TECHNICAL FIELD 
       [0001]    The present invention relates to a cartridge-type dispenser suitable for extruding viscous materials such as composite resin used for dental treatment or the like, wherein a contents-extruding piston does not fall easily out of a dispenser main body and yet is capable of extruding the contents with a smooth action. 
       BACKGROUND ART 
       [0002]    Paste materials with high viscosity, such as composite resin and adhesive agent, are often used in dental treatment or the like and generally extruded from a syringe-type dispenser or a cartridge-type dispenser. In the syringe-type dispenser, contents are extruded from a nozzle by applying a push-in force to a plunger fitted in a syringe main body to slide the plunger in the syringe main body. On the other hand, in the cartridge-type dispenser wherein a piston is arranged in a cartridge main body (referred to below simply as a “main body”), the contents are extruded by fitting the main body to a separately prepared push-in tool (trigger-type push-in tool) and pushing in the piston. (Refer, e.g., to Patent Document 1 identified below.) 
         [0003]    In particular, in the cartridge-type dispenser, the contents filled in the main body are disposed once used in a single treatment, and from perspectives of quality assurance and hygiene, a dispenser of this type is frequently used in recent years. However, since there is a variation and the like in molding accuracy of the piston and the main body which constitute the dispenser, in case where a diameter size of the piston is small, the piston can be easily disengaged from the main body. On the other hand, in case where the diameter size of the piston is large, although this case has an advantage of improved airtightness, the piston can be disengaged from the main body due to an increase in an inner pressure when the piston is installed after the contents have been filled in, and as a result, assembly becomes laborious, and moreover, an operation of extruding the contents can be slowed down, which sometimes causes malfunction in which the contents cannot be smoothly extruded. 
       (Patent Document) 
       [0000]    
       
         Patent Document 1: JP 2001-057987A 
       
     
       DISCLOSURE OF THE INVENTION 
     Technical Problem 
       [0005]    It is an objective of the present invention to provide a cartridge-type dispenser suitable for extruding viscous materials, wherein a piston does not easily fall out and yet is capable of extruding contents with a smooth action. 
       Solution to Problem 
       [0006]    The present invention provides a cartridge-type dispenser comprising: a cartridge main body defining a filling space for contents and provided with an extrusion nozzle at a front end portion of the cartridge main body; and a piston slidably arranged in the filling space of the cartridge main body and configured to cause the contents of the filling space to be extruded from an end of the extrusion nozzle in response to application of a push-in force. The piston includes: a trunk portion having an outer circumferential wall arranged opposite to an inner circumferential wall of the cartridge main body so as to define a gap therebetween; a front end wall portion provided at one end portion of the trunk portion in direct contact with the contents of the filling space; and a rear end wall portion provided at another end portion of the trunk portion to be pressed with a rod of a push-in tool for the application of the push-in force to the piston itself. The outer circumferential wall of the trunk portion is provided with a protrusion, which is in contact with the inner circumferential wall of the cartridge main body to form a sliding part of the piston without sealing off an interior of the cartridge main body. 
       Advantageous Effects of the Invention 
       [0007]    Provision of a protrusion on the trunk portion of the piston, as a sliding part of the piston, makes it possible to reduce the contact area between the piston and the inner circumferential wall of the main body. Accordingly, even when the piston is firmly fitted to the main body at the sliding part to prevent the piston from being easily disengaged from the main body, the piston is able to slide smoothly, thereby improving operability. Furthermore, a tiny gap is formed between the piston and the main body in a part of the piston other than the sliding part. As a result, when the piston is assembled after the contents are filled in, air remaining in the main body is discharged to the outside through the gap, and the inner pressure is not increased. 
         [0008]    With an hourglass shape of the trunk portion of the piston having the reduced diameter at the middle portion in the push-in direction of the piston, it is possible to further reduce the contact area between the piston and the inner circumferential wall of the main body. 
         [0009]    Furthermore, when the sizes of the one end portion and the other end portion of the trunk portion of the piston are D 1  and D 2  (where D 1  is closer to a front end portion, and D 2  is closer to a rear end portion), and the size of the interior of the main body is X, it is preferable that a difference between the sizes D 1 , D 2  and X, i.e. the gap formed between the main body and the trunk portion of the piston, satisfies the conditions X−D 1 ≦1 mm and X−D 2 ≦1 mm, because in this case the contents are in a good state of preservation for a longer period of time. Here, from the perspective of securing the gap, it is preferable that 0.01 mm≦X−D 1 ≦1 mm and 0.01 mm≦X−D 2 ≦1 mm. It is further preferable that 0.02 mm≦X−D 1 ≦0.5 mm and 0.02 mm≦X−D 2 ≦0.5 mm. It is most preferable that 0.04 mm≦X−D 1 ≦0.2 mm and 0.04 mm≦X−D 2 ≦0.2 mm. 
         [0010]    Moreover, when the sizes of the one end portion and the other end portion of the trunk portion of the piston, inclusive of the protrusion, are D 3  and D 4  (where D 3  is closer to the front end portion, and D 4  is closer to the rear end portion), it is preferable that a difference between the sizes D 3 , D 4  and X, i.e. the size when the protrusion is fitted to the main body, satisfies D 3 −X≧0.01 mm and D 4 −X≧0.01 mm, because in this case the piston is further prevented from disengagement. Here, from the perspective of achieving a smooth sliding movement, it is preferable that 0.5 mm≧D 3 −X≧0.01 mm and 0.5 mm≧D 4 −X≧0.01 mm. It is further preferable that 0.3 mm≧D 3 −X≧0.05 mm and 0.3 mm≧D 4 −X≧0.05 mm. It is most preferable that 0.26 mm≧D 3 −X≧0.10 mm and 0.26 mm≧D 4 −X≧0.10 mm. On the other hand, when a total length of the piston in the push-in direction is D 5 , the length D 5  needs to be determined as appropriate, because an appropriate length of D 5  depends on a length of the interior of the main body in the push-in direction, as well as a stroke of the push-in tool. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0011]      FIG. 1A  shows a general embodiment of a dispenser according to the present invention in a cross sectional lateral view, and  FIG. 1B  shows the general embodiment of the dispenser according to the present invention when the dispenser shown in  FIG. 1A  is viewed from a rear end wall thereof. 
           [0012]      FIG. 2  is a perspective appearance view of a piston arranged in the dispenser shown in  FIGS. 1A and 1B , with three protrusions provided on a cylindrical trunk portion thereof. 
           [0013]      FIG. 3  shows another example of the piston favorably applicable to the present invention in which middle sections of the protrusions provided in the piston are removed to divide the ribs into upper and lower end sections so as to further reduce the contact area between the piston and the main body. 
           [0014]      FIG. 4  shows another example of the piston favorably applicable to the present invention in which the protrusions are provided in a portion of the piston other than the end portions thereof with respect to  FIG. 3 . 
           [0015]      FIG. 5  shows another example of the piston favorably applicable to the present invention in which a single protrusion is helically formed. 
           [0016]      FIG. 6  shows another example of the piston favorably applicable to the present invention in which the trunk portion of the piston is depressed so that the piston does not make contact with the main body even when the piston itself is deformed. 
           [0017]      FIG. 7  shows another example of the piston favorably applicable to the present invention in which the protrusions provided in the upper and lower end portions of the piston shown in  FIG. 6  are joined. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0018]    The present invention will be described below in further detail, with reference to the accompanying drawings. 
         [0019]      FIGS. 1A and 1B  show a general structure of a dispenser according to the present invention in a cross sectional view, and  FIG. 2  is a perspective appearance view of a piston arranged in the dispenser shown in  FIG. 1A  and  FIG. 1B . 
         [0020]    Reference numeral  1  in the drawings denotes a main body defining a filling space M for contents. The main body  1  can be made of high-density polyethylene (HDPE), low-density polyethylene (LDPE), polypropylene (PP), acryl, polyethylene terephthalate (PET), polyoxy-methylene (POM), polybutylene terephthalate (PBT), and the like. The main body  1  includes at a front end portion thereof an integrally formed extrusion nozzle  1   a  and also includes around an outer circumferential wall of a rear end opening  1   b  thereof a flange  1   c  for linkage to a separately prepared push-in tool (not shown). The main body  1  is preferably made of PBT, in particular. 
         [0021]    Reference numeral  2  denotes the piston arranged in the main body  1 . The piston  2  has a trunk portion  2   a  having an outer circumferential wall arranged opposite to an inner circumferential wall of the main body  1  so as to define a gap therebetween, a front end wall portion  2   b  provided at one end portion of the trunk portion  2   a  in direct contact with the contents of the filling space M, and a rear end wall portion  2   c  provided at another end portion of the trunk portion  2   a  to be pressed with a rod (not shown) of a push-in tool for application of a push-in force to the piston  2  itself. As the trunk portion  2   a  of the piston  2 , a cylinder having sizes (diameters) D 1  and D 2  at the one end portion and the other end portion (where D 1  is closer to the front end wall portion  2   b , and D 2  is closer to the rear end wall portion  2   c ) and a size D in the middle portion of the trunk portion  2   a  in a push-in direction is used, where D 1 , D 2 , and D are equal. 
         [0022]    Reference numeral  3  denotes longitudinal ribbed protrusions integrally formed on the trunk portion  2   a  of the piston  2 . The protrusions  3  are provided at three positions on the outer circumferential wall at regular intervals such that an end of each protrusion  3  contacts the inner circumferential wall of the main body  1  for sliding in the main body  1  in conjunction with a movement of the piston  2 . Furthermore, sizes D 3  and D 4  of the piston  2  at the one end portion and the other end portion (where D 3  is closer to the front end wall portion  2   b , and D 4  is closer to the rear end wall portion  2   c ), inclusive of the protrusions  3 , are larger than an inner diameter X of the main body  1 . The material for the piston  2  may comprise elastomer, rubber, LDPE (such as linear low density polyethylene (LLDPE)), HDPE, PP, acryl, and the like. Among others, LDPE is preferably used. 
         [0023]    Reference numeral  4  denotes a nozzle cap mounted on the extrusion nozzle  1   a  to close off an extrusion orifice of the extrusion nozzle  1   a . In the embodiments illustrated in the drawings the nozzle cap  4  is linked by engagement means, such as undercuts. Alternatively, however, the nozzle cap  4  may be screwed by a thread. A fitting effect provided by the above engagement means or screw structure allows positioning of the cap, to thereby facilitate the assembling and prevent disengagement of the cap when the cap is loosely mounted and also prevent damage to the cap when the cap is tightly or deeply mounted. 
         [0024]    In case where the nozzle cap  4  is provided with the fitting effect, it is preferable to employ a hardly deformable nozzle cap  4 . The reason is that, although an easily deformable nozzle cap  4  has an advantage that it can be mounted even when the size is not precisely set, such an easily deformable cap tends to be disengaged or to be damaged. In order to restrict deformation of the nozzle cap  4  and provide the nozzle cap  4  with the fitting effect with the extrusion nozzle  1   a , HDPE, LDPE, PP, acryl, and the like can be used as the material of the nozzle cap  4 , and in particular, HDPE is preferably used. 
         [0025]    An outer diameter of the extrusion nozzle  1   a  is preferably as small as possible in the dental treatment, and therefore the inner diameter D 6  is necessarily small. As for the inner diameter D 6  of the extrusion nozzle  1   a , it is preferable that 0.01≦D 6 /X≦0.60, and it is further preferable that 0.25≦D 6 /X≦0.55. 
         [0026]    In the dispenser with the above structure, when the piston  2  is fitted to the main body  1 , a contact part between the piston  2  and the main body  1  is formed only by the protrusions  3 . Accordingly, the piston  2  is able to slide smoothly even in a fitting state (i.e. the state in which the piston  2  cannot undergo disengagement) where the protrusions  3  are in a tight contact with the inner circumferential wall of the main body  1 . 
         [0027]    A tiny gap P is formed between the piston  2  and the main body  1  in a region where the protrusions  3  are not provided (refer to  FIG. 1A ). When the piston  2  is gradually pushed in to be fitted to the main body  1  after the contents have been filled in, air remaining in the main body  1  is discharged through the gap into the outside, and an inner pressure is not increased. As a result, the piston  2  is prevented from being disengaged from the main body  1  due to an increase in the inner pressure. 
         [0028]      FIGS. 3 to 7  show different embodiments of the piston favorably applicable to the cartridge-type dispenser according to the present invention. 
         [0029]      FIG. 3  shows the piston  2  in which the protrusions  3  are provided at the end portions of the trunk portion  2   a  of the piston  2  while a shape of the trunk portion  2   a  remains cylindrical. In this case, contact of the piston  2  with the main body  1  is made at the protrusions  3  provided at a front end portion and a rear end portion of the piston  2 . Accordingly, a contact area is further reduced, and operability is further improved. 
         [0030]      FIG. 4  shows the piston  2  in which the protrusions  3  are provided in a portion of the trunk portion  2   a  other than the end portions thereof while the shape of the trunk portion  2   a  remains cylindrical. In this case, as in  FIG. 3 , the contact area is further reduced, and operability is further improved. 
         [0031]      FIG. 5  shows the piston  2  in which the single protrusion  3  is helically formed while the shape of the trunk portion  2   a  remains cylindrical, which shows an example of the smallest number of the protrusion still satisfying demand characteristics of the present invention. 
         [0032]      FIGS. 6 and 7  show the pistons  2  in each of which the size (diameter) D in the middle portion of the trunk portion  2   a  in a longitudinal direction thereof is minimized with respect to the sizes (diameters) D I , D 2  (D 1 =D 2 ) at the end portions of the trunk portion  2   a , so that the trunk portion  2   a  has an hourglass-shaped cross section tapering from the end portions toward the middle portion in the longitudinal direction. 
         [0033]    In each of the pistons  2  shown in  FIGS. 6 and 7  with the hourglass-shaped trunk portion  2 , as in  FIG. 3 , the contact of the piston  2  with the main body  1  is made at the protrusions  3  provided at the front end portion and the rear end portion of the piston  2 . Accordingly, the contact area is further reduced, and operability is further improved. 
         [0034]    The protrusions  3  may be provided only at the end portions of the trunk portion  2   a , over an entire length of the trunk portion  2   a , or even alternately provided at the front end portion and the rear end portion of the trunk portion  2   a  of the piston  2 . Particularly, when the protrusions  3  are provided alternately at the front end portion and the rear end portion of the trunk portion  2   a  of the piston  2 , intrusion of light through the gaps is better prevented, and therefore quality of the contents is maintained in a more stable manner. 
         [0035]    In addition, although the drawings show the examples of one, three, and six protrusions  3  provided along a circumference of the trunk portion  2   a , the number of the protrusions is not specifically limited. 
       Examples 
       [0036]    The following describes examples of the present invention. However, the present invention is not limited to the examples described below. 
         [0037]    As the cartridge-type dispensers, the bodies made of PBT and the pistons made of the materials described below were prepared, and a vibration test, a depressurization test, a loading test, and a curing test were conducted to evaluate performance of each dispenser. 
         [0038]    Vibration Test 
         [0039]    The main body fitted with a piston was laid on a pan secured to a vibration machine (TUBE MIXER TRIO HM-2F manufactured by As One Corporation) and vibrated for one minute at a dial reading of 10, and presence of disengagement of the piston from the main body was checked. 
         [0040]    Depressurization Test 
         [0041]    An extrusion nozzle was filled with CLEARFIL™ PHOTO CORE (manufactured by Kuraray Medical Inc.) and irradiated with light for twenty seconds with JET-LITE 3000 (manufactured by Morita Corporation) to be cured, and thus the extrusion nozzle was closed off. Subsequently, the piston was fitted to the main body and laid for three minutes at 70 Torr, and the presence of disengagement of the piston from the main body was checked. 
         [0042]    Loading Test 
         [0043]    A piston was fitted to the main body and pushed in at a cross-head speed of 100 mm/min with AUTOGRAPH (AGS-G 500N manufactured by Shimazu Corporation), and a maximum load value measured before the front end wall portion  2   b  of the piston comes into contact with the inner circumferential wall of the main body was measured as a force required for pushing in the piston. 
         [0044]    Curing Test 
         [0045]    The main body was filled with CLEARFIL PHOTO CORE and fitted with the piston and then irradiated from the direction of the piston with light for one minute with JET-LITE 3000, and presence of curing of the filler was checked. 
       Inventive Example 1 
       [0046]    The main body with X=4.42 mm, and the piston made of acryl and having a shape as shown in  FIG. 3  with D 1  (=D 2 )=4.30 mm, D 3  (=D 4 )=4.60 mm, and D 5 =6.80 mm were molded. Table 1 shows the results of evaluation. 
       Inventive Example 2 
       [0047]    The main body with X=4.42 mm, and the piston made of LDPE and having a shape as shown in  FIG. 7  with D 1  (=D 2 )=4.30 mm, D 3  (=D 4 )=4.60 mm, and D 5 =6.80 mm were molded. Table 1 shows the results of evaluation. 
       Comparative Example 1 
       [0048]    The main body with X=4.42 mm, and the piston made of LDPE without the protrusion and with D 1  (=D 2 )=4.30 mm and D 5 =6.80 mm were molded. Table 1 shows the results of evaluation. 
       Comparative Example 2 
       [0049]    The main body with X=4.42 mm, and the piston made of LDPE without the protrusion and with D 1  (=D 2 )=4.54 mm and D 5  3.40 mm were molded. Table 1 shows the results of evaluation. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
               
             
               
               
             
               
               
               
             
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Inventive 
                 Comparative 
               
               
                   
                 Examples 
                 Examples 
               
             
          
           
               
                   
                 1 
                 2 
                 1 
                 2 
               
               
                   
                   
               
             
          
           
               
                 Material of main body 
                 PBT 
               
               
                 Inner diameter size (X) of main 
                 4.42 
               
               
                 body (mm) 
               
             
          
           
               
                 Material of piston 
                 Acryl 
                 LDPE 
               
             
          
           
               
                 Shape of piston 
                 FIG. 3 
                 FIG. 7 
                 Without protrusion 
               
             
          
           
               
                 Diameter (D 1 ) of trunk portion 
                 4.30 
                 4.30 
                 4.30 
                 4.54 
               
               
                 of piston (mm) 
               
               
                 Diameter (D 3 ) of trunk portion 
                 4.60 
                 4.60 
                 N/A 
                 N/A 
               
               
                 of piston, inclusive of 
               
               
                 protrusion (mm) 
               
               
                 X − D 1  (mm) 
                 0.12 
                 0.12 
                 0.12 
                 −0.12  
               
               
                 D 3  − X (mm) 
                 0.18 
                 0.18 
                 N/A 
                 N/A 
               
               
                 Piston total length (D 5 ) 
                 6.80 
                 6.80 
                 6.80 
                 3.40 
               
               
                 in push-in direction (mm) 
               
               
                 Vibration test 
                 No 
                 No 
                 Yes 
                 No 
               
               
                 (piston disengagement from 
               
               
                 main body) 
               
               
                 Depressurization test 
                 No 
                 No 
                 No 
                 Yes 
               
               
                 (piston disengagement from 
               
               
                 main body) 
               
               
                 Loading test (N) 
                 16    
                 14    
                 0   
                 106    
               
               
                 Curing test 
                 No 
                 No 
                 No 
                 No 
               
               
                 (occurrence of curing of filler) 
               
               
                   
               
             
          
         
       
     
         [0050]    It will be appreciated from Table 1 that, in the cartridge-type dispenser in which the piston is not provided with the protrusion on an outer surface thereof and the piston has a diameter smaller than the inner diameter of the main body, the piston easily underwent disengagement in response to the vibration. Furthermore, in the cartridge-type dispenser in which the piston is not provided with the protrusion on the outer surface thereof and the piston has a diameter larger than the inner diameter of the main body, although the piston was not disengaged in response to the vibration, the piston inevitably underwent disengagement when the inner pressure increased, and moreover, the load measured at the time of pushing in the piston was remarkably large. On the other hand, in the cartridge-type dispenser according to the present invention, the piston did not undergo disengagement even when the vibration was applied and when the inner pressure was increased, and the load measured at the time of pushing in the piston was confirmed to be small. 
       INDUSTRIAL APPLICABILITY 
       [0051]    The present invention provides a cartridge-type dispenser suitable for extruding viscous materials that is capable of ensuring a smooth operation of extruding contents while preventing a piston from being easily disengaged from a main body. 
       REFERENCE NUMERALS 
       [0000]    
       
         
           
               1  Main body 
               1   a  Extrusion nozzle 
               1   b  Rear end opening 
               1   c  Flange 
               2  Piston 
               2   a  Trunk portion 
               2   b  Front end wall portion 
               2   c  Rear end wall portion 
               3  Protrusion 
               4  Nozzle cap 
             M Filling space 
             P Gap formed by piston and main body
A cartridge-type dispenser has a cartridge main body with a filling space, an extrusion nozzle on the front, and a piston slidably disposed in the filling space. The piston extrudes contents of the filling space from the extrusion nozzle by applying a push-in force. The piston includes a trunk portion having an outer circumferential wall running along an inner wall of the main body with a gap therebetween, a front wall on one end of the trunk portion directly contacting the contents of the filling space, and a rear wall on the other end whereon a rod of a push-in tool applies the push-in force on the piston. On the outer circumferential wall of the trunk portion, protrusions are provided, which contact the inner circumferential wall of the main body. The points of contact form sliding parts of the piston but do not seal the main body interior.