TPR (Thermoplastic Rubber) is a widely used soft elastomer material in the manufacturing of daily necessities, tool handles, grips, toys, sealing components, protective sleeves and electronic accessories. Due to its low melt viscosity, good fluidity, and the tendency to release plasticizers, softening oils and trace acidic additives during processing, TPR injection molding puts forward special and strict requirements for mold steel. If the steel is improperly selected, problems such as mold corrosion, rusting, sticky demolding, excessive flash, rapid cavity wear and short service life will frequently occur. Therefore, the selection of TPR mold steel must follow the principles of corrosion resistance, high polishability, appropriate hardness, wear resistance and dimensional stability, and be matched with product appearance requirements, production volume and processing conditions.

Key Performance Requirements for TPR Mold Steels

TPR materials will precipitate trace oily components and chemical additives during long-term injection molding, which will continuously corrode the mold surface and lead to rust, blackening or pitting if ordinary carbon steel is used. Therefore, corrosion resistance is one of the core indicators. At the same time, TPR products usually require a smooth, soft and delicate surface, so the mold must have excellent polishing performance to avoid pinholes, orange peel lines and texture marks on the product surface. In addition, TPR has strong fluidity and is prone to flash; therefore, the mold needs high hardness and good wear resistance to maintain dimensional stability and gap consistency during long-term production. The steel should also have good machinability to reduce processing difficulty and ensure the accuracy of complex structures such as ribs, inserts and sliders.

Commonly Used Steels for TPR Molds and Application Scenarios

NAK80NAK80 is a pre-hardened plastic mold steel with a hardness of HRC 38–42. It has uniform structure, good polishing performance and machinability, and can reach a surface finish of Ra ≤ 0.05 μm after polishing. It does not require overall heat treatment, so it has small deformation and high dimensional stability. It is suitable for medium and small TPR molds with medium output, moderate appearance requirements and non-transparent products. It is a cost-effective choice for conventional TPR parts.

718H718H is also a pre-hardened steel with a hardness of HRC 32–36. It has high toughness, good wear resistance and easy processing performance. It is suitable for TPR products with low to medium surface requirements, large structural parts and low-cost molds. However, its corrosion resistance is general, and it needs strict anti-rust maintenance during production shutdown and storage. It is mostly used for low?demand, low-cost TPR mold applications.

S136 / S136HS136 is a martensitic stainless steel refined by ESR (electroslag remelting), which has excellent corrosion resistance and can effectively resist the corrosion caused by TPR additives. It can achieve ultra-mirror polishing up to Ra ≤ 0.02 μm, making it very suitable for high-gloss, matte and transparent TPR products. After heat treatment, its hardness can reach HRC 50–54, with high wear resistance and long service life. S136H is its pre-hardened version, which can be directly processed and polished without quenching, reducing deformation risk. It is the preferred material for high-end, high-volume and high-appearance TPR molds.

SKD61 / 8407SKD61 is a heat-resistant mold steel with high temperature stability, high hardness and excellent wear resistance. It is suitable for TPR materials added with glass fiber, mineral filling or other highly abrasive components. It can withstand long-term production wear and maintain mold accuracy. However, its corrosion resistance is insufficient, so it is usually used with surface treatments such as chrome plating, nitriding or passivation when applied to TPR molds.

Hardness Matching and Surface Treatment Requirements

The hardness of TPR mold steel needs to be matched with production volume and service life. For small-volume and general?purpose molds, pre-hardened steels such as NAK80 and 718H with HRC 32–42 are sufficient. For high-volume, long-life and high-wear molds, stainless steels such as S136 with HRC 48–54 are more appropriate. Proper hardness can reduce flash, wear and deformation, and stabilize product dimensional accuracy.

In terms of surface treatment, mirror polishing is the basic requirement for TPR molds. High-grade polishing can significantly improve demolding performance and reduce sticking. For high-demand molds, surface treatments such as nitriding, hard chrome plating and DLC coating can further improve surface smoothness, wear resistance and corrosion resistance, reduce maintenance frequency and extend mold life. All surface treatments must be clean and pollution-free to avoid affecting the appearance and performance of TPR products.

Summary

The selection of TPR mold steel is a key factor affecting product quality, production stability and mold life. For most TPR products, especially those with long production cycles, high appearance requirements and strict stability demands, stainless steels represented by S136/S136H are the most reliable choice, with excellent corrosion resistance, polishing performance and durability. For conventional and low-cost products, NAK80 can be used as a balanced option. Through scientific steel selection and reasonable surface treatment, TPR molds can achieve long-term stable production, reduce sticking, rust, wear and other failures, and ensure high-quality and efficient molding of soft rubber products.