1. Field of the Invention
The present invention relates to a stencil printing apparatus that performs printing using a stencil base sheet (hereinafter referred to also as a “master”) in which perforations are created by platemaking in accordance with the image data of an original mounted on the outer circumferential surface of a plate cylinder, and more particularly to a stencil printing apparatus capable of performing printing on a coated sheet.
2. Description of the Related Art
Stencil printing apparatuses that comprise a stencil platemaking device (hereinafter referred to simply as a “platemaking” device) for perforating a large number of independent holes in a master using a thermal head to manufacture a master plate, a cylindrical plate cylinder on the outer circumference of which the master made by platemaking is windingly mounted and which is rotationally driven around its axis, an ink supply device provided in the plate cylinder for supplying ink to the inner surface of the plate cylinder, a sheet-feed device for separately feeding out printing sheets, a printing-pressure device for pressing the fed printing sheets onto the outer circumferential surface of the aforementioned plate cylinder, and a discharge device for conveying and discharging the printed sheets, and that employ a system for forming a printed image based on ink having passed through the perforations made by platemaking in the master being transferred onto the printing sheet are already widely known.
The large number of independent holes formed by the platemaking device are produced as a result of the perforation of a thermoplastic resin film of the master based on selective heating thereof by very small heat-emitting bodies of a thermal head in accordance with image information. Ink that passes through the perforations formed in the master is directly transferred onto the surface of the printing sheet to form a printed image.
In addition, predetermined printing images are formed using this kind of stencil printing apparatus as a result of printing sheets (sheet-like recording medium) separated into individual sheets and conveyed by a sheet-feed portion being pressed, while being conveyed, by the printing-pressure device onto the outer circumferential surface of a printing drum on the outer circumferential surface of which a master manufactured by platemaking is windingly mounted so that ink passing through the perforations of the master is selectively transferred onto the printing sheet surface.
The essential conditions required of sheets used in conventional stencil printing apparatuses are a surface of large undulations and ink permeability. This is because the drying system employed in these apparatuses is based on permeation into the sheet. That is to say, rather than natural drying, a system of pseudo-drying the ink transferred onto a sheet based on permeation thereof between the fibers of the sheet and evaporation of the liquid phase of the ink is fabricated.
Accordingly, the surface of the sheet pressed onto the surface film of the master windingly mounted onto the outer circumference of the printing drum possesses a certain degree of undulations between which an air layer exists.
By virtue of this, and the resilient strength of the sheet, a printing sheet can be comparatively easily peeled from the outer circumferential surface of the printing drum immediately following printing by a powerful sheet discharge peeling means. Furthermore, even if the number of printed copies printed using a single master is of the order of 5000 copies and a slight amount of stretch of the master itself has occurred, non-permissible level of creasing and tear will not be generated.
Printing using a stencil printing apparatus employing a master used in a platemaking device such as this on a so-called coated sheet on which a coating has been administered on the surface thereof has hitherto hardly even been implemented because it has conventionally been regarded as impossible.
However, there has been a long-held demand in the Japanese domestic market for printing to be able to be performed on a coated sheet. This is because the method employed by manufacturers for the bulk printing of, for example, flyers has concentrated on printing by offset color printing and has involved the flyers being distributed to regional branch shops, and the branch shops printing additional shop map and independent information on a partial blank area of the flyers and distributing them to individual households.
While in this case a lightweight coated sheet is employed as the optimum sheet for offset color printing and, as the optimum printer for the additional printing performed at each branch shop an inexpensive and simple digital platemaking stencil printer able to be used by anyone anytime is employed, regrettably, ink is unable to be used with a coated sheet because it does not dry and fix thereto.
However, stencil printing employing a UV ray-curable ink is already widely known and, if this system is employed, the ink of a printed image can be dried and cured on even non-permeable sheets, and the stencil printing on the aforementioned coated sheet is regarded as being able to be readily performed.
Known UV ray-curable ink-employing stencil printing apparatuses are described in Japanese Utility Model Publication No. 4-35188 and Japanese Unexamined Patent Application No. H5-64878.
In addition, Japanese Unexamined Patent Application No. 2006-327023 discloses, in a stencil printing apparatus in which both a UV ray-curable ink and a non-UV ray-curable ink, ink attached to a drum being detected as either a UV ray-curable ink or a non-UV ray-curable ink, and a warning being issued when an incompatibility exists. This application similarly discloses a master mounted on a platemaking device being detected as a master suitable for UV ray curing or a master suitable for non-UV ray curing, and a warning being issued when an incompatibility exists.
In addition, Japanese Unexamined Patent Application No. 2004-351757 discloses a configuration in which a permeable drying ink and a UV ray-curable ink are selected by switching a supply valve, and which employs a UV ray-curable ink for printing on a coated sheet.
In addition, Japanese Unexamined Patent Application Nos. H11-227229 and 2006-168370 disclose, in an ink jet system, detection of a recording medium material type and judgment of whether a first ink or a second ink is to be used in accordance therewith.
In addition, Japanese Unexamined Patent Application No. 2001-130120 discloses, in a stencil printing apparatus, a configuration in which two types of master are supported of which one is selectively used in accordance with the printing sheet type.
In addition, Japanese Unexamined Patent Application No. 2001-315291 discloses, in a stencil printing apparatus, the adjustment of platemaking energy in accordance with the printing sheet type.
In addition, Japanese Unexamined Patent Application No. 2001-328332 discloses, in a stencil printing apparatus, automatic selection of printing conditions in accordance with the printing sheet type.
In addition, Japanese Unexamined Patent Application No. 2005-186357 discloses selection of ink jet recording conditions based on a discrimination result of sheet type, and the issue of a warning when an incompatibility exists.
In addition, Japanese Unexamined Patent Application No. 2006-281658 discloses, in a stencil printing apparatus, control of UV lamp flashing according to the use of a UV ink drum unit or a non-UV ink drum unit.
In addition, Japanese Patent No. 3734247 discloses the discrimination of a coated sheet based on optical detection.
Repeated implementation of printing tests on coated sheets employing a conventional stencil printing apparatus carried out by the inventor of the present invention with a view to understanding the merits and problems inherent thereto led to an awareness of additional new problems.
In stencil printing, a large number of very small perforations are provided by a platemaking process in the film surface of a master windingly mounted on the outer circumference of a printing drum, and ink passing through these perforations is pressed onto the printing sheet by a printing pressure portion and transferred on to the printing sheet surface to form a printed image.
At this time, ink is interposed and perfectly closely adhered between the film surface of the master and the surface of the printing sheet. For a coated sheet, because the smoothness of the sheet surface thereof is immeasurably greater than the smoothness of a conventional normal sheet, this adherence force is very large. While the printing sheet is subsequently peeled from the printing drum and discharged, because the printing sheet sticks to the printing drum due to the adhesive strength of the ink, a very large peeling force is required at this time.
For a conventional normal sheet, because of its air permeability and the large undulations in the surface thereof, it can be peeled from the printing drum with a moderate level of peeling force. In addition, using a conventional emulsion ink, because both the adhesive strength and the tack value of the ink itself are very low and the ink cohesion force is also low, the ink tears easily and the printing sheet can be peeled off with a moderate level of force.
However, these conditions are significantly different when the printing sheet is a coated sheet. That is to say, because the surface smoothness of a coated sheet is very high and the sheet sticks to the printing drum due to the adhesive strength of the ink, a markedly greater force is required to peel the sheet from the surface of the master film of the outer circumference of the printing drum because. As a result, it was clear that the master film rather than the sheet was lacking in resistance, and that slippage thereof resulting in creasing and tear thereof sometimes occurred.
It was also apparent that a switch of the printing ink from a conventional emulsion inks to a UV ray-curable inks of which the principal component is a monomer led to an increase in the adhesion strength and tack value of the ink itself and, in turn, to a magnification of the drawbacks described above.
As the master film, a very thin film configured from a thermoplastic synthetic resin of which the thickness thereof allows for the manufacture therein of perforations by platemaking at high speed using a thermal head is used. More specifically, a PET film of thickness of the order of 1.5 to 2 microns is used. A marked drop in strength occurs in the solid image portion in which a large number of perforations are formed by platemaking in this very thin film and, as a result, it separates and creases and tears easily due to the peeling performed immediately following printing.