Mold steel directly determines mold service life, molded part surface quality and mass production stability. Raw material corrosivity, production batch volume, appearance specification and working environment are core reference factors for steel grade selection. Based on practical production conditions, this article elaborates targeted steel selection rules under six typical molding scenarios.
1. Small-batch Production for Non-corrosive Common Plastics
For ABS, PP, PE and other non-corrosive resins with monthly output below tens of thousands and ordinary matte appearance requirement, pre-hardened P20, 45# carbon steel and S50C are preferred cavity materials. These steels are pre-tempered with hardness ranging from 28HRC to 32HRC, available for direct CNC machining without secondary heat treatment, featuring low processing cost and small post-processing deformation. S50C is widely applied for standard mold base to bear clamping load. This economical matching is suitable for small-batch electronic bobbin and housing molds, yet poor wear resistance causes cavity abrasion and dimensional drift once production exceeds 100,000 shots.
2. Medium-batch Production for High-gloss Transparent Plastic Products
For transparent PC, PMMA requiring mirror finish with monthly output from 100,000 to 500,000 pieces, NAK80 and quenched S136 stainless steel serve as mainstream cavity material. Pre-hardened NAK80 reaches 38~42HRC, which can be polished up to 8000# mirror surface with few internal impurities to avoid pinholes on transparent molded parts. Quenched S136 gains hardness of 48~52HRC with outstanding rust resistance to prevent cavity rust during long-time transparent part production, widely adopted for high-end precision plastic housing molds. SKD61 is configured for ejector pins and inserts to balance toughness and wear resistance.
3. Molding Condition for PVC and Flame-retardant Corrosive Plastics
PVC and flame-modified ABS release acidic corrosive gas under molten high temperature, which corrodes ordinary carbon steel and P20 rapidly and forms surface pits and black stains on finished parts. Pre-hardened S136H and 2083 stainless steel are primary anti-corrosion options. Homogeneous metallographic structure of S136H effectively resists acid erosion during intermittent or continuous production; cost-sensitive low-end projects adopt economical 2083 with slightly weaker anti-corrosion performance. Cooling channels and tiny mold inserts also use stainless steel to prevent pipeline blockage from internal rust.
4. High-abrasion Mass Production for Glass-fiber Reinforced Plastics
Glass-filled PA and PBT bring strong abrasive scouring to mold cavity under high-speed melt flow, requiring high-wear steel for over 500,000 monthly shots. Quenched DC53 and SKD11 reach hardness of 55~62HRC with superior anti-abrasion performance to slow cavity inner wall scratch and dimensional shrinkage. High-gloss fiber-filled products select polished S136 to balance rust prevention and wear resistance; SUJ2 bearing steel is used for guide pillars and sleeves to extend mold maintenance cycle.
5. Small-batch Trial Production with Unfixed Material Types
New product trial molding alternates corrosive and non-corrosive plastics frequently, so 718H pre-hardened steel of 32~36HRC becomes optimal prototype cavity material. It features easy machining and convenient local welding repair after design revision, with basic rust resistance to adapt short-period flame-retardant plastic trial runs. Once prototypes switch to formal mass production, steel grade can be replaced according to practical working condition to lower early development cost.
6. High-temperature Molding for Special Engineering Plastics
PPS, PEEK and LCP are molded above 300℃, leading to repeated thermal expansion and contraction fatigue on mold steel. Heat-treated SKD61 and H13 hot work steel with hardness of 46~50HRC feature excellent thermal fatigue and anti-crack property; surface nitriding further improves surface hardness and high-temperature stability for long-cycle high-temperature forming.
Conclusion
Reasonable mold steel selection depends on molding material, production batch and appearance requirement. Surface nitriding or PVD coating on economical base steel becomes cost-effective optimization to balance production cost and mold durability.
