Mold Steel
Mold steels are the pre-hardened and stainless die steels used for injection-mold cores, cavities and die-casting inserts. The pre-hardened Cr-Mo grades (P20 / 1.2311 at 28–32 HRC, 718 / 1.2738 with added nickel at 30–36 HRC) machine, polish and texture without further heat treatment, so a cavity goes from block to tryout with no quench distortion; the stainless grades (420 Mod Mold, 2083, 2316) resist corrosion from PVC and flame-retardant resins, and NAK80 is the age-hardened steel that polishes to a mirror.
Popular mold steel grades
Compare in the explorer| Grade | Yield, MPa | Tensile, MPa | Density, g/cm³ | Machinability | Weldability | Best for |
|---|---|---|---|---|---|---|
| P20 | 440 | 700 | 7.8 | Good | Fair | General cavities and cores at 28–32 HRC |
| 718H | 900 | 1,080 | 7.8 | Fair | Fair | Thick blocks, higher volume; Ni-modified P20 |
| 1.2738 | 470 | 760 | 7.8 | Good | Fair | EN twin of 718; uniform hardness in thick plate |
| NAK80 | 700 | 900 | 7.8 | Very good | Fair | Mirror-polish and fine-texture cavities, 40 HRC |
| 2083 | 480 | 760 | 7.7 | Fair | Poor | Stainless mold steel for PVC and medical tools |
| 420 Mod Mold | 470 | 760 | 7.75 | Fair | Poor | Corrosion-resistant cavities, hardened 48–52 HRC |
| 1.2344 | 500 | 800 | 7.8 | Fair | Fair | H13 for abrasive fill, sharp cores, die-casting |
Choose mold steel for
Injection-mold cavities, cores, slides and inserts; blow-mold and compression tooling; die-casting inserts and sprue bushings; prototype and bridge tooling that must be machined and polished quickly; medical, optical and food-contact molds that need a corrosion-resistant, high-polish surface.
Look elsewhere when
The tool is a cold-forming die or punch (A2, D2), the insert sees extreme abrasion from glass-filled fill and even H13 washes out (D2 or CPM 10V), the job is a machined fixture with no polish or texture requirement (pre-hardened 4140 gives the same hardness), or the part must run above about 600 °C (hot-work H21 or nickel alloys). Aluminum mold plate (7075 class) suits prototype molds under a few thousand shots.
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Mold steel by application
- General injection molds
- P20 / 1.2311 up to about 300 mm thick, 718H / 1.2738 above that or when the tool must run past a few hundred thousand shots; both machine and texture directly and take nitriding for extra life.
- High-gloss and optical parts
- NAK80 polished to a mirror finish (SPI A1) — the aged 40 HRC structure has no carbide banding to show through the polish; 2083 / 420 Mod Mold hardened to 50 HRC when the part is also corrosive or medical.
Working with mold steel
- Machining
- P20 and 718 are machined at their delivered 28–36 HRC with coated carbide: high-speed milling of cavities, then EDM for ribs and sharp corners, with no quench step to distort the finished shape. NAK80 at 40 HRC machines slower but cleanly. The stainless grades and 1.2344 are roughed annealed, hardened, then finished by hard milling or EDM with 0.1–0.3 mm stock.
- Polishing and texturing
- Uniformity is what mold grades are made for: low sulfur and remelted (ESR) stock mean fewer pits under a mirror polish. P20 polishes to SPI B1 easily and A2 with care; 718H and 1.2738 slightly better; NAK80, 2083 and 420 Mod Mold reach A1. Chemical and laser texturing etch evenly on all of them; texture depth is limited by hardness on the pre-hardened grades.
- Welding
- Cavities are repair-welded routinely: preheat P20 / 718 to 200–300 °C, TIG with matching filler (P20 or a Ni-modified rod), peen and stress-relieve at about 500 °C; NAK80 welds with its dedicated filler and is re-aged. H13 and the 420-type stainless grades need 300–400 °C preheat and a full re-temper. Weld before final polish and texture.
- Heat treatment
- P20, 718 and 2316 are used as delivered; nitriding (500–520 °C, 0.1–0.3 mm at 650–1,000 HV) or PVD hardens the working faces without distortion. NAK80 is supplied aged at 40 HRC. 1.2344: 1,020–1,050 °C, vacuum quench, double temper for 44–52 HRC. 420 Mod Mold and 2083: 1,000–1,050 °C, temper at 200–250 °C for 48–52 HRC — tempering above 500 °C costs corrosion resistance.
Other steel series
- Low-Carbon and Mild Steel
- Medium and High-Carbon Steel
- Free-Machining Steel
- Alloy Steel
- Case-Hardening Steel
- Tool Steel
- Spring and Bearing Steel
- Structural and HSLA Steel
- Wear Plate and Advanced High-Strength Steel
- Pressure Vessel and Chromium-Molybdenum Steel
- Electrical Steel and Soft Magnetic Alloys
- Aerospace and Maraging Steel
- Cast Steel
- Cast Iron
- Steel Welding Consumables
Mold Steel — frequently asked
What is mold steel?
Mold steels are the pre-hardened and stainless die steels used for injection-mold cores, cavities and die-casting inserts. The pre-hardened Cr-Mo grades (P20 / 1.2311 at 28–32 HRC, 718 / 1.2738 with added nickel at 30–36 HRC) machine, polish and texture without further heat treatment, so a cavity goes from block to tryout with no quench distortion; the stainless grades (420 Mod Mold, 2083, 2316) resist corrosion from PVC and flame-retardant resins, and NAK80 is the age-hardened steel that polishes to a mirror.
Which grade of mold steel should I use?
P20 (1.2311) for most cavities and cores in the 28–32 HRC range: large blocks, general molds for ABS, PP and PA, low to medium volume. 718 / 718H (1.2738, P20+Ni) when the block is thick — the nickel keeps the hardness uniform to the center of 400 mm plate — and for higher-volume tools at 33–37 HRC. NAK80 (40 HRC, age-hardened) for optical and high-gloss parts that need a mirror polish or fine texture. 420 Mod Mold, 2083 and 2316 for PVC, POM, glass-filled and flame-retardant resins that corrode plain steel, and for medical molds that run in humid clean rooms — hardened to 48–52 HRC, or 2316 pre-hardened at about 30 HRC. 1.2344 (H13, 8407) hardened to 48–52 HRC for long-run abrasive resins, sharp cores and die-casting inserts; 3Cr2Mo and 5CrMnMo are the GB equivalents of P20 and a hot-work hammer-die steel.
Which grade of mold steel is the strongest?
P21 has the highest yield strength in the series at 1,150 MPa; 718H follows at 900 MPa. Values are for each grade's default condition.
Which grade of mold steel is easiest to machine?
P5 (machinability very good, 72/100), then NAK55 (70/100).
Is mold steel heat-treatable?
Some grades are. Check the conditions listed on each grade page.
P20 or 718H?
P20 (1.2311) is the base grade at 28–32 HRC; 718H (1.2738) adds about 1 % nickel and is delivered at 33–37 HRC, so it is stronger, holds its hardness through thick blocks (P20 goes soft in the middle of plate over about 300 mm), polishes a little better and lasts longer in production. Use P20 for small and medium tools or prototypes; 718H for large cavities, high-volume programs or when the polish spec is SPI B1 or finer.
Do I need a stainless mold steel?
Yes if the resin releases corrosive by-products — PVC, POM (formaldehyde), flame-retardant ABS or PC, some fluoropolymers — or if the mold runs in a humid environment, is water-cooled with untreated water, or is a medical or optical tool that cannot tolerate rust spots in a mirror polish. 420 Mod Mold, 2083 and 2316 answer those cases at 30–52 HRC; otherwise P20 or 718H with plated or nitrided surfaces is enough.