Under trend of energy conservation and emission reduction, more and more attention is being paid to replacing steel with aluminum to achieve lightweighting of traditional vehicles and development of new energy vehicles. As key tooling of die-casting process, effective life of die-casting mold is very important for reducing product costs. Thermal alternating stress fatigue caused by temperature imbalance on mold forming surface has become main factor in mold failure. Aiming at problem that super-point cooling commonly used in die-casting is limited by space of mold structure, a new super-point cooling method is designed, which can expand use space of spot cooling, solve defects such as burns and shrinkage holes in local narrow locations of the product, and ultimately improve product qualification rate.
Graphical results

(1) For die casting, temperature difference is reduced by effectively outputting input energy (heat emitted by molten aluminum from 650℃±10℃ to 200~300℃). This mode is mostly used for geometric parts with simple product structures. By designing internal cooling pipes arranged in line on mold, cooling water is used to directly take away heat to achieve temperature balance. Partial adjustments can also be made by adjusting cooling water flow rate. This method is currently widely used, but it also has some problems.
(2) Based on temperature field simulation, targeted local cooling is designed to achieve temperature balance. Among them, point cooling method is widely used in protruding heated parts in mold forming cavity. Combining practical experience, a new design model is proposed.

Traditional spot cooling design method is shown in Figure 1. Cooler and workpiece are threaded, and spot cooler is screwed into workpiece. Since PT thread is used, there is no leakage between spot cooler and workpiece, and spot cooling pipe goes deep into cooling hole of workpiece. When cooling water is connected, water is ejected through central tube of spot cooler, allowing cooling water to circulate in cooling holes of workpiece. A water circulation is formed inside spot cooler, and cooling water takes away heat from bottom of hole, which can reduce size of hot spots on workpiece. This method can effectively achieve temperature balance in local areas and process is mature. However, it cannot be effectively applied to parts with limited outer diameter. When it is necessary to ensure strength of mold wall thickness, bottom hole can only use φ (2 ~ 4) mm super-point cooling. In practice, it is found that effect is not ideal due to influence of structural space, water pressure, etc., see Figure 2.

 

Figure 1 Traditional spot cooling method

 

Figure 2 Mold cooling wall thickness

Choose graphene material with excellent one-way thermal conductivity as conductor. Since graphene is relatively brittle, silver with good thermal conductivity was chosen as substrate. Graphene was deposited on metallic silver through physical vapor phase. Following device was designed and a variety of materials were screened. Process was recorded with a thermometer, as shown in Figure 3. Table 1 shows time required to heat cooling water to 100℃ for different materials. It can be seen that silver with graphene has the best thermal conductivity.
Figure 3 Thermal conductive material screening test device

 

1. Glass tube 2.Sealing ring 3.Conductor 4.Test tube 5. Heating device 6. Boiling water 7. Cooling water

Table 1 Comparison of thermal conductivity properties of different materials

Based on test results, a mold insert was selected for verification. Schematic diagram of local spot cooling design on mold is shown in Figure 4. A graphene-coated metal silver sleeve is embedded into workpiece, a spot cooler is inserted into metal sleeve, spot cooler cools graphene conductor, and graphene conductor cools workpiece, thus achieving purpose of cooling workpiece. It can be used to cool small workpieces. Even if insert is damaged, cooling water will not flow into product, causing product to be scrapped. On-site use found that effective life of this insert has been increased from original 15,000 to 20,000 mold times to 30,000 to 40,000 mold times, and effect is good.

 

Figure 4 New super-point cooling structure
1. Spot cooler 2. Conductor 3. Workpiece