Mold Steel (P20 / 718 / NAK80 / H13 / S136)

Mold steel is a specialised category of alloy steel engineered specifically for the fabrication of plastic injection moulds, die casting dies, blow moulds, and compression moulds — the precision tooling that produces polymer and light alloy components. Mold steel differs from standard structural steel (S355, Q345) in several fundamental ways: mold steel is produced to much tighter chemical composition tolerances with strict control of residual elements, undergoes secondary refining (LF/VD vacuum degassing, or ESR for premium grades) to achieve high inclusion cleanliness essential for polishability, and is supplied in pre-hardened condition (HRC 28–43) or with guaranteed through-hardening response — properties entirely absent from structural steel. Compared to general tool steels (H13, D2, O1), mold steels are specifically optimised for the unique combination of properties required by the mould making process: pre-hardened condition enabling direct CNC machining of complex cavity geometries without post-hardening distortion, superior polishability to achieve VDI 0 mirror finish for optical applications, weldability in pre-hardened condition for mould cavity repair without cracking, and adequate toughness under cyclic injection pressure loading (typically 50–200 MPa cavity pressure in injection moulding). The pre-hardened supply condition is the defining commercial characteristic of mold steels — it eliminates the heat treatment step from the mould making workflow, reduces distortion risk in finished cavities, and allows mould makers to begin cavity machining immediately upon material receipt without additional processing.

These four grades represent distinct positions in the mold steel hierarchy, each optimised for different application requirements and performance levels. P20 (AISI P20, DIN 1.2311, GB 3Cr2Mo) is the baseline general-purpose pre-hardened mold steel with 0.35–0.45% C, 1.80–2.10% Cr, 0.30–0.55% Mo, supplied at HRC 28–34 for medium-sized injection moulds of standard engineering thermoplastics (PP, ABS, PA, POM) in production volumes of 200,000–500,000 cycles. P20 is the most economical and widely available mold steel and is the benchmark against which all other grades are compared. 718 (DIN 1.2738, also called P20+Ni) adds 0.85–1.15% nickel to the P20 composition, significantly improving toughness and core hardness uniformity in large cross-section mould blocks (thickness >400mm) where P20 core hardness falls below acceptable minimums — 718 is specified for large automotive bumper, instrument panel, and exterior trim moulds where block thicknesses exceed 400mm and uniform mechanical properties through the entire block cross-section are required. NAK80 (Daido Steel proprietary grade) is a precipitation-hardening pre-hardened mold steel containing copper (0.80–1.20%) and aluminium (0.05–0.10%) in addition to the chromium-molybdenum base, achieving HRC 37–43 pre-hardened condition with exceptional mirror polishability (VDI 0–1 achievable), excellent weld repair characteristics in pre-hardened condition without preheating, and superior resistance to surface etching during EDM — making it the preferred grade for high-appearance automotive headlight, tail light, and Class A interior trim moulds. S136 (ASSAB designation, equivalent DIN 1.2083, AISI 420 modified) is a high-chromium (12–14% Cr) stainless mold steel providing genuine corrosion resistance against PVC, fluoropolymer, and flame-retardant resin processing environments, supplied annealed for full hardening to HRC 48–52 or in pre-hardened S136H condition at HRC 30–35, with excellent mirror polishability for optical and cosmetic mould applications in corrosive environments.

ESR (Electroslag Remelting) is a secondary refining process that dramatically improves mold steel cleanliness, homogeneity, and polishability beyond what standard EAF + LF/VD production can achieve, and represents the quality standard for premium optical and cosmetic mould applications. Standard quality mold steel is produced by electric arc furnace melting, ladle refining with vacuum degassing, and ingot casting followed by forging — providing acceptable quality for general injection moulding of opaque engineering thermoplastics where VDI 12 or coarser surface finish is sufficient and tool life of 500,000 cycles is adequate. ESR grade mold steel subjects the standard ingot to secondary remelting through a controlled liquid slag bath, achieving: dramatically reduced sulfur content (typically ≤0.003% vs ≤0.030% standard), creating fewer and smaller sulfide inclusions that cause pitting and scratching during polishing; reduced non-metallic inclusion content rated A0 per ASTM E45, enabling polishing to VDI 0 (Ra ≤0.01μm) without inclusion-induced surface defects interrupting the mirror finish; more uniform carbide distribution eliminating carbide banding visible on polished surfaces; and improved ultrasonic testability achieving EN 10228-3 quality class 4, enabling full volumetric verification of large mould blocks. ESR grade is specified when: mirror polish to VDI 0–3 is required for optical or cosmetic applications; mould cavities will be used for transparent or translucent parts (clear PC, PMMA, PET) where any sub-surface imperfection reflects as a visual defect in the moulded part; photo-etching or texturing of mould cavities requires uniform steel microstructure for consistent texture depth and appearance; electrical discharge machining (EDM) of fine cavity detail requires homogeneous steel structure for uniform EDM performance and surface quality; or extended mould life exceeding 1,000,000 cycles is required for high-volume automotive production tools. ESR grade commands a 40–80% price premium over standard quality but is economically justified by the extended polishing life, reduced polishing labour cost, and elimination of costly tool rework from inclusion-induced polishing defects.

Mold steel grade selection involves evaluating five key parameters: resin type, mould size, required surface finish, production volume, and budget. Resin type is the first filter — if processing corrosive resins (PVC, fluoropolymers, flame-retardant ABS, halogen-containing compounds), S136 / 1.2083 stainless mold steel is mandatory regardless of other factors to prevent cavity pitting corrosion that ruins part surface quality. For abrasive resins with high glass fibre or mineral filler content (PA66+GF30%, PPS+GF40%, glass-filled PEEK), higher hardness grades (NAK80 HRC 37–43, or fully hardened S136 HRC 48–52) reduce cavity wear and extend mould life compared to softer P20 HRC 28–34. Mould size is the second critical factor — P20 can be through-hardened to HRC 28–34 in cross-sections up to approximately 400mm; for larger mould blocks (automotive bumper moulds, instrument panel moulds, large exterior trim moulds with cavity block thickness 400–800mm), 718 / 1.2738 with nickel addition is specified to ensure adequate core toughness and uniform hardness through the full block cross-section. Required surface finish drives the polishability requirement — standard engineering thermoplastic parts with VDI 12–18 texture can use P20 standard quality; glossy appearance parts (VDI 3–6) require 718 or NAK80 standard quality; mirror finish transparent or optical parts (VDI 0–1) require NAK80 ESR or S136 ESR grade. Production volume determines the minimum hardness and quality level — prototype and short-run moulds (< 50,000 cycles) may use lower-hardness P20 or even pre-hardened mild steel; medium production (50,000–500,000 cycles) suits standard P20 or 718; high production (500,000–2,000,000+ cycles) justifies NAK80, 718H, or S136 grades. Budget is the final constraint — P20 is most economical, 718 carries 15–25% premium, NAK80 25–50% premium, S136 50–80% premium, and ESR grades add 40–80% to standard grade pricing.

Mold steel is supplied in multiple product forms precisely matched to the mould making workflow. Pre-Hardened Plates are the dominant product form for cavity blocks and core blocks, available in standard thicknesses of 10, 15, 20, 25, 30, 40, 50, 60, 70, 80, 90, 100, 120, 150, 160, 180, 200, 250, 300, 350, and 400mm, widths from 400mm to 2000mm, and lengths from 400mm to 3000mm (standard mill lengths), certified to specified hardness at surface and verified at block centre. Custom saw-cut dimensions to within ±2mm tolerance are available from standard plate stock, with minimum 5–10 working days processing time for standard grades. Round Bars are produced in diameters from 20mm to 600mm in standard mill lengths up to 6 metres and custom cut lengths, in pre-hardened or annealed condition, for round inserts, core pins, cylindrical cavity components, and rotating cores in stack moulds. Flat Bars (rectangular bars) in thickness from 10mm to 250mm and width from 50mm to 1200mm serve as blanks for rectangular mould inserts, side actions, and cavity components where the flat bar cross-section closely matches the finished component geometry, minimising machining waste. Custom Forged Blocks are produced to customer-specified dimensions for very large single-piece mould cavity blocks (automotive bumper moulds, instrument panel moulds) that exceed standard plate dimensions, with forging dimensions up to 2500×1800×700mm and piece weights up to 50 tons. Custom forging with rough machined faces (5–10mm machining allowance per face), guaranteed through-hardness at centre cross-section, and full ultrasonic testing is available. Value-added services including CNC face milling to specified flatness (≤0.1mm over full face), precision surface grinding (Ra 0.8–1.6μm), mirror polishing, and EDM surface preparation are available for projects requiring minimal machining work by the mould maker. Custom marking including laser etching of grade, hardness, heat number, and customer part number on each piece is available for supply chain traceability in high-volume mould making operations.