U.S. patent application number 09/942649 was filed with the patent office on 2002-04-25 for vented lead blade.
Invention is credited to Buchanan, John G., Pitt, Richard E., Van Essen, Roy.
Application Number | 20020046821 09/942649 |
Document ID | / |
Family ID | 9898876 |
Filed Date | 2002-04-25 |
United States Patent
Application |
20020046821 |
Kind Code |
A1 |
Buchanan, John G. ; et
al. |
April 25, 2002 |
Vented lead blade
Abstract
A vented lead blade for use in a paper making machine located so
that the stock jet impinges the forming fabric at, or near to, its
trailing edge. The vented lead blade serves to bend the forming
fabric before it enters the forming section, and to vent at least a
substantial proportion of any air which becomes trapped in the
wedge shaped space between the surface of the forming fabric and
the surface of the stock jet, together with at least some of any
liquid carried with the air. The vented lead blade of this
invention thus enables the forming fabric to be positioned so that
the angle of impingement of the stock jet onto the forming fabric
can be minimized to become substantially zero. It also makes it
possible to reduce the unsupported length of the stock jet. The
vented lead blade of this invention can be used in both an open
surface forming section with one forming fabric, or in the forming
section of a two fabric paper making machine.
Inventors: |
Buchanan, John G.; (Ottawa,
CA) ; Pitt, Richard E.; (Almonte, CA) ; Van
Essen, Roy; (North Gower, CA) |
Correspondence
Address: |
Robert A. Wilkes
SHAPIRO COHEN
Stn. D
P.O. Box 3440
Ottawa
ON
K1P 6P1
CA
|
Family ID: |
9898876 |
Appl. No.: |
09/942649 |
Filed: |
August 31, 2001 |
Current U.S.
Class: |
162/353 |
Current CPC
Class: |
D21F 1/483 20130101;
D21F 9/02 20130101; D21F 9/003 20130101 |
Class at
Publication: |
162/353 |
International
Class: |
D21F 001/58 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 5, 2000 |
GB |
0021767.9 |
Claims
What is claimed is:
1. A curved vented lead blade for use in a paper making machine
upstream of, and immediately adjacent to, the point of impingement
of a stock jet ejected from a head box slice onto a moving forming
fabric carrying an amount of liquid, which lead blade has a leading
edge, a shaped trailing edge portion, including a trailing edge,
and a convexly curved surface located between the leading edge and
the trailing edge portion, over which the forming fabric moves in
sliding contact and wraps through an angle of wrap .theta., the
convexly curved surface and the trailing edge portion including a
plurality of grooves which begin at the leading edge and end at the
trailing edge constructed and arranged to vent at the lead blade
trailing edge at least a substantial proportion of any air trapped
in the forming fabric from a wedge shaped space between the stock
jet and the forming fabric surface, and at least some of the liquid
carried by the forming fabric.
2. A papermaking machine, having a machine direction and a cross
machine direction, which includes: a forming fabric moving in the
machine direction; a head box including a head box slice which
provides a jet of paper making stock which impinges at an angle of
impingement onto the forming fabric at a point of impingement; a
roll, having a cylindrical surface whose axis is in the cross
machine direction, about which the forming fabric passes, and which
is located upstream of the head box slice; a forming section,
located downstream of the point of impingement, including static
support elements which define a fabric path through which the
forming fabric passes; and a vented lead blade, located upstream
of, and immediately adjacent to, the point of impingement of a
stock jet ejected from a head box slice onto a moving forming
fabric, about which a forming fabric wraps through a wrap angle
.theta.; wherein the vented lead blade has a leading edge, a shaped
trailing edge portion, including a trailing edge, and a convexly
curved surface located between the leading edge and the trailing
edge portion, over which the forming fabric moves in sliding
contact and wraps through an angle of wrap .theta., the convexly
curved surface and the trailing edge portion including a plurality
of grooves which begin at the leading edge and end at the trailing
edge constructed and arranged to vent at the lead blade trailing
edge at least a substantial proportion of any air trapped in the
forming fabric from a wedge shaped space between the stock jet and
the lead blade curved surface, and at least some of the liquid
carried by the forming fabric.
3. A papermaking machine, having a machine direction and a cross
machine direction, which includes: a first and a second forming
fabric each moving in the machine direction; a head box including a
head box slice which provides a jet of paper making stock which
impinges at an angle of impingement onto at least one of the
forming fabrics at a point of impingement; two rolls, each having a
cylindrical surface whose axis is in the cross machine direction,
about each of which one of the forming fabric passes, and which are
located adjacent the head box slice; a forming section, located
downstream of the point of impingement, including static support
elements which define a fabric path through which the forming
fabric passes; and two vented lead blades, each located upstream
of, and immediately adjacent to, the point of impingement of the
stock jet, about the first of which the first forming fabric wraps
through a wrap angle .theta..sub.1, and about the second of which
the second forming fabric wraps through a wrap angle .theta..sub.2;
wherein each vented lead blade has a leading edge, a shaped
trailing edge portion, including a trailing edge, and a convexly
curved surface located between the leading edge and the trailing
edge portion, over which the forming fabric moves in sliding
contact and wraps through an angle of wrap, the convexly curved
surface and the trailing edge portion including a plurality of
grooves which begin at the leading edge and end at the trailing
edge constructed and arranged to vent at the lead blade trailing
edge at least a substantial proportion of any air trapped in the
forming fabric from a wedge shaped space between the stock jet and
the lead blade curved surface, and at least some of the liquid
carried by the forming fabric.
4. A curved vented lead blade according to claim 1 wherein the
convexly curved surface is essentially circular, with a constant
radius.
5. A curved vented lead blade according to claim 1 wherein the
shaped trailing edge portion conforms to the convexly curved
surface.
6. A curved vented lead blade according to claim 1 wherein the
convexly curved surface is not circular, and the radius of
curvature increases towards the trailing edge.
7. A curved vented lead blade according to claim 1 wherein the
convexly curved surface is not circular, the radius of curvature
increases towards the trailing edge, and the shaped trailing edge
portion is essentially flat.
8. A curved vented lead blade according to claim 1 wherein the
convexly curved surface has an effective radius of curvature that
is from about 50% to about 100% of the radius of the adjacent
upstream roll.
9. A curved vented lead blade according to claim 1 wherein the
angle .theta. is from about 10.degree. to about 40.degree..
10. A curved vented lead blade according to claim 9 wherein the
angle .theta. is from about 15.degree. to about 30.degree..
11. A curved vented lead blade according to claim 10 wherein the
angle .theta. is from about 20.degree. to about 25.degree..
12. A paper making machine according to claim 3 wherein the two
vented lead blades are the same, and their convexly curved surfaces
are the same shape.
13. A paper making machine according to claim 3 wherein the two
vented lead blades are not the same, and their convexly curved
surfaces are not the same shape.
14. A paper making machine according to claim 3 wherein the two
vented lead blades are the same, the angles .theta..sub.1 and
.theta..sub.2 are the same, and the convexly curved surfaces are
the same shape.
15. A paper making machine according to claim 3 wherein the two
vented lead blades are not the same, the angles .theta..sub.1 and
.theta..sub.2 are not the same, and the two convexly curved
surfaces are not the same shape.
16. A paper making machine according to claim 2 including a vented
lead blade according to claims 4, 5, 6, 7, 8, 9 or 10.
17. A paper making machine according to claim 3 including a first
vented lead blade according to claims 4, 5, 6, 7, 8, 9 or 10.
18. A paper making machine according to claim 3 including a second
vented lead blade according to claims 4, 5, 6, 7, 8, 9 or 10.
Description
FIELD OF THE INVENTION
[0001] This invention concerns a vented lead blade for use in a
paper making machine.
BACKGROUND OF THE INVENTION
[0002] In the initial portion of the forming section of either a
single or two fabric papermaking machine, an unsupported jet of
highly aqueous stock is ejected from the head box slice onto the
surface of a moving forming fabric. The unsupported jet will
typically traverse a distance of from about 6 cm to about 40 cm
before impinging the surface of the forming fabric at the point of
impingement. The angle of impingement, .alpha., formed between the
stock jet and the plane of the forming fabric at the point of
impingement will typically be from about 40.degree. to about
10.degree.. It is well known that improved paper formation can be
obtained by minimizing both the angle a and the length of the
unsupported free jet.
[0003] As the angle .alpha. increases, the magnitude of the
pressure exerted by the jet on the surface of the forming fabric
also increases.
[0004] For a forming fabric moving at a speed of 20 m/sec and a jet
of pure water, the impingement pressure I in kPa can be shown to be
given by the following relationship:
I=200 sin .alpha..
[0005] Typical peak impingement pressures for different values of
the angle a are shown in Table 1.
1TABLE 1 Peak Impingement Pressure Angle of Impingement,
.alpha..degree. Pressure (I, kPa) 1 3.49 2 6.98 4 13.95 6 20.90 8
27.83 10 34.73
[0006] Impingement angles greater than about 5.degree. will create
peak impingement pressures that may cause sheet marking, low
retention of paper making fines and fillers, and plugging of the
forming fabric. Therefore, the angle .alpha. should be made as
small as possible so that, ideally, the unsupported stock jet
impinges on the fabric substantially tangentially.
[0007] As the length of the stock jet increases, its outside
surface begins to break up into ridges and furrows, which will
eventually cause sheet basis weight variations. Further, finely
dispersed fibers in the stock start to reflocculate rapidly in the
unsupported jet prior to the point of impingement. Therefore, the
unsupported stock jet should be made as short as possible to
minimize these effects.
[0008] Due to the competing space requirements of both the head box
slice lip structure and the adjacent upstream rolls such as a
breast roll, it is difficult to shorten the unsupported stock jet
length without increasing the angle .alpha.. Even if the head box
slice lips can be located so that the angle a is very small, and
the free jet is nearly tangential to the forming fabric, air that
is trapped in the small wedge shaped space between the surface of
the forming fabric and the surface of the unsupported stock jet
becomes entrained into the stock forming bubbles which are
detrimental to sheet formation.
[0009] Several proposals have been made to overcome these
difficulties. Nelson et al, U.S. Pat. No. 3,440,136 discloses a
method of avoiding air entrainment by evacuating the air from the
forming zone, and flooding this area with water. However this
proposal has been found to be difficult to realize in practice.
Irwin et al., U.S. Pat. No. 4,734,164, disclose a forming board for
a single fabric machine in which the first blade is slightly curved
to permit the breast roll to be lowered slightly. The difficulty
with this proposal is that air is trapped in the shallow wedge
space between the jet and the forming fabric as it passes over the
curved first blade. This air is forced into the stock as bubbles
which cause formation defects. Malashenko, U.S. Pat. No. 4,802,954,
discloses a lead-in blade located ahead of the curved blade element
proposed by Irwin et al., which is said to reduce the amount of
fluid pumped by the forming fabric into the wedge shaped space
between the jet and the fabric. However, a small wedge shaped air
space remains. The pressure in this space is controlled by a vacuum
pump to reduce the jet disturbance. Ewald in U.S. Pat. No.
5,084,138 addresses the problem of excessive free jet length by
using curved turning bars to replace large diameter breast rolls
and a solid curved blade, but does not avoid air entrapment at the
wedge between the jet and the fabric.
[0010] The present invention seeks to provide a lead blade for use
in the forming shoe or forming board of a high speed paper making
machine which will eliminate or at least substantially reduce all
of the aforementioned deficiencies of the prior art by means of a
construction and arrangement which at least in part adopts these
recommendations.
SUMMARY OF THE INVENTION
[0011] This invention seeks to provide a vented lead blade for use
downstream of the breast roll, between the breast roll and the
fabric supporting elements in the forming section, and located so
that the stock jet impinges the forming fabric at, or near to, its
trailing edge. The vented lead blade does two things. First, it
serves to bend the forming fabric before it enters the forming
section. Second, it serves to vent at least a substantial
proportion of any air which becomes trapped in the wedge shaped
space between the surface of the forming fabric and the surface of
the stock jet. The vented lead blade of this invention thus enables
the forming fabric to be positioned so that the angle .alpha. can
be minimized, and, if desired, by allowing the breast roll to be
repositioned to create space into which the head box can be moved,
thus reducing the unsupported length of the stock jet. The vented
lead blade of this invention can be used in both an open surface
forming section with one forming fabric, or in the forming zone of
a two fabric paper making machine.
[0012] Thus in a first broad embodiment this invention seeks to
provide a curved vented lead blade for use in a paper making
machine upstream of, and immediately adjacent to, the point of
impingement of a stock jet ejected from a head box slice onto a
moving forming fabric carrying an amount of liquid, which lead
blade has a leading edge, a shaped trailing edge portion, including
a trailing edge, and a convexly curved surface located between the
leading edge and the trailing edge portion, over which the forming
fabric moves in sliding contact and wraps through an angle of wrap
.theta., the convexly curved surface and the trailing edge portion
including a plurality of grooves which begin at the leading edge
and end at the trailing edge constructed and arranged to vent at
the lead blade trailing edge at least a substantial proportion of
any air trapped in the forming fabric from a wedge shaped space
between the stock jet and the forming fabric surface, and at least
some of the liquid carried by the forming fabric.
[0013] In a second broad embodiment this invention seeks to provide
a papermaking machine, having a machine direction and a cross
machine direction, which includes:
[0014] a forming fabric moving in the machine direction;
[0015] a head box including a head box slice which provides a jet
of paper making stock which impinges at an angle of impingement
onto the forming fabric at a point of impingement;
[0016] a roll, having a cylindrical surface whose axis is in the
cross machine direction, about which the forming fabric passes, and
which is located upstream of the head box slice;
[0017] a forming section, located downstream of the point of
impingement, including static support elements which define a
fabric path through which the forming fabric passes; and
[0018] a vented lead blade, located upstream of, and immediately
adjacent to, the point of impingement of a stock jet ejected from a
head box slice onto a moving forming fabric, about which a forming
fabric wraps through a wrap angle .theta.;
[0019] wherein the vented lead blade has a leading edge, a shaped
trailing edge portion including a trailing edge and a convexly
curved surface, located between the leading edge and the trailing
edge portion, over which the forming fabric moves in sliding
contact and wraps through an angle of wrap .theta., the convexly
curved surface and the trailing edge portion including a plurality
of grooves which begin at the leading edge and end at the trailing
edge constructed and arranged to vent at the lead blade trailing
edge at least a substantial proportion of any air trapped in the
forming fabric from a wedge shaped space between the stock jet and
the lead blade curved surface, and at least some of the liquid
carried by the forming fabric.
[0020] In a third broad embodiment this invention seeks to provide
a papermaking machine, having a machine direction and a cross
machine direction, which includes:
[0021] a first and a second forming fabric each moving in the
machine direction;
[0022] a head box including a head box slice which provides a jet
of paper making stock which impinges at an angle of impingement
onto at least one of the forming fabrics at a point of
impingement;
[0023] two rolls, each having a cylindrical surface whose axis is
in the cross machine direction, about each of which one of the
forming fabric passes, and which are located adjacent the head box
slice;
[0024] a forming section, located downstream of the point of
impingement, including static support elements which define a
fabric path through which the forming fabric passes; and
[0025] two vented lead blades, each located upstream of, and
immediately adjacent to, the point of impingement of the stock jet,
about the first of which the first forming fabric wraps through a
wrap angle .theta..sub.1, and about the second of which the second
forming fabric wraps through a wrap angle .theta..sub.2;
[0026] wherein each vented lead blade has a leading edge, a shaped
trailing edge portion including a trailing edge and a convexly
curved surface, located between the leading edge and the trailing
edge portion, over which the forming fabric moves in sliding
contact and wraps through an angle of wrap, the convexly curved
surface and the trailing edge portion including a plurality of
grooves which begin at the leading edge and end at the trailing
edge constructed and arranged to vent at the lead blade trailing
edge at least a substantial proportion of any air trapped in the
forming fabric from a wedge shaped space between the stock jet and
the lead blade curved surface, and at least some of the liquid
carried by the forming fabric.
[0027] Preferably, the convexly curved surface is essentially
circular, with a constant radius. More preferably, the shaped
trailing edge portion conforms to the convexly curved surface.
Alternatively, the convexly curved surface is not circular, and the
radius of curvature increases towards the trailing edge.
Conveniently, the convexly curved surface is not circular, the
radius of curvature increases towards the trailing edge, and the
shaped trailing edge portion is essentially flat.
[0028] Preferably, the convexly curved surface has an effective
radius of curvature that is from about 50% to about 100% of the
radius of the adjacent upstream roll. The radius of curvature will
not generally be greater than that of the adjacent upstream
roll.
[0029] Preferably, the angle .theta., .theta..sub.1 or
.theta..sub.2 through which the forming fabric wraps the lead blade
is from about 10.degree. to about 40.degree.. More preferably, the
angle of wrap is from about 15.degree. to about 300.degree.. Most
preferably, the angle of wrap is from about 20.degree. to about
25.degree..
[0030] Preferably, in a paper making machine having two forming
fabrics, the two vented lead blades are the same, and their
convexly curved surfaces are the same shape. Alternatively, in a
paper making machine having two forming fabrics, the two vented
lead blades are not the same, and their convexly curved surfaces
are not the same shape.
[0031] Preferably, in a paper making machine having two forming
fabrics, the two vented lead blades are the same, the angles
.theta..sub.1 and .theta..sub.2 are the same, and their convexly
curved surfaces are the same shape. Alternatively, in a paper
making machine having two forming fabrics, the two vented lead
blades are not the same, the angles .theta..sub.1 and .theta..sub.2
are not the same, and their convexly curved surfaces are not the
same shape.
[0032] The invention will now be described with reference to the
attached schematic drawings in which:
[0033] FIG. 1 shows part of an open surface paper making machine
including a vented lead blade;
[0034] FIGS. 2 and 3 show parts of two different twin fabric paper
making machines;
[0035] FIGS. 4, 5 and 6 show the effects of wrap angle and vented
lead blade effective radius;
[0036] FIGS. 7, 8 and 9 show three different groove arrangements
for the vented lead blade top surface; and
[0037] FIG. 10 shows four different groove shapes.
[0038] In FIGS. 1-6 only the parts of the paper making machine
required for an understanding of this invention are shown.
[0039] FIG. 1 shows a paper making machine which includes a vented
lead blade according to the teachings of this invention. The slice
lips 1A and 1B of the head box deliver a jet of stock 2 onto the
forming fabric 3 at the impingement point I. The forming fabric 3
passes around the roll 6, wraps the vented lead blade 5, and then
passes over the first static support element 4 in the forming
section. As the moving forming fabric 3 and the stock jet 2
converge, air 8 in the wedge shaped space between the surface of
the stock jet 9, the surface 10 of the head box lip 1B, and the
surface of the forming fabric 11 becomes trapped and enters the
forming fabric 3. This air, together with some of the liquid in the
forming fabric at this point, passes through the forming fabric 3
into the grooves 12 in the top surface of the vented lead blade
5.
[0040] In this arrangement, since the axis 15 of the roll 6 is now
located out of the fabric path 7, and since the vented lead blade 5
occupies far less space than the roll, it is possible to maintain
the angle of impingement close to zero, and to shorten the
unsupported length D of the stock jet 2 significantly. Further,
since the trailing edge 14 of the vented lead blade 5 is closely
adjacent the impingement point I, the trapped air is prevented from
interfering in the formation process within the stock in the
forming section. The grooves 12 are located and dimensioned so that
at least a substantial proportion, and preferably all, of the air
passing through the forming fabric, together with any liquid
carried with the air from the liquid on the forming fabric at this
point, is vented as at 13 from the vented lead blade at its
trailing edge 14.
[0041] FIGS. 2 and 3 show a paper making machine which has two
forming fabrics, and which incorporates two vented lead blades
according to the teachings of this invention. In describing these
two Figures the terms "upper" and "lower" refer only to the
orientation shown: in practice in a twin fabric machine the forming
section is often oriented vertically, or at some angle thereto.
[0042] Referring first to FIG. 2, the slice lips 1A and 1B deliver
a jet of stock 2 onto the lower forming fabric 31 at the
impingement point I. The forming fabric 31 wraps the lower vented
lead blade 51. As the moving forming fabric 31 and the stock jet 2
converge, air 81 in the wedge shaped space between the surface of
the stock jet 91, the surface 101 of the head box lip 1B, and the
surface of the forming fabric 111 becomes trapped and enters the
forming fabric 31. This air 81, together with some of the liquid in
the forming fabric 31 at this point, passes through the forming
fabric 31 into the grooves 121 in the lower vented lead blade 51.
The accumulated air and liquid 131 is vented from the lower vented
lead blade 51 at its trailing edge 141. The construction as regards
the lower forming fabric 51 is thus essentially the same as FIG. 1.
Downstream of the impingement point I a second forming fabric 32
converges with the upper side 92 of the stock 2 on the lower
forming fabric 31. Further air is trapped in the space 82 between
the lower face 112 of the upper forming fabric 32 and the surface
92 of the stock 2. This additional air, and some of the liquid in
the forming fabric 52, passes through the forming fabric 52 into
the grooves 122 and is vented at the trailing edge 142 of the upper
vented lead blade 52.
[0043] FIG. 3 shows an alternative arrangement to that of FIG. 2.
The arrangement of the lower forming fabric 31 is the same. The
upper forming fabric 32 converges with the stock jet 2 at the same
point as the lower forming fabric 31, more or less at the
impingement point I. For the upper forming fabric air is now
trapped in the space bounded by the upper surface 92 of the stock
jet, the lower surface 112 of the upper forming fabric 32, and the
upper surface 102 of the head box lip 1A. All of the trapped air,
together with some of the liquid in the two forming fabrics, is
vented through the grooves 121, 122 at the trailing edges 141, 142
of the two vented lead blades 51, 52.
[0044] In both of these twin fabric machines, the two vented lead
blades will often be the same, and have the same convexly curved
shape. However in some circumstances it may be desirable to use two
different vented lead blades, which may have different convexly
curved shapes. It is also then possible that the two wrap angles
.theta..sub.1 and .theta..sub.2 will likely be similar, they may
not be the same.
[0045] FIGS. 4, 5 and 6 show the relationship between the angle
.theta. through which the forming fabric 3 wraps about a vented
lead blade, the effective radius of curvature of the vented lead
blade, and the radius of the adjacent upstream roll. Each of these
Figures uses the same numbers as FIG. 1, and thus considers a
single fabric machine. Similar considerations apply to a twin
fabric machine.
[0046] In each of these Figures, the convexly curved surface 16 of
the vented lead blade 5 has a constant radius, the forming fabric 3
is received tangentially from the adjacent upstream roll 6, and the
impingement point I is adjacent the trailing edge 14 of the vented
lead blade 5. These three Figures are also shown to essentially the
same scale.
[0047] In FIG. 4, the radius R.sub.4 of the vented lead blade
surface 16 is the same as the radius R.sub.1 of the roll 6, which
is some distance from the head box slice lips 1A, 1B. The vented
lead blade 5 then has to be relatively wide in order to bend the
forming fabric 3 into the forming path 7. If R.sub.1 and R.sub.4
are both approximately 46 cm, the angle of wrap .theta..sub.4 is
about 18.degree.. This large radius also implies that the length
D.sub.4 of the unsupported stock jet 2 cannot be shortened very
much.
[0048] In FIG. 5, the radius R.sub.5 of the vented lead blade
surface 16 is smaller than the radius R.sub.1 of the roll 6, which
is still some distance from the head box slice lips 1A, 1B. The
vented lead blade 5 then still has to be relatively wide in order
to bend the forming fabric 3 into the forming path 7. If R.sub.1 is
approximately 46 cm, and R.sub.5 is approximately 23 cm, the angle
of wrap .theta..sub.5 is about 36.degree.. This smaller radius
R.sub.5 allows the length D.sub.5 of the unsupported stock jet 2 to
be shortened.
[0049] In FIG. 6, the radius R.sub.6 of the vented lead blade
surface 16 is smaller than the radius R.sub.1 of the roll 6, which
is now closer to the head box slice lips 1A, 1B. The vented lead
blade 5 is much narrower in order to bend the forming fabric 3 into
the forming path 7. If R.sub.1 is approximately 46 cm, and R.sub.6
is approximately 23 cm, the angle of wrap .theta..sub.6 is about
18.degree.. This smaller radius R.sub.5 allows the length D.sub.6
of the unsupported stock jet 2 to be shortened still further.
[0050] It can thus be seen that the vented lead blade of this
invention provides considerable flexibility in the geometry of the
machine wet end.
[0051] The curved surface of the lead in blade is vented by means
of grooves which begin at the extreme upstream edge of the blade
and are continuous to its downstream edge. Three groove
arrangements are shown in FIGS. 7, 8 and 9. In each Figure, the
forming fabric moves in the direction of the arrow A. In FIG. 7,
the grooves 17 are regularly spaced the same distance apart, and
are essentially parallel to the arrow A. The disadvantage with this
arrangement is that there is some risk of the forming fabric
tracking across the blade. This is avoided in FIG. 8, where
although the grooves 18 are still regular and parallel, they are at
an angle .beta. to the arrow A. Alternatively, two sets of angled
grooves 18A, 18B can be used as shown in FIG. 9. Within each set,
the grooves are parallel to each other and regularly spaced, and
each set is at an angle .beta. to the arrow A. Typically, the angle
.beta. in FIGS. 8 and 9 will be from about 20 to about 300.
[0052] The purpose of the grooves in the vented lead blade surface
is to provide spaces through which at least a substantial
proportion of the air is vented which enters the forming fabric
from the wedge shaped space essentially between the forming fabric
and the jet surface. At the same time, at least some of the liquid
in the forming fabric as it enters the forming section will also be
vented.
[0053] FIG. 10 shows four possible groove structures for this
purpose. For simplicity, the blade surface 17 has been shown flat;
in practice it will be curved. Groove 19 is of constant width for
its entire length from the leading edge 17A to the trailing edge
17B. As the fabric passes over the vented lead blade surface 17,
air is continuously entering the forming fabric. It is thus often
advantageous to use a groove with an increasing cross section, such
as the groove 20 which widens, or the groove 21 which deepens, or
the groove 22 which both widens and deepens, in each case from the
leading edge 17A to the trailing edge 17B. Typically, the grooves
19, 20, 21 and 22 are from 2-8 mm in width, and 1-8 mm in
depth.
[0054] In operation, the forming fabric moves in sliding contact as
it wraps the vented lead blade. At the point of contact of the
forming fabric with the upstream leading edge of the lead blade,
for example at 17A in FIG. 10, the fabric may form a small contact
angle that is ideally 0.50 or less. It is preferred that the
forming fabric is tangential to the vented lead blade surface at
this point. The leading edge of the blade can be curved to minimize
any frictional effects at this point; a suitable edge radius is
from about 0.0025 mm to about 0.0065 mm. The forming fabric should
exit the vented lead blade tangentially to the trailing edge
portion of the blade surface, substantially in the path defined by
the downstream fabric support elements. The downstream path of the
forming fabric should be parallel to and in the same plane as the
underside of the stock jet, so that the angle of impingement of the
stock jet onto the forming fabric is substantially zero.
* * * * *