The present invention relates to a process and an apparatus for scraping excess coating substance off a running web, such as a paper or cardboard web, of the kind employing a pivotable scraper blade to scrape off the excess material. Such a process and apparatus are used when it is desired to coat a web with a substance as evenly as possible. The web may run over a rotatable roller or over a fixed support device at the point where the excess coating substance is scraped off. If the web is coated on both sides, it may run between two symmetrically arranged scraping devices to remove the excess material from each side.
Apparatus of this kind is known, for example, from the Journal "Wochenblatt fuer Papierfabrikation", No. 16, (1978), pages 619 to 624 (published in English in the publication Voith-Press p 2371e), as well as from U.S. Pat. Nos. 3,131,092 and 3,882,817. In the known apparatus, there is a blade holder that is generally attached to a so-called scraper beam. Both the blade holder and the scraper beam extend transverse to the direction in which the web runs. The scraper blade is pressed against the paper web with adjustable force by means of an adjustable pressure device. During this process the scraper blade is deformed to a greater or lesser extent by the force exerted on it by the pressure device. Moreover, the blade is worn down by friction between it and the coating it scrapes off the web.
In the design described on page 622 of the cited Journal the scraper beam may be mounted for rotation as a unit with the blade holder around each of two different pivot axes. A first pivot axis is parallel to the blade and is located in the vicinity of the point of the blade, i.e. the end of the blade that is directed toward the web. By pivoting the scraper beam, the blade holder and the scraper blade around the first pivot axis located at or near the point of the blade, the scraper blade can be brought into different angular positions relative to the running web. This is necessary for adaptation to different types of webs, different coating substances and different coating thicknesses. The angle between the blade and the web, known as the blade angle, is determined by the angular position of the scraper beam about the first pivot axis and by the degree of deformation of the scraper blade caused by the force with which the blade is pressed against the web.
A second pivot axis is also provided, about which the scraper beam can be rotated between a rest position and an operative position. Both pivot axes are parallel to the surface of the running web and are perpendicular to the direction of the motion of the web.
The desired coating thickness can be obtained by setting the force exerted by the pressure device on the blade. If this force varies, the bowing of the blade changes and with it also the blade angle. In addition, the scraping surface of the blade, i.e. the oblique surface located at the point of the blade or that surface formed by wear during operation, no longer lies parallel to the surface of the web, as before. A disadvantage of the known apparatus is that after the force on the blade is altered, some time passes before the scraping surface of the blade is worn down parallel to the web again. Until the blade is worn down by friction in this manner, the coating will not be scraped to a uniform thickness.
To maintain the desired coating thickness as the blade scrapes the coating substance from the web after the force urging the blade against the web is changed, the blade angle must remain the same as before the force was changed. Although the machine operator could manually rotate the scraper beam and blade holder about the first pivot axis until the blade angle resumes its previous value, this procedure requires a high degree of skill and would takes the machine operator away from other tasks.
On the other hand, the apparatus known from the cited Journal reference has the advantage that the blade angle can be set at any required value over a very wide range. For example, it is possible to pivot the scraper beam and blade holder sufficiently far for the blade angle to be zero, while simultaneously urging the blade sufficiently strongly against the web. When the blade angle is zero, a side surface of the blade, rather than the oblique blade scraping surface, is pressed against the web. This reduces the amount of coating substance the blade scrapes off the web, making it possible to apply larger amounts of coating substance to the web and thus to obtain a greater coating thickness.
Finally, German Pat. No. 24 35 527 suggests locating the blade holder pivot axis between the blade point and the point of application of the force on the blade in such a way that the force can be varied as desired by pivoting the blade holder, without the blade angle being altered. As long as the blade angle remains constant, the scraping surface of the blade remains in a substantially unchanged position parallel to the direction of motion of the web. If the arrangement of this patent is employed, therefore, no wearing-down of the blade is required after the magnitude of the force is reset. With this arrangement it is not possible, however, to vary the blade angle by rotating the blade holder around its pivot axis. In particular, it is not possible to achieve the especially desirable operating condition in which the blade angle is zero, without providing a second pivot axis for rotation for the purpose of changing the blade angle. The additional axis would be located, as in the known apparatus first described above, in the vicinity of the point of the blade.