Method for improving service life of die casting mould
Due to the long production cycle, large investment, and high manufacturing precision, die-casting molds have a high cost, so it is hoped that the molds will have a higher service life. However, due to the influence of a series of internal and external factors such as materials and mechanical processing, the molds are prematurely invalidated and scrapped, resulting in great waste.
The failure modes of die-casting molds are: sharp corners, corner cracking, splitting, hot cracking (cracking), wear, and erosion. The main causes of die-casting mold failure are: defects in the material itself, processing, use, maintenance and heat treatment.
First, the material itself has defects
It is well known that the conditions of use of die-casting molds are extremely harsh. Taking aluminum die casting as an example, the melting point of aluminum is 580-740°C. When used, the temperature of aluminum liquid is controlled at 650-720°C. In the case of die casting without preheating of the mold, the surface temperature of the cavity rises from room temperature to the liquid temperature, and the surface of the cavity is subjected to a great tensile stress. When molding the top piece, the cavity surface is subjected to great compressive stress. Thousands of die castings have cracks and other defects on the mold surface.
It can be seen that the conditions for die-casting are quenching. Mold materials should use hot and cold fatigue resistance, fracture toughness, high thermal stability of the hot die steel. H13 (4Cr5MoV1Si) is currently widely used materials, according to reports, 80% of foreign cavity H13 are used, and now the country is still heavily used 3Cr2W8V, but 3Cr2W8VT_art performance is not good, poor thermal conductivity, high linear expansion coefficient, The work generates a lot of thermal stress, resulting in mold cracking or even cracking, and easy to decarburization when heating, reducing the wear resistance of the mold, it is a phase out steel. Maraging steel is suitable for molds that are resistant to thermal cracking and do not require high wear resistance and corrosion resistance. Tungsten-molybdenum and other heat-resistant alloys are limited to small inserts with severe thermal cracking and corrosion. Although these alloys are brittle and notched, they have the advantage of good thermal conductivity and the need for cooling without the need to set the thickness of the water channel. Die casting die casting mold has good adaptability. Therefore, under a reasonable heat treatment and production management, H13 still has satisfactory performance.
The materials used to make the die-casting molds shall meet the design requirements from any aspect, and ensure that the die-casting molds reach the designed service life under their normal conditions of use. Therefore, prior to being put into production, a series of inspections should be conducted on the materials to prevent them from being defective, resulting in the early retirement of molds and the waste of processing costs. Common inspection methods include macroscopic corrosion inspection, metallographic inspection, and ultrasonic inspection.
(1) Macroscopic corrosion inspection. Mainly inspect the porosity, segregation, cracks, cracks, non-metallic inclusions, cracks, and joints on the surface of the material.
(2) metallographic examination. Mainly check the segregation of carbides on the grain boundary, the distribution state, the degree of crystal grains, and inclusions between grains.
(3) Ultrasound examination. Mainly check the internal defects and size of the material.
Second, the die casting mold processing, use, repair and maintenance
The problems that should be paid attention to in the design of die casting molds have been introduced in detail in the die design manual. However, when determining the shot speed, the maximum speed should not exceed 100 m/s. The speed is too high, which promotes corrosion of the mold and increased deposits on the cavity and the core; but too low can easily cause defects in the casting. Therefore, the corresponding minimum injection speeds for magnesium, aluminum, and zinc are 27, 18, and 12 m/s. The maximum shot rate of cast aluminum should not exceed 53 m/s and the average shot rate is 43 m/s.
During processing, thicker stencils cannot be superimposed to ensure their thickness. Because the steel plate is 1 times thicker and the bending deformation is reduced by 85%, the stack can only play a superposition. The same two plates with the same thickness as the veneer have 4 times the bending deformation of the veneer. In addition, special attention must be paid to ensuring concentricity in the processing of cooling waterways. If the head corners are not concentric with each other, the corners of the connection will crack during use. The surface of the cooling system should be smooth and it is best to leave no trace of machining.
EDM is more and more widely used in the mold cavity processing, but the hardened layer remains on the surface of the processed cavity. 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 at the time of processing, deep during rough machining, and shallow during finishing. Regardless of the depth, the mold surface has a great stress. If the hardened layer is not removed or the stress is eliminated, cracks, pitting, and cracking will occur on the mold surface during use. Elimination of hardened layers or removal of stress can be used: 1 removal of the hardened layer by whetstone or grinding; 2 lower stress than the tempering temperature without reducing the hardness, which can significantly reduce the cavity surface stress.
The mold should strictly control the casting process during use. Within the allowable range of the process, the temperature of the casting and casting of the aluminum liquid, the shot speed, and the preheating temperature of the mold are minimized. The preheating temperature of the aluminum die casting mold is increased from 100 to 130°C to 180 to 200°C, and the die life can be greatly improved.
