The effect of machine conditions and furnish properties on paper CD shrinkage profile

All conventional paper machines exhibit a profile of crossmachinedirection (CD) shrinkage which is developed during thedrying stage of production. This paper suggests a new approachto the understanding of these profiles by suggesting that CDshrinkage at a point in the paper depends on the distance of thatpoint from both edges of the sheet and also on the length of the unsupported draws in the dryer section. The nature of the function is unimportant as long as it can be made to fit data for one combination of machine width and effective draw length; substituting other values will then change the shape appropriately. In this paper, a simple exponential decay of shrinkage with distance from the edge is used successfully.This approach is initially demonstrated for laboratory results from the literature, where it successfully predicts changes produced by varying the length, width and by reversing the MD/CD orientation of samples. It is then shown to be consistent for changes of paper machine furnish, press section draw and sheet grammage produced in a series of trials on M-real, New Thames PM6. It is finally shown that theses ideas explain the effect on CD shrinkage results from the literature for splitting the sheet at the press section of a newsprint machine and for reduction of dryer section restraint by deactivation of the blow-boxes.