EP2212471B1

EP2212471B1 - Forming section

Info

Publication number: EP2212471B1
Application number: EP08854819.3A
Authority: EP
Inventors: Hannu Turpeinen; Antti Poikolainen; Kari RÄISÄNEN
Original assignee: Valmet Technologies Oy
Current assignee: Valmet Technologies Oy
Priority date: 2007-11-28
Filing date: 2008-11-04
Publication date: 2018-05-23
Anticipated expiration: 2028-11-04
Also published as: CN101878338B; FI20075851L; WO2009068728A1; EP2212471A1; CN101878338A; EP2212471A4; FI20075851A0

Description

FIELD OF INVENTION

The invention relates to a method according to the preamble of claim 1.
The invention also relates to a forming section according to the preamble of claim 10.

PRIOR ART

The task of a forming section is to remove water from fibre suspension fed by the headbox. The consistency of fibre suspension fed onto the forming section is usually 1% and, after the forming section, the consistency of the web formed on the forming section is again 18-20%.
When the web is manufactured of watery wood fibre stock, water in the pulp is removed on the forming section through a forming wire or forming wires for starting the formation of the web. Wood pulp fibres remain randomly distributed 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, fibres and fill materials in the finished product.
Many types of devices are known on the forming section i.e. the former of the web, such as foil strips, suction boxes, hitch rolls, suction rolls, and rolls provided with an open surface, which have been used in many different arrangements and arrays when trying to optimise the volume, time and location of water being removed when forming the web. The manufacture of the web is still partly art and partly science in simply that removing water as quickly as possible does not produce an end-product of best quality. In other words, manufacturing a high-quality end-product especially with great speeds 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 maintain or improve the quality of the end-product when transferring to higher production speeds, many times unforeseeable problems are created as the result of which either the production volume has to be decreased for maintaining the desired quality or the desired quality has to be sacrificed for achieving the greater production volume.
A forming section known from prior art is a hybrid former, which consists of a single-wire section and a twin-wire section following it, whereby the lower wire forms a second wire of the twin-wire section. The headbox feeds a pulp suspension jet at the beginning of the single-wire section, after which a pulp layer, having received its initial forming on the lower wire, moves onto the twin-wire section on which the formation of the web is continued. On the single-wire section, the web is dewatered only in one direction i.e. through the lower wire and, on the twin-wire section, the web is dewatered in both directions.
The hybrid former can be used in a relatively large basis-weight range, whereby it is possible by means of it to e.g. manufacture fine paper the basis weight of which is in the range of 40-300 g/m². With a gap former, it is usually not possible to manufacture a web the basis weight of which exceeds the value of 200 g/m². Thus, there are still a lot of hybrid formers in use and some old fourdrinier-wire formers are modified into hybrid formers.
A problem related to the hybrid former is that the residual variation of the web formed is dependent on the speed of the machine. The upper limit of the speed range of the best hybrid formers today is about 1,300 m/min. If the speed of the hybrid former is increased to a value of over 1,300 m/min, also the residual variation of the web formed increases strongly. A web having too large a residual variation is not a saleable product.
Fig. 3 of WO specification 2007/096467 describes a hybrid former. A single-layer headbox feeds a pulp suspension jet onto a lower wire at the beginning of a single-wire section on top of a forming board after a breast roll. The forming board is provided with underpressure and there are two successive dewatering zones. Dewatering starts in the first dewatering zone and continues in the second i.e. the latter dewatering zone. The second dewatering zone consists of cross-machine directional strips with which pulsating dewatering is induced in pulp suspension travelling on the lower wire. On the single-wire section, the web is dewatered only in one direction i.e. downwards. The single-wire section is followed by a twin-wire section at the beginning of which an upper wire loop forms a gap with the lower wire. Within the upper wire loop, there is a suction box, which is divided into three successive compartments in which unequal underpressures can be used. The lower surface of the first compartment of the suction box following the gap of the twin-wire section is constituted of a curvilinear, stationary forming shoe provided with thorough holes.
FI utility model 6487 again describes another hybrid former. A multi-layer headbox feeds a pulp suspension jet onto a lower wire at the beginning of a single-wire section immediately after a breast roll. On the single-wire section, there is a suction box which can comprise dewatering strips either with or without suction, various suction boxes, forming shoes or equivalents. The single-wire section is followed by a twin-wire section at the beginning of which an upper wire loop forms a gap with the lower wire. Within the upper wire loop, there is a suction box, which is divided into three successive compartments in which unequal underpressures can be used. The lower surface of the first compartment of the suction box following the gap of the twin-wire section is constituted of a curvilinear, stationary forming shoe provided with thorough holes. The distance between a slice opening of the headbox and a closing point of the gap of the twin-wire section is in the range of 0.5-8 m, advantageously 0.3-3 m.
US patent 4154645 describes a web-forming unit for manufacturing a paper web. Within a forming wire loop, there are a breast roll and a forming roll. On a section between the breast roll and the forming roll below the forming wire, there are a forming board and a combination of a wet suction box and a wire guiding shoe following it. A single-layer headbox feeds a pulp suspension jet on top of a fourdrinier wire on a section following the breast roll. The cover structure of the forming board below the forming wire can be closed, perforated or strip covered. The surface of the forming board is most suitably planar. Dewatering with an open-surfaced forming board takes place most suitably freely, but also a suction effect can be combined with this.
Fig. 1 of WO specification 2005/078187 shows a hybrid former in which two separate single-layer headboxes are used for forming a twin-layer web. The first single-layer headbox feeds a pulp suspension jet at the beginning of a fourdrinier-wire section, onto a first, stationary forming shoe located immediately after a breast roll. The pulp suspension jet fed by the first single-layer headbox falls on a section immediately after a leading edge of the first forming shoe. At the beginning of the twin-wire section, a second single-layer headbox feeds a new pulp suspension layer onto a second, stationary forming shoe which is located below the fourdrinier wire at the beginning of the gap of the twin-wire section. At the beginning of the twin-wire section, within an upper wire loop, there is a suction box which is divided into three successive compartments in which unequal underpressures can be used. The lower surface of the first compartment of the suction box following the gap of the twin-wire section is constituted of a third, stationary forming shoe provided with thorough holes. In the first, second and third forming shoe, there is a curvilinear cover setting against the forming wire in which there are a leading edge and a trailing edge. In the cover, there is an open surface which consists of holes extending through the cover. The holes can consist of openings, grooves, slots or equivalents. Under the cover is arranged underpressure with which water is removed from the web. With such a forming shoe, non-pulsating dewatering is applied to the web.
A problem related to arrangements according to prior art is that formation and tensile strength ratio of the web are strongly dependent on a jet-wire ratio. Then, an optimum has to be searched for the characteristics of the web in relation to both the formation and the tensile strength ratio and usually the situation is such that the optima of both factors are not realised with a certain jet-wire ratio. Then, one ends up with a compromise in which with higher tensile strength ratios one has to be satisfied with weaker formation.
In the arrangements according to prior art, it is important that an impact point of a slice jet of the headbox can be accurately adjusted to the same point with each run speed. The slice jet impacts in the arrangements according to prior art on the section of the wire in which there are no dewatering strips below the wire, whereby one has to be able to guide the slice jet accurately on the section in question. As the location of the headbox cannot be moved in the machine direction, the location of the impact point of the slice jet is adjusted by adjusting the position of the upper slice of the headbox in the machine direction.
When manufacturing a web of two or more different pulps, either a multi-layer headbox combined to a gap former or a hybrid former and two separate headboxes are usually employed. A problem of the first combination are high investments. The latter combination gives good layer purity, because a first partial web formed on the fourdrinier-wire section has had time to drain sufficiently when a second partial web is brought onto it at the beginning of the twin-wire section, but the production speed of the hybrid former remains low due to the above reasons. If again a hybrid former according to prior art employs a multi-layer headbox at the beginning of the fourdrinier-wire section, layer purity suffers in the pulsating dewatering of the fourdrinier-wire section.
US 4,154,645 A also discloses a web forming section having a headbox and a forming shoe. However, US 4,154,645 A further provides an arrangement for forming a multi layer paper board.

SUMMARY OF INVENTION

The arrangement according to the invention provides a surprising effect as the result of which the layer purity of the multi-layer web is improved and the production speed of the machine can be increased. In the invention, the impact of the slice jet on the forming wire of the forming section is controlled better.
The principal characteristic features of the method according to the invention are presented in the characterising part of claim 1.
The principal characteristic features of the forming section according to the invention are presented in the characterising part of claim 10.
The other characteristic features of the invention are presented in the dependent claims.
The forming section according to the invention utilises a multi-layer headbox and a forming board formed of a stationary, straight-covered forming shoe provided with suction and a strip cover provided with suction following it. By using the non-pulsating, straight-covered forming shoe with suction at the beginning of the forming board, the take-off and beading (stock jump) of the pulp jet can be substantially decreased, because the pulp jet lands on the non-pulsating surface having a large open surface. The immediate start of dewatering directly at the impact point damps impact energy. The head of the forming board does not doctor water and does not, for its part, induce the "stock jump". Also the direction of the jet is flexible. When the slice jet formed of several different pulp suspension layers fed by the multi-layer headbox falls on the section of the non-pulsating forming shoe, layer purity remains good because the initial dewatering of the web occurs solely by means of underpressure. Then, two-directional pressure pulses having a layer mixing effect formed by pulsating dewatering elements are avoided. Dewatering occurring on the section of the non-pulsating forming shoe is so great that, after it, pulsating dewatering can be applied to the web without the layer purity suffering. The layer purity of the web is particularly good on that surface which is on the side of the forming shoe i.e. the wire surface.
The open surface of the cover of the forming shoe receiving the slice jet of the headbox is 30-90%, advantageously 40-70% of the section between the leading edge and the trailing edge of the cover. Then, sufficient dewatering is provided on the section of the forming shoe and the support surface of the wire remains large enough for avoiding the deflection of the wire.
The arrangement according to the invention enables an extremely good formation of the web in a wide jet-wire ratio range. The straight-covered forming shoe "freezes" the slice jet of the headbox and differences in the speeds of the slice jet/wires of the headbox will not affect formation so strongly. Then, the formation does not weaken with jet-wire ratios which differ a lot from a so-called equal headbox situation in which the speed of the slice jet of the headbox and the run speed of the wires are equal. This means that it is possible to run on the forming section with a jet-wire ratio diverging in one or the other direction from the equal headbox situation without weakening the formation. Again, the use of a jet-wire ratio diverging from the equal headbox ratio has been discovered to improve layer purity. Particularly the web surface setting against the wire travelling on the forming board becomes very clean i.e. the upper pulp layers will not be mixed with the lowest pulp layer in considerable amount. The invention can advantageously utilise a jet-wire ratio diverging for ±5% or even ±10% from the equal headbox ratio.
The forming board provided with the non-pulsating, underpressurised forming shoe enables, compared to a traditional forming board provided with a sole pulsating strip cover, a greater speed difference between the pulp suspension jet fed by the headbox and the forming wire. This property in itself facilitates and/or improves the forming of layer structure in the web being manufactured. Dewatering intensified by means of underpressure keeps single fibres fixed by layering particularly well fast in a layer drained earlier. When the drained layer increases or it is increased on the section of a suitable cutting field occurring in the machine direction and simultaneously all pulsation occurring in dewatering is minimised, particularly good conditions are provided, inter alia, for layering different fibre materials.
The poor quality of layering is visually seen as unevenness i.e. mottling when the colour differences of layered pulps are great. When using the twin-layer headbox on the fourdrinier-wire section, it is not possible to provide simultaneously good formation and keep layers separate with the conventional pulsating forming board with a strip cover. The forming board provided with a non-pulsating, underpressurised forming shoe utilised here enables the use of cutting forces considerably greater than normal without breaking the web. From this follow both good formation and good layer purity. Layer purity is maintained because there is no need to perform formation with two-directional turbulence like a conventional forming board with a strip cover requires.
Furthermore, the arrangement according to the invention has been discovered to have an edge wave reducing effect with large slice openings. The straight-covered, underpressurised forming shoe "freezes" the slice jet of the headbox, whereby the slice jet will not impact edge rulers on the edges of the wire part. The forming of an edge wave can thus be minimised or totally eliminated.
In the arrangement according to the invention, the impact point of the slice jet of the headbox can vary in the machine direction in the range of 50-200 mm, whereby the upper slice moving in the machine direction in the headbox is not necessarily required for adjusting the impact point of the slice jet. This simplifies the structure of the headbox and makes it sturdy and durable. Furthermore, a headbox provided with an upper slice stationary in the machine direction is cheaper to manufacture than a headbox provided with an upper slice moving in the machine direction. After the change of the slice opening, the operator is not required to go to adjust the impact point of the slice jet on the wire. Thus, the changes of the slice opening can be done from the control room like with a gap former. The slice jet impacts the wire in the arrangement according to the invention on the section of a perforated cover having a certain length located below the wire. Thus, the slice jet would impact a similar landing surface even though it came down 50-200 mm later. The later impact of the slice jet on the wire naturally affects a little the dewatering capacity of the perforated cover, but it can be compensated with a suitable length dimensioning of the cover.
The arrangement according to the invention can be employed on all such forming sections in which the beginning of the forming section consists of a single-wire former or a hybrid former.
The use of the arrangement according to the invention in a hybrid former enables an extremely short single-wire section, because dewatering is not tried to be maximised on the single-wire section. The web can be guided relatively wet onto the twin-wire section. Smaller dewatering on the single-wire section also affects the fact that the residual variation of the web decreases.
In an advantageous embodiment according to the invention, the dewatering of the twin-wire section of the hybrid former is both structurally and process-technically a combination of two dewatering elements. The arrangement according to the invention enables a forming section of a multi-layer web more cost-effective of its investment costs than prior art, by means of which forming section, it is possible to manufacture a multi-layer web in the high speed of 1,400-1,800 m/min. Furthermore, the arrangement according to the invention enables the shortening of fourdrinier wire sections due to great dewatering capacity.
The first dewatering element of the twin-wire section of the hybrid former is a stationary forming shoe provided with a curvilinear cover and holes extending through the cover in which underpressure can be used for adjusting and intensifying dewatering. The aim is that the forming shoe will not induce pulsating dewatering even when dewatering is intensified with underpressure. It is possible to consider that the forming shoe is a curve of a "stationary roll" provided with an open surface. The cover has a large open surface and it is connected by means of holes to an underpressure chamber within the forming shoe. The holes in the cover of the forming shoe are formed so that pulsating dewatering is avoided, which would have been induced if the holes were constituted of cross-machine directional elongated slots. For providing this substantially constant pressure, these holes are either openings, slots arranged substantially in the machine direction, waved slots, embossed machine-directional contact surfaces for supporting the fabric above the cover of the shoe etc. The cross-section of the holes can be circular, quadratic, elliptical or polygonal.
The second dewatering element of the twin-wire section of the hybrid former is a pulsating dewatering fitting which comprises stationary cross-machine directional dewatering strips provided with slots, installed on one side of the forming wires. In connection with the stationary strips, it is possible to use underpressure which affects the pulp between the forming wires via slots between the strips. Into the slots between the stationary dewatering strips, it is additionally possible to position adjustably loaded dewatering strips on the opposite side of the forming wires in relation to the dewatering strips. With these adjustable dewatering strips, the pulsating effect applied to the web is further intensified.
With the non-pulsating forming shoe, it is possible to remove water from a very wet web without breaking the structure of the web, because no peak of underpressure occurs on the delivery side of the stationary forming shoe. With the underpressure connected to the forming shoe, very effective dewatering is provided and, with adjusting the underpressure level, it is possible to affect the dewatering distribution between the upper and lower surface of the web, whereby it is possible to control, inter alia, the fines distribution between the upper and lower surface of the web and the Z-directional symmetry of the web.
The great dewatering capacity of the non-pulsating forming shoe enables the fact that the consistency of the web going onto the twin-wire section can be optimised according to the end-product being manufactured. In the headbox, it is possible to use consistency lower than normal and a slice hole larger than normal. Lower feeding consistency improves the formation of the web being formed.
The radius of the non-pulsating forming shoe and the machine directional length of the shoe can be varied according to each intended use within a very large range. The stationary forming shoe can also be constituted of several curves e.g. so that the radius of the forming shoe is larger at the inlet end, but shortens progressively as a spiral curve towards the outlet end. In such a case, the dewatering pressure is no longer constant over the forming shoe, but it still remains non-pulsating. The possibility to vary the radius in both above ways and the length of the shoe means that non-pulsating dewatering is quite easily designed suitable for each application.
After the non-pulsating dewatering zone, the web is guided to the pulsating dewatering zone in dry content in which the formation of the web can be improved with pulsating dewatering.
In the combination of the non-pulsating and the pulsating dewatering zone, the balance between formation and retention can be adjusted better and the strength characteristics of the web can be optimised.
By using a forming board constituted of a non-pulsating forming shoe and a pulsating strip cover on the single-wire section and dewatering constituted of a non-pulsating forming shoe and a pulsating strip cover on the twin-wire section, the one-sidedness of the web can be well controlled. The volume of water being removed through the forming shoes can be adjusted by adjusting the underpressure prevailing in the forming shoes. The control of one-sidedness (particularly the lower surface) is important for specific paper grades. The adjustability of dewatering gives a good opportunity to optimise the symmetry of the end-product. The controlled compression of the web is provided with underpressure affecting the surface of the web.
In an advantageous embodiment of the invention, a dilution-adjusted headbox is used by means of which it is possible to further decrease the residual variation occurring on the single-wire section. The breast roll of the single-wire section has been additionally transferred away from the customary position below the slice channel of the headbox to the delivery side of the headbox and it has been lifted so that the height difference of the upper surface of the lower wire travelling on top of the breast roll and the lower surface of the slice opening of the headbox is in the range of 0-10 mm measured at the topmost point of the breast roll. The horizontal distance between the vertical plane drawn through the midpoint of the breast roll and the slice opening of the headbox is in the range of 10-250 mm. The free flight in the air of the pulp suspension jet discharging from the slice channel of the headbox is in the range of 100-500 mm. The impact angle of the pulp suspension jet on the lower wire is in the range of 0-4 degrees. The pulp suspension jet impacts the lower wire at the point of the stationary forming shoe at the beginning of the forming board. With such an arrangement between the headbox and the breast roll and with the stationary, straight-covered forming shoe at the beginning of the forming board, it is ensured that the pulp suspension jet will not be thrown in the air or become beaded (stock jump) when it impacts the lower wire. The straight-covered forming shoe enables a small impact angle of the slice jet of the headbox on the forming wire.
Applying the arrangement according to the invention in a hybrid former enables the increase of speed to the range of 1,500-1,800 m/min without the residual variation of the web increasing too much or the formation weakening too much. The arrangement according to the invention is also well suitable in a situation in which webs of a large range of basis weights are manufactured on the forming section.
The invention will now be described with reference to the figures of the accompanying drawings.

BRIEF DESCRIPTION OF FIGURES

Fig. 1 shows a schematic side view of a hybrid former.
Fig. 2 shows an enlargement of the beginning of a forming section on which the impact of a pulp suspension jet fed by a headbox on a forming board is visible.
Fig. 3 shows an enlargement of the beginning of a forming section on which the mutual positioning of a headbox, a breast roll and a forming board is visible.
Fig. 4 shows an enlargement of the beginning of a twin-wire section of the hybrid former of Fig. 1.
Fig. 5 schematically shows a side view of a former in which there are two separate partial web forming units.
Fig. 6 schematically shows another former in which there are two separate partial web forming units.

DESCRIPTION OF ADVANTAGEOUS EMBODIMENTS

Fig. 1 shows a hybrid former in which there is a first single-wire section T1 and a first twin-wire section K1 following it.
The first single-wire section T1 consists of a lower wire loop 11 and dewatering fittings 40, 50, 60 arranged below the lower wire 11. A first headbox 30, which is a twin-layer headbox, feeds from its slice channels 32a, 32b a pulp suspension jet on top of the lower wire 11 onto a first forming shoe 40 located at the beginning of the first single-wire section, immediately after a breast roll 12. The travel direction of the lower wire 11 is designated with arrow S1 which is also the machine direction.
The first single-wire section T1 is followed by a first upper wire unit in which there is a first upper wire 21 which forms the first twin-wire section K1 with the lower wire 11. The first upper wire 21 has been formed an endless wire loop by means of hitch and guide rolls 22a, 22b, 22c, 22d. The first guide roll 22a of the first upper wire loop 21 is arranged above the lower wire 11 so that the first upper wire 21 and the lower wire 11 constitute a wedge-shaped gap G at the beginning of the first twin-wire section K1. The web, which has received its initial forming on the first single-wire section T1, is guided after this between the lower wire 11 and the first upper wire 21 of the first twin-wire section K1. At the end of the first twin-wire section K1, the lower wire 11 and the first upper wire 21 are separated from each other. The travel direction of the first upper wire 21 is designated with arrow S2.
On the first single-wire section T1, there are two dewatering zones Z1, Z2.
The first dewatering zone Z1 of the first single-wire section T1 is located immediately after the breast roll 12 and it is constituted of the non-pulsating first forming shoe 40 and a pulsating strip cover 50 following it which together constitute a forming board. In the non-pulsating first forming shoe 40, there is a cover provided with holes which sets against the inner surface of the lower wire 11. The first forming shoe 40 is connected to a source of underpressure (not shown in the figure), whereby an underpressure effect P is applied to the web via the holes in the cover of the first forming shoe 40. The cover of the first forming shoe 40 is straight at least on the section between the impact point of the pulp suspension jet fed by the headbox and the trailing edge of the cover. The first forming shoe 40 induces non-pulsating dewatering in the stock passing on top of the lower wire 11. With the first forming shoe 40, a lot of water can be removed from the stock.
The second dewatering zone Z2 of the first single-wire section T1 is located at the point of the gap G of the first twin-wire section and it consists of a pulsating strip cover 60. The strip cover 60 is connected to a source of underpressure (not shown in the figure), whereby an underpressure effect P is applied to the web passing on top of the lower wire 11 via slots between the cross-machine directional strips of the strip cover 60.
At the beginning of the first twin-wire section K1, two successive dewatering zones Z3, Z4 are formed.
The first dewatering zone Z3 of the first twin-wire section K1 consists of a second forming shoe 70 in which there is a cover provided with holes which sets against the inner surface of the upper wire 21. The second forming shoe 70 is connected to a source of underpressure (not shown in the figure), whereby an underpressure effect P is applied to the web via the holes in the cover of the second forming shoe 70. The second forming shoe 70 is further arranged so that the stock setting to the gap G of the twin-wire section K on the lower wire 11 will not impact the leading edge of the second forming shoe 70 but will be guided to the section of the cover of the second forming shoe 70 after the leading edge. Thus, the leading edge of the second forming shoe 70 will not remove water from the stock. The second forming shoe 70 induces non-pulsating dewatering in the stock passing between the wires 11, 21. With the second forming shoe 70, a lot of water can be removed from the stock.
The second dewatering zone Z4 of the first twin-wire section K1 consists of stationary and adjustably loadable cross-machine directional dewatering strips 81, 83. The stationary dewatering strips 81 are arranged within the upper wire 21 and between them there are slots 82 via which underpressure P can be applied to the partially formed web between the upper wire 21 and the lower wire 11 for removing water from it. Below the lower wire 11 are arranged the adjustable dewatering strips 83 loaded against the inner surface of the lower wire 11 which strips are located at the points of the slots 82 between the stationary dewatering strips 81. The dewatering strips 81, 83 induce pulsating dewatering to the pulp passing between the wires 11, 21. With this second strongly pulsating dewatering zone Z4 of the first twin-wire section K1, the formation of the web being formed can be improved.
The second dewatering zone Z4 of the first twin-wire section K1 is followed by a strip cover 14 causing pulsating dewatering arranged below the lower wire 11 and a transfer suction box 13 following it by means of which it is ensured that the formed web W follows the lower wire 11 after the twin-wire section K from which it is picked up at a pick-up point (not shown in the figure) to further processing.
The arrangement according to Fig. 1 is particularly well suitable for manufacturing twin-layer liner board in which better stock is brought onto the lower layer of the multi-layer headbox than onto the upper layer. Then, this lower surface forms the outer surface of exterior package board in the end-product.
Fig. 2 shows an enlargement of the beginning of the first single-wire section T1 in which the first headbox 30, the breast roll 12, the first forming shoe 40, and the strip cover 50 are visible. The pulp suspension jet of the first headbox 30 impacts the upper surface of the lower wire 11 at the point of the beginning of the first forming shoe 40. On the cover 41 of the first forming shoe 40, there are a leading edge 43 and a trailing edge 44. On the leading edge 43 of the cover 41, there is a first section without holes 41A and, on the trailing edge 44 of the cover 41, there is a second section without holes 41B. Between the sections without holes 41A, 41B of the cover 41, there is an open surface which consists of holes 42 extending through the cover 41. The holes 42 can consist of openings, grooves, slots or equivalents. The open surface defined by holes 42 of the cover 41 of the forming shoe 40 is 30-90%, advantageously 40-70% of the section with holes 42 between the section without holes 41A of the leading edge 43 of the cover 41 and the section without holes 41B of the trailing edge 44 of the cover 41. Below the cover 41, underpressure P is arranged by means of which the dewatering of the pulp is intensified. The impact point of the pulp suspension fed by the first headbox 30 is located at the beginning of the section with holes 42 after the section without holes 41A of the leading edge 43 of the cover of the first forming shoe 40. The forming shoe 40 can also have been divided into two or more blocks, whereby different underpressures can be used in the blocks and/or the open surface of the cover 41 of the forming shoe 40 can be different in the blocks. Then, the underpressure level can e.g. increase in the machine direction and equivalently also the open surface increase in the machine direction. In some embodiments, this also ensures the lubrication of the cover surface when water being removed from the web lubricates the cover surface.
The trailing edge 44 of the first forming shoe 40 is followed by the pulsating strip cover 50 in which there are cross-machine directional strips 51 between which there are holes 52. Below the strip cover 50 is also arranged underpressure P which affects through the holes 52 of the strip cover and intensifies the dewatering of the pulp. Air A passing along the lower wire 11 is guided from the holes 42 at the beginning of the section with holes of the first forming shoe 40 within the first forming shoe 40 and from the other holes 42 is guided water WT within the first forming shoe 40. For minimising the impact angle of the pulp suspension jet fed by the twin-layer headbox 30, it is possible to use a small angular distortion on the leading edge 43 of the cover 41 of the first forming shoe 40. At the impact point of the pulp suspension jet fed by the first headbox 30 and after it, the surface of the cover 41 of the first forming shoe 40 is however straight. The first forming shoe 40 and the strip cover 50 following it together constitute the forming board. The first forming shoe 40 receives the pulp suspension jet of the first headbox 30 and quickly slows it down onto the surface of the lower wire 11. Simultaneously, the first forming shoe 40 effectively removes water from the web and after this the web can be exposed to the pulsating dewatering of the strip cover 50.
Fig. 2 also illustrates with curve PP the non-pulsating dewatering induced by the forming shoe 40 and the pulsating dewatering induced by the strip cover 50.
Fig. 3 shows another enlargement of the beginning of the first single-wire section T1 in which the mutual positioning of the first headbox 30, the breast roll 12, the first forming shoe 40, and the strip cover 50 following it is visible. In the first headbox 30, there are two separate slice channels 32a, 32b which join in a common slice opening 33. The breast roll 12 has been transferred away from the customary position below the lower slice channel 32a of the first headbox 30 to the delivery side of the headbox 30 and it has been lifted so that the height difference H between the upper surface of the lower wire 11 travelling on top of the breast roll 12 and the lower surface 31 of the slice opening 33 of the first headbox 30 is in the range of 0-10 mm measured at the topmost point A of the breast roll 12. The horizontal distance S1 between the vertical plane Y-Y drawn through the midpoint of the breast roll 12 and the slice opening 33 of the first headbox 30 is in the range of 0-250 mm. The free flight in the air S2 of the pulp suspension jet discharging from the slice opening 33 of the first headbox 30 is in the range of 100-500 mm. The impact angle of the pulp suspension jet on the lower wire 11 is in the range of 0-4 degrees. The pulp suspension jet impacts the lower wire 11 at the beginning of the section with holes of the first forming shoe 40. With such an arrangement between the headbox 30, the breast roll 12, the first forming shoe 40, and the strip cover 50 following it, it is assisted that the pulp suspension jet will not be thrown in the air or become beaded (stock jump) when it impacts the lower wire 11.
Fig. 4 shows an enlargement of the beginning of the first twin-wire section K1 of the hybrid former shown in Fig. 1 in which the gap G of the first twin-wire section K1 and the stationary second forming shoe 70 are visible. In the second forming shoe 70, there is a curvilinear cover 71 setting against the inner surface of the upper forming wire 21 in which cover there are a leading edge 73 and a trailing edge 74. On the leading edge 73 of the cover 71, there is a first section without holes 71A and, on the trailing edge 74 of the cover 71, there is a second section without holes 71B. Between the sections without holes 71A, 71B of the cover 71, there is an open surface which consists of holes 72 extending through the cover 71. The holes 72 can consist of openings, grooves, slots or equivalents. Below the cover 71, underpressure, which is illustrated with an arrow with designation P, is arranged by means of which water is removed from the pulp between the wires 11, 21. The holes 72 are arranged on the cover 71 of the second forming shoe 70 so that the open surface of said cover 71 is large, most advantageously 40-90%, and so that they do not induce pressure pulses on the web because of their design and/or arrangement. Pressure pulses can be induced on the web if the forming wire 11, 21 passing on top of the cover 71 is not uniformly supported for the whole surface of the cover 71. Pressure pulses are not induced if the holes are constituted of openings or slots substantially in the longitudinal direction of the machine. When the holes 72 are constituted of openings, they are most advantageously arranged against the travel direction S of the wire 11, 21 passing over the cover 71 obliquely in relation to the cover 71 so that water is guided to them better. The angle α between the central axis of the holes 72 and the tangent of the outer surface of the cover 71 is in the range of 30-60 degrees.
The cover 71 of the second forming shoe 70 is formed curvilinear so that the radius of curvature R of the cover 71 is in the range of 1-50 m. The overlap angle of the wire 21 on the section of the cover 71 is in the range of 3-45 degrees, advantageously 5-30 degrees. The machine directional length S3 of the cover 71 is in the range of 200-1,000 mm. The underpressure level used in the second forming shoe 70 is in the range of 2-15 kPa, advantageously in the range of 2-5 kPa. The cover 71 can also consist of several parts having a different radius of curvature R. By varying the radius of curvature R of the cover 71 of the second forming shoe 70 and/or by varying the underpressure P prevailing in the second forming shoe 70 and/or the length S3 of the second forming shoe, the volume and distribution of water removed from the web by the second forming shoe 70 can be adjusted.
Fig. 5 schematically shows a side view of a former in which there are two separate partial web forming units. The beginning of the forming section is for its substantial parts equivalent to the hybrid former shown in Fig. 1. The first twin-wire section K1 of the hybrid former is followed by a second upper wire unit in which a second upper wire 111 forms a second fourdrinier-wire section T2. A second headbox 130, which is a single-layer headbox, feeds a pulp suspension jet on the second upper wire 111 onto a breast roll 112 or immediate after the breast roll 112. On the second fourdrinier-wire section T2, water is removed from pulp suspension travelling on top of the second upper wire 111 with dewatering fittings 140 located below the second upper wire 111 which fittings can be suction boxes provided with a strip cover. A second partial web W2 formed in the second upper wire unit is guided after this to a joining point Y formed by an inner roll 113 of the second upper wire loop 111 of the second upper wire unit and the lower wire loop 11 of the hybrid former in which point the second partial web W1 is joined in the first partial web W1 formed in the hybrid former. The joining point Y is followed by a short, second twin-wire section K2 on which the joining of the partial webs W1, W2 is ensured. Water is removed from the web W joined on this second twin-wire section K2 downwards with dewatering fittings 15 arranged below the lower wire 11 of the hybrid former. The second twin-wire section K2 is followed again by a single-wire section on which water is further removed from the joined web W with dewatering fittings 16 arranged below the lower wire 11 of the hybrid former. The single-wire section is followed by a pick-up point P in which the joined web W is picked up from the lower wire 11 of the hybrid former by a pick-up suction roll 90 and brought on a pick-up felt 91 to the presser section.
Fig. 6 schematically shows another former in which there are two separate partial web forming units. A first wire unit consists of a lower wire 11 on which is formed a first fourdrinier wire unit T1. A first headbox 30, which is a twin-layer headbox, feeds a pulp suspension jet onto a first forming board 40, 50 which is totally equivalent to the forming board shown in Fig. 1. Water is removed from the web travelling on top of the lower wire 11 on the fourdrinier-wire section following the first forming board 40, 50 with dewatering fittings 55 arranged below the lower wire 11 which can be suction boxes provided with a strip cover. The first fourdrinier wire unit T1 is followed by a first upper wire unit in which there is a first upper wire 211. A second headbox 230, which is a single-layer headbox, feeds a pulp suspension jet on the first upper wire 211 onto a second forming board 240, 250 after a breast roll 212. Also this second forming board 240, 250 is totally equivalent with the forming board 40, 50 shown in the embodiment of Fig. 1. On a second fourdrinier-wire section T2 following the second forming board 240, 250, water is removed from the pulp suspension with dewatering fittings 260 located below the first upper wire 211 which can be suction boxes provided with a strip cover. A second partial web W2 formed in the upper wire unit is guided after this to a joining point Y formed by an inner roll 213 of the first upper wire loop 211 and the lower wire loop 11 in which point the second partial web W1 is joined in the first partial web W1 formed in the lower wire unit. The joining point Y is followed by a short twin-wire section K1 in which the joining of the partial webs W1, W2 is ensured. Water is removed from the joined web W on this twin-wire section K1 downwards with dewatering fittings 15 arranged below the lower wire 11 of the lower wire unit. The twin-wire section K1 is followed again by a single-wire section on which water is further removed from the joined web W with dewatering fittings 16 arranged below the lower wire 11 of the lower wire unit. The single-wire section is followed by a pick-up point P in which the joined web W is picked up from the lower wire 11 of the lower wire unit by a pick-up suction roll 90 and brought on a pick-up felt 91 to the presser section.
The embodiment shown in Fig. 6 can also employ a twin-layer headbox as the second headbox 230. A straight-covered forming shoe 240 provided with holes and suction at the beginning of the upper wire unit induces non-pulsating dewatering in the pulp suspension travelling on top of the first upper wire 211 so that layer purity remains good particularly on the lower surface of the web. With both twin- layer headboxes 30, 230, it is thus possible to feed pulp in which a first layer setting against the forming wire 11, 211 consists of pulp of better quality and a second layer, setting on top of the first layer consists of pulp of lesser quality. On the surfaces of the joined web comes then the pulp of better quality i.e. having higher strength and better surface properties and to the middle comes the pulp of lesser quality. The surface layers of the joined web remain pure i.e. the lesser-quality pulp of the middle layer is not able to mix in them at least in considerable amount.
The structure of the first forming shoe 40 on the first single-wire section T1 is equivalent to the second forming shoe 70 on the first twin-wire section K1 with the difference that the cover of the first forming shoe 40 is straight. By using underpressure in the forming board 40, 50 in the range of 2-10, advantageously in the range of 5-7 kPa, and by using a jet-wire ratio diverging from the equal headbox ratio for ±5%, advantageously ±10%, good layer purity is provided. The layer purity is formed particularly good on the surface of the web setting against the lower wire 11. The pulp suspension layer fed by the upper slice channel 32b of the first headbox 30 will not thus mix very much with the pulp suspension layer fed by the lower slice channel 32a of the first headbox 30.
From the lower slice channel 32a of the first headbox 30 is fed the pulp of better quality and from the upper slice channel 32b of the first headbox 30 is fed the pulp of lesser quality. The lower surface of the web setting against the lower wire 11 is then formed of better quality than the opposite upper surface of the web. The lower surface of the web can then be used as the outer surface of board in which product information is possibly printed and the inner surface of the web as the inner surface of board.
The machine-directional length of the first single-wire section T1 is in the range of 0.5-10.0 m and the consistency of the pulp suspension fed by the twin-layer headbox 30 is in the range of 0.1-3.0%. With high speeds, the first single-wire section T1 has to be short i.e. in the range of 0.5-3.0 m. In refurbishings, the first single-wire section T1 is usually long due to the existing structure i.e. in the range of 8-10 m and then it is rarely shortened. A long first single-wire section T1 weakens the residual variation of the web. With the arrangement according to the invention, it has been possible to run at the speed of more than 1,600 m/min without the residual variation considerably increasing on a hybrid former with a single-wire section of 8-10 m.
In the embodiments shown in Figs. 1 and 5, it is possible to remove from the water volume included in the pulp suspension fed by the first headbox 30 on the first single-wire section T1 about 40-60% downwards and on the first twin-wire section K1 about 30-50% upwards and about 5% downwards. In a hybrid former applying the arrangement according to the invention, it is possible to achieve relatively uniform dewatering from both surfaces of the web. With the stationary forming shoe at the beginning of the first twin-wire section K1, it is possible to remove a lot of water from a relatively wet web, whereby there is no need to remove so much water on the single-wire section. With the forming shoe, it is possible to remove about half of the total volume of water being removed upwards on the first twin-wire section K1.
The embodiments of the figures show only one forming shoe in connection with the forming board of the first single-wire section and at the beginning of the first twin-wire section of the hybrid former, but there can also be several forming shoes, whereby it is possible to e.g. use different underpressure levels in different forming shoes.
In the embodiments shown in Figs. 1 and 5, the second dewatering zone Z4 of the first twin-wire section K1 consists of the stationary 81 and the adjustably loadable 83 dewatering strips. The second dewatering zone Z4 of the first twin-wire section K1 can also consist solely of the stationary dewatering strips 81. The stationary dewatering strips 81 can form a straight path to the wires travelling on top of them. With underpressure prevailing in the slots 82 of the stationary dewatering strips 81, the path of the wires is slightly deflected in said slots 82, whereby pulsating dewatering is provided in the web between the forming wires. The stationary dewatering strips 81 can also be positioned so that they form a curvilinear path to the wires travelling on top of them. The dewatering strips 81 are then at a small angle of about 0.5-2 degrees in relation to each other. With such an arrangement, intensified pulsating dewatering is provided in the web between the forming wires passing over the dewatering strips. In both cases, the pulsating effect is further intensified if both the stationary 81 and adjustably loadable 83 dewatering strips are used.
The first headbox 30 shown at the beginning of the first single-wire section T1 in Figs. 1, 5 and 6 is advantageously a twin-layer headbox, but it can also be a multi-layer headbox. The second headbox 230 shown at the beginning of the second single-wire section T2 in Fig. 6 is advantageously a single-layer headbox, but it can also be a twin-layer headbox or a multi-layer headbox. In the headbox, there can be N pieces of slice channels where N is a whole number which is greater than or equal to two.
In the embodiment shown in Fig. 5, there is only one upper wire unit after the first twin-wire section K1, but the invention allows employing N pieces of upper wire units whereby N is a whole number which is greater than or equal to 1.
In the embodiment shown in Fig. 6, there is only one upper wire unit after the first fourdrinier-wire section T1, but the invention allows employing N pieces of upper wire units whereby N is a whole number which is greater than or equal to 1.
The forming shoe 40 shown in Fig. 2 and the strip cover 50 following it can utilise the same underpressure. With webs having low basis weight or pulps being easily drained, very low underpressure can be used in the strip cover 50 in order to not remove too much water from the web. With webs having high basis weight or pulps being poorly drained, the web can be clogged too much on the section of the forming shoe 40. Then, pulsation is intensified and dewatering increased on the section of the strip cover 50 by increasing the underpressure of the strip cover 50, whereby the clogged lower surface of the web will be unclogged.
Above were described only some advantageous embodiments of the invention and it is evident to those skilled in the art that several modifications can be made to them within the scope of the enclosed claims.

Claims

A method on a forming section, comprising the following steps:
- forming a first single-wire section (T1) by a lower wire (11) circulating a breast roll (12),

- forming at the beginning of the first single-wire section (T1) immediately after the breast roll (12) a first dewatering zone (Z1) which consists of at least one stationary first forming shoe (40), in which there are a leading edge (43) and a trailing edge (44), a cover (41) provided with through holes (42) setting against an inner surface of the lower wire (11) and underpressure (P) prevailing through the holes (42) of the cover (41), which holes (42) are formed of openings or slots substantially in the longitudinal direction of the machine, whereby non-pulsating dewatering is applied to stock travelling on top of the lower wire (11) of the first single-wire section (T1) on the section with holes (42) of the cover (41) of the first forming shoe (40),

- performing non-pulsating dewatering with the first forming shoe (40) of which the open surface determined by the holes (42) of the cover (41) is 30-90%, advantageously 40-70%, of the section with holes (42) between a section without holes (41A) of the leading edge (43) of the cover (41) and a section without holes (41B) of the trailing edge (44) of the cover (41),

- feeding with a first headbox (30) a pulp suspension jet at an impact point after the leading edge (43) of the first forming shoe (40),

- using as the first headbox (30) a multi-layer headbox,
characterised by the method further comprising the following steps:
- forming the cover of the forming shoe (40) straight at least on the section between the impact point of the pulp suspension jet and the trailing edge (44) of the forming shoe (40),

- arranging in the first dewatering zone (Z1) immediately after the forming shoe (40) a pulsating strip cover (50).
A method according to claim 1, characterised by performing non-pulsating dewatering with the first forming shoe (40) by locating the holes (42) going through the cover (41) obliquely against a travel direction of the lower wire (11) so that an angle (α) between a central axes of the holes (42) and a tangent of the outer surface of the cover (41) is 30-60 degrees.
A method according to claim 1 or 2, characterised by using a dilution-adjustable multi-layer headbox as the first headbox (30), which is located in relation to the breast roll (12) so that a vertical height difference (H) of an upper surface of the lower wire (11) travelling on top of the breast roll (12) and a lower surface (31) of a slice opening (33) of the first headbox (30) is in the range of 0-10 mm, and that a horizontal distance (S1) of a vertical plane Y-Y passing through a central axis of the breast roll (12) and a slice opening (33) of the first headbox (30) is in the range of 10-250 mm.
A method according to any one of claims 1-3, characterised by the method further comprising the following steps:
- forming above the lower wire (11), after the first single-wire section (T1) a first upper wire unit with a first upper wire (21), which first upper wire (21) forms with the lower wire (11) a first twin-wire section (K1) in which there is a beginning in which the lower wire (11) and the first upper wire (21) form a closing gap (G) and an end in which the first upper wire (21) is separated from the lower wire (11),

- guiding the web initially formed on the first single-wire section (T1) to the first twin-wire section (K1),

- forming at least two successive dewatering zones (Z3, Z4) on the first twin-wire section (K1),

- forming the first dewatering zone (Z3) of the first twin-wire section (K1) from at least one stationary second forming shoe (70) located at the beginning of the first twin-wire section (K1), which shoe has a leading edge (73) and a trailing edge (74), a curvilinear cover (71) provided with through holes (72) setting against the upper wire (21) of the first twin-wire section and underpressure (P) prevailing through the holes (72) of the cover (71), which holes (72) consists of openings or slots substantially in the longitudinal direction of the machine, whereby non-pulsating dewatering is applied on the stock travelling between the forming wires (11, 21) of the first twin-wire section (K1) on the section with holes (72) of the cover (71) of the second forming shoe (70),

- forming a latter, second dewatering zone (Z4) of the first twin-wire section (K1) of stationary cross-machine directional dewatering strips (81) setting against one side of the first twin-wire section between which strips there are slots (82), whereby pulsating dewatering is applied on the stock travelling between the forming wires (11, 21) of the twin-wire section (K1) with the stationary dewatering strips (81) and underpressure (P) on the section of the stationary dewatering strips (81).
A method according to claim 4, characterised by forming adjustably loadable dewatering strips (83) on the second dewatering zone (Z4) of the first twin-wire section (K1), which strips are located in relation to the stationary dewatering strips (81) on the opposite side of the first twin-wire section (K1), at the point of the slots (82) between the stationary dewatering strips (81).
A method according to claim 4 or 5, characterised by performing non-pulsating dewatering with the second forming shoe (70), the open surface determined by the holes (72) of the cover (71) of which is 30-90%, advantageously 40-70% of a section with holes (72) between a section without holes (71A) of a leading edge (73) of the cover (71) and a section without holes (71B) of a trailing edge (74) of the cover (71).
A method according to any one of claims 4-6, characterised by performing non-pulsating dewatering with the second forming shoe (70) the holes (72) going through the cover (71) of which are located obliquely against a travel direction of the upper wire (21) so that an angle (α) between a central axes of the holes (72) and a tangent of an outer surface of the cover (72) is 30-60 degrees, preferably so that an overlap angle of the upper wire (21) travelling over the second forming shoe (70) on the section of the cover (71) of the second forming shoe (70) is 3-45 degrees, most advantageously 5-30 degrees.
A method according to any one of claims 1-3, characterised by forming a second upper wire unit with a second upper wire (211) above the lower wire (11) after the first single-wire section (T1), which second upper wire (211) forms a second single-wire section (T2) at the beginning of which a second headbox (230), which is a single-layer headbox, feeds a pulp suspension jet for forming a second partial web (W2) and which second upper wire (211) further forms a second twin-wire section (K2) with the lower wire (11), and on which second twin-wire section (K2) there is a beginning in which the lower wire (11) and the second upper wire (211) form a joining point (Y) of the partial webs and an end in which the second upper wire (211) is separated from the lower wire (11), whereby to the first partial web (W1) formed on the first fourdrinier-wire section (T1) of the beginning of the forming section is joined the second partial web (W2) formed in the second upper wire unit at the joining point (Y).
A method according to any one of claims 4-7, characterised by forming a second upper wire unit with a second upper wire (111) above the lower wire (11) after the first single-wire section (K1), which second upper wire (111) forms a second single-wire section (T2) at the beginning of which a second headbox (230), which is a single-layer headbox, feeds a pulp suspension jet for forming a second partial web (W2) and which second upper wire (111) further forms a second twin-wire section (K2) with the lower wire (11), and on which second twin-wire section (K2) there is a beginning in which the lower wire (11) and the second upper wire (111) form a joining point (Y) of the partial webs and an end in which the second upper wire (111) is separated from the lower wire (11), whereby to the first partial web (W1) formed on the first single-wire section (T1) and the first twin-wire section (K1) is joined the second partial web (W2) formed in the second wire unit at the joining point (Y).
A forming section which comprises:
- a lower wire loop (11) which constitutes a first single-wire section (T1) following a breast roll (12),

- the beginning of the first single-wire section (T1) comprises a first dewatering zone (Z1) which consists of at least one stationary first forming shoe (40), in which there are a leading edge (43) and a trailing edge (44), a cover (41) provided with through holes (42) setting against the inner surface of the lower wire loop (11), and underpressure (P) prevailable through the holes (42) of the cover (41), which holes (42) consist of openings or slots substantially in the longitudinal direction of the machine, whereby non-pulsating dewatering can be applied on the stock passing on top of the lower wire (11) in the section provided with holes (42) of the cover (41) of the first forming shoe (40), wherein the open surface of the cover defined by the holes (42) in the cover (41) of the first forming shoe (40), meant for performing non-pulsating dewatering, is 30-90%, advantageously 40-70%, of the section with holes (42) between a section without holes (41A) of the leading edge (43) of the cover (41) and a section without holes (41B) of the trailing edge (44) of the cover (41),

- a first headbox (30) by means of which a pulp suspension jet can be fed at an impact point after the leading edge (43) of the first forming shoe (40),
wherein the first headbox (30) is a multi-layer headbox, characterised in that

- the cover of the first forming shoe (40) is straight at least on the section between the impact point of the pulp suspension jet and the trailing edge (44) of the forming shoe (40), and

- a pulsating strip cover (50) is provided immediately following the first forming shoe (40).
A forming section according to claim 10, characterised in that the holes (42) going through the cover (41) of the first forming shoe (40) performing non-pulsating dewatering are located obliquely against a travel direction of the lower wire (11) so that an angle (α) between a central axis of the holes (42) and a tangent of an outer surface of the cover (41) is 30-60 degrees, preferably the first headbox (30) is a dilution-adjustable multi-layer headbox which is located in relation to the breast roll (12) so that a vertical height difference (H) of an upper surface of the lower wire (11) travelling on top of the breast roll (12) and a lower surface (31) of a slice opening (33) of the first headbox (30) is in the range of 0-10 mm, and that a horizontal distance (S1) of a vertical plane Y-Y passing through a central axis of the breast roll (12) and a slice opening (33) of the first headbox (30) is in the range of 0-250 mm.
A forming section according to any one of claims 10-11, characterised in that it further comprises:
- a first upper wire unit which is formed above the lower wire (11), after the first single-wire section (T1) on a first upper wire (21), which first upper wire (21) forms with the lower wire loop (11) a first twin-wire section (K1) after the first single-wire section (T1) in which first twin-wire section (K1) there is a beginning in which the lower wire (11) and the first upper wire (21) form a closing gap (G) and an end in which the first upper wire (21) is separated from the lower wire (11),

- at least two successive dewatering zones (Z3, Z4) on the first twin-wire section (K1),

- the first dewatering zone (Z3) of the first twin-wire section (K1) consists of at least one stationary second forming shoe (70) located at the beginning of the twin-wire section (K1), which shoe has a leading edge (73) and a trailing edge (74), a curvilinear cover (71) provided with thorough holes (72) setting against the inner surface of the upper wire loop (21) and underpressure (P) prevailing through the holes (72) of the cover (71), which holes (72) are formed of openings or slots substantially in the longitudinal direction of the machine, whereby non- pulsating dewatering is applied on the stock travelling between the forming wires (11, 21) of the first twin-wire section (K1) on the section with holes (72) of the second forming shoe (70),

- the latter, second dewatering zone (Z4) of the first twin-wire section (K1) is formed of stationary cross-machine directional dewatering strips (81) setting against one side of the first twin-wire section (K1) between which strips there are slots (82), whereby pulsating dewatering is applied on the stock travelling between the forming wires (11, 21) of the first twin-wire section (K1) with the stationary dewatering strips (81) and underpressure (P) on the section of the stationary dewatering strips (81), preferably the second dewatering zone (Z4) of the first twin-wire section (K1) further comprises adjustably loadable dewatering strips (83) which are located in relation to the stationary dewatering strips (81) on the opposite side of the twin-wire section (K), at the point of the slots (82) of the stationary dewatering strips (81).
A forming section according to claim 12, characterised in that the open surface determined by the holes (72) of the cover (71) of the second forming shoe (70) performing non-pulsating dewatering is 30-90%, 40-70% of a section with holes (72) between a section without holes (71A) of a leading edge (73) of the cover (71) and a section without holes (71B) of a trailing edge (71) of the cover, preferably the holes (72) going through the cover (71) of the second forming shoe (70) performing non-pulsating dewatering are located obliquely against a travel direction of the upper wire (21) so that an angle (α) between a central axes of the holes (72) and a tangent of the outer surface of the cover (71) is 30-60 degrees, more preferably an overlap angle of the upper wire (11, 21) travelling over the second forming shoe (70) performing non-pulsating dewatering on the section of the cover (71) of the second forming shoe (70) is 3-45 degrees, most advantageously 5-30 degrees.
A forming section according to any one of claims 10-11, characterised in that it comprises a second upper wire unit which is formed with a second upper wire (211) above the lower wire (11) after the first single-wire section (T1), which second upper wire (211) forms a second single-wire section (T2) for forming a second partial web (W2) and which second upper wire (211) further forms a second twin-wire section (K2) with the lower wire (11), and on which second twin- wire section (K2) there is a beginning in which the lower wire (11) and the second upper wire (211) form a joining point (Y) of the partial webs and an end in which the second upper wire (211) is separated from the lower wire (11), whereby to the first partial web (W1) formed on the first fourdrinier-wire section (T1) of the beginning of the forming section is joined the second partial web (W2) formed in the second upper wire unit at the joining point (Y).
A forming section according to any one of claims 12-13, characterised in that it comprises a second upper wire unit which is formed with a second upper wire (111) above the lower wire (11) after the first twin-wire section (K1), which second upper wire (111) forms a second single-wire section (T2) for forming a second partial web (W2) and which second upper wire (111) further forms a second twin-wire section (K2) with the lower wire (11), and on which second twin-wire section (K2) there is a beginning in which the lower wire (11) and the second upper wire (111) form a joining point (Y) of the partial webs and an end in which the second upper wire (111) is separated from the lower wire (11), whereby to the first partial web (W1) formed on the first single-wire section (T1) is joined the second partial web (W2) formed in the second wire unit at the joining point (Y).