Posted July 01, 2016
By Jack Tetrault
Up-Front Factors that Achieve a Fantastic Final Cast Aluminum Finish
Getting excellent results with hard coat and anodizing cast aluminum articles? “No way!” is a common, but mistaken response. After more than 40 years in the field, DCHN has successfully worked with many companies to achieve beautiful cast finishes—top-quality finishes that are achieved when anodizing cast aluminum because the final finish is the result of many important up-front decisions that eliminate many sources of poor quality.These include:
Only aluminum anodizes and forms the coating. Therefore, choosing an alloy that has a low silicon content whenever possible is the way to go. Otherwise, the silicon, which preferentially migrates to the surface, will leave unanodized areas in the coating. These bare spots will show up as a mottled finish, especially when the parts are subsequently dyed.
When anodizing cast aluminum, there are a number of casting methods that may be used to produce aluminum products. Many of the reasons for selecting a casting method are centered on economic, physical properties, and complexity of fabrication concerns, among others. Too often the anodizability of a particular alloy is not a concern. Casting alloys have a wide spectrum of anodizability—ranging from good to acceptable—to poor to unanodizable. The better alloys for anodizing are generally those that are investment or sand cast, while the more difficult are those that are die cast. But, beware of exceptions! Consult your casting company and your anodizer.
Generally, a cast part with minimum porosity at the surface will anodize better than a part with more porosity.
Machining of Cast Surface
Everything else equal, a cast part that is subsequently machined usually finishes better than a part without machining.
When selecting your aluminum anodizer, ask them to show you examples of work that they are currently doing as they may claim capability that they don’t have. To get a superior casting finish, how the anodizer prepares the surface of the alloy when anodizing cast aluminum is a key step to achieve a beautiful result—as all of the impurities on the cast surface have to be removed before anodizing. This requires know-how and special chemistry that is typically not found in most anodizers’ facilities, due to the set-up requirements to safely use this chemistry. DCHN has the treatment methods to address a wide variety of cast alloys.
Forming an integral coating that can be evenly and consistently dyed, and which protects the article, requires experience; and in the case of hard coat, unique low voltage, high current density rectification technology that Sanford Process possesses. With this proprietary technology, + 2 mil coatings can be consistently applied, allowing for cast articles to be used in even the most demanding of applications.
So, do not shy away from electing a cast process to form aluminum articles out of fear that they will not be properly finished. Instead, work with an experienced anodizer that has demonstrated capability to produce beautiful cast parts and work through all of the design and engineering steps to achieve the quality and cost that meets your goals.
At DCHN, we know that many questions arise when considering aluminum anodizing, hardcoat, and other metal finishing jobs. Our white paper, “12 Proven Tips to Save Time & Money for Aluminum Anodizing, Hardcoat, and Other Metal Finishing Services," is a guide full of great tips to help you save time and money. Download it now.