Runner

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.

A cold runner is an unheated channel system in the mold that carries the melt from the sprue to each cavity. Because the runner is not heated, the plastic in it cools and solidifies along with the parts every cycle, so the runner and sprue are ejected as a connected skeleton and become scrap (usually reclaimed as regrind). It is the simpler, cheaper alternative to a hot runner. ## How it works Each shot fills the cold runner first, then the cavities through the gates. When the part solidifies, the runner does too; the whole runner-and-part assembly is ejected, then the runner is degated, separated and reground. Layouts are kept balanced so every cavity fills evenly. ## Cold runner vs hot runner - Cold runner: unheated; runner freezes and is ejected each cycle → runner scrap/regrind, but low tooling cost, simple, easy color/material changes, tolerant of many resins. - hot runner: heated manifold keeps the runner molten → no runner scrap, faster cycles, automation-friendly, but higher mold cost, more maintenance and harder color changes. ## Trade-offs and use Cold runners suit lower volumes, frequent color/material changes, and shops that can regrind their runner economically. The downsides are the recurring runner scrap, extra material per shot, degating labor, and the heat-history hit each time the runner is reground. Good cold-runner design minimizes runner volume while keeping fill balanced. ## Related terms - See also: runner, sprue, hot runner, regrind, cavity ## What is a cold runner in injection molding? An unheated runner system that delivers melt from the sprue to the cavities; the plastic in it solidifies with the part each cycle and is ejected as runner scrap, usually reground and reused. ## What is the difference between a cold runner and a hot runner? A cold runner is unheated and freezes into scrap every cycle (cheap, simple, easy color changes); a hot runner is heated to stay molten, eliminating runner scrap and speeding cycles, but costs more and is harder to maintain. ## What happens to the cold runner after molding? It is ejected attached to the parts, then degated (separated from the parts) and typically granulated into regrind that is blended back with virgin resin at a controlled ratio.

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

Hot Runner is an assembly of electrically heated nozzles and a manifold that distributes molten plastic from the injection unit to the mold cavities, keeping the material at processing temperature along the whole path. ## Why use a hot runner It eliminates the runner scrap that a conventional cold-runner mold would generate. Each nozzle injects directly into the cavity through a gate, removing the need to trim and recycle material every cycle and enabling full automation without sprue removal. ## Typical parameters - Manifold temperature: 200 – 320 °C depending on resin - Differential vs. barrel: ±5 – 15 °C - Cycle-time reduction: 5 – 20 % vs. cold runner - Material saving: 10 – 30 % per part - Service life of a well-maintained hot runner: >1 million cycles ## Hot runner types - Thermal gate: nozzle always open, relies on melt freeze for closure - Valve gate: mechanical pin closure driven by servo or pneumatic actuator, ideal for PP, PE and parts demanding cosmetic surfaces - External bushing (cold sprue eliminator): economical hybrid - Naturally or rheologically balanced manifold ## Common issues Drooling at open gates at end of injection, stringing of cold material, burn marks from over-temperature, cavity imbalance from differences in heated zones, and leakage from poorly torqued manifold seals.

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.