Cold‑Runner Mold Gate Types: Selection, Advantages and Disadvantages

Cold‑runner systems widely apply in injection molds, transferring molten plastic from sprue to mold cavities via runner channels on parting surfaces. As the transition section between runner and finished part, gate structure directly influences melt filling behavior, surface appearance, internal stress, gate residue and post‑processing difficulty. Improper gate selection causes filling shortage, burn mark, flow mark, gate whitening and stress cracking. Reasonable gate choice based on appearance requirement, polymer property, wall thickness and production scale cuts mold‑trial and modification workload and stabilizes mass‑production performance.

Side Gate

Side gate, also called edge gate, locates on mold parting surface at part sidewall. It is one of the most frequently‑used gate forms for cold‑runner molds. Side‑gate machining and modification are convenient, and gate dimension adjustment can be completed with simple workshop operations. It supports considerable shear rate and smooth melt flow, compatible with most general‑purpose plastics for medium‑small housing components. Gate vestige stays on product lateral surface without spoiling main visible faces, and gate removal processing is straightforward for high‑volume production. Side‑gate generates relatively large cold‑runner scrap. Gate position is restricted by part geometry and cannot be arranged on enclosed inner areas. Thin long parts may suffer insufficient filling pressure, and brittle materials tend to develop stress concentration and cracking near gate spots.

Pin‑Point Gate

Pin‑point gate connects runner and cavity through tiny circular cross‑section, requiring three‑plate mold construction. Gate automatically separates from molded parts during mold opening movement. Pin‑point gate leaves minimal vestige for premium‑appearance parts, and gate can be placed on top surface free from side‑structure limitation. Melt gains enhanced fluidity after shear heating through narrow gate orifice, performing well for multi‑cavity molds. Three‑plate design increases mold thickness and tooling cost. Small gate cross‑section freezes rapidly, bringing high flow resistance for high‑viscosity and glass‑filled polymers, which may result in incomplete filling. Intensive shear heat raises burning risk for heat‑sensitive resins, and pin‑point gate is not suitable for thick‑wall large‑size products.

Submarine Gate

Submarine gate, also known as tunnel gate, sets gate tunnel underneath cavity or core insert. Gate gets sheared off by ejector force during part ejection, achieving automatic separation without manual gate trimming under two‑plate mold configuration. Gate mark remains concealed to preserve surface quality, widely adopted for plastic housings and small accessories aiming for automated production. Submarine‑gate tunnel structure brings higher machining difficulty, and gate‑size modification is inconvenient. Burrs and notches often appear at shear positions. Soft flexible polymers may fail to achieve clean gate breakage. Glass‑fiber‑reinforced materials cause gate orifice abrasion. Gate dimension expands after long‑time mass production, aggravating burr defects. Thick‑wall large‑scale components are not fit for submarine‑gate solution.

Fan Gate

Fan gate is a variant evolved from side gate. Gate channel widens and thins gradually toward cavity inlet. Melt enters cavity in broad thin‑layer status with uniform and stable flow pattern. Fan gate effectively reduces weld lines and lowers internal stress, improving filling performance for thin flat pieces and transparent sheet‑like parts. Large gate area brings heavy post‑trimming workload and obvious residual marks. Abundant cold‑runner sprue scrap raises material waste. Wide gate footprint occupies parting‑surface space and limits maximum cavity quantity, so fan‑gate structure is not recommended for tiny miniature components.

Diaphragm / Tab Gate

Tab gate diverts melt into tab protrusion feature before plastic flows into target cavity. Shear‑induced stress and jetting phenomenon concentrate on the sacrificial tab section instead of finished parts, greatly relieving warpage and silver‑streak risk. Tab gate finds typical use for stress‑sensitive transparent polymers such as PC and PMMA. Extra trimming procedure is required to remove tab sections after molding. Material consumption rises, mold complexity increases and cycle time extends. Tab‑gate solution is only worthwhile for high‑optical‑grade parts and rarely used for ordinary mass‑production goods.

Sprue Gate

Sprue gate, or direct gate, feeds molten plastic directly from main sprue into cavity. Large gate cross‑section provides low flow resistance and pressure loss, ideal for big thick‑wall articles and high‑viscosity hard‑flow polymers. Two‑plate simple‑structure mold lowers tooling and maintenance cost. Sprue gate solidifies together with finished component. Massive sprue material prolongs cooling cycle and total molding time. Gate removal is labor‑intensive, leaving prominent vestige on part surface. Sprue gate can only be arranged on non‑cosmetic surfaces, seldom adopted for appearance‑critical products.

Gate Selection Guidelines

Gate selection needs comprehensive consideration of polymer characteristics, cosmetic grade, wall thickness, cavity quantity and automation requirement. Pin‑point gate or submarine gate is prioritized for high‑appearance demands. Fan gate matches thin flat workpieces. Tab gate serves stress‑sensitive transparent plastics. Sprue gate applies to large thick‑wall parts. Submarine gate realizes automation under two‑plate molds. Defects originating from improper gate design can hardly be eliminated merely by adjusting injection parameters. Gate confirmation finished in mold‑design phase saves repeated trial‑and‑modify costs.