The quality of the die-casting mold directly affects the quality of the die-casting parts. After the die-casting mold is manufactured and delivered to the die-casting factory, as a die-casting factory, it is difficult for us to change the properties of the die-casting mold, but we can delay the mold through the scientific use and maintenance. Failure, reduce the occurrence of die casting defects and maximize the life of the mold.
1. The relationship between the failure form of the die casting mold and the product defect
The main failure form of die-casting mold in use is thermal fatigue, and the failure mold is often accompanied by failure forms such as thermal melting loss and deformation. Forms of thermal fatigue failure: The die-casting temperature of the production process of aluminum alloy castings is about 650 ° C. When spraying, low-temperature paint is sprayed on the high-temperature mold surface. During the die-casting process of a product, there will be multiple cold and heat alternations. , A large thermal stress is formed on the surface of the product and then cracks occur. Most of the cracks are straight, and some are at an angle of 45 ° to the surface, and there are wedges formed by aluminum filling, which accelerates the crack development. The scouring force at the gate is the largest, so the product is prone to dense and fleshy at the gate. It is arranged in a dot-like dense arrangement when it is slight, and it is arranged in a mesh or strip form when it is severe. In the cracks, there are tempered initiate and granular carbides, as well as slight carbide segregation. When the segregated carbides intersect with the profile, the cracks will propagate to the segregation zone. Within the cavity of the failed mold, there is a tissue softening zone. Microscopic analysis of the crack fracture shows that there are corrosion pits on the fracture surface. Deodorization of the mold surface, hard and brittle layers on the nitrated surface and high surface roughness of the mold will all reduce the thermal fatigue resistance of the mold surface.
Hot melt loss effect form: Hot melt loss refers to the severe erosion of the working surface of the mold after a period of work, which makes the mold lighter. It is a concomitant state of thermal fatigue failure and a failure form after severe welding. Aluminum alloy molds will be welded during use. Welding refers to the reaction between the mold and the die-casting alloy. After the welding of the mold surface, a complex and hard Fe-AL intermetallic compound phase will be generated, which will cause fleshy defects on the surface of the casting during the next die-casting cycle. The hard alloy interphase will also accumulate on some surfaces of the mold, increasing the roughness, and in severe cases, it will cause the liquid metal to stick to the mold and the casting will lack meat.
The first type of welding is called impact welding, that is, welding occurs at the entrance of the mold surface toward the cavity or at the injector. These areas have a harsh working environment, and the protective layer of the mold is likely to fail and then expose the metal substrate, aluminum alloy, and substrate material. The reaction occurs to produce a hard intermetallic compound. The compound cracks and falls off during the die-casting process to expose the matrix material, and then starts again and again. The mold surface is corroded and the mold material is damaged. The second type of welding is called deposition welding. Some areas in the mold cavity are where surface treatment or mold lubricant cannot reach, such as back to the gate or away from the runner. Their surface state, temperature distribution and pressure The situation is different than elsewhere. After the aluminum alloy liquid flows here, the fluidity is poor, and the mold solidifies first. The mold in this area is heated for a long time, and the intermetallic compounds are easily deposited here, and finally, the weld is formed. When the mold has a melting loss effect, the working surface is severely eroded at this time, making the overall quality of the mold lighter. Correspondingly, the mean thickness of the castings increased, resulting in fleshy castings and substandard sizes. It is one of the causes of fleshy and lack of meat.
Deformation failure: the parts of the mold are not heated uniformly during the work process, so the parts expand unevenly, there is large stress between them, and then there is deformation, the mold surface is convex, which causes flashing, and even runs in severe cases. water. Sometimes when the flash is very thick, we have to add a cutting die or add a cutting process to remove the flash and increase production costs.
2. Correct use and maintenance of die casting mold
1) Under the conditions of satisfying product quality, try to use a low temperature and low speed. Die casting temperature and die casting speed are as low as possible.
2) The mold is preheated slowly, and the preheating temperature is gradually increased to 150-220 ° C. It is recommended to use a die-casting temperature controller for preheating. Die-casting hot molds are prone to deformation and thermal fatigue failure of the molds, so they cannot rely on die-casting hot molds.
3) The use of cooling water can effectively prevent mold deformation and readjustment, adjust the cooling sequence of the castings so that the first condensed part can be replenished with metal liquid to achieve sequential solidification. Product with excellent performance.
4) Clear the deposits in the cavity and core in time. The removal shall be carried out by grinding or mechanical removal. It cannot be removed by heating with a torch, as this will cause local soft spots or bureaucratization spots in the mold, resulting in burning. During grinding or mechanical removal, the principle of not damaging the mold surface and causing dimensional changes in the casting is adopted.
5) After the new mold is tested, whether it is qualified or not, the stress and fire should be performed without the mold cooling to the room. When the mold is used for 50,000 molds, maintenance can be performed every 2-3 million molds to reduce the speed and time of cracks due to thermal stress.
6) When the erosion and cracking are serious, the surface of the mold should be appropriately nitrated. After nit-riding, the hardness and wear resistance of the mold surface can be improved. The hardness of the nit-riding substrate is controlled between 38 ~ 42HRC, and the nitriding layer generally does not exceed 0.14mm. When nitriding, wipe off the oily dirt and gradually and uniformly nitriding the layer.
7) When it is necessary to repair the mold surface cracks, the surface defects of the mold must be removed by machining or grinding before welding. After drying the electrode, preheat it together with the mold to the temperature of the mold surface and core to 480 ° C, then perform welding repair under protective gas, and then perform cavity trimming and finishing after air cooling. Then heat and temper treatment to eliminate welding stress.
3. Summary
The scientific use and maintenance of the mold can not only improve the producing life of the mold, save production costs, but also improve product quality and increase product quality.
