Details:
Summary:
Printing on corrugated board is a complex process; many input variables affect the results to a varying extent. Not only the printing process itself has an influence on print quality; the pre-conditions of the substrate affect it as well. The topography of the liner surface is one of many important influence factors. As a first step, laboratory tests concerning the influence of the corrugated board production process on the liner surface topography were carried out (Rehberger et al., 2006). The result was that the movement of the liner on a hot plate, as compared to unmoved sheets, is the major criteria in surface roughness changes on coated and uncoated liners. Pilot trials have been carried out, since laboratory tests cannot be scaled up to real conditions. The first pilot trial with an uncoated liner did not result in any surface topography changes in conjunction with gloss, even though the corrugator was set to extreme temperature, pressure and speed conditions. These settings were adjusted to the pre-heater and double facer of the corrugator. The second pilot trial with coated liners, though, showed a clear impact on the topography of the liner surface. Using STFI-MicroGloss meter, the visually perceivable gloss lines have been analyzed and, as result, the average gloss line values computed. The results showed that production speed has the highest influence. The topographical measurements with AFM, FRTMicroProf (r) and CLSM disclosed that these glossy stripes have a much lower nano-scale surface roughness as compared to the raw material. An extreme condition occurs when the corrugator is restarted after a full-stop. One collected sample from the start-up showed longish bubbles across the flute. Not only lowspeed causes gloss lines, so do also the standard settings set by the operator for optimum corrugated board quality.
Finally, printing trials in flexography and ink-jet were performed to determine the gloss influence of the substrate and whether the gloss lines still appear in the print. The print images were measured with the STFI-MicroGloss. The result for the flexographic printed images is that none of the gloss lines from the substrate appears in the print. The same is valid for the ink-jet printed images. Only the gloss from the print is recognizable. Further trials are necessary to shed light on the interrelation between substrate, gloss and print quality.