Improper ink viscosity and mesh selection contribute more to press stoppage than you might think.
One of the things that bothers me the most when I visit production shops is press operators who are obsessed with the constant need to make print adjustments. They can't seem to leave the machine alone, and any adjustment seems to be good for only a few minutes. While there are certainly plenty of times when it is appropriate to make adjustments, many of them are necessary only because we haven't taken the time to understand how ink flows through the mesh onto the substrate. Whether we're printing UV inks for graphics or plastisol inks for garments, we choose to use different mesh counts for different situations. Garment screen printers have a much greater selection than the graphics printers. Nevertheless, each has a choice of at least three different mesh counts over the range of work being produced. Garment printers can easily have half a dozen or more. We must ask ourselves, "Is this really necessary?" Let's consider what causes ink to flow at different rates. One of the most basic of these factors is heat. Heat has an effect on the viscosity of ink, whether it's UV or plastisol, and screen-printing presses pump huge amounts of heat into their platens. This heat radiates into the screens, progressively warming the ink. Since both types of ink are essentially plastic-resin based, the viscosity progressively changes as the temperature increases. The management of heat in the printing system is not the primary focus here, but it must be considered as a major contributing factor to ongoing press adjustment. The major area to concentrate on is the role of the mesh in controlling the flow of ink to the substrate and how we can equalize this flow over the range of meshes we use. Feeling the heat Let me begin by addressing the heat issue in a very simplified way. When we look at any printing system, we need to consider the range of heat variation over the life of the print run. This is something most printers never even think about. The cooler the ink, the thicker it is. As it warms, the viscosity continuously drops, which necessitates constant adjustment to maintain color consistency as the ink-film thickness and dot gain change. There is a point at which the ink's viscosity begins to stabilize and does not significantly change with additional heat. Only when the ink begins to cure does the viscosity dramatically rise again as the ink either crosslinks (UV) or gels (plastisol). It is during this flat stage, when ink flow is constant, that you will not need to make any press adjustments. Most inks reach this stage when the ink temperature reaches about 90
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