DE102023136504A1 - Printhead arrangement, printing device and method for operating a printhead arrangement - Google Patents

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) on the basis of changes in the resistance and/or the capacitance between the conductor structures (17, 18).

Description

• - eine Düse zur Ausgabe eines zu druckenden Materials, wobei die Düse einen Düsenkörper mit einer Austrittsöffnung aufweist, und
• - einen Sensor zur Detektion von außen um die Austrittsöffnung auftretenden Materialansammlungen des zu druckenden Materials.

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, and
• - a sensor for detecting accumulations of material to be printed that occur outside 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 prior 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. Generally, print heads for such printing variants comprise a nozzle through which the material is to be dispensed by means of a suitable actuator, in particular ejected dropwise or continuously. While the actuators can comprise piezo actuators, for example, particularly for low-viscosity inks as the material to be printed, a multitude of mechanical, pneumatic and other actuator variants are known, particularly for materials of higher viscosity. For example, pistons that eject the material can be used for materials of high viscosity.

Such printing processes are used not only in two-dimensional printing, but also in three-dimensional printing. An important application area of 3D printing is printed electronic components. In this case, print heads can be used to apply conductive paths to a base body, especially one that has been printed on its own. For this purpose, special inks, for example, containing conductive particles, can be used as the printing material.

The print quality in these processes is heavily dependent on the accuracy achieved. A problem in this regard can arise when material to be printed accumulates on the nozzle body, particularly in the area of the outlet opening (so-called "wetting"). This must be distinguished from a blockage of the nozzle that occurs within the nozzle, particularly in a nozzle channel. If the accumulation of material around the outlet opening exceeds a certain size, the precise dispensing of the material to be printed, for example as droplets, can be impaired. It has therefore been suggested that cleaning devices be used to remove, or in particular wipe off, such accumulations of material at regular intervals. For this to happen, however, the printing process must be stopped.

The problem is that the time intervals at which such material accumulations occur vary considerably. For example, time intervals can range from 10 seconds to 10 minutes. To reduce the necessary cleaning times and avoid excessively frequent cleaning cycles, it has been proposed to use sensors to detect material accumulations and perform cleaning processes when they are actually necessary. In contrast to the detection of nozzle blockages, only a few solutions have been proposed in the prior art to detect material accumulations on the outside of the nozzle body.

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 trapped in the print head, other error cases through the self-sensing of piezo actuators. These other error cases include temperature control errors, backpressure errors, wetting, and nozzle clogging. The principle is ultimately based on sensing and analyzing returning pressure waves through the material to be printed. This is extremely time-consuming and complex and only applicable to low-viscosity materials. In addition, changes on the outside of the nozzle head, especially next to the outlet opening, are hardly or not at all detectable due to their extremely small effects. Approaches for detecting other error cases caused by vibrations/returning pressure waves are, for example, in EP 3 369 575 A1 and EP 1 092 543 A2 called.

The invention is therefore based on the object of providing a particularly simple and space-saving, reliable possibility 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 emerge from the dependent patent claims.

In a print head arrangement of the type mentioned at the outset, the invention provides 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 connected to the conductor structures Device comprising a measuring means for measuring the resistance and/or capacitance between the conductor structures and configured to detect material accumulations due to changes in the resistance and/or capacitance between the conductor structures. In preferred embodiments, both conductor structures are arranged around the outlet opening.

In the context of the present invention, the term "print head assembly" is to be broadly understood as all material ejection assemblies for the targeted, structured ejection of a material at desired positions on an object. While the print head assembly is preferably an inkjet print head assembly and/or a print head assembly of a 3D printer, it also encompasses material ejection assemblies for devices for applying solder pastes or other pasty materials. In particular, it can be a print head assembly for 3D printing, and/or the print head assembly can have exactly one nozzle per object or slide to be printed.

It is proposed to provide at least one conductor structure, in particular two conductor structures, on the outside of the nozzle body, adjacent to the outlet opening, wherein the relationship between the conductor structures is measurably influenced by material accumulations. It is preferred to perform a resistive measurement, in which the electrical resistance of a distance between the two conductor structures is changed, in particular reduced, by the material accumulation, so that a current flow is permitted, in particular through the material to be printed. In this context, experiments have shown that even poorly or generally non-conductive materials to be printed cause 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 conducted with isopropanol/acetone (a frequently used solvent in inks), which demonstrated feasibility. Alternatively, a capacitive measurement is also conceivable, since the material to be printed, in particular dielectric, influences the capacitance of a capacitor formed by the conductor structures.

The arrangement of at least one of the conductor structures around the outlet opening is to be understood as being arranged on the outside of the nozzle body and/or formed as part of the nozzle body. In general, the nozzle body can also be formed in multiple parts, for example, by having a non-conductive, particularly ceramic, attachment providing the outlet opening, 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 is formed from copper and/or silver. The copper is in particular coated with gold to prevent oxidation. Other conceivable materials for the conductor structures include nickel and/or platinum.

While conductive surfaces are particularly suitable for capacitive measurement, conductive tracks are preferred for resistive measurement. These can be finely formed, since, as will be discussed in more detail below, the material to be printed usually represents the limiting factor with regard to electrical conductivity. Thus, the conductive structures, in particular the conductive tracks, can have a width of 0.1 to 1.5 mm. In particular, the sensor concept described here is also applicable to smaller nozzles. Widths of less than 0.1 mm are also conceivable.

Particularly in the case of resistive measurement, it is preferred to provide the at least one conductor structure, specifically the spacing distance over which a current flow through the material to be printed is to take place in the event of a material accumulation, as close as possible to the outlet opening, but not so close that it lies in a region in which material to be printed can also be present during a normal, undisturbed printing process without the presence of a material accumulation to be detected. In other words, it can be provided that, in a resistive measurement, a spacing distance between the conductor structures to be bridged by the deposition of the material to be printed is arranged at a distance from the outlet opening at least partially by a tolerance range in which material to be printed can be present even 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.

As already mentioned, experiments have shown that even for nominally non-electrically conductive printing materials, especially inks, material accumulations can be detected robustly and reliably even in a simple resistive setup. However, it should be noted that even nominally conductive inks, especially in an application for printed circuit boards, exhibit resistances in the megaohm range across the distance in the liquid state. In connection with such low conductivity For materials to be printed that have a high resistance, a useful development of the invention can provide for the evaluation device to include an amplifier, particularly for resistance measurements. Such an amplifier can, for example, comprise a Darlington circuit. Generally speaking, its use ensures that a sufficient, robustly measurable signal is generated even with high resistances, i.e., low conductivities, of the material to be printed.

Due to different electrical conductivity and/or 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 is configurable and/or calibrated for different materials to be printed, in particular inks. If, for example, a material accumulation is detected based on a threshold value, in particular based on a resistance threshold value measurable due to the current flow or an occurring voltage, the threshold value can be adjusted directly by calibration or, if suitable threshold values are already known, simple configuration by selecting 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 memory means of the evaluation device and/or a control device connected to the evaluation device.

In summary, a novel sensor concept is proposed for the resistive or capacitive detection of material deposits on the outside of the nozzles of printhead assemblies. Its use is particularly advantageous in inkjet printing systems (inkjet printhead assemblies) and/or in printing systems for printed electronics produced using inkjet printing processes or 3D printing in general, such as the printing of printed circuit boards. However, the invention can also be used for similar ejection processes, such as solder paste applications in printed circuit board production or other applications using printable inks and printable pasty materials.

Particularly with regard to circuit board printing and other 3D printing applications, viscous, i.e., highly viscous, materials are often used for printing. Printing systems with only one nozzle are also often used, which must be brought close to the print target for printing. Especially in this context, but also with other printing systems, the invention offers the advantage of a compact, integrated implementation of the sensor, thus avoiding larger, space-consuming sensors. The changes to the nozzle itself are minimal with the concept proposed here and do not result in any significant change in the size of the nozzle itself. The invention is therefore low-cost and can be implemented in a small installation space.

Nevertheless, robust detection of material buildup is enabled, which in turn makes it possible to detect material buildup during the ongoing printing process and, accordingly, to interrupt the printing process only when the sensors detect ink residue on the nozzle. Then, an appropriate cleaning process can be initiated. Due to the external placement of the sensor on the nozzle, only actual material buildup on the outside of the nozzle is detected, and this is independent of the ejection process.

As already mentioned, the present invention can also be applied 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 for ejecting the material, can comprise a piston, particularly for such highly viscous materials. This piston can, for example, be moved at high speed to eject the material to be printed, in particular dropwise.

In a specific 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, a metal, it can itself form one of the conductor structures. Then, only one additional conductor structure is required, which can, for example, be provided on a non-conductive portion, such as a ceramic attachment, or can be electrically insulated from the rest of the nozzle body by a coating.

In general, and in particular when two additional conductor structures are applied, 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 lacquer. Thus, in any case, existing surfaces and/or electrically conductive portions of the nozzle body are used. used on which or through 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 up to the remaining part of the spacing section on the outside of the nozzle body that is not bridged without material accumulation. In other words, the material to be printed can be understood as a "continuation" of the conductor structure up to the actual measuring section. If material accumulates, this last part of the spacing section is then closed. Such an embodiment has the advantage that only one conductor structure needs to be provided on the outside of the nozzle body and is particularly advantageous when the material to be printed has high electrical conductivity.

Preferably, at least one of the conductor structures can be applied by printing or by laser structuring. Vacuum processes such as vapor deposition, in particular PVD (Physical Vapor Deposition) and/or CVD (Chemical Vapor Deposition), are also conceivable, as are known for particularly fine structures in semiconductor manufacturing. Known and common methods for 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 can be provided that they are first printed wider and then the desired width is produced by laser ablation. Preferably, in particular in a print head arrangement for conductor track printing, the material to be printed and the material used for printing, in particular 3D printing, the conductor tracks are different. This prevents solvents and/or other components of the material to be printed from attacking the at least one conductor structure. In this context, regardless of this, particularly when using the same material for the conductor structures and as the material to be printed, it may also be expedient for the at least one conductor structure to be burned and/or cured into the underlying surface. For example, a particularly long sintering time may be provided to ensure the longevity of the conductor structures. The material of the at least one conductor structure is ideally anchored in the nozzle body in a mechanically robust manner by the sintering process and is significantly more difficult to remove than the "fresh" material to be printed, particularly ink.

Particularly when printing on ceramic surfaces of the nozzle body, it may be advantageous to apply at least one of the conductor structures directly to the ceramic surface, particularly a black or blackened one, using LDS (laser direct structuring). This creates particularly durable, resistant conductor structures that enable a long sensor lifespan.

Within the scope of the present invention, it may be advantageous to provide the at least one conductor structure directly on the nozzle body. However, it is also conceivable within the scope of the invention for at least one of the conductor structures to be 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 are provided as a cost-effective and easy-to-manufacture additional part, which can, for example, be stretched over the nozzle body in order to easily provide the additional functionality for detecting material accumulations as needed. The film with the at least one conductor structure is also easily replaceable. To allow simplified positioning, the through-opening can be at least slightly larger than the outlet opening. For example, a size of 1.5 to 5 times is conceivable.

In a specific embodiment of this design, the film can have a deep-drawn portion that matches the shape of the nozzle body around the outlet opening. In this way, the additional part formed by the film with the at least one conductor structure is easily positioned correctly and is also otherwise easy to attach and remove. The fit, particularly combined with the mechanical tension, effectively prevents the material to be printed from entering between the film and the nozzle body.

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 assembly by means of at least two fastening means, in particular screws. The fastening means can provide the mechanical tension. This allows for easy assembly, disassembly, and thus interchangeability. In this context, it is particularly advantageous if the electrically conductive fastening means are arranged to electrically connect the at least one conductor structure of the film to the evaluation device. The fastening means, in particular the screws, then serve a dual function: firstly, to fasten and provide the tension of the film via the nozzle body, and secondly, to transmit the electrical signals to be measured to the evaluation device. Generally speaking, the fastening means can interact with an adapter of the print head, particularly when a non-deep-drawn film is used, which is to be tensioned over the nozzle body at the outlet opening.

A variety of materials are conceivable for the film. The film is preferably made of polyimide. However, other materials, particularly those that are at least partially flexible but only slightly stretchable, can also be used. Polycarbonate or polyethylene terephthalate can be advantageous as film materials, particularly if deep-drawing is to be performed.

In a first specific exemplary embodiment of this embodiment, it can be provided that a two-foil structure is used, wherein the supply lines of the two conductor structures are arranged on different foils, or that a single foil is used, in which the supply 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 relative to the outlet opening, in which a conductor track of the star pattern is alternately plated through to a connecting conductor of the foil or surface facing away from the nozzle body to form the first conductor structure, and a conductor track of the star pattern is alternately plated through to the other foil or surface facing the nozzle body and connected to a connecting conductor to form the second conductor structure. With a single foil, through-contacts (vias) are used. This can also be expedient with other configurations of the conductor structures, for example to bridge a structure of a surface without crossing. Short circuits with metallic components when using a single foil can be prevented by solder mask, which can be provided, for example, anywhere outside the sensor-relevant conductor surfaces and the contact points.

In a second specific embodiment of this embodiment, a single film is formed with one of the conductor structures, while an electrically conductive portion of the nozzle body provides the other conductor structure and/or this is arranged within the nozzle and/or the feed for the material to be printed. In this case, a simpler design can 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.

Generally, at least one of the conductor structures can have a conductor track or conductor surface extending in a circle around the particularly circular exit opening. This makes it particularly easy to cover a complete 360° around the exit opening as a detection area, both for resistive and capacitive measurements.

In a specific embodiment, it can be provided that both conductor structures extend concentrically, in particular with a distance of 0.25 to 2.5 mm, in a circle around the outlet opening, with in particular an inner one of the conductor structures directly adjoining 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, or 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, are preferred as long as part of the 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.

Alternatively, as already explained with regard to the design with a film, 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, excellent angular coverage is also achieved, wherein, in addition, the radial length of the structures still enables robust detection even when the material accumulation only affects 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 design The star-shaped arrangement is preferred because tests have shown earlier detection of material accumulation than with concentric circles.

In general, it can be said that the evaluation device can be implemented on a printed circuit board housed in a housing. The printed circuit board or the section of the printed circuit board that implements the evaluation device can be kept small, for example, with an area of approximately 1 ^cm² . This means that the evaluation device can also be implemented as a separate component, or in combination with another component of the print head assembly, in a space-saving manner.

In a resistive measurement, a preferred embodiment provides that the evaluation device comprises a signal generating unit which, when the current through a circuit connecting the conductor structures exceeds a threshold value, outputs a signal indicating the presence of a buildup. For example, the sensor can form a voltage divider as a measuring means through the conductor structures, the spacing section, and the signal generating unit, in particular together with an amplifier. A voltage is applied to the voltage divider, a larger portion of which drops across the signal generating unit as the resistance across the spacing section decreases, in particular after amplification, so that this reduction in resistance can be measured there and the signal can be generated. The signal can be purely optical, for example, with an LED as the signal generating unit, which begins to glow as a signal due to the resulting current flow. However, the generation of an electrical, digital, or analog signal that can be further processed is preferred.

Thus, in a particularly preferred development of the invention, the print head arrangement can comprise a cleaning device for cleaning the nozzle, wherein the evaluation device or a control device connected to the evaluation device is designed to control the cleaning device for cleaning the nozzle when a material accumulation is detected, in particular when the signal is present. In this way, cleaning is preferably always carried out (and only when) it is necessary, i.e. when there is a material accumulation that could impair the print result. For example, the signal generation unit for controlling the cleaning device can be directly connected to the latter; however, control is preferably via a control device to which the signal generation unit forwards the signal indicating the detected material accumulation. The control device can then interrupt a printing process while the cleaning is in progress.

The control device can, as is generally known for printing devices, be or include a CNC (Computerized Numerical Control) device of the printing device, to which the print head assembly belongs. It can control the operation of the print head assembly or the printing device as a whole, for example, to achieve a print target. During a printing process, which can be controlled by a corresponding printing program, the sensor can be used to monitor whether a material buildup is present. If this is present, the printing process can be interrupted and the cleaning device can be activated for cleaning.

The cleaning device can, for example, comprise a paper strip, particularly one that is absorbent and/or contains a solvent, that wipes off the accumulated material and is moved past the outlet opening in a wiping action. Other variants of the cleaning device, for example, with brushes, sponges, or spraying agents, are also conceivable within the scope of the present invention. In principle, all designs for cleaning devices known in the prior art can also be used within the scope of the present invention.

The evaluation device can expediently have a light source, in particular an LED, which indicates the presence of a material accumulation and which is operated, particularly in the case of a resistive measurement, by a current through a measuring section connecting the conductor structures. The light source can therefore be the signal generating unit, or preferably one of the signal generating units. It is of course also conceivable that, when the electrical signal indicating the presence of a material accumulation is present, the evaluation device and/or the control device activates the light source. This also enables a visual display and thus improved information for a user monitoring the operation of the printing arrangement. If the cleaning device is not automatically activated, the user can activate it accordingly. It should be noted that an acoustic indication can also be issued alternatively or additionally. Additionally or alternatively, logging is also possible, for example in an error memory.

In an expedient further development, it can be provided that the evaluation device is at least partially integrated in a control unit for controlling a material output by the The actuator that causes the nozzle is integrated. In this way, several functionalities are combined in the control unit, creating an integrated, space-saving solution. In particular, the evaluation device can be implemented on a circuit board of the control unit, which also carries the corresponding control means, and/or can use a connection interface of the control unit to a control device.

In addition to the print head assembly, the invention also relates to a printing device, in particular a 3D printer, comprising a print head assembly according to one of the preceding claims. All embodiments of the print head assembly according to the invention can be applied analogously to the printing device according to the invention, so that the aforementioned advantages can also be achieved with it. In particular, the printing device, for example as a 3D printer and/or printing device for electrical conductor tracks, can have exactly one printing assembly per object to be printed, in particular per object slide, which printing assembly 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 capacitance between the conductor structures is measured by means of the sensor and accumulations of material of the material to be printed occurring outside the outlet opening are detected due to changes in the resistance and/or capacitance between the conductor structures. All statements regarding the print head arrangement and the printing device can also be applied accordingly to the method. In particular, the method can be used for 3D printing, for example for printed circuit board printing. Upon detection of an accumulation of material, a printing process can be interrupted, as described above, and a cleaning device for cleaning the nozzle can be activated. Alternatively or additionally, a message can be output to a user, in particular by controlling/operating a lighting device.

• 1 schematisch eine erfindungsgemäße Druckeinrichtung,
• 2 eine Prinzipskizze eines Druckkopfs der Druckeinrichtung der 1,
• 3 eine Aufsicht auf den Düsenkörper des Druckkopfes,
• 4 eine alternative, zweite Ausgestaltung von Leiterstrukturen,
• 5 ein Ausführungsbeispiel eines Schaltplans eines Sensors der Druckeinrichtung,
• 6 eine Materialansammlung auf einem Düsenkörper,
• 7 eine Prinzipskizze zur Erläuterung eines kapazitiven Messprinzips,
• 8 eine alternative, dritte Ausgestaltung von Leiterstrukturen für eine kapazitive Messung,
• 9 ein erstes Ausführungsbeispiel eines Druckkopfes bei Verwendung einer Folie,
• 10 ein zweites Ausführungsbeispiel eines Druckkopfes bei Verwendung einer Folie,
• 11 eine alternative, vierte Ausgestaltung von Leiterstrukturen,
• 12 schematisch eine Realisierung der Leiterstruktur der 11 in einer Zweifolienstruktur,
• 13 schematisch eine Realisierung der Leiterstruktur der 11 mit einer einzigen Folie, und
• 14 eine alternative, fünfte Ausgestaltung von Leiterstrukturen.

Further advantages and details of the present invention will become apparent from the exemplary embodiments described below and from the drawings.

• 1 schematically shows a printing device according to the invention,
• 2 a schematic diagram of a print head of the printing device of the 1 ,
• 3 a top view of the nozzle body of the print head,
• 4 an alternative, second design of conductor structures,
• 5 an embodiment of a circuit diagram of a sensor of the printing device,
• 6 a material accumulation on a nozzle body,
• 7 a schematic diagram to explain a capacitive measuring principle,
• 8 an alternative, third design of conductor structures for capacitive measurement,
• 9 a first embodiment of a print head using a foil,
• 10 a second embodiment of a print head using a foil,
• 11 an alternative, fourth design of ladder structures,
• 12 schematic representation of the conductor structure of the 11 in a two-foil structure,
• 13 schematic representation of the conductor structure of the 11 with a single foil, and
• 14 an alternative, fifth design of ladder structures.

1 shows a functional schematic diagram of a printing device 1 according to the invention. The printing device 1, in this case a 3D printer, comprises a print head assembly 3 assigned to a specimen slide 2, which has a print head 4 with a single nozzle 5. Using a movement actuator 6 (only indicated), the print head 4 can be positioned with high precision relative to the specimen slide 2 or an object thereon. The operation of the printing device 1, in particular with regard to the printing processes to be performed, 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 printing materials, in particular highly viscous inks. For example, conductive inks can be used as the printing material to print circuit tracks.

During a printing process, it can happen that material to be printed accumulates on the outside of the nozzle body next to the nozzle outlet opening (so-called "wetting"). Such material accumulations can negatively influence the printing process. Therefore, the print head assembly 3 has a sensor 8 to detect material accumulations on the outside of the nozzle 5 and a cleaning device 9 to remove such material accumulations. If the sensor 8 detects a If material accumulates on the outside of nozzle 5, i.e., "wetting," it informs control device 7, which interrupts the current printing process and activates cleaning device 9 to clean nozzle 5. In this case, cleaning device 9 comprises a paper that wipes off the accumulated material, particularly a paper that is absorbent and/or contains a solvent, and which is moved past the outlet opening in a wiping action. A light source 10, here an LED 11, is also activated to inform the user.

2 shows a schematic of the structure of the print head 4 in more detail. In addition to the nozzle 5 with the outlet opening 15, the latter has an ejection actuator 12 which, in this case, has a piston (not shown in detail) in order to be able to eject even highly viscous materials to be printed as droplets, which can be operated, for example, by piezo elements and/or hydraulically. To control the actuator 12, the print head 4 further has a control unit 13. As part of the sensor 8, the print head 4 also has an evaluation device 14, which will be explained in more detail later and which can be arranged in its own housing, but can also be integrated with the control unit 13. In this case, the evaluation device 14 can be formed on a circuit board of the control unit 13 and also share an interface with the control device 7.

The evaluation device 14 has a measuring device for measuring the resistance and/or capacitance between two electrically insulated conductor structures arranged outside the outlet opening 15. Based on changes in the resistance and/or capacitance between the conductor structures, the evaluation device 14 is designed to detect material accumulations.

3 shows a top view of the nozzle body 16 of the nozzle 5, showing a first possible configuration of the conductor structures 17, 18. For clarity, the dimensions are not to scale. Both conductor structures 17, 18 comprise a conductor track 19, 20 extending annularly and concentrically around the outlet opening 15. A respective connecting conductor 21, 22 connects the conductor track 19, 20 to the evaluation device 14. The connecting conductor 21 is electrically insulated from the outer conductor track 20, for example, by a bridge or separating layer. 4 shows in this regard an alternative, second design of the conductor structures 17, 18, which does not require a crossing 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 to first print them wider and then produce the desired width by laser ablation. The conductor tracks 19, 20, 21, 22 can be burned into the underlying surface and/or cured. Particularly when printing on ceramic surfaces of the nozzle body 16, it can be expedient to apply the conductor tracks 17, 18, 19, 20 by LDS directly to the, in particular black or blackened, surface of the ceramic. If the nozzle body 16 is designed to be electrically conductive all around the outlet opening 15, an electrically insulating coating can be provided to 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. In this case, the connecting conductor 21 is not necessarily required, since the connection can be made in another way.

In the resistive measurement provided here, a spacing section 23 is provided between the conductor structures 17, 18. When the spacing section 23 is covered for at least one angular range, a more electrically conductive connection, i.e., one with a lower resistance, is formed between the conductor structures 17, 18. This effect is measurable. For this purpose, the evaluation device 14 has a measuring device 24, an exemplary embodiment of which 5 can be seen.

In this case, the measuring device 24 is essentially designed as a voltage divider, which, however, also includes an amplifier 25 to amplify the current flowing across the spacing path 23 between the conductor structures 17, 18, which are only indicated here. The amplifier 25 in this case comprises a Darlington circuit with two bipolar NPN transistors T1, T2, and 26, respectively, but can also be implemented in another way, for example, via a corresponding operational amplifier.

Due to the amplifier 25, even with poorly (or nominally even non-) conductive materials to be printed, a current flowing through them also produces a measurable effect in a measuring branch 27, which comprises a signal generation unit 28. A voltage source 30 provides an operating voltage, for example in the range of 5 to 24 V. If the resistance across the distance 23 between the conductor structures 17, 18 drops, in particular below a threshold value, sufficient voltage is available for a current flow in the measuring branch 27, so that the signal generation unit 28 outputs a signal indicating the presence of a material accumulation on the outside of the nozzle body 16. This signal is transmitted to the control device 7 so that it can interrupt the printing process and activate the cleaning device 9 to clean the nozzle 5. After the cleaning process, the printing process continues.

A situation with a material accumulation 29 relating to circular conductor tracks 19, 20, which can be detected, is shown in 6 schematically indicated. It should also be noted that the conductor structures 17, 18 are generally placed very closely around the exit opening 15, but nevertheless in such a way that at least part of the spacing section 23 lies outside a tolerance range around the exit opening 15 in which material to be printed can be present during an undisturbed, normal printing process.

Generally speaking, the evaluation device 14 can also be configured or calibrated if different materials are used for printing. For example, in the purely illustrative example of the 5 Resistors and/or the signal generation unit 28 itself can be adapted. The components of the evaluation device 14, in particular of the measuring device 24, can be provided at least partially on a printed circuit board, in particular in the case of an integrated 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. 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 for example in 6 As shown, the electric field changes, whereby the correspondingly designed measuring means 24 of the evaluation device 14 detects this change in the electric field or the capacitance.

8 In this context, FIG. 1 shows an alternative, third configuration of the conductor structures 17, 18 as parallel strips or bars of opposing conductor surfaces 31, 32 for capacitive measurement. The electric field spans the exit opening 15.

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 stretched over the front region of the nozzle body 16. For this purpose, the film 33 initially has a deep-drawn portion 35 in the area around the through-opening 34. Two fastening means 36, here screws 37, fasten the film 33 to the print head 4. Since the fastening means 36 themselves are electrically conductive, they also serve to electrically connect the conductor structures 17, 18 to the evaluation device 14, correspondingly indicated here.

The conductor structures 17, 18 are applied, in particular again as conductor tracks 19, 20, 21, 22, to the (electrically non-conductive) foil 33. The conductor structures 17, 18 and the foil form an easily replaceable, cost-effective additional part.

10 shows a opposite 9 slightly modified, second embodiment of the embodiment with a film 33. In this case, no deep-drawn portion 35 is provided, but adapters 38 to be arranged around the nozzle 5 are used to stretch 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 foil 33 can be used for both resistive measurement and capacitive measurement.

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 alternately assigned to one of the conductor structures 17, 18 and connected to the corresponding connecting conductor 41, 42. While the embodiment of the 11 In principle, of course, it can 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 be used, as is shown for example in 12 is shown. Here, the connecting conductor 41 is located on an outer foil 33a, while the connecting conductor 42 is located on an inner foil 33b, so that the corresponding conductor tracks 40 are connected by means of vias 44.

According to 13 A star pattern can also be implemented on a single foil 33c if the connecting conductors 41, 42 are provided on opposite surfaces of the foil 33c. 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.

14 Finally, FIG. 1 shows a fourth possible implementation of the conductor structures 17, 18, wherein only one conductor structure 17 is provided as a conductor track 19, which in this case is circular and closely surrounds 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 implemented within the nozzle 5 as a metallic portion 46 of the nozzle body 16. Alternatively, a metallic portion 47 of the feed 48 can form the second conductor structure 18, or a conductor element, for example, as a rod, can be present in the nozzle 5 or the feed 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 reduced. This is shown in 14 also an example of an ejection actuator 12 is a ceramic piston 50 driven by piezo elements.

QUOTES CONTAINED IN THE DESCRIPTION

This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• EP 3 369 575 A1 [0007]
• EP 1 092 543 A2 [0007]

Cited non-patent literature

• Kye-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 (15)

A print head assembly (3), in particular for 3D printing, comprising - 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 that occur externally around the outlet opening (15), characterized in that the sensor (8) comprises two conductor structures (17, 18) that are electrically insulated from one another, at least one of which is arranged around the outlet opening (15), 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 for detecting the material accumulations (29) due to changes in the resistance and/or the capacitance between the conductor structures (17, 18). is designed. 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 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 varnish. 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 gas phase 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 particularly 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 with a distance of 0.25 to 2.5 mm, in a circle 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 the Claims 1 until 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), the conductor tracks (39, 40) being alternately assigned to the conductor structures (17, 18) in the circumferential direction. Print head arrangement (3) according to one of the preceding claims, characterized in that the evaluation device (14), in particular in the case of a resistance measurement, comprises an amplifier (25). 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 generating unit (28) which, when the current through a circuit connecting the conductor structures (17, 18) exceeds a threshold value, outputs a signal indicating the presence of a material accumulation (29). 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 to control the cleaning device (9) for cleaning the nozzle (5) when a material accumulation (29) is detected, in particular when the signal is present. 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), which indicates 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 in a Control unit (13) for controlling an actuator (12) which causes the material ejection through the nozzle (5) is integrated. Printing device (1), in particular 3D printer, comprising a print head arrangement (3) according to one of the preceding claims. Method for operating a print head arrangement (3) according to one of the Claims 1 until 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 to be printed (49) occurring outside the outlet opening (15) are detected due to changes in the resistance and/or the capacitance between the conductor structures (17, 18).

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