How to Choose Mold Steel for Injection Molding: P20 vs NAK80 vs S136

For most injection molds, P20 covers general-purpose work up to about 300,000 shots, NAK80 is the choice when you need a mirror finish and high cavity counts, and S136 stainless is what you specify for medical, food-contact and corrosive or glass-filled resins. The decision is not about which steel is best — it is about matching hardness, polishability and corrosion resistance to your volume, your resin and your surface specification. The comparison table below puts the five grades we see most often on customer drawings side by side.

Key takeaways

  • Hardness, not price, decides tool life: P20 at 28–32 HRC and 1.2344 (H13) at 48–52 HRC are not interchangeable.
  • Specify stainless such as S136 when the resin is corrosive, filled, or when the part is medical or food-contact.
  • NAK80 exists for one reason: mirror polish plus dimensional stability in multi-cavity tools.
  • European designations (1.2344, 1.2738HH, 1.2842) and Asian trade names (P20, NAK80, S136) describe overlapping steels — match on properties, not on the label.
  • Tool life claims are conditional: up to 1,000,000 shots depends on steel grade and maintenance.

In this guide

  • 1. The short answer: a comparison table
  • 2. P20 tool steel
  • 3. NAK80
  • 4. S136 stainless steel
  • 5. The European grades on your drawing
  • 6. Matching steel to volume, resin and finish
  • 7. What we stock and machine
  • 8. Choosing the right steel for your project

1. The short answer: a comparison table

Mold steel is a trade between three properties: hardness for wear life, cleanliness and structure for polishability, and alloy content for corrosion resistance. No single grade is best at all three, which is why the same shop will quote you different steels for two parts that look similar.

Grade Hardness Strength Best suited to
P20 28–32 HRC, pre-hardened Machinability, cost General-purpose tools, low to medium volume up to ~300,000 shots
NAK80 38–42 HRC, pre-hardened Mirror polish, stability High-cavity and optical tools, 500,000–1,000,000 shots
S136 (1.2083) 48–52 HRC after hardening Corrosion resistance Medical, food-contact, corrosive and glass-filled resins
1.2344 (H13) / 1.2343 (H11) 48–52 HRC after hardening Hot strength, toughness High-volume production, hot runners, abrasive fills
1.2738HH ~33–36 HRC, pre-hardened Large-section consistency Large molds where deep hardening matters

If your volume is under 300,000 and your resin is ordinary, P20 or 1.2738HH will do the job and cost less. Everything above that line is where the harder, cleaner, more expensive grades earn their price.

2. P20 tool steel

P20 is supplied pre-hardened at 28–32 HRC, which means it can be machined and put into production without a separate heat-treat step. That is the source of its two real advantages: lower cost, and a shorter lead time, because nothing has to leave the shop for hardening and come back to be finished.

Its limits follow from the same number. At 28–32 HRC the cavity is soft enough that abrasive resins wear it, and soft enough that a shut-off or a gate detail can deform under pressure. P20 is a sensible choice up to roughly 300,000 shots on unfilled resins, and a poor choice for a glass-filled engineering resin running five years.

Where P20 is genuinely underrated is on large, simple parts at moderate volume — the case where the part geometry is stable, the tolerances are reasonable, and the money is better spent on cavity count than on steel grade.

3. NAK80

NAK80 is a premium pre-hardened grade at 38–42 HRC, developed around polishability and dimensional stability. It takes a mirror finish far more reliably than P20 because it is cleaner and more uniform through the section, and it holds that finish without the micro-pitting that softer steels develop.

That combination is why NAK80 shows up on high-cavity tools and optical parts. In a 32 or 48 cavity tool, a variation in hardness between cavities becomes a variation in part dimension, and the cost of sorting good parts from bad ones quickly exceeds the steel price difference. The same logic applies to transparent parts, where any polish defect reads as a visual defect on the finished product.

NAK80 typically supports 500,000 to 1,000,000 shots, again depending on the resin and on how the tool is maintained.

4. S136 stainless steel

S136 (1.2083) is a martensitic stainless steel, hardened to 48–52 HRC. It is specified for one dominant reason: corrosion resistance. That matters in three situations.

  • Medical and food-contact parts, where a rusting cavity is both a quality and a compliance problem.
  • Corrosive resins, particularly PVC and other grades that release acidic by-products during processing.
  • Humid or poorly controlled plant conditions, where a carbon steel tool will bloom with surface rust between runs.

S136 also holds a good polish, which makes it a common choice for medical parts that need both hygiene and surface quality. Expect a million-plus shots with proper maintenance, and budget for the heat treatment cycle, which adds lead time compared with a pre-hardened grade.

5. The European grades on your drawing

If your drawings come from a European OEM or Tier 1, the labels will differ from the Asian trade names even when the steel behaves similarly. The four you are most likely to see:

  • 1.2344 (H13) — hot-work steel, 48–52 HRC, the standard for high-volume production tools and for hot-runner manifolds.
  • 1.2343 (H11) — close relative of 1.2344, slightly tougher, often chosen where thermal cycling is severe.
  • 1.2738HH — pre-hardened around 33–36 HRC, with the uniform hardness through large sections that big molds need.
  • 1.2842 (O1) — oil-hardening cold-work steel, common for cores, inserts and wear plates rather than for a whole cavity block.

ESR (electro-slag remelted) versions of 1.2343 and 1.2344 appear on drawings where cleanliness matters — high-gloss surfaces, thin walls, or large tools that must not show polish variation. ESR costs more and is worth it in exactly those cases.

6. Matching steel to volume, resin and finish

Three questions resolve most grade decisions:

  1. How many shots? Under 300,000, a pre-hardened grade is usually sufficient. Above that, move to a hardened hot-work or stainless grade.
  2. Is the resin abrasive or corrosive? Filled grades and PVC push you toward S136 or a hardened hot-work steel regardless of volume.
  3. What surface finish is specified? A mirror or optical finish pushes you toward NAK80 or an ESR grade; a texture or a matte finish does not.

Where the answers conflict — high volume plus a glass-filled resin plus a mirror finish — the steel usually resolves to an ESR stainless or a hardened ESR hot-work grade, and the honest conversation is about which of the three requirements can be relaxed, because the specification you can afford is not always the one on the drawing.

7. What we stock and machine

Our answer: the grades we run most often are 1.2344 (H13), 1.2343 (H11), 1.2738HH, S136, P20 and 1.2842, including ESR versions of 1.2343 and 1.2344 where the drawing calls for them. Standard frames come from LKM, DME or HASCO, or are built in-house; hot runner systems come from HRS, Yudo, Husky, Incoe and Mold-Masters.

All of it is cut in our own toolroom: 13 CNC machining centres with travels up to 1,600 mm, six EDM machines including Sodick mirror-finish and twin-head units, three wire EDMs and five surface grinders, with dimensional inspection on CMM. We machine critical mold dimensions to ±0.005 mm, and a tool cut to that tolerance holds ±0.01 mm on critical molded dimensions, subject to the resin and the part geometry. Tools run to 1,600 × 1,000 × 500 mm and 15 t, with cavity counts up to 48.

When you send a drawing, you get a written DFM report within three working days at no cost, modelled in UG and checked through Moldflow. That report states which grade we recommend, why, and what the alternative would cost — including the cases where we recommend a cheaper steel than the one specified. Lead time to first article sample runs 35 days for a simple tool, 42 for a medium one and 50 for a complex one; 30 days is achievable when the design is frozen and components are in stock.

8. Choosing the right steel for your project

If you take one rule from this page, take this one: match the steel to the resin and the volume first, and treat the surface specification as the thing you negotiate afterwards. A tool specified in the wrong steel does not fail loudly — it wears quietly, produces marginal parts for a year, and then explains why your reject rate crept up.

Send us the drawing, the resin and the volume. We will tell you which of these grades fits, and what it changes about the quote.

Related guides in this series

This article is part of our Injection Molding Materials & Mold Steel Guide — a full walkthrough of the topic with the numbers and checklists behind each decision.

Get an engineering answer, not a sales pitch

Send us the part drawing, the resin grade and your expected annual volume. We will come back with a steel recommendation, a design review and an itemised quote — and if a cheaper grade will do the job, we will say so. Talk to a manufacturing engineer.