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Corrosion is not a wear mode — it is a chemistry mode
Corrosion failures look like nothing else. The cavity surface shows a dull, pitted discolouration, the polish disappears early, and the failure is uniform across the face where the corrosive vapour condenses. A tool built from H13 or 2344 will pass first article, then degrade over weeks or months of production as PVC or flame-retardant by-products settle on the cavity.
Corrosion trumps volume, polish and shot life. Once the resin is identified as corrosive — PVC (vinyl chloride monomer release), many halogenated flame-retardant grades, some medical polymers that hydrolyse under mould heat to release hydrochloric acid, and any resin run in a humid or steam-heated tool — the steel family moves to stainless before any other selection is considered.
The three stainless workhorses
S136 (AISI 420 stainless, DIN 1.2083) is the baseline. Martensitic stainless hardened to roughly 48–52 HRC, it handles the corrosive resins mentioned above, polishes well and is widely available. The trade is hot strength — it softens above roughly 150 °C cavity, so a high-temperature corrosive resin needs a different grade. S136H is the pre-hardened variant that arrives ready to machine without a heat-treatment step.
2316 (DIN 1.2316, AISI 420F) is a free-machining variant with added sulphur for better chip control. The corrosion resistance is slightly lower than S136 and the steel does not polish to a mirror standard. It is the choice when the programme needs a stainless grade but the polish requirement is cosmetic rather than optical.
420SS is the supplier-specific variant — typically S136 with a supplier-specific heat-treatment procedure that guarantees a more consistent hardness through the block. Corrosion resistance and polishability match S136; the premium is for the certification and the consistency, not for a different chemistry.
Corrosion resistance and what "good enough" means
"Corrosion resistant" does not mean "immune". Even S136 will show rust pitting if left wet overnight in a tool room. The steel grade reduces the rate at which corrosion progresses during production; it does not eliminate the need for a protection routine after every run — dry the cavity, apply a protective film, flush the cooling circuit.
The practical threshold: run a 100-shot trial of the corrosive resin at production mould temperature, then inspect the cavity under grazing light. If discolouration or pitting shows on a stainless grade, the resin is more corrosive than the grade and the recommendation moves up to a higher-chromium or nickel-based specialty grade.
Heat treatment and polish on stainless
Martensitic stainless steels harden through the section by quenching in oil or nitrogen. Tempering follows; for S136 the target is roughly 48–52 HRC. The higher chromium content affects the hardening curve compared with plain chromium steels, so using a standard hot-work temper cycle on S136 produces the wrong hardness.
Polish on S136 and S136H is straightforward — the steel polishes to a mirror standard. On 2316 the finish is cosmetic at best, the free-machining sulphur introduces inclusions that interfere with polish. Specify the polish requirement together with the stainless grade; upgrading both at the same time is cheaper than upgrading one and reworking the other.
Medical and food-contact considerations
Medical device and food-contact programmes need more than a stainless grade — they need traceable stainless grade. The mill certificate for S136 or 2316 is part of the compliance trail, and the passivation record is part of the quality system. Any welding or re-polishing after passivation nullifies the passivation and has to be re-done.
The honest mistake to avoid: specifying a general-purpose stainless grade for a medical device and hoping it survives. Medical-grade stainless grades are produced to a cleaner cleanliness specification, and the lower inclusion count matters for both polish and corrosion resistance.
Common mistakes
- Putting PVC or a flame-retardant grade into a non-stainless hot-work steel because "it only runs a few thousand shots". Corrosion does not care about shot count — it cares about chemistry.
- Welding the cavity face on a stainless tool and not re-passivating. The weld area is active and will corrode before the rest of the tool.
- Choosing 2316 for a cosmetic or optical polish. The free-machining sulphur inclusions show at the surface under grazing light.
Frequently asked questions
How do I know this category is the right starting point?
Start with the failure mode. If the resin is corrosive, go to the corrosion category before volume. If the resin is abrasive, go to the wear category before polish. If the cavity has to run above 150 °C, go to high-temperature before anything else. The category you land in decides the steel family, and the grade is then chosen within that family.
Can a tool span multiple categories?
Yes — a PEEK tool with a glass-filled resin runs both the high-temperature and abrasive categories at the same time. Where categories overlap, pick the more restrictive one first. A corrosive resin with abrasive filler starts with corrosion resistance; the wear requirement is then layered on top with inserts or coating.
Why not just pick the hardest grade in this category?
Hardness buys wear resistance and indentation. It does not buy toughness, hot strength or corrosion resistance, and it costs schedule and money. The categories deliberately trade one property for another — that trade is what each is designed around.
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