Method for calendering a paper or cardboard web coated at both sides

To achieve desired treatment results at both sides of a material web to be calendered, the material web is initially guided through a first nip formed between yieldable elastic surfaces confronting the throughpassing material web. In the first nip there is predominantly applied a relatively high pressure to the material web. Then the material web is guided through a successively arranged second nip formed between practically non-yieldable hard surfaces confronting the throughpassing material web. In the second nip there is applied, apart from pressure, in particular heat to the material web. The treatment result achieved in the first nip, namely the smoothness and glaze values of the treated material web, are augmented at both sides of the material web to achieve desired treatment effects.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a new and improved method of calendering a 
material web, in particular, a paper or cardboard web preferably, although 
not necessarily coated at both sides or faces thereof. 
In its more specific aspects, there is disclosed a method of calendering, 
that is to say, smoothing and glazing a material web, in particular, a 
paper or cardboard web which is preferably, although not necessarily 
coated at both sides or faces within at least two treatment nips or gaps 
arranged in succession with respect to a predetermined direction of travel 
of the material web. This material web has imparted to both of its web 
sides or faces a desired smoothing effect or smoothness and a desired 
glaze or gloss. 
2. Discussion of the Background and Material Information 
The calendering or calender treatment usually constitutes the last process 
stage during the manufacture of paper or cardboard. For this purpose there 
are employed apparatuses such as, for example, so-called calender 
installations or supercalenders. They serve for the calender treatment of 
the paper or cardboard web at both sides or also even at one side thereof. 
There are available calender installations having so-called "hard" and 
"soft" treatment nips or gaps. In the case of calender installations 
containing the hard nips, the surfaces which bound or delimit both sides 
of the treatment nip are practically non-yieldably hard, and for those 
calender installations containing soft nips there is usually employed a 
yieldable elastic surface at least at one side of the treatment nip. The 
hard surfaces are frequently heated. 
The commercially available calender of the assignee of the present 
invention, sold under the trademark "NipcoMat", constitutes a calender 
having soft treatment nips. This calender can be provided with one or a 
number of treatment nips. The surfaces bounding the treatment nips are 
typically formed by rolls or cylinders having hard metal roll or cylinder 
surfaces or elastically yieldable coated roll or cylinder surfaces. 
There have also been proposed calender installations or calenders where the 
treatment nip is formed between two revolving bands or belts. Significant 
in this regard is the commonly assigned German Patent Publication No. 
3,920,204, published May 10, 1990. The dual-sided calendering of paper or 
cardboard coated at both sides or faces usually is accomplished in at 
least two successively arranged soft treatment nips, and each web side or 
face is processed at both hard and also elastically soft rolls. When 
operating with only two treatment nips, then, in the first treatment nip 
the one side or face of the material web comes into contact with a hard 
roll and the opposite side or face with a soft roll. In the second 
treatment nip, the web side or face which was previously processed by the 
hard roll now comes into contact with a soft roll, and conversely, the 
other web side or face, which was previously in contact with a soft roll, 
now comes into contact with a hard roll. A drawback which has been found 
to exist with this calendering method, particularly when processing 
high-glaze types of paper or cardboard, resides in the fact that the high 
degree of glaze or gloss which has been attained at the one web side or 
face due to web contact with the hot, hard surfaces of the heated rolls, 
is again diminished in the subsequent treatment nip when the web comes 
into contact with rolls having yieldable elastic surfaces. 
SUMMARY OF THE INVENTION 
Therefore, with the foregoing in mind, it is a primary object of the 
present invention to provide an improved method of calendering a material 
web, especially, although not exclusively, a paper or cardboard web which 
is coated at both sides or faces, which is not afflicted with the 
aforementioned drawbacks or limitations of the prior art. 
Another and more specific object of the present invention aims at the 
provision of a method of calendering a material web, especially a paper or 
cardboard web which is coated at both sides or faces, wherein at the 
finished product there is obtained an enhanced smoothing and as great as 
possible degree of glaze or gloss, and specifically, to the same extent at 
both sides or faces of the treated material web. 
Still a further noteworthy object of the present invention, and in keeping 
with the immediately preceding object, is the provision of a method of 
calendering a material web, especially a paper or cardboard web which is 
preferably, although need not always be, coated at both sides or faces, to 
achieve the aforenoted results at the finished product, while allowing the 
use of apparatus structure which enables optimizing the treatment method 
during operation of the apparatus. 
Now in order to implement these and still further objects of the present 
invention, which will become more readily apparent as the description 
proceeds, the web calendering method of the present development for the 
purpose of achieving the desired treatment effect at both sides of the 
material web as concerns the desired web smoothness and the desired web 
glaze or gloss is manifested, among other things, by the features of 
initially movably guiding the material web through a first nip formed 
between yieldable mutually adjustable elastic surfaces which confront the 
throughpassing material web, and thereafter movably guiding the material 
web through a successively arranged second nip formed between hard 
mutually adjustable surfaces which confront the throughpassing material 
web. In the first nip there is produced at both sides of the material web, 
through application of pressure, an increase in web smoothness and gloss, 
and in the second nip there is produced at both sides of the material web, 
through application of a lesser pressure than applied in the first nip and 
high thermal energy, a desired final value of web smoothness and gloss 
without any appreciable loss in the web smoothness and gloss achieved 
through treatment of the material web in the first nip. 
Through the use of two nips there is achieved in the aforedescribed manner, 
in the first "soft" nip a surface quality of the treated material web at 
both sides or faces thereof which already possesses appreciable values as 
concerns web smoothness and gloss, through the utilization of surfaces 
covered with a material having a comparatively high modulus of elasticity 
and through the use of a high treatment pressure. The dynamic modulus of 
elasticity of the elastic material in radial direction should 
advantageously possess a value amounting to between 2,000 and 10,000 
N/m.sup.2. In the second "hard" nip there are appreciably increased the 
smoothness value and gloss value of the treated material web which have 
been attained in the first nip and such smoothness and gloss values are 
beneficially retained at the finished product. In desired manner, the 
material web possesses at both sides thereof an equally high or great 
smoothness and an equally high or great glaze or gloss. 
Suitable materials for forming the cover layer for the surfaces of the 
first "soft" nip comprise, for instance, rubber, polyurethane, polyester 
and epoxy resins, which, if desired, can contain a suitable filler. Steel 
comprises a preferred material for forming the surfaces of the second 
"hard" nip. The pressure typically applied in the first "soft" nip is in 
the range of 5 N/mm.sup.2 to 50 N/mm.sup.2 and in the second "hard" nip in 
the range of 5 N/mm.sup.2 to 30 N/mm.sup.2. The temperature which prevails 
in the second "hard" nip lies in the order of about 100.degree. C. to 
350.degree.0 C. 
A further aspect of the invention contemplates providing in the first nip, 
as the yieldable mutually adjustable elastic surfaces, two rolls having 
elastic surfaces which are yieldable in a direction substantially 
perpendicular to the material web, and providing in the second nip, as t 
he hard mutually adjustable surfaces, two rolls having hard essentially 
non-yieldable surfaces. 
The diameters of the rolls in the first "soft" nip are less than about 
1,000 mm. and preferably below 700 mm. and the diameters of the rolls in 
the second "hard" nip are greater than 800 mm. and preferably greater than 
1,000 mm. 
Still further the material web in the first nip is advantageously processed 
between the elastic surfaces of the two rolls in the first nip which 
possess a high modulus of elasticity. Moreover, there can be provided as 
the two rolls in the second nip respective rolls of sufficiently large 
roll diameter for achieving a large heat transfer to the material web in 
the second nip. 
The calendering method of the present invention contemplates producing a 
predetermined linear or line pressure in the first and second nips by 
providing at least predetermined ones of the rolls of the first and second 
nips with pressure-regulatable support or pressure elements. Also, the 
material web in the second nip can be heated by the pressure-regulatable 
support or pressure elements provided for at least predetermined ones of 
the rolls of the second nip. 
Still further, there can be provided in the first nip, as the yieldable 
mutually adjustable elastic surfaces, two revolving belts having elastic 
surfaces which are yieldable in a direction substantially perpendicular to 
the material web. Furthermore, there can be adjusted the action of the two 
revolving belts upon the material web by support or pressure elements. The 
material web is desirably processed in the first nip between the two 
revolving belts whose elastic surfaces possess a high modulus of 
elasticity. 
Additionally, there can be provided in the second nip, as the hard mutually 
adjustable surfaces, two revolving steel belts. Moreover, there can be 
employed support or pressure elements for adjusting the action of the two 
revolving steel belts upon the material web. The material web can be 
heated in the second nip by the support or pressure elements. But it is 
also possible to inductively heat the revolving steel belts in the second 
nip in order to apply thermal energy to the material web in the second 
nip. 
A further aspect of the inventive web treatment method conceives 
asymmetrically heating the material web in order to achieve a reduction in 
the web smoothness and gloss at one of both sides of the material web in 
relation to the other side of the material web.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Describing now the drawings, it is to be understood that only enough of the 
construction of the various embodiments of calenders for the treatment or 
calendering of a material web have been depicted therein, in order to 
simplify the illustration, as needed for those skilled in the art to 
readily understand the underlying principles and concepts of the present 
invention. 
Turning attention first to the calender depicted in FIG. 1, it will be 
understood that a material web, and specifically, a paper or cardboard web 
1 which is to be treated at both web sides or faces A and B and here, for 
instance, coated at both such web sides A and B, is movingly guided 
through a first nip 2 at a suitable travel velocity. This first nip 2 is 
formed between two rotatable rolls or cylinders 4 and 6 possessing 
yieldable elastic cylindrical roll surfaces 5 and 7, respectively. These 
yieldable elastic cylindrical roll surfaces 5 and 7 are constituted by the 
surfaces of a respective elastic yieldable material layer or covering 5a 
and 7a which possess a high modulus of elasticity in order to be able to 
work in the first nip 2 with a correspondingly high surface compression, 
and materials suitable for this purpose have been heretofore given. For 
the same reason, it is advantageous if the diameters of the rolls or 
cylinders 4 and 6 are chosen to be as small as possible, and typical 
diameter values have been heretofore discussed. The yieldable cylindrical 
roll surfaces 5 and 7 are advantageously adjustable, by means of any 
suitable conventional roll adjustment or positioning means, in a direction 
substantially perpendicular to the throughpassing material web 1, in order 
to produce the high pressure in the first nip 2 in the order of about 5 
N/mm.sup.2 to 50 N/mm.sub.2 which acts upon the throughpassing material 
web 1. 
At least one of the rolls or cylinders 4 and 6 can be provided with 
conventional pressure-regulatable support or pressure elements 12, for 
instance, those pressure elements sold under the trademark "NIPCO" in 
order generate a desired linear or line force profile in the cross-machine 
direction. Suitable types of pressure-regulatable support or pressure 
elements have been disclosed in, for example, U.S. Pat. No. 3,802,044 and 
the cognate German Patent Publication No. 2,230,139, published Jan. 25, 
1973 and equally in U.S. Pat. No. 3,885,283 and the cognate German Patent 
No. 2,254,392, published May 9, 1974, to which reference may be readily 
had and the disclosures of which are incorporated herein in its entirety 
by reference. 
One of the main advantages of the first "soft" nip 2 resides in the 
essentially uniform compaction or compression of the treated material web 
1 between the yieldable elastic covering layers or covers 5a and 7a of the 
rolls 4 and 6, respectively. As a result thereof, there do not arise any 
excessive compression of the material web 1 at locations thereof having 
higher surface weight. 
The material web 1 treated in the first nip 2 is movably guided, with the 
same travel velocity with which it previously moved through this first nip 
2, through a further or second nip 3 arranged in succession after or 
downstream of the first nip 2 with respect to the predetermined direction 
of travel of the material web 1. This second nip 3 is formed between two 
rotatable rolls or cylinders 8 and 10 which are formed, for instance, from 
a hard steel casting and possessing the respective hard cylindrical 
surfaces 9 and 11. In this second nip 3 heat and pressure are applied to 
both sides A and B of the material web 1. As heretofore explained, the 
temperature prevailing in the second nip is in the order of 100.degree. C. 
to 350.degree. C. and the pressure lies between about 5 N/mm.sup.2 to 30 
N/mm.sup.2. In particular, for augmenting the web glaze or gloss, as large 
as possible quantity of heat or thermal energy should be applied from the 
rolls or cylinders 8 and 10 to the material web 1. To that end, the heat 
transfer surfaces at the rolls or cylinders 8 and 10 are maintained as 
large as possible by selecting as large as possible diameter of each of 
the rolls 8 and 10, as previously discussed. At least one of these rolls 8 
and 10 can be likewise equipped with pressure elements 12, for instance, 
those pressure elements sold under the trademark "NIPCO". The other roll 
or roller could be, for instance, constituted by a conventional 
regulatable thermo-roll. These rolls or cylinders 8 and 10 are thus 
regulatably heatable. For this purpose, there also could be conventionally 
employed the "NIPCO"-type support or pressure elements 12 through which 
there then can be regulatably supplied a suitable heat-carrying 
pressurized fluid medium, such as oil, as is well known in this 
technology. 
The regulatability of the heating of the rolls or cylinders 8 and 10 
affords the possibility of accommodation of the web treatment method for 
achieving desired web treatment results. For instance, by asymmetrically 
heating opposites sides or faces A and B of the material web 1 there can 
be attained a reduction in the smoothing and glaze at the two sides A and 
B of the material web i in relation to one another. 
The second stage of web calendering in the second nip 3 results in an 
increase in the quality of the material web 1 at the two sides A and B 
which has been previously achieved in the upstream situated first nip 2. 
Following such further web treatment there can be ascertained increased 
values of the smoothness and glaze of the thus processed material web 1 in 
contrast to the smoothness and glaze values of the material web 1 attained 
in the first nip 2, and moreover, these increased web smoothness and glaze 
values are retained in the final or end product. The regulatability of the 
heating of the rolls or cylinders 8 and 10 also affords correction 
possibilities over the web width by adjusting the heating action in zones 
in the cross-machine direction. 
Within the second nip 3 the pressing pressure or compression applied to the 
material web 1 should be maintained as small as possible, in order to 
preclude local compression or squeezing of the material web 1. However, 
there should be applied a relatively high temperature. It is for this 
reason that, as already mentioned, there also should be provided a large 
heat transfer surface in the second nip 3. Large roll surfaces enhance, 
apart from the web smoothness, also the web glaze or gloss. 
It is here to be remarked that the invention further contemplates a 
calender construction where there are successively arranged more than one 
nip of each of the nip types constituted by the aforedescribed treatment 
nips 2 and 3. In any event, there should be first arranged the "soft" nip, 
such as the heretofore described first nip 2, followed by the "hard" nip, 
such as the heretofore described second nip 3 
It is also contemplated that, when necessary or desired, to advantageously 
arrange along the treatment path or predetermined direction of travel of 
the material web 1 suitable known means, for instance, for moistening or 
heating the material web 1 upstream or the treatment nips or for 
ventilating, cooling or suctionally removing the vapors after a treatment 
nip, which further enhance the attainment of still greater web smoothness 
and glaze values. 
Based upon the showing of FIG. 2 there will be considered a second 
embodiment of calender or calender installation suitable for the practice 
of the inventive method. Here, the surfaces delimiting or bounding the 
relevant first nip 2 and second nip 3 are constituted by revolving endless 
belts or bands 13, 14 and 15, 16, respectively, which are guided through 
the associated treatment nip 2 and 3. The material web 1 is movingly 
guided through the first nip 2 between surfaces of the belts or bands 13 
and 14 which revolve during operation of the calender and which are guided 
through the first nip 2. These revolving belts or bands -3 and 14 are 
elastically yieldable at least at the sides thereof confronting the 
throughpassing material web 1. Due to the elastic yieldability of the 
revolving belts 13 and 14 there are extensively avoided the formation of 
localized over-compression of the material web 1. Both of the revolving 
endless belts 13 and 14 can be similarly constructed and appropriately 
driven to revolve at the same speed. In this first nip 2 there is 
predominantly applied pressure to the throughpassing material web 1. This 
pressure is advantageously produced by means of pressure-regulatable 
support or pressure elements 12, as previously considered. 
After departure from the first nip 2 the material web 1 is subsequently 
movingly guided through the next following second nip 3 which is 
successively arranged with respect to the predetermined direction of 
travel of the material web 1. In this second nip 3 the material web 1 is 
movingly guided between the endless steel belts or bands 15 and 16, which 
revolve during operation of the calender and move through the second nip 
3, and possess the confronting hard belt surfaces which contact the 
throughpassing material web 1. The travel velocity of the material web 
within the second nip 3, dictated by the circumferential velocity of the 
revolving steel belts or bands 15 and 16, is the same as the travel 
velocity of the material web 1 passing through the upstream arranged first 
nip 2. Heat and pressure are applied to the material web 1 within the 
second nip 3. The pressure is exerted by means of the regulatable support 
or pressure elements 12, the forces of which are applied to the revolving 
belts or bands 15 and 16. Also, here, there are advantageously employed 
pressure elements, for instance, those pressure elements sold under the 
trademark "NIPCO". Once again, the heating of the revolving belts or bands 
15 and 16 can be achieved with these support or pressure elements 12 
which, for this purpose, are supplied with a suitable heat-carrying fluid 
medium, oil for instance. However, the heating of the revolving belts 15 
and 16, and thus, the throughpassing material web 1 in contact therewith, 
can be accomplished in a different manner, for instance there can be used 
suitable inductive heaters 100. Also the second nip 3 can be extended in 
length in the direction of travel of the material web 1. 
Through the provision of a "long" or "extended" second nip 3 there results 
an increased residence time of the material web 1 when travelling through 
the second nip 3. As a result, it is possible to work with a reduced 
pressure in the second nip 3 in comparison to the pressure employed in the 
first nip 2, so that there is further reduced the risk of localized 
over-compression of the material web 1 within this second nip 3. The 
selection of a lower web pressing or compression pressure is also possible 
because the heat transfer surfaces afford an advantageous transfer of 
thermal energy to the throughpassing material web 1, so that there is 
enhanced the web smoothness and especially the formation of web glaze or 
gloss. Within the second nip 3 there does not arise an loss in the glaze 
of the treated material web 1. Here also, by asymmetrically heating the 
revolving belts or bands 15 and 16 there can be achieved a different 
treatment of the opposite sides or faces A and B of the material web 1. 
The revolving endless belts 13 and 14 forming the first nip 2 can be formed 
of materials like those employed for the rolls or cylinders 4 and 6 of the 
embodiment of FIG. 1. 
Finally, in FIG. 3 there is depicted a third embodiment of calender or 
calender installation for the practice of the inventive web calendering 
method. It can be advantageous to form the first nip 2 between two 
yieldable elastically coated or covered rolls or cylinders 4 and 6, as 
previously described for the first embodiment of FIG. 1, and to form the 
second nip 3 between two revolving endless belts or bands 15 and 16 each 
possessing a hard, practically non-yieldable surface, as such has been 
described in conjunction with the second embodiment of calender depicted 
in FIG. 2. 
However, it is here mentioned that still a further variant construction of 
calender is possible likewise constituting a combination of the calenders 
considered heretofore in conjunction with FIGS. 1 and 2. Specifically, the 
first nip 2 could be formed, in the manner as shown in FIG. 2, between 
elastic yieldably coated or covered belts or bands, and the second nip 3 
then, as depicted in FIG. 1, would be formed between two heated hard rolls 
or cylinders of larger diameter. Since this modified embodiment can be 
easily conceived from the explanations given and the illustrations of the 
calenders shown in the drawings, it has not been specifically depicted 
herein. 
For certain technological fields of application, it can be advantageous to 
also heat to a modest temperature with conventional heating means the 
elastic yieldable rolls and/or the coverings or cover layers of the 
revolving endless bands or belts. 
By way of completeness, it is noted that for the embodiments of FIGS. 2 and 
3, the operating conditions prevailing in the first "soft" nip and second 
"hard" nip are like those given for the embodiment of FIG. 1. 
The inventive method is not solely limited to the calendering of 
double-sided coated paper or cardboard webs. It can be employed to 
advantage, with the realization of good results, also for calendering 
uncoated material webs and material webs which have only been coated at 
one side or face thereof. 
While there are shown and described present preferred embodiments of the 
invention, it is distinctly to be understood the invention is not limited 
thereto, but may be otherwise variously embodied and practiced within the 
scope of the following claims.