July 14, 2020
Let’s start at the beginning. There are two basic types of specifications: MIL Specs, and Customer or OEM Specs. Most people are familiar with what a MIL Spec is: they are managed by the United States Department of Defence and exist to provide a standard level of quality and workmanship to products purchased by the US government. However, since they are essentially open-source code for anodizing (and many, many other things!), designers and manufacturers frequently choose to call out MIL Specs in their designs because the results are predicable. Customers Specs, on the other hand, are developed in-house by OEMs or their suppliers and are intended to ensure that the process they describe maintains a consistent level of quality. They are usually tightly focused on the type of product a company produces. Interestingly, many Customer Specs are almost identical to MIL Specs and in many cases can be directly substituted with the relevant MIL Spec!
Therefore, unless your customer has required that you use their internal specification (in which case they may have approved suppliers which you may be required to use) there is really only one spec for anodizing you need to know: MIL-A-8625. It is the most common anodizing specification and is used in industries ranging from aerospace to electronics manufacturing. You see it everywhere from window frames to exterior signage to key chains. But within this spec there are at six different Types, and two different Classes!
Don’t panic – we are here to help!
Let’s cover the easy part first: the two Classes. Class 1 means the parts are processed without any colour added during the process. This is often referred to a “clear anodize” because the aluminum substrate is not tinted by a dye. If you want your parts to have a specific colour, that’s Class 2, and you have to specify what colour you want, ie: black anodize. Anotek only offers a limited range of colours: black, red and blue. You are strongly advised to consult with us – or whomever you have selected as your supplier – to ensure they offer the colour you want. It’s also highly advisable to run a sample to ensure the alloy and process your supplier is using provides the desired outcome. Results can vary widely!
Now for the Types. The six Types of MIL-A-8625 break down into two categories: 1) chromic acid anodize and, 2) sulfuric acid anodize. However, the spec itself does not define these Types according to the type of solution they utilize. It defines them according to the thickness of the anodic coating that gets created during processing. This is where it gets a bit tricky because thicker coatings do not always mean better results!
Type I, Type IB and Type IC are all very thin films which have slightly different characteristics. The main difference is that Type I and Type IB are created using chromic acid, which contains chromate and is nasty. Most manufacturers are reducing the use of chromate wherever possible. Type IC is also a thin film but it does not use chromic acid. Sulfuric acid is a popular alternative, but there are other methods as well which are too in depth to cover here.
This is by far the most common type of anodizing. Unless your project has some wacky application, it’s almost certain that Type II can meet your needs. There is a sub-category called Type IIB which functions as a non-chromate alternative to Type I and Type IB, but it is not very common and your engineer should be ready to explain why he or she would want it.
Often called hardcoat, many customers are lured by the siren call of its greater thickness relative to Type I or Type II. Don’t be deceived. All the Types must meet the same quality control requirements, including corrosion resistance. The only additional test for Type III is abrasion resistance because it is intended for use in circumstances where the anodized surface will come into regular, abrasive contact with another surface, which is actually quite rare.
As you can see, understanding the requirements for your anodizing job is something that, with a little advice, is not as difficult as it seems. If you have any questions about this article or your project, please call me at + 1 (604) 459 2868 for a free consultation and we will help you determine how to achieve the best possible anodizing results.
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