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Effect of moisture amount on HSWO waviness
Karlstad University, Faculty of Technology and Science. (Paper Technology)
2004 (English)In: International Printing and Graphic Arts Conference 2004, 2004, 29-33 p.Conference paper, Published paper (Other academic)
Abstract [en]

Waviness or fluting occurs in the hot-air drying oven of the heatset web offset (HSWO) printing press and reduces the visual quality of the print. In order to evaluate a theory that waviness results from stretching of the material in one direction while being prevented from shrinking in the perpendicular direction, the reaction of a paper web under tension to moisture was observed. A paper web was stretched under tension using three rolls, the spacing of which could be varied. The web was wetted using humid air of 60-85% RH and the web observed using the STFI Opti-Topo optical topography technique. Images were obtained using a charge coupled device (CCD) camera. Two 70gsm medium weight coated (MWC) papers were used, one prepared using a gap former with online coating and calendering, the other prepared with a hybrid former followed by online coating and offline calendering. The paper tensile properties were determined. Although no ink was involved, the typical waviness pattern was seen to develop. Higher moisture uptake resulted in a greater waviness. The two papers had similar bending stiffness, but the paper with the highest tensile stiffness showed the lowest waviness.

Place, publisher, year, edition, pages
2004. 29-33 p.
Identifiers
URN: urn:nbn:se:kau:diva-6321OAI: oai:DiVA.org:kau-6321DiVA: diva2:347914
Available from: 2010-09-03 Created: 2010-09-03 Last updated: 2010-11-15Bibliographically approved
In thesis
1. Laboratory method for the study of moisture-induced waviness in paper
Open this publication in new window or tab >>Laboratory method for the study of moisture-induced waviness in paper
2004 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Paper that is subjected to moisture undergoes dimensional changes. It expands during moistening and shrinks during drying. When the paper is under tension between rollers, the effects are complex since shrinkage and expansion are restricted in the width direction. Waves can then appear on the paper web. This can be a problem in heatset web offset printing. The problem is known as waviness or fluting. The printed papers exhibit a wavy shape, which is visually disturbing due to light reflections which create glossy streaks. The aim of the work described in this thesis was to develop a method suitable for studying the moisture- and tension-induced waviness. Experiments were carried out on a laboratory scale to study how such waves develop during moistening and drying. The experimental setup was based on a modified tensile tester. A CCD camera and image analysis based on the STFI-OptiTopo technique was used to characterise the waviness. Moistening and drying were achieved by changing the surrounding air humidity. The method was used to study the effect of moisture uptake by the paper, and to evaluate the effect of tension on the waviness. It was found that increasing moisture resulted in a higher waviness amplitude, but that the web tension controlled the wavelength of the waviness. A high tension gave rise to a shorter wavelength. The measured wavelength was compared with a previously suggested model and the predicted wavelength was about twice as high as the measured wavelength.

Abstract [en]

When paper becomes damp it can develop waves which can be a problem with heatset web offset printing. Paper with waves may have glossy streaks after printing. Laboratory experiments studied the effect of moisture uptake by the paper. Readings were taken at a range of air humidities. The effect of tension on the waviness was also measured. The waviness was characterised using a charge coupled device (CCD) camera and the STFI-Packforsk OptiTopo technique to analyse the images. When the humidity was increased the amplitude of the waves increased. A high tension resulted in shorter wavelengths. Results from a previous model were used as a comparison.

Place, publisher, year, edition, pages
Karlstads universitet: Karlstad, 2004. 33 p.
Series
Karlstad University Studies, ISSN 1403-8099 ; 2004:42
Keyword
method, moisture, paper, paper web, wavelength, waviness
National Category
Paper, Pulp and Fiber Technology
Research subject
Chemical Engineering
Identifiers
urn:nbn:se:kau:diva-6425 (URN)91-85335-11-8 (ISBN)
Presentation
2004-11-05, 10:00 (Swedish)
Supervisors
Available from: 2010-10-04 Created: 2010-09-28 Last updated: 2010-10-04Bibliographically approved

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