Thermal printing head and method of manufacturing the same

Abstract: The present invention provides a thermal head capable of preventing any chipping between a glaze layer and a protective film, increasing an angle of separation in a thermal transfer ribbon and eliminating any step of abrading the protective film to prevent the separation of the thermal transfer ribbon and the darkened background of the heat-sensitive sheet, and a method of making such a thermal head. The thermal head includes a glaze layer 12 formed as a heat-accumulating layer on a substrate 11, resistance and electrode layers 13, 14 and 15 formed on the glaze layer 12 and a protective film 16 formed to cover the entire surface of the thermal head. The edge of the glaze layer 12 is totally cut with the upper cut edge thereof being rounded. The protective film 16 is formed directly on the cut edge of the glaze layer and the substrate. After the glaze layer has been formed, the edge portion of the glaze layer 12 is cut until reaching the substrate 11. Thereafter, the glaze layer is heated at a temperature equal to or higher than its softening point to round the upper cut edge of the glaze layer. The other layers are then formed. The formed layers are then covered with the protective film 16. Finally, a portion of the protective film 16 adhered to the substrate is cut at a position adjacent to the upper rounded edge of the glaze layer.

Description: The present invention relates to a thermal head suitable for use as a print head in various recording instruments such as printers. Conventional recording systems such as page printers, word processors, facsimiles and the like have all used thermal print heads. Fig. 3 shows a thermal head, the detail of which is further shown in Fig. 5 by fragmentary cross-section. Fig. 6(b) is another cross-sectional view, enlarged in scale, of the thermal head, taken along a line B-B in Fig. 5. In these figures, the edge of a substrate 1 of alumina ceramic (dielectric) is formed with a glaze layer 2 which is a heat accumulating layer. For each dot, a resistance layer 3, individual lead electrode 4, common electrode 5 and protective film 6 are formed on the substrate 1 over the glaze layer 2 in the order as just described. If a signal is selectively applied to an individual lead electrode 4 through the common electrode 5, the resistance layer 3 located between these electrodes will be heated and so cause the transfer of the ink of a thermal transfer ribbon 8 onto a recording sheet. Such thermal heads are generally used with a resinous ink such that the thermal transfer ribbon 8 can be used to print or any recording sheet even if it has a rough printing surface. In order to prevent the resinous ink from being re-fused to the recording sheet, therefore, the thermal transfer ribbon 8 must be separated from the recording sheet as rapidly as possible after the transfer of ink has been completed. As shown in Fig. 6(b), thus, the chamfered edge of the conventional thermal head includes a printing portion 100 (above the resistance layer 3 between the individual electrode 4 and the common electrode 5). More particularly, after the glaze layer 2, resistance layer 3, individual lead electrode 4, common electrode 5 and protective film 6 have been formed sequentially on the substance 1 as shown in Fig. 6(a), the substrate 1 will be cut by means of a diamond disc as shown in Fig. 6(b). Such a cutting operation comprises two separate steps, that is, one step for cutting the glaze layer 2 and protective film 6 which are of glass material by the use of a cutting edge having a fine particle size and another step for cutting the substrate 1 of alumina ceramic by the use of a cutting edge having a rough particle size. In the conventional thermal head as shown in Fig. 6(b), however, so-called "chipping" may occur which results in that the protective film 6 is separated from the glaze layer 2 by an impact which is produced by the diamond cutting disc. This is because the surface of the glaze layer 2 is very smooth and makes less contact with the protective film 6 which is made of a material similar to the glass material such as tantalum pentoxide and the like. The chipping causes the moisture barrier properties to be lowered so that the common electrode 5 may become corroded, leading to various problems such as failure in conductivity amongst other things. In order to overcome the problems, the prior art has proposed a thermal head which comprises a protecting and braking layer (7 in Fig. 6(c)) or a bonding layer formed outside the common electrode 5 (or at a position adjacent to the edge of the thermal head), as disclosed in Japanese Patent Laid-Open No. Sho 62-208961, 62-124962 or 63-267564. Such a layer serves to improve the adhesion between the glaze layer 2 and the protective film 6. However, the provision of the protecting and braking layer 7 as shown in Fig. 6(c) increases the spacing between the printing portion 100 and the cut edge portion of the thermal head such that the angle of separation ϑ of the thermal transfer ribbon 8 will be reduced. This makes the printed letters obscure. If the thermal head is subjected to the cutting operation after the protective film 6 has been formed as shown in Fig. 6(b), the upper edge 200 of the thermal head will be formed with a very sharply angled corner. When such a sharp edge 200 is brought into contact with the thermal transfer ribbon 8 or a heat-sensitive sheet on slide movement of the thermal head, there may be created various problems such as separation of ink from the ribbon 8, darkened background of the heat-sensitive sheet and others. Thus, the upper sharp edge 200 of the thermal head or protective film 6 must be abraded to form a gentle curvature. JP-A-1304960 discloses a thermal head comprising a glaze layer formed on a substrate, a heat generating resistor layer formed on the glaze layer and a protective film formed on the resistor layer. This document aims to reduce manufacturing costs by extending a common electrode to the substrate. JP-A-1257064 discloses a thermal printing head comprising: a) a substrate; b) a glaze layer formed, as a heat accumulating layer, on a portion of a surface of said substrate, said glaze layer terminating on said surface at a cut edge, the corner between said cut edge and a surface of said glaze layer remote said surface of said substrate being rounded; c) a resistance layer formed on an area of said surface of said glaze layer adjacent said cut edge; d) an electrode layer formed at given locations on a surface of said resistance layer remote said surface of said glaze layer; e) a printing portion comprising an area between said locations on said resistance layer; and f) a protective film deposited on said electrode layer, said printing portion and said cut edge of said glaze layer. However neither of these documents are concerned with the problem of preventing chipping between the glaze layer and the protective film whilst providing an increased angle of separation of the thermal transfer ribbon. It is therefore an object of embodiments of the present invention to provide a thermal head which can prevent the chipping between the glaze layer and the protective film and which can provide an increased angle of separation of the thermal transfer ribbon without any abrading operation for the protective film, and a method of producing such a thermal head. Another object of embodiments of the present invention is to provide a thermal head which can prevent the separation of the thermal transfer ribbon and the darkened background of the heat-sensitive sheet from being created without any abrading operation for the protective film. To this end, the present invention provides a thermal printing head comprising: a) a substrate; b) a glaze layer formed, as a heat accumulating layer, on a portion of a surface of said substrate, said glaze layer terminating on said surface at a cut edge, the corner between said cut edge and a surface of said glaze layer remote said surface of said substrate being rounded; c) a resistance layer formed on an area of said surface of said glaze layer adjacent said cut edge; d) an electrode layer formed at given locations on a surface of said resistance layer remote said surface of said glaze layer; e) a printing portion comprising an area between said locations on said resistance layer; and f) a protective film deposited on said electrode layer, said printing portion, and said cut edge of said glaze layer; characterised by g) anti-separation means for preventing separation of said protective film from said glaze layer, said anti-separation means comprising said protective film deposited on said cut edge of said glaze layer and on an area of said substrate adjoining said cut edge. The present invention also provides a method of making a thermal printing head, comprising the steps of: a) depositing a glaze layer as a heat accumulating layer on a substrate; b) cutting said glaze layer to form a cut edge and removing said glaze layer from said substrate on one side of said cut edge; c) rounding the corner between said cut edge and a surface of said glaze layer remote said substrate; d) depositing a resistance layer on an area of said glaze layer adjacent said cut edge; e) depositing an electrode layer at given locations on said resistance layer; and f) depositing a protective film on said electrode layer, said printing portion, and said cut edge of said glaze layer; characterised in that the step of depositing said protective film includes the step of forming anti-separation means by depositing said protective film on said cut edge of said glaze layer and on an area of said substrate adjoining said cut edge. Since the protective film is formed directly on the substrate having a relatively rough surface adjacent the cut edge, the adhesion between the protective film and the substrate can be improved. Further, the chipping between the glaze layer and the protective film can be highly reduced since the glaze layer is cut at the step of forming the glaze layer on the substrate. In accordance with the method of the present invention, the glaze layer is cut and rounded at its corner to have a given curvature, for example, by heating the glaze layer to its softening point. When the protective film is further formed on the substrate, a further increased curvature will be formed on the corner of the protective film. In a further embodiment the substrate is cut to form a groove at a surface portion formerly covered by the glaze layer before cutting; the groove having a width equal to about 0.2mm. In another embodiment the substrate and the protective film are cut in the groove to form a cut edge. Examples of the present invention will now be described with reference to the drawings, in which: Fig. 1 is a view illustrating various steps of making a thermal head in accordance with one embodiment of the present invention; Fig. 2 is a fragmentary cross-sectional view of the thermal head shown in Fig. 1 when it is used together with a thermal transfer ribbon; Fig. 3 is a cross-sectional view of a printer in which the thermal head of one embodiment of the present invention is used; Fig. 4 is a view illustrating the thermal transfer of a recording sheet from the thermal head of one embodiment of the present invention; Fig. 5 is a top plan view of a thermal head constructed in accordance with the prior art; and Fig. 6 is a cross-sectional view of the thermal head shown in Fig. 5, illustrating various working steps in accordance with the prior art. Referring first to Fig. 1, there is shown a process of making a thermal head in accordance with the principle of the present invention, Fig. 1(a) shows a glaze layer forming step in which a glaze layer 12 of SiO₂ is partially printed on a substrate 11 of alumina ceramic which is a dielectric material, The glaze layer 12 serves as a heat accumulating layer. Fig. 1(b) shows a first cutting step carried out after completion of the glaze layer forming step. In the first cutting step, the glaze layer 12 is cut, together with the substrate 11, by a diamond cutting disc to form a groove 11a in the substrate 11, the groove having a width equal to about 0.2 mm. Alternatively, only the glaze layer 12 may be cut to expose the surface of the substrate 11, without cutting the substrate 11. The process then proceeds to a heat treating step shown in Fig. 1(c) wherein the glaze layer 12 is heated at a temperature equal to or higher than its softening point, for example, at about 850 °C for about ten minutes. This heat treatment causes the upper cut edge 201 of the glaze layer 12 to have a radius of curvature equal to about 10 microns. The rounded edge of the glaze layer 12 is preferably formed by the above heat treatment, but may be formed by any other suitable machining or abrading operation. On termination of the heat treating step, a resistance layer 13 is then formed over the substrate 1 and glaze layer 12, as shown in Fig. 1(d) on the left side thereof. An individual lead electrode 14 is then formed over the resistance layer 13 by means of printing and etching techniques. At the same time, a common electrode 15 is formed on the upper portion of the resistance layer 13 in the same manner, spaced away from the individual lead electrode 14 toward the groove 11a. The entire surface of the thermal head is finally covered with a protective film 16 to complete a printing portion 101 on the top of the thermal head for each dot. Since the upper cut edge 201 of the glaze layer 12 is rounded into a given radius of curvature, the protective film 16 will be also formed to have a radius of curvature 202 as shown. After the protective film 16 has been formed, the process proceeds to a second cutting step shown in Fig. 1(e). In the second cutting step, the substrate 11 is cut, together with the protective film 16, by the diamond cutting disc. Since the protective film 16 is adhered directly to the substrate 11 of alumina ceramic having its roughed surface, the adhesion between the substrate 11 and the protective film 16 is strengthened so no chipping occurs as a result of the cutting impact. Referring now to Fig. 2, there is shown the thermal head produced by such a process as shown in Fig. 1, which is placed in contact with a thermal transfer ribbon 18. Since the need for any means for preventing the chipping, such as the braking and protecting layer 7 (see Fig. 6) or the like is eliminated, an angle ϑ by which the thermal transfer ribbon 18 is separated from a recording sheet 17 can be increased, as shown in Fig. 2. Modern printing devices are required to print clear letters even if a recording sheet used has a rough surface. The thermal transfer ink has some degree of viscosity so that it can be thermally adhered to the recording sheet, such as a sheet of transfer paper. On the other hand, the thermal transfer ribbon must be rapidly separated from the transfer sheet immediately after the thermal transfer printing has been carried out, such that the printed ink can be prevented from returning from the transfer sheet to the thermal transfer ribbon since the thermal print ink is more easily adhered to the thermal transfer ribbon. In brief, the process requires two steps, that is, one step of rapidly separating the ink from the thermal transfer ribbon 18 when the printing portion 101 of the thermal head is heated and another step of separating the thermal transfer ribbon 18 from the transfer sheet 17 immediately after the thermal transfer printing has been made. The thermal transfer ribbon 18 can be rapidly separated from the transfer sheet 17 immediately after the thermal transfer printing has been performed, since the aforementioned angle of separation ϑ is increased. From combination of the increased angle of separation ϑ with the upper rounded edge 202 of the protective film 16, a distance between a position at which the thermal transfer ribbon 18 is separated from the transfer sheet 17 and the end of the printing portion 101 adjacent to the upper rounded edge 202 of the protective film 16, which will be called simply "separation distance", will approach to zero. Therefore, the heating of the printing portion 101 will not deviate from the time at which the ink is separated from the thermal transfer ribbon 18. As a result, the ink can be properly transferred from the thermal transfer ribbon 18 to the transfer sheet 17 at all times. In such a manner, the thermal head can perform the printing operation clearly. Although the aforementioned embodiment has been described as to the upper edge of the glaze layer 12 rounded by heating, the process may proceed directly to the head shaping step shown in Fig. 1(d) without any heating step. Even in such a case, the upper edge 202 of the protective film 16 can be more or less rounded to provide some degree of effectiveness since the protective film 16 is formed after the cutting of the glaze layer 12. Fig. 3 shows the internal structure of a printer in which the thermal head of one embodiment of the present invention is used. The printer comprises an original supply section, a recording sheet supply section, a thermal transfer printing section, a system control substrate 32 and a source of power 33. The original supply section comprises feed and platen rollers 31a and 31b both for feeding an original 30, and an image sensor 31c. The recording sheet supply section includes a recording platen roller 36 for feeding a recording sheet 17. These functions are not part of the present invention and so will not be described in detail. The recording sheet 17 fed from the recording sheet supply section is then subjected to the thermal transfer at the thermal head 37 in the thermal transfer printing section. This will now be described in detail with reference to Fig. 4. Referring to Fig. 4, the thermal head 37 comprises a rubber platen 38, a heating projection 39 energized through the lead 40 and substrate 11, and a thermal transfer ribbon 18. The recording sheet 17 fed by the recording platen roller 36, having a rough surface, is sandwiched, together with the thermal transfer ribbon 18 incorporated into the thermal head 37, between the rubber platen 38 and the heating projection 39 under the optimum pressure. As a result, the rough surface of the recording sheet 17 will be smoothed so that the ink can be thermally transferred more easily onto the smoothed surface of the recording sheet 17 under the heating action of the heating projection 39. Furthermore, the thermal transfer ribbon 18 can be rapidly separated from the recording sheet 17 without return of the printed ink to the thermal transfer ribbon 18 since the angle of separation ϑ is increased Consequently, the printer in which the thermal head is used can print very clearly. Since the entire edge of the glaze layer is cut and the protective film 16 is formed directly over the cut glaze layer edge and the substrate, the protective film will be formed on the relatively rough surface of the substrate to greatly improve the adhesion between the protective film and the substrate. Fig. 4b shows the method of Fig. 4a which is applied to such a reading device as is used in copying machines, facsimiles and the like. Thus, the rubber platen 38 is of a roller-shaped configuration. However, the thermal head may be similarly applied to a printer having no reading device. In addition to such a printer in which the thermal head 37 as shown in Fig. 4a is fixedly mounted to feed the recording sheet 17 by means of the recording platen roller 36, embodiments of the present invention may be similarly applied to a serial printer in which the thermal head 37 and ribbon cassette 40 are moved on the recording sheet 17 as shown in Fig. 4c and wherein the ink on the ribbon 18 is thermally transferred to the recording sheet 17 by cooperation of the plate-like rubber platen 38 with the heating projection 39 of the thermal head. In accordance with the method of the present invention, moreover, the glaze layer is cut at a stage when the protective film is still not formed over the glaze layer. This eliminates any chipping between the glaze layer and the protective film to improve the adhesion therebetween and also to improve the adhesion between the protective film and the substrate as described above. The improvement of the adhesion between the glaze layer and the protective film and between the protective film and the substrate removes the need for any means to prevent chipping, such as the braking and protecting layer of the prior art. This increases the angle of separation of the thermal transfer ribbon from the recording sheet and so improves the print quality. Since the protective film is formed after the glaze layer has been formed, the upper edge of the thermal head can be slightly rounded without any abrading operation. This eliminates the separation of the ink from the thermal transfer ribbon and the darkened background of the heat-sensitive sheet. Since the upper edge of the glaze layer is cut and rounded preferably by the heat treatment, the separation of the ink from the thermal transfer ribbon and the darkened background of the heat-sensitive sheet can be overcome more effectively.

Claims: A thermal printing head comprising: a) a substrate (11); b) a glaze layer (12) formed, as a heat accumulating layer, on a portion of a surface of said substrate (11), said glaze layer (12) terminating on said surface at a cut edge, the corner between said cut edge and a surface of said glaze layer (12) remote said surface of said substrate (11) being rounded; c) a resistance layer (13) formed on an area of said surface of said glaze layer (12) adjacent said cut edge; d) an electrode layer (14, 15) formed at given locations on a surface of said resistance layer (13) remote said surface of said glaze layer (12); e) a printing portion (101) comprising an area between said locations on said resistance layer (13); and f) a protective film (16) deposited on said electrode layer (14, 15), said printing portion (101), and said cut edge of said glaze layer (12); characterised by g) anti-separation means for preventing separation of said protective film (16) from said glaze layer (12), said anti-separation means comprising said protective film (16) deposited on said cut edge of said glaze layer (12) and on an area of said substrate (11) adjoining said cut edge. A thermal printing head as claimed in Claim 1 characterised in that the rounded corner of said glaze layer (12) has a radius of curvature equal to about 10 microns. A thermal printing head as claimed in Claim 1 or claim 2 characterised by a groove (11a) formed in said substrate (11) adjacent said cut edge, said groove (11a) having a width equal to about 0.2 mm; and in that said protective film (16) is deposited on said cut edge and on said groove (11a). A thermal printing head as claimed in Claim 3 characterised in that said substrate (11) and said protective film (16) terminate at a cut edge in said groove (11a). A thermal printing head as claimed in any preceding claim characterised in that said substrate (11) is made of alumina ceramic which is a dielectric material and wherein said glaze layer (12) is made of SiO₂. A method of making a thermal printing head, comprising the steps of: a) depositing a glaze layer (12) as a heat accumulating layer on a substrate (11); b) cutting said glaze layer (12) to form a cut edge and removing said glaze layer (12) from said substrate (11) on one side of said cut edge; c) rounding the corner between said cut edge and a surface of said glaze layer (12) remote said substrate (11); d) depositing a resistance layer (13) on an area of said glaze layer (12) adjacent said cut edge; e) depositing an electrode layer (14, 15) at given locations on said resistance layer (13); and f) depositing a protective film (16) on said electrode layer (14, 15), said printing portion (101), and said cut edge of said glaze layer (12); characterised in that the step of depositing said protective film (16) includes the step of forming anti-separation means by depositing said protective film (16) on said cut edge of said glaze layer and on an area of said substrate (11) adjoining said cut edge. A method as claimed in Claim 6 wherein the step of rounding the corner of the glaze layer (12) comprises a heat treating step between the steps of cutting the glaze layer (12) and forming the resistance layer (13), said heat treating step being used to round the corner of said glaze layer (12) by heating said glaze layer (12). A method as claimed in Claim 7 characterised in that said heat treating step is carried out at a temperature of about 850°C for ten minutes. A method as claimed in any one of Claims 6 to 9 characterised in that the step of cutting said glaze layer (12) includes the step of cutting a groove (11a) in said substrate (11) at a surface portion of said substrate (11) adjacent said cut edge. A method as claimed in Claim 9 characterised in that said groove (11a) is cut to have a width equal to about 0.2 mm. A method as claimed in Claims 9 or 10 characterised by the step of cutting said protective film (16) and said substrate (11) at a position in which said protective film (16) is deposited directly on said substrate (11) in said groove (11a). A printer using the thermal printing head as claimed in any one of Claims 1 to 5.

Inventors: TATSUMI YUTAKA, TATSUMI, YUTAKA
Applicants: ROHM CO LTD, ROHM CO., LTD.
Application year: 1992
Main classification: B41J2
IPC codes: B41J2