Stock material or miscellaneous articles – Structurally defined web or sheet – Physical dimension specified
Reexamination Certificate
1999-08-30
2003-10-14
Hess, Bruce H. (Department: 1774)
Stock material or miscellaneous articles
Structurally defined web or sheet
Physical dimension specified
C428S297400, C428S304400, C428S365000, C428S401000, C428S913000
Reexamination Certificate
active
06632515
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a heat-sensitive stencil sheet, and more particularly, to a heat-sensitive stencil sheet comprising a thermoplastic resin film of constant surface smoothness laminated on a porous support even when the sheet is wound up into a roll or a plurality of sheets are stacked for a long period of time, and which provides excellent printing images.
2. Background Art
A heat-sensitive stencil sheet is produced, for example, by laminating a thermoplastic resin film having a thickness of approximately 2 &mgr;m, directly or via an adhesive, to an ink-permeable porous support such as Washi (Japanese paper), and applying a releasing agent to the surface of the film to prevent sticking.
In order to make a stencil master, a thermal printing head (TPH) is typically employed. In this case, if the surface of the stencil sheet is rough, uniform contact cannot be attained between the TPH and a film laminated on a porous support, resulting in some parts being easily perforated and other parts being difficult to perforate. Accordingly, the resultant perforations do not necessarily reflect the original text or drawing faithfully, and satisfactorily printing images cannot be obtained.
To avoid this shortcoming, attempts have been made to secure surface smoothness of a stencil sheet by use of a porous support having excellent surface smoothness. Particularly, in recent years, stencil sheet rolls, i.e., stencil sheets wound up into a roll around a core such as a paper tube, are often employed so as to enhance operation efficiency of stencil printing. In this case, even though the stencil sheet has excellent surface smoothness when assuming the form of a flat sheet, winding pressure applied to the sheet during winding to form a roll deteriorates surface smoothness with the passage of time, to thereby fail to obtain excellent printing images.
In order to solve the above problem, Japanese Patent Application Laid-Open (kokai) No. 6-239048 discloses a method for preventing deterioration of the surface smoothness of a stencil sheet by adjusting the winding density of a roll. However, even though winding density is controlled, pressure is applied to a stencil sheet when the sheet is pulled from the roll set in a stencil printing apparatus, to thereby cause “tight winding” of the stencil sheet roll, with the result that surface smoothness of the stencil sheet cannot be maintained, particularly in the vicinity of the core of the roll. If winding pressure is reduced so as to avoid this drawback, another problem is caused in that handling of the roll becomes poor due to phenomena such as “core separation”, “dishing,” and “telescoping.”
As has been conventionally performed, when a porous support is directly subjected to measurement of surface smoothness by use of an air-leak tester such as an Ohken smoothness tester or a PPS (Parker Print Surf) smoothness tester, the smoothness cannot be measured correctly, due to leakage of air through the pores, and measured smoothness values disadvantageously vary widely. In addition, a laser surface profile tester cannot measure surface smoothness correctly, since the measured values are affected by the thickness of the porous support.
Japanese Patent Application Laid-Open (kokai) No. 8-67081 discloses that surface roughness of a thermoplastic resin film laminated onto a porous support is measured by use of surface roughness tester (Model SE-3E, product of Kosaka Seisakusho, stylus surface roughness tester) in accordance with JIS B-06101. The publication has proposed a method for improving performance in perforation of a stencil sheet by use of a thermal head, through the selection of a porous support having a surface roughness of 10 &mgr;m or less based on the above measurement. However, since the surface smoothness of the porous support of a stencil sheet is not specified, deterioration of surface smoothness cannot be prevented if the stencil sheet is wound up into a roll or stored in a stacked state for a long period of time. In addition, according to this prior art method, measurement of surface smoothness is performed after production of a stencil sheet by heat-pressing a porous support and a film. Therefore, surface smoothness of the produced sheet product cannot be estimated before production of the stencil sheet.
SUMMARY OF THE INVENTION
In view of the foregoing, the present inventors have conducted earnest studies on the mechanism of deterioration of surface smoothness of stencil sheets stored in a rolled state or in a stacked state for a long time, and have found that the above-mentioned problems can be removed by correctly measuring, without large variation, surface smoothness of the porous support of a heat-sensitive stencil sheet and by specifying and determining the surface roughness. The present invention was achieved based on this finding.
Accordingly, an object of the present invention is to provide a heat-sensitive stencil sheet which comprises a thermoplastic resin film of constant surface smoothness laminated on a porous support even when the sheet is wound up into a roll or a plurality of sheets are stacked for a long period of time, and which provides excellent printing images by preventing deterioration of characteristics related to perforation by a thermal head.
Accordingly, in a first aspect of the present invention, there is provided a heat-sensitive stencil sheet comprising a thermoplastic resin film laminated onto a porous support, in which the PPS surface smoothness of the surface of the porous support of the stencil sheet is 1.2 &mgr;m or less as measured while a film having PPS surface smoothness of 0.0 &mgr;m and a thickness of 0.1-10 &mgr;m is pressed against the surface of the porous support.
Preferably, in the first aspect of the invention, the PPS surface smoothness of another surface of the porous support to be laminated with a thermoplastic resin film is 0.9 &mgr;m or less as measured while a film having a PPS surface smoothness of 0.0 &mgr;m and a thickness of 0.1-10 &mgr;m is pressed against the other surface of the porous support.
Preferably, the porous support has an average fiber diameter of 2-20 &mgr;m.
In stencil sheet rolls and stencil sheets in a stacked state, the film surface of the stencil sheets adheres to a surface of a porous support of other stencil sheets due to tension during winding or pressure during storage or transportation. The adhesion causes transfer of the rough profile of the surface of the porous support to the film surface of the contacting stencil sheet. For example, an unsupported part of the film on a porous support is dented due to pressure from stacked stencil sheets as time passes, to thereby deteriorate surface smoothness of the film. Therefore, time-elapsed deterioration of surface smoothness of the film surface of a stencil sheet can be prevented by providing a porous support having a smooth surface which maintains contact with other film surface of a stencil sheet.
In the present invention, PPS surface smoothness of the surface of a porous support that maintains contact with the film surface of a stencil sheet is measured while a film having a PPS surface smoothness of 0.0 &mgr;m and a thickness of 0.1-10 &mgr;m is pressed against the surface of the porous support. Therefore, air leakage in the thickness direction of the porous support is eliminated, to thereby enable correct measurement of surface smoothness of the surface of the porous support of a stencil sheet without large variation. Furthermore, the above measurement enables selection of a porous support having a PPS surface smoothness of 1.2 &mgr;m or less, so as to surely and effectively prevent time-elapsed deterioration of surface smoothness of the film surface of stencil sheets.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
In the present invention, a PPS surface smoothness of the porous support surface of a heat-sensitive stencil sheet is measured while a film having a PPS surface smoothness of 0.0 &mgr;m and a thickness of 0.1-10 &mgr;m, preferably 0.1-5.
Fay Sharpe Fagan Minnich & McKee LLP
Hess Bruce H.
Riso Kagaku Corporation
Shewareged B.
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