Total Weight Required

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.

Runner is the set of channels through which molten plastic flows from the sprue to each cavity gate. In multi-cavity molds its design determines fill balance and the amount of scrap generated per cycle. ## Runner types - Cold runner: in-mold cold channel that fills every cycle and is separated from the part as scrap. Simple and inexpensive, ideal for thermally sensitive resins. - Hot runner: heated channel that keeps plastic fluid, no scrap but higher tooling cost. See the hot-runner entry. - Insulated runner: rare hybrid, no external heating, frozen outer skin acts as insulation. ## Cross sections - Trapezoidal: the most common cold-runner section, easy to machine. - Full round: requires both halves of the mold, best area-to-perimeter ratio. - Half round: only one side, less efficient than full round. - Modified parabolic: compromise between flow area and machinability. ## Balanced design - Natural balance: equal flow lengths from sprue to each cavity (H, X, star layouts). - Artificial balance: diameters adjusted to compensate unequal lengths. - Typical diameters: 4 – 10 mm in cold runner, 8 – 20 mm in hot-runner manifold. ## Common issues Cavity imbalance (some with flash, some short), excessive scrap from oversized channels, premature freeze in undersized channels, and degradation of heat-sensitive resins on long runners.

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.

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.