Bead blasting aluminum before clear anodizing. Cosmetic problem.

A. The defect is due to bead blasting not good enough (uneven, in order to blast the small hole ID, areas close to the hole will end with thick blasting if the rest area not properly being compensated). Actually you could find out the uneven defect right after etching and desmut but before your anodizing.
John hu

A. Sir,

I run 2 large, job shop anodize plants.

The problem could be with the anodize or anodizer as we have never really had such problems. We do lots of cosmetic work.

A. It sounds like a racking problem to me. Dark areas on clear anodized parts could be a result of parts losing contact and alloying elements plating back onto the parts, particularly if the knobs are a high copper alloy. Either the anodizer doesn't have a good rack for holding these knobs, or they are working loose somewhere in processing. The "bullseye effect" could even be from arcing due to intermittent contact of loose parts on the rack. Maybe the anodizer is trying to bulk anodize the parts, where a certain percentage of non-anodized parts would be encountered.

If the problem is indeed with the anodizer, as it appears, I must say that no GOOD anodizer would let finished parts go out the door with 30% rejects, particularly when the cause for rejects is highly visual. In these days of quality-oriented production, any good anodizer needs to have an edge over his competitors, and usually the only way of showing customers how good his shop is, is to deliver a quality finish on any parts that he processes. I believe your supplier needs to find another anodizer or demand a major shake-up of the present anodizer's quality methods.

Phil Johnson
- Madison Heights, Michigan
1998

A. I think you may have problems with the smoothness of the part. It may be the anodizing is highlighting the imperfection of the part. I have run anodizing lines for 30 years. I have seen problems like this before.

adv.
If you would like to send me some of the parts to view, I might be able to help you.

I must commend the response directed towards any "GOOD Anodizer" would never ship parts with a 30% reject trend. I have noticed that with today's market of quick turn orders meshed with cost of operating reductions that some outside process vendors are having troubles balancing operations and cut too many corners when cutting costs. One of the biggest mistakes American Industry might make would be to fall short on their "BQS", Basic Quality Standards. Shipping parts to your customer with a 30%...20%...10% failure rate is never ok. Businesses that continuously accept deliveries like this are just as guilty as the vendor shipping the parts in such unacceptable conditions. If American Industry wants to be leaders in manufacturing we need to quit finger pointing, quit accepting lame excuses and most of all address issues with intent to resolve them.

I 100% agree that the Anodizer shipping 30% rejects shouldn't have the opportunity to be so irresponsible and degrade your products.

A. Gus,

One way to even the blasted surface is to perform a two step process. Run parts through your normal caustic etch before anodizing. Anodize parts until you have reached an approx. thickness of .0003 then strip parts again in the caustic bath. If you can follow up with a Nitric/HF bath after the caustic strip is even better. After stripping the parts re-anodize & dye. This process will smooth out the high and lows of poor and uneven blasting.

A. Be sure that the beads are designated aluminum only. If they use the same beads on iron, copper, or magnesium, or any other metal, then you pound giblets of foreign metal into the aluminum which will lighten the shade of the black dye. Again, as the man said above, etch the hell out of it to remove the beaded-in non-aluminum.

A. Two problems.
(1) The manual held bead blasting wand is not leaving a uniform finish. It looks great to the naked eye, but the anodizing magnifies the varying splotches.
(2) If the media has been used on any other metal (iron, brass, etc) then you are imbedding that metal into the soft (Kleenex-like) aluminum. Imbedded iron in an aluminum anodizing tank is oxidized to reddish color.
(3) There is a possible third problem in that where the bend is located the surface is different from the other flat surfaces and this skews the (otherwise) vertical pore structure.

A. Dear Sir,
Buffing and blasting are 2 different processes with different purpose. Buffing is generally used for gloss finish and also for removal of deep scratches.
While blasting with glass beads is used for satin matte finish and also for removal of extrusion lines. Just make sure what bead size you are using; generally 80 or 60 is been used. Make sure the blasting is at even psi. Anodizing is good on 6063 alloys. Also check the alloy which you are using.

Happy Anodizing

A. To get a matte black finish on 6061 T6 aluminum, bead blast with 110-170 glass bead at around 60 psi. We've done 1000's of parts this way.

A. Let's look (1) for imbedded iron from the tumble media and (2) for work hardening in the particular area which may have migrated the zinc into grain boundaries near the surface where the anodizing layer cut all the way thru to the zinc rich spot and fell out. Then let's look at the agitation pattern and see if air is hitting that spot and displacing sulfuric acid.

A. Pictures of the parts as blasted, and after anodizing would be very helpful.

A. If the surface contamination isn't too excessive, then a good long strip-etch and anodizing again will fix the parts, run few of them as a test before doing the whole batch.

What usually happens is that blasting person will turn the pressure up near max available to speed things up - and this breaks up media and embeds the dust into the surface of the soft aluminum part - and normal etching/desmut times simply may not be enough to remove that dirty layer - even more problems may arise if they used that same blasting media to clean a rusty steel part before - and none of them will not admit any of this - but that is what ruins the parts.

My personal opinion - blasting is the last possible option to prep aluminum part for anodizing, and even then it has to be done properly and complicates desmutting process a little, I usually turn these orders away and send them to paint shops - because the result is simply too unpredictable.