Title of Invention

"APPARATUS AND METHOD FOR FABRICATING TUBE-SHAPED GLASS MONOLITH USING SOL-GEL PROCESS"

Abstract

APPARATUS FOR FILTERING VISCOUS MATERIAL, IN PARTICULAR THERMOPLASTIC SYNTHETIC PLASTIC MATERIAL The invention relates to an apparatus for filtering viscous material, in particular thermoplastic synthetic plastic material, comprises a screw (24), which presses the material to be cleaned from an upstream conduit (3) of a supply housing (2) towards a band—shaped screen (4). The screen (4) is abutted on its downstream side by a perforated plate (6) against the pressure of the supplied material. The band-shaped screen (4) can be displaced for replacement of a soiled band section by a fresh band section. Far facilitating this displacement a device is provided, which comprises a closure means (30) that is disposed on the upstream side of the screen (4), by which closure means the upstream conduit (3) for the material to be filtered can be closed at least substantially. On the upstream side of this closure means (30) a storage space (36) for material to be cleaned is connected to the upstream conduit (3). Within this storage space (36) a piston (38) is sealingly guided, which is recippocable by a drive means and closes in its projected position the storage space (36) against the upstream conduit (3).

Full Text

-1A- BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber forming glass rube fabricating method using a sol-gel process, and in particular, to a tube-shaped glass monolith fabricating method using a sol-gel process, which can direct a moist gel to unidirectionally dry and shrink. 2. Description of the Related Art In general, an optical fiber preform is fabricated by inside deposition such as MCVD (Modified Chemical Vapor Deposition) or outside deposition such as VAD (Vapor phase Axial Deposition) and OVD (Outside Vapor Deposition). Among them, MCVD is most generally used in fabrication of high-quality optical fibers. In the MCVD, a preform is fabricated using a high-purity glass tube by inside deposition and overcladding. The glass tube essentially used in the MCVD is more pure and more cost-competitive in a sol-gel process than that fabricated in a conventional technology. A glass tube is fabricated in a general sol-gel process as follows. A first sol is formed by dispersing fine fumed silica particles in water to prevent cracking. The first, gel is gelled and dried. The dried first gel becomes powder through grinding and classification, is thermally treated, and is re-dispersed in water. Thus, a second sol is formed. The second sol is gelled, dried, and sintered. To dry a tube-shaped moist gel in this glass tube fabrication process, the gel is first removed from a mold and dried for a long time at constant temperature and humidity. The mold for forming the tube-shaped gel is divided into an upper portion, a lower portion, an outer portion, and a support. The inner surfaces of the upper and lower portions are planar, and each of the inner surfaces has a hole for fixing the support. That is, the upper surface of the lower portion is planar. However, the conventional gel drying method is limited in molding a dried gel to a thickness of 15mm or above due to the difference between shrinkage rates of the inner and outer surfaces of the tube-shaped gel when it is dried. It is very difficult to mold the gel into a tube shape because the outer and inner surfaces of the moist gel are concurrently dried and thus get shrinkage stresses. In addition, the moist gel should be dried for a long time at constant temperature and humidity, thereby increasing fabrication cost and making it impossible to extend the gel lengthwise. The dried gel is vulnerable to cracking even at a slight impact, and should be at least about lm long to mold an optical fiber forming glass tube. However, the conventional tube-shaped glass forming mold, of which the lower portion has a planar upper surface, cannot relieve the stresses of the gel caused by uniformless shrinkage in the lower portion of the gel due to longitudinal load of the moist gel. Hence, the dried gel is highly vulnerable to cracking, cannot be further extended - 2 - lengthwise, and is unsuitable for an optical fiber forming glass tube. SUMMARY OF THE INVENTION An object of the present invention is to provide a tube-shaped glass monolith fabricating apparatus using a sol-gel process, which can fabricate a large crack-free glass tube for use in fabricating an optical fiber by minimizing stresses on a gel caused by the shrinkage of the gel during drying a moist gel. Another object of the present invention is to provide a tube-shaped glass monolith fabricating apparatus using a sol-gel process, which can extend a gel lengthwise by distributing and reducing a shrinking force imposed on a lower portion of a moist gel and the load of the gel. Still another object of the present invention is to provide a tube-shaped glass monolith fabricating method using a sol-gel process, which can fabricate a large glass tube by unidirectionally drying a tube-shaped moist gel. A further object of the present invention of the present invention is to provide a tube-shaped glass monolith fabricating method using a sol-gel process, which can prevent a gel from cracking during drying the gel by directing the gel to shrink from outside to inside. A yet another object of the present invention is to provide a tube-shaped glass monolith fabricating method using a sol-gel process, which can fabricate an optical fiber forming overcladding tube. - 3 - To achieve the above objects, there is provided a tube-shaped glass monolith fabricating apparatus using a sol-gel process. The tube-shaped glass monolith fabricating apparatus using a sol gel process including sol formation, gelation, and drying steps has an upper mold having cylindrical portions of different sizes and a first fixing hole in the center thereof. A lower mold has a second fixing hole in the center thereof, a first lower cylindrical portion inclined toward the center thereof to minimize stresses generated during the gelation or drying step, and a second lower cylindrical portion having a vacuum releasing hole to prevent vacuum-induced stresses during the drying step. An outer mold is disposed between the upper mold and the lower mold, for performing the sol formation, gelation, and drying steps therein. A rod-shaped central support is installed along the central longitudinal axis of the outer mold, for molding a gel into a tube after the drying step. According to another aspect of the present invention, there is provided a tube-shaped glass monolith fabricating method using a sol-gel process. In the tube-shaped glass monolith fabricating method using a tube-shaped glass monolith fabricating apparatus which has an upper mold including cylindrical portions of different sizes, a lower mold including a first lower cylindrical portion inclined toward the center thereof by a predetermined degree and a second lower cylindrical portion having a vacuum releasing hole, a cylindrical outer mold for forming a tube-shaped glass, and a rod-shaped central support, the lower mold, the outer mold, and the central support are assembled and a sol is filled in the outer mold. Then, an unmixable liquid is filled on the sol, and the upper mold is assembled to the outer mold. The sol is gelled in the outer mold, the upper mold is removed, a cap is opened to release vacuum, and the central support is removed. A small amount of - 4 - the unmixable liquid remains in a gel hole formed in the center of the gel and the vacuum releasing hole is filled with the cap. An upper portion of the gel hole is sealed with a sealing paper. Subsequently, the gel is dried for a predetermined time, the outer mold is removed from the gel, and the gel is dried until the gel completely shrinks. Finally, the sealing paper and the unmixable liquid are removed from the gel. Thus, a tube-shaped dried gel is formed. BRIEF DESCRIPTION OF THI^RAWINGS ^ The above objects and advantages of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the attached drawings in which: FIG. 1 is an exploded perspective view of a mold for forming a tube-shaped glass monolith in a sol-gel process according to a preferred embodiment of the present invention; FIG. 2 is a side sectional view of the mold assembled for fabricating a tube-shaped glass monolith in a sol-gel process, referred to for depicting a gelation step of a tube-shaped glass monolith fabricating method using a sol-gel process, according to the preferred embodiment of the present invention; FIG. 3 is a side sectional view of the mold, for depicting a drying step of the tube-shaped glass monolith fabricating method using a sol-gel process according to the preferred embodiment; and FIG. 4 is a plan view of the mold shown in FIG. 3. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS - 5 - Preferred embodiments of the present invention will be described in detail with reference to the attached drawings. It should be noted that like reference numerals denote the same components in the drawings and a detailed description of related known function and structure of the present invention will be avoided if it is deemed to obscure the subject matter of the present invention. FIG. 1 is an exploded perspective view of a mold for forming a tube-shaped glass monolith in a sol-gel process according to a preferred embodiment of the present invention, and FIG. 2 is a side sectional view of the mold assembled for fabrication of a tube-shaped glass monolith in a sol-gel process according to the preferred embodiment of the present invention. Referring to FIG. 1, a mold being a tube-shaped glass monolith fabricating apparatus using a sol-gel process is divided into an upper mold 10, a lower mold 26, .--- and th£ support 24 \^JS 26friave smooth inner surfaces in contact with fumed silica in a sol state and are formed of polystyrene, polypropylene, teflon, or steel in order to mold a thick-wall tube. The upper mold 10 is comprised of a first upper cylindrical portion 12 to be mounted on the outer mold 18 and a second upper cylindrical portion 16 to engage with the outer mold 18. That is, the first and second upper cylindrical portions 12 and 16 are integrally formed and have a first fixing hole 14 in the centers thereof, for inserting the central support 24. The outer mold 18 is cylindrical and has a hole 22 for retaining the fumed silica in the sol state during sol formation, gelation, and drying steps. The upper surface of the outer mold 18 is a first mounting surface 20 - 6 - for mounting the first upper cylindrical portion 12 of the upper mold 10 thereon. The central support 24 is rod-shaped to enable a gel to be formed into a tube and installed in the center of the outer mold 18. The lower mold 26 is comprised of a first lower cylindrical portion 28 to be inserted into the hole 22 of the outer mold 18, and a second lower cylindrical portion 32 for mounting the outer mold 18 thereon. That is, the first and second lower cylindrical portions 28 and 32 are integrally formed and have a second fixing hole 30 in the centers thereof, for inserting the central support 24. The upper surface of the first lower cylindrical portion 28 is an inclined surface 28a inclined toward the center of the lower mold 26 by about 5-45 ° to niinimize the downward shrinkage force of a gel 100 and pressure generated from the inside of the gel 100 to the outside thereof during a gelation or drying step. Here, the inclined surface 28a may be conical within about 5-45 °. A vacuum releasing hole 34 is formed into one side surface of the second lower cylindrical portion 32, communicating with the second fixing hole 30, so that vacuum-induced stresses are not generated inside a tube-shaped gel when the central support 24 is removed to dry a moist gel molded from a sol. The vacuum releasing hole 34 is generally closed by a cap 36. To fabricate a large glass tube for use in fabrication of an optical fiber in the mold as constituted above, a sol is formed by mixing fumed silica with deionized water. Then, as shown in FIG. 2, the vacuum releasing hole 34 of the second lower cylindrical portion 32 is filled with the cap 36, and the central support 24 is fixedly inserted into the second fixing hole 30 in the center of the lower mold 26. Then, the outer mold 18 is mounted on the second mounting surface 32a of the second lower cylindrical portion 32, and the sol is filled in the hole 22 of the outer mold 18 to a - 7 - predetermined height. An unmixable liquid 102 unmixable with water and having a lower specific gravity than water is filled on the sol to prevent the sol from contacting with outside air. Here, the unmixable liquid 102 is kerosine. The second upper cylindrical portion 16 of the upper mold 10 is fixedly inserted into the hole 22. Here, the first upper cylindrical portion 12 is mounted on the first mounting surface 20 of the outer mold 18 and, simultaneously, the central support 24 is fixedly inserted into the first fixing hole 14 in the center of the upper mold 10. Then, the sol is gelled in the outer mold 18 for about 24-72 hours. At this time, to accelerate the gelation, about 1% or less of an organic polymer is added to the sol, or the pH of the sol may be controlled. After the gelation, the upper mold 10 is removed, the vacuum releasing hole 34 in the second lower cylindrical portion 32 is opened by removing the cap 36, and the central support 24 is slowly retracted upward. At this time, air is introduced into the vacuum releasing hole 34 and the second fixing hole 30, that is, vacuum is released. The unmixable liquid 102 flows into the gel hole 106 and comes out through the vacuum releasing hole 34, as shown in FIG. 3. Then, the unmixable liquid 102 is continuously evacuated by a predetermined amount, and the vacuum releasing hole 34 is filled with the cap 36 when the unmixable liquid 102 remains in the gel hole 106 to a height of 10mm in order to prevent a lower portion of the gel 100 from contacting outside air. As shown in FIGs. 3 and 4, an upper portion of the gel hole 106 is sealed with a sealing paper 104 to prevent shrinkage inside the gel hole 106, direct only the outside shrinkage of the gel hole 106, and prevent the gel 100 from cracking. In addition, the sealing paper 104 is formed of wax paper, wrap film, or vinyl to be - 8 - large enough to cover the gel hole 106. When the outer surface of the moist gel 100 is detachable from the outer mold 18 after the gel 100 is dried for a predetermined time, the outer mold 18 is removed from the lower mold 26 and dried for about 3-7 days until the moist gel 100 is strong enough to resist cracking. Then, the sealing paper 104 is removed from the upper portion of the gel 100, and the remaining unmixable liquid 102 is /df/^ removed from the gel hole 106 by opening the vacuum releasingynole loSpfhen, the lower mold 26 is removed from the gel 100, thereby obtainin§~titecrack-free dried tube-shaped gel 100. Subsequently, the dried gel 100 is thermally treated at about 600-900 °C to be used for fabricating an optical fiber, and sintered at about 1300-1450°C. Thus, an optical fiber forming overcladding tube is completed. As described above, the tube-shaped glass monolith fabricating method using a sol-gel process directs the gel to shrink from outside to inside during the drying step and unidirectionally dries the gel, thereby minimizing stresses of the gel caused by the gel shrinkage, and enabling a large crack-free glass tube for forming an optical fiber to be fabricated. In addition, a high-purity optical fiber forming glass tube can be molded and product cost is reduced. This method can be applied to large, high-purity monolith fabrication. - 9 - WE CLAIM: 1. ft tube-shaped glass monolith fabricating apparatus using ,a sol-gel process which comprises sol formation, gelation, and drying steps, said apparatus comprising: an upper mold having cylindrical portions of different sizes and a first fixing hole in the center thereof; a lower mold having a second fixing hole in the center thereof, a first lower cylindrical portion having upper surface inclined toward the center thereof to minimize stresses generated during the gelation or drying step, and a second lower cylindrical portion having a vacuum releasing hole communicating with the second fixing hole to prevent vacuum-induced stresses during the drying step; an outer mold disposed between the upper mold and the lower mold, for performing the? sol formation, gelation, and drying steps therein; and a rod-shaped central support installed along the central longitudinal axis of the outer mold, and engaged with the first and the second fixing holes, for molding a gel into a tube after the drying step. - 10 - 2- The tube-shaped glass monolith fabricating apparatus using a sol-gel process as claimed in claim 1, wherein said upper ^surface of first lower cylindrical portion is inclined toward the penter thereof by almLrt 5-45°. 3. The tube-shaped glass monolith fabricating apparatus using a sol-gel process as claimed in claim 1, wherein the upper surface of said first lower cylindrical portion is conical. 4. The tube-shaped glass monolith fabricating apparatus using a sol-gel process as claimed in claim 3, wherein the cone shape is inclined toward the center thereby by ateea* 5-45°. 5. The tube-shaped glass monolith fabricating apparatus using a sol-gel process as claimed in claim 1, wherein the inner surface of the respective molds in contact with a sol are formed smooth to make the surface of a tube-shaped glass smooth. 6. The tube-shaped glass monolith fabricating apparatus using a sol-gel process as claimed in claim 1, wherein the upper mold, the outer mold, the lower mold, and the central support are formed of polystyrene. 7. The tube-shaped glass monolith fabricating apparatus using a sol-gel process as claimed in claim 1, wherein the upper mold, - il the outer mold, the lower mold, and the central support are formed of polypropylene. B. The tube-shaped glass monolith fabricating apparatus using a sol-gel process as claimed in claim 1, wherein the upper mold, the outer mold, the lower mold, and the central support' are formed of teflon. 9. The tube-shaped glass monolith fabricating apparatus using a sol-gel process as claimed in claim I, wherein the upper mold, the outer moid, the lower maid, and the central support are formed of steel. 10. A tube-shaped glass monolith fabricating method using a sol- gel process in a tube-shaped glass monolith fabricating apparatus which has an upper mold including cylindrical portions of different sizes,a lower mold including a first lower cylindrical portion inclined toward the center thereof by a predetermined degree and a second lower cylindrical portion having a vacuum releasing hole, a cylindrical outer mold -for forming a tube- shaped glass,and a rod-shaped central support, comprising the steps of : (1) assembling the lower mold, the outer mold, and the central support, and filling a sol in the outer mold; - 12 - (2J filling liquid unmixable with water on the sol, and assembling the upper mold to the outer mold; (4) removing the upper mold, opening a cap of a vacuum releasing hole to release vacuum, and removing the central support, thereby draining out said unmixable liquid through the hole ; (5) maintaining a small amount of the unmixable liquid in a gel hole formed in the center of the gel, and filling the vacuum releasing hole with the cap; (6) sealing an upper portion of the gel hole with a sealing paper; (7) drying the gel for a predetermined time, removing the outer mold from the gel, and drying the gel until the gel completely shrinks? and (8) removing the sealing paper and the unmixable liquid from the gel, thereby forming a tube-shaped dried gel. 11. The tube-shaped glass monolith fabricating method using a sol-gel process as claimed in claim 10, wherien said unmixable liquid has a. Jtower specific: gravity than water. 12. The tube-shaped glass monolith fabricating method using a sol-gel process as claimed in claim 11, wherein said unmixable liquid is keriisine. - 13 - 13. The tube-shaped glass monolith fabricating method using a sal-gel process as claimed in claim 10, wherein said gelling step is performed for about 24-72 hours. 14. The tube-shaped glass monolith fabricating method using a sol-gel process as claimed in claim 10, wherein 1%. or less of an organic polymer is added to the sol during said gelling step to accelerate gelation. 15. The tube-shaped gletss monolith fabricating method using a sol-gel process as claimed in claim 10, wherein said sealing paper is formed of wax paper. 16. The tube-shaped gl sol-gel process as claimed in claim 10, wherien said sealing paper is formed of wrap fiLlm. 17. The tube-shaped glass monolith fabricating method using a sol-gel process as claimed in claim 10, wherein said sealing paper is formed of vinyl. 18. The tube-shaped glass monolith fabricating method using a sol-gel process as claimed in claim 10, said drying eefp is performed for afcrcrot 3-7 days. 14 - 19- The tube-shaped glass monolith fabricating method using a sol-gel process as claimed in claim 10, wherein said dried gel obtained from said removing step is thermally treated and sintered at a predetermined temperature, and then becomes an optical fiber forming overcladding tube. 20. The tube-shaped glass monolith fabricating method using a sol-gel process as claimed in claim 19, wherein said dried gel {obtained from said removing step is thermally treated at /abauir j t t\g To ' ® °- - 15 - 600-900 C, and then sintered at 1300-1450 C, to thereby form an optical fiber farming overcladding tube. APPARATUS FOR FILTERING VISCOUS MATERIAL, IN PARTICULAR THERMOPLASTIC SYNTHETIC PLASTIC MATERIAL The invention relates to an apparatus for filtering viscous material, in particular thermoplastic synthetic plastic material, comprises a screw (24), which presses the material to be cleaned from an upstream conduit (3) of a supply housing (2) towards a band—shaped screen (4). The screen (4) is abutted on its downstream side by a perforated plate (6) against the pressure of the supplied material. The band-shaped screen (4) can be displaced for replacement of a soiled band section by a fresh band section. Far facilitating this displacement a device is provided, which comprises a closure means (30) that is disposed on the upstream side of the screen (4), by which closure means the upstream conduit (3) for the material to be filtered can be closed at least substantially. On the upstream side of this closure means (30) a storage space (36) for material to be cleaned is connected to the upstream conduit (3). Within this storage space (36) a piston (38) is sealingly guided, which is recippocable by a drive means and closes in its projected position the storage space (36) against the upstream conduit (3).

Documents:

00387-cal-1998-abstract.pdf

00387-cal-1998-claims.pdf

00387-cal-1998-correspondence.pdf

00387-cal-1998-description(complete).pdf

00387-cal-1998-drawings.pdf

00387-cal-1998-form-1.pdf

00387-cal-1998-form-2.pdf

00387-cal-1998-form-3.pdf

00387-cal-1998-letters patent.pdf

00387-cal-1998-p.a.pdf

00387-cal-1998-priority document others.pdf

00387-cal-1998-priority document.pdf

00387-cal-1998-reply f.e.r.pdf

387-cal-1998-granted-abstract.pdf

387-cal-1998-granted-claims.pdf

387-cal-1998-granted-description (complete).pdf

387-cal-1998-granted-drawings.pdf

387-cal-1998-granted-form 2.pdf

387-cal-1998-granted-specification.pdf

387-cal-1998-priority document.pdf

387-cal-1998-translated copy of priority document.pdf