Inkjet printing machine for printing individual sheets

Abstract

An inkjet printing machine for printing individual sheets comprises at least one printing station and a transport system defining a transport track (101) for transporting the individual sheets through the printing station, along a transport direction. The transport system comprises a plurality of gripper conveyors (150) running along the transport track (101) for holding the individual sheets during a printing process in the printing station. At least one of the plurality of gripper conveyors (150) comprises a gripper mechanism (175) for gripping a leading edge of one of the individual sheets and at least one of the plurality of gripper conveyors comprises a gripper mechanism (175) for gripping a trailing edge of the individual sheet. The transport system further comprises a linear motor (116, 165) being controllable in such a way that movement of each of the gripper conveyors (150) along the transport track (101) is individually controllable. The printing machine 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.

Claims

1. An inkjet printing machine for printing individual sheets, comprising: at least one printing station; and a transport system defining a transport track for transporting an individual sheet through the at least one printing station and along a transport direction, wherein: the transport system includes a first gripper conveyor and a second gripper conveyor running along the transport track for holding the individual sheet during a printing process in the at least one printing station, the first gripper conveyor includes a first gripper mechanism for gripping a leading edge of the individual sheet, the second gripper conveyor includes a second gripper mechanism for gripping a trailing edge of the individual sheet, and the transport system further includes a linear motor configured to individually control a movement of each of the first and second gripper conveyors along the transport track.

2. The inkjet printing machine as recited in claim 1, wherein: the first gripper mechanism and/or the second gripper mechanism includes a clamping bar, the clamping bar includes a first cross member and a second cross member, and a relative distance of the first cross member and the second cross member is adjustable to clamp the leading edge and/or the trailing edge of the individual sheet.

3. The inkjet printing machine as recited in claim 1, wherein the transport system is configured to transport a plurality of individual sheets and includes: a circulating track, a plurality of gripper conveyors that run along the circulating track, the plurality of gripper conveyors including the first gripper conveyor and the second gripper conveyor, the transport track forms a section of the circulating track, the circulating track extends in a first plane, the plurality of gripper conveyors are guided along the circulating track so that, along the transport track, main surfaces of the individual sheets held by the plurality of gripper conveyors extend in a second plane, and the second plane is perpendicular to the first plane and oriented along the transport direction.

4. The inkjet printing machine as recited in claim 1, wherein the transport system further includes at least one support conveyor provided between the first gripper conveyor and the second gripper conveyor for supporting the individual sheet in a central portion thereof.

5. The inkjet printing machine as recited in claim 1, wherein the at least one printing station includes a plurality of inkjet print bars, the inkjet print bars cover a print area extending across the transport track for the individual sheet, and further comprising an absorbing conveyor for absorbing excess ink when moved to an absorbing position opposite at least one of the inkjet print bars, the absorbing conveyor being movable along the transport track, and the absorbing conveyor includes a sponge-like element.

6. The inkjet printing machine as recited in claim 5, wherein the inkjet print bars are individually and dynamically movable in an adjustment direction perpendicular to a main surface of the individual sheet to be printed, such that a distance between the inkjet print bars and the individual sheet is dynamically adjustable, and further comprising: a detection unit for recording a profile of the individual sheet, wherein the recording the profile of the individual sheet includes recording a bending along the transport direction, and a control unit for controlling a movement of the inkjet print bars in the adjustment direction, based on the recorded bending along the transport direction, the detection unit comprising a distance sensor for recording the profile.

7. The inkjet printing machine as recited in claim 1, further comprising: a plurality of successively arranged modules, each of the plurality of successively arranged modules comprising a section of a rail of the linear motor, neighboring modules being mechanically linkable to each other; a first robot for destacking the individual sheet from a stack, the first robot being arranged upstream the transport track; and a second robot for stacking the printed individual sheet, the second robot being arranged downstream the transport track.

8. The inkjet printing machine as recited in claim 1, further comprising a feeding station for feeding the individual sheet to the first and second gripper conveyors, the feeding station including: two groups of belts running in the transport direction for pinching one individual sheet in between the first and second conveyors, and a vacuum system, wherein at least one of the groups of belts overlaps with a feeding section of the transport track.

9. The inkjet printing machine as recited in claim 1, wherein the first and second gripper conveyors each include a compressed air reservoir for operating the first and second gripper mechanisms, respectively.

10. The inkjet printing machine as recited in claim 9, the transport system comprising a supply station for compressed air to supply the compressed air reservoir of each of the first and second gripper conveyors, the supply station being arranged in a region of a circulating track outside the transport track for the individual sheet.

11. The inkjet printing machine as recited in claim 10, the supply station including a movable air supply interface for moving with one of the first or second gripper conveyors, along a section of the circulating track.

12. The inkjet printing machine as recited in claim 1, the first and second gripper conveyors including permanent magnets forming a rotor of the linear motor, a stationary rail of the linear motor forming a stator of the linear motor.

13. The inkjet printing machine as recited in claim 1, further comprising: a detection device for capturing positions and orientations of the first and second gripper conveyors; and a control device for controlling the at least one printing station to compensate for positional inaccuracies of the individual sheet transported by the first and second gripping conveyors.

14. The inkjet printing machine as recited in claim 13, wherein the detection device includes detectors for capturing two positions on each of the first and second gripper conveyors, the two positions being distant from each other.

15. The inkjet printing machine of claim 1, wherein the linear motor is configured to individually control a speed, in the transport direction, of each of the first and second gripper conveyors.

16. An inkjet printing process for printing individual sheets, comprising: gripping an individual sheet along a leading edge by a first gripper conveyor and along a trailing edge by a second gripper conveyor, the gripper conveyors running along a transport track in a transport direction; and transporting, by the gripper conveyors, the individual sheet along the transport track and through a printing station, wherein movement of each of the first gripper conveyor and second gripper conveyor in the transport direction are individually controllable by a linear motor.

17. The inkjet printing process as recited in claim 16, wherein the individual sheet is held by the first gripper and the second gripper conveyor in such a way that the individual sheet extends across a distance between the first conveyor and the second conveyor in the transport direction of the transport track.

18. The inkjet printing process as recited in claim 16, further comprising: recording a profile of the individual sheet; and individually controlling a vertical position of each of a plurality of inkjet print bars of the printing station such that the vertical position of each of the plurality of inkjet print bars corresponds to the recorded profile.

19. The inkjet printing process as recited in claim 16, further comprising: after gripping, controlling a distance between the first gripper conveyor and the second gripper conveyor to apply a tensioning force to the individual sheet for straightening the individual sheet.

20. An inkjet printing machine for printing individual sheets, comprising: at least one printing station; and a transport system defining a transport track for transporting individual sheets through the at least one printing station and along a transport direction, wherein: the transport system includes a plurality of gripper conveyors running along the transport track for holding the individual sheets during a printing process in the at least one printing station, the plurality of gripper conveyors includes a first gripper conveyor including a first gripper mechanism for gripping a leading edge of one of the individual sheets and a second gripper conveyor including a second gripper mechanism for gripping a trailing edge of the individual sheet, the first gripper mechanism and/or the second gripper mechanism includes a clamping bar to clamp the leading edge and/or the trailing edge of the sheet, and the transport system further includes a linear motor, the linear motor being controllable to cause movement of each of the plurality of gripper conveyors along the transport track to be individually controllable.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The drawings used to explain the embodiments show:

(2) FIG. 1A An oblique view of a printing machine according to the invention;

(3) FIG. 1B, 1C detail views of FIG. 1A showing the starting and end sections of the machine, respectively;

(4) FIG. 2A, 2B an oblique view of a track module with closed and open shutters;

(5) FIG. 3 a schematic side view of the circulating track of the machine;

(6) FIG. 4 an oblique view of a gripper conveyor;

(7) FIG. 5 a side view of the clamping bar of the gripper conveyor;

(8) FIG. 6A, 6B two oblique views of the base part of the gripper conveyor;

(9) FIG. 7 a cross-sectional view illustrating the interaction between the gripper conveyor and the track;

(10) FIG. 8A, 8B two oblique views of the supply station for compressed air; and

(11) FIG. 9 a schematic illustration of the process of gripping of a sheet.

(12) In the figures, the same components are given the same reference symbols.

PREFERRED EMBODIMENTS

(13) The FIG. 1A is an oblique view of a printing machine according to the invention, the FIGS. 1B, 1C are detail views of FIG. 1A showing the starting and end sections of the machine, respectively.

(14) 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.

(15) 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.

(16) 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).

(17) 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.

(18) 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.

(19) 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.

(20) The FIGS. 2A, 2B show an oblique view of a track module with closed and open shutters, respectively. The circulating transport system 100 is composed of a number of such modules. In addition to the track modules 110, one of them shown in FIGS. 2A, 2B the transport system 100 comprises end modules 120 (cf. FIGS. 1B, 1C). Each of the track modules 110 provides two straight sections of the transport track, an upper section 111 and a lower section 112. The end modules 120 provide a curved section of the transport track, linking the lower track to the upper track, turning by 180°.

(21) The track modules 110 comprise a machine frame 115 carrying the upper section 111 and the lower section 112. Both sections 111, 112 comprise a straight carrying rail 113 and a straight guide rail 114 arranged parallel to the carrying rail 113, in a predetermined distance. Furthermore, the sections 111, 112 comprise a number of electromagnets 116 each. The totality of upper sections 111, lower sections 112 and track sections of the end modules 120 constitute the linear motor for transporting the gripper conveyors along the circulating track as described in more detail below. The machine frame 115 further comprises structures extending along the upper lateral edges, for securely attaching stations or elements of stations, such as print bars or the elements of the feeding and removal stations.

(22) The machine frame further carries a number of shutters, including slide shutters 117, hatches 118 and doors 119. As shown in FIG. 2B, these shutters allow for easy access to the interior of the track module 110. One of the doors 119 includes control and power electronics for the individual track module 110. Each module features its own power supply and data connection. Adjoining modules are connected by screw connections, ensuring that the tracks of the two modules fall in line. Once these screw connections are undone, each of the modules may be moved out of the line in a lateral direction for maintenance, repair or replacement. Furthermore, the modular buildup allows for easily constructing printing machines of different lengths or even for changing the length of a printing machine at a later stage, especially if stations are added to or removed from the machine.

(23) The FIG. 3 is a schematic side view of the circulating track of the machine. The FIG. 4 shows an oblique view of a gripper conveyor, the FIG. 5 a side view of the clamping bar of the gripper conveyor, and the FIGS. 6A, 6B two oblique views of the base part of the gripper conveyor. The FIG. 7 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.

(24) 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. As described above, the upper straight section 102 and the lower straight section 103 are provided by the track modules 110, the turning sections 104, 105 are provided by the end modules 120. As described in connection with FIGS. 2A, 2B, 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. 3). 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 are simultaneously interacting with the track 101. The gripper conveyors 150 (and further modules) 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.

(25) An air supply station 130 is provided in the lower straight section 103. It is described in more detail below, in connection with FIGS. 8A, 8B.

(26) The gripper conveyor 150 includes a base part 151 and a clamping bar 171 mounted on top of the base part 151. The FIGS. 4, 5 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.

(27) 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 175a 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 175b of the clamping spring 175 extends from the first section 175a, bent to the inside of the main profile 172 by an angle of about 45°. A third section 175c 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 175c extends parallel to the upper surface (support surface 173). Attached to the free end of the third section 175c are L-shaped clamping elements 175d, 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 175d being supported on the support surface 173, i.e. on the outside of the main profile 172.

(28) 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 175c of the clamping spring 175. In the deflated state shown in FIG. 5, 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.

(29) 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 175c 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 175d 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.

(30) 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.

(31) 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.

(32) Attached to the housing 152 is a holding part 158 for mounting a clamping bar 171 (as shown in FIGS. 4, 5). 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.

(33) 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:

(34) TABLE-US-00001 line reservoir - line tube - control pin tube exterior effect not operated closed open tube is deflated operated (pressed) open closed tube is inflated

(35) 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.

(36) 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. 7. 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.

(37) 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 velocity.

(38) The FIGS. 8A, 8B are two oblique views of the supply station for compressed air. The air supply station 130 features a compressor and a tank 131 for storing compressed air. The tank 131 is connected to a supply pin 132 arranged on a carriage 133 that may be moved along a linear path by a belt drive 134 driven by a drive motor 135. The hose (not displayed) linking the tank 131 to the supply pin 132 is guided by a guide chain 136 such that high speed movements of the carriage 133 are enabled.

(39) The supply pin 132 is mounted on the carriage 133 by means of a pneumatic cylinder 137, which allows for extending or retracting the supply pin 132 with respect to the carriage 133 in a direction perpendicular to the linear path. The free end of the supply pin 132 is provided by a valve, which is opened if a force acts against a valve tip extending from the supply pin 132. The geometry of the supply pin 132 is adapted to the air interface 162 of the base part 151 of the gripper conveyor 150 (cf. FIG. 6A).

(40) Prior to a gripper conveyor 150 entering the air supply section of the circulating track 101, the carriage 133 is moved to its initial position. As soon as the gripper conveyor 150 is aligned with the carriage 133, the supply pin 132 is extended by means of the pneumatic cylinder 137. 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 132. Next, the carriage 133 with the air supply pin 132 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 132 is retracted by means of the pneumatic cylinder 137, and the air supply is automatically stopped as soon as the valve tip loses mechanical contact with the air interface. Finally, the carriage 133 moves back to its initial position, in order to interact with the next guide conveyor.

(41) The FIG. 9 is a schematic illustration of the process of gripping of a sheet. 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. 9 (a), prior to feeding the sheet 5, the first gripper conveyor 150.1 is positioned along the circulating track 101 in a receiving position, a transport speed of the gripper conveyor 150.1 is less than a feeding speed 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 175d 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 175d 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. 9 (b).

(42) 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 buildup as the gripper conveyors 150, however there is no gripping mechanism and therefore no air reservoir or tube. The support conveyor 190 supports a middle section of the sheet 5 as shown in FIG. 9 (c).

(43) Next, a second gripper conveyor 150.2 is moved along the track 101 with a transport speed bigger than the transport speed of the first gripper conveyor 150.1 with the sheet 5. Again, the clamping elements 175d are opened due to interaction of the control pin 164 with the cam. The trailing edge of the sheet 5 is received in between the clamping elements 175d 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. This leads to the situation depicted in FIG. 9 (e).

(44) For the further processing of the sheet 5, the two gripper conveyors 150.1, 150.2 and the support conveyor 190 are moved along the track 101 essentially with identical speeds. 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 and/or the support conveyor 190 may be provided with a vacuum system for aspirating the middle portion of the sheet 5.

(45) 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.

(46) 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.

(47) The handover of the sheets from the gripping conveyors to the removal station essentially corresponds to the feeding of the sheets. After handover, the gripper 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.

(48) 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.

(49) The printing station 50 features a mechanism for individually setting the height of its four print bars relative to the (nominal) feeding height of the sheets. This allows for precisely adjusting the distance of the nozzles of the printbars and the sheets. Thus, the printing station 50 is easily adapted to different sheet thicknesses. Furthermore, an encoder station with a distance sensor in the form of a laser curtain is provided upstream of the printing station 50, recording the profile of the sheet along the transport direction. The vertical adjustment of the print bars is controlled according to the recorded profile, such that local adjustment of the distance is enabled.

(50) Furthermore, the four corners of the gripper bars are provided with visual markings. They are captured by a video system affixed to the stationary part of the print machine. Based on this data, the position and possible mis-orientations of the respective sheet are determined, and the print data is processed to compensate for the determined imprecisions.

(51) In addition to the gripper and support conveyors, the printing machine may feature an absorbing conveyor for absorbing excess ink, especially ink used for flushing the inkjet nozzles. This buildup of this conveyor essentially corresponds to that of the gripper conveyors, however instead of a gripper mechanism there is a sponge like element for absorbing ink.

(52) The transport system of the described printing machine allows for precise positioning and transport of sheets of different thicknesses, widths and lengths. All necessary adjustments may be effected dynamically, without substantially reducing the throughput. The passive gripper conveyors allow for a simple setup, low conveyor weight and reliable operation even at high operating speeds.

(53) The invention is not restricted to the described embodiment. 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.

(54) In summary, it is to be noted that the invention creates a printing machine for printing individual sheets that allows for an increased flexibility and high throughput.