EP4636157A1

EP4636157A1 - Method for forming a multilayer board web and a forming section for forming a multilayer board web, and a fiber web production line for producing multilayer board webs

Info

Publication number: EP4636157A1
Application number: EP25165924.9A
Authority: EP
Inventors: Hannu Turpeinen; Maarit Lahtinen; Maarit Tukiainen; Teuvo Virkkunen
Original assignee: Valmet Technologies Oy
Current assignee: Valmet Technologies Oy
Priority date: 2024-04-18
Filing date: 2025-03-25
Publication date: 2025-10-22

Abstract

The invention relates to a method for forming a multilayer board web (W), preferably a three-layer board web (W), more preferably a folding box board (FBB) web (W) or a solid bleached (sulfate) board (SBS) web (W) or a cup board web (W) in a forming section comprising a multilayer headbox (130) and a forming unit (140). In the method the layers of the multilayer board web (W) are first formed of pulp suspension in the multilayer headbox (130) and fed to the forming unit (140), which is a hybrid type former for water removal and for joining the layers of the multilayer board web (W) and comprising a single-wire part (10A, 10C) and a twin-wire part (10B), and in the method fibers of the pulp suspension fed from the multilayer headbox (130) are oriented in the beginning of the forming unit (140) by a breast roll (12A) provided with a shaker (R) and the fiber orientation is stabilized by a forming board (17) provided with suction. The invention also relates to a forming section for forming a multilayer board web and to a fiber web production line for producing multilayer board webs.

Description

Technical field

The invention relates generally to producing multilayer board webs. Particularly the invention relates to a method for forming a multilayer board web according to the preamble of the independent method claim and to a forming section for forming a multilayer board web according to the preamble of the independent method claim. The invention also relates to a fiber web production line for producing multilayer board webs.

Background

As known from the prior art in fiber web machines, especially in paper and board machines, the fiber web is produced and treated in an assembly formed by a number of apparatuses arranged consecutively in a process line. A typical production and treatment line comprise a forming section comprising a headbox and a forming unit and a press section as well as a subsequent drying section and a reel-up. The production and treatment line can further comprise other devices and sections for finishing the fiber web, for example, a size press, a calender, a coating section. The production and treatment line also comprise typically at least one winder for forming customer rolls as well as a roll packaging apparatus.
The task of the headbox is to supply fiber suspension for the fiber web production into the forming unit. In a multilayer headbox more than one fiber suspension flows are discharged from the headbox via flow channels for pulp suspension layers, each for forming one layer of a multiply fiber web.
The task of a forming unit is to remove water from fiber suspension fed by the headbox. When the web is manufactured of watery fiber stock, water in the stock is removed on the forming section through a forming wire or forming wires for starting the formation of the web. Fibers remain on the forming wire or between the forming wires moving together. Depending on the grade of the web being manufactured, different types of stocks are used. The volume for which water can be removed from different stocks for achieving a web of good quality is a function of many factors, such as e.g. a function of the desired basis weight of the web, the design speed of the machine, and the desired level of fines, fibers and fill materials in the finished product and refining level of used stocks. Many types of devices are known on the forming unit such as foil strips, suction boxes, turning rolls, suction rolls, and rolls provided with an open surface, which have been used in many different arrangements and arrays when trying to optimize the volume, time and location of water being removed when forming the web. The manufacturing a high-quality end-product of desired grade is a function of the volume of dewatering, the dewatering method, the duration of dewatering, and the location of dewatering. When it is desired to improve the water removal capacity and to maintain or improve the quality of the end-product, many times unforeseeable problems are created as the result of which either the water removal volume has to be decreased for maintaining the desired quality or the desired quality has to be sacrificed for achieving the greater water volume.
A commonly used method of making a multiply board web is based on the use of several separate web forming units in which the different layers of the web are caused to be drained in a layer-by-layer fashion either onto one another or onto separate wires, in which case they are combined with one another after partial dewatering. Typically in multiply / multilayer fiber web production i.e. when producing a fiber web having more than one layers term "multiply" is used when the layers are formed separately in the forming section and term "multilayer" is used when a multilayer headbox is used for feeding suspension layers to the forming unit even though these terms multiply / multilayer are used very often synonymously and thus the difference can be defined by the context only.
Fiber webs, especially paper and board are available in a wide variety of types and can be divided according to basis weight in two grades: papers with a single ply and a basis weight of 25-300 g/m² and boards manufactured in multiply technology and having a basis weight of 150-600 m/m². It should be noted that the borderline between paper and board is flexible since board grades with lightest basis weights are lighter than the heaviest paper grades. Generally speaking, paper is used for printing and board for packaging. The present invention relates especially to production of multilayer board webs. In particular the invention relates to forming of folding box boards (FBB) and solid bleached (sulfate) boards (SBS) and cup boards. In general, these grades are typically used for different kinds of packaging of consumer goods. The surface layers are typically of chemically treated pulps and the middle layer is typically of mechanically treated pulp. Generally, FBB has the highest bulk thanks to the mechanical or chemi-mechanical pulp used in the middle layer of the base board, whereas SBS is made from chemical pulp, exclusively. FBB's bulk typically is between 1.1-1.9 cm³/g whereas SBS 0.95-1.3 cm³/g. The PPS-s10-smoothess is respectively for FBB between 0.8 - 2.1 µm and for SBS 0.7 - 2.1 µm. FBB is used for various types of packaging purposes, for example for packaging food, confectionery, cosmetics, pharmaceuticals. The basis weight of FBB is 160-450 g/m² and it has typically three layers, each surface layer typically has the basis weight of about 45 g/m². The middle layer is of mechanical pulp to have high bulkiness and the surface layers are made of at least partially bleached chemical pulp. SBS is also used for various types of packaging purposes similar to those of the FBB, especially for chocolate and cigarettes. The SBS has also typically three layers. The middle layer is of hardwood and/or softwood sulfate pulp to have high bulkiness and the surface layers are made of at least partially bleached chemical pulp. Cup board is a bleached virgin-fiber board with a three layer fiber construction and with CTMP (chemi-thermomechanical pulp) in the middle layer. Basis weight of cup board is 170-350 g/m². It is used for example in paper cups manufacturing. The production lines for FBB, SBS and cup boards have typically a separate headboxes and forming units for each ply. A disadvantage of these types of forming sections is high energy consumption and high investment costs (building and multiply forming units). Often in production of multilayer board grades problems exists as not strong enough bonding strength between the layers of the multilayer board web, which have been tried to be solved by internal sizing with pulp size and by adding starch or other sizing agent between the layers. Middle layer bulkiness and bonding strength between requirements of the layers are typically opposites of each other's, so if middle layer bulkiness is improved typically bonding strength between the layers is weaker. It is known from prior art to add sizing agent to improve the bonding strength between the layers, but this has proven problematic as it causes fouling of devices and elements of the forming section, which in turn affects negatively to their operation, and therefore causes need to clean up and to stop the production, which leads to decreased capacity. The fouling may also cause quality problems on the multilayer board web.
In publication EP2212471A1 is disclosed a forming section which comprising a lower wire loop which constitutes a first single-wire section following a breast roll, in which the beginning of the first single-wire section comprises a first dewatering zone, which consists of at least one stationary first forming shoe, whereby non-pulsating dewatering is applied on the stock passing on top of the lower wire on the first forming shoe, a pulsating strip cover following the first forming shoe, a first headbox, which is a multilayer headbox. This forming section further comprises a first upper wire unit which is formed above the lower wire, after the first single-wire section on a first upper wire, which first upper wire forms with the lower wire loop a first twin-wire section after the first single-wire section.
In publication EP3757287A1 is disclosed a method and a forming section for forming a three-layer board web, preferably a folding box board (FBB) or a solid bleached (sulfate) board (SBS), in a forming section comprising a multilayer headbox and a forming unit, wherein the layers of the three-layer board web are first formed of pulp suspension in the multilayer headbox and fed to only one forming unit, which only one forming unit is a gap former, in which gap former pulp suspension from a multilayer headbox is fed to a gap between lower and upper wires. The gap former has proven to be well suitable for thin multilayer board web grades but not for thick multilayer board web grades, i.e. for multilayer board web grades having grammage over 150 g/m², due to the reason that the gap former does not have enough water removal capacity. If the water removal capacity of the gap former is tried to be increased, it will have a negative effect on bulkiness and strength of the multilayer board web.
In publication US11441268B2 is disclosed a process for making the multilayer paper product, the process comprising: providing a forming fabric carried by a breast roll and at least one additional roll and providing a multilayer headbox, wherein the forming fabric is caused to oscillate in a direction transverse to the machine direction.
An object of the invention is to create a method for forming a multilayer board web and a forming section for a multilayer board web, in which the disadvantages and problems of prior art are eliminated or at least minimized.
An object of the invention is to create a method for forming a folding box board (FBB) web and a forming section for a folding box board (FBB) web, in which the disadvantages and problems of prior art are eliminated or at least minimized.
An object of the invention is to create a method for forming a solid bleached (sulfate) board (SBS) web and a forming section for a solid bleached (sulfate) board (SBS) web, in which the disadvantages and problems of prior art are eliminated or at least minimized.
An object of the invention is to create a method for forming a cup board web and a forming section for a cup board web, in which the disadvantages and problems of prior art are eliminated or at least minimized.
A particular object of the invention is to create a method for forming a multilayer board web and a forming section for a multilayer board web, in which the disadvantages and problems of prior art relating especially to forming of thick multilayer board webs are eliminated or at least minimized.

Summary

In order to achieve the above-mentioned objects, the forming section according to the invention is mainly characterized by the features of the characterizing clause of the independent forming section claim and the method according to the invention is mainly characterized by the features of the characterizing clause of the independent method claim. Advantageous embodiments and features are disclosed in the dependent claims.
It has been surprisingly noted that a method for forming and a forming section comprising a multilayer headbox and a forming unit comprising a hybrid former having a single-wire section with a substantially horizontal run, in which water removal direction is downwards and followed by a twin-wire section with water removal also upwards, wherein the beginning of the single-wire section is provided with a breast roll connected to a shaker in the beginning of the horizontal run and the breast roll connected with the shaker is followed by a forming board with suction, provides an excellent forming section for forming thick multilayer board webs for packaging grades, especially for FBB, SBB and cup board. The breast roll with the shaker improves the fiber orientation by the cross-directional shaking provided by the shaker such, that bonding of the fibers is improved and thus, the bonding strength is improved. The breast roll shaker also improves the formation of the web. Further, the forming board with suction provides for stabilizing, i.e. calming, the fiber orientation created by the breast roll with the shaker. Surprisingly, it has been found out that the forming board with suction and the breast roll shaker together prevents the mixing of multilayer headbox stocks and at the same time it is possible to affect fiber orientation profile. The forming unit further comprising the hybrid forming unit having the single-wire section followed by the twin-wire section ensures effective water removal. Thus, forming of the multilayer board web according to the invention provides for significantly improved layer bonding properties combined with good water removal capacity without negatively affecting the bulkiness. Also, water removal capacity downwards is increased and thus, improved properties for the bottom layer of the multilayer board web are achieved.
According to the invention in the method for forming a multilayer board web, preferably a three-layer board web, more preferably a folding box board web or a solid bleached (sulfate) board web or a cup board web in a forming section comprising a multilayer headbox and a forming unit, the layers of the multilayer board web are first formed of pulp suspension in the multilayer headbox and fed to the forming unit, which is a hybrid type former for water removal and for joining the layers of the multilayer board web and comprising a single-wire part and a twin-wire part, and fibers of the pulp suspension fed from the multilayer headbox are oriented in the beginning of the forming unit by a breast roll provided with a shaker and the fiber orientation is stabilized by a forming board provided with suction.
According to an advantageous feature of the invention in the single-wire part the pulp suspension is run on a length of 5 -15 m before the twin-wire part.
According to an advantageous feature of the invention in the twin-wire part water is removed by a water removal device comprising at least one suction chamber.
According to an advantageous feature of the invention in the twin-wire part water is removed by a water removal sector of a forming roll and by a sleeve roll due to the pressure between wires of the twin-wire part.
According to an advantageous feature of the invention in the method white-water is fed between at least two layers of the multilayer board web in an Aqua-headbox.
According to an advantageous feature of the invention in the method the pulp suspension from the multilayer headbox is first fed onto a bottom wire in the single-wire part on the areas of the forming board provided with suction.
According to the invention the forming section for forming a multilayer board web, preferably a three layer board web, more preferably a folding box board web or a solid bleached (sulfate) board web or a cup board web, comprises a multilayer headbox and a forming unit, and the forming unit is a hybrid type former for water removal and for joining the layers of the multilayer board web and comprises a single-wire part and a twin-wire part, in the beginning of the forming unit a breast roll provided with a shaker is located and configured to orient fibers of pulp suspension fed by the multilayer headbox and a forming board provided with suction is located directly after the breast roll with the shaker and configured to stabilize the fiber orientation.
According to an advantageous feature of the invention length of the single-wire part before the twin-wire part is 5-15 m.
According to an advantageous feature of the invention the twin-wire part water comprises a water removal device comprising at least one suction chamber.
According to an advantageous feature of the invention the twin-wire part water comprises a forming roll with a water removal sector and a sleeve roll.
According to an advantageous feature of the invention the multilayer headbox is an Aqua-headbox comprising at least one Aqua-manifold connected to a corresponding distribution header for feeding white-water between at least two layers of the multilayer board web.
According to an advantageous feature of the invention the single-wire part is configured as a bottom wire and the multilayer headbox is configured to feed the pulp suspension onto the bottom wire on the area of the forming board provided with suction.
According to an advantageous feature of the invention the modified forming section is modified from an existing forming section to a forming section according to the invention.
According to the invention the fiber web production line for producing multilayer board webs comprises a forming section according to any of the invention and the forming section is followed by a press section comprising a center roll.
According to an advantageous aspect of the invention in the hybrid forming unit pulp suspension from a multilayer headbox is first fed to a single-wire section followed by a twin-wire section, wherein the length of the single-wire section in the wire running direction is 5 -15 m. The length of the single-wire section ensures that the multilayer board web thickness has decreased considerable and effective dewatering in the twin-wire section is ensured. The single-wire section and the two-wire section further comprise, as such known, water removal devices support elements, such as foils, suction boxes, rolls etc.
According to an advantageous aspect of the invention in the forming board provided with suction, underpressure level is configured to be low, advantageously 100 - 5000 Pa, and the suction is provided by a vacuum pump or by a suction leg.
According to an advantageous aspect of the invention by the use of the Aqua-type multilayer headbox the bonding strength between the layers can be controlled and by that way it is possible to use less refined middle layer raw material.
According to an advantageous aspect of the invention the forming section according to the invention is comprised by a fiber web production line for production of multilayer board webs, in which fiber web production line the forming section is followed by a press section comprising a center roll. Advantageously, in the press section in connection with the center roll at least two press nips are arranged: one between the center roll and a suction roll and one between the center roll and an extended nip roll. More advantageously in the press section a press nip is arranged between the suction roll and a deflection compensated roll.
Sometimes, there exists a need to modify an existing forming section with a hybrid former for production of multilayer board webs for packaging products, especially FBB, SBB and cup board. The invention provides an improved effect also in this respect, as the existing forming section can easily be modified to be in accordance with the invention, as no significant layout or structural changes are needed.
By the method of forming and the forming section according to the invention many advantages are achieved: Very good bonding strength between the layers is achieved as improved fiber orientation is achieved by the breast roll with the shaker and the improved fiber orientation is stabilized, i.e. calmed, by the forming board with suction. Improved water removal capacity without negative effect on the bulkiness is achieved. Further, the need to feed sizing agent between the web plies is eliminated and thus, overall capacity and improved operational conditions are achieved. Also, in view of forming sections with separate headboxes for each layer as well as separate forming units for each layer, significant energy savings, as well as cost savings are achieved.
Furthermore, very importantly the multilayer board web can be formed without limitations in view of water removal capacity.

Brief description of the drawings

In the following the invention is explained in detail with reference to the accompanying drawing to which the invention is not to be narrowly limited.

In figure 1 is shown schematically an advantageous example of a multilayer headbox of a forming section according to the invention and a fiber web production line comprising the forming section and a press section.
In figure 2 is shown schematically another advantageous example of a multilayer headbox of a forming section according to the invention.

During the course of the following description like numbers and signs will be used to identify like elements according to the different views which illustrate the invention and its advantageous examples. In the figures some repetitive reference signs have been omitted for clarity reasons.

Detailed description

In figure 1 is shown an example of a forming section for a multilayer board web W, in this example for a three-layer board web W. The forming section comprises a multilayer headbox 130, from which the stock suspension M in layer-by-layer structure is fed to a forming unit 140 of a hybrid type, which forming section 140 is followed by a press section 150 and a drying section 160.
The multilayer headbox 130 in the example is a multilayer headbox for three layers, which layers can be of same or of different stock. The multilayer headbox comprises headers 31, 32, 33 from which the pulp suspensions for each layer of a multilayer board web W are fed to manifold tubes to an equalization chamber and further via a turbulence generator to slice channels ending to a slice opening in the multilayer headbox 130. Alternatively, the manifold tubes are connected directly to the turbulence generator without equalization chamber (not shown). Advantageously, the header 32 for the middle layer of the multilayer board web W comprises a dilution control device 34. Also, the headers 31, 33 for the top and bottom layer of the multilayer board web W may comprise dilution controls. The multilayer headbox 130 can be also a so called Aqua-headbox, of which one example is described in EP-patent publication 2784213 . In Aqua-headbox white-water is fed between the pulp suspensions, which is fed from the headers 31, 32, 33 onwards. Additionally, the white-water may contain chemicals, fillers and/or fine substances. By the Aqua layer, the layer purity is improved. Additionally, the inner strength of the multilayer board web W can be improved ty the chemical, filler and/or fine substance additions between the layers of the multilayer board web W. The aqua layer is combined to the stock suspension after the separation elements and by that way white-water prevents mixing of the stock suspensions.
The forming unit 140, which is a hybrid type forming unit with a single-wire part 10A and a twin-wire part 10B, begins as a single-wire part comprising a first wire 10 with a substantially horizontal run, i.e. a bottom wire 10, for beginning of forming the multilayer board web W and thereafter a twin-wire part formed between the first wire 10 and a second wire 20, a top wire 20, followed by a short single-wire run 10C. Length of the single-wire part 10A is 5-15 meters before the twin-wire part 10B depending on the multilayer board web W grade to be produced. The forming unit 140 comprises rolls 12, 22 for guiding and driving the bottom and the top wires 10; 20 as endless loops.
The layered stock suspension M is first fed onto the bottom wire 10, advantageously on the area of a forming board 17 with suction. A breast roll 12A is located before the forming board 17 in the machine direction and multilayer headbox stock suspension hits the wire after the breast roll 12A. The breast roll 12A is connected to a shaker R, as shown in the figure by the arrow. The breast roll 12A provided with the shaker R provides improved fiber orientation and formation of the stock suspension. In more detail the shaker R is described for example in DE utility model publication 202008015357U1 .
The stock suspension is advantageously guided from the multilayer headbox directly onto the bottom wire 10 onto the area of a forming board 17 with suction, which is the first water removal means of the forming unit 140, and provides for stabilization, i.e. calming, of the fiber orientation achieved by the breast roll 12A with the shaker R. The suction provided in the forming board 17 is low , advantageously 100 - 5000 Pa, and is created for example by a vacuum pump or by a suction leg.
Thereafter the stock on the wire 10 is guided past inside the loop of the first wire 10 located further water removal means 11, which can comprise suction devices and/or pulsating or non-pulsating water removal means. The run of the wire 10 during this water removal on the single-wire part is substantially horizontal. The beginning of the forming unit 140 with the substantially horizontal single-wire part comprising the breast roll 12A with the shaker R and the forming board 17 removing water by suction provides for exact control of the headbox flow, so that water is sucked through the wire 10. In the area of the forming board 17 the stock suspension M from the headbox 130 meets the first wire 10 and by the forming board 17 the stock suspension M, which orientation is effected by the breast roll 12A with the shaker R, is calmed on the upper surface of the bottom wire 10 for providing good formation and desired fiber orientation to the board web W.
The stock forming the board web W is then guided to the twin-wire part, where the board web W runs between the two wires 10, 20. Inside the loop of the second, the upper wire 20 a water removal device 26 is located to remove water upwards. The water removal device 26 can be a suction box 26 and it may comprise one or more suction chambers 26A-C, which may have same or different underpressure levels. The water removal device 26 has advantageously forming lists / forming foils (not shown) in the bottom of the water removal device 26, which lists / foils are in contact to the top wire inner surface. The forming lists / forming foils create pressure pulses to the web W for helping dewatering and improving formation of the web W. The suction device 26 may have a curved bottom surface towards the multilayer board web W to improve the run of the web W.
At the end of the twin-wire part 10B a substantially horizontal end part 10C run of single-wire part of the bottom wire 10 is located. At this end run 10A of single-wire part water removal devices 13 may be located inside the loop of the bottom, first wire 10.
In the beginning and in the end of the forming unit 140 during the single-wire parts 10A, 10C water removal is downwards by the water removal devices 11 and 13. In the twin-wire part 10B water is removed also upwards by the water removal device 26.
Then, the multilayer board web W is guided by a suction roll 12B with a suction sector 12V and a pick-up roll 41 to a press section 150. The press section 150 can be a center roll -based press section 150 as shown in the example of the figure 1 or another type of press section such as a linear-type press section.
The press section 150 comprises a center roll 60, in connection with which two press nips are arranged: one with a suction roll 43 provided with a suction sector 43 V and one with an extended nip roll 73. The press section 150 further comprise a press nip between the suction roll 43 and a deflection compensated roll 53. The press section 140 comprises press fabrics 40, 50, 70 with rolls 42, 52, 72, for guiding and driving the bottom and the press fabrics 40, 50, 70 as endless loops. From the forming unit 140, the multilayer board web W is guided to support of a first press fabric 40, inside of which the suction roll 43 is located, and between the first press fabric 40 and a second press fabric 50, inside of which the deflection compensated roll 53 is located. After the 1^st press nip between the suction roll 53 and the deflection compensated roll 53 the multilayer board web W is released from the second press fabric 50 by the suction sector 43V of the suction roll 43 and guided onto surface of the center roll 60. On the surface of the center roll 60 the multilayer board web W first enters to the 2^nd press nip between the suction roll 43 and the center roll 60 and thereafter to the 3^rd press nip between the center roll 60 and the extended nip roll 70, located inside the loop of the third press fabric 70. The smooth surface of center roll 60 is on the side of the bottom surface of the multilayer board web W. From the surface of the center roll 60 the multilayer board web W is guided to a drying section 160, to support of a drying wire 80, running as a loop and guided and supported by rolls 82. The drying section 160 in this example is a single-wire drying section, in which the multilayer board web W is dried on surfaces of drying cylinders 84, located outside the loop of the drying wire 80, and is run meandering from the drying cylinders and reversing rolls 85, located inside the loop of the drying wire 80. Thereafter the multilayer board web W continues its run in the fiber web production line, which may have sections for further treatment of the multilayer fiber web W and a reel-up for reeling the multilayer board web W into a parent roll.
In figure 2 is shown another example of a forming section for a multilayer board web W, in this example for a three-layer board web W. The forming section comprises a multilayer headbox 130, from which the stock suspension M in layer-by-layer structure is fed to a forming unit 140 of a hybrid type, which forming section 140 is followed by a press section 150 and a drying section 160.
The multilayer headbox 130 in the example is a multilayer headbox for three layers, which layers can be of same or of different stock. The multilayer headbox comprises headers 31, 32, 33 from which the pulp suspensions for each layer of a multilayer board web W are fed to manifold tubes to an equalization chamber and further via a turbulence generator to slice channels ending to a slice opening in the multilayer headbox 130. Alternatively, the manifold tubes are connected directly to the turbulence generator without equalization chamber (not shown). Advantageously, the header 32 for the center layer of the multilayer board web W comprises a dilution control device 34. Also, the headers 31, 33 for the top and bottom layer of the multilayer board web W may comprise dilution controls. The multilayer headbox 130 can also be a so called Aqua-headbox, of which one example is described in EP-patent publication 2784213 . In Aqua- headbox white-water is fed between the pulp suspensions, which is fed from headers 31, 32, 33 onwards. Additionally, the white-water may contain chemicals, fillers and/or fine substances. By the Aqua layer, the layer purity is improved. Additionally, the inner strength of the multilayer board web W can be improved ty the chemical, filler and/or fine substance additions between the layers of the multilayer board web W. The aqua layer is combined to the stock suspension after the separation elements and by that way white-water prevents mixing of the stock suspensions.
The forming unit 140, which is a hybrid type forming unit with a single-wire part 10A and a twin-wire part 10B, begins as a single-wire part comprising a first wire 10 with a substantially horizontal run, i.e. a bottom wire 10, for beginning of forming the multilayer board web W and thereafter a twin-wire part formed between the first wire 10 and a second wire 20, a top wire 20, followed by a short single-wire run 10C. Length of the single-wire part is 5-15 meters depending on the multilayer board web W grade to be produced and it ensures that multilayer board web W thickness has decreased to certain level before twin-wire section. The forming unit 140 comprises rolls 12, 22 for guiding and driving the bottom and the top wires 10; 20 as endless loops.
The layered stock suspension M is first fed onto the bottom wire 10, advantageously on the area of a forming board 17 with suction, A breast roll 12A is located before the forming board 17 in the machine direction and multilayer headbox stock suspension hits the wire after the breast roll 12A. The breast roll 12A is connected to a shaker R, as shown in the figure by the arrow. The breast roll 12A provided with the shaker R provides improved fiber orientation and formation of the stock suspension. In more detail the shaker R is described for example in DE utility model publication 202008015357U1 .
The stock suspension is advantageously guided from the multilayer headbox 130 directly onto the bottom wire 10 onto the area of a forming board 17 with suction, which is the first water removal means of the forming unit 140, and provides for stabilization, i.e. calming, of the fiber orientation achieved by the breast roll 12A with the shaker R. The suction provided in the forming board 17 is low, advantageously 100 - 5000 Pa, and is created for example by a vacuum pump or a suction leg.
Thereafter the stock on the wire 10 is guided past inside the loop of the first wire 10 located further water removal means 11, which can comprise suction devices and/or pulsating or non-pulsating water removal means. The run of the wire 10 during this water removal on the single-wire part is substantially horizontal. The beginning of the forming unit 140 with the substantially horizontal single-wire part comprising the breast roll 12A with the shaker R and the forming board 17 removing water by suction provides for exact control of the headbox flow, so that water is sucked through the wire 10. In the area of the forming board 17 the stock suspension M from the headbox 130 meets the first wire 10 and by the forming board 17 the stock suspension M, which orientation is effected by the breast roll 12A with the shaker R, is calmed on the upper surface of the bottom wire 10 for providing good formation and desired fiber orientation to the board web W.
The stock forming the board web W is then guided to the twin-wire part, where the board web W runs between the two wires 10, 20. In the twin-wire part first water removal means is a forming roll 23 located inside the loop of the upper wire 20. The forming roll 23 is advantageously provided with open surface i.e. the outer surface of the forming roll 23 comprises grooves or borings, to which the water removed from the board web W moves. The grooves or borings of the forming roll 23 can extend through the mantle of the forming roll 23, whereby the forming roll 23 is advantageously connected to a vacuum source (not shown). The forming roll 23 also comprises an open water removal sector 23V. Diameter of the forming roll 23 is advantageously 1400 - 2000 mm to provide low enough water removal pressure in the beginning of the twin-wire part.
Thereafter the board web W is guided to a onto a sleeve roll 50 with decreasing radius for further removal of water. The sleeve roll 50 with decreasing radius typically comprises a stationary support shaft on which support elements are supported at a distance from each other, an impermeable belt loop which is led to circle around the stationary support shaft supported by the support elements. The sleeve roll with decreasing radius typically further comprises at least one curvilinear dewatering zone via which the wires are led to travel supported by the belt loop. The degree of curvature of the curve of the curvilinear dewatering zone increases in the travel direction of the belt such that increasing dewatering pressure is applied to the stock suspension travelling between the wires on said at least one curvilinear dewatering zone. Radius of curvature of the curvilinear dewatering zone consists of two partial curves such that the radius of curvature of a first partial curve is greater than the radius of curvature of a second partial curve following the first partial curve in the travel direction of belt loop. Radius of curvature of the curvilinear dewatering zone can contain several curves such that the radius of curvatures decreases in the running direction of the wires. In patent application publication WO 2010046527 A1 is disclosed a forming section comprising two wire loops which form a twin-wire zone which comprises a dewatering element performing initial dewatering and a dewatering device following it. The dewatering device comprises a stationary support shaft on which are supported support elements around which circles an impermeable belt loop. The dewatering device further comprises a curvilinear dewatering zone over which the wires travel supported by the belt loop. The degree of curvature of the curve of the curvilinear dewatering zone increases in the travel direction of the belt such that increasing dewatering pressure is applied to stock suspension travelling between the wires on the curvilinear dewatering zone. Said at least one curvilinear dewatering zone of said at least one dewatering device, consists of two partial curves such that the radius of curvature of a first partial curve is greater than the radius of curvature of a second partial curve following the first partial curve in the travel direction of the web W. The sleeve roll 50 is located inside the loop of the bottom wire 10. Wires 10; 20 wrap angle over the sleeve roll 50 is advantageously 40 - 90 degrees. Advantageously dewatering pressure on the sleeve roll area is maximum 50-150 kPa.
At the end of the twin-wire part a water removal device 13 and after the twin-wire part 10B ends a substantially horizontal end part 10C run of single-wire part of the bottom wire 10 is located, whereafter the board web W is guided by a pick-up roll 41 to a press section 150. The press section 150 can be a center roll -based press section as shown in the example of figure 1 or another type of a press section, for example a linear-type press section. The press section 150 is followed by a drying section as shown in the example of figure 1. Thereafter the multilayer board web W continues its run in the fiber web production line, which may have sections for further treatment of the multilayer fiber web W and a reel-up for reeling the multilayer board web W into a parent roll.
In the beginning of the forming unit 140 during the single-wire part water removal is downwards by the water removal devices 11. In the beginning of the twin-wire part water is removed upwards by the water removal sector 23V of the forming roll 23 and at area of the sleeve roll 50 upwards due to the pressure between the bottom and the top wire 10, 20 through the top wire 20.
The sleeve roll 50 with the decreasing radius improves strength of the board web W and improves the layer stability of the multilayer board web W.
In the description in the foregoing, although some functions have been described with reference to certain features and examples, those functions may be performable by other features and examples whether described or not. Although features have been described with reference to the certain examples, those features may also be present in other examples whether described or not.
Above only some advantageous examples of the inventions have been described to which examples the invention is not to be narrowly limited and many modifications and alterations are possible within the invention.

Claims

Method for forming a multilayer board web (W), preferably a three-layer board web (W), more preferably a folding box board (FBB) web (W) or a solid bleached (sulfate) board (SBS) web (W) or a cup board web (W) in a forming section comprising a multilayer headbox (130) and a forming unit (140), characterized in that in the method the layers of the multilayer board web (W) are first formed of pulp suspension in the multilayer headbox (130) and fed to the forming unit (140), which is a hybrid type former for water removal and for joining the layers of the multilayer board web (W) and comprising a single-wire part (10A, 10C) and a twin-wire part (10B), and that in the method fibers of the pulp suspension fed from the multilayer headbox (130) are oriented in the beginning of the forming unit (140) by a breast roll (12A) provided with a shaker (R) and the fiber orientation is stabilized by a forming board (17) provided with suction.
Method according to claim 1, characterized in that in the single-wire part (10A) the pulp suspension is run on a length of 5 -15 m before the twin-wire part (10B).
Method according to claim 1 or 2, characterized in that in the twin-wire part (10B) water is removed by a water removal device (26) comprising at least one suction chamber (20A-C).
Method according to claim 1 or 2, characterized in that in the twin-wire part (10B) water is removed by a water removal sector (23V) of a forming roll (23) and by a sleeve roll (50) due to the pressure between wires (10, 20) of the twin-wire part (10B).
Method according to any of claims 1-4, characterized in that in the method white-water is fed between at least two layers of the multilayer board web (W) in an Aqua-headbox.
Method according to any of claims 1-5, characterized in that in the method the pulp suspension from the multilayer headbox (130) is first fed onto a bottom wire (10) in the single-wire part (10A) on the area of the forming board (17) provided with suction.
Forming section for forming a multilayer board web (W), preferably a three layer board web (W), more preferably a folding box board (FBB) web (W) or a solid bleached (sulfate) board (SBS) web (W) or a cup board web (W), which forming section comprises a multilayer headbox (130) and a forming unit (140), characterized in that the forming unit (140) is a hybrid type former for water removal and for joining the layers of the multilayer board web (W) and comprises a single-wire part (10A, 10C) and a twin-wire part (10B), that in the beginning of the forming unit (140) a breast roll (12A) provided with a shaker (R) is located and configured to orient fibers of pulp suspension fed by the multilayer headbox (130) and that a forming board (17) provided with suction is located directly after the breast roll (12A) with the shaker (R) and configured to stabilize the fiber orientation.
Forming section according to claim 7, characterized in that length of the single-wire part (10A, 10C) before the twin-wire part (10B) is 5 - 15 m.
Forming section according to claim 7 or 8, characterized in that the twin-wire part (10B) water comprises a water removal device (26) comprising at least one suction chamber (20A-C).
Forming section according to claim 7 or 8, characterized in that the twin-wire part (10B) water comprises a forming roll (23) with a water removal sector (23V) and a sleeve roll (50).
Forming section according to any of claims 7-10, characterized in that the multilayer headbox is an Aqua-headbox comprising at least one Aqua-manifold connected to a corresponding distribution header for feeding white-water between at least two layers of the multilayer board web (W).
Forming section according to any of claims 7-11, characterized in that the single-wire part (10A) is configured as a bottom wire (10) and that the multilayer headbox (130) is configured to feed the pulp suspension onto the bottom wire (10) on the area of the forming board (17) provided with suction.
Modified forming section, characterized in that the modified forming section is modified from an existing forming section to a forming section (140) according to any of the previous claims 7-12.
Fiber web production line for producing multilayer board webs, characterized in that the fiber web production line comprises a forming section (140) according to any of the previous claims 7-13, and that the forming section (150) is followed by a press section (150) comprising a center roll (60).