Meta description: Choosing plastic mold steel? Compare P20, 1.2311, 1.2738, 1.2312, 1.2085, 1.2316, 1.2083 and NAK80 grades by hardness, corrosion resistance and polishability.
The grade of steel used for the cavity, core and mold base of a plastic injection mold influences almost everything that follows. It determines how easily the mold can be rough-machined and finished, how well it accepts a mirror or texture finish, how long it survives in production, and how effectively it resists the corrosive chemistry of certain plastics. Choose a grade that is too soft and the gates, cores and shut-offs wear, flash develops and the finish degrades. Choose a grade that is more expensive than the application requires and the mold costs more and takes longer to build without producing any extra value.
Plastic mold steel is also a category in which the number of grades can be confusing. A typical distributor stocks P20, 1.2311, 1.2738, 1.2312, 1.2085, 1.2316, 1.2083, NAK80-type and other designations, each in several delivery conditions and hardness ranges. Many grades are considered equivalent across the DIN, AISI, JIS and GB standards, but equivalent does not mean identical: composition tolerances, melting practice and heat treatment all vary between producers, and those differences show up in the mold shop.
This guide explains how to approach plastic mold steel selection systematically. It covers the criteria that actually decide the outcome (plastic type, filler content, production volume, surface finish, mold size), the difference between pre-hardened and through-hardened steel, a comparison of the most common injection mold steel grades, the role of ESR quality, and the documentation a buyer should expect from any supplier. In Europe and parts of Asia the same material is often written "plastic mould steel"; the grades and the logic of selection are the same.
Why the Grade Decision Matters More Than Its Price
Steel is rarely the largest line item in a mold budget. Machining, polishing, design and assembly dominate the cost of a finished tool, and the grade of steel influences all of them. A free-machining grade can cut roughing time noticeably; a cleaner, more uniform grade can reduce polishing from days to hours; a steel with poor hardenability may force the purchase of a larger block to achieve the required hardness in the core; and a grade that corrodes in service can require the cavity to be welded and re-polished after a fraction of the expected production volume.
The economic argument for careful plastic mold steel selection is therefore built on total cost rather than price per kilogram. A mold that wears prematurely, loses its gloss or develops rust staining produces scrap parts, unplanned maintenance and late deliveries. Upgrading from a standard pre-hardened grade to a corrosion-resistant variant in an aggressive resin, or inserting hardened blocks at the gates of a glass-filled mold, is usually far cheaper than repeated rework during the mold's life.
Key Criteria for Plastic Mold Steel Selection
No single property defines the right grade. Selection is a matter of matching several requirements at once — and the plastic compound itself usually ranks first.
The Plastic Being Molded
Unfilled commodity resins such as PP, PE, PS and ABS are chemically gentle on steel, and standard pre-hardened grades handle them well for most production runs. Problems appear with resins that release aggressive by-products during processing. PVC is the classic example: at processing temperatures it can release hydrogen chloride, which attacks unprotected steel and causes pitting and rust staining. Halogenated flame retardants in ABS, PC/ABS and similar blends can have a comparable effect, and the risk increases when moisture or recycled material is present.
For these materials, corrosion-resistant grades are the standard answer: 1.2316 pre-hardened stainless or 1.2083 (also sold as S136 or 4Cr13) for through-hardened cavities, with the machinable stainless 1.2085 for mold bases and cores that do not need a high polish. Medical, pharmaceutical and food-contact molds are frequently specified in stainless grades for similar reasons of hygiene and cleanability, even when the resin itself is not aggressive.
Filler Content and Abrasiveness
Glass fiber is the most common abrasive filler, used at loadings from roughly 10% to more than 50% by weight, with mineral fillers and carbon fiber also common. These materials erode gates, hot runners, cores and textured surfaces, and the first evidence is usually gate wash — enlarged gates that cause inconsistent filling and flash. Unfilled or lightly filled materials rarely require more than a good pre-hardened grade at approximately 30 HRC. Heavily filled compounds are typically handled with hardened inserts of 1.2344 (H13) around 48–52 HRC, cold work grades such as 1.2379 (D2) at 58–62 HRC, or powder metallurgy tool steels where wear is extreme. Nitriding and hard coatings are also applied to localized wear areas. Designing a replaceable insert at the gate is considerably cheaper than replacing a cavity.
Production Volume and Expected Mold Life
Volume expectations set the minimum grade rather than the maximum. A pre-hardened 1.2311 mold at approximately 30 HRC, properly maintained, is typically adequate for general commodity molding; a hardened stainless cavity or an insert-based construction is normal practice when the tool must run for years with minimal interruption. Prototype and bridge tooling — hundreds to a few thousand parts — is often cut from free-machining 1.2312 or, for very short runs, aluminum. The practical point is that the cavity and core wear out years before the mold base does, so money is usually better spent on the working surfaces than on the frame around them.
Required Surface Finish
Surface finish requirements often decide the grade on their own. Visible high-gloss parts (appliance housings, automotive interior trim) and transparent or optical parts (light guides, lenses, PET preforms) demand an SPI A1–A2 finish. Achieving that consistently requires steel that polishes cleanly — low in inclusions and segregation, uniform in hardness — and often a working hardness of 38–42 HRC in NAK80-type grades or 48–54 HRC in hardened 1.2083/S136. Softer steels may take a high polish initially but lose it quickly in production. Texture etching (SPI C and D finishes) also favors homogeneous ESR steel, because inconsistent structure etches unevenly and produces a mottled pattern that polishing cannot correct.
Mold Size, Section Thickness and Hardness Uniformity
Plate thickness matters because the hardness achieved in the center of a thick block can differ from the surface. For cavities in the 250–300 mm thickness range, a standard 1.2311 or 1.2738 block is often adequate; beyond that, nickel-containing 1.2738 is preferred because its hardenability keeps the core of the block closer to the specified hardness. For very large molds, ESR material is sometimes specified to reduce segregation in the center of the original ingot. Buyers of thick plates should ask for delivery hardness measured at the core as well as near the surface.
Working Hardness and Wear Resistance
Hardness is a proxy for several behaviors at once: wear resistance, polish retention, dent resistance and load-bearing capacity. The common bands for plastic mold steel are approximately 28–34 HRC for pre-hardened P20-type steels, 38–42 HRC for age-hardened grades such as NAK80, and 48–54 HRC for through-hardened stainless or tool steel cavities. Higher hardness generally means better wear resistance and longer gloss retention, but also higher machining cost, greater sensitivity to cracking in thin sections, and stricter heat treatment control.
Machinability
Machinability affects schedule as much as cost. Free-machining 1.2312 contains a controlled sulfur addition that improves chip formation and can meaningfully reduce roughing time compared with 1.2311; the trade-off is slightly lower polishability and poorer weldability, which is why it is common in mold bases and less common in mirror-polished cavities. Pre-hardened steel at 30–40 HRC is still machined with carbide tooling, but shops should plan spindle load, tool wear and EDM requirements accordingly. For through-hardening materials, machining is done in the annealed condition, and finishing after heat treatment usually means grinding, jig grinding or hard milling.
Lead Time and Availability
Lead time is a selection criterion in its own right. Standard pre-hardened plates are usually available from stock and can be cut to size quickly; ESR grades, stainless through-hardening steels and non-standard thicknesses are more often mill-order items. On critical schedules, confirming stock availability before finalizing the design — or designing around standard stock sizes — is more effective than expediting later.
Pre-Hardened vs Through-Hardened Plastic Mold Steel
This is the fundamental fork in the road for mold steel selection. Pre-hardened steel is delivered ready to machine at its working hardness, typically 28–34 HRC for P20-type grades and 38–42 HRC for age-hardened types such as NAK80 and its equivalents. The mold maker machines it, polishes it and puts it into service without a hardening cycle. Dimensions stay stable, repairs are straightforward, and the risk of heat-treatment distortion or cracking disappears. The limits are the achievable hardness and, in thick sections, hardness uniformity; wear resistance is moderate.
Through-hardened steel — 1.2083/S136/4Cr13, 1.2344, 1.2379 and others — is supplied annealed, machined with an allowance for growth and distortion, then hardened and tempered to its final working hardness, commonly 48–54 HRC for stainless cavity grades and 58–62 HRC for cold work grades. This route achieves the hardness, wear resistance and polish retention that pre-hardened grades cannot, at the cost of an extra process step, grinding after heat treatment, and greater dependence on heat treatment quality. Vacuum hardening with controlled gas quenching is the usual specification for cavity-grade stainless steel.
A third option, widely used in practice, is the hybrid: a pre-hardened frame (1.2311, 1.2738) combined with hardened inserts for gates, small cores, slides and any area of concentrated wear. This captures most of the wear benefit at a fraction of the cost and complexity of a fully through-hardened mold.
Comparing Common Plastic Mold Steel Grades
The table below summarizes the grades most often specified for injection molds, blow molds and mold bases, with typical delivery hardness and the applications each handles well. Values are typical for the grades as commonly supplied; always confirm the specific mill's data before finalizing a design.
Two notes on reading the table. First, "equivalent" grades are close but not identical — a P20 produced to ASTM A681 and a 1.2311 produced to DIN specifications share a family resemblance, but carbon, chromium and molybdenum ranges can differ, and melting practice varies widely. Second, the HB-to-HRC conversions shown are approximate; for acceptance testing, use the hardness scale stated on the purchase order and certificate.
The second table matches grade families to common production scenarios, which is how the decision is usually made in practice.
Why ESR Quality Matters for Polishability and Consistency
Two 1.2083 blocks from different producers can polish very differently, and the explanation usually lies in melting practice rather than chemistry. Conventional ingot casting leaves macro-segregation, oxide and sulfide inclusions and, in large sections, scattered porosity. During polishing, hard inclusions tear out of the surface and drag soft matrix with them, producing pits and comet tails that must be polished away — sometimes at the cost of the cavity's geometry. Soft and hard bands caused by segregation polish at different rates and produce the waviness known as orange peel.
Electro-slag remelting (ESR) addresses this at the source. A consumable electrode of the base heat is remelted through a molten slag layer; non-metallic inclusions are absorbed by the slag, and the ingot solidifies in a controlled, directional manner, producing a markedly cleaner and more uniform structure. In practice, ESR steel polishes faster and to a higher standard, etches uniformly for texture, and performs more consistently in fatigue-sensitive cores where thermal or mechanical cycling is severe. It should be specified when the requirement is an A1–A2 finish, transparent or optically functional parts, fine textures, or large mirror surfaces where any defect is visible.
For pre-hardened grades, cleanliness is complemented by a second requirement: hardness uniformity. If delivery hardness is correct at the surface but low at the core of a large block, polishing exposes softer material and the finish degrades or "sinks" as the mold runs. This is one reason to order ESR plastic mold steel or nickel-added grades for large cavities, and to ask for hardness measured through the thickness rather than at the surface only.
Typical Stock Sizes and Cut-to-Size Options
Mold steel is sold both as standard stock and as cut-to-size plate. In the Chinese and wider Asian market, common stock dimensions are thickness 2–600 mm, width 210–810 mm and round bars 20–500 mm in diameter, with lengths up to 5,800 mm; European and American distributors stock broadly similar ranges. A block cut from stock with a bandsaw is normally the fastest and most economical way to buy. Many integrated mills and distributors also offer milling, turning, grinding, straightening and chamfering, so the block arrives closer to finished size and the mold shop's roughing work is reduced.
Manufacturers such as Hubei Keyspark Special Steel Co., Ltd. (Keyspark Steel), based in Huangshi City, Hubei Province, China, produce plastic mold steel in these size ranges and machine it to order — bandsaw cutting, milling, turning and grinding, with export packing for shipment. For a buyer, the practical questions are whether the required grade and thickness are held in stock, what the cut and surface tolerances are, and whether the surface supplied (as-rolled, milled or ground) suits the first machining operation.
Quality Documentation: What a Mill Test Certificate Should Show
Plastic mold steel is bought on specification, and the specification is verified at delivery through documentation. A mill test certificate (MTC) should accompany every shipment and typically records the heat number, the grade, the chemical composition of the actual heat, delivery hardness and — for through-hardening grades — mechanical properties from a test piece. For plate, the certificate should also reference ultrasonic testing results and product dimensions or tolerances. Under EN 10204, a 3.1 certificate issued by the manufacturer's inspection department independently of production is the usual requirement for tooling steel; some buyers additionally request micro-cleanliness ratings or grain size results for critical parts.
Ultrasonic testing deserves particular attention. Standards such as SEP 1921 or ASTM A388 define inspection methods and acceptance classes; a plate that passes a severe class is a different product from one that meets only a basic class, even though both carry test documents. For thick plates, buyers increasingly ask for hardness verification at the core as well as the surface, since two blocks of the same grade and size can differ in through-hardness depending on hardenability and quench practice.
Keyspark Steel, for example, includes an MTC with every shipment; its testing covers chemical composition, hardness, ultrasonic inspection, surface defects, dimensions and mechanical properties. Whatever the supplier, the requirement is the same: the certificate must describe the exact heat that was shipped, and the piece must remain traceable to it after cutting and processing.
Questions to Ask Any Plastic Mold Steel Supplier
The answers reveal more about a supplier than any brochure. A competent plastic mold steel supplier should be able to answer all of the following without hesitation:
- Which melting route is used — EAF with vacuum degassing, ESR, or both — and can ESR be certified on the material certificate?
- What is the guaranteed delivery hardness, and is it verified at the core of thick plates?
- What documentation accompanies the material, and is ultrasonic testing included or optional?
- Which UT standard and acceptance class apply to plate and blocks?
- Which grades and sizes are available from stock, and what are the cut and surface tolerances?
- Can blocks be supplied pre-machined, milled or ground to reduce in-house roughing?
- What is the lead time for non-stock sizes, stainless grades and ESR material?
- How is the heat number marked and traced through cutting and processing?
- For through-hardening grades, is recommended hardening and tempering data supplied?
- How will the material be protected against rust for transport and storage?
Conclusion
Choosing plastic mold steel comes down to a sequence of questions. What plastic is being molded, and is it corrosive or abrasive? How many parts must the mold produce? What surface finish must the part have? How large is the mold, and how thick are the plates? What hardness does the application require, and can the shop machine it? And finally — what is available, at what lead time, with what documents?
Answering those questions narrows the field quickly. Most commodity applications come down to a pre-hardened grade in the 1.2311–1.2738 family. Corrosive resins push the decision to 1.2316, 1.2085 or an ESR stainless through-hardening grade such as 1.2083/S136. Abrasive compounds call for hardened inserts. High-gloss and optical work calls for clean, uniformly hard steel polished to SPI A1–A2.
Whatever the outcome, two habits pay off. First, verify the specification in writing: grade, delivery hardness, cleanliness route, UT class and certificate type. Second, buy from a plastic mold steel supplier that melts, forges, heat treats and machines under its own quality system, because that is where the equivalence between "the same grade" from different sources actually comes from. Integrated producers such as Keyspark Steel, which handles steelmaking, forging, heat treatment and machining in-house, can answer those questions with certificates rather than claims.
Frequently Asked Questions
What is the most common plastic mold steel?
P20-type pre-hardened plastic mold steel — 1.2311, 1.2738 and their equivalents — is the most common choice, typically supplied at approximately 30 HRC. It machines well, requires no further heat treatment, and suits most unfilled commodity resins at moderate to high volumes when properly maintained.
Which steel is best for PVC or flame-retardant plastics?
Corrosion-resistant grades such as 1.2316 (pre-hardened) or 1.2083/S136 (through-hardened to approximately 48–54 HRC) are the usual choice, because PVC and halogenated flame retardants release aggressive by-products that pit unprotected steel. Mold bases and non-polished components can use 1.2085, a machinable, corrosion-resistant stainless grade.
Is pre-hardened steel better than through-hardened steel?
Neither is better in absolute terms. Pre-hardened steel at 28–42 HRC removes heat treatment from the schedule and eliminates distortion risk, while through-hardened steel reaches 48–62 HRC for better wear resistance and polish retention. The choice depends on the plastic, the surface finish requirement and the production volume.
What hardness should a plastic injection mold be?
General-purpose molds are commonly supplied at 28–34 HRC in pre-hardened steel; high-gloss appearance molds often use 38–42 HRC age-hardened grades; through-hardened stainless cavities typically run at 48–54 HRC; and inserts exposed to glass-filled resins are often hardened to 52–62 HRC.
How can I be sure the steel will polish to an SPI A1 finish?
Specify ESR-remelted steel from a controlled melting route — for example ESR 1.2083/S136 or an ESR NAK80-type grade — and request hardness uniformity and cleanliness information on the certificate. A polishing trial on an offcut before machining the cavity is the most reliable confirmation.
What plate and bar sizes are commonly available?
Typical stock ranges of plastic mold steel are thickness 2–600 mm, width 210–810 mm, round bars 20–500 mm, and lengths up to 5,800 mm, with bandsaw cutting to size and optional milling or grinding. Non-standard sizes are usually a mill order with a longer lead time.