Coating processes – With post-treatment of coating or coating material – Heating or drying
Reexamination Certificate
1999-01-08
2001-01-09
Beck, Shrive (Department: 1762)
Coating processes
With post-treatment of coating or coating material
Heating or drying
C427S356000, C427S358000, C427S361000, C427S398400, C427S428010, C427S211000, C427S350000
Reexamination Certificate
active
06171653
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus for applying a liquid or viscid coating medium onto a moving material web, especially one made of paper or cardboard.
2. Description of the Related Art
An exemplification of a known apparatus for applying a liquid or viscid coating medium onto a moving material web is illustrated in FIG.
2
a
. The coating apparatus includes an applicator roll
12
and a counter roll
14
. The two rolls,
12
and
14
, form a nip through which material web
16
runs in the direction of feed. Applicator roll
12
is coated with a liquid or viscid coating medium by use of a coating mechanism
18
. This medium is subsequently transferred onto the material web
16
in the area of the coating location S. Applicator roll
12
and counter roll
14
are driven in opposite directions around their respective axes A and B as indicated by arrows P and P′. Such coating apparatuses are applied, for example, in film coating processes, roll coating processes and size presses.
Coating apparatus
10
usually experiences problems in the area Sp—located adjacent to the coating location S, on the downstream side relative to the movement of the material web
16
—as the material web
16
separates from the outer surface
12
a
of roll
12
. The pre-metered coating of coating medium
20
being applied onto the outer surface
12
a
is only partially transferred in the nip onto the material web
16
. A part
20
b
of the coating medium
20
adheres to the outer surface of roll
12
, and therefore, returns back to the coating mechanism
18
. Those experienced in the art, refer to this effect as “film splitting”.
In the process of film splitting, “coating medium threads” F (FIG.
2
b
) develop between the coating medium
20
being applied onto the material web
16
and the remaining layer
20
b
of coating medium on roll
12
—similar to the honey threads that are formed between a spoon that is pulled from a honey jar. When these threads F are torn, it is possible to develop droplets or particles
20
c
of the coating medium
20
, which is reflected in the formation of spray mist N. Accordingly, it can be said that the spray mist is formed by the particles and droplets
20
c
of the coating medium
20
, which, for example, are torn from the previously applied coating
20
a
by the adhesive force of applicator roll
12
.
Due to the aforementioned formation of the spray mist, the quality of the coated material web can be compromised in several ways. For example, small craters can form on the outer surface of the coated material web. Furthermore, the remaining parts of the torn-off coating medium threads F can cause a so-called “orange skin effect”. Additionally, the re-deposit of droplets or particles
20
c
in an area removed from the spray area Sp can lead to an irregular structure of the material web's coating surface. The spray mist also causes other drawbacks such as a contamination of the coating apparatus.
The risk of encountering the above-stated disadvantages increases as the feed speed of the material web
16
increases, as the area specific coating weight on the material web
16
increases, and as the absorbency capability and/or the bibulousness of the material web
16
decreases. To avoid the aforementioned disadvantages of the state of the art, it was suggested in a summary report issued by the “Tappi Metered Size Press Forum” by Dr. Hans W. Maurer (weekly publication for paper manufacturing 17, 1997, page 827) to vary the composition of the coating medium. This approach, however, has proven to be impractical.
SUMMARY OF THE INVENTION
The present invention provides a coating apparatus of the type mentioned above, which eliminates the spray mist from the coating medium, or, at a minimum, reduces its adverse effects on the coating result, so that a uniform coating can be achieved.
A coating mechanism first applies the medium onto an applicator element or an applicator roll, which subsequently transfers the coating medium at a coating location onto the material web in the form of a coating layer, or it is entered into a coating sump, formed between the material web and a limiting element or limiting roll in which the coating medium is deposited onto the material web. If so desired, a doctoring device can be positioned at a doctor station, adjacent to the coating location, on the downstream side relative to the movement of the material web, for the purpose of smoothing and/or metering the coating.
This problem is solved by providing an apparatus to generate and/or supply fluid, preferably steam. The apparatus is positioned adjacent to the coating location or doctor station on the downstream side relative to the movement of the material web.
For the sake of simplicity, further elaboration of this invention will be conducted by using steam as the fluid that is being introduced in the area of the coating location. It is, however, understood that other fluids can also be successfully used, not only in the area of the coating location, but also in the area of a doctor station at which the coating is smoothed and/or metered by use of a roll doctor, which also has the tendency to develop spray mist.
In accordance to this invention, a device provided to surround the spray area Sp, located adjacent to coating location S on the downstream side relative to the movement L of the material web, with a steam-saturated atmosphere, preferably steam produced from water. This causes, on one hand, a reduction in the surface tension of the coating medium, which tends to favorably affect the thread and droplet formation. On the other hand, the steam-saturated atmosphere causes a slowing of the drying process of the coating medium, as well as the medium droplets that are being applied onto the material web, so that the structures on the coating that are being produced by tearing threads and droplets can bleed into the coating without leaving any marks. This results in a more uniform coating.
It has been proven to be advantageous to use steam that no longer absorbs any water, i.e., saturated and superheated steam.
Regardless of whether the steam is produced near the coating apparatus or whether the steam is produced in a remote area relative to the coating apparatus and subsequently piped to the spray area, it is desirable for the steam-generating apparatus and/or the steam supply apparatus to point the steam directionally towards the coating location.
For example, the steam-generating apparatus and/or the steam supply apparatus can include a steam supply line in the area of the coating apparatus. The line has at least one steam discharge opening facing the coating location.
In accordance to a first design variation, the aforementioned advantages provided by this invention can be realized with a steam-saturated atmosphere which is, by and large, stationary in the spray area, if one ignores the effects of the material web movement, the steam supply effects and any residual spray mist effects.
According to an alternative design variation, it is, however, also possible to obtain these same advantages with a dynamic steam-saturated atmosphere, which is achieved by blowing the steam into the spray area, past the steam-generating apparatus and/or the steam supply apparatus, and then exiting past the spray area. For that purpose, the steam discharge opening that forms the wall sections of the steam supply line, or the wall sections downstream of the steam discharge opening, can be formed nozzle-like, preferably with the smaller nozzle area on the downstream side.
The movement of the steam that is discharged can be affected by appropriate shaping of the outer surfaces of the steam-generating apparatus and/or the steam supply apparatus in such a way that a further smoothing of the coating on the material web can be achieved. For example, the steam-generating apparatus and/or the steam supply apparatus can be designed as a flow restrictor unit in such a way that the steam-saturated atmosphere, which is being discharged in the area adjace
Hannen Karl-Heinz
Hess Harald
Hoffmann Hans Peter
Kohl Bernhard
Kurtz R{umlaut over (u)}diger
Beck Shrive
Kolb Jennifer
Taylor & Aust P.C.
Voith Sulzer Papiertechnik Patent GmbH
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