Paper machine twin-wire former comprising a dewatering device

Abstract: Object: To prevent lowering of yield of fibers of stock which is pinched by two wires and being transferred, by use of dewatering blades disposed on each of the wires. Dissolving means: A blade set is disposed respectively on two wires 1, 2 forming loops to pinch stock 7 and running therealong and the blades of one blade set are dewatering limiting blades 20, each having a plane portion 20a to support the wires 1, 2 and an inclined face 20b disposed on the wire entering side of said plane portion 20a and forming a space of wedge-shape facing to the wire face and enlarging toward the upstream side in the wire running direction; the blades of the other blade set are dewatering blades 21, each having a plane portion 21a to support the wires 1, 2 and an edge 21b to scrape water toward the upstream side of said plane portion 21a; and said plane portion 21a having said edge 21b is disposed opposingly to said space of wedge-shape.

Description: BACKGROUND OF THE INVENTION: Field of the Invention: The present invention relates to a paper machine twin-wire former comprising a dewatering device to be used for paper layer forming therein. Description of the Prior Art: In a twin-wire former as a paper layer forming device in a conventional paper machine, each of two wires forms a loop, stock is pinched therebetween and as the stock is transferred, it is dewatered by various dewatering devices so that a fiber mat is gradually grown and a web is formed. In Fig. 10, a construction of a typical twin-wire former is shown, and a paper layer forming device of the twin-wire former is described with reference to Fig. 10. Stock 7 injected upwardly from a headbox 6 is pinched in a gap of wedge-shape 5 formed by two wires of a top wire 1 and a bottom wire 2 guided by a forming roll 4 and a breast roll 3, respectively, and as the stock is transferred with the same velocity as the wires 1, 2, the gap 5 is narrowed, and further as the stock is transferred along an approximate curve R on a plurality of dewatering blades 9 arranged on a certain curvature R with intervals between one another on the side of the bottom wire 2, the stock is dewatered toward both sides by a dewatering pressure generated by the dewatering blades 9, a fiber mat is gradually grown and a web is formed. Next, at a suction box 11 and a suction couch roll 12, dewatering by vacuum is done, and at the suction couch roll 12, the web 13 is transferred onto the bottom wire 2 and then is transferred to a next press part by a suction pick-up roll (not shown). A water deflector 10 is disposed within a wire loop of the top wire 1, so that a white water which runs accompanying with the top wire 1 is discharged outside the system therefrom. Further, in the construction of the twin-wire former shown in Fig. 10, as a countermeasure to meet various problems accompanying with the dewatering being done toward both sides at a same time, an employment of dewatering limiting shoes (dewatering limiting blades) of the Japanese laid-open patent application No. Hei 2(1990)-133689 (JP 2133689) as shown in Fig. 11 and a combination of dewatering limiting shoes (dewatering limiting blades) and dewatering shoes (dewatering blades) of the Japanese laid-open patent application No. Hei 4(1992)-194093 (JP 4194093) as shown in Fig. 12 are disclosed. Fig. 13 shows another means of dewatering device used for paper layer forming in the twin-wire former shown in Fig. 10. That is, Fig. 10 shows an example wherein a dewatering device in which dewatering blades 9 are disposed within one wire loop, or a loop of the bottom wire 2, is mainly used, Fig. 11 shows an example wherein a dewatering device 39 incorporating dewatering limiting blades is provided within one wire loop 32, and Fig. 12 shows an example wherein dewatering limiting blades 59 are disposed within a loop of a first wire 51 and downstream thereof both-side dewatering blades 60 are disposed within a loop of a second wire 52. In the arrangement of the. respective dewatering device shown in Figs. 10, 11 and 12, the dewatering pressure generated between the wires is decided by the curvature R mainly on which the dewatering blades 9, 39, 59, 60 are arranged; the intervals with which the dewatering blades 9, 39, 59, 60 are disposed; the tensile force of the top wire 1, 31, 51 or of the bottom wire 2, 32, 52; and the dewatering resistance of the fiber mat layer formed between the two wires, and there is no function of adjusting the dewatering pressure from outside during operation. So, what is shown in Fig. 13 is a dewatering device having a function to adjust dewatering pressure, in which dewatering blades 21 which can adjust pressing force given to the wires from outside during operation are disposed opposingly to the conventional dewatering blades 9 as shown in Fig. 10 via the bottom wire 2 and the top wire 1. In case where a conventional shape of the dewatering blade is used in the mutually opposing dewatering devices, the fiber mat layers formed between the wires receive a reaction force via the wires when the wires bend or jerk at the front edge portion of the dewatering blade and the fibers between the fiber mat layers are further moved and dispersed by said force. But at the same time short fibers (or fine fibers) lose binding with long fibers and there occurs a high possibility that the short fibers are washed off together with water to be dewatered by the pressure acting on the mat layers, thus a yield of the short fibers on the dewatering blade side tends to become worse. For this reason, countermeasures are taken such that the bending of the wire is made smaller or is shared by the front and rear edge portions, but in this case the fiber dispersion ability by the dewatering blades is lowered and as a result there is a disadvantage that the formation becomes worse. Further, as the dewatering is done toward both sides, there is a large problem that the short fibers of the central portion in the thickness direction of the paper layer move to the outer layer portion to become less in the middle layer portion, thus the binding between fibers becomes weaker and the strength in the thickness direction is lowered. In order to prevent the lowering of said strength in the thickness direction, a countermeasure is taken wherein the dewatering ratio toward both sides is changed by use of the dewatering limiting blades as shown in Figs. 11 and 12, but there is still no adjusting means to correspond to paper making conditions such as changes of fiber length in the stock, paper making velocity, alterations of grammage (basis weight). That is, in the device shown in Fig. 11, it is possible to prevent a washing-off of short fibers but it is difficult to enhance the dewatering pressure, and for this reason there is a disadvantage that the fiber dispersion ability within the mat layers is not enhanced. Further, in the device shown in Fig. 12, a paper in which a difference in nature between front and rear faces is small can be made by a construction that both-side dewatering blades are disposed downstream of the dewatering limiting blades disposed immediate downstream of the stock being supplied, but reversely short fibers within the mat layers move to both sides and there is a disadvantage that the inner binding strength is lowered. The prior art dewatering device disclosed in US-A-5 078 835 has blades or ledges alternatingly arranged on both sides of a pair of wires. The blades of this prior art respectively have inclined surfaces facing the downstream side of the wire running direction and are inclined such that the wedge shaped space enlarges towards the downstream side in the wire running direction. This prior art essentially corresponds to the prior art shown in Figure 13 and is focussing on a structure of resilient blades or ledges of relatively narrow structural shape and having an improved guide structure for their resilient movement. US-A-5 061 347 forming the preamble of claim 1, addresses the problem of improving the paper formation by optimising the pressures in the sheet forming zone and for this purpose discloses a structure where dewatering blades and wedge-shaped one-sided dewatering blades are provided in opposing positions on the two wires with the paper stock pinched therebetween. SUMMARY OF THE INVENTION It is the object of the present invention to provide a paper machine twin-wire former comprising a dewatering device which can'dissolve the above-mentioned disadvantages in the prior art and remarkably enhance the quality of paper by improving the yield of short fibers and enhancing the fiber dispersion ability and strength in the thickness direction. In order to attain this object, the present invention provides a paper machine twin-wire former comprising a dewatering device as defined by claim 1. Preferred embodiments of the paper machine twin-wire former are defined in the dependent claims. Further, each said dewatering blade is supported so that the position of the wire supporting plane is changeable, and is elastically pressable to said dewatering limiting blades. Further, each said dewatering limiting blade is supported so that the position of the wire supporting plane is changeable, and is elastically pressable to said dewatering blades. Further, said dewatering blades are movable in the wire running direction along the curvature on which said dewatering limiting blades are arranged. Further, each said dewatering limiting blade and each said dewatering blade are detachable in the width direction. In the dewatering device according to the present invention, as the dewatering limiting blades and the dewatering blades are arranged with wires inbetween, the bending of wires at each blade end becomes larger than that of the device of Fig. 11, the dewatering pressure caused thereby becomes larger and the fiber dispersion within the mat layers is enhanced. The dewatering is done toward one direction only and there occurs no lowering of the inner binding strength. And as the dewatering is done toward one direction, the moving resistance of the short fibers is large and the yield of the short fibers is enhanced. Further, in the dewatering device according to the present invention, as both-side dewatering blades are disposed at a loop portion having a certain curvature, formed immediate downstream of the stock'being supplied and the above-mentioned dewatering device is provided further downstream thereof, dewatering is done quickly toward both sides, strong mat layers are formed quickly, and then dewatering is done toward one direction with a large dewatering pressure by the above-mentioned dewatering device of the downstream, hence the fiber dispersion within the paper layers is enhanced, washing-out of short fibers does not occur and papers with a large binding strength between paper layers can be obtained. BRIEF DESCRIPTION OF THE DRAWINGS: In the accompanying drawings: Fig. 1 is a structural view of a twin-wire former according to the present invention. Fig. 2 is a sectional view showing an example of a dewatering device for explaining certain features of preferred embodiments according to the present invention. Fig. 3 is a sectional view showing a preferred embodiment according to the present invention. Fig. 4 is a sectional view showing another preferred embodiment according to the present invention. Fig. 5 is a sectional view showing a further preferred embodiment according to the present invention. Fig. 6 is an explanatory graph on the adjustability of dewatering pressure of the present invention. Fig. 7 is an explanatory graph showing effects of formation improvement according to the present invention by comparison. Fig. 8 is an explanatory graph showing examples of enhancement of fiber yield rate. Fig. 9 is an explanatory graph showing effect of inner binding strength according to the present invention by comparison. Fig. 10 is a structural view of a twin-wire former in the prior art. Fig. 11 is a structural view of a twin-wire former in the prior art in which dewatering blades are replaced with dewatering limiting blades (Japanese laid-open patent application No. Hei 2-133689). Fig. 12 is a structural view of a twin-wire former in the prior art in which dewatering blades and dewatering limiting blades are combined (Japanese laid-open patent application No. Hei 4-194093). Fig. 13 is a sectional view showing a conventional example. DESCRIPTION OF THE PREFERRED EMBODIMENTS: Fig. 1 shows an entire structural view of a paper machine twin-wire former including a dewatering device according to the present invention, Fig. 2 shows a detailed view of a dewatering device as an example for explaining certain features of preferred embodiments of the present invention, Fig. 3 shows a detailed view of a dewatering device of a preferred embodiment of the present invention, Fig. 4 shows a detailed view of a dewatering device of another preferred embodiment of the present invention and Fig. 5 shows a detailed view of a dewatering device of a further preferred embodiment of the present invention. Figs. 6 to 9 are explanatory graphs showing effects of working the present invention. Below is a description on the entire structure with reference to Fig. 1 and on the details of a dewatering device according to the present invention with reference to Fig. 2. One feature of the present invention is that two blade sets are disposed opposingly each other and each blade of one blade set has an inclined face forming a space of wedge-shape enlarging toward the upstream side. Stock 7 injected upwardly from a headbox 6 is pinched in a gap of wedge-shape 5 formed by two wires of a top wire 1 and a bottom wire 2 guided by a breast roll 3 and a forming roll 4, respectively, and as the stock is transferred with same velocity as the wires 1, 2, the gap 5 is narrowed, and further as the stock is transferred along an approximate curve R on a plurality of dewatering blades 9 arranged on a certain curvature R with intervals between one another on the side of the bottom wire 2, the stock is dewatered toward both sides in an approximately equal amount by dewatering pressure generated by the dewatering blades 9. Then, a dewatering device of the present invention comprises dewatering limiting blades 20, each having a plane portion 20a to support the wires within a loop of the top wire 1 and an inclined face 20b disposed on the wire entering side of said plane portion 20a and forming a space of wedge-shape facing to the wire face and enlarging toward the upstream side in the wire running direction; and dewatering blades 21, each having a plane portion 21a to support the wires within a loop of the bottom wire 2 and an edge 21b to scrape water toward the upstream side of said plane portion 21a, and the dewatering blades 21 are elastically pressable to the dewatering limiting blades 20 which are disposed opposingly to the dewatering blades 21 via the wires 1, 2. By use of the dewatering blades 21, the pressing force is controlled from outside so that an appropriate fiber dispersion action is given and fiber mat layers are gradually formed between the two wires. Then, a web 13 is dewatered by vacuum at a suction box 11 and a suction couch roll 12 and is further transferred to a next press part by a suction pick-up roll (not shown). In Fig. 2, the wires 1, 2 pinching the stock 7 bend at the line position where the inclined face 20b forming the space of wedge-shape enlarging toward the upstream side and the wire supporting plane 20a make crossing, so that a pressure (static pressure) is generated between the wires 1, 2, dewatering is done through the bottom wire 2 and a white water (shown in the figure) flows out. The white water then accompanies with the wire 2 and is discharged outside the system by the dewatering blades 21, having the edge 21b to scrape water toward the upstream side, disposed within the loop of the bottom wire 2. Said dewatering blades 21 are supported so that the position to the wire is changeable and the pressing force can be changed by adjusting the pressure of fluid (air, water, oil etc.) to be supplied to flexible tubes 24. Thereby, the stock 7, pinched between the wires 1, 2 and being transferred, is elastically pressed, the bending of the wires 1, 2 is freely changed from outside and the pressure (static pressure) between the wires can be controlled. The white water which is discharged to the side of the top wire 1 enters to the wedge portion of the upstream side of the dewatering limiting blade 20 disposed within the loop of the top wire 1 and generates a wedge pressure, thus dewatering to the side of the top wire 1 by a static pressure generated at the dewatering limiting blades 20 can be limited. Fig. 3 shows a construction in which the dewatering limiting blades 20 are arranged without intervals therebetween within the loop of the top wire 1, and there occurs no dewatering to the side of the top wire 1. As each dewatering limiting blade 20 and each dewatering blade 21 is constructed so as to be detachable in the width direction, the blade intervals in the dewatering limiting blades 20 or the dewatering blades 21 are changed, the pressing force of the dewatering blades 21 is adjusted, thereby sharing of dewatering amount by the top wire 1 side and the bottom wire 2 side or their dewatering ability can be easily adjusted. Further, as it is apparent as mentioned above that a generation of dewatering pressure by these mutually opposing dewatering limiting blades 20 and dewatering blades 21 is greatly influenced by the degree of bending of the wires, the dewatering blades are moved in the wire running. direction along the curvature on which the dewatering limiting blades are arranged, thereby also the degree of bending of the wires is changed and sharing of dewatering amount and dewatering ability can be adjusted. The shape of the inclined face having the space of wedge-shape of the dewatering limiting blade can be a plane, or a face constructed by a concave or convex curve or a compound curvature in the running directional cross section. Another preferred embodiment according to the present invention is shown in Fig. 4. The dewatering limiting blade 20 has two inclined faces forming spaces of wedge-shape enlarging toward the upstream side. Said two spaces of wedge-shape disposed at these dewatering limiting blades generate a pressure acting on the stock 7 by the bending of the wires there and can prevent an action that the fibers are going to be flocculated each other. There can be two or more of said inclined faces and said spaces of wedge-shape. And a further preferred embodiment according to the present invention is shown in Fig. 5, wherein, contrary to Fig. 4, the dewatering blades 21 are fixed and the dewatering limiting blades 20 are movable. As described above, the present invention has a feature that a blade set is disposed respectively on two wires and the blades of one blade set are dewatering limiting blades, each having a plane portion to support the wires and an inclined face disposed on the wire entering side of said plane portion and forming a space of wedge-shape facing to the wire face and enlarging toward the upstream side in the wire running direction; the blades of the other blade set are dewatering blades, each having a plane portion to support the wires and an edge to scrape water toward the upstream side of said plane portion; and said plane portion having said edge is disposed opposingly to said space of wedge-shape, thereby dewatering pressure can be made larger, fiber dispersion ability within the mat layers is enhanced, dewatering is done toward one direction only, inner binding strength is high and yield of short fibers is enhanced. And both-side dewatering blades are disposed at a loop portion, having a certain curvature, of the immediate downstream of the stock being supplied and the above-mentioned dewatering device is provided further downstream thereof, thereby the above-mentioned effects can be further clearly obtained. Further, as one of the dewatering limiting blade or the dewatering blade is made elastically pressable, the dewatering pressure between the two wires can be adjusted within the black colored area shown in Fig. 6 wherein the upper limit is the case of the dewatering blade being pressable and the lower limit is the case of the dewatering blade being not pressable, or the case of the dewatering limiting blade only being disposed, hence, according to the present invention, more flexible operation to meet paper making velocity, grammage (basis weight) and stock conditions becomes possible. As a result, in the present invention, dewatering on one side is limited and a high pressure pulse can be obtained even with a small bending of the wires at the blade edge portion, hence yield ratio of fibers is remarkably enhanced as shown in Fig. 8 wherein preferred embodiments according to the present invention and a conventional twin-wire former in which no pressable dewatering blade is disposed opposingly to the dewatering limiting blade are compared. Thus, according to the present invention, there is no lowering of yield of short fibers. Further, an evaluation of formation of a preferred embodiment according to the present invention as compared with those of said conventional twin-wire former and of another conventional twin-wire former in which pressable dewatering blades are disposed, although not opposingly to dewatering limiting blades, is shown in Fig. 7 and it is found that a good formation (good fiber dispersion state) can be obtained according to the present invention and movement of short fibers within mat inner layers to the outer layers becomes less. Furthermore, as shown in Fig. 9, as compared with said conventional twin-wire in which pressable dewatering blades are not disposed opposingly to dewatering limiting blades, an enhanced inner strength can be obtained by the present invention, and all these effects that the present invention brings on are revealed by a series of researches and tests performed by the inventors here. While the preferred form of the present invention has been described, variations thereto will occur to those skilled in the art within the scope of the present inventive concepts which are delineated by the following claims.

Claims: A paper machine twin-wire former comprising a dewatering device, the twin-wire former having two wires (1,2) forming loops, the loops having a wire running direction and a direction opposite thereto, and said dewatering device comprising: blades (20,21) disposed respectively on the two wires (1,2) for pinching stock (7) running between the loops, said blades'comprising dewatering limiting blades (20) disposed on one of the two wires (1,2), each dewatering limiting blade (20) comprising a first plane portion (20a) engaging and supporting said one of the two wires (1,2), a first inclined face (20b) disposed upstream of said first plane portion (20a) and, together with said one of the two wires (1,2), forming a first wedge shaped space that increases in size in the opposite wire running direction, and a second inclined face disposed downstream of said first plane portion (20a) which, together with said one of the two wires (1,2), forms a second wedge shaped space that increases in size in the wire running direction, said blades (20,21) further comprising dewatering blades (21) disposed on the other one of the two wires (1,2), each dewatering blade (21) comprising a second plane portion (21a) engaging and supporting said other one of the two wires (1,2), and a water scraping edge portion (21b) disposed upstream of said second plane portion (21a), wherein said second plane portions (21a) of said dewatering blades (21) are in contact with said other one of the two wires (1,2) such that there are non-contact areas therebetween that are not contacted by said second plane portions (21a), characterized in the dewatering limiting blades (20) being arranged without intervals therebetween, so that said first and second wedge shaped spaces of.adjacent dewatering limiting blades form a coextensive space, and in said first plane portions (20a) of said dewatering limiting blades (20) contacting said one of the two wires (1,2) at positions opposite to said non-contact areas of said other one of the two wires (1,2) such that said first and second plane portions (20a,21a) are not positioned opposite to each other and such that each of said second plane portions (21a) is opposite one of said coextensive spaces formed by said dewatering limiting blades (20). A paper machine twin-wire former comprising a dewatering device as claimed in claim 1, wherein each said dewatering blade (21) is supported so that the position of the wire supporting plane (21a) thereof is changeable and is elastically pressable onto said other one of the two wires (1,2). A paper machine twin-wire former comprising a dewatering device as claimed in claim 1 or 2, wherein each said dewatering limiting blade (20) is supported so that the position of the wire supporting plane (20a) thereof is changeable and is elastically pressable onto said one of the two wires (1,2). A paper machine twin-wire former comprising a dewatering device as claimed in any one of claims 1 to 3, wherein said dewatering blades (21) are movable in the wire running direction along a curvature on which said dewatering limiting blades (20) are arranged. A paper machine twin-wire former comprising a dewatering device as claimed in any one of claims 1 to 4, wherein one or more of said dewatering limiting blades (20) comprise(s) additional adjacent third and fourth inclined faces which are provided on the dewatering limiting blade (20) so as to form additional wedge shaped spaces with said one of the two wires (20,21) and so as to form an additional coextensive space.

Inventors: BANDO TAKASHI, IWATA HIROSHI, MASUDA HIROSHI, BANDO, TAKASHI, IWATA, HIROSHI, MASUDA, HIROSHI, Iwata, Hiroshi, c/o Mitsubishi Jukogyo K.K.
Applicants: MITSUBISHI HEAVY IND LTD, MITSUBISHI JUKOGYO KABUSHIKI KAISHA
Application year: 1996
Main classification: D21F9
IPC codes: D21F9, D21F1