Composition and properties of domestic plastic mold steel
Domestic plastic mold steel is a key material supporting the development of my country’s injection mold industry. Its composition and performance optimization must meet the specific mold requirements of plastic molding, such as wear resistance, polishability, corrosion resistance, and processability. In recent years, with advances in domestic steelmaking technology, the quality of domestic plastic mold steel has approached internationally advanced levels and is widely used in mold manufacturing for home appliances, automobiles, electronics, and other fields. For example, a large home appliance company replaced imported material with domestic 718H mold steel, increasing mold life from 500,000 to 600,000 cycles and reducing costs by 30%. Domestic plastic mold steel is based on iron, with properties adjusted by adding alloying elements such as carbon, chromium, nickel, and molybdenum. Different grades of steel are available to meet specific molding requirements, forming a comprehensive product range and providing mold companies with diverse options.
Pre-hardened plastic mold steel is the most widely used type in China. Its composition is primarily based on medium-carbon alloying, ensuring a hardness of 30-45 HRC upon shipment. It requires no subsequent heat treatment and can be directly machined and formed. A typical grade, such as 3Cr2Mo (P20), contains 0.3%-0.4% carbon, 1.8%-2.2% chromium, and 0.2%-0.4% molybdenum. This compositional combination provides excellent hardenability and toughness, with polishability reaching Ra0.4μm. It is suitable for medium-volume production (100,000-500,000 molds) of plastic parts, such as TV housing molds. Improved pre-hardened steels, such as 718H, add nickel (1.0%-1.5%) to 3Cr2Mo, further enhancing toughness and polishability. Hardness uniformity is controlled within ±1 HRC, making them suitable for large, complex molds, such as automotive bumper molds. Its excellent machinability allows for high-speed milling to achieve complex curved surfaces. The room temperature impact toughness of pre-hardened mold steel is generally 20-30J/cm², which can meet the impact resistance requirements of most plastic molds and avoid cracking due to unexpected stress during the mold closing process.
Age-hardening plastic mold steel achieves hardness improvement through precipitation of strengthening phases during aging treatment. Its composition is characterized by low carbon and high alloy, usually with a carbon content of ≤0.2%, and the addition of aging elements such as aluminum, copper, and titanium. Typical grades include 18Ni (250). Domestic 18Ni mold steel contains 17%-19% nickel, 2.8%-3.2% molybdenum, and 0.7%-1.0% aluminum. After solution treatment at 820℃ and aging treatment at 480℃, the hardness can reach 50-52HRC. It also has extremely high purity (non-metallic inclusions ≤0.5 grade) and excellent polishing performance, which can achieve a mirror effect (Ra0.02μm). It is suitable for high-transparency plastic molds, such as PC lenses and acrylic lampshades. The outstanding advantage of this type of steel is that the dimensional change rate after aging treatment is extremely small (<0.01%), ensuring the dimensional accuracy of precision molds. For example, after using 18Ni steel for mobile phone lens molds, the cavity size tolerance can be controlled within ±0.002mm. However, the cost of age-hardened steel is relatively high and it is mainly used for extremely high-precision molds, with annual usage accounting for about 10% of the total plastic mold steel.
Corrosion-resistant plastic mold steel is designed for molding corrosive plastics like PVC and POM. Its chromium content is ≥13%, forming a passive film to resist corrosion. Typical grades include 4Cr13 (S136 corresponds to the domestic grade). Domestic 4Cr13 steel contains 12%-14% chromium and 0.3%-0.4% carbon. After quenching and tempering, it has a hardness of 45-48 HRC and excellent corrosion resistance. It remains rust-free after 24 hours of immersion in a 5% NaCl solution. It is suitable for PVC pipe fitting molds, protecting the mold cavity from HCl gas corrosion. To enhance polishing properties, some modified corrosion-resistant steels, such as 7Cr17Mo , have added molybdenum (0.7%-1.0%) to refine the grain size and improve wear resistance. Polishing properties can reach Ra 0.1μm, making it suitable for demanding corrosion-resistant molds. The disadvantage of this type of steel is that it has relatively low toughness (impact toughness 15-20J/cm²), is difficult to process, and requires special cutting tools. The processing cost is 20%-30% higher than that of ordinary pre-hardened steel.
Free-cutting plastic mold steel improves processing performance by adding elements such as sulfur and lead. It is suitable for molds with complex shapes and high processing volumes. A typical grade is Y3Cr2Mo (sulfur-added). Its composition is based on 3Cr2Mo with 0.08%-0.15% sulfur added. This creates MnS inclusions, which reduce cutting resistance, improve milling efficiency by 30%, and extend tool life by 50%. It is particularly suitable for cavity machining in large household appliance molds. The hardness of free-cutting steel is comparable to that of ordinary pre-hardened steel (30-35 HRC), but its polishability is slightly lower (Ra 0.8μm), making it unsuitable for plastic parts with high aesthetic requirements. In recent years, lead-free free-cutting steel (with selenium or tellurium additions) has been gradually introduced to meet environmental protection requirements. Its processing performance is comparable to that of lead-based steels, while avoiding the toxicity of lead. Domestic free-cutting plastic mold steel holds approximately a 15% market share and is primarily used in mid- and low-end molds requiring high processing efficiency, balancing cost and production cycle time.
