The working conditions of plastic molds are different from those of cold stamping molds, which generally require working at temperatures ranging from 150 degrees Celsius to 200 degrees Celsius. In addition to being subjected to certain pressure, they also have to withstand temperature effects. Based on the different usage conditions and processing methods of plastic forming molds, the basic performance requirements of steel for plastic molds are roughly summarized as follows:
1、 Excellent cutting performance
Most plastic molding mold accessories, such as thimbles, driver barrels, limit clamps, inclined top sliding seats, secondary ejectors, and telescopic covers. In addition to EMD processing, certain cutting and fitter repairs are also required. To extend the service life of cutting tools, enhance cutting performance, and reduce surface roughness, the hardness of steel used for plastic molds must be appropriate.
Picture of dual color injection molding
2、 Good thermal stability
The shape of plastic injection mold parts is often complex and difficult to process after quenching. Therefore, it is recommended to choose parts with good thermal stability as much as possible. When the dual color mold is formed and processed through heat treatment, due to low linear expansion coefficient, small heat treatment deformation, small size change rate caused by temperature differences, stable metallographic structure and mold size, it can be reduced or no longer processed to ensure the accuracy of mold size and surface roughness requirements.
3、 Good polishing performance
High quality dual color injection molded plastic products require a small roughness value on the surface of the mold cavity. For example, the surface roughness value of the injection mold cavity is required to be less than the level of Ra0.1~0.25, while the optical surface requires Ra<0.01nm. The cavity must be polished to reduce the surface roughness value. The steel selected for this purpose requires minimal impurities, uniform microstructure, no fiber orientation, and no pitting or orange peel like defects during polishing.
4、 Adequate surface hardness and wear resistance
The hardness of plastic molds is usually below 50-60HRC, and the heat treated molds should have sufficient surface hardness to ensure that the molds have sufficient stiffness. Due to the high compressive stress and friction force that plastic filling and flowing in the mold during operation, it is required that the mold maintain shape accuracy and dimensional stability to ensure sufficient service life. The wear resistance of the mold depends on the chemical composition and heat treatment hardness of the steel, so enhancing the hardness of the mold is beneficial for enhancing its wear resistance.
5、 Carbon steel of grade 50 has a certain degree of strength and wear resistance, and is often used as mold material 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 deformation during heat treatment.
With the development of plastic industry, the requirements for the complexity and precision of plastic products are becoming higher and higher, and higher requirements are put forward for the materials of two-color Injection mold construction. For plastic molds with complex manufacturing, precision and corrosion resistance, prehardening 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 preventing scratches and bonding. If there is relative motion between the two surfaces, try to avoid selecting materials with the same organizational structure. In special circumstances, one side can be coated or nitrided to make both sides have different surface structures.
