Plastic mold material selection skills

At present, plastic products are increasingly widely used in daily life, of which injection molding technology accounts for about 80%. Injection molding is widely used in automobile, construction, household appliances, food, medicine and many other fields due to its one-shot molding, accurate size, insertable parts, high productivity, easy modernization, and less post-processing. The selection of plastic molds is very critical to whether plastic industrial production can receive good economic benefits. Therefore, it is necessary for mold designers to understand the basic requirements of mold materials and select appropriate materials.

The working conditions of plastic molds are different from those of cold stamping dies. Generally, they must work at 150°C-200°C. In addition to a certain pressure, they must also be affected by temperature. According to the different use conditions and processing methods of plastic molding molds, the basic performance requirements of steel for plastic molds are roughly summarized as follows:

1. Sufficient surface hardness and wear resistance

The hardness of the plastic mold is usually below 50-60HRC, and the heat-treated mold should have sufficient surface hardness to ensure that the mold has sufficient rigidity. Due to the filling and flow of plastic, the mold is required to bear large compressive stress and friction during work, and the mold is required to maintain the stability of shape accuracy and dimensional accuracy to ensure that the mold has a sufficient service life. The wear resistance of the mold depends on the chemical composition of the steel and the hardness of heat treatment, so increasing the hardness of the mold is beneficial to improve its wear resistance.

2. good thermal stability

The shape of the parts of the plastic injection mold is often complicated, and it is difficult to process after quenching. Therefore, it should be selected as far as possible with good thermal stability. When the mold is formed and processed after heat treatment, the linear expansion coefficient is small, the heat treatment deformation is small, and the dimensional change caused by temperature difference. The rate is small, the metallographic structure and the mold size are stable, and the processing can be reduced or no longer required to ensure the mold size accuracy and surface roughness requirements.

3. Carbon steel of grade 50 has certain strength and wear resistance, and is mostly used for mold base material 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 the manufacture of formed parts with small size and simple shape due to its large deformation during heat treatment.

With the development of the plastics industry, the requirements for the complexity and precision of plastic products are getting higher and higher, and higher requirements are also placed on the mold materials. For the manufacture of 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 better performance. Machining, heat treating and polishing properties and high strength.

4. Excellent machinability

Most plastic molding molds require certain cutting processing and fitter repair in addition to EMD processing. In order to prolong the service life of cutting tools, improve cutting performance, and reduce surface roughness, the hardness of plastic mold steel must be appropriate.

5. good polishing performance

High-quality plastic products require a small roughness value on the surface of the cavity. 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<0.01nm. The cavity must be polished to reduce the surface roughness value. The steel selected for this purpose requires less material impurities, fine and uniform structure, no fiber orientation, and no pitting or orange peel defects during polishing.

In addition, when selecting materials, it is necessary to consider preventing scratches and gluing. If there is relative movement between the two surfaces, try to avoid selecting materials with the same structure. In special cases, one side can be plated or nitrided to make the two sides different. surface structure.