Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
BACKGROUND OF THE INVENTION
The prescnt invention relates in general to methods and
apparatus for forming non-woven fabrics; and, more particularly,
to methods and apparatus for improving the fiber throughput
capacity of 2-dimensional systems for forming air-laid webs
of dry fibers on a high-speed production basis; yet, wherein
the web being formed is characterized by a random dispersion
of essentially undamaged, uncurled, individualized fibers
disposed in a controlled cross-directional profile and is
substantially devoid of nits, pills, rice and other aggregated
fiber masses so as to result in a web of aesthetically
pleasing appearance and increased tensile strenqth irrespective
of the basis weight of the web which can range from at least
as low as 13 lbs./2880 ft.2 suitable for bath tissue or the
lS like to heavier webs suitable for facial tissues, components
for feminine napkins, diaper fillers, toweling, wipes, non-
woven fabrics, saturating paper, paper webs, paperboard, et
cetera.
Conventionally, materials suitable for use as disposable
tissue and towel products have been formed on paper-making
equipment by water-laying a wood pulp fibrous sheet.
Conceptionally, such equipment has been designed so that the
configuration of the resulting sheet approaches a planar
structure. This allows continuous operation at high speeds;
and, such sheets may be formed at speeds of 3,000 to 4,000
feet per minute. Indeed, recent developments have allowed
sustained production at speeds of up to 5,000 feet per
minute.
Following formation of the sheet, the water is removed
either by drying or by a combination of pressing and drying.
As water is removed during formation, surface tension forces
of very great magnitude develop which press the fibers into
contact with one another, resulting in overall hydrogen
bonding at substantially all fiber intersections; and a
~hin, essentially planar sheet is formed. It is the hydrogen
bonds between fibers which provide sheet strength and, such
bonds are produced even in the absence of extensive additional
pressing. Due to this overall bonding phenomenon, cellulosic
sheets prepared by water-laid methods inherently possess
very unfavorable tactile properties (e ~., harshness,
stiffness, low bulk, and poor overall softness) and, additionally,
possess poor absorbency characteristics rendering such
sheets generally unsuitable for use as sanitary wipes, bath
and facial tissues, and toweling.
To improve these unfavorable properties, water-laid
sheets are typically creped from the dryer roll--i.e., the
paper is scraped from a dryer roll with a doctor blade.
Creping reforms the flat sheet into a corrugated-like structure,
thereby increasing its bulk and simultaneously breaking a
significant portion of the fiber bonds, thus artifically
~0 improving the tactile and absorbenGy properties of the
material. But creping raises several problems. Conventional
creping is only effective on low basis weight webs (e.g.,
webs having basis weights less than about 15 lbs./2800 ft.2),
and higher basis weight webs, after creping, remain quite
stiff and are generally unsatisfactory for uses such as
quality facial tissues. Because of this, it is conventional
practice to employ at least two plies of creped low basis
weight paper sheets for such uses. Only by doing this can a
suficiently bulky product with acceptable softness be
prepared. However, even this process does not completely
overcome the detrimental effects of the initial overbondin~
'7~ ~
in a water-laid paper sheet.
Sanford _ al. U.S. Pat. No. 3,301,246 proposes improving
the tactile properties of water-laid sheets by thermally
predrying a sheet to a fiber consistency substantially in
excess of that normally applied to the dryer surface of a
paper machine and then imprinting the partially dried sheet
with a knuckle pattern of an imprinting fabric. The sheet
is thereafter dried without disturbing the imprinted knuckle-
pattern bonds. ~hile this method may somewhat improve the
softness, bulk and absorbency of the resulting sheet, the
spaces between the knuckle bonds are still appreciably
compacted by the surface-tension forces developed during
water removal, and considerable fiber bonding occurs.
Creping is still essential in order to realize the maximum
advantage of the proposed process; and, for many uses, two
plies are still necessary.
As will be apparent from the foregoing discussion,
conventional paper-making methods utilizing water are geared
towards the high speed formation of essentially planar
sheets; yet, such methods inherently possess the inefficient
attribute of initial "overbonding," which then necessitates
a creping step to partially "debond" the sheet to enhance
the tactile properties. Also, the extreme water requirements
limit the locations where paper-making operations may be
carried out. Such operations require removing a large
quantity of the water used as the carrier, and the used
process water can create an associated water pollution
problem. Still further, the essential drying procedures
consume tremendous amounts of enexgy.
Air forming of wood pulp fibrous webs has been carried
out for many years; however, the resulting webs have been
7~ (
used for applications where either li~tle strength is
required, such as for absorbent products--i.e., pads~-or
applications where a certain minimum strength is required
but the tactile and absorbency properties are unimportant--
i.e., various specialty papers. ~.S. Pat. No. 2,447,161 to
Coghill, V.S. Pat. No. 2,810,940 to Mills, and British Pat.
No. 1,088,991 illustrate various air-forming techniques for
such applications.
In the late 1940's and early 1950's, work by James D'A.
Clark resulted in the issuance of a series of patents
directed to systems employing rotor blades mounted within a
cylindrical fiber "disintegrating and dispersing chamber"
wherein air-suspended fibers were fed to the chamber and
discharged from the chamber through a screen onto a forming
wire--viz., J.D'A. Clark U.S. Pat. Nos. 2,748,429, 2,751,633
and 2,931,076. However, Clark and his associates encountered
serious problems with these types of forming systems as a
result of disintegration of the fibers by mechanical co-action
of the rotor blades with the chamber wall and/or the screen
mounted therein which caused fibers to be "rolled and formed
into balls or rice which resist separation"--a phenomenon
more commonly referred to today as "pilling". These problems,
inter alia, and proposed solutions thereto, are described
in, for example. J.D'A. Clark U.S. Pat. No. 2,827,668,
J.D'A. Clark et al. U.S. Pat. Nos. 2,714,749 and 2,720,005;
Anderson U.S. Pat. No. 2,738,556; and, Anderson et al. U.S.
Pat. No. 2,738,557. However, prior to the advent of the present
invention, it is not believed that systems of the type
disclosed by J.D'A. Clark and his associates which employed
cylindrical fiber disintegrating and dispersing mechanisms
with and/or without rotors, have been suitable for use in
production type, air-laid, dry fiber, web forming systems,
principally because problems of pilling have not been
resolved, and because of severe fiber damage due t~ the
disintegrating action of the rotor in Clark's cylindrical
chamber.
It should be noted that the aforesaid Clark et al. U.S.
Pat. No. 2,720,005 discloses an air scrabbler system having
a foraminous separating wall wherein slots may be formed in
the wall rather than relatively small openings such as are
employed with conventional woven square-mesh screens. The
Clark et al. patent is silent as to the orientation of the
slots. However, in the aforesaid Clark U.S. Pat. No. 2,748,429
which also contemplates the use of a slotted separating
wall, the slots are shown and described as "circumferentially
extending laterally spaced slots~ (See, Col. 3, lines 22-
23). Such slot orientation has been found to be substantially
inoperable when utilizing 2-dimensional formers of the type
employing a horizontally disposed rotor assembly.
A second type of system for forming air-laid webs of
dry cellulosic fibers which has found limited commercial use
has been developed by Karl Kristian Kobs Kroyer and his
associates as a result of work performed in Denmark.
Certain of these systems are described in: Kroyer U.S. Pat.
Nos. 3,575,749 and 4,014,635; Rasmussen U.S. Pat. Nos. 3,581,706
~5 and 3,669,778; Rasmussen et al. U.S. Pat. No. 3,769,115;
Attwood et al. U.S. Pat. No. 3,976,412; Tapp U.S. Pat.
No. 4,060,360; and, Hicklin et al. U.S. Pat. No. 4,074,393.
In general, these systems employ a fiber sifting chamber or
head having a planar sifting screen which is mounted over a
forming wire. Fibers are fed into the sifting chamber where
they are mechanically agitated by means of a plurality of
--6--
mechanically driven rotors mounted for rotation about vertical
axes. Each rotor has an array of symmetrical blades which
rotate in close proximity to the surface of the sifting
screen. The systems described in the aforesaid Kroyer and
related patents generally employ two, three, or more side-
by-side rotors mounted in a suitable for~ing head.
This type of sifting equipment suffers from poor productivity
and other inherent disadvantages, especially when making
tissue-weight webs. For example, the rotor action concentrates
most of the incoming material at the periphery of the blades
where the velocity is at a maximum. Most of the sifting
action is believed to take place in these peripheral zones,
while other reglons of the sifting screen are either covered
with more slowly moving material or are bare. Thus, a large
percentage of the sifting screen area is poorly utilized and
the system productivity is low. Moreover, fibers and agglomerates
tend to remain in the forming head for extended periods of
time, especially in the lower velocity, inner regions beneath
the rotor blades. This accentuates the tendency of fibers
to roll up into pills. Conse~uently, if the forming head is
to be cleared of agglomerated material, it is necessary to
remove 10~ or more by weight of the incoming material from
the forming head for subsequent reprocessing or for use in
less critical end products. The separating method used
(See, e.g., the aforesaid Kroyer ~.S. Pat. No. 4,014,635)
entrains a large number of good fibers with the agglomerates
leaving the forming head. The severe mechanical action of
the hammermills in the secondary processing system damagès
and shortens such otherwise good fibers, while breaking up
the agglomerates. Another inherent shortcoming of these
systems is a tendency to form webs having a non-uniform
f~ 7~
weight profile across their width. (See, e.g., the aforesaid
Tapp U.S. Pat. No. 4,060,360~. This is a condition which is
very difficult to overcome. It is especially troublesome
when making webs in the towelling and lightweight tissue
ranges.
~ he inventors have found that, when using high quality
fibers--i.e., long, straight fibers, in a sifting type
system--the above difficulties were aggravat2d. The rate of
pill rormation increased and it was necessary to remove and
l~ recycle more than 50% by weight of the incoming fibrous
material to produce good guality tissue-weight webs. Pro-
ductivity was unacceptably low and excessive damage was done
to otherwise qood fibers duxing the secondary hammermilling
step. The tensile strength of the webs produced was decreased.
Moreover, the circular movement of the rotors above the screen
causes corresponding air and fiber movement in the formin~
region below the screen. Strong, unstable cross-flow forces
are present and contribute to non-uniform ~ormation of the web.
Efforts to compensate for the low throughput of si ting type
syste~s involve increasing the area of the screens and the
forming surface. Thus, fiber is more thinly distributed over
the forming surface and is not held in place as rirmly by the
suction box. The fibers are easily disturbed at higher speeds
and wave patterns are formed~
Fibers are also
disturbed by the seal rollers which are required to maintain
the forming region at sub-atmospheric pressure. The difficulties
described above compound each other and are especially
troublesome when forming lightwei~ht webs at acceptable
production speeds.
In an effort to overcome the productivity problem,
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complex production sys~ems have been devised utilizing
multiple forming heads--for example, up to eight separate
spaced forming heads associated with multiple hammermills
and each employing two or three side-by-side rotors. The
most recent sifting type systems employing on the order of
eighteen, twenty or more rotors per forming head, still
require up to three separate forming heads in order to
operate at satisfactory production speeds--that is, the
syst~ms employ up to fifty-four to sixty, or more, separate
rotors with all of the attendant complex drive systems, feed
arrangements, recycling equipment and hammermill equipment.
Moreover, it has been found that the foregoing sifting
systems are also deficient in that there is only limited
control of cross-directional uniformity of the web being
produced--see, e.g., the aforesaid Tapp U.S. Pat. No. 4,060,360--
thereby imposing severe constraints when attempting to scale
the equipment up to make webs of 96 inches, 120 inches, 200
inches, or moret in width. The tensile properties of the
web may suffer as a result of excessive mechanical action in
the forming heads and non-uniformities in web weight and
formation. The aesthetic appearance of the webs is often
less than optimum as a result of wave patterns on the web
surface resulting from the closely spaced rotor blades which
are rotating in a horizontal plane just above the forming
wire` and the other factors described above. To date/ the
foregoing problems have been so significant that this type
of sifting system has been found totally unsuitable for
making relativel~ light weight webs at acceptable production
speeds--e.g., webs having basis weights of from 13 lbs./2880 ft.2
to 18 lbs./2880 ft. suitable for use as bath or facial
tissues--although such equipment can produce low basis
_g_
5~7~ `
weight webs at low forming wire speeds. Rather, the equipment
has generally found application in forming heavier basis
weight webs suitable for use in making towels or paperboard
where the web imperfections inherently produced can be
either tolerated or masked because of the bulk and thickness
of the web.
During the 1970's a series of patents were issued ~o
C.E. Dunning and his associates which have been assigned to
the assignee of the present invention; such patents describing
yet another approach to the formation of air-laid dry fiber
webs. Such patents include, for example: Dunning U.S. Pat.
Nos. 3,692,622, 3,733,234 and 3,764,451; and, Dunning et al.
U.S. Pat. Nos. 3,776,807 and 3,825,381. This development
has been found to resolve a number of the problems that have
heretofore plagued the industry. For example, high productivity
rates have been achieved and fiber webs can easily be formed
at high machine speeds. However, the system requires preparation
of pre-formed rolls o~ fibers having high cross-directional
uniformity and is not suitable for use with bulk or baled
fibrous materials. Because of this, problems are experienced
when attempting to scale the equipment up to produce wide
webs--i.e., webs on the order of 120 inches in width or
greater--and the requirement for pre-formed special web
rolls having the requisite uniformity in cross-directional
profile has bee~ such that, to date, the system has found
only limited commercial application.
Indeed, heretofore it has not been believed that air-
forming techniques can be advantageously used in high speed
production operations to prepare cellulosic sheet material
that is sufficiently thin, and yet has adequate strength,
together with softness and absorbency, to serve in applications
--10--
such as bath tissues, facial tissues and light weight toweling.
SUMMARY OF THE INVENTION
It is a general aim of the present invention to provide
methods and apparatus which overcome all of the foregoing
disadvantages which are characteristic of the prior art, yet
which enable significant improvements in terms of productivity.
In one of its principal aspects, it is an object of the
invention to provide improved methods and apparatus for air
deposition of dry fibers to form webs having any selected
one of a wide range of basis weights and wherein the speed
of web formation is no longer limited by low screening
efficiency or productivity.
It is a further general objective of the invention to
provide dry air-laid web forming methods and apparatus
characterized by their simplicity, yet which permit of high
capacity operation with high fiber throughput and wherein
the product produced is characterized by improved properties
in terms of strength, tactile properties, freedom from nits,
uniformity, and general aesthetis appearance. It is a more
specific object to provide methods and apparatus capable of
producing high-quality webs at speeds in the range of 300 to
2,000 feet per minute and, even at speeds in excess of 2,000
feet per minute;
It is an object of thc present invention to provide
methods and apparatus which are e~ually suitable for mass
production on a high-speed basis of webs such as those used
as bath or facial tissues, diaper fillers, feminine napkin
components, towels, wipes, non-woven fabrics, appliques on
non-woven substrates, smooth paper webs, laminated paper
webs, paperboard, and similar products, all of which have
7~
physical properties at least equal to, and in some cases
better than, those obtained by known dry forming systems.
In this connection, it is an object of the invention to
provide improved methods and apparatus for the air deposition
S of dry fibers in the manufacture of both relatively thin
webs--e.~., webs having basis weights on the order of 13
lbs./2880 ft.2 to 18 lbs./2880 ft.2 suitable for bath and
facial tissues--and relatively thick webs--e.g., webs having
basis weights on the order of 19 lbs./2880 ft.2 to 40 lbs./28~0 ft.2,
and even heavier, suitable for toweling and other uses--yet
wherein the resulting product, irrespective of its basis
weight, is characterized by its uniformity, tensile strength,
freedom from nits, and generally pleasing aesthetic appearance
despite having been formed at speeds in the range of 300 to
2000 feet per minute or higher.
In another of its important aspects, it is an object of
the present invention to provide a versatile and highly
tolerant system for the air deposition of dry fibers which
is characterized by its ability to handle wide ranges o
pulp and other fibers to form both thin and thick webs or
batts, and which is capable of handling fibers having lengths
in the 1-5 mm. range--e.g., wood, cotton linters, rayon or
synthetic fibers, leather, hemp, thermo-mechanical, secondary
and, perhaps, inorganic fibers such as glass microfibers and
asbestos--as well as synthetic fibers of considerably greater
length and, blends of the foregoing fiber types; yet, wherein
the fibers are subjected to only minimal mechanical disintegrating
forces and, consequently, are not shortened or otherwise
damaged.
In another of its aspects, it is an object of the
invention to provide improved methods and apparatus for
permitting hlgh throughput of fibers at relatively high speeds,
yet wherein there is only a minimal tendency to form pills,
nits or the like and, consequently~ where the amount of
undesired materials separated and/or recycled can be
substantially reduced.
In another of its important aspects~ it is an
object of the invention to provide an improved screening
arrangement for permitting high fiber throughput with
effective screening of undesirable materials such as nits or
the like and without subjecting the system to undesired
screen plugging. It is an important object of the invention
to provide a rotational screening system for a dry forming
fiber deposition process wherein provision is made for
maintaining a proper balance between rotor speed and both
air supply and velocity so as to maintain acceptable cross-
dimensional uniformity in the mass flow rate of air-suspended
fibers being delivered to the forming wire so as to form an
air-laid web characterized by its cross-directional basis
weight uniformity which equals or exceeds the cross-directional
uniformity of the fibers entering the former.
Therefore, in accordance with one aspect of the
present invention there is provided the method of forming a
quality web of air-laid dry fibers on a high speed production
basis comprising the steps of: a) delivering dry fibrous
materials to a forming head positioned over a forming surface;
b) conveying the dry fibrous materials through the forming
head in a rapidly moving aerated bed of individualized
fibers, soft fiber flocs and aggregated fiber masses and in
an environment maintained substantially free of fiber grinding
and disintegrating forces; c) continuously separating f~om
1% to 10~ of the fibrous materials delivered to the forming
head from the aerated bed with the materials being separated
- 13 -
including those haviny a bulk density in excess of .2g/cc.
so as to maximize the separation of aggregated fiber masses
from the aerated bed; d) discharging such separated fibrous
materials including the aggregated fiber masses contained
therein from the forming head; e) discharging the
individualized fibers and soft fiber flocs through a high
capacity slotted screen; f) conveying the individualized
fibers and soft fiber flocs discharged through the slotted
screen at a fiber throughput rate anywhere in the range o
.5 lbs./hr./in.2 to at least 1.50 lbs./hr./in.2 through an
enclosed forming zone towards the moving foraminous forming
surface in a rapidly moving air stream; g) air-laying the
individualized fibers and soft fiber flocs on the moving
foraminous forming surface so as to form an air-laid web of
randomly oriented dry individualized fibers and soft fiber
flocs on the forming surface with such web having a~nit level
of from "0" to "3"; and, h) moving the foraminous forming
surface at a controlled and selected speed so as to produce
an air-laid web having a nit level of from "0" to "3" and
any specific desired basis weight in lbs./2880 ft~2 ranging
from at least as low as 13 lbs./2880 ft.2 to in excess of
40 lbs./2880 ft.2.
In accordance with a further aspect there is
provided an apparatus for producing a quality web of air-laid
dry fibers on a high speed production basis comprising, in
combination: a movable foraminous forming surface; a forming
head mounted over and forming surface; means for delivering
dry fibrous materials to the forminy head; means for
conveying the dxy fibrous materials through the forming head
in a rapidly moving aerated bed of individualized fibers, soft
fiber flocs, and aggregated fiber masses while maintaining the
forming head substantially free of fiber grinding and
disintegrating forces; means for continuously separating from
- 13a -
1% to 10% of the fibrous materials delivered to the forming
head from the aerated bed with the materials being separated
including those having a bulk density in excess of .2g./cc.
so as to maximize the separation of aggregated fiber masses
from the aerated bed and discharging such separated fibrous
materials from the forming head; a discharge opening formed
in the forming head; a slotted screen mounted in the
discharge opening; means defining an enclosed forming zone
mounted between the discharge opening and the forming surface;
means for conveying the individualized fibers and soft fiber
flocs from~the forming head through the slotted screen at
a fiber throughput rate anywhere in the range of
.5 lbs./hr./in.~ to at least 1.50 lbsO/hr./in.2 and through
the forming zone towards the movable foraminous forming
surface in a rapidly moving air stream and for air-laying
the individualized fibers and soft fiber flocs on the movable
foraminous forming surface so as to form an air-laid web of
~andomly oriented dry individualized fibers and soft fiber
flocs on the surface during movement thereof with such web
having a nit level of from "0" to "3", and, means for
controllably moving the foraminous forming surface at a
selectable speed so as to produce an air-laid web having a
nit level of from "0" to "3" and any specific desired basis
weight in lbs./2880 ft 2 ranging from at least as low as
13 lbs./2880 ft.2 to in excess of 40 lbs./ 2880 ft~2.
DESCRIPTION OF THE DRAWINGS
These and other objects and advantages of the
present invention will become more readily apparent upon
reading the following detailed description and upon
reference to the attached drawings, in which:
FIGURE 1 is a schematic view, in side elevation,
of one form of apparatus which may be employed for the air
- 13 ~
deposition of dry fibers to form a web continuum in
accordance with the present invention;
FIG. 2 is a schematic view here illustrating
an exemplary
- 13c -
t~ ~
air-laid, dry fiber, web forming system utilizing two substantially
identical cylindrical flow cvntrol and forming heads disposed
in side-by-side relationship above the foraminous for~ing
wire;
FIG. 3 is an oblique view, partially cut away, here
schematically illustrating details of an exemplary novel
fiber feed, eductor, flow control, screening, and fiber
forming arrangement embodying features of the present
invention;
FIG. 4 is a fragmentary front elevational view, partly
in section, of the rotor assembly shown in FIG. 3;
FIG. 5 is an end view of a modi~ied rotor assembly
similar to that shown in FIG. 3, but here depicting a rotor
employing only four rotor ~ars;
FIG. 6 is a diagramatic plan view indicating in
schematic, idealized fashion fiber movement through a
conventional woven square-mesh screen under the influence OL
air movement and rotor action;
FIG. 7 is a view similar to FIG. 6 but here depicting
movement of fibers through a high capacity slotted screen in
which the slots are oriented parallel to the axis of the
rotor in accordance with the invention;
FIG. 8 is a view similar to FIG. 7, but here illustrating
the undesirable plugging action that occurs when the slots
of à slotted screen are oriented in a direction generally
perpendicular to a plane passing through the axis of the
rotor;
FIG. 9 is a photograph illustrating the plugging of a
slotted screen that occurs when the slots are oriented at an
angle of approximately 45 to a plane passing through the
axis of the rotor;
-14-
FIG. 10 is an enlarged, fragmentary side elevational
view here depicting in diagramatic form the air/fiber stream
as it moves through the rotor housing where an annular
moving aerated bed of fibers is created and maintained and,
thereafter, as it moves through the screening means and
forming zone an~ is air-laid on the forming wire to form an
air-laid web of fibers and, further, depicting the pressure
relationships and the air velocity and rotor bar velocity
relationships that are believed to exist when operating the
system of the present invention at a desired one of several
selectable sets of adjustablP parameters in terms of ratio
of air-to-fiber supply, rotor speed, and recycle balance;
FIG. 11 is a highly enlarged view of a portion of the
system shown diagramatically in FIG. 10, here depicting how
the differential relative velocities of the rotor bars and
air stream serve to gencrate a rapidly moving full-width
zone of negative pressure in the wake of each rotor bar,
thereby lifting fibrous materials off the screen in the
region beneath th~ moving negative pressure zone, while
permitting individual fibers to dive axially or end-wise
through the openings in the screen in those regions of
positive pressure drop across the screen between successive
negative pressure zones;
FIG. 12 is a graphic representation of a typical set of
curves indicative of the functional relationships existing
with air-laid web forming systems e~bodying features of the
present invention between fiber throughput for specific
representative screen designs and rotor assembly operating
parameters--viz., rotor RPM and the number of rotor bars
-employed;
FIG. 13 is a graphic representation of the functional
--15--
relationships existing between nit levels in a finished air-
laid web made in accordance with the present invention,
fiber throughput, and the percentage of fibrous materials
separated and/or recycled prior to deposition on a moving
forming wire,
FIGS. 14 through 20 are photographs of exemplary air-
laid fiber webs having increasing nit levels suitable for
subjectively evaluating and rating web guality in accordance
with subjective visual standards as to product acceptability
established by the assignee of the present invention; and,
FIG. 21 is a graphic representation depicting the
relationship between fiber delivery rates expressea as fiber
throughout in pounds per square inch per hour ~lbs./in.2/hr.)
and both woven square-mesh screens and slotted screens
having screen openings ranging from about 0.03`' in at least
one direction to about 0.08" in at least one direction.
While the invention is susceptible of various modifications
and alternative forms, specific embodiments thereof have
been shown by way of example in the drawings and will herein
be described in detail. It should be understood, however,
that it is not intended to limit the invention to the
particular forms disclosed, but, on the contrary, the
intention is to cover all modifications, eyuivalents and
alternatives falling within the spirit and scope of the in-
vention as expressed in the appended claims.
DETAILED DESCRIPTION
A. Definitions
To facilitate an understanding of the ensuing description
and the appended claims, definitions of certain selectedterms and phrases as used throughout the specification and
-16-
clai~s are set forth below.
The phrase "pulp lum?" is herein used to describe a
dense, bonded clump of fibers in the incoming fiberized
supply which is most conventionally caused by hard pressing,
non-uniform application of debonding agents, and/or inadequate
opening or hammermilling. Pulp lumps are present in ordinary
commercial grades of pulp.
The words "pill" and~or "rice" are herein each used to
describe a dense, rolled up bundle of fibers, often including
bonded fibers, which are generally formed by mechanical
action during fiber transport or in a rotor chamber where
the fibers are commonly, and often intentionally, subjected
to mechanical disintegrating action.
The word "nit" is herein used to generically refer to
pulp lumps, pills and/or rice. Nits are considered to be an
unacceptable defect in light-weight tissues such as bath and
facial tissues having basis weights of from 13 lbs./2880 ft 2
to 18 lbs./2880 ft. , and generally result in decreased
tensile strength in webs of these, or even of heavier, basis
weights.
The terms "floc" and "soft floc" are h~rein used to
describe soft, cloud-like accumulations of fibers which
behave like individualized fibers in air; i.e., they exhibit
relatively high co-efficients of drag in air.
The phrase "aggregated fiber masses" is herein used to
generically embrace pulp lumps, pills, rice and/or nits, and
to describe aggregations of bonded and/or mechanically
entangled fibers generally having a bulk density on the
order of greater than .2 grams per cubic centimeter (g./cc.).
Aggregated fiber masses are to be distinguished from flocs
and/or soft flocs whose bulk density is generally less than
.2 g./cc. Moreover, aggregated fiber masses have a relatively
low coefficient of drag in air.
"Bulk density" in the weight in grams of an uncompressed
sample divided by its volume in cubic centimeters.
The phrase "semi-cylindrical" is used herein to describe
a portion of the rotor chamber wall and/or forming screen,
and is intended to mean that wall portion from the upstream
leading edge of the screen to and including the full~width
separator slot. In the various exemplary embodiments herein
described, the phrase "semi-cylindrical" embraces a peripheral
wall portion having an included angle of less than 180.
However, such phrase is used herein in a descriptive sense
and is not intended to be construed in a limiting sense
since those skilled in the art will appreciate as the
ensuing description proceeds that the rotor chamber could be
cylindrical, or substantially cylindrical, in which event
the phrase "semi-cylindrical" would be intended to embrace
peripheral wall portions having an included angle of greater
than 180.
The phrase "2-dimensional" is used to describe a system
for forming a web wherein: i) the cross-section of the syste~
and the flows of air and fiber therein are the same at all sections
across the width of the system; and ii), where each increment
of system width behaves essentially the same as every other
increment of system width; thereby permitting the system to be
scaled up or down to produce high quality webs of any suitable
and commercially useful widths on a hi~h-speed production basis
and wherein a web's cross-directional profile in terms of
basis weight can be controlled and, preferably, can be
maintained uniform.
The phrase "coefficient of variation" is used herein to
-18-
describe variations in the cross-directional basis weight
profile of both the web being formed and the fibrous materials
input to the system, and comprises the standard deviation
(u) expressed as a percent of the mean. The coefficient of
variation should not vary more than 5% and, preferably, should
vary less than 3~ in the cross-machine direction. The basis
weight profile in the cross-machine direction of the web
being formed may, for example, be determined by weighing
strips of the web which are three inches in width (3" C.D.)
by seven inches in length (7" M.D.).
The phrases "uniform cross-directional profile",
"uniform mass quantum of fibers in the cross-machine
direction", and similar phrases, are herein used to describe a
condition in the web being formed, as well as in the fiber
delivered to the forming apparatus, wherein the coefficient of
variation does not vary more than 5% and, preferably, varies
less than 3% in the cross-machine direction.
~he phrases "controlled cross-directional deposition",
"controlled cross-directional profile", "controlled mass
quantùm of fibers in the cross-machine direction", and similar
phrases, are herein used to describe a condition wherein the
cross-directional profiles of the fiber feed and the web
being formed are not necessarily uniform but, rather, may
intentionally be non-uniform; and, because the system is
substantially devoid of cross-directional flows, the cross-
directional profile of the finished web is controlled so as
to be similar in profile to the cross-directional profile of
the fiber feed --e.g., if the fiber feed has twice the mass
quantum of fibers at its center as it does along its marginal
edges, the basis-weight of the web produced will also be
_ ~9 _
csm/~,~
approximately twice as great a-t its center than at i-ts
marginal edges when viewed in cross-directional profile.
One convenient way of delivering fibers to the system is
to form a feed mat having a controlled and/or uniform cross
directional weight profile. Such systems are described in
detail in subsequent portions of this specification. However,
other means of delivering fibers to the system at a
controlled weight rate in the cross direc-tion may be devised
and are within the scope of this invention. While the
invention will herein be described in large part in terms of
a fibrous material input to the system wherein the coefficient
of variation is not more than 5% and, preferably, is less than
3~, and the formation of air-laid webs having a uniform
cross-directional profile with a coefficient of variation
of not more than 5% and, preferably, of less than 3~, since
there is presently a significant demand for such products--
particularly in the case of relatively low basis weight webs
on the order of 13 lbs./2880 ft. 2 to 18 lbsu/2880 ft. 2 --it
should be understood that the invention is not limited to the
formation of webs having uniform cross-directional profiles
but, rather, is equally useful in the manufacture of webs
having controlled cross-directional profiles, both uniform
and non-uniform.
The term "throughput" and the phrase "rate of web
formation" are herein used generally interchangeably and are
to be distinguished from the phrase "ratè of fiber delivery".
Thus, the phrase "rate of fiber delivery" is intended to mean
the mass quantum or weight rate of feed of fibrous materials
delivered to the forming head, and may be expressed, for
example, in units of pounds per hour per inch of former width
(lbs/hr./in.), pounds per minute per foot of former width
- 2 ~-
csm/~
'7~
(lbs./min.Jft.), or in any other suitable units. "Throughput",
on the other hand, is intended to describe the screening rate
for fibrous materials discharged from the forming head--i.e.,
the mass quantum or weight rate of fiber delivery through the
former screen per unit area of screen surface--and may be
expressed, for example, in units of pounds per hour per square
inch of effective screen surface
- 20a -
csm/~
5~
area (lbs./hr./in.2), pounds per minute per s~uare foot of
effective screen surface area (lbs./min./ft.2), or any other
suitable units. The fiber "throughput" achieved is refdected
directly in the "rate of web formation" and may be calculated
by multiplication of fiber throughput by the effective
length of ihe former screen. "Rate of web formation"--i.e.,
the rate at which the air-laid web is formed on the moving
forming wire or other formlng surface--may be expressed, for
example, in units of pounds per hour per inch of former
width (lbs./hr./in.), pounds per minute per foot of former
width (lbs./min./ft.), or in any other suitable units.
The words "up", "down", "above" and/or "below" are
used in a relative, non-limiting sense to describe, merely by
way _ example, a relationship of one structural element to
a forming wire or to another structural element.
B. Overall System Descriptlon
Briefly, and referring first to FIG. 1, there is
illustrated an exemplary system for forming an air-laid web
60 of dry fibers and comprising: a fiber metering section,
generally indicated at 65; a fiber transport or eductor
section, generally indicated at 70; a forming head, generally
indicated at 75, where provision is made for controlling air
and iber flow, and where individual fibers are screened
rom undesirable aggregated fiber masses and, thereafter, are
air-laid on a foraminous forming wire 80; a suitable bonding
station, generally indicated at 85, where the web is bonded
to provide strength and integrity; a drying station, generally
indicated at 87, where the bonded
csm/~
. .
5~
web 60 is dried prior to storage; and, a take-up or reel-
type storage station, generally indicated at 90, where the
air-laid web 60 of dry fibers is, after bonding and drying 7
formed into suitable rolls 95 for storage prior to delivery
to some subsequent processing operation (not shown) where
the web 60 can be formed into specifically desired consumer
products.
In order to permit continuous removal of aggregated
fiber masses, the forming head 75 includes a separator
system, generally indicated at 76. Such separated aggregated
fiber masses and individualized fibers entrained therewith
are preferably removed from the forming area by means of a
suitable conduit 77 maintained at a pressure level lower
than the pressure within the forming head 75 by means of a
suction fan (not shown). The conduit 77 may convey the
masses to some other area (not shown) for use in inferior
products, for scrap, or, alternatively, the undesirable
aggregated fiber masses may be recycled via conduit 78 to a
hammermill, generally indicated at 100, where the masses are
subjected to secondary mechanical disintegration prior to
reintroduction into fiber meter 65. Finally, the forming
head 75 also includes a forming chamber, generally indicated
at 79, positioned immediately above the foraminous forming
wire 80. Thus, the arrangement is such that individual
fibers and soft fiber flocs pass through the forming chamber
79 and are deposited or air-laid on the forming wire 80 to
form a web 60 characterized by its controlled cross-directional
profile and basis weight.
C. Fiber Metering Section
-
30While various types of commercially available fiber
metering systems can, with suitable modifications, be employed
-22-
with equipment embodying the features of the present invention,
one system which has been found suitable and which permits
of the necessary modifyiny adaptations is a RANDO-~EEDER~ (a
registered trademark of the manufacturer, Rando Machine
Corporation, Macedon, New York). The fiber metering section
65 shown by way of example in FIG. 1 is such a system. Indeed,
a RANDO-FEEDER~ is ideally suited for us~ with the present
invention when attempting to work with synthetic fibers.
As here shown, the fiber metering section 65 is mounted on the
mezzanine floor level lO1 of a suitable paper mill. Fibers
may be fed to the fiber separator hopper 102 in any of a
variety of conventional ways. For example, pre-opened
fibers may be ~anually introduced in bulk through inlet
chute 103 which is provided with a closure member 104 so as
to maintain an enclosed chamber. Alternatively, batts or
other compacted fibers may be introduced through inlet 105
of hammermill lO0 (which is here shown only in diagrammatic
block-and-line form and may take any well known conventional
form). The compacted batts are fiberized within the hammermill
and, after fiberization, the individualized fibers are
delivered to the fiber separator hopper 102 via inlet 106.
A fan 107 is provided for removing excess air fro~ the fiber
separator hopper 102, thereby permitting the fibers to form
a loose fiber bed 108 at the bottom of the hopper 102. Thus,
the fan 107 functions to withdraw excess air from the hopper
102 and such excess air, together with some escaping fibrous
materials, are thereafter discharged into a suitable waste
air filter or cyclone separator (not shown~. If desired, a
conventional pre-feeder and opener-blender (not shown) can
0 be used to feed individualized fibers to the fiber meter 65.
In operation, fibers fall from the fiber separator
-23-
~ (
~ 57;Z
hopper 10~ and form a loose bed 108 of open fibers carried
by a floor apron conveyor 109. ~n anti-static spray system
llO may be provided to minimize adherence of the fibers to
portions of the system. The fibers are conveyed by the
5 floor apron conveyor 109 to an elevating apron conveyor 111
havin~ conventional pins and slats (not shown). Fibers are
carried upwardly by the elevator apron conveyor to a rotating
stripper apron 112 which serves to remove excess fiber stock
and return such excess stock to the bed 108. The arrangement
is such that a controlled, metered quantity of small opened
tufts of fiber remains on the pins of elevator apron conveyor
111 and is carried over the top thereof uniformly across the
entire width of pron 111 into an area 113 known as an air
bridge.
Fibers delivered to the air bridge 113 are doffed from
the pins on apron 111 by means of air flow under the control
of a suitable air volume controller 114-. As a result of the
flow rate of air movement, a controllable quantity of fibers--
uniform throughout the full width of air bridge 113- are
deposited on a rotating condenser screen llS, thus forming a
full-width uniform feed mat 116 conveyed by roller CQnVeyor
118 to a feed plate 119. The arran~ement is such that as
the feed mat 116 takes shape, the resistance of the mat on
condensor screen 115 serves to reduce air flow through the
screen and, conseguently, proportionally less doffing occurs
at apron 111 until a condition of equilibrium is reached.
At the equilibrium point, a sufficient guantity of fibers
are doffed to form a continuous uniform feed mat 116, with
the balance of unused fibers being returned by the pins on
elevator conveyor 111 to the fiber bed 108.
The full-width uniform feed mat 116 is then conveyed
-24-
over feed plate ll9 by means of feed roller 120 and into the
path of teeth formed on an opening roll or lickerin 121.
The lickerin 121 serves to comb individual fibers from the
feed mat 116 with the individualized fibers being picked up
and carried by a full-width air strea~ passing under feed
plate 119 and generated by fan 124 and eductor 70. From this
point, the entrained stream of individualized air~suspended
fibers is introduced into the main air supply strea~ generated
by fan 124 and carried through eductor 70 and the forming
head 75, with the fibers exiting the forming head 75 passing
through the forming chamber 79 and being uniformly deposited
across the full-width of forming wire 80 in a uniform, but
completely random, fiber pattern, thereby forming web 60.
D. ~eb Forming, Com~acting, Bonding,
Dryin~ & Storage Section
As heretofore indicated, fibers are air-laid on the
foraminous forming wire 80 at the forming station by means
of an air stream generated primarily by fan 124. In addition,
a vacuum box 126 positioned immediately below the forming
wire 80 and the web forming section 79 serves to maintain a
positive downwardly moving stream of air which assists in
collecting the web 60 on the moving wire 80. If desired, a
second supplementary vacuum box 128 may be provided beneath
the forming wire at the point where the web 60 exits from
beneath the forming cha~ber 79, thereby insuring that the
web is maintained flat against the forming wire.
After for~ation, the web 60 is passed through calender
rolls 129 to lightly compact the web and give it sufficient
integrity to permit ease of transportation to conveyor belt
130. A light water spray can be applied from nozzle 131 in
order to counteract static attraction between th~ web and
the wire. An air shower 132 and vacuum box ~34 serve to
-25-
7~2 ~
clean loose fibers from the wire 80 and thus prevent fiber
build-up.
After transfer to the belt 130, the web 60 may be
bonded in any known conventional manner such, merely by way
of example, as i) spraying with adhesives such as latex, ii)
overall calendering to make a saturating base papex--i.e., a
bulky web with a controlled degree of hydrogen bonding-~
adhesive print pattern bonding, or other suitable process.
Such bonding processes do not form part of the present
invention and, therefore, are neither shown nor described in
detail herein, but, such processes are well known to those
skilled in the art of non-woven fabric manufacture. For
example, the web 60 may be pattern bonded in the manner
described in greater detail in the aforesaid Dunning U.S.
Pat. No. 3,692,622 assigned to the assignee of the present
invention. Briefly, in this bonding process, the moisture
content of the web is adjusted to 6~ to 3596 by a water spray
135 and, thereafter, the web is bonded by passing it through
the nip between a small hard roll 136 and a patterned steel
roll 138. Subsequently, the bonded wf~b 60 is transferred to
conveyor belt 139 and transported thereby through the drying
station 87 to the storage station 90 where the web 60 is
taken up on a driven reel 140 to form roll 95 which may
thereafter be either stored for subseguent use or unwound at
a sùbsequent web processing station (not shown) ~o form any
desired end product. The drying station 87 may take any
suitable conventional form such, for example, as a pair of
closely spaced heated plates 88, 89, or an oven or heated
roll (not shown).
Referring to FIG. 2, there has been diagrammatically
illustrated a typical system employing multiple forming
--26--
57~
heads for increasing overall productivity of the air-laid
dry fiber web forming system. As here shown, multiple
forming heads 75A - 75N are positioned over the foraminous
forming wire 80, with each forming head being supplied with
a full-width uniform supply of air-suspended fibers fed from
respective ones of a multiplicity of hammermills and fiber
mèters (not shown in FIG. 2, but respectively similar to the
hammermill 100 and fiber meter 65 shown in FIG. 1). Of
course, while only two forming heads 75A and 75N have been
shown for illustrative purposes in FIG. 2, those skilled in
the art will appreciate that any desired number of forming
heads could be used dependent upon the productivity desired
in terms of the web's basis weight, forming wire speed, and
the speed at which the bonding station can be effectively
operated. Thus, it will be appreciated that the air-laid
web 60 is formed by a first layer of fibers 60A deposited by
forming head 75Aj and _ (where n = any whole integer) successive
layer(s) 60N deposited by _ downstream forming head(s) 75~J.
As a consequence of this construction, the speed of the
forming wire ~ay be increased by a multiple of the number of
forming heads employed.to form a composite web 60 of a
selected basis weight for a given forming wire speed.
E. Full-Width Metered Fiber Feed
In carrying out the present invention, provision is
madè for forming a full-width feed mat of fibers having a
controlled cross-directional profile in terms of the mass
quantum of fibers constituting the mat. To this end, and as
best illustrated in FIG. 3, feed mat 116 may be formed in
the manner previously described in connection with the fiber
metering section 65 shown by way of example in FIG. 1. Such
feed mat 116 has been found to meet the preferred conditions
s~ ~
of full-width uniformity in terms of the mass quantum of
fibers forming the mat and the coefficient of variation of
the fibrous materials input to the system. The mat thus
formed--e.g., mat 116--is then fed across feed plate 119 by
means of a feed roller 120 into the teeth on lickerin 121
~hich serves to disaggregate the fibers defining the mat by
combing such fibers (along with any pulp lumps, nits and
other aggregated fiber masses which are present) out of the
mat and feeding such materials directly into a high volume
air stream generated by fan 124 (FIG. 1~ and eductor 70
(FIGS. 1 and 3).
In order to permit attainment of the objectives of the
invention, the air-to-fiber ratio preferably employed when
working with cellulosic wood fibers is on the order of 200-
600 cubic feet of air (at standard temperature and atmospheric
pressure conditions) per pound of fiber--viz., 200-600
ft.3/lb. Moreover, when employing the exemplary equipment
herein described such air is supplied at relatively high
volumes which vary dependent upon the operational speed of
the rotor assembly and the types of fibers being worked with--
i.e., ~olumes ranging from 1,000 to 1,800 ft.3/min./ft. of
former width are conventional when working with cellulosic
wood fibers. For example, when employing an 8-bar rotor
operating at 1432 RPM, the volume of air supplied is preferably
on the order of 1500-1650 ft. /min./ft. of former width. On
the other hand, when working with synthetic f ibers or cotton
linters, f example, considerably higher volumes of air per
pound of fiber may be employed--e.g., the air-to-fiber ratio
may range from 1,000 to 3,000 ft.3/lb., or even higher.
In operation, the air-suspended fiber stream is conveyed
through a suitable fiber transport duct 170 (FIG. 3) from
-28-
.1.'~.'`1~7~
the full-width eductor 70 to a full-~idth inlet slot 171
formed in the upper surface of, and extending fully across,
a generally cylindrical housing 172 which here defines the
2-dimensional flow control, screening and separating zone
75. To insure that full-width mass quantum fiber control is
maintained, the exemplary duct 170 is preferably subdivided
into a plurality of side-by-side flow channels separated by
partitions 174 extending the full length of the duct. It
has been found that the desired coefficient of variation
constraint in the web being formed can be obtained by
spacing the partitions 174 apart by approximately four
inches so as to form a plurality of adjacent flow channels
extending across the full axial length of housing 172. It
has also been found that a partitioned duct arrangement of
the type shown in FIG. 3 can be advanta~eously used to
accommodate width differences between the feed mat 116
formed in the fiber metering section 65 and the final air-
laid web 60 deposited on the foraminous forming wire 80.
For example, excellent results have been obtained when
attempting to form a web 60 forty-eight inches in width,
utilizing a feed mat 116 only forty inches in width. Thus,
in such a system the duct 170 may diverge from a full-width
at its upper end of forty inches to a full-width at its
lower end of forty-eight inches, with the individual flow
channels defined by partitions 174 diverging from approximately
three and one-third inches in width to approximately foux
inches in width. This insures that cross-flow forces are
substantially eliminated, and the mass quantum of air-borne
fibrous materials delivered from the lower end of duct 170
to the flow control, screening and separating zone 75
remains substantially unchanged across the full width of the
o29--
3~7;2
system.
F. Flow Control, Screening and Separat.ion
In carrying out the invention, a 2-dimensional cylindrical
rotor former is provided which serves to control flow of the
air-suspended fi~er stream through a separation zone while
minimizing mechanical disintegration of fibrous materials,
and which is designed to provide an acceptable level of fine
scale air turbulence while insuring that the system is
substantially devoid of eddy currents and other undesired
cross-flow forces so as to maintain a controlled mass
quantum of fibers across the full width of the forming head
75. To accomplish this, the exemplary forming head 75
includes a rotor assembly, generally indicated at 175 in
FIGS. 3 and 4, mounted for rotation within housing 172
about a horizontal axis defined by shaft 176. The arrangement
is such that the air-suspended fibrous materials introduced
radially into housing 172 through inlet slot 171 are conveyed
by co-action of the air stream and the rotor assembly 175
through the housing 172 for controlled and selective discharge
either a) through a full-width discharge opening, generally,
indicated at 178 in FIG. 3, and into forming zone 7~ for
ultimate, air-laid deposition on forming wire 80 or, alternatively,
b) through a full-width tangential separator slot ~79 formed
in housing 172 downstream of the discharge opening 178. The
separator slot 179, which here forms part of the separation
and/or recycle zone 76 (FIGS. 1 and 3), is preferably on the
order of from 3/16" to 3/8" in circumferential width when
working with wood ~ibers and, if desired, may he adjustable
in any conventional manner ~not shown) so as to permit
circumferential widening or narrowing of the slot 179 to
optimize separation conditions~
-30-
35~;~
~ o per~it controlled, selective discharge of individualized
fibers and soft fiber ~locs through opening 178 and into
forming zone 79~ while at the same ti~e precluding discharge
of nits and other undesired aggregated fiber masses there-
through, suitable screening ~eans, generally indicated at180 in FIG. 3, is mounted within discharge opening 178.
Such screening means 180 may,
simply take the form of a conventional woven square-
mesh wire screen of the ty~e shown at 180A in ~IG. 6 and
having openings sized to preclude passage of aggregated
fiber masses~ 2 ~ the screen may take the form o~ an 8x8
mesh screen having 64 openings per square inch, a lOxlO mesh
screen, a 12x12 mesh screen, or other commonly available
woven mesh screens; provided only tha~ the screen openings
do not exceed 0.1" open space from wire-to-wire in at least
one direction and ha~e between 30~ and 55~ open area and,
preferably, between 38~ and 46~ open area. As bes~ shown in
FIG. 3, screening means 180 is formed with the same radius
of curvature as the se~i-cylindrical portion of housing 172
~ithin which discharge opening 178 is formed~
As best illustrated by reference to ~IGS. 3 and 4
conjointly, rotor assembly 175 comprises a plurality of
transversely extending rotor bars 181, each fixedly mounted
on the outer periphery of a plurality o~ closely spaced
spiders 182. ~he spiders 182 are, in turn, fixedly ~ounted
on shaft 176 which is journalled for rotation in outboard
bearing housings 183, 184 (FIG. 4) and which is coupled ~o
drive shaft 185 driven by any suitable means ~not shown~.
The arrangement is such that the high volume air-suspended
-31-
5'7~
stream of fibrous materials passing through duct 170 is
introduced radially into housing 172 through inlet slot 171
and such stream tends to pass across the rotationally driven
rotor bars 181--viz., the bars 181 move through the radially
entering stream of air-suspended fibers. As a result of
rotor bar movement and the high velocity movement of the air
stxeam, the air and fibers tend to move outwardly towards
th~ wall of housing 172, thus forming an annular, rotating,
aerated bed of fibrous materials, best illustrated at 186 in
FIG. 10. Such annular-aerated bed 186 of fibrous materials
is believed to be on the order of one-half inch to one and
one-half inches thick (dependent upon actual operating
parameters), and is believed to be moving rotationally at
about half the spe~d of the rotor bars 181. For example, in
a cylindrical 'ormer having an inside housing diameter of
24" where the rotor assembly 175 is being driven at 1432
RPM, the tip velocity of the rotor bars 181 is on the order
of 150 f.p.s. (feet/second) and, consequently, it is believed
that the velocity of the aerated bed 186 is on the order of
80 f.p.s. ~hus, since the rotor bars 181 are moving at 150
f.p.s. through an aerated bed of fibers moving in the same
direction at approximately 80 f.p.s., the relative velocity
between the aerated bed 186 of fibers and the rotor bars 181
is on the order of 70 f.p.s.
In keeping with the invention, the rotor assembly 175
is preferably designed a) to minimize pumping action which
tends to reduce the relative speed differential between the
rotor bars 181 and the aerated bed 186, thus causing the
fibers to move over and beyond the screening means 180, and
b) so as to minimize mechanical action between the rotor
bars 181 and both the housing 172 and screening means 180,
which action tends to disintegrate fibers and aggregated
fiber masses carried in the air stream and to generate
pills. To this end, the rotor bars 181 are generally of
relatively small cross-section--e.g., in the case of the
exemplary rectangular bars shown in FIG. 10, such bars are
on the order 3/4" in radial height by 3/8" in thickness,
such thickness dimension being desired only for purposes of
structural integrity--and are moun~ed so as to provide a
clearance between the outer edges of the bars 181 and the
inner wall surface of the housing 172 and screening means
180 of from 0.10 inches to 0.25 inches and, preferably, from
0.18 inches to 0.20 inches, at least during transit of the
rotor bars from the upstream edge 188 of screening means 180
through separator slot 179. In terms of "pumping action",
therefore, the signficant bar area is only 3/4" times the
width in inches of the forming head 75. To avoid generation
of cross-flow forcest it is important that the rotor bars
181 are continuous, extend the full width of the rotor
chamber, and are oriented parallel to the axis of the rotor
assembly 175.
In carrying out the present invention, the rotor
housing 172 is preferably semi-cylindrical in cros~-section
throughout at least the arcuate span ranging from the
upstream edge 188 of screening means 180 through the tangential
separator slot 179, thereby insuring proper clearance between
the rotor bars 181 and the inner periphery of both the
screening means 180 and housing 172 as the rotor assembly
175 is driven rotationally. The remaining upper segment of
the housing 172 may be of any desired shape, including
substantially semi-cylindrical, but is preferably relieved
immediately adjacent the downstream edge of the inlet slot
-33-
171 as indicated at 189, thereby preventing the tendency of
those fibers passing the separator slot 179 from impinging
against the vertical edge 190 of inlet slot 171 and causing
consequent blockage, or partial blockage, of the inlet slot.
Referring again to FIG. 3, it will be apparent from the
description as thus far set forth, that as air-suspended
fibers are introduced radially into the rotor housing 172
through inlet slot 171, they are moved rapidly through the
housing under the influence of the air stream and movement
of the rotor bars 181, thus forming the moving annular
aerated bed 186 of fibers (FIG. 10) about the lnner periphery
of the housing wall. As the aerated bed--which contains
individualized fibers, soft fiber flocs, nits and other
aggregated fiber masses--passes over the screening means
180, some, but not all, of the individualized fibers and
soft fiber flocs pass through the screening means into the
forming æone 79, while the balance of the individualized
fibers and soft fiber flocs, together with nits and other
aggr~gated fiber masses, pass over the screen without exiting
from the rotor housing 172. The undesired pills, rice and
nits--i.e., aggregated fiber masses--have a bulk density
generally in excess of .2 g./cc. and tend to be separated
along with some individualized fibers and soft fiber flocs
from the aerated bed 186 at the tangential separator slot
179, Wit]l those separate~ materials being centrifugally
expelled through the slot 179 where they are entrained in a
recycle or separating air stream generated by any suitable
means Inot shown) coupled to manifold 191 with the air=
suspended separated particles moving outward through a full-
width discharge passage 192 coupled to separator slot 179
and, ultimately, to conduit 77 (FIG. 1). Such separation is
-34-
aided by a positive air outflow from housing 172 through
separator slot 179.
In keeping with the invention, provision is made for
insuring positive separation of undesired nits and aggregated
fiber masses from individualized fibers and soft fiber
flocs, and for preventing movement of the latter through
separator slot 179 to the full extent possible, thereby
insuring that individualized fibers and soft fiber flocs are
retained within rotor housing 172 and move with the aerated
bed 186 back to the area of screening means 180 where such
desirable materials have successive opportunities to pass
through the screening means 180 into the forming zone 79.
To accomplish this, a full-width classifying air jet 194 is
provided upstream of the separator slot 179 and downstrear:~
of screening means 180; such air jet being positioned to
introduce a full-width air stream generated by any conventional
source (not shown) radially into rotor housing 172 just
ahead of the separator slot 179. As a consequence, the
positive classifying air stream introduced radially into
housinq 172 through air jet 194 tends to divert individualized
fibers and soft fiber flocs within the aerated bed 186
radially inward as a result of tlle relatively high drag
coefficients of such materials and their relatively low bulk
density (which is generally on the order of less than .2
g./cc.). Since the nits and aggregated fiber masses have a
relatively high bulk density in excess of .2 g./cc. and
relatively low drag coefficients, the classifying air stream
introduced through the full-width air jet 194 does not
divert such materials to any significant extent and, therefore,
such undesired materials tend to be centrifugally expelled
through the tangential separator slot 179. It has been
--35--
"~ t~;~
found that the introduction of classifying air through the
full-width classifying air jet 194 into housing 172 at
pressures on the order of from 50" to 100" H2O and at
volumes ran~ing from 1.5 to 2.5 ft.3/min./in. provides an
energy level adequate for deflecting a significant portion
of the indi~idualized ~ibers and soft fiber flocs. The
energy level of the classifying air jet is most conveniently
controlled by adjusting its pressure.
In operation, it has been ound that excellent results
io are obtained by li~iting the amount of fibrous material
removed from the system through separator slot 17g to less
than 10~ by weight and, preferably, to between 1% and s% by
weight, o~ the fibrous material introduced into the housing
172 tllrough inlet slot 171. Stated differently, at least
90~ of the ~ibrous materials introduced and, preferably
between 95% and 99% thereof, ultimately pass through screening
means 180 into the forming zone 79 and are air-laid on the
foraminous forming wire 80 without requiring any secondary
hammermilling operations and without being subjected to any
significant mechanical disintegrating forces~ The quantity
o~ material separated may be controlled by the operator by
varying the voiume of recycle air supplied through manifold
191 and/or by adjusting the circumferential extent of full-
width se~arator slot 179 in any suitable manner tnot shown).
AIR-LAID DRY FIBER WEB FO~IATION IN
` ACCORDANCE WITH THE PRESENT INVENTION
Thu~ far, the environment of the invention has been
described in connection with methods and apparatus wherein
-36-
.. i .1 ~,,f1 ~t 7~
a conventional woven square-mesh screen of the type shown at
180A in FIG. 6 is mounted in the discharge opening 178 in
forming head 75. While such a web ~orming system has
provided significant advantages in terms of fiber throughput
capacity for a 2-dimensional forming system, particularly
when contrasted with conventional sifting systems of the
type disclosed in the aforesaid Kroyer patents, the present
inventors have discovered that even yreater improvements can
be achieved in fiber throughput capacities when using 2-
dimensional web forming systems.
_ High Throughput Screening
In accordance with one of the important aspects of the
present invention, provision is made for substantially
increasing the fiber throughput capacity of a 2-dimensional
fiber forming system, yet wherein aggregated fiber masses
present within the forming head are effectively precluded
from entering the forming zone. Rather, such aggregated
fiber masses are separated from the aerated fiber bed 186 in
the forming head 75 (FIG. 3) and are discharged through
full-width separator slot 179. To accomplish this, a high-
capacity slotted screen 180B of the type shown in FIG. 7 is
mounted within discharge opening 178 with the screen slots
oriented with their long dimensions parallel to the axis of
rotor assembly 175.
When utilizing a slotted type screen 180B with a 2-
dimensional rotor assembly 175 mounted for rotation about a
horizontal axis, it has been found essential that the screen
slots be oriented with their long dimensions parallel to the
axis of the rotor assembly. When so oriented, individualized
fibers tend to move through the screen slots while nits and
aggregated fiber masses--e.g., the aggregated fiber masses
-37-
.
3~
195 shown in FIG. 7--are precluded from passing through the
screen since they are generally larger in size then the
narrow dimensions of the slots which may range between .02"
and 0.1" open space froln wire-to-wire in at least one
direction and, preferably, ranges between .045" and .085"
open space from wire-to-~ire in at least one direction.
Such wire-to-wire dimensions are particularly critical when
the system is being used to make high quality, lightweight
tissue webs--e.g., webs having low nit levels and basis
weights ranging from 13 lbs./2880 ft.2 to 18 lbs./2880 ft.2
and, in some instances, up to 22-25 lbs~/2880 ft.2. However,
when the slots of a slotted screen 180B are oriented with
their long dimensions perpendicular to a plane passing
through the rotor axis as shown in FIG. 8, it has been found
that the screen tends to rapidly plug--indeed, when operating
under commercial production conditions, it has been found
that the screen tends to become completely plugged almost
instantaneously. It is believed that such plugging action
results from the tendency of individual fibers to "staple"
or "hair-pin" and otherwise hang up or collect within the
narrow confines at the end of each slot as best indicated at
196 in the lower right-hand corner of FIG. 8; and, as soon
as a few fibers have collected, other fibers and aggregated
fiber masses 195 almost instantaneously agglomerate on the
screen as depicted in the balance of FIG. 8. This plugging
phenomenon is more clearly visible upon reference to the
photograph reproduced as FIG. 9--such photograph illustrating
a slotted screen 180B wherein the slots are oriented at an
angle of 45 to a plane passing through the rotor axis--and,
under these conditions, the screen 180B plugged almost
completely and instantaneously.
-38-
7~ ~
On the other hand, it has been found that a conventional
woven square-mesh screen of the type shown at 180A in FIG. 6,
and a slotted screen 180B with the slots oriented as shown
in FIG. 7, exhibit little or no tendency to plug undex
normal operating conditions. Rather, while individualized
fibers still have a tendency to "staple" or "hair-pin", as
indicated at 197 in FIGS. 6 and 7, there seems to be adequate
time and room for the suspended fibers to disengage themselves
from the screen; whereas in the arrangements shown in FIGS.
8 and 9, the suspended fibers ~end to catch and congregate
in the closely proximate confined corners of the screen slot
and, as a result, other fibers and aggregated fiber masse~
195 rapidly accumulate, thus plugging the screen and rendering
the system inoperative.
The exemplary system herein described has been depicted
in FIGS. 3 and 4 as including a rotor assembly 175 having
eight rotor bars 181. However, the number and/or shape of
the rotor bars may be varied, provided that such ~odifica-
tions are consistent with mechanical stability and low
rotor "pumping" action. That is, the rotor assembly 175
must be a dynamically balanced assembly and, therefore, it
must include at least two rotor bars. However, it will be
appreciated that it can include fewer or more than the eight
bars illustrated in FIGS. 3 and 4--for example, excellent
results have been achieved with a 4-bar rotor assembly of
the type indicated at 175' in FIG. 5. On the other hand,
care must be taken to insure that the number of rotor bars
employed--e.g., _ rotor bars where n equals any whole integer
greater than "1"--and the shape of the rotor bars are such
that pumping action is minimized. Otherwise, the rotor
assembly 175 will tend to sweep the aerated fiber bed 186
-39-
9~;~
over and beyond the screening means 180 rather than permitting
and, indeed, assisting fiber movement through the screening
means.
In the illustrative form of the invention, the rotor
bars 181 have a rectangular cross-section, and pumping
action is minimized by keeping the effective rotor bar area
relatively small~ ~ 3/4" times the length of the bars
which extend across the full width of the rotor housing
172--and by spacing the bars apart circumferentially by 45
(there being eight equally spaced bars) and from the housing
172 by on the order of 0.18" to 0.20". However, the rotor
bars 181 need not be rectangular in cross-section. Rather,
they can be circular, vane-shaped, or of virtually any other
desired cross-sectional configuration not inconsistent with
the objective of minimizing rotor pumping action. For
example, rotor bars having a circular cross-section would,
because of their shape, be even more effective than rectangular
bars in terms of minimizing rotor pumping action. However,
thè primary function of the rotor assembly as employed in
the present invention is, as more fully described in Section I,
page 45 _ seq., infra, of this specification, to lift
individualized fibers, soft fiber flocs, and aggregated
fiber masses off the surface OL the former screening means
and, thereby, to prevent plugging of the screen, to prevent
2~ layering of fibers on the screen, and to reopen apertures in
the screen so as to permit passage of the air-suspended
fiber stream therethrough. This desirable result is achieved
by the negative pressure 20nes created in the wakes of the
moving rotor bars; and, the negative pressure zones in the
wakes of rotor bars having a rectangular cross-section have
been found to be as effective for this purpose as those
-40-
.
,t~57~ ~
created by rotor bars of circular cross-section.
It is significant to a complete understanding of the
present invention that one understand the difference between
the primary function of the rotor assembly here provided--
viæ., to lift fibrous materials upwardly and off the screenor, stated differently, to momentarily disrupt passage of
the air-suspended fiber stream through the screen--and that
stated for conventional cylindrical rotor systems of the
type disclosed, for example, in the aforesaid 3.D'A. Clark
patents where the rotor chamber functions as a "disintegrating
and dispersing chamber" (See, e.~., col. 4, line 53, J.D'A. Clark
~.S. Pat. No. 2,931,076)--~iz., where the rotor blades
mechanically act upon the fibrous materials to "disintegrate"
such materials and propel them through the screen.
H. Forming Zone
In keeping with another important aspect of the present
invention, provision is made for insuring that individualized
fibers passing through the screening means 180 shown in
FIG. 3 are permitted to move directly to the foraminous
forming wire 80 without being subjected to cross-flow forces,
eddy currents or the like, thereby maintaining cross-directional
control of the mass quantum of fibers delivered to the
forming wire through the full-width of forming zone 79. To
accomplish this, provision is made for insuring ~hat the
upstream, downstream and side edges of the forming zone--
i.e., the boundaries of the zone 79-~are formed so as to
define an enclosed forming zone and to thereby preclude
intermixing of ambient air with the air/fiber stream exiting
housing 172 through screening means 180. It has been found
that the air/fiber stream exiting from housing 172 through
screening means 180 does not exit radially but, rather, at
an acute angle or along chordal lines or vectors which, on
average, tend to intersect a line tangent to the mid~point of
the screening means 180 at an includ~d angle cr. In the
exemplary form of the invention where the screening means
180 covers an arc of approximately 86--i.e., an arc extending
clockwise as viewed in FIG. 3 from a point (indicated at 198
in FIG. 3) a~proximately 15~ from the center of inlet slot
171 to a point 188 approximately 245 from the center of
inlet slot 171--and, where an 8-bar rotor is being operated
at a rotor speed on the order of 1400-1450 RPM, it has been
found that the angle cr is generally on the order of 11.
Consequently, the forming zone 79 is preferably provided
with sidewalls (a portion of one such sidewall is shown at
199 in FIG. 3), a full-width downstrea~ forming wall 200,
and a generally parallel full-width upstream forming wall
201, which are respectively connected to rotor housing 172
at the downstream and upstream edges of screening means 180,
and which respectively lie in parallel planes which intersect
a linP tangent to the mid-point of the screening means 180
at included angles on the order of 11. The upstream end of
forming wall 201 is bent as indicated at 201A, 201B so as to
form a shaped portion which generally accommodatas the
air/fiber flow pattern exiting the upstream portion of
screening means 180. The walls 199, ~00 and 201 serve to
enclose the forMing zone 79 and to thereby preclude disruption
of the air/fiber stream as a result of mixing between ambient
air and the airtfiber stream. The enclosed forming zone 79
is preferably maintained at or near atmospheric pressure so
as to prevent inrush and outrush of air and to thereby
assist in precluding generation of cross-flow forces within
the forming zone. Those skilled in the art will appreciate
-42-
that angle a can vary with changes in operating parameters
such, for example, as changes in rotor RPM. ~owever, for
operation at or near optimum conditions, it is believed that
the angle a will generally lie within the range of 5 to
20 and, preferably, will lie within the range of 8 to 15.
The lower edges of forming walls 200, 201 terminate slightly
above the surface of foraminous forming wire 80--generally
terminating on the order of from one-quarter inch to one and
one-quarter inches above the wire.
In the exemplary form of the invention shown in FIG. 3,
when the angle a is on the order of 11 and when the
forming zone 79 is positioned over a horizontal forming
surface 80, the upstream and downstream forming walls lie in
planes which intersect the horizontall~ disposed forming
surface 80 at included acute angles ~ ~here ~ is on the
order of 33. Howevcr, those skilled in the art will appreciate
that the angular value of ~ is not critical and can vary
over a wide range dependant only upon the orientation of the
forming surface 80 relative to the forming ~.one 79. For
example, one advantage to positioning the forming surface 80
in a horizontal planè as shown in FIG. 3 is that an acute
angle ~? of approximately 33 tends to optimize the fiber
deposition surface area of the forming surface 80. That is,
assuming the forming walls 200, 201 to be parallel and
spaced apart by approximately 9" as measured in a direction
normal to the walls, and assuming an angle ~ on the order
of 33, the lower edges of the forming walls will he on the
order of 16" apart in a horizontal plane just above the
forming surface 80, thereby providing a total fiber deposition
area equal to 16" times the width of the forming æone 79.
Moreover, fiber deposition is optimized by virtue of the
-43-
fact that the fibers approach the forming surface 80 at an
acute angle ~ of about 33 while moving in the direction
of forming surface movement.
As the angle ~ is increased--e.g., towards an angle
of 90--the area of fiber deposition is reduced, approaching
a total deposition surface area equal to only 9" times the
width of the for~ing zone 79 under the assumed conditions;
- and, at the same time, the vector component of fiber movement
in the direction of movement of the forming surface 80 is
also reduced until at an angle ~ of 90, the fibers have
no component of movement in the direction of forming surface
movement. Such an increase in the angle ~ can be readily
achieved by the simple expedient of mounting the forming
surface 80 in an inclined plane--viz., inclined upwardly and
towards the right as viewed in FIG. 3. Conversly, reduction
in the angle ~ below 33 tends to further increase the
total area of fiber deposition on the forming surface 80.
However, it is believed that optimum results are attained
where angle ~ is on the order of 33 when angle ~ is
on the order of 11.
The foregoing arrangement insures that the upstream and
downstream boundaries of forming zone 79 generally coincide
with the upstream and downstream boundaries of the air/fiber
stream exiting the rotor housing 172 through screening means
180, consequently preventing mixing of ambient or room air
with the moving air/fiber stream, minimizing impingement of
the air/fiber stream on the walls of the forming zone and,
thereby preventing the setting up of eddy currents or other
gross cross-flow forces which would interfere with the
cross-directional mass quantum dispersion of fibers being
conveyed through the forming zone 79 in the air stream
-44-
5~
across the full-width of the system. Moreover, since
constraining walls 200, 201 are parallel, there is no
te~dency to decelerate the flow las would be the case where
the walls diverge). This fact again aids in preventing
eddy currents and other unwanted cross-flow forces. There
is, of course, some deceleration of the air/fiber stream
as it exits tne housing 172 through screening means 180;
but, such deceleration occurs immediately upon exit from the
screening means and produces only a fine scale turbulence
effect which does not induce gross eddy currents or cross-
flow forces.
The foregoing zone is preferably dimensioned so that
under normal adjustment of variable system operating parameters,
the velocity of the fiber/air stream through the forming
zone is at least 20 f.p.s. and the fibers are capable of
traversing the entire length of the forming zone 79 from
screen 180 to forming wire 80 in not more than .1 second.
While the forming zone 79 in the exemplary form of the
invention has been depicted as including physical walls 199,
200 and 201, those skilled in the art will appreciate that
the boundary layer confining means could take other forms if
desired without departing from the scope of the invention--
for example, the confining boundary walls could take the
form of air curtains tnot shown). Moreover, in some cases
it might be desirable to have the walls 200, 201 converge
slightly so as to accelerate and, therefore, stabilize the
flow.
I. Overall System Operation
-
Numerous system parameters may be varied in the operation
of a forming system embodying the features of the presen~
invention in order to form an air-laid web of dry fibers
-45-
having specific desired characteristics. Selected repre
sentative and optimum parameter settings are set forth in
greater detail in Section K, page 65 et seq., infra, of this
specification where specific examples have been delineated.
Such variable parameters include, for example: air-to-fiber
ratio (which is, preferably 200-600 ft.3/lb. when working
with cellulosic wood fibers, and preferably 1000 to 3000 ft.3/lb.,
and perhaps higher, when working with cotton linters and
relatively long synthetic fibers); air pressure within
housing 172 (which preferably varies from +0.5" to ~3.0"
H2O); rotor speed (which preferably varies from 800 to 1800
RPM); the number, orientation and shape of rotor bars employed;
the quantity of air supplied per foot of former width (which
is, preferably, on the order of 1500 to 1650 ft.3/min. with
an 8-bar rotor operating at 1432 RPM); the energy level of
classifying air supplied (which preferably ranges from 1.5
to 2.5 ft.3/min.tin. or, stated in terms of pressure, preferably
ranges from 50" to 100" H2O); recycle or separation balance
(which is less than 10% by weight of the fiber supplied and,
preferably, fxom 1% to 5% by weight of the fiber supplied);
screen design--viz., whether the screen is a woven square-
mesh screen or a slotted screen, the size of the screen
openings (which ranges between .02" and 0.1" wire-to-wire
open space in at least one direction and, preferably, ranges
between .045" and .085" open space from wire-to-wire in at
least one direction), the wire diameter used (which preferably
varies from on the order of .023" to .064") and, the percentage
of open screen area (which is between 30% and 55% and,
preferably, varies from 38% to 46%); air pressure within the
enclosed forming zone 79 (which is preferably atmospheric);
as well as the physical dimensions of the forming head 75
-46-
7~ ~
(which, in the exemplary form of the invention, comprises a
generally cylindrical housing 172 having an inside diameter
of 24").
Moreover, the rate of production of the web being formed
can also be varied by altering numerous other system parameters
such, merely by way of example, as the number of forming
heads 75 used, the position of the forming head relative to
the forming wire--i.e., whether the forming head is mounted
in the cross-direction, the machine-direction, or at some
angle therebetween--forming wire speed, and the type of
fibers used. Still other variable parameters under the
control of the operator include the cross-directional profile
of the feed mat delivered to the forming head 75. Thus,
where it is desired to produce a web having a uniform cross-
directional profile with an acceptable coefficient of variation,the feed mat~ ~ feed mat 116 in FIG. 3--preferably will
have a uniform cross-directional profile in terms of the
mass quantum of fibers present. On the other hand, if one
desires to produce an air-laid web having a specific non-
uniform cross-directional profile--e.g., an absorbent
filler web having a central portion with a relatively high
basis weight and marginal edges of relatively low basis
weights--it is merely necessary to form either a single feed
mat or multiple side-by-side feed mats having the requisite
cross-directional profile and, since the present system is
substantially devoid of cross-directional forces, the cross-
directional profile of the input feed mat(s) will control
the cross-directional profile o~ the air-laid web.
Reco~nizing the foregoing, let it be assumed that the
operator wishes to form an air-laid web 60 one foot (1'~ in
width (all ensuing assumptions are per one foot of width of
-~7-
5~
the forming head 75) having a controlled uniform cross-
directional profile and a basis weight of 17 lbs./2880 ft.2.
Assume further:
a) Air-to-fiber ratio supplied through inlet
slot 171 equals 350 ft.3~1b.
b) Inlet slot 171 is 51 in circumferential
width--i.e., the dimension from edge 190
~FIG. 3) to edge 202.
c) Rotor housing 172 is 24" I.D.
d) Rotor asse~bly 175 employs eight egually
spaced rectangular rotor bars 181, each 3~4"
in radial hsight by 3/8" in circumferential
thickness and extending parallel to the axis
of the rotor assembly continuously throughout
the full width of rotor housing 172 and, each
spaced from the rotor housing 172 by 0.18".
e) Rotor assembly 175 is driven at 1432 RPM.
f) Rotor bar 181 tip velocity equals 150 f.p.s.
g) Relative velocity between the rotor bars 181
and the aerated bed 186 is approximately 70
~ .p.s .
h) Screening means 180 defines an arc o~ 86,
and has 40% open area.
i) Separation and/or recycle through separator
slot 179 comprises 5% by weight of fibrous
materials supplied through inlet slot 171.
j) The quantity of classifying air introduced
through air jet 194 is between 1.5 and 2.5
ft.3/min./in. at pressures between 50" and
100" H2O.
k) Forming walls 200, 201 are parallel and spaced
-48
~. :
~$~57~:
9" apart in a direction normal to the parallel
walls 200, 201 and 16" apart in a horizontal
plane passing through their lower extremities
just above the plane of the forming wire 80.
l) Forming wire speed equals 750 f.p.m.
All of the foregoing operating parameters are either
fixed and known, or can be pre-set by the operator, except
for the relative velocity between the xotor bars 181 and the
aerated bed 186 of fibers within the rotor housing 172. The
actual speed of the aerated bed 186 is not known with certainty;
but, it is believed to be substantially less than the rotor
bar tip velocity of 150 f.p.s.; and, more particularly, it
is believed to be on the order of half the tip velocity of
the rotor bars 181. For convenience, it is here assumed to
- 15 be approximately 80 f.p.s., an assumption believed to be
reasonably accurate based upon observation of overall system
behavior, thereby resulting in a relative velocity between
the rotor bars 181 and the aerated bed 186 of approximately
70 f.p.s. (see assumption "gn, supra).
Accordingly, supply and velocity relationships within
the foregoing ;exemplary system can be readily calculated as
ollows; and, such relationships have been illustrated in
FIG. 10:
17 x 750 = 4.43 lbs./min.--Rate of formation [I]
2880 of web 60.
4.43 x 1.05 = 4.65 lbs./min.--Rate of fiber [II]
supply through inlet slot 171.
4.65 x 350 = 1627 ft.3/~in.--Vol. of air sup- [III]
plied through inlet slot 171.
2~r x 86 = 1.5 ft.--Screen circumference. lIV]
360
1.5' x 1' x = 216 in.2--Screen area. ~V]
144 in.2/ft.2
-49-
, ~ .
i7;~ '
4.43 x 60 min.= 1.23 lbs./hr./in.2--Fiber [VI]
216 in.2 through~ut of former screen 180.
1.5ft.2 x 40% = 0.6 ft.2--Amount of open area in [VII~
screen 180.
1627 = 65 f.p.s.--Velocity of air and [VIII]
55/12 x 60 fiber stream entering rotor hous-
ing 172 through inlet slot 171.
1627 = 18 f.p.s.~-Velbcity approaching [IX]
1.5 x 60 the screen 180 (i.e., normal to
the screen).
1627 = 45 f.p.s.--Velocity throug~ screen [~]
- 0.6 x 60 openings.
1627 = 36 f.p.s.--Velocity in forming [XI]
9/12 x 60 zone 79.
1627 _ = 20 f.p.s.--Velocity normal to [XII]
16~12 x 60 forming wire 80.
150 - 70 = 80 f.p.s.--Velocity vector [XIII]
parallel to the screen 180.
~ = 82 f.p.s.--Air velocity vector [XIV]
composite within housing 172.
4.65 - 4.43 = .22 lbs./min.--Amount of fiber [XV]
removed through separator slot
17g .
Keeping the foregoing supply and velocity relationships
in mind, and upon consideration of FIGS. 3 and 10 conjointly,
it will be appreciated that the individualized fibers, soft
fiber flocs, and any aggregated fi~er masses present in the
feed mat 116 (FIG. 3) will be disaggregated and dispersed
within the air stream passing through fiber transport duct
170 with essentially the same cross-directional mass quantum
relàtionship as they occupied in feed mat 116. Under the
assumed conditions, the air/fiber stream enters rotor
housing 172 (FIG. 3) at approximately 65 f.p.s. [Eq. VIII~ and
at a fiber feed rate of 4.65 lbs./min. [EqO II]. The volume
of air supplied to rotor housing 172--vlz., 1,627 ft.3/min.
[Eq. III]--is such that a positive pressure of approximately
1.5" H2O is maintained within the housing 172. Since the
-50-
7;2
forming zone 79 is maintained at atmospheric pressure, there
exists a pressure drop on the order of 1.5" H2O across the
screening means 180 through which the air-suspended fibers
pass.
Although the air/fiber stream entering rotor housing
172 through inlet slot 171 is moving radially initially,
rotation of the rotor assembly 175 (counterclockwise as
viewed in FIGS. 3 and 10) tends to divert the fibers outwardly
towards the periphery of housing 172 so as to form an
annular aerated bed of fibers, as best illustrated at 186 in
FIG. 10. Movement of the rotor bars 181 through the annular
aerated bed 186 of fibers at a rotor bar tip velocity of
150 f.p.s. tends to accelerate the air-fiber stream from its
entry velocity of 65 f.p.s. [Eg. VIII] to approximately
80 f.p.s., thus resulting in a relative velocity of 70 f.p.s
between the rotor bars 181 and the aerated bed 185 of fibers
However, because of the clearance of 0.18" between the rotor
bars 181 and housing 172, and the relatively small effective
area of the rotor bars, only minimal pumping action occurs
and there is little or no tendency to roll fibers between
the rotor bars 181 and either housing 172 or screening means
180. Therefore, there is little or no tendency to form
pills; and, since only minimal mechanical disintegrating
action occurs, curlin~ or shortening of individualized
fibers is essentially precluded. Rather, the rotor bars 181
sweep through the aerated bed 186 and across screening means
180, thus causing at least certain of the individualized
fibers and soft fiber flocs within the aerated bed 186 to
move through the screening means--such air-suspended fibers
have a velocity vector normal to the screening means 180 of
approximately 18 f.p.s. [Eq. IX] and a composite velocity
-51-
5~72
vector of approximately 82 f.p.s. lEq. XIV] directed towards
scr ening means 180 at an acute angle--while, at the same
time, sweeping nits and aggregated fiber masses over and
beyond the screening means 180.
Since the rotor bars 181 are moving throuyh the aerated
bed 18~ of fibers at a relative speed 70 f.p.s. faster than
movement of the aerated bed, a negative suction zone of 1.7"
H2O is generated in the wake of each rotor bar 181, as best
illustrated at 204 in FIG. 10. Each such negative suction
zone extends the full-width of the rotor housing 172 and is
parallel to the axis of the rotor assembly 175. In the case
of rotor bars having a circular cross-section (not shown),
the negative suction generated would be on the order of
3.0" H2O. In either case, the negative suction generated is
sufficient to momentarily overcome the pressure drop of
approximately 1.5" H2O across the screening means 180 and,
as a consequence, normal flow of the air/fiber stream through
screening means 180 ceases momentarily in the region of the
screen beneath the negative suction zone 204. The full-
width negative suction zones 204 are, of course, also sweeping
across the screening means 180 at the same velocity as the
rotor bars 181--viz., 150 f.p.s.--and, as a consequence, the
rapidly moving spaced, full-width negative suction zones 204
tend to establish spaced full-width lifting forces which
serve two i~portant functions--viz., the generated lifting
forces i) tend to lift individualized fibers and soft fiber
flocs off screening means 180 in the wakes of the rotor bars
across the full-width of rotor housing 172, thus preventing
layering of fibers on the screen which tends to plug the
screen openings and thus inhibits free movement of fibers
through the screen; and ii), tend to lift nits and other
-52-
Z
aggregated fiber masses off the screening means 180 so as to
facilitate their peripheral mov~ment over and beyond the
screening means and towards the full-width separator slot
179. Such peripheral movemen~ results from the movement of
the annular aerated bed 186 and the sweeping action of the
rotor bars 181.
It will be apparent that in the exemplary case employing
an 8-bar rotor assembly 175 moving at 1432 RPM with a rotor
bar tip velocity of approximately 150 f.p.s., approximately
one hundred and ninety-one spaced full-width negative
pressure zones 204 or negative impulses are generated per
second, each of which sweep across the surface of the screening
means 180 throughout the full-width thereof at a velocity equal
to that of the rotor bars which generated such zones--viz.,
150 f.p.s. Moreover, each full-width negative pressure zone
204 persists over an arcuate span on the order of several
times the height of the rotor bars 181; although the actual
distance through which the negative pressure zones 204
persist will vary dependent upon the specific operating
parameters selected such, for example, as rotor speed, the
relative speea differential between tha rotor bars 181 and
the aerated bed 186 of fibers, an~ the actual pressure
conditions established. However, assuming a ull-width
negative pressure zone 204 persisting on the order of three
times the height of the rotor bars 181 (which are here 3/4`'
in height), then the arcuate extent x (FIG. 10) of the
negative pressure zone 204 will be on the order of 2.25".
That is, each negative pressure zone 204 will span approximately
24% of the circumferential space of approximately 9.3"
between two adjacent rotor bars 181 (assuming an 8-bar rotor
assembly 175 with an outside diameter of 23.64"--i.e., 24"
--53--
.
7~
I.D. for housing 172 minus 2 x .18" clearance). Therefore,
the balance of the circumferential region from the trailing
edge of each zone 204 to the next succeeding rotor bar 181--
viz., a circumferential arc ~ (FIG. 10~ spanning approximately
76% of the circumferential distance between two adjacent
rotor bars 181, or a distance of approximately 7"--constitutes
a region of positive pressure drop of approximately 1.5" H2O.
Thus, it will be appreciated that in operation, approximately
one hundred and ninety-one full-width negative pressure
zones 204 alternating with an equal number of full-width
zones of positive pressure drop which are each on the order
of three times as extensive in duration ~i.e., ~ - 3x), will
sweep across screening means 180 each second. As indicated
above, in those rapidly sweeping regions beneath the negative
pressure zones 204, flow of the air/fioer stream through
screening means 180 at 45 f.p.s. [Eq. X] ceases momentarily
and, the fibrous material in those regions tends to be
lifted off the screening means. Considering any given fixed
area of the screening means 180, immediately upon passage of
each negative pressure zone 204, the positive pressure drop
conditions of approximately 1.5" H2O are restored until the
next rotor bar 181 passes thereover; thus permitting the
individualized fibers and soft fiber flocs to again move
toward the screening means 180 at a velocity of 18 f.p.s.
[Eq. IX~ normal to the screen and at a composite velocity
vector of 82 f.p.s. [Eq. XIV] directed towards the screen at
an acute angle and, ultimately, through the screen openings
at approximately 45 f.p.s. [Eq. X]. Thus, the arrangement
is such that plugging of the screen is effectively precluded,
while individualized fibers tend to dive end-wise through
the screen openings in the regions of positive pressure
~L~ 7~
drop, as best illustrated in FIG. 11--i.e., in those regions
between the trailing end of each full-width negative pressure
zone 204 and the next succeeding rotor bar 181.
Those individualized fibers, soft fiber flocs, and
aggregated fiber masses within the aerated bed 186 of fibers
which do not pass through the screening means 18U the first
time they are presented thereabove are swept over and beyond
the screening means 180 and, thereafter, past classifying
air jet 194 (FIG. 3). I~nder the assumed conditions, the
individualized fibers and soft fiber flocs tend to be diverted
radially inward by the classifying air jet 194, while the
undesired aggregated fiber masses are centrifugally and
tangentially separated from the aerated bed 186 through
full-width separator slot 179 at the rate of .22 lbs./min.
[Eq. XV]. Those individualized fibers and soft fiber flocs
remaining in the aerated bed 186 after transit of separator
slot 179 are then returned to the region overlying screening
means 180, where they are successively acted upon by the
rapid succession of pressure reversal conditions from full-
width negative pressure zones 204 alternating with full-
width zones of positive !?ressure drops until all such materials
pass through t~e screening means 180 into forming zone 79.
The airtfiber stream tends to exit from housing 172
non-radially through screening means 180--indeed, as previously
indLcated, under the assumed operating conditions the air/fiber
stream tends to exit at an acute angle which, on average
over the full extent of discharge opening 178, intersects a
line tangent to the midpoint of screening means :180 at an
included angle Q' of from 5 to 20 and, preferably, on the
order of 11. 'lne exiting air/fiber stream decelerates
almost immediately to approximately 36 f.p.s. [Eq. XI]
7~
within forming zone 79 and moves through the forming zone
toward the foraminous forming wire 80 which is here moving
at 750 ft./min. The fibers are air-laid or deposited on
forming wire 80 at the rate of 4.43 lbs./min. [Eq. I]--i.e.,
the difference between the rate of fiber supplied [Eq. II]
and the 5% of fibrous ~aterials supplied which are separated
and removed through separating slot 179--to form web 60.
The fibers deposited on the forming wire 80 are held firmly
in position thereon as a result of suction box 126 (FIG. 3)
and its associated suction fan and ducting which serve to
accomodate and remove the high volume of air supplied.
Following formation of web 60 on foraminous forming
wire 8Q in the manner above described, the web, carried by
forming wire 80 at a speed of 750 ft./~in., exits from
beneath forming zone 79. An auxiliary suction box 128 may,
if desired, be provided to insure that the web remains flat
on the forming wire as it exits from beneath forming zone 79
where the web has bèen subjected to the holding action
provided by suction box 126 which accomodates the main air
stream. The thus formed web 60 may then be further processed
in the manner previously described in detail in Section D,
pages 25-27 supra, of this specification. That is, and
as best illustrated in FIG. l, the web 60 is preferably
passed through calender rolls 129 where it is compacted
lightly, and is then transferred to a conveyor belt 130.
The web is thereafter bonded in any suitable bonding station
85, dried at a drying station 87, and formed into a storage
roll 95 at a suitable storage station 90.
Those skilled in the art will appreciate that there has
herein been described a typical set of operating para~eters
for forming an air-laid web 60 of dry fibers at a relatively
-56-
,
3~
high production speed-- _z , 750 ft./min.--utilizing only a
single forming head 75 (FIGS. 1 and 3). The exemplary web
thus formed has a basis weight of 17 lbs./2880 ft.2 and is
essentially devoid of nits and other aggregated fiber masses
which have been removed through separator slot 179. Because
the forming head 75 and forming zone 79 have been designed
so as to essentially preclude induced cross-flow forces
and/or eddy currents therein, the controlled mass quantum
dispersion of fibers remains substantially unchanged throug'nout
the system, thereby permitting the system to be scaled up or
down to form air-laid webs of virtually any desired width
and with a controlled coefficient of variation.
The web 60 deposited on forming wire 80 has more than
adequate integrity to permit rapid movement of the forming
wire. Indeed, if one desires to further increase productivity,
n additional forming heads 75A-75N (FIG. 2--where n equals
any whole integer) may be utilized and the speed of foraminous
forming wire 80 may be increased by a factor equal to the
number of separate forming heads used--e.g., under the
assu~ed operating conditions, two heads would permit operation
at 1,500 f.p.m.; three heads would permit operation at
2,250 f.p.m.; et cetera. Indeed, with the present invention,
_
forming wire speed is no lonyer limited by the speed of web
~ormation but, rather, by the speed of such subsequent
processing steps as bonding in the web bonding station 85
(FIG. 1).
Experimentation with air-laid, dry fiber, web forming
systems embodying the features of the present invention has
indicated that a wide range of results are attainable dependent
30` upon the particular operating parameters selected, as
reference to the representative experimental data set forth
-57-
57~2
in Section K, page 65 et seq., infra, of this specification
indicates. Particularly importan~ are such design and operating
parameters as: i) rotor design; ii) rotor speed; iiiJ recycle
or separation percentage; iv) screen design; and v),
air-to-fiber ratio. of the foregoing, rotor design and
screen design represent fixed parameters which, once selected,
are not normally subject to operator control; whereas the
remaining parameters may be varied over wide ranges to
provide virtually an infinite range of possible permutations
and combinations which can, and will, affect the characteristics
o~ both the web produced and the rate of web productivity.
For example, as indicated in Section G at pages 39-40,
supra, of this specification, the rotor assembly 175 may be
formed with n rotor bars 181 where n equals any whole integer
greater than ~ln. However, it has been ascertained that fiber
throughput--a limiting constraint when attempting to maximize
productivity--is a function of rotor speed multiplied by the
square root of the number of rotor bars employed--i.e.,
fiber throughput: f (RPM x ~No. of rotor bars 181). This
relationship wili, of course, vary with the particular
screen employed; and, has been graphically illustrated in
FIG. 12 wherein fiber throughput in lbs./in./hr. (the ordinate)
has been plotted at various rotor speeds for each of a 2-
bar, 4-bar, and 8-bar rotor assembly (the abscissa) when
using both a coarse wire screen (lOx2.75; .047" wire dia.;
.059" screen opening; and 46.4~ open screen area) and a fine
wire screen (16x4; .035" wire dia.; .032" screen opening;
and 38.8% open screen area). As here shown, the circular
points 205 are each representative of f iber throughput at a
given rotor speed multiplied by the square root of "2" and
are, therefore, indicative of throughput for a 2-bar rotor.
58-
7~ `
Similarly, the triangular points 206 are each indicative of
fiber throughput for a 4-bar rotor, while the square points
208 are indicativ.e of fiber throughput for an 8-bar rotor.
Thus, the line 209 (FIG. 12) represents the Regressor,
or "line-of-best-fit", from which functional relationships
between throughput and rotor speed can be determined when
using a coarse wire screen of the type described aboveO
`Similarly, the line 210 represents the same functional
relationships when using a fine wire screen of the type
described above. The data thus corroborates experimental
findings that rotor RPM can be reduced while fiber throughput
is maintained, or even increased, by going, for example,
from a 4-bar rotor assembly 175' (FIG. 5) to an 8-bar rotor
assembly 175 (FIG. 3). However, when using an 8-bar rotor
assembly 175, the forming system seems to be less tolerant
of mismatches between forming air and rotor speed; and,
where such mismatches occur, fibers tend to accumulate on
the sidewalls 199 of the forming zone 79. This is readily
corrected by reducing rotor speed, normally by less than
10~, while maintaining forming air constant.
It has further been discovered that both nit levels in
the air-laid web 60, and fiber throughput in lbs./hr./in.2,
are a function of the percentage of fibrous materials
removed from the aerated bed 186 ~FIG. 10) through the full-
width separator slot 179 (FIG. 3). Thus, referring toFIG. 13, line 211 graphically portrays the decreasing
relationship of nit level (the ordinate~ with increasing
separation/recycle percentages (the abscissa); while, at the
same time, increasing separation/recycle percentages are
accompanied by increased fiber throughput in lbs./hr./in.2
The graph is here representative of a system in which tile
-59-
s~ '
rotor assembly 175'--a 4-bar rotor assembly--was driven at
1700 RPM and fibers were introduced into the rotor housing 172
(FI~. 3) in an air stream supplying air at approximately 106
ft.3/min./in. When the percentage of fibrous material
separated through separator slot 179 was 1~, fiber
throughput was 0.62 lbs./hr./in.2, and the air-laid web
60 exhibited a nit level of "3"--a level deemed to be
"poor", or border-line between acceptable and non-acceptable.
As described in more detail in Section J, pages 64 and 65
infra, of this specification, numerical nit levels range
from "0" ("excellentn), to "1" ("good"), to "2" ("adequate"),
to "3~ ~"poor"), to 4" through ~6" ("inadequate" to "non-
acceptable"). Such numerical ratings are subjective ratings
based upon visual inspection of the formed web 60 and subjective
comparison thereof with pre-established standards.
As the pressure of the recycle air supplied through
manifold 191 (FIG. 3) is decreased and/or as separator slot
179 is widened, thereby modulating the pressure conditions
within discharge conduits 192 (FIG. 3) and 77 (FIG. 1) which
are maintained at a pressure level below that within the
forming head 75 by means of a suction fan (not shown), the
amount of fibrous material removed from rotor housing 172
through separator slot 179 is increased. Other means such,
for example, as venturi passageways (not shown) could also
be used to insure a controlled outflow of materials through
separator slot 179.
As the percentage of fibrous materials separated and/or
recycled incxeases, nit level in the formed web 60 decreases.
At the operating conditions under which FIG. 13 was prepared,
when the separation percentage was increased to approximately
2.5~, a web having a nit level of "2" (i.e., an "adequate"
~60-
. .
72
nit level rating) was produced; at a separation percentage
of 3~, the web's nit level decreased to approximately "1.6"
(i.e., approximately midway between "adequate" and "good");
at a separation percentage of approximately 3.8%, nit level
dropped to "1" ("good"); and, at a separation percentage of
5%, nit level dropped to approximately "0" ("excellentn).
FIG. 19 also shows that the throughput of the forming
system was increased from .6~ lbs./hr./in.2 to .96 lbs./hr./in.2
while at the same time improving web quality from ~poorn to
"excellentn. The total amount of fiber delivered to the
forming system was increased by an even greater percentage
to compensate for the increased remov~l of fiber and aggregate
for recycling. Productivity of the forming system was thus
increased about 55% even though the screen was more heavily
loaded with fiber; a very significant improvement. These
CQmpariSOnS were made while running good quality pulp (Northern
Softwood Kraft) having a low content of pulp lumps and being
well fiberized in the hammermill. Poorer quality pulps or
less effective fiberization would require higher recycle
rates of up to 10~ to maintain an acceptable nit level in
the web being formed. When making less critical webs or
thick batts, nit level and recycle rate become less critical.
Those skilled in the art will, of course, appreciate
that the experimental data set forth in FIG. 13 is only
representative for one given set of operating parameters;
and, such data will vary with changes in, e.g., air-to-fiber
ratio, type of fiber used, rotor speed, rotor design, air
supply, and/or screen characteristics. However, experiments
have indicated that recycle percentage is critical and, for
cellulosic fibers, should exceed 1%, is preferably between
about 1% and 5~, and should be less than on the order of
10%.
61-
'
9S~Z
It has been found that a 2-dimensional air-laid web
forming system embodying features of the present invention
will, when operating at a proper balance of fiber supply,
forming air supply, and rotor speed, not only deliver
maximum fiber throughput with minimum recycle, but, moreover,
will exert a "healing effect" on basis weight non-uniformities
entering the forming head 75 (FIG. 3). That is, the screen
180, when properly loaded with a moving or transient aerated
bed 186 of fibers (FIG. 10), acts as a membrane which tends
to equalize or even out the passage of fibers through adjacent
incremental widths of the screen. Such "healing effect" is
only operative over distances of six inches (6!') or less.
However, the "healing effect" will tend to reduce the coefficient
of variation within a forming head 75 supplied with an air/fiber
stream delivered through a partitioned duct 170 of the type
shown in FIG. 3--viz., the effect of non-uniformities present
within each four inch wide segment of the air stream exiting
the paxtitioned duct 170 will tend to be minimized. The "healing
effectl' will not function to even out gross irregularities in
fiber basis weight over a wide expanse of former widths. Stated
differently, a forming head 75 embodying the features of the
present invention can even out either low or high non~
uniformities of up to approximately three inches in width,
but it cannot even out gross non-uniformities of eight,
twelve, or more, inches in width. It has further been found
that if insufficient fiber loading occurs--i.e., if the air-
to-fiber ratio increases to substantially above 600 ft.3/lb.
when working, for example, with cellulosic wood fibers--
then, i) the aerated bed 186 tends to be starved of fibers;
ii) the "healing effect" is reduced because of an inadequate
transient membrane over the screen 180; and ~ input non-
-62-
,
.
uniformities tend to be replicated in the finished web 60,
thus deleteriously affecting the coefficient of variation of
the finished web.
J. Forminq Capacities and Web Characteristics
With 2-Dimensional Systems Embodying
Features of the Present Invention
Referring to Table I, it will be observed that a
single forming head 75 embodying the features of the present
invention--e.g., the type shown in FIGS o 1 and 3--and having
a ~emi-cylindrical screen 18" in circumferential length, is
capable of producing webs having basis weights ranging from
14-40 lbs./2880 ft.2 at forMing wire speeds ranging from
about 911 f.p.m. to about 319 f.p.m.
2-DIMENSIONAL FORMER CAPACITIES
IN ACCORDANCE WITH THE INVENTION
- _ _
Forming Wire Speed-- t./min
Basis Weight Product No. of Formin~ Heads
lbs./2880 ft.2 Type ~ -- ~ 2 ~ 3
14 Bath Tissue 911 1821 2737
17 Facial Tissue 750 1500 2250
26 Towel 490 981 1471
34 Towel 375 750 1125
_
Towel 319 638 _956
1. The data set forth in this Table I is based up~n a
fiber throughput capacity of 1.23 lbs./hr./in. for
a single forming head of the type shown at 75 in FIG. 1
and 9, and which uses a relatively fine screen 180
18" in circumferential length and having a screen
openinq of 0.050".
TABLE I
These realistically attainable for~ing wire speeds may be
doubled, tripled, or even further multiplied by using two,
three or more forming heads 75A-75N in the manner shown in
FIG. 2. Consequently, the formation of air-laid webs of dry
fibers is no longer limited to low forming wire speeds; and,
~ 34~ 57 ~
this is believed to be a direct result of the fiber throughput
capacity of each forming head 75 which is capable of delivering
in the order of ten times the mass quantum of fibers per
square inch of former screen as can be delivered by known
prlor art forming systems.
Not only does the present invention permit significant
increased rates of productivity at considerable savings in
terms of energy consumption and space requirements, but,
moreover, the webs produced are not constrained in terms of
width limitations, can be formed in an essentially nit-free
condition, and are comprised of individualized fibers which
have not been disintegrated, shortened, curled, rolled into
pills, or otherwise seriously damaged by excessive mechanical
action within either the rotor housing 172 (FIG. 3) or in
excessive secondary hammermilling operations.
Standards have been established by the assignee of the
present invention for subjectively classifying the nit
levels in air-laid webs formed of dry fibers. Such subjective
standards are based upon visual inspection of the webs and
comparison thereof with existing webs having differing nit
levels which have been subjectively rated as ~0", "1", "2",
"3", "4", "5" and "6". Photographs representative of webs
having nit levels of "0", "1", "2", "3", "4", "5" and "6"
are here reproduced as FIGS. 14-20, respectively. FIG. 14
portrays a web having a nit level of "0" which is indicative
of a web rated "excellent" and which is essentially free of
nits and can, therefore, be used for the highest quality
tissue products. FIG. 15 portrays a web having a nit level
of "1" which is indicative of a high quality web having only
a minimal level of nits and which is classified as "good".
Again, such a web is suitable for use in premium grade,
-64-
7~
quality bath and/or facial tissues. FIG. 16 photographically
depicts a web having a nit level of "2" which is indicative
of a web having a higher percentage of nits; yet which is
"acceptable" for usage in quality bath and/or facial tissues.
FIG. 17 comprises a photograph o~ a web having a nit level
of "3" which is considered "poor", but which is suitable for
occasional usage in quality tissues or for usage in medium
grade tissue products. FIGS. 18-20 photographically portray
webs having nit levels of "4~, "5" and "6", respectively,
and are indicative of webs of inferior quality which are
generally not suitable for usage in bath and/or facial
tissues. As the ensuing descxiption proceeds, the reader
may find it convenient to refer to FIGS~ 14-20 so that the
nit levels given in connection with the descriptions of
Examples I-X will have greater meaning and significance.
K. Examples--Comparative Representative
and/or Optimum Operating Parameters
for Air-Laid Dry Fiber Web Forming
Systems
and Woven Square-Mesh Sceens
The ensuing portion of the present specification includes
a discussion of the effects of varying various system parameters
when utilizing slotted screens in accordance with tho present
invention, as well as when utilizing woven square-mesh
screens. The Examples given are of actual experimental runs
made with the equipment and have been randomly selected
solely for the purpose of illustrating the effect of varying
one or more of the operating parameters. No effort has been
made to optimize operating conditions for each different
given Example; although, certain of the Examples do reflect
sets of operating parameters which either approach optimized
conditions, are at or about optimized conditions, or somewhat
exceed optimized conditions. Data for the various parameters
-65-
for each of the Examples given are set forth in tabular form
in Tables II and III, inclusive. Examples I-III, represent
operating parameters when utilizing woven square-mesh screens;
whereas Examples IV-X represent operating parameters for a
web forming system utilizing slotted screens in accordance
with the present invention.
Referring first to Examples I, II and III (Table II,
page 67), it will be noted that in the forming systems used
to generate the wabs of such Examples, the screens employed
were woven square-mesh screens, respectively lOxlO, 12x12,
and 8x8, and respectively having 42.3~, 51.8% and 38.9~ open
screen area. In all three cases only a single-forming head
was used, here having an 8-bar rotor assembly 175 (FIG. 3).
In the case of Examples I, II and III, recycle percentages
were 10.2%, 7.5% and 7.9%, respectively. The coefficients
of variation for Examples I, II and III were 3.1%, 1.8% and
2.2~, respectively, while the nit levels were "1" ("good")
for Examples I and III, and "0" ("excellent~) for Example
II. Thus, the webs produced were suitable for use in high
quality lightweight tissue products.
It should be noted that in Table II under the category
"Product Made", Examples I, II and III have been designated
as "Exp."--i.e., "Experimental". This designation has been
used simply because the system parameters were not set with
any specific product or end use in mind; rather, the web
being formed was considered to be an "experimental" web.
However, reference to the data for web basis weight reveals
that the experimental webs of Examples I and II are suitable
for facial tissue, while the experimental web of Example III
is suitable for toweling.
Forming wire speeds and fiber throughput--the principal
-66-
-- ' ' . .
357~ '
. _ ,
Example No. I II III IV V
~ ::
Run No. 2899 2940 2942 1035 1025
_ . .
Fiber Type~ ) N~ N~ N5~ N~ NS~
_ _
Fiber Feed Rate--lbs /in /hr ~2) 9.~ 4.6 20.3 17.1 17.0
53. . . _
Top Air Supply--ft. /min./in. 112 115 115 107 107
_ . _
Air-to-Fiber Ratio -ft.3/lb. 689 1500 331 375 377
_ _ _ _ . ~
No. of Rotors 1 1 1 1
.
No. of Rotor Bars/Rotor 8 8 8 8 8
. _ . _ _
Rotor Speed--RP~I 1200 1550 1600 1400 1800
.' . _ . .
Screen Type 10x10 12x12 8x8 11x2.5 11x2.5
Screen Openin~--Inches .065 .060 .078 .050 .050
_ .
% Open Screen Area 42.3 51.8 38.9 43.6 43.6
_ _
Former Pressure--Inches H~O 1.85 1.5 3.0 1.1 1.6
%Fiber Recycled 10.2 7.5 7.9 5.8 5.3
_ . _ _ .
Amount Fiber Recycled--lbs./in./hr.1.0 0.35 1.6 1.0 0.9
_ _ _ _ ~ .
Fiber Throughput--lbs./hr./in.2 .49 .24 1.04 .89 .89
_ _ . ~.-
Classifying Air--ft.~/min./in. 1.3 1.4 2.1 2.2 2.2
Forming Wire Speed--ft./min. 300 150 500 525 500
_ ~ Facial ~acial
Product Made Exp. Exp. Exp. Tissue Tissue
. _
Basis Weight--lbs./2880 ft.c 16.9 17.6 22.7 17.7 18.6
. ~ .
Coefficient of Variation--C.D.% 3.1 1.8 2.2 2.1_ 7.1 _
_ _ _ _ _ _
Tensile--Gms./3" C.D. Width 505 357 763 335 371
_ _ . _ ~ _
Nit Level 1.0 -0- 1.0 1.1 1.6
_ _ - =
1. NSWK is Northern Softwood Xraft.
2. Fiber feed rates as stated represent maximum former
capacity for the operating parameters established.
TABLE II
S72
indicators of productivity--are of particular interest when
evaluating the forming process used to form the webs of
Examples I, II and III. In the case of Example I, for
example, fiber throughput of 0.49 lbs./hr./in. and forming
S wire speed of 300 f.p.m. were achieved utilizing a single
forming head 75. Both parameters are approximately 40% of
the anticipated average maximum production capacity set
forth in Table I, page 63, suPra. In the case of the web
formed in Example II, fiber throughput of 0.24 lbs./hr./in.2
and forming wire speed of 150 f.p.m. represent approximately
~0% of the anticipated average maximum production capacity
set forth in Table I. In the case of Example III, the web
produced was substantially heavier than the webs of Examples
I and II discussed above, having a basis weight of 22.7
lS lbs./2880 ft.2. Forming wire speed of 500 and thxoughput of
1.04 lbs./hr./in.2 are significantly improved over the
comparable parameters for Examples I and II. While the
throughput and forming wire speed data set forth in Example
III is for a web having a basis weight of 22.7 lbs.~2880 ft.2,
such data is equivalent to forming a web of 17 lbs./2880 ft.2
at approximately 668 ft./min.
Thus, it is apparent that the operating parameters used
in formation of the web of Example III are such that the
system is approaching the anticipated average maximum
production capacity set forth in Table I, page 63, ~
That is, according to Table I it is anticipated that a web
having a basis weight of 26 lbs./2880 ft. can be formed by
a single head 75 at an average maximum forming wire speed of
490 f.p.m.; while a 17 lb./ 2880 ft.2 basis weight web can
be formed at an average maximum speed of 750 f.p.m. Consequently,
the average maximum forming capacity for forming a web
_, 6 ~
having a basis weight of 22.7 lbs./2880 ft.2--i.e., a web
identical to that of Example III--would be on the order of
562 f p.m. Therefore, since the web of Example III was
formed at 500 f.p.m., it is evident that the actual rate of
productivity was approximately 89% of the anticipated
average maximum production capacity. In short, the operating
parameters used in forming the web of Example III approach
optimum settings for forming an air-laid web of dry Northern
Softwood Kraft (NSWK) fibers when utilizing an 8x8 woven
square-mesh screen and a single forming head 75 having an 8-
bar rotor assembly such as that shown in FIG. 3. Production
rate may, of course, be further increased by the simple
expedient of utilizing two, three or more tandem forming
heads 75A-75N in the manner suggested in FIG. 2; an arrangement
which would, under the operating parameters set forth for
Example III, permit the formation of a web having a basis
weight of 22.7 lbs./2880 ft.2 suitable for toweling at
forming wire speeds of 1124 f.p.m. (two heads), 1686 f.p.m.
(three heads), et cetera. Alternatively, and assuming all
other operating parameters remain unchanged, a web having a
basis weight of 17 lbs./2880 ft.2 suitable for use as a
facial tissue could be formed at 668 f.p.m. (one head),
1336 f.p.m. (two heads), 2004 f.p.m. (three heads), et
cetera.
25 ` When employing a slotted screen in accordance with the
present invention such, for example! as that shown in FIG.
7, the results in terms of increased productivity are
dramatic. This may be readily demonstrated by reference to
Examples IV a~d V (Table II, page 67), and Examples VI
through X (Table III, page 70), and comparing the data there
given with that set forth in connection with Examples I-III
-69-
'. .
3~
Example No. VII ~II IX X
.___ " . . _
Run No 2717 2861 2908 2909 2946
. __ _
Fiber Type( ) ~WK _ NSWK N~ N5WX
=.=~ , _ ~
Fiber Feed Rate--lbs./in./hr.(2) 26.3 28.9 18.4 18.3 26.0
~
Top Air Supply--ft.~/min./in. 133 131 129 12~ 119
. ,~ ........ . .
Air-to-Fiber Ratio--~t.J/lb. 312 271 420 423 275
- _ ,
No. of Rotors 1 1 1 1
No. of Rotor Bars/Rotor _ 4 8 8 8 8
Rotor Speed--RPM 1700 1600 1000 1000 1550
_, .
Screen Type 1~2.75 9x2.5 1~2.5 11x2.5 11x2.5
10 Screen Opening--Inches 059 .063 050 050 050
% Open Screen Area 46.4 45.5 43.6 43.5 43 6
_ . _
Former Pressure--Inches ~ O 1.6 2.0 0.95 0.95 1.7
_ ~ . ___
~Fiber Recycled 2.7 3.1 5.4 4.9 4.6
Amount Fiber Recycled--lbs./in /hr. 0.7 _ 0.9 1.0 0.9 1.2
Fiber Throughput--lbs./hr.!in. __ 1.a2 _1.55 97 97 1 37
Classifying Air--ft.3/min./in. 2.6 1.6 1.6 1.4 1.8
15 Forming Wire Speed--ft./min. 800 590 375 225 640
- _ _ H.D.
Product Made _ ~ _ Exp. Tbwel Tbwel Ex~.
Basis ~eight--lbs./2880 ft. 17.0 27.3 26.7 44.5 22.3
Coefficient of Variation--C.D.% 4.8 3.5 3.9 4.4 1.1
Tensile--Gms./3" C.D. Width 521 1045 265 559 705
Nit Level 2.0 0.3 1.0 -0- 2~.0
. . - . ~ ., _ _ _
1. NSWK is Northern Softwood Kraft.
2. Fiber feed rates as stated represent maximum former
capacity for the operating parameters established.
TABLE III
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i~ 43~2
(Table II). Thus, in Examples IV-X the recycle percentages
range from a high of 5.8% (Example IV) to a low of 2.7%
(Example VI). In Examples IV through VI, facial tissue
grade webs were produced in accordance with the invention
having basis weights ranging from 17.0 lbs.l2880 ft.2
(Example VI) to 18.6 lbs./2880 ft 2 (Example V); while in
Examples VII through X, toweling grade webs were produced
having basis weights ranging from 22.3 lbs./2880 ft.2
(Example X) to 44.5 lbs./2880 ft.2 (Example IX~. Fiber
throughput for the webs of Examples IV through X ranged from
.89 lbs./hr./in.2 (Examples IV and V) to 1.55 lbs./hr./in.2
(Example VII).
In terms of formed web characteristics~ the nit levels
of "0" ("excellent") "0.3" ("excellent"), "1.0" and "1.1"
("good") for Examples IX, VII, VIII and IV, respectively,
compare favorably to the nit levels for Examples I-III. Nit
levels for Exa~ples V, VI and X were "1.6", "2.0" and "2.0n,
respectively; and, as such, those webs were rated "adequate",
although nit level was not qùite as good as in the case of
Examples I-III. Coefficients of variation for Examples IV
through X were 2.1~, 7.1~, 4.8%~ 3.5~, 3.9~, 4.4%, and 1.1%,
respectively, as compared with Examples I-III where the
coefficients of variation were 3.1~, 1.8% and 2.2%. The
coefficient of variation for Example V of 7.1~ is relatively
~5 poor and would not generally be acceptable for premium grade
facial tissues.
Comparisons of the results attained at the parameter
settings for Examples VI and VII (Table III, page 70) with
the anticipated average maximum forming capacities set forth
in Table I, page 63, supra, reveals that in both cases the
rate of productivity attained substantially exceeded the
anticipated average maximum capacity for the forming system
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of the present invention. Thus, while it would normally be
anticipated that a single forming head 75 could produce a
web having a basis weight of 17 lbs./2880 ft.2 at a forming
wire speed of 750 f.p.m. (See, Table I, page 63) in the case
of Example VI a 17 lb./2880 ft.2 basis weight web was
produced at a forming wire speed of 800 f.p.m.--l.e,
approximately 6.6% faster than the average maximum productivity
rate anticipated. Nevertheless, the resulting air-laid web
was entirely satisfactory for use as a premium grade quality
facial tissue. Similarly, the web of Example VII, which has
a basis weight of 27.3 lbs./2880 ft.2 suitable for toweling,
was actually produced at 590 f.p.m. on a single forming head
75, whereas the anticipated average maximum forming speed
for such a web would normally be on the order of 467 f.p.m.
(Cf., Table I, page 63)--i.e., the actual rate of productivity
acheived exceeded the anticipated average maximum capacity
by approximately 26.3~6. In the case of Examples VI and VII,
the fact that productivity rates actually achieved somewhat
exceed the average anticipated maximum ratès set forth in
Table I is believed to be attributable in large part to the
fact that relatively coarse screens were used in making the
webs of such Examples--viz., relatively coarse screens
having .059" (Example VI) and .063" (Example VII) openings,
rather than fine screens having .050" openings and which
formed the basis for the data set forth in Table I. Experimental
data such as that set forth in Table III suggests that for
heavyweight towel products, relatively coarse screens will
tend to improve productivity rates without giving rise to
any serious problems in terms of operation or web characteristics.
The characteristics of the Example VII web in terms of nit
level, coefficient of variation and basis weight are again
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such that the web produced was of excellent quality suitable
for use in premium grade toweling.
It is apparent that the particular parameters used in
connection with Examples VI and VII exceed, or at the very
least, closely approximate optimum settings, although so~e
fine tuning might be required in an effort to further reduce
the coefficient of variation and nit level for Example VI.
For example, a reduction in screen opening size--e.g., from
the .059" opening used in Example VI to a screen opening on
the order of .050n--might well result in optimizing the
membrane characteristics of the transient aerated bed 186 of
fibers (FIG. lO) so as to produce an increased "healing
effect" of the type described in Section I at pages 62-63,
supra, of this specification, thereby reducing the coefficient
of variation. Similarly, rPduction of rotor speed might
produce the same result. And, an increase in the recycle
percentage of 2.7% is likely to further reduce the nit level
as heretofore described in connection with FIG. 13 (Section I,
pages 60-61, supra). For example, comparison of Examples IV
and V (Table II, page 67) reveals that the operating parameters
established for both Examples were, with the exception of
rotor speed, essentially the same. Rotor speed, however,
was only 1400 RPM in the case of Example IV, whereas in
Example V it was 1800 RPM. Thus, a decrease in rotor speed
of 400 RPM was accompanied by and, presumably, at least in
part resulted in, reduction of the coefficient of variation
in the formed web from 7.1% (Example V) to 2.i% (Example IV),
and a reduction in nit level from "l.6" (Example V) to "l.l"
(Example IV).
As in the case of the woven sguare-mesh screen comparisons
(Examples I, II and III, Table II, page 67); where the best
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7~
result in terms of productivity was achieved with the coarsest
screen--viz., an 8x8 woven square-mesh screen having screen
openings .078" in width (Example III)--in the slotted screen
comparisons the best result in terms of productivity was
also achieved when using a relatively coarse slotted screen--
viz., a 9x2.5 screen having screen openings of .063" in
width (Example V).
Examples III lTable II, page 67), and VII-X (Table III,
page 70), are of interest principally for their showing of
typical operating parameters suitable for forming relatively
heavy basis weight webs which can be used for toweling
products. Considering Example III, it will be noted that
when utilizing utilizing an 8x8 woven square-mesh screen, a
web having a basis weight of 22.7 lbs./2880 ft.2 was produced
at a for~ing wire speed of 500 f.p.m. Considering Examples VII-
X (Table III, page 70, supra), it will be noted that the
webs there formed in accordance with the invention had basis
weights ranging from 22.3 lbs./2880 ft 2 (Example X) to
44.5 lbs./2880 ft.2 (Example XI), coefficients of variation
ranging from 1.1~ (Example X) to 4.4~ (Example IX), and nit
levels of "0", "0.3n, "1.0" and "2.0" for Examples IX, VII,
VIII and X, respectively; all of such basis weights, coefficients
of variation and nit levels being entirely suitable for
commercial grade, high quality toweling products. The webs
of Examples VIII and IX were formed at productivity rates of
approximately 78.5% of the average maximum productivity
rates anticipated (Cf., Table I, page 63). The web of
Example VII (as previously described) was formed at a speed
approximately 26.3% in excess of the anticipated average
maximum capacity; and, the web of Example X was formed at a
speed approximately 12% in excess of the anticipated average
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maximum capacity.
It is believed that the numerical data set forth in
this Section K in connection with Examples I through X
clearly evidences the significant improvement obtained in
fiber throughput--i.e., productivity rate--when utilizing
slotted screens in accordance with the present invention as
contrasted with using conventional woven square-mesh screens
of the type shown in FIG. 6. However, the dramatic improvement
in throughput is made even more evident upon inspection of
that data as reproduced in graphic form in FIG. 21. Thus,
as here shown fiber throughput for each of Examples I through
X in lbs./hr.~in.2 (the ordinate in FIG. 21) has been plotted
versus the screen opening size in inches used with each
Example (the abscissa in FIG. 21). The line 216 is thus
representative of fiber throughput when using woven square-
mesh screens in a 2-dimensional web forming system and has
been generated from the throughput data given in Table II
for Examples I, II and III.
As heretofore indicated, remarkably improved throughput
rates are attained when utilizing a slotted screen with a 2-
dimensional former in accordance with the present invention.
Such results are reflected by the line 218 which has here
been generated using the throughput data recorded for
Examples IV and V (Table II) and VI-X (Table III).
It will be appreciated by those skilled in the art upon
consideration of the data in this Section K and in the
preceeding Section J of this specification, that the present
invention is uniquely suited for forming high quality webs
having virtually any desired basis weight in lbs./2880 ft.2
at relatively high forming wire speeds. Indeed, such extremely
high productivity rates may be readily set forth as follows:
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5~Z
A web having a basis weight of (x) (17 lbs./2880 ft.2)
where "x" is equal to any desired whole or fractional value,
can be produced at a forming wire 80 speed of 750 f.p.m.
divided by "x"; or,
(x) (17 lbs./2880 ft.2) = forming wire speed (750 f.p.m.) [XVI]
Similarly, where N forming heads 75A-75N are used
(See, e.g., FIG. 2), the foregoing relationship of web basis
weight to forming wire 80 speed may be expressed as follows:
(x) (17 lbs./2880 ft.2) = forming wire speed (-)~750xf P ~ XVII]
Based on the experimental data reported herein, it is
evident that the present invention provides a dramatic
improvement in fiber throughput capacity for the forming
head. Thus, the data reflects fiber throughputs ranging from
15 somewhat in excess of .5 lbs./hr.in.2 (Example IV) to in excess
of 1.50 lbs./hr./in.2 (Example VII) when working with cellulosic
wood fibers and a former 75 24" in diameter. Moreover, it
should be noted that the foregoing range of from .5 lbs.~hr./in.2
to at least 1.50 lbs./hr./in.2 reflects efforts made to form
high quality, lightweight tissue and/or towel grade products.
Where product quality in terms of, for exam~le, nit level
can be accepted at lower quality levels, it can be expected
that fiber throughput will exceed and, may substantially
exceed, the level of 1.50 lbs./hr./in.2. Similarly, when
actual production experience has been acquired, it can be
expected that fiber throughputs will be regularly achieved
which do exceed the level of 1.50 lbs./hr./in.2, and such
improved results may also be achieved when the system is
scaled up in size~ ~ to rotor assemblies on the order of
36 in diameter. Therefore, the phrase "to at least 1.50
lbs./hr./in.2~ as used herein and in the appended claims is
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not intended to place an upper limit on throughput capacity.
- Those skilled in the art will appreciate that there has
herein been described a novel web forming system characterized
by its simplicity and lack of complex, space-consuming,
fiber handling equipment; yet, which is effective in forming
air-laid webs of dry fibers at commercially acceptable
production speeds irrespective of the basis weight of the
web being formed. At the same time, the absence of cross-
flow forces insures that the finished web possesses the
desired controlled C.D. profile which may be either uniform
or non-uniform.
17-
