JP2000141725A - Thermal transfer printer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the density irregularity derived from the paper feeding pitch irregularity by forming an image with the relative position of a recording paper with respect to a thermal head at least by specific times in a specific area wherein the pixel density along the main scanning direction changes from a predetermined high density to a predetermined low density in the sub scanning direction. SOLUTION: In a thermal transfer printer 10, an image is formed with a recording paper displaced by at least twice, for example twice by a 1/n of an ordinary conveyance pitch, for example by a 1/2 pitch in a specific area wherein the pixel density along the main scanning direction changes from a predetermined high density to a predetermined low density in the sub scanning direction. In the specific area, high density dots are printed after being resolved into low density dots. Accordingly, drastic change of the total density per one line in the sub scanning direction can be alleviated so that a pitch irregularity can hardly be generated in paper feed. As a result, a gap or a blank part cannot exist among the resolved dots so that generation of the density irregularity can be prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in a thermal transfer printer, and more particularly to a thermal transfer printer in which density unevenness caused by unevenness in the pitch of paper feed is reduced.

[0002]

2. Description of the Related Art A thermal transfer printer includes a thermal head in which a plurality of heating elements are arranged in a main scanning direction, and a platen roller which presses the thermal head via a recording paper and an ink sheet. The recording paper is intermittently moved in a sub-scanning direction orthogonal to the main scanning direction by rotating the platen roller by a predetermined pitch. The ink sheet is heated by the thermal head in accordance with the movement timing of the recording paper, and an image is formed on the recording paper. The ink sheet is sequentially fed out from a supply reel as the recording paper moves, and is taken up by a take-up reel.

As is well known, there are two types of thermal transfer printers: a sublimation type thermal transfer printer and a fusion type thermal transfer printer due to the difference in the transfer system. Type thermal transfer printers are frequently used.

In a conventional general sublimation type thermal transfer printer, the recording paper transport pitch is set to the size of one pixel of an image forming pixel, and the time for one pixel is set to the maximum value of the pulse. The gradation is expressed by the pulse width modulation described above. That is, the gradation control of each dot constituting one pixel is performed by controlling the current application time to each heating element of the thermal head within the time range of one pixel. Since the density of the pixel is proportional to the temperature of the thermal head, if a current is applied to the heating element for a long time, the temperature of the thermal head becomes higher and the density of the formed pixel becomes higher.

[0005]

However, in the conventional sublimation type thermal transfer printer, the recording paper conveyance pitch is set to a constant value at a pitch corresponding to one pixel. In practice, pitch unevenness in paper feed may occur due to vibration of the thermal head and the like. When the paper feed pitch unevenness occurs in this manner, a gap or a blank portion is generated between adjacent pixels in the sub-scanning direction, and there is a problem that fine density unevenness appears in a formed image.

In particular, in a specific area where the pixel density of one line along the main scanning direction rapidly changes from high density to low density in the sub-scanning direction, pitch irregularity in paper feeding easily occurs, and density irregularity tends to appear.

Here, the reason why the pitch irregularity of the paper feed easily occurs in the specific area is as follows. That is, where the printing rate in one line is high and the pixel density in the entire line is high, the load of sticking the ink sheet to the thermal head is large, and the force in the direction that hinders the movement of the recording paper in contact with the ink sheet Is working. Further, the thermal head is pulled by the ink sheet when the recording paper is conveyed. This is because if the pixel density of the entire line suddenly decreases from this state, the friction between the thermal head and the ink sheet becomes small, and the amount of movement of the recording paper slightly increases due to the recoil when the ink sheet comes off. Furthermore, the printing position in the sub-scanning direction shifts due to vibration when the state where the thermal head is pulled by the ink sheet is released.

In order to reduce the density unevenness caused by the pitch unevenness of the paper feed, the transport control of the recording paper is performed by taking into account the change in the state of the ink sheet adhered to the thermal head to improve the transport accuracy. It is possible. However, under the current situation where the frictional resistance between the thermal head and the recording paper varies depending on the type of the ink or the sheet base material, the conveyance control of the recording paper in consideration of the change in the sticking state is extremely complicated. Become.

It is also conceivable to increase the rigidity or assembling accuracy of the thermal head to reduce the vibration or displacement of the head. However, the size of the displacement is several tens of μm, which is smaller than the size corresponding to one pixel. Therefore, it is extremely difficult to reduce the positional deviation with a mechanical configuration.

SUMMARY OF THE INVENTION The present invention has been made to solve the problems associated with the above prior art, and is caused by unevenness in the paper feed pitch while maintaining the recording paper conveyance accuracy and the mechanical structure of the thermal head as they are. An object of the present invention is to provide a thermal transfer printer that can achieve reduction in density unevenness.

[0011]

According to the first aspect of the present invention, an ink sheet is heated by a thermal head in which a plurality of heating elements are arranged in a main scanning direction, so that the ink sheet is printed on recording paper. In a thermal transfer printer for forming an image, driving means for moving the recording paper at a predetermined pitch in the sub-scanning direction relative to the thermal head,
In a specific area where the pixel density along the main scanning direction changes from a predetermined high density to a predetermined low density in the sub-scanning direction, the relative position of the recording paper with respect to the thermal head within the range of the predetermined pitch is determined. Control means for forming an image with the thermal head at least two times shifted by the drive means.

According to a second aspect of the present invention, there is provided a thermal transfer printer for forming an image on a recording sheet by heating an ink sheet by a thermal head having a plurality of heating elements arranged in a main scanning direction. Drive means for moving the recording paper relative to the thermal head at a predetermined pitch in the sub-scanning direction; and drive interval variable means for changing the amount of movement of the recording paper by the drive means within the range of the predetermined pitch. A pixel density detecting means for detecting a pixel density along the main scanning direction, and a specific area where the pixel density detected by the pixel density detecting means changes from a predetermined high density to a predetermined low density in the sub-scanning direction. Determining means for determining whether or not the recording area is the specific area, and when the determining means determines that the area is the specific area, the driving interval varying means determines the size of the recording paper. Slide least twice the relative position with respect to the thermal head, a thermal transfer printer characterized by having a control means for forming an image by the thermal head.

Further, the invention according to claim 3 further comprises a print interval varying means for varying a time for heating each heating element of the thermal head, wherein the control means comprises: When it is determined that the above-mentioned condition is satisfied, an image is formed by changing the heating time by the printing interval varying means.

[0014]

Embodiments of the present invention will be described below with reference to the drawings. Embodiment 1 FIGS. 1A and 1B are a schematic configuration diagram showing a printing section of a thermal transfer printer and a perspective view showing a thermal head.

As shown in FIGS. 1A and 1B, a printing section 11 of a sublimation type thermal transfer printer according to the present embodiment has a line in which a plurality of heating elements 12 are linearly arranged at a predetermined pitch in the main scanning direction. Type thermal head 13 and recording paper 1
4 and the thermal head 13 via the ink sheet 15
And a rotatable platen roller 16 which is pressed against the roller. The platen roller 16 is driven to rotate by a pulse motor M. The recording paper 14 and the ink sheet 15 held between the platen roller 16 and the thermal head 13
It is intermittently moved at a predetermined transport pitch in the sub-scanning direction.
The ink sheet 15 is formed by applying a heat sublimable ink to a sheet base material, is sequentially fed out from a supply reel 17 as the recording paper 14 moves, and is taken up by a take-up reel 18. Reference numeral “19” in FIG. 1A indicates a take-up roller that is driven during non-printing and conveys the ink sheet 15. The take-up roller 19 is free to rotate during printing.

Each of the heating elements 12 is supplied with a current and generates heat according to the gradation of the pixel. The ink sheet 15 is heated by this heat, and the sublimated ink is trapped on the recording paper 14 to form an image for one line. In the sublimation type thermal transfer printer, by changing the amount of energy generated by each heating element 12, the density of the transferred pixel can be changed to express a subtle gradation. The amount of energy generated by the heat generating element 12 is controlled by the power supply time to the heat generating element 12, and if the power supply time is short, the pixel density becomes low,
If the energization time is long, the pixel density becomes high.

When the formation of an image for one line is completed, the platen roller 16 is rotated by one pitch by the pulse motor M to move the recording paper 14 in the sub-scanning direction, and an image for the next one line is formed. By alternately repeating the movement of the recording paper 14 and the heat generation of the heating element 12, an image of one screen is formed on the recording paper 14.

When a color image is formed, an ink sheet is used in which three color ink layers of yellow, magenta and cyan are applied to a sheet substrate in this order, and a yellow image, a magenta image and a cyan image are used. Are transferred in a superimposed manner by a frame sequential method.

FIG. 2 is a block diagram showing a control system of the thermal transfer printer to which the present invention is applied.

The thermal transfer printer 10 has a CPU 20 for controlling each part of the printer, a RAM 21 for developing data, and an EPR storing an operation program.
OM22 and EE which is a rewritable nonvolatile memory for storing density unevenness correction values and other various set values.
A PROM 23, an interface (I / F) 25 for exchanging data (image data, control data, etc.) with an external computer 24, etc., and an interface 2
5, an image buffer memory 26 for storing the image data sent through the
An application energy control unit 27 for applying energy corresponding to the image data to the control unit 2; a lookup table 28 referred to in the control unit 27 for obtaining the applied energy corresponding to the density of the image data; An I / O port 31 for exchanging signals with the components 29 and the sensor group 30, an operation panel 32 having a display unit for displaying a print status and the like and inputting various setting values, and various parts of the printer are required. And a timer 34 for supplying a reference clock signal to the CPU 20. Each of these units is a CPU 2
A control line 35 for transmitting a control signal of 0 and an image data line 36 for carrying image data are connected to each other.
The electromagnetic component 29 includes a pulse motor M for driving the platen roller 16, a motor for driving the take-up roller 19 for cueing the ink sheet 15, and other motors and solenoids. There are a sensor for detecting the position of the recording paper 14, a sensor for detecting the position of the ink layer of the ink sheet 15, and the like.

In this embodiment, the pulse motor M functions as a driving unit. Further, the CPU 20 functions as a control unit, a drive interval variable unit, a pixel density detection unit, a determination unit, and a print interval variable unit.

Next, the operation of the first embodiment will be described.

An outline of the operation of the thermal transfer printer 10 in the first embodiment is as follows. In a specific area where the pixel density along the main scanning direction changes from a predetermined high density to a predetermined low density in the sub-scanning direction, the recording paper 14 is normally used. The image is formed by the thermal head 13 while being moved at a pitch (p / 2) that is half the transport pitch p. In the following description, paper feeding at a preset conveyance pitch p is referred to as “one-step paper feeding”, and paper feeding at a half pitch (p / 2) is referred to as “1 step paper feeding”.
/ 2 step paper feed. "

First, a procedure for determining whether or not a line to be printed is in the specific area will be described. FIG. 3 is a flowchart illustrating a procedure for determining whether or not the area is a specific area.

As shown in FIG. 3, in step S11,
From the image data stored in the image buffer memory 26, the total (ΣA) of the density of each pixel of the current one line to be printed is calculated, and then the density of each pixel of the next one line to be printed is calculated. Is calculated (ΣB). Then, it is determined whether or not the difference between the total density ΣB of the next line and the total density ΣA of the current line is larger than a preset reference value. In the first embodiment, it is determined whether the total density ΣB of the next line is equal to or less than 50% of the total density 現在 A of the current line (S12).

As a result of the determination, when 結果 B / ΣA <0.5, that is, when the pixel density along the main scanning direction changes from a predetermined high density to a predetermined low density of 50% or less in the sub-scanning direction. Is determined to be a specific area from the current line to the next line (S12 "YES").

In this specific area, after the paper is fed by a 1/2 step (S13), the heating for realizing the density of each pixel of the current line to be printed is performed by １ ／ of the current line. Is formed (S1).
4). Specifically, the “加熱 heating” is performed by applying a current to each heating element 12 for half the heating time for realizing the density of each pixel of the current line to be printed. Do. Subsequently, after the paper is fed in a 1/2 step (S15), the heating is performed at the 1/2 and an image for one line of the same line as the current line is formed (S16).

On the other hand, if it is not a specific area, that is, if the density change in the sub-scanning direction is relatively gentle (S12 "N
O ") After one-step paper feeding (S17), a current is applied to each heating element 12 for one line by applying a current to each heating element 12 during a heating time for realizing the density of each pixel of the current line to be printed. An image is formed (S18).

The above steps S11 to S18 are repeated until printing for all lines is completed (S11 to S1).
9) The formation of one image is completed.

The effect of printing while reducing the paper feed pitch in a specific area as described above will be described with reference to FIG. 4 schematically showing the effect.

FIG. 4A shows the originally intended printing state and the state of application of current to the heating element 12, and FIG.
Indicates a printing state in which density unevenness has occurred, and FIG.
3 shows a printing state and a state of applying a current to the heating element 12 according to the first embodiment. 4A to 4C, the dots d1 to d4 are shown in a size corresponding to one pixel, and the amount of one movement of the recording paper 14 is taken as one pitch. Is shown. Each dot d1 to d
The center line 4 is an auxiliary line representing the pitch of the paper feed.
The total density ΔB for one line including the dot d3 is
It is assumed that the total density １A of one line including the dot d2 is 50% or less.

4A and 4C, the vertical axis represents the amount of current applied to the heating element 12 and the horizontal axis represents time. In the diagram of the state where these currents are applied, the recording paper 14
The time interval of moving by one dot, that is, one pitch, is defined as time t, and when a current is applied to the heating element 12 for this time t, the density of the dot is represented as the maximum density. Although the timing of paper feeding is indicated by f1 to f4, the time required for paper feeding is not shown in the figure because the paper feeding time is so short as to be negligible compared to the heating time.

FIG. 4A shows that the thermal head 13 is moved after the recording paper 14 is moved by one pitch at the paper feed timing f1.
Current is applied to the heating element 12 for a time t1 (= t) to form a dot d1, and then the paper feed timing f
After the recording paper 14 is moved by one pitch in step 2, the heating element 12
At time t2 (= t), a current is applied to form a dot d2, and then the recording paper 14 is fed at paper feed timing f3.
Is moved by one pitch, a current is applied to the heating element 12 for a time t3 to form a dot d3. Then, after the recording paper 14 is moved by one pitch at a paper feed timing f4, a current is applied to the heating element 12 for a time t4. Is applied to form a dot d4. The density of each of the dots d1 to d4 is determined by the current application time, and the time t
1, t2 is equal to time t, dots d1, d2 have the highest density, and times t3, t4 are shorter than time t.
3, d4 has a low concentration proportional to the application time.

Even if the total density of one line sharply decreases from high density to low density in the sub-scanning direction, it is desirable that the paper feed pitch be uniform as shown in FIG. State.

However, as described above, in the region where the total density of one line greatly changes from high density to low density in the sub-scanning direction, the thermal head 13 and the ink sheet 1
5 greatly changes. For this reason, FIG.
As shown in (B), the actual paper feed between the dot d2 and the dot d3 becomes one pitch or more, and the pitch unevenness of the paper feed indicated by the symbol Δp occurs. Since the feed pitch is one pixel, if the paper feed pitch unevenness occurs, a gap or a blank portion 40 is generated between the adjacent dots d2 and d3 along the sub-scanning direction, and fine density unevenness appears. There is.

Therefore, in the first embodiment, as described above,
In an area where the total density of one line greatly changes from high density to low density in the sub-scanning direction, the high density dot d2
Is divided into two dots d2a and d2b of low density for printing. That is, as shown in FIG.
At the paper feed timing f2a, first, the recording paper 14 is moved at a half pitch of one half of one pitch, which is a normal single feed amount, and then the recording paper 14 is moved to the heating element 12 for a half of the time t2 which should be originally applied. A current is applied for a time t2a (= t2 / 2 = t / 2) to form a dot d2a. Next, after the recording paper 14 is further moved at a ピ ッ チ pitch at the paper feed timing f2b, a current is applied to the heating element 12 for a time t2b (= t2 / 2 = t / 2) which is a half of the time t2, and a dot is formed. d2b is formed.

Since the times t2a and t2b are both half of the time t2 which should be originally applied, each of the dots d2a and d2b printed after being decomposed has a low density proportional to the application time.

Therefore, the total density ΣB of one line including the dot d3 is larger than 50% of the total density ΣA of one line including the dot d2b, and the total density of one line sharply increases in the sub-scanning direction. , And a sharp change in friction between the thermal head 13 and the ink sheet 15 is suppressed. As a result, no pitch unevenness occurs in the actual paper feed between the dots d2a and d3, and the paper feed pitch between the dots d2a and d3 is reduced to the normal one.
Can be maintained on the pitch. Alternatively, both dots d2a, d3
Even if pitch unevenness occurs in between, the pitch unevenness can be made extremely small. As a result, there is no gap or blank portion between the dot d2b and the dot d3, and the occurrence of density unevenness can be prevented.

Here, each of the dots d2a and d2b has half the current application time as compared with the dot d2 to be originally printed, so that the density is accordingly reduced. However, blank portions 40 (see FIG. 4)
Compared to the case where (B)) occurs, the difference in density is less noticeable, and the effect of reducing density unevenness is remarkably high. Therefore, the gradation characteristics of the formed image are not greatly affected.

Further, since the paper feed pitch is halved only in the specific area as described above, it is possible to suppress as much as possible the time required for forming one image. .

Since the feed pitch of the ink sheet 15 becomes uniform as the actual paper feed pitch becomes uniform, the ink layer on the ink sheet 15 can be used without gaps. Waste can be reduced. Furthermore, since the feed pitch of the ink sheet 15 becomes uniform, the time required to heat the ink sheet 15 at the same position is reduced, and problems such as wrinkles and expansion that may occur on the ink sheet 15 due to the influence of heat can be reduced. There are advantages too. The fact that wrinkles and elongation of the ink sheet 15 can be reduced is very advantageous in forming a high-quality image in a thermal transfer printer.

In the thermal transfer printer 10 according to the first embodiment, the specific area where the pixel density along the main scanning direction changes from a predetermined high density to a predetermined low density in the sub-scanning direction is within a predetermined paper feed pitch range. While the image is formed while shifting the position of the recording paper 14 twice to reduce the density unevenness, the present invention is not limited to this case.
For example, the number of times of shifting the position of the recording paper 14 may be any number as long as the predetermined paper feed pitch is equally divided by 1 / n (where n is a natural number of 2 or more), and is not limited to two times.

Embodiment 2 FIG. 5 is a schematic diagram showing a printing state and a state of applying a current to the heating element 12 according to Embodiment 2. The meaning of the reference numerals in FIG.
Therefore, the description is omitted.

The second embodiment is the same as the first embodiment in that an image is formed while the position of the recording paper 14 is shifted twice within the range of a predetermined paper feed pitch in the specific area. Is not equally divided into two parts, and is different from the first embodiment in which the parts are equally divided into two parts.

As shown in FIG. 5, in the second embodiment, the paper feed timing f2b is set to 60% of the maximum time t which can be set as the application time, and the dot d2b is formed between the dot d2a and the dot d3. are doing. Dots d2a, d
2b, the current application times t2a and t2b are set to 0.
6t and 0.4t. For example, one pixel is t = 12 ms
ec, t2a is 7.2 mse
c and t2b are 4.8 msec. Therefore, the density of each of the dots d2a and d2b is determined by the current application time t.
The density corresponds to 2a and t2b, and the density of the dot d2b is lower than the density of the dot d2a.

Therefore, the total density ΣB of one line including the dot d3 is larger than 50% of the total density ΣA of one line including the dot d2b.
Is smaller than in the first embodiment. Therefore, the sudden change in the total density of one line in the sub-scanning direction is further alleviated. As a result, the pitch shift is reduced, and there is no gap or blank between the dots d2b and d3. The occurrence of unevenness can be prevented.

Paper feed timing f in the specific area
The set value for determining 2b is not limited to 60% of the maximum time t, and can be appropriately set to a value at which pitch unevenness is least noticeable. Further, if the set value is made variable in accordance with the type of the ink sheet 15 or the recording paper 14, the density unevenness can be reduced more precisely.

In the first and second embodiments, when the time interval in which the recording paper 14 is moved by one dot in the sub-scanning direction is set to time t, the pixel density is changed when the current is applied for the time t. Exemplifies the case where the maximum concentration is reached. However, the present invention is not limited to this case, and in the sublimation type thermal transfer printer, the pixel density changes according to the application time, so that the time is shorter than the time t, for example, 90% of the time t. You may set so that it may reach the highest density.

Further, it is determined whether or not the area is a specific area based on a comparison between the total density of the current one line to be printed and the total density of the next one line to be printed thereafter. However, it may be determined whether or not the area is a specific area using the current total density of one line and the total density several lines ahead.

When the density unevenness is reduced by a combination of the timing of moving the recording paper 14 and the timing of applying a current to the thermal head 13, only a sublimation type thermal transfer printer that can express the density by the pulse width of the applied current is used. Although possible, the invention is not limited in this case. For example, the present invention is applicable to a fusion-type thermal transfer printer as long as it is a printer that performs gradation expression by selectively changing the number of heating elements to which a current is applied among a plurality of heating elements forming one pixel. It is possible to apply

[0051]

As described above, according to the thermal transfer printer according to the first and second aspects of the present invention, it is possible to reduce the pitch unevenness while keeping the recording paper conveyance accuracy and the mechanical structure of the thermal head unchanged. Density unevenness appearing in the sub-scanning direction can be reduced.

According to the third aspect of the present invention, it is possible to provide a sublimation-type thermal transfer printer in which density unevenness that appears in the sub-scanning direction due to pitch unevenness is reduced.

[Brief description of the drawings]

FIGS. 1A and 1B are a schematic configuration diagram showing a printing unit of a thermal transfer printer and a perspective view showing a thermal head.

FIG. 2 is a block diagram illustrating a control system of the thermal transfer printer to which the present invention is applied.

FIG. 3 is a flowchart showing a procedure for determining whether or not the area is a specific area.

FIG. 4A shows an originally intended printing state and a state in which a current is applied to a heating element, and FIG. 4B shows a printing state in which density unevenness has occurred; FIG. FIG. 3 is a schematic diagram showing a printing state and a state of applying a current to a heating element according to the first embodiment.

FIG. 5 is a schematic diagram showing a printing state and a state of applying a current to a heating element according to a second embodiment.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 10 ... Thermal transfer printer 12 ... Heating element 13 ... Thermal head 14 ... Recording paper 15 ... Ink sheet 20 ... CPU (control means, drive interval variable means,
Pixel density detecting means, discriminating means and printing interval variable means) M: pulse motor (driving means)

Claims (3)

[Claims] 1. A thermal transfer printer in which an ink sheet is heated by a thermal head in which a plurality of heating elements are arranged in a main scanning direction to form an image on recording paper, wherein the recording paper is moved relative to the thermal head. A driving means for moving at a predetermined pitch in the sub-scanning direction; and a specific area where the pixel density along the main scanning direction changes from a predetermined high density to a predetermined low density in the sub-scanning direction. And a controller for shifting the position of the recording sheet relative to the thermal head at least twice by the drive unit to form an image with the thermal head. 2. A thermal transfer printer in which an ink sheet is heated by a thermal head in which a plurality of heating elements are arranged in a main scanning direction to form an image on recording paper, wherein the recording paper is moved relative to the thermal head. A driving unit for moving the recording paper at a predetermined pitch in the sub-scanning direction; a driving interval changing unit for changing a moving amount of the recording sheet by the driving unit within the range of the predetermined pitch; and a pixel density along the main scanning direction. A pixel density detecting means for detecting the pixel density, and a determining means for determining whether or not the pixel density detected by the pixel density detecting means is a specific area where a predetermined high density changes from a predetermined high density to a predetermined low density in the sub-scanning direction. When the determining means determines that the specific area,
A thermal transfer printer comprising: a control unit for forming an image by the thermal head by shifting a position of the recording sheet relative to the thermal head at least twice by the drive interval variable unit. 3. A print interval varying means for varying a time for heating each heating element of the thermal head, wherein the control means is configured to, when the determination means determines that the area is the specific area, 3. The thermal transfer printer according to claim 2, wherein an image is formed by changing the heating time by a printing interval variable unit.