Abstract:
The invention provides a spinnerette assembly for forming multicomponenthollow fibers of the sheath/core type. The spinnerette assembly contains an extrusion orifice and a core forming material passage in communication with the extrusion orifice. A hollow needle extends through the extrusion orifice in a concentric manner to define an annular passage surrounding the needle. The core forming material passage includes a core forming material inlet port extending from a surface of the spinnerette assembly to an interior of the assembly and a transverse passage extending from the core forming material port to the annular passage surrounding the needle. A bore forming fluid passage communicates with the interior of the needle. The spinnerette assembly contains at least one sheath forming material passage also in communication with the extrusion orifice.

Description:
FIELD OFTHE INVENTION  
         [0001]    This invention relates to spinnerette assemblies for forming hollow fibers. It particularly relates to an improved spinnerette for more efficient and precise production of multi-component hollow fibers.  
         BACKGROUND OFTHE INVENTION  
         [0002]    It is well known to use various hollow fibers, generally made of one or more polymer materials, for various applications. For example, hollow fibers are used in carpets, as fill materials for pillows, as insulation materials for blankets and garments, and as membranes for gas separation, blood dialysis, purification of water, and other filtering applications. For membrane applications, the hollow fibers may be composed of a single component or a plurality of components, such as a hollow structured core with a sheath disposed around the core acting as a separating layer. The fibers can be bundled together and disposed in a tubular housing to provide a separation device known as a permeator. Ordinarily, the hollow fibers are relatively small, having a diameter on the order of 30 to 1000 micrometers. Accordingly, the apparatus and method for manufacturing hollow fibers must be very precise to be able to control the diameter of the fiber, and the concentricity of the core and sheath around the bore.  
           [0003]    Numerous spinning assemblies have been devised for the production of single-component hollow fibers and multi-component hollow fibers of the sheath/core type. Particularly, devices have been proposed for ensuring uniform supply of the fiber-forming fluid or fluids to the orifices of a spinnerette with the object of producing hollow fibers identical in diameter, composition, and concentricity. These spinnerettes use a means for supplying the bore fluid positioned in the spinning orifice for forming the hollow fiber. Usually, a tube or needle is used for this purpose and a gaseous or liquid fluid is injected from the tube, thus forming the bore of the fiber as it is being extruded from the spinnerette orifice. For melt spinning, the nascent fiber can be solidified by cooling in a gaseous or liquid cooling fluid. For solution spinning, the nascent fiber can be solidified by evaporation of the solvent or by contacting the fiber with a solvent-extracting liquid that results in coagulation of the polymer solution(s) to form the fiber wall.  
           [0004]    A typical spinning assembly, such as described in U.K. Patent No. 830,441 for a multi-component fiber, comprises a front and back plate spaced apart from but facing each other so as to provide a liquid channel there between. The front plate is provided with an extrusion orifice therethrough, and at least one of the plates, on the side facing the other plate, is provided with a plateau-type protrusion so as to constrict the liquid channel in a region surrounding the extrusion orifice entrance and, thus, cause the stream of the sheath-forming material to converge substantially radially towards the orifice entrance. A tube is positioned in the orifice entrance to supply the bore fluid. However, a continuing problem is the uniform supply of core-and sheath-forming material during the formation of the multi-component fiber. Most spinnerettes of this type are made largely by hand, one at a time. As a result, parts made for one spinnerette will not always fit another spinnerette. When parts are not interchangeable, any damage to one part of the spinnerette assembly may render the entire assembly useless. In assembling or cleaning conventional bicomponent or hollow-fiber spinnerettes, it is very easy to slightly bend the fluid-injection tube or needle, such that it is off center of the spinning orifice. When this happens, the spinnerette cannot be used until repaired.  
           [0005]    Another related problem of conventional spinnerettes for multi-component hollow fibers is that the sheath and core of the hollow fiber are not concentric. Concentricity of the sheath and core are important to obtain uniform fibers. Concentricity of the sheath and core was customarily obtained by adjustment of metering surfaces to regulate (meter) the flow of the polymers. The metering surfaces are produced by closely machining two surfaces so as to produce a narrow opening which will effectively meter polymer at a uniform pressure and rate as it is being extruded. U.S. Pat. No. 3,458,615 discloses a method for maintaining sheath/core concentricity by circumferential metering of the polymer sheath to be extruded about the polymeric core involving the creation of an annular wedged-shaped flow of sheath polymer around the axially-contained fluid stream.  
           [0006]    In order to maintain the concentricity of fiber diameter and the bore diameter, other spinnerettes have been provided with members for centering the tube and the bore of the spinnerette plate. For example, U.S. Pat. No. 4,493,629 describes a modular spinnerette assembly fitted with multiple screws threaded through the spinnerette plate to center the tube and orifice of the spinnerette. These adjusting screws are unreliable and are prone to error when the spinnerette is disassembled, cleaned and then reassembled. Many devices have been proposed for ensuring uniform supply of homogenous sheath-forming liquid to the orifices of a multi-orifice spinnerette with the object of producing multiple hollow fibers with concentric layers, identical in denier and other characteristics. Such devices usually involve variations in the diameter or the location of orifices and single or multiple spinnerette plates. Lack of concentricity and uniformity remain a problem in the manufacture of such single-component and multi-component fibers.  
           [0007]    Another problem with existing spinnerettes is the ability to deliver the polymer fluid or fluids uniformly around the tube or needle within the spinnerette. U.S. Pat. No. 5,320,512 discloses a spinneret that has a plurality of discrete material passages formed around the needle to deliver the polymer fluid around the needle. The polymer fluid from these individual passages must converge and meld together to form a singular annular flow around the tube or needle as the polymer fluid traverses through the main polymer fluid passage. If complete melding is not attained, seams may develop down the length of the fiber at the interfaces where the individual flows did not fully converge.  
           [0008]    Therefore, it would be desirable to have a spinnerette design which would permit the production of concentric and uniform fibers without the risk of seaming.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention provides an improved spinnerette for the production of hollow fibers.  
           [0010]    It is an object of the invention to overcome the limitations of conventional spinnerettes.  
           [0011]    It is another object of the invention to reduce imperfections in hollow fibers.  
           [0012]    It is another object of the invention to extend hollow-fiber production run times.  
           [0013]    It is another object of the invention to reduce the time for spinnerette maintenance.  
           [0014]    It is another object of the invention to simplify spinnerette fabrication.  
           [0015]    It is another object of the invention to produce high quality composite fibers having one or more sheath layers in an efficient manner.  
           [0016]    To achieve these objects, a first aspect of the invention is a spinnerette assembly for forming a composite hollow fiber comprising:  
           [0017]    at least one extrusion orifice formed in said spinnerette assembly;  
           [0018]    a hollow needle extending through each said extrusion orifice in a concentric manner to define an annular passage around said needle in said extrusion orifice;  
           [0019]    a bore forming fluid passage communicating with an interior of each said needle;  
           [0020]    at least one core forming material passage formed in said spinnerette assembly, wherein each said core forming material passage comprises a core forming material inlet port extending from a surface of said assembly to an interior of said assembly and at least one transverse passage extending from said core forming material port to each said annular passage, wherein a portion of said transverse passage entirely surrounds each said needle in a continuous manner; and at least one sheath forming material passage, wherein each said sheath forming material passage comprises a sheath forming material port extending from a surface of said assembly to each said annular passage.  
           [0021]    A second aspect of the invention is a method of forming a composite hollow fiber comprising the steps of:  
           [0022]    delivering a core forming material to each annular passage in the spinnerette assembly, said core forming material entering the spinnerette assembly through one or more core forming material inlet ports and passing through the interior of said assembly to a transverse passage, a portion of said transverse passage entirely surrounding each needle in a continuous manner, and through an annular passage in communication with an extrusion orifice, and  
           [0023]    delivering at least one sheath forming material concentrically around the core forming material as it traverses through each said annular passage,  
           [0024]    extruding the layered core forming material and at least one sheath forming material through the extrusion orifice and around each said needle,  
           [0025]    injecting a bore forming fluid into each needle to thereby provide a layered composite fiber comprising a bore forming fluid, a core forming material, and a sheath forming material as it exits the spinnerette assembly through the extrusion orifice,  
           [0026]    optionally passing the nascent extruded hollow fiber through an air gap, and  
           [0027]    solidifying the hollow fiber by cooling, solvent evaporation, or solvent extraction. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]    The invention is described through a preferred embodiment and the attached drawings in which:  
         [0029]    [0029]FIG. 1 is a top view of a spinnerette according to a first preferred embodiment of the invention;  
         [0030]    [0030]FIG. 2 is a sectional view of the first preferred embodiment taken along line A-A in FIG. 1 showing one extrusion arrangement;  
         [0031]    [0031]FIG. 3 is an alternative construction of spinnerette body of the first preferred embodiment;  
         [0032]    [0032]FIG. 4 is a top view of a second preferred embodiment of the invention for spinning multiple filaments from a single core forming material passage; and  
         [0033]    [0033]FIG. 5 is a sectional view of a third preferred embodiment of the invention for spinning multiple-sheath hollow fibers. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0034]    In the first preferred embodiment of the invention for the extrusion of multiple hollow fibers, as illustrated in FIGS. 1 and 2, a spinnerette assembly  100  comprises a spinnerette body  110 , bottom plate  120 , and needles  130 . The specific arrangement shown in FIG. 1 is for simultaneous extrusion of twelve hollow fibers, but the spinneret assembly  100  can be modified to produce a single filament or any number of multiple filaments as may be required. A proximal end of each needle  130  is secured in a respective needle mounting hole  111  formed in spinnerette body  110  by drilling or another machining process. The outer diameter of the proximal end of needle  130  and the diameter of mounting hole  111  preferably are sized such that the proximal end of needle  130  can be pressure fitted into needle mounting hole  111  to secure needle  130  to spinnerette body  110 . Needle  130  can be secured to spinnerette body  110  in any appropriate manner that permits access of the bore at the proximal end of needle  130  to the bore forming fluid passages  112  in the spinnerette body  110 . Bottom plate  120  is secured to spinnerette body  110  by fasteners  131 , such as bolts or the like threaded through holes  132 . Channels  144  and  145  formed in the bottom surface of spinnerette body  110  are in communication with gap  141  defined between spinnerette body  110  and bottom plate  120 . Shim  140  disposed between spinnerette body  110  and bottom plate  120  defines the dimension of gap  141  that provides uniform delivery of the sheath forming material around the core forming material in a concentric fashion as the core forming material flows through annular passages  153  and  154  respectively. Flared recesses  156  are formed in bottom plate  120  to permit the multiplicity of extruded fibers to exit spinnerette assembly  100  without interference. In the preferred embodiment, the multiple extrusion arrangements are situated in a linear or circular fashion.  
         [0035]    Bore forming fluid passage  112  is formed in spinnerette body  110  and extends through spinnerette body  110  to a respective needle mounting hole  111  to be in communication with the passage formed through needle  130 . Each bore forming fluid passage  112  includes a bore forming fluid inlet port  113  at the surface of spinnerette body  110 . This structure permits a bore forming fluid to be introduced into an extruded fiber to maintain the hollow structure of the extruded fiber in the manner described below.  
         [0036]    Core forming material passages  150  are formed in spinnerette body  110  through which a core forming material, such as a polymer material, is delivered to the extrusion orifices  155 . Each core forming material passage  150  includes an inlet port  151  that is a hole extending in a direction that is substantially parallel to needle  130 . Each core forming material passage  150  also includes a transverse passage  152  that extends from core forming inlet port  151  to a top portion of annular passage  153  that defines the upper portion of extrusion orifice  155 . Transverse passage  152  is defined by a backcut portion formed in spinnerette body  110  by a tool inserted through core forming material port  151 . Transverse passage  152  extends entirely around needle  130  to permit core forming material to be evenly distributed around needle  130  and evenly introduced into annular passages  153  and  154 .  
         [0037]    Sheath forming material inlet port  142  and sheath forming material passage  143  are formed in spinnerette body  110  to be in communication with channels  144  to permit sheath forming material to be delivered through sheath forming material inlet port  142  and sheath forming material passage  143 , through channels  144  and  145 , and through gap  141 . As sheath forming material exits gap  141 , it is distributed evenly around core forming material at the intersection of gap  141  and annular passage  154 .  
         [0038]    In operation, spinnerette assembly  100  is mounted to a spinning machine through mounting holes  115  using an appropriate fastening mechanism such as bolts or the like. A bore forming fluid supply, a core forming material supply, and a sheath forming material supply of the machine are coupled respectively to the bore forming fluid inlet port  113 , the core forming material inlet port  151 , and the sheath forming material inlet port  142 . Note that there is one bore forming material inlet port  113  and one core forming material inlet port  151  for each extrusion orifice  155 ; whereas, one sheath forming material inlet port  142  provides delivery of sheath forming material to each extrusion orifice through sheath forming material passage  143  and channels  144  and  145  and gap  141 . These ports can be arranged in any way and can be of any number as is appropriate to deliver the materials to the proper passages. For the spinning of hollow fibers, a core forming material, a sheath forming material, and a bore forming fluid are simultaneously delivered into spinneret  100  at known pressures and flow rates to extrude (i.e., spin) hollow fibers. Typically, the core forming material is injected at about 300-500 psig, the sheath forming material is injected at about 150-300 psig, and the bore forming fluid is injected at about 4-5 psig.  
         [0039]    Core forming material travels through core forming material inlet port  151 , through the core forming material passage  150 , into transverse passage  152 , and into upper annular passage  153 . At the same time, sheath forming material travels through sheath forming material inlet port  142 , through sheath forming material passage  143 , and into channel  144 . It should be noted that the dimensions of channel  144  are designed to provide sheath forming material at the entrance to each channel  145  at essentially the same pressure so as to provide uniform delivery of sheath forming material through gap  141 . At the exit of gap  141 , the flow of sheath forming material circumferentially intersects with the flow of core forming material at the gap between the upper and lower annular passages  153  and  154  respectively, thus forming a uniform layer or coating of sheath forming material concentrically around the outer surface of the core forming material. Further, simultaneous with the delivery of the core forming material and the sheath forming material through the spinnerette assembly  100 , a bore forming fluid is injected into the bore forming fluid inlet port  113 , through bore forming fluid passage  112 , and into needle  130 . The bore fluid emerges from the distal end of needle  130  at a position within or just downstream of extrusion orifice  155 . Since the core forming material and sheath forming material are being simultaneously extruded through the lower annular passage  154  and out of the extrusion orifice  155  concentrically around needle  130  and the emerging bore forming fluid therefrom, the resultant extrudate is a fiber comprised of a bore forming fluid at the center, concentrically surrounded by a core forming material that is concentrically coated with a sheath forming material.  
         [0040]    As best illustrated in FIGS. 1 and 2, transverse passage  152  is a backcut portion having a terminal portion that entirely surrounds needle  130  in a continuous manner and is in communication with upper annular passage  153 . This construction eliminates the problem of uniform distribution of core forming material around needle  130 . It also eliminates the problem of longitudinal seaming down the fiber wall due to incomplete melding of a plurality of core forming material streams within the annular passage as disclosed in U.S. Pat. No. 5,320,512. Also, since the core forming material passage  150  and transverse passage  152  are readily accessible when the spinnerette assembly is removed from the spinning machine, cleaning of the spinnerette is relatively easy. This facilitates cleaning and reduces turnaround time for the spinnerette. Also, core forming material passage  150  can be easily machined in spinnerette body  110  by drilling, and transverse passage  152  can be easily and precisely formed by EDM techniques using an angular electrode. Further, since needle  130  is securely fixed to the spinnerette body into mounting hole  111 , alignment of the needle concentrically within upper and lower annular passages  153  and  154  is assured and thus laborious and intricate alignment processes are obviated, thereby further reducing turnaround time.  
         [0041]    Spinnerette assembly  100  of the preferred embodiment has fewer parts and is more easily manufactured as compared to conventional spinnerettes. FIG. 3 illustrates an alternative construction of the spinnerette body of the first preferred embodiment that further simplifies the spinnerette manufacturing process. In the first preferred embodiment depicted in FIG. 2, bore forming fluid passage  112  must be machined, e.g. drilled, at an angle and with a high degree of precision to accurately meet and communicate with needle mounting hole  111  without damaging the integrity of needle mounting hole  111 , which has a relatively small diameter. The spinnerette body of FIG. 3 has an alternative design that obviates this intricate machining step and thus reduces the cost of manufacturing a spinnerette. In particular, a secondary bore forming fluid passage  114  is machined substantially parallel to core forming material passage  150  and extends from the surface of spinnerette body  110  to needle mounting hole  111 , and is concentric with needle mounting hole  111 . Since secondary bore forming fluid passage  114  is coaxial with needle mounting hole  111 , the machining process is greatly simplified. Bore forming fluid passage  112 ′ is machined in spinnerette body  110  so as to originate at bore forming fluid inlet port  113  and intersect with secondary bore forming fluid passage  114  at a point substantially removed from needle mounting hole  111 . Angled bore forming fluid passage  112 ′ is readily machined to communicate with secondary bore forming fluid passage  114  (that can be machined prior to machining angled passage  112 ′) because the diameters of angled passage  112 ′ and passage  114  are relatively large as compared to the diameter of needle mounting hole  111 . The opening of secondary bore forming fluid passage  114  at the face of spinnerette body  110  can be plugged or otherwise sealed prior to or during mounting of the spinnerette assembly  100  onto the spinning machine to avoid leakage of the bore forming fluid. Other aspects of the alternative design of FIG. 3 are similar to the design of FIG. 2 described above.  
         [0042]    A second preferred embodiment in accordance with the invention is illustrated in FIG. 4, which depicts a method for increasing the number of fibers per spinnerette by a factor of two. For illustrative purposes and clarity, the numeration used in FIG. 4 is 100 greater than for corresponding components in FIGS. 1, 2, and  3 . FIG. 4 is a top view of spinnerette body  210 . The distinguishing feature between this embodiment and the one depicted in FIGS. 1, 2, and  3  is that for each core forming material passage  250  are formed two transverse passages  252   a  and  252   b , each in the form of a backcut portion. Each traverse passage  252   a  and  252   b  are provided with an annular passage  253   a  and  253   b  respectively (corresponding to annular passage  153  in FIGS. 2 and 3), and needles  230   a  and  230   b  (corresponding to needles  130  in FIGS. 2 and 3). Similarly, other features in spinnerette body  110  and bottom plate  120  depicted in FIGS. 1, 2, and  3  are provided as corresponding features in spinnerette body  210  and bottom plate  220 . Thus, with reference to FIGS. 3 and 4 the transverse passages  252   a  and  252   b  extend from core forming material passage  250  to the edge of the annular passages  253   a  and  253   b  around each needle  230   a  and  230   b  of the corresponding extrusion orifice. Plural needles  230   a  and  230   b  are disposed in spinnerette body  210  and are in communication with bore forming fluid passage  214   a  and  214   b . Each needle extends through a corresponding extrusion orifice  255   a  and  255   b  defined by the cylindrical upper and lower annular passages  253   a  and  253   b , and  254   a  and  254   b  respectively. Channels  244  and  245  formed in the bottom surface of spinnerette body  210  are in communication with gap  241   a  and  241   b  defined between spinnerette body  210  and bottom plate  220 . Shim  240  disposed between spinnerette body  210  and bottom plate  220  defines the dimension of gap  241  that provides uniform delivery of the sheath forming material around the core forming material in a concentric fashion at the intersection of gaps  241   a  and  241   b  and annular passages  254   a  and  254   b  respectively. Sheath forming material inlet port  242  and sheath forming material passage  243  are formed in spinnerette body  210  to be in communication with channels  244  to permit sheath forming material to be delivered through sheath forming material inlet port  242  and sheath forming material passage  243 , through channels  244  and  245 , and through gaps  241   a  and  241   b . The bore forming fluid is supplied to spinnerette body  210  through bore forming fluid inlet port  213 . The bore forming fluid is distributed from each bore forming fluid inlet port  213  through channels  212 ′ a  and  212 ′ b  to each bore forming fluid passages  214   a  and  214   b . Accordingly, the spinnerette assembly  200  can be attached to the same spinning machine to produce twice as many fibers. One skilled in the art can envision additional embodiments to spin fibers in multiples greater than two (e.g., 3, 4, 5) based on the above embodiments.  
         [0043]    [0043]FIG. 5 illustrates a third preferred embodiment of the invention adapted to make hollow fibers of three components. For illustrative purposes and clarity, the numeration used in FIG. 5 for corresponding components in FIGS. 1, 2, and  3  is 200 greater than that in FIGS. 1, 2, and  3 ). Spinnerette assembly  300  comprises spinnerette body  310 , bottom plate  320 , plural needles  330  (one of which is visible in FIG. 5), and middle plate  360  disposed between spinnerette body  310  and bottom plate  320 . Shim  340   a  is disposed between spinnerette body  310  and middle plate  360 , and shim  340   b  is disposed between middle plate  360  and bottom plate  320 . Spinnerette body  310 , middle plate  360 , bottom plate  320 , and shims  340   a  and  340   b  are assembled into a unitary body during use as described in detail below. A proximal end of each needle  330  is secured, e.g., by a pressure fit, in a respective needle mounting hole  311  formed in spinnerette body  310  by drilling or another machining process. Bottom plate  320  and middle plate  360  are secured to spinnerette body  310  by fasteners  331 , such as bolts or the like threaded through holes  332 , with shim  340   a  defining a gap  341   a . Additionally, shim  340   b  defines a gap  341   b  between middle plate  360  and bottom plate  320 .  
         [0044]    Apertures corresponding to annular passages  353 ,  363 , and  354  are formed in spinnerette body, middle plate, and bottom plate respectively through which a core forming material, a core material plus a first sheath forming material, and a core forming material plus a first and second sheath forming material are extruded respectively. This nascent three-component hollow-fiber ultimately exits the spinnerette assembly through extrusion orifice  355 . Flared recesses  356  are formed in a bottom surface of the bottom plate  320  to permit each extruded fiber to exit spinnerette assembly  300  without interference for further processing by a spinning machine on which spinnerette assembly  300  is mounted. Channels  344   a  and  345   a  are formed in a bottom surface of spinnerette body  310 , and are in communication with gap  341   a . Channels  344   b  and  345   b  are formed in a top surface of bottom plate  320 , and are in communication with gap  341   b.    
         [0045]    Core forming material passages  350  are formed in spinnerette body  310  through which a core forming material, such as a polymer material, is introduced into each extrusion orifice. Each core forming material passage  350  includes core forming material inlet port  351  which is a hole extending in a direction that is substantially parallel to the extrusion orifice. Each core forming material passage  350  also includes transverse passage  352  which extends from core forming material inlet port  351  to a top portion of annular passage  353  which defines an upper portion of the extrusion orifice. Transverse passage  352  is defined by a backcut portion formed in spinnerette body  310  by a tool inserted through core forming material inlet port  350  and extends entirely around needle  330  to permit the core forming material to be evenly introduced into annular passage  353 .  
         [0046]    Bore forming fluid inlet port  313  and bore forming fluid passages  312  and  314  are formed in spinnerette body  310  to be in open communication with needle mounting hole  311 . This arrangement permits a bore forming fluid to be introduced into an extruded fiber to maintain the hollow structure of the extruded fiber in the manner described below.  
         [0047]    A first sheath forming material inlet port  342   a  is formed in spinnerette body  310  to be in communication with channels  344   a  to permit a first sheath forming material, such as a polymer material, to be introduced into the first sheath forming material inlet port  342   a , through first sheath forming material passage  343   a , into channels  344   a  and  345   a , through gap  341   a , and into annular passage  363 .  
         [0048]    Similarly, a second sheath forming material inlet port  342   b  is formed in bottom plate  320  to be in communication with channel  344   b  to permit a second sheath forming material, such as a polymer material, to be introduced into the second sheath forming material inlet port  342   b , through second sheath forming material passage  343   b , into channels  344   b  and  345   b , through gap  341   b , and into annular passage  354 . Other aspects of the third preferred embodiment are similar to the first preferred embodiment and like elements are labeled with similar numerals having a prefix of  
         [0049]    Operation of the third preferred embodiment is similar to the first preferred embodiment. However, the second sheath forming material travels through second sheath forming material inlet port  342   b , through second sheath forming material passage  343   b , into channels  344   b  and  345   b , through gap  341   b , and into annular passage  354 . As second sheath forming material enters annular passage  354 , it is deposited uniformly around the material flowing from annular passage  363  (e.g., core forming material coated with first sheath forming material) into annular passage  354 . Accordingly, a hollow fiber having a core, a first sheath deposited uniformly on the core, and a second sheath deposited uniformly on the first sheath plus core, all in a concentric manner, is formed.  
         [0050]    The various ports, channels, and passages in the spinnerette assemblies described above can be formed in any manner and can be of any number to produce fibers having plural sheaths and core. For example, the core forming material passage can be of any shape or configuration and can comprise plural channels or a single channel. The spinnerette assemblies can be machined using any known techniques such as drilling, electronic discharge machining (EDM), or any other suitable process or processes. There can be any number of extrusion orifices. The invention can be used to make hollow fibers of any type and of any material amenable to extrusion. The various angles and dimensions can be varied to suit the particular application. The spinnerette assemblies can be manufactured of any suitable material such as steel, monel, titanium, aluminum, or alloys thereof. The core forming material and the sheath forming material(s) can be of any type amenable to extrusion such as polymer melts or solutions, ceramic pastes, and the like. The bore forming fluid can be an inert gas or liquid for example.  
         [0051]    The invention has been described through preferred embodiments. However, various modifications can be made without departing from the scope of the invention as defined in the appended claims.