Gate is the only channel for molten plastic to enter mold cavity. Incorrect gate selection can directly lead to batch defects such as weld lines, warping, bubbles, gate marks, and insufficient strength. Four most commonly used gate types are: side gate, point gate, submerged gate, and fan gate.
Intuitive Understanding of Basic Structure and Function of Four Gate Types

The most common side-entry structure, with gate located on side wall of product. Mold processing is simple, and it is a universal gate type.

 

Side gate: A gate method where molten metal enters product's sidewall from mold parting surface.
Product sidewall injection: Molten metal entering product's sidewall helps to conceal gate marks and improves appearance.
Filling Characteristics: Plastic flows into mold cavity from only one direction, forming only one weld line; however, material flow path is long, with high shear friction throughout, making material prone to orientation differences.
Advantages: Low mold processing difficulty and low manufacturing cost; suitable for most conventional shells and structural parts; small gate size, allowing for easy automatic detachment of molten material after mold opening.
Disadvantages: Clear gate marks will be left on sides of product; high unidirectional shear stress, making unevenly thick products prone to warping; not recommended for thin-walled parts.
Molding Principle: Pressure continuously decreases from gate towards distal end, resulting in poor pressure holding at distal end; uneven cooling and shrinkage, unidirectional molecular orientation, residual internal stress after molding, leading to deformation over long-term use; unobstructed venting channels prevent air accumulation.
Suitable Products: Products with general appearance requirements, medium to large general-purpose structural parts, cost-priority products, and products without stringent appearance standards.

 

Molten material enters vertically from directly above product. Gate size is small, almost invisible, is often used with three-plate molds and hot runner systems.

 

Filling Characteristics: Melt radiates outwards from small gate, with multiple streams converging, creating numerous dispersed weld lines inside product.
Advantages: Minimal gate marks, best appearance among four types; allows for multi-point simultaneous injection with hot runners, offering flexible layout; highly adaptable to automated production.
Disadvantages: Extremely high shear rate at gate location, resulting in significant localized frictional heat generation, easily causing spray marks and cold slug spots; multiple weld lines directly reduce the overall strength of plastic part; stress concentration in gate area makes stressed parts prone to cracking.
Molding Principle: Sufficient pressure near gate, but insufficient pressure at far end of cavity, making thick-walled products prone to shrinkage and sinking; multiple flow paths converging and trapping air, easily causing air bubbles to accumulate internally; localized heat accumulation reduces toughness of gate area, making it prone to breakage under stress.
Suitable Products: Small precision exterior parts, digital casings, plastic parts requiring high surface finish, and hot runner multi-cavity mass production molds.

 

A concealed injection gate, hidden below parting line, automatically breaks off upon mold opening, leaving no obvious gate mark on product surface, suitable for automated mass production.

 

Filling Characteristics: Melt enters cavity obliquely from below, flow tumbles upwards, and flow front tends to float, making it difficult for gas to escape smoothly.
Advantages: No gate marks are visible on product's outer surface, eliminating need for manual trimming; gate is automatically cut off during mold opening, adapting to fully automated production lines; runner is hidden inside mold, preserving product's appearance.
Disadvantages: Narrow inlet, high flow rate, and high shear heat generation; rising material compresses air, easily causing bubbles, scorching, and silver streaks; complex internal mold structure, increasing processing and repair costs; thin and small parts may experience short-shot defects.
Molding Principle: Pressure transmission path is tortuous and has high resistance, resulting in insufficient pressure holding at far end, easily leading to shrinkage and depressions; rising material flow traps gas, resulting in poor venting and frequent defects; continuous high shear generates a large amount of heat, causing heat-sensitive plastics to decompose and yellow.
Suitable Products: Appliance casings, multi-cavity mass-production parts, high appearance standards, unmanned automated injection molding production lines.

A wide, thin, gradually tapering gate that spreads horizontally like a fan, allowing melt to be evenly pushed into mold cavity over a large area. Specifically designed for thin-walled, long plastic parts.

 

Filling Characteristics: Wide, thin fan-shaped cross-section simultaneously pushes melt, resulting in a flat, evenly distributed flow front, low shear rate, and virtually no localized high pressure or high temperature.
Advantages: Low flow resistance, excellent filling balance; fewer weld lines and higher bond strength; extremely low pressure loss, uniform overall pressure holding, significantly reducing risk of warpage and shrinkage; smooth venting, allowing gas to escape evenly.
Disadvantages: Large gate width, requiring manual trimming after demolding; complex mold gate area structure, high processing costs; unsuitable for small, precision parts.
Molding Principle: Balanced temperature and cooling rate throughout mold, low molecular orientation disorder, minimal residual stress after molding; gentle flow reduces weld lines, resulting in optimal mechanical stability.
Suitable Products: Large, thin-walled shells, long strips, low warpage requirements, and high-strength plastic parts.

 

Selection Reference
1. Consider Appearance Requirements: For appearance parts, prioritize submarine gates and spot gates; if side marks are not a concern and cost control is prioritized, choose side gates; for long, thin-walled, high-gloss parts, choose fan gates.
2. Consider Product Strength: For load-bearing structural parts, prioritize fan gates; spot gates have many weld lines and are the weakest, so avoid using them for load-bearing parts.
3. Consider Automation Requirements: For large-volume, unmanned production, choose submarine gates; for hot runner multi-point production, choose spot gates.
4. Consider Product Wall Thickness: For thick-walled, large parts, side gates are generally used; for thin-walled, long parts, fan gates are standard; for small, thin appearance parts, use spot gates.
5. Consider Budget Costs: Mold costs from low to high: side gate < spot gate < submarine gate < fan gate.
Summary: There are no absolutely good or bad gates, only suitable ones. When selecting a gate, six factors need to be considered: material properties, product wall thickness, size, appearance standards, automation capacity, and mold cost.
If low-cost, general-purpose mass production is priority, a side gate is basic choice; for a superior appearance and small, precision parts, a point gate is more suitable; for automated, high-volume production with no exposed gates, a submarine gate is appropriate option; for thin-walled, long strips with low warpage and high strength, a fan-shaped gate is irreplaceable. Properly matching gate can reduce molding defects such as weld lines, warpage, bubbles, and gate damage by 80% at source, thus stabilizing production yield.