Cold Runner

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.

Projected area is the sum of the flat areas that a part —and the runners in cold-runner molds— occupies when projected onto the parting plane. It is the key input for calculating the clamp tonnage of the injection press. ## What it is used for The required clamp force comes from multiplying projected area by the specific cavity pressure (tonnage factor) of each resin: > Tonnage (t) = Projected area (cm²) × Tonnage factor (t/cm²) A safety margin of 10 – 20 % is applied to avoid flash at end of fill or due to imbalance between cavities. ## Typical tonnage factor by resin - PE / PP: 2.5 – 4 t/cm² - PS / ABS: 3 – 5 t/cm² - PA / PC: 4 – 6 t/cm² - POM: 4 – 6 t/cm² - Fiber-filled grades: +20 – 50 % - Thin walls (<1 mm) or long flow: +50 – 100 % ## How to measure projected area - In CAD: project the 3D model onto the mold's XY plane, export as a sketch and sum areas. - In 2D: planimetry on the parting plane. - In flow analysis (Moldflow, Moldex3D, Cadmould): automatic calculation with the resin factor. ## Common mistakes Forgetting to include runners in cold-runner molds (underestimates tonnage by 5 – 15 %), using the flat area instead of the projected one on parts with sloped walls, and changing resin without recomputing the factor.

Regrind is plastic material recovered by grinding runners, sprues, defective parts or purges, then blended with virgin resin and reintroduced into the process. It is a key sustainability and cost-reduction tool in injection molding. ## Why use regrind Recovers the ~20 – 30 % unavoidable scrap from cold runners and cuts raw-material cost by 5 – 25 %, with a lower carbon footprint. On non-critical parts, pure or near-pure regrind is fully viable. ## Typical proportions (regrind/virgin blend) - Cosmetic / engineering parts: 10 – 20 % - Non-visible structural parts: 20 – 50 % - Internal / non-critical: 50 – 100 % - Some resins (PVC, PE): up to 100 % in approved applications ## Regrind system equipment - Granulator: rotating blades, sizing screen - Pellet size: 3 – 8 mm for homogeneous mix with virgin - Beside-the-press: granulator next to the machine, regrind returns to the hopper via blower - Magnet + metal detector: mandatory to avoid screw damage - Volumetric or gravimetric blender: meters regrind and virgin in a defined ratio ## Limitations and issues - Cumulative thermal degradation each pass (lowers viscosity, properties) - Cross-contamination with another resin causes delamination or breakage - Yellowing or graying on unpigmented resins - Property loss in multi-generation regrind - FDA / medical / automotive restrictions forbid uncertified regrind ## Disallowed applications FDA food-contact, class II/III medical devices, structural crash parts in automotive, and certain children's toys under specific regulation.

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.