（1） Steel for carburized plastic molds
In order to facilitate cold extrusion forming, Dongguan Machiko Injection Mold Factory mainly uses steel for decarburized plastic molds, which must have high plasticity and low deformation resistance in the hot state. Therefore, low carbon content is required for this type of steel. In order to improve the wear resistance of injection molds, this type of steel is generally subjected to carburization and quenching and tempering treatment after cold extrusion forming, with a surface hardness of 58~62HRC Foreign steel grades such as Sweden's 8416 and the United States' P2 and P4 are introduced below. The newly developed cold extrusion forming special steel is introduced.
New steel material used for injection molding, LJ steel has a low carbon content, excellent plasticity, and low deformation resistance. The chemical composition of the steel is shown in Table 5-22. The main alloying element is chromium, and the auxiliary elements are nickel, molybdenum, vanadium, etc. The main function of the alloying element is to improve the hardenability and carburizing ability, and increase the hardness, wear resistance, and toughness of the carburized layer. LJ steel has good forging performance and heat treatment processability. Forging process: Heating temperature is 1230 ℃, initial forging temperature is 1200 ℃, and final forging temperature is 900 ℃. Annealing process: 880 ℃ x2h, after furnace cooling to 650 ℃, air cooling is taken out of the furnace. Annealing hardness is 100-105HBW, and cold extrusion forming can be carried out smoothly. Carburization process: 930 ℃ x (6-8) h, oil quenched at 850-870 ℃, followed by low-temperature tempering at 200-220 ℃ for x2 h. After heat treatment, the surface hardness is 58-60HRC, and the core hardness is 27-29HRC, with minimal deformation during heat treatment. The carburizing speed of steel is fast, and the depth of the carburizing layer is twice that of 20 steel. The cold formability of LJ steel is similar to that of industrial pure iron, and the mold cavity formed by cold extrusion method has clear, smooth, and high accuracy. LJ steel is mainly used to replace precision plastic molds formed by cold extrusion of 10, 20 steel and industrial pure iron. Due to the deep carburizing hardening layer and high matrix hardness, there will be no surface collapse or internal wall bite of the mold cavity, and the use effect is good.

（2） Steel for hardened plastic molds
The commonly used steels for hardened plastic molds include carbon tool steel, low alloy cold work mold steel, Cr12 type steel, high-speed steel, and some hot work mold steel. The heat treatment of these steels is generally quenching and low-temperature tempering, with a few using medium or high temperature tempering. The hardness after heat treatment is usually above 45HRC. 6CrNiSiMnMoV steel is a new type of quenched plastic mold steel that has been widely used in recent years. Due to its high toughness, good hardenability and wear resistance, small quenching deformation, and low price, using this steel to replace Cr12MoV steel to manufacture large, high wear resistant, and high-precision plastic molds not only reduces costs, but also improves the service life of injection molds.

（3） Steel for pre hardened plastic molds
The steel used for injection molding is a new material, and most of the pre hardened plastic mold steel developed in China in recent years is based on medium carbon steel, with an appropriate amount of chromium, manganese, nickel, molybdenum, vanadium and other elements added. In order to solve the problem of difficulty in cutting at higher hardness, elements such as sulfur, calcium, lead, and selenium are added to the steel to change its cutting performance. The so-called pre hardened steel refers to the pre heat treated steel that has been supplied to achieve the hardness of the injection mold as used. The characteristic of this type of steel is that it can be directly machined for turning, milling, drilling, carving, and precision filing at a hardness of 30-40HRC. After precision machining, it can be directly delivered for use, completely avoiding the impact of heat treatment deformation, thereby ensuring the manufacturing accuracy of injection molds. Some pre hardened steel can be nitrided after injection molding to improve the surface hardness and wear resistance of the mold without reducing the hardness of the substrate. The following is an introduction to two typical pre hardened plastic mold steels by Dongguan Machiko Injection Mold Factory.

Phase transition point of 1.3Cr2Mo steel: Ac, 770 ℃, Ac3825 ℃; Ar,640℃、Ar3760℃; M300℃、M120℃. Forging process: heating temperature 1100~1150 ℃, initial forging temperature 1050~1100 ℃, final forging temperature ≥ 850 ℃, air cooling after forging. Annealing: Hold at 850 ℃ for 2-4 hours, then hold at 720 ℃ for 4-6 hours, and cool the furnace to 500 ℃ for air cooling. Quenching and tempering: 860-870 ℃, oil quenching, 540-580 ℃ tempering. Pre hard hardness of 30-35 HRC The room temperature mechanical properties of 3Cr2Mo steel quenched at 850 ℃ and tempered at 550 ℃ are shown in Tables 5-24 P20 steel is suitable for manufacturing large plastic molds such as casings for televisions, large tape recorders, and panel covers for washing machines. Its cutting performance and polishing performance are better than 45 steel. Under the same conditions, the surface roughness is 1-3 levels lower than 45 steel. The plastic mold for the TV casing made of this steel can be molded 200000 to 300000 times, and the injection mold is still intact.

2.3Cr2NiMo (P4410) steel 3Cr2NiMo steel is an improved version of 3Cr2Mo steel, and the actual composition of P4410 steel is consistent with the P20 steel improved 718 steel produced in Sweden. Phase transition point: Ac, 725 ℃, Ac3810 ℃; Ms280℃. The steel has a dense structure, good mirror polishing performance, and a surface roughness Ra value of 0.05-0.025 μ m. After tempering at 860 ℃, P4410 steel has good processing properties such as turning, milling, and grinding. The steel can also be quenched by flame local heating, and the local surface hardness can reach 56~62 HRC, which can extend the service life of injection molds. After surface chrome plating, the surface hardness can be increased from 370-420HV to 1000HV, improving the wear and corrosion resistance of injection molds.

3. Corrosion-resistant plastic mold steel. Most of the corrosion-resistant plastic mold steel is stainless steel. A new type of precipitation hardening stainless steel developed in China can be machined with a hardness of 32-35HRC, and after aging treatment at 460-480 ℃, good comprehensive mechanical properties can be obtained. Forging process: heating temperature 1180-1200 ℃, initial forging temperature 1150-1100 ℃, final forging temperature ≥ 1000 ℃, air cooling or sand cooling after forging. During forging, sufficient heat penetration should be achieved. When forging, light and fast hammering should be used first, and then heavy hammering can be used to prevent forging cracks. Solution treatment: 1050 ℃, air-cooled, with a hardness of 32-35HRC, suitable for cutting processing. Aging treatment: The general treatment temperature is 460 ℃, and the hardness after aging is 42-44HRC.