IMM 12/97

We recently built a mold for an HDPE part that was constructed from a tooling-grade ASTM/AA 7075 aluminum and featured some local fixed and local sliding inserts made from P20 tool steel, which we nitrided. We trialed the tool and molded approximately 1000 shots before shipping it. The customer molded 2000 additional shots and returned the tool to us complaining of corrosion on the parting faces. I have never seen anything like it. Thankfully, the cavity areas seem to have been spared, but the entire parting face- with the exception of a 1-inch-wide margin around the cavity-has deep pitting from corrosion. Even the faces of inserts that bed off against the opposite tool half are corroded. We sprayed only a reputable "mold saver" onto the parting faces. Does anyone have any ideas as to why this corrosion has happened and how to stop it?

If the rust is occurring on the P20, note that nitriding by itself will only provide a low level of corrosion resistance. We have a treatment called Dyna-Blue that provides 60 to 96 hours of salt spray resistance, as well as wear resistance and better release for molds.
-L. Epler, Dynamic Metal Treating, Canton Township, Ml, (313) 459-8022.