“Chasing” The Finish
Q. Our company does all types of metal spinning. We do spinning on a jobbing basis for others’ products. We also play an active role in the design and development of some of these products. We are currently engaged in product development of spun aluminum chasers. Chasers are also called “plate liners.” These decorative liners, which are used under dinner plates, are a great application for spun aluminum. We know how to produce the chasers, but they have to be finished in a way that gives them a warm, exotic, sort of high-tech look. We are thinking about anodizing and before we get any further into that we would like your opinion about that type of finish on this product. Is it practical to do this? What are some of the things we should be aware of as we look for an anodizer? Any information that you can offer would be
A. To start I can tell you that most alloys of aluminum can be anodized, but, as you know, there are only a few that can be successfully spun into shapes. Alloy 1100-0 (zero temper, dead soft) has the best elongation characteristics, 60%. This is followed by 3003 and 5052 alloys at 30% and 25% elongation respectively. You don’t say what alloy you are planning to use to make these chasers, but 1100-0 is an obvious choice. So, if you have made this choice, you are off to a good start because even though 3003 and 5052 are quite anodizable, 1100 is by far the best because it is high purity, i.e., 99+% aluminum.
As the aluminum is spun, its work hardens. I don’t know what temper you might end up with, probably a “half hard” version of the “H1” temper known as “strain hardening,” something akin to H14 perhaps. Since the temper of the aluminum can make a difference in the final color, temper is important. I think the most important point with temper is to have it be fairly consistent. I suspect this would be the case in these spinnings since all commercial spinning these days is performed on CNC lathes that apply forming pressures to all the parts equally. We can make the assumption that the parts will anodize and dye well. If there were different tempers, it could influence the anodic coating thickness and, therefore, the final dyed color. Consistent color match is important to this product, I’m sure.
Hopefully, you have an experienced anodizer in your area that you can rely on to produce high- quality anodizing on these parts. Here are some of the things to note about the actual anodizing. The real skill in the anodizing and, as I see it, the biggest challenge in anodizing these parts is to rack them so there is no (obvious) rack mark. Unfortunately, the unexposed areas where you would prefer to place the rack mark are very limited. It would be ideal if on the flat part of the underside you could form a small lip where the parts could be held during anodizing. If that can’t be done, then the only choice for the anodizer is to rack on the exposed edge. You mentioned in your email that the parts are 14-inch diameter, so they will have to be racked with enough space in between the parts for the anodic coating to form evenly. The anodizer will know about this.
Finishing involves the steps of cleaning with an aqueous cleaner, usually a mild alkaline bath that removes general dirt and oils. Then the parts are either etched, with an alkaline or an acid solution and desmutted. These parts might look spectacular with a bright, or semi-bright, finish. To do this the parts can skip the etching step or can be lightly etched and then brightened either chemically or electrolytically. After desmutting again the parts are ready to be anodized. I would recommend that the coating thickness be at least 0.0005 inch (half a mil). This would give them some degree of wear resistance, which they are going to need, and would form the basis of excellent dyeing to get deep lustrous colors that will last virtually forever. After anodizing, the parts are rinsed in deionized (DI) water before going into the dye bath. You may know that these parts may be dyed virtually any color you can think of. They could even be subjected to special finishes like splash anodizing (a whole other subject). After dyeing the parts are again thoroughly rinsed in DI water and sealed in a mid-temp nickel acetate bath.
As you can imagine, when this process is done properly you should get a spectacular looking product that has a nice sheen, wonderful depth of color and great durability. One final note: this product must not be put in the dishwasher. Hand wash only.
This important first step can help prepare the metal for subsequent surface finishing.
The following anodizing process overviews are provided as a means of introduction to aerospace anodizing
Anodizing for pre-prep bonding bridges the gap between the metallic and composite worlds, as it provides a superior surface in many applications on aluminum components for bonding to these composites.