Abstract:
A board machine and method for making a multilayer cardboard web, in which a first layer of the web having a printable surface is formed in a first forming unit and carried on an extended forming wire thereof through a couching unit where the first layer is couched with one or more additional layers, the multilayer web then being carried on the extended wire to a pick-up point. An upper press felt of a double-felted press picks up the web at the pick-up point such that the printable surface faces downward and contacts the lower press felt through the nip of the press. The lower felt is smoother than the upper felt, and the lower felt contacts the lower press roll for a minimum sector angle beyond an exit of the nip to ensure that the web follows the lower felt.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Application No. PCT/SE00/01647 filed Aug. 28, 2000, which designated the United States and was published in English under PCT Article 21, and which claims priority from provisional application Ser. No. 60/159,281, filed Oct. 13, 1999. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a board machine and method for manufacturing a multilayer cardboard web with a printable surface layer, comprising a wet section and a press section, which wet section includes a first forming unit for forming a first layer, which first forming unit has at least one forming wire, running in an extended loop up to the press section to form a pick-up point for the multilayer cardboard web, and one or more further forming units for forming one or more further layers and for couching the same with said first layer on said extended forming wire of the first forming unit to form the multilayer cardboard web, which press section includes at least one double-felted press, having an upper press element, a lower press element in the shape of a press roll, which press elements create a press nip with each other, an upper press felt, running in a loop around a plurality of guide rolls and a pick-up roll, arranged at said pick-up point for transferring the multilayer cardboard web to the upper press felt, and a lower press felt, running in a loop around a plurality of guide rolls. 
     As used herein, the expression “the 0 line of the press” is defined, for a roll press, as the tangent perpendicular to a straight line intersecting the centers of the press rolls and, for a shoe press, as the tangent of the transition from the concave curvature to the convex curvature of the shoe at the exit of the press nip. 
     BACKGROUND OF THE INVENTION 
     One side of a multilayer cardboard web is often used for printing. This side, denoted the front side of the finished cardboard product, is formed by a surface layer that must have a high degree of surface smoothness to provide good printability. Special pulps are used for manufacturing the surface layer. Short-fiber pulps result in surface layers with improved printability. The pulp intended for the printable surface layer is preferably, but not necessarily, bleached. It may consist of a mixture of short-fiber and long-fiber pulps, in which the short-fiber proportion of the pulp may constitute 50-70 per cent by weight of the mass. However, the short-fiber proportion may constitute 100 per cent. The layer to be printed may also be made of 100 per cent bleached long-fiber pulp. Short-fiber pulp can be pulp from birch or eucalyptus, for instance, while long-fiber pulp can be pulp from pine, for instance. 
     A number of methods and machines for manufacturing multilayer cardboard webs are described in patent literature and the following are mentioned by way of example: EP-0 511 186, WO 92/06242, U.S. Pat. No. 4,961,824, EP-0 511 185, U.S. Pat. No. 5,074,964, EP-0 233 058 and SE-506 611. 
     U.S. Pat. No. 5,639,349 (corresponding to DE-4401761) describes a method for improving the quality of multilayer papers in the wet section of a paper machine by recirculating the drainage water within each forming unit. The outer layer of the paper web is made of stock of higher quality than the stock for the core. The patent specification does not mention cardboard or board and the problem associated with providing a printable surface layer on a multilayer cardboard web. Neither does the patent specification touch upon the problem relating to the press section and the web run in the same, and in particular does not address the problem of pressing of a multilayer cardboard web with a printable surface layer. 
     U.S. Pat. No. 4,957,778 describes a paper machine for manufacturing two-layer carbonless copy paper. The paper machine has upper and lower fourdrinier formers, the layers of which are combined by couching to form a coherent paper web, which is pressed in a press with two single-felted press nips, created by two press rolls and a counter roll shared by the same. Multilayer cardboard webs are not touched upon in this patent specification and, consequently, neither are the problems associated with pressing a multilayer cardboard web. 
     In practice, the predominant technique for manufacturing a multilayer cardboard web is to manufacture the surface layer with a forming unit, for instance an upper fourdrinier former, arranged relative to at least one other forming unit, for instance a lower fourdrinier former, in such a way that the surface layer is couched with a subjacent layer and the cardboard web emerges from the wet section with the surface layer facing upwards. This in turn dictates the configuration of the press section. In accordance with conventional techniques, a double-felted roll press is employed as the first press. It is also known to use a double-felted shoe press with the shoe in a top or bottom position as the first press. A first double-felted press of known kind has an upper felt acting as a pick-up felt to transfer the cardboard web to the press nip, while the lower felt is intended to carry the cardboard web subsequent to its passage through the press nip. The surface layer of the cardboard web thus comes into direct contact with the upper felt. Accordingly, to be able to satisfy the requirement of high surface smoothness of the surface layer, the structure of the web-contacting surface of the upper felt must not be too rough. If, on the other hand, the structure of the web-contacting surface of the lower felt were to be too smooth or fine to ensure the correct web run after the press nip, the lower felt will not be sufficiently open to allow permeation of water and will relatively quickly become clogged with fibers, which means that reconditioning of the lower felt cannot be accomplished with the desired result and that the service life of the lower felt becomes relatively short. In practice, the two contradictory requirements for the properties of the upper felt and the lower felt result in the requirement that the differences between their surfaces structures with respect to roughness or smoothness become relatively small and there is, therefore, a risk of the cardboard web sometimes having a tendency to accompany the upper felt after the press nip instead of the lower felt as intended, even if the lower felt has the smoother surface. To ensure the correct web run in a shoe press with the shoe in the bottom position the lower felt must be passed over the downstream edge of the shoe and the upper felt passed approximately in the direction of the so-called 0 line, but this is not an acceptable solution as the web is then subjected to detrimental shear forces during its passage over the shoe edge. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide an improved board machine and an improved method of manufacturing a multilayer cardboard web. The invention thus enables the manufacture of a multilayer cardboard web having a printable surface layer with a desired high degree of surface smoothness and maximum dry-solids content after the press section, while safeguarding the web run in the press section. 
     The board machine, in accordance with the invention, is characterized in that the first forming unit is arranged to form the printable surface layer and arranged with its extended forming wire to transfer the multilayer cardboard web to the upper press felt of the press with the printable surface layer facing downwards to contact the lower press felt in the press nip. The lower press felt has a finer web-contacting surface to exert a greater adhesion force on the multilayer cardboard web than the upper press felt, and the lower press felt at the discharge side of the nip is arranged to encompass the lower press roll by a pre-determined minimum sector angle α measured from a point in the press nip intersected by the 0 line of the press, as defined herein for a roll press and a shoe press, respectively. 
     The method, in accordance with the invention, is characterized in that the printable surface layer is formed in the first forming unit and the extended forming wire transfers the multilayer cardboard web to the upper press felt of the press with the printable surface layer facing downwards so that it is in contact with the lower press felt in the press nip. The lower press felt exerts a greater adhesion force on the multilayer cardboard web than the upper press felt by virtue of its finer web-contacting surface, and the lower press felt is caused to encompass the lower press roll by a pre-determined minimum sector angle α measured from a point in the press nip intersected by the 0 line of the press, as defined herein for a roll press and a shoe press, respectively. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features, and advantages of the invention will become more apparent from the following description of certain preferred embodiments thereof, when taken in conjunction with the accompanying drawings in which: 
     FIG. 1 shows schematically parts of a board machine for manufacturing a multilayer board web in accordance with a first embodiment of the invention. 
     FIG. 2 shows schematically parts of a board machine for manufacturing a multilayer cardboard web in accordance with a second embodiment of the invention. 
     FIG. 3 is a cross section along the line III—III in FIG.  1 . 
     FIG. 4 shows schematically a part of a shoe press used in the board machines shown in FIGS. 1 and 2. 
     FIG. 5 shows schematically a roll press in a board machine for manufacturing a multilayer cardboard web in accordance with a third embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. 
     FIGS. 1 and 2 show schematically parts of a board machine for manufacturing a cardboard web  1 , consisting of a first layer  2  and a further layer  3 . In the embodiment shown and in accordance with the present invention, the first layer  2  forms a surface layer in the finished two-layer cardboard web, while the further layer  3  forms its core. Alternatively, a cardboard web is manufactured, consisting of said first layer and several further layers, one of which is said core. 
     The board machines comprise a wet section  4 , a press section  5  and a drying section  6 . 
     The wet section  4  comprises a first forming unit  7  for manufacturing the first layer  2  and a second forming unit  8  for manufacturing the second layer  3 . 
     In the embodiment shown in FIG. 1, the two forming units  7 ,  8  consist of a first fourdrinier former, located upstream, and a second fourdrinier former, located downstream, while in the embodiment in accordance with FIG. 2, they consist of a first twin-wire former or gap former located upstream, and a second twin-wire former or gap former located downstream. In this context, the expressions “upstream” and “downstream” indicate the relative locations of the forming units viewed in the machine direction. 
     The first fourdrinier former  7 , located upstream according to FIG. 1, is extended in the machine direction and has a fourdrinier wire  9 , running in a loop around an upstream breast roll  10 , a downstream suction couch  11 , a wire turning roll  12  and a plurality of other types of guide rolls  13 , such as alignment rolls and tension rolls. The upper part  14  of the fourdrinier wire  9 , dewatering the stock and forming the layer and web, between the breast roll  10  and the suction couch  11  is plane and horizontal. The first fourdrinier former  7 , located upstream, further comprises a headbox  15 , arranged close to the breast roll  10  to emit a jet of stock onto the upper part  14  of the fourdrinier wire  9 , and dewatering members  16  for dewatering the stock to form the first layer  2 . 
     The second fourdrinier former  8 , located downstream according to FIG. 1, has a fourdrinier wire  17 , running in a loop around a breast roll  18 , an upper guide roll  19  and two lower guide rolls  20 , which lower guide rolls are arranged in close proximity to the upper part  14  of the fourdrinier wire  9  of the first fourdrinier former  7  for couching the formed second layer with the formed first layer. The second fourdrinier former  8  comprises a headbox  21 , arranged close to the breast roll  18  to emit a jet of stock onto the upper, plane part  22  of the fourdrinier wire  17 , and dewatering members  23  for dewatering the stock to form the second layer. 
     The first twin wire former, located upstream according to FIG. 2, has first and second forming wires  25 ,  26 , which run together in a forming zone. The first forming wire  25  runs in an upper loop around a plurality of guide rolls  27 . The second forming wire  26  runs in a lower loop around an upstream forming roll  28  and a downstream suction couch  29 , a wire turning roll  30  and a plurality of other guide rolls  31 , comprising alignment rolls and tension rolls. The lower forming wire  26  is extended up to the press section so that the suction couch  29  is located downstream of the second twin wire former  8 . In the loop of the first forming wire  25 , dewatering means  32  are arranged within said forming zone. A headbox  33  is arranged to emit a jet of stock into a gap defined by the forming roll  28  and a guide roll  27  located adjacently to the same in the upper wire loop  25 . 
     The second twin wire former  8 , located downstream according to FIG. 2, has first and second forming wires  34 ,  35 , which run together in a forming zone. The first forming wire  34  runs in a loop around a plurality of guide rolls  36  and has a lower, linear part  37 , passing along the lower forming wire  26  of the first twin wire former  7  to create a couching zone. The second forming wire  35  runs in a loop around a forming roll  38  and, two guide rolls  39 . In the loop of the first forming wire  34 , dewatering means  40  are arranged within said forming zone. A headbox  41  is arranged to emit a jet of stock into a gap defined by the forming roll  38  and a guide roll  36  located adjacently to the same in the first wire loop  34 . 
     The first forming unit  7 , located upstream, is arranged to create a surface layer  2  suitable for printing in the finished cardboard web, while the second forming unit  8 , located downstream, is arranged to create a core  3 , which encounters the surface layer so that the two layers are couched together with each other to a coherent two-layer cardboard web, see FIG. 3, which leaves the forming wire  9 ,  26  of the first forming unit  7  with the surface layer facing downwards. 
     The press section  5  in the board machines shown in FIGS. 1 and 2 comprises a first double-felted press  45  and a second double-felted press  46 , which presses  45 ,  46  are arranged directly one after the other. The first press  45  comprises an upper press element  47  and a lower press element  48 , which press elements create a press nip with each other. The first press  45  further comprises an upper press felt  49 , which runs in a loop around a plurality of guide rolls  50 , comprising a pick-up suction roll  51  for transferring the multilayer cardboard web  1  to the upper press felt  49 , and a lower press felt  52 , which runs in a loop around a plurality of guide rolls  53 , and which together run through the press nip with the web  1  enclosed therebetween in a sandwich construction. The second press  46  comprises an upper press element  54  and a lower press element  55 , which press elements create a press nip with each other. The second press  46  further comprises an upper press felt  56 , which runs in a loop around a plurality of guide rolls  57 , comprising a pick-up suction roll  58  for transferring the multilayer cardboard web  1  to the upper press felt  56 , and a lower press felt  59 , which runs in a loop around a plurality of guide rolls  60 , and which together run through the press nip with the web  1  enclosed therebetween in a sandwich construction. 
     The lower press felt  52 ,  59  of each press  45 ,  46  has a finer surface structure than the upper press felt  49 ,  56  with the purpose of ensuring that the web  1  adheres to the lower press felt  52 ,  59  and not to the upper press felt  49 ,  56  after the press nip. This difference in surface structure or adhesive capability is a first parameter to assist in safeguarding the correct web run. 
     The lower press element  48 ,  55  in each press is a press roll, around which the lower press felt  52 ,  59  runs in contact with the envelope surface of the press roll after the press nip by a pre-determined minimum sector angle α measured from a certain point in the press, depending on which type of press is used, as explained below. The web has a tendency to accompany the one of the two press felts that has the greater part in contact with the press roll after the press nip. This circumstance is a second parameter to assist in safeguarding the correct web run. At least the first parameter, and preferably both the first and second parameters, are utilized in the press, while the printable surface layer  2  simultaneously faces downwards. This enables an increased difference between the degrees of surface smoothness of the lower and upper felts. At the same time, the lower press felt is caused to maintain contact with the lower press roll downstream of the nip for a predetermined sector angle. Thus, the proper web run is facilitated. 
     The press sections  5  shown in FIGS. 1 and 2 are alike and their presses consist of a first shoe press  45  with a press shoe  63  and a subsequent, second shoe press  46  with a press shoe  64 . Each shoe press  45  has a shoe roll  47 ,  54  in the upper position and a counter roll  48 ,  55  in the lower position. Each counter roll  48 ,  55  can have a blind-drilled, grooved or smooth envelope surface. Each shoe roll or one of the shoe rolls has an envelope surface  65 , see FIG. 4, in the shape of a press belt that is smooth, blind-drilled or grooved. From the point of view of operability, a blind-drilled or grooved press belt  65  is preferable, as this provides a large open volume behind the upper press felt  49 ,  56  so that the cardboard web acquires a high dry-solids content while the upper press felt simultaneously remains open towards the open surface behind the upper press felt to enable ventilation of the same. Such high dry-solids content is further improved by employing a blind-drilled or grooved counter roll, thus providing a large open volume behind the lower press felt  52 ,  59 . In especially difficult operating conditions, such as high web speed and low surface weight, a counter roll with a smooth envelope surface is used because the large open volume is not required, as smaller quantities of water (low surface weight) need to be removed and an extra great vacuum pulse is created in the lower press felt, which results in the “attraction” of the web to the lower felt being increased still further. Placing the shoe rolls in a top position creates enhanced possibilities for guiding the cardboard web to the lower press felt by arranging the lower felt to encompass the counter roll to a greater extent. 
     The lower press felt  52  is arranged to encompass the counter roll  48  with a pre-determined minimum sector angle α of 10° measured from a point (denoted a 0 point herein) on the periphery of the shoe  63  at which the concave curvature of the shoe transitions into a convex curvature, the tangent of this point being denoted the 0 line  61  of the shoe. The part of the upper press felt  49  surrounding the counter roll  48  is adjustable within a range from +5° to −5° measured as an angle β between the upper press felt  49  and the 0 line  61 , positive angle values being located below and negative angle values above this 0 line  61 . 
     Alternatively, the first press  45  can consist of a roll press as shown in FIG.  5 . The second press  46  in such a press section can be a similar roll press or a shoe press as described above. The upper and lower press rolls  47 ,  48  of the roll press can have smooth, blind-drilled or grooved envelope surfaces. The lower press felt  52 , see FIG. 5, is arranged to encompass the lower press roll  48  by a pre-determined minimum sector angle α of 10° measured from a 0 point on the periphery of the lower press roll  48  that is tangent to the periphery of the upper press roll. Stated differently, the 0 point is located on the periphery of the lower press roll  48  at a point intersected by a straight line connecting the centers of the press rolls  48 ,  47 . The tangent to this 0 point is perpendicular to the straight line intersecting the centers of the press rolls, which tangent is denoted as the 0 line  62  of the roll press. The sector angle α is normally in the range 10°-25° for a roll press. The part of the upper press felt  49  surrounding the lower press roll  48  is adjustable within a range from +10° to −5° measured as an angle β between the upper press felt  49  and the 0 line  62 , positive angle values being located below and negative angle values above this 0 line  62 . 
     Many modifications and other embodiments of the invention will come to mind to one skilled in the art to which this invention pertains having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.