JPS6354261A - Thermal head - Google Patents

Abstract

PURPOSE:To facilitate control of a heat generating area through varying applied energy and facilitate representing multiple gradations through multiple-valued recording, by setting the width of an electrode at the point of connection with a heat generating element to be not more than the effective recording width of the element, in a thermal head comprising a heat generating element array. CONSTITUTION:An electrode 2 is split into parts 2a and 2b in the vicinity of the point of connection with a heat generating element 1, with an intermediate part 2c between the parts 2a and 2b cut out. As a result, the width of the electrode 2 at the connection points 2a, 2b for the heat generating element 1 is smaller than the width of the element 1. Since the electrodes 2 are connected to both ends of the heat generating element 1, paths of electric currents in the element 1 are as indicated by arrows 3. Thus, some currents flow in diagonal directions in the element 1. Accordingly, the area 81 over which an ink is melted is varied as the duration of a pulse voltage impressed is gradually increased, whereby intermediate gradations can be represented.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a thermal head capable of expressing halftones by multilevel recording in, for example, thermal transfer or heat-sensitive recording devices.

[Prior Art] There are many recording methods conventionally used in color printers, including an inkjet method, a thermal transfer method, a wire dot method, and the like.

In these systems, binary recording is common in principle, and to express gradation using conventional technology, multiple dots are treated as one unit, and halftones are determined by the on/off ratio of the dots in that unit. A method is used to reproduce halftones in a pseudo manner using an area gradation method such as a dither method.

[Problems to be Solved by the Present Invention] However, when this method is used, the number of dots required per unit increases in order to express many gradations, resulting in a decrease in pixel resolution. For example, in order to obtain an image quality of about 6 pixels/mm with 64 gradations, a recording head resolution of about 48 dots/mm is required. A thermal printer requires a thermal head of 48 dots/mm, but such a high-density thermal head cannot be manufactured with current technology. Even if it could be made, it would be impractical to drive a thermal head with a huge number of pixels, since it would require a very large-scale drive circuit.

In other words, there is a limit to high-quality gradation recording with binary recording, and if we could somehow perform multi-value recording that expresses the size of one dot in multiple gradations, this problem could be solved. can.

In recording by heat-melting transfer using a thermal head, which is commonly used in the past, density changes due to changes in applied energy are as shown in Figure 1 (a), and the recorded density has a large slope and variation with respect to applied energy. Therefore, it was extremely difficult to achieve intermediate concentrations. The reason for this will be explained below. In a conventional thermal head as shown in Fig. 1(b), the heating element 1 and the electrode 2 are shaped to have the same width, so the distribution of the current flowing through the heating element 1 is almost constant everywhere. . For this reason, the temperature distribution of the heating element 1 is such that the middle portion, where the amount of heat dissipated is small, is slightly high. As shown in FIG. 1(c), by changing the application time of the pulse voltage applied to the electrode 2, the temperature distribution of the heating element is changed to 11a, 12a (the application time is 11a<12
Figure 1 (a) shows whether the temperature is higher than the melting point Tm of the thermal transfer ink or not.
), a small difference in applied energy can make a large difference. That is, if the applied energy is slightly smaller than Tm, no ink will be melted and transferred, and conversely, if the applied energy is slightly larger than Tm, most of the ink that comes into contact with the heating element will be melted and transferred.

For this reason, the heat-melting transfer method according to the prior art could practically only perform binary recording. However, if it is possible to reduce the slope of the curve representing the relationship between applied energy and recording density as much as possible, and if a method can be found that can reduce the variation in this relationship, then multilevel recording will be possible even with thermal melt transfer. Become.

[Means for Solving the Problems] In order to achieve the above object, the thermal head of the present invention includes a heating element array including a heating element and an electrode for supplying power to the element, the thermal head comprising: The width of the connection point of the electrode to the heating element is set to be equal to or less than the effective recording width of the heating element.

A further feature is that the shape of the heating element is approximated to a heat distribution curve when large electrical energy is applied.

Further, it is further characterized in that each electrode is configured to be in contact with the heating element at at least two points.

[Operation] In this configuration, the heat distribution of the heating element is controlled by changing the electric energy applied to the electrodes, and halftone expression is performed by multilevel recording.

[Example] Hereinafter, embodiments will be described in detail according to the drawings.

FIG. 2 is a partially enlarged view showing the shape relationship between the heating element and the electrodes of the thermal head according to the present invention. Heat generating element 1
are lined up in a line, and the width of the electrode 2 connected to the heating element 1 is narrower than the width of the heating element 1. FIG. 3 is a conceptual diagram of a recording apparatus using this thermal head. A platen 34 is provided to face the heat generating part of the thermal head 31, and an ink sheet 32 and a recording medium 33 are passed between them as shown in the figure. Recording is performed by moving in the direction of the arrow and applying a pulse voltage to the electrode 2 of the thermal head 31 at this time.

FIG. 4(a) shows how current flows when a pulse voltage is applied to the thermal head 31. Current flows most easily in the shortest distance where the resistance is lowest, so if the current value is below a certain value, most of the current flows within the width of the heating element 1, which corresponds to the width of the electrode 2, and the current flow path 3 generates heat. It is difficult to spread over the entire width of the element 1. Therefore, the temperature distribution of heating element 1 is
As shown in Figure (b), only the temperature of the portion of the heating element 1 corresponding to the width of the electrode 2 becomes particularly high, and the temperature difference with the area outside this range becomes large. By gradually lengthening the application time of the pulse voltage in order to change the applied energy, the temperature distribution changes as shown by temperature distribution curves 42, 43, and 44. At this time, if the melting point of the ink on the ink sheet is Tm, the ink melts in a temperature range higher than Tm and is transferred to the recording medium, so the area of the transferred dots is 42a and 43a in FIG. 4(c). , 44a changes like the temperature distribution region. As a result, the recording density has a slope as shown in FIG. 5 with respect to the applied energy, resulting in a relationship in which variations in density are reduced. Therefore, halftones can be expressed by controlling the time for applying the pulse voltage, that is, the applied energy, and the variation in density is relatively small and stable.

Next, the other side will be explained according to FIG.

The heating element of the embodiment shown in FIG. 6 approximates the heat distribution region 44a and has a rounded element width larger than t8i2.

Since the shape is close to the temperature distribution curve when the large applied energy of this heating element 1 is applied to the electrode 2, the shape of the melted and transferred ink is also almost the same as the shape of this heating element 1, and the concentration is This has the effect that the outline of the dots becomes clearer when high transfer is performed, and density variations are reduced.

FIG. 7 shows yet another embodiment, in which the electrode 2 is connected to the heating element 1.
The structure is such that it is divided into 2a and 2b near the connection point, and the middle 2c between 2a and 2b is cut out. Therefore, =g! 2 is smaller than the width of the heating element at the connection points 2a and 2b connected to the heating element 1. In this embodiment, since the electrodes 2 are connected to both ends of the heating element 1, the current flow path within the heating element is as shown by the arrow 3. For this reason, a certain amount of current also flows in the diagonal direction of the heating element 1, so as the pulse voltage application time is gradually lengthened, as shown in FIG. 8(a).
The melting area 81 of the ink changes as shown in ~(d), thereby making it possible to express halftones.

Since the heating elements and electrodes of the thermal head of this embodiment have simple shapes, the manufacturing process of the head is simple and low cost. Further, since it becomes possible to easily express halftones by multilevel recording, it becomes possible to perform gradation recording without sacrificing resolution, and it is possible to provide a recording device with high image quality and low cost.

[Effects of the Invention] As explained above, according to the present invention, by making the width of the electrode narrower than that of the heating element, the heating area can be easily controlled by changing the applied energy. It is possible to provide a thermal head that can easily realize multi-gradation expression.

[Brief explanation of drawings]

Fig. 1 (a) is a diagram showing the relationship between applied energy and recording density of a conventional thermal head, Fig. 1 (b) and (c) are diagrams showing the temperature distribution of a conventional thermal head and a heating element, and Fig. 2 The figure is a partially enlarged front view of the thermal head according to the present invention. Figure 3 is a diagram showing the recording concept by the thermal head according to the present invention. Figures 4 (a), (b), and (C) are the thermal head according to the present invention. The flow path of the current flowing through the heating element, the temperature distribution,
Figure 5 shows the energy applied when halftone recording is performed using the thermal head of the present invention. 6 is a partially enlarged front view of a thermal head according to a second embodiment of the present invention. FIG. 7 is a partially enlarged front view of a thermal head according to a third embodiment of the present invention. FIGS. 8(a) to 8(d) are diagrams showing the relationship between applied energy and ink melting distribution in the third embodiment. In the figure, 1... Heat generating element, 2... Electrode, 3... Current flow path, 31... Thermal head, 42, 43.44
...Temperature distribution curve, 42 a, 43 a, 4
4 a ・"Temperature distribution area. Patent applicant Canon Co., Ltd. Double 1 Koto O Enerki'- (m3) 1st! 11 Brain e4 Chi trace 4 Fig. 4 Fig. 6 (o) (b> 8 Figure 7 (C) (”d) Figure

Claims (3)

[Claims] (1) In a thermal head that includes a heating element array that includes heating elements and electrodes for supplying power to the elements, the width of the connection point of the electrode to the heating element should be less than or equal to the effective recording width of the heating element. Features a thermal head. (2) The thermal head according to claim 1, further characterized in that the shape of the heating element is approximated to a heat distribution curve when large electric energy is applied. (3) The thermal head according to claim 1, further characterized in that each electrode is configured to be in contact with the heating element at at least two points.