Mold steels can be divided into cold work mold steels, hot work mold steels and plastic mold steels. Plastic mold steel has accounted for more than 50% of the consumption of mold steel, has been used for the manufacture of plastic mold materials with different requirements. Therefore, many countries have formed their own codes of plastic mold steel, which is commonly used for the AISI P20-P39 low carbon, high alloy plastic mold Steel), German DIN 1.2738, 1.2311, Japan NAK301 and so on. These plastic mold steels can be divided into carbon structural steel, carburizing plastic mold steel, pre-hardening plastic mold steel, aging hardening plastic mold steel, corrosion-resistant plastic mold steel, easy cutting plastic mold steel, overall hardening plastic mold steel, maraging steel and mirror polished plastic mold steel.
Carbon plastic mold steel
Carbon plastic mold steel is characterized by good processing performance, low price, wide source of raw materials and so on. It is widely used in small molds with simple shapes or molds with low precision and short service life, such as C1045, C1050, etc., with low S and P content, good purity, the narrow fluctuation range of carbon content, and more stable mechanical properties. For thermosetting plastic molds with high hardness requirements and small size, high carbon tool steels with WC = 0.7% ~ 1.3% are generally used.
Carburized plastic mold steel
Some plastic mold cavities with complex shapes adopt the cold extrusion method, which eliminates the cutting process of the mold cavity and can be used for mass production of the mold. Mold processing after carburizing, quenching, low temperature tempering with high hardness, the high wear resistance of the surface and good toughness of the core organization, can make a variety of requirements for high wear resistance surface, good toughness of the core of the mold.
Carburized plastic mold steel has a very low carbon content, generally, 0.1% ~ 0.2%, its plastic deformation resistance is very small, softening annealing hardness is less than 160HBW, complex cavity less than 130HBW. Typical material is Sweden’s 8416, the United States of P2 and P4, China 12CrNi3A and 12Cr2Ni4A, 20Cr2Ni4A, etc., can be used to manufacture complex shape and high load-bearing plastic forming mold.
Prehardened plastic mold steel
Prehardened plastic mold steel is a kind of carbon low alloy steel with wide applications, forging steel block after heat treatment to the mold in advance when using the hardness of 23 ~ 48 (HRC) to avoid due to heat treatment of die deformation and crack problems, and is suitable for manufacturing of large and medium and high precision plastic mold, effectively avoid the deformation in the process of quenching heat treatment.
In order to improve the machining property of prehardening plastic die steel, the addition of free cutting elements such as S, Ca, Pb and Sb significantly improve the cutting property of steel under higher hardness. Prehardened free-cutting plastic mold steels can be used not only for the manufacture of large and medium precision injection molds but also for the manufacture of precision and complex cold work molds. Typical steel materials are P20, P21, 4240, 445 and ect.
Age hardening plastic mold steel
Although the pre-hardened plastic mold steel solves the problem of mold deformation during heat treatment, its high hardness increases the difficulty of processing. At this time, the age-hardening plastic mold steel is developed. This kind of steel has a low carbon content and the addition of Ni, Al, Ti, Cu, Mo and other alloying elements, the die blank is processed under low hardness after solid solution. After forming, aging treatment is processed. Because the aging temperature is low, the deformation is small and regular, age hardening plastic mold steel is suitable for the manufacture of complex shape, high precision, super mirror and large plastic molds, especially for thermoplastic products.
The aging hardening plastic mold steels include Martensitic aging hardening steels and precipitation hardening steels. Typical material such as AISI 18 Ni (200), 18 Ni (250), 18 Ni (300), 18Ni (350), China’s PMS (LNi3Mn2CuAlMo), 06Ni (06Ni6CrMoVtial), 25CrNi3MoAl, Sm2 (20CrNi3AlMnMo), etc.
Corrosion-resistant plastic mold steel
Some plastic products such as PVC, fluorine plastic, flame retardant plastic and so on in the processing process can corrode the mold, at this time we can use Martensitic stainless steel and precipitation hardening stainless steel. Low requirements of plastic products can be chromed on the surface of the mold protective layer, but most cases need to use corrosion-resistant steel, commonly used corrosion resistant plastic die steel: 420, 420 SS, 420J1, 420J2, 440C, etc., this kind of steel is suitable for manufacturing requirements of high wear resistance, high precision and corrosion resistance of plastic mold, can greatly prolong the service life of the mold.
Magnetic plastic products give birth to the development of non-magnetic plastic die steel, austenitic type die steel through aging hardening treatment can meet the requirements of hardness, strength and low magnetic permeability, such as Nak301 and Hitachi HPM75, etc. Considering the complex structure and shape of plastic mold, many varieties, low surface roughness, manufacturing difficulty, so in the choice of plastic mold, steel should comprehensively consider its working conditions, failure, performance and other aspects to improve the service life and manufacturing cost of plastic mold.