REGISTRATION OF DUPLEX PRINTED SHEETS IN A SHEET STACKING DEVICE

Abstract

A method of printing and stacking sheets improves the relative alignment of images in a stack of printed sheet. The method includes feeding a sheet at least twice past a printhead assembly for printing an image on both sides of the sheet, wherein in the first and second passes the respective front and back images are aligned with respect to a first edge of the sheet by detecting a skew angle between the first and second edges and applying the skew angle to align the front and back images; and stacking the sheet, wherein the sheet is registered to a predetermined stacking reference direction by one of its edges, and wherein during the stacking, the sheet is registered by its first edge.

Claims

1. A method of printing and stacking sheets comprising the steps of: feeding a sheet at least twice past a printhead assembly for printing an image on both sides of the sheet, wherein in the first and second passes, respective front and back images are aligned with respect to a first edge of the sheet by detecting a skew angle between the first and second edges of the sheet and applying the skew angle to align the front and back images; and stacking the sheet, wherein the sheet is registered to a predetermined stacking reference direction by one of the first and second edges thereof, wherein during the step of stacking, the sheet is registered by the first edge thereof.

2. The method according to claim 1, further comprising the step of detecting a first angle of the first edge and a second angle of the second edge with respect to a reference direction.

3. The method according to claim 2, wherein the step of detecting the first and second angles is performed before the sheet arrives at the printhead assembly on the first pass.

4. The method according to claim 2, further comprising the step of flipping the sheet, such that the first and second edges each take the relative position of the other.

5. The method according to claim 4, wherein on the second pass the second image is aligned with respect to a leading edge of the sheet, the leading edge having been a trailing edge of the sheet on the first pass.

6. The method according to claim 5, wherein on the first pass the first image is aligned with respect to a trailing edge of the sheet.

7. The method according claim 6, wherein the sheet is registered during stacking to the leading edge after the second pass.

8. The method according to claim 7, wherein the first edge is opposite the second edge of the sheet.

9. A duplex printer comprising: a print surface; a printhead assembly facing the print surface; a sheet conveying system arranged to feed sheets over the print surface and past the printhead assembly, the sheet conveying system including a duplex loop; a skew angle correction system arranged to rotate the sheets relative to images to be printed thereon; a detection system arranged to detect an edge of the sheet; and an electronic controller receiving signals from the detection system and controlling the printhead assembly, the sheet conveying system and the skew angle correction system, wherein the controller is configured to perform the method according to claim 1.

10. A computer program embodied on a non-transitory computer readable medium and comprising instructions to cause the printer according to claim 9 to execute the steps of the method.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The present invention will become more fully understood from the detailed description given herein below and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein:

[0034] FIG. 1 is a schematic side view of a duplex sheet printer;

[0035] FIG. 2 is a schematic top view of a sheet in the step of detecting the angle of the leading edge on the first pass;

[0036] FIG. 3 is a schematic top view of the sheet in the step of detecting the skew angle between the leading and trailing edges;

[0037] FIG. 4 is a schematic top view of the sheet in the step of rotating the sheet so that the first edge is aligned with reference direction;

[0038] FIG. 5 is a schematic top view of the sheet in the step of printing the first image on the first side of the sheet aligned with the reference direction;

[0039] FIG. 6 is a schematic top view of the sheet in the step of flipping the one side-printed sheet;

[0040] FIG. 7 is a schematic top view of the sheet in the step of detecting the angle of the leading edge of the flipped sheet in the second pass;

[0041] FIG. 8 is a schematic top view of the sheet in the step of rotating the sheet so that the first edge is aligned with the reference direction;

[0042] FIG. 9 is a schematic top view of the sheet in the step of printing the second image on the second side of the flipped sheet aligned with reference direction; and

[0043] FIG. 10 is a schematic top view of the sheet in the step of registering the sheet by its first edge contacting a registration element of a sheet stacking device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0044] The present invention will now be described with reference to the accompanying drawings, wherein the same reference numerals have been used to identify the same or similar elements throughout the several views.

[0045] Printer FIG. 1 shows schematically an embodiment of a printing system 1 according to the present invention. The printing system 1, for purposes of explanation, is divided into an output section 5, a print engine and control section 3, a local user interface 7 and an input section 4. While a specific printing system is shown and described, the disclosed embodiments may be used with other types of printing system such as an ink jet print system, an electrographic print system, etc.

[0046] The output section 5 comprises a first output holder 52 for holding printed image receiving material, for example a plurality of sheets. The output section 5 may comprise a second output holder 55. While 2 output holders are illustrated in FIG. 1, the number of output holders may include one, two, three or more output holders. The printed image receiving material is transported from the print engine and control section 3 via an inlet 53 to the output section 5. When a stack ejection command is invoked by the controller 37 for the first output holder 52, first guiding means 54 are activated in order to eject the plurality of sheets in the first output holder 52 outwards to a first external output holder 51. When a stack ejection command is invoked by the controller 37 for the second output holder 55, second guiding means 56 are activated in order to eject the plurality of sheets in the second output holder 55 outwards to a second external output holder 57.

[0047] Preferably the output section 5 comprises a sheet stacking device for one or more output holders 52, 55. Sheet stacking devices are known from e.g. U.S. Pat. No. 9,457,980 B2. The sheet stacking device preferably comprises a registration element against which the leading of the sheets are pressed, such that these leading edges become aligned in the same stacking reference direction. This ensures that the edges of the different sheets 41 are aligned with respect to one another at the output holders 52, 55. This results in a neatly stacked pile of sheets and the aligned edges are preferably applied in the further processing of sheets, such as cutting or binding for aligning the processing equipment to the sheets.

[0048] The print engine and control section 3 comprises a print engine and a controller 37 for controlling the printing process and scheduling the plurality of sheets in a printing order before they are separated from input holder 44, 45, 46. The controller 37 is a computer, a server or a workstation, connected to the print engine and connected to the digital environment of the printing system, for example a network N for transmitting a submitted print job to the printing system 1. In FIG. 1 the controller 37 is positioned inside the print engine and control section 3, but the controller 37 may also be at least partially positioned outside the print engine and control section 3 in connection with the network N in a workstation N1. The controller 37 comprises a print job receiving section 371 permitting a user to submit a print job to the printing system 1, the print job comprising image data to be printed and a plurality of print job settings. The controller 37 comprises a print job queue section 372 comprising a print job queue for print jobs submitted to the printing system 1 and scheduled to be printed. The controller 37 comprises a sheet scheduling section 373 for determining for each of the plurality of sheets of the print jobs in the print job queue an entrance time in the paper path of the print engine and control section 3, especially an entrance time for the first pass and an entrance time for the second pass in the loop in the paper path according to the present invention.

[0049] Resources may be recording material located in the input section 4, marking material located in a reservoir 39 near or in the printhead or printhead assembly 31 of the print engine, or finishing material located near the printhead or printhead assembly 31 of the print engine or located in the output section 5 (not shown).

[0050] The paper path comprises a plurality of paper path sections 32, 33, 34, 35 for transporting the image receiving material from an entry point 36 of the print engine and control section 3 along the printhead or printhead assembly 31 to the inlet 53 of the output section 5. The paper path sections 32, 33, 34, 35 form a loop according to the present invention. The loop enables the printing of a duplex print job and/or a mix-plex job, i.e. a print job comprising a mix of sheets intended to be printed partially in a simplex mode and partially in a duplex mode.

[0051] The printhead or printhead assembly 31 is suitable for ejecting and/or fixing marking material to image receiving material. The printhead or printhead assembly 31 is positioned near the paper path section 34 which comprises the print surface opposite the printhead assembly 31. The printhead assembly 31 may comprise a page-wide array of inkjet printheads. Upstream of the printhead assembly 31 is preferably a skew angle correction system, as known for example from U.S. Pat. No. 5,957,598 A. The skew angle correction system comprises a detection system to detect the angle of the sheet and a correction mechanism to re-orient the sheet into alignment with the printhead assembly 31.

[0052] While an image receiving material is transported along the paper path section 34 in a first pass in the loop, the image receiving material receives the marking material through the printhead or printhead assembly 31. A next paper path section 32 is a flipping unit 32 for selecting a different subsequent paper path for simplex or duplex printing of the image receiving material. The flipping unit 32 may be also used to flip a sheet of image receiving material after printing in simplex mode before the sheet leaves the print engine and control section 3 via a curved section 38 of the flipping unit 32 and via the inlet 53 to the output section 5. The curved section 38 of the flipping unit 32 may not be present and the turning of a simplex page has to be done via another paper path section 35.

[0053] In case of duplex printing on a sheet or when the curved section 38 is not present, the sheet is transported along the loop via paper path section 35A in order to turn the sheet for enabling printing on the other side of the sheet. The sheet is transported along the duplex loop of paper path section 35 until it reaches a merging point 34A at which sheets entering the paper path section 34 from the entry point 36 interweave with the sheets coming from the paper path section 35. The sheets entering the paper path section 34 from the entry point 36 are starting their first pass along the printhead or printhead assembly 31 in the loop. The sheets coming from the paper path section 35 are starting their second pass along the printhead or printhead assembly 31 in the loop.

[0054] When a sheet has passed the printhead or printhead assembly 31 for the second time in the second pass, the sheet is transported to the inlet 53 of the output section 5.

[0055] The input section 4 may comprise at least one input holder 44, 45, 46 for holding the image receiving material before transporting the sheets of image receiving material to the print engine and control section 3. Sheets of image receiving material are separated from the input holders 44, 45, 46 and guided from the input holders 44, 45, 46 by guiding means 42, 43, 47 to an outlet 36 for entrance in the print engine and control section 3. Each input holder 44, 45, 46 may be used for holding a different kind of image receiving material, i.e. sheets having different media properties. While 3 input holders are illustrated in FIG. 1, the number of input holders may include one, two, three or more input holders.

[0056] The local user interface 7 is suitable for displaying user interface windows for controlling the print job queue residing in the controller 37. In another embodiment a computer N1 in the network N has a user interface for displaying and controlling the print job queue of the printing system 1.

Stacking of Duplex Printed Sheets

[0057] FIG. 2 illustrates a sheet 41 as it is being transported towards the printhead assembly 31 on its first pass along the printhead assembly 31. The sheet 41 has non-parallel leading and trailing edges LE, TE. In the example in FIG. 2 the lateral edges of the sheet 41 are also non-parallel. Non-parallel herein being defined as oriented with respected to one another at a non-zero angle, preferably within the same plane, being the plane defined by the sheet 41. The first edge E1 of the sheet 41 in FIG. 2 forms the trailing edge, while the second edge E2 forms the leading edge LE. The leading and trailing edges LE, TE are defined with respect to the transport direction X, while the first and second edges E1, E2 are defined with respect to the sheet 41 itself.

[0058] In FIG. 2 the sheet 41 is being transported to the printhead assembly 31, but first moves towards the skew angle correction system 62, which is positioned upstream of the printhead assembly 31. The skew angle correction system 62 is provided with a detection system 60 for detecting the angle of the sheet 41 with respect to a predetermined direction, in this example the reference direction Y. The reference direction Y is preferably parallel to an angle of the printhead assembly 31. The detection system 60 in FIG. 2 measures the angle of the leading edge LE of the sheet 41, for example by determining the moment of passage of two spaced apart points on the leading edge LE and comparing it to the velocity of the sheet 41. Thereby, the angle of the sheet 41 with respect to the reference direction Y can be determined. In FIG. 2, the leading edge LE is skewed with respect to the reference direction Y by the second angle A2.

[0059] The skew angle correction system 62 further comprises sheet rotation means to adjust the angle and/or position of the sheet 41. In FIG. 2, the sheet rotation means comprise a pair of drive rollers, which can be driven at different speeds and/or oriented at different angles with respect to one another, thereby causing a controlled rotation and/or shifting of the sheet 41.

[0060] In FIG. 3, the sheet 41 moves further across the sheet registration device 62. The trailing edge TE reaches the detection system 60, such that its angle can be determined. In FIG. 3, it is determined that the trailing edge TE is at a first angle A1 with respect to the reference direction Y. Between the measurements of the leading and trailing edge LE, TE the angle of the sheet remained constant, so that from the first and second angles A1, A2 the skew angle A3 of the leading edge LE with respect to the trailing edge TE can be determined via:

A3=A2?A1

[0061] The skew angle A3 corresponds to the angle between the first and second edges E1, E2 of the sheet 41. In case the skew angle A3 is non-zero, the first and second edges E1, E2 of the sheet 41 are not parallel to one another.

[0062] FIG. 4 illustrates the re-orientation of the sheet 41. The skew angle correction system 62 is controlled by the controller 37 based on the determined angle of the first edge E1 to rotate the sheet 41, so that the first edge 41 is brought parallel to the reference direction Y. Thereto, the sheet 41 is rotated by an angle equal to the first angle A1. The rotation is often controlled based on the leading edge E1, so the controller 37 drives the skew angle correction system 62, so that the leading edge LE is at angle with the reference direction Y equal to the skew angle A3. Thereby, the trailing edge TE is brought parallel to the reference direction Y. The detection and rotation are preferably performed while the sheet 41 is in constant movement in the transport direction X, which is perpendicular to the reference direction Y.

[0063] After passing the skew angle correction system 62, the sheet 41 arrives at the printhead assembly 31. The printhead assembly 31 preferably comprises a page-wide printhead array, which extends parallel to the reference direction Y. Since the skew angle correction system 62 rotated the first edge E1, being the trailing edge TE in FIG. 5, parallel to the reference direction Y, the first edge E1 extends parallel to the printhead array in this example. In consequence, the first image 11 is printed on the first side of the sheet 41 perpendicular to the first edge E1. It will be appreciated that the printhead array may also be at an angle with respect to the reference direction Y and/or the detection system, which can be corrected by the skew angle correction system 62 rotating the sheet 41 to compensate for any angular difference(s).

[0064] In FIG. 6, the sheet 41 is flipped by a sheet flipping unit in the paper path section 35A. The flipping is performed around an axis extend in the reference direction Y, such that the first and second edges E1, E2 trade places. The first edge E1 becomes the leading LE and the second edge E2 the trailing TE. The second, unprinted side of the sheet 41 is positioned so that it will face the printhead assembly 31 on the second pass.

[0065] The flipped sheet 41 is returned to the skew angle correction system 61 via the paper path section 35, which is commonly referred to as the duplex path or loop. FIG. 7 illustrates the sheet 41 arriving at the skew angle correction system 62 on its second pass. There, the angle of the first edge E1, now positioned on the leading side of the sheet 41, is detected by the detection system 60. In case the angle of the sheet 41 is preserved perfectly on the paper path section 35 this detection is not required, but in practice the angle of the sheet 41 is altered during transport on the duplex loop.

[0066] Based on the determined angle of the first edge E1 in FIG. 7, the sheet 41 is rotated by the skew angle correction system 62, as shown in FIG. 8. The first edge E1, being the leading edge LE on the second pass, is brought parallel to the reference direction Y. The sheet 41 is aligned in FIG. 8 with respect to the same first edge E1 as in FIG. 4. Thereby, the first image 11 which is now on the side of the sheet 41 facing away from the printhead assembly 31 is perpendicular to the reference direction Y.

[0067] FIG. 9 illustrates the printing of the second image 12 on the second side of the sheet 41. The second image 12 printed in parallel alignment with the first image 11, since on both passes the sheet 41 has been aligned with respect to the printhead assembly 31 by the same first edge E1. The lateral positions of the images 11, 12 may be aligned by the sheet alignment mechanism 62 positioning the sheet 41 with respect to a lateral reference position and applying the lateral dimension of the images 11, 12 as well as their margins with respect to the lateral reference position. Similarly, the images 11, 12 may be aligned in the transport direction X by determining the margins between the respective leading and trailing edges of the sheet 41 and the images 11, 12.

[0068] In FIG. 10, the double-sided printed sheet 41 arrives at the sheet stacking device. The sheet stacking device comprises a registration element 66 which defines a stacking reference direction, which in this example is parallel to the reference direction Y. The sheet 41 is driven against the registration element 66, so that the registration element 66 contacts the sheet at at least two spaced apart points. Thereby, the edge of the sheet 41 in contact with the registration element 66 is oriented parallel to the stacking reference direction Y. In FIG. 10, the leading edge LE contacts the registration element 66. Since the leading edge LE here is the same, first edge E1 with respect to the which the images 11, 12 were aligned all images 11, 12 for all sheets 41 in a stack are aligned parallel to the stacking reference direction Y. In plan-view all images 11, 12 on all the sheets 41 in a stack overlap, so that at least their leading edges are parallel to one another. In a further step, the stack is transported to a stack processing device, such a cutter, book binder, folder, etc. The side of the stack formed of the leading edges is used to align the stack in the stack processing device. Since all the images in the stack are aligned with respect to one another, these will also be aligned in the processed print product. For example, in case of a book all lines and/or images will be in parallel alignment.

[0069] Although specific embodiments of the invention are illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations exist. It should be appreciated that the exemplary embodiment or exemplary embodiments are examples only and are not intended to limit the scope, applicability, or configuration in any way. Rather, the foregoing summary and detailed description will provide those skilled in the art with a convenient road map for implementing at least one exemplary embodiment, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope as set forth in the appended claims and their legal equivalents. Generally, this application is intended to cover any adaptations or variations of the specific embodiments discussed herein.

[0070] It will also be appreciated that in this document the terms comprise, comprising, include, including, contain, containing, have, having, and any variations thereof, are intended to be understood in an inclusive (i.e. non-exclusive) sense, such that the process, method, device, apparatus or system described herein is not limited to those features or parts or elements or steps recited but may include other elements, features, parts or steps not expressly listed or inherent to such process, method, article, or apparatus. Furthermore, the terms a and an used herein are intended to be understood as meaning one or more unless explicitly stated otherwise. Moreover, the terms first, second, third, etc. are used merely as labels, and are not intended to impose numerical requirements on or to establish a certain ranking of importance of their objects.

[0071] The present invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the present invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.