Prediction of Curl Behaviour

Abstract

Curl is unwanted dynamic behaviour that appears already in the manufacturing process and evolves through the end-use of paper or board due to moisture content changes and mechanical treatments. In this paper, the analytical and numerical approaches are used to reveal the sensitivity of the curl tendency to fundamental variables affecting deformation behaviour of paper. Good agreement between the measured curvatures induced into the sheet by the photocopying process and the simulated curvatures is achieved. Paper is treated as a multi-layered material, and finite element simulations are performed by using hygro-elasto-plastic and hygro-viscoelastic material models. Also analytical calculations are carried out to support the conclusions. The results show that the prediction and control of curl is not straightforward; curl depends not only on the fibre orientation structure but interacts in a complex way with the through-thickness dry solids content profiles during moisture content changes and external mechanical forces acting on paper. Despite the complexity of the phenomenon, the simplified computational approaches presented in this paper can be used for analysing and optimising the paper structure and process parameters to prevent detrimental curl.