The runner is a part you throw away
Take the tool in the form: a trapezoidal runner 6 mm across the parting line, 4.4 mm at the bottom, 4.5 mm deep, running 180 mm in total to four gates, fed by a sprue that tapers from 3.5 mm to 7 mm over 60 mm. The runner section works out to 23.4 mm2, so the runner is 4.21 cm3 and the sprue cone another 1.35 cm3. At 1.05 g/cm3 that is 5.8 grams of resin per cycle that never becomes a part.
Against four 22.6 gram mouldings, the shot is 96.2 grams and 6.1 percent of it is runner. Put another way, the whole runner weighs 25.8 percent of one part. Nobody would tolerate a 6 percent scrap rate on parts, and this is the same material.
Where the money actually is
At 1.45 a pound for virgin and 20 cents a pound for scrapped runner, and with none of it going back through the machine, the material in one part costs 7.63 cents. Per thousand parts that is 76.27 dollars, and the 5,000 part run in the form is 381.35 dollars of resin.
Granulate the whole runner and feed it back and the same thousand parts costs 72.25 dollars, a saving of 4.02 dollars per thousand or 20.10 over the run. That is the number worth arguing about, and it is far smaller than most people expect, because the scrap value was doing some of the work already — the runner was never worth nothing.
Section shape matters more than length
Runner volume goes as the section area, and area goes as the square of the size. Trimming the trapezoid from 6 mm wide to 5 mm, keeping the same depth and the same taper, drops the section from 23.4 to 18.9 mm2 and takes about 0.85 grams a shot out of the waste. Shortening the layout by 20 mm, which is usually a much harder change to make, saves about 0.5 grams. The section is the lever.
Whether the smaller section still fills the part is not a question arithmetic answers. Runner sizing is a flow problem with a pressure drop and a freeze time attached to it, and it belongs to whoever is doing the filling analysis. All this page will tell you is what a given section costs once it is decided.
What is not counted
Gates, cold slug wells and the little that stays in the nozzle are all outside the model, and on some tools they are a meaningful fraction of the waste. Start up scrap is outside it too, and on a short run that can dwarf everything here: fifteen shots to get the process settled on a 5,000 part job is 1.4 kg of resin, against 7.3 kg of runner across the whole run, so on a shorter order it stops being a rounding error quickly.
The way to close the gap is to weigh things. Weigh one full shot as it comes out of the tool, weigh the four parts after trimming, and the difference is your real waste per shot including everything the geometry misses. Put that against the figure here and the size of the gap tells you which of the omissions matters on this tool.
Questions people ask
How do I work out runner volume?
Cross section area times total length, then add the sprue as a truncated cone. A full round runner is pi times the diameter squared over four; a half round is half that; a trapezoid is the average of the two widths times the depth. The sprue is pi times the length times the sum of the two radii squared plus their product, over three. This page does all four and turns the volume into grams through the density from the resin datasheet.
What percentage of a shot is normally runner?
It is not a number anybody can give you, because it depends on the part weight, the number of cavities, the layout distance and the section a filling analysis called for. The default tool here comes to 6.1 percent, and the same runner on a tool moulding 5 gram parts instead of 22.6 gram ones would be 23 percent. What is worth doing is measuring your own: weigh a full shot, weigh the trimmed parts, and divide.
Does regrind save money?
At normal prices yes, because runner you feed back displaces virgin you would otherwise buy, and it is worth more inside the machine than a reprocessor pays for it. On the default job, returning all of it saves 4.02 dollars per thousand parts. Whether the resin, the part and the customer will accept regrind is an entirely separate question and one this page has no view on at all.
Is the runner counted in the shot weight?
Yes, and it has to be. The barrel delivers the runner every cycle whether or not you keep it, so it takes screw stroke, it takes residence time and it takes drying capacity. The only thing that changes when you regrind is what you buy, not what the machine moves.
Why does the cost show four decimal places?
Because the material in one moulded part is a few cents, and rounding it to the nearest cent throws away the part of the number people are actually comparing. The page carries the money as whole cents per thousand parts and divides down for display, so the per part, per hundred and per run figures are all derived from one integer rather than rounded separately and drifting apart.