Plastic mold steel with P represented mainly divided into five categories. Carburizing plastic mold steel: P1, P2, P3, P4, P5, P6. Such low carbon steel, mainly the United States and by early extrusion molding method, requires cold plastic, and has a high performance extrusion, molding surface after carburizing to improve the surface hardness and long service life. Quenched and tempered plastic mold core: P20, P21. Currently the amount of P20 plastic mold large, has become the main, most use in the pre-hard state. Carbon alloy tool steel for a thermosetting plastic. Steel are H13, and L2 and S7, O1 and A2 are also applied. Stainless steel for corrosion resistance demanding plastic, mainly steel are 420,414L, 440,416. ◆ maraging steel is the result of the aging process to obtain high performance.