Properties of commonly used plastic mold steel
The mechanical properties of commonly used plastic mold steels are the core indicators for measuring their applicability. These mainly include strength, hardness, toughness, and wear resistance. These properties directly affect the service life of the mold and the quality of the molded plastic parts. Cold work mold steels, such as Cr12MoV, have high hardness and wear resistance. After quenching and tempering, the hardness can reach HRC58-62. It can effectively resist the erosion and wear of plastic melts and is suitable for molding plastic parts containing reinforcing materials such as glass fiber. However, its toughness is relatively low and it is prone to cracking when subjected to large impact loads. Therefore, it is not suitable for molds with large or complex cavities. Hot work mold steels, such as H13, have an excellent combination of strength and toughness, with a hardness of HRC45-50. They also have high high-temperature strength and thermal fatigue resistance. They are suitable for molding plastics that require high-temperature melts, such as PC and PA, and can maintain stable performance during repeated heating and cooling processes. Pre-hardened plastic mold steel, such as 718H, has a hardness of HRC30-35 when it leaves the factory. It does not require subsequent heat treatment and can be directly processed and formed. It has good cutting and polishing properties and is suitable for making plastic molds with high surface requirements. However, its wear resistance is slightly lower than that of cold-working mold steel and is suitable for molding non-reinforced plastics.
The processing properties of commonly used plastic mold steels are key considerations during mold manufacturing, including cutting, polishing, and weldability, which directly impact mold manufacturing efficiency and precision. Pre-hardened mold steels, such as P20, offer excellent cutting properties due to their moderate hardness (HRC 28-32). They can be turned, milled, and planed using standard tools. The resulting low surface roughness reduces the workload in subsequent polishing steps, making them suitable for mass production of small and medium-sized molds. Age-hardening mold steels, such as NAK80, can reach a hardness of HRC 38-42 after aging treatment. They can be precision machined even in the pre-hardened state and offer excellent polishing properties, achieving a mirror finish (Ra ≤ 0.02μm). These steels are suitable for molds for transparent or high-quality finishes, such as optical lens molds. However, these steels have poor weldability, making repair welding difficult if defects occur during mold processing, requiring specialized welding processes and equipment. Cold working die steels such as Cr12 require carbide tools or high-speed steel tools for processing due to their high hardness and large cutting resistance, and the processing efficiency is low. They are suitable for making small molds with simple structures and high precision requirements.
Corrosion resistance is a key performance indicator for certain plastic mold steels. For molds containing corrosive additives or used in corrosive environments, the mold steel’s corrosion resistance directly determines its service life. Martensitic stainless steels, such as 4Cr13, offer a certain degree of corrosion resistance and are resistant to attack by weak acids and bases. This makes them suitable for molding PVC parts, as PVC releases hydrogen chloride gas during the molding process, which is corrosive to molds. 4Cr13 mold steel can effectively reduce surface defects caused by corrosion. Austenitic stainless steels, such as 304, offer superior corrosion resistance to martensitic stainless steels and are resistant to attack by a variety of chemical media. They are suitable for molding parts containing corrosive additives such as flame retardants and plasticizers, or for molds operating in humid and high-temperature environments. However, austenitic stainless steels have a lower hardness (typically HRC 20-25) and poor wear resistance, requiring surface hardening treatments (such as nitriding) to increase their surface hardness and enhance wear resistance. In addition, some specially treated mold steels such as STAVAX (S136) have excellent corrosion resistance and polishing properties through optimized composition and process. The hardness can reach HRC48-52 after quenching and tempering. It is suitable for making high-demand transparent plastic parts molds, especially in situations where release agents or cleaning agents are required during the molding process, and can maintain the smoothness of the mold surface.
The polishing performance and surface quality of commonly used plastic mold steels have a direct impact on the appearance quality of plastic parts. Steel purity and structural uniformity are particularly crucial for molds used to produce high-gloss, transparent parts. High-quality plastic mold steels, such as 718HH, have extremely low levels of impurities like sulfur and phosphorus, a uniform microstructure, and no significant segregation or inclusions. They offer excellent polishing properties and can achieve a mirror-like finish after fine polishing, ensuring a smooth, mirror-like surface finish. This makes them suitable for molds for parts such as automotive headlights and appliance panels. Common carbon tool steels, such as T8A, are lower in purity and contain a high number of non-metallic inclusions. This makes them susceptible to surface defects such as pinholes and scratches after polishing, making them suitable only for parts requiring less demanding surface quality. The grain size of mold steel also affects polishing performance. Fine-grained steels, such as NAK80, have uniform, fine grains, resulting in low surface roughness after polishing, while coarse-grained steels are prone to orange peel-like defects after polishing. In addition, the heat treatment process of mold steel has a great influence on the surface quality. If oxidation, decarburization and other phenomena occur during the quenching and tempering process, it will lead to uneven surface hardness and affect the polishing effect. Therefore, protective atmosphere heat treatment or vacuum heat treatment is required to ensure the excellent surface quality of the mold steel.
Dimensional stability is a key performance of plastic mold steel to maintain accuracy during use, especially for precision molds. Factors such as the thermal expansion coefficient and aging stability of steel will affect the dimensional accuracy of the mold. Pre-hardened mold steels such as P20 do not require heat treatment after processing and forming, which avoids deformation during the heat treatment process. They have good dimensional stability and are suitable for making small and medium-sized molds with high precision requirements. Age-hardening mold steels such as S136H are hardened through aging treatment, with small deformation and a small thermal expansion coefficient caused by temperature changes during use. They can maintain the dimensional accuracy of the mold and are suitable for making molds for precision gears, connectors and other plastic parts. Cold-working die steels such as Cr12MoV, although they have high hardness and good wear resistance, are prone to internal stress during the quenching process, which causes deformation of the mold. Multiple tempering is required to eliminate internal stress and improve dimensional stability. They are suitable for making molds with high precision requirements but simple structures. In addition, the structural stability of mold steel is also very important. If a structural transformation occurs during long-term use (such as the transformation from pearlite to austenite), it will cause dimensional changes. Therefore, it is necessary to select steel with stable structure. For example, mold steel with added alloy elements such as molybdenum and vanadium can effectively inhibit the structural transformation and ensure dimensional stability during long-term use.
