Thick film thermal head

Abstract: A thick film thermal head includes a plurality of first (4a) and second (4b) electrodes which extend in a sub-scanning direction (Y) and are alternately arranged in a main scanning direction (X) intersecting the sub-scanning direction at predetermined spaces. A resistance heater (1) continuously extends in the main scanning direction across the first and second electrodes. Parts of the resistance heater between adjacent first and second electrodes form a plurality of resistance heater elements which are arranged in the main scanning direction and generate heat when energized through the first and second electrodes. The spaces (P) between the first and second electrodes are smaller than the widths (Q) of the first and second electrodes in the main scanning direction.

Description: This invention relates to a thick film thermal head according to the preamble of claim 1. Such thermal head is used for making a heat-sensitive stencil master. Description of the Related Art Thermal heads for making a heat-sensitive stencil master generally comprise an array of resistance heater elements and imagewise perforate a stencil master material by selectively energizing the heater elements and are broadly classified into thin film thermal heads and thick film thermal heads by the structure. A thermal head defined by the preamble of claim 1 is known from JP-61 068264A. Figure 9 shows an example of a conventional thick film thermal head. In Figure 9, the conventional thick film thermal head comprises a ceramic substrate 95, a heat insulating layer 92 formed on the ceramic substrate 95, a plurality of comb-tooth electrodes 94 which are formed on the heat insulating layer 92 and arranged in a row in a direction perpendicular to their longitudinal directions, and a resistance heater 91 which extends in the direction of the row of the comb-tooth electrodes 94. The resistance heater 91 is formed over the comb-tooth electrodes 94 to continuously extend across the electrodes 94. with this arrangement, when an electric voltage is applied across a pair of adjacent electrodes, the part of the resistance heater 91 between the electrodes 94 generates heat. That is, parts 91a of the resistance heater 91 between adjacent electrodes 94 form resistance heater elements. The resistance heater elements 91a are arranged in the direction of the row of the comb-tooth electrodes 94. This direction will be referred to as "the main scanning direction", hereinbelow. When making a stencil master by a thermal head, it is generally necessary to form, on a stencil master material 71, perforations 72 which are separated from each other in both the main scanning direction X and the sub-scanning direction Y (the direction substantially perpendicular to the main scanning direction) as shown in Figure 7. If the perforations 72 are continuous in the main scanning direction X as shown in Figure 8, ink is supplied to the printing paper through the perforations in an excessive amount, which results in offset and/or strike through. If each of the resistance heater elements 91a is thermally insulated from the parts of the resistance heater 91 on opposite sides of the resistance heater element 91a in Figure 9, the width W in the main scanning direction of the area which generates heat when the element 91a is energized will be substantially equal to the space P between adjacent electrodes 94. However, actually each of the resistance heater elements 91a is not thermally insulated from the parts of the resistance heater 91 on opposite sides of the resistance heater element 91a, and accordingly, heat generated from each resistance heater element 91a propagates to the part adjacent thereto, whereby, as shown in Figure 10, an area 101 wider than the resistance heater element 91a, or the space P between adjacent electrodes 94, becomes hot. When adjacent two resistance heater elements 91a are simultaneously energized, the actual heat generating areas 101 of the adjacent resistance heater elements 91a become closer to each other as shown in Figure 10, which can result in continuous perforations. Especially in the case of a thick film thermal head, since the resistance heater 91 is large in thickness, heat generated from each resistance heater element 91a spreads over a wide area while it propagates to the surface of the element 91a and the width W of the heat generating area more tends to become larger than the space P between the electrodes 94 at the surface of the resistance heater 91. Accordingly, in the case of a thick film thermal head, the aforesaid problem is more serious. SUMMARY OF THE INVENTION In view of the foregoing observations and description, the primary object of the present invention is to provide a thick film thermal head which can form perforations which are separated from each other in the main scanning direction without thermally insulating the resistance heater elements from each other. The thick film thermal head in accordance with the present invention is defined by the features of claim 1. Prefered embodiment are defined by the dependent claims. It is preferred that the spaces between the first and second electrodes are smaller than the width of the resistance heater in the sub-scanning direction. The first and second electrodes are preferably formed of metal having a thermal conductivity not lower than 100W/mK at 100°C. For example, the electrodes may be formed of silver (422W/mK in thermal conductivity and 2.08ρ/Ωm in resistivity at 100°C), copper (395W/mK in thermal conductivity and 2.23 ρ/Ωm in resistivity at 100°C), gold (313W/mK in thermal conductivity and 2.88ρ/Ωm in resistivity at 100°C), aluminum (240W/mK in thermal conductivity and 7.8ρ/Ωm in resistivity at 100°C), beryllium (168W/mK in thermal conductivity and 5.3 ρ/Ωm in resistivity at 100°C), tungsten (163W/mK in thermal conductivity and 7.3ρ/Ωm in resistivity at 100°C), magnesium (154W/mK in thermal conductivity and 5.3ρ/Ωm in resistivity at 100°C), iridium (145W/mK in thermal conductivity and 6.8 ρ/Ωm in resistivity at 100°C), molybdenum (135W/mK in thermal conductivity and 7.6ρ/Ωm in resistivity at 100°C), brass (128W/mK in thermal conductivity and 6.3ρ/Ωm in resistivity at 100°C), or zinc (112W/mK in thermal conductivity and 7.8 ρ/Ωm in resistivity at 100°C). It is preferred that a radiator for dissipating heat of the electrodes is provided in a close contact with the electrodes. In the thick film thermal head of the present invention, since the spaces between the first and second electrodes are smaller than the widths of the first and second electrodes in the main scanning direction, that is, since the widths of the resistance heater elements are smaller than the widths of the electrodes, which have a heat dissipation effect, the heat generated by the resistance heater elements is effectively dissipated through the electrodes. Accordingly, the widths in the main scanning direction of the heat generating areas which become hot when the resistance heater elements are energized can be approximated to the spaces between the electrodes, whereby perforations formed in the stencil master material adjacent to each other can be surely separated from each other and offset and/or strike through can be effectively prevented. When the resistance heater elements are larger in width in the main scanning direction (the spaces between the first and second electrodes) than width in the sub-scanning direction (the width of the resistance heater in the sub-scanning), heat are more apt to spread in the main scanning direction. Accordingly, when the spaces between the first and second electrodes are smaller than the width of the resistance heater in the sub-scanning direction, adjacent perforations can be more surely separated from each other. Further, when the picture element density in the sub-scanning direction is not lower than that in the main scanning direction, perforations can be prevented from being continuous in the sub-scanning direction by making the width of the resistance heater in the sub-scanning direction smaller than the perforation pitches, i.e. the pitches of the resistance heater elements, in the main scanning direction. Further when the electrodes are formed of metal having a thermal conductivity not lower than 100W/mK at 100°C, the electrodes can contribute to dissipation of heat and prevention of excessive heat accumulation in the resistance heater. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a fragmentary perspective view showing a part of a thick film thermal head in accordance with a first embodiment of the present invention, Figure 2 is a cross-sectional view taken along line A-A in Figure 1, Figure 3 is a schematic view showing a heat generating area for each resistance heater elements in the thermal head of the first embodiment, Figure 4 is a plan view of the thermal head shown in Figure 1, Figure 5 is a plan view showing a thermal head in accordance with a second embodiment of the present invention, Figure 6 is a cross-sectional view taken along line B-B in Figure 5, Figure 7 is a view showing a stencil master in which perforations are regularly separated from each other, Figure 8 is a view showing a stencil master in which perforations are continuous in the main scanning direction, Figure 9 is a cross-sectional view taken in the main scanning direction of a conventional thick film thermal head, and Figure 10 is a schematic view showing a heat generating area for each resistance heater elements in the conventional thermal head shown in Figure 9. DESCRIPTION OF THE PREFERRED EMBODIMENTS In Figures 1 to 4, a thick film thermal head 10 in accordance with a first embodiment of the present invention comprises a ceramic substrate 5, a heat insulating layer 2 formed on the ceramic substrate 5, a plurality of first and second electrodes 4a and 4b which are alternately formed on the heat insulating layer 2 and arranged in a row in a main scanning direction X (a direction perpendicular to their longitudinal directions), and a resistance heater 1 which extends in the main scanning direction X. The resistance heater 1 is semicylindrical in cross-section and is formed over the first and second electrodes 4a and 4b to continuously extend across the electrodes 4a and 4b in contact therewith. With this arrangement, when an electric voltage is applied across a pair of first and second electrodes 4a and 4b, the part of the resistance heater 1 between the electrodes 4a and 4b generates heat. That is, parts 1a of the resistance heater 1 between adjacent electrodes 4a and 4b form resistance heater elements. The resistance heater elements 1a are arranged in the direction of the main scanning direction. Reference numeral 9 in Figure 4 denotes an equivalent circuit for the resistance heater element 1a. The space P between each pair of first and second electrodes 4a and 4b is smaller than the width Q in the main scanning direction of the electrodes 4a and 4b. The electrodes 4a and 4b are the same in the width Q in the main scanning direction. The first and second electrodes 4a and 4b may be formed of metal high in thermal conductivity such as silver paste, aluminum, copper, or gold, and is preferably 0.5 to 10µm in thickness. When the resolution is to be 400dpi, the width Q of the electrodes 4a and 4b is preferably 30 to 50µm, and more preferably 40 to 45µm. The space P between the electrodes 4a and 4b is preferably 5 to 30µm, and more particularly 15 to 25µm. For example, when the resolution is 400dpi, the center-to-center pitch of the first and second electrodes is 63.5µm. Accordingly, when the width Q of the electrodes 4a and 4b is set at 40mm, the space P between the electrodes 4a and 4b can be 23.5µm. The resistance heater 1 may be formed, for instance, by coating a paste-like mixture of ruthenium oxide powder, glass powder and a solvent over the electrodes 4a and 4b by screen printing. If necessary, the surfaces of the resistance heater 1 and the electrodes 4a and 4b may be covered with wear-resistant coating (e.g., of glass material in thickness of 2 to 20µm). When adjacent first and second electrodes 4a and 4b are energized, the resistance heater elements 1a therebetween generates heat. Due to a large thickness of the heater 1, the heat generated from the resistance heater element 1a spreads over a wide area while it propagates to the surface of the element 1a and the width W of the effective heat generating area 8 becomes larger than the width of the heater element 1a, i.e., the space P between the electrodes 4a and 4b, as shown in Figure 3. However, in this embodiment, since the widths Q in the main scanning direction of the electrodes 4a and 4b are larger than the space P between the electrodes 4a and 4b, the heat is dissipated through the electrodes 4a and 4b as indicated by arrows in Figure 4 and the effective heat generating area 8 can be confined in a relatively narrow area. Further since the parts of the effective heat generating area 8 projecting over the electrodes 4a and 4b cannot be heated to a temperature sufficient to thermally perforate the stencil master material, the width in the main scanning direction of the perforations formed can be substantially equal to the space P between the electrodes 4a and 4b and accordingly the perforations can be surely separated from each other in the main scanning direction. Further, in this particular embodiment, the space P between the first and second electrodes 4a and 4b, i.e., the width W in the main scanning direction X of the resistance heater elements 1a, is smaller than the width R in the sub-scanning direction Y of the resistance heater 1 or the resistance heater elements 1a as clearly shown in Figure 4. When the former is larger than the latter, the heat is more apt to propagate in the main scanning direction and the effective heat generating area 8 is apt to be enlarged in the main scanning direction. Further, in this particular embodiment, the width R in the sub-scanning direction Y of the resistance heater 1 is smaller than the perforation pitches S in the main scanning direction X as clearly shown in Figure 4. This is because if the former is larger than the latter, perforations are apt to be continuous in the sub-scanning direction Y when the picture element density in the sub-scanning direction Y is not lower than that in the main scanning direction X. A thick film thermal head 20 in accordance with a second embodiment of the present invention where a radiator for dissipating heat of the electrodes is provided will be described with reference to Figures 5 and 6, hereinbelow. In Figures 5 and 6, the elements analogous to those shown in Figures 1 to 4 are given the same reference numerals and will not be described here. The thermal head 20 of this embodiment differs from the thermal head 10 of the first embodiment in that a pair of radiators 7 for dissipating heat of the electrodes 4a and 4b are provided on opposite sides of the heater 1 near the heater 1. Each radiator 7 extends in the main scanning direction X in a close contact with the electrodes 4a and 4b with an insulating layer 6 intervening between the electrodes 4a and 4b and the radiators 7 so that the electrodes 4a and 4b are not short-circuited. The insulating layer 6 may be, for instance, of glass and may be about 1 to 20µm in thickness. It is preferred that the radiator 7 be as large as possible in surface area so that heat can be effectively dissipated. For example, it is preferred that the radiator 7 be provided with fins or a plurality of grooves. Further, it is preferred that the radiator be formed of a material which is high in thermal conductivity. Though the electrodes 4a and 4b are in contact with the radiator 7 longer on one side of the heater 1 than on the other side of the same in the embodiment shown in Figure 5, the electrodes 4a and 4b may extend beyond the radiator 7 on both sides of the heater 1.

Claims: A thick film thermal head (10, 20) comprising a plurality of first and second electrodes (4a, 4b) which extend in a sub-scanning direction (Y) and are alternately arranged in a main scanning direction (X) intersecting the sub-scanning direction at predetermined spaces, and a resistance heater (1), which continuously extends in the main scanning direction (X) across the first and second electrodes (4a, 4b), parts of the resistance heater (1) between adjacent first and second electrodes (4a, 4b) forming a plurality of resistance heater elements (1a) which are arranged in the main scanning direction (X) and generate heat when energized through the first and second electrodes (4a, 4b), wherein the spaces (P) between the first and second electrodes (4a, 4b) are smaller than the widths (Q) of the first and second electrodes (4a, 4b) in the main scanning direction (X) characterized in that the width (R) of the resistance heater (1) in the sub-scanning direction (Y) is smaller than the pitch of the resistance heater elements (1a) in the main scanning direction. A thick film thermal head (10, 20) as defined in Claim 1 in which the spaces (P) between the first and second electrodes (4a, 4b) are smaller than the width (R) of the resistance heater (1) in the sub-scanning direction (Y). A thick film thermal head (10, 20) as defined in Claim 1 in which said first and second electrodes (4a, 4b) are formed of metal having a thermal conductivity not lower than 100W/mK at 100°C. A thick film thermal head (20) as defined in Claim 1 further comprising a radiator (7) for dissipating heat of the electrodes (4a, 4b) provided in a close contact with the electrodes (4a, 4b). The use of a thick film thermal head according to any of claims 1 to 4 for making a heat-sensitive stencil master.

Inventors: IMAI RYOICHI, IMAI, RYOICHI, Imai, Ryoichi, Riso Kagaku Corp. R &amp; D Center
Applicants: RISO KAGAKU CORP, RISO KAGAKU CORPORATION
Application year: 2000
Main classification: B41J2
IPC codes: B41C1, B41J2