According to the molding method, the plastic can be divided into thermoplastic and thermosetting plastics. Thermosetting plastics, such as bakelite powder, are pressed and permanently molded under heating and pressure. The bakelite mold is periodically subjected to pressure and continuously heated at 150-200 degrees. Thermoplastic plastics such as polyvinyl chloride are usually molded by injection molding. The plastic is heated in a separate heating chamber and then injected in a softened state to a colder one. In the mold cavity, pressure is applied to make it chill and mold.
 
The cost of manufacturing plastic molds is high, and the cost of materials only accounts for a small part of the cost of the molds. Therefore, when selecting steel materials, priority should be given to steel grades with good process performance, stable performance and long service life. The following requirements are specifically made for plastic molds:
1. Excellent machining performance: The geometry of the cavity of the plastic mold is very complex, and it is required to machine deep-drilled, deep-groove and other difficult-to-machine parts. The steel must have easy-cutting performance. When selecting steel, if other conditions are satisfied This is the basis for the final selection.
2. Excellent polishing performance: regardless of the plastic mold, the allowable surface roughness of the cavity is minimal, almost all require a mirror gloss, and to achieve a mirror gloss, the steel must have a good hardness, the first steel in the miscellaneous The material should be as few as possible, and there should be no bubbles, and the microstructure is uniform. In the molds used in mass production, the surface roughness is also an important condition.
3. Good surface corrosion processability: Many molds require that the surface of the molded plastic part is a pattern of various other materials, such as leather, silk, cloth, fine patterns, etc., which requires the fine and uniform texture of the steel, otherwise the corrosion After the poor results.
4. Good wear resistance and toughness: Plastic molds are subjected to thermal alternating load for a long time, and they are subject to long-term friction. For example, consider only the side of friction, and do not consider the side of the alternating heat load, use cold stamping. When the mold is made of high-carbon, high-alloy steel, cracks due to insufficient toughness are inevitable.
5. Quenching performance is good, deformation is small: most of the cavity part of the plastic mold must be heat treatment, which requires the steel has a good hardenability and very small deformation, general thermoplastic molding die, using quenching, pre Hardening and other treatment can be, while the thermosetting plastic molding mold must be hardened.