1, sufficient surface hardness and wear resistance.
The hardness of plastic mold is usually below 50-60HRC, and the mold after heat treatment should have enough surface hardness to ensure the mold has enough rigidity. Because the filling and flowing of plastic will produce great compressive stress and friction, it is required that the mold maintain the stability of shape accuracy and size accuracy, and ensure that the mold has sufficient service life. The wear resistance of die depends on the chemical composition and heat treatment hardness of steel, and improving the hardness of die is beneficial to improve its wear resistance.
2. Excellent machinability
Most plastic molding dies need cutting and fitter repair besides EMD machining. In order to prolong the service life of tools, improve the cutting performance and reduce the surface roughness, the hardness of steel used for plastic molds must be appropriate.
3, good polishing performance
High-quality plastic products require a small roughness value of the cavity surface. For example, the surface roughness of injection mold cavity should be less than Ra 0. 1 ~ 0.25, and the optical surface should be Ra.
4. Good thermal stability
The parts of injection mold are often complex in shape and difficult to process after quenching, so try to choose the ones with good thermal stability. When the mold is processed after heat treatment, the linear expansion coefficient is small, the heat treatment deformation is small, the dimensional change rate caused by temperature difference is small, the metallographic structure and the mold size are stable, and the processing can be reduced or stopped, thus ensuring the requirements of mold size accuracy and surface roughness.
Grade 45 and 50 carbon steels have certain strength and wear resistance, and are often used as mold base materials after quenching and tempering. High carbon tool steel and low alloy tool steel have high strength and wear resistance after heat treatment, and are mostly used for forming parts. However, high-carbon tool steel is only suitable for manufacturing small-sized and simple-shaped parts because of its large deformation during heat treatment.
With the development of plastic industry, the requirements for the complexity and accuracy of plastic products are getting higher and higher, and higher requirements are also put forward for mold materials. To manufacture complex, precise and corrosion-resistant plastic molds, pre-hardened 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, we should also consider anti-scratch and anti-gluing. If there is relative movement between two surfaces, materials with the same structure should be avoided as far as possible. Under special circumstances, one surface can be electroplated or nitrided to make the two surfaces have different surface structures.