JPH10129054A - Method and apparatus for color thermal printing - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make a color drift not to conspicuous when print timing is decided based on a set recording material conveying speed by slightly altering a recording material feeding amount per one line of partial lines of all lines in a recording material feeding direction. SOLUTION: A line specific pitch number to be corrected is calculated by a correcting amount calculation processor 72 based on a pulse rate set value to be input to a programmable interval timer 64, and a standard pulse rate set value from a system controller 28. A print timing signal from a print timing generator 70 is counted by a line counter 71. When a counted value coincides with the line specific pitch number to be corrected, a correcting timing signal is output. A pulse number setter 73 outputs a pulse number indicating a line length of the line to be corrected according to the signal. The generator 70 generates the print timing signal based on the pulse number for the line to be corrected.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color thermal printing method and apparatus for recording a full-color image in a single pass of a recording material using a plurality of thermal heads.

[0002]

2. Description of the Related Art In a color thermal printing method, for example, a color thermal printing method, a color thermal recording material which develops a color by heating is used, and a thermal head and a color thermal recording material are relatively moved while the thermal head is relatively moved. To record a full-color image. In this color thermosensitive recording material, at least a cyan thermosensitive coloring layer, a magenta thermosensitive coloring layer, and a yellow thermosensitive coloring layer are sequentially formed on a base. In order to selectively color each thermosensitive coloring layer, the thermosensitive coloring layers have different thermal sensitivities, the thermal sensitivity of the cyan thermosensitive coloring layer at the bottom is the lowest, and the thermal sensitivity of the yellow thermosensitive coloring layer at the top. Highest sensitivity. Also, when recording the next thermosensitive coloring layer, so that the recorded thermosensitive coloring layer on it is not recorded again,
The recorded thermosensitive coloring layer is irradiated with a specific electromagnetic ray to fix them.

In such a color thermal printing method,
In order to perform high-speed printing, three thermal heads are arranged in the pass area of the color thermal recording material, and while the color thermal recording material is passed once from the upstream side to the downstream side, a yellow color is formed on the print stage by each thermal head. A one-pass three-head color thermal printing method for sequentially recording an image, a magenta image, and a cyan image to form a full-color image is known.

[0004]

In this one-pass three-head color thermal printing method, for example, the recording material is fed at the same speed by setting the pulse rate of a pulse motor for driving the capstan of each print stage to be the same. I have to. Even when the printing speed is set to the same value, when continuous printing is performed using a roll-shaped recording material, differences in the capstan diameter, differences in the slip ratio of the capstan due to variations in the transport load based on the behavior of the thermal head, etc. During recording of each color, the transport speed of the recording material slightly changes, and as a result, the recording material is pulled or loosened between the printing stages of each color. When the recording material is pulled, stripe-shaped density unevenness, color unevenness, color misregistration and the like occur, and there is a problem that the printing quality is reduced.
Also, the occurrence of excessive slack causes a jam.

Therefore, it is conceivable to change the pulse rate of each recording material transporting section so as to eliminate the machine difference in consideration of the machine difference of the recording material transporting section in the printing stage of each color. However, when determining the print timing for recording one line based on the actual recording material transport speed, a device that accurately measures the actual recording material transport speed is required, and the configuration becomes complicated. Problem. For this reason, in practice, the print timing is determined based on the number of drive pulses obtained from the set pulse rate. Therefore, since there is a difference between the actual recording material transport speed and the set recording material transport speed, there is a problem that the length of one frame in the recording material transport direction is different and color shift occurs. is there. Such a problem occurs not only in a thermal color thermal printer but also in a sublimation type or a fusion type transfer type color thermal printer.

According to the present invention, a set recording material conveying speed is provided.
It is an object of the present invention to provide a color thermal printing method and apparatus in which the actual recording material transport speed is different from the actual recording material transport speed, and when the print timing is determined based on the set recording material transport speed, the color shift is made inconspicuous. Things.

[0007]

According to a first aspect of the present invention, there is provided a color thermal printing method, wherein each recording material is conveyed so as to eliminate a variation in a feeding amount due to a machine difference of each recording material conveying unit. Change the recording material feed speed of
During printing of each color, the actual recording material feed speed in each recording material transport unit is made the same, and during this feeding, the recording material feed amount per line of some of the lines in the recording material feed direction Is slightly changed so that the overall print length in the recording material feed direction of each color is the same. In the color thermal printing method according to the present invention, a part of lines in which the recording material feed amount per line is slightly changed is arranged at a predetermined pitch and dispersed in all lines. . According to a third aspect of the present invention, there is provided the color thermal printing method, wherein a slack is formed in the recording material between the first to third printing stages, and the slack is controlled so as to be maintained within a predetermined range. The actual recording material feeding speed in the material transport unit is the same. Further, in the color thermal printing method according to the present invention, the change of the recording material feed speed for maintaining the slack within a predetermined range is performed during a period in which a margin between the recorded images passes through the thermal head. Things.

According to a fifth aspect of the present invention, in the color thermal printing apparatus, each of the recording material transporting units is a pulse motor drive system, and the pulse rate of the pulse motor of each recording material transporting unit is changed to change the recording material transporting unit. The recording material is fed based on the change value of the pulse rate while keeping the actual recording material feed speed the same, and during this feeding, the pulse motor for one line of some of the lines in the recording material feed direction is used. By changing the number of drive pulses of
The total print length in the recording material feed direction of each color is the same.

According to a sixth aspect of the present invention, there is provided a color thermal printing apparatus according to the fifth aspect, wherein a period of a driving pulse serving as a reference for determining a print timing for recording one line is T1, and a pulse rate is changed. The period of the driving pulse is T2, the number of driving pulses for forming one line is N, the number of driving pulses to be corrected for one line is Nc, and the difference in the slip ratio between each transport unit and the recording material is corrected. When the coefficient to be corrected is k and the pitch of the line to be corrected is PC, PC = [T2 / ｛| (T1-T2) | · N · Nc}] ×
k The pitch PC of the line to be corrected is determined.

[0010]

FIG. 2 shows a color thermal printer of a linear movement type. The color thermosensitive recording material 10 wound up in a roll shape is pulled out by a paper feeding mechanism 9 (not shown). Passing area (pass) of the color thermosensitive recording material 10
, Three platen rollers 11, 12, 13 are rotatably arranged at appropriate intervals. A thermal head 14 for yellow, a thermal head 15 for magenta, and a thermal head 16 for cyan are disposed on each of the platen rollers 11 to 13 so as to face each other. , Cyan print stage 19 are formed. Each of the platen rollers 11 to 13 is selectively set by a roller shift mechanism 20 at a pressure bonding position where the thermal heads 14 to 16 of the thermal heads 14 to 16 are pressed against the heating element arrays 14a to 16a and at a separated retreat position.

Each of the print stages 17 to 19 has a recording material conveying section 22, 23, 24 and a thermal head 1
Recording units 25, 26, 27 for driving the 4 to 16 to record the respective color images on the recording material 10 are provided, and these are controlled by the system controller 28. Further, a yellow fixing device 29 is provided between the yellow print stage 17 and the magenta print stage 18, a magenta fixing device 30 is provided between the magenta print stage 18 and the cyan print stage 19, and a cyan print stage 19 and a paper ejection. Bleachers 32 are arranged between the mechanism 31 and the mechanism 31. In each of the print stages 17 to 19, an image of each color is recorded line by line. After the yellow recording, near yellow ultraviolet rays of 420 nm are irradiated by the yellow fixing unit 29, and the yellow thermosensitive recording layer is optically fixed. After magenta recording, near-ultraviolet light of 365 nm is irradiated by the magenta fixing device 30 to optically fix the magenta thermosensitive recording layer. Further, after the cyan recording, near-ultraviolet rays of 365 nm, which are the same as those of the magenta fixing device 30, are irradiated, and the unprinted portion is bleached. In this way, the yellow, magenta, and cyan images are sequentially recorded in the same recording area in three colors, and a full-color image is obtained. This full-color image has a cutter 33 in a margin between the images based on the detection signal of the positioning mark.
, And is discharged to a discharge tray by the discharge mechanism 31.

The yellow transport section 22 includes an auxiliary roller pair 41, a leading edge sensor 42, and a mark sensor 43 which are arranged in this order on the upstream side in the recording material feeding direction with respect to the yellow thermal head 14; And a pair of conveying rollers 44 arranged on the downstream side. The lower roller of each roller pair 41, 44 is a drive roller 41a and a capstan 44a, and is rotated by a pulse motor 45y. Further, each roller pair 41, 44
The upper rollers 41b, 44b of the roller shift mechanism 46,
47 selectively sets between a nip position and a nip release position. The nip pressure of the auxiliary roller pair 41 is set smaller than the nip pressure of the transport roller pair 44, so that the transport by the transport roller pair 44 is mainly performed. The other magenta transport unit 23 and cyan transport unit 24 have the same mechanical configuration, and the same components are denoted by the same reference numerals. Although the pulse motors of the transport units 22 to 24 have the same configuration, the reference numerals 45y, 45m, and 45c are used to identify which transport unit. Each of the sensors 42 and 43 is connected to
3, 24, the paper feed mechanism and the paper discharge mechanism are not shown.

Each of the pulse motors 45 of each of the transport sections 22 to 24
y, 45m, 45c are motor control units 50, 51, 52
The rotation is controlled by. In the present embodiment, the pulse motor 45m of the magenta transport unit 23 is driven at a standard pulse rate, and the pulse rates of the pulse motors 45y and 45c of the other yellow and cyan transport units 22 and 24 are changed. This change is performed so that the slack amounts of the slacks 53 and 54 formed between the print stages 17 to 19 are kept constant. FIG. 3A shows an example of a drive pulse train (drive pulse period T1) having a standard pulse rate.
(B) shows an example of the corrected driving pulse train (period T2 of the corrected driving pulse).

The magenta transport section 23 is composed of a motor controller and a driver. The motor controller generates a drive pulse at a standard pulse rate and rotates the pulse motor 45m at a constant speed via a driver.

As shown in FIG. 2, the leading edge sensor 42 detects the leading edge of the recording material 10 and sends it to the system controller 28. The mark sensor 43 detects a positioning mark printed on the back surface of the recording material 10 at a predetermined pitch, and sends it to the system controller 28. The system controller 28 tracks the leading edge position of the recording material 10 based on the leading edge detection signal from the leading edge sensor 42 and the number of drive pulses of the pulse motor 45y of the yellow transport unit 22, and based on the tracking result, each roller pair 4
After the leading end has passed through the pair of rollers 41 and 44,
To the nip state in order. The system controller 28 also determines the recording material 10 based on the mark detection signal from the mark sensor 43 and the number of drive pulses of the pulse motor 45y.
Recording start position and recording material 1 on the thermal head 14
The compression start position of 0 is specified, and the yellow recording unit 25 and the roller shift mechanism 20 are controlled based on this.

The pulse motor 4 of the yellow transport section 22
The rotation of 5y is controlled by the yellow motor control unit 50. As shown in FIG. 1, the yellow motor control unit 50
Are the slack amount controller 56, the driver 57, and 1
And a line length correction unit 58. The slack amount controller 56 includes an amplifier 61, an A / D converter 62,
The yellow print stage 1 comprises an arithmetic processing circuit 63 and a programmable interval timer 64.
The slack 53 formed between the magenta print stage 7 and the magenta print stage 18 is measured by the slack amount sensor 65, and the pulse rate is changed based on the slack amount signal when the margin is fed after the completion of yellow printing. , The amount of sag is kept within a certain range. The slack 53 is formed by slightly delaying the start of conveyance of the magenta print stage 18 when the leading end of the recording material 10 is sent to the magenta print stage 18. Further, in addition to delaying the transport start timing, the transport speed V between the yellow transport unit 22 and the magenta transport unit 23
The slack 53 may be formed by providing a speed difference (Vy> Vm) between y and Vm.

The slack amount sensor 65 is constituted by a micro displacement meter, and detects a change in the slack amount Ly by measuring a distance dy from the sensor main body to a maximum hanging portion of the slack 53. The distance detection signal from the slack amount sensor 65 is amplified by the amplifier 61 to a voltage level suitable for the next A / D converter 62, and this is amplified by the A / D converter 62.
After that, it is sent to the arithmetic processing circuit 63.
The arithmetic processing circuit 63 performs a constant conversion after performing the average value operation and the correction of the non-linearity, and calculates a pulse rate set value for keeping the slack amount Ly constant. The pulse rate setting value is sent to the programmable interval timer 64 when sending the margin after the completion of yellow printing, and the recording material feeding speed is changed when the thermal head 14 passes through this margin.

By the way, each of the print stages 17 to 19
Pulse motors 45y, 45m, 4 of the transport units 22 to 24
Even if 5c is set to the same pulse rate, capstan 4
A difference occurs in the actual feeding speed of the recording material due to a diameter error due to the machine difference of 4a and other machine differences in the transport system. However, by changing the pulse rate so as to keep the slack amount constant, the actual recording material feeding speed can be made substantially the same.

A programmable interval timer 64
Counts up the reference clock until the pulse rate reaches the set value, and inverts the output value (pulse motor drive pulse) when the reference clock reaches the set value. After the inversion, the counter is counted up from “0” again. Hereinafter, by repeating these steps, a pulse motor drive pulse having a duty ratio of 50% is created as shown in FIG.

For example, as shown in FIG. 1, when the slack amount Ly of the slack 53 is larger than the reference slack amount, the increase is detected by the slack amount sensor 65, and based on the slack amount, the arithmetic processing circuit 63 A set value obtained by reducing the pulse rate from the standard pulse rate is output to the programmable interval timer 64. As a result, the drive pulse rate of the pulse motor 45y is reduced, and the transport speed Vy of the transport roller pair 44 of the yellow transport unit 22 is set to a low value when the margin is fed, whereby the speed is changed. After changing the speed, printing is performed at the changed transport speed Vy. Due to this speed change, the amount of slack Ly during printing is
Gradually decreases, and after recording one frame worth of images,
The slack amount is set to a reference value. When the slack amount Ly decreases, the operation is reversed, and the transport speed Vy of the transport roller pair 44 is set high, and printing is performed at this speed. As a result, the slack amount Ly is increased to a predetermined range of the reference slack amount. As described above, the yellow transport unit 22 is controlled so that the slack amount is maintained within a certain range.
, The actual transport speeds Vy and Vm between the yellow transport unit 22 and the magenta transport unit 23 can be made substantially constant even when unpredictable transport load fluctuations occur. FIG. 4 is a flowchart showing this processing procedure.

The other motor control units 52 for cyan have substantially the same configuration as the motor control unit 50 for yellow.
However, while the yellow motor control unit 50 forms the sag 53 on the downstream side and maintains it in a certain range, the cyan motor control unit 52 forms the sag 54 on the upstream side, and these slacks 53 and 54 are formed. Is maintained in a fixed range, and the configuration is the same except that the sign of the control amount is inverted based on this difference. Therefore, the pulse motor 45m of the magenta transport unit 23 is driven at a standard pulse rate, and the pulse rate of the cyan transport unit 24 is changed to drive the pulse motor 45c so as to maintain the slack 54 within a certain range. 23 and cyan transport unit 24
And the actual transport speeds Vm and Vc can be made substantially constant.

By the way, as described above, the yellow transport section 22
By changing the pulse rate set values of the pulse motors 45y and 45c of the cyan transport section 24,
Recording material feed speeds Vy, Vm, V
The print timing of each line is determined by the pulse motors 45y, 45m, and 45c.
Is determined by counting the number of drive pulses. Originally, it is preferable to detect the actual recording material transport speeds Vy, Vm, and Vc and determine the print timing of each line based on the actual recording material transport speeds Vy, Vm, and Vc.
In order to detect m and Vc, it is necessary to arrange a high-precision rotary encoder and the like in each of the print stages 17 to 19, and there is a problem that the configuration becomes complicated.
For this reason, in this embodiment, each of the print stages 17 to
The print timing of each pixel in 19 is performed by counting the number of pulses of the drive pulse train from the programmable interval timer 64. Therefore, a slight shift occurs between the actual recording material feed amount and the print timing, and by accumulating this, the recording length of one frame of each color differs, and in a portion separated from the positioning mark, Color shift occurs.

In order to prevent the color misregistration, in the present embodiment, a correction target line having a predetermined pitch is determined for all the lines for one frame, and the length of the correction target line in the recording material feed direction is determined. Is changed so that the entire length of one frame is the same for each color. The specification of the line to be corrected and the change of the line length are performed by the one-line length correction unit 58. In the present embodiment, with respect to the magenta transport unit 23, the other yellow transport units 22,
Since the transport speed of the cyan transport unit 24 is changed, the above color shift correction is performed in the yellow print stage 17 and the cyan print stage 19.

As shown in FIG. 1, one line length correction unit 5
8 includes a print timing generation circuit 70, a line counter 71, a correction amount calculation processing circuit 72, and a pulse number setting circuit 73. The print timing generation circuit 70 includes a programmable interval timer 6
4 is input. The print timing generation circuit 70 counts the number of pulses in the drive pulse train, and sends a print timing signal to the print controller 75 and the line counter 71 when this count matches the pulse number set value for one line.

Image data for one frame of yellow is input to the print controller 75 in advance, and each of the thermal heads 14 based on one line of image data based on a print timing signal from the print timing generation circuit 70 A drive pulse for driving the heating element 14a is generated. As is well known, the driving pulse is composed of a predetermined number of bias pulses and a number of gradation pulses corresponding to the image data, and causes the corresponding pixel to develop a color at a density corresponding to the image data. As a result, a yellow image is thermally recorded one line at a time.

The line counter 71 counts a print timing signal from the print timing generating circuit 70 and specifies a line CL to be corrected for line length (see FIG. 5). For this reason, the line counter 71 stores the correction target line specific pitch number PC in the correction amount calculation processing circuit 7.
2 has been entered. Then, the line counter 71 counts the print timing signal, and sends a correction timing signal to the pulse number setting circuit 73 when the count value matches the preset correction target line specific pitch number PC. The pulse number setting circuit 73 outputs the pulse number N + nC indicating the line length of the correction target line CL to the print timing generation circuit 64 when the correction timing signal is sent.

The correction amount calculation processing circuit 72 uses the pulse rate set value input to the programmable interval timer 64 at the time of printing and the standard pulse rate set value input from the system controller 28 to specify the correction target line specific pitch. The number PC is calculated. The correction target line specific pitch number PC is used to specify the number of the correction target line, and is calculated by the following equation.

PC = [T2 / ｛| (T1-T2) | · N
Nc｝] × k T1: cycle of reference drive pulse (see FIG. 3A) T2: cycle of correction drive pulse (see FIG. 3B) N: number of drive pulses for forming one line Nc : Number of correction drive pulses for the correction target line (for one line) k: Correction coefficient (capstan 4 of each transport roller pair 44)
4a for correcting the difference in slip ratio between the recording material 10 and the recording material 10)

The specific pitch number PC of the line to be corrected is
It is sent to the line counter 71. The line length data of the correction target line is sent to the pulse number setting circuit 73. The pulse number setting circuit 73 converts the value into the pulse number setting value (N + Nc) of the correction target line, and sends it to the print timing generation circuit 64. As a result, the print timing generation circuit 64 generates a print timing signal for the correction target line CL based on the corrected pulse number setting value (N + Nc).

FIG. 3 is a timing chart showing the relationship between the pulse motor drive pulse and the print timing signal. FIG. 3A shows a case where the line length is not changed. In this case, the pulse motor drive pulse 2 is not changed.
One print timing signal is output for zero. FIG. 3B shows the case where the line length is changed, and the correction target line CL is the n-th line, and the n-th line is recorded as the number of drive pulses of 21. In addition, the other n-1 and n +
One line or the like is recorded as 20 pulse motor drive pulses. As described above, the increase / decrease in the number of pulses of the correction target line CL increases when the correction pulse rate is higher than the standard pulse rate, and decreases when the correction pulse rate is lower than the standard pulse rate. FIG. 5 is an enlarged view of the image recorded in this manner, and shows a state in which the correction target lines CL are evenly dispersed at the correction target line specific pitch number PC. Note that the same color misregistration correction processing is also performed in the cyan print stage.

Incidentally, in the case of color thermal printing, the relative friction coefficient between the heating elements 14a to 16a and the recording material 10 differs due to the difference in the amount of heat generated when recording each color of yellow, magenta, and cyan. Yellow, magenta, and cyan decrease in this order. Therefore, each transport roller pair 44
Are different from each other, and the slip ratio of each capstan 44a changes. Accordingly, when printing one frame, the recording material feed amount tends to increase in the order of yellow, magenta, and cyan. As a correction for this, a correction coefficient k1 is set in advance for each print stage, and correction is performed by multiplying the above equation by this correction coefficient k1. This makes it possible to correct the print length due to the difference in the friction coefficient at the time of recording each color.

As described above, in the above embodiment,
The amount of slack of the recording material 10 during each of the print stages 17 to 19 is measured, and the amount of slack is set to be constant.
The transport speeds Vy and V of the yellow transport unit 22 and the cyan transport unit 24 are based on the transport speed Vm of the magenta transport unit 23.
c has been changed. During printing, the pulse motor rotates at a constant speed at the set pulse rate in each of the transport units 22 to 24. Then, the amount of slack is detected after printing, and the pulse rate is changed so that the amount of slack in the next print is constant in a margin where no image is recorded. Therefore, the control is performed to eliminate the speed fluctuation in the next print based on the result of the speed fluctuation in the previous print, which is particularly effective when printing the same picture many times.

In the above-described embodiment, the transport units 22, 24 of the other yellow and cyan print stages 19 are based on the transport unit 23 of the magenta print stage 18.
However, the pulse rates of the remaining print stages may be changed based on one of the transport units 22 and 24 of the yellow print stage 17 and the cyan print stage. . In this case, in the adjacent print stage, control is performed to detect the amount of slack and make the amount of slack constant as described above. In addition, in the transport unit two adjacent to the reference transport unit, the variation in the first slack amount and the variation in the second slack amount are added, and control is performed based on the added variation. I do.
In the above-described embodiment, the slack amount controller 56 and the one-line length correction unit 58 are configured by hardware, but may be implemented by software.

In the above embodiment, the change in the slack amount is detected using the micro displacement meter.
A guide roller that comes into light contact with the recording material 10 may be attached to an arm, a slider, or the like, and the amount of slack may be detected based on the displacement of the arm or the slider. The displacement amount may be detected by a photoelectric switch, a potentiometer, or the like in addition to a mechanical switch such as a microswitch.

In the above embodiment, three transport roller pairs 4
Although the three-capstan method using 4 is employed, the present invention may be implemented by a method in which a platen roller is rotationally driven to feed a recording material. In the above embodiment,
Although the color thermal printer has been described, although not shown, the present invention may be applied to a sublimation type thermal transfer printer using yellow, magenta, and cyan color ink ribbons. In this case, a color ink ribbon is interposed between each heating element array of the thermal head and the recording material, and the ink from each ink ribbon is thermally transferred to the recording material. Instead of the sublimation type, an area gradation type heat melting type may be used.

In the above-described embodiment, in addition to the transport roller pair 44, the auxiliary roller pair 4 is provided upstream of the thermal heads 14-16.
1 is provided, but this may be omitted. In this case, the tip sensor 42 and the mark sensor 43 are connected to the thermal heads 14 to
16 may be arranged downstream. The auxiliary roller pair 4
1 is rotated in synchronization with the transport roller pair 44, but it may be free.

In the above embodiment, each print stage 1
Although slacks 53 and 54 were formed between 7 and 19, in addition to this, between the yellow print stage 17 and the paper feed mechanism,
And between the cyan print stage 19 and the paper discharge mechanism,
A slack may be formed so that a change in the transport load of the recording material in the paper feeding mechanism or the paper discharging mechanism is not transmitted to the thermal head.

[0038]

According to the present invention, printing is performed by slightly changing the recording material feed amount per line when printing some of the lines in the recording material feed direction. Since the entire print length in the recording material feeding direction is the same, the occurrence of color misregistration can be reduced without changing the total print length. That is, when printing each color, the set recording material feed speed of each recording material transport unit is changed so as to eliminate the feed amount fluctuation based on the machine difference of each recording material transport unit. If the recording material feed speed of
Since the print timing is determined based on the set recording material feed speed, the actual feed speed is the same in each color print, and the print timing of each pixel is different. Therefore, due to this, the total print length changes and color misregistration occurs,
In the present invention, such a color shift can be reduced. Further, since the correction lines are arranged at a predetermined pitch and are dispersed in all the lines, the color shift can be evenly dispersed, so that the color shift can be made less noticeable.

Each recording material transporting section is of a pulse motor drive system, and the actual recording material feeding speed in each recording material transporting section is made the same by changing the pulse rate of the pulse motor of each recording material transporting section. By changing the number of drive pulses of the pulse motor per line when printing some of the lines in the recording material feed direction, the entire print length in the recording material feed direction for each color is the same. Therefore, the color shift can be made inconspicuous with a simple configuration. Moreover, the cycle of the drive pulse as a reference is T1, the cycle of the drive pulse whose pulse rate is changed is T2, the number of drive pulses for forming one line is N, and the number of drive pulses corrected for one line Is Nc, and k is a coefficient for correcting the difference in the slip ratio between each transport unit and the recording material. PC = [T2 / ２ | (T1-T2) | · N · Nc}] ×
Since the pitch PC of the correction target line is obtained from k, the color shift can be evenly dispersed, and the color shift can be made less noticeable.

Further, since slack is formed in the recording material between the print stages, a change in the transport load caused by the behavior of the thermal head is not transmitted to the other print stages, and a streak caused by the change in the transport load is prevented. It is possible to suppress the occurrence of uneven density, uneven color, and color shift.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a yellow motor control unit of a color thermal printer embodying the present invention.

FIG. 2 is a schematic view showing the color thermal printer.

3A is a diagram illustrating a drive pulse train at a standard pulse rate and a print timing signal determined based on the drive pulse train. FIG. 3B is a diagram illustrating a correction pulse rate when one-line length correction is performed. FIG. 3 is a diagram showing a drive pulse train and a print timing signal according to the first embodiment.

FIG. 4 is a flowchart showing a processing procedure of a main part in the color thermal printer.

FIG. 5 is an explanatory diagram showing a correction target line in the present invention.

[Explanation of symbols]

 Reference Signs List 10 color thermal recording material 11-13 platen roller 14-16 thermal head 17-19 print stage 22-24 transport section 53,54 slack 65 slack amount sensor 50-52 motor control section Vy transport speed of yellow transport section Vm magenta transport section Transport speed Vc The transport speed of the cyan transport unit

Claims (7)

[Claims] At least three first to third thermal heads for recording a specific color line by line along a pass area of a recording material, and at least three platens against which each thermal head is pressed. At least three recording material transport sections for transporting the recording material to each thermal head are arranged in order from the upstream side, and at least three printing stages of first to third are configured in order, and each of the recording material transport sections comprises When the recording material is fed from the upstream side to the downstream side, the first thermal head on the upstream side is pressed against the recording material in order and then energization is started, each color is sequentially recorded, and the recording material is moved by one movement. In the thermal printing method, which records a full-color image on the printer, the recording material feed speed of each recording material transport unit is changed to eliminate fluctuations in the feed amount due to machine differences in each recording material transport unit. During printing of each color, the actual recording material feeding speed in each recording material transport unit is made the same, and during this feeding, recording material per line of some of the lines in the recording material feeding direction is used. A color thermal printing method characterized by slightly changing the feed amount so as to make the entire print length in the recording material feed direction of each color the same. 2. The color thermal printing method according to claim 1, wherein a part of the recording material feeding amount per line is slightly changed and arranged at a predetermined pitch, and this is dispersed in all the lines. A color thermal printing method, characterized in that: 3. The color thermal printing method according to claim 1, wherein a slack is formed in the recording material between the first to third printing stages, and the slack is controlled to be maintained within a predetermined range. A color thermal printing method wherein the actual recording material feed speed in the recording material transport section is made the same. 4. The color thermal printing method according to claim 3, wherein the change of the recording material feed speed for maintaining the slack within a predetermined range is performed during a period in which a blank portion between the recording images passes through the thermal head. A color thermal printing method, characterized in that: 5. At least three first to third thermal heads for recording a specific color line by line along a pass area of a recording material, and at least three platens against which each thermal head is pressed. At least three recording material transport sections for transporting the recording material to each thermal head are arranged in order from the upstream side, and at least three printing stages of first to third are configured in order, and each of the recording material transport sections comprises When the recording material is fed from the upstream side to the downstream side, the first thermal head on the upstream side is pressed against the recording material in order and then energization is started, each color is sequentially recorded, and the recording material is moved by one movement. In a color thermal printing apparatus for recording a full-color image on a recording medium, each of the recording material transport units is driven by a pulse motor, and a pulse rate of a pulse motor of each recording material transport unit is changed. The same west actual recording material feeding speed of the recording material conveying section by sends a recording material based on the change value of the pulse rate,
During this feeding, the number of drive pulses of the pulse motor per line of some of the lines in the recording material feed direction is changed so that the overall print length of each color in the recording material feed direction is the same. A color thermal printing apparatus. 6. The color thermal printing apparatus according to claim 5, wherein a period of a drive pulse serving as a reference for determining a print timing for recording one line is T1, and a period of a drive pulse having a changed pulse rate is T2. N is the number of drive pulses for forming one line, Nc is the number of drive pulses to be corrected for one line, and k is a coefficient that corrects the difference in the slip ratio between each transport unit and the recording material. When the pitch of the correction target line is PC, PC = [T2 / {| (T1-T2) | · N · Nc}] ×
k. A color thermal printing apparatus for determining a pitch PC of a line to be corrected. 7. The color thermal printing apparatus according to claim 5, wherein a slack is formed in the recording material between the first to third printing stages, and the slack is maintained within a predetermined range. By changing the pulse rate, the actual recording material feed speed in the recording material transport unit is the same,
A color thermal printing apparatus wherein the pulse rate is changed during a period in which a blank portion between recording images passes through a thermal head.