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What is the Runner System in an Injection Mold?
- Ramez Atallah
- Apr 9
- 6 min read
Updated: Apr 13
The runner system in an injection mold refers to the system that directs molten plastic from the injection molding machine's nozzle to the mold cavity. Its design and function significantly influence the efficiency of the injection molding process and the quality of the finished product.
There are two main injection mold types based on whether they use a heated runner system (hot runner) or an unheated runner system (cold runner). This article discusses hot runners and cold runners, details their components, and examines the advantages and disadvantages of each.
TABLE OF CONTENTS
What is a Cold Runner System in an Injection Mold?
A cold runner in an injection mold is a system with runners that are at the same temperature as the mold cavities. This system, which can consist of two or three plates, transfers molten plastic injected through the mold’s sprue, through cold (unheated) runners, and into the mold’s cavity. When the plastic cools, it takes the shape of the mold cavity, with the solidified plastic in the runner attached.
A cold runner is suitable for working with any type of plastic, is less expensive than a hot runner, and is easy to maintain. However, it generates waste and offers less control over temperature and pressure.
The Components of a Cold Runner System
Sprue: A passage in the mold that connects directly to the nozzle of the injection molding machine, allowing the molten plastic to enter the mold cavity. Once the finished product is ejected from the mold, the sprue can be either manually cut or ejected from the mold to be reused. This depends on whether the runner system consists of two or three plates.
Runners: Channels that direct the molten plastic from the sprue to different cavities within the mold, allowing the molding of several parts.
Gates: Narrow openings between the runner and the mold cavity through which molten plastic passes to fill the cavity. The finished product will usually have gate marks from where it was cut off.
Cold Slug Well: A space that is located at the end of the sprue. It captures the initial portion of the molten plastic as it is channeled to the mold cavity. As it enters the mold cavity, this initial portion usually cools and solidifies more quickly and could potentially cause defects in the finished product.
The 2 Types of Cold Runners
1. Two Plate Cold Runner: This system is comprised of two plates; the fixed plate (attached to the machine platen) and the A plate. The A plate contains the sprue, runners, gate, and cavities.
The two plate cold runner system is simpler, faster and less costly than the three plate design. After ejection, however, the runner and gates are attached to the final product and must be cut off.
2. Three Plate Cold Runner: This system uses a third plate called the runner plate, which separates the fixed plate and the sprue from the A plate.
A three plate cold runner system eliminates the need to remove the runner from the finished product, making it ideal for items with intricate designs. While it is a more affordable option compared to hot runner systems, it is more complex, has longer cycle times, and is more expensive than the two-plate design.
Advantages and Disadvantages of Cold Runners
Advantages
Lower investment cost: Less expensive to manufacture.
Lower repair and maintenance costs: Reduced need for inspection and maintenance.
Suitable for all types of plastics: Well-suited for thermally sensitive plastics.
Easier to change colors: The plastic material in the runners is cooled and ejected so it does not interfere with a color change.
Easy to use: Requires less skill and experience to operate.
Disadvantages
Slower cycle times: Reduced plastic flow due to the cold runners and the need to remove the runner and sprue make it less suitable for high-volume production.
Material waste: The molten plastic solidifies in the runners and sprue and can be wasted, leading to more material usage.
More manual labor: Time-consuming processes such as trimming, waste recycling, and removal of runners require more manual labor.
Design limitations: Since there is less control over temperature and pressure, it cannot be used for products with complex designs.
What is a Hot Runner System in an Injection Mold?
A hot runner in an injection mold utilizes a heated manifold system for heating the runner, which, in turn, heats the plastic material flowing through it. It is usually mounted between the back plate and the A plate in an injection mold.
Hot runners offer better control over temperature and pressure in the injection molding process. Furthermore, only the finished products are cooled and ejected, eliminating wasted plastic from the runner.
The Components of a Hot Runner System
Manifold: A heated component located within the hot runner plate and directly linked to the injection mold's sprue. It is positioned between the nozzle of the injection molding machine and the nozzles at each injection point. A manifold contains heated channels that maintain thermal balance as they distribute the molten plastic material to nozzles which feed each cavity in the mold.
Nozzles: Components that connect the manifold’s channels to each cavity in the injection mold. They are attached to heat conductors, which keep the molten plastic material heated as it passes through them to the mold cavities.
Gates: Components located at the entry point for the molten plastic material from the nozzles to the mold cavities. They maintain the molten plastic’s flow to the mold cavity and can shut it off when necessary.
Manifold Heaters: Heating elements made from high-resistance wire that are electrically insulated on the outside. They heat the manifold channels, ensuring proper temperature control.
Nozzle Heaters: Compact heaters with a coiled heating element designed to fit into the nozzle. They provide uniform heat distribution to the molten plastic as it passes through the nozzle.
Thermocouples: Sensors that measure the hot runner’s temperature. They consist of two different metal wires joined at one end. Their tips are fitted into a grooved channel.
Temperature Controller: A specialized device that precisely regulates the temperature of the hot runner system. It maintains a consistent and uniform temperature throughout the manifold's channels.
How Does a Hot Runner Work?
This video shows how this system works.
The injection mold’s sprue provides an opening for the molten plastic to pass from the machine’s nozzle and into the manifold.
The manifold distributes the molten plastic to each nozzle and into the mold cavities. Heaters embedded into the manifold keep the molten plastic material at the required temperature.
From the manifold, the molten plastic enters the nozzles, which are heated by the nozzle heaters to control the plastic’s temperature as it enters the cavities.
From the nozzles, the molten plastic passes through the gates and enters the mold cavities. It is then cooled, and the finished product is ejected from the mold.
Advantages and Disadvantages of Hot Runners
Advantages
Faster production: The runner does not need to solidify and cool down, eliminating the need for time-consuming processes such as trimming, waste recycling, and removal of runners. This results in faster cycle and processing times.
Reduced material waste: The plastic in the heated runners does not solidify, meaning little material is wasted.
Better quality products: Better control over temperature and pressure results in products of higher quality and fewer defects.
Design flexibility: Better control over temperature and pressure makes it suitable for products with more intricate designs and complex geometries.
Long-term cost savings: Reduced material wastage, faster production, and better-quality products can increase profits in the long run.
Disadvantages
High initial investment: More complex components make it more expensive to produce.
Higher maintenance and repair costs: More electrical and mechanical parts increase maintenance and repair costs.
Skilled labor required: Operators must have more technical skills and experience, which can affect quality control and labor costs.
Not suitable for all types of plastics: Not suitable for thermally sensitive plastics.
Harder to change colors: The plastic material in the runners remains molten, so the runners have to be cooled first, then cleaned before changing colors.
Final Thoughts on Selecting the Right Runner System
Understanding the difference between hot runners and cold runners in an injection mold is crucial. Each system offers unique advantages and disadvantages, which can impact your project's results in various ways.
At MOLDEX, we can provide runner design solutions that are tailored to fit your project’s specific requirements. We source our hot runner systems from industry leaders like Husky, Yudo, and Mold Master to provide the best quality results for your project. You can request a quote or contact us and we’ll be happy to assist you in any way we can.
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