Patent Publication Number: US-2003235373-A1

Title: Multi-core ferrule and metallic die assembly for making the same

Description:
TECHNICAL FIELD  
       [0001] The present invention relates to a multi-core ferrule for mounting two or more optical fiber cores and a metallic die assembly for manufacturing such a multi-core ferrule by injection molding, and in particular to a multi-core ferrule suitable for use in optical semiconductor modules and for butt joining a pair of optical fibers and a metallic die assembly for manufacturing such a multi-core ferrule.  
       BACKGROUND OF THE INVENTION  
       [0002] A ferrule of this type, be it a single core type or a multiple core type, requires a high level of dimensional precision. In particular, the front end of the ferrule is required to have a highly precise outer diameter and a high level of coaxiality with respect to the axial center thereof. For this reason, zirconia is preferred as the material for such a ferrule. When making a ferrule out of zirconia, typically, a mixture of zirconia powder and plastic material is injection molded or press molded, and the molded product is sintered and finished by using abrasive material such as diamond or the like.  
       [0003] A zirconia ferrule is generally satisfactory in terms of performance such as dimensional precision, and can adequately meet the various requirements, but is not suitable for mass production and cost reduction because of the difficulty in manufacturing. Because a demand for ferrules is expected to rise sharply in the future, there is a growing desire to develop plastic ferrules so that they can be made by injection molding thermoplastic plastic material in a mass production process in an economical manner.  
       [0004] A plastic ferrule is suitable for mass production and cost reduction as compared to a zirconia ferrule because of ease in manufacturing, but is known to have problems in achieving a required level of performance such as dimensional precision because of the thermal shrinkage of the plastic material following the injection molding process which impairs the dimensional precision and coaxiality of the front end of the ferrule. Also, the core pin for forming the bore for receiving a fiber core is so thin that it is prone to breaking, bending or other damages due to the injection pressure of the plastic material.  
       [0005] To improve the dimensional precision which is the problem with such a plastic ferrule, Japanese patent laid open publication (kokai) No. 2000-84974 and Japanese patent laid open publication (kokai) No. 2001-96570 disclose a ferrule having a metallic insert pipe disposed around a front end thereof and extending at least up to a flange portion provided in an intermediate part thereof, the front end having a core wire receiving bore for receiving an extremely fine optical fiber core in an axial center thereof, and a method for making such a ferrule.  
       [0006] According to this prior art, the plastic ferrule is produced by placing an insert pipe having communication holes for passing plastic material therethrough in a part corresponding to a flange in a metallic die assembly, placing a core pin for forming a bore for receiving a core wire and a sheath at an end of an optical fiber in an axial center thereof, and injecting plastic material from a gate provided adjacent to the communication holes so that the plastic material is filled into the insert pipe from the communication holes and an entire ferrule including the flange is integrally molded by injection molding.  
       [0007] According to this plastic ferrule, the outer circumferential surface of the front part is given with a high level of circularity and coaxiality as well as a high level of outer dimensional precision owing to the use of an insert pipe in the injection molding process. Also, a precise alignment is achieved when connecting the ferrule to an opposing ferrule via a sleeve fitted onto the outer circumferential surface thereof, and a high precision in the positioning of an optical fiber core wire can be achieved when fitting the optical fiber core wire into the core wire receiving bore in the axial center. This contributes to the reduction in the transmission loss.  
       [0008] Various multi-core ferrules and methods for producing such ferrules have been proposed as can be found in Japanese patent No. 3,005,754 and Japanese patent laid open publication Nos.  2001-96570  and  2002-14254.    
       [0009] Optical communication is being extended to home uses, and an increase in the amount of data transmission and a more wide-spread use of two-way communication are expected to occur in the immediate future. This will trigger an increase in the demand for multi-core and more compact ferrules, and this will in turn require an increased level of precision in the manufacturing technology while enabling mass production and cost reduction at the same time. Use of zirconia for the ferrule according to the prior art prevents such mass production and cost reduction as it requires a highly complex manufacturing process.  
       [0010] For instance, if the prior art disclosed in Japanese patent laid open publications (kokai) Nos. 2000-84974 and 2001-96570 is directly applied to multi-core ferrules, because a plurality of core receiving bores for receiving a corresponding number of optical fiber cores are required to be arranged in parallel with each other in a mutually spaced relationship, there is a limit to compact design, and pursuing further development in multi-core and compact design may be prevented.  
       [0011] Because a plurality of core pins are required to be arranged in parallel relationship for forming a plurality of core receiving bores by the molding process, the core pins are more prone to breakage and bending than is the case with a single core pin. Also, when the core pins are not parallel to each other to a required precision, not only the dimensional precision is impaired but also the performance of the products varies from one unit to another. When a rigorous quality control is applied, the production efficiency will be impaired.  
       [0012] A SC type ferrule comprises a core receiving bore for receiving an optical fiber core in an axial center of a part ahead of a flange portion and an optical fiber sheath receiving bore in an axial center of a part behind the flange portion. When a SC type ferrule having such a structure is injection molded as a single piece, the molding cavity increases in size and the length of the path of the flow of the plastic molding material increases, and various problems in terms of cost and performance are produced as a result.  
       [0013] As the part ahead of the flange portion requires a high dimensional precision, it is preferably made of liquid crystal polymer which is relatively more expensive than other plastic materials. As the part behind the flange portion does not require so high a dimensional precision, it may be made of relatively inexpensive common plastic material. Therefore, making the entire ferrule in one piece out of the expensive liquid crystal polymer unnecessarily increases the manufacturing cost.  
       [0014] Increasing the length of the passage for the molding plastic material in the die assembly increases an unevenness of temperature in the molding plastic material and die assembly, and this in turn causes an unevenness in the fluidicity and thermal shrinkage of the plastic material, thereby impairing dimensional precision. In particular, the front end of the ferrule which corresponds to the front end of the flow of the molding plastic material contains a relatively large amount of slag, and this could cause flow marks and sink marks in this part, thereby causing a distortion of the inner circumferential surface of the core receiving bore. In particular, the coaxiality of the core receiving bore with respect to the insert pipe defining the outer diameter tends to be impaired.  
       [0015] In the case of the prior art disclosed in Japanese patent No. 3,005,754 and Japanese patent laid open publications Nos. 2001-356240 and 2002-14254, because a pair of core receiving bores for receiving a corresponding number of optical fiber cores are arranged in parallel with each other in a mutually spaced relationship, there is a limit to compact design, and pursuing further development in multi-core and compact design may be prevented.  
       [0016] Furthermore, according to the prior art disclosed in Japanese patent No. 3,005,754 and Japanese patent laid open publication No.  2001-356240 , the ferrule is made either by forming a cylindrical sleeve by metallic injection molding process using ternary alloy, and press fitting a capillary formed by sintering zirconia powder into the cylindrical sleeve or by forming the entire ferrule by metallic injection molding process using ternary alloy. Therefore, a substantial amount of time and cost becomes necessary in the work of sintering, removing oil and grinding following the molding process, and the resulting ferrule is therefore substantially more expensive than a plastic ferrule.  
       [0017] According to the ferrule disclosed in Japanese patent laid open publication No. 2002-14254, because a molded plastic ferrule is fitted into a metallic sleeve, there are a number of problems that need to be addressed such as the breakage and bending of the core pin when forming the core receiving bore of the capillary by plastic molding, loss of dimensional precision owing to thermal shrinkage and difficulty in achieving the required dimensional precision and bonding strength in installing the separately prepared capillary into the metallic sleeve.  
       BRIEF SUMMARY OF THE INVENTION  
       [0018] In view of such problems of the prior art, a primary object of the present invention is to provide a multi-core ferrule free from such problems of the prior art.  
       [0019] A second object of the present invention is to provide a multi-core ferrule which is suitable for compact design.  
       [0020] A third object of the present invention is to provide a multi-core ferrule which is easy and economical to manufacture.  
       [0021] A fourth object of the present invention is to provide a multi-core ferrule which can be manufactured at a high precision.  
       [0022] A fifth object of the present invention is to provide a metallic die assembly for injection molding such a multi-core ferrule.  
       [0023] According to the present invention, such objects can be accomplished by providing a multi-core ferrule, comprising: a ferrule main body including a cylindrical molded portion having a front end and a rear end, the molded portion defining an optical fiber receiving bore extending between the front end and rear end in an axial center thereof; the optical fiber receiving bore including a core receiving bore formed adjacent to the front end and a sheath receiving bore having a substantially larger inner diameter than the core receiving bore formed adjacent to the rear end, the core receiving bore being defined by a plurality of bore parts each having a circular cross section which are arranged in mutually parallel and contiguous relationship.  
       [0024] Because the core receiving bore of this ferrule is formed by closely arranging a corresponding number of core receiving bore parts each having a circular cross section one next to another, the size of the outer diameter of the ferrule main body is minimized, and the ferrule is therefore adapted for a larger number of cores and a more compact design. Preferably, the bore parts communicate each with an adjacent one of the bore parts via a longitudinally extending narrow gap although the presence of a thin wall of plastic material between the adjacent bore parts is consistent with the spirit of the present invention. Preferably, the optical fiber receiving bore further comprises a tapered bore which smoothly connects the core receiving bore with the sheath receiving bore.  
       [0025] An insert pipe may be insert molded onto an outer circumference of the cylindrical molded part so as to define an outer circumferential surface of the ferrule main body. The insert pipe is made of any hard material suitable for achieving a high dimensional precision of the outer circumferential such as stainless steel. The insert pipe may be provided with an engagement portion consisting of a recess or projection formed around a rear end of the insert pipe for the convenience of handling. Furthermore, when a base portion is insert molded onto the rear end of the ferrule main body to provide a SC type or other multi-core ferrule, the engagement portion provides a means for firmly attaching the ferrule main body to the base portion.  
       [0026] The insert pipe may comprise a thick walled portion surrounding the core receiving bore and a thin walled portion surrounding the sheath receiving bore so that the wall thickness of the molded portion can be made relatively uniform over its entire length. This minimizes the change in the cross sectional area of the passage for the injected plastic material, and this contributes to the production of a ferrule main body having a uniform quality and a high dimensional precision.  
       [0027] Preferably, a front end of the insert pipe comprises a reduced diameter portion having a reduced outer diameter, and the molded portion comprises a slag reservoir portion formed around the reduced diameter portion. According to such a multi-core ferrule, the slag contained in the front end of the flow of the molding plastic material is trapped in the reservoir defined around the reduced diameter portion so that the generation of flow marks and sink marks around the core receiving bore is avoided and the distortion of the inner circumferential surface of the core receiving bore is minimized.  
       [0028] The present invention also provides a metallic die assembly for injection molding a multi-core ferrule of this type, comprising: outer die assembly components defining a cylindrical cavity having a front end and a rear end for forming an outer circumferential surface of the ferrule main body; a plurality of first core pins slidably fitted into the cylindrical cavity in an axial center thereof from the front end thereof; and a second core pin slidably fitted into the cylindrical cavity in the axial center thereof from the rear end thereof; the first core pins being disposed in mutually parallel and contiguous relationship.  
       [0029] The second core pin may be provided with a tapered free end, and a recess provided in the tip of the tapered free end for receiving opposing ends of the first core pins so that the first core pins can be firmly supported by the second core pin and are thereby positively prevented from being bent, broken or otherwise damaged by the pressure of the molding plastic member.  
       [0030] According to a preferred embodiment of the present invention, the cylindrical cavity is adapted to receive an insert pipe defining an outer surface of the ferrule main body, and an end of the insert pipe corresponding to the front end of the cylindrical cavity comprises a reduced diameter portion having a reduced outer diameter, and a gate for injecting molding plastic material is provided in the rear end of the cylindrical cavity. Thereby, the front end of the molding plastic member which contains a relatively large amount of slag is trapped in a slag reservoir formed around the reduced diameter portion, and loss of dimensional precision and distortion of the molded portion can be minimized.  
       [0031] If the insert pipe comprises a thick walled portion surrounding the core receiving bore and a thin walled portion surrounding the sheath receiving bore, the wall thickness of the molded portion can be made substantially uniform over the entire length thereof. This is beneficial because a highly uniform flow of the molding plastic material can be achieved, and this contributes to a uniform property of the molded portion. Also, the uniform thickness also contributes to a favorable thermal shrinkage property which in turn provides a high dimensional precision and absence of distortions.  
       [0032] According to this metallic die assembly for making a multi-core ferrule of the present invention, because the passage for the molding plastic material is narrowed toward center in the tapered portion, the adjoining first core pins are urged by the molding plastic material toward each other and are thereby supported by each other so that the first core pins are effectively prevented from breaking or being bent by the injection pressure of the molding plastic material.  
       [0033] By defining the thick-walled portion cavity with a narrow passage defined between the inner circumferential surface of a front part of the insert pipe and the first core pins, the injection speed can be increased without increasing the injection pressure, and the first core pins are thereby prevented from being bent, broken or otherwise damaged by the injection pressure. Also, the temperature distribution of the metallic die assembly can be made uniform and the adverse effect of thermal shrinkage can be minimized. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0034] Now the present invention is described in the following with reference to the appended drawings, in which:  
     [0035]FIG. 1 is a longitudinal sectional view of a multi-core ferrule embodying the present invention;  
     [0036]FIG. 2 is a longitudinal sectional view taken along line II-II of FIG. 1;  
     [0037]FIG. 3 is an end view as seen from the direction indicated by arrow III of FIG. 2;  
     [0038]FIG. 4 is a longitudinal sectional view of the multi-core ferrule attached to a mounting member;  
     [0039]FIG. 5 is an SC type ferrule embodying the present invention;  
     [0040]FIG. 6 is an end view as seen from the direction indicated by arrow VI of FIG. 5;  
     [0041]FIGS. 7 a  and  7   b  are a longitudinal sectional view and an end view of a modified embodiment of the insert pipe;  
     [0042]FIGS. 8 a  and  8   b  are a longitudinal sectional view and an end view of another modified embodiment of the insert pipe;  
     [0043]FIGS. 9 a  and  9   b  are a longitudinal sectional view and an end view of yet another modified embodiment of the insert pipe;  
     [0044]FIG. 10 is a fragmentary vertical sectional view of the metallic die assembly for injection molding the ferrule main body  1  illustrated in FIG. 1.  
     [0045]FIG. 11 is an enlarged fragmentary vertical sectional view of a part of the die assembly illustrated in FIG. 10;  
     [0046]FIG. 12 is an exploded perspective view showing the first core pins which are components of the metallic die assembly illustrated in FIGS. 10 and 11 to illustrate how they are mounted to the pin holder;  
     [0047]FIG. 13 is a longitudinal sectional view of another embodiment of the multi-core ferrule according to the present invention;  
     [0048]FIG. 14 is a longitudinal sectional view taken along line XIV-XIV of FIG. 13;  
     [0049]FIG. 15 is an end view as seen from the direction indicated by arrow XV of FIG. 14;  
     [0050]FIG. 16 is a longitudinal sectional view of yet another embodiment of the multi-core ferrule according to the present invention;  
     [0051]FIG. 17 is a longitudinal sectional view taken along line XVII-XVII of FIG. 16;  
     [0052]FIG. 18 is an end view as seen from the direction indicated by arrow XVIII of FIG. 17; and  
     [0053] FIGS.  19  to  21  are end views showing other modified embodiment of the arrangement of the core receiving bore parts. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0054] The multi-core ferrule and method of making the same according to the present invention are described in the following in terms of concrete embodiments illustrated in the appended drawings. Referring to FIGS.  1  to  3 , a ferrule is formed by a ferrule main body  1  by itself, and comprises an inner tube consisting of a molded portion  2  for receiving an optical fiber in an axial center thereof, and an outer tube consisting of an insert pipe  3  for defining the outer diameter of the ferrule main body  1 . The inner and outer tubes are integrally joined to each other by placing the insert pipe  3  in a metallic die assembly and injection molding the molded portion  2  therein as will be described hereinafter. In the illustrated embodiment, the insert pipe  3  is not provided with any communication holes, but, if desired, may also be provided with communication holes as disclosed in Japanese patent laid open publication (kokai) No. 2000-84974 and Japanese patent laid open publication (kokai) No. 2001-96570.  
     [0055] The molded portion  2  is axially centrally provided with a core receiving bore  4   a  in a front end thereof for receiving the core of an optical fiber, a sheath receiving bore  4   c  in a rear end thereof for receiving the sheath of the optical fiber, and a tapered bore  4   b  communicating the core receiving bore  4   a  and sheath receiving bore  4   c  with each other. The tapered bore  4   b  provides a smooth transition between the core receiving bore  4   a  and sheath receiving bore  4   c . The core receiving bore  4   a , sheath receiving bore  4   c  and tapered bore  4   b  jointly form an optical fiber receiving bore  4 .  
     [0056] The core receiving bore  4   a  of the fiber receiving bore  4  is adapted to receive two or more cores of an optical fiber which are arranged in parallel with each other. In the embodiment illustrated in FIGS.  1  to  3 , the core receiving bore  4   a  comprises a pair of adjoining core receiving bore parts  4 a- 1  and  4 a- 2 , each having a substantially circular cross section, so as to be compatible with a two-core optical fiber. These two bore parts  4 a- 1  and  4 a- 2  are arranged in a both parallel and contiguous relationship. Preferably, the two bore parts  4 a- 1  and  4 a- 2  communicate with each other via a longitudinally extending narrow gap G (FIG. 3).  
     [0057] As the core receiving bore  4   a  is defined by a pair of adjoining or contiguous cylindrical spaces each having a circular cross section, the outer diameter of the ferrule is minimized so that the ferrule may be made suitable for even more compact design and a larger numbers of cores. Also, during the injection molding process, the extremely fine core pins for forming the core receiving bore are prevented from being bent or broken by the pressure of the injected plastic molding material as they are arranged one next to the other so as to support each other.  
     [0058] The molded portion  2  is formed by injection molding plastic material which is selected from various thermoplastic materials and provided with desired mechanical strength and dimensional precision. In particular, liquid crystal polymers are preferred among other engineering plastic materials as they are favorable in terms of mechanical strength and dimensional stability, and are suited for machining.  
     [0059] The insert pipe  3  comprises a relative thick-walled main portion, a reduced diameter portion  5  having a reduced outer diameter in a front end thereof, a thin-walled portion  6  having an enlarged inner diameter which is enlarged from the inner diameter of the remaining part of the insert pipe  3  in a front end thereof and an engagement portion  7  provided on the outer circumferential surface of the rear end of the thin-walled portion  6 . The core receiving bore  4   a  having a uniform and small diameter extends across the thick-walled main portion and reduced diameter portion  5 . The thick-walled main portion and thin-walled portion  6  have a uniform outer diameter. The tapered bore  4   b  corresponds to the transition between the thick-walled main portion and thin-walled portion  6 . In the embodiment illustrated in FIGS.  1  to  3 , the engagement portion  7  consists of an annular groove formed around the outer circumferential surface of the rear end of the thin-walled portion  6  of the insert pipe  3 .  
     [0060] The molded portion  2  comprises a tapered tip portion  2   a  formed on the front end of the reduced diameter portion  5 , a slag reservoir  2   b  formed around the outer circumferential surface of the reduced diameter portion  5 , a small diameter cylindrical portion  2   c  defining the core wire receiving bore  4   b  in the axial center thereof inside the thick-walled main portion of the insert pipe  3 , a tapered portion  2   d  provided inside the transitional part between the thick-walled main portion and thin-walled portion  6  of the insert pipe  2 , and a large diameter cylindrical portion  2   e  defining the sheath receiving bore  4   c  in the axial center thereof inside the inner circumference of the thin-walled portion  6 .  
     [0061] The insert pipe  3  allows the dimensional precision of the outer diameter and circularity of the assembly to be increased by being fitted on the outer circumferential surface of the molded portion  2 , and the bonding strength between the molded portion  2  and the insert pipe  3  is reinforced by the reduced diameter portion  5  being embedded between the outer and inner parts of the tapered tip portion  2   a  of the molded portion  2 . Also, the slag reservoir  2   b  for the molding plastic material is provided around the reduced diameter portion  5  so that the dimensional precision of the inner diameter of the core receiving bore  4   a  is improved as will be described hereinafter.  
     [0062] The molded portion  2  surrounding the sheath receiving bore  4   c  and tapered bore  4   b  is provided with a substantially uniform thickness with the aid of the thin-walled portion  6  on the rear part of the insert pipe  3 . The engagement portion  7  formed around the rear end of the insert pipe  3  is useful both when using the ferrule main body  1  as an optical semiconductor module as it is, when manufacturing ferrules for butt connection, and when using the ferrule main body as a part of SC type or ST type ferrules.  
     [0063] Referring to FIG. 4, for instance, a ferrule main body  1  having an optical fiber  8  mounted thereto may be passed through a mounting hole  9   a  of a mounting member  9  of an optical semiconductor module, and the ferrule main body  1  may be turned and moved axially with respect to the optical lens system (not shown in the drawing) of the optical semiconductor module so as to adjust the optical center line before the ferrule main body  1  is integrally and permanently attached to the mounting member  9 .  
     [0064] The ferrule main body  1  can be held as desired by gripping the engagement portion  7  with a manipulator or the like when aligning the optical center line of the ferrule main body  1 . The ferrule main body  1  can be integrally attached to the mounting member  9  either by welding or by using a bonding agent following the alignment and positioning of the optical axial line. In particular, when the insert pipe  3  is made of metallic material such as stainless steel, the insert pipe  3  can be integrally attached to a mounting member  9  by laser welding or the like as denoted by numeral  10 .  
     [0065] Referring to FIGS. 5 and 6, a SC type ferrule  14  can be produced by using the ferrule main body  1  as an insert component and injection molding a base portion  13  thereon, the base portion  13  comprising a flange portion  11  and a communication hole  12  communicating with the sheath receiving bore  4   c  in an axial center of the ferrule main body  1 . A ST type ferrule (not shown in the drawing) can also be similarly manufactured by using the ferrule main body  1  as an insert component.  
     [0066] When producing a SC type or ST type in this manner, the engagement portion  7  ensures a strong attachment between the ferrule main body  1  and base portion  13  by providing a means for holding them together. The base portion  13  can be injection molded economically by using relatively inexpensive common engineering plastic material such as polybutylene terephthalate.  
     [0067] When the ferrule is adapted for optical semiconductor modules, butt connectors for optical fibers and ferrules of other types, the cost of each ferrule main body can be reduced through mass production, and other advantages such as ease of procurement and stocking can be achieved.  
     [0068] The insert pipe  3  may be made of such materials as hard metals such as stainless steel, titanium and fiber reinforced metals (FRM), ceramics such as zirconia, and high performance engineering plastics such as polyimide resin. In particular, a metallic pipe is desirable as it can be laser welded when used in a module, and stainless steel is highly desirable in view of its cost, heat resistance, rigidity and dimensional precision.  
     [0069] In the embodiment illustrated in FIGS.  1  to  3 , the insert pipe  3  consisted of a relatively thick-walled metallic pipe. When it is made of stainless steel, forming the reduced diameter portion  5  and thin-walled portion  6  can be accomplished by machining of the outer circumferential surface of the front end, enlarging the inner circumferential surface of the rear end in both economically and at high precision.  
     [0070] It is also possible to form the insert pipe  3  by using a thin walled pipe member and press form the same in such a manner that a reduced diameter portion  5  is formed at the front end and an annular engagement portion  7  is formed by bulging out the outer circumferential surface of the rear end of the insert pipe  3  while doing away with the variations in the wall thickness as illustrated in FIGS. 9 a  and  9   b.    
     [0071] The insert pipe  3  may be provided with the engagement portion  7  by forming localized recesses on the outer circumferential surface of the insert pipe  3  as denoted by numeral  7  in FIGS. 7 a  and  7   b , by providing an annular projection on the outer circumferential surface of the insert pipe  3  as denoted by numeral  7  in FIGS. 8 a  and  8   b , or by forming localized projections (not shown in the drawings).  
     [0072] The method of manufacturing the ferrule main body  1  is described with reference to FIGS.  10  to  12 . FIG. 10 shows a metallic die assembly for injection molding, which comprises a moveable metallic die component  15 A and fixed metallic die component  15 B opposing each other on either side of a parting line PL. The moveable metallic die component  15 A comprises a movable die plate  16 , and the fixed metallic die component  15 A comprises a fixed die plate  17 .  
     [0073] The movable die plate  16  is incorporated with a moveable core  18  which is in turn centrally provided with a second core pin  19  for forming the tapered bore  4   b  and sheath receiving bore  4   c  of the ferrule main body  1 . A pair of ejector pins  20  and  21  are provided on either side of the second core pin  19  for pushing out the molded product or the ferrule main body  1 .  
     [0074] The fixed metallic die component  15 B is provided with a lower core  22  for receiving the insert pipe  3  and an upper core  25  fitted with a pin holder  24  supporting a pair of first core pins  23  for forming the core receiving bore  4   a  of the ferrule main body  1 , and a holding block  26  for holding the upper core  25  from above.  
     [0075] The first core pins  23  are extremely fine pins made of cemented carbide. The upper ends or base ends of the first core pins  23  are supported by the pin holder  24  while the lower ends or free ends thereof consisting of pointed ends  23   a  (FIG. 12) are supported by being detachably fitted into a slot formed in the upper end of the fixed lower core  22 . The pin holder  24  is engaged by the upper surface of a cavity block  27  mounted to the fixed lower core  22 , and is thereby fixedly secured.  
     [0076] The pin holder  24  is provided with a mounting hole  24   a  consisting of a pair of mutually adjoining holes each having a circular cross section for receiving the first core pins  23  adapted to form the core receiving bore  4   a  in an axial center thereof. As the inner diameter of the mounting hole  24   a  is required to have a high dimensional precision, it is desirable to achieve the required dimension by applying electroplating to the surface of the base material and perform electroforming thereon. To minimize the shrinkage of the molded product or prevent the generation of sink marks, highly thermally conductive nickel or nickel alloy which also provides a high hardness may be electrodeposited onto base material also having a high thermal conductivity such as copper.  
     [0077] The first core pins  13  are fitted into the mounting hole  24   a , and are fixedly secured therein by applying an epoxy or other two-part bonding agent from above. The first core pins  13  are thereby bonded not only to the mounting hole  24  but also to each other via their contact surfaces. If there is any excess bonding agent on a surface other than the contact surfaces, it should be wiped off.  
     [0078] The first core pins  13  are required to be integrally attached to the mounting hole  24   a , but may not be necessarily attached to each other. It is preferable, however, to attach them to each other in terms of mechanical strength, but they may be simply placed closely to each other in a mutually parallel relationship. Other bonding means such as welding may also be used instead of a bonding agent.  
     [0079] The cavity block  27  which prevents the pin holder  24  received in the fixed upper core  25  from coming off is formed with an inner bore  27   a  for defining a tapered tip cavity  28   a  for forming the tapered tip  2   a  of the molded portion  2  and a reservoir cavity  28   b  for forming the slag reservoir  2   b.    
     [0080] In a front part of the interior of the insert pipe  3 , a thick-walled portion cavity  28   c  for forming the small diameter cylindrical portion  2   c  of the molded portion  2  is defined between the inner circumferential surface  3   a  of the thick-walled main portion of the insert pipe  3  and the first core pins  23 . In a rear part of the interior of the insert pipe  3 , a large diameter portion cavity  28   e  for forming the large diameter cylindrical portion  2   e  of the molded portion  2  is defined between the inner circumferential surface  3   c  of the thin-walled portion  6  of the insert pipe  3  and the second core pin  19 . In an intermediate part of the interior of the insert pipe  3 , a tapered portion cavity  28   d  is defined between the inner circumferential surface  3   b  of the tapered portion of the insert pipe  3  and the tip  19   a  of the second core pin  19 .  
     [0081] Thus, a cavity  28  for injection molding the molded portion  2  comprised of the tapered tip cavity  28   a , reservoir cavity  28   b , thick-walled portion cavity  28   c , tapered portion cavity  28   d  and large diameter portion cavity  28   e  is formed by closing the metallic die assembly  15  for the injection molding process.  
     [0082] To the large diameter portion cavity  28   e  of the cavity  28  are connected a plurality (four, for instance) of gates  29  extending radially along the parting line PL of the metallic die assembly. Each gate  29  communicates with a spool (not shown in the drawing) provided above the fixed metallic die component  15 B via an annular runner  30  so that the molding plastic material injected from the spool may be filled into the cavity  28  and the molded portion  2  of the ferrule main body  1  may be injection molded.  
     [0083] The molding plastic material first flows into the large diameter portion cavity  28   c , and then flows into the thick-walled portion cavity  28   c  via the tapered portion cavity  28   d  which narrows the flow of the molding plastic material radially inwardly. (The wall thickness of the molded portion is substantially uniform over its entire length, but the molded portion corresponding to the large diameter portion cavity  28   e  has a somewhat larger cross sectional area than that corresponding to the thick-walled portion cavity  28   c  owing to the difference in the average diameter.) Thereafter, the molding plastic material flows towards the front end through the narrow passage (thick-walled portion cavity  28   c ) defined between the small diameter inner circumferential surface  3   a  and the first core pins  23 , and eventually reaches the reservoir cavity  28   b  after passing through the tapered tip cavity  28   a  and around the tip of the reduced diameter portion  5 .  
     [0084] According to this process of injection molding, the first core pins  23  for forming the core receiving bore  4   a  for a two-core optical fiber are prevented from being bent or broken due to the injection pressure of the molding plastic material by being supported by each other as the adjoining first core pins are disposed in parallel to each other in contact with each other at their opposing surfaces.  
     [0085] Because the molding plastic material flowing into the thick-walled portion cavity  28   c  is narrowed in cross section radially inwardly by the tapered portion cavity  28   d , the flow of the molding plastic material urges the first core pins  23  toward each other so that the first core pins  23  are prevented from being bent or broken due to the injection pressure of the molding plastic material.  
     [0086] Also, because the insert pipe  3  surrounding the first core pins  23  is provided with a large wall thickness and the thick-walled portion cavity  28   c  is formed by the narrow passage defined between the small diameter inner circumferential surface  3   a  and first core pins  23 , the injection speed can be increased without increasing the injection pressure, and this prevents the bending, curving or otherwise damaging the first core pins  13 . This also contributes to making the temperature distribution in the metallic die assembly more uniform, and thereby minimizing the adverse effect of thermal shrinkage.  
     [0087] At the front end of the reduced diameter portion  5 , the flow of the plastic material passes around it and reaches the reservoir cavity  28   b  on the outer circumference of the assembly remote from the axial center where the core receiving bore  4   a  is located. Therefore, the part of the molding plastic material at the front end of the flow which contains a relatively large amount of slag is trapped in the reservoir cavity  28   b  so that the generation of flow marks and sink marks around the core receiving bore  4   a  is avoided and the distortion of the inner circumferential surface of the core receiving bore  4   a  is minimized.  
     [0088] FIGS.  13  to  15  show a modified embodiments of the multi-core ferrule. Whereas the core receiving bore  4   a  in the two-core ferrule main body  1  in the preceding embodiment consisted of a pair of core receiving bore parts  4 a- 1  and  4 a- 2  having an identical diameter disposed one next to the other, the core receiving bore  31  of the two-core ferrule main body  1 A illustrated in FIGS.  13  to  15  comprises a pair of core receiving bore parts  31   a  and  31   b  having different diameters disposed one next to the other. The embodiment illustrated in FIGS.  16  to  18  show a three-core ferrule main body  1 B wherein the core receiving bore  32  is defined by three core receiving bore parts  32   a ,  32   b  and  32   c  of an identical diameter disposed one next to the other along a straight line.  
     [0089] These ferrules  1 A and  1 B can be injection molded in a similar fashion as the ferrule main body  1  of the previous embodiment simply by changing the shape and number of the first core pins  23  mounted to the holder  24 , and changing the shape of the receiving slot formed in the front end of the second core pin  19 , and provide similar advantages.  
     [0090] The present invention is by no means limited to these embodiments, but may include any other embodiments in which a plurality of circular core receiving bore parts are placed one next to another to jointly define a core receiving bore. This can be accomplished by using a plurality of first core pins adapted for the desired core receiving bore which are disposed so as to support each other by being in mutual contact at their opposing surfaces.  
     [0091] For instance, the core receiving bore parts may be arranged radially at the interval of 120 degrees around the axial center line to define a core receiving bore  33  of a three-core ferrule as illustrated in FIG. 19. Also, the core receiving bore parts may be arranged radially at the interval of 90 degrees around the axial center line to define a core receiving bore  34  in the case of a four-core ferrule as illustrated in FIG. 20. In the example illustrated in FIG. 21, six core receiving bore parts each having an identical circular cross section are arranged along a pair of straight lines to define a pair of core receiving bores  35  each consisting of three of the core receiving bore parts arranged along a straight line.  
     [0092] Although the present invention has been described in terms of preferred embodiments thereof, it is obvious to a person skilled in the art that various alterations and modifications are possible without departing from the scope of the present invention which is set forth in the appended claims.