Abstract:
A sheet discharge device neatly and uniformly aligns edges of print sheets irrespective of the thickness of a print sheet, and exerts the same effect even at a low discharge speed. The sheet discharge device includes: sheet information sensing means for detecting thickness of a print sheet to judge the sheet as a thick sheet, a regular sheet, or a thin sheet; conveying means for discharging the print sheet bearing a printed image in a predetermined direction; a discharge tray that accommodates a stack of the print sheets discharged by the conveying means; wings arranged at a tail end portion of the conveying means, coming in contact with two side edges in a widthwise direction of the print sheet so as to give a U shape curl by curving the sheet in a rough U shape during discharge; control means for commanding, based on the judgment by the sheet information sensing means, the wing to take a predetermined shift mode during discharge of the print sheet; and drive means for moving the wing in a shift mode designated by the control means.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates in general to a printing apparatus, more specifically, to a sheet discharge device for discharging a print sheet toward a discharge tray. 
   2. Description of the Related Arts 
   In general, a printing apparatus has a sheet feed section to send out a top print sheet of a stack of paper loaded on a sheet feed tray, a printing section to carry out printing on the print sheet supplied from the sheet feed section, a sheet discharge device to discharge a print sheet, and a discharge tray to receive the print sheet discharged by the sheet discharge device. 
   Moreover, it takes some time for wet ink on the print sheet to dry. Therefore, when sheets of printing paper are printed successively, a print sheet being discharged now may brush a printed surface of a previously print sheet that is stacked on the top of the discharge tray, thereby smearing a printed image thereon. 
   Thus, in order to stack up print sheets on the discharge tray, it is desired to make a print sheet fall at right angles to the print sheet(s) on the discharge tray. 
   For instance, Japanese Unexamined Patent Application Publication No. 2001-240290 discloses a sheet discharge device includes a pair of flying induction members that function as jump wings for flying a print sheet having been carried by a conveyor belt (or transfer belt), while maintaining the print sheet in a bent form (roughly U shape). 
   In order to give a roughly U shape curl in the sheet, the flying induction member pair shift in the widthwise direction of the sheet according to information (size of the sheet) of a print sheet, and the two opposite faces come in contact with two side edges in the widthwise direction of the print sheet. Accordingly, a print sheet being discharged maintains a horizontal state with respect to a sheet transfer direction, and ends of the discharged sheets are aligned neatly. 
   However, the thickness of a print sheet is expressed in terms of weight per unit area (g/m 2 ). For example, a print sheet is described as ‘regular’ if the thickness falls within the range of 58 g/m 2  to 105 g/m 2 , ‘thick’ if the thickness is greater than the range, and ‘thin’ if the thickness is less than the range. A typical sheet discharge device determines the distance between sheet discharge wings (flying induction members) corresponding to a regular print sheet of various sizes. 
   What happens in the case of discharging a thick print sheet more elastic than a regular sheet is that, when an attempt is made to give a rough U shape curl to the print sheet (thick sheet), the repulsive elasticity of the thick sheet wins over a nip force (the dead weight of a sheet discharge roller, or its dead weight+spring pressure) and a suction force of a suction section maintaining a print sheet on a conveyor belt, so the thick sheet is not curved in a roughly U shape but floats instead. As a result, the print sheet may not be conveyed. This problem occurs more easily as the repulsive elasticity of a sheet increases even more toward a bottom of the sheet in a cantilever state where the dead weight for pulling the sheet down is greater. If the nip force and the suction force are increased to keep a print sheet from floating, however, the sheet may be damaged during conveying, or problems like ink transfer contamination, a blurred looking image etc., may occur. 
   Therefore, when thick sheets are used for printing in a conventional printer, it is necessary to broaden the width of a sheet discharge wing (or lower the installation height of the sheet discharge wing) so as to give a small U shape curl to the sheet. However, because a sheet width during discharge increases correspondingly to a weak curl of a print sheet into the U shape, the width of a side fence of a V-shaped discharge tray shown in  FIG. 2  has to be broadened greater than the width of loaded sheets. In consequence, a stack of loaded print sheets on the discharge tray may not be evenly arranged in the widthwise direction, thereby impairing alignment accuracy of sheet discharge. 
   In contrast, thin sheets are much less elastic than the regular sheet. Therefore, a conventional sheet discharge device sometimes fails to maintain a thin print sheet in a U shape during discharge, and the thin print sheet could sag downward from an exit port due to its dead weight. If the U shape curl of such print sheet is made in large scale to prevent any sagging of the print sheet, the width of a print sheet falling down onto the discharge tray becomes extremely narrower than the width defined by the side fences of the discharge tray. Hence, the print sheet cannot maintain its U shape and alignment of discharged sheets is not stable and uniform. Moreover, with a conventional sheet discharge device, a print sheet falling on the discharge tray comes in contact with the side fence thereof and the falling speed of the print sheet decreases by that. However, in the case of thin print sheets, no contact is made between the thin print sheets and the side fences of the discharge tray, so the falling speed of the sheets are not decreased. Rather, the thin print sheets may fly up while falling due to air resistance and jump over the end fence of the discharge tray. 
   Especially, the problems with thin sheets are more evident in ink jet printing or double-sided printing where a sheet discharge speed is set to lower than offset printing because print sheets at the exit port stay longer in the cantilever state. 
   SUMMARY OF THE INVENTION 
   In view of the foregoing problems, it is therefore an object of the present invention to provide a sheet discharge device capable of discharging a print sheet in an optimum shape corresponding to the thickness of the print sheet to neatly align edges of the discharged sheets, which is achieved irrespective of sheet thickness and even in inkjet printing with a relatively low discharge speed. 
   The above-mentioned object will be described with the accompanying drawings corresponding to the embodiment. According to a first aspect of the present invention, a sheet discharge device includes: sheet information sensing means for detecting thickness of a print sheet to judge the sheet as a thick sheet, a regular sheet, or a thin sheet; conveying means for discharging the print sheet bearing a printed image in a predetermined direction; a discharge tray that accommodates a stack of the print sheets discharged by the conveying means; wings arranged at a tail end portion of the conveying means, coming in contact with two side edges in a widthwise direction of the print sheet so as to curve the sheet in a rough U shape during discharge; control means for commanding, based on the judgment by the sheet information sensing means, the wing to take a predetermined shift mode during discharge of the print sheet; and drive means for moving the wing in a shift mode designated by the control means. 
   According to a second aspect of the present invention, if the print sheet is judged as a thick sheet by the sheet information sensing means, the control means controls the drive means to enter a shift mode for broadening the print sheet curved in a rough U shape in the widthwise direction, concurrently with passage of the print sheet at the wing. 
   According to a third aspect of the present invention, if the print sheet is judged as a thin sheet by the sheet information sensing means, the control means controls the drive means to enter a shift mode for narrowing the print sheet curved in a rough U shape in the widthwise direction, concurrently with passage of the print sheet at the wing. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an explanatory diagram showing the entire configuration of a stencil printing apparatus to which a sheet discharge device according to a preferred embodiment of the present invention is applied; 
       FIG. 2  is a cross-sectional view taken along line A-A in  FIG. 1 ; 
       FIG. 3  is a plan view of a sheet discharge mechanism of the sheet discharge device shown in  FIG. 1 ; 
       FIG. 4  is a B-B arrow view of  FIG. 3 ; 
       FIG. 5  is a block diagram of means in the sheet discharge device shown in  FIG. 1 ; 
       FIG. 6  is a time chart showing a relationship between a sheet discharge sensor and a wing in the sheet discharge device shown in  FIG. 1 ; 
       FIGS. 7A and 7B  are explanatory diagrams showing the performance of a wing in the case of using a thick print sheet; and 
       FIGS. 8A and 8B  are explanatory diagrams showing the performance of a wing in the case of using a thin print sheet. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   A preferred embodiment of the present invention will now be described with reference to the accompanying drawings.  FIG. 1  is an explanatory diagram showing the entire configuration of a stencil printing apparatus to which a preferred embodiment of the present invention is applied,  FIG. 2  is a cross-sectional view taken along line A-A in  FIG. 1 ,  FIG. 3  is a plan view of a sheet discharge mechanism,  FIG. 4  is a B-B arrow view of  FIG. 3  (excluding the moving mechanism of a wing), and  FIG. 5  is a block diagram of control means. 
   As shown in  FIG. 1 , a stencil printing apparatus  1  includes a housing (or casing)  4  where a manuscript tray  3  whose upper face is covered with a manuscript tray cover  2  is installed; a printing drum  6  which is rotatably installed in the housing  4  and holds a stencil sheet  5  on its surface to supply ink from its inside to the stencil sheet  5 ; a press roller  7  installed opposedly to a printing surface of the printing drum  6 ; a sheet feed tray  8  installed at one side of the housing  4 ; a feed roller  9  which takes out print sheets P′ from the sheet feed tray  8  one by one toward the printing drum  6  to convey the print sheet P′ between the printing drum  6  and the press roller  7  one after another; a sheet discharge mechanism  11  which discharges a print sheet P″ toward a discharge tray  10  installed at the other side of the housing  4 ; an optical read-out section (not shown) which optically reads out a manuscript set on the manuscript tray  3 ; a thermal transfer mechanism (not shown) which engraves image information from the data having been read out by the optical read-out section to a stencil sheet  5 ; a clamp mechanism  12  which cuts the stencil sheet  5  into a proper size and adheres it onto the surface of the printing drum  6 ; an operator panel (not shown) installed outside the housing  4 ; and a control means  14  for controlling each component. 
   As shown in  FIGS. 1 and 2 , the discharge tray  10  is roughly constituted of a bed plate  15 , a pair of side fences  16  mounted opposedly on the bed plate  15  in the widthwise (X) direction, and an end fence  17  erecting on a front stage side of a sheet transfer direction (Y) on the bed plate  15 . As the cross-sectional view of  FIG. 2  illustrates, the bed plate  15  in this embodiment is formed in a rough shallow V-shape, seen from the sheet transfer direction (Y). In addition, the side fences  16  are slidably installed along the widthwise (X) direction, and each one of the side fences  16  has on its inner wall face plural guide bars  16   a  being downwardly convex with respect to a flying direction of the print sheet P, thereby keeping the print sheet P from flying up or lifting off. Therefore, print sheets P falling into the discharge tray  10  are stacked and layered on the bed plate  15 , while still formed in a rough U shape. 
   Referring to  FIGS. 1 ,  3 , and  4 , the sheet discharge mechanism  11  is roughly constituted of a conveyor belt  20  which conveys a print sheet P being fed or drawn out from between the printing drum  6  and the press roller  7  in the sheet transfer direction (Y); a suction section  19  which is disposed at the bottom of the conveyor belt  20  to adsorb a print sheet P onto the conveyor belt  20 ; and a sheet discharge unit  21  mounted at a tail end portion of the conveyor belt  20 . 
   The sheet discharge unit  21  is roughly constituted of a table  22  disposed at an exit port, the tail end position of the conveyor belt  20 ; a pair of discharge rollers  23  disposed at roughly a middle portion of the widthwise (X) direction on the table  22 ; and a pair of discharge wings  24  which are slidably installed along guide slits  22   a  elongated in the widthwise (X) direction on both sides of the discharge rollers  23 , respectively, and which are arranged opposedly to each other along the widthwise (X) direction. 
   The discharge wings  24 , as shown in  FIGS. 3 and 4 , have opposed faces in the widthwise (X) direction that slope downwardly towards each other, and upwardly oblique guide faces  24   a  along the sheet transfer direction (Y), respectively, wherein each of the guide faces  24   a  comes in contact with the bottom face on both ends of a print sheet P in the widthwise (X) direction. When seen from the sheet transfer direction (Y), the print sheet P is curved and given in a roughly U shape curl. In general, the distance between a pair of discharge wings  24  is defined according to each size of a regular print sheet P. 
   With reference to  FIG. 3 , the following briefly explains about a mechanism for slide movement of the discharge wings  24 . 
   As shown in  FIG. 3 , racks  25  are installed at the bottom of their corresponding guide slits  22   a . Each of the racks  25  is held to be movable in the widthwise (X) direction by a guide mechanism (not shown). Moreover, the racks  25  are connected to the discharge wings  24  via a connection member (not shown) which insertedly passed through the guide slits  22   a . Also, each of the racks  25  is engaged with a pinion  26  so that the discharge wings  24  interoperate through their common pinion  26  and move along the widthwise(X) direction, moving relatively close to each other or apart from each other. 
   In addition, a pulley  27  is attached to a shaft of the pinion  26 . Another pulley  29  is attached to a shaft of a stepping motor  28  functioning as drive means. These two pulleys  27  and  29  are connected with a belt  30 . Hence, if the stepping motor  28  starts operating, the pinion  26  rotates to move the discharge wings  24 . 
   Although this embodiment introduced a configuration to obtain a rotating-linear movement by the racks  25  and the pinion  26 , other mechanisms such as links may also be used. Needless to say, other mechanisms may also be employed for the rotation transfer mechanism constituted of pulleys  27  and  29  and a belt  30 . 
   Further, the discharge roller  23  presses a print sheet P by applying its dead weight or its dead weight plus a spring pressure. 
   Besides, a sheet discharge sensor  44  (refer to  FIG. 5 ) is mounted on the tail end portion of the conveyor belt  20  in order to monitor the discharge state of a print sheet P and as a part of sheet information sensing means. 
   The sheet discharge sensor  44  as the sheet information sensing means in  FIG. 5  is provided with a light transmitting element and a light receiving element, so that it may detect the thickness of a print sheet P based on the transmission amount of X-rays irradiated from the light transmitting elements arrayed on the top of the tail end portion of the conveyor belt  20  and judge the type of the corresponding sheet P as a thick, regular, or thin sheet. The judgment result of the sheet discharge sensor  44  is outputted to the control means  14  as a detection signal. Based on this detection signal and contents (or commands) input by a user through the operator panel, the control means  14  controls an operation of the stepping motor  28 . 
   As depicted in  FIG. 5 , the control means  14  includes a CPU  40 , a RAM  41 , and a ROM  42 , and connected, through an I/O port  47 , to all kinds of panel keys  43  having plural input keys on the operator panel, e.g., ‘copies’ to set up the number of copies and a start key to start a printing operation, the sheet discharge sensor  44 , a sheet width sensor  45  (this detects the size of both A- and B-series) composing the sheet information sensing means together with the sheet discharge sensor  44 , and a motor drive circuit  46  of the stepping motor  28 . The control means  14  controls each section of the printing apparatus  1  to repeat its operation in a given sequence until the preset number of copies is completely made after the start key among the panel keys  43  is pressed. 
     FIG. 6  is a time chart showing the relationship between the sheet discharge sensor  44  and wings  24  among the constitutional elements of the printing apparatus. At the operation start point, the sheet discharge sensor  44  has not detected anything yet, but the wings  24  shift to their initial positions corresponding to the size of a print sheet P. And, after completion of printing, the print sheet P passes through the sheet discharge sensor  44  where the thickness of the print sheet P is detected and discriminated as thick, regular, or thin. Then, the wings  24  start moving in a predetermined shift mode corresponding to the detected thickness of the print sheet P. When discharge of the print sheet P is over, the wings  24  return to their initial positions and enter a standby state waiting for a next print sheet P to be conveyed. 
   With reference to  FIGS. 7A ,  7 B,  8 A and  8 B, the following will now explain shift modes of the wings  24  depending on the thickness of a print sheet P. 
   First,  FIGS. 7A and 7B  show the operations of the wings  24  in the case of a thick print sheet P. As depicted in  FIG. 7A , if a thick print sheet P (P 1 ) is used, the control means  14  (refer to  FIG. 5 ) outputs a control signal to drive the stepping motor  28  (refer to  FIG. 3 ) so that the wings  24  may shift to their initial positions coming in contact with both side edges in the direction of width (X) of the print sheet P 1 . Accordingly, the front end of the print sheet P 1  in the sheet transfer direction (Y) is largely reformed into a U shape in an early discharge stage. 
   Next, as depicted in  FIG. 7B , the control means  14  outputs again a control signal to drive the stepping motor  28  so that the wings  24  shift outward from each other in the widthwise (X) direction. As a result, the print sheet P 1  spreads more toward the rear end of the sheet and the U shape curl slowly becomes weak and is eventually released. When the print sheet P 1  fully passes through the sheet discharge unit  21 , the control means  14  outputs yet another signal to drive the stepping motor  28  so that the wings  24  return to their initial positions and enter a standby state waiting for a next print sheet P 1  to be conveyed. 
     FIGS. 8A and 8B  show the operations of the wings  24  in the case of a thin print sheet P. As depicted in  FIG. 8A , if a thin print sheet P (P 2 ) is used, the control means  14  outputs a control signal to drive the stepping motor  28  so that the wings  24  halt at their current positions (i.e., initial positions) being set according to the size of a regular sheet. Since the wings  24  are already at proper positions corresponding to the size of the print sheet P 2  in response to an input command from the operator panel for example, they stop at the initial positions. Therefore, the front end of the print sheet P 2  in the sheet transfer direction (Y) is given a U shape curl similarly to a regular sheet of the same size in an early discharge stage. 
   Next, as depicted in  FIG. 8B , the control means  14  outputs another control signal to drive the stepping motor  28  so that the wings  24  shift inward toward each other in the widthwise (X) direction. As a result, contrary to the thick print sheet P 1 , the thin print sheet P 2  becomes narrower toward the rear end of the sheet in the widthwise (X) direction and the U shape curl becomes gradually evident. And, when the print sheet P 2  passes through the sheet discharge unit  21  completely, the control means  14  outputs yet another control signal to drive the stepping motor  28  so that the wings  24  may return to their initial positions and enter a standby state waiting for a next print sheet P 2  to be conveyed. 
   Meanwhile, if a regular print sheet P is used, the control means  14  outputs a control signal to drive the stepping motor  28  so that the wings  24  remain at their initial positions. Accordingly, the print sheet P is given an even U shape curl. 
   According to the sheet discharge device of this embodiment, with the wings  24  in a predetermined proper shift mode corresponding to the thickness of the print sheet P, it becomes possible to maintain the sheet P in a rough U shape and obtain an optimal discharge state where ends of the discharged sheets are aligned neatly. For instance, if a thick print sheet P 1  is used, the wings  24  shift inward toward each other along the widthwise (X) direction and the front end of the thick print sheet P 1  is given a large U shape curl appropriately for the width defined by the side fences  16  of the discharge tray  10 , the width being set smaller than the width of loaded sheets. Thereafter, as the wings  24  slowly shift outward away from each other in the widthwise (X) direction, the thick print sheet P 1  spreads more toward its rear end and the U shape curl slowly becomes weak and is eventually released. In this way, the strong repulsive elasticity of the thick print sheet P 1  is suppressed to prevent a problem like a non-transfer state. 
   On the other hand, if a thin print sheet P 2  is used, the wings  24  are first controlled to stay at their current positions. As a result, the front end of the thin print sheet P 2  is given a U shape curl similarly to a regular sheet of the same size, and the thin sheet P 2  is discharged properly within the width of the side fences  16  of the discharge tray  10 . Thereafter, as the wings  24  slowly shift inward toward each other in the widthwise (X) direction, the U shape curl of the thin print sheet P 2  is reinforced toward the rear end of the sheet. In this way, the exterior strength of the sheet seen from the transfer direction (Y) is increased, thereby preventing the sheet from sagging downward from the exit port. Consequently, a uniform alignment of the discharged sheets can be accomplished even at a low discharge speed such as in an inkjet printing operation. 
   In this embodiment, the wings  24  slid in the widthwise (X) direction corresponding to the size of a print sheet P used, but they can also move vertically to give the print sheet P a rough U shape curl. That is, if a thick print sheet P 1  is used, the wings  24  are first lifted up to come in contact with both end edges of the print sheet P 1  in the widthwise(X) direction and then slowly descend, spreading or releasing the U shape curl of the print sheet P 1 . 
   Meanwhile, if a thin print sheet P 2  is used, the wings  24  are elevated upward to give the thin print sheet P 2  a U shape curl similarly to a regular sheet of the same size and go up further to reinforce the curl of the print sheet P 2  into a U shape. 
   In this embodiment, although the sheet discharge sensor  44  was employed to detect the thickness of a print sheet P, the present invention is not limited thereto and may incorporate an edge sensor (a secondary sheet feed sensor in double-sided printing) or a top end sensor disposed at the front stage of a conveyor roller in the printing apparatus  1  to do the same, that is, detecting the thickness of a print sheet P. Further, the printing apparatus may include a sensor for detecting the floating degree of the sheet discharge roller  23  caused by the repulsive elasticity of a print sheet P so as to discriminate sheet thickness by judging whether the floating degree is above a reference one. Also, the thickness of a print sheet P may be input in the beginning through the operator panel. 
   In this embodiment, the size of a print sheet P is automatically detected by the sheet width sensor  45 , but it may be input through the operator panel. It is also possible to use the aforementioned sensors in the printing apparatus  1  to detect sheet size based on the passing time of a print sheet P. Sensors arranged at the sheet feed tray  8  may detect sheet size as well. 
   As has been explained so far, the following advantages and effects can be obtained by the present invention. 
   The sheet discharge device according to the present invention makes it possible to keep a print sheet in a rough U shape by selectively setting the wings in a predetermined shift mode corresponding to the thickness of the print sheet during discharge. Therefore, the edges of discharged sheets can always be aligned in an optimal, neat state, irrespective of whether print sheets used are thick, regular or thin. As a result, a print sheet drops perpendicularly to the bottom face of the discharge tray and the print sheet is suppressed from being scattered on the discharge tray in the widthwise direction and the transfer direction of the sheet, thereby stacking neatly onto one another in a uniform manner. Moreover, even if there are already discharged print sheets on the discharge tray, the printed surface of a top sheet in a stack is no more damaged. 
   Especially, if a thick print sheet is used, the front end of a print sheet is first given a large U shape curl. In this way, the sheet width is not broadened but fittable within the width defined by side fences of the discharge tray, the width being set smaller than the width of loaded sheets. Thereafter, the thick print sheet spreads more toward its rear end and the U shape curl is slowly released. Hence, the strong repulsive elasticity of the thick print sheet is suppressed to prevent a problem like a non-transfer state. 
   Meanwhile, if a thin print sheet is used, the front end of a thin print sheet is given a U shape curl similarly to a regular sheet of the same size, and the thin sheet is discharged properly within the width of the side fences of the discharge tray. Thereafter, the U shape curl of the thin print sheet is reinforced toward the rear end of the sheet. In this way, the exterior strength of the sheet seen from the transfer direction is increased, and the sheet is prevented from sagging downward from the exit port, while creating a stable U shape during falling. Consequently, a uniform alignment of the discharged sheets can be accomplished even at a low discharge speed such as in an inkjet printing operation.