Die Casting Mold Sprue Design
The sprue design of a die-casting mold is an important component of the gating system design. It is a vertical channel connecting the sprue sleeve and the runner. Its main function is to guide the molten metal from the sprue sleeve into the runner. The rationality of its design directly affects the flow state of the molten metal, pressure transmission, and the service life of the mold. The design of the sprue needs to be determined according to the type of die-casting machine, the size of the casting, and the characteristics of the alloy to ensure that the molten metal has little energy loss and flows smoothly during the flow process, while avoiding excessive scouring of the mold. For example, for large die-castings, the sprue needs to have a sufficient cross-sectional area to ensure the flow rate and pressure of the molten metal to meet the needs of rapid filling of the cavity; for small castings, the cross-sectional area of the sprue can be appropriately reduced to increase the flow rate of the molten metal.
There are three main types of sprues: cylindrical, conical, and stepped, each suited to different die-casting scenarios. The cylindrical sprue is simple and easy to machine, making it suitable for small, simple castings. Its inner diameter is typically 10-20mm, and its length depends on the mold thickness, generally not exceeding 100mm. A disadvantage of the cylindrical sprue is that it can easily form eddies during the flow of the molten metal, leading to air entrainment. Therefore, it must be used with a diverter cone to improve flow. The inner diameter of the tapered sprue tapers gradually from top to bottom, guiding the molten metal’s flow smoothly and reducing eddies. It is suitable for medium to large castings, with a taper of 2°-5°, ensuring a gradual increase in molten metal flow while minimizing pressure loss. The stepped sprue, composed of multiple cylindrical segments of varying diameters, is suitable for taller molds. The gradual change in diameter ensures smoother molten metal flow and facilitates machining and assembly.
The sprue’s dimensions must meet the required molten metal flow rate. Its cross-sectional area can be calculated based on the casting’s volume and filling time. The calculation formula is: sprue cross-sectional area = casting volume ÷ (filling time × molten metal flow rate). The molten metal flow rate depends on the alloy type; aluminum alloys typically have a flow rate of 30-50 m/s, while zinc alloys have a flow rate of 20-40 m/s. For example, for an aluminum alloy casting with a volume of 500 cm³, if the filling time is 0.1 seconds and the molten metal flow rate is 40 m/s, the sprue’s cross-sectional area must be 500 ÷ (0.1 × 40 × 100) = 1.25 cm² (1 m = 100 cm, unit conversion is required), corresponding to a diameter of approximately 12.6 mm. The sprue’s length should be minimized to minimize heat and pressure losses during the molten metal flow. Generally, the sprue’s length-to-diameter ratio should not exceed 10:1. Otherwise, the molten metal flow rate will decrease, affecting the filling effect.
The design of the connection between the sprue, the gate sleeve and the runner is also crucial. It is necessary to ensure a smooth transition and avoid right angles or sharp angles to reduce flow resistance and the generation of eddy currents. The connection between the sprue and the gate sleeve should adopt an arc transition, and the radius of the fillet is generally 3-5mm, so that the molten metal can flow smoothly from the gate sleeve into the sprue. The connection between the sprue and the runner should adopt an inclined transition, and the transition angle is generally 30°-45°, so that the molten metal can smoothly enter the runner, avoid direct impact on the bottom of the runner, and reduce wear on the mold. In addition, a diverter cone is usually set at the bottom of the sprue. The top of the diverter cone should be concentric with the sprue, and its cone angle should match the taper of the sprue to ensure that the molten metal can be evenly diverted to each runner.
The material selection and heat treatment process for the sprue also require significant attention. Since the sprue comes into direct contact with hot molten metal, it must possess excellent heat resistance, wear resistance, and erosion resistance. Sprue inserts are typically made of hot work die steel, such as H13 steel. After forging and heat treatment, the hardness reaches 44-48 HRC to withstand the high temperatures and erosion of the molten metal. Furthermore, the surface roughness of the sprue must be controlled below Ra0.8μm to reduce flow resistance to the molten metal and prevent surface roughness from causing poor flow or eddy currents. During use, the sprue should be regularly inspected for wear. If severe wear or deformation occurs, the insert should be replaced promptly to ensure proper functioning of the gating system. For example, a die-casting company producing large aluminum alloy wheels experienced severe wear after a period of use due to improper sprue material selection. This resulted in disrupted molten metal flow and increased porosity in the casting. Replacing the sprue with H13 steel inserts and performing appropriate heat treatment significantly extended the sprue’s service life by three times, significantly improving casting quality.
