| 1. |
- Rydefalk, Cecilia, Civilingenjör, et al.
(author)
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Effects of the structural morphology on the compressive response of paperboard
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Other publication (other academic/artistic)abstract
- The paper structure heavily influences the initial part of the ZD-compression curve of paperboard, and due to this, the initial part is often overlooked or disregarded. In the context of contact printing, however, surface roughness is vital to attain print uniformity. The present study delves into the effect of surface roughness of commercial paperboard and how it affects the compression response. A finite element (FE) model of a compression test has been constructed. The approach exercised in this study systematically varies surface morphology amplitudes and wavelengths of a simulated paperboard in the FE-model. The simulations are carried out with commercial software and material models, and the focus is on the structural variations in the sheet. The initial curvature of the compression curve is quantified using the initial energy. The simulations also enable the study of the stress variations underneath the compression probe. The study shows that both the surface roughness’ magnitude and spatial characteristics, as well as the relative placement of the roughness on top and bottom, will affect the initial energy. Additionally, the stress variations show that cases with similar initial energy can have their cause in different mechanisms. Therefore, the local contact conditions and pressure uniformity might not be visible in the global compression response.
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| 2. |
- Rydefalk, Cecilia, Civilingenjör, et al.
(author)
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Separating the effects of maximum pressure and printing nip length on flexographic print quality
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Other publication (other academic/artistic)abstract
- When adjusting the impression in a printing press both the maximum pressure induced and the contact length between the print form and the substrate are simultaneously altered. In the present study, lab printing was performed with controlled load cases. The load cases vary depending on the nip length or the maximum pressure. A lab-scale printing press was augmented with a pressure sensor that measures the width of the print over a square area. By altering the print forms and the force settings in the machine, the curve of the print nip pressure was controlled. Printing was performed in both full-tone and half-tone, and the printed result was evaluated for mottle, density, and dot-gain. By increasing the maximum pressure, the color density increases. By increasing the nip length at a fixed maximum pressure, the color density decreases. The variation within the settings in the present study is small and appears to originate from the split pattern. The change in the nip exit angle with increased nip length is sufficient to alter the ink split point and, thereby, the density. A higher maximum pressure can instead enable a higher ink transfer.
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| 3. |
- Rydefalk, Cecilia, Civilingenjör, et al.
(author)
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Simulations of the lateral stress variations in a flexographic print nip
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Other publication (other academic/artistic)abstract
- Contact printing, calendaring, and coating of packaging paperboard are standard industry processes that utilize rolling nips. The pressure pulse, maximum pressure, and duration have been extensively studied regarding its effect on the substrate and how it can be changed and controlled to achieve the desired effect. The present study considers lateral variations of the stress in a rolling process. A parametric study of the surface roughness, substrate stiffness, cylinder cover stiffness, and changed nip engagement or impression is performed using Finite Element Modelling. The simulation shows that a smooth surface does not completely negate the effects of the structural thickness. The impression has the most significant impact, and the combination of roughness and non-linear material means that the pressure distribution can change drastically, not just the maximum pressure pulse. Additionally, different combinations of settings can achieve the same mean pressure pulse but have very different stress distributions. E.g. changing the surface roughness will have a significant effect on the pressure variations, but the effect on the pressure profile shape is negligible.
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| 4. |
- Rydefalk, Cecilia, Civilingenjör
(author)
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Structural and mechanical aspects in the print nip and their effect on the ink transfer in flexographic packaging printing
- 2024
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Doctoral thesis (other academic/artistic)abstract
- Flexographic packaging printing on paperboard is a simple yet complicated process. The working principle of flexographic printing is simple to understand, especially if one has done potato printing as a child. A raised pattern is cut out of half a potato, dipped in paint and pressed to a paper. The same basic principle is true for industry scale printing too. A raised pattern on a soft polymeric print plate is covered in ink. The ink is transferred by bringing the surfaces together with force. Unlike the kitchen-table version, the print speed can be in the scale of 600 m/min. And the print plate can carry detailed patterns or text that have to be rendered visually attractive and legible. In the present work the material considered in a printing context is mainly commercial liquid packaging paperboard. The type of paperboard one would find in e.g. a milk carton. The thesis explores a few aspects that affect the ink transfer in a print nip. How fast the paperboard is compressed, what happens with a non-smooth surface and how the pressure profile in the nip affects the print.The compression of the paperboard in a print nip is many times faster than any standard method for measuring the compressive properties of paperboard. Paper and paperboard are known to be rate-dependent materials, they creep under long-time loads. Information on the material behavior under rapid compression has, however, been lacking. The present work therefore investigates what happens when the paperboard is compressed at the rate in a print nip. The main effect is seen in the stiffening of the coating layer.A well-known way to improve the ink transfer is to increase the impression. The impression is increased by decreasing the distance between the cylinders in the print press. However, there are limits to how far increasing the impression is possible or still makes an improvement. When printing on sensitive materials and increased impression could lead to damage. The wear on the print plates is also higher with increased impression. The parts of the print that consists of raster dots will be more deformed with a higher impression, leading to a deterioration in print quality. In the present work simulations are performed where different surface roughness and material stiffnesses are combined to explore their interaction and effect on the stress distribution in rolling compression. Unsurprising, the surface roughness has the largest effect. However, since the material have a non-linear response to increased loads the lateral stress distribution changes position with increasing impression. Additionally, large hills and valleys on the bottom side of the paperboard can have an effect on the stress distribution on a smoother top-side.Increasing the impression in a print nip is performed by bringing the cylinders in the print press closer together. The effect is that both the maximum pressure and the contact between paperboard and print plate, the nip length, increases simultaneously. The present work separates the two, and considers the print result. By increasing the maximum pressure while keeping the nip-length constant, the ink transfer to the paperboard is increased. However, by increasing the nip-length while keeping the maximum pressure constant, the ink transfer to the paperboard is decreased.
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| 5. |
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| 6. |
- Thorman, Sofia, 1979-
(author)
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Absorption non-uniformity characterisation and its impact on flexographic ink distribution of coated packaging boards
- 2015
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Licentiate thesis (other academic/artistic)abstract
- 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.
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| 7. |
- Thorman, Sofia, et al.
(author)
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Dynamic out-of-plane compression of paperboard — Influence of impact velocity on the surface
- 2024
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In: TAPPI Journal. - : Technical Assoc. of the Pulp and Paper Industry Press. - 0734-1415. ; 2024-February, s. 113-122
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Journal article (peer-reviewed)abstract
- Processes that convert paperboard into finished products include, for example, printing, where the paperboard is subjected to rapid Z-directional (ZD) compression in the print nip. However, measuring and evaluating the relevant properties in the thickness direction of paperboard are not necessarily straightforward or easy. Measuring at relevant, millisecond deformation rates further complicates the problem. The aim of the present work is to elucidate some of the influences on the compressive stiffness. Both the initial material response and the overall compressibility of the paperboard is studied. In this project, the effect on the material response from the surface structure and the millisecond timescale recovery is explored. The method utilized is a machine called the Rapid ZD-tester. The device drops a probe in freefall on the substrate and records the probe position, thus acquiring the deformation of the substrate. The probe is also allowed to bounce several times on the surface for consecutive impacts before being lifted for the next drop. To investigate the time dependent stiffness behavior, the probe is dropped several times at the same XY position on the paperboard from different heights, thus achieving different impact velocities. The material response from drops and bounces combined allows study of the short-term recovery of the material. The material in the study is commercial paperboard. The paperboard samples are compared to material where the surface has been smoothed by grinding it. Our study shows that there is a non-permanent reduction in thickness and a stiffening per bounce of the probe, indicating a compaction that has not recovered in the millisecond timescale. Additionally, a higher impact velocity has an initial stiffening effect on the paperboard, and this is reduced by smoothing the surface.
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| 8. |
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| 9. |
- Thorman, Sofia, et al.
(author)
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Impact of non-uniform water absorption on water-interference print mottle in offset printing
- 2018
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In: Nordic Pulp & Paper Research Journal. - : Walter de Gruyter GmbH. - 0283-2631 .- 2000-0669. ; 33:1, s. 150-163
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Journal article (peer-reviewed)abstract
- 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. .
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| 10. |
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| 11. |
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| 12. |
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| 13. |
- Thorman, Sofia, et al.
(author)
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Rapid, Out-Of-Plane Compression of Paperboard – Influence of Impact Velocity on The Surface
- 2023
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In: <em>TAPPICon 2023 - "Rock the Roll: Unleashing the Harmonies of the Paper Industry"</em>. - : TAPPI Press.
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Conference paper (peer-reviewed)abstract
- Industry processes, such as printing, subjects paperboard to rapid, Z-directional compression. However, measuring and evaluating the relevant properties in the thickness direction are not necessarily straight forward or easy. Measuring at relevant, millisecond, deformation rates complicate the problem further. The aim of the present work is to elucidate on some of the influences on the compressive stiffness. Both the initial material response and the overall compressibility of the paperboard is studied. In this project the effect on the material response from the surface structure and the millisecond time-scale recovery is explored. The method utilized is a machine called the Rapid ZD-tester. The device drops a probe in free fall on the substrate and records the probe-position, thus acquiring the deformation of the substrate. To investigate the time dependent stiffness behavior the probe is dropped several times at the same xy-position on the paperboard from different heights, thus achieving different impact velocities. The probe is also allowed to bounce several times on the surface before lifted for consecutive drops. The drop-bounce cycle allows study of the short-term recovery of the material. The material in the study is commercial paperboard. The paperboard samples are compared to material where the surface has been smoothed by grinding it. Our study shows that there is a non-permanent reduction in thickness and a stiffening per bounce of the probe, indicating a compaction that has not recovered in the millisecond timescale. Additionally, a higher impact velocity has an initial stiffening effect on the paperboards, and that this is reduced by smoothing the surface.
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| 14. |
- Thorman, Sofia, et al.
(author)
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Simultaneous determination of absorption mottle and white-top mottle in the same area on coated boards
- 2013
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In: Proceedings of the 40th International Research Conference of iarigai. - Darmstadt, Germany : International Association of Research Organizations for the Information, Media and Graphic Arts Industrie (IARIGAI). - 9783981270440 ; , s. 225-232
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Conference paper (other academic/artistic)abstract
- 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.
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| 15. |
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| 16. |
- Thorman, Sofia, et al.
(author)
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Studying the impact of non-uniform absorption on print mottle of coated boards with barrier patterns
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Other publication (other academic/artistic)abstract
- 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.
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| 17. |
- Thorman, Sofia, et al.
(author)
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The impact of non-uniform ink absorptionon flexographic print mottle
- 2018
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In: Journal of Print and Media Technology Research. - Darmstadt, Germany : International Association of Research Organizations for the Information, Media and Graphic Arts Industrie (IARIGAI). - 2223-8905 .- 2414-6250. ; 7:1, s. 7-18
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Journal article (peer-reviewed)abstract
- 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.
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| 18. |
- Thorman, Sofia, et al.
(author)
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Uniformity of liquid absorption by coatings : Technique and impact of coating composition
- 2012
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In: Nordic Pulp & Paper Research Journal. - 0283-2631 .- 2000-0669. ; 27:2, s. 456-465
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Journal article (peer-reviewed)abstract
- 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.
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| 19. |
- Thorman, Sofia, 1979-
(author)
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Where did the ink go? : The effect of liquid absorption on ink distribution in flexography
- 2018
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Doctoral thesis (other academic/artistic)abstract
- 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.
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| 20. |
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| 21. |
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