Incremental printing of symbolic information – Thermal marking apparatus or processes – Specific resistance recording element type
Reexamination Certificate
2000-08-30
2002-09-17
Tran, Huan (Department: 2861)
Incremental printing of symbolic information
Thermal marking apparatus or processes
Specific resistance recording element type
Reexamination Certificate
active
06452621
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a thermal head for making a stencil by thermally perforating a heat-sensitive stencil material, and more particularly to an inexpensive thermal head formed by a thick film process.
2. Description of the Related Art
In stencil making apparatuses which have been put into practice, a heat-sensitive stencil material is used, and there have been known two stencil making systems. One of the stencil making systems is so-called a flash system in which an original having a printing area containing therein carbon is brought into a close contact with a heat-sensitive stencil material is perforated by heat when a printing area of the original is exposed through the stencil material to flashlight from a flash bulb, a xenon flashtube or the like. The other stencil making system is so-called a digital system in which a stencil material is thermally perforated by selectively energizing heater elements of a thermal head according to an image signal read out from an original through an image sensor or the like, or an image signal representing a document and/or an image created through a computer or the like. The digital system is now prevailing over the flash system since the digital system permits the document editing and the image processing. Though the thermal head was once a device exclusively used in facsimiles, thermal recording printers or the like, recently the thermal head has been modified so that it can be used in thermal stencil making. Recently, the modified thermal head has come to be used in a thermal stencil making apparatus of the digital system. As the stencil material, there have been known one comprising thermoplastic resin film laminated to a porous base sheet and one comprising thermoplastic resin film with no base sheet.
Specific structures of thermal heads to be used in thermal stencil making are disclosed, for instance, in the following patent publications.
In Japanese Unexamined Patent Publication Nos. 63(1988)-191654 and 6(1994)-191003, the thickness of the protective layer of the thermal head is defined. In Japanese Unexamined Patent Publication Nos. 2(1990)-67133, 4(1992)-71847, 4(1992)-265759, 5(1993)-345401, 5(1993)-345402, 5(1993)-345403 and 6(1994)-115042, the length in the main scanning direction of the heater element and/or the length in the sub-scanning direction of the same is defined for the pitch of the heater elements in each direction. In Japanese Unexamined Patent Publication Nos. 4(1992)-45936, 7(1995)-68807 and 7(1995)-171940, there is disclosed a thermal head in which the heater element is not rectangular in shape. In Japanese Unexamined Patent Publication Nos. 4(1992)-314552 and 8(1996)-142299, there is disclosed a thermal head in which a cooling member is disposed between each pair of adjacent heater elements. In Japanese Unexamined Patent Publication Nos. 4(1992)-369575 and 8(1996)-132584, the shape or thickness of the glaze layer is defined. Further, in Japanese Unexamined Patent Publication No. 5(1993)-185574, the ratio of the length in the main scanning direction to the length in the sub-scanning direction of the heater element is defined.
Though not clearly described in the above patent publications, the thermal heads disclosed in the above patent publications can be considered to be of a thin film type except those disclosed in Japanese Unexamined Patent Publication Nos. 5(1993)-345401, 5(1993)-345402 and 5(1993)-345403. Actually, at present almost all the thermal stencil making apparatuses using a thermal head use a thin film thermal head, and those using a thick film type thermal head are limited to those for a postcard, those which function also as a word processor printer and those which function also as a heat transfer labeler. Only a very small fraction of the digital system thermal stencil making apparatuses uses a thick film type thermal head.
As pointed out by many of the aforesaid patent publications, it is preferred that the thermoplastic resin film of the stencil material be perforated in such a manner that perforations are discrete and adjacent perforations are not connected to each other. This is because of inherent characteristics of stencil printing that ink is viscous fluid and spreads wider than the area of the perforations when transferred to the printing paper through perforations of the stencil, and when the perforations are connected, the amount of ink transferred to the printing paper and the thickness of printed ink layer on the printing paper are acceleratedly increased and offset is caused. The thermal head for thermal stencil making differs in this point from that for thermal recording in which that recorded pixels overlaps each other is preferred.
In the digital system thermal stencil making, it is preferred that the perforations be separated from each other, the proportion of open area (the proportion of the area of the perforations per unit area of the thermoplastic film of the stencil material) be in a predetermined range (generally about 30 to 40% though depending upon the viscosity of the ink, the kind of the printing paper, the pressure at which the stencil is pressed against the printing paper, and the like) in order to ensure a proper printing density, and the shapes and the areas of the perforations be substantially uniform so that the unperforated portions between the perforations are arranged in a regular pattern and the densities of large printing areas such as solid parts are uniformed.
Typically, the thin film thermal head comprises a heat radiating plate of metal, an electrical insulating substrate and a glaze layer formed on the heat radiating plate in this order, a plurality of strip-like resistance heaters which are formed on the glaze layer to extend in one direction (the sub-scanning direction) and are arranged in a direction transverse to said one direction (the main scanning direction), and a plurality of electrodes each superposed on one of the strip-like resistance heaters with a part of the resistance heater exposed through a gap formed in the electrode. The exposed part of each strip-like resistance heater forms a heater element. That is, a pair of electrodes are formed on each resistance heater with their inner ends opposed to each other in the sub-scanning direction with a gap between. One of the electrodes is connected to a switching element for discretely energizing the heater element and the other electrode are integrated with the corresponding electrodes for the other heater elements to form a common electrode. When producing such a thin film thermal head, an electrical insulating substrate and a glaze layer are superposed on a heat radiating plate and a solid resistance heater layer and a solid electrode layer are formed in this order on the glaze layer. Then the electrode layer is removed along a line extending in the main scanning direction, thereby exposing the resistance heater layer in a line extending in the main scanning direction, and the resistance heater layer and the electrode layer are both removed in the sub-scanning direction at regular intervals in the main scanning direction. Thus, a plurality of strip-like resistance heater layers are formed each covered with a pair of electrode layers opposed to each other in the sub-scanning direction with a gap between. One of the electrode layer is connected to a switching element and forms a discrete electrode for discretely energizing the part of the resistance heater layer free from the electrode layer. The other electrode layer is integrated with the corresponding electrode layer for the other strip-like resistance heater layers to form a common electrode. A protective layer is formed to cover the discrete electrodes, the exposed part of the resistance heater layer and the common electrode. When an electric potential different from the common electrode is applied to a discrete electrode, the exposed part of each of the strip-like resistance heaters between the discrete electrode and the common electrode is energized and generates heat. That is,
Nixon & Peabody LLP
Riso Kagaku Corporation
Strudebaker Donald R.
Tran Huan
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