Performance Differences Between S136 and SKD61 for Plastic Molds
S136 and SKD61 are two high-grade mold steels widely used in plastic injection molding, but they have distinct performance orientations and application scenarios. Confusing their use can lead to mold damage, product defects, and increased production costs. This article details their core performance differences, heat treatment characteristics, and typical applications, providing clear guidelines for material selection to match different production needs and plastic types.
Basic Material Classification & Core Orientation
The fundamental difference between S136 and SKD61 lies in their material composition and design purpose:
S136: A martensitic stainless steel with high chromium content (13.0%-14.5%), designed primarily for corrosion resistance, rust prevention, high polishability, and dimensional stability. It is tailored for molds requiring superior surface quality and resistance to corrosive plastics.
SKD61: A hot-work tool steel alloyed with chromium (4.5%-5.5%), molybdenum (1.0%-1.5%), and vanadium (0.8%-1.2%), optimized for high hardness, wear resistance, thermal fatigue resistance, and impact strength. It is intended for high-load, high-volume production and abrasive plastic materials.
There is no absolute "better" material—selection depends entirely on specific application requirements.
Hardness & Heat Treatment Characteristics
Both steels have similar hardness ranges after heat treatment, but their heat treatment processes and distortion performance differ significantly:
S136: Available in two forms: pre-hardened (HRC 30–34) for direct machining, or quenched and tempered (HRC 48–52) for higher hardness. It has extremely low heat treatment distortion (≤0.05 mm/m), eliminating the need for complex straightening processes and ensuring high dimensional precision.
SKD61: Must undergo quenching and tempering (cannot be used pre-hardened), with a typical working hardness of HRC 48–53 (up to HRC 55 for special applications). Its heat treatment process is more complex, requiring 2–3 tempering cycles to eliminate internal stress. Distortion is slightly higher (≤0.08 mm/m) than S136, and some complex molds may need post-heat treatment straightening.
A key advantage of SKD61 is its superior high-temperature hardness retention: at 200℃, it maintains hardness above HRC 45; at 300℃, it remains above HRC 40, while S136’s hardness decreases significantly above 250℃.
Corrosion Resistance
Corrosion resistance is the most significant difference between the two steels and a key factor in material selection:
S136: Offers excellent corrosion resistance and rust prevention. Its high chromium content forms a dense chromium oxide protective film, effectively resisting erosion by acidic gases (released by PVC, POM, flame-retardant plastics), moisture, and weak acids. It is suitable for humid production environments, food/medical grade molds, and any application involving corrosive plastics.
SKD61: Has almost no corrosion or rust resistance. Its low chromium content cannot form an effective protective film, so it will quickly rust, pit, or peel when exposed to acidic gases or moisture. It must never be used for molds processing corrosive plastics such as PVC.
Polishability & Surface Quality
Polishability directly determines the surface quality of injection-molded parts, making it critical for appearance-focused products:
S136: Features high material purity, uniform structure, and minimal impurities, enabling it to achieve a super-mirror finish (Ra ≤ 0.025 μm) with fine polishing. It is ideal for transparent parts (PC, PMMA), high-gloss shells, and products requiring flawless surfaces, as it avoids flow marks, haze, and blemishes.
SKD61: Contains hard carbides due to its molybdenum and vanadium content, resulting in average polishability. It can only achieve matte to semi-gloss surfaces (Ra ≤ 1.6 μm) and cannot be polished to a mirror finish. It is not suitable for transparent or high-gloss parts, as it tends to leave fine scratches or grainy surfaces.
Wear Resistance & Service Life
Wear resistance is crucial for high-volume production and abrasive plastic materials:
S136: Has moderate wear resistance, suitable for non-abrasive plastics such as PP, PE, ABS, and PVC. Its service life ranges from 300,000 to 800,000 shots for standard applications, but it will wear quickly if used for glass-filled plastics.
SKD61: Exhibits significantly higher wear resistance than S136, thanks to its molybdenum-vanadium carbides. It is ideal for glass-filled resins (PA+GF, PBT+GF, PP+GF) and high-volume, high-load production. Its service life can exceed 1 million shots, even reaching 2 million shots for some structural parts, making it the preferred choice for durable, long-running molds.
Typical Applications & Selection Guidelines
S136 Typical Applications
Transparent parts (PC, PMMA, transparent PVC) and high-gloss appearance parts.
Corrosive plastics (PVC, POM, flame-retardant materials) and humid production environments.
Food/medical grade molds and precision components requiring strict dimensional accuracy.
Small-to-high volume production with high surface quality requirements.
SKD61 Typical Applications
Glass-filled plastics (PA+GF, PBT+GF) and abrasive materials.
High-volume, continuous production and heavy-load structural parts (automotive components, large crates).
High-temperature injection molding and hot runner molds.
Structural parts, tooling, and components with no strict surface quality requirements.
Conclusion
S136 and SKD61 serve distinct roles in plastic injection molding: S136 excels in corrosion resistance, polishability, and dimensional stability, making it ideal for appearance-focused, corrosive, or precision applications; SKD61 leads in wear resistance, thermal fatigue, and durability, suited for high-volume, abrasive, or high-load scenarios. By matching the steel to the plastic type, product requirements, and production scale, manufacturers can ensure mold longevity, stable product quality, and optimal production efficiency.
