Injection Mold Steel Selection: P20 vs 718H vs H13 vs S136 (2026)

Two quotes land on your desk for the same part. One is 40% cheaper. The supplier says the difference is mold steel: one tool is cut from P20, the other from S136. Now you have to justify the budget decision to your own team without going back to materials science. This guide gives you the framework — steel grades compared on hardness, mold life, corrosion resistance, polish and cost — so you specify the right cavity steel before any steel is cut.

Steel Is Not a Material Purchase — It Is a Tooling Investment

Most buyers treat the steel grade as a line item to negotiate down. That is the single most expensive mistake in mold sourcing. Mold steel sets four things that you cannot change later without scrapping and re-machining the tool: wear life, surface finish capability, corrosion resistance and maintenance cost.

Once the block is roughed out, changing grades means starting over — a mistake that typically costs several thousand dollars and two to four weeks. The steel decision is made once, at the quotation stage, and it is locked in for the entire life of the tool.

The Four Grades That Cover Most Molds

Globally, the vast majority of production molds are cut from one of four families. They differ mainly in hardness, polishability, corrosion resistance and price.

P20 / 3Cr2Mo — the general-purpose default

P20 is a pre-hardened alloy steel, supplied at roughly 28–34 HRC. It machines fast, welds and repairs easily, and needs no heat treatment after machining. It is the correct choice for general-purpose thermoplastics — PP, PE, ABS, PS, standard nylon — at medium volumes. Its weaknesses are wear rate and zero corrosion resistance: abrasive fillers wear it quickly, and acidic resins pit it within weeks.

718H / 1.2738 — P20 with better polish

718H is a nickel-modified P20, supplied pre-hardened at roughly 33–38 HRC. It polishes to a higher gloss, holds hardness more uniformly across a large block, and costs around 10–20% more than P20. It is the pragmatic middle step when you need better surface quality or a longer-running tool but cannot justify stainless.

H13 / 1.2344 — the high-volume workhorse

H13 is a hot-work tool steel, hardened to roughly 48–52 HRC by vacuum heat treatment after rough machining. It resists abrasive wear far better than P20 and tolerates high melt temperatures without softening — the right call for glass-filled resins, high cavitation and engineering plastics processed above 280 °C. It is not corrosion-resistant, and the extra heat-treatment cycle adds time and cost to the build.

S136 / 1.2083 (AISI 420) — stainless, for corrosion and optical parts

S136 is a high-chromium stainless tool steel, hardened to roughly 48–54 HRC. The chromium content gives genuine corrosion resistance against the hydrochloric acid released by PVC and many flame-retardant compounds, and its clean, low-inclusion microstructure polishes to a true optical (mirror) finish. It is the only safe choice for PVC, food-contact and transparent parts — and the most expensive of the four.

Steel Grade Comparison at a Glance

Grade	Hardness (HRC)	Corrosion	Polish	Typical Mold Life	Relative Steel Cost
P20 / 3Cr2Mo	28–34 (pre-hard)	Poor	Good (SPI B/C)	300,000–500,000 shots	1.0× (baseline)
718H / 1.2738	33–38 (pre-hard)	Low	Very good (SPI A2–B1)	500,000–800,000 shots	1.1–1.2×
H13 / 1.2344	48–52 (heat treated)	Medium	Fair–good	1,000,000+ shots	1.3–1.5×
S136 / 1.2083	48–54 (heat treated)	Excellent	Mirror (SPI A1)	800,000–1,500,000+ shots	2.0–2.5×

Read these ranges as industry-typical values under well-maintained production conditions, not guarantees. Real mold life depends as much on design, cooling, ejection and maintenance as on the grade itself. Note also that the cost multiplier does not scale linearly with life: a tool that costs 2× more but delivers 3× the cycles is usually the cheaper option per part.

Step 1 — Decide by Production Volume

Volume alone eliminates most of the options before you look at anything else.

Expected Total Volume	Recommended Cavity Steel
Under 100,000 shots	P20 — cost-optimal, fast to machine
100,000–500,000 shots	718H, or P20 if finish requirements are modest
500,000 shots and above	H13 (or S136 where corrosion or optics apply)
Prototype / bridge tooling	P20 or aluminium — do not invest in hardened steel

Beyond roughly 500,000 shots, hardened steel almost always pays for itself. Below 100,000 shots, spending on S136 buys corrosion resistance and polish your application may never need.

Step 2 — Decide by Resin

This is where a wrong grade becomes a scrap tool, not just a slower one.

• Standard resins (PP, PE, ABS, PS): P20 or 718H are sufficient.
• Corrosive resins (PVC, CPVC, POM, halogenated flame retardants): S136 or 1.2316 stainless is mandatory. Running PVC in P20 can produce visible cavity erosion within tens of thousands of shots.
• Abrasive resins (glass-filled PA, PBT, PP with 15–50% GF): H13 is effectively the minimum specification; consider a surface treatment such as nitriding or a PVD coating for the most aggressive grades.
• High-temperature engineering resins (PEEK, PPS, PEI, LCP): H13 or a premium hot-work grade, processed well above 300 °C.

Step 3 — Decide by Surface Finish

Finish requirements are usually expressed in SPI grades, from A1 (mirror) to D3 (matte). Only steels with a clean, uniform, low-inclusion microstructure can hold a mirror finish after polishing.

• SPI A1 / A2 (mirror, transparent, optical): S136, or 718H for a slightly less demanding gloss.
• SPI B1 / B2 (semi-gloss, textured, etched): 718H or P20 — texture etching works perfectly well on both.
• SPI C / D (matte, functional, hidden): P20 is fully adequate; paying for polishability here is wasted budget.

What the Steel Upgrade Actually Costs You

The steel price premium is real, but it is rarely the biggest line on the quote. Machining, EDM and polishing typically make up a much larger share of total mold cost, and harder steels machine more slowly. A steel upgrade from P20 to S136 can add 30–60% to the steel line while adding a smaller percentage to the total tool price — plus time for the heat-treatment cycle. To see how steel choice sits inside the full cost picture, read our breakdown of what drives silicone injection molding cost.

The right comparison is not "which tool is cheaper" but "which tool is cheaper per usable part, including maintenance." A tool that needs a cavity refurbishment halfway through your production run is not cheaper — it is a schedule risk on top of a cost.

What Harder Steel Does Not Fix

Upgrading the grade is not a substitute for good design. Hardened steel will not rescue a badly located gate, an unbalanced cooling layout, a weak ejection plan or a part with insufficient draft. Three common mistakes we see in review:

• Upgrading steel to compensate for a poor part design. Fix the geometry in the DFM stage instead.
• Over-specifying hardness. A harder steel on a complex cavity means longer machining, higher cost and a greater risk of cracking — for a life the program will never reach.
• Specifying a grade without checking resin compatibility. The volume may say P20; the resin may say S136. The resin always wins.

How Steel Choice Affects Your Lead Time

Through-hardened grades such as H13 and S136 must be rough-machined in the annealed state, vacuum heat-treated, then finish-machined to final dimensions. That extra heat-treatment cycle typically adds one to two weeks to the build compared with a pre-hardened P20 tool. Pre-hardened steel is the faster path to first samples; hardened steel is the faster path to a stable multi-year production run. If schedule is your binding constraint, our guide to injection mold lead time from China breaks down every stage.

Specifying Steel as an Overseas Buyer

If you are sourcing from China and shipping to Russia, the CIS or elsewhere, put the steel grade in writing. Vague quotes that say only "high-quality steel" leave you with no recourse if the tool is cut from the wrong block. Require four things in the quotation and final documentation:

• The exact grade and standard — e.g. "S136 / DIN 1.2083", not "stainless steel".
• Steel mill certificate for the cavity and core blocks, showing grade and heat number.
• Hardness test report after heat treatment, measured on the actual blocks.
• Named steel for working components too — slides, lifters, inserts and ejector plates wear out faster than the cavity.

A supplier that will not name the grade is not necessarily cutting corners — but without the grade in the contract, you cannot prove anything later. Precimold designs and builds silicone and plastic injection molds and issues the steel mill certificate and hardness report with every tool, so the grade on the drawing is the grade in the cavity.

Steel Selection Checklist

• ☐ Annual and total (lifetime) volume estimated — not just first-order quantity?
• ☐ Resin identified as standard, corrosive, abrasive or high-temperature?
• ☐ Cosmetic zones defined against SPI grades?
• ☐ Steel grade written into the quotation with the DIN/AISI standard?
• ☐ Steel mill certificate and post-heat-treatment hardness report required?
• ☐ Working components (slides, lifters, inserts) specified, not just cavity and core?
• ☐ Steel choice compared on cost per usable part — not on tool price alone?
• ☐ Heat-treatment time factored into the delivery schedule?

Get the Right Steel Specified From the Start

At Precimold we review the resin, volume and cosmetic requirements first, then recommend the cavity and core steel that fits — and quote it by name, with the certificate and hardness report included in the delivery package. Send us your 3D model, resin and expected annual volume, and we will return a steel recommendation with the mold life and cost implication of each option. We reply within 12 hours.