Abstract:
Apparatus for delivering a sheet to a surface, comprising a rotating gripper that receives and grips the sheet at or near a leading edge thereof at a first position and delivers the leading edge to a second position, the first and second positions being situated along a circular path along which the rotating gripper travels; a surface having a stop at the second position; and a blower adapted to blow air at the sheet as it is held by the rotating gripper and after it is released by the gripper at the second position, the blowing of the air being in a direction that urges the sheet away from the circular path. The sheet is not positively held on the circular path except by the leading edge grippers.

Description:
RELATED APPLICATIONS 
   The present application is a continuation-in-part of PCT application PCT/EL03/00403, filed on May 15, 2003, which is a continuation in part of PCT/IL03/00086 filed Feb. 3, 2003 the disclosures of both of which are incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The field of the invention is printers and copiers and particularly mechanisms and methods for flipping substrates on which images are printed. 
   BACKGROUND OF THE INVENTION 
   When a printer or copier does two-sided copying, it generally prints one side of the paper first, then flips the paper over and prints the other side. A large number of patents and published patent applications exist for paper-flipping mechanisms, including GB 2 168 688 A to Xerox Corp., U.S. Pat. No. 4,969,641 to Fukushima et al., U.S. Pat. No. 5,201,517 to Stemmle, U.S. Pat. No. 5,692,740 to Holtje, U.S. Pat. No. 5,692,747 to Guerrero et al., U.S. Pat. No. 5,720,478 to Carter et al., U.S. Pat. No. 6,199,858 to Wyer, and PCT publication WO 02/060794 A1 to Hallmark Cards Inc. These mechanisms typically work by moving the paper into an intermediate position, and then moving it back out along a different path, so that its orientation is changed. To prevent the paper from going back along the same path when its motion is reversed, something irreversible is done to it before moving it back. In some publications, the irreversible action is falling or sliding under the influence of gravity. Using gravity to move the paper reduces the need for active mechanisms such as motor-driven rollers or pneumatics, reducing cost and increasing reliability. However, the falling or sliding paper must be guided fairly closely to make sure it ends up in the right place without wrinkling or jamming. This leads to the possibility that the first printed side of the paper will be scratched or smudged as a result of rubbing against a surface that is guiding it. 
   WO 02/060794 A1 discloses a mechanism for flipping card stock for two-sided printing, in which the surface is never rubbed. The card stock is held rigidly in a frame, and the whole frame is flipped over. 
   U.S. Pat. No. 6,199,858 to Wyer illustrates the difficulty of designing a flipping mechanism using rollers, that does not allow the paper to rub. The device disclosed in Wyer has the primary purpose of forming a shingled stack of paper, but it also flips the paper when forming the stack. In this device, the paper is fed through a first pair of rollers, and the lead end drops straight down to an intermediate position where it is caught between a second pair of rollers. If the paper were very compliant and with no memory of its curvature, then there would be no need for guide walls, and the paper would drop directly between the second pair of rollers without rubbing against anything. But since paper is not completely compliant and does retain a curl, guide walls are needed to make sure the leading edge of the paper is fed into the gap between the second pair of rollers, and the paper can rub against the guide walls. 
   Other mechanisms for flipping print media, too numerous to mention, are also known in the art. 
   SUMMARY OF INVENTION 
   An aspect of some embodiments of the invention concerns a mechanism which flips a sheet of paper (or other media, herein referred to as “paper”) over without allowing it to rub against any surface. 
   In an embodiment of the invention, a gripper grips the leading edge of the sheet and transports it along a circular path to a second position on a surface at which it is released. A blower blows air at the sheet as it is being transported, and after it reaches the second position, to urge it away from the circular path and general toward the surface. 
   In an exemplary embodiment of the invention, the sheet is not held against the circular path as it moves toward the second position by any mechanism other than the gripper. 
   There is thus provided, in accordance with an exemplary embodiment of the invention, an apparatus for delivering a sheet to a surface, comprising: 
   a rotating gripper that receives and grips the sheet at or near a leading edge thereof at a first position and delivers the leading edge to a second position, the first and second positions being situated along a circular path along which the rotating gripper travels; 
   a surface having a stop at the second position; 
   a blower adapted to blow air at the sheet as it is held by the rotating gripper and after it is released by the gripper at the second position, the blowing of the air being in a direction that urges the sheet away from the circular path, 
   wherein the sheet is not positively held on the circular path except by the leading edge grippers. 
   Optionally, the gripper is a vacuum gripper, optionally situated at the end of an arm that rotates about the center of the circle. 
   Optionally, the blower blows air continuously. 
   In an embodiment of the invention, the stop includes a second gripper that holds the leasing edge at the second position after its release by the rotating griper. Optionally, the second gripper includes a vacuum nipple that holds the paper. 
   In an embodiment of the invention, the surface is substantially tangent to the circular path at said second position. 
   Optionally, the surface is substantially horizontal. 
   Optionally, stop is a conditional stop that can be opened or closed. 
   In an embodiment of the invention, the surface is the surface of a belt. Optionally, after the sheet comes to rest on the belt, the belt is operative to remove the sheet from the second position. Optionally, the belt can be selectively driven in either direction so that it can selectively transport the sheet in, either one of two opposite directions. 
   Optionally, when driven in one direction the belt delivers the printing media in a flipped orientation to be printed on the side opposite a side that was previously printed on. Optionally, when driven in a direction opposite to the one direction the belt delivers the printing media in a flipped orientation to an exit stacker. 
   In an embodiment of the invention, the surface is the surface of a stacker or the surface of previous sheets stacked in the stacker. 
   There is further provided, in accordance with an embodiment of the invention, a method of delivering a sheet to a surface, comprising: 
   gripping at or near a leading edge of the sheet at a first position and delivering the leading edge to a second position by releasing it thereat, said first and second positions being situated along a circular path; 
   stopping motion of the leading edge by abutting the leading edge against a stop at the second position; 
   blowing air at the sheet during at least part of the time of travel between the first and second positions and after as it is released at the second position, the blowing of the air being in a direction that urges the sheet away from the path of travel of the leading edge, 
   wherein the sheet is not positively held during travel between the first and second positions except by gripping at or near said leading edge. 
   Optionally, the path of the leading edge between said first and second positions is a generally circular path. 
   Optionally, the sheet is held at or near said leading edge during travel from said first to said second position by at least one vacuum gripper. 
   Optionally, the blowing of air is continuous. 
   Optionally, the method includes gripping the sheet at the second position after it is released thereat. 
   Optionally, a surface onto which the sheet is released at the second position is substantially horizontal. 
   In an embodiment of the invention, the method includes utilizing a belt to remove the sheet from the second position. 
   Optionally, the method includes selectively driving the belt in either direction so that it can selectively transport the sheet in either one of two opposite directions. Optionally, when driven in one direction the belt delivers the printing media in a flipped orientation to be printed on the side opposite a side that was previously printed on. Optionally, when driven in a direction opposite to the one direction the belt delivers the printing media in a flipped orientation to an exit stacker. 
   Optionally, the sheet is stacked on previously delivered sheets at said second position. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1 through 5  are schematic cross-sectional views of an exemplary embodiment of the invention, showing a time sequence of five stages in flipping a piece of paper or other printing media; 
       FIGS. 6 through 9  are schematic cross-sectional views of another exemplary embodiment of the invention, showing a time sequence of four stages in flipping a piece of paper or other printing media, corresponding respectively to  FIGS. 1 ,  2 ,  3  and  4  in that embodiment of the invention; 
       FIG. 10A  is a schematic cross-sectional view of an exemplary embodiment of the invention, with a bypass mechanism disabled, and  FIG. 10B  shows the same embodiment with the bypass mechanism activated; 
       FIGS. 11-13  are schematic cross-sectional views of different exemplary embodiments of the invention with bypass mechanisms; and 
       FIG. 14  is a schematic cross-sectional view of another exemplary embodiment of the invention with a bypass mechanism. 
   

   DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     FIG. 1  is a cross-sectional view of a paper flipping mechanism  100 , according to an exemplary embodiment of the invention. A piece of paper  102 , or other printing media, goes through a pair of feeder rollers  104 , which direct the paper so that its leading edge enters a nip  106  between a drive roller  108  and a pinch roller  110 . Drive roller  108  is driven in a clockwise direction, optionally at a constant speed, by motor  112 . 
   Optionally, one or both of drive roller  108  and pinch roller  110  do not have solid surfaces going all the way across the width of paper  102 , but comprise one or more rims, covering only a portion of the width of the paper. As long as there is some overlap between the one or more rims of drive roller  108  and the one or more rims of pinch roller  110 , then the drive roller and the pinch roller will form nip  106 . 
   Optionally, feeder rollers  104  are an integral part of flipping mechanism  100 . Alternatively, feeder rollers  104  are the output of another mechanism which performs some process before the paper is flipped over, for example printing a first side of the paper. Optionally, at least one of feeder rollers  104  is motor driven, turning at the same surface speed as drive roller  108 . Alternatively, feeder rollers  104  are passive, or their speed easily changes in response to any torque exerted on them by paper  102 , so their speed adjusts to match the speed of drive roller  108  once paper  102  is taken up by drive roller  108 . Alternatively, feeder rollers  104  are motor driven until paper  102  is taken up by drive roller  108 , at which time feeder rollers  104  are decoupled from the motor driving them and then move passively at the speed that paper  102  is moving. In some embodiments of the invention, there are no feeder rollers  104 , and there is another mechanism for feeding the paper into flipping mechanism  100 , for example, a pneumatic mechanism. 
   To avoid damaging a printed image on one side of the paper, feeder rollers  104 , or an alternative mechanism, are optionally positioned close enough to nip  106 , and at an orientation, such that the leading edge of paper  102  finds nip  106  and is taken up by drive roller  108  and pinch roller  110 , without ever rubbing against drive roller  108  or pinch roller  110 . Alternatively, paper  102  does not find the exact position of nip  106  every time, but paper  102  is moving at the same speed as the surface of drive roller  108  and pinch roller  110 , due to feeder rollers  104 , so even if drive roller  108  or pinch roller  110  has to deflect the leading edge of paper  102  to guide it into nip  106 , the paper will not rub against drive roller  108  or pinch roller  110 . 
   A sensor  114 , located just past nip  106 , senses when the lead edge of paper  102  has passed through nip  106 . Optionally, there is a leading edge guide  116  attached to pinch roller  110 , and sensor  114  is located just past the leading edge guide. Pinch roller  110  is attached to an angular driver  118 , which is capable of moving pinch roller  110  back and forth around drive roller  108 . Optionally, angular driver  118  is driven by its own motor, not shown in  FIG. 1 , separate from motor  112 . Alternately, angular driver  118  is driven by motor  112 , the same motor which drives drive roller  108 , but there are gears and/or cams, not shown in  FIG. 1 , which allow angular driver  118  to disengage from motor  112 , or to be driven in reverse by motor  112 . Optionally, the gears also allow angular driver to be driven at different speeds, without changing the speed of drive roller  108 . 
   In an exemplary embodiment of the invention, drive roller  108  rotates at a constant speed. When sensor  114  senses that the leading edge of paper  102  has passed nip  106  and/or leading edge guide  116 , then sensor  114  sends a signal to a controller, not shown in  FIG. 1 , which causes angular driver  118  to start rotating at the same angular speed and direction as drive roller  108 . Hence, pinch roller  110  and leading edge guide  116  (if leading edge guide  116  exists) stay close to the leading edge of paper  102 , as drive roller  108  turns. 
   Alternatively, drive roller  108  starts to rotate only when the paper approaches the nip between the drive roller and the pinch roller. However, this alternative is less advantageous than having the drive roller rotating continuously, since it requires control of and quick changes in the rotational velocity of the drive roller. 
   Eventually, as shown in  FIG. 2 , pinch roller  110  and the leading edge of paper  102  are near the bottom of drive roller  108 . When the leading edge of paper  102  reaches a stop  202 , optionally guided into the narrow space beneath drive roller  108  by leading edge guide  116 , then the leading edge of paper  102  cannot go any further. At this time, an optional leading edge clamp  204  pushes paper  102  down, holding paper  102  at a point close to its leading edge, pulling paper  102  away from contact with drive roller  108 , at least in the region between pinch roller  110  and the leading edge of paper  102 . Leading edge clamp  204  holds this leading portion of paper  102  down against the bottom of an output tray  206 , or against the top paper in a stack of papers that are already resting in output tray  206 . This prevents paper  102  from rubbing against drive roller  108 . Alternatively, there is no leading edge clamp  204 , and the leading portion of paper  102  simply falls away from drive roller  108 , once the leading edge of paper  102  reaches stop  202 . Having the leading portion of paper  102  fall away from drive roller  108  is optionally facilitated if, as explained below, pinch roller  110  starts to move back up drive roller  108  even before the leading edge of paper  102  has quite reached stop  202 , but when the leading edge of paper  102  is close enough to stop  202  that pinch roller  110  and optionally leading edge guide  116  are no longer needed to guide the leading edge of paper  102  toward stop  202 . 
   At the same time as the leading edge of paper  102  reaches stop  202 , or optionally slightly before as discussed above, angular driver  118  reverses direction, bringing pinch roller  110  back up drive roller  108 , as shown in  FIG. 3 . The motion of the pinch roller is preferably a rolling motion along paper  102  so that there is no rubbing of the paper on the pinch roller. Optionally, angular driver  118  goes back up drive roller  108  at a different angular speed than drive roller  108 , for example it goes at a faster angular speed than drive roller  108 . A trailing portion  302  of the paper, up to pinch roller  110 , continues to move at the speed of drive roller  108 , while a leading portion  304 , between clamp  204  and the leading edge, is not moving. As a result, a middle portion  306  of the paper begins to buckle, separating from drive roller  108  between pinch roller  110  and clamp  204 . 
   At first, shortly after the leading edge has stopped moving, middle portion  306  is very short, and cannot buckle very much without creasing. At this time, however, the paper has only been buckling for a short time, so the degree of buckling is small, and can be accommodated within the short distance of middle portion  306  without bending too sharply and creasing. As the paper continues to buckle, it needs a greater length to accommodate the buckling, but a greater length is available in middle portion  306  because pinch roller  110  is moving away from the leading edge. Depending on the characteristics of the paper, creasing can be avoided if pinch roller  110  is moving back up drive roller  108  faster than some minimum speed, for example at least as fast as drive roller  108  is turning. 
   Optionally, leading edge guide  116 , if it exists, moves back out of the way as pinch roller  110  moves back up drive roller  108 , as seen in  FIG. 3  and  FIG. 4 . In this way, leading edge guide  116  does not rub against the back of the paper as it buckles and moves further away from drive roller  108 . For example, the angular guide rotates counterclockwise around the axis of pinch roller  110 , so that it clears the stack of papers. 
   Alternatively, leading edge guide  116  is fixed in position relative to angular driver  118 , at a position such that it effectively guides the leading edge of paper  102  to stop  202 , but does not rub against middle portion  306  of the paper as it buckles. 
   When the trailing edge of paper  102  goes through nip  106  and is free of drive roller  108 , the trailing portion of the paper falls down, as shown in  FIG. 5 . Optionally, instead of the trailing portion of the paper falling under the influence of gravity, it is forced downward by other means, for example by moving air. If gravity is not used, then the direction shown as down in the drawings need not be directly down. It should be understood that references herein to the paper falling may also refer to these other means of moving the paper. 
   Because the leading portion of the paper is resting against stop  202 , there is no need to guide the paper into a correct position as it falls, so there is nothing for the trailing portion of the paper to rub against as it falls. If leading edge clamp  204  exists and is holding down the leading portion of paper  102 , this prevents the leading edge of paper  102  from drifting even slightly away from stop  202  as it falls. After falling, the paper lies in output tray  206  in a flipped over orientation, relative to its orientation before feeding into the flipping mechanism. From output tray  206 , paper  102  is optionally moved to a place where it is further processed, for example the other side of the paper is printed. Optionally, a plurality of papers are stacked in the output tray, with the bottom of the output tray moving down to accommodate the stack, before any papers are moved out of the output tray. Alternatively, flipping the paper is the last process done to the paper, and paper  102 , or a stack of flipped papers, remains in the output tray. 
     FIGS. 6 ,  7 ,  8  and  9  show a time sequence, corresponding to  FIGS. 1 ,  2 ,  3  and  4 , according to another exemplary embodiment of the invention. This embodiment of the invention differs from the embodiment described above, because pinch roller  110  may be fixed in position near the top of drive roller  108 , and there is no angular driver  118  which moves pinch roller  110  around drive roller  108 . Pinch driver  110  is, of course, free to rotate on its axis. Instead of using pinch roller  110  to hold the leading portion of paper  102  against drive roller  108  as drive roller  108  turns, there is a holder for the leading edge of paper  102 , such as a suction system  616  inside drive roller  108 , which holds the leading portion of paper  102  against drive roller  108 . 
   Optionally, as in the embodiment shown in  FIGS. 1-4 , drive roller  108  does not have a solid surface going all the way across the width of the paper, but has one or more rims which go across only a portion of the width of paper  102 , and has one or more arms with suction cups, located, for example, to the side of the one or more rims, which hold the leading edge of paper  102 , playing the same role as suction system  616 . 
   In one embodiment of the invention, drive roller  108  rotates continuously and the feeding of the paper is synchronized with the arrival of the suction system at nip  106 , between drive roller  108  and pinch roller  110 . 
   In some embodiments of the invention, suction system  616  is activated when sensor  114  detects the leading edge of the paper passing through nip  106 . Alternatively, suction system  616  is active even before the leading edge of paper  102  is picked up by nip  106 , or suction system  616  is turned on at a time when the leading edge of paper  102  is expected to go past nip  106 . In these cases, optionally there is no need for sensor  114 . Although  FIG. 6  shows an opening in suction system  616  only at one azimuthal position at the surface of drive roller  108 , optionally there are openings at the surface of drive roller  108  to provide suction at more than one azimuthal position around drive roller  108 , since the leading edge of paper  102  optionally does not always contact drive roller  108  at the same azimuthal position. Just before the leading edge of paper  102  reaches stop  202 , suction system  616  releases paper  102 , and the leading portion of paper  102  falls away from drive roller  108 , preventing paper  102  from rubbing against drive roller  108  after the leading edge of paper  102  comes to rest against stop  202 . 
   Alternatively, instead of or in addition to suction system  616 , there are grippers, not shown in the drawings, which hold the leading portion of paper  102  against drive roller  108 . However, using a suction system instead of grippers has the advantage that it is not necessary to get the grippers out of the way of stop  202  as drive roller  108  continues to turn after the leading portion of paper  102  is released. 
     FIG. 7  shows the leading edge of paper  102  being released by suction system  616  from drive roller  108  as the leading edge of paper  102  reaches stop  202 .  FIG. 8  shows paper  102  beginning to buckle as drive roller  108  continues to turn, and  FIG. 9  shows the paper buckling further as drive roller  108  continues to turn. The last step of the time sequence looks the same as  FIG. 5 , but without leading edge guide  116  or angular driver  118 , showing the trailing edge of the paper released from nip  106 , and the paper falling down. 
     FIGS. 10A ,  10 B,  11 ,  12 , and  13  show embodiments of the invention in which there is a bypass mechanism, which allows the paper to bypass the paper flipping mechanism.  FIGS. 10A and 10B  show one such embodiment, with the bypass mechanism disabled in  FIG. 10A , and operating in  FIG. 10B .  FIGS. 11 ,  12 , and  13  show other embodiments with alternative bypass mechanisms. 
   In  FIG. 10A , an image is printed on the printing media at impression cylinder  1002 . A transfer rotor  1003  has an arm  1004  with a suction cup  1006  at one end, which picks up the printing media from impression cylinder  1002  after the image is printed. Suction cup  1006  follows a circular path  1008 , carrying the printing media to drive roller  108 .  FIG. 10A  shows arm  1004  extending also to the other side of rotor  1003 , and another suction cup  1010  on the other end of arm  1004 . Optionally, this allows one suction cup to bring one sheet of printing media to drive roller  108 , while the other suction cup is picking up the next sheet of printing media from impression cylinder  1002 . Alternatively, there is only one suction cup associated with transfer rotor  1003 , or there are three or more suction cups associated with transfer rotor  1003 . If there are two or more suction cups, they need not be in the same plane, parallel to the plane of the drawing. Optionally, each suction cup shown in  FIG. 10A  represents a plurality of suction cups, optionally each attached to its own arm, and arranged one behind the other, which attach to different locations across the width of the paper, as is known in the art. 
   Optionally, drive roller  108  does not have a solid surface across the entire width of the printing media, but has at least one solid rim, and there is a suction cup  1012 , mounted on an arm  1014 . Optionally, there are at least two rims, and arm  1014  and suction cup  1012  are located between two of the rims. Alternatively, whether or not there are at least two rims, arm  1014  and suction cup  1012  are located behind or in front of one of the at least one rims. Alternatively, drive roller  108  does have a solid surface across the entire width of the paper, and suction cup  1012  in  FIG. 10A  represents the opening of a suction system inside drive roller  108 . 
   Suction cup  1012  picks up the printing media from suction cup  1010  (or from suction cup  1006  if suction cup  1006  is adjacent to drive roller  108  at the time). Suction cup  1012  then conveys the leading edge of the printing media so that it enters a nip  106  between drive roller  108  and a pinch roller  110 . Optionally, pinch roller  110 , like drive roller  108 , does not have a solid surface all the way across the width of the printing media, but has one or more solid rims. If neither drive roller  108  nor pinch roller  110  have solid surfaces across their entire widths, then there is at least one region across the width of the printing media where drive roller  108  and pinch roller  110  both have rims, in order to form nip  106  between them. 
   The flipping mechanism in  FIG. 10A  works in the same way as the flipping mechanism described in  FIGS. 6-9 , with suction cup  1012  in  FIG. 10A  playing the role of suction system  616  in  FIGS. 6-9 . Suction cup  1012  carries the leading edge of the printing media down and around as drive roller  108  rotates. When the leading edge of the paper reaches leading edge stop  202 , it is released from suction cup  1012 , because the vacuum in suction cup  1012  is reduced, and/or because leading edge stop  202  prevents the leading edge from continuing around drive roller  108  with-suction cup  1012 . A trailing portion of the printing media continues to move through nip  106 , driven by drive roller  108 . As a result, the printing media buckles, and moves out from drive roller  108 . The position of the printing media at successive times, after the leading edge reaches leading edge stop  202 , is shown by curves  1016 ,  1018 ,  1020  and  1022 . When the trailing edge of the printing media passes through nip  106 , the printing media falls down to a transport belt  1024 , in a flipped over orientation, and is carried away from drive roller  108 . 
   Optionally, instead of transport belt  1024 , the printing media falls to a fixed location, similar to output tray  206  in  FIGS. 6-9 , for example. Optionally, there is a leading edge clamp, not shown in  FIG. 10A , similar to leading edge clamp  204  in  FIGS. 6-9 , which holds the leading edge of the printing media in place at leading edge stop  202 . In this case, if the printing media is transported away on transport belt  1024  after being flipped, then the leading edge clamp releases the leading edge of the printing media so that transport belt  1024  can transport the printing media away. Alternatively, the printing media is held against transport belt  1024  with sufficient strength, for example by a vacuum system or by another pinch roller, so as to pull the printing media out of the leading edge clamp. Optionally, instead of suction cup  1012  holding the leading edge of the printing media to drive roller  108  as it continues past nip  106 , pinch roller  110  moves around drive roller  108  and holds the leading edge of the printing media against drive roller  108 , as in  FIGS. 1-4 . 
     FIG. 10A  also shows a bypass mechanism  1026  which is inactivated, raised up and out of the way of the leading edge of the printing media as it emerges from nip  106 . In  FIG. 10B , bypass mechanism  1026  is activated by lowering it to a position such that the leading edge of the printing media enters bypass mechanism  1026 , instead of continuing to travel around drive roller  108 , after emerging from nip  106 . Optionally, the entrance of the leading edge into bypass mechanism  1026  is assisted by having suction cup  1012  release the leading edge after it passes nip  106 . Additionally or alternatively, bypass mechanism  1026  has its own vacuum system, which pulls in the leading edge of the printing media. Once the leading edge is drawn into bypass mechanism  1026 , the printing media follows path  1028 , and lands on transport belt  1024 , without flipping over. Optionally, bypass mechanism  1026  has its own drive roller and pinch roller, with a nip between them, which draws the leading edge of the printing media into bypass mechanism  1026 , once the leading edge of the printing media is directed to it. Optionally, instead of being a slot as shown in  FIGS. 10A and 10B , bypass mechanism  1026  comprises a conveyer belt. 
     FIG. 11  shows a flipping mechanism similar to that shown in  FIGS. 10A and 10B , but with a different method of bypassing the flipping mechanism. In  FIG. 11 , transport belt  1024  extends under path  1008  of transport rotor  1003 . When it is desired to bypass the flipping mechanism, suction cup  1006  or suction cup  1010  (whichever one has picked up the printing media from impression cylinder  1002 ) releases the printing media onto transport belt  1024 , before the printing media reaches suction cup  1012  and drive rotor  108 . Then the printing media follows path  1130 , and ends up lying on transport belt  1024 , without flipping over. 
     FIG. 12  shows another bypass mechanism, similar to that shown in  FIGS. 10A and 10B . Instead of transport rotor  1003  bringing the printing media directly to drive rotor  108 , transport rotor  1003  (and suction cup  1006  or suction cup  1010 ) brings the printing media to a conveyer  1202 . From conveyer  1202 , the leading edge of the printing media either falls down and is caught in nip  106 , if the bypass mechanism is inactive, or continues on path  1204  if the bypass mechanism is active, eventually reaching a conveyer  1206  without flipping over. The flipped over printing media, on conveyer  1024 , is also brought to conveyer  1206 . Alternatively, conveyers  1024  and  1206  form a single long conveyer, and printing media from path  1204  is brought onto the single long conveyer. A potential advantage of using two separate conveyers  1024  and  1206  is that conveyer  1206  can be swung out of the way, for access to parts of the printer or copier that are underneath it for example, without affecting the flipping mechanism. 
   The bypass or flipping mechanism in  FIG. 12  can be selected either by transferring the front end of the sheet to path  1204 , or by diverting it to nip  106 . In general, any method of diverting substrates to one or another path can be used, including movement of rollers (for example a downward movement of the first set of rollers in path  1204  will direct the sheet to nip  106 ), provision of a trap door, where closing the door causes the printing media to follow path  1204 , while opening the trap door disables the bypass mechanism, causing the leading edge of the printing media to fall down and be caught by nip  106 . Alternatively, the leading edge of the printing media is selectively directed to nip  106  or to path  1204  by a different method, for example pneumatically, or by an element which pushes against the printing media or by a diverting element which may be selectively placed in path  1204 , when flipping is desired. 
     FIG. 13  shows a flipping mechanism with some additional features, together with a different bypass mechanism. As in  FIGS. 10A ,  10 B,  11  and  12 , printing media is picked up from impression roller  1002  by suction cup  1006 , which is attached to the end arm  1004  and swung around by transfer rotor  1003 . Suction cup  1006  swings the printing media around clockwise, on circular path  1008 . When the bypass mechanism is not active, suction cup  1006  transfers the printing media to suction cup  1012 , which swings the printing media around counter-clockwise on circular path  1304 . At about the time suction cup  1012  picks up the printing media from suction cup  1006  or optionally a little earlier, the leading edge of the printing media passes through nip  106  between rollers  110  and  1302 . Although it looks as if rollers  110  and  1302  would interfere with the swinging motion of suction cups  1006  and  1012 , in fact the suction cups and rollers are not in the same plane, so they do not interfere with each other. Optionally, each of rollers  110  and  1302  shown in  FIG. 13  represents two rollers, one in back of the other, and the two side edges of the printing media go through nip  106 , while suction cups  1006  and  1012  are located in a plane between the rollers, somewhere in the center of the printing media. Optionally, one or both of cups  1006  and  1012  shown in  FIG. 13  represents two or more cups, one behind the other. 
   When the leading edge of the printing media reaches a stop  202 , suction cup  1012  releases the printing media, and the leading edge of the printing media rests against stop  202 . However, a trailing portion of the printing media, which is still caught in nip  106 , continues moving forward, since one or both of roller  110  and roller  1302  is a drive roller, which continues to push the printing media through nip  106 . As a result, the printing media buckles, and has a configuration similar to  1306 . When the trailing edge of the printing media goes through nip  106 , the printing media falls down to conveyer  1324  in a flipped over orientation, as shown by  1308  and  1310 . Blower  1314  optionally blows a jet of air  1316  at the printing media, helping the media to fall down more quickly. Such an air jet may be particularly useful if the printing media is very light weight, and may prevent the printing media from folding over as it falls. 
   Conveyer belt  1318  optionally can turn in either direction, as desired. When the bypass mechanism is not activated, conveyer belt  1318  turns clockwise, so the lower portion moves to the left. As the printing media falls down, the upper edge may brush against the lower surface of conveyer belt  1318 , which helps to unbend the printing medium and make it lie flat. Conveyer belt  1318  may be particularly useful for a heavy weight or stiff printing media, which does not lose its curl easily. Conveyer belt  1318  and air jets  1316  optionally work together to help make the printing media fall down. Alternatively, only one of them is present or only one of them is operative for some grades of printing media. 
   From conveyer belt  1324 , the printing media is transported to a conveyer belt  1312 . Alternatively, a portion of the printing media may fall directly onto conveyer belt  1312 . Alternatively, there is only one long conveyer belt, instead of belts  1324  and  1312 . Optionally, conveyer belt  1324  and/or conveyer belt  1312  does not start moving until the printing media has fallen down completely, in order to prevent the printing media from folding over as it falls. Alternatively, one or both of belts  1324  and  1312  move continuously, particularly if the printing media falls down quickly enough so that folding is not a problem. 
   If the bypass mechanism is active, then instead of cup  1006  transferring the printing media to cup  1012  near nip  106 , cup  1006  continues to hold the printing media, and releases the printing media only at position  1010 . Although there appear to be two suction cups  1006  and  1010  going around transfer rotor  1003 , the two cups  1006  and  1010  shown in  FIG. 13  are intended to show two different positions of the same cup  1006  as it swings the printing media around. Optionally, however, there are two or even more than two suction cups and arms attached to transfer rotor  1003 , so that one suction cup can be picking up one printing media while another suction cup is transferring another printing media. 
   When suction cup  1010  releases the printing media, a leading portion  1320  of it falls onto conveyer belt  1318 . Optionally, suction cup  1010  holds the printing medium some distance away from the its leading edge, so that the leading portion flops over and is located directly above belt  1318  even before cup  1010  releases it. Alternatively or additionally, air jets  1316  help to push the printing media onto belt  1318 . Alternatively or additionally, belt  1318  extends closer to suction cup  1010  than shown in  FIG. 13 , for example directly under it, but not in a position that would interfere with suction cup  1010 . Alternatively or additionally, a trailing portion of the printing media is still caught in nip  106  between rollers  110  and  1302  when the leading portion is released by suction cup  1010 , and one or both of rollers  110  and  1302  is a drive roller, which continues to push the printing media to the left, causing leading portion  1320  to fall onto belt  1318 . 
   When the bypass mechanism is active, belt  1318  moves counter-clockwise, so the top portion of the belt, where the printing media is resting, moves to the left. The printing media, which is not in a flipped over orientation, goes onto belt  1206  when it reaches the end of belt  1318 . Printing media on belt  1312 , which are in a flipped over orientation, also go onto belt  1206 , when they reach the end of belt  1312 . Alternatively, belt  1206  is not separate from belt  1312 , but belts  1206  and  1312 , and optionally belt  1324  as well, are replaced by one long belt, which printing media from belt  1318  falls onto. Dividing the belt into one than one belt, for example into three belts as shown, has the potential advantage of making it easier to gain access to the machine, since belt  1206 , for example, can be swung up and out of the way, without affecting the other belts. 
   Although  FIG. 13  shows both a bypass mechanism and new features of the flipping mechanism, including belt  1318  which participates both in the bypass mechanism and in the flipping mechanism, optionally only one or some of these features are present. For example, optionally belt  1318  runs only in one direction, and is used only in the bypass mechanism, or only in the flipping mechanism. Optionally, some of the features of  FIG. 13 , such as blower  1314 , can be utilized in the other embodiments. 
     FIG. 14  shows an additional exemplary embodiment of the invention. In the embodiment shown in  FIG. 14 , rollers  110  and  1302  of the embodiment shown in  FIG. 13  are removed. Optionally, suction cup  1012  receives the printing media from suction cup  1006 . As suction cup  1012  guides the printing media toward stop  202  the trailing portion of the printing media is moved toward the rest position of the sheet by air jets  1316  from blowers  1314  instead of being held up by roller  1302 . As shown in  FIG. 14  air jets  1316  ensure the flipping of the printing media while suction cup  1012  guides it to stop  202 . In general, air jets  1316  need not be synchronized with the movement of the sheet and can be operated continuously. 
   It is noted that in the embodiment shown in  FIG. 14 , once the sheet is passed off to suction cup  1012 , it is not held by any elements between element  1002  and element  1304  (which is shown as a path, but which may include rings for supporting the sheet or even no support at all). The inventors have found that despite the fact that the sheet is held only by suction cup  1012  until it reaches stop  202 , the momentum of the sheet and the air flow provided by air jets  1316  are sufficient to provide for reliable landing of the sheet on belt  1324 . 
   It is also noted that while a suction cup  1012  is shown as holding the sheet while it is delivered to belt  1324 , other types of grippers may be used in its place. Furthermore while the sheet is shown as being delivered to the surface of a belt, the present invention can be utilized to deliver sheets to an output stacker. 
   In some embodiments of the invention, stop  202  is a controlled stop, which can be opened or closed, for example by using a mechanical gate or magnetically controlled gate to optionally stop the advance of the printing media In some embodiments of the invention the printing media is stopped at stop  202 . However, after finishing the flipping process, stop  202  is opened to allow the printing media to be transferred to the right on belt  1324 . Optionally transferring to the right allows the printing media to be re-fed to the printing mechanism for printing on the other side or to an output stack. Alternatively, belt  1324  can transfer the printing media to the left to be transferred to another printing mechanism or to be delivered out of the printer in the reverse orientation as if delivered by belt  1318  as described above with respect to the previous embodiment. In some embodiments of the invention, belt  1318  is not used. 
   In some embodiments of the invention, stop  202  is replaced or assisted by a gripper, such as vacuum nipple  204  which receives the printing media from suction cup  1012 . Optionally, vacuum nipple  204  couples the printing media to conveyer belt  1324  releasing it from the grasp of suction cup  1012 . After coupling the printing media to conveyer belt  1324  it follows the direction of movement of conveyer belt  1324 . Vacuum nipple  204  (which may be a suction cup) provides further stability for the sheet from being moved away from the stop by airjets  1316 . 
   Although this description and the claims refer sometimes to paper, the invention may also be used with any other sheet like printing media, such as thin flexible plastic sheets. The invention has been described in the context of the best mode for carrying it out. It should be understood that not all features shown in the drawings or described in the associated text may be present in an actual device, in accordance with some embodiments of the invention. Furthermore, variations on the method and apparatus shown are included within the scope of the invention, which is limited only by the claims. Also, features of one embodiment may be provided in conjunction with features of a different embodiment of the invention. As used herein, the terms “have”, “include” and “comprise” or their conjugates mean “including but not limited to.”