The die casting mold changes violently with the increase and decrease of service temperature. It is directly scoured by high speed and high pressure of liquid metal and various corrosion, which are easy to cause mold defects. Other than the casting defects, what defects are possible in casting molds?

What are the Causes and Solutions of Failures in Molds in Die Casting

1.  Defects of Mold Materials

Mold materials and heat treatment of materials are the main factors affecting die life. Mold materials include cavity materials and sleeve plate materials. Die casting mold parts are mainly divided into parts in contact with liquid metal, sliding mating parts, and mold base structural parts. When the die is not preheated, the cavity surface temperature rises from room temperature to liquid temperature, and the cavity surface bears great tensile stress. When opening the top part, the cavity surface bears great compressive stress. After thousands of die-casting, defects such as cracking will occur on the die surface. The higher the temperature, the more serious the damage and wear of the die casting die. During the production process of metal pressure casting, components such as die-casting mold cavity and sprue are in direct contact with high-temperature, high-pressure, and high-speed metal liquid. Because each component of the die-casting mold works under different conditions, it is also different from the impact and wear of the casting metal. Therefore, the hardness requirements of the die-casting mold should be different according to the use of the parts and the casting metal. For example, parts with severe wear should have higher hardness; Parts working under heating conditions shall have high thermal fatigue performance and high-temperature performance.


How to reduce defects in mold materials:

(1) Inspect the porosity, segregation, cracking, crack, non-metallic inclusion, hammer crack, and joint on the surface of the material.

(2) Check the ability to resist crack propagation and extension to avoid overall cracking of the die;

(3) Check the segregation, distribution state, grain size, and inter grain inclusions of carbides on the material grain boundary. The microstructure determines the performance of steel, not just alloy elements. The finer the metallographic structure, the higher the impact energy. On the one hand, the metallographic structure depends on the characteristics of the material itself, on the other hand, it depends on the heat treatment;

(4) Check the internal defects and size of die-casting die materials, and find and avoid microcracks in time.


2.  Processing, Use, and Maintenance of Die Casting Mold

The defects should be avoided starting from the mold design. If necessary, the reasonable structure of the product shall be discussed with the product design engineer. During mold processing, the formwork shall be of sufficient thickness. Reduce the damage of bending deformation to the die. When processing the cooling water channel, special attention shall be paid to ensure concentricity during processing on both sides. Prevent corners at joints to avoid cracking. The surface of the cooling system should be smooth, preferably without machining marks. EDM is more and more widely used in die cavity machining, but there is a hardening layer on the machined cavity surface, which is caused by self-carburizing and quenching of the die surface. The thickness of the hardened layer is determined by the current intensity and frequency during machining. It is deeper during rough machining and shallower during finish machining. Regardless of the depth, the die surface has great stress. If the hardening layer is not removed or the stress is eliminated, the die surface will produce cracking, pitting corrosion and cracking in the process of use.


Welding repair is a common method in die repair. Before welding, the model of the welded die steel shall be mastered first, and the surface defects shall be eliminated by machining or grinding. The welding surface must be clean and dried. The welding rod used shall be consistent with the composition of die steel, and must also be clean and dried. The mould is preheated together with the welding rod. After the temperature of the surface is consistent with that of the heart, it is welded and repaired under protective gas. During welding, when the temperature is lower than 260 ℃, it shall be reheated. After welding, when the mold is cooled to the touch, it is heated to 475 ℃ and insulated at 25mm/h. Finally, it is completely cooled in still air, and then the cavity is trimmed and finished. Heating and tempering after welding is an important part of welding repair, that is, eliminating welding stress and tempering the thin layer under the heated and quenched welding layer during welding.


After the mold is used for a period of time, there will be deposits on the cavity and core due to the high injection speed and long-term use. These deposits are formed by the combination of release agents, coolant impurities, and a small amount of die-casting metal under high temperature and high pressure. These deposits are quite hard and adhere firmly to the core and cavity surface, which is difficult to remove. When removing deposits, do not heat them with a blowtorch, which may lead to the generation of local hot spots or decarburization points on the die surface, thus becoming the source of hot cracks. It shall be removed by grinding or machinery, but it shall not damage other profiles and cause dimensional changes. Regular maintenance can keep the mold in good condition. After the mold test of the new mold, whether the mold test is qualified or not, stress relief tempering shall be carried out when the mold is not cooled to room temperature. 


In the case of serious erosion and cracking, the die surface can be nitrided to improve the hardness and wear resistance of the die surface. However, the hardness of the nitrided substrate should be 35-43 HRC. When it is lower than 35 HRC, the nitrided layer cannot be firmly combined with the substrate and will fall off after use for a period of time; Higher than 43 HRC, it is easy to cause the fracture of the convex part of the cavity surface. During nitriding, the thickness of the nitriding layer shall not exceed 0.15mm, and excessive thickness will fall off at the parting surface and sharp corners.


3.  Heat treatment

Whether the heat treatment is correct or not is directly related to the service life of the die. Due to the incorrect heat treatment process and process specification, the die is deformed, cracked, and scrapped, and the residual stress of heat treatment leads to the failure of the die in use, accounting for about half of the failure proportion of the die. Die casting mold cavities are made of high-quality alloy steel. The price of these materials is high, coupled with the processing cost, the cost is high. If improper heat treatment or low heat treatment quality leads to scrap or service life not meeting the design requirements, there will be great economic losses.


Note:

(1) The forging should be spheroidized and annealed before it is cooled to room temperature.

(2) Quenching and tempering treatment shall be added after rough machining and before finishing machining. In order to prevent machining difficulties caused by excessive hardness, the hardness should be limited and stress relief tempering is arranged before finishing.

(3) During quenching, pay attention to the critical points of steel and holding time to prevent austenite coarsening. During tempering, the temperature shall be kept at 20mm/h, and the tempering times are generally 3 times. In case of nitriding, the third tempering can be omitted.

(4) During heat treatment, attention should be paid to the decarburization and carburization of the cavity surface. Decarburization will quickly cause damage and high-density cracks; Carburization will reduce the cold and hot fatigue resistance.

(5) During nitriding, it shall be noted that there shall be no oil stain on the nitriding surface. The cleaned surface is not allowed to be touched directly by hand, and gloves should be worn to prevent the nitrided surface from being stained with oil, resulting in an uneven nitrided layer.

(6) Between the two heat treatment processes, when the temperature of the previous process falls to the touch of hands, the next process should be carried out, and it should not be cooled to room temperature.