There have been proposed stencil printers where print is made by driving, for instance, a thermal head according to image data obtained by reading out an original by, for instance, a scanner to selectively melt and perforate stencil material to make a stencil, winding the stencil around a printing drum, supplying ink inside the printing drum, and transferring the ink to printing papers through the stencil by, for instance, a roller.
In the stencil printers described above, when the working environmental temperature changes, the viscosity of the ink changes and the amount of ink transferred to the printing papers through the stencil changes which makes it impossible to make stencil printing at a stabilized density. Accordingly, there has been proposed, in Japanese Unexamined Patent Publication No. 2 (1990)-151473, a method of stencil printing where the pressure of the roller against the printing drum is changed according to the working environmental temperature so that the amount of ink transferred to the printing papers is fixed to make it possible to make stencil printing at a stabilized density.
In the above-mentioned stencil printer, when printing is once interrupted and is resumed, for instance, the ink adhering to the inner side of the printing drum has been dried or the water content or solvent of the ink inside the printing drum evaporates to increase the viscosity of the ink, whereby the ink inside the printing drum becomes hard to be transferred to the printing papers, which reduces the printing density and/or fluctuates the printing density.
In order to avoid this problem, there has been proposed a method where the ceasing time from the preceding printing to resumption of the printing is measured, and the pressure of the roller against the printing drum is increased above that during usual printing according to the length of the measured ceasing time, thereby making it possible to make stencil printing at a stabilized density.
However, in the above-mentioned stencil printer, for instance, when a plurality of kinds of inks different from each other, for instance, in volatility are used, the viscosity of ink inside the printing drum upon start of printing can sometimes differ due to difference in volatility. Accordingly, if the printing pressure is controlled solely according to the length of the ceasing time, the printing density becomes up and down depending on the kind of ink employed, which makes it impossible to make stencil printing at a stabilized density. Further, when a plurality of kinds of inks different from each other in viscosity are used, the viscosity of ink inside the printing drum upon start of printing can sometimes differ and it is impossible to make stencil printing at a stabilized density as described above.
Further, when a stencil comprising thermoplastic film and a porous support sheet such as Japanese paper or non-woven fabric laminated with each other is used, the amount of ink transferred to the printing papers changes according to the kind of porous support sheet since the ink supplied inside the printing drum is transferred to the printing papers through the pores of the porous support sheet and the perforations formed in the thermoplastic film. The reason why the amount of ink transferred to the printing papers differs according to the kind of porous support sheet is that the void volume, the void size, the void structure, the thickness of the support sheet, the material of the support sheet and/or the wetting characteristics differ according to the kind of porous support sheet and the resistance when the ink passes through the pores and/or the thickness of the transferring ink film differ. Accordingly, when a plurality of kinds of stencils are used, it is necessary to empirically obtain the printing pressure by the kind of porous support sheet, and if the printing pressure is controlled solely according to the length of the ceasing time, it is impossible to make stencil printing at a stabilized density.
The object of the present invention is to provide a stencil printing method and system, an ink container, and a stencil material roll which permit the stencil printing to be made at a stabilized density from the start of printing even if a plurality of kinds of inks different from each other in volatility and/or viscosity are used and/or a plurality of kinds of stencils different from each other in the kind of porous support sheet are used in the above-mentioned stencil printer.