Electrohydrodynamic printhead with ink circulation

Abstract

An electrohydrodynamic printhead includes a nozzle carrier (6) and a plurality of nozzles (4) on the nozzle carrier (6). Each nozzle (4) forms a projection on the front side (36) of the nozzle carrier (6). A plurality of ink supply conduits (15) and suction conduits (16) supply ink to and recover ink from the nozzles. Each nozzle (4) comprises an ink supply tube as part of an ink supply conduit terminating in a front supply spout (54a) and an ink suction tube (55) as part of an ink suction conduit terminating in a front suction spout (55a) laterally surrounding the supply spout (54a). The distance from the foremost portion (94) of the nozzle to the supply nozzle (54a) in the ejection direction (X) is not more than half the diameter of the supply nozzle (54a), and the distance from the foremost portion (94) of the nozzle to the suction nozzle (55a) is not more than half the difference between the diameter of the suction nozzle (55a) and the diameter of the supply nozzle (54a). The two nozzles are close to the foremost part (94) of the nozzle, so that the ink can be effectively circulated even at the foremost part of the nozzle, and ink stagnation is prevented.

Description

TECHNICAL FIELD

[0001] The invention relates to an electrohydrodynamic printing head and a method for operating the same. BACKGROUND

[0002] US2018 / 0009223 describes an electrohydrodynamic printing head having a nozzle carrier with a plurality of nozzles. It is designed to eject ink along an ejection direction. The nozzles form protrusions extending along this ejection direction. Ejection electrodes are associated with the nozzles and are located on the target side of the nozzles. An ink supply conduit supplies ink to the nozzles.

[0003] In a printing head of this design, ink can form a pool that "floods" the nozzles and interferes with the operation of the printing head.

[0004] To this end, WO2022 / 152379A1 and WO2022 / 152380A1 suggest using, in addition to the ink supply conduit, an ink suction conduit. The ink suction conduit is adapted to draw ink back from the nozzles and to keep the ink circulating in the printing head. SUMMARY

[0005] The problem addressed by the invention is to make a printing head of this type more reliable.

[0006] The printing head and the method of the independent claims solve this problem.

[0007] The electrohydrodynamic printing head therefore comprises at least the following elements:

[0008] - a nozzle carrier: this is a substrate on which the nozzles are arranged.

[0009] - a plurality of nozzles arranged on the carrier: each such nozzle forms a protrusion (i.e. a bulge) arranged on the "front side" of the nozzle carrier, i.e. the side that faces the printing target during operation. The nozzles extend along the ejection direction of the printing head, i.e. along the direction along which the printing head is designed to eject ink. Advantageously, the nozzles extend parallel to the ejection direction, but it can also extend at a small angle with respect to the ejection direction, in particular at an angle of less than 45°.

[0010] - a plurality of ejection electrodes associated with the nozzles and located on the front side of the nozzles: the ejection electrodes serve to eject ink from their associated nozzles.

[0011] - a plurality of ink supply conduits for the nozzles: at least one ink supply conduit terminates at each nozzle and supplies it with ink.

[0012] - a plurality of ink suction conduits: at least one ink suction conduit terminates at each nozzle and recovers ink from the nozzle.

[0013] Furthermore, each nozzle comprises at least the following elements:

[0014] - an ink supply tube: the ink supply tube forms part of the ink supply conduit and ends at its front end in the supply jet.

[0015] - an ink suction tube. The ink suction tube forms part of the ink suction conduit and ends at its front end in the suction jet.

[0016] One of these jets laterally surrounds the jet of the other.

[0017] Both the supply jet and the suction jet are close to the front end of the nozzle, in terms of the ejection direction (X),

[0018] - the distance from the frontmost part of the nozzle to the supply jet is less than the diameter of the supply jet, and

[0019] - the distance from the frontmost part of the nozzle to the suction jet is less than the difference between the diameter of the suction jet and the diameter of the supply jet.

[0020] Placing both jets close to the frontmost part of the nozzle is based on the understanding that this makes it possible to effectively circulate the ink even at the frontmost part of the nozzle, thereby preventing the formation of stagnant ink zones at the front end of the nozzle. Stagnant ink is prone to creating "spots" due to the progressive evaporation of the ink carrier liquid, and such spots can affect the formation of ink drops when printing.

[0021] Having one of the jets laterally surround the other jet provides a well-distributed flow of ink, thereby reducing asymmetries.

[0022] Advantageously, the suction jet laterally surrounds the supply jet in order to facilitate the formation of a meniscus with a "high" center point. However, the supply jet can also laterally surround the suction jet, in particular for low-viscosity inks in which capillary forces, wetting, and surface tension are the main factors in the formation of the meniscus.

[0023] Advantageously, the suction jet is at least as close to the frontmost part of the nozzle as the supply jet when measured along the ejection direction. In other words, the two jets are the same distance from the frontmost part, or the supply jet is further from the frontmost part of the nozzle than the suction jet. This allows the front edge of the wall of the supply tube to be easily immersed in the ink, thereby preventing the ink from stagnating there.

[0024] However, in another embodiment, the supply jet can be closer to the frontmost part of the nozzle than the suction jet along the ejection direction. This helps to center the apex of the ink on the nozzle.

[0025] The suction jet and the supply jet can be located at the same distance from the foremost part of the nozzle along the jetting direction. In this case, the wall of the supply jet can still remain in a submerged state and the manufacturing of both nozzles from the same layer of material is simpler than in embodiments where the two nozzles terminate at different positions along the jetting direction.

[0026] In one embodiment, the suction jet and / or the supply jet can form the foremost part of the nozzle. In this case, it is easy to prevent stagnant ink at the foremost part, since the foremost part is close to the actively circulating ink.

[0027] In another embodiment, each nozzle can comprise a nozzle tip forming the foremost part of the nozzle. The nozzle tip is arranged forward from the center of the supply jet in the jetting direction. The tip helps to center the ink meniscus on the nozzle, making the ejection of ink drops more predictable.

[0028] For a symmetrical distribution of the ink, the supply jet and the suction jet are advantageously concentric.

[0029] For obtaining a uniform distribution of the ink flow, the supply jet can be circular and the suction jet can be annular, and the suction jet surrounds the supply jet.

[0030] It has to be noted that the printhead can comprise further nozzles than the "multiple nozzles" for which protection is claimed, and such further nozzles do not necessarily need to fulfill the above conditions.

[0031] The invention also relates to a method for operating such a printhead. The method comprises at least the following steps:

[0032] - supplying ink to the nozzles by means of an ink supply conduit.

[0033] - withdrawing ink from the nozzles by means of an ink suction conduit.

[0034] This allows to maintain a flow of ink between the supply jet and the suction jet, thereby reducing the risk of forming stagnant ink. BRIEF DESCRIPTION OF DRAWINGS

[0035] The invention will be better understood and purposes other than those set forth above will become apparent when consideration is given to the following detailed description of the invention and the attached drawings, in which:

[0036] Figure 1 a schematic partial cross-sectional view of a printhead and a target is shown,

[0037] Figure 2 is a vertical cross-sectional view through a first embodiment of a nozzle and surrounding parts, the ink flow being indicated by arrows,

[0038] Figure 3 It is along Figure 2 Horizontal section view of line III-III

[0039] Figure 4 It is along Figure 2 Horizontal cross-section of line IV-IV

[0040] Figure 5 Only shown Figure 2 The nozzle,

[0041] Figure 6 This is a vertical cross-sectional view of the second embodiment of the nozzle.

[0042] Figure 7 This is a vertical cross-sectional view of the third embodiment of the nozzle.

[0043] Figure 8 It is along Figure 7 The horizontal cross-sectional view of line VIII-VIII.

[0044] Figure 9 This is a vertical cross-sectional view of the fourth embodiment of the nozzle.

[0045] Figure 10 This is another embodiment of the printhead, and

[0046] Figure 11 It shows Figure 10 Horizontal cross-sectional views of layers A2, A3 and M1-M4 of the device.

[0047] Note: The accompanying drawings are not to scale. In particular, in most of the drawings, the extension of the component along the injection direction X is exaggerated compared to its extension perpendicular to the injection direction X. See the section on nozzle geometry below for more details. Detailed Implementation

[0048] definition

[0049] "Forward" defines the direction in which the printhead is designed to eject ink. For example, the ejection electrode moves forward from the nozzle.

[0050] "Backward" defines the opposite direction. For example, the nozzle is arranged to face backward from the injection electrode.

[0051] "In front" and "in back" are understood to indicate a position horizontally in front of or behind other things.

[0052] "Front" and "back" refer to the sides that face forward and backward, respectively.

[0053] The ejection direction X of the printhead defines a "vertical" upward direction, i.e. by definition the printhead is designed to eject ink upward. (In operation it can of course be at any angle to the direction of gravity.) Definitions such as "up" and "down" should therefore be understood with reference to the definition of "vertical".

[0054] "Horizontal" is any direction perpendicular to the vertical direction.

[0055] "Lateral" refers to something that is horizontally offset from something else.

[0056] The "nozzle" of a tube is the opening at the end of the tube.

[0057] Advantageously, the nozzle is circular to distribute the ink evenly. However, for non-circular nozzles, the "diameter" of the nozzle is defined as the largest diameter of the nozzle.

[0058] When referring to the pressures applied to the supply and suction nozzles, reference is made to the pressures pi and p0 in the reservoir 22 and suction tank 28, respectively, assuming that the pressure drop in the conduits leading to and from the individual nozzles can be neglected, and that all significant pressure drop occurs at the individual nozzles. If the pressure drop in the conduits leading to and from the individual nozzles is not negligible, the pressure values must be corrected accordingly.

[0059] Printhead

[0060] Figure 1 A schematic cross-sectional view of an embodiment of a printhead 1 is shown. It is configured to eject ink along an ejection direction X onto a target 2.

[0061] Printheads of this type have been described in more detail in WO 2022 / 152380 Al, see Figure 1 and the corresponding description there.

[0062] The printhead comprises a plurality of nozzles 4 located at a front side of a nozzle carrier 6. The nozzles 4 can be arranged in a one- or two-dimensional array.

[0063] The printhead has a plurality of ejection electrodes (not shown in Figure 1 ) for ejecting ink from the nozzles 4 along the ejection direction X, and optional further electrodes arranged on the support structure 8, the design of which is described in more detail below. Further electrodes can be provided in electrical contact with the ink to set the ink to a defined potential.

[0064] The nozzle carrier 6 comprises a front layer 10 to which the nozzles 4 are mounted at a front side and form protrusions thereon. It further comprises a back layer 12 located at a back side of the front layer 10.

[0065] The internal structure of the front layer 10 is described in Figure 1The back layer 12 can be of an insulating semiconducting material, or it can be of a dielectric, in particular a polymer.

[0066] The back layer 12 can be of an insulating semiconducting material, or it can be of a dielectric, in particular a polymer.

[0067] The electrical vias 14 are connected to the ejection electrodes and extend through the front layer 10 and the back layer 12 for connecting the ejection electrodes to a voltage source 17. Advantageously, at least one via 14 is provided per nozzle 4. Further vias can be provided to connect further electrodes to the voltage source 17. Alternatively, part or all of the supply lines can be located at the front side of the printhead.

[0068] The ink conduits 15, 16 supply ink to the nozzles 4 and recover ink from the nozzles 4. They are partly located in the front layer 10 and they extend through the peripheral area of the back layer 12. Their design is described in more detail below.

[0069] Figure 1 An embodiment of a printhead is shown, which has a supply conduit 15 for ink and a suction conduit 16.

[0070] At least one pump 18 and / or a further pressure or vacuum source is provided to supply ink to the supply conduit 15 and to recover ink from the suction conduit 16.

[0071] Advantageously, the printhead comprises a first pressure controller 20 for generating a first defined pressure pi at the input of the supply conduit 15, for example in a reservoir 22.

[0072] The ink is supplied to the nozzles 4 through an optional filter 24 and the supply conduit 15.

[0073] The suction conduit 16 is connected to a suction system, which can comprise a second pressure controller 26 for generating a second defined pressure p0 at the outlet of the suction conduit 16, for example in a suction tank 28. The suction system can also comprise a pump. This can in particular be the pump 18 as described above, in which case the pump 18 functions as a circulation pump.

[0074] The pump 18 can be controlled by level sensors in the reservoir 22 and the suction tank 28.

[0075] As Figure 1 As further shown in the figures, the printhead can comprise a circuit carrier 30, for example a PCB, which is arranged at the back side of the nozzle carrier 6.

[0076] An optional interposer layer 32 may be disposed between the circuit carrier 30 and the nozzle carrier 6 to match the denser resolution of the vias 14 with the circuit resolution of the circuit carrier 30. This interposer layer is used, for example, in flip-chip designs where semiconductor chips are applied to a PCB.

[0077] The circuit carrier 30 carries the control circuit 33, which can, for example, implement at least a portion of the voltage source 17, such as a driver stage of the voltage source, which connects the voltage source to the various electrodes of the printhead.

[0078] In the illustrated embodiment, ink conduits 15, 16 extend through the intermediary layer 32 (if present) and the circuit carrier 30.

[0079] If the vias 14 have a sufficiently large mutual spacing (e.g., greater than 0.4 mm), they can directly interface with the circuit carrier 30 without the need for the interposer layer 32.

[0080] Advantageously, the target 2 is arranged on the accelerating electrode 34, which is connected to the voltage source 17 to generate an accelerating electric field between the printhead 1 and the target 2.

[0081] As described below, pressure controllers 20 and 26 can be used to maintain pressure. Advantageously, they allow for the separate adjustment of pressure levels in the supply conduit 15 and the pressure conduit 16.

[0082] Nozzle design

[0083] Figures 2-5 A first embodiment of nozzle 4 and surrounding elements is shown. (As described above, with...) Figure 1 on the contrary, Figure 2 The jet direction X points upwards.

[0084] from Figure 2 As can be seen, nozzle 4 forms a protrusion on the front side 36 of nozzle carrier 6, for example, on the front side of its front layer 10. It is located at the outlet channel 5, through which ink can be ejected toward target 2.

[0085] Figure 2 Various electrodes that can be associated with nozzle 4 are also shown.

[0086] The injection electrode 38 is located on the front side of the nozzle 4. Figure 2 In this embodiment, it is annular with a central opening 39 through which the ejected ink passes. It is connected to one of the through-holes 14 extending through the support structure 8 and the nozzle carrier 6. Regarding the through-hole 14 (which has been...) Figures 2-4 For more details on the design of the injection electrode (omitted), refer to WO2022 / 152380A1 and section “Nozzle Design 1” of that document.

[0087] A shield electrode 40 can be located at the front side of the jet electrode 38 at a distance from the jet electrode 38, i.e. the jet electrode 38 is closer to the nozzle carrier 6 than the shield electrode 40. Advantageously, there is one continuous shield electrode 40 extending over the front of the printhead 1, but there can also be several such shield electrodes. For details, again refer to WO 2022 / 152380 Al.

[0088] The shield electrode 40 is provided to control the field between the printhead 1 and the target 2. For each nozzle 4, an opening 41 in the shield electrode 40 allows the jetted ink to pass through.

[0089] As Figure 2 indicated, a guard electrode 42 can be located at the back of the jet electrode 38 at a distance from the jet electrode 38, but in front of and at a distance from the nozzle carrier 6. It can also be ring-shaped. Alternatively, it can also extend over several nozzles.

[0090] An opening 43 in the guard electrode 42 above the nozzle 4 allows the jetted ink to pass through.

[0091] The function of the guard electrode 42 is described below.

[0092] The nozzle 4 of this embodiment comprises a tip section 46 and a base section 50 Figure 3 , the tip section 46 being arranged in front of the base section 50.

[0093] The nozzle 4 further comprises an ink supply tube 54 and an ink suction tube 55, which extend coaxially in the jetting direction X of the printhead.

[0094] The ink supply tube 54 terminates at its front end in a front supply jet 54a. In the embodiment shown, the front supply jet 54a is circular. The ink supply tube 54 is connected to the supply conduit section 15a of the supply conduit 15 and directly feeds ink to the front end of the nozzle 4.

[0095] The ink suction tube 55 terminates at its front end in a front suction jet 55a. In the embodiment shown, the front suction jet 55 is ring-shaped and arranged around the supply jet 54a. The ink suction tube 55 is connected to the suction conduit section 16a of the suction conduit 16 and directly withdraws ink from the front end of the nozzle 4.

[0096] Advantageously, for all embodiments shown here, the supply tube 54 and the suction tube 55 are cylindrical and concentric.

[0097] The base section 50 connects the tip section 46 to the nozzle carrier 6. It is surrounded by the ring 52.

[0098] The base section 50 mechanically connects the supply tube 54 and the suction tube 55 to the carrier 6, while providing the connection between the supply conduit section 15a and the supply tube 54 and between the suction conduit section 16a and the suction tube 55.

[0099] In Figure 2 and Figure 3 the suction conduit section 16a is shown to consist of several individual conduits, for example four conduits. This number can vary. As shown in the "Further Embodiments" section below, there can also be only a single annular suction conduit section 16a.

[0100] Furthermore, as shown in Figure 2 and Figure 3 one or more (optional) radial lateral outlet conduits 56 can extend laterally from the supply tube section 15a to the suction conduit section 16a, transverse to the jet direction X.

[0101] An annular groove 58 is formed between the base section 50 and the ring 52.

[0102] In the embodiments of Figure 2 and 3 the lateral conduits 56 extend through the base section 50.

[0103] The lateral conduits 56 are useful when initially filling the suction conduit section 16a and the overlying suction jet 55a with ink. If the ink is initially supplied only through the supply conduit section 15a, and if there are no lateral outlet conduits 56, the ink will need to flow from the supply jet 54a to the suction jet 55a, which can not happen without pressurizing the ink supplied through the supply conduit section 15a, as an ink wetting line can form at the wall of the ink supply tube 54, where the ink supply tube 54 can act as a retainer preventing further spreading of the ink. To push the ink past the wall of the ink supply tube, the surface energy of this ink wetting interface needs to be overcome by pressurizing the ink. Once this "activation energy" is provided, the ink can rapidly propagate into the suction jet 55a and in fact create such a high flow that it overcomes the suction tube 55 and floods the plenum 71. During this filling process, applying a partial vacuum to the suction conduit section 16a can help prevent this flooding, but applying a partial vacuum to the suction conduit section 16a introduces an air flow into the suction jet 55a. If the ink reaches the supply jet 54a, and if this ink contains volatile components, this air flow can cause the ink to dry almost immediately. By introducing the lateral conduits 56, the ink can be introduced from below into the suction jet 55a by virtue of the ink automatically flowing through the lateral conduits 56 and from there up to the suction jet 55a.

[0104] Advantageously, the transverse conduit 56 has a smaller cross section than the supply pipe section 15a and the suction conduit section 16a, so that the pressure drop between the supply pipe section 15a and the suction conduit section 16a mainly occurs within the transverse conduit 56, and for example the pressure in the suction conduit section 16a is substantially not influenced by the pressure applied to the supply pipe section 15a.

[0105] The ring 52 forms an ink holder, the purpose of which is to laterally hold the ink, similar to the ink holder 66 described in WO 2022 / 152380 A1, and it can be provided with the additional features described therein, such as a hydrophobic or oleophobic coating and / or a ledge facing away from the nozzle 4, and it is advantageously shielded by the guard electrode 42.

[0106] The guard electrode 42 is located in front of the front end of the nozzle 4 (i.e. further away from the nozzle carrier 6 than the front end of the nozzle 4), or at the same height as the front end of the nozzle 4, thereby reducing the electric field in the front lateral circumferential area of the nozzle 4.

[0107] As can be seen from Figure 2 , a gas-filled cavity 71 forming a spacing is provided between the guard electrode 42 and the ink holder 66, which prevents the ink from reaching the guard electrode 42.

[0108] Figure 2 A support structure 8 is also shown, which connects the individual electrodes 38, 40, 42 to the nozzle carrier 6. It is arranged on the front side 36 of the nozzle carrier 6 and comprises a plurality of support elements 76, 78 as well as electrode carrier layers 80, 82, 84, which are described in WO 2022 / 152380 using the same reference numerals, and the description of these parts is hereby incorporated by reference.

[0109] Ink control

[0110] As mentioned above, the supply conduit 15 and the suction conduit 16 are provided to supply the nozzles 4 with ink and to recover ink from the nozzles. This allows a fresh flow of ink to be maintained at each nozzle 4.

[0111] In the embodiment of Figures 2-5 , the pressures at the supply conduit 15 and the suction conduit 16 are adjusted to produce ink surfaces 90a, 90b as indicated by the dotted lines in Figure 2 and 5 .

[0112] As shown, the ink forms a central apex 92 in front of the center of the nozzle 4. This apex 92 can be produced by adjusting the pressures and flows at the supply orifice 54a and the suction orifice 55a. It can also be controlled by adding a bias voltage to the firing electrode 38, which is sufficient to pull the ink forward, but not sufficient to eject ink droplets.

[0113] During operation, the pressure p1 applied to the suction nozzle 55a is advantageously lower than the ambient atmospheric pressure, thereby preventing ink from overflowing the inflation chamber 71. The pressure p1 applied to the supply nozzle 54a can be higher or lower than the ambient atmospheric pressure, but higher than pressure p0, thereby generating an ink flow directed from the supply nozzle 54a to the suction nozzle 55a. The levels of pressures p0 and p1 control not only the recirculation flow but also the flow rate directed onto the substrate 2 during printing. For example, p1 can be set to -50 mbar or 0 mbar relative to atmospheric pressure, while p0 can be set to -100 mbar relative to atmospheric pressure. If p1 is -50 mbar, both the printing and circulation flow rates will be lower than when p1 is set to 0 mbar. However, the details of the applied pressure depend on many parameters, which will be explained in more detail in the "Ink Viscosity" section.

[0114] exist Figures 2-5 In this embodiment, the pressure and flow of ink in the supply tube 54 and the suction tube 55 are adjusted such that not all the ink supplied through the supply tube 54 is drawn back by the suction tube 55. Instead, a portion of the ink overflows from the leading edge 55b of the suction tube 55 and flows back into the groove 58, thereby forming an inclined ink surface as shown in line 90a. This prevents ink from stagnating at the leading edge 55b. This process may also be affected by the wetting properties of the ink and capillary forces, for example, by appropriately coating the surface of the component, for example, with a hydrophobic or oleophobic or hydrophilic or oleophilic coating, depending on the ink used.

[0115] The groove 58 is connected to the suction conduit section 16a, and thus the excess ink is drawn back from the nozzle 4.

[0116] Therefore, in an advantageous embodiment, the method for operating the printhead includes the following steps:

[0117] -More ink is supplied via the supply tube 54 than via the suction tube 55, even when no printing occurs.

[0118] -Retract some ink using the suction tube 55.

[0119] - Allow excess ink (the difference between the supplied ink and the ink drawn back through the suction tube 55) to flow back along the outside of the suction tube 55, and

[0120] - Excess ink is collected in the groove 58 surrounding the rear base of the nozzle 4 and drawn back through the suction conduits 16a, 16.

[0121] In the embodiment shown, the ink flow is kept above both the front edge 54b of the feed tube 55 and the front edge 55b of the suction tube 55, thereby avoiding that ink stagnates at a location where the drying of the ink can strongly influence the formation of ink drops. As explained below, in other geometries, the ink flow is kept above at least one of the front edges 54b, 55b. Advantageously, therefore, the method for operating the printhead comprises the step of transporting the ink above the front edge 54b, 55b of at least one of the feed tube 54 and the suction tube 55.

[0122] Figure 6 A second embodiment of ink control is shown. Here, no ink flows past the front edge 55b of the suction tube 55. Instead, the ink is laterally retained at the front edge 55b. To support this retention, the front edge 55b can be equipped with a flange 96 protruding laterally over the rest of the outer surface of the suction tube 55.

[0123] Therefore, in this embodiment, the method for operating the printhead comprises the step of recovering all the ink fed by the feed tube 54 through the suction tube 55 if no printing occurs.

[0124] Figure 6 It is also shown that, if no ink flows past the front edge 55b of the suction tube 55, the wide cavity 71 shown in Figure 2 can be dispensed with. Instead, the support element 78 of the electrode can be positioned closer to the nozzle 4. (Note: for simplicity, Figure 6 Only the guard electrode 42 is shown, without showing any structure from it forwards.

[0125] Nozzle geometry

[0126] Figures 2-5 An embodiment is shown that illustrates another advantageous aspect of the present technology, already mentioned above. It can be seen that both the feed jet 54a and the suction jet 55a are close to the front end of the nozzle 4.

[0127] As Figure 5 best shown in

[0128] Although in the embodiment of Figure 5 the feed jet 54a and the suction jet 55a are arranged at the same height, they can also be located at different heights.

[0129] For example, in Figure 6In an embodiment of the nozzle 4, the foremost portion 94 is formed by the front end 55b of the suction tube 55, while the front end 54b of the supply tube 54 is arranged a distance di rearward (in the direction X). This makes it easier to immerse the front end 54b of the supply tube 54, to prevent dry ink from depositing there.

[0130] However, the distance di is advantageously small compared to the diameter Dsof the suction jet 55a, in order to easily form the apex 92 by means of the ink flow that surges forward from the supply jet 54a. In terms of quantity, the distance di is advantageously less than half the diameter Dsof the suction jet 55a.

[0131] The distance di can also be chosen in relation to the maximum size of the particles present in the ink. Advantageously, di should increase to at least the same size as the particle diameter. In absolute terms, the distance di is advantageously no more than 100 microns for a typical ink.

[0132] However, alternatively, as shown in Figure 9 the foremost portion 94 of the nozzle 4 can also be formed by the front end 54b of the supply tube 54, while the front end 55b of the suction tube 55 is arranged a distance d2 rearward (in the direction X). This makes it easier to maintain the apex 92.

[0133] However, the distance d2 is also advantageously small compared to the diameter Dsof the suction jet 55a, in order to maintain a stable, predictable ink flow at the front end of the nozzle 4. In terms of quantity, the distance d2 is advantageously less than half the diameter Dsof the suction jet 55a.

[0134] Figure 9 The version of the nozzle 4 of the type shown can in particular be used with inks that contain no particles or only very small particles, and inks that are not prone to evaporation and clogging.

[0135] Figure 7 and 8 Another means for forming and controlling the apex 92 of the ink surface is shown. Here, the foremost portion 94 of the nozzle 4 is formed by a nozzle tip 98 in addition to the supply tube 54 and the suction tube 55. The nozzle tip 98 is arranged at the center of the supply jet 94a.

[0136] The nozzle tip 98 comprises a tip section 100 that is mounted on a stem section 102, which is advantageously tapered toward its front end, the stem section 102 being arranged concentrically within the supply tube 94. The stem section 102 rests on a support 104 of the base section 50. Alternatively or in addition, the stem section 102 can be connected to the supply tube 54 using lateral supports (not shown).

[0137] Along the direction X, the nozzle tip 98 is located at a distance d5 in front of the suction jet 55a and at a distance d6 in front of the supply jet 54a. Advantageously, they are small compared to the diameter of the jets, so that it is easier to immerse the nozzle tip 98 in the ink.

[0138] This is not only the case for the embodiment of Figure 7 but also for the other embodiments. Along the direction X, the distance between the frontmost part 94 of the nozzle 5 and the suction jet 55a and the distance between the frontmost part 94 of the nozzle 5 and the supply jet 54a are advantageously not more than 50 micrometer.

[0139] As already mentioned, the various different distances between the components are advantageously adapted to the size parameters of the nozzle 4, in particular to the diameter Df of the supply jet 54a and / or to the diameter Ds of the suction jet 55a. This makes it easier to prevent stagnant ink portions at the front end of the nozzle 4.

[0140] In particular, the distance from the frontmost part 94 of the nozzle 4 to the supply jet 54a (zero in Figure 5 , equal to d1 in Figure 6 , equal to d6 in Figure 7 , zero in Figure 9 ) should be smaller than the diameter Df of the supply jet 54a, in particular smaller than half the diameter Df of the supply jet 54a.

[0141] The distance from the frontmost part 94 of the nozzle 4 to the suction jet 55a (zero in Figure 5 and 6 , equal to d5 in Figure 7 , equal to d2 in Figure 9 ) should be smaller than, in particular smaller than half the difference Ds-Df between the diameter Ds of the suction jet 55a and the diameter Df of the supply jet 54a.

[0142] As already mentioned, most of the figures show the extension of the components along the jetting direction X in an exaggerated manner.

[0143] Figure 9 More to scale (however, the following applies to all embodiments). The height H of the nozzle along the jetting direction X from the nozzle carrier 6 to its frontmost part 94 is advantageously smaller than the diameter Ds of the suction jet 56, in particular H < Ds / 2.

[0144] Advantageously, H is between 5 micrometer and 50 micrometer and Ds is between 20 micrometer and 500 micrometer. However, these parameters depend on the viscosity of the ink, see next section.

[0145] Ink viscosity

[0146] The dimensions of the nozzle geometry are advantageously optimized in view of the expected ink viscosity and the required printing resolution.

[0147] The viscosity between different inks can vary by many orders of magnitude, and the flow characteristics inside the nozzle will be strongly affected by it. For example, in a tubular flow channel, the volumetric flow rate of the ink is linearly proportional to the applied pressure difference (~p), linearly inversely proportional to the viscosity (~1 / η), and inversely proportional to the fourth power of the tube radius (~1 / r 4 ). Thus, for a given tube diameter (e.g. the diameter of the ink feed tube 54), the volumetric flow rate of an ink with a viscosity of 10 Pa-s will be one thousand times lower than the volumetric flow rate of an ink with a viscosity of 10 mPa-s. Or, in other words, if the drop size of both inks is the same, the higher viscosity ink will result in a one thousand times lower ejection frequency.

[0148] However, in general, the resolution requirements of the industry will be lower for high viscosity inks than for low viscosity inks, while at the same time the volume throughput requirements will be higher for high viscosity inks than for low viscosity inks.

[0149] Therefore, from an industrial application point of view, the above scaling behavior is not optimal. As described herein, this dilemma can be offset by increasing the geometry of the nozzle (e.g. the diameter of the ink feed tube 54) and potentially by increasing the pressure difference between p0and p1and the absolute pressure.

[0150] For example, for a 10-fold increase in the diameter of the feed tube 54, one would expect a 10,000-fold increase in the volumetric flow rate (in view of the above 1 / r 4 scaling law). For a 10-fold increase in the nozzle size, one would also expect a roughly 10-fold increase in the drop size, i.e. a 1,000-fold increase in the volume carried by each drop. Thus, at this adjusted drop size, one would expect a ten-fold higher ejection frequency. However, as the drop size gets larger, the electric stress gets lower, and thus this value can be lower. The electric stress generated by the applied voltage is proportional to the Laplace pressure inside the ejected drop and inversely proportional to the drop radius (~1 / r). Because the generated electric stress must be equal to or larger than the static Laplace pressure, the electric stress is also proportional to (~1 / r). Thus, for a 10-fold increase in the nozzle size, one would expect a 10-fold decrease in the voltage. Whether this decrease in electric stress also results in a 10-fold decrease in the volumetric flow rate depends on the value applied to the feed tube 54 in particular.

[0151] Importantly, in embodiments where the combination of supply and suction is absent from the device, the pressure within the supply tube 54 is strongly limited by the Laplace pressure of the ink at the nozzle outlet. If too much pressure is applied, the ink will simply be pushed out of the nozzle and drop-on-demand operation is essentially impossible. In the present device, however, the pressure within the supply tube 54 can be significantly increased, because any liquid pushed out of the nozzle can be carried away by the suction tube 55, so the system can reach an equilibrium state without flooding the printhead with ink. This is highly advantageous, especially for large nozzles where the electrical stress is typically below 100 mbar. For conventional nozzles, this would result in a slow response of the ink to the applied voltage. Especially for high viscosity inks, the delay between voltage application and printing can be in the range of several seconds. With the present invention, the ink flow can be driven at a pressure above the electrical stress, and this allows to reduce the response time of the ink to any electrical excitation by a factor of 10 or more and makes the nozzle compatible with drop-on-demand operation.

[0152] It should be noted, however, that in case the system is operated under atmospheric conditions, the maximum pressure that can be applied to the suction tube 55 is 1 atmosphere (atm) below atmospheric pressure (corresponding to a perfect vacuum). The maximum pressure that can be applied to the suction tube 55 also limits the pressure that can be applied to the supply tube 54. In case the flow resistance of the supply tube 54 and the suction tube 55 are equal, the maximum pressure that can be applied to the supply tube 54 is also about 1 atm above atmospheric pressure, otherwise the ink would flood the printhead. If the flow resistance in the suction tube 55 (including all subsequent suction conduits) is smaller than the flow resistance in the supply tube 54 (including all supply conduits), the pressure that can be applied to the supply tube 54 can be more than 1 atm above atmospheric pressure. It is, however, advantageous that the pressure applied to the supply tube is below 10 bar.

[0153] In the present example, a pressure of 10 mbar applied to the supply tube 54 can be a good operating pressure in case of a low viscosity ink with a viscosity of 10 mPa-s, because the electrical stress for a corresponding small nozzle will be strong enough to drive the ink flow by itself.

[0154] For a high viscosity ink with a viscosity of 10 Pa-s in a nozzle that is ten times larger, a higher value of the pressure pi of the supply tube 54 is desirable, for example a pressure of 1 bar. Finally, using this pressure in the present example, when comparing the 10 Pa-s ink to the 10 mPa-s ink, the nozzle diameter is increased by a factor of 10, while the droplet diameter is increased by the same factor, which will result in a decrease of the ejection frequency by a factor of 10. For example, this decrease can be from 100 kHz to 10 kHz.

[0155] If higher flow rates are required, the size of the nozzle can be further increased. It is worth noting that for large nozzles, the reduction in volumetric flow rate and the corresponding reduction in the size ratio electrostress due to further increases in diameter are generally no longer relevant, since the pressure applied to the feed pipe 54 is expected to be significantly greater than the pressure from the electrostress, so the volumetric flow rate will be driven primarily by the pressure applied to the feed and suction pipes.

[0156] Therefore, it is advantageous for the diameter of the nozzle to be adjusted according to the viscosity of the ink expected.

[0157] For inks with a viscosity lower than 100 mPa s, the maximum diameter Df of the feed pipe 54 should not exceed 50 microns, advantageously it should not exceed 20 microns.

[0158] For inks with a viscosity higher than 1 Pa s, the diameter Df of the feed pipe 54 should not exceed 500 microns, advantageously it should not exceed 200 microns, but it should be at least 20 microns, in particular at least 50 microns.

[0159] Therefore, in an advantageous embodiment, the method for operating the present print head comprises the step of printing an ink with a viscosity lower than 100 mPa s, wherein the diameter Df of the feed pipe 54 is less than 50 microns, in particular less than 20 microns.

[0160] On the other hand, advantageously, the method comprises the step of printing an ink with a viscosity higher than 1 Pa s, wherein the diameter Df of the feed pipe 54 is at least 20 microns, in particular at least 50 microns.

[0161] Any other size parameter of the nozzle can be adjusted proportionally according to the same procedure.

[0162] As for the diameter Ds of the suction pipe 55, it is approximately linearly proportional to the diameter Df of the feed pipe 54, if the thickness of the various walls is neglected. The same applies to all the other size parameters of the nozzle, in particular the parameters dl, d5 and d6.

[0163] In order to be able to draw back all the ink fed through the feed pipe 54, the cross section Ss offered by the suction pipe 55 to the ink is advantageously at least equal to the cross section Sf of the feed pipe 54. However, it is advantageous for it to be greater, at least 1.5 times greater, since the annular geometry of the ink space in the suction pipe 55 produces a greater flow resistance for a given cross section compared to the flow resistance of a circular cross section of the same area. In other words, advantageously Ss > Sf, in particular Ss > 1.5 Sf. Another reason for making the ink cross section Ss of the suction pipe 54 greater is that, as mentioned above, the suction pressure p0 is limited to be lower than atmospheric pressure 1 atm.

[0164] In Figure 9In the example of Fig. 2, for rotational symmetry, the cross section Sf of the feed pipe 54 is

[0165] Sf = Df 2 · π / 4.(1)

[0166] The cross section Ss of the suction pipe 55 is

[0167] Ss = (Ds 2 – Dx 2 ) · π / 4.(2)

[0168] where Dx is the outer diameter of the feed pipe 54 (see Fig. 2). Figure 9 ).

[0169] However, advantageously, the suction jet 55a should not be too large in order to maintain a small meniscus and a high printing resolution. Therefore, advantageously, the cross section Ss of the suction pipe 55 for the ink is advantageously less than twice the cross section Sf of the feed pipe 54 for the ink.

[0170] Recirculation rate

[0171] Another parameter that can be used to optimize the printing performance is the recirculation rate, i.e. the flow rate between the feed jet 54a and the suction jet 55b.

[0172] In the present device, where both jets are close to the front end of the nozzle 4, the recirculation occurs in the region of the ink meniscus, for example compared to the printhead of WO2022152380A1. This allows not only to reduce the drying problems due to solvent evaporation, but also to produce ink drops or jets faster when an electric field is applied to the meniscus by the ejection electrode, because the ink is already in motion.

[0173] Therefore, advantageously, the method for operating the printhead comprises, while printing, advantageously continuously, feeding the ink from the feed jet 54a to the suction jet 55b, i.e. while producing ink drops and / or ink jets by means of the ejection electrode 38 associated with the nozzle 4.

[0174] The recirculation rate can be influenced by the pressure pi applied to the feed conduit 54 and the pressure p0 applied to the suction conduit 55, and it increases with the difference P1-P0.

[0175] Furthermore, the absolute values of the pressures p0 and pi, not only their difference, also influence the flow rate. By increasing these pressures, for example by increasing the value (p0 + pi) / 2, while keeping the pressures low enough to prevent the meniscus from overflowing the front edge of the nozzle, the volume of the meniscus and the flow cross section increase, which reduces the flow resistance through the meniscus and thus increases the recirculation rate.

[0176] Note that the nozzle can also be operated when both p0 and p1 are below atmospheric pressure, as long as the capillary force in the nozzle is sufficient to pull the ink to the tip of the nozzle.

[0177] Therefore, advantageously, the method for operating the printhead may further include the step of selecting the difference and / or sum of the pressures p0, p1 applied to the supply tube 54 and the suction tube 55 according to the viscosity of the ink to be printed, so as to optimize droplet formation as described above.

[0178] Further Examples

[0179] Figure 10 and 11 A further, more detailed embodiment of the printhead is shown, in which... Figure 11 It shows along Figure 10 Cross-sectional views of each layer A2, A3, M1, M2, M3, and M4.

[0180] Here, layers A2 and A3 are the two topmost layers of the nozzle carrier 6. They form the topmost portions of the supply conduit section 15a and the suction conduit section 16a.

[0181] Layer A3 further forms a transverse outlet conduit 56 (in Figure 10 (They are not visible because they extend perpendicular to the cross-sectional plane). The transverse outlet conduit 56 is long and narrow to provide high flow resistance, for reasons mentioned in the "Nozzle Design" section above.

[0182] Layer M1 covers the transverse outlet conduits 56 from above to keep their cross-sections small. It also forms the bottom portion of the groove 58 and the rear segment 52a of the ring 52.

[0183] Layer M2 defines the supply pipe 54 and the suction pipe 55. Alternatively, it can form the front section 52b of the ring 52, thereby extending the groove 58 forward.

[0184] Layer M3 extends the suction tube 55 forward but not the supply tube 54; that is, in this embodiment, the supply nozzle 54a is located behind the suction nozzle 55a.

[0185] Layer M4 forms the end 100 and the substrate for protecting the electrode 42. The end 100 tapers towards its leading edge. This shape can be formed, for example, by applying a photosensitive layer to the top of layer M4 and then 3D structuring the photosensitive layer using grayscale lithography. Subsequently, the 3D structure is transferred to layer M4 by dry etching.

[0186] Notice

[0187] It can be seen that in all current embodiments, the supply jet 54a and the suction jet 55a are arranged in front of the nozzle carrier 6, i.e. they form part of the protruding section of the nozzle 4. This allows shaping and controlling the front surface 90b of the ink at a distance from the nozzle carrier 6 and better laterally holding the ink.

[0188] Furthermore, due to the partial vacuum being applied to the suction jet 55a, the front surface 90b of the ink becomes flatter in the area of the suction jet 55a than it does in the area of the supply jet 54a, which means that the shape of the front surface 90b of the ink in its appearance approximates the case where only the supply jet 54a is present, i.e. without the suction tube 55, as in WO 2022 / 152380 A1. This means that the increase in the diameter of the nozzle 4 due to the addition of the suction tube 55 does not result in the nozzle 4 obtaining a significantly worse print resolution than without such a suction tube 55.

[0189] Furthermore, as shown, at least the suction jet 55a is advantageously set back a distance d3 from the protection electrode in the direction X, as Figure 6 shown, i.e. the protection electrode 42 is arranged between the ejection electrode 38 and the suction jet 55b in the direction X. This allows the front edge 55b of the suction tube 55 to be at least partially shielded from the electric field of the ejection electrode 38. The distance d3 can also be zero, i.e. the front edge 55b of the suction jet 55a is at the same height as the protection electrode 42.

[0190] However, it is advantageous for the lateral distance d4 between the front edge 55b and the protection electrode 42 to be small in order to reduce the field strength at the front edge 55b. Advantageously, d4 is not greater than half the difference Ds - Df, i.e. d4 < (Ds - Df) / 2 (see Figure 9 ). In this case, the protection electrode 42 strongly couples to the front edge 55b but weakly to the front edge 54b, i.e. it prevents the ejection voltage from affecting the ink at the front edge 55b of the suction jet 55a without significantly affecting the ejection of the ink at the front center of the nozzle.

[0191] The relationship between the distance d4 and Ds - Df is particularly important in the embodiments shown in the figures, in which the outer diameter Do of the suction tube 55 is smaller than the inner diameter Dg of the protection electrode 42, since in this case the protection electrode 42 cannot cover the suction tube 55 from above.

[0192] As mentioned above, the supply tube 54 and the suction tube 55 are advantageously cylindrical and concentric. However, they can also have a non-circular cross-section, for example a hexagonal or octagonal cross-section. However, a rounded geometry is better in order to avoid areas of field elevation and to achieve a symmetrical, stable front surface 90b of the ink.

[0193] The supply pipe 54 needs to transport ink to the front end of the nozzle 4, and therefore it advantageously has an axial inner space 110 Figure 5 which extends from the base of the nozzle 4 towards the supply spout 54a and is laterally enclosed by walls on all sides.

[0194] Similarly, advantageously the suction pipe 55 forms a space 112 between the enclosing outer wall and the supply pipe 54, which extends between the base of the nozzle 4 and the suction spout 55a.

[0195] As indicated by the dashed lines in Figure 7 , radial struts 108 can extend between the supply pipe 54 and the suction pipe 55 in order to mechanically mount them to each other. Although only shown in Figure 7 , such struts can be used in any of the embodiments shown here. However, advantageously the struts 108 do not extend all the way up to the suction spout 55a in order to have a more uniform ink distribution at the front end of the nozzle 4.

[0196] While the current preferred embodiments of the application have been shown and described, it is to be clearly understood that the application is not limited to the same but can be variously otherwise embodied and practiced within the scope of the following claims.

Claims

1. An electrohydrodynamic printhead, comprising: Nozzle carrier (6), A plurality of nozzles (4) are arranged on the nozzle carrier (6), wherein each nozzle (4) forms a protrusion arranged on the front side (36) of the nozzle carrier (6) and extending into the jetting direction (X) of the printhead. A plurality of injection electrodes (38) are associated with the nozzle (4) and located on the front side of the nozzle (4). A plurality of ink supply conduits (15) for the nozzle (4), wherein at least one ink supply conduit (15) terminates at each nozzle (4). Multiple ink suction conduits (16), wherein at least one ink suction conduit (16) terminates at each nozzle (4), Each nozzle (4) includes: An ink supply tube (54), which is part of an ink supply conduit, terminates at its front end in a supply nozzle (54a), and An ink suction tube (55) is part of an ink suction conduit, and the ink suction tube terminates at its front end in a suction nozzle (55a). One of the nozzles (55a, 54a) laterally surrounds the other nozzle (54a, 55a). Wherein, along the jet direction (X), The distance from the foremost portion (94) of the nozzle (4) to the supply nozzle (54a) is less than the diameter (Df) of the supply nozzle (54a), and The distance from the foremost portion (94) of the nozzle (4) to the suction nozzle (55a) is less than the difference between the diameter (Ds) of the suction nozzle (55a) and the diameter (Df) of the supply nozzle (54a).

2. The printhead according to claim 1, wherein, Along the jet direction (X), the suction nozzle (55a) is at least as close as, and in particular closer to, the foremost portion (94) of the nozzle (4) than the supply nozzle (54a).

3. The printhead according to claim 1, wherein, Along the jetting direction (X), the suction nozzle (55a) and the supply nozzle (54a) are at the same distance from the foremost portion (94) of the nozzle (4).

4. The printhead according to claim 1, wherein, Along the jetting direction (X), the supply nozzle (54a) is closer to the foremost portion (94) of the nozzle (4) than the suction nozzle (55a).

5. The printhead according to any one of the preceding claims, wherein, Along the jetting direction (X), the front end (54b) of the ink supply tube (54) is positioned a distance d1 backward from the front end (55b) of the ink suction tube (55), wherein the distance d1 is less than half the diameter (Ds) of the suction nozzle (55a).

6. The printhead according to any one of the preceding claims, wherein, At least one of the suction nozzle (55a) and the supply nozzle (54a) forms the foremost portion (94) of the nozzle (4).

7. The printhead according to any one of claims 1 to 5, wherein, Each nozzle (4) also includes a nozzle tip (98) arranged forward from the center of the supply nozzle (54a) and forming the foremost portion (94) of the nozzle (4).

8. The printhead according to any one of the preceding claims, wherein, The supply nozzle (54a) and the suction nozzle (55a) are concentric.

9. The printhead according to any one of the preceding claims, wherein, The supply nozzle (54a) is circular, and the suction nozzle (55a) is annular.

10. The printhead according to any one of the preceding claims, wherein, Both the suction nozzle (55a) and the supply nozzle (54a) are arranged in front of the nozzle carrier (6).

11. The printhead according to any one of the preceding claims, wherein, Each nozzle (4) also includes a flange (96) disposed at the front end of the ink suction tube (55) at the suction nozzle (55a) and extending laterally away from the ink suction tube (55).

12. The printhead according to any one of the preceding claims, comprising a protective electrode (42), wherein, At a given nozzle (4), the protective electrode (42) is arranged between the injection electrode (38) and the suction nozzle (55a) along the injection direction (X), and the lateral distance (d4) between the leading edge (55b) of the suction nozzle (55a) and the protective electrode (42) is not greater than half the difference between the diameter (Ds) of the suction nozzle (55a) and the diameter (Df) of the supply nozzle (54a).

13. The printhead according to claim 12, wherein, The outer diameter (Do) of the ink suction tube (55) is smaller than the inner diameter (Dg) of the protective electrode (42).

14. The printhead according to any one of the preceding claims, wherein, Along the jetting direction (X), the distance (d1) between the supply nozzle (54a) and the suction nozzle (55a) is no greater than 100 micrometers.

15. The printhead according to any one of the preceding claims, wherein, Along the jetting direction (X), the distance between the foremost portion (94) and the suction nozzle (55a), and between the foremost portion (94) and the supply nozzle (54a), is no greater than 50 micrometers.

16. The printhead according to any one of the preceding claims, wherein, The ink supply tube (54) has an axial internal space (110) that extends from the base of the nozzle (4) to the supply nozzle (54a) and is laterally closed on all sides by the wall.

17. The printhead according to any one of the preceding claims, wherein, The ink suction tube (55) forms a space (112) between the wall and the ink supply tube (54), wherein the annular space extends between the base of the nozzle (4) and the suction nozzle (55a).

18. The printhead according to any one of the preceding claims, wherein, The cross-section (Ss) for ink provided by the ink suction tube (55) is larger than the cross-section (Sf) of the ink supply tube (54), especially by at least 1.5 times.

19. The printhead according to any one of the preceding claims, wherein, The suction nozzle (55a) is laterally positioned around the supply nozzle (54a).

20. A method for operating a printhead according to any one of the preceding claims, comprising the following steps: Ink is supplied to the nozzle (4) through the ink supply conduit (15), and Ink is drawn back from the nozzle (4) through the ink suction conduit (15).

21. The method of claim 20, further comprising the step of delivering ink onto the leading edge (54b, 55b) of at least one of the ink supply tube (54) and the ink suction tube (55).

22. The method according to any one of claims 20 or 21, comprising the following steps: More ink is supplied via the ink supply tube (54) than via the ink suction tube (55), even if no printing occurs. A portion of the ink is drawn back through the ink suction tube (55). Allow excess ink to flow back along the outside of the ink suction tube (55), and The excess ink is collected in a groove (58) around the rear base of the nozzle (4) and drawn back through the suction conduit (16a, 16).

23. The method according to any one of claims 20 or 21, comprising the following steps: If no printing occurs, all ink supplied through the ink supply tube (54) is recovered by means of the ink suction tube (55).

24. The method according to any one of claims 20 to 23, comprising the step of printing ink with a viscosity greater than 1 Pa·s, wherein the diameter (Df) of the ink supply tube (54) is at least 20 micrometers, particularly at least 50 micrometers.

25. The method according to any one of claims 20 to 24, comprising supplying ink from the supply nozzle (54a) to the suction nozzle 55b during printing.

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