Gating system is complete flow channel for molten plastic to be transported from injection molding machine nozzle to mold cavity. Its design directly determines filling integrity, dimensional accuracy, and surface quality of injection molded product. Industry data shows that over 80% of injection molding defects (such as weld lines, shrinkage depressions, and warpage) are directly related to improper gating system design. A typical gating system consists of four core functional modules, which work together to ensure stable melt delivery.

 

1. Main Runner (Sprue)
Core Function: As primary channel for melt to enter mold, it connects injection molding machine nozzle and runners, ensuring that melt is quickly and without stagnation flowed into subsequent runners.
Key Design Considerations: Cross-section adopts a conical structure, with a draft angle α of 1~3° (1°~1.5° for low-viscosity, easily flowing materials such as PE/PP, and 2°~3° for medium-to-high viscosity, difficult-to-flow materials such as PC/PVC), facilitating smooth demolding of solidified material. Inlet diameter d and outlet diameter D must meet requirement of a gradual change of D-d=0.5~1.0mm to avoid formation of stagnant zones in runner. Radius R of gate ball must be larger than radius r of injection molding machine nozzle ball; a design difference of R=r+0.5~1mm is recommended to ensure a tight seal during injection and prevent melt leakage.
Size Selection Reference (Classified by Product Weight):

2. Runner
Core Function: Changes melt flow direction, evenly distributing melt transported by main runner to each cavity, while minimizing pressure loss and temperature decay.
Key Design Parameters
Cross-section Shape Selection: A circular cross-section is optimal solution (lowest surface area/volume ratio, slowest heat dissipation, and lowest flow resistance), followed by U-shaped and trapezoidal cross-sections (better manufacturability). Diameter Calculation Formula: D=√(4W√L/π) (where W is mass of a single product in g; L is the total length of runners in mm). Calculation result needs to be corrected for viscosity characteristics of plastic. Layout Selection: A balanced layout is preferred (runners of equal length and diameter for each cavity to ensure consistent filling time); when mold structure is limited and equal-length runners cannot be achieved, an unbalanced layout is used (gradually adjusting runner diameter to compensate for filling imbalance caused by length differences). Surface Quality Requirements: Surface roughness of runner inner wall needs to be controlled at Ra=1.6μm to reduce melt flow resistance and prevent melt adhesion.
3. Gate
Core Function: As a key control node for melt entry into cavity, accurately controlling melt entry speed, pressure, and flow state is core factor determining quality of molded product.
Comparison of Common Types and Application Scenarios

Dimension Calculation Method (Point Gate): d=n×√(A×t)/30 (where A is cavity surface area, in mm²; t is average wall thickness of product, in mm; n is material property constant: PS 1.2, ABS 1.5, PC 2.0).
Core Principles for Location Selection: Prioritize placement at point of maximum wall thickness in product to ensure sufficient melt filling and avoid incomplete filling. Avoid critical stress areas (such as assembly clips and load-bearing surfaces) to reduce stress concentration at gate, which can lead to decreased mechanical properties. Avoid areas visible to product's exterior and prevent melt from forming multiple converging flow streams within cavity, reducing weld lines.
4. Cold Slug Well:
Core Function: Stores cold slug at the front of melt flow (portion of melt that cools and solidifies during transport), preventing it from entering cavity and causing defects such as short runs, cold spots, and insufficient weld line strength.
Standard Design Requirements: Diameter of cold slug well should be 1-1.5 times diameter of main runner's large end, and its depth should be 1-1.5 times its own diameter. A pull rod structure is required to ensure that solidified and cold slugs are removed smoothly and synchronously during demolding.

Balanced Filling Principle: Multi-cavity molds must ensure that all cavities are filled and pressurized simultaneously. This can reduce product deformation by more than 50%, significantly improving consistency in mass production.
Shortest Runner Principle: Within limits of mold structure, minimize melt flow path to reduce pressure loss and temperature decay, especially suitable for high-viscosity engineering plastics.
Efficient Pressure Transmission Principle: Runner cross-section should use a gradual design, avoiding sharp corners to reduce turbulence and pressure loss during melt flow, ensuring effective transmission of holding pressure to cavity.
Sufficient Venting Principle: Venting grooves (0.03~0.04mm depth, 5~10mm width) or vent plugs must be installed at the end of last filled cavity and in weld line area to ensure smooth air removal from cavity.
Smooth Demolding Principle: Runner design must coordinate with demolding mechanism to ensure synchronous demolding of solidified material and product, avoiding demolding failures such as solidified material residue and gate breakage.

Hot runner systems maintain molten state of melt within runner through built-in heating devices, offering significant technological advantages over traditional cold runner systems. They are widely used in mid-to-high-end injection molding production:
Core Technological Advantages: Material utilization rate increased by over 30% (no runner solidification loss), molding cycle shortened by 20%~30%, minimal gate marks, and ability to achieve automated continuous production.
Main Types and Applicable Scenarios: Single-cavity insulated runner: Relies on melt itself for insulation, with a simple structure and low cost, suitable for large, thick-walled products (longer molding cycles, slow melt cooling). Multi-cavity heated runner: Divided into external heating type (heating coil surrounds runner, simple structure, easy maintenance) and internal heating type (heating element built-in, good temperature uniformity, high temperature control accuracy), suitable for production of multi-cavity precision products. Valve-gate hot runner: Controls gate opening and closing sequence through a needle valve mechanism, enabling step-by-step filling, effectively reducing weld lines and improving molecular orientation, suitable for complex structural parts with extremely high appearance requirements.
Typical Application Scenarios: Mass production of automotive interior parts, electronic and electrical housings, medical devices, and high-value engineering plastic products, where stringent requirements for appearance quality and production efficiency are placed.

 

Improper gating system design is primary cause of injection molding defects. Following are causes of frequently occurring defects in industry and targeted solutions:

In summary, design of plastic mold gating system is a key focus of mold design teaching, directly determining quality and production efficiency of injection molded products. This article outlines four core modules of gating system, five design principles, special hot runner design, and solutions to common defects, constructing a teaching knowledge system of "principle-parameter-practice," aligning with teaching needs of combining theory with practical application.
For learners, it is necessary to follow logic of "understanding principle-memorizing parameter-practice application," thoroughly understanding core knowledge points, memorizing key parameters, flexibly adjusting design based on material properties and product requirements, paying attention to practical details, avoiding common defects, consolidating foundation, strengthening abilities, and laying a solid foundation for future career development.