Pressure casting can cast complicated components having high precision, low surface roughness and good mechanical properties. Die casting mould should have the following performances.
1. High temperature strength and toughness
Die casting mould
may easily deform, even craze, when suffering from high temperature, high pressure and thermal stress during molten metal injected. Therefore, mold materials should have sufficient high temperature, toughness and higher hardness under operating temperature.
2. Good high temperature wear resistance
When molten metal is injected into mold at a high speed or strip mold after casting, higher friction may be occurred. For the service life of mold, the mold should have higher wearing resistance under high operating temperature. Die casting for used for mass continuous production should maintain its high hardness under high temperature for a long time. Moreover, it cannot stick mold or cause oxide skin. For this, mold should have good oxidation resistance and temper resistance.
3. Good thermal fatigue property
The surface of die casting mould constantly swells, shrink and cause alternating thermal stress when it is repeatedly heated by high temperature or cooled. When stress exceeds elastic limit of mold materials, reduplicative plastic deformation is caused, resulting in thermal fatigue further. Meanwhile, the surface of mold can also gradually cause micro cracks when it is corroded and oxidized by molten metal for a long time. In most situations, thermal fatigue is the most important factor having influences on service life of mold.
4. High melting damage resistance
With die casting machine upsizing, die casting pressure is also growing, having already improved from low pressure 20~30MPa to high pressure 150~500MPa. High temperature high pressure casting can cause obvious molten damage, and the mold should have higher resistance for this. The mold materials must have higher high temperature strength, lower affinity to molten metal and low surface roughness of mold. The materials also should be accompanied with appropriate oxidation film, nitride layer and other protective layers while decarburized layer should not exist.
5. Good hardenability and less deformation of heat treatment
The common manufacturing mold of die casting mold is that carve materials in annealing into cavity, then heat treating it to the required hardness, or heat treat the materials to required hardness firstly, then, carve into cavity. Carving cavity before heat treatment can acquire high hardness and strength, and cannot easily cause melting and thermal fatigue. No matter which heat treatments are used, acquiring homogeneous hardness is necessary. Particularly for carving cavity before heat treatment, materials have less deformation of heat treatment, which is particularly important for mold having large dimension.
6. Better machinability and grindability
Cavity of die casting mold is formed through cutting. For this, mold materials should have better machinability. It must be pointed that materials having good abrasive resistance while have poorer machinability, such as die steel. Although it is in annealing, matrix is still hard. It is difficult for being cut when it is added with hard carbide.
For smooth die castings, the surface roughness of cavity of mold should be low. So, mold materials should have better polishing performance.
7. Internals of materials should not have impurities and defects
The internals of materials should be homogeneous, having no defects and little directivity. Impurities cannot only influence cracks, strength and thermal fatigue of mold, but also influence deformation of heat treatment.