Technical Field

The invention relates to an inkjet printing system, in particular an electrohydrodynamic inkjet printing system, with a plurality of ink nozzles on a print head and with gas ducts arranged at least partially in the print head. The invention also relates to a method for operating such a printing system.

Background Art

W0202 1008817 describes an electrohydrodynamic inkjet printing system with a print head having a plurality of ventilation ducts located at the ink noz- zles. The ventilation ducts are used to feed dry gas to the space between the print head and the target, thereby expediting a uniform drying of the ink on the target.

US4829325 describes an electrohydrodynamic inkjet printing sys- tem with a print head having a single nozzle and a gas duct ending at the nozzle. The gas is used to convey the ink drops towards the target.

Disclosure of the Invention

The problem to be solved by the present invention is to improve the reliability of inkjet printing systems. This problem is solved by the printing system of claim 1.

Hence, the invention relates to an inkjet printing system comprising at least the following elements:

- A print head.

- A plurality of ink nozzles: These are the nozzles from where the ink droplets are ejected. They are arranged on the print head.

- A first gas source: This is a source of a gas, and it is adapted and structured to feed gas in to the first gas ducts of the next paragraph.

- First gas ducts arranged at least partially in the print head. These first gas ducts have at least a first end connected to the first gas source and a plurality of second ends arranged at the ink nozzles. In this context, “arranged at the ink noz- zles” is advantageously to be understood such that the distance between the second ends and the closest nozzle is less than 100 mhi, in particular less than 50 pm, in par- ticular less than 10 mhi.

- At least one evaporator arranged along first gas ducts before the nozzles: This evaporator is adapted to evaporate at least one fluid into the gas before it arrives at the nozzles. Thus, it is possible to increase the concentration of said fluid in the gas.

The invention is based on the understanding that one factor impact- ing the reliability of inkjet printing systems is the deposition of dried ink residuals at the nozzles.

Further, it is based on the idea that such drying can be reduced or even avoided by feeding a gas to the ink ducts and by using an evaporator to increase the concentration of at least one substance in the gas before it reaches the nozzles, thereby creating an atmosphere, at the nozzles, into which ink is less likely to evapo- rate.

The evaporator may in particular be used to evaporate a solvent used in the, or a substance similar to the solvent, thereby more efficiently reducing the amount of ink solvent that evaporates at the nozzles.

As mentioned, the first gas ducts are arranged at least partially in the print head. The first gas source is, however, advantageously a part separate from (i.e. not integrally connected to) the print head.

The print head is advantageously an electrohydrodynamic print head and comprises ejection electrodes located at the nozzles. They are positioned to eject ink from the nozzles by means of electrical fields acting on the ink.

Alternatively, though, the print head may also be based on another “drop on demand” (DOD) ejection principle, such as on thermal DOD printing or pie- zoelectric DOD printing,

Advantageously, the print head further comprises:

- A front surface: This is the surface of the system facing the target during operation. Advantageously, it is planar for being used with planar targets.

- A plurality of recesses arranged in the front surface with at least one of the ink nozzles and at least one of the second ends of the first gas ducts ar- ranged in each of the plurality of recesses: Hence, the first gas ducts feed the gas di- rectly into the recesses, which allows to maintain a well-defined atmosphere within the recesses, making it less likely that potentially dry gas from the region beyond the front surface reaches the nozzles. (Note that there may be further recesses in the front surface, in addi- tion to said “plurali ty of recesses” that do not fulfill the conditions of the previous paragraph.)

If the print head is an electrohydrodynamic print head, the ejection electrodes may be arranged around the recesses between the front surface and the ink nozzles.

At a location below the ejection electrode, the diameter of the recess (in a direction perpendicular to the nozzle axis) may, in this case, be larger than the inner diameter of the nozzle to form a widened pocket for receiving the nozzle. Such a design reduces the risk of ink reaching the walls of the recess.

In particular, the print head may further comprise a nozzle carrier forming the base (i.e. read end) of the recesses and extending parallel to the front sur- face. The nozzles are mounted to the nozzle carrier. The first gas ducts have duct sec- tions that extend parallel to the nozzle carrier in a region between the nozzle carrier and the front surface. Hence, the region between the nozzle carrier and the front sur- face is used to accommodate at least part of the first gas ducts.

Advantageously, at least some said duct sections extend along sev- eral of the nozzles, in particular along a row of the nozzles in a two-dimensional array of nozzles.

The printing system may further comprise the following elements:

- A second gas source. This is a source of a gas, and it is adapted and structured to feed gas into the second gas ducts mentioned in the next paragraph. The second gas source may be the same unit as, or share components with, the first gas source.

- Second gas ducts arranged at least partially in the print head:

These second gas ducts have at least a first end connected to the second gas source and a plurality of second ends arranged at the ink nozzles, with the second ends of the first gas ducts being closer to the ink nozzles than the second ends of the second gas ducts.

The second gas ducts do not communicate with the evaporator(s), i.e. the gas passing from their first to their second ends does not pass along any evap- orator. Hence, the gas emerging from the second ends of the second gas ducts is dry er than the gas emerging from the second ends of the first gas ducts.

This design al lows to feed dry gas, by means of the second gas ducts, to the area between the nozzles and the target, expediting the drying of the ink on the target while the first gas ducts reduce the drying of ink at the nozzles. The second ends of the second gas ducts are advantageously located at the front surface of the print head while the second ends of the first gas ducts are located in the recesses, thereby making it even less likely that dry gas from the second gas duct reaches the nozzles. The printing system may further comprise the following elements:

- A gas sink: This is a sink of a gas, and it is adapted and structured to suck gas from the third gas ducts mentioned in the next paragraph.

- Third gas ducts arranged at least partially in the print head: These third gas ducts have first ends located at the nozzles and at least a second end con- nected to the gas sink.

These third gas ducts allow remove at least part of the gas that has been conveyed into the region between the print head and the target by the first and/or second gas ducts.

Without the third gas ducts, the excess gas from the first and/or see- ond gas ducts to this region would have to escape in lateral direction (i.e. in a direc- tion parallel to the front surface of the print head), thereby generating a lateral flow of gas that would be stronger at the periphery of the print head than at its center, which would tend to deflect the ink drops.

Advantageously, the second ends of the first gas ducts are closer to the ink nozzles than the first ends of the third gas ducts, which prevents the gas from the first gas ducts from being conveyed off before it can reach the nozzles.

The invention also relates to a method for operating a printing sys- tem as described herein. It comprises at least the following steps:

- Feeding ink having a solvent to the ink nozzles: In this context, the term “solvent” designates a part of the ink that is able to evaporate. The solute of the ink, i.e. the solid portion remaining on the substrate once the solvent has dried, is/are typically dissolved, dispersed or otherwise suspended in the solvent.

- Feeding a gas from the first gas source through the first gas ducts to the ink nozzles: This gas defines at least part of the atmospheric conditions at the location of the ink nozzles.

- E vaporating, by means of the evaporator, at least one component of the solvent into the gas fed through the first gas ducts. Thus, the saturation of the gas with the solvent (or part thereof) is increased, which helps to reduce the evapora- tion rate at the location of the ink nozzles. Preferably the temperature of the gas at the position of the evaporator is at the same magnitude as it is at the printhead, particu- larly at the nozzle, in order to prevent condensation or incomplete saturation which again would promote evaporation from the nozzle. Advantageously, the temperature of the gas at the position of the evaporator is within 5°, in particular within 2°, of the temperature at the nozzle. if the printing system has second and third gas ducts as mentioned above, the total gas flow through the first and second gas ducts is advantageously equal to a total gas flow through the third gas ducts. In this context, two gas flows are advantageously considered to be equal if they differ by less than 20%, in particular by less than 10%.

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 de- tailed description thereof. Such description makes reference to the annexed drawings, wherein:

Fig. I shows a schematic view of a printing system with a partially sectional view of a print head and of the target,

Fig. 2 is a vertical sectional view through an embodiment of the print head at the location of a nozzle,

Fig. 3 is a horizontal sectional view along line A

Fig. 4 is a horizontal secti onal view along line B

Fig. 5 is a horizontal sectional view along line C

Fig. 6 is a horizontal sectional view along line D

Fig. 7 is a horizontal sectional view along line E

Fig. 8 is a horizontal sectional view along line F

Fig. 9 is a horizontal sectional view along line G

Fig. 10 is a horizontal sectional view along line

Fig, 11 is a horizontal sectional view along line

Fig. 12 is a horizontal sectional view along line

Fig. 13 is a horizontal sectional view along line

Fig. 14 is a horizontal sectional view along line

Fig. 15 is a horizontal sectional view along line

Fig. 16 is a horizontal sectional view along line

Fig. 17 is a horizontal sectional view along line

Fig. 18 is a view of the front surface of the print head,

Fig. 19 is a schematic representation of the shape of the duct sec- tions across the print head, Fig. 20 is a sectional view of an embodiment of an evaporator along line A-A of Fig. 22,

Fig. 21 is a sectional view of the evaporator of Fig. 20 in a plane perpendicular to the one of Fig. 20 along line B-B of Fig. 22, with Fig. 21 being at a smaller scale than Fig. 20, and

Fig. 22 are sectional views along lines a - f of Fig. 20.

Note that in Figs. 3 - 10, 12, 13, 15, 16, 19, and 22 black parts de- note hollow regions or (for vias) metal regions and white parts denote solid regions. In Figs. 11, 14 and 17, black parts denote metallic or conductive parts and white parts denote dielectric, non-conductive parts.

Fig. 2 does not have the same scaling as Figs. 3 - 18 in order to show several nozzles at the same time in Figs. 3 - 18.

Modes for Carrying Out the Invention

Definitions

“Forward” or “font” defines the direction into which the print head is designed to eject ink. For example, the ejection electrodes are forward from and in front of the nozzles.

“Backward” or “behind” defines the opposite direction. For exam- ple, the nozzles are arranged backward from or behind the ejection electrodes.

“At the front” and “at the back” are understood to designate a loca- tion at levels forward from or backward from something else.

“Front” and “back” are the forward and backward sides.

Properties “at a given nozzle” are advantageously understood as properties that are true for a majority, in particular for least at 90%, of the nozzles.

For example, if it is said that “at a given nozzle, the guard electrode is arranged be- tween the ejection electrode and the ink retainer”, this advantageously means that this is true for a majority of the nozzles, in particular for at least 90% of them. It may e.g. be that there are some nozzles that do not have ejection electrodes and/or guard elec- trodes, such as nozzles at the edges of the print head and/or unused nozzles.

The ejection direction X of the print head defines the “vertical” up- wards 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 defi- nition of “vertical”. “Horizontal” is any direction perpendicular to the vertical direction,

“Lateral” designates something that is horizontal from something else, Printing System

Fig. 1 shows a schematic view of an embodiment of the printing system 1. It is depicted above a target 2, and it is structured to eject ink along an ejec- tion direction X onto the target.

The print head comprises a plurality of nozzles 4 located at the front side of a nozzle carrier 6. The nozzles 4 are advantageously arranged in a one- or two-dimensional array in recesses 5 in a front surface 7 of print head 3, in particular with more than 10 nozzles per row and/or column.

The printing system has a print head 3 with a plurality of ejection electrodes (not shown in Fig. 1) for ejecting ink from the nozzles 4 and optional fur- ther 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 die ink to set the ink to a defined electrical potential.

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.

The internal structure of front layer 10 is not shown in Fig. 1 and will be described in more detail below. It may e.g. be of a dielectric, in particular comprising several sublayers of polymer and/or glass.

Backing layer 12 may e.g. be an insulated semiconductor material or it may be a dielectric. Advantageously, backing layer 12 is, at least partially, of glass.

Electrical vias 14 are connected to the ejection electrodes and ex- tend through front layer 10 and the backing layer 12 for connecting the ejection elec- trodes 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.

Ink ducts 15, 16 supply ink to the nozzles 4 and (optionally) recycle ink back from the nozzles 4. They are located, in part, in front layer 10, and they e.g. extend through peripheral regions of backing layer 12. Their design is described in more detail below. At least one pump 18 and/or another pressure source or vacuum source is provided to supply ink to the supply ducts 15 and, if there are suction ducts, to retrieve ink from the suction ducts 16.

Advantageously, the printing system comprises a first pressure con- trol 20 for generating a first defined pressure pi at the input of the supply ducts 15, e.g. in a reservoir tank 22.

The ink is supplied through an optional filter 24 and the supply ducts 15 to the nozzles 4,

If there are suction ducts 16, they are connected to a suction system, which may comprise a second pressure control 26 for generating a second defined pressure p2 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.

A suitable pump design is e.g. shown in US 6631983.

Providing ink supply ducts 15 to feed ink to the nozzle and ink suc- tion ducts 16 to retrieve ink from the nozzles, allows to maintain a reservoir of fresh ink at each nozzle 4.

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.

An optional interposer layer 32 may be provided between circuit earner 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. imple- ment at least part of voltage supply 17, such as the driver stage of such a voltage sup- ply, which connects voltage sources to the various electrodes of the print head.

In the shown embodiment, the ink ducts 15, 16 extend through in- terposer layer 32 (if present) as well as circuit carrier 30.

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

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.

The pressure controls 20, 26 advantageously allow to separately ad- just the pressures in the supply ducts 15 as well as in the pressure ducts 16. The printing system further comprises a first gas source 33 adapted to feed a gas to first gas ducts 34. At least one evaporator 35 is arranged along first gas duct 34 to saturate (or at least partially saturate) the gas with a suitable liquid as mentioned above and described in more detail below. The first gas ducts 34 have at least one first end 34-1 at first gas source 33 and feed gas to second ends 34-2 located at the nozzles 4.

The system may further comprise a second gas source 36 adapted to feed a gas to second gas ducts 37. The gas provided by second gas source 36 may be the same kind of gas as or a different kind of gas than the gas provided by first gas source 33, but second gas ducts 37 do not pass along the evaporator(s) 35. The second gas ducts 37 have at least one first end 37-1 at gas source 36 and feed gas to second ends 37-2 located at front surface 7 of print head 4.

Finally, the system may comprise a gas sink 38 adapted to suck gas from third gas ducts 39. The third gas ducts 39 retrieve gas from first ends 39-1 lo- cated at front surface 7 and feed it to at least one second end 39-2 at gas sink 38.

Both, the second gas source 36 and second gas ducts 37 as well as the gas sink 38 and the third gas ducts 39 are optional.

The functions of the gas sources 33, 36, the gas sink 38, and the various gas ducts 34, 37, 39 are described in the following section.

Ventilation System

The gas sources 33, 36, the gas sink 38, and the gas ducts 34, 37, 39 form part of a ventilation system of the printing system.

The function of first gas source 33 and first gas ducts 34 is to bring saturated gas to the nozzles 4, i.e. gas that has a large amount of liquid dissolved therein, such that the evaporation of ink solvent at the nozzles 4 is reduced as de- scribed above.

For this purpose, first gas source 33 may e.g. be a pump or a pres- surized vessel adapted to feed a gas into the first end(s) 34-1 of the first gas ducts 34. This gas may e.g. be air. It may also be a gas specifically designed to withstand the high electric fields between the electrodes of print head 3, such as SF^ or C4pg, or a mixture thereof.

In more general terms, the present invention advantageously relates to a first gas source 33 providing a gas quenching electric discharges, e.g. by having a breakdown voltage, relative to air, of at least 2. For example, the breakdown voltage of SFg is 3 relative to air, the one of CqFg is 3.6. The invention also relates to feeding such a gas, by means of at least some of the gas sources 33, 36, into the first and/or second gas ducts 34, 37.

Typically, the first gas ducts 34 branch and end in a plurality of second ends 34-2, one of which is shown in Fig. 1. These second ends 34-2 are lo- cated at the recesses 5, thereby filling the recesses 5 with the saturated gas and pro- tecting the nozzles 4 from evaporation by displacing non-saturated gas entering the recesses 5 from region 9. Advantageously, the saturated gas is loaded with gaseous solvent at an equivalent temperature as the one the nozzle is kept at. In this way, the concentration of gaseous solvent contained in the saturated gas is equivalent to the vapor pressure of the solvent at the nozzle location.

To prevent condensation of solvent from the saturated gas, the por- tions of the first gas ducts 34 that follows the location of the evaporator 35 are prefer- ably kept at a temperature that is not lower than the temperature at which the gas loading is executed within the evaporator 35. The second gas ducts 37 (if present) branch, too, and end in a plu- rality of outlets 37-2 in front surface 7, one of which is shown in Fig. 1.

Similarly, the third gas ducts 39 (if present) branch and end in a plu- rality of inlets 39-1 in front surface 7, one of which is again shown in Fig. 1.

The second gas ducts 37 can be used to feed dry air into the region 9 between print head 3 and target 2. Such dry gas (i.e. gas that has low saturation for the solvents in the used ink), expedites the drying of the ink on target 2.

The third gas ducts 39 can be used to withdraw the gas that the first and second gas ducts have introduced into region 9, thereby reducing the amount of lateral gas flow in region 9 as mentioned above. Advantageously, the invention therefore relates to a method com- prising the step of feeding a first flow of gas through a first subset of the gas ducts (namely the subset of the gas ducts 34 and, if present, 37) to the region 9 and retriev- ing a second flow of gas through a second subset of the gas ducts (namely the third gas ducts 39), with the first and the second gas flows being equal, in particular within an accuracy better than 5%.

The gas fed from second gas source 36 through the second gas ducts 37 is, advantageously, again a gas with a high breakdown voltage as defined above, thereby reducing the risk of electrical breakdown between electrodes of the print head. Alternatively, or in addition, the gas from second gas source 36 can be an inert gas or it can be a reactive gas that reacts with solute. Such gas will not re- act with solute at the nozzle region if the gas introduced from the second ends 34-2 is an inert gas and displaces the reactive gas introduced from the second gas ducts 37 from tire recesses 5. Instead, the reaction will only occur on the substrate or during droplet flight.

Hence, advantageously, the method of the present invention advan- tageously comprises the following steps:

- Feeding a first gas from first gas source 33 through the first gas ducts 34 to the nozzles (4);

- Feeding a second gas from second gas source 36 through the sec- ond gas ducts 37 to the region 9 between print head 3 and target 2.

In this case, the gasses from first gas source 33 and second gas source 36 may be different (i.e. different even before the first gas passes the evapora- tors) 35).

Advantageously, the first gas is an inert gas for the ink (i.e. there is no chemical reaction at the printing conditions between the first gas and the ink) while the second gas chemically reacts with the ink (i.e., at the printing conditions, there is a chemical reaction between the second gas and the ink).

For example, the first gas may consist of at least one of: nitrogen, carbon dioxide, and a noble gas. The second gas may comprise oxygen. When an oxi- dizing ink is used, diying by oxidation can be used to expedite the drying process. Oxidizing inks are known to the skilled person, see e.g. http://printwiki.org/Oxida- tion. A reaction on the substrate may be further enhanced by increased the substrate temperature, while the print head can be kept at a reference temperature.

Print Head Design Overview

Fig. 2 shows a sectional view of print head 3 at a nozzle 4. (As mentioned above, in contrast to Fig. 1, the ejection direction X in Fig. 2 points up- wards.) Figs. 3 - 17 show sectional views, perpendicular to ejection direction X, along the lines A- A to 0-0 of Fig. 2.

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 sits at the base of its recess 5.

Fig. 2 also shows the various electrodes that may be associated with the nozzles 4. They are described in the following.

The ejection electrode 40 is located on the front side of the nozzl e 4. In the embodiment of Fig. 2 and 14, it is annular with a central opening (at the loca- tion of recess 5) for the passage of the ink. Each ejection electrode 40 is connected, e.g. by means of a lead 40’ (see Fig. 14), to one of the vias 14, which extend through support structure 8 and nozzle carrier 6 as described on more detail below. Fig. 14 also shows electrical leads 103a, arranged at the same level as the ejection electrodes 40, for feeding a voltage to the deflection electrodes 41c mentioned below in the next paragraph by means ofvias 103b shown in Fig. 15. The vias 103b are coated with dielectric layers 103c to reduce the risk of electrical break- down.

Turning to Fig. 2 and 17, a shielding electrode 41 may be located at the front side of and at a distance from the ejection electrodes 40, i.e. the ejection electrodes 40 are closer to nozzle carrier 6 than the shielding electrode 41.

As shown in Fig, 17, shielding electrode 41 may comprise several electrically separate sections 41a, 41b, 41c, which allows to laterally deflect the ink drops exiting from recess 5. Alternatively, it may be one continuous electrode, with the main purpose to prevent the fields from the ejection electrodes from propagating into region 9 between print head 3 and target 2 and to generate a well-defined gradi- ent field between print head 3 and target 2.

If several separate sections 41a, 41b, 41c are used, they may be ap- plied to different potentials, e.g. by applying a voltage gradient across the array of nozzles by means of a voltage divider, which e.g. allows to gradually deflect the ink over a cross section of the print head.

Shielding electrode 41 is provided to control the field between print head 1 and target 2, For each nozzle 4, an opening in shielding electrode 40 allows for the passage of the ink. This opening corresponds to recess 5.

As shown in Figs. 2 and 11 , a guard electrode 42 may be located, at each nozzl e 4, behind and at a distance from ejection electrode 40 but in front of and at a distance from nozzle carrier 6. As shown in Figs. 2 and 11, it may again be annu- lar. Alternatively, it may also extend over several nozzles, Since all guard electrodes 42 are on the same potential, they can e.g. be interconnected, with leads 42’ as shown in Fig. 11, and be connected to voltage supply 17 e.g. by means of separate vias in the periphery of print head 3.

An opening in guard electrode 42 above nozzle 4 (corresponding to recess 5) allows for the passage of the ink.

The guard electrodes 42 reduce the electrical fields at the base of nozzle 4, thereby reducing the tendency of the ink to cross the retainer described be- low, which allows to localize it more securely in base of recess 5. This is described in more detail below. Ink System

In the present embodiment, ink arrives at the nozzles through the ink supply ducts 15, which comprise ink duct sections 15a- 15d located in sublayers 10a - lOd of front layer 10. And ink is retrieved from the nozzles through the ink suc- tion ducts 16, which comprise ink duct sections 16a, 16b in sublayers lOd and 10c.

As shown in Figs. 2 and 3, the fist ink duct sections 15a are located in sublayer 10a and are separated by walls 17a and extend horizontally through sub- layer 10a. in the shown embodiment, it is assumed that they feed ink from reservoir sections 15 u, 15v at the left and right of the shown figure, with each such reservoir section 15u and 15v. which interconnect the ends of several of the first ink duct sec- tions 15a. The geometry of this design is illustrated in Fig. 19, which shows the reser- voirs 15u, 15v as well as the first gas duct sections 15a (named 15’a and 15”a in Fig. 19) branching off from the reservoirs 15u, 15v.

When viewing print head 3 from above, the array 4a of nozzles 4 (shown in dotted lines in Fig. 19) is located laterally between the reservoirs 15u, 15v

The cross sections of the first ink duct sections 15a change along their length: With increasing distance from the reservoir 15u, 15v they are connected to, the cross sections of the first ink duct sections 15a decrease, thereby taking into account that the ink flow in the ink duct sections 15a decreases as ink is branched off from the first ink duct sections 15a into the second ink duct sections 15b.

The reservoirs 15u and 15v are connected, by means of larger diam- eter ink duct sections (not shown), to the ink source, such as reservoir tank 22 of Fig.

1.

Note that the principle depicted in Figs. 3 and 19, with the narrow- ing ducts sections, can also be applied for other duct sections the print head 3, be they ink duct sections or gas duct sections. Examples will be provided below.

Hence, in a more general formulation, the present invention advan- tageously also relates to a printing system having a print head with duct sections for gas or ink. The duct sections extend parallel to the front surface 7 of the print head and branch off from at least one common reservoir 15u, 15v, with the common reser- voirs) !5u, 15v also arranged in the print head. The cross section of the duct sections decreases with increasing distance from the reservoir(s), either continuously (as shown in Figs. 3 and 19), or in a step-wise manner (e.g. wherever another second duct section 15b branches off).

In particular, there is at least a first reservoir 15u and a second res- ervoir 15v arranged at opposite lateral sides of the array 4a of ink nozzles. When seen from above and as shown in Fig. 19, a first set 15”a of ducts extends from the first reservoir 15u into the array 4a and a second set 15’a extends from the second reser- voir 15u into the array 4a, with the two sets arranged interdigitaUy (i.e. altematingly).

Note that the narrowing of the duct sections 15a in Figs. 3 and 19 is shown in exaggerated manner. Sublayer 1 Oa is ad vantageously comparatively thick, advanta- geously at least 20 pm, in order to provide large cross sections for the ink duets.

Fig. 3 also shows the vias 14 that are connected to the ejection elec- trodes 40. In the present example, there is one such via for each nozzle, which allows to control the ejection electrodes 40 individually. The vias 14 in Fig. 3 (and in the other figures showing horizontal sectional views) may e.g. be implemented as solid, vertical metal rods or as a metal coatings on the inner wail of the surrounding dielectric.

As can be seen from Fig. 3, each electrical via 14 is laterally sur- rounded by a non-conducting wall 14a formed by sublayer 10a, which is in turn sur- rounded by a cavity 14b and yet another wall 14c.

Laterally surrounding the vias 14 first by a dielectric wall 14a and then by a cavity 14b increases the electr ical breakdown threshold of the structure. Most of the electrical potential (e.g. in respect to a neighboring via and/or the neigh- boring ink) will drop over the cavity (because it has a lower dielectric permittivity than the dielectric). Hence, the dielectric is protected from structural damage. On the other hand, if there were no wall, there would be a risk of an ionic breakdown through the gas. Hence, the combination of a dielectric coating of the electrode and a cavity surrounding it is advantageous.

In sublayer 10a, this structure is particularly important since there might be a substantial potential difference between the vias 14 and the ink in the duct sections 15a. It is particularly advantageous if the duct sections 15a are in addition covered with an electrode (not shown) that charges the liquid contained within to a defined electric potential.

As will be seen below, similar structures are provided in other lay- ers of pr int head 3.

Hence, in more general terms, print head 3 advantageously com- prises electrically conductive vias 14. Each of these vias 14 is laterally enclosed by a non-conductive first wall 14a, which is laterally enclosed by a cavity 14b. The cavity 14b may in turn also be laterally enclosed in a non-conductive second wall 14c. In the case of Fig. 3, second wall 14c prevents the ink in the duct sections 15a from entering the cavities 14b. Advantageously, the wall(s) 14a, 14c and cavity 14b have, in a plane perpendicular to the extension of via 14, circular cross-section, thereby avoid- ing comers that would lead to locally increased electrical field strengths.

To simplify manufacture, the vias 14 extend advantageously along the ejection direction X, i.e. perpendicular to front surface 7 of print head 3.

Note that there may be other vias in the print head, in particular vias without strong electrical fields in their neighborhood, that do not have such a cavity.

Nor do these vias 14 need to have such cavity structures along their whole length. For example, and as shown in Fig. 4, which shows a sectional view of sublayer 10b above sublayer 10a of Fig. 3, there is no cavity around the vias 14. Hence, first and second wall 14a, 14c can be anchored, at their upper ends, in sub- layer 10b.

Cavities may be filled with vacuum or with a dielectric gas that quenches dielectric beakdowns, e.g. SF6 or C4F8. The same strategy may be used for any other enclosed cavity in the printhead.

Fig. 4 further shows the second ink duct sections 15b, which are also shown in Fig, 2, with one such second ink duct section 15b branching off from the first duct sections 15a below each nozzle 4.

Sublayer 10b can be thinner than sublayer 10a, e.g. approximately 5 pm.

Figs. 2 and 5 show the ink duct sections 16b of ink suction duct 16 formed in sublayer 10c. The design of this sublayer 10c is similar to the one of sub- layer 10a in that the ink duct sections 16b, separated by walls 17b, extend horizon- tally to feed ink to lateral reservoirs 16 and 16v. Again, and as depicted for the ink duct sections 15a in Fig. 19, their cross sections decrease with increasing distance from the reservoirs 16u, 16v, and they are arranged interdigi tally.

Sublayer 10c is, again, advantageously comparatively thick, advan- tageously at least 20 pm, in order to provide large cross sections for the ink duct sec- tions 16b. Fig. 5 shows that the vias 14 are again surrounded by first and sec- ond walls 14a, 14c and a cavity 14b.

Fig. 5 further shows the duct sections 15c with surrounding walls 17c that communicate with the duct sections 15b of the ink supply ducts 15.

As can be seen from Figs. 2 and 6, sublayer lOd forms a wall 17d between ink duct section 15d of ink supply duct 15 and ink duct section 16a of ink suction duct 16. Sublayer lOd can be thinner than sublayers 10a and 1 Oc, e.g. ap- proximately 5 pm.

Nozzle. Design The design of the nozzles 4 can best be seen in Figs, 2 and 7 - 10.

Nozzl e 4 of this embodiment comprises a tip section 46, a shaft sec- tion 48a, 48b, and a base section 50a, 50b, 52, with tip section 46 arranged in front of shaft section 48a, 48b, and base section 50a, 50b, 52 arranged behind shaft section 48.

The shown nozzle design relies on the ink wetting the lateral sur- face of nozzle 4 and passing through a channel of nozzle 4, but a design e.g. as shown in WO 2016/169956 can be used as well.

The present nozzle 4 comprises a radial channel 56 in its base as well as vertical channels 58a, 58b extending into through its shaft section 48a, 48b to the rear end of tip section 46. (Note that, in Fig, 2, radial channel 56 is rotated by 45° as compared to Fig. 7.)

From the upper end of channel 58, the ink wets sides of tip 46 and forms a meniscus at the top 60 of tip 46. In this way, a sharp meniscus-like ink geom- etry is formed already before applying any voltages to the print head, merely by the action of surface tension. Accordingly, the tip is preferably rendered wettable to the solvent being used. This can be achieved, for example, by activating the tip surface with an oxygen plasma.

Radial channel 56 guides ink outwards to an annular opening 62 at the top of base 50a, 50b, 52. Annular opening 62 is arranged between a central sec- tion 50a and a peripheral section 50b of the base of nozzle 4. From annular opening 62, ink can wet the shaft section of nozzle 4, further adding to the ink that is available at tip 46.

Peripheral section 50b may be coated with an anti-wetting coating 64 and forms an ink retainer 66 that prevents ink from laterally spreading over the nozzle. Depending on the ink to be used, coating 64 may be hydrophobic and/or oleo- phobic. For example, it may be formed, at least in part, of Teflon and/or PTFE, which are hydrophobic and oieophobic. Depending on the scope of inks to be used, it may also be only hydrophobic (e.g. HMDS, i.e. Bis(trimethylsilyl)amme) or only be oieo- phobic (e.g. based on polymers). In particular, the surface of ink retainer 66 is advan- tageously more hydrophobic and/or oieophobic than a surface of shaft 48a, 48b of nozzle 4. As mentioned above, guard electrode 42 keeps the electric fields at the location of retainer 66 low, thereby further reducing the tendency of the ink to spread over retainer 66.

Each nozzle 4 is advantageously surrounded by the opening or openings 16a of one or more suction ducts. This may e.g. be a single annular opening

(such as formed by duct section 16a of Fig. 2), or it may be an annular series of suc- tion openings, e.g. circular openings.

Further, each nozzle 4 of the shown embodiment is surrounded by an annular wall 70 of support structure 8, Further support elements 72 may be pro- vided to support the upper parts of support structure 8. For example, these further support elements 72 may be structured in a hexagonal pattern of walls 72 surrounding cavities 74. This kind of wall design minimizes mechanical stress in the structure.

Support Structure As mentioned, a support structure 8 is provided for connecting the various electrodes 38, 40, 42 to nozzle carrier 6. It is arranged on front side 36 of noz- zle carrier 6.

Support structure 8 comprises a plurality of support elements, such as the support elements 70, 72, arranged between the nozzles 4. Ink retainer 66 is advantageously designed to prevent ink from reaching these support elements of the support structure and to prevent it from wet- ting them, thereby reducing the tendency of the ink to submerge the nozzles.

Support structure 8 advantageously comprises at least one electrode carrier layer. In the embodiment of Fig. 2, there are three such carrier layers 80, 82, 84. Each electrode carrier layer comprises at least one of the electrodes 38, 40, 42, and it extends parallel to top surface 36.

Typically, the electrode 38, 40, 42 is embedded within its electrode earner layer 80, 82, 84 and covered on its front and back sides by at least one dielec- tric sublayer 80a, 80b or 82a, 82b or 84a, 84b. In the embodiment shown here, support elements 70, 72, 76 and/or

78 are provided between each of the electrode carrier layers 80, 82, 84 as well as be- tween the backmost electrode carrier layer 80a and nozzle carrier 6. They may, how- ever, also only be provided between a subset of these structures. Gas Ducts

In the present embodiment, the gas ducts 34, 37, 39 of the ventila- tion system (as described above) comprise gas duct sections arranged in the upper sublayers of support structure 8. In the shown embodiment, a set of first set of gas duct sections, in the following called the “primary gas duct sections” 34a, are arranged a sublayer 7G of support structure 8 between the carrier layers 80 and 82, This is shown in Figs. 2 and 12.

These primary gas duct sections 34a extend through print head 3 parallel to front surface 7, The e.g. feed gas from larger duct sections (not shown) at the periphery of print head 3, which are in turn connected to first gas source 33.

In the shown embodiment, there is a plurality of primary gas duct sections 34a, which extend parallel to each other and parallel to rows of the array of nozzles 4. At each of the nozzles 4, they branch into secondary gas duct sec- tions 34b 1, 34b2 and feed gas to a mixing region 90 located in recess 5.

The cross sections of the secondary gas duct sections 34b 1, 34b2 are at least 5 times, in particular at least 10 times, smaller than the cross sections of the primary gas duct sections 34a in order to provide substantially the same gas flow to all nozzles 4 along the length of the primary duct sections 34a by rendering the maximum pressure drop over the whole primary duct sections 34a much smaller (in particular at least 10 times smaller) than the maximum total pressure drop over the secondary duct sections 34b 1 , 34b2.

To expedite this, the secondary duct sections 34b 1, 34b2 have a to- tal (combined) length L (see inset X of Fig. 12) from the primary duct sections 34a to the mixing region 90 (i.e. to the recess 5) that is a substantial fraction k of the dis- tance D between two neighboring nozzles 5, i.e. L = k-D, with k at least 0.1, in partic- ular at least 0.25.

Furthermore, the length L2 of the primary duct sections 34a is pref- erably chosen smaller than the squared ratio of the cross-sections Cl and C2 of pri- mary duct sections 34a and secondary ducts sections 34b 1, 34b2, multiplied by L, i.e,

12 < (C1 C2) ² · L (For a more accurate estimate for elongate cross sections, the

Darcy-Weisbach equation may be used.) For example, if the cross-sections Cl of the primary duct sections 34a are ten times larger than the cross-sections C2 of the secondary duct sections 34b 1 , 34b2, then L2 is preferably shorter than 100L, In this case, by additionally choosing k = 0.1, one would preferably arrange less than ten nozzles between two neighboring reservoirs 15u, 15v.

The number of nozzles can be extended, though, by laterally at least duplicating the arrangement shown in Fig, 19. In this case, a given reservoir may also feed primary duct sections on two opposite sides thereof.

The ends of the secondary gas duct sections 34b, where they enter the mixing regions 90, form the “second ends” 34-2 of the first gas ducts 34 as men- tioned above.

Advantageously, at least two of the second ends 34-2 end in each recess 5, and they are arranged in rotational symmetry around nozzle axis 100 in or- der to generate a symmetric flow of gas that does not laterally deflect the ink.

Hence, in more general terms, the first gas ducts 34 comprise pri- mary and secondary duct sections 34a and 34b 1, 34b2, wherein:

- the primary duct sections 34a extend through print head 3 in a di- rection parallel to front surface 7 and parallel to each other, and

- from each primary duct section 34a, a plurality of secondary duct sections 34b l, 34b2 branches off to connect the primary duct section 34 to a plurality of the recesses 5.

A particularly compact design can be achieved, as shown, if the pri- mary duct sections 34a and, advantageously, also the secondary duct sections 34b 1, 34b2, are incorporated into support structure 8, i.e. in ejection direction X they are lo- cated between surface 7 and nozzle carrier 6.

Advantageously, the primary duct sections 34a are located between the ejection electrodes 40 and nozzle carrier 6.

Also advantageously, the primary and the secondary gas duct sec- tions 34a, 34bl, 34b2 are all located in the same plane parallel to surface 7, which ob- viates the need to manufacture vertical passages, such as passages through the carrier layers 80, 82, 84.

Fig. 2 and 15 illustrate the locations of the gas duct sections for the second and third gas ducts 37, 39.

Again, there are primary gas duct sections 37a, 39a as well as sec- ondary gas duct sections 37bl, 37b2 and 39bl, 39b2, with the duct sections 37a,

37b I, 73b2 forming part of the second gas ducts 37 and the duct sections 39a, 39bl, 39b2 forming part of the third gas ducts 39. Again, the primary gas duct sections 37a, 39a extend parallel to sur- face 7 and are located between surface 7 and nozzle carrier 6 to use the space availa- ble in support structure 8. Advantageously, they are arranged between surface 7 and the ejection electrodes 40. The primary gas ducts 37a and 39a of the second and third gas ducts

37, 39 extend parallel to each other and to the rows or columns of the nozzle array. They are arranged altematingly in a common plane, i.e. each primary gas duct 37a of the second gas duets 37 is arranged immediately between two primary gas ducts 39a of the third gas ducts 39 and vice versa (with the exception of the primary gas ducts and the edges of the nozzle array).

In order to explain the arrangement of the secondary ducts 37bl, 37b2, 39b 1, 39b2, reference is first made to Fig, 18, which shows a view of front sur- face 7 of print head 3 as seen from the target. As shown, in the present embodiment, each recess 5 and therefore each nozzle 4 is surrounded by two second ends 37-2 of the second gas ducts 37 and two first ends 39-1 of the third gas ducts 39, with the for- mer blowing gas into the region between print head 3 and target 2 and the latter suck- ing gas away from this region.

In order to avoid a lateral flow of gas across one of the recesses 5, the two second ends 37-2 of the second gas ducts 37 and two first ends 39-1 of the third air ducts 39 are altematingly arranged on. the comers of a rectangle, in particular a square, centered on recess 5. The resulting air gas flow pattern is shown by arrows in Fig. 18. The longer arrows show the gas flows between the second ends 37-2 and the first ends 39-1, and the shorter arrows show the gas flows between the gas from the second ends 34-1 of the first gas ducts 34 and the first ends 39-1. This kind of design - without the flow from the recesses 5 - is de- scribed in more detail in in WO 2021/008817.

Turning back to Fig. 15, in order to obtain this kind of pattern of the second ends 37-2 and the first ends 39-1, neighboring secondary duct sections 37bi, 37b2 branch off on different sides of their primary duct section 37a of second gas duct 37. Similarly, neighboring secondary duct sections 39b 1, 39b2 branch off on dif- ferent sides of their primary duct section 39a of third gas duct 39. Along a line 102 lo- cated between two neighboring primary ducts 37a, 39a and extending parallel to the primary ducts 37a, 39a, the secondary duct sections (37bl, 37b2) of the second gas ducts (37) alternate with the secondary duct sections (39b 1, 39b2) of the third gas ducts (39).

This is shown in Fig. 15 for the first parts 37b 1, 39b 1 of the second- ary duet sections, with these first parts being arranged on a common plane with the primary duct sections 37a, 39a. And it is shown in Fig. 16 for the second parts 37b2, 39b2 of the secondary duct sections, with these second parts extending along ejection direction X and connecting the first parts 37b 1, 39bl to the ends 37-2 and 39-1, re- spectively.

Again, the secondary duct sections 37bl, 37b2 and 39b 1, 39b2 are designed such that they have a much higher flow resistance than the primary duct sec- tions 37a, 39a, which allows to make the flow through the secondary duct sections along the length of the primary duct sections 37a, 39b more uniform.

Fig. 15 also shows chambers 104 located above the leads 103 a (see Fig. 14). Each chamber 104 is located between two neighboring primary duct sections 37a, 39a and separated from the same by means of walls 106a, 106b. These chambers reduce the risk of electrical breakdown caused by the voltages carried by the leads 103a.

Note that the vias 103b of Fig, 16 continue through the sublayer of Fig. 16 to connect to the shielding electrodes 14c of Fig. 17.

Evaporator

As mentioned, at least one evaporator 35 is arranged along the first gas ducts 34 to saturate (or at least partially saturate) the gas with a suitable liquid as mentioned above.

This evaporator 35 may e.g. be a bubble system where the gas is led, in bubbles, through a pool of the liquid. This type of evaporator is typically ar- ranged outside print head 3.

Alternatively or in addition thereto, evaporator 35 may also be lo- cated in print head 3. An embodiment of an evaporator that may be integrated into the layer structure of a print head is shown in Figs. 20 - 22.

This evaporator formed by layers 110 - 120 of print head 3. These layers may e.g, be sublayers of support structure 8 and/or of nozzle carrier 6.

Advantageously, print head 3 comprises a plurality of such evapora- tors 35.

The gas from gas source 33 extends through a duct section 34x, which forms part of first gas duct 34. A layer 116 forming one of the side walls of duct section 34x comprises a plurality of openings 122, with each opening connecting duct section 34x to a chamber 124 on the other side of layer 116.

Chamber 124 is filled with the liquid to be evaporated. Duct section 34x is under a slight overpressure as compared to the liquid filled into chamber 124. This is due to the fact that that the duct section 34x contains gas that is under pressure so it flows out of the second ends 34-2, which leads to a region of atmospheric pres- sure inside the recesses 5. Liquid fills the openings 122 by surface tension and from an interface 126 (shown in dotted lines in Fig. 20). In order for the interface 126 not to be pushed back into the openings 122 by the pressured gas in the duct section 34x, the opening diameters are chosen small enough such that pressure stored inside the liquid interface due to sur face tension is large than the pressure of the liquid.

A suitable pressure difference between the liquid and duct section 34x can be calculated, for round openings, from the radial diameter d of the openings 122. For d = 5 pm and a liquid with the surface tension of water, the Young Laplace equation yields a pressure difference of less than 144 mbar before the liquid exists from the openings 122. For a liquid with the surface tension of alkane, the pressure difference would have to be less than 40 mbar.

Advantageously, at least the openings 122 should be well wettable. When using a polymer laminate for layer 1 16, this can e.g. be achieved by treating it with an oxygen plasma white or after structuring it.

As can also be seen from Figs. 20 - 22, the openings 122 are advan- tageously surrounded, on the side of duct section 34x, by edges 128 with an undercut 130 beyond the edges 128. This structure is advantageously provided with a poorly wettable surface, such as with one of the materials mentioned for coating 64 above. It pins any ink that may pass through the openings 122.

In more general terms, the printing system may have an evaporator 34 in print head 3 which comprises

- A duct section 34x of the first gas duct 34: This duet section 34x guides the gas from gas source 33 towards the nozzles 3. - A liquid chamber 124: This chamber contains the liquid to be evaporated.

- A wall 116 separating the duct section 34x and the liquid chamber 124: This wall prevents the liquid from freely flowing into the duct section 34x.

- A plurali ty of openings 122 in the wall 1 16: These openings con- nect the duct section 34x and the liquid chamber 124 and provide passage for the liq- uid to reach the duct section.

Advantageously, the smallest diameter of each of the openings is 10 pm or less for allowing to form a liquid interface as described above. Manufacturing the Print Head

The present print head can be manufactured using techniques as they are e.g. knows from semiconductor manufacturing and packaging, e.g. as de- scribed in WO 2013/000558, WO 2016/120381, WO 2016/169956, and in WO 2021/008817..

Operating the Printing System

In operation, i.e. while printing, ink is fed to the nozzles 4 by means of the supply ducts 15. This ink is restricted to the region between the nozzles 4 and the ink retainers 66.

To eject ink drops, the voltage at the desired ejection electrode(s)

(in respect to the voltage of the ink) is increased temporarily. For example, a voltage pulse of 400 V may be generated. While not printing, the voltage at the ejection elec- trodes is maintained at a level where no ink is ejected. Advantageously, it is non-zero, though, e.g. at 200 V.

As mentioned above, the electric field at ink retainer 66 is advanta- geously kept low, e.g. at less than 50%, in particular at less than 10%, of the field strength at the forward end 70 of the nozzle. Since high electric field strengths reduce the surface tension of the ink, this procedure reduces the tendency of the ink to wet the ink retainer and to cross it.

The suction ducts 16, if present, are used to retrieve ink from the nozzles.

Gas is sent from first gas source 33 into the first gas ducts 34, mois- turized by evaporator 35, and used to provide a “wet” atmosphere at the location of the nozzles 4.

While printing, dry gas is sent from second gas source 36 through the second gas ducts 37 to the region 9 between print head 3 and target 2. It expedites the drying of the ink on target 2, but the flow of gas from the first gas ducts 34 pre- vents this dry gas from entering the recesses 5 and reaching the nozzles 4. The flow of gas from the first gas ducts 34 also prevents air from entering the recesses when the print head is moved quickly along the target.

The sum of the gas flows through the first and second gas ducts 34, 37 is advantageously equal to the flow of gas through the third gas ducts 39 for the reasons mentioned above. Notes

Figs. 3 - 6 above illustrate a specific embodiment how ink can be fed to the nozzles 4 and (optionally) be retrieved therefrom. It must be noted that this design of duct sections is only one possible embodiment of handling the ink, and other geometries of ducts may be used as well, such as e.g. described in WO 2021/008817.

Similarly, other designs can be used for connecting the gas ducts to the recesses 5 and front surface 7, e.g. as described in WO 2021/008817.

Even though the present invention is advantageously employed for electrohydrodynamic ink jet printing systems, where liquid ink must be present at the top of the nozzles and is therefore very prone to evaporation, it can also be used in other types of printing systems as mentioned above.

The dielectric sublayers of the print head typically have a thickness between 5 and 30 pm while the metallic sublayers are typically much thinner. in most of the embodiments shown so far, each nozzle is sur- rounded by an ink retainer, which defines a restricted area where the ink can flow from the nozzle.

In the examples, each nozzle is surrounded by its own ink retainer. Alternatively, several nozzles may be surrounded by a common ink retainer, i.e. one ink retainer may surround several nozzles.

Alternatively or in addition thereto, each support element of support structure 8 may be surrounded by an ink retainer, which defines an ink-free area around the support element, preventing the ink to reach the support element. This may be particularly advantageous if the support elements are forming individual, iso- lated pillars.

In the embodiments described so far, three electrodes at three differ- ent vertical levels have been mentioned: the ejection electrodes, the guard electrodes, and the shielding electrodes. It must be noted, though, that there may also be other electrodes, such as: - Electrodes may be provided in nozzle carrier 6, e.g. at the supply ducts 15 and/or suction ducts 16, and/or at the nozzle and/or at the ink retainers for defining the potential of the ink. Such electrodes allow e.g. to keep the ink at a similar or the same potential as the guard electrodes 42. Advantageously, such electrodes are of platinum and/or gold. - Further electrodes (e.g. between ejection electrode 38 and shield- ing electrode 40 of Fig. 2) may be provided if differently sized nozzles are present on the print head. This is particularly useful for nozzles where the distance between the ejection electrode and the nozzle is considerable smaller than the distance between the shielding electrode and the ejection electrode.

- In the embodiments described so far, primary gas duct sections 34a, 37a, 39a were formed within the support structure 8. This simplifies manufactur- ing because primary gas duct sections 34a, 37a, 39a can be formed in the same layer as the secondary gas duct sections 34b 1 , 34b2, 37bl, 39bl that extend parallel to the nozzle carrier 6, and they can be formed within layers that are needed to create the distance between electrodes anyways. At the same time, controlling pressure drop can be more difficult because the thickness of the layer is fixed and a higher pressure drop of secondary gas duct sections 34b 1, 34b2, 37bl, 39bl can only be obtained by re- ducing duct width. Since the latter is limited by manufacturing constraints, primary duct sections 34a, 37a, 39a may be formed within a thicker layer than the layer that forms secondary gas duct sections 34b 1, 34b2, 37bl, 39bl. Also, since the thickness of the layers contained in support structure 8 is affected also by electric considera- t ions, e.g. by desired distances between the different electrodes, thickness of these layers cannot be freely adjusted to achieve a desired flow resistance. To solve this is- sue, additional layers may therefore be formed within front layer 10 and then the sec- ondary gas duct sections may be at least partly be formed by vertical channels which, due to their length, may in tact create the required pressure drop as well. Particularly, they may be formed within hollow-type electrical vias 14.

While there are shown and described presently preferred embodi- ments of the invention, it is to be distinctly understood that the invention is not lim- ited thereto but may be otherwise variously embodied and practiced within the scope of the following claims.