There are high demands on flexographic print quality to be sufficiently high and consistent in order to create a competitive packaging. At the same time the production efficiency need to be high. Printers thus need to achieve the same quality every time and quickly start-up new printing jobs. To accomplish this, one needs to gain a thorough understanding of how the liquid packaging board interacts with the ink and impacts the print quality.

This thesis focuses on water-based ink absorption of liquid packaging boards and particularly on a) how uniformity of ink absorption can be measured and b) to what extent the absorption characteristics contribute to print mottle in flexographic printing.

The work encompasses two parts. First, an absorption non-uniformity test method has been developed using a staining technique. This method is unique as it measures how unevenly an aqueous solution is absorbed, in a short time period and without impact from surface roughness. Moreover, the contributions from white-top mottle and absorption non-uniformity can be quantified simultaneously from one single measurement.

Second, a method to independently study the effects of absorption non-uniformity on print quality has been established. This is achieved by introducing artificial absorption non-uniformities with well-controlled barrier patterns. A barrier pattern may modify local pore structure and/or surface energy, hence lead to lateral absorption variations. By these means, it is possible to produce a substrate property-matrix; encompassing absorption non-uniformity and for example surface roughness. It was demonstrated that non-uniform absorption indeed has a negative impact on print quality, both on smoother and rougher boards. Low absorption made print density decrease and uneven absorption caused print mottle. This was the case when other properties of the samples were kept within a narrow range; otherwise surface roughness appeared to determine print mottle ranking.

The appearance of a print is affected by the individual ink layers. If the ink is unevenly distributed on the substrate it lowers the quality. This thesis puts focus on how the liquid absorbency of a coated substrate impacts on the ink distribution in flexographic printing. It is well known that a smooth surface increases the chances of a uniform print, whereas the influence from an uneven absorption is not established and has even been difficult to measure. If the ink is applied directly onto the substrate, or as an overprint onto already present ink layers, the outcome is even more complex. Ink trapping behaviour affects the uniformity of overprint layers. As of yet, this been largely overlooked in flexography.

The work includes several trials, from monochrome laboratory printing at 0.5 ms^-1 to multicolour printing at 10 ms^-1 in production-scale. These studies showed that ink absorption interacted directly with monochrome ink layers and that pore-structures with larger pores and greater liquid uptake generated more uniform prints. The tolerance of uneven pore-structure, and thereby absorption, varied between samples.

In multicolour printing, the overprint layer interacted directly with the preceding ink and indirectly with the absorbency (rate and uniformity) of the substrate. Overprint layers became thicker when the first ink layer was thinner and, consequently, turned uneven when the first layer was uneven. Moreover, the time between the applications of the two inks was important. When immobilisation of the first ink was too slow or uneven, it disturbed the ink trapping so that the overprint layer became uneven.

Output from this project offers a palette of tools to use when studying liquid absorption and its impact on print quality: a) experimental approach to separate the influence of uneven absorption from surface roughness, b) aqueous staining technique to characterise absorption non-uniformity, and c) technique to characterise ink trapping non-uniformity.

The appearance of a print is affected by the individual ink layers, and when the ink is unevenly distributed it will bring negative consequences. As an extension, print quality can affect how packaging is perceived and may even create ideas about the product inside the packaging. Therefore, it is important to understand the reasons behind poor and uneven print and to ensure that the board substrate provides a stable quality.

This thesis puts focus on how the absorbency of a coated substrate impacts the ink distribution in flexographic printing. Both monochrome and multicoloured areas are dealt with. To generate an understanding of these interactions, two new measurement techniques were developed where the non-uniformities in liquid absorption and ink trapping can be characterised. Surface roughness often has a strong influence on the print and this may hide interactions from ink absorbency. Therefore, the materials used here were carefully selected (or designed) to highlight the way liquid uptake affects the ink layers. Interactions between substrate and printing ink were studied in several ways, from monochrome printing at 0.5 ms^-1 in laboratory-scale to multicolour printing at 10 ms^-1 in production-scale.