I would wager that most career powder coaters are not physicists. I certainly am not.
And yet, success in our occupation relies upon our ability to understand – and harness – certain electrostatic phenomena, including the all-important concept of grounding. Failure to do so can cause major problems for our coatings.
I’ll talk more about grounding best practices in a future blog, but I want to start with this: three signs that you’re grounding your parts improperly, from one non-physicist to another.
- Poor transfer efficiency
Imagine you’re spraying ten pounds of powder. If, when you’re done spraying, the part is fully coated and ten pounds heavier, you’ve achieved 100% transfer efficiency. This never happens in the real world, of course – a more realistic scenario would involve powder falling off of corners and welds, or struggling to penetrate recesses, all of which are signs of less-than-perfect grounding. A little bit of powder on the floor is unavoidable. Too much, and it’s time to revisit your grounding.
- Not enough film thickness
Each pass of the spray gun adds a layer of powder to the part, and each layer of powder acts as an insulator. Eventually, grounded parts become so insulated that they can no longer accept powder. This is totally normal. You’ve simply reached maximum film thickness for that coat. Poorly-grounded parts, however, will stop accepting powder much sooner than you’d like them to. If, after a single pass, your transfer efficiency declines dramatically, it’s unlikely that you’ll be able to achieve the film thickness you need. The powder simply won’t cling to the part. It’s time to check your grounding.
- Surface defects
Grounding-related surface defects tend to come in two forms: orange peel, and back ionization.
Unless it’s intentionally textured, powder coating is meant to be smooth. Peaks, valleys, and craters are a sign that powder is amassing in some areas, but not others. Poorly-grounded parts are particularly prone to this: if the charge is unevenly distributed across the part’s surface, the powder will follow suit, leaving behind orange peel after curing.
If the powder appears to be dancing over the part’s surface as you’re spraying, it means there’s too much charge on the part. This phenomenon is known as back ionization, or starbursting. The powder’s movement will leave behind some strange patterns after curing. A well-grounded part gives the charge somewhere to go (namely, into the ground), preventing back ionization altogether.
Thankfully, proper grounding technique is not hard to achieve – there’s no reason to suffer through any of these problems for long. Stay tuned for more on this topic.