Note: Descriptions are shown in the official language in which they were submitted.
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Applicator apparatus and method for film-transfer coating
The present invention relates to an applicator apparatus
according to the preamble of claim 1 for use in a film-
transfer coater employed in coating paper or board, in
particular concerning a lubrication arrangement of the
doctoring member adapted to the front edge of the press-
urize~d applicator chamber of said applicator apparatus.
The invention also concerns a method for coating paper or
board, said method being capable of reducing the friction
occurring between the film of coating material remaining
after the applicator roll press nip and the member
sealing the front edge of said applicator chamber. The
chamber front-edge sealing member can be a doctor blade
or rod. The lubricant cleans the blade or rod free from
foreign matter and size material residues that are then
allowed to drip into a collection trough adapted below
said chamber front-edge sealing member.
The invention relates generally to methods and appara-
tuses used far metering coating material on a moving base
web of paper or board in a manner comprising first
spreading the coating or size on the applicator roll of a
size press. Next, the coat film formed on the applicator
roll surface is transferred further to the moving web in
a nip between the applicator roll and a backing roll.
Conventional coating equipment such as short-dwell appli-
cator apparatuses employed for coating a moving web of
paper are understood to include all the arrangements
required for feeding the coating material under a posi-
tive pressure into the applicator chamber which at its
one end is bordered by a doctoring member such as a
doctor blade and at its other end by the applicator
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chamber front edge, whereby the application zone is
formed between the web being coated and the applicator
chamber front edge. By feeding the coating material into
the applicator chamber with a sufficiently high volu-
metric rate and positive pressure, an overflow or reverse
flow is created thus forming a flow directed reverse to
the travelling direction of the web. Herein, a flow
running opposite to the web movement is passed via the
gap between the applicator chamber front edge and the
applicator roll or the web, whereby a liquid seal is
provided capable of preventing the passage of air into
the applicator chamber.
These kinds of conventional coaters have not been entire-
ly satisfactory in operation. FI patent no. 61,534 dis-
closes a coater in which the coating is applied to the
web being coated under a positive pressure with a con-
trolled volumetric rate. When cited method has been used
for coating a web of paper, it has been found problematic
to prevent the entry of air into the applicator chamber
via the gap under the chamber front edge member. Entry of
air into the applicator chamber has caused an uneven
cross-machine coat profile on the web.
Though it appears possible to prevent air from entering
the applicator chamber by way of loading the chamber
front edge member against the web, this approach causes
fiber and other foreign matter separating from the web to
accumulate in the applicator chamber, wherefrom such
foreign matter adheres to the doctoring blade thus giving
rise to streaking in the applied coat. When the coating
line comprises a size press with a roll or a number of
rolls, excessive linear loading of the chamber front edge
member against the roll may cause wear marks on the roll
surface, because the foreign matter released from the web
surface tends to remain in front of the chamber front
edge member due to the sealing effect of the excessive
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linear loading of the member.
Another prior apparatus for coating a moving web is dis-
closed in US Pat. No. 4,250,211. In this construction,
the sealing of the applicator device front edge against
entry of air into the applicator chamber is implemented,
in regard to the moving web, by means of allowing the
coating material to flow past the gap under the front
edge member reverse to the direction of the web travel,
whereby the reverse flow entirely seals off the gap. As
the width of the gap between the moving web and the
chamber front edge member typically is in the range of
3-6 mm, the reverse coating material flow must have a
volumetric rate of about 20-30 times larger than the
portion of coating adhering to the web surface in order
to achieve complete sealing off of the gap. When the
coating material is first applied to the surface of a
roll, the excess coating material flow seals off the gap.
Even if the overflow portion could be smaller than what
is needed for sealing off the gap with the coating mate-
rial, it still is relatively large as compared to the
amount adhering to the roll surface as a coating material
film.. If the gap is made wide, the positive pressure of
the applicator. chamber cannot be easily increased due to
the small flow resistance formed by the wide gap to the
passage of the coating material therethrough. Hence, any
increase bf the positive pressure in the applicator
chamber also causes a corresponding increase of overflow
or, respectively, reverse-direction flow through the gap.
On the other hand, if the gap is decreased to, e.g.,
1.5 mm, even the smallest variations in the cross-machine
profile of the gap due to, e.g., manufacturing defects,
will be seen as respective cross-machine defects, or
profile defects, in the applied coat. Furthermore, if the
gap is made very small, the positive pressure of the
applicator chamber may rise due to the increased flow
resistance through the reduced gap so high as to cause
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problems in the sealing of the applicator chamber side
edges.
To avoid the above-described drawbacks in such arrange-
s ments, improvements have been made in the manner dis-
closed in US Pat. No. 4,250,211. According to this
patent, the coating material is adhered to the web
surface by means of hydrostatic forces acting over a
limited area or zone. Herein, reference is made to
arrangements described in US Pat. No. 4,250,211 and US
Pat. No. 4,839,201 as well as in German patent publica-
tion no. 3,338,095. It must be noted that even such sys-
tems that utilize hydrostatic forces for the application
of a coating material it has been necessary to use an
overflow or reverse-direction flow of the coating mate-
rial past the gap between the application zone front edge
and the moving web.
US Pat. No. 4,839,201 describes an apparatus in which the
applicator chamber is sealed off by means of two blades
or, alternatively, two rods. The coating material is
firsts applied by the apparatus onto an applicator roll
and therefrom further to the web being coated. Herein, no
actual reverse-direction flow takes place via the gap
between the applicator chamber blade or rod and the
opposed applicator roll, but instead, a homogeneous layer
of water, size that is used or other liquid lubricant is
passed in front of said blade or rod, e.g., via holes
made to the dam blade. Hence, a lubricant pond is formed
in front of the ingoing-side edge member that prevents
the friction otherwise occurring between a dry edge
member and the applicator roll from causing damage to the
roll or the edge member. Although this apparatus con-
struction requires no actual reverse-direction flow to be
provided, the zone in front of said blade or rod must
anyhow be cleared free from the used lubricant and the
coating material or size scraped away from the surface of
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the applicator roll by said edge member. As the lubricant
in this system is advantageously utilized the same mate-
rial as is being applied to the web, because then any
amount of the lubricant possibly passing into the appli-
5 cator chamber will not dilute the material contained
therein. While the coating material or size used as the
lubricant could be replaced by water, its access into the
applicator chamber could dilute the material contained
therein and, moreover, it can form a film on the appli-
cator_ roll that prevents the coating material or size
from adhering evenly to the roll surface. Hence, lubrica-
tion with water may cause caating defects. The handling
of water or other liquid lubricant is clumsy due to
splashing and, further, since the used lubricant must be
collected away from the point of lubrication and returned
to the coating material or size circulation, or when
using water as the lubricant, to the waste water treat-
ment system.
In the case that the lubricant is a size used for surface
sizing of paper or board or coating mix, the above-
desc:ribed apparatus cannot serve in a coater of the
nonreturn type, because the lubricant applied to the
blade or rod acting as the front edge member of the
applicator apparatus always needs a return circulation
for the lubricant. Even if the amount of applied lubri-
cant could be minimized, the implementation of the return
circulation causes extra costs and the coating material
returned back from the circulation to the applicator is
always of a lower quality as compared to the fresh
coating material or size.
It is an object of the present invention to provide a
nonreturn type of applicator apparatus for a film-
transfer coater.
The goal of the invention is achieved by lubricating the
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front edge member of the applicator chamber using injec-
tion of steam into the gap between said member and the
applicator roll.
More specifically, the applicator apparatus according to
the invention for use in a film-transfer coater is
characterized by what is stated in the characterizing
part of claim 1.
Furthermore, the application method according to the
invention is characterized by what is stated in the
characterizing part of claim 5.
Furthermore, the application method according to the
invention is characterized by what is stated in the
characterizing part of claim 8.
' The invention offers significant benefits in the art of
coating techniques inasmuch the lubrication of the front
edge member of an applicator apparatus by means of steam
injected from a steam injector facilitates the manu-
facture of a final product with improved smoothness of
its surface profile. Furthermore, the occurrence of blade
streaking in the coated final product is reduced by
virtue of the condensing steam that cleans the front edge
member of the applicator chamber free from any foreign
matter such as fiber and foreign matter adhering to the
applicator roll surface. However, the most important
benefit of the invention is that the coating mix
circulation can be designed as a nonreturn system, which
means that all the coating mix fed into the applicator
chamber will be transferred onto the applicator roll.
Hence, the applicator apparatus can be run without any
coating mix return circulation and the applicator chamber
is continually filled with fresh coating mix. The quality
of the coating mix stays good because no air or foreign
matter can become entrained with the mix and the
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equipment will remain cleaner. The amounts of coating mix
to be handled are smaller inasmuch no excess coating mix
or size is needed in the application step. Surface sizing
of paper or board can be accomplished by a system of
absolutely no return circulation, since the surface size
need: no doctoring after application. Obviously, the
exce:~s coating material removed by means of doctoring or
other smoothing technique from the web surface must be
treated in a suitable manner.
In the following, the invention will be examined in
greater detail by making reference to the appended
drawings in which
Figure 1 is a cross-sectional side view of an embodiment
of t:he invention, and
Figure 2 is a cross-sectional side view of another
embodiment of the invention.
The present invention relates to a coating method and
apparatus disclosed in US Pat. No. 4,839,201 to which
reference is also made herein. This type of apparatus is
suited for applying surface size or different kinds of
coating materials to a moving web and here such agents
and compositions to be applied to a web are generally
named as treating agents.
Referring to Fig. 1, therein is shown an applicator
apparatus 10 according to the invention. The apparatus 10
comprises an applicator chamber 2 covering area B on an
applicator roll 1. The area B is bordered by a doctor
blade 4 and a dam blade 3. The doctor blade 4 is an
acute-angle blade which is loaded by means of a pneumatic
tube S. In addition to the loading of the doctor blade,
the position and angle of the blade support beam 8 in
regard to the applicator roll 1 can be adjusted in a
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conventional manner to control the operation of the
apparatus. A detailed description of these adjustment
arrangements are omitted herein.
The volume of the applicator chamber 2 is very small,
which means that the turbulence caused by the surface of
the roll 1 rotating therein is minimal. The coating mix
is fE:d under a positive pressure and with a desired
volumetric rate into the applicator chamber 2. In the
embodiment according to the invention, the infeed rate of
mix can be kept equal to the amount of mix being
transferred to the surface of the roll 1, because there
is no need for a lubricating reverse-direction flow. To
assure homogeneous spreading of the coating mix film on
the rotating roll 1 and an even profile of the applied
coat on the web, entry of air into the applicator chamber
2 must be prevented. In the prior art this has been ac-
comp:Lished by passing a portion of the coating mix flow
in front of the dam blade 3, reverse to the direction of
rotation of the roll 1. Thus, the reverse flow acts as a
lubricating flow that simultaneously seals off the dam
blade 3. In the embodiment according to the present
invention, the applicator chamber 2 is directly sealed
against the roll 1 by means of the dam blade 3 which is
loaded against the surface of the applicator roll with
the help of the blade holder assembly 18. In this case,
there is no lubricating flow outward from the applicator
chamber 2 onto the dam blade 3, and running the dam blade
3 unlubricated under these circumstances would destroy it
quickly, simultaneously also damaging the surface of the
roll 1 into an unusable condition. Hence, the gap between
the dam blade 3 and the roll 1 must be lubricated in some
alternative manner. In the embodiment according to the
invention, lubrication is accomplished by virtue of
injecting steam by means of a steam injector 7 into the
gap between the dam blade 3 and the applicator roll 1.
Herein, a steam injector of a tubular shape is used
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having an elongated nozzle slot provided to its side.
On the surface of the applicator roll 1, there remains a
thin film of coating mix or size even after the roll
surface has passed the press nip formed between the
applicator roll 1 and its backing roll. So, while the
major portion of the coating mix is transferred in the
press nip to the web being treated, a thin film of mix
always adheres to the surface of the web. The steam
injecaed to the gap between the applicator roll 1 and the
dam blade 3 can effectively lower the friction occurring
between said dam blade 3 and the thin film on the roll 1.
The reduction of friction is partly due to the softening
of the film under moisture and heat and partly due to the
conde=nsing water. The condensing water can under
favourable conditions form under the dam blade 3 a thin
condensate film that separates the dam blade 3 from the
roll 1. As the moisture content of the injected steam is
smal:L, also the amount of condensate water is minimal
thus having no detrimental effect on the film being
applied to the surface of the applicator roll 1 in the
applicator apparatus 10, thus making lubrication with
steam capable of overcoming the problems hampering
lubrication with water. The water condensing from the
steam and the injected hot steam itself clean the dam
blade 3 free from adhering foreign matter and
accumulating size or coating mix residues. The condensate
water and removed foreign matter can be collected into a
collection trough adapted to the stem of the dam blade.
Condensation of steam can be promoted by cooled
structures, e.g., providing the blade holder with a
cooling water circulation.
In the above example, the coating mix leveling member is
a doctor blade and the applicator chamber front edge is
formed by a similar member called a dam blade. It must be
noted that depending on the actual design the chamber
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front edge can be angled in a desired manner, whereby the
dam blade angle with the roll may be varied, e.g., in the
range 0-180°.
5 In Fig. 2 is shown an asseriibly in which the doctoring
member is a rotating rod 12 mounted on a rod cradle 11.
The :rod 12 and the cradle 11 are supported to the appli-
cator apparatus frame 13 by means of a pressurized tube
14 adapted to control the linear doctoring force of the
10 rod 12. Depending on the application, the doctor rod 12
may :be a smooth or grooved rod. Also the front edge of
the applicator chamber 2 is bordered by means of a rod 15
adapted in a rod cradle 16 and supported to the appli-
cator apparatus frame by means of a pressurized tube 17.
The diameter of the front edge rod 15 is made slightly
smaller than that of the doctor rod 12 and, advantageous-
ly, the front edge is provided with a smooth rod, because
the lubricating effect of steam is better on a smooth-
surfaced rod than when using a grooved doctor rod.
Moreover, a grooved rod tends to pass size or coating
furnish via the grooves to the zone in front of the rod.
The front edge rod called the dam rod 15 is lubricated by
steam injected from a nozzle 7 that both cleans the rod
and lubricates the gap between the applicator roll 1 and
the dam roll 15. The linear loading of the dam rod 15
against the surface of the applicator roll 1 is set to a
desired value by means of a pressurized tube 17. The rod
15 must be loaded against the applicator roll 1 so
strongly that size, coating mix or other web treatment
agent cannot pass from the applicator chamber 2 reverse
to the direction of rotation of the applicator roll. As
the system operates without any reverse flow sealing the
gap between the dam rod 15 and the applicator roll, it is
extremely essential that the rod is loaded tightly
against the applicator roll 1 in order to prevent entry
of air into the applicator chamber. Owing to the
otherwise sealed structure of the applicator chamber 2,
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the treating agent pumped therein can leave the chamber
but by being transferred to the applicator roll via the
doctor rod 12, whereby no reverse flow occurs. Hence, the
apparatus operates without any return circulation. To
improve the condensation of steam, the dam rod 15 can be
cooled by lubrication water circulated in the rod cradle
or by arranging a cooling water circulation into the
interior of the rod or in the rod cradle.
In either of the above-described embodiments, the steam
injection can be sealed from the machine hall enviroment
with the help of, e.g., sufficiently tight hoods. The
need for enclosures depends on the amount of steam used,
whereby minor amounts of steam may be released into the
machine space.
In addition to those described above, the invention may
have alternative embodiments. Far instance, the
lubricating steam can be fed to the chamber front edge
dam member using the above-mentioned slotted nozzle or
with the help of a set of separate nozzles mounted on
said injection tube, using nozzles having a separate
steam feed for each, or any other suitable manner. When
desired, a doctor blade and a rod-like dam member, or
alternatively, a dam blade and a doctor rod may be
combined in the same apparatus. Multiple different
manners are known in the art for implementing the blade
and rod holders and their loading assemblies.
