Printhead arrangement, printing device and method for operating a printhead arrangement

Abstract

Print head arrangement (3), in particular for 3D printing, comprising - a nozzle (5) for dispensing a material to be printed, in particular a high-viscosity ink, wherein the nozzle (5) has a nozzle body (16) with an outlet opening (15), and - a sensor (8) for detecting accumulations of material (28) of the material to be printed occurring outside the outlet opening (15), wherein the sensor (8) has two conductor structures (17, 18) arranged around the outlet opening (15) which are electrically insulated from one another and an evaluation device (14) connected to the conductor structures (17, 18), which has a measuring means (24) for measuring the resistance and / or the capacitance between the conductor structures (17, 18) and is designed to detect the accumulations of material (28) due to changes in the resistance and / or the capacitance between the conductor structures (17, 18).

Description

The invention relates to a print head arrangement, in particular for 3D printing, comprising:a nozzle for dispensing a material to be printed, the nozzle having a nozzle body with an outlet opening, anda sensor for detecting material accumulations of the material to be printed occurring from the outside around the outlet opening.In addition, the invention relates to a printing device and a method for operating a print head arrangement.Various methods are known in the art in which a material, for example an ink, is used for printing. Examples include material jetting, piezo jetting, valve jetting, binder jetting, and conventional ink jetting. In general, print heads for such printing variants comprise a nozzle through which the material is to be dispensed by means of a suitable actuator system, in particular ejected dropwise or continuously dispensed. While the actuator system can comprise piezoelectric actuators, for example, in particular in the case of low-viscosity inks as the material to be printed, a multiplicity of mechanical, pneumatic and other actuator system variants are known, in particular in the case of materials of relatively high viscosity. For example, for high viscosity materials, the material expelling pistons may be employed.Such printing methods are used not only in two-dimensional printing, but also in three-dimensional printing. An important field of application of 3D printing is printed electronic components. In this case, print heads can also be used in particular to apply conductor tracks to a base body, in particular likewise printed, for which purpose special inks comprising, for example, conductive particles can be used as the material to be printed.The print quality in these methods is highly dependent on the accuracy achieved. A problem in this regard can occur when material to be printed accumulates on the nozzle body, in particular in the region of the outlet opening (so-called "wetting"). This is to be distinguished from a blockage of the nozzle occurring within the nozzle, in particular in a nozzle channel; starting from a specific size of a material accumulation around the outlet opening, the precise dispensing of the material to be printed, for example as droplets, can be impaired. It has therefore been proposed to use cleaning devices which remove, in particular ripen, such material accumulations at regular intervals. However, the printing process must be stopped for this purpose.It is problematic that the time intervals in which such material accumulations occur are greatly scattered. For example, time intervals between 10 seconds and 10 minutes may occur. In order to reduce the required cleaning times and to avoid excessively frequent cleaning cycles, it has been proposed to use sensor systems in order to detect material accumulations and to carry out cleaning processes when they are actually necessary. In contrast to the detection of blockages of the nozzles, only a few solutions for the detection of material accumulations on the outside of the nozzle body have been proposed up to now in the prior art.In an article by Kye-Si Kwon et al, "Inkjet jet failures and their detection using piezo self-sensing", Sensors and Actuators A: Physical 201 (2013), pages 335 to 341, it is proposed to detect, in addition to the occurrence of bubbles enclosed in the print head, further fault cases by the self-sensing of piezo actuators. These other failure cases include temperature control failure, back pressure failure, wetting, and nozzle plugging. The principle is ultimately based on sensing and analyzing return pressure waves through the material to be printed. This is extremely complicated or complex and can only be used for low-viscosity materials. Moreover, changes on the outside of the nozzle head, in particular next to the outlet opening, are hardly detectable or are not detectable at all on account of the extremely low effects. Approaches for detecting other fault cases due to vibrations / returning pressure waves are mentioned, for example, in EP 3 369 575 A1 and EP 1 092 543 A2.The object of the invention is therefore to specify a reliable possibility, which can be implemented in a particularly simple and space-saving manner, for detecting material accumulations on the outside of a nozzle.This object is achieved according to the invention by a print head arrangement, a printing device and a method, in particular a computer-implemented method, according to the independent patent claims. Advantageous further developments are evident from the dependent patent claims.In a print head arrangement of the type mentioned at the beginning, it is provided according to the invention that the sensor has two conductor structures which are electrically insulated from one another, at least one of which is arranged around the outlet opening, and an evaluation device which is connected to the conductor structures and has a measuring means for measuring the resistance and / or the capacitance between the conductor structures and is designed for detecting the material accumulations on the basis of changes in the resistance and / or the capacitance between the conductor structures. In preferred exemplary embodiments, both conductor structures are arranged around the outlet opening.In this context, the term print head arrangement within the scope of the present invention is to be broadly understood as all material ejection arrangements for targeted, structured ejection of a material at desired positions of an object. While the print head arrangement is preferably an inkjet print head arrangement and / or a print head arrangement of a 3D printer, material ejection arrangements for devices for applying solder pastes or other pasty materials are therefore also included. In particular, it can be a print head arrangement for 3D printing and / or the print head arrangement can have exactly one nozzle per object or object carrier to be printed.It is proposed to provide at least one conductor structure, in particular two conductor structures, on the outer side of the nozzle body, adjacent to the outlet opening, wherein the ratio of the conductor structures to one another is influenced in a measurable manner by material accumulations. In this case, it is preferred to carry out a resistive measurement in which the electrical resistance of a distance segment between the two conductor structures is changed, in particular lowered, by the material accumulation, such that a current flow, in particular through the material to be printed, is permitted. In this connection, it has been found in experiments that even materials to be printed classified poorly or generally as non-conductive bring about a sufficient reduction in the resistance between the conductor structures and thus a measurable current flow, so that a measurement is permitted. For example, experiments were performed with isopropanol / acetone (frequently used solvent in inks) which showed feasibility. Alternatively, a capacitive measurement is also conceivable, since the material to be printed, in particular dielectric material, influences the capacitance of a capacitor formed by the conductor structures.In this case, the arrangement of at least one of the conductor structures around the outlet opening is to be understood such that it is arranged on the outside of the nozzle body and / or is formed as part of the nozzle body. In general, the nozzle body can also be formed in multiple parts, for example have a non-conductive, in particular ceramic, attachment which provides the outlet opening and on which at least one of the conductor structures can be provided.In particular, it can be provided that at least one of the conductor structures comprises conductor tracks and / or conductor surfaces and / or are formed from copper and / or silver. In this case, the copper is coated in particular with gold in order to prevent oxidation. Further conceivable materials for the conductor structures comprise nickel and / or platinum.While conductor surfaces are suitable in particular for capacitive measurement, conductor tracks are preferred in the case of a preferred resistive measurement. These can be fine, since, as will be discussed in more detail below, the material to be printed usually represents the limiting factor with regard to the electrical conductivity. It can thus be provided that the conductor structures, in particular the conductor tracks, have a width of 0.1 to 1.5 mm. In particular, the sensor concept described here can also be used for smaller nozzles. Widths of less than 0.1 mm may also be conceivable here.In particular in the case of resistive measurement, it is preferred in this case to provide the at least one conductor structure, specifically the distance via which a current flow through the material to be printed is intended to take place in the event of a material accumulation, indeed as close as possible to the outlet opening, but not so close that it comes to lie in a region in which, in the case of the customary, undisturbed printing operation, without the presence of a material accumulation to be detected, material to be printed can also be present. In other words, it can be provided that in a resistive measurement, a distance between the conductor structures to be bridged by the deposition of the material to be printed is arranged at least partially spaced apart from the outlet opening by a tolerance range in which material to be printed can also be present without material accumulation. For example, the tolerance range for a nozzle for printing conductor tracks, the outlet opening of which can have a diameter of, for example, 50 μm, can be 0.1 to 2.5 mm.As already mentioned, it was found in experiments that material accumulations could be detected in a robust and reliable manner even for nominally non-electrically conductive materials to be printed, in particular inks, even resistively in a simple construction. However, it should be noted here that even nominally conductive inks, in particular in an application for printed conductor strip printing, have resistances in the megaohm range over the distance path in the liquid state. In connection with such materials to be printed having low conductivity, a practical further development of the invention can provide that the evaluation device comprises an amplifier, in particular in the case of a resistance measurement. Such an amplifier may comprise, for example, a Darlington circuit. Generally speaking, its use ensures that a sufficiently, robust measurable signal is produced even in the case of high resistances, i.e. low conductivities, of the material to be printed.Due to different electrical conductivity and / or due to different dielectric properties of materials to be printed, when different materials to be printed are used with the print head arrangement, it can be provided that the evaluation device can be configured and / or calibrated to different materials to be printed, in particular inks. If, for example, a material accumulation is detected on the basis of a threshold value, in particular on the basis of a resistance threshold value that can be measured on the basis of the current flow or on the basis of a voltage that arises, the threshold value can be set directly suitably by calibration or, if suitable threshold values are already known, simple configuration by selection of the material to be printed and / or threshold value, in order to allow robust, reliable detection. For this purpose, threshold values can be stored, for example assigned to materials to be printed, in a storage means of the evaluation device and / or a control device connected to the evaluation device.In summary, a novel sensor concept for resistive or capacitive detection of material deposits on the outside of nozzles of print head arrangements is therefore proposed. Use in inkjet printing systems (inkjet print head arrangements) and / or in printing systems for printed electronics (printed electronics) to be produced by means of inkjet printing methods or generally 3D printing, such as for example the printing of conductor tracks, is particularly advantageous in this case. However, the invention can also be used for similar ejection methods, for example solder paste applications in printed circuit board production or other applications of printable inks and printable paste materials.Particularly with regard to the printed conductor line pressure and other 3D printing applications, viscous, i.e. highly viscous, materials to be printed are frequently used. Also, printing devices with only one nozzle are often used, which must be brought close to the print target for printing. Especially in this context, but also in other printing systems, the invention offers the advantage of a compact, integrative implementation of the sensor, so that larger sensor systems requiring more space can be avoided. The changes to be made to the nozzle itself are minimal in the concept proposed here and do not lead to an appreciable change in size at the nozzle itself. The invention is therefore low in complexity and can be implemented with a small installation space.Nevertheless, a robust detection of material accumulations is made possible, which in turn creates the possibility of detecting material accumulations during the running printing process and accordingly interrupting the printing process only if ink residues have been detected by the sensor system at the nozzle. A corresponding cleaning process can then take place. Due to the arrangement on the outside of the nozzle, on the one hand only actual material accumulations on the outside of the nozzle are detected, and on the other hand also independently of the ejection process.As already mentioned, the present invention can be used even when a high viscosity is present. For example, the material to be printed can have a viscosity of more than 20 mPa*s, in particular more than 50 mPa*s, in particular more than 1000 mPa*s. An ejection mechanism, i.e. the actuator system for ejecting the material, can have a piston, in particular for such highly viscous materials. This can be moved, for example, at high speed in order to expel the material to be printed, in particular in droplets.In a concrete embodiment of the invention, it can be provided that one of the conductor structures is formed by an electrically conductive, in particular metallic, portion of the nozzle body and / or that at least one of the conductor structures is applied to a non-conductive surface of the nozzle body. If the nozzle body itself consists at least partially of electrically conductive material, for example of a metal, it can itself form one of the conductor structures. Then, only one additional conductor structure is required, which can be provided, for example, on a non-conductive part, such as a ceramic attachment, or can be electrically insulated from the rest of the nozzle body by a coating.In general, i.e. in particular also in the case of two additionally applied conductor structures, it can be provided that the non-conductive surface is formed by a ceramic portion of the nozzle body surrounding the outlet opening and / or an electrically insulating coating of the nozzle body, in particular a paint. Thus, in any case, surfaces and / or electrically conductive portions of the nozzle body that are present in any case are used, on which or by means of which the conductor structures can be implemented with little effort.An embodiment of the invention is also conceivable in which one of the conductor structures, for example formed by an electrically conductive, in particular metallic, portion of the nozzle body, is arranged within the nozzle and / or even the feed for the material to be printed. For example, in addition to a metallic portion of the nozzle body, this conductor structure can then be realized by a metal wall of the feed and / or a conductor element in the feed and / or the nozzle. The material to be printed then continues this second conductor structure towards the outlet opening as far as the remaining part of the spacing section, which is not bridged without material accumulation, on the outside of the nozzle body. In other words, the material to be printed can be understood as a "continuation" of the conductor structure up to the actual measurement path. In the event of a material accumulation, this last portion of the distance segment is then still closed. Such a configuration has the advantage that only one conductor structure is to be provided on the outside of the nozzle body, and is particularly advantageous to implement if the material to be printed has a high electrical conductivity.Preferably, at least one of the conductor structures can be applied by printing or by laser structuring. In addition, vacuum processes such as vapor deposition, in particular PVD (physical vapor deposition) and / or CVD (chemical vapor deposition), are conceivable, as are known for particularly fine structures from semiconductor production. Known and common methods of applying conductor structures to a substrate can therefore also be used within the scope of the present invention in order to realize them with low complexity. To obtain printed conductor tracks as conductor structures, it may be provided that they are first printed wider and then produced the desired width by means of laser ablation. In this case, in particular in the case of a print head arrangement for printed conductor tracks, the material to be printed and the material used for printing, in particular 3D printing, the printed conductor tracks are preferably different. In this way, it is avoided that solvents and / or other components of the material to be printed attack the at least one conductor structure. In this context, it may also be expedient, independently of this, in particular when using the same material for the conductor structures and as material to be printed, if the at least one conductor structure is burnt and / or cured in the underlying surface. For example, a particularly long time can be provided for sintering in order to ensure a long inventory of the conductor structures. The material of the at least one conductor structure is ideally mechanically robust anchored in the nozzle body as a result of the sintering process carried out and is significantly harder to dissolve than the "fresh" material to be printed, in particular ink.In particular when printing on ceramic surfaces of the nozzle body, it may be expedient to apply the at least one of the conductor structures directly to the, in particular black or blackened, surface of the ceramic by LDS (laser direct structuring). In this way, particularly long-lasting, resistant conductor structures are created, which allow a long lifetime of the sensor.Within the scope of the present invention, it may be advantageous to provide the at least one conductor structure directly on the nozzle body. It is also conceivable, however, within the scope of the invention that at least one of the conductor structures is formed on an electrically insulating film stretched over the nozzle body, wherein the film has a through-opening corresponding to the outlet opening. Such a configuration has the advantage that the at least one conductor structure, preferably both conductor structures, of the sensor is provided as an add-on part which can be produced cost-effectively and easily and which can be tensioned over the nozzle body, for example, in order to provide the additional functionality for detecting material accumulations in a simple manner if required. The foil with the at least one conductor structure is also easily replaceable. In order to allow simplified positionability, the through opening can be, at least slightly, larger than the outlet opening. For example, a 1.5 to 5 times size is conceivable.In a concrete embodiment of this embodiment, it can be provided that the film has a deep-drawn portion which corresponds to the shape of the nozzle body around the outlet opening. In this way, the additional part formed by the foil with the at least one conductor structure is easily correctly positioned and is also otherwise easy to attach and remove. The fit combined in particular with the mechanical tension prevents the entry of material to be printed between the film and the nozzle body in an improved manner.Furthermore, in an expedient development of this embodiment, it can be provided that the film is detachably fastened to a base body of the print head arrangement by means of at least two fastening means, in particular screws. The fastening means can provide the mechanical tension in this case. Simple assembly, removability and thus exchangeability can be achieved. In this context, it is particularly advantageous if the electrically conductive fastening means are arranged for electrically connecting the at least one conductor structure of the film to the evaluation device. The fastening means, in particular the screws, then thus serve a dual function, on the one hand for fastening and providing the voltage of the film via the nozzle body, and on the other hand for forwarding the electrical signals to be measured to the evaluation device. Generally speaking, the fastening means can cooperate with an adapter of the print head, in particular when a non-deep-drawn film is used, which is to be tensioned over the nozzle body at the outlet opening.A large number of materials are conceivable for the film. Preferably, the film may be made of polyimide. However, other materials, in particular at least partially flexible but only slightly stretchable, can also be used. In particular when deep drawing is to be carried out, polycarbonate or polyethylene terephthalate may be advantageous as the film material.In a first specific exemplary embodiment of this embodiment, it can be provided that a two-foil structure is used, wherein the feed lines of the two conductor structures are arranged on different foils, or a single foil is used, wherein the feed lines of the two conductor structures are provided on different, opposite surfaces of the foil. For example, the conductor structures can form a star pattern of conductor tracks running radially with respect to the outlet opening, in which a conductor track of the star pattern is always alternately plated through with a connecting conductor of the film or surface facing away from the nozzle body to form the first conductor structure and a conductor track of the star pattern is plated through with the other film or surface facing the nozzle body and is connected to a connecting conductor to form the second conductor structure. In the case of a single film, vias (vias) are used. This can also be expedient in other configurations of the conductor structures, for example in order to bridge a structure of a surface without crossing. Short circuits with metallic components when using a single foil can be prevented by solder resist, which can be provided, for example, everywhere outside the sensor-relevant conductor surfaces and the contact points.In a second specific embodiment of this embodiment, a single foil is formed with one of the conductor structures, while an electrically conductive portion of the nozzle body provides the other conductor structure and / or is arranged within the nozzle and / or the feed for the material to be printed. In this case, a simpler configuration can therefore be achieved by using the electrically conductive nozzle itself as a conductor structure and / or by continuing a conductor structure arranged within the nozzle and / or the feed by means of the material to be printed. In particular, the electrically insulating film can provide the electrical insulation, if necessary.In general, it can be provided that at least one of the conductor structures has a conductor track or conductor surface extending in a circle around the in particular circular outlet opening. In this way, it is possible in a particularly simple manner to cover a full 360° around the outlet opening as a detection region, both for the resistive and for the capacitive measurement.In a specific exemplary embodiment, it can be provided that both conductor structures extend concentrically, in particular at a distance of 0.25 to 2.5 mm, in a circular manner around the outlet opening, wherein in particular an inner one of the conductor structures directly adjoins the outlet opening. It is conceivable that the inner one of the conductor structures is formed by an electrically conductive portion of the nozzle body, as well as that two applied, for example printed, conductor tracks extend concentrically around the outlet opening. In the case of a resistive measurement, smaller distances, i.e. a shorter distance distance, are preferred as long as a part of the distance distance lies outside the tolerance range, as discussed. For example, the distance between the concentric, annular conductor structures can be less than 2 mm, preferably less than 1 mm, particularly preferably less than 0.1 or even 0.05 mm.As an alternative to this, as already explained with regard to the embodiment with a foil, it can be provided that the conductor structures comprise radial conductor tracks running in a star shape with respect to the, in particular circular, outlet opening, wherein the conductor tracks are alternately assigned to the conductor structures in the circumferential direction. In this way, an excellent angular coverage is likewise achieved, wherein, in addition, robust detection is nevertheless made possible by the radial length of the structures even in the case of a material accumulation relating to only one angular range. Specifically, it can be provided that 10 to 20, in particular 16, radial conductor tracks are provided and / or that the conductor tracks begin at a distance of 0.25 to 1 mm from the outlet opening. In particular in such a configuration, the star-shaped arrangement is preferred, since tests have shown more early detection of the material accumulation than in the case of concentric circles.In general, it can be said that the evaluation device can be realized on a printed circuit board accommodated in a housing. In this case, the printed circuit board or the printed circuit board section realizing the evaluation device can be kept small, for example with an area of approximately 1 cm 2. This means that the evaluation device can also be realized in a space-saving manner as its own component or else in combination with another component of the print head arrangement.In the case of a resistive measurement, a preferred configuration provides that the evaluation device comprises a signal generation unit which outputs a signal indicating the presence of a collection when a current exceeds a threshold value through a circuit connecting the conductor structures. For example, the sensor can form a voltage divider as a measuring means by the conductor structures, the distance path and the signal generating unit, in particular together with an amplifier. A voltage is present at the voltage divider, from which a greater proportion falls across the signal generating unit as the resistance decreases across the distance path, in particular after amplification, so that this reduction in the resistance can be measured there and the signal can be generated. The signal can be purely optical, for example in the case of an LED as signal generating unit, which begins to light up as an indication by the resulting current flow. However, it is preferred to generate an electrical, digital or analog signal which can be further processed.Thus, in a particularly preferred development of the invention, it can be provided that the print head arrangement comprises a cleaning device for cleaning the nozzle, wherein the evaluation device or a control device connected to the evaluation device is configured for controlling the cleaning device for cleaning the nozzle in the event of a detected material accumulation, in particular in the presence of the signal. In this way, cleaning is preferably carried out whenever necessary (and only when it is necessary, i.e. there is also a material accumulation which could impair the printing result. For example, the signal generating unit for controlling the cleaning device can be connected directly to the cleaning device; however, the control is preferably carried out via a control device to which the signal generating unit passes the signal indicating the detected material accumulation. The control device can then interrupt a printing operation as long as the cleaning is running.As is known in principle in the case of printing devices, the control device can be or comprise a CNC device (computerized numerical control device) of the printing device to which the print head arrangement belongs. It can control the operation of the print head arrangement or the printing device as a whole, for example in order to achieve a print target. During a printing process, which can be controlled by a corresponding printing program, it is possible to monitor by means of the sensor whether a material accumulation is present, and when they are present, the printing process can be interrupted and the cleaning device can be controlled for cleaning.The cleaning device can have, for example, a paper which strips off the material accumulation, in particular absorbent paper and / or paper which is provided with a solvent, and which is moved past the outlet opening in a stripping manner. Other variants of the cleaning device, for example with brushes, sponges or spray-off means, are also conceivable within the scope of the present invention. In principle, all configurations for cleaning devices known in principle in the prior art can also be used within the scope of the present invention.Expediently, the evaluation device can have a lighting means, in particular an LED, which indicates the presence of a material accumulation and is operated in particular during a resistive measurement by a current through a measurement section connecting the conductor structures. The lighting means can therefore be the signal generating unit or preferably one of the signal generating units. It is of course also conceivable that, in the presence of the electrical signal indicating the presence of a material accumulation, the evaluation device and / or the control device activates the lighting means. This also allows a visual display and thus improved information of a user monitoring the operation of the printing arrangement. If no automatic actuation of the cleaning device takes place, the user can activate it accordingly. It should be noted that an acoustic indication can also be output alternatively or additionally. Additionally or alternatively, logging, for example in a fault memory, is also possible.In an expedient development, it can be provided that the evaluation device is at least partially integrated in a control unit for controlling an actuator causing the material discharge through the nozzle. In this way, a plurality of functionalities are combined in the control unit and an integrative solution saving installation space is created. In particular, the evaluation device can be realized on a printed circuit board of the control unit, which also carries corresponding control means, and / or can also use a connection interface of the control unit to a or the control device.In addition to the print head arrangement, the invention also relates to a printing device, in particular a 3D printer, having a print head arrangement according to one of the preceding claims. All embodiments of the print head arrangement according to the invention can be transferred analogously to the printing device according to the invention, so that the already mentioned advantages can likewise be obtained with the latter. In particular, the printing device, for example as a 3D printer and / or printing device for electrical conductor tracks, can have exactly one printing arrangement per object to be printed, in particular per object carrier, which has exactly one nozzle.Finally, the invention also relates to an automatic method for operating a print head arrangement according to the invention, in which the resistance and / or the capacitance between the conductor structures is measured by means of the sensor and material accumulations of the material to be printed which occur outside around the outlet opening are detected on the basis of changes in the resistance and / or the capacitance between the conductor structures. All the designs relating to the print head arrangement and to the printing device can also be transferred accordingly to the method. In particular, the method can be used for 3D printing, for example for printed conductor strip printing. When a material accumulation is detected, a printing process can be interrupted, as described above, and a cleaning device can be controlled for cleaning the nozzle. Alternatively or additionally, an indication can be output to a user, in particular by activation / operation of a lighting means.Further advantages and details of the present invention are evident from the exemplary embodiments described below and on the basis of the drawing. The following are shown: FIG. 1 schematically shows a printing device according to the invention, FIG. 2 shows a schematic diagram of a print head of the printing device of FIG. 1, FIG. 3 is a plan view of the nozzle body of the print head, FIG. 4 shows an alternative, second embodiment of conductor structures, FIG. 5 shows an exemplary embodiment of a circuit diagram of a sensor of the printing device, FIG. 6 shows a material accumulation on a nozzle body, FIG. 7 shows a schematic diagram for explaining a capacitive measuring principle, FIG. 8 shows an alternative, third embodiment of conductor structures for capacitive measurement, FIG. 9 shows a first exemplary embodiment of a print head when using a film, FIG. 10 shows a second exemplary embodiment of a print head when using a film, FIG. 11 shows an alternative, fourth embodiment of conductor structures, FIG. 12 schematically shows a realization of the conductor structure of FIG. 11 in a two-foil structure, FIG. 13 schematically shows a realization of the conductor structure of FIG. 11 with a single foil, and FIG. 14 shows an alternative, fifth embodiment of conductor structures.FIG. 1 shows a functional schematic diagram of a printing device 1 according to the invention. The printing device 1, in the present case a 3D printer, comprises a print head arrangement 3 assigned to an object carrier 2 and having a print head 4 with a single nozzle 5. By means of a movement actuator system 6 which is only indicated, the print head 4 can be placed thereon with high precision with respect to the object carrier 2 or an object. The operation of the printing device 1, in particular with respect to printing processes to be carried out, is controlled by a control device 7 which is designed as a CNC device.The printing device 1 is designed to also use highly viscous material to be printed, in particular highly viscous inks. For example, conductive inks may be used as the material to be printed to print conductive traces.During a printing operation, it can happen that material to be printed accumulates on the outside of the nozzle body next to the outlet opening of the nozzle (so-called "wetting"). Such material accumulations can have a negative influence on the printing process. Therefore, the print head arrangement 3 has a sensor 8 for detecting material accumulations on the outside of the nozzle 5 and a cleaning device 9 for removing such material accumulations. If the sensor 8 detects a material accumulation on the outside of the nozzle 5, i.e. "wetting", it informs the control device 7, which interrupts a current printing operation and controls the cleaning device 9 accordingly for cleaning the nozzle 5. The cleaning device 9 in the present case comprises a paper which strips off the material accumulation, in particular absorbent paper and / or paper which is provided with a solvent and which is moved past the outlet opening in a stripping manner. In addition, a lighting means 10, here an LED 11, is controlled in order to inform a user.Fig. 2 schematically shows the construction of the print head 4 in more detail. In addition to the nozzle 5 with the outlet opening 15, this has an ejection actuator system 12 which, in the present case, in order to be able to expel highly viscous materials to be printed as droplets, has a piston, not shown in more detail, which can be operated, for example, by piezoelectric elements and / or hydraulically. For controlling the actuator 12, the print head 4 further has a control unit 13. As part of the sensor 8, the print head 4 in the present case also has an evaluation device 14, explained in more detail below, which can be arranged in its own housing, but can also be formed integrated with the control unit 13. In this case, the evaluation device 14 can be formed on a printed circuit board of the control unit 13 and can also use an interface to the control device 7.The evaluation device 14 has a measuring means for measuring the resistance and / or the capacitance between two conductor structures which are electrically insulated from one another and arranged on the outside around the outlet opening 15. Based on changes in the resistance and / or the capacitance between the conductor structures, the evaluation device 14 is configured to detect the material accumulations.FIG. 3 shows a first possible configuration of the conductor structures 17, 18 in a plan view of the nozzle body 16 of the nozzle 5. In the present case, both conductor structures 17, 18 comprise a conductor track 19, 20 extending annularly and concentrically around the outlet opening 15. FIG. 4 shows in this respect an alternative, second configuration of the conductor structures 17, 18 which manages without an intersection of conductor tracks 20, 21.The conductor tracks 19, 20, 21, 22 can be applied, for example, by printing or by laser direct structuring (LDS). To obtain printed conductor tracks 19, 20, 21, 22 as conductor structures 17, 18, it can be provided that they are first printed wider and then produced the desired width by means of laser ablation. The conductor tracks 19, 20, 21, 22 can be burnt and / or cured in the underlying surface. In particular when printing on ceramic surfaces of the nozzle body 16, it may be expedient to apply the conductor tracks 17, 18, 19, 20 directly to the, in particular black or blackened, surface of the ceramic by LDS. If the nozzle body 16 is designed to be electrically conductive overall around the outlet opening 15, an electrically insulating coating can be provided, onto which the conductor tracks 17, 18, 19, 20 are applied.It should be noted at this point that instead of the inner conductor track 19, the conductor structure 17 can also be formed by an electrically conductive portion of the nozzle body 16. The connecting conductor 21 is then also not necessarily required, since the connection can be effected in another way.In the case of the resistive measurement provided in the present case, a distance path 23 is provided between the conductor structures 17, 18. When the distance path 23 is overlapped for at least one angular range, an electrically more conductive connection, i.e. having a lower resistance, is formed there between the conductor structures 17, 18. This effect is measurable. For this purpose, the evaluation device 14 has a measuring means 24, from which an exemplary embodiment can be seen in FIG. 5.The measuring means 24 is in the present case substantially designed as a voltage divider, which however also comprises an amplifier 25 in order to amplify the current flowing over the distance segment 23 between the conductor structures 17, 18 only indicated here. In the present case, the amplifier 25 comprises a Darlington circuit with two bipolar NPN transistors T1, T2 and 26, but can also be implemented in another way, for example via a corresponding operational amplifier.Due to the amplifier 25, even in the case of poorly (or nominally even non-) conductive materials to be printed, a measurable effect is also present in a measurement branch 27 which comprises a signal generating unit 28 when current flows through said materials. A voltage source 30 provides an operating voltage, for example in the range of 5 to 24 V. If the resistance falls above the distance segment 23 between the conductor structures 17, 18, in particular below a threshold value, sufficient voltage is available for a current flow in the measuring branch 27 so that the signal generating unit 28 outputs a signal which indicates the presence of a material accumulation on the outside of the nozzle body 16. This is transmitted to the control device 7 in the present case, so that the latter can interrupt the printing process and actuate the cleaning device 9 for cleaning the nozzle 5. After the cleaning operation, the printing operation is continued.A situation with a material accumulation 29 with respect to circular conductor tracks 19, 20 that can be detected is schematically indicated in FIG. 6. It should also be noted here that, although the conductor structures 17, 18 are in principle placed very closely around the outlet opening 15, they are nevertheless placed in such a way that at least a part of the spacing section 23 lies outside a tolerance range around the outlet opening 15, in which tolerance range material to be printed during undisturbed, normal printing operation can be present.Generally speaking, the evaluation device 14 can also be configurable or calibrateable if different materials to be printed are used. For example, in the purely illustrative example of FIG. 5, resistors and / or also the signal generating unit 28 itself can be adaptable. The components of the evaluation device 14, in particular of the measuring means 24, can be provided at least partially on a printed circuit board, in particular in the case of integrative design with the control unit 13 on a printed circuit board there.In addition to a resistive measurement, a capacitive measurement is also conceivable, in which the capacitance between the conductor structures 17, 18 is changed by the material to be printed, in particular dielectric material. FIG. 7 schematically indicates an electric field between conductor structures 17, 18 comprising ring-shaped conductor tracks 19, 20. In the case of a material accumulation 29, as illustrated for example in FIG. 6, the electric field changes, wherein the correspondingly configured measuring means 24 of the evaluation device 14 detects this change in the electric field or the capacitance.FIG. 8 shows in this context an alternative, third embodiment of the conductor structures 17, 18 as parallel strips or beams of mutually opposite conductor surfaces 31, 32 for a capacitive measurement. The electric field spans the outlet opening 15.FIG. 9 shows a first exemplary embodiment of an embodiment of the print head 4, in which the conductor structures 17, 18 only indicated here are provided on a film 33, which has a through-opening 34 corresponding to the outlet opening 15 and is tensioned over the front region of the nozzle body 16. For this purpose, the film 33 initially has a deep-drawn portion 35 in the region around the through-opening 34. 2 fastening means 36, here screws 37, fasten the film 33 to the print head 4 in a clamping manner.The conductor structures 17, 18 are applied, in particular again as conductor tracks 19, 20, 21, 22, to the (non-electrically conductive) foil 33. The conductor structures 17, 18 and the foil form an easily replaceable, cost-effective additional part.FIG. 10 shows a second exemplary embodiment of the embodiment with a film 33 that is slightly modified in comparison with FIG. 9, in which case no deep-drawn portion 35 is provided, but adapters 38 to be arranged around the nozzle 5 are used to tension the film 33 over the nozzle head of the nozzle body 16 and to fasten it by means of the fastening means 36.The embodiment with a film 33 can be used both for a resistive measurement and for a capacitive measurement.FIG. 11 shows a further, advantageous, fourth possible embodiment of the conductor structures 17, 18 which here form a star pattern of conductor tracks 39, 40 running radially with respect to the outlet opening. These conductor tracks 39, 40 are each alternately assigned to one of the conductor structures 17, 18 and are connected to the corresponding connecting conductor 41, 42. While the embodiment of FIG. 11 can in principle also be used when the conductor tracks 39, 40, 41, 42 are applied directly to a non-electrically conductive surface of the nozzle body 16, in the case of the embodiment with foils 33, a two-foil structure 43 can expediently be used, as is shown by way of example in FIG. 12. In this case, the connecting conductor 41 is located on an outer foil 33 a, while the connecting conductor 42 is located on an inner foil 33 b, so that the corresponding conductor tracks 40 are connected by means of plated-through holes 44.As shown in FIG. 13, a star pattern may also be formed on a single sheet 33 cwhen the connection conductors 41, 42 are provided on opposite surfaces of the sheet 33 c. The connecting conductor 42 is located on the inner surface and is insulated from potential metallic components by solder resist. The conductor tracks 40 are also connected here via vias.FIG. 14 finally shows a fourth possible realization of the conductor structures 17, 18, wherein only one conductor structure 17 is provided as the conductor track 19 here which surrounds it in a circular and tight manner as the outlet openings 15. The conductor track 19 is applied to a ceramic insert 45 of the nozzle 5. The second conductor structure 18 is realized within the nozzle 5 as a metallic portion 46 of the nozzle body 16. Alternatively, a metallic portion 47 of the supply 48 can also form the second conductor structure 18, or a conductor element, for example as a rod, can be present in the nozzle 5 or the supply 48. The material 49 to be printed, for example an electrically conductive ink, continues the conductor structure 18 so to speak, so that when a material accumulation 29 is formed, i.e. when the ink reaches the conductor ring formed by the conductor track 19, the circuit of the sensor 8 is closed, since the resistance between the first and second conductor structures 17, 18 is lowered. Also shown in FIG. 14 by way of example for an ejection actuator 12, is a ceramic piston 50 drivable by piezoelectric elements.References included in the specificationThis list of documents cited by the applicant has been produced in an automated manner and is only included for the better information of the reader. The list is not part of the German patent application or utility model application. The DPMA does not take any adhesion for any faults or omissions.Patent Literature citedEP 3 369 575 A1

[0007] EP 1 092 543 A2

[0007] Cited Non-Patent LiteratureKye-Si Kwon et al, "Inkjet jet failures and their detection using piezo self-sensing", Sensors and Actuators A: Physical 201 (2013), pages 335

[0007]

Claims

Printhead arrangement (3), in particular for 3D printing, having - a nozzle (5) for dispensing a material (49) to be printed, in particular a highly viscous ink, wherein the nozzle (5) has a nozzle body (16) with an outlet opening (15), and - a sensor (8) for detecting material accumulations (29) of the material (49) to be printed occurring from the outside around the outlet opening (15), characterized in that the sensor (8) has two conductor structures (17, 18) which are electrically insulated from one another and at least one of which is arranged around the outlet opening (15), and an evaluation device (14) which is connected to the conductor structures (17, 18) and has a measuring means (24) for measuring the resistance and / or the capacitance between the conductor structures (17, 17, 18) and for detecting the material accumulations (29) due to changes in the resistance and / or the capacitance between the conductor structures (17, 18).Print head arrangement (3) according to Claim 1, characterized in that one of the conductor structures (17, 18) is formed by an electrically conductive, in particular metallic, portion (46) of the nozzle body (16), and / or in that at least one of the conductor structures (17, 18) is applied to a non-conductive surface of the nozzle body (16).Print head arrangement (3) according to Claim 2, characterized in that the non-conductive surface is formed by a ceramic portion (45) of the nozzle body (16) surrounding the outlet opening (15) and / or an electrically insulating coating of the nozzle body (16), in particular a paint.Print head arrangement (3) according to Claim 2 or 3, characterized in that at least one of the conductor structures (17, 18) is applied by printing or by laser structuring or by vapor deposition.Print head arrangement (3) according to one of the preceding claims, characterized in that at least one of the conductor structures (17, 18) is formed on an electrically insulating film (33, 33a, 33b) stretched over the nozzle body (16), wherein the film (33, 33a, 33b) has a through-opening (34) corresponding to the outlet opening (15).Print head arrangement (3) according to one of the preceding claims, characterized in that at least one of the conductor structures (17, 18) has a conductor track (19, 20) extending in a circle around the in particular circular outlet opening (15) or a strip-shaped conductor surface (31, 32).Print head arrangement (3) according to Claim 6, characterized in that both conductor structures (17, 18), in particular conductor tracks (19, 20), extend concentrically, in particular at a distance of 0.25 to 2.5 mm, in a circular manner around the outlet opening (15), wherein in particular an inner one of the conductor structures (17, 18) directly adjoins the outlet opening (15).Print head arrangement (3) according to one of Claims 1 to 5, characterized in that the conductor structures (17, 18) comprise radial conductor tracks (39, 40) which run in a star shape with respect to the, in particular circular, outlet opening (15), wherein the conductor tracks (39, 40) are assigned to the conductor structures (17, 18) in an alternating manner in the circumferential direction.Print head arrangement (3) according to one of the preceding claims, characterized in that the evaluation device (14) comprises an amplifier (25), in particular in the case of a resistance measurement.Print head arrangement (3) according to one of the preceding claims, characterized in that, in the case of a resistive measurement, the evaluation device (14) comprises a signal generation unit (28) which, in the event of a current exceeding a threshold value, outputs a signal indicating the presence of a material accumulation (29) through a circuit connecting the conductor structures (17, 18).Print head arrangement (3) according to one of the preceding claims, characterized in that the print head arrangement (3) comprises a cleaning device (9) for cleaning the nozzle (5), wherein the evaluation device (14) or a control device (7) connected to the evaluation device (14) is designed for controlling the cleaning device (9) for cleaning the nozzle (5) in the event of a detected material accumulation (29), in particular in the presence of the signal.Print head arrangement (3) according to one of the preceding claims, characterized in that the evaluation device (14) has a lighting means (10), in particular an LED (11), indicating the presence of a material accumulation (29).Print head arrangement (3) according to one of the preceding claims, characterized in that the evaluation device (14) is integrated in a control unit (13) for controlling an actuator system (12) which brings about the material ejection through the nozzle (5).Printing device (1), in particular 3D printer, having a print head arrangement (3) according to one of the preceding claims.Method for operating a print head arrangement (3) according to one of Claims 1 to 13, in which the resistance and / or the capacitance between the conductor structures (17, 18) is measured by means of the sensor (8), and material accumulations (29) of the material (49) to be printed which arise outside around the outlet opening (15) are detected on the basis of changes in the resistance and / or the capacitance between the conductor structures (17, 18).

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