19483 Fraser Way, Pitt Meadows, BC, V3Y 2V4, Canada
July 27, 2020
Getting good results from anodizing starts on the drafting table. An engineer who gives a bit of thought to the design of a part is going to have a much more successful end product than the engineer who completely ignores the final, most visible part of the manufacturing process – one that will determine the product’s cosmetic appeal and provide a major defence against damage and corrosion.
At Anotek Anodizing, we understand that parts are designed for specific purposes, so while you are creating your next innovation, here are some things to remember about getting it finished properly:
This one easily made the top of the list. Anodizing is an electrochemical process and, therefore, we need to be able to run an electric current through your part. In order to accomplish that, we need to mount your parts on a rack, normally made out of aluminum or titanium. It does not take much imagination to understand that racking metal parts on a metal rack is a delicate operation if you wish to avoid scratching the parts. Hence, when designing a part, it would be nice if some consideration was given to exactly how that might be accomplished. Imagine sticking your fingers into an empty water glass and spreading your fingers to apply enough pressure to lift it up. That kind of physics is what we use to rack parts. So we look for holes, bores, edges, lips and other physical features we can leverage against to create enough pressure for parts to stay on our racks. If your part has none of these features, or all surfaces are high tolerance or cosmetic, then we have a problem that could have been designed out of the part right at the beginning.
Because we physically attach parts to racks, there is by definition places where the trained eye will be able to detect rack marks, or contact marks as they are also called. A well-designed part will have not only left us a place to rack the parts, but that place will be somewhere that does not affect the cosmetic appearance or function of the part. Imagine an aluminum brick – it’s easy enough to rack, but since there are no holes, interior surfaces or other features we can use to try and hide the rack marks, they will be right there staring you in the face. And they will probably be ugly.
Studs and trims
We recently processed a large part for a sign company. It was a cosmetic part and therefore the rack marks had to be hidden. However, it was 32 sq ft in size and cut out of 0.032” aluminum sheet, making it very wobbly. This customer was smart though… he called us in advance to ask how we would rack it and he followed our advice of attaching aluminum studs on various points on the back of the sign. Then he just cut them off afterwards. Similarly, he could have left a small excess of metal in a couple spots where we could rack the part and just trimmed the rack marks off afterwards. Both these methods have pros and cons which we would discuss with you if you call Anotek for a free consultation.
Take that water glass from the earlier example and plunge it upside down into a sink full of water. There will be a bubble trapped inside the glass. That’s air entrapment. Because your part will get immersed in various solutions, and circulation of those solutions around your parts is essential to the anodizing process, parts need to be designed to facilitate this process and air entrapment is frequently a problem. A customer once sent us pair of solid aluminum boots. It was a critical, high tolerance part used in deep sea diving and meant to be absolutely water proof and pressure resistant. Our problem was that it didn’t leak! No matter how we oriented the part, it would either hold solution when it was right side up, or entrap air in the toe when it was upside down. Neither of those conditions are conducive to getting a good anodic coating. Due to the nature of this part, the function of the part absolutely required this design and we managed to complete the job albeit with a deviation allowing a slight amount of air entrapment inside the boot. However, for many parts there is no reason for such unvented cavities or recesses, other than just lazy design.
We get that holes are a great way to insert fasteners and hold things together. I’ve already talked about how helpful they are for racking, so we love holes. Except blind holes. This is a variation on the theme of air entrapment. This time, imagine a straw. Dunk it in the sink and put your thumb over one end and when you lift it out of the water, some liquid will magically stay trapped in the straw, apparently defying gravity. The same thing happens to all the tiny blind holes your engineer put all over your parts. Of course, it’s not possible to outlaw small holes, but where possible, especially in cosmetic locations, make then as large as possible or drill them right through to prevent solution entrapment. The reason for this is that many of the solutions we use are corrosive and although the solution stays entrapped for a while it will eventually leak out and stain your parts. This is especially noticeable on Class II parts (ie coloured anodizing).
First of all, never design a part on which all surfaces are cosmetic! Just writing "No Contact Marks" on your engineering drawing or purchase is lazy design. See the section on racking, above. Thanks. Secondly, be sure to indicate on your drawings (and on your purchase order!) which surfaces are cosmetic. Believe it or not, it is not always obvious to your anodizing supplier; we frequently have no idea what parts are even for. And make sure there is somewhere for us to rack!
Sometimes I think engineers are just messing with us. On a couple of occasions, we have been asked to anodize a certain surface of a part, and apply conversion coating to a different surface. Any engineer who thinks this is a good idea deserves to have the cost of the inevitable rework deducted from his paycheque. While technically possible, it is also technically very difficult, error-prone and, therefore, costly. Of course, the more it costs the more a customer expects perfection and the angrier they get when there are rejects requiring re-processing, the cost of which the high price is intended to offset. Instead of designing a good part, that engineer just designed a vicious circle.
One of the main reasons for anodizing things is to make paint stick better. Logically, a part that is to be painted should preferably be designed in a fashion where it’s easy to apply paint on all surfaces. Therefore, fayed surfaces – meaning two surfaces that are bent such that they touch but are not actually attached – frankly, suck. We have to pry the surfaces apart to get the paint in there and that could damage the anodic coating or even the part itself by bending it out of configuration. Similarly, some parts only require paint on certain surfaces. A well-designed part will make that relatively straightforward.
It’s a shame that not all parts can be designed to make life easier for anodizers. But now that we have aired our grievances, perhaps some engineers, somewhere, will have a brief eureka moment and realize there might be a better way to design the project he or she is working on! Regardless, you can count on Anotek Anodizing to address whatever issues arise with processing your parts in order to give you the best results possible.
For a free consultation on your anodizing, chem conversion, NDT or primer requirements,
please call me at +1 (604) 459-2868.