Peter Rättö, Erik Blohm, Gary Chinga-Carrasco

Abstract
The filament pattern during printing was studied. The printing was performed in a Prüfbau laboratory printing press with IGT medium viscosity oil. The filament pattern was recorded with a high speed camera. Newsprint handsheets with similar surface roughness but different amounts of fine material were used during the trials. Different amount of fine material was obtained by fractionising the TMP pulp and additional beating of kraft pulp. Both qualitative and quantitative analyses of the filament pattern were performed.

The filament pattern at the nip exit  appeared as a bright area. The density of the filament pattern was further analysed with computerized image analysis. The intensity and spreading of the brighter area were quantified. An increased amount of fine material resulted in a denser filament pattern. Calendering densified the filament pattern further.

A dense filament pattern indicates a later oil splitting point from the nip centre and a higher splitting force due to a higher splitting velocity. A dense and smooth paper would thus be subjected to higher tensional forces at the nip exit than a porous and relatively rough paper.

Keywords: film splitting, ink film, porosity, roughness, printing
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Ulrich Hirn, Markus Lechthaler, Wolfgang Bauer

Abstract
The paper related reasons for print unevenness are usually due to inhomogeneities in the paper structure. Instead of measuring overall paper parameters like e.g. PPS roughness we are evaluating high resolution measurements of local paper properties. These 2D paper property maps are linked to images of the print and the effect of local paper property variations on local print density variations is analyzed using statistical models.

The statistical evaluation of rotogravure printed SC papers showed, that for both industrial and laboratory print three paper properties are the governing factors of print unevenness: local brightness variations, beta formation and local variations of printing ink penetration. All three are relevant in low and high tones, however local brightness variations are highly dominant in light tone values and local printing ink penetration is highly dominant in dark tone values. The other parameters examined in this study (Surface topography, local opacity, local refractive index and local thickness variations) were of minor importance. The modeling results presented in our work are stable and reproducible.

The introduced methodology is also applicable to analyze paper related reasons of print unevenness for other paper grades and other printing technologies. Measurement of local paper properties and linking these data to local print density variations can also be applied to offset-, flexo- or digital printing. Thus it might be applied to other research aimed to identify paper properties governing print unevenness in different printing technologies.

Keywords: print quality, local paper properties, statistical model, rotogravure, SC paper
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Laurette Vieille-Grosjean, Alice Vermeulin

Abstract
Ink misting has become a major issue for high speed offset presses. It typically occurs when an ink film splits between rollers and creates ink filaments, which eventually break up into tiny droplets, causing severe degradation of the press environment. Extensive research has helped to better understand misting mechanisms.

An attempt was made to develop a comparative and predictive method to evaluate misting based on physical simulations correlated to rheological properties of a panel of coldset and sheetfed inks. As a complex phenomenon, ink’s tendency to mist cannot be defined by only one rheological parameter, such as tack or elasticity. And a predictive method may include a combination of several data coming from both ink and ink-water emulsion results. Simulating misting on Inkometer seemed to be a good first approach for sheetfed inks, as absence of misting was correlated to no result on a small-scale offset press (Apollo). A lot of observations were done during this study especially on misting simulations of coldset inks and influence of ink-water emulsions on rheological profiles of sheetfed inks. Further experiment shall be carried out to larger series of inks and ink-water emulsions to complete this approach.

Keywords: ink, misting, offset printing, rheology, comparative tool
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