Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
The subject invention concerns a multi-layer dewatering medium for
forming paper sheets. Each layer of the medium comprises polymer material
strands at least in the machine direction. These strands may consist o~
S in~erwoven thread material but could also be extruded strands alone or
com~ined with woven thread materials.
In the forming of paper sheets two different kinds of techniques are
used today. The con~entional and most common technique involves
discharging an acqueous suspension of cellulose fibers onto a forming
10 fabric which travels horizontally and in a fla~ condition during the `
forminy phase proper. The water flows through the for~ing fabric and the
restricted mesh size of the fabric prevents the fiber material from
penetrating into and through the forming fabric. The fibers remain on ~ -
the surface of the fabric, evenly distributed thereon, whereas the
dewatering elements, such as suction boxes and foils, facilitate the
water through-flow. At the end of the flat, horizontal part o~ the -
forming fabrîc loop the fibers have formed a continuous fibrous web.
This type of forming ls generally re~ferred to as Fourdrinier forming.
Aocording to the second applied technique a jet of fibrous
; 20 suspension is discharged into the gap between two mo~ing forming elementsat~l~east one~of whlch~consists of~a forming fabricO The ~ater is removed
by~the~fo~rming elements whereas the fiber material is retained by sald
elements. This ~ype of formation~ls generally referred to as ~win wire -
orming.
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In both forming methods multilayer forming fabrics are generally
used as the forming elements. The stability of multi-layer forming
fabrics is largely superior to that of single-layer fabrics and
particularly in Fourdrinier machines it is desirable to be able to
combine a fine-mesh Forming surface layer with a bottom surface layer
having sufficient wearability. These properties are difficult to combine
in a single-layer fabric. In double-layer forming fabrics the layer
closest to ~he material to be formed could be made from a fine-diameter
thread material, resulting in a fine mesh density whereas the layer
closest to the dewatering elements may be made ~rom a coanser and thus
more abrasion-resistant material.
- In the bottom layer of the fabric which is intended to travel in
contact with the dewatering elements of the machine, it is also customary
to use a weave pattern of a kind which improves the wearability of the
fabric.
To improve weara~ility, it is sugges~ed in SE 74 12 722-b to bring
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the warp knuckles or lengthwise knuckles to a posi~ian in~eriorly of the
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weft knuckles, i.e. the trdnsverse knuckles. Thus, it is the latter
transverse knuckles which ~orm ~he contacting layer against which ~he
20 dewatering elements exert an a~rasive effect.
Also~EP~0 046 899 dlscloses a double-layer forming fabric the weft -~
ayer~or ~ransverse layer of which forms the outer layer which is in
contact with~the dewatering elemènts.~
A number of German Patent Publications describe double-layer forming
25~ fabrics, e.g. DE 30 36 409,~ DE 31 45 385g DE 32 24 187~ DE 42 24 236,
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DE 33 01 810, DE 33 05 713. From all of these publications it is apparent
that the weft or transverse yarn forms the outer layer against which the
dewatering elements exert their abrasive effect.
The technique of designing and constructing mul~i-layer ~abrics has
always resided in exposing the transverse weft threads on the rear face
~o the abrasive effects. The purpose of the lengthwise threads is to
absorb the wire tension and for this reason these threads may be worn
only to a limited degree. However, as no load is exerted on the fabric
in the transverse direction, abrasion of the transverse threads may
continue until close to the wear-through point of these threads before
the fabric is considered to be worn out. This is the reason why all
modern forming fabrics are manufactured wi~h transverse bottom threads
serving as the abrasive medium. Identical structures have also been used
in the twin-wire forming method a1though in this case the abrasion is
not quite so strong.
~hen the speed of twin-wire machines is increased, minute holes may
for- in the paper web at a certain speed level. This has long been~a -
; ~ problem in sertain types of twin-wire machines and has reduced the --
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production speed of~such machines. A large number of experiments, none of
20~ them success~ul, were made before the conventional fabric structure was
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replaced by~the fabric~structure l~n accordance with the ~eachings o~ the ;~
subject inventi~on.~
SUMMARY OF THE::INVENTION
The~dewatering~medium;for~forming paper she~ts in accordance with
25~ the subject~invenh~on~i;s o~f mul~1-layer~type and~con~i~sts of strands of
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polymeric material, which medium has on its one side a first surface
cons~ituting the forming surface of ~he medium, which sur- face in
position of use oF the medium faces the material to be formed, and on its
opposite side a second surface. The dewa~ering mediu~ is characterized
therein that at least in the machine direction the layer closest to the
second surface is made up from strands, that in the machine direction
continuous or discontinuous lengths (floats~ of said strands abut on said
second surface, that the longest ones of said len~ths of strands in the
machine direction are longer than discontinuous lengths (floats) of
strands whlch are repeated in the cross-machine direction and which abut
10 on said second face of the medium. ;
The above problem of hole formation in the paper web in twin wire
machines ;s entirely eliminated in the ~ewatering medium in accordance
with the subject invention and the machine speed may be increased w;thout
difficulty. An ana1ysis o~ the reasons For the hole formation found in
prior-art technology medium has proved to be that the transverse knuckles,
between the forming roller and the separation roller,act as "water wheel
buckets" and ~plash water between the separation roller and the fabrics.
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The water, which is in the form of droplets, is forced through the
fabrics by the separatlon roller and into paper sheet, causing holes to
O form therein~ At lower machine speeds $he splashing is less pronounced
and does not cause any noticeable problems but the splashing increases
w1~h increasing machlne speeds.
Once the problem had been solved in twin-wire machines attemps were
made ~o find ou~ whether a~correspondi~ng structural change would affect
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the forming process in Fourdrinier machines. For this reason a forming
fabric ~as manufactured in the conventional manner with the abutment
knuckles extending transversely. From the same piece of fabric was cut a
fabric extending in the other direction, i.e. such that the former
~ransverse abutment knuckles beca~e lengthwise abutment knuckles. Quite
unexpectedly, it was found that the paper sheet which was formed on the
latter fabric exhibited less marking. Also in this case the elimination
of the splashing caused by the "water wheel phenomenon" is believed to be
the reason for the improvement. The transverse "water wheel knuckles" ~-
splash water back against the rear side of the fabric, thus dis~urbing
the paper sheet forming process. The problem will be discussed in full in
connection with the following description of the drawing figures.
BRIEF DESCRIPTION OF THE DRAWINGS
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The invention will be described in closer detail in the following
with reference to the accompanying drawings, wherein
Fig. 1 is a general schematical view showing a twin-wire papermaking
machine of roll-~ormer type,
FigO 2 is a general schematical view showing a Fourdrin;er-type of
papermaking machine9
2~ Fig. 3 is a perspective view of a forming medium in the form of a
forming fabric comprising two transversely extending thread systems which
are interconnected by one lengthwise ex~ending thread system,
Fig. 4 ls a plan view of the;bottom face of the forming fabric of
Fig~ 3
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Fig. 5 is a perspective view of a second forming medium in the form
of a forming fabric comprising doub1e thread systems in its lengthwise as
well as in its transverse direction,
Fig. 6 is a plan view of the bottom face of the forming fabric in
accordance with Fig. 5, and
Fig. 7 illustrates a forming med;um consisting of an extruded
bottom layer on which is superposed a woYen forming layer.
DETAILED DESCRIPTON OF PREFERRED EMBODIMENTS
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Fig. 1 illustrates a twin-wire machine 11 of roll-former type.
10 From the head box 12 the fiber stock i5 discharged into the gap between -
the upper fabric 13 and the bottom fabric 14. The fabrics 13 and 14 with
' the fiber stock between them i5 carried over part of the cylinder face of
a forming roller 15 for removal of the water through the bo~tom fabric
14 down into the vat 16. When the fabrics leave the forming roller 15,
the forming process should, in principle, have been completed. The moist
paper sheet, still positioned intermediate the two fabrics, is then
carried to a separat~on roller 17. When a bot~om fabric 14 of a
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conventional type is used, this system operates without problem up to a
certain speed limit. At higher speeds, however, a certain amount of water
~ 20 accompanies the fabric up to the separdtion roller 17. This has proved to
- ~ be due ~o the "wa~er wheel bucket ef~ect" of the ~r~nsverse threads on
he rear face o~ the bottom fabric 14. These transverse threads "splash"
water against the separatio~ roller 17. When ~he two fabrics move into
contact with the face of the separation roller 17, the wa~er droplets on .
25~ the~rear face of the~bottom fabric 14 are forced thr~ugh the fabric 14
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and against ~he paper sheet intermediate the fabrics~ causing minute
holes to form in the paper sheet. When the bottom fabric 14 is
manufactured in accordance with the invention with predominantly
lengthwise extending strands the "water wheel splashing effect" will be
eliminated and the rear face of the fabric will be free of water droplets
when the two fabrics are carried into contact with the separation roller
17. In case a small amount of water should nevertheless accompany the
fabric this water will collect intermediate the lengthwise strands
withcut being forced through the fabric and against ~he paper web.
The problem caused by the above "water wheel splashing effect" has ~--
been discovered ~nd remedied by the subject invention primarily with -
regard to twin-wire machines. However, it has been found that a similar
"water wheel splashing effect" may cause problems also in machines of
Fourdrinier type. This wlll be explained in the following with reference
to Fig. 2. In a Fourdrinier-type of machine 21 an endless fabric 22
travels in a continuous path around a series of rollers. From ~he head
box 23 the~s~ock is dlscharged onto a horizontally travelling part of the -
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fabric 22. The pap~r web forming process takes place on this horizontal ~-part of the fabric~loop. The fabrlc 22 travels in contact with dewatering
: ~ ~20~ elements~ such as table rolls 24,;foil~s 25 and suction boxes 26. Owing ~o
the ~"water~wheel splashing effect" water is thrown în between these
elements and~the~form~ng~fabric~Water may then be forced back through
the fabric~and~agai;nst~the paper sheet being formed. Thus, the forming - -
proces~s~is~disturbed,~ resulting ln marking of ~he paper web.
25~ The for~ing~me~i~u- in~ the f-rm of the forming fabric 30 shown in
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Fig. 3 consists of one system of threads 32 which extend at right angles
to the direction of travel 31 of the fabric and which threads 32 abut on
the first surface 33 of the fabric facing the paper web. A second, system
of cross-machine direction threads 34 extend in parallel with the system
of threads 32 and abut on the secon~ surface 35 of the fabric. A third
system of threads 36 extend in the machine direction 31 and at right
angles to the systems of cross-machine direction threads 32 and 34,
interconnecting the latter in to a weave and alternately running between
the first surface 33 facing the paper web and the second surface 35
facing in the opposite direction.
The invention is restricted to the layer closest ~o the second
surface 35. As appears from the plan view of Fig. 4 each
machine-direction thread 36 extends externally across four consecutive
cross-machine direction threads 349 forming discontinuous strand lengths
(floats) 37 in the machine direction of the forming medium. Each
cross-machine direction thread 34 extends externally across one or two
machine-direction threads 36, forming discontînuous short strand lengths
i 38 and sligh~ly longer s~rand lengths 39 in the cross-machine direction
of the forming fabric.
In accordance with a second embodiment the formin~ fabric 40
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comprises a system of cross-machine directions threads 42 which extend at
right angles to ~he direction of travel 41 of the fabric and which system
of threads abut on the first surface 43 of the fabric which faces the ~ ~;
paper web being formed. A second system of cross-machine direction
threads~44 extend in parallel with the first system of threads 42 and
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abut on the second surface 45 of the wire. The forming fabric also
comprises two systems of threads 46 and 47 which extend in the lengthwise
direction of the machine. The system of threads 46 faces the first
surface 43 of the fabric and the sys~em of threads 47 faces the second
S fabric surface 45.
The plan view of Fig. 6 illustrates the layer of threads which abut
on the second surfa~e 45 of the fabric. Each machine-direction khread 47
ex~ends externally across three consecutive cross-machine direction
threads 44, forming discontinuous lengths ~floats) of strands 48 in the
machine direction 41 of the forming fabric. Each cross-machine direction
thread 44 extends externally across only one machine-direction thread 47,
forming discontinuous strand lengths tfloats) 49 in the cross-machine
direction of the forming medium.
A further embodiment is illustrated in Fig. 7. The forming medium
50 shown in this drawing figure consists of a weave Sl comprising machine
direction threads 52 and cross-machine direc~ion ~hreads 53 which, when
interwoven, make up the first forming fabric surface 54 which faces the
paper web. The o~her side of the fabric exhibits continuous strands 55
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which are extruded in the machlne direction and which abut on the second
surface 56 of the medium. These continuous lengthwise strands may be
interconnected by~means of.transversely extending interconnection strands
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The invention is not limited to the embodiments which are
llustrated~and~descri~bed herein but a~number of modifications are
25 possible within the scope of the appended claims. Thè invention is ~ :
applicable to other types of dewatering media than weaves, e.g. to a
structure comprising a per~orated foil and underlying strands. - -
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