Thermal head and controller for controlling the same

Abstract: A thermal head includes a metal substrate; an insulating layer (3) formed on the surface of the metal substrate (1); a plurality of heating elements (14,24) disposed on the surface of the insulating layer (3), the heating elements (14,24) being arranged with a predetermined pitch along a plurality of lines in a main scanning direction, the plurality of lines being spaced from each other in a paper feeding direction perpendicular to the main scanning direction; and a heat radiating element (2) projecting from the surface of the metal substrate (1) to the side where the insulating layer is disposed. In this structure, although most of heat generated by the respective heating elements is transferred to an ink ribbon or print paper, residual partial heat is absorbed by heat radiating means via the insulating layer and radiated into the atmosphere. This suppresses thermal interference among the heating elements. The invention also discloses a thermal head controller for controlling a thermal head for use in a printer.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thermal head capable of printing two lines at the same time using two lines of heating elements, and to a thermal head capable of performing preheating using one of two lines of heating elements while performing printing using the other one of two lines of heating elements thereby achieving a high-speed printing operation. The present invention also relates to a thermal head controller, and more particularly, to a thermal head controller for controlling a thermal head including a preheating heater and a printing heater. 2. Description of the Related Art Fig. 23 illustrates a thermal head disclosed in Japanese Unexamined Patent Application Publication No. 64-58566, wherein Fig. 23A is a top view of the thermal head and Fig. 23B is a cross-sectional view taken along line XXIIIB of Fig. 23A. In Fig. 23, reference numerals 501a and 501b denote ceramic substrates, and reference numeral 517 denotes a common electrode formed of a bulk material. Fig. 24 illustrates a thermal head disclosed in Japanese Unexamined Patent Application Publication No. 10-151784, wherein Fig. 24A is a top view of the thermal head and Fig. 24B is a cross-sectional view taken along line XXIVB of Fig. 24A. In Fig. 24, reference numeral 602 denotes a metal substrate having a projection 603, and reference numerals 608 and 611 denote heating resistors. Fig. 25 illustrates a conventional thermal head, wherein Fig. 25A is a top view of the thermal head and Fig. 25B is a cross-sectional view taken along line XXVB of Fig. 25A. In Fig. 25, reference numeral 701 denotes a substrate formed of single silicon crystal, and reference numeral 707 denotes a common electrode. Reference numeral 702 denotes a through-hole formed in the common electrode 707, wherein the inner surface of the through-hole is plated with a conductive metal 703. Reference numerals 704 and 705 denote heating resistors. Fig. 26 illustrates another conventional thermal head, wherein Fig. 26A is a top view of the thermal head and Fig. 26B is a cross-sectional view taken along line XXVIB of Fig. 26A. In Fig. 26, reference numerals 858, 854, 863 and 864 denote heating resistors, and reference numeral 852 denotes glaze glass. In the case of the thermal head shown in Fig. 23, there is a difference in thermal expansion coefficient between the common electrode 517 and the substrate 501a or 501b, and the difference in thermal expansion coefficient can cause partial removal of the common electrode 517. Thus, performance degradation occurs as the thermal head is used for a long period of time. In the case of the thermal head shown in Fig. 24, the substrate 602 is heated by a common current flowing through the projection 603 which is a part of the substrate 602. As a result, thermal interference occurs between the heating element 608 and the heating element 611. This makes it difficult to control the heating elements 608 and 611 independently of each other. In the case of the thermal head shown in Fig. 25, a complicated process is needed to form the through-hole 702 through the substrate of single silicon crystal. In the case of the thermal head shown in Fig. 26, if the heating resistors 853 and 854 are located very close to the heating resistors 863 and 864, interference due to heat storage in a partial glaze occurs because the heating resistors 853, 854, 863 and 864 are formed on the same partial glaze. The interference can cause the thermal head to become thermally uncontrollable. Although the above problem can be avoided by increasing the distance between two lines of heating resistors, the contact condition between the thermal head and a platen roller (not shown) which urges print paper against the thermal head becomes poor. To improve the contact condition, it is needed to increase the diameter of the platen roller or increase the force applied to the platen roller. In the conventional thermal head, the thermal head heater is continuously energized until a needed intensity of color is obtained each time a line is printed. In this technique, the printed color intensity increases as the temperature (amount of heat) of the thermal head increases. When respective colors of yellow, magenta, and cyan are printed on paper, no color appears during a particular period after turning on the thermal head, wherein the period in which no color appears varies depending upon the color. If, each time a line is printed, the thermal head is energized during a period in which no color appears plus a period needed to obtain a desired intensity of color, a long printing time is needed, that is, the printing speed becomes low. One technique to avoid the above problem is to preheat paper using a preheating heater (preheater) to a temperature which is very close to but lower than a minimum temperature needed to develop a color. In this technique, a printing thermal head heater is used to further heat the paper to develop a color. Thus, color is developed with no delay and thus the problem of the reduction in the printing speed is avoided. Although most of the heat generated by the preheater is consumed to preheat print paper, the heat is partially accumulated in the preheater and parts in the vicinity of the preheater. As a result, when the same amount of heat is generated by the thermal head heater over the entire surface of paper, the printed color intensity is low at a line (first line) at which the printing is started and the printed color intensity increases as the printing operation advances toward a final line as shown in Fig. 27. That is, nonuniformity in printed color density occurs. Furthermore, the preheating can cause a color to be developed in a white-data area in which any color should not appear. In the case where the intensity specified by print data varies across print paper, the preheating can cause a deviation in color intensity from the specified intensity. In the case where printing is performed using both a printing thermal head and a preheating heater, if the printing and the preheating are performed at the same time, a high-capacity power supply capable of supplying a high current with a high voltage is needed to drive the thermal head, and a complicated configuration is needed. WO 9 958 340 discloses a thermal head comprising a substrate, insulating layers having raised portions, and exothermic resistors formed on said raised portions. Common electrodes are provided on the substrate, which protrude from the surface of the substrate, penetrate through the raised portions and are connected to use exothermic resistors. SUMMARY OF THE INVENTION In view of the above, it is an object of the present invention to provide a thermal head which is formed of a material which does not cause removal, which can be produced without needing complicated processing, and which has less thermal interference. According to an aspect of the present invention, there is provided a thermal head comprising: a metal substrate; an insulating layer formed on the surface of the metal substrate; a plurality of heating elements disposed on the surface of the insulating layer, the heating elements being arranged with a predetermined pitch along a plurality of lines in a main scanning direction, the plurality of lines being spaced from each other in a paper feeding direction perpendicular to the main scanning direction; and a heat radiating element projecting from the surface of the metal substrate to the side where the insulating layer is disposed. Wherein current does not flow through said heat radiating element to said heating elements. Note that the heat radiating element does not include a member serving as a path for supplying a current to the heating elements. In this structure, although most of heat generated by the respective heating elements is transferred to an ink ribbon or print paper, residual partial heat is absorbed by heat radiating means via the insulating layer and radiated into the atmosphere. This suppresses thermal interference among the heating elements. In this thermal head according to the present invention, a part, in contact with one line of the heating elements, of the insulating layer and a part, in contact with a directly adjacent line of the heating elements, of the insulating layer may be separated from each other by the heat radiating element. This further suppresses thermal interference among the heating elements. In this thermal head according to the present invention, preferably, the heat radiating element is disposed at least in a part of a region between the metal substrate and a gap between one line of the heating elements and an adjacent line of the heating elements, and a part, in contact with one line of the heating elements, of the insulating layer and a part, in contact with a directly adjacent line of the heating elements, of the insulating layer are connected to each other in a region in contact with the gap so that heat can be conducted therebetween. In this structure, it is possible to prevent print paper preheated by one of two lines of the heating elements from being cooled when it passes over the intermediate part between the two lines of the heating elements. In this thermal head according to the present invention, the heat radiating element may be formed integrally with the metal substrate. This structure allows heat absorbed by the heat radiating element to be transferred more easily into the substrate and radiated. As a result, the effective radiating area increases, and thus a greater amount of heat is radiated into the atmosphere. In the thermal head according to the present invention, portions, in contact with the heating elements, of the insulating layer may protrude in a direction toward the heating elements. This structure ensures that heat is transferred to print paper in a more reliable fashion. In the thermal head according to the present invention, the heating elements may be disposed such that the location, in the main scanning direction, of each heating element is coincident with the location of one of heating elements arranged in an adjacent line. In this structure, it is possible to simultaneously generate heat in two lines of heating elements, and thus an increase in the printing speed is achieved. In the thermal head according to the present invention, the heating elements may be disposed such that the location, in the main scanning direction, of each heating element is shifted by 1/2 pitch relative to the location of one of heating elements arranged in an adjacent line. In this structure, a greater dot density can be achieved, and thus higher-precision printing becomes possible. In the thermal head according to the present invention, the metal substrate may include a fin formed on a side opposite to the side on which the insulating layer is formed. In this structure, a greater heat radiating area is provided to radiate a greater amount of heat into the atmosphere. In the thermal head according to the present invention, two conductor patterns for supplying a current to each heating element to generate heat are connected to each heating element, on the side opposite to the insulating layer. Fig. 1 is a block diagram illustrating the structure of a thermal head according to an embodiment of the present invention; Fig. 2 is a block diagram illustrating the structure of a thermal head according to another embodiment of the present invention; Fig. 3 is a block diagram illustrating the structure of a thermal head according to still another embodiment of the present invention; Fig. 4 is a block diagram illustrating the structure of a thermal head according to still another embodiment of the present invention; Fig. 5 is a block diagram illustrating the structure of a thermal head according to the first embodiment of the present invention; Fig. 6 illustrates thermal head heaters and associated circuit components according to the first embodiment of the present invention; Fig. 10 illustrates a preheater heat storage control table provided in the preset value setting/correcting circuit according to the first embodiment of the present invention; Fig. 15 illustrates the relationship between the printed color intensity and the energizing time of the thermal head according to the first embodiment of the present invention; Fig. 16 illustrates a preheater heat storage control table provided in the preset value setting/correcting circuit according to the second embodiment of the present invention; Fig. 22 illustrates thermal head heaters and associated components according to the third embodiment of the present invention; Fig. 23 is a block diagram illustrating a structure of a conventional thermal head; Fig. 24 is a block diagram illustrating a structure of a conventional thermal head; Fig. 25 is a block diagram illustrating a structure of a conventional thermal head; Fig. 26 is a block diagram illustrating a structure of a conventional thermal head; and Fig. 27 is a graph illustrating the relationship between the printing line and the intensity of a color printed by a thermal head according to a conventional technique. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 illustrates the structure of a thermal head according to a first embodiment of the present invention, wherein Figs. 1A and 1B are a cross-sectional view and a top view thereof, respectively. This thermal head has a structure symmetrical about a center line QQ'. In Fig. 1, reference numeral 1 denotes a stainless steel substrate having a substrate projection 2 and a radiating fin (not shown) for radiating heat generated by heating elements 14 and 24. The substrate projection 2 is formed integrally with the stainless steel substrate 1 so that heat is transferred from a glaze glass layer formed directly on the stainless steel substrate 1 and the substrate projection 2 to the radiating fin (not shown) via the substrate projection 2 and the stainless steel substrate 1. The glaze glass 3 is an insulating element serving to absorb heat remaining in the heating elements 14 and 24 and transfer the absorbed heat to the stainless steel substrate 1. The glaze glass 3 is formed on the substrate 1, for example, by coating glass paste on the substrate 1 and then baking it. In the example shown in Fig. 1, a part of the glaze glass on the side of the heating element 14 and a part on the side of the heating element 24 are connected to each other via a connection part 3a so that heat can travel between them. Reference numeral 14 denotes a heating element consisting of a pair of heater segments 13a and 13b forming one dot. Reference numeral 15 denotes an intermediate electrode connected to the heater segments 13a and 13b. Reference numeral 16 denotes a common electrode connected to a conductor pattern 17b of the heating element 14a and also to a power source (not shown). Reference numeral 17a denotes a conductor pattern connected to the heater segment 13a of the heating element 14 and also to a bonding wire 18. Reference numeral 17b denotes a conductor pattern connected to the heater segment 13b of the heating element 14 and also to the common electrode 16. Reference numeral 19 denotes a control IC which is connected to the conductor pattern 17a via the bonding wire 18. The control IC 19 is connected to the power supply (not shown) and serves to control an on-off operation of the heating element 14 in accordance with a printer control signal. Reference numeral 24 denotes a heating element consisting of heater segments 23a and 23b formed on the glaze glass 3, for preheating print paper. In the case where the heating element 24 is used for preheating, the amount of heat generated by the heating element 24 is set to be slightly lower than a threshold above which thermal transfer of a subliming dye or thermal development of a color occurs. Reference numeral 29 denotes an insulating layer. Reference numerals 25 to 28 correspond to reference numerals 15 to 18, respectively. Now, the operation is described below. If print paper is set on a printer including this thermal head, the print paper is fed onto the heating element 24 by a transport mechanism (not shown). When the heating element 24 receives the print paper thereon, the heating element 24 generates heat corresponding to a current supplied from a controller (not shown). In this heating operation, because the amount of heat generated by the heating element 24 is set to a value for preheating, printing on the print paper does not occur. The print paper is then transported to the heating element 14 by the transport mechanism (not shown). If the heating element 14 receives the print paper thereon, the heating element 14 generates-heat corresponding to a current supplied from the controller (not shown). The sum of heat generated by the heating element 14 and heat generated in the preheating process causes thermal transfer of a subliming dye or thermal development of a color to occur and thus a color with a particular intensity is printed on the print paper. Although most of the heat generated by the heating elements 14 and 24 is consumed to thermally transfer a subliming dye or thermally develop a color, residual heat travels to the radiating fin of the substrate 1 via the glaze glass 3 and is radiated into the atmosphere. Herein, a greater amount of heat is absorbed by the substrate projection 2 and thus thermal interference between the heating element 14 and the heating element 24 is suppressed. Because the connection part 3a is disposed between the heating element 14 and the heating element 24, when the print paper preheated by the heating element 14 or the heating element 24 passes by the middle between the heating element 14 and the heating element 24, the print paper is prevented from being brought into direct contact with the connection part 3a and thus heat removal due to thermal conduction via the connection part 3a is suppressed. Furthermore, because the heating elements are grouped into a set of heating elements used for preheating and a set of heating elements used for printing, it is not needed to pass a large current through all heating elements in a short time as is needed when all heating elements are used for printing. This suppresses degradation of the heating elements. Fig. 2 illustrates the structure of a thermal head according to a second embodiment of the present invention, wherein Figs. 2A and 2B are a cross-sectional view and a top view thereof, respectively. This thermal head has a structure symmetrical about a center line QQ'. In Fig. 2, similar parts to those in Fig. 1 are denoted by similar reference numerals, and they are not described in further detail herein. The second embodiment shown in Fig. 2 is different from the first embodiment shown in Fig. 1 in that the glaze glass has protrusions 52a and 62a and the glaze glass is separated into two parts by a substrate projection 42. In Fig. 2, reference numeral 41 denotes a stainless steel substrate including the substrate projection 42 and a radiating fin (not shown) for radiating absorbed heat. The substrate projection 42 receives heat transferred into the glaze glass 52 from the heating element 14 and transfers it to the radiating fin (not shown). In the example shown in Fig. 2, the protrusion 52a of the glaze glass 52 is formed under the heating element 14. As a result of formation of the protrusion 52a, the heating element 24 is protruded upward and thus it is ensured that print paper can come into contact with the heating element 24 in a highly reliable fashion thereby making it possible to apply a precise amount of heat to the print paper. Reference numeral 69 denotes a control IC which is connected to a conductor pattern 18a via a bonding wire 28. The control IC 69 is connected to a power supply (not shown) and serves to control an on-off operation of the heating element 24 in accordance with a printer control signal. Reference numerals 62 and 62a correspond to reference numerals 52 and 52a, respectively. The operation is described below. If print paper is set on a printer including this thermal head, the print paper is fed onto the heating element 24 by a transport mechanism (not shown). If the heating element 24 receives the print paper thereon, the heating element 24 generates heat under the control of the controller (not shown). The amount of heat generated by the heating element 24 in this heating operation is set such that printing on the print paper does not occur. The print paper is then transported to the heating element 14 by the transport mechanism (not shown). If the heating element 14 receives the print paper thereon, the heating element 14 generates heat under the control of the controller (not shown) to print on the print paper. Heat generated by the heating elements 14 and 24 travels to the radiating fin of the substrate 1 via the glaze glass 52 and 62 and is radiated into the atmosphere. Herein, heat flowing between the heating elements 14 and 24 is transferred to the radiating fin via the substrate projection 42. As a result, the amount of heat flowing between the glaze glass 52 and the glaze glass 62 is limited. Thus, thermal interference between the heating elements 14 and 24 is suppressed. In the present embodiment, because there is no glaze glass on the substrate projection 42 and the heat flowing between the glaze glass 52 and the glaze glass 62 is suppressed, the thermal interference between the heating elements 14 and 24 is more effectively prevented than in the first embodiment. Furthermore, the protrusions disposed under the heating elements make the heating elements protrude upward thereby ensuring heat transfer to print paper. In the first and second embodiments, the time needed for the heat generated by the heating elements 14 and 24 to reach the substrate projection 42 is determined by the length of the path from the heating elements 14 and 24 to the substrate projection 42. Therefore, the cooling characteristic of the heating elements 14 and 24 is determined by the length L shown in Fig. 1 or 2. That is, the cooling rate increases with decreasing length L. The length L is usually selected in the range of several µm to several mm. The thermal head according to the present invention may be produced, for example, according to a technique disclosed in Japanese Unexamined Patent Application Publication No. 10-138541. Although in the first and second embodiments, each of the heating elements 14 and 24 includes heater segments 13a and 13b, or 23a and 23b, each heating element may be formed of a heater segment in a C-like shape. Alternatively, each heating element 14, 24 may be formed into a shape such as that shown in Fig. 4A. More specifically, a curved current path such as a shaded portion shown in Fig. 4A is disposed in an area to be heated, and similar heating elements each having such a curved current path are uniformly arranged in a particular area so that the amount of heat generated in respective portions in this area becomes uniform. That is, it is desirable that each single heating element be formed so as to have an electrical path having as small a width as possible and curved in an area to be heated, and a plurality of similar heating elements be uniformly distributed in the particular area. The shapes of the heating element 24 is not necessarily needed to be the same as that of the heating element 14. For example, the heating element 24 may be formed to have a shape represented by shading in Fig. 4B. That is, the heating elements may have an arbitrary shape, although it is needed that two conductor patterns 17a and 17b connected to a heating element should extend in the same direction and two conductor patterns 27a and 27b connected to another heating element should also extend in the same direction. Furthermore, the heating elements are not necessarily needed to be symmetrical about the center line QQ'. In the first and second embodiments, the heating element 24 is used for preheating, the heating element 24 may be used for printing. In this case, the heating elements 14 and 24 may be controlled at the same time such that they generate as much heat as is necessary to develop colors on respective lines with which the respective heating elements are in contact thereby printing two lines at a time. In this case, if paper is fed at the same rate, printing two lines at a time results in an increase in the printing speed by a factor of two. Furthermore, the arrangements of the heating elements on respective sides of the center line QQ' may be shifted from each other by an amount corresponding to one-half the pitch as shown in Fig. 3. In the construction, the dot density (the number of dots per unit length) in the main scanning direction becomes twice that obtained by the thermal head shown in Fig. 1, and thus a high-resolution printer can be realized.

Claims: A thermal head comprising: a metal substrate (1); an insulating layer (3) formed on the surface of said metal substrate; a plurality of heating elements (14, 24) disposed on the surface of said insulating layer, said heating elements (14, 24) being arranged with a predetermined pitch along a plurality of lines in a main scanning direction, said plurality of lines being spaced from each other in a paper feeding direction perpendicular to the main scanning direction; and characterised by a heat radiating element (2) projecting from the surface of said metal substrate (1) to the side where said insulating layer (3) is disposed wherein current does not flow through said heat radiating element to said heating elements. The thermal head according to Claim 1, wherein a part, in contact with one line of said heating elements, (14, 24) of said insulating layer (3) and a part, in contact with a directly adjacent line of said heating elements (4, 24), of said insulating layer (3) are separated from each other by said heat radiating element (2). The thermal head according to Claim 1 or Claim 2, wherein said heat radiating element (2) is disposed at least in a part of a region between said metal substrate (1) and a gap between one line of said heating elements (14, 24) and an adjacent line of said heating elements (14, 24), and wherein a part, in contact with one line of said heating elements (14, 24), of said insulating layer (3) and a part, in contact with a directly adjacent line of said heating elements (14, 24), of said insulating layer (3) are connected to each other in a region in contact with said gap so that heat can be conducted therebetween. The thermal head according to any one of the preceding Claims, wherein said heat radiating element (2) is formed integrally with said metal substrate (1). The thermal head according to any one of the preceding claims, wherein portions, in contact with said heating elements (14, 24), of said insulating layer (3) protrude in a direction toward said heating elements (14, 24). The thermal head according to any one of the preceding claims, wherein said heating elements (14, 24) are disposed such that the location, in the main scanning direction, of each heating element (14, 24) is coincident with the location of one of heating elements (14, 24) arranged in an adjacent line. The thermal head according to any one of the preceding claims, wherein said heating elements (14, 24) are disposed such that the location, in the main scanning direction, of each heating element (14, 24) is shifted by 1/2 pitch relative to the location of one of heating elements (14, 24) arranged in an adjacent line. The thermal head according to any one of the preceding claims, wherein said metal substrate (1) includes a fin formed on a side opposite to the side on which said insulating layer (3) is formed. The thermal head according to any one of the preceding claims, wherein two conductor patterns for supplying a current to each heating element to generate heat are connected to each heating element (14, 24), on the side opposite to said insulating layer (3). The thermal head according to any one of the preceding claims, wherein one of said heating elements (14,24) in one line and one of said heating elements (14,24) in an directly adjacent line of the one line are separated from each other. The thermal head according to any one of the preceding claims, wherein an electrode for supplying a current to each heating element (14,24) to generate heat is insulated from said metal substrate.

Inventors: KAWAMURA SIGEYUKI, KUBOTA TAKASHI, MASUKAWA KAZUNORI, MATSUDA HIROSHI, MIYASHITA YOSIKAZU, SUGIYAMA HAYAMI, KAWAMURA, SIGEYUKI, KUBOTA, TAKASHI, MASUKAWA, KAZUNORI, MATSUDA, HIROSHI, MIYASHITA,YOSIKAZU, SUGIYAMA, HAYAMI, Kawamura, Sigeyuki, c/o Shinko Electric Co., Ltd., Kubota, Takashi, c/o Shinko Electric Co., Ltd., Masukawa, Kazunori, c/o Shinko Electric Co., Ltd., Matsuda, Hiroshi, c/o Shinko Electric Co., Ltd., Miyashita,Yosikazu c/o Shinko Electric Co., Ltd., Sugiyama, Hayami, c/o Shinko Electric Co., Ltd.
Applicants: SHINKO ELECTRIC CO LTD, SHINKO ELECTRIC CO. LTD.
Application year: 2001
Main classification: B41J2
IPC codes: B41J2