Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND 0~ THE INVENTION
In the production of paper in papermachines,
a slurry of fibres is discharged onto a fabric having
a network-like structure The water is drained off
from the fibrous mass through the opsnings in the
fabric. On the upper face of the fabric, the fibras
are formed into a sheet. As the flow-through of the
water takes place at the points of the fabric where
the yarn material of the fabric meshes does not bar the
; 10 flow, it is of utmost importance that these through-
-flow points are evenly distributed over the entire
fabric area. The permeability of the forming fabric must
be of a certain magnitude while at the same time the
fabric surface must be very fine-meshed in order to
" 15 avoid marking of the sheet being formed and loss of
fibres. A fine-mesh fabric comprising thin threads is,
however, less resistant to wear and 1BSS stable and as
a consequence thereof its useful life is greatly
reduced Up to the beginnings of the 1960's, only
single layer fabrics of metal were used for the sheet
formation. To some extent, metal screens wsre replaced
by single-layer fabrics of synthetic fibre threads, the
so-called synthetic fabrics, which are more wear-
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-resistant but less stable. On account of the
reduced stability of such fabrics, these single-layer
synthetic fabrics could not be used in large and
high-speed machines. Only with the arrival nf
so-called double-layer forming fabrics could synthetic
fibre materials oe used to a larger extent in these
large and high-speed paper machines. In machines of
this kind, the forming fabric is exposed to
considerable tensile stresses which the fabric must
be capable of absorbing without stretching lengthwise
or contracting crosswise to such a degree that the
fabric can no longer be used for its intended purpose.
The so-called doubls-layer forming fabrics that
- are in practical use consist of two layers of synthetic
weft threads as well as synthetic warp threads inter-
connecting the weft layers. In fabrics of this kind,
the conflicting requirements on the one hand of a
fine-mesh forming face comprising fine threads, and
on the other hand of a wear face wherein the threads
are coarser and more wear-resistant are met to some
extent. This could be achieved by using threads of
differing dimensions and/or different fibre materials
in the two weft layers. However, these cunflicting
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requirements cannot be met by the single thread
layer in the opposite direction, viz. the warp thrsads
that interconnect the weft layars. From a sheet-
-forming point of view these warp threads should
preferably be as fine as possible but in their
capacity as machins direction threads they must be
capable of absorbing any tensile stresses that might
occur without stretching to an unduly high degree.
Every expert designer in the field aims at
providing a forming fabric which consists of two
layers of complete fabrics, each one with its individual
ssts of warp threads and weft threads, which fabrics
are joined together. The part of the fabric that is
closest to the material to be formed as a rule consists
of fine threads arranged in a fine-mesh structure
whereas the bottom part consists of coarse and more
wear-resistant threads arrangsd in a more coarse-mesh
structurs. The reason that this aim has been
unattainable in practice depends on the difficulties
2û to interconnect the two wire parts without disturbing
the arrangement of the fine-mesh fabric part to such a
degree that the disturbance of the latter will reflect
on the paper sheet in the form of marking.
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From the US Patent Specification No, 3 127 308
is already known a forming fabric which comprises one
fine-mesh face closest to ths paper web and a coarse-
-mesh face serving as the wear side. These two fabric fa-
ces are structured as complete weaves which arewoven together with one another. The Patent Speci-
fication referrsd to suggests interweaving by inter-
connection of the two fabric parts 61 and 62 by means
of the coarse warp thread 61a. Another prior-art
suggestion is to interweave the two complete fabrics
by means of separate warp binder threads.
In these prior-art structures the two weaves
thus are interconnected either by warp threads that
are a part of one of the weaves, or by separate warp
threads. During the manufacture of weaves of forming
fabric type, the weft threads normally extend in a
straight condition inside the weave body, whereas the
warp threads have a sinuously curved configuration and
form the very face of the fabric. However, by
stretching the fabric it is possible to change the
curvature in the subsequent heat-fixation process,
whereby the sinuosity of the warp threads is reduced
while the curvature of the weft threads is enhanced.
A process of this kind cannot, however, but to a small
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sxtent eliminate the unevenness of the warp threads
given to thsse threads during the weaving opsration.
When a warp thread in the fine-mesh fabric part is
used as the interconnecting thread, this causes
unevenness on the sensitive paper-facing side of
the fabric, when this warp thread is to bind, at
equal intervals, with the bottom side of the fabric.
If a warp thread in the coarse-mesh part of the fabric
or a separate warp thread is used as the interconnecting
thread, unevenness occurs on the paper-facing side of
the fabric, when this thread is to bind with the
fine-mesh fabric part which faces the paper web. These
unevenness which is caused by interconnection with
the aid of warp threads, is sufficient to cauæe a marking
effect on paper of qualities that are particularly
sensitive thereto, such as e g. newsprint paper.
In addition, interconnection by means of warp
threads is considerably more complicated from a
manufacturing-technical point of view, sincs the binder
threads has a size in woven condition that differs
from the rest of the warp threads, on account of the
difference in geometrical configuration. ~inder warp
threads therefore must be warped in a particular
warp-beam and be driven at a different speed from
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that of the r~st of th~ warp threads.
SUMMARY OF THE INVENTION
The subjsct invention concerns a forming fabric
for papermaking, cellulose and similar machines and
which fabric is of the kind comprising a first set of
warp threads and a first set of weft threads which
are interwoven to form a first complete weave, and a
second set of warp threads and a second set of weft
threads which are interwoven to form a second complete
weave. It is characteristic of the invention that
separats binder weft threads are used to interconnect
said first weave and said second weave, which binder
wsft threads are positioned between the two weaves
and interweave with threads from said first and said
second sets of warp threads.
9ecause the weft threads lie comparatively straight
inside the weave body after and during the weaving,
as mentioned abovs, the binder weft threads will also
be in the same position, and the sinuous warp threads
on both sides of the fabric will twine about these
binder threads. The interweaving points proper
therefore will be positioned inside the weave body and
not, like in warp-bound structures, on the surface
thereof. The surfaces of the fabric, and particularly
the facs which is turned towards the web of material
to be formed, will not be effected by the inter-
~` weaving points. One has also found that fabrics
wherein th~ face that is turned towards the material
web is even and which are free of binding (or inter-
weaving) point disturbancs in ths raw weave stage of
the fabric, have a face that is more even and smoother
also after exposure of the fabric to stretching during
; the heat-fixation process. The function of the binder
, 10 threads in the fabri~ is merely to join together the
top and bottom cloths. Their dimensions and material
therefore could be different from the rest of the
thread material. Prsferably, a finer and softer thread
material is used for the binder thread.
ORIEF DESCRIPTION OF THE DRAWINGS
The invention will be described in closer detail
` in the following with reference to the accompanying
schematical drawing of which Figs. 1 to 5 are sectional
views along the warp threads and illustrate different
2û binding patterns.
DETAILED DESCRIPTION OF A PREFERRED EMSODIMENT
The forming wire in accordance with the invention
is woven endless in the usual manner of the art or
- joined endless. In the structure of the fabric when
the fabric is wovan endless, the machine direction
threads of the weave are wsft threads whereas the
warp threads form the crosswise yarn system in the
position of use of the fabric. In the fabric structure
joined endless the situation is the reverse.
The embodiments shown in Figs. 1 to 3 illustrate
a comparatively fine-mesh weave 1 which in position
of use of the fabric is to face the material to be
formed. This weave - in the following referred to as
- 10 the top cloth - is woven in a two-shaft pattern which
means that each warp thread 2 binds over every other
weft thread 3a and beneath every other weft thread 3b.
A second and mora coarse-mesh weave 4, which is woven
in a four-shaft twill weave pattern, forms the bottom
cloth of the fabric. This bottom cloth 4 likewise
comprises two complete thread systems, viz. warp thread
5, which weaves above three weft threads 6a in
succession and below the fourth weft thread 6b. Between
these two cloths 1 and 4 are woven weft threads 7 which
bind or weave with the warp threads 2, 5 from the top
as well as from the bottom cloths 1 and 4, respectively.
These binder weft threads 7 can be placed thinly or
densely. In Fig. 1, the binder weft threads are
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- comparatively thinly placad, more precisely following
each twelfth w~ft thread in the top cloth 1.
Fig. 2 exemplifies a more dense position and
Fig, 3 a further increased density of the binder weft
threads 7.
The patterns of the top cloth 1 and the bottom
cloth 4 may be varied in many ways, some of which are
shown in Figs 4 and 5 by way of example. In Fig. 4,
both cloths 1 and 4 are woven in a four-shaft pattern.
The embodiment of Fig. 5 is distinguished from thoss
shown as examples earlier in that both cloths 1, 4
are woven in a three-shaft pattern and in that the
; ratio of weft density between the top and bottom cloths
is 3 to 1 instead of 2 to 1 as illustrated in the
other drawing figures.
` The binding patterns and the density ratios of
the top and bottom cloths 1, 4 thus may be varied in
a number of ways, and the density ratios of the cloths
; may also differ. The common feature of all is that
the two cloths are anchored in a system of weft
binder threads 7 that are interwoven with and between
the cloths 1, 4. As these binder weft threads 7, like
the other weft threads 3a, 3b, 6a, 6b, are comparatively
straight, wh=rsas the w,rp threads 2, 5 ar~ sinuously
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curved, the binder weft threalis 7 will, after weaving,
be positioned entir0ly inside the body of the double-
-layer fabric. These binder weft threads 7 thersfore
will not b~ in contact with the sxternal parts of
the fabric and therefore cannot, contrary to warp
binder threads, disturb the regularity of the inter-
weaving of the two yarn systems 2 and 3a, 3b of the
top cloth 1.
