Q. We are in the process of adding a 2,000-L paint dip tank to our paint finishing operation for waterborne paint. The paint will have a solids-by-volume content of 43 percent. We plan to circulate the paint using two air-driven diaphragm pumps (moving about 3 L/sec). What is the best circulation pattern for this paint to obtain optimum consistency and to minimize or prevent settlement? Should we pump the paint in at the top and out the bottom, or vice versa? S.G.
A. Many pigments in paint formulations are heavier than their vehicles, and this causes them to settle to the bottom of paint cans, drums and dipping tanks. Paint in cans is mixed by hand using a stirrer in a swirling motion, moving the stirrer upward with each swirl. With this in mind, I would agitate the tank by pumping into its side or sides close to the bottom, thereby moving the solids upward and having the paint flow out of the top. This action is the best way to obtain optimum consistency and prevent settling.
It is important that you monitor the paint temperature and adjust your pumping speed accordingly or add cooling capacity if necessary. As in any painting operation, a temperature increase will lower the paint’s viscosity, thereby reducing the film thickness of an applied paint.