Cavity

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.

Former holes (hole-forming features, core pins) are the parts of the mold whose job is to create the holes, bores and openings called for by the part design. Where the cavity forms the outer shape, a former hole is the positive steel — a pin or core — that the plastic flows around, leaving a hole in the finished molded part. ## How they are built - Integral (same formation): machined directly into the core or cavity block when the hole runs in the mold's opening direction, so it clears as the mold opens. - Sliders / side actions: when a hole runs across the opening direction (a side hole or undercut), the hole-forming pin is mounted on a slider (side-action core) that retracts sideways before ejection, then returns for the next shot. ## Why they matter - Function & assembly: molded-in holes for screws, snaps, shafts and ports avoid secondary drilling and support clean design for assembly. - Flow & defects: melt splitting around a pin rejoins on the far side, forming a weld line; pins also need support, or they deflect under melt pressure and shift the hole. Worn or mismatched pins let plastic seep and create flash around the hole. - Cooling & wear: thin pins run hot and wear fastest, so they are a common maintenance and dimensional-control point. ## Design notes Holes are placed and sized with draft and steel strength in mind; through-holes are often formed by a pin meeting the opposite face ("shut-off"), while blind holes use a single supported pin. Their location relative to the parting line decides whether a simple integral pin or a slider is needed. ## Related terms - See also: cavity, molded part, parting line, design for assembly, flash ## What are former holes in a mold? Mold features — pins or cores — that form the holes and openings in a molded part; the plastic flows around the steel pin, leaving a hole when the part is ejected. ## How are holes made in injection-molded parts? By hole-forming pins or cores in the mold: integral pins for holes in the opening direction, and sliders (side-action cores) that retract sideways for holes or undercuts running across the opening direction. ## Why do molded holes sometimes have weld lines or flash? The melt splits around the pin and rejoins on the far side, leaving a weld line; if the pin is worn, unsupported or mismatched to its mating face, plastic seeps past it and forms flash around the hole.

Gate is the final section of the feed channel where molten plastic enters the mold cavity. Its geometry —shape, size, location— drives fill behavior, surface quality, visible marks and balance across cavities. ## Most common gate types - Edge gate: simplest, on the parting line. Medium parts - Submarine / tunnel gate: self-shears at ejection, no manual cut needed. Diameter 0.5 – 2 mm - Pin gate: small point on the part face. Needs three-plate or hot runner - Direct / sprue gate: sprue feeds directly. For large single-cavity parts - Fan / film gate: for wide flat parts, avoids flow lines - Diaphragm gate: for cylindrical parts, uniform radial fill - Hot tip (valve gate): hot runner with mechanical shut-off, ideal cosmetics - Ring gate: annular, for long cylindrical parts - Tab gate: intermediate tab that is later cut off ## Typical parameters - Diameter: 0.5 – 4 mm depending on part and resin - Land length: 0.5 – 1.5 mm (short, to avoid premature freeze) - Thickness (fan / edge): 30 – 80 % of wall thickness - Gate vs. wall ratio: <0.5 to minimize visible mark ## How to select a gate - Cosmetic: pin gate or valve gate (minimal mark) - Cost-effective: edge gate (easy to machine, simple maintenance) - Self-separation: submarine or tunnel gate - Large single-cavity part: direct / sprue gate - Balanced multi-cavity: pin gate with hot runner ## Common issues - Jetting: jet stream when gate is too open and velocity too high - Premature gate freeze: gate too small → gate seal not reached - Gate vestige (mark): on cosmetic parts, requires smaller gate or valve gate - Gate wear: with fiber-reinforced resins, requires hardened inserts

A molded part is the finished plastic component produced by injection molding, its shape formed by the mold cavity. It is the deliverable of the whole process — one part per cavity, per shot. ## Part vs shot - Molded part: a single finished component (its mass is the part weight, see cavity weight). - shot: everything injected in one molding cycle — all parts plus runners and sprue. A 4-cavity mold yields 4 molded parts per shot. ## What defines a good part A molded part is judged against the print on several axes: - Dimensions: within tolerance, allowing for contraction (shrinkage) and dimensional stability over time. - Weight: stable shot-to-shot — the simplest health check of the process. - Appearance: free of sink, flash, short shots, splay, weld lines and burns. - Mechanical / functional: strength, fit and function as designed. A part that fails any of these becomes scrap. ## From mold to inspection After cooling, the part is freed by part ejection and removed by free-fall, robot or operator, then it may be degated, inspected and packed. ## Related terms - See also: cavity, shot, molding cycle, part ejection, dimensional stability ## What is a molded part in injection molding? It is the finished plastic component shaped by the mold cavity, produced one per cavity each shot, and judged on dimensions, weight, appearance and function. ## What is the difference between a molded part and a shot? A molded part is one finished component; a shot is everything injected in a cycle — all the parts plus the runners and sprue. ## How is molded-part quality checked? By dimensions against tolerance, stable part weight, cosmetic appearance (no sink, flash or short shots) and mechanical/functional performance; failures are scrapped.

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.