Patent Publication Number: US-9902173-B2

Title: Thermal printer

Description:
RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. § 119 to Japanese Patent Application No. 2015-255055 filed on Dec. 25, 2015, the entire content of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a thermal printer. 
     2. Description of the Related Art 
     Hitherto, as a printer configured to perform printing on a recording paper (heat-sensitive paper), there has been known a thermal printer. As the thermal printer, there have been known various printers in which a paper cover is coupled to a casing for receiving a roll sheet in an openable and closable manner so that the roll sheet can easily be set. 
     The thermal printer is sometimes used in an environment where drip-proof performance is required, such as outdoors and a kitchen of a restaurant. In such an environment where the drip-proof performance is required, there is a risk in that liquids, such as rainwater during outdoor work, moisture adhesion on a user&#39;s hand, water splashed in the kitchen, and the like may enter the printer through a boundary portion between the casing and the paper cover to adhere to the roll sheet, a circuit board, and the like. When the liquid adheres to the roll sheet or the circuit board, a printing failure or a malfunction of electric components may be caused. Therefore, the printer needs to have a feature to protect the printer against the liquid, such as the structure which prevents entry of water thereinto, and the structure which can cause water having entered the printer to be discharged without adhesion of the water to the roll sheet or the circuit board. 
     In view of the above-mentioned circumstances, in this technical field, there has been demanded a thermal printer capable of preventing a situation where liquid in a housing wets a printing paper. 
     SUMMARY OF THE INVENTION 
     According to one embodiment of the present invention, there is provided a thermal printer, including: a thermal head; a platen roller opposed to the thermal head; a platen frame configured to support the platen roller; and a housing for receiving the thermal head, the platen roller, and the platen frame, wherein the platen frame includes: an opposing surface formed below the platen roller in a gravitational direction in an assumed carriage posture, the opposing surface being opposed to the platen roller; and a liquid absorbing sheet arranged on the opposing surface. 
     In the above-mentioned thermal printer according to the one embodiment of the present invention, wherein the liquid absorbing sheet is made of a porous material. 
     In the above-mentioned thermal printer according to the one embodiment of the present invention, wherein the opposing surface of the platen frame has a groove. 
     In the above-mentioned thermal printer according to the one embodiment of the present invention, wherein the platen frame further includes a retaining portion for retaining a liquid having entered the housing on a lateral side of the platen frame in the assumed carriage posture, wherein the groove communicates with the retaining portions. 
     In the above-mentioned thermal printer according to the one embodiment of the present invention, wherein the platen frame further includes a locking member configured to lock the liquid absorbing sheet. 
     In the above-mentioned thermal printer according to the one embodiment of the present invention, wherein the locking member includes a plurality of claw members configured to lock the liquid absorbing sheet, wherein the plurality of claw members are arranged at intervals in an extending direction of the platen roller. 
     In the above-mentioned thermal printer according to the one embodiment of the present invention, wherein heights of top portions of the plurality of claw members are common to a height of an upper surface of the liquid absorbing sheet in the gravitational direction in the assumed carriage posture. 
     In the above-mentioned thermal printer according to the one embodiment of the present invention, wherein each of the plurality of claw members has an inclined portion extending from an upper side to a lower side in the gravitational direction in the assumed carriage posture. 
     In the above-mentioned thermal printer according to the one embodiment of the present invention, wherein the platen frame further includes: a rear surface portion formed on a rear surface side of the platen roller in the assumed carriage posture; and a plurality of reinforcing ribs arranged between the plurality of claw members which are adjacent to each other on the rear surface portion, and configured to press the upper surface of the liquid absorbing sheet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a thermal printer, for illustrating a state in which a paper cover takes a closed position. 
         FIG. 2  is a perspective view of the thermal printer, for illustrating a state in which the paper cover takes an opened position. 
         FIG. 3  is a perspective view for illustrating a platen unit of a printing unit when viewed from above. 
         FIG. 4  is a front view for illustrating the platen unit of the printing unit when viewed from the front. 
         FIG. 5  is a rear view for illustrating the platen unit of the printing unit when viewed from the rear. 
         FIG. 6  is an enlarged perspective view for illustrating relevant parts of a platen unit  32 . 
         FIG. 7A  is a sectional side view of the platen unit. 
         FIG. 7B  is an enlarged front view for illustrating a side portion of the platen unit. 
         FIG. 8A  is a schematic sectional side view for illustrating a relationship between a platen roller and a platen frame when a liquid absorbing sheet is not arranged. 
         FIG. 8B  is a schematic sectional side view for illustrating the relationship between the platen roller and the platen frame when the liquid absorbing sheet is arranged. 
         FIG. 9  is an explanatory view for illustrating a flow of liquid in a casing. 
         FIG. 10  is a perspective view for illustrating a platen unit according to a second embodiment of the present invention when viewed from above. 
         FIG. 11  is a perspective view for illustrating a platen unit according to a third embodiment of the present invention when viewed from above. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     A first embodiment of the present invention is described.  FIG. 1  is a perspective view of a thermal printer  1 , for illustrating a state in which a paper cover  3  takes a closed position.  FIG. 2  is a perspective view of the thermal printer  1 , for illustrating a state in which the paper cover  3  takes an opened position. In the following description, for easy understanding of the invention, the illustrations are simplified, for example, by omitting a part of structural components, simplifying shapes of the structural components, and modifying scales of the structural components, as appropriate. Further, in the drawings, FR represents a forward direction. LH represents a leftward direction. UP represents an upward direction. Further, the thermal printer  1  is sometimes used while being carried by a user, and hence a vertical direction of the thermal printer  1  may not be determined in such a case. In this embodiment, a state in which the thermal printer  1  is oriented with the forward and backward directions, the rightward and leftward directions, and the upward and downward directions that are illustrated in  FIG. 1  represents an assumed carriage posture that is assumed as a posture during carriage, and the thermal printer  1  in this assumed carriage posture is described. 
     As illustrated in  FIG. 1  and  FIG. 2 , the thermal printer  1  includes a casing  2  being a housing. The casing  2  has an opening portion  2   a  formed therein. The thermal printer  1  further includes a paper cover  3  serving as an opening and closing cover configured to open and close the opening portion  2   a . The paper cover  3  is rotatably supported by the casing  2 . The thermal printer  1  further includes a printing unit  4  received in the casing  2 . 
     The casing  2  is made of a resin material, e.g., polycarbonate, or a metal material. An upper portion of the casing  2  is formed into a rectangular parallelepiped shape having a front wall  10 , whereas a lower portion of the casing  2  is formed into a box shape having the opening portion  2   a  being opened forward. An operation unit  11  configured to perform various operations of the thermal printer  1  is arranged on an upper portion of the front wall  10  of the casing  2 . As the operation unit  11 , various function switches  12  and various lamps  13  are arranged. The various function switches  12  include a power switch, a FEED switch, or other switches. The various lamps  13  are arranged adjacent to the function switches  12  and include a POWER lamp for notifying information on an ON/OFF state of the power switch, an ERROR lamp for notifying the error or the like of the thermal printer  1 , or other lamps. Further, an open button  18  for opening the paper cover  3  is arranged between the front wall  10  and a side wall  15  of the casing  2 . 
     In the lower portion of the casing  2 , there is defined a roll sheet receiving portion  21  for receiving a roll sheet R through the opening portion  2   a . The roll sheet receiving portion  21  includes a guide plate  22  for holding the roll sheet R, and holds the roll sheet R between the guide plate  22  and an inner surface of the paper cover  3  so as to cover the roll sheet R. The guide plate  22  has an arc-shaped cross section when viewed in the rightward and leftward directions. The guide plate  22  holds the roll sheet R in a state in which an outer peripheral surface of the roll sheet R is held in contact with an inner peripheral surface of the guide plate  22 , and guides a recording paper P drawn out from the roll sheet R to the printing unit  4 . The recording paper P employed in this embodiment is a heat sensitive paper and suitably used for printing of various types of labels, receipts, and tickets and the like. This recording paper P forms the roll sheet R having a hollow hole by being wound into a roll. Then, the printing unit  4  performs printing on a region of the recording paper P, which is drawn out from the roll sheet R. 
     The paper cover  3  is made of a resin material, e.g., polycarbonate. A hinge structure (not shown) configured to pivotally support the paper cover  3  is formed in the lower portion of the paper cover  3 . The paper cover  3  is rotatable with respect to the casing  2  by virtue of the hinge structure. The hinge structure is formed such that a hinge shaft arranged in the casing  2  and a hinge plate arranged in the paper cover  3  are rotatably supported. Further, the paper cover  3  is formed such that an upper end thereof is lockable with the casing  2  through intermediation of a platen unit  32  described later. Through push of the open button  18 , the casing  2  and the paper cover  3  are unlocked, and the paper cover  3  rotates from the closed position illustrated in  FIG. 1  to the opened position illustrated in  FIG. 2 . Further, in the state in which the paper cover  3  takes the closed position, a clearance formed between an upper end edge of the paper cover  3  and a lower end edge of the front wall  10  of the casing  2  serves as a delivery slot  24  through which the recording paper P to be printed by the printing unit  4  is delivered. 
     Cutting blades  25 , which are configured to cut the recording paper P delivered through the delivery slot  24 , are arranged at an opening edge of the delivery slot  24 . The cutting blades  25  are integrally arranged at the lower end edge of the front wall  10  of the casing  2  (portion located on an upper side of the opening edge), and at an upper end edge of the paper cover  3 , respectively. The recording paper P is pulled and moved toward the cutting blades  25  so that the recording paper P is cut. 
     Further, a strap or a hook is mountable to an upper portion on a back surface of the casing  2 . When a user or the like carries the thermal printer  1 , it is assumed that the user may often carry the thermal printer  1  while putting a strap around the user&#39;s neck or shoulder, or mounting a hook to a waist belt. Therefore, the state in which the thermal printer  1  is oriented with the forward and backward directions, the rightward and leftward directions, and the upward and downward directions that are illustrated in  FIG. 1  represents the assumed carriage state. 
     The printing unit  4  includes a head unit  31  and the platen unit  32 . The head unit  31  is arranged in a lower end portion of the front wall  10  of the casing  2 . The platen unit  32  is arranged in an upper end portion of the paper cover  3  and is removably coupled to the head unit  31  in accordance with an opening and closing operation of the paper cover  3 . As illustrated in  FIG. 2 , the platen unit  32  includes a platen frame  35 , which is mounted to the paper cover  3 , and a platen roller  36  rotatably supported by the platen frame  35 . 
       FIG. 3  is a perspective view for illustrating the platen unit  32  of the printing unit  4  when viewed from above.  FIG. 4  is a front view for illustrating the platen unit  32  when viewed from the front.  FIG. 5  is a rear view for illustrating the platen unit  32  when viewed from the rear. Further,  FIG. 6  is an enlarged perspective view for illustrating relevant parts of the platen unit  32 .  FIG. 7A  is a sectional side view of the platen unit, and  FIG. 7B  is an enlarged front view for illustrating a side portion of the platen unit.  FIG. 7B  is an illustration of a state in which the opening portion  2   a  of the casing  2  is closed with the paper cover  3 . In  FIG. 3  and  FIG. 4 , the platen roller  36  is illustrated by imaginary lines. 
     As illustrated in  FIG. 3  to  FIG. 5 , the platen roller  36  includes a platen shaft  41  extending along the rightward and leftward directions and a roller main body  42  made of a rubber or the like, which is externally mounted to the platen shaft  41 . Bearings  43  configured to rotatably support the platen shaft  41  are externally mounted at both end portions of the platen shaft  41 . Each bearing  43  is held by the platen frame  35  (see  FIG. 2 ), and the platen roller  36  is rotatably supported by the platen frame  35  through intermediation of the bearings  43 . 
     A platen gear  45  is mounted to a right end portion of the platen shaft  41 . Further, the head unit  31  includes a gear train mechanism (not shown) brought in mesh with the platen gear  45  and a motor (not shown) connected to the gear train mechanism. When the platen unit  32  and the head unit  31  are coupled to each other, the platen gear  45  is brought in mesh with the gear train mechanism arranged on the head unit  31  side, to thereby transmit a rotational driving force of the motor to the platen roller  36 . In addition, when the platen unit  32  and the head unit  31  are coupled to each other, a thermal head of the head unit  31  is brought into press contact with an outer peripheral surface of the platen roller  36 . 
     As illustrated in  FIG. 6  and  FIG. 7A , the platen frame  35  is arranged below the platen roller  36  in a gravitational direction. The platen frame  35  includes a lower surface portion  51  being an opposing surface to be opposed to the platen roller  36  and a rear surface portion  52  arranged on a rear surface side of the platen roller  36 . In front of the lower surface portion  51 , there is formed a groove  53  extending along the rightward and leftward directions corresponding to an axial direction of the platen roller  36 . The groove  53  is formed to extend to end portions in the rightward and leftward directions of the platen frame  35 . Further, a liquid absorbing sheet  60  is arranged on the lower surface portion  51  of the platen frame  35 . With this, the platen roller  36  is arranged above the liquid absorbing sheet  60 , and a separation distance between the liquid absorbing sheet  60  and the platen roller  36  is not so large. 
     A plurality of upper claw members  54  and lower claw members  55  are arranged in front of the rear surface portion  52 . The plurality of upper claw members  54  and lower claw members  55  are paired respectively and arranged apart vertically in the gravitational direction. The upper claw members  54  are positioned higher than the lower surface portion  51 , and the lower claw members  55  are positioned lower than the lower surface portion  51 . An upper surfaces of the lower claw members  55  are positioned slightly higher than a surface of the lower surface portion  51 . 
     The liquid absorbing sheet  60  is arranged between the upper claw members  54  and the lower claw members  55 . The liquid absorbing sheet  60  is fitted between the upper claw members  54  and the lower claw members  55 , and is locked with the upper claw members  54  and the lower claw members  55 . The liquid absorbing sheet  60  has a width substantially equal to a width of the lower surface portion  51  or the rear surface portion  52  of the platen frame  35  in the rightward and leftward directions. 
     The upper claw member  54  includes an upper claw main body  56  having a shape of an approximately hexagonal columnar body in front view of the platen unit  32 . As illustrated in  FIG. 6 , the upper claw main body  56  includes an upper surface portion  56 A and a lower surface portion  56 B. A width of the upper surface portion  56 A in the rightward and leftward directions is set to be smaller than a width of the lower surface portion  56 B in the rightward and leftward directions. 
     Further, a left inclined surface portion  56 C and a left side surface portion  56 D are formed between a left side of the upper surface portion  56 A and a left side of the lower surface portion  56 B. An upper side of the left inclined surface portion  56 C and a left side of the upper surface portion  56 A are common, and a lower side of the left side surface portion  56 D and a left side of the lower surface portion  56 B are common. Further, a lower side of the left inclined surface portion  56 C and an upper side of the left side surface portion  56 D are common. 
     Similarly, a right inclined surface portion  56 E and a right side surface portion  56 F are formed between a right side of the upper surface portion  56 A and a right side of the lower surface portion  56 B. An upper side of the right inclined surface portion  56 E and a right side of the upper surface portion  56 A are common, and a lower side of the right side surface portion  56 F and a right side of the lower surface portion  56 B are common. Further, a lower side of the right inclined surface portion  56 E and an upper side of the right side surface portion  56 F are common. 
     An upper claw protruding portion  56 H is arranged on the lower surface portion  56 B of the upper claw main body  56  of the upper claw member  54 . The upper claw protruding portion  56 H protrudes downward from the lower surface portion  56 B of the upper claw main body  56 . The upper claw protruding portion  56 H has a rectangular parallelepiped shape. In plan view, a size of the rectangle of the upper claw protruding portion  56 H is set to be smaller than a size of the lower surface portion of the upper claw main body  56 . Further, a width of the upper claw protruding portion  56 H in the rightward and leftward directions is set to be smaller than a width of the lower surface portion  56 B of the upper claw main body  56  in the rightward and leftward directions, and set to be larger than a width of the upper surface portion  56 A in the rightward and leftward directions. Further, a position of the upper claw protruding portion  56 H in the forward and backward directions is set on a rear side region of the upper claw main body  56 . 
     The lower claw member  55  has a rectangular parallelepiped shape. A width of the lower claw member  55  in the rightward and leftward directions is set substantially equal to a width of the lower surface portion  56 B of the upper claw member  54  in the rightward and leftward directions. Further, in plan view, the lower surface portion  56 B of the upper claw member  54  and the lower claw member  55  are arranged so that positions of end portions in a width direction match with each other. 
     A plurality of reinforcing ribs  58  are formed on the rear surface portion  52 . The reinforcing ribs  58  are formed along a surface of the rear surface portion  52  and reinforce the platen frame  35 . Further, in front view of the platen unit  32 , the plurality of reinforcing ribs  58  are each arranged between the adjacent upper claw members  54 . The reinforcing rib  58  has a distal end portion protruding forward with respect to the rear surface portion  52 . The reinforcing rib  58  is formed such that a height of the distal end portion is substantially equal to a height of the lower surface portion  56 B of the upper claw member  54 . With this, the distal end portion of the reinforcing rib  58  is held in abutment against the liquid absorbing sheet  60 . 
     The liquid absorbing sheet  60  arranged on the lower surface portion  51  of the platen frame  35  is made of a porous material, e.g., a nonwoven fabric. Through use of the porous material, the liquid absorbing sheet  60  attains a high liquid-absorbing performance. As the liquid absorbing sheet  60 , there may be used a foam material, e.g., a polyurethane, other than the nonwoven fabric. Further, a material other than the porous material may be used. 
     The liquid absorbing sheet  60  has the high liquid-absorbing performance and absorbs a liquid, e.g., water, having entered the casing  2 . Further, the liquid having been absorbed is adsorbed and retained by a capillary phenomenon. When the amount of liquid exceeds a retainable amount of the liquid absorbing sheet  60 , the liquid leaks from the liquid absorbing sheet  60 . The liquid having leaked from the liquid absorbing sheet  60  drops mainly toward the groove  53 . With this, the amount of leakage of the liquid from a surface of the liquid absorbing sheet  60  is little or extremely small. 
     As illustrated in  FIG. 3 , the liquid absorbing sheet  60  includes a main body portion  61  having an elongated shape. The liquid absorbing sheet  60  is arranged so that a longitudinal direction of the main body portion  61  corresponds to the rightward and leftward directions. There are formed a plurality of fitting recessed portions  62 , which are arranged at intervals in the longitudinal direction of the main body portion  61 , on the rear side of the main body portion  61 . The liquid absorbing sheet  60  has a comb-like shape as a whole. 
     A thickness of the main body portion  61  of liquid absorbing sheet  60  is set substantially equal to a distance between the lower surface portion  56 B of the upper claw member  54  and the upper surface of the lower claw member  55 . Further, a width of the fitting recessed portion  62  of the liquid absorbing sheet  60  is set substantially equal to a width of the upper claw protruding portion  56 H of the upper claw member  54 . The liquid absorbing sheet  60  is locked by being sandwiched between the upper claw members  54  and the lower claw members  55  of the platen unit  32 . Further, the upper claw protruding portions  56 H of the upper claw members  54  are fitted into the fitting recessed portions  62  of the liquid absorbing sheet  60  so that the liquid absorbing sheet  60  is positioned. 
     Retaining tanks  70  are integrally formed with the platen frame  35  on right and left sides of the groove  53  below the groove  53  in the gravitational direction. The groove  53  communicates with the retaining tanks  70  formed on the right and left sides of the groove  53 . Further, the right and left ends of the groove  53  have inclinations declining as approaching outer sides. With this structure, the liquid having been retained in the groove  53  is guided into the respective retaining tanks  70  on the right and left sides of the groove  53 . The liquid having been guided into the retaining tanks  70  drops along outer sides of the roll sheet receiving portion  21  to be discharged to an outside through, for example, the clearance between the casing  2  and the paper cover  3 . 
     Further, as illustrated in  FIG. 7B , on an inner side surface of the paper cover  3 , there is arranged a protruding beam  3 A arranged between the lower claw members  55  and the groove  53  when the opening portion  2   a  of the casing  2  is closed with the paper cover  3 . The protruding beam  3 A is arranged along the rightward and leftward directions and has a substantially uniform thickness. Cutout portions  3 B are formed at end portions of the protruding beam  3 A in the rightward and leftward directions. Further, a cutout portion (not shown) is also formed at a center of the protruding beam  3 A in the rightward and leftward directions. 
     Next, description is made of a process of discharging the liquid having entered the casing  2  of the thermal printer  1 , and the action and effect of the thermal printer  1  according to this embodiment. In the thermal printer  1  according to this embodiment, when the liquid enters the casing  2  to wet the roll sheet R, for example, there arises an adverse effect on the recording paper P. In order to avoid such a situation, it is preferred to keep the liquid having entered the casing  2  away from the roll sheet R. 
     Most of the liquid having entered through the delivery slot  24  illustrated in  FIG. 1  is retained on the lower surface portion  51  of the platen frame  35  positioned below the platen roller  36  in the gravitational direction. When the liquid having been retained on the lower surface portion  51  becomes water droplets, there is a fear in that an upper portion of the water droplet may be brought into contact with a lower side of the platen roller  36  to wet the platen roller  36 . 
     In this regard, the liquid absorbing sheet  60  is arranged on the lower surface portion  51  of the platen frame  35 . With this, the liquid having been retained on the lower surface portion  51  of the platen frame  35  is absorbed into the liquid absorbing sheet  60 , and thus is prevented from becoming the water droplets on the lower surface portion  51  of the platen frame  35 . Thus, the water droplets on the lower surface portion  51  are reduced, thereby being capable of more suitably preventing the situation where the liquid in the casing  2  wets a printing paper. 
     For example, as illustrated in  FIG. 8A , a separation distance between the platen roller  36  and the lower surface portion  51  of the platen frame  35  may be set higher than a height of the water droplet in a case where the liquid is assumed to become the water droplet. With this, a liquid adhesion to the platen roller  36  can be prevented even when the water droplet is retained on the lower surface portion  51 . 
     However, in this case, the separation distance between the platen roller  36  and the lower surface portion  51  of the platen frame  35  needs to be set large. Thus, there is a fear in that the platen unit  32  is increased in size. Further, when the thermal printer  1  is carried while the water droplet is retained, the water droplet may splash toward the platen roller  36  to wet the platen roller  36 . 
     In this regard, as illustrated in  FIG. 8B , the liquid absorbing sheet  60  is arranged on the lower surface portion  51  of the platen frame  35 . With this, adhesion of the liquid to the platen roller  36  may be reduced even when the separation distance between the liquid absorbing sheet  60  and the platen roller  36  is set smaller than the height in the case where the liquid is assumed to become the water droplet. Further, the liquid is retained in the liquid absorbing sheet  60 . Thus, the liquid can be prevented from splashing even when the thermal printer  1  is carried. Therefore, it is possible to more suitably prevent the situation where the liquid in the casing  2  wets the printing paper with downsizing of the platen unit  32 . 
     Further, the liquid absorbing sheet  60  is made of the nonwoven fabric being the porous material. The liquid absorbing sheet  60  attains the high liquid-absorbing performance because the liquid absorbing sheet  60  is made of the porous material. Therefore, the liquid in the casing  2  can be more suitably absorbed, thereby being capable of being more suitably preventing the situation where the liquid in the casing  2  wets the printing paper. 
     Further, a liquid absorption amount of the liquid absorbing sheet  60  has an upper limit, and thus the liquid having been absorbed in the liquid absorbing sheet  60  gradually leaks from the liquid absorbing sheet  60  to be discharged. Here, in the lower surface portion  51  of the platen frame  35 , there is formed the groove  53  which extends along the rightward and leftward directions corresponding to the axial direction of the platen roller  36 . Through formation of the groove  53 , there can be formed a path for discharging the liquid having leaked from the liquid absorbing sheet  60  to the outside. Further, in the liquid absorbing sheet  60 , a liquid absorption capacity can be secured again through leakage of the liquid, and thus the liquid having newly entered the casing  2  can be absorbed. In this manner, it is possible to more suitably prevent the situation where the liquid in the casing  2  wets the printing paper. 
     Further, the retaining tanks  70  for retaining the liquid having entered the casing  2  are formed on the lateral sides of the platen frame  35 . Through formation of the retaining tanks  70 , even when a large amount of liquid enters the casing  2  and is prolonged to be discharged from the casing  2 , for example, the liquid in the retaining tanks  70  can be retained while the liquid is discharged. Thus, the large amount of the liquid can be prevented from remaining on the lower surface portion  51  of the platen frame  35 . 
     Further, the groove  53  of the platen frame  35  communicates with the retaining tanks  70 . With this, the liquid having leaked from the liquid absorbing sheet  60  to be discharged to the groove  53  is guided into the retaining tanks  70  on the lateral sides. In this manner, the liquid retained in the lower surface portion  51  can be reduced. Therefore, it is possible to more suitably prevent the situation where the liquid in the casing  2  wets the printing paper. 
     Further, as illustrated in  FIG. 9 , the protruding beam  3 A of the paper cover  3  is sandwiched between the lower surface portion  51  of the platen frame  35  and the groove  53 , and the cutout portions are formed at both the right and left ends and at the center of the protruding beam  3 A. With this, the liquid having leaked from the liquid absorbing sheet  60  can flow into the groove  53  while avoiding the protruding beam  3 A. 
     Further, the upper claw members  54  and the lower claw members  55  are arranged on the lower surface portion  51  of the platen frame  35 . The liquid absorbing sheet  60  is locked with the upper claw members  54  and the lower claw members  55 . With this, the liquid absorbing sheet  60  can securely be arranged on the lower surface portion  51  of the platen frame  35 . Further, the liquid absorbing sheet  60  has a thickness substantially equal to the distance between the lower surface portion  56 B of the upper claw member  54  and the upper surface portion of the lower claw member  55 . With this, the liquid absorbing sheet  60  can securely be locked with the upper claw members  54  and the lower claw members  55 . The thickness of the liquid absorbing sheet  60  may be a thickness other than the above-mentioned thickness, for example, a thickness being slightly larger than the distance between the lower surface portion  56 B of the upper claw member  54  and the upper surface portion of the lower claw member  55 . 
     Further, the plurality of upper claw members  54  are arranged at intervals in an extending direction of the platen roller  36 . The plurality of lower claw members  55  are arranged in pair with the upper claw members  54 , respectively. The liquid absorbing sheet  60  is locked with the plurality of upper claw members  54  and lower claw members  55 , and thus the liquid absorbing sheet  60  can more securely be arranged on the lower surface portion  51  of the platen frame  35 . 
     Further, clearances are formed between the adjacent upper claw members  54 , and formed between the adjacent lower claw members  55 . The liquid absorbing sheet  60  is arranged at the clearances. Further, both the upper claw members  54  and the lower claw members  55  are held in contact with the liquid absorbing sheet  60 . With this, the liquid to adhere to the upper claw members  54  and the lower claw members  55  can easily flow into the liquid absorbing sheet  60 . 
     Further, the upper claw member  54  has the inclined surface portions  56 C and  56 E. Through formation of the inclined surface portions  56 C and  56 E, the liquid to be retained on the upper claw member  54  can be guided downward. Further, the liquid absorbing sheet  60  is positioned at lower end portions of the inclined surface portions  56 C and  56 E. With this, the liquid having been guided downward can securely be absorbed by the liquid absorbing sheet  60 . In this manner, the liquid is less liable to be retained on the claw members. 
     Further, the lower surface portion  51  of the upper claw member  54  has the upper claw protruding portion  56 H protruding downward. The liquid absorbing sheet  60  has the fitting recessed portion  62  having the width substantially equal to the width of the upper claw protruding portion  56 H. With this, the liquid absorbing sheet  60  can securely be positioned with respect to the lower surface portion  51  of the platen unit  32 . 
     Further, the plurality of reinforcing ribs  58  are formed on the rear surface portion  52  of the platen frame  35 . Each of those reinforcing ribs  58  has the distal end portion protruding forward with respect to the rear surface portion  52 . The reinforcing rib  58  is formed such that the height of the distal end portion is substantially equal to the height of the lower surface portion  56 B of the upper claw member  54 . The distal end portion of the reinforcing rib  58  is held in abutment against the liquid absorbing sheet  60 . With this structure, the reinforcing rib  58  suppresses upward swelling of the liquid absorbing sheet  60  or the like by the distal end portion protruding forward. In this manner, the liquid absorbing sheet  60  can be prevented from coming off without an increase in number of components. Moreover, the liquid absorbing sheet  60  can be prevented from moving upward so that the separation distance between the liquid absorbing sheet  60  and the platen roller  36  can be prevented from becoming smaller. As a result, it is possible to more suitably prevent the situation where the liquid in the casing  2  wets the printing paper. 
     In the above-mentioned embodiment, the upper claw main body  56  of the upper claw member  54  is formed to be a columnar body having the approximately hexagonal section. However, the section of the upper claw main body  56  of the upper claw member  54  may be formed to have other shapes. For example, the upper claw main body  56  of the upper claw member  54  may be a columnar body having no side surfaces and having an approximately trapezoidal section, or a columnar body having no side surfaces and upper surface and having an approximately triangular section. In any of those cases, it is preferred that the inclined surfaces be formed. 
     Second Embodiment 
     Next, description is made of a second embodiment of the present invention. A structure of a thermal printer according to this embodiment is mainly different in the structure for locking the liquid absorbing sheet, as compared to the thermal printer  1  according to the above-mentioned embodiment. Now, the thermal printer according to this embodiment is described with a focus on differences with the above-mentioned thermal printer  1 . 
       FIG. 10  is a perspective view for illustrating a platen unit according to the second embodiment when viewed from above. As illustrated in  FIG. 10 , a platen unit  80  according to this embodiment, similarly to the platen unit  32  according to the above-mentioned first embodiment, includes upper claw members  81  and lower claw members  82 . As in the above-mentioned upper claw member  54 , the upper claw member  81  includes an upper surface portion  81 A, a lower surface portion  81 B, inclined surface portions  81 C and  81 D, and side surface portions  81 E and  81 F. There is no protruding portion like the upper claw protruding portion  56 H of the above-mentioned upper claw member  54 . The lower claw member  82  has the same shape as the above-mentioned lower claw member  55 . 
     Further, a separation distance between the upper claw member  81  and the lower claw member  82  is set smaller than that in the first embodiment. A liquid absorbing sheet  83  is arranged between the upper claw members  81  and the lower claw members  82 . The liquid absorbing sheet  83  is locked with the upper claw members  81  and the lower claw members  82 . The liquid absorbing sheet  83  includes a main body portion  83 A, and fitting recessed portions  83 B are formed at positions corresponding to the upper claw members  81  on an upper surface of the main body portion  83 A. With this, the fitting recessed portions  83 B are sandwiched between the upper claw members  81  and the lower claw members  82  so that the liquid absorbing sheet  83  is locked with the upper claw members  81  and the lower claw members  82 . The fitting recessed portion  83 B has a width substantially equal to a width of the upper claw member  81 . 
     Further, a thickness of the main body portion  83 A of the liquid absorbing sheet  83  is set to be substantially equal to a distance between the upper surface portion  81 A of the upper claw member  81  and an upper surface portion of the lower claw member  82 . With this, under a state in which the liquid absorbing sheet  83  is locked with the upper claw members  81  and the lower claw members  82 , the upper surface of the liquid absorbing sheet  83  and an upper surface of the upper claw member  81  are approximately flush with each other. Further, a space having an approximately triangular section is formed above the inclined surface portions  81 C and  81 D of the upper claw member  81 . In other respects, the thermal printer according to this embodiment is common mainly to the above-mentioned first embodiment. 
     The thermal printer having the above-mentioned configuration, which includes the platen unit  80  according to this embodiment, has the same action and effect as the above-mentioned first embodiment. Further, the upper surface of the upper claw member  81  and the upper surface of the liquid absorbing sheet  83  are approximately flush with each other. Thus, a distance between the upper claw member  81  and the platen roller  36  and a distance between the liquid absorbing sheet  83  and the platen roller  36  are uniform. Therefore, when viewed from the platen roller  36 , the upper claw member  81  is arranged at the same or lower level of the liquid absorbing sheet  83 , with the result that the liquid retained on the upper claw member  81  is less liable to be brought into contact with the platen roller  36 . In this manner, the platen roller  36  is less liable to be wet, thereby being capable of more suitably preventing the situation where the liquid in the housing wet the printing paper. 
     Further, in the thermal printer according to this embodiment, the upper surface of the upper claw member  81  and the upper surface of the liquid absorbing sheet  83  are approximately flush with each other. With this, a space to arrange the upper claw member  81  is not needed above the liquid absorbing sheet  83 . Therefore, the platen unit  80  can be downsized. 
     Third Embodiment 
     Next, description is made of a third embodiment of the present invention. As compared to the thermal printer  1  according to the above-mentioned embodiment, a thermal printer according to this embodiment is mainly different in a structure for arranging the liquid absorbing sheet with respect to the lower surface portion of the platen unit. In the following, the thermal printer according to this embodiment is described with a focus on differences with the above-mentioned thermal printer  1 . 
       FIG. 11  is a front view for illustrating a platen unit according to the third embodiment when viewed from the front. As illustrated in  FIG. 11 , upper claw members and lower claw members as those in the platen unit according to the first embodiment are not arranged in a platen unit  90  according to this embodiment. In the platen unit  90 , there is formed a flat upper step portion  91  and a groove  92  descending one step from the upper step portion  91 . The upper step portion  91  and groove  92  are both formed along the extending direction of the platen roller  36 . 
     Further, the groove  92 , similarly to the first embodiment, communicates with the retaining tanks  70  formed on the lateral sides. Moreover, liquid absorbing sheets  93  are bonded to the upper step portion  91 . The liquid absorbing sheets  93  are bonded to the upper step portion  91  with, for example, adhesive. In other respects, the thermal printer according to this embodiment is common mainly to the first embodiment. 
     The thermal printer having the above-mentioned configuration, which includes the platen unit  90  according to this embodiment, has the same action and effect as the above-mentioned first embodiment. Further, in the thermal printer according to this embodiment, the liquid absorbing sheets  93  are bonded to the upper step portion  91  with the adhesive to be arranged on the platen unit  90 . In this manner, the liquid absorbing sheets  93  can easily be arranged on the platen unit  90 .