Patent Description:
<CIT> describes an electrohydrodynamic inkjet printing system with a print head having a plurality of ventilation ducts located at the ink nozzles. 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.

<CIT> describes an electrohydrodynamic inkjet printing system 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.

<CIT> describes an electrohydrodynamic inkjet printing system with a print head temperature control device adapted to cool or heat print head or target.

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 <NUM>.

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

The invention is based on the understanding that one factor impacting 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 evaporate.

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 piezoelectric DOD printing,
Advantageously, the print head further comprises:.

(Note that there may be further recesses in the front surface, in addition to said "plurality 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 surface. The nozzles are mounted to the nozzle carrier. The first gas ducts have duct sections 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 surface is used to accommodate at least part of the first gas ducts.

Advantageously, at least some said duct sections extend along several 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:.

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 evaporator. Hence, the gas emerging from the second ends of the second gas ducts is dryer than the gas emerging from the second ends of the first gas ducts.

This design allows 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.

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 second gas ducts to this region would have to escape in lateral direction (i.e. in a direction 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 system as described herein. It comprises at least the following steps:.

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 <NUM>%, in particular by less than <NUM>%.

The invention will be better understood and objects other than those set forth above will become apparent when consideration is given to the following detailed description thereof.

Note that in <FIG>, <FIG>, <FIG>, <FIG>, <FIG>, <FIG>, and <FIG> black parts denote hollow regions or (for vias) metal regions and white parts denote solid regions. In <FIG>, <FIG> and <FIG>, black parts denote metallic or conductive parts and white parts denote dielectric, non-conductive parts.

<FIG> does not have the same scaling as <FIG> in order to show several nozzles at the same time in <FIG>.

"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 example, the nozzles are arranged backward from or behind the ejection electrodes.

"At the front" and "at the back" are understood to designate a location 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 <NUM>%, of the nozzles. For example, if it is said that "at a given nozzle, the guard electrode is arranged between 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 <NUM>% of them. It may e.g. be that there are some nozzles that do not have ejection electrodes and/or guard electrodes, 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" upwards direction, i.e. the print head is, by definition, designed to eject ink upwards. (In operation, it may, of course, be under any angle to the direction of gravity. ) Hence, definitions such as "above" and "below" are to be understood in reference to this definition of "vertical".

"Horizontal" is any direction perpendicular to the vertical direction.

"Lateral" designates something that is horizontal from something else.

<FIG> shows a schematic view of an embodiment of the printing system <NUM>. It is depicted above a target <NUM>, and it is structured to eject ink along an ejection direction X onto the target.

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

The printing system has a print head <NUM> with a plurality of ejection electrodes (not shown in <FIG>) for ejecting ink from the nozzles <NUM> and optional further electrodes arranged on a support structure <NUM>, the design of which is described in more detail below. Further electrodes may be provided in electrical contact with the ink to set the ink to a defined electrical potential.

Nozzle carrier <NUM> comprises a front layer <NUM>, with the nozzles <NUM> being mounted to the front side of front layer <NUM> and forming projections thereon. It also comprises a backing layer <NUM> located at the back side of front layer <NUM>.

The internal structure of front layer <NUM> is not shown in <FIG> 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 <NUM> may e.g. be an insulated semiconductor material or it may be a dielectric. Advantageously, backing layer <NUM> is, at least partially, of glass.

Electrical vias <NUM> are connected to the ejection electrodes and extend through front layer <NUM> and the backing layer <NUM> for connecting the ejection electrodes to a voltage supply <NUM>. Advantageously, there is at least one via <NUM> for each nozzle <NUM>. Further vias may be provided to connect other electrodes to voltage supply <NUM>.

Ink ducts <NUM>, <NUM> supply ink to the nozzles <NUM> and (optionally) recycle ink back from the nozzles <NUM>. They are located, in part, in front layer <NUM>, and they e.g. extend through peripheral regions of backing layer <NUM>. Their design is described in more detail below.

At least one pump <NUM> and/or another pressure source or vacuum source is provided to supply ink to the supply ducts <NUM> and, if there are suction ducts, to retrieve ink from the suction ducts <NUM>.

Advantageously, the printing system comprises a first pressure control <NUM> for generating a first defined pressure p1 at the input of the supply ducts <NUM>, e.g. in a reservoir tank <NUM>.

The ink is supplied through an optional filter <NUM> and the supply ducts <NUM> to the nozzles <NUM>.

If there are suction ducts <NUM>, they are connected to a suction system, which may comprise a second pressure control <NUM> for generating a second defined pressure p2 at the exit of the suction ducts <NUM>, e.g. in a suction tank <NUM>. The suction system may also comprise a pump. This may in particular be pump <NUM> as mentioned above, in which case pump <NUM> acts as a circulating pump.

A suitable pump design is e.g. shown in <CIT>.

Providing ink supply ducts <NUM> to feed ink to the nozzle and ink suction ducts <NUM> to retrieve ink from the nozzles, allows to maintain a reservoir of fresh ink at each nozzle <NUM>.

As further shown in <FIG>, the print head may comprise a circuit carrier <NUM>, such as a PCB, arranged at the back side of nozzle carrier <NUM>.

An optional interposer layer <NUM> may be provided between circuit carrier <NUM> and nozzle carrier <NUM> for matching a denser resolution of the vias <NUM> to the circuit resolution of circuit carrier <NUM>. Such interposer layers are e.g. used in flip-chip designs where semiconductor chips are applied to PCBs.

Circuit carrier <NUM> carries control circuitry <NUM>, which may e.g. implement at least part of voltage supply <NUM>, such as the driver stage of such a voltage supply, which connects voltage sources to the various electrodes of the print head.

In the shown embodiment, the ink ducts <NUM>, <NUM> extend through interposer layer <NUM> (if present) as well as circuit carrier <NUM>.

If the vias <NUM> have a large enough mutual spacing (e.g. larger than <NUM>), they may directly interface with circuit carrier <NUM> without an interposer layer <NUM>.

Advantageously, target <NUM> is arranged on an acceleration electrode <NUM>, which is connected to voltage supply <NUM> to generate an accelerating electrical field between print head <NUM> and target <NUM>.

The pressure controls <NUM>, <NUM> advantageously allow to separately adjust the pressures in the supply ducts <NUM> as well as in the pressure ducts <NUM>.

The printing system further comprises a first gas source <NUM> adapted to feed a gas to first gas ducts <NUM>. At least one evaporator <NUM> is arranged along first gas duct <NUM> 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 <NUM> have at least one first end <NUM>-<NUM> at first gas source <NUM> and feed gas to second ends <NUM>-<NUM> located at the nozzles <NUM>.

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

Finally, the system may comprise a gas sink <NUM> adapted to suck gas from third gas ducts <NUM>. The third gas ducts <NUM> retrieve gas from first ends <NUM>-<NUM> located at front surface <NUM> and feed it to at least one second end <NUM>-<NUM> at gas sink <NUM>.

Both, the second gas source <NUM> and second gas ducts <NUM> as well as the gas sink <NUM> and the third gas ducts <NUM> are optional.

The functions of the gas sources <NUM>, <NUM>, the gas sink <NUM>, and the various gas ducts <NUM>, <NUM>, <NUM> are described in the following section.

The gas sources <NUM>, <NUM>, the gas sink <NUM>, and the gas ducts <NUM>, <NUM>, <NUM> form part of a ventilation system of the printing system.

The function of first gas source <NUM> and first gas ducts <NUM> is to bring saturated gas to the nozzles <NUM>, i.e. gas that has a large amount of liquid dissolved therein, such that the evaporation of ink solvent at the nozzles <NUM> is reduced as described above.

For this purpose, first gas source <NUM> may e.g. be a pump or a pressurized vessel adapted to feed a gas into the first end(s) <NUM>-<NUM> of the first gas ducts <NUM>. 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 <NUM>, such as SF<NUM> or C<NUM>F<NUM>, or a mixture thereof.

In more general terms, the present invention advantageously relates to a first gas source <NUM> providing a gas quenching electric discharges, e.g. by having a breakdown voltage, relative to air, of at least <NUM>. For example, the breakdown voltage of SF<NUM> is <NUM> relative to air, the one of C<NUM>F<NUM> is <NUM>. The invention also relates to feeding such a gas, by means of at least some of the gas sources <NUM>, <NUM>, into the first and/or second gas ducts <NUM>, <NUM>.

Typically, the first gas ducts <NUM> branch and end in a plurality of second ends <NUM>-<NUM>, one of which is shown in <FIG>. These second ends <NUM>-<NUM> are located at the recesses <NUM>, thereby filling the recesses <NUM> with the saturated gas and protecting the nozzles <NUM> from evaporation by displacing non-saturated gas entering the recesses <NUM> from region <NUM>. 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 portions of the first gas ducts <NUM> that follows the location of the evaporator <NUM> are preferably kept at a temperature that is not lower than the temperature at which the gas loading is executed within the evaporator <NUM>.

The second gas ducts <NUM> (if present) branch, too, and end in a plurality of outlets <NUM>-<NUM> in front surface <NUM>, one of which is shown in <FIG>.

Similarly, the third gas ducts <NUM> (if present) branch and end in a plurality of inlets <NUM>-<NUM> in front surface <NUM>, one of which is again shown in <FIG>.

The second gas ducts <NUM> can be used to feed dry air into the region <NUM> between print head <NUM> and target <NUM>. 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 <NUM>.

The third gas ducts <NUM> can be used to withdraw the gas that the first and second gas ducts have introduced into region <NUM>, thereby reducing the amount of lateral gas flow in region <NUM> as mentioned above.

Advantageously, the invention therefore relates to a method comprising the step of feeding a first flow of gas through a first subset of the gas ducts (namely the subset of the gas ducts <NUM> and, if present, <NUM>) to the region <NUM> and retrieving a second flow of gas through a second subset of the gas ducts (namely the third gas ducts <NUM>), with the first and the second gas flows being equal, in particular within an accuracy better than <NUM>%.

The gas fed from second gas source <NUM> through the second gas ducts <NUM> 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 <NUM> can be an inert gas or it can be a reactive gas that reacts with solute. Such gas will not react with solute at the nozzle region if the gas introduced from the second ends <NUM>-<NUM> is an inert gas and displaces the reactive gas introduced from the second gas ducts <NUM> from the recesses <NUM>. Instead, the reaction will only occur on the substrate or during droplet flight.

Hence, advantageously, the method of the present invention advantageously comprises the following steps:.

In this case, the gasses from first gas source <NUM> and second gas source <NUM> may be different (i.e. different even before the first gas passes the evaporator(s) <NUM>).

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 oxidizing ink is used, drying by oxidation can be used to expedite the drying process. Oxidizing inks are known to the skilled person, see e.g. http://printwiki. org/Oxidation. 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.

<FIG> shows a sectional view of print head <NUM> at a nozzle <NUM>. (As mentioned above, in contrast to <FIG>, the ejection direction X in <FIG> points upwards. ) <FIG> show sectional views, perpendicular to ejection direction X, along the lines A-A to O-O of <FIG>.

As can be seen from <FIG>, nozzle <NUM> forms a projection on the front side <NUM> of nozzle carrier <NUM>, e.g. on the front side of its front layer <NUM>. It sits at the base of its recess <NUM>.

<FIG> also shows the various electrodes that may be associated with the nozzles <NUM>. They are described in the following.

The ejection electrode <NUM> is located on the front side of the nozzle <NUM>. In the embodiment of <FIG> and <FIG>, it is annular with a central opening (at the location of recess <NUM>) for the passage of the ink. Each ejection electrode <NUM> is connected, e.g. by means of a lead <NUM>' (see <FIG>), to one of the vias <NUM>, which extend through support structure <NUM> and nozzle carrier <NUM> as described on more detail below.

<FIG> also shows electrical leads 103a, arranged at the same level as the ejection electrodes <NUM>, for feeding a voltage to the deflection electrodes 41c mentioned below in the next paragraph by means of vias 103b shown in <FIG>. The vias 103b are coated with dielectric layers 103c to reduce the risk of electrical breakdown.

Turning to <FIG> and <FIG>, a shielding electrode <NUM> may be located at the front side of and at a distance from the ejection electrodes <NUM>, i.e. the ejection electrodes <NUM> are closer to nozzle carrier <NUM> than the shielding electrode <NUM>.

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

If several separate sections 41a, 41b, 41c are used, they may be applied 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 <NUM> is provided to control the field between print head <NUM> and target <NUM>. For each nozzle <NUM>, an opening in shielding electrode <NUM> allows for the passage of the ink. This opening corresponds to recess <NUM>.

As shown in <FIG> and <FIG>, a guard electrode <NUM> may be located, at each nozzle <NUM>, behind and at a distance from ejection electrode <NUM> but in front of and at a distance from nozzle carrier <NUM>. As shown in <FIG> and <FIG>, it may again be annular. Alternatively, it may also extend over several nozzles. Since all guard electrodes <NUM> are on the same potential, they can e.g. be interconnected, with leads <NUM>' as shown in <FIG>, and be connected to voltage supply <NUM> e.g. by means of separate vias in the periphery of print head <NUM>.

An opening in guard electrode <NUM> above nozzle <NUM> (corresponding to recess <NUM>) allows for the passage of the ink.

The guard electrodes <NUM> reduce the electrical fields at the base of nozzle <NUM>, thereby reducing the tendency of the ink to cross the retainer described below, which allows to localize it more securely in base of recess <NUM>. This is described in more detail below.

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

As shown in <FIG> and <FIG>, the fist ink duct sections 15a are located in sublayer 10a and are separated by walls 17a and extend horizontally through sublayer 10a. In the shown embodiment, it is assumed that they feed ink from reservoir sections 15u, 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 sections 15a. The geometry of this design is illustrated in <FIG>, which shows the reservoirs 15u, 15v as well as the first gas duct sections 15a (named <NUM>'a and <NUM>"a in <FIG>) branching off from the reservoirs 15u, 15v.

When viewing print head <NUM> from above, the array 4a of nozzles <NUM> (shown in dotted lines in <FIG>) 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 diameter ink duct sections (not shown), to the ink source, such as reservoir tank <NUM> of <FIG>.

Note that the principle depicted in <FIG> and <FIG>, with the narrowing ducts sections, can also be applied for other duct sections the print head <NUM>, be they ink duct sections or gas duct sections. Examples will be provided below.

Hence, in a more general formulation, the present invention advantageously 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 <NUM> of the print head and branch off from at least one common reservoir 15u, 15v, with the common reservoir(s) 15u, 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 <FIG> and <FIG>), 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 reservoir 15v arranged at opposite lateral sides of the array 4a of ink nozzles. When seen from above and as shown in <FIG>, a first set <NUM>"a of ducts extends from the first reservoir 15u into the array 4a and a second set <NUM>'a extends from the second reservoir 15u into the array 4a, with the two sets arranged interdigitally (i.e. alternatingly).

Note that the narrowing of the duct sections 15a in <FIG> and <FIG> is shown in exaggerated manner.

Sublayer 10a is advantageously comparatively thick, advantageously at least <NUM>, in order to provide large cross sections for the ink ducts.

<FIG> also shows the vias <NUM> that are connected to the ejection electrodes <NUM>. In the present example, there is one such via for each nozzle, which allows to control the ejection electrodes <NUM> individually.

The vias <NUM> in <FIG> (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 wall of the surrounding dielectric.

As can be seen from <FIG>, each electrical via <NUM> is laterally surrounded by a non-conducting wall 14a formed by sublayer 10a, which is in turn surrounded by a cavity 14b and yet another wall 14c.

Laterally surrounding the vias <NUM> first by a dielectric wall 14a and then by a cavity 14b increases the electrical breakdown threshold of the structure. Most of the electrical potential (e.g. in respect to a neighboring via and/or the neighboring 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 <NUM> 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 layers of print head <NUM>.

Hence, in more general terms, print head <NUM> advantageously comprises electrically conductive vias <NUM>. Each of these vias <NUM> 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>, 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 <NUM>, circular cross-section, thereby avoiding corners that would lead to locally increased electrical field strengths.

To simplify manufacture, the vias <NUM> extend advantageously along the ejection direction X, i.e. perpendicular to front surface <NUM> of print head <NUM>.

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 <NUM> need to have such cavity structures along their whole length. For example, and as shown in <FIG>, which shows a sectional view of sublayer 10b above sublayer 10a of <FIG>, there is no cavity around the vias <NUM>. Hence, first and second wall 14a, 14c can be anchored, at their upper ends, in sublayer 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> further shows the second ink duct sections 15b, which are also shown in <FIG>, with one such second ink duct section 15b branching off from the first duct sections 15a below each nozzle <NUM>.

Sublayer 10b can be thinner than sublayer 10a, e.g. approximately <NUM>.

<FIG> and <FIG> show the ink duct sections 16b of ink suction duct <NUM> formed in sublayer 10c. The design of this sublayer 10c is similar to the one of sublayer 10a in that the ink duct sections 16b, separated by walls 17b, extend horizontally to feed ink to lateral reservoirs <NUM> and 16v. Again, and as depicted for the ink duct sections 15a in <FIG>, their cross sections decrease with increasing distance from the reservoirs 16u, 16v, and they are arranged interdigitally.

Sublayer 10c is, again, advantageously comparatively thick, advantageously at least <NUM>, in order to provide large cross sections for the ink duct sections 16b.

<FIG> shows that the vias <NUM> are again surrounded by first and second walls 14a, 14c and a cavity 14b.

<FIG> further shows the duct sections 15c with surrounding walls 17c that communicate with the duct sections 15b of the ink supply ducts <NUM>.

As can be seen from <FIG> and <FIG>, sublayer 10d forms a wall 17d between ink duct section 15d of ink supply duct <NUM> and ink duct section 16a of ink suction duct <NUM>.

Sublayer 10d can be thinner than sublayers 10a and 10c, e.g. approximately <NUM>.

The design of the nozzles <NUM> can best be seen in <FIG> and <FIG>.

Nozzle <NUM> of this embodiment comprises a tip section <NUM>, a shaft section 48a, 48b, and a base section 50a, 50b, <NUM>, with tip section <NUM> arranged in front of shaft section 48a, 48b, and base section 50a, 50b, <NUM> arranged behind shaft section <NUM>.

The shown nozzle design relies on the ink wetting the lateral surface of nozzle <NUM> and passing through a channel of nozzle <NUM>, but a design e.g. as shown in <CIT> can be used as well.

The present nozzle <NUM> comprises a radial channel <NUM> 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 <NUM>. (Note that, in <FIG>, radial channel <NUM> is rotated by <NUM>° as compared to <FIG>.

From the upper end of channel <NUM>, the ink wets sides of tip <NUM> and forms a meniscus at the top <NUM> of tip <NUM>. In this way, a sharp meniscus-like ink geometry 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 <NUM> guides ink outwards to an annular opening <NUM> at the top of base 50a, 50b, <NUM>. Annular opening <NUM> is arranged between a central section 50a and a peripheral section 50b of the base of nozzle <NUM>.

From annular opening <NUM>, ink can wet the shaft section of nozzle <NUM>, further adding to the ink that is available at tip <NUM>.

Peripheral section 50b may be coated with an anti-wetting coating <NUM> and forms an ink retainer <NUM> that prevents ink from laterally spreading over the nozzle. Depending on the ink to be used, coating <NUM> may be hydrophobic and/or oleophobic. For example, it may be formed, at least in part, of Teflon and/or PTFE, which are hydrophobic and oleophobic. Depending on the scope of inks to be used, it may also be only hydrophobic (e.g. HMDS, i.e. Bis(trimethylsilyl)amine) or only be oleophobic (e.g. based on polymers). In particular, the surface of ink retainer <NUM> is advantageously more hydrophobic and/or oleophobic than a surface of shaft 48a, 48b of nozzle <NUM>.

As mentioned above, guard electrode <NUM> keeps the electric fields at the location of retainer <NUM> low, thereby further reducing the tendency of the ink to spread over retainer <NUM>.

Each nozzle <NUM> 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>), or it may be an annular series of suction openings, e.g. circular openings.

Further, each nozzle <NUM> of the shown embodiment is surrounded by an annular wall <NUM> of support structure <NUM>. Further support elements <NUM> may be provided to support the upper parts of support structure <NUM>. For example, these further support elements <NUM> may be structured in a hexagonal pattern of walls <NUM> surrounding cavities <NUM>. This kind of wall design minimizes mechanical stress in the structure.

As mentioned, a support structure <NUM> is provided for connecting the various electrodes <NUM>, <NUM>, <NUM> to nozzle carrier <NUM>. It is arranged on front side <NUM> of nozzle carrier <NUM>.

Support structure <NUM> comprises a plurality of support elements, such as the support elements <NUM>, <NUM>, arranged between the nozzles <NUM>.

Ink retainer <NUM> is advantageously designed to prevent ink from reaching these support elements of the support structure and to prevent it from wetting them, thereby reducing the tendency of the ink to submerge the nozzles.

Support structure <NUM> advantageously comprises at least one electrode carrier layer. In the embodiment of <FIG>, there are three such carrier layers <NUM>, <NUM>, <NUM>. Each electrode carrier layer comprises at least one of the electrodes <NUM>, <NUM>, <NUM>, and it extends parallel to top surface <NUM>.

Typically, the electrode <NUM>, <NUM>, <NUM> is embedded within its electrode carrier layer <NUM>, <NUM>, <NUM> and covered on its front and back sides by at least one dielectric sublayer 80a, 80b or 82a, 82b or 84a, 84b.

In the embodiment shown here, support elements <NUM>, <NUM>, <NUM> and/or <NUM> are provided between each of the electrode carrier layers <NUM>, <NUM>, <NUM> as well as between the backmost electrode carrier layer 80a and nozzle carrier <NUM>. They may, however, also only be provided between a subset of these structures.

In the present embodiment, the gas ducts <NUM>, <NUM>, <NUM> of the ventilation system (as described above) comprise gas duct sections arranged in the upper sublayers of support structure <NUM>.

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 <NUM>' of support structure <NUM> between the carrier layers <NUM> and <NUM>. This is shown in <FIG> and <FIG>.

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

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 <NUM>.

At each of the nozzles <NUM>, they branch into secondary gas duct sections 34b1, 34b2 and feed gas to a mixing region <NUM> located in recess <NUM>.

The cross sections of the secondary gas duct sections 34b1, 34b2 are at least <NUM> times, in particular at least <NUM> 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 <NUM> 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 <NUM> times smaller) than the maximum total pressure drop over the secondary duct sections 34b1, 34b2.

To expedite this, the secondary duct sections 34b1, 34b2 have a total (combined) length L (see inset X of <FIG>) from the primary duct sections 34a to the mixing region <NUM> (i.e. to the recess <NUM>) that is a substantial fraction k of the distance D between two neighboring nozzles <NUM>, i.e. L = k·D, with k at least <NUM>, in particular at least <NUM>.

Furthermore, the length L2 of the primary duct sections 34a is preferably chosen smaller than the squared ratio of the cross-sections C1 and C2 of primary duct sections 34a and secondary ducts sections 34b1, 34b2, multiplied by L, i.e. <MAT>.

(For a more accurate estimate for elongate cross sections, the Darcy-Weisbach equation may be used.

For example, if the cross-sections C1 of the primary duct sections 34a are ten times larger than the cross-sections C2 of the secondary duct sections 34b1, 34b2, then L2 is preferably shorter than <NUM>. In this case, by additionally choosing k = <NUM>, 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>. 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 <NUM>, form the "second ends" <NUM>-<NUM> of the first gas ducts <NUM> as mentioned above.

Advantageously, at least two of the second ends <NUM>-<NUM> end in each recess <NUM>, and they are arranged in rotational symmetry around nozzle axis <NUM> in order to generate a symmetric flow of gas that does not laterally deflect the ink.

Hence, in more general terms, the first gas ducts <NUM> comprise primary and secondary duct sections 34a and 34b1, 34b2, wherein:.

A particularly compact design can be achieved, as shown, if the primary duct sections 34a and, advantageously, also the secondary duct sections 34b1, 34b2, are incorporated into support structure <NUM>, i.e. in ejection direction X they are located between surface <NUM> and nozzle carrier <NUM>.

Advantageously, the primary duct sections 34a are located between the ejection electrodes <NUM> and nozzle carrier <NUM>.

Also advantageously, the primary and the secondary gas duct sections 34a, 34b1, 34b2 are all located in the same plane parallel to surface <NUM>, which obviates the need to manufacture vertical passages, such as passages through the carrier layers <NUM>, <NUM>, <NUM>.

<FIG> and <FIG> illustrate the locations of the gas duct sections for the second and third gas ducts <NUM>, <NUM>.

Again, there are primary gas duct sections 37a, 39a as well as secondary gas duct sections 37b1, 37b2 and 39b1, 39b2, with the duct sections 37a, 37b1, 73b2 forming part of the second gas ducts <NUM> and the duct sections 39a, 39b1, 39b2 forming part of the third gas ducts <NUM>.

Again, the primary gas duct sections 37a, 39a extend parallel to surface <NUM> and are located between surface <NUM> and nozzle carrier <NUM> to use the space available in support structure <NUM>. Advantageously, they are arranged between surface <NUM> and the ejection electrodes <NUM>.

The primary gas ducts 37a and 39a of the second and third gas ducts <NUM>, <NUM> extend parallel to each other and to the rows or columns of the nozzle array. They are arranged alternatingly in a common plane, i.e. each primary gas duct 37a of the second gas ducts <NUM> is arranged immediately between two primary gas ducts 39a of the third gas ducts <NUM> 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 37b1, 37b2, 39b1, 39b2, reference is first made to <FIG>, which shows a view of front surface <NUM> of print head <NUM> as seen from the target. As shown, in the present embodiment, each recess <NUM> and therefore each nozzle <NUM> is surrounded by two second ends <NUM>-<NUM> of the second gas ducts <NUM> and two first ends <NUM>-<NUM> of the third gas ducts <NUM>, with the former blowing gas into the region between print head <NUM> and target <NUM> and the latter sucking gas away from this region.

In order to avoid a lateral flow of gas across one of the recesses <NUM>, the two second ends <NUM>-<NUM> of the second gas ducts <NUM> and two first ends <NUM>-<NUM> of the third air ducts <NUM> are alternatingly arranged on the corners of a rectangle, in particular a square, centered on recess <NUM>. The resulting air gas flow pattern is shown by arrows in <FIG>. The longer arrows show the gas flows between the second ends <NUM>-<NUM> and the first ends <NUM>-<NUM>, and the shorter arrows show the gas flows between the gas from the second ends <NUM>-<NUM> of the first gas ducts <NUM> and the first ends <NUM>-<NUM>.

This kind of design - without the flow from the recesses <NUM> - is described in more detail in in <CIT>.

Turning back to <FIG>, in order to obtain this kind of pattern of the second ends <NUM>-<NUM> and the first ends <NUM>-<NUM>, neighboring secondary duct sections 37b1, 37b2 branch off on different sides of their primary duct section 37a of second gas duct <NUM>. Similarly, neighboring secondary duct sections 39b1, 39b2 branch off on different sides of their primary duct section 39a of third gas duct <NUM>. Along a line <NUM> located between two neighboring primary ducts 37a, 39a and extending parallel to the primary ducts 37a, 39a, the secondary duct sections (37b1, 37b2) of the second gas ducts (<NUM>) alternate with the secondary duct sections (39b1, 39b2) of the third gas ducts (<NUM>).

This is shown in <FIG> for the first parts 37b1, 39b1 of the secondary duct sections, with these first parts being arranged on a common plane with the primary duct sections 37a, 39a. And it is shown in <FIG> 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 37b1, 39b1 to the ends <NUM>-<NUM> and <NUM>-<NUM>, respectively.

Again, the secondary duct sections 37b1, 37b2 and 39b1, 39b2 are designed such that they have a much higher flow resistance than the primary duct sections 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> also shows chambers <NUM> located above the leads 103a (see <FIG>). Each chamber <NUM> 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> continue through the sublayer of <FIG> to connect to the shielding electrodes 14c of <FIG>.

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

This evaporator <NUM> 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 arranged outside print head <NUM>.

Alternatively or in addition thereto, evaporator <NUM> may also be located in print head <NUM>. An embodiment of an evaporator that may be integrated into the layer structure of a print head is shown in <FIG>.

This evaporator formed by layers <NUM> - <NUM> of print head <NUM>. These layers may e.g. be sublayers of support structure <NUM> and/or of nozzle carrier <NUM>.

Advantageously, print head <NUM> comprises a plurality of such evaporators <NUM>.

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

Chamber <NUM> is filled with the liquid to be evaporated. Duct section 34x is under a slight overpressure as compared to the liquid filled into chamber <NUM>. 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 <NUM>-<NUM>, which leads to a region of atmospheric pressure inside the recesses <NUM>. Liquid fills the openings <NUM> by surface tension and from an interface <NUM> (shown in dotted lines in <FIG>). In order for the interface <NUM> not to be pushed back into the openings <NUM> 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 surface 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 <NUM>. For d = <NUM> and a liquid with the surface tension of water, the Young Laplace equation yields a pressure difference of less than <NUM> mbar before the liquid exists from the openings <NUM>. For a liquid with the surface tension of alkane, the pressure difference would have to be less than <NUM> mbar.

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

As can also be seen from <FIG>, the openings <NUM> are advantageously surrounded, on the side of duct section 34x, by edges <NUM> with an undercut <NUM> beyond the edges <NUM>. This structure is advantageously provided with a poorly wettable surface, such as with one of the materials mentioned for coating <NUM> above. It pins any ink that may pass through the openings <NUM>.

In more general terms, the printing system may have an evaporator <NUM> in print head <NUM> which comprises.

Advantageously, the smallest diameter of each of the openings is <NUM> or less for allowing to form a liquid interface as described above.

The present print head can be manufactured using techniques as they are e.g. knows from semiconductor manufacturing and packaging, e.g. as described in <CIT>, <CIT>, <CIT>, and in <CIT>.

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

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 <NUM> V may be generated. While not printing, the voltage at the ejection electrodes is maintained at a level where no ink is ejected. Advantageously, it is non-zero, though, e.g. at <NUM> V.

As mentioned above, the electric field at ink retainer <NUM> is advantageously kept low, e.g. at less than <NUM>%, in particular at less than <NUM>%, of the field strength at the forward end <NUM> 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 <NUM>, if present, are used to retrieve ink from the nozzles.

Gas is sent from first gas source <NUM> into the first gas ducts <NUM>, moisturized by evaporator <NUM>, and used to provide a "wet" atmosphere at the location of the nozzles <NUM>.

While printing, dry gas is sent from second gas source <NUM> through the second gas ducts <NUM> to the region <NUM> between print head <NUM> and target <NUM>. It expedites the drying of the ink on target <NUM>, but the flow of gas from the first gas ducts <NUM> prevents this dry gas from entering the recesses <NUM> and reaching the nozzles <NUM>. The flow of gas from the first gas ducts <NUM> 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 <NUM>, <NUM> is advantageously equal to the flow of gas through the third gas ducts <NUM> for the reasons mentioned above.

<FIG> above illustrate a specific embodiment how ink can be fed to the nozzles <NUM> 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 <CIT>.

Similarly, other designs can be used for connecting the gas ducts to the recesses <NUM> and front surface <NUM>, e.g. as described in <CIT>.

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 <NUM> and <NUM> while the metallic sublayers are typically much thinner.

In most of the embodiments shown so far, each nozzle is surrounded 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 <NUM> 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, isolated pillars.

In the embodiments described so far, three electrodes at three different 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:.

Claim 1:
An inkjet printing system comprising
a print head (<NUM>),
a plurality of ink nozzles (<NUM>) arranged on the print head (<NUM>),
a first gas source (<NUM>),
first gas ducts (<NUM>) arranged at least partially in the print head (<NUM>), wherein at least a first end (<NUM>-<NUM>) of the first gas ducts (<NUM>) is connected to the first gas source (<NUM>) and a plurality of second ends (<NUM>-<NUM>) of the first gas ducts (<NUM>) is arranged at the ink nozzles (<NUM>),
wherein the printing system is characterized by at least one evaporator (<NUM>) arranged along the first gas ducts (<NUM>) before the nozzles (<NUM>) and adapted to evaporate at least one fluid into the gas before it arrives at the nozzles (<NUM>).