COMPARATIVE STUDY OF AN INDUSTRIAL ROTARY BELL SPRAY PAINTING ATOMIZATION PROCESS USING LASER DIFFRACTION AND TIME-SHIFT TECHNIQUES

When it comes to quality and efficiency, droplet diameters have a substantial impact on paint application processes. To determine droplet diameter distributions, Malvern-type laser diffraction and phase Doppler anemometry are common techniques. This has changed since 2014, as SpraySpy-type time-shift instruments became available. In this study, an industrial spray-painting atomization process using a rotary bell atomizer and an automotive clearcoat paint will be individually characterized using laser diffraction and time-shift measurements. Atomizer rotational speed and paint flow rate are varied in the experiments, while the remaining parameters are kept constant. The commonly used measurement setups are adapted and their respective outputs are converted to allow for a direct comparison. Analyzing the volumetric and number distributions of single measurements, it turns out that the laser diffraction device measures far more fine droplets than the time-shift instrument. The measurements of the latter indicate the presence of a considerable volume fraction in droplets larger than 100 μm, which has not been pointed out before. Analyzing the measurements of variations in atomizer rotational speed and paint flow rate, the laser diffraction device shows notably smaller droplet diameters, and its metrics are more strongly influenced by changes in atomizer rotational speed than those of the time-shift instrument, which is more sensitive to changes in paint flow rate with some metrics. Concerning the differences in absolute values, biases influencing the measurements are being discussed. For the time-shift instrument, a procedure to correct the biased measurements is introduced.