JPH0761018A - Thermal printer - Google Patents

Abstract

(57) [Summary] (Correction) [Purpose] To provide a thermal printer having a head tip structure that realizes smooth dot gradation for printing a beautiful image. In the print head, a number of heating elements 6 corresponding to the maximum number of data for one main scanning line of an image are arranged at the same intervals as a printing pitch DOP _{M in the} main scanning direction. The lateral dimension D ₁ of the heating element 6 is 2 / from the printing pitch DOP _{M in the} main scanning direction.
3 small, vertical dimension D ₂ is print pitch DOP in the sub-scanning direction
It is formed to be 2/3 smaller than _S. Both ends of the heating element 6 in the sub-scanning direction are connected to the common electrode 7 and the segment electrode 8, respectively, and applied energy (applied voltage pulse) is selectively supplied by both electrodes to generate heat, and this heat energy is transferred to the thickness of the base film. Is transmitted to the ink ribbon configured to have a size of 5 μm or less, and the complete separation of the ink according to the gradation is realized from the low density gradation stage.

Description

Detailed Description of the Invention

[0001]

[Industrial application] Involved in a thermal printer that prints by ink transfer based on heat control of a heat generating head,
The present invention relates to a thermal printer having a head tip structure that realizes a smooth dot gradation for printing a particularly beautiful image.

[0002]

2. Description of the Related Art Conventionally, a thermal printer heats a large number of heating elements provided at the front end of a print head to selectively color a thermal paper for each pixel D shown in FIG. Is transferred (melt transfer or sublimation transfer) to print an image. The size of the pixel D shown in the figure schematically represents the maximum color development dimension of one print dot.
In addition, the spread d of the transferred ink (hereinafter, the print pixel d
Is changed according to the density gradation. In the figure, the width m of the pixel D matches the printing pitch in the main scanning direction (horizontal direction of the printing operation), and the length n of the pixel D in the sub-scanning direction (vertical direction of the printing operation). It matches the print pitch.
Usually, the printing pitch of a thermal printer is often about 8 dots per 1 mm (millimeter). If the printing pitch of the pixel D shown in the figure is 8 dots / 1 mm in both the main and sub scanning directions, the width and length of the pixel D are both 125 μm.
m (micron).

By the way, the printing pitch m in the main scanning direction is determined by the arrangement interval of the heating elements 1 as shown in FIG. 11 (a). Then, the number of these heating elements 1 is equal to the number of image main scans 1
It corresponds to the maximum number of data lines. The print pitch n in the sub-scanning direction is determined by the paper feed amount (paper feed distance) and the print timing.

Conventionally, the horizontal width m ′ of the heating element 1 is almost equal to the printing pitch m in the main scanning direction (m′≈m), and the vertical length n ′ is the printing pitch n in the sub scanning direction. It is formed so as to be equal or large (n '≧ n). Both ends of the heating element 1 in the sub-scanning direction are respectively connected to a common electrode 2 and a segment electrode 3 having the same width as the lateral width m ′ of the heating element 1, and both electrodes selectively apply applied energy (usually applied voltage pulse). ) Is supplied to generate heat, and heat energy based on this heat is transmitted to the thermal paper or the ink ribbon.

FIG. 12 is a cross-sectional view schematically showing a state in which printing is performed by transferring ink from an ink ribbon using the print head having such a structure. As shown in the figure, a heating element 1 connected to an electrode (not shown) wired on the head substrate 4 is provided at the tip of the head substrate 4 of the print head. The paper P and the ink ribbon r pass on the heating element 1 in a state of being in contact with each other and are then separated, the paper P is conveyed in the upward direction shown by the arrow A in the figure, and the ink ribbon r is shown by the arrow B in the figure. It is discharged downward as indicated by. When passing over the heating element 1, the ink r ₁ on the base film r ₂ is transferred to the surface of the paper P according to the amount of heat transferred from the heating element 1 via the base film r ₂ of the ink ribbon r. The extent of the spread of the transferred ink r ₁ (see FIG.
Pixel density (density of print dots) changes due to (change in size of print pixel d). That is, when viewed in the sub-scanning direction (conveying direction of the paper P and the ink ribbon r) shown in FIG.
The transfer amount of the ink r ₁ per dot onto the paper P changes in the range from zero to the maximum n based on the amount of heat transferred from the heating element 1. The amount of heat transferred from the heating element 1 changes according to the applied energy supplied to the heating element 1. Therefore, if the applied energy supplied to the heating element 1 is controlled according to the density gradation of the pixel, printing with dot gradation (change in dot density due to change in size of print pixel d) can be performed.

As shown in FIG. 11 (b), there is a heating element provided at the tip of the head substrate 4 which is composed of a single heating element 1'extending in the main scanning direction. This is configured such that the common electrodes 2'and the segment electrodes 3'which are thin wirings alternately arranged in the main scanning direction are in contact with the heating element 1 '. In this case, one segment electrode 3'and the two non-contact portions 1'a of the heating element 1'between the two common electrodes 2'on both sides thereof generate heat by the applied energy from both electrodes. . The heat generating portion consisting of two non-contacting portions 1'a between the two electrodes corresponds to the heat generating element 1 of FIG. Also in this case, the dimensions of the horizontal width m'and the vertical length n'of the heat generating portion composed of the two non-contact portions 1'a are the same as those of the heat generating element 1 of FIG. It is substantially equal to or larger than the print pitches m and n in the main and sub-scanning directions.

Conventionally, as described above, the heating portion of the heating element (the heating element 1 in FIG. 11A or the non-contact portion 1'in FIG. 11B).
The shape of a) is configured to be substantially equal to the print pitches m and n in the main and sub-scanning directions of the pixel D, or the heat generating portion is made larger, and the heat of the heat generating portion depends on the magnitude of the applied energy supplied to the heat generating portion. The spread is controlled to perform printing with dot gradation.

[0008]

[Problems to be Solved by the Invention] However, in the past,
Printing was performed by the heat generating portion having the above-mentioned shape, and the following various problems occurred. Hereinafter, these problems will be described by taking up the case of the fusion type thermal transfer system.

First, in the fusion type thermal transfer system, the density gradation is adjusted from 0 density printing (print dot ink spread is 0) to maximum density print (print dot ink spread is maximum, print pixel d = pixel D). The applied energy supplied to the heating element 1 is divided stepwise. This is carried out by changing the applied voltage pulse width or changing the applied voltage per unit time so as to match the experientially obtained ink spreading pattern and density curve.
In this way, gradation control is performed by gradually changing the applied energy supplied according to the number of dot gradations of the printed image.
At this time, the heat distribution of the heating element 1 is as shown in FIGS. 13 (a) to 13 (i).

FIG. 3A shows a state in which the applied energy corresponding to zero density printing is zero, and FIGS. 2B to 2I show applied energy corresponding to the dot gradation from the minimum density to the maximum density. The heat distribution 5 of the heating element 1 is extracted in 8 stages. The figure is a schematic representation. For example, the fact that the heat distribution 5 shown in FIG. 2B does not reach the upper end of the heating element 1 means that the thermal energy does not reach the upper end of the heating element 1. Not that. If there is a heat distribution, the heat energy reaches the upper end of the heating element 1. For example, in the figure (b),
The distribution chart of (c) shows that the heat distribution is low, that is, the heat energy transferred from the heating element 1 to the ink ribbon r is low,
It also shows that the transferred thermal energy is uneven. These irregularities occur due to the fact that the heating element 1 is not composed of a single material and the problem in the production technology for the fine structure of the heating element.

In addition, FIGS. 3D to 3H show that in the heating element 1, heat first spreads laterally and then spreads upward. Further, FIG. 6 (i) shows the heat distribution state at the upper limit of the applied energy. At this time, as will be described later, the dot gradation becomes maximum, and the supply of applied energy is no longer needed.

The transfer state of the ink r ₁ transferred from the ink ribbon r to the paper P based on the heat energy transmitted from the heat generating element 1 according to the heat distribution in the heat generating element 1 is roughly classified into 2 There is, and this is shown in FIG. As shown in the figure, the ink r ₁ on the ink ribbon r forms a layer having a constant thickness. When the thermal energy supplied to the ink ribbon r is small, the ink r ₁ is peeled off from the middle of the layer (hereinafter referred to as ink-like peeling), and becomes peeled ink r ₃ that is transferred to the surface of the paper P. If the thermal energy is not less than a certain level, it is peeled off from the lowermost layer (base film r ₂ surface) (hereinafter referred to as surface peeling), and becomes peeling ink r ₄ and is transferred to the paper P surface.

As described above, the density gradation is expressed not by the density of the ink itself but by the spread of the ink in the pixel D, that is, the size change of the print pixel d. Therefore, the printed ink itself must have no shade and must have maximum density. From this, it can be said that the ink r _{4 that} has been peeled off in a sheet form is in an ideal peeled state because it has the maximum density. On the other hand, the ink r peeled off
_{Since 3} is not the maximum density, it cannot be said that the density gradation can be faithfully expressed.

Conventionally, such states of ink-like peeling and surface peeling are shown in FIG. 15 corresponding to the heat distribution of the heating element 1 shown in FIG. 13, that is, corresponding to changes in applied energy. It occurred in such a state. The three states of the ink peeling inks r _{3-10 to} r _3-30 and the state of the plane peeling ink r _4-10 shown in FIG. 15A are, for example, as shown in FIGS. 13B to _13D .
This corresponds to the heat distribution state of the heating element 1 shown in (e). In FIG. 15A, the ink is peeled off in the ink-like peeling state at the applied energy stage corresponding to the low density gradation, and the ink-like peeling and the surface state at the applied energy stage corresponding to the intermediate density gradation. It shows that the ink is peeled in the state where the peeling is mixed. Then, the five states of the _sheet- like peeling inks r _{4-10 to} r _4-50 shown in FIG.
This corresponds to the heat distribution state of the heating element 1 shown in (i). FIG. 15B shows that at the stage of applied energy corresponding to a density gradation of a certain density or higher, the peeling area is gradually expanded in the state of planar peeling and the ink is peeled off.

FIG. 16 is a density characteristic curve showing the relationship between the applied energy supplied to the heating element 1 and the density (dot gradation) in ink peeling based on such heat spread (heat distribution). As described above, the generation of the thermal energy that causes the ink peeling is based on the heat generated by the applied energy supplied to the entire heating element 1 having substantially the same size as the size of the print pixel D. Therefore, as shown in FIG. On the other hand, the start of ink peeling that causes density (r in Fig. 15 (a))
_3-10 )), the minimum applied energy J (th) is large, and on the other hand, the thermal diffusion to the heat storage material (not shown) around the heating element 1 is small, so the maximum concentration D (m) is reached. The upper limit applied energy J (m) for causing (see r _{4-50 in} FIG. 15B) becomes relatively small. Therefore, the width of change in the applied energy from the minimum applied energy J (th) to the upper limit applied energy J (m) is small, and the characteristic curve α of the concentration D (n) with respect to the applied energy J (n) has a relatively large inclination angle. , The curve is represented as a vertical shape as a whole.

Since the width of change in applied energy from the minimum applied energy J (th) to the upper limit applied energy J (m) is small in this way, a minute applied energy control is necessary to obtain the dot gradation. Also, the concentration characteristic curve α
Therefore, the density variation ΔD (n) increases due to variations in head resistance, variations in applied energy (variations in voltage, pulse width), and variations in curves due to variations in environment such as α ₁ and α ₂ . Therefore, the gradation density becomes unstable, and correction control of these becomes extremely difficult. For these reasons, the dot gradation is generally unstable.

The heat distribution of the heating element 1 in the vicinity of the minimum applied energy J (th) is as shown in FIG. 13 due to the minute resistance value distribution of the heating element 1 (material, thickness, bubbles, impurities, variations). As shown in (b), it becomes non-uniform over the entire area of the heating element 1. Therefore, since the size of the heating element 1 and the size of the pixel D are substantially the same, non-uniform dot printing is performed over the entire area of the pixel D. Not only that, but the same thing occurs for each heating element 1, resulting in non-uniform dot print distribution in the entire image. As described above, in the low density gradation area, the image quality is deteriorated due to the non-uniformity of dot printing.

In addition, as to how the human eyes perceive a change in density, it is easier to feel a change in low density than a change in high density. Therefore, in order to express a beautiful image, it is possible to obtain an image of high print quality by smoothly expressing the low density (finely expressing the density change in the low density area).

However, in the above-described shape of the conventional heating element 1, since the size of the pixel D and the size of the heating element 1 are almost the same, the entire heating element (entire area of the pixel D) generates heat at one time, which is why The minimum applied energy J (th) is large, and heat spread has already occurred at this minimum applied energy J (th), and it is difficult to obtain low density printing (ink peeling in a small range). That is, it is difficult to obtain smooth gradation in the low density region. In order to obtain smooth gradation, theoretically, very small applied energy control may be performed, but since the dot gradation is unstable as described above, it is practically possible to control low density gradation. Is almost impossible.

Further, as shown in FIG. 15 (b), the peeling of the ink is changed from the ink-like peeling to the surface-like peeling in the high density region, whereby the density of the ink itself changes from the unstable state to the maximum density state. Switch. In this way, the tone of printing changes depending on the density, so that the printing quality deteriorates.

Further, in the ink-like peeling, the surface of the adhered ink transferred to the paper P is in an uneven state. On the other hand, in surface peeling, the surface of the deposited ink is flat because it is peeled from the surface of the base film r ₂ . The printed state of the printed surface looks different depending on the two kinds of surface states of the adhered ink. This difference appears between the low-density area and the high-density area as shown in FIGS. 15 (a) and 15 (b), and both different states occur in the intermediate-density area, and the print quality is greatly deteriorated. .

In order to improve the above-mentioned various drawbacks, FIG.
Although the shape of the heat-concentrated heating element 1 ″ as shown in 1 (c) has been proposed, the shapes of the heating element 1 ″ and the electrodes (common electrode 2 ″ and segment electrode 3 ″) having such a curve are However, because of the fine structure, it is difficult to obtain the accuracy as designed, and the resistance variation becomes large, and as a result, stable applied energy corresponding to the dot gradation cannot be supplied. Not only that, but the head price becomes extremely expensive.

In view of the above conventional problems, the present invention provides an inexpensive thermal printer which prints by ink transfer using a heating head and can output a high quality image with smooth dot gradation. With the goal.

[0024]

The structure of the thermal printer according to the present invention will be described below. The present invention has a plurality of heating elements arranged in the main scanning direction, the energization time to the heating elements is changed to change the melting area of the ink on the ink ribbon, and the melted ink is transferred to a sheet. It is premised on a thermal printer for forming a gradation image.

First, the size D of the heating element in the main scanning direction.
₁ has a relationship of D ₁ ≦ DOP _M × (2/3) with respect to the array pitch DOP _{M of} the heating elements in the main scanning direction, and the size D ₂ of the heating elements in the sub-scanning direction is the heating value. Pitch D determined by the paper feed amount of the body in the sub-scanning direction and the print timing
It is configured to have a relationship of D ₂ ≦ DOP _S × (2/3) with respect to OP _S.

Then, for example, as described in claim 2, the thickness of the base film constituting the ink ribbon is 5 μm.
m or less.

[0027]

According to the present invention, the ink on the ink ribbon is melted by the heat generation corresponding to the energization time by the heating element having a shape smaller than the printing pitch in the main and sub-scanning directions by 2/3 or less. As a result, a small range of ink is completely peeled off in accordance with the applied energy when the density is low,
A smooth dot gradation can be obtained.

[0028]

Embodiments of the present invention will be described in detail below with reference to the drawings. FIG. 1A is a partial configuration diagram of a print head according to an embodiment. As shown in the figure, the heating element 6 is provided in the print head in the main scanning direction in the pixel D shown in FIG.
Are arranged at the same intervals as the print pitch DOP _{M in} the main scanning direction. The total number of these heating elements 6 arranged is the same as the maximum number of data for one main scanning line of the image.

The lateral dimension D ₁ of the heating element 6 is formed to be 2/3 smaller than the printing pitch DOP _{M in the} main scanning direction. The vertical dimension D _{2 is} also formed to be ⅔ smaller than the printing pitch DOP _{S in} the sub-scanning direction.

Both ends of the heating element 6 in the sub-scanning direction are respectively connected to a common electrode 7 and a segment electrode 8 having the same width as the lateral width D ₁ of the heating element 6, and the applied energy is selectively applied by both electrodes. In the example, an applied voltage pulse, which will be described later, is supplied to generate heat, and the heat energy due to this heat generation is transmitted to the ink ribbon.

FIG. 6B is a partial configuration diagram of a print head of another embodiment. The print head shown in the figure comprises one heating element 9 extending in the main scanning direction, and a common electrode 10 and a segment electrode 11 that are in contact with the heating element 9 and are alternately arranged in the main scanning direction. Has been done. The two segment non-contact portions 9a of the heating element 9 between one segment electrode 11 and two common electrodes 10 on both sides thereof in FIG. This heating portion functions similarly to the heating element 6 shown in FIG. Further, the horizontal dimension D ₁ and the vertical dimension D ₂ of the heat generating portion composed of the two non-contact portions 9a are the same as the horizontal dimension and the vertical dimension of the heating element 6 of FIG.

FIGS. 2 (a) to 2 (h) correspond to the applied energy supplied to the heating element 6 in accordance with the density gradation from the printing of the density 0 to the printing of the maximum density by the print head having such a structure. 2 schematically shows the state of heat distribution that occurs.
As described above, the heating portion of the heating element 9 shown in FIG. 1 (b) also functions in the same manner. However, in the following description, the heating element 6 shown in FIG.

The above-mentioned FIG. 2 (a) shows a mark corresponding to 0 density printing.
The state of zero applied energy is shown in Fig. 6 (b) to (h).
Applied energy corresponding to the dot gradation from density to maximum density
The heat distribution 13 by the heating element 6 based on Rugie is divided into 7 stages.
It has been extracted. Then, (b) and (c) of Fig.
Width D of heating element 6₁Is the print pitch DOP in the main scanning direction _M
Since it is smaller, inside the heating element 6 at the stage of low density dot gradation
The spread of the unevenness of the heat distribution 13 generated in the
Character pitch DOP_MSmaller than (hence pixel D)
Is shown. In addition, the same drawings (d) and (e) show the heating element 6 as well.
Width dimension D₁Is smaller than the conventional one, so the low-density dot floor
The heat distribution 13 propagates above the heating element 6 at an early stage.
It is shown that. In addition, (f) to (h) of FIG.
At the dot gradation stage of, the heat distribution 13
Via the heat storage material 14 that is disposed in contact with the substrate 12
It shows that it propagates.

FIG. 3 shows a peeled state of the ink which is peeled off in response to such a change in thermal energy propagation. As shown in the figure, the ink-like peeling r _{3-5 to} r _3-15 generated on the ink r ₁ transferred from the ink ribbon r to the paper P at a low density stage is caused by the small width dimension D ₁ of the heating element 6. Corresponding to the above, it occurs only in a narrow range, and does not occur in a range larger than this range. As described above, the heat distribution to the upper side is performed at an early stage, and therefore, the state of the surface peeling r _4-5 is switched to at an early stage at the low concentration stage. After that, as the applied energy increases, the state of the surface peeling gradually expands to the maximum density (maximum area) (the surface peeling ink r in FIG.
_{4-40 to} r _4-50 ).

FIG. 4 is a density characteristic curve showing the relationship between the applied energy supplied to the heating element 6 and the density (dot gradation). As described above, the vertical and horizontal dimensions of the heating element 6 are smaller than the printing pitch in the main and sub-scanning directions, respectively. That is, the volume of heat conduction is smaller than before. For this reason, effective thermal energy that starts the peeling of the ink is generated at the early stage of the change in the applied energy. Therefore, the minimum applied energy J (th) required for this is smaller than in the conventional case as shown in FIG. 16 (see FIG. 16). On the other hand, the thermal energy for causing the maximum peeling propagates through the heat storage material 14 having a relatively large specific heat around the heating element 6. The area of the heat storage material 14 around the heating element 6 is increased as much as the heating element 6 is smaller than the conventional one. For this reason, the upper limit applied energy J (m) required for maximum peeling becomes larger than in the past. Therefore, the range in which the applied energy changes from the minimum applied energy J (th) to the upper limit applied energy J (m) from the density generation to the maximum density D (m) becomes large. In other words, the characteristic curve α of the concentration D (n) with respect to the applied energy J (n) has a small inclination angle, and the curve is expressed as a horizontally long shape as a whole.

As described above, since the range of change in applied energy from the minimum applied energy J (th) to the upper limit applied energy J (m) is large, the amount of change in applied energy for each gradation to obtain the dot gradation is changed. Can be large. Therefore, gradation control is easy. Further, since the concentration characteristic curve α lies horizontally long, the curve variation α ₁ , due to variations in the head resistance value, variations in applied energy (variations in voltage and pulse width), variations due to environmental changes, etc.
The density variation ΔD (n) due to α ₂ is small. Therefore, the gradation density is stable and correction control is easy. From these things, the dot gradation is extremely good.

The unevenness of the heat distribution of the heating element 6 near the minimum applied energy J (th) is within the width of the heating element 6 as shown in FIG. 2 (b). Therefore, the area of the heating element 6 is approximately 4/9 of the area of the pixel D, and
Stage where the above unevenness is extremely low (ultra-low concentration range)
Since it occurs only in the above case, it is visually imperceptible and has no influence on the image quality.

Since the ink peeling is switched from the ink-like peeling state to the planar peeling state even at a low-concentration stage, the low-concentration region is smooth, that is, the density change in the low-concentration region is faithful. Not only can it be expressed in detail, but a uniform printing tone can be obtained at any density gradation. Therefore, a beautiful image with high print quality can be obtained.

Next, FIG. 5 is provided with the above-mentioned print head.
It is a block diagram of the structure of the obtained thermal printer. In the figure
The image data output section K₁Temporary image data
It is a circuit housed in
8-bit wide image data S_DThe gradation data control unit K₂To
Output. Gradation data control unit K₂Pre-programmed
Gradation architecture, for example as shown in FIG.
Based on the density curve table, the image data output unit K₁Or
Image data S input from_DThe grayscale data included in
The image signal after this correction is serially corrected.
Print data S_FourAs serial / parallel converter K_FourTo
Output. Control pulse generator K₃Controls the entire device
Transfer clock S₀Image data output section K
₁To the data reading clock S₁The gradation data control unit
K₂And serial / parallel converter K_FourTo latch black
Click S₂And strobe signal S_FiveSerial / parallel
Converter K_FourTo the motor feed signal S₃The motor
Liver M₁Output to each. Motor driver M₁
Is the motor feed signal S₃By stepping motor M ₂To
To drive. The serial / parallel converter K_FourIs the gradation
Data control unit K₂Serial print data S input from
_FourParallel data S_Four ′ Converted to head driver
L₁Output to. Head driver L₁Is a parallel drive
Print head L by outputting motion signal Dd₂To drive.

FIG. 6 is a schematic diagram showing the main part of the thermal printer centering on the print head L ₂ . As shown in the figure, the print head L ₂ is arranged so as to contact the back surface of the ink ribbon r. The ink ribbon r is held by a pair of ribbon rolls r ₅ and r ₅ ′ and wound up by the ribbon roll r ₅ ′ to the left in the sub-scanning direction of the print image, which is indicated by arrow C in FIG. The paper P comes into contact with the surface of the ink ribbon r, which is the opposite surface with which the print head L ₂ comes into contact, and is conveyed in the left direction, which is also the sub-scanning direction of the print image, as indicated by arrow D in FIG. The platen j is intermittently driven by the stepping motor M ₂ described above in the counterclockwise direction indicated by arrow E in the figure in synchronism with the print timing in the sub-scanning direction, and conveys the paper P. The ink ribbon r is also wound by the ribbon roll r ₅ ′ in synchronization with the intermittent driving of the platen j. Print head L ₂
Is urged by a spring 15 to the lower end of the drawing, which is provided with a heating element, to press the ink ribbon r and the paper P against the platen j surface appropriately.

FIG. 7 shows such a print head L ₂ , FIG.
_{3 is} a circuit block diagram of a head driver L ₁ and a serial / parallel conversion unit K ₄ shown in FIG. As shown in the figure,
The serial / parallel conversion unit K ₄ includes a shift register K
_4-1 , a latch circuit K _4-2 , and a NAND circuit K _4-3 .

Shift register K_4-1To the terminal CLK
Input data read clock S ₁In sync with the terminal
Serial print data S to DIN_FourAre input in sequence. Schiff
Register K_4-1Is the serial print data S_FourSequentially
And O₀~ O_Q-1Converted to parallel data of
Parallel data O₀~ O_Q-1Latch circuit K_4-2Go out
Force

Latch circuit K_4-2Is the shift register K
_4-1Parallel data output from ₀~ O_Q-1A
Clock S₂Latch in synchronization with the input of
Parallel data O₀~ O_Q-1NAND circuit K
_4-3Output to.

In the NAND gates of the NAND circuit K _4-3 , the number of which corresponds to the parallel data O _{0 to} O _Q-1 , the strobe signal S ₅ input to one input terminal is active ("1"). During a certain period, the corresponding signals of the parallel data O _{0 to} O _Q-1 input from the latch circuit K _4-2 to the other input terminal are inverted and output to the head driver L ₁ .

The head driver L ₁ uses the N amplifiers (amplifiers) corresponding to the respective NAND gates to make the NAN.
The inverted data O _{0 to} O _Q-1 input from each NAND gate of the D circuit K _4-3 are amplified and inverted, and output to the print head L ₂ as a parallel print head output Dd.

No. corresponding to the above Q amplifiers of the print head L ₂ . 1, 2, ..., Q-1, Q, and Q heat generating elements 6 generate heat when data "1" (pulse) is input from the corresponding amplifier. The amplifier output is the voltage of the segment electrode 8 or 11 shown in FIGS. 1 (a) and 1 (b), and the input S ₆ to the print head L ₂ shown in FIG. It is the voltage of the common electrode 7 or 10 shown in (b). The output of the amplifier corresponding to the above-mentioned one-time latch clock S ₂ is print data (applied energy) corresponding to one gradation.

Therefore, for example, if the image data consists of 8-bit width gradation data, 128 gradations can be obtained in the density expression. In this case, the serial print data S described above is obtained.
₄ are converted to parallel data O ₀ ~O _Q-1, the latch circuit K _4-2, NAND circuit K _4-3 and the head driver L ₁
A series of processes until output to the print head L ₂ via the process corresponds to 1/128 of the process for one line in the main scanning direction. Then, the serial print data S ₄ is input 128 times to the serial / parallel converter K ₄ for the processing for one line in the main scanning direction.

Next, the print control operation of the thermal printer having such a configuration will be described with reference to the timing chart of FIG. In the figure, (a) is the timing period T,
(b) is the data read clock S ₁ , (c) is the serial print data S ₄ , (d) is the latch clock S ₂ , (e) is the strobe signal S ₅ , (f) is the print head output Dd, and (g) )
Is the motor feed signal S ₃ .

In the timing period T shown in FIG. 9A, the timings of the maximum print period T _W for one line in the main scanning direction and the paper feed period T _M alternate. Within the timing of this maximum printing period T _W , in the first unit printing period t _W , in synchronization with the first 128 clocks S ₁₁ of the data reading clock S ₁ (see FIG. 7B), one gradation The serial print data S ₄₁ (see (c) in the figure) is input to the shift register K _4-1 and is synchronized with the latch clock S ₂ (see (d) in the figure) to the latch circuit K _4-2 . When the strobe signal S ₅ is latched and is active (see (e) in the figure), it is output as the initial print head output Dd for one line through the NAND circuit K _4-3 and the head drive L ₁ ( See Fig. (F). The processing of the unit printing period t _W is 1
It is repeated 28 times and the serial print data S ₄₁ is integrated 128 times, and this integrated data S ₄₂ becomes the print head output Dd of the maximum print period T _W. Serial print data S
_The data ("1") corresponding to the print dot of the lowest gradation (white is not included) of ₄₁ is output only for the first unit printing period t _W , and the data corresponding to the print dot of the highest gradation is all It is output in the unit printing period t _W , that is, 128 unit printing period. The data corresponding to the print dots of the intermediate gradation is continuously output for the number of unit printing periods t _W corresponding to the number of gradations from the first unit printing period t _W. And non-printing (white)
In the case of, "0" is output.

As described above, in the printing process for one line in the main scanning direction, gradation control is performed from the lowest gradation to the highest gradation, and in accordance with this control, as described above, from the low gradation stage to the surface condition. With the ink that is peeled off, the spread of the print pixel d in the pixel D, which is faithful to the gradation, can be obtained.

Thereafter, during the paper feed period T _M , the motor P feed signal S3 conveys the paper P and the ink ribbon r by the pitch DOP _{S in the} sub-scanning direction, and the maximum printing period T _W again.
At the timing of, the printing for the next one line in the main scanning direction is performed. The paper feed period T _M is the stepping motor M ₂
If the feeding operation is fast, it will be short, and if the feeding operation is slow, it will be long. That is, the feeding operation of the stepping motor M ₂ affects the slow speed of the printing process as a whole.

Here, the ink ribbon used with the heating element 6 having the above-described structure will be considered. As shown in FIG. 9A, when the base film of the ink ribbon has a certain thickness or more, the thermal energy spreads laterally in the base film r ₂ , and therefore the thermal conductivity also spreads to the ink r ₁ . (See heat distribution 16, thermodynamic line 17, ink-like peeling r _3-5 , r _{3-10 in} FIG. 3A). For this reason, it becomes difficult to cope with low density gradation, and ink-like peeling easily occurs over a wide range, and gradation becomes unstable. According to an experiment by the inventor, the thickness of the base film of the ink ribbon is 6
When it is at least μm, wide-range ink-like peeling as described above occurs.

In this embodiment, as shown in FIG.
The thickness of the base film of the ink ribbon is set to 5 μm or less in order to achieve the same good ink peeling as shown in FIG. As a result, the spread of the ink faithful to the dot gradation due to the above-mentioned surface separation can be obtained, and extremely high quality image printing can be realized.

In this embodiment, the gradation data of the image is 8
Although the bit width is used, the data width is not limited to 8 bits and can be increased or decreased according to the required printing accuracy. Further, although the printing ink is described as one color, it can be applied to printing with three primary colors corresponding to a color image.

[0055]

As described in detail above, according to the present invention, the horizontal dimension of the heating element of the print head is formed to be 2/3 smaller than the printing pitch in the main scanning direction, and the vertical dimension is printed in the sub scanning direction. Since it is formed 2/3 smaller than the pitch,
The range in which the applied energy changes from the lowest applied energy to the upper limit applied energy is large, and therefore the applied energy for obtaining the dot gradation can be largely changed, and therefore gradation control becomes easy. In addition, since the density characteristic curve is horizontally long, density variations due to curve variations due to variations in head resistance value, variations in applied energy, variations due to environmental changes, etc. are small, and gradation density is stable and correction control becomes easy. Also, since the state of ink-like peeling changes to the state of surface peeling at the low-concentration stage, it is not only possible to express low-concentration areas smoothly (finely change the density in low-concentration areas), but also which density level Even in tone, a uniform print tone can be obtained at all times, so that a beautiful image with high print quality can be obtained. Furthermore, since the thickness of the base film of the ink ribbon is 5 μm or less, the above function can be exerted strongly.

[Brief description of drawings]

1A is a partial configuration diagram of a print head according to an embodiment of the present invention, and FIG. 1B is a partial configuration diagram of a print head according to another embodiment.

2 (a) to 2 (h) are schematic diagrams showing a state of heat distribution corresponding to energy applied to a heating element of the print head of FIG. 1 (a).

FIG. 3 is a diagram showing a peeled state of ink corresponding to a change in thermal energy propagation due to a heating element.

FIG. 4 is a density characteristic curve diagram showing the relationship between applied energy and dot gradation.

FIG. 5 is a configuration block diagram of a thermal printer according to an embodiment.

FIG. 6 is a schematic configuration diagram of a main part centering on a print head of a thermal printer.

FIG. 7 is a circuit block diagram of a print head, a head driver, and a serial / parallel conversion unit.

FIG. 8 is a timing chart illustrating a print control operation by the thermal printer.

9A is a diagram showing an example of ink peeling in the case of a conventional thick base film, and FIG. 9B is a diagram showing an example of ink peeling in the case of a thin base film of 5 μm or less in Example.

FIG. 10 is a diagram illustrating gradation printing of pixels D, a printing pitch in the main scanning direction, and a printing pitch in the sub scanning direction.

11 (a), (b), and (c) are diagrams showing a configuration of a heating element of a conventional print head.

FIG. 12 is a cross-sectional view schematically showing a state in which printing is performed by ink transfer from an ink ribbon by a print head.

13 (a) to (i) are diagrams showing an example of heat distribution corresponding to applied energy by a conventional heating element.

FIG. 14 is a diagram illustrating roughly two types of ink transfer states in which the ink ribbon is transferred to a sheet.

15 (a) and 15 (b) are diagrams for explaining a state of occurrence of conventional ink-like peeling and surface peeling.

FIG. 16 is a density characteristic curve diagram showing a conventional relationship between applied energy and dot gradation.

[Explanation of symbols]

6, 9 heating element D ₁ lateral dimension of heating element (heating part) D ₂ vertical dimension of heating element (heating part) 7, 10 common electrode 8, 11 segment electrode 9a non-contact part 12 substrate 13 heat distribution 14 heat storage material 15 Spring D Pixel d Print Pixel DOP _M Main Scan Direction Print Pitch DOP _S Sub Scan Direction Print Pitch P Paper r Ink Ribbon r ₁ Ink r ₂ Base Film r _3-5 , r _3-10 , r _3-15 Ink _Stripping ink r _4-5 , r _4-40 , r _4-50 Surface stripping ink J (th) Minimum applied energy J (m) Upper limit applied energy J (n) Applied energy ΔD (n) Concentration variation S _D image Data K ₁ Image data output unit K ₂ Gradation data control unit K ₃ Control pulse generation unit K ₄ Serial / parallel conversion unit S ₀ Transfer clock S ₁ Data read clock S ₂ Latch clock S ₃ Motor feed signal S ₄ Serial print data S ₄ ' Parallel data S ₅ strobe signal M ₁ motor driver M ₂ stepping motors L ₁ head driver L ₂ printheads Dd parallel drive signals (print head output) r _5, r ₅ 'ribbon roll j platen K _4-1 shift register K _{4 -2} Latch circuit K _4-3 NAND circuit O _{0 to} O _Q-1 Parallel data T _W Maximum printing period T _M Paper feeding period t _W Unit printing period S ₁₁ 128 clocks S ₄₁ Serial printing data S ₄₂ Integrated data

Claims (2)

[Claims] 1. A plurality of heating elements arranged in the main scanning direction are provided, and the energization time to the heating elements is changed to change the melting area of the ink on the ink ribbon, and the melted ink is printed on a sheet. In a thermal printer that transfers and forms a gradation image, the size D ₁ of the heating elements in the main scanning direction is D ₁ ≦ DOP _M × with respect to the arrangement pitch DOP _M of the heating elements in the main scanning direction.
The size D ₂ of the heating element in the sub-scanning direction has a relationship of (2/3), and D ₂ ≦ DO with respect to the pitch DOP _S determined by the sheet feed amount of the heating element in the sub-scanning direction and the print timing.
A thermal printer having a relationship of P _S × (2/3). 2. The thermal printer according to claim 1, wherein the base film forming the ink ribbon has a thickness of 5 μm or less.