Print Quality of Laser-Engraved Cliches for Pad Printing

摘要

Polymer and steel are widely used materials for making pad-printing cliches. Polymer cliches are typically produced in house using a photographic method involving exposure with film positives followed by development. There are many steps in the process, which affects consistent quality. The production of steel cliches is usually outsourced to professional etching houses because aggressive chemicals are used in the etching process. It is difficult to get straight-walled etching because pooling of the chemicals causes uneven edges and rough bottoms. Computer to plate (CTP) systems utilizing laser engraving to make cliches are available in the market today. They eliminate film and chemicals. They ensure direct image output and eliminate the myriad variables and time-consuming steps required in conventional methods. However, little research has been done yet to evaluate the print quality of laser-engraved cliches. In this study, a CO_2 laser cutter from Universal Laser Systems was used to engrave the image areas on polymer cliches. The laser system has a power of 150 watts and the laser beam has a spot size of 0.001 inch, which ensures fine detail reproduction. Image resolutions are determined by pulses per inch (ppi) in the lateral direction and dots per inch (dpi) in the downward direction. Highest available resolution was used, which was 1000 ppi by 1000 dpi. Laser power and engraving speed determines the depth of the ink wells. These two engraving parameters were varied to obtain optimum results. It was found that 100% speed with 11% power created an optimum etch depth. A vector image and images with large open areas that were screened with different settings were laser-engraved on cliches and printed on a pad printer Model Sealcup 60 from Trans Tech. The engraved sidewalls appeared to be straight and pooling of ink did not occur in the printing process, thus produced high print quality of line arts. Images with large open areas that were screened at a maximum angle of 45° with the doctoring direction and a high screen frequency of 200 lpi had the best print quality and highest print density. Differences between dot shapes were only observed when the screen frequency was low.