Abstract: Tightening force of die castings always changes dynamically from cooling of aluminum alloy in mold to ejection of castings. Focus on analysis that mold materials and alloy materials have different thermal expansion coefficients as temperature changes. When designing molds, choose different shrinkage rates according to different die-casting alloys. Ejection casting failures caused by temperature changes in die-casting factories during production can be solved by temperature adjustment.
During solidification process of die-casting alloy, shrinkage of casting produces 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 has a certain temperature after being ejected from mold, and will shrink when it cools to room temperature. Mold design manual recommends a shrinkage rate of 0.4% to 0.7% for 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. This topic only analyzes shrinkage and thermal expansion coefficient of molds and castings, and provides parameters for selecting mold shrinkage when designing molds. It also provides an effective countermeasure for die-casting manufacturers when encountering difficulties in ejecting castings caused by temperature changes.

Thermal expansion coefficients involved in die-casting process are mainly two aspects: mold steel and die-casting materials.

Commonly used die-casting mold materials include 8407, DIEVAR, H13, SKD61, etc., which all belong to same type of metal materials. For aluminum alloy die-casting, mold temperature range during normal production after mold preheating is between 75 and 425℃ (see Figure 1). There are differences in mold usage temperatures of different die-casting companies. Companies can conduct statistical confirmation based on big data to select corresponding mold thermal expansion coefficient based on mold usage temperature.

 

Figure 1: Relationship between mold material temperature and thermal expansion coefficient

Thermal expansion coefficient of die-cast aluminum alloy and magnesium alloy is mainly related to Si content. At the same temperature, the higher Si content, the smaller thermal expansion coefficient. Figure 2 shows thermal expansion coefficients of three commonly used die-casting materials ADC12, A380, and AM50. Vertical line is median value of material composition requirements. Die-casting manufacturers can query thermal expansion coefficient based on their company's commonly used Si content.

 

Figure 2: Thermal expansion coefficient of die-cast alloys

When selecting shrinkage rate in die-casting mold design, it is necessary to comprehensively select mold material, aluminum alloy type, and temperature of mold during die-casting.

Definition of shrinkage used in mold design is change in dimensions of casting when it cools to normal temperature from moment it is ejected from mold. Measured temperature change is when die-cast aluminum alloy is ADC12 and mold is 8407. Maximum temperature of Ar1 area of mold measured by thermal imager is 289.5 ℃ (mold is still cooling rapidly when casting is ejected). Measured external temperature of casting is 350 ℃ and internal temperature is about 400 ℃ (there is a delay in measuring temperature with this thermal imager. Temperature is about 50 ℃ lower than actual temperature) (see Figure 3).

 

Figure 3: Thermal imaging temperature of mold

Thermal expansion coefficients of the three commonly used die-casting materials are ADC12, A380 and AM50, as shown in Table 1. Taking die-casting aluminum alloy ADC12 (melting point 580℃) as an example, mold material is 8407. Temperature of die-casting part in mold starts from 550℃ and drops to 500℃, 450℃, 400℃, until room temperature is 20℃, respectively. Corresponding thermal expansion coefficients r are 25.3, 24.8, 24.3, 23.8, etc., which are defined as 0 at normal temperature. Corresponding to temperature of casting, thermal expansion coefficient of mold steel 8407 at temperatures of 450℃, 400℃, 350℃, and 300℃ (100℃ lower than casting) is 12.89, 12.69, 12.51, 12.33, etc.

Table 1: Correspondence table of thermal expansion coefficient changes between die casting and mold temperature changes

It can be seen from Table 1 that when casting is ejected from mold, when temperature inside casting is 350~400℃, corresponding mold temperature is 280~300℃. At this time, nominal dimensions of ADC12 casting and 8407 mold are close. After testing temperatures of molds and castings, counting dimensional and temperature changes of molds in production, reasonable recommended values in Table 1 (data outside black box are for reference only) are determined as reference data needed for mold design. When producing continuously, temperature changes of mold and casting tend to be stable. Thermal expansion coefficients of ADC12 when ejection casting temperature is 370 ℃ and mold temperature is 290 ℃ are 23.6 and 24.1*10-6 ℃-1 respectively. Based on this, shrinkage rate of ADC12 die casting mold design is calculated according to Table 1, which is 0.005. Theoretical calculation shows that A380 and AM50 are 0.006 and 0.007 respectively.

When it is difficult to eject die-casting part from mold, method of baking casting to heat it up and then ejecting casting is adopted. When it is difficult to eject a casting from mold, the most common way to deal with problem on site is to apply a layer of grease to joint between casting and mold, then bake casting with natural gas before ejecting casting again. Method is simple and effective.
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 increases to a reasonable area B or even area A, which is larger than size of mold, reducing tightness of casting. However, temperature rise cannot be too high, as this will reduce strength of casting and lead to penetration of casting. During this process, you can use method of trying to eject while baking. This method is quick, simple and effective. Otherwise, mold needs to be removed by mold worker, which is time-consuming and labor-intensive.

Thermal expansion slopes of die castings and mold steels are different, intersection point is point where thermal expansion amounts of the two are equal. When die-casting enterprises encounter difficulties in ejecting casting molds during production, they use method of baking castings to heat up, so that when thermal expansion of castings reaches near intersection point of slope, wrapping force of castings can be greatly reduced, so that castings can be ejected smoothly. .