An Attempt to Rationalize the Roughness Change i nthe Soft Nip Calendering of Coated Papers

Calendering is a well-established process that has been used in paper manufacturing for many years. Despite this fact, there is no theoretical model in the literature for predicting the roughness of calendered paper as a function of sheet characteristics and operating conditions. Instead, the response of paper to calendering is determined largely through pilot or mill trials that are costly and time consuming. In this work, an attempt is made to rationalize the soft nip calendering process using a phenomenological model. Paper is approximated by a rough viscoelastic-plastic continuum as represented by the Feygin’s element. The coating layer compression is assumed to be elastic, while a Maxwell element is used to approximate the viscoelastic behaviour of roll cover material. Using this model, a parametric study is conducted to estimate the relative effect of various calendering parameters on the roughness of coated papers. The effect of base sheet rheological characteristics, coating layer thickness and elastic modulus, as well as soft cover properties on the sheet smoothness are examined and comparison is made with the published calendering data. It is found that the present model predicts reasonable trends and that estimated roughness values are of similar magnitude compared to the experimental results.