Ink jet print head with water protection

A method for operating a printhead of a continuous inkjet printer comprising: producing at least one ink jet in a cavity of the print head; electrostatically separating drops or sections of one or more of the jet intended for printing from drops or sections that do not serve for printing; exiting from the cavity drops or sections of ink intended for printing, through a slot open on the outside of the cavity; and circulating at least one flow of air along the outlet slot of the cavity in a direction essentially perpendicular to at least one jet of ink emitted by the printhead and intended for printing. The air having a water vapor pressure lower than the water vapor pressure defined by 100% relative humidity at the coldest temperature of the printer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from French Patent Application No. Ep 18248284.4 filed on Dec. 28, 2018. The content of this application is incorporated herein by reference in its entirety.

PRIOR ART AND TECHNICAL PROBLEMS

The invention notably applies to print heads of printers or to deviated continuous ink jet printers or to binary continuous ink jet printers provided with a multi-nozzle drop generator.

Ink used in known ink jet print head have a certain concentration of water. In a CIJ printer, said concentration may vary in a narrow range, for example from 0.5% to 5%. Beyond that range the ink can no longer be properly used to print.

The added water results from exchange with humid air from outside the print head and/or results from condensation of water vapor in the hydraulic system connected to the print head.

In some embodiments, the ink not used for printing is recirculated with help of a pump pumping both said ink and air, with a pumping rate of at least 10 l (air)/h.

CIJ print heads are fabricated to work in environments comprising 10% to 90% relative humidity, associated to temperature from 5° C. to 45° C., which is a source of water entering inside the head and the recirculation circuit.

Indeed, temperature variations to which the print head is subject will result in water condensation inside circuit.

More precisely, the vapor flow rate Qvaploaded by air can be calculated as follows:

where:Q is the air flow rate (l/h);Mwis the molar mass of water (g/mol);Psat(T) is the saturation vapor pressure (kPa);Patmis the atmospheric pressure (kPa);Vm(T) is the molar volume at temperature T, calculated as follows:

This shows that for T=45° C. and HR=90%, an air flow rate of 10 l/h loads 0.58 g/h water vapor. If the temperature drops a few degrees, for example 5° C., which is a very realistic situation for an ink jet printer, liquid water will condense. Thus, based on the saturated vapor pressure curve, for an air flow rate of 10 l/h, at a temperature of 45° C. and 90% relative humidity, a temperature reduction of 5° C. results in 10% condensation, which means 0.06 g/h or 0.07 cm3/h of water added into the ink.

Furthermore, the volume of an ink circuit of a CIJ printer is about 11, which means about 850 g for an ink density of 0.85. The initial mass concentration of water being for example 0.5% (=4.25 g of water, or 0.43% of the volume). After 70 h of operation
70×0.06=4.2gwater have been added, which meansawater mass concentration of 1% (the upper acceptable limit).

In real conditions, a printer is operated nearly continuously and the above calculations give results which are underestimated even though each printed drop is loaded with a certain quantity of water (if the printed ink flow rate is 1 l/month for 10 h daily use (200 h/month), the average printed ink flow rate is 5 cm3/h).

An overall balance of the quantity of water in the circuit takes into account the condensation of water (0.07 cm3/h, according to the above example) and the quantity of ink added into the circuit (with a volume concentration of 0.43% water according to the above example) whereas water is consumed by printed ink.

The evolution of the volume of water in the circuit is given by:
V(t+Δt)C(t+Δt)=V(t)C(t)+QwaterΔt+QinkC0Δt−QinkC(t)Δt
Or:
V(t+Δt)C(t+Δt)=V(t)C(t)+QwaterΔt−Qink(C(t)−C0)Δt

where:V(t) is the total volume in the circuit at time t;C(t) is the volume concentration of water at time t;V(t) C(t) is the total volume of water in the circuit at time t;Qwater(resp. Qink) is the water (resp. ink) flow rate.

Assuming that V(t)=V(t+Δt)=V, one can write:

The solution of this last equation is:

Based on this curve, a water mass concentration of 1% in the ink is obtained after 80 h of printing (which means about 8 days of operation), which is not acceptable by the user of the printing machine in particular for some technical inks.

Of course, the above results can vary, depending on the initial values. But, even if the water flow rate is half (0.03 g/h instead of 0.06 g/h), the upper limit of acceptable water mass concentration will be reached after 200 h, which is also not acceptable by the user.

One solution is to pressurize the printing head, which prevents entry of air from the outside atmosphere into the head. But this increases the evaporation of solvent in the printing head, generates turbulences and disturbs the drops which are deviated from their trajectory.

These problems are amplified in multi-jets print heads, where the pumping rate can reach 60 l (air)/h or more, in which case the water concentration can reach the sustainable values after only some hours of operation.

FIGS. 9A and 9Bshow the water mass concentration in a circuit of a known printer for a pumping rate of 10 l/h (FIG. 9A) and for a pumping rate of 100 l/h (FIG. 9B); for the lower, resp. upper, pumping rate a water mass concentration of 1% is reached at 100 h, resp. at about 10 h; in other words, the upper limit of acceptable water mass concentration can be reached much faster for a high pumping rate, which makes the problem even more acute.

SUMMARY OF THE INVENTION

The invention first concerns a method for operating a printhead of a continuous inkjet printer, wherein said method comprises:producing at least one ink jet in a cavity of said print head,electrostatically separating drops or sections of one or more of said jet intended for printing from drops or sections that do not serve for printing,exiting or releasing from said cavity drops or sections of ink intended for printing, through a slot open on the outside of the cavity or of the print-head.

In a method according to the invention, the local atmosphere at the inlet and/or at the exit of said outlet slot is dry and cold, and prevents humid air from the atmosphere outside the print head to flow into said print head. A method according to the invention preferably circulates at least one first flow of air, preferably dry and cold air, along at least part of said outlet slot of said cavity or of said printhead, more preferably along at least part of the inlet and/or of the exit of said outlet slot; preferably said at least one first flow of air circulates in a direction perpendicular or essentially perpendicular to at least one jet of ink emitted by said printhead and intended for printing.

Said air preferably has a water vapor pressure lower than the water vapor pressure defined by 100% relative humidity at the coldest temperature in said printer.

The air thus circulated will not condense inside the head and will not add water to the ink. The concentration of water in the ink will therefore remain in a narrow range, for example from 0.5% to 5%.

Said at least one first flow of air circulated along at least part of the outlet slot may comprise dry air (or dry and cold air) that is provided by means for generating dry air from ambient air.

In an embodiment, air extracted from said cavity is recirculated through a recirculation circuit and is injected into the cavity of said print head, said recirculation circuit comprising for example at least one condenser.

In an example, part (for example 50%) of said recirculated air may be circulated along at least part of the slot without being mixed with air of said at least one flow of air (for example dry air that is provided by means for generating dry air from ambient air) which is also for circulation along at least part of the slot, whereas another part, for example 50%, of said recirculated air is injected into said cavity without being circulated along said slot.

In another embodiment, part of said flow of air extracted from said cavity and recirculated through a recirculation circuit is mixed with at least part of said flow which is circulated along at least part of said outlet slot.

For example, part (for example 50%) of said recirculated air may be mixed with at least one flow of air (for example dry and cold air that is provided by means for generating dry air from ambient air) which is for circulation along at least part of the slot, said mixture being then circulated along at least part of the slot, whereas another part, for example 50%, of said recirculated air is injected into said cavity.

The temperature and/or the hygrometry can be measured, for example with at least one temperature and/or at least one hygrometry sensor, inside and/or outside said cavity and/or in a recirculation circuit, for example at the outlet of a condenser of said recirculation circuit, said condenser being for condensing solvent vapors.

The temperature and/or the hygrometry of the air circulated along said at least part of said outlet slot can be estimated and/or calculated and/or regulated so that the water vapor pressure of said air is lower than the water vapor pressure defined by 100% relative humidity at the coldest temperature of said printer.

Said coldest temperature of said printer can be estimated based on a preset temperature belonging to a temperature working range of said printer and/or said water vapor pressure can be estimated based on a temperature working range of said printer and/or on a hygrometry working range of said printer. This is particularly useful when the printer does not have any sensor.

Preferably, said flow of air (circulated along at least part of said outlet slot of said cavity or of said printhead) is at a temperature which is lower than or equal to the coldest temperature inside the printhead and/or inside the recirculation path. In particular, the temperature inside said cavity can be measured—for example with a temperature sensor—and compared with a temperature measured—for example with a temperature sensor—at the outlet of the condenser of a recirculation circuit, in order to confirm that the outlet of the condenser is colder than the cavity.

In a method according to the invention, at least the temperature and the hygrometry can be measured outside said cavity, and at least another temperature is measured in a recirculation circuit, preferably at the outlet of a condenser of said recirculation circuit, the temperature and/or hygrometry of air recirculated by said recirculation circuit and supplied to said print head (for circulation along at least part of said outlet slot of said cavity or of said printhead) being adapted according to said measurements (temperature and hygrometry) outside said cavity and in (temperature) said recirculation circuit.

In a preferred embodiment of a method according to the invention:said coldest temperature of the printer is estimated based on a preset temperature belonging to a temperature working range of said printer;and/or said water vapor pressure is estimated based on a temperature working range of said printer and/or on a hygrometry working range of said printer.

In order not to interfere with the ink jet(s) emitted by said print head, said flow of air circulates air along at least part of the outlet slot at a speed less than 2 m/s.

In a particular embodiment, said flow of air circulates along at least part of the outlet slot outside of the cavity or of the printhead or between at least part of the outlet slot of the cavity and at least part of an outlet slot of said printhead.

In a particular embodiment, said flow of air is injected into the printhead or into the cavity and circulates inside or outside the head or the cavity along the outlet slot, preferably in a straight direction and/or without deviation, from one side of the cavity or of the printhead with respect to the jet(s) direction (or from one side of the jet(s)) to the other side.

Said flow of air circulates along at least part of the outlet slot, preferably in a straight direction and/or without deviation, from one side of the cavity or of printhead (with respect to the jet(s) direction) until it has passed the slot, and in some embodiments to the other side of the cavity or of printhead. It flows first along said outlet slot and, in some embodiments, can then be deviated, for example by another flow flowing in the opposite direction, in which case both flows form an atmosphere of dry and cold gas at the outlet slot.

More generally, in a method according to the invention, the local atmosphere at the inlet and/or at the exit of said outlet slot is dry and cold, and prevents humid air from the atmosphere outside the print head to flow into said print head. In some embodiments, two flows of air can circulate along the outlet slot, or along at least part of it (or meet at the outlet slot) preferably in a straight direction and/or without deviation, from both sides of the cavity or of printhead (with respect to the jet(s) direction).

The invention also concerns a print head of a binary continuous jet printer comprising:a cavity for circulating at least one ink jet,means for producing at least one ink jet in said cavity,means for electrostatically separating drops or sections of one or more of said jet intended for printing from drops or sections that do not serve for printing,a slot, open on the outside of the cavity or of said printhead and enabling the exit of drops or sections of ink intended for printing,at least one gutter for recovering drops or sections not intended for printing.

The print head according to the invention comprises, or is connected to, a circuit for forming dry and cold air, at least locally at the inlet and/or at the exit of said outlet slot, in order to prevent humid air from the atmosphere outside the print head to flow into said print head.

Said circuit preferably comprises means for forming or circulating at least one flow of air, preferably dry and cold air, along at least part of said outlet slot of said cavity or of said printhead, preferably in a direction perpendicular or essentially perpendicular to at least one jet of ink emitted by said printhead and intended for printing.

Said circuit can comprise means for generating dry and cold air from ambient air, said dry and cold air being then circulated so as to flow along at least part of said outlet slot.

A printhead according to the invention may comprise means for implementing a method to the invention.

Preferably a printhead according to the invention comprises means to control and/or to regulate the temperature and/or the hygrometry inside at least a portion of said circuit for circulating air along at least part of said slot.

Preferably said temperature and/or hygrometry is controlled and/or regulated such that air in said circuit has a water vapor pressure lower than the water vapor pressure defined by 100% relative humidity at the coldest temperature of said printer.

A print head according to the invention may further comprise a recirculation circuit of air and/or ink not used for printing, said recirculation circuit possibly comprising at least a condenser, air from said recirculation circuit being injected into said cavity of said printhead.

Said circuit for circulating air along at least part of said slot may comprise means for circulating air from said recirculation circuit and air from said means for generating dry air from ambient air, said circuit comprising means for mixing at least part of the air from said recirculation circuit and at least part of the air from said means for generating dry air from ambient air.

A print head according to the invention may comprise means for mixing part (for example 50%) of said recirculated air with air of said circuit for forming or circulating at least one flow of air which is for circulation along at least part of the slot, said mixture being then circulated along at least part of the slot.

A print head according to the invention may comprise means for circulating part (for example 50%) of said recirculated air along at least part of the slot, in parallel to the flow of dry and cold air also circulated along at least part of said slot. The other part, for example 50%, of said recirculated air can be injected into the cavity of said print head (said other part not being circulated along at least part of the slot).

At least one sensor may be implemented to measure the temperature and/or hygrometry inside and/or outside said cavity and/or in a recirculation circuit of air extracted from said cavity or said printhead, for example at the outlet of a condenser of said recirculation circuit.

A sensor can be implemented to measure a temperature inside said cavity, said print head further comprising means for comparing said temperature inside said cavity with a temperature measured at the outlet of the condenser of a recirculation circuit in order to confirm that temperature measured at the outlet of the condenser is colder than in the cavity.

The means which can be implemented to control and/or to regulate the temperature and/or the hygrometry of at least a portion of said circuit for circulating air along said slot may comprise a controller or a computer specially programmed for maintaining air injected into the cavity at a target temperature and/or hygrometry and/or for maintaining the water vapor pressure of air in said circuit lower than the water vapor pressure defined by 100% relative humidity at the coldest temperature of said printer.

For example a print head according to the invention may comprise means for, or programmed for, calculating or estimating or selecting the temperature and/or the hygrometry and/or a water vapor pressure lower than the water vapor pressure defined by 100% relative humidity at the coldest temperature of the printer.

Said temperature and/or hygrometry and/or water vapor pressure can be estimated based on measurements of one or more temperature and/or hygrometry inside and/or outside said cavity or said printhead and/or in a recirculation circuit, for example at the outlet of a condenser of said recirculation circuit, and/or based on one or more temperature and/or hygrometry of a range of a temperature working range and/or hygrometry working range for said printer. Said circuit for circulating air along at least part of said slot comprises means for circulating said air along the outlet slot at a speed preferably less than 2 m/s.

Said circuit can be, or can comprise means, for circulating air along at least part of the outlet slot outside and/or inside the cavity or the printhead.

In a particular embodiment, said head comprises a 1stgutter fixed with respect to the head, a 2ndgutter movable with respect to the head, said 2ndgutter being located between said cavity and a cover comprising an outlet slot, said circuit comprising means for circulating said air between said 2ndgutter and said cover.

In a further particular embodiment, said circuit is for circulating said air inside or outside the head or the cavity along at least part of the outlet slot, preferably in a straight direction and/or without deviation, from one side of the cavity or of the printhead with respect to the jet(s) direction (or from one side of the jet(s)) to the other side.

In some embodiments, said circuit is for circulating said flow of air along at least part of the outlet slot, preferably in a straight direction and/or without deviation, from one side of the cavity or of printhead (with respect to the jet(s) direction) until it has passed the slot, and in some embodiments to the other side of the cavity or of printhead.

In some embodiments, said circuit is for circulating two flows of air, each along at least part of the outlet slot, preferably in a straight direction and/or without deviation, from both sides of the cavity or of printhead (with respect to the jet(s) direction).

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 1is an example of a print head to which the invention can be applied.

The head shown includes a drop generator11. An integer number n of nozzles4are aligned on a nozzle plate2along an X axis, between a first nozzle41and a last nozzle4n.

The first and the last nozzles (41,4n) are the nozzles with the greatest distance between them.

Each nozzle has a jet emission axis parallel to a Z direction or axis (located in the plane ofFIG. 1), perpendicular to the nozzle plate and to the X axis mentioned above. A third axis, Y, is perpendicular to each of the X and Z axes, the two X and Z axes extending in the plane ofFIG. 2.

Each nozzle is in hydraulic communication with a pressurized stimulation chamber. The drop generator comprises one stimulation chamber for each nozzle. Each chamber is provided with an actuator, for example a piezo-electric crystal. An example design of a stimulation chamber is described in document U.S. Pat. No. 7,192,121.

There are sort means or a sort module6downstream from the nozzle plate, that will be used to separate drops used for printing from drops or jet segments not used for printing. Said means or sort module6may comprise one or more electrodes, which can be formed against, or in, a wall10which delimits the cavity in which the jets are produced. At least one electrode may be flush with the surface of the wall in question. Thus the drops or sections that do not serve for printing are deviated by electrostatic effect of at least one electrode on the drops.

This separation or deviation may be done without charging of the deviated drops or the deviated sections of jets, as explained in the document FR2906755 or U.S. Pat. No. 8,162,450. In other words, in such case, the cavity does not contain an electrode for charging drops or sections of ink. The ink which is deviated to the gutter is thus not charged.

More precisely, drops or jet segments emitted by a nozzle and that will be used for printing follow a trajectory a along the Z axis of the nozzle, and then strike a print support8, after having passed through the outlet slot17(shown in dashed lines inFIG. 2). The slot is open to the outside of the cavity and ink drops to be printed exit through it; it is parallel to the X direction of nozzle alignment, the Z direction axes of the nozzles passing through this slot, that is on the face opposite the nozzle plate2. Its length is equal to at least the distance between the first and the last nozzle.

Drops or jet segments emitted by a nozzle and not intended for printing, are deviated by means6(they follow a trajectory such as trajectory b) and are recovered in a gutter7and then recycled. The length of the gutter along the X direction is equal to at least the distance between the first and the last nozzle.

For example, document U.S. Pat. No. 8,540,350 (FR 2 952 851) that describes a method of avoiding crosstalk between jets from nozzles adjacent to each other, could be referred to particularly for information about the formation of jets and breaking the jets to form drops, and about the deviation of drops. Reference could also be made to prior art described in U.S. Pat. No. 7,192,121 (FR 2 851 495) describing jet breaking positions depending on whether a drop formed by breaking the jet will or will not strike the print support.

In the present application, the term “cavity” designates the zone of space in which ink flows between the nozzle plate2and the outlet slot17(or the lower wall which contains said slot) of drops intended for printing or between the nozzle plate and the recovery gutter. The nozzle plate2in fact forms an upper wall of the cavity. Laterally, the cavity is delimited by lateral walls (see walls9,10onFIGS. 3A-3D, 4A, 4B), substantially parallel to the curtain of jets constituted by the different jets emitted by the nozzles. One of these walls has already been evoked above, in relation with a jet deviation electrode.

The curves ofFIG. 2show the evolution of the vapor pressure of water as a function of the temperature, for different levels of hygrometry; in order to avoid condensation, the vapor pressure for a given temperature is preferably selected under curve I.

If a print head is operated in an atmosphere at 30° C. the black horizontal line gives the water vapor saturation pressure corresponding to the saturation vapor pressure at 30° C.

For example, if a print head is operated in an atmosphere at 30° C., air inside the print head:having a water vapor pressure less than about 4500 Pa will not condense;at a temperature less than 30° C. will not condense, whatever its relative humidity;at a temperature higher than 30° C. could condense or not depending on its relative humidity; it does not condense if its water vapor saturation pressure is less than the water vapor saturation pressure defined by 100% at the coldest temperature (which is 30° C. in this case).

If the same print head is connected to a recirculation circuit which comprises a condenser, the temperature at which outset being for example 20° C., the dotted horizontal line gives the water vapor saturation pressure corresponding to the saturation pressure at 20° C.

For the same print head operated in an atmosphere at 30° C., but with a condenser with an outset temperature of 20° C., air inside the print head:having a water vapor pressure less than about 2500 Pa will not condense; in other words, air at the temperature T and with a relative humidity HR will not condense, if the point identified by coordinates (T, HR) in the plane ofFIG. 2is located below the dotted horizontal line (corresponding to 2500 Pa);air at a temperature less than 20° C. will not condense, whatever its relative humidity;air at a temperature of 30° C. will not condense, if its relative humidity is less than 54.7%;more generally, air having a water vapor pressure lower than the water vapor pressure defined by 100% relative humidity at the coldest temperature of the printer or of the system will not condense.

Based on curves like those ofFIG. 2, and in order to avoid undesirable water condensation, air injected into the print head (respectively into a system including a print head and a recirculation circuit which sucks air and ink from the gutter of the cavity, extracts solvent from the air and injects the air (after solvent extraction) into the print head), preferably has a temperature and hygrometry providing water vapor pressure lower than the water vapor pressure defined by 100% relative humidity at the coldest temperature in the print head (respectively lower than the water vapor pressure defined by 100% relative humidity at the coldest temperature in the system (print head and recirculation circuit))).

Alternatively, it is possible to implement a temperature sensor and/or a hygrometry sensor, which may help in selecting proper relative humidity and temperature of air injected into the print head.

According to a first example, a system includes a print head and a recirculation circuit which comprises a condenser. A first temperature sensor is implemented and located at the outlet of said condenser. The value (T1) measured by this first sensor is considered to be the lowest temperature in the whole system. A second sensor, measuring temperature (T2) and hygrometry is located outside the print head, for example on a cover of the print head. Air which must be injected into the print head (and which is pumped from the atmosphere outside the print head) thus has a water vapor pressure (VP2) given by the temperature T2and by the hygrometry measured by said second sensor. A target vapor pressure (VP1) corresponds to 100% humidity at temperature T1measured by said first sensor (located in said recirculation circuit). So, air has to be dried and/or cooled to exhibit vapor pressure lower than VP1.

If, for example, the second sensor measures T2=30° C. and a hygrometry RH=90% (which corresponds to a water vapor pressure equal to 3780 Pa (VP2)) and the first sensor measures T1=20° C., which corresponds to 2300 Pa (VP1), the system will have to transform air having a vapor pressure VP2into air having a vapor pressure VP<VP1; for example air initially at 30° C. and having RH=90% must be transformed, for example by a membrane air dryer and/or a condenser, into air at 20° C. and RH=50%. It has to be noted that, without the information concerning the hygrometry RH given by the outside sensor, the assumption that the hygrometry RH=100% has to be made.

According to a second example, a system includes a print head and a recirculation circuit which comprises a condenser. A temperature sensor is implemented and located at the outlet of said condenser. The value (T1) measured by this sensor is considered to be the lowest of the whole system. As there is no sensor to measure the temperature and the hygrometry outside the print head, the maximum value given by the printer datasheet is considered, for example 40° C. and RH=90% (or more generally, it can be a set of values (T, HR) memorized in the system, for example in the controller, and which is taken as a set of reference values for both the temperature and the relative humidity). It is assumed that air which must be injected into the print head (and which is pumped from the atmosphere outside the print head) thus has a water vapor pressure (VP2) at the maximum temperature/hygrometry given by said datasheet (more generally, a water vapor pressure (VP2) given by said set of values (T, HR) memorized in the system). Target vapor pressure (VP1) value is given by 100% humidity at the temperature provided by said sensor located in said recirculation circuit. So, air has to be dried and/or cooled to exhibit vapor pressure lower than VP1.

For example, 40° C./90% RH corresponds to a water vapor pressure equal to 6660 Pa (VP2). The sensor located at the outlet of the condenser measures T1=20° C. corresponding to 2300 Pa (VP1). The system thus has to transform, for example by a membrane air dryer and/or a condenser, air at VP2into air at VP<VP1, for example into air at 20° C. and 50% RH.

According to a third example, a system includes a print head and a recirculation circuit which comprises a condenser, but no sensor is implemented: no information can be measured concerning the temperature and/or the hygrometry outside the print head or the temperature in the recirculation circuit. We have to manage the whole range of temperature/hygrometry given for example by the datasheet of the printer, for example a temperature range of 10° C.-40° C. and a hygrometry range of 10%-90% RH (or more generally, it can be a set of values (T, HR) or a range of temperatures T and a range of hygrometry HR memorized in the system, for example in the controller, and which is taken as a set of reference values or a set of reference of ranges for both the temperature and the relative humidity). If the temperature of the recirculation circuit can be at maximum 10° C. colder than temperature outside print head, air injected into the print head should exhibit a vapor pressure lower than the vapor pressure at T=0° C. and HR=100%; this vapor pressure is VP2=600 Pa.

At any rate a printer according to the invention can comprise means, for example a membrane air drier and/or a condenser, to transform air (taken from ambient air) which is to be injected into the print head and/or into its cavity. It is possible to control said means, for example by the controller of the printer; in particular:the pressure difference between both sides of the membrane,and/or the power of a condenser, can be controlled in order to adapt the efficiency of the membrane air drier and/or of the condenser according to the needs and depending on the thermodynamic conditions (temperature and/or hygrometry).

FIGS. 3A-3Dare section views of different examples of printing head (multi-jet or CIJ) to implement the invention.

In these figure references identical to those ofFIG. 1designate identical technical elements.

Aspects common to these different embodiments, and to the embodiments of FIGS.4A1-4D, will firstly be explained. These sections are taken along a plane parallel to the plane YZ, and containing the axis Z of a nozzle4. The representation of each section keeps the same shape over the distance going, along the direction X (perpendicular to the plane of each of theFIGS. 3A-3B), from the first nozzle41to the final nozzle4n. In these figures, only the cavity5in which the jets circulate is represented.

P0designates the plane that goes through the nozzle4xand which is parallel to the plane XZ. This plane is perpendicular to each ofFIGS. 3A-3Cand goes through all the nozzles, which are aligned along X. It also goes through the slot17. A plot of this plane is represented inFIGS. 3A-3Din broken lines.

The upper part of the cavity is delimited by the 1stwall2, also called upper wall, which also forms, or comprises, the nozzle plate or comprises nozzles. The lower part of the cavity is delimited by a 2nd wall21, also called lower wall, traversed by the slot17, and by a part of the gutter7. Walls9and10limit the lateral extension, along the axis Y.

The cavity comprises in addition, on one side of the plane P0, a lateral wall9, preferably parallel to the plane P0and contiguous with the nozzle plate2. The wall10, situated on the other side of the plane P0, faces the wall9. The cavity is thus delimited, on either side of the plane P0, by these 2 walls9and10. By convention, the side of the plane P0where the wall10and the gutter7are located is called first side of this plane, the other side (where the wall9is located), is called second side.

The wall10has ends, along the direction X, which are contiguous with the nozzle plate2. In the part which is close to the nozzle plate2and over a length that is, preferably, slightly greater than the distance between the first41and the final nozzle4n, this wall may comprise a slot14, which will make it possible to suck up ink that is deposited on the nozzle plate or in its vicinity.

At the bottom of this wall10is located the inlet slot of the recovery gutter7to make it possible to recover drops that are deviated in order that they do not pass through the slot17.

The gutter may be placed in hydraulic communication with the slot14, by means of a conduit13that emerges in, or is in connection with, the gutter and which is situated to the rear of the wall10with respect to the plane P0.

The means6for selecting and deviating drops not intended for printing are flush on the wall10or are attached to said wall. These means mainly comprise electrodes. They are intended to be connected to powering up means, not represented in the figure.

Preferably, the distance between the wall10and the plane P0, measured along the direction Y, perpendicular to the plane P0, is, going from the plate2, firstly constant; this corresponds to a 1stpart101of the wall10, which is substantially parallel to P0.

Then, in a particular embodiment, in a second part102, further from the plate2than the 1stpart101, from a point61of incline of the wall10, the distance between the wall10and the plane P0increases with the moving away from the nozzle plate.

In this example, the wall10is close to the plane P0, and parallel thereto, in a 1stpart of the cavity situated in the vicinity of the nozzles4x, in the place where the path of the drops is hardly modified, even when drops situated more downstream on this path are deviated to enter into the recovery gutter7.

This is what may be seen inFIGS. 3A-3D, where a path of drops is deviated to the gutter7: the upper part of the jet is not, or is only very slightly, deviated, whereas, from a point61of inclination of the wall10, the jet moves away more and more, almost linearly, from the plane P0. This could be termed a ballistic path of the jet downstream of the electrostatic field area.

A lower part of the wall10and a wall12, situated to the rear of the wall10with respect to the plane P0, defines, facing a wall11, a conduit, or gutter7for evacuating drops that will not be used for printing.

The walls10and12are, preferably, contiguous with each other, the reference18designating the junction line of these two walls10and12; this line is parallel, or substantially parallel, to the direction X. They form an upper wall of the gutter.

The wall11forms a lower wall of the gutter. It comprises a 1stpart111, the most upstream in the sense of circulation of the drops in the conduit7and a second part112, the most downstream.

The potential conduit13may emerge in the upper wall12and hydraulically connect the recovery gutter7to a conduit141hydraulically connected to the slot14.

The reference28designates a junction line of the parts111and112of the wall11; this line is parallel, or substantially parallel, to the direction X and to the line18.

The part111the most upstream, at the inlet of the conduit7of the lower wall11, terminates by an end part15, which, advantageously, constitutes its apex (or summit). It is the point of the surface11that is the closest to the plane P0.

Preferably, this apex15(which is the point the most upstream of the gutter) is in a same plane as wall16that is parallel to the plane P0and which forms one of the walls surrounding or delimiting the outlet slot17. In other words, the point the most upstream of the gutter is directly in line with the outlet slot17of the cavity. This makes it possible to optimise the recovery of drops: thanks to this configuration, any drop deviated, even slightly, will be recovered by the gutter.

The slot17constitutes an opening of the cavity5through which pass drops intended for printing. The intersection of plane P0with the plane ofFIG. 3Ais a materialization of the axis of the nozzle4x. This axis goes through the centre of the slot17.

Another wall of the cavity is constituted by the wall21: it is substantially parallel to the plate2, but the furthest away therefrom in the cavity5. In other words, it is situated on the side of the outlet slot17. An end of this wall may form an inlet edge of the slot17, facing the wall16already mentioned above.

A wall210, substantially perpendicular to the wall21, delimits, with the wall16, the outlet slot17: the drops are going to circulate between these 2 walls, before exiting the slot17and being crushed on the printing support8.

Finally, the reference211designates the exterior surface of the cavity, into which the outlet of the slot17emerges.

An example of operation of these cavities is as follows. A continuous ink jet is emitted by the print head. The deflection of this jet is commanded by electrodes6to create, as a function of the pattern to print and the position of the support8, drops intended or not for printing.

Drops intended for printing move along the axis Z (in the plane P0) and pass through the slot17.

Drops not intended for printing are deviated from the axis Z (or from the plane P0), and along a trajectory that brings them to strike the lower wall11of the gutter7.

Since the gutter is connected to a vacuum source, the ink of these drops, which have stricken the wall11, exit, with air, the cavity5via the gutter.

Furthermore, the conduit13and the slot14can maintain a slight low pressure at the level of the nozzle plate2. This low pressure makes it possible to absorb ink which, by capillarity, is deposited on the nozzle plate2.

InFIG. 3Ais represented a particular aspect of an embodiment of the invention.

The reference7designates a recovery gutter, for example of the type known from the prior art according to the teaching of document WO 2012/038520. Pumping means (not represented in the figure) may be connected to the gutter to suck up ink that enters into the latter.

A 1stlateral conduit20enables the cavity5to be placed in communication with a source of gas, preferably air, not represented.

One of the walls of this conduit20is the wall21; a 2ndwall22, which faces the 1stwall and which is parallel to it, re-joins the wall9, in which an opening enables the conduit to emerge in the cavity5. The conduit20is thus arranged laterally, at the bottom of the cavity, that is to say, along the axis Z, on the side opposite to the plate2. It is also arranged, laterally, on the side opposite to that in which the gutter7emerges. This conduit20is going to make it possible to make circulate, in the direction of the cavity5and substantially parallel to the wall21, a flow of air or gas, as represented by the arrow2001. This flow of air or gas is injected into the print head, for example with help of a pump, preferably so that the air sweeps (circulates in) a portion of the print head just along the outlet slot17, or along at least part of the inlet of said outlet slot, in order to limit the exchange of from/toward the outside of the head and the contact between the injected air and the jet(s). Said flow of air circulates inside the head along the outlet slot, preferably in a straight direction, without deviation, from one side of the cavity (or from the jet(s)) to the other side.

Said flow of air has a temperature and/or a hygrometry such that it does not condensate inside the print head; preferably it is drier and colder than air in said cavity.

Thus additional air is injected into the cavity, said air not condensing in the head.

In an embodiment air is injected, for example laterally (in particular, it can be a vertical and/or horizontal injection as shown by arrows2011and2021onFIGS. 3A-3D) through one or more ducts20b,20cmade in the head and then flows directly into conduit20or is deviated to flow into conduit20to sweep the lower portion of the head. Said duct(s) can be connected to a pump to inject air into it/them.

In the embodiments ofFIG. 3A-3Dor4A1-4D air is circulated so as not to disturb the trajectory of the ink jet emitted by said print head. In particular, the flow of air is preferably kept at a value less than 2-3 m/s, for example about 1 m/s or less. This air comprises:preferably dry and cold air, obtained for example from ambient air flowing through a condenser and/or a membrane air drier;and/or air recirculated from the print head.

FIG. 3Bis another example of printing head (multi-jet or CIJ) to implement the invention.

The head is identical to the print head ofFIG. 3Abut the flow of dry and cold air is circulated outside the cavity, just below surface211so that the air flows just below the outlet slot17(along at least part of the exit171of said outlet slot), thus also limiting the exchange of air from/toward the outside of the cavity and the contact between the injected air and the jet(s). Air flows for example along a conduit20′, a wall211′ of this conduit facing wall211.

The thickness e of this flow of air sweeping the outside of the head is for example equal to 2 mm or 3 mm or more generally between 1 mm and 5 mm. e is also the distance between walls211and211′;

This configuration increases the distance, preferably limited to less than 20 or 30 mm, between the nozzle plate2and the substrate8on which printing is performed.

Alternatively, as illustrated onFIG. 3C, a 2ndinjection of fluid symmetrical to the injection made through the 1stconduct20′ can be performed through a 2ndconduct20′a. The 2ndflow of dry and cold air is circulated in a direction opposite to the flow circulation inside conduct20′, just below surface211so that the air of this 2ndflow also flows just below or along part of the outlet slot17(along at least part of the exit171of said outlet slot), thus also limiting the exchange of air from/toward the outside of the cavity and the contact between the injected air and the jet(s). Air flows for example along a conduit20′a, a wall212of this conduit facing wall211.

As illustrated onFIG. 3D, an additional element, for example a plate21, can be added to the bottom of a print head so as to implement an air flow circulating outside the cavity, just below surface211so that the air flows just below or along the outlet slot17or along part of it (or along at least part of the exit171of said outlet slot).

Said additional plate21comprises a frame comprising a central hole213adapted to receive at least part of a printing head. Lateral thicker connecting portions211and212comprise connection means, for connecting one or two ducts20b1,20′b1to inject air.

For example each of the connecting portions211and212comprises connection means for a hose barb (or fir tree) connection, made of a tube with a diameter slightly higher than that of inside the hose, this tube being equipped with concentric barbs having a low angle in the insertion direction of the hose and a sharp angle in the extraction direction, the hose is thereby retained during an extraction.

The cover comprises inner ducts20′1,20′2for circulating the air from the lateral injection duct(s) to a central opening217which faces the outlet slot17of the cavity when the print head is positioned in the hole213.

FIG. 3E, shows a perspective view of said additional plate21, with the lateral thicker connecting portions211and212. The height h (FIGS. 3D, 3E) is for example between 1 mm and 3 mm, and the width d (FIG. 3D) is for example between 5 mm and 10 mm.

As illustrated on the embodiment ofFIG. 3A, but also in those ofFIGS. 3B-3D, a further duct or conduit225can be implemented in the print head to inject a 2ndflow of air into the cavity5of the print head. This 2ndflow of air is preferably for “feeding” the jet or the jets curtain (“feeding” meaning, more precisely, replacing the air which is sucked by the gutter); the pressure effect (by the injected gas) can be made more or less equal to, or is to compensate more or less, the suction effect by the gutter7. This gaseous flow does not bring about any perturbation of the jet(s). Preferably:this 2ndflow of air is or comprises air recirculated from the print head;while air injected through ducts20′ (FIGS. 3B, 3C) and/or20′a(FIG. 3C) or20′1,20′2(FIG. 3D) is or comprises dry air, obtained for example from ambient air flowing through a condenser and/or a membrane air drier.

In the embodiments ofFIGS. 3A-3E, air is injected perpendicularly to the direction of slot17. In a variant of any of these embodiments it can be injected along a direction parallel to the slot17.

FIGS.4A1-4D show another example of printing head (multi-jet or CIJ) to implement the invention.

On the figures references identical to those of the preceding figures designate identical technical elements (electrode(s)6, 1stgutter7, outlet slot17).

The head of FIGS.4A1-4D comprises a 1st, fixed, recovery gutter and a 2nd, movable, for example sliding, recovery gutter70, located between said surface211and a cover215. Said cover forms a cavity213aunder surface211and has an outlet slot219aligned with the outlet slot17so that a jet intended for printing flows first through outlet slot17and then through outlet slot219.

A flow of dry and/or cold air is injected into the print head, for example from a lateral side of the head, and then this air is oriented so as to circulate in the lower part, under the 2ndgutter so that the air is directed just above or along the outlet slot219, or of at least part of it (or along at least part of the inlet of said outlet slot), with the advantages explained above. In the open position of the 2ndgutter (see below), the air is also directed just below or along the outlet slot17, or of at least part of it (or along at least part of the exit171of said outlet slot).

Means are implemented to move this 2ndgutter, for example in translation (according to a direction approximately perpendicular to the direction z of flow of the jets in the cavity), between a closed position (as on FIGS.4A1-4A3and4C,4D), in which its inlet slot71is in the continuation of the outlet slot17of the cavity, and an open position (as onFIG. 4B), in which the outlet slot17of the cavity is free. The 2ndgutter may be moved in translation in one direction, until it is closed, then in the opposite direction, from the closed position to the open position. For example a motor147(located in the print head), through transmission means, may move the 2ndgutter in both directions. Reference146on FIGS.4A1-4D is a transmission axis of the motor (the transmission means comprising further transmission elements). In a particular embodiment return means, for example a spring80(FIGS.4A1-4B,4D), keep the 2ndgutter in one of the closed or open positions; for example, said spring is pre-tensioned and keeps the 2ndgutter in the open position (FIG. 4B). This spring can be wound on an axis146, for example the transmission axis of the motor, an end81of this spring being linked with the 2ndgutter (as shown on FIGS.4A1-4D).

In the closed position (as on FIGS.4A1-4A3,4D), the inlet slot71of the 2ndgutter, is against the outside surface211of the cavity, so that the inlet slot71is in the continuation of the outlet slot17of the cavity; preferably, the 2ndgutter comprises sealing means (not shown on the FIGS.4A1-4B,4D) around slot71so that a liquid cannot flow between the outside surface211and the 2ndgutter; for example it comprises one or more joints which bear against said outside surface211, close to the outlet slot17of the cavity.

This 2ndgutter may recover, upon starting the print head, both the initial solvent then the curtain of ink jets.

The 2ndgutter can be connected to suction means, for example a pump, through a suction channel74; preferably, suction means of the 2ndgutter are the same as those of the 1stgutter, for example a common pump. One or more solenoid valve(s) allows individual activation of each of the gutters. The 2ndgutter, when closed (as on FIGS.4A1-4A3and4C,4D), also forms means for suction of cleaning solvent that otherwise would flow outside the cavity.

The 2ndgutter may be guided in translation by guiding means76, for example studs, which guide the gutter when it is sliding against the outside surface211of the cavity. Other guiding means77, for example studs, located under the 2ndgutter, guide the 2ndgutter when it is sliding against the inside surface of a cover215. Laterally, the 2ndgutter can be guided in translation by further guiding means, for example studs, which slide against lateral walls, for example of the cover215, the gutter moving along said lateral walls between its open and its closed positions.

In an embodiment air is injected laterally (for example vertically and/or horizontally as shown by arrows2011and2021on FIGS.4A1-4A3and4B) through one or more ducts20b,20cmade in the head and then flows under the 2ndgutter for example between the 2ndgutter and the cover213(see arrow2001on FIGS.4A1-4C) to sweep the lower portion of the head. Said duct(s) can be connected to a pump to inject air into it/them. The air thus injected flows along the outlet slot219and remains for a certain time in the cavity213abetween the lower portion of the head and the cover215.

A further duct223, similar to the duct22ofFIG. 3A, can be added to inject air, for example a mixture of air coming from both ducts20b,20cdirectly inside the cavity.

In a variant (FIG.4A2), an extra duct or conduit225can be implemented in the print head to inject a 2ndflow of air into the cavity of the print head.

In a further variant (FIG.4A3), said extra duct or conduit225is connected to duct20bby a duct227so that a 2ndflow of air can be injected through duct225, part of said 2ndflow being mixed with the flow injected through duct20band the rest of said 2ndflow being injected into the cavity5(through duct223).

This 2ndflow of air (or the part of said 2ndflow injected into the cavity5) is preferably for “feeding” the jet or the jets curtain; the pressure effect (by this 2ndflow of injected gas) can be made more or less equal to, or is to more or less compensate, the suction effect by the gutter7. The gaseous flow does not bring about any perturbation of the jet(s). Preferably:this 2ndflow of air is or comprises air recirculated from the print head;while air injected through duct20b(see FIGS.4A2-4A3) is or comprises dry and cold air, obtained for example from ambient air flowing through a condenser and/or a membrane air drier; in the variant of FIG.4A3, this air injected through duct20bis mixed with part of the air injected through duct225.

The air injected through duct20b(possibly mixed with part of air injected through duct225) is circulated so as not to disturb the trajectory of the ink jet emitted by said print head. In particular, the flow of air circulating under the 2ndgutter is preferably kept at a value less than 2-3 m/s, for example about 1 m/s or less.

According to an embodiment (FIG. 4C) the outlet face of the cavity is inclined with respect to the flow direction of the jets in the cavity (or to the z axis), for example with an angle β comprised between 10° and 80°; the inlet face of the 2ndgutter is also inclined, approximately with the same angle, so that both faces contact with each other, or face each other, when the 2ndgutter is closed (as onFIG. 4C).

Just like in the embodiments of FIGS.4A1-4A3and4B, a flow of dry and/or cold air can be injected into the print head, preferably from a lateral side of the head and then the air is oriented so as to circulate under the 2ndgutter (see arrows201), between it and the cover215of the head, so that the air is directed just above the outlet slot219with the advantages explained above; it is also possible to inject a 2ndflow of air through a further duct225(see FIGS.4A1-4A3) and possibly to combine part of said 2ndflow with the flow of dry and/or cold air.

Preferably, the 2ndgutter comprises the same features, in particular geometrical features, as the 1stgutter.

As illustrated onFIG. 4C, the 2ndgutter70may comprise:a 1stpart which begins at an inlet slot71for drops in the gutter;a restriction or an elbow72; the 1stpart may be inclined from the inlet slot until this restriction; in a particular embodiment the section, or the width, of the 1stpart, reduces, preferably progressively, on moving away from the plane P0and the plate2, from inlet slot71to an elbow72, which makes it possible to confer to the flow of air that circulates in the gutter a velocity that increases from the inlet of the gutter;a 2ndpart74follows on from 1stpart, for example from the elbow72, in the sense of circulation of drops recovered by the gutter70; in a preferred embodiment the section of this 2ndpart, or its width, increases, preferably, on moving away from the plane P0and on coming closer to the plate2; which makes it possible to create a Venturi effect. The flow of air that circulates in this part of the gutter has a velocity that decreases. A constant section of this 2nd part, or its width, is possible, but then without creation of Venturi effect.

As illustrated onFIG. 4Dthe additional element21ofFIG. 3Ecan be adapted to the print heads of FIGS.4A1-4C so as to implement the air flow circulating outside the cavity, just below the cover215and below the 2ndgutter70. The print head ofFIG. 4Dis that of FIG.4A1but a similar combination can be made with a print head of any of FIGS.4A2-4A3.

Preferably, air injected between the gutter70and the cover215(via ducts20b,20c) and through ducts20′1,20′2is or comprises dry air, obtained from ambient air flowing through a condenser and/or a membrane air drier.

Thus air flows:above or along the outlet slot219, or of at least part of it (or along at least part of the inlet of said outlet slot), with the advantages explained above; in the open position of the 2ndgutter, the air is also directed just below or along the outlet slot17, or of at least part of it (or along at least part of the exit171of said outlet slot);above or along at least part of the exit of said outlet slot219, just below cover215.

These air flows have the advantages already mentioned above. In a specific embodiment, a further internal duct225is implemented, like on FIG.4A2or4A3, preferably for injecting into the cavity5air recirculated from said cavity. If said duct225is connected to duct20bthrough a duct227(like on FIG.4A3), part (for example 50%) of the recirculated air can be mixed with air injected through duct20b, which is preferably dry and cold air, the mixture being circulated between the gutter70and the cover215.

FIGS. 5A-5Cshow examples of a circuit for injecting air according to the invention; in the examples ofFIGS. 5B and 5C, the circuit includes a recirculation circuit, which, here and in this application comprises means for recovering air from the printhead cavity and ink not used for printing, means for recovering solvent—for example with help of a condenser—and means for sending air back to the print head. The air recirculated by this recirculation circuit can be used to inject filtered and dry air through ducts or conduits225(FIG. 3B-D,4A2,4A3). The active element(s), for example a condenser, of this recirculation circuit can be controlled depending on the thermodynamic conditions (temperature and hygrometry).

The print head can be any of the examples described above, in particular in connection withFIGS. 3A-4D.

FIG. 5Ashows the print head1and the gutter7. The print head1is supplied with dry and cold air by a device370for drying ambient air371, said device comprising for example a compressor and/or a membrane air dryer. As already explained, both the compressor and the membrane can be controlled depending on the thermodynamic conditions (temperature and hygrometry). A pump can be implemented at the outlet of device370to supply print head1with air from device370. One or more sensor(s)73can be implemented, for example against the outside wall of a cover containing the print head, to measure the temperature and/or humidity of the ambient air in which the print head is located. Device370is implemented in the other embodiments disclosed in connection withFIGS. 5B and 5C. The dry and cold air provided by device370can be used to inject dry air through ducts or conduits20(FIG. 3A),20′ (FIG. 3B, 3C),20′1,20′2(FIGS. 3D and 4D),20b(FIG.4A1-4D).

Reference100designates an ink reservoir into which ink not consumed during printing will be directed from the gutter7through a pump530(for example a diaphragm pump).

The reservoir100can supply the head1with ink; the supply circuit of the head can comprise a pump570and two filters590,630, the second filter630preferably being close to the print head. With this circuit gas can be recirculated to the print head from the reservoir100. A sensor610measures the temperature and/or the hygrometry in the supply pathway to the head1.

In a variant, the reservoir100is not used to supply the head1with gas; in other words, only device370supplies the print head with gas.

FIG. 5Bshows additional elements for recirculating air from the print head and means for recovering solvent.

References100again designates an ink reservoir into which ink not consumed during printing will be directed from the gutter7through a pump530(for example a diaphragm pump).

A flow110of vapors from this reservoir100can be directed to a filter200. In return, a liquid flow25that is condensed on the inlet surface210of the filter can be carried to the reservoir100by a duct.

At the outlet from the filter, the flow270of filtered vapors is directed to solvent extraction means260(for example condensation means), that will condense solvent vapors and produce clean and dry gas350that can be returned to the print head1. It is said that the filter is positioned upstream from the means260, since the vapors110to be treated firstly pass through the filter, and the filtered flow270is then directed to the means260. A sensor261can be implemented to measure the temperature and/or humidity of the air in, or at the outlet of, the condenser260.

The solvent extracted (for example by condensation) can then be carried to the reservoir100through an evacuation line290that could be provided with a pump280. The solvent extraction means260used may be any means of denaturing a solvent in a gas flow containing it, or any means of extracting a solvent from a gas flow or lowering the concentration of solvent in such a flow, for example by membrane separation or adsorption. Another example of condenser is given in connection with FIGS. 16A and 16B of US-2018-0050543. The remainder of this description applies to condensation means (or a condenser) but all these other examples of solvent extraction means can be used to produce solvent extracted from the gas flow and a gas flow with a reduced solvent concentration. Reference261designates a temperature sensor to measure the temperature of the gas at the outlet of said solvent extraction means260.

Device370(already described above) can be included in the circuit, dry and cold air produced by said device can be provided to the print head, as explained above in connection withFIGS. 3B-4D.

FIG. 5Cshows another circuit comprising 2 filters200,200a, for example made of glass fibers; in an embodiment, they can be used in alternation.

On this figure, references identical to references in the previous figures designate identical elements or elements performing the same technical function.

Each of filters200,200ais connected to a solvent buffer tank101,100aby a duct110a,110b. On this figure, the reference500designates a buffer volume in which condensation products from the solvent extraction means260are recovered. Preferably a temperature sensor261is implemented to measure the temperature of the gas at the outlet of said solvent extraction means260. This volume500can use a pump300to supply filters200,200aready to clean them. A pump670can pump solvent from the tanks101,100ato add to the ink in the reservoir100. The atmosphere of both tanks communicate (for example through a duct102) so that they operate at a same pressure. Solvent from filter200is supplied to buffer tank101.

The reservoir100can be supplied with recovered ink pumped using a pump530(for example a diaphragm pump) from the gutter in the print head1. The flow in the recovery line is two-phase, with a flow equal to, for example, between 0.3 and 10 liters/hour of liquid, and between 10 and 10000 liters/hour of gas, for example 1000 l/hour. This two-phase flow is generated by the pump530.

The reservoir100can supply the head1with ink through the pump570and a first filter590then a second filter630, close to the print head. A sensor610measures the pressure in the supply pathway to the head1.

The reservoir100is connected to tank100aby a duct100c. A separator can be placed between the reservoir100and the tank100a. For example, this separator functions by inertial precipitation. It can separate the largest particles contained in the atmosphere arriving from the reservoir100. Thus, vapors from which the largest particles or pollutants have been removed are sent to the filter200,200a.

The gas flow from tanks101,100ais carried due to the positive pressure in the reservoir100, to the filter200or200awhich can be connected with the open pathway of a 3-way valve450. This valve may for example be controlled using a predefined clock.

A separator can be placed between reservoir100aand the filter200aand/or a separator can be placed between reservoir101and the filter200. For example, this separator functions by inertial precipitation. It can separate the largest particles contained in the atmosphere arriving from the corresponding reservoir100aor101. Thus, vapours from which the largest particles or pollutants have been removed are sent to the corresponding filter200,200a.

The gas flow is filtered in the selected filter200or200aand is then directed to the condenser260through the open pathway of the valve450. A mechanism for separation of condensates from desaturated air carries the condensates in the buffer volume500, and air through the return line690, to the print head1.

Another pathway starting from the buffer volume500directs a calibrated quantity of condensates through a pump300and controlled valves470, to the filter200,200awaiting for maintenance (this is the filter for which the pathway from the 3-way valve450is closed). Therefore this solvent flow follows a path opposite the path followed by vapors output from the tank101,100aand that have to be treated by one of the filters200,200a: it passes firstly through the downstream side of the filter200a(resp.200) and then through the filter body, and is then directed to the upstream side of the same filter, cleaning particles deposited on the downstream surface and in the depth of the filter.

After the liquid has passed through the filter(s) during rinsing, another pump320connects the desaturated gas pathway to the filters; this gas is directed by two valves470, for example controlled according to the preconfigured clock. This drying mechanism can also open pores of the filter membrane after having rinsed it.

The desaturated gas thus drawn off is returned to the separator, then to the filter that is not in the maintenance phase.

Consequently, the air flow used starting from line690to dry one of the filters in maintenance, circulates in a local loop, which will not have any impact on the net flow transferred to the head1. Air drawn off by the pump320will generate a surplus flow through the filter in maintenance, and is then transferred to the condenser260and returned to the line690, which compensates for the deficit generated by the pump320. Air drawn off by the pump320also generates an overpressure in the reservoir100, but also in the other filter, through which a higher flow rate circulates since both filters communicate with the same atmosphere. As a variant, air can be brought in from the exterior and then transferred by pumping to the required filter in preparation for drying.

The intensity of this gas flow in the local loop is preferably controlled to minimize the pressure fluctuation in the reservoir100and in the gas flow to the return from the print head1.

As in the system illustrated onFIGS. 5A and 5B, device370(already described above) can be included in the circuit, comprising for example a compressor and a membrane air dryer. Air from said device370can be provided to the print head, as explained above in connection withFIGS. 3B-4D. Preferably a temperature and hygrometry sensor263is implemented to measure the temperature and the hygrometry of the gas at the outlet of said device370for producing dry and cold air.

More generally a circuit to recirculate the ink can comprise means to recover solvent, for example as disclosed in US-2018-0050543. Such a circuit can comprise means for injecting air according to the invention, for example like means370ofFIGS. 5A-5C. Air from said source of dry air can be mixed with air from the recirculation circuit either in the print head (as onFIG. 5C) or upstream of the print head.

Preferably air from said extra source is drier and/or colder than air in any other part of the circuit and of the print head.

In any of the above embodiments of a print head or of a circuit, one or more sensor(s)73,610,261,263may be implemented to measure the temperature and/or the hygrometry of the atmosphere around the print head and/or of the air in the recirculation circuit, preferably at the coldest place. Practically, such a sensor73can be located close to the print head (for example close or against a cover containing the print head1) and/or a sensor261can be located at the outlet of means260(FIGS. 5B and 5C) or in the recirculation loop (sensor610,FIG. 5A) and/or a sensor263can be located at the outlet of means370(FIGS. 5A-5C).

Based on the measured temperature(s) and/or hygrometry(ies), for example the temperature measured by sensor261, the temperature and/or hygrometry of air injected into the print head or along the outlet slot of the print head, for example air supplied by device370(FIGS. 5A-C), can be adapted or controlled or regulated. For example, an automatic control based on the partial pressure curve (a curve giving the partial pressure as a function of the temperature, for example the curve ofFIG. 2) is implemented with help of the controller of the printer to control the hygrometry and/or the temperature of the air at the outlet of device370. Preferably the hygrometry and/or the temperature of the air supplied to the print head (said air being injected into the print head or along the outlet slot of the print head) has a temperature and/or hygrometry such that the water vapor pressure is lower than the water vapor pressure defined by 100% humidity at the coldest temperature in the print head and/or in the recirculation circuit; said coldest temperature can be given by the sensor at the outlet of solvent extraction means260; alternatively, it can be assumed that the coldest temperature in the print head and/or in the recirculation circuit has a predefined difference with respect to a predefined temperature, said predefined temperature being for example a temperature belonging to an operating range of the print head.

A sensor can be implemented to measure a temperature inside the cavity5and a sensor can be implemented to measure a temperature at the outlet of the condenser260of the recirculation circuit in order to confirm that the temperature measured at the outlet of said condenser is colder than in the cavity. If the temperature measured at the outlet of said condenser is higher than in the cavity, the feeding power of the condenser can be regulated, for example by the controller of the printer.

The volume of a print head according to the invention is of about some cm3, for example between 1 and 2 cm3. The flow of air injected into the cavity or sweeping along the outside of the cavity is adapted accordingly.

A test was made over 300 h in a very humid atmosphere (35° C., 80% water). As can be understood fromFIG. 6(which represents the water concentration of ink as a function of time) the ink circuit has kept a stable water concentration during the 300 h. For this test, a head structure as illustrated onFIG. 3Band a recirculation circuit as illustrated onFIG. 5Cwere implemented, air being recirculated after inertial precipitation, filtration and condensation. The measurements were made by regular sampling (every 1 or 2 days) then by a KarlFisher method performed with help of a laboratory device.

A structure of a printer comprising a multi-nozzle ink jet print head according to the invention is illustrated onFIGS. 7 and 8.

Regardless of what embodiment is envisaged, the instructions to activate the print head and to produce ink jets and the gutter pumping means530and/or the means (for example a membrane air drier and/or a condenser) forming part of the device370for producing dry and cold air and/or the means570for sending ink into the print head and/or the means300,320of cleaning the filter are produced and sent by the control means (also called the “controller”) and/or the recirculation circuit (in particular a condenser forming part of said recirculation circuit). These are the instructions that, in particular, cause:circulation of ink under pressure towards the print head,then generate jets as a function of motifs or patterns to be printed on a support8(FIG. 1),800(FIG. 7),activate and/or regulate the elements forming part of the device370and/or of any recirculation circuit in order to regulate the temperature and/or hygrometry of the print head based for example on measurements of the outside temperature and/or hygrometry, as already explained above.

These control means may for example be made in the form of a computer or a processor or a chip, or a programmable electric or electronic circuit, or a microprocessor programmed to implement a method according to the invention.

This controller also controls opening and closing of valves on the path of the different fluids (ink, solvent, gas), and operation of the means of circulating a fluid in the filter means (for example valves450and470inFIG. 5C), or pumps300,320. The control means can also memorize data, for example data for measurement of ink levels in one or more reservoirs, and process these data. The control means can also memorize data of curves like those ofFIG. 2, representing the water vapor pressure as a function of temperature.

The control means can receive information or data from one or more sensor(s) measuring temperature and/or humidity and/or water vapor pressure in a part of the circuit or of the head or of the environment (or ambient air) and:compare said measured information or data with data of one or more data of the water vapor saturating pressure as a function of temperature; for example one or more data representative of the temperature inside the cavity or the print head can be compared with one or more temperature data of the temperature at the outlet of a condenser inside a recirculation circuit,and/or control or regulate the temperature and/or humidity and/or water vapor pressure of air injected into the head (like onFIGS. 3A, 3B) or close to the head (like onFIG. 3Cor4A1-4D), in particular air for flowing along at least part of the outlet slot as explained above, so that temperature and/or humidity and/or water vapor pressure is adapted in order not to condense in the cavity or elsewhere in the circuit; this can be achieved by controlling the pressure difference between both sides of the membrane of a membrane air drier and/or the power of a condenser (for example in device370). The control means can be specially programmed for keeping air injected into the cavity and/or air flowing along the outlet slot at a target temperature and/or hygrometry and/or water vapor pressure based on measured temperature and/or humidity data and/or on data concerning the vapor saturating pressure of the air (seeFIG. 2for example) at one or more temperature(s).

FIG. 7shows the main blocks of an inkjet printer (for example a continuous inkjet printer or CIJ printer) that can implement one or several of the embodiments described above.

Such a printer comprises a print head1(that can also have the structure illustrated onFIG. 2) and means200,300,400of supplying printing ink to the head. The print head is connected to a recovery circuit like that described above.

A printer according to the invention may comprise a console300, a compartment containing particularly the ink and solvent conditioning circuit400, and reservoirs for ink and solvents (in particular, the reservoir to which ink recovered by the gutter is delivered). In general, this compartment is in the lower part of the console. The top part of the console comprises the control and instrumentation electronics and display means. The console is hydraulically and electrically connected to a print head1through an umbilical200.

Means for maintaining the head, for example a portal frame not shown, are used to install the print head facing a print support800, which moves along a direction materialized by an arrow. This direction is perpendicular to an alignment axis of the nozzles. Preferably, these means are controlled, through the controller, so that printing can be performed on surfaces which are not flat, for example cables or bottles or cans. In a preferred embodiment, these means can maintain the distance (for example at least between 4 mm and 5 mm, in particular for a CIJ printer) between a printing head and the substrate which must be printed higher than in conventional desk printers.

Examples of print heads that can be used with a device or a method according to the invention are illustrated inFIGS. 3A-4Cand have been described above.

An example of a fluid circuit400of a printer to which the invention can be applied is illustrated inFIG. 8. This fluid circuit400comprises a plurality of means100,500,111,220,310, each associated with a special function. There is also the head1and the umbilical200.

This circuit400is associated with a removable ink cartridge130and a solvent cartridge140that is also removable.

Reference100designates the main reservoir that collects a mix of solvent and ink.

Reference111designates means of drawing off and possibly storing solvent from a solvent cartridge140and providing solvent thus drawn off to other parts of the printer, either to supply solvent to the main reservoir100, or to clean or maintain one or several other parts of the machine.

Reference310designates all means of drawing off ink from an ink cartridge130and providing ink thus drawn off to supply the main reservoir100. As can be seen on this figure, according to the embodiment presented herein, these same means310are used to send solvent to the main reservoir100and from the means111.

At the outlet from the reservoir100, a set of means globally designated as reference220applies pressure to the ink drawn off from the main reservoir and sends it to the print head1(these means can comprise particularly the pump570,590inFIG. 5Cabove). According to one embodiment illustrated herein by the arrow250, it is also possible to use these means220to send ink to the means310, and then again to the reservoir100, which enables recirculation of ink inside the circuit. This circuit220is also used to drain the reservoir in the cartridge130and to clean connections of the cartridge130.

The system shown on this figure also includes means500of recovering fluids (ink and/or solvent) that return from the print head, more precisely from the gutter7of the print head or the head rinsing circuit. Therefore these means500are arranged downstream from the umbilical200(relative to the direction of circulation of fluids that return from the print head). In particular, they include means530inFIG. 5C, but they can also include a solvent vapors treatment circuit according to one embodiment of the invention.

As can be seen inFIG. 8, the means111can also be used to send solvent to these means500directly without passing through the umbilical200or through the print head1or through the gutter.

The means111can comprise at least 3 parallel solvent supplies, one to the head1, the 2nd to the means500and the 3rd to the means310.

Each of the means500,111,210,310described above can be provided with a pump to treat the fluid concerned (namely 1st pump, 2nd pump, 3rd pump, 4th pump respectively). These different pumps perform different functions (the functions of each of their means) and are therefore different from each other, even though these different pumps may be of the same type or similar types (in other words none of these pumps performs 2 of these functions).

Such a circuit400is controlled by the control means described above that are usually contained in the console300(FIG. 7).

The invention is particularly useful in applications in which air or a gas flow injected into the cavity in the print head and in the recirculation circuit is high since the risk of entry of humid air into the print head is higher.

For example, the flow may be of the order of several tens of l/h or several hundred l/h, for example between 10 l/h and 1000 l/h (or 5000 l/h), or for example between about 300 l/h (or 500 l/h) and 1000 l/h. These values are particularly applicable to the case of a print head with 64 jets, but the invention is also applicable to the case of a print head with a smaller number of jets, for example 16, or even only 1 jet, or to the case of a print head with a larger number of jets, for example 128.

The printers concerned by the invention are industrial printers, for example which have the ability to print on surfaces which are not flat, for example cables or bottles or cans. Another aspect of such printers is that the distance between the printing head and the substrate which must be printed is higher than in conventional desk printers. For example that distance is at least between 4 mm and 5 mm for a CIJ printer.

Another aspect of such printers is their speed: their maximum speed is up to 10-15 m/s.

Another aspect of such printers is that they can print on very different surfaces, for example glass, or metal or blisters or packaging materials.