Patent Publication Number: US-6220155-B1

Title: Stencil making device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a stencil making device for perforating a stencil sheet supplied from a rolled stencil sheet. 
     2. Description of the Related Art 
     FIG. 7 illustrates a conventional stencil making device. A rolled base sheet S for stencil printing (hereinafter referred to as a stencil sheet S) is supported in a master holder  106 . The stencil sheet S supplied from the rolled stencil sheet S is in a shape of a continuous strip. The stencil sheet S changes conveying direction while turning on a set guide shaft  100 . Then, the sheet is sandwiched between a thermal head  101  and a platen roller  102 , and subsequently held between an upper roller  104  and a lower roller  105  after turning on a roller  103 , and then conveyed to an initial stop position. 
     When the stencil sheet S is conveyed, tension in an opposite direction to the conveying direction is exerted on the stencil sheet S by a back-tension unit  107  disposed to the master holder  106 , so that a portion of the sheet sandwiched between the thermal head  101  and the platen roller  102  does not wrinkle. The rolled stencil sheet S is attached to the back tension unit  107 . 
     While being perforated by the thermal head  101 , the stencil sheet S is conveyed only by a driving force of the platen roller  102  holding the sheet on the thermal head  101 . The perforated stencil sheet S is guided downward by the roller  103  to move to a reservoir box  108 . The perforated stencil sheet S is stored in the reservoir box  108  until the perforation by the thermal head  101  finishes. 
     On completion of perforating, the thermal head  101  moves upward to release the stencil sheet S held on the platen roller  102 . The perforated stencil sheet S is conveyed by the upper and the lower rollers  104 ,  105  until a leading end thereof reaches a clamp plate  110  of a printing drum  109 . The leading end of the stencil sheet S is held on the printing drum  109  by the clamp plate  110 . When the printing drum  109  rotates simultaneously with rotation of the upper and the lower rollers  104 ,  105 , the stencil sheet S is conveyed and wound around the circumferential surface of the printing drum  109 . After the sheet is wound around the drum, a cutter unit  111  cuts the stencil sheet S. 
     The present device combines stencil printing function with the stencil making function. A part of the constitution for stencil printing is not illustrated in the drawings. A part of a circumferential wall of the printing drum  109  is ink-permeable. Ink supplying means is disposed inside the printing drum  109  for supplying ink to an inner surface of the circumferential wall. Pressing means is disposed under the printing drum  109  for pressing printing sheet against the printing drum  109 . When the printing drum  109  rotates, the printing sheet is supplied between the printing drum  109  and the pressing means at a predetermined timing, so that the printing sheet is pressed against an image area of the stencil sheet S. The ink supplied to the inner surface of the printing drum  109  passes through the circumferential wall of the printing drum  109 , and then transferred to the printing sheet through perforations of the stencil sheet S to form an image thereon. 
     According to the conventional stencil making device as illustrated in FIG. 7, the back tension unit  107  comprises a silicon oil damper. The rolled stencil sheet S is perforated by a perforating section comprising the platen roller  102  and the thermal head  101 , and conveyed to the downstream side. Then, the back tension unit  107  exerts back tension on the stencil sheet S held between the back tension unit  107  and the perforating section. 
     The tension changes according to a diameter of the rolled stencil sheet S. The tension is small when the diameter of the rolled stencil sheet S is long, and it increases as the diameter decreases. Torque required to rotate the silicon oil damper for exerting the back tension is constant. A product of the tension exerted on the stencil sheet unrolled from the most outer portion of the rolled stencil sheet and the diameter of the rolled stencil sheet is balanced with torque required to rotate the silicon oil damper. Accordingly, when the diameter of the rolled stencil sheet decreases, the tension exerted on the unrolled stencil sheet increases. 
     The increased tension exerted on the stencil sheet may cause a slip between the platen roller  102  and the stencil sheet S. If such a slip occurs, the stencil sheet moves in shorter distance than rotational distance of the platen roller  102 . The slip results in shortening the size of the perforated area in the stencil sheet than a predetermined size (a size of an original), thereby causing shrinkage in print-image. 
     If torque of the silicon oil damper is set at smaller level to lower the tension exerted on the stencil sheet when the diameter of the rolled stencil sheet is shorter, the stencil sheet may wrinkle in perforating when the diameter of the rolled stencil sheet is longer since the tension exerted on the stencil sheet is excessively small. 
     The present invention is made in view of the aforementioned problems. An object of the present invention is to provide a stencil making device capable of exerting appropriate tension on the stencil sheet S in perforating to avoid wrinkling the stencil sheet. 
     SUMMARY OF THE INVENTION 
     A stencil making device as defined in the first aspect of the present invention comprises supporting means for rotatably supporting a rolled stencil sheet, perforating means for perforating the stencil sheet conveyed from the supporting means to form an image therein, a pair of rollers rotatably disposed between the supporting means and the perforating means to hold the stencil sheet therebetween, and torque applying means attached to one of the rollers to apply a predetermined torque to the roller when the roller rotates. 
     According to a stencil making device as defined in the second aspect of the present invention, in the stencil making device of the first aspect, the torque applying means is a torque limiter. 
     According to a stencil making device as defined in the third aspect of the present invention, in the stencil making device of the first aspect, the torque applying means is a damper. 
     According to a stencil making device as defined in the fourth aspect of the present invention, in the stencil making device of the first aspect, the pair of rollers comprises a tension roller rotatably disposed at a predetermined position and a nip roller vertically movably and rotatably disposed over the tension roller, the nip roller is urged downwardly to contact the tension roller, and the torque applying means provides the predetermined torque to the tension roller. 
     According to a stencil making device as defined in the fifth aspect of the present invention, in the stencil making device of the fourth aspect, the nip roller is urged to the tension roller by its self weight. 
     According to a stencil making device of the sixth aspect of the present invention, in the stencil making device of the fourth aspect, the stencil making device further comprises an elastic member to elastically urge the nip roller to the tension roller. 
     According to the aforementioned stencil making device, back tension exerted on the stencil sheet in perforating is constant regardless of the diameter of the rolled stencil sheet. Accordingly, the stencil sheet does not wrinkle in perforating and the size of perforated area in the stencil sheet does not shrink. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of one embodiment of the present invention illustrating the entire constitution thereof, 
     FIG. 2 is a perspective view illustrating a constitution in the vicinity of a thermal head (TPH) in one embodiment; 
     FIG. 3 is a perspective view illustrating a constitution in the vicinity of a tension roller and a nip roller in one embodiment; 
     FIG. 4 is a sectional view illustrating a constitution in the vicinity of a tension roller and a nip roller in one embodiment; 
     FIG. 5 is an elevation view illustrating a constitution of a shaft-end of a tension roller in one embodiment; 
     FIG. 6 is a perspective view illustrating a constitution in the vicinity of a tension roller and a nip roller in another embodiment; and 
     FIG. 7 is a sectional view of an example of a conventional stencil making device. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     One embodiment of the present invention will be explained referring to FIGS. 1 to  6 . 
     A stencil making device  1  of the present embodiment combines a perforating function of thermally perforating a stencil sheet S by using a thermal head and a printing function of conducting stencil printing to printing paper by using the perforated stencil sheet S. As shown in FIG. 1, a printing drum  3  is placed inside a main frame  2 . The printing drum  3  comprises a partially ink-permeable circumferential wall  4 , a clamp plate  5  disposed to the outer circumferential surface of the circumferential wall  4  as clamping means, and ink supplying means (not shown) disposed to the inside of the circumferential wall  4  for supplying ink to the inner circumferential surface of the circumferential wall  4 . The printing drum  3  is driven to rotate around a rotation axis disposed at a predetermined position parallel with a central axis  6  thereof. Pressing means (not shown) is placed under the printing drum  3  for pressing printing paper against the printing drum  3 . 
     Inside the main frame  2 , there is disposed a roll supporting section, i.e. a master holder  7 . In the master holder  7 , a rolled stencil sheet S is rotatably stored. The rolled stencil sheet S is fitted on a rotatable shaft  7   a . The stencil sheet S is composed of a thermal film and a porous substrate attached to each other. A partial opening is formed in an upper surface of the main frame  2  of the present stencil making device. A pressing frame  8  is openably attached thereto by a support axis  9 . Between the master holder  7  and the printing drum  3 , from the master holder  7  side, successively aligned are a nip roller  10  and a tension roller  11 , set guide shaft  12 , an intermediate roller  13 , a platen roller  14  and a thermal head  15  (hereinafter referred to as TPH  15 ), a guide roller  16 , an upper load roller  17  and a lower load roller  18 , and a cutter unit  19 . The stencil sheet S is conveyed while passing through the rollers and so on, and perforated by the TPH  15 . Inside the main frame  2 , a reservoir box  20  for storing a perforated stencil sheet S therein is disposed below the platen roller  14  and the TPH  15  or the guide roller  16 . 
     As shown in FIG. 1, the TPH  15  is attached to the pressing frame  8 . As shown in FIG. 2, the TPH  15  has a shape of a plate that is longitudinal in a width-direction of the stencil sheet S. The TPH  15  is placed parallel with the width-direction of the stencil sheet S and contacted with an upper surface of the stencil sheet S. Further, the TPH  15  can approach or withdraw from the platen roller  1  when being driven by a driving mechanism not shown. The TPH  15  contacts the platen roller  14  while perforating the stencil sheet S, and withdraws from the platen roller  14  on completion of the perforating. The TPH  15  has a plural heat-emitting elements aligned parallel with the width direction of the stencil sheet S. The stencil sheet S is thermally perforated by the heat-emitting elements. In the present embodiment, the TPH  15  and the platen roller  14  constitute perforating means. 
     The platen roller  14  is disposed below the TPH  15  in the main frame  2 . When the pressing frame  8  closes, the TPH  15  of the pressing frame  8  contacts the platen roller  14  of the main frame  2 . The stencil sheet S sandwiched between the TPH  15  and the platen roller  14  is thermally perforated by the TPH  15  while being conveyed by the platen roller  14 . 
     As shown in FIG.  1  and FIG. 2, a claw  30  is attached to each end of the TPH  15 . As illustrated in FIG. 1, it may be recommended that the platen roller  14  be positioned to the TPH  15  when the claw  30  engages a shaft  36  of the platen roller  14 . This claw  30  has a vertical slit  31  formed therein. A shaft  32  of the intermediate roller  13  movably engages the slit  31 . The intermediate roller  13  can move vertically along the slit  31 . Before the intermediate roller  13  in a direction of conveying the stencil sheet S, the set guide shaft  12  is placed as guiding means for guiding the stencil sheet S. The set guide shaft  12  is rotatably disposed to the main frame  2  side. The stencil sheet S supported between the set guide shaft  12  and the platen roller  14  is pressed downwardly to be convex by the self-weight of the intermediate roller  13 . 
     As shown in FIG. 1, vertical slits  34  and  35  are formed in the pressing frame  8 . The guide roller  16  and the upper load roller  17  are attached to the slits  34  and  35  respectively. The guide roller  16  and the upper load roller  17  are vertically movable and rotatable. 
     Referring to FIGS.  3 , 4 , and  5 , the nip roller  10  and the tension roller  11  will be explained. The pair of the rollers can exert an appropriate tension on the stencil sheet S in perforating, thereby preventing occurrence of wrinkles in the stencil sheet. 
     The nip roller  10  and the tension roller  11  are attached to the main frame  2  side. On a circumferential surface of the tension roller  11 , a high-friction material such as rubber is disposed. Otherwise, the circumferential surface may be treated to gain high friction. Accordingly, the stencil sheet S is not likely to slip on the tension roller  11 . As shown in FIG. 4, a shaft  21  of the tension roller  11  is rotatably supported by a bearing  40  at a predetermined position of the main frame  2 . 
     As shown in FIG. 3, torque applying means, i.e. a torque limiter  22  is disposed to the shaft  21  of the tension roller  11 . As shown in FIG.4, the torque limiter  22  comprises a cylindrical outer casing  22   a , hollow and cylindrical inner casing  22   b , and fixing claw  22   c . The torque limiter  22  is fixed to the main frame  2  by the fixing claw  22   c.    
     The shaft  21  of the tension roller  11  is coaxially fitted into the inner casing  22   b . On one end portion of the inner casing  22   b , two notches  22   d  are formed. As shown in FIG. 5, on one end portion of the shaft  21  of the tension roller  11 , two protrusions  21   a  are formed. The protrusions  21   a  engage the notches  22   d  by inserting the end portion of the shaft  21  into the end portion of the inner casing  22   b . The end portion of the shaft  21  protrudes outward from another end portion of the inner casing  22   b . The end portion of the shaft  21  is rotatably supported by the bearing  40  at a predetermined position of the main frame  2 . 
     The inner casing  22   b  rotates relative to the outer casing  22   a  when torque applied to the inner casing exceeds a predetermined value. Torque transmission mechanism of the torque limiter  22  may utilize a spring, friction between friction plates, or attraction and repulsion of a magnet. Such torque transmission mechanism loads a predetermined torque on the tension roller  11  when it rotates. A constant torque is applied to the stencil sheet S since the stencil sheet S does not slip on the tension roller  11  when being conveyed. 
     As shown in FIGS. 3 and 4, the nip roller  10  is disposed over the tension roller  11 . A horizontal support plate  23  is fixed to the main frame  2 . On both ends of the support plate  23 , downward side plates  24  are disposed respectively. In each of the side plates  24 , a vertical slot  25  is formed. Each end of a shaft  26  of the nip roller  10  is movably engaged with the slot  25 . The nip roller  25  is vertically movable along the slot  25 . In the present embodiment, the nip roller  10  is urged downwardly by its weight to contact the tension roller  11 . 
     As shown in FIG. 6, urging means  29  comprising a compression spring  27  and a pressing member  28  may be disposed to or around each of the side plates  24 . Hence, the shaft  26  of the nip roller  10  is urged downwardly, and the nip roller  10  contacts the tension roller  11 . 
     Since the torque limiter  22  is disposed on the shaft of the tension roller  11 , the stencil sheet S held between the tension roller  11  and the nip roller  10  is conveyed between the platen roller  14  and the TPH  15  with a constant tension applied thereto regardless of diameter of the rolled stencil sheet S. 
     Next, perforating operation of the present device will be explained. 
     The stencil sheet S will be set through said rollers and so on. Firstly, the pressing frame  8  is opened. A continuous strip of the stencil sheet S is unrolled and provided from the rolled stencil sheet S stored in the master holder  7 . The stencil sheet S is drawn to pass between the nip roller  10  and the tension roller  11 . Further, the stencil sheet S is placed to contact each upper peripheral surface of the set guide shaft  12 , the platen roller  14  and the lower load roller  18 . And, the pressing frame  8  is closed. The stencil sheet S is then sandwiched between the TPH  15  and the platen roller  14 . 
     In perforating, image information is given to the TPH  15 . The TPH  15  forms perforated images in the stencil sheet S held against the platen roller  14  according to the image information. During the perforation, power for conveying the stencil sheet S is exerted only by the platen roller  14 . The perforated stencil sheet S is guided downwardly by the roller  16  into the reservoir box  20 . Until the perforation is finished in one operation, the perforated stencil sheet S is successively stored in the reservoir box  20 . 
     In said perforating operation, the stencil sheet S provided from the rolled stencil sheet S passes through the tension roller  11  with the torque limiter  22  and the nip roller  10  urged toward the tension roller  11 . Accordingly, a constant tension is always applied to the stencil sheet S moving toward the TPH  15 . 
     The intermediate roller  13  deforms the stencil sheet S supported between the set guide shaft  12  and the platen roller  14  to be convex with self-weight thereof As stated above, a constant tension is applied to the stencil sheet S by the nip roller  10  and the tension roller  11  during perforating. Further, the intermediate roller  13  is vertically movable and contacts the upper surface of the stencil sheet S. Accordingly, the stencil sheet S with a constant tension exerted thereon lifts up the intermediate roller  13  while being conveyed, so that the shaft  32  of the intermediate roller  13  floats over the bottom of the slit  31 . Therefore, the shaft  32  of the intermediate roller  13  is automatically adjusted parallel with the axis of the platen roller  14 . The stencil sheet S does not slack while being conveyed. The stencil sheet S does not wrinkle after being perforated. 
     After perforating, printing is performed. The TPH  15  moves upward to leave the platen roller  14  below. The stencil sheet S held between the TPH  15  and the platen roller  14  is released. A leading end of the perforated stencil sheet S is transferred to the clamp plate  5  by the upper roller  17  and the lower roller  18 . The leading end of the stencil sheet S is held on the printing drum  3  by the clamp plate  5 . When the printing drum  3  rotates simultaneously with rotation of the upper and the lower rollers  17 ,  18 , the stencil sheet S is conveyed and wound around the circumferential surface of the printing drum  3 . After the sheet is wound around the drum, the cutter unit cuts the stencil sheet S. 
     When the printing drum  3  rotates, a printing sheet is supplied between the printing drum  3  and the pressing means at a predetermined timing. The printing sheet is pressed against an image area of the stencil sheet S by the pressing means. Ink supplied to the inner surface of the printing drum  3  passes through the circumferential wall of the printing drum  3 , and then transferred to the printing sheet through perforations of the stencil sheet S. An image is formed on the printing sheet 
     In the embodiment explained before, the torque limiter is used as torque applying means. A damper may be used as torque applying means as well. As the damper, a silicon-oil damper or a damper using viscous fluid or powder may be used. 
     The stencil sheet, while being conveyed by the platen roller, changes its speed in a predetermined range according to factors such as perforation rate. According to the changes in the speed, rotational speed of the pair of rollers, namely of the tension roller and the nip roller, also changes in a predetermined range. Accordingly, torque gained from the torque applying means, namely by the torque limiter, changes as well in the strict sense of the meaning. But such torquechange is only within a narrow range; the effect of the present invention is fully attained. 
     In the aforementioned embodiment, the rolled stencil sheet S is rotatably supported in the master holder  7 , and the silicon-oil damper as illustrated in FIG. 7 is not used there. The stencil making device with the torque applying means of the present invention may also include the torque damper as illustrated in FIG. 7 for preventing the rolled stencil sheet from slacking. In this case, if the torque of the silicon-oil damper is lower than that of the torque applying means (namely below 50%, preferably below 30%), the problems occurred in the case where only the conventional silicon-oil damper is used can be prevented. Namely, when the present invention includes the silicon-oil damper as illustrated in FIG. 7, this silicon-oil damper changes the tension in the stencil sheet according to the changes in the diameter of the rolled stencil sheet; therefore, if adequately low torque of the silicon-oil damper is employed, changes in the tension exerted on the stencil sheet can be restrained in a predetermined range. 
     According to the present invention, in the stencil making device, a pair of rollers is disposed between the supporting means and the perforating means for holding the stencil sheet therebetween, and the torque applying means is disposed to one roller of the pair. Hence, the back tension exerted on the stencil sheet during perforation is maintained constant regardless of the diameter of the rolled stencil sheet. Accordingly, the stencil sheet does not wrinkle during perforation, and the size of the perforated area does not shrink.