Sheet transport device for a duplex printing system

Abstract: A sheet transport device (100) for a duplex printing system having an endless transport belt (110) which cooperates with an acceleration nip (102) and retime nip (120) to allow duplexing of sheets (48) without loss of productivity. As each sheet to be duplex printed is removed from the printing process path after first side imaging, it is accelerated by the acceleration nip to create a space between it and a subsequent sheet. The sheet is then stalled or slowed by the retime nip, and the transport belt slips past the stalled sheet. Concurrently, a subsequent sheet to be duplexed is driven by the same transport belt (110) to the retime nip. The first mentioned sheet is reinserted into the printing process path at the proper time for receiving the second side image before the arrival of the second sheet at the retime nip. Subsequent duplex sheets are handled in the same manner so that duplex copies are interleaved or otherwise reinserted into the printing process path with first side copies.

Description: This invention relates generally to a sheet transport device, and more particularly concerns a system for printing of duplex (printed on both sides) copy sheets with an endless loop duplexing path. In printing machines where input page imaging and printing on both sides of the copy sheets (duplex printing) is provided, it is important to try to fill every available imaging panel or pitch space on the photoreceptor in order to maintain a constant copy output and the highest possible machine productivity. Eliminating a conventional intermediate sheet stacking duplexing buffer tray, and its re-separating feeder, eliminates sheet jams and jam clearances associated therewith. It eliminates the sheet feeder/separator hardware and the space it requires as well as associated hardware such as sheet stackers, edge joggers, set separators, bail bars, and tray edge guide resetting means for different sheet sizes. Sheet waste due to discarded unfinished duplex sheets in a machine jam situation should also be miniimized. It is therefore desirable to provide a system in which the printer does not normally have to wait (skip one or more copying pitches) for the time required to turn over and return to the transfer station a copy sheet for copying its other side in the desired sequence, yet collation of the copy sheets is provided at their output. US-A-5,337,135 discloses a printing system having a high productivity trayless duplex printer with a duplex loop having a variable path velocity. The duplex path is provided with a variable speed duplex drive for driving a major portion of the duplexing path sheet feeders to at least two different sheet feeding speeds so as to avoid skipped pitches. US-A-5,184,185 describes a printer in which gaps which naturally exist in the output of printed copy sheets from a duplex paper path due to duplex printing are selectably combined with inner set interval skipped pitches so as to provide an appropriate interset interval between each set of printed copy sheets while minimizing the number of skipped pitches which actually need to be scheduled. US-A-5,159,395 discloses a method of scheduling copy sheets in a dual mode duplex printing system in which the scheduling is selectably conducted in one of a first mode where for each set of copy sheets to be printed the copy sheets are consecutively outputted from the copy sheet paper path without any skipped pitches between each consecutively outputted copy sheet and a second mode where for each set of copy sheets to be printed the copy sheets are output from the copy sheet paper path with skipped pitches between at least some of the consecutively outputted copy sheets. The second mode results in copy sheets being outputted from the imaging device at a lower frequency than in the first mode. US-A-5,095,342 discloses a method for scheduling sheets in an imaging system having an endless duplex paper path loop. The method involves consecutively inserting copy sheets to be imaged into the duplex loop without placing any skipped pitches therebetween regardless of set or job boundaries. Duplex side ones from subsequent sets or jobs are used to fill any gaps in the duplexed side one sheet stream of earlier sets or jobs. US-A-4573789 describes a duplex copying apparatus in which a copy sheet having a first image is supplied to a variable speed feed belt which is then controlled selectively to supply the sheet for duplex copying. This requires relatively complex control of the operation of the feed belt. In accordance with a first aspect of the present invention, we provide a method for transporting copy sheets having an image on one side which are received from a duplex printing machine having a printing process path and a predetermined process speed through an endless duplex loop in order to re-insert the copy sheet into the process path for placement of an image on the other side of the copy sheet, comprising: accelerating a first sheet having an image on a first side thereof out of a process path into a duplex path loop; transporting the first sheet with a transport mechanism to a timing mechanism for reinsertion into the process path; positively driving a second sheet toward the timing mechanism with the transport mechanism while the first sheet simultaneously slips with respect to the transport mechanism while captured within the timing mechanism; and reinserting the first sheet into the process path before the second sheet reaches the timing mechanism. In accordance with a second aspect of the present invention, we provide apparatus for transporting copy sheets having an image on one side which are received from a duplex printing machine having a printing process path and a predetermined process speed through an endless duplex loop in order to re-insert the copy sheet into the process path for placement of an image on the other side of the copy sheet, comprising: means for accelerating a first sheet having an image on a first side thereof out of a process path into a duplex path loop; a timing mechanism; and a transport mechanism for transporting the first sheet to the timing mechanism for reinsertion into the process path, the transport mechanism being adapted to positively drive a second sheet toward the timing mechanism while the first sheet simultaneously slips with respect to the transport mechanism while captured within the timing mechanism, and to reinsert the first sheet into the process path before the second sheet reaches the timing mechanism. In accordance with one example, there is provided a sheet transport device for a duplex printing system, comprising: a first sheet drive; an endless transport belt having a first end adjacent said first drive so that a copy sheet fed from said first drive and is then driven by said transport belt, said transport belt contacting a first side of the copy sheet which has no image thereon and frictionally transporting the sheet; a biased idler contacting said transport belt and sheet thereon to maintain the copy sheet in contact with said transport belt; a timing mechanism adjacent a second end of said transport belt to delay and then sequence the copy sheet into a process path, said transport belt positively driving the sheet to said timing mechanism and then slipping relative to the first side of the sheet during the sheet delay, when the first sheet drive feeds a second copy sheet into the belt transport, said transport belt simultaneously drives the second copy sheet toward said timing mechanism and slips relative to the first copy sheet while the first copy sheet is being delayed by the timing mechanism, so that the speed of the first copy sheet is altered in order that it may be fed in the proper timed relation into the process path. Pursuant to another example, there is provided a sheet transport device for an electrophotographic printing machine having a given sheet printing rate in which copy sheets are selectively printed in a duplex mode having printed image data printed on both sides of a sheet the sheet transport device providing a duplex path in which a sheet having an image on one side thereof is received from an output portion of a process path and transported back into the process path to have a second image transferred to the second side thereof, comprising: a first sheet drive adjacent the output portion of the process path operating at a speed substantially higher than the sheet printing rate; a second sheet drive adjacent said first drive operating at a speed substantially equal to the sheet printing rate so that a copy sheet is accelerated from the process path into the duplex path and then controlled by said second drive; a timing mechanism adjacent said second drive to sequence the copy sheet into the process path to the transfer station of the printing machine to complete duplex printing, said second drive positively driving the sheet to the timing mechanism and then slipping relative to the sheet while simultaneously driving a second sheet toward the timing mechanism so that the speed of the first sheet is altered to achieve proper reimage timing and the first sheet is then forwarded into the process path. Pursuant to yet another example, there is provided a method for transporting copy sheets having an image on one side which are received from a duplex printing machine having a printing process path and a predetermined process speed through an endless duplex loop in order to re-insert the copy sheet into the process path for placement of an image on the other side of the copy sheet,comprising: accelerating a first sheet having an image on a first side thereof out of a process path into a duplex path loop; transporting the first sheet to a timing mechanism for reinsertion into the process path; positively driving a second sheet toward the timing mechanism with a transport mechanism while the first sheet simultaneously slips with respect to the transport mechanism while captured within the timing mechanism; and reinserting the first sheet into the process path before the second sheet reaches the timing mechanism. The invention provides more efficient duplexing of multipage collated jobs with reduced skipped printer pitches, for more closely spaced or continuous production of duplex copy sheets with relatively simple and inexpensive machine paper path modifications, and while remaining fully compatible with normal printing of simplex (one side printed) sheets in the same printing apparatus. The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a is an elevational view of the of one example of the endless loop duplex path of the present invention, which is part of an exemplary duplex printer; and Figure 2 is an isolated view of the duplex path of Fig. 1. Figure 1 schematically depicts an electrophotographic printing machine incorporating the features of the present invention therein. An original document is positioned in a document handler 27 on a raster input scanner (RIS) 28. The RIS contains document illumination lamps, optics, a mechanical scanning drive and a charge coupled device (CCD) array. The RIS captures the entire original document and converts it to a series of raster scan lines. This information is transmitted to an electronic subsystem (ESS) or controller 29 which controls a raster output scanner (ROS) described below. The printing machine employs a photoconductive belt 10. Preferably, the photoconductive belt 10 is made from a photoconductive material coated on a ground layer, which, in turn, is coated on an anti-curl backing layer. Belt 10 moves in the direction of arrow 13 to advance successive portions sequentially through the various processing stations disposed about the path of movement thereof. Belt 10 is entrained about stripping roller 14, tensioning roller 16 and drive roller 20. Initially, a portion of the photoconductive surface passes through charging station A. At charging station A, a corona generating device 22 charges the photoconductive belt 10 to a relatively high, substantially uniform potential. At an exposure station, B, a controller or electronic subsystem (ESS) 29, receives the image signals representing the desired output image and processes these signals to convert them to a continuous tone or greyscale rendition of the image which is transmitted to a modulated output generator, for example the raster output scanner (ROS) 30. Preferably, ESS 29 is a self-contained, dedicated minicomputer. The image signals transmitted to ESS 29 may originate from a RIS as described above or from a computer, thereby enabling the electrophotographic printing machine to serve as a remotely located printer for one or more computers. Alternatively, the printer may serve as a dedicated printer for a high-speed computer. The signals from ESS 29, corresponding to the continuous tone image desired to be reproduced by the printing machine, are transmitted to ROS 30. ROS 30 includes a laser with rotating polygon mirror blocks. The ROS illuminates the charged portion of photoconductive belt 10 at a resolution of about 300 or more pixels per inch to record an electrostatic latent image thereon corresponding to the continuous tone image received from ESS 29. After the electrostatic latent image has been recorded on photoconductive surface 12, belt 10 advances the latent image to a development station, C, where toner, in the form of liquid or dry particles, is electrostatically attracted to the latent image using commonly known techniques. The latent image attracts toner particles from the carrier granules forming a toner powder image thereon. As successive electrostatic latent images are developed, toner particles are depleted from the developer material. A toner particle dispenser 44, dispenses toner particles into developer housing 46 of developer unit 38. After the electrostatic latent image is developed, the toner powder image present on belt 10 advances to transfer station D. A print sheet, represented by dashed arrow 48, is advanced to the transfer station, D, by a sheet feeding apparatus, 50. Preferably, sheet feeding apparatus 50 includes a feed roll 52 contacting the uppermost sheet of stack 54. Feed roll 52 rotates to advance the uppermost sheet from stack 54 into vertical transport 56. Vertical transport 56 directs the advancing sheet 48 of support material into registration transport 57, past image transfer station D, to receive an image from photoreceptor belt 10 in a timed sequence, so that the toner powder image formed thereon contacts the advancing sheet 48 at transfer station D. Transfer station D includes a corona generating device 58 which sprays ions onto the back side of sheet 48. This attracts the toner powder image from photoconductive surface 12 to sheet 48. After transfer, sheet 48 continues to move in the direction of arrow 60 by way of belt transport 62 which advances sheet 48 to fusing station F. Fusing station F includes a fuser assembly 70 which permanently affixes the transferred toner powder image to the copy sheet. Preferably, fuser assembly 70 includes a heated fuser roller 72 and a pressure roller 74 with the powder image on the copy sheet contacting fuser roller 72. The pressure roller is cammed against the fuser roller to provide the necessary pressure to fix the toner powder image to the copy sheet. After passing through fuser 70, a gate 80 either allows the sheet to move directly via output 16 to a finisher or stacker, or deflects the sheet into the duplex path 100, after first deflecting the sheet into single sheet inverter 82. That is, if the sheet is either a simplex sheet, or a completed duplex sheet having both side one and side two images formed thereon, the sheet will be conveyed via gate 80 directly to output 16. However, if the sheet is being duplexed and is then only printed with a side one image, the gate 80 will be positioned to deflect that sheet into the inverter 82 and into the duplex loop path 100, where that sheet will be inverted and then fed to acceleration nip 102 and belt transport 110, described in further detail below, for recirculation back through transfer station D and fuser 70 for receiving and permanently fixing the side two image to the backside of that duplex sheet, before it exits via exit path 16. After the print sheet is separated from photoconductive surface 12 of belt 10, the residual toner/developer and paper fiber particles adhering to photoconductive surface 12 are removed therefrom at cleaning station E. Subsequent to cleaning, a discharge lamp (not shown) floods photoconductive surface 12 with light to dissipate any residual electrostatic charge remaining thereon prior to the charging thereof for the next successive imaging cycle. The various machine functions are regulated by controller 29. The controller is preferably a programmable microprocessor which controls all of the machine functions hereinbefore described. In Figure 2, there is illustrated a detailed elevational view of the high productivity duplex path 100 of the present invention. The duplexing path includes an acceleration drive nip 102, which is made up of a drive roll 106 and an idler roll 104, which initially forwards a sheet from the process path into the duplex path at a speed substantially higher than the process speed of the printing machine. The sheet then moves along into the duplex path where it is transported by an endless belt transport 110 from the acceleration nip to the retime nip 120. There are biased load cells 130 located along the path to maintain the sheet in contact with belt transport 110 as the sheet moves from the acceleration nip to the retime nip. The load cells are freewheeling rotating balls 132 or idlers which contact the previously imaged first side of the sheet. The load cells 130 are biased by a weight 134 or spring so that the idler end 132 is urged into contact with the belt portion 112 of belt transport 110. The belt transport 110 contacts the blank second side of the sheet to be imaged. Once the sheet reaches the retime nip 120 it is stalled and the belt transport 110 continues to drive the sheet but slips against the backside of the sheet. The load cells 130 allow the sheet to stall without damaging or smearing the previously imaged first side. Meanwhile, a second sheet for duplexing is accelerated by the acceleration nip 102 out of the process path and into the belt transport 110 of the duplex path. As the second sheet approaches the retime nip 120, which is made up of a driven roll 121 with clutch mechanism 122 and an idler roll 124, the retime nip is actuated by the clutch mechanism 122 under the control of the machine controller 29 to forward the first sheet back into the process path ahead of the transfer station D for subsequent transfer of the duplex image to the second side of the sheet. The second sheet is then stalled in the retime nip 120 until properly timed for duplexing. Thus, the slip transport in the duplex path allows a one-sided copied sheet to be transported out of the process path, accelerated, and then stalled for timing purposes while still allowing a second and subsequent sheets to be duplexed to follow the same route to allow for maximum productivity for duplex copying. The sheet is then reinserted into the process path in an interleaved manner or otherwise inserted with other sheets having images transferred to the first side thereof. In this manner, there is no need for skipped pitches on the photoreceptor to allow for the delay in the duplex path for the second sided copying of each sheet. As a result of the belt transport being allowed to slip against the backside of a copy sheet, there is no need for a complicated variable speed duplex path as a result of the invention herein. Only one extra acceleration nip which creates a spacing between sheets to be duplexed by accelerating a sheet out of the process path and into the duplex path at which the sheet is then slowed back down to substantially the process speed, stalled, and timed back into the process path for subsequent transfer of the second side image. As a result of the slip mechanism, one sheet can be stalled in the retime nip while a second sheet is accelerated out of the process path and into the duplex path where it is forwarded towards the retime nip and then stalled after the first sheet is fed back into the process path. While the invention herein has been described in the context of a trayless duplex loop it will be readily apparent that the transport can be utilized in any sheet feeding situation which requires sheets to be delivered in a timed relationship. Accordingly, there is provided a duplex path loop 100 having a acceleration nip 102 cooperating with a belt transport 110 and retime nip 120 to allow duplexing of sheets while minimizing skipped pitches on the photoreceptor or photoconductive belt 10. As each sheet to be duplex printed is removed from the process path, after first side imaging, it is accelerated to create a gap between it and subsequent sheets. The sheet is then stalled in a retime nip, while subsequent sheets are simultaneously driven by the same transport, and reinserted into the process path at the proper time for receiving the second side image. Subsequent duplex sheets are handled in the same manner so that duplex copies are interleaved into the process path with first side copies so that skipped pitches on the photoreceptor are minimized.

Claims: A method for transporting copy sheets having an image on one side which are received from a duplex printing machine having a printing process path and a predetermined process speed through an endless duplex loop in order to re-insert the copy sheet into the process path for placement of an image on the other side of the copy sheet, comprising: accelerating a first sheet having an image on a first side thereof out of a process path into a duplex path loop; transporting the first sheet with a transport mechanism to a timing mechanism for reinsertion into the process path; positively driving a second sheet toward the timing mechanism with the transport mechanism while the first sheet simultaneously slips with respect to the transport mechanism while captured within the timing mechanism; and reinserting the first sheet into the process path before the second sheet reaches the timing mechanism. Apparatus for transporting copy sheets having an image on one side which are received from a duplex printing machine having a printing process path and a predetermined process speed through an endless duplex loop in order to re-insert the copy sheet into the process path for placement of an image on the other side of the copy sheet, comprising: means (104, 106) for accelerating a first sheet having an image on a first side thereof out of a process path into a duplex path loop; a timing mechanism (121, 122, 124); and a transport mechanism (110) for transporting the first sheet to the timing mechanism for reinsertion into the process path, the transport mechanism being adapted to positively drive a second sheet toward the timing mechanism while the first sheet simultaneously slips with respect to the transport mechanism while captured within the timing mechanism, and to reinsert the first sheet into the process path before the second sheet reaches the timing mechanism. Apparatus according to claim 2, wherein the transport mechanism comprises an endless transport belt (110) defining a duplex loop having a first end adjacent said accelerating means so that a copy sheet (48) fed from said accelerating means is driven by said transport belt, said transport belt contacting a first side of the copy sheet which has no image thereon and frictionally transporting the sheet, wherein the timing mechanism includes a biased idler (132) contacting said transport belt and sheet thereon to maintain the copy sheet in contact with said transport belt, and wherein the timing mechanism (122) is adjacent a second end of said duplex loop to delay and then sequence the copy sheet into a process path. Apparatus according to claim 3, wherein the accelerating means comprises: a drive roll (106); and a member (104) in tangential contact with said drive roll to form a drive nip therewith. Apparatus as claimed in any of claims 2 to 4, wherein said timing mechanism comprises: a second drive roll (121); a member (124) in tangential contact with said second drive roll so as to form a drive nip therewith, the nip capturing the sheet and stopping the sheet thereat; and an actuator mechanism (122), attached to said second drive roll to activate said second drive roll to properly time the copy sheet. Apparatus as claimed in claim 5, wherein said actuator comprises: a clutch connected to said second drive roll; and a controller (29) which sends a timing signal to said clutch so that the copy sheet is transported at the proper time.

Inventors: BALDWIN LEROY A, GRAMLICH JOHN D, HOWER JOHN D JR, MARTIN KATHLEEN M, BALDWIN, LEROY A., GRAMLICH, JOHN D., HOWER, JOHN D., JR., MARTIN, KATHLEEN M.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1995
Main classification: G03G15
IPC codes: B41J13, B65H85, G03G15, G03G21, H04N1