What are the considerations for die casting mould design?

Die-casting mould is one of the three elements of die-casting production, the correct and reasonable structure of the mould is a prerequisite for the smooth running of die-casting production, and plays an important role in ensuring the quality of the castings (under the machine pass rate).

Due to the characteristics of the die-casting process, the correct choice of process parameters is the decisive factor in obtaining high quality castings, and the mould is able to correctly select and adjust the process parameters of the premise, mould design is essentially a comprehensive reflection of the various factors expected to occur in die-casting production. If the mould design is reasonable, the actual production problems encountered in less, castings under the machine qualified rate is high. On the contrary, the mold design is unreasonable, example a casting design when the dynamic fixed mold wrapping force is basically the same, and the pouring system is mostly in the fixed mold, and placed in the pressure injection after the punch can not feed the filling die-casting machine on the production, can not be normal production, the casting has been stuck in the fixed mold. Although the cavity of the fixed model of the finish hit very light, because the cavity is deeper, still appear sticky on the fixed mold phenomenon. So in the mold design, must be a comprehensive analysis of the structure of the casting, familiar with the operation process of the die-casting machine, to understand the die-casting machine and the possibility of adjusting the process parameters, to master the filling characteristics in different cases, and consider the mold processing methods, drilling and fixed form, in order to design a realistic, to meet the production requirements of the mold.

The service life of the mould is usually through careful design and manufacture, in normal use conditions, combined with good maintenance under the natural damage, in can no longer repair and scrap before, the number of die-casting die (including the number of scrap in die-casting production).

In actual production, mould failure mainly has three forms: ① thermal fatigue cracking damage failure; ② cracking failure; ③ corrosion failure.

Causes the mould failure of many factors, both external factors (such as casting temperature high or low, whether the mould is preheated, the amount of water agent coating spraying amount, die-casting machine tonnage size is matched, die-casting pressure is too high, the internal gate speed is too fast, cooling water open not synchronized with the die-casting production, the type of casting material and composition Fe high or low, casting size and shape, wall thickness size, coating type, etc.). There are also internal causes (e.g. the metallurgical quality of the mould itself, the forging process of the billet, the rationality of the mould structure design, the rationality of the pouring system design, the internal stresses generated during the machining of the mould (electrical machining), the heat treatment process of the mould, including various fit accuracy and finish requirements, etc.).

If early failure of the mould occurs, it is necessary to find out which internal or external causes are responsible for future improvement.

① Die thermal fatigue cracking failure

Die-casting production, die repeatedly by the role of cold and heat, forming surface and its internal deformation, mutual involvement and repeated cycles of thermal stress, resulting in tissue structure two damage and loss of toughness, triggering the emergence of micro-cracking, and continue to expand, once the crack is expanded, there is molten metal liquid extrusion, coupled with repeated mechanical stress are to accelerate the expansion of the crack.

For this reason, on the one hand, die-casting must be fully preheated at the beginning of the mould. In addition, during the die-casting production process the mould must be kept in a certain working temperature range to avoid early cracking failure. At the same time, to ensure that the mould before production and manufacturing in the internal factors do not occur problems. In practice, the majority of mould failures are thermal fatigue cracking failures.

② Cracking failure

Under the action of the pressure injection force, the mould will sprout cracks at the weakest point, especially the mould forming surface scribing traces or electric machining traces are not beaten grinding light, or forming the clear corner will be the first to appear fine cracks, when the grain boundary exists brittle phase or grain coarse, that is easy to fracture. And brittle fracture when the expansion of the crack is very fast, which is a very dangerous factor for the mold failure is broken. For this reason, on the one hand, where the mould surface scratches, electrical processing traces, etc. must be played grinding light, even if it is in the pouring system parts, must also be played light. In addition, the mould material used is required to have high strength, good plasticity, good impact toughness and fracture toughness.

③Melt failure

As already mentioned, the commonly used die-casting alloys are zinc alloy, aluminium alloy, magnesium alloy and copper alloy, there are also pure aluminium die-casting, Zn, Al, Mg are more active metal elements, they have a good affinity with the mould material, especially Al is easy to bite the mould. When the mould hardness is higher, the corrosion resistance is better, and if there are soft spots on the molding surface, it is not good for corrosion resistance. However, in actual production, the corrosion is only local to the mould, for example, the parts directly flushed by the inner gate (core, cavity) are prone to corrosion, and the hardness of the soft parts are prone to aluminium alloy sticking to the mould.