Injection mold steels are critical materials in mold manufacturing. They directly determine mold service life, dimensional accuracy, surface quality, and overall production cost. During injection molding, molds endure high-temperature melts (typically 150–350°C), high pressures (tens to over 100 MPa), repeated opening/closing impacts, and wear or corrosion from glass fibers, flame retardants, and other fillers in plastics. Material selection must therefore balance hardness, toughness, wear resistance, heat resistance, polishability, corrosion resistance, and processing cost.
Poor selection can lead to early cracking, galling, corrosion, or deformation-raising maintenance costs, compromising part appearance and accuracy, or even causing complete mold failure. Below is a detailed overview of the most common grades, covering performance characteristics, typical applications, supporting data, and practical examples to help engineers make better decisions.
4Cr13 Steel (Chinese Stainless Mold Steel)
4Cr13 is a medium-carbon, high-chromium martensitic stainless steel, roughly equivalent to AISI 420 or DIN 1.2083. It offers good hardenability, relatively high hardness, and moderate corrosion resistance. After quenching and tempering, hardness typically reaches HRC 48–52. A dense oxide film forms on the surface, providing protection in mildly corrosive environments.
It is suitable for molds that must withstand significant pressure and moderate wear, especially where cost sensitivity is high and corrosion is not severe-such as ordinary appliance housings or household goods. Polishability and corrosion resistance are inferior to premium mirror-finish stainless grades (e.g., S136). It is less ideal for highly corrosive plastics (PVC, halogenated flame-retardant resins) or demanding optical surfaces. With appropriate surface treatment, it can support medium-volume production.
SKD61 Steel (Japanese Hot-Work Tool Steel)
SKD61 is a Japanese JIS-standard hot-work tool steel, corresponding to AISI H13 (USA), DIN 1.2344 (Germany), and Chinese 4Cr5MoSiV1. Its key strengths are excellent high-temperature strength, toughness, and resistance to thermal fatigue. After quenching and tempering, hardness reaches HRC 50–54, retaining good strength and wear resistance at elevated mold temperatures.
It is particularly well suited to high-temperature engineering plastics (PPS, LCP, glass-fiber-reinforced PA, etc.) and high-wear conditions. In practice, SKD61 molds used with glass-filled plastics often achieve 500,000–1,000,000+ shots when combined with surface treatments such as nitriding. It is a frequent choice for automotive structural components and high-filled plastic gears due to its resistance to heat checking and impact fatigue.
H13 Steel (American Hot-Work Tool Steel)
H13 is closely related to SKD61 and is one of the most widely used hot-work tool steels worldwide. It provides good toughness, thermal-fatigue resistance, and wear resistance. Heat-treated hardness is typically HRC 48–52 (or higher).
It is appropriate for molds subjected to high stress and wear, such as those for plastic gears, tableware, and automotive structural parts. When production volumes are large or plastics contain glass fiber or mineral fillers, H13 performs especially well. Industry data show that in high-volume tools (≥1 million shots), H13 combined with proper cooling design and surface hardening significantly reduces thermal cracking risk and extends mold life. Many automotive and mechanical-component molds standardize on H13.
718 Steel (Swedish Pre-Hardened Plastic Mold Steel)
718 is an ASSAB (Uddeholm) pre-hardened steel, an improved P20-type grade (approximately 1.2738) with added nickel for better hardenability, toughness, and polishability. It is supplied pre-hardened at HRC 30–36 (or HB 290–330), eliminating the need for secondary heat treatment and the associated risk of distortion.
It is suitable for medium-to-large molds requiring good pressure and stress resistance, such as automotive bumpers, instrument panels, and electronic housings. Polishability is superior to standard P20, enabling better surface finishes. In practice, large automotive interior/exterior molds made from 718 commonly achieve 150,000–400,000 shots and offer an excellent performance-to-cost balance. Compared with basic P20, it delivers more uniform hardness and improved wear resistance.
2311 Steel (German Pre-Hardened Plastic Mold Steel)
2311 corresponds to DIN 1.2311 and belongs to the P20 family of pre-hardened steels. Supplied hardness is approximately HRC 29–34. It offers good toughness, fatigue resistance, and machinability at a relatively low cost.
It is suitable for medium-requirement plastic molds such as ordinary appliances, consumer goods, and toys. When mold size is moderate, production volume is medium, and plastic corrosivity is low, 2311 is an economical and reliable choice. It shares the pre-hardened advantages of 718 but has somewhat lower polishability and wear resistance, making it better for applications without extreme surface demands.
NAK80 Steel (Japanese Daido Pre-Hardened Mirror Steel)
NAK80 is a precipitation-hardening pre-hardened plastic mold steel developed by Daido Steel (Japan). It is supplied at HRC 37–43 and requires no quenching, providing excellent dimensional stability. Its standout features are outstanding mirror polishability (capable of Ra ≤ 0.01 μm) and good EDM performance.
It is ideal for transparent and high-gloss plastic parts-mobile-phone housings, acrylic light covers, optical lenses, cosmetic packaging, and LCD panel molds. In industry use, NAK80 is commonly applied to precision appearance tools with volumes up to around 800,000 shots. Its polishing efficiency and surface consistency often reduce secondary finishing costs and improve part appearance yield compared with conventional grades.
S136 Steel (Swedish ASSAB Stainless Mirror Mold Steel)
S136 is ASSAB's classic stainless mold steel (modified 420 / 1.2083 ESR). High chromium content gives excellent corrosion resistance and superior mirror-polish capability. It is supplied annealed and can be heat-treated to HRC 48–52. It is especially suitable for corrosive plastics (PVC, POM, flame-retardant grades) and for medical, food-contact, and optical applications that demand high cleanliness.
In transparent and mirror-finish molds it performs exceptionally well, achieving optical-grade surfaces. Medical devices, preforms for beverage bottles, and precision optical components frequently use S136. It effectively prevents cooling-channel corrosion and cavity attack, extending mold life while ensuring contamination-free parts. Compared with 4Cr13, it offers higher purity and more consistent polishability.
8407 Steel (Swedish ASSAB Premium Hot-Work Tool Steel)
8407 is ASSAB's upgraded version of H13 (ORVAR series). Through higher purity and improved isotropy, it delivers better toughness and thermal-fatigue resistance than standard H13. Heat-treated hardness reaches HRC 44–52, allowing higher hardness without sacrificing toughness.
It is used for demanding hot-work and injection molds involving high-temperature engineering plastics, highly abrasive filled materials, or precision automotive structural parts. In both die-casting and high-quality plastic tooling, 8407 often extends mold life beyond standard H13, making it suitable for long, stable production runs.
Comprehensive Selection Guidelines and Key Factors
Steel selection is not simply a matter of choosing the hardest or most expensive grade; it must match the actual operating conditions:
By plastic type: Ordinary ABS/PP/PE → P20 or 718; transparent/high-gloss → NAK80 or S136; corrosive (PVC, etc.) → prioritize S136; high glass-fiber content → H13, SKD61, or 8407.
By production volume: Low volume (≤ 500,000 shots) → P20 or 4Cr13; medium volume → 718 or NAK80; high volume (≥ 1 million shots) → H13, SKD61, or 8407.
By surface requirement: Optical or true mirror finish → S136 or NAK80; standard polish → 718 is often sufficient.
Other considerations: Mold size, cooling design, need for nitriding/PVD coatings, budget, and lead time must all be evaluated.
Correct material selection can substantially reduce total cost of ownership. For example, one automotive parts manufacturer upgraded glass-filled PA molds from ordinary pre-hardened steel to H13 and roughly doubled mold life while lowering cost per part. Optical lens molds switched to S136 achieved higher finished-part yields and far fewer repairs.
In summary, selecting injection mold steel is a systems decision that requires combining material property data, production experience, and appropriate heat-treatment or surface-engineering processes. The grades covered above span the range from economical to premium options. Before final tooling, consult steel suppliers or experienced mold engineers and validate the choice against the specific product requirements to achieve the best overall performance and cost.
FAQ
Q1: What is the difference between pre-hardened steels (such as 718, 2311, NAK80) and through-hardening steels (such as H13, SKD61, S136, 8407)?
Pre-hardened steels are delivered already heat-treated to a usable hardness (typically HRC 30–43) and can be machined directly without further hardening, reducing the risk of distortion and shortening lead time. Through-hardening steels are usually supplied annealed, machined in the soft state, and then quenched and tempered to higher hardness (HRC 48–52 or more). They offer superior wear resistance and thermal fatigue performance for high-volume or abrasive applications but require careful heat-treatment control.
Q2: Which steel should I choose for glass-fiber-reinforced plastics?
For plastics containing significant glass fiber or mineral fillers, prioritize high-wear-resistance hot-work grades such as H13, SKD61, or 8407. These steels maintain hardness and toughness under abrasive conditions and elevated temperatures. Surface treatments (nitriding, PVD coatings) further extend life. Pre-hardened grades like 718 or NAK80 are generally less suitable for high-filler content over long production runs.
Q3: When is stainless mold steel (S136 or 4Cr13) necessary?
Stainless grades are recommended whenever the plastic is corrosive (PVC, flame-retardant compounds that release acidic gases, certain POM grades) or when the application requires high cleanliness (medical devices, food-contact parts, optical lenses). S136 is preferred over standard 4Cr13 for better purity, polishability, and long-term corrosion resistance, especially in cooling channels.
Q4: How does production volume affect steel selection?
Low volume (under ~300,000–500,000 shots): Economical pre-hardened steels such as P20/2311 or 4Cr13 are often sufficient.
Medium volume (500,000–1 million shots): 718 or NAK80 provide a good balance of cost and performance.
High volume (over 1 million shots) or abrasive/high-temperature resins: H13, SKD61, or 8407 are preferred for longer tool life and lower cost per part.
Q5: Can I polish H13 or SKD61 to a true mirror finish?
These grades can achieve good polish levels (SPI A2–B1 range), but they do not match the optical-grade mirror capability of S136 or NAK80. For transparent or high-gloss cosmetic parts requiring Ra ≤ 0.01 μm, S136 or NAK80 are the safer choices.
Q6: What surface treatments are commonly used to improve mold life?
Nitriding is widely applied to H13/SKD61/8407 to increase surface hardness and wear resistance. PVD coatings (TiN, CrN, DLC) further reduce friction and abrasion. For stainless grades, careful polishing and, if needed, passivation help maintain corrosion resistance. Over-nitriding of pre-hardened steels such as NAK80 should be avoided because it can reduce toughness or cause edge chipping.
Q7: Is 8407 significantly better than standard H13?
Yes, in demanding applications. 8407 (ASSAB's premium H13 equivalent) offers higher purity and better isotropy, allowing 1–2 HRC higher hardness without loss of toughness and improved resistance to thermal fatigue cracking. It typically delivers longer service life in high-volume or high-temperature tools compared with conventional H13.
Q8: How important is steel purity (ESR / vacuum-remelted grades)?
Electroslag remelting (ESR) or vacuum remelting greatly reduces non-metallic inclusions, improving polishability, toughness, and dimensional stability. Grades such as S136 ESR, 8407 Supreme, and high-quality H13 ESR are preferred for optical, medical, or high-stress molds where surface quality and fatigue life are critical
