Racking: Function and Location (Part 1 of 2)

Can you anodize a part without leaving a rack mark?

The simple answer is no, BUT, you can help reduce the potential of having rack marks become a problem by helping the anodizer understand the product or by providing a location where the mark will not be objectionable.

Racking serves two functions. 1) It provides a way to hold the part during the process and 2) It is the means by which the current (electricity) enters the part. It is the latter of the two that is often overlooked by designers and engineers and it’s the most important function because supplying the correct current is crucial to obtaining successful anodizing quality. Anodic coatings are applied as a direct relation to the surface area of the part. The contact area (rack mark) must be able to accept 10- 50 amps per square foot (ASF) depending on whether the anodic coating applied is type II and type III (hard coat). This means that the size of the mark cannot always be a pinpoint. For either case it is imperative that the contact (rack) be very tight.

 There are many, many racking methods and here are a few common practices:

1)    Squeezing the part between two or more contacts that wish to remain closed.

2)    Squeezing the contacts that wish to remain open and inserting in a hole or opening in the part.

3)    Threading a bolt into an existing threaded hole or using a nut and bolt combination in a hole and tighten to make contact.

4)    Clamping with a C-clamp device against an aluminum or titanium bar 

Often, anodizers are forced to take an educated guess as to where an acceptable rack location would be because we have not been provided any information. Providing a good place for a solid rack contact is tantamount to a successful anodic coating.

This post was submitted by AAC Member Jack Tetrault, President of the Sanford Process Corporation.  (Part two will be posted on 2-19-14.  Please return to check it out.)