The present invention relates to a thermal head.
As illustrated in
In addition, an electrically insulating protection film 4 (hereinafter referred to as an “insulating protection film 4”) made of a PbO—SiO2—ZrO2 based glass material, for example, coveres substantially over the entire surface of the insulating substrate 1, including the partial glaze layer 2, the heating resistor 3 and the electrodes. A sheet of thermal paper 8 serving as a printing medium is conveyed while it is pressed by a platen roller 7 against the insulating protection film 4 so that a color is developed with heat generated by the heating resistor 3 and then transferred through the insulating protection film 4.
Moreover, as illustrated in
Patent Document 1: Japanese Patent No. 3603997
Patent Document 2: Japanese Patent Unexamined Publication No. 2001-232838
However, the above-described thermal head has the problem that, depending on the type of thermal paper as the printing medium, the insulating protection film 4 is significantly abraded due to friction with the thermal paper, including a pigment, etc. contained therein, and that the mechanical strength and the electrical insulation performance of the insulating protection film 4 are degraded.
Further, some thermal paper, e.g., label paper, has a relatively large thickness. In view of that type of thermal paper, there is a tendency to set a pressing pressure of the platen roller 7 to a higher lever for the purpose of improving a following capability with respect to the thermal head.
In that case, the abrasion of the insulating protection film 4 is accelerated with the higher pressing pressure of the platen roller 7. Meanwhile, it is also experimentally proved that abrasion resistance of the insulating protection film 4 against the above-mentioned friction with the thermal paper is greatly affected by a coverage rate at which printing is performed by the thermal head on the thermal paper.
Thus, an amount of abrasion tends to become larger at a higher coverage rate than at a lower coverage rate. Such a tendency is attributable to the influence, described below, that is imposed on the thermal head at the higher coverage rate rather than the lower coverage rate. More specifically, when the heating resistor generates heat, there occurs a temperature distribution having a peak in a central portion. At the higher coverage rate, the heat generated in the central portion is more apt to accumulate as a result of a synergistic effect with heat generated in adjacent portions of the heating resistor, particularly when the printing operation is continuously repeated. Accordingly, temperature reaches neat the transition point of the insulating protection film 4, whereby the insulating protection film 4 can no longer maintain the inherent hardness and becomes more susceptibly reactive to mechanical stresses, e.g., friction.
Under such a condition, the abrasion resistance of the insulating protection film 4 is easily degraded because the thermal paper 8 is conveyed over the insulating protection film 4 while the thermal paper 8 is pressed by the platen roller 7.
Particularly, in an environment when the thermal head is used at a printing speed of 200 mm/s or higher, the heat generated by the heating resistor is more apt to accumulate, thus raising a problem how to maintain the abrasion resistance.
To cope with that problem, a thermal head is proposed in which an electrically conductive protection film having good thermal conductivity is disposed as an additional protection layer on the insulating protection film (see Patent Document 1).
However, even when the insulating protection film 4 is made not so susceptibly reactive to mechanical stresses, e.g., friction, by averaging the heating temperature inside the heating resistor, there still remains a possibility that the abrasion resistance of the insulating protection film may be degraded depending on the thermal paper used.
With the view of solving the problems described above, the present invention provides a thermal head in which an insulating protection film can maintain high abrasion resistance while maintaining satisfactory efficiency of color development in printing performance.
A level difference is provided between an insulating protection film covering a heating resistor, which is disposed on a partial glaze layer, and a flat portion of the insulating protection film over an area outside the partial glaze layer. The level difference is set such that a platen roller can press thermal paper against the flat portion of the insulating protection film as well at the side close to the partial glaze layer.
A pressing pressure of the platen roller is distributed without being concentrated onto the insulating protection film positioned on the heating resistor, whereby mechanical abrasion of the insulating protection film can be suppressed.
As illustrated in
Further, as illustrated in
The heating resistor 3 is disposed on the partial glaze layer 2 in an overlapped relation to the comb-shaped electrode 6a and the individual electrodes 5 (
There is a level difference t between an insulating protection film 4a positioned on the surface of the heating resistor 3, which is disposed on the partial glaze layer 2, and a flat portion 4b of the insulating protection film in an area outside the partial glaze layer 2. A size of the level difference t is set such that the platen roller 7 presses the thermal paper 8 against surfaces 10a and 10b of the flat portion 4b of the insulating protection film as well, the surfaces 10a and 10b being each positioned at the side close to the partial glaze layer 2.
In this respect, the flat portion 4b of the insulating protection film 4 is formed as follows. After forming the insulating protection film 4 so as to cover the heating resistor 3 and the power feed electrodes as illustrated in
As one example, when thermal paper having a thickness of 65 μm is used as a printing medium as described later, the height of the flat portion 4b is set to a level lowered by 15 μm (i.e., t=15 μm) such that the platen roller 7 presses the thermal paper against the surfaces 10a and 10b (
With the presence of the level difference t, the platen roller 7 can press the thermal paper against not only the insulating protection film 4a on the heating resistor 3, but also the surfaces 10a and 10b of the flat portion 4b of the insulating protection film 4. Therefore, a pressing force of the platen roller is distributed to the surfaces 10a and 10b without being concentrated onto the insulating protection film 4a on the heating resistor 3, whereby mechanical abrasion of the insulating protection film 4 can be suppressed.
Further, the surface 10a of the insulating protection film, which is positioned on the entry side of the thermal paper 8 with respect to the partial glaze layer 2, causes an action to improve smoothness of the thermal paper 8 at the same time. Such an action contributes to further suppressing the mechanical abrasion of the insulating protection film 4a on the heating resistor 3.
An example of a method of forming the insulating protection film 4 providing the above-mentioned level difference t will be described below.
After forming the insulating protection film 4 to extend in a state riding across the partial glaze layer 2 as illustrated in
Further, other forming methods may be practiced, as illustrated in
In still another embodiment, as illustrated in
In still another embodiment, as illustrated in
In one example, the electrically conductive protection film 11 is made of a material having thermal conductivity of 9.628 W/mK, and the insulating protection film 4 as the lower layer is made of a material having thermal conductivity of 1.616 W/mK. Because the electrically conductive protection film 11 has high thermal conductivity and provides a structure having superior heat transference, it can momentarily transfer heat to the thermal paper with a good thermal response and can contribute to averaging a heat distribution generated by the heating resistor 3. Therefore, stresses generated in the electrically conductive protection film 11 are suppressed and a mechanical abrasion resistance characteristic is improved as a result of the synergistic effect with the provision of the level difference.
Here, Table 1 indicates a nip width (i.e., a size of the width over which the platen roller contacts the thermal head with the thermal paper interposed between them) when the thermal paper having the thickness of 65 μm is actually pressed by the platen roller under 19.6 N/the thermal head (under 0.27 N/mm (size of the printing width)) in the embodiment of the present invention illustrated in
<Conditions> Thickness of the thermal paper: 65 μm,
Pressing pressure of the platen roller: 19.6 N/printing width
When the flat portion 4b of the insulating protection film is formed to provide the level difference of 15 μm in the embodiment illustrated in
As a result, the pressing pressure of the platen roller per unit length is reduced to (1/4.22) to the contrary, and mechanical stresses caused by the pressing pressure of the platen roller are reduced correspondingly. Therefore, abrasion of the insulating protection film 4a formed on the partial glaze layer 2 is suppressed.
Numerical values indicated in Table 1 represent the results obtained by coating ink over the insulating protection film of the thermal head, causing the platen roller to press the insulating protection film, and then measuring an area where the coated ink has been removed (in terms of size in the widthwise direction).
1 . . . ceramic substrate, 2 . . . partial glaze layer, 3 . . . heating resistor, 4 . . . insulating protection film, t . . . level difference
Number | Date | Country | Kind |
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2010-017643 | Jan 2010 | JP | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/JP2010/060274 | 6/17/2010 | WO | 00 | 6/26/2012 |