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Characterisation of gravure cylinders
The shape and volume of engraved cells are critical to the performance of the gravure printing process. A slight discrepancy between the desired and actual cell volume, can lead to a significant change in volume of ink transferred, forcing the press crew to apply compensation to the ink in order to achieve the correct colour balance. Traditionally the volume of the cells engraved in a gravure cylinder have been estimated by optically measuring the width of the cell on the surface (by optical microscope, usually combined with image processing) and inferring the cell volume from a knowledge of the engraving tool geometry. This can lead to significant differences in ink transfer from notionally identical cylinders. As part of a fundamental study into the release of ink from cells in gravure printing process, an accurate method of characterising the engraved cell geometries was required. The angles of the sides of the engraving preclude the use of stylus techniques to obtain this information and also limit the ability to make accurate casts of the surface. White light interferometry was used to characterise both the engraving and the surface finish at several stages during the production of gravure cylinders. It was used to calculate the cell volume, which was compare with that estimated using traditional means and with the performance on the press. A 2mm difference in depth was found between identical engravings on three cylinders. This was sufficient to change the printing characteristics of the cylinder to the extremes of the adjustments available to the press crew. The cells where also examined using a montaging microscope. The microscope achieves high magnification with depth of field capable of viewing from the whole height of the cell, by scanning the microscope and then recombining the in focus areas of the images to produce a complete picture. This offers the ability to examine the cell walls, which will also aid the understanding of the ink release mechanism. The white light interferometer was used both in the laboratory and in the field to characterise gravure cylinders. Despite its high capital cost, the press time, which this method could save by improving the predictability of the process, could eventually lead to its adoption as a routine means of quality assurance. This study of the methods of characterising the engraved cells and the fundamental principles of ink release also has implications for other processes such as flexography and pad printing.