WO2012038520A1

WO2012038520A1 - Binary continuous ink-jet printing device, with reduced consumption of volatile compounds such as solvents.

Info

Publication number: WO2012038520A1
Application number: PCT/EP2011/066552
Authority: WO
Inventors: Bruno Barbet
Original assignee: Imaje SA; Markem Imaje SAS
Current assignee: Markem Imaje SAS
Priority date: 2010-09-24
Filing date: 2011-09-23
Publication date: 2012-03-29
Anticipated expiration: 2013-03-24
Also published as: FR2965215A1

Abstract

The invention relates to a solution for limiting the consumption of volatile compounds, such as solvents, initially present in ink during the ejection of the latter in a binary continuous ink-jet printer. According to the invention: - it is selected an ink which volatile compounds have a Schmidt number Sc equal to one or superior; - the gutter (7) for recovery and collection of ink not intended to print and issued from a jets curtain is judiciously positioned to swallow the air flow laden by vapour of volatile compounds entrained by the ink jets curtain; - a depression level is set in the gutter to remove the volumetric flow of the swallowed flow of air; - air is injected in the vicinity of the nozzle plate of the print head to compensate recovered volumetric air-flow.

Description

BINARY CONTINUOUS INK-JET PRINTING DEVICE,

WITH REDUCED CONSUMPTION OF VOLATILE COMPOUNDS SUCH AS SOLVENTS. DESCRIPTION

TECHNICAL FIELD

The invention relates to a binary continuous ink-jet printing device.

It relates to a solution which allows a reduction in the consumption of volatile compounds, such as solvents initially present in the ink.

It relates more specifically to printers which include this type of printing device and which are used in the field of industrial printing.

PRIOR ART

The fundamental principle behind an ink-jet printer is that it forces liquid to pass through calibrated orifices. The liquid is usually ink made up of several components (resins, colouring agents, salts etc.) and mainly of one or more solvents which are by their nature volatile.

Two major groups of printing technologies are based on this concept:

- drop-on-demand technology in which ink is stored in a liquid circuit called the ink circuit and is in contact with the surrounding air only at the ejection nozzles. The liquid is ejected directly through the nozzles in the form of drops. This means that the exchange surface area between the drops of liquid and the surrounding air is usually very small. As a result the problems associated with the loss of volatile compounds, with denaturing of the ink and with pollution/contamination of the environment are minimal.

- continuous jet technology involves maintaining ink under pressure so that the liquid (essentially ink) forms one or more continuous jets downstream of the nozzle plate (a plate in which the calibrated orifice (s) is/are formed) . In order for printing to take place, the continuous jet(s) is/are then continuously or intermittently broken up into drops The drops intended for printing are directed towards the media to be printed and the remainder of the liquid is collected by a return gutter which is usually at a sub- atmospheric pressure in order to draw the collected liquid through an umbilical towards the printer's ink circuit (for recycling, re-processing etc.) . The continuous jet technology has to deal with the problem of managing the products of the evaporation of the liquid not only in order to guarantee the composition and the proportions of the constituents in the ink, but also to minimise the production of volatile species in the environment, as well as the means of reprocessing the returned ink. This problem is in fact present in deviated continuous jet technology (usually based on a few jets) but it becomes of prime importance in binary continuous jet technology (multiple jets, usually several tens of jets) . In effect the phenomenon of evaporation of volatile species is amplified in this binary continuous jet technology because of its high level of productivity: in effect, in order to guarantee rapid drying of the printing that is carried out, inks are selected which, although they vary greatly in their composition, usually have a volatile solvent base (and other volatile compounds) .

In continuous jet technology, the evaporation of volatile species (solvents or other compounds) mixed with the ink mainly takes place in two zones :

- inside the printing head, where the losses through evaporation are significant. In this first zone, the ejected ink is continually replaced and driven at a speed of several metres to several tens of metres per second, which produces a strong ventilation effect on the free surface of the jet(s);

- inside the recovery gutter and the umbilical (the name usually given to the flexible connection system which includes the electrical input cables and liquid line and which links the body of the printer to the print head: the term "umbilical" used in the context of the invention therefore refers to the liquid line of the flexible connection system) . In this second zone, the diphasic mixture (ink-air) is collected and drawn into the gutter and undergoes stirring in the umbilical which promotes evaporation until the air (drawn in) is saturated with volatile species coming from the liquid ink.

The prior art only offers techniques for recycling the air coming from the gutter through the umbilical: the printer ink circuit is designed so that liquid is separated from the gas (air laden by vapour) coming from the umbilical. The gas laden by volatile compounds is sent back to the printing head, that is, just upstream of the ink recovery gutter (as proposed in patent EP 0 123 523 by the WILLETT INTERNATIONAL company) or into the printing head (as proposed in patent EP 0 560 332 from the HITACHI LTD company) . The technical solution common to these patents therefore involves recycling the gas to form an almost closed circuit made up of the head, the umbilical and the ink circuit.

These patents do not deal with the problem of solvent (or other volatile compounds) lost by evacuation at the outlet of the printing head. In effect, as stated above, losses (or consumption) of volatile compounds occur through evaporation inside the printing head through the ventilation effect of the continuous jet(s) . These losses are even more significant in binary continuous jet technology in which the volumetric air flow caused by the multiple numbers of jets is greater.

Therefore, ideally, in order to minimise or even eliminate losses by the removal of solvents (or other volatile compounds) which have already evaporated inside the printing head, the following two contradictory design needs must be reconciled:

- in order to allow the passage of drops which are intended for printing onto the media to take place, the printing head must be equipped with an opening towards the external surroundings, aligned with the continuous jet or jets; this is one of the paths favoured for the evacuation of air entrained by the continuous jet or jets;

- in order to minimise the exchange between the interior of the head and the external surroundings where the media to be printed is found, it is a priori essential to confine the head (closed and sealed head) .

Until today, no solution has been found which controls the exchanges between the interior and the exterior of the printing head through the outlet slot or opening which allows the final printed drops to pass through, in terms of limiting or even eliminating the consumption of solvents which have already evaporated inside the printing head.

The inventor has thus searched a solution for recovering the solvents, or other volatile compounds, inside the print head. He has concluded that such a solution could not be implemented in a print head according to the deviated continuous jet technology: indeed, in that case, a print head includes necessarily electrodes (charge electrodes and deflection electrodes downstream said charge electrodes) into its cavity through which the ejected ink jet goes. These electrodes imply a certain encumbrance. In fact, the presence of these electrodes causes turbulences of air containing solvent (s) or other volatile species around the ink jet. In other words, the solvent (s) or other volatile species can be agitated only in a random way, which renders impossible their recovery.

The general purpose of the invention is thus to propose a solution for recovering the volatile compounds, such as solvents initially present in ink, during ejection of the jets (curtain) of the latter and in the interior of a print head of a binary continuous jet printer. A specific purpose is to propose such a solution that is simple and effective.

PRESENTATION OF THE INVENTION

To this end, the invention relates to a recovery method of volatile compounds initially present in an ejected ink under the form of a curtain in a print head of a binary continuous ink-jet printer, comprising:

• an ink drop generator whose lower edge includes a nozzle plate including multiple nozzles adapted to the ejection of a curtain of jets simultaneously both along the first direction Z and along a second direction X perpendicular to the first direction Z,

• a sorting block, arranged downstream of the nozzle plate and offset in relation to the nozzle(s), along a third direction Y perpendicular to the first direction Z, and to each jet, where the sorting block includes means for selective deviation of the drops issued of the break-up of the jets,

· one gutter which includes a single recovery tray whose inlet includes a apex which is offset from the sorting block in a third direction Y, arranged downstream of the sorting block and at a set height L from the lower edge of the nozzle plate in the first direction Z and which extends along the second direction X in order to recover the ink issued from the jets curtain not to be used for printing.

According to the invention, the method comprises the following steps: a/ selecting an ink which volatiles compounds have a Schmidt number of the order of 1 or more;

b/ adjusting the distance which separates the jets curtain from the gutter apex along the third direction Y such that it is equal to at least the air thickness δ₂ entrained by the jets curtain, where the

thickness satisfies the equation 6₂ = , where Vj is the speed of the jets curtain, a is a numerical coefficient between 3 and 5, typically 3; v_a is the kinematic viscosity of air equal to 2.1CT⁵ m².s^~1;

c/ adjusting the depression that is maintained within the gutter during the ejection of a jets curtain not intended for printing, to a value which allows both the drops from the jets and the volumetric flow of the air entrained by the ink jets curtain at the inlet of the gutter to be evacuated;

d/ introducing air close to the nozzle plate to compensate for the volumetric air-flow recovered and evacuated through the gutter, steps c/ and d/ being such that when the distance along the third direction Y between the jets curtain and the sorting block is:

- equal to at least 6₂ over the height L, then the volumetric flow of air entrained by the jets curtain and removed through the gutter is, per unit width of the printing head in the second direction X, equal to at least Q_VT =Vj x 6₂.

- less than 6₂ over the height L, then the volumetric flow of air entrained by the jets curtain and discharged through the gutter is, per unit width of the printing head in the second direction X, equal to at least Q_VT =V_j x [ (δι+δ₂ ) /2 ] , where δι is the average distance over the height L separating the jets curtain from the sorting block at the gutter apex in the third direction Y.

In the context of the invention, the first Z, second X and third Y directions define a three- dimensional system of three axes which are perpendicular in pairs .

In the context of the invention it must be understood that the air entrained by the ink jets (curtain) is the layer of air which may be laden by the volatile compounds and which surrounds it or them.

The purpose of the invention is therefore to reduce losses of solvent and other volatile species in a binary continuous jet printing head. According to the invention, simple and effective means are implemented in order that volatile compounds produced by evaporation from a curtain of ink jets are collected and evacuated without escaping from the printing head. The means of the invention thus implemented are used to collect both the ink which has to be recycled and the vapour from the ink jets without modifying the outlet opening or slot through which the ink drops intended to print emerge.

The inventor has thus identified a solution to the problem of high levels of solvent consumption by using means which are already implemented in a binary continuous ink jet printer: only the position of the inlet of the gutter or the dimensions of its inlet opening are altered and the level of the sub-atmospheric pressure is adjusted so that the air flow collected is in some way collected. In order to determine the position of the inlet of the gutter or the dimensions of its inlet opening, the inventor has first analyzed the physical mechanisms for volatile compounds, such as solvents, when evacuated by a jets curtain. He thus considered that two transport mechanisms act simultaneously and influence the travel of volatiles compounds:

- the molecular diffusion mechanism according to which volatile compounds present in the jets curtain may escape radially to this latter towards the surrounding air (concentration gradient effect) in a diffusive boundary layer. This diffusive boundary layer is characterized by the molecular diffusion coefficient, also called mass diffusive coefficient,

- the hydrodynamics draw mechanism according to which the jets curtain draws the surrounding air in a hydrodynamics boundary layer. This hydrodynamics boundary layer is characterized by the dynamic viscosity .

The ratio between the dynamic viscosity and the mass diffusive coefficient is defined by the dimensionless number called Schmidt number Sc.

Consequently, in function of the prevalence of one boundary layer compared with the other, it is possible to distinguish the three different cases as follows :

1/ the volatile compounds are confined at the surface of the jets curtain. It does that mean that the Schmidt number Sc is superior or very superior to one, 2/ the hydrodynamics boundary layer increases as fast as the diffusive boundary layer (migration of volatile compounds) . It does that mean that the Schmidt number Sc is of the order of one,

3/ the diffusive boundary layer increases very quickly in the radial direction of the jets curtain and pierces namely the hydrodynamics boundary layer. It does that mean that the Schmidt number Sc is inferior or very inferior to one.

The inventor has thus concluded that it would particularly efficient to intercept the air flow drawn by the jets curtain if the ink has volatile compounds with a Schmidt number of the order of one.

Otherwise said, it has to be checked that the used ink transports volatile compounds (solvents) which diffusion is substantially according to the same asymptotic profile of boundary layer as the air hydrodynamics boundary layer drawn by the curtain of ink jets .

In other words, the solution according to the invention involves:

- providing either a sufficiently distant gutter position;

- then adjusting the level of the depression maintained in the recovery gutter in order, on the one hand, to collect then evacuate practically all the boundary layer of air laden by volatile compounds (solvents) entrained by the jets curtain.

To determine the positioning of the gutter entry, the inventor used the rules for carrying out fluid calculations on boundary layers of air which surround a liquid, and which are disturbed (Couette profile) by a physical component (sorting block) or not (Blasius profile) .

It should be noted at this point that the boundary layer of air entrained by a jet exhibits, in accordance with the theory for fluid dynamics, an asymptotic profile, and that consequently collection by means of the gutter which is the aim of the invention cannot achieve a complete collection of the air-flow.

As is detailed below, the layer of entrained air may have an identical profile or different profile on either side of one jet, depending on the printing head environment close to each jet; that is, depending on the obstacles or physical barriers encountered by the air along its journey from the nozzle to the inlet of the gutter.

In other terms, the layer may also exhibit:

- a Couette profile on one side of the jet and a Blasius profile on the other side of the jet: this may be the case where the distance between the curtain of jets and the sorting block is such that the air comes into contact with the block of electrodes;

- or a Couette profile on either side of the jet: this may be the case with an environment which disturbs the layer of air entrained by the curtain of jets on either side of it, that is, a layer of air in contact both with the sorting block and with another physical element causing an obstacle on the other side of the curtain of jets in a given plane.

Even if, the invention is less interesting for volatile compounds with a Schmidt number very superior to one because there is no necessity to control their consumption in that case, the collection of the hydrodynamics boundary layer by the gutter implies also the collection of volatile compounds.

The invention is particularly efficient with inks containing alcohol and/or ketone as solvents. Indeed, the alcohol has a Schmidt number Sc of the order of 1.4. Ketone has a Schmidt number Sc in the order of 1.7.

Preferably, step b/ is achieved such that the distance between the gutter apex and a jets curtain recovered by said gutter is along the third direction Y at least equal to 380pm, preferably at least 700pm.

Preferably, the height L between the lower edge of the nozzle plate and the apex is comprised between 7 and 14 mm whilst the speed Vj of the jets curtain is comprised between 10 m/sec and 16 m/sec.

The invention concerns also a print head such as described above in which the recovery gutter is adapted to be displaced at least along the third direction Y.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will be more apparent at the reading of the following description with reference to the appended drawings, which are given for illustrative purposes only and are in no way restrictive. Of these drawings:

- Figure 1 is a general schematic view of a printing head in accordance with the invention and which implements binary continuous jet technology, - Figure 2 is a side schematic view of a printing head in accordance with existing technology and which implements binary continuous jet printing technology,

- Figure 3 is a side schematic view of a printing head in accordance with the invention and which implements binary continuous jet printing technology,

- Figures 4A and 4B show a side schematic view of the adjustment steps to implement the invention in a binary continuous jet printing head.

DETAILED DESCRIPTION OF SPECIFIC CONSTRUCTION OPTIONS

A printing head which implements binary continuous jet printing technology and which is in accordance with the invention includes a generator known as a drop generator 1 equipped with a nozzle plate 2. In the printing phase, the ink 3 pressurised in the generator 1 flows, through means for stimulation, through the calibrated nozzles 4 to form a jets curtain. Downstream of the nozzle plate 2, along the direction of flow of the ink (along the Z axis) there is a sorting block 5 which includes a functional part 6 with electrodes. The function of this part with electrodes 6 is to place the portions of jets formed by break-up into different trajectories, one volume of which is collected by the recovery gutter 7 (recycled and non-printed ink), whilst the other is directed towards the media to be printed 8 (figure 1) . These printed drops are generally spherical and follow the fluid trajectory of each jet (direction given by the axis of a given nozzle) . The reverse configuration could also be envisaged: the printed drop is deviated and the non-deflected drop is collected by the gutter.

The terms «lower» and «upper» are to be considered with the printing head pointing downwards (direction of flow of jets along z), that is with the generator 1 above partly in line with the electrode block 5 according to the invention.

The terms «upstream» and «downstream» are relative to the direction of flow of the jets of liquid (ink with any volatile compounds) .

The three directions X, Y and Z defined according to the invention form a three dimensional axis system with the directions perpendicular to each other in pairs .

As shown schematically in figure 2, a jets curtain 10, operated at a speed Vj , typically of the order of 12 m/sec (generally between 10 and 20 m/sec in a binary continuous jet technology printer), entrains the surrounding air from the nozzle 4 plate 2 to the recovery gutter, otherwise called the recycling gutter 7. At the nozzle plate 2, the jets create a draught that is fulfilled by the air 11 coming from an injection zone Z.I. which is preferentially located close to the nozzle plate. The air driven at a speed which is close to that of the jets 10, remains confined very close to the jets curtain. Along the direction of the movement of the jets, the entrainment effect (under the effect of viscosity) spreads on either side of the jets curtain so that it is at maximum width at the gutter 7. In combination with this entrainment effect, the air becomes laden by solvent vapour and vapour of the various volatile species, as shown schematically in 13, 14, coming from the liquid surface of the jets deflected (see parts 12) by the electrode part 6. The physical phenomenon by which the air becomes laden by solvents or other volatile species is due to an evaporation phenomenon .

The inventor has revealed the following phenomenon. In a printing head which is in accordance with existing technology, as shown schematically in figure 2, by its construction the air transported by the jets curtain 10 is principally divided into two distinct flows :

- a flow 13 drawn in by the gutter 7, the vapours in which may be recycled since they are collected and removed by the said gutter;

- a flow 14 in some way split by the apex 15 of the gutter and which follows a trajectory along the direction of the printed drops 16 and through their outlet slot.

The sum of these two flows 13, 14 may reach a volumetric air flow of several hundred litres per hour. Thus, according to the state of the art, the flow of air 14 split off by the apex 15 and which is laden or even saturated by solvent is not recycled via the gutter 7: this loss may represent a level of consumption of several tens of cm³/hr of liquid volatile solvent such as MEK, widely used in industrial ink-jet printing. Furthermore, such losses cause pollution of the atmosphere outside the printing head, or in other words the external environment of the media to be printed 8. In order to remedy this drawback involving consumption and pollution, the inventor has judiciously- thought to position the gutter (whose prime function is to recover and remove the ink) and to adjust its depression level, so that it intercepts and removes all the air laden by solvents (or other volatile species) which is transported by the jets curtain 10.

The inventor considers that an ink containing alcohol and/or ketone is particularly appropriated.

Figure 3 shows the solution according to the invention with jets curtains 10, 12. This figure shows that the part of the jets 12 deflected by the sorting block 6 (electrodes for example for electrostatic action) , drops intended to print and produced by the break-up of the jet 10 upstream are not shown.

Here the level of depression in the gutter 7 is adjusted so that the volumetric flow of air drawn in by the latter is at least equal to the volumetric flow of the air transported by the jets curtain 12. This flow is estimated approximately in the following manner (at the gutter 7, the flow regime is regarded as being established, that is, stationary and laminar) . It is known first of all that the flow of air 17 entrained by the jets curtain 10 upstream of the gutter 7 may be shown schematically as two distinct zones on either side of the jets curtain 10, respectively zone 1 and zone 2, that is, respectively, to the left and to the right of the curtain 10 in accordance with the convention in figure 3. Here it can be considered that the spacing between jets 10 is less than the value of the thickness of the boundary layer 17 of entrained air so that the air located between two adjacent jets moves at the same speed as the jet irrespective of its distance along the Z direction.

More precisely, the zone 1 is located between the jets curtain 10 and the sorting block 6 which forms a physical barrier to the flow of air 17. In this zone 1, the speed of the air at the surface of the jet varies more or less linearly from 0 in contact with the face 18 (face opposite the jets curtain) of the sorting block 6 to a speed Vj (typically of the order of 12 m/sec), with a co-called Couette profile 19 along direction Y.

Thus the unit volumetric air flow in zone 1, Qvi, that is, per unit width along direction X, may be calculated as follows:

where δι represents the average distance between the face 18 of the electrode block 6 and the jets curtain 10. This value is typically of the order of 300 pm.

Zone 2 is located outside the jets curtain 10 on the side not facing the latter. In this zone 2, the speed of the air changes from Vj to a zero speed as it moves away from the jets curtain 10 along the direction Y: the decrease in the speed of the entrained air schematically follows a so-called Blasius profile 22

The thickness 6₂ of the boundary layer 17 (air flow in which the air is moved at a significant speed) may be calculated as follows:

where a represents a numerical coefficient of between 3 and 5, typically 3, that is, the thickness 6₂ represents between 90 and 99 % of the thickness of the boundary layer,

v_a is the kinematic viscosity of the air, typically equal to 2.1CT⁵ m².sec^_1,

L is the distance separating the nozzle plate 2 from the gutter input, that is, precisely between the lower edge 20 of the nozzle plate 2 and a horizontal passing through the apex 15 of the gutter 7.

In order to determine the coefficient a and therefore the thickness 6₂ which corresponds to a value of between 90 and 99% of the boundary layer, those working in this field may quite naturally turn to a text book on fluid mechanics, such as that entitled « AN INTRODUCTION TO FLUID DYNAMICS », G.K. Batchelor, page 311, 1970 edition - Cambridge Press.

Typically, L is equal to 10 mm, which gives 6₂ = 380 μπι for a jets curtain 10 speed of the order of 12 m/ s .

Thus the unit volumetric air flow in zone 2, Qv₂ (per unit width along direction X) may be calculated as a first approximation, as follows:

Qv2 = (Vj / 2) x δ₂.

The total volumetric air-flow rate Qv_T of air entrained by the jets curtain 10 both in zone 1 and zone 2 and collected by the gutter 7 (per unit width of the printing head along direction Y) is therefore equal to :

Thus, using all the numerical values given above (Vj = 12 m/s; δι = 300 μπι; δ₂ = 380 μπι) , for a printing head of the order of 1 inch wide, a value of the total volumetric flow of air Q entrained by the jets curtain 10 and collected by the gutter 7 which is of the order of 370 litres per hour is obtained.

The apex 15 of the gutter is then positioned so as to intercept and collect the entire flow of air entrained by the jets curtain 10.

This gives a distance along the direction Y, d = δι + δ₂. With the numerical values given above, this gives a value equal to about 680 pm (300 + 380 pm) .

In both the embodiment shown, the volumetric air flow Q drawn in by the gutter 7 (and therefore removed from the printing head) is compensated for by a natural or forced addition of air, introduced close to the nozzle plate. An air injection zone Z.I. could therefore be sized close to the nozzle plate 2 in order to achieve the desired equivalent addition of flow.

Figures 4A and 4B show the adjustment steps for implementing the invention in a binary continuous ink jet printing head.

The drop generator 1 includes multiple nozzles 4 allowing a jets curtain to be formed by pressurization of the ink. In the nominal position and in the absence of deflection (absence of a voltage in the electrodes part 6), the jets curtains 10η is said to be in the hydraulic position as the ink jets are guided individually by each nozzle 4 (section view in figure 4A) . The jets curtain 10η is the nominal point at distance YO along direction Y.

According to the invention, so-called «factory settings» can be used to ensure that the thickness δ₂ of the boundary layer of air 17 outside (in the positive direction Y) the deflected jets curtain 10 in printing operation is effectively collected by the gutter 7 made up of a single tray.

These «factory settings» usually involve a calibration of the electrical potential applied to the deflection electrodes 6 and of the position of the apex 15 of the gutter 7 in the Y direction. The calibrations must therefore compensate/allow for:

- the chain of functional dimensions between the various mechanical elements of the print head (generator 1 with nozzle plate 2, sorting block with electrode part 6, gutter 7);

- any spread of the orientation between one jet and another in the same curtain 10: indeed, although the mechanical alignment of the various nozzles 4 by drilling is practically defect free, it could be that the orientation of a jet ejected from a given nozzle is not strictly identical to that of another jet ejected from another nozzle.

Thus, for a given desired jet ejection speed Vj (which corresponds to the speed of the printed drops) :

- an electrical potential is usually applied (voltage level) to the deflection electrodes 6 so as to deflect the jets with an amplitude of value Dl, typically 530 pm in the negative Y direction. This amplitude of value Dl, typically 530 pm, is the nominal value for the deflection checked on an optical bench (display assembly: monitor-zoom-camera-micrometer translation movement units) with a measurement precision better than 50 pm;

- the position of the gutter 7 in the Y direction is adjusted using a device that is not shown whose function is to move the gutter forward (movement along Y - positive direction) or retract it (movement in the Y direction - negative direction) . Through its construction, the stroke of the gutter 7 can be calibrated so that all the deflected ink-jets are intercepted by the gutter 7 and so that the apex 15 no longer intercepts any intended to print which may be dispersed from one jet to another of the same curtain. To do this one can initially cause the gutter apex 15 to be tangential to the jets curtain which undergoes no deflection (figure 4A) . Concretely, this tangential point may readily be seen by an operator since the touch contact between the jets curtain 10 and the apex 15 slightly soils the latter.

So considering the required speed Vj of the jet 12 and the thickness 6₂ of the boundary layer of entrained air, one can further check that the factory setting of the gutter 7 is satisfactory for the collection required by the: in other terms, the movement stroke of the gutter 7 is adjusted so that the distance D2 between its inlet apex 15 and the dimension Y0, typically 150 pm, allows air of thickness 6₂ (which corresponds to the difference D1-D2 ^ 6₂) to be collected . The invention provides a technical solution to a problem which has received little or no attention in the existing technology, whilst offering the following advantages:

^ the ability to recover all or part of the volatile species emitted by evaporation from the ink jet(s) in the printing head;

^ the absence of additional design constraints for the outlet slot through which drops intended for printing pass;

^ the collection and evacuation of the air that is made to move continuously by the ink jets through the gutter as part of the continuity of movement that is created; this flow does not generate any turbulence which might destabilise (add noise to) the flight of drops intended to print;

^ the option of further treatment of the air collected in this way and removed using ad hoc means (condensation device, for example);

a particularly easy implementation of the invention since it does not require any additional components to the print-head, no volume constraints, no additional active subsystem (blower systems etc.) . The cost of the function is low for maximum effectiveness;

^ denaturing or changes to the physical properties of the ink associated with the loss of less volatile compounds: the usual devices in an ink-jet printer which, in the event of modification of the printed ink, act on the ink quality/composition through a feedback system, are therefore less stressed; ^ reduced operating costs for ink-jet printers because of lower solvent consumption and a better maintained environment (for the operator, the object to be printed etc.) .

Claims

1. A recovery method of volatile compounds initially present in an ejected ink under the form of a curtain in a print head of a binary continuous ink-jet printer, comprising:

• an ink drop generator (1) whose lower edge (20) includes a nozzle plate (2) including multiple nozzles (4) adapted to the ejection of a curtain of jets (10) simultaneously both along the first direction Z and along a second direction X perpendicular to the first direction Z,

• a sorting block (6), arranged downstream of the nozzle plate (2) and offset in relation to the nozzle(s), along a third direction Y perpendicular to the first direction Z, and to each jet, where the sorting block includes means for selective deviation of the drops issued of the break-up of the jets,

• one gutter (7) which includes a single recovery tray (7) whose inlet includes a apex (15) which is offset from the sorting block in a third direction Y, arranged downstream of the sorting block and at a set height L from the lower edge (20) of the nozzle plate in the first direction Z and which extends along the second direction X in order to recover the ink issued from the jets curtain not to be used for printing, the method comprising the following steps:

a/ selecting an ink which volatiles compounds have a Schmidt number of the order of 1 or more;

b/ adjusting the distance which separates the jets curtain (10) from the gutter apex (15) along the third direction Y such that it is equal to at least the air thickness δ₂ entrained by the jets curtain where the thickness satisfies the equation 6₂ = oc

where Vj is the speed of the jets curtain, a is a numerical coefficient between 3 and 5, typically 3; v_a is the kinematic viscosity of air equal to 2.1CT⁵ m².s^~1;

c/ adjusting the depression that is maintained within the gutter during the ejection of a jets curtain not intended for printing, to a value which allows both the drops from the jets and the volumetric flow of the air entrained by the ink jets curtain at the inlet (15) of the gutter (7) to be evacuated;

d/ introducing air close to the nozzle plate to compensate for the volumetric air-flow recovered and evacuated through the gutter, steps c/ and d/ being such that when the distance along the third direction Y between the jets curtain (10) and the sorting block (6) is :

- equal to at least 6₂ over the height L, then the volumetric flow of air entrained by the jets curtain (10) and removed through the gutter (7) is, per unit width of the printing head in the second direction

X, equal to at least Q_VT =Vj x 6₂.

- less than 6₂ over the height L, then the volumetric flow of air entrained by the jets curtain

(10) and discharged through the gutter (7) is, per unit width of the printing head in the second direction X, equal to at least Q_vT =Vj x [ (δι+δ₂ ) /2 ] , where δι is the average distance over the height L separating the jets curtain (10) from the sorting block at the gutter apex in the third direction Y.

2. A method according to claim 1, in which it is used an ink containing alcohol and/or ketone as solvent .

3. A method according to claim 1 or claim 2, in which step b/ is achieved such that the distance between the gutter apex (15) and a jets curtain (10) recovered by said gutter is along the third direction Y at least equal to 380pm, preferably at least 700pm.

4. A method according to any preceding claim, in which the height L between the lower edge (20) of the nozzle plate (2) and the gutter apex (15), is comprised between 7 and 14 mm whilst the speed VJ of the jets curtain (10) is between 10 m/sec and 16 m/sec.