Choosing Mold Steel by Plastic Type

Three attack mechanisms

Plastics damage molds in three distinct ways — corrosion, abrasion, and surface replication (every cavity defect prints onto the part). Each resin family stresses them differently.

Corrosive resins → stainless mold steel

• PVC and CPVC release hydrogen chloride at processing temperature; flame-retardant grades can release corrosive halogen compounds as well. Unprotected P20-type cavities rust and pit, sometimes in days.
• Use martensitic stainless mold steels: 1.2316 where corrosion dominates, 1.2083/S136-type where polish also matters.
• Acetal (POM) releases formic acid vapours — mildly corrosive; stainless or well-protected cavities are the safe choice for long runs.

Abrasive resins → hardness and wear resistance

• Glass- and mineral-filled compounds (GF-PA, GF-PBT, filled PP) sandblast the cavity, especially at gates and parting lines.
• Use hardened steels at the upper end of their range, generous gate design, and consider surface treatments or coatings at wear points; replaceable gate inserts are cheap insurance.
• Prehardened 1.2311/1.2738 suffices for unfilled commodity resins, but filled grades shorten its polishing life noticeably.

Optical and high-gloss → polishability first

• PMMA, PC and optical parts replicate sub-micron cavity defects. Cleanliness rules: ESR-remelted stainless (1.2083 ESR and similar) is the standard base, with fine, inclusion-free structure for SPI A-class finishes.

The everyday middle

Unfilled PP, PE, ABS, PS are chemically gentle. Prehardened P20-family steels (1.2311, 1.2738 for thick blocks) remain the cost-effective default; money is better spent on cooling and ejection than on steel upgrades.

Resin	Main attack	Steel direction
PVC, flame-retardant	Corrosion	1.2316 / stainless
POM (acetal)	Mild corrosion	Stainless preferred
Glass/mineral filled	Abrasion	Hardened steel, coatings, inserts
Optical PC/PMMA	Surface replication	ESR stainless, high polish
Unfilled commodity	None severe	P20 family prehardened

Temperature changes the answer

High-temperature engineering resins — PEEK, PSU, PPS and similar — run melt temperatures in the 340–400 °C class and mold temperatures that can exceed 150–200 °C. Sustained heat softens prehardened steels' advantage and accelerates corrosion and wear; hot-work-type and stainless hot-work mold steels enter the discussion, and tempering resistance becomes a specification item. Conversely, cold-running commodity molds fail mostly from condensation and cooling-water corrosion, not resin chemistry: rust in water channels and on cavity backs during humid storage is the everyday enemy, which is why shutdown drying, protective coatings and water treatment are specified alongside steel for all resin families.

A note on texture and release

Chemically aggressive resins also attack etched textures and photo-etched grain preferentially, since etching opens micro-crevices. For textured corrosive-resin tools, the stainless choice outranks the small extra polish or hardness a non-stainless grade might offer; texture loss is usually the first visible failure, long before dimensions drift.