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Förslaget inkom 2004-01-26

New techniques for analysing film splitting in offset printing

OBS! ANSÖKNINGSTIDEN FÖR DETTA EXJOBB HAR LÖPT UT.
In offset printing, the ink and fountain solution is transferred from the print cylinder to an intermediate ¿offset¿ rubber blanket cylinder and in turn to the surface of the paper or board.
The splitting of the ink film in the cylinder-paper nip is a complex phenomenon, in which the film is rapidly subjected to a compression,/extension- and shear-pulse which induces cavitation
and gives rise to ink filaments which rupture at the nip exit. Simultaneously, the mobile phase of the ink is pressed into the porous substrate, resulting in an increase in its viscosity. This ink
setting process continues after the printed paper leaves the nip, and its rate determines to what extent the ink film roughness due to ruptured filaments is able to level-out and build gloss prior
to immobilisation. These interrelated events determine both the runnability of the printing press and the final quality of the printed material after oxidative drying.
The micro-mechanisms of film splitting are, despite their enormous industrial importance, relatively poorly understood. Ink parameters, such as shear- and extensional-viscosity and viscoelasticity, and substrate parameters, including topography, pore structure, and mechanical properties, have been shown to empirically correlate to certain print characteristics. However, these measures, determined in the rest-state or steady-state, lack direct coupling to the true
conditions of the rapid pressure pulse in the printing nip.
A novel approach recently studied at YKI involves the use of acoustic measurements in the printing nip to obtain the sound spectrum from the film cavitation and splitting events. The sound spectrum contains a wealth of information, and is sensitive to variations in printing
conditions. The challenge is to understand these links by systematically decomposing the acoustic signal to quantify its dependence on the measurable input parameters characterising the ink,
paper and press conditions. The further development of the acoustic technique, and analysis of the spectra obtained, are the subject of this Master¿s Thesis project. The technique will be applied to film splitting between rolls, between roll and paper, and to tackiness of the printed paper. A background in data analysis, and in particular, signal processing, would be ideal but not a prerequisite.

Utförs vid Skogsindustriella Sektionen, YKI, Ytkemiska Insitutet i Stockholm.

Kontaktperson:
Sektionschef Docent Andrew Fogden, YKI
[email protected] / Tel. 08-790 99 49

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