The right finish

Special design techniques come into play when zinc die castings get a surface finish Tool Engineer and Program Manager Chromates meet a wide range of corrosion-resistant requirements: from olive drab and bright yellow with up to 96 hr of protection in a 5% continuous salt-spray environment and clear with up to 24 hr. Trivalent chromate-based coatings with up to 240 hr of salt-spray protection have replaced hexavalentchromium coatings to meet environmental regulations for the automotive industry, but they are also an inexpensive, environmentally friendly protection solution for nonautomotive components. The operating environment is the primary factor that dictates what sort of surface finishes zinc alloy die castings should have. Key selection drivers include the need for corrosion protection, conductivity, surface hardness, wear resistance, and solderability. But aesthetics and cost may also be important as are other considerations including the type of alloy, the design and complexity of the component, tolerance specifications, and finishing processes. Some finishes are surface-conversion processes. These provide uniform protection but don't build up on the casting surfaces so dimensions and tolerances won't change. Other finishes add more than 0.001 in. per side to the size of the component. Certain design features can also be problematic. They can result in even more thickness at critical points, reduce the corrosion properties of the finish, or make it impossible to give adequate protection to certain areas of the part. Fortunately, an experienced die caster can help designers circumvent these issues by suggesting alternative design strategies. Early involvement by the die caster helps ensure the correct interaction between the casting and plating operations. The type of zinc alloy often dictates the choice of surface finish. Zamak 3 and Zamak 5, for example, can be chemically milled to remove surface irregularities to promote more uniform plating. Although this preplating process can improve