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Calender provided with an extended nip
The invention concerns a calender provided with an extended nip for
calendering of
a paper or board web, which calender comprises at least one calendering nip,
which
is formed between a heatable hard roll, on one hand, and a calendering belt
which
has been formed as an endless loop and which is loaded against the heatable
hard
roll by means of a backup roll or a press shoe, on the other hand, the paper
or board
web to be calendered being arranged to run through said calendering nip.
When it has been desirable to raise the level of calendering, with the present-
day
solutions, .practically the only possibility has been to increase the number
of calen-
dering nips. This results in a more complex construction of the calender and
in more
difficult control and threading of the paper web. In particular in the case of
on-line
machines, it must be possible to solve the contradictions arising from high
running
speed and full-speed web threading. Attempts have been made to solve these
problems by means of various belt and shoe calenders, by whose means the calen-
dering nip is extended and, thereby, the operation of the nip is enhanced. In
shoe
calenders, as a rule, the technique very well known from extended-nip presses
is
employed, according to which the extended nip is produced so that the paper
web is
pressed against the backup roll by means of a shaped glide shoe and an endless
belt
running over the shoe. In a belt calender, by means of the belt, attempts are
made
to provide the necessary resilience in the calendering nip when the paper web
is
pressed between the rolls that form the calendering nip. Belt calenders in
themselves
are relatively new constructions, and they can also be formed so that the
paper to be
calendered is passed, by means of the endless belt, into a preliminary contact
with
a hot calender roll, in which case it is possible to provide a steep
temperature
gradient, which is favourable from the point of view of calendering. In such a
case,
by means of the belt, the effective length of the nip is increased because of
the
preliminary contact and because, as the belt material, it is possible to
employ
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considerably softer polymers than in roll coatings without problems arising
from
deformations related to heat. With a nip more extended than in a supercalender
or
soft calendar, the press impulse applied to the paper can be increased so that
the
pressure peak does not become excessively high and that the bulk is not
reduced.
With respect to the prior art related to belt calendars, reference is made,
for
example, to the Finnish Patent No. 95, 061 and to the equivalent US Patent No.
S, 483, 873.
The belt and shoe calendars that have been used so far have involved quite a
signi-
ficant problem, which has arisen from the structure of the belt that has been
used.
The belts that have been used have been almost completely non-compressible,
and,
as a rule, their structure has been such that there is a support fabric in the
middle of
the belts, and a polyurethane layer at both sides of the support fabric. Even
though
the belt in itself may have been elastic, its volume has not been
compressible. Such
a belt of substantially non-corr<pressible volume behaves in a calendeting
nip, in
which it is pressed with a high pressure to make it thinner, so that the
material of
the belt has to flow parallel to the face of the belt away from the nip. This
has,
among other things, the effect that a wave is formed ahead of the nip out of
the belt
material. Owing to the non-compressibility, in the belt material, forces
parallel to the
belt face are formed, which promote a collapse of the fibrous strucitrre in
the paper,
i.e. a reduction in the thickness of the paper. In a nip, reversible and non-
reversible
compression of paper always take place. Shear forces in the nip increase the
amount
of non-reversible compression, which has a detrimental effect on the bulk of
the
paper. Further, a shear force that changes its direction abruptly in the nip
tends to
vibrate the paper, which mechanism can have quite s considerable effect on the
calendered density of the paper. Thus, employment of a non-compressible belt
makes
the fibrous structure of the paper collapse and increases the strains applied
to the belt
face, the wear and the fatigue.
The present invention is directed to the provision of an essential improvement
over
the existing calendars provided with extended tips.
In accordance with one aspect of the present invention, there is provided a
calendar
provided with an extended nip for calendering of a paper or board web, which
calendar comprises at least one calendering nip which is formed between a
heatable hard roll, on one hand, and a calendering belt which has been formed
as
an endless loop and which is loaded against the heatable hard roll by means of
a
backup roll or a press shoe, on the other hand, the paper or board web to be
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calendered being arranged'to run through said calendering nip,~~whereby,
through
the nip, at the side of the backup roll or of the press shoe, respectively, in
relation
to the web, a belt has been passed which is, in respect of at least a part of
its
thickness, made of a material of compressible volume, wherein the belt of a
~ material of compressible volume is either the calendering belt provided with
a
mufti-layer structure, which comprises a support layer of compressible volume
and, at least on its face to be placed against the material web to be
calendered, a
tight polymer layer or a support belt formed as an endless loop and running at
the
side facing the backup-roll in relation to the calendaring belt.
By means of the invention, a significant advantage is obtained over the prior
art, and
of the advantages obtainable by means of the invention, in this connection;
among
other things, the following can be stated. Thus, it is an essential feature of
a first
embodiment of the invention that the belt that is used in the calender is made
of a
material of substantially compressible volume. Similarly, it is an essential
feature of
a second embodiment of the invention that, in belt calendering, through the,
calender-
ing nip, a calendering belt, favourably a conventional polyurethane belt, is
passed,
and additionally a separate, substantially fully compressible support belt is
passed,
which performs the major part of the compression work in the nip. In studies
that
have been carried out a~ in test runs, it has been noticed that, by means of
such a
belt of compressible volume, and, similarly, by means of a solution in which,
in
addition to a polyuret3tane belt, a support belt of compressible volume is
used, the
forces parallel to the face of the paper andlor belt can be eliminated from
the nip
almost completely. Owing to' the compressibility of the calendering belt or of
the
support belt placed underneath the calendering belt, the extent of non-
reversible
compression of the paper in the nip is reduced essentially, in which case the
thick-
ness and the bulk of the paper can be maintained as well as possible. Further,
it is
to be considered a significant additional advantage that the so-called
"toleration of
fibre string" is substantially beuer than in earlier solutions, because, owing
to the
compressibility of the calendering belt or of the support belt placed bath the
calendering belt, respectively, fibre strings can pass through the nip readily
without
causing a shear load. From the point of view of manufacturing technology, a
multi-
layer belt of the novel type is considerably easier to manufacture than the
earlier belt
provided with a support fabric, and, for example, the belt can be, provided
with
uniform thickness more easily than in the prior art.
Similarly, a solution with two belts can be accomplished very easily, because,
in the
manufacture, it is possible to employ.conventional prior-art techniques, in
particular
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in the case that a polymer belt is used as the calendering belt and the
support belt is
made of a felt material. As the support belt, it is also possible to use other
com-
pressibie materials, for example, cellular rubbers, cellular plastics, and
equivalent.
Uniform thickness can be achieved easily in a unit composed of a calendering
belt
and of a support belt in particular made of a felt material. On the other
hand, it can
also be stated as an advantage that, owing to the compressibility of the belt,
the
calendering process is not so sensitive to little variations in thickness of
the belt as
it was earlier. The further advantages and characteristic features of the
invention will
come out from the following detailed description of the invention.
In the following, the invention will be described by way of example with
reference
to the figures in the accompanying drawing. _
Figure 1 is a fully schematic side view of a single-nip calender in which a
calender-
ing belt is employed.
Figure 2 shows an enlarged detail from the area of the calendering nip in a
calender
as shown in Fig. 1.
Figure 3 is an illustration corresponding to Fig. 1 of a shoe calender.
Figure 4 is an enlarged detail from the nip area in a calender as shown in
Fig. 3.
Figure 5 is a schematic side view of a belt calender in which, in accordance
with the
second embodiment of the invention, a combination consisting of a calendering
belt
and a support belt is employed.
Figure 6 shows an enlarged detail from the area of the calendering nip in a
calender
as shown in Fig. 5.
Thus, Figs. 1 and 2 illustrate a belt calender, in which the calendering nip N
is
formed between a hot calender roll 1 and a backup roll 3. Further, through the
nip
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N, a calendering belt 10 has been passed to provide the nip N with the
necessary
elasticity so that the paper web W runs through the nip N between the
calendering
belt 10 and the face of the hot roll 1. Fig. 2 is a more detailed illustration
of the
conduct of the calendering belt 10 in the nip, whose length is denoted with
the
5 reference a. Thus, in the case of a belt calender, the calendering belt 10
consists of
a support layer 12 of compressible volume and of a tight polymer layer 11
fitted on
one face of said support layer 12. As is shown in Fig. 2, the polymer layer 11
is
placed on the face of the support layer 12 placed facing the paper web W. The
support layer 12 itself is placed against the backup roll 3. Thus, it is an
essential
feature of the belt 10 that the structure of the support layer 12 is such that
it can be
compressed in the nip N, i.e. its volume is reduced, in which case no wave of
the
sort formed in prior-art solutions can be formed ahead of the nip. The support
layer
12 of compressible volume is favourably a felt, even though other materials
can also
be employed in the support layer 12. Of these other materials can be
mentioned, for
example, cellular rubbers, cellular plastics, and equivalent which are
compressible.
The material of the tight polymer layer to be placed against the paper is, for
example, thermosetting or thermoplastic resin, favourably polyurethane.
Figs. 3 and 4 illustrate a shoe calender, in which the extended calendering
nip N is
formed between a hot hard roll 1 and a shoe roll 2. The shoe roll 2 again
comprises
a press shoe 24 supported by a stationary beam 25 as well as a calendering
belt 20,
which has been passed around the press shoe 24 and the beam 25 and which has
been formed as an endless loop. By means of the press shoe 24, the necessary
load
is produced in the nip N. As is illustrated in Fig. 4, in this embodiment the
calen-
dering belt 20 consists of three layers fitted one above the other, i.e. of a
support
layer 22 of compressible volume placed in the middle and of tight polymer
layers
21, 23 placed at both sides of the support layer. The materials of the support
layer
22 and of the polymer layers 21, 23 can be the same as in the embodiment shown
in
Figs. 1 and 2, so that the polymer layers are preferably of polyurethane, and
felt,
cellular rubber, cellular plastic, or equivalent is employed as the support
layer 22 of
compressible volume. In the embodiment of Figs. 3 and 4, a tight polymer layer
23
is also required at the side of the press shoe 24, because it must be possible
to make
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the friction between the press shoe 24 and the calendering belt 20
sufficiently low.
The mode of reduction of friction that is known best and that is employed most
commonly is producing an oil film between the press shoe and the calendering
belt
20. Thus, the face of the calendering belt 20 that is placed against the press
shoe 24
must be tight, in which case a polymer layer 23 is necessary. A similar belt
can, of
course, also be employed in connection with the embodiment shown in Figs. 1
and
2. As has been illustrated by means of Figs. 2 and 4, the support layer 12; 22
can
be compressed over the length a of the nip N without causing any wave ahead of
the
niP.
Thus, Figs. 5 and 6 illustrate a belt calender, in which the calendering nip N
is
formed between a hot calender roll la and a backup roll 3a. Through the nip N,
a
calendering belt 30 as well as a support belt 40 of a material of compressible
volume
have been passed so as to provide the nip N with the necessary resilience, so
that the
web W runs through the nip N between the calendering belt 30 and the face of
the
hot roll la. Both the calendering belt 30 and the support belt 40 have been
formed
as endless loops by means of alignment and tensioning rolls. The calendering
belt 30
and the support belt 40 are passed into the nip N between the calendering
rolls la
and 3a by means of a first alignment roll 4, and similarly they are passed out
of the
nip N by means of a second alignment roll 5, said first and second alignment
roll 4
and 5 being, thus, common of the calendering belt 30 and of the support belt
40.
Thus, the calendering belt 30 and the support belt 40 run from the first
alignment
roll 4 through the nip N to the second alignment roll 5 as a joint run and
while fitted
one above the other. After the second alignment roll 5, the calendering belt
30 and
the support belt 40 are, however, separated from one another so that they form
separate loops of their own.
From the second alignment roll 5, the support belt 40 is, thus, guided onto
the
alignment roll 6 of the support belt, from it onto the support belt tensioning
roll 7,
and from it further onto the first alignment roll 4, which is common of the
support
belt and the calendering belt, as was already described above. Similarly, the
calendering belt 30 is guided from the second alignment roll 5 onto the belt
align-
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ment roll 8, from it further onto the belt tensioning roll 9, from which the
calender-
ing belt 30 is passed, in the case shown in Fig. 5, onto the support belt 40
at the
support belt tensioning roll 7. In this respect, the arrangement can, however,
also be
such that the calendering belt 30 is passed from the belt tensioning roll 9
directly
onto the first alignment roll 4 common of the calendering belt 30 and of the
support
belt 40. For each loop, i.e. for the calendering belt loop 30 and for the
support belt
loop 40, alignment rolls 6,8 and tensioning rolls 7,9 of their own are needed
in
order that the tension of the calendering belt 30 and of the support belt 40
could be
regulated appropriately. A solution with two belts is in itself known, for
example,
from presses, but there the paper web is always placed against a porous belt
for the
purpose of removal of water, and the smooth support belt is placed underneath
the
porous belt.
Fig. 6 illustrates the conduct of the calendering belt 30 and of the support
belt 40 in
the nip N in more detail, the length of the nip being denoted with the
reference a.
Thus, through the nip N, the non-compressible calendering belt, favourably a
polymer belt 30, and the support belt 40 of compressible volume, favourably a
felt
or a support belt of a similarly compressible material, are passed. In the way
shown
in Fig. 6, the polymer belt 30 has been arranged in the nip N at the side of
the
healable calendering roll la, i.e. at the side of the web W to be calendered.
The
material of the polymer belt 30 is favourably polyurethane. The structure of
the
calendering belt 30 is such that it is provided with a thin fibre
reinforcement and
with a polyurethane layer at least at the side placed facing the healable
calendering
roll la. The structure of the belt can also be such that the fibre
reinforcement is
placed, in relation to the thickness of the calendering belt 30, in the middle
of the
belt, and the smooth polyurethane layers are placed at each side of the
calendering
belt. Further, in the way shown in Fig. 6, the support belt that runs through
the nip
N, favourably a felt 40 or equivalent, is placed in the nip against the backup
roll 3a.
Thus, it is an essential feature of the invention that the belt 40 or
equivalent can be
compressed in the nip N, in other words, its volume is reduced, in which case
a
wave corresponding to that formed in the prior-art solutions cannot be formed
ahead
of the nip N. As was stated earlier, in stead of a felt material, it is also
possible to
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use other materials, e.g. cellular rubbers, cellular plastics, and equivalent
which are
compressible.
In the illustration in Figs. 5 and 6, the web W to be calendered is introduced
into the
nip N so that it does not enter into contact with the calendering belt 30
until in the
nip N, and similarly the web W is taken out from the nip N so that it is
separated
from the calendering belt 30 directly. The web W can also be passed into the
nip so
that it is brought onto the calendering belt 30 before the nip N, and
similarly, after
the nip, the web is transferred on the calendering nip 30, from which it is
taken to
further processing. Thus, the running of the web W can be accomplished as a
what
is called closed draw.
With a calendering belt of compressible volume in accordance with the
invention,
and similarly in an arrangement in which a tight calendering belt, in
particular a
polymer belt, and a support belt of compressible volume, such as a felt or
equival-
ent, are employed, by means of a calender a thicker and stiffer paper or board
can
be calendered so that the gloss and smoothness of the calendered web are at an
equally high level as with a non-compressible calendering belt alone. Also, it
can be
established that the runnability with a felt or equivalent of compressible
volume is
better, because, in particular with thin paper grades, the frequency of web
breaks is
substantially lower than earlier. In the case of a compressible multi-layer
belt, the
improved runnability was ascertained in test runs. It can be suggested as a
reason for
this that, when a non-compressible calendering belt is used alone, the flow of
the
belt material parallel to the belt face strains the paper to such an extent
that the
frequency of web breaks becomes higher.
Above, the invention has been described by way of example with reference to
the
figures in the accompanying drawing. The invention is, however, not confined
to the
exemplifying embodiments illustrated in the figures alone, but a belt of
compressible
volume in accordance with the invention can also be used in other connections.
As
examples of such other applications can be mentioned various shoe presses,
size
presses that employ a belt, belt coating devices, belt winders, and
equivalent. Also
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in the other respects, it must be stated that the invention can be varied
within the
scope of the inventive idea defined in the accompanying claims.
*rB
