JPH0671922A - Thermal print head - Google Patents

Abstract

PURPOSE:To prevent the decrease of the recording speed and eliminate the density irregularity of the recording result by adjusting the electric resistance value of each heat generating element so that the electricity supplied via a common electrode is equal for all the heat generating elements. CONSTITUTION:A heat storing layer 21 of SiO2 glass or the like is formed on arm electrically insulating substrate 20 formed of alumina ceramics or the like. Moreover, a heat generating resistor layer 22 of tantalum nitride or the like is formed by sputtering over the heat storing layer 21. After a metallic thin film of Al, Cu or the like is formed on the heat generating resistor layer 22 by sputtering or the similar method, the metallic thin film is selectively removed by photolithography and, a common electrode 12 and an individual electrode line 15 are separated. At the same time, a heat generating resistor element 14 is formed. A protecting layer 23 of silicon nitride, etc., is provided on the surface. The width (d) or length of the heat generating element is increased/decreased by correcting end parts 12a, 12b of the common electrode 12 which are selectively removed, thereby to adjust the electric resistance value.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used in a recording device such as a facsimile machine and a printing device such as a printer.
Regarding a thermal print head.

[0002]

2. Description of the Related Art Conventionally, a facsimile apparatus has been shown in FIG.
The illustrated thermal print head 1 is used for thermal recording. The thermal print head 1 is formed on an electrically insulating substrate having heat resistance such as ceramics. On the surface of this substrate, the common electrode 2 is formed along one side parallel to the main scanning line direction. Common electrode lines 3 and 4 extending in a direction perpendicular to the longitudinal direction of the common electrode are connected to both ends of the common electrode 2, respectively. A plurality of heating elements 6a and 6b are formed on the heater line 5, which is an imaginary line parallel to the longitudinal direction of the common electrode 2. Each heating element 6a, 6b
Has one end commonly connected to the common electrode 2 and the other end connected to the plurality of individual electrode lines 7a and 7b, respectively. The plurality of individual electrode lines 7a, 7b include a plurality of driving ICs 8a,
8b is connected. Each driving IC 8a, 8b includes a driving circuit for driving each heating element 6a, 6b. When the drive circuit is activated, the individual electrode lines 7a and 7b are driven so that current flows from the power source to the heating elements 6a and 6b through the common electrode lines 3 and 4. The driven heating elements 6a and 6b generate heat due to Joule heat, and perform thermal recording using thermal recording paper or thermal transfer ribbon.

[0003]

In the conventional thermal print head 1 as shown in FIG. 8, the individual electrode lines 7a and 7b are formed to have substantially the same length, width and thickness, and have an electric resistance value. Are designed to be approximately equal. Also, the width and thickness of the common electrode 2 are constant,
The resistance value of each heating element 6a, 6b is also designed to be constant. Therefore, due to the voltage drop in the common electrode 2,
A constant power is not supplied from end to end, but the power supplied to the heating element 6b near the center is reduced. In particular, in a printer or the like, unlike a facsimile machine or the like, the printing rate is high, and all the heat generating elements 6a and 6b are often in a state close to heat generation.
Then, the heat generation amount unevenness of the heat generating element 6b on the inner side tends to be conspicuous as the density unevenness of the recording result.

In order to solve such a problem, the following measures have been considered.

The width of the common electrode 2 is increased to reduce the electric resistance and reduce the influence of voltage drop.

Not all the heating elements 6a and 6b are driven at the same time, but the driving current is reduced by the division driving.

The division drive is also used to adjust the position of the printing energy.

The above-mentioned countermeasures cause the following problems and cannot be effective countermeasures. That is, as for the measure 1, the widening of the width of the common electrode 2 causes the thermal print head 1 to be upsized and the manufacturing cost of the thermal print head 1 to be increased. In the divided printing, the time required to print one line becomes long, and the thermal recording cannot be speeded up. The adjustment of the printing energy increases the cost of the control drive IC and the like, and increases the manufacturing cost of the thermal print head.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a thermal print head which prevents an increase in manufacturing cost and a decrease in recording speed and which does not cause uneven density in a recording result.

[0010]

According to the present invention, a plurality of heating elements are linearly arranged, one end of each heating element is connected to a common electrode extending in the arrangement direction, and the other end of each heating element is In the thermal print head connected to the individual electrodes provided for each heating element, each heating element has its electric resistance value adjusted so that the electric power supplied through the common electrode becomes the same for all the heating elements. The thermal print head is characterized by being formed as follows.

According to the present invention, power is supplied from at least one end of the common electrode, and the width of each heating element or the width of each heating element becomes a quadratic function along the arrangement direction of the heating elements. It is characterized in that the length is adjusted.

[0012]

According to the present invention, the thermal print head is provided with a plurality of heating elements arranged linearly. One end of each heating element is commonly connected to a common electrode extending in the arrangement direction. The other end of each heating element is connected to each of the plurality of individual electrodes. Each heating element is formed by adjusting the electric resistance value so that the electric power supplied via the common electrode becomes equal for all the heating elements. Since the electric power supplied via the common electrode is the same for each heating element, the difference in the amount of heat generated for each heating element is small, and uneven density does not occur in the result of thermal recording. Since it is possible to prevent the occurrence of density unevenness simply by adjusting the electric resistance of each heating element, it is possible to prevent a decrease in recording speed and an increase in manufacturing cost.

According to the invention, the width or length of each heating element is adjusted so that the electric resistance value of the heating element becomes a quadratic function along the arrangement direction of the heating elements. As a result, when the common electrode has a constant cross-sectional area, the electric resistance value per unit length is constant, and the electric resistance value of the conductive path connecting the individual electrode and the heating element is also constant, the electrodes are arranged at regular intervals. The same driving power can be supplied to the heat generating elements.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a cross-sectional structure near a heating resistor element 14 of a thermal print head 11 according to an embodiment of the present invention. A heat storage layer 21 made of SiO ₂ glass or the like is formed on an electrically insulating substrate 20 made of alumina ceramic or the like. On the electrically insulating substrate 20 on which the heat storage layer 21 is formed, a heating resistor layer 22 such as tantalum nitride Ta ₃ N ₄ is formed by a sputtering method. A metal thin film such as aluminum Al or copper Cu is formed on the heating resistor layer 22 by a sputtering method or the like. This metal thin film is selectively removed by the photolithography method to separate into the common electrode 12 and the individual electrode line 15, and the heating resistance element 14 is formed. A protective layer 23 made of silicon nitride Si ₃ N ₄ or the like is formed on the surface. It should be noted that the end portion 1 of the common electrode 12 that is selectively removed so that the electric power supplied to the heating element 14 via the common electrode 12 is equal for all the heat generations 14.
By correcting 2a and 12b, the width d of the heating element
Alternatively, the length is increased or decreased to adjust the electric resistance value.

FIG. 2 shows the outer shape of the thermal print head 11 shown in FIG. The thermal print head 11 is
It is composed of a head substrate portion 30 on which the heater line 5 is formed and a printed substrate portion 31 for electrical connection to the recording apparatus. The printed circuit board portion 31 has screw holes 3 for mounting.
2 is also provided.

FIGS. 5 and 6 show changes in characteristic values in the longitudinal direction of the thermal print head 11 using the heating element 14 of the embodiment shown in FIG. 3 shows a change in voltage drop in the common electrode 12 of the head substrate, FIG. 4 shows a change in resistance value of the common electrode 12, and FIG.
6 shows the change in the electrical resistance value of FIG. In FIG. 3, when electric power is supplied from the common electrode lines 3 and 4 on both sides of FIG. 7, the voltage drop due to the common electrode 12 has a parabolic shape in which the voltage drop is maximum in the central portion. this is,
This is because the resistance value of the common electrode lines 3 and 4 is represented by a linear function as shown in FIG. 4, and the value becomes the maximum value in the central portion. Each heating element 14 arranged in a straight line
4 is changed according to a straight line as shown in FIG. 5 which is symmetrical to the straight line shown in FIG. 4, the electric power supplied to the common electrode 12 and the heating element 14 becomes a constant value as shown in FIG. .

FIG. 7 shows a process in which the voltage drop of the common electrode 2 is compensated by adjusting the resistance value of the heating resistance element 27. As shown in FIG. 7, the heater line 5 has a length L, is line-symmetric with respect to the center line passing through the center point A, and the current I is supplied from the common electrode lines 3 and 4 on both sides. Suppose. That is, the current I supplied from the point B is subjected to a voltage drop due to the resistance value R0 in the common electrode lines 3 and 4 and reaches the points C and C at both ends of the heater line 5. The electric resistance value of the common electrode 2 between the points C is formed to have a constant width and thickness so as to be r (Ω / mm) per unit length. Moreover, since it is axisymmetric with respect to the point A, only the portion of the length L / 2 on the left side in FIG. 7 need be considered. The electric resistance value of the common electrode 2 at the position separated from the left side point C by x is represented by the following expression (1).

[0018]

[Equation 1]

Here, R0 is the electric resistance value of the common electrode lines 3 and 4. Assuming that the current flowing through the heating elements 6a and 6b per unit length has a constant value i, the value of the current I (x) flowing through the common electrode 2 at the position of x is given by the following equation (2).

[0020]

[Equation 2]

Since I (x) becomes 0 at the point A in the central portion, I (x) is expressed by the following equation 3 under such boundary conditions.

[0022]

[Equation 3]

Since the common electrode 2 has a resistance value of r per unit length, its voltage drop E (x) is expressed by the following equation (4).

[0024]

[Equation 4]

In order to flow a current of i per unit length from the common electrode 2 whose voltage changes as shown in the equation 4, the equivalent resistance W (x) of the heating element and the individual electrode is given by the following equation 5.
Must be represented by the formula

[0026]

[Equation 5]

Here, a and b are constants. Equivalent resistance W
If (x) changes as a quadratic function as shown in FIG. 5, the current i per unit length becomes constant. In order to make the amount of heat generated by Joule heat uniform with this constant current i, the electric resistance value of the heating element must also be constant. Therefore, W
The change in electric resistance required as (x) is caused by the heating element 6
It is necessary to achieve it by adjusting the electric resistance values of a and 6b.

For example, in the case of adjusting the electric resistance value by the width d of the heating element 14, the central portion 1a of the thermal print head as shown in FIG. 8 generates heat more than the thermal print head end portions 1b and 1c. When the width d of the element 14 is gradually widened and the electric resistance value is adjusted by the length of the heating element 14, the thermal print head central portion 1
In a, the length of the heating element 14 is gradually shortened as compared with the both ends 1b and 1c of the thermal print head.

In this way, the electric resistance of each heating element 14 is supplied to the common electrode 2 and the heating element 14 only by adjusting the electric resistance to a linear function corresponding to the resistance value of the common electrode lines 3 and 4. The power supplied can be kept constant.

[0030]

As described above, according to the present invention, the electric resistance value of the heating element is changed, and the electric power supplied through the common electrode is adjusted to be the same for all the heating elements.
It is possible to prevent uneven density in the thermal recording result. Even if a plurality of heating elements are driven at the same time, density unevenness does not occur, so there is no need for divided driving, and heat-sensitive recording can be performed quickly. The configuration of the drive circuit and the like may be simple, and it is not necessary to make the electric resistance value of the common electrode particularly small. This can prevent an increase in the manufacturing cost of the thermal print head.

Further, according to the present invention, the electric resistance value of the heating elements is formed by adjusting the width or length of each heating element so as to be a quadratic function along the arrangement direction of the heating elements. The electric power supplied via the common electrode can be made equal for all the heating elements, and the occurrence of uneven density in thermal recording can be prevented.

[Brief description of drawings]

FIG. 1 is a sectional view showing a structure in the vicinity of a heating element 14 of a thermal print head 1 according to an embodiment of the present invention.

FIG. 2 is a schematic plan view of a thermal print head according to the embodiment shown in FIG.

FIG. 3 is a graph showing changes in voltage drop in the common electrode 12 of the embodiment shown in FIG.

FIG. 4 is a graph showing changes in electric resistance of common electrode lines 3 and 4 of the thermal print heads 1 and 11 in FIGS. 1 and 7.

5 is a thermal print head 1 of the embodiment shown in FIG.
It is a graph which shows the change of the electric resistance of the heating element 14 of No. 1.

6 is a graph showing changes in the electric power supplied by the heating element 14 and the common electrode 12 of the embodiment shown in FIG.

FIG. 7 is a diagram showing an equivalent resistance for calculating a voltage drop due to a common electrode 2.

FIG. 8 is a plan view showing a schematic electrode arrangement of a conventional thermal print head 1.

[Explanation of symbols]

 1, 11 Thermal Print Head 2, 12 Common Electrode 12a, 12b End of Common Electrode 3,4 Common Electrode Line 5 Heater Line 14, 6a, 6b Heating Element 20 Electrical Insulating Substrate 21 Heat Storage Layer 22 Heating Resistor Layer 23 Protection layer

Claims (2)

[Claims] 1. A plurality of heating elements are linearly arranged and formed, one end of each heating element is connected to a common electrode extending in the arrangement direction, and the other end of each heating element is provided for each heating element. In the thermal print head connected to each of the electrodes, each heating element is formed by adjusting the electric resistance value so that the electric power supplied via the common electrode becomes equal for all the heating elements. Thermal print head. 2. The width or length of each heating element is supplied with electric power from at least one end of the common electrode so that the electric resistance value of the heating element becomes a quadratic function along the arrangement direction of the heating elements. The thermal print head according to claim 1, wherein the thermal print head is formed by being adjusted.