Stencil printer

Printing – Stenciling – Rotary machines

Reexamination Certificate

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Details

C101S126000

Reexamination Certificate

active

06389964

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a stencil printer.
Stencils for thermal printing include one having a thermoplastic resin film, a porous thin sheet or similar base permeable to ink and adhered to the resin film, and an anti-stick layer formed on the resin film for preventing the film from sticking to a thermal head. The porous sheet is, in many cases, implemented by flax fibers or a mixture of flax fibers, synthetic fibers, and wood fibers.
The above conventional stencil, however, has the following problems (a) and (b) left unsolved because the fibrous base just overlies the resin film.
(a) A great amount of adhesive gathers in the from of webs in regions where the overlapping portions of fibers and the resin film contact each other, obstructing the perforation of the stencil by the thermal head.
(b) Fibers themselves check the passage of ink and make printing irregular.
To solve the above problems (a) and (b), some different stencils for thermal printing have been proposed in the past. Japanese Patent Laid-Open Publication No. 3-193445, for example, teaches a porous thin sheet or base implemented by fibers as thin as 1 denier. This kind of base solves the problem (b), but cannot solve the problem (a).
Japanese Patent Laid-Open Publication No. 4-7198 discloses a printing method using a stencil produced by applying a mixture solution of fine particles of polymer dispersion and colloidal silica to the surface of a film and then drying it to form a porous layer. The stencil is perforated by a master making machine Print Gokko (trade name) available from Riso Kagaku Corporation to thereby make a master. The master is used to print images on papers with ink HG-4800 available from EPSON. The above porous layer, however, does not allow ink to smoothly pass therethrough and cannot implement satisfactory image density when use is made of conventional ink for thermal printing. Moreover, the porous layer itself lacks a sufficient heat insulating ability and prevents the stencil from being desirably perforated.
Japanese Patent Laid-Open Publication No. 54-33117 proposes a stencil consisting substantially only of a heat-sensitive resin film, i.e., not including a base. While this kind of stencil solves the previously stated problems (a) and (b) at the same time, it brings about other problems, as will be described hereinafter.
When the film constituting the stencil is 10 &mgr;m thick or less, the stencil fails to have sufficient stiffness and cannot be easily conveyed. In light of this, Japanese Patent Publication No. 5-70595 proposes to wrap an elongate film around a print drum without cutting it and cause the entire film to rotate together with the print drum during printing. This scheme, however, increases the turning moment because the film and a master attaching and detaching unit rotate together with the print drum during printing. This, coupled with the noticeable offset of the center of gravity from the axis of rotation, requires a printer to have a heavy, bulky configuration.
On the other hand, when the film is 5 &mgr;m thick or more, its heat sensitivity is reduced and obstructs perforation by the thermal head. Moreover, heat applied from heating means is transferred to a platen via the stencil and lost in a substantial amount, limiting energy available for the perforation of the film. Japanese Patent Laid-Open Publication No. 10-230690 discloses a stencil made up of a thermoplastic resin film and a porous resin film or base provided on one surface of the resin film, i.e., a stencil without a filler.
Specifically, the porous resin film included in the above stencil is formed by precipitating a resin solved in a solvent and, e.g., solidifying it. Comparing the film to a floor, the porous resin film is a wall-like film implemented as an assembly of a number of cells with or without a ceiling, a foam-like film implemented by an assembly of open cells, or a film implemented by an assembly of resin in the form of particles or fibers. The cells may be fully closed or partly open. Openings appear on the surface of the porous resin due to the bursting of cells occurring in the drying step.
Open cells, a resin in the form of particles or fibers and cells without a ceiling, which constitute the porous resin film in combination, are connected together. This configuration provides the resin film with sufficient tensile strength and stiffness and thereby provides the stencil with sufficient tensile strength while allowing it to be smoothly conveyed.
Generally, the porous resin film has a mean cell size of 1 &mgr;m or above, but 50 &mgr;m or below. Mean cell sizes less than 1 &mgr;m obstruct the smooth passage of ink. Should ink with low viscosity be used to allow it to pass through the resin film in a sufficient amount, it would blur images and would ooze out from the sides of the print drum and the trailing edge of a master wrapped around the drum. In addition, the void content of the resin film would decrease and would therefore further obstruct the perforation by the thermal head.
Mean cell sizes greater than 50 &mgr;m reduce the ink regulating effect available with the porous resin film. As a result, ink is forced out from the print drum to a paper or similar recording medium in an excessive amount, smearing the rear surfaces of papers and blurring images.
The porous resin film should only have a number of voids therein side and on the surface thereof. To promote the passage of ink, the voids should preferably be communicated to each other in the direction of thickness of the film and extend, comparing the film to the floor, throughout the ceiling. Alternatively, at the boundary between the porous resin film and the thermoplastic resin film, the former may cover the latter so long as it does not obstruct perforation by the thermal head. The resin constituting the porous resin film so covering the thermoplastic resin film should generally be 7 &mgr;m thick or less inclusive of the thermoplastic resin film, although dependent on the kind of the resin, heat sensitivity of the thermoplastic resin film, etc.
The total area of openings having diameters of 5 &mgr;m or above in terms of a true circle, as measured on the surface of the porous resin film, is 4% to 80%, preferably 10% to 60%, of the entire surface area. If this radio is less than 4%, the resin film is apt to obstruct the perforation by the thermal head and the passage of ink. If the ratio is greater than 80%, the tensile strength and stiffness of the resin film decrease.
The porous resin film is entirely different in structure from the conventional porous portion of a stencil for thermal printing. Specifically, solid portions included in the porous resin film have various shapes including a rod-like shape, a spherical shape and a branch-like shape. The configuration of the solid portions is determined by conditions for the fabrication of the resin film, e.g., the kind of the resin, the solid content of a liquid, the kind of a solvent, the amount of deposition of a resin solution, the temperature of the resin solution, temperature for drying the resin solution, and ambient temperature and humidity for application. Among them, the temperature of the resin solution and ambient temperature and humidity for application have critical influence on the configuration of the solid portions.
For example, when the temperature of the resin solution is 10° C. or below, the resin solution easily gels and is difficult to apply. Conversely, when the above temperature exceeds 30° C., it is difficult to form the porous resin film. It follows that the temperature for application should also preferably be between 10° C. and 30° C. The ambient humidity for application above 50% RH would cause the surface of the thermoplastic resin film to adsorb a great amount of water and would thereby lower wettability with respect to the solution, weakening adhesion between the porous resin film and the thermoplastic resin film.
The stencil with the above porous resin film realizes attractive images when used with a stencil printer PREPOR

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