3-D Web Deformations from Calendering in a Hard Nip, 1996 Finishing and Converting Conference Proceedings

M.F. Gratton

Paprican

The main purpose of calendering paper is to develop surface properties, but permanent changes in the dimensions of the paper also occur in this operation. The thickness reduction caused by calendering is accompanied by an increase of in-plane dimensions. We describe a method that is suitable for measuring cross-machine and machine direction dimension changes on sheets of newsprint calendered in a hard nip. We have found that single nip calendering to the same bulk reduction leads to smaller CD elongations as did multi-nip calendering. The nip to nip variation of the CD elongation coupled to the concomitant thickness reduction in multi-nip calendering has allowed us to demonstrate that early on in the calendering process, large thickness reductions are accompanied by large CD elongations, but in the lower more heavily loaded nips, only smaller reduction of bulk and in-plant elongation occurred. We have also monitored the recoil of paper in the z-direction after calendering. Although we have measured that the greater percentage of recoil takes place within the first hour following calendering, it continues for up to 24 hours. Experience in a mill allowed us to measure in-plane and z-direction sheet properties. With in-plane elongations and z-direction recoil, we were able to confirm the high propensity for in-plane dimensions expansion which lead to slack edges. Those findings should help design a strategy to minimize web instabilities in dry end operations.