METHOD AND APPARATUS OF MANUFACTURING MULTILAYER BOARD

Abstract: A method of manufacturing a thick multilayer board. Inter­ mediate layers are formed by two consecutive head boxes disposed on a fourdrinier wire, in which layers the pulp suspension consistency is so high that the forming pulp layer reaches in a head box slice or immediately after it such a fibrage degree that fibres cannot significantly move themselves as regards to one another. Dewatering of both pulp layers takes place though a fourdrinier wire.

Description: The present invention relates to a method of manufacturing a multilayer board web, wherein superposed pulp layers are formed by headboxes disposed on a fourdrinier wire with two intermediate headboxes feeding intermediate high consistency stock layers being fed between two surface layers formed by headboxes providing low consistency stock layers; and wherein the consistency of the high consistency stock layers is so high that, in a slice channel of the intermediate headboxes or after the stock has been discharged from each intermediate headbox slice, the forming pulp layers reach such a degree of felting that the fibres of the pulp cannot significantly move on the wire in relation to one another, and that dewatering of the high consistency layers takes place through the wire, and to an apparatus therefor. When manufacturing multilayer boards, layers of different qualities are used. The core layer that gives sturdiness and rigidity to the board is often manufactured from pulps made of waste paper, whilst the surface layers or at least one surface layer is usually made from bleached pulp of better quality. With multilayer board machines in which several pulp layers are bonded by wet pressing, a strong and rigid packaging board that has good printing qualities and a grammage of 200-600 g/m² can be produced with low costs for raw materials. A very old method of manufacturing multilayer board involves manufacturing the layers with cylinder moulds. The driving speed of cylinder moulds is, however, relatively slow and high stock consistencies cannot be used in them which thus involves large dewatering quantities. In addition, a plurality of cylinder moulds are needed if the board is required to be very thick. Fourdrinier wire machines can be driven at higher speeds than cylinder moulds and the sheet formation can be better controlled in them. Multiwire fourdrinier machines are most suitable when manufacturing thin board qualities. High investment and running costs, however, limit the use of these machines. In the closest known prior art WO 85/05387 of A. Ahlstrom Corporation there is disclosed a method for manufacturing multilayer board in which the pulp suspension forming a surface layer 12 is fed onto the horizontal section of a first, inner wire 1. The pulp suspension forming an intermediate layer 11 is supplied onto said web layer, which is already formed, in or in front of a tapering gap 6 constituted by the first wire and a second, outer wire 4. Both of the wires and the web layers 11, 12 between them are guided over a curved surface 3. After this the first wire and both of the web layers are guided against a third, lower wire 8 onto the horizontal section of which the pulp suspension forming the other surface layer 13 is supplied. The consistency of the pulp suspension forming the intermediate layer 11 is higher than that of the pulp suspensions forming the surface layers. Mention is made on page 4 of this document of the possibility of the provision of a fourth headbox between the turning roll 7 and the headbox 14 on the lower wire 8 (or the headboxes 9 and 10) to supply another intermediate layer on the horizontal section of the lower wire 8 although the nature of the fourth headbox is not specifically mentioned. In this arrangement the formation of one or two layers on a lower wire 8 naturally requires apparatus for such and consequentially has a greater space requirement in the lower region of the apparatus. The main object of the present invention is to provide a method of manufacturing high grammage (500 - 700 g/m²) multilayer board with simple means. This is achieved according to the present invention by the provision of a method of manufacturing a multilayer board web, wherein superposed pulp layers are formed by headboxes disposed on a fourdrinier wire with two intermediate headboxes feeding intermediate high consistency stock layers being fed between two surface layers formed by headboxes providing low consistency stock layers; and wherein the consistency of the high consistency stock layers is so high that, in a slice channel of the intermediate headboxes or after the stock has been discharged from each intermediate headbox slice, the forming pulp layers reach such a degree of felting that the fibres of the pulp cannot significantly move on the wire in relation to one another, and that dewatering of the high consistency layers takes place through the wire; wherein the pulp layers are formed on a single wire, and the formed pulp layers between the two intermediate high consistency headboxes is smoothed by a dandy roll. Also according to the present invention there is provided an apparatus for manufacturing a multilayer board according to the method of claim 1, wherein headboxes are disposed on a fourdrinier wire for forming superposed pulp layers with two of the headboxes being provided to form two surface layers formed from low consistency stock and with two of the headboxes being intermediate headboxes for feeding intermediate high consistency stock layers between said two surface layers, wherein the two intermediate headboxes are such that the stock consistency supplied thereby is so high that, in a slice channel of the intermediate headboxes or after the stock has been discharged from each intermediate headbox slice, the forming pulp layers reach such a degree of felting that the fibres of the pulp cannot significantly move on the wire in relation to one another, and that dewatering of the high consistency layers takes place through the wire; and wherein a single wire is provided for the formation of the superposed layers, and a dandy roll is provided between the two intermediate high consistency headboxes to act to smooth the formed pulp layer between said headboxes. The invention will be described further, by way of example, with reference to the accompanying drawing which is a schematic illustration of a section of a web forming section in a board machine according to the invention. A web forming section of a board machine is illustrated in Fig. 1 and is a twin wire former comprising a first wire arrangement or first wire 1 having a main run which generally extends horizontally from a breast roll 2 to a forming roll 3 and a second wire arrangement or wire 4 which forms with the first wire a gap 5 that tapers towards the forming roll. After the forming roll 3 both wires run diagonally downwards and the second wire 4 is detached from the first wire. Under the horizontal section of the first wire 1 are disposed known (not shown), wire supporting dewatering devices such as vacuum boxes and foils which effect dewatering downwardly through the wire. The stock forming the first outer ply of the board is fed onto the first wire 1 by a headbox 6 which is disposed at the breast roll 2 and operates at a normal consistency range of 0.1 to 1.2%. The layers forming the core of the multilayer board are produced by feeding stock onto the wire on top of the already formed web layer by two intermediate high consistency headboxes 7 and 8 disposed at a distance from each other in which the stock consistency is substantially higher than the normal headbox consistency, e.g. 3%. A headbox of this type is described in e.g. Finnish Patent Publication No. 56221. When the slice spray reaches the wire at this consistency, the drainage of the pulp does not take place in the same way as at a normal headbox consistency, but rather the web is formed in such a way that the fibres form a continuous network across the whole thickness of the web in the slice channel of the headbox or immediately after it. The web structure becomes felted and there is a relatively strong z-orientation of the fibres in the web. The structure of a web formed at a normal consistency is on the contrary stratified. During the web formation the felted structure remains more open than the stratified structure which means that dewatering of such a web is easier than that of a web formed at a normal consistency which again makes it possible to drain water from the pulp layer formed by the headbox 8 downwards through the pulp layer formed by the headbox 7. Since the quantity of water to be drained from the pulp layers is considerably smaller than that to be drained from pulp layers formed at a normal consistency, both layers can be thick and a thick core can be produced by two headboxes. Due to the fibre orientation, the core also has a higher bulk than a core formed at a normal consistency. The web formation of a multilayer board can be improved by treating the web by means of a dandy roll 9 disposed between the headboxes 7 and 8. The second thin outer ply is supplied by a headbox 10 which is disposed in front of the gap formed by the first and second wires 1 and 4 and which operates at a normal consistency range. The web layers on the forming roll 3 are pressed between the wires 1 and 4 and due to the centrifugal force water is drained upwards through the wire 4. Subsequently the thus formed 4-ply web is guided by known methods to the press and dryer sections of a board manufacturing machine for further treatment. The stock consistency in the high consistency headboxes 7 and 8 is 1.5 - 6% or higher, preferably 2 - 4%. The grammage of the core layers formed at a high consistency can be 100 - 300 g/m² or higher, preferably 150 - 250 g/m². Thus a 4-ply board with a grammage of ca. 600 g/m² can be manufactured on a relatively short twin wire former. The invention is not only limited to the described embodiment presented as example, but several modifications and applications may be made of it within the scope of the invention concept defined by the appended claims. It will be understood by persons skilled in the art that the term "fourdrinier wire" throughout the specification and claims, relates to and is generally understood to mean an endless belt arrangement as is used in fourdrinier machines which is sometimes referred to as a "cloth" and which may, e.g. be made of a wire mesh (mesh of metal filaments) or of a plastics material mesh.

Claims: A method of manufacturing a multilayer board web, wherein superposed pulp layers are formed by headboxes disposed on a fourdrinier wire with two intermediate headboxes feeding intermediate high consistency stock layers being fed between two surface layers formed by headboxes providing low consistency stock layers; and wherein the consistency of the high consistency stock layers is so high that, in a slice channel of the intermediate headboxes or after the stock has been discharged from each intermediate headbox slice, the forming pulp layers reach such a degree of felting that the fibres of the pulp cannot significantly move on the wire in relation to one another, and that dewatering of the high consistency layers takes place through the wire, wherein the pulp layers are formed on a single wire, and the formed pulp layer between the two intermediate high consistency headboxes is smoothed by a dandy roll. A method according to claim 1 characterised in that the stock consistency is 2 - 4%. A method according to claims 1 or 2 characterised in that the grammage of each of the pulp layers is 100 - 300 g/m². A method according to any one of claims 1 to 3 characterised in that the high consistency pulp layers form the core of the multilayer board. An apparatus for manufacturing a multilayer board according to the method of claim 1, wherein headboxes (6, 7, 8, 10) are disposed on a fourdrinier wire (1) for forming superposed pulp layers with two of the headboxes (6, 10) being provided to supply two surface layers formed from low consistency stock and with two of the headboxes (7, 8) being intermediate headboxes (7, 8) for feeding intermediate high consistency stock layers between said two surface layers, wherein the two intermediate headboxes (7, 8) are such that the stock consistency supplied thereby is so high that, in a slice channel of the intermediate headboxes (7, 8) or after the stock has been discharged from each intermediate headbox slice, the forming pulp layers reach such a degree of felting that the fibres of the pulp cannot significantly move on the wire (1) in relation to one another, and that dewatering of the high consistency layers takes place through the wire; and wherein a single wire (1) is provided for the support of the superposed layers, and a dandy roll (9) is provided between the two intermediate high consistency headboxes (7, 8) to act to smooth the formed pulp layer between said headboxes (7, 8).

Inventors: WARIS, TAPIO
Applicants: VALMET-KARHULA INC.
Application year: 1987
Main classification: D21F11
IPC codes: D21F11