Patent Publication Number: US-7903132-B2

Title: Thermal printhead

Description:
TECHNICAL FIELD 
     The present invention relates to a thermal printhead used for printing on e.g. thermal paper. 
     BACKGROUND ART 
       FIG. 7  shows an example of conventional thermal printhead (see Patent Document 1 given below). The illustrated thermal printhead X includes a substrate  91  and a heating resistor element  93  extending on the substrate in the primary scanning direction. The heating resistor element  93  is covered by a protective film  94 . The heading resistor element  93  is connected to an electrode  92  and another electrode (not shown) whose polarity is opposite to that of the electrode  92 . When current is applied to the heating resistor element  93  via these electrodes, heat is generated. The heat is transferred to thermal paper through the protective film  94 , whereby an image or letter is formed on the thermal paper. 
     Patent Document 1: JP-A-7-186429 
     Generally, to enable clear printing, the surface of thermal paper is made smooth. Examples of such surfacing techniques include the application of coating agent to thermal paper. Conventionally, however, the thermal paper having a smooth surface tends to stick to the protective film  94  when pressed against the thermal printhead X. When such a phenomenon (called “sticking”) occurs, the thermal paper cannot be smoothly slid relative to the thermal printhead X, which may result in deterioration in printing quality. 
     Moreover, the above-described coating agent is generally hydrophilic and tends to absorb moisture in the air. Thus, when the thermal paper is pressed against the protective film  94 , the moisture which has been absorbed in the coating agent may seep out between the thermal paper and the protective film  94 . Conventionally, such moisture also causes the sticking of the thermal paper to the protective film  94 . 
     DISCLOSURE OF THE INVENTION 
     The present invention has been proposed under the circumstances described above. It is, therefore, an object of the present invention is to provide a thermal printhead which is capable of preventing sticking. 
     According to a first aspect of the present invention, there is provided a thermal printhead comprising a substrate and a heating resistor element formed on the substrate and elongated in the primary scanning direction. The thermal printhead further includes an electrode for applying current to the heating resistor element, and a protective film covering the heating resistor element and the electrode and including a contact surface for coming into contact with a recording medium. The contact surface of the protective film is made irregular to reduce contact area with the recording medium. 
     Preferably, the protective film includes a first layer directly covering the heating resistor element and the electrode, a second layer formed on the first layer, and a third layer formed on the second layer to come into contact with the recording medium. For instance, in this case, the first layer is made of glass, the second layer is made of porous glass including a plurality of pores, and the third layer is made of a water repellent material. The third layer partially enters each of the pores of the second layer. 
     Preferably, the third layer is made of polyimide resin. 
     In a thermal printhead according to a second aspect of the present invention, the protective film includes a first layer directly covering the heating resistor element and the electrode and a second layer formed on the first layer. The second layer comprises a plurality of projecting elements spaced from each other. 
     Preferably, each of the projecting elements has a rectangular cross section, and a diagonal of the rectangular cross section is parallel to the secondary scanning direction which is perpendicular to the primary scanning direction. 
     Preferably, in the thermal printhead according to the second aspect of the present invention, the protective film includes a third layer covering the second layer and having water repellency. For instance, in this case, the second layer is made of either of SiC and a composite material of C and SiC, whereas the third layer is made of polytetrafluoroethylene. 
     Other features and advantages of the present invention will become more apparent from the detailed description given below with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view showing a principal portion of a thermal printhead according to a first embodiment of the present invention. 
         FIG. 2  is a sectional view taken along lines II-II in  FIG. 1 . 
         FIG. 3  is a sectional view showing the structure of a protective film of the thermal printhead of the first embodiment. 
         FIG. 4  is a perspective view showing a principal portion of a thermal printhead according to a second embodiment of the present invention. 
         FIG. 5  is a sectional view taken along lines V-V in  FIG. 4 . 
         FIG. 6  is a plan view showing projecting elements of the thermal printhead according to the second embodiment. 
         FIG. 7  is a sectional view showing a principal portion of a conventional thermal printhead. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Preferred embodiments of the present invention will be described below with reference to the accompanying drawings. 
       FIGS. 1-3  show a thermal printhead according to a first embodiment of the present invention. The illustrated thermal printhead A 1  includes an insulating substrate  1 , electrodes  2 A and  2 B, a heating resistor element  3  and a protective film  4 . The heating resistor element  3  is elongated in the primary scanning direction (x direction in  FIG. 1 ). In printing, recording paper such as thermal paper is transferred in the secondary scanning direction (y direction in  FIG. 1 ) relative to the thermal printhead A 1 . 
     The substrate  1  is made of e.g. a ceramic material. A glaze layer (not shown) is formed on the substrate  1  to provide a smooth surface. The glaze layer also functions to prevent heat from escaping from the heating resistor element  3  to the substrate  1 . 
     The electrodes  2 A and  2 B are made of a metal such as Au and have different electrical polarities. The electrode  2 A includes a plurality of comb-teeth-shaped extensions  21  extending in the secondary scanning direction y, and the electrode  2 B also has similar extensions  22 . The extensions  21  and  22  are alternately arranged in the primary scanning direction x. The electrodes  2 A and  2 B are connected to a non-illustrated drive IC. The electrodes  2 A and  2 B may be formed by printing Au resinate paste into a predetermined shape and then baking the paste. 
     The heating resistor element  3  is made of e.g. ruthenium oxide. The heating resistor element  3  extends in the primary scanning direction to cross the extensions  21  and  22 . In this arrangement, the heating resistor element  3  includes a plurality of portions (unit heating portions) each sandwiched between adjacent extensions  21  and  22 . When current is applied to a selected one of the unit heating portions by the drive IC, the unit heating portion generates heat. Due to the heat, a region of the thermal paper corresponding to one dot is colored, whereby printing is performed. The heating resistor element  3  may be formed by printing paste containing ruthenium oxide into a predetermined shape and then baking the paste. 
     The protective film  4  protects the electrodes  2 A,  2 B and the heating resistor element  3 . As shown in  FIG. 2 , the protective film  4  has a laminated structure made up of a first layer  41 , a second layer  42  and a third layer  43 . The first layer  41  is a dense layer directly covering the electrodes  2 A,  2 B and the heating resistor element  3  and made of e.g. glass. The first layer  41  has a thickness of e.g. about 4 μm. The first layer  41  is formed by printing glass paste containing SiO 2 , B 2 O 3  and PbO to cover the electrodes  2 A,  2 B and the heating resistor element  3  and then baking the paste. The softening point of the glass paste is e.g. about 680° C. 
     The second layer  42  is made of e.g. glass and laminated on the first layer  41 . As shown in  FIG. 3 , the second layer  42  has a porous structure including a plurality of pores  42   a . The thickness of the second layer  42  is e.g. about 4 to 6 μm. The diameter of the pores  42   a  is e.g. about several tens of μm. The second layer  42  may be formed as follows. First, conductive paste is uniformly printed on the first layer  41 . As the conductive paste, use is made of a mixture of glass paste (base paste) containing SiO 2 , ZnO, CaO as the main components and resistor paste. The resistor paste is prepared by adding 0.3 to 30 wt % of ruthenium oxide particles having a particle size of about 0.001 to 1 μm to glass made of e.g. PbO, SiO 2 , B 2 O 3 . The softening points of the base paste and the resistor paste are 785° C. and 865° C., respectively. To form the second layer  42 , the conductive paste is then baked at a temperature of e.g. 760° C. This baking temperature is lower than both of the softening temperature of the base paste and that of the resistor paste. Thus, the conductive paste does not flow considerably during the baking. In the baking process, bubbles are formed around the ruthenium oxide contained in the conductive paste. These bubbles finally form the pores  42   a , whereby the porous second layer  42  is obtained. 
     As shown in  FIG. 3 , the third layer  43  covers the second layer  42  and portions of the first layer  41  which are not covered by the second layer  42 . The third layer  43  is made of e.g. polyimide resin and has water repellency. The third layer  43  has a thickness of about 1 to 10 μm. Each pore  42   a  of the second layer  42  is filled with the third layer  43  at least partially. Due to the existence of the pores  42   a , the upper surface of the third layer  43  (and hence the protective film  4 ) is not a smooth surface but an irregular surface including recesses  4   a  at locations corresponding to the pores  42   a . The third layer  43  may be formed by printing or transferring a water-repellent resin onto the second layer  42 . 
     The advantages of the thermal printhead A 1  will be described below. 
     According to the embodiment described above, since the surface (which is to come into contact with paper) of the protective film  4  is formed with recesses  4   a , the contact area between the protective film  4  and the thermal paper is small. As a result, the conventional problems of sticking and deterioration in printing quality are prevented. Further, by preventing the sticking, the feed speed of thermal paper (and hence the printing speed) can be increased. 
     Moreover, even when moisture which has been absorbed in the coating agent of the thermal paper seeps out, the moisture is retained in the recesses  4   a . This prevents the protective film  4  and the thermal paper from strongly sticking to each other due to moisture. Particularly, the use of polyimide resin, which has water repellency, as the material of the third layer  43  is advantageous for preventing moisture from being retained at the contact portion between the protective film  4  and the thermal paper. Alternatively, as the material of the third layer  43 , a material which has an appropriate level of water repellency and provides a smooth surface may be used instead of polyimide resin. 
       FIGS. 4-6  show a thermal printhead according to a second embodiment of the present invention. In these figures, the elements which are identical or similar to those of the first embodiment are designated by the same reference signs as those used for  FIGS. 1-3 . 
     As shown in  FIGS. 4 and 5 , the thermal printhead A 2  according to the second embodiment includes an insulating substrate  1 , electrodes  2 A and  2 B, a heating resistor element  3  and a protective film  4 . The substrate  1  is made of e.g. a ceramic material. A non-illustrated glaze layer is formed on the substrate  1 . The electrodes  2 A and  2 B are made of e.g. Au and include a plurality of extensions  21  and  22  extending in the secondary scanning direction y. The extensions  21  and  22  are alternately arranged in the primary scanning direction x. The heating resistor element  3  is made of e.g. ruthenium oxide. The protective film  4  protects the electrodes  2 A,  2 B and the heating resistor element  3  and has a laminated structure made up of a first layer  41 , a second layer  44  and a third layer  45 . The first layer  41  is a dense layer directly covering the electrodes  2 A,  2 B and the heating resistor element  3  and made of e.g. glass. The first layer  41  has a thickness of e.g. about 4 μm. The second layer  44  is made of SiC or a composite material (C—SiC) of C and SiC. 
     As shown in  FIG. 4 , the second layer  44  includes a plurality of projecting elements  44   a . The projecting elements  44   a  are arranged to be spaced from each other in a plane including the primary scanning direction x and the secondary scanning direction y. Each of the projecting elements  44   a  is rectangular in horizontal cross section. As shown in  FIG. 6 , each projecting element  44   a  has a diagonal  44   d  which is parallel to the secondary scanning direction y. Each of the projecting elements  44   a  has a height of e.g. 4 to 6 μm. For instance, the second layer  44  may be made by forming a uniform film of the above-described material (SiC or C—SiC) by sputtering and then subjecting the film to patterning by etching. In another method, the portions of the first layer  41  on which the projecting elements  44   a  are not to be formed are covered by patterning a photosensitive resist. Then, a film of the above-described material is formed by sputtering to cover the photosensitive resist and the first layer  41 . By subsequently removing the photosensitive resist, the second layer  44  including the projecting elements  44   a  is obtained. 
     As shown in  FIG. 5 , the third layer  45  covers the second layer  44  (i.e., the projecting elements  44   a ) and the upper surface of the first layer  41  (the portions which are not covered by the projecting elements  44   a ). The third layer  45  fills only part of the space between adjacent projecting elements  44   a  and does not fill the space completely. Thus, the surface (which is to come into contact with paper) of the protective film  4  is irregular. The third layer  45  is made of e.g. polytetrafluoroethylene (hereinafter referred to as “PTFE”) and has water repellency. The thickness of the third layer  45  is e.g. about 2 to 3 μm. The third layer  45  may be formed by e.g. printing, transferring or sputtering. 
     In the thermal printhead A 2  having the above-described structure, the contact area between the protective film  4  and the thermal paper is small, similarly to the first embodiment. This is advantageous for preventing the sticking. Although dust may be formed due to the rubbing between the protective film  4  and the thermal paper, such dust is retained in the space between adjacent projecting elements  44   a . Thus, deterioration in printing quality is prevented. 
     In the thermal printhead A 2 , the diagonal  44   d  of each projecting element  44   a  is parallel to the secondary scanning direction y, and any side of the rectangular cross section is not parallel to the primary scanning direction x. Thus, the projecting element  44   a  comes into contact (via the third layer  45 ) with the thermal paper, which is being transferred in the secondary scanning direction y, from its apex. This is suitable for achieving smooth feed of the thermal paper. 
     Moreover, since the second layer  44  is made of SiC or C—SiC, the carbon content is relatively large. The larger the carbon content of a material is, the more likely PTFE, which forms the third layer  45 , adheres to the material. Thus, the third layer  45  strongly adheres to the second layer  44 . Further, since SiC and C—SiC has a high thermal conductivity, the heat from the heating resistor element  3  is efficiently transferred to the thermal paper. It is to be noted that, in the present invention, the third layer  45  of the protective film  4  according to the second embodiment can be eliminated. In this case, the projecting elements  44   a  constituting the second layer  44  directly come into contact with the thermal paper. In this variation, the formation density of the projecting elements  44   a  (i.e., the number of projecting elements per unit area) is so set that the thermal paper is not damaged by the projecting elements  44   a  when the paper is being transferred. Further, even when any of the projecting elements  42   a  has a defect (e.g. breakage or release from the first layer  41 ), it does not have an adverse effect on other projecting elements  42   a.    
     The projecting elements  44   a  are not limited to those having a rectangular cross section. For instance, projecting elements which are polygonal or circular in cross section may be employed. The materials of the second layer  44  and the third layer  45  are not limited to those described above. For instance, the second layer  44  may be made of silane coupler, whereas the third layer  45  may be made of polyimide resin. The third layer  45  made of polyimide resin exhibits good water repellency and achieves smooth sliding relative to the thermal paper. Polyimide resin and silane coupler can be bonded strongly to each other.