METHOD AND DEVICE FOR FORMING A MULIPLY WEB

Abstract: Method for forming multiply paper or board product, where all the plies are formed by means of roll forming of the type where the fibre suspension jet is introduced in the twin-wire nip at such a speed that the outer wire of the tensioned wires (2; 10; 17) is deflected while maintaining a substantially constant tension during the deflection of the outer wire, by the fact that the outer wire is supported by a rotating support (5; 14; 19), at least one of which being resiliently or displaceably mounted to compensate for said deflection. The forming of each ply occurs over only one forming roll (4; 11; 18), wherein the wire winding angle ( alpha ) over the forming rolls is at least 110 DEG , that the forming rolls are located alternatingly in two planes, that for the forming roll (rolls) in the upper plane the wires leave the roll in a lower quadrant (G, H) where the paper web follows the outer wire, and for the forming roll in the lower plane the wires leave the roll in an upper quadrant (E, F), where in this case the paper web also follows the outer wire.

Description: METHOD AND DEVICE FOR FORMING A MULTIPLY WEB This invention concerns a method for high speed forming of a multi-ply paper or board, and a forming apparatus utilizing the method according to the invention. Background of the invention Multi-ply forming enables the cost performance relationship of the product to be optimized by using different furnishes in different plies. For many paper products multi-ply technology makes it possible to increase the content of recycled paper and high yield pulps which are interesting both for cost reducing reasons and for environmental reasons. Existing technology fails however to accomplish the task of high speed forming of multi-ply paper or board with superior mechanical properties as well as favourable ply coverage characteristics. The need for a forming technology applicable to high production rates is stressed by recent developments in wet pressing technology. Shoe presses giving high press impulses and high pressing efficiency are now being installed in the production of most major paper and board grades. Good ply coverage characteristics, i.e. good forming and purity of the individual plies, is an obvious requirement in order to fully utilize the potential of a multi-ply product. The need for a forming technology yielding superior mechanical properties is stressed by the growing interest to use raw materials with a relatively low strength potential such as recycled fibres and high yield pulps. Multi-ply forming technology may be grouped into three main categories: 1. Forming each ply in a separate forming unit before couching the plies together. 2. Simultaneous forming of all plies in one forming unit using a multi-layer head box. 3. Forming the web plies on top of each other in a sequential mode, i.e. forming the second ply on top of the first ply and the third ply on top of the second ply etc. The present invention belongs to this category. Separate forming is commonly carried out with a multi-fourdrinier machine. Hybrid forming or twin-wire forming (cf. e.g. DE 44 02 273 C2) may also be applied. The increase in dewatering capacity given by two or more separate forming units may be utilized for increased production rates and/or lowered forming consistency for improved sheet properties. All variants of separate forming have one problem in common, however, viz. the ply bonding which generally limits the Z-direction strength of the multi-ply product. Often starch or some other bonding agent have to be sprayed on the plies before couching them together. While twin-wire forming would be preferable for speeds above 1000 m/min, avoiding free surface instabilities and providing higher dewatering capacity, the problem of ply- bonding then becomes worse. This is because a twin-wire-formed sheet ply has two wire sides with poor ply-bonding ability, in contrast to a fourdrinier ply which has one wire side and one top side with a better ply-bonding ability. Simultaneous forming of a multi-ply product with a multi-layer head box may also be employed. Examples of multi-layer head boxes are found in EP 0 681 057 N2 and in GB 2 019 465. With this method, however, the dewatering capacity is limited to that given by a single dewatering unit. Hence, this principle is not suitable for high speed forming of moderate to high grammages at low forming consistency. Hitherto, it has moreover proved difficult to accomplish acceptable ply coverage characteristics with simultaneous forming. Multi-ply forming in a sequential mode has traditionally been applied in the forming of two-ply liner board using a secondary head box placed some distance downstream a fourdrinier wire with dewatering of the top ply through the primary web ply formed upstream of the secondary head box. The problem of ply-bonding is essentially avoided by depositing a fibre suspension onto the pre-formed web. This means that the Z- direction strength of the multi-ply product is often determined by the Z-direction strength of the individual plies rather than by the ply-bonding. Forming a top ply onto a primary web ply on a fourdrinier wire involves several disadvantages, however. It suffices to mention the drawback regarding dewatering capacity and the severe grammage variations due to the free surface occurring especially above 1000 m/min. Dewatering in a twin-wire zone created by the wire carrying the primary web ply and an additional, web-free wire through which the top ply is essentially dewatered has been applied shortly after the secondary head box in units relying extensively on vacuum generated dewatering (see e.g. Nttwood (1991) "Multi-ply forming", Pulp and Paper manufacture Vol. 7 Paper Machine Operations, TNPPI & CPPN; p.250-251). Dewatering through the previously formed web is thus essentially avoided whereby an improved dewatering efficiency is achieved. Dewatering of the top ply through a web- free wire is moreover advantageous with regard to purity and forming of the top ply, because a separate handling of the white-water of the top ply is possible and because any influence of the primary web ply structure on the top ply is substantially avoided. The capacity of this kind of units is still limited, however, and they are typically used on multi-ply board machines running slower than 600 m/min. Roll forming according to Webster's method was introduced in its basic sense some four decades ago (US 3,056,719), and is well-known in the field of high speed production of single-ply paper. The forming occurs between two wires running over a rotating forming roll, by introducing the fibre suspension jet into the twin- wire nip at such a velocity that the outer one of the tensioned wires is deflected while maintaining a substantially constant tension during the deflection of the outer wire by fact that the outer wire is supported by a rotating support, at least one of which one is resiliently or displaceably mounted to compensate for the deflection. The dewatering pressure is determined by the tension of the outer wire divided with the instantaneous radius of curvature, and during the roll dewatering the pressure rises steeply during an initial phase, after which it levels out to a plateau. In WO 99/09249 a method and an apparatus for multi-ply forming are shown, where roll forming according to Webster's method is used for forming a top-ply on top of a primary base ply. Roll forming according to Webster is preferably applied to form all plies including the primary web ply. Two forming and dewatering rolls are shown for each forming unit. According to one of the machine configurations shown, a long base wire runs through all the dewatering units, which may imply problems concerning the wire stability as several dewatering units are used. According to another configuration shown, each wire runs over two forming units at the most. A problem with the latter configuration concerns the separation of one of the wires from the paper web and the second wire after the second roll in the forming units, where the second dewatering roll lies over the first roll. The separation herein occurs by means of the outer wire over the second roll leaves this, while the web and the inner wire continues substantially horizontally to next forming unit. At the separation of one of the wires from the paper web and the second wire, there is in the actual dry soilds area a risk that pieces come loose from the paper web. With the actual configuration pieces of the paper web may be thrown out of the nips and due to gravity fall down on the paper web, which may lead to damages on the finished product and to paper web breakage. Another drawback with the previous machine constructions concerns the difficulty to obtain a sufficiently short free jet length after the head boxes for the second and the following plies. The incoming wire runs substantially horizontally while the centre of rotation of the chest roll is located substantially vertically over/under the centre of the forming roll. The angle between the incoming wire and the connection line between the centre of rotation of the forming roll and centre of rotation of the chest roll ("the aperture angle"), is thus 90° in the construction described above. With a long free jet length the jet tends break up by instabilitiess before it hits the nip, at which the forming is impaired. Summary of the invention The object of the present invention is to achieve a method for multi-ply forming of a paper or board product at high speed, implying a low investment cost with maintained high dewatering capacity and avoiding runnabihty problems referring to pieces coming loose from the paper web at the separation of one wire of the paper web and the second wire. According to the invention, this has been achieved by means of that forming of each ply occurs over only one forming roll, that the wire winding angle over the forming rolls is at least 110°, that the forming rolls are located alternatingly in two planes, that for the forming roll (rolls) in the upper plane where the wires leave the roll in a lower quadrant, where the paper web follows the outer wire, and for the forming roll (rolls) in the lower plane the wires leave the roll in an upper quadrant, where in this case the paper web also follows the outer wire. The wire winding angle should preferably be at least 120° for enabling a high dewatering capacity. At the forming rolls being used for forming of ply two and the following plies according to a preferred embodiment the wire with the incoming paper is lead to the respectively forming roll, at an angle of at least 10°, preferably at least 15° relative to the horizontal plane. In this way space for the head boxes are given for avoiding long free jet lengths. Further the invention refers to an apparatus for forming a multi-ply paper or board product applying the method defined above. Further details and characteristics of the invention are stated in the following description and in the dependant claims. Brief description of the drawings The invention will be closer described in the following with reference to an embodiment shown in the enclosed drawing. Figure 1 shows schematically an example of an apparatus for forming a three-ply paper web. Figure 2 schematically illustrates the sectors where the wires leave the roll. Figure 3 schematically illustrates the geometry around the head box relative to the roll and incoming wire. Description of embodiments A fibre furnish which is fed out from the head box 1 is introduced into a nip created between two tensioned wires, an inner wire 2 and an outer wire 3, both of which wrap around a rotating forming roll 4. The fibre suspension jet is introduced into the twin-wire nip at such a high speed that the outer wire 3 of the tensioned wires is deflected such as is described in US-A-3,056,719, the content of which should be considered be contained as a reference in the present application. The tension of the outer wire 3 is maintained substantially constant during the deflection, by the fact that it is supported by a rotating support 5, being resiliently or displaceably mounted to compensate for said deflection. In order ro reduce the cost for the machine investment the principal dewatering occurs in each forming unit over only one forming roll 4, i.e. the wires runs with the fibre suspension/the paper web between over only one roll- the forming roll 4. For obtaining a sufficient dewatering capacity, the roll winding angle α of the outer wire 3 should be at least 110°and preferably at least 120°. The diameter of the roll should be at least 1,2 m, preferably approx. 2 m. According to the invention, the forming phase is completed during the roll dewatering. Thereafter the fibre network structure is essentially fixed so that any significant rearrangement of the fibres does not occur as the paper web passes over further dewatering elements. Further consolidation of the paper web may then be accomplished according to well-known methods such as couch rolls, suction boxes and the like, before the paper web enters the press section or another forming unit. The wires 2,3 leave the roll 4 in an upper quadrant (see fig. 2), in the example shown, the upper upward quadrant E, where the paper web follows the outer wire 2. The separation of the outer wire 2 with accompanying paper web from the inner wire 3 occurs by means of a suction box 6, or in another way, accomplishing that the paper web follows the outer wire after the forming roll 4. The paper web here follows the outer wire 2 upwards and is later turned over a turn roll 7. By the fact that the paper web and the outer wire 2 is now transported above the inner wire 3 the risk for runnabihty problems have been reduced, since possible paper web pieces which have come loose at the separation follow the inner wire 3, from which they are removed by a wire sprout or the like. The wires 2, 3 may also leave the roll in the upper downward quadrant F. Afterwards the wire 2 transfer the formed primary web ply to a second forming unit. Further dewatering may occur over suction boxes. The second forming comprises a head box 9, a twin-wire nip formed by the wire 2, which now may constitute the inner wire, and an outer wire 10 and also a forming roll 11. For avoiding high machine constructions the forming rolls are placed alternatingly in two planes. Thereby the difference in height between the extreme points of the forming rolls are limited to preferably maximum 4-5 m. The first forming roll 4 over which the primary web-ply is formed, thus, is located in a lower plane while the second forming roll 11 is located in an upper plane. In the second forming unit a second web-ply is formed on the moist primary web-ply, which preferably has a dryness of 7-15 % by weight. The head box 9 delivers a fibre suspension jet in to the twin-wire nip created by the wires 2 and 10. In this case the jet impingement also occurs at such a speed that the outer wire 10 is deflected. The tension of the outer wire 10 is maintained substantially constant during the deflection, by the fact that it is supported by a rotating support 12, being resiliently or displaceably mounted to compensate for said deflection. The forming occurs in an equivalent way as in the first forming unit. The wires 2, 3 leave the roll 4 in a lower downward quadrant H (see fig.2), where the paper web follows the outer wire 10 downwards. The separation of the outer wire 10 with accompanying paper web of the inner wire 2 occurs by means of a suction box 13, or in another way accomplishing, that the paper web follows the outer wire after the forming roll 11. The paper web here follows the outer wire 10 downwards to later be lead over a turn roll 14. Possible pieces of the paper web which have come loose at the separation and followed the outer wire 10 are removed downwards as the outer wire 10 and the paper web are deflected over the turn roll 14. Additional dewatering of the paper web may occur over suction boxes 15. It is also possible that the wires 2, 10 leave the roll a certain distance inside the lower upward quadrant G. For a roll rotating in counterclockwise direction in the upper plane the wires leave the roll, in this case preferably in the downward quadrant G (Fig.2) . In the third forming unit a third web-ply is formed on top of the both other formed plies. The third forming unit is located on the same plane- the lower plane- as the first forming unit. The head box 16 delivers a fibre suspension jet in the twin- wire nip being created by the wires 10 and 17. In this case the jet impingement also occurs at such a speed that the outer wire 17, being supported of a resiliently or displaceably support 19, is deflected. In a corresponding way as in the first forming unit the wires 10,17 leave the roll 18 in an upper quadrant E (se fig. 2), where the paper web follows the outer wire 17. The separation of the outer wire 17 with accompanying paper web from the inner wire 10 occurs by means of a suction box 20, or in another way accomplishing, that the web follows the outer wire 17 after the forming roll 18. The paper web follows the outer wire 17 upwards to later be lead over a turn roll 21. Possible pieces of the paper web which have come loose at the separation follow the inner wire 17, from which they are removed by a wire sprout or the like. For the most head box constructions a requirement of short free jet length implies an aperture angle of at least 110°. Hence, in all the forming stations an increased space has been created for the head boxes 4, 11 and 18 by means of the fact that the wire with the incoming paper web is angled in relation to the horizontal plane. Thus, the wire forms an angle β to the incoming paper web of at least 10° with respect to the horizontal plane. By this long free jet lengths are avoided at the same time as the decrease of the wire winding angle on the forming roll is reduced. The invention is of course not limited to the embodiment shown and described, but several modifications thereof are possible within the scope of the claims. The invention also comprises that one or several of the head boxes may be a multi-layer head box.

Claims: Claims 1. Method for forming multi-ply paper or board product, in which all the plies are formed by means of roll forming of the type where the fibre suspension jet is introduced into the twin- wire nip at such a speed that the outer wire of the tensioned wires (2; 10; 17) is deflected while maintaining a substantially constant tension during the deflection of the outer wire, by the fact that the outer wire is supported by a rotating support (5; 14; 19), at least one of which being resiliently or displaceably mounted to compensate for said deflection, characterised in that the forming of each ply occurs over only one forming roll (4; 11 ; 18), that the wire winding angle (α) over the forming rolls is at least 110°, that the forming rolls are located alternatingly in two planes, that for the forming roll (rolls)(l 1) in the upper plane the wires (2,10) leave the roll in a lower quadrant (G, H), where the paper web follows the outer wire (10), and for the forming roll (rolls) (4; 18) in the lower plane (2,3; 10, 17) of the wires leave the roll in an upper quadrant (E, F), where the paper web also in this case follows the outer wire (2; 17). 2. Method as claimed in claim 1, characterised in that the wire winding angle (α) over the forming rolls (4;ll;18)isatleastl20°. 3. Method as claimed in claim 1 or 2, characterised in that at least at the forming rolls (11 ; 18) being used for forming ply two and the following plies the wire (2; 10) with the incoming paper web is lead in to the respectively forming roll (11;18) at an angle (β) of at least 10°, preferably of at least 15° relative to the horizontal plane. 4. Method as claimed in any of the previous claims, characterised in that for the forming roll (rolls) (4; 18) in the lower plane the wires (22,3; 10, 17) leave the roll in an upper upward quadrant (E). 5. Method as claimed in any of the previous claims, characterised in that for the forming roll (rolls) (11) in the upper plane the wires (2,10) leave the roll in a lower downward quadrant (H). 6. Apparatus for forming a multi-ply paper or board product, in particular for forming a top ply on top of a primary web ply, for carrying out the method according to claim 1, and which as the only forming unit for forming each ply comprises a roll forming unit of the type where the fibre suspension jet introduced into the twin- wire nip between two tensioned wires (2,3; 2, 10; 10, 17) at such a speed that the outer (2; 10; 17) of the tensioned wires is deflected, whereby the apparatus further comprises means for maintaining a substantially constant tension during the deflection of the outer wire, by the fact that the outer wire is supported by a rotating support (5; 14; 19), at least of which one being resiliently or displaceably mounted to compensate for said deflection, characterised in that only one forming roll (4; 11 ; 18) is arranged for forming each ply, that the wire winding angle (α) over the forming rolls is at least 110°, that the forming rolls are placed alternatingly in two planes, that for the forming roll (rolls)(l 1) in the upper plane the wires are arranged to leave the roll in a lower quadrant (G, H), where the paper web follows the outer wire (10), and for the forming roll (rolls) (4; 18) in the lower plane the wires are arranged to leave the roll in an upper quadrant (E, F), where also in this case the paper web follows the outer wire (2; 17). 7. Apparatus for forming a multi-ply web as claimed in claim 6, characterised in that the wire winding angle (α) over the forming rolls (4;ll;18)isatleastl20°. 8. Apparatus as claimed in claim 6 or 7, characterised in that at least at the forming rolls ( 11 ; 18) being used for forming ply two and the following plies the wire (2; 10) with the incoming paper web at the respectively forming roll (11;18) makes an angle (β) of at least 10°, preferably at least 15°, relatively to the horizontal plane. 9. Apparatus as claimed in claim 6-8, characterised in that for the forming roll (rolls) (4; 18) in the lower plane the wires (2,3; 10,17) are arranged to leave the roll in an upper upward quadrant (E). 10. Apparatus as claimed in claim 6-9, characterised in that for the forming roll (rolls) (11) in the upper plane the wires are arranged to leave the roll in a lower downward quadrant (H).

Inventors: NORDSTROEM BENGT, NORDSTROEM, BENGT
Applicants: NORDSTROEM BENGT, SCA HYGIENE PROD AB, NORDSTROEM, BENGT, SCA HYGIENE PRODUCTS AB
Application year: 2000
Main classification: D21F11
IPC codes: D21F9, D21F11