Ink-jet print heads protecting system in a digital printing machine

Abstract

The present invention relates to an ink-jet print heads protecting system in a digital printing machine, wherein the module comprises a chassis (1), rollers (2), and a plurality of digital printing units (10), each with a bridge (11) with guides (12) and a sliding carriage (13) with ink-jet print heads (14), one part of each guide overlapping the band and another part projecting beyond the band path, the carriage being movable between the printing and maintenance positions; and in that each bridge is attached to the chassis by of a position adjustment device (20) and an articulation (25) that are spaced apart or by two position adjustment devices (20) that are spaced apart.

Claims

1. An ink-jet printing heads protection system in a digital printing machine, the system comprising: a plurality of printing units, each comprising one bridge with linear guides and a carriage including a group of ink-jet printing heads slidably connected to the linear guides, the bridge being supported above a band path transversely to two side edges of the band path, the linear guides including a first portion located above the band path between the side edges thereof and a second portion extending beyond one side edge of the band path, the carriage being actuated by a second actuator configured to move the carriage along the linear guides between an operative position, where the ink-jet printing heads are above the band path facing a printing segment thereof, and a maintenance where the carriage is in the second portion of the linear guides and the ink-jet print heads are non-opposed to the band path permitting access to the ink-jet print heads from bellow for maintenance without interrupting the printing process of other printing units in the operative position; a chassis comprising a first sub-chassis and a second sub-chassis, the first sub-chassis supporting one or more supports defining the band path as an arched band path or as a polygonal band path for guiding a sheet material in a taut manner, and the second sub-chassis supporting the plurality of printing units, the first sub-chassis being moveable by a first actuator between a printing position where the printing segments of the band path are spaced apart less than 5 mm from the ink-jet print heads of the carriages placed in the operative position, and a secure position in which the distance between the printing segments and the ink jet print heads of the carriages placed in the operative position is bigger than in the printing position; a thickening presence signal generator configured to generate a thickening presence signal when a thickening of the sheet material approaches the printing units in a conveyance direction; the first actuator is connected to the thickening presence signal generator and is configured to move the first sub-chassis from the printing position to the secure position in response to the thickening presence signal; each bridge includes a first anchoring point attached to the second sub-chassis through an adjustment device and a second anchoring point attached to the second sub-chassis through an articulation or through another adjustment device, each adjustment device being configured to provide a precisely moving the anchoring point connected thereto for correctly aligning the bridge with respect to the corresponding printing segment and with respect to the other bridges of other plurality of printing units.

2. The ink-jet printing heads protecting system according to claim 1 wherein the thickening presence signal generator is selected among: a thickening detector configured to detect a thickening of the sheet material placed in a first position upstream of the printing units in the conveyance direction; and/or a metering device configured to determine a correspondence between a given portion of the sheet material placed in a first position upstream of the printing units in the conveyance direction and a pre-stored thickening position data; and/or a thickener device configured to create a thickening on the sheet material in a first position upstream of the printing units in the conveyance direction.

3. The ink-jet printing heads protection system according to claim 2 wherein the thickener device is an automatic splicer configured to adhere one final end of one partial band of sheet material to one initial end of another successive partial band of sheet material to create the continuous band of sheet material, creating a thickened splice.

4. The ink-jet printing heads protection system according to claim 2 wherein the thickening detector is selected among: an optical detector directed towards the band of sheet material; a thickness meter for metering the thickness of the band of sheet material; a roughness detector in contact with at least one face of the band of sheet material.

5. The ink-jet printing heads protection system according to claim 1 wherein the one or more supports are a single rotary roller defining the arched band path thereon, or a plurality of parallel rotary rollers defining the polygonal arched band path thereon, each printing segment is defined coincident with a single rotary roller, or each printing segments is defined between two successive rotary rollers.

6. The ink-jet printing heads protecting system according to claim 1 wherein the digital printing machine further comprises an image detector facing a freshly printed surface of the band of sheet material downstream from the printing segments configured to capture images thereof and configured to send the captured images to a control unit, the control unit is configured to perform an automatic analysis of the captured images to detect rubbing printing defects and/or ink-jet obturation printing defects, and the control unit is configured to activate the first actuator to move the first sub-chassis away from the ink-jet print heads and/or to move printing units from the operative position opposed to a printing segment to the maintenance position in response to the detection of rubbing printing defects and/or ink-jet obturation printing defects.

7. The ink-jet printing heads protecting system according to claim 6 wherein the control unit comprises stored data with ink-color information of an ink-color printed by each of the ink-jet print heads of each printing unit, and the control unit is configured to determine ink-color information of the rubbing printing defects and/or ink-jet obturation printing defects in the captured image and, considering the stored ink-color information and the deemed ink-color of the defects, to determine the printing unit printing with the rubbing printing defects and/or ink-jet obturation printing defect, and the control unit being further configured to activate the first actuator and/or the second actuator to move the first sub-chassis away from the ink-jet print heads, and/or to move printing units from a position opposed to a printing segment to the maintenance position in response to the detection of rubbing printing defects and/or ink-jet obturation printing defects.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The foregoing and other advantages and features will be better understood based on the following detailed description of an embodiment in reference to the attached drawings which must be interpreted in an illustrative and non-limiting manner, in which:

(2) FIGS. 1a and 1b shows a longitudinal section of the system according to an embodiment in which the supports are six rollers arranged in succession, defining an arcuate path for the band of sheet material, and provided with six printing units, each provided with digital ink-jet print heads arranged opposite a printing segment supported on one of said rollers, and all the rollers being supported on a first sub-chassis attached in a sliding manner to a second sub-chassis supporting the printing units, FIG. 1a showing the supports being placed in the printing position, and FIG. 1b showing the supports being placed in the secure position;

(3) FIGS. 2a and 2b shows a digital printing module similar to the one shown in FIG. 1a but provided with 5 printing units, each provided with a digital ink-jet print head arranged opposite a printing segment located in a portion of sheet material located in the interspace existing between two adjacent rollers, the digital printing module being additionally provided with a conveyor belt, and all the rollers being supported on a first sub-chassis attached in a swiveling manner to a second sub-chassis supporting the printing units, FIG. 2a showing the supports being placed in the printing position, and FIG. 2b showing the supports being placed in the secure position;

(4) FIGS. 3a and 3b shows an alternative embodiment of the digital printing module in which one of the rollers is of a diameter that is sufficient so that multiple printing units can have the corresponding digital ink-jet print heads arranged opposite printing segments of the sheet material supported on said roller, FIG. 3a showing the roller being placed in the printing position, and FIG. 3b showing the roller being placed in the secure position;

(5) FIGS. 4a and 4b show schematic perspective views of a portion of the system including two parallel rollers, a portion of the band path including a printing section supported on said rollers, and a schematic drawing of a printing unit showing a guide extending in the transverse direction, a carriage including ink-jet printing heads, wherein FIG. 4a shows the ink-jet printing heads being placed in the printing position and FIG. 4b shows the ink-jet printing heads being placed in the maintenance position;

(6) FIG. 5 shows a cross-section of the digital printing module parallel to the rollers, according to the embodiment shown in FIG. 1, the digital ink jet print heads and the corresponding carriage in a printing position being arranged opposite the printing segment, on a roller;

(7) FIG. 6 shows the same view as FIG. 4 but with the digital ink jet print heads and the corresponding carriage in the maintenance position not being arranged opposite the path for the band of sheet material, and with a head cleaning unit or a hermetic closure cover coupled to said digital ink jet print heads for cleaning or protection;

(8) FIG. 7 shows a view equivalent to the one shown in FIG. 4 but according to an embodiment in which the bridge and the corresponding guides are inclined with respect to the printing segment of the band of sheet material;

(9) FIG. 8 shows an enlarged detail of an embodiment similar to the one shown in FIG. 2 but lacking a conveyor belt, in which perforated plates connected to suction equipment through air ducts have been included, such that the band of sheet material is suctioned against the plate through said perforations as it passes over said plate, thereby obtaining complete flatness in the printing segment coinciding with said plate. Optionally, said plates can also be heated to increase the ink drying speed;

(10) FIG. 9 shows a schematic drawing of a possible embodiment of the adjustment mechanism which allows precise adjustment in the transverse, longitudinal, and perpendicular directions by three mechanisms, each provided with a spindle operated by a motor, the three spindles being orthogonal to one another, each parallel to one of the three transverse, longitudinal, and perpendicular directions.

(11) In the drawings showing several printing segments 6, the transverse, longitudinal, and/or perpendicular directions of at least some of said printing segments 6 have been indicated by a coordinate arrow system; therefore, in some cases various coordinate systems are included in one and the same drawing, one for each printing segment.

(12) An arrow indicates the direction of movement of the sheet material along the path for the band of sheet material defined by the rollers.

DETAILED DESCRIPTION OF AN EMBODIMENT

(13) The attached drawings show illustrative, non-limiting embodiments of the present invention.

(14) The present invention relates to a ink-jet print heads protecting method and system for digital printing machines.

(15) The proposed system comprises multiple rollers 2 parallel to one another provided for supporting and conveying a band of sheet material 4 in a taut manner, defining a path for the band of sheet material 4. Parts of said band path, corresponding with printing segments 6, are arranged opposite digital ink jet print heads 14 which are heads provided for projecting microdroplets of ink in precise positions of a printing segment of the sheet material in correspondence with the movement of the sheet material along the band path, thereby printing images stored in digital format on the sheet material in a quick, clean, and precise manner.

(16) Each digital ink jet print head 14 is integrated in a printing unit 10.

(17) Each printing unit 10 has its corresponding digital ink jet print heads 14 located opposite, adjacent, and orthogonal to a printing segment 6 of the sheet material 4. Each printing segment defines, in coincidence with the plane defined by the sheet material 4, a transverse direction DT which will be tangent to the band of sheet material in the printing segment 6 and perpendicular to the direction of conveyance of said band of sheet material in that printing segment 6, and a longitudinal direction DL which will be parallel to the direction of conveyance of the sheet material in that printing segment 6 and tangent to the band also in that printing segment 6. A perpendicular direction DP will be that direction perpendicular to the transverse direction DT and longitudinal direction DL, and therefore also perpendicular to the sheet material 4 in the printing segment 6.

(18) In the embodiment shown in the Figures a few of said rollers 2 are arranged in an arcuate configuration, therefore also defining an arcuate path for the band of sheet material at least in the segment supported on said rollers 2, where the printing segments 6 are located.

(19) As shown in FIGS. 1a, 1b and 2a, 2b the rollers 2 defining the arcuate portion are supported on a first sub-chassis 1a which is attached to a second sub-chassis 1b in a movable manner, allowing relative movement between both, said first and second sub-chassis 1a and 1b being constituents of the chassis 1.

(20) In the embodiment shown in FIG. 1, both first and second sub-chassis 1a and 1b are attached by guide elements, in this example vertical guides, which allow moving the first sub-chassis 1a and the rollers 2 it supports in a direction away from the digital ink-jet print heads 14, which therefore allows modifying the distance between these elements to adapt to different thicknesses of the band of sheet material.

(21) In the embodiment shown in FIG. 2, both first and second sub-chassis 1a and 1b are attached by an articulated attachment allowing the first sub-chassis 1a to swivel with respect to the second sub-chassis 1b.

(22) A first actuator 1c provided for actuating the movement of the first sub-chassis 1a with respect to the second sub-chassis 1b has been included in the two embodiments.

(23) The activation of said first actuator 1c is activated by a control unit 30 as a response to a thickening presence signal produced by a thickener device 33. a thickening detector 34 or a metering device 35 located upstream of the printing units 10.

(24) FIGS. 1a and 1b shown a first embodiment provided with a thickener device 33, for example a splicer configured to adhere one final end of a partial band of sheet material with one initial end of another partial band of sheet material to generate a continuous band of sheet material, but creating a thickening in the splice. Said thickener device 33 communicate a thickening presence signal when a splice is created to the control unit 30 and said control unit 30 activate the first actuator 1c previous to the arrival of the thickening to the printing segments 6.

(25) FIGS. 2a and 2b shown a second embodiment provided with a thickening detector 34 represented in these examples as a camera oriented towards the band of sheet material 4 for detecting the approach towards the printing units 10 of a thickening, for example a splice or a defect of the band of sheet material, which may damage the ink-jet print heads 14. In such case, a control unit 30 receiving a thickening presence signal from the thickening detector 34 actuate the first actuators 1c to move the first sub-chassis 1a in a direction away from the ink-jet print heads 14, thereby making the space between the printing segments 6 and said ink jet print heads 14 larger, allowing the passage of the detected thickening without causing any damage. The first actuators 1c will then return the sub-chassis 1a and 1b to their initial position to resume printing.

(26) In the embodiment shown in FIGS. 1a and 1b, the printing segments 6 coincide and are supported on the rollers 2, whereas in the embodiment shown in FIGS. 2a and 2b, the printing segments 6 are arranged in the interspaces existing between successive rollers 2.

(27) FIG. 8 shows how there may be arranged, in said interspaces, other supports in the form of flat plates 60 which can be heated to increase the drying speed of the ink printed on the sheet material 4 and can also be perforated and connected by the back thereof to a suction unit through air conduits, thereby getting the band of sheet material to adhere perfectly on the surface of the plate 60 as it passes over same, in this case coinciding with the printing segment 6, thereby getting the printing segment 6 to have a sheet material with a perfectly flat surface.

(28) Alternatively, to said heating of the plates, thermal energy can be provided to said flat plates 60 by an installation with IR lamps, or by providing a hot air flow aimed at the flat plates 60, or other heating solutions well known in the sector.

(29) FIGS. 2a and 2b furthermore shows how, at least in part of said band path, the sheet material is supported on a continuous conveyor belt 61 having a portion supported on said rollers 2 defining the arcuate path. The conveyor belt 61 may be made of a flexible material such as rubber, for example, and may go through a heating unit that allows increasing its temperature above room temperature, such that the temperature of the sheet material supported thereon also increases, thereby accelerating the drying of the printed ink.

(30) FIGS. 3a and 3b shows an alternative embodiment in which multiple printing units 10 are all arranged around a single roller 2, the corresponding digital ink-jet print heads 14 being arranged opposite multiple printing segments 6 of the sheet material 4, all supported on the periphery of the mentioned single roller 2, and further including a metering device 35 upstream of the printing segments 6. The metering device 35 determines which specific portion of the band of sheet material 4 is in the first position where the metering device 35 is placed at every moment. Said specific portion of the band of sheet material 4 placed in the first position is compared with a register of portions of the band of sheet material including thickenings, for example splices, wrinkles or other defects, and when the presence of one portion of the band of sheet material, coincident with a portion stored as a thickened portion, is determined to be in the first position a thickening presence signal is generated, triggering the activation of the first actuator 1c by the control unit 30. Said register of portions of the band of sheet material including thickenings can be stored in the control unit 30, for example including therein the known defects of a particular laminar material roll unwind to feed the digital printing machine.

(31) Both the conveyor belt 61 of FIGS. 2a and 2b and the single central roller 2 of FIGS. 3a and 3b allow preventing the band of sheet material from sustaining elongations or deformations during conveyance which may cause distortions in the printed images if it is made of an elastic material, for example.

(32) Where FIGS. 1a, 1b and 2a and 2b showed longitudinal section views of different embodiments of the proposed digital printing unit, FIGS. 5, 6, and 7 show a cross-section view of the embodiment shown in FIGS. 1a and 1b and FIGS. 4a and 4b shown a partial and simplified perspective view of the embodiment shown in FIGS. 1a, 1b, 2a and 2b.

(33) In said cross-sections of the digital printing unit, it can be seen how a printing unit 10 is located above the rollers 2 and above the band path for the band of sheet material, in coincidence with a printing segment 6 which is supported on one of the rollers 2 in this example.

(34) The printing unit 10 consists of a bridge 11 supported on the chassis 1, said bridge 11 being arranged transversely with respect to the band path and having a length of at least twice the width of the band path defined by the length of the rollers 2, half of the bridge 11 being located above the printing segment 6 and the other half projecting laterally form the band path, overlapping a user-accessible maintenance passageway 50.

(35) Specifically, in the example shown in FIGS. 5 and 6, the bridge 11 is arranged parallel to the transverse direction DT with respect to the corresponding printing segment 6.

(36) The bridge 11 includes two guides 12 also parallel to the transverse direction DT.

(37) A carriage 13 is attached in a sliding manner to the guides 12, which allows moving said carriage 13 along the bridge 11 between a printing position shown in FIG. 5, in which the digital ink jet print head 14 of the printing unit 10 is arranged opposite and adjacent to the printing segment 6, and a maintenance position, in which the digital ink-jet print head 14 of the printing unit 10 is not arranged opposite the printing segment 6, being located above the maintenance passageway 50. The size of the carriage 13 will the same as or smaller than the width of the band path defined by the length of the rollers 2.

(38) The sliding attachment between the carriage 13 and the guides 12 will be carried out, for example, by a system of wheels, and will be actuated by an actuation mechanism, provided with motors and belts, not shown in the drawings, for example.

(39) The carriage 13 will include the digital ink-jet print head 14, although it is understood that said digital ink-jet print head 14 may be formed by a plurality of digital ink jet print heads 14 combined so as to cover the entire printing segment 6. Therefore, the movement of the carriage 13 along the guides 12 of the bridge 11 allows moving the digital ink-jet print head 14 of the printing unit 10 from the printing position to the maintenance position or vice versa.

(40) The lateral movement of the ink jet print heads 14 in the transverse direction DT prevents any damage to them when a major thickening exists in the band of sheet material, for example when said band of sheet material breaks and wrinkles.

(41) In the example shown in FIG. 7, the bridge 11 and the guides 12 are not arranged parallel to the transverse direction DT of the printing segment 6 but rather form an upward angle, in this case of 10°, with respect to said transverse direction, such that the movement of the carriage 13 from the printing position to the maintenance position causes the digital ink-jet print head 14 to raise up with respect to the plane defined by the printing segment 6, given that the segment of the bridge located on the maintenance passageway 50 is located at a height greater than the segment of the bridge 11 located on the rollers 2.

(42) Likewise, to assure that the carriage 13 is correctly positioned with respect to the bridge 11 when it reaches the printing position, it is proposed to include a first centering element 41 attached to the carriage 13, and a second centering element 42 attached to the bridge, both centering elements being aligned in the direction of movement of the carriage 13, and positioned for being perfectly coupled with one another when the carriage 13 reaches the printing position. By way of example, it is proposed for said first centering element 41 to include a protruding semi-spherical protruding convex shape complementary to a semi-spherical concave shape having complementary dimensions provided in the second centering element 42. Other geometric shapes such as cone shapes, frustoconical shapes, pyramidal shapes, frustopyramidal shapes, etc., for example, are also contemplated. This allows improving the precision of the positioning of the carriage 13 and the digital ink-jet print head 14 in the printing position.

(43) Despite the foregoing, the positioning of the digital ink-jet print head 14 may be incorrect due to tolerances, expansions, etc.

(44) If all the digital ink jet print heads 14 of all the printing units 10 integrated in the digital printing module are not correctly aligned, the print may include imperfections, so an improvement which allows an independent correct alignment of each printing unit 10 is proposed.

(45) To achieve the foregoing, it is proposed to attach a first anchoring point of each bridge 11 to the chassis 1 through an adjustment device 20, and to attach a second anchoring point of each bridge 11 to the chassis 1 through another adjustment device 20 or through an articulation 25, the first anchoring point and the second anchoring point being spaced apart.

(46) The embodiment shown in FIGS. 5 and 6 shows how the first anchoring point is located at one end of the bridge 11, on one side of the path for the band of sheet material, whereas the second anchoring point is located in an intermediate portion of the bridge 11, on the other side of the band path, the rollers 2 being arranged below the bridge segment 11 comprised between the first and second anchoring points. According to this embodiment, part of the bridge 11 extends in cantilever fashion beyond the second anchoring point, projecting in cantilever fashion through the side of the band path over a maintenance passageway 50.

(47) In the example of FIGS. 5 and 6, the second anchoring point includes the reference numbers referring both to the adjustment device 20 and to the articulation 25, thereby showing that either of the two elements could be located at this point.

(48) In contrast, FIG. 7 shows an embodiment in which the first anchoring point and second anchoring point are located at the two opposite ends of the bridge 11.

(49) Each adjustment device 20 allows precisely moving the anchoring point to which it is connected. When the bridge 11 only has one adjustment device 20, it allows correcting the inclination of the bridge 11 from one of the anchoring points, pivoting the entire bridge 11 with respect to the other anchoring point for correctly aligning same with respect to the corresponding printing segment 6 and with respect to the other bridges 11.

(50) When both anchoring points have adjustment devices 20, in addition to the inclination thereof, the separation of the entire bridge 11 with respect to other bridges 11 or with respect to the corresponding printing segment 6 can be modified. The combination of the precise movement of both ends of the bridge allows modifying the position of the digital ink jet print head 14 held by said bridge 10 to obtain its ideal position.

(51) To achieve said movement of the bridge 11, each adjustment device 20 will provide one, two, or three degrees of freedom, for example, providing movement of the end of the bridge 11 in one or more of the transverse direction DT, longitudinal direction DL, or perpendicular direction DP in relation to the respective printing segment 6.

(52) When the bridge 11 is provided with an adjustment device 20 and an articulation 25, said articulation must have several degrees of freedom compatible with the degrees of freedom of the adjustment device, i.e., if the adjustment device has only one degree of freedom, the articulation 25 must also have at least one degree of freedom in the same direction as that of the adjustment device 20. Therefore, the articulation can be a simple rotary attachment rotating about an axis, allowing rotation in the perpendicular direction DP or in the longitudinal direction DL, for example, or a ball joint, allowing simultaneous rotation in both perpendicular direction DP and longitudinal direction DL, or an articulated attachment which allows sliding in the transverse direction DT.

(53) According to a preferred embodiment one point of the bridge 11 includes an articulation 25 and the other point of the bridge includes an adjustment device 20 comprising a screw or spindle in the longitudinal direction DL configured to produce the precise movement of said point of the bridge 11 in the longitudinal direction DL when rotates. The rotation of said screw or spindle can be manual or automatic. It is further proposed to attach the bridge 11 to the second sub-chassis 1b by fixation screws directed in the perpendicular direction DP, so that different gauges can be interleaved between the bridge 11 and the second sub-chassis 1b to modify the position of that point of the bridge 11 in the perpendicular direction DP.

(54) The example shown in FIG. 8 shows a schematic mechanism making up an adjustment device 20 providing three degrees of freedom in the three directions, i.e., transverse direction DT, longitudinal direction DL, and perpendicular direction DP, by three successively connected mechanisms 21, each providing one degree of freedom.

(55) In this example, each mechanism consists of a servomotor 23 connected to a spindle 22, the spindle being arranged parallel to the transverse direction DT, longitudinal direction DL, or perpendicular direction DP.

(56) There is attached to the spindle 22 a threaded element which will move axially along the spindle 22 as said spindle 22 rotates.

(57) If more than one mechanism 21 is included, the successive mechanisms 21 are suspended from the threaded elements of the preceding mechanisms, forming a chain of mechanisms, each determining a movement in one of the directions, such that the actuation of one mechanism 21 causes the movement of all the successive mechanisms 21 in one direction.

(58) A first mechanism 21 will be connected to the chassis 1, for example, and the last mechanism 21 of the chain of mechanisms 21 will have the corresponding threaded element connected to the bridge 11. Obviously, a reverse construction is also possible.

(59) In the example shown in this drawing, a first mechanism 21 is held on the chassis 1 by clamps with bearings surrounding the ends of a spindle 22 which is parallel to the perpendicular direction DP.

(60) The threaded element of the first mechanism 21 is attached to a support holding a second mechanism 21 identical to the first mechanism 21 but having the corresponding spindle 22 oriented in a direction parallel to the transverse direction DT.

(61) The threaded element of the second mechanism 21 is attached to a support holding a third mechanism 21 identical to the first and second mechanisms 21 but having the corresponding spindle 22 oriented in a direction parallel to the longitudinal direction DL. The threaded element of the third mechanism 21 is connected to the bridge 11.

(62) The precise and coordinated actuation of the three mentioned mechanisms 21 making up the adjustment device 20 will allow moving an end of the bridge 21 in any of the three directions, i.e., transverse direction DT, longitudinal direction DL, or perpendicular direction DP, and the precise movement of both ends of the bridge 11 will also allow introducing rotations to the bridge 11 in at least two additional degrees of rotational freedom.

(63) Obviously, it will be understood that this is a schematic embodiment and that the adjustment device 20 may be different, or may include other movement transmission mechanisms, for example, those based on gears, in addition to including other supplementary elements as guide elements to assure a correct movement in each of the directions.

(64) To achieve a correct alignment of the digital ink jet print heads 14 of the multiple printing units 10, it is proposed for the servomotors 23 of all the adjustment devices 20 to be controlled through a control device.

(65) Said control device will receive information through a first or second measuring device 31, 32 which allow determining the deviations existing in the position of each digital ink-jet print head 14, as well as calculating the control orders required for controlling the necessary adjustment devices 20 in order to correct said detected deviations.

(66) FIGS. 1, 2, and 3 show a second measuring device 32 in the form of an optical sensor or a camera located downstream of the printing units 10 and directed towards the sheet material 4. In this embodiment, said optical sensor or camera will be the aforementioned second measuring device 32.

(67) The optical sensor or camera will capture images of the motifs printed on the sheet material 4, sending said images to the control system. Said control system will process the images, detecting any misalignment between the different colors printed by the different printing units 10, for example, observing print marks or misalignments existing between the colors of the images.

(68) The misalignment between the printed colors will correspond with misalignments of the digital ink jet print heads 14 to be corrected. Based on that information, the control system will generate control orders to be transmitted to the adjustment devices 20, correcting the position of each of the bridges 11 and of the corresponding printing units 10.

(69) According to an additional embodiment shown in FIG. 7, the second measuring device 32 will consists of position sensors, for example laser meters, which will precisely measure the position of each of the digital ink jet print heads 14, such that they allow detecting any deviation existing in the actual position of each digital ink-jet print head 14 with respect to a pre-established ideal position stored in the control system, or with respect to other successive ink-jet print heads, which allows generating the control orders for the adjustment devices 20 from said information.

(70) The control system can consist of one or more computers, programmable logic controllers, or other similar devices.

(71) FIG. 7 also illustrates an embodiment of the first measuring device 31, which is shown in this embodiment as a laser meter that precisely measures the distance between each ink jet print head 14 and the corresponding printing segment 6.

(72) According to an alternative or additional embodiment, the digital printing machine can also include an image detector 36, such a camera or a video camera, facing the freshly printed surface of the band of sheet material downstream from the printing segments 6, as shown on FIGS. 1a, 1b, 2a, 2b, 3a and 3b.

(73) This image detector 36 capture images from the motifs printed on the surface of the band of sheet material and send said images to the control unit 30. The control unit 30 analyzes said captured images and determine if a rubbing printing defect and/or if an ink jet obturation printing defect exist on said captured images, and which colors are affected by said detected defects.

(74) This determination performed by the control unit 30 can be obtained, for example, by comparing the captured images with a digital image containing the motif to be printed or containing the motif to be printed on each color. Any difference between the captured image and the stored digital image will be considered a printing defect.

(75) A programed or trained algorithm executed by said control unit 30 can further determine which type of defect has been detected.

(76) Said control unit 30 will also be aware of the color in which the ink jet printing heads contained in each printing unit 10 are printing, for example because this information has been previously stored in the control unit 30.

(77) Comparing the information obtained relative to which color suffers from a printing defect with the information relative to which printing unit 10 prints on each color, the control unit 30 can determine which printing unit 10 comprises the ink jet printing units 14 responsible of the detected printing defect, permitting the control unit 30 to actuate only on the affected printing unit 10 to solve the detected printing problem, for example increasing the distance between the printing segment 6 and the printing unit 10 responsible from this particular problem, even without interrupting the printing process, of moving the printing unit 10 affected with the detected problem to the maintenance position, where a cleaning operation can be performed.

(78) It is also considered to include multiple first sub-chassis 1a each supporting only a portion of the supports adjacent some of the printing segments 6, and each being movable independently by a first actuator 1c, permitting to adjust the distance between each printing segment 6 and the correspondent printing unit 10 independently.

(79) It will be understood that the different parts making up the invention described in one embodiment can be freely combined with the parts described in other different embodiments even though said combination has not been explicitly described, provided that said combination is not detrimental to the invention.