In injection molding, the runner system is an essential part of the mold that allows the flow of molten plastic from the injection molding machine’s nozzle to the cavities where the parts are formed. The runner system ensures the even distribution of plastic material, and it is later removed as scrap. There are several types of runner systems used in injection molding, each with its advantages and disadvantages. Here’s a comprehensive guide to the most common runner types:
- Cold Runner System: The cold runner system is the most traditional and widely used runner type in injection molding. In this system, the molten plastic is injected into a set of channels (runners) that remain at room temperature during the injection process. The plastic flows through the runners and into the cavities where it solidifies to form the parts. After the injection cycle, the solidified plastic in the runners is removed as waste, and it can be recycled or reprocessed. The cold runner system is simple, cost-effective, and suitable for a wide range of part sizes and geometries. However, it generates more waste material, which may increase material costs and waste management.
- Hot Runner System: The hot runner system is an advanced runner type that addresses some of the limitations of the cold runner system. In this system, the runners are heated to maintain the plastic in a molten state, even when the molding machine is not injecting. This prevents the generation of waste material, as the plastic in the runners remains in a usable state for subsequent shots. The hot runner system reduces material wastage, shortens cycle times, and allows for faster production. However, it is more complex and expensive to set up and maintain compared to the cold runner system. The initial investment can be higher, but it can lead to cost savings in the long run for high-volume production.
- Insulated Runner System: The insulated runner system is a variation of the hot runner system that uses insulation material to retain heat within the runners. This helps to reduce energy consumption and improve thermal efficiency, resulting in consistent part quality and reduced cycle times.
- Valve Gate System: The valve gate system is a specialized hot runner system that uses mechanical valves to control the flow of plastic into the cavities. This allows for precise control over the injection process, leading to high-quality parts with minimal gate vestige or witness marks. The valve gate system is often used in applications where gate appearance is critical, such as in automotive interior components or consumer electronics.
- Sequential Valve Gating: Sequential valve gating is a technique that uses multiple gates in the hot runner system, which are activated one after the other in a predetermined sequence. This allows for better control over the flow of plastic into the cavities, reducing internal stress and improving part quality, especially in large and complex parts.
Selecting the appropriate runner system depends on various factors, such as part design, material, production volume, cost considerations, and required part quality. Manufacturers should carefully analyze these factors and consult with mold experts to determine the most suitable runner type for their specific injection molding project. Each runner system has its advantages and trade-offs, and the right choice can significantly impact the efficiency and success of the injection molding process.
Injection molding is a widely used manufacturing process that produces plastic parts by injecting molten material into a mold cavity. The mold cavity is shaped like the desired part and can have multiple cavities for producing multiple parts at once. The molten material is fed into the mold cavity through a system of channels called runners.
Runners are an essential component of injection molding, as they affect the quality, efficiency, and cost of the process. Different types of runners have different advantages and disadvantages, depending on the part design, material properties, and production requirements. In this article, we will discuss the main types of runners in injection molding, their characteristics, benefits, and drawbacks, and how to choose the best runner type for your project.
Table of Contents
- What are runners in injection molding?
- Types of runners in injection molding
- Cold runners
- Hot runners
- Insulated runners
- How to choose the best runner type for your project
What are runners in injection molding?
Runners are the channels that connect the injection unit to the mold cavity. They allow the molten material to flow from the nozzle of the injection machine to the gate of the mold cavity. The gate is the point where the material enters the mold cavity and forms the part.
Runners can have different shapes, sizes, and configurations, depending on the part design and mold layout. The most common shapes are circular, trapezoidal, and rectangular. The size of the runner affects the pressure drop, flow rate, and cooling time of the material. The configuration of the runner depends on the number and location of the cavities in the mold. The most common configurations are single-cavity, multi-cavity, and family molds.
Runners are usually classified into two main types: cold runners and hot runners. Cold runners are made of the same material as the part and solidify after each injection cycle. Hot runners are heated by an external source and keep the material in a molten state throughout the process. There is also a third type of runner called insulated runner, which is a hybrid between cold and hot runners.
Types of runners in injection molding
Cold runners are the simplest and most common type of runners in injection molding. They are made of the same material as the part and solidify after each injection cycle. They have to be separated from the part manually or automatically and either reused or discarded.
The main advantages of cold runners are:
- They are easy to design and manufacture.
- They are compatible with most materials and mold designs.
- They do not require additional heating or cooling equipment.
- They reduce material waste and energy consumption.
The main disadvantages of cold runners are:
- They increase cycle time and labor cost due to the need to remove and handle them.
- They reduce part quality due to potential defects such as weld lines, sink marks, and jetting.
- They limit design flexibility due to geometric constraints and pressure loss.
Hot runners are a type of runners that are heated by an external source and keep the material in a molten state throughout the process. They do not solidify after each injection cycle and do not need to be separated from the part. They consist of two main components: a manifold and a nozzle. The manifold distributes the material from the injection unit to multiple nozzles that feed each cavity. The nozzles can have different types of gates such as edge gate, pin gate, valve gate, or fan gate.
The main advantages of hot runners are:
- They eliminate cycle time and labor cost associated with cold runners.
- They improve part quality by reducing defects such as weld lines, sink marks, and jetting.
- They increase design flexibility by allowing complex geometries and large parts.
- They reduce material waste and energy consumption by eliminating cold runner scrap.
The main disadvantages of hot runners are:
- They are more expensive and complex to design and manufacture.
- They require additional heating and cooling equipment and maintenance.
- They are not compatible with some materials that are sensitive to thermal degradation or contamination.
- They can cause thermal expansion and contraction issues that affect mold alignment and dimensional accuracy.
Insulated runners are a type of runners that are made of a thermally insulating material that prevents heat transfer from the molten material to the mold. They act as a buffer between cold and hot runners, keeping the material at a temperature close to its melting point. They solidify partially after each injection cycle but do not need to be separated from the part.
The main advantages of insulated runners are:
- They combine some benefits of both cold and hot runners.
- They reduce cycle time and labor cost compared to cold runners.
- They improve part quality compared to cold runners.
- They reduce material waste and energy consumption compared to hot runners.
The main disadvantages of insulated runners are:
- They are more difficult to design and manufacture than cold runners.
- They require special materials and mold coatings that are thermally insulating and wear-resistant.
- They are not suitable for some materials that have a narrow processing window or high viscosity.
- They can cause material degradation and color change due to prolonged exposure to heat.
How to choose the best runner type for your project
Choosing the best runner type for your project depends on several factors, such as:
- Part design: The shape, size, and complexity of the part affect the choice of runner type. For example, cold runners are more suitable for simple and small parts, while hot runners are more suitable for complex and large parts.
- Material properties: The type, grade, and color of the material affect the choice of runner type. For example, hot runners are more suitable for materials that have a high melting point, low viscosity, and high thermal stability, while cold runners are more suitable for materials that have a low melting point, high viscosity, and low thermal stability.
- Production requirements: The quantity, quality, and cost of the production affect the choice of runner type. For example, hot runners are more suitable for high-volume, high-quality, and low-cost production, while cold runners are more suitable for low-volume, low-quality, and high-cost production.
Runners are an important component of injection molding, as they affect the quality, efficiency, and cost of the process. Different types of runners have different advantages and disadvantages, depending on the part design, material properties, and production requirements. Choosing the best runner type for your project requires a careful analysis of these factors and a trade-off between them. In general, cold runners are more suitable for simple, small, and low-volume parts made of low-melting-point, high-viscosity, and low-thermal-stability materials. Hot runners are more suitable for complex, large, and high-volume parts made of high-melting-point, low-viscosity, and high-thermal-stability materials. Insulated runners are a hybrid between cold and hot runners that combine some benefits of both types.