Proper Sieve Size for Powder Coating's Smooth Results

Q: Which size sieve is normal for production of high glossy and smooth powder coating?

A: High glossy and smooth powder coatings require a narrow particle size distribution. The top end “scalp” is critical to ensure the desired smoothness. Good milling conditions are also important to make sure there is minimal waste, so it is wise to manage the grinding process first. The rotor speed and classifier wheel settings need to be optimized to minimize both the fines (<10 microns) and coarse fractions (>100 microns).

As for sieve size, I would start with a 125-micron screen, then check particle size distribution and, most importantly, film smoothness. If this does not produce the desired smoothness, then move to a 100-micron screen. This should yield the desired smoothness; however, you may have a large quantity of “overs” (>100 microns). This fraction can be reintroduced to the mill to manage the yield. If the 100-micron screen doesn’t produce the desired smoothness, then you must work on reducing the viscosity of the resin system. In addition, you may also have to slow down the cure rate of the resin/crosslinker.