Patent Publication Number: US-7212222-B2

Title: Thermal head and thermal printer

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a thermal head having a heating element array with a line of a plurality of heating elements, and a thermal printer having the thermal head. 
     2. Background Arts 
     A thermal printer is known to record images on a recording paper by using a thermal head having a line of a plurality of heating elements (resistors). As the thermal printers, there are a color direct thermal printer that develops colors by directly heating a thermosensitive recording paper by means of the heating elements, and a thermal transfer printer that transfers the ink of the ink ribbon to the recording paper by putting the ink ribbon on the recording paper and heating the rear face of the ink ribbon by means of the heating elements. 
     Japanese Patent Laid-Open Publication Number H10-138543 discloses a thermal head that has the heating elements comprising first heating resistors disposed at the upstream side of a transportation path of the recording paper and second heating resistors disposed at the downstream side of the first heating resistors. The first heating resistors supply the recording paper with a minimum energy necessary to develop color, while the second heating resistors supply the recording paper with the energy to develop color at predetermined gradations. This thermal head is capable of reducing the driving electrical current to be supplied to the second heating resistors. 
     Another thermal head is capable of extending the lives of the heating elements by having the first heating resistors and the second heating resistors, which are disposed at a predetermined distance from the first heating resistors in the transporting direction of the recording paper, and using them alternately (for example, disclosed in Japanese Patent Laid-Open Publication Number H10-278329). One of the methods for recording fine lines and minute dots with the above thermal heads is to partially heat the recording paper by using the heating elements shortened in the transporting direction of the recording paper. However, the thermal head, disclosed in Japanese Patent Laid-Open Publication Number H10-138543, is not capable of recording fine lines and minute dots, regardless of that the first and the second heating resistors have different lengths. This is because this thermal head performs color developing twice, first with the first heating resistors then with the second heating resistors. The thermal head disclosed in Japanese Patent Laid-Open Publication Number H10-278329 may extend the lives of the heating elements, but it is not capable of recording fine lines and minute dots either, for having the heating elements of the same length. 
     Although the thermal head disclosed in Japanese Patent Laid-Open Publication Number H10-138543 is possibly capable of recording fine lines and minute dots by selectively heating the first heating resistors that is shorter than the second heating resistors, it needs to have electrode layers threading between the first and the second heating resistors. This results not only in complicating the constitution of the thermal head that leads to increase the manufacturing cost, but also in inhibiting the downsizing of the thermal head because the adjacent heating elements need to be spaced to each other. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a thermal head and a thermal printer with the simple constitution that are capable of recording fine lines and minute dots as well as regular images. 
     In order to achieve the above object and other objects, a heating element in the thermal head of the present invention comprises a first electrode layer formed on an insulation substrate, a second electrode layer formed in the position opposite to the first electrode layer, a first insulation layer formed on the first electrode layer to expose both ends of the first electrode layer in the transporting direction of a recording material, a third electrode layer formed on the first insulation layer to expose both ends of the first insulation layer in the transporting direction, a heating resistor layer formed across the first to the third electrode layers and the first insulation layer, and at least one protective layer covering all of those layers. 
     In addition, the thermal printer of the present invention is provided with the thermal head having the first, the second, the third and a fourth electrode layers, and a controller for selectively supplying the electricity to among the first to the third electrode layers or among the first to fourth electrode layers based on the image data to record. 
     According to the present invention, it is possible to record fine lines and minute dots by the thermal head with the simple constitution. Thereby, it provides a print of higher quality and resolution. Furthermore, it is not necessary to extend the thermal head in the width direction of the recording paper, since it is possible to arrange the heating elements without widely spacing because the second electrode layer forms a common electrode, whereas the first and the third electrode layer form discrete electrodes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       One with ordinary skill in the art would easily understand the above-described objects and advantages of the present invention when the following detailed description is read with reference to the drawings attached hereto. 
         FIG. 1  is a schematic view illustrating the constitution of a color thermal printer of the present invention; 
         FIG. 2  is a cross sectional view illustrating the schematic constitution of a heating element; 
         FIG. 3  is a plane view illustrating the schematic constitution of the heating element; 
         FIG. 4  is a cross sectional view illustrating the schematic constitution of the heating element in another embodiment of the present invention; and 
         FIG. 5  is a plane view illustrating the schematic constitution of the heating element shown in  FIG. 4 . 
     
    
    
     PREFERRED EMBODIMENTS OF THE INVENTION 
     As shown in  FIG. 1 , a color direct thermal printer  2  of the present invention uses a continuous color thermosensitive recording paper (hereinafter abbreviated as recording paper)  10  as a recording material. The recording paper  10  is rolled into a recording paper roll  11  and set in the color direct thermal printer  2 . 
     The recording paper  10  comprises a support, a cyan thermosensitive coloring layer, a magenta thermosensitive coloring layer, a yellow thermosensitive coloring layer, and a protective layer, stacking in the stated order. The yellow thermosensitive coloring layer formed on top of the other two coloring layers is the most thermosensitive among the three thermosensitive coloring layers and develops yellow color with low thermal energy. The cyan thermosensitive coloring layer formed under the other two coloring layers is the least thermosensitive among the three thermosensitive coloring layers and needs high thermal energy to develop cyan color. 
     The yellow thermosensitive coloring layer loses its ability to develop yellow color when exposed to near ultraviolet rays of 420 nm. The magenta thermosensitive coloring layer develops magenta color by thermal energy with a level between the one to develop yellow color in the yellow thermosensitive coloring layer and the other to develop cyan color in the cyan thermosensitive coloring layer, and loses its ability to develop magenta color when exposed to ultraviolet rays of 365 nm. Note that it is also possible to use a recording paper comprising four thermosensitive coloring layers formed of the above three color thermosensitive coloring layers and a black thermosensitive coloring layer. 
     A feed roller  13 , rotated by a transport motor  12 , contacts with the periphery of the recording paper roll  11 . The transport motor  12  is a stepping motor driven by a driving pulse from a motor driver  14 . When the feed roller  13  rotates counterclockwise, the recording paper roll  11  is rotated clockwise by the feed roller  13  and the recording paper  10  is drawn from the recording paper roll  11 . When the feed roller  13  rotates clockwise, the recording paper roll  11  is rotated counterclockwise by the feed roller  13  and the recording paper  10  is rewound to the recording paper roll  11 . 
     The recording paper  11  drawn out from the recording paper roll  11  is fed into a horizontal transportation path. In the transportation path, a transport roller pair  15  and a discharge roller pair  16  are disposed to nip and transport the recording paper  10 . The transport roller pair  15  comprises a capstan roller  15   a  and a pinch roller  15   b  pressed against the capstan roller  15   a , and the discharge roller pair  16  comprises a capstan roller  16   a  and a pinch roller  16   b  pressed against the capstan roller  16   a . Each transport roller pair  15  and discharge roller pair  16  is rotated by the transport motor  12  to reciprocate the recording paper  10  both in the transporting direction A and in the rewinding direction B. A discharge opening  17  is disposed at the downstream side of the discharge roller pair  16  to discharge the recorded recording paper  10 . 
     A thermal head  18  and a platen roller  19 , which is disposed opposite to the thermal head  18  across the transportation path, are disposed between the feed roller  13  and the transport roller pair  15 . The surface of a head substrate  30  of the thermal head  18  facing to the recording paper  10  is provided with a heating element array  32  with a line of a plurality of heating elements  31  (see  FIG. 2  and  FIG. 3 ). 
     The platen roller  19  rotates in response to the transportation of the recording paper  10  and firmly puts the recording paper  10  on the heating element array  32 . The platen roller  19  is movable upward and downward and biased by a spring (not shown) in the direction to the heating element array  32 . The platen roller  19  is operated to descend by a shift mechanism (not shown) comprising a cam and a solenoid, to stop nipping the recording paper  10  with the thermal head  18 . 
     An optical fixer  22  is disposed facing the recording area of the recording sheet  10  at the downstream side of the transport roller pair  15 . A cutter  23  is provided between the optical fixer  22  and the discharge roller pair  16  to cut the recording paper into a sheet of a predetermined print size. The optical fixer  22  comprises a yellow-fixing light source  22   a  for fixing the yellow thermosensitive coloring layer by radiating near ultraviolet rays having an emission peak at 420 nm, and a magenta-fixing light source  22   b  for fixing the magenta thermosensitive coloring layer by radiating ultraviolet rays having the emission peak at 365 nm. Both light sources  22   a  and  22   b  are driven by a lamp driver  24 . 
     The following is an explanation of an operation of the color direct thermal printer  2  having the above constitution. When an operation to start image recording is performed, the feed roller  13  is rotated counterclockwise by the rotation of the transport motor  12  and draws out the recording paper  10  from the recording paper roll  11  to feed it in the A direction. The front end of the recording paper  10  is nipped by the transport roller pair  15  after transported through the transportation path, and transported further to downstream side in the A direction. 
     When the recording paper  10  reaches a start position for image recording, the transport motor  12  is stopped rotating temporarily. Subsequently, the platen roller  19  is shifted upward by the shift mechanism to nip the recording paper  10  with the heating element array  32 . In this state, the transport motor  12  is driven again to transport the recording paper  10  in the A direction, and a yellow image is recorded on the yellow thermosensitive coloring layer of the recording paper  10  by the heating element array  32 , which is heated based on the driving data input to the head driver  21 . 
     After the record of the yellow image, the recording paper  10  is transported until the rear end of the recorded image faces the yellow-fixing light source  22   a  of the optical fixer  22 , and then the transport motor  12  is stopped rotating. At this point the platen roller  19  is shifted downward by the shift mechanism to release the nip of the recording paper  10  with the thermal head  18 . Subsequently, the yellow-fixing light source  22   a  is lit up by the lamp driver  24  and optically fixes the recorded yellow thermosensitive coloring layer while the transport motor  12  counterrotates rewinding the recording paper  10  in the B direction. 
     After the optical fixation of the yellow thermosensitive coloring layer, the recording paper  10  is transported until the front end of the recorded image faces the heating element array  32 , and then the transport motor  12  is stopped rotating. At this point the platen roller  19  is shifted upward by the shift mechanism in the same way as the record of the yellow image to nip the recording paper  10  with the heating element array  32 . In this state, the transport motor  12  is driven again to transport the recording paper  10  in the A direction, and a magenta image is recorded on the magenta thermosensitive coloring layer of the recording paper  10 . 
     After the record of the magenta image, the recording paper  10  is transported until the rear end of the recorded image faces the magenta-fixing light source  22   b  of the optical fixer  22 , and then the transport motor  12  is stopped rotating. Subsequently, the magenta-fixing light source  22   b  is lit up by the lamp driver  24  in the same way as the fixation of the yellow image and optically fixes the recorded magenta thermosensitive coloring layer while the transport motor  12  counterrotates rewinding the recording paper  10  in the B direction. 
     After the optical fixation of the magenta thermosensitive coloring layer, the recording paper  10  is transported until the front end of the recorded image faces the heating element array  32 , and then the transport motor  12  is stopped rotating. Subsequently, a cyan image is recorded on the cyan thermosensitive coloring layer of the recording paper  10  in the same way as the record of the yellow and the magenta images. 
     Then, the recording paper  10  is transported in the A direction by the transport roller pair  15 , and discharged by the discharge roller pair  16  out of the color direct thermal printer  2  via the paper discharge opening  17  after cut into a sheet of the predetermined print size by the cutter 
     As shown in  FIG. 2  and  FIG. 3 , a glaze layer  33  formed of glazed glass, the heating element  31  comprising a first to a third electrode layers  34  to  36 , a first insulation layer  37 , a heating resistor layer  38 , and a protective layer  39  are stacked upon the head substrate  30 . A partial glaze  40  is formed to cylindrically protrude in a part of the glaze layer  33 . The partial glaze  40  extends in the width direction of the recording paper  10 . To avoid complication, the protective layer  39  is not shown in  FIG. 3 . 
     The first electrode layer  34  and the second electrode layer  35  are disposed opposite to each other along the transporting direction of the recording paper  10 . The first insulation layer  37  is formed on the first electrode layer  34  to expose both ends of the first electrode layer  34  in the transporting direction. The third electrode layer  36  is formed on the first insulation layer  37  to expose both ends of the first insulation layer  37  in the transporting direction. The heating resistor layer  38  is formed across the first to the third electrode layers  34  to  36  and the first insulation layer  37 . The protective layer  39  is formed to cover all of those layers except one end of each of the first electrode layer  34 , the third electrode layer  36 , and the first insulation layer  37 . Note that the protective layer  39  can be multilayered. 
     Each of the first, the second and the third electrode layers  34 ,  35  and  36  is connected to a print board (not shown) having a driver IC mounted thereon for controlling the current applied to the heating element  31 . The second electrode layer  35  forms a common electrode, while the first and the third electrode layers  34  and  36  respectively form discrete electrodes. A lead line (not shown) is connected to the ends of the first and the third electrode layers  34  and  36 , which are not covered with the protective layer  39 . Each of the heating elements  31  is selectively driven by the driver IC. 
     The heating element array  32  is heated based on the driving data input to the head driver  21  from the system controller  20  to develop colors on each thermosensitive layer of the recording paper  10 . Based on the image data to record, the system controller  20  selectively applies the current to among the first, the second, and the third electrode layers  34 ,  35  and  36 . In particular, the system controller  20  selects the third electrode layer  36  to heat the heating resistor layer  38  between the third electrode layer  36  and the second electrode layer  35  in order to record a regular image, while it selects the first electrode layer  34  to heat the heating resistor layer  38  between the first electrode layer  34  and the second electrode layer  35  in order to record fine lines and minute dots. Thereby, the length of the heating element  31  in the transporting direction of the recording paper  10  becomes shorter than that in the regular image recording. That is, the heating element  31  is capable of recording fine lines and minute dots on the recording paper  10 , since a pixel is shortened in a sub-scanning direction in comparison with that to record the regular image. 
     Note that it is also possible to form a second insulation layer  52  on the second electrode layer  35  to expose one end of the second electrode layer  35  in the transporting direction of the recording paper  10  and to form simultaneously a fourth electrode layer  53  on the second insulation layer  52  to expose one end of the second insulation layer  52  in the transporting direction. In this case, the second and the fourth electrode layers  35  and  53  form common electrodes, and the heating resistor layer  38  is heated when the current is supplied to the combination of the first electrode layer  34  and the fourth electrode layer  53  or the combination of the third electrode layer  36  and the fourth electrode layer  53 , in addition to the combination of the first electrode layer  34  and the second electrode layer  35  or the combination of the second electrode layer  35  and the third electrode layer  36 . Thereby the length of the heating element  50  is more gradually changed in the transporting direction of the recording paper  10 , which enables to record the image of higher resolution based on the image data to record. 
     Note that the heating element with a plane glaze is also applicable to the present invention instead of the heating elements  31  and  50  with the partial glaze  40  in the above embodiment. In addition, a one-path system color direct thermal printer, in which yellow, magenta and cyan images are thermally recorded on the recording paper while the recording paper is once passing three thermal heads for each color, is also applicable to the present invention instead of the color direct thermal printer  2  in the above embodiment that is a reciprocating transport print type to thermally record and optically fix yellow, magenta and cyan images by reciprocating the recording paper  10  under a single thermal head  18  and the optical fixer  22  alternately. 
     Furthermore, the present invention is applicable not only to the color direct thermal printer  2  in the above embodiment by way of example but also to other thermal printers such as a thermal transfer printer and the like. 
     Although the present invention has been described with respect to the preferred embodiments, the present invention is not to be limited to the above embodiments but, on the contrary, various modifications will be possible to those skilled in the art without departing from the scope of claims appended hereto.