Hot Runner

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

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.

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.