During solidification process of die casting, shrinkage of alloy generates a tightening force on mold, and mold material also shrinks during temperature changes. Coordinated changes between casting and mold cause dynamic changes in tightening force. Casting also shrinks when it is ejected from mold and cools to room temperature. Mold design manual recommends a shrinkage rate of 0.4% to 0.7% for aluminum alloy die castings. This dynamic change creates difficulties for mold designers in selecting shrinkage ratios. In fact, tightness of casting is closely related to draft angle, mold finish, ejector pin layout, release agent concentration, mold temperature, casting temperature, etc. Tightening force of aluminum alloy die castings always changes dynamically from cooling in mold to ejection of castings. Molds and alloys have different thermal expansion coefficients as temperature changes. When designing molds, different shrinkage rates should be selected based on different die-casting alloys. Ejection casting failures caused by temperature changes in die-casting production can be solved by adjusting temperature. It analyzes shrinkage rate and thermal expansion coefficient of mold and castings, provides parameters for selecting shrinkage rate of mold during design, and provides an effective countermeasure to difficult ejection of castings caused by temperature changes.

Commonly used materials for die-casting molds are 8407, DIEVAR, H13, SKD61, etc. For aluminum alloy die-casting, temperature of mold during normal production is between 75 and 425℃ (see Figure 1). There are differences in use temperature of different die-casting molds. Enterprises can conduct statistical confirmation based on big data and determine corresponding mold thermal expansion coefficient based on use temperature of mold.

 

Figure 1 Relationship between mold temperature and thermal expansion coefficient

 

Figure 2 Thermal expansion coefficient of die-cast aluminum and magnesium alloys

 

Figure 3 Thermal imaging temperature of 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) Correspondence table between mold size, casting size D and temperature change: Calculation formula: D2=D1[1+r(t2-t1)]; (2) When mold is at room temperature of 20℃, shrinkage rate α = [mold size/casting size]-1. Among them, nominal size of product at normal temperature is D1, mold change caused by thermal expansion is D2, t2 is temperature during thermal expansion, and t1 is starting temperature.

 

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

One of reasons for difficulty in ejecting casting mold is that pause in production causes temperature of casting to drop to near C zone. Shrinkage of casting makes size smaller than size of mold, resulting in a greater tightening force. Baking method is used to increase temperature of casting, and thermal expansion is increased to a reasonable area B or even area A, so that tightness is reduced. However, temperature rise cannot be too high, which will reduce strength of casting and lead to penetration. During this process, you can try to eject while baking. This method works better.