Stencil printer having ink leakage preventing construction

Abstract: In a stencil printer having a printing drum 10 which has a circumferential wall portion for mounting a stencil sheet made of a flexible sheet member 20 and including a perforated portion 20c, an inking roller 12 provided inside of the printing drum for supplying ink to the printing drum from its inside with radially shifting out the perforated portion, and a back press roller 14 for supporting a print sheet against the printing drum by holding a leading end of the print sheet by a clamp 25, a means is provided for restricting the radial shifting out of the inking roller 12 when the inking roller is opposed by the clamp 25 to prevent ink leakage from the perforated portion 20c of the printing drum due to insufficient support of the print sheet by the back press roller 14 at the portion of the clamp 25. As an embodiment, an annular cam follower 23 mounted on the inking roller 12 is engaged by a cam 48 mounted on the back press roller.

Description: Background of the Invention Field of the Invention The present invention relates to a stencil printer, and more particularly to a construction for preventing ink leakage from a printing drum of the stencil printer. Description of the Prior Art A stencil printer comprising a printing drum having a perforated construction at a circumferential portion thereof except two annular edge portions extending along opposite axial ends of a cylindrical configuration thereof and a strip of stencil sheet leading end mounting portion extending between said two annular edge portions along a generatrix of the cylindrical configuration, an inking roller provided inside of said printing drum to be shiftable in radially outward and inward directions of said printing drum so as to supply ink to said perforated circumferential portion of said printing drum from the inside thereof while selectively pushing a part of said circumferential portion of said printing drum extending along a generatrix thereof radially outwardly, and a back press roller having a transverse groove formed at a portion of an outer circumferential surface thereof along a generatrix thereof, said printing drum and said back press roller being arranged in proximity and parallel to one another and adapted to rotate in synchronization with one another in opposite directions such that said stencil sheet leading end mounting portion is aligned with said transverse groove when they meet one another, thereby executing a stencil printing such that the ink supplied to said perforated portion of said printing drum by said inking roller is selectively transferred through a stencil sheet mounted around said printing drum to provide a printed image of ink on a print sheet supplied between said printing drum and said back press roller, is shown in Japanese Patent Application 63-28553 (Laid-open Publication 1-204781), Japanese Patent Application 1-47029 (Laid-open Publication 2-225078) and Japanese Patent Application 2-223550 (Laid-open Publication 4-105984) filed by the same assignee as the present application. Further, in a stencil printer having the above-mentioned construction, it is also known to provide a clamp at the back press roller for holding the leading end of a print sheet so as to remove the print sheet from the perforated circumferential portion of the printing drum at a condition that an ink layer contained in the perforated portion is retained from moving by the inking roller so that a clear uniform print image is available with no dim print portion being caused due to an insufficiency of ink, while avoiding that the back of a print sheet laid above another is stained by the ink image of the lower print sheet, as described in Japanese Patent Application 3-162218 (Laid-open Publication 4-361043) filed by the same assignee as the present application. Still further, there has been proposed a stencil printer having a construction by which it is prevented that ink leaks from the perforated portion to an adjacent non-perforated portion of the printing drum even when the relative rotational position between the printing drum and the back press roller is changed within a predetermined range for adjusting the longitudinal position of the print image relative to a print sheet mounted on the back press roller with its leading end being held to the back press roller by a clamp, as described in Japanese Patent Application 6-66708 (Laid-open Publication 7-179014) filed by the same assignee as the present application. The stencil printer according to the above-mentioned Japanese Patent Application 6-66708 is constructed such that, in order to prevent the above-mentioned ink leakage, the circumferential length of each of the perforated portion of the printing drum and the transverse groove of the back press roller and the relative rotational phase between said perforated portion and said transverse groove are so determined that said perforated portion and said transverse groove do not lie one over the other, so that therefore the perforated portion of the printing drum is always firmly supported by the cylindrical outer circumferential surface of the back press roller when the perforated portion is applied with the ink supply pressing by the inking roller from the inside thereof, or in other words, it is avoided that the perforated portion of the printing drum is pressed by the inking roller from the inside thereof in a condition not supported at the outside thereof as it is exposed to the transverse groove of the back press roller. However, although the clamp provided at the outer circumferential surface of the back press roller adjacent the transverse groove for holding the leading end of a print sheet is constructed to present an outer surface following as good the cylindrical configuration of the outer circumferential surface of the back press roller as possible, there remains necessarily a clearance around the clamp since it is a movable member, and therefore the back press roller can not present a perfect cylindrical outer surface in this area. Particularly when the clamp is of an inclining type, a relatively large clearance is required to allow the clamp to incline to its open position. This situation will be described with reference to the drawing. Fig. 1 of the attached drawing is the same as Fig. 1 of the above-mentioned Japanese Patent Application 6-66708, showing diagrammatically the basic construction of an embodiment of the stencil printer of said prior application. In this figure, 10 is a printing drum, and 14 is a back press roller. The printing drum 10 is constructed to have a pair of annular portion 16 at opposite axial ends thereof which are connected with one another by a transverse bar member 18 extending along a generatrix of the cylindrical configuration of the printing drum, thereby constructing a frame of the printing drum. A flexible sheet member 20 having a rectangular configuration in development is mounted such that opposite side edge portions thereof are laid over the outer circumferential surface of the pair of annular portions 16, while its leading end portion 20a and its trailing end portion 20b are respectively mounted to the transverse bar member 18. Although in the figure the mounting of the leading end portion 20a and the trailing end portion 20b of the flexible sheet member to the transverse bar member 18 is diagrammatically shown as simply laid one over the other at a corresponding portion thereof, particular mounting constructions of the trailing end portion 20b to the transverse bar member 18 are described in the above-mentioned Japanese Patent Application 1-47029 and also in Japanese Patent Application 5-306028 (Laid-open Publication 7-137415) and Japanese Patent Application 5-306029 (Laid-open Publication 7-137416) filed by the same assignee as the present application. A stencil sheet 19 is mounted around the cylindrical surface of the printing drum 10 defined by the flexible sheet member 20 with its leading end portion being mounted to the transverse bar member 18 by a clamp 21. Since the construction of mounting the leading and trailing end portions of the flexible sheet member to the transverse bar member is not concerned with the gist of the present invention, further description and illustration of such mounting construction will be omitted. The flexible sheet member 20 has a non-perforated construction at both the leading end portion 20a and the trailing end portion 20b and has a perforated construction to pass ink therethrough at an intermediate perforated portion 20c. In the assembled construction, the non-perforated leading and trailing end portions 20a and 20b of the flexible sheet member 20 cooperate with the transverse bar member 18 to provide a strip of non-perforated stencil sheet leading end mounting portion 10a extending between the opposite axial ends of the printing drum 10 along a generatrix thereof. On the other hand, the back press roller 14 is formed with a transverse groove 22 extending along a generatrix thereof. The printing drum 10 and the back press roller 14 are of the same diameter with one another and are adapted to be rotated in mutually opposite directions in synchronization with one another such that the stencil sheet leading end mounting portion 10a of the printing drum aligns with the transverse groove 22 of the back press roller when they meet one another. The rotational direction of the printing drum 10 is anti-clockwise, while that of the back press roller 14 is clockwise, as viewed in Fig. 1. Inside the printing drum 10, an inking roller 12 is provided so as to be rotated by a shaft 13 about a central axis thereof, with its outer circumferential surface contacting an inner circumferential surface of the printing drum 10. In order to avoid that during the rotation of the printing drum the inking roller 12 percussively contacts a rigid portion including the transverse bar member 18 in traversing, a cam 24 is provided at each of the pair of annular portions 16 over a region covering the transverse bar member 18, while an annular cam follower 23 to engage the cam 24 is provided on the shaft 13 of the inking roller 12. The back press roller 14 is provided with a clamp 25 adjacent a trailing edge of the transverse groove 22 as viewed in the direction of rotation of the back press roller for holding the leading end of a print sheet (not shown), so that the print sheet is tightly held on the circumferential surface of the back press roller 14 starting from the clamped leading end when the back press roller rotates clockwise in the figure, so that the print sheet is passed through a nip region between the back press roller and the printing drum to be provided with a print image thereon by the ink urged through the flexible sheet member 20 radially outwardly at the perforated portion 20c from its inside to its outside by the inking roller 12 and passed through perforated portions of the stencil sheet 19. As will be appreciated from Fig. 1, according to the invention of Japanese Patent Application 6-66704, the outer circumferential length of each of the perforated portion 20c of the printing drum 10 and the transverse groove 22 of the back press roller 14, indicated by P and G, respectively, and herein called as the length along a cylindrical outer configuration of the printing drum or the back press roller, and the relative rotational phase between the perforated portion 20c and the transverse groove 22, are determined such that the perforated portion 20c does not overlap with the transverse groove 22, so as thereby to provide an outer circumferential portion 30 remained between a point (actually a line) 28 on the back press roller 14 corresponding to a point (actually a line) 26 bordering the perforated portion 20c and the non-perforated leading end portion 20a of the flexible sheet member and the transverse groove 22, while an outer circumferential portion 36 is also remained between a point (actually a line) 34 on the outer circumferential surface of the back press roller 14 corresponding to a point (actually a line) 32 bordering the perforated portion 20c and the non-perforated trailing end portion 20b and the transverse groove 22. By these outer circumferential portions 30 and 36 being remained on the back press roller 14, when the bordering point 26 or 36 and a part of the perforated portion 20c adjacent thereto pass the position of contact with the inking roller 12, even though ink would leak from the perforated portion 20c to the adjacent non-perforated portion by the squeezing action applied by the inking roller 12, such a leaking out of the ink is suppressed by the rigid cylindrical outer surface of the back press roller 14 being tightly pressed against the flexible sheet member, thereby effectively preventing such a leaking out of the ink onto the outside surface of the non-perforated portion 20a or 20b. In Japanese Patent Application 5-306033 (Laid-open Publication 7-137419) filed by the same assignee as the present application, it was proposed to shift the relative rotational position between the printing drum 10 and the back press roller 14 from a standard position such as shown in Fig. 1 at which the transverse bar member 18 is just in alignment with the transverse groove 22, in opposite directions, for the purpose of adjusting the longitudinal position of a print image on a print sheet. When such a longitudinal print position adjustment mechanism is incorporated in the stencil printer, the point 28 on the back press roller 14 corresponding to the bordering point 26 on the printing drum 10 and the point 34 on the back press roller 14 corresponding to the bordering point 32 on the printing drum 10 move in biasing ranges 38 and 40, respectively. Therefore, if the circumferential length of the portions 30 and 36 are determined by taking those biasing ranges 38 and 40 into consideration, when such a longitudinal print position adjustment means is incorporated into the printer, the bordering point 26 or 32 and a part of the perforated portion 20c adjacent thereto will be retained from being exposed to the transverse groove 22 in traversing the position of contact with the inking roller 12, regardless of changes of the relative rotational phase between the printing drum 10 and the back press roller 14 effected by the longitudinal position adjustment means, so that the bordering point 26 or 32 and the part of the perforated portion 20c adjacent thereto are applied with the firm support of the rigid cylindrical outer circumference of the back press roller 14, so that no ink leakage from the perforated portion 20c to its adjacent non-perforated portion should occur. However, when the clamp 25 is provided at the outer circumferential portion 30 of the back press roller 14, since there is necessarily formed a certain clearance around the clamp 25, as it has a movable construction, and particularly when the clamp is of a pivotally inclining type, a relatively large clearance 27 is formed, as shown in Figs. 2 and 3, so that the back press roller can no longer present a perfect cylindrical outer surface at the portion 30. Therefore, when the perforated portion 20c is pressed by the inking roller 12 against the portion formed of clearance, the outside support to the flexible sheet member becomes insufficient, whereby a press out of ink from the inside to the outside of the perforated portion 20c occurs as shown in Fig. 3. When such a press out of ink once occurs, the highly viscous ink does not return by itself to the inside of the perforated portion 20c even when the press action by the inking roller has been removed. Therefore, when such a process is repeated along with rotations of the printing drum, the ink flows unidirectionally across the perforated portion 20c at such a poor support portion from its inside to its outside, such that there is generated a substantial accumulation of ink between the flexible sheet member 20 and the stencil sheet 19. If such an accumulation of ink progresses, there will occur an ink leakage therearound, causing a contamination by ink and even a damage of the stencil sheet by it being strongly pressed against an edge of the clamp or a notch for mounting the clamp. Summary of the Invention In view of the above-mentioned problems caused by the unidirectional transfer of ink through the perforated portion of the printing drum due to a pressing by the inking roller at a clearance formed around the clamp of the back press roller in the stencil printer having the above-mentioned construction, it is a principal object of the present invention to provide an improved stencil printer in which the contamination by ink and the damage of stencil sheet caused by the above-mentioned mechanism are effectively avoided. According to the present invention, the above-mentioned object is accomplished by a stencil printer comprising a printing drum having a perforated construction at a circumferential portion thereof except two annular edge portions extending along opposite axial ends of a cylindrical configuration of the printing drum and a strip of stencil sheet leading end mounting portion extending between said two annular edge portions along a generatrix of the cylindrical configuration, an inking roller provided inside of said printing drum to be shiftable in radially outward and inward directions of said printing drum so as to supply ink to said perforated circumferential portion of said printing drum from the inside thereof while selectively pushing a part of said circumferential portion of said printing drum extending along a generatrix thereof radially outwardly, and a back press roller having a transverse groove formed at a portion of an outer circumferential surface thereof along a generatrix thereof and a clamp for mounting a leading end of a print sheet thereto, said printing drum and said back press roller being arranged in proximity and parallel to one another and adapted to rotate in synchronization with and directions opposite to one another such that said stencil sheet leading end mounting portion is aligned with said transverse groove when they meet one another, thereby executing a stencil printing such that the ink supplied to said perforated portion of said printing drum by said inking roller is selectively transferred through a stencil sheet mounted around said printing drum to provide a printed image of ink on a print sheet supplied between said printing drum and said back press roller with a leading end portion thereof being held by said clamp, wherein the printer further comprises a means adapted to act at said inking roller when said clamp of said back press roller aligns with said inking roller, so as to restrict said radially outward shifting of said inking roller not to press the circumferential portion of said printing drum against said clamp. By such an inking roller shifting restriction means being provided, in spite of an adjustment of the longitudinal position of the print image, it is definitely avoided that the perforated portion of the printing drum is pressed by the inking roller from the inside thereof in a condition that it is laid over the clamp or the clearance formed therearound of the back press roller, whereby the contamination by ink or the damage of stencil sheet according to the above-mentioned mechanism is effectively avoided. The above-mentioned means for restricting said radially outward shifting of said inking roller may comprise an annular cam follower provided coaxially on said inking roller and a cam means provided on said back press roller to engage said cam follower. Said annular cam follower may have a diameter not greater than that of said inking roller, and said cam means may have an arcuate cam face projecting out of the cylindrical configuration of said back press roller. In this case, said cam means may be movably mounted on said back press roller so as to be selectively retracted within the cylindrical configuration of said back press roller. Brief Description of the Drawing In the accompanying drawing, Fig. 1 is a diagrammatic view showing the construction of the essential portions of a prior art stencil printer having the problems solved by the present invention; Fig. 2 is a perspective view showing an example of the back press roller of the stencil printer shown in Fig. 1; Fig. 3 is an enlarged view of a part of the stencil printer shown in Fig. 1, illustrating the problems solved by the present invention; and Fig. 4 is a diagrammatic view of a stencil printer similar to that shown in Fig. 1, incorporating the present invention in the form of an embodiment thereof. Description of the Preferred Embodiment Fig. 4 is a diagrammatic view showing an embodiment of incorporating the present invention into the prior art construction of Fig. 1, wherein the above-mentioned inking roller shifting restriction means is constructed as a cam provided at the back press roller to cooperate with the prior art cam follower. In Fig. 4, the portions corresponding to those shown in Fig. 1 are designated by the same reference numerals as in Fig. 1. In this embodiment, a cam 48 is provided at the back press roller 14 to extend over a region including the transverse groove 22 and certain opposite side portions thereof, so as to present an arcuate cam face which engages the annular cam follower 23 provided on the shaft 13 of the inking roller 12 to perform the same function as the cam 24 in the construction of Fig. 1 and also to perform as a means to prevent the inking roller 12 from being pressed against the circumferential portion of the printing drum when the inking roller 12 aligns with the clamp 25 provided at the back press roller for holding the leading end portion of a print sheet. By such a cam 48 being constructed to have an appropriate angular extension, when the printing drum 10 and the back press roller 14 rotate further from the position shown in the figure such that the relative position of the inking roller 12 to the back press roller 14 changes as shown by phantom circle 12' or 12'', regardless of any change of the relative angular position between the printing drum 10 and the back press roller 14 for an adjustment of the longitudinal position of a print image, the inking roller 12 is never pressed against the circumferential portion of the printing drum as aligned with the clamp 25, and therefore it is of course avoided that the perforated portion 20c of the printing drum is pressed against the clamp 25 or the clearance left therearound. In the shown embodiment, the cam follower 23 has a diameter smaller than that of the inking roller 12, and corresponding thereto the arcuate cam face of the cam means 48 projects out of the cylindrical configuration of the back press roller 14. When the cam 48 provided at the back press roller 14 projects outside of the cylindrical configuration of the back press roller as shown in Fig. 4, if the stencil printer is constructed such that the printing drum 10 is axially moved relative to the back press roller 14 so as to be axially drawn out of the housing of the stencil printer for the purpose of maintenance or exchange of the printing drum as proposed in Japanese Patent Application 5-306032 (Laid-open Publication 7-137418) by the same assignee as the present application, there is an inconvenience that the cam 48 hinders the axial movement of the printing drum 10. In such a case, it will be desirable that the cam 48 is mounted to be shiftable along a guide groove 50 as shown by arrows in the figure and moved by an appropriate drive means (not shown) so that, when the printing drum is axially drawn out of the housing of the printer, the cam 48 is retracted along the groove 50 so as to be housed within the cylindrical configuration of the back press roller 14. Although the present invention has been described in detail in the above with respect to an embodiment thereof incorporated into the stencil printer diagrammatically shown by Fig. 1 of Japanese Patent Application 6-66708, it will be apparent for those skilled in the art that various modifications are possible within the scope of the present invention as defined in the appended claims.

Claims: A stencil printer comprising a printing drum (10) having a perforated construction (20c) at a circumferential portion thereof except two annular edge portions extending along opposite axial ends of a cylindrical configuration of the printing drum and a strip of stencil sheet leading end mounting portion (10a) extending between said two annular edge portions along a generatrix of the cylindrical configuration, an inking roller (12) provided inside of said printing drum (10) to be shiftable in radially outward and inward directions of said printing drum (10) so as to supply ink to said perforated circumferential portion (20c) of said printing drum (10) from the inside thereof while selectively pushing a part of said circumferential portion of said printing drum (10) extending along a generatrix thereof radially outwardly, and a back press roller (14) having a transverse groove (22) formed at a portion of an outer circumferential surface thereof along a generatrix thereof and a clamp (25) for mounting a leading end of a print sheet (5) thereto, said printing drum (10) and said back press roller (14) being arranged in proximity and parallel to one another and adapted to rotate in synchronization with and directions opposite to one another such that said stencil sheet leading end mounting portion (10a) is aligned with said transverse groove (22) when they meet one another, thereby executing a stencil printing such that the ink supplied to said perforated portion (20c) of said printing drum (10) by said inking roller (12) is selectively transferred through a stencil sheet (19) mounted around said printing drum (10) to provide a printed image of ink on a print sheet (5) supplied between said printing drum (10) and said back press roller (14) with a leading end portion thereof being held by said clamp (25), characterised in that the printer further comprises a means adapted to act at said inking roller (12) when said clamp (25) of said back press roller (14) aligns with said inking roller (12), so as to restrict said radially outward shifting of said inking roller (12) not to press the circumferential portion of said printing drum against said clamp. A stencil printer according to claim 1, wherein said means for restricting said radially outward shifting of said inking roller comprises an annular cam follower (23) provided coaxially on said inking roller (12) and a cam means (48) provided on said back press roller (14) to engage said cam follower (23). A stencil printer according to claim 2, wherein said annular cam follower (23) has a diameter not greater than that of said inking roller (12), and said cam means (48) has an arcuate cam face projecting out of the cylindrical configuration of said back press roller (14). A stencil printer according to claim 3, wherein said cam means (48) is movably mounted on said back press roller (14) so as to be selectively retracted within the cylindrical configuration of said back press roller (14).

Inventors: MOTOE KATSURO, NEGISHI HIDEO, MOTOE, KATSURO, NEGISHI, HIDEO, Motoe, Katsuro, c/o Riso Kagaku Corp. R &amp; D Center, Negishi, Hideo, c/o Riso Kagaku Corp. R &amp; D Center
Applicants: RISO KAGAKU CORP, RISO KAGAKU CORPORATION
Application year: 1996
Main classification: B41L13
IPC codes: B41L13