Quality of die-casting parts is closely related to control of die-casting mold surface temperature. Mucous membranes are often encountered during die-casting process, causing strain and damage to the surface of die-casting parts. If cooling is too slow, shrinkage cavities and shrinkage porosity will easily occur inside die casting, resulting in a decrease in mechanical properties and sealing. These are related to excessive mold temperature. Therefore, based on FLOW-3D CAST software, die-casting mold was simulated under air cooling, spray cooling, and water channel cooling conditions, effects of the three cooling methods on mold temperature were analyzed, compare it with actual measured mold temperature, compare difference between traditional spraying and motion spraying, and compare it with actual mold temperature, aiming to provide a reference for solving related problems.
Graphical results
Three-dimensional model of automobile engine block casting is shown in Figure 1. Outline size is 395mm*380mm*230mm. Weight is 14kg. It is a complex and thick piece with many local thick-walled areas. Therefore, reasonable control of mold temperature is very important to quality of die castings. Pouring temperature of die castings is 600~680℃, and heat is transferred to mold through thermal conduction. The longer casting stays in mold cavity, the more heat mold absorbs and the higher mold temperature. Heat transfer of mold can be mainly divided into internal heat transfer and surface heat transfer. Internal heat transfer mainly includes heating and cooling systems. External cooling includes air cooling, high-speed air cooling, release agent spray cooling, etc.

 

Figure 1 Three-dimensional diagram of automobile engine block

 

Figure 2 Schematic diagram of location of temperature monitoring points

 

Figure 3 Temperature change curve
Mold area is roughly divided into three temperature ranges: medium, medium and low. Temperature monitoring points P1, P2 and P3 are selected in three areas respectively, as shown in Figure 2. Change curve of simulated output temperature is shown in Figure 3. During natural cooling, temperature drop in high-temperature area is about 1.5℃/s for a period of time before mold opening, then gradually decreases, and slowly rises again. In low temperature areas, there is little or no change in temperature. This analysis proves that without forced spray cooling, natural cooling rate is very slow.

 

Figure 4 Comparison of mold temperatures among three simulation schemes
(Plan A) With cooling and spraying (Plan B) Without cooling and spraying (Plan C) Without cooling and spraying

 

Figure 5 Effect of spraying with different flow rates on mold temperature
For thermal mold cycle simulations, in order to accurately predict temperature distribution in mold, spatial variation in spray cooling should be taken into account. A kinematic spray cooling model was developed for this purpose. This model shows calculation of cooling amount for each spray, whereas traditional spray simulation defines a constant heat transfer coefficient for the entire mold cavity and all surfaces of mold are sprayed. However, during actual spraying, it was found that some deep rib structures in complex molds cannot be sprayed. Software calculates movement of nozzle, spray area on mold surface will be continuously calculated and updated, also taking into account spray cooling obstruction due to spray angle and mold surface shape, see Figure 6.

 

Figure 6 Simulation of spraying area

 

Figure 7 Schematic diagram of spraying parameters

 

Figure 8 Comparison between simulated spraying and actual spraying
Compared with temperature measured by actual imager (see Figure 8), this area is prone to strain defects in actual production, which is also caused by high mold temperature. Therefore, flow rate of spraying will be increased in this area to reduce mold temperature. By observing thermal imager picture after spraying, it can be seen that mold temperature in this area has decreased. Motion spray simulations show that temperatures in this area also decrease. However, temperature in this area is higher using traditional spraying simulation. Therefore, mold temperature simulated by motion spraying is more accurate and can better reflect actual spraying effect. Local errors require setting spray path more accurately and adjusting spray area. Die-casting mold cools down very slowly under natural cooling. Heating and cooling water channels have a slight impact on mold surface temperature. Large-flow spraying has a great impact on mold surface temperature, and temperature drops significantly. Motion spraying simulation can simulate mold temperature more accurately.