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  • Current Density QUestion

    I see on the 720 Rule Calculator that the Current Density is set to 6.0 amps. Yet in the Caswell instructions, 4.5 amps is specified.
    With the part sizes I am going to anodize - at 6.0 current density, it comes out at 3.48 amps for 120 mins (1 mil) and at the 4.5 amp current density - it comes out to 2.61 amps for 160 mins.
    I am using the 3:1 water/acid ration and a constant current PS. Material is 6061 or 7075.
    Thoughts? (first time doing this BTW...)

    Thanks in advance.

  • #2
    If you are running with controlled current, then changes in bath chemistry have much less of an effect on the process. The conductivity of H2SO4 depend on the concentration which in turn effects the overall resistance of the entire anodizing circuit(bath + busing +connections). If you are running by constant current, the resistance change will be seen as a increase or decrease in the power supply's output voltage. As long as you are not capping the power supply's output voltage, small changes in concentration don't matter. That's is the primary reason for running constant current.

    In commercial shops, the goal is to do as many anodizing runs as possible in a day to maximize profitability. To achieve that, the Type II process is typically run at 12 ASF. Since Type II is done at ~65F, running much higher than 12 ASF may cause burning on the part, but running anything less will decrease the number of runs in the day. So why is 6 ASF the accept norm here? Because most people here are not commercial shops and don't have the equipment needed to handle the addition heat generated by 12ASF (as opposed to 6ASF). That said, my recommendation is to run as close to 12 ASF as your setup allows. Be aware though that if your bath temperature get much past 75-80F, you coating will most likely be [email protected] and you may damage the part.

    Certainly if you are new to anodizing 6061 is recommended because it is one of the easier alloys to anodize.
    Last edited by KevinB; 04-25-2020, 08:42 AM.
    Process control doesn't give you good quality, it gives you consistent quality.
    Good quality comes from consistently doing the right things.

    Process control systems for Anodizers
    If a post helps you out LIKE the post
    I'm not an Amateur Metal Finisher. I've just been around the industry for a dozen years or so helping and consulting when and where I can.

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    • #3
      My PS is rated at 10A / 30 VDC max. An equal concern is the ability to dye black. Time isn't really an issue - really looking for a decent anodize layer (1 mil) and the proper cell structure to accept the dye. My take away from your info (thank you btw) is that I can keep bath temps down with the lower current density. I read somewhere that running below 3 amps results in poor quality. (true?) At the 6 ASF it translates to 3.48 amps at a peak voltage of 15. Thoughts? and would this also apply (quality wise) to 7075?
      Last edited by opsoff1; 04-24-2020, 11:40 AM.

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      • #4
        A few things here...
        For a dyed part, there is no reason to exceed 0.5 - 0.7 mil. Typically dyes cannot penetrate beyond that so going to 1 mil is at least 30% more than needed.

        Heat in the bath comes directly from the power out of the supply. Power = output voltage * output current. Some of this power is lost in the busing and connection in the form of heat, but most of it is converted to heat at the part during the chemical change of AL into AL2O3. If you are running 15V @ 4A, that equates to 60W of power. Put your hand on a 60W light bulb and you will know just how much heat that is (on second thought don't; it's pretty hot). The take away here is the higher the current density, the more voltage is needed and the more heat is generated.

        That said, running too low of a current density is no good either. Although I don't know for absolute certain, I have read studies that indicate pore size is inversely proportional to the anodizing voltage. So the lower the voltage the bigger the pore and if the voltage is too low, I would guess the pore can be too big to contain the dye and it will simply leech out in the post dye rinse. Also, the lower the voltage that longer time in the bath you will be. This by itself will tend to soften the coating which will ruin a Type III coating (i.e.hard coat) but I'm not sure what effect if any that will have for Type II coating.
        Last edited by KevinB; 04-25-2020, 08:39 AM.
        Process control doesn't give you good quality, it gives you consistent quality.
        Good quality comes from consistently doing the right things.

        Process control systems for Anodizers
        If a post helps you out LIKE the post
        I'm not an Amateur Metal Finisher. I've just been around the industry for a dozen years or so helping and consulting when and where I can.

        Comment


        • #5
          Great info - thanks! My plan is: .7 mil 6.0 ASF, Current set at 3.48 A, 84 mins, peak voltage 15.0 DC. Hopefully this will result in good hardness and excellent dyeing capability.

          Also running a water heating rough calculation - that set of parameters should see about an 8-10 deg temp rise from 60 deg starting bath temp.
          Last edited by opsoff1; 04-24-2020, 01:19 PM.

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          • #6
            Hi all ! I just came across this forum since I want to start anodizing the parts I machine on my CNC mill.

            I´ve been trying to figure out how to use a "720 rule calculator" but have some doubts about the concept of "Current Density". At first I thought about dividing a chosen amperage by the surface of the part to anodize, however, that gives me crazy numbers (high voltages).

            While searching the Web, I came across this post which makes me wonder if CD is a given constant, as some users mention using a value of 6. Is that so ? Should I keep the same value for all my calculations ?

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            • #7
              Cvitanic, please start a new thread for a new topic.
              Process control doesn't give you good quality, it gives you consistent quality.
              Good quality comes from consistently doing the right things.

              Process control systems for Anodizers
              If a post helps you out LIKE the post
              I'm not an Amateur Metal Finisher. I've just been around the industry for a dozen years or so helping and consulting when and where I can.

              Comment

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