Twin-wire former in a paper machine

Abstract: In a twin-wire former in a paper machine, an operation of a hybrid former mode and that of a gap former mode are conducted, and papers of quality conforming to the kind of papers can be made in a wide range of basis weight with regard to multiple kinds of papers. In the twin-wire former, a forming roll (4), which guides a top wire (2) so that it can approach to a bottom wire (1) to pinch the stock (14) of paper and run approximately horizontally, is made adjustable in an up and down direction. Upper surfaces of blades of a forming board (6) and a forming shoe (7) provided inside a loop of the bottom wire (2), are upwardly convex so that dewatering can be effected mainly downward.

Description: The present invention relates to a twin-wire former in a paper machine which pinches the stock between horizontal running portions of the upper and lower wire loops to dewater the stock. In a twin-wire former of a conventional paper machine, each of two wires respectively form a loop, and while the stock is pinched between the wires, the stock is dewatered by various dewatering devices so that a fiber mat is gradually grown and a web is formed. In Figs. 6 and 7, two typical twin-wire formers are shown. In the twin-wire former shown in Fig. 6, a top wire 2' is engaged with a bottom wire 1' corresponding to a conventional long net used for a Fourdrinier wire so that upward dewatering can be additionally performed, and this type of former is referred to as an on-top-former or a hybrid-former. Stock 14 injected from a head box 5' is landed on a position between a breast roll 3 and a forming board 6'. While conveyed on the forming board 6', a foil 11, and a vacuum foil box 12, the stock 14 is dewatered downward and a mat is formed from the down side. After that, the stock 14 is pinched by the two wires in a gap 13' which is formed by the top wire 2' and the bottom wire 1' on a forming shoe 7', and then the stock 14 is dewatered mainly upward by tension caused by the two wires and pulse pressure generated in a portion of the forming shoe 7'. Advantages of the aforementioned former is in that since the former is provided with a Fourdrinier wire type of preforming zone before the stock 14 is pinched between the top wire loop 2' and the bottom wire loop 1', a paper of high quality can be made which has high strength in a thickness direction so that a degree of orientation (a ratio of longitudinal to lateral tensile strength) is low. On the other hand, the aforementioned former has the following drawbacks. The stock 14 in the preforming zone has a free surface. Accordingly, when the stock 14 is conveyed at high speed, air resistance and agitation effect caused by the foil become excessive, so that the surface of the stock is disturbed and jumping would occur. Therefore, the operation becomes difficult, and deterioration in paper quality such as degradation in the formation and increase in the air permeability, would be caused. In the former shown in Fig. 7, a bottom wire 1'' and a top wire 2'' are engaged with each other immediately after a breast roll 3 and a forming roll 4 so that a wedge-shaped gap 13 is formed. Accordingly, this type of former is referred to as a gap former or a true twin-wire former because it is not provided with a Fourdrinier wire type preforming zone. Stock 14 injected from a head box 5 is pinched by the two wires in a gap 13 located immediately after the breast roll 3 and the forming roll 4. Then, the stock 14 is dewatered simultaneously upward and downward due to the squeezing effect by the wire tension and pulse pressure acting on the stock 14 in a forming shoe 7'' arranged after the gap 13. Advantages of this former is in that pulse-like dewatering pressure acts on the stock from when the stock concentration is low, so that a mat of good formation can be formed. Further, this former is characterized in that the operation can be conducted at high speed since the stock injected from the head box 5 is immediately pinched by the two wires; and the stock does not flow laterally in the preforming zone, so that the angle of orientation of the stock injected from the head box can be maintained to obtain a paper in which the angle of orientation in the width direction is not fluctuated. On the other hand, in this former, when the thickness of stock is large (that is, when the concentration is low), the stock is pinched by the two wires, so that the degree of orientation (the ratio of longitudinal to lateral tensile strength) becomes high by the pressure generated when the stock is pinched. Further, this former has a drawback that since the stock is dewatered toward both sides by pulse-like pressure from an initial stage of dewatering, the ratio of dewatering to the upper and lower sides becomes approximately half, so that the binding strength in a middle layer of the mat is lowered, and the strength in the thickness direction of the paper becomes low. As explained above, the twin wire former which has been used as a mainstream of formers nowadays has merits and demerits. Therefore, the type of a former is selected in accordance with the kind of papers and the object. That is, in the case of a newspaper, the formation and speed are important, so that a gap former is utilized. A hybrid former is mainly used in the case of a middle and high class paper such as an information paper (a PPC paper) in which high lateral rigidity is required (a low ratio of tensile strength is required) in order to improve running property when the paper is used in a copier, and in which a low curling property (a difference in nature between its front and rear surfaces is small, and an angle of orientation is small) is required, and such as a coating paper in which a high strength in the thickness direction is required in order to mitigate a problem caused by blisters. Especially in the latter case, the required quality to which priority is given, is different case by case, and at the same time the formation, which is an essential quality of paper, and high speed to improve productivity are required. Consequently, when these various kinds of papers are made by one former, the former must be provided with a performance to make a compromise with various quality required for each paper. US-A-4 714 521 discloses a twin-wire former for a paper machine in which two forming boards and deflectors are disposed in the rear of a breast roll within a bottom wire loop. The forming boards serve to perform initial dewatering in the downward direction and the deflectors serve as a white water wiping-off unit for this initial dewatering. Although not explicitly mentioned it is to be presumed that the forming boards and the deflectors are flat with respect to their upper surfaces. To further restrain initial dewatering in the upward direction, a water-impermeable belt is arranged in the top wire loop along almost half of its total contact length with the bottom wire loop. The breast roll of the bottom wire loop and a breast roll of the top wire loop are vertically adjustable, however, this feature only serves to adjust the converging angle between the top and the bottom wire loops to allow smooth and gradual pinching of the raw material jet. Consequently, since in said known former the dewatering in only the downward direction is performed prior to sandwiching of the feedstock between the wire loops, said former fundamentally is a single-mode hybrid type forming system similar to the prior art system described in connection with fig. 6. DE-A-39 10 892 describes a twin-wire former for a paper machine, in which a top wire loop engages with a bottom wire loop on a forming roll of the open roll type. Neither the forming roll nor the guide roll in the top wire loop are movable. Downstream of the forming roll there is arranged within the bottom wire loop a forming board and a forming shoe in consecutive order. While the forming board is formed by a plurality of parallel narrow-width blades which are arranged so as to inform a curvature in the running direction with their upper surface extending along the entire length of the board, the forming shoe is formed by a plurality of blades which have flat upper surfaces and are arranged to form a straight plane extending along the entire length of the shoe. Additionally, deflector blades are arranged within the top wire loop and reach into gaps between adjacent blades of the shoe. Consequently, since the feedstock is directly fed into the gap between the wire loops and initially dewatered while sandwiched between the top and bottom wire loops, said known former represents a single-mode forming system of the gap-type similar to the system already described in the application in connection with fig. 7. It is an object of the present invention to provide a twin-wire former in which an operation of a gap former mode and that of a hybrid former mode can be conducted so that various kinds of papers can be made under the most optimum condition. Said object is achieved by a twin-wire former in a paper machine as defined by claim 1. By means of this invention, the forming roll can be adjusted in the up and down direction to move an engagement position, i.e. the gap at the point of convergence of the top and bottom wire loops, either to the forming board or to the forming shoe. Due to this feature in combination with the specific structure of the forming board and of the forming shoe, namely that the forming board has a first blade with an upwardly convex upper surface of a curvature R1 and a plurality of successive blades with flat upper surfaces and that the forming shoe has a first blade with an upwardly convex upper surface of a curvature R2 and a plurality of successive blades being arranged so as to form an upwardly convex surface of a curvature R3, the former of the invention can be switched over between operation modes, i.e. can be used either as a gap-type former or as a hybrid-type former simply by moving the forming roll up or down to move the converging point along the length of the wire loops. BRIEF DESCRIPTION OF THE DRAWINGS In the attached drawings: Figure 1 is a schematic view showing a general construction of the first embodiment of the present invention; Figure 2 is a view showing the detail of an essential portion of the first embodiment of the present invention; Figure 3 is a longitudinal sectional front view of the second forming shoe of the first embodiment of the present invention; Fig. 4 is a longitudinal sectional front view of the second forming shoe of the second embodiment of the present invention; Fig. 5 is a longitudinal sectional side view of the second forming shoe of the second embodiment of the present invention; Fig. 6 is a schematic view showing a general construction of a conventional hybrid type of twin-former; and Fig. 7 is a schematic view showing a general construction of a conventional gap former. DESCRIPTION OF THE PREFERRED EMBODIMENTS Now the present invention will be described in greater detail in connection to the preferred embodiments of the invention illustrated in Fig. 1 to Fig. 3. Numerals 1 and 2 are respectively a loop-shaped bottom and top wire. The bottom wire 1 is wound around a breast roll 3, and the top wire 2 is wound around a forming roll 4. While stock 14 is pinched between the two wires, the bottom wire 1 and top wire 2 run approximately horizontally at an equal speed. Numeral 5 is a head box which injects the stock 14 toward a space between the aforementioned breast roll 3 and forming roll 4. A forming board 6 and a first forming shoe 7 are provided in order in a loop of the bottom wire 1 on the downstream side of the breast roll 3 in the wire running direction (referred to as a downstream side or the upstream side, omitting the term of "in the wire running direction", hereinafter). A second forming shoe 8 and a suction box 9 are provided in order in a loop of the top wire 2 on the downstream side of the first forming shoe 7, and a suction box 10 is provided in a loop of the bottom wire 1 on the downstream of the suction box 9. As illustrated in Fig. 2, the aforementioned forming roll 4 can be adjustably moved between a solid line and one-dot-chain line position in the up and down direction by a drive means not shown in the drawing. The forming board 6 is composed of a first blade 6a located on the upstream side and a plurality of detachable narrow blades 6b provided on the downstream. The upper surface of the first blade 6a is upwardly convex (protruded upward) forming a curved surface of radius of curvature R₁. The upper surfaces of the plurality of blades 6b are on the same plane so that they form a flat surface. The first forming shoe 7 is composed of a first blade 7a on the upstream side and a plurality of narrow detachable blades 7b provided on the downstream side. The upper surface of the first blade 7a is curved in such a manner that it is upwardly convex (protruded upward) and the radius of curvature is R₂, and the upper surfaces of the plurality of blades 7b are curved in such a manner that they are upwardly convex (protruded upward) and the radius of curvature is R₃. The values of R₁, R₂ and R₃ are set such that their relation can be R₁ < R₂ < R₃. The first forming shoe 7 is supported by an adjusting device 7c so that it can be rotated around a portion close to the tip of the first blade 7a. Further, a vacuum acts on the forming board 6 and the first forming shoe 7. As shown in Fig. 3, the second forming shoe 8 is provided with a plurality of shoe blades 15. Each shoe blade 15 is provided with a tip 15a of the upstream side which comes into contact with the top wire 2, an inclined portion 15d which is located in the downstream of the aforementioned tip 15a being connected with it to form a wedge-shaped space 15c between the inclined portion 15d and the top wire 2 in such a manner that the wedge-shaped space 15c is reduced as it goes to the downstream side, and a rear end portion 15b which is located in the downstream of the inclined portion 15d being connected with it and coming into contact with the top wire 2, and each blade 15 is structured in such a manner that the top wire 2 which runs coming into contact with the aforementioned tip 15a and end portion 15b, is curved by angle ϑ protruding downward. A space is formed between the blades 15 which are adjacent to each other, and a minimum vacuum acts on the second forming shoe 8 so that the wires 1, 2 and the blade 15 can be sealed at the tip 15a. Each blade 15 is detachably inserted into a T-bar 16, as shown in Fig. 3. Function and effect in each operation mode of this embodiment having the aforementioned structure will be explained as follows. 1. In a case where newsprint are made, the important quality of which is the formation and difference in nature between the front and rear surfaces. When the forming roll 4 is set to a lower position shown by a solid line in Fig. 2, the wires 1 and 2 are converged upon the surface of the first blade 6a of the forming board 6 so that a gap 13 is formed. Consequently, the stock 14 injected from the head box 5 is pinched between the wires 1 and 2 before its surface is not disturbed, and a free surface of the stock 14 disappears, so that the operation can be conducted at high speed. A relative speed generated between an upper outer layer of the stock 14 and the top wire 2 and that generated between a lower outer layer and the bottom wire 1 are the same at a gap 13, so that a paper can be made having a little difference in the degree of orientation. In this case, the first blade 6a of the forming board 6 is protruded upward forming a curved surface having radius of curvature R₁, so that the stock 14 can be smoothly pinched between the wires 1 and 2. When the stock pinched between the two wires 1, 2 passes through on the detachable blades 6b which are arranged approximately horizontally, and the upper surfaces of which are flat, the reoccurrence of floc of fiber can be prevented because the upper surfaces of the blades 6b are flat and the stock 14 is appropriately agitated by a vacuum actuated on the forming board 6. Next, when the stock 14 passes through on the detachable blades 7b which are arranged on the first forming shoe 7 in such a manner that the blades 7b form a curved surface of which the radius of curvature is R₃, fibers in the stock 14 are dispersed again by pulse-like pressure given to the stock, so that a good formation can be formed. The pressure given to the stock can be changed when a vacuum is made to act on the forming shoe 7 or every other blade 7b is arranged as shown by a hatched portion in Fig. 2. Since the first forming shoe 7 is supported in such a manner that it can be rotated around a portion close to the tip of the first blade 7a by the adjusting device 7c, the rear end position can be changed in accordance with the thickness of the stock advancing to the second forming shoe arranged in the downstream. As shown in Fig. 3, in a portion around the second forming shoe 8, upward dewatering is facilitated by a suction effect generated by vacuum, so that a fine fiber distribution can be obtained which has a little difference in nature between both surfaces of paper. Each shoe blade 15 forms the space 15c which is reduced as it goes to the downstream, and the wires 1, 2 are bent at the rear end portion 15b. Accordingly, as shown in Fig. 3, a pressure pulse (shown by sign + in Fig. 3) directed toward the bottom wire 1 side is generated at the tip of the rear end portion 15b, and water which has once sucked from the stock 14 to the wedge-shaped space 15c, runs together with the wire and is pushed back to the wire side by the action of the wedge-shaped space 15c, so that downward dewatering can be effected and a yield of fibers can be improved. An amount of fine fibers in portions close to the top and bottom surface can be finely adjusted by controlling a vacuum value inside the suction boxes 9 and 10. 2. In a case where a low areal weight coat paper is made at high speed in which strength in the thickness direction and low air-permeability are important. In order to operate the machine at high speed, a gap forming mode operation is conducted in the same manner as the aforementioned case 1. However, in the case of a coat paper, priority is given to strength in the thickness direction and low air-permeability more than the formation and difference in nature between both surfaces of paper. Accordingly, the following points are effective in this embodiment. That is, after the stock 14 injected from the head box 5 has been dewatered upward on the first blade 6a of the forming board 6 by the top wire tension, the stock 14 is hardly dewatered upward since the upper surfaces of the blades 6b are arranged on a line and dewatering is effected only by the squeezing effect of the two wires 1 and 2. In the following first forming shoe 7, radius of curvature R₂ of the upper surface of the first blade 7a is larger than that R₁ of the upper surface of the first blade 6a of the forming board 6, so that upper dewatering is decreased. When all the following blades 7b are utilized, a peak value of pulse-shaped pressure generated on the blades 7b can be reduced, so that upper dewatering can be also decreased here in addition to the decrease in upper dewatering caused by radius of curvature R₃ of the upper surface which is larger than radiuses of curvature R₁, R₂ of the upper side blades 6a, 7a. Consequently, even though the stock 14 passing through the forming board 6 and the first forming shoe 7 is pinched by the two wires, an operation can be performed in which upper dewatering is reduced to the minimum. Next, in the second forming shoe 8 which is a main paper layer forming place, only a vacuum of a necessary minimum pressure is actuated in a space between the blades 15 so that the sealing between the wire and blade can be ensured at the front edge 15a of the blade 15. On the other hand, the wires 1, 2 are bent at the rear end 15b of the blade 15, and therefore dewatering pressure acts on the stock between the two wires. In the same manner as the aforementioned case 1, the wedge-shaped space 15c formed between the top wire 2 and blade 15 is filled with water which has been obtained during dewatering, so that the generated water pressure acts as a back pressure. Therefore, dewatering toward the blade 15 side can be inhibited. Consequently, dewatering is mainly effected downward in the second forming shoe 8, too. As described above, in spite of a twin-wire type of former, dewatering is mainly effected downward so that a dewatering ratio can be biased. Accordingly, a paper of which strength in the thickness direction is high, can be obtained. In a portion of the second forming shoe 8, it is necessary to dewater downward using a pulse having a high peak value in order to facilitate to redisperse fibers. At that time, a mat has already been formed by a gentle downward dewatering action conducted on the upstream side, and further the bottom wire 2, which is on the dewatering side, is not rubbed by the shoe plate 15, so that fine fibers do not fall off and a paper of low air-permeability can be obtained. In this case, the function and effect of the suction boxes 9, 10 are the same as the aforementioned case 1. 3. (a) In the case where information papers (PPC papers) are made in which low tensile strength, a little difference in nature between both surfaces of paper, and a small angle of orientation are important paper qualities. 3. (b) In a case where high basis weight coat papers are made in which strength in the thickness direction is important paper quality. When the forming roll 4 is set to an upper position indicated by a one-dot-chain line in Fig. 2, the top wire 2 is engaged with the bottom wire 1 on the first blade 7a of the first forming shoe 7, so that a gap 13' can be formed. On the forming board 6, the top wire 2 is completely separated from the stock 14 and a Fourdrinier wire type preforming board is formed, and an operation of hybrid mode is performed. In this case, in order to avoid a difference in relative speed between the wire and the stock which is caused when the stock 14 is pinched by the two wires 1 and 2 immediately after the stock 14 has been injected from the head box 5 and landed on the wires, a J/W ratio (a ratio of wire speed to jet speed), which is one of the parameters to control the fiber orientation at a stock landing point, is made to be 1.00 (that is, the jet speed is made to coincide with the wire speed). In the aforementioned manner, papers of a small degree of orientation (a ratio of longitudinal tensile strength and lateral tensile strength) can be made. When thick coat papers are made, it is necessary to make a lip open large. In this case, when the operation is conducted according to the gap mode, the stock 14 is not sufficiently taken into the gap by the wires, and the difference in relative speed between the wires and the stock becomes too large, so that a harmful effect would be brought about. In this case, the operation should be conducted in the aforementioned hybrid mode, and the stock, the thickness of which is reduced by facilitating downward dewatering at the forming board section 6, should be sent to the gap portion. Function and effect of each unit in the aforementioned operation mode will be further explained as follows. When the stock 14 injected from the head box 5 is landed onto the first blade 6a of the forming board 6 in such a manner that the stock jet is tangent to the blade surface, a reaction force in landing can be minimized, and disturbance in the stock can be also minimized. Since the upper surfaces of the successive blades 6b are arranged on one straight line, the stock 14 is not separated from the bottom wire 1 and conveyed to the first forming shoe 7. When the stock 14 is conveyed into the gap 13 without disturbing the free surface as above, it is effective to prevent defects such as entrainment of air in the gap. After that, a dewatering pressure caused by top wire tension acts on the stock 14 pinched by the two wires in the gap 13, at the first blade 7a of the first forming shoe which is upwardly convex, and the first dewatering takes place. The successive blade 7b and the second forming shoe 8 are utilized as follows. (a) In the case of information papers (PPC papers), the setting is conducted in the same manner as the case of newsprint. (b) In the case of thick coat papers, the setting is conducted in the same manner as the case of thin coat papers so that required paper quality can be obtained. Referring now to Fig. 4 and Fig. 5, the second embodiment of the present invention will be explained. In this embodiment, the second forming shoe in the aforementioned first embodiment is structured as follows. That is, a plurality of blades 15' composing the second forming shoe, are detachably supported, wherein a T bar 16' is used for a guide. The section of the blade 15' is rectangular. As shown in Fig. 5, the blade 15' is provided with a cut-out portion in the wire width direction, the length of which corresponds to the width of the stock. When the wires 1, 2 run along the blade 15', a space 15'c is formed in it. On the other hand, T bar 16' is provided with a narrow long rectangular groove 16'a in the wire width direction, and a sealing groove 16'b having a sealing member 17 surrounds the rectangular groove 16'a. The space 15'c and the groove 16'a are connected by an opening 15'd which is formed in the blade 15' in the wire width direction at appropriate intervals. A supply port 16'c used for supplying fluid (water or air) to prevent dewatering, is provided at the end of T bar 16'. In this embodiment, when fluid is injected from the supply port 16'c formed in T bar 16', the groove 16'a and the space 15'c are filled with the fluid, and then the fluid is sealed by a fiber mat pinched by the two wires 1, 2, and the stock, so that pressure of the fluid is maintained to an amount which has been set in the outside. The blade 15' is pressed toward the T bar 16' by wire tension and hydraulic pressure caused by the difference in area between the pressure chambers, so that a mating face of the aforementioned blade 15' and T bar 16' is sealed by the sealing member 17 provided in the sealing groove 16'b. When the stock pinched by the wires 1 and 2 passes through on the blade 15', the wires are bent by the rear end 15'b of the blade. Dewatering into the space 15'c caused when the wire is bent in the manner described above, is inhibited by the enclosed fluid, so that the same effect as that in the case of the blade 15 can be achieved. When clean water is supplied from the fluid supply port 16'c in the case where the former is not in operation, the inside can be cleaned without demounting the blade 15'. According to the present invention, a horizontal type of twin-wire former in a paper machine in which a loop-shaped top wire and bottom wire pinch the stock of paper and run at an equal speed being approximately horizontally to each other, is characterized in that, as described in claim 1, a forming roll which guides the top wire is adjustably in an up and down direction, and as described in claims 2 and 3, a forming board and a forming shoe on the downstream side in the running direction are provided in a loop of the bottom wire. Accordingly, the following effects can be provided: 1. An operation of a gap former mode and that of a hybrid former mode can be conducted by one former. 2. Paper quality can be obtained in accordance with the kind of paper. 3. Various kinds of papers, and papers of wide range basis weight can be made by one machine.

Claims: A twin-wire former in a paper machine, comprising an adjacent wire running section in which a looped top wire (2) and a looped bottom wire (1) approach to each other and run approximately horizontally at an equal speed pinching the stock (14) of paper injected from a head box (5) so as to dewater said stock, a forming roll (4) for guiding said top wire (2) to said adjacent wire running section, said forming roll (4) being movable in an up and down direction, and a forming board (6) provided in a loop of the bottom wire (1), characterized in that said twin-wire former includes the forming board (6) and a forming shoe (7) on a downstream side of said forming board (6) in the wire running direction, each of which is provided in said adjacent wire running section in a loop of the bottom wire (1), the upper surface of a blade (6a) located at a tip of said forming board (6) on an upstream side in the wire running direction being upwardly convex, and the upper surfaces of a plurality of successive blades (6b) being formed flat, and the upper surface of a blade (7a) located at a tip of said forming shoe (7) on an upstream side in the wire running direction and a surface formed by upper surfaces of a plurality of successive blades (7b) being respectively upwardly convex, the radius R₃ of curvature of the surface formed by the successive blades (7b) being larger than that (R₂)of the upper surface of the blade (7a) located at the tip of the forming shoe (7). The twin-wire former in a paper machine according to claim 1, characterized in that a relation between radius of curvature R₁ of the upper surface of the blade (6b) located at the tip of said forming board (6), and radiuses of curvature R₂, R₃ of the upper surface of the blade (7a) located at the tip of said forming shoe (7) and that of the surface formed by the successive blades (7b), can be expressed by R₁ < R₂ < R₃.

Inventors: BANDO TAKASHI C O MIHARA MACHI, FUCHIOKA HIROYUKI C O MIHARA M, MASUDA HIROMU C O MIHARA MACHI, SAKAMOTO KAZUHIDE C O MIHARA M, BANDO, TAKASHI, C/O MIHARA MACHINERY WORKS, FUCHIOKA, HIROYUKI, C/O MIHARA MACHINERY WORKS, MASUDA, HIROMU, C/O MIHARA MACHINERY WORKS, SAKAMOTO, KAZUHIDE, C/O MIHARA MACHINERY WORKS
Applicants: MITSUBISHI HEAVY IND LTD, MITSUBISHI JUKOGYO KABUSHIKI KAISHA
Application year: 1991
Main classification: D21F9
IPC codes: D21F9, D21F1