WO2010020707A1

WO2010020707A1 - Board machine

Info

Publication number: WO2010020707A1
Application number: PCT/FI2009/050653
Authority: WO
Inventors: Kari RÄISÄNEN
Original assignee: Metso Paper, Inc.
Priority date: 2008-08-22
Filing date: 2009-08-11
Publication date: 2010-02-25
Also published as: AT509113B1; AT509113A2; FI20085782A0; AT509113A3; FI20085782L; CN102131981B; DE112009002741T5; CN102131981A

Abstract

A board machine comprises a forming section where a lower wire loop (11) forms a fourdrinier-wire section (Tl) at the beginning of which is located a forming board (200) which consists of at least one stationary, non-pulsating forming shoe (210). In the forming shoe (210), there is a cover (211) provided with thorough holes (212), setting against the inner surface of the lower wire loop (11). A first headbox (100), which is a multi-layer headbox, feeds a pulp suspension jet onto the forming shoe (210). The board machine further comprises a press section (Pl) where a surface (Wa) of the web (W) having been against the lower wire loop (11) is pressed against a smooth surface, a dryer section (Dl) where the web (W) is dried with at least one cylinder-dryer group and a coating section (400) where at least the surface (Wa) of the web (W) having been against the lower wire (11) is coated.

Description

Board machine

FIELD OF INVENTION

The invention relates to a method according to the preamble of claim 1.

The invention also relates to a board machine according to the preamble of claim

8.

PRIOR ART

Liner board is used as the surface layers of corrugated board. The typical basis- weight range of liner board is 100-350 g/cm². Liner board consists of a top layer and a back layer and possibly of a protective layer between them. Liner board is manufactured with various top layers and of different raw materials. Both virgin and recycled fibre are used in manufacturing liner board. Liner board mainly made of virgin fibre is called kraft liner and liner board made of recycled fibre is called testliner. Recently, almost all liner board is based on recycled fibre. As the raw material of the back layer can be used e.g. old corrugated containers (OCC) and as the raw material of the top layer can be used e.g. de-inked pulp (DIP) or more expensive, e.g. virgin bleached, fibre.

As its name suggests, white top liner (WTL) board comprises a top layer manufactured of bleached fibre and a back layer manufactured of unbleached fibre. Between the top and back layer, there is often also a protective layer. The ratio of the basis weights of the top layer and the back layer is typically 30/70. WTL board is typically used in packing materials which require good printing properties of the surface of the board. The share of bleached fibre is considerable in costs whereby, for saving the costs, WTL board tends to be manufactured using the so-called protective layer which is a coloured fibre layer between the brown and the white one. Then, the share of the expensive, totally white fibre can be decreased. The share of the protective layer of the board basis weight is usually about 20%.

WTL board composed of two layers can be manufactured in a fourdrinier-wire machine provided with a twin- layer headbox or in a hybrid former, whereby the unbleached fibre forms the back layer against the fourdrinier wire and the bleached fibre forms the top layer. Dewatering occurs on the fourdrinier wire solely downwards, whereby the fibre in the top layer can be mixed with the fibre lower in the back layer. Due to this, the bleached fibre was chosen for the top layer in the arrangements according to prior art.

On the other hand, WTL board composed of two layers can also be manufactured with two single-wire units, whereby the back layer (unbleached fibre) is manufactured on the lower wire unit and the top layer (bleached fibre) on the upper wire unit. Dewatering occurs on the lower wire solely downwards, whereby the fibre of the upper layer of the combined web can be mixed with the fibre in the lower layer. Due to this, the bleached fibre was chosen for the top layer in the arrangements according to prior art.

In addition to the twin-layer headbox and the fourdrinier-wire machine or the hybrid former, manufacturing WTL board composed of three layers requires a separate upper wire unit with its headbox, whereby the top layer is formed in the upper wire unit.

It is also possible to manufacture WTL board composed of two layers such that the light layer is manufactured totally by coating with the air-brush or curtain- coating technique. Then, the coating layer becomes considerably thick, whereby its drying requires a lot of energy. The required strength of the board has then to be provided with a lower basis weight. For instance, the strengths of a board web of the basis weight of 140 g/cm² have to be provided in a board web of the basis weight of 120 g/cm², because the portion of clay coating in the basis weight of the board web is 20 g/cm². Providing strength then requires both better fibre and beating energy which again increase the total costs.

WO specification 2008/000900 describes a forming section of a web-forming machine which comprises a lower wire loop which forms a single-wire section after a breast roll. The beginning of the single-wire section comprises a first dewatering zone which consists of at least one stationary first forming shoe, in which there are a leading edge and a trailing edge, a cover provided with thorough holes setting against the inner surface of the lower wire, and underpressure prevailing through the holes of the cover, which holes consist of openings or slots substantially in the longitudinal direction of the machine, whereby non-pulsating dewatering is applied on the stock passing on top of the lower wire on the section provided with holes of the cover of the first forming shoe. The first forming shoe is followed by a pulsating strip cover. A headbox feeds a pulp suspension jet at an impact point after the leading edge of the first forming shoe. The cover of the first forming shoe is straight at least on the section between the impact point of the pulp suspension jet and the trailing edge of the first forming shoe.

EP patent 1218593 describes a paper machine for manufacturing liner board composed of at least two layers. The paper machine comprises a lower wire unit for manufacturing the base layer of liner board and an upper wire unit for manufacturing the top layer of liner board. The forming section is followed by a press section where the upper surface of the top layer of liner board sets at least in the last nip against a smooth-surfaced transfer belt. The upper surface of the top layer of the web then acquires sufficient smoothness for printing. In the embodiment shown in Fig. 1, the web composed of the base layer and the top layer is transferred from the forming section at a pick-up point onto a pick-up felt and is taken supported by the pick-up felt and a lower press felt into a first double- felted press nip which consists of a lower press roll and a first upper press roll. After the first press nip, the web follows on the outer surface of the pick-up felt running around the first upper press roll into a second press nip which is formed between the first upper press roll and a middle roll. After the second press nip, the web follows on the outer surface of the smooth press felt running around the middle roll into a third press nip which is formed between the middle roll and a second upper press roll. In the third press nip, the web passes between the smooth press felt running around the middle roll and a smooth-surfaced transfer belt being against the second upper press roll. After the third press nip, the web follows the smooth-surfaced transfer belt from which the web is transferred at a first transfer point onto the press felt or a transfer wire and on that further to a second transfer point in which the web is transferred onto the support of a drying wire and on that further to a dryer section. The upper surface of the top layer of the web formed in the upper wire unit sets in the third nip against the transfer belt, whereby sufficient smoothness for printing is provided on the upper surface of the top layer of the web.

SUMMARY OF INVENTION

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 board machine according to the invention are presented in the characterising part of claim 8.

The board machine according to the invention comprises a forming section in which there is a first fourdrinier-wire section at the beginning of which is located a forming board which consists of at least one stationary, non-pulsating forming shoe. In said at least one forming shoe, there is a cover provided with thorough holes, setting against the inner surface of the lower wire which holes are formed of openings or slot substantially in the longitudinal direction of the machine. A pulp suspension jet is fed onto the forming shoe with a first multi-layer headbox in which there are a lower slice channel and at least one upper slice channel. The forming section is followed by a press section where the web surface having been against the lower wire is pressed against a smooth surface and the press section is followed by a dryer section where the web is dried with at least one cylinder-dryer group. The board machine further comprises a coating section where at least the web surface having been against the lower wire is coated.

Said smooth surface of the press section can consist of a transfer belt, i.e. a so- called transbelt which is a fabric impermeable to water, a smooth roll surface, e.g. a smooth middle roll, or a metal belt.

By using a non-pulsating forming shoe 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". The direction of the jet is also flexible. When the slice jet formed of several different pulp suspension layers fed by the multilayer headbox falls on the section of the non-pulsating forming shoe, layer purity remains good because the initial dewatering of the web occurs through a non- pulsating, supported surface. Then, two-directional pressure pulses having a layer mixing effect formed by pulsating dewatering elements are avoided. 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 dewatering of the non-pulsating forming shoe can further be intensified by using underpressure in the forming shoe. Dewatering occurring on the section of the non-pulsating forming shoe can be obtained so great with underpressure that, after it, also pulsating dewatering can be applied to the web when required without the layer purity suffering.

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 with holes 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 non-pulsating forming shoe "freezes" the slice jet of the headbox and differences in the speeds of the headbox slice jet/wires 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. It is thus 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 passing on the forming board becomes very clean i.e. the upper pulp layers will not be mixed with the lowest pulp layer in a considerable amount. The invention can advantageously utilise a jet-wire ratio diverging for ±5% or even ±10% from the equal headbox ratio.

As the non-pulsating forming shoe "freezes" the slice jet of the headbox, the slice jet will not either impact edge rulers on the edges of the wire section. The forming of an edge wave can thus be minimised or totally eliminated.

The forming board provided with the non-pulsating forming shoe enables, compared to a traditional forming board provided solely with a 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 the 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 twin-layer headbox on the fourdrinier-wire section and the conventional pulsating forming board with a strip cover do not enable good formation and layer purity simultaneously. The poor quality of layering is visually seen as unevenness i.e. mottling when the colour differences of layered pulps are great. The forming board provided with a non-pulsating forming shoe 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 the conventional forming board with a strip cover requires.

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 web surface, whereby it is possible to control, inter alia, the fines distribution between the upper and lower web surface and the Z-directional symmetry of the web.

In the non-pulsating forming shoe, it is possible to use either a straight or slightly curved cover. The straight cover would seem to slow the slice jet down on the surface of the cover of the forming shoe best.

The arrangement according to the invention can additionally employ a dilution- controlled headbox, whereby residual variation occurring on the single-wire section further decreases. The breast roll of the single-wire section can additionally be transferred away from the customary position below the slice channel of the headbox to the delivery side of the headbox and it can be lifted such that the height difference of the upper surface of the lower wire passing 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. Such an arrangement between the headbox and the breast roll and with the stationary forming shoe at the beginning of the forming board ensures that the pulp suspension jet will not be thrown in the air or become beaded (stock jump) when it impacts the lower wire. The non-pulsating forming shoe enables a small impact angle of the headbox slice jet on the forming wire.

The board machine according to the invention utilises a press section where water is removed from the web in at least one such press nip in which the lower web surface sets against a smooth surface. Said smooth surface can be formed of a transfer belt impermeable to water, e.g. a transfer belt known with the trademark Transbelt, a smooth-surfaced roll, e.g. a middle roll of a middle roll press, or a metal belt. The lower web surface is thus obtained smooth and having good printing properties. As the press, it is possible to use e.g. a press known with the applicant's trademark SymPress B in which there is a smooth-surfaced middle roll. In such a press section, the web is pressed against the smooth- surfaced middle roll with a so-called long nip roll, whereby the dewatering capacity of the press is high and, together with the forming section according to the invention, high run speed is provided.

The investment and operating costs of the board machine according to the invention are lower compared to prior-art arrangements, because it does not necessarily require the upper wire unit, whereby the costs of at least one headbox and wire section are saved. The use of a press section simple of its structure, e.g. a SymPress B press, leads, for its part, to low investment and operating costs. The board machine according to the invention thus also consumes considerably less energy compared to the arrangements according to prior art. In the board machine according to the invention, it is possible to use a cheaper raw material, e.g. recycled fibre, instead of pulp on the lower surface of multi-layer boards, because the lower surface will be coated.

For ensuring good brightness, the lower web surface is coated with a separate coating device. As the coating method, any coating method can be used, such as e.g. film coating which is a simple and versatile coating method.

The board machine according to the invention is particularly well applicable to manufacturing white top liner board lighter of its lower surface.

The invention will now be described with reference to the figures of the accompanying drawings.

BRIEF DESCRIPTION OF FIGURES

Fig. 1 shows a schematic view of a board machine according to the invention.

Fig. 2 shows a schematic view of an alternative forming section of a board machine according to the invention.

Fig. 3 shows an enlargement at the beginning of the forming section of the board machine shown in Fig. 1.

Fig. 4 shows another enlargement at the beginning of the forming section of the board machine shown in Fig. 1. DESCRIPTION OF ADVANTAGEOUS EMBODIMENTS

Fig. 1 shows a schematic view of a board machine according to the invention, in which there are two separate forming sections of a partial web. A first wire unit consists of a lower wire 11 on which is formed a first fourdrinier-wire section Tl.

A first headbox 100, which is a twin-layer headbox, feeds a pulp suspension jet onto a first forming board 200 after a first breast roll 12. Water is removed from the web passing on top of the lower wire 11 on the first fourdrinier-wire section Tl following the first forming board 200 with dewatering fittings 15 arranged below the lower wire 11 which can be suction boxes provided with a strip cover.

The first wire unit is followed by a second wire unit which consists of an upper wire 21 on which is formed a second fourdrinier-wire section T2. A second headbox 110, which is a single-layer headbox, feeds a pulp suspension jet on the upper wire 21 onto a second forming board 300 after a second breast roll 22. On the second fourdrinier-wire section T2 following the second forming board 300, water is removed from the pulp suspension with dewatering fittings 25 located below the upper wire 21 which can be suction boxes provided with a strip cover. The second fourdrinier-wire section T2 is followed by a twin-wire section Kl where the upper wire 21 is in contact with the lower wire 11. On the twin- wire section Kl, there is a beginning where a roll 23 within the upper wire loop 21 guides the upper wire into contact with the lower wire 11 at a joining point Y of partial webs and an end where the upper wire 21 is separated from the lower wire 11. A second partial web W2 formed in the second, upper wire unit is joined at the joining point Y into a first partial web Wl formed in the first, lower wire unit.

The joining point Y is followed by the short twin-wire section Kl between the lower wire 11 and the upper wire 21 on which the joining of the first Wl and the second W2 partial web is ensured. Water is removed from the joined web W on this twin- wire section Kl downwards with dewatering fittings 16 arranged below the lower wire 11 of the lower wire unit which fittings can be suction boxes provided with a strip cover. The twin- wire section Kl is followed by a third single-wire section T3 formed on the lower wire 11 where water is further removed from the joined web W with dewatering fittings 17 arranged below the lower wire 11 of the lower wire unit which fittings can be suction boxes provided with a strip cover. The third single-wire section T3 is followed by a pick-up point P where the joined web W is picked up from the lower wire 11 of the lower wire unit with a pick-up suction roll 32 and brought on a pick-up felt 31 to a press section Pl.

On the press section, the web W is first brought supported solely by the pick-up felt 31 and after this between the pick-up felt 31 and a lower press felt 41 into a first double-felted press nip Nl which is formed between a first, upper press roll 33 and a lower press roll 42. After the first press nip Nl, the web W detaches from the lower press felt 41 and follows the pick-up felt 31 on the surface of the first, upper press roll 33 into a second single-felted press nip N2 which is formed between the first, upper press roll 33 and a middle roll 70. After the second press nip N2, the web W detaches from the pick-up felt 31 and follows on the surface of the middle roll 70 into a third, single-felted press nip N3 which is formed between the middle roll 70 and a second upper press roll 52. The second upper press roll 52 is advantageously a so-called long-nip roll by means of which long dwell time is provided and, along with it, high dewatering capacity in the press nip. After the third press nip N3, the web follows on the surface of the middle roll 70 to a transfer point Sl where the web W is transferred onto the support of a drying wire 61 and on that further to a dryer section. A lower surface Wa of the web W i.e. the web surface W having been against the lower wire 11 sets on the press section against the smooth surface of the middle roll 70, whereby the lower surface Wa of the web W becomes sufficiently smooth for the later coating of the lower surface Wa of the web W. After the dryer section, the web W is guided into a film-coating device 400 where the web W passes between two rolls 410, 420. In the film-coating device 400, at least the lower surface Wa of the web W is coated, which lower surface Wa was on the forming section against the lower wire 11 and on the press section against the middle roll 70. As coating material is used light coating material in order to be able to provide desired brightness on the lower surface Wa of the web W for printing. The coating can here be done lightly, whereby the basis weight of the coating layer is in the range of 4-12 g/cm². In the film-coating device 400, an upper surface Wb of the web W is also advantageously coated with starch size by means of which the strength of the web W can be improved.

Fig. 2 shows a schematic view of an alternative forming section of a board machine according to the invention. The forming section according to this embodiment differs from the forming section shown in Fig. 1 such that the second wire unit solely comprises the upper wire loop 21 located above the lower wire 11 by means of which loop the twin- wire section Kl is created with the lower wire 11. On the twin- wire section Kl, there is a beginning where the lower wire 11 and the upper wire 21 join and an end where the upper wire 21 is separated from the lower wire 11. On the twin- wire section Kl in question is located within the upper wire loop 21 a dewatering element 80 in which there is a non-pulsating forming shoe 81 in which there are a cover with holes setting against the inner surface of the upper wire 21 and underpressure Pl prevailing through the holes of the cover. The non-pulsating forming shoe 81 is followed by a pulsating strip cover 82. Therefore, this is a so-called hybrid former. The structure of the non-pulsating forming shoe 81 described here is in principle equivalent to the one of the first non-pulsating forming shoe 200 shown in Fig. 1. The dewatering element 80 on the twin-wire section Kl increases dewatering capacity and improves formation. The increased dewatering capacity again enables the use of higher run speed. The press section, dryer section and coating shown in Fig. 1 are also applicable for use with the forming section shown in Fig. 2. Fig. 3 shows an enlargement at the beginning of the forming section of the board machine shown in Figs. 1 and 2, which shows the beginning of the first fourdrinier-wire section Tl i.e. the first headbox 100, the first breast roll 12 and the first forming board 200 which comprises a non-pulsating forming shoe 210 and a pulsating strip cover 220. The pulp suspension jet of the first headbox 100 impacts the upper surface of the lower wire 11 at the point of the beginning of the first forming shoe 210. On a cover 211 of the first forming shoe 210, there are a leading edge 213 and a trailing edge 214. On the leading edge 213 of the cover 211, there is a first section without holes 21 IA and, on the trailing edge 214 of the cover 211, there is a second section without holes 21 IB. Between the sections without holes 211 A, 211B of the cover 211, there is an open surface which consists of holes 212 extending through the cover 211. The holes 212 can consist of openings, machine-directional grooves, machine-directional slots or equivalents. The open surface defined by the holes 212 of the cover 211 of the forming shoe 210 is 30-90%, advantageously 40-70% of the section with holes 212 between the section without holes 21 IA of the leading edge 213 of the cover 211 and the section without holes 21 IB of the trailing edge 214 of the cover 211. Below the cover 211, 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 100 is located at the beginning of the section with holes 212 after the section without holes 21 IA of the leading edge 213 of the cover of the first forming shoe 210. The forming shoe 210 can also be divided into two or more blocks, whereby different underpressures can be used in the blocks and/or the open surface of the cover 211 of the forming shoe 210 can be different in the blocks. Then, the underpressure level can e.g. increase in the machine direction and equivalently also the open surface increases 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 214 of the first forming shoe 210 is followed by the pulsating strip cover 220 in which there are cross-machine directional strips 221 between which there are holes 222. Below the strip cover 220 is also arranged underpressure P which prevails through the holes 222 of the strip cover and intensifies the dewatering of the pulp. Air A passing along the lower wire 11 is guided from the holes 212 at the beginning of the section with holes of the first forming shoe 210 within the first forming shoe 210 and from the other holes 212 is guided water WT within the first forming shoe 210. For minimising the impact angle of the pulp suspension jet fed by the twin-layer headbox 100, it is possible to use a small angular distortion on the leading edge 213 of the cover 211 of the first forming shoe 210. At the impact point of the pulp suspension jet fed by the first headbox 100 and after it, the surface of the cover 211 of the first forming shoe 210 is advantageously straight. The straight cover 211 would seem to slow the slice jet down on the surface of the cover 211 of the first forming shoe 210 extremely well, but the surface of the cover 211 of the first forming shoe 210 can also be slightly curved upwards. The first forming shoe 210 and the strip cover 220 following it together constitute the forming board. The first forming shoe 210 receives the pulp suspension jet of the first headbox 100 and quickly slows it down onto the surface of the lower wire 11. Simultaneously, the first forming shoe 210 effectively removes water from the web and after this the web can be exposed to the pulsating dewatering of the strip cover 220.

Fig. 3 also illustrates with curve PP the non-pulsating dewatering induced by the forming shoe 210 and the pulsating dewatering induced by the strip cover 220.

The forming board 300 at the beginning of the second fourdrinier-wire section T2 is totally equivalent to the forming board 200 at the beginning of the first fourdrinier-wire section Tl.

Fig. 4 shows another enlargement at the beginning of the forming section of the board machine shown in Figs. 1 and 2, which shows the beginning of the first fourdrinier-wire section Tl i.e. the mutual location of the first headbox 100, the first breast roll 12, the first forming board 210, and the strip cover 220 following it. In the first headbox 100, there are two separate slice channels i.e. a lower slice channel 102a and an upper slice channel 102b which join in a common slice opening 103. The breast roll 12 has been transferred away from the customary position below the lower slice channel 102a of the first headbox 100 to the delivery side of the headbox 100 and it has been lifted such that the height difference H between the upper surface of the lower wire 11 passing on top of the breast roll 12 and the lower surface 101 of the slice opening 103 of the first headbox 100 is in the range of 0-10 mm measured at the topmost point A of the breast roll 12. The horizontal distance Sl between the vertical plane Z-Z drawn through the midpoint of the breast roll 12 and the slice opening 103 of the first headbox 100 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 103 of the first headbox 100 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 210. Such an arrangement between the headbox 100, the breast roll 12, the first forming shoe 210, and the strip cover 220 following it enables that the pulp suspension jet will not be thrown in the air or become beaded (stock jump) when it impacts the lower wire 11.

From the lower slice channel 102a of the first headbox 100 is fed pulp of better quality i.e. bleached pulp and from the upper slice channel 102b of the first headbox 100 is fed pulp of lesser quality i.e. unbleached pulp. The lower web surface setting against the lower wire 11 is then formed of better quality than the opposite upper web surface. The lower web surface can then be used as the outer surface of board, where product information is possibly printed, and the inner web surface as the inner surface of board.

The machine-directional length of the first fourdrinier-wire section Tl is in the range of 0.5-10.0 m and the consistency of the pulp suspension fed by the twin- layer headbox 100 is in the range of 0.1-3.0%. With high speeds, the first fourdrinier-wire section Tl has to be short i.e. in the range of 0.5-3.0 m. In refurbishings, the first fourdrinier-wire section Tl is usually long due to the existing structure i.e. in the range of 8-10 m and thus it is rarely shortened. A long first fourdrinier-wire section Tl weakens the residual variation of the web.

The embodiments of the figures show only one forming shoe at the beginning of the fourdrinier-wire sections Tl, T2 in connection with the forming board 200, 300, but there can also be several forming shoes, whereby it is possible to e.g. use various underpressure levels in different forming shoes.

In the embodiments shown in the figures, the first headbox 100 shown at the beginning of the first fourdrinier-wire section Tl is advantageously a twin-layer headbox, but it can also be a multi-layer headbox. The second headbox 110 shown at the beginning of the second fourdrinier-wire section T2 is advantageously a single-layer headbox, but it can also be a twin-layer headbox or a multi-layer headbox. In the headboxes 100, 110, there can be N pieces of slice channels where N is a whole number which is greater than or equal to two.

In a situation in which both headboxes 100, 110 are twin-layer headboxes, it is possible to feed pulp with them in which the first layer setting against the forming wire 11, 21 consists of pulp of better quality and the second layer, setting on top of the first layer, consists of pulp of lesser quality. On the surfaces of the joined web then sets the pulp of better quality i.e. having higher strength and better surface properties and to the middle sets the pulp of lesser quality. The surface layers of the joined web remain pure due to the structure of the forming board

200, 300 i.e. the lesser-quality pulp of the middle layer is not able to mix in them at least in a considerable amount.

In the embodiment shown in Fig. 1, the upper wire unit follows directly after the first fourdrinier-wire section Tl, but between the first fourdrinier-wire section Tl and the upper wire unit can also be an upper wire unit forming a hybrid former shown in Fig. 2. At its simplest, the forming section can be implemented such that it comprises a fourdrinier wire unit implemented solely with the lower wire 11.

In the embodiment shown in Fig. 1, there is only one upper wire unit after the first fourdrinier- wire section Tl, 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 210 shown in Fig. 3 and the strip cover 220 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 forming shoe 210 and the strip cover 220 in order not to 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 210. Then, pulsation is intensified and dewatering increased on the section of the strip cover 220 by increasing the underpressure of the strip cover 220, whereby the clogged lower web surface will be unclogged.

In the embodiments shown in the figures, the non-pulsating forming shoe 210 is followed by the pulsating strip cover 220. In some situations, the non-pulsating forming shoe 210 is sufficient i.e. there is no need for the pulsating strip cover 220. Furthermore, the non-pulsating forming shoe 210 does not necessarily always require underpressure for intensifying dewatering.

The embodiments shown in the figures utilise film coating, but it is also possible to use e.g. curtain coating here.

The embodiments shown in the figures employ a middle roll press in which the web surface having been against the lower wire sets against the middle roll. Around the middle roll, it is also possible to use a transfer belt, whereby the web surface having been against the lower wire sets against the smooth surface impermeable to water of the transfer belt. Instead of the middle roll press, it is also possible to use a separate press in which two press rolls form a press nip solely by themselves. Around one press roll of the separate press is guided a transfer belt or a metal belt, whereby the web surface having been against the lower wire sets against the transfer belt or the metal belt.

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

1. A method for forming a multi- layer web, comprising the following steps of:

- forming on a lower wire (11) running around a breast roll (12) a first fourdrinier- wire section (Tl), at the beginning of which a forming board (200) is located,

- feeding a pulp suspension jet with a first headbox (100) onto said forming board (200),

- guiding the web (W) onto a press section (Pl) where water is pressed from the web (W) in at least one press nip, - guiding the web (W) onto a dryer section (Dl) where the web (W) is dryed with at least one drying group applying cylinder drying, characterised by the method further comprising the following steps of:

- forming the forming board (200) of at least one stationary, non-pulsating forming shoe (210) in which there is a cover (211) provided with through holes (212), setting against the inner surface of the lower wire (11) which holes (212) consist of openings or slots substantially in the longitudinal direction of the machine,

- forming the first headbox (100) as a multi-layer headbox in which there are a lower slice channel (102a) and at least one upper slice channel (102b), - pressing a surface (Wa) of the web (W) having been against the lower wire (11) on the press section (Pl) against a smooth surface,

- coating at least the surface (Wa) of the web (W) having been against the lower wire (11) in a coating section (400).

2. A method according to claim 1, characterised by performing non-pulsating dewatering with a forming shoe (210) the open surface determined by the holes (212) of the cover (211) of which is 30-90%, advantageously 40-70% of the section with holes (212) of the cover (211) and guiding underpressure (P) through the holes (212) of the cover (211).

3. A method according to claim 1 or 2, characterised by performing pulsating dewatering with a strip cover (220) located after the non-pulsating forming shoe (210).

4. A method according to any one of claims 1-3, characterised by forming the press section of a middle roll press where pressing the surface (Wa) of the web (W) having been against the lower wire (11) against the smooth metal surface of a middle roll (70).

5. A method according to any one of claims 1-4, characterised by forming a coating section of a film coater (400) where the web (W) passes between two rolls (410, 420), whereby coating the surface (Wa) of the web (W) having been against the lower wire (11) with light coating material and coating an opposite surface (Wb) of the web (W) with starch size.

6. A method according to any one of claims 1-5, characterised by forming an upper wire unit above the lower wire (11) where on an upper wire (21) forming a second fourdrinier-wire section (T2), at the beginning of which a pulp suspension jet is fed with a second headbox (110), which is a single-layer headbox, for forming a second partial web (W2), and which upper wire (21) additionally formes a twin- wire section (Kl) with the lower wire (11) such that there is a beginning on the twin- wire section (Kl) where the lower wire (11) and the upper wire (21) form a joining point (Y) of the partial webs and an end where the upper wire (21) is separated from the lower wire (11), whereby a second partial web (W2) formed on the second fourdrinier-wire section (T2) is joined with a first partial web (Wl) formed on the first fourdrinier-wire section (Tl) at said joining point (Y).

7. A method according to any one of claims 1-5, characterised by forming a twin- wire section (Kl) with the lower wire (11) and an upper wire (21) above it, in which twin- wire section (Kl) there is a beginning where the lower wire (11) and the upper wire (21) join and an end where the upper wire (21) is separated from the lower wire (11), whereby non-pulsating and pulsating dewatering is applied to the web (W) formed on the first fourdrinier-wire section (Tl) on the twin- wire section (Kl) with a dewatering element (80) within the upper wire loop (21).

8. A board machine which comprises:

- a forming section where a lower wire loop (11) forms a fourdrinier-wire section (Tl) after a first breast roll (12) at the beginning of which is located a forming board (200),

- a first headbox (100) with which a pulp suspension jet is fed onto the forming board (200),

- a press section (Pl) where water is pressed from the web (W) in at least one press nip,

- a dryer section (Dl) where the web (W) is dried with at least one cylinder-dryer group, characterised in that:

- the forming board (200) consists of at least one stationary, non-pulsating forming shoe (210) in which there is a cover (211) provided with through holes (212), setting against the inner surface of the lower wire (11) which holes (212) consist of openings or slots substantially in the longitudinal direction of the machine, - the first headbox (100) is a multi- layer headbox which comprises a lower slice channel (102a) and at least one upper slice channel (102b),

- on the press section (Pl), there is a smooth surface against which a surface (Wa) of the web (W) having been against the lower wire (11) is pressed,

- the board machine further comprises a coating section (400) where at least the web surface (Wa) having been against the lower wire is coated.

9. A board machine according to claim 8, characterised in that the open surface determined by the holes (212) of the cover (211) of the forming shoe (210) is 30- 90%, advantageously 40-70% of the section with holes (212) of the cover (211) and underpressure (P) prevails through the ho les (212).

10. A board machine according to claim 8 or 9, characterised in that the non- pulsating forming shoe (210) is followed by a pulsating strip cover (220).

11. A board machine according to any one of claims 8-10, characterised in that the press section comprises a middle roll press where the surface (Wa) of the web (W) having been against the lower wire (11) is pressed against the smooth surface of a middle roll (70).

12. A board machine according to any one of claims 8-11, characterised in that the coating section comprises a film coater (400) where the web (W) passes between two rolls (410, 420), whereby the surface (Wa) of the web (W) having been against the lower wire (11) is coated with light coating material and the opposite surface (Wb) of the web (W) is coated with starch size.

13. A board machine according to any one of claims 8-12, characterised in that the board machine further comprises an upper wire unit located above the lower wire (11) in which the upper wire (21) forms a second fourdrinier-wire section (T2), at the beginning of which a second headbox (110), which is a single-layer headbox, feeds a pulp suspension jet for forming a second partial web (W2) and which upper wire (21) further forms a twin-wire section (Kl) with the lower wire (11) and on which twin- wire section (Kl) there is a beginning where the lower wire (11) and the upper wire (21) form a joining point (Y) of partial webs and an end where the upper wire (21) is separated from the lower wire (11), whereby the second partial web (W2) formed on the second fourdrinier-wire section (T2) is joined with a first partial web (Wl) formed on the first fourdrinier-wire section (Tl) at said joining point (Y).

14. A board machine according to any one of claims 8-12, characterised in that the board machine further comprises an upper wire (21) located above the lower wire (11) which forms an twin- wire section (Kl) with the lower wire (11), in which twin- wire section (Kl) there is a beginning where the lower wire (11) and the upper wire (21) join and an end where the upper wire (21) is separated from the lower wire (11), and a dewatering element (80) within the upper wire loop (21) with which non-pulsating and pulsating dewatering is applied on the twin- wire section (Kl) to the web (W) formed on the first fourdrinier-wire section (Tl).