Note: Descriptions are shown in the official language in which they were submitted.
31~36~
AIR KNIFE COATER
The invention relates to an air Icnife coater for coating
at least one side of a running paper or cardboard web.
In known air knife coaters, coating substance is first
applied onto a running web at a position rather ar away
from the air knife device. Due to this, the coating sub-
stance on the web has time to undergo changes before it is
treated by the air knife~ Thereby, a part of the liquid
component of the coating substance usually penetrates into
the web and another part of it vaporizes. This increases
the viscosity and the dry solids content of the coating
substancey which reduces the effectiveness of the air knife.
Due to this~ air knife coaters have previously been used
only at relatively slow web speeds, about 300...400 m/min,
the dry solids content of the coating substance being
relatively small, usually less than 40 %. Furthermore,
the use of an air knife frequently causes foaming prob-
lems because of-the high moisture content of the coating
substance combined with a high level of excess coating
substance applied onto the web.
The object of the invention is to create an air knife
coater, which operates effectively even at a high web
speed and when the dry solids content of the coating
substance is high.
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la
According to the invention there is provided a device for
coating at least one side of a running paper or cardboard
web, comprising in combination: an air knife device; an
application zone disposed prior to said air knife device
in the moving direction of the running web for applying
coating substance onto the web by means of a coating sub-
stance applicator; said application zone being situated
so close to said air knife device that said web, when
running at normal coating speed, leaves the application
zone and reaches the air knife device in a time of at
the most 5 milliseconds, preferably at the most 3 milli-
seconds; said application zone being so limited in the
longitudinal direction of the running web that said web,
when running at normal coating speed, enters into the
application zone and reaches the air knife device in a
time of at the most 15 milliseconds, preferably at the
most 9 milliseconds.
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In a preferred embodiment o~ the invention, coating
substance is applied onto the web over a distance in the
longitudinal direction of the web that is substantially
equal to the distance between the air knife and the rear end
5 of the coating substance application zone. This usually
gives the best coating result. It is also favourable that
the cross section o~ the coating substance application duct
continuously decreases in a direction towards the web.
Hereby, the coating sub~tance flow accelerates in the
applicator, which eliminates places with low flow velocity
where impurities could accumulate.
The applicator normally applies an excess amount o~ coating
~ubstance of about 20...50 % onto the web and this excess
amount is removed by the air knife. If the desired coating
quantity is 10~g/m2, the applicator applies about
12...15 g/m onto the web. However, even if the coating
substance feed is so adjusted, that no ~ubstantial excess of
coating ~ubstance is provided, the the air knife still has a
favourable e~fect on the quality of the coating, even when
it does not blow away excess coating substance. This being
the case, it is important that the function of the
applicator is so adjustable, that the excess amount o~
coating subskance applied may approach zero.
The running web passes the coating substance applicator,
whereby coating substance is spread orto the web. That side
of the applicator, from which the web is entering the
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application zone, is called the ~ront side of the
applicator, and the side, from which the web leaves the
applicator, is called its back side. In a preferred
embodiment of the invention, there is an adjustable slot or
gap at the back side of the appIicator, between the
applicator and the web. By varying the height of this slot
the quantity of coating ~ubstance spread onto the web i5
adjusted. In a coater according to the invention also other
kinds of applicators may be used, for example, applicators
~here a flexible coating blade or a coating bar is urged
against the web at the back edge of tha application zone.
Different kind~ of extrusion applicator~ may also be used.
It has proved to be ~avourable to arrange a coating
substance overflow slot or gap at the front edge of the
applicator. This gives the advantage, that impurities
accumulated in the applicator may depart through the
overflow slot~ To use an overflow applicator o~ this kind in
a web coating process is known per se.
A coater according to the invention operates in the best
manner, i~ the web is supported on the side which is to be
left uncoated by a support drum at the position of the
coating substance applicator and the air knife. It is also
fea~ible that no support is used and instead a device
according to the invention is used at both sides of the web
for double sided coating of the web. Thereby, the coating
substance applicators and the air knives are preferably
arranged symmetrically at opposite sides of the web. If a
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support drum is used, its most favourable diameter is
uYually at least 400 mm and at most 1300 mm. The most
~avourable web speed is higher than 400 m/min, preferably
about 600 m/min. However, the invention gives favourable
results also at considerably smaller web speeds.
The invention will now be described, by way of example, with
reference to the accompanying drawing, in which the single
Figure is a sectional view o~ an embodiment of a coater
according to the invention.
In the drawing, numeral 1 indicates a running paper or
cardboard web~ The web 1 is supported by a rotating support
drum 2, the diameter of which may be, for example9 0.5
meter. Coating substance is spread onto the web by means of
an applicator 3, into which coating substance is pumped
through a conduit 4, which at the same time acts as a
tilting shaft for the applicator 3. The tilting motion of
the applicatGr is accomplished by means o~ one or several
power cylinders 5. The accurate operating position of the
applicator is determined by means of an adjustable support
6. The power cylinder 5 presses the applicator 3 against the
support 6. The applicator has a feed duct continuously
narrowing in a direction towards the web 1 in order to
obtain a continuously accelerating coating substance flow.
At the back edge of the applicator there is a blade 7, which
i i~ not in contact with the web 1 , but leavs a small gap 8
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between itself and the web 1. Coating substance 9 fed from
the applicator is taken up by the web and moves along with
the web to the position of the nozzle of an air knife device
10, where an excess of coating substance is blown away from
the web. The distance a between the back edge of the
applicator 3 and the nozzle of the air knife device 10 i3 SO
smallg that the web 1 normally moves this distance i~ less
than 5 milliseconds, preferably less than 3 milliseconds.
Further, the distance between the front and back edge of the
applicator is so small, that the time of the entire coating
process, that is, the time it takes for a point on the web
to move from the ~ront edge 12 of the appliator to the air
knife 10, is shorter than 15 milliseconds, preferably
shorter than 9 milliseconds. At high web speeds, the web may
move the di3tance a in 0,3 milliseconds and the time of the
entire caoting process may corr~spondingly be 0,6...0,9
milliseconds. The viscosity of the coating substance is
reduced due to the strong shearing stress caused on the
coating substance layer by the blade 7 or by any other
corresponding element. This condition of the coating layer
~ill not change substantially before the coating layer is
subject to the influence of the air knife 10, which improves
the e~ficiency of the air knife function.
At the front edge of the applicator 3, there is an
adjustable edge member 11, which is located at a
considerable distance from the web 1, thereby forming a
rather wide gap 12 between it3elf and the web. Through this
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gap coating substance is continuously flowing ~rom the
applicator. This overflow is adjusted to a relatively low
level by keeping the pressure in the applicator low. The
coating substance flowing out through the gap 12 is
collected in a collecting tank 13, ~rom which it is sucked
back into the coating substance flow system through an
outlet tube 14. The walls of the collecting tank 13 are
double, and cooling water is lead to the space between them.
Due to the water cooling, the inner surface of the tank 13
remainY cold and water condenses thereon, so that coating
substance does not so easily stick to the surface. Inside
the collecting tank 13, the pressure is kept at a level
sllghtly below atmospheric pressure by means of the suction
` in the outlet tube 14. This reduces spreading o~ coating
substance drops to the environment. At the outer side o~ the
blade 7 there is a water cooled cover 15, which diminishes
soiling of the blade 7 with coating substance in the same
manner as the water cooling of the walls of the tank 13
keeps them clean.
The quantity of coating substance that the applicator
applies onto the web 1 is under normal conditions greater
than the desired final coating quantity. The excess amount
is usually about 20.~.50 %, but it is also possible to
perform the coating substance application so, that no
~ubstantial excess of coating substance is provided. This is
carried Ollt by pressing the coating blade 7 against the web
1 with a suitable force.
~23~65;
The invention is not limited to the embodiment ~hown, but
several modi~ications therof are feasible within the scope
of the attached claims.
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