Today, as energy problems and environmental pollution problems continue to intensify, development of new energy has become an important strategy to implement national energy conservation and emission reduction requirements, develop a low-carbon economy. Against such a background, new energy automobile industry is developing rapidly, large-scale integrated die-cast structural parts are also the hottest topic at the moment and the most concerned topic.
Different from traditional die castings, focus of simulation simulation has also undergone tremendous changes. From original focus on pores and shrinkage cavities, it now needs to more aspects such as temperature, stress, thermal balance, production rhythm, lightweight product structure, topology optimization, etc. Among these concerns, analysis and control of mold temperature are top priority.
Graphical results

Basic information of casting is as follows: outline size 1300*1500*535, casting blank weight 45kg, using a new type of heat-free alloy (Si content 7%), F-state mechanical properties meet yield strength 120Mpa, tensile strength 250Mpa, and elongation 12% , and input its material card into simulation software.
Without a temperature control system, perform an initial temperature analysis on mold. As shown in Figure 1, temperature at pouring end of mold is the highest, followed by temperature at overflow end. Mold temperature in remaining areas is relatively low, and the overall temperature difference is large.

 

Figure 1 Initial mold temperature
Corresponding heat balance data statistics show that after a single cycle, the overall mold intake heat is 73,365.51KJ, but overflow heat is only 37,941.08KJ, which is basically twice difference (as shown in Figure 2).
Therefore, there are two major problems. The first aspect is that difference in heat intake and discharge is too large, causing a large amount of heat to be retained in mold; the second aspect is local high temperature of mold.

 

Figure 2 Initial heat balance data analysis

 

Figure 3 Initial product deformation state and Mises stress distribution state

According to initial simulation analysis results, a temperature control system is added to control mold temperature and heat balance. Temperature control mainly includes two aspects. The first aspect is oil and water channels of mold. Temperature is controlled from inside mold. Initial heat transfer coefficients of oil channels and water channels are C1037.01 and C5693.73 respectively; the second aspect is to control mold surface temperature through traditional spraying. Heat transfer coefficient is C2000-10000 in spraying state and 1000-2700 in blowing state (shown in Figure 4).

 

Figure 4 Temperature control layout

 

Figure 5 Analysis and comparison of filling results

 

Before adding temperature control

 

After adding temperature control
Figure 6 Comparison of heat data after adding temperature control
Heat data shows that after increasing temperature control, heat intake of mold is 89,446.41KJ, and heat discharged is 83,183.58KJ. Compared with previous one, difference is almost double. Now from perspective of quantity and value, it has basically reached a balanced state (shown in Figure 6 ). Although the overall heat value of mold has reached equilibrium, as shown in Figure 7, the overall temperature distribution of mold is uneven. Temperatures at pouring end and tail end are relatively high, around 390° and 300° respectively, while mold temperatures at other locations are relatively low, especially in the middle position of mold corresponding to product and slider positions on both sides, where temperatures are around 160° and 100° respectively.

 

Figure 7 Mold temperature comparison after adding temperature control

 

Figure 8 Product deformation state after adding temperature control

 

Figure 9 Mises stress state of product after adding temperature control
After increasing air density control, although the overall heat value is almost balanced, due to uneven distribution of mold temperature field, deformation state and Mises stress distribution state of product have not been improved, but have shown a trend of increasing. Maximum deformation position of product increased from 7.904mm to 8.969mm, and maximum Mises stress increased from 174.8Mpa to 195Mpa (shown in Figures 8 and 9).

Different from traditional die-casting parts, the overall wall thickness of large-scale integrated die-casting structural parts is relatively average and thin. It can be observed from Figure 10-1 that except for injection end and tail end of mold, which have higher temperatures, the rest of mold can only reach about 150° after alloy releases temperature, which results in insufficient innate energy absorption. Secondly, by observing working process of spraying and oil-water, it can be found that mold did not receive sufficient heat exchange in the early stage. Temperature control system even lowered temperature of mold. Temperature in the lowest temperature area was only about 70°.

 

Figure 10-1 Heat source analysis

 

Figure 10-2 Heat source analysis

 

Figure 10-3 Heat source analysis
Based on above analysis, temperature control system was adjusted, and spraying was changed from traditional to microscopic spraying (which can also greatly improve production cycle). Secondly, oil and water circuits were optimized to control the best heat transfer coefficient.

 

Figure 11 Optimization of temperature control scheme

After optimization, the overall mold temperature changes evenly, with an average temperature of about 250℃. Temperature in each local area is higher, about 310℃. Subsequent optimization still needs to be continued (Figure 12). After mold temperature is optimized and balanced, deformation and stress concentration of product must also be greatly optimized.

 

Figure 12 Mold temperature comparison after optimization

 

Figure 13 Product deformation state after optimization

 

Figure 14 Mises stress state of optimized product

Above observation and research process is numerical statistics of mold temperature balance and overall mold heat balance. However, in actual production process, while focusing on the overall situation, attention is also paid to observing state of local point temperature and heat balance.
As shown in Figure 15 and Figure 16, in a single production cycle, if temperature difference between head temperature and tail temperature at same point is less than 5℃, then local temperature point will be considered to have reached point thermal equilibrium. This critical criterion value can provide standards for setting number of simulated preheating cycles and setting process in actual production process.

 

Figure 15 Comparison of the first and last temperatures of mold in same cycle

 

Figure 16 Complete data chart

According to actual production conditions, heat source and heat direction of mold are analyzed in detail, controllable factors that can affect temperature and heat changes are found, specific adjustments are made according to goals and needs. While product structure remains unchanged, adjusting and optimizing mold temperature will have a significant improvement effect on product deformation and product stress distribution.