Ink Stability During Printing

Abstract

It was found that particle sizes of ink pigments change after several hours of printing in such a way, that it affects printability properties. This was demonstrated on several gravure and flexo printing trials lasting for several hours. Both rotogravure toluene-based process inks and flexo water-based inks exhibited changes in pigment particle size. Decrease of rotogravure inks particle size was most obvious in black (from 482nm to 231nm), while the decrease of particle size in all other colors was rather similar: from 257 to 173 (yellow), from 270 to 193nm (magenta) and from 282 to 207nm (cyan). Differences in color, print density, and specular gloss measured on printed substrates at the beginning of trial and after seven hours of printing were found. Color changes were calculated from L*a*b * color coordinates as ∆E values. ∆E was higher than 3.0 for each process color rotogravure ink. Such color difference can be observed by human eye. Further measurements indicate that specular gloss increases with the decrease of the particle size. On the other hand, density mottle decreased with diminished particle size. There was no dot gain change observed. Particle size of Pantone Brown flexo water based spot color decreased from 250nm to 178nm after 270 minutes of printing, producing a change in ∆E value of 3.2.