Shot Weight

Cavity weight is the mass of plastic in a single cavity — the weight of one molded part as it comes out of the mold. It is the basic building block for sizing a shot, estimating material use and balancing a multi-cavity tool, and it is usually found simply by weighing a good part on a scale. ## How it fits the shot A full shot is more than the parts: > Shot weight = (cavity weight × number of cavities) + runner + sprue So cavity weight scales up into the shot weight and feeds the total weight required used to plan material per run. If you know the part's volume and the resin's specific weight (density), you can also estimate cavity weight before the first shot. ## Why it matters - Material planning & cost: cavity weight × cavities × shots gives resin consumption and part cost. - Process monitoring: a stable part weight shot-to-shot is one of the clearest signals of a stable process; a drop signals a short shot, a rise signals flash or overpacking. - Cavity balance: in a multi-cavity mold, comparing each cavity's weight reveals fill imbalance — heavy and light cavities mean the runner or gates need balancing. ## Related terms - See also: molded part, shot weight, total weight required, cavity, specific weight ## What is cavity weight in injection molding? The weight of the plastic in one cavity — i.e. one molded part — usually measured by weighing a finished part; it is the basis for shot size, material planning and checking cavity-to-cavity balance. ## How do you calculate cavity weight? Weigh a good part on a precise scale, or estimate it from the part's volume times the resin's density (specific weight). Multiply by the number of cavities and add runner and sprue to get the full shot weight. ## Why monitor part (cavity) weight during production? Because a consistent part weight shot-to-shot indicates a stable process; a falling weight points to short shots, a rising weight to flash or overpacking, making weight a simple, powerful quality check.

A cold runner is an unheated channel system in the mold that carries the melt from the sprue to each cavity. Because the runner is not heated, the plastic in it cools and solidifies along with the parts every cycle, so the runner and sprue are ejected as a connected skeleton and become scrap (usually reclaimed as regrind). It is the simpler, cheaper alternative to a hot runner. ## How it works Each shot fills the cold runner first, then the cavities through the gates. When the part solidifies, the runner does too; the whole runner-and-part assembly is ejected, then the runner is degated, separated and reground. Layouts are kept balanced so every cavity fills evenly. ## Cold runner vs hot runner - Cold runner: unheated; runner freezes and is ejected each cycle → runner scrap/regrind, but low tooling cost, simple, easy color/material changes, tolerant of many resins. - hot runner: heated manifold keeps the runner molten → no runner scrap, faster cycles, automation-friendly, but higher mold cost, more maintenance and harder color changes. ## Trade-offs and use Cold runners suit lower volumes, frequent color/material changes, and shops that can regrind their runner economically. The downsides are the recurring runner scrap, extra material per shot, degating labor, and the heat-history hit each time the runner is reground. Good cold-runner design minimizes runner volume while keeping fill balanced. ## Related terms - See also: runner, sprue, hot runner, regrind, cavity ## What is a cold runner in injection molding? An unheated runner system that delivers melt from the sprue to the cavities; the plastic in it solidifies with the part each cycle and is ejected as runner scrap, usually reground and reused. ## What is the difference between a cold runner and a hot runner? A cold runner is unheated and freezes into scrap every cycle (cheap, simple, easy color changes); a hot runner is heated to stay molten, eliminating runner scrap and speeding cycles, but costs more and is harder to maintain. ## What happens to the cold runner after molding? It is ejected attached to the parts, then degated (separated from the parts) and typically granulated into regrind that is blended back with virgin resin at a controlled ratio.

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 size is the volume of melt the screw meters and injects each cycle — set as a screw-position stroke (mm) or a volume (cm³). It is the volumetric twin of shot weight, which is the same material expressed as mass. ## How it is set - During recovery (dosing) the screw rotates and retracts to a set position; that retraction distance defines the shot size. - Set it so the first-stage (velocity-controlled) fill reaches about 95–99 % of the part, then hold packs it out — leaving a stable cushion so the screw never bottoms out. - The shot should use roughly 20–80 % of the barrel's rated capacity (see barrel occupancy) to keep residence time in range. ## Why it matters Shot size drives part-weight repeatability and where the transfer position cut off lands. Too small and you cannot fill the part and still keep a cushion; too large and you waste material, stretch residence time and risk degradation. ## Shot size vs shot weight - Shot size: the volume or screw stroke the machine delivers per cycle. - shot weight: the mass of that same shot (parts + runners + sprue), read on a scale. Shot size × melt density ≈ shot weight. ## Related terms - See also: shot weight, cushion, recovery, barrel occupancy, transfer position cut off ## What is shot size in injection molding? It is the metered volume (or screw stroke) of melt injected per cycle, set during recovery so the part fills on first stage and a cushion remains. ## How do you set shot size? Dose to a screw position that fills about 95–99 % on first stage and leaves a small, stable cushion, keeping the shot within roughly 20–80 % of barrel capacity. ## What is the difference between shot size and shot weight? Shot size is volumetric (screw stroke or cm³); shot weight is the mass of the same shot in grams. Multiplying shot size by melt density gives approximately the shot weight.

Total weight required is the total mass of plastic needed to produce an order — the material-planning figure a molder calculates before a run so the right amount of resin is dried, dosed and purchased. It builds directly on the shot weight: how much each shot consumes, multiplied across all the shots the job needs. ## How it is calculated Start from one shot and scale up: > Shot weight = (cavity weight × number of cavities) + runner + sprue > Shots needed = required good parts ÷ (cavities × yield) > Total weight required = shot weight × shots needed + allowances Allowances cover purge, start-up scrap, rejects and a safety margin, so the real material order is a bit above the theoretical minimum. ## Why it matters - Material purchasing & inventory: tells you how much resin (and color/additive) to buy and dry for the run, avoiding both shortages and costly leftover lots. - Costing & quoting: total resin mass × price is a core input to part cost; runner/sprue waste and regrind recovery shift the real figure. - Drying & logistics: the amount drives dryer capacity, number of hopper loads and delivery scheduling. Reducing runner and sprue mass, recovering regrind, and improving yield all lower the total weight required for the same number of good parts. ## Related terms - See also: shot weight, cavity weight, runner, regrind, specific weight ## What is total weight required in injection molding? The total mass of plastic needed to fill an order — shot weight multiplied by the number of shots, plus allowances for purge, start-up scrap and rejects — used to plan how much resin to dry and buy. ## How do you calculate total material for a molding run? Find the shot weight (cavity weight × cavities + runner + sprue), divide the required good parts by cavities and yield to get the shots needed, multiply the two, then add purge and scrap allowances. ## How can you reduce the total weight of resin required? Trim runner and sprue mass, recover and reuse regrind, raise first-pass yield, and right-size the shot — each cuts the resin consumed per good part without changing the part itself.