Metal Finishing and Buffing of Zinc

Zinc:  In most manufacturing operations Zinc Die Cast parts are not made from only zinc, but a blend of Zinc, Aluminum, Magnesium and Copper and often other metals as well.  The blended material provides a good balance for the part to be molded, machined and buffed.  Because of the relatively low temperatures to die cast (~750 °F), zinc is usually formed into complex shapes often making it difficult to buff.  Because of these complex geometries zinc parts are most often buffed in a Mush Buff Operation.  If zinc parts are exposed to temperatures approaching 350 °F it is possible for some of the impurities in the metal to migrate toward the surface causing blemishes that show through the plating. The lower temperatures used in Mush Buffing (< 140 °F) help prevent this from occurring. 

Zinc Die Casts have been described as “A lot of air held together by a little bit of metal.”  This definition catches some of the problem with polishing and buffing a thin skinned casting with a porous interior just below the surface of the skin.  The porous layer has air pockets (often seen as pin holes) that are very difficult to cover in the plating process and often requires scraping the parts when holes appear after buffing.  Continuing to buff usually just drags out the hole (like a comet tail) instead of removing it.  Sometimes it is possible to put a thin acid copper plate over the part then buff the copper before the part goes to plating.  Most often it is less expensive to re-melt the zinc and start over rather than repair the defect with buffing.

When Zinc is buffed it exposes a bare metal surface that is active (reactive) and oxidizes quite easily.  If the surface finish is important then zinc parts are always either plated or painted to protect the surface.  If there is no protection applied to the surface of the part and the time between buffing and plating is too long then the part will oxidize on the surface making it impossible to properly plate the part.  It is possible to use an anti-oxidant in the buffing compound that can protect the part for several days (and maybe even weeks) before the part is plated.