Aluminum Anodizing Process: Techniques for Minimizing Rack Marks
- Jack Tetrault, President Sanford Process Company
Woonsocket, RI (December 5, 2013)
Can you anodize an aluminum part without leaving a rack mark? The simple answer is no – but you may have more choices than you can imagine that will help make the marks unobjectionable. This post explores why rack marks are inherent to the aluminum anodizing process, your choices in deciding where to rack the parts, and what directions to give to your anodizer to help produce the best result.
Racking serves two functions:
1) It provides a way to physically hold the part.
2) It is the means by which electrical current enters the part.
To complete a generic color anodizing process, a part has to travel over four major process stages (pre-clean, process, dye, and seal) each comprised of one or more chemical tanks and usually multiple rinse tanks. There are many potential additional process steps and endless permutations in an aluminum anodizing process, but the point is – the parts are moving up and down as they enter and exit tanks, side to side in chemistry and water – and they therefore need to be secure. No one wants their parts lost in the tank or damaged by accidentally falling off the rack! An important implication of this rule is that the heavier the part, the sturdier the rack that is required, and the larger the rack marks. Also, to keep a large part stable, the part needs to be fixtured to restrict movement, and this might mean several contact places.
It is the latter of the two rack functions of racking that is often overlooked by designers and engineers, and it’s an essential one because supplying the correct current is crucial to developing an anodic coating. Anodizing is an electro-chemical process where the coating is formed by converting the aluminum on the surface of the part to aluminum oxide. The amount of current required to drive this process is in direct relation to the surface area of the part. The electrical contact (i.e., where the rack touches the part) must be able to accept 10- 50 amps per square foot (ASF) of surface area, depending on whether the anodic coating being applied is Type II (decorative) and or Type III (hard coat). So for a part with a total surface area of 1 ft2, a 35-amp hard coat process will require a contact that can transmit 35 amps current. There are a number of important implications of this basic requirement:
1) Type II anodizing leaves smaller rack marks than Type III, as less current is required.
2) The size of the rack mark is a function of the surface area of the part. Larger surface area parts have larger rack marks as more total current is applied.Larger surface area parts also may need to have more than one rack location to create a uniform coating – as the coating grows from the racking location until there is a consistent coating thickness across the part.
3) The rack mark depends also on the conductivity of the rack. Racks can be made out of aluminum or titanium. Titanium, which is a more electrically resistant metal, generally requires more surface area to conduct the same amount of electrical current than aluminum racks. (The benefits of titanium racks however will be discussed in a separate post.)
This means that the size of the mark cannot always be a pinpoint. In addition, the contact between the rack and the part needs to be very tight, as loose racking creates higher resistance and heat, which can lead to burned parts.
There are many racking methods to achieve a good mechanical fixture and an electrical contact between the part, the rack, and the source of the electrical current.
Here are just a few common practices:
a)Squeezing the part between two or more contacts that wish to remain closed.
b)Squeezing the contacts that wish to remain open and inserting in a hole or an opening in the piece.
c)Threading a bolt or rod into an existing threaded hole or using a nut and bolt combination in a hole and tighten to make contact.
d)Clamping with a C-clamp type device against an aluminum or titanium bar
When engineering a part that will be anodized, it is best to identify where the part will be racked and not leave it up to the anodizer to guess what location will be acceptable. Typically, the racking location is chosen to minimize the visibility of the mark after the part is assembled.
There are a number of common areas to rack:
a)Within any hole or opening including threaded holes
c)Using corresponding surface features larger than the rack
Play around with the part and think about where you would hold it to both support the weight, what surface will be hidden or less noticeable, and whether the location is likely to give enough electrical current to drive the process.
Completing a finishing review early on during the engineering process as certain choices may be made more easily at the beginning than later on in the process often arrives at the best solutions. If you have more questions, remember to ask Jack Tetrault (firstname.lastname@example.org)
© 2014 Jack Tetrault, Sanford Process Co., All Rights Reserved