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TypeIII Hardcoat current density questions - Caswell manual very inconsistent

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  • TypeIII Hardcoat current density questions - Caswell manual very inconsistent

    Trying to work out the correct current density for Type III hardcoat on 6061-T6 aluminum.

    Parameters:
    Square footage of the part = 0.11sqft
    Desired thickness = 0.0015"
    Acid bath concentration = 2:1 battery acid to distilled water as per the Caswell manual
    Temperature set to 37-40F, regulated by water circulated through a separate circuit in a titanium aquarium chiller
    No agitation currently.

    We bought the Caswell LCD & Type III kits and have read the manuals over and over, but they state grossly different amps per square foot in different places (and no, I'm not confusing the Type II instructions with Type III).
    Page 122 of the Caswell Plating Manual Version X, under Type III Anodizing, says "current densities range between 18-24 amps per square foot".
    Meanwhile, Page 4 of the "Hardcoat Anodizing System - Type III" manual says "The optimum current requirement is 4.5 amps per square foot, or 30 milliamps per square inch".
    However, Page 6 of the same "Hardcoat Anodizing System - Type III" manual says "Current at 6 amps per square foot for 120 mins".

    No two specifications line up. There is a considerable difference between 4.5 and 24. Frankly, the manual is terrible. At some point in the hard anodizing manual linked above (page 11 onwards) it goes into all kinds of things that are nothing to do with Type III anodizing (eg chrome plating).

    So my questions are:
    1. What is the ACTUAL amps/sqft we should be using? I searched around the forums and did not come up with a clear answer. Many people seemed to just refer to the 720 rule, which brings me to my next question...
    2. Is the 720 rule relevant/valid for Type III? We need the hardness, these are a functional part susceptible to wear.
    3. How necessary is agitation?
    4. What methods of agitation are people mostly using? Just pumping compressed air through the tank or mechanical agitation? Looking for something cheap and easy for our 5gal tank.

    Thanks.
    Last edited by Steve M; 10-05-2021, 09:59 PM.

  • #2
    First I'd like to say that I've never read the manual, nor do I work with anyone using the LCD method. The information below is based on how commercial shops run.

    For the most part, Type III is governed by mil-spec 8625. It basically says what tests the coating must pass (coating weight, Rockwell hardware, abrasion, etc.) to be compliant with the spec. It says nothing about the process itself nor how it is run. That leave the door open to running many different ways.

    1. For Type III the norm is 24ASF at 28-32F although I've seen Type III done as low as 18ASF and as high as 36ASF. I've seen some shops run at 18ASF at 40F and call it "commercial hard coat" although whether it meets the requirements of 8625 is doubtful.

    2. IMHO the 720 rule should be the de facto standard when it comes to anodizing either Type II or Type III. It essential says if you run at 12ASF for 1 hour you will get 1 mil coating thickness. From that we can also say that running at 24ASF for an hour will result in 2 mils which is the nominal thickness for Type III coatings. We (my company) monitors this by measuring the number of amp-hours consumed during the run. If you know the amp-hours, the 720 rule can be arranged into the following equation,

    thickness[mil] = Amp-hours/(12*area)

    3. For Type III agitation is very important to prevent burning. Assume you are running 1 SQFT at 24 ASF. You will be running 24 amps, and after about 30 minutes, your voltage should be approaching 40-50 volts or more. Assuming 50 volts X 24Amps = 1,200 Watts, much of which is dissipated in the form of heat localized on the part. Without adequate agitation to pull the heat off the part, it will certainly burn.

    4. Do not use compressed air. Even with multiple oil filters, you will still end up with oil in the bath. Even a single drop of oil in any tank can contaminate the chemistry in all downstream tanks. For large tanks you would be looking for a high volume, low pressure air pump. For small tanks you might try a fish tank pump if you can find one big enough. As for the agitation in the tank, we usually see a manifold made of 1/2" pvc pipe with 3/16" holes drilled every 2-3 inches. The pipe should be placed in the tank directly under the parts hole side down and weighted so it won't float. While agitation is a must, too much is not good either. Agitation raises the overall bath resistance which in turn creates a high voltage demand on your power supply.

    GL
    Kevin
    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


    • #3
      Originally posted by KevinB View Post
      First I'd like to say that I've never read the manual, nor do I work with anyone using the LCD method. The information below is based on how commercial shops run.

      For the most part, Type III is governed by mil-spec 8625. It basically says what tests the coating must pass (coating weight, Rockwell hardware, abrasion, etc.) to be compliant with the spec. It says nothing about the process itself nor how it is run. That leave the door open to running many different ways.

      1. For Type III the norm is 24ASF at 28-32F although I've seen Type III done as low as 18ASF and as high as 36ASF. I've seen some shops run at 18ASF at 40F and call it "commercial hard coat" although whether it meets the requirements of 8625 is doubtful.

      2. IMHO the 720 rule should be the de facto standard when it comes to anodizing either Type II or Type III. It essential says if you run at 12ASF for 1 hour you will get 1 mil coating thickness. From that we can also say that running at 24ASF for an hour will result in 2 mils which is the nominal thickness for Type III coatings. We (my company) monitors this by measuring the number of amp-hours consumed during the run. If you know the amp-hours, the 720 rule can be arranged into the following equation,

      thickness[mil] = Amp-hours/(12*area)

      3. For Type III agitation is very important to prevent burning. Assume you are running 1 SQFT at 24 ASF. You will be running 24 amps, and after about 30 minutes, your voltage should be approaching 40-50 volts or more. Assuming 50 volts X 24Amps = 1,200 Watts, much of which is dissipated in the form of heat localized on the part. Without adequate agitation to pull the heat off the part, it will certainly burn.

      4. Do not use compressed air. Even with multiple oil filters, you will still end up with oil in the bath. Even a single drop of oil in any tank can contaminate the chemistry in all downstream tanks. For large tanks you would be looking for a high volume, low pressure air pump. For small tanks you might try a fish tank pump if you can find one big enough. As for the agitation in the tank, we usually see a manifold made of 1/2" pvc pipe with 3/16" holes drilled every 2-3 inches. The pipe should be placed in the tank directly under the parts hole side down and weighted so it won't float. While agitation is a must, too much is not good either. Agitation raises the overall bath resistance which in turn creates a high voltage demand on your power supply.

      GL
      Kevin

      Thank you very much for the detailed answer Kevin.

      What's the reasoning behind that ASF range? I'm guessing higher ASF burns the parts, lower ASF just takes too long? If time is not a big concern, is there any downside to using say 9ASF?

      Thanks again.
      Steve

      Comment


      • #4
        The longer the parts are in the sulfuric bath and the warmer it is the softer the coating gets. Given that you need to build the Type III coating as quickly as possibly without burning. 24ASF seems to be a happy medium, and it also works easily with the 720 rule insofar as the math goes. I have also read some technical papers published by the smart kids which elude to a correlation between higher anodizing voltages and the smaller coating pore size. For Type III you need small pores.
        Kevin
        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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