WO2007038987A1

WO2007038987A1 - Printing cell and printing head for printing of molten metals

Info

Publication number: WO2007038987A1
Application number: PCT/EP2005/054967
Authority: WO
Inventors: Martin Suter
Original assignee: Inspire Ag Fur Mechatronische Produktionssysteme Und Fertigungstechnik
Priority date: 2005-09-30
Filing date: 2005-09-30
Publication date: 2007-04-12
Also published as: DE112005003693T5

Abstract

The invention is directed to a printing head for the dispensing of Electrically conductive liquids at high temperatures, in paticular molten metals. The operating principle is based on magneto-hydrodyanmic pumping, applying a magnetic and an electric field. The printing head comprises a helically shaped coil for generating a manetic field and at least one supply channel arranged parallel to the coil axis. Dispensing channels are interconnected to at least one supply channel and are arranged perpendicular to the coil axis and the supply channel . Each dispensing channel has two electrodes aligned to each other and perpendicular to the coil axis and the dispensing channel. To dispense a specific amount of liquefied material through the dispensing channel a first voltage is applied to the coil to create a magnetic field and a second voltage is applied to the electrodes. In a preferred embodiment the printing head has a layered setup .

Description

PRINTING CELL AND PRINTING HEAD FOR PRINTING OF MOLTEN METALS

FIELD OF THE INVENTION The invention relates in general to a system and a method for manufacturing three- dimensional metal objects, e.g. such as used in solid freeform fabrication. More particularly, the invention is related to a printing head for printing liquefied metals such as aluminum, magnesium, bronze, titan or steel in the form of droplets or as continuous stream onto a substrate to make 2D or 3D objects from the printed materials. By controlling the motion of the printing head and the substrate relative to each other a part can be built layer by layer. This is especially useful in Rapid Manufacturing and Solid Freeform Fabrication.

BACKGROUND OF THE INVENTION

Liquid droplet ejection techniques are well known in the field of ink-jet printers and metal spray devices. Ink-jet printers typically create a printed image by ejecting water-based inks or hot-melt waxes through an orifice at an ambient or relatively low temperature. A wide variety of materials is used to manufacture the ejection mechanisms for these printers because of the low temperature of the printing process. For example, piezoelectric crystals are used to convert an electrical signal into an acoustical signal to eject a low temperature droplet of ink. In another example, a bundle of fine wires is used to transmit an ultrasonic pulse to an ink meniscus to eject an ink droplet at a low temperature. At temperatures greater than 600 ⁰C piezoelectric materials and bundles of fine wires decompose or cease to function and are therefore inappropriate. GBl 472939 was filed in 1977 by Osprey Metals Ltd. and is directed to a metal spray device. The described device ejects droplets which follow erratic trajectories and cannot be used to build precision structures.

US5257657 was filed in 1993 by lncre Inc. (US) and describes the making of a free-form, three-dimensional, solid-phase object from droplets of liquid-phase material having appreciable surface tension and well-defined solidification properties. The liquid-phase material is ejected from an ejection head in discrete droplets onto a substrate. The temperature, frequency, size, and trajectory of the droplets and the relative speed of motion between the substrate and the ejection head are adjusted to compensate for the physical properties of the liquid-phase material and the heat dissipation characteristics of the growing object to form a desired object.

US51 71360 was filed in 1992 by the University of Southern California and describes a method of manufacture of a product by directing a stream of liquid from a nozzle onto a collector of the shape of the desired product. By applying a disturbance to the stream of liquid a droplet stream is produced. The described apparatus for manufacturing a net form product comprises a source of molten material under pressure, a support for positioning a product collector in a chamber with the collector defining a desired product, a droplet stream generator positioned within the chamber. The apparatus further comprises a conduit for conducting molten material from the material source to the generator nozzle, a mecha- nism, typically a modulator, for disturbing the droplet stream, and a drive mechanism for relative movement of the nozzle and support.

US5598200, filed by David Gore in 1997, is directed to a relatively complicated apparatus for ejecting on demand a discrete droplet of liquid at a high temperature along a predetermined trajectory by transferring a physical impulse from a low temperature environment to a high temperature environment. The ejector apparatus includes a vessel having an interior that contains a high-temperature liquid, such as liquid metal (Aluminum (Al), Tin (Sn), Zinc (Zn)). The interior includes an inlet end with a thermally isolative impulse transmitting device, a feed supply for the droplet material and a discharge region having an orifice through which the discrete droplets are ejected. An inert gas is feed through the inlet end and into the vessel to create an overpressure over the liquid. By increasing the overpressure the droplet size is increased. A heater heats the material contained within the interior. An impulse generator is connected and imparts a physical impulse to the impulse transmitting device to produce an ejection pressure at the orifice to eject a discrete droplet of the high-temperature liquid.

US6149072 was filed in 2000 by the University of Arizona and is directed to a droplet selection system and a method for freeform fabrication of three-dimensional objects. Molten metal is ejected as a continuous stream under the influence of gravitational forces. The stream is broken up into droplets by a disturbing vibration induced mechanically or acousti- cally in the melt pool. To be able to build mechanical parts, the continuous stream of droplets has to be turned on and off - or alternatively the 'unwanted' droplets have to be deflected by electrostatic forces to a collector which prevents that unwanted droplets reach the target. Many droplets can be created side by side, forming a line. Also a matrix printing head with individually actuated nozzles is presented in the drawings.

US5960853 was filed in 1999 by Aeroquip Corp (US) and describes an apparatus for creating a free-form three-dimensional article using a layer-by-layer deposition of a molten metal. In general the same method as described in US6149072 is used to generate droplets. The making of a three dimensional part is described in detail. US6202734 was filed in 2001 by Sandia Corp. (US) and describes a relatively bulky and complicated apparatus to supply molten metal droplets for manufacture of parts made out of metal. The apparatus comprises a device for supplying molten metal, a device for heating molten metal such that it remains fluid while within the apparatus, a device for applying a magnetic field in a direction perpendicular to the device for supplying molten metal, a device for transmitting electric current directly through the molten metal to exert a force in a first direction on molten metal. The magnetic field is generated by a permanent magnet thus constraining the application to temperatures below 500⁰C. The patent shows the design of a single cell using a layer strategy.

PROBLEM TO BE SOLVED

It is an object of the present invention to provide a printing head suitable to eject liquid metal having a simple and robust setup. It is a further objective to enable the control of the ejection process, such as the moment of ejection, the amount of material dispensed, the ejection speed and other relevant process parameters. It is a further objective to provide a printing head capable of dispensing molten metals, such as liquid steel or aluminum, and other melts at very high temperatures. It is a further object of the present invention to provide a printing head having a modular setup with several printing cells which can be activated and controlled individually. It is still a further object to provide a printing head having a setup suitable to miniaturize such that very small amounts of liquefied material be dispensed. SUMMARY OF THE INVENTION

The present invention is directed to a printing cell and a printing head consisting out of several individual printing cells each made such that an on demand ejection of a specific amount of liquid metal is possible when an external command is received.

The operating principle of a preferred embodiment of a printing cell is based on magneto- hydrodynamic pumping (from now on MHD pumping). In MHD pumping a Lorenz Force acts onto a moving charge located in the effective range of a magnetic field. By two electrodes, which are in contact with an electrically conductive fluid in a channel, a voltage is applied to the electrically conductive fluid in the channel such that current flows between the two electrodes and thus moving charges are present. In that a magnetic field is applied in general perpendicular to the general direction of the current and to the flow channel, a force exerts onto the fluid in the channel such that the fluid is pumped through the channel.

A printing head in general comprises a helically shaped coil for generating a magnetic field and at least one supply channel arranged substantially parallel to the coil axis. Dispensing channels are interconnected to at least one supply channel and are arranged substantially perpendicular to the coil axis and the supply channel. Each dispensing channel has two electrodes aligned to each other and being in contact with the liquefied material present in the dispensing channel ready to be dispensed. The electrodes are arranged in general perpendicular to the coil axis and the dispensing channel. To dispense a specific amount of liquefied material through the dispensing channel a magnetic field is generated by the coil in that a first voltage is applied to the coil. By applying a second voltage to the electrodes a current and thus moving charges are present in the liquefied material present in between the electrodes. Thereby a Lorenz Force is generated parallel to the dispensing channel which acts onto the liquefied material such that the liquefied material is ejected in a defined man- ner out of the dispensing channel. Depending of the size of the dispensing channel and the dispensing head it is possible to eject droplets having a diameter of e.g. 100 μm or to dispense material as a continuous stream of material.

If appropriate the dispensing channel is equipped with an ejector nozzle to precisely form the jet ejected from the dispensing channel such as required. Depending on the field of ap- plication the ejector nozzle has a convergent, narrowing down from a wide diameter to a smaller diameter in the direction of the flow, or a divergent, expanding from a smaller diameter to a larger one, fluid channel or a combination of both (convergent-divergent). The side walls of the nozzle may be straight, concave or convex. E.g. by a convergent nozzle with convex side walls it is achieved that a fast jet of liquid results from the outlet of the nozzle. Depending on the material to be dispensed it is advantageous when the nozzle outlet opening has sharp edges or a chamfer such that a precise tear-off edge results. The ejector nozzle preferably is made out of a thermally insulating material such that no unnecessary thermal energy is extracted from the material to be dispensed. If appropriate the ejector nozzle may protrude above the outer contour of the printing head defining the distance between the printing head and the substrate.

If appropriate, one ore more dispensing channels can be connected to the outside environment or a gas supply by a relieve channel. Through the relieve channel gas or another suitable fluid can be applied to / sucked into the dispensing channel when the printing material is accelerated. Thus a separation of the fluid between the electrodes and the supply fluid can be achieved in the area of the where the relieve channel meets the dispensing channel such that only a small mass of the fluid to be ejected has to be accelerated. After the ejection of an amount of fluid, the supply fluid again replaces the sucked in gas and the dispensing channel is completely filled with working material. To avoid freezing of the material present in the supply and the dispensing channels the material has to be heated. This can be realized in that the coil is made such that it supplies sufficient residual heat and/or by additional heating means arranged inside and/or outside the coil. By modifying the height and orientation of the first and the second voltage is pos- sible to precisely adjust the force acting onto the material to be ejected and thereby the behavior of the ejection.

Due to the fact that a printing cell is capable in printing metals such as aluminum, steel, bronze, titan or other metallic materials in their liquid form, the materials used for the printing cells and the printing head must be thermally stable at and a certain range above the melting point of the materials to be dispensed. Suitable electrically conductive materials to be used for the electrically conductive part of the device are e.g. tungsten, which has a melting point of 3422 ⁰C, or tantalum which has a melting point of 301 7 ⁰C. It has to be noted that depending on the filed of application other materials may be suitable. Suitable electrical insulating materials are out of the group of ceramics such as aluminum oxide (AI₂O₃), which is also commonly referred to as alumina. Alternatively other materials having high temperature stability may be appropriate. All materials which are in contact with the liquid metal to be dispensed should be thermally stable at high temperatures and insoluble by the liquid metal.

A preferred embodiment of a printing cell according to the present invention preferably has a setup made out of layers comprising electrically conductive and non-conductive materials combined in a single layer. The single layers of material may be made e.g. by punching out different shapes out of thin layer of materials. Then the single layers are assembled in that at least one piece of electrically conductive material is surrounded by an electrically non- conductive material. For assembly of the device the single layers of material are put on top of each other such that between the single layers in distinct area electrical contact is estab- lished. The single layers may be bonded to each other e.g. by sintering or another appropriate process. Alternatively or in addition the layered setup offers the opportunity to clamp the single layers together which allows a simple assembly and disassembly of the device.

By the layered setup it is possible to assemble a printing head comprising a helically shaped three-dimensional coil and several printing cells for the ejection of liquefied material. Thereby coil axis of the three-dimensional coil is normally arranged parallel to at least one supply channel for liquefied material and parallel to a dispensing channel interconnected to the supply channel.

An embodiment of a printing cell according to the present invention has the following four layered setup:

a) A first layer comprises a layer made out of insulating material surrounding an interconnecting area made out of electrically conductive material suitable to guide electrical current between two adjacent layers. The layer further comprises a pass- through opening arranged in the insulating material and being part of a supply channel suitable to supply material to be dispensed from a material supply to a dispensing channel arranged in a neighboring layer.

b) A second layer also having a pass-through opening for liquefied material being part of the supply channel. The pass-through opening is normally arranged in the insulating material and aligned to the pass-through opening of the first layer. In the second layer the pass-through opening is interconnected to a dispensing channel in the insulating material which is interconnected to a first and a second lateral electrode aligned to other and arranged in general perpendicular to the dispensing channel. The second layer further comprises an interconnecting area made out of electrically conductive material which electrically interconnects the interconnecting area of the first with a further interconnecting area of a third layer. The interconnecting area is at least partially surrounded by insulating material.

c) A third layer in general corresponds to the first layer in that it also comprises a pass through opening for liquefied material arranged in the insulating material. A interconnecting are made out of electrical material is arranged such that in an assembled position of the device it is electrically interconnected to the interconnecting area of the second and a coil area of a forth layer.

d) A fourth layer comprises an area made out of insulating material having a pass- through opening aligned with the pass-through opening of the other layers. The fourth layer further comprises a coil area made out of electrically conductive material in arcuated or, if appropriate, square shape. In the assembled device, the coil area is electrically connected to the interconnection areas of neighboring layers.

In a preferred embodiment the layers of a printing cell have a typical thickness in the range of 0.1 mm to 1 mm. In a preferred embodiment the helical coil has a typical cross-section in the range of 0.5 to 5 mm². However, depending on the field of application other dimensions may be appropriate.

In a preferred embodiment of an assembled printing head, when several printing cells are put on top of each other, the coil area of a last (fourth) layer and the interconnecting areas of the other layers are forming an electrical coil having an in general helically curved shape with the axis of the coil arranged in general perpendicular to the electrodes and the dispensing channel and passing through the centre of the area between the electrodes. When supplied with a sufficient amount of current the electrical coil establishes a magnetic field ar- ranged in general perpendicular to the dispensing channels and the electrodes of the printing cells. By waste heat of the electrical coil and/or an additional heating element metal arranged in the supply channel and in the dispensing channel is kept in liquid state such that it can be dispensed through the dispensing channel(s). The single layers of a printing cell may have the same or a different thickness. If appropriate, further layers may be foreseen to add additional functions.

A preferred printing cell according to the present invention comprises at least one supply channel which is interconnected to at least one dispensing channel arranged in general perpendicular to the pass-through channel. The at least one dispensing channel is intercon- nected to a first and a second electrode. The pass-through channel may be located inside or outside of a helically shaped electrical coil suitable to establish a magnetic field in general perpendicular to the dispensing channel and the electrodes. If appropriate the supply channel is pressurized, e.g. by gas pressure, to force the material in the direction of the dispensing channel. In certain embodiments the dispensing channel comprise a gate, e.g. in the form of a lateral slide, to control the shape and the opening of the dispensing channel or to close the dispensing channel completely.

To be able to build a printing cell in small sizes a simple design is employed. All features are designed in a way so that they can be fabricated using a layer manufacturing method. In this way a single printing cell can be built out of four different layers as described above. A dispensing layer comprises the dispensing channel and the two electrodes. Two spacing layers (see above items a) and c)) form insulating walls of the liquid metal channels. The magnetic field is generated by a coil incorporated into the layers. Common to all layers is a pass-through channel which is connected to a reservoir of liquid material. Stacked upon each other, layers one through four build a printing cell. An arbitrary number of printing cells can be stacked to build a printing head. In a printing head, each cell can be actuated individually by sending current through the electrodes. The coils of the individual printing cells are connected together to build a single coil with multiple windings.

A printing head may comprise more than one coil, e.g. to achieve special magnetic fields or effects or to dispense several materials at the same time. The coils are normally arranged parallel or next to each other. As mentioned above the printing head may comprise more than one supply channel arranged in general parallel to each other and to the coil axis of one or several coils. The dispensing channels may be arranged in the same or in different coils each generating an individual field for each supply and the interconnected dispensing channels. Each supply channel is interconnected to a series of dispensing channels arranged next to each other or in an array. A printing head with more than one supply channel offers the opportunity to dispense different materials at the same time out of one printing head.

The printing head may be used to dispense (print) liquid metal onto a powder bed. The re- gion of the powder bed surface, where liquid metal is printed onto, is joined together by the liquid metal to form a fully dense, solid layer of metal. After a new layer of powder is spread on top of the previous one, the printing of the next layer can begin. By this method a solid freeform metal part can be fabricated. Loose powder remains in place as support structure and is removed after the part is built.

A preferred embodiment of a control unit for controlling the dispensing of material by a printing head according to the present invention is made such that current and voltage applied to the electrodes and/or the coil are controllable synchronously such that dispensing of the material droplets may be precisely adjusted. BRIEF DESCRIPTION OF THE DRAWINGS

The herein described invention will be more fully understood from the detailed description of the given herein below and the accompanying drawings which should not be considered limiting to the invention described in the appended claims.

Fig. 1 shows a setup of a first printing cell and a first printing head;

Fig. 2 shows the inside of a printing head according to Fig. 1 without insulating material;

Fig. 3 shows a second embodiment of a printing head in a perspective view;

Fig. 4 shows the printing head according to Fig. 3 without external housing.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A better understanding of the present invention may be obtained by the present detailed description which, when examined in connection the accompanying drawings sets forth preferred embodiments of the inventions described herein. It should be understood that corresponding elements in the various figures are generally identified with corresponding refer- ence numbers.

Figure 1 is showing a first embodiment of a printing head I ₁ a printing cell 2 and several layers of a printing cell 2 in perspective view. The printing head 1 consists out of several printing cells 2 laterally aligned and functionally interconnected to each other. A printing cell 2 of the described embodiment comprises substantially four layers 3, 4, 5, 6 with areas made out of insulating material 7 acting as housing and areas made out of conductive material 8, 14, 15, 1 6. A first layer 3 comprises an area made out of insulating material 7 surrounding an interconnecting area 8 made out of electrically conductive material suitable to guide electrical current between two adjacent layers 4, 6. The first layer further comprises a pass-through opening 9 arranged in the insulating material and being part of a supply channel 10 (general x- direction) suitable to supply material to be dispensed from a material supply (see Figure 2) to a dispensing channel 1 1 arranged in a neighboring layer 4.

A second layer 4 is also having a pass-through opening 9 for liquefied material being part of the supply channel 10. The pass-through opening 9 is normally arranged in an area of insulating material 7 and aligned to the pass-through opening 9 of the first layer 3. In the second layer 4 the pass-through opening 9 is interconnected to a dispensing channel 1 1 in the insulating material 7 which is interconnected to a first and a second lateral electrode 1 2, 13 arranged in y-direction and aligned to each other. As it can be seen the electrodes 12, 13 are arranged in general perpendicular to the dispensing channel 1 1 which runs in general z-direction. The second layer 4 further comprises an interconnecting area 14 made out of conductive material which electrically interconnects the interconnecting area 8 of the first with a further interconnecting area 1 5 of the third layer 5. As it can be seen the second layer 4 comprises in the shown embodiment a gate in the form of a lateral slide 27. The slide 27 allows the modification of the cross-section or the complete closing of the dispensing channel in that it is moved into the dispensing channel or out of it. The second layer 4 here further comprises a relieve channel 26 which interconnects the dispensing channel 1 1 with the outside. By the relieve channel 26 gas can be sucked into the dispensing channel 1 1 when the liquefied material is accelerated. Thus a separation of the fluid between the electrodes and the supply fluid can be achieved resulting in that only a small mass of the fluid to be ejected has to be accelerated. After the ejection of an amount of fluid, the supply fluid again replaces the sucked in gas and the dispensing channel is completely filled with working material. The relieve channel of the here shown embodiment is arranged at an an- gle with respect to the dispensing channel 1 1 such that a good separation of the material is achieved. It becomes clear that, depending on the field of application, the lateral slide 27 and the relieve channel 26 need not to be present in a printing cell.

A third layer 5 in general corresponds to the first layer 3 in that it also comprises a pass through opening 9 for liquefied material surrounded by insulating material 7. The interconnecting area 15 made out of electrically conductive material is arranged such that in an assembled position of the device it is electrically interconnected to the interconnecting area 14 of the second and a coil area 1 6 of a forth layer 6.

The fourth layer 6 comprises an area made out of insulating material 7 surrounding a pass- through opening 9 aligned with the pass-through opening of the other layers (alignment indicated by dashed lines). The fourth layer further comprises a coil area 1 6 made out of electrically conductive material. The coil area 16 has in the plane of the layer a substantially arcuated shape encircling the pass-trough opening 9 at a certain distance.

Figure 2 is showing an assembled printing head 1 without the insulating material 7 (see Figure 1 ) which acts as housing, holding the areas made out of conductive material 8, 14, 15, 1 6 and which forms the supply channel 10. Thereby it is possible to see the three dimensional helical shaped coil 1 7 which is formed by the electrical interaction of the interconnecting areas 8, 14, 15 and the in general circularly coil areas 16. Inside the coil 1 7 liquefied material 18 is schematically visible as it would be arranged in the supply channel 10 and dispensing channel 1 1 (see Figure 1 ). The dispensing of the material 18 is controlled by the electrodes 12, 13 arranged laterally to each dispensing channel 1 1 . The supply channel 10 is interconnected to a material supply reservoir 19 in which material to be dispensed by the printing head 1 is present. During operation the material to be dispensed 18 is kept - by heating at least in the supply channel 10 - at a temperature above the melting point.

During operation voltage from a first power supply 20 is applied to the coil 17 such that a current flows through the coil 17. Thereby a magnetic field is generated which is schemati- cally indicated by arrow B (x-di recti on). The coil 1 7 and the power supply 20 may be designed such that the coil 1 7 generates sufficient waste heat to keep the material 18 to be dispensed at a liquefied state in the supply channel 10 and the dispensing channels 1 1. The dispensing of the material 18 through the dispensing channels 1 1 is controlled by the electrodes 1 2, 13 in that voltage of a second power supply 21 is applied to each of the elec- trades 12, 13 by a switch 22 (here only shown in connection with the first two electrodes - all the other electrodes would have a similar activation). By closing the switch 22 electric charges flow through the electrodes 12, 13 and the liquefied material arranged between the electrodes 1 2, 13 as schematically indicated by arrow Q (y-direction). Due to that a force F acts on the material arranged in the effective range of the electrodes 12, 13 causing dis- pensing of a precise amount of material from the dispensing channel 1 1 . The ejection of material droplets 23 is demonstrated in a simplified manner. The amount and the speed of material ejected mainly depends from the voltages of the first and the second power supply 20, 21 , the length of the impulse applied, respectively the amount of moving charges exchanged between the first and the second electrodes 12, 13. Further parameters are given by the shape of the impulse applied to the electrodes 12, 13. Alternatively or in addition the dispensing of the material 18 can be controlled by the shape of the voltage of the first power supply 20.

Figure 3 and Figure 4 are showing a further embodiment of a printing head 1 in assembled manner. Figure 3 is showing the printing head 1 in a first perspective view from below with housing and Figure 4 is showing the same printing head 1 in a second perspective view from bellow without housing (without layers of insulating material 7).

The shown embodiment further comprises eject nozzles 25 arranged at the end of each dispensing channel 1 1 which serve to form droplets of precise shape. The eject nozzles 25 are made out of a material which is thermally stable even at temperatures above the melting temperature of the materials to be dispensed. In difference to the embodiment of Figures 1 and 2 the coil areas 1 6 of this embodiment are in general rectangularly shaped whereby a "longer" magnetic effect results in the area where the electrodes are arranged.

Claims

PATENT CLAIMS

1 Printing head for the dispensing of an electrically conductive liquid, comprising at least one helical coil having a coil axis, at least one supply channel arranged in gen- eral parallel to a coil axis and at least one dispensing channel arranged in general perpendicular to a coil axis whereby the at least one dispensing channel is interconnected to a first and a second electrode aligned to each other and arranged in general perpendicular to the coil axis and the dispensing channel.

2 The printing head according to claim 1 , wherein the printing head comprises a heat- ing means to keep the printing head at a certain temperature above a melting temperature of a material to be dispensed.

3 The printing head according to claim 2, wherein the heating means is the at least one helical coil.

4 The printing head according to one of the previous claims, wherein the printing head comprises at least two helical coils.

5 The printing head according to claim 4, wherein the at least two helical coils are arranged parallel to each other.

6 The printing head according to claim 4, wherein the at least two helical coils are arranged concentrically to each other.

7 The printing head according to one of the previous claims, wherein the printing head comprises at least two supply channels each interconnected to at least one dispensing channel.

8 The printing head according to claim 7, wherein at least two supply channels are arranged inside the same or different helical coil. The printing head according to claim 7, wherein at least one supply channel is arranged outside of at least one helical coil.

The printing head according to one of the previous claims, wherein at least one dispensing channel has an ejector nozzle.

The printing head according to claim 10, wherein the ejector nozzle has a convergent and/or a divergent shape.

The printing head according to one of the previous claims, wherein the dispensing channel has a dispensing opening with a round or a square cross-section.

The printing head according to one of the previous claims, wherein the dispensing channel is interconnected to a relieve channel such that the material to be dispensed is separated during ejection.

The printing head according to claim 13, wherein the material is separated in that gas flows through the relieve channel.

The printing head according to claim 13 or 14, wherein the relieve channel intercon- nects the dispensing channel with the outside.

The printing head according to one of the previous claims, wherein the supply channel is pressurized.

The printing head according to one of the previous claims, wherein a voltage applied to the coil and/or the electrodes results in a force acting on the fluid to be dispensed in the direction either towards or away from the dispensing opening.

The printing head according to one of the previous claims, wherein the dispensing channel comprises a gate to modify the cross-section of the dispensing channel. The printing head according to claim 18, wherein the gate is a lateral slide.

The printing head according to one of the previous claims, wherein the printing head is made out of interconnected printing cells arranged next to each other, each comprising a part of the supply channel, a part of the helical coil and a dispensing chan- nel.

The printing head according to claim 20, wherein the printing cells comprise a layered setup made out of insulating material and out of electrically conductive material.

The printing head according to claim 21 , wherein the printing cell comprises

a) a first layer comprising an area made out of insulating material comprising a pass-through opening and a first interconnecting area made out of an electrically conductive area;

b) a second layer comprising an area made out of insulating material comprising a pass-through opening aligned to the pass-through opening of the first layer, a dispensing channel and an second interconnecting area, made out of electrically conductive material interconnecting the first interconnecting area of the first layer with a third interconnecting area of a third layer;

c) a third layer comprising an area made out of insulating material, a pass- through opening aligned to the pass-through opening of the second layer and a third interconnecting area made out of electrically conductive material interconnecting the second interconnecting area of the second layer to a coil area of a fourth layer;

d) a fourth layer comprising an area made out of insulating material comprising a pass-through opening aligned to the pass-through opening of the third layer and a coil area, made out of electrically conductive material, intercon- necting the third interconnecting area of the third layer with an interconnecting area of a next layer.

The printing head according to one of the claims 21 or 22, wherein the layers are bonded to each other by sintering.

The printing head according to one of the claims 21 or 22, wherein the insulating material consists out of the group of ceramics and the electrically conductive material consist out of the group of tungsten or tantalum.

The printing head according to claim 22, wherein the coil area has an arcuated shape.

The printing head according to claim 22, wherein the coil area has a rectangular shape.

The printing head according to one of the claims 13 to 26, wherein each dispensing channel comprises an ejector nozzle to eject material.

The printing head according to claim 27, wherein the material of the nozzle comprises sapphire.