Technical Field

The invention relates to an inkjet printing machine for printing individual sheets, the machine comprising at least one printing station and a transport system for transporting the individual sheets through the printing station, along a transport direction. The invention further relates to an inkjet printing process for printing individual sheets.

Background Art

Inkjet printing machines for the printing of individual sheets, such as sheets of corrugated cardboard, are known. 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.

Summary of the invention It is the object of the invention to create a printing machine pertaining to the technical field initially mentioned, that allows for flexible processing of different sized sheets as well as for increased dynamics and throughput.

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, wherein the supported individual sheet and the support conveyor are movable along the transport direction with respect to each other. 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 printed on is primarily required in the printing region of the printing station.

The sheet and the support conveyor are movable at least along the transport direction with respect to each other 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.

Instead of having a rather bulky and heavy carrier supporting the sheeting 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 improves the flatness of the individual sheet. This is particularly advantageous in the case of large substrates or substrates with low inherent stability. 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 printing 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 printing region of the printing station.

Accordingly, an inkjet printing process for printing 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 printing station;

c) during transport through the printing station, supporting a region of the gripped sheet by a support conveyor running in the transport direction, wherein a distance between the gripper conveyor and the support conveyor is changed during the transport through the printing station in order to ensure that the supported region coincides with a printing region of the printing 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 printing region. If no gripper conveyor is in the printing region or immediately adjacent thereto, the location of the support conveyor will be generally opposite the printing station, in the printing region. Preferably, the longitudinal extension of the support conveyor is chosen such that the sheet is supported with perfect flatness in the printing region.

Preferably, during transport through the printing 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 printing station. The inventive printing machine allows for a corresponding mode of operation.

Preferably, the support conveyor comprises a pneumatic system for supporting the individual sheet on an interacting surface of the support conveyor. In the present context, "support" includes contact support (by mechanical contact) as well as non-contact support (e. g. by means of an air cushion). Pneumatic systems may help reducing the immediate contact between the sheet and the support conveyor and thus negative mechanical impacts to the sheet. Furthermore, the effect of pneumatic systems may be easily controlled by adjusting the amount of air supplied, pressure and/or 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 printing process, respectively. Advantageously, the pneumatic system comprises a cushioning device for providing a support cushion between the interacting surface and the individual sheet supported by the support conveyor. Air cushions reduce friction between the interacting surface and the individual sheets. In a preferred embodiment, the cushioning device comprises a plurality of Bernoulli cups for holding the sheet in a predetermined distance of the interacting surface of the support conveyor. 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 printing device (i. e. print bars). This ensures invariable high printing quality. For their operation the Bernoulli cups need pressurized air, no vacuum supply is required. Instead of or in addition to Bernoulli cups, the cushioning device may include other elements such as hollow pads connected to a compressed air supply.

In a preferred embodiment, the pneumatic system comprises a vacuum system comprising a plurality of orifices in the interacting surface.

In a particularly preferred embodiment, an cushioning device, in particular an air cushioning device comprising a plurality of Bernoulli cups, is combined with a vacuum system. This 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.

Preferably, the support conveyor comprises a mechanism for adapting the pneumatic system to a sheet width of the processed individual sheet. In particular, individual elements of the pneumatic system (such as Bernoulli cups and/or vacuum orifices or chambers and/or groups of Bernoulli cups and/or 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.

Preferably, the machine includes a supply mechanism comprising a movable supply element that may be temporarily coupled to the support conveyor for the supply of compressed air and/or vacuum. Accordingly, during transport through the printing station, the support conveyor is temporarily coupled to the movable supply element of the supply mechanism for the supply of compressed air and/or vacuum. This allows for feeding required amounts of compressed air (or providing vacuum) as long as needed. No local storage is required on the support conveyor.

The movable supply element may include a flexible hose with a quick-action coupling that is coupled with a suitable counterpart element on the support conveyor prior to operation of the pneumatic system, in particular before contacting the sheet. After removal of the sheet, the quick-action coupling is decoupled and moved back to the coupling site. More than one hose for coupling to subsequent support conveyors may be used in order to increase throughput. Instead of a flexible hose, a movable carriage carrying a substantially rigid coupling mechanism may be used. This carriage may be passively moved in the transport direction due to the mechanical connection to the support conveyor. The carriage or the hose may be moved back to the starting position by a suitable mechanism, e. g. by a pneumatic mechanism or a pusher driven by a linear drive.

Instead of Bernoulli cups, air bearings may be used, in particular aerostatic bearings. In principle, vacuum systems are also applicable, however measures need to be taken to ensure that relative movement between the sheet and the support conveyor is not prevented.

Alternatively, the support conveyor mechanically supports the individual sheets. In applications where the individual sheets are fully or partially made from a ferromagnetic material, magnetic support systems may be used.

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 printing 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 printing 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 printing 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 printing 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 printing 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, 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. 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. 1 A An oblique view of a printing machine according to the invention;

Fig. 1 B, 1 C 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. 5 A, 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 support conveyor;

Fig. 8 a schematic illustration of the process of gripping and transporting of a sheet; and

Fig. 9 A, B diagrams showing the speeds of the gripper and support conveyors as a function of location on the machine's track.

In the figures, the same components are given the same reference symbols.

Preferred embodiments

The Figure 1 A is an oblique view of a printing machine according to the invention, the Figures 1 B, 1 C are detailed views of Fig. 1 A 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 x 1.3 m (width x 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 2017/01 1923 A1 and WO 2017/01 1924 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 Figure 2 is a schematic side view of the circulating track of the machine. The Figure 3 shows an oblique view of a gripper conveyor, the Figure 4 a side view of the clamping bar of the gripper conveyor, and the Figures 5A, 5B two oblique views of the base part of the gripper conveyor. The Figure 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 1 10, the turning sections 104, 105 are provided by the end modules 1 20 (see Fig. 1 B, 1 C). As shown in Figure 6, the main components of the circulating track 101 are the carrying rail 1 13, the guide rail 1 14 and the electromagnets 1 16 (not shown in Fig. 2). The described track has a length of about 2x10m plus the two turning sections, along the track the linear motor features about 90 electromagnets 1 16, 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 1 13 at two points of contact and with the guide rail 1 14 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 Figures 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 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.

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. 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 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 1 2 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 Figure 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 Figures 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:

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 1 13, the guide rail 1 14 and the electromagnets 1 16 of the circulating track 101 is discussed in connection with Figure 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 1 13. 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 1 13. 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 1 16. The support roll 155 runs on a lateral surface of the guide rail 1 14 and supports the gripper conveyor 150 against tilting about an axis in the transport direction. By appropriately switching the electromagnets 1 16, 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. Figure 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 16 1 on the gripper conveyor 1 50. 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 Figure 7 is an oblique view of a support conveyor. The support conveyor 190 includes a base part (not shown in Figure 7) and a support bar 191. The base part essentially corresponds to the base part of the gripper conveyor as shown in Figures 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 193a and a trailing region 193b, separated by a slit 194 extending from one lateral end of the support bar 191 to the other. The leading region 193a 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 vents 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 193b of the upper surface 193 is provided by a number of circular holes. They help reducing the weight of the support bar 191.

The Figure 8 is a schematic illustration of the process of gripping and transporting of a sheet. The Figures 9A, B are diagrams showing the speeds of the gripper and support conveyors as a function of location on the machine'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 Figure 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, 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 175d of the gripper conveyor 150.1. Field 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 1 50. As soon as this has happened, the cam ends, the control pin 164 extends and the tube 18 1 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 Figure 8 (b).

The first gripper conveyor 150. 1 is further moved along the track 1 01 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 Figure 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, of the first gripper conveyor 150.1. Immediately after placing the support conveyor 1 90 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 vent, 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 1 90 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,=v _s of the first gripper conveyor 150. 1 with the sheet 5. Again, the clamping elements 1 75d are opened due to interaction of the control pin 164 with the cam, see Figure 8 (d) showing a rear region of the track. 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 1 81 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 1 50. 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 Figures 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,=v ₂ =v _s of the two gripper conveyors 150. 1 , 150.2 and the sheet 5, such that the support conveyor 1 90 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 Figure 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 vi=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 Figures 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, 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 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.

As mentioned above, in another embodiment, the support conveyor includes a vacuum system in addition to the Bernoulli cups. The vacuum system is constituted by vacuum chambers that are arranged immediately below the upper surface of the support bar. The latter is provided by a large number of evenly distributed orifices connecting the respective vacuum chamber with the space above the upper surface. 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. 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.