Patent Publication Number: US-8967789-B2

Title: Spreader/transfix system for handling tabbed media sheets during duplex printing in an inkjet printer

Description:
TECHNICAL FIELD 
     This disclosure relates to inkjet printers and, more particularly, to transferring ink images to media in these printers. 
     BACKGROUND 
     Drop on demand inkjet printing systems eject ink drops from printhead nozzles in response to pressure pulses generated within the printhead by either piezoelectric devices or thermal transducers, such as resistors. The ink drops are ejected toward an image receiving surface where each ink drop forms a pixel of an ink image on the image receiving surface. The printheads have a plurality of inkjet ejectors that are fluidly connected at one end to an ink supplying manifold through an ink channel and at another end to an aperture in a face plate of the printhead. 
     In some phase change or solid ink printers, known as direct printers, the printer ejects ink drops directly onto a print medium such as a paper sheet. After ink drops are printed on the print medium, the printer moves the print medium through a nip formed between two rollers that apply pressure and optionally heat to the ink drops and print medium. One roller, referred to as the “spreader roller” contacts the printed side of the print medium. The spreader roller is heated and coated with a release agent that prevents ink drops on the print medium from transferring onto the spreader roller. The second roller is referred to as a “pressure roller.” This roller presses the media against the spreader roller. The pressure roller may be optionally heated to facilitate the fixing of the ink to the sheet of print medium. The heat and pressure applied through the nip flattens the ink drops and secures the printed ink image to the print medium in a process known as “fixing.” 
     In an indirect printing embodiment, the printheads eject ink drops onto the surface of an intermediate image receiving member such as a rotating drum or endless belt. A “transfix” roller is positioned against the intermediate image receiving member to form a transfix nip. As a media sheet passes through the transfix nip in synchronization with the ink image on the intermediate image receiving member, the ink image transfers and fixes to the media sheet under pressure and heat in the transfix nip. The transfer and fixation of the ink image are well known to the art and are referred to as a transfix process. 
     Both direct and indirect inkjet printers are capable of producing either simplex or duplex prints. Simplex printing refers to production of an image on only one side of a print medium. Duplex printing produces an image on each side of a media sheet. In duplex direct printing, an ink image is formed on a first side of the media sheet, which then passes through the spreader nip to fix the ink image onto the first side of the media sheet. The medium is then inverted and sent along a path that passes the second side of the media sheet by the printheads for the formation of a second ink image on the second side. The sheet then returns to the spreader nip where the second ink image is fixed to the second side of the media sheet. A similar process is used with indirect printing, except the image is initially formed on an intermediate drum and then transferred to the media and fixed in the nip at the same time. 
     In both direct and indirect printing systems, having significant levels of oil on the media before imaging is undesirable, as the release agent can prevent ink from properly adhering or transferring to the media. Therefore, in a duplex printing process, preventing the release agent from transferring to the back side of a sheet during printing of the first side image is desirable. To achieve this goal, current printing systems slow down the transfix process and use special sheet and nip formation sequencing during duplex printing to prevent release agent from being transferred to the back of a sheet during front side printing. One technique for minimizing this problem is synchronizing the transfix or pressure rollers with the media sequencing so that the portion of the roller that contacts the back of the media sheet only contacted another media sheet on the previous revolution. The portion contacted was thus not in direct contact with the intermediate drum or the spreader roller, which would have transferred excess oil to the transfix or pressure roller surface and thus to the back of the present sheet. Unfortunately, synchronization of the rollers may not prevent release agent from transferring to media sheets having non-uniform edges, such as media sheets having extended tabs, pre-punched holes, or different sizes. Consequently, improved operation of direct and indirect printers that addresses the limited ability of current printers to keep release agent from tabbed, hole punched, and other non-uniform sized media sheets would be beneficial to higher throughput and image quality in such printers. 
     SUMMARY 
     In one embodiment, a method of operating a printer to avoid release agent being transferred to non-uniform structured or sized media has been developed. The method includes operating a media transport to move media sheets through a nip formed between a first roller and a second roller; applying release agent with an applicator to the first roller only; and adjusting operation of the media transport to insert a leading edge of a media sheet into the nip as a first portion of the second roller on which release agent transferred from the first roller exits the nip in response to the media sheet being different than a previous media sheet that passed through the nip to enable the media sheet to be interposed between the first roller and a second portion of the second roller bearing substantially less release agent than the first portion as the media sheet passes through the nip. 
     In another embodiment, a printer that avoids release agent being transferred to non-uniform structured or sized media has been developed. The printer includes a media transport, a release agent applicator, and a controller. The media transport includes a plurality of actuators, each actuator configured to drive a roller in the media transport to move media sheets through a nip formed between a first roller and a second roller. The release agent applicator is configured to apply release agent to the first roller only. The controller is operatively connected to the plurality of actuators of the media transport, and is configured to generate electrical signals to adjust operation of the media transport to insert a leading edge of a media sheet into the nip as a first portion of the second roller on which release agent transferred from the first roller exits the nip in response to the media sheet being different than a previous media sheet that passed through the nip to enable the media sheet to be interposed between the first roller and a second portion of the second roller bearing substantially less release agent than the first portion as the media sheet passes through the nip. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects and other features of a media path in a printer that controls the distribution of release agent between rollers that engage media sheets is explained in the following description taken in connection with the accompanying drawings. 
         FIG. 1  is a schematic view of a direct printer. 
         FIG. 2  is a schematic view of a spreader roller and a pressure roller of the direct printer depicted in  FIG. 1 . 
         FIG. 3A  is a view of a pitch of a pressure roller in a printer showing a position of a first tabbed media sheet on the pitch. 
         FIG. 3B  is view of the pitch of  FIG. 3A  showing a position of a second media sheet positioned on the pitch. 
         FIG. 3C  is view of the pitch of  FIG. 3A  showing a position of a third media sheet positioned on the pitch. 
         FIG. 4A  is a view of a pitch of another pressure roller in a printer showing a position of a first tabbed media sheet on the pitch. 
         FIG. 4B  is view of the pitch of  FIG. 4A  showing a position of a second media sheet positioned on the pitch. 
         FIG. 4C  is view of the pitch of  FIG. 4A  showing a position of a third media sheet positioned on the pitch. 
         FIG. 5  is a schematic view of an indirect printer. 
         FIG. 6  is a schematic view of an imaging drum and a transfix roller in the printer depicted in  FIG. 5 . 
         FIG. 7  is a schematic view of a single-pass indirect printer. 
         FIG. 8  is a block diagram of a process for operating a printer in a duplex printing mode. 
     
    
    
     DETAILED DESCRIPTION 
     For a general understanding of the environment for the system and method disclosed herein as well as the details for the system and method, reference is made to the drawings. In the drawings, like reference numerals have been used throughout to designate like elements. As used herein, the term “printer” encompasses any apparatus that produces images on media with one or more colorants for any purpose, such as a digital copier, bookmaking machine, facsimile machine, a multi-function machine, or the like. The systems and methods described below may be used with various printer embodiments. A direct printer ejects ink drops directly onto print media to form ink images on the media and subsequently fixes the ink image to the media sheet. An indirect printer forms an ink image on an intermediate image receiving member, such as a drum or endless belt, and transfers the ink image to a media sheet in a “transfix” operation that is well-known in the art. A single-pass indirect printer ejects images onto the intermediate image receiving member with no portion of the image passing by the ejectors more than once. A multi-pass indirect printer ejects portions of an image onto the image receiving member with each revolution, such that a complete image is ejected onto the image receiving member in two or more revolutions of the image receiving member. 
     A “media sheet” or “print medium” as used in this description may refer to any type and size of medium on which printers in the art produce images, including printer paper of various sizes. Each media sheet includes two sides, and each side may receive an ink image corresponding to one printed page. As used herein, the term “tabbed media sheet” refers to a media sheet containing a tab extending from one edge of the media sheet. The tabbed media sheet can be any size, including A 4 , letter, legal, or tabloid, and is generally the same size as other media sheets in a print job to enable the tab to extend from the completed print job to identify a section of the printed media. 
     As used herein, a “print job” or “document” is a set of related sheets, usually one or more collated copy sets copied from a set of original print job sheets or electronic document page images. A print job can contain data corresponding to a single size of media sheet, or multiple sizes of media sheets, some of which can be tabbed or hole-punched media sheets. An image generally includes information in electronic form, which is to be rendered into data used to generate signals that operate inkjet ejectors to form an ink image on an image receiving surface and can include text, graphics, pictures, and the like. 
     As used herein, the term “image receiving member” refers to any member having a surface that is configured to receive an ink image. In a direct printer, the image receiving member is typically print media, such as a paper sheet or continuous media web. In an indirect printer, the image receiving member is typically a rotating drum or endless belt that receives ink ejected by one or more printheads to form ink images. In a direct printer, a media transport carries print media along a media path past printheads in a print zone, while in an indirect printer the image receiving member rotates or moves past the printheads in a repeating manner. As used herein, the term “roller” refers to any cylinder or belt used in image fixation or transfer processes, for example, an image drum, image receiving belt, spreader roller, pressure roller, transfix roller, offset cylinder, impression cylinder, or fuser roller. 
     Phase change ink printers use phase change ink, also referred to as a solid ink, which is in a solid state at room temperature but melts into a liquid state at a higher temperature. The liquid ink drops are printed onto an image receiving member in either a direct or indirect printer. As described in more detail below, both direct and indirect printers apply a coating of release agent to selected components in the printer to prevent phase change ink from adhering to the printer components instead of the print medium. In one embodiment, the release agent is an oil such as silicone oil. 
       FIG. 1  depicts a direct inkjet printer  100  that controls a transfer of release agent between two rollers  132  and  136  while printing in a duplex mode. Printer  100  includes media supplies  104  and  108 , a media path  112 , a print zone  120 , a media sheet conveyor  114 , a spreader roller  132 , a pressure roller  136 , a media output tray  110 , and a controller  190 . The media supplies  104  and  108  are each configured to hold a plurality of media sheets and supply the media sheets to the printer via the media path  112  for printing. In the embodiment of printer  100 , the media supplies  104  and  108  can hold media sheets of different sizes. For example, the media supply  104  holds letter size (215.9 mm×279.4 mm) media sheets, while the media supply  108  holds letter size tabbed media sheets. In alternative configurations, either or both media supplies  104  and  108  hold media sheets having A4 size (210 mm×297 mm), legal size (216 mm×356 mm), tabloid size (279 mm×432 mm), letter, legal, A 4 , or tabloid size tabbed media sheets, or various other sheet sizes. Other embodiments can include more than two media supplies to enable the printer to store and print a variety of media sizes and types. Various printer embodiments move the media sheets in either a length or width orientation during printing. Thus, the “length” of a media sheet in the process direction can be either of the length or width dimensions commonly used to describe a media sheet size. For example, the length of a letter size media sheet in the process direction can be either 215.9 mm or 279.4 mm depending on the orientation of the media sheet as a media transport moves the media sheet in a process direction through the printer. Furthermore, a tabbed media sheet can have the tab extending from an edge of the length or width of the media sheet, and therefore can be inserted into the nip as a leading edge, trailing edge, or on an edge on a side of the media sheet. 
     During a print job, media sheets from one or both of the media supplies  104  and  108  move along the media path  112 . The media path  112  is a media transport that includes a plurality of guide rollers, such as guide rollers  116 , which engage each media sheet and move the media sheets through the printer  100 . In  FIG. 1 , the media path  112  guides each media sheet past a print zone  120  in a process direction for imaging operations on a first side of each media sheet. A portion of the media path  112 ′ reverses an orientation of the media sheets and directs the media sheets through the print zone  120  a second time in the process direction to enable the print zone  120  to print ink images during imaging operations on the second side of each media sheet. As described in more detail below, a portion of the media path  112  between the print zone  120  and the rollers  132  and  136  includes a series of variable speed conveyors  114 . 
     The print zone  120  includes a plurality of printheads arranged in a cross-process direction across a width of each media sheet. In  FIG. 1 , the print zone  120  includes a total of eight marking stations configured to print color images using a combination of cyan, magenta, yellow, and black (CMYK) inks. In the print zone  120 , marking stations  122 A and  122 B print magenta ink, marking stations  124 A and  124 B print cyan ink, marking stations  126 A and  126 B print yellow ink, and marking stations  128 A and  128 B print black ink. Various alternative configurations print with a single color of ink, or include different ink colors including spot colors. Each of the marking stations  122 A- 128 B includes a plurality of printheads, each one of which includes a plurality of inkjets. 
     The printheads in each set of marking stations  122 A- 122 B,  124 A- 124 B,  126 A- 126 B and  128 A- 128 B are arranged in interleaved and staggered arrays to enable printing over the entire cross-process width of a media sheet. For example, marking station  122 A includes one array of staggered printheads that print images at a resolution of 300 drops per inch (DPI) in the cross-process direction over a media sheet. Each printhead in the staggered array covers a portion of the width of the media sheet, and the printheads are aligned end-to-end in the cross-process direction to print a continuous line of ink drops across the media sheet. Marking station  122 B includes a second staggered array of printheads that are interleaved with the printheads in the marking station  122 A to enable both of the marking stations to print magenta ink with a combined resolution of 600 DPI in the cross-process direction. 
     In the print zone  120 , the printheads in each marking station eject liquid drops of a phase change ink. In one embodiment, the ink is supplied as a series of solid ink sticks to each of the marking stations  122 A- 128 B. A heater positioned in each marking station melts the ink to supply liquefied ink to the corresponding printhead array. As depicted in  FIG. 1 , each marking station includes a set of supporting electronics  123 . The electronics  123  include driver electronics, which generate the signals that operate the printheads in the marking station  122 A. The printheads are also supplied with ink from a supply. In one alternative configuration, two marking stations that print a single color of ink receive melted solid ink from a single supply. In another alternative configuration, the phase change ink is supplied in a plurality of granular pastilles rather than in the form of ink sticks. While printer  100  is depicted as using a phase-change ink, the methods described herein can also be used in xerographic printers using oiled fuser systems, to offset printers using oiled offset systems, and to inkjet printers using alternative forms of ink including aqueous, gel, solvent based, and UV curable inks. 
     A media sheet moves through the print zone  120  to receive an ink image and the media path  112  moves the media sheet out of the print zone  120  in the process direction. The printheads in marking stations  122 A- 128 B print ink drops onto a predetermined area of the surface of the media sheet as the media sheet moves through the print zone to form an ink image on the media sheet. A section of the media path  112  located after the print zone  120  includes one or more conveyors  114 . The conveyors  114  are configured to control the velocity of the media sheet in the process direction as the media sheet approaches a nip  134  formed between spreader roller  132  and pressure roller  136  and to shift the media sheet in the cross-process direction. As described in more detail below, the printer  100  controls the rotation of the rollers  132  and  136  and the movement of media sheets on the conveyors  114  to enable each media sheet to pass through the nip  134  with minimal re-transfer of release agent to a non-imaged side of the media sheet during duplex print operations. 
       FIG. 2  depicts the rollers  132  and  136  in the printer  100 . Media sheets pass through the nip  134  formed between the rollers  132  and  136 . In the embodiment of printer  100 , both the spreader roller  132  and pressure roller  136  apply pressure to media sheets as the media sheets pass through the nip  134 . The spreader roller  132  engages the side of the media sheet that carries the ink drops formed on the sheet in the print zone, and the pressure applied to the media sheet spreads and fixes the ink to the media sheet. An actuator  133  rotates the spreader roller  132  to move media sheets in the process direction, and the friction between the rollers generates a counter-rotation in the pressure roller  136 . In other embodiments, a separate drive motor rotates the pressure roller  136  to position the pressure roller  136  accurately during periods when the nip is split or opened, for example, between print jobs. The side of each media sheet holding an ink image printed in the print zone  120  contacts the spreader roller  132 , while pressure roller  136  contacts the opposite side of the media sheet. The rollers  132  and  136  apply pressure, and optionally heat, to the media sheet as the media sheet moves through the nip  134 . The pressure and heat flatten individual ink drops formed on the media sheet so that the ink image formed on the media sheet is “fixed” to the sheet in a durable manner. A release agent  152  coats the surface of the spreader roller  132  that contacts the ink image on each media sheet. The release agent  152  is typically an oil, such as silicone oil, which prevents ink from adhering to the surface of the spreader roller  132 . A drum maintenance unit  140  includes a reservoir holding the release agent. In the configuration of  FIG. 2 , two applicator rollers  144  and  148  apply the release agent  152  in a coating formed around the spreader roller  132 , although alternative embodiments use different mechanisms to apply the release agent. 
     During operation, the rotational position of the pressure roller  136  can optionally be monitored by a rotational sensor including an optical encoder disk  160  and a sensor  164 . The optical encoder disk is axially mounted to the pressure roller  136  and rotates with the pressure roller  136 . As the optical encoder disk  160  rotates, the encoder  160  interrupts a light beam generated in the sensor  164 , which generates signals corresponding to the interruptions in the light beam. The signals generated in the sensor  164  can identify both the rotational velocity of the pressure roller  136  and the rotational position of the pressure roller  136 . In an alternative embodiment, the optical encoder disk includes a predetermined pattern of light and dark segments that alter the reflection of light from the surface of the optical disk to the sensor  164  as the optical encoder rotates. In still another embodiment, the pressure roller  136  is configured with a Hall Effect sensor. In an embodiment without a rotational sensor, the system uses the known diameter of the pressure roller and the timing used in the system to identify the rotational position of the roller and oil free areas of the roller. 
     During a print job where a series of media sheets pass through the nip  134 , a portion of the release agent  152  formed on the spreader roller  132  transfers to the pressure roller  136  at areas of the rollers outside the width of the media sheet and when the rollers rotate in contact with each other in gaps that separate consecutive media sheets. In  FIG. 2 , release agent forms patches  168  and  172  on two portions of the surface of the pressure roller  136  as the pressure roller  136  contacts the spreader roller  132  between media sheets. The circumferential distance between the two patches corresponds to the length of the media sheets. As used herein, both of the terms “patch of release agent” and “portion of a surface roller having release agent” refer to an area on a roller that has a significantly greater amount of release agent than the other portions of the roller. As used herein, the term “portion on a surface of a roller bearing substantially less release agent” refers to a portion of a roller that has a lesser amount of release agent than the portion of the roller having release agent because the majority of the release agent in that portion has been transferred to a previous media sheet. A small amount of release agent may be present across the entire surface of the roller. In addition, while the patches  168  and  172  contain release agent across a longitudinal length of the pressure roller  136 , the entire pressure roller  136  includes significant amounts of release agent on the outer sections of the pressure roller  136 , outside the areas ordinarily contacted by media sheets  156 . 
     In the configuration of  FIG. 2 , the pressure roller  136  has an outer circumference that is greater than twice the length of each media sheet  156  in the process direction, and the pressure roller  136  engages two different media sheets during each rotation in a “two-pitch” configuration. As used herein, the term “pitch” refers to a portion of a surface of a roller that engages a media sheet and a gap between one media sheet and a subsequent media sheet during a single rotation of the roller. The term pitch is often referenced in conjunction with a numerical designation. For example, in a single-pitch configuration, a roller engages one media sheet during a single rotation. The roller has a circumference that is longer than a length of the sheet in the process direction, so a section of the single-pitch does not engage the media sheet. As described below, the section of the pitch that does not engage the media sheet can contact another roller and accumulate a patch of release agent. 
     A roller with an integer, non-fractional, number of pitches engages the entire length of an integer number of media sheets during a single rotation. In a two-pitch embodiment, the pressure roller has a circumference that is larger than two times a length of a letter size media sheet in the direction of roller rotation. The two-pitch roller engages two media sheets during a single rotation with gaps on the roller separating the two media sheets. Rollers having different circumferences and media sheet sizes can accommodate three or more pitches as well. A single roller can operate as a single-pitch or multi-pitch roller for different sizes of media sheets and gaps between the media sheets in various print modes. In one print mode, the media transport in the printer  100  is operated in a two-pitch configuration to insert a leading edge of a next letter size media sheet into the nip as the identified portion of the surface of the pressure roller bearing substantially less release agent enters the nip. 
     The printer  100  controls the rotation of the rollers  132  and  136  and the speed of the media sheets  156  in the media path  112  to position a leading edge of each media sheet in the nip as one portion of the pressure roller  136  carrying the release agent exits the nip  134 . For example, in  FIG. 2 , a leading edge  157  entered the nip  134  as the release agent patch  168  exited the nip. The media sheet  156  primarily contacts one portion of the pressure roller  136  that is between the release agent patches  168  and  172 . In a duplex print mode, the spreader roller  132  fixes the first printed side of the media sheet  156 , and the second side of the media sheet  156  exits the nip  134  receiving minimal release agent from the pressure roller  136 . A subsequent media sheet  158  enters the nip  134  as the release agent patch  172  exits the nip  134 . Consequently, the print zone  120  prints an ink image on the second side of the media sheets in a duplex mode with minimal dropout or reductions in image quality due to release agent contamination on the second side of each media sheet. 
     The printer controller is configured to operate the media transport to position a media sheet that is different than a previous media sheet at a position to enable the portions of the second side of the media sheet that are to receive ink drops in the second-side printing operation to receive minimal release agent transfer during the first-side imaging operation. The controller operates a plurality of actuators in the media transport to position the media sheet at the desired position longitudinally on the pressure or transfix roller. The actuators move the media sheet into the nip to enable the media sheet to enter the nip at a location that minimizes the potential for pixel dropout on the second side of the media sheet. 
     As discussed in detail below, the release agent transfer to a tabbed media sheet can be minimized by positioning the tab of the media sheet at an edge of the portion bearing substantially less release agent, enabling the second side of the tab and the majority of the media sheet to not receive release agent. For a media sheet having a size different from the previous media sheet printed, the controller can be configured to analyze the image data corresponding to the placement of the image on the second side of the media sheet. During the first-side printing operation the media sheet is positioned to enable most or all of the areas that receive ink in the second-side imaging operation to contact only the portion of the pressure or transfix roller bearing substantially less release agent. A similar media placement algorithm can be used for media sheets having holes punched or having irregular edges or shapes, and to place media sheets following the irregular sheet to minimize pixel dropout. Alternatively, the controller can be pre-programmed with instructions to place particular sizes and types of media sheets in predetermined positions corresponding to known printing patterns and typical image coverage, without reference to image data for the current media sheet. The controller can also be configured to receive user instructions corresponding to sheet placement and areas of the image to receive high priority as the controller determines the optimal image placement. 
       FIG. 3A-3C  illustrate one pitch  200  of a roller, such as the pressure roller  136  or transfix rollers  319  or  632 , and the longitudinal and circumferential position of media sheets on the roller. The vertical direction in  FIG. 3A-3C  represents the circumferential length of the pitch on the roller, while the horizontal direction represents the axial length of the roller. In a multi-pitch roller two or more pitches, such as pitch  200 , are positioned around the circumference of the roller, or stacked vertically in the representation of  FIG. 3A-3C . The pitch  200  includes an area that was contacted by a previous media sheet in a nominal position  240  ( FIG. 3B-3C ), represented by areas  212 ,  216 ,  220   a , and  220   b , and an area containing release agent  204  and  208  that was transferred to the roller by contact with another roller containing release agent, such as spreader roller  132  or imaging drums  312  or  628 . The areas contacted by the previous media sheet  212 ,  216 ,  220   a , and  220   b  contain substantially less release agent than the other areas of the pitch, as the previous media sheet contacting the pitch  200  prevented the roller from receiving release agent and collected the majority of release agent on the roller in the areas  212 ,  216 ,  220   a , and  220   b.    
     When the controller receives an image to be printed on a tabbed media sheet having a tab on an edge on a side of the media sheet in a duplex print job, the controller generates signals to operate the actuators of the media transport to prevent the second side of the tab from receiving release agent. One or more actuators of the media transport shift the media sheet to align an edge of the tab with an edge of the area contacted by the previous media sheet  212 ,  216 ,  220   a , and  220   b . The leading edge of the media sheet is fed through the nip as the area contacted by the previous media sheet  212 ,  216 ,  220   a , and  220   b  enters the nip. The second side of the tabbed media sheet therefore contacts areas  208 ,  212 , and  216  on the pitch  200 , as shown in  FIG. 3A , enabling the tab to contact only the area on the roller that contacted the previous media sheet to prevent transfer of release agent to the second side of the tab. Release agent transfers only to area  208  on an edge of the tabbed media sheet opposite the extended tab. In general, tabbed media sheets are part of a print job that is bound or three-hole punched near the edge opposite the tab, and therefore ink is usually not ejected on the edge of the media opposite the tab. Thus, ink can be ejected onto the second side of the tab of the media sheet without pixel dropout, while the pixel dropout on the opposite edge of the media sheet is minimal. 
       FIG. 3B  illustrates the placement of a second media sheet in the pitch  200  after the first tabbed media sheet contacts the pitch  200 . The second media sheet is the same size as the first media sheet, but does not include a tab. The area contacted by the first tabbed media sheet  232   a ,  232   b ,  236   a , and  236   b  is essentially clean of release agent, as the release agent in areas  232   a ,  232   b ,  236   a , and  236   b  was transferred to the first media sheet. Thus, significant amounts of release agent are only present on the pitch in areas  224 ,  228   a , and  228   b . In order to move the alignment of media sheets back to the nominal position  240 , the controller operates the media transport to position the second media sheet slightly toward the nominal position  240  from the area contacted by the edge of the first media sheet opposite the tab. Thus, the second sheet contacts areas  228   a ,  228   b ,  232   a , and  232   b  on the roller, collecting release agent only from areas  228   a  and  228   b . The size of areas  228   a  and  228   b  can be selected such that the areas of the media sheet that collect release agent are outside the printed region of the second side of the second media sheet. In one practical embodiment, the width of the areas  228   a  and  228   b  is approximately two millimeters, although different widths can be used in other embodiments depending on the characteristics of the print job and the width of the tab on the first media sheet. 
       FIG. 3C  depicts the placement of a third media sheet on the pitch  200  of the roller. As the third media sheet is fed into the nip, the pitch  200  contains release agent on areas  244  and  248 , while areas  252  and  256  that were contacted by the second media sheet are substantially clean of release agent. The media transport again aligns the media sheet slightly toward the nominal position  240  from the area clean of release agent  252  and  256 , to enable the third media sheet to contact areas  248  and  252 . The controller operates the media transport with reference to the nip to control the size of area  248  so release agent is transferred only to areas that are not be printed on the second side of the third media sheet. Alternatively, the controller can operate the media transport to keep the width of area  248  the same as the width of areas  228   a  and  228   b  and move the sheets uniformly toward the nominal position  240 . The controller continues to instruct the media transport to shift subsequent media sheets toward the nominal position  240  until a media sheet is aligned with the nominal position, at which point the following media sheets are positioned at the nominal position on the pitch until another media sheet having a tab, hole punch, or different size is printed. In the illustrated embodiment, the media transport is operated to return the media sheets to the nominal position after approximately three media sheets. In other embodiments, the media transport can be operated to align the sheets to require more or fewer media sheets to return to the nominal position depending on the width of the tab and the characteristics of the print job. In one practical embodiment, printing on a tabbed media sheet with a twelve millimeter wide tab, the media sheets are returned to the nominal position after printing six sheets, each media sheet being shifted toward the nominal position approximately two millimeters from a previous media sheet. 
       FIG. 4A-4C  illustrate a single pitch  500  for a roller configured to print on a tab that is on the leading edge of a media sheet. In  FIG. 4A-4C , the vertical direction represents the circumference of the pitch in the process direction, with the leading edge of the paper contacting the pitch at the bottom portion of the figure, while the horizontal direction represents the longitudinal length of the roller. When the controller receives an image to be printed on a tabbed media sheet in a duplex print job with the tab on the leading edge of the media sheet, the printer operates the media transport to keep the second side of the tab from receiving release agent. The controller operates the media transport to alter the velocity of the media sheet as the media sheet approaches the nip to time the insertion of the leading edge of the tab with an edge of the area contacted by the previous media sheet  512 ,  516 ,  520   a , and  520   b , which is in a nominal position  540  ( FIG. 4B-4C ). The nominal position refers to the position media sheets are placed on the pitch in print jobs not containing tabbed media sheets, and can be centered across the longitudinal length of the roller and along the circumferential length of the pitch. The leading edge of the tab is fed through the nip as the area contacted by the previous media sheet  512 ,  516 ,  520   a , and  520   b  enters the nip to enable the second side of the tabbed media sheet to contact areas  508 ,  512 , and  516  on the pitch  500 , as shown in  FIG. 4A . The tab contacts only the area on the roller that contacted the previous media sheet, enabling the media sheet to pass through the nip with minimal transfer of release agent to the second side of the tab. Release agent transfers only to the media sheet from area  508  on an edge of the tabbed media sheet opposite the extended tab. In general, tabbed media sheets are part of a print job that is bound or three-hole punched near the edge opposite the tab, and therefore ink is usually not ejected on the edge of the media opposite the tab. Thus, ink can be ejected onto the second side of the tab of the media sheet without pixel dropout, while the pixel dropout on the opposite edge of the media sheet is minimal. 
       FIG. 4B  illustrates the placement of a second media sheet in the pitch  500  after the first tabbed media sheet passes through the nip. The second media sheet is the same size as the first media sheet, but does not include a tab. The area contacted by the first tabbed media sheet  532   a ,  532   b ,  536   a , and  536   b  is essentially clean of release agent, as the release agent in areas  532   a ,  532   b ,  536   a , and  536   b  was transferred to the first media sheet. Thus, significant amounts of release agent are only present on the pitch in areas  524 ,  528   a , and  528   b . In order to move the alignment of media sheets back to the nominal position  540 , the controller operates the actuators of the media transport to adjust the velocity of the second media sheet and time the entrance of the second media sheet into the nip slightly after the nominal position  540 . Thus, the second sheet contacts areas  528   a ,  528   b ,  532   a , and  532   b  on the roller, collecting release agent only from areas  528   a  and  528   b . The size of areas  528   a  and  528   b  can be selected such that the areas of the media sheet that collect release agent are outside the printed region of the second side of the second media sheet. In one practical embodiment, the width of the areas  528   a  and  528   b  are approximately two millimeters, although different widths can be used in other embodiments depending on the characteristics of the print job and the width of the tab on the first media sheet. 
       FIG. 4C  depicts the placement of a third media sheet on the pitch  500  of the roller. As the third media sheet is fed into the nip, the pitch  500  contains release agent on areas  544  and  548 , while areas  552  and  556  are substantially cleaned of release agent. The controller adjusts operation of the media transport to regulate the velocity of the approaching third media sheet so the sheet enters the nip slightly after the nominal position  540 , but slightly before the portion contacted by the second media sheet  552  and  556  to enable the third media sheet to contact areas  548  and  552 . The operation of the media transport is controlled so area  548  has a size that transfers release agent only to areas that are not be printed on the second side of the third media sheet. Alternatively, the width of area  548  can be the same as the width of areas  528   a  and  528   b  to move the media sheets uniformly toward the nominal position  540 . The controller continues to instruct the media transport to time subsequent media sheets to enter the nip closer to the nominal position  540  until a media sheet coincides with the nominal position  540 , at which point the following media sheets are inserted into the nip as the nominal position enters the nip until another tabbed, hole-punched, or differently sized media sheet is printed. In the illustrated embodiment, the media sheets return to the nominal position after approximately three media sheets. In other embodiments, the media transport is operated by the controller to time the sheets to require more or fewer media sheets to return to the nominal position depending on the width of the tab and the characteristics of the print job. In one practical embodiment, printing on a tabbed media sheet with a twelve millimeter wide tab, the media sheets are returned to the nominal position after printing six sheets, each media sheet being positioned approximately two millimeters closer to the nominal position from the previous media sheet. 
     In another embodiment, where no ink is to be ejected on an area of the edge of the second side of the next media sheet having a width equal to the width of the tab, the next media sheet can be returned to the nominal position, with subsequent media sheets then printed at the nominal position without release agent transfer. In other embodiments, the distance of the offset of subsequent media sheets can be selected by the user. In still other embodiments, the controller determines the optimal placement of subsequent media sheets to reduce the number of media sheets needed to return to the nominal position without release agent transfer issues based on image content, ink locations, and media sheet types. 
     While  FIG. 4A-4C  were described above with reference to the tab on the media sheet being on the leading edge of the sheet, the media transport can be operated to position a media sheet including a tab on a trailing edge of the media sheet such that the tab does not collect release agent. Instead of operating the media transport to time the entrance of the leading edge of the media sheet into the nip as the clean area on the pitch enters the nip, the media transport times the entrance of the sheet to enter the nip before the clean area on the pitch enters the nip to enable the trailing edge of the tab to exit the nip as the trailing edge of the clean area exits the nip. Subsequent sheets are then inserted into the nip after the clean area enters the nip until the clean area returns to the nominal position. 
     In another embodiment, the media transport is configured not to return the media sheets to the previous nominal position. Instead, a new nominal position is established in the area contacted by the rectangular portion of the tabbed media sheet, for example, areas  508  and  512  of  FIG. 4A . Subsequent media sheets are timed to enter the nip and positioned to contact the areas of the new nominal position. In a print job including successive tabbed media sheets, the tabs can be positioned to contact the area contacted by the previous tab. Alternatively, if the tab is not at the same position on the media sheet as the previous tab, then the tab can be aligned at the edge of the new nominal position to enable minimal transfer of release agent to the tab, and another new nominal position is established for the subsequent media sheet. 
     In some multi-pitch configurations, the printer is operated by the controller to provide an alternating sequence of media sheets to the nip to further control the transfer of release agent to a roller, such as pressure roller  136  or transfix rollers  319  or  632 , in a duplex print mode. Referring to  FIG. 2  and  FIG. 6 , the media sheets pass through the nip in an interleaved order where one sheet passes through the nip during a first side imaging operation and the following media sheet passes through the nip during a second side imaging operation. The alternating sequence of first and second side media sheets continues during the print job. For example, in  FIG. 2 , a first side image formed on the media sheet  156  is fixed to the sheet as the sheet passes through the nip  134 . The next media sheet  158  has previously undergone first side imaging, and a second side image is fixed to the second sheet  158  as the second media sheet  158  passes through the nip  134 . In  FIG. 6 , the ink image  420  transfixes to the first side  443  of media sheet  440  as the media sheet  440  passes through the transfix nip  318 , and the ink image  424  transfixes to a second side  448  of the next media sheet  446 . Various configurations of the direct printer  100  and the indirect printer  300  sequence media sheets in an alternating first side and second side order. During the beginning of a print job, the printer operates in a reduced throughput print mode for a first number of media sheets until a sufficient number of media sheets with a first side image have been printed to enable the printer to provide the alternating sequence of first and second side media sheets to the nip. 
     The alternating media sheet sequence prevents a transfer of accumulated release agent from the pressure roller to an unprinted side of a media sheet during a duplex printing operation. During the second side printing, the previously printed first side of a media sheet contacts a pressure roller, for example, pressure roller  136  or transfix rollers  319  or  632 . Release agent that transferred to the media sheet during the imaging of the first side transfers to the roller as the media sheet passes through the nip a second time. While the amount of the release agent transferred to the roller is typically less than the amount of release agent present in the release agent patches on the roller, the release agent can still transfer to a second side of a media sheet prior to printing the second side. The alternating sequence of the media sheets ensures that the section of the pressure roller that accumulates release agent from the first sides of duplexed media sheets only contacts the previously printed sides of duplexed media sheets, while a separate section of the pressure roller only contacts blank sides of media sheets that are free of release agent during first-side printing. 
     During a print job, the pressure roller  136  contacts the spreader roller  132  and remains in contact with roller  132  as multiple media sheets pass through the nip  134 . An actuator  138  removes the pressure roller  136  from contact with the roller  132  between print jobs and during maintenance operations in the printer  100 . A cleaning process removes release agent and other contaminants from the pressure roller  136  when the pressure roller  136  is removed from contact with the spreader roller  132 . The actuator  138  moves the pressure roller  136  into engagement with roller  132  at the beginning of a print job. This engagement can be done quickly to minimize the transfer of release agent to the pressure roller  136 . 
     In the printer  100 , the controller and user interface  190  is operatively connected to various components and subsystems, including the media path  112 , the print zone  120 , the actuators  133  and  138 , and the sensor  164  that senses the rotation of the pressure roller  136 . The controller  190  receives and processes print job data that include image data and print job parameters. Exemplary print job parameters include the number of copies of the image data to be generated, the image and color quality levels of the printed images, and whether the printer should print the media pages in a simplex or duplex print mode. In some configurations the controller  190  receives the print job data through a network interface module  196 , while in alternative configurations, such as a photocopier, an optical scanner generates image data corresponding to one or more pages. One or more print job parameters may be entered via user input controls  192 , and a visual display  194  displays information about the status of a print job, ink and print media supply levels, and errors or other diagnostic information that pertain to the status of the printer  100 . 
     The controller  190  can be implemented with general or specialized programmable processors that execute programmed instructions, for example, printhead operation. The instructions and data required to perform the programmed functions can be stored in memory associated with the processors or controllers. The processors, their memories, and interface circuitry configure the controllers to perform the processes that enable the printer  100  to control the transfer of release agent during duplex printing. These components can be provided on a printed circuit card or provided as a circuit in an application specific integrated circuit (ASIC). Each of the circuits can be implemented with a separate processor or multiple circuits can be implemented on the same processor. Alternatively, the circuits can be implemented with discrete components or circuits provided in VLSI circuits. Also, the circuits described herein can be implemented with a combination of processors, ASICs, discrete components, or VLSI circuits. 
     During operation, the controller  190  generates electronic firing signals to operate individual inkjets in the printheads in each marking station  122 A- 128 B as the media sheet moves through the print zone  120 . The inkjets in the marking stations  122 A- 128 B eject individual ink drops in response to each firing signal to form an ink image on each media sheet. To generate color images, the printer  100  ejects ink drops of different colors in close proximity to one another on the media sheet to form “dithered” patterns that the human eye perceives as a wide gamut of colors. 
       FIG. 5  depicts an embodiment of an indirect phase change inkjet printer  300  including a multi-color printhead assembly  332  and multi-color printhead assembly  334 , rotating imaging drum  312 , transfix roller  319 , optical encoder disk  335  and controller  380 . As illustrated, the printer  300  includes a frame  311  to which the operating subsystems and components described below are mounted directly or indirectly. The indirect phase change inkjet printer  300  includes an intermediate image receiving member  312  that is shown in the form of an imaging drum, but in other embodiments is in the form of a supported endless belt. The imaging drum  312  has an image receiving surface  314  that is movable in the direction  316 , and on which phase change ink images are formed. A drum maintenance unit  394  includes a supply of release agent and applicators including rollers and metering blades that distribute a thin layer of release agent on the surface of the imaging drum  312 . A transfix actuator  341  moves the transfix roller  319  into and out of engagement with the imaging drum  312 . The transfix roller  319  rotates in the direction  317  when placed against the surface  314  of drum  312  to form a transfix nip  318  within which ink images formed on the surface  314  are transfixed onto a heated media sheet  349  that passes through the transfix nip  318 . 
     During operation, the rotational position of the transfix roller  319  is monitored by a rotational sensor including an optical encoder disk  335  and a sensor  337 . The optical encoder disk is mounted on an axle of the transfix roller  319  and rotates with the transfix roller  319 . The optical encoder disk  335  and optical sensor  337  operate in the same manner as the optical encoder disk  160  and sensor  164  depicted in  FIG. 2 . The controller  380  identifies the rotational position and rotational velocity of the transfix roller  319  with reference to the signals generated by the optical sensor  337 . 
     A media transport, depicted as media path  350 , includes a plurality of rollers, some of which are driven by actuators operatively connected to a controller  380 , and media guides that control the movement of media sheets such as media sheet  349  through the transfix nip  318  in a process direction  362  and a cross-process direction. The media path  350  further includes a duplex process direction  362 ′. In a duplex print mode, the printer  300  transfixes an ink image to a first side of a media sheet, and the media sheet moves through the media path  350  in the duplex process direction  362 ′ to invert the media sheet. The inverted media sheet passes through the transfix nip  318  a second time and the printer  300  transfixes a second ink image to the second side of the media sheet. 
     Operation and control of the various subsystems, components and functions of the printer  300 , including the media path  350  and printhead assemblies  332  and  334 , are performed with the aid of a controller or electronic subsystem (ESS)  380 . The ESS or controller  380 , for example, is a self-contained, dedicated computer having a central processor unit (CPU)  382  with a memory  383 , and a display or user interface (UI)  386 . The ESS or controller  380 , for example, includes a sensor input and control circuit  388  as well as an ink drop placement and control circuit  389 . In addition, the CPU  382  reads, captures, prepares and manages the image data flow associated with print jobs received from image input sources, such as the scanning system  376 , or an online or work station connection  390 , and controls the printhead assemblies  332  and  334 . As such, the ESS or controller  380  is the main multi-tasking processor for operating and controlling all of the other printer subsystems and functions. 
     The controller  380  can be implemented with general or specialized programmable processors that execute programmed instructions, for example, printhead operation. The instructions and data required to perform the programmed functions can be stored in the memory  383  associated with the processors or controllers. The memory  383  includes one or more digital data storage devices including, but not limited to, static and dynamic random access memory (RAM), magnetic and optical disk storage devices, read-only memory (ROM), and solid state data storage devices including NAND flash data storage devices. The processors, their memories, and interface circuitry configure the controllers to perform the processes, described more fully below, that enable operation of the imaging drum  312 , transfix roller  319 , optical sensor  337 , and media path  350  to perform duplex printing while controlling the transfer of release agent to media sheets. These components can be provided on a printed circuit card or provided as a circuit in an application specific integrated circuit (ASIC). The CPU  382  can be implemented as a special-purpose VLSI circuit, or can be a general purpose microcontroller or processor, for example, processors in the x86 and ARM families. Each of the circuits can be implemented with a separate processor or multiple circuits can be implemented on the same processor. Alternatively, the circuits can be implemented with discrete components or circuits provided in VLSI circuits. The circuits described herein can also be implemented with a combination of processors, ASICs, discrete components, or VLSI circuits. 
     The phase change ink printer  300  also includes a phase change ink delivery subsystem  320  that has multiple sources of different color phase change inks in solid form. Since the phase change ink printer  300  is a multicolor printer, the ink delivery subsystem  320  includes four (4) sources  322 ,  324 ,  326 ,  328 , representing four (4) different colors CMYK (cyan, magenta, yellow, and black) of phase change inks. The phase change ink delivery subsystem also includes a melting and control apparatus (not shown) for melting phase change ink from a solid state to a liquid state. Each of the ink sources  322 ,  324 ,  326 , and  328  includes a reservoir used to supply the melted ink to the printhead system  330 . In the example of  FIG. 3 , ink sources  322 ,  324 ,  326 , and  328  supply cyan, magenta, yellow, and black inks, respectively, to the multi-color printhead assemblies  332  and  334 . In some configurations, the imaging drum  312  completes two or more rotations as the printhead assemblies  332  and  334  form ink images on the imaging drum  312  in a multi-pass printing configuration. 
     The phase change ink printer  300  includes a substrate supply and handling subsystem  340 . The substrate supply and handling subsystem  340 , for example, may include sheet or substrate supply sources  342 ,  344 , and  348 , of which supply source  348 , for example, is a high capacity paper supply or feeder for storing and supplying image receiving substrates in the form of cut sheets  349 . In one configuration, the supply sources  342 - 348  store media sheets of different sizes such as letter, A 4 , legal, and tabloid media sizes, some of which can include tabs or punched holes. The printer  300  executes print jobs that specify the various media sheet sizes and types and the media supply path  350  extracts media sheets from one of the media sources  342 - 348  according to the media size and type specified in each print job. The substrate supply and handling subsystem  340  also includes the substrate media path  350  that has a substrate heater or pre-heater assembly  352 . The phase change ink printer  300  as shown can include an original document feeder  370  that has a document holding tray  372 , document sheet feeding and retrieval devices  374 , and a document exposure and scanning subsystem  376 . 
     In operation, the printer  300  receives a print job containing image data for one or more images from either the scanning subsystem  376  or via the online or work station connection  390 . Additionally, the controller determines and/or accepts related subsystem and component controls, for example, from operator inputs via the user interface  386 , and accordingly executes such controls. During a warm up operation at the beginning of the print job, the controller  380  can activate one or more heaters in the ink delivery subsystem  320  and the printhead assemblies  332  and  334  to provide molten ink to each of the printheads and inkjets in the printer  300 . The printer  300  performs a warm up operation subsequent to leaving a deactivated state or a low power sleep mode prior to commencement of the print job. 
     Printhead assemblies  332  and  334 , when activated by firing signals generated by the controller, eject ink drops onto selected locations of the imaging surface  314  to form ink images corresponding to the image data. Media sources  342 ,  344 , and  348  provide image receiving substrates that pass through substrate media path  350  to arrive at transfix nip  318  formed between the image receiving member  312  and transfix roller  319  in timed registration with the ink image formed on the image receiving surface  314 . As the ink image and media travel through the nip  318 , the ink image is transferred from the surface  314  and fixedly fused to the image substrate within the transfix nip  318 . During the imaging and transfixing operations, the controller  380  identifies the rotational position of the transfix roller  319  with reference to signals generated by the optical sensor  337  in response to rotation of the optical encoder disk  335 . The controller  380  identifies one or more sections of the transfix roller  319  that do not carry release agent using the optical sensor  337  and information stored in memory corresponding to placement of previously printed media sheets. The controller  380  also operates the actuators of the media path to regulate the position of the media sheets as described above with reference to  FIG. 3A-FIG .  4 C as the media sheets are supplied to the transfix nip  318 . 
       FIG. 6  depicts the imaging drum  312  and transfix roller  319  of  FIG. 5  in a two-pitch configuration where the printer  300  transfixes ink images to two media sheets during a single rotation of the transfix roller  319 . In the example embodiment of  FIG. 6 , the printer  300  forms two latent ink images  420  and  424  on a thin layer of release agent  432  that covers the surface of the imaging drum  312 . The transfix roller  319  engages the imaging drum  312  to form a transfix nip  318  with the transfix roller engaging the imaging drum  312  in an inter-document gap  433  formed between the ink images  420  and  424 . As used herein, the term “inter-document gap” refers to a portion of the surface of an image receiving member that is positioned between ink images corresponding to two different pages in a print job, or to a portion of the surface of the image receiving member that is positioned between two ends of a single ink image when a single ink image is formed on the image receiving member. 
     The imaging drum  312  rotates in direction  316  and the transfix roller  319  rotates in direction  317  as a first media sheet  440  approaches the transfix nip  318 . A patch of release agent  434  transfers from the imaging drum  312  to the transfix roller  319  as the transfix roller  319  rotates through the inter-document gap  433 . The leading edge  444  of a first media sheet  440  enters the transfix nip  318  according to the timing sequence discussed above with reference to  FIG. 3A-4C . The imaging drum  312  and transfix nip  319  rotate to transfix the ink image  420  to a first side  443  of the media sheet  440 . 
     In a single-pass printing configuration, the transfix roller  319  remains in contact with the imaging drum  312  through a second inter-document gap  435  that contacts the transfix roller  319  at the location of a second release agent patch  436  formed on the transfix roller  319 . A second media sheet  446  enters the transfix nip  318  as the second release agent patch  436  exits the transfix nip  318 , and the imaging drum  312  and transfix roller  319  transfix the second ink image  424  to the first side  448  of the media sheet  446 . 
     In a multi-pass configuration, the transfix roller  319  remains in contact with the imaging drum  312  through a portion of second inter-document gap  435  and the transfix actuator  341  subsequently disengages the transfix roller  319  from the imaging drum  312 . The printhead assemblies  332  and  334  form ink images on one or more defined areas of the imaging receiving surface  314  as the imaging drum  312  completes two or more rotations. The transfix actuator  341  re-engages the transfix roller  319  with the imaging drum  312  in a position within one of the inter-document gaps on the imaging drum  312  after the images are formed on each area of the image receiving surface  314  of the imaging drum  312 . Some multi-pass printer configurations include a transfix roller actuator that is operated by a controller that is configured to rotate the transfix roller  319  to engage a patch of release agent on the transfix roller  319  with the imaging drum  312  after ink images are formed on the imaging drum  312 . 
     In the embodiment of  FIG. 6 , the imaging drum  312  has approximately the same circumference as the transfix roller  319 . Alternative embodiments, however, include imaging drums with a wide range of sizes. The imaging drum can be the same size as the transfix roll or the drum can be sized such that an integer number of images can be formed around the circumference of the imaging drum.  FIG. 6  is referred to as a two-pitch configuration where two areas with minimal release agent are formed on the transfix roller  319 . Alternative transfix roller and media sheet sizes can operate with one, three, or more pitches around the transfix roller. The controller  380  identifies the rotational position of the transfix roller  319  with the optical sensor  337  and identifies the portions of the transfix roller  319  that carry the release agent patches  434  and  436  and the portions of the transfix roller  319  that do not carry release agent. The portions of the transfix roller  319  that do not contain release agent are determined by the controller from a combination of the information obtained from the optical sensor  337  and information stored in the controller memory corresponding to the placement of the previous media sheet that contacted a particular pitch. The controller  380  adjusts the rotation of the imaging drum  312  and the timing of the media path  350  as described above with reference to  FIG. 3A-4C  to enable tabbed and untabbed media sheets to be positioned on the transfix roller  319  where minimal release agent is transferred to the media sheet. Consequently, the second side of each of the media sheets  440  and  446  is substantially free of release agent prior to a duplex imaging operation. In the printer  300 , the transfix actuator  341  removes the transfix roller  319  from engagement with the imaging drum  312 . A transfix roller actuator  339  rotates the transfix roller  319  to a rotational position that enables a release agent patch formed on the transfix roller  319  to contact an inter-document gap on the imaging drum  312  at the beginning of another transfix operation. 
       FIG. 7  illustrates a single-pass indirect printer  600  including printheads  624 A- 624 H, a rotating imaging drum  628 , a transfix roller  632 , media supplies  604  and  608 , a media output tray  644 , and a controller  660 . The imaging drum  628  rotates in direction  680 , and has an image receiving surface on which ink images are formed. A drum maintenance unit  648  includes a supply of release agent and applicators including rollers and metering blades that distribute a thin layer of release agent on the surface of the imaging drum  628 . The transfix roller  632  is fixed in place and configured to contact the imaging drum  628  and rotate in direction  684  as the imaging drum  628  rotates in direction  680 , forming a transfix nip  636  within which ink images formed on the drum surface are transfixed onto a media sheet that passes through the transfix nip  636 . During operation, the rotational position of the transfix roller  632  is monitored by the controller  660 , which identifies the position of the transfix roller  632  from the known diameters of the roller  632  and drum  628  and the rotation of the imaging drum  628 . 
     In the single-pass printer  600 , printheads  624 A- 624 H eject one or more complete ink images onto the imaging drum  628  with each rotation of the imaging drum  628 . Each complete ink image is then transferred to a media sheet in the nip  636  as the drum rotates. The drum receives a complete image with every rotation, enabling the transfix roller  632  to remain in a fixed position engaged with the image drum  628 . The transfix roller  632  in the embodiment of  FIG. 7  is smaller than the imaging drum  628 , although in other embodiments, the transfix roller  632  can be the same size or larger than the imaging drum  628 . 
     Printer  600  includes a media transport, which removes media sheets from the media supplies  604  and  608  and delivers the media sheets through the nip  636  and to the output tray  644 . The media supplies  604  and  608  can include different sizes and types of media sheets, some of which can include tabs or punched holes. In other embodiments the printer can include more than two media supplies to enable the printer to print on a wide variety of media types and sizes. The media transport includes a plurality of rollers  612 , some of which are driven by actuators  614  operatively connected to a controller  660 , and media guides that control the movement of media sheets in a process direction  616  and a cross-process direction as the media sheets approach and pass through the transfix nip  636 . The media path further includes a duplex process direction  620  and an inverter  640 . In a duplex print mode, the printer  600  transfixes an image to a first side of a media sheet, and the media sheet is then inverted by the media inverter  640  and guided in the duplex process direction  620  back to the transfix nip  636 . The inverted media sheet passes through the transfix nip  636  a second time and the printer  600  transfixes a second image to the second side of the media sheet, which is then deposited in the media output tray  644 . 
     Operation and control of the various subsystems, components and functions of the printer  600 , including the media path actuators  614  and printheads  624 A- 624 H, are performed with the aid of a controller or electronic subsystem (ESS)  660 . The ESS or controller  660 , for example, is a self-contained, dedicated computer having a central processor unit (CPU) with a memory, and a display or user interface (UI)  386 . The CPU reads, captures, prepares and manages the image data flow associated with print jobs received from image input sources and controls the media transport actuators  614  to align and time the insertion of media sheets into the transfix nip  636  as described above. The CPU generates electric signals that operate ink ejectors in the printheads  624 A- 624 H with reference to the timing of the insertion of the media sheets into the nip  636 . 
       FIG. 8  depicts a process  700  for printing to media sheets in a duplex mode while reducing transfer of release agent to an unprinted side of a tabbed media sheet. In this figure, the term pressure roller is used to describe a transfix roller or pressure roller like those described in  FIG. 1 ,  FIG. 2 ,  FIG. 5 ,  FIG. 6 , and  FIG. 7 . In the discussion below, a reference to the process performing a function or action refers to a controller executing programmed instructions stored in a memory to operate one or more components to perform the function or action. Process  700  begins as the printer receives a print job to print images on tabbed and untabbed media sheets. The print job can be received from an optical scanner attached to the printer or from a computer or other electronic device through an interface. 
     Process  700  identifies a portion of the pressure roller bearing substantially less release agent (block  708 ). The rotational position of the pressure roller is determined with reference to signals from the rotational sensor as the pressure roller engages a second roller, for example, an imaging drum or transfix roller. Alternatively, the rotational position can be identified without a sensor from stored data corresponding to previous printed sheets, the speed of the rollers, and the diameters of the rollers. The controller determines the lateral position of the portion bearing substantially less release agent from a memory associated with the controller, which stores the lateral alignment and media type of the previous media sheet. Alternatively, the lateral position of the portion bearing substantially less release agent can be sensed with an optical sensor configured to sense release agent on the roller. In a roller including more than one pitch, such as the roller of  FIG. 2 , the identification of the portion bearing substantially less release agent refers to the portion on the pitch that enters the nip next as the roller rotates. 
     Process  700  continues as the controller determines if the next media sheet to be printed includes an extended tab (block  712 ). If the next media sheet to be printed includes an extended tab, the controller operates the media transport to position an edge of the tab at an edge of the portion bearing substantially less release agent to enable the tab to contact only the portion of the first roller bearing substantially less release agent (block  716 ). If the tab is at a leading or trailing edge of the media sheet, the media transport is operated to change the velocity of the media sheet to time the insertion of the edge of the tab to coincide with the leading or trailing edge, respectively, of the portion of the roller bearing substantially less release agent entering the nip. If the tab is on a lateral edge of the media sheet, then the media transport laterally shifts the edge of the tab to align the edge of the tab with an edge of the portion bearing substantially less release agent. The media transport then aligns the edges of the media sheet adjacent to the edge having the tab with edges of the portion of the roller bearing substantially less release agent (block  720 ) to minimize the transfer of release agent to the second side of the other portions of the media sheet. The leading edge of the media sheet is then inserted into the nip as the portion bearing substantially less release agent enters the nip (block  736 ). For a tabbed media sheet having a tab on the leading edge, the edge of the tab is inserted into the nip as the portion bearing substantially less release agent enters the nip. Tabbed media sheets having a tab on the trailing edge are inserted to enable the edge of the tab of the media sheet to contact the trailing edge of the portion bearing substantially less release agent. If the tab is on a lateral edge, the media transport alters the velocity of the media sheet to insert the leading edge of the media sheet into the nip at the same time as the portion bearing substantially less release agent enters the nip, to enable the tab and the majority of the media sheet to pass through the nip without collecting release agent. 
     If the next sheet to be printed does not include an extended tab (block  712 ), the controller determines if the portion bearing substantially less release agent is at the nominal position (block  724 ). As described above with reference to  FIG. 3A-4C , the nominal position refers to the position on the roller of the portion bearing substantially less release agent prior to printing. If the portion of the surface of the first roller bearing substantially less release agent is at the nominal position, the controller instructs the media transport to position the media sheet at the nominal position (block  732 ) and to alter the velocity of the media sheet to insert the leading edge of the media sheet into the nip as the area bearing substantially less release agent enters the nip (block  736 ). 
     If the portion bearing substantially less release agent is not at the nominal position (block  724 ), then the controller operates the media transport to position an edge of the media sheet a predetermined distance from an edge of the portion of the surface of the roller bearing substantially less release agent (block  728 ). If the portion bearing substantially less release agent is shifted in the cross-process direction from the nominal position, then the media transport shifts the media sheet laterally from the portion bearing substantially less release agent by the predetermined distance in the direction of the nominal position. In one practical embodiment the predetermined distance is approximately two millimeters, although other distances can be used in alternative embodiments. If the portion of the roller bearing substantially less release agent is shifted in the process direction from the nominal position, then the media transport alters the velocity of the media sheet to enable the media sheet to contact the pressure roller in the nip the predetermined distance from the edge of the portion of the surface of the pressure roller bearing substantially less release agent, while aligning the lateral edges with the portion bearing substantially less release agent to enable the media sheet to pass through the nip collecting a minimal amount of release agent while shifting toward the nominal position. The controller then operates the media transport to insert the media sheet into the nip as the portion bearing substantially less release agent enters the nip (block  732 ). 
     The controller next determines if there is more image data in the print job ready for printing (block  736 ). If there is additional image data ready, then the process continues (block  708 ). If there is no more image data, then the process terminates (block  740 ). 
     It should be appreciated that while the process  700  is described with reference to tabbed media sheets, a similar process can apply to hole-punched media and to sheets of a different size than the media used for the bulk of the print job, for example, 9 inch by 11 inch covers mixed into a print job of primarily 8.5 inch by 11 inch sheets. The different sizes and types of media sheets are aligned to enable minimal transfer of release agent to areas of the second side of the media sheet being printed. Shifting media back to the nominal position can be accomplished in the same manner as described above in process  700 . 
     It will be appreciated that variations of the above-disclosed and other features and functions, or alternatives thereof, may by desirably combined into many other different systems or applications. Also, that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.