generous for trials and never trimmed afterward, so every tool carries insurance volume forever. Material gets quoted on part weight only, and then yield variance runs 4 percent and nobody knows why. Regrind gets counted as 100 percent recovery while the granulator sits down half the year and the gaylords quietly go out the door at 30 percent of virgin price. And standards drift: a 12 g runner becomes 14 g after a weld repair opens the sprue, a 17 percent increase nobody catches because nobody has weighed a runner since week one.

Deploy it simply. Daily, operators drop one full runner per shift on the scale and log it; anything 5 percent over standard flags a setup review. Weekly, the material handler reports regrind generated versus consumed by tool. Monthly, rank tools by runner kilograms and review the top five for diameter trims or hot runner conversion. World class is a plantwide runner ratio under 15 percent, every tool with a weighed standard, regrind loops balanced within 10 percent, and a conversion review that actually retires two or three cold runner systems a year.

Published 2026-07-02.