Cavity Weight

Cavity is the hollow region inside the mold that shapes the exterior of the molded part. Together with the core, it defines the final geometry: whatever the melt fills is exactly what becomes the part after cooling. ## Cavity vs. core - Cavity (female side): usually the fixed half of the mold, defines the cosmetic / outer surface. - Core (male side): usually the moving half, defines the inside and houses the ejector pins. The line where the two halves meet is the parting line. ## Single- vs. multi-cavity molds - 1 cavity: prototypes, large parts, low-volume technical work - 2, 4, 8, 16 cavities: medium production (containers, caps) - 32, 64, 96, 128 cavities: high production (PET closures, preforms) - Family molds: different cavities in the same tool to produce a set of parts ## Critical design considerations Balanced runners so all cavities fill simultaneously, symmetrical cooling, draft angle on every vertical wall, surface finish (texture, VDI or SPI polish), and replaceable inserts in high-wear areas (gates, cores). ## Typical cavity defects Imbalance in multi-cavity molds (some parts with flash, others short), scratches from misalignment, ejector marks from poor pin placement, and localized wear at gates of cavities with weaker cooling.

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.

Mold (Tool) is the mechanical assembly of plates, cavities, injection system and cooling that shapes the molded part. It is the single most expensive asset of the operation (10,000 – 500,000 USD) and its design dictates everything: cycle, quality, productivity and unit cost. ## Main components - Cavity plate (fixed half): nozzle side, usually houses the cavity - Core plate (moving half): ejector side, holds the core and ejector pins - Injection system: sprue, runners, gates (cold or hot) - Cooling system: water/glycol channels, conformal in premium molds - Ejection system: pins, sleeves, stripper plates, slides for undercuts - Standard components: leader pins, bushings, retainers, sensors - Replaceable inserts in wear areas ## Mold types - Single-cavity: prototypes, large parts, low production - Multi-cavity (2/4/8/16/32+): mass production - Family mold: different cavities for parts of the same assembly - Cold runner: with cold runners separated each cycle - Hot runner: no runner scrap, shorter cycles - Stack mold: two levels of cavities to double capacity - Two-shot / multi-material: two resins in the same part ## Mold materials - P20 (pre-hardened steel): standard for medium production, easy to machine - H13: hardened inserts, high thermal-wear resistance - S136 (stainless): polished cavities, corrosion resistance (PVC, PET) - Aluminum (7075): prototype or low-volume molds - NAK80: mirror polish without distortion ## Typical service life - Aluminum: 5,000 – 50,000 cycles - P20: 100,000 – 1,000,000 cycles - Hardened H13: 1 – 10 million cycles - Carbide / TZM in hot-runner gates: up to 50 million ## Critical maintenance Cleaning after every production run, vent inspection, lubrication of pins and guides, monitoring of cooling channels (scaling), and repair of cavity damage before it spreads.

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.