EP3429857A2

EP3429857A2 - Method of printing by an ink jet printer

Info

Publication number: EP3429857A2
Application number: EP17712069.8A
Authority: EP
Inventors: Bruno Barbet
Original assignee: Dover Europe SARL
Current assignee: Dover Europe SARL
Priority date: 2016-03-15
Filing date: 2017-03-15
Publication date: 2019-01-23
Also published as: WO2017158048A3; WO2017158048A2

Abstract

The invention concerns a print head of a binary continuous jet printer comprising: -a cavity (23) for circulating jets, -means (5, 203, 400), for producing through nozzles (9, 190, 290) a plurality of liquid jets in said cavity (23),and for forming drops by natural breaking of said jets; -a slot (25), which opens on the outside of the cavity (23) and enables the exit of liquid drops or sections of jet intended for printing, -a catcher (19a) for recovering drops or sections not intended for printing.

Description

METHOD OF PRINTING BY AN INK JET PRINTER

DESCRIPTION

TECHNICAL FIELD AND PRIOR ART

The invention relates to a device and a method of printing by a binary continuous ink jet printer.

In a binary continuous ink jet printer, ink is ejected in the form of jets by nozzles of Z axes parallel with each other, aligned on a nozzle plate along an alignment X axis. On coming out of the nozzles, the ink circulates in a cavity in the form of jets. The jets from the different nozzles are broken up in a controlled manner to produce drops therefrom. Also in a controlled manner, the drops are directed either to a printing support or to a recovery gutter. The selection between drops of ink that are going to be printed and those that are recovered is made in the cavity. The drops intended for printing leave the cavity, going through an outlet slot. The drops that do not serve for printing are recycled through the recovery gutter. Reference may be made, with regard to the formation of jets, their break up to form drops, the deflection of the drops, to the "prior art" paragraphs of patents assigned to the present applicant. For example, the patent US 8,540,350 (FR 2 952 851). Reference may also be made to the prior art in the patent US 7,192,121 (FR 2 851 495).

As an illustration, in figure 1 is represented, in a very schematic manner, the elements useful for the control of drops of ink formed from one of the nozzles of a multi-nozzle print head of a binary continuous ink jet printer. Figure 1 comprises the parts A and B. Part A represents a section of a print head along a plane comprising an X axis of alignment of nozzles on a nozzle plate, and the Z axes of the different nozzles of the plate. In order not to overload the figure, the section is truncated such that only a single nozzle appears on this part A. Part B is a section of a print head along a plane perpendicular to the plane XZ, and comprising a Z axis of a nozzle. The directions X and Z are materialised by arrows on part A. The directions Y and Z are materialised by arrows on part B. The print head 1 represented in figure 1 comprises a drop generator 3. The drop generator 3 comprises a pressurisation chamber 5, a conduit 7 between the pressurisation chamber 5 and a nozzle 9 present on a nozzle plate 11. The pressurisation chamber 5 is mechanically connected with a stimulator 13. The role of this stimulator 13 is to cause on command a vibration in the ink contained in the pressurisation chamber 5. Most of the time it is a piezoelectric crystal. Other stimulation means are also known and described summarily in the "state of the prior art" part for example of the patent already cited US 7,192,121 (FR 2 851 495). Downstream of the nozzle plate, the print head comprises charge electrodes 15, deflection electrodes 17, and a recovery gutter 19. Often the charge 17 and deflection 19 electrodes are flush with the surface of a wall 20 of a cavity 23. This cavity comprises a slot 25 through which passes ink intended for printing.

Such a structure is complicated and there is the problem of finding a structure of a print head which is easier to build, and preferably comprising less components.

The operation of a print head structure described above is as follows: ink is introduced and pressurised in each of the pressurisation chambers 5. Due to the pressure, a jet of ink of Z axis is present at the outlet of each nozzle 9. The speed of the jet Vj is mainly a function of the pressure exerted on the ink of each chamber, the viscosity of the ink, and the diameter of each nozzle 9. A jet is broken into drops in a controlled manner in the following manner. An electrical pulse is sent to the piezoelectric crystal 13_x which is in mechanical communication with the pressurisation chamber 5_X. The index X is a whole number comprised between 1 and n, n being the number of nozzles 9 present on the nozzle plate 11. The number n is also the number of pixels that are printed simultaneously for each position of the print head above a printing support. For a head for which the distance between the first nozzle 9i and the last nozzle 9_n is for example an inch, the number n is of the order of one hundred or so. The vibration of the crystal 13_x produces a pressure wave in the chamber 5_X. This wave propagates to the jet from the chamber 5_X. Hence, the surface of the jet is deformed and has bulges and contractions. If a contraction is greater than or equal to the radius of the jet, it leads to a breakage of the jet. If two breakages of the same jet are consecutive, a section of jet is formed which, under the action of the surface tension of the ink, is transformed into a drop of ink. A drop may be formed more or less far from the nozzle plate 11. Two successive pulses spaced apart by a duration Ti make it possible to form drops at a spot that is at the same electrical potential as the ink. Successive pulses spaced apart by a duration T₂ different to Ti make it possible to form drops at a spot at a distance from the nozzle plate where the electrical potential is different to that of the ink. Hence these latter drops are electrically charged. Thus it is possible to form charged and non-charged drops of ink. The difference in charges makes it possible to differentiate the trajectories of the drops by application, to these drops, of an electric field. This is the function of the deflection electrodes 17. Under the action of the electrode 17, the charged drops have a trajectory deflected from the Z axis of the nozzle 9 from which they come out. The deflection of the jet takes place in the direction Y. This provides a means for separating drops to direct to the recovery gutter 19 from those that are going to pass through the slot 25 to then strike a printing support. In general, but not always, in binary printers it is the non-deflected drops that are going to strike the printing support. The ink recovered by the gutter 19 is recycled in an ink recovery circuit. A first example of circuit for recovering ink and solvent is described in patent application WO 2013/173200 assigned to Videojet in relation with figure 1 thereof. A second example of recovery circuit is described in relation with figure 1 of patent application FR 2 954 216 assigned to the present applicant.

Such a method is complicated and there is the problem to find a method of operating a print head, preferably comprising less components, which is easier to implement.

As regards the printing command means, they include in general a keyboard and a visualisation screen. These means make it possible to command and to visualise the pattern to print by the print head. Digital transformation means, including notably generators of usual characters and symbols of different fonts, transform the instructions coming from the keyboard into differentiated pulse command sequences. The pulses are created by pulse generators and transmitted by distribution circuits to stimulators 13 which are mechanically connected with the pressurised chambers. A print head may comprise around one hundred or so nozzles, and as many stimulation chambers and stimulators. The distribution circuits comprise notably gates having commanded opening and closing. The command of the gates makes it possible to direct the pulses to the stimulators so as to form drops of ink intended for printing and drops of ink intended for recovery. For most printers, the drops directed to the printing support to form a black pixel are not electrically charged drops. Charged drops are deflected and directed to the recovery gutter. Due to the fact that these drops do not reach the support, the corresponding pixel is white. For each relative position of the head and the printing support, an information is transmitted to the pulse command means. These means also receive an information on the colour, white or black, of the pixel to print for this position. The printing of a pixel may be done with one or more drops. The duration of formation of the drop or drops intended for printing of a black pixel must be at the most equal to the duration of scrolling of a pixel so as not to interfere with the formation of the following pixel.

For each relative position of the support and the print head, a stimulator of a chamber 5 in hydraulic communication with a nozzle 9 of order x having to print a black pixel, receives pulses that break up the jet from the nozzle so as to form drops at a distance from the nozzle plate which is at a potential equal to that of the print head. One or several non-electrically charged drops are formed throughout the whole duration of scrolling of the black pixel. These drops are not deflected and are directed to the support. For the same relative position of the support and the print head, a chamber 5 in hydraulic communication with a nozzle 9 of order y having to print a white pixel, receives pulses which break up the jet from the nozzle so as to form drops at a distance from the nozzle plate which is at a potential different to that of the print head. One or several electrically charged drops are formed throughout the duration of scrolling of the white pixel. These drops are deflected by a deflection electrode and are directed to the gutter.

For a print head comprising for example a hundred or so nozzles, the command means and the pulse formation and distribution circuits comprise a large number of elements because each crystal must be able to be commanded individually. BRIEF DESCRIPTION OF THE INVENTION

As explained above, a structure of a known print head is complicated and there is the problem of finding a structure of a print head which is easier to build, and preferably comprising less components.

There is also the problem to find a method of operating a print head, preferably comprising less components, which is easier to implement.

Another problem, as may be noted on reading the prior art recalled above, is that a binary continuous ink jet printer groups together in a small space a large number of components. The control of the different elements entering into the formation and the selection of drops to direct to the printing support or the gutter requires taking into account various complications which result from the close proximity between all these elements. For example, crosstalk can be created between adjacent stimulation chambers. This phenomenon perturbs the circulation and the intensity of the wave (resulting from stimulation) between adjacent chambers, and consequently the distance of formation of drops from the nozzle(s). This problem of crosstalk is for example discussed in EP 2504172. Another problem is that the trajectories of neighbouring charged drops may be perturbed by phenomena of repulsion or attraction between drops to the point of perturbing the trajectory of the drops, including that of non-charged drops.

A first print head according to the invention, for example for a binary continuous liquid jet printer, comprises at least a nozzle in hydraulic connection with a pressurisation chamber not provided with a stimulator, and preferably a deviation electrode. Such a print head has no charge electrode.

In other words, a first print head according to the invention is able or is conceived to form, or comprises means for forming, drops by natural break up of a jet from said at least one nozzle.

Such a structure of a print head is easier to build than print heads of the prior art, and comprises less components: since there is no stimulator there is also no circuit to trigger any stimulator. A method to operate a structure of a print head according to the invention is also easier to implement than known methods of the prior art: since there is no stimulator there is no need to trigger any stimulator and to control pulses applies to such a stimulator.

The invention also concerns a print head, for example as already mentioned above, of a binary continuous jet printer comprising:

- a cavity for circulating jets,

- means, for producing through nozzles a plurality of liquid jets in said cavity, and for forming drops by natural breaking of said jets;

- a slot, which opens on the outside of the cavity and enables the exit of drops or sections of liquid intended for printing.

In a specific embodiment, said print head further comprising one or more deflection electrode(s).

The invention also concerns an assembly of print heads, comprising at least one 1^st print head according to the invention, in particular as described above, and at least a 2^nd print head and a 3^rd print head, each comprising a cavity for circulating jets, means for producing through nozzles a plurality of liquid jets in said cavity, a slot, which opens on the outside of the cavity and enables the exit of drops or sections of liquid intended for printing, said 1^st, 2^nd and 3^rd print heads being fixed with respect to each other.

In one embodiment of an assembly of print heads according to the invention, said 1^st, 2^nd and 3^rd print heads, or their nozzles, are either aligned along a same 1^st axis or said 2^nd and 3^rd print heads, or their nozzles, are aligned along a same 2^nd axis parallel to said 1^st axis.

For example, said 2^nd and 3^rd print heads can be aligned along a same 2^nd axis parallel to said 1^st axis, the extreme left and right jets produced by said 1^st print head being on the left hand side, respectively on the right hand side, of the extreme jets produced by said 2^nd print head, respectively said 3^rd print head.

In an assembly of print heads according to the invention: - at least one of said 2^nd and 3^rd print heads can be a print head according to the invention, for example as already described above;

- and/or at least one of said 2^nd and 3^rd print heads can comprise one or more stimulator(s) for producing a plurality of liquid jets in its cavity.

Preferably, in an assembly of print heads according to the invention, the diameters of the nozzles of the 2^nd and 3^rd print heads are decreasing from the most extreme nozzle to the nozzle the closest to a nozzle of the 1st print head.

The present invention also relates to a print head, or an assembly of print heads, of a binary continuous liquid jet printer, comprising a whole number of nozzles preferably aligned along a same direction X, for example in a nozzle plate, the series of n nozzles of the assembly comprising:

- a central series of central nozzles comprising ni nozzles,

- a first lateral series of lateral nozzles, a so called left lateral series of nozzles, comprising n₂ nozzles situated on one first side, for example the left side, of the central series;

- and a second lateral series of lateral nozzles, comprising n3 nozzles situated on the other second side, for example the right side, of the central series.

In an embodiment, each nozzle of the central series of nozzles is in hydraulic connection with a pressurisation chamber which has no stimulator, whereas each nozzle of the first and second lateral series of lateral nozzles is in hydraulic connection with a pressurisation chamber which has a stimulator. The presence of one or more stimulators in one or more chambers or connected with one or more chambers makes it possible to control the breakup of the jet and the volume of the drops at the outlet of a nozzle in hydraulic communication with this chamber or the corresponding chamber(s).

Each nozzle is associated with, or in hydraulic communication with, a pressurisation chamber which can be common to several nozzles; in particular a chamber can be common to the nozzles of the central series of nozzles.

For any embodiment of the invention, the average volume of the drops, emitted by a nozzle in hydraulic connection with a pressurisation chamber not provided with a stimulator, is adjustable on the one hand by construction (by selecting the geometric parameters of the head and of that nozzle) and on the other hand by selecting the pressure in the pressurisation chamber. The jet is desintegrated into droplets which form by natural break up of a jet from said nozzle (capillary instability modeled by Lord Rayleigh in 1889).

A print head according to the invention preferably includes a recovery gutter or catcher (which can be common to several nozzles or to all nozzles of a same print head) for drops or sections of fluid not directed to a printing support; the print head can include means for moving said recovery gutter along a direction perpendicular to the direction of the jets emitted by the nozzles. Said recovery gutter is in this case movable along said direction.

A print head or in an assembly of print heads according to the invention may comprise at least one deflection electrode (which can be common to several nozzles or to all nozzles of a same print head). Preferably it does not comprise any charge electrode. Even without charge electrodes, deflection electrodes make it possible to deflect the jets (of a fluid or liquid which is electrically conductive) by attraction because, in the vicinity of the deviation electrode, electrical charges of sign opposite to that of the electrode are formed on the part of the jet that faces the electrode. For example, in an assembly of print heads according to the invention at least one of the 2nd and 3rd print heads may comprise at least one deflection electrode.

A nozzle of a print head according to the invention has a diameter which can be, for example, between 10 μιη and 100 μιη.

The present invention also relates to a printing method implementing a print head or an assembly of print head according to the invention.

In particular, in a printing method according to the invention, throughout the duration of printing or during at least part of the duration of printing and for at least one, or for any, nozzle of the assembly:

- a constant ratio is maintained between the quantity of liquid directed to the printing support and the total quantity of liquid output by said nozzle; - and/or a constant ratio is maintained between the quantity of liquid directed to the printing support and the total quantity of liquid output by said nozzle, the flow rate coming from nozzles of one at least a 2^nd print head and a 3^rd print head, being greater than the flow rate of liquid coming from nozzles of the central series.

In a particular embodiment, the ratio between the quantity of liquid directed to the printing support and the total quantity of liquid output is 1 for all the nozzles.

In other particular embodiments:

- the ratio between the flow rate of liquid directed to the support and the total flow rate of liquid of nozzles of at least the 2^nd print head and the 3^rd print head, is greater than the ratio between the flow rate of liquid directed to the support and the total flow rate of ink of nozzles of the 1^st print head;

- and/or the frequency of formation of drops from at least the 2^nd print head and/or the 3^rd print head is controlled;

- and/or the flow rate from the nozzles of at least the 2^nd print head and/or the 3^rd print head, is decreasing from each of the nozzles of the 2^nd print head and/or the 3^rd print head which are the farthest from the 1^st print head to the nozzles of the 2^nd print head and/or the 3^rd print head which are the closest from the 1^st print head.

The present invention also relates to a method for printing with a printing head or an assembly of one or more print heads according to the invention, for example in a binary continuous liquid jet printer, each print head of the assembly including a recovery gutter for liquid not directed to a printing support, comprising a nozzle plate provided with a whole number of nozzles aligned along a direction, the series of n nozzles of the assembly comprising a central series of central nozzles comprising ni nozzles, a first left lateral series of left lateral nozzles comprising n₂ nozzles situated on the left of the central series and a second right lateral series of right lateral nozzles comprising n3 right lateral nozzles situated on the right of the central series, wherein the printing support and the assembly of print heads have a relative movement (they are moveable with respect to each other along a direction Y perpendicular to the direction X). Printing is performed during said relative movement. Preferably, during printing, for example throughout the printing duration, for any one of the nozzles of the assembly, a constant ratio is maintained between the quantity of liquid directed to the printing support and the total quantity of liquid output by said any nozzle.

n is the total number of nozzles of the assembly and n₂ + ni + n3 = n.

Preferably, the flow rates of liquid coming from each of the nozzles of the assembly are constant during printing, for example throughout the printing duration.

The ratio between the quantity of liquid directed to the support and the total quantity of liquid output over a given duration varies in a discontinuous manner and changes each time that an additional drop is directed to the support or to the gutter. A constant ratio between the quantity of liquid directed to the printing support and the total quantity of liquid output by a nozzle throughout the duration of printing means that the ratio is constant over any quite long duration so that the duration of formation of an additional drop is negligible compared to said quite long duration. The quantity of liquid deposited on the support to be printed is not continuous but varies by steps; the quantity of liquid contained in this additional drop is also negligible with respect to the quantity of liquid already deposited. For example, if out of each group of 12 consecutive drops 9 are directed to the support, the constant ratio is 0.75. In this example, for any group of 4 drops, three consecutive drops are directed to the support and the fourth is directed to the gutter. In general, the ratio is defined by the fact that out of each group of k consecutive drops, a number ki of drops is directed to the support whereas a number (k - ki) is directed to the gutter, k and ki are such that ki/k has a constant value when printing, in particular when printing a strip S . Thus, for example, for any group of k consecutive drops, k-1 consecutives drops are directed to the support and the drop following these k-1 drops is not directed to the support.

More generally, k can be defined as the quantity of ink emitted by a jet during a period of time, ki is the quantity of liquid directed to the support (during the same period of time), and k-ki the quantity of liquid directed to the gutter. The ratio ki/k is constant during printing, for example during printing of a strip S. In an embodiment the flow rates of liquid coming from each of the nozzles of the assembly are constant, preferably throughout the printing duration.

The step between two consecutive nozzles is the step between orthogonal projections of the centres of said two consecutive nozzles on a same X axis. The distance between two nozzles of the assembly is the distance of the orthogonal projections of the centres of these nozzles on a same X axis.

A device or a method according to the invention is/are particularly suited to the printing of strips.

Thanks to the method according to the invention, the command of a print head is greatly simplified. In addition, in certain configurations the drop generator and the electrodes can also be simplified.

According to one aspect, all the liquid coming from the central and left and right lateral nozzles is directed to the printing support. The ratio between the quantity of liquid directed to the printing support and the total quantity of liquid output is 1 for all the nozzles of the assembly.

According to further aspects:

- the flow rate of liquid coming from one at least of the left and right lateral series of lateral nozzles, is greater than that coming from nozzles of the central set;

- and/or the ratio between liquid directed to the support and the total flow rate of liquid of lateral nozzles is greater than the ratio between liquid directed to the support and the total flow rate of liquid of central nozzles, for example two times greater.

Each of the two latter aspects has the consequence that the distances between consecutive impacts of drops coming from a lateral series are smaller than those between consecutive impacts of drops coming from a central series. The notched aspect of the longitudinal limits is thus reduced.

According to another aspect, at the moment of start-up of a print head of the assembly and before its extinction, or even during the whole printing process, a vibration is imposed by a vibrating system (for example a piezo-electric element or actuator or an electro-acoustic actuator or transducer) on said print head along the direction X. This thus randomises the real position in X, at least at the start of printing and at the end of printing, that is to say for impacts coming from nozzles of the print heads of the assembly that are located in a terminal zone of a strip to print. Preferably, the vibration imposed is such that the difference between the real position of an impact and its nominal position is maintained less than 25% of the step between consecutive nozzles.

The method according to the invention is intended for the printing of strips S, which may have different widths, as is explained in the description of detailed exemplary embodiments given hereafter.

The width L of a strip formed by a single passage of an assembly of print heads above a support is at the most equal to the distance between extreme nozzles of the assembly increased by the diameter of the radiuses of the discs formed by the impact, on the printing support, of drops situated at the edge of the strip. In the remainder of the present application, the width of a printed strip will be assimilated with the distance between the nozzles the furthest away from each other that send liquid to the printing support, while neglecting the increase in width resulting from the dimension of the impact of one drop on the support. The length of the strip is only limited by the length of the support on which the strip is printed. As a general rule, the print heads are fixed. The relative movement in the direction Y of the head and the printing support stems from the fact that the support is moveable in the direction Y. In the case of the printing of strips, it may be advantageous to leave the support fixed and to move the assembly of print heads. Preferably, the assembly will also be moveable in the direction Z so as to be able to follow, preferably at constant distance, a potential relief of the printing support.

In one embodiment, the flow rate of ink coming from the lateral nozzles is decreasing from each of the extreme nozzles up to each of the lateral nozzles the nearest to a central nozzle. This may be obtained, for example, by the fact that the diameters of the extreme nozzles are decreasing from the most extreme nozzle up to the nozzle the closest to a central nozzle of the assembly.

In one embodiment, only the stimulation chamber(s) of one or several heads (but not all of them), for example one or two extreme heads, or of the lateral groups of nozzles, in the direction X, are provided with stimulators. In one embodiment, only the stimulation chambers of the two extreme heads, or of the lateral groups of nozzles, in the direction X are provided with deflection electrodes, making it possible on command to deflect or not the jets from the nozzles of this head. This characteristic makes it possible to vary the width of the printed strip. It is thus possible to vary the width of the printed strip, by steps equal to the distance between extreme nozzles of a print head, by adding or removing said print head, for example a head provided with central nozzles and/or by steps equal to a distance between consecutive nozzles, for example extreme nozzle of a head, for example of a lateral head. The width is thus adjustable in a nearly continuous manner.

Finally, the invention also relates to a kit for assembling several print heads, for example for forming an assembly of print heads according to the invention, comprising means for maintaining said several print heads in such a way that the nozzles of each of the print heads are all aligned along a same direction, at least one, and preferably each, print head being provided with a 1^st mark, for example a left mark, making it possible to visualise the position, in a direction of alignment of the nozzles, of the interval between two of its nozzles, for example the extreme left nozzles and/or a 2^nd mark, for example a right mark, making it possible to visualise the position, in said direction, of the interval between two other nozzles, for example the extreme right nozzles.

Usually the development of a print head is difficult because the printer is designed so that it can print all the patterns that a user will want to print. Part of the invention concerns a simplified print head and a corresponding simplified operating method, adapted to print a pattern, in particular a strip S, in a recurring manner.

A device and a method according to the invention are adapted to printing with a liquid, for example ink or a paint, having for example a viscosity of between 1 cP and 50 cP. In this application the examples given for ink also apply to other liquids, for example a paint. BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the method according to the invention as well as simplified print heads suitable for carrying out the method will now be described in relation with the appended drawings in which:

Figure 1 already described is relative to the prior art.

Figure 2 illustrates strip to be printed on a support.

Figures 3A and 3B are each a schematic section along a plane perpendicular to the plane XZ, and comprising a Z axis of a nozzle of a first embodiment of a simplified print head according to the invention.

Figures 4A, 4B and 4C are views from above of groups of nozzles of print heads according to the invention.

Figures 4D and 4E illustrate how to print a pattern with various width, implementing a device according to the invention.

Figures 5A and 5B are each a schematic section along a plane perpendicular to the plane XZ, and comprising a Z axis of a nozzle of another embodiment of a simplified print head according to the invention.

Figure 6 represents in an enlarged view of impacts of drops on a printing support.

Figures 7A and 7B represent enlarged views of drops on a printing support.

Figures 8A and 8B each represent an example of circuit for distributing pulses to actuators.

Figure 9 represents the main units of an ink jet printer.

Figure 10 represents a structure of an ink jet printer to which the present invention may be applied.

In all the figures, including in figure 1, identical reference numbers designate elements having the same function. DETAILED DESCRIPTION OF EMBODIMENTS

I n this description :

- the surface formed by the impact of a drop on the support is assimilated with a disc, the perimeter of which is a circumference; figures 6 and 7A-7B illustrate examples of such impacts. The expression "meeting points of two impacts" means the meeting points of the circumferences surrounding the discs. Distance or spacing between impacts is taken to mean the distance between impact centres;

- a "strip" S is ta ken to mean a rectangle having small sides 52i, 52₂ with a small width W compared to its length L of its long sides 51i, 51₂, as illustrated on figure 2. The long sides 51i, 51₂ of the rectangle will be ca lled "longitudinal limits". The zones of the strip close to a longitudina l limit will be ca lled longitudinal zones. The sma ll sides 52i,

52₂ of the rectangle will be called "terminal limits". The zones of the strip close to a terminal limit will be called terminal zones.

A print head and a printing method according to the invention are particularly well suited for printing such a strip, in particular from small side 52i to small side 52₂. , the support and the printing machine having a relative velocity parallel to the long sides 51i, 51₂.

Figure 1, described above, illustrates the structure of a print head comprising a stimulator 13.

Figure 3A is a schematic section of one print head 1 simplified by deletion of the stimulators 13, the charge electrodes 15 a nd the deflection electrodes 17 represented in figure 1.

I n this embodiments represented in figures 3A and 3B, each head comprises, or uniquely comprises, one or more pressurisation chamber 5, a nozzle plate 11 comprising one nozzle or a plurality of nozzles 9 (a ligned along an axis X perpendicular to figure 3A or 3B), emerging on a cavity 23, a recovery gutter 19, and an outlet slot 25 for the ink to exit from the cavity 23 for printing on a substrate 30. I n this embodiment, a pressurisation chamber 5 may be common to several nozzles 11. The cavity 23 can be delimited by latera l walls 29i, 29₂, and by an upper wall 293 and a lower wall 29₄. The slot 25 (which can be the same for several nozzles), which can pass through the lower wall, opens on the outside of the cavity and enables the exit of drops or sections of ink intended for printing.

The structures of figures 3A, 3B, 5A, 5B extend along an axis X perpendicular to each of the figures, in particular when they comprise several nozzles aligned along said X axis.

The embodiment represented in figure 3B differs from that of figure 3A uniquely by the fact that the gutter has a rounded part 27 suitable for producing a Coanda effect of attraction of the ink from the moment that part of the rounded part enters into contact with the jets. A potential smearing effect at the end of printing, for example of a strip, is thus reduced.

The Coanda effect is defined as the tendency of a jet of fluid emerging from a nozzle to follow an adjacent flat or curved surface. The application of this effect to ink jet recovery has been described in EP0805039.

These embodiments have no stimulator, and in particular no piezoelectric means, no means for thermal stimulation (by heating) of the liquid, no electrodes for an electro-hydrodynamical stimulation of the liquid.

I n these embodiments, since there are no stimulators 13, there is no stimulator command circuit either. The command of printing can thus begin at a start command, in which liquid is directed to the support, and a stop command in which the gutter is advanced to recover ink. The drops of ink form by natural break-up of the jet. The fundamentals of natural break-up of a jet a re explained in the article by J.W. Strutt and Lord Rayleigh "On the instability of jets", proceedings of the London mathematical society, vol.1, p;4-13 and in J. Eggers, E. Villermaux, "Physics of liquid jets", Rep. Prog. Phys. 71 , 036601 (2008). I n this embodiment there are no charge and deflection electrodes either. It is thus not possible to stop printing by deflection of the drops to the gutter. The "start" command may for example be triggered by a first signal, each first signal indicating the arrival of one of the heads of the assembly at the start of a pattern, for example a strip S, to print. Similarly, second signals may indicate the passage of a head of the assembly at the end of a pattern, for example a strip S, to print and thus the stoppage of printing for said head.

Means, for example a motor, may be included to move the gutter along an axis Y perpendicular to a jet in the cavity.

Such means may also be used for example during the cleaning of a head. During cleaning phases, ink is replaced by solvent. The solvent being in general non- electrically conducting, the jets of solvent cannot be deflected by electrodes. For this reason, during these phases, the gutter is advanced in the direction Y in a sufficient manner so that the gutter recovers the solvent that flows from the nozzles 9. To stop printing, the gutter 19 may be moved in the direction Y in a sufficient manner so that the gutter recovers the ink coming from the nozzles 9. When printing starts again, the gutter is moved back so as to allow the ink to pass to the printing support 30.

This simplified embodiment is recommended in particular if it is provided to use the assembly with non-electrically conducting inks or paints.

Figure 4A represents a schematic top view of a series 90i of ni nozzles 9i,... 9_ni, aligned along an axis X, for example according to figure 3A or 3B. This series of nozzles can be combined with a plurality of pressurization chamber 5, or have a common pressurization chamber. In a particular embodiment, none of said chamber(s) has a stimulator for any of the nozzles.

Figure 4B represents the same head or series 90i of ni nozzles along a same axis X.

A first other head or series 90₂ of n₂ nozzles 190i,...190_n2, also extends along the same X axis, for example on the left hand side, forming a left lateral series or group.

A second other head or series 903 of n3 nozzles 290i,...290_n3, also extends along the same X axis, for example on the right hand side, forming a right lateral series or group.

Each other head or series of nozzles can have the structure disclosed in connection with any of figures 3A or 3B. The head or group or series 90i of ru nozzles forms a central head or group of nozzles.

Each such group of nozzles can form part of a print head. In this case, the reference 90k (k=i, 2, 3) is used to designate the corresponding series of nozzles but also the print head to which said series of nozzles belong. For example, each of the 3 series 90k (k=i, 2, 3) of nozzles can belong to an individual print head or to a same print head. The 3 series assembled together as illustrated on figure 4B thus represents 3 printing heads assembled together. Each of the examples of figures 4B -4E has 3 heads or groups of nozzles, but the invention also applies to any number of heads or groups of nozzles.

The extreme nozzles of such a print head or of such an assembly of print heads are the nozzles of this series which are the furthest away from each other. The extreme nozzles of the print head or of the assembly of print head according to figure 4B are the nozzles 190i and 290i.

The extreme nozzles of the left lateral series are the nozzles 190i and 190n2.

The extreme nozzles of the right lateral series are the nozzles 290i and

290_n2.

Figure 4C represents the same series of nozzles or heads 90ι, 9Ο2 and 9Ο3, but not aligned along a same axis: the heads 9Ο2 and 9Ο3 are aligned along an axis X' which is parallel to X; the distance between X and X' is approximately equal to the width w of head 90i. The 3 heads are fixed with respect to each other but are offset, along the axis X (or X') such that, on a same axis X" parallel to X:

- the orthogonal projection Pi of the Z axis of the extreme left nozzle 9i, of head 90i is on the left side of the orthogonal projection P_n2, on said same axis X", of the projection P_n2 of extreme right nozzle 190_n2 of head 902;

- the orthogonal projection P_n3 of the Z axis of the extreme left nozzle 290n3 of head 9Ο3 is on the left side of the orthogonal projection P_ni, on said same axis X", of the projection P_ni of extreme right nozzle 290_n3 of head 9Ο3.

This embodiment of figure 4C allows an overlap of the prints made by the different heads: - in a zone corresponding to the space between the drops printed by nozzle 9i of head 90i and by nozzle 190_n2 of head 90₂;

- and in a zone corresponding to the space between the drops printed by nozzle 9_ni of head 90i and by nozzle 290_n3 of head 9Ο3.

This embodiment thus contributes to a more uniform printing.

At least 2 print heads, but preferably each of the print heads of a device according to the invention, for example a head according to figure 4C, can have one or more alignment mark(s) 91, 191, 192 to align and to position at least 2 heads, and preferably all of them, with respect to each other so that the appropriate positioning is obtained in particular with respect to the resulting printing. This can be checked based on the impacts of the drops on the printing support (examples of such impacts are illustrated on figures 6 and 7A-7B).

As already mentioned the invention also applies to any number N of heads or groups of nozzles (for example N=2 or N>3).

In an example of assembly of N print heads - or groups of nozzles - according to the invention, in particular intended to carry out a method according to the invention, each head or group includes a plate comprises nozzles of axes parallel with each other (and parallel to a direction Z), all nozzles being aligned along a same direction X; each head or group of nozzle of the assembly has an extreme left nozzle and an extreme right nozzle. For two successive heads or groups of nozzles of this assembly, the orthogonal projection on a same axis X" (parallel to direction X), of the Z axis of the extreme left nozzle of a following head is left of the orthogonal projection on this same axis X" of the Z axis of the extreme right nozzle of the preceding head.

An assembly of several print heads according to the invention, in particular the preceding example, may include at least three print heads, wherein the nozzles of the head the most to the left constitute the lateral left nozzles and the nozzles of the head the most to the right constitute the lateral right nozzles.

Preferably, the diameters of the nozzles of the extreme heads (the heads the most to the left and to the right) are decreasing from the most extreme nozzle to the nozzle the closest to a central nozzle of the assembly. In a specific embodiment, only the stimulation of the two extreme heads in the direction X are provided with stimulators.

In other specific embodiments :

- only the extreme right or left head is provided with deflection electrodes making it possible on command to deflect or not the jets from the nozzles of this head;

- only the chambers of the central heads are provided with stimulators. In another specific embodiment, only the central heads are provided with deflection electrodes.

In an assembly of several print heads according to the invention, each print head can be provided with a first mark making it possible to visualise the position, in the direction X, of the interval between two nozzles, for example the two extreme left nozzles and/or a second mark making it possible to visualise the position, in the direction X, of the interval between two nozzles, for example the two extreme right nozzles.

In a specific embodiment of an assembly of several print heads according to the invention, a vibrator is applied on each of the heads of the assembly making it possible on command to vibrate the head to which it is applied in the direction X.

In any assembly of heads or groups of nozzles according to the invention, all nozzles of the different heads or groups are along a same axis X or in a same plane.

The heads can be assembled together by any fastening or fixing means, for example one or more screw(s) or glue.

Any head or group of nozzles can have any number N of nozzles, N>1, for example 5, or 10 or more (20 or 50 or 100) nozzles.

In any of the embodiments disclosed above, in particular in connection with figure 4B or figure 4C, the nozzles of all heads or of all groups of nozzles have a same diameter. But, in an alternative embodiment, different nozzles may have different diameters. For example, in a specific embodiment: - the diameter of the nozzles of the left lateral series 90₂ is decreasing from the nozzle 190i to the nozzle 190_n2 (which is the closest to the series 90i of ni nozzles);

- and/or the diameter of the nozzles of the right lateral series 9Ο3 is decreasing from the nozzle 290i to the nozzle 290_n3 (which is the closest to the series 90i of ni nozzles).

In a more general embodiment:

- the diameter of the nozzles of the head or of the lateral series of nozzles which is most to the left is decreasing from the most extreme left nozzle to the most extreme right nozzle of the same head or of the same lateral series of nozzles;

- and/or the diameter of the nozzles of the head or of the lateral series of nozzles which is most to the right is decreasing from the most extreme right nozzle to the most extreme left nozzle of the same head or of the same lateral series of nozzles.

In a particular embodiment the decrease in diameter of the nozzles of each of the right and left lateral series or groups or heads (or of the most extreme right and left lateral series or groups or heads) is such that the lateral nozzles having the smallest diameter have a diameter equal to that of the nozzles of the central group 90i (the diameter of the nozzles of the central group being the same). In other words, the diameter of the nozzles of each lateral group or of each most extreme lateral group or head can be decreasing from each of the extreme right and left nozzles to the central nozzles. In one application of this embodiment, when printing a strip as illustrated on figure 2, the quantity of ink received by the support can be decreased, preferably in a progressive manner, from each of the longitudinal limits to the central zone of the printed strip.

It is pointed out that the nozzles of a same group or head are all aligned on an axis of direction X. However the X axes of different groups or heads may be different, as illustrated on figure 4C.

The step (or the distance) between two consecutive nozzles (which is also the distance between the centres of these consecutive nozzles) is, or corresponds to, the step between orthogonal projections of the centres of said two consecutive nozzles on an axis X" parallel to said X axis. The distance between two nozzles of the assembly is the distance of the orthogonal projections of the centres of these nozzles on a same X" axis.

In an alternative embodiment one or more (or each) of the nozzles of one or each of the groups of nozzles (for example the groups of nozzles 90i, 90₂, 903 of figure 4B) or of one or each of the heads forms part of, or is in communication with, a chamber 5 which is comprises a stimulator 13 intended to control the break-up into drops of the jet emitted by the nozzle. A print head comprising a nozzle 190i associated with a stimulator 13 is illustrated on figure 5A. The nozzle 190i represented is from the left lateral group 90₂, but it could be any other nozzle from the same group or head or any other nozzle from any other group or head.

In one specific embodiment of the print head structure of figure 4B, each of the nozzles of only the right and left lateral groups 90₂, 9Ο3 of nozzles or heads is provided with a stimulator 13 (or of only the most right and left lateral groups of nozzles or heads). The nozzles of the central group of nozzles form part of, or are in communication with, a chamber 5 (or chambers) which does (do) not comprise any stimulator 13.

In particular, the use of stimulators 13 makes it possible to obtain a more regular distribution of the impacts on the support than the distribution obtained by natural break up.

In another specific embodiment, all the chambers are provided with stimulators.

In another specific embodiment, only each of the chambers in communication with the central groups 90i of nozzles is provided with a stimulator (or stimulators) 13. The nozzles of each lateral groups of nozzles form part of, or are in communication with, a chamber (or chambers) which does (do) not comprise any stimulator 13.

It is sometimes advantageous to provide a head or a group of heads of an assembly according to the invention, notably according to any of figures 3A - 3B, or 4A - 4C, or 5, with one or more deflection electrodes, like electrode 17 of figure 1. This is in particular the case if the fluid used is electrically conducting, for example an electrically conducting ink or paint.

Even without charge electrodes, at least one deflection electrode makes it possible to deflect the jets by attraction because, in the vicinity of the at least one deflection electrode, electrical charges of sign opposite to that of the deflection electrode(s) are formed on the part of the jet that faces said deflection electrode. This applies also to jets from nozzles which form part of, or are in communication with, a chamber (or chambers) which does (do) not comprise any stimulator 13.

Any such deflection electrode can be connected to voltage supply means in order to activate it (placing it at a potential different to that of the ink).

Such a deflection electrode may command the stoppage or the starting of the head(s) when:

- they are all activated, that is to say placed at a potential different to that of the ink);

- or non-activated, that is to say placed at a potential equal to that of the ink.

Activating only some of the deflection electrodes makes it possible to selectively deflect jets to the recovery gutter. This may be advantageous to regulate in a certain measure the width of a printed pattern, for example a strip S according to figure 2. For example activating the deflection electrode(s) located facing the jet(s) corresponding to one of the most extreme lateral nozzle(s) (they bear the references 190i and 290i on figures 4B and 4C) and possibly to one or more consecutive nozzle(s) close to said most extreme lateral nozzle(s) makes it possible to broaden or reduce the width of the printed pattern.

I n a preferred embodiment, one or more of the deflection electrode(s) ca n be positioned so as to deviate a jet (before formation of drops), thereby avoiding the need to control the parameters of the break up distance and time of the drops.

As already explained above, in one embodiment, only the stimulation chambers of the two extreme heads (heads 90₂ and 903 of figure 4B and 4C) or of the lateral groups of nozzles, in the directions X, X' are provided with deflection electrodes, making it possible on command to deflect or not the jets from the nozzles of these head or groups of nozzles. This makes it possible to vary the width of the printed pattern, for example a printed strip S (figure 2). It is thus possible to vary the width of the pattern, by steps equal to the distance between extreme nozzles, by adding or removing a print head provided with central nozzles, and/or by steps equal to the distance between consecutive nozzles of the extreme right or extreme left nozzle. The width is thus adjustable in a practically continuous manner.

Figures 4D and 4E illustrate examples based on the structure of figure 4B, wherein the print heads 90i and 90₂ are not provided with deflection electrodes, whereas print head 903 is provided with deflection electrodes for each of its nozzles:

- activating both heads 90i and 90₂ and part of the head 9Ο3 (thanks to the deflection electrodes) makes it possible to print a pattern having a width equal to 2 print heads and 5 nozzles of head 9Ο3 (figure 4D);

- activating head 90i (but not head 90₂) and part of the head 9Ο3 (thanks to the deflection electrodes) makes it possible to print a pattern having a width equal to 1 print head and 2 nozzles of head 9Ο3 (figure 4E);

- activating only some of the nozzles of head 9Ο3 (but none of heads 90i, 90₂ ) makes it possible to print a pattern having a width less than 1 print head (not represented on the figures).

The above examples are given for 3 heads, but any number of heads or groups of nozzles can be taken and adapted to print a pattern with any width.

If n_n is the number of nozzles in a print head, and N_p the number of heads, including Np-1 heads not comprising deflection electrodes (non « adjustable » heads), and one « adjustable » head (comprising deflection electrodes), then the width L of a printed pattern can vary from 0 a n_n.N_p according to formula :

L = (a.n_n + b). p, where : p is the pitch between consecutive nozzles,

a is the number of printing non « adjustable » heads (0≤a≤N_p -1), b is the number of printing jets of the adjustable heads (0≤b≤ n_n). On figures 4D and 4E, n_n = 10 et N_p = 3.

In another example, N_p = 9, 7 heads not comprising deflection electrodes (non « adjustable » heads), and 2 « adjustable » lateral heads (comprising deflection electrodes). It is possible to print:

- over a width equal to 9 heads;

- or, by selecting only 6 out of the 7 non adjustable heads, over a width equal to 8 heads ;

- or, by activating the deflection electrodes of the adjustable heads, over a width equal to a number of the adjustable heads plus a the number of printing jets of the adjustable heads.

Figure 5A is a schematic section of one print head 1 as on figure 3, including a stimulator 13, but without any charge electrode. The other elements on this figure have already been described above, in particular in connection with figure 3A.

The embodiment represented in figure 5B differs from that of figure 5A uniquely by the fact that the gutter has a rounded part 27 suitable for producing a Coanda effect of attraction of the ink from the moment that part of the rounded part enters into contact with the jets. Reference is made to the above explanation concerning this effect.

Ways of forming and distributing drops of ink on a support, for example with a print head or an assembly of print heads according to the invention, so as to blacken the whole surface comprised between the longitudinal limits 51i, 51₂ of a strip S (figure 2) will be discussed hereafter.

Again, it is assumed that the impact formed by a drop arriving on a support is a disc. The covering power of a drop is thus proportional to the square of the diameter of this disc. The covering power increases with the volume of the drop. If, through simplification, the volume of the drop is assimilated with a sphere of radius R, the volume of a drop is proportional to the cube of the radius R. If it is assumed that the drop spreads out on the support according to a disc, the radius of the impact r, verifies the equality e.r² = 4/3. R³ in which e is a proportionality coefficient. The presence of stimulators 13 (according to figure 5A or 5B for example) mechanically connected with a chamber 5 makes it possible to control the volume of the drops at the outlet of a nozzle 9 in hydraulic communication with this chamber. The control of the volume of the drops may be of interest, because the covering power of a drop is, as has just been explained, an increasing function of the volume of the drop.

The covering power is not however uniquely a function of the volume of each drop. It also depends on the value of the surface tension of the ink, the volatility of the solvent of the ink, the absorbency of the support, the surface condition of the support.

Finally, due to the relative speed V_s of the support and the print head in the direction Y, the shape of the impact of a drop on the printing support may take a shape akin to that of an ellipse, the large axis of which has the direction Y. In many cases, the faster the speed V_s, the greater the ratio between the large axis and the small axis of the ellipse. It may be noted that this phenomenon is favourable in the case of printing of strips S as illustrated on figure 2. It is also noted that the optimum speed between the support and the print head is a function of the flow rate of the ink coming from each of the nozzles.

For each ink and for each support one may thus carry out experiments to determine or estimate the optimum size of the drops, the relative speed of the support and the head.

The different ways of spacing the impacts of drops on the support to cover the whole surface of a strip with the least ink possible will now be considered.

The impacts formed by print heads comprising neither stimulators 13, nor charge electrodes nor deflection electrodes will firstly be considered.

When the jets are from nozzles in hydraulic connection with one or more pressurisation chamber(s) not provided with a stimulator, the volume of the drops is adjustable on the one hand by construction (by selecting the geometric parameters of the head) and on the other hand by selecting the pressurisation pressure of the pressurisation chamber(s). By construction, it is for example possible to select : -the step p between consecutive nozzles,

- and/or the characteristics of the nozzles, for example the diameter of the outlet orifice of the nozzle, and/or the conicity of the nozzle in the nozzle plate;

- and/or the length of the conduit between the pressurisation chamber(s) and the nozzle plate.

Tests can be carried out in an experimentation phase to determine the optimum diameter of the impacts as a function of the relative speeds envisaged between the printing support and the assembly of print heads. For example during this experimental phase, a sample of the support to print is moved with respect to the print head at an experimental speed V_se sufficiently fast so that the impacts of drops formed by natural break up of the jets are without overlap in the direction Y. It is possible to determine or estimate:

- an average value of the diameter D_e of these impacts;

- an average value of the distance between centres of consecutive impacts i.e. L_e.

In the relationship T = L_e/V_se., T is the average time interval between consecutive drops coming from a same nozzle.

In order that the entire surface between impacts of consecutive drops is covered, it is preferable that:

- the diameter of the impacts of the drops is greater than the step p between consecutive nozzles;

- and the spacing between drops is such that a cord common to two consecutive impacts is at least equal to the step p.

This general case is represented in figure 6. In this figure, as in figures 7A and 7B which will be commented on hereafter, the y axes are spaced apart by p, each representing a straight line obtained by the series of intersection points of the Z axis of a nozzle with a printing support during the relative movement of the assembly of heads and the support. Circles represent drop impacts. The circles have a diameter greater than the step p between drops and the spacing between consecutive impacts of drops is chosen so that the cord Cm common to these two impacts is at least equal to p. The hatched parts represent, each, the overlap surfaces between impacts. Compared to reality, figures 6 and 7A and 7B are greatly enlarged since the real step between nozzles is less than one mm, for example 0.254 mm for a resolution of 100 DPI (DPI = dots per inch). To minimise the overlap surfaces, it is preferable to choose the step p and the spacing between consecutive impacts such that p > De/V2. To cover the surface, it is preferable to choose the step p and the spacing between consecutive impacts such that p < De/V2.

An optimal value of p is hence given by :

D_e/V2 = p (1)

This is the case that is represented in figures 7A and 7B. The relationship (1) makes it possible to determine the step p between consecutive nozzles when the diameter of the impacts that form by natural break up of a jet from a nozzle are known by tests. In the direction Y, the distance L_e between consecutive impacts is also equal to D_e/V2. This thus gives the relationship De/V2 = TVs.

This relationship, when only V_s is unknown, makes it possible to determine the value of the relative speed V_s to have total coverage of the zone to blacken given the characteristics that have been retained for the nozzle.

In figures 7A and 7B the diameter of the impacts is equal to V2p. The spacing between centres of consecutive impacts is equal to p. The cord C_m common to two consecutive impacts is equal to the step p. The hatched surfaces represent the overlap of impacts.

In one example of an embodiment of an assembly suitable for realising projections of impacts by natural break up of jets, all the nozzles have the same diameter, and their pressurisation chambers are subjected to the same pressure. In figures 6 and 7A-7B, the impacts are represented as regularly spaced apart and of same diameter. It has been seen that it is possible to form the chamber(s) 5, the nozzles 9, and the step p between consecutive nozzles and to regulate the speed V_s between the support and the assembly of print heads so that the entire surface of the support between consecutive nozzles is covered.

However, in this embodiment, and as represented in figures 6 and 7A- 7B, the extreme edges have a notched aspect. The notched aspect may be decreased if the flow rate of ink or paint of the extreme left and right lateral nozzles is increased with respect to the flow rate of the central nozzles, for example doubled. For a same relative speed of the support and the print head, the spacings between impacts are then reduced by half. An intercalary or intermediate impact is represented in dotted lines on the right hand parts of figures 7A and 7B. A portion be of an impact reduces the depth d of the hollow between consecutive impacts be. This intercalary impact is obtained, for example, by the dimensioning of the lateral nozzles, selected or carried out in such a way that, for the same pressure applied to all the stimulation chambers, the flow rate of the left and right lateral nozzles is greater than that of the central nozzles, for example double as represented in dotted lines in figures 7A-7B.

In an alternative embodiment, the instants of formation of drops in jets coming from a part at least of the nozzles of the assembly is controlled. To this end, one or more chambers of the assembly comprise stimulators 13 intended to control the break-up of the jets into drops. A stimulator 13 is represented on figures 5A and 5B. In one embodiment, only the instants of formation of drops from lateral nozzles (those of groups 90₂ and 9Ο3 of figures 4B and 4C) are controlled by stimulators, the central nozzles 90i printing the central part of the strip by natural break up. The consecutive impacts of a same nozzle overlap and impacts between consecutive nozzles also overlap (as illustrated on figure 6). The probability that a central part of the strip is not covered is low. Conversely, in the vicinity of the lateral limits, and in particular for the most extreme nozzles 190i and 290i of groups 90₂ and 9Ο3, there is no overlap by impacts of consecutive nozzles. In one embodiment, only each of the chambers in communication with the groups of right and left lateral nozzles 90₂, 9Ο3 (see figure 4B) are provided with stimulators 13. The use of stimulators 13 makes it possible to obtain a more regular distribution of the impacts on the support than the distribution obtained by natural break up. The notched aspect of the longitudinal limits of the strip is thus decreased.

In the case mentioned above where the diameter of the nozzles is decreasing from each of the extreme right and extreme left nozzles 190i, 290i (figure 4B) to the central nozzles 9i, 9_ni, the frequency of repetition of the pulses applied to the stimulators of the different chambers in communication with each of the groups of lateral nozzles can be increased, preferably in a regular manner. In other words, in order to reduce the notch effect, the frequency of the stimulators associated with the most extreme nozzle(s) 190i, 290i (which have the largest diameters) is increased with respect to the frequency of the nozzle(s) located closer to the central group 90i of nozzles. The spacing between consecutive impacts of the different lateral nozzles can thus be increased, preferably progressively, in a controlled manner. For the groups of left and right lateral nozzles the closest to a central nozzle, the spacing between consecutive impacts may be the same as that of the central nozzles.

In one embodiment disclosed above where the flow rate of the lateral nozzles is decreasing, preferably regularly, from a first value for an extreme nozzle 190i, 290i to a second value for the nozzle 190_n2, 290_n3 the closest to an extreme central nozzle 9i, 9_ni, the chopping frequency of the jet is decremented, preferably regularly, from a value F2 for the extreme lateral nozzle 190i, 290i to a value Fl for the nozzle 190_n2, 290_n3 the closest to the extreme nozzles of the central group 90i of nozzles.

For example, the decrement is, for the group 903 (respectively 90₂) of extreme right (respectively left), nozzles (F2 - Fl)/n3 (respectively (F2 - Fl)/n₂). If the flow rate of the extreme lateral nozzles 190i, respectively 290i is for example double that of the central nozzles 9i - 9_ni, the chopping frequency applied to the stimulators corresponding to these extreme lateral nozzles 190i, respectively 290i , is double the frequency Fl applied to the central nozzles 9i - 9_ni. The frequency decrement between consecutive lateral nozzles is then Fl/n₂, respectively Fl/n3.

If, given the inks or paints employed, coverage defects are present in the central part, then the instants of formation of drops of jets from central nozzles are also controlled. This means that, preferably, all the stimulation chambers are provided with stimulators, including those of the central group 90i (figure 4B) of nozzles. It may be noted that the use of stimulators enables not only control of the size of each drop and thus of each impact but also the instant of formation of the drop. It is thus possible and preferable, as explained in relation with figure 8B, to separate by a half period the formation pulses applied to the chambers in communication with nozzles of even line and those applied to the chambers in communication with nozzles of odd line. Although the chamber(s) 5 of the head are, in these embodiments, equipped with stimulators 13, the stimulator command circuit is simplified.

A first exemplary embodiment of stimulators command circuit is represented in figure 8A. This example represents a command circuit 133 of stimulators 13 mounted on consecutive chambers 5. The consecutive chambers 5 provided with stimulators 13 are, for example:

- either those communicating with the lateral series 90₂, 903 of nozzles of the structure of figure 4B;

- or those communicating with the lateral series 90₂, 903 of nozzles and those communicating with the central series of nozzles 90i of the structure of figure 4B.

The circuit comprises a pulse generator 2. The repetition frequency of the generator 2 is adjustable by a command F (which can be generated by the controller of the printer). An outlet 4 of the generator is coupled to each of the stimulators 13 stimulating the corresponding chambers, for example those of the nozzles of the lateral series 90₂, 9Ο3 of nozzles. A connection of the outlet 4 of the generator 2 with each of the stimulators 13 of the central series of nozzles 90i has been represented in dotted lines because, depending on the embodiments, the stimulators of said central series are present or absent. The length of the sections of jet formed by a nozzle associated with a stimulator to which consecutive pulses are applied is equal to VjT, where T designates the duration between two consecutive pulses.

As illustrated on figure 8B, a delay 6 is preferably placed between the outlet 4 and the connections to stimulators 13 of even order (respectively of odd-order), no delay being between the outlet 4 and connections to stimulators 13 of odd order (respectively of even order). This delay 6 makes it possible, on the one hand, to reduce crosstalk between jets from nozzles adjacent to each other and, on the other hand, to offset the formation of drops from consecutive nozzles. Preferably, the offset is a half of the period of formation of the drops. Preferably, the presence of the delay 6 may be commanded by a command R (which can be generated by the controller of the printer). The drop formation pulses applied to stimulators in communication with nozzles of even line can thus be offset by a half period with respect to those applied to stimulators in communication with nozzles of odd line. This can be useful in particular when a head is located above a non-terminal zone of the strip to print.

An example of operation of a device according to the invention is the following.

Pressurised ink is introduced into the pressurisation chamber(s) 5. Jets of ink are ejected by each of the nozzles 9. Without stimulators on any of the chambers, the jets break up in a natural manner and form drops that pass through the slot 25 and are going to strike a printing support 30. Preferably, the speed V_s of the support with respect to the assembly of print head(s) is regulated as explained above so that the impacts of consecutive drops formed from each of the nozzles are secant to each other and have a common cord at least equal to the step P between consecutive nozzles.

When the chambers 5 in hydraulic communication with the nozzles of, for example, the lateral series 90₂, 903 of figure 4B are provided with stimulators 13, the period of the jet cutting pulses is preferably regulated so that the impacts of consecutive drops formed from each of the nozzles are secant to each other and have a common cord at least equal to the step P between lateral consecutive nozzles. Preferably, the period of the pulses is regulated so that the impacts have a diameter substantially equal to V2p. In a specific embodiment, the speed of the support can be regulated so that the spacing between consecutive impacts coming from a same nozzle is substantially equal to p. The effect described in solid lines in relation with figure 7A is then reproduced for the part of strip S formed by the lateral series of nozzles.

It may be noted that, in particular in the operating mode that has just been described, if 100% of the flow rate of a nozzle is sent to the support, the speed of the support is a function of the flow rate of the nozzles (the distance between consecutive printed impacts depends on the speed). The greater the flow rate of the nozzles, the faster the speed. The presence of stimulators makes less irregular the formation of the drops and thus the distribution of the impacts.

To reduce the effect of notching of the longitudinal limits of the strip, it has been seen, in relation with figures 6 and 7A, 7B, that it is advisable to increase the frequency of the drops, and thus of the impacts; this increase is preferably combined with an increase of the flow rate, which further reduces the effect of notching. If all the nozzles are identical by construction, the consumption of ink is considerably increased, for example doubled, if the speed of the support is reduced by half. In order to reduce the notching effect, without increasing too much the consumption of ink, in one embodiment the left and right lateral nozzles of the lateral series 90₂, 903 of figure 4B are constructed to have a same pressurisation pressure in all the pressurisation chambers (or all nozzles of one of these series are associated with a single pressurisation chamber, in which case the pressure is the same for all these nozzle), and a flow rate greater than that of the central nozzlesfor example a double or triple flow rate. Thus the impacts are larger and/or are more frequent. In one embodiment, the flow rate of the lateral nozzles is regularly decreasing from an extreme lateral nozzle 190i and/or 290i to the extreme right, respectively left, central nozzle 9i, 9_ni and the chopping frequency of the stimulators is preferably adapted as explained above.

The embodiments that have just been described are applicable not just to electrically conducting inks or paints but also to those which are not electrically conducting.

Whatever the embodiment of the invention envisaged, the instructions, for activating the means or stimulators 13 for producing liquid jets and/or the means for pressurising the chamber(s) 5 and/or for controlling pumping means to pump liquid from the gutter and/or the opening and the closing of valves in the path of the different fluids or liquids (for example ink, solvent), and/or for controlling any pumping means of the system are sent by control means (also called "controller"). These control means are for example realised in the form of a processor or of an electric or electronic circuit or of a microprocessor or of a micro-computer or of a computer, each of them being programmable or programmed to implement a method or steps according to the invention.

In figure 9 is represented the main units of an ink jet printer that can implement one or more of the embodiments described above. The printer comprises a console 300, a compartment 400 containing notably the circuits for conditioning the liquid(s) (for example ink and solvent), as well as reservoirs for the liquid(s) (for example ink and the solvent(s)) (in particular, the reservoir to which the liquid recovered by the gutter is bought back). Generally, the compartment 400 is in the lower part of the console. The upper part of the console comprises the command and control electronics as well as visualisation means. The console is hydraulically and electrically connected to one or more print head(s) 1 by an umbilical 203.

A gantry, not represented, makes it possible to install the print head facing a printing support 30, which moves along a direction materialised by an arrow. This direction is perpendicular to an axis of alignment of the nozzles.

A printer according to the invention is an industrial printer, for example which has the ability to print on surfaces which are not flat, for example cables or bottles or cans or spare parts of a car body, or pieces of metal or plastic which offer some curvature. Preferably the printer includes means for displacing the print head(s) along an axis perpendicular to the surface on which printing is performed, for example in order to keep a constant distance between said surface and the print head.

The invention is adapted to industrial painting, or to industrial coating, or to industrial printing on textile.

The invention is also adapted to post- treatments and/or pre- treatments, for example for forming plane surfaces, in order to form a layer or a couch to perform further depositions (with ink or any other liquid) on it, with a good resistance and/or a good contrast, or in order to mask a pattern or a marking or a code.

An example of fluidic circuit 400 of a printer to which the invention may be applied is illustrated in figure 10. This fluidic circuit 400 comprises a plurality of means 410, 500, 110, 220, 310, each associated with a specific functionality. The head 1 and the umbilical 203 are also illustrated.

With this circuit 400 are associated a removable ink cartridge 131 and a solvent cartridge 141, also removable.

The reference 410 designates a main reservoir, which makes it possible to receive a mixture of solvent and ink.

The reference 110 designates a set of means that make it possible to withdraw, and potentially to store, solvent from a solvent cartridge 141 and to provide the solvent thereby withdrawn to other parts of the printer, whether it involves supplying the main reservoir 410 with solvent, or cleaning or maintaining one or more of the other parts of the machine.

The reference 310 designates the set of means that make it possible to withdraw ink from an ink cartridge 131 and to provide the ink thereby withdrawn to supply the main reservoir 410. As may be seen in this figure, according to the embodiment presented here, the sending, to the main reservoir 410 and from the means 110, of solvent, goes through these same means 310.

At the outlet of the reservoir 410, a set of means, globally designated by the reference 220, makes it possible to pressurise the ink withdrawn from the main reservoir, and to send it to the print head(s) 1. According to an embodiment, illustrated here by the arrow 250, it is also possible, by these means 220, to send ink to the means 310, then again to the reservoir 410, which enables a recirculation of ink inside the circuit. This circuit 220 also makes it possible to empty the reservoir in the cartridge 131 as well as to clean the connectors of the cartridge 131.

The system represented in this figure also comprises means 500 for recovering fluids (ink and/or solvent) that return from the print head, more exactly from the gutter 19a of the print head(s) or the rinsing circuit of the head(s). These means 500 are thus arranged downstream of the umbilical 203 (with respect to the sense of circulation of the fluids that return from the print head(s)).

As may be seen in figure 10, the means 110 may also make it possible to send solvent directly to these means 500, without going either through the umbilical 203 or through the print head 1 or through the recovery gutter.

The means 110 may comprise at least 3 parallel solvent supplies, one to the head 1, the 2^nd to the means 500 and the 3^rd to the means 310.

Each of the means described above is provided with means, such as valves, preferably electromagnetic valves, which make it possible to orient the fluid concerned to the chosen destination. Thus, from the means 110, it is possible to send the solvent exclusively to the head 1, or to the means 500 or to the means 310. Each of the means 500, 110, 210, 310 described above may be provided with a pump which makes it possible to treat the fluid concerned (respectively: 1^st pump, 2^nd pump, 3^rd pump, 4^th pump). These different pumps assure different functions (those of their respective means) and are thus different to each other, even if these different pumps may be of the same type or of similar types (in other words: none of these pumps assures 2 of these functions).

In particular, the means 500 comprise a pump (1^st pump) that makes it possible to pump fluid, recovered, as explained above, from the print head, and to send it to the main reservoir 410. This pump is dedicated to the recovery of fluid coming from the print head and is physically different to the 4^th pumping means 310 dedicated to the transfer of ink or the 3^rd pumping means 210 dedicated to the pressurisation of ink at the outlet of the reservoir 410.

The means 110 comprise a pump (the 2^nd pump) that makes it possible to pump solvent and to send it to the means 500 and/or to the means 310 and/or to the print head 1.

Such a circuit 400 is controlled by the control means described above, these means are in general contained within the console 300 (figure 9).

This circuit is well adapted to ink. For other fluids like a paint, the circuit can be adapted and will comprise for example a cartridge or can 131 of liquid, a circuit like circuit or means 310 (described above), provided with a pump, to withdraw liquid from said cartridge or can and to send it either to reservoir 410 or to the print head 1 downstream of the umbilical 203. Such a circuit is controlled by the control means described above, these means being in general contained within the console 300 (figure 9).

Claims

1. Print head of a binary continuous jet printer comprising:

- a cavity (23) for circulating jets,

- means (5, 203, 400), for producing through nozzles (9, 190, 290) a plurality of liquid jets in said cavity (23), and for forming drops by natural breaking of said jets;

- a slot (25), which opens on the outside of the cavity (23) and enables the exit of liquid drops or sections of jet intended for printing,

- a catcher (19a) for recovering drops or sections not intended for printing.

2. Print head according to claim 1, further comprising one or more deflection electrode(s) (17).

3. An assembly of print heads, comprising at least one 1^st print head according to claim 1, and at least a 2^nd print head (90₂) and a 3^rd print head (9Ο3), each comprising a cavity (23) for circulating jets, means (5, 203, 400) for producing through nozzles (190, 290) a plurality of liquid jets in said cavity (23), a slot (25), which opens on the outside of the cavity and enables the exit of drops or sections of liquid intended for printing, a catcher (19a) for recovering drops or sections of liquid not intended for printing, said 1^st, 2^nd and 3^rd print heads being fixed with respect to each other.

4. An assembly of print heads according to claim 3, said 1^st, 2^nd and 3^rd print heads being either aligned along a same 1^st axis (X) or said 2^nd and 3^rd print heads being aligned along a same 2^nd axis (Χ') parallel to said 1^st axis (X).

5. An assembly of print heads according to claim 4, said 2^nd and 3^rd print heads being aligned along a same 2^nd axis (Χ') parallel to said 1^st axis (X), the extreme left and right jets produced by said 1^st print head being on the left hand side, respectively on the right hand side, of the extreme jets produced by said 2^nd print head, respectively said 3^rd print head.

6. An assembly of print heads according to any of claims 3 to 5, said 2^nd and 3^rd print heads being according to claim 1.

7. An assembly of print heads according to any of claims 3 to 6, at least one of said 2^nd and 3^rd print heads comprising one or more stimulator(s) (13) for producing a plurality of liquid jets in its cavity (23).

8. An assembly of print heads according to claim 7, at least one of said 2^nd and 3^rd print heads comprising several stimulators (13) for producing a plurality of liquid jets in its cavity (23), the frequency of said stimulators varying, for example increasing, from one end of said print head to its other end.

9. An assembly of several print heads according to any of claims 3 to 8, wherein the diameters of the nozzles of the 2^nd and 3^rd print heads are decreasing from the most extreme nozzle to the nozzle the closest to a nozzle of the 1st print head.

10. An assembly of several print heads according to any of claims 3 to 9, wherein at least one of the 2nd and 3rd print heads comprises at least one deflection electrode.

11. Printhead or assembly according to any of claims 1 to 10, wherein at least one of the catcher(s) comprises a curved wall (27) to collect drops or sections not intended for printing, and direct them as a flow of ink along its surface to the opening (19b) of the return line (500).

12. A printing method implementing a head according to claim 1 or an assembly of print heads according to any of claims 2 to 11, wherein throughout at least part of the duration of printing, for at least part, or for any, of the nozzles of the assembly, a constant ratio is maintained between the quantity of liquid directed to the printing support and the total quantity of liquid output by said nozzle.

13. A printing method implementing an assembly of print heads according to any of claims 2 to 11, wherein throughout at least part of the duration of printing, and for at least part, or for any, of the nozzles of the assembly, a constant ratio is maintained between the quantity of liquid directed to the printing support and the total quantity of liquid output by said nozzle, the flow rate coming from nozzles of one at least a 2^nd print head (90₂) and a 3^rd print head (9Ο3), being greater than the flow rate of liquid coming from nozzles of the central series.

14. A printing method according to claim 12 or 13, wherein the ratio between the quantity of liquid directed to the printing support and the total quantity of liquid output is 1 for all the nozzles.

15. A printing method according to any of claims 12 to 14, wherein the ratio between the flow rate of liquid directed to the support and the total flow rate of liquid of nozzles of at least the 2^nd print head (90₂) and the 3^rd print head (9Ο3), is greater than the ratio between the flow rate of liquid directed to the support and the total flow rate of liquid of nozzles of the 1^st print head (90i).

16. A printing method according to any of claims 12 to 15, wherein the frequency of formation of drops from at least the 2^nd print head (90₂) and/or the 3^rd print head (9Ο3), is controlled.

17. A printing method according to any of claims 12 to 16, wherein the flow rate from the nozzles of at least the 2^nd print head (90₂) and/or the 3^rd print head (9Ο3), is decreasing from each of the nozzles of the 2^nd print head (90₂) and/or the 3^rd print head (9Ο3) which are the farthest from the 1^st print head (90i) to the nozzles of the 2^nd print head (90₂) and/or the 3^rd print head (9Ο3) which are the closest from the 1^st print head (90i).