The present invention relates to a melting type thermal transfer recording device and recording method wherein multi-gradation representation of high resolution and high quality is achieved by using thermal melting type ink.
The multi-gradation representation using thermal melting type ink is generally performed by a dither method employing a plurality of pixels (matrix), or by a heat concentration method employing a thermal head with special heating resistors (see, for instance, "Imaging" Part I, Photographic Industry Publishing. Co., Pages 103 to 108).
However, the method employing a plurality of pixels has a drawback that the resolution is degraded so that the image quality is remarkably worsened, and also the heat concentration method wherein a special thermal head is used as disclosed in JP-A-60-58877 has a drawback that printing costs are increased even though the thermal head is a conventional one.
For solving these problems, a melting type thermal transfer recording system which enables the achievement of a multi-gradation image of high quality employing a thermal head as used in ordinary thermal transfer recording devices was proposed in JP,A, 6-286181 and U.S. Pat. No. 5,521,626.
This proposal is directed to a melting type thermal transfer printing system comprising: an ink ribbon comprising a thin film and a thermal melting type ink applied on the thin film at an amount of not more than 2.5 g/m.sup.2, a surface porous recording medium comprising a base material and a surface porous layer formed on the base material and having a pore diameter of 1 to 10 .mu.m, a thermal head comprising a plurality of heating resistors arranged in a line at intervals of 8 dot/mm or less, each of the resistors, when generating heat, showing such a temperature gradient that the temperature is highest at the central portion thereof and decreases toward the periphery thereof, and a gradation control circuit for controlling an area of the ink melted with the heating resistors by controlling an amount of electricity supplied to the thermal head, wherein the system is arranged such that while the ink of the ink ribbon is brought into close contact with the porous ink receiving layer of the surface porous recording medium whereupon the thermal head is pressed from the thin film side of the ink ribbon, the area of melted ink of the ink layer is controlled by the gradation control means to form a multi-gradation image on the surface porous recording medium.
This recording system is capable of forming outstandingly excellent multi-gradation images as compared with the conventional dither method.
However, it often happens in the above recording system that the amount of ink that is transferred to the surface porous recording medium is not stable due to factors such as conditions for exfoliating the ink ribbon, thereby giving uneven images. Further, the image reproducibility in low density regions is inferior as compared with the above-mentioned heat concentration method using a special thermal head. On the other hand, improvements in techniques for obtaining high quality images in recording methods such as ink jet systems and sublimation type thermal transfer systems wherein the gradation is achieved by a density gradation are quite remarkable. At the present stage the above-mentioned conventional recording system is inferior in image quality to the image forming systems using such methods.
In view of the foregoing, it is an object of the present invention to provide a recording device and recording method which employ melting type thermal transfer system providing images excellent in light resistance and water resistance at low cost, and which are capable of providing multi-gradation images of high resolution and high image quality that are identical to or superior to those obtained by ink jet recording systems or sublimation type thermal transfer recording systems.