Abstract:
Duplex printing apparatus for printing on two sides of a sheet, the apparatus including: 
     an expression roller on which the sheet is held during printing; 
     a imager which prints an image on a first side of the sheet while it is being held on the impression roller; and 
     a sheet inverter which removes the sheet from the impression roller, inverts the sheet and returns it to the impression roller for printing on a second side of the sheet by imager, wherein the sheet it to held on said impression roller referenced to a first edge thereof during the printing of the first side thereof and is also held on the impression roller referenced to said first edge during printing of the second side thereof.

Description:
RELATED APPLICATION 
     The present application is a U.S. national stage application of PCT/IL98/00553, filed Nov. 11, 1998, and a continuation of PCT/IL98/00235, filed May 24, 1998. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to printing systems and more particularly to duplex printing systems for printing variable information on one or both sides of a sheet. 
     BACKGROUND OF THE INVENTION 
     Apparatus for duplex copying of documents and for duplex printing by means of laser printers are known in the art. U.S. Pat. No. 4,949,949 to Holmes et al. describes a “Hybrid Sequencing Duplex Automatic Document Handling System” which includes apparatus for handling document sheets both sides of which are to be copied and for making duplex (i.e. double-sided) copies of such document sheets. The apparatus involve the use of one or more pairs of reversible rollers, lengthy inversion paths, and buffer trays for the handling of the documents and the copy paper prior to and in the course of making duplex copies. U.S. Pat. No. 4,884,794 to Dinatale et al. describes a document handler for duplex photocopying having first and second inverting path segments, which are utilized to re-orient the copy paper prior to duplex copying. U.S. Pat. No. 5,003,355 to Tanzawa describes a sheet transport control apparatus for use in a duplex unit of a laser printer, the apparatus including a transport system and a switchback system, and a series of driving motors and sensors. All these systems described in the prior art share the common feature of being mechanically complex, and they all involve transporting the paper through relatively lengthy and convoluted paths after printing on the first side so as to be able to print on the second side. Other systems for duplex printing are described in U.S. Pat. Nos. 4,806.079; 4,814,822; 4,568,169; 4,639,126; 4,428,667; 4,607,940; 4,375,326 and 5,020,788 and EP publication 0342704. 
     PCT publication 93/04409 describes a switchback system with a much shorter path than older systems, which allowed for on demand duplex printing without storage of large numbers of sheets. 
     Systems which utilize the same impression roller and/or the same printing engine for printing both sides of a web are known in the art. However, even in those systems the two sides of the web are printed at different printing positions in the printer and the web is not indexed at an edge. 
     Also known are systems for reversing sheets between printing stations. One such system is called a “perfecta” type system and comprises a roller that acts to turn over the sheet. Such systems, unlike those used for laser printers, reference the printing sheet from the same edge for printing on both sides. 
     A prior art perfecta system  10  is shown in FIGS. 1A and 1B. This system comprises a first impression roller  12 , which holds a sheet  14  for printing thereon by a print roller (not shown). Sheet  14  is transferred to roller  16  where it is held by a front edge clamp  20 . Roller  16  continues to rotate and the front edge of sheet  14  passes an inverting roller  18 . When the trailing edge reaches inverting roller  18 , a clamp  20  on roller  18  catches the trailing edge of sheet  14  and, as shown in FIG. 1B inverts the sheet prior to its being clamped to a second impression roller  22 . 
     An advantage of perfecta systems is that while the leading edge for printing the first and second sides of the sheet are reversed, the same edge is used as a reference position for printing both sides. Another advantage of perfecta systems, which is related to the first advantage, is that the sheets are always positively held by the system during inversion of the sheet. Positive holding of sheets distinguishes “perfecta” systems from systems which utilize a single printing engine and which generally do not positively hold the sheets during the entire process of transfer and reversal. 
     However, inverting systems which provide the advantages of perfecta systems are not known in a printer using the same impression roller and printing engine for printing both sides of the sheet. 
     SUMMARY OF THE INVENTION 
     One aspect of some preferred embodiments of the present invention provides apparatus and a method for duplex printing of sheets, utilizing the same edge of the sheet for reference for printing both sides thereof, while utilizing the same impression roller and/or the same printing engine. 
     One aspect of some preferred embodiments of the present invention provides apparatus and a method for duplex printing of sheets utilizing an impression roller for printing both sides of a sheet, while positively holding the sheet during the entire process of reversal and transfer of the sheet. Preferably, this means that the sheet is positively held from the start of the printing process to its end. 
     In a preferred embodiment of the invention only one sheet, other than a sheet held in the impression roller is in the inverting system. 
     In a preferred embodiment of the invention, the same set of one or more printing engines is used in the printing of both sides of all the sheets. 
     In preferred embodiments of the present invention a perfecta-like system is used. This system includes rollers and/or belts which receive the sheet from one position on the circumference of an impression roller and, after reversing the sheet. delivers the sheet to a second position on the impression roller. Preferably, the path traveled by the sheet between the two positions holds an integral number of sheets. Preferably, the impression roller holds a plurality of sheets and presents them seriatim to one or more print engines. Preferably, the engine or engines are electrographic or other engines providing programmable images such as electrophotographic engines, ink or bubble jet print heads thermal printing heads or any other suitable printing engines. 
     Other aspects of some preferred embodiments of the invention are concerned with high speed printing engines, especially with high speed electrographic printing engines. In such engines special care must be taken in charging a photoreceptor and, when liquid toner is utilized, in treating and transport of the image. Some aspects of some preferred embodiments of the present invention deal with improvements in such engines especially useful for high speed printing. 
     There is thus provided, in accordance with a preferred embodiment of the invention duplex printing apparatus for printing on two sides of a sheet, the apparatus comprising: 
     an impression roller on which the sheet is held during printing; 
     imager which prints an image on a first side of the sheet while it is being held on the impression roller; and 
     a sheet inverter which removes the sheet from the impression roller, inverts the sheet and returns it to the impression roller for printing on a second side of the sheet by the imager, wherein the sheet is held on said impression roller referenced to a first edge thereof during the printing of the first side thereof and is also held on the impression roller referenced to said first edge during printing of the second side thereof. 
     Preferably, the sheet inverter positively controls the position of the sheet during the inversion thereof, without releasing the sheet during the inversion. 
     There is further provided, in accordance with a preferred embodiment of the invention a duplex printing apparatus for printing on two sides of a sheet, the apparatus comprising: 
     a surface, on which an image to be printed is selectably formed; 
     an impression roller on which the sheet is held during printing, referenced to a first edge thereof; 
     an imager which prints an image on a first side of the sheet while it is being held on the impression roller; and 
     a sheet inverter which removes the sheet from the impression roller, inverts the sheet and returns it to the impression roller for printing on a second side of the sheet by the imager, wherein the sheet inverter positively controls the position of the sheet from the removal of the sheet from the impression roller to the return of the sheet thereto after the inversion thereof, without releasing the sheet. 
     in a preferred embodiment of the invention the sheet inverter comprises: 
     a paper pick-off system which removes the sheet from the impression roller, after printing of the first side of the sheet, while the sheet is held referenced to said first edge; 
     an inverting transport past which the first edge is carried while the sheet remains referenced to said first edge; and 
     a sheet pick-off on said inverting transport which captures a second edge of the sheet, opposite the first edge while the sheet is still being held referenced to the first edge, such that said capture is made referenced to the first edge, 
     said inverting transport transporting the second edge to the impression roller for capture by the impression roller, such that the second side of the sheet is presented for printing by the imager. 
     Preferably, the apparatus includes at least one intermediate transport which receives the sheet from the sheet pick-off system and transports it to the inverting transport while the sheet remains referenced to the first edge. Preferably, the at least one intermediate transport comprises at least one roller. Preferably, a sheet path in the paper pick-off, sheet pick-off and intermediate transport is at least the length of a plurality of sheets. 
     In a preferred embodiment of the invention, the inverting transport comprises a transport roller. 
     Preferably, the page inverter comprises a perfecta system. 
     In a preferred embodiment of he invention, the page inverter stops the motion of the sheet while positively holding it referenced to the printing on the first side and then moves the sheet in a reverse direction for capture by the impression roller. 
     Preferably, the paper pick-off comprises at least one vacuum pick-off that picks the sheet off the impression roller and holds it while it is being inverted. Preferably, the at least one vacuum pick-off comprises two sets of vacuum pick-offs each comprising at least one pick-off, wherein the vacuum pick-offs pick offs alternate in picking sheets off the impression roller. 
     Preferably, the apparatus includes a belt transport that receives a sheet from the at least one vacuum pick-off and transports it to the impression roller while positively holding the sheet referenced to the image printed on the first side. 
     Preferably, the vacuum pick-offs rotate about an axis and have a radial extent from the axis and the apparatus has at least one at least partial disk concentric with the axis and the disk has a radial extent equal to the radial extent of the vacuum pick-offs. 
     In a preferred embodiment of the invention, the impression roller and the inverting system hold no more than two sheets at any one time. 
     Preferably, the imager comprises a plurality of imaging stations each of which transfers an image of a different color to the sheet. 
     In a preferred embodiment of the invention, the imager includes an image forming surface on which the image is formed prior to transfer to the sheet. Preferably, the imager includes at least one intermediate transfer member to which images are transferred from the image forming surface and from which the images are transferred to the sheet. 
     In a preferred embodiment of the invention the imager provides different images to the sides of the sheet. 
     According to one preferred embodiment of the invention the imager is an electrographic imager. 
     The imager can be a powder toner imager or a liquid toner imager. 
     The imager can be an ink-jet or bubble jet imager. 
     In a preferred embodiment of the invention the impression roller is adapted to hold a plurality of sheets at one time. 
     There is further provided, in accordance with a preferred embodiment of the invention, a duplex printing method for printing on two sides of a sheet, the method comprising: 
     printing an image on a first side of the sheet at a printing position, the sheet and thus said printing being referenced to an edge of the sheet; 
     inverting the sheet and returning it to the printing position while it remains referenced to said edge; and 
     printing an image on a second side of the sheet at said printing position while the sheet and thus said printing is referenced to said edge. 
     Preferably, the position of the sheet is positively controlled during inversion thereof, without releasing the sheet between printing of the first and second sides thereof. 
     There is further provided, in accordance with a preferred embodiment of the invention a duplex printing method for printing on two sides of a sheet, the method comprising: 
     printing an image on a first side of the sheet at a printing position; 
     inverting the sheet and returning it to the printing position; and 
     printing an image on a second side of the sheet at said printing position, 
     wherein the position of the sheet is positively controlled during printing and inversion thereof, without releasing the sheet. 
     Preferably, the sheet is delivered to said printing position by a moving member on which it is held while being referenced to said edge. 
     In a preferred embodiment of the invention the sheet is printed while being moved by the moving surface, past the printing position and wherein the sheet is held at said edge during printing of one side thereof and held by an opposite edge of the sheet during printing of the other side thereof. 
     Preferably, the sheet is printed while being moved past the printing position with said edge passing the position first during printing of one side of the sheet and wherein said edge passes the printing position after the rest of the sheet during the printing of the other side of the sheet. 
     Preferably the method includes printing different images on the two sides of the sheet. 
     In a preferred embodiment of the invention, inverting the sheet includes: 
     moving the sheet to a first position while holding it referenced to the first edge; and 
     stopping it at the first position: and 
     returning it to the printing position while it remains referenced to the first edge. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more completely understood and appreciated from the following detailed description of preferred embodiments of the invention, taken in conjunction with the drawings. Corresponding structures in different drawings are indicated with the same reference numeral. The drawings are: 
     FIGS. 1A and 1B illustrate schematically a prior art multi-station (multi-impression roller) duplex printing apparatus; 
     FIG. 2 is a schematic cross-sectional view of a single impression roller duplex printing apparatus in accordance with a preferred embodiment of the invention; 
     FIG. 3 is a schematic cross sectional view of a portion of the apparatus of FIG. 2, showing a portion the mechanism by which a sheet is inverted; 
     FIG. 4 is a schematic cross sectional view of an alternative apparatus for inverting a sheet in accordance with a preferred embodiment of the invention; 
     FIG. 5 is a very schematic cross-sectional illustration of a printing engine in accordance with a preferred embodiment of the invention; 
     FIG. 6 is a schematic cross-sectional view of a second single impression roller duplex printing apparatus in accordance with a preferred embodiment of the invention; 
     FIG. 7 is a schematic isometric view of a portion of a sheet inverter of the preferred embodiment of FIG. 6; 
     FIGS. 8A-8H schematically shows the progress of sheets in the preferred embodiment of FIG. 6; 
     FIG. 9 is a schematic cross sectional view of a dual duplex printer in accordance with a preferred embodiment of the invention; 
     FIG. 10 illustrates a photoreceptor charging system, especially suitable for high speed printing, in accordance with a preferred embodiment of the invention; 
     FIG. 11 illustrates a developing station in accordance with a preferred embodiment of the invention; 
     FIG. 12 illustrates an intermediate transfer member and associated apparatus, in accordance with a preferred embodiment of the invention; and 
     FIG. 13 is a cross-sectional representation of a cleaning station in accordance with a preferred embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Reference is now made to FIGS. 2 and 3, which illustrate a multi-color duplex printing system  40  in accordance with a preferred embodiment of the present invention. 
     System  40  includes an impression roller  42  that rotates in a direction indicated by arrow  44 . Situated around the periphery of roller  42  are one or more print engines  46 . In a preferred embodiment of the invention, each of engines  46  transfers a single color image to substrate sheets  48  that are held on- and travel with-impression roller  42 . Thus, as illustrated in FIG. 2, four color separations may be printed on a sheet as it sequentially passes the four engines shown. If it is desired to print a greater or lesser number of colors, more or fewer engines may be provided. While in a preferred embodiment of the invention engines  46  are a particular type of electrophotographic engine described below, any suitable electrophotographic engine or a printing engine of another type may be used. Especially suitable for use in the present invention are printing engines which print a variable image, such as a computer generated image. This allows for different images to be printed on the front and back of the sheet and for different images to be printed on sequential sheets. 
     Also situated around the periphery of impression roller  42  are a source of sheets  50  and associated sheet feeding apparatus  52 , a sheet take-off apparatus  54 , a stacker for printed sheets  56  and a sheet inverting system  58 . A portion of inverting system  58 , illustrating various stages in the inversion of a sheet, is shown in FIG.  3 . 
     The following discussion describes the progress of a single sheet  48  as it is printed on both sides. As shown in FIG. 2, one edge of each of sheets  48  is held by a clamp  60  of conventional design. A sheet  48  is synchronously fed from source  50 , by feeding apparatus  52  such that its leading edge is captured by one of clamps  60 . Impression roller  42 , which is preferably driven by a motor (not shown) carries sheet  48  past print engines  46  such that by the time it passes the last engine, printing of a first side of the sheet is complete. Alternatively, fewer engines may be used and each engine may print a plurality of colors in one of several rotations of impression roller  42 . The sheet then approaches sheet take-off mechanism  54 . Since only the first side of sheet  48  has been printed, mechanism  54  is not activated and sheet  48  passes it. A controller (not shown), which controls the printing and sheet transportation determines which path the sheet should take. As the leading edge of the sheet held by clamp  60  passes a first roller  64  of inverting system  58 , the leading edge of sheet  48  is handed off to a similar clamp  62  on roller  64 . The leading edge of the sheet is then successively handed off to a clamp  66  on a roller  68  and a clamp  70  on a roller  72 . During each hand-off the sheet is held between two rollers and or by a clamp such that registration of the leading edge is preserved. 
     When the leading edge of the sheet approaches a roller  74 , the leading edge is captured by a clamp  76  and carried toward roller  74 . Roller  74  receives the sheet and a clamp  76  holds the sheet on the roller. 
     When the leading edge of sheet  48  reaches an inverting roller  78 , the trailing edge is fed to a clamp  80  on roller  78  (shown more clearly in FIG. 3.) preferably utilizing by a lifter  82 . Lifter  82  may lift the trailing edge of the sheet by air pressure or mechanically. Lifter  82  can also utilize a vacuum to hold the sheet to the roller. It should be understood that when clamp  80  captures the trailing edge of sheet  48 , the position of the sheet is still determined by its leading edge, held by clamp  76 . Clamp  76  releases sheet  48  as or just after it is captured by clamp  80 . 
     However, while sheet  48  has reversed direction (as well as having been turned over), and is traveling with the (former) trailing edge first, its position remains referenced to the leading edge, which reference has been preserved during the various hand-offs of the sheet from roller to roller. 
     FIG. 3 shows a number of stages of transfer of sheet  48  from roller  74  to impression roller  42  by roller  78  and clamp  80 . As can be seen from FIG. 3, the sheet has now been reversed and, when it is transferred to impression roller  42  it is ready for having its second side printed. 
     Returning again to FIG. 2, sheet  48  again passes printing engines  46  whereat an image is printed on the second side of the sheet. 
     The sheet now approaches take-off apparatus  54 . Since both sides of the sheet have now been printed, the sheet is ready for removal. As clamp  60  (holding the edge of the sheet) approaches apparatus  54 , a clamp  84  on a belt  86  receives the sheet and removes it to stacker  56 . 
     When the blank space in the inverter system reaches the impression roller another sheet is fed to impression roller  42  from source  50  and placed in the position vacated by the sheet which was removed by apparatus  54 . It should be understood that whenever no sheet is available from inverter  58  to fill a clamp  60 , a new sheet is preferably fed from paper source  50 . 
     While the system has been shown with an inverter having a path that holds three sheets at one time and an impression roller that has four sections for holding sheets, a greater or lesser number of sheets and positions can be provided. One major consideration is the amount of room taken by the print engines and other apparatus situated around the periphery of the impression roller. Furthermore, while separate engines for each color are shown, a single multicolor engine may be provided. Furthermore, stacker  56  may be replaced by a finisher which produces booklets directly from the sheets as they are printed. 
     FIG. 4 shows an alternate inverting system in which rollers  64  and  68  have been replaced by a belt mechanism which receives the sheets from the front end of take-off apparatus  54 . 
     FIG. 5 shows a very schematic representation of a preferred printing engine  100  (corresponding to one of engines  46  of FIG.  2 ), in accordance with a preferred embodiment of the invention. While preferred engine  100  is especially suitable for a high speed duplexing system as shown in FIGS. 2-4, as indicated above, the duplexing system can operate with a wide variety of print engines. Similarly, engine  100  may operate with other types of duplexing systems or in a single sided printer. 
     Engine  100  includes a photoreceptor drum  102 , a charger  104  which charges the photoreceptor, an imagewise discharge system, such as a scanning laser  106  which forms a latent image on charged drum  102  and a developer  108  which develops the latent image. The developed image is preferably transferred to an intermediate transfer member  110 . After the image is transferred to intermediate transfer member  110 , photoreceptor  102  is cleaned of residual toner by a cleaning station  112 . 
     For slow speed systems, intermediate transfer members as described below can operate without any drying systems. In these systems the heat of the intermediate transfer member dries the image somewhat and removes some of the liquid carrier in the image, to improve the transfer of the image to sheet  48  on impression roller  42 . For some systems, liquid is removed prior to transfer of the image to the intermediate transfer member. For high speed imaging a dryer  114  is preferably used to dry the image on the intermediate transfer member. After transfer of the image to sheet  48 , a further dryer  116  removes some liquid which remains on or is solvated by the intermediate transfer member to improve transfer of the next image to the intermediate transfer member. 
     The duplexing mechanisms shown in FIGS. 2-4 operate in a synchronous manner with all of the rollers and/or belts moving in a synchronous manner. Thus, these duplex mechanisms are basically limited to one size sheet of paper or other printing material. 
     FIGS. 6-8 illustrate a printer having a duplexing mechanism which can handle a large variety of sheet sizes. 
     FIG. 6 shows a general layout of a printer  200  in accordance with a preferred embodiment of the invention. In addition to the differences in the duplex mechanism described below, printer  200  differs from printer  40  in other ways. For example, in printer  40  a plurality of printing engines  46  (shown as engine  100  in FIG.  5 ), each including a photoreceptor  102  and associated components and an intermediate transfer member  110  and associated components. Printer  200  of FIGS. 6-8, utilizes a single photoreceptor  102  and intermediate transfer member  110 . However, situated about photoreceptor  102  are a plurality of developers  108 . Each developer develops an electrostatic image on photoreceptor  102  with a different color image. In a preferred embodiment of the invention, a sheet  48  on impression roller  110  rotates once for each color and the different color images are transferred to the sheet seriatim. Preferably, photoreceptor  102  is large enough so that a plurality or all of the individual color images are developed during a single rotation of photoreceptor  102 . 
     It should be understood that the printing arrangement shown in FIG.  6  and that shown in FIGS. 2-5 are substantially interchangeable and can be used with any of the sheet inverting apparatus shown in this application or with sheet inverting apparatus of the prior art. As indicated above, the sheet inverting apparatus disclosed herein can be used with any suitable printer system known in the art. 
     Printer  200  shows sheet inverting apparatus  258  which is different from sheet inverting apparatus  58  of FIGS. 2-4. In particular, inverting apparatus  258  is capable of handling sheets of different sizes. It can invert sheets all of which are the same size and also invert sheets which have different sizes and which are interleaved. 
     In a preferred embodiment of the invention, when a sheet  48  is fed by feeding apparatus  52 , a detector detects the leading and trailing edges of the sheet being fed. The length of the sheet thus determined is transferred to a controller (not shown) which also controls the movement of the other elements of the printer as described below. 
     After being fed, sheet  48  is acquired by clamp  60  on impression roller  42 . Impression roller  42  may carry a single sheet or multiple sheets (a single sheet version is shown in FIGS. 6-8) at one time. After capture, individual color images are generated on photoreceptor  102  and transferred to the sheet, seriatim. In general, the photoreceptor may have a diameter many times that of the impression roller, such that the images can all be developed on the photoreceptor. during a single rotation of photoreceptor  102 , which corresponds to multiple rotations of impression roller  42 . Of course, the images on photoreceptor  102  are spaced such that, when transferred to sheet  48 , they are overlaid in an aligned manner. 
     When all the color images have been transferred to sheet, it is acquired by a first transfer mechanism  202 . Mechanism  202  is shown in perspective in FIG. 7 and a portion of inverting mechanism  258  is shown in FIG.  8 . As shown in FIG. 7, mechanism  202  preferably comprises two sets of vacuum pick-up arms A and B. Arms A are attached to and rotate with a central shaft  204 , driven by a motor  206 . Arms B are attached to and rotate with a series of elements  208  which can rotate about shaft  204 . Arms B are made to rotate together by a series of belts  210  and a shaft  212  which is driven by a motor  214 . As is clear, arms A rotate together and arms B rotate together as sets; however, the set of arms A and the set of arms B can rotate independently of each other. 
     Also shown in FIG. 7 is a disk  215  preferably mounted on shaft  204 . The radial extent of disk  215  is substantially the same as that of arms A and B. In a preferred embodiment of the invention, a plurality of such disks are present, where the disks are placed between the arms A, preferably one between each pair of arms. Only one is shown for clarity of presentation. 
     FIG. 8A shows mechanism  258  at a moment when gripper  60  which grips the leading edge of sheet  48  has just reached a point at which arms A can grip the sheet. (The trailing edge of the sheet is shown by reference number  49 ). Arm B is also shown, however, it does not yet play any part in the operation. At this point gripper  60  release sheet  48  arms A which are suction arms acquire the sheet. It should be understood that at this point sheet  48  is still held in a nip  216  between intermediate transfer member  110  and impression roller  42 . The tip of arms A rotate together with impression roller such that the sheet is removed from the impression roller gradually as portions of the sheet reach the 8 o&#39;clock position on impression roller  42 . 
     At a later period shown in FIG. 8C, trailing edge  49  of sheet  48  is free of nip  216 , (shown in FIG.  8 B). At this point sheet  48  is no longer held by impression roller  42  and is free to move under the influence of arms A. However, it remains on the impression roller due to its being held by arms A and by friction with impression roller  42  and disks  215 . It should be noted that at this point a new sheet  48 ′ is approaching gripper  60  which meanwhile has rotated to about  11  o&#39;clock. Arm A then advances the sheet so that it “falls off the edge” of impression roller onto a belt  218 . This position is shown in FIG.  8 D. It is noted that the image printed on sheet  48  is facing away from the belt. It is noted that at this point gripper  60  has gripped new sheet  48 ′, having a trailing edge  49 ′, and carried it into nip  216 . 
     Arms A hold sheet  48  in the position shown in FIG. 8D while the impression roller (together with sheet  48 ′ rotates a number of times required to transfer color images to it from intermediate transfer member  110 . At that time it is ready to be removed and replaced by sheet  48  so that the unprinted side of the sheet is printed. 
     FIG. 8E shows this condition. Arms B have now gripped sheet  48 ′ after its release by grippers  60 . At this point arms A start rotating counterclockwise such that trailing edge  49  of sheet  48  reaches gripper  60  when it reaches belt  218  at which point it acquires sheet  48 . This is shown in FIG.  8 F. Arms A then release sheet  48 , which is carried along by impression roller  42  for printing of the reverse side of sheet  48 . It should be noted that while gripper  60  has gripped the trailing edge of the sheet, it remains referenced to its leading edge, since this edge (or at least the sheet referenced to this edge) are held throughout by arms A and/or belt  218 . 
     In a continuation of the operation, the situation shown in FIG. 8B is reached with A replacing B and with trailing edge being held by gripper  60  and the leading edge lying on belt  218 . 
     In alternate preferred embodiments of the invention, belt  218  is provided with vacuum grippers or other mechanical grippers which acquire the sheet such that it is carried towards gripper  60  by the belt and not by the arms. In this situation, it is possible to provide only a single set of arms. 
     Alternatively or additionally the impression roller is enlarged somewhat so that the length of sheet  48  is only about 70% or less than the circumference of impression roller  42 . Under these conditions, the single set of arms will have enough time to drop one sheet and move to the position at which it has to pick-up the sheet. 
     Alternatively or additionally, the impression roller is enlarged such that it holds two sheets at the same time. For this configuration, only a single arm is necessary, especially if belt  218  is supplied with grippers. 
     After the reverse of sheet  48  is printed, the reverse of sheet  48 ′ is to be printed. The attachment of sheet  48 ′ to impression roller  42  is as described above for sheet  48 . Sheet  48  is now removed from the printer according to the process whose start is shown in FIGS. 8G and 8H. In FIG. 8G sheet  48 ′ is advancing toward nip  216  and sheet  48  has been gripped by arms A. In general, the position of grippers  60  and the arms is similar to that shown in FIG. 8D, except that arm A does not stop at this point but continues carrying sheet  48  to the position shown in FIG.  8 H. 
     As shown in FIG. 8H, in a preferred embodiment for the invention, sheet  48  is advanced so that it meets gripper arms  220 , which may be of the same type as arms A and B. Gripper arms  220  transfer the sheet directly to exit stack  56 . See FIG.  6 . Alternatively, if it is desired to invert the sheet before stacking, an addition set of gripper arms  222  receives the sheet from arms  220  and delivers it to the stack. Alternatively, a belt may receive the sheet from arms A and transfer it to the stack. Alternatively or additionally, stacker  56  may be replaced by a finisher which produces booklets directly from the sheets as they are printed. 
     FIG. 9 shows a tandem printer  300  for duplex printing. Printer  300  is comprised of first printer section  301 , second printer section  302  and transfer section  303 . After one side of a sheet is printed in first printer section  301  the sheet is removed from impression roller  42  by a vacuum gripper arm  306 . Gripper arm  306  transfers the sheet to a second gripper arm  308 . The sheet is then transferred to gripper arms  310 , which in turn transfer the sheet to gripper arms  312 . The sheet is then transferred to an impression roller  42 . It should be noted that the sheet is image side down on arms  306 , image side up on arms  308 , image side down on arms  310  (note the reversal of direction of the sheet), image side up on arms  312 . The sheet is placed image side down on the second impression roller such that the inverse side of the sheet is printed in the second printer. 
     In each of the above embodiments, grippers  60  are shown as mechanical grippers. However, in alternative preferred embodiments of the invention, air suction grippers may be used on impression roller  42 . 
     The elements of engine  100  or the printer section of FIGS. 2-9 may be purely conventional as has been described in numerous patents, patent applications and patent publications assigned to the assignee of the present application, Indigo, Nev. and Spectrum Sciences B.V. In addition certain parts of the preferred embodiment of the invention including intermediate transfer blankets, photoreceptor sheets, etc. are available from Indigo, Nev. 
     Some of such elements are described, for example. in PCT publications WO 94/23347, WO 96/17277, WO 97/07433, in U.S. Pat. No. 4,684,238, PCT Publication WO 90/04216, U.S. Pat. No. 4,974,027 and WO 93/01531 and in other patents and applications referred to therein. The disclosures of all these documents are incorporated herein by reference. 
     FIG. 10 shows a preferred embodiment of a charger  120  corresponding to charger  104  of FIGS. 5 and 6. The charger shown comprises six corotrons or scorotrons, each comprising a charging surface such as a charged wire  122  and grid  124  for scorotrons, although a greater or lesser number may be used as required. Each pair of scorotrons is preferably housed in a housing  126  including a chamber  128  into which air is pumped. This air is forced by pressure past wires  122  and onto the surface of photoreceptor  120 . This flow of air carries away evaporated carrier liquid which otherwise has a tendency to coat the wires and reduce their life. In addition, this flow also caries away ozone which is generated by the charging surface. 
     In order to prevent the air (now containing some carrier liquid and/or ozone) from contaminating the surroundings, both inside the printer and outside of it, chambers  130  are provided, beside the scorotrons. These chambers are connected to suction pumps, such that air fed to chambers  128  and passing wires  122  to the surface of drum  102  is immediately removed from the environment. In a preferred embodiment of the invention, carrier liquid and/or ozone are removed from the air suctioned via chambers  130 , for example by catalytic action. 
     FIG. 11 shows a preferred embodiment of a developer  140  corresponding to developer  108  of FIGS. 5 and 6. This developer corresponds generally to developers whose structure and operation is shown and described in WO 93/01531 and WO 95/10801, the disclosures of which are incorporated herein by reference. Developer  108  comprises a toner inlet  142  which feeds toner concentrate to a toner chamber  144 . Toner is fed from chamber  144  to a rotating developer roller  146 . The rotation of developer roller  146  pumps the toner past an electrode  148 . A voltage difference between electrode  148  and roller  146  preferably coats roller  146  with a concentrated layer of toner. A squeegee  150  preferably removes additional liquid from the toner layer which layer is then selectively transferred to develop a latent image on photoreceptor  102 . Toner remaining on developer  146  is preferably removed by a charged roller  152  (see for example element  174  in FIG. 7B of WO 93/01531). Toner is preferably removed from roller  152  by the combined action of a scrapper  154  and a counter rotating sponge roller  156 . A squeegee  158  preferably compresses sponge roller  156  and removes excess material from it into a waste chamber  159 . Other designs of liquid development systems or powder toner systems may be substituted for developer  140  if desired. 
     FIG. 12 shows further details of print engine  100  and printer  200 . In preferred embodiments of the invention, especially where the printing speed is high, it is desirable to dry the image somewhat while heating it on intermediate transfer member  110 . To this end, a dryer  160  (corresponding to dryer  114  of FIG. 5 or  6 ) is preferably provided. To minimize the amount of pollution generated, dryer  160  preferably comprises a chamber  162  into which air is pumped via an inlet  164 . The air exits chamber  162  via an exit slit  166  onto the surface of transfer member  110 . The air which exits slit  166  preferably forms an air knife. A second chamber  168 , open to the surface of the transfer member, is provided with an exit for air through which air is withdrawn via an exit port  169 . Thus, excess carrier liquid that is withdrawn from the image on intermediate transfer member  110  is immediately removed without polluting the internal environment of the printer. 
     To improve transfer of images and to provide more consistent transfer, intermediate transfer member  110  is preferably provided with a further dryer  170  (corresponding to dryer  116  of FIG. 5 or FIG.  6 ), which dryer operates in a similar manner to dryer  160 , in that air is forced onto the surface of the intermediate transfer member and is removed therefrom by suction. 
     In preferred embodiments of the invention, carrier liquid removed by dryers  160  and  170  is removed from the air stream, for example by catalytic action and the air is recirculated for drying. 
     FIG. 13 shows a cleaning station  180  corresponding to cleaning station  112  of FIG.  5 . Cleaning station  180  comprises three stages. In a first stage cooled liquid (for example carrier liquid) is supplied to the surface via a chamber  182 . A roller  184  is operative to keep the liquid from leaking out of the cleaner and for pumping it in the upstream direction of photoreceptor  102 . The cooled liquid flows along the surface of the photoreceptor to a counter-rotating sponge roller  184  which removes adhering toner particles. These particles and liquid picked up by the sponge roller are squeegeed out of sponge roller  184  by a squeegee roller  186 . A scrapper blade  188  completes the cleaning process by scrapping any remaining toner from the surface and keeping excess carrier liquid from leaving the cleaning station. 
     While preferred printing engines have been shown and described, it should be understood that duplex printers of the type described above may use other types of electrographic printers as are known in the art. Thus, the printing engines may be of any suitable type. Preferably, the engines are of a type which produces images under control of a computer such that the images may be changed from print to print. Such printers are generally known as “digital” printing engines. Furthermore, while in the preferred embodiment of the invention, image transfer utilizing an intermediate transfer member is described, such transfer may be replaced by direct transfer from an imaging surface. 
     While the present invention has been described with respect to preferred embodiments thereof, these embodiments are presented by way of example only and are not meant to limit the scope of the invention which is defined by the claims. Furthermore, embodiments of the invention may incorporate some but not all features of the above preferred embodiments and may include combinations of features from different embodiments. As used in the claims the terms “comprise” or “include” and their conjugations shall mean “including but not necessarily limited to.”