Patent Publication Number: US-4549189-A

Title: Thermal printing head

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a thick film type thermal printing head which is used for a thermal printer. 
     FIG. 1 is a plan view of a typical conventional thick film type thermal printing head and FIG. 2 is a sectional view as seen along line A--A in FIG. 1. In this thermal head, electrodes 2-1, 2-2, 2-3, . . . are arranged in a zigzag manner on a substrate 1. A heating resistor is formed to connect the electrodes 2-1, 2-2, 2-3, . . . . Further, the electrodes 2-1, 2-2, 2-3, . . . and the heating resistor 3 are covered with a protecting layer 4. The substrate 1 is formed of alumina, the electrodes 2-1, 2-2, 2-3, . . . are formed of gold and the heating resistor 3 are formed of ruthenium. The sub scanning direction (i.e., feeding direction of a heat-sensitive sheet) is designated by an arrow B in FIG. 1. 
     The printing of image on a heat-sensitive sheet by the thermal printing head is performed as follows: The heat-sensitive sheet is provided to contact with the protecting layer 4 on the heating resistor 3, and is scanned in the direction designated by the arrow B. Then, electric current in the form of pulse is applied through the electrodes 2-1, 2-2, 2-3, . . . to a desired portion (e.g., a heating portion 3-1) of the heating resistor 3, and the corresponding portion of the heat-sensitive sheet is colored by Joule heat which is produced in the above portion of the heating resistor 3, whereby the image is reproduced on the heat-sensitive sheet as desired. 
     FIG. 3 shows a temperature distribution on the surface of the heating portion 3-1 when the above portion of the resistor 3 is energized by the electric current through the electrodes 2-1 and 2-2 in the conventional thermal printing head. As evident from FIG. 3, the temperature distribution on the surface of the heating portion 3-1 is of cone shape with its peak at the center between the electrodes 2-1 and 2-2. Such temperature distribution is formed because of the heat dissipating effect through the electrodes 2-1 and 2-2. 
     To color a heat-sensitive sheet, it is generally required that a thermal printing head has a predetermined effective coloring length. This effective coloring length is shown as L in FIG. 3, and depends upon the lowest necessary temperature T which is necessary to generate a color on the heat-sensitive sheet. However, the temperature over the temperature T is not necessary for the coloring, causing the loss of electric power. The amount of this loss is shown by the shaded part P which is surrounded by the curve of the temperature distribution and the line of the lowest necessary temperature T in FIG. 3. Since the temperature distribution curve is cone shaped, the area of the shaded part P, i.e., the amount of loss of the electric power becomes large, with the result that large electric power is required to obtain necessary effective coloring length L, thereby deteriorating the efficiency of the conventional thermal printing head. 
     Further, because of the cone shape in the temperature distribution, the maximum temperature Tmax in the heating portion is high. This generates large thermal stress in the heating portion of the heating resistor 3, which causes the heating resistor 3 to be cracked, thereby shortening the lifetime of the thermal printing head. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of this invention is to provide a thermal printing head which can eliminate the aforementioned drawbacks and disadvantages of the conventional thermal printing head and by flattening the temperature distribution of a heating portion thereof, thereby achieving high efficiency and long lifetime. 
     According to this invention, a heating resistor is divided into a plurality of resistor segments. With this structure, each of the heating resistor segments generates peak temperature at the center of the heating portion, and the separating portion between the segments are heated almost to the peak heating temperature of the two heating resistor segments. In this manner, the temperature distribution on the surface of the heating portion becomes substantially a flat shape with the maximum temperature being suppressed, and the loss of electric power is accordingly reduced, whereby the efficiency and the lifetime of the thermal head can be enhanced. 
     An embodiment of the invention will now be described in detail with reference to the attached drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
     FIGS. 1 and 2 are plan and sectional views of a conventional representative thick film type thermal printing head; 
     FIG. 3 illustrates the temperature distribution of the heating portion of a conventional thermal printing head; 
     FIGS. 4 and 5 are plan and sectional views of an embodiment of a thermal printing head according to the present invention; 
     FIG. 6 show the temperature distribution of the heating portion of the thermal printing head shown in FIG. 5; 
     FIG. 7 is a sectional view of a screen for forming a heating resistor of the thermal head in FIG. 5; and 
     FIG. 8 is a sectional view of another embodiment of the thermal printing head according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIGS. 4 and 5 wherein same reference numerals in FIGS. 1 and 2 are used to designate equal or equivalent parts to those in FIGS. 1 and 2, the thermal printing head of this embodiment of the present invention is constructed so that the heating resistor 3 is divided into two resistor segments 3a and 3b. 
     In this structure of the thermal printing head, each of the resistor segments 3a and 3b has a peak temperature in the heating portion, with the result that the temperature distribution on the surface of the heating portion substantially becomes a trapezoidal shape as shown in FIG. 6. In this manner, an area of the shaded part P which is surrounded by the curve of the temperature distribution and the line of the lowest necessary temperature T for coloring a heat-sensitive sheet and hence the loss of the electric power is reduced. In other words, less power consumption is necessary to obtain the same effective coloring length L. Further, the maximum temperature Tmax of the heating portion becomes low, and thermal stress which is produced in the heating portion is accordingly reduced, thereby causing the heating portion to be less likely to be cracked. 
     FIG. 7 shows a screen mask for forming heating resistor segments 3a and 3b in FIG. 6 which are separated from each other. A screen mask as shown in FIG. 7 is formed by coating emulsions 6-1, 6-2 and 6-3 on a stainless steel mesh 5. The heating resistor segments 3a and 3b are formed by the screen mask by means of a screen printing machine (not shown) using a screen printing technique. The emulsion 6-2 is used for separating the heating resistor into two segments. 
     In this embodiment of the thermal printing head, the temperature distribution on the surface of the heating portion varies depending upon the material and the thickness of the film of the heating resistor as well as the width w of the emulsion 6-2 shown in FIG. 7. The temperature distribution may be formed in a trapezoidal shape in which the center is slightly recessed as shown in FIG. 6 by suitably selecting the above described factors. 
     FIG. 8 shows another embodiment of the present invention. In this embodiment, an intermediate layer 7 which is formed of a glass material is arranged on the intermediate portion between the separated heating resistor segments 3a and 3b. In this embodiment, a protecting layer which covers the electrodes 2 and the heating resistor segments is not employed. In other words, the layer 7 operates as a wear resistant protecting layer. In the structural advantages of this embodiment, a heat-sensitive sheet is contacted directly with the heating resistor segments 3a, 3b since no protecting layer is provided, with the result that the thermal efficiency of the thermal head can be further improved. 
     According to the present invention as described above, the heating resistor is divided into two segments, whereby the temperature distribution curve of the heating portion in the thermal printing head is substantially flat. Accordingly, the loss of electric power can be reduced, and the maximum temperature can be decreased, thereby causing the heating portion not to create large thermal stress and therefore lengthening the lifetime of the thermal printing head.