Presses – Methods – With heating or cooling
Reexamination Certificate
1999-09-30
2002-07-16
Gerrity, Stephen F. (Department: 3721)
Presses
Methods
With heating or cooling
C100S306000, C100S311000, C100S313000, C100S153000, C100S161000
Reexamination Certificate
active
06418840
ABSTRACT:
FIELD OF THE INVENTION
The invention concerns a calendering method, in which the material web to be calendered, in particular a paper or board web, is passed through the calender, in which at least one calendering nip is formed by means of two rolls provided with resilient roll coatings, which rolls are loaded towards each other so that; owing to the resilient nature of the roll coatings, the nip between the rolls becomes an extended
The invention also concerns a calender that makes use of the calendering method, which calender comprises at least one calendering nip, which is formed by means of two rolls provided with resilient roll coatings, and through which calendering nip the material web to be calendered, in particular a paper or board web, has been passed, in which connection the rolls provided with resilient roll coatings are loaded towards each other so that, owing to the resilient nature of the roll coatings, the nip between the rolls becomes an extended nip.
BACKGROUND OF THE INVENTION
When it is desirable to improve the standard of calendering, with the present solutions, in actual fact, the only possibility is to increase the number of calendering nips. This results in a more complicated construction of the calender and in more difficult control and tail threading of the paper web. Especially in the case of on-line machines, it must be possible to solve the contradictions arising from high running speed and from threading at full speed. Attempts have been made to solve these problems by means of various belt and shoe calenders, by whose means the calendering nip is extended and, thus, the operation of the nip is made more efficient. For example, in belt calenders, which in themselves are relatively recent constructions, the paper to be calendered is passed by means of an endless belt into a preliminary contact with a hot calender roll, in which case it is possible to create a steep temperature gradient, which is favourable from the point of view of calendering. By means of the belt, the effective length of the nip is increased, owing to the preliminary contact and because, as the belt material, it is possible to use considerably softer polymers than in roll coatings without problems arising from deformations related to heat. With a nip longer than in a supercalender or soft calender, the press impulse applied to the paper can be increased so that the pressure peak does not become excessively high and that the bulk does not start decreasing.
One belt calender solution has been described in the prior art, for example, in the Finnish Patent No. 95,061. A calender embodiment in accordance with said publication is illustrated schematically in
FIG. 1A
in the drawing, which figure represents the prior art. Thus,
FIG. 1A
is a schematic illustration of a prior-art calender, in which the calendering nip N is formed between a heatable hard roll
1
and a calendering belt, in particular a metal belt
5
, supported by a roll
2
with resilient coating. The metal belt
5
is an endless belt, and its material can be, for example, steel. The belt is passed over a nip roll
2
provided with a resilient coating
3
and over a reversing roll
4
. As was already stated above, in this prior-art calender, the calendering nip N is, thus, formed between a heatable hard roll
1
and said metal belt
5
, which is supported by a calender roll
2
provided with a resilient coating. Such a solution is, in fact, quite extensively similar to a nip in a soft calender, in which, however, by means of the metal belt
5
, both faces of the paper W can be subjected to a substantially equal treatment and, thus, the glazing can be made to take place at both sides of the paper W at the same time.
Further,
FIG. 1B
illustrates a further development of the prior-art calender as shown in FIG.
1
A. In the illustration in
FIG. 1B
, the calender has been extended to be a calender with two nips, so that the calender comprises two heatable hard-faced calender rolls
1
A and
1
B, two calender rolls
2
A,
2
B provided with resilient roll coatings
3
A,
3
B, and an endless metal belt
5
. The endless metal belt
5
is passed over said rolls
2
A,
2
B with resilient coatings, and said rolls with resilient coatings form calendering nips N
1
, N
2
with the heatable hard-faced rolls
1
A,
1
B. More correctly, the calendering nips are formed, in each particular case, between the heatable hard-faced roll
1
A,
1
B and the metal belt
5
, which metal belt
5
is loaded by means of a corresponding roll
2
A,
2
B with a resilient coating against the heatable hard-faced roll
1
A,
1
B. As is shown in
FIG. 1B
, the paper web W is passed through the first nip N
1
, after which the web is spread and reversed by means of the take-out leading rolls
4
A,
4
B and guided into the second nip N
2
. The construction and the operation of the rolls
2
A,
2
B with resilient coatings are similar to the illustration in FIG.
1
A. The prior-art calenders as shown in
FIGS. 1A and 1B
are suitable for use with paper grades that do not require a long nip time to be glazed. Such paper grades are, for example, coated grades in which the glass transition temperature of the coating paste is low and in which, therefore, the glazing is rapid.
In view of on-line operation, a belt calender provides a significant advantage, among other things, in respect of the clear and linear running of the paper web, which again permits tail threading taking place at a high speed. In a belt calender supported by means of a glide shoe, the nip is formed between an endless belt and a steel roll. Owing to the glide shoe, the press zone becomes wider than in the belt calender described above. In a nip in a shoe calender, owing to the wider nip, the maximal pressure remains lower than in other present-day calenders, for which reason it is best suitable for paper grades in which retaining of the bulk has a high importance. As regards its construction, such a shoe calender is quite extensively similar to extended-nip presses, which have already been in use for a rather long time.
In respect of the prior art, as an example that represents shoe calendering, reference can be made, e.g., to the Published German Patent Application No. 43 44 165, wherein a smoothing method is described in which the fibrous web is passed in between two heatable faces which have been fitted at both sides of the web and which can be pressed against the web. The compression pressure can be regulated both in the running direction of the web and in the cross direction of the web in a way that has been chosen in advance. The prior art also includes the U.S. Pat. No. 5,163,364, which concerns a similar equipment provided with a glide shoe.
In respect of belt-supported calender concepts, reference is made additionally to the U.S. Pat. No. 4,596,633, in which a web finishing process is described, wherein the surface portions of the web to be finished are first moistened to a high degree of moisture (dry solids content 50% . . . 70%), and the web is then passed, on support of a belt, into a long finishing zone of low pressure, which zone comprises more than one roll nips. In said method, as the belt, prior-art paper machine fabrics are employed, such as felts, wires or polymer belts, while the surface treatment proper is applied exclusively to the side of the web placed facing the backup roll.
In surface treatment devices provided with a glide shoe, it can be considered that the, at least partly dragging, contact between the belt and the glide shoe is a problem, which contact applies quite a high strain both to the glide shoe and also to the belt. When a technology commonly employed in initial drying of a fibrous web has been applied, it has been realized that the quality of a belt that is well suitable for initial drying and that operates well in initial drying is inadequate in conditions of finishing of the web surface, in particular in respect of its resistance to the higher strains applied to the belt. Also, of course, the high local strains applied to the belt in web break situati
Gerrity Stephen F.
Metso Paper Inc.
Steinberg & Raskin, P.C.
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