Method for improving the service life of die casting mold
Due to the long production cycle, large investment, and high manufacturing precision, the die-casting mold has a high cost, so it is desirable that the mold has a high service life. However, due to a series of internal and external factors such as materials and machining, the mold is prematurely invalidated and scrapped, resulting in great waste.
The failure modes of die-casting molds mainly include: sharp corners, cracks at the corners, splitting, hot cracks (cracks), wear, erosion, and the like. The main causes of failure of the die-casting mold are: defects in the material itself, processing, use, maintenance, and heat treatment.
First, the defects of the material itself
It is well known that the conditions of use of die casting molds are extremely poor. Taking an aluminum die-casting mold as an example, the melting point of aluminum is 580-740 ° C, and when used, the temperature of the aluminum liquid is controlled at 650-720 ° C. Die casting without preheating the mold, the surface temperature of the cavity rises from room temperature to liquid temperature, and the surface of the cavity is subjected to great tensile stress. When the top part is opened, the surface of the cavity is subjected to extremely large compressive stress. After thousands of times of die casting, cracks and other defects are formed on the surface of the mold.
It can be seen that the conditions of die casting use are hot and cold. The mold material should be hot work die steel with high thermal fatigue resistance, fracture toughness and high thermal stability. H13 (4Cr5MoV1Si) is a widely used material. According to reports, 80% of foreign cavities use H13. Nowadays, 3Cr2W8V is still widely used in China, but 3Cr2W8VT_ has poor performance, poor thermal conductivity and high linear expansion coefficient. A large thermal stress is generated in the work, which causes the mold to crack or even rupture, and is easy to decarburize when heated, and reduces the anti-wear performance of the mold, so it is a steel type that is eliminated. Maraging steel is suitable for molds that are resistant to heat cracking and that do not require high wear resistance and corrosion resistance. Heat-resistant alloys such as tungsten and molybdenum are limited to small inlays with hot cracking and corrosion. Although these alloys are brittle and notched sensitive, they have the advantage of good thermal conductivity and thicker water channels that require cooling. Die-casting die-casting molds have good adaptability. Therefore, under reasonable heat treatment and production management, H13 still has satisfactory performance.
The material of the die-casting mold should meet the design requirements in any aspect to ensure that the die-casting mold reaches the design service life under its normal use conditions. Therefore, before putting into production, a series of inspections should be carried out on the materials to prevent defective materials, resulting in early scrapping of the mold and waste of processing costs. Commonly used inspection methods include macroscopic corrosion inspection, metallographic examination, and ultrasonic inspection.
(1) Macroscopic corrosion inspection. The main materials are examined for porosity, segregation, cracks, cracks, non-metallic inclusions, and hammer cracks and joints on the surface.
(2) Metallographic examination. Mainly check the segregation, distribution state, crystal grain size and intergranular inclusions of carbides on the grain boundary of the material.
(3) Ultrasound examination. Mainly check the defects and size inside the material.
Second, the processing, use, maintenance and maintenance of die-casting mold
The problems that should be paid attention to in the design of die-casting molds have been detailed in the mold design manual, but the maximum speed should not exceed 100m/s when determining the injection speed. The speed is too high, causing corrosion of the mold and increased deposits on the cavity and core; however, too low a defect in the casting. Therefore, for magnesium, aluminum, zinc, the corresponding minimum injection speed is 27, 18, 12m / s, the maximum injection speed of cast aluminum should not exceed 53m / s, the average injection speed is 43m / s.
Thicker stencils cannot be layered to ensure their thickness during processing. Because the thickness of the steel plate is 1 times and the amount of bending deformation is reduced by 85%, the laminate can only act as a superposition. The bending deformation of the two plates of the same thickness as the veneer is four times that of the veneer. In addition, when processing the cooling water channel, special attention should be paid to ensuring the concentricity in the two-sided processing. If the corners of the head are not concentric with each other, the corners of the joint will crack during use. The surface of the cooling system should be smooth, preferably without machining marks.
EDM is more and more widely used in mold cavity machining, but the surface of the cavity after processing has a hardened layer. This is due to the carburizing and quenching of the mold surface during processing. The thickness of the hardened layer is determined by the current intensity and frequency during processing, deeper during roughing, and shallower during finishing. Regardless of the depth, the mold surface has great stress. If the hardened layer is not removed or the stress is removed, cracks, pitting and cracking will occur on the surface of the mold during use. To eliminate the hardened layer or to remove stress, it is possible to use: 1 to remove the hardened layer with oil stone or grinding; 2 to lower the stress below the tempering temperature without lowering the hardness, which can greatly reduce the surface stress of the cavity.
The casting process should be strictly controlled during the use of the mold. Within the scope of the process, try to reduce the casting temperature of the aluminum liquid, the injection speed, and increase the preheating temperature of the mold. The preheating temperature of the aluminum die-casting mold is increased from 100 to 130 ° C to 180 to 200 ° C, and the mold life can be greatly improved.