Injection Mould Steel Selection: P20, H13 and S136 Compared

Injection Mould Steel Selection: P20, H13 and S136 Compared

P20, H13 and S136 are the three steels most buyers see on injection mould quotes. Pick by resin, yearly volume and surface finish, not by which grade sounds premium. P20 is the economical pre-hardened workhorse for unfilled plastics at moderate volumes. H13 is the hardened hot-work steel for abrasive, glass-filled and high-cycle work. S136 is the stainless choice for corrosion resistance or a mirror polish. Getting this wrong either wastes money on steel you do not need, or wears the cavity out before the product life is finished.

Key facts

• P20 (roughly 1.2311) is usually supplied pre-hardened at about 28 to 34 HRC. It machines quickly, needs no post-machining heat treatment, and suits many ABS, PP, PE and PS tools for a few hundred thousand shots of unfilled resin.

• H13 (roughly 1.2344) is machined soft, then hardened and tempered, commonly to about 48 to 52 HRC. It resists abrasive wear and thermal fatigue, so it is the usual upgrade for glass-filled nylon, PBT and high-volume programmes.

• S136 (roughly 1.2083 / 420-type stainless) hardens to a similar range and takes an excellent polish. Specify it for clear PC or PMMA, medical and cosmetic cavities, PVC, and flame-retardant grades that attack ordinary steel.

• Shot-life numbers on quotes are estimates. Resin, gate design, cooling, maintenance and storage change the real life more than the grade name alone.

• Many production moulds mix steels: a P20 or 718 mould base with H13 wear inserts and S136 cavities where polish or corrosion matter.

What do P20, H13 and S136 actually mean?

They are common trade names, not unique proprietary alloys. Mills use different designations for related grades, so ask for the exact specification and a material certificate, not only the nickname on the quote.

• P20 / 1.2311. A pre-hardened mould steel, typically about 28 to 34 HRC. It can be machined and polished without a separate hardening step, which shortens lead time and avoids heat-treatment distortion. Related grades such as 718 / 1.2738 are often chosen when a cleaner structure and better polish are needed on larger cavities.

• H13 / 1.2344. A hot-work tool steel, usually rough-machined annealed, then vacuum hardened and tempered, finish-ground and polished. Working hardness for inserts is often about 48 to 52 HRC. Its strength is wear resistance and resistance to heat checking under repeated thermal cycling.

• S136 / 1.2083. A martensitic stainless mould steel in the 420 family. Hardened to a similar range as H13, with high chromium for corrosion resistance and excellent polishability when the steel is clean. ESR-quality stock is preferred for optical and medical work.

Hardness alone does not decide the job. Polishability, toughness, weldability for repairs and cooling all matter. A soft cavity with good cooling and a replaceable gate insert can outlast a hard cavity that is poorly designed.

How should you choose mould steel?

Answer these four questions before the steel is ordered. Changing grade after the cavities are cut is expensive.

1. What resin, and with what filler? Unfilled ABS, PP and PE are gentle. Glass fibre, mineral fill and some flame-retardant packages abrade gates, vents and sharp corners. PVC and some FR grades are corrosive.

2. How many shots over the product life? A bridge tool for 5,000 parts does not need the same steel as a million-shot tool. State yearly volume and expected total life on the RFQ.

3. How cosmetic is the surface? A painted or textured housing can live with a good P20 or 718 polish. A clear lens, piano-black bezel or medical tray shows every pit, so S136 is the safer cavity steel.

4. Where will the mould run and sit idle? Humid shops, coastal shipping and long storage favour stainless cavities or strict anti-rust packing, even when the resin itself is not corrosive.

Treat published shot-life ranges as starting points for discussion with the mould maker, not as a guarantee.

P20 vs H13 vs S136 at a glance

Typical industry ranges for well-designed moulds. Confirm hardness, polish grade and life target on the written quote.

P20 (1.2311)

• Usual condition: Pre-hardened ~28–34 HRC

• Post-machining heat treat: Usually not required

• Relative cost: Lowest

• Abrasion resistance: Low to medium

• Corrosion resistance: Low

• Mirror polish: Good

• Typical unfilled-resin life: ~300,000–500,000 shots

• Best fit: General housings, trays, bridge and medium-volume tools

• Weak point: Soft against fibre and corrosive gases

H13 (1.2344)

• Usual condition: Hardened ~48–52 HRC

• Post-machining heat treat: Required (vacuum harden + temper)

• Relative cost: Higher

• Abrasion resistance: High

• Corrosion resistance: Low to moderate

• Mirror polish: Very good

• Typical unfilled-resin life: ~500,000–1,000,000+ shots

• Best fit: Glass-filled resins, high-wear inserts, long runs

• Weak point: Needs controlled heat treatment; not stainless

S136 (1.2083)

• Usual condition: Hardened ~48–52 HRC

• Post-machining heat treat: Usually required for full performance

• Relative cost: Highest of the three

• Abrasion resistance: Medium to high

• Corrosion resistance: High

• Mirror polish: Excellent (optical / SPI A-1 class when steel is clean)

• Typical unfilled-resin life: ~500,000–1,000,000+ shots

• Best fit: Clear, medical, cosmetic, PVC and FR resins

• Weak point: Cost and careful heat treatment / polishing

Life figures fall sharply with abrasive or corrosive resins, thin steel sections, aggressive gates and poor maintenance. Ask what volume and resin the estimate assumes.

When is P20 enough?

P20 is enough for a large share of consumer and industrial moulds. Use it when the resin is not abrasive, the surface is not optical, and the volume is moderate.

Good fits

• ABS, PP, PE, PS and many unfilled engineering plastics for housings, covers, trays, caps and appliance shells.

• Prototype, bridge and low-to-medium production tools where lead time and cost matter more than million-shot life.

• Mould bases and plates even when the cavities themselves are a harder grade.

Watch-outs

• Glass-filled PA, PBT or PP will erode gates, shut-offs and sharp corners in P20 much sooner than the unfilled life numbers suggest.

• PVC, POM off-gassing and some flame-retardant packages can pit and rust ordinary steel.

• High-gloss or clear faces may need a cleaner grade such as 718H, NAK80 or S136 even at modest volumes.

P20 is also the easiest of the three to weld and rework after a T1 trial, which helps when the design is still moving.

When do you need H13?

Choose H13 when wear or heat will beat a pre-hardened cavity before the product life is over.

Good fits

• Glass-filled and mineral-filled resins (for example PA66-GF, PBT-GF, PP-GF), where fibres scrub the steel at the gate and along the flow path.

• High yearly volumes and multi-cavity tools that must hold parting-line and shut-off dimensions for hundreds of thousands of shots.

• Local inserts: gate inserts, thin cores, slides, lifters and aggressive shut-offs inside an otherwise P20 or 718 tool.

• Higher melt or mould temperatures that put thermal fatigue on the steel.

Practical notes

• Ask for the hardness target, heat-treatment record and a hardness check on the finished insert.

• Leave machining stock for post-hardening grind and polish. Skipping this causes soft spots and size drift.

• H13 is not stainless. In humid storage or with corrosive resins, protect it or combine it with stainless cavities.

A full H13 cavity block is not always required. A P20 body with H13 inserts in the wear zones often gives most of the life benefit at lower cost, with easier later repair.

When should you specify S136?

Specify S136 when corrosion or a high polish would make ordinary steel fail cosmetically or chemically, even if the shot count is not extreme.

Good fits

• Clear polycarbonate and PMMA lenses, light pipes, windows and see-through covers, where every pit shows.

• Medical and laboratory parts that need a cleanable, corrosion-resistant cavity and a stable polish.

• Cosmetic packaging and high-gloss consumer faces that demand an SPI A-class or equivalent finish.

• PVC and many flame-retardant electrical resins that release corrosive by-products during moulding.

• Moulds that will sit idle in humid plants or travel through coastal ports between production lots.

Practical notes

• Ask for clean, preferably ESR-quality steel for optical and medical work.

• Agree the polish standard in writing (for example SPI A-1 / A-2 or an approved sample plate) before steel cutting finishes.

• S136 still needs correct resin drying, good venting and careful ejection; stainless steel does not forgive a wet PC shot or a burn mark.

For a black textured ABS housing at low volume, S136 is usually over-specified. Spend the money where the surface or the chemistry actually needs it.

Can one mould use more than one steel?

Yes, and that is often the best engineering answer. A hybrid tool matches steel to the local job instead of upgrading the whole block.

Typical combinations:

• P20 or 718 mould base for plates, ejector housing and non-critical structure.

• H13 inserts at gates, thin cores, slides and shut-offs that see abrasion or high contact pressure.

• S136 cavities or cores on the cosmetic or corrosion-critical faces only.

• Replaceable wear inserts so a worn gate or core can be swapped without scrapping the mould.

Ask the quote to list the steel grade for the cavity, the core, the slides and the mould base separately. A single line that says “P20 mould” without that breakdown hides important decisions.

What reduces mould life regardless of steel?

Steel grade sets the ceiling. These factors decide whether you reach it.

• Abrasive fillers. Glass and mineral fibres wear gates, vents and sharp corners first. Design generous radii, use hardened inserts, and plan spare gate tips.

• Corrosive resins and idle time. PVC, some FR packages and condensation rust unprotected cavities. Clean, dry and oil the mould before storage, and use desiccant packing for shipment.

• Poor cooling. Hot spots warp parts and thermally fatigue the steel. Balanced channels and baffles or bubblers in deep cores matter as much as hardness.

• Aggressive gates and thin steel. Tiny high-speed gates erode quickly; thin sections between holes or ribs crack.

• Missed maintenance. Flash in vents, worn ejector pins and dry slides accelerate damage on any grade.

• Wet hygroscopic resin. Moisture leads to splay, burns and process abuse that marks the cavity.

Agree a maintenance interval and a spare-parts list with the first article pack. That paperwork often saves more money than upgrading one steel grade.

What should you ask before the steel is ordered?

1. Which exact grade and mill specification for the cavity, core, slides and mould base, with material certificates?

2. What working hardness (HRC) is specified after heat treatment, and will it be checked on the finished inserts?

3. What resin grade, filler percentage and yearly / lifetime volume did the quote assume?

4. What surface finish is promised (SPI / VDI or an approved sample), and is the steel clean enough to reach it?

5. Are wear parts designed as replaceable inserts, and are spares included?

6. Who does the heat treatment, and are vacuum-hardening records provided for H13 and S136?

7. How will the mould be protected against rust for sea freight and idle storage?

8. Who owns the mould drawings and the tool after payment?

Frequently asked questions

Is P20 good enough for a production mould? For many unfilled ABS, PP and similar parts at moderate volumes, yes. It is not the default for glass-filled resins, corrosive plastics, optical surfaces or million-shot tools.

Is H13 always better than P20? No. H13 costs more, takes longer because of heat treatment, and is harder to rework. Use it where abrasion or heat justify it, often as local inserts rather than the whole cavity.

Why does S136 cost more? It is a stainless mould steel with stronger corrosion resistance and superior polish potential. The premium is worth it when the part or the resin needs those properties.

Can aluminium be used instead of these steels? Aluminium moulds suit prototypes and short runs because they machine fast and cool well. They are not a substitute for hardened steel on abrasive resins or long production lives.

Does steel grade change cycle time? Indirectly. Cooling design and wall thickness dominate cycle time. Steel choice mainly affects how long the cavity holds size and finish.

What documents should arrive with the mould? Material certificates, hardness reports where heat-treated, steel list on the mould drawing, first-article dimensions, trial samples and a maintenance sheet.

Need a mould steel recommendation and quote?

HongYing Tooling manages mould making and production through a network of verified manufacturing partners in China, and we act as one engineering contact for the whole project. Send us the 3D model, the resin grade (including any glass fill or flame rating), the surface finish you need, and your yearly volume. We will recommend a practical steel combination for the cavity, core and wear inserts before any steel is cut, and give you a quote with a clear schedule. Start on our injection mould page, see our electronics or medical industry pages, or email jayson@hongyingtooling.com.