Abstract:
A printer including: a platen roller; a housing that holds the platen roller; and a print head that is mounted on the housing and performs printing on a sheet; wherein the platen roller includes a columnar straight part, and a first taper part that is formed on both ends of the straight part and has a diameter which changes towards an outside from the straight part along an axial direction.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2016-133770 filed on Jul. 5, 2016, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    A certain aspect of the embodiments is related to a printer. 
       BACKGROUND 
       [0003]    There has been known a card conveying device that holds a card between a print head and a platen roller and conveys the card by the rotation of the platen roller (e.g. see Patent Document 1: Utility Model Application Laid-Open Publication No. 6-30856). By making the surface of this platen roller into a hand drum shape, a conveying amount per one rotation of both ends of the platen roller becomes large and the conveying amount per one rotation of the center of the platen roller becomes small. Therefore, a centering force occurs against the card. By making the surface of the platen roller into a taper shape and pressing the card to a guide in the side of a small conveying amount, the meandering of the conveyance of the card is suppressed. 
       SUMMARY 
       [0004]    According to an aspect of the present invention, there is provided a printer including: a platen roller; a housing that holds the platen roller; and a print head that is mounted on the housing and performs printing on a sheet; wherein the platen roller includes a columnar straight part, and a first taper part that is formed on both ends of the straight part and has a diameter which changes towards an outside from the straight part along an axial direction. 
         [0005]    The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
         [0006]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0007]      FIG. 1  is a perspective view of a platen roller unit and a platen roller included in a thermal printer according to a present embodiment; 
           [0008]      FIG. 2A  is a diagram illustrating a position relationship of a platen miler  20  and a sheet  2 ; 
           [0009]      FIG. 2B  is a perspective view illustrating the position relationship of a thermal head  11 , the platen roller  20  and the sheet  2 ; 
           [0010]      FIG. 3A  is a diagram illustrating the position relationship of the platen roller  20  and the sheet  2 ; 
           [0011]      FIG. 3B  is a diagram illustrating the state transition of the thermal head  11 , a taper part  20 B of the platen roller  20  and the sheet  2 ; 
           [0012]      FIG. 4A  is a top view illustrating a first variation of the platen roller  20 ; 
           [0013]      FIG. 4B  is a perspective view illustrating the first variation of the platen roller  20 : 
           [0014]      FIG. 4C  is a diagram illustrating a normal deformation state of the taper part  20 B at a lowest point; 
           [0015]      FIG. 4D  is a diagram illustrating an abnormal deformation state of the taper part  20 B at the lowest point; 
           [0016]      FIG. 5A  is a top view illustrating a second variation of the platen roller  20 ; 
           [0017]      FIG. 5B  is a perspective view illustrating the second variation of the platen roller  20 ; 
           [0018]      FIG. 5C  is a partial cross sectional view of the thermal head  11 , the platen roller  20  according to the second variation, and a right end R of the sheet  2 ; 
           [0019]      FIG. 6A  is a diagram illustrating the position relationship of the sheet  2  and the platen roller  20  according to the second variation; 
           [0020]      FIG. 6B  is a diagram illustrating the state transition of the thermal head  11 , the sheet  2  and a taper part  20 C of the platen roller  20  according to the second variation; 
           [0021]      FIG. 7  is a top view illustrating a third variation of the platen roller  20 ; 
           [0022]      FIG. 8A  is a diagram illustrating the position relationship of the sheet  2 , the thermal head  11 , and the platen roller  20  and a shield  31  which are mounted on a shaft  21 ; 
           [0023]      FIG. 8B  is a top view illustrating a part of the platen roller  20  and the shield  31  which are mounted on the shaft  21 ; 
           [0024]      FIG. 8C  is a side view illustrating the platen roller  20  and the shield  31  which are mounted on the shaft  21 ; 
           [0025]      FIG. 9  is a diagram illustrating the state transition of the shield  31 , the thermal head  11 , the taper part  20 B of the platen roller  20  and the sheet  2 ; 
           [0026]      FIG. 10A  is a diagram illustrating the position relationship of the sheet  2 , the thermal head  11 , and the platen roller  20  of the second variation and a shield  32  which are mounted on the shaft  21 ; 
           [0027]      FIG. 10B  is a top view illustrating a part of the platen roller  20  of the second variation and the shield  32  which are mounted on the shaft  21 ; 
           [0028]      FIG. 10C  is a side view illustrating the platen roller  20  of the second variation and the shield  32  which are mounted on the shaft  21 ; 
           [0029]      FIG. 11  is a diagram illustrating the state transition of the shield  32 , the thermal head  11 , the sheet  2  and the taper part  20 C of the platen roller  20  according the second variation; 
           [0030]      FIG. 12A  is a diagram illustrating a variation of a platen roller unit  1 ; 
           [0031]      FIG. 12B  is a diagram illustrating a first variation of a shield  40 ; and 
           [0032]      FIG. 12C  is a diagram illustrating a first variation of a shield  41 . 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0033]    In the card conveying device of the Patent Document 1, when a sheet softer than the card is used, a wrinkle might occur in the center of the width direction of the sheet. Moreover, when the sheet skews in a printer, the sheet contacts a guide part of a platen roller unit, and folding of the sheet occurs. 
         [0034]    A description will now be given of an embodiment according to the present invention with reference to drawings. 
         [0035]      FIG. 1  is a perspective view of a platen roller unit and a platen roller included in a thermal printer according to a present embodiment. 
         [0036]    A platen roller unit  1  includes a housing  10  and a thermal head  11  that performs printing on a sheet by heat. Concave parts  13  are formed on right and left wall parts  10 A of the housing  10 , and a shaft  21  of a platen roller  20  is fitted in and fixed to the concave parts  13 . The shaft  21  is a rotational shaft which passes through the center of the platen roller  20 . 
         [0037]    The thermal head  11  is mounted on the housing  10  so as to move in front and rear directions. Moreover, a spring  12  is fixed between the housing  10  and the thermal head  11 , and the spring  12  energizes the thermal head  11  in a front direction (i.e., a direction toward the platen roller  20 ). When the shaft  21  of the platen roller  20  is attached to the concave parts  13 , the platen roller  20  is opposite to and contacts the thermal head  11 . When a roll thermal paper (hereinafter referred to as “a sheet”) is inserted between a base  10 B of the housing  10  and the platen roller  20 , for example, the sheet is conveyed towards the thermal head  11  by the rotation of the platen roller  20 . The sheet is nipped between the thermal head  11  and the platen roller  20  and is printed by the thermal head  11 . The printed sheet is discharged in an upper direction of  FIG. 1 . Here, an A-direction of  FIG. 1  indicates a conveyance direction of the sheet. 
         [0038]      FIG. 2A  is a diagram illustrating a position relationship of the platen roller  20  and a sheet  2 .  FIG. 2B  is a perspective view illustrating the position relationship of the thermal head  11 , the platen roller  20  and the sheet  2 . 
         [0039]    The platen roller  20  is made of a heat-resistant rubber which does not melt at a temperature of the thermal head  11 . Moreover, the platen roller  20  includes a columnar straight part  20 A, and taper parts (a first taper part)  20 B that are formed on both ends of the straight part  20 A and have a diameter which continuously or linearly increases towards the outside from the straight part  20 A along an axial direction. Each taper part  20 B includes an inclined plane  22  formed so that the diameter of the taper part  20 B continuously increases. The diameter of the straight part  20 A is constant. The taper parts  20 B are formed integrally with the straight part  20 A, but may be formed separately from the straight part  20 A. When the taper parts  20 B are formed separately from the straight part  20 A, for example, members corresponding to the taper parts  20 B cover both ends of a member corresponding to the straight part  20 A, and the whole shape becomes the shape of the platen roller  20  illustrated in  FIG. 2 . A width W 1  of the straight part  20 A is equal to or more than a width W 2  of the sheet  2 . 
         [0040]      FIG. 3A  is a diagram illustrating the position relationship of the platen roller  20  and the sheet  2 .  FIG. 3B  is a diagram illustrating the state transition of the thermal head  11 , the taper part  20 B of the platen roller  20  and the sheet  2 . 
         [0041]    Codes “ 1 A” to “ 1 E” of  FIG. 3A  indicate positions of the platen roller  20  opposite to the sheet  2 , respectively. Codes “ 1 A” to “ 1 E” of  FIG. 3B  illustrate partial cross sectional views of right ends of the thermal head  11 , the platen roller  20  and the sheet  2  at positions of the codes “ 1 A” to “ 1 E” of  FIG. 3A . It is assumed that a part of the sheet  2  protrudes to a position opposite to the taper port  20 B in  FIG. 3B . 
         [0042]    First, at the position “ 1 A” of  FIG. 3B , the taper part  20 B begins the deformation and lightly contacts the thermal head  11 , but the platen roller  20  does not touch the sheet  2 . At the position “ 1 B” of  FIG. 3B , the taper part  20 B continues the deformation and contacts a right end R of the sheet  2 . When the taper part  20 B contacts the right end R of the sheet  2 , a force that pushes out the sheet  2  to the outside of the platen roller  20  is applied to the right end R of the sheet  2 , by a frictional force between the taper part  20 B and the sheet  2  and the deformation of the taper part  20 B. However, the taper part  20 B contacts the thermal head  11 , and therefore the sheet  2  does not jump out to the outside of the platen roller  20 . 
         [0043]    A position “ 1 C” of  FIG. 3B  is a lowest point of the platen roller  20 . The taper part  20 B is crushed, the sheet  2  is nipped between the platen roller  20  and the thermal head  11 , and printing is performed. Also, the taper pan  20 B contacts the thermal head  11 , and therefore the sheet  2  does not jump out to the outside of the platen roller  20 . At a position “ 1 D” of  FIG. 3B , the deformation of the taper part  20 B begins to be restored, and the inclined plane  22  of the taper part  20 B contacts the right end R of the sheet  2 . When the inclined plane  22  of the taper part  20 B contacts the right end R of the sheet  2 , a force that returns the sheet  2  to the inside of the platen roller  20  is applied to the right end of the sheet  2 . At the position “ 1 E” of  FIG. 3B , the deformation of the taper part  20 B is restored, the taper part  20 B lightly contacts the thermal head  11 , and the platen roller  20  does not contact the sheet  2 . 
         [0044]    As described above, the platen roller  20  includes the taper parts  20 B that are formed on both ends of the straight part  20 A and have the diameter which increases towards the outside from the straight part  20 A along the axial direction. Therefore, even when the part of the sheet  2  protrudes to the position opposite to the taper part  20 B, the sheet  2  does not jump out to the outside of the platen roller  20 . It is possible to prevent the sheet  2  from folding. Moreover, since the platen roller  20  includes the columnar straight part  20 A which has no irregularity and is not a hand drum shape, a force in which the right and left ends of the sheet  2  advance to a central part of the platen roller  20  does not occur in the sheet  2 , and it is possible to prevent a wrinkle from occurring on the sheet  2 . 
         [0045]      FIG. 4A  is a top view illustrating a first variation of the platen roller  20 .  FIG. 4B  is a perspective view illustrating the first variation of the platen roller  20 .  FIG. 4C  is a diagram illustrating a normal deformation state of the taper part  20 B at a lowest point.  FIG. 4D  is a diagram illustrating an abnormal deformation state of the taper pan  20 B at the lowest point. 
         [0046]    In  FIGS. 4A and 4B , the taper pan  20 B is formed so that the diameter increases towards the outside from the straight part  20 A along the axial direction, as with  FIG. 2A , and further a plurality of grooves  25  in parallel with the axial direction are formed on the surface of the taper part  20 B. 
         [0047]    When the platen roller  20  is mounted on the concave parts  13 , the lowest point of the taper part  20 B usually deforms toward the outside of the platen roller  20 , as illustrated in  FIG. 4C . 
         [0048]    On the other hand, when the platen roller  20  is mounted on the concave parts  13 , the lowest point of the taper part  20 B may deform toward the inside of the platen roller  20  as illustrated in  FIG. 4D  by a mounting failure. Thus, even when the lowest point of the taper part  20 B deforms in an unintentional direction, since the plurality of grooves  25  in parallel with the axial direction are formed on the taper part  20 B, the taper part  20 B deformed in the unintentional direction can return to an original shape by the rotation of the platen roller  20 . 
         [0049]      FIG. 5A  is a top view illustrating a second variation of the platen roller  20 .  FIG. 5B  is a perspective view illustrating the second variation of the platen roller  20 .  FIG. 5C  is a partial cross sectional view of the thermal head  11 , the platen roller  20  according to the second variation, and the right end R of the sheet  2 . 
         [0050]    As illustrated in  FIGS. 5A and 5B , the platen roller  20  according to the second variation includes the columnar straight part  20 A, and taper parts  20 C that are formed on both ends of the straight part  20 A and have a diameter which decreases towards the outside from the straight part  20 A along the axial direction. As illustrated in  FIG. 5C , an end part of each taper part  20 C at a side of the straight part  20 A includes a wall part  23  substantially perpendicular to the thermal head  11 . A diameter of the wall part  23  is larger than the diameter of the straight part  20 A, and a diameter of an opposite end of each taper part  20 C is the same as the diameter of the straight part  20 A. The taper parts  20 C are formed integrally with the straight part  20 A, but may be separated from the straight part  20 A. Moreover, the plurality of grooves  25  in parallel with the axial direction are formed on the taper parts  20 C. 
         [0051]    When the plurality of grooves  25  are not formed on the taper parts  20 C, the wall part  23  contacts the right end R of the sheet  2  and prevents the sheet  2  from skewing so that the sheet  2  does not protrude to the outside of the straight part  20 A, as illustrated in  FIG. 5C . 
         [0052]    On the other hand, when the plurality of grooves  25  are formed on the taper parts  20 C, the sheet  2  cannot be sufficiently nipped between the grooves  25  and the thermal head  11 , and hence the sheet  2  may protrude to the outside of the straight part  20 A. 
         [0053]      FIG. 6A  is a diagram illustrating the position relationship of the sheet  2  and the platen roller  20  according to the second variation.  FIG. 6B  is a diagram illustrating the state transition of the thermal head  11 , the sheet  2  and the taper part  20 C of the platen roller  20  according to the second variation. Codes “ 2 A” to “ 2 E” of  FIG. 6A  indicate positions of the platen roller  20  opposite to the sheet  2 , respectively. Codes “ 2 A” to “ 2 E” of  FIG. 6B  illustrate partial cross sectional views of right ends of the thermal head  11 , the platen roller  20  according to the second variation and the sheet  2 . It is assumed that a part of the sheet  2  protrudes to a position opposite to the taper part  20 C in  FIG. 6B . 
         [0054]    First, at the position “ 2 A” of  FIG. 6B , the taper part  20 C begins the deformation and lightly contacts the thermal head  11  via the sheet  2 . At the position “ 2 B” of  FIG. 6B , the taper part  20 C continues the deformation and contacts a right end R 1  of an upper surface of the sheet  2  in a surface contact manner. When the taper part  20 C contacts the right end R 1  of the upper surface of the sheet  2  in the surface contact manner, a force that pushes out the sheet  2  to the inside of the platen roller  20  is applied to the right end R of the upper surface of the sheet  2 , by the frictional force between the taper part  20 C and the sheet  2  and the deformation of the taper part  20 C. However, the sheet  2  is nipped between the taper part  20 C and the thermal head  11 , and therefore the sheet  2  does not jump out to the outside of the platen roller  20 . 
         [0055]    A position “ 2 C” of  FIG. 6B  is the lowest point of the platen roller  20 . The taper part  20 C is crushed, the sheet  2  is nipped between the platen roller  20  and the thermal head  11 , and printing is performed. Also, the sheet  2  is nipped between the taper part  20 C and the thermal head  11 , and therefore the sheet  2  does not jump out to the outside of the platen roller  20 . At a position “ 2 D” of  FIG. 6B , the deformation of the taper part  20 C begins to be restored, and the taper part  20 C contacts the right end R 1  of the upper surface of the sheet  2 , and a force that returns the sheet  2  to the outside of the platen roller  20  is applied to the right end R 1  of the upper surface of the sheet  2 . At the position “ 2 E” of  FIG. 6B , the deformation of the taper part  20 C is restored, the taper part  20 C lightly contacts the thermal head  11  via the sheet  2 . 
         [0056]    As described above, the platen roller  20  includes the taper parts  20 C that are formed on both ends of the straight part  20 A and have the diameter which decreases towards the outside from the straight part  20 A along the axial direction. Therefore, even when the part of the sheet  2  protrudes to the position where it is nipped between the taper part  20 C and the thermal head  11 , the sheet  2  does not jump out to the outside of the platen roller  20 . It is possible to prevent the sheet  2  from folding. Moreover, since the platen roller  20  includes the columnar straight part  20 A which has no irregularity, the force in which the right and left ends of the sheet  2  advance to the central part of the platen roller  20  does not occur in the sheet  2 , and it is possible to prevent the wrinkle from occurring on the sheet  2 . 
         [0057]      FIG. 7  is a top view illustrating a third variation of the platen roller  20 . 
         [0058]    The platen roller  20  includes the columnar straight part  20 A, first taper parts  20 D that are formed on both ends of the straight part  20 A and have a diameter D 1  which increases towards the outside from the straight part  20 A along the axial direction, and second taper parts  20 E that are formed on the outside of the first taper parts  20 D along the axial direction and have a diameter D 2  which continuously increases towards the outside from the straight part  20 A along the axial direction and is larger than the diameter D 1  of the first taper parts  20 D. The straight part  20 A, the first taper parts  20 D and the second taper parts  20 E are formed integrally with each other, but may be formed separately from each other. Here, each of the first taper parts  20 D and the second taper parts  20 E may include the plurality of grooves  25  in parallel with the axial direction. 
         [0059]    In the structure of  FIG. 7 , even when the part of the sheet  2  protrudes to a position opposite to the second taper part  20 E over the first taper part  20 D, the sheet  2  does not jump out to the outside of the platen roller  20 . It is possible to prevent the sheet  2  from folding. 
         [0060]      FIG. 8A  is a diagram illustrating the position relationship of the sheet  2 , the thermal head  11 , and the platen roller  20  and a shield  31  which are mounted on the shaft  21 .  FIG. 8B  is a top view illustrating a part of the platen roller  20  and the shield  31  which are mounted on the shaft  21 .  FIG. 8C  is a side view illustrating the platen roller  20  and the shield  31  which are mounted on the shaft  21 .  FIG. 9  is a diagram illustrating the state transition of the shield  31 , the thermal head  11 , the taper part  20 B of the platen roller  20  and the sheet  2 . Here, the platen roller  20  of  FIGS. 8A to 8C and 9  is the same as that of  FIG. 2A . 
         [0061]    Codes “ 3 A” to “ 3 E” of  FIG. 9  correspond to positions “ 3 A” to “ 3 E” of  FIG. 8C , respectively, and indicate the state of the shield  31 , the thermal head  11 , the taper part  20 B of the platen roller  20  and the sheet  2  at the respective positions. 
         [0062]    As illustrates in  FIGS. 8A and 8B , the shaft  21  axially passes through the center of the platen roller  20  and the shield  31 , and the shield  31  is provided on both ends of the platen roller  20 . The shield  31  includes a ring-like mounting part  31 A and a plate-like cover part  31 B. The mounting part  31 A is fixed to the shaft  21 . The cover part  31 B extends in a radial direction of the platen roller  20  from the mounting part  31 A and is bent along the shape of the taper part  20 B so as to cover a part of the taper part  20 B. The cover part  31 B is made of an elastic body such as a plate spring. An apical part  31 C of the cover part  31 B is nipped between the platen roller  20  and the thermal head  11  or the sheet  2 , and is elastically deformable. The apical part  31 C of the cover part  31 B prevents the sheet  2  from contacting the taper part  20 B in order to prevent the sheet  2  from moving toward the outside from the straight part  20 A. Even when the apical part  31 C of the cover part  31 B contacts the sheet  2 , the frictional force does not almost act between the apical part  31 C and the sheet  2 . 
         [0063]    As illustrated in  FIG. 8C  and  FIG. 9 , the apical part  31 C of the cover part  31 B is formed at positions corresponding to the positions “ 3 A” and “ 3 B”. That is, the apical part  31 C of the cover part  31 B covers a domain of the inclined plane  22  of the taper part  20 B between a position where the taper part  20 B begins the deformation by the pressure from the thermal head  11  and a position just before the lowest point “ 3 C” of the taper part  20 B. 
         [0064]    At the position “ 3 A” of  FIGS. 8C and 9 , the taper part  20 B begins the deformation. The taper part  20 B lightly contacts the apical part  31 C of the cover part  31 B and does not the sheet  2 . At the position “ 3 B” of  FIGS. 8C and 9 , the taper part  20 B continues the deformation and the apical part  31 C of the cover part  31 B contacts the right end R of the sheet  2 . Even when the apical part  31 C of the cover part  31 B contacts the sheet  2 , the frictional force does not almost act between the apical part  31 C and the sheet  2 , and hence the force that pushes out the sheet  2  to the outside of the platen roller  20  does not occur. 
         [0065]    A position “ 3 C” of  FIGS. 8C and 9  is the lowest point of the platen roller  20 . The apical part  31 C of the cover part  31 B does not cover the taper part  20 B. The taper part  20 B is crushed, the sheet  2  is nipped between the platen roller  20  and the thermal head  11 , and printing is performed. At a position “ 3 D” of  FIGS. 8B and 9 , the deformation of the taper part  20 B begins to be restored, and the inclined plane  22  of the taper part  20 B contacts the right end R of the sheet  2 . When the inclined plane  22  of the taper part  20 B contacts the right end R of the sheet  2 , the force that returns the sheet  2  to the inside of the platen roller  20  is applied to the right end R of the sheet  2 . At the position “ 3 E” of  FIGS. 8B and 9 , the deformation of the taper part  20 C is restored, the taper part  20 B lightly contacts the thermal head  11  and the platen roller  20  does not the sheet  2 . 
         [0066]    Thus, since the apical part  31 C of the cover part  31 B covers the domain of the taper part  20 B between the position where the taper part  20 B begins the deformation by the pressure from the thermal head  11  and the position just before the lowest point “ 3 C” of the taper part  20 B, only the force that returns the sheet  2  to the inside of the platen roller  20  is applied to the sheet  2 , as illustrated at the position “ 3 D” of  FIGS. 8C and 9 . Therefore, it is possible to prevent the sheet  2  from skewing and prevent the sheet  2  from folding. Moreover, since the platen roller  20  includes the columnar straight part  20 A which has no irregularity, the force in which the right and left ends of the sheet  2  advance to the central part of the platen roller  20  does not occur in the sheet  2 , and it is possible to prevent the wrinkle from occurring on the sheet  2 . 
         [0067]      FIG. 10A  is a diagram illustrating the position relationship of the sheet  2 , the thermal head  11 , and the platen roller  20  of the second variation and a shield  32  which are mounted on the shaft  21 .  FIG. 10B  is a top view illustrating a part of the platen roller  20  of the second variation and the shield  32  which are mounted on the shaft  21 .  FIG. 10C  is a side view illustrating the platen roller  20  of the second variation and the shield  32  which are mounted on the shaft  21 .  FIG. 11  is a diagram illustrating the state transition of the shield  32 , the thermal head  11 , the sheet  2  and the taper part  20 C of the platen roller  20  according the second variation. Here, the platen roller  20  of  FIGS. 10A to 10C and 11  is the same as that of  FIG. 5A . 
         [0068]    Codes “ 4 A” to “ 4 E” of  FIG. 11  correspond to positions “ 4 A” to “ 4 E” of  FIG. 10C , respectively, and indicate the state of the shield  32 , the thermal head  11 , the taper part  20 C of the platen roller  20  and the sheet  2  at the respective positions. 
         [0069]    As illustrated in  FIGS. 10A and 10B , projects  26  and the grooves  25  are alternately formed on a periphery of the taper part  20 C. The shaft  21  axially passes through the center of the platen roller  20  and the shield  32 , and the shield  32  is provided on both ends of the platen roller  20 . The shield  32  includes a ring-like mounting part  32 A and a plate-like cover part  32 B. The mounting part  32 A is fixed to the shaft  21 . The cover part  32 B extends in a radial direction of the platen roller  20  from the mounting part  32 A and is bent along the shape of the taper part  20 C so as to cover a part of the taper part  20 C. 
         [0070]    The cover part  32 B is made of the elastic body such as the plate spring. An apical part  32 C of the cover part  32 B is nipped between the platen roller  20  and the thermal head  11  or the sheet  2 , and is elastically deformable. The apical part  32 C of the cover part  32 B prevents the sheet  2  from contacting the taper part  20 C in order to prevent the sheet  2  from moving toward the outside from the straight part  20 A. Even when the apical part  32 C of the cover part  32 B contacts the sheet  2 , the frictional force does not almost act between the apical part  32 C and the sheet  2 . 
         [0071]    As illustrated in  FIG. 10C  and  FIG. 1 , the apical part  32 C of the cover part  32 B is formed at positions corresponding to the positions “ 4 A”, “ 4 D” and “ 4 E”. The apical part  32 C of the cover part  32 B covers a domain of the taper part  20 C between a position just behind the lowest point of the taper part  20 C and a position where the taper part  20 B finishes the deformation, and a part of the projection  26  adjacent to the domain via the groove  25  in the conveyance direction of the sheet. That is, the apical part  32 C of the cover part  32 B covers a domain of the taper part  20 C generating the force that pushes out the sheet  2  to the outside of the platen roller  20 . 
         [0072]    At the position “ 4 A” of  FIGS. 10C and 11 , the deformation of the taper part  20 C is finished. The apical part  32 C of the cover part  32 B lightly contacts the sheet  2 . At a position “ 4 B” of  FIGS. 10C and 11 , the apical part  32 C of the cover part  32 B does not cover the taper part  20 C. The taper part  20 C resumes the deformation and contacts the right end R 1  of the upper surface of the sheet  2 , and the force that returns the sheet  2  to the inside of the platen roller  20  is applied to the right end R 1  of the upper surface of the sheet  2 . 
         [0073]    The position “ 4 C” of  FIGS. 10C and 11  is the lowest point of the platen roller  20 . The apical part  32 C of the cover part  32 B does not cover the taper part  20 C. The taper part  20 C is crushed, the sheet  2  is nipped between the platen roller  20  and the thermal head  11 , and printing is performed. At the position “ 4 D” of  FIGS. 10C and 11 , the apical part  32 C of the cover part  32 B covers the taper part  20 C. The deformation of the taper part  20 C begins to be restored and the apical part  32 C of the cover part  32 B contacts the sheet  2 , but the frictional force does not almost act between the apical part  32 C and the sheet  2 . Therefore, the force that pushes out the sheet  2  to the outside of the platen roller  20  does not occur. At the position “ 4 E” of  FIGS. 10C and 11 , the apical part  32 C of the cover part  32 B covers the taper part  20 C. The deformation of the taper part  20 C is restored, and the apical part  32 C of the cover part  32 B lightly contacts the sheet  2 . 
         [0074]    Thus, since the apical part  32 C of the cover part  32 B covers the domain (i.e., the positions  4 A,  4 D and  4 E) of the taper part  20 C generating the force that pushes out the sheet  2  to the outside of the platen roller  20 , only the force that returns the sheet  2  to the inside of the platen roller  20  is applied to the sheet  2 , as illustrated at the position “ 4 B” of  FIGS. 10C and 11 . Therefore, it is possible to prevent the sheet  2  from skewing and prevent the sheet  2  from folding. Moreover, since the platen roller  20  includes the columnar straight part  20 A which has no irregularity, the force in which the right and left ends of the sheet  2  advance to the central part of the platen roller  20  does not occur in the sheet  2 , and it is possible to prevent the wrinkle from occurring on the sheet  2 . 
         [0075]      FIG. 12A  is a diagram illustrating a variation of the platen roller unit  1 .  FIG. 12B  is a diagram illustrating a first variation of a shield  40 .  FIG. 12C  is a diagram illustrating a first variation of a shield  41 . 
         [0076]    As illustrated in  FIGS. 8B and 10B , each of the shields  31  and  32  is fixed to the shaft  21 . 
         [0077]    As illustrated in  FIGS. 12A and 12B , a pair of shield  40  fixed to the right and left wall parts  10 A of the housing  10  may be provided to prevent the sheet  2  from contacting the taper part  20 B, as substitute for the shield  31 . In this case, a domain where the shield  40  covers the taper part  20 B is the same as the domain where the shield  31  covers the taper part  20 B. 
         [0078]    As illustrated in  FIGS. 12A and 12C , a pair of shield  41  fixed to the right and left wall parts  10 A of the housing  10  may be provided to prevent the sheet  2  from contacting the taper part  20 C, as substitute for the shield  32 . In this case, a domain where the shield  41  covers the taper part  20 C is the same as the domain where the shield  32  covers the taper part  20 C. 
         [0079]    Thus, also when the shield  40  or  41  is fixed to the wall parts  10 A of the housing  10 , it is possible to prevent the sheet  2  from skewing and prevent the sheet  2  from folding. 
         [0080]    All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various change, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.