Gates

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.

Hold Time is the duration of the packing/hold phase during which controlled pressure is applied to the material in the cavity to compensate for shrinkage during initial cooling. It ends when the gate freezes and no more material can flow into the cavity. ## How to determine the optimum — gate seal study The most reliable method is weighing parts at increasing hold times: 1. Mold parts with hold of 0.5, 1, 2, 3, 5, 8, 12 s 2. Weigh each (scale with 0.01 g precision) 3. Plot weight vs. hold time 4. Weight rises until it plateaus once the gate freezes 5. Optimal time = first point of plateau + 10 % margin ## Typical values - Small parts (<10 g), wall <2 mm: 1 – 3 s - Medium parts, 2 – 4 mm wall: 3 – 8 s - Large parts, >4 mm wall: 8 – 20 s - Thick parts (>6 mm): up to 60 s - Hot runner: depends on gate type (valve gate shorter) ## Why it matters - Too short (before gate seal): material backs out → sinks, low dimensions - Optimum (at gate seal): maximum weight, repeatable dimensions - Too long (after gate seal): does not affect the part, wastes cycle time ## Relation to other parameters - Wall thickness: thicker wall → longer hold - Gate diameter: bigger gate → longer hold - Mold temperature: colder → gate freezes faster → shorter hold - Gate type: valve gate closes mechanically, time is not freeze-dependent ## Common mistakes - Time "by feel" with no gate seal study, usually over-sized - Not re-validating when changing resin or batch - Multi-cavity: same time for all, but freeze may be asymmetric - Confusing hold time with cooling time (they often overlap)

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.

Sprue is the main channel of the mold that receives molten plastic directly from the machine nozzle and conducts it to the runner (or directly to the cavity in single-cavity molds). In cold-runner molds it is the first piece of scrap generated each cycle. ## Sprue geometry - Conical shape with 2 – 4° per side (4 – 8° included) for clean ejection - Entry radius equal to or larger than the nozzle radius - Entry diameter: 2 – 6 mm depending on part - Exit diameter: 4 – 12 mm - Length: as short as possible, typically 30 – 80 mm ## Mold types - Standard sprue bushing: hardened-steel insert (H13, P20) bolted to the plate - Hot sprue: heated, eliminating the scrap cone - Direct gate: sprue feeds directly into the cavity, no runner (single cavity) - Cold sprue eliminator: hybrid with extended nozzle ## Sprue extraction In cold-runner molds the sprue must come out with the runner via: - Sprue puller (Z-pin, reverse-taper retention) - Robot with cutter - Gravity drop if mold geometry allows ## Common issues Sprue sticking to the nozzle (insufficient radius or draft), drooling at open, gating of the cone to the stripper plate, and premature wear of the sprue bushing with glass-fiber-reinforced resins.