Mold Steel Selection Guide — How Do You Choose Between P20, H13, S136 and NAK80 for Your Injection Mold, and What Are the Trade-offs?
Last updated: 2026-09-03 · Reading time: about 5 min
P20, H13, S136 and NAK80 are not interchangeable — each is chosen by production volume, resin aggressiveness and the surface finish the part needs. You match the steel to three inputs: how many shots the mold must run, whether the resin is abrasive or corrosive, and what SPI polish grade the part calls for.
See also related article.
Key facts
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Ejector pin — A steel pin that pushes the molded part out of the cavity after cooling. Typical diameter: 2–10mm. Pin marks are visible on the part surface — position at non-cosmetic areas. Pin count and placement affect ejection force and part warpage. See also related guide.
What are the properties of each steel — and when should you use them?
The four steels differ in hardness, mold life, corrosion resistance, polishability and cost. The table below is the selection baseline; the resin and the finish then narrow it to one grade.
| Property | P20 | H13 | S136 | NAK80 |
|---|---|---|---|---|
| Hardness (as delivered) | HRC 28–32 pre-hardened | HRC 48–52 hardened + tempered | HRC 52–56 hardened + tempered | HRC 38–42 pre-hardened |
| Mold life (shots) | 50,000–100,000 | 500,000–1,000,000+ | 500,000–1,000,000+ | 200,000–500,000 |
| Corrosion resistance | Low (not stainless) | Low (not stainless) | High (martensitic stainless) | Low (not stainless) |
| Polishability (SPI) | A2–B1 | B1–B2 | A1–A2 | A1 (mirror) |
| Wear resistance | Low | High | High | Moderate |
| Typical use | Prototype, low volume, non-abrasive resin | Glass-filled resin, high volume, high temperature | PVC, flame-retardant resin, optical parts | Mirror surfaces, cosmetic housings, lenses |
| Steel cost (relative) | 1.0× baseline | 1.5–2.0× | 2.5–3.5× | 2.0–3.0× |
Three factors drive the choice: production volume, resin type and surface finish. A 50,000-shot prototype in ABS needs only P20. A 1,000,000-shot run in glass-filled nylon forces H13. An optical lens in PC needs NAK80 or S136 for the mirror finish.
How do you match the steel to the resin?
The resin is usually the deciding constraint. Abrasive, corrosive and high-temperature grades rule out the baseline steel and force a specific one.
Match the steel to the worst condition the tool will see, not the average. A mold that runs mostly ABS but occasionally a glass-filled or flame-retardant grade should be built to the aggressive grade — swapping inserts mid-program costs far more than the steel upgrade at the start.
“We name the steel grade on every line of the mold breakdown — P20, H13, S136 or NAK80 — with its hardness range and the mill it comes from. If your resin is abrasive or corrosive, we tell you which steel it forces and why, before the deposit. You should never have to guess what is inside your tool.”
— Project Engineer, aCatMould
Frequently asked questions
Can you coat a P20 insert to extend mold life instead of switching to H13?
Yes, within limits. A TiN, TiCN, CrN or DLC coating raises the wear life of P20 by 2–5× — a TiN-coated P20 insert can 150,000–200,000 shots in GF30 nylon, approaching uncoated H13. Coating runs $500–2,000 per insert versus $2,000–5,000 to upgrade the steel.
The limits matter: coatings add no corrosion resistance (PVC still attacks a coated P20 insert), they can chip at sharp corners, and they need a polished substrate. For abrasive resins at moderate volume, a coated P20 insert is a sound alternative to H13.
Does imported P20 (LKM, ASSAB, Daido) differ from domestic Chinese P20?
It differs in cleanliness, homogeneity and through-hardening. Imported P20 has a lower inclusion count, more uniform carbon distribution and consistent hardness through a thick block. Domestic P20 can be softer at the core of a large section.
For thin-wall molds and small parts the gap is negligible. For blocks above 400 mm thick or tolerances below ±0.02 mm, imported steel is the safer call — it warps less in heat treatment and wears evenly. The cost gap is 30–50%.
How do you verify the steel grade — and what documentation should the mold maker provide?
Three documents. First, the steel mill certificate — grade, heat/lot number, composition and hardness, issued by the mill (LKM, ASSAB, Daido), not the mold maker; check it matches the supplier named in the quote.
Second, a hardness test report — each insert measured on Rockwell C against the grade range (P20 28–32, H13 48–52, S136 52–56, NAK80 38–42). Third, for critical molds, a third-party XRF or OES analysis ($100–300 per insert) confirms composition.
Key takeaways
- P20 is the low-volume, standard-resin baseline at 1.0× cost; H13 handles abrasive and high-temperature resins above 500,000 shots.
- S136 is the stainless choice for corrosive resins and optical surfaces; NAK80 is the mirror-finish steel for cosmetic parts and lenses.
- Verify with the mill certificate, a hardness report and — for critical molds — a third-party composition analysis before the tool ships.
Steel selection sets the mold life, the surface quality and the maintenance cost for the whole program. Choosing the grade that matches the resin and the finish prevents premature wear, corrosion and mid-program insert replacement.
Sources
ASSAB — tool steel solutions, reference for stainless and pre-hardened mold steel grades
Böhler — tool steel data and heat-treatment guidelines
MatWeb — material property database, reference for mold steel hardness and composition
Plastics Technology — mold steel selection and wear practice
LKM — mold base and steel supplier, reference for imported P20 grade standards
Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet