Bearing mesh: heat treatment of plastic mold steel

      Plastic products have been widely used in industry and daily life. Plastic molds have been developed in the direction of precision and large-scale, and the performance requirements for plastic mold steel are getting higher and higher. The properties of the molded steel should be determined according to the type of plastic, the use of the product, the molding method and the size of the production batch. Generally, plastic mold steel has good comprehensive performance. The strength and toughness of the mold material are not as high as that of cold work and hot work, but the processing performance of the material is high, such as simple heat treatment process, small deformation or no deformation. The pre-hardened state has good cutting performance, excellent mirror polishing performance and pattern etching performance.

The basic properties required for plastic mold steel are as follows:
1) Comprehensive mechanical properties Molding molds are subject to different temperatures, pressures, erosions and wear during operation. Therefore, the material of the mold material is required to be uniform, no mesh and banded carbide appear, and the heat treatment process should have a small tendency of oxidation, decarburization and distortion, and should have a certain strength after heat treatment. In order to ensure sufficient wear resistance, many plastic molds are tempered and then subjected to surface strengthening treatment such as nitriding or chrome plating.
2) Machining performance For large, complex and precise injection molds, it is usually pre-hardened to between 28 and 35 HRC, and then cut and ground to the required size and precision before being used. Defects in oxidation and decarburization. Therefore, the free-cutting elements S, Co and rare earth are often added to improve the pre-hard cutting performance.
3) Mirror surface processing The surface roughness of plastic products such as optical discs and plastic lenses can be very high, which is mainly ensured by the roughness of the mold cavity. Generally, the roughness of the mold cavity is one level higher than that of the plastic product. The mirror finish of die steel is related to the purity, texture, hardness and mirror finish of steel. High hardness, small and uniform microstructure, and low non-metallic inclusions are all beneficial to the improvement of mirror polishing. Plastic mold steels with high mirror polishing requirements are often vacuum smelted and vacuum degassed.
4) Pattern etching performance Some plastic products require a clear and full-fledged pattern on the surface. Large leather embossed plates in the leather industry require good pattern lithography properties. The requirements of the pattern lithography performance are similar to those of the mirror polishing. The purity of the steel is high, the structure is dense, and the hardness is high.
5) Corrosion resistance When a resin containing chlorine and fluorine and a flame retardant are added to the ABS resin, a corrosive gas is released during the molding process. Therefore, such plastic molds should be made of corrosion-resistant plastic mold steel, or chrome-plated, or nickel-phosphorus amorphous coating.
In addition, plastic mold steels are required to have good pre-hardening properties, high cold-press properties and repair welding properties.

       Plastic mold steel can be divided into: carburized type, pre-hardened type, corrosion-resistant type, age-hardening type and cold extrusion type according to chemical composition and performance. Their heat treatment is described as follows.
1. Heat treatment of carburized plastic mold steel
Plastic mold parts that are subject to high impact require a hard surface and a tough core. Usually, a carburizing process or a carbonitriding process is used to achieve this purpose.
Generally, carburized parts can be alloyed carburized steel of structural steel, and the heat treatment process is basically the same as that of structural parts. For plastic mold forming parts with high surface quality requirements, special steel should be used. The key to heat treatment is to select advanced carburizing equipment and strictly control the process to ensure the microstructure and performance requirements of the carburized layer.
The specification of carburizing or carbonitriding process can refer to the process standard of heat treatment industry: JB/T3999-1999 "Carburizing and carbonitriding quenching and tempering treatment of steel".
Commonly used carburized rubber mold steels are 20, 20Cr, l2CrNi2, l2CrNi3, l2Cr2Ni4, 20Cr2Ni4 steel.
       1) l2CrNi3 steel l2CrNi3 steel is a traditional medium hardenability alloy carburized steel with a cold forming property of medium. The steel has a low carbon content and is added with alloying elements such as nickel and chromium to improve the hardenability of the steel and the toughness of the carburized layer. Especially after the addition of nickel, the solidification of the steel can be significantly improved while solid solution strengthening is produced. The steel has good forging performance, the forging heating temperature is 1200 ° C, the initial forging temperature is 1150 ° C, the final forging temperature is greater than 850 ° C, and the forging is slow and cold.
l2CrNi3 steel is mainly used for cold extrusion forming complex shallow cavity plastic molds, or for large and medium-sized plastic molds for cutting and forming. The plastic mold is manufactured by cutting, and in order to improve the cutting performance, the mold blank needs to be normalized. The cavity is formed by cold extrusion, and the softening annealing process must be performed after forging.
When 12CrNi3 steel adopts gas carburizing process, the heating temperature is 900~920°C, and the heat preservation is 6~7h, and the carburized layer of 0.9~1.0mm can be obtained. After carburizing, it can be pre-cooled to 800~850°C for direct oil cooling or air quenching. After quenching, the hardness of the surface layer can reach 56~62HRC, and the hardness of the core is 250~380HBS.
2) 20Cr2Ni4 steel 20Cr2Ni4 steel is a high-strength alloy carburized steel with good comprehensive mechanical properties, and its hardenability and toughness exceed l2CrNi3 steel. The forging performance of the steel is good, the forging heating temperature is 1200 ° C, the initial forging temperature is 1150 ° C, the final forging temperature is greater than 850 ° C, and the forging is slow and cold.https://www.supplyforever.com/