Note: Descriptions are shown in the official language in which they were submitted.
WO91/04374 2 0 6 6 2 ~ 6 PCT/CAgo/00304
PRESS SECTION D~WATER-NG FABRIC
This invention relates to dewatering fabrics used in the press
section of a paper making machine, and is particularly concerned with such
a fabric including flattened monofilaments configured to provide improved
water removal and redùced paper marking.
In the press section of a paper making machine a thin, wet, self
supporting web of matted paper fibers, having a consistency of from about
15% to about 25% (that is a wet paper web containing from about 15~ to
about 2570 of fibers and other solids and from about 75% to 85% water), is
passed though a series of pressure rollers whilst supported on a series of
endless belts of permeable felts. In each of the sets of rollers, some of
the water in the paper web is transferred to the felt by the action of
line nip pressure between the press rolls. At the end of the press
section, the wet paper web will have a consistency of from about 30% to
about 50%. Generally in the press section pressure rolls are used in
pairs. One roll usually is smooth, and may be provided with an
elastomeric (typically rubber) surface. The other roll has a contoured
surface usually made also of an elastomeric material adapted to provide
voids into which water can be transported from the press felt. A roll
having a grooved surface wherein the grooves are around the roll and
essentially perpendicular to the roll axis is commonly used. The press
felt acts as an intermediary between these grooves (or other receptacles,
such as perforations) and the wet paper web. As the paper web carried on
the felt enters the nip between the press rolls, water is squeezed from
the paper web by the smooth roll into the compressed felt and ultimately
into the roll grooves. As the felt and wet paper web leave the nip, some
of the water remaining in the felt can be transferred back to, and be
reabsorbed by, the wet paper web.
Generally, a press felt comprises a combination of a base cloth
having needle punched to it a staple fiber batt. In some press felts a
single layer of batt is used, needle punched to the paper side of the base
cloth. In others, two layers of batt are used, one on each side of the
base cloth, to which they are both needle punched. It is known that the
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batt fibers tend to be aligned in the direction the batt is laid on the
base cloth. If the batt is cross lapped, that is, laid essentially in the
cross-machine direction, then in addition to the batt fibers being aligned
across the machine, a cross lap line exists between successive strips of
batt. These join areas can result in mass variations in the press felt
which, in extreme cases, can generate vibration effects in the roll stands
which will damage the machinery. In more recent practise, the batt can be
laid substantially in the machine direction using, for example, the method
described by Dilo, in U.S. 3,508,307, which both eliminates the
cross-machine mass variations and provides better drainage due to the
fiber alignment in the batt being in the machine direction.
The base fabrics of modern press felts can include a pin seam
and are typically woven of synthetic, circular cross-section monofilaments
as both the warp and weft, as typified by Lilja in U.S. 4,601,785. The
machine direction yarns, which form the pin-receiving loops of these
felts, must be monofilaments for the loops to retain their shape, thereby
ensuring that the fabric may be easily seamed during its installation on
the paper making machine. However, it is difficult to reliably needle a
batt to a fabric that is woven of all round monofilaments. The needles
will tend to deflect the round yarns rather than penetrate them. A
machine side batt must then be used to assist adhesion of the paper side
batt. A further disadvantage of press felt base fabrics woven of all
round monofilaments is that they tend to form prominent knuckles at warp
and weft intersection points. A further disadvantage is that the area of
contact between warp and weft cross-overs is limited to a point. The
fabric is thus susceptible to diagonal distortion or sleaziness.
It has been proposed by Miller et al. in U.S. 4,414,263 to
improve the properties of the base cloth, and thereby of the press felt,
by incorporating into the base cloth fabric monofilaments of a flattened
cross-section. Miller et al. define their improved press felt as
"comprising an open-mesh fabric woven of a plurality of synthetic
filaments extending in both the lateral and longitudinal directions, and
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at least one batt of staple fibers needled thereto, characterised in that
at least some of the filaments extending in the lateral direction are
monofilaments having a flattened cross-section, the long axis of which
lies parallel to the plane of the fabric". Miller et al. incorporate
these flattened monofilaments into a conventional weave pattern. Miller
et al. recommend that the aspect ratio (that is the ratio of width to
thickness) should be from 1.2:1 to 3:1, with a value of about 2:1 being
preferred, for these flattened monofilaments.
It has also been proposed to use similar flattened monofilaments
in a paper maker's forming fabric by both Johnson (U.S. 4,815,499) and
Kositzke (U.S. 4,142,557). Although such paper maker's forming fabrics
may be superficially similar to those used in the press section,
nevertheless they are actually quite different, as is dictated by the
conditions under which they are used. Kositzke, for example, is
primarily concerned with improving a forming fabric which is woven as a
continuous run of flat fabric, and seamed to provide the required loop,
and in which the weave pattern used is the four harness satin weave. In
order to improve such a forming fabric, Kositzke advocates the use of a
flattened monofilament in which the ratio of width to height is of the
order of 1.2:1 to 1.3:1. Flat monofilaments have been proposed for dryer
fabrics to reduce air permeability (Buchanan et al., U.S. 4,290,209) or to
increase surface contact (Malmendier, U.S. 4,621,663). It is also known
to extrude such flat monofilaments with contoured surfaces (Langston et
al., U.S. 4,643,119).
Flattened monofilaments of this general type have been used in
other woven fabrics. In these fabrics, a much higher aspect ratio,
usually above 10:1, is used, for example in carpet backing (U.K. Patent
1,362,684 assigned to Thiokol Chemical Corporation) and in geotextiles,
webbing, and bulk containers (Langston et al., U.S. 4,643,119), in a woven
fabric using a fiber-reinforced flat tape as both warp and weft, and
intended for use as a plastics reinforcement (Binnersley et al., U.S.
4,816,327) and in a sail cloth (Mahr, U.S. 4,590,121). It has also been
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proposed to use a flattened monofilament in a woven filter fabric intended
to be used for sludge dewatering (E.P. O 273 892, assigned to Scandiafelt
AB).
It has also been proposed to dewater a paper web in a press
section using a fabric to which no batt is attached. Such a procedure is
described by Kufferath, in West German Patent 3,426,264. However, it has
been found that the dewatering fabric described by Kufferath is only
successful when making thicker grades of paper.
The key feature of the Miller et al. press felt is the use in
the base fabric of a flattened monofilament. It has now been discovered
that similar flattened monofilaments can be used to provide an improved
press dewatering fabric offering both improvements in web dewatering and
resistance to paper marking by either the dewatering fabric or the press
roll grooves. Furthermore, the paper side batt required by Miller et al.
can be omitted in some applications.
According to this invention, flattened monofilaments are used
both at a high fill factor and in a weave pattern that provides a long
float surface on the paper side of the fabric. These features of the
dewatering fabrics of this invention appear to impart to the fabric a
relatively flat, smooth, almost platform-like surface on the paper side of
the fabric. This relatively flat surface appears to transfer the
mechanical loads imposed by the press rolls in a way that provides
improved pressure uniformity. It is also believed that the improved paper
web dewatering capabilities and the resistance to paper marking shown by
the fabrics of this invention may be directly related to the pressure
uniformity characteristics of these fabrics under compressive loading.
Thus, in its broadest aspect, this invention provides a woven
dewatering fabric for the press section of a paper making machine having
opposed side edges, the fabric having a cross machine direction extending
between the side edges and a machine direction extending perpendicularly
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to the cross-machine direction, and having a fabric weave pattern that
provides long exposed floats on the paper side of the fabric of a
monofilament warp yarn having a flattened cross-section with an aspect
ratio of at least 1.5:1, having a fill factor for the flattened
monofilament of at least 45%, and having a float ratio for the exposed
floats of the flattened monofilaments expressed by the formula of a/b
wherein:
(i) "a" represents the number of paper side surface layer weft
yarns in a single weave pattern repeat of a flattened
monofilament warp which are underneath and in contact
with that warp;
(ii) "b" represents the total number of paper side surface
layer weft yarns in the single weave pattern repeat;
and further wherein for a majority of the long exposed floats:
(iii) "a" is greater than 1; and
(iv) "a" is greater than one half of "b".
Preferably, the fill factor for the flattened monofilaments is
at least 60%. More preferably, the fill factor is at least 80%; most
preferably the fill factor is about 85%.
Preferably, the dewatering fabric is of a single layer
construction but the benefits of this invention can also be obtained with
more complex fabrics. Preferably the float ratio for the flattened
monofilaments, calculated as detailed above, is at least 5/8 and more
preferably is from 3/4 to 7/8.
Preferably the aspect ratio of the flattened monofilament is at
least 1.6:1 and most preferably at least about 2:1.
If a paper side batt of staple fibers is used it is preferred
that it be applied substantially perpendicularly to the flattened
monofilaments. It is also contemplated that a batt layer may be applied
to the roll side of the dewatering fabric.
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A press dewatering fabric can be woven in several ways. It can
be woven as a closed endless loop of the desired length, and which may
include a pin seam. Alternatively, the fabric can be woven as a
continuous run of flat fabric, a suitable length of which is then seamed,
for example with a pin seam, to provide the required endless loop.
The main difference between these methods is the orientation in
the woven fabric loop of the warp and weft yarns:
(a) in a fabric woven as a closed endless loop (with
or without a seam), the warp yarns lie in the
cross machine direction, and comprise the flattened
monofilaments of this invention; and
(b) in a fabric woven as a continuous run, which is seamed
to provide a closed endless loop, the warp yarns are
in the machine direction, and comprise the flattened
monofilaments of this invention.
It is also known to flat weave a fabric in which the weft yarns
are flattened monofilaments. However, special weaving and post-weaving
processing techniques may be necessary to properly expose the floats of
flattened monofilaments.
If the fabric is to be used in the press section with a needled
batt applied either to the paper side, or to both sides of the fabric, a
fabric woven as a closed endless loop is preferred. The endless loop may
also include a pin seam.
If the fabric is to be used in the press section without a
needled batt applied to it, as this invention contemplates, either one of
the previously described weavi~g techniques may be employed. If the
fabric is to be seamed to facilitate its installation on the paper making
machine, it is advantageous to use a pin seam.
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Generally, press felts are constructed from nylon monofilaments,
with nylon staple fibers as the batt, although polyester and other
materials have been used. It is preferred to use nylon monofilaments and
staple fibers for this invention, but this invention is not limited to
this material.
The invention will now be described in more detail with
reference to the attached figures wherein:
Figure 1 shows in schematic form a dewatering fabric according
to this invention;
Figure 2 shows the fabric of Figure 1 with a paper side battj
Figure 3 shows the fabric of Figure 1 with batts on both sides;
Figure 4 shows typical flattened monofilament cross-sections;
Figure 5 illustrates some alternative weave patterns for single
and double layer fabrics;
Figures 6 and 7 illustrate pin seam structures; and
Figure 8 illustrates the weave structures used in the examples.
In Figure 1, one example of a dewatering fabric according to
this invention is shown schematically, generally at 1. The arrow 2
indicates the cross-machine direction, and the arrow 3 indicates the
machine direction. As shown, the fabric is thus one made as a closed loop
by endless weaving. For a fabric woven as a continuous flat run, arrows 2
and 3 are interchanged: 2 becomes the machine direction and 3 becomes the
cross-machine direction. In this Figure, a single layer fabric is shown
comprising essentially parallel weft yarns 4 and essentially parallel
flattened warp monofilaments 5. The weft yarns 4 can be any of those
commonly used in such a fabric, including monofilaments, spun yarns and
braided yarns.
.,~
As is shown in Figures 2 and 3, porous layers, such as a needle
punched fiber batt, may be attached to the fabric. A paper side batt is
shown generally at 6A, and a machine side batt generally at 6B. It is
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preferred that a paper side batt 6A be applied substantially perpendicular
to the flattened monofilament warps 5, that is, substantially parallel to
the arrow 3.
As shown in Figure 4, which represents a cross-section of the
flattened filament 5, these filaments have a width W and a thickness T.
The aspect ratio of such a filament is defined as the ratio W:T. For the
filament shown the aspect ratio is 4:1. For the purposes of this
invention, the aspect ratio should be greater than 2:1, and preferably of
the order 2:1 to 20:1. If T is made too low, the filament becomes too
thin and too flexible to prevent both the knuckle pattern of the weft
yarns 4 and the groove, or other pattern, in the press roll from being
transmitted to the paper in the press roll nip. Such marking of the paper
surface is not desirable. A suitable lower limit for T appears to be at
about 0.1 mm.
The lowest value for the aspect ratio is 1:1; that is, a
substantially square monofilament. In the fabrics of this invention it is
intended that the long exposed floats of the flattened monofilament
provide something approximating to a flat surface to support the wet paper
web. If the aspect ratio is made too small, it becomes difficult to
create such a fabric with currently available machinery, even at the high
flattened monofilament fill factors used in this invention. An
undesirable degree of twisting of the flattened monofilaments appears to
occur if the aspect ratio is less than about 1.3:1. In view of this, an
aspect ratio of 2:1 or higher is preferred. The upper limit for the
aspect ratio appears to be determined by the weaving equipment. A
practical upper limit appears to be at about 100:1.
When a batt is needle punched to the dewatering fabric, many of
the needles will puncture the flattened monofilaments. Even though this
inherently means that the punched monofilaments are damaged, this appears
to be of no consequence provided the amount of needling used is not
excessive. Further, it appears that the high fill factors used herein
WO 91/04374 PCT/CA90/00304
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lead to better attachment of a batt to the dewatering fabric as fewer
needles fail to encounter a warp or weft yarn. Additionally, at least in
part due to the high fill factor, split monofilaments tend to pinch the
batt fibers and hold them in place.
It is also possible to control to some degree the point at which
the needles will penetrate the flattened monofilaments. The monofilament
shown generally at 7 in Figure 4(b) with an aspect ratio of 4:1 has four
flat faces 8 separated by three grooves 9 on each side. It is found in
practice that the needles will tend to punch through in the grooves 9
rather than the faces 8 with such a monofilament.
It was noted above that the orientation of the flattened
monofilaments is of importance in the context of pin seams. Due to the
fact that the wefts used are of a substantially circular cross-section, in
a needle punching operation few of the wefts are punctured by the needles:
in most cases the weft is simply deflected a little sideways by the
needle. It is therefore advantageous to form the pin seam from the
undamaged weft yarns. Since the wefts are in the machine direction in a
fabric woven as a closed loop, it is preferred to use such a fabric if a
batt is to be applied. Alternatively, if no batt is to be used, then it
may be advantageous to form the pin seam using the flattened monofilament
warp yarns.
In Figures 5, 6(b), 7(b) and 8 are shown diagrammatic
cross-sections for various possible dewatering fabric constructions, of
which three, as is discussed in more detail below, are outside the scope
of this invention (Figures 5(d), 8(a) and 8(e)) and are given for
comparison purposes.
Two features of the dewatering fabrics of this invention are
particularly important, one of which is shown in these diagrams. One is
the "float ratio", the other is the "fill factor".
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The float ratio represents the proportion of a flattened
monofilament warp which provides a long, exposed float on the paper side
of the fabric, as at 10 in this group of Figures. The float ratio is
expressed as a "ratio" a/b in which a and b are integers, and
(i) "a" represents the number of paper side surface layer weft
yarns in a single weave pattern repeat of a flattened
monofilament warp which are underneath and in contact with
that warp; and
(ii) "b" represents the total number of paper side surface layer
weft yarns in the surface of the fabric in the pattern
repeat.
For such an exposed float to satisfy the requirements of this
invention the observed float ratio must satisfy two further limitations:
(iii) "a" is greater than 1; and
(iv) "a" is greater than one half of "b".
Applying these principles first to Figure 5 gives the following
float ratios for Figures S(a), (b) and (d):
- for Figure 5(a): a = 4, b = 5, and the float ratio is
4/5: the group of four wefts 11 is beneath and in
contact with the warp 12, and there is one additional
weft 13 in the repeat pattern X.
- for Figure 5(b): a = 5, b = 8, and the float ratio is
5/8: only the five wefts 14 under warp 15 and the three
wefts 16 above warp 15 are counted; the weft 17 although
above, and the seven wefts 18 although below are not in
the surface layer and are not counted in determining
either a or b.
- for Figure 5(d): a = 3, b = 6, and the float ratio is
3/6 and thus outside the scope of this invention, since
the subsurface layer is not counted.
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In a similar fashion, float ratios are calculable for all of the
remaining diagrams:
Figure 6(b) : a = 5, b = 8: float ratio: 5/8;
Figure 8(a): a = 4, b = 8: float ratio: 4/8 (comparison);
Figure 8(b): a = 6, b = 8: float ratio: 6/8;
Figure 8(c): a = 5, b = 6: float ratio: 5/6;
Figure 8(d): a = 7, b = 8: float ratio: 7/8;
Figure 8(e): a = 3, b = 6: float ratio: 3/6 (comparison);
Figure 8(f): a = 5, b = 8: float ratio: 5/8.
The float ratio in a given fabric need not be constant either
along a given flattened monofilament, or for all of the monofilaments in a
given weave. Further, not all of the exposed floats need have a float
ratio in accordance with the limitations placed on a and b in this
invention, although maximum benefit will be obtained if all of the
flattened monofilaments do have a float ratio in accordance with those
limitations. Figure 5(c) shows a fabric with varying float ratios: from
the top downwards the float ratios are: 7/8, 5/8, 6/8, 1/8, 4/8, 6/8, 3/8
and 5/8. In a similar way, it is also possible to change the float length
periodically along the length of the warps, to provide, for example, in
sequence a 1/2 unit, and then a 5/6 unit. In such a case, for determining
the float ratio the overall length of the full pattern repeat should be
used: the preceding example gives a float ratio of 6/8. However, as the
proportion of the flattened monofilament warps woven with float ratios in
which both a and b are small numbers, or in which a is close to one half
of b, then the dewatering properties of the fabric will be impaired, and
the risk of the fabric imparting knuckle marks to the paper will increase.
It is also possible to include in the dewatering fabric warps
which are not flattened monofilaments. Th;s ;s not recommended, as the
dewatering capabilities of the fabric will likely be impaired, and
increase the risk of paper marking.
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There is however a limit to the float factor beyond which fabric
structural integrity becomes questionable. In view of the varieties of
weave possible it is difficult to be precise. For a simple fabric as
shown in Figures 1 and 5(a) this structural limitation appears to be
reached at a float factor of about 9/10. A float factor of 7/8 appears to
be a suitable practical limit.
The fill factor expresses essentially just how much of the space
in the fabric is taken up by the yarns from which it is constructed It can
be measured for both the warp and the weft yarns. It is given by:
N x W
fill factor % = x 100
where: (i) N is the number of yarns in a given distance D; and
(ii) W is the maximum lateral width of the yarn.
In this formula N is also known as the yarn count.
For a yarn of essentially circular cross-section, for
example as used in the wefts of Figure 1, W is the yarn diameter.
For the flattened monofilaments, W is the monofilament width as
indicated in Figure 4(a). Both D and W are measured in the same
units. For the purposes of this invention, the fill factor for the
flattened monofilaments should be above 45%, preferably at least 60%,
and more preferably is about 80%, with a value of 85% being most
preferred. As noted earlier, this high fill factor aids in batt
reter,tion when these are used.
For the other yarns, the yarn count, N, to a degree
determines the amount of support provided to the long exposed floats.
These need to be supported enough to substantially prevent them from
sagging under the pressure applied to the dewatering fabric in the
press roll nip. If the flattened monofilaments in a warp are
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relatively thick, have a high fill factor, and the press roll line
pressure is relatively low, then the weft yarn count can be
decreased. Generally, it is found that the yarn count should be
relatively high for the yarns other than the flattened monofilaments.
The previously described advantages of increased dewatering
capability are also realized in pin-seamed fabrics that are woven
either as closed loops or continuous runs.
Typical pin seams of largely conventional construction are
shown in Figures 6 and 7. In Figure 6 the fabric is one woven as a
closed loop, with the exposed warp floats 10 in the cross machine
direction. In Figure 7 the fabric is one woven as a continuous flat
run, with the exposed warp floats 10 in the machine direction. In
Figure 6 the batt 6A is shown only in Figure 6(c) for clarity.
Referring first to Figure 6, the fabric shown in face view in Figure
6(a) is shown in section along the lines I-I in Figure 6(b), and
along the lines II-II in Figure 6(c). The fabric, which has a float
ratio of 5/8, comprises flattened warps 20, two layers of wefts 21
and 22, a single surface batt 6A, and a pin seam pin 23. The pin
seam is constructed by providing loops as at 24 in the weft yarns
which may be a plain bend (Figure 6(c)) or a more or less complete
loop (Figure 6(d)). When prepared for installation on the paper
making machine, the pin is removed, the fabric is fed through the
press section, and the loop is closed by reinterdigitating the fabric
butt end loops and reinserting the pin.
In Figure 7 the construction is substantially the same.
The fabric weave is essentially the same, including flattened
monofilament warps 20, two layers of wefts 21 and 22, and a pin seam
~ pin 23. For clarity only one side of the seam is shown in Figure
7(a). In creating the seam essentially the same procedure is used,
however in this case allowance has to be made for the fact that the
crimped flattened monofilaments are forming the loops of the pin
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seam. Accordingly, it is desirable to incorporate a twist in the
warps used to make the loops, as at 25, and in those woven back into
the fabric ends but not used to provide loops, as at 26. In this
way, the warp end 27 can be re-entered into the weave to form an
overlapped joint in a fashion that is well known in the art, since it
will then be crimped to fit the existing weave pattern.
The flattened monofilaments in either configuration provide
a surface that offers excellent paper side batt adherence, possibly
eliminating the need for a machine side batt. This is because the
flat monofilaments split when needled, trapping the batt fibers and
anchoring them in the base fabric. Flattened monofilaments reduce
paper marking because the warp and weft cross-over points are not as
prominent as those formed of all-round monofilaments. Pin-seamed
fabrics woven according to the invention are also more resistant to
diagonal distortion because the area of contact between the flat and
round monofilaments at cross-overs is greater than that found at
cross-overs of two round yarns.
In order to establish the float ratio lower limits,
comparative tests were made, using single layer and double layer
weaves, at various float ratios. Changes in the consistency of the
paper as it passes through the press section are used as a measure of
water removal efficiency. "Consistency" is defined as the percentage
of dry paper solids (fibers, fillers, and so forth) in the wet paper
web. A typical consistency entering a press section is 22%. In the
last pressure nip of a press section, the exiting consistency will
typically be from about 38% to about 41%.
The effect of float ratio on water removal was tested using
a laboratory press, and following generally the technique described
by Jackson, Tappi Journal, 72(9), 103-107. This laboratory procedure
appears to give a good relative indication of press felt performance.
In Table I is given the data for four single layer felts with the
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same nominal batt design and weft type, but differing float ratios.
In Table II similar data is given for two double layer felts. The
weave patterns are shown diagrammatically in Figure 8. In these
Figures, the wefts 30 are 6-strand cabled monofilaments, and the
wefts 31 are multifilament yarns.
Table I
Weft: 0.2 mm/2/3 cabled monofilament; woven at a pitch of 1.1 mm
Warp: 0.2 mm by 0.4 mm without grooves; fill factor: 80%
No. Weave Pattern in Figure 8 Float Ratio Consistency after
Pressing
MA3 (a) 4/8 (0.5) 53.3%
MA2 (b) 6/8 (0.75) 54.2%
MA26 (c) 5/6 (0.83) 55.6%
MA1 (d) 7/8 (0.875) 55.9%
Table II
Weft: upper layer (31): 350 tex multifilaments; woven at a pitch
of 1.1 mm
lower layer (30): as in Table I
Warp: 0.2 by 0.4 mm without grooves; fill factor: 80%
No. Weave Pattern in Figure 8 Float Ratio Consistency after
Pressing
MA25 (e) 3/6 (0.50) 56.1
- MA15 (f) 5/8 (0.625) 56.9
- For each set of tests, the ingoing consistency of the paper
was 35%. The data for MA3 and MA25 is included for comparison
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purposes. In each case, the float ratio is one in which a is one
half of b, and thus is outside this invention.
These results show clearly that as both a and b increase,
and as a approaches b, water removal is improved. In the paper
making industry, a 1% increase in consistency is generally considered
to be industrially significant.
It has been noted above that the woven dewatering fabric of
this invention may not require a porous structure such as a batt on
the paper side surface. If a batt is used, some thought needs also
to be given to the direction in which it is to be laid. If the
dewatering fabric is an endless woven loop with flattened
monofilament warps in the cross machine direction then the batt
should be laid in the machine direction using, for example, the Dilo
method (see U.S. 3,508,307). If the dewatering fabric is a flat
woven fabric in which the warps are the flattened monofilaments then
a cross lapped batt structure is preferred. But if the dewatering
fabric is a flat woven fabric in which the wefts are the flattened
monofilaments, then the batt should be preferably laid in the machine
direction. Again, the Dilo method can be used.
