Process of and apparatus for making a nonwoven web

[0002] This application claims priority from U.S. Provisional Application No. 60/257,584 filed Dec. 22, 2000.

[0003] This invention is directed to a method and apparatus for controlling fiber or filament distribution and orientation in the manufacture of nonwoven fabrics, including spunbond nonwovens, as well as to the resulting nonwovens having a desired fiber or filament distribution and orientation. More particularly, this invention is directed to a controlled application of an electrostatic field in combination with specific target electrode deflection means acting on fibers or filaments prior to deposition on a forming wire or other web forming means. The design of the deflector means located below fiber drawing means, when combined with the controlled application of electrostatics provides separation of the fibers or filaments and directional distribution on the forming surface to result in webs with desired preferential orientation and resulting web properties. The invention also includes a method of producing spunbond and other nonwoven fabrics that can be tailored to achieve a wide variety of physical and other properties for numerous applications in personal care, health care, protective apparel and industrial products.

[0004] Nonwoven fabrics or webs constitute all or part of numerous commercial products such as adult incontinence products, sanitary napkins, disposable diapers and hospital gowns. Nonwoven fabrics or webs have a physical structure of individual fibers, strands or threads which are interlaid, but not in a regular, identifiable manner as in a knitted or woven fabric. The fibers may be continuous or discontinuous, and are frequently produced from thermoplastic polymer or copolymer resins from the general classes of polyolefins, polyesters and polyamides, as well as numerous other polymers. Blends of polymers or conjugate multicomponent fibers may also be employed. Methods and apparatus for forming fibers and producing a nonwoven web from synthetic fibers are well known, common techniques and include meltblowing, spunbonding and carding. Nonwoven fabrics may be used individually or in composite materials as in a spunbond/meltblown (SM) laminate or a three-layered spunbond/meltblown/spunbond (SMS) fabric. They may also be used in conjunction with films and may be bonded, embossed, treated or colored. Colors may be achieved by the addition of an appropriate pigment to the polymeric resin. In addition to pigments, other additives may be utilized to impart specific properties to a fabric, such as in the addition of a fire retardant to impart flame resistance or the use of inorganic particulate matter to improve porosity. Because they are made from polymer resins such as polyolefins, nonwoven fabrics are usually extremely hydrophobic. In order to make these materials wettable, surfactants can be added internally or externally. Furthermore, additives such as wood pulp or fluff can be incorporated into the web to provide increased absorbency and decreased web density. Such additives are well known in the art. Bonding of nonwoven fabrics can be accomplished by a variety of methods typically based on heat and/or pressure, such as through air bonding and thermal point bonding. Ultrasonic bonding, hydroentangling and stitchbonding may also be used. There exist numerous bonding and embossing patterns that can be selected for texture, physical properties and appearance. Qualities such as strength, softness, elasticity, absorbency, flexibility and breathability are readily controlled in making nonwovens. However, certain properties must often be balanced against others. An example would be an attempt to lower costs by decreasing fabric basis weight while maintaining reasonable strength. Nonwoven fabrics can be made to feel cloth-like or plastic-like as desired. The average basis weight of nonwoven fabrics for most applications is generally between 5 grams per square meter and 300 grams per square meter, depending on the desired end use of the material. Nonwoven fabrics have been used in the manufacture of personal care products such as disposable infant diapers, children's training pants, feminine pads and incontinence garments. Nonwoven fabrics are particularly useful in the realm of such disposable absorbent products because it is possible to produce them with desirable cloth-like aesthetics at a low cost. Nonwoven personal care products have had wide consumer acceptance. The elastic properties of some nonwoven fabrics have allowed them to be used in form-fitting garments, and their flexibility enables the wearer to move in a normal, unrestricted manner. The SM and SMS laminate materials combine the qualities of strength, vapor permeability and barrier properties; such fabrics have proven ideal in the area of protective apparel. Sterilization wrap and surgical gowns made from such laminates are widely used because they are medically effective, comfortable and their cloth-like appearance familiarizes patients to a potentially alienating environment. Other industrial applications for such nonwovens include wipers, sorbents for oil and the like, filtration, and covers for automobiles and boats, just to name a few.

[0005] It is widely recognized that properties relating to strength and barrier of nonwoven fabrics are a function of the uniformity and directionality of the fibers or filaments in the web. Various attempts have been made to distribute the fibers or filaments within the web in a controlled manner. These attempts have included the use of electrostatics to impart a charge to the fibers or filaments, the use of spreader devices to direct the fibers or filaments, the use of deflector means for the same purpose, and reorienting the fiber forming means. However, it remains desired to achieve still further capability to gain this control in a way that is consistent with costs dictated by the disposable applications for many of these nonwovens.

[0006] The present invention includes the use of electrostatics in combination with a segmented target electrode deflector plate below the fiber drawing means acting on fibers or filaments prior to laydown on a forming surface to control the distribution and orientation of the fibers or filaments in the resulting web. Particularly when used in a spunbond process, the resulting web can be made to achieve widely varying degrees of physical and barrier properties, including a very high degree of uniformity if desired. The invention is applicable to spinning a wide variety of polymers in monocomponent, biconstituent or conjugate filaments and using many different bonding steps, such as patterned thermal or ultrasonic bonding as well as adhesive bonding. Also, the filaments or fibers may vary widely in denier, cross-sectional shape and the like and may be combined as mixtures of the foregoing. Single layer nonwoven webs or multilayer laminates may be formed in accordance with the invention.

[0007] The invention provides a process for forming a nonwoven web includes the steps of:

[0008] a. providing a source of fibers and/or filaments;

[0009] b. subjecting the fibers and/or filaments to an electrostatic charge;

[0010] c. directing the fibers and/or filaments to a deflector device while under the influence of the electrostatic charge; and

[0011] d. collecting the fibers and/or filaments on a forming surface to form a nonwoven web.

[0012] In one embodiment the fibers and/or filaments are provided by melt spinning. In a further aspect the meltspun filaments may be continuous and subjected to pneumatic draw forces in a fiber draw unit prior to being subjected to said electrostatic charge. In a specific embodiment the deflector device includes a series of teeth separated by a distance determined by the desired orientation of the fibers and/or filaments in the nonwoven web. Also, in one aspect the teeth are oriented at an angle with respect to the directed fibers and/or filaments, the angle determined by the desired orientation of the fibers and/or filaments in the nonwoven web. The invention also includes the apparatus and resulting nonwoven webs.

[0013] FIG. 1 is a schematic illustration of a spunbond process including the fiber or filament control of the invention.

[0014] FIG. 2 is an enlarged view of the combined electrostatics and segmented target electrode deflector device in accordance with the invention.

[0015] FIG. 3 is a detailed view of a target electrode deflector device in accordance with the invention.

[0016] As used herein and in the claims, the term “comprising” is inclusive or open-ended and does not exclude additional unrecited elements, compositional components, or method steps.

[0017] As used herein the term “nonwoven fabric or web” means a web having a structure of individual fibers or threads which are interlaid, but not in an identifiable manner as in a knitted fabric. Nonwoven fabrics or webs have been formed from many processes such as for example, meltblowing processes, spunbonding processes, and bonded carded web processes. The basis weight of nonwoven fabrics is usually expressed in ounces of material per square yard (osy) or grams per square meter (gsm) and the fiber diameters useful are usually expressed in microns. (Note that to convert from osy to gsm, multiply osy by 33.91).

[0018] As used herein the term “microfibers” means small diameter fibers having an average diameter not greater than about 75 microns, for example, having an average diameter of from about 0.5 microns to about 50 microns, or more particularly, microfibers may have an average diameter of from about 2 microns to about 25 microns. Another frequently used expression of fiber diameter is denier, which is defined as grams per 9000 meters of a fiber and may be calculated as fiber diameter in microns squared, multiplied by the density in grams/cc, multiplied by 0.00707. A lower denier indicates a finer fiber and a higher denier indicates a thicker or heavier fiber. For example, the diameter of a polypropylene fiber given as 15 microns may be converted to denier by squaring, multiplying the result by 0.89 g/cc and multiplying by 0.00707. Thus, a 15 micron polypropylene fiber has a denier of about 1.42 (15²×0.89×0.00707=1.415). Outside the United States the unit of measurement is more commonly the “tex”, which is defined as the grams per kilometer of fiber. Tex may be calculated as denier/9.

[0019] As used herein the term “spunbonded fibers” refers to small diameter fibers which are formed by extruding molten thermoplastic material as filaments from a plurality of fine, usually circular capillaries of a spinneret with the diameter of the extruded filaments then being rapidly reduced as by, for example, in U.S. Pat. No. 4,340,563 to Appel et al., and U.S. Pat. No. 3,692,618 to Dorschner et al., U.S. Pat. No. 3,802,817 to Matsuki et al., U.S. Pat. Nos. 3,338,992 and 3,341,394 to Kinney, U.S. Pat. No. 3,502,763 to Hartman, and U.S. Pat. No. 3,542,615 to Dobo et al. Spunbond fibers are generally not tacky when they are deposited onto a collecting surface. Spunbond fibers are generally continuous and have average diameters (from a sample of at least 10) larger than 7 microns, more particularly, between about 10 and 20 microns. The fibers may also have shapes such as those described in U.S. Pat. No. 5,277,976 to Hogle et al., U.S. Pat. No. 5,466,410 to Hills and 5,069,970 and 5,057,368 to Largman et al., which describe fibers with unconventional shapes.

[0020] As used herein the term “meltblown fibers” means fibers formed by extruding a molten thermoplastic material through a plurality of fine, usually circular, die capillaries as molten threads or filaments into converging high velocity, usually hot, gas (e.g. air) streams which attenuate the filaments of molten thermoplastic material to reduce their diameter, which may be to microfiber diameter. Thereafter, the meltblown fibers are carried by the high velocity gas stream and are deposited on a collecting surface to form a web of randomly dispersed meltblown fibers. Such a process is disclosed, for example, in U.S. Pat. No. 3,849,241 to Butin et al. Meltblown fibers are microfibers which may be continuous or discontinuous, are generally smaller than 10 microns in average diameter, and are generally tacky when deposited onto a collecting surface.

[0021] As used herein “multilayer laminate” means a laminate wherein some of the layers may be spunbond and some meltblown such as a spunbond/meltblown/spunbond (SMS) laminate and others as disclosed in U.S. Pat. No. 4,041,203 to Brock et al., U.S. Pat. No. 5,169,706 to Collier, et al, U.S. Pat. No. 5,145,727 to Potts et al., U.S. Pat. No. 5,178,931 to Perkins et al. and U.S. Pat. No. 5,188,885 to Timmons et al. Such a laminate may be made by sequentially depositing onto a moving forming belt first a spunbond fabric layer, then a meltblown fabric layer and last another spunbond layer and then bonding the laminate in a manner described below. Alternatively, the fabric layers may be made individually, collected in rolls, and combined in a separate bonding step. Such fabrics usually have a basis weight of from about 0.1 to 12 osy (6 to 400 gsm), or more particularly from about 0.75 to about 3 osy. Multilayer laminates may also have various numbers of meltblown layers or multiple spunbond layers in many different configurations and may include other materials like films (F) or coform materials, e.g. SMMS, SM, SFS, etc.

[0022] As used herein the term “polymer” generally includes but is not limited to, homopolymers, copolymers, such as for example, block, graft, random and alternating copolymers, terpolymers, etc. and blends and modifications thereof. Furthermore, unless otherwise specifically limited, the term “polymer” includes all possible geometrical configurations of the molecule. These configurations include, but are not limited to isotactic, syndiotactic and random symmetries.

[0023] As used herein, the term “machine direction” or MD means the length of a fabric in the direction in which it is produced. The term “cross machine direction” or CD means the width of fabric, i.e. a direction generally perpendicular to the MD.

[0024] As used herein the term “monocomponent” fiber refers to a fiber formed from one or more extruders using only one polymer. This is not meant to exclude fibers formed from one polymer to which small amounts of additives have been added for color, antistatic properties, lubrication, hydrophilicity, etc. These additives, e.g. titanium dioxide for color, are generally present in an amount less than 5 weight percent and more typically about 2 weight percent.

[0025] As used herein the term “conjugate fibers” refers to fibers which have been formed from at least two polymers extruded from separate extruders but spun together to form one fiber. Conjugate fibers are also sometimes referred to as multicomponent or bicomponent fibers. The polymers are usually different from each other though conjugate fibers may be monocomponent fibers. The polymers are arranged in substantially constantly positioned distinct zones across the cross-section of the conjugate fibers and extend continuously along the length of the conjugate fibers. The configuration of such a conjugate fiber may be, for example, a sheath/core arrangement wherein one polymer is surrounded by another or may be a side by side arrangement, a pie arrangement or an “islands-in-the-sea” arrangement. Conjugate fibers are taught in U.S. Pat. No. 5,108,820 to Kaneko et al., U.S. Pat. No. 4,795,668 to Krueger et al., U.S. Pat. No. 5,540,992 to Marcher et al. and U.S. Pat. No. 5,336,552 to Strack et al. Conjugate fibers are also taught in U.S. Pat. No. 5,382,400 to Pike et al. and may be used to produce crimp in the fibers by using the differential rates of expansion and contraction of the two (or more) polymers. Crimped fibers may also be produced by mechanical means and by the process of German Patent DT 25 13 251 A1. For two component fibers, the polymers may be present in ratios of 75/25, 50/50, 25/75 or any other desired ratios. The fibers may also have shapes such as those described in U.S. Pat. Nos. 5,277,976 to Hogle et al., U.S. Pat. No. 5,466,410 to Hills and 5,069,970 and 5,057,368 to Largman et al., which describe fibers with unconventional shapes.

[0026] As used herein the term “biconstituent fibers” refers to fibers which have been formed from at least two polymers extruded from the same extruder as a blend. The term “blend” is defined below. Biconstituent fibers do not have the various polymer components arranged in relatively constantly positioned distinct zones across the cross-sectional area of the fiber and the various polymers are usually not continuous along the entire length of the fiber, instead usually forming fibrils or protofibrils which start and end at random. Biconstituent fibers are sometimes also referred to as multiconstituent fibers. Fibers of this general type are discussed in, for example, U.S. Pat. Nos. 5,108,827 and 5,294,482 to Gessner. Bicomponent and biconstituent fibers are also discussed in the textbook Polymer Blends and Compositesby John A. Manson and Leslie H. Sperling, copyright 1976 by Plenum Press, a division of Plenum Publishing Corporation of is New York, IBSN 0-306-30831-2, at pages 273 through 277.

[0027] As used herein the term “blend” means a mixture of two or more polymers while the term “alloy” means a sub-class of blends wherein the components are immiscible but have been compatibilized. “Miscibility” and “immiscibility” are defined as blends having negative and positive values, respectively, for the free energy of mixing. Further, “compatibilization” is defined as the process of modifying the interfacial properties of an immiscible polymer blend in order to make an alloy.

[0028] “Bonded carded web” refers to webs that are made from staple fibers which are sent through a combing or carding unit, which breaks apart and aligns the staple fibers in the machine direction to form a generally machine direction-oriented fibrous nonwoven web. Such fibers are usually purchased in bales which are placed in a picker which separates the fibers prior to the carding unit. Once the web is formed, it then is bonded by one or more of several known bonding methods. One such bonding method is powder bonding, wherein a powdered adhesive is distributed throughout the web and then activated, usually by heating the web and adhesive with hot air. Another suitable bonding method is pattern bonding, wherein heated calender rolls or ultrasonic bonding equipment are used to bond the fibers together, usually in a localized bond pattern, though the web can be bonded across its entire surface if so desired. Another suitable and well-known bonding method, particularly when using bicomponent staple fibers, is through-air bonding.

[0029] As used herein, “ultrasonic bonding” means a process performed, for example, by passing the fabric between a sonic horn and anvil roll as illustrated in U.S. Pat. No. 4,374,888 to Bornslaeger.

[0030] As used herein “thermal point bonding” involves passing a fabric or web of fibers to be bonded between a heated calender roll and an anvil roll. The calender roll is usually, though not always, patterned in some way so that the entire fabric is not bonded across its entire surface, and the anvil roll is usually flat. As a result, various patterns for calender rolls have been developed for functional as well as aesthetic reasons. One example of a pattern has points and is the Hansen Pennings or “H&P” pattern with about a 30% bond area with about 200 bonds/square inch as taught in U.S. Pat. No. 3,855,046 to Hansen and Pennings. The H&P pattern has square point or pin bonding areas wherein each pin has a side dimension of 0.038 inches (0.965 mm), a spacing of 0.070 inches (1.778 mm) between pins, and a depth of bonding of 0.023 inches (0.584 mm). The resulting pattern has a bonded area of about 29.5%. Another typical point bonding pattern is the expanded Hansen Pennings or “EHP” bond pattern which produces a 15% bond area with a square pin having a side dimension of 0.037 inches (0.94 mm), a pin spacing of 0.097 inches (2.464 mm) and a depth of 0.039 inches (0.991 mm). Another typical point bonding pattern designated “714” has square pin bonding areas wherein each pin has a side dimension of 0.023 inches, a spacing of 0.062 inches (1.575 mm) between pins, and a depth of bonding of 0.033 inches (0.838 mm). The resulting pattern has a bonded area of about 15%. Yet another common pattern is the C-Star pattern which has a bond area of about 16.9%. The C-Star pattern has a cross-directional bar or “corduroy” design interrupted by shooting stars. Other common patterns include a diamond pattern with repeating and slightly offset diamonds with about a 16% bond area and a wire weave pattern looking as the name suggests, e.g. like a window screen, with about a 19% bond area. Typically, the percent bonding area varies from around 10% to around 30% of the area of the fabric laminate web. As in well known in the art, the spot bonding holds the laminate layers together as well as imparts integrity to each individual layer by bonding filaments and/or fibers within each layer.

[0031] As used herein, the term “personal care product” means diapers, training pants, swimwear, absorbent underpants, adult incontinence products, and feminine hygiene products. It also includes absorbent products for veterinary and mortuary applications.

[0032] As used herein, the term “protective cover” means a cover for vehicles such as cars, trucks, boats, airplanes, motorcycles, bicycles, golf carts, etc., covers for equipment often left outdoors like grills, yard and garden equipment (mowers, rototillers, etc.) and lawn furniture, as well as floor coverings, table cloths and picnic area covers.

[0033] As used herein, the term “outdoor fabric” means a fabric which is primarily, though not exclusively, used outdoors. Outdoor fabric includes fabric used in protective covers, camper/trailer fabric, tarpaulins, awnings, canopies, tents, agricultural fabrics and outdoor apparel such as head coverings, industrial work wear and coveralls, pants, shirts, jackets, gloves, socks, shoe coverings, and the like.

[0034] Turning to FIG. 1, there is shown an example of a spunbond nonwoven forming process in accordance with the invention. As illustrated, spinplate 10 receives polymer from a conventional melt extrusion system (not shown) and forms filaments 12 which may be monocomponent, conjugate or biconstituent as described above. Fiber draw unit 14 includes a source of drawing air from chambers 16 directed at high velocity pulling filaments 12 causing orientation of the filaments, increasing their strength properties. Below the fiber draw unit 14 there is shown electrostatics unit 18 including rows 20 of pins producing a corona discharge against target electrodes 22 and deflector 24. The charged filaments 12 then are directed to the forming wire 26 moving around rolls 28, one or both of which may be driven. A compaction device such as air knife 30 may be used to consolidate web 32 prior to bonding nip 34 between calender rolls 36, 38 (one or both of which may be patterned as described above) which form bonded web 40. If desired, conventional means 15 for removing or reducing the charge on the web may be employed such as applying an oppositely charged field or ion cloud. Such devices are known and described, for example, in U.S. Pat. No. 3,624,736 to Jay, incorporated herein in its entirety by reference.

[0035] It will be recognized by those skilled in the art that various combinations of charge polarity may be used in carrying out the invention. For example, with reference to FIG. 1, the following chart illustrates exemplary alternatives. A charge of zero indicates the device is connected to ground.

[0036] Turning to FIG. 2, there is shown a view of one corona discharge arrangement 201 useful in accordance with the invention. The exit from fiber draw unit 14 is indicated at 203 and is separated by insulation 205, 225 from ammeter 207 connected to power supply 209 forming target 235 including plate 211. The electrode array 229 is comprised of multiple bars, for example four bars 213, 215, 217, 219, each of which contains a plurality of recessed emitter pins 221 connected through ammeter 227 to power supply 223. Also forming part of the target 235 is deflector 231 attached by conductive means such as bolt 233 to plate 211. The deflector target can be isolated from or connected to the target plate by a conductive means.

[0037] Turning to FIG. 3, there is shown a perspective view of one target electrode deflector 231 in accordance with the invention. The deflector is segmented by grooves 301 formed by teeth 303 is mounted by bolts 305 to support 307. Although not apparent from the drawing, teeth 303 may be separated by a spacing of, for example, about one eighth inch to provide for additional control of fiber distribution. The shape and spacing of the teeth 303 may be varied to produce intended degrees of fiber separation and orientation on laydown.

[0038] While the invention will be illustrated by means of examples, the examples are only representative and not limiting on the scope of the invention which is determined in reference to the appended claims.

[0039] Electrode

[0040] Emitter pins are spaced apart at ¼ inch, and recessed at ⅛ inch in a cavity of 0.5 inch high×0.25 inch deep. These 26 inch wide rows (24 effective inch) of pins are stacked up in four, and the distance between pins is ¾ inch (See FIG. 2). The row of pins was manufactured by The Simco Company, Inc., 2257 North Penn Road, Hartfield, Pa. 19440. These electrodes were connected to a high voltage DC source through a single 100 mega ohm resistor to measure the discharge current via the corresponding voltage. The power supply was Model EH3OR3, 0-30 KV, 0-3 MA, 100 watt regulated, reversible with respect to chassis ground, but the negative voltage was applied here although opposite charge may also be used. It was manufactured by Glassman High Voltage, Inc., PO Box 551, Route 22 East, Salem Park, Whitehouse Station, N.J. 08889.

[0041] Target

[0042] Two target objects were used: a target plate and target deflector. The plate was 3 inches high×26 inches wide conducting steel plate. The deflector was comprised of a multitude of 60 degree angle×⅜ inch wide×1.88 inches long, conducting steel teeth. They were stacked at an angle 32 degrees with respect of the center line of the fiber draw unit with a spacing of ⅛ inch (see FIG. 3). Their steel surfaces were coated with ceramic PRAXAIR LA-7 coating 0.002-0.005 inch thick. This abrasion resistant coating had very little surface resistance of 7 ohms over approximately ¾ inch distance, while the corresponding value of the uncoated steel resistance was close to 0.0002 ohms. These two targets were joined with conducting steel bolts to each other, and connected to another power supply through another 100 mega-ohm resistor. The power source was the same Glassman power supply, but with different, positive sign, polarity. Thus, the net current between the value at the electrode and that at the target indicates the amount of discharge in the air borne fiber stream, and estimated the amount of charge in the fibers.

[0043] Spinning Condition

[0044] A 17 inches effective wide spin plate of 130 holes/inch was used at 0.65 grams/hole to obtain 0.5 ounce/yd²web of approximately 2 denier/filament spunbond polypropylene fibers. The equipment used was generally in accordance with above-described Matsuki U.S. Pat. No. 3,802,817, incorporated herein in its entirety by reference, except as specifically described herein.

[0045] As shown in Table 1, the electrostatic charging in this bias circuitry at −20 to −23 improved formation, but much greater improvements were made with target deflector plate with a high voltage bias circuitry.

[0046] While this invention is not limited to any theory of operation, it is believed that such dramatic improvement has been made as follows. Typically the fibers are easily moved around in the flowfield due to local fluctuations in velocity which is a characteristic of turbulent flow. As fibers are charged, the resulting electrostatic repulsion force prevents the fibers from roping or clumping together. A typical velocity at the exit of the fiber draw unit is of the order of 6000 m/min. Assume the turbulent fluctuation in velocity is of the order of 10% of the mean velocity, i.e., 6000×10/100=600 m/min. Further assume this fluctuating velocity component is directed perpendicular to the fiber axis. The drag force acting on the fiber due to this fluctuation in velocity would be of the order of 1 dyne. This force would correspond to a filament spacing of 0.02 cm for two 2 dpf and 1 cm long fibers with 3.3 microcoulomb/gram charge according to the Coulombic Law. Essentially there is a balance between the electrostatic force and turbulence induced forces at a length scale of 0.02 cm. Strictly speaking the electrostatic forces insure filament separation on a small length scale.

[0047] On the other hand the mechanical deflector provides mixing that helps improve formation defects that are of the order of 1.2 to 2.5 cm in scale. Coupling the electrostatics with the mechanical deflector insures fiber uniformity over a length scale of 0.02 to 2.5 cm. Consider the following analogy. A sand box contains sand of varying depth resulting in a bumpy surface. Dragging a rake across the sand would help reduce surface texture on a length scale equal to the spacing of the tines. Dragging a screen across the sand would help smooth the surface on a length scale of the mesh in the screen. For this analogy the mechanical deflector acts as the rake and electrostatics acts like the screen.

[0048] While the invention has been described in terms of its best mode and other embodiments, variations and modifications will be apparent to those of skill in the art. It is intended that the attached claims include and cover all such variations and modifications as do not materially depart from the broad scope of the invention as described therein.