Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Composite having voids in a component
Reexamination Certificate
1999-09-14
2004-11-02
Shewareged, B. (Department: 1774)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Composite having voids in a component
C428S195100, C428S409000, C428S913000
Reexamination Certificate
active
06811866
ABSTRACT:
The present invention relates to a heat-sensitive stencil sheet, and more particularly to a heat-sensitive stencil sheet which causes no jamming in a stencil printing machine during feeding and no creasing at the time of winding around or loaded on a printing drum, and can provide sharp images.
Conventional heat-sensitive stencil sheets are not necessarily satisfactory in sharpness of printed images, especially, in uniformity of solid portions of the images. There are various causes therefor, and one of them is due to fibers constituting a porous substrate of the stencil sheet.
That is, thin papers comprising natural fibers, which have been most widely utilized as the substrate, have relatively thick and non-uniform fiber diameter and are flat. Therefore, ink is apt to unevenly pass therethrough, and especially passing of ink is often hindered by the fibers present just below the perforated parts of the stencil sheet to cause fading of the printed images. Furthermore, smoothness of the surface of a film laminated on the substrate is deteriorated by the thick fibers, and contact with thermal head at the time of perforation is poor to often cause deficient perforations. Thus, voids are formed in solid printing.
Some measures to solve these problems have been proposed. That is, it is proposed to use papers or nonwoven fabrics made from a mixture of synthetic fibers such as polyester fibers with natural fibers in place of the above-mentioned thin paper, thereby to make thinner the fibers of substrates or reduce the basis weight of the fibers as much as possible. See JP-A-59-2896, JP-A-59-16793, JP-A-2-67197, and the like.
However, although sharpness of images is improved by thinning the fiber diameter of the substrate or reducing the basis weight, the following new problems occur. That is, the stencil sheet is deteriorated in running property to cause jamming in the printing machine, or creases when the unperforated or perforated stencil sheet is wound around and loaded on a printing drum, resulting in deterioration of print quality.
In order to solve these problems, it is proposed to specify tensile strength and flexural rigidity of stencil sheets, namely, strength and nerve of stencil sheets. See JP-A-8-67080.
However, even if the conditions of the strength and the nerve of the stencil sheet are satisfied, the jamming of stencil sheets in the printing machine still occurs or creases are generated at the time of winding an unperforated or perforated stencil sheet around a printing drum depending on the state of dispersion or lamination of the fibers of the substrate, though the basis weight of the substrate is large.
The object of the present invention is to provide a heat-sensitive stencil sheet which is excellent in carrying properties and can be wound around a drum with forming no creases.
The inventors have made an intensive research on “running mechanism of stencil sheets”, “creasing mechanism of stencil sheets during a drum winding operation”, and “bending characteristics of stencil sheets” in a printing machine, and have found that a heat-sensitive stencil sheet satisfying a specific residual torque is excellent in carrying properties and winding properties. Thus, the present invention has been accomplished.
That is, the present invention relates to a heat-sensitive stencil sheet, which comprises a laminate of a thermoplastic resin film and a porous substrate mainly composed of synthetic fibers, said stencil sheet satisfying 0.150≦T−H wherein T means an arithmetic average value (g·cm/cm) of absolute values of KES bending torque in lengthwise direction of the stencil sheet at curvatures of +2.3 and −2.3 (cm
−1
), H means a bending hysteresis (g·cm/cm), and T−H means a residual torque (g·cm/cm).
Hereupon, the residual torque (T−H) is a numerical value relating to bending characteristics of the stencil sheet, and especially it specifies a numerical value relating to recovery from bending, and the lengthwise direction means the running direction of the stencil sheet fed to the printing machine. Furthermore, the KES is an abbreviation of KAWABATA'S EVALUATION SYSTEM FOR FABRICS, and is a method widely employed as a method for measurement of physical quantity of the texture of woven or knitted fabrics, which was devised by Prof. Sueo Kawabata of the Kyoto University in Japan.
The creasing mechanism of stencil sheets at the time of being wound around or loaded on a drum is considered as follows. That is, the stencil sheet is wound around the circumferential surface of the rotated printing drum to load it on the drum, while it is pressed by a press roller. In this case, bubbles are sometimes taken into the portion between the stencil sheet and the surface of the printing drum in the area between the back end portion of the stencil sheet and the press roller. In the region where bubbles are present, the stencil sheet is apart and raised from the surface of the printing drum.
When the printing drum is further rotated, the bubbles gather in the vicinity of the press roller to form large bubbles and the portion where the stencil sheet is apart and raised from the surface of the printing drum becomes larger. The raised stencil sheet is finally buckled and bent, and this bent portions form creases.
In the above-mentioned creasing mechanism, formation of creases of the stencil sheet has a close relation with the bending characteristics (buckling characteristics) of the stencil sheet. That is, the bending torque (bending stress) generated when the stencil sheet is raised increases with further raising of the stencil sheet, but bending hysteresis (loss of stress) at the time of recovery from bending also gradually increases.
When the raised stencil sheet is buckled as mentioned above, the bending hysteresis tends to become extremely great, and the bending torque tends to become extremely small. That is, whether the buckling readily occurs or not can be judged by the property values of the bending torque and the bending hysteresis. However, these property values are strongly affected by the shape and the basis weight of the substrate. For example, a stencil sheet having a large basis weight is great in bending hysteresis so that a large loss of bending torque is caused, but since the bending torque is inherently great, the bending torque is still large even if the loss is deducted.
That is, “residual torque” which is a value obtained by deducting “bending hysteresis” from “bending torque” of the stencil sheet can be employed as an indication of whether the creases readily occur or not.
According to the conventional techniques which employ only the KES bending rigidity value B (average bending rigidity) as the indication, even if the KES bending rigidity value B is in a specified range, there is a possibility of the residual torque being smaller than the above limitation owing to the loss of balance of the bending torque or the bending hysteresis depending on a state of dispersion or lamination of fibers of the substrate. In this case, there is a problem that the stencil sheet is buckled and jams because of weak recovery power from buckling to make the running impossible or creases are formed to deteriorate the quality of print.
On the other hand, since the residual torque employed as an indication in the present invention is a value including the influence of dispersion or lamination state of fibers of the substrate, buckling of the stencil sheet can be effectively inhibited, and failure in feeding of stencil sheets and generation of creases can be effectively inhibited.
Thus, according to the present invention, even if raising of a stencil sheet (bending of a stencil sheet) occurs due to the bubbles at the time of winding of the stencil sheet around the drum as mentioned above, when the residual torque (T−H) in the lengthwise direction which is the same as the feeding direction of the stencil sheet is 0.150 (g·cm/cm) or more, preferably 0.180 (g·cm/cm) or more, the stencil sheet has a power to recover even when it is nearly buckled, and thu
Kinoshita Hideyuki
Watanabe Hiroshi
Riso Kagaku Corporation
Shewareged B.
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