Tool / Mold

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.

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

Parting Line is the line or surface where the two halves of the mold (cavity and core) meet when closed. It defines how the mold opens to eject the part and leaves a linear mark on the plastic that is usually visible. ## Importance in design - Determines the mold's opening plane - Defines which surfaces are cosmetic and which can accept the mark - Conditions the location of gates, vents and ejectors - Affects mold complexity and cost ## Parting-line types - Flat: simplest, single horizontal or vertical plane - Stepped: with offsets to accommodate geometries - Curved / 3D: follows the part contour, requires 5-axis machining - Multiple: when there are slides for undercuts ## Design rules - Place the parting line on the part's "natural" edges (corners, flanges) - Avoid crossing cosmetic surfaces - Guarantee accessibility for machining and polishing - Maintain minimum draft of 0.5° on both sides - Plan venting through the parting line ## Associated defects - Flash: the most common, material exits through a poorly sealed line - Visible mark: noticeable on cosmetic parts; mitigated with texture or hidden location - Asymmetric wear: if pressure is poorly distributed across cavities - No venting: if the line is polished perfectly with no air paths ## Maintenance - Visual inspection every 100,000 cycles - Re-grind plates if the line shows recurring flash - Vent cleaning: 0.02 – 0.05 mm standard depth - Plate flatness check: <0.01 mm in precision molds

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.