Dye problems with bead blasted finishes

[Easttex]

I've been doing polished finishes with great success and virtually no problems that were not of the operator related variety. I was doing some step-wedge calibrations and color blends for color densities tonight when I noticed problems with the back side of my tests. The front side was roughly polished while the back was bead blasted. The back side (bead blasted side) of each of the pieces did not take color at the same rate as the polished side of the test piece. The bead blasted side was very flat and uncolored looking, where as the oposite (polished) side which recieved the same dye duration in the same step area showed an time appropriate color density. How can I get good color on a bead blasted finish? I'm not having much luck.......... Thanks,
EasTTex

[Fibergeek]

I'm suprised to hear that, my experience has been that beadblasted surfaces dye the easiest.

I use glass beads only, 150 -200 mesh (fine) what's important is they are used on aluminum only. The presence of steel particles or other metals will cause anodizing defects, but this doesn't sound like your case. You must have an effective oil trap in your air line, the presence of compressor oil on the work can cause what you describe. Another possibility is wrong cathode placement, what happens if you reverse the orientation of the work in your tank? Did the reverse side anodizing look OK before you started beadblasting?

[Easttex]

The beads I am using do not have a rating on them. The packaging lists the ingredients as "glass oxide." I obtained the blast media from a friend who has a radiator shop, they use the beads to clean aluminum radiators without scarring them. If I were to compare the beads to anything size wise, it would probably be the media in a small hour glass found in board games(not a very scientific comparison.) The grains would probably be, say 20% the size of granulated sugar. I have a moisture/oil trap on my airline at my compressor. The blasted surface just seems to have a grayish appearaance to it and just doesn't seem to take dye at all, or very little at best. I thought blasted finishes were supposed to be easier. The possible oil problem should be resolved by the simple green degreaser I am using not to mention the alkaline cleaner I use as my final cleaner shouldn't it? The pieces I was running the tests on were .25x 1.5 x 6" long. The aluminum is just ordinary bar stock. I was running twelve of them at a time with two racks zip tied together and the parts kicked out to each side at about a 30* angle to each other. I ran the parts at about 10.5 amps. My aluminum cathode area is about 144sq" running down each long dimention of my tank with .75" solid copper bus bars.The tank is holding about twenty gallons of sulceric @5% with wetting agents and mist suppressents and running at 70*. As mentioned, I am having great results with the polished finishes, it is just the bead blasted side of the test pieces that are giving me problems. I have ran considerably larger runs in the past with flawless results. Thanks,
EasTTex

[NeoMoses]

What happens when you strip the parts and re-anodize them without bead blasting again? Do they turn out as expected?

I, too, have seen this problem of poor dye absorbtion with colors. Although I haven't determined a root cause, stripping and re-anodizing the parts usually fixes it for me. It's not a good solution, but it is a solution. See if it works for you.

[Fibergeek]

It still sounds like the beadblasting is contaminating the work, most likely with oil. If this is the case, the oil has been blasted into the now rough surface of the work, its in deep. I doubt simple green, alkaline, or whatever will take it off easily. Elevated temperature and vigorous scrubbing would be in order.

The oil/moisture trap that came with my compressor was an ineffective POS, I replaced it.

Since radiator shops aren't noted for their oil-free environment, get your own glass beads from MSC or where ever, you then will know that they're clean. Your blasting cabinet and the gun have to be clean too, are they?

I'm also assuming you haven't changed anything else besides adding the beadblasting.

[Easttex]

Hey guys,
I appreciate the input. The beads I purchased were freshly delivered and in a sealed 50# bag. I am trusting that they were oil free. I am going to install an auxiliary oil trap and moisture collector at my blast cabinet, which is an the other end of a about 40 foot of air line, as well as, putting in some fresh beads. I'll let you know how it comes out. Thanks again,
EasTTex

[M_D]

I haven't had any experience anodizing bead blasted parts, but all of this sounds a lot like anodizing parts tumbled in ceramic media. Simple cleaning isn't sufficient like it is for smooth machined surfaces. I found that the stripped parts (previously tumbled) did better the second time around. I asked myself why, and the only thing I could think of was they were likely way cleaner and down to pure aluminum alloy after stripping.

So, I began trying to clean the parts better in the first place. It took a while, but I discovered they needed to soak in 165º + SP cleaner for several minutes at the minimum.

Things go a lot better when the parts are cleaned in stripper or lye either before or after. Since the lye introduces some level of smut into the equation, they need to be treated in the desmut. Then when they are rinsed of the desmut, they get cleaned a few more minutes in the hot SP cleaner.

Since the parts are actually clean in the most common sense before starting the cleaning, I am going straight to the lye solution first, desmut second, and cleaner last, with thorough rinsing in between.

With the aggressive and extended cleaning process, I have eliminated virtually all uncertainty about if the tumbled parts will turn out. I really think Fibergeek is right about the parts being contaminated by dirty beads and/or air, and better cleaning will be the cure.

[Easttex]

Cool,
Thanks for the input. I'm on my way out to the shop right now to put in fresh beads and clean out my hose and blast gun. I am putting in a coalescing oil trap on before I run parts tonight. I'll post the results.
EasTTex

[Easttex]

Hey,
I'm still not getting good bead blasted results, a bit better, but not what I expect. I have put in an oil trap right outside of the blast cabinet and changed out the beads. I'm running 200 grit glass at 90 psi, I'm getting a much better peening effect, but still having a very dull gray'ish cast to the parts. I've checked for continuity and all is well. The other side of the test piece, which is polished, seems to be coming out fine. I've done several tests with nearly the same results. The test pieces are always lacking in color saturation. When doing step density tests, there is nearly no difference in the time steps on the blasted side, where as on the polished side the steps are very recognizable and very vivid in color. Any ideas?
Thanks,
Easttex

[Fibergeek]

I'm beginning to wonder if what you are seeing is actually a problem, or the normal visual dyeing differences between matte and polished surfaces.

The "no change between time steps" makes me consider this; the matte has more surface per unit area than the polished, it will require more time in the dye than polished to get the same shade. If you reduce the PSI and/or use smaller beads; so that the matte surface is somewhat less matte (smoother) do things improve?