IL43094A - Method of making filaments of small cross section from a molten material - Google Patents

Description

ηιο noma jep inn ^ so ΟΌΤΠ nann1? η»·» Method of making filaments of small cross section from a molten material Battelle Development Corporation Background of the Invention The present invention relates to a method of making filamentary material by the movement of a heat-extracting member in contact with the free surface of a pool-like source of molten material.

The present invention is an improvement over the method States cation a method of producing filamentary material is disclosed as the rotating of a disk-like heat-extracting member on the surface of a pool of molten material with such a disk-like member disposed to solidify and form a filamentary product on its contacting surface and spontaneously release such a product after its formation. This prior art also contemplates the use of multiple projections in contact with the melt surface to form a plurality of filaments.

The prior art does not contemplate the form of the disk-like projection being the extremity of a helical machine thread. The present invention, as will be hereinafter disclosed, reduces the effect of surface-borne impurities by continually moving the point of material extraction in relation to the molten material and also inducing a sweeping action across the melt surface that removes objectionable surface-borne impurities from the area of filamen formation and extraction.

The present invention is particularly suited to the production of filamentary material of small cross section at high rotational speeds of the member having the helical projections. At the higher speeds the filamentary material The object of the present invention is to provide a method of producing filamentary material, heretofore only available as a wrought product, directly from the molten material.

A second object of the present invention is to provide an inexpensive method of making filamentary material with very small cross-sectional area.

A third object of the invention is to produce controlle lengths of filament without the lengths of the filament being dependent on the diameter of the rotating heat-extracting member.

While filament of small cross section (as for example from 50 to 1500 microns effective diameter) has utility for a number of commercial processes, the present invention is one embodiment produces filament having an effective diameter in the range of from 15 microns to 400 microns directly from the molten material in commercially useful quantities per unit time. Such a product was heretofore unavailable without utilizing extensive post-forming size reductions or forming the product through very small orifices both of which are inherently complex and costly. The present invention inexpensively forms both metals and nonmetals that behave like metals near their melting point into a filamentary product. Such products inherently have many uses due to their large surface-to-volume ratio, as for example: a gaseous getter. Such fibers could be treated much like metal powders in metallurgical forming operations. The product in the desired length could also be used for the fiber reinforcement of bulk materials.

The present invention allows materials heretofore excluded from applications due to problems in the forming of filaments by prior art techniques to be included in materials Brief Summary of the Invention The present invention consists of a method of making filamentary materials by the introduction of helically arranged heat-extracting projections to the free surface of a pool of molten material. Such a molten material will be in most cases a metal, however, the invention is applicable to materials having properties in the molten state near their melting points similar to that of molten metals. Such properties are specifically set out in the previously cited prior art. The shape of the projection of the heat extracting in contact with the melt surface is the crux of the present invention. The cylindrical heat-extracting member has on its outer radial surface helical machine threads that when introduced to the surface of the molten material at relatively high speeds produce high quality filament having a small effective diameter. The present invention is limited to production of filaments no longer than the total helical length of one thread as goes from one end of the rotating member to the opposite end. In the production of very small fiber (less than 100 microns) the length of the fiber" is less than the total helical thread length and has been observed to range between .5 and 12 inches in the absence of specific process controls on the fiber length.

Brief Description of the Drawing Figure 1 is a cross-sectional view of the heat-extracting member at its point of contact with the melt surface.

Figure 2 is a frontal view of a heat-extracting Detailed Description of the Invention Figure 1 shows an embodiment of the present invention where a cylindrical heat-extracting member 30 is rotated while in contact with the surface 15 of a pool of molten material 10. The axis of rotation of the member 30 is parallel to the equilibrium surface 15 of the molten material 10. The member 30 is characterized in this embodiment by the presence of multiple projections from the surface in the form of machine threads 31. These threads are formed on the portion of the cylindrical member 10 at the curved surface a constant distance from the axis of rotation or more definitely the outer radial surface of the cylinder.

The fact that the projections 31 in contact with the surface 15 of the melt 10 are not simply circular concentric projections but have a helical pitch yields several advantages and allows the production of small diameter filaments in large quantities. The first advantage is that the pitch of the projection necessitates that the point of formation of the filament moves across the surface of the molten material. This minimizes the effect of temperature differences in the~"melt on the size of the final filamentary product since no one projection is continually forming filament from the same location on the melt surface. The second advantage is that the pitch of the projections tends to sweep impurities across the surface of the melt, and, therefore, if surface impurities are eliminated from one edge 12 of the member at the melt surface, the projections 31 will form filaments from a continually clean melt surface. The edge at which the impurities should be eliminated is that where the direction of rotation and direction of the itch induce material flow from the edge to the center of the member 30. In Figure 1 this would be the area shown as 12 assuming the member 30 is rotated in the direction of the arrow and the projections 31 are conventional right-handed threads. The third advantage is that where it is desired to make large quantities of filament from a plurality of projections into the melt a helical machine-screw thread is a particularly inexpensive embodiment to fabricate. Furthermore, repair or machine refinishing of the helical projections can be carried out as an inexpensive continuous process rather than treating a plurality of concentric projections individually The magnitude of the thread pitch does not seem to be critical to the operability of the process at ordinary ranges of pitch. However, the yield of filamentary product is maximized by the greatest number of threads per inch but at some point if the projections 31 are too close the formation of the filament at the melt surface would disturb the formation of the adjacent filament. This point definitely arises where the spacing between projections is less than twice the width of the filament produced. At the other extreme, if the pitch were extremely large it would be expected that the lateral force induced by the rotation used would disturb the formation of filaments as well as induce turbulence to the surface of the melt. While the subsequent range of pitch does not define the operable limits of the invention, good quality filament can be produced in a preferred embodiment when the magnitude of the pitch, as measured in threads per inch, is in the range of from 4 to 20.

It should be understood that while the invention is defined and described in terms of pitch, there are configurations that incline the projections 31 in relation to the direction of rotation of the member 30 without having the projections being in a helical configuration. Such configurations will be specifically disclosed in a subsequent portion of the specification but in any case they will be considered the functional equivalent of a helical machine thread. It should also be understood that in discussing process parameters in terms of pitch, we mean the distance between two adjacent thread roots (or two adjacent thread projections) lying on different portions of the same helical path defined by the thread roots (or projections) . Pitch is most easily quantized by referring to the threads per inch. Therefore when we refer to large pitch, it means few threads per inch and therefore small pitch means many threads per inch.

Since the most inexpensive way to produce projections on the outer radial surface of a cylinder where the projections are counted from a plane perpendicular to the axis of symmetry is by conventional machine threads, that embodiment of the invention is preferred.

The present invention is only operable where the projections 31 have a depth of insertion below the surface 15 of the melt less than the root depth of the helical thread.

The exact insertion depth would be very difficult to measure; however, from careful observation of the process, it is apparent that the projections merely contact the surface of the melt with the depth of insertion not believed to exceed 250 microns. It should be noted that under some conditions . the movement of the member 30 or system vibration, may induce some surface turbulence. The present invention may operate with the ro ections 31 above the e uilibrium surface 15 of the melt using the surface turbulence to make momentary material contact with the projections 31, and, therefore, produce short lengths of filament.

During operation of the present invention in the embodiment where the rotational speed of the threaded member is such that the threaded projections move at a linear velocity in excess of 30 ft/sec in relation to the melt, the central portion 13 of the member 30 has a higher rate of filament production than the edge portions of the member.

A preferred embodiment of the present invention would have the width of the rotating member 30 in excess of one inch.

The rotation of the member 30 at high speeds induces some air flow parallel to the direction of rotation adjacent to the projections 31. This air flow can create perturbations on the surface of the molten material which combine with the normal depth of insertion of the projections 31 to exceed the root depth of the thread and therefore momentarily disrupt the formation of filament. A means of deflecting this induced air flow at the point where the projections 31 rotate into contact with the surface of the melt will alleviate this problem when it arises.

The diameter of the member 30 has not been found to be critical to the process and the diameter of the member may be from 5 to 30 inches with no indication that the limits of the range define the only operable embodiments. Generally, however, where high linear velocities of the projections 31 at the melt surface are desired, the member 30 is not small since prohibitively high rotational speeds must be used.

A referred embodiment of the resent invention would have The present invention is inherently limited to the production of discontinuous filament. Theoretically the longest filament that can be produced would have a length equal to the total helical length of the projections 31 on the member. Such a fiber would normally be produced at relatively slow speeds (e.g., 3 to 30 ft/sec). During operation of the present invention at higher speeds (in excess of 30 ft/sec) the filament is generally produced in random lengths of from about .5 inch to 12 inches in length.

If it is desired to make controlled length filament, then indentations on the extremities of the projections will interrupt the filament formation yielding filament with a length equal to the distance between indentations. The exact shape of the indentation does not appear to be critical and indentations in the shape of a semicircle having depths in excess of the insertion depth have been used with success.

The shape of the projections is most conveniently triangular since conventional machining methods facilitate the formation of such a shape. However, the only requirements for the shape of the projection are: that they be formed at an inclination on the outer radial surface of the member 30, a discrete distance one from another; and they must present a narrow elongated shape on the surface of the melt. The specific curvature of that shape may be that formed by conventional machine thread forming techniques. It should be recognized that there are means of machining thread-like projections where the pitch of the thread does not control the number of threads per unit length. An embodiment of that type would have a plurality of arallel ro ections canted from the direction f tat n of the member. It would also be possible to combine the helical and parallel configurations by having a plurality of parallel projections formed in a helical pattern on the surface of the rotating member.

Figure 2 shows an additional embodiment of the invention where a heat-extracting member 30' has on its outer radial surface, in separate segments 16 and 17 left hand and right hand machine threads 31 „ If such a member 30' were in the same relationship to the melt as member 30 shown in Figure 1 with areas 16 and 17 in contact with the surface 15 of the melt 10 and rotating in the same direction as shown on member 30, then the lateral flow induced by the pitch of the projections 31 would sweep the melt surface 15 from the central portion 18 toward the edges of the member 30*.

This would prevent the movement of surface impurities at one edge across the entire region of filament formation where the member 30 has a single pitch direction.

The speed at which the member 30 is rotated, and, therefore, the linear velocity of the projections as they contact the surface of the melt has proved to be directly related to the size" of the filament to be produced and the pitch of the projections in contact with the melt. In general, the larger the filament to be produced the slower the rotational speed, the greater the insertion depth' and the more separation is required between the projections in contact with the surface of the melt0 For the production of filamentary material having an effective diameter less than 100 microns, it has been found that the rotational speed must yield a linear velocity of the ro ections at the melt surface in excess of 30 ft sec.

The present invention is limited in the size of the filamentary product that can be produced. The pitch of the projections necessitate minimal insertion below the equilibrium surface level of the melt to avoid gross turbulence and material flow perpendicular to the axis of rotation. The minimal insertion thereby limits the contact area of the projections to the melt surface and as a result only small filamentary products can be formed by the present invention. The maximum size is as hereinbefore disclosed is produced at slower rotational speeds with a practical upper limit on size being a filament having an effective .diameter of 1500 microns. The present invention is particularly applicable to the production of filaments having an effective diameter less than 100 microns.- By effective diameter it is meant that the non- circular filament has a cross-sectional area equal to a circular cross-sectional wire of that diameter independent of the width and thickness of the noncircular filament.

The present invention is operable with the area of the projection in contact with the molten material having a surface finish that is smoother than 50 micro inches (CLA) . The consistency of the filamentary product is improved as the surface finish becomes smoother and a surface finish smoother than 20 micro inches (CLA) is a preferred embodiment. This preferred surface finish can be produced on a copper member by application of 600 grit abrasive paper.

The present invention has been shewn to be operable in forming metal filaments as will be set out in subsequent examples, however, the invention is not limited solely to metals. The present invention should be operable with any material possessing properties, in the molten state at temperatures reasonably close to its melting point, similar to those of molten metals. U.S. Patent Application Serial No. 251,895 specifically sets out the properties used to determine material operability and such a criterion is applicable in full to the present invention.

Mode of Operation of the Invention The present invention was used to produce filamentary materials out of tin, aluminum, and cast iron. Three specific examples in conjunction with the teachings of the cited prior art are sufficient to enable one skilled in the art to carry out the present invention.

Example 1 A cylindrical aluminum heat-extracting member two inches in width and seven inches in diameter having 18 triangular machine threads per inch on its outer radial surface was rotated at a speed of 2000 rpm (61 ft/sec) .

This member was made to contact the free surface of a pool of molten tin at a temperature of 500°F. The projections on the member were in vertical-point contact with the surface and filamentary tin having an effective diameter of from 50 to 100 microns was produced from each of the projections on the member. Filamentary material having a distribution of lengths from .5 to 12 inches was produced.

Example 2 The same embodiment was used to determine the effect of the rotational speed keeping all other parameters identical to those in Example 1. At rotational speeds less than 500 rpm (15 ft/sec) the molten tin would not form individual filaments since the molten metal bridged between the filaments to form a s - t Example 3 A cylindrical copper member one inch wide and eight inches in diameter having 18 triangular threads per inch was rotated at a speed of 1000 rpm (35 ft/sec) in contact with the free surface of a pool of molten cast iron at from 2500 to 2600°F. Again there was minimal insertion of the projection below the equilibrium surface level of the molten material. Filamentary iron of an effective diameter of from 75 to 150 microns was produced.

The present invention has been disclosed as specific embodiments and in terms of observations and hypotheses on the operation of the present invention based on observation of the invention as it is practiced, however, the scope of the invention is not limited to the specific embodiments disclosed but solely by the appended claims.

Claims (12)

1. A method of making a plurality of filaments having an effective diameter less than 1500 microns comprising: contacting the free surface of a pool of molten material with the circumferential extremities of a plurality of machine threads on the outer radial surface of a cylindrical heat-extracting member rotating at a rate yielding a linear speed at said circumference in excess of 3 ft/sec.

2. The method of Claim 1 wherein the filaments produced have an effective diameter less than 100 microns including: contacting said free surface with .a plurality of triangular cross-section helical machine threads on the outer radial surface of said heat-extracting member rotating at a rate yielding a linear speed at said circumference in excess of 30 ft/sec.

3. The method of Claim 2 wherein said extremities consist essentially of metal and said molten material consists of a material selected from the group consisting of tin, aluminum, or ironc

4. The method of Claim 3 wherein said threads have a pitch of approximately 18 threads per inch and a surface finish smoother than 20 microinches (CLA') .

5. The method of Claim 1 wherein the width of said cylindrical heat-extracting member exceeds one inch.

6. The method of Claim 1 wherein said threads have a pitch of from 4 to 20 threads per inch'.

7. ; 7. The method of Claim 1 wherein said circumferential extremities have indentations thereon disposed to attenuate the length of the filament produced to equal the distance between said indentations.

8. The method of Claim 1 wherein circumferential air ' flow induced by rotation of said member is deflected away from said molten material at the point where said member rotates into contact with said material.

9. The method of Claim 1 wherein said cylindrical member has a diameter in the range of from 6 to 12 inches.

10. The method of Claim 1 wherein said machine threads have, on different portions of said member; opposite pitch directions with said member rotating in such a direction that the pitch of said threaded portions induces molten material flow in a, direction away from the central portion of the outer radial surface of said member.

11. A method of making a plurality of filaments, substantially as hereinbefore described with reference to the Examples and to the accompanying drawings.

12. Filaments whenever produced by the method according to any of Claims 1 to 11. For the Applicant Dr. Yitzhak Hess