CA1244697A - Twin wire former - Google Patents

Abstract

TWIN WIRE FORMER
ABSTRACT OF THE DISCLOSURE
A method and apparatus for forming a sheet of paper from a slurry of paper making fibres by applying the slurry between a pair of forming wires and gently draining in a first drainage zone formed by passing said wires under tension over a first forming shoe having a solid forming surface, such drainage being substantially only in one direction through one of the forming wires to form a first thin layer of fibres adjacent the one wire, then draining through the other wire which is in contact with the opposite surface of the wet slurry to form a second thin layer on the second wire in a second forming zone formed by passing the wires in tensioned relationship over a second forming shoe having a solid forming surface thereby to substantially complete the "formation" of said web as said wire traverses the second shoe.

Description

~4~697 TWIN WIRE FORMER
FIELD OF THE INVENTION
The present invention relates to a twin wire former particularly adapted to improve formation under high speed forming conditions by forming two discrete layers one on each of the wires by draining substantially solely in one direction and then substantially solely in the opposite direction each drainage occurring over a discrete forming shoe.
BACKGROUND OF THE PRESENT INVENTION
Many different concepts for twin wire formers have evolved over the years. For example, a twin wire former sold by Dominion Engineering Works under the trade mark Papriformer ~ is a roll type former wherein the jet issuing from the headbox is trapped between a pair of wires at the on-coming nip between the wires formed on a roll and drainage is substantially completed on this roll. Drainage is relatively fast and formation is completed in a very 2U short period of time or distance along the roll surface thereby freezing the fibres in their relative positions as they will eventually appear in the finished web very quickly.
The formation achieved using this equipment is adequate for some purposes.
Other twin wire formers incorporate a ~ '.

~2~4697 forming shoe or shoes which reduced the drainage rate since the curvature of the shoe surface has a much longer radius than the radius that could practically be used for a forming roll. Such use of the longer radius reduced the unit pressure on the slurry between the wires, which thereby reduced the rate of drainage and had a tendency to improve formation. Such a machine was marketed by Beloit under their trade mark ~el Baie ~ and was followed by the Bel Baie II and Bel Baie III formers all of which used forming shoes of one form or another with drainage either mainly in one direction or simultaneous drainage in two directions. All apply pulses to the stock by passing over spaced elements forming the surface of at least one shoe.
seloit also sell selbond ~ formers which are basically a modified fourdrinier machine having a forming shoe mounted above the fourdrinier wire at a distance spaced from the headbox. The forming shoe forms a twin wire section on the machine and is provided with suction boxes etc., so that drainage occurs in the area of the shoe in both directions, i.e., through the top wire and bottom wire simultaneously, the main sequence of drainage being drainage downwardly only, followed by a first shoe which normally permits drainage in both directions and a second or main shoe positioned within the upper wire to force water downwardly through the main fourdrinier wire while at the same time drawing water up through the top wire via an auto slice and suction boxes.
This particular device is primarly used to form multi-layer board grades a low speeds.
Dominion Engineering Works Limited, within the last several years, has sold a ~ynaformer which is a roll type former primarily used as fourdrinier re-build wherein no shoes were necessary.

~2~4697 Such a machine is disclosed in Canadian Patents 1,144,795, issued April 19, 1983 and 1,157,302 issued November 22, 1983 to Malashenko. A number of these machines have been sold and are currently operating to permit the formation of paper at high speed. It is energy efficient in that only rolls are used and forms a sheet having relatively good formation but the machine has limited adjustments to ensure optimum formation when the stock to the machine changes.
Similar type formers to the Dynaformer have been sold under the trade mark Duo-Former H ~ by Voith and Symformer ~ by Valmet oy.
The Black Clawson Company has recently sold a twin wire former under the trade mark Top Flyte R
which is similar in some respects to the former shown in the above referred to Canadian Patents of Malashenko, however, instead of formation occurring on the roll, formation takes place as the twin wires pass between blades that alternately engage opposite wires and apply pulses to the stock and force drainage in two directions at each of the blades. This part~cular former apparently produces paper web having reasonably good formation and provides some adjustability by changing the location of the blades, i.e., the relative position of the edges of the blades to the normal path of travel of the wire that they contact provides an adjustment that may be made if required when the stock is changed.
U.S. Patent 4,433,812, issued July 5, 1977, to Riihinen discloses a machine especially adapted for the manufacture of tissue paper wherein formation is completed very quickly since tissue stock is relatively free draining and the thickness of the sheet being formed is relatively thin. In this particular device drainage occurs downwards from the bottom wire then upwards through the top wire and ~Z~4697 continues to drain upward through the top wire until formation is completed. Obviously since the machine relates to a tissue machine, formation is not a prime objective and the considerations for drainage etc., and formation are significantly different than for a newsprint sheet or a fine paper sheet.
Attention is also directed to U.S. Patent 4,154,645, issued May 5, 1979 to Kankaapaa. The device disclosed in this patent is a machine for making a multi-layered paper boards. The formation of each layer is accomplished by a drainage sequence wherein drainage initially takes place through a first or lower wire, is followed by two way drainage which in turn is then followed by one wa~ drainage through the top wire to complete formation.
Generally the above described devices in one manner or another apply sharp pulses to the stock during formation for example as the stock passes the doctor like element of the Top Flyte ~ former or over spaced element forming the surface of the shoes of most of the other formers.
It has also been recognized in the art that relatively slow drainage is important to formation thus drainage over the initial section of a fourdrinier wire wherein the thin layer is formed, is taught for example in Canadian Patent 983,750, issued February 17, 1975, to Kankaapaa. In this particular patent, drainage initially is downward through the wire at a relatively gentle rate until formation of the total thickness of the web is completed and then dewatering takes place over a pair of consecutive shoes which drive the water first in one direction and then the other from the web, i.e., formation is completed by slow drainage in one direction only, namely through the bottom wire on a fourdrinier section.

3 Z~4697 It is also known from Canadian Patent 930,580 issued July 24, 1973 to McCormick to form a web by forcing water through the top wire of a twin wire former over an extended length of soild shoe having progressively decreasing radius of curvature and scooping the water forced from the web from the top wire without applying pulses. In this case formation takes place against one wire only and the pressure is gradually increased as required to drive the water through the layer of formed pulp as this layer increases in thickness.
BRIEF DESCRIPTION OF THE PRESENT INVENTION
It is an object of the present invention to provide an improved twin wire former particularly suited for the manufacture of fine paper, and newsprint or the like.
Broadly the present invention relates to a twin paper forming machine comprising a first forming wire, forming a first continuous loop and a second forming wire forming a second continuous loop, means to feed low consistency paper making stock containing paper making fibres and water between said forming wires, a first forming shoe having a solid forming surface within the loop of said first forminy wire and engaging the inner surface of said first forming wire, said second forming wire pressing said stock between said wires against said first shoe to cause water to drain from said stock through said second forming wire and form first formed layer of stock thereon, a second forming shoe having a solid forming surface mounted within the loop of said second wire and engaging the inner face of said second wire said second shoe being spaced in front of said first shoe in the direction of travel of said wires, said solid surface of said first shoe having a curvature of at least 50 inches radius and having a length sufficient so that between 25 and 70% of the water in the stock is drained over said first shoe without completing formation, said shoes being arranged so that said wires curve smoothly over each said shoe and between said shoes with no major abrupt changes in direction thereby to gently drain said stock without subjecting said stock to significant pulses.
Preferably the lenyths of the working wraps of said shoes measured in the machine direction will be between about 8 and 15 inches and the spacing between adjacent shoes will be between about 1/2 and 3 inches so that formation can be completed on the second shoe in a forming zone less than about 30 inches in length measured from the beginning of the first shoe to the end of the second shoe.
BRIEF DESCRIPTION OF THE DRAWINGS
Further features, objects and advantages will be evident from the following detailed description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings in which:
Figure 1 is a schematic side elevation view of one form of the invention;
Figure 2 is a view similar to Figure 1 showing a modified forming section;
Figure 3 is a view similar to Figure 1 showing a modified dewatering section;
Figure 4 is an enlarged schematic illustration showing how formation is believed to take 30 place when using the present invention.
Figure 5 is a schematic arrangement of another form of the invention.
DESCRIPTION OF THE PXEFERRED EMBODIMENTS
Before describing the present invention it should be noted that terms such as formation, formed and forming as used in the disclosure and claims ~Z~4697 unless otherwise indicated are intended to be read in their paper making sense, that is designate that the fibres are fixed in their relative positions substantially as they will be in the finished or dried sheet, i.e. the random distribution of fibres in the finished sheet is fixed when "formation~ is completed. After formation has been completed the sheet is simply dewatered with only minimal changes in fibre location, for example by stretching and the like as the sheet traverses the driers, calendars etc in the paper making process. Generally formation is completed when the stock reaches about 4% consistency.
As shown in Figure 1, former 10 of the present invention comprises an upper forming wire 12 and a lower forming wire 14 each of which forms a continuous loop. These upper and lower wires pass over a pair of breast rolls 16 and 18 respectively which may be either solid or open rolls and stock is ejected between these wires from the headbox generally indicated at 20.
In the arrangement shown in Figure 1 the wires 12 and 14 pass from the breast rolls 16 and 18 and converge over a shoe 22 having a solid forming surface 24 which deflects the wires 12 and 14 so that the tension in the wire 14 applies pressure against the stock between the wires 12 and 14 and enforces drainage into the saveall 26 appropriately positioned to receive this draining water. If desired a forming board shown in dotted lines at 28 maybe provided adjacent the lower breast roll 18 however this is not essential and is dependent to a degree on the length of the incoming run from the breast roll 18 to the shoe 22. The majority of drainage through the bottom wire will occur as the wires traverse the shoe 22.
From the shoe 22 the wires pass onto a second shoe 30 having a solid forming surface 32 ~Z~4~97 curved in the opposite direction to the surface 24 over which the wires 12 and 14 pass with the tension in the wire 12 forcing water up through the wire 12 to be scrapped from the wire 12 via scrappers or doctors generally indicated at 34 that preferably just contact the wire and into a saveall generally indicated at 36.
The wires pass from the forming shoe 30 to a solid roll or as illustrated to an open roll 38 which causes drainage in two directions namely out through the wire 12 and into the saveall 40 and into the roll 30 and down into the saveall 42. The open roll will only be used where the amount of dewatering demands d~ainage in two directions. The wrap on the roll 38 is not critical but will generally be in the order of 20 to 60 degrees.
From the roll 38 the wires pass over a roll 44 which will normally be a solid roll although in certain circumstances might be a suction roll or open roll provided with suitable water clearing equipment such as savealls and suction glands etc.
The use of such more expensive rolls will normally be avoided since it is not believed to be necessary.
Generally the roll 44 will be a solid roll and drainage will take place through the wire 14 into the saveall 42.
After the roll 44 the wires 12 and 14 are separated with the web following with the wire 14 to be carried to the normal couch roll etc (not shown).
It will be apparent that suitable guide roils such as those indicated at 46 will be provided for the upper wire 12 and those indicated at 48 for the lower wire 14 to complete the loops.
In the illustrated arrangement the shoe 22 is mounted on leading and trailing variable length pillars 50 and 52 so that the angle of the shoe 22 may be adjusted as well as its vertical position relative ~L244697 to the wire path i.e. it can be bodily raised or lowered. Similarly the shoe 30 will be mounted on leading and trailiny adjustable length pillars 54 and 55 which permits its angle of orientation to be adjusted and operating both the pillers in unison its height or position relative to the breast rolls may be changed.
It is important that there be no major abrupt change in direction of the wires as they approach or leave the shoes 22 and 30 and thus the leading end 58 of the shoe 30 must be positioned relative to the trailing end 60 of the shoe 22 so that there is no major abrupt change in direction of the wires as they pass from the shoe 22 to the shoe 30.
It has been found that the leading end 58 of shoe 30 may deflect the wires but up to 1/16 of an inch i.e.
"dig into" the wire by distance of up to 1/16 of an inch measured substantially perpendicular to the direction of wire travel and that such a defection will tend to increase drainage with little effect on formation. Slightly greater deflection has been seen to tend to crush the paper web being formed.
The curvatures of the surfaces 24 and 32 will be such that they will have radiuses of at least 50 inches probably not over 250 inches and generally closer to 100 inches say between 80 and 150 inches. Obviously, the precise curvature of the shoe will depend on the rate of the drainage desired (the greater the radius the slower the rate of drainage for any given tension in the outside wire passing over the shoe and this determines the unit pressure applied by this outside wire to cause drainage therethrough).
If desired the surface 32 of the trailing shoe 30 may be divided into a first section 62 that is substantially planar and has a length of about 2 to 4 inches followed by a second section 64 that is curved ~Z~697 as above described and provides the forming section of this shoe. The working wrap of the shoe will be about 8 to 15 inches in length measured in the path of wire travel. The working wrap or working length of a shoe is that portion of the wrap of the shoe by the wires in which the tension in wire 14 drives water from the stock through the outside wire.
The surface 24 of the shoe 22 may be similarly formed with a straight or planar lead ln section substantially equivalent to the section 62 at its lead in end and a curved forming section equivalent to the section 64 i.e. both of the shoes 22 and 30 may be provided with similar formi~g surfaces 24 and 32 respectively. The working wrap of shoe 22 will also be between about 8 to 15 inches in length measured in the path of wire travel. More generally the length of working wrap of the wires on the shoe 22 will be such that the total drainage over the shoe 22 will drain 25 to 70~ of the water in the stock leaving the headbox without completing formation.
A further embodiment of the present invention in shown in Figure 2. In this embodiment the first forming shoe 22' is substantially equivalent to the shoe 22 but is located within the loop of the bottom wire 14 so that drainage is upward through the top wire 12 first, with the water pressed from the slurry via the wire 12 passing up the ramp generally indicated at 66 into the saveall 68 substantially equivalent to the saveall 36, as described hereinabove. Mounted below the saveall 66 is shoe 30' that may be substantially equivalent to the shoe 30 described hereinabove but located within the top wire 12 to force drainage through the bottom wire 14.
Because the water does not form a puddle on the bottom wire as it does on the top wire when drainage is downward the length of shoe 30' in the machine ~Z44697 direction may be extended beyond the above described 15 inch maximum length.
Another embodiment of the present invention is shown in Figure 3. This particular arrangement is substantially the same as that shown in Figure 1 however instead of the roll 38 being located within the loop of the bottom wire 14 a similar roll 70 is located within the loop of the top wire 12 to cause drainage downwardly into the saveall 72 and if an open roll is used upwardly into the saveall 74 positioned behind the second roll 76. This second roll 76 is equivalent to the roll 44 but it is located within the loop of the bottom wire 14 and forces water up through the top wire 12 into the saveall 74.
Obviously, the arrangement in Figure 2 may also be used with the roll arrangement shown in Figure 3 i.e. with the rolls 70 and 76 replacing the rolls 38 and 44 respectively and located within the loops of the different wires 12 and 14.
In the above described embodiments the rolls 38 and 44 or 70 and 76 have been used for dewatering the stock after the forming shoes, i.e.
after formation has been completed. These rolls could be replaced by a second pair of solid forming shoes that further dewater the stock. The advantage of such shoes is reduced cost and reduction in shear that accompanies wrapping of the two rolls by the two wires.
An example of an arrangement using shoes in place of the rolls is illustrated in Figure 5 in combination with the Figure 2 arrangement but wherein the forming shoes 22' and 30' are followed by dewatering shoes 100 and 120 which dewater the formed web while keeping the wires in a more or less straight line path.
Obviously be repositioning the shoes 100 and 102 the shoes may be used to replace rolls in the ~LZ~46~7 Figure 1 embodiment.
Referring now to Figure 4 the operation of the device will be described particularly for the drainage over the two shoes 22 and 30 wherein formation is substantially completed before the wires leave the shoe 30.
As shown in Fiyure 4 stock issues from the headbox 20 as indicated by the arrow 80 between the two wires 12 and 14 as they converge on the shoe 22.
Water is driven through the wire 14 and drained into the saveall 26 over the curved section of the surface 24 of the shoe 22 to form the first layer 82 of formed fibres adjacent the wire 14. The two wires then pass through an open stretch generally indicated at 84 in Figure 4 which normally will be between approximately half an inch to about 3 inches in length (there being little point in extending the spacing 84 much beyond three inches) to flow onto the surface 32 of the shoe 30 without, as above defined, any major change in direction of wire travel. As the wires pass over the shoe 30 the tension in the wire 12 forces the wire 12 towards the shoe 30 in the curved section 64 thereof to force water to drain through the wire 12.
This water is then scraped by a doctor blade 34 which just contacts, or as above described may defect the wire up to 1/16 of an inch, the inner surface of the wire 12 to deflect the water into the saveall 36. The water draining through the upper wire 12 forms a second layer 86 adjacent the wire 12. As this layer increases in thickness due to drainage through the wire 12 the amount of water throughout substantially the whole thickness of the slurry between the wires is reduced and consistency increases so that formation is substantially completed as the wires leave the shoe 30 as schematically illustrated at 88 in Figure 4 i.e.
the formed layers span the gaps between the wires 12 :~L2~4697 and 14.
The total drainage over the first shoe 22 or 22' should not complete formation of the total thickness of the web and normally should only result in drainage of between 25 and 70% of the water in the stock leaving the headbox.
It will noted that as the wires traverse the respective shoes 22 and 30 initially there is no or substantially no build up of fibres or formed layer on the wire through which drainage is forced as the respective shoe is traversed i.e. as the shoe 22 is traversed initially there is substantially no layer 82 and this layer gradually builds up as the two wires with the slurry therebetween pass over the shoe 22.
The shoe 22 has sufficient curvature that the tension in the wire 14 gradually drives water through the wire 14 into the saveall 26. Obviously this drainage is also aided by the influence of the gravity, thus the curvature of the forming section or curved section 64 may be of a greater radius than the equivalent curved section 64 of the shoe 30 since drainage over the shoe 30 must overcome gravity and be forced through the upper wire 12.
It will be evident since the working length or working wrap in the machine direction of the curved section 64 of shoes 22 and 30 will be in the range of about 8 to 15 inches preferably 10 to 14 inches and that the total forming length from the beginning of the shoe 22 to the end of the shoe 30 may be as short as about 2 feet i.e. ~formation" rnay be completed in about 2 feet of running length of the wires yet relatively gentle drainage obtained with the resultant formation being equivalent or better than that obtained in using much more extensive forming sections or that obtained using rolls.
Having described the invention modifications ~Z44~i9'7 will be evident to those skilled in the art without departing from the spirit of the invention as defined in the appended claims.

Claims (14)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A twin wire paper former comprising a first forming wire forming a first continuous loop and a second forming wire forming a second continuous loop; a first forming shoe having a solid forming surface within the loop of said first forming wire;
means to feed low consistency stock containing paper making fibres and water between said wires as said wires converge on said first forming shoe, means for tensioning said second wire pressing said stock toward said first shoe to cause said stock to drain through said second forming wire and form a first layer of formed fibres from said stock; a second forming shoe having a solid forming surface; said second forming shoe being positioned within the loop of said second forming wire;
means for tensioning said first forming wire pressing said stock against said second forming shoe to drain said stock through said first forming wire and form a second formed layer of fibres from said stock on said first wire; said solid surface of said shoes having curvatures of at least 50 inches in radius and said first shoe having a length in the direction of wire travel correlated with its curvature and tension in said second wire so that drainage over said first shoe drains between 25 and 75% of the water in the stock fed between the wires but insufficient water to complete formation; said shoes being arranged to gently drain said stock first in one direction and then in the other with initial drainage in each direction being substantially unobstructed.

2. A machine as defined in claim 1 wherein said shoes are arranged such that stock between said wires is not subjected to any major abrupt change in direction of wire travel as said layers are formed.

3. A machine as defined in Claim 1 wherein each said shoe is between 8 and 15 inches in working wrap length measured in the direction of wire travel and wherein said second shoe is spaced from said first shoe in the direction of travel by a distance of between 1/2 and 3 inches.

4. A forming machine as defined in Claim 1 wherein said first wire is an upper forming wire and wherein a scrapper trails said second shoe and directs water expressed from said stock through said first wire into a saveall located within the loop of said first wire.

5. A machine as defined in Claim 3 wherein said first wire is an upper forming wire and wherein a scrapper trails said second shoe and directs water expressed from said stock through said first wire into a saveall located within the loop of said first wire.

6. A machine as defined in Claim 3 wherein said shoes are arranged such that stock between said wires is not subjected to any major abrupt change in direction of wire travel as said layers are formed.

7. A machine as defined in Claim 2 wherein said first wire is an upper forming wire and wherein a scrapper trails said second shoe and directs water expressed from said stock through said first wire into a saveall located within the loop of said first wire.

8. A machine as defined in Claim 6 wherein said first wire is an upper forming wire and wherein a scrapper trails said second shoe and directs water expressed from said stock through said first wire into a saveall located within the loop of said first wire.

9. A machine as defined in Claim 1 wherein said first forming wire is the bottom forming wire wherein a scrapper leads said second shoe and directs water separated through said second forming wire over said first shoe into a saveall.

10. A machine as defined in claim 2 wherein said first forming wire is the bottom forming wire wherein a scrapper leads said second shoe and directs water separated through said second forming wire over said first shoe into a saveall.

11. A machine as defined in claim l wherein said first shoe is between 8 and 15 inches in working wrap length measured in the direction of wire travel.

12. A machine as defined in claim 2 wherein said first shoe is between 8 and 15 inchdes in working wrap length measured in the direction of wire travel.

13. A machine as defined in claim 9 wherein said first shoe is between 8 and 15 inches in working wrap length measured in the direction of wire travel.

14. A machine as defined in claim 10 wherein said first shoe is between 8 and 15 inches in working wrap length measured in the direction of wire travel.