Injection Mold Steel Selection: P20, H13, S136, and NAK80
Injection Mold Steel Selection: P20, H13, S136, and NAK80
Injection mold steel selection determines whether your tool runs 500,000 cycles or falls apart at 50,000. Get it wrong and you are looking at premature wear, surface defects, or a full resteel that costs as much as the original build. In our shops, the four grades that cover roughly 90% of all decisions are P20, H13, S136, and NAK80. This guide gives you the numbers to choose correctly the first time.
Why Steel Grade Drives Total Tool Cost, Not Just Material Cost
The raw material price difference between P20 and H13 looks small on a purchase order. A P20 block for a mid-size core might run $1.80 to $2.10 per pound. A comparable H13 block runs $2.60 to $3.20 per pound. That delta looks manageable until you factor in what happens downstream.
H13 requires stress-relief cycles and vacuum heat treatment to reach working hardness of 44 to 52 HRC. That adds $800 to $2,500 per mold half in a typical offshore shop, depending on block size. P20 ships pre-hardened to 28 to 32 HRC and goes straight to the machining center. If your part geometry is simple and your volume is under 500,000 shots, paying for H13 processing is waste.
Steel grade also controls cycle time. A core block with poor thermal conductivity forces longer cooling, which adds seconds per shot. At 100 million annual shots across a four-cavity tool, two extra seconds per cycle costs you roughly 2,300 machine hours per year. That number turns a “small” steel decision into a six-figure production cost.
Mold Steel Comparison: Composition and Hardness
Understanding what each grade is made of explains why it behaves the way it does under heat, pressure, and abrasive resins. The table below covers the four primary grades used in injection tooling sourced from China-based suppliers.
| Grade | Standard / Equivalent | Carbon % | Chromium % | Hardness (HRC) | Approx. Cost (USD/lb) |
|---|---|---|---|---|---|
| P20 | AISI P20 / DIN 1.2311 | 0.35 | 1.9 | 28 to 32 (pre-hardened) | $1.80 to $2.10 |
| H13 | AISI H13 / DIN 1.2344 | 0.39 | 5.2 | 44 to 52 (after heat treat) | $2.60 to $3.20 |
| S136 | ASSAB S136 / DIN 1.2083 | 0.38 | 13.6 | 30 to 34 (pre-hardened); up to 52 (hardened) | $4.10 to $5.50 |
| NAK80 | Daido NAK80 / DIN 1.2767 approx. | 0.15 | 3.0 | 37 to 43 (age-hardened) | $3.40 to $4.20 |
Cost figures reflect offshore mill pricing as of 2024. Domestic US pricing runs 30% to 60% higher depending on supplier and block size. Hardness ranges for S136 reflect two common supply conditions; most offshore shops order it pre-hardened and then finish-harden after rough machining.
P20 vs H13: The Core Decision for High-Volume Tools
The p20 vs h13 debate comes down to three variables: resin abrasiveness, required surface finish, and shot volume. P20 handles general-purpose resins well at volumes under 500,000 shots. It machines fast, welds easily, and does not require post-machine heat treatment. For a 250,000-shot consumer goods tool running ABS or PP, P20 is the right call every time.
H13 earns its place when you are running glass-filled nylon (GF30 PA66), glass-filled PBT, or any abrasive resin above 20% filler loading. At 44 to 52 HRC, H13 resists gate erosion and parting line wear long enough to justify the heat treatment cost. According to ASTM International, H13 is classified as a chromium hot-work tool steel specifically because its 5.2% chromium content maintains hardness under thermal cycling, which mirrors the gate and core conditions in a high-cavitation mold running short cycle times.
H13 also handles thermal fatigue better than P20. When you are running a 12-second cycle in an 8-cavity tool, the gate area sees rapid heating and cooling thousands of times per hour. P20 at 28 to 32 HRC will show heat checking at the gate land after roughly 100,000 cycles in those conditions. H13 at 48 HRC typically shows no heat checking through 500,000 cycles under the same conditions, based on published data from Uddeholm’s mold steel technical guides.
S136 Mold Steel: When Corrosion Resistance Is Non-Negotiable
S136 mold steel is the correct choice for two situations: corrosive resins and optical-quality surface finishes. With 13.6% chromium, S136 qualifies as a stainless mold steel, which means it resists the hydrochloric acid off-gassing produced by PVC and the moisture absorption issues that plague flame-retardant ABS and PC/ABS blends. No other commonly available tooling grade gives you that combination of corrosion resistance and polishability in a single material.
Polishability is where S136 separates from the field. The fine carbide structure in S136 allows polishing to SPI A1 (Society of Plastics Engineers designation, equivalent to Ra 0.012 to 0.025 micron) without pull-out or pitting. P20 polishes to SPI A2 reliably but struggles at A1. H13 can reach A1 but the harder matrix makes polishing labor-intensive, adding $400 to $900 per cavity in finishing labor at offshore rates.
The cost premium for S136 is real. Expect to pay 90% to 150% more per pound compared to P20. For a single-cavity lens mold or a medical device part requiring Class VI biocompatibility traceability, that premium is justified. For a cosmetic housing with a semi-gloss texture finish, you are paying for performance you will never use.
NAK80 Mold Steel: The Polishability and Weldability Compromise
NAK80 mold steel occupies a specific niche that neither P20 nor S136 fills cleanly. It is an age-hardening maraging steel from Daido Steel that reaches 37 to 43 HRC without conventional heat treatment. You machine it in the annealed state, then age it at 490 degrees Celsius for several hours to reach working hardness. That process produces very low distortion, which matters on complex core geometry with tight tolerances of plus or minus 0.01 mm or tighter.
The weldability of NAK80 is exceptional. Our project managers on Class A automotive surface programs use NAK80 specifically because engineering changes late in the tool build require weld repairs that must blend invisibly into a polished or textured surface. NAK80 welds accept polish at the same rate as the parent material. H13 weld repairs, by contrast, often read differently under texture or high polish, requiring secondary work to correct.
NAK80 is not a stainless mold steel. At only 3% chromium, it offers no meaningful corrosion resistance. If you are running PVC or halogen-containing flame retardants, S136 is the correct choice regardless of weldability requirements. Combining NAK80 with a cavity insert of S136 is a valid architecture for programs that need both polishability and localized corrosion resistance at the gate area.
Injection Mold Steel Selection Decision Matrix
The table below is the filter we apply at the start of every new program. Run your job through these criteria and the steel grade becomes obvious in most cases. The variables are shot volume, resin type, required SPI finish class, and corrosion risk.
| Scenario | Shot Volume | Resin | Finish Class | Recommended Grade |
|---|---|---|---|---|
| General-purpose prototype or bridge tool | Under 100K | ABS, PP, PE | SPI B2 or lower | P20 |
| High-volume commodity part | 500K to 2M | ABS, PP, HIPS | SPI B1 to A3 | P20 (nitrided at gate) |
| Abrasive-filled resin tool | 500K+ | GF Nylon, GF PBT, mineral-filled PP | SPI B1 or lower | H13 (48 to 52 HRC) |
| Optical or Class A surface tool | Any | PC, PMMA, ABS | SPI A1 to A2 | S136 or NAK80 |
| PVC or corrosive resin tool | Any | PVC, FR-ABS, POM | Any | S136 (stainless mold steel) |
| Late-change complex core, Class A surface | Under 500K | ABS, PC/ABS | SPI A2 to A1 | NAK80 |
One important note on nitriding: for P20 tools running non-abrasive resins at volumes between 300,000 and 700,000 shots, a plasma nitride treatment on gate inserts and core tips adds a surface hardness of 65 to 70 HRC at a depth of 0.1 to 0.3 mm. Cost is typically $150 to $350 per treated component. That treatment extends P20 tool life significantly without the full cost of an H13 build.
Frequently Asked Questions
What is the practical difference between P20 and H13 for a 1-million-shot program?
For non-abrasive resins like ABS or PP, a properly built P20 tool with nitrided gate inserts will run 1 million shots without major maintenance. For glass-filled or mineral-filled resins, H13 at 48 to 52 HRC is required to prevent gate erosion and parting line wear from degrading part quality before you reach that shot count. The cost difference on a mid-size tool is typically $4,000 to $9,000 more for H13 including heat treatment, which is recoverable through reduced maintenance costs over the tool life.
Is S136 the same as 420 stainless steel?
No. S136 is a proprietary grade from ASSAB (a subsidiary of Voestalpine) with tighter alloy controls and a finer carbide distribution than standard 420SS (AISI 420 stainless). Both are martensitic stainless steels with roughly 13% chromium, but S136 achieves better polishability and more consistent hardness response. If a supplier quotes you 420SS as a substitute for S136, it is not an equivalent substitution for optical or medical tooling applications.
Can you mix steel grades in a single mold?
Yes, and we do it regularly. A common architecture pairs a P20 mold base with H13 cavity inserts for abrasive-resin tools, or S136 gate and core inserts inside a NAK80 cavity block for complex optical parts. The key engineering constraint is matching thermal expansion coefficients. P20 and H13 have similar coefficients (approximately 11.5 to 12.5 micron per meter per degree Celsius), so interference fits remain stable across processing temperatures. Always confirm the fit calculation with your toolmaker before finalizing the insert stack.
How does offshore injection mold steel selection differ from domestic?
The grade names are often the same but the supply chain is not. In China, P20 equivalent is typically supplied as DIN 1.2311 or branded grades like ASSAB 618. Hardness certification is common but third-party chemical composition verification is not always standard practice. On any tool valued above $25,000, we specify third-party PMI (positive material identification) testing of mold steel blocks before machining begins. That step costs roughly $300 to $600 per tool and has caught misgraded material on 4 out of every 100 tools we have audited over the past five years.
What is the cycle life difference between P20 and S136?
For non-corrosive, non-abrasive resins, the cycle life difference between P20 and S136 is minimal if both are properly hardened and maintained. The real advantage of S136 is not raw cycle count but surface retention. S136 holds SPI A1 polish through 500,000 or more cycles on optical parts where P20 would show micro-pitting from moisture and corrosive outgassing well before that point. Use cycle life as a secondary metric; use surface retention and corrosion resistance as your primary selection filters for S136.
Use our injection molding consulting service to get a steel grade recommendation and mold specification review before you send an RFQ to any offshore supplier. Getting the spec right on paper costs nothing; correcting a misgraded tool after machining starts costs real money.