Die casting production, mold repeatedly by the effect of cold heat, forming surface and internal deformation, involved and repeated cycle of thermal stress, lead to tissue structure damage and loss of toughness, cause the emergence of micro cracks, and continue to expand, once the crack expand, and molten metal squeeze, and repeated mechanical stress make crack accelerated expansion. For this reason, on the one hand, the mold must be fully preheated at the beginning of the die-casting. In addition, the mold must be kept in a certain working temperature range in the die casting process to avoid early cracking failure. At the same time, to ensure that the mold before the production and manufacturing internal causes do not have problems. In the actual production, most of the mold failure is thermal fatigue cracking failure.
Under the action of the pressure ejection force, the mold will produce cracks at the weakest place, especially the marking marks or electrical processing marks on the mold forming surface have not been polished, or the clear corner of the molding will first appear fine cracks, when the brittle phase or the grain boundary is large, it is easy to fracture. The expansion of the crack when brittle fracture is fast, which is a very dangerous factor for the fracture failure of the mold. For this reason, on the one hand, the scratches on the surface of the mold must be polished, even if it is in the pouring system, it must be polished. In addition, the mold materials used requires high strength, good plasticity, impact toughness and fracture toughness.
As mentioned above, the commonly used die casting alloys are zinc alloy, aluminum alloy, magnesium alloy and copper alloy, as well as pure aluminum die casting, Zn, Al, Mg are more active metal elements, they have good affinity with mold materials, especially Al easy to bite mold. When the mold hardness is high, the corrosion resistance is good, and if the molding surface is soft point, the corrosion resistance is adverse.
Cause mold failure of many factors, both external factors (example casting temperature height, mold by preheating, water coating spraying amount, whether the size of the die casting machine tonnage matching, die casting pressure is too high, the pouring speed is too fast, cooling water open not synchronized with die casting production, casting material types and composition of Fe, casting size, wall thickness, coating type, etc.). There are also internal causes (such as the metallurgical quality of the mold itself material, the forging process of the billet, the rationality of the mold structure design, the rationality of the pouring system design, the internal stress generated by the mold machine (electrical processing) processing, the heat treatment process of the mold, including each Mix accuracy and finish requirements, etc.). If the mold fails early, it is necessary to find out which internal or external causes are, for future improvement. However, in the actual production, dissolution is only a local part of the mold. For example, the direct erosion of the inner gate (type core and cavity) is easy to dissolve, and the adhesive mold of aluminum alloy is easy to appear where the hardness is soft.