Coating processes – Spraying – Moving the base
Patent
1998-06-02
2000-05-16
Beck, Shrive
Coating processes
Spraying
Moving the base
427421, 118314, 118315, 118324, B05B 1302, B05D 102
Patent
active
060634499
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention is directed to a method and apparatus for coating a moving web of paper or paperboard in which an applicator or levelling apparatus applies a coat layer to the web without contacting the web.
BACKGROUND OF THE INVENTION
To improve the printability of paper, the paper may be coated with a coating formulation containing mineral pigment and binder components. Over the years, application and levelling of the coat have been carried out using a variety of apparatuses. Higher web speeds and increased demands on process efficiency and paper quality in combination constitute the stimulus driving the development of applicator equipment.
Initially, paper coating with a pigment-containing formulation was performed using coaters of the gate roll type, in which the coating mix was first metered with the help of furnish rolls to a set of transfer rolls, and therefrom further to the moving web of paper. However, the function of such a coater is impaired at web speeds exceeding 400 m/min. The nips of the rolls start to throw out splashes of the coating mix, and the coating process lacks the stability required to achieve an acceptable coat quality. Furthermore, well-behaved control of coat weight is difficult to achieve when using the above-described technique.
Particularly for surface sizing, sizing presses have been used in which the downward, running web is passed through a coat mix pond sealed by the rolls. Herein, a problem arises from the strong increase of moisture content in the web and difficult controllability of the correct amount of applied size.
In the kiss-coating technique, the coating mix is metered directly in a nip from the casting roll to the surface of the paper web. In the early days, and in paperboard coating even today, excess coat is doctored away with the help of an air knife. At web speeds above 500 m/min, however, the impact force of the air flow from the slot orifice of the air knife is insufficient for effective doctoring of the coat layer applied to the web surface.
An essential increase in coating speed was facilitated by the adoption of the doctor blade levelling technique for controlling the final coat weight. In the first generation of blade coaters, the web was arranged to run from above downward, and the coating mix was pumped into a pond formed in the recess between the backing roll and the blade. In fact, the same technique is still being used in two-sided coating.
The actual break-through of the blade coating technique occurred along with the adoption of the transfer coating method. Herein, the coat is applied directly to the web surface in the nip between a transfer roll and a backing roll. Excess coat is removed by means of a doctor blade extending aver the entire web width. This kind of coating technique makes it possible to increase the web speed to about 1300 m/min. At web speeds above this, splashing of the coat at the nip and the air film which is entrained in the nip along with the moving web, thereby causing skip marks on the coated web, make the use of this method extremely complex if not impossible. The higher the web speed, the fewer degrees of freedom will be available in the selection of coat mix components. Herein, the coating mix formulations must be selected under the constraints of web runnability, sometimes even compromising the quality of the end product.
Due to the poor runnability of transfer coaters, a short-dwell doctor blade coater was developed to provide an alternative technique for applying light coats to thin-caliper paper grades. In this type of coater, the web is guided past a slot orifice box which is formed by a short-dwell application chamber and the doctor blade and is adapted to operate against a backing roll. This method has been extremely popular in the art and facilitated effective on-machine coating. Also in this method, the maximum practicable web speed has turned out to be the limiting factor for further development. At web speeds above 1300 m/min, striping will appear at coat weights higher than
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Kinnunen Jorma
Kirveskari Erkki
Koskinen Jukka
Luomi Seppo
Nissinen Vilho
Beck Shrive
Calcagni Jennifer
Valmet Corporation
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