Plastic can be divided into thermoplastic and thermosetting plastics according to the molding method. Thermosetting plastics, such as bakelite powder, are pressed and permanently molded under heating and pressure. Bakelite molds are periodically subjected to pressure and continue to be heated at 150-200 degrees. Thermoplastic plastics such as PVC are usually molded by injection moulds. Plastics are heated in a separate heating chamber and then injected into a cooler mold chamber in a softening state to exert pressure so that they are cold-hardened.

The cost of making plastic molds is high, and the cost of materials only accounts for a small part of the cost of molds. Therefore, steel grades with good technological properties, stable performance and long service life should be preferred when choosing steel products. Specifically, the requirements for plastic molds are as follows:

1. Excellent machinability: the geometry of plastic mold cavity is very complex, and requires the processing of deep holes, deep ditches and other difficult parts, steel must have free-cutting performance, in the selection of steel, if other conditions can be met, this is the basis for the final timing.

2. Excellent polishing performance: no matter what kind of plastic mold, the allowable surface roughness of its cavity is very small, almost all require to be able to achieve a mirror gloss, and to achieve a mirror gloss, steel must have good hardness, first of all, steel impurities to be as little as possible, and can not have bubbles, and the microstructure is uniform, in large quantities. The surface roughness of dies used in production is also important.

3. Good surface corrosive machinability: many molds require the surface of the plastic parts formed for a variety of other material patterns, such as leather, silk, cloth patterns, fine patterns, and so on, which requires that the texture of steel is fine and uniform, otherwise the effect of corrosion is not good.

4. Wear resistance and toughness are good: plastic mold is long-term bear heat alternating load, but also long-term bear friction, if only consider the friction side, without considering the alternating heat load, the use of cold stamping die of high-carbon high-alloy steel manufacturing, will inevitably lead to lack of toughness and cracking.

5. Hardening performance is good, deformation is small: most of the cavity part of the plastic mold must be heat-treated, which requires the steel has a good hardenability and very small deformation, thermoplastic molding mold, using tempering, Pre-hardening and other treatment can be used, and thermosetting plastic molding mold must be hardened.