What are the materials to choose for injection molds?
Introduction to Mold Steel
Mold steel can be broadly divided into 3 categories: cold work mold steel, hot work mold steel, and plastic mold steel, which are used for forging, stamping, cutting, and die casting. Since the uses of various molds are different and the working conditions are complex, the mold steel used for the manufacture of the molds should have high hardness, strength, wear resistance, sufficient toughness, as well as high hardenability, quenchability, and other process performance according to the working conditions. Due to the different uses and complex working conditions, the performance requirements for mold steel are also different.
Cold work mold steel includes cold stamping dies, wire drawing dies, deep drawing dies, embossing dies, thread rolling dies, roll forming dies, cold heading dies, and cold extrusion dies. Cold work mold steel, according to its manufacturing working conditions, should have high hardness, strength, wear resistance, sufficient toughness, as well as high hardenability, quenchability, and other process performance. The alloy tool steel used for these purposes generally belongs to high-carbon alloy steel, with a carbon mass fraction of over 0.80%. Chromium is an important alloying element in this type of steel, and its mass fraction is usually not greater than 5%. However, for some mold steels that require high wear resistance and very small deformation after quenching, the maximum chromium mass fraction can reach 13%, and in order to form a large amount of carbides, the carbon mass fraction in the steel is also very high, up to 2.0%~2.3%. Cold work mold steel has a relatively high carbon content, and its microstructure mostly belongs to hypereutectoid steel or ledeburite steel. Commonly used steel types include high-carbon low-alloy steel, high-carbon high-chromium steel, chromium-molybdenum steel, and medium-carbon chromium-tungsten-manganese steel.
Hot work molds are mainly divided into hammer forging, die forging, extrusion, and die casting types, including hot forging dies, press forging dies, stamping dies, hot extrusion dies, and metal die casting dies. Hot work molds are subjected to huge mechanical stress during work, and must also withstand the effects of repeated heating and cooling, which causes large thermal stresses. In addition to high hardness, strength, red hardness, wear resistance, and toughness, hot work mold steel should also have good high-temperature strength, thermal fatigue stability, thermal conductivity, and corrosion resistance. Furthermore, it is required to have high hardenability to ensure uniform mechanical properties throughout the entire cross-section. For die casting mold steel, it should also have the performance to withstand repeated heating and cooling on the surface layer without cracking, and to withstand the impact and erosion of liquid metal flow. This type of steel generally belongs to medium-carbon alloy steel, with a carbon mass fraction of 0.30%~0.60%, which is hypoeutectoid steel, and some steel becomes eutectoid or hypereutectoid steel due to the addition of more alloying elements (such as tungsten, molybdenum, vanadium, etc.). Commonly used steel types include chromium-manganese steel, chromium-nickel steel, chromium steel, and chromium-tungsten steel.
Plastic molds include thermoplastic plastic molds and thermosetting plastic molds. The steel for plastic molds is required to have certain properties such as strength, hardness, wear resistance, thermal stability, and corrosion resistance. In addition, it is also required to have good process performance, such as small heat treatment deformation, good machinability, good corrosion resistance, good grinding and polishing performance, good repair welding performance, high roughness, good thermal conductivity, and stable working condition size and shape. Generally, hot work mold steel can be selected for injection molding or extrusion molding dies; cold work mold steel can be selected for thermosetting molding and dies that require high wear resistance and high strength.