Sheet-forming apparatus for a paper machine

Abstract: A sheet-forming apparatus for a paper machine having at least two fixed hydroextractors (9) which have separate locations for drainage to the hydroextractor side from locations for dispersion of fibres. These hydroextractors (9) are arranged alternately about two endless wire screen loops (1,2).

Description: The present invention relates to a sheet-forming apparatus for a paper machine. In a twin wire former type of sheet-forming apparatus for a paper machine, two sheets made of woven wire form respective endless wire screen loops, between which the raw material liquid mixture (liquid plus fibres) is held and run; free water is removed from the raw material liquid mixture by various hydroextractors whereby a fibre mat grows gradually and a web is formed. The above description is illustrated further in accompanying Figure 5 for the case of a sheet-forming apparatus having a fixed hydroextractor of a conventional shoe type. In this apparatus, two wire screens 1,2, guided by rolls 3,4, respectively, form a wedge shaped gap 5. Thereafter the wire screens come together so that they mutually overlap one another and pass over shoe blades 9 arranged on a prescribed curvature R as a part of a first fixed hydroextractor 8, running in a bent path along the curve of approximate radius R. A jet of raw material mixture (liquid plus fibres) 7 is injected from a headbox 6 towards the gap 5 and is held between the two wire screen sheets 1,2 so as to run at the same speed as the wire screens. Initial drainage starts when the raw material jet 7 is held between the two wire screens 1, 2 as a result of squeezing caused by the wire tension. However, most of the drainage is done subsequently on the shoe blades 9 by the pressure applied to the raw material mixture held between the wire sheets. After leaving the shoe blades 9, further draining of the raw material mixture occurs at a suction roll 12 forming a second hydroextractor and the resulting wet sheet 13 is transferred on the wire 2 to a subsequent press operation (not shown). In Fig. 5, the reference number 10 denotes a water deflector and the reference number 11 denotes a low vacuum box for draining by vacuum. The structure and drainage effect of the known hydroextractors are now explained. Figs. 6 and 7 of the accompanying drawings show two examples of typical prior art arrangements of the shoe blades and model curves in respective examples illustrating the pressure applied to the raw material mixture disposed between the wire screens. In Figs. 6 and 7, each shoe blade 9 is detachably mounted to the hydroextractor 8 by means of a guide on a supporter 14 fixed to the hydroextractor 8 and is arranged so that the surface on the centre line has a prescribed curvature R. Therefore, the number of blades and the pitch between them are adjustable. The angles ₁ and ₂ at which the wire screens 1 and 2 bend at the leading edge and at the trailing edge of the shoe, vary depending on the pitch (refer to Fig. 7). The greater the pitch is, the greater become the angles; the peak value of the generated pressure becomes greater accordingly. By virtue of the resulting pressure, fibres in the raw material mixture held between the wire screens 1 and 2 are moved and dispersed further; simultaneously water is drained through both of the two wire screens. Drainage is achieved in both directions with respect to the wire 1 side and the wire 2 side in the path extending between the shoe blades. On the other hand, drainage to only the wire 1 side occurs during passage over the shoe blade 9 because draining to the wire 2 side is inhibited by the shoe blade 9 itself, as illustrated by arrows in Figs. 6 and 7. Fibres in the raw material mixture can be divided into those which mat to form a sheet and those which are washed out with "white water" (mixture of drained water and some fibres). It has been understood that the prior art fibre mat, formed by the pressure applied to the raw material mixture held between the two wire screens 1 and 2 during drainage in the region of the shoe blades 9, is the same both on the wire 1 side and in the wire 2 side. However, the actual effects of the shoe blade 9 on the mat formed on the wire 1 surface and on the wire 2 surface are not always the same. That is, the spacing between the two wire screen sheets 1 and 2 swells at the leading edge and at the trailing edge by respective applied pressure P₁ and P₂ as shown in Fig. 8. In this condition, fibres near the boundary of the wire screen 1 and the fibre mat 15' formed on the wire 1 side run with the wire while being fixed as the mat. On the other hand, the fibre mat 15" on the wire screen 2 side is subjected to a reaction force through the wire screen 2, when the wire screen 2 is scraped by the front edge of shoe blade 9. As a result of that force, fibres in the mat are moved further and dispersed, whereby short fibres lose connection with long fibres and tend to be washed out with water drained by the pressure P₁ to the mat 15", resulting in lower yield tendency of short fibres compared with the wire 1 side. In the prior art sheet-forming apparatus shown in Figure 5, a fixed hydroextractor 8 of the above mentioned drainage characteristics is located only inside the wire screen 2 loop. Thus, the top side tends to differ from the back side in the paper sheet formed by this apparatus. In order to mitigate these problems, paper industries are now managing by adopting different specifications for the wire screen 1 and for the wire screen 2. For example, the mesh of wire screen 2 is made to be finer than wire screen 1, that is, wire screen 2 has more weaves. In the known apparatus of US-A- 4 790 909 two fixed hydroextractors are arranged alternately on the two sides of a pair of wire dewatering screens between which the web is formed. The two hydroextractors are each of convex shape and consist of a plurality of identical blades each of which presents a uniform radiused surface to the wire screen passing over that blade. As a result, the wire screen tends to be bent over the leading end of each blade and fiber dispersion and drainage tend to take place together at the locations of these bends. Such drainage tends to carry out fine elements of the raw material of the web in large amounts, thereby resulting in a relatively poor yield rate. In the known apparatus of our EP-A- 369296, a convex dewatering suppression shoe is disposed within the loop of an upper of the two wire dewatering screens. This shoe comprises a plurality of serrated shoe members each having a land portion for supporting the wire screen and an inclined portion forming a wedge-shaped hollow space between the surface of the wire screen and the surface of the shoe, upstream of said land portion. No other shoe portions exist between each land portion and the next wedge-shaped hollow space. Thus, the wire-screen passes from one said land portion to the next and no provision is made for scraping up water upstream of each hollow wedge shaped portion. With the latter arrangement drainage can be obtained at one side only, thereby leading to reduced drainage performances. It is an object of the present invention to provide a sheet-forming apparatus for a paper machine which overcomes the problems of the prior art apparatus explained above, improving the fibre yield and also reducing the difference between topside and underside of the paper produced. In accordance with the present invention, there is provided a twin wire paper-sheet forming machine comprising two moving wire screen loops which cooperate together over part of their lengths to define a common line of travel, means for introducing a raw material of water and fibres to be formed into paper between the wire screens, and fixed hydroextractor means for removing the water from the raw material, the hydroextractor means having a convex surface defined by a plurality of spaced-apart shoe blades with land portions which contact a respective one of the wire screens, the hydroextractor means comprising at least two hydroextractors, the land portions of one of said at least two hydroextractors contacting one wire screen and the land portions of the other of said at least two hydroextractor contacting the other wire screen, the at least two hydroextractors being spaced alternately from one another along and on opposite sides of the line of travel of said wire screens and the lands of the shoe blades being contoured, each land having a front leading portion , a trailing back portion and a mid-portion located therebetween, and wherein said front leading portion is flat and coincides with said line of travel of the wire screens, and said mid-portion is formed by a wedge-shaped trough, with the depth of the trough decreasing from the front to the back of the land, the at least two hydroextractor being structured and arranged such that on contact of a respective wire screen with the front leading portion of each land, the wire screens proceed onto the respective front leading portions of each land without bending at the leading edge thereof, and the wire screens bending on the trailing back portion of each land. In one advantageous embodiment of the invention, the trailing back portion of each land slope away from the respective wire screen along said line of travel of the wire screens. In another advantageous embodiment of the invention the second hydroextractor in said line of travel has an adjustable support means to adjust the angle of contact of the leading portion of the lands thereof with the wire screen so as to avoid bending of the wire screen upon making contact with the shoe blades of the second hydroextractor. Since the front edge of the shoe blade is formed so that the wire screens proceed without bending at that edge, the front edge functions only for scraping white water as in the case of a foil blade of a fourdrinier paper machine. By bending the wire screens at an intermediate position or at the back edge of the shoe blades, pressure in a pulse form necessary for further dispersion of the fibers in the raw material mixture is generated in the same manner as in a conventional apparatus; drainage to the shoe blade side at this location is restricted. Thus the location where drainage to the hydroextractor is made and the location where fibres are dispersed are separated. The fixed hydroextractors equipped with shoe blades are arranged alternately within the wire loop. Thereby, the effects of the shoe blades are directed alternately to the two sides of the mat being formed and thus there develops no difference between the top side and the back side of the resulting paper. Initial set for the first fixed hydroextractor and for the second hydroextractor is made so that the two wire screens should not be bent at the back edge position of the last end of the first hydroextractor and at the front end position of the second fixed hydroextractor. However, the wire tends to bend during operation due to added thickness of the raw material. As a countermeasure, the structure of the second fixed hydroextractor is arranged to include said adjustable support means whereby, in a preferred arrangement, the second hydroextractor can be made so as to move rotationally around a centre near its back end whereby the wire screen can be supported without bending by adjusting the position of the front end in accordance with the thickness of the proceeding raw material. The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings, in which:- Fig. 1 is a side view of one example of a sheet-forming apparatus embodying the present invention; Fig. 2 is a detailed lateral sectional view of a first embodiment of a hydroextractor for an apparatus according to the present invention; Fig. 3 is a detailed lateral sectional view of a second embodiment of a hydroextractor for an apparatus according to the present invention; Fig. 4 is a detailed lateral sectional view of the second fixed hydroextractor in the embodiment illustrated in Fig. 1; Fig. 5 is a side view of sheet-forming apparatus having a known fixed hydroextractor of the shoe type; Fig. 6 is a detailed lateral sectional view of a known fixed hydroextractor; Fig. 7 is another detailed lateral sectional view of a known fixed hydroextractor; and Fig. 8 is an enlarged side view of a shoe blade part of a known fixed hydroextractor. Figure 1 shows one example of a sheet-forming apparatus in a paper machine having hydroextractors according to the present invention. Operating characteristics of the apparatus are shown in Fig. 2 and Fig. 3. Fig. 4 is a detailed drawing of the device indicating how the decision is made as to the location of the front edge of the second fixed hydroextractor. Constituent parts shown by the reference numbers 1 to 7 in Fig. 1 are identical with those used in Fig. 5 and function in essentially the same way. Thus, detailed explanations of these parts are omitted here. In Fig. 2, the front (leading) edge 9′a of a shoe blade 9′ is located so as to be in the same plane as the wire screen 2. Therefore, wire screens 1 and 2, between which raw material mixture is held, proceed to shoe blade front edge 9′a without any bending of wire screens 1 and 2 at this front end. Thus, only a small pressure (P₁′) due to the collision reactive force of white water is generated, unlike the large pressure arising at the front edge of prior art shoe blade 9. The shearing force applied to mat between the wire screens is also small. Vacuum is applied to the space between shoe blades 9′. Therefore, drainage in the region between adjacent shoe blades 9′ is nearly the same as static drainage. Thus, the drainage V in this latter region is performed separately from the location of fibre dispersion, that is, in nearly the same manner as for static drainage, with a high yield. Wire screens 1 and 2, holding the raw material mixture 15 that has passed the front edge 9′a of the shoe blade, bend at the leading side of back edge 9′c with the angle of ₂′. The shape of the back edge 9′c is made so that wire screens 1 and 2 must bend in this way. With this arrangement, a pressure pulse is generated due to the same action as in the prior art shoe blade whereby further dispersion of fibres in the mat is promoted. The peak pressure value is adjustable by providing a first land portion 9′b of inclined, generally concave shape between the front edge 9′a and the back edge 9′c of the shoe blade 9′ and by selecting the parameters (l,α) governing the wedge-shaped space formed by the inclined bottom surface and the wire screen 1. The part near back edge 9'c of the shoe blade in said wedge-shaped space, which is associated with a positive pressure P₂' on the raw material mixture between the wire sheets, is filled with the white water which has drained, as taught in said prior art disclosure. Thus, dropping out of short fibres, which often occurs at prior art shoe blades, is avoided and the yield at the shoe blade side is improved. Fig. 3 shows another example of a shoe blade attaining the object of the present invention. The functions of parts 9"a to 9"c in Fig. 3 are the same as those of 9'a to 9'c in Fig. 2. Shoe blade 9" has a second land part 9"d declining towards the downstream side, in a similar manner to a file blade of a Fourdrinier paper machine. Vacuum force generated in the space formed by the second land part 9"d and wire screen 1 removes water, saving the vacuum force. The drainage capacity is adjustable by changing angle β as is the case of a Fourdrinier paper machine. The raw material mixture held between the wire screens 1 and 2 passing through the first hydroextractor 8' towards the downstream side, reaches the front edge of No. 1 shoe blade 9'a mounted on the second fixed hydroextractor 11'. The second fixed hydroextractor 11' is supported, as shown in Fig. 4, by a rotatable support device 16 whose supporting point 11'a is located near the back end and is set so that the wire screen 2 proceeds without bending at the front edge position of No. 1 (the front end) shoe blade 9'a by making the wire 2 lie at a distance from the shoe-blade corresponding to the thickness of raw material. The rotatable position is adjusted by detecting the white water taken out at said front edge. Thus, white water is taken out at the front end without scraping the formed mat on the wire screen 2 side. Further, on the second fixed hydroextractor 11′, the wire screen 2 side (that was outside at the first fixed hydroextractor 8′ part) of raw material liquid held between wire screens 1 and 2 is subjected to a draining action as explained above while running on the surface of shoe blade 9′. Thus, the mat running after the second hydroextractor 11′ has the same history at both sides and the difference between the top side and the back side is smaller, resulting in the achievement of a good yield of fine fibres. The mat is despatched to the suction roll 12 under such conditions. The function of the downstream equipment is the same as that of the prior art. It will be clear that the drainage at both sides of high yield rolls does not impair the characteristics of the formed mat. However, by consideration of the treatment of white water (shown by arrow mark A in Fig. 1) drained towards the outside of the roll, a more inclined wire run (in the direction of the wire screen movement) at the contact point of wire 2 on suction roll 12 makes the treatment easier. (For reference, γ′ [Fig.1] > γ [Fig.5]). By use of the present invention, the yield is improved by separating the drainage locations to the machine side from the locations for fibre dispersion during drainage by the fixed hydroextractors. Furthermore, drainage zones for both sides of paper are separated and respective drainage control is possible. Moreover, by arranging the hydroextractors alternately in the two endless wire screen loops, difference between the top side and the back side of the paper is improved and the operational life of both wire screens become nearly the same because both wire screens run along similar fixed hydroextractors. Therefore, the life of both wire screens becomes nearly the same and the shut down period of the machine is shortened.

Claims: A twin wire paper-sheet forming machine comprising two moving wire screen loops (1,2) which cooperate together over part of their lengths to define a common line of travel, means (6) for introducing a raw material of water and fibres to be formed into paper between the wire screens (1,2), and fixed hydroextractor means for removing the water from the raw material, the hydroextractor means having a convex surface defined by a plurality of spaced-apart shoe blades (9') with land portions which contact a respective one of the wire screens (1,2), the hydroextractor means comprising at least two hydroextractors (8',11') the land portions of one (8') of said at least two hydroextractors contacting one wire screen (1) and the land portions of the other (11') of said at least two hydroextractors contacting the other wire screen (2), the at least two hydroextractors (8',11') being spaced alternately from one another along and on opposite sides of the line of travel of said wire screens (1,2), characterised in that the lands of the shoe blades are contoured, each land having a front leading portion (9'a), a trailing back portion (9'c) and a mid-portion (9'b) located therebetween, and in that said front leading portion (9a') is flat and coincides with said line of travel of the wire screens, and said mid-portion (9'b) is formed by a wedge-shaped trough, with the depth of the trough decreasing from the front to the back of the land, the at least two hydroextractors (8',11') being structured and arranged such that on contact of a respective wire screen (1,2) with the front leading portion (9'a) of each land, the wire screens (1,2) proceed onto the front leading portion (9'a) of each land without bending at the leading edge thereof, and with the wire screens (1,2) bending on the trailing back portion (9'c) of each land. A machine according to claim 1, wherein the trailing back portion (9'c) of each land slopes away from the respective wire screen (1,2) along said line of travel of the wire screens. A machine according to claim 1 or claim 2, wherein the second hydroextractor (11') in said line of travel has an adjustable support means (16) to adjust the angle of contact of the leading portion of the lands thereof with the wire screen (2) so as to avoid bending of the wire screen (2) upon making contact with the shoe blades of second hydroextractor (11').

Inventors: BANDO TAKASHI, IWATA HIROSHI, SUZUMURA HIROSHI, BANDO, TAKASHI, IWATA, HIROSHI, SUZUMURA, HIROSHI, BANDO, TAKASHI, C/O MIHARA MACHINERY WORKS, Iwata, Hiroshi, c/o Hiroshima Technical Inst., Suzumura, Hiroshi, c/o Hiroshima Technical Inst.
Applicants: MITSUBISHI HEAVY IND LTD, MITSUBISHI JUKOGYO KABUSHIKI KAISHA
Application year: 1991
Main classification: D21F9
IPC codes: D21F1, D21F9