Patent Publication Number: US-9889701-B2

Title: Writing implement

Description:
TECHNICAL FIELD 
     The present invention relates to a writing implement that enables tilting of the writing tip. 
     BACKGROUND ART 
     Hitherto, in technology related to writing implements that enable tilting of the writing tip and improve the writing sensation, writing implements such as those described in Patent Document 1 are known in which an elastic body is interposed between a shaft body and a tip member, such that the writing tip is tilted by flexing of the elastic body according to the force during writing. 
     PRIOR ART DOCUMENTS 
     Patent Document 1: JP-A No. 2002-331789 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, issues with the invention described in Patent Document 1 are that the amount of flexing of the writing tip is limited by the gap between the shaft body and the tip member, and that wobbling and changes in texture arise with degeneration of an elastic body, such as rubber or elastomer. Moreover, due to the elastic body being installed at the outer periphery of the shaft body, there is also an issue of the writer touching the elastic body and promoting deterioration of the elastic body. 
     Thus the present invention addresses the provision of a writing implement in which the amount of flexing in the pen tip can be changed according to the force applied during writing, deterioration of the elastic body suppressed, wobbling is less liable to occur, and a consistent writing sensation can be obtained. 
     Solution to Problem 
     First Aspect 
     In order to solve the above problems, a first aspect of the present application includes a writing tip, an ink supply section that supplies ink to the writing tip, a joint that connects the writing tip and the ink supply section together, and a shaft that internally houses the writing tip, the ink supply section, and the joint, wherein the joint includes a center member that is connected to at least the writing tip, a surrounding member that is separated from the center member, and an interposing member that is formed so as to connect the center member and the surrounding member, and wherein the interposing member is made of a different material from that of the center member and the surrounding member. 
     According to the type of ink, the writing implement may be a ballpoint pen or a marker pen by making the writing tip a ballpoint pen tip or a fiber bundle for use in a marker. The shaft may have ink directly housed inside, or may have a structure in which a refill filled with ink is housed inside the shaft. The ink supply section may supply the writing tip with ink housed inside the shaft using an intermediate member, such as a collector, or may supply the writing tip with ink housed in an ink housing tube, such as a refill, that is a separate body to the shaft. 
     The present aspect enables a soft writing sensation like that of a fountain pen to be obtained due to the joint flexing according to the force applied during writing. 
     Second Aspect 
     In addition to the features of the first aspect, in a second aspect of the present application the joint is molded by two color molding, the center member and the surrounding member are molded by primary molding, and the interposing member is integrally molded onto the center member and the surrounding member by secondary molding. 
     The two color molding referred to here is molding of a single molded item by molding two or more types of resin over two or more times of molding, and includes molding using the same type of resin. 
     The present aspect, similarly to the first aspect, enables a soft writing sensation like that of a fountain pen to be obtained due to the joint flexing according to the force applied during writing. Moreover, due to the integral molding, the center member and the surrounding member can be securely engaged by the interposing member. 
     Third Aspect 
     In addition to the features of the first aspect or the second aspect, in a third aspect of the present application the interposing member is molded from a softer material than that of the center member and the surrounding member. 
     The soft material referred to here is not limited as long as it is softer than the center member and the surrounding member; however, a thermoplastic elastomer is preferably employed in order to perform a molding process using a mold at a high temperature. 
     The present aspect, similarly to the first aspect, results in a softer writing sensation due to employing a soft material for the interposing member such that the joint flexes more according to the force applied during writing. 
     Fourth Aspect 
     In addition to the first, second, or third aspect, in the fourth aspect of the present application the center member of the joint has a substantially cylindrical shape, and the surrounding member of the joint is formed in a substantially cylindrical shape at an outer periphery of the center member, with a leading end of the ink supply section being inserted into an annular space formed between the center member and the surrounding member. 
     Advantageous Effects of Invention 
     Due to configuration as described above, the invention of the present application is able to make the joint flex according to the force applied during writing, and, accompanying such flexing, is able to make the writing tip flex so as to obtain a soft writing sensation like that of a fountain pen. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1A  is a face-on view of a ballpoint pen of a first example of the present invention, and  FIG. 1B  is a vertical cross-section thereof. 
         FIG. 2  is an enlargement of a fitting zone (X) of the shaft in  FIG. 1B . 
         FIG. 3A  is a face-on view of a tip shaft, and  FIG. 3B  is a vertical cross-section thereof. 
         FIG. 4  is a vertical cross-section illustrating a state in which an ink supply section, a joint, and a ballpoint pen tip have been inserted into a tip shaft. 
         FIG. 5  is a vertical cross-section of a collector. 
         FIG. 6  is a vertical cross-section illustrating an enlargement of a tip end of the ballpoint pen. 
         FIG. 7A  is a perspective view of a joint,  FIG. 7B  is a face-on view thereof, and  FIG. 7C  is a vertical cross-section thereof. 
         FIG. 8A  is a perspective view of a primary molded body,  FIG. 8B  is a face-on view thereof, and  FIG. 8C  is a vertical cross-section thereof. 
         FIG. 9A  is a perspective view of a secondary molded body,  FIG. 9B  is a face-on view thereof, and  FIG. 9C  is a vertical cross-section thereof. 
         FIG. 10A  is a face-on view of a ballpoint pen according to a second example and  FIG. 10B  is a vertical cross-section thereof. 
         FIG. 11  is an enlargement of a fitting zone (Y) of the shaft in  FIG. 10B . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Explanation follows using a ballpoint pen  1  having a collector inside as an example of a writing implement  1  of an embodiment of the present invention, with reference to the drawings. Note that in the present invention, “front” with reference to a writing implement  1  and its configuring components means the tip side for cases in which a writing tip  40  serves as the tip of the writing implement  1 , and “rear” is the opposite side thereto. 
     First Example 
     Overall Structure 
     As illustrated in  FIG. 1A  and  FIG. 1B , the ballpoint pen  1  includes a ballpoint pen tip  40  with a writing ball  50  clasped at the leading end, an ink supply section  30  that supplies ink to the ballpoint pen tip  40 , a joint  60  that connects the ballpoint pen tip  40  and the ink supply section  30  together, a shaft  10  internally housing the ballpoint pen tip  40 , joint  60 , and ink supply section  30 , and an outer member  20  that is installed to the leading end of the shaft  10  and covers the outer periphery of the ballpoint pen tip  40 . 
     Shaft Structure 
     As illustrated in  FIG. 1B , the shaft  10  is configured from a ink tank  13  which is formed in a cylindrical shape and has a closed rear end, a tip shaft  11  which is formed in a cylindrical shape with a leading end having a tapered profile, and a rear shaft  12  which is formed in a cylindrical shape and has a closed rear end. A leading end of the ink tank  13  is fitted at the inner periphery of a rear end of the tip shaft  11 . A rear end of the tip shaft  11  is fitted at the inner periphery of a leading end of the rear shaft  12 . 
     As illustrated in  FIG. 1B , the outer diameter of the ink tank  13  is substantially the same diameter as the inner diameter of the rear end of the tip shaft  11 . As illustrated in  FIG. 2 , at the outer periphery of the ink tank  13 , an ink tank outer projection  13   a  which projects outwards at a location approximately ⅓ of the way along the ink tank  13  from the leading end is provided. The ink tank  13  is filled with non-illustrated ink. 
     As illustrated in  FIG. 1B , the tip shaft  11  is substantially circular cylinder shaped with the leading end having a tapered profile. A tip shaft flange  15  is provided at a location on the outer periphery of the tip shaft  11  approximately ⅓ of the way along the tip shaft  11  from the rear end. The tip shaft flange  15  has an outer diameter which is substantially the same as the outer diameter of the rear shaft  12 , and the leading end of the rear shaft  12  abuts the tip shaft flange  15 . The tip shaft  11  is configured from a substantially cylindrical tube shaped tip shaft rear portion  14  at the rear, and a tip shaft front portion  16  having a tapered profile at the front, with the tip shaft flange  15  serving as a boundary therebetween. As illustrated in  FIG. 2 , a tip shaft inner projection  11   a  is provided on the inner periphery of the tip shaft rear portion  14  so as to project inward immediately to the rear of the ink tank outer projection  13   a  when the ink tank  13  has been fitted into the tip shaft  11 . In front of the tip shaft inner projection  11   a , there are plural tip shaft outer projections  11   b  provided on the outer periphery of the tip shaft rear portion  14  so as to project outward. As illustrated in  FIG. 3B , at the inner periphery of the tip shaft front portion  16 , a tip shaft front inner projection  16   a , which is formed in a ring shape and engages with the ink supply section  30  at a location where the tapering starts, and thickened portions  16   b , where a portion of the inner periphery of the tip shaft front portion  16  is thickened so as to make contact with the joint  60 , are formed. Moreover, at the front of the tip shaft front portion  16 , a tip shaft front reduced diameter hole  16   c , having a reduced diameter so as to enable the leading end of the joint  60  to be inserted therein, and an outer insertion hole  16   d , which is contiguously provided to the front of the tip shaft front reduced diameter hole  16   c  and has the outer member  20  inserted therein, are provided. A groove is provided on the outer periphery of the tip shaft front portion  16  in the vicinity of the leading end thereof, so as to enable mounting of another member, and a grip member  16   e  having an anti-slip action is mounted in the groove. 
     As illustrated in  FIG. 1B , the inner diameter of the rear shaft  12  is substantially the same diameter as the outer diameter of the tip shaft rear portion  14 . As illustrated in  FIG. 2 , at the inner periphery of the rear shaft  12 , a rear shaft taper portion  12   a , which is formed with a slightly reduced diameter at a position covering the rear end of the tip shaft  11  when the tip shaft  11  is fitted into the rear shaft  12 , and rear shaft inner projections  12   b , which project inward, are provided. The same number of rear shaft inner projections  12   b  are provided as the number of tip shaft outer projections  11   b , with the rear shaft inner projections  12   b  each provided directly in front of the respective tip shaft outer projection  11   b  so as to be paired with the respective tip shaft outer projection  11   b.    
     The above-described ink tank outer projection  13   a  and the tip shaft inner projection  11   a  engage with each other. The tip shaft outer projections  11   b  and the rear shaft inner projections  12   b  also engage with each other. Due to such engagement, the ink tank outer projection  13   a , the tip shaft inner projection  11   a , the tip shaft outer projections  11   b , and the rear shaft inner projections  12   b  act as stoppers, such that the ink tank  13 , the tip shaft  11 , and the rear shaft  12  are not readily moved apart from each other. The ink tank outer projection  13   a  and the tip shaft inner projection  11   a  are each ridges formed with a ring profile. Moreover, the tip shaft outer projections  11   b  and the rear shaft inner projections  12   b  are, similarly, each ridges formed with a ring profile. A configuration may be adopted in which only one out of the ink tank outer projection  13   a  or the tip shaft inner projection  11   a  is formed as a ridge in a ring shape along the circumference. Moreover, a configuration may be adopted in which, similarly, only one out of the tip shaft outer projections  11   b  or the rear shaft inner projections  12   b  are each formed as a ridge in a ring shape along the circumference. 
     Ink Supply Section 
     As illustrated in  FIG. 4  and  FIG. 5 , the ink supply section  30  is configured from a collector  31  which is formed in a substantially tube shape and whose outer periphery is formed with plural comb shaped fins  33 , and a collector core  32  which is made from polyester fiber formed into a rod shape and is inserted through the collector  31  along the axial direction, with the leading end of the collector core  32  fitted into the rear end of the joint  60 . As illustrated in  FIG. 5 , a tip retainer  34  having an enlarged inner diameter is provided at the leading end of the collector  31 . 
     As illustrated in  FIG. 4  and  FIG. 5 , at the outer periphery of the tip retainer  34 , a collector flange  34   a  which is formed in a ring shape immediately in front of the tip shaft front inner projection  16   a  to engage with the tip shaft front inner projection  16   a  formed to the inner periphery of the tip shaft front portion  16 , a collector step  34   b  which is contiguously provided in front of the collector flange  34   a  with a smaller diameter than the collector flange  34   a , and a collector contact portion  34   c  contiguously provided in front of the collector step  34   b  with a smaller diameter than the collector step  34   b , are provided. 
     The rear end of the collector  31  contacts the ink tank  13 . The rear of the collector core  32  projects inside the ink tank  13 , and the front of the collector core  32  is inserted from the rear end of the joint  60  to a position approximately ½ the way along the joint  60 . 
     Ballpoint Pen Tip 
     As illustrated in  FIG. 6 , the ballpoint pen tip  40  is configured from a holder  42  which is formed in a circular cylinder shape, a center core  41  which is formed in a rod shape, projects out from the rear end of the holder  42  and is inserted into the holder  42 , and the writing ball  50  held by the holder  42 . A swaged section  42   a  is formed to the leading end of the holder  42  by pressing and deforming the tip end edge of the holder  42  inwards to give a reduced diameter. The leading end of the writing ball  50  is configured so as to be exposed through the leading edge of the swaged section  42   a . The center core  41  is formed from polyester fiber similarly to the collector core  32 . The rear end of the center core  41  fits together with the leading end of the collector core  32 , and the leading end of the center core  41  reaches to immediately behind the writing ball  50 , absorbs ink that has permeated into the collector core  32 , and supplies the ink to the writing ball  50 . Furthermore, the rear end of the ballpoint pen tip  40  to approximately ⅔ of the way along the ballpoint pen tip  40  is held by the joint  60 . 
     The collector core  32  and the center core  41  are formed by appropriately selecting the porosity and surface profile of the polyester fiber according to characteristics, such as the viscosity, of the ink employed. 
     Joint 
     As illustrated in  FIG. 4  and  FIG. 7 , the joint  60  is configured from a center member  61  which is formed in a substantially cylindrical shape, a surrounding member  62  which is formed in a substantially cylindrical shape from approximately ¼ from the rear end of the center member  61  to approximately ⅓ from the rear end of the center member  61  such that a gap section  64  of the thickness of the tip retainer  34  of the collector  31  is present at the outer periphery of the rear of the center member  61 , and an interposing member  63  which is formed so as to cover the center member  61  from the leading end of the surrounding member  62  to a Position Approximately ⅓ Back from the Leading End of the Center Member  61 . The collector core  32  is inserted inside the center member  61  from the rear, and the ballpoint pen tip  40  is inserted inside the center member  61  from the front. The tip retainer  34  fits into the gap section  64  formed between the center member  61  and the surrounding member  62 . 
     As illustrated in  FIG. 8C , at the inner periphery of the center member  61 , there are a rear insertion hole  61   a  of substantially the same diameter as the collector core  32  provided at the rear end, a center insertion hole  61   b  of substantially the same diameter as the center core  41  provided at a position from the vicinity of the rear end of the center core  41  to the rear end of the holder  42 , and a front insertion hole  61   c  of substantially the same diameter as the holder  42  provided at a position from the rear end of the holder  42  to the leading end of the center member  61 . At the outer periphery of the center member  61 , there is a center member flange  61   e  which is formed in a ring shape further to the rear than ⅓ of the way back from the leading end of the rear insertion hole  61   a  of the center member  61 . The center member flange  61   e  is formed at a position in front of the leading end of the peripheral member  62  and has a smaller diameter than the outer diameter of the tip retainer  34 . The outer periphery of the center member  61  is configured from a rear circular columnar rear section  61   d  provided at a position from the rear end of the center member  61  up to the center member flange  61   e , a rear circular columnar front section  61   f  provided at a position from the center member flange  61   e  further to the rear than the leading end of the rear insertion hole  61   a , a central circular columnar section  61   g  provided at a position from the leading end of the rear circular columnar front section  61   f  to slightly to the front of the rear end of the center insertion hole  61   b , and a front circular columnar section  61   h  provided at a position from the leading end of the central circular columnar section  61   g  to the leading end of the center member  61 . The outer diameters of the rear circular columnar rear section  61   d  and the rear circular columnar front section  61   f  are substantially the same diameter as the inner diameter of the tip retainer  34 , the outer diameter of the central circular columnar section  61   g  is substantially the same diameter as the diameter of the tip shaft front reduced diameter hole  16   c , and the outer diameter of the front circular columnar section  61   h  is substantially the same diameter as the diameter of a rear insertion hole  24  provided in the rear of the outer member  20 . 
     As illustrated in  FIG. 8C , the inner diameter of the surrounding member  62  is substantially the same diameter as the outer diameter of the tip retainer  34  of the collector  31 . On the inner periphery of the surrounding member  62 , a surrounding member wide diameter portion  62   a  with a slightly wider diameter located at a position at the rear end of the surrounding member  62 , and a surrounding member inner projection  62   b  formed in a ring shape located at a position approximately ½ the way along the surrounding member  62  from the rear end of the surrounding member  62 , are provided. The diameter of the surrounding member wide diameter portion  62   a  is set smaller than the diameter of the collector contact portion  34   c  so as to facilitate insertion of the tip retainer  34  therein. The surrounding member inner projection  62   b  is provided to clamp the tip retainer  34  of the ink supply section  30 , using the surrounding member  62  and the center member  61 . The outer periphery of the surrounding member  62  is configured from a surrounding member rear outer peripheral portion  62   c  at positions from the rear end of the surrounding member  62  to the surrounding member inner projection  62   b , a surrounding member central outer peripheral portion  62   d  contiguous to the surrounding member rear outer peripheral portion  62   c , and a surrounding member front outer peripheral portion  62   e  contiguous to the surrounding member central outer peripheral portion  62   d  to a position as far as the leading end of the surrounding member  62 . The diameter of the surrounding member rear outer peripheral portion  62   c  is substantially the same diameter as the collector step  34   b , the diameter of the surrounding member central outer peripheral portion  62   d  is slightly smaller than the diameter of the surrounding member rear outer peripheral portion  62   c , and the diameter of the surrounding member front outer peripheral portion  62   e  is slightly smaller than the diameter of the surrounding member central outer peripheral portion  62   d . A ring shaped surrounding member outer projection  62   f  is provided from the leading end of the surrounding member front outer peripheral portion  62   e  to a position about ½ way along the surrounding member front outer peripheral portion  62   e . A cutout  62   g  is provided to the surrounding member outer projection  62   f  at at least one location in the circumferential direction. The outer diameter of the cutout  62   g  is the same as the outer diameter of the surrounding member front outer peripheral portion  62   e.    
     As illustrated in  FIG. 7  and  FIG. 9C , the interposing member  63  is configured from a interposing member rear circular columnar section  63   a  of substantially circular cylinder shape covering the peripheral member  62  and having substantially the same outer diameter as the diameter of the peripheral member outer projection  62   f , a coupling section  63   b  which is contiguously provided to the interposing member rear circular columnar section  63   a  and is formed in a substantially circular conical shape covering as far as the center member flange  61   e , and an interposing member front conical section  63   c  which is contiguously provided to the coupling section  63   b  and is formed in a gently-sloping circular conical shape covering as far as the vicinity of the center of the central circular columnar section  61   g . The interposing member rear circular columnar section  63   a  covers from the rear end of the surrounding member front outer peripheral portion  62   e , across the cutout  62   g  of the surrounding member  62 , as far as the leading end of the surrounding member front outer peripheral portion  62   e , and, as a result, engages with the surrounding member outer projection  62   f . The inner periphery of the interposing member  63  is formed so as to follow along the outer periphery of the center member  61  and the surrounding member  62 , has an inner diameter at the position of the rear end of the coupling section  63   b  that is the same as the inner diameter of the surrounding member  62 , and has a taper such that an inner diameter at the position in front of the coupling section  63   b  gradually decreases from the inner diameter of the interposing member  63  to the outer diameter of the center member flange  61   e.    
     Two Color Molding 
     The joint  60  is a component molded by two color molding, the center member  61  and the surrounding member  62  are molded by primary molding, and the interposing member  63  is integrally molded onto the center member  61  and the surrounding member  62  by secondary molding. In order to obtain flexing, the interposing member  63  is formed from an elastic resin material at the coupling section  63   b . A thermoplastic elastomer is preferably employed as the elastic material in order to perform a molding process using a mold at high temperature. The material of the center member  61  and the surrounding member  62  configuring the primary molded body is preferably a hard resin, such as a polyacetal resin. 
     Outer Member 
     As illustrated in  FIG. 6 , the outer member  20  is configured from an outer member fixing portion  21  which is formed in a circular cylinder shape, and an outer member taper portion  22  which is contiguously provided to the outer member fixing portion  21  and has a substantially circular conical shaped profile. A beveled outer member tip portion  23  is provided at the leading end of the outer member taper portion  22 . A hole is formed through from the rear end toward the leading end of the outer member  20 . A rear insertion hole  24  is provided at the inner face of the outer member fixing portion  21 , and a front insertion hole  25  of smaller diameter than the rear insertion hole  24  is provided at the inner face in the vicinity of the leading end of the outer member taper portion  22 . The outer member fixing portion  21  is fixed to the tip shaft  11  by fitting into the outer member insertion hole  16   d . The outer member  20  covers the ballpoint pen tip  40 . Then, the front circular columnar section  61   h  of the joint  60  is inserted into the rear insertion hole  24 , and the ballpoint pen tip  40  projecting out from the joint  60  is inserted into the front insertion hole  25 . As illustrated in  FIG. 6 , the writing ball  50  and the swaged section  42   a  covering the writing ball  50  are exposed from the outer member tip portion  23 . 
     Ballpoint Pen Characteristics 
     In the ballpoint pen  1 , the joint  60  is formed from 3 members, and although the surrounding member  62  and the center member  61  are formed by the same primary molded body, they are not connected to each other as the primary molded body. Deformation accordingly occurs in the vicinity of the coupling section  63   b  of the interposing member  63 , giving a structure in which the joint  60  readily flexes. 
     Ballpoint Pen Assembly 
     The ballpoint pen  1  is assembled in the following manner. First the tip retainer  34  of the collector  31  is inserted into the gap section  64  of the joint  60 , the collector core  32  is then inserted from the rear end of the collector  31  and the ballpoint pen tip  40  clasping the writing ball  50  is inserted from the leading end of the joint  60 , fitting the center core  41  of the ballpoint pen tip  40  together with the collector core  32 . The assembled collector  31 , collector core  32 , joint  60 , and ballpoint pen tip  40  are then inserted from the rear end of the tip shaft  11 . Then the outer member  20  is installed at the inner periphery of the tip shaft  11  so as to cover the ballpoint pen tip  40 . Or assembly may be performed by, after installing the outer member  20  at the inner periphery of the tip shaft  11 , inserting the assembled collector  31 , collector core  32 , joint  60 , and ballpoint pen tip  40  from the rear end of the tip shaft  11 . 
     Next, the rear end of the tip shaft  11  is fitted over the outer periphery of the ink tank  13  filled with ink until the leading end of the ink tank  13  contacts the rear end of the collector  31 . 
     Finally, the rear end of the tip shaft  11  is inserted at the inner periphery of the leading end of the rear shaft  12  so as to cover the ink tank  13 . When inserting the tip shaft  11  into the rear shaft  12 , insertion is performed until the leading end of the rear shaft  12  contacts the tip shaft flange  15 . 
     Then, positioning is performed on the ballpoint pen tip  40  and the ink supply section  30  by the tip shaft front inner projection  16   a  provided at the inner periphery of the tip shaft  11  and by the collector flange  34   a  of the collector  31 . There is accordingly a need to insert the joint  60  and the ballpoint pen tip  40  through from the tip shaft front inner projection  16   a  as far as the outer tip portion  23 . Then, due to the interposing member  63  of the joint  60  being an elastic member, even if there were to be a difference in dimension in the vertical direction of the tip retainer  34  and the outer fixing portion  21 , housing in the outer tip portion  23  through from the tip shaft front inner projection  16   a  can be achieved by contraction of the elastic member. 
     Fitting Method of the Tip Shaft, the Rear Shaft and the Ink Tank 
     The method of fixing the ink tank  13  and the tip shaft  11  is by, when inserted such that the leading end of the ink tank  13  contacts the rear end of the collector  31 , positioning the ink tank outer projection  13   a  immediately in front of the tip shaft inner projection  11   a  such that each of the projections acts as a stopper. 
     When the ink tank  13  has been inserted into the tip shaft  11 , a gap develops between the inner peripheral face at the rear end of the tip shaft  11 , and the outer peripheral face at the leading end of the ink tank  13 , depending on the height of the tip shaft inner projection  11   a.    
     Note that there are no particular limitations to the method of fixing the rear shaft  12  and the ink tank  13 . For example, a method may be adopted in which the inner diameter of the rear shaft  12  is formed so as to be smaller at the rear end than the outer diameter of the ink tank  13 , and the ink tank  13  is press-fixed to the rear shaft  12 , or a method may be adopted in which the ink tank outer projection  13   a  and the rear shaft inner projections  12   b  are provided to engage, with one of the projections riding over the other projection, such that each of the projections acts as a stopper. 
     Then, when the rear shaft  12  covers the tip shaft  11 , the rear shaft inner projections  12   b  are positioned immediately in front of the tip shaft outer projections  11   b , each of the projections acts as a stopper, and, at the same time, the rear end of the tip shaft  11  is compressed inwards by the rear shaft taper portion  12   a , the tip shaft inner projection  11   a  acts as a pivot point and gaps are compressed, and the rear end of the tip shaft  11  is pressed against the ink tank  13 , such that the rear shaft  12  and the tip shaft  11  are fixed. 
     As a result, the rear end of the tip shaft  11  is pressed against the outer peripheral face of the ink tank  13 , and in addition the tip shaft inner projection  11   a  and the ink tank outer projection  13   a , and the tip shaft outer projections  11   b  and the rear shaft inner projections  12   b , act as stoppers, enabling strong fixing to be achieved. 
     Moreover, due to a reaction to the rear end of the tip shaft  11  being pressed by the rear shaft taper portion  12   a  with the tip shaft inner projection  11   a  acting as a pivot point, positions at the front side of the tip shaft inner projection  11   a  are pressed toward the outside, raising the stopper action of the tip shaft outer projections  11   b  and the rear shaft inner projections  12   b , and enabling stronger fixing to be achieved. 
     Second Example 
     Explanation follows regarding a second example according to an embodiment of the present invention, with reference to  FIG. 10  and to  FIG. 11 . Note that the  FIG. 5  to  FIG. 9  of the first example are common to the second example. Moreover, configuration elements in the second example similar to those of the first example are allocated the same reference numerals as the reference numerals employed in the first example. Explanation is omitted of parts duplicating those of the first example, with explanation focusing on the features of the second example. 
     The second example is an embodiment in which the tip shaft outer projections  11   b  exhibit a screw thread profile, and fixing is by screwing the tip shaft outer projections  11   b  together with the rear shaft inner projections  12   b.    
     Overall Structure 
     As illustrated in  FIG. 10A  and  FIG. 10B , a ballpoint pen  1  includes a ballpoint pen tip  40  with a writing ball  50  clasped at the leading end, an ink supply section  30  that supplies ink to the ballpoint pen tip  40 , a joint  60  that connects the ballpoint pen tip  40  and the ink supply section  30  together, a shaft  10  internally housing the ballpoint pen tip  40 , joint  60 , and ink supply section  30 , and an outer member  20  that is installed to the leading end of the shaft  10  and covers the outer periphery of the ballpoint pen tip  40 . 
     Tip Shaft Structure 
     As illustrated in  FIG. 11 , a tip shaft  11  of the second example features tip shaft outer projections  11   b  exhibiting a screw thread profile formed along the outer peripheral face of a tip shaft rear portion  14 . Note that the configuration of other parts of the tip shaft  11  is common to that of the first example, and so explanation thereof is omitted. 
     Fitting Method of the Tip Shaft, the Rear Shaft and the Ink Tank 
     The method of fixing the ink tank  13  and the tip shaft  11  together is common to that of the first example, and so explanation thereof is omitted. 
     The method of fixing the rear shaft  12  and the ink tank  13  together is common to that of the first example, and so explanation thereof is omitted. 
     Then, the tip shaft  11  is inserted into the rear shaft  12  so that the tip shaft outer projections  11   b  are screwed together with the rear shaft inner projections  12   b , and, at the same time, the rear end of the tip shaft  11  is compressed inward by the rear shaft taper portion  12   a , the tip shaft inner projection  11   a  acts as a pivot point and a gap is compressed, and the rear end of the tip shaft  11  is pressed against the ink tank  13 , such that the rear shaft  12  and the tip shaft  11  are fixed. 
     This invention may be employed as a writing implement, such as a ballpoint pen or a marker pen.