The present invention relates to a calendering method for web-like material according to the preamble of claim 1, for example, wherein the web is brought to a calender that comprises at least two rolls arranged in nip contact, at least one of which is a heated thermo roll, the web being calendered in the nip between the rolls.

The invention also relates to a calender for implementing the method.

When calendering paper and cardboard, the web is treated in the nip formed of two opposite rolls. In addition to the rolls, the nip can also be formed of other opposite members, such as the shoes of a shoe calender. Generally, calendering is carried out by a machine calender, a soft calender or multiroll calender, such as a super calender. All these calender types use heat, moisture, and the pressure exerted by the nip on the web to improve the polish and the smoothness of the paper. The nips are usually formed of both hard and soft rolls. The surface of the soft roll is made of paper or other suitable fibre material or, at present, more frequently, of polymeric materials developed for this purpose. The hard rolls are generally made of cast iron and they can be heated with oil, steam or in some other ways, fore example, by induction heating.

The purpose of calendering is to increase the smoothness and polish of the paper or cardboard, and to improve other characteristics of the printing surface. Before calendering, the paper is moistened to a sufficient level to accomplish a good calendering result. At present, the paper is calendered fairly dry; the moisture content in calendering is typically about 10 %. The wetter the paper to be calendered, the easier it is to calender, and the lower the nip pressure needed. When using low nip pressure, the rolls and many other parts of the calender can be made more compact. A problem with wet calendering is the post graining of the paper surface. The paper is calendered damper than normally by using a so-called braker- stack calender located in the drier part of the paper machine, whereby the moisture content is about 10-25%. To improve the smoothness of the surface, however, the dry web must be calendered one more time before rolling on, which is why braker- stack calendering is hardly ever used.

As much as half of the heat brought to the thermo rolls can evaporate from the mantle of the roll directly into the environment without participating in the calendering process. Typically, the thermal power brought to the thermo roll is 50- 70 kW/meter in the lateral direction. Heat loss can be reduced by the solution disclosed in the U. S. A. patent No. 5,613,307, wherein a thermo roll is partly surrounded by a heat-insulating jacket, which is placed at a small distance from the surface of the thermo roll. One disadvantage of the solution described above is its high price. In addition, the space between the thermo roll and the heat-insulating jacket must be cooled to prevent overheating of the roll, which decreases the efficiency.

The purpose of this invention is to provide a new kind of calendering method and a calender, which can be used to reduce the heat loss from the thermo roll.

The invention is based on the fact that evaporation of heat from the thermo roll is reduced by making the web pass along the surface of the thermo roll after and/or before the nip, so that it surrounds at least half of the circle of the thermo roll mantle surface.

More specifically the calendering method according to the invention is characterized by what is stated in the characterizing part of claim 1, for example.

Furthermore, the calender according to the invention is characterized by what is stated in the characterizing part of claim 5, for example.

The invention provides considerable advantages.

In the solution according to the invention, the web that is to be calendered is in contact with the surface of the thermo roll for a considerably longer period of time than in traditional calendars, and therefore, the web dries more effectively. At the same time, heat loss from the thermo roll into the environment shows a considerable reduction, which is why almost all the energy fed into the thermo roll can be utilized. Because of effective drying, the web can be calendered wetter than normal, decreasing the amount of nip pressure needed. As the thermo roll preheats the web

before calendering, the amount of nip pressure can also be reduced in applications, in which the web is brought to the nip along the surface of the thermo roll. As the moving web conveys heat away from the roll surface, the thermo roll does not become overheated. Furthermore, the solution according to the invention has a very simple structure, which is easy to implement.

In the following, the invention is described in detail with the aid of the appended drawing, which presents a diagrammatic side-view of one calender according to the invention.

In addition to the rolls used in conventional calenders, the term"roll"in connection with this invention also refers to other members that can be used to form the nip of the calender, such as the shoe or belt of a long nip calender.

The calender according to the invention comprises at least two rolls 1,2 arranged in nip contact, at least one of the rolls being a thermo roll 2 that can be heated. A web 3 to be calendered is brought to the calender and calendered in the nip between the rolls 1,2. In the drawing, the direction of motion of the web 3 is marked with an arrow.

The web 3 that is calendered in the nip between the rolls 1,2 is directed to travel on the mantle surface of the thermo roll 2 so that the web 3 partially surrounds the thermo roll. The web 3 is directed to travel against the mantle surface of the thermo roll 2 by using control members, such as a guide roll 4 attached to the body of the calender.

In order to effectively utilize the energy fed into the thermo roll 2 in the calendering process, the thermo roll 2 is preferably surrounded by the web 3 as perfectly as possible. Typically, the web 3 surrounds at least half, preferably 60-80 %, of the mantle surface circle of the thermo roll 2.

The invention also has embodiments that deviate from the above description. The direction of motion of the web can also be opposite to the embodiment in the figure, whereby the web travels on the surface of the thermo roll before reaching the nip. In

that case, the heat of the thermo roll preheats the web before calendering, reducing the amount of nip pressure needed. The web can also be directed to travel on the surface of the thermo roll both before and after the nip, whereby the thermo roll preheats the web before calendering and dries it after calendering.