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Type III (hard coating)?

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  • Type III (hard coating)?

    I have read on may sites that Type III anodizing is very hard to do (most small shops use type II).

    The only notes I seem to see repeatedly: current density is much higher and temperature is much lower.

    If these are the only differences between type II and type III, I should be able to overcome this.

    I have ac and dc welders capable of 300 amps. Low temperatures can be obtained by using dry ice.

    Is there something I am missing

    Also, I read about the LCD kit. Is this process true type III or just a good type II process?

    Thanks .... Mike

  • #2
    Yes, you are missing something. There is more to this than just adding dry ice and hooking up an arc welder.
    The current needed for this kind of process is in the 30 amps per sq. ft. range I believe, and that kind of power will really heat up the electrolite. Your power source is not appropriate for anodizing, and the temperature control would be very difficult to monitor.
    The LCD method is a proven and safe type II process.
    I do things.


    • #3
      Acidrain is quite right.

      Since Xtal01 is looking to Hardcoat a piece 0.56 sq.ft. (I visit other websites too) I can provide some approximate numbers.

      Using Acidrain's ASF figure, figuring on 7075 alloy, and a coating thickness of 3 mils:
      DC current; 17A
      Peak voltage 29VDC
      Anodizing time; 72 min.
      Average power dissipaton in the tank; 320W
      These figures assume PERFECT electrical connections. The voltage and power can be much higher if they're not.

      The cooling problem is not getting the electrolyte cold enough, but its keeping it in a narrow temp range for the full 72 minutes, with the mentioned power dissipation. You will fail if you don't, and we haven't even mentioned the very vigorous agitation required.

      Assuming you actually formed a Type III layer, dyeing it will be problematical, you probably want it jet black, right?

      Other than true milspec M-16 lowers that have to pass govt. specs for govt. procurement, commercial lowers are more likely a Type II layer grown to 2-3 mils thick. The dyeing is too good for it to be Type III. Yes I know what you have read on the internet. Claims of a Type III coating are easy to make, it takes a well equiped test lab to tell the difference between Type III and Type II grown to the same thickness.


      • #4
        And this is why I came to this site!

        Exactly what I wanted to know.

        Although a mechanical engineer by trade (never taught us much about hands on work ... like how to anodize), I have a small shop (mill, lath, ...).

        I am very much a "do it yourself" person (always building something ... wife says at least she can find me in the shop and not out drinking with the boys).

        It does look like a type II process would be fine for my AR-15 lower (and a few other small projects I am working on ... front of a pulse jet engine for one).

        Just for my own curiosity, what is the correct temperature for type III anodizing?

        I see your point with dry ice as a coolant. I did however build a a/c unit from scrap parts twice int he past, I suppose this would be what I need.

        Thanks .... Mike


        • #5
          Electrolyte temperature for Type III is 32-36 deg. F. for electrolytes with no additives, to as high as 45-50 deg. F. for electrolyte with proprietary additives. These additives can cause other problems however, you have to know what you are doing.


          • #6

            I appreciate all the information.

            I definitely think type II will be enought to try in my little shop (at least till I get some experience).

            Thanks again for all the information!