Printing – Stenciling – Stencils
Reexamination Certificate
2000-06-23
2002-03-19
Funk, Stephen R. (Department: 2854)
Printing
Stenciling
Stencils
C428S195100
Reexamination Certificate
active
06357347
ABSTRACT:
The present invention relates to a stencil sheet, and more particularly to a stencil sheet which does not expand even if it has been used for continuous printing of a large number of copies and can give sharp printed images faithful to originals.
Stencil sheets generally comprise a thermoplastic resin film such as polyester film, polyvinylidene chloride film or polypropylene film and a porous support of a thin paper, a nonwoven fabric or a gauze made of natural fibers or synthetic fibers, the film and the porous support being laminated to each other with an adhesive (JP-A-57-182495, JP-A-58-147396, JP-A-59-115898, etc.).
However, printed images obtained using these conventional stencil sheets (hereinafter sometimes referred to as “stencil printing sheets”) are not necessarily satisfactory in sharpness of the printed images. Various causes are considered for the sharpness of the printed images being unsatisfactory, and one of them relates to the fibers constituting the porous support (hereinafter sometimes referred to as merely “supports”). That is, when the commonest thin paper comprising natural fibers is used as a porous support, passing of ink therethrough is apt to become uneven because the fibers are thick, non-uniform and flat. If ink is hindered from passing through perforations of the film, the prints are faded or voids are produced in the solid prints. Moreover, if large foreign matters coming from the natural fibers are not sufficiently removed at the production step of supports, these foreign matters hinder the passing of ink to cause generation of voids.
For the improvement of these defects, it has been proposed to use a thin paper made of natural fibers and synthetic fibers in admixture or a nonwoven fabric comprising thin synthetic fibers such as polyester fibers or polypropylene fibers as a porous support, thereby to reduce the basis weight of the fibers as much as possible (See, for example, JP-A-59-2896, JP-A-59-16793, and JP-A-2-67197).
Furthermore, for the improvement of sharpness of printed images, it is effective to enhance perforation sensitivity of the thermoplastic resin films, and for this purpose, a heat-sensitive stencil printing sheet comprising a film of thin thickness has been proposed.
However, if the fibers of the support are made thin, the basis weight is reduced or the thickness of the film is made thin, there are the following problems, namely, running property of the stencil sheet is deteriorated to cause jamming in the printing machine, and when the perforated sheet is wound around a printing drum, creases occur in the sheet (creasing at winding) to cause distortion or blurring of the printed images at the creasing portion, resulting in deterioration of sharpness of the printed images. Furthermore, in continuous printing of a large number of copies, expansion of the sheet occurs (expansion at printing) to cause decrease in reproducibility of original or creasing occurs during the printing (creasing at printing) to cause deterioration in sharpness of the printed images.
In order to solve these defects, it has been proposed to carry out printing with a stencil sheet having a given tensile strength in machine direction and a given flexural rigidity (JP-A-8-67080) and to carry out printing with a stencil sheet having a given wet elongation under a certain tensile load (JP-A-5-104875). These stencil sheets are satisfactory in that they are excellent in running property and creases hardly occur in winding around a printing drum, but are still not satisfactory in that they show expansion at printing and crease at printing, and are not sufficient in reproducibility of originals and sharpness of the printed images.
The object of the present invention is to solve the above problems in conventional techniques, and to provide a stencil printing sheet which is inhibited from expanding at printing of a large number of copies and from creasing at printing and thus can faithfully reproduce originals and give sharp prints.
The above object can be attained by a stencil sheet comprising a laminate of a thermoplastic resin film and a porous support mainly composed of a synthetic fiber, characterized in that the stencil sheet has a wet tensile strength in longitudinal direction of 200 gf/cm or more and a breaking strength under shear of 400 gf/cm
2
or more.
At the stencil printing, the printing paper in contact with the stencil sheet gives an external stress to the stencil sheet in the direction of the stencil sheet being expanded. This external stress is caused by back tension generated by printing paper feeding rollers. Under a certain external stress, the expansion at printing decreases with increase of wet tensile strength of the stencil sheet in longitudinal direction, and hence the greater wet tensile strength of the stencil sheet in longitudinal direction is preferred. If the wet tensile strength of the stencil sheet in longitudinal direction is less than 200 gf/cm, the expansion of the stencil sheet at printing is great and besides creases occur in the stencil sheet at printing to deteriorate reproducibility of originals. Moreover, the stencil sheet sometimes cannot be smoothly carried if the stencil sheet is insufficient in tensile strength because a tension is applied to the stencil sheet in the running direction during being carried in a printing machine. Furthermore, when the stencil sheet is wound around a printing drum, the stencil sheet creases. Therefore, in the present invention, the stencil sheet is needed to have a wet tensile strength in longitudinal direction of 200 gf/cm or more, preferably 300 gf/cm or more.
However, even in the case of stencil sheets of 200 gf/cm or more in wet tensile strength in longitudinal direction, some of them are great in expansion at printing or are apt to crease at printing depending on the kind of the stencil sheets. Therefore, the inventors have conducted intensive research on the mechanism of occurrence of expansion at printing and occurrence of creases at printing, and, as a result, it has been found that the heat-sensitive stencil printing sheets satisfying the above-mentioned wet tensile strength decrease in expansion at printing with increase of breaking strength under shear. That is, it has been found that if the breaking strength under shear in longitudinal direction of the stencil sheets is less than 400 gf/cm
2
, creases occur at printing. Therefore, in the present invention, the stencil sheets are required to have a breaking strength under shear in longitudinal direction of 400 gf/cm
2
or more, preferably 600 gf/cm
2
or more.
Thus, according to the present invention, occurrence of expansion of the stencil sheet at printing of many copies can be inhibited and occurrence of creases at printing can also be inhibited and, hence, sharp printed images faithful to originals can be obtained by using a stencil sheet which satisfies simultaneously the above requirements of wet tensile strength and breaking strength under shear.
The stencil sheet of the present invention is formed by laminating a thermoplastic resin film and a porous support mainly composed of synthetic fibers and is needed to have the above-mentioned wet tensile strength and breaking strength under shear. The “longitudinal direction” in the present invention means the peripheral direction when the sheet is wound around a drum, and is usually the same as the longer direction of a roll stencil sheet and the running direction in the stencil printing apparatus.
As the thermoplastic resin films used in the present invention, there are used those which are suitable for thermal perforation by a thermal head or the like, and examples are conventionally known films such as those of polyester, polyamide, polypropylene, polyethylene, polyvinyl chloride, polyvinylidene chloride and copolymers thereof. From the point of perforation sensitivity, polyester films are preferred.
As the polyesters, there may be preferably used polyethylene terephthalate, copolymer of ethylene terephthalate and ethylene isophthalate, polyethylene-2,6-naphthalate, polyhexame
Fitch Even Tabin & Flannery
Funk Stephen R.
Riso Kagaku Corporation
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