A method for evaluating ink mileage in gravure printing

Abstract

Researchers have studied ink mileage curves for many years, and they have reported several models for curve fitting. Regression coefficients derived from curve fitting have been found to be very useful for comparing different inks and have been related to various properties of ink and paper. However, these models were based on experimental data from prints made on IGT and Prüfbau printability testers using offset inks. The quantity of ink transferred, and hence the amount of ink on paper, was determined by the weight difference of the printing disk before and after printing. These models have not been tested on other ink types or on commercial printing presses. The challenge has been that it is difficult to accurately measure the coat weight of gravure or flexographic ink film. In this study, eight coated rotogravure papers were printed on a Cerutti rotogravure web press. The ink film coat weights at different tonal values were calculated from the copper (Cu) concentrations in both liquid ink and printed paper samples, which we analyzed using an atomic absorption (AA) spectrometer after digestion. The ink film coat weight and relative reflection density data were fitted by different models using OriginPro® 7.5 software. The degree of fit was determined by the sum of the squares of residuals and the distribution of residuals around zero. Both the Oittinen and Calabro-Savagnone models fitted the experimental data equally well. The regression coefficients derived from curve fitting could be related to paper properties such as surface roughness, air permeability, and pore size. We found that the saturation density Ds had higher correlations with air permeability and with pore size than with surface roughness. The parameter m, which determines the steepness of the density curve in the region of very thin film (also called density smoothness), was highly correlated with paper roughness as measured by the profilometer and at a less significant level with air permeability. The parameter n was well correlated with both profilometer roughness and air permeability.