Wood containing paper pilot calendering : obtained results through supercalendering, soft nip calendering and extended nip concept

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 in the heated rolls. Calendering parameters are more complicated to set when the bulk of the substrate and consequently its thickness must be controlled. Thus, pilot plants may be very helpful to understand the phenomena thanks to their flexibility. 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 maybe 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. In this work, a wood containing substrate, pigmented on both sides through film transfer is considered. Calendering trials were performed on a supercalender, a soft nip calender and an extended nip concept under various temperature, specific pressure and dwell time conditions. Paper properties, such as bulk, smoothness, gloss and gloss heterogeneity, and the quality of the printed papers were scrutinised. Comparison of the 3 calendering processes shows the great impact of the soft roll coverage and the dwell time on the paper properties. The extended nip concept leaves bulky calendered papers with a remaining high gloss homogeneity, even at high calendering temperature (250 °C). Thickness reduction during calendering has been modelled with respect to the soft roll coverage on the substrate considered. Heat transfers which occur in the calender nip were also modelled in order to quantify the penetration of temperature into the thickness of the web.