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Printing Humidification Static build-up, web breaks and paper curl are all problems caused by dry air in the printing industry and are particularly prevalent during the winter months. By humidifying the atmosphere around printing presses, reel- stand areas and paper stores, problems like these can be dramatically reduced. Printing Humidity Issues Paper is a natural material and is hygroscopic, which means that it is very susceptible to changes in relative humidity (RH). For optimum printing and paper storage conditions the humidity should be kept between 50-60% RH. Low humidity environments are typically created in cold periods, as heat generated by machinery and heaters dries the air. In the depths of winter, RH levels as low as 15- 20% RH are not uncommon. In dry conditions, moisture is drawn from the exposed surfaces of stacks of paper and they shrink, but the centre of the paper still maintains its original moisture content. This causes "tight edges" and the paper begins to curl. As a result, the paper misfeeds and creasing occurs. Changes in humidity will cause the physic al dimensions of the paper to change which results in mis-registration of c olours in multi- pass jobs. Any change in the dimensions of the paper, however small, between runs of two or more colour processes and the colours will not line up. When the relative humidity drops below the electrostatic threshold of 40% RH, the build-up of static is enhanced. This results in sheets of paper sticking and misfeeding, not lying Hat on each other or stacking properly as they emerge from the other end of the press. In the lay-down and reelstand areas of large scale web-based newspaper printers, the exposed edges of the reels dry. As the web runs at speed through the press, the dry edges are under tension compared with the centre. The slightest disruption of the edge will result in a web break, causing the paper to tear right across its width. Re-feeding through the press results in consider able down-time and production loss.


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