(1) High Wear Resistance
To increase the rigidity and mechanical properties of plastic parts, glass fiber and inorganic fillers are widely used in plastic formulations for reinforcement. After these additives are added, they enter the mold cavity at high speed along with the molten material, causing significant friction within the mold. If the mold material is not wear-resistant, the cavity is easily subjected to excessive wear. Therefore, the material used to make such molds should have particularly good wear resistance.
(2) High Corrosion Resistance
Many plastic additives, such as flame retardants, and even the chemical action of the molten material itself, can corrode the mold cavity. When the plastic raw material contains abrasive fillers and reinforcing materials, mold damage is further aggravated. Therefore, it is best to use corrosion-resistant steel or perform surface electroplating treatment on the mold, such as chrome plating or nickel plating.
(3) Good Thermal Conductivity
During injection molding, good mold temperature control has a significant impact on the quality of plastic parts, especially when processing semi-crystalline thermoplastics. Generally, copper alloys have much better thermal conductivity than alloy steels, but their elastic modulus, hardness, and durability are lower. The insufficient thermal conductivity of steel can be compensated for by a cooling system.
(4) Good Dimensional Stability
During injection molding, the temperature of the mold cavity reaches over 300℃. Such a large temperature difference easily causes microscopic changes in the cavity structure, resulting in mold dimensional changes and unstable plastic part dimensions. Therefore, tool steel with appropriate tempering treatment, i.e., heat-treated steel, can be selected. To improve hardness and wear resistance, molds generally undergo heat treatment. However, this heat treatment should have minimal impact on mold dimensions. Pre-hardened steel that can be machined can be used; this type of steel does not require heat treatment after machining.
(5) Good Polishing Performance
The inner wall of the mold cavity requires high precision and very low roughness to meet the requirement of good gloss on the surface of the plastic part. Therefore, the mold cavity generally needs to be polished, and the brighter the surface, the better. For this reason, the mold material should be easy to polish, and the selected material should not contain impurities or defects such as pores.
