Mold steel selection: where to use P20, H13, S136, NAK80

Mold steel selection is based on part quantity (expected life), the plastic being molded (abrasive/corrosive or not), the required surface finish, and budget. The wrong choice either wears the mold out early or inflates cost unnecessarily.

The four most common options

• P20 (pre-hardened, ~30 HRC): The standard choice for medium/large base and cavity blocks. Easy to machine, low cost; but limited life for high-polish or abrasive materials (glass-filled).
• H13 (hot-work tool steel, ~48-52 HRC after hardening): Resists high temperature and pressure; preferred for aluminum die-casting molds and high-cycle injection molding. Harder to weld/repair after heat treatment.
• S136 (stainless, ~50-54 HRC): Preferred when working with corrosive additives (flame-retardant, antibacterial) or moisture-sensitive plastics (PVC, some engineering plastics) to resist rust and chemical wear. Suitable for mirror polishing (optical/cosmetic part molds).
• NAK80 (pre-hardened, ~40 HRC, high Ni): Needs better polishability and more uniform hardness than P20; a balanced choice for cosmetic-surface, medium-to-high volume parts.

What should the choice be based on?

1. Quantity/life: P20 may be enough for prototypes and small series; H13/S136 extend life at high volumes.
2. Abrasive or corrosive feedstock? Glass/mineral-filled plastics are abrasive (hardness matters); PVC and halogenated flame retardants release corrosive gas (stainless is preferred).
3. Surface expectations: For mirror/optical quality, a homogeneous microstructure (S136, NAK80) gives better results than coarser cast steels.
4. Heat treatment and distortion risk: If there's a risk of mold distortion/dimensional shift after hardening, pre-hardened options (P20, NAK80) are more predictable.

Practical note

There is no single "best steel"; the right question is which steel gives less maintenance and longer life for this part, this quantity and this feedstock. Asking your steel supplier for a certified hardness/cleanliness (inclusion) report reduces surprises, especially for optical/cosmetic molds.