GB2113264A - Method and apparatus for improving formation on a paper making machine - Google Patents

Abstract

A system of foils (16, 17, 18) for a Fourdrinier paper making machine has periodic variability in their surfaces across the width of the machine so as to cause ridges, in the surface of the liquid stock, which oscillate as they travel down the forming section of the machine causing mixing of the stock and improved paper formation. <IMAGE>

Description

SPECIFICATION Method and apparatus for improving formation on a paper making machine This invention relates to a process and apparatus for improving the formation of a paper web on a paper making machine.

In the operation of a typical paper making machine, the pulp stock which is a thin suspension of fibres and fillers, containing generally about 99.5% water, is flowed from a headbox slice onto the upstream surface of a moving endless screen belt (forming fabric) which is made of woven metal or plastic filaments. The fabric passes over a breast roll at one end of the forming section of the machine and a couch roll at the other end and between these two rolls the fabric travels in contact with spaced apart transverse dewatering foils and then over suction boxes where water is withdrawn from the pulp stock leaving a thin, selfsupporting formation of matted fibres on the surface of the fabric.This sheet of formed fibres is lifted off the fabric at the couch roll at the downstream end of the forming section and is transferred to a press section and then to a dryer section to complete the dewatering. After travelling around the couch roll, the fabric belt is returned through a series of return rolls to the upstream end of the forming section where it travels around the breast roll and again passes under the slice to complete the cycle.

In spite of attempts to thoroughly mix the pulp stock in the headbox of the paper machine so that the fibres thereof will be uniformly dispersed, the fibres tend to agglomerate as they emerge from the slice and deposit on the fabric in clumps or flocs, If these flocs remain undispersed the finished paper will not be of uniform density. Also, there is a tendency for stock fibres to become aligned in the machine direction which is detrimental to cross machine strength of the paper.

Several methods have been proposed and used to redistribute fibres in the pulp stock after it has been transferred to the forming wire and during the early stages of dewatering. A common method that has been used for many years to reduce flocculation is to provide a rapid shaking motion to the upstream end of the forming section of the machine. In other methods air and/or water jets are sprayed on the wet stock either from above or below to rewet and agitate the stock and so redistribute the fibres.These methods have not proven entirely satisfactory particularly in the case of large, high speed machines where machinery required to shake the forming section is expensive and power consuming or when jets of air and/or water tend to force excessive amounts of the fibres and solids through the forming fabric and increases the amount of water which ultimately has to be withdrawn from the stock, therefore, requiring additional dewatering equipment which is usually power consuming.

It is known that flocculation occurs almost continuously in the formation zone whiie the pulp fibres are still in suspension and that deflocculation of the fibres is most effectively accomplished on slow running machines in which the wet pulp stock is subjected to almost continuous cross-machine shear by a shake mechanism during the forming cycle. On large machines running at high speed it is impossible to attain adequate continuous cross-machine shear due to rapid passage of the wet pulp through the forming zone and to difficulty in overcoming the inertia of the large mass of machinery to provide sufficiently rapid cross-machine oscillation.

It has been observed also that when ridges and gullies form in the sheet of stock discharged from the slice and produce thick and thin sections alternately in the cross-machine direction,-the ridges tend to dissipate when the stock lands on the forming fabric and, as they do, create a flow of stock in the cross-machine direction which produces shear action within the layer of stock at the dissipating ridge also in this direction. This condition is most apparent in cases where intermittent ridges are produced by the action of a rectifier roll in the headbox immediately preceding the slice.

Further, and possibly due to the greater energy contained in the ridges, a wave action is promoted and there occurs an interchange where a ridge becomes a gully and vice versa as the wet pulp moves down the forming table of the machine.

This interchange of phase is enhanced as the stock on the fabric passes over a dewatering element such as a table roll or a foil blade where the stock is subjected to the vacuum created thereon and even a more vigorous shearing action is promoted within the layer of stock. While this action is almost instantaneous in a high speed machine it does tend to counteract the continual flocculation of pulp fibres with the result that in the vicinity where variations in stock thickness have occurred, and particularly where there has been a longitudinal (machine direction) change of phase as described above, agglomeration of pulp fibres is reduced and the formation of the paper becomes more uniform.

Another known method of redistributing fibres in the pulp stock attempts to utilize the shearing action promoted by ridges by providing a serrated slice lip to induce a regular flow of ridges in the headbox discharge. This measure has not been entirely effective because the ridges tend to dissipate too soon.

It is a feature of this invention to utilize the above-mentioned phenomenon and induce controlled variations in thickness in the wet sheet of pulp stock on the forming fabric throughout the forming zone of a paper making machine to create cross-machine shear in the wet sheet for the purpose of deflocculating agglomerated fibres of stock.

According to one aspect of the invention there is provided one or a plurality of dewatering foil blades that are specifically designed to provide cross-machine variations in dewatering of the pulp stock to promote like variations in the thickness of the sheet of wet pump stock on the forming fabric.

The blade of the invention is generally similar to those normally installed in a fixed position to extend crosswise under and in contact with the forming fabric in the dewatering zone. The blade has a leading edge, a flat fabric supporting surface and a foiling surface set at a small divergent angle from the supporting surface. The foiling surface of the blade of the invention, however, is provided with repetitive machine direction non-foiling portions at intervals crosswise of the machine direction so that uniform drainage induced by the foiling angle is interrupted and sections of wet pulp on the fabric passing over a non-foiling portion will not be dewatered to the same extent as sections passing over a foiling portion.The resulting interrupted drainage produces thick and thin sections in the wet pulp in the cross-machine direction and promotes cross-direction shear to beneficially influence fibre distribution.

The foiling surface of a foil blade is defined as a surface diverging from the forming fabric by an angle of less than approximately 5 degrees.

The blade of the invention may be constructed of any suitable material that is chemically inert and which provides a wear resistant, low friction surface. A preferred material is high density polyethylene which is easily machinable. The preferred way of mounting the blade is generally referred to as a T-bar attachment and is described in detail in U.S. Patent No. 3,337,394.

Standard drainage foils may be replaced by blades of the invention at any location in the forming section to improve deflocculation thus providing an efficient, inexpensive and flexible method of improving formation on the paper making machine.

According to another broad aspect of the present invention there is provided a fabric supporting blade for use in a wet section of a paper making machine and positionable in supporting relationship to the forming fabric in the wet end of the paper making machine and extending thereacross transversely, of the direction of fabric travel. The blade comprises a body provided with an upstream side, a leading edge, a transverse continuous flat fabric supporting surface, a discontinuous foiling surface and a downstream side. The blade is characterised In that the foiling surface, is provided in the cross-machine direction, with a plurality of repetitive non-foiling portions interspersed with repetitive non-foiling portions lying adjacent to each other and extending in the machine direction.

The said foiling portions slope downward from the downstream edge of the flat fabric supporting surface at a conventional foiling angle of less than 5 degrees. The non-foiling sections are either coplanar with the fabric supporting surface or slope downward from the downstream edge of the fabric supporting surface at an angle greater than 5 degrees and open in the downstream side of the blade body so as to provide intermittent dewatering in the cross-machine direction to induce longitudinal ridges and gullies in a layer of wet stock as it is being dewatered for the purpose of causing lateral shear in the stock to effect a more thorough interlacing of the stock fibres to thereby improve formation.

According to a still further broad aspect of the present invention there is provided a method of forming a web of paps. on a forming fabric of a paper making machine. The method comprises positioning at least one fabric supporting blade under and in contact with the forming fabric in a wet section thereof.The blade has upstream and downstream sides, a leading edge, a transverse continuous flat fabric supporting surface, a downstream surface and in said downstream surface a plurality of uniform foiling surfaces interspersed in the cross-machine direction by uniform non-foiling surfaces, said foiling surfaces extending from and convergent with the downstream edge of said flat fabric supporting surface and diverging therefrom at a foiling angle, said interspersing non-foiling surfaces being either coplanar with said flat fabric supporting surface or diverging therefrom at an angle greater than a foiling angle and opening into the said downstream side.The blade induces in the layer of wet stock deposited on the said forming fabric longitudinal alternate ridges and gullies as the pulp on the fabric is conveyed over the said blade for causing lateral shear in the wet stock to effect a more thorough interlacing of fibres in the wet stock to thereby improve formation.

The invention will be better understood by the following description and the accompanying drawings which illustrate preferred embodiments and in which: Fig. 1 is a perspective view, illustrating schematically the forming section of a conventional paper making machine equipped with foil blades, some of which are modified according to the invention; Fig. 2 shows end and side elevations of a portion of a foil blade modified according to one embodiment of the invention: Fig. 3 shows end and side elevations of a foil blade portion modified according to another embodiment of the invention; and Fig. 4 shows end and side elevations of a foil blade portion modified according to yet another embodiment of the invention.

Referring to Fig. 1, 10 is the side rail of the machine, 11 is the breast roll over which the forming fabric 12 passes, 1 3 is the headbox and 14 is the slice. Numeral 1 5 designates a forming board and 16, 17 and 18 are detachable foil blades of the invention which extend the width of the machine. Numerals 1 9 and 20 are conventional detachable foil blades shown here as they might be placed in a secondary dewatering stage. Numeral 21 denotes a conventional Tshaped rail upon which the individual blades may be mounted. At 22 is shown, generally, the layer of wet pulp stock on the fabric and ridges in the stock produced by the foil blades of the invention are shown as strokes running in the direction of the fabric which is shown by arrow 23.An attempt has been made to indicate phase changes in the ridges as they decay and reappear as gullies with adjacent new ridges on either side. A phase change is most likely to be seen at or just after a blade where a slight vacuum is produced which promotes the interchange as explained previously.

For example, as the stock 22 issues from the slice, small ridges are usually formed which soon dissipate. As the wet pulp passes over blade 1 6 which is slotted according to the invention, variations in dewatering are promoted and, as a result, ridges and gullies form in the stock. The location of ridges will generally correspond to the location of non-foiling portions in the blade. As the ridged pulp approaches the next blade in line, numeral 17 the interchange of ridges and gullies (phase change) has commenced and is accelerated by the dewatering action as it passes over this blade.Blade 1 7 may be a conventional blade or, as is shown in the drawing, another blade of the invention, in which case, it would be preferable to offset the foiling portions in the cross-machine direction from those of the preceding blade to enhance the above-mentioned phase change. Other blades at locations 1 8,1 9 and 20 may be blades of the invention or they may be conventional foil blades.

The preferred T-rail method of attachment allows considerable flexibility to be exercised in the deployment of blades to achieve optimum dewatering efficiency and fibre formation for any given stock and operating condition.

Referring to Fig. 2, numeral 20 is a blade of the invention which has an upstream side 23, a leading edge 21, a flat fabric supporting surface 22 and, commencing at the downstream edge 25 of the fabric supporting surface, a plurality of uniform foiling surfaces 26 interspersed in the cross-machine direction with a plurality of uniform non-foiling surfaces 27. The foiling surfaces 26 diverge downward from the downstream edge of the fabric supporting surface at an angle not exceeding 5 degrees to meet the downstream side of the blade body 24. The non-foiling portions 27 in this embodiment are slots which have been cut at even spacing in the cross-machine direction in such a way that their bottom surfaces 28 diverge downwardly from the downstream edge of the fabric supporting surface at an angle exceeding the divergent angle of the foiling surfaces.The non-foiling slots open into the downstream side 24 of the blade. Numeral 29 is a T-shaped recess in the bottom of the blade having opposed flanges for slidable attachment to a T-bar mounted on the frame of the machine. Numeral 30 is a section of the forming fabric which runs in contact with the support surface 22 in the direction of arrow 31. .

Numeral 32 depicts the layer of wet pulp on the fabric and 33 a ridge forming in the pulp over a non-foiling slot in the blade.

Preferably, but not exclusively, the cross-machine width of the land areas of the foiling surfaces 26 may range from 8 of an inch to 2 inches and the cross-machine width of the slots may range from 8 of an inch to T of an inch. The overall width of the blade, in the machine direction, will be the same as any conventional foil blade and the blade will extend in the cross-machine direction a few inches beyond each edge of the forming fabric.

In Fig. 3 the flat fabric supporting surface 42 extends to the downstream side of the blade and a plurality of identical foiling slots 47 is cut in the fabric supporting surface, each slot having a flat sloping bottom 48 which diverges from the supporting surface at a common alignment shown at 49 which is parallel to and some distance downstream of the leading edge. The extensions of the fabric supporting surface provide intermittent non-foiling portions. The slope of the bottoms 48 of the foiling slots will be less than 5 degrees in order to produce the foiling action required to induce intermittent dewatering in the cross-machine direction.Preferably, but not exclusively, the cross-machine width of the land areas of the non-foiling portions may range from 8 of an inch to 2 inches and the cross-machine width of the foiling slots may range from 8 of an inch to i of an inch.

In Fig. 4 the downstream portion of the fabric supporting surface 52 of blade 50 is slotted with a plurality of identical flat, slanting foiling surfaces 57 which slope in the cross-machine direction and extend from a short distance downstream from the leading edge 51 to the downstream side 54 where the slots present a saw tooth appearance.

The slanting slots provide for intermittent dewatering in uniformly varying degrees in the cross-machine direction. In this particular embodiment of the invention the maximum slope in the machine direction of each of the slanted slots would be less than 5 degrees and preferably, but not exclusively, the width of the slots in the cross-machine direction at the downstream side of the blade could range from about 2 an inch to 2 inches.

In the preferred concept of this embodiment alternate blades may have slots which slope in the opposite direction.

It is not intended to limit the invention to foiling slots having rectangularly or triangularly oriented flat surfaces. The slots in the downstream surface of the foil of the invention may have curved surfaces to provide for varying degrees of dewatering crosswise of each slot.

In order to minimize uneven wear at the downstream portion of the transverse fabric supporting surface which is continuous, it is recommended that the foil blades of the invention be provided with wear resistant inserts as described in detail in U.S. patent No. 3,446,702.

It is within the ambit of the present invention to cover any obvious modifications of the examples of the preferred embodiment described herein, provided such modifications fall within the scope of the appended claims. As a typical example of modifications, it is conceivable that the crosssection of the slots may take other configurations than those shown in the drawings. For example, the foiling or non-foiling slots may be of triangular cross-section, semi-circular cross-section, or any other suitable shape and they may be angled with respect to the machine direction. Also, these blades may be intermixed with conventional foil blades, throughout the wet section and the slot configuration of these blades may also vary one from the other.

Claims (17)

1. A fabric supporting blade for use in a wet section of a paper making machine and positionable in supporting relationship to the forming fabric in the wet end of said machine and extending thereacross transversely of the direction of fabric travel, said blade comprising a body provided with an upstream side, a leading edge, a transverse continuous flat fabric contacting and supporting surface, a discontinuous foiling surface downstream of said continuous flat surface and a downstream side, characterized in that the said foiling surface is provided, in the cross-machine direction, with a plurality of repetitive divergent foiling portions interspersed with repetitive non-foiling portions lying adjacent to each other and providing intermittent dewatering in the cross-machine direction to induce longitudinal ridges and gullies in a layer of wet stock as it is being dewatered for the purpose of causing lateral shear in said stock to effect a more thorough interlacing of the stock fibres to thereby improve formation.

2. A fabric supporting surface as claimed in Claim 1, wherein the unfoiling surfaces are slots cut in the said foiling surface, said slots having bottoms which diverge from the flat support surface at an angle greater than 5 degrees.

3. Afabric supporting surface as claimed in Claim 1, wherein the unfoiling surfaces are coplanar with the said flat fabric support surface and the said foiling portions are slots having bottoms which diverge from the flat support surface at an angle less than 5 degrees.

4. A fabric supporting blade as claimed in Claim 2 or 3, wherein said slot bottoms are flat and further define opposed side walls, said bottoms and side walls tapering from said upstream end.

5. Afabric supporting blade as claimed in Claim 3, wherein said slot bottoms are flat and are slanted in said cross-machine direction and define a single side wall.

6. A fabric supporting blade as claimed in Claim 4, wherein said side walls are perpendicular side walls.

7. A fabric supporting blade as claimed in Claim 5, wherein said single side wall is perpendicular to said downstream surface.

8. A fabric supporting blade as claimed in Claim 5, wherein said openings of said slots are positioned one adjacent the other, said slots extending from and convergent with the downstream edge of said transverse flat fabric supporting surface.

9. A fabric supporting blade as claimed in Claim 1, wherein a plurality of said blades are disposed in a parallel spaced apart configuration under and in contact with said forming fabric in said wet end where fibres in said pulp stock are in suspension.

10. A fabric supporting blade as claimed in Claim 9, wherein said foiling portions of consecutive ones of said blades are offset from one another to enhance a machine direction change of phase of said gullies and ridges where said stock passes over said blades.

11. A fabric supporting blade as claimed in Claim 5, wherein a plurality of said blades are disposed in a parallel spaced apart configuration under and in contact with said forming fabric in said wet end where fibres in said pulp stock are in suspension.

12. A fabric supporting blade as claimed in Claim 5, wherein a plurality of said blades are disposed in a parallel spaced apart configuration under and in contact with said forming fabric in said wet end where fibres in said pulp stock are in suspension, consecutive ones of said blades having their slot slanted bottoms in opposite direction from the slanted bottoms of the slots of adjacent blades.

13. A method of forming a web of paper on a forming fabric of a paper making machine, said method comprising (i) positioning at least one fabric supporting blade under and in contact with said forming fabric in a wet section thereof; said blade having a leading edge, a transverse flat fabric supporting surface, a downstream surface and, in said downstream surface, a plurality of uniform foiling surfaces interspersed by non-foiling surfaces in the cross-machine direction, said foiling surfaces extending from and convergent with the downstream edge of said transverse flat fabric supporting surface and diverging therefrom at an angle less than 5 degrees.

(ii) depositing a layer of wet paper stock on said forming fabric upstream of said blade, (iii) inducing, in said layer of wet stock, longitudinal alternate ridges and gullies as said pulp on said fabric is conveyed over said blade for causing lateral shear in said wet stock to effect a more thorough interlacing of fibres in said wet stock to thereby improve formation.

14. A method as claimed in Claim 13 wherein said step (i) comprises positioning a plurality of said fabric supporting blades under and in contact with said forming fabric with said foiling surfaces of consecutive ones of said blades being offset from one another to enhance a machine direction change of phase of said gullies and ridges where said wet stock passes over said blades.

1 5. A method as claimed in Claim 14 wherein said blades re removably securable under said forming fabric whereby one or more of said blades may be interchanged while said paper making machine is operative and said wet stocks are being conveyed over the remaining support blades.

1 6. A fabric supporting blade substantially as described herein with reference to Fig, 1, 2, 3 or 4 of the accompanying darwings.

17. A method of forming a web of paper on a forming fabric of a paper making machine substantially as described herein.