This article will introduce you to the concept and principle of injection mold runner design, which is an important aspect of plastic injection molding. You will learn what a runner is, how it affects the quality and efficiency of your injection molded parts, and how to design a runner system that suits your needs.
Table of Content
- What is a runner?
- Why is runner design important?
- What are the types and shapes of runners?
- How to design a balanced and unbalanced runner system?
- What are some tips and best practices for runner design?
What is a runner?
A runner is the melt flow passageway between the end of the main runner (the channel that connects the nozzle of the injection machine to the mold) and the gate (the opening that allows the molten plastic to enter the cavity). The runner is used to change melt flow direction and allocate it to each cavity in a stable and balanced way.
injection molding, a runner is a crucial component of the mold that serves as a pathway for the molten plastic material to flow from the injection molding machine’s nozzle into the mold cavity. The runner system is an intricate network of channels that distribute the molten plastic to multiple mold cavities, allowing for the simultaneous production of multiple parts in one injection cycle. Runners play a pivotal role in the injection molding process by ensuring a smooth and balanced flow of the molten plastic throughout the mold, thereby facilitating the formation of consistent and high-quality parts.
There are different types of runner systems used in injection molding, each offering specific advantages depending on the requirements of the molding project. The most common types include the conventional runner system, hot runner system, and cold runner system.
In a conventional runner system, the molten plastic flows through the runners, filling the mold cavities with the desired parts. Once the molding process is complete, the runners are ejected along with the parts as scrap. Conventional runner systems are relatively simple and cost-effective, but they do generate waste material, which can increase production costs and material usage.
The hot runner system, on the other hand, is designed to eliminate the need for runners and minimize waste material. In a hot runner system, the channels that deliver the molten plastic to the mold cavities are internally heated, keeping the plastic in the runners in a molten state during the entire injection molding cycle. This allows for precise control of the flow and temperature of the molten plastic, resulting in reduced cycle times, lower material wastage, and improved part quality. Although the initial investment for a hot runner system is higher, the long-term cost savings and improved efficiency make it an attractive choice for high-volume production.
In contrast, the cold runner system is similar to the conventional runner system, but the runners and sprue are not heated, leading to the solidification of the plastic material in these channels after each injection cycle. The solidified material in the runners is then removed along with the parts as scrap. Cold runner systems are simple and cost-effective but can generate more waste material compared to hot runner systems.
The design and selection of the runner system depend on various factors, including the part design, material, production volume, and cost considerations. Engineers and mold designers carefully analyze these factors to determine the most suitable runner system that maximizes efficiency, minimizes waste, and ensures the production of high-quality plastic parts.
In conclusion, a runner is an integral component of the injection molding process, serving as the pathway for the molten plastic material to flow from the injection molding machine into the mold cavity. The runner system plays a crucial role in the efficient and balanced distribution of the molten plastic, allowing for the simultaneous production of multiple parts in one injection cycle. The choice of runner system, whether conventional, hot runner, or cold runner, is a critical decision that impacts the efficiency, cost-effectiveness, and quality of the injection molding process. By carefully considering the specific requirements of the molding project, engineers can optimize the runner system design and ensure the successful production of high-quality plastic parts.
Why is runner design important?
Runner design can have a significant impact on the quality and efficiency of your injection molded parts. Here are some reasons why:
- Material usage: The runner design can affect how much plastic material is required to produce the parts. A poorly designed runner system can lead to excessive material waste, resulting in increased production costs.
- Cycle time: The runner system’s design can also affect the cycle time of the injection molding process. A longer or thicker runner can increase the cooling time and reduce the productivity. A shorter or thinner runner can cause insufficient filling or pressure loss, resulting in defective parts.
- Part quality: The runner design can influence the filling pattern, pressure distribution, temperature gradient, and shear stress of the molten plastic in the mold. These factors can affect the dimensional accuracy, surface finish, mechanical properties, and aesthetic appearance of your injection molded parts.
What are the types and shapes of runners?
There are two main types of runners: cold runners and hot runners. Cold runners are made of the same material as the part and solidify after each injection cycle. They need to be separated from the part and recycled or discarded. Hot runners are heated channels that keep the plastic molten throughout the injection cycle. They do not produce any waste material, but they require more complex and expensive mold design and maintenance.
The shape of the runner cross-section can vary depending on the mold design and material properties. Some common shapes are:
- Circular: This shape has the minimum specific surface area (runner surface area/runner volume), which reduces heat loss and pressure drop. However, it requires both sides of the mold plate to be aligned during production.
- Trapezoid: This shape is easy to process and allows for easy ejection of the runner from the mold. It also provides a flat surface for attaching a gate.
- U-shape: This shape is similar to trapezoid, but with rounded corners that reduce stress concentration and improve flow characteristics.
- Semicircle: This shape combines the advantages of circular and trapezoid shapes, but it is more difficult to process.
- Rectangle: This shape has a larger specific surface area and high resistance to flow, thus it is seldom applied.
How to design a balanced and unbalanced runner system?
- Balanced: This layout ensures uniform filling, with each cavity concurrently filled. The length and diameter of each branch runner are equal or proportional to each other. This layout minimizes pressure loss, material waste, and cycle time, but it may require more complex mold design and processing.
- Unbalanced: This layout allows for different lengths and diameters of each branch runner, depending on their position relative to the main runner. This layout is simpler and cheaper to design and process, but it may cause uneven filling, pressure variation, and part quality issues.
What are some tips and best practices for runner design?
Here are some general guidelines for designing an optimal runner system for your injection molding project:
- The size of the runner should be determined by the plastic part size/type, injection speed, and length of the runner. Generally, when the diameter of a branch runner is smaller than 5 – 6mm, runner size will have a greater influence on fluidity; when the diameter is greater than 8mm, there will be little influence on fluidity.
- The internal surface roughness of the runner should be 1.6. In this case, the outer layer melt flows faster than the inner layer melt inside the branch runner, which tends to cool down and thus form a heat insulation layer.
- The runner should be designed on either half of the plastic injection mold. When considering the layout of the cavity and the branch runner, it is better to ensure that when projected on the parting surface, the geometric center of the total projection area of the cavity and the runner overlap with the center of the clamping force.
- If the branch runner is very long, it might be better to further extend the branch runner along the flow direction to form a cold slug well, so that the cold materials will not enter the cavity.
- The runner should be designed to avoid sharp turns, dead ends, or sudden changes in cross-section, which can cause turbulence, shear stress, or material degradation.