Patent Publication Number: US-6213014-B1

Title: Stencil printer having printing drum and retainer roller

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a stencil printer which comprises a printing drum, an internal press roller provided inside the printing drum to be brought into contact with the inner surface of the printing drum and a pinching means which is provided outside the printing drum and is pressed against the peripheral wall of the printing drum with a printing paper pinched therebetween, and more particularly to such a stencil printer in which the peripheral wall of the printing drum is prevented from being deformed inward through contact with the internal press roller. 
     2. Description of the Related Art 
     The structure of the conventional stencil printer and the drawbacks of the stencil printer will be described with reference to FIG. 12, hereinbelow. As shown in FIG. 12, the conventional stencil printer comprises a printing drum  200  having a cylindrical peripheral wall. The peripheral wall is permeable to ink and a stencil master is wound around the peripheral wall. The printing drum  200  is rotated about its longitudinal axis. Am arm  202  which is rotatable about a fixed shaft  201  is provided inside the printing drum  200 . An internal press roller  203  is mounted for rotation on the arm  202 . A doctor roller  205  is disposed near the internal press roller  203 . The doctor roller  205  forms an ink layer of a predetermined thickness on the outer surface of the internal press roller  203 . The arm  202  is rotated in synchronization with rotation of the printing drum  200  so that the internal press roller  203  presses outward the peripheral wall of the printing drum  200 . A paper pinch drum  206  is supported for rotation at a predetermined distance from the printing drum  200 . When a printing paper is supplied between the printing drum  200  and the paper pinch drum  206 , the internal press roller  203  deforms outward the peripheral wall of the printing drum  200  and the printing paper is conveyed pinched between the stencil master on the deformed part of the peripheral wall and the paper pinch drum  206 , whereby ink supplied from the internal press roller  203  to the inner surface of the peripheral wall of the printing drum  200  passes through the peripheral wall of the printing drum  200  and the stencil master and is transferred to the printing paper to form an image. 
     In the conventional stencil printer shown in FIG. 12, there has been a problem that a part of the peripheral wall of the printing drum  200  adheres to the internal press roller  203  and is deformed inward as indicated at D in FIG.  12 . This phenomenon occurs on the downstream side of the contact line between the outer surface of the internal press roller  203  and inner surface of the peripheral wall of the printing drum  200  and occurs due to the fact that a part of the inner surface of the peripheral wall of the printing drum  200  cannot be properly separated from the outer surface of the internal press roller  203  after the part passes through the contact line between the internal press roller  203  and the paper pinch drum  206 . 
     Accordingly, this phenomenon is more apt to occur when the peripheral wall of the printing drum  200  is deformable or when the ink has a high viscosity. This phenomenon is especially apt to occur under a low temperature where the viscosity of the ink increases. This phenomenon can occur not only in the conventional stencil printer where the peripheral wall of the printing drum  200  is flexible but also in stencil printers where the peripheral wall of the printing drum is rigid. This is because the thickness of the rigid peripheral wall of the printing drum is made as thin as possible in order to reduce the amount of ink held by the printing drum so that printing quality is improved. For example, when the peripheral wall of the printing drum is formed of a stainless steel plate which is about 0.15 mm in thickness, the aforesaid deformation of the peripheral wall can occur depending on the conditions. 
     The aforesaid deformation of the peripheral wall of the printing drum can cause the following problems. That is, when the peripheral wall of the printing drum is deformed, the stencil master attached on the outer surface of the peripheral wall can be stretched or displaced in the circumferential direction of the printing drum. Further when the peripheral wall is repeatedly deformed and is repeatedly subjected to stress, the peripheral wall can be broken at the part where the stress is concentrated. Further repeated deformation of the peripheral wall can finally result in plastic deformation of the peripheral wall, e.g., the peripheral wall can corrugate as seen in a cross-section parallel to the longitudinal axis of the printing drum. Further when the peripheral wall is finally moved away from the internal press roller under its resiliency, ink splashes mechanisms in the printing drum. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing observations and description, the primary object of the present invention is to provide a stencil printer in which deformation of the peripheral wall of the printing drum can be suppressed, whereby the stencil master wound around the peripheral wall of the printing drum is stabilized, durability of the peripheral wall is increased and ink is prevented from splashing. 
     In accordance with the present invention, there is provided a stencil printer comprising 
     a printing drum which has an ink-permeable peripheral wall, around which a stencil master is wound and to the inner surface of which ink is supplied, and is rotated about its longitudinal axis, 
     an internal press roller which is disposed inside the printing drum and is in contact with the inner surface of the peripheral wall of the printing drum, 
     a pinching means which is disposed outside the printing drum and carries a printing paper with pinching the printing paper between the pinching means and the stencil master wound around the peripheral wall, and 
     a retainer roller which is disposed in a predetermined position near the internal press roller and the inner surface of the peripheral wall downstream of the contact line between the internal press roller and the inner surface of the peripheral wall in the direction of rotation of the printing drum. 
     In one embodiment of the present invention, the internal press roller is mounted on a rotatable member disposed inside the printing drum to be movable, in response to rotation of the rotatable member, between an inoperative position where it is away from the peripheral wall of the printing drum and an operative position where it presses outward the peripheral wall, the pinching means is a rotatable paper pinch drum which is disposed outside the printing drum at such a distance from the printing drum that the printing paper is pinched between the peripheral wall of the printing drum and the paper pinch drum when the internal press roller deforms outward the side, and the retainer roller is mounted for rotation on the rotatable member. 
     In another embodiment of the present invention, the internal press roller is mounted on a rotatable member disposed inside the printing drum to be movable, in response to rotation of the rotatable member, between an inoperative position where it is away from the peripheral wall of the printing drum and an operative position where it presses outward the peripheral wall, the pinching means is a rotatable paper pinch drum which is disposed outside the printing drum at such a distance from the printing drum that the printing paper is pinched between the peripheral wall of the printing drum and the paper pinch drum when the internal press roller deforms outward the side, and the retainer roller is mounted for rotation on a rotatable arm which is mounted for rotation on the rotatable member and is urged by an urging means toward the inner surface of the peripheral wall of the printing drum. 
     In still another embodiment of the present invention, the pinching means is an external press roller which is movable up and down between an operative position in which it is pressed against the peripheral wall of the printing drum with a printing paper pinched therebetween and an inoperative position where it is away from the printing drum, and the retainer roller is supported for rotation. 
     In still another embodiment of the present invention, the pinching means is an external press roller which is movable up and down between an operative position in which it is pressed against the peripheral wall of the printing drum with a printing paper pinched therebetween and an inoperative position where it is away from the printing drum, and the retainer roller is movable upward from the predetermined position and is urged toward the predetermined position. 
     Generally the retainer roller is normally spaced from the inner surface of the peripheral wall. 
     The retainer roller may be rotated in the same direction as the internal press roller. 
     The retainer roller may be of various shapes. For example, the retainer roller may be a cylindrical member having a uniform outer diameter or a spindle-shaped member. Further the retainer roller may comprise a plurality of roller sections which are coaxially mounted. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic side view of a stencil printer in accordance with a first embodiment of the present invention in a state where the squeegee roller is in the operative position, 
     FIG. 2 is a schematic side view of the stencil printer of the first embodiment in a state where the squeege roller is in the inoperative position, 
     FIG. 3 is a perspective view partly cut away showing the printing drum and the paper pinch drum of the stencil printer of the first embodiment, 
     FIG. 4 is an enlarged side view of the printing drum of the stencil printer of the first embodiment, 
     FIG. 5 is an enlarged perspective view of the squeegee roller and the support roller of the stencil printer of the first embodiment, 
     FIG. 6 is an enlarged perspective view of the support roller of the stencil printer of the first embodiment, 
     FIG. 7 is an enlarged perspective view of a modification of the support roller, 
     FIG. 8 is an enlarged perspective view of another modification of the support roller, 
     FIG. 9 is an enlarged perspective view of still another modification of the support roller, 
     FIG. 10 is an enlarged side view of the printing drum of a stencil printer in accordance with a second embodiment of the present invention, 
     FIG. 11 is a schematic side view of a stencil printer in accordance with a third embodiment of the present invention, and 
     FIG. 12 is a schematic side view of a conventional stencil printer. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A stencil printer in accordance with a first embodiment of the present invention will be described in detail with reference to FIGS. 1 to  6 , hereinbelow. 
     In FIGS. 1 to  6 , the stencil printer of this embodiment has a printing drum  1  comprising a pair of disc-like rigid end plates  3  opposed to each other in the longitudinal direction of the printing drum  1 , a rigid clamp base plate  5  which extends in parallel to the longitudinal axis of the printing drum  1  and connects the end plates  3 , and a cylindrical peripheral wall  7  wound around the side plates  3 . 
     The peripheral wall  7  is formed by weaving wire such as of stainless steel into a mesh-like structure and accordingly is flexible and ink-permeable. By virtue of such a structure, the peripheral wall  7  can be deformed outward in a radial direction of the printing drum  1 . 
     A clamp plate  11  which clamps the leading end portion of a stencil master is mounted for rotation on the clamp base plate  5 . The stencil master is wound around the peripheral wall  7  of the printing drum  1  with its leading end clamped by the clamp plate  11 . 
     The printing drum  1  is supported for rotation on a tubular drum shaft  13  which extends through the printing drum  1  along its longitudinal axis. Each of the end plates  3  is formed with a gear  15  on its circumferential surface and the gear  15  is in mesh with a drive gear of a printing drum drive motor (not shown). The printing drum  1  is rotated in the counterclockwise direction as seen in FIG. 1 by the printing drum drive motor about the drum shaft  13  which is fixed. 
     An in-drum frame  17  is fixedly supported in the printing drum  1  by the drum shaft  13 . A pair of roller support arms  21  are connected to the in-drum frame  17  by way of pivot shafts  19  at their one ends so that the roller support arms  21  are rotatable up and down. A squeegee roller  23  (an internal press roller) is supported for rotation on intermediate portions of the roller support arms  21 . The squeegee roller  23  extends in parallel to a generatrix of the printing drum  1  and the side surface of the squeegee roller  23  is brought into contact with the inner surface of the peripheral wall  7  of the printing drum  1 . 
     A doctor rod  25  extends in parallel to the squeegee roller  23  at a slight distance therefrom and is fixed to the roller support arms  21  at its opposite ends. An ink fountain  27  is formed between the squeegee roller  23  and the doctor rod  25 . An ink delivery pipe  29  supplies ink to the ink fountain  27 . The ink delivery pipe  29  is connected to an ink hose  31  which extends through the tubular drum shaft  13  to the outside of the printing drum  1  and is connected to an ink source (not shown). 
     Ink in the ink fountain  27  is caused to pass the narrow space between the squeegee roller  23  and the doctor rod  25  by counterclockwise rotation of the squeegee roller  23  and is metered. Thus ink adheres to the outer surface of the squeegee roller  23  in a layer of a predetermined thickness and is squeezed into the inner surface of the peripheral wall  7  as the squeegee roller  23  rotates. 
     As shown in FIG. 4, an ink guard plate  24  is fixed to the roller support arms  21  to extend in parallel to the longitudinal axis of the squeegee roller  23 . The ink guard plate  24  is disposed downstream of the contact line between the printing drum  1  and the squeegee roller  23  in the direction of rotation of the printing drum  1 . The ink guard plate  24  is substantially equal to the squeegee roller  23  in length. Ink film is formed between the inner surface of the peripheral wall  7  of the printing drum  1  and the outer surface of the squeegee roller  23  and when the ink film is broken, ink splashes. The ink guard plate  24  is disposed to extend obliquely across splashing path of the ink. 
     As shown in FIG. 4, a pair of mounting pieces  22  are fixed to the roller support arm  21  at its opposite ends and a retainer roller  20  is supported for rotation by the mounting pieces  22 . In FIG. 3, the roller support arm  21  is omitted for the purpose of simplicity of the drawing. 
     The retainer roller  20  retains the peripheral wall  7  not to be deformed upward by the squeegee roller  23 . As shown in FIG. 6, in this particular embodiment, the retainer roller  20  is uniform in thickness and has a continuous circumferential surface in its longitudinal direction. The retainer roller  20  is substantially the same as the squeegee roller  23  in length and extends in parallel to the squeegee roller  23 . The retainer roller  20  is disposed near the squeegee roller  23  and the inner surface of the peripheral wall  7  downstream of the contact line between the squeegee roller  23  and the inner surface of the peripheral wall  7  in the direction of rotation of the printing drum  1 . The retainer roller  20  is not in contact with the peripheral wall  7  but is at an adequate distance from the peripheral wall  7 . The retainer roller  20  may be either set free or driven in the same direction as the squeegee roller  23 . The distance between the retainer roller  20  and the peripheral wall  7  is set to prevent deformation of the peripheral wall  7  in such a degree that can cause a problem. 
     When the retainer roller  20  is in contact with the peripheral wall  7 , ink can sometimes leak outside the peripheral wall  7  where the retainer roller  20  is in contact with the peripheral wall  7 . Since the retainer roller  20  employed in this particular embodiment is substantially cylindrical and is uniform in thickness over the entire length thereof, the retainer roller  20  will be brought into contact with the peripheral wall  7  over the entire length thereof when there is no distance between the retainer roller  20  and the peripheral wall  7 , whereby the pressure applied to the peripheral wall  7  becomes too high and ink will leak outside the peripheral wall  7 . 
     A cam shaft  33  is supported for rotation on the in-drum frame  17 . The in-drum frame  17  is provided with a cam mechanism formed by a double-heart-shaped cam plate  35  fixed to the cam shaft  33  and a cam follower  39  mounted on a yoke member  37 . The yoke member  37  is connected to the end of the roller support arm  21  by way of a pivot  41 . Each time the cam plate  35  is rotated by 90°, the cam plate  35  alternately takes a printing position shown in FIG. 1 where it moves the squeegee roller  23  to an operative position where the squeegee roller  23  presses outward the peripheral wall  7  of the printing drum  1  and a non-printing position shown in FIG. 2 where it moves the squeegee roller  23  to an inoperative position where the squeegee roller  23  is kept away from the peripheral wall  7 . 
     The cam shaft  33  is connected to a driven side of an electromagnetic clutch  43  and a drive side of the electromagnetic clutch  43  is connected to a cam shaft drive gear  45 . The cam shaft drive gear  45  is in mesh with an in-drum main gear  47  and is driven by the main gear  47  in response to rotation of the printing drum  1 . 
     A cam switch (a limit switch)  49  is mounted on the in-drum frame  17  and the cam switch  49  is actuated by an actuator  51  mounted on the yoke member  37  as shown in FIG. 3, thereby detecting whether the cam plate  35  is in the printing position or the non-printing position. 
     As shown in FIG. 1, a roller drive arm  53  is supported for rotation on the drum shaft  13  at an intermediate portion thereof. An intermediate gear  55  is supported for rotation on one end portion of the roller drive arm  53  and the other end portion of the roller drive arm  53  is connected to a tension spring  57  so that the roller drive arm  53  is urged in the counterclockwise direction. The intermediate gear  55  is in mesh with the in-drum main gear  47  and a gear  58  formed on one end portion of the squeegee roller  23  coaxially with the squeegee roller  23  under the force of the tension spring  57  and is rotated in response to rotation of the printing drum  1  to rotate the squeegee roller  23  in the same direction as the printing drum  1 , i.e., in the counterclockwise direction. 
     When the squeegee roller  23  is rotated in the counterclockwise direction while the cam plate  35  is in the printing position and the squeegee roller  23  is held in the operative position shown in FIG. 1, the squeegee roller  23  presses outward the peripheral wall  7  of the printing drum  1  toward a paper pinch drum  63  to be described later. 
     On the other hand, when the cam plate  35  is in the non-printing position and the squeegee roller  23  is held in the inoperative position shown in FIG. 2 where the squeegee roller  23  is kept away from the peripheral wall  7 , the peripheral wall  7  is not deformed even if the squeegee roller  23  is rotated. 
     As shown in FIG. 3, a cam follower  59  is mounted on the squeegee roller  23  and is brought into contact with a cam  61  formed on the inner side surface of the printing drum  1 . In an angular position where a recess  65  on the paper pinch drum  63  is faced toward the printing drum  1 , the cam follower  59  abuts against the cam  61 , whereby the cam follower  59  lifts, i.e., moves inward, the squeegee roller  23  away from the peripheral wall  7  of the printing drum  1  when the clamp base plate  5  comes to be below the squeegee roller  23  and the clamp base plate  5  is prevented from colliding against the edge of the recess  65 . Thus the peripheral wall  7  is protected. 
     The paper pinch drum  63  is substantially the same as the printing drum  1  in outer diameter and is supported for rotation on a shaft  62  at a predetermined distance from the printing drum  1  in parallel thereto. The paper pinch drum  63  is rotated in the clockwise direction about the shaft  62  at a speed equal to the printing drum  1  by a synchronized drive unit (not shown). The paper pinch drum  63  is provided with the recess  65 , which is for avoiding interference with the stencil master clamp mechanism of the printing drum  1 . 
     The paper pinch drum  63  functions as the pinching means for pressing the printing paper against the printing drum  1  during printing. 
     As shown in FIG. 1, the part of the peripheral wall  7  deformed by the squeegee roller  23  is pressed against the paper pinch drum  63  with the stencil master and the printing paper P sandwiched therebetween. When the squeegee roller  23  is in the inoperative position, a space is formed between the printing drum  1  and the paper pinch drum  63  and the printing paper P can pass between the printing drum  1  and the paper pinch drum  63 . 
     The paper pinch drum  63  is provided with a paper clamp member  67 . The paper clamp member  67  is rotatably mounted on the paper pinch drum  63  by a pivot  69 . A clamping piece  71  which is associated with the outer side surface of the paper pinch drum  63  to clamp a printing paper P is provided on one end of the clamp member  67  and a cam follower  73  is formed on the other end of the paper clamp member  67 . The cam follower  73  is in contact with a fixed cam  75  and the clamping piece  71  is moved in synchronization rotation of the paper pinch drum  63  to clamp a leading end portion of the printing paper P, supplied from a paper supply section  77  to be described later, in an angular position of the paper pinch drum  63  indicated at a (paper clamping position) in FIG.  2  and to release the printing paper P in an angular position of the paper pinch drum  63  indicated at b (paper releasing position). 
     The stencil printer further comprises a paper supply section  77  and a paper discharge section  79 . The paper supply section  77  comprises a paper supply table  81  on which a stack of printing papers P is placed, a pair of paper supply rollers  83  and a paper separator roller  85  for taking out the printing papers P from the paper supply table  81  one by one, a paper guide  87 , a pair of timing rollers  89  which feeds at a predetermined timing the printing paper P to the paper clamping position a where the clamping piece  71  of the paper pinch drum  63  clamps the printing paper P, and an optical paper supply sensor  91  which detects that the printing paper P is fed to the paper clamping position a. 
     The paper discharge section  79  comprises a discharge pinch roller  93  which is disposed in the paper release position b and is associated with the paper pinch drum  63  to convey and discharge the printing paper P from the paper pinch drum  63 , a paper scraper  95  which removes the printing paper P from the paper pinch drum  63 , a pair of paper discharge pinch rollers  99  which discharge the printed paper to a paper chute  97 , a paper discharge table  101  on which the printed papers are stacked, and an optical paper discharge sensor  103  which optically detects that the printed paper P is chuted from the paper chute  97  toward the paper discharge table  101 . 
     The discharge pinch roller  93  and the upper one of the discharge pinch rollers  99  are brought into contact with the upper surface of the printing paper P, bearing thereon a printed image, only at opposite margins of the printing paper P. The positions of these rollers are automatically adjusted according to the size of the printing papers P on the paper supply table  81  detected by a paper size sensor (not shown) so that the rollers are brought into contact with the upper surface of the printing paper P only at opposite margins irrespective of the width of the printing paper P. 
     Operation of the stencil printer will be described, hereinbelow. First a stencil master is wound around the peripheral wall  7  of the printing drum  1 . Then when a print start key on a control panel (not shown) is depressed, the printing drum  1  and the paper pinch drum  63  are start to rotate. As the printing drum  1  and paper pinch drum  63  start to rotate, a printing paper P is taken out from the paper supply table  81  by the paper supply rollers  83  and the paper separator roller  85  and fed toward the timing rollers  89  under the guidance of the paper guide  87 . 
     When the printing drum  1  and the paper pinch drum  63  are rotated to a predetermined angular position, the timing rollers  89  feed the printing paper P to the paper clamping position a at a predetermined timing. 
     When the cam switch  49  is not on, the electromagnetic clutch  43  is energized for a predetermined time interval, whereby the cam plate  35  is rotated to the printing position. At this time, the actuator  51  actuates the cam switch  49 , whereby that the cam plate  35  is in the printing position is detected. When the cam plate  35  is rotated to the printing position, the squeegee roller  23  is moved downward into abutment against the inner surface of the peripheral wall  7  of the.printing drum  1  as shown in FIG.  1 . Then as the printing drum  1  is further rotated, the squeegee roller  23  presses the peripheral wall  7  radially outward and deforms the same toward the paper pinch drum  63 . 
     When the printing paper P is supplied from the paper supply section  77  in synchronization with rotation of the paper pinch drum  63 , the leading end of the printing paper P is clamped by the clamping piece  71  in the paper clamping position. As the paper pinch drum  63  rotates, the printing paper P is wound around the paper pinch drum  63  and is carried to the contact area of the printing drum  1  and the paper pinch drum  63 , i.e., to the deformed part of the peripheral wall  7 . Thus the printing paper P is pinched, together with the stencil master on the printing drum  1 , between the deformed part of the printing drum  1  and the paper pinch drum  63  under a predetermined pressure. Printing is made on the printing paper P while the printing paper P is conveyed by rotation of the printing drum  1  and the paper pinch drum  63 . 
     When the leading end of the printing paper P reaches the paper release position b, the printing paper P is released from the clamping piece  71  and delivered to the paper discharge pinch roller  93 . Thereafter, the printing paper P is removed from the paper pinch drum  63  by the paper scraper  95  and is discharged to the paper chute  97  by the paper discharge pinch rollers  99 . Thereafter the printing paper P is chuted onto the paper discharge table  101  with its printed surface facing upward. 
     In this embodiment, when a part of the peripheral wall  7  of the printing drum  1  is pulled inward as the squeegee roller  23  is rotated, the retainer roller  20  disposed near the inner surface of the peripheral wall  7  downstream of the contact line between the squeegee roller  23  and the inner surface of the peripheral wall  7  in the direction of rotation of the printing drum  1  prevents deformation of the peripheral wall  7 . 
     Accordingly, behavior of the stencil master wound around the peripheral wall  7  is stabilized and the stencil master is prevented from being stretched or displaced in the circumferential direction of the printing drum  1 . Further breakage of the peripheral wall and/or corrugation of the peripheral wall due to repeated stress can be prevented and durability of the peripheral wall  7  is increased. Further since the situation where the peripheral wall  7  resiles away from the squeegee roller  23  under its resiliency can be avoided, ink cannot splash the mechanisms in the printing drum  1 . Further the ink guard plate  24  and the retainer roller  20  guard the mechanisms in the printing drum  1  from ink even if some ink should accidentally splash. 
     FIGS. 7 to  9  show some modifications of the retainer roller  20  which can be employed in the present invention. The retainer roller  20   a  shown in FIG. 7 is substantially spindle-shaped. That is, the retainer roller  20   a  has a maximum thickness at the middle thereof and is tapered toward opposite ends. Since deformation of the peripheral wall  7  is maximized at the middle thereof, deformation of the peripheral wall  7  at the middle thereof can be effectively suppressed by use of the retainer roller  20   a  shown in FIG.  7 . The retainer roller  20   a  shown in FIG. 7 may be shorter than the retainer roller  20  shown in FIG.  6 . Further, in the retainer roller  20  shown in FIG. 7, since it is tapered toward the opposite ends, the opposite end portions are less apt to contact with the inner surface of the peripheral wall, whereby ink becomes less apt to leak at opposite ends of the peripheral wall  7 . 
     The retainer roller  20   b  shown in FIG. 8 comprises a plurality of roller sections mounted coaxially with each other. In this retainer roller  20   b , since the roller sections are spaced from each other and the outer surface of the retainer roller  20   b  is discontinuous, the retainer roller  20   b  contacts with the peripheral wall  7  over a smaller area, whereby leakage of ink can be suppressed. 
     The retainer roller  20   c  shown in FIG. 9 comprises a plurality of roller sections mounted coaxially with each other as in the retainer roller  20   b  shown in FIG.  8 . The retainer roller  20   c  differs from the retainer roller  20   b  in that each roller section is spindle-shaped. This shape of the roller section contributes to further reducing the contact area between the retainer roller  20   c  and the peripheral wall  7 , whereby leakage of ink can be suppressed more effectively. The contact area between the retainer roller  20   c  and the peripheral wall  7  can be further reduced when the length of the maximum diameter portion of each roller section is shortened. 
     A stencil printer with an internal press mechanism in accordance with a second embodiment of the present invention will be described with reference to FIG. 10, hereinbelow. 
     FIG. 10 shows only the printing drum  1   a  of the stencil printer of this embodiment. The part of the stencil printer of this embodiment not shown in FIG. 10 is the same as that in the first embodiment. The printing drum  1   a  and the paper pinch drum  63  of the stencil printer of this embodiment are basically the same in structure as those of shown in FIG.  4  and accordingly the elements analogous to those shown in FIG. 4 are given the same reference numerals and will not be described here. Further the difference of the printing drum  1   a  of this embodiment from that of the first embodiment will be mainly described, hereinbelow. 
     In this embodiment, the mounting structure of the retainer roller  20  differs from that in the first embodiment. As shown in FIG. 10, mounting pieces  22  are mounted for rotation on the roller support arm  21  by pivots  26  at their one ends. The pivot  26  is positioned between the squeegee roller  23  and the pivot shaft  19  below the ink guard plate  24 . The retainer roller  20  is supported for rotation on the other ends of the mounting pieces  22 . A coiled torsion spring  28  which is connected to the ink guard plate  24  at one end and to the mounting piece  22  at the other end is fitted on the pivot  26  and urges downward the retainer roller  20 . The retainer roller  20  is the same in structure as that employed in the first embodiment. The retainer roller  20  is normally held near the squeegee roller  23  and the inner surface of the peripheral wall  7  downstream of the contact line between the squeegee roller  23  and the inner surface of the peripheral wall  7  in the direction of rotation of the printing drum  1   a  under the force of the coiled torsion spring  28 . 
     As in the first embodiment, the retainer roller  20  suppresses deformation of the peripheral wall  7  of the printing drum l a . However in this embodiment, the retainer roller  20  can be displaced upward while pressing the peripheral wall  7  under the force of the spring  28  when the force applied from the peripheral wall  7  is large. 
     FIG. 11 is a view similar to FIG. 10 but shows a stencil printer in accordance with a third embodiment of the present invention. In this embodiment, an external press roller  304  is moved up and down toward and away from printing drum  300  to pinch therebetween the printing paper P. 
     The printing drum  300  has a rigid and ink-permeable peripheral wall  301 . A squeegee roller  302  and a doctor roller  303  are disposed inside the printing drum  300 . An external press roller  304  as a pinching means is disposed below the printing drum  300  to be movable up and down and is rotated in a direction opposite to the direction of rotation of the printing drum  300  in synchronization therewith. 
     Printing is effected while the printing paper P is conveyed pinched between the printing drum  300  and the external press roller  304 . 
     In this embodiment, a retainer roller  20  is mounted on a frame (not shown) in the printing drum  300  in the same manner as in the first embodiment. The retainer roller  20  may be mounted for rotation as in the second embodiment.