Links Between Ink Rheology, Drop-on-Demand Jet Formation, and Printability

This article links measurements of ink jetting performance in drop-on-demand printing with the high-frequency rheological properties of model viscoelastic fluids containing linear polymers with various molecular weights. Jet formation and evolution were studied for solutions of polystyrene in diethyl phthalate. Ligament length, initial jet ejection speeds, and ligament extension and retraction rates were determined by high-resolution imaging with high time resolution. For these fluids, the viscosity measured under low shear-rate conditions showed no correlation with their jetting performance. The jetting behavior was, however, well correlated with high frequency rheological properties measured at 5 kHz using a piezoelectric axial vibrator rheometer. This study shows that high frequency rheometry can provide useful predictive data about the jettability of fluids, and differentiate between inks that have similar low shear-rate viscosity yet show different jetting behavior. A phenomenological model has been proposed and fitted to the evolution of the average ligament length from emergence, through break-up and into the final state of unmerged drops and associated satellites in order to help discuss the influence of viscoelastic behavior on the fixed speed drop-on-demand jetting and printability of fluids. The values of the parameters of this model obtained from the fitting are shown to have a consistent correlation with the rheological properties of the jetted fluids.