Abstract
In order to study the powder coating process of metal substrates, a comprehensive, numerical 3D Eulerian-LaGrangian model, featuring two particle sub-models, has been developed. The model considers the effects of electro-static, fluid-dynamic and gravity forces. The code has been implemented in C++ within the open source CFD platform OpenFoam®, is transient in nature with respect to the applied LaGrangian particle implementation and the electro-static field calculation and is stationary regarding fluid-dynamic phenomena. Qualitative validation of the developed solver has already been achieved by comparison to simple coating experiments and will hereby be presented alongside a thorough description of the model itself. Upon combining knowledge of the relevant dimensionless groups and the numerical model, a dimensionless chart, representing all possible states of coating, was populated with comprehensive, exemplary cases, which are shown here as well.
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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