Patent Description:
<CIT> describes a method of duplex printing, comprising the steps of:.

wherein step a) comprises printing on the sheet at least two marks which define a reference direction of the front side image, and said reference direction is used as the reference direction in step b).

In a print process it is generally desired that a leading edge of the printed front side image, which edge defines a reference direction of the image, is aligned with the leading edge of the sheet. If the sheet is fed in a skewed position, it is common practice to rotate the sheet so as to align the leading edge of the sheet with the reference direction of the image. Since the leading edge of the sheet and the leading edge of the image are normally separated by a certain margin, a slight misalignment of the two edges is hardly perceptible with the naked eye. Consequently, the skew angle correction needs to be performed only with a limited accuracy.

In duplex printing, however, a "shadow" of the back side image is in many cases visible from the front side of the sheet because the sheet has a certain translucency. Consequently, the positions of the edges of the back side image can be compared directly with the positions of the corresponding edges of the front side image, and even minor misalignments become visible and are found disturbing. Even when the sheet is totally opaque, a misalignment of the front side and back side images may be found disturbing, for example, when a multi-page duplex document is bound into a booklet, so that a front side image and a back side image are visible simultaneously. Likewise, when a multi-page duplex document is scanned-in and the scanned document is scrolled on a computer screen, the differences between the reference directions of the front side images and back side images become perceptible.

If the leading edge and the trailing edge of the sheet are not exactly parallel to one another, and if the skew angle of the leading edge is corrected before printing the front side image in the first pass, and, in the second pass, the skew angle of the leading edge, which has formerly been the trailing edge, is corrected by rotating the sheet, then the front side image and the back side image will be rotated relative to one another by a relatively large angle which is twice the angle between the leading and trailing edges of the sheet.

Thus, the angle of the first edge of the sheet, which becomes the leading edge in the second pass, is related directly to the reference direction of the image that has been printed in the first pass. This permits to calculate a rotation which precisely corrects the skew angle difference between the front side and the back side image, and when, in the second pass, the angle of the sheet (or the image) is adjusted to the angle that has been calculated in this way, a perfect registry of the front side and back side images will be obtained.

By comparison, if the direction of the second edge of the sheet, which is the leading edge in the first pass, would be taken as the reference direction, then the accuracy of the correction would be inferior, because it cannot always be taken for granted that the front side image is really aligned with the second edge (leading edge in the first part) of the sheet which sufficient accuracy.

This also permits to eliminate all mounting tolerances that may affect the alignment of the printhead assembly (and hence the reference direction of the printed images) with the detection system that is used for measuring the skew angles.

The duplex printed sheets are generally further stacked by sheet stacking device, which registers the received sheets by one of their edges to a predetermined angle. It was found that in the completed stack, the images of different sheets were not aligned with respect to one another. Since the stack is usually aligned with respect to processing equipment for cutting, book binding, etc. by the registered edges, the images in the final print product may become misaligned with respect to one another.

<CIT> discloses an image forming device which acquires a detection signal of a side end detection sensor for detecting a side end point of a sheet between a first roller and a second roller to calculate a first angle showing an angle of inclination of the sheet. The image forming device acquires a detection signal of a leading end detection sensor for detecting a front end point of the sheet between the side end detection sensor and the second roller to calculate a second angle showing an angle of inclination of the sheet. The image forming device controls driving of an attitude change motor for changing the attitude of the second roller so that pre-operation for changing the attitude of the second roller is performed and that after the pre-operation, main operation of changing the attitude of the second roller is performed on the basis of a remaining angle of the first angle.

<CIT> discloses an image forming device, wherein skew detecting means <NUM> and <NUM> are provided upstream of an image forming part to detect skew at a tip 107a and a rear end of a sheet <NUM>, skew quantity at the tip 107a of the sheet <NUM> and skew quantity at the rear end are determined based on signals from them, and skew correcting action by a skew correcting means <NUM> is controlled in accordance with determined skew quantity of the sheet <NUM>. The skew quantity at the rear end of the sheet <NUM> after the skew is corrected by the skew correcting means <NUM> and the skew quantity at the tip of the sheet <NUM> as it is reversed are respectively detected, and skew correcting action by the skew correcting means <NUM> is controlled in accordance with difference between the skew quantity of the sheet <NUM> and the skew quantity at the tip of the reversed sheet.

<CIT> discloses that in supplying recording medium hold means with printing paper S again where images are recorded on a surface thereof by switchback for recording images on a reverse surface, positions of printed images when performing double-sided printing on both of the surface and the reverse surface of the printing paper S match by acquiring shape information about a rear end of the printing paper S when recording images on the surface of the printing paper S and by performing corrected positioning with respect to a slope at the end of the printing paper S by an advance registering part.

It is an object of the invention to provide an improved method of duplex printing and stacking that improves the alignment of the images on different sheets in the stack.

In accordance with the present invention, a method of printing and stacking sheets according to claim <NUM> or claim <NUM>, a duplex printer according to claim <NUM>, a computer program according to claim <NUM>, and a computer readable medium according to claim <NUM> are provided.

It is the insight of the inventor that by registering the sheets in the stack to the same edge of the sheet as the edge that was used for aligning the front and back images on the sheet, all images in the stack will be aligned with respect to one another. The front and back images are aligned (i.e. positioned at the same angle) with respect to the same, first edge of the sheet, wherein the skew angle is applied to correct for non-parallelism of the leading and trailing edge. The same, first edge is also applied in the sheet stacking device to register the sheet. The first edge is preferably opposite the second edge of the sheet. All the images on all sheets in the stack are aligned in the same angle and thus with respect to one another. Thereby, the object of the present invention has been achieved.

According to claim <NUM>, the first edge is the trailing edge on the first pass and the leading edge on the second pass, and the sheet is registered during stacking on the first edge being the leading edge as the sheet moves towards a sheet stacking device. According to claim <NUM>, the first edge is the leading edge on the first pass and the trailing edge on the second pass, and the sheet is flipped after printing the back image, so that the sheet is registered during stacking on the first edge being the leading edge as the sheet moves towards a sheet stacking device. When aligning the front image with respect to the trailing edge on the first pass, the first edge becomes the leading edge on the second pass, and thereby also the edge which is used to register the sheet during stacking. The same result may also be achieved when aligning the front image with respect to the leading edge on the first pass and flipping the sheet for a second time after printing the back image, so that the first edge is again the leading edge.

The method further comprises the step of detecting a first angle of the first edge and a second angle of a second edge with respect to a reference direction. Preferably, the first angle of the first edge is detected with respect to the reference direction as well as the second angle of the second edge. By comparison and/or subtraction of the first and second angles, the skew angle between the first and second edges can be determined. The detection may be performed by a detection system configured to detect the passage of two spaced apart points on an edge of the sheet. The reference direction is preferably defined with respect to the printhead assembly to facilitate easy alignment of the sheets with the images. The reference direction is the same for both passes of the sheet, but does not need not be parallel to the stacking reference direction.

The step of detecting the first and second angles is performed before the sheet arriving at the printhead assembly on its first pass. The skew angle between the first and second edges has been detected and/or determined upstream of the printhead assembly. This allow the sheet to be rotated so that the first edge is aligned with and thus preferably parallel to the reference direction before printing on the sheet. Thereby, the first edge is aligned with and preferably parallel to the direction of the printhead assembly, so that the first image is printed perpendicular to the first edge.

The method further comprises the step of flipping the sheet, such that the first and second edges each take the relative position of the other. The sheet is flipped around an axis perpendicular to its transport direction and/or substantially parallel to the reference direction. The sheet is preferably flipped after printing on the first side of the sheet on the first pass and before returning the sheet to the printhead assembly.

According to claim <NUM>, on the second pass the second image is aligned with respect to a leading edge of the sheet, which respective edge was a trailing edge of the sheet on the first pass. On the second pass, the first edge has become the leading edge due to the flipping of the sheet. The first image has been aligned with respect to the first edge and in the second pass the sheet is rotated, such that the leading edge is aligned with the reference direction. The second image is thereby printed in parallel alignment to the first image, preferably perpendicular to the leading edge. After duplex printing, the sheet continues towards the sheet stacking device with the first edge as it leading edge, so that the first edge comes into contact with the registration element. In another aspect, the sheet is registered during stacking to its leading edge after its second pass. All sheets in a stack are printed and registered in this manner. As both front and back images have been aligned to the first edge, all images in the stack are in parallel alignment with one another.

More specific optional features of the invention are indicated in the dependent claims.

According to claim <NUM>, on the first pass the first image is aligned with respect to a trailing edge. On the first pass, the first edge is the trailing edge and the first image is aligned with respect to the trailing edge by rotating the trailing edge into alignment with the reference direction, preferably parallel to the reference direction. Preferably, the sheet is re-orientated, such that the first image is printed perpendicular to the trailing edge. On the second pass, the sheet is flipped so that the first edge becomes the leading edge, making it easy to utilize the first edge in registering the sheet during stacking. It will be appreciated that it is common for a skew angle correction system to control the rotation of the sheet based on detection of its leading edge. The skew angle is applied to ensure that the trailing edge is aligned to the reference direction even when the rotation is controlled based on the leading edge. The leading edge may for example be controlled to be skewed at angle with respect to the reference direction, so that the trailing edge is parallel to it.

The present invention further relates to a duplex printer comprising a print surface, a printhead assembly facing the print surface, a sheet conveying system arranged to feed media 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 above described method.

The invention further relates to a computer program comprising instructions to cause the printer to execute the steps of the above described method and to a computer-readable medium having stored thereon the computer program.

Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration only.

<FIG> shows schematically an embodiment of a printing system <NUM> according to the present invention. The printing system <NUM>, for purposes of explanation, is divided into an output section <NUM>, a print engine and control section <NUM>, a local user interface <NUM> and an input section <NUM>. 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..

The output section <NUM> comprises a first output holder <NUM> for holding printed image receiving material, for example a plurality of sheets. The output section <NUM> may comprise a second output holder <NUM>. While <NUM> output holders are illustrated in <FIG>, 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 <NUM> via an inlet <NUM> to the output section <NUM>. When a stack ejection command is invoked by the controller <NUM> for the first output holder <NUM>, first guiding means <NUM> are activated in order to eject the plurality of sheets in the first output holder <NUM> outwards to a first external output holder <NUM>. When a stack ejection command is invoked by the controller <NUM> for the second output holder <NUM>, second guiding means <NUM> are activated in order to eject the plurality of sheets in the second output holder <NUM> outwards to a second external output holder <NUM>. Preferably the output section <NUM> comprises a sheet stacking device for one or more output holders <NUM>, <NUM>. Sheet stacking devices are known from e.g. <CIT>. 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 <NUM> are aligned with respect to one another at the output holders <NUM>, <NUM>. 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.

The print engine and control section <NUM> comprises a print engine and a controller <NUM> for controlling the printing process and scheduling the plurality of sheets in a printing order before they are separated from input holder <NUM>, <NUM>, <NUM>. The controller <NUM> 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 <NUM>. In <FIG> the controller <NUM> is positioned inside the print engine and control section <NUM>, but the controller <NUM> may also be at least partially positioned outside the print engine and control section <NUM> in connection with the network N in a workstation N1. The controller <NUM> comprises a print job receiving section <NUM> permitting a user to submit a print job to the printing system <NUM>, the print job comprising image data to be printed and a plurality of print job settings. The controller <NUM> comprises a print job queue section <NUM> comprising a print job queue for print jobs submitted to the printing system <NUM> and scheduled to be printed. The controller <NUM> comprises a sheet scheduling section <NUM> 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 <NUM>, 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.

Resources may be recording material located in the input section <NUM>, marking material located in a reservoir <NUM> near or in the printhead or printhead assembly <NUM> of the print engine, or finishing material located near the printhead or printhead assembly <NUM> of the print engine or located in the output section <NUM> (not shown).

The paper path comprises a plurality of paper path sections <NUM>, <NUM>, <NUM>, <NUM> for transporting the image receiving material from an entry point <NUM> of the print engine and control section <NUM> along the printhead or printhead assembly <NUM> to the inlet <NUM> of the output section <NUM>. The paper path sections <NUM>, <NUM>, <NUM>, <NUM> 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.

The printhead or printhead assembly <NUM> is suitable for ejecting and/or fixing marking material to image receiving material. The printhead or printhead assembly <NUM> is positioned near the paper path section <NUM> which comprises the print surface opposite the printhead assembly <NUM>. The printhead assembly <NUM> may comprise a page-wide array of inkjet printheads. Upstream of the printhead assembly <NUM> is preferably a skew angle correction system, as known for example from <CIT>. 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 <NUM>.

While an image receiving material is transported along the paper path section <NUM> in a first pass in the loop, the image receiving material receives the marking material through the printhead or printhead assembly <NUM>. A next paper path section <NUM> is a flipping unit <NUM> for selecting a different subsequent paper path for simplex or duplex printing of the image receiving material. The flipping unit <NUM> 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 <NUM> via a curved section <NUM> of the flipping unit <NUM> and via the inlet <NUM> to the output section <NUM>. The curved section <NUM> of the flipping unit <NUM> may not be present and the turning of a simplex page has to be done via another paper path section <NUM>.

In case of duplex printing on a sheet or when the curved section <NUM> 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 <NUM> until it reaches a merging point 34A at which sheets entering the paper path section <NUM> from the entry point <NUM> interweave with the sheets coming from the paper path section <NUM>. The sheets entering the paper path section <NUM> from the entry point <NUM> are starting their first pass along the printhead or printhead assembly <NUM> in the loop. The sheets coming from the paper path section <NUM> are starting their second pass along the printhead or printhead assembly <NUM> in the loop. When a sheet has passed the printhead or printhead assembly <NUM> for the second time in the second pass, the sheet is transported to the inlet <NUM> of the output section <NUM>.

The input section <NUM> may comprise at least one input holder <NUM>, <NUM>, <NUM> for holding the image receiving material before transporting the sheets of image receiving material to the print engine and control section <NUM>. Sheets of image receiving material are separated from the input holders <NUM>, <NUM>, <NUM> and guided from the input holders <NUM>, <NUM>, <NUM> by guiding means <NUM>, <NUM>, <NUM> to an outlet <NUM> for entrance in the print engine and control section <NUM>. Each input holder <NUM>, <NUM>, <NUM> may be used for holding a different kind of image receiving material, i.e. sheets having different media properties. While <NUM> input holders are illustrated in <FIG>, the number of input holders may include one, two, three or more input holders.

The local user interface <NUM> is suitable for displaying user interface windows for controlling the print job queue residing in the controller <NUM>. 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 <NUM>.

<FIG> illustrates a sheet <NUM> as it is being transported towards the printhead assembly <NUM> on its first pass along the printhead assembly <NUM>. The sheet <NUM> has non-parallel leading and trailing edges LE, TE. In the example in <FIG> the lateral edges of the sheet <NUM> 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 <NUM>. The first edge E1 of the sheet <NUM> in <FIG> 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 <NUM> itself.

In <FIG> the sheet <NUM> is being transported to the printhead assembly <NUM>, but first moves towards the skew angle correction system <NUM>, which is positioned upstream of the printhead assembly <NUM>. The skew angle correction system <NUM> is provided with a detection system <NUM> for detecting the angle of the sheet <NUM> 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 <NUM>. The detection system <NUM> in <FIG> measures the angle of the leading edge LE of the sheet <NUM>, 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 <NUM>. Thereby, the angle of the sheet <NUM> with respect to the reference direction Y can be determined. In <FIG>, the leading edge LE is skewed with respect to the reference direction Y by the second angle A2.

The skew angle correction system <NUM> further comprises sheet rotation means to adjust the angle and/or position of the sheet <NUM>. In <FIG>, 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 <NUM>.

In <FIG>, the sheet <NUM> moves further across the sheet registration device <NUM>. The trailing edge TE reaches the detection system <NUM>, such that its angle can be determined. In <FIG>, 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: <MAT>.

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

<FIG> illustrates the re-orientation of the sheet <NUM>. The skew angle correction system <NUM> is controlled by the controller <NUM> based on the determined angle of the first edge E1 to rotate the sheet <NUM>, so that the first edge <NUM> is brought parallel to the reference direction Y. Thereto, the sheet <NUM> 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 <NUM> drives the skew angle correction system <NUM>, 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 <NUM> is in constant movement in the transport direction X, which is perpendicular to the reference direction Y.

After passing the skew angle correction system <NUM>, the sheet <NUM> arrives at the printhead assembly <NUM>. The printhead assembly <NUM> preferably comprises a page-wide printhead array, which extends parallel to the reference direction Y. Since the skew angle correction system <NUM> rotated the first edge E1, being the trailing edge TE in <FIG>, parallel to the reference direction Y, the first edge E1 extends parallel to the printhead array in this example. In consequence, the first image I1 is printed on the first side of the sheet <NUM> 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 <NUM> rotating the sheet <NUM> to compensate for any angular difference(s).

In <FIG>, the sheet <NUM> 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 <NUM> is positioned so that it will face the printhead assembly <NUM> on the second pass.

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

Based on the determined angle of the first edge E1 in <FIG>, the sheet <NUM> is rotated by the skew angle correction system <NUM>, as shown in <FIG>. The first edge E1, being the leading edge LE on the second pass, is brought parallel to the reference direction Y. The sheet <NUM> is aligned in <FIG> with respect to the same first edge E1 as in <FIG>. Thereby, the first image I1 which is now on the side of the sheet <NUM> facing away from the printhead assembly <NUM> is perpendicular to the reference direction Y.

<FIG> illustrates the printing of the second image I2 on the second side of the sheet <NUM>. The second image I2 printed in parallel alignment with the first image I1, since on both passes the sheet <NUM> has been aligned with respect to the printhead assembly <NUM> by the same first edge E1. The lateral positions of the images I1, I2 may be aligned by the sheet alignment mechanism <NUM> positioning the sheet <NUM> with respect to a lateral reference position and applying the lateral dimension of the images I1, I2 as well as their margins with respect to the lateral reference position. Similarly, the images I1, I2 may be aligned in the transport direction X by determining the margins between the respective leading and trailing edges of the sheet <NUM> and the images I1, I2.

In <FIG>, the double-sided printed sheet <NUM> arrives at the sheet stacking device. The sheet stacking device comprises a registration element <NUM> which defines a stacking reference direction, which in this example is parallel to the reference direction Y. The sheet <NUM> is driven against the registration element <NUM>, so that the registration element <NUM> contacts the sheet at at least two spaced apart points. Thereby, the edge of the sheet <NUM> in contact with the registration element <NUM> is oriented parallel to the stacking reference direction Y. In <FIG>, the leading edge LE contacts the registration element <NUM>. Since the leading edge LE here is the same, first edge E1 with respect to the which the images I1, I2 were aligned all images I1, I2 for all sheets <NUM> in a stack are aligned parallel to the stacking reference direction Y. In plan-view all images I1, I2 on all the sheets <NUM> 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.

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.

Claim 1:
A method of printing and stacking sheets (<NUM>) comprising the steps of:
- feeding a sheet (<NUM>) in a transport direction (X) at least twice past a printhead assembly (<NUM>) for printing an image (I1, I2) on both sides of the sheet (<NUM>), wherein in the first and second passes the respective front and back images (I1, I2) are aligned with respect to a trailing edge (TE) of the sheet (<NUM>) on the first pass by detecting a skew angle (A3) between the leading and trailing edges (LE, TE) by means of detecting a first angle (A1) of the trailing edge (TE) and a second angle (A2) of the leading edge (LE) with respect to a reference direction (Y) before the sheet (<NUM>) arrives at the printhead assembly (<NUM>) on its first pass;
- applying the detected skew angle (A3) to align the front and back images;
- flipping the sheet (<NUM>), such that the leading and trailing edges (LE, TE) each take the relative position of the other before printing the back image (I2),
wherein on the second pass the second image (I2) is aligned with respect to a leading edge (LE) of the sheet (<NUM>), which leading edge (LE) was a trailing edge (TE) of the sheet (<NUM>) on the first pass; and in that the method further comprises the step of stacking the sheet (<NUM>), wherein the sheet (<NUM>) is registered to a predetermined stacking reference direction (Y) by the edge which was the trailing edge (TE) on the first pass.