正确的曝光时间及设备极限.pdfVIP

Correct exposure time of stencil and Equipment limitations by Wolfgang Pfirrmann, KIWO, Inc. Most of the problems that involve the screen or stencil are related to exposure of the stencil. Mainly underexposure represents the biggest threat for the screen maker. Underexposure can lead to a multitude of difficulties during the print run and afterwards when the screen should be reclaimed and reused. The screen maker is not always to blame, as it might be the screen room equipment that dictates the screen quality. This article will expose these equipment limitations and shed some light on the question how to determine exposure times. What happens during exposure? The purpose of exposing the stencil is of course to image the screen, or to reproduce the artwork on the printing plate. That, however, is only one task we have to accomplish. Just as important is, that the emulsion changes its chemistry. It transforms from a water soluble material to a water resistant and solvent resistant material during exposure. Only after exposing or cross-linking the emulsion correctly is the stencil tough enough to withstand the torture of printing. Imagine that underexposure is similar to sawing on the link of a chain. The stencil is only as strong as its chemical bonds of links are, just like a chain. Can your equipment limit the screen quality? Modern stencil systems such as Diazo, Diazo-Photopolymer and SBQ-Photopolymer emulsions can handle all possible requirements. Be that short print runs with plastisol inks, thousands of 4-color posters with water based UV ink or several hundred thousand prints on bottles with solvent based inks. The problems start when the screen maker is limited by the equipment and can not expose the emulsion properly. What are you looking for in a stencil? All you want to see is that the artwork is reproduced as true as possible. However, not all exposure systems are able to do tha