Printing – Stenciling – Stencils
Reexamination Certificate
2003-05-30
2004-07-06
Funk, Stephen R. (Department: 2854)
Printing
Stenciling
Stencils
Reexamination Certificate
active
06758138
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method of and an apparatus for making a stencil by thermally perforating a thermoplastic resin film of heat-sensitive stencil material by a thermal head or the like, and to a heat-sensitive stencil material. More particularly, this invention relates to improvement in shape of perforations, printing quality and stencil making speed.
2. Description of the Related Art
Methods of making a heat-sensitive stencil are broadly divided into a method in which the resin film side of the heat-sensitive stencil material is brought into close contact with an original bearing thereon an image formed of a carbon-containing material and the resin film is perforated by heat generated by the image upon exposure to infra-red rays and a method in which the resin film of the heat-sensitive stencil material is imagewise perforated by two-dimensionally scanning the resin film side of the heat-sensitive stencil material with a device such as a thermal head having an array of micro heater elements. The former method will be referred to as “an analog stencil making method” and the latter method will be referred to as “a digital stencil making method”, in this specification. At the present, the digital stencil making method is prevailing over the analog stencil making method since the former does not require carbon in the original and permits easy image processing.
When the stencil is made by the digital stencil making method, it is preferred that the perforations be discrete by pixel, and be uniform in shape and degree of penetration so that the thin lines and/or edges of the printings show rims faithful to the original, the solid portions of the printings has a sufficient density and the amount of ink to be transferred to each printing sheet can be well controlled not to cause offset (the phenomenon the ink on the surface of a first printed sheet stains the back side of a second printed sheet superposed on the surface of the first printed sheet).
On the other hand, in order to meet the recent demand for higher image quality, highly fine or high resolution thermal heads such as of 400 dpi or 600 dpi have been in wide use as the thermal device for thermally perforating the stencil material. Such high resolution thermal devices are generally lower than low resolution thermal devices in the maximum temperature they can provide. Accordingly, in order to perforate the stencil material in a given size with the high resolution thermal device, the stencil material should be more sensitive to perforation than when it is perforated by the low resolution thermal device. Further, since the number of perforations (pixels) increases as the resolution increases, it is preferred that the time required to form each perforation be shortened, that is, each perforation be formed at a higher speed. Thus, physical properties of the resin film, the structure of the thermal head, and the method of controlling the thermal head for meeting these demands have been searched for.
The thermoplastic resin film for the heat-sensitive stencil material produces shrinkage stress when heated by a heat source such as a thermal head and is perforated by shrinkage. In order to improve sensitivity to perforation of the heat-sensitive stencil material, there has been proposed thermoplastic resin film having a specified heat shrinkage factor as disclosed, for instance, in Japanese Unexamined Patent Publication No. 4(1992)-125190 or thermoplastic resin film having a specified heat shrinkage factor and a specified heat shrinkage stress as disclosed, for instance, in Japanese Unexamined Patent Publication Nos. 7(1995)-52573 and 7(1995)-68964. However, in these patent publications, the heat shrinkage factor or the heat shrinkage stress is specified on the basis of measurement of the heat shrinkage factor or the heat shrinkage stress when the film is heated several to several tens of minutes, which is very long as compared with the time for which the film is heated in the actual perforation. Further, the measurement is static and does not reflect the actual perforation. Further, though the heat shrinkage factor or the heat shrinkage stress measured by, for instance, TMA (thermo-mechanical analysis) under a macroscopic and quasi-static condition where the area to be heated is not smaller than several millimeters (mm) and the temperature change is 10° C./min or so has been reported, the behavior of the perforations under a microscopic and dynamic condition in the actual stencil making process where the area to be heated by the thermal head or the like is several tens of micrometers (&mgr;m) and the temperature change is 1° C./&mgr;s or so has not been reported. Thus the reported heat shrinkage factor or heat shrinkage stress does not conform to the actual perforation.
Further, conventionally, discussion on the perforation in the stencil making process has been made not on the basis of behavior of perforations in course of perforation but on the final state of perforations. In such discussion, physical properties of the resin film and the structure of the thermal head, and the method of controlling the thermal head are generally discussed in order to control the final size and shape of the perforations and the TMA data on the film is employed only to indicate the sensitivity to perforation. Accordingly, the properties of the film concerning to the degree to which the perforations are discrete by pixel and the shape of the perforations is stabilized are generally incompatible with the sensitivity to perforation of film and the speed at which the film is perforated. That is, when a film can be perforated so that the perforations are well discrete and uniform in shape, the film is less sensitive to the perforation and takes a long time to perforate. Naturally the opposition is also true. Accordingly, in the actual design of a stencil making system, a plurality of kinds of thermoplastic resin film are prepared, the sensitivity to perforation of each kind of film is determined by repeating experiments or TMA measurements, and one of the kinds of film which is most close to a target sensitivity is selected.
The general data on the heat shrinkage factor and heat shrinkage stress do not always conform to the evaluation of film obtained in the actual design of a stencil making system with respect to, for instance, discreteness and uniformity of shape of the perforations, the sensitivity to perforation and the perforating speed. As described above, this is because the TMA data and the like are obtained under a macroscopic and quasi-static condition whereas the actual perforation in the actual stencil making process is effected under a microscopic and dynamic condition. Further, it is difficult to read from the TMA data the performance of the film representing the perforating speed, the stability of the shape of perforations and the like except the sensitivity to perforation. Even about the sensitivity to perforation, it is difficult to estimate the difference in the sensitivity to perforation between film samples which are slightly different from each other, for instance, in TMA curve since it is actually impossible to prepare a variety of film samples which are different from each other in one or more particular factor such as the TMA curve with the other factors held to be the same. Accordingly, when a suitable kind of resin film is to be selected, stencils must be actually made using a variety of resin film samples, which adds to the development cost.
As described above, information obtained as a characteristic value in the stencil making experiments is only on the size and shape of the perforations at the time the perforations are completed. Accordingly, it has been very difficult to know, without experience and sense, how the physical properties of the resin film should be changed on the basis of the result of experiment in order to obtain a desirable form of perforation, which has been made difficult development of new products and improvement of the performance of the produ
Nakamura Jun
Ohshima Kenji
Funk Stephen R.
Nixon & Peabody LLP
Riso Kagaku Corporation
Studebaker Donald R.
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