Shot Size

Barrel occupancy is the share of the barrel's rated shot capacity actually used by the shot, expressed as a percentage. It is the single best check that a job is on the right-size machine, because it drives melt quality and residence time. ## How to calculate it Barrel occupancy (%) = shot weight ÷ barrel rated shot capacity × 100 (equivalently, the shot size stroke ÷ maximum screw stroke). Example: a 60 g shot on a barrel rated for 150 g is 40 % occupancy. ## The recommended window Keep occupancy roughly between 20 % and 80 %, with the practical sweet spot around 20–65 %: - Below ~20 %: the shot is tiny for the barrel; the resin sits too long, residence time stretches out and the polymer degrades. - Above ~80 %: too little reserve; you get unmelted material, poor melt uniformity and long screw recovery. ## Why it matters Occupancy is how you sanity-check machine selection without re-deriving residence time every time. If a job falls outside the window, move it to a barrel of a different size rather than fighting splay, color and recovery problems on the wrong press. ## Related terms - See also: barrel, shot weight, shot size, residence time, screw ## What is barrel occupancy in injection molding? It is the percentage of the barrel's rated shot capacity used by the shot — shot weight divided by barrel capacity — used to confirm the job is on a correctly sized machine. ## What is a good barrel occupancy? Generally 20–80 %, with 20–65 % the practical sweet spot for stable melt and acceptable residence time. ## What happens outside the 20–80 % range? Below 20 % the resin over-residences and degrades; above 80 % you get unmelt, poor mixing and slow recovery — both point to the wrong-size barrel.

Recovery (also called plasticizing, dosing or charging) is the stage of the cycle where the screw rotates and retracts to melt and meter the next shot. It runs during cooling, building a reservoir of melt ahead of the screw tip up to the set shot size while leaving a cushion. ## How it works As the screw turns, the flights convey pellets forward; shear plus barrel heat melt them and the new melt collects ahead of the screw (the check valve opens to let it pass), pushing the screw back to the metering position. Two main controls: - Screw rotation speed (RPM): how fast the shot is built. - Back pressure: resistance to the screw's retreat — more back pressure improves mixing, colour dispersion and melt uniformity, but adds shear, heat and residence time. ## Timing — keep it off the critical path Recovery should finish within the cooling time so it is not the cycle-limiting step. If recovery takes longer than cooling, the cycle waits on the screw. Use recovery protect time and rotate delay recovery delay to manage when it starts. ## Why it matters A repeatable recovery time and a stable cushion signal a healthy melt-delivery system; erratic recovery points to feed problems, a worn check valve or wrong back pressure. ## Related terms - See also: screw, shot size, cushion, cooling time, check valve ## What is recovery in injection molding? It is the plasticizing/dosing stage where the screw rotates to melt and meter the next shot during cooling, controlled by screw speed and back pressure. ## What is the difference between recovery and injection? Recovery builds and meters the next shot (the screw rotates and retreats); injection pushes that shot forward into the mold (the screw moves forward without rotating). ## What does back pressure do during recovery? It resists the screw's retreat, improving melt mixing, homogeneity and colour dispersion, at the cost of more shear, higher melt temperature and longer residence time.

A shot is the complete charge of molten plastic injected into the mold in one cycle — and, as a verb, the act of injecting it. One shot equals one molding cycle and fills every cavity plus the runners and sprue. ## What a shot includes - All the molded parts (one per cavity in a multi-cavity mold). - The runner system and sprue that feed them. A shot is the basic production count: "shots per hour" and total shots are how output and tooling life are tracked. ## How a shot is quantified - shot size: its volume, set by the screw stroke that delivers it. - shot weight: its mass on a scale (parts + runners + sprue). The machine never fully empties — a small cushion always stays ahead of the screw. ## Related issues A short shot is a shot that did not fully fill the cavity (a defect). Shot-to-shot consistency — stable weight and cushion — is the core measure of a stable process. ## Related terms - See also: molding cycle, shot size, shot weight, cavity, cushion ## What is a shot in injection molding? It is the full charge of melt injected per cycle — all the parts plus runners and sprue — and it equals one molding cycle. ## Is one shot one part? Not necessarily: a shot fills every cavity, so a 4-cavity mold makes four parts per shot, plus the runners and sprue. ## What is the difference between a shot and shot size? A shot is the actual charge injected each cycle; shot size is the volumetric setting (screw stroke) that determines how big that charge is.

Shot weight is the total mass of plastic injected in one cycle — every cavity's molded part plus the runners and sprue. It is the number you read by weighing one full shot on a scale, and it drives machine sizing, dosing and several derived calculations. ## How to find it - Weigh one complete shot (parts + runners + sprue) on a gram scale — that is the shot weight. - Or estimate it: shot weight = (part weight × number of cavities) + runner and sprue weight. - By volume: shot weight = shot volume × melt density of the resin. ## Why it matters - Machine selection: the shot should sit comfortably inside the barrel's usable range — neither so small that residence time stretches out, nor so close to the maximum that melt quality suffers (see barrel occupancy). - Material planning: shot weight × cycles = resin consumption, including the scrap from runners. - Process setup: it anchors the dosing stroke and the transfer-to-cushion position. ## Shot weight vs related terms - Part weight / cavity weight: only the molded part(s), without runners. - shot size: usually the volumetric stroke (cm³ or mm of screw travel) that delivers the shot weight. ## Related terms - See also: shot size, barrel occupancy, residence time, cavity weight, cushion ## What is shot weight in injection molding? It is the total grams of plastic injected per cycle — all parts plus runners and sprue — found by weighing one full shot. ## How do you calculate shot weight? Multiply part weight by the number of cavities and add the runner and sprue weight, or weigh a complete shot directly on a scale. ## What is the difference between shot weight and part weight? Part weight is only the molded part; shot weight adds the runners and sprue, so it is always equal to or greater than the combined part weight.