Thermal head and electronic equipments

Abstract: A thermal head is suitable for use in electronic equipments such as printers and the like and comprises a partially or entirely trapezoidal partial-glaze layer (22) formed on the top of an insulation substrate (21) at one edge, a resistive film layer (23) formed on the partial-glaze layer (22), a pattern of common and discrete electrodes (25,26) formed on the resistive film layer (23) on the opposite sides of the partial-glaze layer (22) except the area thereof that corresponds to the top of the partial-glaze layer (22), and a protective film layer (27) formed to cover the resistive film layer (23) and electrodes (25,26). In such a manner, a heating section is provided on the top of the trapezoidal partial-glaze layer (22). The heating section extends further upwards from the surrounding area. Unnecessary dispersion of pressure to the surrounding area other than the heating section can be avoided to improve the printing efficiency and speed.

Description: The present invention relates to a thermal head having an improved printing efficiency, electronic equipments with such a thermal head, such as printers, word processors, facsimile machines and plotters, and a process of making such a thermal head. The thermal heads can be classified into partial-glaze type, double-partial-glaze type and true-edge type. As shown in Fig. 1, the partial-glaze type thermal head comprises an insulation substrate 11, a partial glaze layer 12 formed on the substrate adjacent to its edge portion, having a width equal to about 300 - 1200 microns and an outwardly convex configuration, a resistive film layer 13 formed over the partial glaze layer 12, common and discrete electrodes 15 and 16 formed on the resistive film layer 13 at the top positions of the glaze layer 12 opposite to each other to form a heating section 14 on the top of the glaze layer 12 and a protective film 17 covering these layers as a whole. The double-partial-glaze type thermal head is one similar to the partial-glaze type thermal head except that a portion of the glaze layer 12 placed at the heating section 14 is formed into an upwardly convex configuration by glaze etching or the like, as shown in Fig. 2. The true-edge type thermal head is one where in the glaze layer 12 and the heating section 14 are formed to cover the edge of the insulation substrate 11, as shown in Fig. 3. Fig. 4 shows a modification of the thermal head shown in Fig. 3, in which the edge portion of the insulation substrate 11 is slantingly cut to provide a slope 11b adjoining the top face 11a of the insulation substrate 11 and an edge face 11c adjoining the slope 11b and extending perpendicular to the top face 11a. Glaze layers 12a, 12b and 12c are formed over the respective faces 11a, 11b and 11c. The heating section 14 is formed at the slope 11b. As shown in Fig. 5, the thermal transfer is carried out against an ink ribbon 18 and a sheet to be printed on 19 which are held between the glaze layer 12 and a rubber platen 20, with the ink ribbon 18 being urged to the sheet to be transferred 19 by the glaze layer 12. Since the top of the glaze layer 12 exerting the maximum urging force to the ink ribbon 18 includes the heating section 14, the heating and urging of the ink ribbon 18 will be simultaneously made on the thermal transfer. In order to print a rough sheet and to improve the printing efficiency, the thermal head requires that a heating resistor portion concentrates its pressure onto the ink ribbon, the sheet to be printed on and the platen. For example, if a rough sheet is to be printed, a letter pattern is thermally cut away from the ink ribbon 18. The cut letter pattern is then transferred to the sheet to be printed 19 under the urging force from the glaze layer 12. In the partial-glaze and the double-partial-glaze type systems, however, an area at which the glaze layer 12 having the heating section 14 engages the rubber platen 20 is relatively large. This raises a problem in that the pressure from the heating section 14 is not sufficiently concentrated onto the ink ribbon 18. Although the true-edge type system (Fig. 3) has been developed to overcome such a problem, it cannot presently provide a sufficient advantage since the substrate has an undesirable thickness equal to about 2 mm. A common problem with the aforementioned systems of the prior art is that when a substrate is to be worked at its side face to print a plurality of thermal heads thereon, they cannot be formed at the same time. As a result, the manufacturing cost per thermal head will be increased. JP-A-238065 discloses a thermal head wherein an upwardly convex partial-glaze layer is formed on a substrate. Adjacent to the glaze layer a groove is etched into the substrate. A resistive film is then formed over the partial glaze layer and electrodes are selectively formed thereon. A protective film is then formed over the electrodes, the exposed resistive film and the groove. A thermal head of similar structure to that shown in Figure 1 is disclosed in Figure 2 of FR-A-2482007. The disclosed arrangement comprises a thermal head suitable for use in electronic equipments, said thermal head comprising: (a) an insulation substrate; (b) an upwardly convex partial-glaze layer formed over a top surface of said insulation substrate at an edge thereof; (c) a resistive film layer formed over at least a top surface of said partial-glaze layer; (d) an electrode pattern formed over said resistive film layer except on the area thereof that corresponds to the top of said partial-glaze layer; and (e) a protective film layer formed to cover said resistive film layer not covered by said electrode pattern and electrode pattern; wherein a heating section is formed over the top surface of said partial-glaze layer, said heating section being substantially centred over the top surface of said partial-glaze layer. It is therefore an object of the present invention to provide a thermal head which can be inexpensively produced with an improved printing efficiency and which can concentrate the pressure onto the heating section. Another object of the present invention is to provide a process of easily producing a plurality of such thermal heads at a time. The present invention provides a thermal head suitable for use in electronic equipments, said thermal head comprising: (a) an insulation substrate; (b) an upwardly convex partial-glaze layer formed over a top surface of said insulation substrate at an edge thereof; (c) a resistive film layer formed over at least a top surface of said partial-glaze layer; (d) an electrode pattern formed over said resistive film layer except on the area thereof that corresponds to the top of said partial-glaze layer; and (e) a protective film layer formed to cover said resistive film layer not covered by said electrode pattern and electrode pattern; wherein a heating section is formed over the top surface of said partial-glaze layer, said heating section being substantially centred over the top surface of said partial-glaze layer; characterised in that said partial-glaze layer has a first sloping surface adjacent said edge and a second sloping surface remote from said edge, said second sloping surface being steeper than said first sloping surface. In such a thermal head, the steeper slope of the upwardly convex partial-glaze layer is located at a position adjacent to the center of the insulation substrate and formed by glaze etching. Thus, the heating section on the top of the resistive film layer will extend further outwards from the remaining head portions. Unnecessary dispersion of pressure to the head portions other than the heating section can be avoided. As a result, necessary pressure can be concentrated only onto the heating section to improve the printing efficiency. This also improves the transmission of heat to the ink ribbon and other parts such that any unnecessary heat accumulation can be avoided. This speeds up the printing. Since the heating section is formed on the top of the partial-glaze layer covered with the resistive film layer at the edge of the substrate, furthermore, a plurality of thermal heads can be simultaneously formed in the same insulation substrate through the half-cutting process (non-through cutting process) with a reduction of the manufacturing cost for each thermal head. The present invention further provides a process of producing thermal heads, comprising the steps of (a) forming an upwardly convex partial-glaze layer on the top of an insulation substrate at an edge thereof; (b) cutting said upwardly convex partial-glaze layer on a surface remote from said edge to form a sloping surface which is steeper than the slope of the surface of said partial-glaze layer adjacent said edge; (c) rounding the corner where said steeper sloping surface is cut; (d) forming a resistive film layer on said upwardly convex partial-glaze layer; (e) forming an electrode pattern on said resistive film layer except on the area thereof that corresponds to the top of said upwardly convex partial-glaze layer; and (f) forming a protective film layer to cover said resistive film layer not covered by said electrode pattern and said electrode pattern. This process can easily mass-produce thermal heads each having one slope steeper than the other. More particularly, after the upwardly convex partial-glaze layer has been formed on the top of the insulation substrate at one edge, it is cut by for example, glaze etching, to form one slope of the upwardly convex partial-glaze layer steeper than the other. The cut slope of the upwardly convex partial-glaze layer is then rounded by a re-heating (or re-burning) step or by chemical hydrofluoric acid treatment. As a result, the subsequent patterning can be facilitated. Thereafter, the partial-glaze layer is covered with the resistive film layer and common and discrete electrodes such that the resistive film layer and part of the discrete electrode will be coincident with the steeper slope of the partial-glaze layer. Thus, the heating section will apparently extend further upwards from the surrounding area and be subject to a sufficient concentration of pressure. By the use of the half-cutting process (non-through cutting process) and the rounding step, a plurality of thermal heads can be simultaneously produced in the same insulation substrate with a reduction of the manufacturing cost for each thermal head. If the volume of the partial-glaze layer is reduced, its heat capacity is also reduced to cool the heating section more rapidly, leading to a reduction of the energy efficiency. However, the thermal responsiveness will be increased because the heating section can be set at higher temperatures since the heating section can be more rapidly cooled from these higher temperatures. The increase of the thermal responsibility speeds up the printing. If the width of the partial-glaze layer is suitably regulated, the high-speed printing can be carried out while maintaining the thermal head at a higher temperature. However, if the width of the partial-glaze layer is too small, the partial-glaze layer itself cannot function significantly. It is therefore important that the width and height of the partial-glaze layer are appropriately selected for the desired purpose. It has been found that the thermal head could accomplish the desired purpose if the partial-glaze layer had a width ranged between 300 microns and 1000 microns and preferably equal to about 550 microns and a height ranged between 10 microns and 70 microns and preferably equal to about 55 microns. When the thermal head of the present invention including its heating resistor section subject to a sufficient concentration of pressure is incorporated into an electronic printing instrument such as a printer or the like, the instrument can print more clearly and quickly. Since a pattern of ink can be reliably separated from an ink ribbon through the increased concentration of pressure at the heating resistor section of the thermal head, the printing can be appropriately made even for rough sheets. Since the thermal heads of the present invention can be mass-produced inexpensively, electronic instruments such as printers incorporating the thermal heads of the present invention can be also produced correspondingly cheaply. Examples of the present invention will now be described with reference to the drawings, in which:- Fig. 1 is a partial cross-section of a thermal head constructed in accordance with the prior art. Fig. 2 is a partial cross-section of another thermal head constructed in accordance with the prior art. Fig. 3 is a partial cross-section of still another thermal head constructed in accordance with the prior art. Fig. 4 is a partial cross-section of a further thermal head constructed in accordance with the prior art. Fig. 5 illustrates the disadvantage of the prior art. Fig. 6 is a cross-sectional view of the major part of a thermal head which is one embodiment of the present invention. Fig. 7 illustrates a process of making the thermal head of the embodiment of the present invention, in which a glaze layer is formed on a substrate. Fig. 8 illustrates a steeper slope formed on the glaze layer. Fig. 9 illustrates the rounding step of the slope. Fig. 10 is a cross-sectional view of the major part of an electronic instrument (printer) into which a preferred thermal head constructed in accordance with the present inventionn is incorporated. Fig. 11 is a schematic diagram of the major part of the electronic instrument shown in Fig. 10. Referring now to Fig. 6, there is shown one embodiment of a thermal head constructed in accordance with the present invention, which comprises an insulation substrate 421; a partial-glaze layer 422 formed on the top of the insulation substrate 421 at one edge, the partial-glaze layer having one slope steeper than the other slope; a resistive film layer 423 formed on the partial-glaze layer 422, common and discrete electrodes 425, 426 formed on the resistive film layer 423 at the opposite sides of the partial-glaze layer 422; and a protective film layer formed to cover the resistive film layer 423 and common and discrete electrodes 425, 426. The top of the partial-glaze layer 422 not covered with the common and discrete electrodes 425, 426 provides a heating section 424. The thermal head of this embodiment is characterized by the fact that the partial-glaze layer 422 has one slope 431 steeper than the other slope. The partial-glaze layer 422 is of an upwardly convex curvature configuration in this embodiment. The steeper slope 431 may be formed by cutting that side of the upwardly convex partial-glaze layer 422 adjacent to the center of the substrate 421 or opposite to the edge thereof such as by glaze etching. The resistive film layer 423 and part of the discrete electrode 426 are inclined more steeply in the presence of the steeper slope 431 in the partial-glaze layer 422, so that the heating section 424 apparently extends further upwards from the surrounding area. Figs. 7 - 9 illustrate a process of making the thermal head in accordance with this embodiment of the present invention. In order to produce the thermal head of this embodiment, an upwardly convex curved partial-glaze layer 422 is first formed on the top of the insulation substrate 421 at one edge (Fig. 7). As shown in Fig. 8, the steeper slope 431 is formed by cutting that side of the upwardly convex partial-glaze layer 422 nearer the center of the insulation substrate 421 such as by glaze etching. As shown in Fig. 9, the corner 432 of the cut slope 431 is then rounded by re-heating (or re-burning) or chemical hydrofluoric acid treatment. This facilitates the subsequent patterning step. The partial-glaze layer 422 is covered with the resistive film layer 423 and then the common and discrete electrodes 425, 426 are formed on the resistive film layer 423. The resistive film layer 423 and part of the discrete electrode 426 are inclined more steeply in the presence of the steeper slope 431 in the partial-glaze layer 422. The protective film layer 427 is finally formed to cover the resistive film layer 423 and common and discrete electrodes 425, 426 (see Fig. 6). In such a manner, the heating section 424 will be provided on the top of the partial-glaze layer 422 which extends further upwards from the surrounding area. The process can easily produce the thermal head of this embodiment. By the use of half-cutting (non-through cutting) and rounding steps, a plurality of thermal heads can be simultaneously produced in the same substrate with a reduction of the manufacturing cost. In this embodiment, since the partial-glaze layer has one slope steeper than the other slope, it will apparently extend further upwards from the surrounding area. Thus, the heating section can be subject to a sufficient concentration of pressure to improve the transmission of heat to the ink ribbon and other parts, thereby increasing the printing efficiency and speed with improvement of the thermal transfer to rough sheets. The thermal head of this embodiment may substitute for the conventional thermal heads without any particular modification. Since the heating section exists at the edge of the substrate, a plurality of thermal heads can be simultaneously produced in the same substrate with a reduction of the manufacturing cost. If the volume of the partial-glaze layer is reduced, its heat capacity is also reduced to cool the heating section more rapidly, leading to a reduction of the energy efficiency. However, the thermal responsiveness will be increased because the heating section can be set at higher temperatures since the heating section can be more rapidly cooled from these higher temperatures. The increase of the thermal responsiveness speeds up the printing operation. If the width of the partial-glaze layer is suitably regulated, the high-speed printing can be carried out while maintaining the thermal head at a higher temperature. However, if the width of the partial-glaze layer is too small, the partial-glaze layer itself cannot function significantly. It is therefore an important point that the width and height of the partial-glaze layer are appropriately selected for the desired purpose. It has been found that the thermal head of the present invention can accomplish the desired purpose if the partial-glaze layer had a width ranged between 300 microns and 1000 microns and preferably equal to about 550 microns and a height ranged between 10 microns and 70 microns and preferably equal to about 55 microns. The thermal head according to the previously described embodiment has a heating section which can be subject to a sufficient concentration of pressure to improve the transmission of heat to the ink ribbon and other parts, thereby increasing the printing efficiency and speed with improvement of the thermal transfer to rough sheets. The thermal head of the previously described embodiment may be incorporated into various electronic equipments such as printers, word processors, facsimile machines, plotters and so on. Fig. 10 shows a printer incorporating a thermal head which is constructed in accordance with the present invention. The printer 40 comprises an inlet 44 into which an original 42 is to be inserted, a feed roller 46 for feeding the original 42, an image sensor 48 for reading the original 42, a printing section 50 for printing onto a recording sheet 54 and a recording platen roller 52 located adjacent to the printing section 50. The printer 40 is powered by a source of electric power 56. When a plurality of originals 42 are inserted into the printer 40 through the inlet 44, the originals 42 will be Separated by separating means 43 and then transferred to the image sensor 48 one at a time. The pattern on the original 42 is converted into electrical signals by the image sensor 48. These electrical signals are used to cause the printing section 50 to print onto the recording sheet 54. As shown in Fig. 11, an ink ribbon 62 is used to enable the printing for rough sheets in the printer 40. Fig. 10 shows a reading mechanism for a copying or facsimile machine. However, the thermal head of the present invention may be used in a printer having no reading mechanism. The thermal head of the present invention may be also used in a serial printer in which a thermal head 64 moves on a flat plate-like platen plate 79 with a ribbon cassette 77 used therein. Since the thermal head of the present invention has a heating section subject to a sufficient concentration of pressure to improve the transmission of heat to the ink ribbon and other parts to increase the printing efficiency and speed, the printer incorporating the thermal head of the present invention can perform the printing at a speed higher than the prior art. The increased concentration of pressure against the heating section can more effectively cut letter patterns away from the ink ribbon and transfer the cut letter pattern to a recording sheet. The thermal transfer can be more effectively carried out for rough sheets. In order to improve the thermal transfer for rough sheets, it is required that the ink in the ink ribbon has an increased viscosity and that the letter pattern is thermally cut away from the ink ribbon and applied to the recording sheet under the pressure from the thermal head. If the pressure from the thermal head is too low, the cut letter pattern may not be transferred to the recording sheet or may be returned to the ink ribbon after the letter pattern has been temporarily transferred to the recording sheet. The thermal head of the present invention can provide a sufficient concentration of pressure against the heating section such that the just mentioned problem can be overcome to perform thermal transfer to the rough sheets more efficiently. Since the thermal head of the present invention has the heating section which can substitute for the conventional heating heads without any particular modification in design, the conventional printers incorporating thermal heads constructed in accordance with the prior art may be easily replaced by the printer incorporating the thermal head of the present invention. Since the heating section is formed in the substrate at the edge according to the present invention, a plurality of thermal heads can be simultaneously produced in the same substrate more inexpensively. When a printer includes a thermal head constructed in accordance with the present invention, the printer can be inexpensive with an improved performance.

Claims: A thermal head suitable for use in electronic equipments, said thermal head comprising: (a) an insulation substrate (421); (b) an upwardly convex partial-glaze layer (422) formed over a top surface of said insulation substrate (421) at an edge thereof; (c) a resistive film layer (423) formed over at least a top surface of said partial-glaze layer (422); (d) an electrode pattern (425, 426) formed over said resistive film layer (423) except on the area (424) thereof that corresponds to the top of said partial-glaze layer (422); and (e) a protective film layer (427) formed to cover said resistive film layer (423) not covered by said electrode pattern (425, 426) and said electrode pattern (425, 426); wherein a heating section (424) is formed at the top surface of said partial-glaze layer (422), said heating section (424) being substantially centred over the top surface of said partial-glaze layer; characterised in that said partial-glaze layer has a first sloping surface adjacent said edge and a second sloping surface (431) remote from said edge, said second sloping surface (431) being steeper than said first sloping surface. A printing apparatus comprising an image sensor (48) for outputting electrical signals corresponding to patterns on a document (42) to be printed, means (52) for supplying a sheet of printing paper (54), means for feeding an ink ribbon (62) and a thermal head (50) in accordance with Claim 1, said thermal head (50) being responsive to the electrical signals from said image sensor (48) for heating said ink ribbon (62) to print said patterns on the sheet of printing paper (54). A process of making a thermal head as defined in Claim 1, said process comprising: (a) forming an upwardly convex partial-glaze layer (422) on the top of an insulation substrate (421) at an edge thereof; (b) cutting said upwardly convex partial-glaze layer (422) on a surface remote from said edge to form a sloping surface (431) which is steeper than the slope of the surface of said partial-glaze layer adjacent said edge; (c) rounding the corner where said steeper sloping surface is cut; (d) forming a resistive film layer (423) on said upwardly convex partial-glaze layer (422); (e) forming an electrode pattern (425, 426) on said resistive film layer (423) except on the area (424) thereof that corresponds to the top of said upwardly convex partial-glaze layer (422); and (f) forming a protective film layer (427) to cover said resistive film layer (423) not covered by said electrode pattern (425, 426) and said electrode pattern (425, 426).

Inventors: HIROAKI ONISHI, SHIMIZU KATSUHIKO, HIROAKI, ONISHI, SHIMIZU, KATSUHIKO, HIROAKI, ONISHI C/O ROHM CO., LTD., PRINT HEAD
Applicants: ROHM CO LTD, ROHM CO., LTD.
Application year: 1992
Main classification: B41J2
IPC codes: B41J2