Incremental printing of symbolic information – Thermal marking apparatus or processes – Having an intermediate medium for transferring ink
Reexamination Certificate
2001-02-20
2002-04-23
Tran, Huan (Department: 2861)
Incremental printing of symbolic information
Thermal marking apparatus or processes
Having an intermediate medium for transferring ink
C347S176000
Reexamination Certificate
active
06377291
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an image forming technology for forming multicolor images using area gradation (by which gradation is set by the sizes of dots in pixels) by thermal transfer and, more particularly, to an image forming technology which uses a method (to be referred to as a dot-on-dot method hereinafter) which obtains a predetermined color by stacking dots having different colors in substantially the same spot.
A printing method is practically most widely used among other methods of writing images on a medium on the basis of image information. Other technically possible examples of the methods are a thermal transfer method to be described in the present invention, electrophotography method, ink-jet method, thermal destruction method, and various transfer recording methods using photopolymerization recording materials.
Unfortunately, any of these methods has some problems, e.g., difficulty in forming an image directly on a final recording medium (a final product) to which the image is to be given, low mass-productivity, and high cost. In cases like these, an image is formed on an intermediate transfer medium and then transferred from this intermediate transfer medium onto a final product.
When an image forming method is a thermal transfer method using, e.g., a sublimating dye, the operation is performed following a procedure explained below as is well known. First, a thermal transfer ribbon coated with a sublimating dye in a thermally transferable form and a target body as a final recording medium are overlapped on a substrate film. Subsequently, the thermal transfer ribbon is selectively heated by using a thermal head or the like on the basis of image data, thereby recording a desired image on the target body by transfer.
When the faces of different persons are to be separately recorded on different target bodies, for example, the above means can easily record a number of different images as color images having rich gradation on target bodies. This is the advantage which the printing method does not have. That is, if the printing method is used to record the faces of different persons, enormous cost, labor, and time are generally required, resulting in very poor economy.
On the other hand, materials which can be dyed by sublimating materials are limited. That is, it is possible to use only target bodies made of limited materials such as polyester, acrylic resin, and vinyl chloride resin. Hence, when thermal transfer recording using a sublimating dye is to be performed although a material other than these materials is used as a target body, some improvements are necessary as disclosed in, e.g., Jpn. Pat. Appln. KOKAI Publication No. 63-81093. In this reference, an image writing unit using a transfer ribbon of a sublimating dye and a thermal head first writes an image on a film-like intermediate transfer medium having an adhesive layer. Subsequently, a transfer unit transfers the image on this intermediate transfer medium together with the adhesive layer onto a target body by heat and pressure.
The above method is an example using a sublimating dye. In the following description, however, methods which use coloring materials other than a sublimating dye and by which an image is once formed on an intermediate transfer medium and then transferred, together with the layer in which it is formed, from this intermediate transfer medium onto a target body will be generally referred to as indirect transfer methods.
In some cases, however, images cannot be directly formed on target bodies, or enormous cost and time are required if images are to be actually formed. This happens due to various reasons when, e.g., a target body as a final product (a recording medium) has a nonuniform thickness, has a rough surface (a typical example is a contactless IC card), or is a semi-completed product such as a booklet (a typical example is a passport). In such cases, images can be formed only by indirect transfer methods in practice.
If electrophotography is used as a method of writing an image on an intermediate transfer medium on the basis of image information and if the image is a full-color image, an electrophotographic process must be repeated three times (for three colors) or four times (for four colors). The electrophotographic process of each color includes charging of a photosensitive body, formation of a latent image on the charged photosensitive body by exposure, development of a toner image corresponding to the latent image on the photosensitive body, transfer of the image to a transfer member such as a transfer drum for temporarily storing the toner image of the corresponding color, erasure of unnecessary charged portions on the photosensitive body, cleaning of the photosensitive body, and the like. In this case, therefore, the process is time-consuming and, in addition to that, it is necessary to prevent unstable image formation resulting from the use of static electricity which is very unstable.
Furthermore, since the sizes of dots of toner images for forming an image cannot be largely changed, the image is basically a binary image. Accordingly, a density change of an image cannot be expressed without using the method of pseudo area gradation using a dither matrix of Bayer type or Fatton type (including screw type). As a consequence, an image itself is coarse.
When the ink-jet method is used, on the other hand, an image is formed on an intermediate transfer medium by using a liquid ink, so the image must be dried. This also poses a problem of nozzle clogging. Additionally, since the sizes of dots cannot be largely changed even in the ink-jet scheme, the method of pseudo area gradation such as a dither matrix or an error diffusion method is used. This often decreases the resolution of an image.
Note that the thermal destruction method cannot form a full-color image at present.
For the reasons described above, image formation by the thermal transfer method using a sublimating dye is simple and inexpensive and can achieve high image quality and high resolution. Accordingly, this method is superior as an image forming method to other indirect transfer methods.
Unfortunately, this thermal transfer method using a sublimating dye has a large drawback: a sublimating dye itself is a coloring material very inferior in so-called resistances, e.g., heat resistance, light resistance, and solvent resistance. Hence, when a sublimating dye is used, the durability of an image on a target body as a final product significantly lowers. For example, even when a target body is an IC card having a heat resistance of about 120° C., a decrease in image density due to a phenomenon such as thermal decomposition or resublimation of a sublimating dye occurs at about 80° C. That is, no sublimating dye can have a heat resistance exceeding a heat resistance of 120° C. of a target body.
Also, when paper such as a passport is used as a target body, an image transferred onto the paper surface “oozes out” from the back side owing to the ambient of a solvent such as paradichlorobenzene or naphthalene often used as a mothproofing agent. Additionally, a sublimating dye resublimates from the paper fibers at high temperatures, and this lowers the image density.
Furthermore, since the sublimating printing method is in widespread use in the world, if this method is used for a security purpose of, e.g., a passport, the passport is readily forged or altered. In addition, this forgery or alteration cannot be easily found.
To solve these problems unique to a sublimating dye and achieve simplicity, low cost, high image quality, and high resolution of the thermal transfer method, a melt-transfer printing method using area gradation is very effective. This method obtains gradation by changing the sizes of dots to be transferred in accordance with the amount of heat generated by a thermal head used in thermal transfer. That is, area gradation is possible by changing a region in which an ink-ribbon ink is softened or melted, in accordance with the controlled heat amount from the thermal head.
In
Andoh Tomio
Honma Nobuaki
Marugame Tomoyuki
Staas & Halsey , LLP
Toppan Printing Co. Ltd.
Tran Huan
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