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resistance swing while anodizing......

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  • resistance swing while anodizing......

    Im anodizing some parts to try out some new Ti racking. Ive had this issue some during certain runs before using the Ti racking also. Im running a CC PS, and the voltage just goes up, down, up, down....heres how much:

    Time(minutes): Voltage:
    1 8.58
    24 8.85
    30 8.16
    33 6.34
    35 7.65
    38 7.60
    39 6.00
    40 4.27
    41 6.00
    43 8.84

    It smoothly ramps between the voltage, doesnt jump. Sometimes it does this for me, sometimes the voltage slowly rises like normal. Any ideas?

    Also, this batch using the Ti rack seems to have turned my ano bath rather darker then normal. Anybody have an idea why? Thanks for any help!

  • #2
    Up and down swings in the voltage (actually the current) indicate; you guessed it, bad connections. Your spring tension in the Ti isn't high enough.


    • #3
      I find that fairly hard to believe, just due to the fact that I know hard the clips are digging into the metal. If I wiggle the parts on the Ti clips, it actually digs into the metal. The next step would be tightly bolting wire to the parts which, in my case, is totally not possible.

      How is the industry using Ti racks with no problems? The high current anodizing make THAT much of a difference as far as forgiving "bad" connections? If thats the case, Caswell should tell anybody that hopes to use LCD anodizing in any sort of business or somebody that cant drill holes in their products to run bolts through to look elsewhere.

      I hope theres a simple fix for this, it would be disturbing to have bought a CC power supply and changed my ano bath just to have to convert back so that I can anodize without so picky.


      • #4
        to expand on this.....

        While the parts were still anodizing.....I wiggled them around (not real easy....they were pretty tight) while somebody watched the voltage for a jump to to a connection getting better or worse. Nothing happened. The clips simply grinded into the metal when I twisted the part. There were also tons of bubbles pouring off the parts, so they had to be making at least some contact. Are there any other possibilities......or just the connections?

        If bubbles are pouring off the parts, how can the connections be bad enough to cause the current to go up and down so much? Thanks again


        • #5
          You're expecting too much, no connection method short of fusion welding is going to be 100% perfect all of the time. Rack anodizing has some fallout, not all of the pieces will anodize properly all of the time. Any commercial anodizer will tell you that. The fact that you can get the spring pressure right most of the time indicates success, 100% yield isn't going to happen with any mechanical attachment method.

          BTW; this current robbing problem is still there and causes the same problems if high current densities are used, if anything the problem is worse. Most of the Industry completely ignores monitoring electrical parameters while anodizing. When the job turns out bad, they concoct some BS chemistry explanation for something as stupidly simple as a bad connection.

          Edited to add the BTW.


          • #6
            I have seen this problem also. I'm running a CC power supply and have a Ti wire tightened to the part with an aluminum bolt. In previous runs I've seen the steady climb in voltage that is expected, but now when I put the part in the bath and turn the current up I'll see a slow climb up to about 10V and then the voltage will start a slow steady drop. I don't allow the voltage to drop too far before I turn the current up a bit and the voltage will climb again. I was thinking maybe my cathodes (lead ones that came with Caswell kit) needed to be cleaned. I leave them in the acid bath all the time, and they don't look bad at all. When the voltage drops is this the connection getting better or worse?


            • #7
              Lead cathodes don't get cruddy with sulfate deposits like aluminum ones do, they don't require more than occasional cleaning. Have you checked the wire joint connecting the cathodes?

              If you are operating at 4.5 A/sq.ft. and it takes around 60 -90 minutes for this drop to occur, you are seeing PAR. If the drop occurs much faster than that something bad is going on with the connections somewhere. Try using soft aluminum wire in place of the titanium, and crush the wire under the aluminum bolt.

              In CC mode the current remains at a constant value, the voltage increasing means that the total resistance in the setup is increasing. This is good, because most of the resistance is from the bulk resistance of the anodic coating itself. This resistance going up means that the coating is getting thicker. The anode connection to the work has the insidious property of having its resistance increase slowly if the connection will allow electrolyte to penetrate it and anodize the contact point. If your CC power supply can put out enough voltage to compensate for this extra resistance no harm is done, providing that the extra power dissipated (as heat) doesn't raise the temperature too much.

              Problems arise if the connection resistance gets so bad that there isn't sufficient voltage available, or the temperature gets too high.