Printing – Stenciling – Stencils
Reexamination Certificate
2002-09-30
2004-01-20
Funk, Stephen R. (Department: 2854)
Printing
Stenciling
Stencils
Reexamination Certificate
active
06679166
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a stencil plate making method and apparatus, in which a heat shrinkable film of a heat sensitive stencil sheet is perforated with a heating device such as a thermal head or laser beam, and also relates to a stencil plate produced by the method or apparatus. This invention particularly relates to a stencil plate making method and apparatus and a stencil plate, in which properly sized perforations with less irregularity are formed without requiring any severe temperature condition in the heating device.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
The heat sensitive stencil sheet has a thermoplastic resin film (hereinafter also called just “film”) which has a nature that perforations for penetration of ink can be formed by heating with a heating device such as a thermal head or laser. When the stencil sheet is used for printing, ink passes through the perforations and is transferred onto paper. Various materials are proposed hitherto for the film. For example, JP-A-41-7623 proposes polypropylene, polyamides, polyethylene, and vinyl chloride vinylidene chloride copolymers; JP-A-47-1184 proposes propylene copolymers; JP-A-47-1185 proposes chlorinated polyvinyl chloride; JP-A-47-1186 proposes high crystalline polyvinyl chloride; JP-A-49-6566 proposes propylene-&agr;-olefin copolymers; JP-A-49-10860 proposes ethylene-vinyl acetate copolymer; JP-A-51-2512 proposes acrylonitrile resins, JP-A-51-2513 proposes polyethylene terephthalate; Japanese Patent No. 1,669,893 proposes polyvinylidene fluoride; and Japanese Patent No. 2,030,681 proposes polyethylene naphthalate copolymers. Among them, films that are presently used for heat sensitive stencil sheets on the market are heat shrinkable films obtained by biaxially stretching a polyethylene terephthalate film or vinylidene chloride copolymer film, mainly for reasons of perforation sensitivity (i.e., performance to give sufficiently large perforations with small quantity of heat) and machine suitability (i.e., unlikelihood to cause wrinkling, loosening, elongation and deformation when the stencil sheet is produced into a stencil plate and used for printing). Especially for integral stencil printing machines which can automatically produce stencil plates and perform printing, the polyethylene terephthalate film is mainly used.
Alternatively, for forming perforations by means of heat, a film obtained by casting a resin with a low melting point may be used in place of the stretched heat shrinkable film. For example, Japanese Patent No. 1,668,117 and JP-A-62-173296 propose films obtained by casting a synthetic resin solution or emulsion, and JP-A-4-78590 proposes a cast thermoplastic resin film containing a silicone oil. In case of the cast film, it is not thermally shrunken, but since it is made of a resin low in melting point, it can be molten at heated portions to form perforations (hereinafter this film is called “hot-melt film”).
However, at present, the hot-melt film is not practically used on the market as a heat sensitive stencil sheet. The main reasons are considered to be low perforation sensitivity, perforation configuration irregularity and low mechanical strength for printing use.
Heat shrinkable films of the heat sensitive stencil sheets currently used on the market for stencil printing machines are about 1.5 to 3 &mgr;m in thickness, and encounter no difficulty in stable forming and lamination, in contrary to hot-melt films of 10 &mgr;m or less in thickness as disclosed in the Japanese Patent No. 1,668,117 and the like.
In terms of behavior of perforation or migration of molten resins, the hot-melt film relies only on surface tension while the heat shrinkable film relies on heat shrinkage stress which is sufficiently larger than the surface tension. Therefore, the heat shrinkable film has such a higher sensitivity as to allow sufficiently large perforations to be obtained with a smaller heat quantity than the hot-melt film with the same thickness.
The heat shrinkage stress of the heat shrinkable film clearly depends on a temperature, and thus perforations can be obtained faithfully to a temperature pattern formed on the film, for example, by the heating elements of a thermal head. On the other hand, in case where a hot-melt film is heated and perforated due to surface tension, the temperature pattern of heating elements cannot be accurately reflected by the perforation configuration. The reason is that when resins lowered in viscosity due to melting migrate in accordance with surface tension, it does not always migrate toward low temperature portions far away from the center of each heating element, but can be concentrated near fibers of substrates or can flow irregularly due to a shear caused by its motion relative to the heating element. Therefore, even if a heat sensitive stencil sheet using a hot-melt film is processed into a stencil plate with an opening ratio suitable for printing conditions, uniform perforations are hardly obtained. That is, microscopically, large perforations and small perforations exist together, and it is hard to obtain uniform density, for example, in a solid printed portion of an image.
Furthermore, though the hot-melt films are composed of resins of a low melting point, they must be heated by heating elements to a temperature much higher than that for the heat shrinkable film, in order to sufficiently induce the migration of the resins with surface tension in very small areas (e.g., pixel density of 300 to 600 dpi) and in a short time (e.g., sub scanning period ranging from 2 to 4 ins) that are ordinary stencil plate making conditions of stencil plate making devices installed in current stencil printing machines. This causes the heating elements to be deteriorated due to overheating.
Moreover, during printing, the heat sensitive stencil sheet is stressed due to shear between itself and printing paper in the rotating direction of printing drum. A heat sensitive stencil sheet having a cast hot-melt film is generally lower in elastic modulus and rupture strength than a heat sensitive stencil sheet having a stretched heat shrinkable film. Therefore, a heat sensitive stencil sheet having a hot-melt film is more likely to cause deformation of printed images and, as the case may be, more likely to be broken to cause stained images, compared with a heat sensitive stencil sheet having a heat shrinkable film.
For the above reasons, it can be said that heat shrinkable films are and will be mainly used as films for heat sensitive stencil sheets. Therefore, the discussion concerning heat sensitive stencil sheets is hereinafter limited to the heat sensitive stencil sheets using a heat shrinkable film.
The heat sensitive stencil sheet is usually prepared by laminating the above-mentioned film on a porous substrate in order to impart a strength necessary for avoiding elongation, wrinkling (which distorts printed image) and breaking (which stains printed images) due to forces acting when the stencil sheet is mounted to a printing machine and used for printing. The porous substrate provides a heat sensitive stencil sheet with a strength, and allows ink to penetrate through perforations after the stencil sheet has been processed into a stencil plate. It is known that materials for the porous substrate include (1) so-called Japanese paper prepared from natural fibers such as Broussonetia Kazinoki, Edgeworthia chrysantha and Manila hemp, (2) paper-like sheets prepared from regenerated or synthetic fibers of rayon, vinylon, polyester, nylon, etc., (3) mixed paper prepared by mixing the natural fibers of (1) and the regenerated or synthetic fibers of (2), and (4) so-called polyester paper prepared by hot-calendering a thin paper prepared from a mixture of polyester fibers with non-stretched polyester fibers serving as binder fibers.
A heat sensitive stencil sheet prepared by laminating a film and a porous substrate as mentioned above has a strength sufficient to endure the forces caused by printing
Ikejima Shoichi
Nakamura Jun
Fitch Even Tabin & Flannery
Riso Kagaku Corporation
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