In injection molding, multi-cavity molds are widely used in automotive parts, 3C products, and other fields due to their high-efficiency production characteristics. However, the runner balance problem in multi-cavity molds remains a core challenge restricting production stability—uneven filling of each cavity will directly lead to defects such as product dimensional deviations, shrinkage, and weld lines, seriously affecting yield and production costs. Achieving runner balance requires coordinated optimization from three dimensions: mold design, process adjustment, and material selection. Essentially, it ensures that each cavity experiences the same pressure, temperature, and cooling conditions during injection molding production.
The mold design stage is the foundation of runner balance. The path length and cross-sectional dimensions from the main runner to each cavity must be strictly consistent; this is the ideal balance solution. For example, using an "H-type" symmetrical runner layout can ensure equal flow resistance of the melt from the main runner to each cavity, which is especially suitable for multi-cavity products with the same shape. If a symmetrical layout cannot be used due to product shape limitations, the resistance difference must be compensated for by adjusting the runner dimensions: shortening the length of long-path runners or increasing their cross-section to reduce pressure loss. High-end molds can also combine hot and cold runner systems, with cold slug wells at the end of the cold runner to prevent cold slug from entering the cavity; or they can use needle valve-type hot runners, controlling the opening and closing of each gate through timing, such as allowing gates in distant cavities to open earlier to compensate for flow lag.
Gate design is the "key valve" controlling the entry of melt into the cavity. Gates in the same cavity should be located in the same position and have completely consistent dimensions to avoid uneven filling due to differences in gate resistance. For complex cavities, guide ribs or inclined runners can be used to guide the melt to fill evenly, reducing turbulence and stagnation. For example, deep cavity structures have high flow resistance, requiring optimized gate position and size to ensure the melt can quickly fill the cavity; while thin-walled parts require larger gate sizes or increased injection speeds to prevent insufficient filling due to excessive cooling.
The influence of material properties on runner balance cannot be ignored. High-viscosity materials have high flow resistance and require higher runner balance; low-viscosity materials, although having good fluidity, may still experience uneven cooling of the leading slug due to runner temperature differences. For example, high-viscosity materials such as polycarbonate (PC) require higher mold temperatures to reduce flow resistance, while low-viscosity materials such as polyethylene (PE) require controlled mold temperatures to prevent filling interruptions due to excessive cooling. Furthermore, the hygroscopicity of the material also affects the filling effect—highly hygroscopic materials, if not fully dried, will have air bubbles in the melt leading to unstable filling, requiring strict drying before injection molding.
Dynamic matching of process parameters is the "last line of defense" for runner balancing. Through multi-stage injection pressure control, high-pressure, rapid injection is used during the filling stage to ensure the melt fills the cavity; pressure is reduced during the holding stage to avoid overfilling and flash, while also compensating for cooling shrinkage. Matching mold temperature and material temperature is equally crucial: high-viscosity materials require higher mold temperatures, while low-viscosity materials can have lower mold temperatures to accelerate cooling. For example, when a car parts factory used a 16-cavity mold to produce bumpers, zoned temperature control increased the temperature of the molds in the far cavities, slowing down the cooling rate and significantly reducing the difference in shrinkage rates between cavities, resulting in a significant improvement in yield.
The application of intelligent technology provides a more precise solution for runner balancing. By installing pressure sensors in each cavity and collecting the filling pressure curve in real time, the system can automatically adjust the feed rate or injection speed of the corresponding gate. For example, if the pressure in a cavity is abnormal, the system will dynamically adjust the opening of the needle valve gate to ensure that the pressure in each cavity is consistent. Furthermore, the conformal cooling water channels manufactured using 3D printing technology can closely fit the cavity contour, ensuring consistent cooling efficiency in each cavity and reducing shrinkage differences caused by uneven cooling.
The runner balancing of multi-cavity molds is a systematic project requiring coordinated efforts from three dimensions: structural optimization of mold design, dynamic matching of process parameters, and precise control of material properties. For high-precision applications, the introduction of intelligent monitoring and control systems is the future trend. Through scientific analysis methods and continuous process iteration, the production stability of multi-cavity molds can be significantly improved, providing a solid guarantee for efficient and high-quality injection molding production.
