Mould Steel Selection: P20 vs H13 vs S136 vs Aluminium Tooling
Ask a toolmaker what a mould costs and the first question back is usually about the steel. The mould material decides how long the tool holds tolerance, what surface finish it can take, how it survives abrasive or corrosive resins, and how much of the quote is machining time. This guide compares the four workhorse options — aluminium, P20, H13 and S136 — with the numbers buyers need to challenge or confirm a tooling specification.
What does the mould steel actually decide?
Four things follow directly from the steel choice:
- Tool life in shots. Soft materials wear; hardened steel resists. The spread runs from tens of thousands of shots in aluminium to a million-plus in hardened stainless.
- Surface finish capability. A true mirror polish (SPI A1) needs through-hardened steel at 48 HRC or harder; soft steels and aluminium will not hold it.
- Cycle time. Thermal conductivity differs roughly fivefold between aluminium and stainless steel — a tool that pulls heat out faster cools the part faster.
- Quote price. Steel grade drives both material cost and machining time; aluminium machines 30–50% faster than steel.
Steel selection is therefore an economics decision, not just a metallurgical one — it sits alongside cavity count and class in the tool types guide.
The four workhorse options compared
| Material | Hardness | Thermal conductivity | Typical shot life | Relative tool cost | Best for |
|---|---|---|---|---|---|
| Aluminium (7075, QC-10) | ~150–170 HB | 130–170 W/m·K | 10,000–100,000 | Baseline (from €3,000) | Prototypes, bridge, low volume |
| P20 / 1.2311 (pre-hardened) | 28–36 HRC | ~30–35 W/m·K | 300,000–500,000 | 2–4× aluminium | Mid-volume production, unfilled resins |
| H13 / 1.2344 (through-hardened) | 48–52 HRC | ~25–28 W/m·K | 1,000,000+ | +15–30% over P20 | High volume, glass-filled resins |
| S136 / 1.2083 (stainless, hardened) | 48–52 HRC | ~20–25 W/m·K | 1,000,000+ | +30–50% over P20 | Corrosive resins, mirror polish, medical |
Two notes on the table. Shot life assumes routine maintenance; skipped maintenance ends any tool early, whatever the grade. And relative cost reflects the whole tool — the steel block itself is a minority of a mould quote; machining hours dominate, which is why aluminium's machinability matters more than its price per kilogram.
How does the resin choose the steel?
The resin is the steel's working environment, and some resins are hostile:
- Glass-filled grades (PA66-GF30, PBT-GF, PC-GF): glass fibres scour gates, runners and parting lines. Below 48 HRC, wear is measurable within tens of thousands of shots. Specify H13 or S136, hardened gate inserts, and expect more frequent refurbishment.
- PVC and other corrosive resins: PVC releases hydrochloric acid at processing temperature, which pits ordinary tool steel within months. S136 stainless is the standard answer; some shops use plated P20 as a compromise.
- Flame-retardant grades: many FR packages are mildly corrosive and some are abrasive; check the datasheet's tooling recommendation before accepting a P20 quote.
- Optically clear PC, PMMA: clarity demands a flawless, polished cavity — through-hardened stainless holds the polish that soft steel cannot.
- Unfilled PP, PE, ABS, PS: kind to tooling; P20 handles them for hundreds of thousands of shots without drama.
The resin side of this pairing is covered in the materials selection guide.
When is aluminium the right choice?
Aluminium is not a cheap substitute for steel; it is a different tool for a different job. Its thermal conductivity — roughly five times that of steel — means faster cooling and shorter cycles. It machines quickly, so lead times of 3–6 weeks are realistic. And it welds and re-machines easily, which suits parts still evolving.
The limits are mechanical. Aluminium dents from careless handling, wears under abrasive resin, and cannot hold a fine polish or sharp texture through long runs. Above roughly 100,000 lifetime shots, or with any glass-filled resin, steel is the honest answer. Nordmould's Rapid and Bridge tiers run aluminium for exactly the volumes where it wins; Production tier tools are steel.
What about copper-beryllium inserts?
Beryllium-copper alloys conduct heat at 105–130 W/m·K — four times better than H13 — and are used as inserts where a steel tool cannot pull heat out fast enough: deep cores, thick bosses, hot spots that drive cycle time. A BeCu core insert can cut local cooling time substantially, but the alloy is soft (typically 36–42 HRC), expensive and a health hazard to machine, so it appears as inserts, never whole tools. If cycle time is the constraint, this is a tooling conversation worth having — see the cycle time guide.
How do hardness, polish and texture interact?
Surface specification and steel grade are one decision:
| Surface requirement | Minimum sensible steel |
|---|---|
| SPI A1 mirror polish (optical, cosmetic lens) | S136 or H13 at 48–54 HRC |
| SPI B/C semi-gloss, standard cosmetic | P20 or harder |
| Fine etched texture (VDI 24–33) | P20 acceptable; harder steel holds texture longer |
| Heavy texture, deep grain | Any grade; texture hides wear |
Soft steel takes a polish but loses it in production; aluminium loses it faster. If the part's A-surface carries the brand, the steel grade is part of the cosmetic specification — a point also made in the surface finishes guide.
What about repair and modification?
Tools get modified — engineering changes, wear repair, the occasional machining error. P20 welds readily with matching filler; H13 welds well but needs pre-heat and post-weld heat treatment; S136 welds require care to preserve corrosion resistance; aluminium welds easily but the repaired zone is softer than the parent metal. Nitriding can add a hard skin (0.1–0.3 mm case at 60+ HRC equivalent) to P20 wear areas as a middle path. Whatever the grade, the maintenance agreement should say who pays for weld repairs and engineering changes.
A selection checklist for buyers
- State lifetime volume honestly — the shot-life column only works with a real forecast
- Name the exact resin grade — filler content and corrosiveness drive hardness requirements
- Specify the surface finish — mirror polish or fine texture raises the steel grade
- Ask for the grade in writing — "steel" is not a specification; "H13, 48–52 HRC" is
- Check wear items separately — gate inserts, parting-line inserts and ejector components can be hardened even in a P20 tool
Related guides
- Injection mould tool types and SPI classes
- Injection mould tooling cost
- Injection moulding materials: which plastic to choose
Planning a production tool? Send Nordmould your STEP file and annual volume — the steel grade is specified in writing with every tooling quote.
Frequently asked questions
Which steel is best for an injection mould?
There is no universal best — the resin and the volume decide. P20 pre-hardened steel (28–36 HRC) is the default for mid-volume production in unfilled resins. H13 hardened to 48–52 HRC suits high volumes and abrasive glass-filled grades. S136 stainless is for corrosive resins like PVC and for optical or medical tools. Aluminium is the fast, cheap answer for prototypes and low volumes.
How many shots does each mould steel last?
As planning figures: aluminium tools 10,000–100,000 shots, P20 pre-hardened steel 300,000–500,000, and through-hardened H13 or S136 1,000,000 or more with maintenance. Abrasive glass-filled resins can halve any of these numbers, which is why resin and steel must be specified together.
What is the difference between P20 and H13 tool steel?
P20 (1.2311) is delivered pre-hardened at 28–36 HRC: it machines directly, welds easily and costs less, but wears faster. H13 (1.2344) is machined soft and then through-hardened to 48–52 HRC, adding heat-treatment cost and lead time but doubling or tripling tool life, especially with glass-filled resins.
When is S136 stainless mould steel required?
Specify S136 (1.2083) when the resin is corrosive — PVC releases hydrochloric acid when processed — when the tool needs a true SPI A1 mirror polish, or when the application is medical or optical and rust or pitting would be a quality risk. It costs more and machines slower, so it is not the default.
Is aluminium tooling strong enough for production?
For low volumes, yes: modern aluminium alloys such as QC-10 machine 30–50% faster than steel and conduct heat superbly, giving short cycles. The limits are wear, denting and polish retention — aluminium tools typically manage 10,000–100,000 shots and are a poor choice for glass-filled resins or long production runs.
How does a glass-filled resin affect mould steel choice?
Glass fibres are abrasive: a 30% glass-filled PA can wear gates, parting lines and cavities fast enough to halve tool life. With glass-filled resins, specify through-hardened H13 or S136 at 48–54 HRC, harden or nitride wear items such as gate inserts, and budget for more frequent maintenance.
Last reviewed: 2026-08