Under the trend of energy conservation and emission reduction, the lightweighting of automobiles is receiving increasing attention. The use of aluminum alloy instead of steel structure has been increasingly applied in the lightweighting of traditional automobiles and the development of new energy vehicles. As a key fixture in the die-casting process, the effective lifespan of die-casting molds is crucial for reducing product costs. The thermal alternating stress fatigue caused by uneven surface temperature during mold forming is the main factor leading to mold failure.

1. Ways to solve temperature imbalance
(1) For die casting, the temperature difference is reduced by effectively outputting the input energy (the energy emitted by the aluminum alloy melt from 650 ℃± 10 ℃ to 200~300 ℃). This mode is mostly used for geometric parts with standardized product appearance models. By designing internal cooling pipes arranged in a straight line on the mold, cooling water can directly take away heat energy to achieve temperature balance. Local adjustments can also be made by adjusting the flow rate. At present, there are many applications, but there are also some problems.
(2) Design targeted areas based on temperature field simulation, and achieve local cooling to achieve temperature equilibrium. The point cooling method is widely used in the protruding heated parts of the mold forming cavity. Based on practical work experience, propose a new design pattern.

2. The general traditional point cooling design method
The traditional point cooling design method is shown in Figure 1. The cooler is connected to the workpiece with threads, and the spot cooler is screwed into the workpiece. Due to the use of PT threads, there is no leakage between the spot cooler and the workpiece. The spot cooler tube penetrates the cooling hole of the workpiece, and when cooling water is connected, it is sprayed out through the center tube of the spot cooler, allowing the cooling water to circulate in the cooling hole of the workpiece.

     Water circulation is formed inside the point cooler, and the cooling water takes away the heat from the bottom of the hole, which can reduce the size of the hot spot on the workpiece. This method can effectively achieve temperature balance in local areas, and the process is mature. But it cannot be effectively applied to parts where the outer diameter size is limited. When it is necessary to ensure the strength of the mold wall thickness, only super point cooling with a diameter of 2~4 mm can be used for the bottom hole. However, in practical design and manufacturing, the effect is not ideal due to the influence of structural space, water pressure, etc., as shown in Figure 2.
 

3. Choose materials with good heat transfer performance
We chose graphene material with excellent unidirectional thermal conductivity as the conductor. Due to the brittleness of graphene itself, metallic silver with good thermal conductivity was chosen as the substrate. Graphene was deposited onto metallic silver through physical vapor deposition. The following device was designed to screen various materials and record the process using a thermometer, as shown in Figure 3.

The experimental results are shown in Table 1, which indicates that silver with graphene has the best thermal conductivity.

4. Design and verification of a new type of super point cooling structure
Based on the experimental results, a mold insert was selected for specific verification. The schematic diagram of local point cooling design on the mold is shown in Figure 4

A metal silver sleeve with graphene is embedded into the workpiece, and a point cooler is inserted into the metal sleeve. The point cooler cools the graphene conductor, which in turn cools the workpiece, thus achieving the purpose of cooling the workpiece in the cold zone. Can be used for cooling small workpieces, even if the insert is damaged, the cooling water will not flow into the product, causing it to be scrapped. On site use revealed that the effective lifespan of this insert has increased from 15000 to 20000 mold cycles to 30000 to 40000 mold cycles, with good results.

5. Conclusion
The new type of point cooler can effectively solve the problem of mold core leakage caused by mold core cracking, improve production efficiency, save production costs, and increase the range of use of point cooling structures.

Author: Wang Yue, Min Feng, Liang Jiang, Pioneer Yang Yingfeng