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Where did the ink go?: The effect of liquid absorption on ink distribution in flexography
Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Chemical Sciences (from 2013).
2018 (English)Doctoral 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.

Abstract [en]

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.

Place, publisher, year, edition, pages
Karlstad: Karlstads universitet, 2018. , p. 96
Series
Karlstad University Studies, ISSN 1403-8099 ; 2018:52
Keywords [en]
flexographic printing, print mottle, ink trapping, absorbency, absorption non-uniformity, water-interference mottle
National Category
Chemical Engineering Paper, Pulp and Fiber Technology
Research subject
Chemical Engineering
Identifiers
URN: urn:nbn:se:kau:diva-69984ISBN: 978-91-7063-892-3 (print)ISBN: 978-91-7063-987-6 (electronic)OAI: oai:DiVA.org:kau-69984DiVA, id: diva2:1259747
Public defence
2018-12-18, 21A 244, Ljungberg, Universitetsgatan 2, Karlstad, 10:15 (English)
Opponent
Supervisors
Funder
Knowledge Foundation, Dnr 20100268
Note

Samarbete med RISE AB

Available from: 2018-11-28 Created: 2018-10-30 Last updated: 2018-11-28Bibliographically approved
List of papers
1. Uniformity of liquid absorption by coatings: Technique and impact of coating composition
Open this publication in new window or tab >>Uniformity of liquid absorption by coatings: Technique and impact of coating composition
2012 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 27, no 2, p. 456-465Article in journal (Refereed) Published
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.

Keywords
Coated boards, Flexographic printing, Liquid absorption, Test method
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kau:diva-34906 (URN)10.3183/NPPRJ-2012-27-02-p459-465 (DOI)
Available from: 2015-01-15 Created: 2015-01-15 Last updated: 2018-10-30Bibliographically approved
2. Simultaneous determination of absorption mottle and white-top mottle in the same area on coated boards
Open this publication in new window or tab >>Simultaneous determination of absorption mottle and white-top mottle in the same area on coated boards
2013 (English)In: 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, Published 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.

Place, publisher, year, edition, pages
Darmstadt, Germany: International Association of Research Organizations for the Information, Media and Graphic Arts Industrie (IARIGAI), 2013
Series
Advances in Printing and Media Technology, ISSN 2225-6067 ; Vol. XL
Keywords
absorption non-uniformity, absorption mottle, white-top mottle, liquid absorption, flexography
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kau:diva-34908 (URN)978-3-9812704-4-0 (ISBN)
Conference
40th International Research Conference of iarigai, Chemnitz, Germany, September 2013
Available from: 2015-01-15 Created: 2015-01-15 Last updated: 2018-10-30Bibliographically approved
3. The impact of non-uniform ink absorptionon flexographic print mottle
Open this publication in new window or tab >>The impact of non-uniform ink absorptionon flexographic print mottle
2018 (English)In: Journal of Print and Media Technology Research, ISSN 2223-8905, Vol. 7, no 1, p. 7-18Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
Darmstadt, Germany: International Association of Research Organizations for the Information, Media and Graphic Arts Industrie (IARIGAI), 2018
Keywords
liquid packaging board, coated board, surface free energy, surface roughness, image analysis
National Category
Chemical Engineering
Research subject
Chemical Engineering
Identifiers
urn:nbn:se:kau:diva-69995 (URN)DOI 10.14622/JPMTR-1804 (DOI)
Funder
Knowledge Foundation, Registration no. 20100268
Available from: 2018-10-30 Created: 2018-10-30 Last updated: 2018-10-30
4. Impact of non-uniform water absorption on water-interference print mottle in offset printing
Open this publication in new window or tab >>Impact of non-uniform water absorption on water-interference print mottle in offset printing
2018 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 33, no 1, p. 150-163Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
De Gruyter Open, 2018
Keywords
absorption non-uniformity, coated paper, dispersant, offset printing, uncovered area, water-interference mottle, Fountains, Moisture, Dispersants, Non-uniformities, UnCovered Areas, Water interference mottles, Water absorption
National Category
Chemical Engineering
Research subject
Chemical Engineering
Identifiers
urn:nbn:se:kau:diva-68066 (URN)10.1515/npprj-2018-3011 (DOI)000450919200017 ()2-s2.0-85047988264 (Scopus ID)
Available from: 2018-06-26 Created: 2018-06-26 Last updated: 2018-12-06Bibliographically approved
5. Mechanism and characterisation of ink trapping defects in conventional multicolour printing
Open this publication in new window or tab >>Mechanism and characterisation of ink trapping defects in conventional multicolour printing
(English)Manuscript (preprint) (Other academic)
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kau:diva-69985 (URN)
Available from: 2018-10-30 Created: 2018-10-30 Last updated: 2018-11-05
6. Influence of board properties on the flexographic ink distribution in single and multicoloured prints
Open this publication in new window or tab >>Influence of board properties on the flexographic ink distribution in single and multicoloured prints
(English)Manuscript (preprint) (Other academic)
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kau:diva-69987 (URN)
Available from: 2018-10-30 Created: 2018-10-30 Last updated: 2018-11-05

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