Shrinkage of die castings
The shrinkage rate of a die-casting refers to the ratio of the shrinkage of the die-casting’s dimensions from liquid to solid during the solidification and cooling process to its original liquid dimensions. It is a crucial parameter that must be considered in die-casting production. As the molten metal solidifies in the die-casting mold cavity, it undergoes three stages of contraction: liquid phase, solidification phase, and solid phase. These three stages of contraction together contribute to the total shrinkage rate of the die-casting. Different alloy materials have different shrinkage rates. For example, aluminum alloys generally have a shrinkage rate of 0.8%-1.2%, zinc alloys have a shrinkage rate of 0.5%-0.8%, and copper alloys have a relatively large shrinkage rate, typically between 1.2% and 1.5%. Understanding and mastering the shrinkage rate of die-castings is crucial for mold design, process parameter formulation, and dimensional accuracy control of die-castings.
Numerous factors influence the shrinkage of die-castings. The first is the alloy composition; different alloying elements significantly affect the metal’s shrinkage properties. For example, in aluminum alloys, adding silicon can reduce the alloy’s shrinkage, while adding magnesium slightly increases it. The die-casting’s structural shape is also a significant factor influencing shrinkage. For die-castings with uneven wall thicknesses, the shrinkage of thicker-walled areas is typically greater than that of thinner-walled areas. This is due to the slower cooling rate and longer solidification shrinkage time in thicker-walled areas. Furthermore, the size of the die-casting also affects shrinkage. Generally speaking, large die-castings have relatively small shrinkage rates, while small die-castings have relatively large shrinkage rates. This is because large die-castings have poorer heat dissipation conditions, slower cooling rates, and greater shrinkage constraints.
Die-casting process parameters also have a significant impact on the shrinkage of die-cast parts. Die-casting temperature is one of the key parameters affecting shrinkage. The higher the pouring temperature of the molten metal, the greater the liquid and solidification shrinkage, and the greater the overall shrinkage of the die-cast part. Mold temperature also affects the shrinkage of die-cast parts. If the mold temperature is too low, the molten metal cools too quickly, and solidification shrinkage is significantly constrained by the mold, reducing shrinkage. If the mold temperature is too high, the molten metal cools slower, increasing shrinkage. Furthermore, die-casting pressure and holding time also affect shrinkage. Appropriately increasing die-casting pressure and extending holding time can enhance the molten metal’s shrinkage compensation, reduce solidification shrinkage, and thus lower the shrinkage of the die-cast part.
During mold design, the mold cavity dimensions must be determined based on the die-cast part’s shrinkage rate to ensure the die-cast part achieves the desired dimensional accuracy after cooling. The formula for calculating mold cavity dimensions is: Cavity size = die-cast part design dimensions × (1 + shrinkage rate). In actual calculations, the appropriate shrinkage rate should be selected based on the specific parts and dimensions of the die-cast part. For areas requiring high dimensional accuracy, a lower shrinkage rate should be selected; for larger or more complex parts, a higher shrinkage rate should be selected. Furthermore, factors such as mold wear and elastic deformation must be considered, and appropriate margins should be added to the mold cavity dimensions to extend the mold’s lifespan.
To effectively control the shrinkage of die-castings, a series of measures need to be taken during the production process. First, the alloy material should be selected rationally. Based on the performance requirements and dimensional accuracy requirements of the die-casting, an alloy material with a small and stable shrinkage rate should be selected. Second, the die-casting process parameters should be optimized. By adjusting parameters such as pouring temperature, mold temperature, die-casting pressure, and holding time, the solidification process of the molten metal can be controlled to reduce fluctuations in the shrinkage rate. In addition, for die-castings with complex structures, a segmented cooling method can be adopted to control the cooling rate of different parts to ensure uniform shrinkage of the die-casting. At the same time, the inspection and analysis of the die-casting dimensions should be strengthened. By statistically analyzing the changes in the shrinkage rate, the mold cavity size and process parameters can be adjusted in a timely manner to ensure that the dimensional accuracy of the die-casting meets the design requirements.
