Plastic Mold Steel｜Strong, High Polishability, and Reliable for All Injection Mold Applications

In the plastic injection mold industry, one of the most frequent challenges is surface scratches during polishing, mold cracking after heat treatment, or corrosion from gas and additives during injection. These issues become more severe in high-volume production molds and engineering plastic molds that use materials containing fillers or flame retardants. The result surface deterioration, dimensional instability, and shorter mold life.
Key Industry Problems
Conventional mold steels that lack precise microstructural control often suffer from uneven carbide distribution and high residual stress. This leads to warping or cracking after heat treatment. Additionally, steels with poor polishability or insufficient corrosion resistance make it difficult to achieve a smooth plastic surface forcing manufacturers to spend more time on re-polishing and maintenance.
Yamaichi’s Solution
Yamaichi Special Steel (Thailand) offers premium-quality plastic mold steel that meets Japanese Industrial Standards (JIS G4053 / JIS G4404), covering SCM Modify and P21 grades. Manufactured through the High Purity ESR (Electro Slag Remelting) process under strict quality control, each grade is designed to provide an optimal balance of strength, toughness, and polishability. The material supports hardening and surface treatments such as nitriding, PVD, and hard chrome plating, ensuring longer mold life, excellent surface finish, and consistent performance in high-precision plastic molding applications.
Characteristics of YAMAICHI Plastic Mold Steel
1. SCM MODIFY — Pre-Hardened Plastic Mold Steel (Cr–Mo Alloy)
• Chromium–Molybdenum Alloy Steel, pre-hardened from factory
• Hardness: HRC 28–34, uniform and stable
• High toughness, no additional heat treatment required before use
• Fine grain structure and easy-to-polish surface
• Ideal for General injection molds, engineering plastic molds, mold bases, and core inserts
2. P21 — High Polishable Mold Steel (Ni–Cr Alloy Type)
• Nickel–Chromium alloy steel, pre-hardened to HRC 30–36
• Outstanding mirror-finish polishability
• Excellent toughness and corrosion resistance
• Ideal for optical and transparent molds requiring surface perfection
• Ideal for Optical molds, lens molds, transparent covers, and precision electronic housing
Why Choose Yamaichi Plastic Mold Steel
• Japan Quality Manufacturing — certified under ISO 9001:2015
• High Purity ESR Process — clean steel with minimal inclusions
• Superior Polishability & Dimensional Accuracy
• Cut-to-Length and Heat Treatment Services available in-house
• Suitable for all mold types: Injection, Blow, and Compression Molds
Conclusion
SCM MODIFY and P21 plastic mold steels from Yamaichi Special Steel (Thailand) are designed to provide an ideal balance of strength, toughness, and high surface finish. Backed by Japan-quality production and comprehensive machining services, these steels deliver exceptional durability, polishability, and corrosion resistance ensuring longer mold life and consistent product quality for every molding application.
※ For more information, please contact us below or download our brochure.
Glossary
• Pre-Hardened: Steel that has been hardened at the factory and ready for use without further heat treatment
• Polishability: The ability of steel to achieve a smooth, mirror-like finish
• Inclusion: Non-metallic impurities in steel that affect polish quality
• Nitriding / PVD: Surface treatments that improve hardness and corrosion resistance
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