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  • 1.
    Thorman, Sofia
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Chemical Sciences.
    Absorption non-uniformity characterisation and its impact on flexographic ink distribution of coated packaging boards2015Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    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.

  • 2.
    Thorman, Sofia
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Chemical Sciences (from 2013).
    Where did the ink go?: The effect of liquid absorption on ink distribution in flexography2018Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    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.

  • 3.
    Thorman, Sofia
    et al.
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Chemical Sciences (from 2013). RISE Bioeconomy.
    Ström, Göran
    RISE Bioeconomy.
    Gane, Patrick. A. C.
    OmyaInternationalAG, Switzerland; Aalto University, Finland.
    Impact of non-uniform water absorption on water-interference print mottle in offset printing2018In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 33, no 1, p. 150-163Article in journal (Refereed)
    Abstract [en]

    Print mottle is a serious and yet common print defect in offset printing. An imbalance between the feed of fountain solution and the ability of the paper substrate to absorb and transport this water away from the surface can cause moisture/water interference problems. In the study presented here, we have investigated the uniformity of aqueous absorption and coating structure of pilot-coated papers with different types and dosages of dispersants and linked this to print mottle and uncovered areas (UCA). In earlier studies, the print quality of these papers indicated that a moderate addition of excess dispersant caused ink refusal, ink-lift-off (ink-surface adhesion failure) and water-interference mottle when printing at elevated fountain feed. In the present study, we have shown that a majority of the samples with uneven water/moisture absorption and an uneven burn-out reflectance tended to have more severe printing problems related to surface-moisture/water.An aqueous staining technique was used to characterise the absorption non-uniformities. This method has been developed previously with focus on absorption of flexographic water-based inks but can clearly give relevant information also for offset printing, when it comes to moisture/water interference mottle.

  • 4.
    Thorman, Sofia
    et al.
    Innventia AB, Stockholm.
    Ström, Göran
    Innventia AB, Stockholm.
    Hagberg, Anni
    Innventia AB, Stockholm.
    Johansson, Per-Åke
    Innventia AB, Stockholm.
    Uniformity of liquid absorption by coatings: Technique and impact of coating composition2012In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 27, no 2, p. 456-465Article in journal (Refereed)
    Abstract [en]

    The interaction between a liquid and a paper surface is important for a number of paper treatment processes, where absorption is of special significance during printing. Many absorption measurement techniques use a large available volume of liquid to characterise absorption, when compared to the volume of the coating. The uniformity of the absorption is also seldom characterised. We have developed a new technique, which is presented in this article, to study the uniformity of absorption of a small amount of liquid. This technique is based on the short-time absorption (tenth of a second) of a coloured liquid, the blotting of excess liquid and a characterisation of the pattern of the stain. This method made it possible to detect differences among coating layers with different compositions. In many cases, the absorption non-uniformity could be linked to variations in the coating thickness and/or wettability. The thinner and thicker areas of the coating layers were interpreted as having different pore structures. Neither the coating thickness nor the wettability could provide a full explanation, which showed the need to develop a method to characterise absorption uniformity instead of only relying on measuring the total absorption potential.

  • 5.
    Thorman, Sofia
    et al.
    Innventia AB, Stockholm.
    Yang, Li
    Innventia AB, Stockholm .
    Hagberg, Anni
    Innventia AB, Stockholm.
    Simultaneous determination of absorption mottle and white-top mottle in the same area on coated boards2013In: Proceedings of the 40th International Research Conference of iarigai / [ed] Enlund, N. & Lovreček, M., Darmstadt, Germany: International Association of Research Organizations for the Information, Media and Graphic Arts Industrie (IARIGAI), 2013, p. 225-232Conference paper (Other academic)
    Abstract [en]

    The proposed measurement technique offers a number of advantages compared to other absorption methods. It enables one to study liquid absorption in a time period that is relevant to printing. Moreover, with this technique one can quantify the contributions of white-top mottle and absorption mottle at the same location and in great detail. This makes this technique a useful tool for quality control of paper board products and for predicting print mottle of flexographic printing. The reproducibility and reliability of the technique were very promising, as suggested by the Round Robin tests.

  • 6.
    Thorman, Sofia
    et al.
    Innventia AB, Stockholm.
    Yang, Li
    Innventia AB, Stockholm.
    Hagberg, Anni
    Innventia AB, Stockholm.
    Ström, Göran
    Innventia AB, Stockholm.
    Studying the impact of non-uniform absorption on print mottle of coated boards with barrier patternsManuscript (preprint) (Other academic)
    Abstract [en]

    Absorption and surface roughness of a substrate are believed to impact flexographic print mottle. However, their respective contributions are not that well recognized as they always co-exist. In addition to average absorption rate, absorption uniformity is of higher importance and even more difficult to separate from surface roughness. Therefore we propose a method to independently study the effects of absorption non-uniformity on print quality. This is achieved by introducing artificial absorption non-uniformity with well-controlled barrier patterns. The barrier patterns are added onto coated boards by means of flexographic printing with appropriate chemicals. These patterns modify not only surface chemistry but also pore structure by closing the surface pores.

    This article provides proof of the concept, which has successfully been applied to seven coated liquid boards. For the first time, this technique enables one to create a property-matrix, absorption non-uniformity vs. e.g. surface roughness. From this matrix the impact from either of the properties on print mottle can be studied independently. Experimental investigations showed that the level of absorption non-uniformity in a coated board had clear impact on print mottle. In most cases the relationship appeared to be linear, though the degree of impact differed from one board to another. This suggests that the print was more sensitive to uneven absorbency on some samples. This may be contributed to relative importance of other properties including the original non-uniformity of the board surface. When absorption non-uniformity is at a constant and low level, the effects of surface roughness becomes more pronounced.

  • 7.
    Thorman, Sofia
    et al.
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Chemical Sciences (from 2013). RISE AB.
    Yang, Li
    RISE AB.
    Hagberg, Anni
    RISE AB.
    Ström, Göran
    RISE AB.
    The impact of non-uniform ink absorptionon flexographic print mottle2018In: Journal of Print and Media Technology Research, ISSN 2223-8905, Vol. 7, no 1, p. 7-18Article in journal (Refereed)
    Abstract [en]

    Absorption non-uniformity and surface roughness of coated packaging boards are believed to have an impact on flexographic print mottle. Yet, their respective contributions are not well recognised due to their co-existence. Therefore, we propose a method that can solely study the effects of absorption non-uniformity on print mottle. This is achieved by artificially introducing uneven absorption, through well-controlled barrier patterns. The barrier patterns were added onto board surfaces using flexographic printing. By applying barrier patterns of several area coverages on board substrates of different intrinsic surface roughness it is possible to create a property-matrix, absorption non-uniformity versus for example surface roughness. With this matrix, the impact on print mottle from either of the properties can be studied independently. The results showed that surface roughness had a dominant effect on the print mottle, but mainly when comparing samples that spanned a broad roughness range. On the other hand, within a limited range of surface roughness, uneven ink absorption governed print mottle instead. This may explain why printing problems are sometimes encountered despite smooth board surfaces. Although the impact of absorption non-uniformity on print mottle differed from one board to another, the results indicated that a change towards more uneven absorption will have a negative impact on the print quality for most of the coated boards. The results give a better understanding of flexographic print quality and thereby can enable more reliable print mottle predictions.

  • 8. Thorman, Sofia
    et al.
    Yang, Li
    Lestelius, Magnus
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Chemical Sciences (from 2013).
    Influence of board properties on the flexographic ink distribution in single and multicoloured printsManuscript (preprint) (Other academic)
  • 9. Thorman, Sofia
    et al.
    Yang, Li
    Lestelius, Magnus
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Chemical Sciences (from 2013).
    Mechanism and characterisation of ink trapping defects in conventional multicolour printingManuscript (preprint) (Other academic)
1 - 9 of 9
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