JP2001246771A - Thermal print head - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress variation of printing densities by uniformizing thermal accumulation at a portion in the vicinity of a heating element. SOLUTION: A thin glass layer as a circuit substrate 22 having an insulation property is formed on an upper side of a glass substrate as an insulation substrate 21 to form a two-layer structure. A heating element 13 and a wiring electrode 14 are provided to the top face of the circuit substrate 22 and the top of the heating element is covered with a protection film 15 by glass coating. When the heating element 13 is activated by a print signal using the wiring electrode 14, the heating element 13 heats. The heat is transferred to a printing medium through the protection film 15 and to a heat-accumulating body 23 through the circuit substrate 22. The thermal conductivity of the heat- accumulating body 23 is high so that the heat is quickly transferred to the heat-accumulating body 23 even when each heating element has a different time calorific value, thereby uniformizing a temperature of the whole accumulation body. The temperature influence to each of the heating elements is made uniform and the variation in the printing density is eliminated. The power consumption on the heating element is made half.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thermal print head.

[0002]

2. Description of the Related Art A printer using a thermal print head has a simple mechanism and no noise, and a printer having a large number of heating elements as recording elements can be easily manufactured. Therefore, high-speed printing is made possible and widely used for business use. Further, since the printer can be reduced in size and weight, its use has been expanded to portable use.

FIG. 5 is a sectional view showing an example of the structure of a conventional thermal print head. The thermal print head has a two-layer structure in which glass is glazed on the entire upper surface of a ceramic substrate 11 to form a thermal print head substrate, and a heating element 13 and a wiring electrode 14 are provided on the upper surface of the glass layer 12. . One end of the heating element 13 is connected to the common terminal 14a, and the other end is connected to the individual terminal 14b.

[0004] A DC voltage is supplied to the common terminal 14a. When the individual terminal 14b is connected to the ground potential based on a print signal, a current flows through the heating element 13 to generate Joule heat. The generated heat is transmitted to the print medium through the protective film 15 and printed. Further, the heat remaining in the glass layer 12 is conducted to the ceramic substrate 11, thereby reducing the influence of the heat remaining in the glass layer 12 on the next printing. The heat conducted to the ceramic substrate is further radiated to an external metal plate. As described above, the thermal print head performs the thermal printing while dissipating excess heat near the heating element.

[0005]

In recent years, with the introduction of digital printers, improvement in printing quality has been required. Since a thermal print head of high printing quality has a large number of heating elements at high density, excess heat is accumulated near the heating element as printing continues. At this time, a temperature distribution due to heat storage occurs near the heating element due to a difference in the printing history of each heating element. Since the temperature distribution due to this heat storage affects the heat generation temperature of each heating element, a distribution occurs in the print density on the print medium. For this reason, the print density varies, which causes a decrease in print quality.

As a solution to this, a temperature sensor is provided on a substrate on which a heating element is formed to detect the temperature of the substrate.
A method has been developed to control the voltage applied to the heating element by using a temperature signal.However, since the thermal time constant is as long as several seconds to several milliseconds of the printing cycle, it takes time to correct the variation in printing density. There was a problem that it took.

A method has also been proposed in which the same number of temperature sensors as the number of heating elements are provided in the vicinity of each heating element to detect the heat storage temperature, and the applied voltage is corrected for each heating element. The circuit configuration has become complicated, and it has not been put to practical use because it contradicts the demand for downsizing and cost reduction of the thermal print head. Instead of equipping each heating element with a temperature sensor, a method of correcting the next applied voltage to an appropriate value based on heat storage temperature data calculated with reference to a print history immediately before each heating element has also been proposed. However, accurate correction requires reference to a long-time print history, and data calculation by the CPU is required to perform accurate temperature control, which further complicates the circuit configuration. Absent.

The present invention has been made in view of such a conventional problem, and even in a small-sized thermal print head having a high-density multi-element heating element, the heat storage near the heating element can be made uniform. Accordingly, it is an object of the present invention to provide a thermal print head having good print quality by suppressing variations in print density.

[0009]

In order to solve the above-mentioned problems, the substrate of the thermal print head of the present invention is a thin plate having a two-layer structure composed of an electrically insulating circuit substrate and a low heat conductive substrate. A heating element and a wiring electrode are provided on the upper surface of the circuit substrate of the layer, and a concave portion is provided on a lower heat conductive substrate of the lower layer in a portion which is in contact with the circuit substrate and is located below the heating element. In this case, means for filling the heat storage body is taken. In addition, a temperature sensor is provided close to the heat storage element, and in order to maintain a constant print density by controlling the current flowing through the heat generation element based on the heat storage temperature signal detected by the temperature sensor,
A means for providing a heat control circuit including a basic application time setting device, a room temperature compensator, a heat storage temperature detector, a voltage control oscillator, a printing cycle generator, a drive pulse generator, and a heating element driver is provided.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The thermal print head of the present invention uses a substrate having a low thermal conductivity as a base of a substrate.
The heat generated from the heating element is hardly radiated to the outside of the thermal print head other than the print medium. Furthermore, by providing a heat storage element in contact with the circuit substrate of the low heat conductive base material and in the vicinity of the heating element, heat generated from the heating element, transmitted through the circuit base material, and conducted to the opposite side of the print medium can be reduced. Most will be stored in the thermal storage inside the thermal printhead substrate.

When printing is performed by the thermal print head of the present invention, first, each heating element generates heat in accordance with a print signal. Most of the heat generated by the heat generating element is conducted to the print medium and used for printing, but a part of the generated heat is transmitted to the circuit base material and stored in the heat storage body. Since the heat storage body is made of a material having a high thermal conductivity, heat is transferred to the entire heat storage body in a very short time, so that the entire heat storage body has substantially the same temperature and no temperature distribution occurs. Since the entire heating element and the heat storage element are in a thermally coupled state with the circuit base material in between, the temperature effect from the heat storage element on each heating element is the same, and the print density varies between the heating elements. Acts like no.

Further, by disposing the temperature sensor close to the heat storage element, the temperature of the heat storage element can be accurately measured, and by controlling the energy applied to the heating element according to the temperature of the heat storage element. It functions to appropriately correct the heat generation temperature of the heat generation element and keep the print density constant.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A typical embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a cross-sectional view showing the structure of a thermal print head provided with a thermal storage. In FIG. 1, a glass substrate is used as the low thermal conductive base material 21 and has a two-layer structure in which a thin glass layer is formed as an electrically insulating circuit base material 22 on the upper side of the glass substrate. The heating element 13 and the wiring electrode 14 are disposed on the upper surface of the circuit substrate 22 by a thin film technique or a thick film technique, and the upper part of the heating element is covered with a protective film 15 made of glass coating or the like by the thin film technique or the thick film technique. I have.

The wiring electrode 14 includes a common terminal 14a and an individual terminal 14b. When the common terminal 14a is connected to a power source and the individual terminal 14b is connected to the ground by a print signal, a current flows through the heating element of the heating element 13. Fever. This heat is conducted to the print medium through the protective film 15 to perform printing. Further, heat generated by the heating element is also transmitted to the heat storage element 23 through the circuit substrate 22.

Since the periphery of the heat storage body 23 is a glass substrate (low heat conductive base material 21), the heat dissipation to the outside is smaller than that of a conventional ceramic substrate, and the heat generated by the heating body 13 is transferred to the printing medium. Most of the heat consumed and conducted to the circuit board 22 is stored in the heat storage unit 23. Since the heat storage body 23 is made of a material such as copper having a high thermal conductivity, even if there is a difference in the amount of heat generated by each heating element over time, the heat storage body 23 is quickly conducted to the heat storage body and makes the temperature of the entire heat storage body uniform. Therefore, since the temperature distribution of the entire heat storage body is uniform, the temperature influence on each heating element is also uniform, and the print density does not vary.

FIG. 2 is a cross-sectional view showing the structure of a thermal print head provided with a temperature sensor in contact with a heat storage element.
In FIG. 2, a temperature sensor 31 is provided on the surface of the heat storage unit 23. When the thermal printer operates, the heat storage temperature of the heat storage unit is always measured, and the current flowing through the heating element is controlled with reference to the temperature signal. Thus, the heating temperature of each heating element can be kept constant, and the printing density can be optimized.

FIG. 3 is a block diagram showing an example of a heat control circuit for appropriately controlling the electric energy applied to the heating element based on the heat storage temperature signal from the temperature sensor 31.
In FIG. 3, an output voltage tb from a basic application time setting unit 42 for setting a basic printing time for each heating element.
The output voltage to = t obtained by adding the storage temperature voltage ta output from the storage temperature detecting unit 43 via the temperature sensor 31 and the room temperature voltage tr from the room temperature correcting unit 49 by the adder 44 to = t = t
The frequency F of the output signal of the voltage controlled oscillator 45 is determined by b + ta + tr. Here, the basic application time setting section 42 sets the current drive time for the heating element at a standard room temperature (for example, 15 ° C.), and the room temperature correction section 49 corrects the current drive time to the optimum current drive time for the actual room temperature. Is what you do. The output signal of the voltage control oscillator 45 is input to the application pulse generator 46, and outputs a heating element driving time signal Tc that is inversely proportional to the frequency F for each trigger signal Ti generated by the print cycle generation unit 41. The heating element driving unit 47 is a driver that supplies current to the heating elements only when the heating element driving time signal Tc is at the Lo level, and selects each of the heating elements according to the print data signal 48 to drive the current. A heating element driving signal Tp corresponding to the print data signal 48 is output from the heating element driving section 47 to each heating element.

FIG. 4 is a time chart showing a waveform of an electric signal of each block in FIG. In FIG. 4, the temperature of the heat storage unit 23 is room temperature before printing, but rises by printing, and the heat storage temperature voltage ta and the output voltage to = t = t of the adder 44.
b + ta + tr increases with time. The output voltage to of the adder changes the frequency F of the rectangular wave output of the voltage controlled oscillator 45 as the temperature of the heat accumulator rises. The drive time signal Tc is output in synchronization with the trigger pulse Ti from the print cycle generation unit 41. While the heating element drive time signal Tc is at the Lo level, a current flows through the heating element, and the heating element generates heat and prints. As the printing proceeds, the heat storage temperature of the heat storage body 23 gradually increases, and the heat storage temperature voltage ta from the temperature sensor.
Since the output voltage to of the adder 44 also increases, the output frequency F of the voltage controlled oscillator increases. Then, the pulse width of the output Tc of the application pulse generator 46 becomes smaller and the amount of heat generated by the heating element decreases, but the amount of heat stored in the heat storage element is added to the heating element, so that a constant amount of printing heat is maintained. Since the energy obtained by subtracting the amount of heat stored in this manner is controlled so as to be applied to the heating element, appropriate energy is always applied to the heating element.

Each signal waveform in FIG. 4 shows a case in which printing is continuously performed from the start of printing. However, when printing is stopped halfway, the heat storage temperature decreases, and the heat storage temperature voltage ta from the temperature sensor and the output voltage of the adder 44 are output. Because to also descends,
The output frequency F of the voltage controlled oscillator will decrease. Then, the pulse width of the output Tc of the application pulse generator 46 is increased, the current driving time for the heating element is increased, the heat generated by the heating element is increased, and control is performed so as to maintain a constant printing heat. In the thermal print head of the present invention, since the temperature distribution of the entire heat storage body is substantially uniform, and the temperature influence from the heat storage body to each heating element is also substantially uniform, the temperature sensor and the heat control circuit are realized by one circuit. It is possible.

[0020]

Since the present invention is configured as described above, it has the following effects.

According to the thermal print head of the present invention, there is no difference in the heat storage temperature due to the printing history of each heating element, and the temperature distribution near the heating element can be made uniform.
Variations in print density can be eliminated, and high print quality can be obtained.

Further, the structure in which the periphery of the heat storage body is covered with glass reduces heat dissipation, so that power consumption can be reduced to half or less as compared with a conventional thermal print head.

Further, by providing a temperature sensor and measuring the temperature of the heat storage body, the printing density can be always made uniform. Further, since the heat generation of each heating element can be realized by one temperature sensor, simplification, miniaturization and cost reduction of the print head can be realized.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view illustrating a structure of a thermal print head including a heat storage element.

FIG. 2 is a cross-sectional view illustrating a structure of a thermal print head including a heat storage body and a temperature sensor.

FIG. 3 is a block diagram illustrating an example of a heat control circuit.

FIG. 4 is a time chart showing a waveform of an electric signal of each block in FIG. 3;

FIG. 5 is a cross-sectional view showing a structure of a conventional thermal print head.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 ... Ceramic substrate 12 ... Glass layer 13 ... Heating body 14 ... Wiring electrode 14a ... Common terminal 14b ... Individual terminal 15 ... Protective film 21 ... Low heat conductive base material (glass substrate) 22 ... Circuit base material 23 ... Heat storage body 31 ... Temperature Sensor 40: Thermal control circuit 41: Printing cycle generating unit 42: Basic application time setting unit 43: Heat storage temperature detecting unit 44: Adder 45: Voltage controlled oscillator 46: Applied pulse generator 47: Heating element driving unit 48: Print data Signal 49: room temperature correction unit

Claims (3)

[Claims] 1. In a two-layer thin-film thermal printhead substrate comprising a low thermal conductive base material (21) and an electrically insulating circuit base material (22), a plurality of heat sources are formed on the surface of the circuit base material (22). A heating element (13) composed of an element and a wiring electrode (1)
4) and a heat storage body (23) made of a high heat conductive material is provided inside the low heat conductive base material (21), and the heat storage body (23) is provided with a heating element (13) of the circuit base material (22). A thermal printhead, wherein the thermal printhead is disposed in close proximity to a part that has been set. 2. A temperature sensor (31) is disposed adjacent to the heat storage element (23), and the heat generation element (13) is controlled to keep the printing density constant by a heat storage temperature signal from the temperature sensor (31). The thermal print head according to claim 1, further comprising a heat control circuit (40) for controlling a current flowing through each heating element. 3. The heat control circuit (40) comprises a basic application time setting section (42), a heat storage temperature detection section (43), a room temperature correction section (49), a voltage controlled oscillator (45), and a print cycle generation section (41). ), An application pulse generator (46) and a heating element driving section (47), and a basic application time setting section (4).
2) The output frequency of the voltage controlled oscillator (45) is controlled by the output voltages from the heat storage temperature detector (43) and the room temperature corrector (49), and the applied pulse generator (46) is a print cycle generator (46). A single pulse having a time length inversely proportional to the output frequency of the voltage controlled oscillator (45) is output as a heating element driving time signal in synchronization with the trigger pulse from 41), and
3. The thermal printhead according to claim 2, wherein the heating element drive section (47) selects each of the heating elements according to the heating element drive time signal and the print data signal (48) to drive current.