CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application is a continuation-in-part of U.S. patent application Ser. No. 10/030,202 which is a continuation of copending International Application No. PCT/EP00/05703 filed Jun. 21, 2000 which designates the United States, and claims priority to German application no. 199 29 709.6 filed Jun. 24, 1999. 
     
    
     TECHNICAL FIELD OF THE INVENTION  
       [0002]     The present invention relates to a device for spinning materials forming threads, from melts or solutions at temperatures above the ambient temperature.  
       BACKGROUND OF THE RELATED TECHNOLOGY  
       [0003]     Microthreads of this kind, but usually microfibres of finite length, have for many years been made by a hot-air spun-blown method, the so-called melt-blown method, and today there are different apparatuses for this. A common feature of all of them is that, in addition to a row of melt holes—several rows parallel to each other have also become known—hot air which draws the threads escapes. By mixing with the colder ambient air, there is cooling and solidification of these threads or fibres, for often, usually of course undesirably, the threads break. The disadvantage of this melt-blown method is the high expenditure of energy to heat the hot air flowing at high speed, a limited throughput through the individual spinning nozzles or spinning bores (even though these have been set increasingly closer together in the course of time, down to a spacing of below 0.6 mm with 0.25 mm in hole diameter), that with thread diameters of less than 3 μm breaks occur, which leads to beads and protruding fibres in the subsequent textile composite, and that due to the high air temperature necessary to produce fine threads the polymers are thermally damaged well above the melt temperature. The spinnerets, of which a large number have been proposed and also protected, are elaborate injection moulding dies which have to be made with high precision. They are expensive, operationally susceptible and tedious to clean.  
         [0004]     Spinnerets containing spinning nozzles for the manufacture of continuous filaments or threads from spun raw materials of a natural or synthetic origin as a rule are operated at temperatures significantly above the ambient temperature. The melt-spun threads of polymers, i.e. plastics, are located at the upper end of the temperature region of 250 to 400° C., and others, which are spun in solution, like those of cellulose masses, so-called lyocell threads at around 100° C., and a multitude of threads of a natural and synthetic origin are located between these temperatures. The spinnerets as delivery tools for the thread formation accordingly need to be heated so that the spinning material melt or solution is not cooled to below the desired spinning temperature or so that it indeed does not even freeze. An overheating of the liquid spinning materials in order to compensate the losses at the supply conduit to the spinnerets and at these nozzles is generally ruled out since the spinning materials do not tolerate being molecularly degraded too greatly.  
         [0005]     It is common to externally heat the spinnerets containing the spinning nozzles just as the conduits leading to them from the outside, and then to insulate this heating with respect to the surroundings, in each case with the aim of keeping the temperature of the liquid spinning material constant with a lowest expense of energy, actively by way of accompany heating and passively by way of thermal insulation.  
         [0006]     Additionally to the desire to keep the temperature of the spinning material constant there also exists the desire for a greater heating shortly before the exit of the threads from the spinneret for improving the subsequent thread results, for example by way of an increased molecular orientation or for producing finer threads, which however should only be effected for an as short as possible period of time due to the undesirable molecular degradation.  
         [0007]     Spinnerets for producing threads for application in textile, medical and sanitary as well as technical-industrial fields, according to the state of the art are heated externally by way applying them in heated chambers. The heating is effected by way of electrical resistance heating, sometimes also inductively, above all however via a heat transfer medium in the liquid or vapour-like condition, the latter by way of known diphyl/dowtherm heaters. These are tried and tested systems which fulfil their task with regard to keeping the spinning temperature constant, but less however with regard to an increased heating over short paths. The expense with regard to heat transfer medium heaters is considerable, in particular with vaporous media which otherwise are quite favourable, where the condensation product must be led back to the heat source in an expensive manner. A fire hazard and even an explosion hazard sometimes exist with these media. An air gap always exists between the spinneret to be heated and the heating housing in which it is installed, which accordingly needs to be overcome technically by way of an accordingly higher temperature of the heater housing, thus with more energy expense. Here, particularly large heat losses may also occur on account of the known chimney effects.  
         [0008]     A direct heating of the spinnerets or spin packs by way of installing heating rods, so-called heating cartridges, perpendicularly from above into these nozzles at a certain distance from one another also exists. In particular with long spinnerets, as are required for spunbond nonwoven manufacturing methods, here in particular the melt-blown method, the heating in this manner is unequal in a waved manner over the length of the spinnerets, which although capable of being alleviated by way of a suitably greater mass of the spinneret, however on account of this leads to an increased material expense and an extended pre-heat time of the spinneret before this may begin with the production. The expense for the individually, perpendicularly introduced heating cartridges is also considerable.  
         [0009]     Spinnerets for spunbond nonwoven which to sometimes extend over several metres, are held in heated housings, so-called spinning heads, and together with these and the means for the distribution of the melt, form the so-called spinning beam. With the melt-blown method with which the threads are drawn by way of hot air, the above-described electrical heating rods with the larger widths are inserted as individual cartridges in a parallel manner at certain distances to one another into the spinneret or spin pack from above or into a heated chamber, with those disadvantages cited above. They themselves are thus directly or indirectly heated via these heating chambers.  
       SUMMARY OF THE INVENTION  
       [0010]     It is therefore the object of the present invention to provide an apparatus for the manufacture of essentially endless threads/filaments, which requires less expenditure of energy, does not cause thread damage on account of excessive temperature and use a spinning tool of simple construction.  
         [0011]     Another object of the invention is to provide a device for spinning materials forming threads or continuous filaments from melts or solutions, with which the channels leading the melt or solutions (in the following only the term “melt” is used, which is also to include the solutions), the distribution chambers and where appropriate also the individual spinning capillaries or nozzles with their fore-bores are uniformly heated without dissipating too much waste heat to the surroundings, and with which one may make do without special heating chambers.  
         [0012]     This object can be achieved according to the invention by an apparatus comprising a spinning head connected to a feeder for the melt, a spinneret assembly which is held in the spinning head and comprises at least one spinning bore and which spins a melt monofilament, a plate which is located below the spinning head and which comprises a Laval nozzle arranged in a fixed geometrical relationship to the spinning bore, wherein between plate and spinning head is formed a closed first space provided with a supply of gas and below the plate is provided a second space, and wherein the throughput of the melt per spinning bore, the temperature of the melt and the pressure in the first and second spaces are adjusted in such a way that the spun melt monofilament carried by the flow of gas in or after leaving the Laval nozzle before solidification thereof attains a hydrostatic pressure which is greater than the gas pressure surrounding it, such that the thread bursts and splits into a plurality of fine threads.  
         [0013]     The spinneret assembly can be insulated from the first space in the region of the at least one spinning bore by an insulating assembly and/or is heated in the region of the at least one spinning bore. The pressure ratios in the first and second spaces can be adjusted in such a way that the gas flow in the Laval nozzle attains speeds up to the speed of sound and over. The second space can be at ambient pressure or a few mbar over. The supplied gas can be at ambient temperature or the temperature of its feeder. The outlet opening of the at least one spin hole in the region of the Laval nozzle can be located at the level of the upper edge of the plate, a few mm above the upper edge of the plate, or extends a few mm into the Laval nozzle. The spinning nozzle assembly may comprise a plurality of spinning bore which are if occasion arises provided with nipples and which form a row or several parallel rows. The plate may comprise at least one elongate Laval nozzle. The plate may comprise a plurality of rotationally symmetrical Laval nozzles. A delivery belt can be provided for deposition of the threads and formation of a non-woven fabric. A winding device can be provided for winding the threads. The Laval nozzles can be designed to provide for a gas flow around the at least one thread which is laminar.  
         [0014]     The object can also be achieved by a device for spinning materials forming threads, from melts or solutions at temperatures above the ambient temperature, with a spinneret comprising at least two plate-like parts arranged over one another, and at least one melt supply bore and distribution conduits for distributing the melt, and a plurality of spinning bores, wherein electrical heating conductors are arranged in the region of the boundary surface of the at least two plate-like parts in a manner such that they surround the distribution conduits and compensate heat losses to the outside.  
         [0015]     The distribution conduits may comprise melt channels which at the boundary surface are machined into the surface at least of one of the plate-like parts. The distribution conduits may comprise a melt distribution space which is arranged in the region of the boundary surface. One of the plate-like parts can be designed as a spinneret&#39;s lower part with the spinning bores, wherein the spinneret&#39;s lower part comprises a holed plate which is applied into a recess and which is applied over the spinning bores. Three plate-like parts can be provided, wherein electrical heating conductors are arranged in the region of the respective boundary surface between two plate-like parts. The electrical heating conductors can be arranged parallel to the respective boundary surface. The electrical heating conductors can be designed in an annular manner. At least one groove can be incorporated in the region of the respective boundary surface, into which the respective tubularly formed heating body is applied or pressed. Further electrical heating conductors for increasing the temperature of the melt can be arranged in the region of the spinning bores transverse to their longitudinal directions. The electrical heating conductors can be formed as cartridges which are applied into bores or fittings. Several cartridges can be applied behind one another over the length of the arrangement of the spinning bores. The cartridges may have different heating outputs. The spinning bores can be formed in a web which is connected to the plate-like spinneret&#39;s lower part as one piece, wherein the electrical heating conductors are arranged on both sides of the web transversely to the spinning bores.  
         [0016]     The object can furthermore be achieved by an apparatus comprising a spinning head connected to a feeder for a melt, a spinneret comprising at least two plate-like parts arranged over one another, and at least one melt supply bore and distribution conduits for distributing the melt, and a plurality of spinning bores, wherein electrical heating conductors are arranged in the region of a boundary surface of the at least two plate-like parts in a manner such that they surround the distribution conduits and compensate heat losses to the outside, and a Laval nozzle arranged in a fixed geometrical relationship to the spinning bores to provide a gas flow around the spinning bores, wherein the throughput of the melt through the spinneret, the temperature of the melt is adjusted in such a way that a spun melt monofilament exiting a spinning bore is carried by the flow of gas in or after leaving the Laval nozzle before solidification thereof and attains a hydrostatic pressure which is greater than the gas pressure surrounding it, such that the thread bursts and splits into a plurality of fine threads.  
         [0017]     The supplied gas can be at ambient temperature or the temperature of its feeder. The arrangement may comprise at least one elongate Laval nozzle. The arrangement may comprise a plurality of rotationally symmetrical Laval nozzles. A delivery belt can be provided for deposition of the threads and formation of a non-woven fabric. A winding device can be provided for winding the threads.  
         [0018]     According to the invention polymer melt can be pressed out of spin holes, which are arranged in one or more parallel rows or rings, into a chamber having a given pressure which is filled with gas, as a rule with air, and which is separate from the environment, wherein in the molten state the threads pass into a region with rapid acceleration of this gas at the outlet from the chamber. The forces transmitted to the respective thread on the way there by shear stress increase, its diameter decreases greatly and the pressure in its still liquid interior increases to a corresponding extent in inverse proportion to its radius due to the effect of the surface tension. Due to the acceleration of the gas, its pressure drops by the laws of flow mechanics. In the process, the conditions of the melt temperature, gas flow and its rapid acceleration are coordinated with each other in such a way that the thread before solidification thereof attains a hydrostatic pressure in its interior which is greater than the surrounding gas pressure, so that the thread bursts and divides into a plurality of fine threads. Due to a gap at the bottom in the chamber, threads and air leave the latter. Bursting takes place after the gap and under otherwise unchanged conditions with surprising stability at a given fixed location. In the region of great acceleration, gas and thread streams run parallel, the flow interface around the threads being laminar. Continued splitting of the original thread monofilament occurs without bead formation and breaks. From a monofilament is produced a multifilament of very much finer threads using a gas stream having ambient temperature or gas stream slightly above this.  
         [0019]     The new threads arising from splitting are considerably finer than the original monofilament. They may even still be drawn slightly after the splitting point until they are solidified. This happens very quickly because of the greater thread area suddenly created. The threads are endless. But more to a minor extent they can be threads of finite length due to deviations in the polymer, individual speed or temperature disturbances, dust in the gas and the like disturbances in real industrial processes. The process of splitting thread-forming polymers can be adjusted in such a way that the numerous very much finer single filaments produced from the monofilament are endless. The threads have a diameter of well below 10 μm, mainly between 1.5 and 5 μm, which in the case of polymers corresponds to a titre of between about 0.02 and 0.2 dtex, and are referred to as microthreads.  
         [0020]     The area of great acceleration and pressure drop in the gas stream is according to the invention realized in the form of a Laval nozzle with convergent contour to a narrowest cross-section and then rapid widening, the latter already so that the newly formed single threads running adjacent to each other cannot stick to the walls. In the narrowest cross-section, with a suitable choice of pressure in the chamber (in the case of air, about twice as high as the ambient pressure behind), the speed of sound can prevail, and in the wider portion of the Laval nozzle supersonic speed prevails.  
         [0021]     For the manufacture of non-woven thread fabrics (spun-bonded fabrics), spinning nozzles in row form and Laval nozzles of rectangular cross-section are used. For the manufacture of yarns and for special kinds of non-woven fabric manufacture, round nozzles with one or more spin holes and rotationally symmetrical Laval nozzles can also be used.  
         [0022]     The method borrows from methods for the manufacture of metal powders from melts, from which it was developed. According to DE 33 11 343, the molten metal monofilament in the region of the narrowest cross-section of a Laval nozzle bursts into a large number of particles which are deformed into pellets by the surface tension and cooled down. Here too the result is a liquid pressure in the interior of the melt monofilament which outweighs the surrounding laminar gas flow. If the pressure drop takes place so rapidly that solidification is not yet close, the pressure forces can outweigh the forces of cohesion of the molten mass, mainly viscosity forces, and bursting into a plurality of filament pieces (ligaments) occurs. The crucial factor here is that the thread must remain liquid at least in the interior so that this mechanism can set in. It has therefore also been proposed to further heat the monofilament after its emergence from the spinning nozzle.  
         [0023]     Automatic bursting of a molten metal thread is also named the “NANOVAL effect” after the firm which uses it.  
         [0024]     Defibration by bursting has become known in the manufacture of mineral fibres, thus in DE-A-33 05 810. By interfering with the gas flow in a rectangular channel arranged below the spinning nozzle by means of fittings which generate cross flows, as stated there the result is defibration of the single melt monofilament. In a not quite clear account there is mention of defibration by static pressure gradient in the air flow, and in fact in EP 0 038 989 drawing from a ‘loop or zigzag movement . . . after the fashion of a multiple whiplash effect’. The fact that the actual ‘defibration’ is caused by an increase in pressure in the interior of the thread and decrease in the surrounding gas flow was not recognized, nor any control mechanisms in this direction.  
         [0025]     For polymers, this finding from mineral fibre manufacture was obviously made use of by the same applicant firm. In DE-A-38 10 596 in an apparatus according to  FIG. 3  and description in example 4 the melt stream of polyphenylene sulphide (PPS) is ‘defibrated by a high static pressure gradient’. The gas streams are hot, even heated beyond the melting point of the PPS. A static pressure gradient in the gas flow, decreasing in the direction of thread travel, cannot on its own defibrate the thread. It was not recognized that, for this, the melt stream must remain liquid in its interior, at least in an adequate portion. But by using hot air in the region of the polymer melt temperature, this happens by itself. It is not a ‘pressure gradient occurring after the outlet holes’, column 1, lines 54-55 that draws the melt streams into fine fibres, but a static pressure gradient between melt stream and surrounding gas flow that causes it to split or defibrate. The threads produced are of finite length and amorphous.  
         [0026]     The threads of the method according to the invention on the other hand are endless or essentially endless. They are produced by selectively controlled bursting of a still molten monofilament in a laminar gas flow surrounding them, that is, without turbulence-generating cross flows. Basically all thread-forming polymers are considered, such as polyolefins PP, PE, polyester PET, PBT, polyamides PA 6 and PA 66 and others such as polystyrene. Here, those such as polypropylene (PP) and polyethylene (PE) are to be regarded as favorable because surface tension and viscosity are in a ratio which readily allows the build-up of an internal thread pressure against the surface tension force of the thread skin, while the viscosity is not so high that bursting is prevented. The ratio of surface tension to viscosity can be increased by increasing the melt temperature in most polymers. This takes place in a simple manner in melt manufacture and can be reinforced by heating the spinning nozzles shortly before emergence of the threads. Heating the threads afterwards by hot gas streams does not however take place according to the present invention.  
         [0027]     It can be established that controlled splitting of a polymer thread drawn with cold air into a plurality of finer single threads of endless or essentially endless single threads, has not yet been found. This takes place by the automatic effect of bursting of the melt thread due to a positive pressure difference between the hydraulic pressure in the thread, arising from the surface tension of the thread envelope, and the gas flow surrounding it. If the pressure difference is so great that the strength of the thread envelope is no longer sufficient to hold together the interior, then the thread bursts. Splitting into a plurality of finer threads occurs. The gas, usually air, can be cold, i.e. does not have to be heated, only the process conditions and the apparatus must be such that the melt monofilament in its critical diameter which depends on the melt viscosity and the surface tension of the polymer concerned is not cooled to such an extent that it can no longer burst due to the internal liquid pressure building up. Also the melt holes must not be cooled by the gas so greatly that the melt cools down too greatly, let alone already solidifies there. The process and geometrical conditions for producing this splitting effect are relatively easy to find.  
         [0028]     The advantage of the present invention lies in that, in a simple and economical manner, very fine threads within a range of well below 10 μm, mainly between 2 and 5 μm, can be produced, which in the case of pure drawing for example by the melt-blow method can be accomplished only with hot gas (air) jets heated above melting point, and so requires considerably more energy. Moreover, the threads are not damaged in their molecular structure by excessive temperatures, which would lead to reduced strength, with the result that they can then often be rubbed out of a textile structure. Another advantage lies in that the threads are endless or quasi-endless and cannot protrude from a textile structure such as a non-woven fabric and come away as fuzz. The apparatus for carrying out the method according to the invention is simple. The spin holes of the spinneret can be larger and so less susceptible to breakdowns, and the Laval nozzle cross-section in its precision does not need the narrow tolerances of the lateral air slots of the melt-blown method. For a given polymer one need only coordinate the melt temperature and the pressure in the chamber with each other, and with a given throughput per spin hole and the geometrical position of the spin holes relative to the Laval nozzle splitting occurs.  
         [0029]     By way of the fact that the device comprises a spinneret with at least two plate-like parts arranged over one another, e.g. the lower part with the nozzles, and electrical heating conductors are arranged in the region of the boundary surface of the at least two plate-like parts in a manner such that they surround the distribution conduits and compensate heat losses to the outside, one achieves a uniform heating of the melt and essentially no waste heat is dissipated to the outside.  
         [0030]     It is particularly advantageous for the distribution conduits to comprise melt channels and at least one melt distribution space which are incorporated (machined) into at least one boundary surface, or are arranged in the region of the boundary surface. With longer paths of the spinning material melts, the problem exists that dead corners arise in the spinneret where the melt does not flow or no optimal conduits with regard to flow technology having a uniform resistance and a uniform melt distribution are present. If the distribution conduits are provided in the region of the boundary surface of the second plate-like parts, then the melt conduits, disregarding the supply and discharge conduits, are completely accessible on manufacture and may thus be checked with regard to the nature of the surface and dead corners.  
         [0031]     The electrical heaters are essentially applied parallel to the melt conduits or melt channels and melt distribution space and surround them in their entirety. At the same time one uses tubular heating conductors which are applied pressed into fittingly calibrated grooves or inserted into the grooves, wherein the grooves in the region of the respective boundary surfaces are incorporated (machined) into the plate-like part or parts.  
         [0032]     It is particularly advantageous for further electrical heating conductors, for increasing the temperature of the melt, to be arranged in the region of the spinning bores, i.e. essentially perpendicular to their longitudinal directions. By way of these measures the melt, before exit from the spinneret, is heated again to higher temperatures, wherein this heating is only effected over a short path so that the thread result is improved and the melt is not thermally damaged, which would have a negative effect on the subsequent mechanical properties of the thread and thus of the nonwoven or yarn. This increased heating is realized by way of groove heating or heating cartridges. In one advantageous manner, the device according to the invention may be used with the methods and means as are disclosed in DE 199 29 709 C1 and WO 02/05 2070 A2. Here, on the one hand it is a question of an as uniform as possible heating of the spinneret over the width if spunbond nonwovens are to be manufactured, since all temperature differences are noticeable due to the changes in viscosity of the exit flow quantity from the individual spinning bores and thus unequal web or nonwoven weights result over the web width. In order to distribute a polymer melt, the temperature should not be too high and thus the viscosity should not be too low. On the other hand however with regard to the requirements of this method with which a melt monofilament is converted into a multifilament of individual threads by way of splicing, the melt must exceed a certain temperature so that a splicing also indeed takes place. The device according to the invention may however also be applied to other spinning methods. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0033]     Practical examples of the invention are shown in the drawings and described in more detail in the description below. They show:  
         [0034]      FIG. 1 a  schematic sectional view of an apparatus for the manufacture of microthreads by means of cold gas jets by bursting of a melt stream into a plurality of single threads according to a first embodiment of the invention,  
         [0035]      FIG. 2 a  perspective partial view of the apparatus according to the invention in an embodiment with row nozzle and spin holes in nipple form for the manufacture of non-woven fabrics from microthreads, and  
         [0036]      FIG. 3 a  partial view in section of the spinning nozzle and Laval nozzle according to a third embodiment of the invention.  
         [0037]      FIG. 4 a  longitudinal section through a spinneret according to the invention, with a rectangular shape,  
         [0038]      FIG. 5 a  cross section through the spinneret according to the invention, according to  FIG. 4 ,  
         [0039]      FIG. 6-7  the views of the upper and lower plates used for the spinneret, from below ( FIG. 6 ) and from above ( FIG. 7 ),  
         [0040]      FIG. 8 a  plan view of a further embodiment example of a spinneret for a wide thread curtain with several infeeds for the supply of melt.  
         [0041]      FIG. 9 a  perspective partial view of the apparatus according to the invention with a Laval nozzle arrangement and a spinneret according to  FIG. 4  and  FIG. 5 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0042]     In  FIG. 1  is shown an apparatus for the manufacture of essentially endless fine threads from melt-spinnable polymers, which comprises a spinning head or spinning beam  11 , not described in more detail, in which is held a spinneret  1  containing one or more spinning nozzles. The spinning head  11  and the spinneret are in a generally known manner composed of different parts, so that a description of them is omitted. The spinning head or spinning beam  11  is surrounded by a heating system  12  which is designed as a system for liquid or steam heating via chambers or as an electric band heating system. The spinning head or spinning beam is connected to melt-proportioning devices, not shown, such as spinning pumps and extruders, which are the usual devices for the manufacture of synthetic fibres, so that these are not described further either.  
         [0043]     The spinneret comprises a nozzle orifice  3  which comprises one, or usually several spin holes arranged in a row. Several parallel rows are possible too. Below the spinning head  11  is located a plate  6 ′ with a gap  6  which is of convergent-divergent construction and widens greatly due to a space  7  located below it and forms a Laval nozzle. Depending on the shape of the spinneret  1 , the Laval nozzle  6  is constructed rotationally symmetrically similarly to a stop in the case of a single nozzle or as a longitudinal gap in the case of a row nozzle. The spinneret or the spin holes of the spinneret end just above the Laval nozzle  6  or in the plane of the plate  6 ′, but the spinning nozzle can also extend slightly into the Laval nozzle  6 .  
         [0044]     Between spinning head and plate  6 ′ is located a closed chamber  8 , to which gas is supplied in the direction of the arrows  5  for example by a compressor. The gas is usually at ambient temperature, but can be at a slightly higher temperature, for example 70 to 80°, on account of the heat of compression from the compressor.  
         [0045]     The nozzle orifice  3  is surrounded by an insulating assembly  9  which protects the nozzle orifice  3  against excessive heat losses due to the gas flow  5 . In addition an electric band heating system  10  can be arranged between insulating assembly  9  and orifice  3 .  
         [0046]     The space  7  is usually at ambient pressure, while the gas in the chamber  8  is at an elevated pressure compared with the space  7 . In the case of directly following further processing into a non-woven fabric or other thread structures, the space  7  can be at a slightly elevated pressure compared with ambient pressure, i.e. atmospheric pressure, for example a few mbar higher, which is needed for the further processing, such as non-woven fabric laying or other thread collecting devices.  
         [0047]     The polymer melt is pressed in the direction of the arrow  2  out of the nozzle orifice  3  out of the spin hole or opening  4  as a melt monofilament, and is picked up by the gas jets  5  and tapered to smaller diameters by shear stresses at its circumference. As the basically cold gas streams, which can be air streams, cool it down, after a few millimeters it must pass into the narrowest region of the Laval nozzle and so into an area of lower pressure. As soon as the taper has progressed so far and due to the effect of the surface tension of the melt at the thread envelope the pressure in the interior has increased so far that it is above that of the gas flow, bursting of the monofilament occurs, namely when the thread envelope can no longer hold the melt thread together against the internal pressure which has increased with thread constriction. The melt monofilament is divided into single threads which, on account of the temperature difference between melt and cold gas or air and the suddenly greatly increased surface area of the single threads referred to the thread mass, cool down rapidly. Hence a given number of very fine essentially endless single threads or filaments is produced.  
         [0048]     It follows from the nature of such bursting processes similar to explosions that the number of threads produced after the splitting point, which can be for example 5 to 25 mm below Laval nozzle  6 , cannot be constant. Because of the short distance which thread and gas together cover up to the splitting point, the flow interface around the thread is laminar. Preferably also the air from the supply pipes is conducted in laminar fashion as far as possible to the region of splitting. This has the advantage of the lower flow losses and hence the lower energy requirements which distinguish laminar flows compared with turbulent ones, but also a more uniform time curve of splitting, because disturbances due to turbulent changes are absent. The accelerated flow such as is present in the cross-section of the Laval nozzle  6  remains laminar and can even be laminarised if a certain turbulence prevailed before.  
         [0049]     The added advantage of laminar drawing of the melt monofilament up to the splitting point and also beyond it leads to splitting into more uniform single threads, because there are not greater differences in the flow rate and hence in the sheer stress acting on the melt monofilament and resulting single threads and in the pressure of gas flow. The distribution of the thread diameters is, as it turned out surprisingly, very narrow, e.g. propylene threads whose diameters are all between 2 and 4 μm can be produced.  
         [0050]     As already stated above, the speed of gas flow in the direction of the arrows  5  increases constantly towards the spin hole  4  and then in the Laval nozzle  6 . In the narrowest cross-section of the Laval nozzle it can increase up to the speed of sound if the critical pressure ratio, depending on the gas, is reached, and in the case of air the ratio between the pressure in the chamber  8  and the space  7  is around 1.9.  
         [0051]     The threads move downwards in the direction of the arrows  21  and for example can be deposited on a belt, not shown, to form a non-woven fabric, or otherwise further treated.  
         [0052]     In  FIG. 2  is shown a further example of the present invention, in which the spinneret  1  containing the spinning nozzle is constructed as a row nozzle. Here are shown in particular the outlet points of the spinneret  1  with orifice  3  which comprises nipples  25 . This form allows concentric access of the gas to the melt monofilament, which proved to be advantageous for splitting, both with respect to the obtainable fineness of the threads and with respect to the width of fluctuation of their diameters.  
         [0053]     By contrast with the band heating system  10  according to  FIG. 1 , here round heating elements  26  are shown for the supply of heat to the nozzle orifice  3  which is shielded by insulators  27  from the gas flow  5 . The threads leave the Laval nozzle in a wide curtain together with the gas, and move in the direction of the arrows  21  towards a collecting belt  20  and are deposited in the region  22  to form a non-woven fabric  23 . The non-woven fabric  23  leaves the area of its manufacture in the direction of arrow  24 .  
         [0054]     A further embodiment of the spinning and splitting apparatus according to the invention is shown in  FIG. 3 . Here again the melt monofilament is expelled from an insulated nozzle orifice  3  with one or more melt holes  4  and picked up by the laterally applied gas stream  5  and drawn in length into thinner diameters by shear stress forces. In the plate  6 ′ in the region of the Laval nozzle  6  a heating device  30  is incorporated. On the way to the narrowest cross-section of the Laval nozzle  6 , therefore, the melt monofilament has heat supplied to it by radiation. As a result, cooling by the basically cold air/gas streams is delayed. The melt monofilament passes, drawn to a smaller diameter, into the partial-pressure area of the Laval nozzle  6  and can split into even finer single threads.  
         [0055]     The following examples describe methods and apparatuses with the essential process data used with different raw materials and the thread results.  
       EXAMPLE 1  
       [0056]     By means of a laboratory extruder (screw with a diameter of 19 mm and L/D=25) for the processing of polymers, polypropylene (PP) with a MFI (melt flow index) of 25 (230° C., 2.16 kg) was melted and supplied via a gear spinning pump to a spinning head with a nozzle orifice  3  comprising seven holes  4  arranged in a row at equal intervals of 4.5 mm each, with a diameter of the holes  4  of 1 mm. The melt-conducting pipes were heated from the outside by electric band heating systems. The nozzle orifice  3  was insulated according to  FIG. 1  at its flanks by a ceramic insulator  9  (calcium silicate) against the gas flow below it and heated with electric heating systems. Below the spinneret  1  was located the chamber  8  for gas supply. Air which was taken from a compressed air network and fed into the latter by a compressor was taken as the gas in this and the other examples. The chamber  8  was defined at the bottom by a plate which had a slot forming the Laval nozzle  6  with a width of 4 mm at its narrowest cross-section. The lateral supply cross-sections for the air in the chamber had a height of 32 mm, measured from the upper edge of the Laval nozzle plate  6 ′. The outlet openings of the holes  4  were arranged exactly at the level of the upper edge of the Laval nozzle plate  6 ′ and had a distance of 10 mm from the narrowest cross-section of the Laval nozzle  6 .  
         [0057]     Melt pressure and temperature between spinning pump and connecting piece to the spinning nozzle  1  were measured with a strain-gauge pressure measuring device (Dynisco, model MDA 460) or a thermoelement.  
         [0058]     Characterization of the threads obtained is by the thread diameter d50 averaged over 20 single measurements, and if required also by the standard deviation s.  
         [0059]     Splitting is characterized by the theoretical thread count N. This indicates how many single filaments of the measured average thread diameter d50 must move at the maximum possible speed through the narrowest cross-section of the Laval nozzle  6  in order to convert the measured melt mass to single filaments. The maximum possible speed is the gas speed in the narrowest cross-section of the Laval nozzle  6 , which is either the speed of sound which can be calculated from the conditions in the chamber  8  or, in the event that the critical pressure ratio which is needed to attain the speed of sound is not attained, can be calculated from these conditions with Saint-Venant and Wantzell&#39;s formula. If the theoretical thread count N is more than 1, the thread diameter observed cannot have been produced simply by drawing, this would be contrary to the law of preservation of mass. For the observed theoretical thread counts N well over 1, only splitting is possible as an explanation. A multiple whiplash effect can perhaps explain values just over 1 to 10, but not the observed values of up to 627. As the actual thread speed must be below the maximum, the single filament number actually obtained will be above the theoretical number.  
         [0060]     At a melt temperature of 340° C. and a melt pressure between spinning pump and connecting piece to the spinneret  1  of approximately 1 bar above the pressure in the pressure  8 , which was approximately also the pressure in front of the spinning capillaries, and with a quantity proportioned via the spinning pump of 43.1 g/min, that is, 6.2 g/min per hole, the following thread values resulted at the different pressures in the chamber  8  above the atmospheric pressure in space  7 :  
                                                       0.25 bar   d50 = 7.6 μm, N = 123            0.5 bar   d50 = 4.4 μm, N = 276            1.0 bar   d50 = 3.9 μm, N = 283                      
 
         [0061]     If only 4.6 g/min are passed through instead of 6.2 g/min per hole, at a pressure in the chamber  8  of 0.5 bar a d50 of 3.0 μm instead of 4.4 μm is obtained.  
         [0062]     How important exact coordination of melt temperature, melt quantity and gas flow is, is shown by the following example of a distance between the holes  4  of 15 mm, a throughput per hole  4  of 4.6 g/min and a pressure in the chamber  8  of 0.5 bar:  
                                                       melt temperature 340° C.   d50 = 3.0 μm, s = 0.8 μm, N = 187           melt temperature 305° C.   d50 = 8.2 μm, s = 4.7 μm, N = 25.                      
 
         [0063]     Clearly conditions are such that around the monofilaments there has already formed a cold envelope which greatly hinders splitting. Not the whole monofilament is split open, but only a portion, which can be seen by the fact that, although the minimum observed thread diameter has not changed (some therefore split open), some single filaments with a diameter of more than 10 μm occur. Thus no splitting has occurred there. At the higher temperature, on the other hand, all single filaments are between 1.6 μm and 4.8 μm. The greater variance of thread diameters is reflected in the much greater standard deviation.  
         [0064]     A design of the orifice  3  with nipples  25  according to  FIG. 2  allows the manufacture of much finer threads with a smaller width of fluctuation and/or a distinct increase in throughput. Thus, for a temperature of 370° C., a distance between holes  4  of 15 mm, a distance from outlet openings of the holes  4  to the narrowest cross-section of the Laval nozzle of 8.5 mm (the outlet openings are submerged 1.5 mm in the imaginary plane of the Laval nozzle plate) and a pressure in the chamber  8  of 0.75 bar, the following thread values are obtained:  
                                                        6.2 g/min per hole   d50 = 2.1 μm, s = 0.30 μm, N = 445           12.3 g/min per hole   d50 = 2.5 μm, s = 0.60 μm, N = 627                      
 
       EXAMPLE 2  
       [0065]     With the apparatus from example 1, polyamide 6 (PA6) with a relative viscosity 0rel=2.4 was fed to a nozzle orifice  3  with 58 holes  4  at intervals of 1.5 mm and with a diameter of 0.4 mm. The distance from the outlet openings of the holes  4  to the narrowest cross-section of the Laval nozzle was 12.0 mm (the outlet openings ended 2.0 mm above the imaginary plane of the Laval nozzle plate). With a throughput per hole  4  of 0.25 g/min and a pressure in the chamber  8  of 0.02 bar above the environment, filaments with a mean diameter d50 of 4.1 μm were produced.  
       EXAMPLE 3  
       [0066]     With the apparatus from example 1, polypropylene (PP) with a MFI of 25 (230° C., 2.16 kg) was fed to a nozzle orifice  3  with three holes  4  at intervals of 15 mm and with a diameter of 1.0 mm. Individual rotationally symmetrical Laval nozzles  6  were arranged in the Laval nozzle plate  3  coaxially with the three holes  4 . The outlet openings of the holes  4  were arranged exactly at the level of the upper edge of the Laval nozzle plate and had a distance of 4.5 mm from the narrowest cross-section of the Laval nozzles  6 . At a pressure in the chamber  8  of 0.75 bar above the environment  7  and with a throughput per hole  4  of 9.3 g/min, single filaments with a mean diameter d50 of 4.9 μm were produced. In this case a theoretical thread count of 123 results.  
         [0067]     Of interest in this manner of operation is the observation that the bursting point compared with example 1 has clearly shifted in the direction of the narrowest cross-section of the Laval nozzles  6 . Whereas in the case of the slot-like Laval nozzle  6  this point is about 25 mm below the narrowest cross-section, the distance in the case of the rotationally symmetrical Laval nozzle  6  is only about 5 mm. The observation is explained by the fact that, due to the rotationally symmetrical enclosure of the melt thread, higher shear stresses were transmitted to it, and therefore it is drawn more rapidly to the smaller diameter yielding the bursting point. Moreover, the pressure in the free jet after leaving the Laval nozzle  6  does not drop suddenly to the ambient pressure, but only after a certain running length. The free jet characteristics are however in the planar case different to those in the rotationally symmetrical case.  
         [0068]     The spinneret  100  shown in  FIG. 4  which can be used in the arrangements of FIGS.  1  to  3  is composed of several plates  104 ,  107 ,  112  arranged above one another, wherein the lower plate  112  forms the lower part of the spinneret with the nozzles. A bore  103  is provided in the upper plate  104  and this is connected to a supply  102  of melt. The bore  103  enters into melt channels  113 ,  113 ′ which in the region of the boundary surface between the plates  104  and  107  on the one hand are incorporated into the surface of the plate and on the other hand in the surface of the intermediate plate  107 . In another embodiment example they may either be incorporated into the lower side of the upper plate  104  or in the upper side of the intermediate plate  107 , wherein however in this case they are open towards the boundary surface, i.e. are designed in a groove-like manner. With this, the groove may be semicircular, rectangular or profiled in another manner, and the surface of the respective other plate within this context remains unmachined which creates lower costs. Since the individual plate surfaces are completely accessible on manufacture and with a later disassembly, the particularly critical locations of the melt channels at their ends before entry into the supply conduits  106 ,  106 ′ which pass perpendicularly through the intermediate plate  107  may be monitored in their shaping by way rounding-off and in their surface constitution on manufacture.  
         [0069]     The spinneret&#39;s lower part  112  comprises a recess for accommodating a holed plate  109  with a multitude of perforating holes. The supply conduits  106 ,  106 ′ merge into a melt distribution space  108  which is arranged between the holed plate  109  as a surface of the spinneret&#39;s lower part  112  and the lower surface of the intermediate plate  107 , i.e. in the region of the boundary surface between the two plate-like parts  107  and  112 , wherein a melt sieve which is carried by the holed plate  109  and is indicated dashed by the reference numeral  110  is provided in the melt distribution space  108 , which serves for filtering the melt and for increasing the pressure resistance for an improved distribution.  
         [0070]     Spinning bores or nozzles  111  arranged in a row are machined into the spinneret&#39;s lower part  112 , wherein a nozzle web  114  is formed on the spinneret&#39;s lower part  112  forming one piece, and contains the spinning bores  111  and at its lower end comprises the flow exit capillaries  116 . Of course the spinning bores  111  may also be provided in the spinneret&#39;s lower part  112  in another manner. According to the subsequent method they may be arranged in several rows parallel to one another, but may also be arranged in a curved alignment. Electrical heating conductors are provided in the region of the boundary surfaces between the upper plate  4  and the intermediate plate  107  or the intermediate plate  107  and spinneret&#39;s lower part  112 , and these are formed as tubular heating bodies  123 ,  126  and are applied or pressed in the grooves  122 ,  125  machined in the surface of the plates  104 ,  112  respectively. The electrical heating conductors or the tubular heating bodies  123 ,  126  surround the melt channels  113 ,  113 ′ or the melt distribution chamber  108  in an essentially circular manner and are arranged essential parallel to them, wherein their electrical connections to the outside are indicated in  FIGS. 6 and 7  at  124  and  128 . Generally, a plate carries the circulatory heating and the oppositely lying one likewise obtains its heat form here. Several heating systems, e.g. two for two plates may also be applied. The design of the plate surfaces in its finished dimensional quality is essential for a sealing against leakage flows of the melt and for the heating of two plates with only one heating, so that the necessary pressure for sealing may be mustered by way of the screws of which only a few are shown here for the bores  127  in  FIG. 6  envisaged for them for the plate  104 .  
         [0071]     The spinneret&#39;s lower part  112  without the holed plate  109  is shown in  FIG. 7 , wherein the spinning bores  111  arranged in a row may be recognised.  
         [0072]     In operation, the melt is supplied to the upper plate  104  from an extruder or likewise via the melt supply conduit  102  through the bore  103 , whereupon it flows in the melt channels  113 ,  113 ′ to both ides in the direction of the arrows  105 ,  105 ′ via the supply conduits  106 ,  106 ′ into the melt distribution space  108  and reaches the spinning bores  111  via the melt sieve  110  and the holes of the holed plate, from whose flow exit capillaries  116  the respective melt threads exit.  
         [0073]     Additionally in accordance with  FIG. 8  which shows a section transversely through the spinneret, a heating of the nozzle web  114  which contains the spinning bores may be provided. For this, a heater pair  117 ,  117 ′ is arranged on both sides of the web  114  which for example comprises heating cartridges. With such heating cartridges the heating conductor  119  is located in the inside about which an insulation of lining material  120  and then an outer casing  121  is arranged, as may be recognized from  FIG. 7 . The cartridge is fitted into a body  115  which conducts well, e.g. of copper. Individual heating cartridges of a certain length may be applied behind one another in a stepped manner along the row(s) of spinning nozzles. The electric current supply may be attached at the ends of the row of spinning bores, with more than two cartridges after another they may be led out in between. Should a lacking heating output between the cartridges unallowably compromise the supply of heat to the spinning bores here, the produced electrical heating output of the cartridges may be distributed unequally over the length, wherein an increase may be carried out at the location concerned.  
         [0074]     The melt conduits  113 , and  113 ′ after the central supply through the bore  103  in  FIG. 7  may also feed more than only two supply conduits  106  and  106 ′ in  FIG. 7  if the nozzle row is longer than about 40-50 cm, so that, as in  FIG. 8  four of such supply conduits are here, which for example may be fed by a simple fourfold spinning pump. The melt channels  113  and  113 ′ may then be machined into the nozzle plates such that they are equally long in each case, even better that they have the same resistance taking diversions into account. The outer ones are diverted directly onto the outer supply conduits  129  by way of a deflection section, and the inner two with roughly the same resistance in the straight paths are likewise led to the outside and then diverted inwards to the supply conduits  130  by way of equally large deflection sections. The groove heaters  123  and  126  are located next to the melt channels  113  and heat these externally. For maintaining a constant melt temperature, the heater only needs to make up for the losses to the outside, where the spinneret is usefully insulated. With the supply of heat to the melt such as here at the nozzle web  114 , one heats to a correspondingly greater extent. But here also the heaters  117 , and  117 ′ are usefully insulated to the outside which is not shown in detail in the FIGS.  4  to  8 .  
         [0075]     The flow exit capillaries  116  for the melts may of course also be arranged in circles instead of rows, irregularly or individually in so called spin packs, this is particularly the case if it is not nonwoven which is to be manufactured, but yarn.  
         [0076]     In  FIG. 9  another embodiment for the manufacture of essentially endless fine filaments from melt-spinnable polymers or solutions is shown in which a spinneret  200  according to  FIGS. 4 and 5  is used. Below the spinneret  200  a plate  206 ′ with a Laval nozzle  206  according to  FIG. 1  is arranged separating an upper closed chamber  208  and a lower space  207  from one another, chamber  208  being at a higher pressure than space  207  which is at atmospheric pressure. Gas is applied at  205  as described in connection with  FIGS. 1 and 2 .  
         [0077]     The spinneret  200  is composed of plates  204 ,  207 ,  212  in which heating bodies  223 ,  226  are inserted as described in connection with  FIGS. 4 and 5 . The melt leaves the spinning bores  211  which are heated by heat cartridges  217 , in form of melt threads, respectively. The melt threads are surrounded and drawn by the gas streams  205  and due to a pressure difference between the hydraulic pressure in the melt thread and the gas flow surrounding it, the melt thread bursts in or just after the Laval nozzle  206  and is split into a plurality of fine filaments as shown in  FIG. 9 .
An apparatus comprises a spinning head connected to a feeder for the melt, a spinneret assembly which is held in the spinning head and comprises a spinning bore and which spins a melt monofilament, a plate which is located below the spinning head and which comprises a Laval nozzle arranged in a fixed geometrical relationship. Between plate and spinning head a closed first space is formed provided with a supply of gas and below the plate a second space is provided. The throughput of the melt per spinning bore, the temperature of the melt and the pressure in the first and second spaces are adjusted in such a way that the spun melt monofilament after leaving the Laval nozzle before solidification thereof attains a hydrostatic pressure which is greater than the gas pressure surrounding it, such that the thread bursts and splits into a plurality of fine threads.