Q: What would be the best particle size distribution for any powder? And how is powder charging related to the powder?
A: Particle size distribution is critical to successful application of a powder coating. The “best” particle size depends on what you are coating and how thick you intend to coat the item. For example, if you are trying to coat a relatively simple object around 50 microns (2.0 mils), a median particle size diameter of 35 to 40 microns will suffice.
It’s also very important to consider the distribution. It’s critical to avoid a high concentration of fines or small particles. Fines are defined as particles that are less than 10 microns in diameter. In general, it is best to have a less than 7% concentration of fines.
Fines cause a number of problems. They make it difficult to fluidize the powder, which interferes with transport performance through system hoses. Fines also have a tendency to “overcharge,” meaning they attain a high charge-to-mass ratio. This makes film build difficult, especially in tight inside corners (Faraday cages). Fines also have a tendency to clump and absorb moisture due to their high surface area. Are you convinced that fines are bad news in powder?
Large particles (> 90 microns) can also cause problems. These boulders have trouble making it to the substrate because of their high mass. Gravity pulls them to the bottom of the booth, reducing the transfer efficiency. The ones that do make it to the part will cause increased texture.
Speaking of texture, you will want a much coarser particle size if you are applying a textured powder. Depending on the texture desired, the median particle size may be as high as 70-80 microns. A powder with such a large median size won’t have many fines. You have to pay attention to the large particle size fraction, however. Really big particles (> 200 microns) may cause undue and uneven texture.