Hot runner systems represent a relatively complex technology in the field of injection molding. There are numerous considerations and points that mold designers/manufacturers and mold users need to take into account when selecting and purchasing hot runner systems. 1. Selection of Hot Runner Suppliers
When a mold manufacturing company designs and fabricates a hot runner mold, it is not simply a matter of purchasing a hot runner system from a supplier and installing it on the mold. In fact, between the mold manufacturer and the hot runner supplier, there is extensive and close coordination and cooperation in both technical and commercial aspects—from the conceptual design phase of the hot runner mold, through the actual mold fabrication process, to the final use of the mold. Therefore, mold manufacturers and users should carefully select a qualified hot runner supplier. When choosing a hot runner supplier, two fundamental aspects should be focused on: first, the variety, quantity, and quality of hot runner components produced by the supplier; second, the technical support and after-sales service provided by the supplier in the region where the mold user is located.
1. Variety, Quantity, and Quality of Hot Runner Components
Hot runner systems are primarily classified into three types: Hot Tip (HOT TIP), Sprue Gating (SPRUE GATING), and Valve Gating (VALVE GATING). Each type can be further subdivided into multiple product series. For example, the cross-sectional diameter of the runner in a Hot Tip Nozzle (NOZZLE) typically ranges from 4 mm to 12 mm, and nozzles are accordingly manufactured in different sizes to form product series. Nozzle gate inserts also have numerous variants to meet diverse application requirements. Therefore, the richer the product series offered by the selected hot runner supplier, the better. This allows users to have a wider selection of hot runner components, enabling the production of plastic parts with a broader range of types, sizes, weights, and applications—ultimately optimizing the injection molding process and improving product quality.
During the injection molding of plastic products, hot runner systems and hot runner molds operate under high temperature, high pressure, and dynamic load conditions. There are many factors that can cause the failure of hot runner system components. Moreover, injection molding is mainly used for mass production of plastic parts, so any production downtime can result in significant economic losses. Therefore, the quality and reliability of the hot runner system are critical. Customers should thoroughly investigate the quality and application history of hot runner components produced by suppliers. Some outstanding hot runner manufacturers have obtained ISO quality standard certification from international organizations.
2. Technical Support and After-sales Service
When using hot runner molds, in addition to selecting a reliable hot runner system, users must also consider the crucial factor of whether timely and effective technical support and after-sales service can be obtained if any issues arise with the hot runner. Many hot runner suppliers have robust technical support and service networks in their home countries/regions, equipped with dedicated service personnel who can visit customers at any time to troubleshoot problems. However, in countries/regions far from their headquarters, these suppliers often only establish offices focused on hot runner product sales, resulting in significantly inferior technical support. Users should pay close attention to this factor.
2. Several Technical Issues in Selecting and Purchasing Hot Runner Systems
Selecting and purchasing a hot runner system involves many specific technical aspects for users. If users possess solid technical knowledge related to hot runners, they can more easily choose and purchase the most suitable hot runner system—ensuring smooth subsequent injection molding production and improved product quality.
1. Correct Selection of Hot Runner Product Series
Hot runner suppliers typically categorize their hot runner components into product series based on the size and weight of the plastic parts to be processed. For example, large plastic parts are processed with large-size nozzles, while small parts use small-size nozzles. Therefore, after correctly selecting the hot runner type (either Hot Tip or Valve Gating system), users need to choose the appropriate product series—which in turn determines the mold's structural dimensions and design/fabrication. If the wrong product series is selected and discovered late in the mold fabrication process or during plastic part production, the error will be severe and difficult to rectify. To assist with the correct selection of product series, each hot runner supplier provides guiding technical documents for reference. Users should collaborate closely with suppliers to select the right hot runner product series.
2. Injection Pressure Loss in Hot Runner Systems
Injection pressure loss in hot runner systems cannot be ignored. Many hot runner mold users mistakenly believe that hot runners have much lower injection pressure loss compared to cold runners, assuming the plastic melt in the hot runner remains hot throughout the injection process. In reality, the opposite is often true. Due to structural design requirements of hot runner molds, the flow distance of the melt in the hot runner system is significantly increased—resulting in often larger injection pressure losses. In practical applications, excessive pressure loss in hot runner systems frequently causes difficulties in injection molding. Therefore, for processing plastics with poor fluidity (e.g., PC, POM), long melt flow distances in the hot runner system, or large part weights, CAE software (such as MoldCAE) should be used for runner analysis and calculation—focusing on flow path analysis.
3. Standard vs. Custom Hot Runner Systems
All hot runner manufacturers offer both standard and custom hot runner systems. Whenever possible, users should prioritize standard hot runner systems—opting for nozzles, manifolds, gate inserts, etc., with standard lengths and dimensions. The advantages are that standard systems are more cost-effective and have much shorter lead times than custom systems. Additionally, parts are interchangeable, facilitating future use and maintenance (if a part fails, a new standard part can simply be purchased and installed). Common standard manifold configurations include 2-cavity inline, 4-cavity inline, 8-cavity inline, 4-cavity X-shape, and 8-cavity XX-shape.
4. Selection of Cavity Number and Layout on the Mold
When designing a hot runner mold and selecting the number of cavities, users should not only maximize the number of cavities to improve production efficiency but also consider hot runner design requirements. The selection of cavity number and layout should facilitate balanced melt flow in the hot runner system. For example, if multiple identical cavities are arranged in a line, it is best to choose 2 or 4 cavities instead of 3. Molds with 2 or 4 inline cavities can be designed with a fully naturally balanced hot runner system, whereas 3-cavity molds require artificial flow balancing of the manifold (adjusting runner sizes for different flow paths to achieve balance). The quality of flow balancing depends on the expertise of the hot runner designer. Therefore, users should prioritize cavity numbers that enable natural flow balancing (e.g., 16 cavities instead of 15) to eliminate errors caused by artificial flow balancing design.
5. Limitations on Minimum Cavity Spacing
When designing molds for micro-sized parts, users typically aim to minimize cavity spacing to create a more compact mold and accommodate more cavities. However, the minimum cavity spacing is limited by the minimum distance between hot runner components (e.g., nozzles). Therefore, when designing molds with tight cavity spacing, users must verify the minimum allowable nozzle distance to avoid mold design rework.
6. Type of Plastic to Be Processed
The type of plastic to be processed is a critical factor in selecting a hot runner system. For glass-reinforced plastics (e.g., glass-filled nylon), wear-resistant gate inserts should be used. For plastics prone to thermal decomposition (e.g., PVC), a hot runner system with unobstructed runners and no flow dead spots is essential. For plastics with poor fluidity (e.g., PC), larger nozzle series and increased runner cross-sectional dimensions in the manifold should be considered.
7. Maturity of Hot Runner Products
Different hot runner products vary in maturity and application history. Newly launched hot runner products require a long period of refinement, and suppliers continuously introduce new products while phasing out those proven unsuitable through practice. Therefore, users should prioritize mature, widely used, and time-tested hot runner products. Both suppliers and other users have extensive experience and successful cases with such products, which can be referenced by inexperienced new users.
8. Pre-delivery Status of Hot Runner Systems
Some hot runner suppliers conduct testing on their systems before delivery to users, and for critical applications, even perform actual injection molding trials. However, the scope of pre-delivery testing varies among suppliers. Users should understand this to make informed decisions.
9. Multi-zone Temperature Control of Hot Runners
For users requiring large, complex hot runner systems or processing temperature-sensitive plastics with narrow processing parameter ranges, a hot runner design with independent multi-zone temperature control should be selected. This allows users to locally adjust and control temperature distribution as needed. An ideal hot runner system should have uniform temperature distribution, but in practice, temperature variations occur due to factors such as the quality of heating elements, excessive heat loss at the mold-hot runner interface, and varying shear heat of the melt in different parts of the runner. Larger and more complex hot runner systems require multi-zone temperature control.
10. Applications with Color Change Requirements
Some users produce the same plastic part in different colors using a single mold—these are applications with color change requirements. When ordering hot runners, users should select systems with small runner volumes to accelerate color change and reduce waste. Additionally, all runner bends must be smooth and free of flow dead spots. For Valve Gating systems used in color change applications, special attention must be paid to potential dead spots behind the valve pin (DEAD SPOT).
11. Multi-cavity Molds with Different Part Shapes (Family Molds)
When designing family molds (multi-cavity molds with different part shapes), melt flow balancing must be considered. If part sizes/weights differ significantly (resulting in injection pressure differences exceeding 200-300 BAR between cavities), flow balancing cannot be achieved by merely adjusting runner sizes in the hot runner system. Unbalanced flow in family molds causes issues such as insufficient filling/pack pressure for some parts and over-filling (flash, high residual stress) for others. In such cases, Valve Gating systems should be considered (allowing users to close gates of early-filled cavities to prevent over-filling) or the overall mold design should be revised.
12. Volume Ratio of Cavities to Hot Runner
The runner volume of the hot runner system should not be excessively large relative to the cavity volume—otherwise, the melt will remain in the hot runner for too long, leading to thermal decomposition and defective parts. For extremely small plastic parts, a hybrid hot/cold runner system should be adopted. Using cold runners increases the injection volume, improving the cavity-to-hot runner volume ratio and reducing melt residence time in the hot runner system.
13. Use of Prototype Molds
Hot runner molds are expensive, especially high-cavitation molds (e.g., 96 or 128 cavities). For new application areas with limited experience or when testing new hot runner components (e.g., nozzles or gates), a simple single-cavity prototype mold should be fabricated first to verify feasibility. After gaining sufficient experience, the formal production mold (which is more costly) can be manufactured.
14. Drawing Hot Runner Components on Mold Designs
Hot runner suppliers typically provide electronic libraries of their components for users. Mold designers can select nozzle and other component drawings from these libraries and place them in appropriate positions on the mold design. Since suppliers regularly update their product lines, users should obtain the latest libraries from suppliers. Although 3D mold design is becoming prevalent, most hot runner component libraries remain in 2D format—though some suppliers are developing 3D libraries to adapt to the trend of 3D mold design.
15. Warranty Period
Hot runner suppliers offer warranty periods for their products, ranging from 1 to 5 years (depending on the supplier and specific product). During the warranty period, suppliers provide free replacement for components that fail under normal operation. Longer warranty periods are obviously more beneficial to users. To maintain warranty validity, users must adhere to the supplier's usage and maintenance guidelines and must not modify any hot runner components without prior approval from the supplier.
3. Conclusion
This article discusses key considerations for selecting and purchasing hot runner systems. Like the production and use of traditional injection molds, hot runner molds involve extensive experience and techniques that users accumulate over time.
