Presses – Methods – With heating or cooling
Reexamination Certificate
1999-03-26
2001-05-15
Gerrity, Stephen F. (Department: 3721)
Presses
Methods
With heating or cooling
C100S330000, C100S331000, C100S334000
Reexamination Certificate
active
06230615
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
The present application claims priority under 35 U.S.C. §119 of German Patent Application No. 198 13 640.4, filed on Mar. 27, 1998, the disclosure of which is expressly incorporated by reference herein in its entirety.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a calender and process, in particular for treatment of a paper web, the calendar having a roll stack of at least three rolls, with at least one double-soft nip formed between two rolls having elastic surfaces.
2. Description of Background Information
A calender is known, for example, from European Patent Application EP 0 748 895 A2. Calendars of this type play mainly two roles in paper manufacture. The first role of the calender is to agglomerate the paper web. The second role is to produce specific surface characteristics, for example, the highest gloss possible and the highest smoothness possible.
The calender known from EP 0 748 895 A2 has, in a roll stack of 6 to 12 rolls, alternatingly a “hard” and a “soft” roll. The hard rolls are heated and have a very smooth surface. They are, as a rule, designed as steel or cast-iron rolls. The smooth surface of a hard roll “imprints” itself on the surface of the paper web, and gives the paper web the desired smoothness and, together with the warmth produced by heating, the desired gloss. The “soft” rollers have an elastic surface and serve primarily to agglomerate the paper web. Since the surface of the soft rolls is elastic, the soft rolls avoid crushing of the fibers of the paper web to a certain extent.
As a rule, it is desirable to smooth both sides of the paper web. Accordingly, in the known calender, both sides of the paper web must be passed over a hard roll with a smooth surface. For this, a “change nip”, which is formed by two soft rolls, is required. This change nip has the sole role of altering the sequence of hard and soft rolls (e.g., switching the sides of the web to which the hard roll and soft rolls are applied). The actual effect on the paper web in the change nip is generally considered slight or negligible.
A similar situation also results with other material webs which must be processed in a similar manner, such as a paper or cardboard web.
SUMMARY OF THE INVENTION
In view of the shortcomings of the prior art, an object of the invention is to improve the capacity for action on the material web, i.e., to reduce the number of rolls required to achieve the desired effects and processing.
According to a first aspect of the present invention, a calender for treatment of a material web includes a roll stack of three or more rolls. The roll stack has two or more rolls with an elastic surface forming a double-soft nip. One or more heaters heats the double-soft nip formed by the two rolls with an elastic surface. The double-soft roll nip delimited by the two rolls with an elastic surface, i.e., the two “soft” rolls, thus assumes an additional function beyond that of a “change nip”. In this double-soft nip, because of the elevated temperature, energy can be introduced into the material web such that additional agglomeration is possible. The nip is therefore referred to as a “double-soft nip”. Because of the elevated temperature, the double-soft nip can be used for processing where a change nip would be wasted for the processing of the material web. In a more advantageous case, it is possible in such an embodiment to eliminate one of the remaining nips such that, under certain circumstances, the structural height of the calender may be reduced. This significantly reduces costs.
In one embodiment of the invention, the roll stack has at least three rolls each with an elastic surface, forming at least two double-soft nips. The heater or heaters then heat at least one of the three rolls forming each of two double-soft nips. For example, the calender has at least two double-soft nips and each double-soft nip can have at least one heatable roll. Thus, it is possible to utilize the advantageous effects of the double-soft nip in the calender two or more times, as long as at least one of the soft rolls which form the double-soft nip is heated.
Optionally, at least one of the rolls with an elastic surface forming the double-soft nip has a smooth surface having an average roughness not greater than 0.5 &mgr;m Ra under operating conditions. A smooth surface of this kind is possible even though the surface is elastic. In this manner, it is possible not only to agglomerate the material web in the double-soft nip, but also to smooth the web at least on the side which contacts the soft roll with the smooth surface. The results are further improved if both rolls forming the double-soft nip have the smooth surface. In this case, it is possible to agglomerate the material web in the double-soft nip(s) and to smooth it on both sides. Under certain circumstances, superfluous roll gaps or nips in the calender can thereby be eliminated. The two-sidedness of the material web can also be significantly reduced.
Further optionally, each of the rolls with an elastic surface includes a rigid or hard core and a surface layer formed from an elastic material. In this manner, construction is simplified. The hard core supports the elastic surface layer. Moreover, by an appropriate selection of the thickness of the surface layer, some parameters of the double-soft nip can be influenced.
Still further optionally, the elastic material has a predetermined good heat conductivity, e.g., no less than 10 W/m·K. The thermal conductivity of the elastic material can be improved by a number of measures, for example, through interlayering of high thermal conductivity material with elastic material, or the dispersion of high conductivity material such as metal fiber or metal powder throughout the elastic material in a composite form. It is also possible to use a material which, by itself, has a predetermined good thermal conductivity. In this case, a higher temperature produced, for example, inside the roll, can penetrate to the surface with low losses. Of course, it is also possible to heat the surface directly from the outside.
When a surface layer is used, the surface layer may have a thickness less than approximately 4 mm. More advantageously, the surface layer has a thickness from approximately 0.02 to 2 mm. An appropriately thin layer provides good heat transport from the inside of the roll to the outside with an appropriately low thermal resistance, such that it is possible to obtain the necessary temperatures on the surface of the surface layer very quickly and with low losses. Moreover, a thin surface layer has additional advantages. For example, the thin surface layer enables fibers of the material web, in particular in the case of a paper web, to be pressed locally or superficially against the elastic surface. On the other hand, a roll having a very thin surface layer has almost the operating characteristics of a “hard” roll, i.e., the thinlayer roll yields, in operation, a surface form of the roll which corresponds, at least approximately, to the surface form of a hard roll. This is true in particular when two soft rolls oppose each other in the double-soft nip, since similar conditions are present on both sides of the nip or roll gap. The deformation of the elastic surface layer remains very slight, in many cases even imperceptible, with a thin layer and a material web located in the double-soft nip. Accordingly, it is possible to obtain virtually the same compressive tension conditions as in a roll gap or nip made of one soft and one hard roll or even (almost) made from two hard rolls.
The surface layer may be formed from a plastic material. Plastics are available in a great variety such that it is possible to select the suitable plastic for the specifications. The thinner the layer, the lower the modulus of elasticity can be.
Alternatively, the surface layer is formed from a paint film coating. Thus, it is possible to use a “hard” roll for the roll with an elastic surface layer, i.e., a roll core made of steel or cast iron,
Conrad Hans-Rolf
Zimmermann Lothar
Gerrity Stephen F.
Greenblum & Bernstein P.L.C.
Voith Sulzer Papiertechnik Patent GmbH
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