Electrohydrodynamic print head with ink circulation

Abstract

An electrohydrodynamic print head comprises a nozzle carrier (6) and a plurality of nozzles (4) arranged on said nozzle carrier (6). Each nozzle (4) forms a projection arranged on a front side (36) of the nozzle carrier (6). A plurality of ink supply ducts (15) and suction ducts (16) feed ink to the nozzles and retrieve ink from the nozzles. Each nozzle (4) comprises an ink feed tube, as part of the ink supply ducts, that ends in a front feed muzzle (54a) and an ink suction tube (55), as part of the ink suction ducts, that ends in a front suction muzzle (55a) laterally surrounding the feed muzzle (54a). Along the ejection direction (X), the distance from the frontmost portion (94) of the nozzle to the feed muzzle (54a) is no more than half the diameter of the feed muzzle (54a) and the distance from the frontmost portion (94) of the nozzle to the suction muzzle (55a) is no more than half of the difference between a diameter of the suction muzzle (55a) and the diameter of the feed muzzle (54a). Placing both muzzles so close to the frontmost portion (94) of the nozzle is based on the understanding that this makes it possible to actively circulate the ink even in the frontmost parts of the nozzle, thereby preventing stagnant ink.

Description

[0001] Electrohydrodynamic print head with ink circulation

[0002] Technical Field

[0003] The invention relates to an electrohydrodynamic print head and to a method for operating the same.

[0004] Background Art

[0005] US 2018 / 0009223 describes an electrohydrodynamic print head having a nozzle carrier with a plurality of nozzles. It is designed to eject ink along an ejection direction. The nozzles form projections extending along this ejection direction. Ejection electrodes are associated with the nozzles and located on the target-side of the nozzles. Ink feed ducts feed ink to the nozzles.

[0006] In print heads of this design, ink may form pools that “submerge” the nozzles and disrupt the operation of the print head.

[0007] For this reason, WO 2022 / 152379A1 and WO 2022 / 152380A1 suggest using ink suction ducts, in addition to the ink feed ducts. The ink suction ducts are adapted to withdraw ink from the nozzles and to maintain an ink circulation in the print head.

[0008] Disclosure of the Invention

[0009] The problem to be solved by the present invention is to make this type of print head more reliable.

[0010] This problem is solved by the print head and method of the independent claims.

[0011] Accordingly, the electrohydrodynamic print head comprises at least the following elements:

[0012] - A nozzle carrier: This is a substrate on which the nozzles are arranged.

[0013] - A plurality of nozzles arranged on the carrier: Each such nozzle forms a projection (i.e., a protrusion) arranged on the “front side” of the nozzle carrier, which is the side that faces the print target during operation. The nozzle extends along the ejection direction of the print head, i.e., along the direction into which the print head is designed to eject ink. Advantageously, the nozzle extends parallel to the ejection direction, but it may also extend under a small angle, in particular under an angle smaller than 45°, thereto.

[0014] - A plurality of ejection electrodes associated with the nozzles and located on the front side of the nozzles: The ejection electrodes are used to eject ink from their associated nozzles.

[0015] - A plurality of ink supply ducts for the nozzles: At least one ink supply duct ends at each nozzle and feeds ink to it.

[0016] - A plurality of ink suction ducts: At least one ink suction duct ends at each nozzle and retrieves ink from the nozzle.

[0017] Further, each nozzle comprises at least the following elements:

[0018] - An ink feed tube: The ink feed tube forms part of the ink supply ducts and ends in a feed muzzle at its front end.

[0019] - An ink suction tube. The ink suction tube forms part of the ink suction ducts and ends in a suction muzzle at its front end.

[0020] One of these muzzles laterally surrounds the other muzzle.

[0021] Both the feed muzzle and the suction muzzle are close to the front end of the nozzle in the sense that, along the ejection direction (X),

[0022] - the distance from the frontmost portion of the nozzle to the feed muzzle is smaller than a diameter of the feed muzzle and

[0023] - the distance from a frontmost portion of the nozzle to the suction muzzle is smaller than the difference between the diameter of the suction muzzle and the diameter of the feed muzzle.

[0024] Placing both muzzles close to the frontmost portion of the nozzle is based on the understanding that this makes it possible to actively circulate the ink even in the frontmost parts of the nozzle, thereby preventing regions of stagnant ink forming at the nozzle’s forward end. Stagnant ink tends to create "stains" due to progressive evaporation of ink carrier liquid, and such stains would affect ink drop formation while printing.

[0025] Making one of the muzzles laterally surround the other muzzle provides for a well-distributed ink flow, reducing asymmetries.

[0026] Advantageously, the suction muzzle laterally surrounds the feed muzzle in order to encourage the formation of a meniscus having a "high" center point. However, and in particular for low-viscosity inks where capillary forces, wetting, and surface tensions are the primary factors shaping the meniscus, the feed muzzle may also laterally surround the suction muzzle.

[0027] Advantageously, the suction muzzle is, when measuring along the ejection direction, at least as close to the frontmost portion of the nozzle as the feed muzzle. In other words, the two muzzles may be at the same distance from the front- most portion, or the feed muzzle is further away from the frontmost portion of the nozzle than the suction muzzle. This allows to easily submerge the forward edge of the wall of the feed tube in ink, thereby preventing stagnant ink there.

[0028] In another embodiment, however, the feed muzzle may be closer, along the ejection direction, to the frontmost portion of the nozzle and the suction muzzle. This helps in centering the apex of the ink on the nozzle.

[0029] The suction muzzle and the feed muzzle may be located, along the ejection direction, at the same distance from the frontmost portion of the nozzle. In this case, the feed muzzle wall can still be kept in a submerged state, and manufacturing the two nozzles from the same layer(s) of material is simpler than for an embodiment where the two nozzles end at different locations along the ejection direction.

[0030] In one embodiment, the suction muzzle and / or the feed muzzle may form the frontmost portion of the nozzle. In this case, it is easy to prevent stagnant ink at the frontmost portion because the frontmost portion is close to the actively circulated ink.

[0031] In another embodiment, each nozzle may comprise a nozzle tip that forms the frontmost portion of the nozzle. This nozzle tip is arranged, in ejection direction, forward from the center of the feed muzzle. Such a tip helps to center the ink apex on the nozzle, thereby making ink drop ejection more predictable.

[0032] For a symmetric distribution of the ink, the feed muzzle and suction muzzle are advantageously concentric.

[0033] In order to obtain an evenly distributed flow of ink, the feed muzzle may be circular and the suction muzzle may be annular, with the latter surrounding the former.

[0034] It must be noted that the print head may comprise further nozzles, in addition to the claimed “plurality of nozzles”, and such further nozzles do not necessarily need to fulfill the above conditions.

[0035] The invention also relates to a method for operating such a print head. Said method comprises at least the following steps:

[0036] - Feeding ink through the ink supply ducts to the nozzles.

[0037] - Withdrawing ink through the ink suction ducts from the nozzles.

[0038] This allows to maintain flow of ink between the feed muzzle and the suction muzzle, thereby reducing the risk of the formation of stagnant ink.

[0039] Brief Description of the Drawings The invention will be better understood and objects other than those set forth above will become apparent when consideration is given to the following detailed description thereof. Such description makes reference to the annexed drawings, wherein:

[0040] Fig. 1 shows a schematic, partially sectional view of a print head and of the target,

[0041] Fig. 2 is a vertical sectional view through a first embodiment of a nozzle and the surrounding parts, with the ink flow denoted by arrows,

[0042] Fig. 3 is a horizontal sectional view along line III-III of Fig. 2,

[0043] Fig. 4 is a horizontal sectional view along line IV -IV of Fig. 2,

[0044] Fig. 5 shows only the nozzle of Fig. 2,

[0045] Fig. 6 is a vertical sectional view of a second embodiment of a nozzle,

[0046] Fig. 7 is a vertical sectional view of a third embodiment of a nozzle,

[0047] Fig. 8 is a horizontal sectional view along line VIII- VIII of Fig. 7,

[0048] Fig. 9 is a vertical sectional view of a fourth embodiment of a nozzle,

[0049] Fig. 10 is a further embodiment of the print head, and

[0050] Fig. 11 shows horizontal sectional views of layers A2, A3, and Ml

[0051] - M4 of the device of Fig. 10.

[0052] Note: The drawings are not to scale. In particular, in most of the drawings, the extensions of the components along ejection direction X are exaggerated as compared to their extensions perpendicular thereto. For details, see the section Nozzle Geometry below.

[0053] Modes for Carrying Out the Invention

[0054] Definitions

[0055] “Forward” defines the direction into which the print head is designed to eject ink. For example, the ejection electrodes are forward from the nozzles.

[0056] “Backward” defines the opposite direction. For example, the nozzles are arranged backward from the ejection electrodes.

[0057] “At the front” and “at the back” are understood to designate a location at levels forward from or backward from something else.

[0058] “Front” and “back” are the forward and backward sides.

[0059] The ejection direction X of the print head defines the “vertical” upwards direction, i.e. the print head is, by definition, designed to eject ink upwards. (In operation, it may, of course, be under any angle to the direction of gravity.) Hence, definitions such as “above” and “below” are to be understood in reference to this definition of “vertical”.

[0060] “Horizontal” is any direction perpendicular to the vertical direction.

[0061] “Lateral” designates something that is horizontally offset from something else.

[0062] The “muzzle” of a tube is the opening at the end of the tube.

[0063] Advantageously, the muzzles are circular to evenly distribute the ink. For non-circular muzzles, however, the "diameter" of a muzzle is defined as the largest diameter of the muzzle.

[0064] Where the pressures applied to the feed muzzles and the suction muzzles are mentioned, reference is made to the pressures pl and pO in reservoir tank 22 and suction tank 28, respectively, assuming that the pressure drops in the ducts to and from the individual nozzles are negligible, and all significant pressure drops occur at the individual nozzles. If the pressure drops in the ducts to and from the individual nozzles are non-negligible, the pressure values must be corrected accordingly.

[0065] Print Head.

[0066] Fig. 1 shows a schematic sectional view of an embodiment of the print head 1. It is structured to eject ink along an ejection direction X onto a target 2.

[0067] A print head of this type has been described in more detail in WO 2022 / 152380 Al, see Fig. 1 and the corresponding description there.

[0068] The print head comprises a plurality of nozzles 4 located at the front side of a nozzle carrier 6. The nozzles 4 may be arranged in a one- or two-dimensional array.

[0069] The print head has a plurality of ejection electrodes (not shown in Fig. 1) for ejecting ink from the nozzles 4 along ejection direction X and optional further electrodes arranged on a support structure 8, the design of which is described in more detail below. Further electrodes may be provided in electrical contact with the ink to set the ink to a defined electrical potential.

[0070] Nozzle carrier 6 comprises a front layer 10, with the nozzles 4 being mounted to the front side of front layer 10 and forming projections thereon. It also comprises a backing layer 12 located at the back side of front layer 10.

[0071] The internal structure of front layer 10 is not shown in Fig. 1 and is described in more detail in WO 2022 / 152380 Al. It may, e.g., be of a dielectric, in particular a polymer. Backing layer 12 may be an insulating semiconductor material, or it may be a dielectric. Advantageously, backing layer 12 is, at least partially, of glass. Backing layer 12, may, however, also be a conductive layer as long as any electrical vias extending therethrough are suitably insulated.

[0072] Electrical vias 14 are connected to the ejection electrodes and extend through front layer 10 and the backing layer 12 for connecting the ejection electrodes to a voltage supply 17. Advantageously, there is at least one via 14 for each nozzle 4. Further vias may be provided to connect other electrodes to voltage supply 17. Alternatively, part or all of the electrical feed lines may be located at the front side of the print head.

[0073] Ink ducts 15, 16 supply ink to the nozzles 4 and recycle ink back from the nozzles 4. They are located, in part, in front layer 10, and they extend through peripheral regions of backing layer 12. Their design is described in more detail below.

[0074] Fig. 1 shows an embodiment of the print head that has supply ducts 15 as well as suction ducts 16 for the ink.

[0075] At least one pump 18 and / or another pressure source or vacuum source is provided to supply ink to the supply ducts 15 and to retrieve ink from the suction ducts 16.

[0076] Advantageously, the print head comprises a first pressure control 20 for generating a first defined pressure pl at the input of the supply ducts 15, e.g., in a reservoir tank 22.

[0077] The ink is supplied through an optional filter 24 and the supply ducts 15 to the nozzles 4.

[0078] The suction ducts 16 are connected to a suction system, which may comprise a second pressure control 26 for generating a second defined pressure pO at the exit of the suction ducts 16, e.g., in a suction tank 28. The suction system may also comprise a pump. This may in particular be pump 18 as mentioned above, in which case pump 18 acts as a circulating pump.

[0079] Pump 18 may be controlled by level sensors in reservoir tank 22 and suction tank 28.

[0080] As further shown in Fig. 1, the print head may comprise a circuit carrier 30, such as a PCB, arranged at the back side of nozzle carrier 6.

[0081] An optional interposer layer 32 may be provided between circuit carrier 30 and nozzle carrier 6 for matching a denser resolution of the vias 14 to the circuit resolution of circuit carrier 30. Such interposer layers are, e.g., used in flip- chip designs where semiconductor chips are applied to PCBs. Circuit carrier 30 carries control circuitry 33, which may, e.g., implement at least part of voltage supply 17, such as the driver stage of such a voltage supply, which connects voltage sources to the various electrodes of the print head.

[0082] In the shown embodiment, the ink ducts 15, 16 extend through interposer layer 32 (if present) as well as circuit carrier 30.

[0083] If the vias 14 have a large enough mutual spacing (such as larger than 0.4 mm), they may directly interface with circuit carrier 30 without an interposer layer 32.

[0084] Advantageously, target 2 is arranged on an acceleration electrode 34, which is connected to voltage supply 17 to generate an accelerating electrical field between print head 1 and target 2.

[0085] The pressure controls 20, 26 can be used to maintain pressures as described below. Advantageously, they allow to separately adjust the pressure levels in the supply ducts 15 as well as in the pressure ducts 16.

[0086] Nozzle Design

[0087] Figs. 2 - 5 show a first embodiment of a nozzle 4 and the surrounding elements. (As mentioned above, in contrast to Fig. 1, the ejection direction X in Fig. 2 points upwards.)

[0088] As can be seen from Fig. 2, nozzle 4 forms a projection on the front side 36 of nozzle carrier 6, e.g., on the front side of its front layer 10. It is located at an exit passage 5 through which ink can be ejected towards target 2.

[0089] Fig. 2 also shows the various electrodes that may be associated with the nozzles 4.

[0090] The ejection electrode 38 is located on the front side of the nozzle 4. In the embodiment of Fig. 2, it is annular with a central opening 39 for the passage of the ejected ink. It is connected to one of the vias 14, which extends through support structure 8 and nozzle carrier 6. For more details on the design of the vias 14 (which have been omitted from Figs. 2 - 4), and the ejection electrodes, reference is made to WO 2022 / 152380 Al and the section “Nozzle Design 1” of that document.

[0091] A shielding electrode 40 may be located at the front side of and at a distance from the ejection electrodes 38, i.e., the ejection electrodes 38 are closer to nozzle carrier 6 than the shielding electrode 40. Advantageously, there is one continuous shielding electrode 40 extending over the front of print head 1, but there may also be several such shielding electrodes. For details, reference is again made to WO 2022 / 152380 Al. Shielding electrode 40 is provided to control the field between print head 1 and target 2. For each nozzle 4, an opening 41 in shielding electrode 40 allows for the passage of the ejected ink.

[0092] As shown in Fig. 2, a guard electrode 42 may be located behind and at a distance from ejection electrode 38 but in front of and at a distance from nozzle carrier 6. It may again be annular. Alternatively, it may also extend over several nozzles.

[0093] An opening 43 in guard electrode 42 above nozzle 4 allows for the passage of the ejected ink.

[0094] The function of guard electrode 42 is described below.

[0095] Nozzle 4 of this embodiment comprises a tip section 46 and a base section 50 (Fig. 3), with tip section 46 arranged in front of base section 50.

[0096] Nozzle 4 further comprises an ink feed tube 54 and an ink suction tube 55, which coaxially extend along ejection direction X of the print head.

[0097] Ink feed tube 54 ends, at its forward end, in a front feed muzzle 54a. In the shown embodiment, front feed muzzle 54a is circular. Ink feed tube 54 is connected to a supply duct section 15a of the supply ducts 15 and directly feeds ink to the forward end of nozzle 4.

[0098] Ink suction tube 55 ends, at its forward end, in a front suction muzzle 55a. In the shown embodiment, front suction muzzle 55 is annular and arranged around feed muzzle 54a. Ink suction tube 55 is connected to a suction duct section 16a of the suction ducts 16 and directly withdraws ink from the forward end of nozzle 4.

[0099] Advantageously, and for all embodiments shown here, feed tube 54 and suction tube 55 are cylindric and concentric.

[0100] Base section 50 connects tip section 46 to nozzle carrier 6. It is surrounded by a ring 52.

[0101] Base section 50 mechanically connects feed tube 54 and suction tube 55 to carrier 6 while providing a connection between supply duct section 15a and feed tube 54 as well as a connection between suction duct section 16a and suction tube 55.

[0102] In Figs. 2 and 3, suction duct section 16a is shown to consist of several individual ducts, e.g., four of them. This number may vary. There may also be only a single annular suction duct section 16a as shown in the section "Further Embodiment" below. In addition, and as shown in Figs. 2 and 3, one or more (optional) radial transversal exit ducts 56 may extend, transversally to ejection direction X, outwards from feed tube section 15a to suction duct section 16a.

[0103] Annular trench 58 is formed between base section 50 and ring 52.

[0104] In the embodiment of Figs 2 and 3, the transversal ducts 56 extend through base section 50.

[0105] The transversal ducts 56 are useful in initial filling of suction duct section 16a and above- situated suction muzzle 55a with ink. If ink were initially only supplied through supply duct section 15a, and if transversal exit ducts 56 are absent, ink would need to pass from feed muzzle 54a to suction muzzle 55a, which may not happen without pressurizing the ink supplied through supply duct section 15a, because an ink contact line may form at the wall of ink feed tube 54, wherein the latter may act as a retainer that prevents ink from spreading further. To push ink over the wall of the ink feed tube, surface energy of this ink wetting interface needs to be overcome by pressurizing the ink. Once this “activation energy” is supplied, the ink may quickly propagate into suction muzzle 55a and in fact create such high flow that it surpasses suction tube 55 and floods the air-filled cavity 71. During this filling process, applying partial vacuum to the suction duct section 16a could help prevent such flooding, but applying partial vacuum to section duct section 16a will introduce a flow of air into suction muzzle 55a. If the ink arrives at feed muzzle 54a and if such ink contains a volatile component, such air flow can lead to almost immediate drying of the ink. By introducing transversal ducts 56, ink can be introduced to suction muzzle 55a from below by means of ink flowing automatically through the transversal ducts 56 and from there up to suction muzzle 55a.

[0106] Advantageously, the transversal ducts 56 have smaller cross-section than feed tube section 15a and suction duct section 16a, such that the drop of pressure between feed tube section 15a and suction duct section 16a primarily takes place within the transversal ducts 56, and, for example, pressure in suction duct section 16a is not substantially influenced by what pressure is applied to feed tube section 15a.

[0107] Ring 52 forms an ink retainer, whose purpose is to retain the ink laterally, similar to ink retainer 66 described in WO 2022 / 152380 Al, and it may be provided with the additional features described therein, such as a hydrophobic or oleophobic coating and / or a ledge facing away from nozzle 4, and it is advantageously shielded by guard electrode 42.

[0108] Guard electrode 42 is located forward from (i.e., further away from nozzle carrier 6 than) the forward end of nozzle 4 or at the same height as the froward end of nozzle 4, thereby reducing the electrical field in the region of the forward lateral circumference of nozzle 4.

[0109] As can be seen in Fig. 2, there is an air-filled cavity 71 forming a gap between guard electrode 42 and ink retainer 66, which prevents the ink from reaching guard electrode 42.

[0110] Fig. 2 also shows support structure 8, which connects the various electrodes 38, 40, 42 to nozzle carrier 6. It is arranged on front side 36 of nozzle carrier 6 and comprises a plurality of support elements 76, 78 as well as electrode carrier layers 80, 82, 84, which are described, using the same reference numbers, in WO 2022 / 152380, and the description of these parts is incorporated herein by reference.

[0111] Ink Control

[0112] As mentioned above, feed ducts 15 and suction ducts 16 are provided to feed and retrieve ink from the nozzles 4. This allows to maintain a flow of fresh ink at each nozzle 4.

[0113] The pressure at the supply ducts 15 and the suction ducts 16 is, in the embodiment of Figs. 2 - 5, adjusted to generate an ink surface 90a, 90b as shown in dash-dotted lines in Figs. 2 and 5.

[0114] As shown, the ink forms a central apex 92 centrally forward from nozzle 4. This apex 92 can be generated by adjusting the pressures at and flows at the feed muzzle 54a and suction muzzle 55a. It can further be controlled by adding a bias voltage to ejection electrode 38 that is sufficient to pull ink forward but insufficient to eject an ink drop.

[0115] During operation, pressure pl applied to suction muzzle 55a is advantageously lower than the surrounding atmospheric pressure such that it prevents ink from flooding air-filled cavity 71. Pressure pl applied to feed muzzle 54a may be higher or lower than surrounding atmospheric pressure but higher than pressure pO, such that an ink flow is created that directs from feed muzzle 54a to suction muzzle 55a. The levels of the pressures pO and pl do not only control recirculation flow but also the flow rate directed onto substrate 2 during printing. For example, pl may be set to either -50 mbar or 0 mbar relative to atmospheric pressure while pO may be set to -100 mbar relative to atmospheric pressure. If pl is -50 mbar, both printing and circulation flow rate will be smaller than in case pl is set to 0 mbar. However, the details of the applied pressure depend on numerous parameters that will be explained in more detail in the section "Ink Viscosity".

[0116] In the embodiment of Figs. 2 - 5, the pressures and flows of the ink in feed tube 54 and suction tube 55 are adjusted such that not all of the ink fed through feed tube 54 is withdrawn by suction tube 55. Rather, part of the ink overflows over the forward edge 55b of suction tube 55 and flows backwards into trench 58, forming a sloped ink surface as indicated by line 90a. This prevents stagnant ink at forward edge 55b. This process may also be influenced by the wetting properties and capillary forces of the ink, e.g., by suitably coating the surfaces of the components, e.g., with hydrophobic or oleophobic or hydrophilic or oleophilic coatings, depending on the inks to be used.

[0117] Trench 58 communicates with suction duct section 16a, and therefore this surplus ink is withdrawn from nozzle 4.

[0118] Hence, in an advantageous embodiment, the method for operating the print head comprises the steps of

[0119] - feeding more ink through feed tube 54 than through suction tube 55, even if no printing takes place,

[0120] - withdrawing part of the ink through suction tube 55,

[0121] - letting surplus ink (the difference between the fed ink and the ink withdrawn through suction tube 55) flow backwards along an outer side of suction tube 55, and

[0122] - collecting the surplus ink in trench 58 surrounding a rearward base of nozzle 4 and withdrawing it through the suction ducts 16a, 16.

[0123] In the shown embodiment, a flow of ink is maintained over both the forward edge 54b of feed tube 55 and the forward edge 55b of suction tube 55, thereby avoiding stagnant ink at a location where dried-up ink deposits may strongly affect droplet formation. As will be seen below, in other geometries, a flow of ink is maintained over at least one of the forward edges 54b, 55b. Hence, advantageously, the method for operating the print head comprises the step of conveying ink over the forward edge 54b, 55b of at least one of the feed tube 54 and the suction tube 55.

[0124] Fig. 6 illustrates a second embodiment of ink control. Here, no ink flows over forward edge 55b of suction tube 55. Rather, the ink is laterally retained at forward edge 55b. To support such retaining, forward edge 55b may be equipped with a ledge 96 projecting laterally over the rest of the outer surface of suction tube 55.

[0125] Hence, in this embodiment, the method for operating the print head comprises the step of retrieving all ink fed through feed tube 54 by means of suction tube 55 if no printing takes place.

[0126] Fig. 6 also illustrates that, if no ink flows over forward edge 55b of suction tube 55, the wide cavity 71 as shown in Fig. 2 may be dispensed with. Rather, the support elements 78 of the electrodes may be located closer to nozzle 4. (Note: for simplicity, Fig. 6 only shows guard electrode 42 but not any structures forward from it.)

[0127] Nozzle Geometry

[0128] Figs. 2 - 5 illustrate an embodiment of another advantageous aspect of the present technique, which has already been mentioned above. As can be seen, both the feed muzzle 54a as well as the suction muzzle 55a are close to the forward end of nozzle 4.

[0129] As best seen in Fig. 5, in the shown embodiment feed tube 54 and suction tube 55 form the frontmost portion 94 of nozzle 4. Hence, there are no other nozzle parts forward from feed muzzle 54a and suction muzzle 55a.

[0130] While, in the embodiment of Fig. 5, feed muzzle 54a and suction muzzle 55a are arranged at the same height, they may also be located at different heights.

[0131] In the embodiment of Fig. 6, for example, the frontmost portion 94 of nozzle 4 is formed by the forward end 55b of suction tube 55 while the forward end 54b of feed tube 54 is set back (along direction X) by a distance dl. This makes it easier to submerge forward end 54b of feed tube 54 in order to prevent dried ink deposits from forming there.

[0132] However, distance dl is advantageously small, compared to the diameter Ds of suction muzzle 55a in order to make it easy to form apex 92 by means of the flow of ink surging forward from feed muzzle 54a. In quantitative terms, the distance dl is advantageously smaller than half the diameter Ds of suction muzzle 55a.

[0133] The distance dl can also be chosen with regard to the maximum size of particles that are present inside the ink. Advantageously, dl should be increased to at least the same size as the diameter of the particles. In absolute terms, for typical inks, distance dl is advantageously no more than 100pm.

[0134] Alternatively, though, and as shown in Fig. 9, the frontmost portion 94 of nozzle 4 may also be formed by forward end 54b of feed tube 54 while forward end 55b of suction tube 55 is set back (along direction X) by a distance d2. This makes it easier to maintain apex 92.

[0135] However, distance d2 is again advantageously small, compared to the diameter Ds of suction muzzle 55a in order to maintain a steady, predictable flow of ink at the forward end of nozzle 4. In quantitative terms, the distance d2 is advantageously smaller than half the diameter Ds of suction muzzle 55a. The version of nozzle 4 of Fig. 9 may 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.

[0136] Figs. 7 and 8 illustrate a further means for forming and controlling apex 92 of the ink surface. Here, frontmost portion 94 of nozzle 4 is formed by a nozzle tip 98 in addition to feed tube 54 and suction tube 55. Nozzle tip 98 is arranged at the center of feed muzzle 94a

[0137] Nozzle tip 98 comprises a tip section 100 that is advantageously tapered towards its forward end mounted on a stem section 102 arranged concentrically within feed tube 94. Stem section 102 rests on struts 104 of base section 50. Alternatively or in addition thereto, stem section 102 may be connected, using lateral struts (not shown), to feed tube 54.

[0138] Along direction X, nozzle tip 98 is at a distance d5 forward from suction muzzle 55a and at a distance d6 forward from feed muzzle 54a. These are advantageously small as compared to the diameter of the muzzles to make it easier to submerge nozzle tip 98 in ink.

[0139] This is not only the case with the embodiment of Fig. 7 but also with the other embodiments. Along direction X, the distance between the frontmost portion 94 of nozzle 5 and the suction muzzle 55a and the distance between the front- most portion 94 of nozzle 5 and the feed muzzle 54a are advantageously no more than 50pm.

[0140] As already mentioned, the various distances between components are advantageously adapted to the size parameters of nozzle 4, in particular to the diameter Df of feed muzzle 54a and / or the diameter Ds of suction nozzle 55a. This makes it easier to prevent sections of stagnant ink at the forward end of nozzle 4.

[0141] In particular, the distance from frontmost portion 94 of nozzle 4 to feed muzzle 54a (which is zero in Fig. 5, equal to dl in Fig. 6, equal to d6 in Fig. 7, and zero in Fig. 9) should be smaller than the diameter Df of feed muzzle 54a, in particular smaller than half the diameter Df of feed muzzle 54a.

[0142] The distance from frontmost portion 94 of nozzle 4 to suction muzzle 55a (which is zero in Figs. 5 and 6, equal to d5 in Fig. 7, and equal to d2 in Fig. 9) should be smaller than, in particular smaller than half of the difference Ds - Df between the diameter Ds of suction muzzle 55a and the diameter Df of feed muzzle 54a.

[0143] As mentioned above, most of the drawings show the extensions of the components along ejection direction X in exaggerated manner.

[0144] Fig. 9 is more to the scale (the following, however, applies to all embodiments). The height H of nozzle along ejection direction X from nozzle carrier 6 to its frontmost portion 94 is advantageously smaller than the diameter Ds of suction muzzle 56, in particular H < Ds / 2.

[0145] Advantageously, H is between 5 pm and 50 pm and Ds is between 20 pm and 500 pm. These parameters depend on the viscosity of the ink, though, see next section.

[0146] Ink Viscosity

[0147] Dimensions of the nozzle geometry are advantageously optimized in view of the expected ink viscosity and the required printing resolution.

[0148] Viscosity can vary by many orders of magnitude between different inks, and the flow characteristics inside the nozzle will be strongly affected by this. For example, in a tube-like flow channel, the volumetric flow rate of ink scales linearly with the applied pressure difference (~p), inversely linearly with the viscosity (~l / r|), and to the inverse fourth order of the tube radius (~ 1 / r4). Hence, for a given tube diameter, e.g., the diameter of ink feed tube 54, the volumetric flow rate of an ink with 10 Pa-s of viscosity will be a thousand times lower than that of an ink with 10 mPa-s of viscosity. Or in other words, if the droplet size is the same for both inks, the higher viscosity ink will generate a thousand times lower ejection frequency.

[0149] Generally, however, the resolution requirement of the industry for high-viscosity inks will be lower than for low-viscosity inks while at the same time the volumetric throughput requirements will be higher for high-viscosity inks than for low-viscosity inks.

[0150] Therefore, the above scaling behavior is not optimal in view of industrial applications. This dilemma can be offset by increasing the nozzle geometry, e.g., the diameter of the ink feed tube 54, and potentially by increasing the pressure difference between pO and pl, as well as the absolute pressure, as described herein.

[0151] For example, for a tenfold increase in the diameter of the feed tube 54, one would expect the resulting volumetric flow rate to increase by a factor 10’000 (given the 1 / r4scaling law mentioned above). For a tenfold increase in nozzle size, one would also expect an approximately tenfold increase in droplet size, i.e., a 1000- fold increase in the volume carried by each droplet. At such adjusted droplet size, one would therefore expect an only tenfold higher ejection frequency. This value, however, is likely reduced by the fact that, with larger droplet size, also electric stress will be lower. Electric stress created by the applied voltages directly scales with the Laplace pressure inside the ejected droplet, which is inversely proportional to the droplet radius (~ 1 / r). Since the created electric stress must be equally large or larger than the static Laplace Pressure, the electric stress scales with (~l / r) as well. Hence, for a tenfold increase in nozzle size one expects a tenfold reduction in the electric pressure. Whether this reduction in electric stress also leads to a tenfold reduction in volumetric flow rate depends on the value applied particularly to the feed tube 54.

[0152] Importantly, in embodiments that lack the combination of feed and suction of the present device, the pressure inside the feed tube 54 is strongly limited by the Laplace Pressure of the ink at the nozzle exit. If too much pressure is applied, the ink is simply pushed out of the nozzle and a drop-on-demand ejection process is essentially impossible. In the present device, however, the pressure inside the feed tube 54 may be substantially increased because any liquid pushed out of the nozzle can be carried off by the suction tube 55, so the system can reach an equilibrium condition without flooding ink over the whole printhead. This is of great advantage, especially for large nozzles where the electric stress on average is well below 100 mbar. For a conventional nozzle, this would result in a slow response of the ink to the applied voltage. Particularly for high-viscosity inks, the delay between voltage application and printing can be in the range of seconds. With the present invention, it is possible to drive the ink flow at pressures that are higher than the electrically induced stresses, and this allows to reduce the response time of the ink to any electric stimulus by a factor of ten and more and make the nozzle compatible with drop -on-demand operation.

[0153] It should be noted though that the maximum pressure that can be applied to the suction tube 55 is 1 atmosphere (atm) below atmospheric pressure (equivalent to full vacuum), in case the system is operated under atmospheric conditions. The maximum pressure applicable to suction tube 55 also limits the pressure applied to feed tube 54. In case the flow resistances of feed tube 54 and suction tube 55 are equivalent, the maximum pressure applicable to feed tube 54 is approximately 1 atm above atmospheric pressure as well — otherwise ink will flood the printhead. If the flow resistance in suction tube 55 (including all subsequent suction ducts) is smaller than that in feed tube 54 (including all feed ducts), then the pressure applicable to the feed tube 54 can be higher than one atm above atmospheric pressure. Advantageously, the pressure applied to the feed tube is lower than 10 bar though.

[0154] In the present example, in the case of low-viscosity ink with 10 mPa-s of viscosity, a pressure of 10 mbar applied to feed tube 54 can be a good operating pressure since electric stress for a respective small nozzle will be strong enough to drive ink flow by itself.

[0155] For a high-viscosity ink with 10 Pa-s viscosity situated in a ten times larger nozzle, a higher value of pressure pl to feed tube 54 is advisable, e.g. a pressure of 1 bar. Ultimately, using this pressure in the present example, an increase of the nozzle diameter by a factor of ten, and a simultaneous increase of the droplet diameter by the same factor, would then result in an ejection frequency that is ten times lower when comparing a 10 Pa-s ink to one having a viscosity of only 10 mPa-s. Such reduction may be from 100 kHz to 10 kHz, for example.

[0156] If higher flow rates are required, the size of the nozzles may be increased further. Noteworthy, for large nozzles, the reduction of volumetric flow rate due to a further diameter increase and the respective size -proportional electric stress reduction are generally not relevant anymore since the pressure applied to the feed tube 54 is expected to be considerably larger than the pressure from electric stress, and hence volumetric flow will be primarily driven by the pressure applied to the feed and suction tubes.

[0157] Therefore, advantageously, the diameters of the nozzles are adjusted in view of the expected ink viscosity.

[0158] For ink with a viscosity below 100 mPa-s, the maximum diameter Df of feed tube 54 should not exceed 50pm, advantageously it should not exceed 20pm.

[0159] For an ink with a viscosity of above 1 Pa-s, the diameter Df of feed tube 54 should not exceed 500 pm, advantageously it should not exceed 200 pm, but it should be at least 20 pm, in particular at least 50 pm.

[0160] Therefore, in an advantageous embodiment, the method for operating the present print head comprises the step of printing an ink having a viscosity of less than 100 mPa-s, wherein the diameter Df of feed tube 54 is below 50pm, in particular below 20pm,

[0161] On the other hand, advantageously, the method comprises the step of printing an ink having a viscosity above 1 Pa-s, wherein the diameter Df of feed tube 54 is at least 20pm, in particular at least 50pm.

[0162] Any other size parameter of the nozzle may be scaled according to the same procedure.

[0163] As to the diameter Ds of suction tube 55, it scales roughly linearly with the diameter Df of feed tube 54 if the thickness of the various walls is neglected. The same applies to all other size parameters of the nozzle, in particular the parameters dl, d5, and d6.

[0164] For being able to withdraw all ink fed through feed tube 54, the cross section Ss provided for the ink by suction tube 55 is advantageously at least equal to the cross section Sf of feed tube 54. Advantageously, though, it is larger, at least by a factor 1.5, because the annular geometry of the ink space in suction tube 55 gives rise, for a given cross section, to a larger flow resistance as 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 suction tube 54 larger is due to the fact that the suction pressure pO is limited, as mentioned, to 1 atm below atmospheric pressure.

[0165] In the example of Fig. 9, for rotational symmetry, the cross section Sf of feed tube 54 is

[0166] Sf = Df2• 7t / 4. (1)

[0167] The cross section Ss of suction tube 55 is

[0168] Ss = (Ds2- Dx2) • 7t / 4, (2) with Dx being the outer diameter of feed tube 54 (see Fig. 9).

[0169] However, advantageously, suction muzzle 55a should not be too large in order to keep the meniscus small and the printing resolution high. Hence, advantageously, the cross section Ss for ink of suction tube 55 is advantageously less than two times the cross section Sf for ink of feed tube 54.

[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 feed muzzle 54a and suction muzzle 55b.

[0172] In the present device where both muzzles are close to the forward end of nozzle 4, e.g. in contrast to the printing head of WO2022152380A1, the recirculation occurs in the region of the ink meniscus. Besides reducing dry-out issues due to solvent evaporation, this also allows to more quickly generate a droplet or jet of ink when applying an electric field to the meniscus by means of the ejection electrode because the ink is already on the move.

[0173] Hence, advantageously, the method for operating the print head comprises feeding ink from the feed muzzle 54a to the suction muzzle 55b, advantageously continuously, while printing, i.e., while generating ink drops and / or an inkjet by means of ejection electrode 38 associated with the muzzle 4.

[0174] The recirculation rate can be influenced by the pressure pl applied to feed duct 54 and the pressure pO applied to suction duct 55, and it increases with the difference pl - pO. In addition, the absolute values of the pressures pO and pl, and not only their difference, affects the flow rate as well. By increasing these pressures, e.g., by increasing the value (pO + pl) / 2, while keeping the pressures low enough to prevent an overflow of the meniscus over the forward rim of the nozzle, the volume and flow-cross-section of the meniscus are increased, which reduces the flow resistance through the meniscus and therefore increases the recirculation rate.

[0175] Note that the nozzles could also be operated with both pO and pl being below atmospheric pressure as long as the capillary forces in the nozzle are sufficient to pull the ink to the front end of the nozzle.

[0176] Hence, advantageously, the method for operating the print head may also comprise the step of selecting the difference and / or a sum of the pressures pO, pl applied to feed tube 54 and suction tube 55 as a function of a viscosity of the ink to be printed in order to optimize the ink drop formation as described above.

[0177] Further Embodiment

[0178] Figs. 10 and 11 shows yet a further, more detailed embodiment of the print head, with Fig. 11 showing sectional views along the various layers A2, A3, Ml, M2, M3, M4 of Fig. 10.

[0179] Here, layers A2 and A3 are the two topmost layers of nozzle carrier 6. They form the topmost parts of feed duct section 15a and suction duct section 16a.

[0180] Layer A3 further forms the transversal exit ducts 56 (not visible in Fig. 10 because they extend perpendicularly to the sectional plane). The transversal exit ducts 56 are long and narrow in order to have a high flow resistance for the reasons mentioned in the section "Nozzle Design" above.

[0181] Layer Ml covers the transversal exit ducts 56 from above to keep their cross section small. It also forms the bottommost part of trench 58 as well as a rear section 52a of ring 52.

[0182] Layer M2 defines feed tube 54 and suction tube 55. In addition, optionally, it may form a front section 52b of ring 52, thereby extending trench 58 forwards.

[0183] Layer M3 extends suction tube 55 forwards, but not feed tube 54, i.e., in this embodiment feed muzzle 54a is located behind suction muzzle 55a.

[0184] Layer M4 forms tip 100 as well as the substrate for guard electrode 42. Tip 100 is tapered towards its forward end. Such a shape can e.g., created by applying a photosensitive layer to the top of layer M4, which photosensitive layer is then 3D-structured using grayscale lithography. Thereafter, the 3D structure is transferred to layer M4 by means of dry etching. Notes

[0185] As can be seen, in all present embodiments, feed muzzle 54a and suction muzzle 55a are arranged forward from nozzle carrier 6, i.e., they form part of the projecting section of nozzle 4. This allows to shape and control the front surface 90b of the ink at a distance from nozzle carrier 6 and to better retain the ink laterally.

[0186] Also, due to partial vacuum being applied to suction muzzle 55a, the front surface 90b of the ink becomes flatter in the region of the suction muzzle 55a than it is in the region of the feed muzzle 54a, which means that shape of the front surface 90b of the ink is close in its appearance to the case in which only a feed muzzle 54a would be present, i.e. at the absence of suction tube 55, like in WO 2022 / 152380 Al. This means that the increase in nozzle 4 diameter due to the addition of the suction tube 55 does not result in the nozzle 4 achieving a printing resolution that is substantially worse than in at the absence of such suction tube 55.

[0187] Also, as shown, at least suction muzzle 55a is advantageously set back, along direction X, from guard electrode, by a distance d3 as shown in Fig. 6, i.e., guard electrode 42 is arranged, along direction X, between ejection electrode 38 and suction muzzle 55b. This allows to at least partially shield forward edge 55b of suction tube 55 from the electrical field of ejection electrode 38. Distance d3 may also be zero, i.e., the forward edge 55b of suction muzzle 55a is at the same level as guard electrode 42.

[0188] However, the lateral distance d4 between forward edge 55b and guard electrode 42 is advantageously small in order to reduce the field strength at forward edge 55b. Advantageously, d4 is not larger than half the difference Ds - Df, i.e., d4 < (Ds - Df) / 2 (see Fig. 9). In this case, guard electrode 42 couples strongly to forward edge 55b but much less so to forward edge 54b, i.e., it prevents the ejection voltage to affect the ink at forward edge 55b of suction muzzle 55a while not significantly affecting the ejection of ink at the forward center of the nozzle.

[0189] This relationship between distance d4 and Ds - Df is of particular importance in embodiments as shown in the drawings where the outer diameter Do of suction tube 55 is smaller than the inner diameter Dg of guard electrode 42 because, in that case, guard electrode 42 cannot cover suction tube 55 from above.

[0190] As mentioned, feed tube 54 and suction tube 55 are, advantageously, cylindrical and co-centric. They may, however, also have non-circular cross section, such as a hexagonal or octagonal cross section. Rounded geometries are better, though, in order to avoid regions of elevated electrical fields and for reaching a symmetric, stable font surface 90b of the ink.

[0191] Feed tube 54 needs to carry the ink to the forward end of nozzle 4, and therefore it advantageously has an axial inner space 110 (Fig. 5) that extends from the base of nozzle 4 towards feed muzzle 54a and that is closed laterally at all sides by a wall.

[0192] Similarly, advantageously, suction tube 55 forms a space 112 between a closed, outer wall and feed tube 54, with the space extending between the base of nozzle 4 and suction muzzle 55a.

[0193] As indicated by dotted lines in Fig. 7, radial struts 108 may extend between feed tube 54 and suction tube 55 in order to mechanically mount them to each other. Even though only shown in Fig. 7, such struts may be used for any of the embodiments shown here. Advantageously, though, the struts 108 do not extend upwards all the way to suction muzzle 55a in order to have a more homogeneous ink distribution at the forward end of nozzle 4.

[0194] While there are shown and described presently preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and practiced within the scope of the following claims.

Claims

Claims1. An electrohydrodynamic print head comprising a nozzle carrier (6), a plurality of nozzles (4) arranged on said nozzle carrier (6), wherein each nozzle (4) forms a projection arranged on a front side (36) of said nozzle carrier (6) and extending into an ejection direction (X) of the print head, a plurality of ejection electrodes (38) associated with said nozzles(4) and located on the front side of said nozzles (4), a plurality of ink supply ducts (15) for the nozzles (4), with at least one ink supply duct (15) ending at each nozzle (4), a plurality of ink suction ducts (16), with at least one ink suction duct (16) ending at each nozzle (4), wherein each nozzle (4) comprises an ink feed tube (54), as part of the ink supply ducts, that ends, at its forward end, in a feed muzzle (54a) and an ink suction tube (55), as part of the ink suction ducts, that ends, at its forward end, in a suction muzzle (55a), wherein one of the muzzles (55a, 54a) laterally surrounds the other muzzle (54a, 55a), wherein, along the ejection direction (X), a distance from a frontmost portion (94) of the nozzle (4) to the feed muzzle (54a) is smaller than a diameter (Df) of the feed muzzle (54a) and a distance from the frontmost portion (94) of the nozzle (4) to the suction muzzle (55a) is smaller than a difference between a diameter (Ds) of the suction muzzle (55a) and the diameter (Df) of the feed muzzle (54a).

2. The print head of claim 1 wherein, along the ejection direction (X), the suction muzzle (55a) is at least as close to, in particular closer to, the front- most portion (94) of the nozzle (4) as the feed muzzle (54a).

3. The print head of claim 1 wherein, along the ejection direction (X), the suction muzzle (55a) is at the same distance from the frontmost portion (94) of the nozzle (4) as the feed muzzle (54a).

4. The print head of claim 1 wherein, along the ejection direction (X), the feed muzzle (54a) is closer to the frontmost portion (94) of the nozzle (4) and the suction muzzle (55a).

5. The print head of any of the preceding claims wherein, along the ejection direction (X), a forward end (54b) of the feed tube (54) is set back by a distance dl from a forward end (55b) of the suction tube (55), wherein the distance dl is smaller than half the diameter (Ds) of the suction muzzle (55a).

6. The print head of any of the preceding claims wherein at least one of the suction muzzle (55a) and the feed muzzle (54a) forms the frontmost portion (94) of the nozzle (4).

7. The print head of any of the claims 1 to 5 wherein each nozzle (4) further comprises a nozzle tip (98) arranged forward from a center of the feed muzzle (54a) and forming the frontmost portion (94) of the nozzle (4).

8. The print head of any of the preceding claims wherein the feed muzzle (54a) and the suction muzzle (55a) are concentric.

9. The print head of any of the preceding claims wherein the feed muzzle (54a) is circular and the suction muzzle (55a) is annular.

10. The print head of any of the preceding claims wherein both the suction muzzle (55a) and the feed muzzle (54a) are arranged forward from the nozzle carrier (6).

11. The print head of any of the preceding claims wherein each nozzle (4) further comprises a ledge (96) arranged at the suction muzzle (55a) at a fro- ward end of the suction tube (55) and extending laterally away from the suction tube (55).

12. The print head of any of the preceding claims comprising a guard electrode (42), wherein, at a given nozzle (4), the guard electrode (42) is arranged, along the ejection direction (X), between the ejection electrode (38) and the suction muzzle (55a), and wherein a lateral distance (d4) between a forward edge (55b) of the suction muzzle (55a) and the guard electrode (42) is not larger than halfof a difference between a diameter (Ds) of the suction muzzle (55a) and the diameter (Df) of the feed muzzle (54a).

13. The print head of claim 12 wherein an outer diameter (Do) of the suction tube (55) is smaller than an inner diameter (Dg) of the guard electrode (42).

14. The print head of any of the preceding claims wherein, along the ejection direction (X), a distance (dl) between the feed muzzle (54a) and the suction muzzle (55a) is no more than 100pm.

15. The print head of any of the preceding claims wherein, along the ejection direction (X), distances between the frontmost portion (94) and the suction muzzle (55a) and between the frontmost portion (94) and the feed muzzle (54a) are no more than 50pm.

16. The print head of any of the preceding claims wherein the feed tube (54) has an axial inner space (110) extending from a base of the nozzle (4) to the feed muzzle (54a) closed laterally on all sides by a wall.

17. The print head of any of the preceding claims wherein the suction tube (55) forms a space (112) between a wall and the feed tube (54), with the annular space extending between a base of the nozzle (4) and the suction muzzle (55a).

18. The print head of any of the preceding claims wherein a cross section (Ss) provided by the suction tube (55) for the ink is larger than, in particular at least 1.5 times larger than, a cross section (Sf) of the feed tube (54).

19. The print head of any of the preceding claims wherein the suction muzzle (55a) laterally surrounds the feed muzzle (54a).

20. A method for operating the print head of any of the preceding claims comprising the steps of feeding ink through the ink supply ducts (15) to the nozzles (4) and withdrawing ink through the ink suction ducts (15) from the nozzles(4).

21. The method of claim 20 comprising the step of conveying ink over a forward edge (54b, 55b) of at least one of the feed tube (54) and the suction tube (55).

22. The method of any of the claims 20 or 21 comprising the steps of feeding more ink through the feed tube (54) than through the suction tube (55), even if no printing takes place, withdrawing part of the ink through the suction tube (55), letting surplus ink flow backwards along an outer side of the suction tube (55), and collecting the surplus ink in a trench (58) surrounding a rearward base of the nozzle (4) and withdrawing it through the suction ducts (16a, 16).

23. The method of any of the claims 20 or 21 comprising the step of retrieving all ink fed through the feed tube (54) by means of the suction tube (55) if no printing takes place.

24. The method of any of the claims 20 to 23 comprising the step of printing an ink having a viscosity above 1 Pa-s, wherein a diameter (Df) of the feed tube (54) is at least 20pm, in particular at least 50pm.

25. The method of any of the claims 20 to 24 comprising feeding ink from the feed muzzle (54a) to the suction muzzle 55b while printing.

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