On- and off-line measurement of paper shrinkage

Shrinkage is reputed to be one of the major causes of paper\´s dimensional instability. The removal of water from the paper web during the "pressing" and the "drying" stages of production gives rise to a differential shrinkage profile in the cross-machine (CD) direction. Forming fabrics, dandy rolls and felts leave regular marks on the wet web. Shrinkage can therefore be measured as the percentage difference in periodicity between the shrunken and the original patterns. This paper presents four algorithms which have the goal of measuring the pattern repetition frequency (PRF) of the above mentioned paper marks. The paper signal, which is their input, is collected by a charge-coupled device (CCD) line scan camera, which scans the paper in the cross-machine direction. The first three algorithms model the paper signal as a stationary process. The fourth algorithm, instead, assumes the paper signal to be nonstationary. Each measurement method has its advantages and its drawbacks, so that a combination of two or more provides the operator with more reliable estimates of shrinkage.