What causes pitting on mold steel?

Basically, it's rust. And rust usually happens because people skip the anti-rust oil.

First, sometimes the mold just never gets any oil at all-bare metal, no film, nothing. If you leave it like that in storage or during shipping, the moisture in the air hits the steel right away and starts doing its thing. At first you don't see it, but those tiny rust spots grow into real pits over time. And if you're in a humid place or near the ocean? Yeah, it's way worse.

Second-this one catches a lot of guys-you pull the mold off the press while it's still hot, and then you don't spray oil on it right away. Here's what happens: that warm steel cools down, and the temperature difference makes water condense on the surface-just like dew on a cold drink. That water just sits there, and if you don't oil it soon enough, it'll eat into the steel. The worst spots are the cavity details, corners, and any EDM-textured or polished surfaces, because water likes to pool there and doesn't dry easily. I call it "sweating corrosion." You see it a lot in rainy seasons or when your shop floor is way warmer than the storage area.

Third, even if you do oil it properly, some steels just aren't made to resist rust. Cheap grades like P20, or steels that don't have enough chromium or molybdenum-they'll rust no matter what if the storage conditions aren't perfect. That's why stainless grades like S136, 1.2085, or 420-based steels hold up better. If you're molding stuff that gives off corrosive gases-like PVC or flame-retardant materials that release HCl or HBr-We would suggest specifying a corrosion-resistant steel at the initial stage, which will help prevent future concerns.

At Vim Steel, we would like to learn more about what grade of steel suits your needs. Let's have a brief discussion and see if we can be of any assistance.

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