Patent Description:
Sheet stackers are applied in printers to form stacks of printed sheets. Certain sheet stackers, as for example disclosed in <CIT>, are provided with a flipping device comprising at least one slot for receiving a leading portion of a sheet. The flipping device rotates the slot with the sheet in it, which results in a flipping motion of the sheet. Thereby the surfaces of the sheet are inverted and the sheet can be quickly deposited on a sheet stack on a stack support in a rapid and controlled manner. It was found however that after flipping deformation or damage to the leading edge portions of sheets could occur.

It is an object of the invention to provide an alternative method for stacking sheets which reduces or prevents damage to sheets.

In accordance with the present invention, a method according to claim <NUM>, a sheet stacker according to claim <NUM>, and a sheet printer according to claim <NUM> are provided. The method is a method for stacking sheets received from a printer by means of a sheet flipping device comprising at least one slot for receiving a leading portion of a sheet, which method comprises the steps of:.

It is the insight of the inventors that the deformation of the leading portion occurred for sheets with a relatively high stiffness, while relatively weaker sheets underwent the stacking process without damage. The inventors further deduced that the deformation was due to the local bending of the leading portion of the stiffer sheets in the at least one slot of the flipping device during the flipping. The inventors had the further insight that this deformation could be prevented by inserting relatively stiffer sheets not fully into the at least one slot. Thereby, the deformation of the leading portion of stiffer sheets is reduced and/or prevented. It was found however that weaker sheets require a relatively deeper insertion into the at least one slot to properly hold them in place during flipping. In consequence, the inventors proposed a method wherein relatively stiffer sheets are inserted into the at least one slot at a shallower insertion depth than relatively weaker sheets. Thus, a wide range of print media sheets can be reliably stacked with a reduced risk of deformation. Thereby the object of the present invention has been achieved.

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

In an embodiment, the method further comprises the step of selecting a media type for a print job, and comparing the selected media type to an insertion depth look-up table to determine the corresponding insertion depth parameter. The to be applied insertion depth of the sheets is automatically determined when a media type for a print job is input. The media type is compared to an insertion depth look-up table, wherein insertion depth parameters are defined or derivable for each media type. Thus, when a media type is selected, the corresponding insertion depth up to which a sheet of said media type is to be inserted into the at least one slot is automatically derived from the insertion depth look-up table. This allows for productive and/or unattended printing and stacking of sheets. It will be appreciated that the designated insertion depth is equal to the designated length of the sheet to be inserted into the at least one slot.

In an embodiment, the insertion depth look-up table is comprised in a media catalogue, wherein an insertion depth parameter has been designated for each media type. The media catalogue defines all relevant media types for use with the respective printer. Generally, such a media catalogue comprises information regarding sheet dimensions, materials, sheet processing parameters, etc. The media catalogue is extended to include an insertion depth parameter for each media type. The insertion depth parameter defines or can be used to derive the length by which a sheet of a certain media type is to be inserted into the at least one slot to avoid deformations in the sheet after flipping.

In an embodiment, the insertion depth of sheets is inversely proportional to their stiffness. The length by which relatively stiffer sheets are inserted into the at least one slot is smaller than the length by which relatively weaker sheets are inserted into the at least one slot. Inversely proportional can include any number of different insertion depths, including a binary division between weak and stiff media, wherein each media type is assigned to one of these two categories and inserted at one of two corresponding different insertion depths.

In an embodiment, the step of flipping the sheet comprises rotating a flipping wheel on which the at least one slot has been provided. The at least one slot is thereby rotated, inverting the sheet, which allows for high speed sheet stacking.

In an embodiment, the insertion depth is determined by controlling the relative velocities of the at least one slot and of the sheet as it is being inserted into the at least one slot. Different insertion depths can be achieved within a single slot by controlling the length by which a sheet is inserted. The insertion of the sheet is stopped when is respective insertion depth has been reached. The inserted length of the sheet is then equal to its designated insertion depth. Weaker media are inserted deeper into the at least one slot, while the insertion depth for stiffer media is relatively shallow.

In an embodiment, the flipping device is provided with two slots, having different depths in an insertion direction. Different insertion depths can also be achieved by two or more different slots having different sizes. Sheets can then be fully inserted into their respective slot while still allowing for different insertion depths. Upon insertion of a sheet of a certain media type, the slot with the corresponding insertion depth is rotated into a receiving position to receive said sheet.

The present invention further relates to a sheet stacker for stacking sheets of printed media, comprising:.

The controller stores a set of insertion depth parameters in the insertion depth look-up table, which define for each media type the insertion depth up to which a sheet of said media type is to be inserted into the at least one slot to avoid deformation of said sheet after flipping. Upon selection of a certain media type for a print job, the controller derives the corresponding insertion depth from the insertion depth look-up table. The flipping device is then controlled, such that sheets of said media type are then inserted into the at least one slot up to a length equal to the insertion depth. The inserted length does not exceed the insertion depth. When a different media type is selected, the controller adjusts the insertion depth accordingly. In this manner, the controller ensures that relatively stiffer media are not inserted too deeply into the at least one slot to avoid deformations, while relatively weaker media types are inserted sufficiently deep to ensure a reliable flipping of these media types. This results in reliable and deformation-free sheet stacking.

In an embodiment, the controller is configured to receive print job information defining a media type via an user interface and to compare the defined media type to the insertion depth look-up table to determine an insertion depth parameter corresponding to a depth by which a sheet of said media type is to be inserted in the at least one slot, and to control the sheet flipping device to insert the sheet of said media type at said depth into the at least one slot. From the input print job information the controller determines the media type for the print job. From the media type, the corresponding insertion depth is derived. In consequence, the workload of the operator is reduced.

In an embodiment, the insertion depth look-up table is comprised in a media catalogue stored on the controller's memory. The controller generally stores a media catalogue defining various parameter or properties of print media types which can be used in the printer. The media catalogue has been extended to include an insertion depth parameter for each of the media types defined in the catalogue. The insertion depth parameter may be expressed as the length by which the sheet is to inserted, or any other suitable parameter from which the insertion depth can be derived, such as stiffness, rigidity, elasticity related parameters.

In an embodiment, the sheet flipping device comprises a rotatable flipping wheel upon which the at least one slot has been provided. In another embodiment, the sheet flipping device further comprises a stop element positioned, such that when contacting the stop element the sheet is released from the at least one slot.

In an embodiment, the sheet flipping device comprises an insertion device, and wherein the controller is configured to control the relatively velocities of the at least one slot and the insertion device to control the depth by which the sheet is inserted into the at least one slot. The insertion depth can varied by controlling how deep a sheet is inserted into the at least one slot. Weaker sheets are for example inserted fully into a slot, while stiffer sheets are only inserted halfway into the slot or less (halfway herein being defined with respect to the total length of the slot).

In an embodiment, the sheet flipping device comprises a pair of slots, wherein the slots have different depths in an insertion direction of the sheet. Different insertion depth can also be achieved by providing different slots on the flipping wheel, wherein each slot has a different total length. The total length of the slot in this case defines the insertion depth. Weaker sheets are fully inserted into a longer or deeper slot, while stiffer sheets are inserted into a smaller or shallower slot.

The present invention further relates to a sheet printer comprising a sheet stacker as described above. The printer is preferably an inkjet printer.

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, since various changes and modifications within the scope of the present invention will become apparent to those skilled in the art from this detailed description.

<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 inkjet 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>.

The output section <NUM> is digitally connected by means of a cable <NUM> to the print engine and control section <NUM> for bi-directional data signal transfer.

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. The sheet scheduling section <NUM> will also be called scheduler <NUM> hereinafter.

The sheet scheduling section <NUM> takes the depth of the loop into account. The depth of the loop corresponds to a loop time duration of a sheet going through the loop dependent on the velocity of the sheets in the loop. The loop time duration may vary per kind of sheet, i.e. a sheet with different media properties.

Resources may be recording material located in the input section <NUM>, marking material located in a reservoir <NUM> near or in the print head or print assembly <NUM> of the print engine, or finishing material located near the print head or print 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 print head or print 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 print head or print assembly <NUM> is suitable for ejecting and/or fixing marking material to image receiving material. The print head or print assembly <NUM> is positioned near the paper path section <NUM>. The print head or print assembly <NUM> may be an inkjet print head, a direct imaging toner assembly or an indirect imaging toner assembly.

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 print head or print assembly <NUM>. A next paper path section <NUM> is a flip unit <NUM> for selecting a different subsequent paper path for simplex or duplex printing of the image receiving material. The flip 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 flip unit <NUM> and via the inlet <NUM> to the output section <NUM>. The curved section <NUM> of the flip 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 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 print head or print assembly <NUM> in the loop. The sheets coming from the paper path section <NUM> are starting their second pass along the print head or print assembly <NUM> in the loop. When a sheet has passed the print head or print 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 stacker <NUM> comprising a sheet flipping device <NUM> for forming a stack of sheets S on a stack support <NUM>. The general operation of such a sheet stacker <NUM> will be explained in <FIG>.

The sheet flipping device <NUM> comprises a flipping wheel <NUM> rotatably provided around a rotation axis <NUM>. A drive (not shown) is provided to rotate the flipping wheel <NUM>. At least one slot <NUM> is provided on the circumference of the flipping wheel <NUM>. The slot <NUM> is fixed with respect to the flipping wheel <NUM>, such that it rotates when the flipping wheel <NUM> is rotated by its drive. The slot <NUM> is configured to hold a leading portion of a sheet S. The sheet S is supplied from an inserting device <NUM>. The inserting device <NUM> comprises a transport path extending towards the slot <NUM>, when in the receiving position shown in <FIG>. The inserting device in <FIG> comprises guide plates <NUM> to guide the sheet S towards the inserting pinch <NUM>. The inserting pinch <NUM> is formed of a pair of rollers, at least one of which is provided with a drive. Since the rollers of the inserting <NUM> are pressed together, rotation of the driven roller transports the sheet S towards the slot <NUM> in a controlled manner.

When the leading portion of the sheet S has been received in the slot <NUM>, the flipping wheel <NUM> is rotated, a shown in <FIG>. This results in a flipping motion of the sheet S, wherein the trailing portion of the sheet S rolls out beyond the flipping wheel <NUM>. After having passed through a predetermined angle, the leading edge of the sheet S contacts one or more stop elements <NUM> adjacent the flipping wheel <NUM>. The one or more stop elements <NUM> are positioned such that the flipping wheel <NUM> and the slot <NUM> can pass by the stop elements <NUM>, while the leading edge of the sheet S is prevented from further passage. In consequence, as shown in <FIG>, the leading portion of the sheet S is released from the slot <NUM> by the continued rotation of the flipping wheel <NUM>. The slot <NUM> is returned to its receiving position for receiving a subsequent sheet. The sheet S completes its flipping motion and is thereby deposited on the stack support <NUM> (or on a top sheet of a stack already present thereon), as shown in <FIG>.

It was found that when flipping sheets of stiffer media types, deformations SD were present in the sheet S after flipping, as shown in <FIG>. It was found that these deformations resulted from the residency of the leading portion in the slot <NUM>. To counteract this damage to the sheet S, the inventors varied the insertion depth D1, D2 by which the leading portion of the sheet S is inserted into the slot <NUM> based on the sheets' stiffness.

As illustrated in <FIG>, different insertion depths are defined for sheets S of different stiffness. The insertion depth D1, D2, is measured from the entry point at the open end of the slot <NUM>, <NUM> in the insertion direction of the sheet S into the slot <NUM>, <NUM>. The insertion direction corresponds to the circumferential direction wherein the flipping wheel <NUM> is rotated during flipping. The first insertion depth D1, which is greater than the second insertion depth D2, is assigned to sheets S of relatively low stiffness. These weak and/or flexible sheets S can be inserted deeply into the slot <NUM>, for example into the substantially full depth of the slot <NUM>, without the risk of deformation, to ensure a reliable holding during flipping. Sheets S with a substantially greater stiffness are designated to be inserted to the second insertion depth D2. The insertion depth D2 for relatively stiffer sheets S is significantly smaller than the insertion depth D1 for relatively weaker sheets S. This prevents deformation of the stiffer sheets S.

<FIG> illustrates the two manners of inserting sheets at different insertion depths D1, D2 dependent on the sheet stiffness. Different insertion depths D1, D2 can be applied to the same slot <NUM> by controlling the insertion of the sheet S by the inserting device <NUM> into the slot <NUM>. The controller <NUM> determines a measure of the stiffness of the sheet S to be used from print job information input to the controller <NUM> via the user interface <NUM>, which can be on the printer <NUM> and/or or on a workstation N1 connected to the network N. The print job information triggers a media type selection, wherein one of a plurality of media types stored in a media catalogue is selected. Additionally, an insertion depth for the to be used media type is determined by the controller <NUM>. The media catalogue may comprise for example a look-up table (<NUM> in <FIG>), which relates insertion depths D1, D2 to media types, or which derives an insertion depth based on a stiffness parameter assigned for each media type. The look-up table <NUM> may be in any suitable format, such as a classic row-by-column table, matrix, formula, graph, etc. When the insertion depth D1, D2 has been determined, the controller <NUM> controls the velocities and/or timings of the sheet insertion by inserting device <NUM> into the slot <NUM>. A weaker sheet S is inserted into the slot <NUM> to the insertion depth D1, while a stiffer sheet S is inserted only partially in the slot <NUM>. The insertion depth D1, D2 is determined by controlling the drive of the inserting pinch <NUM>. The inserting pinch <NUM> is stopped when the insertion depth D1, D2 has been reached, even if the sheet S has not yet reached the end of the slot <NUM>. The drive of the flipping wheel <NUM> is then activated and the sheet S is flipped. Stiffer sheets S are flipped while only partially, e.g. no more than halfway, inserted into the slot <NUM> to avoid deformations SD. Weaker sheets S substantially fill the slot <NUM> in the insertion direction during flipping, until they are released. It will appreciated that the above described insertion operation can also be performed while the flipping wheel <NUM> rotates by adjusting the velocity and timing of the inserting pinch accordingly.

<FIG> further illustrates that different insertion depths D1, D2 can be achieved by providing two slots <NUM>, <NUM> with different slot depths on the flipping wheel <NUM>. When the controller <NUM> determines that the to be stacked sheet S has a relatively high stiffness, it rotates the flipping wheel such that the smaller slot <NUM> is in the receiving position, when said sheet S is present in the inserting device <NUM>. The stiffer sheet S is then inserted into the smaller slot <NUM>, which has a relatively small insertion depth D2. When a relatively weaker sheet S has been selected for use, the controller <NUM> rotates the larger slot <NUM> into the receiving position, when the weaker sheet S is at the inserting device <NUM>. The insertion depth D2 of the larger slot <NUM> is greater than that of the other slot <NUM>. In consequence, the inserted length of the weaker sheet S in the larger slot <NUM> is greater than that of the stiffer sheet S in the smaller slot <NUM>. Thus, different insertion depths D1, D2 can be achieved by differently formed slots <NUM>, <NUM> and/or controlling the level of insertion into a single slot <NUM>.

<FIG> illustrates the advantage of inserting a stiff sheet S only partially into the slot <NUM>. The partial insertion ensures that the radius of curvature A1 at the leading portion of the sheet S remains relatively large. This prevents deformation of the leading portion. Additionally, <FIG> shows that in this manner the radius of curvature A2 of the sheet S during flipping also remains relatively large. By avoiding smaller radii of curvature permanent deformation of the sheet S is avoided.

<FIG> illustrates the steps of the method according to the present invention. In step i, print job information is submitted to the controller <NUM>, from which the controller <NUM> determines which media type is to be applied for the corresponding print job. Said media type is then selected by the controller <NUM> from the media catalogue. In step ii, the selected media type is compared to the insertion depth look-up table <NUM> to determine the insertion depth D1, D2 corresponding to the selected media type. It will be appreciated that step i and ii can be combined into a single step by incorporating the insertion depth look-up table <NUM> into the media catalogue. The insertion depth look-up table <NUM> in <FIG> comprises different rows a-c, each of which is designated to a specific media type. For each row a-c, several parameters relating to the media type are stored in the insertion depth look-up table <NUM>. In the example in <FIG>, each row a-c defines the length L, width W, and the insertion depth parameter D. The insertion depth parameter D may be expressed in any suitable format, such as an insertion depth D1, D2 in mm or cm, designation to one of the slots <NUM>, <NUM>, a release timing for inserting device <NUM> in ms or s, etc. The insertion depth parameter D may also be a measure for the stiffness or rigidity of the media type, from which the insertion depth D1, D2 can be derived by a formula, table, or graph stored on the controller <NUM>. Therein, the length L and width W of the media type may be taken into account.

In step iii, a sheet S of the selected media type arrives at the inserting device <NUM>. In step iv, the controller <NUM> controls the inserting device <NUM> and/or the flipping wheel <NUM> to insert the sheet S at the determined insertion depth D1, D2. The sheet S is inserted such that the inserted length matches the insertion depth D1, D2. In step v, the controller <NUM> controls the flipping wheel <NUM> to rotate with the sheet S inserted at the corresponding insertion depth D1, D2. Thereby, the sheet S is flipped to begin forming a stack on the stack support <NUM>. Steps iii to v are repeated until the controller <NUM> in step vi determines that a different media type is selected for an upcoming print job. The controller <NUM> then executes step ii to determine a new insertion depth D1, D2 corresponding to this different media type before proceeding with steps iii to v to form a subsequent sheet stack.

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. Generally, this application is intended to cover any adaptations or variations of the specific embodiments discussed herein.

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 for stacking sheets (S) received from a printer (<NUM>) by means of a sheet flipping device (<NUM>) comprising at least one slot (<NUM>, <NUM>) for receiving a leading portion of a sheet (S), the method comprising the steps of:
- determining a respective insertion depth parameter (D) for a first and a second sheet (S), wherein the second sheet (S) has a greater stiffness than the first sheet (S);
- inserting a length (D1) of the first sheet (S) into at the least one slot (<NUM>, <NUM>) in correspondence with its respective insertion depth parameter (D), followed by flipping said first sheet (S);
- inserting a length (D2) of the second sheet (S) into at the least one slot (<NUM>, <NUM>) in correspondence with its respective insertion depth parameter (D), followed by flipping said second sheet (S), wherein the inserted length (D1) of the first sheet (S) is greater than that of the second sheet (S).