Abstract:
The invention concerns a device and method for limiting of the width of coating in coating of paper or board. The coating agent is spread onto a moving base to be coated by means of a coating device provided with a grooved coating bar. The coating agent is spread substantially only over the width of the web and any coating agent extending substantially beyond the web width is scraped off the base to be coated with the aid of smooth end areas fitted at both ends of the coating bar. The invention is also related to a device for carrying out the method.

Description:
FIELD OF THE INVENTION 
     The invention concerns a device and method for limiting of the width of coating in coating of paper or board, wherein the coating agent is spread onto the moving base by means of a coating device provided with a grooved coating bar. 
     The device comprises a revolving coating bar, which rests against the moving base to be coated and extends across the machine width. A substantial portion of the revolving coating bar is provided with grooves. The coating bar is fitted to spread and to smooth the coating agent onto the base to be coated. The coating agent is introduced into the coating device in the direction of running of the base to be coated, before the coating bar. 
     BACKGROUND OF THE INVENTION 
     At present, in the coating of paper and board, two alternative methods and devices are commonly used, i.e. a blade coater or a bar coater. The present invention is expressly related to bar coaters, which have proved excellent especially in the surface-sizing technique. A bar coater is employed in coating and surface-sizing for spreading and smoothing the coating agent onto the surface of a paper or board web, to which the coating agent is introduced by means of a spreading roll. 
     In film size presses, the coating agent may also be introduced directly onto the faces of the rolls in the size press, from which, in the roll nip, it adheres to the web that passes through the nip. Coating bars currently in use ar usually made of steel. Coating bars are frequently provided with chromium plating to increase their service life. Bars with fully smooth faces are not used. Rather the bar face is provided with grooves, or steel wire may be wound onto the bar to form a solution similar to grooves on the bar face. 
     When the coating agent is applied by means of such a grooved coating bar, e.g., onto the rolls in the size press, the areas of the roll ends placed outside the web must be scraped clear by mean of lateral doctors. Otherwise the coating agent that remains in the end areas of the rolls causes considerable splashing in the roll nip. 
     The scraping of these lateral areas of the rolls has proved very difficult. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a method and a device which overcomes the drawbacks of prior art coating bars and through which an improved coating result is achieved as compared with prior art. 
     In view of this objective and others, the present invention is directed in part to a method where the coating agent is spread substantially over the width of the web only, and any coating agent extending substantially beyond the web width is scraped off the base to be coated with the aid of smooth end areas fitted at both ends of the coating bar. 
     The device in accordance with the present invention relates to a coating bar wherein the grooves have been formed substantially across the web width, and the end areas of the coating bar that extend substantially beyond the web width have been formed smooth. 
     An important advantage of the invention over the prior art is that pursuant to the invention, it is possible to omit the lateral doctors completely, because they are unnecessary. In the method and device in accordance with the invention, the scraping of the lateral areas is carried out by means of the coating bar itself, the scraping already taking place at the stage of application of the coating. In the case of a size press, the area beyond the web width on the smooth bar portion is so small that the size passes through the nip without splashing. Further advantages and characteristics of the invention are set forth in the following detailed description of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following, the invention will be described in detail with reference to the Figures in the accompanying drawings. The Figures of the accompanying drawings represent preferred embodiments of the invention, but should not be construed to limit the scope of the invention. 
     FIG. 1 is a schematic side view of a film size press in which the method and the device in accordance with the invention are applied. 
     FIG. 2 is a plan view of a simplified illustration of one coating device in the film size press, shown on a larger scale. 
     FIG. 3 is a schematic sectional side view of a bar coater which is employed for coating taking place directly onto the web surface. 
     FIG. 4 is a simplified illustration of FIG. 3, viewed in the direction of the arrow A. 
    
    
     DETAILED DESCRIPTION 
     In FIG. 1, the size press is denoted generally with the reference numeral 1. The film size press 1 comprises size press rolls 2 and 3. The first roll 2 and the second roll 3 form a nip N between them, the paper or board web W being passed through the nip. In the film size press 1, a first size film F1 is metered onto the face 4 of the first roll 2 by means of a first coating device 10 and, in a corresponding way, a second size film F2 is metered onto the face 5 of the second roll 3 by means of a second coating device 20. In the roll nip N, the size films F1 and F2 are transferred onto the paper or board web W running through the nip. In FIG. 1, the coated web is denoted with the reference W&#39;. 
     In the film size press 1 shown in FIG. 1, the coating devices 10 and 20, by whose means the size films F1 and F2 are spread onto the faces 4 and 5 of the rolls 2 and 3 in the size press, are bar coaters, which are equal in size to one another, as is shown in FIG. 1. The coating devices 10 and 20 are of the so-called short-dwell type, in which the coating agent is introduced into a pressurized coating-agent chamber 16, 26 placed before the coating bar 11, 21. Besides being defined by the coating bar 11, 21, the chamber is also defined by the roll face 4, 5, the front wall 14, 24 of the coating-agent chamber, as well as by possible lateral seals, if any (not shown). The coating bar 11, 21 is fitted in a cradle 12, 22, which is made of a suitable material, e.g. polyurethane. The cradle supports the coating bar 11, 21 over its entire length. The coating bar 11, 21 is provided with a drive gear (not shown) which rotates the coating bar 11, 21 in directions opposite to the directions of rotation of the rolls 2, 3. The holders of the cradles of the coating bars in FIG. 1 are denoted with the reference numerals 13 and 23, and the holders of the front wall with the reference numerals 15 and 25. 
     In the invention, the coating bar used is a grooved bar 21, which is shown in more detail in FIG. 2. As shown in FIG. 2, the grooved bar 21 is not provided with grooves extending from end to end, but the end areas a at both ends of the bar remain smooth. The ratio of the grooved portion f and the smooth end areas a on the coating bar 21 has been chosen so that the grooves f have been formed onto the bar 21 to be of a width substantially equal to the width of the web. The outer diameter D of the bar 21 at the smooth end areas a is substantially equal to the outer diameter d of the grooved portion on the bar 21. Owing to this arrangement, on said smooth end areas a of the outer bar, any size extending beyond the width of the web is spread onto face 5 of the roll 3 in the end areas b of the roll so that the size layer is so thin that it passes through the roll nip N. In such a case, substantially no splashing occurs in the roll nip N. 
     In FIGS. 3 and 4, a second embodiment of the invention is shown, which is applied to spreading of the coating agent directly onto the surface of the paper or board web W. In FIG. 3, the coating device is denoted generally with the reference numeral 50. As is shown in FIG. 3, the coating bar 53 is fitted against the paper or board web W that runs on the face of a backup roll 54. The coating device 50 is a coating device of so-called short-dwell type, in which the coating agent is introduced into a coating-agent chamber 51, which is placed in the direction of running of the web W and before the coating bar 53. In addition to the coating bar 53, the chamber is also defined by the web W, by the front wall 52 of the coating-agent chamber, and by lateral seals (not shown). The coating-agent chamber 51 is pressurized in the conventional way, and out of the chamber 51, overflow of the coating agent is provided through the gap 55 between the front wall of the coating-agent chamber and the web W. The coating bar 53 is fitted in a cradle 58 of a suitable material, e.g., polyurethane, said cradle supporting the coating bar 53 over its entire length. The coating bar 53 is provided with a drive gear (not known), by whose means the coating bar 53 is rotated in the direction opposite to the direction of running of the web W. The cradle 58 of the coating bar 53 is fitted on a support 56 and both the cradle 58 and the support 56 are together attached to the holder 59 fitted on the frame of the coating device 50. Moreover, on the support 56, underneath the cradle 58, a loading hose 57 is provided, by whose means the coating bar 53 can be loaded in the desired way against the web W. In FIGS. 3 and 4, the coated web is denoted with the reference W&#39;. 
     As is shown in FIG. 4 the coating bar 53 is a grooved bar similar to that shown in FIGS. 1 and 2. Thus, the bar 53 is provided with grooves across its entire length, but the end areas a of the bar have been allowed to remain smooth so that the grooved portion f of the bar 53 extends substantially across the web width only, or, as is shown in FIG. 4, is slightly narrower than the web width so that dry lateral areas c remain in the web W&#39; which is coated otherwise. Also, in the embodiment shown in FIGS. 3 and 4, the diameter D of the smooth end areas a of the bar 53 is substantially equal to the outer diameter d of the grooved portion of the bar. 
     The invention has been described by way of example with reference to the Figures in the accompanying drawing. The invention is, however, not confined to the exemplifying embodiments shown in the Figures alone, but many variations are possible within the scope of the inventive idea defined in the following patent claims.