Metal belt calendering of uncoated and coated paper, PaperCon 2010 Conference

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The effect of metal belt calendering on quality of uncoated and coated paper was studied in two pilot scale trials.The targets of these studies were to find out 1) is it possible to save fibres of uncoated paper with metal belt finalcalendering and 2) could the finishing process of coated paper be more efficient with metal belt pre-calenderingprior coating.

For uncoated fine paper, a metal belt final calender gives higher bulk and bending resistance values than a soft nipcalender, which makes it possible to reduce the grammage of the paper. When the grammage was decreased by 7 %(70 65 g/m²), the bending resistance of the paper was still on the same level as the soft nip calendered trial pointswith 70 g/m². Another way to take advantage of metal belt calendering is to increasethe filler content of the paper,when fibre amount is decreased. Then total grammage of the paper is the same. In general, the price of the filler issignificantly lower than the price of the fibres. The filler content could be increased by 5 %-units (24 29 %) withmetal belt calendered paper, and the bending resistance of paper was still higher than that produced with a soft nipcalender. Increasing the filler content also gives higher brightness and opacity values. Metal belt calendereduncoated paper also feels smoother than the traditional roughness measurement indicates. The feel of the paper’ssurface roughness was evaluated by trained panellists.

The results show that metal belt pre-calendering reduces the need for final calendering for double-coated fine paper(76 g/m²). Two calendering concepts were compared: 1) new metal belt pre-calendering + 4 soft nip finishing and2) conventional soft nip pre-calendering + 10 roll multinip finishing. The new concept with metal belt pre-calendering can reach the quality level of conventional 10 roll multinip calendered papers. The new concept basedon metal belt pre-calendering gave slightly better bulk in similar end product smoothness, but the bending resistancewas clearly higher. Another benefit of the new concept is a simpler and more efficient finishing lay-out.