Plating options such as anodize, chromate conversion coating, and passivation are popular choices for many precision machined parts, offering benefits such as corrosion resistance, increased durability, and enhanced electrical insulation or conductivity.
However, when it comes to plating, many contract machining customers are unaware of the full range of options available or what each process entails. There are many considerations that must go into plating a precision machined part to ensure a successful outcome.
Reata Engineering relies on a well-vetted network of trusted finishing vendors to perform plating services on our customers’ parts post-machining, so we’re used to having conversations about plating with customers and plating experts alike.
We’re happy to share our expertise with you by providing this primer on different plating types and key considerations for each to help you make the right decision for your next project:
4 Common Plating Types
Here are the plating techniques we often use for our most commonly machined materials.
1. Anodize and hard anodize
Anodizing is an electrochemical cell reaction that converts the surface of an aluminum part into a porous aluminum oxide layer, enhancing corrosion resistance and enabling strong adhesion for glues and paint primers.
It’s possible to create many different anodize colors through subtractive coloring methods such as electrolytic, dip, integral, and interference. However, color results can vary considerably when applied to different aluminum grades.
Anodize both penetrates and adds dimension to a part. The full dimension of the layer ranges from 0.8 to 1.2 mils, and the proportion of penetration to surface buildup dimension is roughly 70:30.
Hard anodize, or hardcoat, is a thicker type of anodize that creates a coating ranging from 1.8 to 2.2 mils. Hard anodizing offers even stronger corrosion resistance, particularly when sealed with a 5% dichromate solution. The proportion of penetration to surface buildup dimension for hard anodize is roughly 50:50.
2. Chromate conversion coating/chemical film
Chromate conversion coating, also known as chemical film or chemfilm, is another method used to plate aluminum. Creating a smooth, consistent finish in hues that range from clear to iridescent golden brown, different varieties of chemical film may be used as a paint base or for parts that require corrosion resistance or conductivity.
A popular option for stainless steel parts, passivation removes foreign metals from the surface of steel and creates a chromium layer that offers excellent corrosion resistance. Parts remain unchanged in appearance and accrue virtually no buildup, making passivation an ideal choice for stainless steel parts with tight tolerance requirements.
4. Electroless nickel plating
Producing parts that are roughly the color of stainless steel, electroless nickel plating is a process used for many materials including aluminum. It provides a uniform surface and smooths out imperfections in the metal. Electroless nickel plating can hold the closest tolerance of any plating type.
What to Know Before Plating Your Parts
With a solid understanding of the benefits and risks of different plating techniques, you can make an informed decision about which plating type is ideal for your parts. Here are a few considerations to keep in mind.
1. Many variables are hard to control
Plating is a tricky and often unpredictable process. Hundreds of variables affect plating results, and your contract machining partner and finisher can only control a few dozen of them.
Buildup is one of the most important variables to consider when choosing a plating method. For example, anodize and hard anodize both create buildup on a part, changing the final part dimensions. Although the amount of total buildup is predictable within a certain range, the proportion of the buildup that adds to the surface dimension (as opposed to penetrating into the part) varies depending on the anodize type.
Precision machine shops must take this information into account during the machining phase to ensure that the part remains in tolerance post-plating. At Reata Engineering, if we know a part will be anodized, we typically counteract the anticipated buildup from the process by machining our parts out of tolerance so they will achieve the desired tolerance after plating.
In some cases, we may determine that a customer’s tolerance requirements are too tight to be compatible with anodize or hard anodize and recommend another method for plating aluminum that does not produce buildup, such as electroless nickel plating.
The appearance of plated parts can also be hard to control. Factors such as the amount of electricity used and the temperature of the treatment tanks for certain processes can greatly affect the color and continuity of the final coating.
Unless you choose a plating method such as passivation, which doesn’t change your part’s appearance, you should anticipate that your part may look slightly different than expected after plating.
2. Poor plating results lead to scrapped parts
If the machinist and the finisher don’t work together to ensure that a correct cumulative tolerance is achieved, the parts may have to be scrapped. Plating can’t be undone, so an unsatisfactory result means you’re headed back to the precision machine shop.
Reata Engineering takes full responsibility for every service we contract. In the unlikely event that your plating results fall short of your expectations, we will cover the cost. And we take every possible measure to prevent such an outcome, which brings us to the last point. . .
3. Communication is key to successful plating
By ensuring that our machinists and our contracted vendors understand our customers’ needs and expectations for plating, we consistently achieve outstanding results.
Although plating precision machined parts can be complex, our commitment to facilitating detailed communication between our team and our trusted plating vendors prevents potential problems before they can develop.
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