Abstract
As a prerequisite for studying and ultimately improving the powder coating process, particle motion and deposition effects within flow- and electro-static fields need to be thoroughly understood and thus characterized. In this context, a range of dimensionless groups is proposed and new means of characterization are presented. Considering the impact of electro-static, fluid-dynamic and gravity forces on coating particle motion, a triangle chart notation to characterize the state of varying particle size classes, is introduced. Furthermore a derivation of the dimensionless particle momentum equation is shown to lead to a dimensionless chart, representing all possible process states of coating. In combination with a Eulerian-LaGrangian, numerical model, the new means of characterization have led to a far better, over all perspective of occurring phenomena and their causes. Some examples are demonstrated here.
Highlights
The quality of electro-static powder coating processes of metal substrates is related to the uniformity of coating layer thickness
In order to create the basis for such improvement efforts, a thorough understanding of particle motion and deposition effects within flow- and electro-static fields is required
Close to the negatively charged electrode, oxygen ionization processes cause the presence of spatial charges of free electrons e, positively charged oxygen ions O+2 and negatively charged oxygen ions O-2 [5]
Summary
A simple evaluation of these dependencies, for an exemplary case (Figure 2, bottom) shows that for small particles (e.g.: case Figure 2, Dp < 20μm) fluid drag forces dominate; that for medium sized particles (e.g.: case Figure 2, 20μm < Dp < 230μm) electric forces dominate; and that large particle motion (e.g.: case Figure 2, Dp > 230μm) is dominated by gravity These initial insights do correspond with findings from practical applications, which report highest coating efficiencies for medium sized particles. By defining the three dimensionless groups as dimensions, the chart can be used to visually characterize the states of particle flow paths for any set of process parameters and any combination of particle properties. Intermediate particle sizes can either be placed within the electro-statically dominated region, or at least have a tendency towards stronger electro-static impact
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