Fig. 1. Working scheme of a roller coater.Figure optionsDownload full-size imageDownload as PowerPoint slide
In rotary printing processes, a very frequently arising printing artifact is the so-called “ribbing” . Ribbing describes the occurrence of ribs in between the gap of two moving rolls as it BYK 191023 can be seen in Fig. 2 (left). These ribs are transferred to the substrate and lead to an inhomogeneous surface topography of the deposited and dried layer as shown in Fig. 2 (right).
Fig. 2. Development of ribs in between the gap of moving rolls (left); printed ribs on a substrate (right).Adapted from .Figure optionsDownload full-size imageDownload as PowerPoint slide
The appearance of this undesired printing artifact is caused by capillary effects between the two rolls and can be evaluated by the dimensionless capillary number Ca (see Eq. (1)) .equation(1)Ca=μU¯σ
The value of the capillary number Ca is dependent on the viscosity μ and on the surface tension σ of the ink as well as on the applied printing speed U¯. The calculation of the capillary number Ca is strictly spoken only suitable for fluids which provide Newtonian flow behavior. Ca is a machine parameter and the calculated absolute values cannot be used for the adaption of an ink to an arbitrary rotary printer. Even if important parameters as the gap distance between the metering roll and the applicator roll or the surface energy of both rolls are not considered, Ca is a useful tool to develop suitable inks for rotary printing and roller coating. The complex flow behavior during the coating process was studied in the past by flow simulation experiments .