| 1. |
- Hagman, Anton, 1984-, et al.
(författare)
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ZD – Directional Testing of Paperboard, Using Refined Methods to Revisit Out of Plane Properties
- 2022
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Ingår i: TAPPICon 2022 Proceedings.
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Konferensbidrag (refereegranskat)abstract
- The ZD-tensile strength is tested by attaching the top and bottom sides of a paperboard to rigid blocks that are pulled apart. In a production laboratory the strength is recorded using a tape as an adhesive. In specialized laboratories a more thorough method is available that also measures the force-displacement curve of the sample. The advanced method involves laminating and gluing the paperboard sample to metal blocks which are mounted in a universal testing machine. In this study the advanced ZD-tensile method was refined by removing the glue step and laminating the paperboard directly to the blocks. The new method was validated against the regular method with adequate results. The limits of the refined method were explored with regards to ZD-strength and paper/paperboard thickness. In an attempt to unify the ZD-tensile and -compressive behaviour of paperboard, samples were laminated and tested in combined compression and tension testing. The compressive properties were compared to non-laminated samples. The laminated samples showed a different behaviour than the non-laminated samples. The flat slope seen in the initial part of the pure compression curve disappeared, replaced by a continuous response passing 0 N. The stiffness in this region resembled the response in tensile testing.
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| 2. |
- Rydefalk, Cecilia, Civilingenjör, et al.
(författare)
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Effects of the structural morphology on the compressive response of paperboard
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Annan publikation (övrigt vetenskapligt/konstnärligt)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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| 3. |
- Rydefalk, Cecilia, Civilingenjör, et al.
(författare)
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On the effect of maximum pressure and contact time in flexographic printing
- 2023
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Ingår i: TAPPICon 2023. - : TAPPI Press. ; , s. 993-
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)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 is altered. In the present study lab-printing has been performed with controlled load cases. The load cases are either varying the time in contact or the maximum pressure. A lab-scale printing press was augmented with a pressure sensor that measures over a square area the width of the print. By altering the print-forms and force 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 evaluated for mottle and dot-gain. The print performance metrics shows the influence of the different load cases.
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| 4. |
- Rydefalk, Cecilia, Civilingenjör, et al.
(författare)
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Separating the effects of maximum pressure and printing nip length on flexographic print quality
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Annan publikation (övrigt vetenskapligt/konstnärligt)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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| 5. |
- Rydefalk, Cecilia, Civilingenjör, et al.
(författare)
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Simulations of the lateral stress variations in a flexographic print nip
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Annan publikation (övrigt vetenskapligt/konstnärligt)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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| 6. |
- Rydefalk, Cecilia, Civilingenjör
(författare)
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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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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)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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| 7. |
- Rydefalk, Cecilia, Civilingenjör, et al.
(författare)
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Z-directional testing of paperboard in combined tensile and compression loading
- 2024
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Ingår i: TAPPI Journal. - Peachtree Corners, GA : TAPPI. - 0734-1415. ; 23:5, s. 268-284
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Tidskriftsartikel (refereegranskat)abstract
- The out-of-plane properties of paperboard are important in several converting applications such as printing, sealing, creasing, and calendering. A juxtaposed tensile and compression curve in the z direction (ZD) will, however, appear to have a kink or discontinuity at 0 stress. The purpose of the present work is to capture the continuous transition between tension and compression and to increase the understanding of the complex ZD properties of paperboard by cyclic testing. In this attempt to unify the ZD tensile and compressive behavior of paperboard, samples were laminated to the testing platens using heat seal laminate film. The method for adhering the samples was compared to samples that were laminated and glued to the testing platens. The edge effects of the cutting method were evaluated in compression testing with samples not attached to the testing platens. The flat slope seen in the initial part of the pure compression curve disappeared when the samples were laminated to the testing platens. The flat slope was instead replaced by a continuous response in the transition across 0 N. The stiffness in the transition region resembled the response in tensile testing. When the testing is cycled, the material exhibits a history dependence. Starting the cycle in either compression or tensile will show an effect on the stiffness at the transition, as well as the compressive stiffness. However, the ultimate tensile strength is unaffected.
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| 8. |
- Thorman, Sofia, et al.
(författare)
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Dynamic out-of-plane compression of paperboard — Influence of impact velocity on the surface
- 2024
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Ingår i: TAPPI Journal. - : Technical Assoc. of the Pulp and Paper Industry Press. - 0734-1415. ; 2024-February, s. 113-122
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Tidskriftsartikel (refereegranskat)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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| 9. |
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| 10. |
- Thorman, Sofia, et al.
(författare)
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Rapid, Out-Of-Plane Compression of Paperboard – Influence of Impact Velocity on The Surface
- 2023
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Ingår i: <em>TAPPICon 2023 - "Rock the Roll: Unleashing the Harmonies of the Paper Industry"</em>. - : TAPPI Press.
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Konferensbidrag (refereegranskat)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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