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.