Paper loses moisture rapidly when exposed to dry air. Sheet and web roll paper is typically produced and wrapped to stay in equilibrium with air at a humidity of 50±5%RH. When the ambient humidity falls below this level, paper loses water to the environment from exposed surfaces, changing its shape, dimensions and physical properties.Below 40%RH, the movement of paper on paper and other surfaces generates electrostatic charges, which cause it to stick together, attract dust and become difficult to handle.
Sheet Fed Offset
Unwrapped paper in a dry press hall will lose moisture from any exposed surface. Tight edges result, making the paper curl, causing misfeed, misregister, dot doubling and creasing as the paper passes between blanket and impression cylinders. Misfeed also occurs due to electrostatic build-up, making stacking difficult. Printed work awaiting another pass continues to change shape, resulting in misregister.During finishing, paper that has low moisture content will crack along folds resulting in a lesser quality product or increased wastage. Maintaining a relative humidity of 50-55%RH in the press hall and bindery resolves these issues.
If air is too dry in the paper store, laydown area and reel stand, the exposed edges of the paper rolls lose moisture and shrink. On the web press, the distorted paper is then under greater tension at its edges than the centre, causing tears. These web breaks mean substantial downtime as the paper is spliced or rethreaded through the press.
In a dry press hall, the rapid passage of paper through the press generates electrostatic charge. This results in problems with stacking, cutting, trimming and folding, due to sticking. Web printing is highly productive and any loss of production is costly, but problems relating to dry air can be addressed by maintaining the relative humidity at 50-55%RH. In flexo, with water-based inks, 55-60% is required.
- Avery Dennison
- Canadian Bank Note Company
- Eclipse Imaging
- Graphic Packaging International
- Harland Clarke
- ING Document Center
- Quad Graphics Inc.
- RR Donnelley
- Staples Print Solutions
Learn more about humidification for printing and paper...