US3788786A - Orifice assembly for extruding low-viscosity melts - Google Patents

Abstract

An improved orifice assembly is provided for extruding filaments from low-viscosity or essentially inviscid melts. An orifice system typified by an extrusion orifice containing plate and a gas plate containing an orifice for effecting filament attenuation is improved by introducing a third orifice containing plate. The third orifice containing plate functions to reduce or eliminate disturbances from developing in the molten jet stream. The third orifice is characterized by having walls which converge towards the orifice exit.

Description

United States Patent [191 Dobo [ ORIFICE ASSEMBLY FOR EXTRUDING LOW-VISCOSITY MELTS Emerick J. Dobo, Cary, NC.

[73]. Assignee: Monsanto Company, St. Louis, Mo.

[22] Filed: Aug. 30, 1972 [21] Appl. No.: 284,790

[75] Inventor:

[52] U.S. Cl 425/72, 164/82, 164/259 [51] Int. Cl B22d 11/00, B29f 3/00 [58] Field of Search 425/72; 164/66, 82, 259; 264/176 [56] References Cited UNITED STATES PATENTS 3,583,027 6/1971 Garrett et al. 425/72 3,613,158 10/1971 Mottem et a1 164/82 X 3,645,657 2/1972 Otstot et al. 425/72 FOREIGN PATENTS OR APPLICATIONS Switzerland 425/72 Jan. 29, 1974 Primary Examiner-R. Spencer Annear Attorney, Agent, or Firm-Russell E. Weinkauf et a1.

[5 7] ABSTRACT An improved orifice assembly is provided for extruding filaments from low-viscosity or essentially inviscid melts.

An orifice system typified by an extrusion orifice containing plate and a gas plate containing an orifice for effecting filament attenuation is improved by introducing a third orifice containing plate. The third orifice containing plate functions to reduce or eliminate disturbances from developing in the molten jet stream. The third orifice is characterized by having walls which converge towards the orifice exit.

3 Claims, 2 Drawing Figures ORIFICE ASSEMBLY FOR EXTRUDING LOW-VISCOSITY MELTS 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 practi' cally 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. Pats. No. 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. This patent, which is incorporated herein by way of reference discloses an orifice assembly having two concentric plates disposed in a stacked relationship, one above theother. 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 upper-most or first plate is generally of straight bore and serves as the melt shaping die or extrusion orifice. The orifice of the second plate is larger in diameter than the extrusion orifice, but not more than 30 times larger. The length of the second plate orifice is not more than 100 times greater than the diameter of the extrusion orifice.

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 facesof the two plates. The gap distance between the two plates in the vicinity of their co-axially aligned orifices is generally less than one-half the diameter of the second plate orifice.

During 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 an atmosphere of reactive gas wherein the stabilizing film is formed about the jet surface.

One of the many advantages imparted by the aforedescribed development is that it eliminates fouling of the extrusion orifice. Previously, the reactive stabilizing gas tended to diffuse into the area of the melt shaping orifice where contact with the emerging stream at the orifice interface resulted in the formation of deposits which caused erratic stream motion. The presence of the inert gas prevents the reactive stabilizing gas from contacting this area. Although this and other advantages are extremely important, it is the provision of a filament attenuation capability which the technique provides which is most significant.

Attenuation of essentially inviscid molten jets cannot be accomplished by stretching or drawing as is the case when viscous high polymers are melt spun to form fibers. It can be achieved by employing the gas plate apparatus and method as above-described. It is thought that the realizable attenuation or reduction in the diameter of the jet is due for the most part to the pressure differential above and below the gas plate orifice and to the viscous drag interaction of the inert gas with the jet. The degree of attenuation can be controlled by such variables as the flow rate of the inert gas entering the system and the geometry of the inert gas chamber as defined by the spacing between the extrusion orifice plate and the gas plate beneath. This ability to attenuate the jet during the course of extrusion adds a new dimension to the capability of forming filaments from essentially inviscid melts. For example, productivity rates can be substantially increased, filament diameter can be controlled, and larger extrusion orifices, which are easier to fabricate, may be utilized.

While the realizable benefits of the method and apparatus as set forth in U. S. Pat. No. 3,613,158 are clearly evident, under certain conditions of practice disturbances of the jet have been noted which cause an undesired sinuous effect on the stream. This has been observed particularly when employing conditions required for the attenuation of the jet diameter to less than half of the orifice diameter. Such large attenuations require a high co-current flow'rate of the inert gas which frequently results in the continuity of the jet being disrupted to the point that only short fiber or staple is formed rather than the desired continuous filament. When continuous filament does form, it is found to be sinuous in appearance and weakened by the stresses of solidification.

This disturbance of the jet stream has been found to be due largely to the high relative velocity differences between the inert gas and the jet. Often the co-current velocity of the inert gas exceeds that of the jet by a factor of ten to twenty times. The interaction of the inert gas and the inherent minor lateral deviation (bends in the jet length) of the jet gives rise to the well-known Bernoulli phenomenon. Thus, the low pressure regions which form adjacent to the bends in the jet tend to increase the amplitude of these bends and irregularities.

This problem of disturbances developing in the jet stream when attenuating the same with a co-current gas flow has been recognized and dealt with in the U. S. Pat. No. 3,613,158. The solution provided therein is to strip the co-currently flowing inert gas from the molten stream a short distance downstream from the initial point of impingement against the jet. This is accomplished by applying suction to an enclosed chamber positioned beneath the chamber into which the inert gas is supplied.

The technique, as described in U. S. Pat. No. 3,613,158 has been found to be relatively effective, but it is not without disadvantages. One is that the pumping means necessary for creating the required suction adds to the complexity and cost of the operational equipment. Moreover, it has been found that effectiveness becomes greatly impaired when operating at high production rates, i.e., in excess of 1,500 feet per minute of filament production.

Accordingly, it is an object of this invention to provide a means for preventing disturbances from developing in the stream when attenuating an essentially inviscid molten jet prior to film stabilization. It is a further object of the invention to provide a simplified, economical means for preventing undesired sinuous effects when attenuating an essentially inviscid molten jet with a co-current flow of inert gas.

It is a still further object of the invention to provide a means for preventing stream disturbances in an essentially inviscid molten jet when attenuating the same at high speeds prior to film stabilization.

SUMMARY OF THE INVENTION The above objects are achieved by introducing an additional orifice containing plate structure into the orifice assembly referred to hereinabove and described in U. S. Pat. No. 3,645,657. The added orifice plate is positioned immediately below the gas plate in a manner such that an enclosed chamber is formed by virtue of a space gap between the opposing faces of the two plates. Means are provided for supplying a reactive gas to this enclosed chamber and into the orifice of the newly introduced plate. The orifice of the added plate is coaxially aligned with the orifice of the gas plate aboveit which in turn is in coaxial alignment with the uppermost extrusion plate orifice.

Considered from a gross structural standpoint, the improved orifice assembly of this invention may be viewed as constituting three concentric plates disposed in a stacked relationship one above the other. The uppermost or first plate defines a first or extrusion orifice, the second or gas plate defines a second or attenuation orifice and the. third or lower most plate defines a third or stream control orifice-Each of the orifices and their throats are in coaxial alignment with the other, the axis of alignment being normal to the transverse planes of the first, second and third plates. Additionally, the first and second plate define a first enclosed chamber by spacing between them. A second enclosed chamber is defined by plates two and three in similar manner. Means are provided for supplying an inert gas to the first chamber and a gas reactive with the filamentary stream to the second enclosed chamber.

In achieving the realizable results of this invention, it has been found that a specialized geometry of the orifice in the newly introduced third plate is of critical importance. That is, the orifice must take the form of a frusto-conical section in which the orifice walls converge away from the entrance and towards the orifice exit. It is of further critical significance that the included angle of convergence between the walls be in the range of from about 7 to 20, and preferably of from 10 to BRIEF DESCRIPTION OF THE DRAWINGS Other features and advantages of this invention will become apparent as the description progresses in connection with the accompanying drawings in which:

FIG. 1 is a vertical cross-section ofa 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.

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 definean essentially enclosed cham her, which can be referred to as'inert gas zone 16.

Beneath gas plate 14 is a third plate 17 hereinafter called a stream control plate. Stream control plate 17 has an orifice or throat 18 which is aligned substantially coaxial with throat 15 (and consequently with orifice 13). The walls of orifice 18 converge in the direction of its exit with the included angle of convergence being between 7 and 20 degrees. Stream control plate 17 and gas plate 14 define a second substantially enclosed chamber 19, which can be referred to as a reactive gas zone.

Pedestal 20 supports the entire apparatus and also defines cavity 21 wherein the molten jet further reacts with a film forming gas.

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 inert gas line 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 selfdistributing toward symmetrical flow. The inert gas then flows co-currently with jet 22 through the gas plate throat l5 and into chamber 19.

Chamber 19 is provided with a quantity of gas reactive with jet 22 via gasline 24. The reactive filmstabilizing gas contacts jet 22 at the entrance of orifice l8 and is at a flow rate' sufficient to penetrate the shroud of inert gas which has been caused to envelope the jet as it issued from gas plate orifice 15. A further quantity of reactive gas is supplied by gas line 25 into cavity 21 for contact with jet 22 proximate to the exit of orifice 18.

FIG. 2 illustrates the general geometrical relationship between plates 12, 14 and 17 together with their respective orifices. Although the relative size of theorifices can vary over a wide range, gas plate throat 15 has a diameter which is larger than that of orifice 13. However, it is less than 30 times greater and preferably less than 10 times greater than the diameter of orifice 13. The length of gas plate throat 15 is maintained at less than about one hundred times and preferably less than 50 times the diameter of extrusion orifice 13. It is generally desirable although not critical that the entrance diameter of orifice 18 be from about one to four times larger than the throat diameterof gas plate orifice l5.

As pointed out in U. S. Pat. No. 3,645,657, the gap distance of gap 26 between orifice plate 12 and gas plate 14 should be less than one-half the diameter of gas plate throat 15. On the other hand, the dimensions of gap 27 between gas plate 14 and stream control plate 17 is not considered to be critical. However, enough space should be provided to accommodate a sufficient quantity of reactive gas to penetrate the inert gas which flows co-currently with jet stream 22 Generally, it has been found that a gap distance of from about 5 to mils between gas plate-14 and stream control plate 17 in the vicinity of their respective orifices is satisfactory.

The following example illustrates the results achievable through the use of the improved orifice assembly of this invention.

EXAMPLE An apparatus such as depicted in FIG. 1 was employed to form filaments by extruding the melt of steel alloyed with 1.0 percent by weight of aluminum at an extrusion rate up to 2,500 feet per minute.

The orifice assembly used was of a design as typified by FIG. 2 of the drawings. Orifice 18 in stream control plate 17 had an exit diameter which was approximately four times the throat diameter of orifice 15. The included angle of convergence between the walls of orifice 18 was 15.

A positive pressure was used to force the melt through the orifice of the extrusion plate and into the gap space between this plate and the underlying gas plate. The gap was supplied with pressurized helium gas which contacted the filamentary jet at an angle normal to its path of movement and then flowed co-currently with the jet through the orifice of thegas plate. Contact was then made with a film-forming gas (carbon monoxide) in the gap space between the gas plate and the stream control plate beneath the gas plate. The jet then passed through the stream control orifice (shown as orifice 18 in FIG. 1 and FIG. 2 of the drawings) with additional carbon monoxide gas being supplied to the area of exit of this orifice.

During the course of this high speed extrusion, the molten jet remained continuous and did not deviate from a straight path.

In order to ascertain the effect of the converging angle made by the walls of orifice in the stream control plate, a series of experiments were made employing a laboratory mockup of the extrusion apparatus as described herein. The plates of the orifice assembly were fabricated from a transparent plexiglass to permit photographs of the extruded jet as it passed through the assembly.

The influence of the converging angle on stream behavior was studied by photographing a stream of mercury while undergoing extrusion through the laboratory apparatus. It was found that when the included angle of convergence lied outside the range of from about 7 to 20 the jet exhibited either severe deviation or breakup. Best results were obtained when the included angle was 15.

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.

The invention is hereby claimed as follows:

1. In an orifice assembly for extruding a filamentary jet from an essentially inviscid melt to form fibers and filaments wherein the orifice assembly is of the type characterized by a first plate containing an extrusion orifice and a second plate spaced beneath said first plate and having an orifice coaxial with said extrusion orifice, said second plate orifice having a throat length of less than times the exit diameter of said first plate extrusion orifice and a throat diameter less than 30 times larger than the exit diameter of said first plate extrusion orifice, and wherein said first and second plates define an enclosed chamber by a gap distance therebetween which in the area of their respective orifices is less than one-half the diameter of said second plate orifice, said orifice assembly being further characterized by means for supplying an inert gas to said enclosed chamber and into said second plate orifice, the improvement which comprises: a third plate orifice having walls which converge towards the orifice exit such that the included angle of convergence is from about 7 to 20; means for supplying a gas reactive with said filamentary jet to said second enclosed chamber and into said third plate orifice.

2. 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 their respective orifices.

3. The orifice assembly of claim 1 wherein the walls of the third plate orifice converge toward the orifice exit to form an included angle of convergence of from 10 to 15.

Claims (3)

1. In an orifice assembly for extruding a filamentary jet from an essentially inviscid melt to form fibers and filaments wherein the orifice assembly is of the type characterized by a first plate containing an extrusion orifice and a second plate spaced beneath said first plate and having an orifice coaxial with said extrusion orifice, said second plate orifice having a throat length of less than 100 times the exit diameter of said first plate extrusion orifice and a throat diameter less than 30 times larger than the exit diameter of said first plate extrusion orifice, and wherein said first and second plates define an enclosed chamber by a gap distance therebetween which in the area of their respective orifices is less than one-half the diameter of said second plate orifice, said orifice assembly being further characterized by means for supplying an inert gas to said enclosed chamber and into said second plate orifice, the improvement which comprises: a third plate orifice having walls which converge towards the orifice exit such that the included angle of convergence is from about 7* to 20*; means for supplying a gas reactive with said filamentary jet to said second enclosed chamber and into said third plate orifice.

2. 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 their respective orifices.

3. The orifice assembly of claim 1 wherein the walls of the third plate orifice converge toward the orifice exit to form an included angle of convergence of from 10* to 15* .

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