In the injection molding process, mold runners and sub-runners are often overlooked by designers, who consider their design relatively simple. However, a well-designed runner and sub-runner system can significantly improve production efficiency, reduce material waste, and shorten cycle times. Therefore, as a mold designer, paying attention to these details and continuously refining your designs will enable you to deliver higher-quality products, enhancing your competitiveness in the market.
What Are the Roles of Mold Runners and Sub-Runners?
A mold runner, simply put, is a channel in the mold that directs the molten plastic from the injection unit to the mold cavity where the final product will be formed. Sub-runners as branch runners or secondary runners, are smaller channels that connect the main runner to individual cavities.
The runner typically starts at the nozzle of the injection molding machine and extends into the mold, branching out to feed multiple cavities if needed. It’s designed to minimize resistance to flow and to ensure even distribution of the plastic material.
Key Considerations for Mold Runner and Sub-Runner Design
The design of mold runners and sub-runners is influenced by a variety of factors, including the complexity of the mold structure, specific parameters of the injection molding process, and the requirements of the product design. While product size and wall thickness can impact runner design, it is not always true that larger cross-sectional runners will effectively enhance the filling process.
Key aspects such as the shape and thickness of the runners, as well as the configuration of cooling channels, play a significant role in determining the flow characteristics of the material and the efficiency of cooling. The length of the runners does not directly affect the viscosity of the plastic; instead, viscosity is primarily dictated by the properties of the material and the processing conditions.
To assist with design, here are recommended runner diameters for common plastic materials (in mm):
- ABS / SAN: 4.8–9.5
- Polyoxymethylene (POM): 3.0–10
- Acrylic Fiber: 1.6–10
- California: 1.6–11
- Nylon 66: 1.6–10
- Polyphenylene Oxide (PPO): 6.4–10
- Polyphenylene Sulfide (PPS): 6.4–13
- Polycarbonate: 4.8–10
- Polyethylene: 4.8–9.5
- Polypropylene: 1.6–10
Ultimately, the design of the runner system significantly affects not only the filling efficiency of the plastic but also the quality of the molded parts and the overall production cost. A well-thought-out runner design can minimize material waste, enhance production efficiency, and ensure that the final products meet quality standards.
To optimize injection molding, consider these key design principles for runners and sub-runners:
- Minimize Distance: Ensure molten plastic enters the cavity quickly, reducing heat loss and pressure drop.
- Simultaneous Feeding: Design multiple gates for uniform temperature and pressure during filling.
- Optimize Cross-Section: Use smaller cross-sectional areas to save material and reduce cooling time.
- Trapezoidal: Enhances filling and reduces pressure drop, wider end connects to the sprue.
- Surface Area Efficiency: Aim for a low surface area to volume ratio in runners to enhance cooling.
- Surface Roughness: Maintain an Ra value of about 1.6 μm to prevent drag from cold material.
- Circular: Ideal for complex parts, offers excellent flow with optimal diameters.
- U-Shaped: Facilitates efficient flow and easy separation from the molded part.
- Smooth Flow: Use slopes and arcs to connect runners and gates, reducing flow resistance.
- Rectangular: Easy to manufacture with adjustable dimensions for uniform flow.
- Semi-Circular: Promotes smooth flow and minimizes pressure drop.
These principles help improve efficiency and part quality in the injection molding process.
Advantages of Well-Designed Runners and Sub-Runners
Well-designed runners and sub-runners in injection molding offer several key benefits:
Improved Flow Efficiency
A well-designed runner system enhances the flow of molten plastic, minimizing pressure drop and ensuring that the material fills the mold cavities uniformly. This optimized flow reduces the risk of incomplete filling and helps achieve consistent part dimensions. By promoting smooth material movement, manufacturers can achieve higher-quality parts with fewer defects.
Reduced Cycle Time
Efficient runner design contributes to shorter cooling times, allowing for quicker production cycles. By minimizing the time the molten plastic spends in the runner system, manufacturers can significantly increase throughput. This is particularly important in high-volume production environments, where even small reductions in cycle time can lead to substantial increases in overall productivity.
Consistent Part Quality
Uniform flow distribution facilitated by well-designed runners helps prevent defects such as weld lines, air traps, and uneven wall thickness. Consistent filling ensures that each part meets quality standards. Improved part quality leads to higher customer satisfaction and reduced rework costs.
Easier Mold Maintenance
Simplified runner systems can significantly ease the maintenance of molds. A well-designed runner setup minimizes the accumulation of debris and residual material, making cleaning more efficient. Reduced downtime for maintenance translates to more productive use of manufacturing equipment, enhancing overall operational efficiency.
Adaptability
A versatile runner design can accommodate a variety of materials and part geometries, providing manufacturers with the flexibility to adapt to changing production needs. A well-engineered runner system can support diverse applications without requiring extensive rework or redesign.
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This article was written by engineers from the BOYI team. Fuquan Chen is a professional engineer and technical expert with 20 years of experience in rapid prototyping, mold manufacturing, and plastic injection molding.