Facing the domestic economic construction situation in the 21st century, mold enterprises must adapt to the development of the market economy, and the automobile industry, which is a national pillar industry, will increase the output of light cars, thus putting forward higher requirements for the precision and quality of mold castings. Due to the long production cycle, high investment cost, and high manufacturing precision of die-casting molds, the cost is also relatively high. Therefore, many die-casting enterprises hope that the die-casting molds have a long service life, thereby reducing the production cost of the enterprise. However, due to the influence of a series of internal and external factors such as raw materials and mechanical processing, the phenomenon of premature failure and scrapping of die-casting molds is common, resulting in a great economic waste of enterprises.
The early failure situations of the mold mainly include: punch breakage, the collapse of the edge of the mold cavity, cracking of the flash by the bridge, cracks at the bottom of the mold cavity, cracking at the corners, wear, and erosion, etc. The main reasons for the failure of die casting molds are The defects of the mold material itself, processing, use, maintenance, and heat treatment of the mold.
- Defects in the mold material itself
As we all know, the conditions of use of die casting molds are extremely harsh. Taking the aluminum die-casting mold as an example, the melting point of aluminum is 580-740℃, and the temperature of the molten aluminum is controlled at 650-720℃during use. In die casting without preheating the mold, the surface temperature of the cavity rises from room temperature to the liquid temperature, and the surface of the cavity bears great tensile stress. When the top part of the mold is opened, the surface of the cavity is subjected to great compressive stress. After thousands of times of die casting, defects such as cracks appear on the surface of the mold. It can be seen that the use conditions of die casting are rapid heating and rapid cooling. The die material should be made of hot work die steel with high thermal fatigue resistance, fracture toughness, and thermal stability. H13 (4Cr5MoV1Si) is currently a widely used material. According to reports, 80% of foreign cavities use H13, and 3Cr2W8V is still widely used in China, but 3Cr2W8VT has poor technical performance, poor thermal conductivity, and high linear expansion coefficient. A lot of thermal stress is generated during the work, resulting in cracks or even cracks in the mold, and it is easy to decarburize when heated, which reduces the wear resistance of the mold, so it is an outdated steel grade. Maraging steel is suitable for molds that do not require high wear resistance and corrosion resistance against thermal cracking. Heat-resistant alloys such as tungsten and molybdenum are limited to small inserts with severe thermal cracking and corrosion. Although these alloys are both brittle and notch sensitive, their advantage is that they have good thermal conductivity. Die casting die has good adaptability. Therefore, under reasonable heat treatment and production management, H13 still has satisfactory performance.
The material used to manufacture the die-casting mold should meet the design requirements in every aspect to ensure that the die-casting mold can reach the designed service life under its normal use conditions. Therefore, before putting into production, a series of inspections should be carried out on materials to prevent early scrapping of molds and waste of processing costs caused by defective materials. Commonly used inspection methods include macroscopic corrosion inspection, metallographic inspection, and ultrasonic inspection.
- Macroscopic corrosion inspection. Mainly check the porosity, segregation, cracks, cracks, non-metallic inclusions, hammer cracks, and seams of the material.
- Metallographic inspection. It mainly inspects the segregation, distribution state, crystallinity, and inter-grain inclusions of carbides on the grain boundaries of the material.
- Ultrasound examination. It mainly inspects the defects and sizes inside the material.
- Processing, use, repair, and maintenance of die-casting molds
When determining the injection speed of the die casting machine, the maximum speed of the die casting process should not exceed 100m/s. If the die casting speed is set too high, it is easy to cause corrosion of the mold and increase deposits on the cavity and core; but the setting of the injection speed is too low. Defects in castings. Therefore, the minimum injection speed of aluminum die casting should be set to 18 m/s, the maximum injection speed of aluminum die casting should not exceed 53 m/s, and the average injection speed should be set to 43 m/s.
In the process of processing, thicker templates cannot be superimposed to ensure their thickness. Because the thickness of the steel plate is doubled, the bending deformation is reduced by 85%, and the lamination can only play a superimposing role. The bending deformation of two boards with the same thickness as the single board is 4 times that of the single board. In addition, when processing cooling water channels, special attention should be paid to ensuring concentricity during processing on both sides. If the head corners are not concentric with each other, then during use, the connected corners will crack. The surface of the cooling system should be smooth and preferably free of machining marks.
EDM is more and more widely used in mold cavity machining, but a hardened layer remains on the surface of the cavity after machining. This is due to the self-carburizing and quenching of the mold surface during processing. The thickness of the hardened layer is determined by the current intensity and frequency during machining, and it is shallower during rough machining. Regardless of the thickness of the hardened layer, the surface of the mold has great stress. After the mold cavity is EDM, the hardened layer must be removed or the stress must be relieved. Otherwise, cracks, pitting, and cracking will occur on the surface of the mold during use.
Hardened layer relief or stress relief available:
- Remove the hardened layer with whetstone or grinding;
- Under the condition of not reducing the hardness, the stress can be relieved below the tempering temperature, which can greatly reduce the surface stress of the mold cavity.
The casting process should be strictly controlled during the use of the mold. Within the scope of the process permits, try to reduce the casting temperature of the molten aluminum, the injection speed, and increase the mold preheating temperature. The preheating temperature of the aluminum die-casting mold is increased from 100-130℃ to 180-200℃, and the life of the mold can be greatly improved.
Welding repair is a common method in mold repair. Before welding, the type of die steel to be welded should be mastered, and surface defects should be eliminated by machining or grinding. The welding surface must be clean and dried. The electrodes used should be of the same composition as the die steel and must also be clean and dried. The mold is preheated together with the electrode (H13 is 450℃), and after the surface temperature is consistent with the core temperature, it is repaired by welding under shielding gas. During the soldering process, when the temperature is lower than 260℃, it is necessary to reheat. After welding, when the mold cools down to the touch, then heat it to 475℃ and keep it warm at 25mm/h. Finally, it is completely cooled in still air, and then the trimming and finishing of the cavity are carried out. Heating and tempering the mold after welding is an important part of welding repair, that is, eliminating welding stress and tempering the thin layer under the welding layer that is heated and quenched during welding.
After the mold is used for some time, due to the high injection speed and long-term use, there will be deposits on the cavity and core. These deposits are formed by the combination of mold release agents, coolant impurities, and small amounts of die-cast metal under high temperature and pressure. These deposits are fairly hard and adhere firmly to the cavity and core surfaces and are difficult to remove. When removing deposits, they cannot be removed by heating with a blowtorch, which may lead to the generation of local hot spots or decarburization spots on the surface of the mold, thus becoming the origin of thermal cracking. Grinding or mechanical removal should be used, but other profiles should not be damaged, resulting in dimensional changes.
Regular maintenance of the mold can keep the mold in good condition. After the new mold is tested, no matter whether the test is qualified or not, the stress relief tempering shall be carried out before the mold has been cooled to room temperature. When the new mold is die-casting 10,000 times, the mold cavity and mold base should be tempered at 450 – 480℃, and the cavity should be polished and nitrided to eliminate internal stress and slight cracks on the surface of the cavity. In the future, the mold must be maintained the same after every 12,000~15,000 mold times. When the mold is used for 50,000 mold times, it can be maintained every 25,000 to 30,000 mold times. With the above method, the speed and time of mold cracking caused by thermal stress can be significantly slowed down.
In the case of severe erosion and cracking, nitriding treatment can be performed on the surface of the mold to improve the hardness and wear resistance of the mold surface. However, the hardness of the nitriding matrix should be 35-43HRC. When the hardness is lower than 35HRC, the nitrided layer cannot be firmly combined with the matrix. After a period of use, it will fall off into large pieces. If it is higher than 43HRC, it is easy to cause fractures in the convex parts of the cavity surface. When nitriding, the thickness of the nitriding layer should not exceed 0.15mm. If it is too thick, it will fall off at the parting surface and sharp corners.
- Heat treatment of molds
Whether the heat treatment is correct or not is directly related to the service life of the mold. Due to the incorrect heat treatment process and process rules, the mold is scrapped due to deformation, cracking, and the residual stress of the heat treatment causes the mold to fail in use, accounting for about 50% of the mold failure ratio.
The die-casting mold cavity is made of high-quality alloy steel. The price of these raw materials is relatively high, plus the processing cost and the combined cost are very high. If due to improper heat treatment or poor heat treatment quality, the scrap or the life does not meet the design requirements, it will cause great economic losses. Therefore, pay attention to the following points during heat treatment:
- The forgings are spheroidized and annealed before being cooled to room temperature.
- Additional quenching and tempering treatment are added after roughing and before finishing. To prevent the hardness from being too high and causing processing difficulties, the hardness is limited to 25-32HRC, and stress relief tempering is arranged before finishing.
- Attention should be paid to the decarburization and increase of carbon on the surface of the cavity during heat treatment. Improper decarburization will cause mold damage and high-density cracks; carbon increase will reduce thermal fatigue resistance.
- When nitriding, it should be noted that there should be no oil stains on the nitriding surface. The cleaned surface is not allowed to be directly touched by hand, and gloves should be worn to prevent the nitrided surface from being stained with oil and causing an uneven nitrided layer.
- Between the two heat treatment processes, when the temperature of the previous process is reduced to the touch, the next process will be carried out, and it should not be cooled to room temperature.
- Pay attention to the critical points Ac1 and AC3 of the steel and the holding time during quenching to prevent austenite from coarsening. When tempering, keep the heat at 20mm/h, and the number of tempering is generally 3 times. When nitriding is present, the third tempering can be omitted.
The above are some superficial insights and analyses of HARSLE on how to improve the service life of die casting molds. In the actual production process, many factors affect the service life of die casting molds and involve a wide range of aspects. How to improve the service life of die casting molds is a complex problem. The comprehensive problem deserves further discussion and research by professional and technical personnel.
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