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Fish eyes in powder coating operation

adv. pointer  Tiger Drylac

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Q. I've been having trouble with many of my coatings, I keep getting fish eyes and I can't figure out why. I switched to an element style water filter, its basically a roll of brown paper just like a roll of toilet paper to absorb water, that is attached directly to my powder gun air connection. that seemed to have helped but now I have them again. Should I be using a special air hose? I'm not sure if the hose I am using contains any silicone and if it did I don't know if that would effect my coating in this use.

I am a small shop and my process is, washing as necessary, sandblasting, most of the time I will pre-bake the parts, then I treat them with Carpenter Chemicals Ecophor A447 Plaforization. After media blasting parts are blown off with compressed air and never handle with soiled hands. I use a Tribo gun for a lot of my coatings because I find that I can get powder into tight areas a lot easier than with my Corona gun. (Mitsuba 700 series) I powdered an engine bracket for a customer a month ago that was done in Tiger Drylac Bengal Blue, it was a badly soiled part when I got it, it was first wiped clean, washed with Dawn and warm water, media blasted, baked for an extensive period at 420F then coated. It came out of the oven with fish eyes everywhere, in the center of many of them was a yellow speck (the previous color I had sprayed) I also disassemble my gun and dipstick and clean thoroughly between color changes.

Any ideas would be greatly appreciated.

Drake Spectre
- Antioch, Illinois, USA


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A. That little thing you're using as an air dryer might only work until it gets saturated, then it will start letting loose whatever is trapped in it. Change it very often or put a real air dryer in your air line.
Are you capturing and reusing the plaforization? If you are you might need to be aware that it traps oils and might be depositing them back on your substrate. It's supposed to convert them but you never know.
Fish eyes can be hard to diagnose.

Sheldon Taylor
   supply chain electronics
Wake Forest, North Carolina

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A. Have you tried an alkaline degreaser? I find that old castings that have been oily need to be cleaned, baked out and cleaned again to remove the oils that come out of the pores when the part is heated.


Larry Johnson
- Key West, Florida


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A. Drake,

Fisheyes can be very difficult. Just for kicks, I would look at that "blow off with compressed air" step. Put a clean white cloth over the outlet and let her blow for a couple of minutes. You may be introducing oils, water or the crap off the inside of you airlines.
We once experienced a bad run of fisheyes and it finally turned out a racker had switched hand lotions. Hard to find.
Good Luck

Trent Kaufman
electroplater - Galva, Illinois


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Q. Dear All,

We do powder coating for aluminium profiles which is used in buildings. We have 9 tanks non chrome pre-treatment, We have some repeated problem of crater (fish eye) appearing on the powder coated surface. If anyone knows what could be the reason for this is welcome.

regards,

A.Srinivas
Supervisor - Mysore, Karnataka, India


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A. Although it might be impossible to tell you what is causing your fisheye problem, I can tell you what causes fisheyes. Oil in the airlines, spray lubricants that contain silicon, make sure there isn't and oil slick on top of the water in your final(or any)rinse tank ... other things of that nature. Check your air dryer, air lines, in line filters, watch what people are doing as they come and go. I had a fisheye problem once and it was traced to a maintenance person that went up on the roof and sprayed oil onto the cooler bearings, the coolers then blew the overspray all down into the shop. Another time we had a problem that turned out to be machine operators that were spraying oil onto their brake tooling to prevent rust even though spray lubes had been out-lawed in the shop. It's easier to spray than to rub oil on with a rag, who cares about quality, right?

Sheldon Taylor
   supply chain electronics
Wake Forest, North Carolina


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A. I agree with Mr. Taylor

Firstly, Check Your Air line thoroughly, Change it completely if required. Also change the air line in your spray equipment completely service them.

Also find if all the powders give this fish eye problem, if it doesn't, change the particular powder supplier, the powder itself may contain moisture and impurities.

Shankar.O.Khandelwal
- Coimbatore, Tamilnadu, India



June 8, 2013

Q. Hello friends,

This is Suresh from India, AP, Tirupati. I am working as a powder coating In-charge. We are facing issue at after powder coating the components are coming with craters/fish eyes effect.

1. components are MS parts.
2. 9 stage pretreatment process (Zinc phosphating).
3. Electrostatic spray powder coating.
4. Diesel fired oven - 200 degrees centigrade.
5. Glossy finish powders only we are facing this issue.
6. This type of issue at only some portion of the component.

So, please suggest how to eliminate this craters issue.

Suresh Peddineni
- Tirupati, Andhra Pradesh, India
  ^- Privately contact this inquirer -^


July 5, 2013

A. We had the same problem - I believe it was due to oil remaining on the parts, not allowing the paint to adhere.
This may have been worsened by a bad reaction between a phosphate based cleaner and a phosphate-free conversion coating, but I have not seen this problem for nearly 2 weeks now.
I recommend checking your soap for oily globs floating on top or around in it, as well as cleaning your rinse tanks & washing the walls down with dawn dish soap prior to refilling.

Fauna Tester
- Seattle, Washington, USA

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