System for controlling operation of the nozzles of a printing head

The present invention concerns the technical field relating to the manufacture of printing- systems which use ink-jet printing heads, the ink-jet printing heads being widely used in various sectors, such as office professional printing, industrial printing and, in particular/ printing on ceramic. Specifically, the invention concerns the technical field regarding systems for controlling the correct operation of printing systems, in particular of nozzles of ink-jet printing heads.

The use of the ink-jet technique for decorating a variety of materials is widespread, although most applications concern printing particularly on paper. However, the numerous nozzles of the printing heads used by the ink-jet technique tend to get clogged and consequently produce results with evident defects. While a print defect has a relatively inexpensive impact if paper is printed, such a defect becomes unacceptable if it occurs in processes, like the process of decorating ceramic tiles, in which the defect becomes visible after the product has undergone other operations and the rejection of the product results in considerable economic loss , The problem of possible partial or total clogging of the nozzles of a printing head, which can be easily experienced by anyone who uses a desktop printer, is frequent also in the industrial field and the consequence of this problem, whether the clogging is partial or total, is that the print results become unusable. This implies serious drawbacks and considerable costs, especially in the industrial field, since an imperfectly printed product must be disposed of, which results in a loss of earning. In order to highlight how much important is that the nozzles of a printing head work correctly, or are constantly controlled, it is important to remember that the industrially used machines, in all the fields mentioned above and in general, are so designed as to

employ advanced automation systems which are suitable for the digital printing system at issue. This means that the general production is of the serial and continuous type, so that the defective operation of the nozzles of a printing head implies a considerable economic waste for a factory. Such economic waste is due both to the loss of time required for making the system working again and to the materials deteriorated while the nozzles of a printing head are only partially working. Moreover, when industrial productions are concerned, the print area is generally inside the production line of the specific product, which inevitably leads to working in cleanliness conditions that are not always optimum, even if specific subgroups may be provided for cleaning purposes. Dusts, vapours, or the printing ink which is not perfectly ejected considerably increase the possibility of clogging of the printing head nozzles, so that this serious problem may occur more frequently in this specific operating field- Currently there is no means which makes it possible to check in advance, as far as printing is concerned, the correct operation of the nozzles of an ink-jet printing head, so that this defect can only be highlighted *eχ post" after the damage is done.

The object of this invention is to avoid these and other drawbacks, by providing a system for controlling the nozzles of a printing head which allows the working efficiency of the nozzles of the printing head to be controlled before printing and/or periodically, thereby making it possible to act before the problem occurs. This result has been reached by adopting the technical solution according to the independent claims , Other features of the invention are set out in the dependent claims . The advantages deriving from this invention are basically that: damages due to badly working of the nozzles can be avoided before printing, thereby safeguarding the production line from material rejects and expensive machine downtimes for restoring the initial conditions; the controlling station

made for this purpose has extremely small dimensions and weight and can be used in existing machines, both in the design phase or added as an important accessory; said controlling station can employ the machine movements to bring the printing heads in a control position; the control operation can be carried out both in a phase of production start of the line and periodically, during the downtimes of the machine to which it is applied, without negatively affecting the hourly production. These and further advantages, objects and features of the present invention will be better understood by any skilled person from the following description and with the help of the attached drawings in which some embodiments of the invention are shown by way of non-limiting example, in which: Figure 1 shows a schematic and non-limiting axonometric view of the components of the controlling station. Any kind of material, having generally a parallelepiped shape, can be used for creating a box structure or housing 1 that is substantially closed, the housing 1 having a top opening through which the ink drop/s 2 ejected by the nozzle/s of the printing head can vertically pass. Said boic structure has the function of making it possible to work in a zone that is protected from light. However, the box structure could also be omitted. The laser lighting source 3 is positioned on the wall of the box structure perpendicularly to the jet 2, The laser lighting source 3 produces a horizontal light beam 4 which, if the printing head nozzles are correctly working, will surely be crossed by the ink drop/s 2. This situation is recorded by video camera 5, that is positioned on the same plane as the laser 3, but perpendicularly to the laser 3. When one or more drop/s pass/es through the lighted zone, the light signal produced by diffusion and/or reflection of the light beam 4 on the drop/s 2 crossing said light beam is detected by the video camera S. This allows the efficiency of each single nozzle of the printing head, to be controlled. If the nozzle is badly working, no ink drop/s is/are ejected,

hence no light is diffused and therefore the video camera does not detect any light signal, thereby indicating the presence of a problem to the controlling system. The latter possibly interrupts production, thereby avoiding production rejects, and warns the operator of the need to replace or clean and in any case, restore the printing head nozzles to working order.

Figure 2 shows a side view of the controlling station, which view further highlights the position of the printing head 6, with the ink jet 2 crossing the light beam 4 generated by the laser 3, and the luminous effect of light diffusion which is detected by the video camera 5.

Figure 3 shows a plan view of the same system, highlighting a possible position (in this example of perpendicularity) between the video camera 5 and the laser 3, which allows optimum signalling of the presence or absence of ink drop/s 2 visible owing to the phenomenon of diffusion of light of the beam 4. The invention relates to a system for controlling operation of the nozzles of an ink-jet or similar printing head.

The system concerned employs the physical phenomenon of light diffusion. If particles are involved, such as aerosol particles, having dimensions ranging from the same order of magnitude of the wavelength of visible light to much larger dimensions, light diffusion is due to different phenomena, such as refraction of rays incident on the surface of particles and reflection by the surface of the particles. Diffraction in particular conditions also produces diffusion. In the system, there is provided lighting the zone adjacent to the printing head nozzle, which is intended to be crossed by the drops of ink coming out of the nozzle if the latter works correctly. The impact between the light rays and the particles of the ink drops having dimensions greater than the wavelength of said rays, leads to diffusion and thus generates a ray of light which can be perceived and detected by a video camera.

The system concerned does not therefore involve lighting the ink drops to make them visible, but involves causing the ink drops to meet the light beam, so as to originate a diffusion which, in turn, creates a "light" which can be recorded by a video camera or a similar device.

The recording system, such as a video camera, provided in the controlling system, is connected to an electronic processor, such as a sensor, a memory device, a computer or another device, which receives the data sent by the video camera, interprets it and carries out further functions for which it is programmed.

In the system, given at least one noz∑sle of a printing head the operation of which is to be controlled, there is arranged: - at least one lighting system 3 directed at lighting the zone adjacent to at least one nozzle, said zone being crossed by an ink jet 2 in correct working conditions; at least one recording system 5 directed at recording the zone adjacent to the at least one nozzle, said zone being crossed by an ink jet 2 in correct working conditions.

Advantageously, the lighting system 3 generates a light beam 4 which is incident on the plane which must be crossed by the jet 2. The incidence of said beam 4 in relation to the zone intended to be crossed by the jet 2 may be of any degree. The system cannot work only if the light beam 4 is parallel to the jet 2 and such as not to intersect said jet, even to the slightest extent, since in that case diffusion does not occur . In an embodiment, which is preferred, said light beam 4 is perpendicular or almost perpendicular to the jet 2.

In an embodiment, the light beam 4 defines a horizontal plane while the jet 2, which drops downwards from the top, draws a vertical line which intersects said plane 4. This reciprocal position is preferable since the drops normally flow out of the nozzle of the printing head 6 and fall vertically.

The lighting system is advantageously realized by using light

sources like LED, LASER, halogen lamp or other sources. Preferably, but without limitation, the light beam is generated by a lighting source consisting of a laser, provided with suitable optics, which generates a lamellar light beam.

The laser 3 generates a horizontal light beam 4 which makes the jet 2 visible. If there is no jet, because the printing head is clogged, nothing is visible and the recording system will record the absence of this jet. Said laser 3 is perpendicular, or in any case, incident, in relation to the direction of the jet 2,

Advantageously the recording system 5 is positioned in such, a way that the light generated by the laser does not strike the recording system 5 » Said recording system must be suitable for recording the effect of the diffusion deriving from the impact between the photons and the ink particles, without receiving the direct light of the light source. In an optimum configuration of the system, the lighting source is placed on the same axis and on the same side as the video camera, in such a way that the lighting source and the video camera are one over the other but not parallel . In the attached figures, to facilitate the representation, said light beam 4 is shown orthogonal with respect to the recording system, but both the light beam 4 and the recording system are on the same plane.

The recording system 5 is on the same plane on which the light beam 4 is formed or, in any case, on a plane not too distant from the above mentioned plane, so that the video camera can record the effect of diffusion. The outlet speed of the drop from the nozzle is high (8-10 m/sec) and an acquisition system based on a video camera having an extremely high acquisition speed would therefore be recjuired. Such a video camera would therefore be much expensive. This invention uses a light source consisting of a laser, provided with suitable optics which generates a "blade" of light 4, incident on the zone through which the

jet must pass and positioned in such a way as not to strike the video camera frontally. The light source thus does not strike the video camera directly and therefore a dark image is acquired in normal conditions. Before the controlled nozzle emits the drop, the video camera is programmed so as to have an acquisition time longer than the time required by the drop to pass through the visual field of the video camera. The result is the following phenomenon: when the drop passes through the "blade" of light produced by the laser source, a flash of light is produced by diffraction. Such flash of light is detected by the video camera. Thus, by checking by means of a special software that the image acquired by the video camera has a clear dot on a dark background, it is possible to measure the actual production of the drop by the controlled nozzle. If this is not the case, it means that the nozzle has not emitted the jet and is therefore clogged.

Advantageously, in order to detect the signal generated by the passage of the ink 2 on the lighted plane 4 a video camera S is provided, the video camera 5 being connected to a processor suitably programmed for transmitting a signal to a controlling system of the machine, generally a PLC, if the jet 2 is detected, thereby allowing the machine to proceed. If this is not the case, the automatic process can be interrupted and the operator can be requested to act to solve the problem.

The system comprises a programmed processor connected to the recording system in such a way that once the signal relative to the absence of the jet is received, further control operations, alarm operations or reset operations can be carried out.

If the nozzle does not emit drops, it means that the nozzle is clogged and therefore the unclogging procedure can be activated. The unclogging procedure can be carried out automatically by the system.

After trying to unclog the nozzle, the test is repeated and even if in. this case correct ejection of drops is not detected, an alarm is activated which requires intervention by the operator, in order to proceed with manual unclogging or replacement of the printing head.

Advantageously, the control is carried out in a dark zone and preferably inside a special chamber 1.

The system can be implemented by means of a controlling device comprising a chamber 1 having: - at least one opening, having shape and dimensions compatible with the shape and dimensions of the printing head, said at least one opening being, at the right moment, at the outlet point at which the ink exits from said nozzle,- - at least one recording system 5 _V suitable for recording the zone intended to be crossed by the ink jet 2; at least one lighting system 3 suitable for lighting the zone intended to be crossed by the ink jet 2. Advantageously, the nozzle of the printing head is moved until it is positioned near said chamber, preferably above it, but, in any case, in such a way that the ink drop can fall into εaid opening.

This movement can be caused directly by the automation mechanism which controls the movement of the printing head to an operating position.

Said movement may also be a movement of the controlling system which moves , together with the chamber, to arrive under the printing head in such a way as to be able to check its correct operation. in an embodiment, before one possible drop, preferably more than one drop, is emitted by the at least one nozzle of the printing head, the recording system, such as a video camera, is set up for acquisition for an acquisition time longer than the time taken by the drop to cross the field of vision of the video camera.

In an embodiment, when the drop passes through the light beam

4 generated by the lighting source, preferably by the laser 3, a flash of light is produced by diffraction. This flash of light is detected by the video camera.

In an embodiment, by checking that the image acquired by the video camera has a clear dot on a dark background, it is possible to detect the actual production of the drop by the specific controlled nozzle.

Advantageously _/ in the latter case, the system provides that a signal is transmitted to a controlling system, generally a PLC or a suitably programmed processor which controls the continuation of the printing process. in an embodiment, by checking that the image acquired by the video camera is completely dark, it is confirmed that the at least one examined nozzle has not emitted the jet and is therefore clogged.

Advantageously, in the latter case, the system provides that an alarm signal is emitted,

Advantageously, in the latter case, the system envisages that a signal is transmitted to a controlling system, generally a PLC or a suitably programmed processor which makes it possible to carry out the interruption of the printing process and /or the unclogging procedure and/or any other operation that may be useful or necessary. In an, embodiment, after trying to clear the nozzle, the test is repeated, and if correct ejection of drops is not detected even in this case, an alarm is activated which requires intervention by the operator, in order to proceed with manual unclogging or replacement of the printing head. Advantageously, the control is carried out in a dark area and preferably inside a special chamber 1.

A device for controlling operation of at least one nozzle, preferably more than one, of a printing head, comprises a box structure 1 having a top opening through which the jet 2 of ink drops passes, at least one lighting source 3 and at least one recording system 5.

In an embodiment, said opening is kept closed in the rest

phase so that no dust or other residue can enter the box structure, and it is opened only near the printing head when operation is to be controlled.

In .an embodiment, the controlling station in which the controlling system and/or the controlling device i works is located at any point of the production line in such a way that the printing head is positioned above said point, emits the jet of ink drops and proceeds with the operation, In an embodiment, the printing head is moved so as to be positioned above the above mentioned controlling station and/or device.

In an embodiment, the above mentioned controlling station and/or device are movable and are positioned below the printing head. The invention further comprises a method which makes it possible to implement parts of the functions provided for by the system.

Specifically, the method comprises at least the following steps s - checking the detection of at least one ink drop by the recording system; if the result is positive, transmitting a signal to a controlling system, generally a PLC or a suitably programmed processor, which allows continuation of the printing process.

In addition to the steps described above, the method also provides the following steps, carried out alone or in addition to the previous steps: checking the detection of at least one ink drop by the recording system,- if the result is negative, emitting at least one alarm signal. The method further comprises the following steps: checking the detection of at least one ink drop by the recording system; if the result is negative, transmitting at least one

signal to a controlling system, generally a PLC or a suitably programmed processor which makes it possible to interrupt the printing process and /or to activate the unclogging procedure and/or to carry out any other poeration that may be useful or necessary.

The method also provides the following step: trying to clear the clogged nozzle and repeating the controlling procedure, and possibly emitting an alarm or other signal, if the controlling procedure is negative. As an example, as one of the forms of implementation of the above mentioned method, the invention comprises a program for an electronic processor suitable for carrying out one or more of the above mentioned steps.

The invention also comprises an electronic processor programmed for carrying out one or more of the steps of the method described above.