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topic 389

Phosphating of zinc die castings


(1996)

I am looking for some information on the process of phosphating for prefinishing zinc die-cast parts. In order to improve the paint adhesion of our die-cast parts we currently sand blast or belt sand. This works fine except it can expose the porosity inside the castings which will eventually lead to blistering of the paint finish. Is it possible to phosphate zinc parts and if so what type (iron-phosphate, zinc-phosphate, etc.)? What may the environmental concerns be of the phosphating material?

Mike Ulinski


Phosphating of Metals
by Rausch

May we safely assume that the die-castings are not chromated? Then, yes, zinc die-castings can be zinc phosphated. You will generate sludges within the zinc phosphating process itself, and there will be zinc in your wastewater that will probably need to be removed. I work more in the plating end of the industry, so I don't know offhand what, if any, federal categorical standards cover phosphating processes; but surely there will be local effluent standards dictated by your POTW.

You may find that some other process, like a wash-primer, can improve adhesion at lower cost.

Hopefully someone who paints die-castings will volunteer some advice.

Ted Mooney, finishing.com Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey

(1996)

There is no straight answer for the treatment of die castings to take paint finishes. It is possible to impregnate after shot blasting to seal the porosity but it is an added cost to production and not 100% certain. In my view phosphating would be the route to take but choice of equipment will dictate level of labour cost involved together with application efficiency and chemical consumption.

Peter D. Young



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