US 3906588 A
An apparatus is disclosed for high speed production of uniform webs by air-laydown of textile fibers. A feed batt of staple fibers is fed to a toothed disperser roll that projects the fibers at high velocity and low angle into an airstream of high uniform velocity and low turbulence to form a thin fiber stream from which the fibers are subsequently separated in the form of a web. The webs are suitable for producing high quality nonwoven fabrics by known fiber-interlocking or bonding treatments.
Claims available in
Description (OCR text may contain errors)
United States Patent 11 1 1111 3,906,588 Zafiroglu 1 Sept. 23, 1975 [5 AIR-LAYDOWN APPARATUS FOR 2,676,364 4/1954 Plummer et a1. 19/156.4
ASSEMBLING FIBERS INTO WEBS Dimitri P. Zafiroglu, Newark, Del.
Assignee: E. I. Du Pont de Nemours and Company, Wilmington, Del.
Filed: Feb. 14, 1974 Appl. No.: 442,404
Related US. Application Data Division of Ser. No. 322,757, Jan. 11, 1973, Pat. No. 3,797,074, which is a continuation-in-part of Ser. No. 241,924, April 7, 1972, abandoned, and Ser. No. 135,734, April 20, 1971, abandoned, and Ser. No. 135,735, April 20, 1971', abandoned.
US. Cl l9/l56.3; 19/97 Int. Cl. DOIG 25/00 Field of Search 19/159, 156-1564,
References Cited UNITED STATES PATENTS 4/1954 Plummer et a1. 19/156.4
FOREIGN PATENTS OR APPLICATIONS 944,854 12/1963 United Kingdom 19/114 1,010,147 11/1965 United Kingdom.... 1,050,832 12/1966 United Kingdom 19/156.4
Primary Examiner-Dorsey Newton  ABSTRACT An apparatus is disclosed for high speed production of uniform webs by air-laydown of textile fibers. A feed batt of staple fibers is fed to a toothed disperser roll that projects the fibers at high velocity and low angle into an airstream of high uniform velocity and low turbulence to form a thin fiber stream from which the fibers are subsequently separated in the form of a web. The webs are suitable for producing high quality nonwoven fabrics by known fiber-interlocking or bonding treatments.
7 Claims, 7 Drawing Figures Sept. 23,1975 Sheet 1 of3 "3,906,588
US Patent US Patent Sept. 23,1975 Sheet 2 0f-3 3,906,588
US Patent Sept. 23,1975 sheet3of 3 3,906,588
FIG-7 AIR-LAYDOWN APPARATUS FOR ASSEMBLING FIBERS INTO WEBS BACKGROUND OF THE INVENTION A This invention relates to an air-laydown apparatus for assembling textile fibers into webs, and is more particularly concerned with improvements in dispersing and transporting textile fibers in an air stream for collection on a moving screen to form webs which are suitable for use in producing high quality nonwoven fabric.
Nonwoven fabrics are produced from fibrous webs by bonding or interlocking the fibers to provide durability and strength. The fibers of the'web may be bydraulically entangled by treatment with high energy liquid streams as disclosed in Evans U.S. Pat.- No. 3,485,706, issued Dec. 23, 1969. When producing relatively heavy weight textile fabric Lauterbach US. Pat. No. 2,910,763, issued Nov. 3, 1959, disclosesthat fiber interlocking may be initiated by treatment with a needle loom and completed. by crimping or shrinking the fibers. Production of bonded nonwoven fabrics may be accomplished as disclosed in Graham US. Pat. No. 2,765,247, issued? Oct. 2, 1956. The quality of fabric produced by these methods depends upon the quality and uniformity of the web which is treated.
Webs suitable for producing high .quality nonwoven fabrics, by treatments of the above type, can'be prepared by air laydown of textile fibers. Prior art airlaydown processes and apparatus are illustrated by Buresh U.S. Pat. No. 2,451,915 issued Oct. :19,- 1948, Plummer et al. U.S. Pat. No. 2,676,363 issued Apr. 27, 1954, and Simison US. Pat. No. 3,381,069 issued Apr.
30, 1968. Staple fibers are shipped as a compacted mass. Conventional picking and carding operations are used to separate the fibers. Theresulting loosely opened fiber lap is fed to a toothed disperser rolland a stream of air is sucked or blown over the roll. The roll is rotated at highispeed to feed the fibers into the air stream, the objective being to feed individual fibers rather than clumps or groups of fibers. The-fibers are carried by the air stream as a spreading cloud through a conduit to the screen surface of a condenser roll or conveyor, where the fibers are deposited over a relatively large surface area to form a layer on the moving screen. The Plummer et al. patent discusses the importance of air turbulence for providing a generally uniform distribution of fibers over relatively large areas throughout the conduit. Turbulence is introduced in the air supply by changing the direction of air flow ad- 1 jacent to the disperser roll, byproviding a restricted air passage past the roll into an expanded duct, and by irregularities in the duct leading to the condenser roll.
A commercial embodiment of such prior art layer forming apparatus is shown in FlGS.- 38 and 40 of Evans US. Pat. No. 3,485,706. As stated at column 20,
line 65, The layer-forming apparatus processesa whereas'the jet-treating apparatus is capable of high speed opc rationl Non-blotchy, uniform webs weighing about 1 ounce per square yard (oz/yd?) have been produced at a maximum rate of about 18 feet per minute'(ft./min. or about-0.63 pounds per inch of disperser-roll width per hour (lb/in. h.r.), when using such layer-forr'ning apparatus. Production of high quality webs at significantly higher speed does not appear to be possible unless basic modifications are made in this layerforming apparatus.
SUMMARY OF THE INVENTION The present invention provides an air-laydown apparatus suitable for high speed production of uniform webs of excellent quality from feed'batts of staple fibers; The new air-'laydown apparatus makes possible the production of such webs at rates in excess of 3 pounds per inch of web width during each hour of operation, even when lightweight webs of 0.5 to 2 oz./yd. are produced from fine, long staple fibers (e.g., 1.25 denier, l /z-inch long fibers). Highly uniform webs weighing 4 to 10 oz.yd. or more can be produced at rates of over 10 lbs/in. hr. and, in some cases, at rates above 15 to 20 lbs/in. hr. The invention provides webs of high' quality when evaluated for blotchiness (i.c., small nonuniformities which may be formed, for example, by depositing clumps of fibers), streakiness (i.e.. lines of different fiber density which may be caused, for example, by local variations in air stream velocity), and other visible defects. A g
The present invention is an apparatus for assembling textile fibers into a web which comprises duct means for conveying fibers in a controlled flow of air, fiber disperser means for projecting fibers into the duct meansto form a thin stream of fibers in air, air supply means for directing a low turbulence flow of air through the duct means, and condcnser means for separating the fibers from the air to form a web. The duct means includes sidewalls and endwalls forming a rectangular cross-section of at least the width of the web. One of the sidewalls has an opening through which the fibers are projected. The walls are substantially straight and parallel. up to this opening to maintain the air in stable flow over the opening. The fiber disperser means comprises a toothed disperser roll, suitable for rotation at a surface speed of at least 3,000 feet per minute, and a stationary disperser plate having a curved surface spaced from the roll teeth. This spacing is less than 0.125-inch from a point where the fibers are picked up by the roll teeth to a point where they are projected into the air stream, to form a narrow slit where the fibers are projected through the opening into the duct by inertia.
The disperser roll may be 5 to 50 inches in diameter and have between 8 and 350 teeth per square inch of surface. The teeth are usually shorter than 0.250-inch. with 0.010 to 0.030-inch clearance between the teeth and the disperser plate. Boundary layer control means in the duct means prior to the above opening may be provided for improving the boundary layer of air through which the fibers are projected. This control means may bea thin boundary obstruction extending across the duct walls to eliminate streamwise vorticity in boundary air flow. Another control means is a suction slot close to the opening in the duct wall for removing t'urbulent'boundary air flow.
The air supply means may comprise a high uniformity wind tunnel of larger cross-sectional area than the duct means; and a flow nozzle connection for providing a controlled flow'of substantially uniform velocity up to where the fiber stream is formed. The duct means may have a substantially uniform rectangular cross section along the path of the fiber stream to the condenser means. or it may converge to accelerate the air stream conveying the fibers. Preferably, the duct means is substantially straight.
GENERAL DESCRlPTlON The fibers are preferably centrifugally doffed from a toothed disperser roll rotating at a surface speed of at least 3,000 ft./min. Supply means feeds a substantially uniform layer of fibers onto the rotating disperser roll and a closely spaced. curved disperser plate holds the fibers close to the roll until a fiber-doffing position is reached at the tip of the disperser plate. At this location the stream of fibers is projected by tangential ejection from the disperser roll through an opening into duct means for supporting the fibers in air. The roll is mounted outside of and adjacent to the wall of the duct means so that a small surface are of the roll is exposed within the duct and fills the opening without appreciably reducing the cross-sectional area defined by the duct walls. a Air supply means directs a uniform velocity. stable. low turbulence, low vorticity flow of air through the duct in the direction of movement of the roll surface so that the fibers are projected into the stream at an angle of less than about 25 and preferably less than 12. This projection. angle is measured between the line tangent to the disperser roll at the point adjacent to the tip of the disperser plate and a line which coincides with the general direction of air flow through the duct. Boundary layer control means may be incorporated in the duct upstream of the exposed roll surface to provide a controlled. low level of turbulence in the air layer through which the fibers must uniformly project to reach the region of the air stream which has uniform flow of low turbulence and low vorticity. The duct configuration downstream 'of the disperser roll is contoured to prevent boundary layer separation and its attendant formation. of vortices and eddies; generally sudden changes in duct cross-section or direction are avoided. Condenser means separates the fibers from the air to form webs weighing from about 0.] to 10 oz.- /yd.". 1
A- conventional type of toothed disperser roll is suitable for projecting the fibers into the uniform flow of air. The curved surface of a disperser plate is arranged at a small distance from the revolving surface of the roll to provide a narrow passage where the fibers are carried to the point of projection into the duct. The spacing between the stationary disperser plate and the tips of the disperser roll teeth. from the point where the fibers are picked up by the teeth to the point where they are projected into the air stream. should be less than one-eighth inch. The disperser roll teeth are usually shorter than one-fourth inch. preferably about oneeighth inch. To the extent that the fibers penetrate between theteeth. the fiber stream projected into the air stream will be thicker than the clearance between the ends of the teeth and the disperser plate. Preferably, that clearance will be 0.010 to 0.030 inch. The disperser roll has a surface speed of from .3000 ft./min. to
a maximum limited-by speeds which damage the fibers or cause excessive vibration; preferably the surface speed is about 10,000 to 20.000 ft./min. The roll diameter should not be excessive. since the fibers are pro- 5 jected tangentially only a'short distance before losing momentum, but a diameter of at least 16 inches may be required to reduce vibration on a wide machine. Preferably the portion of the disperser plate facing the roll has a low-friction surface to facilitate projection of the fibers.
The duct means for conveying fibers provides a conduit through which a uniform velocity. low turbulence stream of air is passedadjaeent to the exposed surface of the disperser roll and on to the fiber condenser means. The duet configuration is such that substantially all of thefibers projected from the disperser roll are supported in the flow as a thin fiber stream (e.g.. ofless than one-fourth inch initial thickness, and preferably less than one-eighth inch initial thickness) which is essentially kept away from layers of high turbulence. nonuniform flow. or separated flow. For economical operation the cross-section of the duct should be the minimum necessary for carrying out the process.
Preferably, the duct has a substantially rectangular cross section. The cross-seetionalarea may increase or decrease as the duct approaches the condenser means. provided that the changes :in cross-section do not disturb the fiber stream. The adverse effects of boundary layer separation in a toorapidly-diverging duct (eg, a diffuser in which the walls diverge at more than a 30 angle) are avoided. Channel curvature through the duct should be kept to a minimum. but a slight curvature can help maintain the fiber stream centered in the duct. The length of the duct means should be sufficient to provide room for the various apparatus components.
The upstream side of the duct means is connected to conventional air supply means. such as is used in wind tunnels. for providing an air flow of low turbulence intensity and minimized eddy size. The required air supply can be furnished by fans blowing air into a highly uniform passage provided with flow distribution devices. such as vanes. perforated plates. honeycomb sections. and turbulence reduction screens. which also act as eddy eliminators. Typically. the air is forced through a multicell structure having uniform cells of regular crosssection. Each cell has diagonals of about one-half inch maximum length (preferably one-eighth inch). a maximum wall thickness of one-sixth inch (preferably 0.010 inch) and a cell length of at least 25 cell widths. Other equivalent'vortex elimination systems are known in the art. The air passage will normally be the same width as the duct into which the fibers are projected and several times greater in the other cross-sectional dimension. Thus. the air velocity in the larger air passage in which the screens. honeycombs and perforated plates are located is much less than the air velocity in the relatively small duct means. The transition between the large air passage and the small duct means is gradually curved in accordance with good flow nozzle design to accelerate the air flow of low turbulence and substantially uniform velocity is supplied from the nozzle to the duct means.
Of key importance to this invention is the condition of the How immediately upstream of the opening in the duct means through which the fibers'are projected. The portion of the air stream which advances into the fiber path is carefully controlled. The air velocity in this region is uniform across the thickness of the r egion as well as across increments of width of the regions Across the thickness "of the region, the velocity in cach0.']- ineh-thick layer (excluding the first 0.1-inch thickness next to the wall) is within $157!,- and preferably within :1 of the average velocity in the region, and is designated hereinafter as AV/V. Values of AV/V in excess of il57 result inexcessive' mixing among the layers which leads to eddies which disturb and expand the fiber stream.-This causes the loss of fiber stream thickness and trajectory control leading to blotchiness in the web product. The velocity variation within any one foot increment across the width of the region of the air flow that advances into the fiber stream is less than 110%,
and preferably less than $571, and is designated hereinafter as AV/W. Values of AV/W in excess of $107: result in eddies which deleteriously affect the fiber stream thickness and trajectory, thus leading to blotchiness in the web product. The average turbulence inten sity (i.e., the standard deviation of velocity variation with time) in the region is less than 157:, preferably less than 771. These numerical values refer only to the portion of the air. stream which advances into the fiber path, and exclude the boundary layer portion within 0.1 inch of the wall. Average turbulence intensities of greater than 15% in this region (as well as in the subsequent path of fiber flow) or large eddies, as indicated by large velocity variations and unstable local turbulence intensities, prevent formation of a thin fiber stream, cause the fibers to disperse as an expanding cloud, and result in excessive blotchiness in the web product. The air passing through the duct means outside the region which advances into the path of fiber flow need not be limited in turbulence intensity or velocity distribution, so long as its influence on the air stream conveying the fibers to the condenser is such that the average turbulence intensity at any crosssection of the fiber path from the disperser to the condenser is less than 15% and preferably less than 7%. Turbulence intensities in excess of l7 in this portion of the fiber path also result in excessive blotchiness in the web product. i
It is recognized that turbulence is produced by fluid friction with the walls of the duct. This can be confined by a boundary layer next to the walls by use of smooth, streamlined duct walls and by supplying the air flow to the duct in a proper condition, as described in the preceding paragraphs. The duct is also arranged to keep the fiber stream essentially centered in the low turbulence, uniform flow region, so that boundary layer turbulence will not disturb the fibers after the thin stream of fibers is formed. However, the fibers must be projected through'the boundary layer turbulence on the disperser roll side of the duct. Thus, gross nonuniformities in that boundary layer should be avoided. A serioud nonuniformity problem may be caused by a phenomenon of streamwise vorticity (i.e., small corkscrewtype vortiees with high energy). These vortiees can be detected by scanning the flow with a hot wire anemometer across a line perpendicular to the side walls of the duct in the planeof the flow cross section. A vortex shows as a localincrease in turbulence intensity and a corresponding decrease in local air velocity. :Fibers projected through such vortiees. are deflected from their intended paths. This results in nonuniformdistribution of fiber in the airstream, which in turn produces streaks in the web pI'OdLlCtw-A SUCtlOlTFSlOt can be used in the boundary layer that develops in the duct means shortly downstream of the location where the air enters the-ductfrom the'curved flow nozzle or in the newly developing boundary layer downstream of a suction slot. It has been found that web streaks due to these vortiees can be eliminated when a very thin obstruction (frequently called a boundary layer trip), such as a strip of sandpaper or a smooth round wire of about l8 gauge'(about 0.04 inch in diameter), is extended across the duct wall, in the boundary layer, just upstream of the laminar-to-turbulent transition zone. The transition to turbulence is then uniformly accelerated to form a uniform, stable, turbulent boundary layer which is penetrated uniformly by the fibers. The turbulence added by these obstructions is usually small and in general does not add significantly to stream mixing over the disperse'r roll or 'to the blotchiness of the final web.- A
combination of a suction slot to remove the boundary layer followed by a very fine wire boundary layer trip to prevent nonuniform redevelopment of the boundary layer, is preferred in cases where the slot cannot be placed sufficiently close'to the disperser roll. because of space limitations.
Large random eddies, arising from lack of proper preparation of the air flow to the duct means. can also cause the fibers to disperse uncontrollably as a cloud rather than as a thin stream, as soon as the fibers leave the disperser roll. in these cases, the air turbulence at a given point in the duct means is unstable. This instability is characterized by random, large, sudden increases in turbulence intensity to values of 1.5 to 2 times the average turbulence intensityat that point. even though the average turbulence intensity may be well below the desired 15% level. These large sudden increases in turbulence cause loss of control of fiber stream thickness and trajectory. These unstable flows are usually also accompanied by large scale, relatively low frequency air velocity nonuniformities. In the operation of this invention, a stable, substantially uniform velocity across the width of the duct means is maintained at lines parallel to the slot where fibers are projected into the air stream. The velocity variation within any one foot distance along these lines (excluding end or wall effects), referred to herein as AV/W, as indicated above, is smaller than il0'/( of the average within that same lfoot distance and preferably within $57!.
The fibers are conveyed to condenser means of a type conventionally used to separate fibers from an air stream in the formof random filoer webs. The fibers are uniformly deposited in a succession of overlapping areas, somewhat analogous to the way shingles are laid. The shingle length is directly proportional to the width of the fiber laydown area and to the apparent thickness of the fiber stream. A.low-turbulence thin stream having a fixed (non-oscillating) trajectory gives a shingle length of about one-half inch. An increased area of fiber-laydown can be provided by placing the condenser screen at a more oblique angle to the fiber path. Alternatively; a modification of the invention accomplishes an equivalent effect with means for controlling the flow of air to increase the effective fiber laydown area. Mixing stream generator means for controllably developing gradually enlarging eddies of low turbulence intensities can be located. in the duct meansbetween the duct entrance and the disperser roll position. For example, a three-fourth inch metal strip perforated with oneeighth inch holes to give 42% open area, and fastened to the end walls of the duct to extendacross the central portion of the duct, was found to increase the width of the effective fiber-laydown area from V2 inch to about 1% inches, thus giving a spread streamv effect. Alter natively, circular or square rods of various diameters or sizes (e.g., rods of one-sixteenth to one fourth inch diameter), depending upon. the flow-mixing and shingle length desired, can be used to provide thiseffeet. By
blocking the upper half of the flowat the nozzle delivery with a solid or perforated plate, the area of laydown can be increased even more. These mixing stream generators initiate stable eddies of large dimensions while leaving the region of the air stream'entering the fiber flow path essentially unaffected. The mixing stream generators of this invention produce stable average turbulence intensity values of less than 15%, preferably less than 7% in the-fiber flow path. By use of mixing stream generators and long ducts (e.g., about 6 feet long) the effective fiber laydown area can be extended to as much as about 12 inches. All these mixing stream generators. cause some lateral mixing within the fiber stream, which may be desirable for eliminating any fine streaks in the web produced by small nonuniformities in the fiber delivery point. These positive effects, however, must be balanced against the increase in blotchiness resulting from an-accompanying increase in turbulence caused by the mixing stream generator.
BRIEF DESCRITPION OF THE DRAWINGS dispersing section showing another embodiment of the invention.
FIGS. 4 and 5 are longitudinal vertical sectionsof a part of the apparatus showing other embodiments-of the invention.
FIG. 6 is a fragmentary section taken on line 66 of FIG. 4 showing another embodiment of the invention.
FIG. 7 is a longitudinal vertical section of a part of the apparatus. showing another embodiment of the invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to FIG. I, there is shown a fiber feeding means consisting, in this embodiment, of a conveyor belt 2, feed roll 3, compressing roll 4 and shoe 5 for supplying fiber l to the disperser roll 8. The fiber feeding means is designed to feed a butt of staple fibers having a weight, in ounces per square yard, which is about 3 to 150 times theweight'of the web to be produced. The disperser roll separates the fibers and carries them mixed with the air adjacent to the roll surface through the space between the roll and disperser plate 10, and discharges this mixture centrifugally into lel walls 16 of the airpassag'e are connected to the duct walls 20 by converging section 18 of the flow nozzle configuration. Screens 38 and 42, and honeycomb structure 40, provide a uniform flow substantially free of turbulence and vorticity. Air is blown into the air passage by one or more fans 36, through a duct system 33, shown diagrammatically.
The fibers'are'deposited to form a web on continuous, moving screen 26 which is driven and supported by rolls 28'and 30. The air-flows through the screen and is withdrawn through vacuum duct 34. The air may be filtered to remove any particles passing screen 26 and then be recirculated to fan 36. Several fans in series or an open air system withone or more fans supplying the air and one or more fans exhausting the air can also be used. The screen 26 is sealed against the fiber duct 20 and the vacuum duct 34'by sealing means 32 such as a plate of polyethylene. i FIG. 2 shows another embodiment of the doff-bar 12 having a rounded edge 13' and an upper surface 15 which slopes. The up of the doffing bar 13 should be less than 0.125 inch from the teeth of the disperser roll and preferably about 0.010 to 0.015 inch away from the teeth. I
FIG. 2 also shows a further embodiment of the invention wherein the disperser plate 10 is provided with a slot 44 connected by conduit 46 to vacuum manifold 48 and a vacuum pump (not shown') In the arrangement shown the walls 50 of the vacuum manifold 48 serve to support the disperser plate 10. The slot may have awidth of from 0.02 to 0.12 inch. The suction slot 44 serves to reduce or remove completely any turbulent boundary laycrin the air strcanrthat may develop on the lower side of theairduct. A vacuum of from 1 to 20 inches of water is suitable. i i I In FIG. 3, dotted line 58 is the tangent to the outer edge of the disperser ,roll teeth 7. The upper edge 54 of disperser plate 10 can be placed on the tangent line 58 or ,can be somewhat below .the tangent line. e.g. onehalf inchbelow. Preferably thedisperser plate tip 52 is rounded with a radius of at least 0.015 inch but less than about 0.06.inch. The face 56 of the disperser plate is essentially concentric with. the disperser roll. The clearance between the face .56 and the teeth, 7 should be less than 0.125 inch in order to avoid premature turbulent mixing of air and fiber under. the plate which would result in fiber clumps. Preferably a clearance of between about 0.01 and 0.03 inch is used.
An additional embodiment of the invention is shown by boundary layer tripping means 62 in FIG. 3. The tripping means consists of a very thin obstruction, such as a round wire of about 18 gauge ora 0.50-inch wide strip of 60 grit sandpaper, locatedon disperser plate top surface 54 about l'to 1 0 inches downstream from the start of the duct means, as determined for the particular --process' conditions. The resulting turbulent boundary layer (of the order of one-fourth inch thick) is relatively uniform ac'rossthe width of the duct.
The centerline of the fiber duct 22 may ex ten d a straight line parallel to the top oft he disperser plate as shown in FIG. 1 or it can be bent as shown in FIG .-4. The angle between the top of the disperser plate and the centerline of the fiber duct at the condenser section is termed the bending angle The bending angle should be less than 30 and is preferably from to 10. FIG. 4 also illustrates reduction of the-cross-sectional area of the duct over the doff-bar 12 to cause the air to ac celerate and provide a flatter velocity-distribution after the fiber stream is formed. i
FIGS. 4 and 6 show a further modification with the provision of eddy generator means 66 located in the air stream prior to the fiber doffing point. Generator 66 consists of a perforated plate or scfreen covering about the middle third of the air duct 20. The plat'e shown in FIG. 6 with a staggered array of holes (e.g., 0.l2 -inch diameter) and an open area of about 42% 'will provide a spread stream effect as fibers are deposited on' rotary condenser screen 26. A solid platelin the upper portion of the air duct, and small diameterisolid rods located in the middle of the air stream upstream of the fiber doffing point are also useful as eddy generator means FIG. 5 shows another arrangement of a suitable machine with the disperser roll named above the fiber duet. FIG. 5 also illustrates the use of a condenser screen 26 moving at a 45 angle to the fiber stream.
FIG. 7 shows another embodiment which has a pair of conveyor belts 70 and 71, a pair of compression rolls 72 and 73 about which the belts run, and a pairof feed folls 74 and 75. The batt of fibers l is carried by the belts 70 and 71 between restraining means 76 and fed through the nip between positively-driven, fluted, feed rolls 74 and 75 to the transfer zone 80 defined by the space between the feed rolls and the teeth of disperser roll 8. Feed roll 74 is mounted in fixed shoe 79 and feed roll 75 is mounted in a spring-loaded shoe 77. The initial loose staple fiber batt, havinga bulk density of no more than about 1 pound per cubic foot and preferably weighing from 40 to 150 ounces per square :yard (or more), is first partially compressed: between the belts,
is further compressed to maximum density of at least 4 pounds per cubic foot in the nip ofthe feed rolls. and then expands to a density of about 2 to 5 pounds per cubic foot in the transfer zone, where it is forwarded in a substantially radial direction to the toothed surface of the disperser roll.
The disperser roll 8 is of conventional design and is usually about 5 to 50 inches diameter. It is usually of hollow construction. The cylindrical outer surface of the roll is usually provided with low rake, fine metallic wire clothing 7 (FIG. 3 formed by spirally winding one or several saw-tooth strips about the roll and anchoring it. The sharp ends of the teeth are located so that the ends lie in a substantially true cylinder about the axis of rotation of roll 8. Typical arrangements include:
Tooth rake Face angle within about 8 from radial direction. I
Tooth length Shorter than one-fourth inch, preferably about one-eighth inch. I Tooth ends Tip width less than 0.030 inch. Tooth density Between about 8 and 350' teeth per square inch of roll surface. I
Roll Diameter Peripheral Speed Acceleration (inches) (feet/minute) (times gravity) .l6 3000 to ZO OOO l 17 to 5200 24 3600 to 24.000 1 12 to 5000 32 4200' to 30,000 1 l5 to 5700 90 or more of the arc of the disperser roll. Although unitary disperser-plates and doff bars are shown in I FIGS. 1 and 2, both parts can be fabricatedofa'number of sections with'suitable attachments.
Inallof the above-described embodiments of this in- 'vention, disperser roll 8 projects fibers into the air 'stream at an initial angle of less than 12 to thegeneral direetion of air flow. Although low angles of fiber projection are preferred, angles as high as about 25 are suitable in some cases. At angles greater than about 25 the roll intrudes excessively into the duct means. so
' thatunstable eddies formjust upstream of the disperser roll. This causes an unstable and nonuniform region of air flow to be advanced into the fiber stream being projected by the disperser roll which results in a thick cloud offibers, rather than a thin stream. being dis persedJThisin turn results in a blotchy. streakyweb product. Moreover,=with angles greater than 25. the fibers'may be impinged against the opposite duct wall leading to agglomeration of the fiberdispcrsion. and
preferred design is that of Rouse andHassan in Mechanical Engineering"; 'Volume: '7l. I No. 3. March. 1949, which is approximated in FIG. 1. This air supply assembly provides a uniform flow of air at the exit of the converging section 18. Exclusive of boundary layers (ile.. layers within about one-halfineh of the duct walls), the flow has a total velocity variation across the cross-section, both vertically and laterally of less than 1071. but-usually and preferably less than 15%. and a stable turbulence intensity of less than about l5 /t. but usually' and preferably well below 7%. The peripheral velocity (V of the disperser roll 'is at least 3.000
ft./min. A'suitable range of air velocity (V) at the edge of the disperser'plat'e in Zone: A is between about 0.5 and 3.5 times V and'preferably about 0.5 to L2 times V,,. A suitable rang'e'of maximum air velocity in Zone A (i.e., over the point ofhigh'est, intrusion of the disperser rollinto the duct)-is between about 0.5 and 3 times V0. but preferably between 0.7 andl .7.times V Suitable ratios of weight rates offlow of air to fiber are at least 25 to I.
Measurement of Air Velocity and Turbulence Intensity The velocity of the air flow at various points is determined with a conventional hot wire ancmometer. A suitable instrument for this purpose. which was used for the measurements reported herein. is a Model 1050 8-4 hot-wire ancmometer,manufactured by Thermal Systems Inc., of St. Paul. Minn. Others are well known in the art. When the output of the anemomcter is also passed to ana-c coupled, root-mean-squarc (RMS) voltmeter, such as a. Model 3400A, manufactured by Hewlett Packard. lnc.. of Loveland, Colo.. the RMS value of the velocity fluctuation in the direction of air flow with time is measured. For the values reported herein. the RMS readings were averaged for about to 10 seconds. The RMS value of the velocity fluctuation, multiplied by 100 and divided by the average velocity at that location is referred to herein as the local turbu lcnce intensity. Further details on the use of hot wire anemometers for measuring velocity and turbulence.
intensity is given in numerous places in the art, such as Bulletin 53, The Hot Wire Anemomcter, of Flow Corporation of Cambridge. Mass. Theoretical discussionsof turbulence intensity are found in H. Schlichting, .Boundary Layer Theory", 6th Ed.. McGraw Hill Book Company, New York. 1968. pages 455457,
538539. 558. etc. i
The velocities and turbulence intensities which are of key importance to this invention are those associated with the air. layers that advance along the fiber path and carry the fibers to the condenser. To determine the path .of the fibers in, the process of this invention is relatively simple. since the processv provides a distinct stream of fibers. One way is to determine the fiber path -optieally through transparent sections of the duct means. For convenience, the fiber stream can be lighted from the top and the thickness clearly seen and measured from the edge of the transparent section of the duct means. Permanent records of the fiber path can be made with regular speed.Polaroid" photos or high speed (near IO second exposure) Polaroid photos. The high speed photos show the instantaneous position of individual fibers-in the duct. Once the fiber path has been determined. the velocity and turbulence intensity measurements are made with no fiber flowing. The resultant measurements without fiber flowing are considered typical of the case with fiber flowing (all other things being equal) because of the very high minimum weight ratio of air to fiber flow (i.e., at least to l j The velocities and turbulence intensities are measured at at least three typical key locations: l) in the flow layers advancing over the disperser plate tip into the fiber path just upstream of the disperser roll; (2) in the fiber path flow just downstream of the disperser roll over the doff bar tip; and (3) in the fiber path flow at the end of the doff barjust upstream of thccondenscr. The velocity and turbulence intensities are measured at each of these locations, in a typical stream position (i.e.. removed from the side walls) at intervals of 0.1 inch, starting 0.l inch away from the wallof the duct containing the disperser-roll opening and continuing through the thickness of the fiber path. The measurements at each location are averaged for the thickness of the path to give an average velocityand an average turbulence intensity. If the fiber path is very wide,.scans of the path should be made at additional points in the width oitbc stream at the threeabove-mcntioned locations. in all embodiments of this invention, the average turbulence intensity at each of these locations along the fiber path is no more than about 15% Average turbulence intensities in excess of 15% result in blotchy webs.
Measurements of velcoity across the width of the fiber path at the same three locations mentioned above (without fiber flowing also provide a measure of large scale, low frequency, velocity nonuniformities. Immediately upstream of the disperser roll. the velocity variation in a foot width of stream advancing into the fiber path is less than il07r for embodiments of this invention, and preferably less than i596. The velocity variation for 0.1 inch thick layers of this advancing stream (excluding the first 0.1 inch from the wall) is within 15% (preferably within 10%) of the average velocity in the region.
If the fiber path from the disperser roll to the condenser is long, more than the three turbulence-intensity measurcing locations mentioned above should be used to characterize the turbulence to which the fiber stream is exposed in its advance to the condenser. It is recommended that the measuring locations be spaced essentially equidistant from each other.
i The following examples, which illustrate specific em bodiments of this invention. are not intended to limit the invention in any way.
EXAMPLE l in this example. an apparatus. similar to that illustrated in FIG. 1,. is used to form 36-inch wide, high quality webs at the following rates:
Production Rates Product Weight lbs/in. hr. yards/min oz/ytl 6.5 l.] 9.1 78 l. I I'll (v8 1 .6
In each of the above runs. the feed to the disperser roll consists of LZS-denier-pcr-fiber. three-fourths inchlong. polyethylene tcrephthalate staple fibers in the form of a loosely opened l00-oz./yd. batt. This is fedto a 24-inch diameter disperser roll having 40 teeth/ square inch. each tooth being 0.09 inch high and 0.009
inch thick. The clearance between the ends of the teeth width of the duct at this location is less than 1% per foot. The approximate height dimensions of 36-inch wide rectangular duct 20 and the average air velocities at various locations in the duct are as follows:
Thickness Velocity Location (inches) (ft/min) X. immediately downstream of nozzle (i.e.. at entry to rectangular duct) 2 9000 l Over plate 10 just upstream oldisperser roll l 3/8 H.000
-Continued Thickness Velocity Location (inches) (ft/min) 2. At point of maximum intrusion of roll into duct 7 7/8 20.600 3. Over plate l2, just downstream of disperser roll I 1/8 16.000 4. Just upstream of collecting screen 26 l 7/8 9.600
The distance between locations X and l is about 9 inches; between 1 and 3, about 8 /2 inches; and between 3 and 4, about I) inches. The fibers are projected into the duct at an initial angle to the airflow ofabout 9 and then conveyed in the air stream in a thin straight path to the collecting screen. At no location along the fiber path is the turbulence intensity greater than about 2%.
EXAMPLE II This example illustrates the effect of turbulence intensity along the fiber path on the production rate of high quality, defect-free webs.
Apparatus similar to that illustrated in FIG. lis used to form a 70-inch wide web weighing 1 oz/yd at a rate of 60 yards per minute. The feed batt consists of I.5-denier-perfiber, threc-fourth-inch rayon staple which is carded into a loosely opened fiber lap. This is fed at a rate of 6.25 lbs/in. hr. to a l6-inch diameter disperser roll rotating at a surface speed of 12,500 feet per minute. Air is supplied through the duct at 5,000 feet per minute under conditions such that the turbulence intensity in the air stream leaving the nozzle is about 0.4%. A uniform web of high quality is produced. When the turbulence intensity leaving the nozzle is increased from 0.4 to 0.7%, equivalent blotch level in the web can be obtained at production speeds of only about 40 yards per minute or less.
EXAMPLE III This example shows the beneficial effects that can be obtained through use of suction as a means of boundary layer control.
Apparatus similarto that illustrated in FIG. 1, but provided with suction means for removing boundary layer" turbulence as shown in FIG. 2, is used to form a 36-inch wide web weighing I oz/yd at a rate of 80 yards per minute. An air-formed batt of 1.5 denier per fiber, three-fourth-ineh polyethylene terephthalate staple is fed at a rate of 8.3 lb/in. hr. to a 24-inch diameter disperser rollrotating at a surface speed of 15,000 feet per minute. Air is supplied through the duct at 8,000 feet per minute under conditions such that the maximum turbulence intensity in the fiber path is about 0.5%. A uniform web is produced which is of high quality and free from streaks. When the above-described conditions are repeated without suction to remove boundary layer turbulence, some streaks are formed due to nonuniformities in the boundary layer.
EXAMPLE IV In this example a series of webs is produced under different conditions of turbulence intensity along the fiber path.
An apparatus similar to that illustrated in FIG. 1 is used to form a series of l l-inch wide webs of about I oz/yd unit weight at a rate of 5 lbs/in. hr. The batts fed to disperser roll 8 are oz/yd. batts of 1.25 denier per fiber, three-fourths inch long polyethylene terephthalate stable fibers prepared from multiple 2 oz.yd layers formed on a Rando-Webber The disperser roll is 16 inches in diameter, has teeth per square inch, each tooth being 0.090 inch high and having a 0.009 inch tip thickness. A 0.02 inch clearance is maintained between the tips of the teeth and plate 10. The disperser roll projects a thin stream of fiber into the duct means at an initial angle of 1 1 with the general direction of air flow and at an initial velocity (V,,) of I 1,600 ft/min. the dimensions of the rectangular duet means at specified locations downstream of the cubic nozzle are as follows:
screen 26 In this apparatus, the moving screen 26 interrupts the air-fiber stream at a 45 angle. The distance between the specified locations X and l, l and 3, and 3 and 4 are respectively 8, l l and I2 inches.
To control the turbulence intensity in the fiber path a series of mixing stream devices were positioned at location X. The webs produced with these devices and under the conditions describe-d above were then arranged in order of increasing blotchiness. The results are summarized in the table below. The designations V V V and V, are used for average air velocities at the above locations I. 2, 3 and 4, respectively. Turbulence intensity measurements (I) were made as described hereinbefore at locations 1, 3 and 4. The velocity variations in the region of the air stream which advances into the fiber path was also measured at location 1 (AV/V).
Without making any changes in the operation of the apparatus except to decrease the speed of the web'- collecting screen, a series of webs with about 3 oz./yd'. unit weight is also produced.
The blotchiness of the l oz./yd. and 3 oz./yd.- webs is measured by means of a Paper Formation Tester (M. N. Davis et .11., Technical Association of the Pulp and Paper Industry, Technical Papers, Series l8. 386-39] 1935 As a standard for determination of formation value (FV), a suitable number of sheets of l oz./yd. onion-skin paper are used to give a unit weight corresponding to the web samples to be examined. The data' reported in the table are averages of three determinations on single samples of each web.
TABLE 1 Incoming Air Stream Over Disperser Path to Collector Run g No. Mixing Stream Generator V, \\'/W V,/\/,, I V. V l, V, I.
a None 8.700 1 4 0.75 1.4 13.600 1.17 1 1.700 1.7 i 9.000 1.2 b l/l6-inch diameter \\'irc* 8.700 1 4 0.75 26 13.600 1.17 1 1.700 2.0 9.000 1.8 c 3/32-inch diameter wire* 8.700 3 0.75 2.9 13.600 1.17 1 1.700 2.8 9.000 2.0 d l/8-inch diameter wire* 8.600 1 3 0.74 3.3 13.500 1.16 1 1.600 3.3 8.900 2.2 e 3/l6-ineh diameter rod* 8.600 2 0.74 6.2 13.500 1.16 1 1.600 4.5 8.900 4.2 f l/4-irlch diameter rod" 8.500 3 3 0.73 8.3 13.400 1.15 11.500 5.6 8.800 4.8 g l-inch gate opening 8,200 7 0.71 I 3.3 12,800 1.10 1 1.100 13.5 8.500 7.2 h 3/4-inch gate opening" 1 8.000 -5 14 0.69 25.1 12.600 1.08 10,900 19.5 8.300 12.1
Notes The wire or rod mixing stream generator is located across the middle of the incoming air stream at location X. The gate is at location X on the sitle ol' the duct op osite the disperser roll (i.e.. the gate opening is on the disperser-roll side of the duct).
V,, lnitial projection velocity oi the lihers. l'L/niin. V Average velocity of incoming air stream. l'L/min.
AV/W Velocity variation across the \\'1L1lh ol" the region of the air stream advancing into the path of liber llo\v. 'Hlhot o1 \vidth. Velocity \ariation in the region of the air stream advancing into the path of fiber flow. 1").
l= 'l'urhulenee intensity z '1. Suhscripts refer to the specified locations 1. 2. 3 and 4.
FORMATION VALUE The webs made under the conditions described in the table above for runs a. I), c. (1 and 0 were of excellent to goodquality having very little blotchiness. The websof run f; although still satisfactory. were more blotchy. Webs of run g. although made within the scope of the invention. were still more blotchy and somewhat marginal in quality. Run I! produced the blotchiest and most streaky web in this series and is considered to be unsatisfactory for the purposes of this invention.
From this series of runs and others it is concluded that low, stable turbulence intensities of less than 15% and preferably less than 7% along the fiber stream path arerequired to produce high quality webs at high rates. Further. a velocity variation of less,than i1571 in the region of air flow that advances into the fiber path is also required and a variation of less than 12107: is preferable.
In a second series of runs without mixing stream generators the velocities ratios V,/V,, and Vg/V were varied between 0.4 and 2.0 and between 0.6 and 3.]. respectively. This series of tests and others showed that the suitable range for V /V is between about 0.5 and 3.5. with 0.5 to 1.2 being preferred. and the suitable range for V is between 0.5 and 3.0. with 0.7 to 1.7 being preferred in order to obtain the most uniform webs at the highest production rates. In addition. these tests showed that the suitable range for the average air velocity at. any cross-section downstream of the location where the thin fiber stream forms is between 0.25 and 3 times and preferably between 0.4 and 1.2 times the initial average air velocity (V EXAMPLEV Good quality dispersions (as judged by high-speed photographs of fiber flying overthe disperser roll or by inspection of thin webs laid down from the dispersion) suitable for making a uniform 1 ounce per square yard web. with no fiber clumps or neps and substantially no breakage of fibers are obtained. when using the conditions given in Table I1.
The fiber for Run No. l consistsofa mixture of 65% 1 inch long. 1.5 dpf acrylic. fibers and 35% /2 inch long. 1.5 dpf rayon fibers. The fiber for Runs 2 to 5 is 1.5 dpf rayon with a length of 1 inch for Run 2 and 0.75 inchfor Runs-3 to 5. I
Batts of well-opened fibers made by cross-lapping a card web in a direction transverse to the dispersion feeding direction arefed to an apparatus similar to vFIG. 7 using a 16 inch diameter disperser roll ,operating at a peripheral speed of 12,500 feet per minute.
The throughput rate of Runs 3 to 5 is the maximum possible under the conditions used which will produce an acceptable degree of dispersion. However. the use of a 60 oz./yd. feed batt under the conditions of Run 3 gives a better dispersion and an 8.0 o7..-/yd. feed batt gives even a larger improvement;-
When webs of from about 1 .to .4 oz./yd. weight are made from the above fiber.dispersions. using the air stream conditions of Example 1. uniform webs are made.
What ls Claimed ls:
1. In an apparatus comprising duct means for conveying fibers in a stream of air, air supply means for directing a stream of air through the duct means, a rotating toothed disperser roll and coacting stationary disperser plate for projecting fibers through an opening in the duct means to form a stream of fibers in the stream of air, and condenser means for separating the fibers from the air to form a web; the improvement for high speed production of uniform webs which comprises, in the air supply means, a highly uniform air passage with a larger cross-sectional area than said duct means connected directly to the duct means by a converging section in the form of a smooth, gradual curve, in combination with screens and a honeycomb structure located in the larger air passage to provide a uniform flow substantially free of turbulence and vorticity.
2. Apparatus as defined in claim 1 wherein said duct means has a substantially uniform rectangular crosssection along the path of the fiber stream to said condenser means.
3. Apparatus as defined in claim 1 wherein said duct means has a substantially uniform crosssection up to the opening at said disperser roll and thereafter reduces in cross-sectional area to cause the air to accelerate after the fiber stream is formed 4. Apparatus as defined in claim 1 wherein boundary layer control means is incorporated in the duct means prior to the opening at said disperser roll for improving the boundary layer of air through which the fibers are projected.
5. Apparatus as defined in claim 4 wherein said boundary layer control means is a thin boundary obstruction extending across the duct walls to eliminate streamwise vorticity in boundary air flow.
6. Apparatus as defined in claim 4 wherein said boundary layer control means is a suction slot close to the opening at the disperser roll for removing turbulent boundary air flow.
20 7. Apparatus as defined in claim 1 wherein eddy generator means extends across the duct means prior to the opening at the disperser roll.
l l =l l