Coating device for coating of a size-press roll, paper or board

The invention relates to a coating device for coating, e.g. a size-press roll, paper or board or of an equivalent moving base. The coating device comprises a revolving coating bar which rests against a moving base. The coating bar extends across the machine width and is supported in a cradle substantially over its entire length. The coating bar spreads and smoothes the coating agent onto the moving base which coating agent is introduced into the coating device in the running direction of the moving base before the coating bar. The coating bar in accordance with the invention is a smooth bar of large diameter arranged against the moving base so that the profile of coating quantity can be controlled.

BACKGROUND OF THE INVENTION 
The invention relates to a coating device for coating a size-press roll, 
paper or board or an equivalent moving base. The coating device comprises 
a revolving coating bar which rests against the moving base and extends 
across the machine width. The coating bar is supported in a cradle 
substantially over its entire length and is fitted to spread and to smooth 
a coating agent onto the moving base. The coating agent is introduced into 
the coating device in the direction of running of the moving base before 
the coating bar. 
The invention is further related to a coating device for coating a 
size-press roll, paper or board or an equivalent moving base, comprising a 
coating-agent chamber including a revolving coating bar supported against 
a moving base and which acts as a coating member. The coating bar extends 
across the machine width, and the coating-agent chamber is defined by the 
front wall of the coating-agent chamber, by the lateral seals of the 
coating device, and by the moving base. The coating agent is arranged to 
be fed into the coating-agent chamber under pressure. 
At present, in the coating of paper or board, two alternative methods and 
devices are commonly used, i.e. a blade coater or a bar coater. The 
present invention is expressly related to the latter, bar coaters, which 
have proved excellent especially in the film size press technique. The 
material of the coating bars currently in use is usually steel, and, in 
view of increasing the service life of the bar, the bars are provided with 
chromium plating. In surface sizing of paper, bars with fully smooth faces 
have not been used. Rather, the face of the coating bar has been provided 
with grooves, or alternatively steel wire is wound onto the bar to form an 
arrangement similar to grooves on the bar face. A grooved bar is used for 
the surface sizing of paper because the thickness of the size film to be 
applied onto the base to be coated is determined by means of the depth of 
the grooves. 
A grooved bar is, however, poorly suitable for the preparation of thin size 
films, because it is very difficult to manufacture grooves of sufficiently 
small scale. On the other hand, such small grooves are easily 
contaminated. It is a further highly significant drawback of grooved bars 
that they are worn rapidly. Pigmenting with a high dry solids content is 
also entirely impossible with grooved bars, because the wear of the bars 
is excessive in this connection. The diameters of the coating bars 
currently in use have been very small. Small-diameter bars are used 
because the bar ought to be able to adapt itself to the shape of the roll 
face in the size press in order that the profile of the size film could be 
made suitable and correct. Generally, the order of magnitude of the 
diameter of currently used coating bar is about 10 mm. 
It has not been possible to use bars of large diameter because, owing to 
their thickness, they have been too rigid to provide an adequate 
profiling. In respect to the prior art, reference is made in particular to 
U.S. Pat. No. 2,970,564, from Which a device is known for spreading of a 
size film or of a pigment coating film onto the rolls in a size press, 
said films being transferred to the paper in the nip in the film size 
press. In the method described therein, smooth or grooved bars are used 
for the metering of the size film. It is stated therein that while it is 
possible to use a smooth-faced bar of small diameter (diameter about 9 mm) 
for surface sizing, such a bar is unsatisfactory. Accordingly, a 
corresponding device has never been applied in practice. In respect of the 
prior art, reference is further made to FI Patent No. 30,147. 
OBJECT AND SUMMARY OF THE INVENTION 
Accordingly, an object of the present invention is to provide a coating bar 
which avoids the drawbacks of the prior art and by whose means a 
significant improvement is produced in particular in surface sizing of 
paper. 
It is another object of the present invention to provide a new and improved 
coating device to coat a web which is travelling in an upward direction 
through the coating device. 
In view of achieving these objects, and others, the invention is mainly 
characterized in that the coating bar is a smooth bar of large diameter, 
which is fitted against the moving base so that the profile of coating 
quantity can be regulated under control. It is an important advantage of 
the present invention that while using a large-diameter coating bar, it is 
now possible to run very thin size films and coating pastes of high dry 
solids content even at high running speeds. With the device in accordance 
with the invention, the level of the coating quantity and the profile of 
the coating have been very good. 
In a preferred embodiment, the paper or board is arranged to move in an 
upward direction through a nip formed by a pair of size-press rolls. In 
this embodiment, a pressurized and closed coating-agent chamber, which is 
inherently air-tight, is placed before the coating bar so that the coating 
agent is introduced onto the moving base, i.e. the pair of rolls, to be 
spread and smoothed by the coating bar. In this manner, the coating agent 
used in the coating process of the present invention can be thicker than 
in prior art processes utilizing coating bars.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 is a schematic illustration of the size press, which is denoted 
generally with reference numeral 1. The film size press 1 comprises size 
press rolls 2 and 3 so that the first roll 2 and the second roll 3 form a 
nip N between them, the paper or board web W being passed through said 
nip. In the film size press 1, a film size F1 is metered onto the face 4 
of the first roll by means of a first coating device 10 and, in a 
corresponding way, a second size film F2 is metered onto the face 5 of the 
second roll by means of a second coating device 20. In the roll nip N, the 
size films F1 and F2 are transferred onto the paper or board web W running 
through the nip. In FIG. 1, the coated web is denoted with reference W'. 
In the film size press 1 shown in FIG. 1, the coating devices 10 and 20, by 
whose means the size films F1 and F2 are spread onto the faces 4 and 5 of 
the rolls 2 and 3 in the size press, are bar coaters, which are 
substantially equal to one another, as is shown in FIG. 1. The coating 
devices 10 and 20 are coating devices of so-called short-dwell type, in 
which the coating agent is introduced into a pressurized coating-agent 
chamber 16, 26 placed before the coating bar 11, 21. Besides being defined 
by the coating bar 11, 21, the chamber is also defined by the roll face 4, 
5, by the front wall 14, 24 of the coating-agent chamber, as well as by 
possible lateral seals, if any (not shown). 
The coating bar 11, 21 is provided with a dedicated drive gear (not shown), 
by whose means the coating bar 11, 21 is rotated in directions opposite to 
the directions of rotation of the rolls 2, 3. Further, in FIG. 1, the 
holders of the cradles of the coating bars are denoted with reference 
numerals 13 and 23, and the holders of the front wall with reference 
numerals 15 and 25. Further, between the coating-bar 11, 21, cradle 12, 22 
and the holder 13, 23, an ordinary loading hose or equivalent (not shown) 
is fitted, by whose means the coating bar 11, 21 can be loaded against the 
roll face 4, 5 to produce the desired loading pressure. 
According to the invention, in the coating devices 10, 20 shown in FIG. 1, 
a smooth-faced coating bar 11, 21 is employed. In addition to the fact 
that the coating bar 11, 21 in accordance with the invention has a smooth 
face, the diameter of the coating bar 11, 21 is substantially larger than 
prior art coating bars. In the coating device in accordance with the 
invention, the diameter of the coating bar 11, 21 is at least 18 mm, and, 
in test runs, very good results have been obtained with a coating device 
in which the diameter of the coating bar was 20 mm. A dimension even 
larger than this is considered to be most appropriate bar diameter, and 
the diameter of the bar is optimally from about 25 to about 80 mm. 
In particular in wide machines, it is necessary to regulate the profile of 
the coating quantity produced in the coating device, such regulation would 
be substantially impossible with a large-diameter coating bar if 
constructions known from the prior art small-diameter bars were employed 
in which constructions the coating-quantity profile can be controlled by 
regulating the loading of the bar locally, in which case the loading 
profile is transferred to the coating process as the coating bar is 
deflected. For this reason, in the device in accordance with the 
invention, alternative arrangements have been developed for the purpose of 
regulating the loading profile. 
FIG. 2 depicts another embodiment of the invention which allows for 
regulation of the profile of the coating quantity to be carried out by 
means of a large-diameter coating bar in accordance with the invention. In 
FIG. 2, the size-press roll is denoted with reference numeral 6. The roll 
is provided with a coating 7 in a conventional manner. The coating 7 may 
be, e.g., rubber or an equivalent. For the purpose in accordance with the 
invention, the most appropriate hardness of the roll 6 face is about 35 
P&J.+-.15 P&J, so that the selection of the material for the coating is 
carried out on this basis to obtain the correct hardness. 
In FIG. 2, the coating bar in accordance with the invention is denoted with 
reference numeral 31 and, comprises a small-diameter body 32, whose 
diameter is, e.g., of an order of 12 mm or less. The body 32 of the bar is 
provided with a suitable outer layer 33, by whose means the diameter of 
the bar 31 can be made sufficiently large. The outer layer 33 may be 
formed, e.g., by winding a band or equivalent around the body 32 of the 
bar. 
On the other hand, the outer layer 33 may also consist of bushings 33a 
fitted on the bar body 32, said bushings being attached to one another in 
a suitable way as non-revolving. Such a embodiment is shown in FIG. 2A, 
wherein the body 32 of the bar threaded bar, onto which the bushings 33a 
have been fitted. The bushings are provided with appropriate means, by 
which their rotation in relation to one another is prevented and in the 
axial direction they are tightened into contact with one another by means 
of a tightening nut 37 threaded onto the bar. To provide adequate 
tightness, the bar 32 may be pre-stretched. The mode of fastening shown in 
FIG. 2A is particularly well suitable for large diameter bars whose 
diameter is larger than 25 mm. Differing from which is shown in the 
Figures, a large-diameter smooth bar can also be made of a tube whose 
material is, e.g., chromium-plated copper or steel. 
In a conventional manner, the bar 31 is installed in order to revolve in 
cradle 34 which is made, e.g., of polyurethane, and is attached to a 
cradle holder 35. Between the cradle holder 35 and the cradle 34, loading 
means, e.g., a loading hose 36 or an equivalent loading member, is fitted 
so that the bar 31 can be loaded as desired against the roll 6. In the 
embodiments shown in FIGS. 2 and 2A, when a small-diameter bar body 32 is 
used, by means of the loading hose 36 the bar 31 can be profiled in a way 
corresponding to the prior art small-diameter bars. 
FIG. 3 shows another embodiment of the invention which provides a manner 
for bringing the profile of the coating quantity on the size-press rolls 
to the correct level by means of coating bars in FIG. 3. Out of the 
coating devices for the rolls 61 and 71, in FIG. 3, the coating bars 41 
and 51 only are shown, while omitting all the other components relates to 
the coating devices in the illustration of FIG. 3. In the embodiment shown 
in FIG. 3, the coating bars 41 and 51 are large-diameter bars, having a 
diameter of at least 18 mm, and preferably about 20 mm. 
In the embodiment shown in FIG. 3, the bars 41 and 51 have a unified, solid 
construction and are consequently quite rigid. Accordingly, the size press 
rolls 61 and 71 shown in FIG. 3 are variable crown rolls, so that they 
comprise a rigid central axle 62, 72, on which the roll mantle 63, 73 is 
arranged in a revolving manner. On the axle 62, 72, hydraulic loading 
means 64, 74 are provided, which are supported against the inner face of 
the roll mantle 63, 73 in the plane of the nip N. In addition to the crown 
variation, the rolls 61 and 71 may be adjustable in zones, even though 
this is not essential in view of the operability of the invention. Thus, 
in the embodiment shown in FIG. 3, the profiling is carried out by means 
of the loading means 64, 74 in the roll nip N itself. For this reason, in 
the embodiment shown in FIG. 3, the coating bars 41, 51 themselves need 
not be profiled, because the roll mantle 63, 73 is straight at the coating 
bar 41, 51. Thus, by means of the embodiment shown in FIG. 3, coating 
films with uniform profiles are produced on the roll 61, 71 faces. 
Instead of the hydraulic means 64, 74 shown in FIG. 3, the crown variation 
in the size-press rolls 61, 71 may be accomplished so that the rolls 61, 
71 are provided with devices fitted inside the roll mantle 63, 73, by 
means of which devices the temperature of the roll mantle 63, 73 can be 
adjusted in zones. In such an embodiment, the regulation of the profiles 
of the size films is carried out by heating or cooling the rolls 61, 71 in 
zones. 
FIGS. 4 and 5 illustrate a further embodiment, by whose means the profile 
of the coating quantity applied to the roll face can be regulated as 
desired. FIGS. 4 and 5 are schematic illustrations in the machine 
direction, showing a part of a roll as well as the coating on the roll 
face. Further, in FIG. 6, a coating bar is shown schematically. In FIGS. 4 
and 5, the roll is denoted with reference numeral 91, the outer face of 
the roll with reference numeral 91a, the roll coating with reference 
numeral 92, and the outer face of the coating with reference numeral 92a. 
As shown in FIGS. 4 and 5, the outer face 91a of the roll is shaped as 
curved in the axial direction, i.e., crowned. In these Figures, the 
crowning is shown as remarkably exaggerated, because generally the 
crowning is of an order of 0.3 mm with a machine width of 7 m. As is shown 
in FIG. 4, the outer face 92a of the roll coating 92 is provided with 
negative crowning, so that at the ends of the roll the coating 92 is 
considerably thicker than at the middle of the roll. 
FIG. 5 shows an embodiment in which a large-diameter coating bar 81 in 
accordance with the invention is fitted against the roll. The coating bar 
81 is made of solid material, being substantially rigid, in which case, 
when the coating bar 81 is loaded against the roll, the coating 92 is 
compressed below the coating bar 81 so that the compression a of the 
coating 92 in the end areas of the roll is considerably larger than the 
compression b at the middle of the roll. As the coating 92 is thinner at 
the middle of the roll than at the roll ends, the compression of the 
coating is smaller at the middle than at the roll ends. Thus, in a 
situation as shown in FIG. 5, the linear load between the coating bar 81 
and the coating 92 is the same across the entire machine width. In this 
way, by means of this embodiment, a uniform profile of coating quantity is 
obtained. Instead of a coating bar 81 made of solid material, in many 
cases it is possible to achieve sufficiently rigidity by means of an 
embodiment in which the bar is made of a hollow tube, e.g., of 
chromium-plated copper or steel. 
It is, of course, also contemplated that, e.g., by combining of embodiments 
shown in FIGS. 3, 4 and 5, a good result is also obtained. In such a 
combination, the size press would be arranged such that one press roll is 
a variable-crown roll or a roll adjustable in zones in respect to 
temperature and the other roll is a owned roll whose coating is provided 
with negative crowning. As such, the regulating means for regulating the 
profile of coating quantity being applied to the web W in the nip N would 
entail at least one of the rolls 61,71 being a variable-crown roll, 
conventionally provided with a roll mantle against which the loading means 
64, 74 engage, wherein a linear contact face located between the coating 
bar 41,51 and the variable-crown roll 61,71, respectively, is adjusted to 
be straight via loading means 64,74. Also, the regulating means might 
consist of one of the rolls 61,71 being a thermally adjustable roll 
wherein a linear contact face between the coating bar 41,51 and the 
variable-crown roll 61,71, respectively, is adjusted to be straight by 
regulating the temperature of the thermally adjustable roll. 
The embodiment shown in FIG. 6 illustrates a coating process which takes 
place directly onto the paper or board web W. In FIG. 6, the roll is 
denoted with reference numeral 110. The paper or board web W is passed 
over the roll face 111, and the coating is carried out by means of the 
coating device 100 directly onto the web W. 
The coating device 100 is a coating device of the so-called short-dwell 
type, which includes a large-diameter coating bar 101 is accordance with 
the invention mounted as revolving in a cradle 103. The cradle 103 is 
mounted in a holder 104 in the normal way. The coating bar 101, together 
with the front wall 102 of the coating device 100, comprises the 
pressurized coating agent chamber 106, into which the coating agent is 
introduced. The front wall 102 is mounted on a holder 105 in the normal 
way. 
In test runs that have been carried out, with a coating device in 
accordance with the invention, excellent results have been obtained. The 
test runs were carried out with an embodiment wherein a smooth coating bar 
of a diameter of 20 mm was mounted as revolving in a coating bar cradle of 
conventional construction. The coating device constructed in this way was 
fitted in a size press. 
By means of the device of this embodiment, test runs were carried out for 
pigmenting. In some test runs, the dry solids content of the coating paste 
was 51% and the viscosity 700 cPs. In the test runs, the coating quantity 
varied in the range of from about 3 to about 10 g per m2 per side, and the 
running speed varied in the range of from about 800 to about 1200 m per 
min. The profile of coating quantity that was obtained was very good. 
Cavitation was so little that almost no cavitation could be noticed. One 
reason for this was that the diameter of the coating bar was substantially 
smaller than in roll coaters proper, in which the large roll diameters 
cause cavitation as the film is split after the nip. The result was 
substantially better than with any method that was described earlier in 
the prior art. 
Further, FIG. 7 is a graphic presentation of the results obtained with 
another paste quality. In FIG. 7, the paste quantity obtained is shown as 
a function of the loading pressure of the coating bar at different running 
speeds. The tests illustrated in FIG. 7 were also carried out by means of 
a smooth bar of a diameter of 20 mm mounted in a conventional cradle. The 
coating paste that was used was PSP kaolin paste, whose dry solids content 
was 50% and viscosity 550 mPas. The roll in the press was provided with a 
polyurethane coating. In the test runs, running speeds of 300-1200 m/min 
were used. The loading pressure indicated in the table expressly means the 
loading pressure of the coating bar against the roll. As can be clearly 
seen from FIG. 7, by means of the construction in accordance with the 
invention, very thin coating quantities were obtained with a high dry 
solids content of the paste. Thus, by means of the invention, a remarkable 
improvement is achieved over the prior art. There is a great demand for 
improvements provided by the method in accordance with the invention in 
pigmenting and surface sizing of newsprint as well as in surface-sizing of 
SC-paper. 
FIG. 8 shows an embodiment of the invention wherein the coating device is 
of the short-dwell type. The coating devices 10,20 are similar to the 
coating devices described with respect to FIG. 1. However, in the 
embodiment illustrated in FIG. 8, the web W' runs in an upward direction 
through nip N formed between the two size press rolls 2,3. The coating 
devices 10,20 will therefore operate and coat the press rolls 2,3 on the 
underside of the rolls 2,3, e.g., on the lower half of the rolls 2,3 when 
viewed in the running direction of the web W'. 
The coating agent is applied from pressurized coating agent chambers 16,26 
in each of the devices 10,20, respectively, onto the rolls 2,3. The 
pressurized chambers 16,26 are closed to prevent overflow of the coating 
agent and assure that a sufficient pressure prevails in the pressurized 
chambers 16,26 to apply the coating agent onto the rolls. In this manner, 
it is not necessary to arrange the coating devices 10,20 to open downward 
toward the rolls in order to apply the coating agent onto the rolls 2,3, 
as illustrated in FIG. 1 wherein the coating devices 10,20 are arranged on 
the topside of the rolls and the coating process utilizes the effect of 
gravity. Rather, the closed and pressurized chambers 16,26 provide 
sufficient force to apply the coating agent onto the rolls 2,3 regardless 
of the positioning of the coating machines 10,20 on the periphery of the 
rolls 2,3. 
A significant advantage of providing a pressurized and closed coating agent 
chamber 16,26 is that a thicker coating agent can be used to coat the 
rolls 2,3 than that used in prior art devices. In addition, the web can be 
arranged to run in an upward direction, as illustrated in FIG. 8. It is 
not possible to simply change the running direction of the prior art 
devices, in which the web runs in a downward direction, to obtain such 
advantages in a device wherein the runs in an upward direction, e.g., the 
use of a thicker coating agent. 
To further assist in the application of the coating agent in the 
pressurized chambers 16,26, in a preferred embodiment, the bar coaters 
12,22 may be inclined downward so that any unrestrained coating agent in 
the chambers 16,26 flows toward the coating bars 11,21 and thereafter onto 
the rolls 2,3 as the coating bars 11,21 revolve. 
The examples provided above are not meant to be exclusive. Many other 
variations of the present invention would be obvious to those skilled in 
the art, and are contemplated to be within the scope of the appended 
claims.