Orifice Assembly For Extruding And Attenuating Essentially Inviscid Jets

Abstract

An improved orifice assembly is provided for extruding filaments from low-viscosity or essentially inviscid melts. By employing an orifice having a nozzle like configuration in the "gas plate" substantial increases in attenuation rates are realized. The orifice is characterized by a convergent entry section, a throat section and a divergent exit section.

Description

BACKGROUND OF THE INVENTION

This invention is concerned with an improvement in apparatus for carrying out an extrusion of molten materials of extremely low viscosity to form filamentary structures. More particularly, the invention is directed to an improved orifice assembly for forming filamentary structures from essentially inviscid melts.

Until quite recent, it was not possible to fabricate filaments and fibers from materials such as metals, metal alloys and ceramics by the method of melt extrusion. The limiting factor was that the melt viscosity of the various metals and ceramics is so low as to be practically negligible. In other words, the melts of metals and ceramics are essentially inviscid.

The problem presented by an inviscid melt when attempting to extrude it to form filaments is that the surface tension of the filamentary jet, as it issues from the shaping die, is so great in relation to its viscosity that the molten stream breaks up before sufficient heat can be transferred for conversion to the solid state

This intractable problem has now yielded to a unique solution as described in U.S. Pat. Nos. 3,216,076 and 3,658,979. In accordance therewith, the nascent molten jet, as it issues from the shaping die, is brought into contact with a gas capable of instant reaction with the jet surface. The result is the formation of a thin film which envelopes the jet surface. This thin film has been found to be capable of holding the jet stream together until sufficient heat can be transferred to effect solidification.

Substantial improvements in the actual practice of this promising method have been made since it was first introduced. Perhaps, most significant to date has been the improved orifice system as described in U.S. Pat. No. 3,613,158. Disclosed therein is an orifice assembly having two concentric plates disposed in a stacked relationship, one above the other. Each plate contains a centrally disposed orifice with the orifice of one plate being in co-axial alignment with that of the other. The orifice of the uppermost or first plate is of straight bore and serves as the melt shaping die or extrusion orifice. The orifice of the second plate is larger in diameter than that of the first and has a straight or a tapered bore. The second plate, referred to as the "gas plate" is provided with gas inlet ports and gas distribution means in the form of a gap space, which defines an essentially enclosed chamber between the opposing faces of the two plates.

In operation, a quantity of inert gas is supplied under pressure through the inlet port of the gas plate and contacts the jet as it emerges from the extrusion orifice in a direction perpendicular to the jet path. The inert gas is then caused to change direction and flow co-currently with the jet through the gas plate exit orifice and thence, into a reactive atmosphere.

Although this improved orifice apparatus provides many advantages, the most significant is that in its application filament attenuation is achieved. Since the nature of essentially inviscid materials precluded attenuation by the conventional practice of drawing, it did not appear that it could be accomplished. With this capability now provided, rates of productivity can be increased, filament diameter can be controlled, and larger extrusion orifices, which are easier to fabricate, may be utilized.

Since the advent of this ability to attenuate the jet during the course of extrusion, filament productivity rates have been raised to a level in the order of 1,300-1,400 feet per minute -- the maximum attainable with known and existing equipment. Yet there has been a desire for even higher rates of productivity in order that over-all process economics may be further improved. To accomplish this it must be made possible to achieve greater attenuation rates than can be attained presently.

Accordingly, it is a principal object of this invention to provide an improved orifice assembly by which greater attenuation rates than previously attainable can be achieved when attenuating an essentially inviscid molten jet prior to film stabilization.

It is a further object of the present invention to provide an improved orifice assembly wherein the inert gas employed to effect attenuation of an essentially inviscid molten jet can be applied at a higher pressure to effect increased attenuation than has been feasible heretofore.

It is a still further object of this invention to provide an orifice assembly wherein there is a more effective utilization of the energy contained in the inert attenuation gas than in previous systems.

SUMMARY OF THE INVENTION

The above objects are achieved by the provision of an orifice assembly comprised of two concentric plates stacked one above the other. Each plate contains a centrally disposed orifice in co-axial alignment one with the other. There is a gap space between the opposing faces of the two plates which defines a substantially enclosed chamber. Means are provided for supplying an inert gas under pressure to this chamber with the gas exiting through the orifice of the lower plate.

The first or upper-most plate contains the extrusion or filament forming orifice. The second or "gas plate" contains a shaped orifice having a nozzle configuration with a convergent entry section, an intermediate throat section and a divergent exit section. The included angle of divergence in the exit section is of particular criticality being in the range of from about 4.degree. to 12.degree., with from about 6.degree. to 8.degree. being preferred for the higher attenuation rates.

It has been found that the specialized geometry of the gas plate orifice is of paramount significance in the realization of the results attainable by this invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of this invention will become apparent in connection with the accompanying drawings in which:

FIG. 1 is a schematic vertical cross-section of a typical filament extrusion 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 an enlarged partial view of the gas plate orifice in the orifice assembly of FIG. 1.

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 a shaped orifice 15 which is aligned substantially co-axial with orifice 13. Plates 12 and 14 define an essentially enclosed chamber 16, which may be referred to as the inert gas zone. Inert gas under pressure is supplied to inert gas zone 16 through inert gas line 17. The nature of the inert gas is not critical as long as the gas is inert to the extruded materials, the orifice plate and other parts of the extrusion apparatus. Helium and argon have been successfully employed with helium being particularly suitable. Pedestal 18 supports the entire apparatus and also defines a cavity 19 for the stabilization of the molten jet 20. A gas reactive with molten jet 20 is supplied to cavity 19 through reactive gas line 21. The nature of the reactive gas is not critical so long as it is capable of forming a film about the surface of molten jet 20. In many instances oxidizing gases such as carbon monoxide and air have been successfully employed.

In operation, a positive pressure is supplied to molten material 11 by an external means (not shown). The jet 20 is thus caused to issue from the extrusion orifice 13 into chamber 16. Chamber 16 is provided with a quantity of inert gas which is supplied under pressure through inert gas line 17. The inert gas is constrained to move laterally between orifice plate 12 and gas plate 14 and thus contacts the emerging jet 20 in a direction initially normal to its path. This flow is in a large measure self-distributing toward symmetrical flow. The inert gas then flows co-currently with jet 20 through gas plate orifice 15 and into cavity 19.

Cavity 19 is provided with a quantity of gas reactive with jet 20 via reactive gas line 21. The reactive film-stabilizing gas contacts jet 20 at the exit of shaped orifice 15 and is at a flow rate sufficient to penetrate the shroud of inert gas which has been caused to envelope the jet as it issues from gas plate orifice 15.

FIG. 2 illustrates the general geometrical relationship between plates 12 and 14 together with their respective orifices. Although the diameter of the throat section (most narrow section) of gas plate orifice 15 may be larger than the exit diameter of extrusion orifice 13, best results are obtained when it is of an equal or lesser diameter than that of the exit of orifice 13. Particularly good results may be obtained when the ratio of the exit diameter of orifice 13 to the throat diameter of orifice 15 lies in the range of from about 1.1 : 1.0 to 1.5 : 1.0. The length of orifice 15 is generally maintained at from about five to 100 times greater than the exit diameter of orifice 13.

The vertical distance of gap space 22 between orifice plate 12 and gas plate 14 should be maintained at less than five times the diameter of the throat in gas plate orifice 15. Arrows 23 and 24 illustrate the respective paths of the inert gas and the reactive stabilization gas.

FIG. 3 illustrates gas plate 14 and its shaped orifice 15 schematically in an enlarged vertical section. The entry area or convergent section 25 is rounded gently to reduce friction. The extent of convergence is not critical, it being merely necessary that the orifice walls converge in some degree at the entry. The convergence terminates at throat section 26 from where the walls diverge to form divergent exit section 27. The included angle of divergence in this section should be between 4.degree. and 12.degree., with from 6.degree. to 8.degree. being of preference for attenuation at the higher speeds. Although not critical for operation, best results are achieved when divergent section 27 is of greater length than convergent section 25, and particularly when the length is from 10 to 20 times greater. Arrows 23 and 24 illustrate the respective flow paths of the inert and reactive stabilization gases.

It has been found that when employing the orifice assembly of this invention filament production rates can be increased in the order of three fold or better over that obtainable by previous systems. This is due for the most part to the unique orifice configuration in the gas plate which permits much greater attenuations of the extruded jet than was possible heretofore. Greater attenuations are realized by virtue of the fact that the inert, attenuating gas can be supplied to the gas plate orifice at substantially higher pressure to give higher pressure drops through the orifice without causing disruption of the molten jet stream. Moreover, the orifice shape provides a more effective utilization of the energy contained in the attenuation gas.

The materials which are utilized in fabricating the plates which comprise the orifice assembly of this invention should be essentially inert, each to the other, under the conditions employed during extrusion. Moreover, the materials must be resistant to thermal shock and have sufficient strength to withstand the substantial mechanical stresses imposed by 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. For high temperature extrusion using ceramic charges, materials such as molybdenum and graphite can be employed. For extrusion processes involving lower temperatures, stainless steel assemblies have been found to perform well.

Claims

I claim:

1. An improved orifice assembly for use in the formation of fibers and filaments by extruding an essentially inviscid melt as a filamentary stream, which comprises in combination:

a. a first plate;

b. a second plate, said second plate being spaced beneath said first plate in a stacked relationship therewith;

c. a first orifice, said first orifice being centrally disposed in said first plate;

d. a second orifice, said second orifice being centrally disposed in said second plate in co-axial alignment with said first orifice, said second orifice having a nozzle configuration with a convergent entry section, an intermediate throat section and a divergent exit section, with the exit section having an included angle of divergence of between 4.degree. to 12.degree.;

e. a substantially enclosed chamber, said chamber being defined by a gap space between the opposing faces of said first and second plates, said gap space having a vertical distance of less than five times the diameter of the throat section of said second plate orifice;

f. a means for supplying an inert gas under pressure to said substantially enclosed chamber and into said second orifice.

2. The orifice assembly of claim 1, wherein the included angle of divergence in the divergent exit section of said second orifice is in the range of from about 6.degree. to 8.degree..

3. The orifice assembly of claim 1, wherein the diameter of the throat section of said second orifice is substantially equal to the exit diameter of said first orifice.

4. The orifice assembly of claim 1, wherein the exit diameter of said first orifice is larger than the diameter of the throat section of said second orifice.

5. The orifice assembly of claim 1, wherein the ratio of the exit diameter of said first orifice to the diameter of the throat section of said second orifice is from about 1.1 : 1.0 to 1.5 : 1.0.

6. The orifice assembly of claim 1, wherein the length of said second orifice is from about five to 100 times greater than the exit diameter of said first orifice.