Strain Release
Dimensional movement occurring when dried-in stresses are liberated from a paper sheet represents a fundamental cause of substrate distortion during processing. This phenomenon, called orientation strain release, happens when moisture or heat softens the fiber network, allowing the frozen-in tensile forces to relax. The sheet will shrink in the direction where the fibers were stretched most during the drying process.
Physical Mechanics
Papermaking fibers undergo tension during the press and dryer sections of the paper machine as the wet web is pulled through the system. This stretching dries a state of elastic strain into the paper structure. When the board later encounters moisture in a printing press or a laminator, the hydrogen bonds between the fibers soften and release this stored energy.
The resulting shrinkage or expansion can cause curl, warp, or misregistration. Monitoring the tension profiles across the paper machine helps minimize the severity of these changes.
Press Stability
Printing operations require a flat and stable sheet to ensure accurate multi-pass registration. Severe orientation strain release on a high-speed press leads to feeding delays or sheet wrinkling. Maintaining uniform moisture levels throughout the storage and printing environment stabilizes the board before and during the run.
