In injection molding, gating system is the first critical factor determining product quality. Statistics show that over 80% of product quality problems are related to improper gating system design:
Weld lines, porosity, and poor appearance; Insufficient filling and shrinkage; Insufficient structural strength; Molecular orientation leading to product warping and deformation; Improper gate placement causing stress concentration.

 

This article will systematically analyze core technical points of gating system design, from basic principles to advanced applications, helping you solve these problems at their source!

Gating System Definition: Channel through which molten plastic flows in mold after exiting nozzle of injection molding machine and before reaching mold cavity.
Four Main Components:
1. Spree: Material flow channel from nozzle of injection molding machine to runners. Main runner is generally circular and requires a draft angle of 2-4°.
2. Runner: Material flow channel between main runner and gate. Common cross-sectional shapes include circular, rectangular, semi-circular, and trapezoidal.
3. Gate: A short section of material flow channel with a small cross-section between runner and mold cavity. Gate design directly affects product quality.
4. Cold Slug Well: End of runner used to store cold material from the front, preventing it from entering mold cavity and affecting product quality.

Basic Forms of Main Runners
Vertical Main Runner: D-d = 0.5~1.0mm (Difference between inlet and outlet diameters); R > r (Gate ball radius is larger than nozzle ball radius); α = 1~3° (Draft angle)
Inclined Main Runner: Single Inclined: α=30° (for PE, PP, PA); α=20° (For PS, SAN, ABS, PC, POM, PMMA) Double-inclined type: α can be up to 15°.
Recommended main runner size:

Note: Injection volume in table refers to injection volume of injection molding machine in one pass. d is main runner inlet diameter, and D is main runner outlet diameter.

Comparison of Runner Cross-Section Shapes

Runner Size Design
Runner Diameter Calculation Formula: D = √(4W√L / π). Where: D - Runner diameter (mm); W - Mass of plastic in product (g); L - Flow channel length (mm)
Recommended flow channel diameters for commonly used plastics:

Runner Layout Principles
Fill balance is one of the most important design principles in plastic injection molding:
Natural Balance Runners: All cavity runners are of equal length, achieving natural balance.
Artificial Balance Runners: Fill balance is achieved by adjusting runner diameter.
Professional Tip: A balanced fill design ensures uniform distribution of pressure, melt temperature, and volume shrinkage, resulting in consistent product quality and a 50% reduction in deformation.

Common Gate Types and Characteristics
1. Pin Point Gate
Advantages: Automatic material cut-off upon mold opening, minimal residual marks
Disadvantages: Difficult to process, significant pressure loss
2. Submarine Gate
Advantages: Automatic material cut-off upon mold opening, minimal residual marks
Disadvantages: Difficult to process, significant pressure loss
3. Horn Gate
Advantages: Automatic material cut-off upon mold opening, minimal residual marks
Disadvantages: Difficult to process, significant pressure loss, difficult ejection, prone to blow marks
4. Fan Gate
Advantages: Reduces orientation stress, suitable for large flat surface products
Disadvantages: Difficult gate removal, noticeable marks
Gate Size Calculation
Gate Diameter Calculation Formula (Pin Point Gate): d = n × √(A × t) / 30
Where: d - Gate diameter (mm); A - Cavity surface area (mm²); t - Product wall thickness (mm); n - Material Constants
Material constant n values: 0.6 for PE, PS; 0.7 for POM, PP; 0.75 for ABS; 0.8 for CA, PMMA, PA, PC; 0.9 for PVC.
Gate Location Selection Principles: Gate should be located at point of maximum wall thickness of product, allowing plastic to flow from thick walls to thin walls. Prevent jetting from gate, which can cause serpentine flow during filling. Gate should be located in main stress direction of product. Consider influence of molecular orientation on product deformation. Gate location should minimize weld lines to avoid weak weld line strength. Gate should be located in a place that does not affect appearance. For narrow and long products, gate is often located at 2/3 of its length. For ribbed products, gate should be aligned with direction of rib and should not be directly opposite rib.
Important Reminder: Avoid placing a small gate directly opposite a large mold cavity, otherwise jetting, serpentine flow, folding, and air trapping may occur.

 

Functions of Cold Slug Well: Stores cold material at initial injection stage when front end is at a lower temperature. Prevents cold material from entering mold cavity and affecting product quality. Aids in runner demolding. 
Cold Slug Well Design Principles: Cold slug well diameter is generally 1-1.5 times diameter of main runner's large end. Cold slug well depth is generally 1-1.5 times diameter. Pull rod should not obstruct flow of cold material into cold slug well.

Characteristics of Hot Runner Systems
Reduced waste: No need for runner solidification, saving material.
Shorter cycle time: No need for ejector runners, shortening molding cycle.
Reduced pressure loss: Maintains melt temperature, reducing injection pressure.
Controlled gate marks: Improves product appearance quality.
Controlled plastic flow: Achieves more precise filling control.
Main Forms of Hot Runner Systems
1. Hot Sprint: The simplest form of hot runner, suitable for single-cavity molds.
2. Single Valve Pin Hot Runner: Gate opening and closing is controlled by a valve pin, enabling timing control.
3. Multiple Hot Runners: Multi-cavity hot runner systems allow for multi-point injection.
Hot Runner Gate Types
Valve Gate: Opening and closing controlled by a mechanical valve pin
Point Gate: Open hot runner gate
Direct Gate: Hot runner application for large products
Edge Gate: Hot runner with side injection

Is main runner size appropriately selected based on type of plastic and product size? Is cross-sectional shape of branch runners appropriate (circular is best)? Are runner dimensions determined based on type of plastic and flow length? Is gate type suitable for product structure and appearance requirements? Has gate size been calculated and determined? Does gate location avoid jetting and serpentine flow? Does gate location consider influence of molecular orientation on deformation? Does gate location minimize weld lines? Is a suitable cold slug well provided? Does runner layout achieve filling balance? Is hot runner system selection appropriate? Is venting system adequately considered?

Common Problems and Solutions
Problem 1: Weld lines appear on product.
Cause Analysis: Improper gate location, melt convergence angle too small.
Solutions: Adjust gate position, increase melt convergence angle, and set up venting channels.
Problem 2: Insufficient Product Filling
Cause Analysis: Insufficient gate size, resulting in significant runner pressure loss.
Solution: Increase gate size, optimize runner layout, and increase injection pressure.
Problem 3: Product Warpage and Deformation
Cause Analysis: Inconsistent molecular orientation, leading to uneven shrinkage.
Solution: Adjust gate position, use multi-point injection, and optimize holding pressure.
Problem 4: Poor Appearance at Gate
Cause Analysis: Inappropriate gate type selection or unreasonable gate size.
Solution Solution: Change gate type, optimize gate size, and adjust injection speed.
Design Optimization Suggestions:
Fill Balance Priority: Ensure simultaneous filling of all cavities for consistent product quality.
Shortest Runner Principle: Reduce pressure and heat loss.
Minimum Number of Gates: Minimize number of gates while ensuring sufficient filling.
Consider Material Properties: Design runner system based on flowability of different plastics.
Utilize CAE Analysis: Optimize gating system design through mold flow analysis.
Professional Insight: Success or failure of trial molding and mass production, as well as associated costs, are 80% determined by design phase. Excellent designers should clearly understand arduous process involved in creating their product and mold designs.

Gating system design is one of the most critical technical aspects of mold design, directly impacting product quality, production efficiency, and mold costs. By systematically mastering design principles, key technical points, and optimization methods of gating systems, you can:
Significantly improve product quality and reduce defects such as weld lines and porosity; Increase production efficiency and shorten molding cycles; Reduce material waste and save production costs; Cope with complex product structures and expand process capabilities
Take immediate action: In your next mold design project, systematically apply these gating system design principles and key technical points, you will see a significant improvement in product quality and production efficiency!