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Aluminum anodizing and blind holes

Our company has recently come across an issue with our anodizing vendor. Our parts are to be anodized Type II per Spec - MIL-A-8625 / MIL-PRF-8625 [on DLA]. Our parts have a blind hole in which we recently noticed that the bottom of the holes are not anodized or have a lighter color. Our vendor is telling us that it is our cleaning process or blasting process. I thought that all parts go through a "bright dip" prior to anodizing to prepare the surface. If this "bright dip" solution is not effectively removed will this leave an un-anodized surface? I am not so sure it is us because we have not changed our processes.

Michele [last name deleted for privacy by Editor]
- Indiana, USA
2006

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First of four simultaneous responses --

A blind hole does not help much. Anodize will gradually decrease as it gets deeper into the hole, and that is with the hole mounted straight up. Actual depth will vary from time to time and with anodizers, but will only go about as deep as the diameter of the hole.
So, how wide and how deep is your "blind hole"?

James Watts
- Navarre, Florida
2006

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Second of four simultaneous responses -- 2006

Deep blind holes will typically not coat evenly, with more coating at the top, and less at the bottom. However, with that being said, it sounds like an air bubble is being trapped (very likely if the blind hole is racked facing down, during the anodizing process, especially if its a shallow blind hole).
The other thought would be that there is crud at the bottom of the hole that isn't being cleaned out prior to coating, therefore preventing the anodic film from developing.

Marc Green
anodizer - Boise, Idaho

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Third of four simultaneous responses --

Michele

You do not tell us whether or not the parts are dyed a color, or how you determine the bottom of the holes are not anodized. Lighter color seems to indicate the parts are dyed.

Blind holes oriented upside down will form an air pocket and little or no anodize will form.

The anodize solution is heavier than water and if not rinsed out, will interfere with the dyeing process- little or no color.

Depending on bath parameters, the 'throwing power' of the anodize solution typically can achieve full thickness at 3x the diameter of the bore. Deeper than that and you will have thinner coating. Thinner coatings will not absorb the same amount of dye, and subsequently yield a lighter color. If required, an auxiliary cathode can be used to achieve full thickness.

If you have not changed your process, and you were getting acceptable results prior, something may have changed in your supplier's process. Blind holes present challenges if not properly addressed in the anodize process.

Willie Alexander
- Colorado Springs, Colorado
2006

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Fourth of four simultaneous responses -- 2006

If the blind hole is too deep or too small diameter, then the cleaning solutions, bright dips,rinses and deoxidizers cannot get in/out. If the drilling hole used wax or paraffin, then the bottom of the hole is partially masked. Finally, anodizing "throws" very well, but depending on the size of the hole, it may not "throw" down into the bottom.

Also, some holes may have to be positioned up-side-down, hence a hydrogen gas trap will prevent solution and electrical current from getting into the hole.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

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Not enough information. If the parts were previously anodized satisfactorily, may have a racking or pretreatment problem. Small diameter, threaded deep holes are difficult, as need to both drain liquid and avoid air pockets. At most, can only anodize blind holes on half of the part's surfaces.

Ken Vlach [deceased]
- Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.
2006

Thank you for your responses. The hole is Ø .078 by .070 deep. These parts get anodized all different colors.

Michele [last name deleted for privacy by Editor]
- Indiana, USA
2006

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Is there any test to check that an air bubble is not forming inside a taper? Immersion testing was mentioned : With the rack fully submerged check each component position by lifting each head product above the fixator but below the liquid surface and rotate the product so that the internal taper is visible and check that there is no air bubble present in the taper bore and the recess. Record results.
Is there any dye that could be put in the blind hole that changes colour when it is fully wetted? Any other options would be considered.

Deb o'd.
work - Europe
December 31, 2010

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Ed. note: please join the community by including your last name and town, Deb. "Real" people usually receive more of the readers' time than semi-anonymous strangers. Thanks.