Tamara Tomašegović

Modifying the qualitative properties of print by surface treatment of flexographic printing plate

In the fast-pace technology development in graphic industry, modern flexography has found its domain mostly in the packaging sector and it is increasingly of interest for functional printing. Functional printing has a relatively low market value but greater interest in research. Due to the new qualitative requirements, workflows and materials used in flexography had to be updated and improved. The application of digitally controlled processes and procedures has taken the place of the analogue production, together with the new methods of material processing and improvements of the materials themselves. This research focuses on the functional modification of photopolymer flexographic printing plate’s properties with the aim of achieving optimal output quality. During the transfer of the printing ink from the anilox to the printing plate and then to the printing substrate, surface properties of the printing plate influence the quality of the print. Therefore, surface properties of the printing plate should be compatible with the used printing ink and the printing substrate, which is especially important when using new formulations of inks and experimenting with different printing substrates and applications, for example in functional printing. In this research, samples of photopolymer flexographic printing plates were exposed to UV-ozone treatment in order to modify the surface properties of the photopolymer material. Results have displayed significant changes in surface free energy of the photopolymer material when the printing plate samples were exposed to the UV-ozone for periods up to 5 minutes. In order to analyze the quality of the prints produced with UV- ozoned flexographic printing plates, test prints were produced. Prints produced with printing plates with longer UV- ozone treatment have displayed the qualitative changes in the reproduction of fine printed elements, i.e. the width of fine lines, coverage values, ink volume on print and the definition of the shape and edges of printed elements. Changes of named properties of the print are significant for conventional, and in many cases for functional printing. Performed research proved that the functional modification of flexographic printing plates with the aim of improving the print quality is possible. The UV-ozone treatment is a procedure where the printing plate is exposed to significantly higher energy than with conventional UVA and UVC tubes. Therefore, the duration of the UV-ozone treatment must be precisely adjusted in order to maximize the quality of the print, while at the same time maintaining printing plate’s functionality

Tailoring the properties of deposited thin coating and print features in flexography by application of UV-ozone treatment

Printed functional materials are a rapidly growing area of interest for low-cost high-speed device manufacture with flexographic printing seen as a route to achieving this. The relationship between surface tension of the ink and surface free energy (SFE) of the photopolymer plate is a key for optimum performance. However, traditional methods of surface tension modification of the ink/coating often cannot be employed for functional inks. In this research, rapid, permanent modification of flexographic printing plate’s SFE is achieved through controlled UV-ozone treatment, and the effects of the treatment on the polar and dispersive component of SFE are analyzed by Fourier transform infrared attenuated total reflectance spectroscopy, swelling experiments, and roughness measurements. Printing trials using the modified printing plates reveal improved print uniformity and control of deposited ink layer thickness, as well as improved print features—particularly track and pad junctions which can be problematic for printed electronic applications. The ability to rapidly tailor printing plate SFE is of benefit to all volume printing applications. Furthermore, it is of critical importance for functional printing and printed electronics where surface tension of the ink is determined by the functional material and chemical modification is not possible or desirable.

Formation of the Printing Elements in the Photopolymer Material Used in Flexography

Starting point of this paper is photopolymer printing plate used for flexographic printing. It is used for transfer of the printing ink onto the printing surface during the reproduction process. Photopolymer printing plate consists of several layers: polyester basis, photo sensitive polymer material and LAMS - Laser Ablation Mask. In the platemaking process the photosensitive material, which will form a printing plate, has to be several time exposed to different radiation in order to obtain a functional printability performance. LAMS layer has a role of masking in the exposure process. Upon pre-exposure, LAMS layer has to be removed by laser ablation only at the surfaces where photopolymer printing plate needs to be exposed. After ablation the exposures to UV lights follows and the plate will be finished with chemical removal of the unexposed parts of the polymer. Functionality of the finished printing plate has to be characterised and monitored in every procedure step because the formed image element on the printing plate has a major influence on the quality of the finished printed product. In this paper observing of the changes in the polymer material which is caused by exposure through LAMS layer will be performed. The aim is to measure the surface openings on the LAMS mask and to measure surface coverage (image elements) on the polymer material formed by exposure through the LAMS. Measuring will be made by image analysis software based on microscopic images of the control fields of differing halftone values. It is assumed that there will be a correlation between the LAMS openings and formed image elements on the printing plates. Preliminary results indicate that certain differences in image elements can be detected and are probably the consequence of the different amount of irradiated surface of the polymer material. Since the polymer material which forms the printing plate should be stable in the graphic reproduction process, results of the paper will explain the influence of UV exposures on polymerisation process and on the functional printability performance of the plates.