Choosing materials for mold making
is very critical. If the materials are not excellent enough, workpiece made by mold has poor performance. So, materials used for mold processing should meet the following six properties:
1. Abrasive resistance
When blanks deforms plastically in cavity of mold, blanks are flowing and sliding along the surface of cavity. Blanks cause drastic friction between surface of cavity and blanks, resulting in mold
losing efficacy due to abrasion. So, abrasive resistance is one of the most basic and important functions. Rigidity is the main factor having influence on abrasive resistance. In general, the higher of hardness of mold parts is, the less abrasion loss is and the better abrasive resistance is. In addition, abrasive resistance is also related to categories, quantities, shapes, dimensions and distributions of carbides contained in materials.
2. Strong tenacity
Most of working conditions of molds are very wicked. Some of molds are usually suffering from larger impact load, which results in brittle rupture further. To prevent mold parts from rupturing suddenly during operation, the mold should have higher intensity and tenacity. Tenacity of mold mainly depends on carbon contents, grain size and texture of materials.
3. Fatigue break
During operation, mold usually leads to fatigue break under long-term action of cyclic stress. It is shown that little energy impacts parts of fatigue break, tensile fatigue break, contact fatigue break and bending fatigue break. Fatigue break of mold mainly depends on intensity, tenacity, hardness and content of impurities in materials.
4. High temperature performance
When working temperature of mold is higher, hardness and intensity of mold will be decreased leading to early abrasion of mold or lose efficacy for causing plastic deformation. Therefore, mold materials should have higher temper stability, ensuring that mold has higher hardness and intensity under high working temperature.
5. Cold and hot fatigue resistance
Some molds are located in a status of being heated and cooled repeatedly, so that surface of cavity is impacted by tensile force, pressure and varying stress, which cause carzing and spalling on surface, increasing friction force, hindering plastic deformation and reducing the dimensional precision, then resulting in mold losing efficacy. Thermal fatigue is one of the main shapes that hot working mold loses efficacy. For this, hot working mold should have higher cold and hot fatigue resistance.
6. Corrosion resistance
When some molds such as plastic injection mold work, chlorine, fluorine and other factors are contained in plastic and they dissolve out HCI, HF and other strong corrosive gases after being heated. Those gases corrode surface of mold cavity, increasing the surface roughness and intensifying abrasion and resulting in losing efficacy further.