During the solidification process of die castings, the shrinkage of the alloy generates a clamping force on the mold, and the mold material also shrinks during temperature changes. The synergistic changes between the casting and the mold cause dynamic changes in the clamping force. The casting will also shrink when it is ejected from the mold and cooled to room temperature. The mold design manual recommends a shrinkage rate of 0.4% to 0.7% for aluminum alloy die castings. This dynamic change makes it difficult for mold designers to choose the shrinkage rate. In fact, the clamping force of castings is closely related to the starting angle, mold smoothness, top rod layout, release agent concentration, mold temperature, casting temperature, etc. The clamping force of aluminum alloy die-casting parts is always dynamically changing from the cooling inside the mold to the process of ejecting the casting. The mold and alloy have different coefficients of thermal expansion corresponding to temperature changes. When designing the mold, different shrinkage rates should be selected based on different die-casting alloys. The failure of ejected castings caused by temperature changes in die-casting production can be solved by adjusting the temperature. Analyze the shrinkage rate and thermal expansion coefficient of molds and castings, provide parameters for selecting mold shrinkage rate during design, and provide an effective solution to the problem of difficult ejection of castings caused by temperature changes.

Graphic and textual results

    The commonly used materials for die-casting molds are 8407, DIEVAR, H13, SKD61, etc. For aluminum alloy die casting, the temperature during normal production of the mold is between 75 and 425 ℃ (see Figure 1). There are differences in the usage temperature of different die-casting molds, and enterprises can use big data to statistically confirm and determine the corresponding thermal expansion coefficient of the mold based on the usage temperature of the mold.

Figure 1 Relationship between Mold Temperature and Thermal Expansion Coefficient

Figure 2 Thermal expansion coefficients of die cast aluminum and magnesium alloys

Figure 3 Thermal imaging temperature of the mold

Table 1 Changes in thermal expansion coefficient of die-casting and mold temperature changes

Table 2 Mold Design Shrinkage Rates Corresponding to Different Casting Sizes

Note: (1) Corresponding table between mold size and casting size D and temperature change: Calculation formula: D2=D1 [1+r (t2 t1)]; (2) Shrinkage rate of the mold at room temperature of 20 ℃ α= [Mold size/Casting size] -1. Among them, the nominal size of the product at room temperature is D1, the mold change caused by thermal expansion is D2, t2 is the temperature at which thermal expansion occurs, and t1 is the starting temperature.

Figure 4 Dimensional changes of castings (nominal size 40mm) and molds at different temperatures

research conclusion

   One of the reasons for the difficulty in ejecting the casting mold is that the production pause causes the casting temperature to drop to the vicinity of zone C, and the shrinkage of the casting causes the size to be smaller than the mold size, resulting in greater clamping force. Using baking method to increase the temperature of the casting and increase the thermal expansion to a reasonable B zone, even A zone, can reduce the clamping force. However, the temperature rise cannot be arbitrarily too high, which will reduce the strength of the casting and cause top penetration. During this process, you can try to push out while baking, which is a better method.

Author of this article:

Tang Heyong, Li Yongjie, Luo Jiulin

Chongqing Construction Yamaha Motorcycle Co., Ltd

Source of this article: Journal of Special Casting and Nonferrous Alloys