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Hot runner mold. Image Credit: Ace Photography

Overview of Hot Running Injection Molding System

Xomety X
By Team Xometry
June 7, 2023
 15 min read
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Plastic injection molding is a widely adopted manufacturing process that offers versatility, precision, and cost-effectiveness for producing complex plastic components. Over the years, advancements in injection molding technology have continually pushed the boundaries of what is possible, enhancing the efficiency and quality of the manufacturing process. One such innovation that has revolutionized plastic injection molding is the introduction of hot runner systems.

Hot runner injection molding systems have emerged as a game-changing solution, allowing for improved control, reduced material waste, and increased productivity. Hot runner systems offer numerous advantages over traditional cold runner systems by eliminating the need for cold runners and optimizing the flow of molten plastic within the mold.

What Is Hot Runner Injection Molding?

Hot runner injection molding is a method of injection molding in which plastic is injected through a heated manifold system to deliver plastic directly into each cavity of a mold. This method differs from traditional cold runner injection molding, in which molten plastic is introduced to the mold via a runner, which is simply a series of plastic channels for the molten plastic to flow through. After an injection cycle, the system needs to be cooled, and the molded part and the runner get ejected. The manifold system used in hot runner injection molding eliminates the need for these disposable runners, and results in less waste, as well as a quicker cycle time.

What Are Examples of Hot Runner Injection Molding Projects?

Hot runner injection molding projects encompass a diverse range of applications, demonstrating the versatility and effectiveness of this technology. In the automotive industry, hot runner systems are utilized to produce interior components, exterior parts, and engine components. These systems ensure the production of high-quality, dimensionally accurate parts essential for the automotive sector.

The packaging industry extensively relies on hot runner systems for manufacturing plastic packaging products. Bottle caps, closures, containers, and lids are efficiently produced with consistent quality and minimal waste using hot runner technology.

Hot runner systems find significant applications in the medical field for the production of intricate and precise parts used in medical devices. Syringes, inhalers, IV components, and surgical instruments benefit from the reduced cycle times and high precision enabled by hot runner injection molding.

Consumer goods manufacturing also benefits from hot runner technology. Electronic components, household appliances, toys, and consumer electronics can be efficiently produced, eliminating cold runners and minimizing waste during mass production.

In the industrial sector, hot runner systems are utilized for the manufacturing of critical components such as connectors, valves, gears, and enclosures. The precise control over molten plastic flow ensures consistent quality and dimensional accuracy for these essential industrial parts.

How Do Hot Running Injection Molding Systems Work?

Hot runners operate by utilizing a temperature controller to heat the entire runner system, enabling the delivery of molten plastic to mold cavities. The temperature can be adjusted as needed to ensure optimal part quality, guaranteeing that only molten plastic flows through the molds.

A hot runner system typically comprises a heated barrel, a manifold, and a collection of nozzles. These components work together to facilitate the injection molding process. As the molten plastic material is injected into the mold, it flows through the heated nozzles, aided by the consistent heat provided by the barrel. This heat ensures that the plastic remains in a liquid state, allowing it to smoothly enter the mold cavity. Once inside, the molten plastic cools and solidifies, taking the shape of the mold cavity. The barrel of the hot runner system is commonly heated using electric heaters, although specific equipment variations may exist.

What Is the Hot Runner System in Plastic Industry?

A hot runner system is a specialized component used in the plastic injection molding process. It is designed to maintain a consistent temperature in the channels or "runners" through which molten plastic flows into the mold cavity. When plastic parts are manufactured through injection molding, the molten plastic material is injected into a mold cavity under high pressure. The plastic material solidifies and takes the shape of the mold, forming the desired plastic product. In a conventional injection molding system, the plastic is injected into the mold through a sprue, which leads to a cold runner system. The cold runner is a set of channels that distribute the molten plastic from the injection molding machine nozzle to the individual mold cavities.

In contrast, a hot runner system eliminates the need for a cold runner by using heated channels. The hot runner system consists of a manifold, which is a heated block with multiple channels, and a series of nozzles that deliver the molten plastic directly into the mold cavities. The channels and nozzles in the hot runner system are heated to maintain the plastic in a molten state throughout the injection molding process. For more information, see our guide on Injection Molding vs. Vacuum Forming - What's the Difference?

What Are the Advantages of Using a Hot Runner Molding System?

Hot runner systems offer numerous advantages, making them increasingly preferred for molding thermoplastics. The key benefits include:

  1. Cooling time is primarily influenced by the thickest section. Injection screw recovery and injection times are also shortened due to smaller shot sizes, further reducing processing costs.
  2. There are no solidified runners to be removed after each shot. Once the system reaches operating temperature, the mold is ready to run. Additionally, hot runner systems enable lower injection pressures, leading to reduced mold and platen deflection. This lessens the likelihood of flash caused by mold component movement.
  3. Maintains a balanced melt flow at a constant temperature, ensuring complete filling and packing of cavities. This enables molders to achieve precise dimensional accuracy and high-quality parts. To prevent cavity damage, it is easy to reduce heat to the hot runner nozzle and prevent the production of faulty parts.
  4. Provide part dimensional consistency and eliminate runners that could interfere with mold mechanisms, robots, conveyors, and assembly machinery, resulting in flash-free plastic parts.
  5. Offer the flexibility to place gates at various points on the part. With hot tip gating, valve gating, or edge gating, gates can be located optimally for ideal filling and aesthetics. This flexibility extends to mold design, allowing for optimized cavity orientation, cooling, and simplified mold structures.
  6. Provide balanced melt flow by utilizing separate externally heated manifolds insulated from the surrounding mold plates. Multi-level hot manifolds ensure equal flow and pressure to all cavities, regardless of the cavity count. This temperature and pressure control is particularly beneficial for resins with narrow processing windows.

What Are the Disadvantages of Using a Hot Runner Molding System?

There are some disadvantages to using a hot runner molding system, including:

  1. More expensive than cold runner injection molding systems, especially the initial investment costs. However, they do save money over time through reduced waste and increased process productivity. 
  2. Require high precision in their production, making them more complex and expensive to maintain.
  3. Requires additional heating sources for operation.

Why Use Hot Runner Injection Molding Instead of a Cold Runner Injection Molding System?

Hot runner injection molding has a number of advantages over cold runner injection molding, making it more suitable for a number of scenarios. A hot runner system can have cost savings due to reduced waste and higher efficiency. These cost savings can outweigh the higher costs of hot runner machines if high production volumes are required. The increased design flexibility of a hot runner system means that it is more suitable to produce intricate or complex parts.

Also, the consistent temperatures in a hot runner system produce improved part quality and surface finish, making it ideal for manufacturing parts in which aesthetics are important. The controlled temperatures also make it suitable for certain heat-sensitive materials which can degrade in the cooling process used by cold runner systems.

What Are the Factors To Consider When Using a Hot Runner Injection Molding System?

When choosing the appropriate hot runner system, several factors should be considered to ensure a successful implementation. Some of them are listed and summarized below:

  1. Consider the Design: Each hot runner system has unique requirements, even within standardized systems. It is crucial to involve the hot runner manufacturer in the system design process to ensure proper operation.
  2. Consider the Application: Hot runner system configurations vary significantly depending on the application. For example, the design will differ based on whether the system needs to gate directly to the part or use a sub-runner attached to the part. Configurations will also change for high-temperature requirements, high-volume projects, or other application-specific needs.
  3. Other Considerations: The type of material being molded and the color requirements have an impact on the type of hot runner system that is suitable. While cost is typically a primary consideration due to the expense of hot runner systems, the benefits provided by these systems often outweigh the initial costs.

What Is the Significance of Precise Temperature Control for Runner Injection Molding Systems?

Precise temperature control is of utmost importance in runner injection molding systems due to several key reasons:

  1. Material Quality: Different types of plastics used in injection molding have specific temperature requirements for optimal processing. Precise temperature control ensures that the molten plastic remains within the correct temperature range, allowing for proper material flow, viscosity, and homogeneity. Maintaining the ideal temperature throughout the molding process helps prevent issues like material degradation, inconsistent part quality, or defects caused by improper material behavior.
  2. Part Quality and Consistency: Temperature control directly affects the quality and consistency of the molded parts. Deviations in temperature can lead to variations in part dimensions, warping, sink marks, or other cosmetic defects. By tightly controlling the temperature, the molder can produce parts that meet the desired specifications consistently, minimizing rejects and ensuring consistent product quality.
  3. Cycle Time Optimization: Efficient temperature control allows for precise timing and control of the cooling and solidification process. Proper cooling is crucial to achieving shorter cycle times in injection molding. By accurately controlling the temperature of the mold and the molten plastic, the cooling time can be optimized, reducing overall production cycle times and increasing productivity.
  4. Energy Efficiency: Precise temperature control helps minimize energy consumption in runner injection molding systems. By maintaining the required temperature within a narrow range, energy wastage due to excessive heating or cooling can be avoided. Energy-efficient systems not only reduce operational costs but also contribute to environmental sustainability.
  5. Process Stability and Control: Temperature fluctuations can have a significant impact on the stability and repeatability of the injection molding process. Precise temperature control enables greater process stability, reducing the likelihood of process variations and deviations. This stability allows for better process control, making it easier to fine-tune the parameters and optimize the molding process for consistent and reliable production.

What Are the Different Types of Hot Runner Systems?

Hot runner systems encompass a variety of types that differ in structure and functionality. Here, we explore distinct types of hot runner systems:

  1. Hot Tip Hot Runner System: This system relies on a hot tip located at the nozzle's front end, coupled with a cooling mechanism. By doing so, it enables precise adjustment and control of the plastic molding temperature at the gate. The material and shape of the nozzle’s hot tip are critical factors in this process. Such systems are suitable for processing a broad range of crystalline and amorphous plastics such as: PP, LCP, PA, PE, PS, PEEK, POM, PEI, PMMA, ABS, PET, PBT, PVC, PSU, PC, TPU, and more. Typically, hot-tip gates are ideal for manufacturing small to medium-sized parts, especially those with smaller dimensions, with gate diameters ranging from 0.5 mm to 2.0 mm. The gate size is determined primarily by the part's weight, wall thickness, material quality, and desired profile.
  2. Sprue Hot Runner System: Designed for medium-sized or heavy injection molding projects, the sprue hot runner system injects plastic into a mold through a sprue, ensuring low pressure at the gate. It offers the advantage of maintaining a low shear rate during plastic flow, resulting in minimal residual stress and deformation post-injection molding. Unlike the hot-tip gate, the sprue gate is larger, potentially leading to larger gate marks. Consequently, it is commonly employed in structural parts that do not require stringent aesthetic standards. Sprue gates are often combined with cold runners, with the sprue acting as the main runner, which allows for a larger sprue to facilitate plastic flow.
  3. Valve Gate Systems: Operating through a needle's mechanical movement to control nozzle opening and closing, the valve gate hot runner system employs mechanical, pneumatic, or hydraulic pressure for needle control. It enables the closing of the hot nozzle opening before the product is fully condensed during the holding time in the injection molding process. This system ensures smooth cavity filling, and balanced melt flow, and eliminates weld marks on the final product. Weld marks occur when two melt fronts meet but do not fully merge, resulting in visible lines. For larger injection molded parts, multiple hot gates are often necessary to ensure proper filling. The valve gate system provides programmatic control over each gate, opening subsequent gates as needed while maintaining fusion between melts, and eliminating weld marks and seam lines on the finished product.

Is There Too Much Waste During the Hot Runner Injection Molding Process?

No. Hot runner injection molding processes are generally known for producing less waste compared to cold runner systems. This is primarily because hot runner systems eliminate the need for a sprue and runner system, which typically generates waste material in the form of runners and gates.

Is a Hot Runner Injection Molding System More Expensive Than a Cold Runner Injection Molding System?

Yes, hot runner injection molding systems are generally more expensive than cold runner injection molding systems. In a cold runner system, the mold is designed with channels or passages called runners, which distribute the molten plastic to the various cavities in the mold. Cold runner systems are relatively simple and cost-effective, making them a popular choice for certain applications. On the other hand, hot runner systems are more complex and require additional components such as heated nozzles, manifold systems, and temperature controllers. The additional components and complexity of hot runner systems make them more expensive upfront. For more information, see our guide on Hot Runner vs. Cold Running Molds in Injection Molding - What's the Difference?

What Is the Difference Between 3D Printing and Injection Molding?

3D printing and injection molding are two different manufacturing processes with distinct characteristics and differences. 3D printing, or additive manufacturing, is a process that produces three-dimensional objects by building them layer by layer from a digital model. Injection molding, on the other hand, involves injecting molten material into a mold cavity to produce a part. Listed below are some key differences between the two:

  1. Process: In 3D printing, objects are built layer by layer from the bottom up using a digital model. On the other hand, injection molding is a manufacturing process in which molten material is injected into a mold cavity and cooled to form the desired shape.
  2. Complexity: 3D printing allows for the creation of highly complex geometric shapes with intricate details, including internal features and undercuts. Injection molding, while capable of producing complex parts, may have limitations due to the need for mold design and ejection considerations.
  3. Materials: 3D printing supports a wide range of materials, including plastics, metals, ceramics, and composites. Injection molding primarily uses thermoplastic and thermosetting polymers that can be melted and injected into the mold.
  4. Production Volume: 3D printing is often used for low-volume production or prototyping due to its flexibility and ability to create customized products. Injection molding is more suitable for high-volume production runs, as the cost per part decreases with larger quantities.
  5. Surface Finish: 3D printed parts typically have visible layer lines, which may require additional post-processing to achieve a smooth surface finish. Injection molding produces parts with a consistently smooth surface, requiring little to no post-processing.
  6. Cost: The cost structure of the two processes differs. 3D printing can be cost-effective for small production runs or highly customized parts, but it may become expensive for large-scale production. Injection molding generally has higher upfront costs due to mold design and manufacturing but becomes more cost-effective for large quantities.

For more information, see our guide on 3D Printing vs. Plastic Injection Molding.


This article presented the hot running injection molding system, explained it, and discussed how it works and its different advantages. To learn more about injection molding systems, contact a Xometry representative.

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Xomety X
Team Xometry
This article was written by various Xometry contributors. Xometry is a leading resource on manufacturing with CNC machining, sheet metal fabrication, 3D printing, injection molding, urethane casting, and more.