Key facts
Mold steel selection guide: P20, 718H, H13, S136, NAK80 and 1.2344 compared by hardness, polishability, corrosion resistance and expected tool life - with our recommendations by project type.
Steel choice decides your mold cost, its life and the surface quality of every part it makes. Here is how we recommend steel – openly, by project type.
The Workhorse Steels
- P20 / 1.2311: pre-hardened ~30 HRC, good machinability, economical – general-purpose tools for 300,000+ shots of unfilled resins
- 718H / 1.2738HH: tougher and more uniform than P20 – larger molds and higher cavitation
- H13 / 1.2344: hardened 48-52 HRC – the choice for glass-filled and abrasive resins, and for die casting dies
- S136 / 1.2083: hardened stainless, corrosion resistant, takes SPI A1 mirror – cosmetic parts, PVC and flame-retardant resins, medical tools
- NAK80: pre-hardened 40 HRC with excellent as-machined finish – cosmetic cores without full polish
Our Recommendations
- Prototype or bridge tool, unfilled resin: aluminum 7075 or P20
- Production tool, commodity resin: 718H
- Glass-filled or high-temperature resin: H13
- High-gloss cosmetic or medical tool: S136 (cavity) + NAK80 (core)
Match Steel to Program, Not Habit
The steel recommendation in every Shiny Mold quote comes with reasoning: expected shots, resin abrasiveness, polish requirements and budget. You can always ask us to quote two grades side by side.
Steel grades in one comparison table
P20 (1.2311): pre-hardened 28-32 HRC, the default for non-abrasive resins up to roughly 300,000 shots, easiest to rework. 718H (1.2738): tougher big-block version of P20 for large cavities. H13 (1.2344) at 48-52 HRC: the choice for glass-filled resins and million-shot programs, slower to machine, holds polish well. S136 (1.2083) stainless at 48-52 HRC: for corrosive resins, medical and optical work, best mirror finish of the group, needs the most careful heat treatment.
Hardness is not the whole story – polishability, weldability for repairs, and thermal conductivity all shift the choice. A conformal-cooled insert in a high-conductivity alloy can cut cycle time 15-20 percent on thick parts, paying back its premium within the first production year. Steel for plates versus cavity detail, and where replaceable inserts make sense, is covered in the design guide; the abrasive resins that demand H13 are listed in the materials guide.
One more practical angle: steel availability and repairability in your region. If the tool will live with a molder overseas, a grade with local machining and welding support – and documented hardness so any shop can match it – reduces your long-term risk. We flag this in the quote when the tool destination is known, and we keep the maintenance documentation complete so a transfer never strands the tool.