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Improvement of dry toner digital print quality for efficient communication

Leistungen

Overviews of toner machines, toner and toner devoted papers available on the market have been realised. Their main properties and characteristics have been summarized. The different testing methods used by the partners of the project to measure toner transfer to paper and toner adhesion have been described and compared. Improved methods have been developed and accepted by all partners as reliable ones for the future. Three methods have been developed to characterise print quality: - Visual examination of a whole test-chart and classification according to a Figure of Merit. - Visual examination of a microscopic view of the target, showing toner squatter. - Measurement of line smoothness and edge sharpness by image analysis. Problems appearing during the converting of printed-paper, due to a lack of toner adhesion have been listed. Tools for requirements on toner adhesion coming from converting were presented. Identification of substances exhausted by printing and converting was also performed. The paper properties to investigate have been defined. Production of paper was performed at different sites at industrial scale and pilot scale. Paper properties as well as printing behaviour were measured and analysed. An experimental fusing device has been built to simulate a hot roll fusing station with independent and reproducible fusing parameters (temperature of the hot roll, pressure and time in the nip). The influence of these parameters on toner print quality has been studied. A comparison between two toners printed in the same machine showed the interest of low viscosity toner to improve toner adhesion to paper. Several printing and converting trials have been organized at different sites to improve paper runability and print quality in web fed printing lines. The interest of remoistening the paper as well as recommendations to avoid toner abrasion problems have been emphasized during those trials. Relations between papers properties and toner printing quality have been investigated. The paper property that mostly governs toner transfer is the ability of the paper surface to be electrostatically charged. The paper property that mostly governs toner adhesion is the paper surface energy. Recommendations for optimised print have been reported in the final report of the project. The improvements can be summed up in the following few statements: - Uncoated paper for electrophotographic printing can be improved by increasing paper surface energy and paper smoothness. - Coated paper for electrophotographic printing can be improved by slightly increasing the moisture content and by changing electrostatic behaviour (for example using salts). - Pre-printing inks with a high temperature stability and fast drying behaviour must be used. Dryness must be controlled before digital printing. - Printers and converting lines must be adjusted individually with respect to the paper. - Controlling the moisture in the paper and the climate in the pressroom is absolutely necessary to avoid problems. As far as the actual outcome of the project is concerned, a new application for flexo inks was developed: Currently, flexographic inks are not used for pre-printing purposes. In the project, a flexo ink was pre-printed on paper at ink coverages of 30 and 100% and toner adhesion was measured. It was found that this water based flexographic ink promoted toner adhesion in a huge way, both on uncoated and coated papers. Thus flexographic ink can be considered as a good alternative to offset ink for pre-printing applications. A new method to determine toner adhesion was also developed: In most of the cases, the rudimentary tests used to characterise toner adhesion only make it possible to establish tendencies and to define a range of hot roll temperatures for which the toner adhesion is considered to be satisfactory, between a "minimal fixing temperature" and a "hot offset temperature", where the layer of molten toner loses its cohesion and partly adheres to the hot roll. In order to be able to characterise toner adhesion in a quantitative and reproducible way, an adhesion testing method has been developed. There is a recent European standard concerning the determination of toner adhesion (EN 12283: 1996, "Paper - Copy papers - Determination of toner adhesion"). The adhesion testing method elaborated within this project is based on this standard. The test is carried out in accordance with EN 12283 standard and requires the use of an IGT apparatus, but in our testing method, toner adhesion is not defined as the ratio of the optical densities before and after pulling up, but as the speed from which the picking of the printed area appears. The precision, the repeatability and the extent of measurements obtained were optimised compared to those obtained with EN 12283 standard.

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