US3613158A - Orifice assembly for spinning low viscosity melts - Google Patents

Abstract

AN IMPROVED ORIFICE ASSEMBLY AND PROCESS IS PROVIDED WHEREBY FINE DIAMETER AND/OR LIGHT FIBERS AND FILAMENTS MAY BE FORMED FROM ESSENTIALLY INVISCID MELTS WITHOUT ATTENDING SINUOUS EFFECTS UPON THE EXTRUDED MOLTEN STREAM.

Description

Oct. 19, 1971 w. MOTTERN ETAL 3,613,158

ORIFICE ASSEMBLY FOR SPINNING LOW VISCOSITY MELTS Filed Dec. 15, 1969 FIG.3.

INVENTORS JOHN W. MOTTERN ROBERT E. CUNNINGHAM ROBERT P. BELL FIG. 2.

ATTORNEY United States Patent US. (ll. l8-8 QM 4 Claims ABSTRACT OF THE DISCLOSURE An improved orifice assembly and process is provided whereby fine diameter and/or light fibers and filaments may be formed from essentially inviscid melts without attending sinuous efiects upon the extruded molten stream.

COPENDI'NG APPLICATIONS The subject matter of this application is related to the subject matter of commonly assigned and copending applications Ser. No. 829,216 filed June 2, 1969, of S. A. Dunn, L. F. Rakestraw, and R. -E. Cunningham and Ser. No. 838,593 filed July 2, 1969, of R. S. Otstot and J. W. Mottern.

BACKGROUND OF INVENTION Field of the invention This invention relates to improvements in the formations of fibers and filaments of materials which are essentially inviscid in the melt by extrusion of the molten material as a free jet into atmospheres which stabilize the nascent molten fiber or filament prior to breakup caused by surface tension pending solidification.

Discussion of the prior art and problems The recent advent of the film stabilization of inviscid jets has been recognized as a practical means for forming filaments and fibers from materials which exhibit extremely low viscosities in the liquid or molten phase. Application Ser. No. 829,216 incorporated by way of reference herein, describes in detail and claims various techniques by which stabilization may be established.

Briefly, where materials such as metals, metal alloys, and ceramics exhibit extremely low viscosities in the molten state of less than about 10 poises and more commonly only a fraction of a poise, the surface tension of such a free molten filamentary stream is so great in relation to its viscosity that the stream tends to break into small spheres or shot before it can be solidified by cooling or quenching by practical means. It has been discovered that the length of the molten inviscid stream or jet, or the time in which such a jet exists as a continuous stream prior to break-up due to its surface tension when extruded at appropriate velocities may be considerably increased by extruding the inviscid jet into an atmosphere which upon contact with the nascent molten jet forms a thin film on the surface thereof. The stabilizing film must, of course, be rapidly formed, be a solid or at least have a viscosity substantially greater than that of the molten jet at the spinning temperatures employed. The film must also be substantially insoluble in the molten jet under the spinning conditions so that substantial, and desirably complete, continuity of the film is achieved and maintained.

The means for film stabilizing inviscid jets are known and are varied as are the materials which may be stabilized. For example, the molten inviscid jet may be extruded into atmopsheres which readily react with the surface of the molten jet to form a film or into atmospheres which decompose upon contact with the molten jet to 3,613,158 Patented Oct. 19, 1971 form films. Thus, a molten aluminum jet which is extruded into an atmosphere containing oxygen is stabilized by the rapid formation of aluminum oxide which is both solid at the optimum extrusion temperature and is substantially insoluble in the molten jet. Aluminum oxide jets, on the other hand, may be extruded into hydrocarbon atmospheres, such as propane, which upon contact with the hot ceramic jet decompose leaving a stabilizing carbon film on the jet. In a special case described in US. Patent 3,216,076 issued to Alber et al., it was noted that the oxides of certain metals, such as iron, silver, and gold, are soluble in their respective melts to the extent that they do not serve to form stabilizing films. Alber et al. suggest that filaments may be formed from such materials by extruding alloys thereof with compatible metals whose oxides are substantially insoluble in the molten jet. Thus, for example, a jet of a ferrous alloy containing a small amount of a metal, such as aluminum, the oxide of which is insoluble in the jet, may be effectively stabilized against surface tension promoted break-up, pending solidification by normal or even accelerated heat transfer phenomena.

Generally, the formation of fibers and filaments by the film stabilized inviscid spinning process is applicable to the extrusion of jets having diameters of less than about 50 mils. It appears that sufiicient transfer of heat out of the molten stream, having a diameter of 50 mils or greater, even though film stabilized, is difiicult to accomplish as a practical matter to prevent break-up even when the jet is extruded into a cooled chamber. On the other hand, when using the film stabilization technique, fine diameter fibers can be adequately cooled prior to break-up at room temperature or greater sothat there is no necessity for elaborate cooling systems for chilling or attempting to supercool the molten jet. In order to provide sufficient jet lengths initially to provide for film stabilization of the molten filamentary shaped jet, the extrusion velocity of jet in a given case should be such that the Rayleigh parameter (Ra), a dimensionless quantity,

Ra=V\/L lies between 1 and 50, preferably 2 to 25, where V is the jet velocity (cm./sec.), D is the jet diameter upon issue (cm.), p and a are the melt density (gm./cm. and surface tension (dynes/cm. respectively, of the molten material. When the velocity is such that the Rayleigh parameter falls below about 1.0, the jet length may be so short that it normally cannot be adequately stabilized prior to breakup. Conversely, when the velocity of the jet is too high, breakup can be caused by aerodynamic effects.

The materials having essentially inviscid melts which are generally employed to form fibers and filaments are generally those normally solid metals and inorganic non-metals having melt viscosities below about 10 poises and usually a fraction of a poise. By normally solid is meant those materials which are in the solid phase at about 25 C. The term metals is meant to include the metals, alloys thereof, and intermetallic compounds. The term inorganic non-metals includes the ceramics, metalloids, and salts.

Among the normally solid metals are beryllium, cobalt, aluminum, thorium, nickel, iron, copper, gold, uranium, zinc, magnesium, tin, and alloys made from such metals. Representative of the low melt viscosity ceramics are alumina, calcia, magnesia, zirconia and mixtures of these and other oxides which exhibit low melt viscosities. Metalloids, such as boron and silicon, salts such as potassium chloride and a variety of other normally solid materials having melt viscosities below about poises may be employed to make filaments and fibers by the stabilization techniques described hereinbefore.

A significant improvement in the extrusion of essentially inviscid materials is described and claimed in application Ser. No. 838,593 which is incorporated by way of reference herein. It is recognized therein that the nature of essentially inviscid materials precludes attenuation of extruded molten jets by drawing as in the case of viscous and polymeric organic materials. Yet attenuation is important in processes for spinning materials primarily because of the difficulty in making true fine diameter orifices in materials which are both substantially inert at high temperatures and strong enough to withstand the extrusion pressure needed to force the melt through small orifices at these same temperatures. The problem was solved by initially directing a quantity of inert gas against the emerging jet in a direction perpendicular to the jet path, and then causing the inert gas to flow co-currently with the jet. It is thought that the resulting attenuation in the jet diameter is primarily due to the viscous drag interaction of the inert gas with the jet. The degree of attentuation may be controlled by a number of variables such as the amount of inert gas entering the system and the geometry of an inert gas chamber which is defined by the orifice plate and a second plate known as the gas plate spaced beneath the orifice plate.

An entirely unrelated problem also noted in application Ser. No. 838,593 is the disruption of the jet by materials which form in the orifice or about the orifice exit. It is recognized that the undesired materials are the reaction or decomposition products formed as a result of the sta bilizing atmosphere contacting the jet in the orifice and/ or in the immediate vicinity of the orifice exit. By blanketing the region immediately beneath the orifice with an inert gas, however, a reaction-free region is maintained in and beneath the orifice thereby allowing an unimpeded extrusion of the molten materials as a jet.

Under some conditions even when utilizing the inert gas certain disruptive effects have been noted, particularly when attempting to extrude small diameter and/ or light filaments which are truly continuous, e.g., 1000 feet or more. The effects appear to be particularly deleterious under conditions necessary for the attenuation of the jet diameter to below about 50% of the orifice diameter. Such large attenuations need a high co-current flow of the inert gas which frequently results in the jet being disrupted and in the formation of short fiber or staple. Often, the filaments or fibers have a wavy appearance instead of being straight. Further, some of the filaments and fibers have been found to be weakened due to the stresses incurred during solidification.

Failure of the stabilizing atmosphere to properly stabilize the jet under conditions necessary for large jet attenuation with the gas plate has also been observed, particularly when the stabilizing reaction or decomposition occurs slowly or the concentration of the stabilizing component in the stabilizing atmosphere is low. Under these conditions, only short, irregularly shaped fiber is formed.

When utilized denser inert gases, e.g., argon as compared to helium, it has been noted that the adverse effects stated above are accentuated. It is therefore desirable to use helium, which is the least dense of the inert gases. Helium, however, is considerably more expensive.

While the advantages of utilizing the process and apparatus of application Ser. No. 838,593 are readily evident, it is also apparent that it would be extremely beneficial to improve thereupon so as to economically permit uninterrupted extrusion of continuous filaments, particularly when attenuating to obtain filaments ha ving small diarneters. It is therefore an object of the present invention to provide the above improvement and concomitantly ensure that proper stabilization of the jet occurs, particularly when large attenuation of the jet is desired.

4 BRIEF STATEMENT OF THE INVENTION It has been discovered that the break-up of the jet continuity or the wavy appearance in the resulting filaments and fibers is largely due to the high relative velocity between the jet and inert gas. Typically, the cocurrent velocity of the inert gas is ten to twenty times as great as the jet velocity, when utilizing the attenuation process and appparaus of application Ser. No. 838,593. The interaction between the inert gas and the inherent minor lateral deviations (bends in the jet length) and surface irregularities of the jet gives rise to the well-known Bernoulli effect. The low pressure regions which form adjacent to the bends in the jet or irregular surface protrusions are analogous to the low pressure regions above the wing of a moving aircraft. The low pressure region's tend to increase the amplitudes of the bends and irregularities, sometimes to the point of disrupting the jet thereby causing the formation of wavy filaments, stape, and/ or filaments with weakened tensile strengths.

Additionally, it has been observed that the co-current inert gas flow may continue downstream into the zone in which stabilization should occur. The inert gas then may act as a barrier or blanket to the stabilization gas, and in extreme cases, prevent proper stabilization from occur ring. By removing the inert gas a short distance downstrea of the initial impingement point against the jet in accordance with the present invention, it has been found that the undesired sinous effect may be prevented while concomitantly providing proper film stabilization.

BRIEF DESCRIPTION OF THE DRAWINGS The features of the present invention which are desired to be protected are pointed out with particularity in the appended claims. The invention itself, together with further objects and advantages thereof, may be best understood with reference to the following description taken in connection with the appended drawings in which:

FIG. 1 is a vertical cross-section of a typical spinning apparatus employing an orifice assembly in accordance with the present invention.

FIG. 2 is an enlarged, partial view of the orifice assembly of FIG. 1.

FIG. 3 is a vertical cross-section of another orifice assembly in accordance with the' present invention.

DESCRIPTION FIG. 1 depicts a crucible 10 enclosing a quantity of molten essentially inviscid material 11. Functionally as part of the base of crucible 10 is an orifice plate 12 having an extrusion orifice 13. Spaced beneath plate 12 is a gas plate 14 having an orifice or gas plate throat .15 which is aligned substantially coaxial with orifice 13. Plates 12 and 14 define a substantially enclosed chamber referred to as the inert gas zone 16.

Beneath gas plate 14 is a plate 17 hereinafter called a suction plate, having an orifice or suction plate throat 18 which is aligned substantially coaxial with throat 15 (and therefore with orifice 13). Suction plate 17 and gas plate 14 define a second substantially enclosed chamber 19.

Pedestal 20 supports the entire apparatus and also defines a large cavity 21 for the stabilization of the molten jet 22.

Under positive pressure supplied to molten material 11 by an external means (not shown), the jet 22 issues from the extrusion orifice 13 into chamber 16. Chamber '16 is provided with a quantity of inert gas which is supplied under pressure through inlet port 23. The inert gas is constrained to move laterally between orifice plate 12 and gas plate 14 and thus contacts the emerging jet 22 in a direction initially normal to the path of jet 22. This flow is in a large measure self-distributing toward symmetrical fiow. The inert gas then flows co-currently with jet 22 through the gas plate throat .15. It is thought that the reduction in the diameter of jet 22 is primarily due to the viscous drag interaction of inert gas with jet 22.

A suction is supplied to outlet port 24 creating a low pressure zone in the chamber 19 and thereby causing the inert gas to be stripped away from jet 22 as it enters chamber 19. Simultaneously, the stabilizing atmosphere which may be introduced into the system in cavity 21 flows into chamber 19 under the influence of the low pressure zone and contacts the jet for proper stabilization.

FIG. 2 illustrates the general geometrical relationship between plates 12, 14 and 17 and their respective orifices. The relative size of the orifices, however, is not to scale in order to promote clarity. Gas plate throat 15 has a diameter which is greater than that of orifice 13. The diameter of the gas plate throat is below 30 times and preferably below times the diameter of the orifice. When the gas plate throat is a straight bore orifice as in the case of gas plate throat 15, it is preferable that the suction plate throat be as large or larger in diameter but it may be utilized when as small as one-half the diameter of the gas plate throat. It should be recognized, however, that when the diameter of the gas plate throat is nearly that of the orifice diameter, it is necessary that the diameter of suction plate be larger. Suction plate throat 18 is illustrated as being larger than throat 15. Arrows 25 and 26 illustrate the respective paths of the inert gas and stabilization atmosphere.

As stated in application Ser. 'No. 838,593, gap 27 between orifice plate 12 and gas plate 14 should be less than fifteen times the diameter of orifice 13 and preferably less than one-half the diameter of gas plate throat 15. The length of gas plate throat is maintained at less than about one hundred times and preferably less than fifty times the orifice diameter. On the other hand, the dimensions of gap 28 between gas plate 14 and suction plate 17 is not considered to be critical although optimum extrusion conditions have been observed when gap 28 is maintained between about five and ten mils. A spacing outside this range tends to make it more difiicult to obtain the low pressure zone.

One geometrical relationship must always be observed. That is, the orifice and throats must be aligned substantially coaxial for optimum extrusion conditions. Nonalignment may cause the stream to deviate to one side of the gas plate and suction plate throats.

FIG. 3 illustrates another embodiment of an orifice spinning assembly in accordance with the present invention. In this embodiment, gas plate 32 has a gas plate throat 33 which takes the form of a tapered orifice with the walls thereof diverging away from extrusion orifice 31 of orifice plate 30. As before, suction plate 34 with suction plate throat 35 is spaced beneath gas plate 32. Orifice 31, throat 33, and throat 35 are essentially coaxial. The inert gas is supplied to the chamber defined by plates 30 and 32 while a suction is applied to the chamber defined by plates 32 and 34.

In the embodiment of FIG. 3, it is preferable that suction plate throat 35 have a diameter greater than the entrance diameter of gas plate throat 33 but less than the exit diameter. This particular construction appears to provide a more complete stripping of the inert gas from the extruded jet. The increased stripping is belived to be the result of a number of factors. The increasing crosssection of throat 33 results in a greater volume which the inert gas can occupy, thus causing the inert gas to become les dense about the jet. There is also a tendency for the moving stream of inert gas to follow the diverging contour of throat 33. Finally, the smaller diameter of throat 35 relative to the exit diameter of throat 33 physically blocks the passage of a portion of the inert gas following the jet.

EXAMPLE I The apparatus depicted in FIG. 1, without the suction plate 17 which was replaced by a spacer element, was placed in a resistance heated melt-spinning assembly. The crucible 10 was charged with an alloy comprising 61.8% wt. lead and 38.2 wt. percent tin. The extrusion orifice 13 had a diameter of 4 mils and an aspect ratio of 1.5. The inert gas plate throat 1 5 had a diameter of 15.9 mils and an aspect ratio of 1.33. The gap between plates 12 and 14 was 8 mils. Cavity 21 was open to the atmosphere.

The alloy was melted under the influence of a vacuum to a temperature of 300 C. and subsequent thereto the melt was extruded through the application of a pressure of 20 p.s.i.g. of argon. Subsequent to the initiation of the molten jet stream, helium was introduced into the inert gas zone 16 at the rate of 3,000 cc./min. The molten jet stream was observed under a strobe light and appeared to have a sinusoidal configuration along a portion thereof. The resultant filaments had a wavy appearance over their entire length which was noted to vary between one and two feet with an average filament diameter of 2 mils. While the jet stream had been attenuated it was impossible to produce a continuous length filament principally due to the Bernoulli distortions of the stream and the absence of sufficient stabilizing gas vicinal to the extrusion orifice to effect complete stabilization thereof.

EXAMPLE II The experiment of Example I was repeated after the suction plate 17 was placed in the apparatus of FIG. 1. The suction plate had a throat diameter of 62 mils with an aspect ratio of approximately 0.33. The gap between the plates 14 and 17 was 10 mils.

Subsequent to the initiation of streaming through the extrusion orifice 13, helium was introduced into the inert gas zone 16 at the rate of 3,000 cc./min. whereupon the jet stream responded as in Example I. Vacuum was then applied to port 24 and as the vacuum was gradually increased the stream was observed to be distortion free at which time shot was produced. A slight increase in suction flow resulted in the stripping of the helium gas from the jet surface with the simultaneous increase in the flow of the atmosphere upward into the low pressure zone whereupon a very stable stream resulted in the production of a smooth, continuous filament having a substantially uniform diameter of 2.1 mils.

EXAMPLE III The apparatus of FIG. 1 was adapted for utilization with an inductively heated melt spinning unit for the production of filaments from EC grade aluminum.

The extrusion orifice had a diameter of 12.0 mils with an aspect ratio of 4. The gas plate had a throat diameter of 40 mils with an aspect ratio of 1.2. The inert gas gap residing between the orifice plate 12 and the gas plate 14 was 15 mils.

The suction plate 17 had a throat diameter of 60 mils with an aspect ratio of 0.6. The suction gas gap residing between the plates 14 and 17 was 15.0 mils.

A charge of EC grade aluminum was placed in the melt crucible and was heated to a temperature of 800 C. under the influence of a vacuum. Subsequent to the attainment of the melt extrusion temperature, argon at a pressure of 6.0 p.s.i.g. was applied to the melt whereby streaming of the melt through the orifice and gas plate assembly was elfected into a stabilizing atmosphere of air.

Helium at a flow rate of 5,000 cc./ min. was introduced to the inert gas zone 16 through port 33. A high velocity helium flow in passing through the throats 15 and 18 cocurrent with the jet stream resulted in the sinuous breakup thereof which prevented the formation of continuous fiber. After observing the disruption of the stream, a vacuum was applied to the exhaust port 24 which resulted in exhausting a mixture of helium and air at a flow rate of approximately 6500 cc./min. whereupon the molten jet stream became steady and long lengths of aluminum filaments were produced having an average diameter of 9.0 mils.

It is apparent from the above examples that a substan tially high rate of inert gas flow is required to effect a reduction in the molten stream diameter along with the prevention of the ditfusion of the stabilizing gas medium upstream to the exit side of the extrusion orifice. This flow rate results in disrupting the molten jet stream such that it can only be stabilized under very closely controlled conditions. Rapid removing the inert gas, however, from about the vicinity of the jet stream to the extrusion orifice obviates the effects of this high rate of flow and simultaneously affords a simplified process for the production of substantially continuous filaments.

'While the drawings and discussions herein relate to certain preferred and simplified gas plate geometries, other arrangements may be employed. Further the combination of the orifice plate, gas plate, and suction plate may be assembled in a variety of ways. For example, the orifice plate, gas plate, and suction plate may be separate or integral members as desired. The materials which are utilized in the fabrication of the plates should be essentially inert, each to the other, under the conditions of the extrusion process. Moreover, the materials must necessarily be resistant to thermal shock and to withstand the inherent mechanical stresses in the extrusion process. For example, in the extrusion of metals such as copper and ferrous alloys, it may be preferable to use ceramic materials such as high density alumina, beryllia, and zirconia or the best resistant materials such as graphite. When extruding high temperature melts such as ceramics, graphite and molybdenum may be employed. In low temperature extrusion processes, stainless steel assemblies have been found to perform adequately. Other materials and combinations commensurate with the practice of the present invention may also be used.

From the foregoing, it should be evident that the objects as set forth have been obtained. Of paramount importance is the substantial elimination of the sinuous disturbances which often occur along a jet when fabricating fine diameter and/ or light fiber directly from an essentially inviscid melt.

Although the description has been limited to particular embodiments of the present invention, it is thought that modifications and variations would be obvious to one skilled in the art in light of the above teachings. It is understood, therefore, that changes may be made in the features of the present invention described herein which fall within the full intended scope of the invention as defined by the following claims.

We claim:

1. An orifice assembly for the formation of fibers and filaments by the extrusion of an essentially inviscid melt as a free molten filamentary stream comprising sequentially disposed (a) a first plate having a first orifice being substantially the size of the initial molten filament to be formed by the extrusion of said melt therethrough;

(b) a second plate having a second orifice;

(c) a third plate having a third orifice, said first and second plates defining a first substantially enclosed chamber, and said second and third plates defining a second substantially enclosed chamber wherein said second orifice connects with said first and second chambers, said orifices being positioned substantially coaxial with respect to each other, said first enclosed chamber having a gap distance therebetween in the vicinity of said first and second orifices of less than one-half the diameter of said second plate orifice, said second orifice having a length less than times the diameter of said first orifice and a diameter less than 30 times the diameter of said first orifice, said third orifice having a diameter equal to or greater than one-half the diameter of said second orifice;

(d) a means for supplying an inert gas to said first substantially enclosed chamber and into said second orifice; and

(e) a means for evacuating said second substantially enclosed chamber of the inert gas which has entered therein through said second orifice.

2. The orifice assembly of claim 1 in which said second orifice takes the form of a tapered orifice having Walls which diverge in the direction of said third orifice.

3. The orifice assembly of claim 2 in which the crosssectional diameter of said third orifice is larger than the entrance diameter of said second orifice but smaller than the exit diameter thereof.

4. The orifice assembly of claim 1 in which said second and third plates are spaced apart a distance of between 5 and 20 mils in the vicinity of said second and third orifices.

References Cited UNITED STATES PATENTS 2,273,105 2/1942 Heckert 18-8 QM X 2,818,461 12/1957 Gruber et a1 164-259 X 2,976,590 3/1961 Pond 16482 3,447,202 6/1969 Kato 18-8 QM J. SPENCER OVERHOLSER, Primary Examiner M. O. SUTTON, Assistant Examiner US. Cl. X.R.

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