Note: Descriptions are shown in the official language in which they were submitted.
BACKGROIJN~ OF I~VENTION
(a) Field of the Invention
This invention relates to fabrics as used in
the dryer sections of paper making machines.
(b) Description of Prior Art
In the mamlfacture of paper on a Fourdrinier
paper making machine, ~or example, an aqueous suspen~ion
of cellulose fibres, comprising one part or less ibres
in 99 parts or more of water by weight, is flowe~ ~n to
an endles~ rotating forming screen woven of metaL or
; synthetic filaments. As this belt, or forming fabric or
"wire", as it is ca~led, passes over water extraction
devices such as table rolls, drainage foils and suction
boxes, the water content of the suspension supported on
the fabric is reduced to about 80 to 85 percent.
The thin web of fibres, now self supporting, is
removed from the forming fabric and passes to a series of
one or more press sections where it is deposited on other
endless kelts of relatively thick fabric, one or both ~ur-
faces of which may be composed of a needled bat of ~yn-
thetic or natural fibres. These belts, called wet ~elts
carry the web through the nips of press rolls where more
of the water remaining in the web is squeezed into th~ -
~ absorbent felts until the water content is lowered to
`; 25 about 65% at which point it is not generally practical to
attempt further water removal by direct extraction ~uch
~ as with pressure or vacuum.
;~ The web of paper is then passed to the dryer
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section of the machine where the remainder of the water is
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removed by an evaporation process accelerated by the appli-
cation of heat. The dryer section consists of a number of
large, hollow cast iron or steel cylinders over which the
paper web passes in a serpentine fashion. The cylinders
are rotated synchronously to facilitate the passage of the
web. ~eat is supplied by steam condensing inside each
cylinder and the web is held in intimate contact with
portions of the heated surfaces of the dryer cylinders by
the dryer fabrics.
To provide sufficient drying capacity a newsprint
dryer section, for example, may consist of about 50 dryer
cylinders each about 5 feet in diameter and set up in an
upper and lower tier in four or five individual sub- ;
sections.
In order to appreciate the magnitude of the dryer
section of a modern paper making machine, the overall size
may be about 200 feet long, up to 40 feet wide and up to
40 feet high. The paper web may pass through the dryer
section at speeds up to 3000 feet per minute so that any
part of the web may only remain in the dryer section for
as little as 15 seconds during which time the web will be
reduced to a normally dry sheet of paper.
The dryer fabrics serve to hold the paper web
against the heated surfaces of the rotating dryer cylinders
to promote more effective heat transfer to the web by
partially eliminating a heat insulating layer of air which
adheres to the surface of the cylinders. The drier fabrics
- also serve to prevent the paper web from wrinklingO
In the conventional dryer section there is an
upper and a lower dryer fabric. The upper fabric wraps
around and holds the paper web against the upper peri~
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pheries of the upper dryer cylinders while the lower fabric
wraps around and holds the paper web against the lower peri-
pheries of the lower dryer cylinders. The fabrics are guided
by intermediate fabric rolls placed between the cylinders.
Dryer fabrics operate in a particularly adverse
environment in which they are alternately exposed to hot and
wet and hot and dry conditions. They must be flexible in
the machine direction so that they can bend around the felt
rolls easily. They must have good dimensional stabili-ty and
durability under the conditions of tension, temperature and
humidity which prevail in the dryer section of a paper machine.
Generally, dryer fabrics are woven from either natural or
synthetic yarns to form a relatively bulky fabric that will
have good absorbent characteristics and high porosity to en-
hance removal of moisture from the web of paper. To attain
these results the yarns are woven closely together and some-
times in several plies to form a comparatively impermeable
fabric~ To decrease permeability further sometimes bulky
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staple fibre yarns, some containing asbestos, are woven in.
These fabrics thus exhibit an undesirable tendency to hold
sufficient water to rewet the sheet. They also become in-
creasingly difficult to clean of various foreign substances
such as sizing agents, clay-like fillers and resins, gums,
waxes and pitch and the fabric becomes plugged up so that it
has to be cleaned frequently or replaced.
i Dryer fabrics are usually woven with approximately
100% warp fill, as shown in the drawings of this application
and as is well known to those skilled in the art. Warp fill
is defined as the amount of warp in a given space relative
to the total space considered. Warp fill can be over 100%
when there are more warp strands jammed into the available
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space than the space can dimensionally accommodate in asingle plane. Fabrics having a nominal warp fill of approx
imately 100% will generally have an actual calculated warp
fill of from 80% to 125% as is the fabric of this invention.
Values over 100% are brought about by crowdlng and lateral
undulation of the warp strands.
Permeability is an important characteristic of a
dryer fabric and is a measure of its air passage capability.
A low permeability fabric will resist the passage of air
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and tend to absorb vapour whereas a high permeability
fabric will allow free passage of air and vapour.
As indicated previously dryer fabrics were con-
ventionally made Erom cotton or wool and somtimes con-
tained asbestos fibres. With the development o~ synthetic
yarn materials the conventional fabrics are gradually being
replaced by fabrics containing synthetic yarns. These may
be woven in simple or in very complex weaves in two or ,
three plies or more of either relatively large diameter
monofilament yarn or of multifilament yarns spun from many
small diameter filaments, ~
Of the new synthetic yarns, monofilaments are - ~;
preferred because the resultant fabric has increased run-
ning life, is easy to clean, does not shed fibre and does
not carry excessive moisture During the part of the
cycle when the fabric is in contact with the sheet over
a dryer cylinder, low moisture content and high perme-
ability enhance transfer of heat to the web, Also,
the high permeability of the fabric can have a beneficial
- 20 effect on ventilation of the dryer pockets, producing
a more even moisture profile in the web. ~Iowever, the
high permeability of fabrics made from all-monofilament
; yarns in some cases is a disadvantage as it causes excessive
air movement in dryer pockets which results in sheet
flut-ter. This problem increases with machine speed and a
;~ point is soon reached when the flutter, particularly in
the first and second dryer sections where the web is wet
~, and weak, is violent enough to cause it to break.
, The effect of fabric permeability on dryer
pocket ventilation and sheet flutter has been described
by Race, Wheeldon, et al (Tappi, July 1968 Vol. Sl No. 7)
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and they have shown that air movement in dryer pockets is
influenced by permeability rather than by the surface
roughness of the fabric as was previously supposed. Air
movement i~ dryer pockets is induced by the fact that a
moving fabric carries with it layers of air. At the sur-
face of the fabric the velocity of the air layer is the
same as that of the fabric and as the distance from the
surface of the fabric increases the velocity of the air
decreases, When the fabric wraps around a roll, the layer
of air on the inside is trapped in the nip between the
roll and the fabric and, if the fabric is sufficiently
permeable, the air from the inside is pumped through, joins
the air stream on the outside of the fabric and the combined
velocity of the two streams is greater than the speed of
the fabric. As the fabric passes around the roll the layers
of air on the outside tend to be thrown outward by centri-
fugal force generating tangential air movement. This results
in a large mass of air moving laterally out of the pockets
when high permeability fabrics are used on high speed
machines.
The Race, Wheeldon et al experiments show that
as fabric speed increases, the air which is pumped through
the fabric by the felt rolls of the dryer increases in
velocity, particularly at speeds above 1500 r.p.m, They
also show that as fabric permeability is reduced, the
amount of air pumped into the dryer pockets is correspond-
ingly reduced. Thus at low speeds a dryer fabric with high
permeability can be tolerated and, in fact, is usefuL in
achieving high drying rates, but at high speeds, parti-
cularly in the first or second dryer sections, it is neces-
sary to have lo~ permeability fabrics in the range of 50 to
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200 cu.ft./min./sq.ft. Thus on high speed machines it is ~ -
often not practical to take advantage of the easy to clean
characteristic of monofilament fabrics because of their
inherent high permeability.
"Permeability" is usually expressed by the number
of cubic feet of air per minute passing through a square
foot of the fabric when the pressure drop across it is 0.5
inches of water. One instrument used to measure air perme-
ability is a Frazier Air Permeometer.
In this instrument air is drawn by a variable
speed fan through a 1 square inch section of fabric to be
tested then through upper and lower chambers joined by one
of a set of replaceable orifices calibrated for measuring
; volume by pressure differential. The speed of the fan is
increased until the upper chamber reaches a vacuum of 0.5
~ inches of water as indicated on a manometer. The vacuum,
; in inches of water, in the lower chamber is then read off
another interconnected monometer and this value is applied
to a reference graph to convert the reading to cubic feet
of air per minute per square foot of fabric.
;,
~hile in the conventional dryer system, the
problem of sheet flutter may be overcome by using a dryer
fabric having low permeability, another method of alleviat-
ing this problem is known as the single fabric dryer system.
;- 25 In this method, a single dryer fabric is used to guide the
web of paper in serpentine fashion through the dryer sec~
tions of the paper machine. The paper, for example, is
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introduced under the fabric at the first upper cylinder
` and passes substantially in contact with the fabric all
through a dryer section so that it lies between the fabric
i and the cylinders in the upper tier and outside the fabric
around the cylinders in the lower tierO
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The main advantage of the single fabric dryer
system is that the web of paper is partially supported by
the fabric as it passes bekween the tiers of dryer
cylinders and sheet flutter is thereby reduced or may be
entirely eliminated. `
Other important advantages of the single fabric
dryer system include reduction of dryer fabric costs and
elimination of felt rolls and one set of stretch and guide
rolls which are no longer required. Also, since the lower
tier of cylinders is not encumbered by a separate lower
dryer felt, the waste paper from paper breaks, or "broke"
as it is called, may be removed more easilyO
A disadvantage of the single fabric system is
that when it is applied to existing dryer sections in which
all the dryer cylinders are the same size and are driven
at the same rotational speed by an interconnected set of
gears, the conventional monofilament fabric, having a high
modulus of elasticity, is quite inextensible and will try
to force the upper cylinders, which have a larger effective
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diameter due to the layer of paper, to turn at a lower
rotational speed. This braking action of the cylinders by
force tending to stretch the fabric, produces considerable
stress on the drive train and even when the web of paper is
fairly thin, the stress has been sufficient to cause
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~ 25 abnormal wear of the gear teeth and bearings and in some
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cases structural failure.
'- The stretch of the fabric, called fabric draw,
; caused by the difference in fabric path lengths over the
cylinders is within the elastic range of the fabric and is pro-
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portional to the thickness of the web of paper. The stress,
expressedintermsoftorque, on thedryer cylinder gears, ispro-
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portional to the product of the paper thickness and the
modulus of elasticity of the fabric. As a practical example,
in a single fabric dryer sèction where the paper web is
only 0,0~2 inches thic~ the calculated torque developed ` -at the drive gear of an upper cylinder will amount to 3000
ft.-lbs. From this it will be apparent that the problem
of gear wear and structural failure will be significantly
alleviatedbyusing a fabric having a lower modulus of
elasticity so that it stretches more easily and can absorb
the stress developed by differentials in dryer cylinder
diameter due to paper thickness.
While the above example illustrates the degree
of stress that can be developed by a relatively thin web
of paper, it will be appreciated that differences in dryer
cylinder diameters caused by wear or by thermal expansion
due to temperature differentials may also have destructive
effects which can be alleviated by using a dryer fabric
having a lower modulus of elasticity.
The stress problem can be overcome in those
cases where it is possible to disconnect the upper gear
train from the lower gear train so tha~ either the upper
or the lower cylinders only are driven, In such cases the
, cylinders which are disconnected are rotated by the dryer
fabric and it doesn't matter if they rotate at a different
speed. There are some installations, however, in which it
is not possible to disconnect some of the drive gears and
it is in these cases where a fabric having low modulus of
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elasticity will be used to advantage. -~
A further disadvantage of the single fabric
dryer system arises because of the relative thickness of
: a conventional fabric, For example, when the wet web of
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paper passes from an upper dryer cylinder, where it lies
under the fabric, to a lower dryer cylinder where it li~s
over the fabric, it is strètched due to the difference in
diameters. mis stretch, or paper draw, is proportional
to the thickness of the fabric. Since it is èasily exten-
sible the wet web of paper will accommodate to the draw.
However, as it progres~ses from a lower dryer cylinder to
an upper dryer cylinder a negative draw is created and
because the wet web of paper is non-elastic it separates
from the fabric and billows out so that it can fold or
overlap on itself before passing under the fabric at the
upper dryer cylinder, thus nullifying the effect of the
support of the fabric. It will be apparent therefore that
it is advantageous to use the thinnest possible dryer
fabric in the single fabric system.
SUMMARY OF INVENTION
The present invention provides a dryer fabric,
for use on a papermaking machine, having reduced perme-
ability and reduced modulus of elasticity. Said dryer
fabric comprises a plurality of interwoven monofilament
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plastic polymeric warp and weft strands wherein at least
the warp strands, which extend in the machine direction,
have a flattened cross-section the long axis of w~ich lies
; parallel to the plane of the fabric. The fabric of this
. .
invention has the advantages of being easy to clean and
being non-absorptive.
An important feature of the flattened warp is
that it has a near rectangular cross-section which has
a lower resistance to bending about its long axis than a
circular cross-section of the same area and therefore,
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for the same stren~th of loom blow during wea~in~, the ~pacin~
of the weft stranc3s can be reduced greatly compared with the
spacing when woven with circular warp. Also, because of the
lower profile of the flattened~ warp,t~.edimensions of diagonal ': ..
apertures in the me~h which allow the passage o air there~iy
reduced.
~ furthe~ feature of the flattened warp is that with
the long axis of the rectangular cross section beiny parallel
to the weft yarns, the fabric i5 made more resistant to dis- ~ '
tortion in its own plane while permitting easy flexing of the
fabric about the axis which is parallel to the weft strands,
thus, making it easier for the fa~ric ~-_flex aroulld dryer
cylinders and smaller diameter rolls in tlle dryer system.
Although reduced permeability is essentially attained
1.5 by using flattened warp, further reduction in permea~ility,
also afeature of the invention, may be attained by the use of
mono~ilament weft strands that are shaped in cross-section so
as to su~stantially conform'to tbe horizontally directed inter-
sticial weft direction passages of the mesh naturally formed by
the woven warp strands to thereby rec3uce the space between
adjacent weft strands.
.~ The invention also features the use of round or
shaped weft which is relatively malleable as compared to the ; :'
warp so that during the weaving process, and su~,sequently under ' ~:
~5 any stressful condition, it will tend to aclapt itself to the
shape of mesh interstices to thereby restrict them and reduce
permeability further still.
A further'feature of the i.nvention is the use of
round or shapec'l polymeric weft, t'hat i5 hollow (tu~ular) so that
it may more easily adapt itself to conform to the shape of the
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mesh interstices.
An important advantage of the flattened monofilament
warp, either with round or with shaped monofilament weft, is
that is provides low permeability in an all-monofilament dryer
abric without the necessity of adding bulked yarns, as des-
cribed in ~anadian Patent No. 861,275, which absorb dirt and
moisture, or adding bulky weft yarns comprising fine staple
fibres which are low in bending resistance and contribute to
reduced resistance of the fabric to distortion in its own
plane.
Another advantage obtained in using flattened warp
strands is that the points of contact, or cross-overs, between
warp and weft (contact area between weft and warp) are in- ;
creased which serves to help stiffen the fabric against dis-
tortion in its own plane.
A still further advantage of the flattened warp
according to this invention is that the fabric from which it
is woven is relatively thin and has been found to have an
elastic modulus that is only about one half that of similar
' 20 fabric woven of conventional round wa,rp. As explained above, ~
low thicXness caliper and low modulus of elasticity is parti- ~ -
cularly advantageous if the fabric is to be used in a single
fabric dryer system.
; According to the above features, from a broad aspect,
the present invention provides a dryer fabric for use in a
papermaking machine comprising a plurality of interwoven warp
and weft monofilament plastic polymeric strands woven with ap-
proximately 100% warp fill. The strands extending in at least
the machine direction have a flattened cross~section with the
long axis of the cross-section extending parallel to the plane
of the fabric. The lowered profile of the Elattened strands
defines restricted diagonal aperturPs in the mesh of the fabric
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to thereby reduce the permeability of the fabric uniformly
throughout. ~-
The weft strands, which extend in the cross-machine
direction, may have either a round cross-section or a cross-
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section shaped to substantially conorm to wef t passayes o~ the
mesh naturally ~ormed by the warp strands to Eurther reduce
permeal~ility. As a furth~r embodimentof the invel-tion sorne or
all o~ the we~t strands may he hollow plastic strands or strands
formed of plastic material which is relatively maLleable as
compared to the material of the warp strands so that they can
adapt to conforrn to the shape of mesh interstices to partially
fill these and still further reduce perrneabilit~ of the fabric.
. The fabric of this invention having the lowest perrn-
eability will have, besides flattened warp, weft strands shaped :
to substantially conform to weft passa~es of the mesh and weft
strands that are relatively rnaLleable as compared to the warp -
strands,
BRIEF DESCRIPTION OF DRAWI~GS
A preferred embodiment of the present invention will .
now be descri~ed with reference to the examples illustrated
by the accompanying drawings in which:
E'IGURE 1 is a schematic view of a typical dryer
section as used in a paperrnaking
.70 machine;
FIG~E 2 is a schematic view of a typ~,cal single
- fabric dryer section;
FIGUR~ 3 is an enlarged sectional view of a
; portion of a dr~er fabric illustrating
?.5 interwoven weft and warp monofilament
circular strands as presently utilized:
FIGURES 3A and 3B are cross-sectional views along
section lines A-A and B-B of Figure 3;
FIGURE 4 is an enlarged sectional view of a
fabric structure sirnil.ar to that as shown
in Figure 3 ~ut utilizing the flattened
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cross-section warp strands forming the
improved dryer fabric of the present
invention,
FIGURES 4A and 4B are sectional views along
cross-section lines A-A and B-B of
Figure 4,
; FIGURE 5 is an enlarged sectional view of an all
monofilament 4-shaft 8 repeat duplex
weave dryer fabric of the prior art;
FIGURES 5A and 5B are sectional views along
'; section lines A-A and B-B of Figure 5,
FIGURE 6 is an enlarged sectional view o~ a dryer : .
` fabric as shown in Figure 5 but utiliz-
ing the flattened warp strands to obtain
the improved dryer fabric of the present
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invention, .. ~.
. FIGURES 6A and 6B are sectional views along - :
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section lines A-A and B-B of Figure 6,
. FIGURE 7 is an enlarged cross-section view of
the flattened monofilament warp strand
: as utilized in the dryer fabric of the
present invention.
: DESCRIPTION OF PREFERRED EMBODIME~TS
Referring to Figure 1 there is schematically
illustrated a sub-section of a typical dryer section in
a papermaking machine (not shown). The top tier dryer
, cylinders are generally indicated at 10 and the bottom
. tier at 11. The paper web 13 passes in a serpentine
. fashion over the top and bottom dryer cylinders as
. . .
. 30 shown. An endless top fabric 14 holds the paper web 13
tightly against the upper cylinders 10 as it passes partially
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around the first upper cylinder around a felt roll 15,
partially around the remaining top cylinders 10 and around
the other intervening felt rolIs 15 then around return roll
16, passing over guide and tensioning rolls 24 and 23,
respectively and over a steam heated dryer roll 17 to
remove some of the residual moisture in the fabric and then
over other return rolls 16, ~efore it passes again over the
first dryer cylinder to complete the cycle. Similarly an :
endless bottom fabric 18 holds the paper web 13 tightly -
against the lower dryer cylinders 11 as it passes around
these and the intervening bottom felt rolls 19, return ;
rolls 21, tensioning roll ~5, guide roll 26, bottom fabric
dryer roll 22 and other return rolls 21, substantially as
shown. The areas, bounded by the paper web 13 both approach-
ing and leaving a dryer cylinder and the dryer fabric as it
leaves the previous cylinder, wraps a felt roll and approaches ~;
the next dryer cylinder, are called pockets 12. It is in
; these pockets 12 that a large quantity of the moisture is
evaporated from the heated web of paper. Proper ventilation
of the pockets 12 provides for removal of the moisture from
the system and maintains the equilibrium of the evaporation
process.
Figure 2 represents, schematically, a typical
dryer section in which all the cylinders are substantially
:
the same diameter and are driven at the same number of
revolutions per minute by interconnected gearing. As in
Figure 1, the upper tier dryer cylinders are generally
indicated at 10 and the lower tier at 11~ A single endless ;
fabric, 14, passes in serpentine fashion around the first
upper cylinder, down around the first lower cylinder, up
around the second upper cylinder, down around the second
lower cylinder and so on, then it passes around a return roll
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16, a guide roll 24, a tensioning roll 23, a steam heated
dryer roll 17 and other return rolls 16, as shown. me
paper web 13 is introduced under the fabric at the first
upper cylinder and follows the fabric, passing between it
and the upper cylinders and outside the fabric at the lower
tier cylinders. It will be seen that in respect to the
fabric, because of the thickness of the paper web, the
effective diameter at the upper cylinders is now larger
than the diameter at the lower cylinders by an amount
equal to twice the thickness o-f the paper web.
Figure 3 shows generally at 30, a plain weave
synthetic fabric structure of the prior art in which numeral
; 31 denotes consecutive warp strands and nu~eral 32 denotes
consecutive weft strands. In this structure each warp
strand 31 passes over a first weft strand 32, under the
second weft strand, over the third and so on. Similarly,
the adjacent warp strand passes under the first weft strand,
; over the second, under the third and so on. Sl denotes the
center-to-center distance between adjacent weft strands 32.
In Figure 3B "x" denotes the shortest distance between
adjacent warp strands 31 in the vertical section ta]cen at
the point of tangency between warp and weft, thus represent-
ing the largest diagonal aperture which permits passage of
~` air through the fabric 30.
Referring now to E'igures 4, 4A and 4B there is
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shown the same fabric structure 30' made with warp mono-
filament strands 31' that have been flattened to the
extent that its short axis "b" (see Figure 7) is only
about half (1/2) the diameter of round warp 31 of corres-
-~ 30 ponding cross-sectional area.
In comparing the fabrics of Figures 3 and 4, it
will be apparent that, due to the lower resistance to
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bending of the rectangular cross-section, the flattened
warp 31' assumes a crimp more easily so that the center-
to-center distance between`weft strands, S2 of Figure 4, -
is smaller than Sl of Figure 3. Also, because of the
flat profile of the flattened warp the distance "y" in
Figure 4B is noticeably less than the corresponding
distance "x" in Figure 3B. Similarly, because of the
reduced spacing of weft strands 32', distance S2, the area
of the roughly triangular interstice based on "y" in
Figure 4B is much smaller than that based on "x" in
Figure 3B.
Figures 5, 5A and 5B aepict an all monofilament
4-shaft 8 repeat duplex weave dryer fabric 40, a type which
is commonly used in the papermaking industry. In Figure 5,
numerals 41, 42, 43 and 44 are consecutive warp strands.
The weft is paired in two layers and numbered ~8 to 57
as shown. In this structure a warp strand 41 passes in ~-
order over a first pair of weft strands 50-51, between the
second pair 52-53, under the third pair 54-55, between the
fourth pair 56-57 and so on. The next consecutive warp
strand passes between the first pair of weft strands, over
the second pair, between the third pair and under the
fourth pair. Similarly, the third and fourth consecutive
warp strands are woven commencing under and between the
first pair of weft strands respectively.
't` S3 denotes the center-to-center distance between -~
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pairs of weft strands, 52,53 and 54,55 and '`x" (see Figure
5B) is again the shortest distance between adjacent warp
strands in the vertical section taken at the point of
tangency between warp and weft. Referring to Figure 5A,
P denotes the shortest distance between crossing pairs of
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warp strands taken in a vertical plane midway between
pairs of weft strands.
Typically the conventional fabrics of Figure 5,
in the mesh ranges commonl~ used, yield air permeabilities
in the range between 400 and 900 cu.ft./min./sq.ft. In
order to reduce permeability in this type of construction
as indicated above, it is common to add bulky yarns
between some of the monofilament weft strands as shown -
at 58 in this figure, Bulky yarns are normally made
from staple fibres which fluff out and fill the space
between the wefts.
Figures 6, 6A and 6B show the same fabric 40' as
illustrated in Figure 5 but with the warp strands 41'-44'
flattened as in Figur~ ~. It will again be apparent that
the distances S4 and llyll in Figures 6 and 6B are less than
the corresponding distances S3 and ~x~i in Figures 5 and 5B.
The distance "q'~ in Figure 6A is not appreciably different
from the corresponding distance "p" in Figure 5A, but ~-
` again due to the reduced spacing S4 the area of the inter-
stice bounding "q" is much less than the area of the
interstice bounding "p",
As also shown in Figure 6, we provide, as an
alternative to bulky staple fibre yarns, extra monofilament
strands 59 woven into the fabric. As further illustrated
2S in Figure 6, the extra strands may have a diamond or rect-
angular shaped cross-section,shown at 60, to further fill
the passages 61 of the fabric without making the fabric sus-
ceptible to picking up more foreign substancesorretaining
more water, Althoughnot shown, when three or more layers of
weft strands 50, 51are provided, two or more passages 61 will
be formed in the area between adjacent pairs of weft strands,
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i.eu in the area delineated across the fabric between the
distance S4, some or all of these passages may be filled
with the shaped weft of thè invention.
Further, all the weft strands may be made
of plastic polymeric material that is more malleable where-
by under stress in the weaving or other treatment of the
... .
fabric, the weft strands will deform to further fill the
interstices of the mesh to still further reduce the perme-
ability of the fabric.
In the case of each of these types of fabric -
the reduction in the dimensions S2 and S4 and "x" to "y" ;
results in a reduction in size of the interstices of the
fabric and, therefore, a reduction in permeability. By ~ -
the use of suitably flattened monofilament warp strands
and with suitably shaped and possibly more malleable weft
strands the permeability of the fabric can be reduced to
- the 50 to 250 cu.ft./min./sq.ft. range without resorting
to the use of flu~fy bulked "stuffer" yarns with their
attendant disadvantages.
Typical conventional monofilament dryer fabric, `~ -
as shown in Figure 4, has a thickness usually greater than ~ `
`~ 0.070 inches and an elastic modulus greater than 5000 lbs
per inch. Experimental fabric woven according to the
invention as shown in Figure 5, having warp strands flat-
tened in the ratio of 2:1 and heat set in the normal way
had an average thickness of 0.058 inches and an average
, modulus of elasticity of 2690 lbs. per inch, In general,
fabric woven according to the invention will have a thick-
; ness within the range 0.035 to 0.070 inches and modulus
of elasticity from 1500 to 3000 lbs. per inch.
. .
. 19
~ 92~1~Q5 ~:
r~he warp yarns and the shaped weft yarns of the :~
present invention may be made by mechanical rolling appa-
ratus for rolling round monofilament strands in the range
of 0.2 mm to 1.0 mm in diameter between pairs of rolls in
order to flatten them or similarly flat or shaped strands
may be extruded from a specially shapad die or made by the
use of slit film to produce ribbons of monofilament-like
material. The flattened cross-sectional shape of a mono-
filament strand is shown at Figure 7, in which "a" is the
width and "b" the thickness. A possible cross-sectional
area range of a flattened monofilament warp strand would
be from 0,07 sq. mm. to 0.5 sq. mm. and a possible ratio
range of a:b would be 1.1:1 to 3:1.
rrhe fabric of the present invention would have .
a warp count preferably in the range of 30 to 100 strancls :~:
per inch and a weft count preferably in the range of 10 to
100 strand~ per inch.
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