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

Controlling the transfer of ink to the substrate is a key requirement of the flexographic printing process. Its ability to transfer ink from the image carrier to the substrate at low pressures enables the process to be used for the production of printed matter using pressure-sensitive materials. 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 action associated with other volume print processes. This paper evaluates the effect of pressure changes on 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 anilox roll specifications, dot pitch, and plate-to-substrate engagement on the reproduction of both a continuous 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 influence on solid density (a parameter used as an indirect measure of ink film thickness) and halftone dot formation; however, the geometrical characteristics of the cells were also shown to have an effect. An initial increase in the pressure within the printing nip resulted in a significant rise in both solid density and tone gain (growth of the halftone dots) due to improved ink transfer from the plate to the substrate. Subsequent increases in pressure produced little further increase of solid density, indicating ink transfer had reached a plateau. The rate of increase of halftone density was found to be reduced as pressure increased, which was attributed to the ink approaching its maximum capability for spreading on the substrate.