Abstract:
A thermal head is supported by a head arm which can move between a position where it is close to a platen and a position where it is apart from the platen. The thermal head is configured such that it can move between a printing position where a sheet and an ink ribbon are pushed against the platen and a retracted position where the ink ribbon is spaced from the platen sot that the ink ribbon is spaced from the sheet. The thermal head moves between the printing position and the retracted position by a cam mechanism, for example. Such a mechanism is effective, for example, in allowing only a sheet to feed, preventing wasteful feeding of an ink ribbon.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the invention  
           [0002]    The present invention relates to improvement of a printing portion in a thermal printer.  
           [0003]    2. Description of the Prior Art  
           [0004]    In a line thermal printer, printing is performed such that, in a state where a platen and a line thermal head are opposed to each other, a heat-sensitive paper is, or a heat-sensitive ribbon and a paper superimposed on each other are, caused to pass in between the platen and the line thermal head. At the time of printing, it is important for maintaining printing quality in a high level that the line thermal head applies proper contacting pressure to the platen via the heat-sensitive paper, or the ink ribbon and the paper. On the other hand, when printing is performed using the ink ribbon, a so-called ribbon saving is performed to prevent a wasteful consumption of an ink ribbon by feeding only a paper, not feeding an ink ribbon, when receiving a printing signal indicating that only a paper be fed without printing.  
           [0005]    As an ink ribbon is pressed against a paper at suitable pressure by a platen and thermal head when printing is performed, a strong friction generates between the ink ribbon and the paper, so that it is impossible to feed only a paper without feeding the ink ribbon.  
           [0006]    For the above reason, the ribbon saving mechanism actuates a solenoid based on the printing signal indicating an empty line or an empty column to separate a line thermal head from the platen via a lever or to separate the platen from the thermal head.  
           [0007]    Conventionally, since such a structure is employed that a whole head arm including a peripheral mechanism for supporting the line thermal head together with the head is also ascended/descended, the machine or apparatus itself is large-sized, which results in difficulty of rapid operation. Also, the ascending/descending of the entire head to be effected by using the solenoid can be performed securely, but the operation is linear and an impact may occur at an end point(retracted position) of operation for separating the line thermal head from the platen or at an end point (printing position) of operation for causing the line thermal head and the platen to approach to each other. As a result, there may occur a drawback that the thermal head supporting mechanism go out of order so that printing quality is adversely affected.  
         SUMMARY OF THE INVENTION  
         [0008]    In accordance with an aspect of the present invention, a thermal printer is provided with a platen, a head arm and a line thermal head. The head arm holds the line thermal head. The head arm can move relative to a main body from between a first position (a set position where printing can be made on a paper) where the line thermal head is caused to approach to the platen and a second position (an open position where loading of a sheet is not obstructed) where the line thermal head is separated from the platen.  
           [0009]    Such a structure can further be employed that, when the head arm is in the first position, the line thermal head mounted on the head arm can move between (1) a printing position where the line thermal head is brought in pressure-contact with the platen through a sheet and an ink ribbon and (2) a retracted position where the line thermal head is separated from a platen by a distance so as to forms a gap or clearance between the ink ribbon and the sheet on the platen.  
           [0010]    In addition to the above-described structure, the following configurations may by applied to the present invention.  
           [0011]    As a specific configuration, the thermal printer is provided with a paper or sheet feeding path for allowing the paper and the ink ribbon to pass in between the platen and the line thermal head in a superimposed manner, a sheet feeding mechanism for feeding a paper, and an ink ribbon feeding mechanism for feeding the ink ribbon. When only the paper is fed, the line thermal head is moved to the retracted position.  
           [0012]    As a configuration for moving the line thermal head between the printing position and the retracted position by using a stepping motor and a cam mechanism, it is preferable to select a cam curve, since impact can be reduced to almost zero at the end point of the movement of the line thermal head between the printing position and the retracted point.  
           [0013]    Also, such a structure can be employed that the line thermal head is slidably supported by a head frame supported by the head arm in upward and downward directions, and the head frame is moved by the cam without employing a structure where the line thermal head and the cam are brought into direct contact with each other. Since mounting work or exchanging work of the line thermal head can be facilitated and there is not any influence of the standard and/or size of the line thermal head, a contacting position of the line thermal head with the cam can be set relatively freely.  
           [0014]    The line thermal head is urged towards the platen by a resilient member such as a compression spring from the head arm to maintain a proper contacting pressure at the printing position. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    The above object and features, and other object and features will become apparent from the following explanation of an embodiment with reference to the accompanying drawings, wherein:  
         [0016]    [0016]FIG. 1 is a schematic front view showing an arrangement of constituent elements of a line thermal printer according to the present invention in a housing from which a cover is removed;  
         [0017]    [0017]FIG. 2 is a perspective view of the line thermal printer shown in FIG. 1, where its head arm is in a first position;  
         [0018]    [0018]FIG. 3 is a perspective view of the line thermal printer shown in FIG. 1, where its head arm is in a second position;  
         [0019]    [0019]FIG. 4 is a front view showing a schematic configuration of a head set when the head arm is in the second position;  
         [0020]    [0020]FIG. 5 is a front view showing a schematic configuration of the head set when the head arm is in the first position;  
         [0021]    [0021]FIG. 6 is a schematic front view for explaining that a line thermal head is put in a printing position close to a platen by a cam of a driving mechanism for ribbon saving; and  
         [0022]    [0022]FIG. 7 is a schematic front view for explaining that the line thermal head is put in a retracted position separated from the platen by the cam of the driving mechanism for ribbon saving. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]    [0023]FIG. 1 is a schematic configuration of a line thermal printer  1  according to an embodiment of the present invention, wherein a cover is removed from a casing  2 . In the line thermal printer  1  shown in FIG. 1, a sheet  3  and a heat-sensitive ink ribbon  4  are loaded.  
         [0024]    In the casing  2 , its space is partitioned into two interior spaces vertically by a partition wall  5 . Supported to this partition wall  5  are a roll supporting shaft  6 , a supporting shaft of a tension roller  7 , a ribbon shaft  10  on the feeding side, and a ribbon shaft  11  on the winding side. Also, a sheet feeding mechanism  8  and a printing section  9  are provided on a front side of the partition wall  5 . Arranged on a back side of this partition wall  5  are a driving mechanism such as gears and the like for driving elements, such as the sheet feeding mechanism  8 , the printing section  9  and the like and a control mechanism.  
         [0025]    A sheet  3  fed from a sheet roll loaded on the roll supporting shaft  6  passes between an upper main body frame  12  and a lower main body frame  13  outside from the housing  2  via the tension roller  7  and the sheet feeding mechanism  8 . In a course of the feeding path of the sheet  3 , various guides (not shown), rollers (not shown) provided at a position for converting a feeding direction of the paper and the like exist in a conventional manner.  
         [0026]    Incidentally, these upper main body frame  12  and lower main body frame  13  are constituting elements for the printing section described later and they are fixed to the partition wall  5 .  
         [0027]    A mechanism for feeding the ink ribbon  4  includes the ribbon shaft  10  on the feeding side and the ribbon shaft  11  on the winding side. Guide rollers and rollers (not shown) for guiding the ink ribbon exist in the feeding path of the ink ribbon  4 .  
         [0028]    The feeding path of the sheet  3  and the feeding path of the ink ribbon  4  are combined at a position where the upper main body frame  12  and the lower main body frame  13  are opposed to each other, and the sheet  3  and ink ribbon  4  are superimposed to pass between the upper main body frame and the lower main body frame.  
         [0029]    The roll formed by rolling the sheet  3  can easily be attached/detached to the roll supporting shaft  6  manually. The tension roller  7  is made of synthetic resin sponge and friction occurs between the tension roller  7  and the sheet  3 . When tension acting on the paper is increased, the tension is absorbed by deformation of the section area of the tension roller  7  due to softness or flexibility thereof, thereby preventing the sheet  3  from being damaged.  
         [0030]    The sheet feeding mechanism  8  is provided with a feed roller, a guide roller, and pinch rollers arranged so as to be opposed to these feed roller and guide roller vertically, (which are not illustrated in detailed). The sheet  3  is fed towards the printing section  9  while passing between the feed roller and the pinch roller.  
         [0031]    The upper main body frame  12  and the lower main body frame  13  are fixed on the partition wall  5  so as to be opposed to each other in a vertical direction. A head set  14  is arranged to the upper main body frame  12  and a platen  15  is disposed on the lower main body frame  13 .  
         [0032]    The upper main body frame  12  is provided with right and left frame walls  16 ,  17  parallel to each other, as shown in FIG. 2. A head arm  18  which is one constituting element of the head set  14  is mounted between the left frame wall  16  and the right frame wall  17  so as to be rotatable about a shaft  33  (FIG. 6). Arc-shaped slots  20  are respectively formed on the left and right frame walls  16 ,  17 , and supporting shafts  18   b  projecting from the left and right sides of the head arm  18  project from these slots  20 . Portions of the supporting shafts  18   b  projecting from the slots  20  to the outside are fixed with operation levers  19 .  
         [0033]    When the operation lever  19  is positioned at a lower end portion of the slot  20 , as shown in FIG. 2, the head arm  18  is put in a “first position” close to the platen  15 . At this position, printing on the sheet  3  can be made possible. Also, when the head set  14  is ascended from the position shown in FIG. 2 to an upper end portion of the slot  20  along the longitudinal direction of the slot  20  by operating the operation lever  19 , as shown in FIG. 3, the head arm  18  comes to a “second position” separated from the platen  15 . At this position, it becomes possible to load the sheet  3 .  
         [0034]    A holding mechanism is provided such that, when the head arm  18  is in the first position and also when it is in the second position, the head arm  18  are held in the respective positions due to the own weight of the head set  14 .  
         [0035]    As shown in FIGS. 4 and 5, supporting pins  21 ,  22  which are spaced from each other in a horizontal direction so as to be slidable downward, namely towards the platen  15 , are provided on the head arm  18 . Fixed to lower ends of the supporting pins  21 ,  22  is a head frame  23 . Stopper rings are respectively fixed to upper portions of the supporting pins  21 ,  22 , so that the supporting pins are prevented from falling off from the head arm  18 .  
         [0036]    Furthermore, a mounting pin  25  is mounted on a central portion of the head arm  18  so as to be slidable downward. A male screw  26  with a small diameter formed at a lower end of the mounting pin  25  penetrates a central portion of the head frame  23  to engage a female screw  24   b  formed on a central portion of the line thermal head  24  arranged on a lower face side of the head frame  23  in a threading manner.  
         [0037]    Incidentally, the length of the female screw  24  is determined such that, when the male screw  26  is screwed into the female screw  24   b  of the line thermal head  24 , a gap or clearance of about 0.5 mm occurs between the head frame  23  and the line thermal head  24  necessarily. As a result, left and right ends of the line thermal head  24  can swing vertically about a portion which is supported by the male screw  26  utilizing this gap with the head frame  23 .  
         [0038]    Furthermore, pressing pins  27 ,  28  are mounted in the vicinities of left and right end portions of the head arm  18  so as to be slidable downward. End portions of the pressing pins  27 ,  28  penetrate through holes formed in the head frame  23  to face the line thermal head  24 . Compression springs  29  are fitted on the pushing pins  27 ,  28 . At a time of printing operation, the compression springs  29  push the pushing pins  27 ,  28  downward so that the lower ends of the pushing pins  27 ,  28  directly push the line thermal head  24  downward. For this reason, the line thermal head  24  is brought in pressure-contact with the platen  15 .  
         [0039]    When the head arm  18  is rotated downward to be positioned at the first position (FIG. 2), the line thermal head  24  is close to the platen  15  and the lower ends of the pushing pins  27 ,  28  abut on the thermal line head  24 , as shown in FIG. 5.  
         [0040]    On the other hand, when the head arm  18  is rotated upward to be positioned at the second position (FIG. 3), the line thermal head  24  is separated from the platen  15  and the lower ends of the pushing pins  27 ,  28  are separated from the line thermal head  24 , as shown in FIG. 4. The head frame  24  put in this state is suspended from the head arm  18  via the mounting pin  25 , as shown in FIG. 4.  
         [0041]    The head frame  23  is formed at its left and right ends with bent portions  30  extending vertically downward in a suspending manner. The right bent portion  30  is provided with a recessed portion  32  engaging a positioning ring  31  arranged at a right end portion of the platen  15 , as shown in FIGS. 4 and 6.  
         [0042]    Also, the line thermal head  24  is mounted on a front face of a front portion of the head frame  23 , and a rear portion of the head frame  23  is engaged with a shaft  33  provided to the head arm  18 . Accordingly, when the head arm  18  is rotated about the shaft  33 , the head frame  23  suspended from the head arm  18  via the mounting pin  25  is also rotated about the shaft  33 .  
         [0043]    As shown in FIG. 6, the driving mechanism  34  for ribbon saving is provided on the lower main body frame  13 , and it has a plate cam  36  and a lever  37  driven by a stepping motor  35 . The lever  37  is rotatably supported to a shaft  38  fixed to the lower main body frame  13 . A first cam follower  39  is provided at a lower end portion of the lever  37 , and a second cam follower  40  is provided at a upper end portion thereof. A portion of the lever  37  which is supported by the shaft  38  is positioned on an upper end rear side, and a portion of the lever which is provided with the second cam follower  40  is positioned on an upper end front side.  
         [0044]    The second cam follower  40  of the lever  37  comes in contact with a lower end of the right side bent portion  30  of the head frame  24 . At this time, biasing or urging forces of the compression springs  29  fitted on the pushing pins  27 ,  28  serve so as to push the bent portion  30  against the second cam follower  40 .  
         [0045]    When the plate cam  36  is rotated, the lever  37  is rotated in a clockwise direction on FIG. 6 so that the second cam follower  40  pushes up the head frame  23 . Also, the pushing pins  27 ,  28  push the line thermal head  24  by the urging forces of the compression springs  29  so that the head frame  23  which has been pushed up is returned back to its original or home position. A cam curve of the plate cam  36  is set so as to allow the above operations. The shape (cam curve) of the plate cam  36  is set to be a smooth curve wholly, thereby preventing impact from occurring on rotation of the lever  37 .  
         [0046]    Since the distance between the rotating shaft  38  and the second cam follower  40  is significantly shorter than the distance between the first cam follower  39  and the rotating shaft  38 , the second cam follower  40  can ascend or elevate a relatively heavy assembly of the line thermal head  24  and the head frame  24  via the right bent portion  30  of the head frame  23  lightly and smoothly.  
         [0047]    When the head arm  18  is in the first position (FIG. 2) and the plate cam  36  in the driving mechanism  34  for ribbon saving is in the rotated position shown in FIG. 6, the head frame  23  is biased by the biasing forces of the compression springs  29  fitted on the pushing pins  27 ,  28  to be positioned at the lowermost position, and the sheet  3  and the ink ribbon  4  are in a pressure-contact state with the platen. That is, the line thermal head  24  is in the “printing position” where printing is performed.  
         [0048]    When the plate cam  36  is rotated by 90° from the rotated position shown in FIG. 6 in a clockwise direction and the lever  37  is in an attitude shown in FIG. 7 obtained when it is rotated by a certain angle from the position shown in FIG. 6 in a clockwise direction, the second cam follower  40  reaches an ascended position (a position higher than the lever rotating shaft  38 ) which is higher than the position from the position shown in FIG. 6 (height which is approximately equal to the height of the lever rotating shaft  38 ). That is, FIG. 7 shows a state where the line thermal head  24  is in the “retracted position” obtained by pushing-up of the head frame  23  conducted by the ascended second cam follower  40  via the right bent portion  30  of the head frame  23 .  
         [0049]    As shown in FIG. 7, a gap d occurs between the line thermal head  24  positioned at the retracted position and the platen  5 , and the pressure-contact between the sheet  3  and the ink ribbon  4  has been released. Therefore, such a ribbon saving operation can be performed that only the sheet  3  is fed by the sheet feeding mechanism  8  and a wasteful feeding of the ink ribbon  4  is prevented.  
         [0050]    The pinch roller of the sheet feeding mechanism  8  is ascended upwardly. Then, the operation lever  19  in the printing section  9  is rotated upwardly to put the head arm  18  in the second position (FIG. 3) so that a gap occurs between the head set  14  and the platen  15 . Subsequently, the sheet  3  is loaded by turning the same as shown in FIG. 1. Furthermore, the rolled heat-sensitive ribbon  4  loaded to the ribbon shaft  10  on the feeding side is caused to pass between the line thermal head  24  in the printing section  29  and the platen  15  to be attached to the winding side ribbon shaft  11  opposed to the shaft  10 .  
         [0051]    Then, the operation lever  19  is rotated downwardly to descend the head set  14  and the head arm  18  is put in the first position (FIG. 2). Thereby, the sheet  3  and ink ribbon  4  are superimposed to be brought into pressure-contact with each other between the line thermal head  24  and the platen  15 . The driving mechanism  34  for ribbon saving puts the plate cam  36  in a position corresponding to the printing position (FIG. 6). In this state, the sheet  3  and the ink ribbon  4  are fed according to a printing signal from the control device (not shown), and simultaneously printing is performed by the line thermal head  24 .  
         [0052]    Then, when a printing signal for an empty line or an empty field or column is output, the stepping motor  35  is driven to rotate the plate cam  36  by 90° so that the line thermal head  34  is positioned in the retracted position shown in FIG. 7. Then, the plate cam  36  is further rotated 90° according to the next real printing signal to put the line thermal head  24  in the printing position where printing is restarted.  
         [0053]    As mentioned above, the printing and the ribbon saving are performed.  
         [0054]    In the above, regarding the ribbon saving, the embodiment where the line thermal head  24  is moved to the printing position and the retracted position has been explained. As another system, such a configuration can be employed that the lowering or descending position of the head frame  23  is restricted and simultaneously a platen shaft is pivoted to an upper end of the lever  37  in the driving mechanism  34  so that the platen  15  is moved by rotation of the plate cam  36  upward and downward (to the printing position and the retracted position) relative to the line thermal head  24  positioned at the above lower limit position.  
         [0055]    Incidentally, the cam for moving the lever  37  is not limited to the plate cam but it may take such a form as a cam groove or an eccentric plate.  
         [0056]    According to the present invention, since the ribbon saving is performed by the cam operation, impact hardly occurs when the line thermal head is moved to the printing position or when it is moved to the retracted position. As a result, there does not occur such a drawback that adjusted position is displaced in the mechanism for ribbon saving, and an excellent printing quality can be maintained.  
         [0057]    Since the mechanism for separating the head arm from the platen when the paper and the ink ribbon are set and the mechanism for slightly separating the head frame from the platen for ribbon saving are separated from each other, the mechanism for ribbon saving can be small-sized and faster operation can be made possible.