Inkjet printing system

Abstract

The present invention relates to an inkjet printing system comprising at least one printhead provided with a nozzle plate formed in a bottom surface of the printhead, the nozzle plate being configured to eject a liquid onto a substrate, the substrate being configured to travel under the printhead in a direction of transportation. An air suction module is arranged downstream of the printhead in the direction of transportation, the air suction module being configured to direct an airflow below the nozzle plate of the printhead, wherein the air suction module comprising an air inlet, an air outlet and a first filter module located between the air inlet and the air outlet.

Description

[0001] INKJET PRINTING SYSTEM

[0002] Field of the invention

[0003] The present invention relates to an inkjet printing system for printing on sheets or web substrates. Such an inkjet printing system may be used in printing machines or converting machines comprising a printing module.

[0004] Background

[0005] Printing machines and converting machines are used in the production of packaging material such as paperboard and cardboard boxes. These machines can be configured to solely print on sheet substrates, or in addition to also provide mechanical operations, such as cutting and creasing the sheet or web substrates.

[0006] Corrugated cardboard substrates tend to display a large variability in their flatness. This results in a variation of the printing gap, which is the distance between the printhead and the substrate. When printing on corrugated cardboard, the printing gap often needs to be relatively large. A large printing gap has implications in the ink drop behavior and mist generation, which may result in poor printing quality and machine contamination.

[0007] To alleviate these problems, it is known to provide a controlled airflow in the printing gap. An example of a printhead with a forced airflow is disclosed in document US11633958. The device in US11633958 comprises an air supply conduit configured to blow air in the printing gap, and a return circuit configured to suck airflow from the printing gap.

[0008] However, during time the airflow in the return circuit is reduced due to build-up of ink. The build-up of ink in the return circuit makes it difficult to achieve a calibrated and homogeneous airflow which leads to a degradation of the printed quality of the motif. Summary

[0009] In view of the above-mentioned drawbacks of the prior art, it is an object alleviate the above-mentioned problem and provide an improved airflow system for an inkjet printing module.

[0010] This object is solved by an inkjet printing system according to claim 1.

[0011] According to a first aspect of the present invention, there is provided an inkjet system comprising at least one printhead provided with a nozzle plate formed in a bottom surface of the printhead, the nozzle plate being configured to eject a liquid onto a substrate, the substrate being configured to travel under the printhead in a direction of transportation, and wherein an air suction module is arranged downstream of the printhead in the direction of transportation, the air suction module being configured to direct an airflow below the nozzle plate of the printhead, wherein the air suction module comprising an air inlet, an air outlet and a first filter module located between the air inlet and the air outlet.

[0012] In an embodiment, the first filter module comprises a rugged surface section having a plurality of wall elements configured to force the airflow in a curved path through the first filter module.

[0013] In an embodiment, the first filter module is configured to conduct the airflow through the suction module at an upwardly sloping angle in relation to the horizontal direction.

[0014] In an embodiment, the angle is selected to be between 80 and 105 degrees, preferably about 90 degrees, whereby the extension of the first filter module in the flow direction is essentially coinciding with the vertical direction.

[0015] In an embodiment, the rugged surface section is upwardly concave.

[0016] In an embodiment, the inclination of the upwardly concave section is non-constant and is increased towards the air outlet. ln an embodiment, the suction module comprises an elongated hollow suction bar. In an embodiment, a plurality of printheads is grouped together as a print bar, and wherein said print bar is received in an elongated print bar carriage, and wherein the suction bar is attached to a lateral side of the print bar carriage.

[0017] In an embodiment, the suction bar has a length equal or longer than the print bar carriage.

[0018] In an embodiment, the first filter module further comprising an ink evacuation channel configured to collect and evacuate liquid, and wherein the evacuation channel extends in the longitudinal direction of the suction bar.

[0019] In an embodiment, the evacuation channel is located between the air inlet and the rugged portion within the suction module.

[0020] In an embodiment, the evacuation channel is connected to a pump mechanism configured to move the collected liquid from the evacuation channel into an ink collection reservoir.

[0021] In an embodiment, the pump mechanism comprises a hydrodynamic screw located inside the evacuation channel. Alternatively, an Archimedes pump can be used.

[0022] In an embodiment, the filter module further comprises a second filter located downstream of the first filter in a flow direction through the suction bar.

[0023] In an embodiment, the second filter forms a separative wall with an air evacuation channel.

[0024] In an embodiment, an air flow module located upstream of the printhead in the direction of transportation of the substrate, said airflow module being configured to direct air under the printhead.

[0025] In an embodiment, an air flow module is located upstream of the printhead in the direction of transportation of the substrate, said airflow module being configured to direct air under the printhead.

[0026] The airflow module is thus configured to provide a “forced airflow” through the printing gap. The forced airflow can be defined as an additional airflow in relation to the airflow already present in the printing gap. The airflow already present in the printing gap comprises the airflow generated by the movement / transportation of the sheet in the direction of transportation. The airflow parameter may be determined in the printing gap. Alternatively, the airflow parameter may be determined upstream and / or downstream of the printing gap and the airflow parameter in the printing gap may be further calculated by the control unit.

[0027] Brief description of the drawings

[0028] The invention will now be described with reference to the appended drawings, in which like features are denoted with the same reference numbers and in which:

[0029] Fig. 1 is a schematic perspective view of a converting machine in the configuration of a printing machine;

[0030] Fig. 2 is a schematic top view of a printed substrate;

[0031] Fig. 3a is a schematic perspective view of a print bar assembly according to an embodiment of the present invention;

[0032] Fig 3b is a schematic perspective view of a printhead according to the present disclosure;

[0033] Fig. 4a is a partial side view of a print bar assembly according to an embodiment of the present invention;

[0034] Fig. 4b is a cross-sectional view of a print bar assembly of Fig. 4a;

[0035] Figs. 5a and 5b are a schematic perspective cut-away views of a suction bar according to an embodiment of the present invention; and

[0036] Fig. 6 is a is a cross-sectional view of a print bar assembly with details of a sensing system according to an embodiment of the present invention; and

[0037] Fig. 7 is a schematic perspective view of an air dispensing bar according to an embodiment of the present invention.

[0038] Detailed description

[0039] The present inkjet printing system can be used for many different types of converting machines, such as inkjet printing machines configured to print on corrugated cardboard in sheet form or web form, rotary die-cutter machines and flexo folder gluer machines. Within the context of this invention, a converting machine can be a machine configured solely as an inkjet printing machine, or a machine comprising an inkjet printing module and converting modules configured to modify the shape of the substrate by e.g. cutting and / or creasing. Machines with converting modules include rotary die-cutting machines and flexo- folder gluer machines.

[0040] For simplicity, the present invention is described with reference to a converting machine 2 in the form of an inkjet printing machine 2 configured to print on sheet substrate 1. As illustrated in figure 1 , the converting machine 2 comprises a feeder 4, a transportation system 6 and a printing module 8. Preferably, the converting machine 2 further comprises a dryer 10 located downstream of the printing module 8. The converting machine 2 also preferably comprises a loader module (not illustrated), which may be a combination of a conveyor belt and lift table. Alternatively, a robotic gripper and loading system may be used. The loader is configured to receive stacks of sheet elements such as cardboard sheets and place batches of the sheet elements into the feeder 4.

[0041] The sheet substrate 1 is transported through the converting machine 2 with the transportation system 6. The transportation system 6 may be gripper bar transportation system 6 such as the one described in document EP3941865 A1. Alternatively, a transportation system 6 comprising a combination of rollers and conveyor belts may be used. Such a system is described in EP4225541 A1. For web substrates 1 , the transportation system 6 may comprise a plurality of drive cylinders around which the substrate 1 is wound.

[0042] Within the context of this disclosure, the term “substrate” is defined as including both sheet elements and web substrates. The substrate may be paper, plastic, textile material or a combination thereof.

[0043] The printing module 8 comprises a plurality of printheads 9, where each printhead 9 comprises a nozzle plate 13 configured to eject ink droplets. The printheads 9 may be arranged in clusters 11 . Such printheads 9 and clusters 11 are described in document EP3325273 A1 , the content of which is hereby incorporated by reference.

[0044] As illustrated in figures 1 , 3 and 4a, the clusters 11 are arranged in elongated print bars 12 where each print bar 12 may be dedicated to a specific ink color or varnish. Each print bar 12 may comprise a plurality of clusters 11 , such as for instance four to six clusters 11 . The distance between the nozzle plates 13 and the top surface of the substrate 1 is defined as the printing gap G. The print bars 12 have a longitudinal extension which is arranged in the lateral direction L, which is perpendicular to the direction of transportation T of the substrate 1.

[0045] The print bar 12 is preferably arranged in a print carriage 14. The print bar 12 and the print carriage 14 form a print bar assembly 16. The print carriage 14 comprises an elongated cavity 18 and electronic connectors (not illustrated) designed to receive and connect to the clusters, or the individual printheads 9. The print carriage 14 is preferably movable in the lateral direction L between a printing position above the trajectory of the substrate 1 and a service position. In the service position, the printheads 11 may be replaced or the nozzle plates 13 of the printheads 11 may be cleaned.

[0046] The transportation system 6 and the movements of the substrate 1 create a localized airflow at the clusters 11. This localized airflow is often a turbulent airflow. The turbulent airflow arises in particular for transportation systems 6 which comprise a front gripper bar configured to hold a front edge of a sheet substrate 1 and a rear gripper bar configured to hold a rear edge of the sheet substrate 1.

[0047] The sheet substrates 1 are transported at a relatively high speed of which may be between 50 to 150 meters / minute. As the gripper bars and the sheet substrates 1 pass under the nozzle plate 13, a significant airflow volume with a turbulent airflow characteristic is often generated. This turbulent airflow tends to a modification of the printing position on the sheet substrate 1 where each ink droplets is supposed to be deposited. Hence, the ink droplets are not deposited in a straight manner in the gravitational direction. The largest error occurs for small droplets also referred to as satellite droplets and mist. The mist typically comprises smaller particles than the satellite droplets. The mist and satellite droplets may be referred to as “unintentional aerosol particles” generated by the printhead. Whereas the “main droplet” is the droplet which should be deposited on the substrate 1 to form the motif.

[0048] The unintentional aerosol particles are often deposited away from the predefined printing location on the substrate 1 and may cause visible errors in the printed motif. A portion of the unintentional aerosol particles also accumulates on the nozzle plate. This also affects the droplet generation and therefore results in print quality deterioration.

[0049] To overcome the effect of unintentional aerosol particles, a forced and controlled airflow FR can be produced under the nozzle plate 13 of the printhead 9. The forced airflow FR is applied with a flow direction F1 coinciding with the direction of transportation T of the substrate 1.

[0050] The forced airflow FR can be achieved by arranging an air suction module 20 adjacent to the print bar 12. In such a way, the suction module can be for achieving a forced airflow FR underneath the printhead 9. The air suction module 20 is thus configured to suck in an extracted airflow F2 containing unintentional aerosol particles.

[0051] It is advantageous to separate the sucked in air from its ink content and create a controlled environment adjacent to the converting machine 2. It is also advantageous to separate the ink from the extracted airflow F2 in a way that clogging in the conduits of the suction module 20 can be avoided.

[0052] As best seen in figures 4b, 5a and 5b, the air suction module 20 may be arranged downstream of the printhead 9 in the direction of transportation T of the substrate 1 . The air suction module 20 comprises suction bar 22 comprising an air inlet orifice 24 an air outlet 26. A suction device 28 is connected to the air outlet. The air suction bar 22 further comprises an air evacuation channel 25 and a first filter module 27 located between the air inlet orifice 24 and the air outlet 26.

[0053] Preferably, the print bar 12 is received in an elongated print carriage 14, and the suction bar 22 is attached to a lateral side of the print carriage 14. Alternatively, the suction bar 22 can be attached inside the carriage. The suction bar 22 may have a length which is equal or longer than the print bar 12. In an advantageous embodiment, the air suction bar 22 is longer than the print bar 12 in the lateral direction L. Preferably, the air suction bar 22 is longer than the print bar 12. Preferably the air suction bar 22 is between 0 and 20% longer than the print bar 12, preferably about 10%. This allows the air suction bar 22 to always be wider in the lateral direction L than a print zone on the substrate 1. In such a way, aerodynamic side effects around the distal edges of the print bar 12 can be avoided and a more homogenous airflow under the nozzle plate 13 can be obtained.

[0054] As illustrated in figures 5a and.5b, the suction bar 22 is preferably formed as an elongated hollow profile. The air inlet orifice 24 into the suction bar 22 can be configured as an elongated slot. Optionally, as illustrated, a plurality of elongated air inlet orifices 24 can be designed as slots separated by wall elements 30 can be provided. In another embodiments, a plurality of round orifices 24 can be used.

[0055] Downstream of the air inlet orifice 24, an air inlet section 32 is provided. The air inlet section 32 is preferably curved such as to vertically collect an extracted airflow F2 underneath the nozzle plate 13. The air inlet section 32 is preferably provided with smooth conduit walls.

[0056] After the air inlet section 32, a filtering section 34 is provided. The filtering section 34 comprises a first filter module 27. The first filter module 27 is located within the suction bar 22 and preferably extends along the longitudinal length L1 of the suction bar 22. The filtering section 34 may be horizontal (i.e. extending in the horizontal direction H). However, it is preferred to provide an upwardly sloping a filtering section 34.

[0057] Preferably, the filtering section 34 may be arranged at an upwardly sloping angle a in relation to the horizontal direction H. The angle a can be between 0 and 90 degrees, degrees, preferably about 90 degrees. Hence, the filtering section 34 may be arranged to coincide with the vertical direction V.

[0058] In the illustrated embodiment, the filtering section 34 is upwardly concave. Preferably, the inclination of the upwardly concave rugged surface S is nonconstant and is increasing in the flow direction F2. The angle a is varying along the flow direction and may be between 30 and 90 degrees. In such a way, the filtering effect by gravitation is increased in a direction towards the air outlet 26.

[0059] The upwardly sloping filtering section 34 allows the use of gravitation to facilitate the separation of ink from the extracted airflow F2.

[0060] The first filter module 27 comprises a rugged surface S and an ink collection channel 29. The rugged surface S comprises surface obstacles 35 which form the rugged surface S. In such a way, the extracted airflow F2 through the suction bar 22 touches the surface obstacles 35, and the ink settles and accumulates on the obstacles 35. Under the effect of gravity, the ink is thereafter directed into the ink collection channel 29.

[0061] The rugged surface S is preferably designed such that the extracted airflow F2 is prevented from following a straight path. Hence, the extracted airflow F2 can be forced in a curved path. The rugged surface S can be designed by a first series of wall elements 35a and a second series of wall elements 35b. In such a way, the extracted airflow F2 is forced into a zig-zag flow path. The wall elements 35 may be formed by a first series of oblique wall elements 35a and a second series of oblique wall elements 35b. The first series of oblique elements 35a may be angled in a first direction D1 and the second series of oblique elements 35b may be angled in a second direction D2, opposite from the first direction. Preferably, there are a plurality of first and second series of wall elements 35a, 35b. In other non-illustrated embodiments, the wall elements 35 may be provided with circular shapes or like.

[0062] Preferably, the filtering section 34 may comprise a smooth surface region 34’ located between the air inlet orifice 24 and the rugged surface S. The smooth surface region 34’ allows the extracted airflow F2 to travel further into the air suction bar 22 before the ink is forced to deposit on the rugged surface S.

[0063] The ink evacuation channel 29 is located between the air inlet orifice 24 and the filtering section 34. The ink evacuation channel 29 extends in the longitudinal direction L of the suction bar 22. The ink evacuation channel 29 is configured to collect and evacuate the ink to a waste collection reservoir (not illustrated). A pump device 23 is connected to the evacuation channel 29. The pump device 23 may be connected to an end of the evacuation channel 29. Alternatively, the ink evacuation channel 29 comprises a plurality of ink outlets along the length of the suction bar 22 which are connected to the pump device 23. The pump device 23 may comprise a hydrodynamic screw 23’ extending within the ink evacuation channel 29.

[0064] The air evacuation channel 25 is configured to evacuate the extracted airflow F2 after the ink has been collected and separated from the extracted airflow F2 in the filtering section 34. The air evacuation channel 25 may be integrally formed in the suction bar 22. The air suction module 20 may further comprise a second filter 40 located downstream of the first filter module 27 in a flow direction F2 through the suction bar 22. The second filter 40 may be elongated and provided with a length equal to the length of the suction bar 22.

[0065] The second filter 40 may form a separative wall with the air evacuation channel 25. In such a way, the elongated second filter 40 and the air evacuation channel 25 are located side by side. The second filter 40 may be a mesh filter, or a nonwoven filter or like. The second filter 40 may be made from metal, paper, textile-based, or a polymeric material. Additionally or alternatively, a centrifugal filter (also referred to as a cyclone filter) can be provided at the air outlet 26 or in a conduit between the suction bar 22 and the air outlet 26. Still in another embodiment, a water trap (e.g. a water separator) can be arranged in a conduit between the suction bar 22 and the air outlet 26.

[0066] The second filter 40 may be received in an elongated slot 42 in the suction bar 22. A top edge of the second filter 40 may be flush with or protrude outside the suction bar 22. In such a way, the second filter 40 can be replaced by pulling it upwards in the vertical direction V.

[0067] As best seen in figure 4a and 6, the print bar 12 can be further provided with an air flow module 44 located upstream of the print bar 12 in the direction of transportation T of the substrate 1. The air flow module 44 is configured to create a forced and controlled airflow FR under the print bar 12 together with the air suction module 20. The air flow module 44 comprises an elongated air dispensing bar 46 and an airflow generator 45. The air dispensing bar 46 is configured to supply an inlet airflow F1 under the nozzle plate 13 Hence, the air dispensing bar 46 is arranged upstream of the print bar 12 and the suction bar 22 is provided downstream of the print bar 12 in the direction of transportation of the substrate 1.

[0068] The elongated air dispensing bar 46 is preferably longer than the print bar 12. Preferably, the air dispensing bar 46 is provided with the same length as the suction bar 22. The air dispensing bar 46 may be fixedly or releasably attached to the print bar 12 or the print carriage 14.

[0069] The air dispensing bar 46 comprises a main airflow channel 50, which is connected to the airflow generator 45. The connection to the airflow generator 45 may be located at a distal end of the air dispensing bar 46. Preferably, the main airflow channel 50 is provided with a non-constant cross section. In such a way, the airflow (volume / time) can be designed such as to be constant along the full length of the air dispensing bar 46. Alternatively, the connection to the airflow generator 45 may be provided at the center of the air dispensing bar 46.

[0070] The main airflow channel 50 is provided with at least one air outlet opening 52. Preferably, a plurality of air outlet openings 52 are provided. Alternatively, in a nonillustrated embodiment, the main airflow channel 50 may be provided with a single opening in the form of an elongated slot. The air outlet openings 52 may have an orifice located on a bottom side of the air dispensing bar 46. In such a way, the inlet airflow F1 is directed vertically downwards against the top surface of the substrate 1 . The air outlet openings 52 may therefore be provided with a curved outlet section.

[0071] The forced airflow FR leads to a control of the airflow under the nozzle plate 13. Hence, the movements of unintentional aerosol particles can be controlled efficiently as their trajectory can be modified by the forced airflow FR. The forced airflow FR thus controls the trajectory of the unintentional aerosol particles such that they are diverted away from the sheet substrate 1 and captured in the air suction module 20. Advantageously, the velocity of the forced airflow FR under the nozzle plate 13 is calibrated such that the main ink droplets are deposited in a predefined printing location. This is possible due to their gravity being larger than that of the mist and satellite droplets which still allows the main droplets to still be deposited on the substrate 1 .

[0072] The forced airflow FR creates a predefined displacement of the main droplets and thus modifies their trajectory and their deposit location on the substrate 1 . In order to achieve a controlled displacement, it is desired to create a calibrated and homogeneous forced airflow FR. With a calibrated airflow, it is meant an airflow having a variable airflow parameter that is either constant, or adapted in a controlled manner such that the printing process is controlled in relation to a variable airflow parameter. With a homogenous airflow, it is meant an airflow having a variable airflow parameter that is essentially the same over time and / or is not providing any local variations along the longitudinal length L1 and the width W of the print bar 12. In this controlled printing process, the operation of the printheads 11 and the airflow module 44 are preferably controlled together.

[0073] Variable airflow parameters include:

[0074] Velocity of the airflow,

[0075] Direction of the airflow,

[0076] Temperature of the airflow, and

[0077] Humidity of the airflow.

[0078] It is thus desired to control one or a plurality of these parameters in the printing gap G.

[0079] In order to achieve a reduction of the unintentional aerosol particles in the printing gap G, while ensuring a controlled displacement of the main ink droplet, the airflow velocity VR in the printing gap G should be laminar, and consistent over the longitudinal length L1 of the print bar. Hence, such that all the nozzle plates 13 located along the print bar 11 are subject to the same variable airflow parameters. It is also desired to prevent any local velocity differences along the exterior edges of the print bar 11 , where velocity differences frequently occur.

[0080] Preferably, the air velocity is homogeneous in width direction (i.e. in the direction of transportation T of the substrate), with an air velocity difference not exceeding 0.5m / s. The air velocity can be sensed in the direction of transportation T of the substrate 1 , such as in the center of the print bar 11 . The air velocity may be about at 2m / s in the center of the print bar 11 .

[0081] The airflow velocity VR of the forced airflow FR in the printing gap G may deteriorate the printing quality mainly due to an uncontrolled displacement of the deposited (main) droplets on the substrate. This can be referred to as a dynamic printing error. This dynamic printing error typically occurs when the calibrated airflow velocity setting is different from the actual airflow velocity. This error may be visually detected as for instance a misregistration between different colors.

[0082] The temperature and relative humidity at the printhead 9 deteriorate the printing quality mainly due to dried ink obstructing nozzles on the nozzle plates. This can be referred to as a static printing error. This static printing error typically occurs for high ambient temperatures and / or low relative humidity at the nozzle plates 13. This error may be visually detected as missing, wrong size main droplets or deviated main droplets on the substrate.

[0083] Some types of inks, and in particular water-based inks are susceptible to increased drying behavior under influence of an airflow. This may cause ink drying on the nozzle plate 13, which can create print issues, such as obstructed nozzles and deviated jets. These problems are particularly visible for low relative air humidity.

[0084] To regulate at least one of these variable airflow parameters, the present inkjet printing system further comprises an airflow control system provided with a sensing system configured to determine at least one variable airflow parameter selected from the group comprising airflow velocity, airflow relative humidity and airflow temperature.

[0085] The sensing system is provided with at least one sensor and a control circuitry 60 configured to detect at least one actual airflow parameter P selected from the group comprising velocity, humidity, and temperature. The control circuitry 60 may be further configured to determine a desired airflow parameter P and calculate a required change in an actual airflow parameter P selected from the same group of parameters.

[0086] Additionally, or alternatively, the control circuitry 60 may be configured to issue an error signal if at least one variable airflow parameter P exceeds a predetermined tolerance threshold. Optionally, the control circuitry 60 may generate a shutdown of the converting machine.

[0087] Additionally, the control circuitry 60 may be further configured to determine a printing parameter such as a substrate transportation parameter. The substrate transportation parameter may be a speed correction such as an overall speed correction, and / or a momentary acceleration or a deceleration of the substrate.

[0088] The control circuitry 60 may further comprise a memory 64 which comprises a default airflow parameter selected from the group comprising airflow velocity, airflow relative humidity and airflow temperature. For the airflow velocity VR , the memory 64 may comprise a default airflow velocity VR. For instance, in an embodiment, 2 to 5 meters per second could be taken as a default airflow velocity in the printing gap.

[0089] The default airflow velocity maybe a previously calibrated airflow velocity. The memory 64 may also comprise previously performed job recipes, wherein the job recipes may include settings related to the size and thickness of the sheet substrate, and transportation speeds.

[0090] Other settings in the memory 64 may be ink properties such as viscosity and density, droplet volume, and printhead waveform. This enables a specific printing calibration as each ink has its own electric signal characteristics to fire the pumps mechanisms inside the printhead 9 to create the targeted drop size and shape.

[0091] The job recipes comprise production parameters for the setup of the converting machine. In such a way, the control unit may calculate a new default airflow velocity VR depending on the determined correspondence or similarity with a previously performed job recipe.

[0092] The relationship between the inlet air velocity supplied to the printing gap G by the air dispensing bar and the outlet air velocity sucked in by the suction bar preferably remains unchanged during the printing process. The airflow velocity VR supplied by the air dispensing bar and the air velocity of the air sucked into the suction bar 22 can be different from each other. However, the airflow velocity VR in the printing gap G is calibrated and controlled overtime. In an embodiment, the inlet air velocity supplied by the air dispensing bar to the printing gap G is preferably the same as the outlet air velocity into the suction bar.

[0093] The airflow generator 45 may be connected to one or a plurality of air dispensing bars. The airflow velocity V1 from the air dispensing bar 46 may be controlled independently for each respective air dispensing bar 46. This is advantageous as the airflow velocity through a first suction bar may be different from the airflow velocity through a second suction bar. The airflow velocity VR in the printing gap G can thus be adjusted by adapting the inlet air velocity V1 from the air dispensing bar 46. Additionally, or alternatively, the air outlet velocity V2 from the suction bar 22 can be adjusted. To this effect, each air dispensing bar may be provided with a valve through which the airflow into the main airflow channel of the air dispensing bar can be regulated. Alternatively, each air dispensing bar may be connected to a separate airflow generator 45.

[0094] To determine the velocity of the actual airflow in the printing gap G, a first airflow sensor 51a may be located on the air dispensing bar 46 and a second airflow sensor 51b may be located on the suction bar 22. The first and second airflow sensor may be pressure sensors.

[0095] As illustrated in figure 7, the first pressure sensor 51a may be located in the inlet air channel 27 of the air dispensing bar 46 and the second pressure sensor 51b may be located in the air inlet air 24 to the air suction bar 22.

[0096] In an advantageous embodiment, a plurality of pressure sensors is located along the printing bar 12 in the lateral direction L. In such a way, an average value of the pressure can be determined. Additionally local problems of debris build-up can be identified.

[0097] The control unit 62 is configured to detect an increase of the resistance in the air suction bar. The control unit 62 may be configured to continuously monitor the resistance in the suction bar and determine the actual air velocity in the printing gap G.

[0098] The control circuitry 60 of the converting machine may be further configured to monitor the air velocity in the printing gap G during printing and automatically adjusting the air velocity in the printing gap by modifying the inlet air velocity and / or the outlet air velocity in the printing gap.

[0099] The control unit 62 is configured to determine the resistance in the air suction device 20 with the second pressure sensor 51b and determine when the resistance exceeds a tolerance threshold. If the sensed pressure in the suction bar exceeds the tolerance threshold, it can be determined that the air suction device is clogged and needs cleaning. Preferably, when the tolerance threshold is exceeded, cleaning of the suction bar is effectuated. In an embodiment, the suction bar can be displaced into a service position (not illustrated) located laterally of the transportation path of the substrate 1. The service station may comprise at least one fountain nozzle configured to supply liquid to the suction bar while the suction bar is sucking. Additionally, or alternatively a cleaning device may be provided in the printing module. The cleaning device may be configured to clean the suction device in the printing position. An example of a printing device is a brush, or a laterally displaceable fluidic nozzle.

[0100] Optionally, a rinsing liquid can be injected inside the suction bar 22. The injection of the rinsing liquid may be performed continuously or sequentially. This injection can occur during the printing process or at the service station.

[0101] The control unit 62 may retrieve a production schedule of planned print jobs / batches and determines a suitable time for cleaning, preferably scheduled between the jobs. The production schedule for the cleaning may be stored in a remote location in for instance a Cloud network. The production schedule may be stored with other production parameters linked to the job. Additional cleaning due to a drop in airflow velocity VR (above the tolerance threshold) may initiate an additional cleaning process.

[0102] The direction of the airflow FR can be controlled by providing the air dispensing bars 46 and air suction bars 22 with a length exceeding the length of the print bar in the lateral direction L. The airflow direction may be further defined given by the outlet channels geometry.

[0103] It is desirable to avoid an excessive temperature which will speed up the drying of the nozzle plate 13. An ideal airflow temperature may be around 23°C.

[0104] To this effect, a temperature sensor 53 may be located in the printing carriage or in the air dispensing bar. In such a way, the temperature in the printing gap can be determined. In an advantageous embodiment, the temperature sensor is located at the air outlet openings 52.

[0105] The temperature sensor is preferably located on the airflow module 20. In an embodiment, the temperature sensor may be located on the air dispensing bar. Alternatively, the temperature sensor may be located on the suction bar. An ideal airflow temperature may be around 23°C. The memory 64 may comprise an allowable temperature range and such that the control unit 62 issues an error signal if the actual temperature is outside of the allowable temperature range. Alternatively, the air dispensing bar may comprise a heater and / or a cooler to modify the temperature of the airflow. The sensing system may further comprise a humidity control system. The humidity control system comprises at least one sensing device and a humidifier.

[0106] The desired humidity is selected such as to obtain a relative humidity which prevents the nozzle plate 13 from drying, while avoiding dilution of the main droplets with water. A relative humidity of between 30-80% RH is preferred. More preferably, the relative humidity in the printing gap may preferably be around 50%.

[0107] The humidity control system comprises a humidifier configured to inject humidity in the forced airflow FR. The humidifier comprises a liquid reservoir and a vaporizer. The vaporizer can be a piezoelectric vaporizer or a thermal vaporizer. The vaporizer is preferably located upstream of the airflow generator 45 or in a conduit between the airflow generator and the airflow generator. Alternatively, natural evaporation may be used.

[0108] Humidity could be monitored with a sensing device in the form of at least one electronics probe 55, such as pressure sensors 55 or relative humidity sensors. Preferably the humidity sensors 55 are located at both the air dispensing bar and the suction bar. As illustrated in figure 7, the humidity sensor may be located upstream of the printheads 9 in the direction of transportation. The humidity sensor may be a relative humidity sensor or a moisture sensor.

[0109] In an advantageous embodiment, a plurality of humidity sensors is located along the longitudinal length of the printing bar. In such a way, an average value of the relative humidity can be determined.

[0110] The print bars may be exposed to different temperatures and different relative humidity in the converting machine. For instance, the distance between a specific print bar and a dryer module may vary, and which may create a different environment for a specific print bar. To this effect, each air dispensing bar may be connected to a separate supply of humidified airflow.

[0111] If the forced airflow velocity VR changes over time, the deposit location of the main droplet on the substrate is modified. This leads to register displacement shifts and leads to a displacement of the printed motif, or at least some colors forming the motif. To control this register displacement in the printed motif on the substrate 1 , the converting machine may further comprise an inspection device is configured to detect register displacements. Such an inspection device is described in W02023001694A1. In such a way, the deposit location of the main droplet on the substrate 1 can be controlled by the present inkjet printing system.

[0112] The register displacements may be corrected by providing a corrective displacement of the substrate 1. The corrective displacement may be a speed correction such as an overall speed correction, and / or a momentary acceleration or a deceleration of the substrate.

[0113] An Initial calibration and calculation of a theoretical register displacements can be provided by determining a correspondence of an actual print recipe with a previous job recipe stored in the memory 64.

[0114] A machine learning process can be implemented by calculating register displacements from similar substrate 1 dimensions (length, width, height) transportation and airflow velocities V1, V2, VR. Optionally, the need for recalibration may be performed by analyzing printing register accuracy with the inspection device.

[0115] The control unit 62 may be configured to calculate a required register correction of the substrate 1 when the airflow velocity in the printing gap is changed.

Claims

1. CLAIMS1 . An inkjet printing system comprising at least one printhead provided with a nozzle plate formed in a bottom surface of the printhead, the nozzle plate being configured to eject a liquid onto a substrate, the substrate being configured to travel under the printhead in a direction of transportation, and wherein an air suction module is arranged downstream of the printhead in the direction of transportation, the air suction module being configured to direct an airflow below the nozzle plate of the printhead, wherein the air suction module comprising an air inlet, an air outlet and a first filter module located between the air inlet and the air outlet.

2. The inkjet printing system according to claim 1 , wherein the first filter module comprises a rugged surface section having a plurality of wall elements configured to force the airflow in a curved path through the first filter module.

3. The inkjet printing system according to claim 1 or 2, wherein the first filter module is configured to conduct the airflow through the suction module at an upwardly sloping angle in relation to the horizontal direction.

4. The inkjet printing system according to the preceding claim, wherein the angle is selected to be between 80 and 105 degrees, preferably about 90 degrees, whereby the extension of the first filter module in the flow direction is essentially coinciding with the vertical direction.

5. The inkjet printing system according to any one of claims 2 to 4, wherein the rugged surface section is upwardly concave.

6. The inkjet printing system according to the preceding claim, wherein the inclination of the upwardly concave section is non-constant and is increased towards the air outlet.

7. The inkjet printing system according to any one of the preceding claims, wherein the suction module comprises an elongated hollow suction bar.

8. The inkjet printing system according to any one of the preeding claims, wherein a plurality of printheads is grouped together as a print bar, and wherein said print bar is received in an elongated print bar carriage, and wherein the suction bar is attached to a lateral side of the print bar carriage.

9. The inkjet printing system according to claim 7 or 8, wherein the suction bar has a length equal or longer than the print bar carriage.

10. The inkjet printing system according to any one of the preceding claims, wherein the first filter module further comprising an ink evacuation channel configured to collect and evacuate liquid, and wherein the evacuation channel extends in the longitudinal direction of the suction bar.11 . The inkjet printing system according to the preceding claim, wherein the evacuation channel is located between the air inlet and the rugged portion within the suction module.

12. The inkjet printing system according to the preceding claim, wherein the evacuation channel is connected to a pump mechanism configured to move the collected liquid from the evacuation channel into an ink collection reservoir.

13. The inkjet printing system according to the preceding claim, wherein the pump mechanism comprises a hydrodynamic screw located inside the evacuation channel.

14. The inkjet printing system according to the preceding claim, wherein the filter module further comprises a second filter located downstream of the first filter in a flow direction through the suction bar.

15. The inkjet printing system according to the preceding claim, wherein the second filter forms a separative wall with an air evacuation channel.

16. The inkjet printing system according to any one of the preceding claims, further comprising an air flow module located upstream of the printhead in the direction of transportation of the substrate, said airflow module being configured to direct air under the printhead.

17. The inkjet printing system according to any one of the preceding claims, further comprising a cleaning device configured to inject a rinsing liquid inside the suction bar.

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