Shot Size

The Shot Size Playbook: Setup, Barrel Utilization, and Residence Time

Shot size decides whether the barrel works for you or cooks resin against you. This playbook covers the part and runner math, the 20 to 80 percent barrel rule, residence time checks, and the setup audits that keep part weight stable.

Shot size errors show up as scrap, degraded resin, or both. Undersize the shot and you run short parts and starve the cushion; oversize it and resin sits in the barrel long enough to degrade, producing black specks, splay, and brittle parts that fail at the customer. The money is not small: a job running 8 cavities at 12 grams per part with a 10 percent unnecessary overfill wastes purge and regrind, but the degradation side is worse, because one lot of embrittled nylon parts in the field can cost more than a year of resin. Shot size is a calculated setup value, and plants that treat it as a knob to twist during startup pay for it in ways the press report never shows.

The setup math: shot size equals part weight times number of cavities, plus runner weight, times an overfill safety factor. An 8 cavity tool making 12 gram parts with an 18 gram cold runner needs 114 grams of melt; apply a 5 percent factor and set up for 120 grams. The Shot Size calculator turns this into a 30 second pre-setup step, and converting grams to screw stroke closes the loop: with a known barrel and resin density, that 120 grams becomes a specific shot stroke, and first-piece part weight confirms it. A full-shot part weight check at setup, all cavities plus runner on a scale, catches cavity imbalance and calculation errors before the first pallet does.

Then check the shot against the barrel, because the same shot can be healthy on one press and destructive on another. The working rule is 20 to 80 percent of barrel capacity, with 25 to 65 percent as the comfortable middle. Residence time makes the rule concrete: multiply barrel capacity divided by shot weight by the cycle time. A 120 gram shot in a 400 gram barrel at a 30 second cycle means 3.3 shots in the barrel, about 100 seconds of residence, which is fine. The same shot in a 1,500 gram barrel at the same cycle is over 6 minutes, and heat-sensitive resins like POM, PVC, and flame-retardant grades start degrading at 5 to 8 minutes at melt temperature.

Levers worth running: hot runners delete the runner weight from every shot, and 18 grams times 1 million cycles is 18 metric tons of resin, worth roughly $40,000 at $1 per pound before regrind handling costs; balanced cavities, verified by individual cavity weights within 2 to 3 percent of each other, let you cut the overfill factor from 10 percent toward 3 to 5; and matching jobs to barrels at scheduling time, not setup time, keeps residence inside the window without heroics. Regrind policy belongs here too: a stable 15 to 20 percent regrind ratio with consistent granule size holds shot-to-shot recovery steady, while wild ratios move melt density and effectively change your shot every hour.

Failure modes to hunt: shots below 20 percent of barrel showing up as intermittent black specks and gels, which teams chase with temperature changes when the real fix is a smaller barrel or shorter cycle exposure; shots above 80 percent where the screw cannot recover melt before cooling ends, so recovery time silently becomes the cycle constraint and melt uniformity collapses; and excessive decompression after recovery pulling air into the nozzle, which reads as splay and gets misblamed on moisture. The subtlest one is setup drift, where each changeover copies the last tech's tweaks instead of the calculation, and after five setups the shot is 15 percent off the engineering number with nobody able to say why.

Cadence: every setup starts from the calculated shot, not the last saved program, and first-piece full-shot weight gets recorded against the target with a plus or minus 1 percent expectation. Daily, part weight SPC on the highest-value jobs, since part weight drift beyond 0.5 percent flags check ring wear, resin lot changes, or heater drift before dimensions do. Weekly, review residence time on any job that moved presses. Monthly, run a cavity balance study on one workhorse tool, five shots with individual cavity weights, and open a mold work order when any cavity strays past 3 percent. This is under an hour a day across a typical 20 press plant.

World class shot management: setup sheets show the calculation, not just the answer; part weight runs a Cpk above 1.33 on controlled jobs; barrel utilization and residence time are checked at scheduling, so heat-sensitive resins never land on oversized barrels; and overfill factors sit at 3 to 5 percent because cavity balance is maintained instead of buffered. The visible results are black speck complaints approaching zero, startup scrap cut by half because first shots land on weight, and changeovers that hit saleable parts in 5 to 10 shots instead of 50 because the shot was right before the press ever cycled.

Published 2026-07-02.