Coating device for coating of a size-press roll, paper or board

Abstract: The invention concerns a coating device for coating of a size-press roll, paper or board or of an equivalent moving base. The coating device comprises a revolving coating bar (11,21), which rests against the moving base (4,5), which extends across the machine width, and which is supported in a cradle (12,22) substantially over its entire length. The coating bar is fitted to spread and to smooth the coating agent onto the moving base (4,5), which said coating agent was introduced into the coating device ( 10,20), in the direction of running of the moving base (4,5), before the coating bar (11,21). The coating bar (11,21) in accordance with the invention is a smooth bar of large diameter, which is fitted against the moving base (4,5) so that the profile of coating quantity can be regulated under control.

Description: The invention concerns a coating device for coating of a size-press roll, paper or board or of an equivalent moving base, comprising a revolving coating bar, which rests against the moving base, which extends across the machine width, which is supported in a cradle substantially over its entire length, and which said coating bar is fitted to spread and to smooth the coating agent onto the moving base so that the profile of coating quantity can be regulated under control, which said coating agent was introduced into the coating device, in the direction of running of the moving base, before the coating bar. At present, in the coating of paper or 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 the latter, bar coaters, which have probed excellent especially in the film size press technique. The material of the coating bars currently in use is usually steel, and, in view of increasing the service life of the bar, the bars are provided with chromium plating. In surface sizing of paper, bars with fully smooth faces have not been used, but the face of the bar has been provided with grooves, or steel wire may have been wound onto the bar to form a solution similar to grooves on the bar face. The use of a grooved bar for surface sizing of paper is based thereon that the thickness of the size film to be applied onto the base to be coated is determined by means of the depth of the grooves. A grooved bar is, however, poorly suitable for the preparation of thin size films, because it is very difficult to manufacture grooves of sufficiently small scale and, on the other hand, these grooves are easily contaminated. It is a further highly significant drawback of grooved bars that they are worn rapidly. Pigmenting with a high dry solids content is also entirely impossible with grooved bars, because the wear of the bars is excessive in this connection. The diameters of the coating bars currently in use have been very little, the order of magnitude of the diameter being, as a rule, about 10 mm. Small-diameter bars are used because the bar ought to be able to adapt itself to the shape of the roll face in the size press in order that the profile of the size film could be made suitable and correct. It has not been possible to use bars of large diameter because, owing to their thickness, they have been excessively rigid to provide an adequate profiling. In respect of the prior art, reference is made in particular to the US Patent No. 2,970,564, from which a solution is known for spreading of a size film or of a pigment coating film onto the rolls in a size press, said films being transferred to the paper in the nip in the film size press. In the method of said publication, smooth or grooved bars are used for the metering of the size film. In said publication, it is possible to use a smooth-faced bar of small diameter (diameter about 9 mm) for surface sizing, but this solution has not proved sufficiently good, and therefore a corresponding solution has never been applied in practice. In respect of the poor art, reference is further made to the FI-A-30,147. WO-A-88/05698, which forms the basis of the preamable of claim 1, discloses a coating bar with an elastic or flexible outer layer. The object of the present invention is to provide a solution by whose means the drawbacks related to the prior art are avoided and by whose means a significant improvement is produced in particular in surface sizing of paper. In view of achieving this objective the invention is characterized by the features stated in the characterizing clause of claim 1. It is the most important advantage of the present invention over the prior art that, by means of a coating device in accordance with the invention and while using a large-diameter coating bar, it has been possible to run very thin size films and coating pastes of high dry solids content even at high running speeds. With the device in accordance with the invention, the level of the coating quantity and the profile of the coating have been very good. Further advantages and characteristic features of the invention come out 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 a coating device in accordance with the invention is applied. Figure 2 is a fully schematic side view of an embodiment of the invention by whose means a good final result is obtained in the regulation of the profile of the coating. Figures 3 and 4 are schematic illustrations of an embodiment of the solution in accordance with the invention for the control of the profile of the coating quantity. Figure 5 is a schematic side view of an embodiment of the invention wherein the invention is employed in coating taking placed directly on the paper or board web. Figure 6 is a graphic presentation of test results obtained with a coating device in accordance with the invention, the coating quantity being shown as a function of the loading of the coating bar. Fig. 1 is a schematic illustration of the size press, which 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 i 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 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. Further, between the coating-bar 11,21 cradle 12,22 and the holder 13,23, an ordinary loading hose or equivalent (not shown) is fitted, by whose means the coating bar 11,21 can be loaded against the roll face 4,5 to produce the desired loading pressure. According to the invention, in the coating devices 10,20 shown in Fig. 1, a smooth-faced coating bar 11,21 is employed. In addition to the fact that the coating bar 11,21 in accordance with the invention has a smooth face, the diameter of the coating bar 11,21 is substantially larger than in the prior-art solutions. In the coating device in accordance with the invention, the diameter of the coating bar 11,21 is at least 18 mm, and, in test runs, very good results have been obtained with a coating device in which the diameter of the coating bar was 20 mm. However, a dimension even larger than this is considered to be the most appropriate bar diameter, and the diameter of the bar is optimally 25...80 mm. In particular in wide machines, it is necessary to regulate the profile of the coating quantity produced in the coating device, which regulation would be substantially impossible with a large-diameter coating bar in accordance with the invention if constructions known from the prior-art small-diameter bars were employed, in which constructions the coating-quantity profile can be controlled by regulating the loading of the bar locally, in which case the loading profile is transferred to the coating process as the coating bar is deflected. This is why, for the device in accordance with the invention, alternative solutions have been developed for the purpose of regulation of the profile. Fig. 2 shows an alternative solution for how to bring the profile of the coating quantity on the size press rolls to the correct level by means of coating bars in accordance with the invention. In Fig. 2, the size press rolls are denoted generally with the reference numerals 61 and 71. The rolls 61 and 71 form a nip N between them, the web W being passed through said nip. In the nip N, the size films are transferred onto the web W surface, and the coated web is denoted with the reference W' in Fig. 2. Out of the coating devices for the rolls 61 and 71, in Fig. 2, the coating bars 41 and 51 only are shown, while omitting all the other components related to the coating devices in the illustration of Fig. 2. In the embodiment shown in Fig. 2, according to the invention, the coating bars 41 and 51 are large-diameter bars, having a diameter of at least 18 mm and advantageously 20 mm. In the embodiment shown in Fig. 2, the bars 41 and 51 have a unified and solid construction and are, consequently, quite rigid. This is why the size press rolls 61 and 71 shown in Fig. 2 are variable-crown rolls, so that they comprise a rigid central axle 62,72, on which the roll mantle 63,73 is arranged revolving. In the axle 62,72, hydraulic loading means 64,74 are provided, which are supported against the inner face of the roll mantle 63,73 in the plane of the nip N. In addition to the crown variation, the rolls 61 and 71 may be adjustable in zones, even though this is not essential in view of the operability of the invention. Thus, in the embodiment shown in Fig. 2, the profiling is carried out by means of the loading means 64,74 in the roll nip N itself. This is why, in the embodiment shown in Fig. 2, the coating bars 41,51 themselves need not be profiled, because the roll mantle 63,73 is straight at the coating bar 41,51. Thus, by means of the solution shown in Fig. 2, coating films with uniform profiles are produced on the roll 61,71 faces. In stead of the hydraulic loading means 64,74 shown in Fig. 2, the crown variation in the size-press rolls 61,71 may be accomplished so that the rolls 61,71 are provided with devices fitted inside the roll mantle 63,73, by means of which devices the temperature of the roll mantle 63,73 can be adjusted in zones. In such an embodiment, the regulation of the profiles of the size films is carried out by heating or cooling the rolls 61,71 in zones. Figs. 3 and 4 illustrate a further embodiment, by whose means the profile of the coating quantity applied to the roll face can be regulated as desired. Figs. 3 and 4 are schematic illustrations in the machine direction, showing a part of a roll as well as the coating on the roll face. Further, in Fig. 4, a coating bar is shown schematically. In Figs. 3 and 4, the roll is denoted with the reference numeral 91, the outer face of the roll with the reference numeral 91a, the roll coating with the reference numeral 92, and the outer face of the coating with the reference numeral 92a. As is shown in Figs. 3 and 4, the outer face 91a of the roll is shaped as curved in the axial direction, i.e. crowned. In these figures, the crowning is shown as remarkably exaggerated, for, as a rule, the crowning is of an order of 0.3 mm with a machine width of 7 m. As is shown in Fig. 3, the outer face 92a of the roll coating 92 is provided with negative crowning, so that at the ends of the roll the coating 92 is considerably thicker than at the middle of the roll. Fig. 4 shows a situation in which a large-diameter coating bar 81 in accordance with the invention is fitted against the roll. The coating bar 81 is made of solid material, being substantially rigid, in which case, when the coating bar 81 is loaded against the roll, the coating 92 is compressed below the coating bar 81 so that the compression a of the coating 92 in the end areas of the roll is considerably larger than the compression b at the middle of the roll. As the coating 92 is thinner at the middle of the roll than at the roll ends, the compression of the coating is smaller at the middle than at the roll ends. Thus, in a situation as shown in Fig. 4, the linear load between the coating bar 81 and the coating 92 is the same across the entire machine width. In this way, by means of this solution, a uniform profile of coating quantity is obtained. In stead of a coating bar 81 made of solid material, in many cases it is possible to achieve sufficient rigidity by means of a solution in which the bar is made of a tube, e.g., of chromium-plated copper or steel. It is, of course, also obvious that, e.g., by combining of solutions shown in Figs. 2, 3, and 4, a good result is also obtained. Thus, in such a combination, the size press would consist of a solution wherein one press roll is a variable-crown roll or a roll adjustable in zones in respect of temperature and the other roll is a crowned roll whose coating is provided with negative crowning. The solution shown in Fig. 5 illustrates coating taking place directly onto the paper or board web W. In Fig. 5, the roll is denoted with the reference numeral 110. The paper or board web W is passed over the roll face 111, and the coating is carried out by means of the coating device 100 directly onto the web W. The coating device 100 is a coating device of the so-called short-dwell type, which includes a large-diameter coating bar 101 in accordance with the invention mounted as revolving in a cradle 103. The cradle 103 is mounted in a holder 104 in the normal way. The coating bar 101, together with the front wall 102 of the coating device 100, defines the pressurized coating-agent chamber 106, into which the coating agent is introduced. The front wall 102 is mounted on a holder 105 in the normal way. In test runs that have been carried out, with a coating device in accordance with the invention, excellent results have been obtained. The test runs were carried out with a solution wherein a smooth coating bar of a diameter of 20 mm was mounted as revolving in a coating-bar cradle of conventional construction. The coating device constructed in this way was fitted in a size press. By means of the device, test runs were carried out for pigmenting. In some test runs, the dry solids content of the coating paste was 51 % and the viscosity 700 cPs. In the test runs, the coating quantity varied in the range of 3...10g per m² per side, and the running speed varied in the range of 800...1200 m per min. The profile of coating quantity that was obtained was very good. Cavitation was so little that almost no cavitation could be noticed. One reason for this was that the diameter of the coating bar was substantially smaller than in roll coaters proper, in which the large roll diameters cause cavitation as the film is split after the nip. The result was substantially better than with any method that had been studied earlier. Further, Fig. 6 is a graphic presentation of test results obtained with another paste quality. In Fig. 6, the paste quantity obtained is shown as a function of the loading pressure of the coating bar at different running speeds. The tests illustrated in Fig. 6 were also carried out by means of a smooth bar of a diameter of 20 mm mounted in a conventional cradle. The coating paste that was used was SPS kaolin paste, whose dry solids content was 50 % and viscosity 550 mPas. The roll in the press was provided with a polyurethane coating. In the test runs, running speeds of 300...1200 m/min were used. The loading pressure indicated in the table expressly means the loading pressure of the coating bar against the roll. As can be seen fully clearly from Fig. 6, by means of the construction in accordance with the invention, very thin coating quantities were obtained with a high dry solids content of the paste. Thus, by means of the invention, a remarkable improvement is achieved over the prior art. There is a high demand for the method in accordance with the invention in pigmenting and surface-sizing of news-print as well as in surface-sizing of SC-paper.

Claims: Coating device for coating of a size-press roll, paper or board or of an equivalent moving base, comprising a revolving coating bar (11,21,41,51,81, 101), which rests against the moving base (4,5,92a,W), which extends across the machine width, which is supported in a cradle (12,22,34,103) substantially over its entire length, and which said coating bar is fitted to spread and to smooth the coating agent onto the moving base (4,5,92a,W), so that the profile of coating quantity can be regulated under control which said coating agent was introduced into the coating device (10,20,100), in the direction of running of the moving base (4,5,92a,W), before the coating bar (11,21,41,51,81,101), characterized in that the coating bar (11,21,41,51,81,101) is a rigid smooth bar having a diameter of at least 18 mm. Coating device as claimed in claim 1, characterized in that the diameter of the coating bar (11,21,41,51,81,101) is 25 to 80 mm. Coating device as claimed in any of the claims 1 and 2, characterized in that the coating bar (11,21,41,51,81,101) is made of a unified solid material. Coating device as claimed in any of the claims 1 and 2, characterized in that the smooth-faced coating bar (11,21,41,51,81,101) is made of a tube. Coating device as claimed in claim 4, characterized in that the material of the tubular coating bar (11,21,41,51,81,101) is chromium-plated copper, steel. Coating device as claimed in claim 1 for the coating of a roll in a size press, said size press comprising a pair of rolls (61,71) that form a nip (N), through which nip (N) the paper or board web (W) is passed and in which pair of rolls the rolls (61,71) are provided with a coating of rubber or equivalent, characterized in that at least one of the rolls (61,71) that form the size press nip (N) is a variable-crown roll or a roll adjustable by means of heat from inside the roll in zones, so that, by means of regulation of the deflection of said variable-crown roll or by regulating the temperature of the thermally adjustable roll in zones, the linear contact face between the coating bar (41,51) and said roll (61,71) is adjusted straight. Coating device as claimed in claim 6, characterized in that both of the rolls (61,71) in the pair of rolls are variable-crown rolls or rolls thermally adjustable in zones. Coating devices as claimed in claim 6, characterized in that the roll (91) that forms a nip (N) with the variable-crown roll (61,71) is a crowned rigid roll, whose coating (92) has been shaped so that its outer face (92a) is shaped as negatively crowned. Coating device as claimed in any of the preceding claims, which is fitted in connection with a size-press roll (2,3,61,71,91), which roll is provided with a coating (92), characterized in that the hardness of the coating (92) is 35 P&J ± 15 P&J.

Inventors: RANTANEN RAUNO, RANTANEN, RAUNO
Applicants: VALMET PAPER MACHINERY INC, VALMET PAPER MACHINERY INC.
Application year: 1991
Main classification: D21H25
IPC codes: D21H23, D21H25