Special design techniques come into play when zinc die castings get a surface finish
Tool Engineer and Program Manager
FisherCast Global Corp.
Peterborough, Ont. Canada
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 RIGHT FINISH
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 the surface finish, it's also possible to plate zinc casting without chemical milling. The higher percentage of aluminum or copper in Zamak 2, ZA-8, AcuZinc, and AZ91D-magnesium makes these alloys unsuitable for chemical milling.
Nickel, chromates, and E-Coat (Phos-E) are used on the bulk of zinc alloy die-cast components that are surface treated. The balance of components are plated with tin, brass, copper, chrome, black oxide, silver, and gold. They can also be coated with paints and powder to meet specific requirements.
Nickel, a popular surface finish for die-cast zinc-alloy parts, applies over copper plate. Designers select it for appearance, wear resistance, lubricity, and some corrosion protection.-Nickel can also improve the natural-EMI-shielding propertiesof zinc. However, the tooling design must compensate for the thickness of the copper and nickel layers.
Electrodeposited (electrolytic)-process is the least-expensive method of nickel plating,but it does have drawbacks. They include material buildup on sharp external edges and thinner deposits on sharp internal corners. However, special design features can help promote improved distribution of the finishing material.
Electroless nickel, while slightly more costly than electrolytic, is a superior plating method for castings. The nickel doesn't build up on corners, providing more accurate coverage on close tolerance components. The trade-off, however, is a final finish that's less durable and softer than with electrolytic processes.
Chromates are a low-cost conversion coating applied through electrochemical treatments. These widely used coatings primarily improve corrosion resistance. In a 5% continuous salt-spray environment, for example, bright yellow and olive-drab chromates provide 96 hr of protection; clear, up to 24 hr. Chromates also give designers a way to color-code parts.
Trivalent chromium is a coating option that serves as a good corrosion inhibitor and is an alternative to hexavalentchromium coatings. It meets environmental regulations for the automotive industry and provides salt-spray protection up to 240 hr. This coating typically applies over a zinc flash for maximum salt-spray protection and more uniform shading. It can also be applied directly onto the component. The bright, aesthetic finish is also an inexpensive, environmentally friendly alternative for nonautomotive components.
E-Coat (Phos-E) provides ultrahigh salt-spray protection (minimum 400 hr), with protection against chipping, cracking, and abrasion. And as a black glossy finish its less costly than powder or liquid spray coatings.
Designers must include a plating allowance in the die-casting cavity to compensate for the additional thickness of the finishing process. The standard thickness for nickel plating is 0.0002 to 0.0004 in. over 0.0003 to 0.0005 in. of copper. For increased salt-spray protection, additional nickel and copper can be applied, but the design must compensate for the total buildup. Some chromates can be applied directly to the casting and don't add to surface thickness, but are usually applied over a zinc flash of 0.0002 in. ECoat's electrolytic process deposits a thickness from 0.0003 to 0.0007 in. per side.
Die-casting suppliers can help with determining plating allowance as well as suggest accommodations in component design to maximize surface finish performance.
- Sharp corners: in the electrolytic nickel plating process, even small radii (R0.2 mm) can help reduce the effect of corner buildup. Conversely, filling in a sharp fillet corner with a small radii can help plate better into the corner. The same requirement applies to edges, ribs, and other projections as well as grooves, serrations, and holes.
- Internal tapped holes must also be cut oversized to accommodate buildup of the plating applied later on.
- Features such as blind holes require special treatment as the electroplating process does not attract copper and nickel into the full length of the hole. A small, nonfunctional pocket or throughhole can be designed at the end or side of the blind hole to let the plating materials flow through.
FisherCast Global Corp., (800) 547-6905, fishercast.com