Printhead unit and method for controlling the pressure of a fluid in a recirculating printhead of a 3D printer

Abstract

The invention, which relates to a printhead unit (15) and a method for controlling the pressure of a fluid (5) in a recirculating printhead of a 3D printer, is based on the objective of providing a solution that ensures a reliable supply of fluid (5) to the recirculating printheads (1) of the 3D printer via the fluid system and reliably prevents uncontrolled leakage of the fluid (5) from the recirculating printheads (1) of the 3D printer. Furthermore, the solution should enable the pressure pU of the fluid in the recirculating printheads to be adjusted to changing atmospheric pressure pA in the environment of the 3D printer.This task is solved by means of an arrangement in which a switchable pressure regulating unit (31) is arranged parallel to the fluid discharge channel (7) to control a pressure pUde of the fluid (5) at interfaces (12) in nozzles (3) of the recirculating pressure head (1), the switchable pressure regulating unit (31) having a throttle valve (36), a pump (37) and a shut-off valve (43), wherein the pump (37) is configured not to pump the fluid (5) or to pump the fluid (5) in the second flow direction (29) or to pump the fluid (5) in a third flow direction (33).

Description

The invention relates to a printhead unit for controlling the pressure pU of a fluid in a recirculating printhead of a 3D printer, wherein the printhead unit has at least one printhead tank and a recirculating printhead, wherein a fluid supply channel for supplying the recirculating printhead with fluid from the printhead tank and a fluid discharge channel for returning the fluid from the recirculating printhead to the printhead tank are arranged between the printhead tank and the recirculating printhead. The invention also relates to a method for controlling a pressure pU of a fluid in a recirculating printhead of a 3D printer, wherein a fluid drainage channel is provided between the recirculating printhead and a printhead tank, and wherein the fluid is supplied at interfaces in nozzles of the recirculating printhead at a pressure pU which corresponds to an atmospheric pressure pA in a region of the nozzles. It is well known that 3D printing, or a 3D printing process, is used to manufacture individual or serial components, workpieces, or molds. In such printing processes, three-dimensional components or workpieces are built up layer by layer. The assembly process is computer-controlled and uses one or more liquid or solid materials according to predefined dimensions and shapes. Specifications for the components or workpieces to be printed can be provided, for example, by computer-aided design (CAD) systems. The printing of 3D structures or 3D components involves physical or chemical hardening processes or a melting process within a particulate building material. Materials used for such 3D printing processes include plastics, synthetic resins, ceramics, unconsolidated sediments such as minerals or sands, and metals. Various manufacturing process sequences are known for the implementation of 3D printing processes. Several of these process sequences, however, include the following process steps, illustrated by way of example: • Partial or full-surface application of particulate building material, also referred to as particle material or powdered build-up material, onto a so-called build area to form a layer of unsolidified particulate material, wherein the partial or full-surface application of particulate building material includes the discharge and smoothing of the particulate building material; • Selective solidification of the applied layer of unsolidified particulate building material in predetermined areas, for example by selective compaction, printing, or application of treatment agents, such as a fluid or binder, using a printhead, a laser, or electron beams;• Repeating the preceding process steps in another layer level to build up the component or workpiece layer by layer. For this purpose, the component or workpiece, which is built up or printed layer by layer on the build area, is lowered by one layer level or layer thickness along with the build area, or the 3D printing device is raised by one layer level or layer thickness relative to the build area before a new layer is applied partially or completely; • Subsequent removal of loose, unsolidified particulate build material surrounding the manufactured component or workpiece. Particulate building materials are generally understood to be an accumulation of individual particles of a substance or mixture, each particle having a three-dimensional shape. Since these particles are predominantly round, oval, or elongated, it is possible to specify an average diameter for such a particle, which is usually in the range of 0.01 mm to 0.4 mm. Such particulate building materials can exhibit fluid properties. It is known from the prior art to selectively print or apply a fluid or binder to the applied particulate building material using a printhead during the process step of selective compaction. Such a printhead typically has a number of nozzles, which are arranged, for example, in a row or preferably in a matrix configuration on the printhead. Each of these nozzles is arranged in a so-called metering unit, which also includes a means for generating a mechanical force. By means of this generated mechanical force, a droplet of the binder is ejected from the metering unit through the nozzle of the metering unit towards the particulate building material. By means of these droplets of binder printed or applied to the particulate building material, the particulate building material is selectively solidified in these areas. To supply the binder to the metering units arranged in a series or matrix within a recirculating printhead, the recirculating printhead is connected to a printhead tank via a fluid supply channel and a fluid discharge channel within a so-called printhead unit, thus ensuring the circulation of the fluid within the recirculating printhead. It is also known that several recirculating printheads within a printhead unit are connected to the printhead tank in a parallel circuit via the fluid supply channel and the fluid discharge channel. It is also known that the printhead tank is connected via a fluid line to a reservoir tank, which has a larger capacity than the printhead tank. This reservoir tank stores the fluid for the 3D printer, which is then supplied to the printhead tank. In 3D printer systems with multiple printhead units, it is common practice to install a distribution element between the reservoir and the printhead reservoirs. Such a distribution element has at least one inlet that connects it to the reservoir, through which the fluid is drawn from the reservoir to the distribution element. The distribution element typically has several outlets, each with a fluid line connecting it to different printhead reservoirs in different printhead units. From EP 3 177 453 B1, a printhead, its use, and a 3D printing process are known. To improve upon the known prior art, a printhead is provided, particularly for the selective application of printing fluid to a particle material during the production of three-dimensional models using layer-by-layer technology. This printhead comprises a printhead tank for receiving the printing fluid, wherein the printhead tank is connected to printhead modules through which the printing fluid is conveyed to pump chambers and to the nozzles. Furthermore, at least one metallic support is provided to which the printhead modules are attached. Additionally, plastic fluid guides are provided for guiding the printing fluid through the metallic support, and these guides are positioned by the metallic support. Thus, a prior art system is disclosed which has at least one storage container, one distribution element and several printhead units, wherein each printhead unit contains a printhead tank and a printhead module with several nozzles. DE 20 2024 100 984 U1 relates to a pulsation dampener, an inkjet printhead for the selective drop-by-drop metering of a liquid in a 3D printer, comprising the pulsation dampener, and a 3D printer comprising the inkjet printhead according to the invention. The problem to be solved is to provide an inkjet printhead and a 3D printer that overcomes the disadvantages of the prior art. To solve this problem, an inkjet printhead is specified which comprises an inkjet print module. In particular, this inkjet print module comprises a liquid inlet for supplying the liquid (F) to the inkjet print module, at least one nozzle for dispensing a metered drop of the liquid (F) in the 3D printer, and a liquid return for returning a return medium (R) from the inkjet print module.The return medium (R) comprises the liquid (F) and / or a contaminant medium (S), in particular air (L) and / or particles (P) drawn from the 3D printer into the nozzle. Furthermore, a liquid tank is provided for storing the liquid (F) and for supplying the liquid inlet of the inkjet printing module with the liquid (F). The liquid tank includes a liquid supply for replenishment of liquid (F) to the liquid tank and a vacuum port for applying a vacuum (U). The resulting vacuum (U) creates an adjustable meniscus equilibrium pressure (MGD) in the nozzle, which prevents uncontrolled leakage of the liquid (F) from the nozzle and / or uncontrolled intake of the contaminant medium (S) from the 3D printer into the nozzle.The system also includes a return fluid supply for returning fluid (F) from the inkjet printing module via the fluid return to the fluid tank and a return pump for pumping the return medium (R) from the inkjet printing module via the fluid return to the return fluid supply, with a pulsation damper arranged between the return pump and the fluid return. German patent DE 10 2021 104 946 A1 discloses a device for printing with ink, comprising an ink reservoir and an arrangement of several identical printing elements, such as printheads, that generate ink droplets and are supplied with ink from a common first line. The device provides for a separate element, such as a pump, whose ink flow rate is controllable or adjustable, to be supplied with ink from the first line in parallel with the printing elements. This is intended to make it possible to largely or even completely reduce undesirable changes in the hydrostatic pressure of the ink. From DE 10 2022 134 640 A1, a printhead unit and a method for the layer-by-layer construction of molded parts are known. A system for applying a printing fluid is disclosed, comprising a printhead unit, an arrangement of printing modules, and a pressure fluctuation compensation module. The printhead and the pressure fluctuation compensation module are coupled together. It is further disclosed that this coupled system is used in a method for manufacturing models. Since the printheads or rotary printheads used in 3D printers are so-called open systems, which are also referred to as thermodynamically open systems, a problem according to the state of the art is to precisely adjust the pressure of the fluid in the fluid system. Such open systems are characterized by their ability to exchange fluids with their environment. Using the example of a recirculating printhead in a fluid system for supplying fluid to a recirculating printhead in a 3D printer, this means that the fluid in the fluid system, particularly the fluid in the area of ​​the nozzles of the recirculating printhead's metering units, can escape uncontrollably into the environment through the nozzles, or that ambient air can enter the fluid system through the nozzles. A transition zone forms in the nozzle area, which is an interface between the fluid in the fluid system, particularly in the nozzles of the recirculating printhead (such as a binder), and the ambient air. In this process, the fluid on one side of the interface has a fluid pressure that is set within the fluid system, particularly in the area of ​​the nozzles of the recirculating printhead. On the other side of the interface is the ambient air, which is subject to atmospheric pressure. It is known that atmospheric pressure, also referred to as mean atmospheric pressure, is approximately 101,325 Pa at sea level, or about 1.013 bar or 1013 mbar. If the fluid pressure at the interface of the fluid system exceeds the atmospheric pressure surrounding the printhead, the fluid will unintentionally escape from the nozzles. This leads to defects in the 3D structure buildup, as the uncontrolled fluid leakage causes selective and faulty solidification of the particulate building material. Furthermore, fluid is lost from the fluid system. If the fluid pressure at the interface of the fluid system is lower than the atmospheric pressure surrounding the printhead, ambient air or other materials may unintentionally enter the fluid system through the nozzles. This leads to contamination of the fluid system and defects in the 3D structure buildup due to the contaminated fluid, which prevents proper selective solidification of the particulate building material. Therefore, when operating the 3D printer, pressure equality in the transition area on both sides of the interface is sought. A 3D printer operating at sea level is exposed to an atmospheric pressure of approximately 1013 mbar, which acts on the 3D printer from all sides, including, for example, the nozzles in the 3D printer's rotary print head. This means that for a 3D printer operating at sea level, the fluid pressure in the fluid system at the nozzles of the recirculating print head must be 1013 mbar to prevent uncontrolled fluid leakage. Alternatively, the fluid pressure in the fluid system can be lower than 1013 mbar, also referred to as negative pressure, to reliably prevent uncontrolled fluid leakage. Such a tolerance or limit value, which ensures that no ambient air, contaminants contained in that ambient air, or other materials unintentionally enter the fluid system through the nozzles, lies, for example, in a range of less than 3 mbar, particularly less than 1 mbar. This value depends, for example, on capillary forces occurring (i.e., a combination of adhesion and cohesion), the static pressure of the fluid, the dynamic pressure of the fluid in both the lines and the recirculating printhead, the movement of the entire printhead unit, and the surface tension of the fluid. To supply the distributor element from the reservoir tank, as well as to supply the printhead tanks in the printhead units from the distributor element, it is almost always necessary to pressurize the fluid, at least in these internal areas of the fluid system. This overpressure can range from 0.5 bar to 3.0 bar, relative to the atmospheric pressure surrounding the 3D printer. For example, assuming an atmospheric pressure of 1013 mbar surrounding the circulating printhead, the pressure in the area between the reservoir tank and the printhead tank would have an absolute value between 1513.0 mbar and 4013.0 mbar. This overpressure is necessary, for example, to supply the distributor element from the storage tank, since the storage tank does not necessarily have to be located at a height sufficiently above the distributor element. The height difference between the storage tank and the distributor element would have to be large enough to allow the fluid to be pumped from the upper storage tank to the lower distributor element with a sufficient volume flow rate solely by the force of gravity acting on the fluid. In fluid mechanics, volume flow rate is a physical quantity that indicates how much volume of a medium is transported through a defined cross-section per unit of time. This overpressure is also necessary, for example, to supply the printhead tanks of the printhead units from the distributor element, since the distributor element cannot always be positioned at a sufficient height above the printhead tanks in the 3D printer to convey the fluid in sufficient quantity or with a sufficient volume flow rate using only the force of gravity acting on the fluid. Since it is necessary to convey predetermined quantities of fluid through the small-diameter connecting pipes within a given time frame, appropriate means of generating overpressure in the fluid system are required. This is because if the height difference is too small to ensure sufficient gravity-driven fluid transport from top to bottom, an adequate volume flow rate cannot be achieved. Additionally, filters, sensors, or friction losses in the lines lead to a pressure drop in the fluid system, which can only be compensated for by a corresponding overpressure. Pressure fluctuations requiring compensation also arise from movements of the entire print head unit in the 3D printer above the build area during the printing process. Thus, within the fluid system, in some areas it is necessary to pressurize the fluid, while at the interface between the fluid and the ambient air with its atmospheric pressure, the pressure of the fluid at the nozzles of the recirculating pressure heads of the fluid system must be adjusted to the current atmospheric pressure or a value, for example, 0.5 mbar below the atmospheric pressure. Another problem stemming from the current state of the art is that 3D printers are not only operated at sea level with an atmospheric pressure of 1013 mbar. If the 3D printer is operated in an area significantly above sea level, it is surrounded by a different atmospheric pressure, as atmospheric pressure is known to decrease with increasing altitude above sea level. While in the example the atmospheric pressure at sea level is expected to be 1013 mbar, the atmospheric pressure at an altitude of about 2000 m is about 800 mbar and at about 4000 m it is only about 600 mbar. In the case described above, where the pressure of the fluid in the fluid system at the interface between the fluid and the ambient air or atmosphere at the nozzles is set to a value of 0.5 mbar below the atmospheric pressure of 1013 mbar at sea level, i.e., to a value of approximately 1012.5 mbar, when operating this 3D printer with this setting at an altitude of 2000 m at approximately 800 mbar atmospheric pressure, there would no longer be pressure equality or negative pressure at the interface in the area of ​​the nozzles of the rotary print head, and the fluid would leak out or escape uncontrollably. This means that it is necessary to adjust the pressure of the fluid at the interfaces of the nozzles of the rotary print heads of a 3D printer to the installation location of the 3D printer in order to ensure maximum accuracy in the creation of the 3D structures in the 3D printer. Another problem is that the weather at the 3D printer's location can change, affecting the prevailing atmospheric pressure. If adequate measures are not taken to prevent weather-related pressure changes from reaching the area around the 3D printer's print head, these pressure fluctuations will also impact the quality of the 3D structures being produced. One such measure could be a chamber surrounding the entire 3D printer, in which the pressure is maintained at a constant level. Such weather-related air pressure differences, for example, range from 970 mbar to 1030 mbar. An air pressure difference of 60 mbar between these exemplary values ​​can therefore also affect the fluid system with its set negative pressure at the nozzle interfaces of the printheads. It is known to arrange at least one printhead unit in a 3D printer, which comprises a printhead tank and at least one recirculating printhead with its metering units. The printhead tank has an inlet fluid line via which the printhead tank is connected to, for example, a distribution element for filling with fluid. The fluid supply channel is located between the printhead tank and the recirculating printhead, through which the fluid is conveyed from the printhead tank to the recirculating printhead. The fluid return channel is also located between the recirculating printhead and the printhead tank, through which the fluid is conveyed from the recirculating printhead back to the printhead tank. To achieve the desired pressure equilibrium at the interfaces in the transition zone between the fluid and the ambient air at the nozzles, thereby reliably preventing uncontrolled fluid leakage from the nozzles, pressure control devices known in the prior art are necessary. Such pressure control devices can change the pressure in the fluid and thus adjust it to a predetermined pressure value. According to the prior art, such a pressure control device is, for example, arranged in a fluid line, i.e., in the inlet to the printhead tank. Alternatively or additionally, such a pressure control device can be arranged in a fluid supply channel, i.e., between the printhead tank and the recirculating printhead. In any case, the pressure of the fluid at the interfaces is adjusted by means of at least one pressure adjustment device so that the pressure of the fluid corresponds to the atmospheric pressure at the interface. The pressure of the fluid at the interfaces, generated by a pressure-regulating agent, is influenced by disturbances such as capillary forces (adhesion), static fluid pressure, dynamic fluid pressure in both the lines and the printhead, movement of the entire printhead unit above the build area, and surface tension of the fluid. Thus, a disadvantage of the known prior art is that fluctuations occur in the pressure of the fluid at the interfaces generated by the pressure-regulating agent.Another disadvantage is that along the longitudinal extent of the printhead, between a fluid supply channel and a fluid discharge channel of a recirculating printhead, there is a difference in fluid pressure, with areas of the recirculating printhead located directly on the fluid supply channel having a higher fluid pressure than areas of the recirculating printhead located away from the fluid supply channel. Furthermore, the fluid requirement in the recirculating printhead fluctuates depending on the 3D structure being produced as the printhead unit passes over the build area. Such fluctuations in demand, or a temporary increase in fluid requirement, particularly in an operating mode of the recirculating printhead where fluid consumption is at its maximum, also lead to fluctuations or pressure reductions in the fluid pressure generated by the pressure control medium at the interfaces. Such adverse pressure fluctuations are only inadequately compensated by conventional pressure setting methods, which affects the accuracy and therefore the quality of the 3D structure produced in the 3D printer. Therefore, there is a need for an improved printhead unit and an improved method for controlling the pressure of a fluid in a recirculating printhead of a 3D printer. The object of the invention is to provide a printhead unit and a method for controlling the pressure pU of a fluid in a recirculating printhead of a 3D printer, thereby ensuring a reliable supply of fluid to the recirculating printheads of the 3D printer via the fluid system and reliably preventing uncontrolled fluid leakage from the recirculating printheads. In particular, pressure differences Δp between the fluid pressure pU and atmospheric pressure pA, which occur in an operating mode of the recirculating printhead with maximum fluid consumption, are to be minimized. Furthermore, the pressure in the fluid should be nearly uniform in all areas of the recirculating printhead. The solution should also enable the fluid pressure pU in the recirculating printheads to be adjusted to changing atmospheric pressure pA in the vicinity of the 3D printer. The problem is solved by an arrangement with the features according to claim 1 of the independent patent claims. Further developments are specified in dependent patent claims 2 to 6. According to the invention, the arrangement with the features according to claim 1 can be combined arbitrarily with one or more features of dependent claims 2 to 6 in order to advantageously further develop the subject matter of the invention. Printhead units for controlling the pressure pU of a fluid in a circulating printhead of a 3D printer are known from the prior art, which have at least one printhead tank and a circulating printhead, wherein a fluid supply channel for supplying the circulating printhead with fluid from the printhead tank in a first flow direction and a fluid discharge channel for returning the fluid from the circulating printhead to the printhead tank in a second flow direction are arranged between the printhead tank and the circulating printhead. The aim is to achieve pressure equality at the interfaces in the transition zone between the fluid and the ambient air at the nozzles of the recirculating printhead, where the fluid pressure pUdes in the 3D printer's recirculating printhead should correspond to the atmospheric pressure pA at the interfaces. If this pressure equality is achieved at the interfaces, uncontrolled fluid leakage from the nozzles of the recirculating printhead, or from the nozzles of multiple recirculating printheads connected to the fluid supply channel, is reliably prevented. In this description, the pressure pU always refers to the fluid pressure pUdes in the recirculating printhead, which is present at the respective interfaces in the transition zone between the fluid and the ambient air at the nozzles of the recirculating printhead, even if this is not always fully explained below. To achieve the desired pressure equilibrium at the interfaces in the transition zone between the fluid and the ambient air at the nozzles, thereby reliably preventing fluid from escaping the nozzles, pressure control devices known according to the prior art are necessary. In one example, such a pressure control device is arranged in the area of ​​the fluid supply channel. It is known that fluctuations in the pressure pUde of the fluid set by the pressure setting agent at the interfaces are caused, for example, by capillary forces occurring, a static pressure of the fluid, a dynamic pressure of the fluid both in the lines and in the recirculating printhead, a movement of the entire printhead unit over the build area, a surface tension of the fluid and, in particular, a fluctuating fluid requirement during the generation of the 3D structure. Such fluctuations in the pressure of the fluid set by the pressure setting agent negatively affect the quality of the 3D structure to be produced and must therefore at least be minimized or completely eliminated. According to the invention, it is therefore provided that, in order to control a pressure pUde of the fluid at interfaces in nozzles of the recirculating pressure head, a switchable pressure regulating unit is arranged parallel to the fluid discharge channel, and that the switchable pressure regulating unit has a throttle valve, a pump and a shut-off valve, wherein the pump is configured not to pump the fluid, or to pump the fluid in the second flow direction, or to pump the fluid in a third flow direction. The object of the switchable pressure regulating unit according to the invention is to minimize or even completely eliminate the pressure difference Δp occurring between the pressure pU of the fluid and the atmospheric pressure pA by using the switchable pressure regulating unit, which, in switch-on mode, can generate a volume flow of the fluid parallel to the fluid discharge channel. In particular, in an operating mode of the recirculating printhead in which the fluid consumption in the printhead is at its maximum, the pressure difference Δp is minimized by the use of the switchable pressure regulating unit in its switch-on mode according to the present invention. According to the invention, the switchable pressure regulating unit is arranged parallel to the fluid discharge channel such that, in its switched-on mode, it generates a volume flow parallel to and independent of the fluid discharge channel. For this purpose, a valve belonging to the pressure regulating unit, also referred to as a shut-off valve, is opened when the pressure regulating unit is switched on. Furthermore, at least one pressure sensor is arranged in or at least near the recirculating pressure head, which determines the current pressure pU and transmits its measured values ​​to a central control unit.In a preferred embodiment, a first pressure sensor is arranged in the area of ​​the inlet of the recirculating printhead and a second pressure sensor is arranged in the area of ​​the outlet of the recirculating printhead in order to determine the current pressure pU in different areas within the recirculating printhead. Furthermore, a third pressure sensor for determining the current atmospheric pressure pA is arranged, for example, in the area of ​​the recirculating printhead. This sensor determines current values ​​of the atmospheric pressure pA and transmits them to the central control unit. Alternatively, a pressure sensor already installed in the 3D printer can be used to determine current values ​​of the atmospheric pressure pA. In the central control unit, the current pressure pU of the fluid in the recirculating printhead is compared with the current atmospheric pressure pA to determine a pressure difference Δp necessary for the present process. If the central control unit detects a pressure difference Δp when comparing the pressure pU with the pressure pA, the central control unit generates a control signal for the switchable pressure regulation unit such that, in its switch-on operation, the volume flow of the fluid is changed in parallel to the fluid discharge channel and, as a result of this change in the volume flow, the detected pressure difference Δp is at least reduced. The switchable pressure regulating unit influences or changes the volume flow of the fluid, which flows to the pressure head tank via an output of the recirculating pressure head, when switched on. The switchable pressure regulating unit alters the flow resistance acting on the fluid between the outlet of the recirculating printhead and the printhead tank. This results in a change in the fluid pressure (pUde) at the interfaces in the nozzles of the recirculating printhead, provided that the pressure regulating device, located, for example, in the fluid supply channel, operates at a constant fluid pressure. If multiple recirculating printheads are arranged in the printhead unit, the switchable pressure regulating unit, when activated, changes the fluid pressure (pUde) at all interfaces in all nozzles of all recirculating printheads.Thus, the switchable pressure regulating unit, in its switch-on mode, changes the pressure of the fluid in the circulating pressure heads in such a way that pressure equality is achieved in the transition area on both sides of the interfaces of all nozzles of all circulating pressure heads. According to the invention, the switchable pressure regulating unit comprises a throttle valve, a pump, and a shut-off valve, wherein the pump is configured to either not pump the fluid, pump the fluid in the second flow direction, or pump the fluid in a third flow direction. Particularly in the operating mode of the recirculating pressure head, in which fluid consumption is at its maximum, the invention provides for pumping the fluid through the switchable pressure regulating unit in a third flow direction. Furthermore, according to the invention, the switchable pressure regulating unit comprises a throttle valve and a pump. For example, in this embodiment, the throttle valve can be fixed and thus provide a constant flow resistance, while the pump is a controllable pump. Thus, by controlling the pump during the switchable operation of the pressure regulating unit, the volume flow through the switchable pressure regulating unit can be influenced. It is also advantageous that the pump can be switched between delivering the fluid in the second flow direction from the printhead to the printhead tank and delivering the fluid in the third flow direction from the printhead tank to the printhead. The shut-off valve of the switchable pressure regulating unit fluidically isolates the switchable pressure regulating unit during normal operation of the printhead unit, so that no fluid can flow through the switchable pressure regulating unit. The shut-off valve of the switchable pressure regulating unit activates the switchable pressure regulating unit during switch-on operation, so that fluid flows through the switchable pressure regulating unit in the second or third flow direction according to the invention. Furthermore, the throttle valve is arranged such that one throttle valve inlet is connected via the shut-off valve of the pressure regulating unit to an outlet of the recirculating pressure head, and one throttle valve outlet is connected to a third inlet of the pressure head tank. The pump is connected with a first pump port to the throttle valve inlet and with a second pump port to either the throttle valve outlet or a fourth inlet of the pressure head tank. The preferred embodiment is the connection according to the invention of the second pump port to the fourth inlet of the pressure head tank. The printhead tank has a first inlet through which it receives fluid via a fluid line, for example, from a distribution element or directly from a storage tank. The printhead tank also has a first outlet to which the fluid supply channel for the recirculating printhead is connected. The printhead tank also has a second inlet to which the fluid return channel is connected, through which the fluid flows from the outlet of the recirculating printhead back to the printhead tank, thus ensuring fluid recirculation within the recirculating printhead. When the switchable pressure regulating unit is activated, i.e., operated in its activation mode, the shut-off valve of the pressure regulating unit is open. In this activation mode, the throttle valve is arranged parallel to the fluid discharge channel, thus creating an additional fluid flow parallel to the discharge channel. If, during the activation mode of the pressure regulating unit, no fluid is being pumped by the pump, the throttle valve alone, through its flow resistance, generates a reduced secondary flow of the switchable pressure regulating unit, parallel to the main flow through the discharge channel in the secondary flow direction of the fluid. This secondary flow can range from 20% to 90% relative to the main flow of the discharge channel arranged parallel to the throttle valve. In an example where the pressure difference Δp is zero and the switchable pressure regulating unit, and therefore the pump, is not active in this so-called normal operation of the printhead unit, the associated pipe diameters and the flow resistance of the throttle valve are designed such that between 50% and 90% of the fluid flows in the secondary flow direction to the printhead tank through the fluid discharge channel as the main flow, and between 50% and 10% of the fluid flows in the secondary flow direction to the printhead tank through the throttle valve as a secondary flow. It is also intended that the cross-sections of the pipes connected to the throttle valve and the pump, as well as the fluid return channel, are essentially the same size. In this description, all exemplary specifications regarding a delivery rate or volume flow in the fluid return channel refer to comparable pressure conditions between one outlet of the recirculating printhead and the second inlet of the printhead tank. It is clear to those skilled in the art that a change in the pressure conditions between the outlet of the recirculating printhead and the second inlet of the printhead tank affects the volume flow and thus the delivery rate of the fluid in the fluid return channel. Advantageously, the pump in the switchable pressure regulating unit is arranged parallel to the fluid discharge channel in such a way that the throttle valve is bypassed by the pump. In the preferred arrangement according to the invention, the pump is connected with its first pump port to the throttle valve inlet and with its second pump port to a fourth inlet of the pressurehead tank. This improves the continuous circulation of the fluid in the area of ​​the fluid return to the pressurehead tank. By means of the pump, which is controlled, for example, by a central control unit, a volume flow of the switchable pressure regulating unit is generated in the switch-on operation of the pressure regulating unit parallel to the fluid discharge channel, regardless of the set volume flow through the throttle valve in the second flow direction or preferably in a third flow direction opposite to the second flow direction. In normal operation of the printhead unit, in which the pressure difference Δp is zero, and the switchable pressure regulating unit and the pump are switched off, the main volume flow flows through the fluid return channel. If, according to the procedure, a pressure difference Δp is detected such that the fluid pressure pUde in the recirculating pressure head is less than the atmospheric pressure pA, the pump in the switchable pressure regulating unit is controlled in switch-on mode such that the switchable pressure regulating unit generates a volume flow in the third flow direction towards the recirculating pressure head, thereby minimizing the resulting pressure difference Δp. For such a volume flow in the third flow direction, the pump of the switchable pressure regulating unit must generate a volume flow in the third flow direction that is greater than the volume flow through the throttle valve of the pressure regulating unit. In this configuration, the throttle valve serves to set an operating point of the pump, as is known from the prior art. In the event that a pressure difference Δp is detected such that the fluid pressure pUde in the circulating pressure head is greater than the atmospheric pressure pA, the pump in the switchable pressure regulating unit is controlled in switch-on mode such that the switchable pressure regulating unit generates a volume flow in the second flow direction towards the pressure head tank, thereby minimizing the resulting pressure difference Δp. For such a volume flow in the second flow direction, the pump of the switchable pressure regulating unit generates an additional volume flow in the same direction as the volume flow through the throttle valve of the pressure regulating unit. According to the invention, the pump is also a controllable pump, which is connected to a central control unit via a control line for control purposes. The plan is to use a control signal generated by a central control unit to change the speed and / or flow rate of the controllable pump of the switchable pressure regulating unit, as well as its flow direction, during the switch-on operation of the pressure regulating unit. The aim of this change in speed, and thus flow rate and flow direction of the controllable pump in the second or third flow direction, is to counteract and minimize the detected pressure differences Δp between the pressure pU of the fluid at the interfaces and the atmospheric pressure pA. It has proven advantageous according to the invention that the throttle valve is a controllable throttle valve which is connected to the central control unit via a control line for control purposes. In a further development of the switchable pressure regulating unit according to the invention, the throttle valve is also designed to be controllable or adjustable. This expands the possibilities for influencing the volume flow by the switchable pressure regulating unit in its switch-on mode. It has proven particularly advantageous to have a pressure sensor arranged in the recirculating printhead or in the area of ​​the output of the recirculating printhead, which is connected to the central control unit via a sensor line. A pressure sensor measures the current pressure pUde of the fluid, and the resulting measurements are transmitted to the central control unit. This allows the central control unit to compare the current pressure pUde of the fluid with the current atmospheric pressure pA and generate the control signal, at least for the pump of the pressure regulating unit. For this measurement of the fluid pressure pUde, the pressure sensor is preferably arranged in the recirculating printhead. Alternatively, the pressure sensor can be arranged at the outlet of the recirculating printhead, since the fluid pressure pUde can also be measured with sufficient accuracy at the outlet. In another alternative, pressure sensors are arranged at both the inlet and outlet of the recirculating printhead, transmitting their measurement signals to the central control unit. This allows, on the one hand, the determination of an average fluid pressure pUde within the recirculating printhead, which is then used to calculate the pressure difference Δp. Alternatively, on the other hand, the pressure difference of the fluid within the recirculating printhead between a section of the inlet and a section of the outlet can be determined and taken into account when calculating the pressure difference Δp.Particularly in the case of longitudinally extended recirculating pressure heads, a pressure drop along the longitudinal extension of the fluid can be detected and corrected in different areas of the recirculating pressure head by generating a volume flow in the third flow direction towards the recirculating pressure head using the pump of the switchable pressure regulation unit. The problem is also solved by a method with the features according to claim 7 of the independent patent claims. Further developments are specified in dependent patent claims 8 to 10. According to the invention, the method with the features according to claim 7 can be combined arbitrarily with one or more features of dependent claims 8 to 10 in order to advantageously further develop the subject matter of the invention. According to the known state of the art, a printhead unit with a recirculating printhead has a fluid supply channel between an outlet of a printhead tank and an inlet of the recirculating printhead, and a fluid discharge channel between the recirculating printhead and a second inlet of the printhead tank. The printhead tank is supplied with fluid via a fluid line through the first inlet, for example, from a distribution element or directly from a storage tank. The fluid supply channel for supplying the recirculating pressure head from the pressure head tank is connected to the first outlet of the pressure head tank, while the fluid discharge channel is connected to the second inlet of the pressure head tank, through which the fluid flows back from the outlet of the recirculating pressure head to the pressure head tank, thus ensuring the circulation of the fluid in the recirculating pressure head. Furthermore, according to the state of the art, a pressure pU of a fluid is generated in the recirculating printhead of the 3D printer in such a way that the fluid is provided at interfaces in nozzles of the recirculating printhead with a pressure pU which corresponds to an atmospheric pressure pA in the area of ​​the nozzles. In this description, the wording - provided with a pressure pU which corresponds to an atmospheric pressure pA in a region of the nozzles - means that the pressure pU is equal to the pressure pA, or deviates from it by a maximum of ± 0.5 mbar. According to the invention, a switchable pressure regulating unit is provided parallel to the fluid discharge channel, and when a pressure difference Δp occurs between the pressure pU of the fluid at the interfaces and the atmospheric pressure pA, the switchable pressure regulating unit generates a volume flow parallel to the fluid discharge channel in switch-on mode, thereby minimizing the pressure difference Δp. The switchable pressure regulating unit includes a throttle valve, a pump, and a shut-off valve, and the throttle valve provides a first partial flow of the fluid, at least partially and continuously throttled, as a component of a secondary volume flow of the switchable pressure regulating unit in switch-on mode, in a second flow direction parallel to a main volume flow of the fluid discharge channel in the second flow direction. A pressure sensor is installed, for example, in the recirculating printhead, which determines current measured values ​​for the fluid pressure pU at the interfaces. Furthermore, another pressure sensor is either already present in the 3D printer or is additionally installed to determine the current atmospheric pressure pA in the area of ​​the recirculating printhead. After the respective measured values ​​are transmitted to the central control unit, the central control unit compares the values ​​of the pressures pU and pA and determines the pressure difference Δp. If this determined pressure difference Δp exceeds a predetermined limit value, the central control unit generates at least one control signal for the switchable pressure regulating unit, in particular the pump of the switchable pressure regulating unit, such that in switch-on operation of the pressure regulating unit the volume flow through the switchable pressure regulating unit arranged parallel to the fluid discharge channel is changed according to the procedure. In normal operation of the printhead unit, where the pressure difference Δp is almost zero or below the specified limit value, the control signal of the central control unit is generated in such a way that the switchable pressure regulation unit and thus the pump are switched off. In the event of a pressure difference Δp occurring which is equal to or above the specified limit value, the control signal of the central control unit is generated in such a way that the switchable pressure regulating unit and thus the pump, depending on the sign of the pressure difference Δp occurring, conveys fluid in the second flow direction or preferably the third flow direction in order to minimize the pressure difference Δp that has occurred. In the present invention, it is provided that a throttle valve, a pump and a shut-off valve are provided in the switchable pressure regulating unit and that the throttle valve provides a first partial flow of the fluid, at least partially and continuously throttled, as a portion of a secondary volume flow of the switchable pressure regulating unit in the switch-on operation of the pressure regulating unit with the shut-off valve open, in a second flow direction parallel to a main volume flow of the fluid discharge channel in the second flow direction. To minimize the pressure difference Δp between pressures pU and pA, a throttle valve and a pump are provided in the switchable pressure regulating unit. It is further provided that at least the pump is controllable, allowing the flow rate through the pressure regulating unit to be varied. A shut-off valve is also provided in the switchable pressure regulating unit. This shut-off valve is only open when the pressure regulating unit is switched on, thus enabling the switchable pressure regulating unit to generate a flow rate parallel to the main flow rate of the fluid return channel in the second flow direction, or preferably in the third flow direction. According to the invention, the direction of the volume flow can be changed by the switchable pressure regulating unit. This means that the total volume flow resulting from the throttle and pump elements arranged in the switchable pressure regulating unit can be directed in the second or third flow direction. Thus, the output volume flow of the circulating fluid at the outlet of the circulating pressure head in the direction towards the pressure head tank is reduced or increased. Assuming a constant inlet volume flow at the inlet of the recirculating printhead, a reduction in the outlet volume flow leads to an increase in the pressure pUde of the fluid at interfaces in nozzles of the recirculating printhead. Assuming a constant inlet flow rate at the inlet of the recirculating pressure head, an increase in the outlet flow rate leads to a decrease in the fluid pressure pUdes at interfaces in the nozzles of the recirculating pressure head. Thus, by switching on the pressure regulating unit in switch-on mode, generating the control signal for the pump, and thereby adjusting the total flow rate of the switchable pressure regulating unit according to the invention, the fluid pressure pUdes at interfaces in the nozzles of the recirculating pressure head is specifically influenced in a controlled manner. The parameters controlled or regulated according to the procedure during the switch-on operation of the pressure regulating unit are the pump speed and the pump direction. For example, the pump flow rate increases with increasing pump speed and decreases with decreasing pump speed. This relationship applies to both flow directions of the pump that delivers the fluid in the secondary flow of the switchable pressure regulating unit. In the area of ​​the so-called return flow of the recirculating pressure head, between the outlet of the recirculating pressure head and the pressure head tank, a main fluid flow occurs in the second flow direction through the fluid discharge channel. This main fluid flow in the second flow direction to the pressure head tank is between 60% and 95% of the fluid conveyed from the recirculating pressure head to the pressure head tank. Furthermore, when the pressure regulating unit is activated, a secondary fluid flow occurs in a second flow direction to the pressure head tank or in a third flow direction to the recirculating pressure head. This secondary flow influences the output flow rate of the circulating fluid at the outlet of the recirculating pressure head. A further advantage of the invention is that, in the switching operation of the pressure regulating unit, the pump is controlled by means of a control signal generated by a central control unit to transport the fluid in the second flow direction or, preferably according to the invention, in a third flow direction. To adjust the flow rate through the switchable pressure regulating unit to the required flow rate, a controllable pump is located within the unit. This pump is connected to the central control unit via a control line to transmit a control signal. Thus, upon detecting a pressure difference Δp, the central control unit can use its control signals to activate the pressure regulating unit, open the shut-off valve, and change the pump's delivery rate and the flow direction of the controllable pump, thereby altering the direction and magnitude of the flow rate through the switchable pressure regulating unit, since the pressure regulating unit's throttle valve is set to a fixed flow resistance. According to the invention, it is particularly provided that the pump in switching operation is controlled by the central control unit in such a way that the pump is switched off if the pressure pUdes of the fluid is equal to the atmospheric pressure pA, that the pump generates a volume flow in the third flow direction if the pressure pUdes of the fluid is less than the atmospheric pressure pA, and that the pump generates a volume flow in the second flow direction if the pressure pUdes of the fluid is greater than the atmospheric pressure pA, and that any pressure difference Δp that occurs is thus minimized. Within the switchable pressure regulating unit according to the invention, the secondary flow generated by the pressure regulating unit during switch-on operation is divided into a first partial flow through the throttle valve and a second partial flow through the pump. The flow resistance of the throttle valve is preferably set to a fixed value. If the second partial flow is generated by the pump in the second flow direction, the secondary flow of the pressure regulating unit is the sum of the first partial flow through the throttle valve and the second partial flow through the pump. If the second partial flow is generated by the pump in the third flow direction, the secondary flow of the pressure regulating unit is the difference between the first partial flow through the throttle valve and the second partial flow through the pump. The method according to the invention controls the switchable pressure regulating unit by means of a control signal generated by the central control unit such that the switchable pressure regulating unit and the pump are switched off during normal operation of the printhead unit, in which the pressure pU of the fluid is equal to the atmospheric pressure pA, the shut-off valve of the pressure regulating unit is closed, and no fluid is conveyed through the switchable pressure regulating unit. The conveyance of the fluid from the outlet of the recirculating printhead to the printhead tank only occurs in the main volume flow of the fluid return channel. The method according to the invention controls the switchable pressure regulating unit by means of a control signal generated by the central control unit in the event of a pressure difference Δp, at which the fluid pressure pU is less than the atmospheric pressure pA, such that the pump generates a volume flow in the third flow direction, i.e., towards the outlet of the recirculating pressure head. This secondary volume flow of the switchable pressure regulating unit in the third flow direction, caused by the pump, opposes the main volume flow through the fluid discharge channel in the second flow direction. Thus, the output volume flow from the recirculating printhead to the printhead tank via the outlet of the recirculating printhead is reduced, leading to an increase in the pressure pUim the recirculating printhead and a reduction in the pressure difference Δp. The method according to the invention controls the switchable pressure regulating unit by means of a control signal generated by the central control unit in the event of a pressure difference Δp, at which the fluid pressure pU is greater than the atmospheric pressure pA, such that the pump generates a volume flow in the second flow direction, i.e., towards the pressure head tank. This secondary volume flow of the switchable pressure regulating unit in the second flow direction, caused by the pump, interacts with the main volume flow through the fluid discharge channel in the second flow direction. Thus, the output volume flow from the recirculating printhead to the printhead tank via the outlet of the recirculating printhead is increased, which leads to a reduction in the pressure pUim the recirculating printhead and a reduction in the pressure difference Δp. By using the arrangement and method according to the invention, the pressure difference Δp that occurs is reduced in such a way that this pressure difference Δp has a value of less than 3 mbar, in particular less than 1 mbar. The features and advantages of this invention, as explained above, can be better understood and evaluated after careful study of the following detailed description of the preferred, non-restrictive exemplary embodiments of the invention with the accompanying drawings, which show: Fig. 1: a rotary printhead with several metering units in a prior art 3D printer, Fig. 2: a fluid system with a prior art printhead unit, Fig. 3: a basic embodiment of the printhead unit according to the invention, and Fig. 4: a detailed embodiment of the printhead unit according to the invention. Fig. 1 shows a sectional view of a rotary printhead 1 with several dosing units 2 in a prior art 3D printer, using an example with three dosing units 2 in the rotary printhead 1. Such dosing units 2 can be arranged in the rotary printhead 1, for example, in the form of a matrix with several rows and several columns. Each metering unit 2 of the rotary printhead 1 has a nozzle 3 which is oriented in one direction of a build area 4. The metering units 2 are at least partially separated from one another, for example by corresponding chamber walls, without impeding the supply of a fluid 5. On this build area 4, a particulate building material (not explicitly shown in Fig. 1) is applied, smoothed, and solidified to build up a 3D structure layer by layer. Methods known in the art for applying, smoothing, and solidifying the particulate building material are used for this purpose. The illustrated recirculating printhead 1 has a fluid supply channel 6 and a fluid discharge channel 7. The fluid 5, such as a binder, is supplied to the recirculating printhead 1 via the fluid supply channel 6 and discharged from the recirculating printhead 1 via the fluid discharge channel 7. Thus, the recirculating printhead 1 and its metering units 2 are circulated by the fluid 5. This circulation 8 of the fluid 5 through the recirculating printhead 1 is represented by several small arrows. The two large arrows above the fluid supply channel 6 and above the fluid discharge channel 7 in Fig. 1 indicate the flow direction of the fluid 5 in the circulation 8. The fluid 5 located in the metering units 2 of the circulation printhead 1 is prevented from uncontrolled escape through the nozzle 3, for example, by a slight vacuum. To meter the fluid 5 via the nozzles 3 of the metering units 2 onto the particulate building material located on the build platform 4, a means 9 for generating a mechanical force is arranged in each metering unit 2. This means 9 generates a mechanical force in the form of a pressure wave, which propagates from the means 9 itself through the fluid 5 in the associated metering unit 2 towards the associated nozzle 3, thus metering a droplet 10 via the corresponding nozzle 3. Such a means 9 could, for example, be a piezoelectric element operating according to the piezoelectric effect. The droplet 10 reaches the surface of the particle-shaped building material located on the building site 4 and causes a selective solidification of the building material at the point of impact, creating a partial area of ​​the 3D structure to be produced. The illustrated rotary printhead 1, with its metering units 2 divided into chambers, is moved across the build area 4 in the usual manner. In this way, the fluid 5 is metered precisely at the points necessary for the formation of a 3D structure or a 3D component. In the example shown in Fig. 1, a filter element 11 is provided in the fluid supply channel 6 and a filter element 11 in the fluid discharge channel 7. Alternatively, only one filter element 11 can be provided in the fluid supply channel 6 or one filter element 11 in the fluid discharge channel 7. These filter elements 11 serve to protect against blockages or contamination. In this way, for example, clogging of the nozzles 3 can be prevented. This recirculating printhead 1, known from the prior art, enables the fluid 5 to be kept in motion within the metering units 2 of the recirculating printhead 1 and to flow through the recirculating printhead 1 in a continuous 8 cycle. This prevents moisture loss, changes in the viscosity of the fluid 5, oxidation, and narrowing of the nozzle 3 due to drying, thus preventing metering errors and printing defects. In the area of ​​the nozzles 3, an interface 12 forms between the fluid 5 in the recirculating printhead 1 and the surrounding ambient air or atmosphere, in which a so-called atmospheric pressure pA prevails. This atmospheric pressure pA acts uniformly on the recirculating printhead 1 from all sides. Thus, the atmospheric pressure pA also acts uniformly on all interfaces 12 at all nozzles 3 of the recirculating printhead 1. The atmospheric pressure pA is represented in Fig. 1 by way of example in a cloud, whereas in practice this atmospheric pressure pA surrounds the recirculating printhead 1 on all sides. The circulating pressure head 1 is connected, for example, to a pressure head tank 13 (not shown in Fig. 1) via the fluid supply channel 6 and the fluid discharge channel 7. Fig. 2 shows a prior art fluid system 14. A printhead tank 13 and at least one circulating printhead 1, comprising several metering units 2, are arranged in a printhead unit 15 of the fluid system 14. The circulating printhead 1 is connected to the printhead tank 13 via a fluid supply channel 6, for supplying the fluid 5, such as a binder, from the printhead tank 13 to the metering units 2 of the circulating printhead 1, and a fluid discharge channel 7, for discharging the fluid 5 from the metering units 2 of the circulating printhead 1 into the printhead tank 13. The fluid 5 is moved from the printhead tank 13 to the circulating printhead 1 and back again in a circulating motion 8 by means not shown, such as pumps. An arrangement of several printheads 1 in one printhead unit 13 is possible, which is not shown in Fig. 2. A distributor element 17 is typically arranged between a storage tank 16 and the printhead tank 13. The distributor element 17 has a distributor inlet 18, which is connected to the storage tank 16 via a fluid supply line 19. The distributor element 17 also has a distributor outlet 20, which is connected to the storage tank 16 via a fluid return line 21. In this way, the fluid 5 can be circulated between the storage tank 16 and the distributor element 17 and back again to prevent flocculation or agglomeration in the fluid 5 or to dissolve any flocculation or agglomeration that has already formed. The distributor element 17 and several printhead units 15, each having a printhead tank 13 and at least one recirculating printhead 1 connected to the printhead tank 13, are arranged in a printing unit 22 which can be moved over a build area of ​​the 3D printer (not shown). The distributor element 17 is connected to the first printhead tank 13 of the first printhead unit 15 via a first fluid line 23. The first fluid line 23 is connected to a first inlet 24 of the printhead tank 13, the first inlet 24 being located in the lower region of the printhead tank 13. To supply the further printhead tanks 13 in the further printhead units 15, the distributor element 17 is connected to the further printhead tanks 13 next to the first fluid line 23 via further fluid lines 23a, 23b and 23c. In the example shown in Fig. 2, the distributor element 17 has four connected fluid lines 23, 23a, 23b, and 23c. The connections of the additional fluid lines 23a, 23b, and 23c to the associated printhead tanks 13 in the further printhead units 15 are not shown in Fig. 2. The further printhead units 15 are shown only schematically. Fig. 3 shows a basic embodiment of the printhead unit 15 according to the invention. Figure 3 shows a printhead unit 15 arranged in a 3D printer, in which a printhead tank 13 and at least one recirculating printhead 1 with its metering units 2 are arranged. The printhead tank 13 has the arranged fluid line 23 at its first inlet 24, by which the printhead tank 13 is connected to, for example, a distributor element 17 (not shown) for filling with the fluid 5. The arrow next to the fluid line 23 illustrates that the fluid 5 is conveyed to the printhead tank 13 in the direction indicated by the arrow. Between the printhead tank 13 and the recirculating printhead 1, the fluid supply channel 6 is arranged, through which the fluid 5 is conveyed from an outlet 25 of the printhead tank 13 to an inlet 26 of the recirculating printhead 1. The arrow shown next to the fluid supply channel 6 represents the first flow direction 27, in which the fluid 5 is conveyed from the outlet 25 of the printhead tank 13 to the inlet 26 of the recirculating printhead 1 via the fluid supply channel 6. A fluid discharge channel 7 is also arranged between the recirculating printhead 1 and the printhead tank 13, through which the fluid 5 is conveyed in a main volume flow from an outlet 28 of the recirculating printhead 1 back to a second inlet 34 of the printhead tank 13 in a second flow direction 29. The resulting circulation 8 is represented by an arrow in the area of ​​the metering units 2 of the circulation printhead 1. Furthermore, Fig. 3 shows, by way of example, some droplets 10 dispensed from the circulating printhead 1 via the nozzles 3 (not shown in detail). Additionally, the interfaces 12 are shown by way of example at two nozzles 3 of the right-hand metering units 2 of the circulating printhead 1. To achieve the desired pressure equilibrium at the interfaces 12 in the transition zone between the fluid 5 and the ambient air, thereby reliably preventing uncontrolled leakage of the fluid 5 from the nozzles 3 or uncontrolled leakage from the recirculating printheads 1 of the 3D printer, pressure adjustment devices 30, known according to the prior art, are necessary. Such pressure adjustment devices 30 can decrease or increase the pressure in the fluid 5 as needed and thus adjust it to a predetermined pressure value. According to the prior art, such a pressure adjustment device 30 is, for example, arranged in the fluid line 23. Alternatively or additionally, such a pressure adjustment device 30 can be arranged in the supply channel 6. In any case, the pressure pUdes fluid 5 at the interfaces 12 is adjusted by means of at least one pressure adjusting device 30 such that the pressure pUdes fluid 5 corresponds to the atmospheric pressure pA at the interface 12. In Fig. 3, the pressure pUdes fluid 5 is shown in the area of ​​the metering units 2 of the recirculating printhead 1. Furthermore, the atmospheric pressure pA, which acts on the recirculating printhead 1 from all sides and thus also on the interfaces 12, is represented by a cloud in the area of ​​the recirculating printhead 1. According to the invention, the aim is to achieve pressure equality at the interfaces 12 in the transition area between the fluid 5 and the ambient air at the nozzles 3 of the recirculating printhead 1, wherein the pressure pU of the fluid 5 corresponds to the atmospheric pressure pA. This means that the pressure pU is equal to the pressure pA, or deviates from it by a maximum of ± 1.5 mbar, and in particular only by ± 0.5 mbar. Fluctuations in the pressure pUde of the fluid 5 set by the pressure setting agent 30 at the interfaces 12 are caused, for example, by capillary forces occurring, a static pressure of the fluid 5, a dynamic pressure of the fluid 5 both in the lines and in the recirculating printhead 1, a movement of the entire printhead unit 15 over the build area 4, a surface tension of the fluid 5 and, in particular, a fluctuating fluid requirement during the generation of the 3D structure. Such fluctuations in the pressure of the fluid 5 set by the pressure setting agent 30 have a negative impact on the quality of the 3D structure to be produced. Therefore, according to the invention, it is provided to minimize a pressure difference Δp occurring between the pressure pU of the fluid 5 and the atmospheric pressure pA by using a pressure regulating unit 31 that can be switched on when a pressure difference Δp occurs. According to the invention, the switchable pressure regulating unit 31 is arranged parallel to the fluid discharge channel 7. For this purpose, a first connection of the switchable pressure regulating unit 31 is connected to the outlet 28 of the recirculating pressure head 1 via a shut-off valve 43. A second connection of the switchable pressure regulating unit 31 is preferably connected to a third inlet 35 of the pressure head tank 13, as shown in Fig. 3. Furthermore, it is provided that at least one pressure sensor 32 is arranged in the recirculating printhead 1, which determines the current pressure pU and transmits its measured values ​​to a central control unit (not shown). Alternatively, the pressure sensor 32 can be arranged in the area of ​​the output 28 of the recirculating printhead 1 to determine the current pressure pU, which is not shown in Fig. 3. A main volume flow of fluid 5 flows through the fluid discharge channel 7 from the outlet 28 of the recirculating printhead 1 to the second inlet 34 of the printhead tank 13. This main volume flow is not affected by the switchable pressure regulating unit 31 during normal operation of the printhead unit 15, in which the pressure pU of the fluid is equal to the atmospheric pressure pA and therefore no pressure difference Δp occurs. During this normal operation of the printhead unit 15, the shut-off valve 43, which is controlled by a control signal from the central control unit (not shown), is closed. In the event of a pressure difference Δp occurring, the switchable pressure regulating unit 31 is switched on and activated, generating a secondary flow of fluid 5 through the switchable pressure regulating unit 31 in parallel to the main flow of fluid discharge channel 7. In this switchable mode, the shut-off valve 43, controlled by the central control unit, is open. The secondary volume flow of fluid 5 can be generated by the switchable pressure regulating unit 31 parallel to the main volume flow flowing in the second flow direction 29, also in the second flow direction 29, and thus increases the output volume flow of fluid 5 at the outlet 28 of the recirculating pressure head 1 in the direction of the pressure head tank 13. In a preferred embodiment of the invention, this secondary flow of fluid 5 can be generated by the switchable pressure regulating unit 31 parallel to the main flow flow in the second flow direction 29 and in the opposite direction in a third flow direction 33, thus reducing the output flow of fluid 5 at the outlet 28 of the recirculating printhead 1 towards the printhead tank 13. Particularly in an operating mode of the recirculating printhead 1 in which the consumption of fluid 5 in the recirculating printhead 1 is at its maximum, the secondary flow of fluid 5 generated according to the invention by the switchable pressure regulating unit 31 in the third flow direction 33 minimizes the pressure difference Δp and thus improves the print image of the recirculating printhead 1 and the quality of the 3D structure produced in the 3D printer. The secondary flow of fluid 5 through the switchable pressure regulating unit 31 is switched off again in the event that the normal operation of the printhead unit 15, in which the pressure pUdes fluid equals the atmospheric pressure pAist, is reached. By influencing or changing the secondary volume flow using the switchable pressure regulating unit 31, the current pressure pUdes fluid 5 at the interfaces 12 is altered. In this way, when a pressure difference Δp between the pressure pUdes fluid 5 and the atmospheric pressure pA is detected, the switchable pressure regulating unit 31 counteracts this and at least reduces or minimizes the pressure difference Δp. The switchable pressure regulating unit 31 is controlled by the central control unit in normal operation of the printhead unit 15 in such a way that the shut-off valve 43 is closed and no secondary flow is generated by the switchable pressure regulating unit 31. In the event that the pressure pUdes of fluid 5 becomes greater than the atmospheric pressure pA, the switchable pressure regulating unit 31 is designed to generate a secondary flow in the secondary flow direction 29. Thus, in addition to the main flow through the fluid discharge channel 7, the secondary flow through the switchable pressure regulating unit 31 is also directed from the recirculating pressure head 1 to the pressure head tank 13. The output flow of fluid 5 at outlet 28 of the recirculating pressure head 1 to the pressure head tank 13 is the sum of the main and secondary flow rates. As a result of this increased output flow compared to normal operation, more fluid 5 is discharged from the recirculating pressure head 1 via outlet 28, thereby reducing the pressure pUdes of fluid 5 in the recirculating pressure head 1 and thus decreasing the pressure difference Δp.The process of generating the secondary volume flow in the second flow direction 29 by the switchable pressure regulating unit 31 is increased in the amount of the secondary volume flow until the pressure difference Δp that occurred has been sufficiently reduced or is zero. In the event that the pressure pU of fluid 5 becomes lower than the atmospheric pressure pA, the switchable pressure regulating unit 31 generates a secondary flow, which is directed in the third flow direction 33. Thus, the secondary flow generated by the switchable pressure regulating unit 31 is directed in a third flow direction 33 and therefore opposite to the main flow through the fluid discharge channel 7. This reduces the output flow of fluid 5 at the outlet 28 of the recirculating pressure head 1 towards the pressure head tank 13. In this case, the output flow is formed by the difference between the main flow and the secondary flow.As a result of this reduced output volume flow compared to normal operation, less fluid 5 is discharged from the circulating pressure head 1 via the outlet 28, thereby increasing the pressure pUde of the fluid 5 in the circulating pressure head 1 and thus reducing the pressure difference Δp. The process of generating the secondary volume flow in the third flow direction 33 by the switchable pressure regulating unit 31 is increased in the amount of the secondary volume flow until the pressure difference Δp has been sufficiently reduced or is zero. In this way, fluctuations in the pressure pUde of the fluid 5 set by the pressure setting means 30 at the interfaces 12 are at least minimized or completely eliminated by the switchable pressure regulating unit 31. Fig. 4 shows a detailed embodiment of the printhead unit 15 according to the invention. Figure 4 also shows a printhead unit 15 arranged in a 3D printer, in which a printhead tank 13 and at least one recirculating printhead 1 with its metering units 2 are arranged. The printhead tank 13 has the arranged fluid line 23 at its first inlet 24, with which the printhead tank 13 is connected to a distributor element 17 (not shown) for filling with the fluid 5. Between the printhead tank 13 and the recirculating printhead 1, the fluid supply channel 6 is arranged, through which the fluid 5 is conveyed from the printhead tank 13 to the inlet 26 of the recirculating printhead 1 via the outlet 25. Also between the recirculating printhead 1 and the printhead tank 13, the fluid discharge channel 7 is arranged, through which the fluid 5 is conveyed from the outlet 28 of the recirculating printhead 1 back to the printhead tank 13 in the main flow. The resulting recirculation 8 is shown by an arrow in the area of ​​the metering units 2 of the recirculating printhead 1. Furthermore, Fig. 4 also shows exemplary dispensed droplets 10. Additionally, the interfaces 12 are shown by way of example at two nozzles 3 of the right-hand metering units 2 of the rotary printhead 1. Fig. 4 also shows two positions at which the pressure setting means 30 known from the prior art can be arranged. In any case, the pressure of the fluid 5 at the interfaces 12 is adjusted by means of at least one pressure adjusting means 30 such that the pressure of the fluid 5 corresponds to the atmospheric pressure at the interface 12. Since fluctuations in the pressure pUdes Fluids 5 set by the pressure setting means 30 have a negative impact on the quality of the 3D structure to be produced, it is provided according to the invention to minimize a pressure difference Δp occurring between the pressure pUdes Fluids 5 and the atmospheric pressure pA by using the pressure regulating unit 31 according to the invention and switchable. In the detailed embodiment shown in Fig. 4, the switchable pressure regulating unit 31 with its associated shut-off valve 43 is also arranged parallel to the fluid discharge channel 7. It is also provided that at least one pressure sensor 32 is arranged in the circulating pressure head 1, which determines the current pressure pU and transmits its measured values ​​to a central control unit (not shown). In the embodiment of Fig. 4, the switchable pressure regulating unit 31 is provided to have a throttle valve 36 and a pump 37. The throttle valve 36 is arranged in the switchable pressure regulating unit 31 such that a throttle valve inlet 38 is connected via the shut-off valve 43 to the outlet 28 of the recirculating pressure head 1, and a throttle valve outlet 39 is connected to the third inlet 35 of the pressure head tank 13. Furthermore, the pump 37 is connected via a first pump port 40 to the throttle valve inlet 38 and the shut-off valve 43, and via a second pump port 41 to a fourth inlet 42 of the pressure head tank 13. Thus, the first pump port 40 is also connected via the shut-off valve 43 to the outlet 28 of the recirculating pressure head 1. The switchable pressure regulating unit 31 influences the output volume flow of the fluid 5 at the outlet 28 of the recirculating pressure head 1 when the pressure regulating unit 31 is switched on. In this process, the output volume flow of the fluid 5 at the outlet 28 of the recirculating pressure head 1 is divided into the main volume flow through the fluid return channel 7 and the secondary volume flow, which is adjustable by the switchable pressure regulating unit 31 when switched on. The secondary flow is divided within the switchable pressure regulating unit 31 into a first partial flow through the throttle valve 36 and a second partial flow through the pump 37. The first partial flow through the throttle valve 36 is fixed. The switchable pressure regulating unit 31 is controlled by the central control unit in normal operation of the pressure head unit 15 such that the shut-off valve 43 is closed and the pump 37 is switched off, so that no second partial flow and no secondary flow is generated. In normal operation of the printhead unit 15, this means that the pressure pUdes fluid 5 in the circulating printhead 1 is generated by a state-of-the-art pressure adjustment means 30 in such a way that no fluctuations of the pressure pU and thus no pressure difference Δp occur between the pressure pUdes fluid 5 at the interfaces 12 and the atmospheric pressure pA. If, during a continuous comparison of the pressures pU and pA by the central control unit, a fluctuation or a pressure difference Δp is detected which exceeds a permissible tolerance, the central control unit generates at least one control signal for controlling the switchable pressure regulating unit 31. In particular, a first control signal is generated which opens the shut-off valve 43. Furthermore, a second control signal for the pump 37 is generated such that, by changing the second partial flow generated by the pump 37, either a secondary flow of the switchable pressure regulating unit 31 is generated in the second flow direction 29 or, preferably, in the third flow direction 33. When the secondary flow is generated by the switchable pressure regulating unit 31 in switch-on mode in the third flow direction 33 by means of the pump 37, the first partial flow must be compensated by the throttle valve 36 in the second flow direction 29. Only after the pump 37 has compensated for this first partial flow through the throttle valve 36 with its second partial flow can a flow acting in the third flow direction 33 be generated by the pressure regulating unit 31. In the event that the pressure pUdes of the fluid 5 becomes greater than the atmospheric pressure pA, the invention provides that a secondary flow resulting from the first partial flow through the throttle valve 36 and the second partial flow through the pump 37 is generated by the switchable pressure regulating unit 31, which is directed in the second flow direction 29 in order to at least minimize the pressure difference Δp that has occurred. In the event that the pressure pUdes of the fluid 5 becomes less than the atmospheric pressure pA, the invention provides that a secondary flow resulting from the first partial flow through the throttle valve 36 and the second partial flow through the pump 37 is generated by the switchable pressure regulating unit 31, which is directed in the third flow direction 33 in order to at least minimize the pressure difference Δp that has occurred. List of reference symbols 1 Circulating printhead 2 Metering unit 3 Nozzle 4 Build area 5 Fluid 6 Fluid supply channel 7 Fluid discharge channel 8 Circulation 9 Means of generating a mechanical force 10 Droplet 11 Filter element 12 Interface 13 Printhead tank 14 Fluid system 15 Printhead unit 16 Storage tank 17 Distributor element 18 Distributor inlet 19 Fluid supply line 20 Distributor outlet 21 Fluid return line 22 Pressure unit 23, 23a, 23b, 23c Fluid line 24 First inlet of the printhead tank 25 Outlet of the printhead tank 26 Inlet of the circulating printhead 27 First flow direction 28 Outlet of the circulating printhead 29 Second flow direction 30 Pressure setting means (state of the art) 31 Pressure regulating unit 32 Pressure sensor 33 Third flow direction 34 Second inlet of the printhead tank 35 Third inlet of the printhead tank 36 Throttle valve 37 Pump 38 Throttle valve inlet 39 Throttle valve outlet 40 First pump connection 41 Second pump connection 42 Fourth inlet of the pressure head tank 43 Shut-off valve

Claims

Printhead unit (15) for controlling the pressure of a fluid (5) in a recirculating printhead (1) of a 3D printer, wherein the printhead unit (15) comprises at least one printhead tank (13) and a recirculating printhead (1), wherein a fluid supply channel (6) for supplying the recirculating printhead (1) with fluid (5) from the printhead tank (13) in a first flow direction (27) and a fluid discharge channel (7) for returning the fluid (5) from the recirculating printhead (1) to the printhead tank (13) in a second flow direction (29) is arranged between the printhead tank (13) and the recirculating printhead (1), characterized in that a switchable pressure regulating unit (31) is arranged parallel to the fluid discharge channel (7) for controlling a pressure pU of the fluid (5) at interfaces (12) in nozzles (3) of the recirculating printhead (1), and that the switchable pressure regulating unit (31) includes a throttle valve (36), a pump (37) and a shut-off valve (43),wherein the pump (37) is configured not to pump the fluid (5) or to pump the fluid (5) in the second flow direction (29) or to pump the fluid (5) in a third flow direction (33). Printhead unit (15) according to claim 1, characterized in that the throttle valve (36) is arranged such that a throttle valve inlet (38) is connected via the shut-off valve (43) to an outlet (28) of the recirculating printhead (1) and a throttle valve outlet (39) is connected to a third inlet (35) of the printhead tank (13) and that the pump (37) is arranged with a first pump port (40) to the throttle valve inlet (38) and with a second pump port (41) to the throttle valve outlet (39) or to a fourth inlet (42) of the printhead tank (13). Printhead unit (15) according to one of claims 1 or 2, characterized in that the pump (37) is a controllable pump (37) which is connected to a central control unit via a control line for control purposes. Printhead unit (15) according to one of claims 1 to 3, characterized in that the throttle valve (36) is a controllable throttle valve (36) which is connected to the central control unit via a control line for control purposes. Printhead unit (15) according to one of claims 1 to 4, characterized in that a pressure sensor (32) is arranged in the circulating printhead (1) or in the area of ​​the output (28) of the circulating printhead (1), which is connected to the central control unit via a sensor line. Method for controlling the pressure pU of a fluid (5) in a recirculating printhead (1) of a 3D printer, wherein a fluid supply channel (6) is provided between an inlet (26) of the recirculating printhead (1) and an outlet (25) of the printhead tank (13), and a fluid discharge channel (7) is provided between an outlet (28) of the recirculating printhead (1) and a second inlet (34) of a printhead tank (13), and wherein the fluid (5) is supplied at interfaces (12) in nozzles (3) of the recirculating printhead (1) at a pressure pU which corresponds to an atmospheric pressure pA in a region of the nozzles (3), characterized in that a switchable pressure regulating unit (31) is provided in parallel to the fluid discharge channel (7).that when a pressure difference Δp occurs between the pressure pU of the fluid (5) at the interfaces (12) and the atmospheric pressure pA, the switchable pressure regulating unit (31) generates a volume flow parallel to the fluid discharge channel (7) in switch-on mode and minimizes the pressure difference Δp in such a way that a throttle valve (36), a pump (37) and a shut-off valve (43) are provided in the switchable pressure regulating unit (31) and that the throttle valve (37) provides at least a partially and continuously throttled first partial flow of the fluid (5) as a component of a secondary volume flow of the switchable pressure regulating unit (31) in switch-on mode in a second flow direction (29) parallel to a main volume flow of the fluid discharge channel (7) in the second flow direction (29). Method according to claim 6, characterized in that the pump (37) in the switching operation of the pressure regulating unit (31) is controlled by means of a control signal generated by a central control unit to transport the fluid (5) in the second flow direction (29) or in a third flow direction (33). A method according to one of claims 6 or 7, characterized in that the pump (37) is controlled by the central control unit such that the pump (37) is switched off if the pressure pU of the fluid (5) is equal to the atmospheric pressure pA, that the pump (37) generates a volume flow in the third flow direction (33) if the pressure pU of the fluid (5) is less than the atmospheric pressure pA, and that the pump (37) generates a volume flow in the second flow direction (29) if the pressure pU of the fluid (5) is greater than the atmospheric pressure pA, and thus any pressure difference Δp is minimized.

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