Links between fluid rheology and drop-on-demand jetting and printability
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NIP24 and Digital Fabrication 2008, page 130--133. The Society for Imaging Science and Technology and The Imaging Society of Japan, (2008)

The amplitude of the actuation stimulus required for a certain drop velocity (say 6 m/s) provides a simple measure of the ability of a system (printhead and fluid) to achieve good jetting (\~ 6m/s). This paper links measurements of drop size and associated ligament length with the viscoelastic rheological properties of model fluids which are solutions of a (linear) polymer of various molecular weights. Jet formation and evolution for solutions of polystyrene in a solvent of diethyl phthalate and dioctyl phthalate was studied by using 20 ns spark flash imaging and off-line analysis techniques. Images recorded under nominally identical conditions were analysed to determine main drop size, ligament length, satellite size and spacing. Viscosities measured under standard, low shear-rate laboratory conditions were found to show no correlation with the level of actuation required for these fluids. The size of the main drop, ligament length and jetting speed were however, found to be well correlated with other rheological properties. Qualitative comparison was made by using a filament-stretching rheometry technique and quantitative analysis was carried out by measuring the high frequency response (> 5 kHz) with a piezoelectric axial vibrator (PAV) rheometer. The paper demonstrates that details of jet and droplet formation for weakly viscoelastic fluids during inkjet printing can be linked with appropriate measurements of the fluid rheology. These techniques would provide useful predictive tools in the design of inkjet fluids, and to differentiate between identical inks that might have similar bulk properties but show different jetting behaviour during printing. This may assist in the investigation of cases where batch variations result in different jetting behaviour and issues with printability.
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