Film thickness prediction in halftone screen-printing

Abstract

The paper presents a numerical model of image transfer in screen-printing that is supported by experimental data. The model focuses on a roller squeegee system. It combines thin film hydrodynamic behaviour with roller squeegee deformation and flow through a porous screen. The work within this investigation has, for the first time, enabled an estimate of deposit thickness at different halftone coverage. For low coverage, ink transfer is governed by hydrodynamic behaviour in the nip contact, and deposited film thickness is represented by an ink spread model. For larger open areas, dependent on squeegee load, the film is removed from the top of the screen and therefore the deposit is likely to be controlled by the screen thickness. The work confirms that a roller squeegee system leads to higher nip pumping capacity in comparison with a sliding squeegee of nominally the same shape. The roller system is therefore appropriate when heavy ink deposits are required. The velocity gradients through the film when ink flows through the screen are reduced as the open area increases. The consequent reduction in shear rate leads to a recovery in viscosity within the nip contact.