Injection Mold Runner Design Guideline

1. Introduction

The runner system plays a crucial role in the injection molding process, influencing the quality and efficiency of production. In this blog, we will provide you with a comprehensive guide to understanding and optimizing the design of injection mold runners.    By the end of this blog, you will have a solid understanding of the various aspects involved in designing an efficient and effective runner system for injection molds. So, let's dive in and explore the world of injection mold runner design!

2. Key Principles of Runner Design

The main principles of runner design in injection molding are as follows:

Uniform Flow: The runner should be designed to achieve a uniform flow of the melt, ensuring proper filling throughout the entire mold and avoiding defects such as bubbles or short shots. Proper runner cross-section and length can help achieve uniform melt flow.

Shortest Path: The runner should be designed to have the shortest possible path, reducing the residence time of the melt in the runner and minimizing energy losses. Short runners contribute to increased injection molding speed and efficiency.

Even Distribution: If the product has multiple gating points, the runner should be designed to achieve an even distribution of the melt through a multi-branch structure. This helps ensure simultaneous filling of all gating points, avoiding uneven shrinkage or deformation.

Sufficient Cooling: The runner should have a well-designed cooling system to ensure proper cooling and solidification of the melt as it flows through the runner. Control of cooling time is crucial in avoiding thermal stress and improving product quality.

Pressure Loss Reduction: The runner's cross-section and length should be designed appropriately based on the flow characteristics of the melt to minimize pressure losses. Reducing pressure losses contributes to improved energy efficiency and production efficiency of the injection molding machine.

Avoidance of Dead Ends and Sudden Section Changes: The runner design should aim to avoid dead ends and sudden changes in section, reducing melt stagnation and energy losses. A smooth runner design helps achieve a stable injection molding process.

Consideration of Material Characteristics: In runner design, the flowability, viscosity, and shrinkage characteristics of the melt need to be taken into account. Adjusting the dimensions and shape of the runner based on the specific material properties allows for optimal injection molding results.

These principles provide a basic guideline for runner design, but actual designs need to consider factors such as product shape, material properties, and the capabilities of the injection molding machine. Runner design optimization can be achieved through simulation software and experimental validation to ensure the best injection molding outcomes.

3. Main Runner Design

Structural Form: The structural form of the Main Runner can be linear, annular or branched, depending on the specific injection molding requirements and product design.

Dimension Design: The dimensioning of the main runner needs to take into account the injection capacity of the injection molding machine, material properties, product size and other factors. A properly dimensioned main runner ensures sufficient melt flow and pressure to ensure adequate filling of the product.

4. Sub runner Design

Positioning: Sub runners should be positioned according to the shape of the product and the injection molding requirements. They are usually connected to specific filling points in the product to achieve uniform melt distribution and filling.

Dimension Determination: The size of the manifold is determined by the size of the main runner and the filling requirements of the product. The size should be chosen to ensure sufficient melt flow and pressure to avoid uneven filling or melt stagnation.

5. Hot runner system

Heating method

External heating: Heat-resistant electric heater is wound on the outer shell of the runner for heating.

Internal Heating: Heat the runner by placing electric heating wire or electric heating tube inside the runner.

Whether external heating or internal heating, need to control the runner temperature above the melting temperature of the material, usually 10-50°C higher.

Temperature control and safety

A thermocouple or ion current detector is installed to monitor the runner temperature in real time and compare it with the set value to precisely control the runner temperature.

At the same time need to set up overheating protection device, when the runner overheating cut off the power supply to ensure safe operation.

6. Cold runner design

Setting purpose

Ensure that the melt from the hot runner into the cavity, still be able to maintain a high temperature and good fluidity, to avoid partial melting in the cavity.

Dimensioning

According to the insulation distance (the furthest distance from the hot runner to the cavity) and the ability of the material to conserve heat energy, determine the diameter and length of the cold runner, so that the temperature drop of the melt after passing through the cold runner is less than 20°C.

7. Gating system design

The form of gate

(1) Straight gate: simple structure, but easy to cause stress concentration.

(2) Ring gate: Stress distribution is uniform, but the structure is complicated.

(3) Spot gate: suitable for thin-walled and small and medium-sized products, with stress dispersion.

Dimension design

According to the wall thickness and structure of the product, determine the appropriate diameter of the gate, so that the melt can smoothly enter the cavity without causing thermal damage or short circuit. At the same time, should be set up to guide the flow ramp or arc, smooth transition.

8. Vents and Venting System

Vent design: In a runner system, vents are designed to expel gases generated during the injection molding process and to prevent gases from being trapped in the product to form cavities. Venting holes are usually located in low pressure areas of the mold or thick walled parts of the product and are sized and distributed in such a way as to effectively facilitate the venting of gases.

Venting System Design: The purpose of the venting system is to ensure a smooth flow of the melt during the injection molding process and to avoid air pockets and excessive compression. A properly designed venting system, consisting of holes, channels and slots, can improve the stability of the injection molding process and the quality of the product.

9. Simplified Runner Design

Simplified Runner Design: Simplified runner design aims to reduce complex branching structures and lengths to reduce manufacturing costs and injection pressure losses. By optimizing the size and shape of the main runner and reducing the number and length of divergent runners, injection molding efficiency and consistency can be improved.

Direct Nozzle Systems: Another way to simplify runner design is to use a direct nozzle system, which injects the melt directly into the product at the fill point, omitting the need for a traditional runner system. This design is suitable for some simple shapes of products, which can reduce the complexity of the runner and the difficulty of mold manufacturing.

By reasonably designing the air holes and exhaust system and simplifying the runner structure, the efficiency of injection molding can be improved, the manufacturing cost can be reduced, and the stability of product quality can be ensured. In the actual design, it is necessary to consider and optimize according to the specific product and injection molding process requirements to meet the specific production needs.

10. Conclusion

Reasonable runner system design is an indispensable aspect of successful injection mold design. Designers must fully consider the product structure characteristics, material performance parameters, filling flow law and other factors, the comprehensive use of runner design principles, the use of reasonable runner structure, to determine the optimal size of each part, so that the melt is fully and uniformly filled. A well-designed runner system can not only directly improve the filling performance of the mold, but also promote the efficiency and stability of the entire injection molding production process, is to achieve high quality, high efficiency production basis.