Ceramic Ink-Jet Printing for Digital Decoration: Physical Constraints for Ink Design

In the last decade, the drop-on-demand (DOD) ink-jet printing has become the leading technology for the decoration of ceramic tiles. The inks employed for such application are colloidal suspensions of oxide particles (0.3 mu m) whose stability (against agglomeration and over time) is fundamental to get successfully the tile decoration. Jettability from the DOD print heads is a key requirement along with proper colour after sintering. This means that a careful set up of inks properties (viscosity, density, surface tension) is required. The phenomena involved in the different stages of the ink-jet printing process (drop ejection from the nozzles, impact and spreading on the substrates) are described through dimensionless numbers as Reynolds, Weber, and Bond numbers, or their combinations. In literature physical constraints, obtained with a semi-theoretical approach on the basis of experimental evidences on other systems, allow to define a reference region in the space of the dimensionless numbers (here called "printable fluid region," PER) where the inks should be suitable for the application. In this paper, 26 inks currently used for ceramic tile decoration were characterized and mapped in the space of dimensionless numbers for several printing conditions. For typical nozzle diameters (20-50 mu m) and drop velocities (6-8 m/s), it has been found that they fall in the region identified by 3 < Re < 30, 27 < We < 160, and 0.6 < 1/Oh <2.5, where Re, We, and Oh, are the Reynolds, Weber and Ohnesorge numbers, respectively. Such experimental region can be taken as reference to tune the colloidal interactions in proper way, though the thresholds delimiting the PFR should be better defined.