Cross-section design of injection molding runners
Injection runners are crucial channels connecting the main runner and the gate. Their cross-sectional design directly impacts the melt’s flow properties, pressure loss, cooling rate, and raw material consumption. A reasonable cross-sectional shape and size can significantly improve injection molding efficiency and part quality. The cross-sectional design of the runner must comprehensively consider factors such as the plastic material’s properties, part structure, and production batch size. While ensuring smooth melt flow, the design should minimize the process flow, pressure loss, and raw material waste.
Common runner cross-sections include circular, trapezoidal, U-shaped, and semicircular, with a circular cross-section being the most ideal. A circular runner has the largest cross-sectional area to circumference ratio, meaning it has the smallest specific surface area. This reduces the heat dissipation area during melt flow, minimizing temperature loss and maintaining melt fluidity. Furthermore, a circular cross-section offers low melt flow resistance and pressure loss, effectively transmitting injection pressure, making it particularly suitable for high-viscosity plastics such as PC and PMMA. The diameter of a circular runner should be determined based on part weight and runner length. For small parts, the runner diameter is 4-6mm; for medium-sized parts, it is 6-10mm; and for large parts, it is 10-16mm. However, a circular cross-section requires semicircular grooves on both sides of the mold, which requires high mold clamping precision. Improper clamping can easily cause flash, increasing mold manufacturing difficulty and cost.
A trapezoidal cross-section is a widely used runner shape in production. It’s easier to process than a circular cross-section and ensures better melt flow properties. The upper base width of a trapezoidal cross-section is generally 6-12mm, the lower base width is 2-4mm smaller, and the height is 0.6-0.8 times the upper base width. For example, a trapezoidal runner with a 10mm upper base would have a 7mm lower base and a height of 6-8mm. One side of the trapezoidal cross-section is machined into the movable mold, while the other side is flat (the fixed mold), eliminating the need for precise alignment and simplifying mold processing and assembly. Compared to a circular cross-section, a trapezoidal cross-section has a slightly larger specific surface area and dissipates heat from the melt slightly faster. It is suitable for low- and medium-viscosity plastics (such as PE, PP, and ABS) and is widely used in small and medium-sized molds.
U-shaped and semicircular cross-section runners are suitable for specific scenarios. The depth-to-width ratio of a U-shaped cross-section is generally 0.5-0.6. Its flow properties are similar to those of a trapezoidal cross-section, but the bottom is a circular arc transition, which reduces dead angles for melt flow, facilitates melt flow and mold cleaning, and is suitable for glass fiber reinforced plastics, reducing glass fiber breakage and accumulation. The radius of a semicircular cross-section is usually 3-8mm. Processing is simple, requiring only a semicircular groove to be machined on one side of the template. However, due to its large specific surface area, the melt cools quickly, and the pressure loss is also large. It is only suitable for simple molds for small-batch production or low-viscosity plastics (such as PE and PS).
The dimensional design of the runners must adhere to the principle of “balanced runners.” Especially in multi-cavity molds, the length and cross-sectional dimensions of each runner must be identical, allowing the melt to reach each cavity simultaneously and ensuring consistent part quality. The runner length should be minimized, generally not exceeding 500mm. Excessive length can lead to excessive melt pressure loss and excessive temperature drop, affecting mold filling. Runner bends should be designed with arc transitions, with a radius of 0.5-1 times the runner diameter, to avoid increased melt flow resistance and localized overheating and degradation caused by right-angle turns. Furthermore, the connections between the runner and the main runner, and between the runner and the gate, must be smooth, avoiding steps or sharp corners to ensure smooth melt flow.
The cross-sectional design of the runner should also consider the cooling system and ease of demolding. Runners should be placed at an appropriate distance from cooling water channels (generally no less than 10mm) to avoid mutual interference. For runners for high-viscosity plastics, cooling water channels can be installed below them to control the melt temperature. A cold slug well should be designed at the end of the runner to collect cold slugs from the front end and prevent them from entering the mold cavity and affecting part quality. The depth and diameter of the cold slug well should be slightly larger than the runner dimensions. Regarding demolding, the cross-sectional shape of the runner should facilitate the release of the slug. Circular and trapezoidal cross-sections make it easy to remove the slug from the mold, while complex cross-sections may increase demolding difficulties. By comprehensively considering melt flow properties, processing difficulty, cost, and production requirements, selecting the appropriate runner cross-sectional shape and size can effectively improve injection molding efficiency and part quality.
