Mold Steel Selection Guide — P20, H13, S136, NAK80

A buyer approves a mold quote, and six months later the tool is producing flash and dimensional drift at 400,000 shots. The mold was specified in P20 — the cheapest steel on the quote. The program needed 2 million shots over three years. The $3,000 saved at tooling buy is now costing $50,000 in downtime, repairs, and rejected parts. The steel selection was wrong, and the error was made before a single block was machined.

Mold steel is the most consequential decision in a tooling project because it cannot be changed later. The cavity and core are machined from the steel — upgrading means cutting a new tool. This guide covers the four workhorse steels plus the specialty grades, how to select by production volume, material, and mold temperature, and when surface treatments make sense.

What Mold Steel Actually Does

A mold steel must do four jobs simultaneously, and no steel is best at all four:

1. Wear resistance — the melt erodes gates, ribs, and thin features shot after shot. Glass-filled resins are abrasive; unfilled commodity resins are not.
2. Hardness — hardness determines both wear resistance and shot life, but excessive hardness makes the steel brittle and difficult to machine.
3. Corrosion resistance — PVC, flame-retardant grades, and some medical resins release corrosive gases during processing that attack unprotected steel.
4. Machinability and polishability — the steel must be machined to ±0.005mm and polished to the required surface finish. High-hardness steels machine slowly; some steels polish to a mirror, others do not.

The selection process matches these four jobs against the program: how many shots, with what resin, at what mold temperature, with what surface finish. The table below is the working reference:

The Four Workhorse Steels

P20 (718H) — The Default

P20 is pre-hardened mold steel at 28–34 HRC, supplied ready to machine without heat treatment. It is the default for a reason: it machines well, polishes acceptably, and costs the least. For programs under 300,000 shots with non-abrasive resins — ABS, PP, unfilled PC — P20 is the economically correct choice, not the cheap choice.

Its limits are equally clear. At 28–34 HRC it wears visibly in gated areas on glass-filled material, its corrosion resistance is minimal, and it cannot hold a mirror polish for optical parts. P20 is a volume-and-material decision: low shot count, benign resin.

Selection rule: P20 for programs under 300K total shots with commodity resins. Above that, the marginal cost of H13 is returned many times over in tool life — see mold maintenance and tool life.

H13 — The Production Workhorse

H13 at 48–52 HRC is the standard production steel: roughly 2× the shot life of P20, higher hot hardness for high-temperature resins (PA66, POM, PEEK all process above 270°C where P20 softens and wears faster), and better resistance to glass-fiber abrasion.

The trade-off is machinability and cost — H13 machines slower than P20 and adds roughly 40% to the steel cost of the tool. On a 1M-shot program that difference is trivial per part. On a 50K-shot program it is wasted money.

Selection rule: H13 for anything above 500K shots, any glass-filled or high-temperature resin, and any program where unscheduled tool maintenance is more expensive than the steel.

S136 (420 Stainless) — For Corrosive and Optical Work

S136 is stainless mold steel at 48–52 HRC. It exists for two situations P20 and H13 cannot handle: resins that off-gas corrosives (PVC, some flame-retardant grades) and parts that require a true mirror finish (optical lenses, light guides, high-gloss cosmetic surfaces).

The corrosion mechanism matters: PVC releases hydrochloric acid during processing, which pits unprotected steel within weeks. A pitted cavity produces a textured part where the drawing calls for gloss. S136 resists the acid; P20 does not. See the surface finish and texture standards for what each finish class demands of the steel.

Selection rule: S136 for PVC, corrosive or high-gloss optical resins, and medical parts where surface integrity is part of the specification.

NAK80 — For Mirror Polish

NAK80 (38–43 HRC) is a precipitation-hardened steel developed for one job: mirror polishing. It polishes to SPI A1 (Ra 0.012μm) faster and more reliably than any other common mold steel, which is why it is the standard choice for optical lenses and high-gloss cosmetic parts.

Its shot life is P20-class, so it is a surface-finish decision, not a volume decision.

Selection rule: NAK80 when the part is optical or mirror-cosmetic and the volume does not justify hardened stainless — otherwise S136.

2344 — The Long-Run Specialist

2344 (44–52 HRC) is the long-run production steel: 800K–1M+ shot life, used for automotive and other programs where the tool must outlive multiple product generations. It is the steel to specify when the program plan says “this tool will run for ten years.”

Selection by Program Profile

Steel selection reduces to three questions, answered in order:

1. How many shots over the tool’s planned life?

• Under 300K → P20 class
• 300K–500K → P20 with surface treatment, or H13 if abrasive
• 500K–1M → H13
• 800K+ / multi-generation → 2344

2. What resin?

• Abrasive (GF15–GF50, mineral-filled) → minimum H13, 2344 for long runs
• Corrosive (PVC, FR grades) → S136
• High-temperature (PA, POM, PEEK, >270°C) → H13 or 2344
• Commodity (ABS, PP, PE, unfilled PC) → P20 unless volume says otherwise

3. What surface?

• Mirror / optical → NAK80 or S136
• VDI texture / standard → any steel in the hardness class
• The polish requirement alone can force a steel upgrade even on a low-volume program

The program profile that fails most often: high-volume consumer electronics housings quoted in P20 because “the part is simple ABS.” The part is simple; the volume is not. 2 million shots in P20 means visible gate wear by year one and dimensional drift by year two — on a program that was supposed to run five.

Surface Treatments: Extending the Steel You Have

Surface treatments modify the cavity surface after machining, adding wear or corrosion performance to a cheaper base steel:

• Chrome plating — adds a hard, corrosion-resistant surface layer to P20. Standard for PVC-free low-cost tools where moderate corrosion or wear protection is needed. Plating thickness is 0.02–0.08mm; it can chip at sharp corners.
• Nitriding — diffuses nitrogen into the steel surface, raising surface hardness without dimensional change. Effective on H13-class steels for abrasive resins; the treated layer is 0.1–0.3mm deep.
• PVD/CVD coatings — thin ceramic coatings for gated areas and high-wear features. Effective but expensive; typically reserved for the highest-wear features of production tools.

The honest economics: a treated P20 tool still does not match an H13 tool on total cost of ownership for a long program. Treatments extend a steel one class, not two. Use them to protect specific features — gates, shut-offs, thin ribs — rather than to substitute for steel selection.

What the Mold Quote Should Tell You

A serious mold quote specifies more than a steel name. Demand:

• Steel grade and hardness range, with the supplier’s standard (28–34 HRC for P20, 48–52 HRC for H13) stated in writing
• Steel certificates with heat numbers, and hardness test reports measured on the finished tool — not the nominal grade
• Shot-life expectation for the specified steel with your resin
• Surface treatment plan if any, with treatment type and coverage areas

The verification comes at tool acceptance: hardness measured on the actual cavity block, certificates matching the quote, and the mold trial process confirming the tool performs as specified. See supplier qualification for how steel specification fits the broader audit.

Frequently Asked Questions

Is P20 ever the right choice for production tooling? Yes — for programs under 300K shots with commodity resins, P20 is the economically correct steel. The failure is not choosing P20; it is choosing P20 for a 2-million-shot program to save 10% on tooling cost.

What is the practical difference between H13 and 2344? 2344 is the long-run variant: 800K–1M+ shot life versus 500K–1M+ for H13, with better toughness for high-pressure automotive molds. The cost premium is modest; specify 2344 when the tool must outlive multiple product generations.

Can a P20 mold be upgraded to H13 later? No. The cavity is machined from the steel block; upgrading means cutting a new tool. Steel selection is irreversible — which is why it belongs in the quoting stage, not the maintenance stage.

Which steel for a 500K-shot medical part in PC? S136. The volume is H13-class, but medical optical-clear PC demands the corrosion resistance and polish of stainless. The specification is driven by the material and the surface, not the volume alone.

How do I verify the steel is what the supplier claims? Request heat-number certificates at tool delivery and measure hardness on the finished cavity block — a hardness tester reading 30 HRC on a tool specified as H13 is a failed acceptance, regardless of what the invoice says. See the mold trial process for the acceptance workflow.

Does glass-filled material really require H13? GF15 and above, yes — glass fiber erodes P20 gated areas within tens of thousands of shots, producing flash and dimensional drift. The upgrade from P20 to H13 on glass-filled programs is the single most reliable tooling decision in injection molding.

The Steel Question in One Line

The steel decision asks one question: what does this tool need to survive — how many shots, of what resin, at what surface — and which steel does that for the lowest total cost, not the lowest purchase price?

Review our mold manufacturing capability — P20, H13, S136, NAK80, and 2344 tooling built in-house with steel certificates and hardness verification on every tool — or submit your part for a tooling quote with a written steel specification.