Injection Mold Steel Selection: P20, H13, S136 and When to Pay More

Steel choice is a volume decision before it is a metallurgy decision

When a mould specification is being written, the steel question usually arrives as "what should we use" and gets answered with a grade name. That is the wrong order. The first question is how many parts the mould will make and what the part has to look like, because those two facts eliminate most of the options before any metallurgy is discussed.

There are three failure modes that steel selection is trying to prevent, and each one dominates at a different volume:

Wear — the cavity erodes, edges round off, and dimensions drift. Dominant in glass-filled resins and high volumes.

Corrosion — PVC, POM, and any resin that releases acidic decomposition products pit the steel. Dominant regardless of volume.

Polishing and detail loss — the steel cannot hold the surface finish or the fine features, so the part looks wrong from the first shot.

The four steel families

Table 1 — The four families. The names differ by supplier and standard (AISI, DIN, JIS) but the families are consistent across them.

The grade naming is a genuine source of confusion in quotations. P20 (AISI) is close to 1.2311 (DIN) and to 718 (ASSAB). S136 (ASSAB) is 1.2083 (DIN) and roughly 420 modified (AISI). When comparing two quotes, compare hardness, delivery condition and heat treatment requirement — not names.

Grade by grade: where each one stops making sense

Table 2 — Grade selection by volume. The cost index is relative to P20 per kilogram of finished cavity block, before heat treatment.

The volume thresholds are not sharp. A P20 mould running unfilled PP at 60 °C will comfortably exceed 100,000 shots. The same P20 mould running 30% glass-filled PA66 at 290 °C will show edge wear at 40,000. The table assumes a moderate resin; adjust up for unfilled commodity resins and down for filled engineering resins.

What the resin does to the steel

This is where most specification errors happen. The steel is chosen for the volume and then the resin quietly destroys it.

Table 3 — Resin-driven steel requirements. POM and PVC are the two that most often catch out a supplier who specified on volume alone.

The POM case is worth spelling out. Polyoxymethylene decomposes to formaldehyde when overheated, and formaldehyde plus moisture plus steel is a corrosion cell. A P20 mould running POM will show pitting in the cavity within a few thousand shots if the melt temperature drifts, and the pits transfer to the part as small raised bumps. It is not a wear problem and no amount of polishing fixes it permanently.

Polishability and why it costs what it does

Polishability is a function of inclusion content and hardness. Inclusions — sulphides, oxides, and in poorly made steel, porosity — are harder or softer than the matrix, so polishing cuts them at a different rate and leaves a surface that looks like a fine orange peel under grazing light.

Table 4 — Surface finish versus steel. Above the gloss line, the steel price stops being a small part of the mould cost because polishing hours dominate.

ESR (electro-slag remelted) steel costs 30–60% more than conventionally cast steel of the same grade and is worth it only when the finish requirement is genuinely optical. For an automotive interior gloss part, the difference between a standard 718 and an ESR grade is often visible in the first sample — a faint haze that no amount of additional polishing removes.

Heat treatment: what changes after machining

Pre-hardened steel arrives at 28–34 HRC and is machined in that condition. Through-hardened grades arrive annealed at around 20 HRC, are machined soft, then hardened, quenched and tempered — which means the mould distorts and has to be finish-machined or EDM'd afterwards.

Table 5 — The two routes through the shop. The through-hardened route is not just harder steel, it is a different manufacturing sequence with different risk.

Distortion is the reason many mould makers push back on hardened steels for large or complex cavities. A 0.4% distortion on a 500 mm block is 2 mm, which has to be removed by EDM or by leaving stock. On a deep ribbed cavity that is expensive and sometimes not possible.

The whole-life cost calculation

The steel price difference between P20 and S136 is real but small compared with what happens if the wrong one is chosen. A mould that fails at 60% of its intended life costs the full mould price plus the production downtime plus the requalification.

Table 6 — Cost per 100,000 shots for the same cavity block. The two lines where the cheap steel is expensive are the glass-filled and the POM cases, and both are common.

The calculation excludes downtime, which usually dominates. A mould that has to come out for repair during a production campaign costs far more in lost output than in steel. That is the argument for choosing one grade above what the volume strictly requires whenever the part is on a critical delivery schedule.

What to write on a mould specification

A steel specification that a mould maker can quote against should state: cavity and core steel grade with hardness range, the heat treatment route, the surface finish requirement with an Ra value and a reference plaque, the expected production volume, the resin and its fillers, and whether the mould will run in a humid or corrosive environment.

Add one line that is usually missing: what happens at end of life. If the part may run for ten years with volume revisions, a mould designed for insert replacement costs more upfront and far less later. That is a steel-and-construction decision, not just a steel decision.