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Release time：2026-09-29 Visits：3
Injection mold steel selection is the cheapest decision on your tooling PO and the most expensive one to get wrong. Cavity and core steel is usually 10–20% of the tool price. The consequences of the wrong grade — a cracked core at 60,000 shots, a rusted cavity after one humid storage season, a polished surface that never reaches the cosmetic spec — arrive two quarters later, buried in your unit cost.
This guide covers the three grades we are asked about most: P20, H13 and S136. It explains how each behaves against resin chemistry, how to size cavity life against your real volume, and what to put in the PO before steel is cut. Xiangqin Plastic is a Dongguan mold maker and injection molder — a 30-machine mold shop and 50 presses from 140T to 500T, running 50–60 new molds a month since 2014.
P20 arrives pre-hardened at roughly 28–32 HRC. You can machine it, drill cooling lines and press it without a heat-treatment cycle. It machines fast, welds well and costs the least of the three.
Use it for ABS, PP, PE and PS housings with moderate cosmetics and volumes. Skip it for filled resins, PVC, and any part that will sit in a humid warehouse between production runs.
H13 is a hot-work tool steel, supplied annealed and hardened to about 48–52 HRC, often nitrided on the cavity surface. It resists abrasion and holds up at high melt temperature.
Use it when the resin attacks the steel: glass-filled or carbon-filled PA and PBT, plus high-temperature grades. H13 is also right for high-volume tools where a P20 cavity would wash out at the gate and the sharp edges.
The trade-off is real. Higher hardness means slower EDM, less forgiving polish, and a heat-treatment step of 2–4 days you must schedule.
S136 is a 13.6% chromium stainless tool steel. It is normally supplied pre-hardened at about 30–34 HRC, and can be hardened to 50 HRC and above for maximum life.
Two things justify the premium. It does not rust — PVC, POM and flame-retardant compounds release corrosive by-products that destroy a P20 tool. And it polishes to a mirror finish, which is what high-gloss cosmetic parts require.
The cost is 1.5–2× the P20 baseline and an extra 3–6 days if you harden after machining. Our mold development team specifies the grade at design freeze, not after the quote.
| Property | P20 (1.2311) | H13 (1.2344) | S136 (1.2083) |
|---|---|---|---|
| Steel family | Pre-hardened Cr-Mo | Hot-work Cr-Mo-V | Corrosion-resistant stainless |
| Typical delivered hardness | 28–32 HRC | 48–52 HRC, often nitrided | 30–34 HRC pre-hardened; 50+ HRC hardened |
| Indicative cavity life | ~200,000–500,000 shots | ~500,000–1,000,000+ shots | ~1,000,000+ shots when hardened |
| Resins it handles well | ABS, PP, PE, PS, general housings | Glass/carbon-filled PA & PBT, PEEK, PPSU | PVC, POM, flame-retardant grades, PC, PMMA |
| Corrosion resistance | Low | Low | High |
| Polishability | Good | Moderate | Excellent — mirror finish |
| Repair / weldability | Good | Good | Moderate |
| Heat treatment after machining | Not needed | Usually yes — 2–4 days | Yes if hardened — 3–6 days |
| Relative steel cost | Baseline 1.0× | 1.2–1.6× | 1.5–2.0× |
The life figures are indicative ranges for tooling budgeting. Actual life depends on part geometry, cooling layout, gate design, resin, machine condition and maintenance. Confirm the final grade against your volume plan with your toolmaker.
Most bad steel decisions come from sizing the tool by volume alone. Resin chemistry rules out grades faster.
Glass-filled PA, carbon-filled PBT, PEEK and PPSU wear out a soft cavity. Glass fibre erodes the gate, runner and insert edges. H13 is the default; our PEEK and PPSU tools get hard steel and generous gate detail because the melt window is narrow.
PVC releases hydrochloric acid and pits a non-stainless cavity within a production run. POM and flame-retardant compounds behave the same way. S136 is the only sensible answer. The screw and barrel matter too: PVC and acidic resins need dedicated corrosion-resistant hardware, not the standard set.
If the part is clear or high-gloss, steel polishability sets your scrap rate. PC and PMMA need S136-level polish; a P20 cavity can leave orange-peel texture or flow marks no process tuning will fix. A second shot or overmolding station needs the same grade discipline on both halves.
Steel grade should follow from arithmetic, not habit. Run this on every project with your mold development team:
That last step wins internal arguments: tooling cost is a one-off, tool failure is recurring. Our injection molding line runs trial molding and small-batch trial production before any tool moves to mass production.
You do not need one grade for the whole tool:
A mixed strategy typically recovers 15–25% of the steel cost increase without touching part quality. What never works is downgrading the gate insert or a sharp edge on a filled-resin tool.
Send this to your toolmaker. Hesitation on items 2, 3, 8 or 9 is a reason to pause.
Two checks catch most substitution problems. First, ask for the mill certificate and match the heat number to the delivered steel. No certificate means you are buying on trust.
Second, request a hardness report. Testing is fast, cheap and non-destructive, and the result is a number you can compare against the spec. Test at least three locations on the cavity block, because spot hardness varies across a hardened block. Our quality management system measures finished parts on 2D and 3D equipment for first-article inspection, and dimensional reports go out with T1 samples — the same discipline applies to incoming steel, and it is what our documented certifications are meant to support.
Either the hardness matches spec or it does not. No supplier should object to the request.
P20 is normally sufficient. At 100,000 shots you are well inside a pre-hardened cavity’s range, and you skip the heat-treatment lead time. Choose H13 or S136 only if the resin is abrasive, corrosive or runs hot.
No. S136 earns its premium in three situations: corrosive resins such as PVC and POM, transparent or high-gloss cosmetic parts, and tools you intend to store and re-run for years. On a plain ABS housing with a 150,000-shot life, skip it.
You can, and you will regret it. Glass fibre abrades the gate and cavity edges, so a P20 tool on 30% glass-filled PA shows measurable wear well before the volume target. Use H13, and expect to maintain the gate area.
Indicatively: H13 adds 1.2–1.6× on steel cost plus 2–4 days of heat treatment. S136 adds 1.5–2.0× plus 3–6 days if hardened after machining. On a 5–8 week program that is the difference between shipping on time and shipping late, so decide the grade before design freeze.
Ask for the mill certificate with the heat number, plus a hardness report taken at three points on the cavity block. Cross-check the HRC against the spec you approved.
Injection mold steel selection is a one-time decision with a multi-year bill. Tell us your resin, annual volume and cosmetic requirement. We will recommend the cavity grade, hardness target and insert strategy — with the cost delta written down. We build 50–60 molds a month, run trial molding and small-batch trial production in-house, then move the same tool to mass production on 140T–500T presses. See the shop yourself on a factory tour.
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Tell us what you are molding and how many you need per year. We reply with a grade recommendation and indicative tooling cost within two working days — contact our team.
