Overflow and exhaust system design
Overflow and exhaust systems are essential components of die-casting mold design. Their primary function is to remove gas, slag, and cold molten metal from the mold cavity, ensuring that the molten metal can smoothly fill the cavity and produce dense die-castings. During the die-casting process, residual gas in the cavity primarily originates from the initial air in the mold cavity, gases generated by lubricant volatilization, and gases volatilized by the molten metal at high temperatures. If these gases are not promptly expelled, they can form defects such as pores and pinholes within the die-casting, affecting the mechanical properties and surface quality of the die-casting. Furthermore, slag and cold molten metal are produced during the flow of molten metal. If not promptly removed, these can lead to defects such as inclusions and cold shuts in the die-casting, reducing the product’s pass rate.
The design of the overflow trough is the core of the overflow system. It should be located at the part where the molten metal is last filled, in the corners where gas easily gathers, and in areas where slag may be produced. For example, at the corners of the die casting, in deep cavities, and at the junctions of thin and thick walls, gas and slag are often easily accumulated, and an overflow trough should be set here. The size and shape of the overflow trough should be reasonably determined according to the structure of the die casting and the flow rate of the molten metal. Generally speaking, the volume of the overflow trough should not be less than the total volume of the residual gas and slag in the cavity, and the cross-sectional area at its entrance should be slightly larger than the cross-sectional area of the ingrowth to ensure that it can smoothly accommodate excess molten metal and impurities. In addition, the bottom of the overflow trough should be set with a certain slope to facilitate the smooth entry of molten metal and impurities into the overflow trough, while preventing the molten metal in the overflow trough from flowing back into the cavity.
The design of the exhaust groove needs to be coordinated with the overflow groove. Its main function is to discharge the gas in the cavity to the outside of the mold through the overflow groove. The exhaust groove is usually set at the end of the overflow groove, and its position should be as close to the edge of the cavity as possible to shorten the gas exhaust path. The depth and width of the exhaust groove need to be determined according to the material of the die-casting and the production process parameters. For aluminum alloy die-castings, the depth of the exhaust groove is generally 0.05-0.1mm and the width is 5-10mm to prevent the molten metal from leaking from the exhaust groove. At the same time, the length of the exhaust groove should be moderate. Too long will increase the flow resistance of the gas, and too short may lead to incomplete exhaust. When designing the exhaust groove, care should also be taken to avoid interference between the exhaust groove and other parts of the mold to ensure that the gas can be discharged smoothly.
The layout of the overflow and exhaust systems should adhere to the principle of “nearby exhaust and centralized overflow” to ensure that gases and impurities in the cavity are discharged in the shortest possible time. For die-castings with complex shapes, multiple sets of overflow and exhaust grooves can be used to exhaust and overflow different areas. For example, overflow and exhaust grooves can be set up separately at the multiple branch locations of a die-casting to ensure that each branch is adequately exhausted and overflowed. At the same time, the layout of the overflow and exhaust grooves should avoid affecting the appearance and dimensional accuracy of the die-casting. For areas that require machining, the overflow and exhaust grooves can be set within the machining allowance to facilitate subsequent removal.
During the actual design process, the overflow and vent system parameters need to be adjusted and optimized through mold trials. During mold trials, the proper design of the overflow and vent systems can be determined by observing the location of defects in the die-casting and the level of molten metal in the overflow trough. If pores or inclusions appear in a specific area of the die-casting, this indicates poor venting or overflow performance in that area, and the vent trough size or position should be appropriately increased. Furthermore, the effectiveness of the overflow and vent systems can be evaluated by measuring the density and mechanical properties of the die-casting. Only by continuously optimizing the overflow and vent system design can defects in die-castings be effectively reduced, and product quality and yield rates improved.
