In injection molding, molten plastic originates from injection molding machine nozzle and passes through main runner and branch runners before reaching mold cavity. Gate is final channel connecting branch runners and mold cavity in gating system (except for direct gates, where melt enters cavity directly from main runner). It is typically designed as a short, narrow section with the smallest cross-sectional dimension in the entire runner system. This channel directly determines melt's orientation, speed, and entry position into mold cavity, thus affecting filling, holding pressure, appearance, and internal stress.
This article will begin by examining function and classification of gates, systematically outlining characteristics, applicable scenarios, and selection principles of six commonly used gate types, and analyzing basic principles of gate placement.

Gate is "throat" through which melt enters mold cavity, a crucial part of gating system.

 

Gate serves four functions:
1. Preventing backflow: Gate is the smallest cross-section section in gating system. After mold filling, during holding pressure stage, it cools and solidifies first due to the fastest heat dissipation, closing channel. After pressure release, it prevents backflow of melt within cavity, holding pressure and shrinkage compensation immediately cease.
2. Improving melt flowability: When melt passes through a narrow gate, it experiences strong shear, generating shear heat that causes localized temperature rise. Simultaneously, polymer chains untangle and orient under shear, causing melt to exhibit a shear-thinning effect, which together reduces local viscosity of the melt, facilitating mold filling.
3. Regulating and controlling feed rate: In multi-cavity molds, adjusting cross-section of each gate allows all cavities to fill simultaneously within approximately same timeframe; this is called balanced feeding.
4. Ensuring molding quality: Position, shape, cross-sectional size of gate directly determine orientation and speed of melt entering cavity. When gate design is unreasonable, finished product is prone to defects such as insufficient filling, shrinkage depressions, serpentine patterns, vibration marks, weld lines, and warping.
Because gate must ensure rapid mold filling and effective pressure holding, while also being as small as possible, easy to separate from product, and leaving no obvious trace, design must comprehensively balance product size, cross-sectional size, mold structure, molding conditions, and material properties. It cannot be selected in isolation from specific product.

There are many types of gates, mainly including direct gates, side gates, overlapping gates, point gates, submarine gates, fan gates, thin-plate gates, as well as claw gates, ring gates, umbrella gates, and secondary gates. Following focuses on six most commonly used types in engineering.
1. Direct Gate
Direct gate, also known as main runner gate, is commonly called a "large gate." Melt enters cavity directly after passing through main runner, without intermediate branch runners or smaller gates.
Advantages: Shortest runner and largest cross-section, minimal flow resistance and pressure loss, good pressure holding and shrinkage compensation effect, and full filling of deep cavity products.
Disadvantages: Large gate cross-section, slow cooling, large gate residue marks requiring manual post-processing removal, and stress concentration near gate, easily leading to residual stress and warping.
Applications: Primarily used in single-cavity molds, suitable for deep-cavity, large, thick-walled products and high-viscosity, poor-flow materials (such as PC, PSF, PPO, etc.); not suitable for shallow, thin-walled products (prone to warping, large gate marks), should be used with caution for materials with significant anisotropy in shrinkage, such as PE and PP (high residual stress, easy deformation).

 

2: Side Gate
Side gates, also known as edge gates, are located on parting surface, allowing material to enter from outer edge of product. Cross-section is usually a small rectangle or trapezoid.
Advantages: Easy to process and trim, easy to remove, particularly suitable for multi-cavity molds, one of the most common forms for small and medium-sized products. By changing width and depth of gate, feed rate and filling of each cavity can be easily adjusted.
Disadvantages: Visible gate marks will be left on the side of part, and melt feeds unidirectionally from edge, which can easily cause flow imbalance and weld lines in large flat parts. It is usually not used directly for front shells with high appearance requirements.

 

3: Overlapping Gate
An overlapping gate can be considered an improvement on side gate. Its characteristic is that gate overlaps on the wall of part, rather than directly facing cavity opening.
Advantages: Melt spreads along wall before entering cavity, effectively avoiding jetting patterns (serpentine patterns) caused by melt directly entering cavity in a jet state, resulting in smoother flow and better surface quality.
Disadvantages: Marks left on the surface of part after removing gate are slightly more obvious than those of a regular side gate, and location needs to be chosen to avoid appearance surface.

 

4: Pin Gate
A pin gate, also known as a fine sprue or needle-point gate, is a typical form of three-plate mold gating system.
Advantages: Melt enters cavity directly from one or more points above product. Gate mark is a very small dot that can be automatically broken off when mold opens, requiring virtually no manual trimming and enabling automated production. Multi-point feeding can also shorten process flow and improve filling balance, making it particularly suitable for large or asymmetrical products. It is applicable to various materials such as PE, PP, PC, PS, POM, AS, and ABS, and is one of the most widely used gate types.
Disadvantages: Gate cross-section is small, resulting in greater pressure loss and injection pressure loss, which is unfavorable for materials with poor flowability. Furthermore, a three-plate mold (or hot runner) structure must be used, leading to complex molds, higher costs, and relatively more solidified material.

 

5: Submerged Gate
Submerged gate, also known as a tunnel gate, enters cavity from end of runner along an oblique or arc-shaped path below parting surface, from a concealed location such as back, side, or rib of product.
Advantages: Gate is automatically sheared during mold opening and ejection, allowing runner material to separate from product. This results in high automation, a short production cycle, and concealed injection points with minimal visible traces. Therefore, it is widely used in casings of home appliances and consumer electronics. Horn gate (arc-shaped submarine gate) is a variant that uses a curved tunnel to inject material from inside or bottom of product, offering better concealment.
Disadvantages: Tunnel processing is more difficult, resulting in greater gate pressure loss. It also has poor adaptability to rigid filler materials such as glass fiber reinforcement (tunnel is prone to wear). Brittle materials (such as PS) are prone to crumbling, with debris clogging runner. Highly resilient materials (such as PA and elastomers) are prone to stringing and cannot be cut. Careful selection is required for both.

 

6: Fan-shaped gate, thin-film gate
Fan-shaped gate gradually expands from end of runner into a fan-shaped cross-section for injection. Thin-film gate (sheet gate) creates a thin, wide slit-like gate along the entire width of product. Both methods involve spreading molten metal evenly laterally at leading edge of mold cavity, then advancing it parallel in a near-surface manner.
Advantages: They allow for uniform feeding and consistent orientation across all parts of large-area flat components, significantly reducing internal stress, warpage, flow marks, and also facilitate venting. They are particularly suitable for flat, thin-walled, large-area products, as well as sheet metal and cover plate parts sensitive to flatness.
Disadvantages: Gate edge is long and narrow, making removal more labor-intensive and leaving a longer mark. They are typically located at the edge of product or in areas that will be obscured during subsequent assembly.

 

In addition to types mentioned above, there are several specialized gates for specific structures: Annular gates feed material evenly along circumference of tubular or coaxial parts, preventing core from being deflected by unidirectional material flow and reducing weld lines; umbrella-shaped (disc-shaped) gates are suitable for disc-shaped parts with a central hole; claw-shaped gates evenly cover slender cores with multiple feed points, suitable for slender tubular parts; secondary gates, through two-stage sequential feeding, combine improved flow and suppression of jetting, are often used for products with limited feed points or those requiring suppression of jetting.

First select gate type, then determine gate location, and finally determine gate cross-sectional dimensions. When selecting a gate type, four factors should be considered together: Product dimensions, shape, wall thickness, and appearance requirements; Material properties, including viscosity, shear sensitivity, and whether it contains fillers such as glass fiber; Mold structure, such as whether a two-plate or three-plate mold is used, and number of cavities; Production volume and automation level.
For large, deep-cavity, and thick-walled parts, direct gates are preferable; for small to medium-sized multi-cavity parts, side gates are preferred; for shells with high appearance requirements and requiring automated production, point gates or submarine gates are suitable; for large-area flat parts, fan-shaped or thin-film gates should be considered.
Gate placement principles: Gate should be placed on parting surface as much as possible for easy mold processing and subsequent cleaning. Distance from gate to each part of cavity should be as equal as possible, with the shortest possible flow path, so that melt fills cavity evenly in the shortest possible time. Feed direction should face spacious, thick-walled parts of cavity to facilitate melt flow and pressure holding/compensation. Flow direction should ensure that cavity gas can be orderly discharged towards venting channel, avoiding gas trapping and scorching caused by flow. Avoid direct impact of flow onto cavity wall, core, or inserts to prevent deformation or displacement. Minimize weld lines on product, or guide weld lines to secondary locations that do not affect strength or appearance. Gate should be located in the easiest-to-clean part of product that minimizes impact on appearance.
Gate Cross-Section Dimensions: A larger cross-section slows down condensation and sealing, disrupts backflow prevention and compensation rhythm, and makes gate marks more noticeable. A smaller cross-section, while shear heating helps reduce viscosity and fill mold, increases pressure loss and may even cause jetting, short shots, premature gate freezing, or interruption of pressure holding. For materials with high viscosity, poor flowability, or added glass fiber, gate should be appropriately enlarged and its length shortened as much as possible; For easily flowing materials such as PE and PP, a smaller cross-section can be used to obtain clear breakpoints and slight marks. In actual design, values are often initially determined based on experience, then gate size and number are gradually corrected through short-shot trial molding and mold flow analysis.
Conclusion: Although small, gate is "master switch" for melt filling behavior. Product's gate needs to match product structure, material properties, mold design, and production method. Understanding injection logic and advantages and disadvantages of various gates, then comprehensively weighing them in conjunction with seven basic principles of gate location, can find a balance between filling quality, appearance marks, mold cost, and production efficiency, reducing number of trial moldings and molding defects from source.