Patent Description:
<CIT> describes an electrohydrodynamic print head having a nozzle layer comprising a plurality of nozzles. It is based on a structure where the nozzles are arranged on one side and feed ducts extend through a feed layer on the other side.

<CIT> describes a print head having a nozzle layer (<NUM>) with a plurality of nozzles and a plurality of ventilation openings extending through the nozzle layer.

Nozzle electrodes are used to accelerate ink away from the nozzles and onto a target.

<CIT> describes a piezoelectric print head having ventilation openings.

The problem to be solved by the present invention is to provide a print head of the type mentioned above that has improved printing quality.

This problem is solved by the print head of claim <NUM>. Accordingly, the print head comprises a nozzle layer. This nozzle layer in its turn comprises at least the following parts:.

The ventilation openings allow to feed a gas to the region between the printing head and the target and/or to convey gas away from said region. Hence, it becomes possible to control or at least modify the composition of the gas in the space where printing takes place, which provides a wealth of options for controlling the printing process. Some of these options are described below.

According to the invention, the print head comprises a core region-with activatable nozzles and ventilation openings. The print head further comprises.

This edge region allows to generate a more homogeneous flow pattern and therefore a better printing result.

The ventilation openings advantageously comprise at least two types of openings: A first type is designed as suction openings for feeding gas away from the region adjacent to the nozzles. The second type is designed as blow openings for feeding gas towards said region.

The invention provides improved printing quality while suffering from less clogging problems. Particularly, the use of many nozzles on a flat multi-nozzle print head can result in the accumulation of evaporated liquid in the region between the print head and a target that is printed on. This problem is particularly pronounced for electrohydrodynamic multi-nozzle print heads like that disclosed in <CIT> where printing resolutions can be in the sub-<NUM>, potentially even in the sub-<NUM> resolution regime. Furthermore, such print heads potentially allow the use of millions of nozzles arranged in large nozzle arrays, for example as disclosed in <CIT>. Different from conventional ink-jet print heads, nozzles may be arranged in much larger numbers and in rectangular arrays that are largely of similar size along both main array axes.

In comparison, most ink-jet print heads are normally built such that nozzles are arranged within narrow rectangular or skewed rectangular areas, wherein a fast movement is exercised essentially in direction of the narrow dimension of said rectangular area. During such movement, a pixel position generally obtains a single or a very small number of droplets. Since the movement generally scans the whole width or length of the target, the residence time of the print head on top of a single droplet is very short and hence the printed droplets can dry while there is no print head on top of the target. The print head may only return for a second printing cycle once the previously printed droplets have fully dried.

In the use of an electrohydrodynamic print head, due to the high printing resolution, movement speeds are preferably smaller than those used for an ink-jet print head. Furthermore, the comparably large extension of the print heads in potentially all movement directions implies that the residence time of the print head above any equally large area on the target is large in comparison to the printing throughput. Hence, the print head may cover a given position on the target for much longer durations than what a single droplet would require drying. Hence, the presence of the print head and the many liquid-filled nozzles, as well as their printing output on the target, can strongly influence the drying behavior of deposited liquid. Given that the distance between the print head and the target is much smaller than the lateral dimension of the print head, this situation will generally result in the saturation of environment between print head and target with evaporated liquid. This can result in an evaporation blockage or at least in non-uniform evaporation due to the fact that the edge regions will have lower concentrations of evaporated liquid than the center positions of the print head. Both problems will strongly affect printing throughput and uniformity. The present invention provides a solution to allow a print head to operate even at long residence times.

As mentioned, the invention is particularly useful for large print heads, i.e. for print heads having an array of nozzles with a diameter that exceeds, in all directions, <NUM>, in particular <NUM>.

Advantageously, the print head comprises an array of nozzles. In said array, there is at least one ventilation opening per nozzle; in particular, there are at least two of them per nozzle. This allows maintaining a controlled microenvironment for each nozzle.

In one embodiment, the array can be divided into a plurality of identical unit cells, with each unit cell comprising at least one nozzle and the same arrangement of ventilation openings. In other words, the relative arrangement of the nozzle and the ventilation opening(s) is the same in each unit cell.

The print head is advantageously an electrohydrodymamic print head and comprises at least one nozzle electrode at each nozzle. The nozzle electrode can be positioned to electrohydrodynamically eject ink from the nozzle.

The print head may further comprise ventilation ducts connected to the ventilation openings in order to convey gas between the ventilation openings and as least one gas source and at least one gas sink.

The print head may further comprise electrically conductive vias extending through at least part of the ventilation ducts, which allows using the ventilation ducts not only for transporting gas but for transporting electricity, too.

The invention also relates to a method for operating the print head that comprises at least the following:.

The printing and conveying advantageously takes place concurrently for increased printing speed.

As mentioned, some of the openings may be blow openings while others may be suction openings. In this case, the method may advantageously comprise:.

This allows locally exchanging the gas at the nozzles. Advantageously, gas is fed away from the region and fed to the region at the same time in order to maintain a steady exchange of gas.

Furthermore, there is advantageously introduced a temperature difference between print head and the target. Because of this temperature difference, there will be a difference in vapor pressure between liquid deposited on the target and liquid contained within the nozzle in the print head. This pressure difference causes a diffusive motion of evaporated liquid from the higher towards the lower pressure region. Advantageously, the higher pressure (i.e. higher temperature) region is on the target and the lower pressure (i.e. lower temperature) region is on the print head. Hence, to cause evaporated liquid to move away from the substrate towards the print head, such print head is advantageously at a lower temperature than the substrate.

However, if evaporated liquid moves from the target towards the print head, due to a temperature difference, this can readily lead to an over-saturated environment at the print head surface and therefore to condensation of liquid on the print head. Especially an electrohydrodynamic print head may be rendered completely non-functional due to this. However, condensation can be prevented if the gas introduced through blowing-type ventilation openings can dissolve evaporated liquid that was previously printed onto the substrate. Such liquid-concentrated gas may then be removed through suction-type ventilation openings.

Advantageously, introduced gas therefore contains less than <NUM>% saturation, more advantageously less than <NUM>% saturation, of the at least one liquid used for printing. In this way condensation of liquid on the print head can be prevented, although through proper choice of temperature difference and gas flow minimal condensation conditions may be upheld. This is viable because condensation does not immediately occur on the print head even at over-saturated conditions, due to thermodynamic energy barriers associated with the formation of a liquid nucleus, i.e. a growth center, on a dry surface. In comparison, condensation into the liquid formed within the nozzle is readily possible. Therefore, the nozzle can compensate for an over-saturated environment by absorbing small amounts of liquid through condensation.

Condensation onset on dry parts is advantageously further reduced by coating the print head surface with Teflon or another liquid repellant material.

The combination of temperature difference and gas flow can result in fast removal of liquid from the target, thereby allowing more ink to be printed per time unit.

Here, the word "ink" advantageously describes a combination of liquid carrier and a contained material to be deposited. The material can be dispersed, dissolved or otherwise stabilized in the liquid. Upon printing of the ink and evaporation of the liquid carrier only the deposited material will remain.

Generally, the contained material is dedicated to be formed into structures of given size, for example into a line of a certain width and height. If ink is deposited into such a line at a high volumetric flow rate, this can result in widening of the line due to liquid accumulation. As a result, a lower volumetric flow rate must be chosen. As an alternative to decreasing volumetric flow rate, it may be advantageous to increase evaporation rate by introducing a gas circulation between the ventilation openings and, advantageously, by additionally introducing a lower temperature at the print head than at the target. The latter additionally allows to specifically control evaporation rates at the nozzle and the target. Advantageously, evaporation at the print head is close to zero or even slightly negative (i.e. slight condensation occurs), which implies that the concentration of contained material within the ink at the nozzle position remains almost constant, thereby reducing the problems associated with clogging or first droplet effects.

Importantly, if each nozzle has associated at least one ventilation opening all nozzles will have very similar environments in terms of the concentration of gas-dissolved liquid. If, for example, gas would be blown underneath the print head, from one side of the print head towards the other, of between ventilation openings that are separated by several nozzles, such gas would have a higher concentration of liquid where it exits as compared to where it entered (because it continuously takes up liquid along its way underneath the print head). Accordingly, the situation for nozzles at the entry and exit point will not be the same and this can result in a different drying speed and different clogging vulnerability and to non-uniform printing results.

Hence, the invention also relates to a printing system including a print head of this type as well as a target holder and at least one temperature control device for heating or cooling the print head and/or the target holder. In particular, the system comprises a print head temperature control device for cooling the print head and/or a target temperature control device for heating the target holder.

Similarly, the method of the present invention may advantageously comprise the step of controlling the temperature of at least one of the print head and the target holder.

Advantageously, the target is maintained at a higher temperature than the print head, in particular with a temperature difference between the target and the print head of at least <NUM>, in particular of at least <NUM>.

Also, the temperature of the target is advantageously heated, such as to at least <NUM>, for supporting in-situ tempering of the deposited material.

The print head is advantageously operated as an electrohydrodynamic print head, i.e. electrical fields from the nozzle electrodes of the print head are used to eject ink from the nozzles during printing.

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. This description makes reference to the annexed drawings, wherein:.

A "unit cell" of an array of nozzles and ventilation openings is the smallest group of nozzles and ventilation openings having the overall symmetry of the array, and from which the entire array can be built up by repetition in two dimensions.

Terms such as above, below, top, bottom are to be understood such that the nozzle layer is arranged at the bottom side of the print head with the nozzles' ejection direction being downwards.

Horizontal designates the directions parallel to the planes of the nozzle layer. Vertical designates the direction perpendicular to the plane of the nozzle and layer.

<FIG> illustrates a general setup of an embodiment of the invention. It shows a print head <NUM>, which is used to print ink onto a target <NUM>.

Print head <NUM> has e.g. the basic design as described in <CIT> and comprises an array of nozzles <NUM> for ejecting inks. As described in more detail in an embodiment below, nozzle electrodes located at the nozzles <NUM> are used to electrohydrodynamically eject ink droplets from the nozzles <NUM> and an acceleration electrode accelerates ink from the nozzles onto target <NUM>.

A target holder <NUM> is arranged below print head <NUM> and adapted to hold target <NUM> at a distance of e.g. <NUM> to <NUM> below print head <NUM>. Target holder <NUM> may e.g. form said acceleration electrode.

In this example the acceleration electrode is associated with the target <NUM> such that between the two flat surfaces of target <NUM> and print head <NUM> a uniform electric can form which accelerates the droplets ejected from any nozzle <NUM> in perpendicular direction to the print head <NUM> surface towards target <NUM> where they are being deposited.

Print head <NUM> may comprise, and/or be thermally connected to, a print head temperature control device <NUM>, and target holder <NUM> may comprise, and/or be thermally connected to, a target temperature control device <NUM> to expedite the drying of the ink on substrate <NUM>, by introducing a temperature gradient between print head <NUM> and target <NUM>.

Temperature control devices <NUM>, <NUM> may include a resistive heater or a Peltier element or remote heating or cooling of a liquid which passes through the temperature control device <NUM>, <NUM>.

In any case, also passive heating or cooling may be employed, e.g. to bring either print head or target, or both, to room temperature.

In an advantageous embodiment, print head temperature control device <NUM> is adapted to cool or heat the ink itself. For example, the ink may be cooled or heated outside print head <NUM> and then fed into print head <NUM>.

For better temperature control, the ink may be recirculated before being printed. In other words, and as illustrated in <FIG>, the printing system may comprise a circulation pump <NUM> connected to print head <NUM> for circulating ink through print head <NUM>, advantageously with the ink being temperature-controlled by print head temperature control device <NUM>. Part of the circulated ink is branched off to the nozzles <NUM> for printing.

There may also be used a combination of techniques for heating and/or cooling. For example, there could be used a Peltier element for heating or cooling a block of metal or other material with good thermal conductivity, such as Aluminum Nitride, with this block being in contact to the feed layer <NUM>. At the same time, this block may be passed by the ink in which case the ink takes up the temperature of the block before entering the feed layer <NUM>.

Advantageously, the print head temperature control device <NUM> sets the print head <NUM> temperature such that it is lower than the temperature set at the target <NUM> by the target temperature control device <NUM>. In this way, higher liquid evaporation rates are created at the target <NUM> than at the print head <NUM>. It is understood that both the absolute temperature at print head <NUM> and target <NUM> may be above or below room temperature, while still the print head <NUM> is at a lower temperature than the target <NUM> in comparison. For example, the temperature at print head <NUM> may be chosen <NUM> while the temperature at the target <NUM> may be chosen to be <NUM>. Such high temperatures at target <NUM> may be chosen in order to not only enhance evaporation of solvent upon droplet impact but in addition the temperature at the target may also introduce some sort of in-situ temperature sintering of the deposited material contained within the ink. Furthermore, it is possible to adjust for solvents of different vapor pressure. For example, a lower boiling point liquid may be operated with a lower median temperature than a higher boiling point liquid, wherein median temperature described the intermediate temperature between target <NUM> and print head <NUM>.

As described in more detail in the following, print head <NUM> comprises a plurality of ventilation openings, which include blow openings <NUM> and suction openings <NUM>.

The blow openings <NUM> are used to feed gas to a region <NUM> below the nozzles <NUM>. The suction openings <NUM> are used to feed gas away from region <NUM>.

A gas source <NUM> may comprise a pump or pressure reservoir, and optionally a mass flow controller 20a connected in series to an inlet <NUM> of the print head. Alternatively or in addition thereto, the gas sink <NUM> may comprise a vacuum pump or low pressure reservoir, and optionally a mass flow controller 22a connected in series to an outlet <NUM> of the print head.

Advantageously, the mass flow controller comprises a mass flow sensor, a pressure regulator, and a fast switching piezo valve 20b, 22b connected in series. The piezo valve is used for fast on/off switching of the gas source to the inlet or gas sink to the outlet. The steady state gas flow is controlled by the pressure regulator, using the mass flow sensor as feedback device. The pressure at the pressure regulator may also be set above the requirements for the steady state flow, to improve transient behavior.

Consequently, the piezo valve is operated in a linear proportional mode or a pulse width modulated mode to limit and control the steady state flow. The pressure regulator can be entirely omitted by using two fast switching piezo valves in a half-bridge configuration and a pressure sensor, applying the aforementioned in a linear proportional or pulse width modulated driving mode.

Fast switching of the gas flow between on-state and idle state of the print head is beneficial because the gas flow is advantageously adjusted to the printing flow rate and drying speed of ink on the target <NUM>. For example, upon finalization of a print, printing flow rate may rapidly go to zero, in which case the gas flow is advantageously reduced to zero as well, in order to not accelerate evaporation of liquid from the nozzle. Commercial mass flow controllers enable precise gas flow control and settling times in the order of <NUM> milliseconds. Faster switching and settling times in the order of <NUM> to <NUM> milliseconds or even below <NUM> millisecond are achieved with piezo valves.

In more general terms, the printing system of the present invention may include at least one mass flow controller for regulating (i.e. measuring and maintaining at a desired level) the mass flow of the gas passing through the ventilation openings <NUM>, <NUM> and/or a valve 20b, 22b for controlling the flow of the gas.

Print head <NUM> comprises a nozzle layer <NUM>, which includes the nozzles <NUM> as well as the nozzle electrodes, and feed layer <NUM>. Feed layer <NUM> forms feed ducts for feeding ink to the nozzles as well as ventilation ducts connecting the ventilation openings <NUM>, <NUM> to their respective gas source <NUM> and gas sink <NUM>.

The arrangement of the nozzles <NUM> and the ventilation openings <NUM>, <NUM> can affect the trajectory of the ink through region <NUM>, as well as the drying behavior of ink at substrate and nozzle and it should therefore be designed with care.

<FIG> illustrates a first embodiment, which corresponds to the design illustrated in <FIG>. Here, each nozzle <NUM> is arranged between one blow opening <NUM> and one suction opening <NUM> (i.e. on the connecting line between the blow opening <NUM> and the suction opening <NUM>). Advantageously, the nozzle <NUM> is arranged at the center between these two ventilation openings <NUM>, <NUM>.

<FIG> shows two rows of nozzles <NUM> and ventilation openings <NUM>, <NUM> extending parallel to each other. It depicts only a small part of the array of nozzles <NUM> and ventilation openings <NUM>, <NUM> of the array.

As can be seen, the array can be divided into a plurality of unit cells <NUM>. <FIG> illustrates two such unit cells <NUM>, each surrounded by a dotted square.

Each unit cell <NUM> advantageously comprises at least one nozzle <NUM>, at least part of a blow opening <NUM>, and at least part of a suction opening <NUM> in order to generate a controlled, local gas flow around the nozzle <NUM>.

In the embodiment of <FIG>, each unit cell <NUM> contains exactly one nozzle <NUM>, its neighboring blow opening <NUM> and its neighboring suction opening <NUM>.

The gas flow generated by the ventilation openings <NUM>, <NUM> is illustrated by dashed arrows <NUM>. In particular, there is a basically linear gas flow across the exit of nozzle <NUM>. It will tend to deflect the ink exiting from the nozzle, but since the gas flow is the same at all nozzles, the deflection caused by it merely results in a linear offset of all the ink deposited on substrate <NUM>.

<FIG> shows another embodiment of the arrangement of nozzles <NUM> and ventilation openings <NUM>, <NUM>.

Here, each unit cell <NUM> consists of two halves a blow opening <NUM> and two halves a suction opening <NUM> alternatingly arranged at the centers of the edges of a square <NUM> (which coincides with the border of unit cell <NUM>) and one nozzle <NUM> in the center of square <NUM>.

In this design, and as illustrated in <FIG>, there is no direct gas flow <NUM> across the nozzle <NUM>, which reduces the deflection of the ink ejected by the nozzle. This is beneficial because even a uniform deflection force as caused in the first embodiment of <FIG> can introduce problems, for example if the distance between print head and substrate is not the same for all nozzles. In this case, some droplets will be deflected for a longer duration than others, which will introduce a relative offset in their impact position on the substrate. Distance variations may be caused by improper alignment between print head and substrate surfaces but potentially also by the existence of surface topography located at the substrate. Hence, the embodiment of <FIG> may be regarded superior to the embodiment of <FIG>.

<FIG> shows a third embodiment of the arrangement of nozzles <NUM> and ventilation openings <NUM>, <NUM>.

Here, each unit cell <NUM> consists of four quarters of suction openings <NUM> at the corner of a first square <NUM>, four halves of blow openings at the middle of the edges of the first square <NUM>, and one suction opening <NUM> in the center of the first square <NUM>. Further, the unit cell includes four nozzles <NUM> at the corners of a second square <NUM>. The first and second squares <NUM>, <NUM> have parallel edges and are concentric, and the first square <NUM> has twice the diameter of the second square <NUM>.

In this case, the gas flow around two neighboring nozzles <NUM> is of opposite direction. However, since there is (similar to the embodiment of <FIG>) no gas flow directly across the nozzles <NUM>, the consequences of this asymmetry are small.

It must be noted that the unit cell <NUM> in the embodiment of <FIG> may also be offset by one nozzle distance (e.g. to the right in the figure). In that case, each unit cell <NUM> consists of four quarters of blow openings <NUM> at the corner of first square <NUM>, four halves of suction openings <NUM> at the middle of the edges of first square <NUM>, and one blow opening <NUM> in the center of the first square <NUM>. In other words, the unit cell can be described in more than one way, with the descriptions being interchangeable in that they describe the same physical arrangement of the nozzles <NUM> and the ventilation openings <NUM>, <NUM>.

This illustrates that there is typically more than one way to describe such a unit cell, and for fulfilling a claimed unit cell type it is sufficient that a given physical arrangement can be divided into unit cells of a claimed unit cell type.

In the embodiments of <FIG> and <FIG>, there are two ventilation openings <NUM>, <NUM> for each nozzle <NUM> while, in the embodiment of <FIG>, there is only one ventilation opening <NUM>, <NUM> for each nozzle <NUM>. Hence, the density of nozzles in the embodiment of <FIG> can be larger than in the embodiments of <FIG> and <FIG>.

<FIG> illustrate a possible design of the nozzle layer <NUM> and the feed layer <NUM> of a print head <NUM>.

The design of the nozzles <NUM> in nozzle layer <NUM> substantially corresponds to the design of the device described in <CIT>.

In particular, each nozzle <NUM> comprises a spout <NUM> arranged in a recess <NUM>. At a level below spout <NUM>, at least one nozzle electrode <NUM> surrounds recess <NUM> and is used to extract ink away from a liquid meniscus formed at spout <NUM>. In the shown embodiment, the nozzle electrodes <NUM> are annular (cf.

In the shown embodiment, an optional shield electrode <NUM> is arranged at a level below the nozzle electrodes <NUM>. It covers substantially all the array of nozzles <NUM> with the exception of openings at the location of each nozzle <NUM> and helps to shield the influence of nozzle electrodes <NUM> of neighboring nozzle <NUM> and to maintain a uniform electrical field in region <NUM>.

Nozzle layer <NUM> may comprise a first dielectric sublayer <NUM> between the electrodes <NUM>, <NUM> and a second dielectric sublayer <NUM> right above the nozzle electrodes <NUM>. A third dielectric sublayer <NUM> forms the spouts <NUM>. A fourth dielectric sublayer <NUM> forms a carrier membrane of the spouts <NUM> for positioning and holding each spout <NUM> at the center of its recess <NUM>.

Feed layer <NUM> forms feed ducts 56a, 56b for feeding ink to the nozzles <NUM>. In the embodiment shown, they include via sections 56a extending vertically upwards from the nozzles <NUM> and horizontal interconnect sections 56b. The latter run e.g. perpendicular to the sectional plane of <FIG>, and each of them interconnects a plurality of the via sections 56a. The interconnect sections 56b may be connected to larger ink terminals of print head <NUM>, where an ink reservoir may be connected to them.

The ventilation openings <NUM>, <NUM> are connected to ventilation ducts 58a, 58b, 58c, which extend through print head <NUM> to ventilation terminals <NUM>, which can be connected to the gas source <NUM> and to the gas sink <NUM>.

In the shown embodiment, the ventilation ducts 58a, 58b, 58c include chimney sections 58a, which extend from the ventilation openings <NUM>, <NUM> vertically through at least part of print head <NUM>, in particular through nozzle layer <NUM> and at least part of feed layer <NUM>.

Further, the shown ventilation ducts 58a, 58b, 58c include two sets of interconnect ducts 58b, 58c, each of which interconnects a plurality or all of the ventilation openings <NUM>, <NUM>.

Advantageously, and as shown, the interconnect ducts 58b, 58c advantageously extend horizontally through print head <NUM>.

In the shown embodiment, the first set 58b of interconnect ducts interconnects the blow openings <NUM>, and the second set 58c of interconnect ducts interconnects the suction openings <NUM> (or vice versa). The two sets form distinct duct systems for feeding gas to the blow openings <NUM> and for feeding gas away from the suction openings <NUM>. Hence, in this example an arrangement of ventilation openings is shown that is essentially equal to that shown in <FIG>.

Advantageously, the first and second set of interconnect ducts 58b, 58c are located at different vertical levels in the print head, which makes it easier to keep them separate. In any case, after forming the interconnect ducts 58b, the 58c, a single vertical ventilation duct may be formed to continue gas flow to the next higher level of the feed layer <NUM>. In this way, space is created for the formation of interconnect ducts of different gas pressure (i.e. dedicated to suction of blowing) or of horizontal interconnect sections carrying ink. This can be important for other embodiments than that shown in <FIG>.

For example, the embodiment of <FIG> would not allow interconnection of the ventilation ducts 58a of either suction or blowing type, without passing over of ventilation ducts 58a of the other type (i.e. blowing or suction) or over feed ducts 56a. To succeed with the interconnection anyway, one can first interconnect via section 56a by horizontal interconnect section 56b as it is shown in <FIG>. By freeing the space previously occupied by the many via sections 56a, one would then be able to interconnect ventilation ducts 58a on a higher sublayer of the feed layer <NUM>.

The reduction in the number of ventilation ducts 58a towards the higher sublayers of the feed layer <NUM> eventually reduces to at least one ventilation terminals <NUM>.

However, such reduction implies that the distance of any two blow openings <NUM> or suction openings <NUM> to the respective inlet <NUM> or outlet <NUM> (see <FIG>) may vary between individual openings. To have equivalent gas flow through the different blow openings <NUM> or suction openings <NUM>, it is advantageous to make sure that the pressure drop takes place majorly through the thin channels of the blow openings <NUM> or the suction openings <NUM> or through any other channel that is uniform across the whole print head <NUM>. In the embodiment of <FIG> this includes chimney sections 58a.

This may e.g. be achieved by adjusting the average cross-section and length of uniform ventilation ducts, e.g. chimney section 58a, as compared to their non-uniform counterparts, e.g. of the interconnect ducts 58b, 58c. A smaller cross-section and longer length of a given ventilation duct thereby implies a higher pressure drop. Hence, one way of achieving appreciable results is by reducing the diameter of the blow openings <NUM> and the suction openings <NUM> until the pressure drop over uniform parts of the ventilation ducts on averages varies less than a certain percentage. Preferably, such percentage is less than <NUM>% across all blow openings <NUM> and suction openings <NUM>, more preferably less than <NUM>%.

In more general terms, operating the print head advantageously comprises at least one of the following steps:.

Advantageously, equivalent procedures are executed also when designing the feed ducts 56a, 56b and the diameter of the nozzles <NUM>. In this case the relevant flow is the flow of liquid through the nozzle <NUM> when ejecting liquid.

In the shown embodiment, feed layer <NUM> comprises several sublayers, which are advantageously of a dielectric material in order to insulate the various electrically conductive tracks within feed layer <NUM> (to be described below).

A first sublayer 62a forms the via sections 56a as well as part of the chimney sections 58a.

A second sublayer 62b forms the interconnect sections 56b of the ink feed ducts for the ink. The chimney sections 58a extend through this second sublayer 62b.

A third sublayer 62c covers second sublayer 62b and closes the interconnect sections 56b from above. The chimney sections 58a extend through this third sublayer 62b.

The third sublayer 62b may also form at least one via section from each interconnect section 56b. The same via section may also extend upwards into each of the higher sublayers until there is formed an opening in the topmost layer which allows contacting to an ink source via an ink terminal. In the present example, one such via section 56c and the respective ink port 56d are illustrated, in dotted lines. This exemplifies that after interconnecting feed ducts only few via sections need to be effectively formed all the way to the topmost layer.

A fourth sublayer 62d forms the first set 58b of interconnect ducts for the gas to the blow openings <NUM>. The chimney sections 58a associated with suction openings <NUM> extend through this fourth sublayer 62d.

A fifth sublayer 62e covers fourth sublayer 62d and closes the interconnect ducts 58b from above. The chimney sections 58a associated with suction openings <NUM> extend through this fourth sublayer 62e.

A sixth sublayer 62f forms the second set 58c of interconnect ducts for the gas from the suction openings <NUM>.

The sixth sublayer 62f may also form at least one chimney section from each interconnect duct of the first set of interconnect ducts 58b. The same chimney section may also extend into each of the following upper sublayers, until there is formed an opening in the topmost layer in the form of a gas terminal <NUM> (not shown).

A seventh sublayer <NUM> covers sixth sublayer 62f and closes the second set of 58c interconnect ducts from above. It may also form one or more of the gas terminals <NUM>.

As mentioned, there are several electrically conductive tracks in print head <NUM> and in particular in feed layer <NUM> in order to connect the various electrodes to one or more voltage sources of the printer.

They may include suitable electrical vias extending through some or all of the sublayers of the print head.

In one particularly advantageous embodiment, print head <NUM> may contain electrically conductive vias <NUM> extending at least through part of the ventilation ducts 58a, 58b, 58c. In particular, such electrically conductive vias <NUM> may extend along at least some of the chimney sections 58a. They may e.g. be formed by an electrically conductive coating extending along at least part of the wall of the respective chimney section 58a.

The conductive vias <NUM> may be connected to the nozzle electrodes <NUM> as shown in <FIG>. In order to have different nozzle electrodes <NUM> connected to at least two different voltage sources, one half of the vias <NUM> may be connected to a first set of electrically conductive interconnect lines 66a, e.g. at top surface of sublayer 62c (see <FIG>), while the other half of the vias <NUM> may be connected to a second set of electrically conductive interconnect lines 66b, e.g. at the top surface of sublayer 62e.

It must be noted that electrically conducting vias <NUM> in chimney sections 58a may also be used, in addition or alternatively to the above application, to feed voltage to any other electrode(s) in print head <NUM>, such as to shield electrode <NUM>.

If the print head is to be brought to a specific temperature, it is advantageous to at least form some of the electrical vias <NUM> as separate vias which are completely filled with metal, e.g. by electroplating or by printing a metal ink into the voids. Particularly, a metal with good thermal conductivity like copper can be used to fill such vias. In this way, the temperature implied onto the print head by a cooling or a heating device is efficiently forwarded to the nozzle layer, across the dielectric layers of the feed layer which by definition do not have very good thermal conductivity properties.

As can be seen, nozzle layer <NUM> of print head <NUM>, as well as feed layer <NUM>, form a single, integral body. They may e.g. be manufactured using masking and etching steps as they are known from semiconductor technology.

Particularly feed layer <NUM> may be made by stacking and patterning of permanent dry film resist, e.g. epoxy-based dry film resist, or from individual patterned glass sheet, particularly laser-patterned glass sheets, which are bonded together, e.g. by adhesive bonding.

The print head is operated by applying suitable voltage pulses to the nozzle electrodes <NUM> in order to eject ink from the nozzles <NUM> onto target <NUM>.

At the same time, or at times different from the actual printing steps, gas is conveyed through the ventilation openings <NUM>, <NUM>.

Advantageously, the steps of printing the ink and conveying the gas are concurrent even though they may also take place intermittently as described for an example below.

Advantageously, the flow rate of gas conveyed through the blow openings <NUM> to region <NUM> (e.g. the gas volume conveyed each second into region <NUM>) and the flow rate of gas conveyed through the suction openings <NUM> from region <NUM> (e.g. the gas volume conveyed each second from region <NUM>) are equal. This helps to keep the gas composition in region <NUM> homogeneous and avoid lateral gas flows towards the edges of region <NUM>, which might deflect the ink segments, e.g. in droplet-form, passing it.

It must be noted, though, that the flow rates through the blow openings <NUM> and the suction openings <NUM> need not be equal. In fact, one type of ventilation openings (either the blow openings or the suction openings) may even be dispensed with completely while it is still possible to ventilate region <NUM> by relying on a global horizontal gas exchange at the edges of region <NUM>. In that case, for example, region <NUM> may be flooded with fresh gas from the blow openings <NUM>, or it may be flooded with fresh gas drawn in horizontally from the edges of region <NUM> while the old gas is removed by means of the suction openings <NUM>. In this embodiment, printing may advantageously be interrupted while operating the gas flow in order to avoid asymmetric deflection caused by the gas flow in region <NUM>.

Also, when printing is interrupted altogether, the gas flow is advantageously deactivated while the temperature difference between print head <NUM> and target <NUM> is upheld. In case gas flow continuous after printing discontinuation, the lack of liquid on the target implies that the gas flow will end up supporting removal of liquid from the nozzle, due to the absence of a diffusive gas flow from the target towards to print head. To support persistence of the nozzle against clogging, a gas may be switched from a partially saturated to a fully saturated species.

The gas conveyed to region <NUM> through the blow openings <NUM> may fulfill one or more of the following functions:.

It is understood that the gas can also be used for both, to support drying and introduce a chemically inert environment.

<FIG> shows another advantageous technique for a print head <NUM> with ventilation openings <NUM>, <NUM>, which does not form part of the invention. It is illustrated, by way of example, for the geometry of nozzles <NUM> and ventilation openings <NUM>, <NUM> of <FIG>, but it can be combined with any of the embodiments described herein.

The figure depicts the edge region of the print head, with reference number <NUM> denoting, symbolically, two edges of the print head. For simplicity, the blow openings <NUM> are denoted by a plus sign and the suction openings <NUM> by a minus sign. The nozzles <NUM> are represented by small, black dots.

<FIG> further shows the outer border <NUM> of the active nozzles <NUM>. In the figure, the nozzles <NUM> to the left of the border <NUM>, in a core region <NUM> of print head <NUM>, are structured thus that they can be activated for printing. When printing, they are being activated to eject ink.

In a region outside border <NUM>, namely in an edge region <NUM>, there are no activatable nozzles, but there is a plurality of blow openings <NUM> and/or suction openings <NUM>.

Advantageously, and as shown, in edge region <NUM>, blow openings <NUM> and/or suction openings <NUM> extend along a row, in particular along a single row, parallel to border <NUM>.

In more general terms, the print head advantageously comprises a core region <NUM> with activatable nozzles <NUM> and ventilation openings <NUM>, <NUM> and an edge region <NUM>, surrounding the core region <NUM>, with ventilation openings <NUM>', <NUM>' but no activatable nozzles <NUM>, with a (virtual) border <NUM> between them.

In the embodiment of <FIG>, there is exactly one row of ventilation openings <NUM>', <NUM>', <NUM>" surrounding core region <NUM>.

These ventilation openings <NUM>', <NUM>', <NUM>" have, in the embodiment of <FIG>, smaller diameter than the ventilation openings <NUM>, <NUM> in core region <NUM>, thereby generating a smaller gas flow rate. This design takes into account that the ventilation openings <NUM>', <NUM>', <NUM>" along the edge have fewer neighboring ventilation openings than those in core region <NUM>. Thus, reducing the gas flow through them makes the flow pattern of the print head more homogeneous at the location of the outmost active nozzles <NUM>.

In general terms, the print head is adapted and structured to generate a smaller gas flow rate through at least some of the ventilation openings <NUM>', <NUM>', <NUM>" in the outmost row than through the ventilation openings <NUM>, <NUM> in the core region <NUM>.

In particular, the outmost ventilation openings <NUM>', <NUM>' along the edges (but not the ventilation openings <NUM>" at the corners) outside core region <NUM> have, in the embodiment of <FIG>, have only three instead of four neighboring ventilation openings, and therefore the gas flow through them is advantageously approximately <NUM>% of the gas flow through the ventilation openings <NUM>, <NUM> in the core region. Similarly, the gas flow through the ventilation openings <NUM>" outside the corners of core region <NUM> should advantageously be adapted to have approximately <NUM>% of the gas flow through the ventilation openings <NUM>, <NUM> in core region <NUM>.

In <FIG>, the gas flow reduction is implemented, as mentioned, by a diameter reduction of the outmost ventilation openings <NUM>', <NUM>', <NUM>". The amount of diameter reduction depends on the length and geometry of ventilation openings and the ventilation ducts and can be calculated using numerical simulation and/or approximating calculations.

Alternatively to reducing the diameter of the ventilation openings <NUM>', <NUM>', <NUM>", the diameters of the ventilation ducts leading to these ventilation openings may be reduced.

In yet another embodiment, separate gas sources and/or gas sinks can be provided for core region <NUM> and edge region <NUM>, with the latter having lower pressure for lower gas flows than the former.

<FIG> shows a design according to the invention for reducing inhomogeneous gas flow at the edge of core region <NUM>.

Here, edge region <NUM> is several rows of ventilation openings deep, but the ventilation openings <NUM>', <NUM>' in edge region <NUM> advantageously have the same gas flow (at least close to border <NUM>) as those in core region <NUM>.

The distance W from the border <NUM> to the outmost ventilation openings <NUM>', <NUM>' of the edge region <NUM> (i.e. those ventilation openings of edge region <NUM> that are farthest away from border <NUM>) is at least two times, in particular at least five times, the average inter-nozzle distance D of in core region <NUM> along the direction perpendicular to border <NUM>.

In the case of the embodiments of <FIG> and <FIG> and any other embodiments having an edge region without activatable nozzles, the method for operating the print head advantageously comprises not ejecting any ink from edge region <NUM> while it does comprise the step of ejecting ink from the nozzles <NUM> in core region <NUM>.

Using such specially designed edge regions <NUM> is based on the understanding that the gas flow pattern generated by the ventilation openings tends to become inhomogeneous at the edge of the area covered by ventilation openings, i.e. leading to a non-homogeneous distribution of evaporated ink, and to non-uniform airflow patterns that may cause slight flight path deviations between the droplets ejected by different nozzles. Hence, for homogeneous printing results, it is advantageous not to have activatable nozzles in edge region <NUM>.

As mentioned, there are no activatable nozzles in the edge region <NUM>. This can e.g. be achieved by one or more of the following measures:.

Advantageously, the print head is adapted and structured to generate a smaller gas flow rate through at least some of the ventilation openings <NUM>', <NUM>' in edge region <NUM> than through the ventilation openings <NUM>, <NUM> in core region <NUM>. This can e.g. be achieved, as illustrated in <FIG>, by providing at least some of the ventilation openings in edge region <NUM> with a diameter smaller, in particular at least <NUM>% smaller, than the ventilation openings <NUM>, <NUM> in core region <NUM>.

In such and similar cases with reduced gas flow ventilation openings at edge region <NUM>, the method for operating the print head advantageously comprises feeding a larger amount of gas through at least some of the ventilation openings <NUM>, <NUM> of core region <NUM> than through at least some of the ventilation openings <NUM>, <NUM> of edge region <NUM>, advantageously with an at least <NUM>% smaller gas flow.

Apart from the nozzles <NUM> arranged in a regular array, print head <NUM> may comprise further nozzles outside said array, e.g. nozzles dedicated to special printing tasks. These further nozzles are advantageously fewer in number (e.g. no more than <NUM>% of all nozzles) and they may or may not be provided with their own ventilation openings.

In the above examples, the unit cells <NUM> are squares. It must be noted, though, that they may also be mere rectangles. There is no strict need to have equal nozzle spacing in two perpendicular horizontal directions even though, depending on unit cell design, the higher geometry of a square over a rectangle may be advantageous for maintaining identical gas flows around the nozzles.

Equally, nozzles must not be placed in square-like fashion but may also be placed in a hexagonal fashion. For example, this would be achieved by adding a nozzle <NUM> to the print head <NUM> in <FIG> at all those positions that form the center of a square which has its edges situated at the position of four neighboring nozzles <NUM>. In this way, the number of nozzles <NUM> on the print head would be doubled while the number of ventilation openings <NUM>, <NUM> remains constant, i.e. there will be only one ventilation opening <NUM>, <NUM> per nozzle <NUM>, and hence there is no more symmetry at the level of a single nozzle, similar to the situation in <FIG>. This is illustrated in <FIG>.

<FIG> finally illustrates a design with the nozzles arranged with <NUM>-fold symmetry.

Claim 1:
A print head for depositing ink on a substrate comprising
a nozzle layer (<NUM>) comprising
a) a plurality of nozzles (<NUM>), and
b) a plurality of ventilation openings (<NUM>, <NUM>) extending through said nozzle layer (<NUM>) and being adapted to feed a gas to a region between the printing head and a target and/or to convey gas away from said region,
wherein the print head comprises a core region (<NUM>) with activatable nozzles (<NUM>) and ventilation openings (<NUM>, <NUM>),
wherein the print head comprises
an edge region (<NUM>) with ventilation openings (<NUM>, <NUM>) but without activatable nozzles (<NUM>),
with a border (<NUM>) extending between the core region (<NUM>) and the edge region (<NUM>),
characterized in that the edge region (<NUM>) is several rows of ventilation openings (<NUM>, <NUM>) deep, wherein a distance (W) from the border (<NUM>) to the outmost ventilation openings (<NUM>, <NUM>) of the edge region (<NUM>) is at least two times, in particular at least five times, an average inter-nozzle distance (D) in the core region (<NUM>) along a direction perpendicular to the border (<NUM>).