Heat-sensitive stencil master sheet

Abstract: A heat-sensitive stencil master sheet comprising a thermoplastic resin film (5) mounted on a permeable substrate (6) for use with an array of thermal elements (2) actuatable selectively to penetrate the film at desired locations, said thermal elements being arranged in a first direction at a first pitch, and the sheet and the array being arranged to be relatively movable so that the thermal elements (2) are selectively actuatable at locations arranged in a second direction at a second pitch, wherein the substrate comprises a mat of fibres defining regions therebetween at least a substantial proportion of said regions each having an area the same or less than an area defined by the product of the first and second pitches. The stencil sheet of the invention has improved perforating and printing properties and provides images having improved resolution.

Description: This invention relates to a heat-sensitive stencil master sheet. Particularly it relates to a heat-sensitive stencil master sheet having superior perforating and printing characteristics, and a process for digital-perforating the heat-sensitive stencil master sheet using a digital stencil-making device having a thermal head. Stencil master sheets used in the past for stencil master printing have generally been prepared by adhering a thermoplastic resin film (hereinafter often abbreviated to film) onto a permeable substrate with an adhesive. In recent years, as such a stencil master sheet, a heat-sensitive sheet for digital perforation by the heat of a thermal head is prevailing, and in order to obtain printed matters having good resolving properties, a heat-sensitive stencil-making and printing device having a thermal head having a high resolution is used. As conditions of the permeable substrate used for the heat-sensitive stencil master sheet, easy handling, no occurrence of wrinkles in the sheet at the time of perforation, durability to printing, etc. are required. Since the quantity of ink passed through the perforated master sheet for the printed image and its quantity retained in the master sheet vary depending upon the kind of the substrate, various regulations of the master sheet such as basis weight, thickness, density, dispersibility of ink, strength (or wet strength), stiffness, etc. have been provided. However, the heat-sensitive stencil master sheet used in the prior art has been chosen based mainly upon the quantity of ink passed and the quantity of ink retained relative to the image properties, but no sufficient consideration has been paid about the effect upon the perforation of film, and there may occur a case of insufficient perforation or a case of connected perforations, where it is difficult to control the quantity of ink passed. Printed matters printed by a perforated stencil master sheet having insufficient perforations or non-perforated parts have drawbacks that they are inferior in the resolving properties and the reproduction of fine letters, to cause parts where no ink is attached, such as white points at a solid part or parts lowered in the ink concentration. Further, printed matters printed by a perforated stencil master sheet having connected or broadened perforations have drawbacks that the quantity of ink transferred increases extremely, and reduction in the resolving properties or increase in the setting off of ink to the surface of the printed matter due to exudation of ink occur. JP-A-60-189496 discloses a stencil formed by adhering a heat-shrinkable resin film of polyethylene terephthalate or the like to one side of a screen regularly woven at 100 mesh from fibres having a diameter of about 30 microns. JP-A-57-182495 discloses a method wherein saturated polyester resin having a strong adhesive power is used as an adhesive agent for a polyester film with a thickness of about 2 to 8 microns to a porous tissue paper having a basis weight of about 5 to 15 g/m² or a meshform sheet which has good perforation properties, provides good resolution in the image which has been printed from the stencil, and avoids excessive transfer of ink thereby to give a good printed image. The object of the present invention is to provide a heat-sensitive stencil master sheet which has good perforation properties, provides good resolution in the image which has been printed from the stencil, and avoids excessive transfer of ink thereby to give a good printed image. The present invention provides a heat-sensitive stencil master sheet comprising a thermoplastic resin film mounted on a permeable substrate comprising a mat of fibers defining clearance areas therebetween in combination with a heat-sensitive stencil-making device including a thermal head having an array of thermal elements actuatable selectively to penetrate the film at desired locations, characterised in that said thermal elements are arranged at a right angle to the advancing direction of the sheet at a first pitch, the sheet and the array of thermal elements arranged to be relatively movable so that the thermal elements are selectively actuatable at locations arranged in the advancing direction of the sheet at a second pitch, and the proportion of said clearance areas which each have an area the same or less than a scanning pitch area defined by the product of the first and second pitches is such that the combined area of such clearance areas is at least 80% of the total of said clearance areas defined by said fibers. The present invention also provides a process for digitally-perforating a heat-sensitive stencil master sheet by the thermal head of a heat-sensitive stencil-making device, wherein said stencil master sheet comprises a sheet of fibers and a thermoplastic resin film adhered thereon, characterized in that the sum of the clearance areas surrounded by the fibers same as or less than the scanning pitch area sought by the product of the primary scanning pitch at a right angle to the advancing direction of the sheet, of the thermal head of the heat-sensitive stencil-making device, by the secondary scanning pitch thereof in the advancing direction of the sheet, occupies 80% or more of the total of the clearance areas surrounded by fibers of the sheet. The present invention further provides a process for preparing a heat-sensitive stencil master sheet for digital perforation thereof by the thermal head of a heat-sensitive stencil making device, comprising the steps of: providing a permeable substrate having a plurality of fibers defining the substrate; determining a scanning pitch area as the product of a primary scanning pitch of the thermal head as measured at a right angle to an advancing direction of the sheet, and a secondary scanning pitch of the thermal head as measured in the advancing direction of the sheet; and adhering a thermoplastic resin film to the substrate at contacting zones of said thermoplastic resin film, said contacting zones being defined as areas of the thermoplastic resin film that are supportedly contacted by said fibers, said contacting zones defining clearance areas of said thermoplastic resin film as being areas of the thermoplastic resin film that are not supportedly contacted by said fibers, wherein at least 80% of the plurality of clearance areas respectively have areas of less than or equal to the scanning pitch area. It should be understood that the term "array of thermal elements" as used in the present specification includes both the use of a single row of thermal heating elements and a plurality of rows of heating elements. The term "secondary pitch" is defined as the distance between the centers of the adjacent rows of heat elements when a plurality of rows of heating elements is present, which distance may also be an advancing pitch of the sheet, or it is defined as the advancing pitch of the sheet which is in contact with the thermal head when a single row of heating elements is used. As shown in Figure 1, a thermal head may be composed of an array of heating elements 2 of Al, for example, aligned on a resistance layer in a single row at right angles to the advancing direction of the sheet 3 (as shown in Figure 3) and a composite ceramic layer supporting the resistance layer. The array of heat elements 2 may also be composed of a plurality of rows of such elements. A dot pitch which is referred to herein as the primary or first scanning pitch is defined as the distance 2A between the centers of the adjacent thermal elements 2 in the same row (as shown in Fig. 1). Usually, the thermal head used for the digital stencil-making device has a dot pitch as fine as 400 dpi (dots per inch), whereas, in the clearance areas surrounded by the fibers of the stencil master sheet, a number of those areas having an area exceeding that of the above dot pitch so that no substrate supporting the film is often present in the clearance areas. Fig. 5 shows a view illustrating the relationship between a conventional heat-sensitive stencil master sheet and a heat element of a thermal head. In Fig. 5, since the clearance area 7 surrounded by the substrate fibers 6 of the stencil master sheet is often broader than the clearance area of dot pitch 2A, the contact of the film 5 to the heating element 2 of the thermal head is inferior, so that non-perforation or insufficient perforation may often occur. Further, the dots of the resulting perforations are easy to broaden, since the substrate fibers to support the perforated film are absent. Thus, the perforated dots might often have been broadened to adjacent dots to form connected perforations. The regions between the fibers (also referred to as "clearance areas surrounded by fibres" throughout this specification) are measured as follows:- When a light is projected onto a heat-sensitive stencil master sheet, holes formed by transmission of the light are observed. By using a printed image-processing system (EXCEL 11, trademark of Japan Avionics Co., Ltd.), ten sites of the sheet to be measured, each 1.5 x 1.5 mm², are taken, each site enlarged up to 50 times the original size so that the center of the thickness of the sheet is focused and measured with the respective clearance areas. In this case, the area of 100 µm² or less are neglected as a noise. Thus, the proportion of the clearance areas of a definite area or less, to the total of the clearance areas of the sheet can be sought by averaging the above measurement values of the ten sites. Examples of the thermoplastic film used in the present invention are those of polyester, polypropylene, polycarbonate, vinylidene chloride-vinyl chloride copolymer, etc. Its thickness is usually 10µm or less, preferably 1 to 6µm. As the permeable substrate, a thin sheet of synthetic fibers such as polyester fibers, polyvinyl alcohol fibers, nylon fibers, etc., natural fibers such as Manila hemp, Kouzo, Mitsumata, pulp, etc. are exemplified. The above fibers may be used alone or in admixture of two or more kinds. The fineness of these fibers is preferably 3 denier or less, the basis weight of the sheet is preferably 6 to 14 g/m², more preferably 8 to 13 g/m². The thickness of the sheet is preferably 10 to 60µm, more preferably 15 to 55µm. The thermoplastic film is adhered onto the substrate sheet having the above specific clearance areas to form a heat-sensitive stencil master sheet of the present invention. The present invention will now be illustrated further, by way of example only, with reference to the accompanying drawings, in which:- Fig. 1 shows a typical view illustrating the relationship between a heat-sensitive stencil master sheet and a thermal head of a digital stencil-making device according to the present invention. Fig. 2 shows a view of illustrating the perforation mechanism of the stencil master sheet by means of a thermal head of a digital stencil-making device. Fig. 3 shows a plane view of a film perforated by the thermal head in the present invention. Fig. 4 shows a plane view of a film perforated by the thermal head in prior art. Fig. 5 shows a view illustrating the relationship between conventional heat-sensitive stencil master sheet and a thermal head of a digital stencil-making device. Fig. 1 shows a relationship between the heat-sensitive stencil master sheet of the present invention and the heat element provided at the thermal head of a heat-sensitive stencil-making device, when a thermoplastic film of the sheet is contacted with the thermal head. This heat-sensitive stencil master sheet consists of a thermoplastic film 5 and a substrate sheet of fibers 6 supporting the film 5. Fig. 2 shows an explanatory view of main parts of a heat-sensitive stencil-making device for illustrating stencil-making mechanism. A heat-sensitive master sheet 3 is fed between a thermal head 1 having many dots of heating elements 2 and a platen-roll 4 by the rotation of the platen-roll 4, and contacted directly with the heating elements 2 of the thermal head 1. The sheet 3 is advanced in an arrow direction with the rotation of the platen-roll 4 continuously or intermittently, while the thermal head 2 is moved at a right angle to the advancing direction of the sheet 3, and at the same time, heat is applied to selective dots of the heating elements 2 of the thermal head so that a selected perforation image is formed on the film of the heat-sensitive stencil master sheet 3. As the substrate sheet 6, a thin sheet is used wherein the sum of the clearance areas 7 surrounded by fibers same as or less than the scanning pitch areas of the thermal head occupies 80% or more of the total of the clearance areas surrounded by fibers of the sheet. The scanning pitch area of the thermal head can be sought by the product of the primary scanning pitch 2A of the thermal head of the stencil-making device, at a right angle to the advancing direction of the sheet, by the secondary scanning pitch thereof in the advancing direction of the sheet. Referring to the figure, since the clearance area 7 is smaller than the scanning pitch area which is proximated to a dot pitch 2A of the heating element 2 of the thermal head, the film 5 is uniformly attached onto the heating element 2, whereby it is possible to easily obtain independent perforations of the film. In the case of using a stencil printing device having a thermal head of 400 or more dots per pitch (63.5 µm or less of dot pitch 2A), a stencil master sheet where the sum of the clearance areas surrounded by fibers being each of 4,000 µm² or less, preferably 500 to 3,500 µm², in the sheet is 80% or more, preferably 90% or more, based upon the total of the clearance areas are,used. If the clearance areas of 4,000 µm² or less is less than 80%, the permeability of ink of the substrate, the resolving properties of printed matters and the setting-off of ink to the surface are not improved. Further, the average clearance areas of the substrate sheet is preferably 500 to 2,500 µm² in view of the permeability of ink. According to the present embodiment, by using as the substrate, a thin sheet having the above specific clearance areas, the contact of the film onto the thermal head is improved so that the perforation of the film is carried out with precision without causing non-perforation or connected perforations; hence the resulting printed image is superior in the resolution without causing setting-off of ink on the surface of printed matters. The present invention will be illustrated in more detail by way of the following Examples. Example 1 Fibers of 100% hemp were subjected to sheet-making according to a wet sheet-making process using a conventional cylyndrical or short net type paper machine to obtain a permeable sheet of a basis weight of 8.8 g/m² wherein the sum of the clearance areas surrounded by fibers same as or less than the scanning pitch areas of the thermal head having 400 dots per inch of heating elements of a heat-sensitive stencil-making device occupied 84.6% of the total of the clearance areas of the sheet, followed by laminating a PET (polyethylene terephthalate) film having a thickness of 2 µm upon the above sheet with an adhesive (ultraviolet curing-type, produced by TOAGOSEI K.K.) and applying a releasing agent onto the surface of the laminated film to obtain a heat-sensitive stencil master sheet of the present invention. Example 2 Example 1 was repeated except that the sheet obtained in Example 1 was replaced by a permeable sheet of a basis weight of 10.0 g/m², prepared by subjecting fibers of 60% hemp and 40% synthetic fibers (mixture of polyester fibers, polyvinylalcohol fibers and natural fibers) to a wet paper-making process, in which sheet the sum of the clearance areas same as or less than the scanning pitch areas ofthe thermal head occupied 81.7% of the total of the clearance areas, to prepare a heat-sensitive stencil master sheet. Comparative example 1 Example 1 was repeated except that the sheet of Example 1 was replaced by a permeable sheet of a basis weight of 9.6 g/m², obtained by subjecting fibers of 100% hemp to wet paper-making process, in which sheet the sum of theclearance areas same as or less than the scanning pitch areas of the thermal head occupied 72.0% of the total of the clearance areas, to prepare a heat-sensitive stencil master sheet. Comparative example 2 Example 1 was repeated except that the sheet of Example 1 was replaced by a permeable sheet of a basis weight of 12.5 g/m², obtained by subjecting fibers of 100% PET to sheet-making according to a wet paper-making process, in which sheet the sum of the clearance areas same as or less than the scanning pitch areas of the thermal head occupied 70.7% of the total of the clearance areas of the sheet, to prepare a heat-sensitive stencil master sheet. Comparative example 3 Example 1 was repeated except that the sheet of Example 1 was replaced by a permeable sheet of a basis weight of 10.0 g/m² obtained by subjecting fibers of 100% hemp to sheet-making according to a wet paper-making process, in which sheet the sum of the clearance areas same as or less than the scanning pitch areas of the thermal head occupied 55.0% of the total of the clearance areas of the sheet, to prepare a heat-sensitive stencil master sheet. The heat-sensitive stencil master sheets obtained in the above Examples and Comparative examples were subjected to stencil-making by means of a heat-sensitive stencil-making device. As a heat-sensitive stencil-making device used in the examples, a heat-sensitive stencil-making and printing device produced by Riso Kagaku Corporation (RISOGRAPH RL 115D, Trademark of Riso Kagaku Corporation) having a thermal head of 400 dpi (dots per inch) was used. The resulting perforated stencil master sheet was then subjected to printing by means of the same device. The perforations of the film on the sheet were examined by means of an optical microscope. In Examples 1 and 2, since a thin sheet wherein the proportion of clearance areas of 4,000 µm² or less was of 80% or more, was used as a substrate, the perforations 8 of the sheet were almost independent and uniform as shown in Fig. 3. On the other hand, in the case of Comparative examples 1 to 3, since the above proportion of clearance areas was of less than 80%, connected perforations 10 and unsuccessful perforations 9 were generated as shown in Fig. 4. Further, the resulting perforation and printed image were evaluated according to the following evaluation standards. The results are shown in Table 1. Evaluation standards: Unsuccessful perforations (visible evaluation by means of microscope) ○---none, ▵---a few, X---many. Connected perforations (visible evaluation by means of microscope) ○---none, ▵---a few, X---many. Concentration: The reflection concentration of the solid part was measured at 10 points by means of a densiometer (DM-400, Produced by Dainippon Screen Kabushiki Kaisha) and the resulting values (relative values) were averaged. Uniformity: The uniformity of the concentration inside the solid part was evaluated according to a printed image-processing system. ○---good, ▵---somewhat inferior, X---inferior. Resolution: (Visible evaluation of a printed letter) ○---good, ▵---the continuity of the letter is somewhat inferior or the letters are thick to a certain extent, X---the continuity of the letter is inferior or the letters are thick. Preventing ink from setting-off to the surface of printed matters: (Visible evaluation) ○---good, ▵---somewhat inferior, X-inferior. Example 1 Example 2 Comp.ex.1 Comp.ex.2 Comp.ex.3 Kind of fiber Hemp Hemp Synthetic fiber Hemp PET Hemp Basis weight (g/m ) 8.8 10.0 9.6 12.5 10.0 Proportion of clearance areas of the sheet same as or less than scanning pitch areas (%) 84.6 81.7 72.0 70.7 55.0 Unsuccessful perforation ○ ○ ▵ ▵ X Connected perforation ○ ○ ▵ ▵ X Concentration 0.99 1.02 1.04 1.05 1.00 Uniformity ○ ○ ▵ ▵ X Resolution ○ ○ ▵ ▵ X Setting-off of ink on the surface ○ ○ ▵ ▵ X As seen from Table 1, according to Examples 1 and 2 wherein a sheet in which the proportion of the clearance areas of the sheet same as or less than the scanning pitch areas of the thermal head was 80% or more of the total of the clearance areas, was used as the substrate, theperforations were almost independent, and the printing concentration became adequate due to controlled quantity of ink passed. Thus, a good printed matter having superior resolving properties and uniformity, and particularly, few setting-off of ink to the surface thereof was obtained. Whereas, according to Comparative examples 1, 2 and 3 wherein the proportion of the clearance areas of the sheet same as or less than the scanning pitch area was less than 80% of the total of the clearance areas, connected perforations and unsuccessful perforation were observed, the resolution was lowered and the setting-off of ink to the surface increased, and in an extreme case, the concentration reduction was observed. According to the present invention, by using a stencil master sheet having a specific clearance areas of the sheet suitable for a thermal head of the stencil-making device, the aimed printed image was perforated with a good fidelity and with a fine and independent perforation of the film. Hence, it is possible to obtain a printed image having a superior resolution and extremely few setting-off of ink to the surface of printed matters.

Claims: A heat-sensitive stencil master sheet (3) comprising a thermoplastic resin film (5) mounted on a permeable substrate (6) comprising a mat of fibers defining clearance areas (7) therebetween in combination with a heat-sensitive stencil-making device including a thermal head (1) having an array of thermal elements (2) actuatable selectively to penetrate the film (5) at desired locations, characterised in that said thermal elements (2) are arranged at a right angle to the advancing direction of the sheet (3) at a first pitch, the sheet (3) and the array of thermal elements (2) arranged to be relatively movable so that the thermal elements (2) are selectively actuatable at locations arranged in the advancing direction of the sheet (3) at a second pitch, and the proportion of said clearance areas (7) which each have an area the same or less than a scanning pitch area defined by the product of the first and second pitches is such that the combined area of such clearance areas (7) is at least 80% of the total of said clearance areas (7) defined by said fibers. A heat-sensitive stencil master sheet and stencil-making device according to claim 1, wherein the array of thermal elements (2) is incorporated in a thermal head (1) having 400 or more dots per inch, and the sum of the said clearance areas (7) between the fibers in the sheet (3) having 4,000 µm² or less is 80% or more based upon the total area of the said clearance areas (7) defined by said fibers. A heat-sensitive stencil master sheet and stencil-making device according to any one of the preceding claims, wherein the average area of the clearance areas (7) defined by the fibers is 500 to 2,500 µm². A heat-sensitive stencil master sheet and stencil-making device according to any one of the preceding claims, wherein the basis weight of the stencil master sheet (3) is 6 to 14 g/m², and the fineness of the fibers in the sheet is 3 denier or less. A process for digitally-perforating a heat-sensitive stencil master sheet (3) by the thermal head (1) of a heat-sensitive stencil-making device, wherein said stencil master sheet (3) comprises a sheet of fibers (6) and a thermoplastic resin film (5) adhered thereon, characterized in that the sum of the clearance areas (7) surrounded by the fibers same as or less than the scanning pitch area sought by the product of the primary scanning pitch at a right angle to the advancing direction of the sheet (3), of the thermal head (1) of the heat-sensitive stencil-making device, by the secondary scanning pitch thereof in the advancing direction of the sheet (3), occupies 80% or more of the total of the clearance areas (7) surrounded by fibers of the sheet (3). A process for digitally-perforating a heat-sensitive stencil master sheet (3) according to claim 5, wherein said stencil-making device has a thermal head (1) of 400 or more dots per inch, and the sum of the clearance areas (7) surrounded by fibers of said sheet (3) having 4,000 µm² or less is 80% or more based upon the total of the clearance areas (7) of the sheet (3). A process for digitally-perforating a heat-sensitive stencil master sheet (3) according to claim 5 or 6, wherein the average clearance area (7) of said sheet is 500 to 2,500 µm². A process for digitally-perforating a heat-sensitive stencil master sheet (3) according to claim 5, 6 or 7, wherein the basis weight of said sheet (3) is 6 to 14 g/m² and the fineness of said fibers of the sheet (3) is 3 denier or less. A process for preparing a heat-sensitive stencil master sheet (3) for digital perforation thereof by the thermal head (1) of a heat-sensitive stencil making device, comprising the steps of: providing a permeable substrate (6) having a plurality of fibers defining the substrate (6); determining a scanning pitch area as the product of a primary scanning pitch of the thermal head (1) as measured at a right angle to an advancing direction of the sheet (3), and a secondary scanning pitch of the thermal head (1) as measured in the advancing direction of the sheet (3); and adhering a thermoplastic resin film (5) to the substrate (6) at contacting zones of said thermoplastic resin film (5), said contacting zones being defined as areas of the thermoplastic resin film (5) that are supportedly contacted by said fibers, said contacting zones defining clearance areas (7) of said thermoplastic resin film (5) as being areas of the thermoplastic resin film (5) that are not supportedly contacted by said fibers, wherein at least 80% of the plurality of clearance areas (7) respectively have areas of less than or equal to the scanning pitch area. A process for preparing a heat-sensitive stencil master sheet (3) according to claim 9, wherein the thermal head (1) provides at least 400 dots per inch. A process for preparing a heat-sensitive stencil master sheet (3) according to claim 9 or 10, wherein the scanning pitch area is 4000 µm². A process for preparing a heat-sensitive stencil master sheet (3) according to claim 9, 10 or 11, wherein the average area of all clearance areas (7) is between 500 µm and 2500 µm². A process for preparing a heat-sensitive stencil master sheet (3) according to any one of claims 9 to 12, wherein the basis weight of said sheet (3) is between 6 g/m² and 14 g/m², and the fineness of said fibers is less than or equal to 3 denier.

Inventors: UJIIE MITSURU, UJIIE, MITSURU, UJIIE, MITSURU, RISO KAGAKU CORPORATION
Applicants: RISO KAGAKU CORP, RISO KAGAKU CORPORATION
Application year: 1992
Main classification: B41N1
IPC codes: B41C1, B41N1