Single Nip Pilot Calendering Using Three Different Soft Covers, 2000 Coating Conference Proceedings

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J.-P. Maume, D. Chaussy, S. Moreau-Tabiche, D. Guerin, P. Piette, V. Morin

Calendering remains a critical step in paper and board production. It may be carried out off-line on a supercalender or on-line on a soft calender depending on the type of paper. Usually, the smoothness and gloss values required are obtained through either pressure control in the nips or temperature adjustment of the heated rolls. Calendering parameters are more complicated to set when the bulk of the substrate and consequently its thickness must be controlled. A soft calender has been installed on-line on the CTP pilot coater. Linear loads from 50 to 525 kN/m, and temperatures from 20 to 270°C may be reached. In addition, a special unit has been installed on the first nip of the soft calender in order to study the extended nip concept through an endless belt. In this work, a wood containing substrate, pigmented on both sides through film transfer is considered. Calendering trials were performed at various temperatures, specific pressures and dwell times. Three different kinds of soft covers were tested: a cotton filled roll, a polymeric soft roll and an endless polymeric belt with very low hardness. Paper properties, such as bulk, smoothness, gloss and gloss heterogeneity, and the quality of the printed papers were scrutinised. Comparison of the 3 nip configurations shows the great impact of the soft roll cover and the dwell time on the paper properties. The belt nip leaves bulky calendered papers with a remaining high gloss homogeneity, even at high calendering temperature (250°C). A model is proposed to describe thickness reduction during calendering and smoothness as functions of pressure and dwell time.