In an effort to raise the level of calendering, the only practical possibility has been to increase the number of calendering nips. This has led to a more complicated calender construction and more difficult control and threading of the paper web. Particularly, in reference to on-line machines, a solution must be found to contradictions arising from high running speed and full-speed web threading. Attempts have been made to solve these problems with various belt and shoe calenders for extending a calendering nip to enhance thereby the nip operation. For example, paper to be calendered with belt calenders is carried by means of an endless belt to a preliminary contact with a hot calender roll, thus enabling the creating of a steep temperature gradient favourable from the viewpoint of calendering. By means of the belt, the effective length of a nip is extended by virtue of the preliminary contact as well as for the reason that the belt material enables the use of polymers which are considerably softer than those used in roll coatings, without problems resulting from deformations related to heat. With a nip more extended than in a supercalender or soft calender, the press impulse applied to the paper can be increased without the pressure peak becoming excessively high and without reducing the bulk.
One solution for a belt calender has been disclosed earlier e.g. in Finnish patent publication No. 95061. A calender embodiment according to this cited publication is shown schematically in FIG. 1 of the drawings, depicting the prior art.
In the prior art calender shown in FIG. 1, a calendering nip N is formed between a heatable hard roll 1 and a calendering belt, preferably a metal belt 5, supported by an elastic surface roll 2. The metal belt 5 is an endless belt and could be e.g. steel in its material. The belt has been extended over the nip roll 2 and a reversing roll 4, the former being lined with an elastic coating 3. In this prior art calender, the calendering nip N is formed between the heatable hard roll 1 and said metal belt 5 supported by the calender roll 2 lined with an elastic coating. Such a solution is largely consistent with the nip of a soft calender, wherein, however, the metal belt 5 enables both sides of a paper W to be subjected to a substantially identical treatment and, thus, the glazing to occur simultaneously on either side of the paper W.