Method for limitation of the width of coating in coating of paper or board and a device intended for carrying out the method

Abstract: The invention concerns a method for limitation of the width of coating in coating of paper or board. The coating agent is spread onto the moving base (5) to be coated by means of a coating device provided with a grooved coating bar (21). It is an essential feature of the invention that the coating agent is spread substantially over the width of the web only and that any coating agent extending substantially beyond the web width is scraped off the base (5) to be coated with the aid of smooth end areas (a) fitted at both ends of the coating bar (21). The invention also concerns a device that carries out the method.

Description: The invention concerns a method for limitation of the width of coating in coating of a paper or board web, the coating agent being spread onto the moving base to be coated by means of a coating device provided with a grooved coating bar. The invention also concerns a device intended for carrying out the method, comprising a revolving coating bar, which rests against the moving base to be coated, which extends across the width of the coating device, and a substantial portion of which is provided with grooves, said coating bar being fitted to spread the coating agent onto the base to be coated. At present, in the coating of a paper or board web, two alternative methods and devices are commonly used, i.e. a blade coater or a bar coater. The present invention is expressly related to the latter, 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, as a rule, 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. The material of the coating bars currently in use is usually steel, and, in view of increasing the service life of the bars, they are frequently provided with chromium plating. Bars with fully smooth faces are, as a rule, not used, but 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 a grooved bar, e.g., onto the rolls in the size press, the areas of the roll ends placed outside the web must be scraped clean by means of lateral doctors, because otherwise this coating agent that remains in the end areas of the rolls causes considerable splashing in the roll nip. The scraping of the lateral areas has proved very difficult. JP-A-59-073073 discloses a coater, in which a grooved applicator roll is both feeding and transferring the coating agent. However, the coating agent transferred to the web has not been smoothed, dry zones of the web remaining between the coated strips. EP-A-0 079 759 discloses a method and an apparatus for surface replication on a coated sheet material. The apparatus comprises a coating station, which is followed by an electron beam curing station having a replicating roll. A web is coated in the coating station by a coating pick-up roll, a coating transfer roll and a nip formed between the transfer roll and a backing roll. However, the apparatus lacks a rotating coating bar, that smooths out the coating agent. Any smoothness formed in the coating layer on the web is destroyed by the replicating roll, which forms a permanent replication in the coating. An object of the present invention is to provide a method and a device which eliminate the drawbacks of prior art as disclosed above. This object is attained by the method defined in the preamble of claim 1, that is based on the publications mentioned above, the method having the features defined in the characterizing clause of claim 1. The above object is also attained by a device defined in the preamble of claim 4 having the features defined in the characterizing clause thereof. The most remarkable advantage of the invention over prior art is that, when the method of the invention is employed, it has been 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 size present beyond the web width on the smooth bar portion is so little that the size passes through the nip without splashing. The further advantages and characteristic features of the invention appear from the following detailed description of the invention. In the following, the invention will be described in detail with reference to the figures in the accompanying drawing. Figure 1 is a fully schematic side view of a film size press in which the method and the device in accordance with the invention are applied. Figure 2 is a simplified illustration of one coating device in the film size press, shown on a larger scale. Figure 3 is a schematic sectional side view of a bar coater which is employed for coating taking place directly onto the web surface. Figure 4 is a simplified illustration of Fig. 3, viewed in the direction of the arrow A. 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 so that the first roll 2 and the second roll 3 form a nip N between them, the paper or board web W being passed through said nip. In the film size press 1, a first size film F1 is metered onto the face 4 of the first roll 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 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'. 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 similar to one another, as is shown in Fig. 1. The coating devices 10 and 20 are coating devices of 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, which chamber is, besides by said coating bar 11,21, also defined by the roll face 4,5, by 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 made of a suitable material, e.g. polyurethane, said cradle supporting the coating bar 11,21 over its entire length. The coating bar 11,21 is provided with a purposeful drive gear (not shown), by whose means the coating bar 11,21 is rotated in directions opposite to the directions of rotation of the rolls 2,3. Further, in Fig. 1, the holders of the cradles of the coating bars 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 is 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 have been allowed to 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 substantially as of a width 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 such an arrangement, on said smooth end areas a of the bar, any size extending beyond the width of the web is spread onto the face(s) 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, before the coating bar 53 and which chamber is, besides by said coating bar 53, 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 purposeful drive gear, 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 a 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'. As is shown in Fig. 4, the coating bar 53 is a grooved bar the same as the one described in connection with Figs. 1 and 2. Thus, the bar 53 is not provided with grooves across its entire axial 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' 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.

Claims: Method for limitation of the width of a coating in coating of a paper or board web (W), the coating agent being spread onto the moving base (4,5,W) to be coated by means of a coating device (10,20,50) provided with a grooved coating bar (11,21,53), characterized in that 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 (4,5,W) to be coated with the aid of smooth end areas (a) fitted at both ends of the coating bar (11,21,53). Method as claimed in claim 1, characterized in that, the coating agent, such as a sizing agent, is spread onto the face (4,5) of a roll in a size press, and by means of the smooth end areas (a) on the coating bar, the coating agent is spread onto the face (4,5) of the roll in the size press outside the width of the web so that the layer of coating agent applied onto the end areas (a) of the roll passes as a sufficiently thin layer without splashing through the nip (N) of the size press. Method as claimed in claim 1, characterized in that the coating agent is spread directly onto the surface of the paper or board web (W), and by means of the smooth end areas (a) of the coating bar, the coating agent is scraped so that the lateral areas of the web (W) remain substantially free from the coating agent. Coating device (10,20,50) intended for carrying out the method as claimed in claim 1, comprising a revolving coating bar (11,21,53), which rests against the moving base (4,5,W) to be coated and extends across the width of the coating device, and a substantial portion of which is provided with grooves (f), said coating bar (11,21,53) being fitted to spread the coating agent onto the base (4,5,W) to be coated, characterised in that the coating agent is introduced into the coating device (10,20,50), in the direction of running of the base (4,5,W) to be coated, upstream of the coating bar (11,21,53), which smoothes the spread coating agent and onto which the grooves (f) have been formed substantially across the web width, and the end areas (a) of the coating bar that extend substantially beyond the web width have been formed smooth. Device as claimed in claim 4, characterized in that the diameter (D) of the smooth end areas (a) of the coating bar (11,21,53) is substantially equal to the outer diameter (d) of the grooved portion of the coating bar (11,21,53).

Inventors: RANTANEN RAUNO, RANTANEN, RAUNO
Applicants: VALMET PAPER MACHINERY INC, VALMET PAPER MACHINERY INC.
Application year: 1991
Main classification: B05C1
IPC codes: B05C3, B05C11, B05C1, D21H23