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So the powder is bonded...Who cares right? Wrong!

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  • So the powder is bonded...Who cares right? Wrong!

    A little Q & A from the new Powder Coating Magazine:

    Solving a problem with metallic powder coating....

    Question: We're having an issue with a metallic powder. After cure, random spots, devoid of pigment, and small crater like spots have been causing rejection of parts. When this first occurred, new powder was used in carefully cleaned equipment, but the problem still persisted. Our pretreatment process checked out. This problem is only occurring with this particular powder. What are your thoughts?
    C.M., Worcester, Mass.

    Answer: I'll tell you right up front that I'm not a chemist, and I don't have any experience with the problem. However, I have a friend who is a chemist in powder coatings. So, I asked him for some assistance. He responded as follows, and I'll take license with his answer and pass it on to you. "It would seem that the metallic powder is not bonded, or not properly bonded. In this case, bonding means that the powder coating particles are impact-fused to the metallic particles, forming a homogeneous powder-metal agglomerate of somewhat larger size than the average powder particles. Non-bonded metallic powders can stratify in a fluidizing hopper. The specific gravity of a transparent powder is usually in the 1.2 to 1.3 range, whereas aluminum flake has a specific gravity of 2.7. You can see that stratification would occur in the fluidizing air. This is why non-bonded metallic powders can cause a splotchy appearance. Non-bonded powders should not be reclaimed because the reclamation separates the metallics even more, so it becomes an ever-decreasing spiral to poorer results. An addition fluidization ring in the bottom of the hopper or some added agitation can usually improve the appearance of a non-bonded powder. Bonded metallic powder will fluidize more uniformly because of encapsulation, and the metallic particles will be more uniform and can be reclaimed. As for the crater, I don't know. It may be a surface-tension issue due to the non-uniformity of the two powder conditions. So there. hope that helps. I understand it, but it's too deep for me to sleep on!

    Now if you have blotches with metallics you know why......Dan (wise) this one is for you bud! (Start buying bonded)

  • #2
    Re: So the powder is bonded...Who cares right? Wrong!

    Originally posted by Harleydad
    I just read that yesterday. I always try to use bonded metallics, all though alot of the time it's dependent on the powder a customer chooses to have their parts coated with. I also spray metallics at a lower kv, you'll get more of the metallic effect if you can get the flake to lay flat.
    The king just posted info on spraying metallics not too long ago, but here's some more: ... wders.html

    There's also a good article in this issue titled "Job Shops: Juggling customers, competitors, compliance, and change".
    Thanks Harleydad for that other link. Yes, the other article was pretty good!