Selection Skills of Plastic Mold Materials

The working conditions of plastic molds are different from those of cold stamping dies. Generally, they must work at 150 ℃ to 200 ℃. In addition to being subject to certain pressure, they must also be subject to temperature. The basic performance requirements of steel for plastic mould are summarized as follows according to different use conditions and processing methods of plastic mould:
1、 Excellent machinability
In addition to EMD processing, most plastic molding dies also need to be cut and fitter repaired. In order to prolong the service life of cutting tools, enhance the cutting performance and reduce the surface roughness, the hardness of steel used for plastic moulds must be appropriate.
2、 Good thermal stability
The shape of plastic injection mold parts is often complex and difficult to process after quenching. Therefore, the parts with good thermal stability should be selected as far as possible. When the two-color mold is formed and processed after heat treatment, the linear expansion coefficient is small, the heat treatment deformation is small, the size change rate caused by temperature difference is small, the metallographic structure and mold size are stable, and processing can be reduced or no longer conducted, which can ensure the mold size accuracy and surface roughness requirements.
3、 Good polishing performance
High quality two-color injection molded plastic products require a small roughness value of the cavity surface. For example, the surface roughness value of the injection mold cavity is required to be less than Ra0.1~0.25, and the optical surface is required to be Ra<0.01 nm. The cavity must be polished to reduce the surface roughness value. The steel selected for this purpose requires less impurities, uniform microstructure, no fiber directivity, and no pitting or orange peel defects during polishing.
4、 Sufficient surface hardness and wear resistance
The hardness of plastic mold is usually below 50-60HRC, and the mold after heat treatment should have sufficient surface hardness to ensure that the mold has sufficient stiffness. Because the filling and flowing of plastic must bear large pressure stress and friction force during the work of the mold, the mold is required to maintain the stability of shape accuracy and size accuracy to ensure that the mold has sufficient service life. The wear resistance of the die depends on the chemical composition and heat treatment hardness of the steel, so increasing the hardness of the die is conducive to enhancing its wear resistance.
5、 Grade 50 carbon steel has certain strength and wear resistance, and is mostly used for mold base materials after quenching and tempering treatment. High carbon tool steel and low alloy tool steel have high strength and wear resistance after overheating treatment, and are mostly used for forming parts. However, high carbon tool steel is only suitable for manufacturing small and simple shaped parts due to its large heat treatment deformation.
With the development of plastic industry, the requirements for the complexity and precision of plastic products are becoming higher and higher, and the requirements for two-color injection mold materials are also higher. For complex, precise and corrosion-resistant plastic molds, prehardened steel (such as PMS), corrosion resistant steel (such as PCR) and low carbon maraging steel (such as 18Ni-250) can be used, all of which have good cutting, heat treatment and polishing properties and high strength.
In addition, when selecting materials, it is also necessary to consider the prevention of scratches and gluing. If there is relative movement between the two surfaces, try to avoid selecting materials with the same structure. Under special circumstances, one side can be plated or nitrided to make the two surfaces have different surface structures.