Abstract:
In order to realize an adjustment mechanism which can adjust the pressing force of a thermal head according to paper thickness, and in addition, which is suitable for a small thermal printer, in a small thermal printer in which a platen roller ( 11 ) is adapted to be detachably attachable to arms for rotatably holding the platen roller and room at the back of a thermal head where a spring ( 16 ) for pressing the thermal head ( 14 ) toward the platen roller is disposed is narrow, an auxiliary plate for receiving the spring is provided at the back of the thermal head, the auxiliary plate ( 13 ) is adapted to be approachable to or separable from a head support plate ( 15 ), and means ( 18, 41 ) which can adjust a position of the auxiliary plate with respect to a position of the head support plate ( 15 ) is provided.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a technique which is effective when used in a printing device, and further, a printing device (printer) with a printhead being in pressure contact with a platen roller and with a printing paper sheet sandwiched between the printhead and the platen roller for conducting printing. More particularly, the present invention relates to a thermal printing device (thermal printer) which can conduct satisfactory printing on printing paper sheets of different thicknesses. 
         [0003]    2. Description of the Related Art 
         [0004]    In a thermal printer which conducts printing with roll thermal paper (hereinafter referred to as roll paper) inserted in between a printhead having a heater element (hereinafter referred to as a thermal head) and a platen roller, because the thermal head is in pressure contact with the platen roller, when the roll paper is loaded, it is necessary to release the thermal head from the platen roller and to insert a leading edge of new roll paper thereinbetween. 
         [0005]    For example, Patent Document 1 (JP 2003-251837 A) and Patent Document 2 (JP 2006-116714 A) disclose a conventional technique for making it easier to insert roll paper in between a thermal head and a platen roller when the roll paper is loaded. In these patents, a cam is disposed at the back of the thermal head. The cam is adapted to be rotated to, against repulsion of a spring, press the thermal head against the platen roller. The cam is also adapted to be rotated in an opposite direction to move, using the repulsion of the spring, the thermal head away from the platen roller to create clearance therebetween into which roll paper is to be inserted. 
         [0006]    Further, as a technique for making it easier to set roll paper between a thermal head and a platen roller, there is a clam shell printer disclosed in, for example, Patent Document 3 (JP 2000-318260 A) in which the platen roller is detachable. 
         [0007]    In order to conduct correct printing by a thermal printer, it is necessary to press a thermal head against a platen roller and to position a heater element of the thermal head in a range where a surface of the platen roller made of rubber becomes flat (which is called a nip). Various kinds of thermal paper for printing are offered in the market, and it is required to conduct high quality printing on any kind of thermal paper. 
         [0008]    However, if the thickness of the thermal paper varies and still the thermal head is pressed against the platen roller with the same pressure, there is a possibility that the width of the nip varies and the quality of the printing is lowered. More specifically, as the thickness of the thermal paper becomes larger and the thermal paper becomes sturdy, the width of the nip becomes smaller, and thus, it is desirable that the pressing force of the thermal head be made to be larger as the thickness of the thermal paper becomes larger. However, in a conventional thermal printer, because a function of changing the pressing force of the thermal head is not provided, there is an inconvenience that the quality of the printing varies depending on the paper thickness. 
         [0009]    A problem of the structures disclosed in Patent Documents 1 and 2 that paper is difficult to insert is solved in the clam shell as disclosed in Patent Document 3.  FIG. 6  illustrates a conventional structure of a unit used in a clam shell printer. In this unit, hemispherical bearing concave portions  12   a  and  12   b  with which a shaft of a platen roller  11  can be engaged is formed at tips of arms  12 A and  12 B of a holder  12  for holding a head support plate  15  having a thermal head  14 . The shaft of the platen roller  11  is adapted to be detachably engaged with the bearing concave portions  12   a  and  12   b . A clam shell printer with such a unit also does not have a pressure changing mechanism as described in the present application, and, pressure necessary for thick paper is applied such that all recommended paper sheets can be used without the need for adjustment. However, the high pressure means that torque transferred by a motor becomes larger, and power consumption of and heat generated by the motor are bad. Also, because load applied to a gear which transfers power becomes heavier, the durability of the gear is decreased. Therefore, it is revealed that to accommodate both thin paper and thick paper with only one model forces a customer who only uses thin paper to put up with a lower specification due to a useless function, and, on the other hand, to set an additional model which accommodates thick paper is not preferable from the viewpoint of inventory control. 
         [0010]    Accordingly, the present inventors studied making adjustable the pressing force of the thermal head according to the paper thickness. In the inventions described in Patent Document 1 and Patent Document 2 described above, a cam is disposed at the back of the thermal head, and the pressing force is adapted to be applied to the thermal head using a repulsion of a leaf spring brought in contact with the cam or of a compression spring inserted between a plate brought in contact with the cam and the thermal head. By using such a cam, it is also possible to change the pressing force of the thermal head. 
         [0011]    However, a thermal printer to which the inventions described in Patent Document 1 and Patent Document 2 are applied is relatively large, and thus, there is room to dispose the cam at the back of the thermal head. On the other hand, in a small thermal printer which the present inventors regard as the target, there is not enough room at the back of the thermal head and it is difficult to dispose the cam. Therefore, if a cam is used to change the pressing force of the thermal head, there is a problem that the apparatus becomes large. 
         [0012]    Further, because, in the printers described in Patent Document 1 and Patent Document 2, the position of the platen roller is not movable with respect to the position of a member for rotatably holding the platen roller, it is necessary to keep enough distance between the platen roller and the thermal head when roll paper is loaded, and thus, the cam is used to move the thermal head a relatively large distance. Therefore, when the thermal head is moved by means other than the cam (for example, a feed screw), there are inconveniences that it takes much time to move the thermal head and the number of parts is increased. 
         [0013]    Further, in the printers described in Patent Document 1 and Patent Document 2, it is also possible that the cam for making it easier to load roll paper is adapted to be used to change the pressing force of the thermal head, but it is revealed that, if such a structure is adopted, because it is necessary to adjust the position of the cam every time a user loads roll paper such that optimum pressure can be obtained according to the thickness of the paper to be used, not only the setting is burdensome but also inappropriate pressure may be set by accident. 
       SUMMARY OF THE INVENTION 
       [0014]    It is an object of the present invention to provide an adjustment mechanism which can adjust the pressing force of a thermal head according to paper thickness, and in addition, which is suitable for a small thermal printer. 
         [0015]    Another object of the present invention is to provide a thermal printer in which the pressing force of a thermal head can be adjusted according to paper thickness, in which it is not necessary to adjust the pressing force of the thermal head every time when the same kind of printing paper sheets are used, which provides ease of use, and in which erroneous setting is less liable to occur. 
         [0016]    In order to attain the above objects, according to the present invention, in a small thermal printer in which a platen roller is adapted to be detachably attachable to arms for rotatably holding the platen roller and room at the back of a thermal head where a spring for pressing the thermal head toward the platen roller is disposed is narrow, an auxiliary plate for receiving the spring is provided at the back of the thermal head, the auxiliary plate is adapted to be approachable to or separable from a head support plate, and means which can adjust a position of the auxiliary plate with respect to a position of the head support plate is provided. 
         [0017]    More specifically, a thermal head having a heater element, a head support plate for holding the thermal head, a holder having a pair of arm portions for rotatably holding the head support plate and a plate portion for connecting respective one ends of the arm portions, a platen roller engaged with concave portions formed in part of the arm portions and disposed so as to be in proximity to the head support plate, an auxiliary plate rotatably provided between the plate portion of the holder and the head support plate, a spring inserted between the auxiliary plate and the head support plate, and adjustment means which can adjust a position of the auxiliary plate with respect to a position of the head support plate are provided. Here, the adjustment means may be, for example, a spacer which can be inserted between the plate portion and the auxiliary plate, or a screw which is inserted into a screw hole provided in the plate portion and a tip of which is adapted to be in contact with the auxiliary plate. 
         [0018]    According to the above-mentioned means, because the repulsion of the spring, and hence, the pressing force of the thermal head against the platen roller can be changed by adjusting the position of the auxiliary plate with respect to the position of the head support plate, satisfactory printing can be conducted by changing the pressing force of the thermal head according to the thickness of the printing paper sheet to be used. Therefore, a user is not required to prepare different printers according to the kind of the printing paper sheet to be used, and one printer can accommodate various kinds of printing paper sheets. Further, a manufacturer can prepare and provide with ease a printer in which the pressing force of the thermal head is optimally set according to the kind of a printing paper sheet to be used by a user. 
         [0019]    It is to be noted that, a printer which is provided at present can also be adapted to accommodate various kinds of printing paper sheets by pressurizing the platen with pressure which is higher than necessary, but, if such is done when the purpose is to use only thin paper, the lifetime of the printer is shortened wastefully. In a printer according to the present invention, because pressure which is optimum for the specifications of paper which a user wants to use can be applied, there is an advantage that, while necessary print quality is maintained, the printer lifetime can be set to be appropriate for the paper. 
         [0020]    Further, when a spacer is used as the adjustment means, it is desirable that a step portion be formed in the plate portion, and the position of the auxiliary plate with respect to the position of the head support plate is adapted to be changed by inserting the spacer between an area where the step portion exists and the auxiliary plate or between an area where the step portion does not exist and the auxiliary plate. This can change the pressing force of the thermal head against the platen roller with a relatively simple structure and with ease. 
         [0021]    Further, when a spacer is used as the adjustment means, it is desirable that a protrusion be provided on the auxiliary plate or the spacer and a positioning hole with which the protrusion can be engaged be provided in the spacer or the auxiliary plate. This can prevent the inserted spacer from being displaced. 
         [0022]    According to the present invention, an adjustment mechanism which can adjust the pressing force of the thermal head according to paper thickness, and in addition, which is suitable for a small thermal printer can be materialized. Further, there is an effect that a thermal printer can be materialized in which the pressing force of the thermal head can be adjusted according to paper thickness, which can thus improve the print quality and can make longer the lifetime of the printer, in which it is not necessary to adjust the pressing force of the thermal head every time when the same kind of printing paper sheets are used, which provides ease of use, and in which erroneous setting is less liable to occur. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    In the accompanying drawings: 
           [0024]      FIG. 1  is a perspective view illustrating a first embodiment of a printer unit to which the present invention is applied; 
           [0025]      FIG. 2  is a perspective view of the printer unit according to the first embodiment illustrating a state where a spacer is inserted between a plate portion of a holder and an auxiliary plate; 
           [0026]      FIG. 3  is a perspective view of the printer unit according to the first embodiment illustrating a state where the spacer is inserted between the plate portion and the auxiliary plate at a step portion of the holder; 
           [0027]      FIG. 4  is a perspective view illustrating a second embodiment of the printer unit to which the present invention is applied; 
           [0028]      FIG. 5  is a sectional side view illustrating an exemplary structure of a preferred printer using the printer unit according to the embodiment; and 
           [0029]      FIG. 6  is a perspective view illustrating an exemplary structure of a printer unit used in a conventional clam shell printer. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0030]    In the following, preferred embodiments of the present invention are described based on drawings. 
         [0031]      FIG. 1  illustrates a first embodiment of a printer unit to which the present invention is applied. The printer unit according to this embodiment is formed of a platen roller  11 , a holder  12  having arms  12 A and  12 B on both sides thereof for holding a shaft of the platen roller  11 , an auxiliary plate  13  disposed between a plate portion  12 C of the holder  12  and the platen roller  11 , a head support plate  15  with a thermal head  14  held on a front surface thereof, a pair of compression springs  16 A and  16 B inserted between the auxiliary plate  13  and a rear surface of the head support plate  15 , and the like. 
         [0032]    Semicircular bearing concave portions  12   a  and  12   b  with which the shaft of the platen roller  11  can be engaged are formed at tips of the arms  12 A and  12 B of the holder  12 . By holding with the two arms  12 A and  12 B the shaft of the platen roller  11  engaged with the bearing concave portions  12   a  and  12   b , the platen roller  11  is rotatably supported. Although not shown in the figure, a plurality of heater elements are provided in a line in a direction in parallel with the shaft in an area of a front surface of the thermal head  14  which is in contact with the platen roller  11 . 
         [0033]    The auxiliary plate  13  and the head support plate  15  are rotatably attached to a rotation spindle  17  which bridges over base portions of the arms  12 A and  12 B, respectively. When the compression springs  16 A and  16 B are inserted therebetween, a rear surface of the auxiliary plate  13  is brought into contact with a front surface of the plate portion  12 C of the holder  12  to be pressed by the springs. Then, the front surface of the thermal head  14  is adapted to be pressed toward the platen roller  11  by the head support plate  15  on a side opposite to the compression springs  16 A and  16 B which is pressed forward by the repulsion of the springs. Although not shown in the figure, a gear is fixedly attached to another end of the platen roller  11 , and rotational force is transferred from a drive motor (not shown) via a gear transfer mechanism to rotate the platen roller  11  according to printing by the thermal head  14 . 
         [0034]    The printer unit according to the first embodiment is adapted to change the pressing force of the thermal head  14  against the platen roller  11  by inserting a spacer  18  between the auxiliary plate  13  and the plate portion  12 C of the holder  12  thereby to change the distance between the auxiliary plate  13  and the head support plate  15  and to change the magnitude of the repulsion of the compression springs  16 A and  16 B. 
         [0035]    Further, in order to change the distance between the auxiliary plate  13  and the head support plate  15  using the same spacer  18 , a step portion formed of a bent piece  12   d  is formed substantially at the center of the plate portion  12 C of the holder  12 . Further, in order to prevent sideways displacement of the spacer  18  inserted between the auxiliary plate  13  and the plate portion  12 C of the holder  12 , a cylindrical protrusion  18   a  is formed on a front surface of the spacer  18 , while positioning holes  19   a  and  19   b  which can engage with the protrusion  18   a  are formed in the plate portion  12 C. One of the positioning holes  19   a  and  19   b  ( 19   a ) is provided at a position corresponding to the bent piece  12   d  and the other ( 19   b ) is provided in an area which does not have the bent piece  12   d.    
         [0036]    In this embodiment, when the spacer  18  is inserted between the plate portion  12 C of the holder  12  and the auxiliary plate  13  at a position where the protrusion  18   a  is engaged with the positioning hole  19   b , as illustrated in  FIG. 2 , the distance between the auxiliary plate  13  and the head support plate  15  is made smaller by the thickness of the spacer  18  compared with that illustrated in  FIG. 1  to make larger the repulsion of the compression springs  16 A and  16 B by one increment. 
         [0037]    Further, when the spacer  18  is inserted between the plate portion  12 C of the holder  12  and the auxiliary plate  13  at a position where the protrusion  18   a  is engaged with the positioning hole  19   a , as illustrated in  FIG. 3 , the distance between the auxiliary plate  13  and the head support plate  15  is made smaller by the thickness of the spacer  18  plus the step of the bent piece  12   d  compared with that illustrated in  FIG. 1  to make larger the repulsion of the compression springs  16 A and  16 B by two increments. 
         [0038]    Accordingly, when printing is conducted on thin paper, as illustrated in  FIG. 1 , the spacer  18  is not inserted, while, when printing is conducted on thick paper, as illustrated in  FIG. 3 , the spacer  18  is inserted at the position corresponding to the bent piece  12   d . Further, when printing is conducted on paper having an intermediate thickness, as illustrated in  FIG. 2 , the spacer  18  is inserted at a position which does not have the bent piece  12   d . This makes it possible to press the thermal head  14  against the platen roller  11  with optimum force according to the paper thickness to conduct satisfactory printing. It is to be noted that the relationship between the protrusion  18   a  and the positioning holes  19   a  and  19   b  may be opposite. More specifically, the positioning holes  19   a  and  19   b  may be provided in the spacer  18  and the protrusion may be provided on the plate portion  12 C of the holder  12 . 
         [0039]    Further, because, in the printer unit according to this embodiment, the platen roller  11  is detachably attached to the tips of the arms  12 A and  12 B of the holder  12 , printing paper sheet is adapted to be loaded by removing the platen roller  11 , pulling out the printing paper sheet at a leading edge thereof, positioning a predetermined area of the paper on the side of the front surface of the thermal head  14 , and then engaging the platen roller  11  again with the concave portions at the tips of the arms  12 A and  12 B. Therefore, even in a small thermal printer where, for example, the length of the platen roller  11  is 5 to 6 cm or smaller, the dimension of the room at the back of the thermal head  14  is 1 cm or smaller, and it is extremely difficult to load printing paper sheet by making larger the distance between the platen roller  11  and the thermal head  14  and inserting a leading edge of the printing paper sheet, printing paper sheet can be loaded with ease. 
         [0040]      FIG. 4  illustrates a second embodiment of the printer unit to which the present invention is applied. The printer unit according to this embodiment is adapted to change the pressing force of the thermal head  14  by providing screw holes for inserting adjustment screws  41 A and  41 B thereinto in the plate portion  12 C of the holder  12  having the arms  12 A and  12 B for holding the shaft of the platen roller  11 , bringing tips of the adjustment screws inserted into the screw holes into contact with the rear surface of the auxiliary plate  13  disposed between the plate portion  12 C and the platen roller  11 , screwing the screws, and thus, changing the distance between the auxiliary plate  13  and the head support plate  15  thereby adjusting the repulsion of the compression springs  16   a  and  16   b.    
         [0041]    It is desirable that the above-mentioned adjustment screws  41 A and  41 B have scales in the vicinity thereof such that the amounts of adjustment of the left and right screws can be set to be the same. The printer unit according to this embodiment has an advantage that the pressing force of the thermal head  14  can be changed in an analog way. 
         [0042]      FIG. 5  illustrates an exemplary structure of a preferred printer using the printer unit according to the above-mentioned embodiment. 
         [0043]    The printer illustrated in  FIG. 5  is provided with the printer unit illustrated in  FIG. 1 , a printer frame body  30  having a housing portion  31  for housing roll paper  20  which is thermal paper wound into a roll, and a cover  33  rotatably attached to one end of the printer frame body  30  about a spindle  32 . The platen roller  11  forming the printer unit is rotatably attached to a distal end side of the cover  33 . Reference numeral  37  denotes a sensor for detecting paper. 
         [0044]    Further, a cam  36  fixedly attached to a shaft  35  is provided at the back of the plate portion  12 C of the holder  12 . By rotating a lever (not shown) provided at an end of the camshaft  35 , the cam  36  is rotated and the plate portion  12 C is rotated forward, whereby the shaft of the platen roller  11  and the arms  12 A and  12 B of the holder  12  are adapted to move out of engagement. Further, in front portions of side walls of the printer frame body  30 , at positions overlapping with the concave portions  12   a  and  12   b  of the arms  12 A and  12 B of the holder  12 , similar concave portions are formed, whereby the shaft of the platen roller  11  is engaged with the concave portions. Normally, the concave portions in the side walls of the printer frame body  30  and the concave portions  12   a  and  12   b  of the arms  12 A and  12 B hold the shaft of the platen roller  11  so as not to pop up. 
         [0045]    Therefore, when, after the cam  36  is rotated to move the shaft of the platen roller  11  and the arms  12 A and  12 B of the holder  12  out of engagement, the cover  33  is rotated upward as illustrated by alternate long and short dashed lines in  FIG. 5 , an upper portion of the printer is wide open. With this state maintained, the roll paper  20  is put in the housing portion  31 , and a leading edge of the roll paper is pulled out to reach in front of the thermal head  14  of the printer unit. After that, the cover  33  is rotated in an opposite direction such that the shaft of the platen roller  11  at the distal end thereof is pressed in and engaged with the arms  12 A and  12 B and with the concave portions in the side walls of the frame body  30 . Then, the leading edge of the roll paper is sandwiched between the thermal head  14  and the platen roller  11  and is in a state where printing can be conducted. 
         [0046]    Further, by, for example, attaching to the spindle  32  of the cover  33  a coil spring for urging at all times the cover in an opening direction, a structure can be obtained where, when the operation lever of the camshaft  35  is operated to move the platen roller  11  out of engagement with the concave portions  12   a  and  12   b  of the arms  12 A and  12 B, the cover  33  automatically opens and the platen roller  11  is removed with a single motion. Further, it is also possible to, for example, provide an operation lever which protrudes outside the unit at an end portion of the head support plate  15  having the thermal head  14  such that the platen roller  11  is adapted to be removed by moving the head support plate  15  to the side of the auxiliary plate against the spring. In this case, it is not necessary to provide the cam  36 . 
         [0047]    It is to be noted that, in a printer having the cam  36  as illustrated in  FIG. 5 , it seems possible to use this cam to change the pressing force of the head, but, because it is the holder  12  that the cam  36  presses, if the holder  12  is moved forward in order to make larger the pressing force of the head, there is an inconvenience that the arm  12 A and  12 B are rotated and the shaft of the platen roller  11  is liable to be removed. Therefore, in order to make larger the pressing force of the head with the cam while avoiding such an inconvenience, it is necessary to insert the cam between the plate portion  12 C of the holder  12  and the auxiliary plate  13 , but, as described in the above, because there is no room for inserting the cam in a small printer unit which is the target of the present invention, it is difficult to use the cam. 
         [0048]    Although the invention made by the present inventors is specifically described in the above based on the embodiments, it goes without saying that the present invention is not limited to the above embodiments and various modifications can be made within the scope not departing from the gist of the present invention. For example, although, in the first embodiment, a structure where the pressing force of the thermal head  14  can be changed in three steps is described, a structure where the pressing force can be changed in four or more steps by providing a plurality of bent pieces having different step heights is also possible. Further, the steps may be formed not only by the bent piece(s) provided at an upper end of the holder but also on the front surface of the plate portion  12 C. 
         [0049]    Further, the structure may be such that a plurality of spacers of different thicknesses are prepared and the pressing force is adjusted by changing the spacer to be inserted, or such that a portion tapered in a direction in parallel with the shaft is formed in the step and in the spacer and the pressing force can be adjusted steplessly. Further, although, in the second embodiment, two adjustment screws are used, the screw may be only one. Further, a leaf spring may be used instead of the compression springs  16   a  and  16   b.    
         [0050]    Although examples where the invention made by the present inventors is applied to a thermal printer which is the field as the background of the invention are described in the above, the present invention is not limited thereto. The present invention can also be used when, for example, in a transfer device for transferring paper sandwiched between two rollers, the pressure of the rollers is required to be changed according to the paper thickness.