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720 rule and PAR

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  • 720 rule and PAR

    Are you supposed to reach PAR using the 720 rule? I?m going back to some notes of some anos I have done (just using the 4.5/144 rule on the Caswell manual) and keying info into the 720 Rule Anodizing Calculator and I wouldn?t be able to leave a part that long with out going past PAR! For example:

    I did a batch with a surface area of 118.17 Sq Inches.

    Using the Caswell?s manual -----4.5/ 144 = Amps / 118.17 so Amps = 3.69
    Using the 3.69 amps this batch reached PAR in 90 Minutes with a voltage of 10.2 volts!
    Successful ano ! Nice color etc.

    I keyed in the same surface area of 118.17 Sq Inches into the 720 Rule Anodizing Calculator (just for comparison purposes)!

    I imputed
    Surface Area = 118.17 in2
    Thickness = 1 mils
    Current Density = 4.5 Amp/Ft2

    It calculates
    Set Current = 3.69 Amps (same as my calculated amps with the 45/144)
    Time = 160 Minutes (70 minutes past PAR)
    Peak Voltage = 11.25 Vdc (peaked at 10.3 volts)

    I?ve keyed in a couple of successful ano notes and all are similar in comparison to 720 Rule Anodizing Calculator. So what do you do once you reach PAR and you still have and other 70 minutes to go according to the 720 Rule Anodizing Calculator. I?ve never used the 720 Rule Anodizing Calculator to ano anything but I?m trying to get familiar with it!


  • #2
    Re: 720 rule and PAR

    it took me a while to get my head around the 730 rule, i'de been experimenting with another method and only dying black, so i was constantly looking for PAR.
    Acidrain gave me some good advice, which i followed and have had constant consistant results since.
    with the 720 rule, if you have worked out your surface area correcly, and you know what the alloy is, ignore your voltmeter readings and just let it run.
    voltage does drop slightly towards the end of the run due to the temp of the electrolyte rising slightly giving a false sense of PAR being reached.
    the following is copied from one of Acidrains earlier posts

    The 720 rule calculator works well for 5xxx and 6xxx alloys, but on 2xxx and 7xxx, reduce the time by 25%.
    For example, if you're anodizing at 6 amps per sq. ft., and want a full 1 mil build, the 720 rule says ano for 2 hrs.
    This is true for 5052 and 6061, but for 2024 and 7075, only ano for 1-1/2 hours.
    Also lack of agitation will cause Desolution and over heating of the part which will also give a false PAR point
    Last edited by spankey666; 03-12-2010, 02:53 PM. Reason: added a bit
    "A stupid man's report of what a clever man says is never accurate because he unconsciously translates what he hears into something he can understand.
    Custom Anodising


    • #3
      Re: 720 rule and PAR

      Maybe this chart will show why PAR does not coincide with the 720 rule.
      Alloy vs. Time, 18 ASF - Caswell Inc Photo Gallery
      The coating continues to grow in thickness even though the resistance may not increase, dependent on material grade, due to increase in pore size and tank temp.
      PAR is not a consistent method to anodize. Some grades of aluminum reach PAR well before the coating is grown to sufficient thickness.