Patent Publication Number: US-8525859-B2

Title: Thermal head and thermal printer

Description:
BACKGROUND 
     1. Technical Field 
     The present invention relates to a thermal head and to a thermal printer that uses the thermal head. 
     2. Related Art 
     Thermal printers that print by conveying thermal paper or other print medium enabling thermal printing over a thermal head having heating elements disposed thereto are known from the literature. See, for example, Japanese Unexamined Patent Appl. Pub. JP-A-2006-88584. 
       FIG. 8  is a section view of the print unit in the thermal printer  201  described in JP-A-2006-88584. The thermal head  220  disposed in this thermal printer  201  is pushed to the platen roller  210  side by a coil spring  206 , and the print medium P is thereby held between the platen roller  210  and the thermal head  220 . This type of thermal printer  201  prints by causing the print medium P to change color by applying heat thereto by means of the thermal head  220 . 
     When this thermal printer  201  according to the related art prints for an extended period of time to a low quality, coarse print medium P with high surface roughness, parts of the common electrodes  225  may wear and fail as a result of the print medium P repeatedly wearing a particular part of the common electrode  225  of the thermal head  220  (see  FIG. 9 ), eventually resulting in an inability to print. 
     To further describe this problem,  FIG. 9  shows a top view of the main parts of a common thermal head  220 . The contact surface  211   a  pressed by the platen roller  210  against the thermal head  220  is indicated by a double-dot dash line in  FIG. 9 . 
     As shown in  FIG. 9 , a heat unit  221  having a plurality of heat elements  221   a  arrayed in a line is formed on the substrate  223  of the thermal head  220 . A plurality of drive electrodes  224  that supply drive current to the heat elements  221   a  are formed on the substrate  223  on one side of the linear heat unit  221 , and are connected to a drive chip not shown. 
     A common electrode  225  that is conductive with each of the heat elements  221   a  is also formed on the substrate  223  on the other side of the heat unit  221 . The common electrode  225  communicates with the drive electrode  224  side through a electrode connection unit  226  that is formed at the end of the heat unit  221  array, and is connected to an external connector not shown. 
     The rotational axis Ax of the platen roller  210  is disposed opposite the thermal head  220  aligned with the alignment axis of the plural heat elements  221   a  so that the print medium P can be efficiently pressed against the heat unit  221 , and is affixed to the frame of the thermal printer  201  not shown. The print medium P is held between the platen roller  210  and the thermal head  220  as a result of the thermal head  220  being pushed to the platen roller  210  side by the coil spring  206 . 
     The width of the platen roller  210  is greater than the width (the left-right direction in  FIG. 8 ) of at least the heat unit  221  so that the print medium P can be reliably pressed against the heat unit  221 . As a result, the platen roller  210  is pressed through the intervening print medium P to the heat unit  221  and the electrode connection unit  226  that is disposed on the axial end  221   c  side of the heat unit  221 . While the thermal head  220  and platen roller  210  meet at the contact surface  211   a , pressure is particularly great on the area  211   b  of the contact surface  211   a  that is closest to the rotational axis Ax because the platen roller  210  is a cylinder centered on the rotational axis Ax. 
     The common electrode  225  including the electrode connection unit  226  is thicker than the drive electrodes  224  and the heat elements  221   a  in order to carry the combined current flowing from the plural heat elements  221   a . A protective coating is also formed over the electrode connection unit  226  and the heat elements  221   a . However, as the protective coating on the electrode connection unit  226  is worn by the print medium P, the electrode connection unit  226 , which is softer than the coating, becomes worn in spots. More particularly, as shown in  FIG. 10 , the part  226   a  of the electrode connection unit  226  that is opposite the pressure area  211   b  of the platen roller  210  becomes worn as shown in  FIG. 10 . 
     As the electrode connection unit  226  continues to wear and the common electrode  225  finally fails in this part  226   a  of the electrode connection unit  226 , conductivity is lost between the external connector and the common electrode  225 , and the heat unit  221  cannot be driven. The thermal printer  201  thus becomes unable to print when a low quality, coarse print medium P is used for a long time. 
     SUMMARY 
     The present invention is directed to solving this problem by providing a thermal head in which the electrodes are not broken even after printing to a low quality, coarse print medium for a long time, and a thermal printer having this thermal head. 
     A first aspect of the invention is a thermal head to which a print medium is pressed through an intervening platen roller, the thermal head including a heating unit having a plurality of heat elements arrayed on an axis, and an electrode unit formed on a linear extension of the axis. A receptive space to which an end part of the platen roller contact surface that is pressed to the thermal head is formed on the axis between the heating unit and the electrode unit. 
     The thermal head according to this aspect of the invention positions the end of the contact (pressure) surface of the platen roller in a receptive space between the heating unit and the electrode unit in the axial direction of the heat elements. The electrode unit is thus not worn by the platen roller, and the electrode unit will not be interrupted. A thermal head that can be used for a long time without electrode disconnections can therefore be provided. 
     In a thermal head according to another aspect of the invention, the receptive space is filled with hard glass. 
     By filling the receptive space of the thermal head with hard glass, direct conductivity between the heat elements and the electrode unit resulting from moisture getting into the receptive space can be prevented, and a more highly reliable thermal head can be provided. 
     In a thermal head according to another aspect of the invention, a dummy heat element that does not produce heat is disposed to the receptive space side end of the heating unit, or in the receptive space. 
     The thermal heads according to these aspects of the invention can improve print quality because the heat elements disposed at the axial end of the array and the heat elements disposed in the middle of the array can be driven to heat uniformly by providing a dummy heat element. In addition, even if the dummy heat element is disposed to the receptive space and is exposed by the platen roller, printing can continue because the dummy heat element does not directly affect the printing operation, and a thermal head with a long service life can be provided. 
     Another aspect of the invention is a thermal printer including a thermal head including a heating unit having a plurality of heat elements arrayed along an axis, and an electrode unit formed on a linear extension of the axis with a receptive space between the electrode unit and the heating unit; and a platen roller that presses a print medium to the thermal head. An end part of the platen roller contact surface that is pressed to the thermal head is positioned in the axial direction to the receptive space. 
     In a thermal printer according to this aspect of the invention, the end part of the platen roller contact surface that is pressed to the thermal head is positioned in the receptive space. The electrode unit is thus not worn by the platen roller, and the electrode unit will not be interrupted. A thermal printer with a thermal head that can be used for a long time without electrode disconnections can therefore be provided. 
     In a thermal printer according to another aspect of the invention the receptive space is filled with hard glass. 
     By filling the receptive space of the thermal head with hard glass in the thermal printer according to this aspect of the invention, direct conductivity between the heat elements and the electrode unit can be prevented, and a more highly reliable thermal printer can be provided. 
     In a thermal printer according to another aspect of the invention, a dummy heat element that does not produce heat is disposed to the receptive space side end of the heating unit, or in the receptive space. 
     The thermal printers according to these aspects of the invention can improve print quality because the heat elements disposed at the axial end of the array and the heat elements disposed in the middle of the array can be driven to heat uniformly by providing a dummy heat element. In addition, even if the dummy heat element is disposed to the receptive space and is exposed by the platen roller, printing can continue because the dummy heat element does not directly affect the printing operation, and a thermal printer with a long service life can be provided. 
     Another aspect of the invention is a thermal printer having a thermal head including a heating unit that extends in a direction perpendicular to a print medium conveyance direction, and an electrode unit formed on a linear extension of the axis on which the heating unit extends; and a platen roller that presses the print medium to the thermal head. The electrode unit is formed on the axis of the heating unit at a position separated from the heating unit so that the platen roller does not press against the electrode unit. 
     Because the electrode unit is formed at a position separated from the heating unit and is not pressed to the platen roller in a thermal printer according to this aspect of the invention, the electrode unit is not worn by the platen roller, and the electrode unit will not be interrupted. A thermal printer with a thermal head that can be used for a long time without electrode disconnections can therefore be provided. 
     In a thermal printer according to another aspect of the invention the receptive space is filled with hard glass. 
     By filling the receptive space of the thermal head with hard glass in the thermal printer according to this aspect of the invention, direct conductivity between the heat elements and the electrode unit can be prevented, and a more highly reliable thermal printer can be provided. 
     In a thermal printer according to another aspect of the invention, a dummy heat element that does not produce heat is disposed to the receptive space side end of the heating unit, or in the receptive space. 
     The thermal printers according to these aspects of the invention can improve print quality because the heat elements disposed at the axial end of the array and the heat elements disposed in the middle of the array can be driven to heat uniformly by providing a dummy heat element. In addition, even if the dummy heat element is disposed to the receptive space and is exposed by the platen roller, printing can continue because the dummy heat element does not directly affect the printing operation, and a thermal printer with a long service life can be provided. 
     Another aspect of the invention is a thermal printer including: a thermal head having an electrode unit formed on an extension of the alignment axis of a plurality of heat elements outside the area of the heat elements; and a platen roller that presses a recording medium to the thermal head. Wherein the platen roller is formed so that, of the axial end of the heat elements and the plural ends of the electrode unit located on an axial extension of the heat elements, an end part of the thermal head contact surface of the platen roller is positioned between the axial end of the heat elements and the end of the electrode unit that is located farthest therefrom. 
     In a thermal printer according to this aspect of the invention, the alignment axis end of the contact surface of the platen roller is positioned between the axial end of the heating unit and the axial end of the electrode unit. More specifically, the contact surface of the platen roller that presses the print medium to the thermal head is not formed to the axial end of the electrode unit. No part of the electrode unit is therefore pressed against the print medium, and the electrode unit is therefore not interrupted. A thermal printer that can be used for a long time without electrode interruptions can therefore be provided. 
     In a thermal printer according to another aspect of the invention, the thermal head has a dummy heat element that does not produce heat on the axial end part of the heating unit; and the platen roller is formed so that the axial end of the contact surface overlaps the area where the dummy heat element is located. 
     The thermal printer according to this aspect of the invention can improve print quality because the heat elements disposed at the axial end of the array and the heat elements disposed in the middle of the array can be driven to heat uniformly by providing a dummy heat element. In addition, even if the dummy heat element is exposed by the platen roller, printing can continue because the dummy heat element does not directly affect the printing operation, and a thermal printer with a long service life can be provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a section view of a thermal printer according to a preferred embodiment of the invention. 
         FIG. 2  is an enlarged front view of part of the thermal head in the thermal printer shown in  FIG. 1 . 
         FIG. 3  is a section view of a print unit used for comparison. 
         FIG. 4  is a section view of a print unit used for comparison. 
         FIG. 5  is a section view of a print unit used for comparison. 
         FIG. 6  is a section view of a print unit used for comparison. 
         FIG. 7  is a section view of the print unit in the thermal printer shown in  FIG. 1 . 
         FIG. 8  is a section view of the print unit in a thermal printer according to the related art. 
         FIG. 9  is an enlarged front view of the thermal head in a thermal printer according to the related art. 
         FIG. 10  is an enlarged front view of the thermal head in a thermal printer according to the related art. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Preferred embodiments of the present invention are described below with reference to the accompanying figures. 
       FIG. 1  is a section view of a thermal printer  1  according to a preferred embodiment of the invention. The thermal printer  1  shown in  FIG. 1  prints by pressing a heat unit  21  that produces heat against a print medium P such as thermal paper that changes color when heat is applied to the print medium. 
     The thermal printer  1  has a housing  2 , a paper compartment  3  for storing the print medium P (thermal roll paper in this example), a print unit  30  including a platen roller  10  and thermal head  20 , and a drive unit (not shown in the figure) including gears and a motor for rotating the platen roller  10  and conveying the print medium P. After printing by the print unit  30 , the print medium P is discharged from a paper exit  5 . 
     The print unit  30  includes a platen roller  10  with a rotational shaft axially supported by the housing  2 , and a thermal head  20  disposed so that the heat unit  21  is opposite the platen roller  10 . The thermal head  20  is a flat member having a pivot shaft  22  that is axially supported by the housing  2  disposed to one end, and the heat unit  21  disposed to a position separated from the pivot shaft  22 . The flat thermal head  20  is constantly urged toward the platen roller  10  by an urging member  6  such as a coil spring having one end fastened to the housing  2 . 
       FIG. 2  is an enlarged front view of parts of the thermal head  20  shown in  FIG. 1 . A plurality of heat elements  21   a  rendering the heat unit  21  are disposed in a line perpendicular to the conveyance direction of the print medium P (left-right as seen in  FIG. 2 ) on a substrate  23  of the thermal head  20 . A plurality of mutually independent drive electrodes  24  extending from the pivot shaft  22  side to the heat element  21   a  side are formed on the substrate  23 , and are conductive with the corresponding heat elements  21   a . The drive electrodes  24  are connected to a drive chip not shown and selectively supply current to the heat elements  21   a  according to the print data, thereby causing the heat elements  21   a  to emit heat and print. 
     A common electrode  25  that is conductive to the heat elements  21   a  is disposed to the heat unit  21  on the opposite side as the pivot shaft  22  of the thermal head  20 . The common electrode  25  has an electrode connection unit  26  (electrode unit) outside the area of the heat unit  21  where the heat elements  21   a  are formed in a line along the axis of the heat unit  21  (outside the axial ends  21   b  shown in the figure). 
     The plural heat elements  21   a  are formed on a glass glaze layer  29  (see  FIG. 7 ) in order to align the heat elements  21   a  to the same height (elevation). A flat glaze layer  29  can be formed on the surface of the substrate  23  by coating the substrate  23  with molten glass. The heat unit  21  can therefore be formed with the heat elements  21   a  aligned to the same height even when there are minute variations in the surface roughness of the substrate  23  by disposing the heat elements  21   a  to the flat top surface of the glaze layer  29 , and a thermal printer  1  with excellent print quality can thus be provided. 
     The common electrode  25  extends through the electrode connection unit  26  to the pivot shaft  22  side, and conducts current supplied from the drive electrodes  24  to the heat elements  21   a  to an outside connector not shown. Because current supplied to the heat elements  21   a  flows together in the common electrode  25 , the common electrode  25  is thicker than the drive electrodes  24  so that sufficient current can be carried. 
     As shown in  FIG. 2 , a receptive space A is formed between the heat unit  21  and the electrode connection unit  26  in the axial direction of the heat elements  21   a . More specifically, the electrode connection unit  26  is formed with the receptive space A between it and the heat unit  21  on an extension of the alignment axis of the heat element  21   a  array. The common electrode  25  and drive electrodes  24  are not formed in this receptive space A because the receptive space A is an area that is worn by the platen roller  10  as described below. 
     A dummy heat element  28  is formed adjacent to the electrode connection unit  26  on the axial end  21   b  side of the heat unit  21 . The dummy heat element  28  is made from the same material as the heat elements  21   a , but is not connected to a drive electrode  24  and does not produce heat. The dummy heat element  28  is provided to achieve a uniform thermal environment by rendering the area surrounding the heat element  21   a  adjacent to the dummy heat element  28  with the same material and shape as that around the heat elements  21   a  in the middle of the heat element  21   a  group. More specifically, by providing this dummy heat element  28 , the heat element  21   a  adjacent to the dummy heat element  28  can output heat in the same way as the heat elements  21   a  in the middle of the array, thereby preventing printing problems at the end of the heat element array. 
     Note that the embodiment shown in  FIG. 2  has only one dummy heat element  28 , but a plurality of dummy heat elements  28  may be provided. Yet further, the dummy heat element  28  may be disposed in the heat unit  21  as shown in  FIG. 2 , or in the receptive space A. 
     The platen roller  10  that presses the print medium P to the thermal head  20  thus comprised is disposed directly above the heat unit  21  with its rotational axis Ax parallel to the alignment axis of the heat unit  21 . The platen roller  10  is also disposed relative to the thermal head  20  so that the end  11   a  of the contact surface with the thermal head  20  is located in the receptive space A in the direction of the alignment axis of the heat elements  21   a . In other words, the electrode connection unit  26  is formed at a position separated from the heat unit  21  with the receptive space A therebetween so that the platen roller  10  does not push against the electrode connection unit  26 . Because the platen roller  10  therefore does not press against the electrode connection unit  26  even when the platen roller  10  is pressed against the thermal head  20 , the electrode connection unit  26  does not wear and there is no danger of the common electrode  25  being interrupted. This effect is further described below with reference to the print unit  130  in other comparison models. 
       FIG. 3  to  FIG. 6  are section views of print units  130  used for comparison. As shown in  FIG. 3 , a coating  27  made of hard glass, for example, is disposed over the heat unit  21  and electrode connection unit  26  (on the platen roller  110  side) to prevent wear by the print medium P. This coating  27  is formed to the same uniform thickness as the heat unit  21  and electrode connection unit  26 . As described above, because the electrode connection unit  26  is thicker than the heat unit  21  in order to carry more current, a bump  27   a  is formed in the surface of the thermal head  20  at the electrode connection unit  26 . 
     A reactive force (pressure) is therefore applied from the platen roller  110  to the thermal head  20  at the contact surface  111  of the platen roller  110  in response to the urging force applied by the urging member  6  to the thermal head  20 . Because the platen roller  110  is made of rubber or other elastic material, the contact surface  111  thereof deforms when this contact pressure is applied as shown in  FIG. 3 . Because the contact surface  111  is compressed by the bump  27   a  in the coating  27  when the contact surface  111  is pressed against the coating  27  formed on the heat unit  21  and electrode connection unit  26 , stress is concentrated on the bump  27   a  as shown in  FIG. 3 . As a result, only the bump  27   a  in the coating  27  is worn by the contact surface  111  of the platen roller  110  or the print medium P pressed to the contact surface  111 . 
     As this wear progresses and only the bump  27   a  is worn down, the top of the coating  27  becomes worn down to a flat surface as shown in  FIG. 4 . When this happens and the entire surface of the coating  27  then wears, the electrode connection unit  26 , which is thicker than the drive electrodes  24  and heat unit  21 , becomes exposed at the top of the thermal head  20  as shown in  FIG. 5 . The electrode connection unit  26  is made of Au, Ag, Cu, or other metal, and has less wear resistance than the coating  27 , which is made of hard glass such as borosilicate glass. Wear is therefore concentrated on the electrode connection unit  26  even if the same stress is applied from the print medium P through the contact surface  111  of the platen roller  110 . 
     Even if the top of the electrode connection unit  26  becomes lower than the top of the heat unit  21  as a result of continued wear of the electrode connection unit  26 , the electrode connection unit  26  continues to be worn by the print medium P pressed thereto by the contact surface  111  of the platen roller  110  because the contact surface  111  of the rubber platen roller  110  elastically deforms and protrudes to the electrode connection unit  26  side. As a result, as the electrode connection unit  26  continues to wear, the common electrode  25  is eventually broken by the electrode connection unit  26  as shown in  FIG. 6 , becomes unable to supply current to the heat unit  21 , and printing becomes impossible. 
     To prevent such concentrated wear of the electrode connection unit  26 , the print unit  30  according to this embodiment of the invention is built so that the contact surface  11  of the platen roller  10  does not push against the electrode connection unit  26 . More specifically, as shown in  FIG. 7 , the receptive space A in which the axial end  11   a  part of the contact surface  11  of the platen roller  10  is positioned is formed between the heat unit  21  and the electrode connection unit  26  in the axial direction of the heat elements  21   a . The contact surface  11  of the platen roller  10  thus pushes against the receptive space A where no electrodes are formed, pressure is not applied to the electrode connection unit  26  by the contact surface  11  of the platen roller  10 , the electrode connection unit  26  therefore does not wear, and the common electrode  25  is not broken. 
     Note that as shown in  FIG. 7  the receptive space A may be filled with borosilicate glass or other hard glass such as used in the coating  27 . By thus separating the heat elements  21   a  and electrode connection unit  26  with a hard glass insulator, shorts between the heat elements  21   a  and electrode connection unit  26  caused by moisture getting into the receptive space A can be prevented. 
     In addition, by aligning the height of the top of the hard glass filler in the receptive space A with the top of the coating  27  formed on the heat unit  21 , the contact surface  11  of the platen roller  10  can be pressed with uniform pressure against the entire surface of the heat unit  21  without concentrating stress only at the axial end  11   a  of the contact surface  11  of the platen roller  10 . The service life of the thermal head  20  can therefore be extended because the coating  27  formed on the heat unit  21  can be made to wear evenly. 
     The dummy heat element  28  may also be disposed to the receptive space A. Because the heat elements  21   a  at the axial ends of the heat element array and the heat elements  21   a  in the middle of the array can be heated in the same way and print quality can be improved by providing a dummy heat element  28 , and the function of the thermal head  20  can be maintained even if the dummy heat element  28  becomes exposed, the service life of the platen roller  10  can be increased. More specifically, while printing is disabled when the coating  27  becomes worn by the platen roller  10  and the electrode connection unit  26  or heat elements  21   a  are exposed, printing can continue even if the dummy heat element  28  becomes exposed because the dummy heat element  28  does not directly affect printing. 
     Furthermore, this embodiment of the invention describes forming a receptive space A to which the end  11   a  of the contact surface  11  of the platen roller  10  is positioned between the heat unit  21  and the electrode connection unit  26  in the axial direction of the heat elements  21   a , but the invention is not so limited. 
     For example, the platen roller  10  may be formed so that, of the axial end of the heat elements  21   a  and the plural ends of the electrode connection unit  26  that are located on an extension of the axis of the heat elements  21   a , the end  11   a  of the platen roller  10  contact surface  11  that is pressed to the thermal head  20  is positioned between the axial end of the heat elements  21   a  and the end  26   b  of the electrode connection unit  26  that is farthest from the heat elements  21   a . This is because the print medium P is not pressed against all of the electrode connection unit  26  because the contact surface  11  of the platen roller  10  does not extend to the axial end  26   b  of the electrode connection unit  26 , and the electrode connection unit  26  will not become completely interrupted. A thermal printer  1  that is protected against such electrode interruptions for a long time can therefore be provided. 
     The invention is described with reference to a preferred embodiment thereof above, but the technical scope of the invention is not limited to the scope of this embodiment. Various modifications and improvements that will be obvious to one skilled in the art are also possible without departing from the scope of the accompanying claims.