Basic form of die casting mold runner
The runner (also known as a diverter) in a die casting mold is a crucial channel connecting the main runner and the ingates. The choice of its basic form directly impacts molten metal flow distribution, pressure transmission, and filling uniformity. The runner’s primary function is to smoothly and evenly distribute the molten metal from the main runner to the individual ingates. It also removes impurities and gasses before the molten metal enters the ingates, minimizing casting defects. Depending on their cross-sectional shape, layout, and functional characteristics, die casting runners come in several basic forms: circular, trapezoidal, U-shaped, symmetrical, and radial. Each type has its own unique application scope and design characteristics.
A circular cross-section runner is an ideal form. Its cross-section is circular and has the characteristics of low flow resistance, low pressure loss, and uniform flow distribution. Under the same cross-sectional area, the circumference of the circular cross-section is the smallest, and the contact area between the molten metal and the runner wall is small, so the friction resistance is small, which helps the molten metal maintain a high flow rate and pressure. It is suitable for castings with high pressure transmission requirements, such as thick-walled castings and parts with high strength requirements. The diameter of the circular cross-section runner is usually determined based on flow calculations and is generally between 8mm and 20mm. For large molds or multi-ingate designs, the diameter can be appropriately increased. However, the processing of circular cross-section runners is more difficult, requiring the use of special tools for milling or grinding, and it is difficult to achieve a completely circular shape on the parting surface. Usually, a combination of two semicircles is used. Therefore, it is subject to certain limitations in actual applications and is more often used in occasions with higher performance requirements.
The trapezoidal cross-section runner is the most widely used form in actual production. Its cross-section is trapezoidal, with a wide upper base and a narrow lower base. It is characterized by simple processing, low cost, and stable flow control. The upper base width of the trapezoidal cross-section is generally 8mm-15mm, the lower base width is 6mm-12mm, and the height is 5mm-10mm. The specific dimensions can be adjusted according to flow requirements. The trapezoidal cross-section design enables the molten metal to form a stable flow state during the flow process, reducing the generation of eddies and air entrainment. It also facilitates the installation of a slag collection bag at the bottom of the runner to collect oxide slag and impurities in the molten metal, thereby improving the purity of the casting. The trapezoidal cross-section runner is suitable for most small and medium-sized castings and multi-cavity molds, especially for commonly used die-casting materials such as aluminum alloys and zinc alloys, and can meet their flow characteristics and pressure transmission requirements. In addition, the trapezoidal cross-section runner is easy to process on the parting surface and can be completed by ordinary milling, which greatly reduces the manufacturing cost of the mold.
The U-shaped runner is a modified cross-section with a U-shaped profile, a rounded bottom, and vertical walls on either side, combining the advantages of both circular and trapezoidal cross-sections. The rounded bottom of the U-shaped cross-section reduces resistance to molten metal flow and improves pressure transmission efficiency, while the vertical walls facilitate machining and positioning, making it suitable for castings requiring high flow stability, such as thin-walled complex parts. The width of the U-shaped cross-section is generally 10mm-20mm, the depth is 5mm-15mm, and the bottom arc radius is 2mm-5mm. The design of the U-shaped cross-section allows the molten metal to maintain a good laminar flow state during flow, reducing turbulence and air entrainment. The rounded bottom structure also encourages impurities and gases to gather on the sides, facilitating their discharge through the overflow troughs located on both sides. The machining difficulty of the U-shaped cross-section is between that of the circular and trapezoidal cross-sections, and can be completed through a combination of milling and grinding. It is suitable for the production of castings with high quality requirements and large batch sizes.
A symmetrical runner is a layout designed based on the cavity distribution characteristics. Its runners are symmetrically distributed around the main runner, ensuring that the length, cross-sectional dimensions, and shape of each branch runner are identical, thus achieving uniform distribution of the molten metal. Symmetrical runners are suitable for multi-cavity molds, especially those with an even number of cavities and a symmetrical arrangement, such as in paired gears, connectors, and other castings. The symmetrical runner design ensures that the molten metal filling speed, pressure, and temperature are essentially consistent within each cavity, reducing variations in casting quality caused by uneven flow distribution and improving product consistency. When designing a symmetrical runner, it is important to ensure that the connection between each branch runner and the main runner uses a smooth, rounded corner to avoid pressure loss and eddy currents caused by right-angle turns. Furthermore, the dimensions of each ingates should be consistent to ensure balanced flow distribution. Symmetrical runners require high machining and assembly requirements, ensuring dimensional accuracy and symmetry on both sides to fully utilize their advantages of uniform distribution.
The radial runner is a layout that is centered on the main runner and radiates outwards. It is suitable for circular or symmetrical castings with center-feed, such as discs and hub parts. The branch runners of the radial runner extend outward from the center of the main runner. They are of equal length or have slight differences depending on the position of the cavity. The cross-sectional size gradually decreases from the center to the end to accommodate the decreasing flow rate of the molten metal. The design of the radial runner enables the molten metal to be quickly and evenly filled from the center to the surrounding areas, reducing filling time and temperature differences. It is suitable for large circular castings or molds with multiple cavities distributed in a circular pattern. When designing the radial runner, it is important to note that the angles between the branch runners should be evenly distributed to ensure balanced flow distribution. At the same time, the inner gate should be set on the inside or edge of the casting so that the molten metal can flow smoothly in the radial direction to avoid eddies and air entrainment. The processing of radial runners is more difficult, especially for complex structures with multiple branches, and CNC machining equipment is required to ensure dimensional accuracy and symmetry. However, for castings of specific shapes, it can significantly improve the filling effect and casting quality.
