Metal finishing Q&As since 1989

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Inserting self clinching fasteners in anodized parts without hardcoat cracking?

We are having an interesting problem with hard coat anodizing. When we press self clinching fasteners into the body of the part the hard coat seems to crystallize and flake off in that area. We are not the design authority and are manufacturing the part per print. Should the plating be able to withstand hardware insertion or is the specification to do so a design flaw?

Russ MacPherson
Quality Assurance Sheet Metal Industry - Londonderry, New Hampshire, USA
May 28, 2010

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Hard coat is a very good wear resistance coating, but is brittle. When you clinch the fastener, it deforms the substrate aluminum and the hard coat will flake to some extent. James Watts - Navarre, Florida Russ, Without seeing the exact design requirements I can't say for certain that there is a design fault. There are some features that do cause problems for hard coat anodising. Hard coat is particularly brittle around sharp edges. If the design does not include a good sized radius, or more preferable a chamfer then the anodising is liable to chip on those edges. Hard coat is normally applied for wear resistance and in a wearing mode/environment works great, it is less resistant to impact damage and if the force is sufficient can chip. Again this is emphasised on sharp edges. Brian Terry Aerospace - Yeovil, Somerset, UK

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If I'm reading your post correctly, and to expand a bit on what James wrote, it sounds to me as if the actual pressing in of the fasteners is what's casing the anodizing to flake, not the function of the fasteners themselves. It almost sounds as if the hole is too small, and what you are doing is expanding the aluminum around the hole during the pressing process. If that's the case, the aluminum will give, but the anodic coating will not, as it's much harder, and as James correctly stated, much more brittle.

It's not uncommon for dowel pins to be inserted/press fit into anodized parts after coating, make sure your design specs are similar to what they'd be for a dowel pin, and also make sure that the hole is masked during the anodizing process, and you should not have problems with a good quality anodic coating.

Marc Green
anodizer - Boise, Idaho
June 2, 2010