Q: We coat wire shelving and baskets using an epoxy/polyester hybrid powder. As the product line is 95% welded wire assemblies, is there a particle size that would lend itself to consistency of film thickness on the wire and flat surfaces?
We currently experience high film build on the wire surfaces to assure specified film requirements on the flat substrate. While I know this is a phenomenon of electrostatics, will a particular particle size help?
A: Particle size is critical for your end use. In fact, everything associated with your powder application system is critical. Let’s tackle the particle size issue first.
You should specify a particle size distribution that is somewhat narrow, avoiding a high concentration of fines and coarse particles. On the low end, you want to have no more than 7% of your particles less than 10 microns. On the coarse side, you want nothing over 90 microns. This will give you a median particle size of roughly 35-40 microns.
Keep in mind that specifying particle size is tricky. Powder coating particle size fluctuates within a range during its production. Singular data points for a large batch of powder coating can be misleading. It is best to require statistical information for the batch, not just an average on these three critical parameters.
Other application parameters are equally important. Next on the list is ensuring that you have an adequate ground. You should have no more than 0.5 megohms resistance from the part to the ground, which means you need to keep all connections (hooks, swivels, etc.) and contact points clean. Buildup not only hurts powder penetration, it can also create a dangerous buildup of current that will eventually arc. Arcing can ignite a powder coating cloud, with dire consequences.
Powder conveyance parts need to be clean and dry. This includes hoses, internal gun parts, powder venturies, and pick-up tubes. Regular cleaning will ensure consistent flow of powder from the fluidizing hopper to the gun tip. Speaking of gun tips, these need to be cleaned regularly to avoid excessive buildup and subsequent impairment of corona ionization.
Fluidizing hoppers need to be kept clean, dry, and properly maintained. The porous plate on the bottom of the hopper should be cleaned regularly with compressed air that is dry and oil free. A combination of a refrigerated air dryer and air/oil separation filters must be in place and be regularly maintained. It’s also very wise to keep meticulous logs of data for the powder you use and the application system performance. This will help you keep track of what works and provide useful data when troubleshooting.