1. Field of the Invention
The present invention relates to a thermal recording apparatus which thermally transfers inks of different colors on a print medium by selectively driving a plurality of heating-elements mounted on a thermal head to generate heat. The invention more particularly relates to a thermal recording apparatus in which odd and even heating-elements are energized at different time points to transfer an ink of one color every time each of the odd and even heating-elements are alternately energized, in order to prevent overlap of the first color ink and the second color ink, and then the third color ink is transferred on the position displaced from the positions where the first and second color inks are transferred, so that the color inks are less overlapped each other to improve color-reproducibility, and the size of apparatus and the manufacturing cost are reduced.
2. Description of Related Art
There have been proposed various thermal recording apparatus to dissolve the problems such as the unevenness of density caused by a difference of transferring efficiency between inks and the mixing failure and fixing failure due to insufficient melted ink. Such the problems are caused when inks of different three colors, namely, cyan, magenta, and yellow are thermally transferred on a print medium by a thermal head so that heating-elements of the thermal head are energized by the same energy to thermally transfer the three color inks, overlapping one another.
For example, Japanese patent (JP) publication No. 3(1991)-54633 discloses a thermal transfer recording apparatus that is provided with a circuit for discriminating the recording history of a recording paper with a recorded picture element as a unit and a thermal head is controlled in accordance with the recording history to energize heating-elements to generate heat. The energy control is executed by the control of pulse width, duration of the power to be supplied to the heating-elements or voltage of the power, and the control of temperature of the thermal head. Accordingly, recording images transferred with uniform density can be obtained by the second and following thermal transfer recording, regardless of whether or not an ink is previously thermally transferred on the recording paper.
JP patent publication, No. 3(1991)-73471 discloses a multicolor thermal recording apparatus provided with means for setting the energy to be supplied to a thermal head for the n-th overlap recording to be larger than that for the (nxe2x88x921)th recording. Concretely, the means sets the amount of energy to be supplied to the. thermal head for second overlap recording to be larger than that for the first recording, and the amount of energy for the third overlap recording to be larger than that for the second recording. Accordingly, the ink used for overlap recording can be melted in and mixed with the ink used before the overlap recording, improving mixability of the inks. Also, the ink used for the overlap recording can be saturated in the recording paper with the ink used before the overlap recording, so that the ink fixation can be enhanced.
However, the above thermal recording apparatus disclosed in JP publications Nos. 3-54633 and 3-73471 require to control the amount of energy to be supplied to each of the heating-elements of the thermal head every time the thermal transfer is performed, and to change the amount of energy according to temperature variations in an operating environment. This results in a complicated control circuit, a large-sized apparatus and an increase in manufacturing cost.
The present invention has been made in view of the above-circumstances and has an object to overcome the above problems and to provide a thermal recording apparatus capable of reducing the amount of overlapping of plural inks having different colors by simple control, of improving color-reproducibility, and of reducing the apparatus size and the manufacturing cost thereof by energizing odd and even heating-elements at different time points to transfer an ink of one color every time each of the odd and even heating-elements are energized so that the first and second color inks are transferred without overlapping each other, and then the third color ink is transferred in a position displaced from the positions of the first and-second color ink.
Additional objects and advantages of the invention will be set forth in part in the description which follows and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the purpose of the invention, there is provided a thermal recording apparatus including a thermal head provided with a plurality of heating-elements, the thermal head being movable in a first direction, a drive device for selectively driving the heating-elements to generate heat, an ink ribbon applied thereon with at least a first color ink and a second color ink, a print medium on which images are to be printed through the ink ribbon by the selected heating-elements, the print medium being fed in a second direction intersecting the first direction, means for dividing the heating-elements into a fist group of odd heating-elements and a second group of even heating-elements, and alternately driving the odd heating-elements and the even heating-elements at different time points, a memory for storing image data corresponding to the first color and image data corresponding to the second color, and control means for operating the drive device to drive one group of the first heating-element group and the second heating-element group to generate heat and, after moving the thermal head by one pitch defined by a print dot in the first direction, operating the drive device again to drive the other group of the first heating-element group and the second heating-element group to generate heat, thereby to print the first color ink on the print medium based on the image data, and for operating the drive device to drive the other group that is different from the one group driven first to print the first color ink thereby to print the second color ink on positions where the first color ink has been printed on the print medium.
In the above thermal recording apparatus according to the present invention, the heating-elements associated with odd dots to be printed and those associated with even dots are driven to generate heat at different time points. To thermally transfer an ink of the first color on a print medium based on the image data, a plurality of heating-elements out of one of the odd and even heating-elements are energized and, after the thermal head is moved by a printing pitch in the predetermined sub-scanning direction, a plurality of heating-elements out of the other of the odd and even heating-elements are energized. In this way, the odd and even heating-elements are alternately energized every time after the thermal head is moved by a printing pitch each.
Sequentially, to thermally transfer an ink of the second color, the odd or even heating-elements that differ from ones used for the thermal transference of the first color ink in a transference start position thereof are energized to generate heat, transferring the ink of the second color.
Accordingly, the first color ink and the second color ink are thermally transferred without overlapping each other, so that images with uniform density can be formed on the print medium. Since the energy to be supplied to a heating-element needs no controlling for respective heating-elements, each of the first and second color inks can be thermally transferred on the print medium by a simple control, enabling the reduction of the size and manufacturing cost of the apparatus. The first color ink and second color ink are thermally transferred in a staggered and latticed arrangement, thereby improving color-reproducibility due to an additive process. Furthermore, the alternate driving of the odd and even heating-elements can substantially prevent the influence of accumulation of heat on each of the heating-elements, which enables printing of color dots with a fine diameter.