Comparison of electrostatic fine powder coating and coarse powder coating by numerical simulations

Powder coating has several key advantages over liquid coating and fine powder coating makes the surface finish quality comparable with liquid coating. This work reports on the numerical simulation of fine powder coating process in comparison with coarse powder coating, using a commercial computational fluid dynamic code, FLUENT v6.1. The purpose of the study is to understand the gas and particle flow fields inside the coating booth for various operating conditions and the effect of reducing particle size on the coating process. The air and powder particle flows in a coating booth were modeled as a three-dimensional turbulent continuous gas flow with solid particles as a discrete phase. The continuous gas flow was calculated by solving Navier-Stokes equations including the standard k-ε turbulence model with non-equilibrium wall function and the discrete phase is modeled based on the Lagrangian approach. In addition to drag force and gravity, the electrostatic force including the effect of space charge due to the free ions was considered in the equation of motion and implemented using user defined scalars and user defined functions. The governing equations were solved using second order upwind scheme. This study demonstrates that the use of finer particles of size 15 μm or lower can give very smooth and uniform surface finish, which may serve the requirement of automotive top clear coat. This also provides useful information about optimum operating conditions such as the airflow rate, the applied external voltage and the powder-spray rate. The numerical model can also be used to optimize the gun-booth design for a better coating efficiency.