Effect of impression pressure and anilox specification on solid and halftone density

Abstract

Controlling the transfer of ink to the substrate is a key requirement of the flexographicprinting process. Its ability to transfer ink from the image carrier to the substrate at low pressuresenables the process to be used for the production of printed matter using pressure-sensitivematerials. These can range from substrates where high pressures can damage surface structure(e.g. corrugated board), to specialist inks, which can be damaged by the high shearing actionassociated with other volume print processes. This paper evaluates the effect of pressure changeson print quality for different anilox specifications and line rulings on the plate.Data collected from an experimental print trial were used to quantify the effects of aniloxroll specifications, dot pitch, and plate-to-substrate engagement on the reproduction of both acontinuous ink film and the formation of discrete halftone dots.The ink-carrying volume of the cells of the anilox roll was shown to have the greatest influenceon solid density (a parameter used as an indirect measure of ink film thickness) and halftonedot formation; however, the geometrical characteristics of the cells were also shown to havean effect. An initial increase in the pressure within the printing nip resulted in a significantrise in both solid density and tone gain (growth of the halftone dots) due to improved inktransfer from the plate to the substrate. Subsequent increases in pressure produced little furtherincrease of solid density, indicating ink transfer had reached a plateau. The rate of increase ofhalftone density was found to be reduced as pressure increased, which was attributed to the inkapproaching its maximum capability for spreading on the substrate