U.S. patent application number 10/131014 was filed with the patent office on 2002-08-29 for method to improve the formation of a paper or paperboard web by pre-pressing in a twin-wire former.
Invention is credited to Honkalampi, Petter, Kaasalainen, Heikki, Pakarinen, Pekka, Soderholm, Nils.
Application Number | 20020117283 10/131014 |
Document ID | / |
Family ID | 23919763 |
Filed Date | 2002-08-29 |
United States Patent
Application |
20020117283 |
Kind Code |
A1 |
Soderholm, Nils ; et
al. |
August 29, 2002 |
Method to improve the formation of a paper or paperboard web by
pre-pressing in a twin-wire former
Abstract
A method and apparatus for improving forming of a web of paper
or paper board employing prepressing during web forming. Forming
wires are brought together over at least one roll or alternatively
two rolls. The web then passes, sandwiched between two wires
through one or more pressure nips which may be one or more roll
nips or an extended nips. The pressure nip may be formed as the web
travels between the two fabrics, one of them being a forming wire,
along a straight path or alternatively as the web travels partially
around a roll. The web is then sandwiched between a wire and a
fabric, which may be a felt or a belt, and passed through another
press nip which may be one or more roll nips or an extended
nip.
Inventors: |
Soderholm, Nils;
(Anjalankoski, FI) ; Honkalampi, Petter; (Muurame,
FI) ; Pakarinen, Pekka; (Jyvaskyla, FI) ;
Kaasalainen, Heikki; (Jyvaskyla, FI) |
Correspondence
Address: |
COHEN, PONTANI, LIEBERMAN & PAVANE
Suite 1210
551 Fifth Avenue
New York
NY
10176
US
|
Family ID: |
23919763 |
Appl. No.: |
10/131014 |
Filed: |
April 24, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10131014 |
Apr 24, 2002 |
|
|
|
09483365 |
Jan 14, 2000 |
|
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Current U.S.
Class: |
162/203 ;
162/202; 162/208; 162/209; 162/301; 162/358.1; 162/360.2;
162/361 |
Current CPC
Class: |
D21F 9/003 20130101 |
Class at
Publication: |
162/203 ;
162/202; 162/208; 162/209; 162/301; 162/358.1; 162/361;
162/360.2 |
International
Class: |
D21F 011/00; D21F
001/00; D21F 003/00 |
Claims
What is claimed is:
1. A method for improving the structure of a paper web in a forming
section of a paper making machine comprising: ejecting a suspension
of paper pulp from a head box between a pair of forming fabrics so
that a paper web starts to be formed between the forming fabrics;
and continuing web formation by pressing the web between two
fabrics by passing the fabrics and the web sandwiched therebetween
through at least one pressure nip formed by a pair of rollers such
that water in the web is partially removed from the web.
2. The method of claim 1, wherein at least one of the forming
fabrics passes through the at least one pressure nip.
3. The method of claim 1, wherein the paper pulp ejected from the
head box between the forming fabrics has a dry content above about
2%.
4. The method of claim 3, wherein the paper pulp ejected from the
head box between the forming, fabrics has a dry content above about
3%.
5. The method of claim 3, wherein the paper pulp ejected from the
head box between the forming fabrics has a dry content of less than
about 12%.
6. The method of claim 5, wherein the paper pulp ejected from the
head box between the forming fabrics has a dry content of less than
about 10%.
7. The method of claim 1, wherein at least one of the fabrics
passing through the at least one pressure nip is a wire.
8. The method of claim 7, wherein both of the fabrics passing
through the at least one pressure nip are wires.
9. The method of claim 1, wherein one of the fabrics passing
through the at least one pressure nip is a water absorbing
felt.
10. The method of claim 1, wherein one of the fabrics passing
through the at least one pressure nip is a non-water absorbing
belt.
11. The method of claim 1, wherein the at least one pressure nip is
formed by a pair of press rolls.
12. The method of claim 1, wherein the a t least one pressure nip
comprises an extended nip press.
13. An apparatus for improving the structure of a paper web in a
forming section of a paper making machine comprising: a pair of
endless first fabrics supported so that a surface of one of said
first fabrics travels proximate a surface of the other of said
first fabrics along a portion of the length of said first fabrics;
a head box positioned to eject a suspension of paper pulp therefrom
between said first fabrics so that the ejected paper pulp starts to
form a web between said first fabrics; a pair of endless second
fabrics supported so that a surface of one of said second fabrics
travels proximate a surface of the other of said second fabrics
along a portion of the length of said second fabrics, said pair of
second fabrics positioned to receive the paper web from said pair
of first fabrics; and at least one pressure nip positioned to press
the web between said second fabrics by passing said pair of second
fabrics and the web sandwiched therebetween through said at least
one pressure nip, the web being pressed such that water in the web
is partially removed from the web, said at least one pressure nip
being formed by a pair of rollers.
14. The apparatus of claim 13, wherein at least one of said second
fabrics is at least one of said first fabrics.
15. The apparatus of claim 13, wherein neither of said second
fabrics is either of said first fabrics.
16. The apparatus of claim 13, wherein at least one of said second
fabrics is a wire.
17. The apparatus of claim 16, wherein both of said second fabrics
are wires.
18. The apparatus of claim 13, wherein one of said second fabrics
is a water absorbing felt.
19. The apparatus of claim 13, wherein one of said second fabrics
is a non-water absorbing belt.
20. The apparatus of claim 13, wherein said at least one pressure
nip is formed by a pair of press rolls.
21. The apparatus of claim 13, wherein said at least one pressure
nip comprises an extended nip press.
22. The apparatus of claim 14, wherein one of said second fabrics
is a water absorbing felt.
23. The apparatus of claim 14, wherein one of said second fabrics
is a non-water absorbing belt.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a method and apparatus for
forming a web of paper or paperboard from stock using a twin-wire
former.
BACKGROUND OF THE INVENTION
[0002] A paper making machine typically includes a forming section
where a head box ejects a flow of a paper or paperboard stock
suspension between adjacent surfaces of a pair of endless fabrics
or metal screens, referred to as wires. As the stock suspension
travels along with and between the wires, water is removed by
gravitational and centrifugal forces. Water removal can be further
increased by applying vacuum or pressure pulses. The forming
section is followed by a press section where the web and at least
one water receiving felt is lead through at least one press roll
nip. The web is then transported to a drying section of the paper
machine where the web is dried and from there to other sections of
the paper machine where the web is further treated, such as by
being coated. Such a forming section is typically referred to as a
twin-wire forming section. Examples of twin-wire forming sections
are disclosed in U.S. Pat. Nos. 3,726,758, 4,056,433, and
4,879,001.
[0003] In papermaking, it is a goal to form a web in a forming
section of a paper machine that has a uniform consistency
throughout its length and width, that is, at all points of the web
the density of the pulp fibers is consistent. One known way of
forming such a uniform web is to increase the water content of the
fiber suspension that is ejected from the head box. Such a fiber
suspension is referred to as having a low consistency, typically
having a fiber content of about 1% or less. By using a low
consistency fiber suspension, the fibers in the suspension are more
free to flow as the wires gradually press them to form the web.
However, using a low consistency fiber suspension requires a great
deal of water which must subsequently be removed before a web of
dry paper or paperboard is obtained. To remove such great amounts
of water requires that the forming section include additional
dewatering elements which increase its cost, both in terms of the
initial investment and in terms of the cost to operate the machine.
Furthermore, pumping equipment must be sufficient to handle the
high volume of water used in forming the fiber suspension and the
high volume of water subsequently removed from the web. The capital
and operating cost of such pumping equipment also increases the
overall cost of the paper machine.
[0004] If instead a high consistency fiber suspension is employed,
that is, having a fiber content of greater than about 1% and as
high as about 2% to about 3% or as high as about 12%, less water is
used and less water has to be removed during the paper making
process, thereby decreasing the capital and operating cost of the
process. One fundamental characteristic of a high consistency
formed web is that its fibers are more or less randomly oriented in
all directions rather than in the plane of the web. A likely reason
for this phenomenon is that during drainage the densely packed
fibers collide with one another and cannot easily change their
orientation from vertical to horizontal. The result is that, rather
than forming a planar structure, the fibers form a felted structure
with high bulk, a random fiber orientation, high porosity, a grainy
formation, increased z-direction strength, and reduced in-plane
strength. Thus, fiber orientation affects the strength of the paper
and additionally determines whether the paper has the undesirable
tendency to curl. Additionally, in high consistency forming, flocks
or agglomerations of fibers tend to form which have typically been
difficult to break up. If these flocks are not broken up, it is
likely that the paper or paperboard ultimately formed will have
varying density, leading to a product without a uniform opacity
and/or thickness. While many of these characteristics are either
desirable or immaterial for some board grades, they are generally
not desirable for paper. High consistency forming has been used to
make 100 g/m.sup.2 or heavier board; however, no one has yet
developed a suitable high consistency former for making paper.
[0005] To remove water from the web in the forming section, foils
have traditionally been used which contact and press the forming
wires together. However, because such foils make a frictional
contact with the forming wires, they will necessarily cause wear to
the wires, thereby decreasing their useful life. In addition,
because the frictional forces applied by the foils to the forming
wires must be overcome by the equipment that move the wires, energy
use is increased. Vacuum is also frequently used to assist in
dewatering in the forming section. However, vacuum devices also
result in wear to the forming wires and require a great amount of
energy for their operation.
[0006] The prior art shows various methods of dewatering the web at
the end of a web forming section. In U.S. Pat. No. 3,285,806, a
press roll is positioned at the end of the forming section and
presses the web between the forming wire and a water receiving
pick-up felt. U.S. Pat. No. 3,671,389 discloses pressing the web
with a press felt in a forming section after the web has been
removed from the forming wire. U.S. Pat. No. 4,056,433 discloses a
twin-wire former in which a bottom wire leads the web into the
press section where the first press nip is between the bottom wire
and a felt. U.S. Pat. No. 4,879,001 discloses a twin-wire former in
which the bottom wire leads the web into a press nip in which a
felt is on one surface of the web and the bottom wire and a second
felt are on the other surface of the web. U.S. Pat. Nos. 5,389,205,
5,522,959 and 5,620,566 and PCT Patent Publication No. WO 98/32917
disclose the use of a roll nip press or an extended nip press at
the end of the forming section in which the web supported by a
bottom wire passes through the extended nip press along with a felt
or wire applied to the upper surface of the web. U.S. Pat. No.
5,820,731 discloses one or two press rolls are positioned at the
end of the forming section to press the web between the forming
wire and another wire. German Patent Document No. DE 196 54 200
discloses the use of prepressing in a forming section with a dry
content of less than or equal to 15 %. PCT Patent Publication No.
WO 97/13030 discloses prepressing the web in a press nip employing
a water impervious transfer belt. However, none of these references
teach that the use of prepressing of the web in the forming section
can be used to improve the characteristics of the web, such as
formation and/or fiber orientation.
SUMMARY OF THE INVENTION
[0007] In accordance with the present invention, a web of paper or
paper board is formed by ejecting a suspension of paper pulp stock
from a head box between a pair of forming wires. The forming wires
are brought together over at least one roll or alternatively two
rolls. The web thus formed then passes, sandwiched between two
wires (which may be the same two forming wires or only one of the
forming wires and a different fabric, that may be a wire, a felt or
a belt) through one or more pressure nips. The pressure nip may be
one or more roll nips or an extended nip in which the web is
pressed along a portion of its length in the extended nip press.
The pressure nip may be formed as the web travels between the two
forming wires along a straight path or alternatively as the web
travels partially around a roll. The web is then sandwiched between
a wire and a felt or a belt and passed through another press nip
which may be one or more roll nips or an extended nip. The linear
load in the press nips is preferably between about 50 kN/m to about
1000 kN/m.
[0008] To aid in contouring the web across the width of the web,
one or more of the press rolls in the nips may optionally be
selected so that the roll's shell is adjustable along its length.
Although foils may be used to aid in forming the web, fewer foils
are required or may even be omitted entirely. Less vacuum is
required to assist in dewatering of the web. Consequently, the
power required to drive the wires may be reduced, and the
frictional wear upon the wires is substantially eliminated, thereby
increasing their useful life.
[0009] Because the press nips are capable of removing the vast
preponderance of the water from the web, the length of the forming
section may be reduced and the subsequent drying section may be
simplified since fewer drying elements are required, thereby
resulting in a cost and space savings.
[0010] The present invention also allows the use of a high
consistency fiber suspension, of from about 2% to about 3% and as
high as about 10% (as compared with a conventional consistency of
about 1% or less), while at the same time producing a web of
substantially uniform consistency and with minimal formation of
flocks or agglomerations of fibers. Consequently, the problems
associated with low consistency fiber suspensions are substantially
eliminated--high water usage, a requirement for high pumping
capacity for the fiber suspension and the water removed from the
web, and a requirement for additional drying equipment in the
drying section of the paper machine.
[0011] The present invention is not only capable of providing
substantial dewatering of the web but in addition providing a web
with an improved structure. By applying pressure with one or more
press nips in the forming section, formation can be considerably
improved to result in more uniform fiber orientation in the plane
of the web, increased strength of the paper in the plane of the
web, and a decrease in the tendency of the finished paper to
curl.
[0012] The present invention can be used in conjunction with the
manufacture of all paper and paper board grades, and can be used in
new paper machines or as a retrofit to or rebuild of existing
machines. Web speeds of about 400 m/min to at least about 2700
m/min are achievable to ultimately form a web with a basis weight
of from about 30 to about 200 gmm.sup.2.
[0013] Other objects and features of the present invention will
become apparent from the following detailed description considered
in conjunction with the accompanying drawings. It is to be
understood, however, that the drawings are intended solely for
purposes of illustration and not as a definition of the limits of
the invention, for which reference should be made to the appended
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] In the drawings, wherein like reference numerals delineate
similar elements throughout the several views:
[0015] FIG. 1 is a schematic drawing of a web forming section of a
paper making machine in accordance with a first embodiment of the
present invention;
[0016] FIG. 2 is a schematic drawing of the embodiment of the
present invention shown in FIG. 1 and showing additional
embodiments thereof;
[0017] FIG. 3 is a schematic drawing of a web forming section of a
paper making machine in accordance with another embodiment of the
present invention;
[0018] FIG. 4 is a schematic drawing of a web forming section of a
paper making machine in accordance with another embodiment of the
present invention;
[0019] FIG. 5 is a schematic drawing of a web forming section of a
paper making machine in accordance with another embodiment of the
present invention;
[0020] FIG. 6 is a schematic drawing of a web forming section of a
paper making machine in accordance with another embodiment of the
present invention;
[0021] FIG. 7 is a schematic drawing of a web forming section of a
paper making machine in accordance with another embodiment of the
present invention;
[0022] FIG. 8 is a schematic drawing of a web forming section of a
paper making machine in accordance with another embodiment of the
present invention;
[0023] FIG. 9 is a schematic drawing of a web forming section of a
paper making machine in accordance with other embodiments of the
present invention;
[0024] FIG. 10 is a schematic drawing of a web forming section of a
paper making machine in accordance with another embodiment of the
present invention;
[0025] FIG. 11 is a schematic drawing of a web forming, section of
a paper making, machine in accordance with another embodiment of
the present invention;
[0026] FIG. 12 is a bar chart showing experimental results
employing prepressing, in accordance with the present
invention;
[0027] FIG. 13 is a graph showing experimental results of formation
employing, prepressing, in accordance with the present invention;
and
[0028] FIG. 14 is a graph showing experimental results of fiber
orientation employing prepressing in accordance with the present
invention.
DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS
[0029] FIG. 1 schematically depicts a web forming section of a
paper making machine in accordance with a first embodiment of the
present invention. The forming section includes an endless first
forming wire 4 and an endless second forming wire 6 which are
disposed adjacent a head box 2 so that paper or paperboard stock
suspension ejected from the head box 2 is fed between the first and
second forming wires 4, 6. The first and second forming wires 4, 6
travel toward the head box 2, as shown by the arrows, over a
plurality of guide rolls 8. The stock suspension is ejected
substantially vertically upward from the head box 2 between the
converging surfaces of the first and second wires 4, 6 as they
travel about rolls 10, 12 which bring the two forming wires 4, 6
together to have a common run. The forming wires 4, 6 with the web
sandwiched therebetween travel from roll 10 to roll 16 in a
substantially vertical path. Rolls 10, 12 and 16 are preferably
perforated or grooved so that water passing through the forming
wires 4, 6 can travel through or about the rolls 10, 12, 16. Rolls
10 and 16 preferably have one or more vacuum zones where vacuum is
applied to forming wires 4 and 6, respectively. The vacuum zones
are preferably individually controlled so that different vacuum
levels may be employed in each vacuum zone. The forming wires 4, 6
are pressed together in their substantially vertical common run due
to the tension in the wires 4, 6 and the path traveled by the
forming wires 4, 6 between roll 10 and roll 16. In particular, as
the pair of forming wires 4, 6 pass around roll 10, the wires are
pressed together, thereby expelling water from the fiber suspension
to form a paper or paperboard web. A plurality of foils 14 are
optionally positioned along the common run of the two wires 4, 6
between roll 10 and roll 16 to further press the wires 4, 6
together to remove water from the web. Some or all of the foils 14
may be backed by blades 20 which aid in pressing the two forming
wires 4, 6 together. In addition, some or all of the foils 14 may
be vacuum foils to further increase the amount of water that is
removed from the web. The wires 4, 6 with the web sandwiched
therebetween are then partially wrapped about roll 16. By having
the pair of wires 4, 6 pass partially around roll 16, the wires 4,
6 are pressed together to expel water from the web by centrifugal
dewatering and in addition the fibers are pressed together to form
a more cohesive web. A press roll 18 is positioned so that it
further presses the two forming wires 4, 6 together, thereby
pressing the web in a press nip N.sub.1 against roll 16, thereby
removing water from the web and improving the fiber structure of
the web.
[0030] After the web, sandwiched between the two forming wires 4,
6, has passed through the press nip N.sub.1 formed by rolls 16 and
18, additional water is removed from the web by suction box 22 and
foils 24, which may be vacuum foils to provide additional water
removal. The first forming wire 4 is then removed from the web by a
guide roll 8, and an endless fabric 26, guided by a guide roll 32,
is applied to the web by a transfer roll 30 backed by a support
roll 28, which is preferably a grooved or perforated roll. The nip
N.sub.1 formed by support roll 28 and transfer roll 30 is
preferably a press nip so that the web W will adhere to the bottom
side of the fabric 26 and can'thus be removed from the second
forming wire 6. In addition, when the nip N.sub.2 formed by support
roll 28 and transfer roll 30 is a pressure nip where pressure is
applied to the web, additional water is removed from the web in the
nip and the smoothness and the structure of the web is improved.
The web W is then removed from the second forming wire 6 by
adhering to the bottom surface of the fabric 26 for further
pressing and drying in the drying section of the paper machine (not
shown). To aid in the transfer of the web from the second forming
wire 6 to the fabric 26, support roll 28 can be perforated or
foraminous and has a plurality of air pressure zones where positive
air pressure is applied through the second forming wire 6 to the
web W so that the web will more easily be removed from the second
forming wire 6. If the fabric 26 is a felt, the transfer roll 30 is
preferably a pick-up roll having one or more vacuum zones.
Alternatively, the fabric 26 may be a transfer belt having a
surface to which the web adheres. Such a transfer belt is
described, for example, in U.S. Pat. No. 5,298,124, which is
incorporated herein by reference. When a transfer belt is used, the
transfer roll 30 preferably has a solid surface.
[0031] One or more of the rolls in one or more of the various nips
may alternatively be covered with a shrink-sleeve fabric jacket or
sleeve to form a shrink-sleeve press. In accordance with the
embodiment, the underlying roll may have a smooth surface (with
either a metal, rubber ore rubber-like surface), a grooved surface
or a perforated surface. Although in FIG. 1 the foils 14 are shown
as being disposed on only one side of the forming wires 4, 6, foils
may alternatively be positioned on both sides of the wires 4, 6.
Furthermore, more or fewer foils and blades than as shown in FIG. 1
may be used in accordance with the present invention. In fact, to
substantially eliminate friction upon the forming wires and thereby
increase their useful life, the foils and blade may be completely
eliminated. In accordance with this embodiment of the present
invention, which is shown in FIG. 2, water removal from the web and
forming of the web is accomplished by the tensioning and the
pressing of the forming wires along their path of travel in
conjunction the pressing of the web between the forming wires as
they pass through the pressure nips, N.sub.1 and N.sub.2.
[0032] The embodiment of FIG. 1 is shown in FIG. 2 in conjunction
with a number of alternative embodiments for pressing the web at
two locations in the forming section of the paper machine. Although
the foils 14, 24 and blades 20 are not employed in the embodiments
of the present invention shown in FIG. 2, these devices may
alternatively be used. The dashed box 100 designates the portion of
the forming section where the web, sandwiched between the first and
second forming wires 4, 6, are pressed in the nip N.sub.1 formed by
rolls 16 and 18. In the alternative embodiments of the present
invention shown in boxes 100a and 100b, the roll nip may instead be
an extended nip. In the embodiment shown in box 100a, roll 116a is
an extended nip press roll which has a press shoe 117a to press the
web along a portion of the length of the web. In the embodiment
shown in box 100b, roll 118b is an extended nip press roll which
has a press shoe 117b. In the embodiment shown in box 100c, two
press rolls 118c form two press nips, N.sub.1', N.sub.1", with roll
116c.
[0033] In FIG. 2 the dashed box 200 designates the portion of the
forming section where the web, sandwiched between the second
forming wire 6 and the fabric 26, are pressed in the nip N.sub.2
formed by rolls 28 and 30. In the alternative embodiments of the
present invention shown in boxes 200a and 200b, the roll nip
N.sub.2 may instead be an extended nip. In the embodiment shown in
box 200a, roll 230a is an extended nip press roll which has a press
shoe 229a to press the web along a portion of the length of the
web. In the embodiment shown in box 200b, roll 228b is an extended
nip press roll which has a press shoe 229b. In the embodiment shown
in box 200c, two press rolls 230c form two press nips, N.sub.2' and
N.sub.2" and with roll 228c.
[0034] In accordance with the present invention, any one of the
embodiments shown in boxes 100, 100a, 100b and 100c of FIG. 2 may
be used in conjunction with any of the embodiments shown in boxes
200, 200a, 200b, and 200c.
[0035] In FIG. 3 is shown an alternative embodiment of the
embodiment shown in FIG. 1. A suction box 34 is positioned along
the common run of the two forming wires 4, 6 between roll 12 and
roll 16. The suction box 34, which is positioned within the
perimeter of the second forming wire 6 and proximate to the surface
of the second forming wire 6, provides suction to the second
forming wire 6 and the web, as indicated by the negative sign, "-".
An air pressure box 36 is positioned within the perimeter of the
first forming wire 4 and extends along the common run of the two
forming wires 4, 6. The air pressure box 36, which also extends
partially around roll 16, has a plurality of air pressure zones
which are preferably individually controlled to capable of
providing varying degrees of air pressure, if desired. In
particular, as shown in FIG. 3, all of the air pressure zones
except for the final zone provide suction or negative pressure to
the surface of the first forming wire 4, while the final zone
provides a positive air pressure, as indicated by the positive
sign, "+". Roll 16 is also provided with a plurality of air
pressure zones which are preferably individually controlled to
capable of providing varying degrees of positive or negative air
pressure, if desired. In FIG. 3, all of the air pressure zones of
roll 16 provide positive air pressure to the second forming wire 6
except for the final zone which provides a negative pressure, or
suction. A press roll 18 is optionally used in conjunction with
roll 16 to press the web sandwiched between the two forming wires
4, 6. In operation, the air pressure zones aid in transporting the
web along the desired path, and the negative pressure (or suction)
zones additionally aid in removing water from the web and forming
wires 4, 6. Alternatively, the optional press roll 18 may be
replaced by an extended nip roll configuration as shown in boxes
100a and 100b of FIG. 2, or there may be two roll nips as shown in
box 100c of FIG. 2. Alternatively, at nip N.sub.1 the press shoe
may be positioned in the lower roll as shown in box 200a of FIG. 2,
the nip may be a press nip as shown in box 200 of FIG. 2, or there
may be two roll nips as shown in box 200c of FIG. 2.
[0036] The embodiment of the present invention shown in FIG. 4 is
substantially the same as that shown in FIG. 3 except that an
initial felt 38 is introduced between roll 10 and wire 4 and
partially wraps about roll 10. The felt 38 is guided by a number of
guide rolls 8. A suction box 40 is positioned within the perimeter
of felt 38 and proximate the surface of roll 10. Suction box 40 has
a plurality of suction zones, designated by a negative sign, "-".
These suction zones are positioned proximate the point where the
felt 38 begins and ends its partial wrap about roll 10. Roll 10 has
a positive air pressure zone proximate the suction box 40. Felt 38
is used to aid in removing water from the web through the first
forming wire 4. The suction zones of the suction box 40 and the
positive air pressure zone of roll 10 aid in removing water that
comes off of the felt 38. If, however, two-sided sheet properties
are preferred, the felt 38 can be replaced by a non-water-receiving
belt. In this case, no pressure or vacuum zones are needed.
Alternatively, the optional press roll 18 may be replaced by an
extended nip roll configuration as shown in boxes 100a and 100b of
FIG. 2, or there may be two roll nips as shown in box 100c of FIG.
2. Alternatively, at nip N.sub.2 the press shoe may be positioned
in the lower roll as shown in box 200a of FIG. 2, the nip may be a
press nip as shown in box 200 of FIG. 2, or there may be two roll
nips as shown in box 200c of FIG. 2.
[0037] The embodiment of the present invention shown in FIG. 5 is
similar to that shown in FIG. 3 with a few differences. The first
forming wire 4, instead of the second forming wire 6, passes
through the second nip N.sub.2. In addition, the fabric 26 (i.e., a
felt or belt) is applied to the web W immediately prior to the
second nip N.sub.2 so that the top surface of the fabric 26 is in
contact with the web, rather than the bottom surface of the fabric
26. The second nip N.sub.2 is formed by an extended nip press where
the press shoe 229b is in the bottom roll 230b. Alternatively, the
optional press roll 18 may be replaced by an extended nip roll
configuration as shown in boxes 100a and 100b of FIG. 2, or there
may be two roll nips as shown in box 100c of FIG. 2. Alternatively,
at nip N.sub.2 the press shoe may be positioned in the upper roll
as shown in box 200a of FIG. 2, the nip may be a press nip as shown
in box 200 of FIG. 2, or there may be two roll nips as shown in box
200c of FIG. 2.
[0038] In the embodiment of the present invention shown in FIG. 6,
the first forming wire 4 passes only through the first nip,
N.sub.1, while the second forming wire 6 passes partially around
roll 16 and through the second nip N.sub.2. Roll 12 is completely
eliminated, and the stock suspension is ejected substantially
horizontally from the head box 2 between the converging surfaces of
the first and second wires 4, 6 as they travel between rolls 10, 16
which bring the two forming wires 4, 6 together. A suction box 40
is positioned within the perimeter of forming wire 4 and proximate
the surface of roll 10. Roll 10 has a positive air pressure zone
proximate the suction box 40. The suction zones of the suction box
40 and the positive air pressure zone of roll 10 aid in removing
water that comes off of the felt 38. In alternatives of this
embodiment, the optional press roll 18 may be replaced by an
extended nip roll configuration as shown in boxes 100a and 100b of
FIG. 2, or there may be two roll nips as shown in box 100c of FIG.
2. Alternatively, in nip N.sub.2 the press shoe may be positioned
in the bottom roll as shown in box 200bof FIG. 2, the nip may be a
press nip as shown in box 200 of FIG. 2, or there may be two roll
nips as shown in box 200c of FIG. 2.
[0039] The embodiment of the present invention shown in FIG. 7 is
substantially the same as that shown in FIG. 6 except that the
first forming wire 4, rather than the second forming wire 6, passes
through the second nip N.sub.2. Consequently, the web is applied to
the top surface of the felt rather than the bottom surface, as
shown in FIG. 6. In addition, the head box 2 is positioned so that
the paper pulp suspension is ejected in a substantially vertical
direction. Finally, at the second nip N.sub.2, the press shoe 229b
is located in the lower roll 228b. In alternatives of this
embodiment, the optional press roll 18 may be replaced by an
extended nip as shown in boxes 100a and 100b of FIG. 2, or there
may be two roll nips as shown in box 100c of FIG. 2. Alternatively,
the press shoe may be positioned in the top roll as shown in box
200a of FIG. 2, the nip may be a press nip as shown in box 200 of
FIG. 2, or there may be two roll nips as shown in box 200c of FIG.
2.
[0040] In the embodiment of the present invention shown in FIG. 8,
only the second forming wire 6 passes through both nips, N.sub.1
and N.sub.2. The first forming wire 4 is used only in initially
forming the web and to aid in transporting the web around roll
116b. The stock suspension is ejected substantially horizontally
from the head box 2 between the converging surfaces of the first
and second wires 4, 6 as they travel between roll 116b and guide
roll 8awhich bring the two forming wires 4, 6 together. Roll 116b
has a plurality of suction zones which aid in removing water from
the web and in ensuring that the web remains in contact with roll
116b as the first forming wire 4 is removed from the web before the
web enters the first nip N.sub.1. After the first forming wire has
been removed from the web, a third forming wire 7 is applied to the
web so that the web is sandwiched between the second forming wire 6
and the third forming wire 7 as they pass through the first nip
N.sub.1. The web sandwiched between the two forming wires 6, 7
passes through the second nip N.sub.2. Then the second forming wire
6 is removed from the web, the fabric 26 is applied to the bottom
surface of the web, and the third forming wire 7 is removed from
the top surface of the web. The web, supported by the fabric 26
then travels to the drying section of the paper machine for further
dewatering and drying. The position of a guide rolls 8a located
along the path of travel of the third forming wire 7 is preferably
adjustable so that the tension in the wires 6, 7 can be adjusted.
The extended nips N.sub.1 and N.sub.2 may alternatively be replaced
with any of the roll configurations shown in boxes 100, 100a or
100c, and 200, 200b or 200c, respectively.
[0041] In the embodiment of the present invention shown in FIG. 9,
both forming wires pass through both nips N.sub.1 and N.sub.2. The
stock suspension is ejected substantially horizontally from the
head box 2 between the converging surfaces of the first and second
wires 4, 6 as they travel between roll 10 and roll 12 which bring
the two forming wires 4, 6 together. In this embodiment, the web,
sandwiched between wires 4 and 6 pass through one or more initial
press nips N.sub.0 before travelling through nip N.sub.1 formed by
rolls 116a and 118a. The initial press nip N.sub.0 may be formed as
a roll nip by two rolls 302a, 304a as shown in box 300a, or as an
extended nip as shown in boxes 300b and 300c, with the press shoe
in either the upper roll 304b and a solid roll 302b in the lower
position, as shown in box 300b, or with the press shoe in the lower
roll 302a and a solid roll 304c in the upper position, as shown in
box 300c. The particular number and type of initial press nips No
is selected to obtain a desired web dryness and structure. Thus,
there may be one, two or more initial press nips N.sub.1 which may
be press nips, extended nips, or a combination thereof. In
alternatives of this embodiment, first nip N.sub.1 may be replaced
by an extended nip with the press shoe in the supper roll as shown
in box 100b of FIG. 2, or there may be a single roll nip as shown
in box 100 of FIG. 2, or there may be two roll nips as shown in box
100c of FIG. 2. Alternatively, in the second nip N.sub.2, there can
be an extended nip as shown in boxes 200a and 200b of FIG. 2, or
there may be two roll nips as shown in box 200c of FIG. 2.
[0042] In the embodiment of the present invention shown in FIG. 10,
the stock suspension is ejected substantially horizontally from the
head box 2 between the converging surfaces of the first and second
wires 4, 6 as they travel between roll 10 and roll 12 which bring
the two forming wires 4, 6 together. In this embodiment, a suction
box 40 is positioned within the perimeter of wire 6 and proximate
the surface of roll 12. Suction box 40 has a plurality of suction
zones, designated by a negative sign, "-". These suction zones are
positioned proximate the point where the wire 6 begins and ends its
partial wrap about roll 10. Roll 12 has a positive air pressure
zone proximate the suction box 40. The suction and pressure zones
are aid in removing water from roll 12 and from the web. A suction
box 34 is positioned along the common run of wires 4, 6 from rolls
10, 12 to roll 16. A suction box 36 is positioned proximate the
surface of roll 16 and has a plurality of suction zones to aid in
removing water from the web and to ensure that the web travels
along with wire 6 to nip N.sub.2. All of the pressure zones in
suction box 36 are suction or negative pressure zones. Roll 16 has
a plurality of pressure zones. The fabric 26 is applied to the
upper surface of the web, and the web, sandwiched between fabric 26
and wire 6 passes through nip N.sub.2. As shown in boxes 100, 100a,
100b and 100b of FIG. 2, the optional nip N.sub.1 may have various
configurations, and nip N.sub.2 may also have various alternative
configurations as shown in boxes 200, 200b and 200c of FIG. 2.
[0043] The embodiment shown in FIG. 11 is similar to that shown in
FIG. 10 except that wire 4 travels through nip N.sub.2 rather than
wire 6. Consequently, fabric 26 is applied to the bottom surface of
the web as it passes through nip N.sub.2 rather than the top
surface of the web. Also, the position of the press shoe in nip
N.sub.2 is reversed from that shown in FIG. 10. The last difference
relates to the pressure zones in roll 16 and in suction box 36. In
FIG. 11, all of the pressure zones in roll 16 are positive except
for the final zone, and all of the pressure zones in suction box 36
are negative except for the final zone which is positive.
[0044] Although in the foregoing embodiments various specific
locations for pressure and suction zones of various rolls and air
pressure boxes are disclosed, the pressure zones may be located in
additional rolls and/or additional pressure boxes may be added.
Furthermore, the specific pressures (positive or negative) of the
pressure zones and the number of such pressure zones may be changed
to improve the transportation of the web and/or to increase water
removal. Also, the angle at which the suspension is ejected from
the head box may be varied as desired. In addition, so that the
tension of the forming wires may be adjusted, the positions of one
or more of the guide rolls 8 within a circuit of the forming wires
may be made to be adjustable. It must be understood that in any of
the embodiments according to the present invention one of the
forming wires can be substituted by a belt that is substantially
not-water-receiving. The surface of such a belt may optionally have
a wire-type structure. This alternative is especially beneficial
when two-sided paper properties, and hence one-sided dewatering, is
preferred.
[0045] In any of the above described embodiments of the present
invention, any one or more of the particular press rolls employed
may be selected so that the pressure applied to the web may be
adjusted at a number of points across the width of the web, that is
in the direction perpendicular to the direction of travel of the
web through the paper machine. By making the pressure being applied
to the web so adjustable, inconsistencies in the thickness of the
web across its width can be substantially removed.
[0046] Preferably, the linear load in the pressure nips are between
about 50 kN/m and about 2000 kN/m, and the web speed is from about
400 m/min to about 2700 m/min. Preferably, the linear load in the
first pressure nip N.sub.1 is less than that in the second pressure
nip N.sub.2.
[0047] FIG. 12 is a bar chart showing experimental results
employing a prepressing unit substantially as the nip N.sub.2 in
FIG. 3 using a shoe press nip. The web was pressed between the wire
and a non-water-receiving transfer belt. Sheets of newspaper grade
paper with a basis weight of 50 g/m.sup.2 were introduced at a web
speed of 16.7 m/sec. The paper was introduced into the first shoe
press nip with an incoming dryness of 10%. The linear load of the
shoe presses for each trial run is shown within the bar of the
chart. For the first run, the reference run, there was no linear
load, that is the shoe press did not press the web to any extent.
The linear load in the shoe press for run No. 11 was 150 kN/m, for
run No. 14 the linear load was 1300 kN/m, for run No. 25 the load
was 1700 kN/m, and for run No. 16.1 the load was 450 kN/m. The
height of the bar, designated "Formation", reflects how well the
web was formed and is used as a measure of how evenly distributed
the fibers of the web are--the lower the formation value, the more
evenly distributed the fibers in the web, and the better the paper
quality is. Formation was determined by measuring the mass of the
web per area (g/m.sup.2) using a conventional formation meter
employing a radiation source and a radiation detector. For each
run, the web was sampled at about 100 random positions. A mean
value was obtained and the standard deviation from that mean value
is shown in the bar graph. FIG. 12 demonstrates that formation was
improved upon the use of the formation configuration employing shoe
presses in accordance with the present invention.
[0048] FIG. 13 shows the effect of web dryness at the press inlet
and the linear load in the shoe press on web formation as tested
using a forming process in accordance with the present invention
Several test runs were performed under different combinations of
conditions, namely, the dryness of the web prior to being pressed
in the shoe press (incoming dryness) and the pressure applied to
the web in the shoe press (linear load). The quality of formation,
reflected by the formation values, was assessed by the same method
described above. As in FIG. 12, the formation value reflects how
well the web was formed in each test case--the lower the formation
value, the more evenly distributed the fibers in the web and the
better the paper quality. The formation quality is, under the test
conditions, improved by decreasing the incoming dryness or
increasing the linear load, or both.
[0049] FIG. 14 shows the results of a study on orientation
anisotropy behavior in thickness direction of the paper sheet
formed in accordance with the present invention. The study was
conducted using the same sample as in Case 2 of the formation
analysis shown in FIG. 13, where the dryness at the press nip inlet
(incoming dryness) was 8%. Orientation anisotropy is defined as the
ratio of major axis to minor axis of the ellipse describing the
fibers' pattern of orientation relative to the machine direction.
This ellipse can be measures in a variety of way, such as, for
example, optically, with ultra sound or with depolarized light. The
anisotropy value reflects the tendency of the fibers to be oriented
in the direction, i.e., in the thickness direction of the web. In
general for paper making, the lower the anisotropy value, the
better the paper quality. The data shown in FIG. 14 shows a
decreasing tendency of orientation anisotropy as a function of
increased linear load.
[0050] The results of these and other trial runs demonstrate that
the evenness of a paper web and thus formation is improved when the
web is pre-pressed in the formation section of a paper making
machine. Those studies also reveal that the improvements are more
pronounced on the side of the web that carries a higher hydraulic
pressure during pressing than the opposing side.
[0051] Thus, while there have been shown and described and pointed
out fundamental novel features of the present invention as applied
to a preferred embodiment thereof, it will be understood that
various omissions and substitutions and changes in the form and
details of the devices illustrated, and in their operation, may be
made by those skilled in the art without departing from the spirit
of the present invention. For example, it is expressly intended
that all combinations of those elements and/or method steps which
perform substantially the same function in substantially the same
way to achieve the same results are within the scope of the
invention. Substitutions of elements from one described embodiment
to another are also fully intended and contemplated. It is also to
be understood that the drawings are not necessarily drawn to scale
but that they are merely conceptual in nature. It is the intention,
therefore, to be limited only as indicated by the scope of the
claims appended hereto.
* * * * *