Thermal transfer type color recording method and apparatus therefor

A thermal transfer color recording method of and an apparatus for recording a color image on a transfer paper by successively transferring inks of different colors from an ink film to the transfer paper at a recording station. A ink of one color is transferred from the ink film to the transfer paper by a thermal head based on an image to be recorded while the ink film and the transfer paper are moved through the recording station in a forward direction in intimate contact with each other. After the transfer of the ink of one color, the transfer paper in intimate contact with the ink film is transported in the forward direction with the ink film not taken up to thereby slacken the ink film. Then, the transfer paper is transported in the reverse direction to position the leading end of the recorded area of the transfer paper at the recording station to transfer an ink of another color from the ink film to the transfer paper from the leading end of the recorded area thereof. Further, the ink film may be moved to position the ink of another color at the recording station after transport of the transfer paper to position a non-recorded area of the transfer paper at the recording station.

FIELD OF THE INVENTION 
The present invention relates to a thermal transfer color recording method 
and an apparatus therefore, and more particularly to a method of and an 
apparatus for recording color images on transfer paper with use of a 
single thermal head by successively transferring inks of different colors 
from an ink film to the paper. 
BACKGROUND OF THE INVENTION 
The thermal transfer color recording method mentioned above is disclosed in 
U.S. Pat. No. 4,505,603 (corresponding to Unexamined Japanese Patent 
Publication SHO 58-140270) and U.S. Pat. No. 4,642,656 (corresponding to 
Unexamined Japanese Patent Publication SHO 61-14973). 
The disclosed method employs an ink film having ink layers arranged in 
succession and different from one another in color (e.g. yellow, magenta 
and cyan). For recording, the ink film and transfer paper in intimate 
contact therewith are transported in one direction relative to a thermal 
head. After an image has been recorded in one color, the ink film is 
transported in the same direction as for recording to position the next 
ink layer of another color in opposed relation to the thermal head, while 
the transfer paper having the ink of the first color transferred thereto 
is transported in a direction opposite to the direction for recording to 
position the leading end of the recording area thereof again in opposed 
relation to the thermal head. 
When thus transported in directions opposite to each other, the transfer 
paper bearing the recorded image and the ink film are held in intimate 
contact with each other, so that the prior-art technique has the problem 
that the drive means are subjected to greatly varying loads due to the 
frictional resistance between the paper and the film. Consequently, the 
paper or the ink film becomes transported improperly to position the image 
of the second color out of register with the image of the first color. It 
is also likely that the transferred ink will be removed from the paper by 
friction. 
SUMMARY OF THE INVENTION 
Accordingly, the main object of the present invention is to provide a 
thermal transfer color recording method for producing satisfactory color 
images and an apparatus for practicing this method. 
Another object of the invention is to provide a thermal transfer color 
recording method and an apparatus therefore wherein after recording with 
an ink of a color has been completed, the transfer paper and the ink film 
can be transported in different directions without producing great 
frictional resistance therebetween. 
These objects are fulfilled by a thermal transfer color recording method 
with use of an ink film and transfer paper comprising transporting the 
transfer paper in the same direction as for recording, with ink film not 
taken up, after recording with an ink of a color has been completed, to 
thereby slacken the ink film, and, thereafter, transporting the transfer 
paper in a direction opposite to the direction for recording. The objects 
are fulfilled also by an apparatus for practicing this method. 
In another aspect of the present invention, the ink film is transported 
while opposed to a non-recorded area of the transfer paper other than the 
recorded area thereof after recording has been completed with an ink of 
the color.

In the following description, like parts are designated by like reference 
numbers throughout the several drawings. 
DESCRIPTION OF THE PREFERRED EMBODIMENT 
An embodiment of the invention will be described below. 
FIG. 1 is a diagram schematically showing the construction of a thermal 
transfer color printer embodying the invention. 
The printer includes a tractor feeder 1 and a platen roller 2 around which 
elongated transfer paper P extends. At the position where the transfer 
paper P is in contact with the platen roller 2, the paper P comes into 
contact with an ink film F extending from a feed roll 3 to a takeup roll 
4. At the position where the paper P and the film F are in contact with 
each other, inks on the ink film F are transferred onto the paper P by a 
thermal head 5 disposed on the rear side of the film F, whereby images are 
recorded on the paper P. The diagram further shows a guide roller 7 for 
stabilizing the tension on the ink film F, and a separating roller 8 for 
moving the ink film F out of intimate contact with the transfer paper P 
after recording. Disposed in the vicinity of the separating roller 8 is a 
sensor 9 for detecting marks SY, SM and SC (FIG. 2) indicating the colors 
of the ink film F. 
The tractor feeder 1, which is coupled to a stepping motor M1, operates to 
transport the paper P in two directions, i.e., forward (direction of arrow 
a) and reversely (direction of arrow b). The amount and direction of 
transport of the transfer paper P are controlled by changing the number 
and polarity of pulses to be given to the stepping motor M1. 
The takeup roll 4 is coupled to a stepping motor M2 through a one-way 
clutch 10 for transporting the ink film F only in the direction of arrow 
c. The amount of transport of the ink film F is controlled by varying the 
number of pulses to be given to the stepping motor M2. The one-way clutch 
10 transmits a drive force from the motor M2 to the takeup roll 4 for 
transporting the ink film F in the direction of arrow c but does not 
transmit any reverse torque. A slip clutch or the like is used as the 
one-way clutch 10 so that an excessive torque, even if acting in the film 
transport direction c, will not break the ink film F. The one-way clutch 
10 gives constant tension to the ink film F at all times, thus permitting 
the takeup roll 4 to wind up the film reliably. 
The thermal head 5 is pivotally movable between a position where the head 5 
presses the transfer paper P and the ink film F against the platen roller 
2 and a position away from the platen roller 2. An eccentric cam 11 in 
contact with a portion of the thermal head 5 is provided for pressing the 
thermal head 5 against the platen roller 2 or moving the head away from 
the roller 2. The eccentric cam 11 is driven by a rotary solenoid 14 (FIG. 
3). When the thermal head 5 is pressed against the platen roller 2, a 
multiplicity of heating elements mounted on the thermal head 5 and 
arranged in a direction parallel to the axis of the platen roller 2 are 
selectively driven, whereby the ink is transferred from the ink film F to 
the transfer paper P. 
With reference to FIG. 2, the ink film F is in the form of a strip having 
approximately the same width as the transfer paper P and is provided 
thereon with ink layers IY, IM and IC of three colors, i.e., yellow, 
magenta and cyan, having a specified length L. The ink film F has the 
color indicating marks SY, SM and SC preceding the respective ink layers 
IY, IM and IC with respect to the direction of transport of the film. The 
marks SY, SM and SC indicate the colors of the respective following ink 
layers and are detectable by the color sensor 9. 
FIG. 3 is a block diagram showing the control circuit of the thermal 
transfer color printer. The printer is controlled in its entirety by a CPU 
12 which has connected thereto a print switch 13, the color sensor 9, the 
motors M1 and M2, the thermal head 5, and the rotary solenoid 14 for 
pivotally moving the thermal head 5. 
The recording operation of the color printer will be described below with 
reference to the flow charts of FIGS. 4A to 4C and the time chart of FIG. 
5. 
When depression of the print switch 13 is detected (step S1), the stepping 
motor M2 is energized to start transporting the ink film F (step S2). In 
this state, the thermal head 5 is away from the platen roller 2. Upon the 
color sensor 9 detecting the yellow mark SY, a counter CT is reset to "0" 
(step S4), and the motor M2 is deenergized to discontinue transport of the 
ink film. The counter CT is advanced every time an image is recorded in 
each of the colors, yellow, magenta and cyan. At this time, the leading 
end of a recording area RA of the transfer paper P where the image is to 
be recorded is positioned at a distance of L2+L5 away from the thermal 
head 5 upstream thereof with respect to the direction of advance of the 
paper P as seen in FIG. 6. To position the leading end of the recording 
area RA in opposed relation with the thermal head 5, the stepping motor M1 
is therefore energized to start advancing the paper P (step S6). 
Subsequently, the number of pulses PP given to the stepping motor M1 
becomes equal to the sum of pulse numbers P2 and P5 for transporting the 
paper P by a distance L2 and distance L5, respectively (step S7), 
whereupon the motor M1 is deenergized to discontinue the transport of the 
paper P. 
In this way, the yellow ink layer IY of the ink film F and the recording 
area RA of the paper P are both positioned opposed to the thermal head 5, 
whereupon the thermal head 5 is pressed against the platen roller 2 (step 
S9). Recording data for one line is transferred to the thermal head 5 step 
S10) to drive heating elements individually according to the recording 
data and thereby record an image portion for one line (step S11). A 
predetermined number of pulses are then fed to each of the stepping motors 
M1 and M2 to advance both the paper P and the ink film F in synchronism by 
one line (step S12). The one-line recording operation of steps S10 to S12 
is repeatedly conducted until the contemplated image is completely 
recorded in the recording area RA for every line. 
When the recording in the yellow ink has been completed in all lines of the 
recording area RA (step S13), both the stepping motors Ml, M2 are 
energized to advance the paper P and the film F in intimate contact 
therewith in synchronism (step S14), and the counter CT is advanced by an 
increment of 1 (step S15). The number of pulses PP given to the stepping 
motor M2 then reaches a pulse number F1 for transporting the film F by a 
distance L1 from the position opposed to the thermal head 5 to a position 
where the film is separated from the paper P by the separating roller 8 
(step S16), whereupon the counter CT is checked as to whether the count is 
smaller than "3". If the count is smaller than "3", indicating that 
recording in all the three colors has not been completed, the stepping 
motor M2 is deenergized (step S18), with the stepping motor M1 held 
energized, advancing the paper P. Consequently, with the thermal head 5 
pressed against the platen roller 2, the ink film F in intimate contact 
with the transfer paper P is transported not by the torque of the stepping 
motor M2 but only by the movement of the paper P. The number of pulses PP 
given to the stepping motor M1 in step S14 and the following steps then 
reaches a pulse number (P1+P3) required for transporting the paper P by 
the distance L1 plus a specified distance L3 (step S19), whereupon the 
thermal head 5 is released from the platen roller 2 (step S20). The ink 
film F has been moved forwardly by contact with the paper P with the 
stepping motor M2 held unenergized and therefore slackens in the vicinity 
of the separating roller 8 without being wound up on the takeup roll 4 
(see FIG. 7). 
After the ink film F has been thus slackened, the stepping motor M1 is 
brought into reverse rotation to start transporting the paper P reversely 
in the direction of arrow b (step S21). During the reverse transport, the 
number of pulses PP given to the stepping motor M1 reaches a pulse number 
(P0+P1+P2+P3) needed for a nonrecording area of the paper P, which is away 
from the leading end of the recording area RA by the distance L2 upstream 
thereof, to retract to the position opposed to the thermal head 5 (step 
S22), whereupon the stepping motor M1 is deenergized (step S23). The 
pulses, P0 in number, are those given to the stepping motor M1 for 
transporting the paper P by a distance corresponding to the length L0 of 
the recording area RA along the direction of transport. 
After the nonrecording area of the transfer paper P has been thus brought 
to the position where it is opposed to the thermal head 5, i.e., the 
position where the area comes into contact with the ink film F, the 
stepping motor M2 is energized to start transporting the ink film F (step 
S24) for recording an image in the magenta ink. Upon the color sensor 9 
detecting the magenta mark SM (step S25), the stepping motor M2 is 
deenergized to discontinue the transport of the film F (step S26). 
Subsequently, to position the leading end of the recording area RA of the 
paper P in opposed relation with the thermal head 5, the stepping motor M1 
is energized (step S27). The number of pulses PP given to the motor then 
reaches the pulse number P2 required for transporting the paper P by the 
distance L2 (step S28), whereupon the motor M1 is deenergized (step S29). 
In this way, the leading end of the recording area RA already bearing the 
yellow record is registered with the leading end of the magental ink layer 
IM at the position opposed to the thermal head 5, and the same procedure 
as the foregoing steps S9 to S29 is executed to produce a magenta record. 
After the completion of recording in magenta, the same steps as steps S9 
to S16 are repeated to produce a cyan record. Consequently, a color image 
is formed in the recording area with the three color inks. 
When the recording in cyan is completed, the count of the counter CT is 
"3". This is confirmed in step S17, which is then followed by step S30 to 
turn off the stepping motor M2 which is energized after the completion of 
cyan recording. The thermal head 5 is released from the platen roller 2 in 
step S31. The number of pulses PP given to the stepping motor M1 after the 
cyan recording then reaches a pulse number (P1+P3+P4) for discharging the 
transfer paper P from the printer (step S32), whereupon the motor M1 is 
deenergized (step S33). The pulse number P4 is set to a desired value for 
transporting the paper P excessively by a distance L4 to discharge the 
paper from the printer after recording in all colors has been completed. 
With the thermal transfer color printer embodying the invention and 
described above, the transfer paper P only is advanced with the stepping 
motor M2 for transporting the ink film F held deenergized after the 
completion of recording in yellow as well as in magenta to thereby slacken 
the ink film F, whereupon the paper P is transported in the reverse 
direction. Accordingly, the paper P can be transported reversely with 
reduced frictional resistance between the paper P and the ink film F since 
the film F is not tensioned. 
Further with the present embodiment, the ink film F is transported while 
opposed to the nonrecording area of the transfer paper P for the color 
sensor 9 to detect the marks SY, SM and SC provided for the respective ink 
layers on the film F, with the result that the ink already transferred 
onto the paper P is precluded from coming into contact with the ink on the 
film F. Investigation of the frictional resistance between the transfer 
paper and the ink film we made has revealed that when the ink on the ink 
film is brought into contact with the ink transferred to the paper, the 
frictional resistance acting between the two ink portions is exceedingly 
great, whereas when the ink film is transported while opposed to the 
nonrecording area of the transfer paper as in the present embodiment, the 
frictional resistance can be substantially diminished. 
With the frictional resistance between the paper and the ink film thus 
diminished, both the paper and the film can be transported with good 
stability to afford satisfactory color images. 
Although the transfer paper used in the foregoing embodiment is a strip of 
paper in the form of a roll, cut sheets of paper are also usable. In this 
case, however, there arises a need to provide in place of the tractor 
feeder a pair of paper transport rollers at each of the upstream side and 
downstream side of the platen roller with respect to the direction of 
transport of the sheet for handling the paper sheet, and to control the 
amount and direction of rotation of the rollers. It is also necessary to 
provide guide plates defining the path of transport of the paper sheet. 
Furthermore, d.c. motors are usable in place of the stepping motors 
employed in the above embodiment as drive means for transporting the 
transfer paper and the ink film. However, since the rotation of the d.c. 
motor is not controllable by varying the number of pulses, there is a need 
to use an encoder, timer or the like for controlling the amount of 
rotation of the motor. 
Although the ink film used for the above embodiment has yellow, magenta and 
cyan ink layers, a black ink layer may be formed on the film in addition 
to these ink layers, or an entirely different combination of colors may be 
used. These ink layers of different colors can be arranged in an optional 
order. 
Although the present invention has been fully described by way of example 
with reference to the accompanying drawings, it is to be noted that 
various changes and modifications will be apparent to those skilled in the 
art. Therefore, unless otherwise such changes and modifications depart 
from the scope of the present invention, they should be construed as being 
included therein.