In the plating business we often are required to plate parts manufactured from cast material. As most are aware there are some inherent difficulties with castings, and the irony is that these issues, which are the reasons casting should be plated, are all the issues that cause all the problems with trying to plate cast materials.
Regardless of the type of casting process, sand, continuous, investment, die, permanent mold etc. they all share similar properties, the most obvious and vexing is porosity. The act of pouring molten metal into a mold will cause all sorts of flow lines, density variants, and trapped gases which all produce pores in the metal. Depending on the type of casting process some of these pores can be very large, or microscopic and for a plater they all cause problems. These pores in the material effectively increase the surface area of the part, and as such increase the propensity for corrosion – more sites for corrosion to initiate, thus requiring a cast material to be subject to some kind of coating or surface treatment to protect the part. The difficulty then arises from these same pores when the part is run through a cleaning process. Typical plating lines run a variety of cleaners, both acidic and caustic, and at varying temperatures which cause pores to fill with different solutions. Carrying one solution to another tank is called “drag out” and can contaminate baths down the line, until finally at the plating stage these solutions can leach out and cause all sorts of problems in the plated deposit. Typically blisters and lack of adhesion in localized areas are the sign of issues with contamination from pores. There are measures to be taken to help minimize this issue – adequate rinsing is essential, hot water rinses also help open pores and remove contaminants. A pre-plate strike with a bridging type plating such as acid copper or sulphamate nickel can help cover the pores and prevent things from leaching out during the final plating, but this is not always viable depending on the part, and the material. In the end, the better the casting, the better the chance of the plating turning out well.
The other issue with casting is contamination in the material itself. For example aluminum oxide particles embedded in a cast part will not plate with most wet chemistry coatings, and thus leave an area unplated on the surface of the parts. This leaves and interface between the plated and unplated areas which can lead to premature corrosion failures in the field. The contaminations sites can be very small pin hole sized sites or can encompass a very large area of the part – even as large as a few square centimeters in some extreme cases. Surface contamination can usually be removed by blasting the as cast surface before plating. Machining the part to remove the as cast surface also helps remove most contamination.
The most unfortunate issue with plating castings in today’s global economy, is the overseas sourcing of many cast parts. I have seen an alarming amount of casting with porosity and contamination issues that do not become readily apparent until we subject the part to a chemical cleaning and plating line. In addition to the issues with trying to plate cast materials these defects also pose serious consequences for the physical properties of the part. Strength, hardness, ductility etc. are all affected by poor quality casting. In the end, the better the casting, the better the part, and the better the plating will turn out. Our experience with all sorts of casting has allowed us to implement some extra steps and checks to try and mitigate the risks when working with castings. Thorough inspection of the surfaces during processing, alternating cleaner temperatures, as well as aggressive rinsing all help to alleviate the problems with processing castings.
When considering using a casting in your part design, make sure you consider all the steps it will go through before the final part is finished. The quality of the casting has a large impact on all facets of manufacturing, and plating in particular exposes many quality issues.
Michael Zuraw