Color thermal printer and color thermal printing method

In a one-pass multi-head type thermal printer, a slack portion is provided in a recording sheet on each transport path from one thermal head to another. The slack portion is provided by a difference in transporting speed between adjacent two transport members, or by guiding the leading end of the recording sheet along a concavely curved guide member between the adjacent two thermal heads. A change in transporting speed caused by a change in load to the recording sheet during transportation is absorbed in the slack portion, and does not have bad influence on recording.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a color thermal printer which has plural 
thermal heads and platen rollers arranged along a one-way path of a 
recording material, and a color thermal printing method for the color 
thermal printer. More particularly, the present invention relates to a 
color thermal printer and a color thermal printing method which are 
effective to avoid occurrence of stripes of irregularly higher or lower 
optical density which could be caused by changing load to the recording 
material being moved along the path. 
2. Background Art 
There are various color thermal printers, of which examples are direct 
thermal printing type and a thermal transfer printing type. Any of the 
types incorporates at least a thermal head in which a great number of 
heating elements are arranged in line. A thermosensitive recording sheet 
for use in the direct thermal printing type includes cyan, magenta and 
yellow recording layers. These recording layers have different thermal 
sensitivities and develop respective colors in different heat energy 
ranges from one another. 
There is a one-pass multi-head type color thermal printer which has a 
plurality of, e.g. three thermal heads and in which a recording sheet is 
passed for one time under the thermal heads. The one-pass multi-head type 
has an advantage in that a shorter time is required for printing a 
full-color image compared with a three-pass one-head type color thermal 
printer which has one thermal head and in which a recording sheet is 
passed three times under the thermal head to record a full-color image. 
FIG. 17 illustrates a direct color thermal printer of the one-pass 
three-head type. A color thermosensitive recording sheet 16 is transported 
along a substantially straight path by means of a plurality of pairs of 
feed rollers 3, 4, 5 and 6 whose axes are parallel to one another. Three 
platen rollers 10, 11 and 12 having parallel axes to the axes of the feed 
rollers 3 to 6 are disposed in series along the transport path, and a 
yellow recording thermal head 13, a magenta recording thermal head 14 and 
a cyan recording thermal head 15 are disposed across the transport path 
from the platen rollers 10, 11 and 12, respectively. The thermal heads 13 
to 15 are movable between an upper retracted position and a lower 
actuating position by solenoids 7, 8 and 9. In the retracted position, the 
thermal heads 13 to 15 are set away from the recording sheet 16. In the 
actuating position, the thermal heads 13 to 15 get into contact with the 
recording sheet 16 and press it onto the platen rollers 10 to 12, 
respectively. 
The recording sheet 16 is transported in the direction of an arrow shown 
rightmost in FIG. 17. When a print area comes to the yellow recording 
thermal head 13, the yellow recording head 13 is moved to the actuating 
position and starts recording a yellow frame of a full-color image to the 
yellow recording layer of the recording sheet 16. Then the yellow 
recording layer is fixed by an optical fixing device 17 for yellow. When 
the print area comes to the magenta recording head 14, the magenta 
recording head 14 comes to the actuating position and starts recording a 
magenta frame of the full-color image to the magenta recording layer. The 
magenta recording layer is fixed by an optical fixing device 18 for 
magenta. Likewise the cyan recording head 15 records a cyan frame of the 
full-color image to the cyan recording layer in the print area. 
Accordingly, in the one-pass three-head type, if a full-color image has a 
length longer than the spacing between the heads 13 and 14 or when a 
plurality of full-color images are printed in continuous succession on the 
recording sheet 16, the magenta recording head 14 comes in contact with 
the recording sheet 16 while the yellow recording head 13 is recording the 
yellow image. As the thermal heads 13 to 15 are moved quickly to the 
actuating position, the recording sheet 16 abruptly receives the pressing 
force from the thermal heads 13 to 15, so that there occurs a rapid 
increase in load to the recording sheet 16 and the platen roller 11 each 
time any of the thermal heads 13 to 15 gets into contact with the 
recording sheet 16. In results, the recording sheet 16 is distorted to a 
certain extent. 
Since the transporting speed of the recording sheet 16 is lowered 
temporarily according to the rapid increase in the load or the distortion 
of the transport system, a stripe SH1 of an irregularly higher optical 
density is provided in a portion of the recording sheet 16 where the 
yellow recording thermal head 13 is making print on the recording sheet 16 
at the moment when the magenta recording thermal head 14 comes into 
contact with the recording sheet 16, as is shown in FIG. 18A. If a 
stepping motor is used for rotating the feed rollers 3 to 6, the rapid 
load change would cause the feed rollers 3 to 6 to stop in positions 
deviated from regularly determined stop positions, because the magnetic 
force and the load in transportation are balanced in unwanted fashion, 
even through the deviation are not so large as to cause the rotor of the 
stepping motor to fall out of step. 
On the other hand, when the yellow frame recording is terminated, the 
yellow recording thermal head 13 is deactivated and is moved back to the 
retracted position. Then, the load of the yellow recording thermal head 13 
to the recording sheet 16 abruptly decreases to zero. Since the abrupt 
decrease in the load also results in a temporary increase in the 
transporting speed of the recording sheet 16, a stripe SL1 of an 
irregularly lower optical density takes place in another portion of the 
recording sheet 16 where the magenta recording thermal head 14 is 
recording at that moment, as is shown in FIG. 18B. Similarly, a stripe of 
a conspicuously lower optical density takes place in a portion where the 
cyan recording thermal head 15 presses the recording sheet 16 at that 
moment. Occurrence of irregularly high or low density stripes is 
conspicuous when the recording sheet 16 is tensed in the transport path. 
If, however, the recording sheet 16 is too loose in the transport path, the 
recording sheet 16 is apt to jam between the rollers. 
Load applied to the recording sheet during the transportation also depends 
on friction between the recording sheet and the thermal heads. The 
friction depends on the temperature of the thermal heads as well as the 
pressure of the thermal head to the recording sheet, because the surface 
of the recording sheet is softened or melted by heat to reduce the 
friction. 
Since the thermal head is adapted to start recording with a delay from the 
contact with the recording sheet, there is a stage when the cold thermal 
head is in contact with the recording sheet. Therefore, a load change 
occurs also at the moment when the thermal head starts heating for the 
first time after coming to the actuating position. That is, the load to 
the recording sheet in transportation is reduced and thus the transporting 
speed is temporarily raised at that moment when the recording sheet starts 
to be heated. 
For instance, when the magenta recording head 14 is moved down to the 
actuating position and then turned on to start heating the recording sheet 
16 while the yellow recording head 13 is making print, a stripe SL2 of an 
irregularly low density is formed following the high density stripe SH1 in 
the yellow image. On the contrary, when the yellow recording head 13 is 
turned off to terminate heating and then moved up to the retracted 
position while the magenta recording head 14 is making print, a stripe SH2 
of an irregularly high density is formed prior to the low density stripe 
SL1. 
Also, dot percentage or coloring density per unit area, e.g. per line or 
per frame, has effect on the temperature of the thermal head, and hence on 
the friction between the thermal head and the recording sheet. 
Accordingly, the transporting speed can fluctuate due to the difference in 
dot percentage between frames to be recorded by the thermal heads. 
The problem of such irregularly high or low density stripes occurs in the 
thermal transfer printers. Even if the stripes are not conspicuous, the 
change or fluctuation in the transportation load has bad effect on the 
color registration. 
OBJECT OF THE INVENTION 
In view of the foregoing, a prime object of the present invention is to 
prevent irregularly high or low density stripes and color registration 
failure which may be caused by the fluctuation in load to the recording 
sheet during transportation. 
The present invention also has an object to provide a thermal printer 
wherein the above preventive measure is achieved with simple construction. 
SUMMARY OF THE INVENTION 
To achieve the above objects in a one-pass multi-head type thermal printer, 
the present invention provides a slack portion in a recording sheet on 
each transport path between adjacent two thermal heads. Since the quantity 
or length of the slack portion changes according to change in transporting 
speed relative to each of the adjacent two thermal heads, a change in load 
to the recording sheet, which is caused by one of the two adjacent thermal 
heads, is not transmitted to the other thermal head. That is, the slack 
portion absorbs the temporary change in transporting speed which is caused 
by the change in load to the recording sheet. Accordingly, the change in 
load does not adversely affect the recording. 
The slack portions may be provided by a difference in transporting speed 
between adjacent two transport members, or may be provided by guiding the 
leading end of the recording sheet along a concavely curved or sagged 
guide member between the thermal heads. 
It is preferable to monitor the quantity of the slack and keep it in a 
predetermined range by slightly changing the transporting speed of one 
transport member disposed before the slack portion, relative to the 
transporting speed of the other transport member disposed behind the slack 
portion. The transport member may be a pair of feed rollers rotated by a 
motor, or may be a platen roller rotated by a motor. When the motor is a 
pulse motor, the fine control of the transporting speed is preferably 
performed by changing pulse rate of motor drive pulses.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 1, three platen rollers 10, 11 and 12 having parallel axes to one 
another are disposed at appropriate intervals along a transport path of a 
color thermosensitive recording sheet 16 (hereinafter referred to as a 
recording sheet 16). Thermal heads 13, 14 and 15 for recording yellow, 
magenta and cyan color separation images are disposed in opposition to the 
platen rollers 10, 11 and 12, respectively. Thee pairs of guide rollers 
19, 20 and 21 having parallel axes to the platen rollers 10 to 12 are 
disposed respectively before the platen rollers 10 to 12 in view of a 
transport direction of the recording sheet 16. Also, three pairs of feed 
rollers 22, 23 and 24 having parallel axes to the platen rollers 10 to 12 
are disposed respectively behind the platen rollers 10 to 12 in the 
transport direction. Each feed roller pair 22, 23 or 24 consists of a 
pinch roller 22a, 23a or 24a and a capstan roller 22b, 23b or 24b. The 
capstan rollers 22b, 23b and 24b are each individually rotated by pulse 
motors 25, 26 and 27. Thus, the recording sheet 16 is withdrawn from a 
supply roll and is fed serially to the thermal heads 13 to 15. 
Between the first guide roller pair 19 and the yellow recording thermal 
head 13 (hereinafter referred to as the first thermal head 13), a leading 
end sensor 30 is disposed for detecting a leading end of the recording 
sheet 16. The leading end sensor 30 may be an optical sensor having a 
light-emitting member and a light-receptive member disposed on opposite 
sides of the transport path of the recording sheet 16, to detect the 
leading end of the recording sheet 16 responsive to an interruption of 
light from the light-emitting member. The leading end sensor 30 may be a 
reflective optical sensor having light-emitting and light-receptive 
members on the same side of the transport path. 
Similar leading end sensors 31 and 32 to the leading end sensor 30 are 
disposed immediately behind the guide roller pairs 20 and 21, 
respectively. By counting motor drive pulses supplied to the pulse motor 
25 from the time when the leading end is detected by the leading end 
sensor 30, the position of the recording sheet 16 in the transport path is 
determined. Also, a print start position on the recording sheet 16 for the 
first thermal head 13 is determined based on the count of the motor drive 
pulses. In the same way, a print start position for the magenta recording 
thermal head 14 (hereinafter referred to as the second thermal head 14) is 
determined by counting motor drive pulses supplied to the pulse motor 26 
upon the leading end being detected by the leading end sensor 31. So is 
determined a print start position for the cyan recording thermal head 15 
(hereinafter referred to as the third thermal head 15) by counting motor 
drive pulses to the pulse motor 27 upon detection of the leading end by 
the leading end sensor 32. 
The thermal heads 13 to 15 have respective heating element arrays 13a, 14a 
and 15a, each array consisting of a great number of heating elements. The 
heating element arrays 13a, 14a and 15a extend axially to the platen 
rollers 10 to 12. Hereinafter, the axial direction of the platen rollers 
10 to 12 will be referred to as a main scan direction, while the transport 
direction perpendicular to the main scan direction will be referred to as 
a sub scan direction. The thermal heads 13 to 15 are movable between an 
upper retracted position and a lower actuating position by solenoids or 
the like. In the retracted position, the thermal heads 13 to 15 are set 
away from the recording sheet 16. In the actuating position, the heating 
element arrays 13a, 14a and 15a of thermal heads 13 to 15 get into contact 
with the recording sheet 16 and press it onto the platen rollers 10 to 12, 
respectively. 
An optical fixing device 17 for yellow is disposed between the first and 
second thermal heads 13 and 14. The optical fixing device 17 is 
constituted of a lamp 34 radiating ultraviolet rays having an emission 
peak at 420 nm, and a reflector 35. An optical fixing device 18 for 
magenta is disposed between the second and third thermal heads 14 and 15. 
The optical fixing device 18 is constituted of a lamp 37 radiating 
near-ultraviolet rays having an emission peak at 365 nm, and a reflector 
38. 
A guide plate 40 and a guide roller 41 are disposed on opposite sides of 
the transport path between the optical fixing device 17 and the guide 
roller 20. On the guide plate 40, the recording sheet 16 forms a slack 43 
due to a difference in transporting speed between the feed roller pairs 22 
and 23. A slack sensor 42 is disposed above the guide plate 40 so as to 
measure a height LSy as a value representative of the magnitude or amount 
of the slack 43. Similarly, a guide plate 44 and a guide roller 45 are 
disposed on opposite sides of the transport path between the optical 
fixing device 18 and the guide roller 21. On the guide plate 44, the 
recording sheet 16 forms a slack 47 due to a difference in transporting 
speed between the feed roller pairs 23 and 24. A slack sensor 46 is 
disposed above the guide plate 44 so as to measure a height LSy as a value 
representative of the magnitude of the slack 47. The slack sensors 42 and 
46 may be micro-displacement gages each having a light-emitting member and 
a light-receptive member disposed for receiving light reflected from the 
recording sheet 16. 
Thanks to the slacks 43 and 47 between the thermal heads 13 to 15, a change 
in load to the recording sheet 16, which is caused by contact and removal 
as well as activation and deactivation of any of the thermal heads 13 to 
15, will not affect the transporting speed of the recording sheet 16 in 
the other recording portions where the other thermal heads are currently 
recording. 
FIG. 2 shows an example of layered structure of the recording sheet 16, 
wherein a thermosensitive cyan recording layer 86, a thermosensitive 
magenta recording layer 87, a thermosensitive yellow recording layer 88 
and a protection layer 89 are formed on a support material 85 in this 
order from reverse to obverse. For easy understanding, "C", "M" and "Y" 
are shown in the thermosensitive cyan, magenta and yellow recording layers 
86, 87 and 88, respectively. The recording layers 86 to 88 have the higher 
thermal sensitivities the closer to the obverse. The order of the 
recording to the recording layers 86 to 88 depends on the thermal 
sensitivity, that is, from the obverse toward the reverse recording layers 
88 to 86. Therefore, if an alternative recording sheet for use with the 
printer has a structure where the yellow recording layer and the magenta 
coloring layer are interchanged, then its recording order is 
"magenta-yellow-cyan". 
As the support material 85, an opaque coating paper or plastic film is 
used. But when making a print for use with an over head projector, a 
transparent plastic film is used as the support material 85. In practice, 
there are not-shown intermediate layers between the recording layers 86 to 
88 so as to adjust the thermal sensitivities of the magenta and cyan 
recording layers 87 and 86. 
As shown in FIG. 3, the yellow recording layer 88 requires a smallest heat 
energy for coloring, whereas the cyan thermosensitive coloring layer 87 
requires a largest heat energy for coloring. For the yellow recording 
layer 88, the coloring heat energy applied thereto is a sum of a constant 
bias heat energy BY and an image heat energy GYj determined by a gradation 
level "J" of each pixel. The bias heat energy BY has such a level that the 
yellow recording layer 88 is about to be colored. Also, the coloring heat 
energy for the magenta recording layer 87 is a sum of constant bias heat 
energy BM and an image heat energy GMj, and the coloring heat energy for 
the cyan recording layer 86 is a sum of constant bias heat energy BC and 
an image heat energy GCj. 
The cyan recording layer 86 contains an electron donating dye precursor and 
an electron accepting compound as main components, and is colored cyan 
when it is heated. The magenta recording layer 87 contains a diazonium 
salt compound having a maximum absorption factor at a wavelength of about 
365 nm and a coupler which acts upon the diazonium salt compound and is 
developed in magenta when it is heated. The magenta recording layer 87 
loses its capacity of color-developing when it is exposed to 
electromagnetic or near-ultraviolet rays of about 365 nm, because the 
diazonium salt compound is photochemically decomposed by this range of 
rays. The yellow recording layer 88 contains a second diazonium salt 
compound having a maximum absorption factor at a wavelength range of about 
420 nm and a coupler which acts upon the second diazonium salt compound 
and is colored in yellow when it is heated. The yellow recording layer 88 
is also optically fixed, that is, loses its capacity of color-developing 
when it is exposed to ultraviolet rays of about 420 nm. 
As shown in FIG. 4, the heating element array 13a of the first thermal head 
13 includes a great number of heating elements 90a, 90b, 90c . . . 
arranged in a line which extends in the main scan direction M. Each of the 
heating elements 90a, 90b, 90c . . . has a length L3 in the main scan 
direction M and a length L4 in the sub scan direction S. For example, the 
lengths L3 and L4 are 140 .mu.m and 100 .mu.m. By moving the recording 
sheet 16 stepwise at a constant stride in the sub scan direction L 
relative to the thermal head 13, a line of pixels 91 extending in the main 
scan direction M and corresponding in number to the heating elements 13a 
are recorded one line after another. The second and third thermal heads 14 
and 15 may have the same construction as the first thermal head 13. 
Referring to FIG. 5 showing the circuitry of the thermal printer, a system 
controller 50 sequentially controls first to third transport sections 51 
to 53 and yellow, magenta and cyan recording sections 55, 56 and 57. The 
first transport section 51 is to transport the recording sheet 16 for the 
yellow frame recording, the second transport section 52 is to transport 
the recording sheet 16 for the magenta image recording, and the third 
transport section 53 is to transport the recording sheet 16 for the cyan 
image recording. The system controller 50 controls start and stop of the 
pulse motor 25 through a motor controller 60 of the first transport 
section 51. The motor controller 60 outputs motor drive pulses with a 
constant cycle to a motor driver 61, thereby to rotate the pulse motor 25 
at a constant speed. 
A counter 62 starts counting the motor drive pulses from the time when the 
leading end sensor 30 detects the leading end of the recording sheet 16, 
so as to determine the position of the leading end of the recording sheet 
16 being in transport. To determine when to move down and move up the 
respective thermal heads 13 to 15, as well as when to activate and 
deactivate the thermal heads 13 to 15, the count of the counter 62 is sent 
to the system controller 50. The second and third transport sections 52 
and 53 are constructed in the same way as the first transport section 51. 
As shown in FIG. 6, a slack controller 64 is connected to the motor 
controller 60 of the first transport section. The slack controller 64 
makes fine control of transporting speed of the recording sheet 16 by 
adjusting pulse rate of the motor drive pulses, in order to form the slack 
43 on the guide plate 40 and keep the height LSy of the slack 43 in a 
range of more than L1 to less than L2, wherein L1 and L2 are predetermined 
reference values. A signal from the slack sensor 42, which is 
representative of the height LSy, is amplified by an amplifier 66 up to a 
voltage level which is suitable for analog-to-digital conversion in an A/D 
converter 67. The digital signal is sent to an operation circuit 68. 
The operation circuit 68 averages the digital slack sensor output signals 
and makes non-linearity correction to convert the average value to a 
length LSy' of the slack 43 which corresponds to the height LSy. Then, the 
operation circuit 68 compares the length LSy' corresponding with reference 
lengths L1' and L2' corresponding to the reference heights L1 and L2. If 
the value LSy' is not less than the value L2', the operation circuit 68 
outputs the excess as a load value to a programmable timer 69. If the 
value LSy' is not more than the value Li', the operation circuit 68 
outputs the shortage as a load value to the programmable timer 69. It is 
alternatively possible to omit conversion of the slack sensor output 
signal to the height LSy, and control the amount of slack directly based 
on the slack sensor output signal. 
Responsive to the load value corresponding to the excess of the slack 43, 
the programmable timer 69 lowered the pulse rate of the motor drive 
pulses. Thus, the transporting speed of the capstan roller 22b for the 
recording sheet 16 is slightly lowered in comparison with that of the 
capstan roller 23b, so that the slack 43 is reduced to the predetermined 
range. When the load value represents the shortage of the slack 43, the 
programmable timer 69 raises the pulse rate of the motor drive pulses, to 
raise the transporting speed of the capstan roller 22b relative to that of 
the capstan roller 23b. Thus, the slack 43 increases till the height LSy 
goes above the height L1. A slack controller 65 having the same 
construction as the slack controller 64 is connected to a not-shown motor 
controller of the second transport section 52, so as to maintain the 
height LSm of the slack 47 in a predetermined range, i.e. L1&lt;LSm&lt;L2. 
FIG. 7 shows a flow chart illustrating the above-described operation of the 
slack controllers 64 and 65. FIG. 8 shows a flow chart illustrating an 
initial operation of the slack controllers 64 and 65 for setting-up the 
slacks 43 and 47 while the leading end of the recording sheet 16 passes 
the first to third thermal heads 13 to 15. 
As shown in FIG. 5, the system controller 50 controls the yellow recording 
section 55, the magenta recording section 56 and the cyan recording 
section 57 with reference to the counts of the respective counters of the 
first to third transport sections 51 to 53. Specifically, the system 
controller 50 controls up-and-down of the thermal heads 13 to 15, and 
outputs a line print start signal to each of the thermal heads 13 to 15 
for designating a start of printing of one line. 
Responsive to the line print start signal, a yellow print controller 70 of 
the yellow recording section 55 starts printing one line of the yellow 
image. Accordingly, the number of lines having been recorded can be 
determined by counting the number of line print start signal having been 
applied to the yellow print controller 70. The magenta and cyan recording 
sections 56 and 57 have the same construction as the yellow recording 
section 55, and operate in the same way as the yellow recording section. 
Therefore, the following description relates only to yellow recording 
section 55, but applies to the magenta and cyan recording sections 56 and 
57. 
A memory 71 stores bias data and other data necessary for yellow recording. 
The bias data is commonly used for every heating elements 90a, 90b, 90c . 
. . of the yellow thermal head 13. Based on the common bias data, bias 
data of one line is formed. A bias line memory 72 is revised each time the 
type of the bias data changes. Unless the type of the bias data is 
changed, the same bias data is commonly used for every line. However, as 
the heating elements have inevitable variations in resistance, heat energy 
generated from one heating element can differ from that from another 
heating element even in response to the same drive pulse. To compensate 
for the resistance variation, it is desirable to predetermine specific 
bias data to each heating element. 
A yellow frame memory 73 stores yellow image data per frame, which is 
inputted through a video camera or a scanner. The yellow frame memory 73 
is read line by line during the yellow image recording. The yellow image 
data per line is sequentially written in an image line memory 74. 
Alternatively, it is possible to write blue frame data in a frame memory 
and convert it line by line into yellow image data after the blue frame 
data is read line by line. 
A selector 75 first selects the bias line memory 72 to serially send the 
bias data of one line to a comparator 76 in the order of the pixels. Next, 
the selector 75 selects the image line memory 74 to serially send the 
yellow image data of one line to the comparator 76. 
A comparative data generator 77 generates a series of comparative data to 
the comparator 76. If the image data have 256 tonal steps, the series of 
comparative data is from "0" to "255" in decimal notation. The comparator 
76 compares the bias data and the image data with the comparative data. As 
a result, the bias data is converted into 256-bit bias drive data per one 
pixel, and the image data is converted into 256-bit image drive data per 
one pixel. 
A head drive circuit 78 provides logical products of the drive data from 
the comparator 76 and strobe pulses from a strobe signal generator 79. 
Thus, a drive pulse having the same width as the strobe pulse is generated 
when the binary drive data has a value "1". When the binary drive data has 
a value "0", no drive pulse is generated. The width of the strobe pulse 
varies depending upon whether it is for bias heating or image heating, and 
according to color to be printed, i.e., depending upon the thermal 
sensitivities of the recording layers 86 to 88. Generally, the strobe 
pulse has a larger width for bias heating than that for image heating. 
An up-down mechanism 80 moves the first thermal head 13 between the 
actuating position where the first thermal head 13 presses the heating 
element array 13a onto the recording sheet 16, and the retracted position 
where the heating element array 13a is removed away from the recording 
sheet 16. A predetermined time after the contact of the first thermal head 
13 with the recording sheet 16, which is taken to get the transport of the 
recording sheet 16 into a stable condition, the heating element array 13a 
start to be supplied with electric power. Also, the first thermal head 13 
is not removed from the recording sheet 16 until the recording sheet 16 
has been transported by a predetermined number of lines after the power 
supply to the heating element array 13a stops. The up-down mechanism 80 
may be a cam mechanism or a solenoid. 
It is possible to start applying bias heating energy to the recording sheet 
16 before the leading margin of the print area reaches under the thermal 
head 13. This makes the change in load to the recording sheet 16 gentler 
at the start of printing. 
Now the operation of the thermal printer as set forth above will be 
described. 
First, three color separation frames of a full-color image are written in 
the respective color frame memories. Thereafter when a print start switch 
(not-shown) is turned on, the system controller 50 turns the optical 
fixing devices 17 and 18 on, and causes a paper feeding mechanism 
(not-shown) to feed the recording sheet 16. The system controller 50 also 
commands the motor controller 60 to start driving the pulse motor 25. The 
motor controller 60 then supplies the motor drive pulses at constant 
intervals so as to rotate the pulse motor 25 at a constant speed. As a 
result, the first pair of guide rollers 22 are rotated at a constant 
speed. The feed rollers of each pair 19 to 21 do not nip the recording 
sheet 16 until the leading end of the recording sheet 16 has passed 
therebetween. The recording sheet 16 is withdrawn from the roll by a pair 
of feed rollers (not-shown) which is rotated by a not-shown motor, toword 
the transporting system. 
While the recording sheet 16 is being transported toward the platen roller 
10 through the guide rollers 19, the leading end sensor 30 detects the 
leading end of the recording sheet 16. Upon the detection signal form the 
leading end sensor 30, the counter 62 starts counting the motor drive 
pulses in order to measure the transported position of the recording sheet 
16. 
The system controller 50 always monitors the count of the counter 62 and, 
when it determines based on the count that the leading end of the 
recording sheet 16 reaches a predetermined position behind the first 
thermal head 13, commands the yellow print controller 70 to move down the 
first thermal head 13. Then, the yellow print controller 70 causes the 
up-down mechanism 80 to move the first thermal head 13 down to press the 
heating element array 13a onto the recording sheet 16. According to the 
present embodiment, once the leading end of the recording sheet 16 has 
passed the first to third thermal heads 13 to 15, the thermal heads 13 to 
15 continue to press the recording sheet 16 while a predetermined number 
of full-color images are successively printed. 
After the first thermal head 13 gets into contact with the recording sheet 
16, the yellow print controller 70 reads out yellow image data of the 
first line from the yellow frame memory 73, and write the yellow image 
data in the image line memory 74. Simultaneously, the bias data is read 
out from the memory 71 and is written in the bias line memory 72. 
The system controller 50 has a memory which is written with a distance from 
the leading end to the leading margin of the print area of the recording 
sheet 16 through a not-shown keyboard or during the manufacture of the 
thermal printer. The system controller 50 commands the yellow print 
controller 70 to start yellow image recording when it determines based on 
the count of the counter 62 that the leading margin of the print area is 
placed under the heating element array 13a of the first thermal head 13. 
Then, the yellow print controller 70 connects the selector 75 to the bias 
line memory 72 to read bias data of one line. For instance, the bias data 
of one pixel has a value "240" in decimal notation. The bias data of one 
line is serially sent to the comparator 76. Simultaneously, the yellow 
print controller 70 resets a counter of the comparative data generator 77, 
so that the comparative data generator 77 first outputs a value "0" in 
decimal notation as the comparative data to the comparator 76. 
The comparator 76 compares the bias data with the comparative data, and 
outputs binary "1" as a bit of the 256-bit bias drive data when the bias 
data is larger than the comparative data. Therefore, the comparator 76 
serially outputs binary "1" for every pixel of one line when the 
comparative data is "0". 
The serial drive data from the comparator 76 is converted into a parallel 
form through a not-shown shift register of the head drive circuit 78. The 
head drive circuit 78 outputs a bias drive pulse for each pixel as a 
logical product of the parallel drive data and the strobe pulse from the 
strobe signal generator 79. Since the drive data is "1" for every pixel of 
one line in its initial bit, a drive pulse O.sub.B having the same width 
as the strobe pulse is simultaneously outputted to every heating element 
of the heating element array 13a. As a result, all the heating elements 
are heated simultaneously. FIG. 9A shows a drive pulse train applied to 
one of the heating elements 90a, 90b, 90c and so forth. 
Next, the comparative data generator 77 outputs "1" in decimal notation as 
the comparative data of one line. Then, the bias data of one line is 
serially read out from the bias line memory 72 for the second time, and is 
compared with the comparative data to produce a second bias drive pulse 
for every heating element of the heating element array 13a, in the same 
way as for the initial bias drive pulse of each train. 
In this way, every heating element of the heating element array 13a is 
supplied with 241 bias drive pulses to radiate the bias heating energy BY 
for the yellow recording layer 88. 
When the bias heating is complete, image heating is started. First, the 
yellow print controller 70 resets the counter of the comparative data 
generator 77 to zero, so that the comparative data generator 77 newly 
starts outputting comparative data from "0" to "255". Simultaneously, the 
selector 75 is switched to the image line memory 74. Next, the yellow 
print controller 70 reads the yellow image data of the first line from the 
image line memory 74 to send it one pixel after another to the comparator 
76. 
The comparator 76 sequentially compares the yellow image data of one line 
with each value of the comparative data "0" to "255" in decimal notation. 
Since the image data of one pixel has a value in a range from "0" to "255" 
in decimal notation, and the comparator 76 outputs "1" only when the image 
data is larger than the comparative data, 256-bit image drive data is 
generated for the yellow image data of one pixel in the same way as for 
the bias data. The image drive data of one line is sent in serial to the 
head drive circuit 78, to be converted into a parallel form, and then 
converted into image drive pulses for the heating element array 13a by 
using strobe pulses of a width predetermined for yellow image recording. 
Thus, the heating elements 90a, 90b, 90c and so forth are supplied with 
different numbers of image drive pulses corresponding to the associated 
yellow image data, to radiate a variable image heating energy GYj each. 
To record a pixel at a highest density, the image data of that pixel is 
"255", and the associated heating element is supplied with 256 image drive 
pulses 0.sub.K to 255.sub.K, as is shown in FIG. 9A. To record a pixel at 
a lowest density, the corresponding image data is "0", and no image drive 
pulse is supplied to the associated heating element. According to the 
applied image heating energy GYj, the yellow recording layer 88 is colored 
to develop a dot at a variable density in each pixel 91. 
After the image heating, each heating element is cooled by not being heated 
for a period. The cooling period varies depending upon the number of 
preceding image drive pulses, and is determined such that even for the 
heating element that has been heated with the largest number "256" of 
image drive pulses, a time enough to cool the heating element down to a 
given temperature at the room temperature may be provided. 
After all of the heating elements of the array 13a get into the cooling 
period, the yellow print controller 70 reads out the yellow image data of 
the second line from the yellow frame memory 73 and writes it in the image 
line memory 74. Also, the selector 75 is switched to the bias line memory 
72. The bias line memory 72 continues to store the same bias data as 
before, except for a case to print a specific line. 
At the end of the cooling period, the system controller 50 newly outputs 
the line print start signal to the yellow print controller 70, to start 
printing the second line of the yellow frame. The second and following 
lines of the yellow frame are sequentially recorded in the same way as for 
the first line. 
While the first thermal head 13 is driven to record the yellow image in 
this way, the recording sheet 16 is transported line by line in the sub 
scan direction through the rotation of the platen roller 10 and the feed 
rollers 22. Then, the portion of the recording sheet 16 where the yellow 
image has been printed is moved under the optical fixing device 17, so 
that the yellow recording layer 88 is optically fixed by the ultraviolet 
rays of about 420 nm. 
The system controller 50 monitors the count of the second transport section 
52 that corresponds to the number of motor drive pulses supplied to the 
pulse motor 25 from the time when the leading end sensor 31 detects the 
leading end of the recording sheet 16. When the system controller 50 
determines based on the count of the second transport section 52 that the 
recording sheet 16 reaches the second thermal head 14, the system 
controller 50 causes the magenta recording section 56 to move the second 
thermal head 14 down to the actuating position. Thereafter when it is 
determined that the leading margin of the print area reaches the second 
thermal head 14, the system controller 50 commands the magenta recording 
section 56 to start printing. Thus, the first line of a magenta frame is 
recorded over the first line of the yellow frame. 
When recording a magenta pixel, one of the heating elements of the second 
thermal head 14 is supplied with 240 bias drive pulses to radiate the bias 
heating energy BM for the magenta recording layer 87. As the bias heating 
energy BM is larger than the bias heating energy BY for the yellow 
recording layer 88, the width of strobe pulses for the magenta recording 
and thus the width of the bias drive pulses for the second thermal head 14 
are longer than those for the first thermal head 13 for the yellow 
recording, as shown in FIG. 9B. 
After the bias heating, the heating elements of the second thermal head 14 
are supplied with 0 to 256 image drive pulses corresponding in number to 
the magenta image data, to record magenta pixels to the magenta recording 
layer 87. The second line of the magenta frame starts to be printed when 
all the heating elements are cooled down to a given temperature after the 
image heating for the first line. The second and following lines are 
recorded in the same way as above. The magenta recording section 56 and 
the second transport section 52 operate similarly to the yellow recording 
section 55 and the first transport section 51. The recorded magenta image 
is successively fixed by being exposed to the near-ultraviolet rays of 
about 365 nm radiated from the optical fixing device 18. 
When the leading end of the recording sheet 16 reaches the third thermal 
head 15, the system controller 50 causes the third thermal head 15 to 
press the recording sheet 16. Thereafter when the leading margin of the 
print area reaches the first thermal head 15, the cyan recording section 
57 starts printing. 
The cyan recording section 57 drives the respective heating elements of the 
third thermal head 15 with a constant number, 240 in this instance, of the 
bias drive pulses and a variable number of image drive pulses, to heat and 
color-develop the cyan recording layer 86. In this way, the first line of 
a cyan frame is recorded over the first lines of the yellow and magenta 
frame. The second and following lines are recorded in the same way as 
above. 
The recording sheet 16 having the full-color images recorded through the 
first to third thermal heads 13 to 15 in continuous succession is cut into 
pieces of each individual image. 
As described so far, the thermal heads 13 to 15 are set in the retracted 
positions before the leading end of the recording sheet 16 passes 
therethrough, and are set in the actuating positions each after the 
leading end of the recording sheet 16 passed. Thereafter, the thermal 
heads 13 to 15 are activated and deactivated. Although the up-down 
movement of the thermal heads 13 to 15 changes the load to the recording 
sheet 16 in the transport path, the change in the load is absorbed by the 
slack 43 or 47, so that the change in the load does not affect the 
recording. 
Since the respective quantities of the slack portions 43 and 47 are 
maintained in the predetermined range by slightly changing the rotational 
speeds of the feed rollers. The slight change in transport speed results 
in a slight change in coloring density through it is very small compared 
with the density change caused by the load change. Therefore, it is 
preferable to adjust the heating energy to the change in the transport 
speed of the feed rollers. 
For example, when the rotational speed of the pulse motor 25 is slightly 
increased by increasing pulse rate of the motor drive pulses in order to 
increase the quantity or amount of the slack 43, as the shorter time is 
taken to transport the recording sheet 16 by one line, it is necessary to 
apply the head drive pulses at a correspondingly higher frequency to each 
heating element. Then, the total cooling period per line is reduced, so 
that heat storage can occur in the first thermal head 13, resulting in a 
higher coloring density. This applies to the pulse motor 26. According to 
an embodiment, while the transport speed of the pulse motor 25 or 26 is 
increased, modified bias data having a smaller value, e.g. "235" in 
decimal notation, is adopted so as to drive each heating element with a 
smaller number, i.e. 236, of bias drive pulses than usual, as is shown in 
FIG. 10B. 
On the other hand, when the rotational speed of the pulse motor 25 or 26 is 
decreased, the total cooling period per line is elongated, so that the 
coloring density would be slightly lowered. Therefore, modified bias data 
having a larger value, e.g. "245" in decimal notation, is adopted, as is 
shown in FIG. 10A, while the transport speed of the pulse motor 25 or 26 
is lowered. As a result, each heating element is driven with a larger 
number, i.e., 246 bias drive pulses than usual. In this way, the coloring 
density is maintained proper even while the transporting speed is slightly 
changed. The modified bias data may be previously stored in the memory 71, 
to be written in the bias line memory 72 according need. 
Instead of changing the number of bias drive pulses, it is possible to 
change the number of image drive pulses or both the numbers of bias drive 
pulses and image drive pulses. 
In case where a bias drive pulse is used per pixel, as is shown in FIG. 11, 
the width of the bias drive pulse may be slightly changed so as to prevent 
the above problem. 
To measure the amount of the slack portions 43 and 47, it is possible to 
dispose a dancer roller 97 in contact with either of the slack portions 43 
and 47, as is shown with respect to the slack 43 in FIG. 12. The dancer 
roller 97 is supported by an arm 96 which is mounted to a shaft of the 
guide roller 41 so as to be capable of swinging independently of the guide 
roller 41. A potentiometer 99 detects the angular position of the arm 96 
as a measure of the slack amount. The arm 96 is gently biased upward by a 
spring 96a. The dancer roller 97 may be supported by a sliding guide 
member or the like so as to be movable up and down in accordance with the 
increase and decrease of the slack 43 or 47. 
It is possible to form a slack portion by controlling the rotational speed 
of feed rollers in a downstream side of the slack portion, instead of 
controlling the rotational speed of feed rollers in an upstream side of 
the slack portion . . . 0082 
Although the thermal printer of the above-described embodiment continuously 
monitors and controls the quantity or amount LS of either of the slack 
portions 43 and 47 so as to keep it in the predetermined range (L1&lt;LS&lt;L2), 
it is possible to control the slack amount LS only in those periods when 
the thermal heads 13 to 15 do not make thermal recording. That is, for 
example, when the blanks between the print areas are placed under the 
heating element arrays 13a, 14a and 15a of the thermal heads 13 to 15. 
FIG. 13 shows the flow chart of this embodiment. In this case, the lower 
limit L1 of the slack amount LS should be large enough to absorb possible 
variation in transport amount during one-frame recording. It may be 
possible to control the slack amount in the cooling period after recording 
one line. Since the fine control of transport speed of the recording sheet 
16 for the slack amount control will not affect the recording, it is 
unnecessary to adjust heat energy from the thermal head to the change in 
the transport speed. 
When a plurality of full-color images are successively printed on the 
continuous recording sheet 16, as in the above-described embodiment, it is 
not always possible to exactly determine the leading margin of the print 
area for each full-color image merely with reference to the leading end of 
the recording sheet. Therefore, the print start positions of the 
respective thermal heads 13 to 15 for the same full-color image can 
deviate from one another, resulting in color registration failure. 
Moreover, the deviation will increase with the number of printed images. 
To avoid this problem, it is preferable to previously provide positioning 
marks at regular intervals along the recording sheet 16, and determine 
with reference to the positioning marks the leading margin of each print 
area or the positions of the recording sheet 16 at which the thermal heads 
13 to 15 begin to press it. It is also possible that the thermal printer 
provides the recording sheet 16 with such positioning marks in the 
vicinity of the guide rollers 19. The positioning marks may be thermally 
recorded, or painted in ink. It is possible to provide the marks as 
notches or punches. In case the positioning marks being painted in ink, it 
is desirable to dispose them in a side edge of the recording sheet 16. The 
positioning marks should preferably be disposed in blanks or spacings 
between the print areas, so that the marks can be cut off with the blanks 
when the recording sheet 16 is cut into individual pieces of the printed 
full-color images. In this case, the positioning marks can also serve as 
indicia for cutting positions. 
FIG. 14 shows another preferred embodiment of the present invention, 
wherein paper guide members 110a, 110b and 110c are disposed respectively 
before the thermal heads 13 to 15 in the transporting direction indicated 
by arrows. The paper guide members 110a to 110c each has a concavely 
curved guide surface relative to a straight horizontal plane between the 
thermal heads, so that a recording sheet 16 curves to provide slack 
portions 115, 116 and 117 before each of the thermal heads 13 to 15. 
Designated by 17 and 18 are optical fixing devices. The thermal heads 13 
to 15 are movable up and down through not-shown up-down mechanisms. 
Three pairs of feed rollers 118, 119 and 120 are respectively disposed 
behind the thermal heads 13 to 15. There are a pair of feed rollers 121 
and a mark sensor 125 in a paper feed section 126. Each pair of the feed 
rollers 118 to 121 is constituted of a pinch roller 118a, 119a, 120a, 121a 
and a capstan roller 118b, 119b, 120b, 121b. The pinch rollers 118a to 
121a are movable up and down through not-shown sliding frames, to be set 
each individually in a pinching position or a releasing position. The 
capstan rollers 118b, 119b, 120b and 121b are rotated by motors 136, 137, 
138 and 139, respectively. 
The recording sheet 16 is provided with not-shown positioning marks at 
regular intervals. To detect the positioning marks, mark sensors 122, 123 
and 124 are disposed behind the thermal heads 13 to 15, respectively. The 
mark sensors 122 to 125 also detects the leading end of the recording 
sheet 16. It is possible to provide leading end sensors separately from 
the mark sensors 122 to 125. 
The paper guide members 110a to 110c are each constituted of a pair of 
arc-shaped guide plates 140 and 141. One guide plates 140 of the pairs are 
respectively supported on shafts of the pinch rollers 121b, 118b and 119b, 
whereas the other guide plates 141 of the pairs are supported on shafts of 
platen rollers 130, 131 and 132. The guide plates 140 and 141 can swing 
independently from the pinch rollers and the platen rollers 130 to 132, 
between guide positions where the paper guide members 110a, 110b and 110c 
are closed, as is shown by chain-dotted lines in FIG. 14, on one hand, and 
retracted positions where the paper guide members 110a, 110b and 110c are 
open, as is shown by solid lines in FIG. 14, on the other hand. Not-shown 
guide plate switching mechanisms switch over the paper guide members 110a 
to 110c between the guide positions and the retracted positions. 
Prior to printing, the paper guide members 110a to 110c are set in the 
guide positions, so the recording sheet 16 is guided along the guide 
members 110a to 110c. When the leading end of the recording sheet 16 is 
detected by the mark sensor 125, the pinch roller 121a is moved down to 
pinch the recording sheet 16 between the rollers 121a and 121b. Thereafter 
when the mark sensor 122 detects the leading end, the recording sheet 16 
is pinched between the feed rollers 118a and 118b, and the paper guide 
member 110a is set in the retracted position. As a result, the recording 
sheet 16 sags between the feed roller pair 121 and the thermal head 13. 
Similarly, upon detection of the leading end by the mark sensor 123 or 
124, the recording sheet 16 is pinched between the feed rollers of one 
pair 119a and 119b, or 120a and 120b, and then the paper guide member 110b 
or 110c is set in the retracted position, respectively. In this way, the 
slack portions 115 to 117 are formed along the curves of the paper guide 
members 110a to 110c. Because of the slack portions 115 to 117, even if 
there occurs any change in load to the recording sheet 16, the change is 
absorbed, and thus have no effect on the coloring density and the color 
registration. 
Distal end faces 140a and 141a of the guide plates 140 and 141 are inclined 
in opposite directions to each other. The guide plates 140 and 141 overlap 
with each other in the guide position such that the inclined distal end 
face 140a of the upstream guide plate 140 is laid on the inclined distal 
end face 141a of the downstream guide plate 141, and that guide surfaces 
140b and 141b of the guide plates 140 and 141 continues to each other 
without any stepped portion. So the leading end of the recording sheet 16 
can smoothly guided over the joint between the guide plates 140 and 141. 
The movable guide plates 140 and 141 may be mounted to specific shafts, 
instead of the shafts of the capstan rollers 121b, 118b, 119b and the 
platen rollers 130 to 132. The movable guide plates 140 and 141 should not 
necessarily be pivotal, but may be retracted from the transport path in a 
lateral or a vertical direction to the recording sheet 16. 
FIG. 15 shows a further embodiment similar to the embodiment shown in FIG. 
14, wherein a recording sheet 16 is nipped between the heating element 
arrays 13a to 15a of the thermal heads 13 to 15 and platen rollers 153, 
154 and 155, and the platen rollers 153 to 155 are rotated by pulse motors 
156, 157 and 158 to transport the recording sheet 16. According to this 
embodiment, the feed roller pairs 118 to 120 are not necessary, but some 
guide rollers 159 and 160 besides the platen rollers 153 to 155 are 
enough, so that the transporting mechanism is simplified. Also, mark 
sensors 122 to 124 are disposed immediately behind the platen rollers 153 
to 155, respectively. 
It may be possible to provide a movable paper pushing member above the 
transport path before each of three color thermal heads, so as to push the 
recording sheet in a direction perpendicular to the transport direction 
after the recording sheet is nipped between a pair of feed rollers which 
are disposed immediately behind each of the color thermal heads. 
FIG. 16 shows a thermal printer according to another preferred embodiment 
of the invention, whose construction is fundamentally equivalent to those 
of the embodiment shown in FIG. 1. The essential feature of this 
embodiment is that motor drive pulses for a pulse motor 214 in a 
transporting section 202 of a magenta print station 191 are fixed at a 
constant pulse rate, whereas pulse rates of motor drive pulses for pulse 
motors 213 and 215 in transporting sections 201 and 203 of yellow and cyan 
print stations 190 and 192 are changed to control the respective amounts 
of slack portions 195 and 196. That is, the slack portions 195 and 196 are 
formed by loosening the recording sheet 16 to sag between the print 
stations 190 to 192. 
As a measure of the amount of the slack 195, a distance Ly from a lowest 
surface or the bottom of the slack portion 195 to a slack sensor 230 is 
detected by the slack sensor 230. The slack sensor 230 is a 
micro-displacement gage or a reflective photosensor, and is disposed under 
the slack 195 in a middle of the transport path between the yellow and 
magenta print stations 190 and 191. In the same way, a distance Lc from a 
lowest surface of the slack 196 to a slack sensor 235 is detected by the 
slack sensor 235 as a measure of the amount of the slack 196. 
A paper feed mechanism 200 has a pair of feed-in rollers 206 which are 
rotated by a not-shown motor to feed a recording sheet 16 to the yellow 
transport section 201. A paper ejection mechanism includes a pair of 
feed-out rollers 204 which are rotated by a not-shown motor to feed out 
the recording sheet 16 after printing. Thereafter, the recording sheet 16 
is cut into pieces of individual images. 
The transport sections 201 to 203 have a pair of feed rollers 210, 211 and 
212 each. Each pair of the feed rollers 210, 211 and 212, consists of a 
capstan roller 210a, 211a or 212a and a pinch roller 210b, 211b or 212b. 
The capstan rollers 210a, 211a and 212a are respectively and independently 
driven by the pulse motors 213 to 215. The pinch rollers 210b, 211b and 
212b are movable up and down between a pinch position and a release 
position through a roller shift mechanism 216. 
Reflective photosensors 220, 221 and 222 are provided as marks sensors 
between the capstan rollers 210a to 212a, on one hand, and respective 
platen rollers 10, 11 and 12, on the other hand, to detect positioning 
marks formed on the recording sheet 16 at regular intervals, and also 
detect the leading end of the recording sheet 16. Detection signals from 
the mark sensors 220 to 222 are sent to a system controller 225. The 
system controller 225 discriminates the detection signals between those 
representative of the leading end and those representative of the 
positioning marks, depending on the magnitude of signal level change. 
The system controller 225 causes the thermal heads 13 to 15 to move into 
respective retracted positions through a head shift mechanism 226, and 
also sets the feed rollers 210 to 212 and the feed-out rollers 207 in the 
release position, while the leading end of the recording sheet 16 passes 
those members. A predetermined time after the start of paper feeding, 
which is previously stored in a ROM 227 of the system controller 225 and 
represents the time necessary for the leading end of the recording sheet 
16 to pass the thermal head 13, the system controller 225 causes the 
thermal head 13 to move down to press its heating element array 13a onto 
the recording sheet 16. It is possible to determine the pass of the 
leading end of the recording sheet 16 on the basis of counting motor drive 
pulses supplied to a not-shown pulse motor for rotating the feed-in 
rollers 206. In the same way as for the thermal head 13, the feed roller 
pair 210 is set in the pinching position when the leading end of the 
recording sheet 16 has passed therethrough. 
In the magenta print station 191, the thermal head 14 is moved down in a 
predetermined time after the leading end detection signal is outputted 
from the mark sensor 220 of the yellow print station 190. So is the feed 
roller pair 211 set in the pinching position. Similarly, in the cyan print 
station 192, the leading end of the recording sheet 16 is determined based 
on time after the leading end detection by the mark sensor 221 of the 
magenta print station 191. Also, the feed-out rollers 204 is set in the 
pinching position when a predetermined time passes away from the output of 
the leading end detection signal of the mark sensor 222. 
It is possible to move down the thermal head and the pinch roller with 
reference to the leading end detection signal from the mark sensor of the 
same print station. 
The thermal heads 13 to 15 start recording the first line of each frame 
when a leading margin of a print area is determined to be placed under the 
heating element array 13a, 14a or 15a based on the positioning mark 
detection signal from the mark sensor 220, 221 or 222 of the same print 
station. 
The pulse motor 213 of the yellow print station 190 is controlled by a 
motor controller 232 through a driver 233. The pulse motor 214 of the 
magenta print station 191 is controlled by a motor controller 228 through 
a driver 229. The pulse motor 215 of the cyan print station 192 is 
controlled by a motor controller 237 through a driver 238. The quantities 
or amounts of the slack portions 195 and 196 are controlled to be in a 
predetermined range in the following manner. First, all of the pulse 
motors 213 to 215 are driven with motor drive pulses of a standard pulse 
rate. Next, pulse rate of the motor drive pulses to the pulse motor 213 of 
the yellow print station 190 is set higher than the standard value, 
whereas pulse rate of the motor drive pulses to the pulse motor 215 of the 
cyan print station 192 is set lower than the standard value. Thus, the 
slack portions 195 and 196 are formed. 
Simultaneously, the distances Ly and Lc are measured by the slack sensors 
230 and 235. If the distance Ly is more than a predetermined maximum value 
L1y, the pulse rate of the motor drive pulses to the pulse motor 213 is 
made higher than the present value. Then, the speed of transportation in 
the yellow transport section 201 is raised to reduce the distance Ly. An 
decrease in the distance Ly means an increase in the amount of the slack 
195. On the contrary, when the distance Ly is less than a predetermined 
minimum value L2y, the pulse rate of the motor drive pulses to the pulse 
motor 213 is made lower than the present value. Then, the speed of 
transportation in the yellow transport section 201 is lowered to increase 
the distance Ly, and thus reduce the amount of the slack 195. 
In the same way as for the slack 195, the amount of the slack 196 is 
controlled based on the distance Lc detected by the slack sensor 235. 
Paper guide members 240 to 244 are disposed along the transport path, as 
is shown by chain-dotted lines. Optical fixing devices 245 and 246 for 
yellow and magenta have each a pair of ultraviolet lamps 247 and 248 which 
extend in parallel to each other and perpendicularly to the transporting 
direction. The optical fixing device 245 or 246 may have an ultraviolet 
lamp of U-shape in place of the pair of ultraviolet lamps 247 or 248. 
It may be possible to form the guide member as a single-piece member and 
retract the same vertically, horizontally or diagonally from the transport 
path. 
It is possible to fix pulse rate of the motor drive pulses to the motor 213 
at the standard value, and change pulse rate of the motor drive pulses to 
the motors 214 and 215 to control the slack amount. It may be possible to 
fix pulse rate of the motor drive pulses to all of the motors 213 and 215 
at the standard value, but change pulse rate of the motor drive pulses to 
the motor 214 so as to control the slack amount after the amounts of the 
slack portions 195 and 196 are once set in the predetermined range. 
As described so far, according to the present invention, if there is any 
change in load to the recording sheet 16, the slack portions absorb the 
fluctuation in transporting speed caused by the load change, so that the 
load change have no influence on the coloring density and the color 
registration. 
Although the present invention has been described in detail with respect to 
some preferred embodiments, the present invention should not be limited to 
those embodiments. 
For example, it is possible to move the thermal heads 13 to 15 
simultaneously down to the actuating position after the leading end of the 
recording sheet 16 has passed the last thermal head 15, to reduce the 
frequency of load change. But it is of course possible to move the thermal 
heads 13 to 15 up and down at the end of each frame recording and at the 
start of the next frame recording. Even in that case, the load change 
caused by the up-down movement of the thermal heads 13 to 15 will not 
adversely affect the recording in the next print station. It is possible 
to fix the thermal heads and move the platen rollers, so that the 
positions of the heating element arrays relative to the recording sheet 
will not fluctuate. 
The recording sheet may be transported in a substantially vertical 
direction, though it is transported horizontally in the above embodiments. 
It is also possible to transport the recording sheet along a curved path. 
In the straight transport path as shown in the drawings, the platen 
rollers may be replaced by platen plates. The recording sheet should not 
be limited to a long continuous sheet withdrawn from a supply roll, but 
may be cut in a length extending over a plurality of print stations. 
It is possible to use a color thermosensitive recording sheet including a 
black recording layer in addition to the yellow, magenta and cyan 
recording layers. In that case, four thermal heads should be disposed 
along the transport path. The recording sheet may further includes another 
color recording layer which can develop a specific color. Five thermal 
heads are necessary for that recording sheet. 
The present invention is applicable not only to color direct thermal 
printers, but also to color thermal wax transfer printers and color 
thermal ink transfer printers. 
Thus, various changes and modifications may be possible to those skilled in 
the art without departing from the spirit and scope of the appended 
claims.