engleski

Effect of the post-treatment of printing plate on the quality of fine printed elements in flexography

With the increasing qualitative requirements present for the products printed in flexography, various improvements in the workflow of flexographic printing are being expolred and utilized. The aim of this paper is to propose and present a method for increasing the quality of fine printed elements in flexography with the post-treatment of the printing plate seen as a tool for improving the definition of the printed features. In the experimental part, two types of flexographic printing plates were used: one solvent-washable photopolymer CtP plate, and one water-washable photopolymer CtP plate. Samples were produced using the procedure recomended by the manufacturer, and by applying the same compensation curves for both types of printing plates. The durations of UVA and UVC post- treatments were varied in order to influence the properties of the printing plates that will affect the transfer of the ink to the pritning substrate. On the printing plates, formed printing elements were monitored by 3D microscopy in order to assess the initial quality of the image transferred onto different types of photopolymer materials. Samples of the post-treated printing plates were then evaluated by the hardness measurements and surface free energy calculations. Test prints were produced by means of the modified printing plate samples, and the quality of the reproduction of fine elements was monitored by optical methods (microscopy, measurement of coverage values). Results of the research proved that the modifications of both UVA and UVC post-treatments significantly affect the quality of the fine printed elements. Specifically, UVA and UVC post-treatments have proven as a useful tool for improving the definition of the thin printed lines due to the combined effect of the changes in the hardness and surface free energy of printing plates. Furthermore, sufficiently increased hardness of the printing plate together with the specific adjustment of the surface free energy of the printing plate enables the adjustment of the dot gain in the shadow areas of the print (90% - 100% coverage value). Therefore, the post- treatment of the flexographic printing plates can be used as a workflow-incorporable method for the fine adjustment of the qualitative properties of the flexographic print.

flexography; photopolymer; UV post-treatment; surface modification; hardness

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