Coating processes – Spraying – Moving the base
Reexamination Certificate
1995-06-16
2001-03-20
Beck, Shrive (Department: 1762)
Coating processes
Spraying
Moving the base
C427S209000, C427S294000, C118S325000, C118S316000, C118S050100
Reexamination Certificate
active
06203858
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method and system for coating a traveling material web, such as a paper web or cardboard web.
2. Description of the Related Art
Coating systems for paper and cardboard are variously known where, for instance according to GB 21 03 115 A, the transfer of coating mixture on the material web, from the shells of two coordinated rolls forming together a press gap, takes place in the press gap. Employed as an applicator system are nozzle applicators or dip rolls. For smoothing and dosing of the applied layer, the applicator system uses a doctor blade.
Known for the coating of traveling webs are also applicator systems which operate in the fashion of a curtain coater and where the uniformity and adhesion of the curtain coat is improved in that the system is preceded by a vacuum chamber arranged on the backing roll that carries the web. A curtain coating method employing a vacuum chamber is described in U.S. Pat. No. 3,735,729, which is not customarily used for paper webs. It serves primarily the coating of photographic paper, since the respective coating mixtures possess a high consistency. Considerable problems are encountered in the coating of paper webs with sizing, which have a very low consistency and viscosity.
All of these inventions are suited for the application of relatively large quantities of coating mixture per square meter (m
2
) of web area. But they fail when an extremely low coating weight per m
2
is desired.
Known from EP 04 35 904 B1 is a method and a device for the coating of paper, cardboard or similar substrates that travel continuously along a path. The coating is transformed to a mist and fed to an applicator nozzle with an applicator surface. A vacuum chamber for pickup of surplus mist is arranged adjacent to the applicator surface. The spray mist created is by a “positive force” (pressure) and by means of the applicator nozzle applied directly on the substrate (paper web). With this method, too, it is doubtful whether an extremely low coating weight per m
2
can be applied on the paper web with the uniformity that is necessary for many paper grades.
Lastly, with so-called air knife systems it is also known to arrange the air knife setup in a large vacuum container. Here, however, the air knife does not apply a coating, but serves to scrape coating mixture off the web in order to dose it.
The problem underlying the invention is to provide an applicator system, or coating system, for traveling material webs involving very light, extremely thin coatings and nonetheless uniform amounts of coating, notably for the application of sizing.
SUMMARY OF THE INVENTION
The present invention provides a spray nozzle for producing a spray mist of coating mixture which is applied to the outside of at least one roll and transferred to the material web from the roll. The spraying of a liquid mixture as such is customary in many technical fields, but is not at all customary for the coating of traveling webs of paper or cardboard.
According to the invention, a coating that is both extremely thin and very uniform, notably also transverse to the direction of travel, is obtained by direct application; that is, spray mist is applied on a bare roll and picked up by the material web. The uniformity is inventionally obtained at the contact point between web and roll, where the web picks the thin liquid film up from the roll. The web may be carried by the roll with a certain tension, the web being deflected. Preference is given though to allowing the web to proceed through the press gap formed by the two rolls, which makes a double-sided coating possible.
The invention makes use of the vacuum chamber known from EP 04 35 904 B1, which is arranged around the spray nozzle. The vacuum chamber serves to suck up surplus coating mixture mist also in the present invention, but part of the mist can settle on the inside wall surface.
According to an important, further idea of the invention, a baffle is provided for part of the surplus spray mist, by arranging within the vacuum chamber a side wall beside the spray nozzle, on the departure side of the pertaining roll surface. The baffle (side wall) is preferably so fashioned, or enlarged, that it forms an inner chamber around the spray nozzle. In this chamber prevails a certain pressure, produced by the spray mist discharging from the spray nozzle.
The effect of the baffle (side wall or inner chamber) is, for one, that the spray mist proceeds better to the roll and, for another, a relief of the vacuum chamber which, similar to EP 04 35 904 B1, safeguards that coating mixture (spray mist) issuing out of the spray nozzle cannot escape from the chamber, that is, through the gap existing between the chamber and backing roll, into the surroundings. The air entering from the surrounding holds the spray mist back in the chamber, allowing it, in the invention, to settle partly on the baffle and partly on the inside wall of the outer chamber. An outlet allows the precipitated coating to drain. Precipitated mist remaining in the outer or inner chamber is passed, for instance by a suction blower, to a separator where the separated mixture and the mixture draining directly from the chamber can be fed to a common collection vessel for recycling.
REFERENCES:
patent: 4448818 (1984-05-01), Hartog et al.
patent: 4944960 (1990-07-01), Sundholm et al.
patent: 5376177 (1994-12-01), Elvidge et al.
patent: 5397601 (1995-03-01), Korhonen
patent: 2 068 781 (1980-12-01), None
patent: 2 103 115 (1982-07-01), None
patent: 2103115 (1983-02-01), None
Beck Shrive
Chen Bret B.
Taylor & Aust P.C.
Voith Sulzer Papiermaschinen GmbH
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