Abstract:
Provided is a thermal printer which provides ease of maintenance, such as ease of replacement of the thermal head, and which can shorten the time required for maintenance work and reduce maintenance cost. The thermal printer is one which performs printing on a sheet sandwiched between a thermal head having a heat generating element and a platen roller, and includes a first frame (outer frame  18 ) for retaining a printing mechanism including the thermal head and the platen roller; a head support member to which the thermal head is secured; a pressure plate that detachably retains the head support member; a joining member for joining the head support member and the pressure plate together; a support shaft (shaft  20 ) passed through a shaft hole provided in each of both side walls of the first frame, the support shaft rotatably supporting the head support member; a second frame (lock arm  17 ) which is rotatable integrally with the pressure plate about the support shaft; and an urging member suspended between the pressure plate and the second frame, for generating a pressing force between the thermal head and the platen roller in a state where the head support member is joined to the pressure plate.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a thermal printer that performs printing on a sheet sandwiched between a thermal head and a platen roller, and more specifically to an attachment and detachment mechanism for a thermal head.  
         [0003]     2. Description of the Related Art  
         [0004]     Thermal printers are often employed as, for example, printers for performing receipt printing at cash registers, portable label printers for printing POS labels on food products or for printing labels for distribution control, or the like. In a thermal printer, a thermal head having a heat generating element is brought into press contact with a platen roller, with printing performed on a heat-sensitive recording sheet sandwiched between the two members.  
         [0005]     In conventional thermal printers, for example, a thermal head is fixed to a head support member also serving as a heat radiation plate, and the head support member is axially supported to an outer frame or the like of the printer so as to be freely rotatable, with the head support member being urged by a spring or the like to be pressed toward the platen roller side (for example, see JP 2000-318260 A).  
         [0006]     In such conventional thermal printers, when, for instance, defects develop in the print dots produced by the thermal printer due to the influence of excessive energization, damage on the sheet, etc., the only way to solve this problem is to dissemble the entire printer assembly to replace the thermal head, and such an operation is extremely troublesome.  
         [0007]     In particular, in the conventional thermal printers, an E-shaped fastening fitting, namely the so-called E-ring, is fastened and secured to a shaft for axially supporting the head support member to the outer frame of the printer so that the shaft is securely fixed and does not slide sideways, or, alternatively, a support shaft provided so as to project from the either right and left side surface of the head support member is press-fitted to a bearing provided on the outer frame to be fixed in place. The above construction of the conventional printers makes it hard to detach the head support member with ease. Further, it is also impossible to detach only the thermal head portion while the head support member is incorporated in the printer main body as it is.  
         [0008]     Generally speaking, in the case of thermal printers for printing letters or the like, a printing failure of a single dot does not really necessitate replacement of the thermal head. In recent years, however, label printers that perform label printing for delivery control are often used for barcode printing, and if, as anticipated, barcodes are to be printed in increasingly thinner lines in the future, even slight chipping or loss of dot may cause trouble. Hence, it is conceivable that replacement of the thermal head due to dot failures, etc. will become more frequent than was previously the case.  
         [0009]     In view of the above, the inventors of the present invention have proposed a thermal printer which includes a lock arm that movably holds a head support member, an urging means suspended between the head support member and the lock arm and adapted to generate a pressing force between a thermal head and a platen roller, and an outer frame that holds the lock arm and the platen roller, in which the thermal head, the head support member, and the urging means are capable of being attached to and detached from the outer frame while being assembled onto the lock arm (JP 2002-283230 B). With the thermal printer, the thermal head portion can be readily removed from the printer main body, facilitating replacement of the thermal head portion.  
         [0010]     In the thermal printer proposed in the above previously filed patent application, however, the thermal head, the head support member, and the urging means are assembled onto the lock arm, which means that when replacing the thermal head, it is also necessary to replace the head support member, the urging means, and the like together with the thermal head, leading to an increase in parts cost. Conceivably, an improvement in this respect will enable a further reduction in maintenance cost.  
         [0011]     It is an object of the present invention to provide a thermal printer which provides ease of maintenance, such as ease of replacement of the thermal head, and which can shorten the time required for maintenance work and reduce maintenance cost.  
       SUMMARY OF THE INVENTION  
       [0012]     According to the present invention, to achieve the above object, there is provided a thermal printer which performs printing on a sheet sandwiched between a thermal head having a heat generating element and a platen roller, the thermal printer including: a first frame (outer frame  18 ) for retaining a printing mechanism including the thermal head and the platen roller; a head support member ( 15 ) to which the thermal head is secured; a pressure plate ( 16 ) that detachably retains the head support member; joining means for joining the head support member and the pressure plate together; a support shaft (shaft  20 ) passed through a shaft hole provided in each of both side walls of the first frame, the support shaft rotatably supporting the head support member; a second frame (lock arm  17 ) which is rotatable integrally with the pressure plate about the support shaft; and urging means suspended between the pressure plate and the second frame, for generating a pressing force between the thermal head and the platen roller in a state where the head support member is joined to the pressure plate.  
         [0013]     Preferably, the head support member has in a lower part of its each side surface an engaging portion ( 15   b ) adapted to engage with the support shaft, the head support member being placed into a predetermined position by rotating the head support member toward the pressure plate by engaging the engaging portion with the support shaft.  
         [0014]     More specifically, the head support member has a locking portion ( 15   a ) for joining the head support member to the pressure plate, and the pressure plate has a rest portion ( 16   a ) on which the locking portion is placed to rest. The head support member and the pressure plate are joined together by fastening with a screw ( 12 ) while the locking portion and the rest portion are held in contact with each other. Here, it is desirable that the locking portion be composed of a U-shaped locking piece capable of engaging with the screw.  
         [0015]     According to the present invention, by arranging the urging means between the pressure plate and the second frame and joining the head support member, to which the thermal head is secured, to the front surface of the pressure plate, it is possible to obtain a pressing force for urging the thermal head toward the platen roller. Also, the head support member is constructed so as to be attachable to and detachable from the pressure plate, facilitating replacement of the thermal head. Further, unlike according to the previously filed application (JP 2002-283230 B), when replacing the thermal head, it is not necessary to replace the whole frame onto which the thermal head, the head support member, and the urging means are assembled, making it possible to avoid an increase in parts cost and to achieve a reduction in maintenance cost. Further, while a connector for connecting to an FPC is provided on the back surface portion of the thermal head, only the head support member can be detached according to the present invention, facilitating attachment and detachment of the FPC as well.  
         [0016]     Further, the engaging portion that engages with the support shaft is provided in a lower part of either side surface of the head support member, and the head support member can be rotated into a predetermined position by engaging the engaging portion with the support shaft, whereby positioning on the head support member can be performed with relative ease. That is, the position in the back and forth direction of the head support member is fixed, whereby the head support member and the pressure plate can be readily brought into intimate contact with each other, which makes it possible to efficiently release the heat of the thermal head.  
         [0017]     Further, the head support member and the pressure plate are joined together with the screws, enabling attachment and detachment of the head support member by means of a relatively simple structure.  
         [0018]     Further, the locking portion is constituted by the U-shaped locking piece, whereby the locking piece of the head support member can be brought into engagement with each screw without fully unscrewing the screw from the rest portion provided in the pressure plate, thereby achieving a marked improvement in operability, and there is no fear of the screws getting lost. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     In the accompanying drawings:  
         [0020]      FIGS. 1A through 1C  are respectively a top view, a bottom view, and a cross-sectional view taken along the line A-A of  FIG. 1A , of a printing mechanism of a thermal printer according to a preferred embodiment of the present invention;  
         [0021]      FIGS. 2A through 2D  are respectively a left-hand side view, a right-hand side view, a rear view, and a front view of the printing mechanism of the thermal printer according to the embodiment of the present invention;  
         [0022]      FIG. 3  is an exploded perspective view of portions (a lock arm  17 , a pressure plate  16 , and a head support member  15 ) that are integrally assembled onto the lock arm  17 ;  
         [0023]      FIG. 4A  is a top view showing the printing mechanism from which the head support member  15  is being detached;  
         [0024]      FIG. 4B  is a cross-sectional view taken along the line A-A of  FIG. 4A ;  
         [0025]      FIG. 5A  is a top view showing the printing mechanism from which the head support member  15  has been detached;  
         [0026]      FIG. 5B  is a cross-sectional view taken along the line A-A of  FIG. 5A ; and  
         [0027]      FIG. 6  is a perspective view showing the printing mechanism and its vicinity in the thermal printer according to the embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]     Hereinbelow, a preferred embodiment of the present invention is described in detail with reference to the drawings.  
         [0029]      FIG. 1A  is a top view showing a printing mechanism of a thermal printer to which the present invention is suitably applied,  FIG. 1B  is a bottom view thereof, and  FIG. 1C  is a cross-sectional view thereof taken along the line A-A of  FIG. 1A .  FIG. 2A  is a left-hand side view showing the printing mechanism of the thermal printer of this embodiment,  FIG. 2B  is a right-hand side view thereof,  FIG. 2C  is a back view thereof, and  FIG. 2D  is a front view thereof.  FIG. 3  is an exploded perspective view of portions (a lock arm  17 , a pressure plate  16 , and a head support member  15 ) that are assembled integrally onto the lock arm  17 .  
         [0030]     The thermal printer according to this embodiment is, although not particularly limited to, a portable label printer for performing printing on a sheet, specifically a recording sheet with attached release paper (hereinafter referred to as the label paper) whose adhesive surface on the back side is exposed by peeling off the release paper. The thermal printer has a horizontally elongate construction so that it can produce relatively broad prints.  
         [0031]     As shown in  FIGS. 1A through 3 , the thermal printer of this embodiment is equipped with: a thermal head  13  having multiple heat generating elements  13   a  arranged horizontally in a row; a platen roller (not shown) adapted to press a sheet against the portion of the thermal head  13  where the heat generating elements  13   a  are arranged and feed the sheet as it rotates; an outer frame  18  surrounding a printing mechanism  10  including the thermal head  13  and the platen roller; a head support member  15  which supports the thermal head  13  and functions as radiating means for radiating heat of the thermal head  13 ; a pressure plate  16  that detachably holds the head support member; a shaft  20  passed through shaft holes provided on both side walls of the outer frame  18  and rotatably supporting the head support member  15  etc.; a lock arm  17  formed so as to be integrally rotatable with the pressure plate  16  about the shaft  20  and locking the platen roller with a bearing for rotatably supporting the platen roller held between the lock arm  17  and the outer frame  18 ; coil springs  14  arranged between the pressure plate  16  and a back surface portion  17 B of the lock arm  17  and serving as urging means for urging the two members so as to repel each other; a stepping motor (not shown) for rotationally driving the platen roller through the intermediation of a gear etc.; and the like.  
         [0032]     The head support member  15  has the thermal head  13  secured to its front surface portion and has locking pieces  15   a  provided in a lower part of its back surface and protruding toward the pressure plate  16  side at a substantially right angle. A U-shaped recess is formed in each locking piece  15   a,  allowing the recess to engage with each of screws  12  without having to unscrew the screws  12  from rest portions  16   a  of the pressure plate  16  which will be described later. Further, a back surface portion of the thermal head  13  secured to the front surface portion of the head support member  15  is exposed from a lower part in the back surface of the head support member  15 , with a connector  13   b  for connecting FPC (Flexible Print Circuit) being arranged in substantially the center of the thus exposed back surface portion of the thermal head.  
         [0033]     Further, provided in a lower part of right and left side walls of the head support member  15  are U-shaped locking portions  15   b  adapted to engage with the shaft  20 . The locking portions  15   b  enable the head support member  15  to rotate about the shaft  20 , and the width of the U-shaped part of the locking portions  15   b  is made substantially the same as the diameter of the shaft  20 , facilitating positioning of the head support member  15  in the back and forth direction.  
         [0034]     Further, the locking portions  15   b  of the head support member  15  protrude down to the lower side of the thermal head  13 , and this configuration of the locking portions  15   b  allows the center of the shaft  20  to overlap the extension line of the surface of the thermal head  13  on which the heat generating elements  13   a  are provided. Accordingly, upon rotating the head support member  15  toward the pressure plate  16  side, the back surface portion of the head support member  15  and the pressure plate  16  can be brought into intimate contact with each other with reliability.  
         [0035]     The pressure plate  16  has openings  16   b  from which the locking pieces  15   a  of the head support member  15  are introduced, with the rest portions  16   a  on which the locking pieces  15   a  are placed to rest being provided on the bottom side of the openings  16   b.  The locking pieces  15   a  of the head support member  15  and the rest portions  16   a  of the pressure plate  16  are fastened together with the screws  12  while held in contact with each other. Further, a rectangular C-shaped cutout portion is formed substantially in the center of the pressure plate  16 , allowing the FPC introduced from the back surface side to be passed on to the connector  13   b  of the thermal head  13 .  
         [0036]     Formed in the right and left side walls of the pressure plate  16  are forwardly-projecting flanges  16   c  provided with shaft holes  16   d  through which the shaft  20  is passed. In addition, provided rearward of the both side walls are overhanging portions  16   e  for supporting the lock arm  17 . Note that when joined to the head support member  15 , the pressure plate  16  also functions as heat radiating means for releasing the heat of the thermal head  13 .  
         [0037]     The lock arm  17  has a rectangular C-shaped configuration as a whole. Provided in the right and left side walls of the lock arm  17  are hook portions  17   a  that lock the bearing member for the platen roller (hereinafter referred to as the “platen-roller bearing member”) securely in place, and shaft holes  17   b  through which the shaft  20  is passed. Further, formed in the lower rearward portion of the lock arm  17  are cutout portions  17   c  so that a back surface portion  17 B of the lock arm  17  can be supported at the overhanging portions  16   e  of the pressure plate  16 . Further, formed in the lower right of the back surface portion is a spring hole  17   d  for hooking on an end of a spring  21  that will be described later. With its other end being fixed onto a spring hole  18   c  provided in an erected wall  18   a  in a lower part of a right side wall  18 R of the outer frame  18 , the spring  21  thus urges the lock arm  17  rearward.  
         [0038]     Further, arranged between the pressure plate  16  and the lock arm  17  are the springs  14  for pressing the pressure plate  16 , causing a pressing force to act between the thermal head  13  and the platen roller. The springs  14  are each secured in place with its both ends respectively engaged with a protrusion (not shown) formed in the pressure plate  16  and a recess (not shown) formed in the lock arm  17 . The springs  14  urge the back surface portion of the lock arm  17  and the pressure plate  16  so as to repel each other; at this time, the overhanging portions  16   e  entering the cutout portions  17   c  of the lock arm  17  serve as stoppers, restraining the back surface portion  17 B of the lock arm  17  and the pressure plate  16  from moving away from each other beyond a certain distance. Further, the back surface portion  17 B of the lock arm  17  is supported by means of the overhanging portions  16   e  of the pressure plate  16 , with the result that the pressure plate  16  and the lock arm  17  are rotated integrally about the shaft  20 .  
         [0039]     The outer frame  18  is composed of a back surface portion  18 B, a bottom surface portion  18 F, a left side wall  18 L, and the right side wall  18 R which are integrally molded, with an opening  18   b  for introducing the FPC being formed substantially in the center of the back surface portion  18 B. Further, provided on the right and left walls  18 R and  18 L of the outer frame  18  are receiving grooves  18   c  for retaining the platen-roller bearing member, and shaft holes  18   d  through which the shaft  20  is passed. Further, a gear support shaft  19  for supporting a gear is provided to the left side wall  18 L, and provided in a lower part of the right side wall  18 R is a spring hole  18   e  for attaching the spring  21  whose one end is fixed onto the spring hole  17   d  of the lock arm  17 .  
         [0040]     The lock arm  17  is normally pulled rearward by the spring  21  into intimate contact with the back surface portion  18 B of the outer frame  18  with the hook portions  17   a  being also pulled rearward, allowing the platen-roller bearing member to be retained between the receiving grooves  18   c  provided in the right and left side walls  18 R and  18 L of the outer frame  18  and the hook portions  17   a  provided in the right and left side walls of the lock arm  17 . Note that by pushing the rear portion of the lock arm  17  forward (or by pulling the front portion thereof), the lock arm  17  is rotated about the shaft  20  to separate the hook portions  17   a  of the lock arm  17  away from the platen-roller bearing member, thus making it possible to detach the platen roller.  
         [0041]     As shown in  FIG. 3 , the shaft  20  is passed through the shaft holes  16   d  and the shaft holes  17   b  provided in the right and left side walls of the pressure plate  16  and the lock arm  17 , respectively. Further, the ends of the shaft  20  are passed through the shaft holes  18   d  provided in the right and left walls  18 R and  18 L of the outer frame  18 , and fastening fittings such as E rings are fastened and secured in place from the outside of the outer frame  18 , whereby those components are assembled onto each other and do not separate from each other.  
         [0042]     On the other hand, when the locking portions  15   b,  which are formed in the lower part of the right and left side walls of the head support member  15 , are engaged with the shaft  20 , and the head support member  15  is rotated toward the pressure plate  16 , the locking pieces  15   a  are introduced from the openings  16   b  provided in the pressure plate  16 . Then, the head support member  15  and the pressure plate  16  are joined together into intimate contact with each other by fastening with the screws  12  with the locking pieces  15   a  of the head support member  15  held in contact with the rest portions  16   a  of the pressure plate  16 .  
         [0043]      FIG. 4A  is a top view showing the printing mechanism from which the head support member  15  is being detached.  FIG. 4B  is a cross-sectional view taken along the line A-A of  FIG. 4A .  
         [0044]     As shown in  FIGS. 4A and 4B , to detach the head support member  15  from the pressure plate  16 , first, each screw  12  is loosened to a degree such that the lower end of the screw  12  does not dislodge from the rest portion  16   a  of the pressure plate  16 , and then the head support member  15  is rotated forward about the shaft  20  to thereby take out the locking pieces  15   a  of the head support member  15  from the openings  16   b.  Since the locking pieces  15   a  of the head support member  15  are each formed in a U-shaped configuration and engaged with the screw at its recessed portion, the locking pieces  15   a  can be pulled out without fully unscrewing the screws  12  from the rest portions  16   a  of the pressure plate  16 . Therefore, a marked improvement can be attained in terms of operability. Further, there is no fear of the screws  12  getting lost. It is to be noted that an operation reverse to the operation described above is performed when attaching the head support member  15  to the pressure plate  16 .  
         [0045]      FIG. 5A  is a top view showing the printing mechanism from which the head support member  15  has been detached, and  FIG. 5B  is a cross-sectional view taken along the line A-A of  FIG. 5A .  
         [0046]     Once the head support member  15  has been detached, only a printing mechanism  10  as shown in  FIG. 5A  remains in the thermal printer. Since the head support member  15  is detachable from the pressure plate  16  in the thermal printer of this embodiment as described above, only the head support member  15  needs to be detached when replacing the thermal head  13 , and other parts attached to the outer frame  18  are not replaced. Therefore, unlike according to the previously filed application (JP 2002-283230 B) described above, it is not necessary to perform such replacement by detaching the entire lock arm  17  to which the head support member  15  is attached, making it possible to avoid an increase in parts cost incurred while replacing the thermal head  13  and therefore to achieve reduced maintenance cost.  
         [0047]      FIG. 6  is a perspective view showing the printing mechanism and its vicinity in the thermal printer of this embodiment.  
         [0048]     Referring to  FIG. 6 , reference numeral  1  denotes a thermal printer main body,  4  denotes a gear box containing a gear for rotating a platen roller  8 ,  5  denotes a sheet guide which guides a sheet into between the platen roller  8  and the thermal head  13 ,  6  denotes a fixture for fixing the outer frame  18  to the printer main body, and symbol P denotes a sheet retaining portion where rolled sheets are retained.  
         [0049]     The outer casing of the thermal printer  1  is constituted by a main housing  2  and a movable cover  3  that opens and closes to expose a part of the printing mechanism  10 . A sheet delivery portion and a sheet cutter are provided between the main housing  2  and the movable cover  3 . A label on which printing has been performed is delivered from the sheet delivery portion. That is, the forward end of the movable cover  3  is located in (or in the vicinity of) the portion where the thermal head  13  and the platen roller  8  come into contact with each other, and is normally covered by the main housing  2 .  
         [0050]     When the thermal head  13  is to be replaced in the thermal printer  1  constructed as described above, the movable cover  3  is opened to detach the platen roller  8 , and then the head support member  15  is detached. At this time, the screws  12  serving as joining means for joining the head support member  15  to the pressure plate  16  is covered by the main housing  1 , which means that opening the movable cover  3  alone does not make the screws  12  visible from the outside. However, the head support member  15 , the pressure plate  16 , and the lock arm  17  are rotatable about the shaft  20 ; hence, rotating them toward the opening side causes the joining portion (screws  12 ) to be exposed, which makes it possible to detach only the head support member  15  by loosening the screws  12 .  
         [0051]     While the present invention as implemented by the inventor has been described above in detail based on the embodiment thereof, the present invention is not to be limited to the above-described embodiment and can be subject to various modifications. For example, while in the above embodiment the U-shaped recess is formed in each of the locking pieces  15   a  of the head support member  15 , a through hole through which the screw is passed may be provided instead. In this case, by forming the locking pieces  15   a  at a slightly upward incline with respect to the horizontal direction, simple fastening with the screws  12  can enhance the intimate contact between the head support member  15  and the pressure plate  16 .