Printhead and methods for starting the printhead

Abstract

This invention relates to a method for activating a nozzle channel of a printhead to process liquid molten metal, the method comprising the steps of: A) preparing the printhead, wherein metal is melted in a crucible to form molten metal, and a piston tip is introduced into a nozzle chamber; B) generating overpressure within the crucible to induce molten metal into the nozzle chamber; C) moving the piston tip within the nozzle chamber, the piston tip reciprocating at a filling frequency via an actuator until molten metal is discharged from the nozzle channel; D) moving the piston tip within the nozzle chamber, the piston tip reciprocating via an actuator, wherein the movement initially occurs with an initial amplitude at which molten metal is discharged from the nozzle channel and exits the printhead, and subsequently the amplitude of the movement gradually decreases, and wherein the movement occurs with a gradually decreasing amplitude until molten metal no longer exits from the nozzle channel and exits the printhead, wherein the amplitude at which molten metal just ceases to exit from the nozzle channel and exits the printhead is defined as the limiting amplitude. The invention further relates to a printhead for processing liquid molten metal, the printhead comprising a control unit designed to perform the method according to the invention.

Description

Technical Field

[0001] This invention relates to a method for activating the nozzle channel of a printhead to process liquid molten metal, the method comprising the following steps:

[0002] A) Prepare the printhead, in which metal is melted in a crucible to form molten metal, and introduce the piston tip into the nozzle chamber.

[0003] B) Overpressure is generated inside the crucible, causing the molten metal to enter the nozzle chamber.

[0004] C) Move the piston tip within the nozzle chamber, wherein the piston tip reciprocates at the filling frequency via an actuator, until molten metal is discharged from the nozzle channel.

[0005] D) A piston tip is moved within the nozzle chamber, the piston tip reciprocating via an actuator, wherein the movement initially occurs with an initial amplitude at which molten metal exits the nozzle channel and leaves the printhead, and subsequently the amplitude of the movement gradually decreases, wherein the movement occurs with a gradually decreasing amplitude until molten metal no longer exits the nozzle channel and leaves the printhead, wherein the amplitude at which the molten metal just ceases to exit the nozzle channel and leaves the printhead is defined as the limiting amplitude. The invention further relates to a printhead for processing liquid molten metal, the printhead including a control unit designed to perform the method according to the invention. Background Technology

[0006] Additive manufacturing is a method of building parts step-by-step by adding small amounts of material in an orderly manner. The advantages of this type of additive manufacturing are that the tooling used to produce the parts incurs little to no expense. Furthermore, additive manufacturing avoids the large amounts of waste or scrap generated during part formation, as is often encountered in metal cutting manufacturing methods. In addition, complex shapes, such as undercuts, can be produced in a single operation in additive manufacturing, which is impossible with other one-step forming, re-forming, or cutting methods. Therefore, additive manufacturing is particularly suitable for the rapid and cost-effective production of prototypes and small batches of parts. For parts made of plastics, additive manufacturing is already well-known, for example, in the form of 3D printing, where three-dimensional physical parts are produced based on virtual data models.

[0007] Simultaneously, metallic materials can be processed in additive manufacturing methods, thereby producing three-dimensional metallic parts. Metallic parts can be produced in a metal powder bed using a beam source, such as a laser. The metal powder is locally melted and bonded by the beam source to form a solid. This method is known, for example, under the name "laser-powder bed fusion (LPBF)". Similarly, a method known as "direct energy deposition (DED)" is suitable for producing three-dimensional metallic parts.

[0008] Furthermore, a method known as "material jetting (MJT)" is known, in which molten liquid metal in droplet form is melted three-dimensionally to form metal parts. Systems employing this complex technology remain difficult to obtain on the market. Currently, further development work is underway for the mass production of such methods. In these methods, a printhead is used, which ejects liquid metal in droplet form and thus continuously produces metal parts in layers. A key component of such printheads is the nozzle through which the liquid metal droplets are ejected. In such nozzles, it is important to prevent liquid metal from adhering to the nozzle's exit area and to prevent the nozzle's exit area from being unintentionally altered or closed. Such alterations to the nozzle will lead to unreproducible behavior in droplet ejection and thus result in undesirable, unreproducible printing results in manufacturing. To address this, nozzles with metal-repellent surfaces, i.e., metal-repellent surfaces, have been developed to prevent adhesion and promote droplet separation. However, if the inner surface of the nozzle channel through which the liquid metal exits the nozzle is also designed to be metal-repellent, then reproducible wetting of the liquid metal will typically not be possible there, making droplet ejection unreproducible.

[0009] This conflict of objectives can be addressed, for example, by designing one part of the nozzle to be metal-repellent and another part, particularly the inner side of the nozzle channel, to be metal-friendly. Such segmented designs of the nozzle surface are described, for example, in DE 10 2018 221752A1. The drawback of this solution is that, at least in the sub-segments of the nozzle, an additional coating must be applied in a locally restricted manner. This additional coating is complex and increases the cost of the nozzle, which, as a wear element, requires periodic replacement.

[0010] Furthermore, to improve wetting inside the nozzle channel, a method for starting the nozzle is known, in which high-frequency vibration is coupled to the liquid metal to remove air bubbles from the molten metal. If the molten metal in the nozzle channel or nozzle chamber contains air bubbles, these air bubbles will act as an air spring under pressure, hindering the directional and reproducible discharge of droplets. For example, such a solution is disclosed in WO 2020 / 200908 A1. A disadvantage of this starting method is that, while effectively improving wetting inside the nozzle channel or nozzle chamber by removing air bubbles, it is difficult to determine whether and when complete wetting occurs in the nozzle channel. Although the known starting method is implemented, it cannot therefore be guaranteed that the nozzle will subsequently discharge liquid metal droplets with good quality and in a reproducible manner. Summary of the Invention

[0011] Therefore, the object of the present invention is to provide a solution by which metal-repellent nozzle channels for printheads used to process liquid molten metal can be more easily prepared for high-quality, reproducible discharge of metal droplets.

[0012] This objective is achieved by a method for activating the nozzle channels of a printhead used for processing liquid molten metal, the printhead comprising the following components:

[0013] - A crucible used to hold the metal to be printed.

[0014] - Heating device for melting metal

[0015] - A nozzle, which is connected to the crucible and includes a nozzle chamber and a nozzle channel extending in the discharge direction, the nozzle chamber and the nozzle channel being merged with each other.

[0016] - An actuator, including a piston having a piston tip that can be introduced into the nozzle chamber, the actuator being configured to cause the piston to reciprocate linearly, particularly in the direction parallel to the discharge direction.

[0017] The method includes the following steps:

[0018] A) Prepare a printhead, wherein metal is melted in a crucible to form molten metal, and a piston tip is introduced into a nozzle chamber, wherein, in the introduced state, there is at least a partial distance between the piston tip and the nozzle chamber in the radial direction relative to the discharge direction.

[0019] B) Overpressure is generated inside the crucible, causing the molten metal to enter the nozzle chamber.

[0020] C) Move the piston tip within the nozzle chamber, wherein the piston tip reciprocates via an actuator at a dispensing frequency greater than or equal to 1 kHz, until molten metal is discharged from the nozzle channel and exits the printhead.

[0021] D) Moving the piston tip within the nozzle chamber, wherein the piston tip reciprocates via an actuator, wherein the movement initially occurs with an initial amplitude at which molten metal is discharged from the nozzle channel and exits the printhead, and subsequently the amplitude of the movement gradually decreases, wherein the movement occurs with a gradually decreasing amplitude until molten metal no longer discharges from the nozzle channel and exits the printhead, wherein the amplitude at which molten metal just ceases to discharge from the nozzle channel and exits the printhead is defined as the limiting amplitude.

[0022] E) Repeat step D) n times consecutively, wherein in each of the n repetitions of step D), the limiting amplitude is determined in step D).

[0023] F) Once the limiting amplitudes determined in the n+1 repetitions of method step D) are all equal to or less than the defined limiting amplitudes, the method for starting the nozzle channel is terminated.

[0024] The method according to the invention is performed to activate the nozzle channels of the printhead. Preferably, the method according to the invention is initially performed once to activate previously unwetted nozzle channels. Alternatively, in the event of a failure during additive manufacturing, the method according to the invention can also be used for nozzle channels that are already in operation. Preferably, the method according to the invention is performed in the chronological order shown for method steps A) to F). However, the order in which the method steps are performed can also be at least partially interchanged or changed.

[0025] First, components or elements of a printhead activated by means of the method according to the invention will be described. The printhead includes a crucible containing metal to be printed in additive manufacturing. The crucible is designed to be heat-resistant, allowing it to continuously contain liquid molten metal, even at high temperatures. A heating device for melting the metal or for at least maintaining its molten state is provided adjacent to or connected to the crucible. The heating device can be implemented as, for example, a resistance heater or an induction heater. The printhead further includes a nozzle connected to the crucible. The nozzle is the component that discharges the liquid molten metal during additive manufacturing. The nozzle may be partially formed by a section of the crucible or may be a separate component mounted in the crucible. The discharge direction is defined as the direction in which the liquid molten metal is discharged during the operation of the printhead. The nozzle includes a nozzle chamber and at least one nozzle channel adjacent to the nozzle chamber. Both the nozzle chamber and the nozzle channel extend along the discharge direction. The nozzle chamber is located on the side of the nozzle facing the interior of the crucible, and the nozzle channel is located on the side of the nozzle chamber facing away from the interior of the crucible, forming the area from which the molten metal is discharged from the nozzle. The nozzle chamber and the nozzle channel are merged with each other and are fluidly connected to each other. The nozzle chamber's inner cross-section perpendicular to the discharge direction is larger than the corresponding inner cross-section of the nozzle channel. The printhead further includes an actuator that constitutes an assembly that causes liquid molten metal to be discharged from the nozzle by converting energy. The actuator is connected to a piston. The actuator's direction of motion and the piston's longitudinal axis extend in or parallel to the discharge direction. The piston has a piston tip on its side opposite to the actuator's connection, which can be introduced into the nozzle chamber. The actuator is a linear actuator; this means it is configured to cause the piston to reciprocate linearly.

[0026] The method according to the invention includes the following steps: In a first step A), a printhead is prepared. For this purpose, the metal to be printed is melted in a crucible to form a melt. For this purpose, the metal introduced into the crucible as a solid can be melted in the crucible by a heating device. Alternatively, at least partially molten metal can also be supplied to the crucible from the outside in a liquid state. The crucible can also be heated first by the heating device before the metal is introduced into it. Once the liquid molten metal is present in the crucible, partially molten metal has already flowed to the connection between the crucible interior and the nozzle chamber. Preferably, the nozzle chamber is connected to the interior of the crucible in the lower section, such that the molten metal moves to, and partially moves into, the nozzle chamber by gravity. Preferably, after the liquid molten metal is already present in the crucible, the piston tip of the piston is introduced into the nozzle chamber. This initial introduction of the piston tip facilitates the molten metal into the nozzle chamber by drawing the liquid molten metal from the piston tip. However, alternatively, the piston tip can also be introduced into the nozzle chamber before the metal is introduced into the crucible. With the piston tip introduced into the nozzle chamber, a distance exists at least partially between the inner wall of the nozzle chamber and the outer surface of the piston tip. This distance can extend, for example, in a radial direction relative to the discharge direction. This distance ensures that the molten metal can move in the direction of the nozzle channel, past the piston tip, and into the nozzle chamber. Subsequently, guiding the molten metal in the direction of the nozzle channel is important for achieving complete filling of the nozzle chamber and the interior of the nozzle channel.

[0027] In step B) of the second method, an overpressure is generated within the crucible. This overpressure induces further penetration of the molten metal into the nozzle chamber and nozzle channel. Since the surfaces of the nozzle chamber and nozzle channel are metalphobic, the surface tension of the molten metal must be overcome to induce wetting of these surfaces, which occurs at least in part through the driving overpressure within the crucible. The level of overpressure depends on the geometry of the crucible and nozzle. If sufficient gravity-driven penetration of the molten metal into the nozzle has been achieved, it is feasible to establish extremely low overpressure or no overpressure at all.

[0028] In the third method step C), the piston tip reciprocates within the nozzle chamber at a filling frequency. This filling frequency is significantly higher than the frequency at which molten metal is expelled as droplets during printhead operation. The filling frequency is preferably higher than 1 kHz and can reach, for example, 4 kHz. However, even higher filling frequencies, such as greater than 20 kHz, can be used. The reciprocating motion of the piston tip is caused by a correspondingly driven actuator. Here, the amplitude of the piston tip in method step C) is preferably greater than the initial amplitude in method step D), which will be further described below. The amplitude of the piston tip in method step C) is also preferably greater than the amplitude of the piston tip's movement in the operating mode where the printhead expels molten metal in droplet form to form a part. In this operating mode, the amplitude of the piston tip can be greater than the initial amplitude in method step D).

[0029] The high-frequency movement of the piston tip within the nozzle channel causes vibration of the molten metal, which in turn leads to the disintegration of bubbles present in the molten metal or in the boundary region between the molten metal and the nozzle wall. If bubbles are present in the molten metal, these bubbles act like air springs, inhibiting energy transfer between the piston tip and the molten metal during printhead operation. For this reason, bubbles in the molten metal result in unreproducible printing results and also hinder initial wetting of the nozzle chamber and nozzle channel. Vibration in the molten metal caused by the piston tip dissolves the bubbles, leading to improved energy transfer between the piston tip and the molten metal. In this way, the molten metal continuously moves into the nozzle chamber and also into the interior of the nozzle channel, resulting in at least partial wetting of the metal-repellent surface. Therefore, the high-frequency movement of the piston tip causes the molten metal to move through the entire nozzle channel and be discharged from the printhead as early as method step C). In method step C), it is possible that the nozzle chamber and / or nozzle channel are not yet completely filled with molten metal, and despite this, molten metal is still discharged when the piston tip moves. In this scenario, for example, only the section of the nozzle channel facing the nozzle chamber may be filled with melt. The melt will then form a meniscus across the nozzle channel cross-section in the boundary region between the filled section and the unfilled section of the nozzle channel. Therefore, in step C), it cannot be reliably determined whether the entire inner surface of the nozzle channel is actually wetted and whether droplet discharge is reproducible.

[0030] In the fourth method step D), which begins after the discharge of liquid molten metal in method step C), the piston tip moves within the nozzle chamber with a reduced amplitude. Also in method step D), a linear reciprocating motion of the piston tip occurs, caused by a correspondingly driven actuator. In method step D), the piston tip initially moves with an initial amplitude. Here, amplitude can also be understood as the stroke length of the piston tip. Ultimately, the amplitude of the piston tip results in a reduction in the available volume of molten metal in the nozzle chamber. Therefore, the amplitude of the piston tip is proportional to the change in volume of liquid molten metal in the nozzle chamber. The initial amplitude is selected to be large enough to achieve reliable discharge of molten metal from the nozzle. The initial amplitude is the maximum amplitude during the implementation of the method according to the invention. Preferably, the actuator is designed to produce an amplitude even larger than the initial amplitude. Therefore, the initial amplitude is preferably smaller than the maximum amplitude that the actuator can generate. For example, the size of the selected initial amplitude can be determined empirically in previous experiments. In method step D), at least when moving at the initial amplitude, the piston tip preferably accelerates at a significantly higher speed than in the operating mode used for producing parts. During the subsequent method step D), the amplitude of the piston tip gradually decreases. For each decrease, it is assessed whether molten metal is discharged from the nozzle channel. To assess whether molten metal is discharged, an associated technical discharge detection device can be placed on or near the printhead. The amplitude of the piston tip continues to decrease, and the discharge of molten metal is verified for each amplitude, until no more molten metal is detected from the nozzle at a set amplitude. The amplitude at which molten metal just stops discharging is defined as the limiting amplitude. Preferably, this limiting amplitude is the amplitude set before the step where molten metal no longer discharges.

[0031] In the fifth method step E), the previously performed method step D) is repeated. Method step D) is performed again, with the piston tip again initially moving at the starting amplitude and then decreasing the amplitude again until no more molten metal is discharged from the nozzle. For this repetition, the limiting amplitude is also determined as previously described. The limiting amplitude determined for this repetition is then compared with the limiting amplitude determined in the first repetition and / or with the limiting amplitude determined for the previous repetition. The comparison of limiting amplitudes allows conclusions to be drawn regarding the degree of wetting of the inner surface of the nozzle channel. If there is a significant difference between the determined limiting amplitudes, it can be assumed that some portions of the nozzle channel have not yet been wetted by molten metal. Conversely, if the limiting amplitudes exhibit low variation and are close to each other, it can be assumed that the interior of the nozzle channel has been fully wetted and is ready for reproducible production of the part. Method step D) is repeated n times. In the case of a new or unstudied nozzle type, n is preferably selected in the range of 1 to 5 to obtain high reliability regarding the wetting assessment. In the case of technical problems, n can also be selected to be greater than 5. For nozzle types or nozzle channel types that have been thoroughly studied and for which multiple defined limiting amplitudes already exist, n can be chosen to be zero, and therefore the entire method step E can be omitted. In this case, after performing method step D) only once, it can be predicted with sufficient certainty that complete wetting of the nozzle channel has occurred based on the determined limiting amplitude. However, n is preferably chosen to be at least equal to 1, and method step E) is performed at least once to enable comparison of at least two defined limiting amplitudes with each other.

[0032] Once all determined limiting amplitudes from the aforementioned method steps D) and E) are equal to or less than the defined limiting amplitude, the method according to the invention terminates at the sixth method step F). This defined limiting amplitude should be understood as a threshold determined empirically beforehand, representing the amplitude at which molten metal is necessarily discharged from the nozzle when the nozzle channel is fully wetted. If the limiting amplitude determined by the initiation method is greater than this defined limiting amplitude, it indicates that the wetting of the nozzle channel is likely insufficient. In this case, the initiation method according to the invention continues to drive molten metal into the nozzle channel by means of a higher amplitude and acceleration than that used in the production component by repeatedly performing method step D), thereby inducing further wetting. The actual completion of the method can be conditional upon various criteria. For example, such a criterion could be that the determined limiting amplitude must be less than or equal to the defined limiting amplitude for n+1 consecutive times. For example, if the determined limiting amplitude is less than the defined limiting amplitude twice and greater than the defined limiting amplitude a third time, then this criterion is not met. In this case, the method must continue to repeat method step E) until all n+1 consecutive repetitions are less than or equal to the defined limiting amplitude. Alternative or additional criteria for completing this method can also be based on, for example, the determination of the variation in the limiting amplitude, comparing the determined variation with the maximum permissible variation.

[0033] Because the method uses the same components and control elements as those used in producing parts via the printhead, the method for activating the nozzle channel of the printhead according to the invention can be implemented in a simple manner. Using the method according to the invention, economically produced, fully metal-repellent nozzle channels can be prepared for the operation of producing parts, enabling high-quality, reproducible discharge of molten metal from the printhead. Furthermore, the activation method according to the invention adapts easily to the level of nozzle channel development and existing expertise. The method can be effortlessly modified and adjusted by changing the number of repetitions n of method step D). After executing the activation method according to the invention, batch production of parts can be repeatedly performed using fully wetted nozzle channels. Moreover, batch production is cost-effective because simple replacement components, particularly those including nozzle channels, can be used. Furthermore, the method according to the invention can also be executed in a simple and automated manner. A control unit or controller can be set up to automatically execute the method steps and save and statistically analyze the determined values, particularly the limiting amplitude. In this way, the results of the method according to the invention can also be archived and recorded in a procedurally reliable manner. Furthermore, the method according to the invention can also be easily adapted to other printhead designs. The principles of the method according to the invention can also be similarly applied to larger or smaller printheads, and therefore can be used for the new development of various additive manufacturing production systems.

[0034] In one embodiment, between method steps B) and C), in method step B1), the piston tip reciprocates within the nozzle chamber with an amplitude at least 10 times greater than the initial amplitude. In this embodiment, the method includes an additional method step that helps wet the inner surfaces of the nozzle chamber and nozzle channel. In this additional method step B1), the piston tip reciprocates within the nozzle chamber with an amplitude significantly greater than the initial amplitude and the amplitude of the piston tip's movement during part production in printhead operation. For example, in method step B1), the piston tip moves at least one-third of the nozzle chamber length parallel to the discharge direction. It is also possible for the piston tip to move half or more of the nozzle chamber length. Similarly, the piston tip can be completely withdrawn from the nozzle chamber and then reintroduced. The amplitude of the piston tip movement in method step B1) can be, for example, 15 mm. Using the large amplitude movement of the piston tip, molten metal moves into the nozzle chamber and moves in the direction of the nozzle channel. With this movement, the piston tip draws in the liquid molten metal, which is pushed against the inner wall of the nozzle chamber during the movement. In step B1), the piston tip preferably enters the nozzle chamber at a higher speed and exits it again at a lower speed. A suitable speed for the piston tip to enter the nozzle chamber is, for example, 15 mm / s. A suitable speed for exiting the nozzle chamber is, for example, 5 mm / s. Due to the higher speed during the piston tip's entry into the nozzle chamber, the molten metal is forced into the nozzle chamber and nozzle channel, and due to the higher speed, the melt can only escape to a limited extent through the distance between the piston tip and the nozzle chamber. When being withdrawn, the piston tip moves at a lower speed to prevent the piston tip from drawing out a large amount of molten metal from the nozzle chamber again, which would hinder good wetting of the nozzle chamber and nozzle channel. At the lower speed during withdrawal, the melt present above the piston tip can flow through the distance between the piston tip and the nozzle chamber wall and is not drawn out of the nozzle chamber. In this way, wetting of the inner wall of the nozzle chamber is promoted. Furthermore, when the piston tip moves in the direction of the nozzle channel, it pushes the molten metal into the nozzle channel, thereby promoting its wetting. Preferably, in method step B1), at least a portion of the piston tip will always remain within the nozzle chamber. The piston tip is not completely withdrawn from the nozzle chamber because, in the case of complete withdrawal, the molten metal already in the nozzle channel might be displaced by air drawn in through the nozzle channel opposite the nozzle's discharge opening, to compensate for the empty volume generated during the withdrawal of the piston tip from the nozzle chamber. Preferably, in method step B1), the amplitude of the piston tip is significantly greater than the initial amplitude. Therefore, in method step B), this amplitude may also be 100 or 1000 times larger than the initial amplitude. To enable the piston tip to move with such a large amplitude, in addition to the actuator, another moving device can be provided to enable the piston tip to move with such a large amplitude.

[0035] Furthermore, it is advantageously conceivable that if no molten metal is discharged after a predetermined residence time in method step C), method step B1) according to the foregoing embodiment is repeated. In this embodiment, if liquid molten metal is not discharged from the nozzle channel in method step C), then the aforementioned method step B1) is repeated. It is possible that no molten metal is discharged because the high-frequency movement of the piston tip in the nozzle chamber in method step C) is performed with a significantly smaller amplitude than the slower reciprocating motion in method step B1). This may occur, for example, if very little molten metal is being delivered into the nozzle channel at this time. In this case, method step B1) is repeated to deliver a larger amount of liquid molten metal into the nozzle channel. For subsequently delivering a larger amount of material, the slow movement of the piston tip with a large amplitude in method step B1) is more suitable than the high-frequency movement with a lower amplitude in method step C). In another alternative embodiment of the method, method step B1) may also be omitted in the first cycle, and method steps B) and C) may be performed directly and continuously. In this case, if no molten metal is discharged in step C), then step B1) is executed for the first time after step C) to deliver a larger amount of molten metal into the nozzle channel and nozzle chamber. The cycle of steps B1) and C) is repeated until the molten metal is discharged from the nozzle channel in step C).

[0036] In another embodiment, it is envisioned that method step C1) is performed between method steps C) and D), wherein the overpressure within the crucible is reduced. In method step B), an overpressure is generated within the crucible, which causes molten metal to be driven into the nozzle channel by this overpressure. This overpressure generated in method step B) can be reduced before the start of method step C). This overpressure reduction can be performed such that the pressure in the crucible is set to the pressure set for operation of the printhead used for producing parts. With this overpressure reduction in the crucible, the applicability of the limiting amplitude determined in method step D) to operation of the printhead used for producing parts is improved, and its informational value is increased.

[0037] Advantageously, it is envisioned that in method step E), the n repetitions of method step D) are performed within the crucible under a reduced overpressure following method step C1). In this embodiment, the n repetitions of method step E) are also performed within the crucible under a reduced overpressure compared to method step B). In this embodiment, due to the reduced overpressure following method step C1), method step E) is performed under pressure conditions within the crucible that substantially correspond to the pressure conditions during component production via the printhead. Therefore, the determined limiting amplitude allows for direct conclusions about the limiting amplitude during normal printing operation. Thus, the method provides particularly relevant and reproducible results regarding the limiting amplitude in this embodiment.

[0038] In an advantageous embodiment, it is conceivable that in method step D), if no molten metal is discharged from the nozzle channel and leaves the printhead during the movement of the piston tip, method step C2) is performed, corresponding to method step C) performed under the reduced overpressure in the crucible according to method step C1), and method step D) is repeated after method step C2). It is possible that during the start-up process, in method step D), no molten metal is discharged from the nozzle channel at the set start amplitude. This could be because, for example, the nozzle channel is not adequately filled with liquid molten metal before method step D) begins. In this case, the starting amplitude can be increased until molten metal discharge occurs. However, the starting amplitude should be selected such that molten metal discharge should occur if the nozzle chamber and nozzle channel are adequately filled with molten metal. Another reason for no material discharge at the starting amplitude in method step D) could be the presence of air bubbles in the molten metal that were not removed in the previous method step C). To this end, an additional method step C2) is performed, in which the piston tip moves at a high frequency as in method step C) to remove bubbles that may remain in the melt by vibration, in the event that no molten metal is discharged at the previously determined initial amplitude in method step D). However, method step C2) is preferably performed in the crucible at a reduced overpressure compared to method step B). Once the molten metal is discharged from the nozzle channel, method step C2) is complete, and method step D) is then performed again. This cycle can also be repeated until the molten metal is discharged when the piston tip moves at the initial amplitude in method step D).

[0039] Furthermore, it is conceivable that the movement of the piston tip in method step D) is performed at a starting frequency, which is at most 10% of the minimum filling frequency of method step C). In this embodiment, the movement of the piston tip in method step D) is significantly slower than that in method step C). In method step C), the piston tip intentionally moves at a high frequency to dissolve air bubbles in the molten metal. Also in method step C), the molten metal is discharged from the nozzle channel by high-frequency movement. This discharge occurs in the form of small droplets with irregular trajectories and not entirely along the discharge direction. The discharge of these smaller molten metal droplets in different flight directions is referred to as "jetting". Various flight directions of the small droplets occur when the inner wall of the nozzle channel is not yet fully wetted and / or the nozzle channel is not yet completely filled with molten metal. One disadvantage of jetting is that some droplets may contaminate the nozzle components by following trajectories other than the discharge direction, which may in turn lead to unreproducible printing results later. To avoid unwanted jetting in method step D), a starting frequency of at most 10% of the filling frequency is chosen. Even at the piston tip frequency, which is between the filling frequency and the start-up frequency in step C), such as 50% of the filling frequency, droplets that do not move in the discharge direction can be expelled, which should be prevented. A start-up frequency of up to 10% of the filling frequency has been found suitable for expelling droplets that only move in the discharge direction. For example, the start-up frequency can be on the order of 10 Hz. At such low frequencies, no vibration is coupled into the molten metal, and droplet expulsion occurs in a substantially organized manner in the discharge direction. Furthermore, the low start-up frequency prevents unwanted contamination of the nozzle or printhead during the start-up process. In step D) or in operating modes where the piston tip moves at a frequency greater than 10% of the filling frequency and the nozzle channel is completely filled and wetted by the molten metal, the expulsion of droplets that do not move linearly in the discharge direction may also occur. After the piston tip's movement for expelling the molten metal, the meniscus on the side facing away from the nozzle chamber, toward the externally restricting discharge opening of the nozzle channel, remains vibrating for a period of time. If further movement of the piston tip is performed while the meniscus is still vibrating due to previous droplet discharge, the next droplet discharge may occur in an unpredictable direction due to the vibrating meniscus. Therefore, at the end of step D) and in the operating mode where a reproducible discharge direction of the droplet is obtained, insufficient time interval between two amplitudes, or in other words, excessive frequency during the movement of the piston tip, should be avoided. Subsequent movement or amplitude of the piston tip should only be initiated when the meniscus is no longer vibrating due to the previous movement or amplitude. The duration of meniscus vibration can depend on the amplitude, the geometry of the nozzle channel, the viscosity of the molten metal, and / or other factors.

[0040] In another embodiment, it is conceivable that in method step D), during the gradual decrease of the piston tip's movement amplitude, starting with an initial amplitude, the amplitude is then gradually reduced by a differential. After each subtraction of the differential from the current amplitude, the piston tip continues its movement for a period of time or a certain number of movements before the amplitude decreases again, determining whether molten metal has been discharged from the nozzle channel and left the printhead during that period of time or during a certain number of movements at a constant amplitude. In this embodiment, starting with the initial amplitude in method step D), the differential is gradually subtracted from the currently set amplitude. After each subtraction of this differential, whether molten metal has been discharged from the nozzle channel is evaluated. If molten metal has been discharged, the current amplitude is reduced by the differential again. This process is performed until no more molten metal is discharged from the nozzle. After each subtraction of the differential, the piston tip moves at a constant amplitude a certain number of times or for a certain period of time to reliably determine whether molten metal has been discharged. The number of times the piston tip moves at a constant amplitude, or the time period during which this movement occurs, depends substantially on the method used to determine whether molten metal has been discharged. The amplitude is reduced in subsequent steps only after it has been reliably determined whether molten metal has been discharged. A constant amplitude difference can be selected, thus reducing the amplitude in constant step sizes. Alternatively, the amplitude difference can be selected to vary in consecutive amplitude reduction steps. For example, the amplitude difference can be selected to be larger at the beginning of method step D) than in subsequent steps of method step D), where the current amplitude approaches the expected limiting amplitude. In this way, the time required to accurately determine the limiting amplitude can be reduced. Alternatively, method step D) can be performed manually, with the printhead operator adjusting the amplitude difference for each step individually. Similarly, the number of piston tip movements or the duration of piston tip movement at a constant amplitude can be selected to be always the same size or alternatively, to vary. Therefore, at the beginning of method step D), the duration of piston tip movement at a constant amplitude can be selected to be shorter because at a larger amplitude, it is possible to determine whether molten metal has been discharged more quickly and reliably. During the period approaching the expected limiting amplitude, this duration can be increased to improve the accuracy of detecting the discharge of molten metal from the nozzle channel.

[0041] Furthermore, it is advantageous to envision that the piston tip is accelerated to a higher degree than in method step D) and / or the operating mode, in method step C) where the printhead ejects molten metal in droplet form to form a part during its reciprocating motion. In this embodiment of the method, the actuator accelerates the piston tip at a higher speed in method step C) than in method step D). This increased acceleration allows for a higher dispensing frequency in method step C), which facilitates the removal of air bubbles from the molten metal. Furthermore, the high acceleration in method step C) promotes the flow of molten metal into the nozzle channel. In method step D), the acceleration of the piston tip is correspondingly reduced, thereby enabling controlled ejection of molten metal from the nozzle channel. The reduced acceleration in method step D) also serves to avoid unwanted jetting in method step D), as it hinders the detection of molten metal ejection. Moreover, the acceleration in method step C) is higher than the acceleration in the operating mode where the printhead ejects molten metal in droplet form to form a part.

[0042] It is possible that during its reciprocating motion in step D), the piston tip accelerates at a higher speed than in the operating mode where molten metal is ejected in droplets from the printhead to form parts. The acceleration of the piston tip in step D) can be selected to be higher than the acceleration of the piston tip in the operating mode of the printhead for producing parts. The higher acceleration in the initiation method causes the entire surface of the nozzle channel to be wetted with molten metal. The acceleration of the piston tip in step D) can be, for example, at least 5 times, preferably at least 10 times, higher than the acceleration in the operating mode. Furthermore, the piston tip may be moved in step D) with the same acceleration as in the operating mode. This is advantageous because the same operating parameters as in the operating mode are used in step D), which facilitates the applicability of the determined limiting amplitude. Finally, at the beginning of step D), a higher acceleration than in the operating mode can also be selected, particularly when using an initial amplitude, to initially induce wetting of the nozzle channel. In further processes of step D), the acceleration can be reduced to a value corresponding to the acceleration of the piston tip in the operating mode, particularly when approaching the limiting amplitude. This variable acceleration advantageously enables good wetting of the nozzle channel at the beginning of method step D), and simultaneously produces conclusive results for the determined limiting amplitude at the end of method step D).

[0043] Furthermore, it is conceivable that after completing step F), the printhead switches to an operating mode that discharges molten metal in droplets to form a part, the discharge of the molten metal in droplets occurring at a standard frequency and / or an amplitude greater than the limiting amplitude. After completing the method for activating the nozzle channel, the printhead is switched to its part-forming operating mode. In this operating mode, the piston tip reciprocates within the nozzle chamber via an actuator, causing droplets of molten metal to be discharged from the nozzle channel in the discharge direction. In this operating mode, the piston tip moves at a standard frequency. Typically, the standard frequency is in the range of 100 Hz to 1 kHz. In this operating mode, the piston tip moves with an amplitude equal to or greater than the limiting amplitude. The limiting amplitude is the amplitude determined in this activation method at which droplet discharge just does not occur. Preferably, this amplitude is selected to be greater than the limiting amplitude in the operating mode to obtain a degree of certainty that molten metal in droplet form is always discharged from the nozzle channel in the operating mode. To determine the amplitude in the operating mode, statistical calculations can be used, for example, based on the determined limiting amplitude and the number of repetitions (n) in steps D) and E). In this way, the amplitude of the operating mode can be calculated, enabling reproducible and high-quality discharge of molten metal from the production of components with high statistical certainty. As the amplitude in the operating mode increases, the diameter of the discharged molten metal droplets may also increase. However, the droplet diameter plays a secondary role during nozzle channel startup.

[0044] In a preferred embodiment, the method is envisioned to be executed in a manner that is at least partially automated, with a control unit controlling the method steps, and the completion of the method being determined by the control unit, with associated notifications output to the operator of the control unit. In this embodiment, a control unit is provided that executes at least a portion of the method in an automated manner. During this process, the actuator is automatically controlled by the control unit, which controls its amplitude, acceleration, and frequency. Furthermore, the limiting amplitude is automatically determined, saved, and analyzed by the control unit. The completion of the method for activating the nozzle channel is also determined by the control unit. Due to this automation, the method can be executed in a particularly reproducible manner, and data can be saved as evidence of the correctly implemented activation method. Of course, the method can also be executed at least partially manually, for example, by manually inputting the amplitude difference via an interface in method step D).

[0045] In another embodiment, it is conceivable that in method steps C) and D) and in the operating mode where molten metal in droplets is ejected from the printhead to form a component, the amplitude of motion of the piston tip is generated by the work provided by the control unit and acting on the actuator. A work receiving element designed to receive a portion of the work is provided, and in the operating mode, the work receiving element receives a portion of the work provided by the control unit, while the actuator only converts the remaining portion of the work. This results in a decrease in the amplitude and / or acceleration of the piston tip compared to the conversion of the entire work. The amplitude of the piston tip depends on the work converted by the actuator. This work is provided by the control unit. The greater the work, the greater the amplitude of the piston tip. In the startup method, particularly in method steps C) and D), the piston tip preferably moves with a larger amplitude and / or higher acceleration than in the operating mode. The work in the startup method and the operating mode may also be equally large. Furthermore, the amplitude of the piston tip may be larger in the operating mode than in the startup method, for example, when droplets with particularly large volumes are ejected from the printhead in the operating mode. However, in most cases, a relatively large amount of power is required for the starting method to effectively drive the molten metal into the nozzle channel and achieve complete wetting. Therefore, preferably, the acceleration of the piston tip is also high in the starting method, which is related to the change in power transferred to the actuator over time. In both the starting method and the operating mode, the control unit must be designed to precisely control the output power, and therefore precisely control the controlled amplitude of the piston tip. Since the power can vary as previously described, this places high demands on the power output of the control unit. Therefore, in this embodiment, a power receiving element is provided that receives a portion of the power output by the control unit in the operating mode where the required power is typically lower. This results in the control unit always outputting power of the same order of magnitude. In this way, the control unit can be designed to be particularly precise within this order of magnitude. In other orders of magnitude of power, the control unit's operating precision may be lower, which reduces the demands on the control unit and thus makes it more cost-effective. In the operating mode, a portion of the power will always flow into a power receiving element that can be connected in parallel with the actuator. By utilizing this parallel connection, the amplitude of the piston tip can be reduced in operating mode, even if the control unit outputs the same order of magnitude of power as in the starting method. For example, if the actuator is implemented as a piezoelectric element, then the power is constituted by the amount of charge. In this case, the power receiving element can be formed by a capacitor connected in parallel with the actuator. The described arrangement of the power receiving element also results in improved controllability of the acceleration and velocity of the piston tip. In the starting method, the piston tip preferably moves with a higher acceleration than in operating mode to drive molten metal into the nozzle chamber and nozzle channel. To achieve high acceleration, the power must be transferred to the actuator in a short time.On the other hand, in operating modes requiring lower acceleration at the piston tip, power should be transferred to the actuator more slowly, meaning over a longer period. The control unit also includes regions where the power transferred over time varies, in which the control unit operates with particularly high precision. Conversely, in other operating ranges, the control unit operates less precisely, and therefore the acceleration at the piston tip can be set less precisely in these regions. Parallel connection of power receiving elements as needed, and the reception of a portion of the transferred power by the power receiving elements as needed, can advantageously be used to precisely set the acceleration and velocity of the piston tip. For example, if a power receiving element with the same storage capacity as the actuator is connected in parallel with the actuator, then, with the control unit settings remaining the same, the power transferred to the actuator per unit time will be halved. In this way, the acceleration of the piston tip by the actuator is also substantially halved.

[0046] The aforementioned arrangement or use of the power receiving element, which can be connected in parallel with the actuator as needed, is also advantageously independent of the method for starting the printhead according to the invention. Therefore, a method for operating a printhead for processing liquid molten metal is also disclosed, the printhead comprising the following components:

[0047] - A crucible used to hold the metal to be printed.

[0048] - Heating device for melting metal

[0049] - A nozzle, which is connected to the crucible and includes a nozzle chamber and a nozzle channel extending in the discharge direction, the nozzle chamber and the nozzle channel being merged with each other.

[0050] - An actuator including a piston having a piston tip that can be introduced into the nozzle chamber, the actuator being configured to cause the piston to reciprocate linearly with a certain amplitude, particularly in the direction parallel to the discharge direction.

[0051] The method includes the following steps:

[0052] I. Start the printhead to prepare it for operation, in which the printhead ejects molten metal in droplets to form a part.

[0053] II. Switch the printhead to operating mode.

[0054] The amplitude of the piston tip's motion in steps I and II is generated by the power supplied by the control unit and acting on the actuator. A power receiving element designed to receive a portion of the power is provided, and the power receiving element receives the power supplied by the control unit in step II. Only the remaining power is converted by the actuator in step II.

[0055] In this method, the printhead can be started in step I, and is not limited to any starting method according to the invention. Here, the parallel connection of the power receiving elements as needed produces the same advantage of more precise actuator control described previously in conjunction with embodiments of the method according to the invention. Therefore, this method also produces improved quality and reproducibility of the print produced by the printhead.

[0056] Cleverly, it is conceivable to use an ejection detection device that determines whether molten metal has exited the nozzle channel and left the printhead. In this embodiment, the ejection detection device determines whether molten metal has exited the nozzle channel manually or automatically. Here, the ejection detection device can be based on various physical operating principles.

[0057] In a preferred embodiment, the discharge detection device is conceivable to include a flash source, a camera, and an electronic detection unit. The detection unit determines whether molten metal has been discharged from the nozzle channel and exited the printhead by automatically analyzing images captured by the camera. In this embodiment, the electronic detection unit optically determines whether molten metal has been discharged from the nozzle channel and exited the printhead. Furthermore, a flash source, also known as a stroboscope, is provided, which emits light flashes at short time intervals and may illuminate the discharged molten metal droplets. Here, illumination can be performed from the same direction as the camera or from a position relative to the droplet and opposite the camera. Therefore, the droplet can be illuminated using either reflected light or transmitted light methods. During each of these light flashes, the camera captures an image of the area near the nozzle channel. The electronic detection unit automatically determines whether the molten metal droplet is visible in the image by analyzing these image data captured by the camera.

[0058] Cleverly, it is envisioned that the amplitude of the piston tip's movement in step C) is between 0.5 μm and 60 μm. Amplitudes within this range have been found to be particularly effective for removing bubbles from the liquid molten metal in the nozzle channel. Preferably, the initial amplitude and / or operating amplitude in step D) are also on this order of magnitude. The amplitude in the operating mode can also be within the indicated range.

[0059] In an advantageous embodiment, the actuator is conceived to include at least one piezoelectric element coupled to the piston, the main expansion direction of which is parallel to the longitudinal axis of the piston. A control unit controls the expansion of the piezoelectric element and thus the reciprocating motion of the piston tip coupled to the piezoelectric element. The backward motion of the piston is at least partially caused by spring force. The amount of work transferred from the control unit to the piezoelectric element in the form of charge is related to the amplitude of the piston tip. In this embodiment, the actuator is based on the operating principle of the piezoelectric element. The piezoelectric element is characterized by high accuracy in amplitude generation. Furthermore, the piezoelectric element can be controlled at extremely high clock frequencies, such that even the high frequencies in method step C) can be achieved by such an actuator. When a defined amount of charge, as work, is applied to the piezoelectric element, the piezoelectric element will expand. This effect is typically used to cause forward motion of the piston and piston tip. The corresponding backward motion is typically achieved by removing the charge from the piezoelectric element and simultaneously pushing the piezoelectric element back to a non-expanded state by a correspondingly positioned set of springs. Alternatively, only a portion of the charge previously transferred to the piezoelectric element can be removed for the backward motion of the piston and piston tip. In this state, the piezoelectric element remains in a partially expanded state, and therefore for a longer period than in the state where there is no charge at all. To control the actuator, the control unit can independently control the amount of charge applied to the piezoelectric element during forward movement and during backward movement, thus achieving different amplitudes. The magnitude of the amplitude generated by the piezoelectric element is directly related to the amount of charge supplied to it by the control unit.

[0060] Cleverly, one can conceive of a control unit that controls the ampere rate at which charge is transferred to the piezoelectric element, the ampere rate being related to the acceleration of the piston tip. The ampere rate affects how quickly a given amount of work or charge can be transferred to the piezoelectric element. To increase the amount of charge transferred, the ampere rate or the transfer time can be increased. Therefore, the ampere rate during charging and / or discharging of the piezoelectric element has a direct impact on the acceleration of the piston tip. When a higher piston tip acceleration is required, the piezoelectric element charges or discharges at a higher ampere rate, thereby reducing the time to reach the target capacitance.

[0061] Furthermore, it is conceivable that the actuator includes a moving device that, in method step B1), moves the piezoelectric element, including a piston coupled thereto, relative to the nozzle in a direction parallel to the longitudinal direction of the piston. This moving device is designed to produce an amplitude at least ten times larger than the maximum amplitude of the piezoelectric element's expansion. In this embodiment, in method step B1), the piston tip is moved by a moving device other than the piezoelectric element. Typically, the maximum possible amplitude of the piezoelectric element is on the order of 60 μm. To significantly promote wetting of the nozzle chamber and nozzle passage, movement of the piston tip on the order of millimeters in method step B1) has been found to be particularly effective. The piezoelectric element cannot produce such a large amplitude. Therefore, an additional moving device is provided as part of the actuator. In a simple embodiment, this moving device can be formed, for example, by a guide parallel to the discharge direction. To perform method step B1), the piezoelectric element, including the piston attached thereto, is separated from the rest of the actuator and manually reciprocated. After performing method step B1), the piezoelectric element is fixed again, and the subsequent method steps are performed. Alternatively, the moving device may also be formed by a linear axis that moves the piezoelectric element and the piston, including the piston tip, back and forth in method step B1). Such a linear axis typically includes at least one guide and a linear actuator controlled by a control unit. The moving device can move the piezoelectric element together with the piston. Alternatively, in method step B1), the piezoelectric element may not be moved, but only the piston, including the piston tip, may be reciprocated by the moving device.

[0062] Furthermore, it is advantageously envisioned that, in method step C1), the overpressure within the crucible is reduced by at least four times compared to the overpressure within the crucible in method step B). Preferably, an overpressure relative to the environment is also maintained in method step C1) to facilitate the subsequent supply of liquid molten metal to the nozzle.

[0063] In an advantageous embodiment, it is envisioned that the overpressure in the crucible in step B) is 20 to 30 mbar, and / or the overpressure in the crucible in step C1) is 3 to 8 mbar. The described overpressure has been found to be particularly advantageous for activating the nozzle channel.

[0064] In another embodiment, it is envisioned that in method step C), the movement of the piston tip couples vibrations into the molten metal, and the inner surface of the nozzle channel is at least partially wetted by the vibrating molten metal. The high-frequency movement of the piston tip in method step C) causes vibrations in the liquid molten metal. Bubbles are dissolved, and these vibrations promote the wetting of the metal-repellent surfaces within the nozzle chamber and nozzle channel.

[0065] Furthermore, it is advantageously envisioned that in method step D), the initial amplitude is generated by the initial charge supplied by the control unit to the piezoelectric element including the actuator, and the effect of the charge supplied to the actuator gradually decreases during the gradual decrease in amplitude. In this embodiment, the amplitude of the piston tip is directly related to the amount of charge transferred from the control unit to the piezoelectric element belonging to the actuator. Therefore, in method step D), the reduction in the amount of charge transferred results in a reduction in amplitude used to determine the limiting amplitude. Alternatively, it is feasible to provide an actuator that does not include a piezoelectric element and is based on another physical operating principle. For example, a pneumatic actuator can be provided, wherein the amplitude is determined by the amount of compressed air supplied by the control unit to the actuator to move the piston tip. In this case, the initial amplitude is determined by a specified amount of compressed air, which is gradually reduced in a further process of method step D).

[0066] In an advantageous embodiment, it is conceivable that, in method step D), the limiting amplitude is generated by a limiting charge supplied to the actuator by the control unit. If the actuator includes a piezoelectric element that moves the tip of the piston, then the limiting amplitude determined in method step D) is generated by the limiting charge. This limiting charge is the charge at which the liquid molten metal just fails to exit the nozzle channel. Then, in the printhead's operating mode, a charge or amount of charge greater than or equal to the limiting charge is used to produce parts.

[0067] In one embodiment, it is conceivable that the work provided by the control unit for moving the piston tip is provided in the form of electric charge, which is at least partially converted in the piezoelectric element of the actuator, and the work receiving element is formed by a capacitor that can be connected in parallel with the piezoelectric element as needed. In method steps C) and D), the work receiving element is not connected in parallel with the piezoelectric element, and in the operating mode, the work receiving element is connected in parallel with the piezoelectric element to receive a portion of the work provided by the control unit in the form of electric charge. In this embodiment of the method, the actuator also includes a piezoelectric element that moves the piston tip in proportion to the charge transferred to the piezoelectric element by the control unit. In this case, the work provided by the control unit is provided as electric charge or a quantity of electric charge. In method steps C) and D), the entire quantity of electric charge provided by the control unit is transferred to the piezoelectric element, causing the piston tip to perform a related motion with a large amplitude. Therefore, all the electric charge output by the control unit is transferred to the actuator, which results in a high acceleration of the piston tip. Furthermore, in this embodiment, the aforementioned work receiving element is formed by a capacitor that can optionally be connected in parallel with the piezoelectric element. To perform this method, and particularly steps C) and D), it is preferable not to use a work-receiving element in the form of a capacitor, such that all charge and / or amperes are transferred to the piezoelectric element. In the operating mode, i.e., after performing the method for activating the nozzle channel, the capacitor is preferably connected in parallel to the piezoelectric element, such that a portion of the charge and / or amperes provided by the control unit is transferred to the capacitor, and only the remainder is transferred to the piezoelectric element. Thus, in this embodiment, the capacitor receives a portion of the work or amperes in the form of charge from the control unit. In this way, the control unit can operate in a manner very similar to performing the activation method, and particularly outputs the charge and / or amperes by a comparable order of magnitude. In this way, the control unit can always operate within its optimal operating range, where the accuracy of control and the reproducibility of the output charge are optimal. In summary, these parallel connections of the piezoelectric element and the capacitor produce a stable and accurate system with a simple design for controlling the printhead during activation and in the operating mode. In the operating mode, it has been found advantageous to move the piston tip in the nozzle chamber with a lower acceleration than in the activation method. Lower acceleration, especially during the return motion of the piston tip after the droplet is expelled from the nozzle channel, prevents ambient air from being drawn into the nozzle channel to compensate for the volume vacated by the piston tip in the nozzle chamber. The acceleration of the piston tip is controlled by the current intensity flowing from the control unit to the actuator, and vice versa. Here, the control unit has a minimum possible ampere for performing the round-trip charge transfer to the piezoelectric element. This minimum possible ampere is related to a minimum limit on the piston tip acceleration.Because a capacitor is connected in parallel as a power receiving element as required, a portion of the transmitted current flows into this capacitor, reducing the effective current flowing to the piezoelectric element to below the minimum possible current intensity of the control unit. In this case, the parallel connection of the capacitor has the additional effect of reducing the acceleration of the piston tip, which can lead to more reproducible printing results in operating mode. Of course, the parallel connection of the capacitor and the piezoelectric element can also be used to reduce the ampere number when charging the piezoelectric element, and thereby reduce the acceleration of the piston tip during droplet ejection. Therefore, the power receiving element formed by the capacitor can be used in conjunction with the settings of the control unit to affect both the amplitude and acceleration of the piston tip. Here, the electrical parameters of charge amount and ampere number can be set and changed independently of each other in the control unit to obtain good results. For example, the capacitor can also be connected in parallel with the piezoelectric element to increase the output charge amount and simultaneously reduce the output current intensity in operating mode. In this case, for example, the piston tip can move with a higher amplitude than in the starting method, however, with a reduced acceleration compared to the starting method in operating mode. The influence of amplitude and acceleration can occur independently of each other.

[0068] Advantageously, it is envisioned that n equals 2 in method steps E) and F). In this embodiment, n = 2 is applied such that method step D) is repeated twice after the first implementation according to method step E). In this way, the limiting amplitude is determined three times before the method according to method step F) is completed. Based on these three determined limiting amplitude values, it can be determined with high certainty whether the nozzle channel has been fully wetted. Of course, n can also take other values, and can be selected, for example, from 0 to 10. Furthermore, n can also dynamically adapt to the determined limiting amplitude values. This dynamic adaptation is understood to mean that if all three determined limiting amplitudes are lower than the previously defined limiting amplitude, then the method is completed according to method step F) when n = 2. If this is not the case, i.e., if one or more of the three determined limiting amplitudes are greater than the previously defined limiting amplitude, then n is increased by 1, and the method continues until a limiting amplitude less than the previously defined limiting amplitude is determined in the three consecutive implementations of method step D). Alternatively, this dynamic adaptation of n can also be performed based on changes in multiple previously determined limiting amplitudes. If there is a large variation in the continuously determined limiting amplitude, this may indicate insufficient wetting of the nozzle channel. In this case, the startup method continues until the variation falls below a previously defined value. For example, the previously defined limiting amplitude or the associated value of the maximum variation of the previously defined limiting amplitude can be determined by previously performed experiments. As an alternative to evaluating wetting by statistically determining the defined limiting amplitude according to the invention, the degree of wetting can also be determined optically. In such optical determination, for example by means of a camera, it is verified whether the area of ​​the inner wall of the nozzle channel adjacent to the discharge opening of the nozzle channel, visible from the outside of the printhead, is actually wetted by molten metal. However, this optical determination is more prone to error and more complex than the described method for starting the printhead.

[0069] The object of the present invention is further achieved by a printhead for processing liquid molten metal, the printhead comprising:

[0070] - A crucible used to hold the metal to be printed.

[0071] - Heating device for melting metal

[0072] - A nozzle, which is connected to the crucible and includes a nozzle chamber and a nozzle channel extending in the discharge direction, the nozzle chamber and the nozzle channel being merged with each other.

[0073] - An actuator, including a piston having a piston tip that can be introduced into the nozzle chamber, the actuator being configured to cause the piston to reciprocate linearly, particularly in the direction parallel to the discharge direction.

[0074] -Control unit for controlling the actuator.

[0075] The control unit is designed to perform the method described according to one of the foregoing embodiments.

[0076] A printhead according to the invention is provided to perform the method according to the invention. Some elements or components of the printhead according to the invention have been described above in conjunction with the method. Referring to the above description, the printhead according to the invention further includes a control unit configured to perform the method according to the invention and correspondingly control other elements or components of the printhead. This control unit may also be referred to as a control module or electronic control unit (ECU). Here, the control unit includes at least one central processing unit (CPU), a memory, and input and output interfaces for exchanging data and information with the components of the printhead and with the operator of the printhead. Furthermore, the control unit may include additional hardware components, such as those for controlling and regulating the amount of charge to be transferred to the actuator. In the memory of the control unit, values ​​and default values ​​are stored in the form of data required to perform the method according to the invention. Furthermore, the control unit can be connected to a data network, enabling long-distance data exchange. The printhead according to the invention is advantageous because, due to its ability to perform the method according to the invention, it makes it possible to produce nozzle channels using more cost-effective metal-repellent components, while simultaneously providing reproducible high-quality results in component production. At the same time, the use of metal-repellent components prevents the nozzle from being undesirably contaminated. The printhead according to the invention has a simple yet robust design and enables a reliable method for activating new, previously unused nozzle channels based on hardware components or components also used in the production of components.

[0077] In one embodiment of the nozzle head, the nozzle is envisioned to include an insert inserted into the crucible, and the nozzle chamber is implemented as a cavity in the insert. The nozzle further includes a nozzle plate that at least partially forms the boundary of the nozzle chamber. The nozzle plate and the insert are releasably connected to each other by clamping elements, and a nozzle channel is configured to extend fully through the cavity of the nozzle plate. In this embodiment, the nozzle is formed of multiple components. The nozzle includes an insert that forms a connection interface with the crucible. The nozzle channel is disposed in the nozzle plate, which is a component other than the insert. The nozzle plate and the insert are connected to each other by clamping elements. Since the nozzle channel is worn during the production of the component and therefore needs to be replaced from time to time, in this embodiment, the nozzle channel is disposed in an easily replaceable nozzle plate. This allows the nozzle channel of the nozzle to be replaced quickly and cost-effectively. Of course, other embodiments of the nozzle are also possible. For example, the insert may not be provided, and portions of the nozzle may be integrally formed with the crucible. In this embodiment, the nozzle channel may also be disposed in a replaceable nozzle plate that is connected to other components of the nozzle or crucible via clamping elements.

[0078] Furthermore, it is envisioned that the nozzle plate is made of a metal-repellent material and / or the surfaces of the nozzle plate and nozzle channel are metal-repellent. As previously mentioned, metal-repellent surfaces of the nozzle are advantageous because they eliminate or prevent undesirable adhesion of molten metal. Therefore, providing a metal-repellent surface for the entire nozzle plate is easy and cost-effective. To avoid wetting difficulties inside the nozzle channel, a method according to the invention is provided.

[0079] Cleverly, it can be envisioned that the nozzle plate is made of graphite or a graphite-based material, and the nozzle channel is implemented as a hole in the nozzle plate. In this embodiment, the nozzle plate is made of a graphite-based material. This material is heat-resistant and can be formed in a simple manner. The nozzle channel is formed by a cylindrical cavity in the nozzle plate. Here, the nozzle channel can have multiple different inner diameters, which are merged with each other via circular or inclined sections.

[0080] Cleverly, the actuator is envisioned to include a piezoelectric element, and the actuator and / or control unit includes a power receiving element formed of a capacitor. The control unit is configured to electrically connect the piezoelectric element and the power receiving element in parallel as needed. In this embodiment, the printhead includes a power receiving element in the form of a capacitor. This capacitor is disposed near the actuator including the piezoelectric element, or near or within the control unit. The power receiving element is configured to receive a portion of the power provided by the control unit in the form of charge in operating mode. To this end, a circuit is further provided that allows the power receiving element to be connected in parallel to the actuator when needed, i.e., typically in the operating mode of the printhead. Furthermore, in a method for activating the printhead and nozzle channels, this circuit enables the interruption of the parallel connection between the power receiving element and the actuator, allowing all the power provided by the control unit to be transferred to the actuator. The control unit is designed to automatically establish the parallel connection between the power receiving element and the actuator when needed.

[0081] Features, effects, and advantages disclosed in connection with the method are also considered to be disclosures related to the printhead. Conversely, features, effects, and advantages disclosed in connection with the printhead are also considered to be disclosures related to the method. Attached Figure Description

[0082] Embodiments of the invention are schematically illustrated in the accompanying drawings. Here,

[0083] Figure 1 A partial cross-sectional perspective view showing an embodiment of the printhead is shown.

[0084] Figure 2 A circuit diagram illustrating an embodiment of the circuitry for the control unit, actuator, and power receiving element.

[0085] Figure 3A flowchart illustrating an embodiment of a method for starting a printhead is shown. Detailed Implementation

[0086] In the accompanying drawings, the same elements are represented by the same reference numerals. Generally, the characteristics of an element described in one figure also apply to other figures. Directional information, such as up or down, refers to the described figure and applies to other figures according to their meaning.

[0087] Figure 1A partial cross-sectional perspective view of an embodiment of printhead 1 is shown. The most relevant components of printhead 1 are schematically shown in the figure. Printhead 1 may include other components not shown or described. Printhead 1 includes a crucible 11 containing metal to be printed by means of printhead 1. Crucible 11 is made of a heat-resistant material, preferably ceramic. A heating device 14 is provided on the outer wall of crucible 11, configured to heat crucible 11 and the metal contained therein. In the illustrated embodiment, heating device 14 is implemented as an induction heater, which generates heat by the flow of current through a resistive conductor. Alternatively, heating device 14 based on another physical operating principle, such as a resistance heater, may also be used. Furthermore, heating device 14 may also be disposed within or inside the wall of crucible 11. The metal to be printed can be supplied to crucible 11 in solid form, such as in pellets or wire. Alternatively, the metal to be printed, already in liquid form as a melt, can also be supplied to crucible 11. A nozzle 13 is disposed in the lowermost section of the crucible 11, from which molten metal is discharged during the operation of the printhead 1 to produce a part. In the illustrated embodiment, the nozzle 13 includes an insert 132 directly connected to the crucible 11. A nozzle chamber 134 is located within the insert 132. Here, the nozzle chamber 134 is designed to be generally cylindrical and extends along the discharge direction AR. The discharge direction AR is the direction in which molten metal is discharged in droplet form during the operation of the printhead 1. The nozzle chamber 134 is circumferentially defined by the insert 132. In an alternative embodiment, the nozzle chamber 134 may also be disposed directly in the crucible 11 and circumferentially defined by the crucible. In this case, the nozzle does not include the insert 132. The downward-facing front side of the nozzle chamber 134 is defined by a replaceable nozzle plate 133. In the nozzle plate 133, a nozzle channel 1331 extending along the discharge direction AR is provided. The nozzle channel 1331 extends through the entire nozzle plate 133. Here, the nozzle channel 1331 has a cross-section that varies along the spray direction AR. On the side facing the insert 132, the cross-sectional area of ​​the nozzle channel 1331 is larger than that on the side facing away from the insert 132. The nozzle channel 1331 narrows along the discharge direction AR, and since the molten metal is accelerated in the narrower lower section of the nozzle channel 1331, droplet formation is promoted during printing. The discharge opening of the nozzle channel 1331 on the lower side is preferably designed with sharp edges, which also promotes droplet separation. Here, the nozzle plate 133 is implemented as a cylindrical disk and made of a graphite-based material. A clamping element 131 detachably connects the nozzle plate 133 and the insert 132 to each other. Here, the clamping element 131 is implemented as a clamping nut, which is screwed onto a thread provided on the outer circumference of the insert 132. Alternatively, the clamping element 131 can also be implemented as a cap, which is clamped onto the unthreaded outer circumference of the insert 132.The connection between the clamping element 131 and the insert 132 can also be achieved by other forced engagement elements, such as a bayonet lock. During printhead operation, the clamping element 131 clamps the nozzle plate 133 onto the lower front side of the insert 132, thereby sealing the nozzle chamber 134. To replace the nozzle plate 133, the clamping element 131 is removed, and the nozzle plate 133 is replaced with a new one. In the illustrated embodiment, all surfaces of the nozzle plate 133 are metal-repellent. The insert 132 and the nozzle plate 133 together define the nozzle chamber 134, which opens into the interior of the crucible 11. The nozzle chamber 134 forms a compression space in which pressure is applied to the liquid molten metal via the piston tip 1211, and in this way, the liquid molten metal is discharged from the nozzle 13 through the nozzle channel 1331.

[0088] The reciprocating motion of piston 121 parallel to the discharge direction AR causes molten metal to be discharged from printhead 1. Piston 121 has a piston tip 1211 at its lower end that can be introduced into nozzle chamber 134. Piston 121 is connected to actuator 12 at its upper end. Due to the high temperature in crucible 11 and nozzle 13, piston 121 is made of ceramic material. The outer cross-section of piston tip 1211 is slightly smaller than the inner cross-section of nozzle chamber 134 in its cylindrical cross-section. Therefore, when piston tip 1211 is introduced into nozzle chamber 134, there is a distance in the radial direction relative to discharge direction AR. This distance causes molten metal to flow from crucible 11 through piston tip 1211 into nozzle chamber 134 in the direction of nozzle channel 1331. Therefore, during the reciprocating motion of piston tip 1211, molten metal is subsequently supplied in the direction toward nozzle channel 1331 through the distance between piston tip 1211 and the wall of nozzle chamber 134. As the piston tip 1211 exits the nozzle chamber 134, molten metal subsequently flows from the crucible 11 across the distance between the nozzle chamber 134 and the nozzle channel 1331, with the piston tip 1211 preferably always partially retained within the nozzle chamber 134. During the movement of the piston tip 1211 from the crucible 11 toward the nozzle channel 1331, a portion of the molten metal present between the downward-facing end of the piston tip 1211 and the nozzle channel 1331 flows upward back across the distance in the direction of the crucible 11. The impulse by which the piston tip 1221 is moved to displace the molten metal from the nozzle channel 1331 can be adjusted by the magnitude of this distance or the cross-sectional area. Here, the distance between the piston tip 1211 and the wall of the nozzle chamber 134 can be constant in the circumferential direction around the piston tip 1211. Alternatively, the magnitude of this distance can also vary in the circumferential direction. For example, other cylindrical piston tips 1211 can be flattened in segments, thereby creating a larger distance between the piston tip 1211 and the nozzle chamber 134 in these regions. Actuator 12 is configured to move piston 121 upward and downward as shown in the figure. Actuator 12 includes piezoelectric element 122 that expands upon charge transfer. Piezoelectric element 122 comprises multiple stacked individual elements. Actuator 12, including piezoelectric element 122, enables piston 121 to move with an amplitude of up to approximately 90 μm. Therefore, piezoelectric element 122 is capable of generating the maximum amplitude required for performing the method and for printing parts through printhead 1. However, this maximum amplitude of piezoelectric element 122 is insufficient if method step B1) is to be performed. Therefore, additional moving means may be provided to enable piston 121 or the entire actuator 12 to reciprocate in the discharge direction AR with an amplitude even greater. In the illustrated embodiment, actuator 12 may be detached from the rest of printhead 1 and manually reciprocated within a guide over a range of a few millimeters. In this case, the moving means is formed by a guide between actuator 12 and the rest of printhead 1.Alternatively, the moving device may also include a linear motor, etc., which can individually cause the actuator 12 including the piston 121 or just the piston 121 alone to move parallel to the discharge direction AR with an amplitude in the range of a few millimeters.

[0089] The printhead 1 further includes a plurality of housing portions 15a, 15b, and 15c connecting the crucible 11 and the actuator 12 to each other. Housing portion 15a supports the crucible 11 and simultaneously provides thermal insulation for the actuator 12, which is heated during operation. Housing portion 15c houses and secures the actuator 12. Furthermore, housing portion 15c can serve as a guide for the actuator 12 when the actuator 12 is manually moved relative to the rest of the printhead 1 with a large amplitude in method step B1. Simultaneously, housing portion 15c serves as a flange through which the printhead 1 can be attached to other machine parts, and this flange is used to house sensors. Housing portion 15b is used to cool housing portion 15c and the actuator 12. Housing portion 15b may include, for example, channels through which coolant, such as water, is guided. In this way, heat leaking from the crucible 11 through the insulating housing portion 15a can be dissipated from the printhead 1. During the operation of the printhead 1, high temperatures exist in the crucible 11 to keep the metal in a molten state. Such high temperatures can damage the actuator 12. Therefore, the crucible 11 and the actuator 12 are arranged in the printhead 1 such that they are thermally insulated from each other, and preferably, a cooling device is arranged between the two components.

[0090] A control unit 123 is schematically shown above the actuator 12. This control unit 123 is connected to the actuator 12 and transfers charge to the actuator 12 to move the piston 121. The control unit 123 is also configured to perform the method according to the invention in a manner that is at least partially automated. For this purpose, the control unit 123 may include other interfaces, such as interfaces for connecting to sensors or other actuators, heating devices 14, etc.

[0091] Below the printhead 1, an ejection detection device 2 is schematically shown. When the method for starting the printhead 1 is executed, this ejection detection device 2 optically determines whether liquid molten metal in droplet form is ejected from the nozzle channel 1331. The determined result from the ejection detection device 2 is transmitted to the control unit 123 and integrated into the control of the execution of the method for starting the printhead 1.

[0092] Figure 2 A circuit diagram of an embodiment of the circuitry for the control unit 123, actuator 12, and power receiving element 3. Figure 2The diagram shows a circuit that influences the work transferred to the actuator 12 in the form of electrical charge via a control unit 123. On the left, the control unit 123 is schematically shown. The control unit 123 is connected to the piezoelectric element 122 of the actuator 12 via two conductors. Here, a work receiving element 3 in the form of a capacitor is connected in parallel with the piezoelectric element 122. Figure 2In the illustrated state, the power receiving element 3 and the piezoelectric element 122 are connected in parallel, and both are connected to the control unit 123. In this state, a portion of the power output in the form of charge through the control unit 123 flows into the piezoelectric element 122, and another portion of the output power flows into the power receiving element 3. Therefore, a portion of the power flows back to the control unit 123 from both the power receiving element 3 and the piezoelectric element 122. The parallel connection between the power receiving element 3 and the piezoelectric element 122 can be interrupted by a relay R. When the parallel connection of the power receiving element 3 is interrupted or suspended, all the power output through the control unit 123 flows exclusively to the piezoelectric element 122. The power transferred from the control unit 123 to the piezoelectric element 122 is directly related to the amplitude generated by the piezoelectric element 122. Therefore, the parallel connection between the power receiving element 2 and the piezoelectric element 122 results in a reduction in the amplitude and / or acceleration of the piston tip generated by the piezoelectric element 122 when the same power is output through the control unit 123. When the parallel connection between the power receiving element 3 and the piezoelectric element 122 is interrupted by the relay R, this results in an increase in amplitude and / or acceleration while the same amount of power is output through the control unit 123. Such a circuit is particularly advantageous for enabling the control unit 123 to operate within its optimal operating range at all times, while simultaneously generating amplitudes and / or accelerations of varying magnitudes at the piezoelectric element 122. To implement the method for starting the printhead 1, a larger amplitude and acceleration are typically required than in the printhead's operating mode. Therefore, during startup, particularly in step C), the parallel connection of the power receiving element 3 is interrupted. Then, for the operating mode, the parallel connection between the power receiving element 3 and the piezoelectric element 122 is established via the relay R. In the operating mode, the control unit 123 outputs the same or similar amount of power as during startup; however, this power is distributed between the power receiving element 3 and the piezoelectric element 122, causing the piezoelectric element 122 to generate a smaller amplitude than during the startup method. To perform method step D), a parallel connection can also be established, particularly to determine the limiting amplitude at the end of this method step. In the operating mode, the piston tip typically moves with an amplitude greater than the limiting amplitude, for example, to expel droplets with a larger diameter during printing. This increase in amplitude in the operating mode can be achieved by controlling the control unit 123 while maintaining the parallel connection of the power receiving element 3. The change in the amount of charge output by the control unit 123 over time corresponds to the output or received amperes. The parallel connection of the piezoelectric element 122 and the power receiving element 3 also affects the amperes transferred between the control unit 123 and the piezoelectric element 122. Here, the parallel connection reduces the transferred amperes, resulting in a decrease in the effective acceleration of the piston tip 1211. In the startup method, a higher acceleration of the piston tip 1211 is required than in the operating mode. The described parallel connection in the operating mode also meets this requirement and thus improves the quality of the print result.For reproducible print results, it is important that the control unit 123 outputs a reproducible and accurate charge and / or ampere value. The precise output of a specific charge and / or ampere value is optimal within the operating range of the control unit 123 and deteriorates as deviations from this optimal operating range increase. Therefore, the circuit shown simultaneously contributes to reproducible print results and a reliable method for starting the print head 1.

[0093] Figure 3 A flowchart illustrating an embodiment of a method for starting printhead 1 is shown. The diagram explains the process sequence of the method for starting printhead 1 and illustrates various alternatives in its flow. Features associated with the various method steps A) to F) have also been described previously, and therefore, reference is made to the above description here as well.

[0094] In the first method step A), the printhead is prepared for starting the method. For this purpose, metal is melted in crucible 11, and piston tip 1211 is introduced into nozzle chamber 134. Even in this state, molten metal can enter nozzle chamber 134 and nozzle channel 1331.

[0095] In the next method step B), an overpressure is generated within crucible 11. This overpressure supports the delivery or transfer of liquid molten metal from crucible 11 into nozzle channel 1331.

[0096] In the following method step B1), the piston tip 1211 now reciprocates within the nozzle chamber 134 along or parallel to the discharge direction AR, with a significantly larger amplitude than the movement of the piston tip 1211 in other method steps. This movement of the piston tip 1211 delivers liquid molten metal into the nozzle chamber 134 and the nozzle channel 1331. The frequency of the piston tip 1211's movement in method step B1) is significantly lower than the frequency in other method steps or in the operating mode. Method step B1) is optional and not mandatory. However, it has been found that the movement of the piston tip 1211 with an amplitude in the range of several millimeters according to method step B1) significantly improves or accelerates the method for starting the printhead 1.

[0097] In the next step C), the piston tip 1211 now reciprocates at a high filling frequency FF higher than 1 kHz. This high-frequency motion dissolves air bubbles in the molten metal, causing a large area of ​​the inner surface of the nozzle channel 1331 to be wetted in this step. However, step C) is performed until the molten metal is discharged (sprayed) from the nozzle channel 1331. Preferably, the discharge detection device 2 is used to detect such discharge of the molten metal.

[0098] exist Figure 3In method step C), the first query Q1 can be seen below. In method step C), the piston tip 1211 moves at a filling frequency FF until molten metal is discharged. However, there are cases where molten metal is not discharged, for example, when wetting is insufficient in this method step. In this case, method step C) is performed for a specified dwell time. After this dwell time expires, method step C) is completed, and then the query Q1 shown occurs. In this query Q1, it is determined whether molten metal was discharged in method step C). If molten metal was discharged, then the method continues to method step D). If no molten metal was discharged, then method step B1) is applied or repeated, in which the piston tip moves slowly but with a large amplitude within the nozzle chamber 134 to deliver additional molten metal in the direction of the nozzle channel 1331.

[0099] Once the molten metal has been discharged in step C), step C1) may optionally be performed, wherein the overpressure in crucible 11 is reduced compared to step B). This pressure reduction in crucible 11 preferably establishes pressure conditions that are also prevalent in the operating mode of printhead 1. Alternatively, the following method steps may be performed at a higher overpressure according to step B).

[0100] In the following method step D), the limiting amplitude in this method is now determined for the first time. This limiting amplitude is the amplitude value of the piston tip 1211 at which molten metal just fails to exit from the nozzle passage 1331. To determine this limiting amplitude, the piston tip 1211 is moved at a starting frequency IF, which is significantly lower than the filling frequency FF in method step C) and lower than the standard frequency SF of the piston tip 1211 movement in the operating mode used for producing parts. In method step D), the piston tip 1211 initially moves with an initial amplitude. Here, it is determined whether molten metal is exited. If so, the initial amplitude is gradually reduced, and it is determined whether molten metal is still exited at the reduced amplitude. By this continuous method of reducing the amplitude and determining whether exiting occurs, the limiting amplitude is finally determined.

[0101] In the next query Q2, it is determined whether molten metal was discharged during the first pass of method step D). If molten metal was discharged and thus the limiting amplitude was determined, the method continues to query Q3. However, if it is determined in query Q2 that no molten metal was discharged during method step D), then method step C2) is performed. In method step C2), similar to method step C), the piston tip 1211 also reciprocates at a high filling frequency FF. Method step C2) is set to dissolve any air bubbles that may still be present in the molten metal. Such remaining air bubbles may be the reason why no molten metal was discharged in method step D), even at the initial amplitude. Here, method step C2) can be performed under an overpressure in crucible 11, which is reduced compared to method step B). Alternatively, method step C2) can also be performed under the overpressure according to method step B). Alternatively, it can also be determined whether the limiting amplitude previously determined in method step D) is greater than the limiting amplitude defined in query Q2. The defined limiting amplitude can be a value determined empirically beforehand, representing the limiting amplitude at which droplet ejection occurs under conditions of good wetting in nozzle channel 1331. If the determined limiting amplitude is greater than the defined limiting amplitude, this allows for the conclusion of insufficient wetting. Therefore, if query Q2 determines that the limiting amplitude is greater than the previously defined limiting amplitude, step C1) of the previously described method can be performed to improve the wetting of the nozzle channel.

[0102] If the discharge of molten metal was previously detected and / or a limiting amplitude less than the defined limiting amplitude was determined in a repetition of method step D), then the method continues to query Q3. In query Q3, it is determined whether the previously determined n repetitions of method step D) have been performed. If this is not the case, i.e., if the number of repetitions of method step D) is less than n, then method step E) is performed. Method step E) is simply a repetition of method step D) which again determines the limiting amplitude. Therefore, in Figure 3 In the flowchart, method step E) is an arrow that leads from query Q3 back to method step D), thus symbolizing the repetition of this method step. After repeating method step D) according to method step E), queries Q2 and Q3 are executed again. With each loop of method step E), the repetition count increases by one. When the actual repetition count determined in query Q3 finally equals the predetermined repetition count n, method step F) is executed after query Q3.

[0103] In method step F), the method for starting printhead 1 is completed. Since method step F) is essentially a query, it is shown as being represented by the same diamond symbol as the previous queries Q1 to Q3. However, the actual completion of the method, as represented by result G), only occurs if the limiting amplitude previously determined in method step D) is equal to or less than the limiting amplitude defined in n consecutive repetitions. In this case, it can be assumed that nozzle channel 1331 is reproducibly wetted, and therefore reproducible and high-quality part production in subsequent operating modes is possible. If the limiting amplitude determined in method step D) is wholly or partially greater than the limiting amplitude defined in the last n consecutive repetitions, then it can be assumed that the wetting of nozzle channel 1331 is still insufficient. In this case, method step E) or D) and another repetition of the following method steps are performed first. Then, in method step F), all limiting amplitudes from the last n consecutive repetitions are evaluated again to see if they are below the defined limiting amplitude. If the situation is that the actual number of repetitions used to determine the limiting amplitude in method step D) increases, then the method is finally completed in method step G). If a limiting amplitude greater than the defined limiting amplitude occurs within the last n consecutive repetitions, then the startup method continues. Alternatively, another condition related to the finally determined limiting amplitude can be checked in method step F). Alternatively or additionally, such another condition could be, for example, an assessment of whether the change exhibited by the finally determined limiting amplitude is less than or equal to the defined allowable change. Typically, various analyses and assessments for the previously determined limiting amplitude can be performed in method step F). Therefore, the method for starting printhead 1 completed in method step G) may be affected by a variety of conditions. In this way, the method can be easily and flexibly adapted to different requirements or applicants. In most cases, the wetting of nozzle channel 1331 can be continuously improved by repeating method steps D) and / or C2) until the startup method is finally determined to be successfully completed in method step F). In rare cases, it may also occur that the method is still impossible to complete according to method steps F) and G), despite the increase in the number of repetitions. In this case, manual or optical inspection of nozzle channel 1331 or nozzle chamber 134 may help to resolve the problem.

[0104] List of reference numerals

[0105] 1. Print head

[0106] 11. Crucible

[0107] 12 Actuators

[0108] 121 Piston

[0109] 122 Piezoelectric element

[0110] 123 Control Unit

[0111] 1211 Piston tip

[0112] 13 Nozzles

[0113] 131 Clamping element

[0114] 132 Insert

[0115] 133 Nozzle Plate

[0116] 1331 Nozzle Channel

[0117] 134 Nozzle Chamber

[0118] 14 Heating device

[0119] 15a, 15b, 15c Casing sections

[0120] 2. Discharge detection device

[0121] 3-Power Receiving Element

[0122] R relay

[0123] AR discharge direction

[0124] SF standard frequency

[0125] FF refill frequency

[0126] A) Methods and Steps

[0127] B) Methods and Steps

[0128] B1) Method and Steps

[0129] C) Methods and Steps

[0130] C1) Method Steps

[0131] C2) Method and Steps

[0132] D) Methods and Steps

[0133] E) Methods and Steps

[0134] F) Methods and Steps

[0135] G) Method and Steps

[0136] Q1 First Inquiry

[0137] Q2 Second Inquiry

[0138] Q3 Third Inquiry

Claims

1. A method for activating a nozzle channel (1331) of a printhead (1) to process liquid molten metal, wherein the printhead (1) comprises the following components: - Crucible (11), used to hold the metal to be printed, - Heating device (14) for melting the metal, - A nozzle (13), which is connected to the crucible (11) and includes a nozzle chamber (134) and a nozzle channel (1331) extending in the discharge direction, wherein the nozzle chamber (134) and the nozzle channel (1331) are merged with each other. - An actuator (12) including a piston (121) having a piston tip (1211) capable of being introduced into the nozzle chamber (134), wherein the actuator (12) is configured to linearly reciprocate the piston (121). The method includes the following steps: A) Prepare the printhead (1) wherein metal is melted in the crucible (11) to form molten metal, and introduce the piston tip (1211) into the nozzle chamber (134), wherein, in the introduced state, there is at least a partial distance between the piston tip (1211) and the nozzle chamber (134) in the radial direction relative to the discharge direction. B) Overpressure is generated in the crucible (11) to induce molten metal to enter the nozzle chamber (134). C) Move the piston tip (1211) in the nozzle chamber (134), wherein the piston tip (1211) reciprocates via the actuator (12) at a filling frequency greater than or equal to 1 kHz until molten metal is discharged from the nozzle channel (1331) and leaves the printhead (1). D) Moving the piston tip (1211) in the nozzle chamber (134), wherein the piston tip (1211) reciprocates via the actuator (12), wherein the movement is initially performed at an initial amplitude at which molten metal is discharged from the nozzle channel (1331) and leaves the printhead (1), and subsequently the amplitude of the movement gradually decreases, wherein the movement is performed at a gradually decreasing amplitude until molten metal no longer discharges from the nozzle channel (1331) and leaves the printhead (1), wherein the amplitude at which molten metal just ceases to discharge from the nozzle channel (1331) and leaves the printhead (1) is defined as the limiting amplitude. E) Repeat method step D) n times consecutively, wherein in each of the n repetitions of method step D), the limiting amplitude is determined. F) The method ends once all the limiting amplitudes determined in the n+1 repetitions of method step D) are equal to or less than the defined limiting amplitude.

2. The method according to claim 1, characterized in that, Between method steps B) and C), in method step B1), the piston tip (1211) reciprocates within the nozzle chamber (134), wherein the amplitude of this motion is at least 10 times greater than the initial amplitude.

3. The method according to claim 2, characterized in that, If no molten metal is discharged after the predetermined residence time in step C), repeat step B1 of the method according to claim 2.

4. The method according to claim 1, characterized in that, Between method steps C) and D), method step C1) is performed, in which the overpressure in the crucible (11) is reduced.

5. The method according to claim 4, characterized in that, In method step E), the n repetitions of method step D) are performed under overpressure reduction in the crucible (11) after method step C1), and / or the piston tip (1211) is accelerated at a higher speed during the reciprocating motion in method step C) than in method step D) and / or in the operating mode in which molten metal in droplet form is discharged from the printhead (1) to form a part.

6. The method according to claim 1, characterized in that, In method step D), the movement of the piston tip (1211) is performed at a starting frequency, which is at most 10% of the filling frequency in method step C).

7. The method according to claim 1, characterized in that, In step D), during the gradual reduction of the movement amplitude of the piston tip (1211), starting from the initial amplitude, and then the amplitude gradually decreases by a differential amplitude, wherein after each subtraction of the differential amplitude from the current amplitude, the movement of the piston tip (1211) continues for a time period or a fixed number of movements, and then the amplitude decreases again by the differential amplitude, wherein during the time period or the fixed number of movements at a constant amplitude, it is determined whether molten metal is discharged from the nozzle channel (1331) and leaves the printhead (1).

8. The method according to claim 1, characterized in that, After completing method step F), the printhead (1) is switched to an operating mode in which the printhead (1) discharges molten metal in droplet form to form a component, wherein such discharge of molten metal in droplet form occurs at a standard frequency and an amplitude equal to or greater than the limiting amplitude, and / or The method step D) is performed in a manner that is at least partially automated, wherein the control unit (123) controls each method step, and the completion of the method is determined by the control unit (123), and associated notifications are output to the operator of the control unit (123).

9. The method according to claim 1, characterized in that, In steps C) and D) of the method, and in the operating mode in which the printhead (1) discharges molten metal in the form of droplets to form a component, the amplitude of motion of the piston tip (1211) is generated by the power provided by the control unit (123) and acting in the actuator (12), wherein a power receiving element (3) designed to receive a portion of the power is provided, and in the operating mode the power receiving element (3) receives a portion of the power provided by the control unit (123), wherein in the operating mode the actuator (12) converts only the remaining portion of the power.

10. The method according to claim 1, characterized in that, Using a discharge detection device (2), the discharge detection device determines whether molten metal is discharged from the nozzle channel (1331) and leaves the printhead (1).

11. The method according to claim 8, characterized in that, The actuator (12) includes at least one piezoelectric element (122) coupled to the piston (121), wherein the main expansion direction of the piezoelectric element (122) is oriented parallel to the longitudinal axis of the piston (121), the control unit (123) controls the expansion of the piezoelectric element (122) and thus controls the reciprocating motion of the piston tip (1211) coupled to the piezoelectric element (122), wherein the rearward motion of the piston (121) is at least partially carried out by spring force, wherein the amount of work in the form of charge transferred by the control unit (123) to the piezoelectric element (122) is related to the amplitude of the piston tip (1211), and / or in method step D), the initial amplitude is generated by the initial charge supplied to the actuator (12) including the piezoelectric element (122) through the control unit (123), and the charge supplied to the actuator (12) gradually decreases during the gradual decrease of the amplitude.

12. The method according to claim 11, characterized in that, Between method steps B) and C), in method step B1), the piston tip (1211) reciprocates within the nozzle chamber (134), wherein the amplitude of this motion is at least 10 times greater than the initial amplitude, wherein the actuator (12) includes a moving device that, in method step B1), causes the piezoelectric element (122), including the piston (121) coupled thereto, to move relative to the nozzle (13) in a direction parallel to the longitudinal direction of the piston (121), wherein the moving device is designed to produce an amplitude at least ten times greater than the maximum amplitude of the expansion of the piezoelectric element (122).

13. The method according to claim 11, characterized in that, In method steps C) and D), and in the operating mode where the printhead (1) discharges molten metal in droplet form to form a component, the amplitude of motion of the piston tip (1211) is generated by the power provided by the control unit (123) and acting in the actuator (12), wherein a power receiving element (3) designed to receive a portion of the power is provided, and in the operating mode, the power receiving element (3) receives a portion of the power provided by the control unit (123), wherein in the operating mode, the actuator (12) converts only the remaining portion of the power, wherein the power provided by the control unit (123) for moving the piston tip (1211) is provided in the form of charge, the power being converted at least partially in the piezoelectric element (122) of the actuator (12), and the power receiving element (3) is formed of a capacitor that can be connected in parallel with the piezoelectric element (122) as needed, wherein the power receiving element (3) in method steps C) and D) The power receiving element (3) is not connected in parallel with the piezoelectric element (122), and the power receiving element (3) is connected in parallel with the piezoelectric element (122) in the operating mode to receive a portion of the power in the form of charge provided by the control unit (123).

14. The method according to claim 1, characterized in that, The actuator (12) is configured to cause the piston (121) to reciprocate linearly in a direction parallel to the discharge direction.

15. A printhead (1) for processing liquid molten metal, comprising: - Crucible (11), used to hold the metal to be printed, - Heating device (14), used for melting metal, - A nozzle (13), which is connected to the crucible (11) and includes a nozzle chamber (134) and a nozzle channel (1331) extending in the discharge direction, wherein the nozzle chamber (134) and the nozzle channel (1331) are merged with each other. - An actuator (12) including a piston (121) having a piston tip (1211) capable of being introduced into the nozzle chamber (134), wherein the actuator (12) is configured to linearly reciprocate the piston (121). - Control unit (123) that controls the actuator (12). The control unit (123) is configured to perform the method according to any one of the preceding claims.

16. The printhead according to claim 15, characterized in that, The actuator (12) is configured to cause the piston (121) to reciprocate linearly in a direction parallel to the discharge direction.

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