Patent Publication Number: US-2022219926-A1

Title: Machine for processing individual sheets

Description:
TECHNICAL FIELD 
     The invention relates to a machine for processing individual sheets, the machine comprising at least one processing station and a transport system for transporting the individual sheets through the processing station, along a transport direction. The invention further relates to a process for processing individual sheets. 
     BACKGROUND ART 
     Machines for the processing of individual sheets, such as sheets of corrugated cardboard, are known. They may be dedicated to different processing methods and comprise e. g. inkjet printing machines or laser processing machines such as laser cutting machines. 
     As an example, DE 10 2014 203 821 A1 (Xerox) discloses an image registration system which helps to accurately produce an image onto an oversized media substrate (e. g. of corrugated cardboard) in a large scale printer, for example an inkjet printer. The system includes a rail support track, a printing zone and a platen cart moveable along the rail support track through said printing zone. The image registration system also includes an image capturing apparatus for capturing a position of the media substrate in relation to the platen cart in order to ensure accurate image on media substrate reproduction. 
     DE 10 2007 014 876 B4 (KBA Metronic) discloses a conveyor system having an elongated guide defining a closed transport path extending through a plurality of treatment stations, especially treatment stations of a printing machine, and a plurality of carriers movable on the guide along the path and each capable of holding a respective workpiece. At least one magnet is provided on each carrier, and an annular row of individually energizeable electromagnets extends along the path and is capable of exerting force on the magnets of the carriers so as to displace the respective carriers along the path. 
     The machines according to the prior art comprise platen carts or carriers, respectively, wherein each of the carts or carriers is capable of holding a workpiece. If workpieces of different dimensions shall be processed, the carts or carriers need to be replaced, or elements for holding the workpieces on the carts or carriers need to be readjusted. This leads to considerable changeover times and may require additional storage space for the different carts or carriers needed. Furthermore, in the case of large individual sheets, suitable carts or carriers are bulky and have a considerable weight, thus reducing the achievable dynamics and throughput with respect to the sheet transport. 
     WO 2012/108870 A1 (Hewlett-Packard Development Company) relates to a media transport assembly for an inkjet printing machine, including a plurality of pallets arranged to circulate on an endless track through a print zone and a handling zone. In the print zone, the pallets are temporarily grouped together to support and move a print media during printing at a substantially constant velocity. In the handling zone, the pallets are spaced apart from each other as they circulate back to the print zone without supporting any print media. The pallets may be provided by a vacuum mechanism to apply negative pressure or vacuum at a vacuum surface portion to draw and removably secure the print media against and relative to a top portion of the pallet. In another embodiment is the vacuum source located remotely from the individual pallets, and the track guiding and constraining the movement of the pallets includes a vacuum conduit cooperating with a vacuum recess extending across the width of the respective pallet. 
     The solution with the integrated vacuum mechanism makes the pallets rather complex and costly. If the alternative solution is chosen, it is required to employ a number of pallets if the vacuum force shall be evenly applied to large media. This makes the handling of the sheets more complicated. 
     SUMMARY OF THE INVENTION 
     It is the object of the invention to create a machine pertaining to the technical field initially mentioned, that allows for evenly supporting large sized sheets in a comparably simple manner. 
     The solution of the invention is specified by the features of claim  1 . According to the invention, the transport system comprises at least one gripper conveyor movable along the transport direction, for gripping one of the individual sheets defining a sheet position in transport direction, and the transport system further comprises at least one support conveyor movable along the transport direction for supporting a region of the individual sheet. The support conveyor comprises a vacuum system for supporting the individual sheet on an interacting surface of the support conveyor, the vacuum system comprising a plurality of orifices in the interacting surface. 
     In particular, the support conveyor supports a region of the individual sheet extending over the entire width of the sheet (across the transport direction) but only over part of the length of the individual sheet (in transport direction). The extension of the supported region in the transport direction is substantially smaller than the corresponding extension of the sheet that is not supported by the gripper conveyor (or the gripper conveyors). After all, support of the sheet to be processed is primarily required in the processing region of the processing station. 
     As discussed in more detail below, it is not required that the vacuum source is comprised by the vacuum system of the support conveyor but it may be arranged separate therefrom, and vacuum lines may connect it to the vacuum system of the support conveyor. 
     Instead of having a rather bulky and heavy carrier supporting the sheet over its entire surface, a gripper conveyor (or two or more gripper conveyors) is combined with at least one support conveyor. The gripper conveyor as well as the support conveyor are substantially less bulky and much lighter. Furthermore, adaptation to different sheet sizes is facilitated. 
     The additional support conveyor and the vacuum system improve the flatness of the individual sheet. This is particularly advantageous in the case of large substrates or substrates with low inherent stability. The vacuum system having a plurality of orifices in the interacting surface allows for evenly supporting also large sized substrates. 
     The effect of vacuum system may be easily controlled by adjusting the underpressure. This allows for flexibly adapting the characteristics of the support to the stages of the process and/or different characteristics such as sheet size, thickness and material or the actual process, respectively. 
     The invention is applicable in particular to inkjet printing machines where the processing station is an inkjet printing station. However, the invention is not restricted to such machines but applies to other machines where large sized sheet-like substrates are processed, including e. g. laser processing (e. g. laser cutting) machines. 
     In a preferred embodiment, the at least one support conveyor comprises a conveyor connection opening, the conveyor connection opening being fluidly connected to at least one of the plurality of orifices in the interacting surface. The machine further comprises a fixed vacuum distribution unit comprising a distribution unit connection opening fluidly connected to a vacuum source. The distribution unit connection opening is arranged such that a sealed connection with the conveyor connection opening is established, when the support conveyor moves through a processing region of the machine. 
     The vacuum distribution unit is fixed with respect to the movement of the gripper and conveyors in the transport direction, i. e. it will not be moved during operation of the machine. Nevertheless, the vacuum distribution unit may be adjustably fixed to the machine, e. g. such that its position may be adjusted to the type of sheets that shall be handled. 
     It is not required that the connection between the distribution unit connection opening and the conveyor connection opening is absolutely tight, but the underpressure generated by the vacuum source should be applied to the vacuum system of the conveyor without a substantial loss of air and (under-)pressure. 
     The processing region is defined by the processing station of the machine. In the case of an inkjet printing machine, it essentially corresponds to the location of the print heads or print bars and includes the area where ink is applied to the sheets. 
     Preferably, the vacuum distribution unit comprises a plurality of distribution unit connection openings arranged along the transport direction, connections to the vacuum source being selectively switchable for at least two subsets of the plurality of the distribution unit connection openings. 
     All subsets or some of the subsets may include one distribution unit connection opening or several distribution unit connection openings that are arranged in a longitudinal and/or lateral distance from each other. As an example, the connections to the vacuum source may be selectively switchable for subsets (groups) of openings that comprise several openings across the distribution unit and support conveyor, respectively, at essentially the same position along the transport direction. In another example, the distribution unit connection openings have an elongated shape, extending primarily in a direction across the distribution unit, and there is only one opening at a given longitudinal position. In this case it is preferred if any of the distribution unit connection openings may be selectively coupled and shut off with respect to the vacuum source, such that only those connections are activated that lead to the distribution unit connection opening(s) that is/are in a cooperating position with respect to the conveyor connection opening(s). 
     Having a plurality of distribution unit connection openings that may be selectively used to connect the support conveyor to the vacuum source allows for supplying the support conveyor in an operation area having an extension along the transport direction that is substantially longer than the support conveyor itself without having to resort to a vacuum connection moving together with the support conveyor. Accordingly, a simple and robust construction may be achieved, and the dynamics of the machine is not compromised. 
     Accordingly, a process for processing individual sheets comprises the steps of:
     a) gripping one of the individual sheets by a gripper conveyor running in a transport direction;   b) the gripper conveyor transporting the gripped sheet along the transport direction, through a processing station;   c) during transport through the processing station, supporting a region of the gripped sheet by a support conveyor running in the transport direction,   d) during transport of the support conveyor through a processing region of the processing station, selectively switching a number of connections between a vacuum system of the support conveyor and a fixed vacuum source, the connections being arranged along the transport direction.   

     Advantageously, the distribution unit connection openings extend along a substantial part of the processing region. Preferably, the distribution unit connection openings (measured from a maximum extension of the first opening upstream to the maximum extension of the last opening downstream, along the transport direction) extend along at least 60% of the processing region, most preferably along at least 75% of the processing region. This ensures that the vacuum may be applied on the support conveyor throughout essentially the entire processing operation. 
     Preferably, the vacuum distribution unit comprises a plurality of chambers arranged along the transport direction, each of the chambers featuring at least one of the distribution unit connection openings and a valve for selectively closing and opening a fluid connection to the vacuum source. In particular, the number of chambers is in the range of 5-30. 
     In addition or as an alternative, the vacuum source may include several vacuum pumps that may be selectively activated and deactivated, respectively, in order to selectively apply a vacuum to subsets of the plurality of the distribution unit connection openings. 
     Advantageously, the distribution unit connection openings are arranged in a planar surface extending across the processing region (i. e. perpendicular to the transport direction), and the conveyor connection openings are arranged in a planar surface of the support conveyor on an opposite side of the interacting surface. In particular, the interacting surface constitutes the upper horizontal surface of the support conveyor, and the planar surface opposite thereof is a lower horizontal surface of the support conveyor. This allows for a simple and efficient airflow within the support conveyor and provides a large surface for the exchange of air between the support conveyor and the distribution unit. 
     Alternatively, the conveyor connection openings are arranged on a lateral surface of the support conveyor or in an edge region of the upper surface, and the distribution unit connection openings are arranged correspondingly, essentially laterally outside of the support area for the sheets. 
     Preferably, the machine comprises a controller for controlling the movement of the support conveyor and the selective switching of the connections to the vacuum source, wherein the controller is configured to control the selective switching in synchronism with the movement of the support conveyor. This means that the connection between the vacuum source and the connection opening(s) of the support conveyor is switched in such a way that it is established only if the support conveyor is in a position where the sealed connection is established. This is the case only in a certain range of the support conveyor&#39;s position along the transport direction, accordingly the switching is synchronized with the support conveyor&#39;s movement in order to avoid air and pressure loss. 
     Preferably, the support conveyor comprises a mechanism for adapting the vacuum system to a sheet width of the processed individual sheet. In particular, individual elements of the vacuum system (such as vacuum orifices or chambers and/or groups of vacuum orifices or chambers) may be selectively switched on and off. This allows for switching off elements lying completely or partially outside the paper width. The switching mechanism may include valves for the closing and opening of supply lines and/or sliders for covering individual or groups of elements. In certain embodiments, adaptation to the sheet width may be obtained by having a number of separate chambers in the support conveyor, each of the chambers connecting one or several conveyor connection openings with a subset of the vacuum orifices on the interacting surface of the support conveyor lying in a certain lateral region, and having corresponding distribution unit connection openings that may be selectively coupled to the vacuum source. 
     By adjusting the active width of the vacuum system, the required power may be reduced because the sucking of air in regions outside the support sheets is avoided. Furthermore, the print quality is improved because air movement along the lateral edges of the sheet is prevented, and accordingly a deflection of ink droplets in those regions due to the vacuum system is avoided. A preferred embodiment of the inventive machine comprises a vacuum distribution unit with the plurality of selectively switchable distribution unit connection openings arranged along the transport direction as well as the mechanism for adapting the vacuum system to the sheet width. This allows for adjusting the operation of the machine to the all dimensions of the processed sheet, and minimizes the required power for the vacuum system as well as unwanted air movement in regions along the edges of the sheet. 
     A cushioning device, in particular an air cushioning device comprising a plurality of Bernoulli cups, may be combined with the vacuum system. Bernoulli cups allow for supporting substrates, especially thin substrates such as sheets, with minimum contact and without substantially hindering relative movement between the sheet and the cups parallel to the sheet plane. They allow for guiding a substrate in a predetermined distance from a surface, which means that they allow for precisely positioning the sheet in a predetermined height and therefore in a predetermined distance from a processing device (e. g. print bars in the case of an inkjet printing machine). This ensures invariable high processing quality. For their operation the Bernoulli cups need pressurized air, no vacuum supply is required. 
     The combination of Bernoulli cups with the vacuum system allows for homogenously supporting the sheet, thus improving the sheet flatness, and at the same time moving the support conveyor relative to the sheet with reduced friction. Furthermore, having a vacuum system as well as a cushioning device improves the flexibility of the device with respect to handling substrate of different sizes and/or materials as operation parameters of the vacuum system as well as of the cushioning device may suitably adjusted. 
     The vacuum system may be constituted by one or several vacuum chambers that are connected to the plurality of orifices in the interacting surface. The Bernoulli cups may be distributed along the width of the support conveyor. Their orifices lead into the interacting surface as well. 
     Instead of Bernoulli cups, air bearings may be used, in particular aerostatic bearings. 
     Preferably, the supported individual sheet and the support conveyor are movable along the transport direction with respect to each other, in particular even in the supported state of the sheet, i. e. the position of the support conveyor with respect to the sheet is not fixed. It is to be noted that relative movement between the sheet and the support conveyor may be in transport direction or against the transport direction. 
     Due to the fact that the support conveyor is movable with respect to the sheet, the optimum support position (e. g. with respect to the processing station) may always be chosen and the longitudinal extension of the support conveyor may be minimized, which reduces the weight and inertia of the support conveyors (thus increasing the achievable dynamics of the machine) and facilitates the handling thereof. 
     Preferably, the machine includes a control system for controlling a movement of the gripper conveyor and the support conveyor such that the supported region coincides with a processing region of the processing station. Accordingly, during operation a distance between the gripper conveyor and the support conveyor is changed during the transport through the processing station in order to ensure that the supported region coincides with a processing region of the processing station. 
     Taking into account the present location of the gripper conveyor(s), the location of the support conveyor is chosen in such a way that the sheet is stably supported in the processing region. If no gripper conveyor is in the processing region or immediately adjacent thereto, the location of the support conveyor will be generally opposite the processing station, in the processing region. Preferably, the longitudinal extension of the support conveyor is chosen such that the sheet is supported with perfect flatness in the processing region. 
     Preferably, during transport through the processing station, a transport speed of the support conveyor is smaller than a transport speed of the gripper conveyor. The control system is designed to control the speeds accordingly. This allows for controlling the movements of the gripper conveyor(s) and of the support conveyor in such a way that in a first stage, the support conveyor is supporting a region close to the leading edge of the transported sheet. Subsequently, the support conveyor “lags” with respect to the gripper conveyor(s) and therefore supports regions of the sheet further away from the leading edge, approaching the trailing edge. 
     In certain cases, especially for the processing of comparably small sheets, it will not be required to change the distance between the gripper conveyor and the support conveyor during the transport through the processing station. The inventive machine allows for a corresponding mode of operation. 
     Preferably, in particular if the supported individual sheet and the support conveyor are movable along the transport direction with respect to each other, the interacting surface of the support conveyor comprises a plurality of rollers having a rotation axis substantially perpendicular to the transport direction, i. e. having a rotation axis that includes an angle of 5° or less with a line that is perpendicular to the transport direction and parallel with the main plane defined by the interacting surface. This reduces the friction between the individual sheet and the support conveyor, in particular when the movement of the support conveyor is decelerated or accelerated with respect to the gripper conveyor. In particular, the interacting surface comprises a number of rollers arranged in parallel. 
     When processing corrugated sheet material it is preferred to have a rotation axis that is oriented parallel to the main plane but slightly inclined to the perpendicular direction, in particular including an angle of 1-15° therewith. This avoids engagement of the rollers with the corrugated board and ensures even support of the corrugated sheet. The width of the support conveyor may be covered by a plurality of groups arranged in succession, along the main extension of the interacting surface, each group including a number of parallel rollers (e. g. 3-12 rollers). 
     Preferably, the transport system comprises a circulating track, wherein the at least one gripper conveyor and the at least one support conveyor are running along the circulating track and wherein a section of the circulating track extends in transport direction. Having a circulating track simplifies the recirculation of the gripper conveyors and support conveyors, no additional recirculation system is needed, and the gripper and support conveyors are always arranged on the track, i. e. during normal operation, no introduction or removal of gripper or support conveyors is required. During operation of the machine, the gripper and support conveyors will usually stand still or move in a single predetermined direction. 
     Advantageously, the circulating track extends in a first plane, and the gripper and support conveyors are guided along the circulating track in such a way that along the transport track a main surface of individual sheets held by the gripper conveyors and supported by the support conveyors extends in a second plane, the second plane being perpendicular to the first plane and oriented along the transport direction. In particular, the first plane is oriented in a vertical direction, the second plane as well as the transport direction are oriented horizontally. This means that the footprint of the machine is not substantially affected by having a circulating track, as the recirculation of the gripper conveyors happens below (preferred) or above the transport track. 
     Preferably, the transport system further comprises a linear motor being controllable in such a way that movements of the at least one gripper conveyor and of the at least one support conveyor along the circulating track are individually controllable. This means in principle that the movement of a given gripper conveyor or support conveyor may be controlled independently from the movement of every other gripper conveyor or support conveyor (or further moveable units interacting with the transport track). It is to be noted that during operation of the machine movements of several gripper conveyors and support conveyors will usually be synchronized, and there may be constraints with respect to the relative positions and movements of several gripper conveyors and support conveyors that have to be taken into account when controlling the movement of the conveyors. Nevertheless, the linear motor and the conveyors are built in such a way that individual control is possible. 
     As an alternative to having a linear motor with a circulating track, a linear motor may be used for a straight track leading through the processing station, and other means are provided for recirculating the gripper conveyors to the start of the straight track. 
     Advantageously, at least one first gripper conveyor comprises a gripper mechanism for gripping a leading edge of one of the individual sheets and at least one second gripper conveyor comprises a gripper mechanism for gripping a trailing edge of the individual sheet. Accordingly, for transporting the individual sheet through the processing station, a leading edge of the sheet is gripped by the first gripper conveyor and a trailing edge of the sheet is gripped by the second gripper conveyor, the at least one support conveyor being arranged between the first gripper conveyor and the second gripper conveyor in the transport direction. 
     Gripping the individual sheets along their leading edge and their trailing edge allows for efficient and flexible handling of individual sheets, in particular large format sheets of materials such as corrugated cardboard or other materials that have a certain degree of inherent stability (such as thick cardboard sheets, plastic sheets, thin metal sheets etc.). Due to the fact that the gripper conveyors are individually controllable, the machine is easily readjusted for different sheet formats. There is no need for having a cart or carrier the dimensions of which matching the dimensions of the sheets to be processed, but the readjustment of the relative distance of the gripper conveyors for gripping the leading edge and the trailing edge, respectively, is sufficient for adapting the machine to different sheet dimensions in the transport direction (length). With respect to the sheet dimension across the transport direction (width), at least in the case of rectangular sheets, it does not matter if the grippers exceed the sheet width. 
     Having support conveyors to support the substrate between the leading and trailing gripper conveyors, the dimensions of the gripping conveyors along the transport direction may be chosen to be very short, in any case much shorter than the length of the individual sheets. Accordingly, the movable units of the transport system are much smaller and lighter than the carts or carriers of the prior art, thus allowing for faster dynamics and higher throughput. At the same time, adequate support of the sheets is ensured. 
     Preferably, after gripping, a distance between the first gripper conveyor and the second gripper conveyor is controlled in such a way that a tensioning force is applied to the individual sheet for straightening the individual sheet. In combination with the supporting action of the support conveyor, this reduces bending of the sheet along the transport direction. Using the individually controllable conveyors, the tensioning force may be precisely controlled. 
     Preferably, in the case of an inkjet printing machine, the printing station comprises a plurality of inkjet print bars, the print bars covering a printing region extending in a direction across the transport direction. In preferred embodiments, the print bars are essentially fixed in a lateral direction, and they cover the whole width of the print area all the time. In other embodiments, scanning print bar arrangements are employed. 
     Nevertheless, in general, the invention may be applied to other kinds of printing systems, especially printing systems for the printing of large sheet-like substrates, as well as to other systems for processing large sheet-like substrates. 
     Other advantageous embodiments and combinations of features come out from the detailed description below and the entirety of the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings used to explain the embodiments show: 
         FIG. 1A  An oblique view of a printing machine according to the invention; 
         FIG. 1B, 1C  detailed views of  FIG. 1A  showing the starting and end sections of the machine, respectively; 
         FIG. 2  a schematic side view of the circulating track of the machine; 
         FIG. 3  an oblique view of a gripper conveyor; 
         FIG. 4  a side view of the clamping bar of the gripper conveyor; 
         FIG. 5A, 5B  two oblique views of the base part of the gripper conveyor; 
         FIG. 6  a cross-sectional view illustrating the interaction between the gripper conveyor and the track; 
         FIG. 7  an oblique view of a first embodiment of a support conveyor; 
         FIG. 8  a schematic illustration of the process of gripping and transporting of a sheet; 
         FIG. 9A , B diagrams showing the speeds of the gripper and support conveyors as a function of location on the machine&#39;s track; 
         FIG. 10A-F  a schematic illustration of the functioning of the vacuum system of the machine according to the invention; 
         FIG. 11  the positions of some of the valves of the vacuum distribution unit of the vacuum system as a function of the position of the support conveyor along the transport direction; 
         FIG. 12  a schematic illustration of the functioning of a width adjustment mechanism of the vacuum distribution unit; and 
         FIG. 13  a top view of a support conveyor featuring a roller arrangement. 
     
    
    
     In the figures, the same components are given the same reference symbols. 
     PREFERRED EMBODIMENTS 
     The  FIG. 1A  is an oblique view of a printing machine according to the invention, the  FIGS. 1B, 1C  are detailed views of  FIG. 1A  showing the starting and end sections of the machine, respectively. 
     The printing machine  1  according to the shown embodiment is a continuously operated single pass inkjet printing machine for printing individual sheets, e. g. from corrugated cardboard. The maximum format of the individual sheets is 2.1×1.3 m (width×length). Typical thicknesses of corrugated cardboard that may be processed with the machine range from 0.7 to 7.0 mm. The achievable speed is 100 m/min (about 1 sheet per second), the printing resolution is 1′200 dpi. The printing machine is capable of printing water-based ink, e. g. for the printing of food packaging. 
     The printing machine  1  includes in succession a destacking robot  10  for destacking individual sheets from an input stack  2 , a feeding station  20 , a precoating station  30 , a first drying station  41 , a printing station  50  for four-colour inkjet printing, a second drying station  42 , a varnishing station  60 , a third drying station  43 , a removal station  70  and a stacking robot  80  for stacking the processed individual sheets onto an output stack  3 . An accommodating space  90  is provided between the removal station  70  and the stacking robot  80 . It may accommodate a further station such as a quality control station. A circulating transport system  100  extends from the feeding station  20  to the removal station  70 . It is described in more detail below. 
     All drying stations  41 ,  42 ,  43  are built alike, in a manner known as such, providing infrared and warm air drying. The destacking robot  10  and the stacking robot  80  are articulated arm robots and built alike, featuring gripper means for gripping partial stacks of individual sheets. The printing station  50  as well as the precoating station and the varnishing station  60  are based on print bars extending over the entire width of the machine. A suitable print bar technology is described in WO 2017/011923 A1 and WO 2017/011924 A1 (filed by Radex AG, now Mouvent SA). 
     The input stack  2  has a typical height of about 2 m. From the input stack  2 , the destacking robot  10  seizes partial stacks having a height of about 20 cm, turns them over and feds them to the feeding station  20 . The feeding station  20  is constituted of a first unit  21  and a second unit  22 . The first unit  21  comprises a sheet lift and a number of manipulators. The sheet lift receives a partial stack from the destacking robot  10 . The sheets of the partial stack are lifted by the sheet lift. The uppermost sheet is seized by a lateral bar, using a vacuum system, the present lateral position is determined and the sheet is positioned in an exact predetermined lateral position. The orientation is ensured by suitable guides. This exact lateral position and orientation of the sheet is maintained until the sheet is seized by the circulating transport system  100 . 
     The sheet is then fed to the second unit  22  comprising in a first stage a set of upper transport bands and a set of lower transport bands. All transport bands extend in the longitudinal direction, parallel to the transport direction of the sheets. In the first stage, the sheets are received between the two sets of transport bands. In a second stage of the second unit  22 , the sheets are attached to the top set of belts only, using a vacuum system. It is from this second stage where the sheets are seized by the circulating transport system  100 . The belt and vacuum system ensures that the sheets are provided in a flat state, their lateral position and orientation corresponding to that defined by the first unit of the feeding station  20 . 
     The removal station  70  basically corresponds to the second stage of the second unit  22  of the feeding station  20 , i. e. the processed sheets are received from the circulating transport system  100  by means of a set of upper vacuum belts. These belts transport the sheets one by one to the next station. 
     The  FIG. 2  is a schematic side view of the circulating track of the machine. The  FIG. 3  shows an oblique view of a gripper conveyor, the  FIG. 4  a side view of the clamping bar of the gripper conveyor, and the  FIGS. 5A, 5B  two oblique views of the base part of the gripper conveyor. The  FIG. 6  is a cross-sectional view illustrating the interaction between the gripper conveyor and the track, along a plane between the housing and elements for interacting with the track, attached to or protruding from the housing. 
     The circulating transport system  100  includes a circulating track  101  constituted by an upper straight section  102 , a lower straight section  103 , a first turning section  104  (input side) and a second turning section  105  (output side), the turning sections  104 ,  105  linking the upper straight section  102  and the lower straight section  103 . The upper straight section  102  and the lower straight section  103  are provided by track modules  110 , the turning sections  104 ,  105  are provided by the end modules  120  (see  FIG. 1B, 1C ). As shown in  FIG. 6 , the main components of the circulating track  101  are the carrying rail  113 , the guide rail  114  and the electromagnets  116  (not shown in  FIG. 2 ). The described track has a length of about 2×10 m plus the two turning sections, along the track the linear motor features about 90 electromagnets  116 ,  30  gripper conveyors  150  and  15  support conveyors  190  are simultaneously interacting with the track  101 . The gripper conveyors  150  and support conveyors  190  interact with the carrying rail  113  at two points of contact and with the guide rail  114  at a further point of contact, as described in more detail below. 
     An air supply station  130  is provided in the lower straight section  103 . An air supply mechanism  140  is provided in the upper straight section  102 , in the region of the printing station  50 . These components are described in more detail below. 
     The gripper conveyor  150  includes a base part  151  and a clamping bar  171  mounted on top of the base part  151 . The  FIGS. 3, 4  show a clamping bar  171  which is designed to clamp a trailing edge of an individual sheet to be processed. The clamping bar  171  features a main profile  172 , which is prismatic and has a basically trapezoid cross-section. The longer of the parallel sides of the trapezoid constitutes the upper surface of the clamping bar  171 , together with extensions extending to both sides. The upper surface is a support surface  173  for the individual sheet to be processed. It features a slit  174  extending from one lateral end of the clamping bar  171  to the other. 
     A clamping spring  175  made of spring steel is attached to one of the extensions of the main profile  172 . In cross section, a first section  175   a  of the clamping spring  175  is supported on the inner face of the extension and mounted to the main profile  172  by a mounting block  176  screwed to the extension. A second section  175   b  of the clamping spring  175  extends from the first section  175   a , bent to the inside of the main profile  172  by an angle of about 45°. A third section  175   c  extends from the second section, bent to the upper surface of the clamping bar  171  by an angle of about 45°, i. e. the third section  175   c  extends parallel to the upper surface (support surface  173 ). Attached to the free end of the third section  175   c  are L-shaped clamping elements  175   d , arranged along the whole length of the clamping spring  175 , and penetrating the slit  174  in the support surface  173 , the shorter leg of the clamping elements  175   d  being supported on the support surface  173 , i. e. on the outside of the main profile  172 . 
     The clamping bar  171  further comprises an elongated inflatable tube  181 . It is attached to the section of the main profile  172  forming the shorter parallel side of the trapezoid and is arranged in between this section of the profile  172  and the third section  175   c  of the clamping spring  175 . In the deflated state shown in  FIG. 4 , the tube  181  does not impact any force on the clamping spring  175 , and due to its geometry and elasticity, the clamping spring  175  exerts a certain clamping force to the support surface  173  of the clamping bar  171 . 
     The inflatable tube  181  is a closed air container and features a single access, linked to a vent. In an uninflated state, the tube  181  has an oval cross-section. By inflating the tube  181  with compressed air, the tube  181  changes its shape to a more circular cross-section, i. e. the height of the tube  181  increases and its width decreases. This has the effect that the third section  175   c  of the clamping spring  175  is contacted by the outer surface of the tube  181  and moved in the direction of the support surface  173 . The clamping elements  175   d  are moved as well and their short legs are raised from the support surface  173 , such that a gap is formed for receiving a sheet edge. The maximum gap height exceeds the maximum thickness of the substrates to be processed. In the shown case, the maximum gap height is 12 mm. 
     If the inflatable tube  181  is deflated again, the force between the tube  181  and the clamping spring  175  decreases to substantially zero, and the clamping force between the clamping spring  175  and the sheet (or the support surface  173 ) is reestablished due to the elasticity of the clamping spring  175 . 
     The base part  151  comprises a housing  152 . The housing  152  mounts two rail guides  153 ,  154 , both including a rotational bearing, on which a guide element for interacting with a guide rail is mounted. In the  FIG. 6B , one of the guide elements is displayed, the other is omitted for illustration purposes. The two rail guides  153 ,  154  are arranged near the upper edge of the housing  152 , on the front as well as on the back end thereof. The rotational axes of the rotational bearings are parallel to each other and run perpendicular to a lateral surface of the housing  152 . In a central section of the lower edge of the housing  152 , a support roll  155  is mounted. The rotational axis of the support roll  155  runs parallel to the lateral surface of the housing  152  and perpendicular to the support surface  173  of a clamping bar  171  mounted to the base part  151 . 
     Attached to the housing  152  is a holding part  158  for mounting a clamping bar  171  (as shown in  FIGS. 3, 4 ). The holding part  158  is connected to the housing  152  by a mounting flange as well as by an adjustment lever  159 , one of the lateral surfaces of the housing  152  and the holding part  158  forming an essentially L-shaped element, the adjustment lever  159  extending from the housing  152  to the free end of the leg forming the holding part  158 . The adjustment lever  159  allows for precisely adjusting an angle between the longitudinal extension of the clamping bar  171  and the plane defined by the two rail guides  153 ,  154  and the support roll  155 . 
     An air reservoir  161  is accommodated in the housing  152 . An air interface  162  is connected to the air reservoir  161  by a line including a check valve. This allows for introducing pressurized air through the air interface  162  into the air reservoir  161 . The air reservoir  161  is further connected to a multiport valve  163 . This valve may be switched between two modes of operation by means of a control pin  164  arranged on an lower surface of the housing  152  as follows: 
     
       
         
           
               
               
               
               
             
               
                   
               
               
                   
                 line reservoir - 
                 line tube - 
                   
               
               
                 control pin 
                 tube 
                 exterior 
                 effect 
               
               
                   
               
             
            
               
                 not operated 
                 closed 
                 open 
                 tube is deflated 
               
               
                 operated (pressed) 
                 open 
                 closed 
                 tube is inflated 
               
               
                   
               
            
           
         
       
     
     Finally, the base part  151  of the gripper conveyor  150  features a permanent magnet bar  165  for interacting with the electromagnets of the stationary part of the linear motor. The magnets are sealed in a slab of synthetic resin. The slab is mounted on a lateral surface of the housing  152 , on the same side as the guide elements of the rail guides  153 ,  154 . 
     The interaction of a gripper conveyor  150  with the carrying rail  113 , the guide rail  114  and the electromagnets  116  of the circulating track  101  is discussed in connection with  FIG. 6 . It shows a part of the circulating track  101  in one of the end modules, where the track is curved. The two rail guides  153 ,  154  on the base part  151  of the gripper conveyor  150  interact with the carrying rail  113 . They are constructed in such a way that lateral as well as normal forces may be transmitted between the gripper conveyor  150  and the carrying rail  113 . There are three points of contact, ensuring a defined position of the conveyor with respect to the track at all times, also in the curved sections. 
     The permanent magnet bar  165  is arranged on the base part  151  in such a way that it aligns with one or two of the local electromagnets  116 . The support roll  155  runs on a lateral surface of the guide rail  114  and supports the gripper conveyor  150  against tilting about an axis in the transport direction. By appropriately switching the electromagnets  116 , the gripper conveyor  150  moves along the circulating track  101  in a predetermined direction with a predetermined individual speed. 
     In order to supply compressed air to the air reservoirs  161  of the gripper conveyors  150 , the supply station  130  features a compressor and a tank for storing compressed air. The tank is connected to a supply pin arranged on a carriage that may be moved along a linear path by a belt drive driven by a drive motor. A hose linking the tank to the supply pin is guided by a guide chain such that high speed movements of the carriage are enabled. 
     The supply pin is mounted on the carriage by means of a pneumatic cylinder, which allows for extending or retracting the supply pin with respect to the carriage in a direction perpendicular to the linear path. The free end of the supply pin is provided by a valve, which is opened if a force acts against a valve tip extending from the supply pin. The geometry of the supply pin is adapted to the air interface  162  of the base part  151  of the gripper conveyor  150  (cf.  FIG. 5A ). 
     Prior to a gripper conveyor entering the air supply section of the circulating track  101 , the carriage is moved to its initial position. As soon as the gripper conveyor  150  is aligned with the carriage, the supply pin is extended by means of the pneumatic cylinder. It enters the air interface  162  of the gripper conveyor  150 , and the flow of compressed air is activated by the mechanical contact between a collar of the air interface  162  and the valve tip of the air supply pin. Next, the carriage with the air supply inserted into the air interface  162  follows the linear movement of the gripper conveyor  150  until a retraction point is reached. During this movement, pressurized air is introduced through the air interface  162  into the air reservoir  161  on the gripper conveyor  150 . The amount of air is sufficient to operate the gripper mechanism of the gripper conveyor  150  during a full cycle on the circulating track. At the retraction point, the air supply pin is retracted by means of the pneumatic cylinder, and the air supply is automatically stopped as soon as the valve tip loses mechanical contact with the air interface. Finally, the carriage moves back to its initial position, in order to interact with the next guide conveyor. 
     The  FIG. 7  is an oblique view of a first embodiment of a support conveyor. The support conveyor  190  includes a base part (not shown in  FIG. 7 ) and a support bar  191 . The base part essentially corresponds to the base part of the gripper conveyor as shown in  FIGS. 5A, 5B . In contrast to the base part of the gripper conveyor, an air reservoir is missing and the air interface is connected to the Bernoulli cups. The interaction of the base part of the support conveyor with the circulating track corresponds to the interaction between the gripper conveyor and the track. 
     The support bar  191  features a main profile  192 , which is prismatic and has a basically trapezoid cross-section, the longer of the parallel sides of the trapezoid constituting the upper surface of the support bar  191 . The upper surface  193  features a leading region  193   a  and a trailing region  193   b , separated by a slit  194  extending from one lateral end of the support bar  191  to the other. The leading region  193   a  features eleven Bernoulli cups  195  which are evenly distributed along the bar. The Bernoulli cups  195  are connected to the air interface in the base part. Using valves arranged in the supply lines, some of the cups in the outer regions of the support bar  191  may be selectively activated or deactivated. 
     The slit  194  provides an escape for the air flow generated by the Bernoulli cups  195  and therefore prohibits the buildup of an extensive air cushion between the support bar  191  and the supported sheet, which would deteriorate the precision of the sheet support. The trailing region  193   b  of the upper surface  193  is provided by a number of circular holes. They help reducing the weight of the support bar  191 . 
     The  FIG. 8  is a schematic illustration of the process of gripping and transporting of a sheet. The  FIGS. 9A , B are diagrams showing the speeds of the gripper and support conveyors as a function of location on the machine&#39;s track according to two alternative operating procedures. 
     As described above, the sheets  5  are fed from the second unit of the feeding station, held by the upper set of belts and a corresponding vacuum system. As shown in  FIG. 8 ( a ) , prior to feeding the sheet  5 , the first gripper conveyor  150 . 1  is positioned along the circulating track  101  in a receiving position in a receiving region  6 , a transport speed v 1  of the gripper conveyor  150 . 1  (dot-dashed line) is less than a (constant) feeding speed v s  of the sheet  5 . In this section, the track  101  features a cam, which interacts with the control pin  164  of the gripper conveyor to inflate the tube  181 . This opens the clamping elements  175   d  of the gripper conveyor  150 . 1 . Held by the upper set of belts, the sheet  5  is inserted with its leading edge in between the clamping elements  175   d  and the upper surface  173  of the gripper conveyor  150 . As soon as this has happened, the cam ends, the control pin  164  extends and the tube  181  is deflated. At this place, the belts end, i. e. the handover of the respective portion of sheet  5  to the conveyors of the transport system is finished. This leads to the situation depicted in  FIG. 8 ( b ) . 
     The first gripper conveyor  150 . 1  is further moved along the track  101  and a support conveyor  190  is moved below the sheet  5 . The support conveyor  190  has the same general buildup as the gripper conveyors  150 , however there is no gripping mechanism and therefore no air reservoir or tube. Initially, the support conveyor  190  supports the sheet  5  in a front region thereof, adjacent to the edge being gripped by the first gripper conveyor  150 . 1 , as shown in  FIG. 8 ( c ) . In this phase, the support conveyor  190  follows the first gripper conveyor  150 . 1  with a speed v 3  corresponding essentially to the feeding speed v s  of the sheet and to the speed v 1  of the first gripper conveyor  150 . 1 . Immediately after placing the support conveyor  190  below the sheet  5 , a contact element coupled to the free end of a movable flexible hose is connected to a quick-action coupling of the support conveyor  190 . This automatically opens a valve, such that pressurized air is supplied to the support conveyor, where it is distributed to those Bernoulli cups that are activated. The hose is coupled until the support conveyor  190  reaches a releasing region  8 . There, the quick-action coupling is released, by a control pin interacting with a cam of the track  101  and the hose is retracted. A carrier of the hose is recirculated to the initial position in the receiving region  6 . 
     Next, a second gripper conveyor  150 . 2  is moved along the track  101  in the receiving region  6 , with a transport speed v 2  bigger than the present transport speed v 1 =v s  of the first gripper conveyor  150 . 1  with the sheet  5 . Again, the clamping elements  175   d  are opened due to interaction of the control pin  164  with the cam, see  FIG. 8 ( d )  showing a rear region of the track. The trailing edge of the sheet  5  is received in between the clamping elements  175   d  and the upper surface  173  of the second gripper conveyor  150 . 2 . Finally, as soon as the cam ends, the control pin  164  extends and the tube  181  is deflated. 
     The sheet  5 , held by both gripper conveyors  150 . 1 ,  150 . 2  and supported by the support conveyor  190  in a front region is further transported until the printing station  50  is reached. The region of the sheet  5  that is processed by the printing station is supported by the first gripper conveyor  150 . 1  and the support conveyor  190 , see  FIG. 8 ( c ) ,  8  ( e ). In a region immediately upstream the printing region  7  as well as within the printing region  7  itself, the speed v 3  of the support conveyor  190  is reduced. The  FIGS. 9A, 9B  show two different possibilities. According to the progression in  FIG. 9A , the support conveyor  190  is standing still in a position facing the printing station  50 , i. e. the speed v 3  goes down to zero. According to the progression in  FIG. 9B , the support conveyor  190  is slowed down to a certain small speed but it is never completely standing still. In both cases, in and around the printing region  7  the speed v 3  is considerably smaller than the speed v 1 =v 2 =v s  of the two gripper conveyors  150 . 1 ,  150 . 2  and the sheet  5 , such that the support conveyor  190  lags with respect to the first gripper conveyor  150 . 1  and the sheet  5  and is approached by the second gripper conveyor  150 . 2 , see  FIG. 8 ( f ) . During transport, in order to further improve the flatness of the sheet  5 , the speeds of the two gripper conveyors  150 . 1 ,  150 . 2  may be adjusted to impart some tensioning force on the sheet  5 . 
     As soon as the second gripper conveyor  150 . 2  approaches the printing region  7  the speed v 3  of the support conveyor  190  is again increased until it reaches the transport speed v 1 =v 2 =v s  of the gripper conveyors  150 . 1 ,  150 . 2  and the sheet  5 . Thus the sheet  5  is transported out of the printing station  50 , a rear region immediately adjacent the gripped trailing edge being supported by the support conveyor  190 , see  FIGS. 8 ( g ) ,  8  ( h ),  8  ( i ). 
     From receiving the sheets, during the entire processing the sheets and up to hand over the sheets to the removal station, the gripper conveyors do not require any energy supply. This is due to the following:
         the actuation of the gripping mechanism is based on a mechanical interaction between the control pin and the cam,   the energy required for actuating the gripping mechanism is provided by the air reservoir on the gripping conveyor, and   the energy for movement of the conveyors is delivered to the stationary electromagnets of the linear motor.       

     The only place where external energy is provided to the conveyors is the air supply station, as described above. Nevertheless, despite the passive nature of the conveyors, their movement along the track may be individually controlled. For this purpose, the control system of the printing machine is connected to appropriate sensors for determining the positions of all the grippers. 
     The handover of the sheets from the gripping conveyors to the removal station essentially corresponds to the feeding of the sheets. This means that after opening the clamping mechanism in a releasing region  8 , the speed v 1  of the first gripper conveyor  150 . 1  is increased such that the leading edge of the sheet is released and the conveyor goes into recirculation. In contrast, after opening the respective clamping mechanism, the speed v 2  of the second gripper conveyor  150 . 2  is temporarily decreased to release the trailing edge of the sheet. As soon as the sheet has been removed, the speed v 2  is increased to recirculate the second gripper conveyor  150 . 2 . This also applies to the support conveyor  190 . 
     After handover, the gripper conveyors and support conveyors are further moved along the track, passing the first turning section, the lower linear section with the air supply station and the second turning section. Along a first part of the lower linear section, the speed of the conveyors is substantially higher than on the upper linear section. This allows for reducing the recirculation speed in the air supply station and ensures that the gripper conveyors are timely supplied for the next cycle. 
     The printing machine may further comprise a cleaning station for cleaning the gripper and support conveyors. It may be arranged in the vicinity of the air supply station. 
     The  FIGS. 10A-F  provide a schematic illustration of the functioning of the vacuum system of the machine according to the invention. The support conveyors  290  according to a second embodiment include a vacuum system instead of the Bernoulli cups. Apart from these elements, the support conveyors  290  correspond to the support conveyors  190  according to the first embodiment as previously described. Again, only the support bars are shown in the  FIG. 10 , not the base part that interacts with the circulating track. 
     The support conveyor  290  comprises connection openings  296  on its lower surface. With respect to the transport direction, they are arranged in a central portion of the support conveyor  290  and extend about half of the length of the lower surface. On its upper surface, the support conveyor  290  features a vacuum orifice array  297  having a large number of vacuum orifices distributed about most of the top surface. As described in more detail below, in connection with  FIG. 12 , within the support conveyor  290 , the connection openings  296  are fluidly connected to groups of orifices of the vacuum orifice array  297  lying above the respective opening. 
     The vacuum system further comprises a vacuum distribution unit  300  which is fixedly attached to the printing machine and extends in the transport direction over substantially the length of the printing station  50 . The vacuum distribution unit comprises a suction line  301 , which is connected to a vacuum source (not shown). The suction line  301  is connected to a number n of vacuum channels  302 . 1  . . . n extending in a vertical direction, upwards from the suction line  301 , where n denotes a number in particular in the range of 5 . . . 30. The number may be smaller or larger, depending on the length of the region that needs to be covered. Preferably it is chosen in such a way that the covered length corresponds to the length of the printing region plus the extra length needed to keep the sheet flat before and after the printing unit. 
     In the  FIG. 10 , not all the vacuum channels are shown, the middle region that is left out is indicated by dot-dashed lines. All vacuum channels  302 . 1  . . . n feature a connection opening  303 . 1  . . . n at their free upper end as well as a valve  304 . 1  . . . n at their lower end, close to the connection with the suction line  301 . Each of the valves  304 . 1  . . . n comprises a flap pivotable about a horizontal axis perpendicular to the transport direction, that is independently controllable to open and close the connection of the suction line  301  to the respective vacuum channel  302 . 1  . . . n. In the starting position shown in  FIG. 10A , all valves  304 . 1  . . . n are closed. 
     The transport path of the support conveyor  290  and the vacuum distribution unit  300  with its vacuum channels  302 . 1  . . . n are arranged such that the lower surface of the support conveyor  290  cooperates with the upper ends of the walls surrounding the vacuum channels  302 . 1  . . . n in such a way that an essentially sealed connection may be established between one or more of the connection openings  303 . 1  . . . n of the vacuum channels  302 . 1  . . . n and the connection opening  296  of the support conveyor  290 . 
     The  FIG. 11  shows the positions of the first five valves  304 . 1  . . .  5  on the vertical axis (0: fully closed, 1: fully open) as a function of the position x of the support conveyor  290  along the transport direction. The correspondence is as follows: 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 first (upstream) valve 304.1 
                 solid line 
               
               
                   
                 second valve 304.2 
                 dot-dashed line (2 dots - 3 dashes) 
               
               
                   
                 third valve 304.3 
                 long dashed line 
               
               
                   
                 fourth valve 304.4 
                 dot-dashed line (10 dots - 1 dash) 
               
               
                   
                 fifth valve 304.5 
                 dotted line 
               
               
                   
                   
               
            
           
         
       
     
     The further valves  304 . 6 , . . .  304 . n  are controlled accordingly. 
     The positions shown in the  FIGS. 10A-10E  are indicated in the  FIG. 11  by the arrows A-E, respectively. 
     As soon as the connection opening  303 . 1  of the first (upstream) vacuum channel  302 . 1  has been sealed by the lower surface of the support conveyor  290 , i. e. if the support conveyor  290  has reached the position along the transport direction shown in  FIG. 10A , the corresponding valve  304 . 1  in the first vacuum channel  302 . 1  is opened, in this position the extension of the opening between the first vacuum channel  302 . 1  and the connection opening  296  along the transport direction is about 20 mm. This establishes a fluid connection between the orifices of the vacuum orifice array  297  and the vacuum source, such that a sheet supported by the support conveyor  290  (not displayed) is firmly pressed against the upper surface of the support conveyor  290 . The situation and the resulting airflow are schematically shown in  FIG. 10B . 
     The support conveyor  290  is further moved along the transport direction. As soon as the second vacuum channel  302 . 2  is sealed by the lower surface of the support conveyor  290 , i. e. if the support conveyor  290  has reached the position along the transport direction shown in  FIG. 10C , the valve  304 . 2  in the second vacuum channel  302 . 2  is opened. Subsequently, for a certain period the first vacuum channel  302 . 1  as well as the second vacuum channel  302 . 2  are connected to the connection opening  296  of the support conveyor as well as to the vacuum source, cf.  FIG. 10D . 
     Before the sealing relationship between the lower surface of the support conveyor  290  and the first vacuum channel  302 . 1  is lost due to the further movement of the support conveyor  290 , the valve  304 . 1  is closed, cf.  FIG. 10E . 
     The process continues, wherein the support conveyor  290  transported along the vacuum distribution unit  300  cooperates successively with the vacuum channels  302 . 2 ,  302 . 3 ,  302 . 4 , . . . ,  302 . n −1,  302 . n . The valve  304 . n  of the last (downstream) vacuum channel  302 . n  is closed before the sealing relationship of the lower surface of the support conveyor  290  with the last vacuum channel  302 . n  is lost. This is the moment when the vacuum support ends (cf.  FIG. 10F  showing the situation immediately prior the closing of the last valve  304 . n ). 
     The  FIG. 12  is a schematic illustration of the functioning of a width adjustment mechanism of the vacuum distribution unit. The illustration shows a cross-section through the vacuum distribution unit  300 , the support conveyor  290 , the sheet  5  supported on the support conveyor  290  and the printing station  50 , in a plane perpendicular to the transport direction. 
     In the vacuum channel  302  a slider  305  is arranged, movable along an axis perpendicular to the transport direction. The slider  305  is designed such that a sealed connection with the contact surface of a support conveyor interacting with the vacuum distribution unit as well as with the floor and the side walls of the vacuum channel  302  is established. 
     The valve  304  linking the vacuum channel  302  to the suction line  301  is arranged in a middle section of the vacuum distribution unit  300 . By sliding the slider  305 , the width of the region of the connection opening  303  of the vacuum channel  302  that is effectively in communication with the suction line  301  may be adjusted. 
     The support conveyor  290  comprises a number of separation walls  298  running in the transport direction, designed in such a way that the plurality of orifices of the vacuum orifice array  297  are connected to connection openings  296  lying essentially below on the lower surface of the support conveyor  290 . 
     Accordingly, by moving the slider  305  the width of the active area of the vacuum orifice array  297  of the support conveyor  290  may be chosen. In a region outside the slider  305 , the orifices of the array are not in communication with the suction line and therefore, no air will be sucked through these orifices. In cases where the sheet width is smaller than the width of the support conveyor the required power of the vacuum system is reduced and air movement around the sheet which might impair the print quality is avoided. 
     It is not required that the separation walls  298  are arranged along the entire width of the support conveyor. In particular, they are not required in a central region that will always be covered with the sheet when the vacuum system is operated. 
     The  FIG. 13  is a top view of a support conveyor featuring a roller arrangement. The top surface of the support conveyor  290  constitutes the interaction surface for contacting the sheet to be supported. The top surface includes six groups  291 . 1  . . .  6  of eight rollers  292  each, wherein each group  291 . 1  . . .  6  covers the entire length of the support conveyor  290  (along the transport direction) and a sixth of the width of the support conveyor  290 . The eight rollers  292  of a group  291 . 1  . . .  6  are freely rotatable and mounted in parallel to each other. The rotation axes of the rollers  292  of the three first groups  291 . 1 ,  291 . 2 ,  291 . 3 , starting from a first side of the support conveyor  290 , are oriented in parallel to the top surface including an angle of 4° with respect to the main extension of the support conveyor  290  (extending perpendicular to the transport direction). The rotation axes of the rollers  292  of the three further groups  291 . 4 ,  291 . 5 ,  291 . 6  are oriented in parallel to the top surface including an angle of −4° with respect to the main extension of the support conveyor  290 . The orifices of vacuum orifice array are arranged below the rollers  292  and air is sucked in between the neighbouring rollers  292 . 
     The invention is not restricted to the described embodiment or to inkjet printing machines. In particular, dimensions of the machine, the number and type of stations or the geometrical design of machine elements may be different from the shown examples. 
     As mentioned above, in another embodiment, the support conveyor includes Bernoulli cups in addition to the vacuum system. In this embodiment of the support conveyor, the Bernoulli cups are arranged in a central line along the width of the support conveyor. They are surrounded by the orifices of the vacuum system, which essentially extends over the entire upper surface of the support bar. 
     Interaction with the vacuum distribution unit is not limited to the support conveyors. In principle, the gripper conveyors may include vacuum systems themselves working similarly as the one of the support conveyor. 
     In summary, it is to be noted that the invention provides an inkjet printing machine that allows for flexible processing of different sized sheets as well as for increased dynamics and throughput.