Compression uniformity measurements on coated anduncoated paper surfaces

Abstract: Paper is exposed to compressive stresses at several occasions in its production process and in subsequent printing operations. In the press section of a paper machine, the fibre web is mechanically dewatered by compression. In calendering its surface properties, such as gloss and smoothness, are enhanced by compressing the paper between rollers or plates. Also in rotogravure printing, paper is pressed against the printing cylinder in order to create good contact and thus good ink transfer. In all situations where paper is compressed, its local structural properties control the compression uniformity and consequently the end-product quality, e.g. in the form of gloss variation, calender blackening, ink absorption variations, printing failure such as missing dots in rotogravure.



This work describes the development and application of a technique to measure local compression uniformity. Local pressure distributions on a paper surface under compression are quantitatively characterized using a custom-made pressure-sensitive film in a hydraulic press, a CCD-camera and image analysis. The technique was tested in the fields of coating, calendering and printing. A measure for the load coverage ability of coating was obtained. It gives insight into how well a coating covers the base paperÕs non-uniformity under compression. In a second study, different calendering techniques were analysed and it was found that the local structure of the load-bearing flocs was changed significantly depending on the calendering conditions. In a third study, the relation between missing dots in rotogravure and the underlying paper structure was outlined. Missing dots predominantly occurred in valleys of a certain critical size, situated in direct proximity to load-bearing structure elements in the paper