FIBROUS STOCK FORMING UNIT

Abstract: There is provided a movable forming band or wire (10) which passes over a dewatering box (15). A pressure former (11) having an exit slice lid or plate (12) is located over the forming band or wire (10) so as to define therebetween a forming zone (13) which is non-linear.

Description: FIBROUS STOCK FORMING UNIT Background of the Invention This invention relates to the manufacture of a web from fibrous stock and in particular to the manufacture of paper and paperboard. Paper and paperboard webs are normally formed by draining a fibrous suspension through a moving foraminous screen or forming band, sometimes called a "wire" or "fabric." The moving screen can be flat, as in the case of a traditional Fourdrinier machine or curved in the case of a traditional cylinder mold machine. Normally, the fibrous suspension is fed in a free state onto the forming screens. However, it is known to feed the fibrous suspension onto the screen by means of a short forming unit known as a "short former" or as a "pressure former." In such a unit the fibrous suspension is supplied to the screen or band t rough a distribution unit followed by a dispersing chamber and then through an exit slice or dispenser having a lid which defines with the forming band a forming zone. Formation takes place under pressure between the lid and the screen or band. Such pressure formers which have an explosion chamber followed by an exit slice are described in U.K. patents 1, 179,847 and 1,548,924. It has previously been proposed that such pressure formers should be used in cooperation with a cylinder mold machine. Such an arrangement is described in U.K. patent 1,548,925. Such prior pressure forming arrangements have disadvantages which the present invention is intended to overcome. The expense and complication of a large cylinder mold can be avoided. The resulting forming unit can be simpler and less costly than a traditional cylinder mold installation. Rotary screens and the problems associated with the effects of centrifugal force at higher speeds can be problematic with cylin¬ der mold machines, and the present invention is de¬ signed to avoid such problems. It is also intended to provide a forming unit which is capable of a wider range of operation to handle a wider variety of con- sistencies and fibre material and to improve the control of water drainage from the web. It has been proposed to use pressure formers with flat wires, and such arrangements are described in UK patent 1,548,925. However, the prior arrangement has a flat exit slice plate cooperating with a flat wire, thereby defining a linear forming zone which is far from ideal. The fragile web formed in the forming zone can easily shear against the surface of the lid of the former causing disruption to the web and there- by disrupting formation. It is one of the objects of the present invention to improve on such formation. Summary of the Invention It is a purpose of the present invention to provide a separate forming unit that can be used in the manufacture of paper and paperboard by modifying existing equipment. According to the present inven¬ tion, there is provided a forming unit for laying a sheet of fibrous material such as paper or paperboard on a moving foraminous fabric or wire, including a pressure former type of headbox having an exit slot through which a liquid suspension of such fibrous material is expelled, said headbox having a top lid or slice plate extending from said slot in the direction of stock movement through said slot for applying the stock of fibrous material to an exposed surface of the moving wire, and in which such wire is guided across an open surface of a dewatering box positioned gener¬ ally in opposed relation to said lid thereby defining with said lid a forming zone which extends over at least a portion of said dewatering box, which forming zone decreases in volume in the direction of wire movement over said box, characterised by the fact that said forming box and/or said lid are configured to define a non-linear said forming zone in said direc- tion of wire movement, the non-linearity of which corresponds generally to the rate at which liquid is removed from said suspension by said dewatering box. Preferably the surface of the dewatering box over which the band moves and/or the exit slice plate is curved to define the non-linear forming zone. In one form of the invention the surface of the dewatering box is curved and the exit slice plate is planar. Thus the pressure former can be provided with a planar exit lid, thereby reducing cost and simplifying the design, and can cooperate with a curved dewatering box over which the forming band runs to define with the planar exit lid a forming zone which has a non-linear variation in cross-section from its upstream to its downstream ends. The variation would more approximate a parabolic section rather than linear. The ideal section is one in which the rate of convergence match- es the rate of dewatering within the forming zone, i.e. matches the variation in solids content of the slurry or stock through the forming zone. While the preferred embodiment of this invention utilises a generally planar exit lid combined with a • | 0 non-planar or curved dewatering box, to define a non¬ linear forming zone, the relative configurations of the suction box and the forming lid may be reverse, i.e., the lid may be formed with a convex curvature in opposed relation to a planar or flat forming box over ,,- which the forming band moves. Because the forming band is pulled under tension over the surface of the forming box, particularly where the forming box is curved, this forming fabric is always wrinkle free and level throughout the critical forming zone.Pressure ?π formers of the type described may be considered as an apparatus for filter forming a fibrous web on a wire, where a well dispersed fibre suspension is progres¬ sively drained through the wire under controlled conditions. This assures that a well-formed level web t - of known bases weight is produced over a wide range of speeds from a wide range of furnishes, i. e., a wide range of paper stock fibre products and consistencies. The former of this invention permits the use of stock suspensions having a higher consistency than that , 0 which can be used in conventional forming equipment. Tensile ratios ranging from 1.5 to 1.0 to 4.0 to 1.0 have been obtained in experimental studies. Such control may be adjusting the throat dimensions at the point at which the fibrous suspension flows from the 35 explosion chamber into the forming zone and onto the forming screen. In each of the embodiments illustrated, a non¬ linear forming zone, which varies in dimension at a changing rate in the direction of wire movement, provides progressive drainage under controlled condi¬ tions and approximates or matches, and in which the rate of convergence approximates or matches the rate at which white water is drained through the forming screen. Since this non-linear relationship matches the variation in solids content of the slurry or stock, at any given position, the rate of progression or movement of the stock within the forming zone can remain substantially unaffected by the necessity in prior art formers to adjust flow rate for volumetric changes, and a higher and more controlled basis weight may be applied to the forming band surface. Prefera- bly, the exit slice plate or lid is provided with means by which it may be adjusted in position relative to the forming band or wire, so as to vary the size or variations in section of the forming zone, to accommo¬ date various conditions. Further, it is within the scope of this invention to provide a non-linear form¬ ing zone in which both the forming box surface and the lid surface are curved, or alternatively, to provide a lid which is partially planar at the exit of the explosion chamber, and which is then curved to define the major portion of a non-linear forming zone. In a preferred embodiment, the dewatering box is a suction or vacuum box adapted to increase water removal by the application of negative pressure. The dewatering box preferably has a plurality of separate dewatering compartments disposed in succession along the length of the forming zone. The dewatering box may extend beyond the forming zone to provide further dewatering to the web on the screen. In addition to or in place of the extended de¬ watering box, a separate dewatering or suction box may be provided downstream of the forming zone to provide further dewatering. The dewatering box may be provided with means for obstructing dewatering over a part of its surface. In a typical application the obstruction will be provided at the upstream end of the forming zone to control or prevent dewatering in that zone. The dewatering box may be provided with an ultra¬ sonic generator to facilitate formation and drainage of the stock. The forming band is preferably an endless band making the forming unit a self-contained unit. A second forming band may be provided to define with the first forming band a twin band dewatering run through which the web may pass to facilitate dewater¬ ing from both surfaces of the web. The apparatus also can incorporate deflectors which induce shear forces to the fibrous web resulting in an improved formation even at high consistencies. The pressure former may be of a multi-channel unit providing at least two exit slices for multiple stock supply to the forming zone. Such a unit is described for example in prior UK patent 2,078,812. The forming unit according to the present inven¬ tion may form part of a paper or paperboard machine in which the web formed on the unit is transferred from the forming band onto the main forming wire of the machine. According to another aspect of the present inven¬ tion, there is provided a multiply paper or paperboard machine incorporating a forming unit according to the present invention. Preferably the unit transfers the web to the underside or top side of the main band of the machine. A paper or paperboard machine may incorporate two or more forming units according to the present inven¬ tion, and the webs from the two units may be combined before the combined web is transferred to the main band of the machine. According to another aspect of the present inven¬ tion there is provided a multiply paper or paperboard machine comprising a plurality of forming units ac¬ cording to the present invention, each supplying a ply to a common machine band. According to a further aspect of the present invention there is provided a cylinder mold machine provided with a forming unit according to the present invention wherein the forming band passes around the cylindrical screen of the cylinder mold machine fol¬ lowing formation on the forming unit whereby the web is transferred to the main forming band of the cylin¬ der mold machine. Brief Description of the Accompanying Drawings The invention will now be described by way of example with reference to the accompanying drawings in which: Fig. 1 is a diagrammatic view of a forming unit according to the present invention; Fig. 2 is a diagrammatic representation of an alternative design of a forming unit according to the present invention; Figs. 3, 3A, 4 and 5 are variations on the form¬ ing unit of Fig. 1; Fig. 6 is a variation on the forming unit of Fig. 1 showing the unit in a complete form to be used with a paper making machine; Figs. 7 and 8 are variations on the forming unit of Fig. 1 ; Fig. 9 shows a modification to the forming unit of Fig. 1 in which the unit is shown complete as a section of a full paper making machine; Figs. 10-12 show forming units according to the present invention applied to a Fourdrinier paper making machine; Fig,. 13 is a multiply paper making machine pro¬ vided with two forming units according to the present invention; Fig. 14 is a cylinder mold machine modified by the provision of a forming unit according to the present invention,- Figs. 15 and 16 are, respectively, alternative embodiments of the forming unit of Fig. 1; and Figure 17 shows a further variation according to the present invention. Description of Preferred Embodiment Referring to Fig. 1, there is shown a forming unit according to the present invention comprising a forming screen or wire 10, a pressure former 11, having an exit lip 12 defining with the forming wire a forming zone 13 and a dewatering box 15 having three dewatering compartments the first two of which extend over the forming zone 13. The dewatering box 15 has a curved upper surface 16, over which the forming wire 10 runs and the curved surface 16 defines with the exit slice 12 a non-linear forming zone 13. The curvature of the surface 16 is preferably such that with the planar slice lip 12, a cross-sectional variation is achieved which in the direction of formation more closely approximates the rate of dewatering of the stock within the forming zone 13. It has been found that such a variation in section is preferable to a linear decreasing section since it avoids the possibility of interference with web formation in the formation due to shear action against the slice lip 12. It is also apparent that the action of pulling under tension the forming wire 10 over the curved surface of the forming box 15 levels out any ripples in the forming wire. This results in a desired level fabric or screen in the forming zone 13. The pressure former 11 comprises inlet pipes 20 and explosion chamber 21 from which the turbulent and deflocculated stock exits to the forming zone 13 where it is formed under pressure onto the wire 10 to form a web of paper or paperboard. The compartments of the dewatering box 15 may be supplied with suction to .facilitate dewatering through the wire 10 and into the dewatering box. The dewater- ing box need not extend beyond the length of the forming zone 13, but preferably does so to improve dewatering thereafter. An alternate form of design is shown in Fig. 2 in which similar components have been given similar reference numerals, but an order of magnitude higher. In this variation the upper surface 116 of the vacuum box 115 is planar and the upper lid of the exit slice 112 is curved to define with the upper surface 116 of the dewatering box a non-linear forming zone 113. The operation of the device and the improved forming within the pressure forming zone 113 is simi- lar to that of the device shown in Fig. 1. However, to improve the run of the wire against the surface of the dewatering box and to avoid the complication of a curved lid 112, the embodiment of Fig. 1 is preferred to that of Fig. 2 in producing forming units according to the present invention. Referring now to Figs. 3 and 3A there is shown a modification of the forming unit of Fig. 1 in which the upper surface 16 of the dewatering box is provided with vanes 16a to improve water removal from the underside of the forming wire 10. Additionally, initial section of the upper surface 16 can be ob¬ structed as shown at 30 in Fig. 3A, whereby water is prevented from entering the first of the compartments of the dewatering box 15. As shown, the first three areas of the dewatering box are obstructed and water cannot enter until the last of the areas within the first section and thereafter dewatering occurs through the balance of the first section and through the second compartment which extends over the forming zone 13. Fig. 4 shows a variation of the forming unit in which a further dewatering box 40 is provided down¬ stream so that further dewatering can be effected which can be particularly applicable for the produc¬ tion of heavier weight paperboard and special material such as roofing felts, etc. Such modification would be required where all the dewatering could not be affected within the length of the forming zone as defined by the lip 12 or in the further sections of the dewatering box 15. The further dewatering box 40 is provided on a further length of the wire 10 at a location downstream of and remote from the dewatering box 15. In Fig. 5 there is shown a further modification to the dewatering box 15. In this modification the upper surface 16 of the dewatering box has been pro- vided with ultrasonic generators 60, 61 which apply shear forces to the fibrous suspension in the critical forming zone 13 in order to improve formation and drainage. Shown in Fig. 6 is a full forming unit. As illustrated, the full extent of the forming wire 10 can be seen to be a continuous wire forming the unit into a self-contained and compact unit used for the formation of paper and paperboard. In this particular embodiment, immediately after the wire leaves the dewatering box 15 passes it around a '-oil 65 about which there is also wrapped a second forming wire 70 to form with the first forming wire 10 a twin wire run from forming roll 65 to a second roll 66. Within the forming run the twin wire formation will effect dewatering from both sides of the web. Deflector blades 67, 68 facilitate water removal from both sides of the web. The blades 67, 68 also induce shear forces to the fibrous web resulting in an im¬ proved formation even at high consistencies. Figs. 7 and 8 show variations on the forming unit design in which the pressure former has been modified to the design shown at 71 and 72, respectively being provided with "back-to-back" explosion chambers as shown at 71 (Fig. 7) or "tandem" explosion chambers as shown at 72 (Fig. 8). In each case the purpose is to facilitate the production of multi-furnish papers, e ' . - / duplex papers and by using the pressure formers of such design, a compact unit can be provided with two or more furnishes being supplied forming two or more webs out of a single forming unit onto the forming wire. Such a design has previously been described in more detail in U.K. patent 2,078,812. Fig. 9 shows the forming unit of Fig. 1 as a complete unit for use as a forming section on a multi¬ ply paperboard making machine. As seen, the wire 10 is a continuous wire but short in length to form a compact forming unit. As shown, the unit forms part of a multi-plyboard making machine in which the main forming wire/felt 80 of the machine receives the formed web from the forming unit as it leaves its dewatering box 15 on wire 10. The web is transferred from wire 10 to the underside of the main machine wire/felt 80 and thereafter is taken by the machine wire/felt 80 down machine to the next forming station. It will be seen from illustration of Fig. 9 how compact the forming unit can be within a multiply making machine. The unit can be used to produce a single ply web or could be used as one of many such units to form a multi-plyboard. Fig. 10 shows that the application of a forming unit of the form shown in Fig. 9 applied to a tradi¬ tional Fourdrinier machine The Fourdrinier machine has a headbox 81 and a main forming wire 82. A form- ing unit 83 of a design according to the present invention and shown for example in Fig. 9 is disposed such that a web formed on this unit is transferred to the upper face of the main forming wire 82 of the Fourdrinier machine onto the upper surface of the web already disposed on that wire and formed from the headbox 81. It will be seen that the forming unit 83 according to the present invention can provide a very simple modification and application to a traditional Fourdrinier machine for the purpose of adding an extra ply and thereby increasing the production of such a machine., Fig. 11 shows an arrangement whereby forming units 84, 85 made according to the present invention are disposed such that the webs formed on each of these units are combined into a single multiply web which is then transferred onto the main machine web 86 of a flat wire machine. In this particular embodiment the web on the flat wire 86 has been produced by a forming unit 87 also of a design according to the present invention. However, it will be appreciated that the forming unit 87 could be replaced by a tradi¬ tional head box such as that shown at 81 in Fig. 10. By the arrangement shown in Fig. 11, however, three forming units according to the present invention have been combined to provide a multiply machine. In Fig. 12 there is shown a modification of the machine of Fig. 11 in which, in addition to the form¬ ing units 85 and 87, the unit 84 has been replaced by a dry forming unit 88. Such a dry forming unit is of well known design in which dry fibres are distributed onto a wire and are formed into a web possibly by the use of heat and moisture, but without the use of traditional methods. The dry formed web is combined with the web formed on unit 85 and the combined dry/wet multiply web is combined with the web on the main machine wire 86. By such a configuration, the outer plies of the triple ply web will be wet formed with the inner ply being dry formed, an arrangement which can produce particularly attractive properties where a relatively bulky central ply is sandwiched between well formed traditional outer plies having the usual paper characteristics of wet formed materials. In Fig. 13 there is shown a configuration for a multiply paperboard machine in which two forming units 90, 91 made according to the present invention are placed in series to cooperate with the main forming wire/felt 92 of the machine The webs made on the forming units 90 and 91 are transferred to the main machine wire/felt 92 to form on that wire a multiply web. The particular formation shown in Fig. 13 is attractive because additional centrifugal dewatering is achieved by wrapping the forming wires 93, 94 of units 90, 91 and the machine wire/felt 92 about common rolls 95, 96 respectively. Further dewatering occurs as the wires combine around these rolls and centrifu¬ gal force assists in the dewatering process. In Fig. 14 there is shown a modification of a traditional cylinder mold machine in which the cylin- drical screen 100 of the cylinder mold machine is utilised as a roll about which the machine wire 10 of the forming unit according to the present invention is wrapped in its wire run. Thus the screen 100 forms part of a forming unit but does not play a part in the formation itself. A web formed in the forming zone 30 over two compartments of the dewatering box 50 passes over a further dewatering box 101 and thereafter the web is transferred to the underside of the main ma¬ chine wire 102 as the wire 10 passes over the screen 100. The configuration of Fig. 14 illustrates a very simple modification to an existing cylinder mold machine in which the formation produced by the tradi¬ tional cylinder mold is replaced by the new fast high speed pressure forming unit according to the present invention providing greatly improved formation in the web and a final product which is of improved charac¬ teristics compared with the traditionally produced cylinder mold product, but utilising the existing machinery to reduce the expense and cost involved with the major rebuilding of such a machine. Such a modi¬ fication provides a cheap and efficient way of modify¬ ing a cylinder mold machine to produce an improved product. Furthermore, the higher speed production achieved by such a modification also overcomes the adverse effects of centrifugal force normally associ¬ ated with the use of such a rotary screen. In Fig. 15, the former 11 is provided with a forming plate or lid with a planar or straight section 12a positioned immediately at the downstream wall of the chamber 21, joined with a curved section 12(b) in cooperative relation with the curved surface of the dewatering box 15. The lid sections 12(a) and 12(b), combined with the curved surface of the forming box 15, define a non-linear forming zone. The curvature of the section 12(b) is concave downwardly toward the curved suction box surface. The embodiment of Fig. 16, in many ways, is similar to that of Fig. 15, except that the upper lip 12(c) of the former is curved throughout its entire length with the curvature being concave downwardly toward the curved surface of the forming box 15. However, the radius of curvature is less than that of the forming and wire-guiding surface of the forming box so that a progressively smaller volume is repre¬ sented in the forming zone, in the direction of wire movement. It will of course be appreciated that it is the positioning of the upper lip 12(c) relative to the forming box, rather than the relative radii of curva¬ ture of the lip and the forming box, which is impor¬ tant in ensuring that the forming zone has a progres- sively smaller volume. The embodiments of Figs. 15 and 16 provide alternative arrangements by which the forming zone defined between the lip and the suction box surface is one which decreases at a rate which approximates the rate at which the stock is dewatered. Forming devices according to the present inven¬ tion can be used for producing a wide range of paper and paperboard products in both single and multiply form. The weight ranges of such products can vary from paper grades to heavy boards including synthetic non-woven materials, high consistency layers, compos¬ ite materials, roofing materials and the like. Units according to the present invention can also be combined to form simple multiply paper making machines which can be less expensive and more effi¬ cient than traditional designs. One example of such an arrangement is shown in figure 17 in which a making felt 130 in the form of a fabric or wire is provided. In figure 17 the making felt 130 carries a previously formed web 131.- which is combined with the web formed on the wire 10 using pressure former 11. The combined webs then proceed to further processing 132 on the making felt 130. Units according to the present invention can be used to form webs from foamed fibrous suspensions. With such an arrangement the fibrous suspension in the carrier liquid is deliberately foamed, and when ap- plied through the forming unit flows onto the draining screen and the application of vacuum to the drainage box collapses the foamed suspension leaving a well dispersed and well formed web. Because the forming unit according to the present invention is relatively simple and inexpensive laboratory units, pilot plans and other simple test equipment can be built using such designs. Many variations on the basic design described above can be effected without departing from the present invention. Thus, the dewatering box 15 need not be of the suction design. The compartments within the box can be flooded to prevent drainage or can be operated with a vacuum or on a syphoning system. Similarly, many variations on wire run and utilisation of such units with traditional forming machine of both flat wire and cylindrical screen design can be uti¬ lised while enjoying the benefits of the present invention. For example, where the. speci ication refers to two forming units operating together it will be appreciated that more than two forming units could be provided, if necessary.

Claims: CLAIMS 1. A forming unit for laying a sheet of fibrous material such as paper or paperboard on a moving foraminous fabric or wire, including a pressure former type of headbox having an exit slot through which a liquid suspension of such fibrous material is expelled, said headbox having a top lid or slice plate extending from said slot in the direction of stock movement through said slot for applying the stock of fibrous material to an exposed surface of the moving wire, and in which such wire is guided across an open surface of a dewatering box positioned generally in opposed relation to said lid thereby defining with said lid a forming zone which extends over at least a portion of said dewatering box, which forming zone decreases in volume in the direction of wire movement over said box, characterised by the fact that said forming box and/or said lid are configured to define a non-linear said forming zone in said direction of wire movement, the non- linearity of which corresponds generally to the rate at which liquid is removed from said suspension by said dewatering box. 2. The forming unit of claim 1 further characterised by the fact that the surface of the dewatering box over which said wire moves is curved towards said lid in said direction of wire movement thereby presenting to said zone a convex surface. 3. The forming unit of claim 1 characterised by the fact that said lid is formed, at least in part, with a curvature which curves toward said wire in said wire direction of movement, thereby presenting to said forming zone a concave surface. 4. The forming unit according to claim 2 in which a portion of the open surface of said dewatering box is curved in the direction of wire movement presenting said convex surface to said forming zone. 5. The forming unit as claimed in any preceding claim characterised by the fact that said exit slice lid is adjustable in the position relative to said forming wire at said forming zone. 6. The pressure former according to claim 3 in which said forming lid is approximately planar in said forming zone. 7. The pressure former according to any preceding claim further characterised by the fact that said dewatering box has a plurality of separate dewatering compartments disposed in succession along the length of said forming zone. 8. A forming unit as claimed in any preceding claim wherein the dewatering box is provided with means for obstructing dewatering over a part of its surface. 9. A forming unit as claimed in any preceding claim wherein the dewatering box extends in the direction of wire movement beyond said lid to support the forming zone to provide further dewatering. 10. A forming unit as claimed in any preceding claim wherein the dewatering box is a vacuum box adapted to increase water removal by the application of vacuum to the box. 11. A forming unit as claimed in any preceding claim wherein the forming band is an endless band. 12. A forming unit as claimed in any preceding claim wherein the pressure former is a multi-channel unit providing at least two exit slices for multiple stock supply to the forming zone. 13. A multiply paper or paperboard machine incorporating a forming unit as claimed in any preceding claim. 14. A forming unit substantially as hereinbefore described with reference to the accompanying drawings.

Inventors: ATTWOOD BRIAN WILLIAM, ATTWOOD, BRIAN, WILLIAM
Applicants: BLACK CLAWSON CO, THE BLACK CLAWSON COMPANY
Application year: 1992
Main classification: D21F1
IPC codes: D21F1