The importance of the measurement of paper differential CD shrinkage

Abstract

During the drying stage of the papermaking process, paper undergoes dimensional changes caused by fiber dehydration, but exhibiting uneven width reduction magnitudes along the cross-machine direction. The degree of cross-direction (CD) shrinkage can be very different at the edges when compared to the center of the web. The highest shrinkage amplitude at the web edges can vary according to the operation conditions and the drying section configuration, as well. The difference in the degree of CD shrinkage gives rise to adverse effects on the finished CD paper sheet properties, particularly strength and hygroelastic properties, which can vary many percents from the edges to the center of the web. Extreme property values lead to different problems, such as sheet differential dimensional instability, which can limit paper quality and converting efficiency, especially in high speed machines (e.g., multi-color offset printing and office copier machines). The issues considered above make clear the advantages of measuring and carefully analyzing the differential CD shrinkage profile of finished paper. This work reviews the CD differential shrinkage profile development in the drying section, its impact on finished paper properties and presents an accurate and low cost measurement method based on image analysis techniques, supported by digital processing through the two-dimensional Fast Fourier Transform (2D-FFT). The experimental results show that image analysis applied to paper samples taken from the jumbo roll represents a reliable method for determining the differential CD shrinkage profile, and a practical tool for monitoring paper quality and evaluating its CD dimensional stability profile.