Note: Descriptions are shown in the official language in which they were submitted.
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TWIN--WIRE WEB FORMING SECTION
This invention is directed to a web forming :
system, and in particular to a system utilizing a twin-
wire web former.
The use of twin-wire formers, particularly in
the manufacture of paper webs such as newsprint has
received an enormous amount of development in recent
years, and considerable progress has been made towards
improvement of web formation, with consequent enhancement
of the product. However, there have remained inherent
unsolved problems in the advance of formers.
One approach by the present inventOr to the problems
of web formation involves the adaptation of a fourdrinier
type machine to a twin wire forming section by the super- :
imposition of a forming roll and subsequent dewatering
15 roll in cooperating relation with the elementsof an existing -
fourdrinier machine, as disclosed in Canadian patent
application Serial No. 350,147 filed April 18, 1980.
The present invention is directed to an improved
forming sectiQn wherein unexpected advantages were found
in the control of web formation, with consistent
achievement in improved web formation, a capability of
operating with a wide spectrum of varying stocks, over a
wide speed range, to produce webs over a wide range of
basis weight.
In carrying out tests on the referred-to
fourdrinier conversion work it was discovered that
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dewatering of the stock in order to form the web should
be interrupted, and stock dewatering reversed in direction
for at least a portion of the thickness of the web, in
order to reform the interior of the web.
A particular and apparently inherent problem of
two-wire forming has been the absence of so-called
Z-strength. Twin wire formers have, up to the present
all tended to produce strong two ply formation with
weakness in the central zone, giving a tendency to
separate due to the low Z-axis strength.
Referring to the prior art, certain machines
come quite close to providing the desired dewatering
sequences. However, most instances, presumably because
of the use of dewatering means other than forming rolls,
such as foils, table rolls and curved shoes, web formation
is not satisfactory, there being a severe retention problem,
with loss of fines and fibre through the wires. This
problem appears to stem from high magnitude forces of short
duration. Such machines include the VertiformaTM and
machines of the type shown in Canadian Patent No. 981,952
E.J. Justus which issued January 20, 1976. Substantially
all of these prior art machines appear to produce webs
afflicted with ply separation, that is to say, there is
weakness in the centre section of the web, in the
Z-direction.
The presently disclosed system thus provides
a method of forming a web between a pair of permeable
dewatering bands passing about a pair of forming rolls
wherein initial dewatering includes bidirectionally
forming the outer faces of the web, adjacent the
constraining bands, to the extent that web formation is
incomplete interiorly of the web, and subsequently
continuing dewatering of the web about the second of the
forming rolls wherein the direction of dewatering flow
through the web is substantially reversed for at least a
significant portion of the thickness of the web, and
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formation is substantially completed, whereby the interior
of the web is effectively reformed. The system further
includes the provision of relative longitudinal displace-
ment between the bands in order to enhance the reformation
of the web interior, with consequent floc modification.
The invention thus provides in one embodiment
a web forming machine having a pair of endless forming
bands to receive stock in dewatering web forming relation
therebetween, comprising a first forming roll located
within the loop of one of the bands and having an open
roll surface to support the one band in water receiving
relation therewith; a second forming roll located within
the loop of the other one of the bands, the bands passing
in tensioned mutually opposed relation from the first roll
to the second roll during operation of the mechine; an
initial web forming zone for receiving a layer of stock
in deposited relation from a suitable source on at least
one of the bands, at least a portion of the initial
forming zone having both of the bands sandwiching the stock
passing about a portion of the periphery of the first
forming roll to provide bidirectional dewatering of the
stock to the extent to only partially form the web, the
bands carrying the partially formed web to a further
formation zone located about the periphery of the second
25 - roll and providing direction reversed dewatering to at
least a portion of the thickness of the web to substantially
complete the formation of the web.
In comparing characteristic web formation provided
by a machine operating in accordance with the teachings
of the present invention, fines retention is excellent,
surpassing that of an equivalent fourdrinier. Furthermore,
the machine is particularly stable in sheet formation
with respect to stock consistency variations, and the
consistency for the entry to the first forming roll is
not critical, unlike the case of the "SymformerTM" which
employs a forming shoe.
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The described embodiment permits the arrangement
of the forming rolls in predetermined spaced apart relation
to provide a stretch therebetween along which the bands
travel in generally tangential relation to the rolls, having
the partially formed web in sandwiched relation therebetween.
Owing to the differences in the respective instantaneous
linear velocities of the bands relative longitudinal band
displacement occurs which correspondingly affects the
substantially unformed interior of the web, to assist in
reforming the web, thus tending to modify floc formation
within the web. It has been found that while at present
the individual quantitative effects of reversed dewatering
and differential longitudinal web displacement may not
readily be determined, the two effects appear to be
lS complementary and lead to significantly reformed flocking,
with enhanced formation.
In one embodiment the provisions of the initial
web forming zone include a first stretch of one of the
bands, on which stock is deposited from a headbox, the
band passing over suitable guide means such as foils or
table rolls to facilitate the initial formation of the
lower face of the web upon the band, prior to sandwiching
by the second band for passage about a portion of the
periphery of the first forming roll.
The provision of an initial dewatering stretch
akin to that of a fourdrinier machine permits initial
dewatering of the stock and commencement of formation,
with beneficial accumulation of fines, prior to the
commencement of bidirectional dewatering.
To clarify the term, formation is generally
accepted as meaning the distribution of fibres and
fibreflocks in a finished sheet viewed when illuminated by
light passing through the sheet i.e. backlighted. As the
web is dewatered, the fibres become fixed in their relative
positions, thereby determining their location in the
finished sheet and such fixed fibres are deemed as "formed"
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in the instant disclosure with those fibres or fiberflocks
still suspended in sufficient water to permit mobility and
change in their relationship or orientation being termed
as "unformed". I
Thus when practicing the present invention there -
will always be as above indicated an "unformed" portion in :
the interior of the web when the.web leaves the first
forming roll.
To further clarify the action of the present
invention, the second forming roll, as above indicated,
causes flow in the reverse direction through a portion
of the web i.e. the balance of the water flow towards the
two wires at the second roll is different from that of
the first forming roll so that at least some of the fibres
in the interior of the web are subject to flow in the
opposite direction which tends to influence their final
disposition in the formed sheet.
Between the two rolls the two wires or web forming
bands are substantially parallel and thus there is a section
between the two rolls where little drainage occurs and the
drainage is in effect interrupted. In this section as
above indicated there is also relative machine direction
movement between.the.wires to apply shear to the forming
web.
A forming machine constructed in accordance with
the presently disclosed arrangement using forming rolls
including a non-suction open surfaced roll normally makes
possible the provision o a substantially suctionless
machine wherein the absence of.drag, previously occasioned
by the widespread use of suction boxes, permits the
driving of one band loop by the other band loop without any
increase in power requirement, and even with a diminution
in drive power requirements considered in relation to an
equivalent fourdrinier machine. The improved machine also
results in improved band life.
.Thus, in the case of typical fourdrinier machines
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using flat boxes, foils and the like, which create
significant drag, the parasitic drag can take up to 60%
of the drive power. The drag of such dewatering devices
is both speed and moisture content sensitive.
The sealing of a band to a suction box surface
is largely influenced by the wetness of the band and web.
~wing to the enhanced dryness of webs upon completion of
dewatering over the second forming roll of the present
invention, the drag over any suction boxes being used is
correspondingly diminished. Additionally, where suction
boxes might be used, they can be operated at correspondingly
less vacuum.
In comparison with an equivalent fourdrinier
machine it appears that for equivalent bone dryness at
the couch, the subject forming section offers the potential
saving of up to 50% power consumption. Compared with a
commercial twin wire machine relying upon a forming shoe,
a saving of about 40~ drive power is expected.
Alternatiuely, operational speed-up for no increase
in power consumption can be achieYed.
Certain embodiments of the invention are described,
reference being made to the accompanying drawings, wherein:
Figure 1 is a schematic side elevation of a
first embodiment;
Figure 2 is a like view of a second embodiment;
Figure 3 is a like view of a third embodiment;
and
Figures 4A and 4B show photomicrographs of paper
made in accordance with the present invention and paper made
by a corresponding fourdrinier machine.
Turning to Figures 1 and 2, the respective
machines 10, 30 each comprise an upper band loop 14 and
a lower band loop 12. The bands 12, 14 are usually so-
called fourdrinier wieres, generally known as forming
fibrics.
A slice portion 18 of a headbox (not shown)
deposits a layer of stock on a first stretch 20 of the
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band 12. This stretch 20 is supported in tensioned relation
upon a plurality of drainage elements such as foil sections
or table rolls, to provide initial landing of the stock
upon the band 12, and to promote satisfactory initial stock
dewatering downwardly.
The bands 12 and 14 converge about the open faced
roll 22 which comprises a grooved roll having a foraminous
wire sleeve shrunk thereover. Such a roll 22 would have
an open area in the order of 80% of its face area.
The angle of wrap of the bands 12, 14 must be
limited in order to preclude total formation on roll 22.
The illustrated approximately 45 wrap angle has been found
to be effective. Operation in the range 60 to 25 in order
to maintain incomplete formation, with compensation by stock
dilution adjustment is anticipated, after which the sand-
wiched stock, passes tangentially from the roll 22 to the
roll 24. After passage about the roll 24 the dewatering of
the web is sufficiently complete that the web is fully formed.
In the stretch between rolls 22 and 24, owing to
the difference in linear speeds of the two bands necessitated
by their respective reversed inner and outer positions on
the two rolls 22, 24, there is relative longitudinal movement
of one band relative to the other band, thereby producing
a corresponding relative displacement between the two facing
formed portions of the web carried by the respective bands.
From the final formation achieved by the machine the
longitudinal relative movement appears to play some part
in reforming the interior of the web by modifying the floc
formation prior to its being dewatered about the roll 24 to
the extent that formation is then completed.
The roll 24 in the embodiments illustrated in
Figures 1 and 2 is a plain roll, whereby dewatering rill
occur substantially solely in an outward direction relative
to the roll surface.
Upon passage from the roll 24 in the Figure 1
embodiment the bands 12, 14 pass by way of guide roll 26
to a table roll 28, whereat the band 14 diverges gradually
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by a small angle in the range 2 to 6 from the web and
the band 12. The table roll 28 assists in maintaining the
web upon the band 12 for passage about a suction couch
roll 30, in known fashion. In the Figure 2 embodiment only
the table roll 28 is necessary, in view of the bands
leaving the underside of the roll 24.
While the second forming roll 24 has been found
to operate effectively as a solid roll, it is contemplated
that in certain instances for heavier grades, or uslng
stocks more difficult to dewater, that a suction forming
roll may be used; or an open roll such as roll 22.
Referring to the Figure 2 embodiment, owing to
the fact of downward dewatering taking place at the
second forming roll, whereby handling of the water is
more readily arranged, this embodiment is suited for a
pulp forming section or for such as heavyweight low speed
kraft grades wherein greater quantities of water are
removed.
In the Figure 3 embodiment the pre-drainage band
stretch is dispensed with. The machine is sufficiently
versatile that it is not very sensitive to stock variation,
and the characteristic improved formation can usually be
achieved in the shortened version, which can be based
upon either the Figure 1 or the Figure 2 embodiment.
The web formation produced by the improved former
has demonstrated superior qualities of symmetry, homo-
geneous structure and Z-direction bond strength, which
particularly suits the requirements for many specialty
papers.
The present invention makes possible the
formation of webs using substantially suctionless equip-
ment, marked by the absence of vacuum forming rolls and
the substantial absence of suction forming boxes and the
like, or the number of such boxes may be reduced and
operated at less vacuum, with reduced drag.
The second wire of the combination does not
require to be independently driven, and can be effectively
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driven by the first wire, while the wire drive power consumption is
effectively diminished, relative to an equivalent fourdrinier machine
due to the reduced wire drag, made possible by the reduced use of
suction boxes or by operation under loss vacuum. The presently
disclosed arrangement also leads to extended life of the woven wires
or fabrics, due to the substantially diminished sliding wear across
the covers of suction boxes, which formerly applied, in the case of
most fourdriniers. This benefit applies equally over those twin wire
machines relying upon forming shoes to dewater the web.
The portion of the forming path, wherein the other face
of the web is formed by predominant drainage towards the open-face
roll, provides an excellent surface formation and good two-sidedness.
The short duration of the passage about the open-faced first forming
roll results in a partially formed web, so that it is believed in
the succeeding portion of the forming path, where significant relative
longitudinal displacement is applied across the thickness of the web
breaks up floc formations are broken up in the unformed interior of
the web, with a marked reduction in floc formations which contributes
to improved formation.
Referring to Figures 4A and 4B, Figure 4A is a photomicro-
graph of a cross-section of newsprint made by a twin wire former built
and operated in accordance with the present invention, and Figure 4B
is a like photograph for the same grade of paper made from the same
stock on a corresponding fourdrinier machine.
It will be noted that Figure 4A shows significantly more
long fires crossing the central plane of the sheet, than for the
fourdrinier sheet, Figure 4B, indicating the superior interply
bonding indicative of enhanced Z-strength, achieved by the present
invention.
In order to further demonstrate the capability of a
machine constructed in accordance with the present invention, operating
in the manner herein taught, the tabulated results below are for a
model test section capable also of operation in a fourdrinier mode
to provide comparison, over several tests, with sheet made by an
equivalent fourdrinier machine.
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In the foregoing test results, under "Test No."
the numeral indicates the sequence number of the test, the
initial "F" indicates the "Fourdrinier" mode machine, and
the initials T/W indicates the Twin Wire machine constructed
according to Figure 1 of the present invention. Also:
"B.W." is basis weight of the sheet.
"M/C Speed FPM" is the respective machine operat-
ing speed in feet per minute.
"H'D Box Cons. %" is the percentage of solids in
the furnish of the headbox.
"Specific Volume" is volume, in cubic centimetres
per gram weight, of the furnish.
"Air Res. Gurley SjmL" is a porosity test of the
bone dry paper involving the percolation through the
thickness of the web in a time of 'S' seconds of a
milli litre of air at standard conditions of~pressure and
temperature.
"Break'g Length MD-m" is the length of sheet
in metres "m", taken in the machine direction "MD", at
which the sheet breaks under its own weight (i.e. a measure
of longitudinal tensile strength).
"Break'g Length CD-m" is the transverse tensile
strength similarly measured, in metres "m" in the cross
machine direction "CD" at which the sheet breaks under its
own weight.
"Tens. Ratio MD/CD" is the ratio of tensile
strength of the web in the machine direction (MD) compared
with the cross machine direction (CD).
"Tear Index MD mN.m2/g" is the tear strength of
the web in the machine direction (MD), measured in milli-
Newtons of force per unit of basis weight, basis weight
being expressed in the numerator, by inversion, as square
metre per grams.
"Tear Index CD mN.m2/g" is the corresponding tear
strength of the web in the cross machine direction (CD).
"Ratio MD/CD" is the ratio of tear strengths, in
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the machine direction (MD) compared with the cross
machine direction (CD).
"Caliper In." is the thickness of the web in
inches.
"Scott Int. Bond kJ/m2" is a pick test
indicative of web internal bond strength (the Z-direction
strength) measured in kilo Jou~les~ per square metre.
Based on the comparative test values in the
foregoing table wherein tests are paired on the basis of
the same sheet caliper,~ ~t the same machine speed
the following comments are made:
In the first and second set of values, tests
lF and 3 T/W, both at 2000 ft/min, there is a slight
difference in basis weight, although the T/W furnish is
less dense.
Web porosity is substantially diminished i.e.
the T/W web is 40% less porous ; the tensile strength of
the twin wire sheet is about 34% greater in the machine
direction; and about 23% greater in the cross machine
direction; the web internal bond strength is about 20%
greater for the twin wire machine web.
It will be seen that most of the tests show
consistent similar improvements of the twin wire sheet
over that of the fourdrinier, except in the case of fine
paper test 7F:15 T/W, where the twin wire sheet was more
porous (however, the furnish used was much lighter
(46 v-s 60) and even then the bond strength is 8% greater
for the twin wire sheet.
It should be remarked that fourdrinier sheet is
usually consistently and markedly superior in bond
strength to sheets from other known twin wire formers, so
that the comparative benefits of the present invention are
quite evident.
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