At the end of die-casting filling, the remaining gas content in the mold cavity directly affects the quality of the final formed casting. The higher the gas content, the more likely it is to cause defects such as porosity, shrinkage, and looseness, resulting in low density and poor mechanical properties of castings. When the metal liquid rapidly fills the mold cavity, in order to avoid the gas in the mold cavity obstructing the smooth filling of the metal liquid and causing die-casting defects, the gas in the mold cavity must be discharged in a timely manner. In the actual production of die-casting parts, opening overflow grooves near the locations where defects are prone to occur can prevent the occurrence of porosity and shrinkage defects.

     Generally, when filling die-casting parts, gas can be discharged from the exhaust duct opened at the parting surface of the mold. But when the sealing degree of the mold parting surface is good and the exhaust space opened at the parting surface is limited, the gas in the mold cavity cannot be discharged in time, and an overflow groove must be set up at this time. The overflow tank can remove gas from the mold cavity and store cold materials mixed with gas, paint, residue, and cold dirty metal liquid. Therefore, using a reasonable overflow groove can quickly extract the gas from the mold cavity, enhance the exhaust effect, effectively control and regulate the flow state of the metal liquid filling, prevent local vortex generation of the metal liquid, eliminate shrinkage, porosity, vortex wrapped gas and cold insulation. At the same time, it can adjust the temperature of various parts of the mold, improve the thermal balance of the mold, and reduce defects such as casting flow marks, cold shuts, and poor filling.

     For castings with thin and deep ribs (such as cylinder heads, air grilles, blower impellers, etc. of air-cooled engines) and thin-walled shell shaped and deep parts, although measures such as opening overflow grooves have been taken in production, due to the limitations of mold space, a large number of air holes and insufficient pouring defects will still occur. This is usually due to the rapid impact of high-speed molten metal flowing into the deep and narrow mold cavity, forming vortices. After the high-speed molten metal is drawn into the gas, the gas that cannot be discharged in time is wrapped in it during solidification. Even if local pressurization, opening large overflow channels, and other methods are used at these locations, the gas drawn in cannot be effectively discharged. In order to reduce various defects in the filling process of castings, a tooth shaped overflow groove structure was directly installed on the original mold. On the basis of not changing the original mold structure, it was found that it can effectively improve the gas pressure and metal liquid filling ability in the mold cavity during filling, and effectively discharge low-temperature metal liquid and floating slag, reducing or eliminating various die-casting defects. Through multiple sets of experimental statistics and comparative analysis, the optimized process parameters obtained can effectively reduce die-casting defects.

Graphic and textual results

    The experimental material is ADC12 aluminum alloy, the mold core material is H13 steel, the mold temperature is 240 ℃, the initial pouring temperature of aluminum liquid is 670 ℃, and the mold cavity is controlled by an oil circuit to balance the temperature. The inlet temperature of the oil circuit is controlled at 200 ℃, and the outlet temperature is 220 ℃. The other parts are cooled with cooling water, with inlet and outlet temperatures of 40 ℃ and 60 ℃, respectively. The slow injection speed is 0.22m/s, the fast injection speed is 2.1m/s, the slow injection interval is 272mm, and the fast injection interval is 74mm. The total mass of the product, including the gate and overflow groove, is 0.787kg. The clamping force of the die-casting machine is 3500kN, the diameter of the pressure chamber is 60mm, the filling rate of the pressure chamber is 29.3%, the length of the material handle is 16.3mm, and the length of the pressurization interval is 10mm.

research conclusion
(1) For thin-walled shell castings, using traditional overflow grooves can result in various defects such as poor filling and porosity, which are caused by poor exhaust of the mold cavity.

(2) The tooth shaped overflow groove has a large cross-sectional area, which can effectively improve the exhaust of the mold cavity, reduce the filling pressure during die-casting filling, improve the filling ability of the metal liquid, and effectively discharge the low-temperature metal liquid and floating slag that filled the mold cavity first. After replacing the traditional straight through overflow groove with a toothed overflow groove, the scrap rate of the sample decreased from 11.0% to 1.5%. By adopting a tooth shaped overflow groove without modifying the basic structure of the mold, the product qualification rate has been improved.

(3) The use of a tooth shaped overflow exhaust device saves mold and production costs, making it easy to replace and reuse.

author
Dai Wei and Liu Jia
State Key Laboratory of Material Forming and Die Technology, Huazhong University of Science and Technology
Cheng Cheng
School of Materials Science and Engineering, Wuhan University of Technology
This article comes from: "Special Casting and Nonferrous Alloys" magazine, "Die Casting Weekly" strategic partner