Die Casting Mold Runner Design
The runner design of the die-casting mold is a key link connecting the sprue and the ingates in the pouring system. Its main function is to distribute the molten metal transported by the sprue to each ingate smoothly and evenly, while maintaining the temperature and pressure of the molten metal during the flow process, providing good conditions for the subsequent filling of the cavity. The rationality of the runner design directly affects the flow state of the molten metal, the pressure loss, and the quality stability of the casting. For example, if the runner is improperly designed, it may cause eddies and air entrainment in the molten metal during the flow process, or it may fail to fill the cavity due to excessive pressure loss, resulting in defects such as cold shut and insufficient pouring. Therefore, the runner design needs to comprehensively consider factors such as the casting structure, alloy characteristics, and production batch to ensure that the molten metal enters the ingates in the best condition.
The cross-sectional shape of the runner is one of the core design parameters. Common shapes include trapezoidal, semicircular, rectangular, and U-shaped, with the trapezoidal cross-section being the most widely used. A runner with a trapezoidal cross-section has the characteristics of low flow resistance and slow heat dissipation. Its top width is greater than its bottom width, with the upper bottom width generally being 1.5-2 times the lower bottom width, and its depth being equivalent to the lower bottom width. This structure enables the molten metal to form a stable flow pattern during flow and reduces turbulence. Runners with semicircular cross-sections are easy to machine and suitable for small, simple castings. However, due to their rapid heat dissipation, they suffer from significant temperature loss in the molten metal and are therefore unsuitable for large or thin-walled castings. While runners with rectangular cross-sections offer a greater flow rate, eddies are prone to forming in the corners, leading to air entrainment. Therefore, they require rounded corners to improve flow. A U-shaped cross-section combines the advantages of both trapezoidal and semicircular shapes, making it easier to machine while reducing flow resistance. It is suitable for applications requiring high fluidity of the molten metal.
The runner’s dimensions must be designed based on the molten metal flow rate, flow rate, and volume of the casting, and its length, width, and depth must match. The runner’s length should be minimized to reduce molten metal flow and heat loss. It’s generally recommended that the length not exceed 1.5 times the maximum casting dimension. For complex castings, the length of a single runner can be shortened by providing multiple runner branches. The runner’s width and depth must be calculated based on the molten metal flow rate using the following formula: runner cross-sectional area = molten metal flow rate ÷ flow rate. The molten metal flow rate must be adjusted based on the type of alloy. The flow rate for aluminum alloys is generally 20-30 m/s, and for zinc alloys, it’s 15-25 m/s. For example, for an aluminum alloy casting with a volume of 300cm³ and a filling time of 0.1 seconds, the molten metal flow rate is 3000cm³/s. If the flow rate is 25m/s (2500cm/s), the cross-sectional area of the runner must reach 3000÷2500=1.2cm². Considering the characteristics of the trapezoidal cross-section, a runner with an upper bottom width of 12mm, a lower bottom width of 8mm, and a depth of 8mm can be designed.
The layout of the runner is determined by the shape of the casting and the location of the ingates. Common types include linear, branched, and circular. Linear runners are suitable for simple castings with a single ingates, offering a simple structure and minimal pressure loss. Branched runners are suitable for complex castings with multiple ingates. Through a combination of the main and branch runners, the molten metal is distributed to each ingate. Arc transitions are required at the branches to avoid eddies and pressure loss caused by right-angle turns. Annular runners are suitable for castings with circular or symmetrical structures. They enable the molten metal to evenly fill the cavity from multiple directions, reducing variations in filling time. However, they are more difficult to process and are therefore suitable for applications requiring higher quality. Furthermore, the runner should be designed to taper from the sprue to the ingates, with a taper of 1°-3°. This allows the molten metal to gradually increase in velocity during flow, improving filling efficiency.
The runner design also needs to consider venting, slag removal, and coordination with other systems. Small overflow troughs can be installed at the ends or branches of the runner to collect cold material and impurities, preventing them from entering the mold cavity and affecting casting quality. The connection between the runner and the ingate should have a smooth transition to avoid sudden dimensional changes. The connection angle is generally 30°-60° to ensure smooth entry of the molten metal into the ingate. Furthermore, the runner should be arranged away from the mold’s cooling water channels to prevent an excessive drop in the molten metal temperature, which could affect fluidity. After the design is completed, the runner’s performance is verified through computer simulation. The flow trajectory, pressure distribution, and temperature changes of the molten metal are observed, and any inconsistencies are optimized. For example, when designing the runner for a multi-cavity die-casting mold, a company discovered through simulation that excessive pressure loss at the branch point resulted in uneven filling of the cavities. By increasing the cross-sectional area of the branch point and optimizing the transition radius, the difference in filling time between cavities was reduced from 0.05 seconds to 0.01 seconds, significantly improving the stability of casting quality.
