5. Material selection of high gloss mold
Factors affecting the surface quality of the product In addition to the cavity temperature, the choice of mold steel and plastic materials is also crucial. The selection of high-gloss mold steel and the choice of plastic materials are introduced below.
(1) Selection of mold steel
The performance and life of the mold are directly related to the steel used in the mold. Plastic mold parts are subject to external influences due to different working conditions. Therefore, in addition to some basic requirements for the performance of steel, there must be some emphasis. For example, large-scale high-gloss plastic mold cavity steels, in addition to good machinability, electrical discharge processability and weldability, also need to have excellent polishing properties and high hardness. This not only can form a bright and beautiful surface, but also can reduce the wear of the plastic on the surface of the cavity, which not only prolongs the service life of the mold, but also reduces the injection pressure and protects the injection molding machine. At present, commonly used high performance mirror plastic mold steels are: POLMAX, LKM838H, LKM818H, S-STAR (A), NAK80, STAVAXS136, STAVAXS136H, OPTIMAX, X13T6W (236), X13T6W (236H) and so on.
(2) Selection of plastic materials
At present, the plastic materials used in the front casing of the television manufacturing are generally HIPS and ABS. As two kinds of common shell materials, although the cost of ABS is larger, the molded products are better than HIPS in impact resistance, surface gloss and hardness, so in the production of high-gloss products, high light is usually used. ABS material.
ABS is an amorphous polymer with no apparent melting point. Due to the wide variety of grades, appropriate process parameters should be formulated according to the grades in the injection molding process, generally above 160 °C, below 270 °C. In the molding process, ABS has good thermal stability, a wide range of options, and is not easy to degrade or decompose, and the melt viscosity of ABS is moderate, and its fluidity is better than that of polyethylene (PS), polycarbonate, etc. And the melt cooling and solidification speed is relatively fast, generally it can be cooled in 5-15 seconds.
The fluidity of ABS is related to the injection temperature and injection pressure, and the injection pressure is slightly more sensitive. For this reason, in the molding process, it is possible to start from the injection pressure to lower the melt viscosity and improve the filling performance. ABS has different properties of water absorption and water adhesion due to different components. The surface adhesion water and water absorption rate are between 0.2% and 0.5%, and sometimes between 0.3% and 0.8%. In order to obtain a more ideal product, Drying before molding, the water content is reduced to less than 0.1%. Otherwise, bubbles on the surface of the part will appear, such as bubbles and silver.