Method and apparatus for stencil printing

Abstract: By preventing a deformation (9) of printing drum (1) of a rotary stencil printing apparatus caused at the time of printing, uniform printed images can be printed even under conditions such as low temperature and high-speed printing and even using an apparatus provided with a large-sized drum. That is, the invention provides a method for stencil printing which comprises winding a perforated stencil sheet (8) around the peripheral wall of a cylindrical printing drum (1) having an ink permeable peripheral wall and rotating around its central axis, supplying an ink to the peripheral wall from the inside of the drum at the time of rotation of the drum, and pressing a printing paper (10) to the outer surface of the peripheral wall of the drum to transfer the ink to the printing paper through the stencil sheet (8), wherein the peripheral wall of the drum has such a strength that the peripheral wall shows a stress of 0.75 kgf/cm 2 or more, preferably 1.4 kgf/cm 2 or more when it is deformed by 0.1 cm in a direction towards its central axis and viscosity of the ink is 330 Pa·s or lower, preferably 150 Pa·s or lower. An apparatus for carrying out the stencil printing is also provided.

Description: Technical Field: The present invention relates to a method and an apparatus for stencil printing according to which images can be uniformly printed by preventing deformation of printing drum at printing. Background & Prior Art: As shown in FIG. 1, a rotary stencil printing apparatus is provided with a cylindrical printing drum 1 which rotates around its central axis, and the drum 1 has an ink passing peripheral wall composed of a material having many pores or a mesh material. This drum 1 can generally be made by a method of rolling a plate material such as a punching metal and an expanded metal into a cylindrical form and holding both ends by flanges, a method of fixing a part of a mesh material such as a screen to a flange to hold it in a cylindrical form, or a method of fitting flanges at both ends of a cylindrical shaped article, and the basic strength of the peripheral wall of the drum is determined by the materials constituting the drum. If necessary, one or more screen layers of micro-reticulate structure (not shown) can be wound around the peripheral surface of the drum 1. Inside the drum 1, is provided an ink supplying means comprising a squeegee roller 4 which contacts with the inner peripheral surface of the drum, a doctor roller 5 which is disposed adjacent the squeegee roller, and others. Outside the drum 1, is disposed an opposing roller 6 at the position opposite to the squeegee roller 4, and the opposing roller 6 functions as a means of carrying a printing paper 10 in cooperation with the drum 1 with nipping the printing paper therebetween at the time of printing. According to FIG. 1, the opposing roller 6 is provided so that it can contact with and leave from the drum 1, and has a function to press the printing paper 10 against the outer peripheral surface of the drum 1 at the time of printing. Alternatively, the opposing roller 6 may have a diameter similar to that of the drum 1, and the squeegee roller 4 may have a function to press the peripheral wall of the drum 1 outwardly to deform the peripheral wall in such a manner that the peripheral wall can contact with and leave from the opposing roller 6, and the printing paper 10 may be carried with being nipped between the deformed drum and the opposing roller 6. At the time of printing, one end of perforated stencil sheet 8 is clamped by a clamping means 2 provided at the outer peripheral surface of the drum 1 and the stencil sheet 8 is wound around the outer peripheral surface of the drum 1. When the drum is rotated, ink is supplied to the peripheral surface from inside of the drum 1 by the squeegee roller 4 and excess ink forms a reservoir of ink 7 between the squeegee roller 4 and the doctor roller 5. In this case, the printing paper 10 is carried with being nipped between the opposing roller 6 and the outer peripheral surface of the drum 1, whereby the ink is transferred to the printing paper through the stencil sheet 8 to perform printing. Under usual printing conditions, the drum 1 undergoes some deformation by pressing force at the nip portion at which the opposing roller 6 and the drum 1 contact with each other, but the apparatus must be designed so that other portions of the drum 1 are not deformed at the time of rotating in order to prevent slippage of the stencil sheet or creasing of the printing paper at the time of carrying the paper. Although the peripheral wall of the drum can keep cylindrical shape under usual printing conditions, it sometimes undergoes deformation under some use conditions such as low temperature and high-speed printing. The similar phenomenon is also seen in an apparatus provided with a large drum designed so as to apply to the use of large-sized printing papers. As shown in FIG. 2, this deformation appears as a phenomenon that at the time of rotation of the drum 1 the periphery wall of the drum 1 deforms in the direction towards the squeegee roller 4 at the position 9 which is slightly downstream in rotation direction from the contact portion of the squeegee roller 4 and the drum 1, and this causes uneveness in the resulting printed images. It has therefore been considered desirable to ascertain the causes for the deformation of the drum as mentioned above, and to prevent the deformation so that uniform printed images can be obtained even under the use conditions such as low temperature and high-speed printing and even by apparatus provided with a large-sized drum. Summary of the Invention: As a result of intensive research conducted by the inventors on the strength of drum and the viscosity of ink, they have considered that the deformation at the position 9 of the drum shown in Fig 2 is caused by the viscosity of ink collected between the portion 9 and the squeegee roller 4, and have found that the deformation of the drum can be prevented when the peripheral wall of the drum has a strength higher than a certain value and viscosity of the ink supplied to the drum is lower than a certain value. Thus, according to one aspect of the present invention, there is provided stencil printing apparatus comprising an ink and a cylindrical printing drum which has an ink permeable peripheral wall and in use is to rotate around a central axis of the drum with a perforated stencil sheet wound around the peripheral wall, ink supply means internally of the drum in use to contact an inner surface of the peripheral wall of the drum and supply said ink to the peripheral wall during rotation of the drum, and an opposing roller disposed externally of the drum opposite the ink supply means in use to carry a printing paper into and through the nip between the opposing roller and the drum, wherein the ink used has a viscosity of 330 Pa·s or less and the peripheral wall of the drum is in use resistant to a deformation force exerted upon the drum by said ink the stress in said peripheral wall due to said deformation force being 0.75 kgf/cm 2 or more upon a radial deformation of the drum's peripheral wall by 0.1cm towards the central axis. Furthermore, according to another aspect of the present invention, there is provided a method of utilising apparatus according to said one aspect of the invention, wherein the ink employed has a viscosity of 330 Pa·s or less, and the deformation force exerted by the ink upon the drum's peripheral wall induces a stress therein of 1.4 kgf/cm 2 or more when the peripheral wall is radially deformed by 0.1cm towards the central axis. According to the present invention, the strength of the drum is specified by the stress generated when the peripheral wall is deformed, and printing is carried out using a drum which shows a stress of 0.75 kgf/cm 2 or more, preferably 1.4 kgf/cm 2 or more, when the peripheral wall is deformed by 0.1 cm in the direction towards its central axis, and using an ink having a viscosity of 330 Pa·s or lower, preferably 150 Pa·s or lower, whereby deformation of the drum can be prevented because the strength of the peripheral wall of the drum at the position indicated by 9 in Fig 2 exceeds a deformation force exerted to the drum by the viscosity of the ink. In the case of a drum formed of usual materials, when the stress is increased to more than 9.00 kgf/cm 2 , thickness of the drum must be increased or opening ratio must be lowered and this causes blurring of printed images or decrease of printing density. Moreover the viscosity of the ink is preferably 9 Pa·s or more. If the viscosity is lower than 9 Pa·s, ink drips onto the drum from the squeegee roller during waiting period or leaks from a gap between the drum and the stencil sheet, or printed images spread. The drum of the present invention may be made of any materials as far as the above strength is satisfied, and thickness, opening pore pattern, opening ratio and processing method of the peripheral wall of the drum can be optionally set. Brief Description of Drawings: The accompanying drawings will be explained below briefly. FIG. 1 is a schematic sectional view illustrating printing mechanism of a rotary stencil printing apparatus. FIG. 2 is a schematic sectional view showing the defect in the conventional stencil printing method. FIG. 3 is an oblique view explaining the method for measurement of strength of the drum in the present invention. Detailed Description of Examples of the invention: Hereinafter, the present invention will be explained in more detail by way of presently preferred working examples. However, it should be understood that the present invention is not limited to the examples. Example 1 Printing was conducted by a stencil printing apparatus (RISOGRAPH GR377 manufactured by RISO KAGAKU CORPORATION) ) having a construction shown in FIG. 1 in which the printing drum had a peripheral wall made of a nickel electroformed material having a thickness of 0.2 mm, a pore diameter of 0.2 mm and an opening ratio of 12%, using inks of various viscosities shown in Table 1, at a printing environmental temperature of 23°C with changing the printing speed from 60 to 130 rpm. The deformation of the drum was visually examined. The results are shown in Table 1. The viscosity of inks was measured using a Brookfield viscometer (VISCOMETER-BH manufactured by TOKI SANGYO CO,. LTD.) with a No.7 rotor under the conditions of 23°C and 20 rpm in rotating speed. As shown in FIG. 3, strength of the drum was determined by measuring a stress caused when a jig 21 of 0.5 cm in diameter was pressed to nearly the center of the peripheral wall of a drum at a speed of 1.0 cm/min to result in deformation of 0.1 cm in the direction towards the central axis of the drum, using a testing apparatus 20, namely AUTOGRAPH (AGS-500D) manufactured by SHIMADZU SEISAKUSHO, LTD. Example 2 Evaluation was conducted in the same manner as in Example 1, except that the peripheral wall of the drum was made of a nickel electroformed material having a thickness of 0.18 mm, a pore diameter of 0.2 mm and an opening ratio of 12%. The results are shown in Table 1. Example 3 Evaluation was conducted in the same manner as in Example 1, except that the peripheral wall of the drum was made of a stainless steel etched material having a thickness of 0.2 mm, a pore diameter of 0.2 mm and an opening ratio of 30%. The results are shown in Table 1. Example 4 Evaluation was conducted in the same manner as in Example 1, except that the peripheral wall of the drum was made of a nickel electroformed material having a thickness of 0.15 mm, a pore diameter of 0.2 mm and an opening ratio of 12%. The results are shown in Table 1. Example 5 Evaluation was conducted in the same manner as in Example 1, except that the peripheral wall of the drum was made of a stainless steel etched material having a thickness of 0.15 mm, a pore diameter of 0.2 mm and an opening ratio of 30%. The results are shown in Table 1. Example 6 Evaluation was conducted in the same manner as in Example 1, except that the peripheral wall of the drum was made of a nickel electroformed material having a thickness of 0.3 mm, a pore diameter of 0.3 mm and an opening ratio of 5%. The results are shown in Table 1. Comparative Example 1 Evaluation was conducted in the same manner as in Example 1, except that the peripheral wall of the drum was made of a stainless steel etched material having a thickness of 0.12 mm, a pore diameter of 0.2 mm and an opening ratio of 30%. The results are shown in Table 1. According to the present invention, stencil printing is carried out using a printing drum having a stress of 0.75 kgf/cm 2 or more, preferably 1. 4 kgf/cm 2 or more when the peripheral wall of the drum is deformed by 0.1 cm in a direction towards the central axis of the drum and using an ink having a viscosity of 330 Pa·s or less, preferably 150 Pa·s or less. As a result, the peripheral wall of the drum is prevented from being deformed in the vicinity of an ink supplying means due to the viscosity of the ink, and therefore uniform printed images free from unevenness can be obtained.

Claims: Stencil printing apparatus comprising an ink (7) and a cylindrical printing drum (1) which has an ink permeable peripheral wall and in use is to rotate around a central axis of the drum with a perforated stencil sheet (8) wound around the peripheral wall, ink supply means (4,5) internally of the drum in use to contact an inner surface of the peripheral wall of the drum and supply said ink (7) to the peripheral wall during rotation of the drum, and an opposing roller (6) disposed opposite the ink supply means (4,5) in use to carry a printing paper (10) into and through the nip between the opposing roller (6) and the drum (1), characterised in that the ink used has a viscosity of 330 Pa·s or less and the peripheral wall of the drum (1) is in use resistant to a deformation force exerted upon the drum by said ink, the stress in said peripheral wall due to said deformation force being 0.75 kgf/cm 2 or more upon a radial deformation (9) of the drum's peripheral wall by 0.1cm towards the central axis. Stencil printing apparatus according to Claim 1, characterised in that the peripheral wall of the drum (1) is in use resistant to a deformation force exerted upon the drum by said ink , the stress in said peripheral wall due to said deformation force being 1.4 kgf/cm 2 or more upon a radial deformation (9) of the drum's peripheral wall by 0.1cm towards the central axis. Stencil printing apparatus according to Claim 1 or Claim 2, characterised in that the peripheral wall of the drum (1) is in use resistant to a deformation force exerted upon the drum by said ink, the stress in said peripheral wall due to said deformation force being 9 kgf/cm 2 or less upon a radial deformation (9) of the drum's peripheral wall by 0.1cm towards the central axis. Stencil printing apparatus according to any preceding Claim, wherein the ink has a viscosity of 150 Pa·s or less. Stencil printing apparatus according to any preceding Claim, wherein the ink has a viscosity of 9 Pa·s or higher. A method of stencil printing utilising apparatus according to Claim 1, characterised in that the ink (7) employed has a viscosity of 330 Pa·s or less, and the deformation force exerted by the ink upon the drum's peripheral wall induces a stress therein of 0.75 kgf/cm 2 or more when the peripheral wall is radially deformed by 0.1cm towards the central axis. A method of stencil printing utilising apparatus according to Claim 2, characterised in that the ink (7) employed has a viscosity of 330 Pa·s or less, and the deformation force exerted by the ink upon the drum's peripheral wall induces a stress therein of 1.4 kgf/cm 2 or more when the peripheral wall is radially deformed by 0.1cm towards the central axis. A method of stencil printing utilising apparatus according to Claim 3, characterised in that the ink (7) employed has a viscosity of 330 Pa·s or less, and the deformation force exerted by the ink upon the drum's peripheral wall induces a stress therein of 9 kgf/cm 2 or less when the peripheral wall is radially deformed by 0.1cm towards the central axis. A method of stencil printing according to any one of Claims 6 to 8, characterised in that the ink employed has a viscosity of 150 Pa·s or less. A method of stencil printing according to any one of Claims 6 to 9, characterised in that the ink employed has a viscosity of 9 Pa·s or higher.

Inventors: ISOZAKI TAKASHI, KATO HIROYASU, OKUDA SADANAO, SUZUKI KENJI, ISOZAKI, TAKASHI, KATO, HIROYASU, OKUDA, SADANAO, SUZUKI, KENJI, Isozaki, Takashi, Riso Kagaku Corp. R &amp; D Center, Kato, Hiroyasu, Riso Kagaku Corp. R &amp; D Center, Okuda, Sadanao, Riso Kagaku Corp. R &amp; D Center, Suzuki, Kenji, Riso Kagaku Corp. R &amp; D Center
Applicants: RISO KAGAKU CORP, RISO KAGAKU CORPORATION
Application year: 1999
Main classification: B41L13
IPC codes: B41M1, B41L13