Die casting mould failure causes and maintenance of die casting moulds

Die-casting die due to the long production cycle, investment, manufacturing precision, so the cost is high, so hope that the die has a high service life. However, due to the influence of a series of internal and external factors such as materials and machining, resulting in premature failure and scrapping of the mould, resulting in a great waste.

Die casting die failure form mainly have: sharp corner, corner cracking, splitting, thermal cracking (cracking), wear, erosion, etc.. The main reasons for die-casting mould failure are: defects in the material itself, processing, use, maintenance and heat treatment problems.

First, the material itself has defects

As we all know, the die-casting die use conditions are extremely poor. Take the aluminum die-casting mold for example, the melting point of aluminum is 580-740 ℃, when using, the aluminum liquid temperature control in 650-720 ℃. In the case of die-casting without preheating the mould, 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. The cavity surface is subjected to great compressive stress when the mould is opened and the top part is opened. Thousands of times after the die-casting, the surface of the mould will produce cracking and other defects.

It can be seen that the die-casting conditions are hot and cold. H13 (4Cr5MoV1Si) is a widely used material, according to the introduction, 80% of foreign cavities are using H13, now the domestic still use a lot of 3Cr2W8V, but 3Cr2W8VT_Art performance is not good, thermal conductivity is very poor, high coefficient of linear expansion, the work of a large Thermal stress, resulting in cracking and even rupture of the mold, and easy decarburization when heated, reducing the mold wear resistance, and therefore belong to the elimination of steel. Maraging steel is suitable for heat cracking and wear resistance and corrosion resistance requirements of the mold is not high. Tungsten molybdenum and other heat-resistant alloys are limited to thermal cracking and corrosion of the more serious small inserts, although these alloys are brittle and notch sensitivity, but the advantage of good thermal conductivity, the need for cooling and can not set up waterways for thick die-casting die casting die has good adaptability. Therefore, under reasonable heat treatment and production management, H13 still has satisfactory performance.

The material used in the manufacture of die casting moulds should meet the design requirements in every respect to ensure that the die casting mould reaches its design life under its normal conditions of use. Therefore, before being put into production, a series of checks should be carried out on the material to prevent defective material, causing early scrapping of the mould and waste of processing costs. Common means of inspection are macro-corrosion inspection, metallographic inspection, ultrasonic inspection.

(1) Macro-corrosion inspection. Mainly check the porosity of the material, segregation, cracking, cracks, non-metallic inclusions and surface hammer cracks, joints.

(2) Metallographic examination. Mainly check the material grain boundary carbide segregation, distribution state, crystal material degree and intergranular inclusions.

(3) Ultrasonic inspection. Mainly to check the internal defects and size of the material.

Second, the die-casting mold processing, use, repair and maintenance

Die design manual has been detailed in the die-casting mold design should pay attention to the problem, but in determining the speed of pressure injection, the maximum speed should not exceed 100m / S. Speed is too high, prompting mold corrosion and cavity and core on the increase in deposits; but too low easy to make castings produce defects. Therefore, for magnesium, aluminum, zinc corresponding to the lowest pressure injection speed of 27, 18, 12m / s, cast aluminum maximum pressure injection speed should not exceed 53m / s, the average pressure injection speed of 43m / s.

During processing, thicker formwork cannot be guaranteed in a stacked way to ensure its thickness. Because the steel plate is 1 times thicker, the bending deformation is reduced by 85% and the stack can only act as a superimposed layer. The bending deformation of 2 plates of the same thickness as the single plate is 4 times greater than that of the single plate. In addition, when processing the cooling water channel, special attention should be paid in the processing of both sides to ensure concentricity. If the heads are cornered and not concentric with each other, then the corners of the connections will crack during use. The surface of the cooling system should be smooth, preferably leaving no machining marks.

EDM is becoming more and more widely used in mould cavity machining, but the surface of the cavity is left with a hardened layer after machining. This is caused by the self-carburising and hardening of the mould surface during machining. The thickness of the hardened layer is determined by the intensity and frequency of the current during machining, and is deeper for rough machining and shallower for finishing. Regardless of the depth, the mould surface is extremely stressed. If the hardened layer is not removed or the stresses are removed, cracking, pitting and cracking will occur on the surface of the mould in the course of use. Removal of the hardened layer or stress relief can be achieved by:

① removing the hardened layer with an oil stone or grinding;

②De-stressing at a temperature lower than the tempering temperature without reducing the hardness, which can substantially reduce the stress on the surface of the mould cavity.

The casting process should be strictly controlled during the use of the mould. Within the scope of the process permitting, try to reduce the burning casting temperature of the aluminium liquid, the speed of pressure injection, and improve the preheating temperature of the mould. Aluminum die-casting mold preheating temperature from 100 ~ 130 ℃ to 180 ~ 200 ℃, mold life can be substantially increased.