Numerical investigation on viscous droplet generation for selective coating processes

Numerical studies on the generation of viscous droplets based on a Drop-on-Demand device have been performed. Interface methods VOF (volume-of-fluid) in a CFD-code were applied to predict the paint droplet formation. A dynamic mesh model was used to simulate the valve tappet movement. Liquid filling process in the nozzle and liquid pinch-off outside the nozzle were investigated in detail. To account for the effects of non-Newtonian behaviors of paint liquids, both shear thinning and elongation viscosity models were applied for the calculation of liquid thread and droplet formation. Validations of used models as well as simulation results were carried out using experimental measurements. Subsequently, a parametric study was performed, to give a better understanding of the individual parameters on the droplet formation. Dimensionless numbers, such as Weber-Number and Ohnesorge-Number were used to describe a reasonable range for a droplet generation and process optimization.