Front page / What is the difference between P20, H13 and S136 mould steel?
What is the difference between P20, H13 and S136 mould steel?
P20 is a pre-hardened steel, supplied at roughly 28 to 32 HRC and machined as delivered, which makes it the common choice for general-purpose and moderate-volume moulds. H13 is a hot-work steel machined soft and then hardened to about 46 to 50 HRC, chosen for wear, heat and glass-filled resins. S136 is a stainless steel of the 420 type, hardened to about 48 to 52 HRC, chosen when the mould needs corrosion resistance or a mirror polish.
A mould quote will usually name the steel for the cavity and core inserts, and sometimes a different one for the mould base or slides. The grade affects how long the mould lasts, how well it polishes, whether it resists corrosive resins, and how long it takes to build.
The three steels side by side
| P20 | H13 | S136 / 420 stainless | |
|---|---|---|---|
| Standard designation | AISI P20; W.Nr 1.2311 or 1.2738 (nickel version) | AISI H13; W.Nr 1.2344 | AISI 420 type; W.Nr 1.2083 |
| Delivery condition | Pre-hardened, machined as supplied | Annealed, hardened after machining | Annealed, hardened after machining |
| Typical working hardness | About 28 to 32 HRC (Uddeholm supplies its P20 grade at 290 to 330 HB) | About 46 to 50 HRC | About 48 to 52 HRC |
| Polish | Good | Good to very good | Excellent; used for optical and high-gloss parts |
| Corrosion resistance | Low | Moderate | High (about 13 percent chromium) |
| Wear resistance with glass-filled resins | Low | High | High |
| Common uses | General cavities, larger moulds, lower volumes | High-wear inserts, slides, cores, filled resins | PVC and other corrosive resins, medical and clear parts |
Hardness ranges are from Axiom Molds' steel selection guide; designations and delivery conditions are from the steelmaker Uddeholm's product pages for its P20, H13 and 420-type grades. Exact figures vary by steelmaker and heat treatment, so treat them as typical rather than specification values.
Why hardness matters
Harder steel resists wear at gates, shut-offs and parting lines, which is why abrasive glass-filled nylons and PBT are usually run in hardened steel. The trade-off is build time and risk: a hardened steel has to go through heat treatment after rough machining, and can move slightly, so the toolmaker finishes it afterwards. A pre-hardened steel such as P20 avoids that step, which is one reason it is popular for faster builds and large moulds.
Axiom Molds' guide also links steel to expected mould life, suggesting pre-hardened P20 for programmes under about 300,000 shots and hardened grades for 1,000,000 shots and more. Those figures overlap with the SPI mould classes, which describe life in shots for different classes of mould.
Why corrosion resistance matters
Some resins release corrosive gases during moulding; PVC is the usual example, and some flame-retardant grades are similar. Cooling water can also corrode channels inside a non-stainless mould. Uddeholm notes that stainless mould steel reduces maintenance because the cooling channels are not attacked by corrosion. Stainless is also common for medical moulds and wherever the polish must stay perfect for a long time.
How to choose
- Start from the resin: filled or abrasive grades point to hardened steel; corrosive grades point to stainless.
- Consider the finish: a lens or high-gloss face points to a premium stainless grade.
- Consider volume: a long life in shots favours hardened steel; a short run may be fine in P20 or aluminium.
- Ask which steel is used for each insert, not just "the mould". Inserts in high-wear areas can be harder than the rest.
- Ask for steel certificates and hardness readings to be supplied with the mould.
Hardness and life figures here are published typical values. The toolmaker and the steel supplier's data sheet set the real specification.
Questions buyers ask
Is S136 the same as 420 stainless?
They are close relatives. S136 is a branded premium grade in the 420 family, made with tighter control of cleanliness for better polish. A generic 420 may be fine for many moulds, but it is not automatically equal for optical work.
What is NAK80?
Another pre-hardened steel, supplied harder than P20 (Axiom gives about 37 to 43 HRC) and known for good polish without heat treatment. It is often quoted for cosmetic parts.
Can a P20 mould be upgraded later?
High-wear areas can sometimes be replaced with hardened inserts. The whole cavity generally cannot be changed to a different steel without rebuilding it.
Where does aluminium fit?
Aluminium moulds machine faster and suit prototypes and short runs, but wear sooner than steel. See Should I choose an aluminium or steel injection mould?
Sources
- Axiom Molds, How to choose injection mold steel
- Uddeholm, Impax Supreme (AISI P20)
- Uddeholm, Orvar Supreme (AISI H13)
- Uddeholm, Stavax ESR (AISI 420 type)