What is a hot runner mold system for plastic injection molds ?
- What is a hot runner mold system for plastic injection molds ?
- What is a hot runner system?
- Why use hot runner injection molds instead of cold runner injection molds?
- Main components of hot runner system
- A simple explanation of the differences between hot runner and cold runner injection mold systems
- Classification of hot runners in injection molds
- Types of hot runner systems
- The main heating method of hot runner in injection molds
- Two main factors for designing successful hot runner injection molds
- Advantages of hot runner systems
- A Brief Overview of Key Design Considerations for Hot Runner Injection Molds
- Factors to consider when selecting equipment for hot runner injection molds
- Future Development Trends of Hot Runner Molds
- in conclusion
Injection molding is a widely used manufacturing technology for mass production of plastic products. This technology reduces labor input, increases efficiency, and lowers costs. Injection molding processes have many technical types and can produce a wide variety of products; different technologies are used for different plastic materials. Hot runner systems are one type of mold used in injection molding; they are often referred to as hot runner molds.
However, this technology is often considered an extension of injection molding. It has its own characteristics, uses, and types, but these are usually determined when selecting materials and mold types for a project. If you need to manufacture plastic products using different material types, you may need to understand in detail the molds and molding-related information using this technology. Therefore, this article aims to provide you with information about hot runner systems and related molds.
What is a hot runner system?
Hot runner systems are the opposite of cold runner systems. Therefore, unlike cold runner systems, hot runner molds have heated components. A hot runner system is a molten plastic delivery unit used in injection molds. It consists of heating components used in plastic injection molds that guide molten plastic from the injection machine barrel into the mold cavity. The dimensions of the hot runner melt channels depend on many factors, such as the type of resin, injection speed, filling rate, and the specific part being molded. A separate controller or a controller built into the injection machine heats the resin in the hot runner system and inside the barrel to the processing temperature before injecting it into the mold. The resin flows through the main sprue, down the manifold, then through the manifold to the individual nozzles, and finally through the injection point (or gate) into the final cavity, where the final part is formed.
Why use hot runner injection molds instead of cold runner injection molds?
Hot runner systems not only improve productivity and system performance, producing injection-molded parts with more refined appearances, but also enhance flexibility and improve process monitoring. They can increase the production efficiency of high-quality plastic products, avoid waste generation, thereby accelerating the production cycle of high-volume plastic products and reducing component costs.
The core principle of hot runner systems is to deliver the melt to the mold, thus enabling high-quality gates, excellent cavity-to-cavity balance, rapid color changes, unlimited design freedom, and higher customization. Injection molds using hot runner solutions can achieve low cost, high quality, and high throughput.
Finally, the hot runner system excels in both hot spot and pin gate configurations and is optimized for specific applications by processors. The benefits of hot runner technology are most pronounced, particularly for high-volume applications with long product lifespans.
Main components of hot runner system
A hot runner system consists of many components, including heating manifolds and heating nozzles. There are typically many nozzles. Their main purpose is to properly distribute and deliver molten plastic to the nozzles, ensuring precise injection into the mold cavity. Furthermore, they must maintain a uniform temperature of the molten plastic. Even during the cooling process of the injection molding machine, the temperature of the molten plastic often remains low. Therefore, a hot runner controller is needed to control the temperature of the hot runner. This helps maintain the uniformity of the molten plastic within the nozzles. Below are the basic components of a hot runner injection mold.
| Heating element | These mainly include heating elements, resistance wires, or heating rods, which are embedded inside the hot runner plate or nozzle to provide the necessary heat to the plastic melt and keep it flowing. |
| Manifold hot runner plate | This is a key component connecting the injection molding machine nozzle and the gates of each cavity. It has multiple channels inside for distributing and conveying molten plastic. |
| Nozzle | Directly connected to the hot runner plate, it is responsible for precisely injecting molten plastic into each cavity. Depending on the design requirements, there are various types of nozzles, such as open type and needle valve type. |
| Temperature control system | Components such as temperature controllers and thermocouples are used to monitor and regulate the temperature of the hot runner system to ensure that the plastic melt flows within a suitable temperature range. |
| Cooling system | While the primary goal is to keep the hot runner system at a high temperature, certain areas still require appropriate cooling to ensure overall mold temperature balance and avoid problems such as plastic degradation caused by localized overheating. |
A simple explanation of the differences between hot runner and cold runner injection mold systems
Currently, there are two types of runner systems for injection molds: hot runner systems and cold runner systems. They are both runner systems, but they differ in some aspects. Some differences are obvious, while others are not.
Hot runners inject molten plastic directly into the mold cavity, while cold runners inject it into the gate first. Hot runners, by injecting directly into the cavity, help maintain the temperature of the gate and runner. In each cycle, only the part needs to be removed. This reduces unloading time and increases product lifespan. However, cold runner injection molding injects plastic into the runner through the gate (a central channel). This process extends the production cycle. Furthermore, the plastic cools on the cold runner. Each production cycle requires cooling and removal of excess plastic. Cold runners are also heavier and more prone to leaving spots on the product surface.
Classification of hot runners in injection molds
Hot runner systems for injection molds are key components in the injection molding process, significantly improving efficiency, quality, and mold design flexibility. Hot runner systems are divided into two main types: full hot runner systems, which provide precise temperature control across all runners; and semi-hot runner systems, which heat only a portion of the runners to balance cost-effectiveness and performance advantages.
Full hot runner system
The fully heated runner system integrates heating elements in each runner. This configuration allows for precise temperature control of the molten plastic, ensuring uniform flow and filling of each mold cavity.
| Precise temperature control | Because each runner is heated, these systems can maintain a consistent temperature, thereby improving the quality of the final product by reducing defects and ensuring uniform filling of all cavities. |
| Equilibrium flow | Individual heating of each channel helps the molten plastic flow in the runner, reducing problems such as material shortages and part quality variations. |
| Reduce material waste | By eliminating cooling channels, fully hot runner systems can significantly reduce material waste, thereby saving costs, especially in high-volume production operations. |
Semi-hot runner system
A semi-hot runner system combines heated and unheated channels within the same system. This design strikes a balance between the advantages of hot runners and the cost-effectiveness of cold runners.
| Cost savings | Using a combination of heated and unheated channels can reduce the total number of heaters and components required, thereby reducing initial setup and operating costs. |
| Flexibility | These systems offer a middle ground, allowing for a degree of design flexibility while still providing the advantages of reduced waste and faster cycle times compared to traditional cold runner systems. |
Types of hot runner systems
As mentioned above, there are two main types of hot runner systems. The difference lies in the type of injection point. Each type has its own uses and characteristics. Types of hot runner systems include:
Hot runner system
The hot runner system is a standard hot runner system. It’s a multi-purpose system, meaning it can be used for various purposes. It’s suitable for mass production of simple parts and primarily for small parts, but can also be used to produce medium-sized parts. When using a hot runner head, gate marks will be left, but the number may be small or minimal. It’s best to place the gate in a concealed location on the part.
Gating-type hot runner system
This hot runner system has an open channel through which plastic flows into the mold cavity. Unlike hot runner heads, which are suitable for medium-sized parts, it is better suited for parts larger than medium-sized parts. It also deforms less and has higher mechanical strength. Additionally, it is larger than a hot runner head, therefore producing gate marks, which are typically larger than those of a hot runner head. However, you can combine it with a cold gate for a better appearance.
Finally, temperature control is crucial in this runner system. You must control both the mold temperature and the plastic temperature simultaneously. Inappropriate temperatures can cause the gate to stop working, for example, by shutting down the plastic flow after the mold has opened.
Valve-type gate hot runner system
Valve gates differ from hot gates; they are a different type of hot runner system. They are equipped with automated mechanical valves: after the machine injects plastic into the mold, the valves automatically close. This allows for a seamless closure, leaving no marks on the product. It is ideal for products where aesthetics are paramount.
Valve gates offer a balance of detail, style, and design. Therefore, for products with extremely high aesthetic requirements, valve gate hot runner systems can achieve better design results. Valve gates are generally more efficient than hot gates. However, higher efficiency also means higher cost. And it’s significantly more expensive than hot gate runner systems. But if you need a stylish look and higher efficiency, then don’t hesitate to spend the extra money on a valve gate.
The main heating method of hot runner in injection molds
Different hot runner systems have different heating methods and components. The following are three heating systems used in hot runner injection molds and their respective advantages.
Heat transfer oil hot runner
The heat transfer oil hot runner uses heat transfer oil as the heat transfer medium, directing heat energy to the mold cavity through heating pipes and heat pipes. Compared with traditional heating methods, it can quickly reduce product defects and improve production efficiency.
Electric heating hot runner
The electrically heated hot runner system transmits electricity to the mold cavity through heater tubes, thereby heating the plastic material. It is characterized by high efficiency and flexibility, and can handle different types of injection-molded products.
Water-heated flow channel
Hydrothermal runners use high-temperature water or steam as the heat transfer medium to control temperature and facilitate the smooth flow of plastic materials into the mold. Due to their environmentally friendly and sustainable advantages, hydrothermal runners are increasingly being incorporated into the production of more and more products.
Two main factors for designing successful hot runner injection molds
Designing a successful hot runner mold application requires ensuring success in several aspects. Among them, two technical factors are most important: control of plastic temperature and control of plastic flow characteristics.
Control of plastic temperature
In hot runner mold applications, temperature control of the plastic is extremely important. Many processing and product quality problems that occur during production stem directly from poor temperature control of the hot runner system. Examples include poor gate quality in torpedo-type hot runner injection molding, difficulty in fully closing the valve pin in valve-type hot runner molding, and inconsistent filling time and quality of parts in multi-cavity molds. If possible, hot runner systems with multi-zone segmented temperature control should be selected to increase flexibility and adaptability.
Control of plastic flow
Plastic must flow in a balanced manner within a hot runner system. Gates must open simultaneously to ensure synchronized filling of all cavities. For parts with significantly different weights, the runner dimensions must be designed for balance. Otherwise, some parts may experience insufficient filling and holding pressure, while others may experience overfilling and holding pressure, resulting in excessive flash and poor quality. The hot runner runner dimensions must be designed reasonably. If the dimensions are too small, the filling pressure loss will be too high. If the dimensions are too large, the hot runner volume will be too large, causing the plastic to remain in the hot runner system for too long, damaging material properties and resulting in parts that fail to meet usage requirements after molding.
Advantages of hot runner systems
Hot runner systems also have their advantages and disadvantages. Hot runner systems offer advantages such as flexible design, short cycle times, and high efficiency. These advantages ultimately improve the molding results.
Efficiency Improvement
Hot runner systems can improve the efficiency of injection molding. They eliminate the need for solid runners that need to be removed and are always operational. That is, they start running as soon as the system reaches its operating temperature. Start-up is very easy. Furthermore, they can operate with lower injection pressures, thus reducing mold deformation.
Shorten cycle time
Hot runners can also shorten cycle time. Because they are thinner and lighter than cold runners, cooling of the thickest parts can be controlled. This helps save molding cycle time.
Flexible design
Hot runners offer greater flexibility in gate placement. Gate positioning can be problematic with other runner systems. However, with hot runners, gate placement is no longer an issue. The nozzles can position the gate in multiple locations. This allows the gate to be placed at the optimal filling point without worrying about the runner not being able to locate it. Therefore, it provides greater freedom in gate design and placement.
Reusable
Unlike cold runners, hot runners can be reused. More modern materials require new processes, which cold runners cannot meet. Typically, cold runners cannot be reused. Or, even if they could, storage costs are high. Cleaning them is noisy and prone to dust accumulation.
Injection Molding Automation
Leading companies are automating their processes in search of ways to produce more with less manpower. Hot runner systems perfectly meet this need. When used in conjunction with automation, connections are very convenient. There are no easily tangled runners. Therefore, hot runners facilitate automation.
A Brief Overview of Key Design Considerations for Hot Runner Injection Molds
Having read this far, you should now have a basic understanding of hot runner systems and related mold technologies. Below, we summarize some key design considerations for hot runner molds.
Thermal insulation design
Good insulation measures must be in place between the runner plate and the template to prevent the runner plate temperature from rising and causing malfunctions.
Mold temperature control
Mold temperature must be accurately controlled, with fluctuations allowed within 5°C. The runner and each nozzle should be temperature-controlled separately.
Flow channel size design
The runner should have a circular cross-section to facilitate melt insulation and filling flow, typically with a diameter of 6–8 mm. A larger value should be used for dimensional fillers. The runner should be free of dead corners and grooves that trap material. The surface roughness of the runner should be consistent with the cavity; chrome plating is preferred to ensure wear resistance.
Nozzle orifice design
The nozzle orifice diameter is generally not less than 4 mm, and it has a 0.5° to 1° inverted cone to facilitate gate demolding.
Flow channel plate design
The flow channel plate should have a parting surface and be equipped with a hook-type opening and closing locking plate to facilitate the removal of the solidified material from the flow channel.
Flow channel volume design
The runner volume should be smaller than the total volume of plastic injected at one time to prevent the plastic melt from staying in the runner for too long and solidifying.
Factors to consider when selecting equipment for hot runner injection molds
In fact, hot runner molds and injection molding are inseparable. Injection molding machines should be adaptable to all automatic cycle capabilities and hot runner systems. Therefore, choosing the right injection molding machine is crucial. When a hot runner mold is designed and manufactured and needs to be put into production, the selection of an injection molding machine should be based on a comprehensive consideration of multiple factors:
1. The thickness of a hot runner mold increases with the addition of the hot runner and runner plate. When selecting an injection molding machine, pay attention to its installation height.
2. When measuring the injection volume of an injection molding machine, the compressibility of the plastic in the large-volume runner should be considered.
3. For the depreciation of hot runner molds, it is recommended to run the injection molding machine continuously and adopt multi-stage start-up to reduce damage.
Future Development Trends of Hot Runner Molds
Current development trends show that hot runner injection molding technology is moving towards intelligence, modularity, and efficiency. In terms of intelligence, temperature control systems integrating sensors can achieve real-time monitoring and adaptive adjustment of temperature and pressure; modular design enhances the interchangeability of mold components, shortening mold change and debugging time; and material adaptability continues to improve, with hot runner solutions for biodegradable plastics and high-performance composite materials constantly emerging.
Furthermore, given the strong market demand for precision parts, miniaturization is one of the main directions for the future development of injection molds, enabling precise injection molding. Hot runner molds, by reducing the size of the nozzle, can achieve multi-cavity or multi-gate filling in a single mold, thereby improving efficiency. Many companies have already successfully developed combined compound nozzles or multi-gate nozzles by leveraging component miniaturization.
In the future, with the deep integration of Industry 4.0 technologies, hot runner injection molds will further realize digital control of the production process, providing stronger support for efficient and precise production in the manufacturing industry.
in conclusion
As discussed in this article, there are many types of hot runners, each with different uses and characteristics. Furthermore, hot runners play a crucial role in modern injection molds, helping manufacturers improve production efficiency, reduce production costs, enhance product quality, and improve the production environment. Selecting the appropriate type of hot runner for injection molds will help manufacturers better meet the demands of different types of plastic injection molded products.
If you are looking to customize hot runner injection molds or want to produce high-volume plastic products using such molds, please contact Elimold. We manufacture a wide range of specialized molds and provide injection molding services, producing large-volume custom parts and products for companies worldwide (offering services such as design optimization, mold making, manufacturing process optimization, and final assembly). Contact us now for a free quote.