Relative humidity and
web offset printing

Prevent web breaks, paper dust, static, and ink drying problems in web offset printing

Paper is a highly moisture sensitive material and will absorb, or lose moisture until in equilibrium with the conditions to which it is exposed. When paper is moved from an atmosphere with one level of relative humidity to another, the moisture content is reduced, or increased by approximately 1% for every 10% change in relative humidity. This change affects the fundamental properties of the paper, ie, its tensile strength, dimension, tear resistance, fold endurance and smoothness.

In web based printing, reels are delivered to the facility in a moisture proof wrapping so that moisture changes during transit and storage cannot occur. Prior to printing, the reels should generally be stored at a similar temperature to that of the press room for a sufficient period of time to allow them to acclimatize. However, as soon as the sides of the reels are exposed, the moisture content at the reel edge will start to change, if the relative humidity in the works is not in equilibrium with the paper. If the surrounding air is relatively dry, the paper edge loses moisture and will tend to shrink, and therefore tighten. When running on a press in this situation, the edge of the paper web will be tighter than the middle, and also less able to resist tearing. Web tension and any slight damage on the edge could result in a web break, which of course means costly downtime.

Other problems found at low levels of relative humidity can include increased levels of airborne paper dust; ink drying problems, particularly with water based inks and loss of print definition. Creasing at the centre of the web can also occur if the dimensional change across the width of the web is significant, or the reel has not had sufficient time to acclimatize.

Correct reel conditioning and press room conditions are of paramount importance if these problems are to be avoided. Reels should be allowed sufficient time to stabilize to the environmental conditions in the press room. A level of relative humidity of between 50% and 60% rh is ideal for both reel conditioning areas and press rooms.