EP0160019B1 - Ink feeding circuit for an ink jet printing head - Google Patents

Abstract

The ink feeding circuit for an ink jet printing head is particularly comprised of a first auxiliary circuit (R1) of which the function is to measure the ink pressure between the feeding line (20) and the ink supply line (18), and a second auxiliary circuit (R2) provided to clear out the ink drops (100) from the ejection orifice (41). This circuit may also comprise a third auxiliary circuit (R3) producing a signal representative of the ink viscosity. The measurement of the pressure and of the viscosity enables to servo control the main parameters governing the operation of the printers fitted with said ink feeding circuit. The invention may be applied to any ink jet printer.

Description

The invention relates to an ink supply circuit for an ink jet print head. It also concerns any printer which is equipped with it.

Several problems arise in the technique of inkjet printing, the main ones of which are recalled below. On the one hand, the orifice used for the formation of the jet is of reduced size (of the order of 75 mm) and it can happen that a dust comes to obstruct it. It is important, in this case to have a rapid means allowing the unclogging of this orifice.

On the other hand, the printing qualities of such a printer are intimately linked to the speed of ejection of the ink from the orifice. The speed can be altered by a variation in ink pressure in front of the orifice, as well as by a variation in the viscosity of the ink. This can be due to a loss by evaporation of the solvent from the ink which is often very volatile.

It is known, in particular from the German patent DE-A-3 043 260, to interpose in the recovery circuit of the unused ink drops and falling in the recovery gutter, a device capable of permanently controlling the viscosity of the ink. Such a device is based on the addition of a certain amount of solvent and fresh ink, the dosage of which is calculated as a function of the number of drops emitted by the print head.

Admittedly, such a device can overcome variations in viscosity but does not solve the problem of possible clogging of the nozzle.

The present invention aims precisely to solve all these problems and relates to a circuit for supplying ink to a printer having a simple architecture and very flexible operation which, with a minimum of components (two pumps and four solenoid valves) allows '' obtain in particular a control of the pressure and the viscosity of the ink as well as a function of unclogging the nozzle.

The invention relates more precisely to an ink supply circuit of an ink jet print head of the type comprising an ink tank (1), the bottom (ab) of which has a first outlet connected to a pipe. (18) for supplying ink to a pressurization pump (5) supplying the body (10) of the print head through a supply line (20), an inlet connected to a supply line delivery (28) through which the unused ink passes, taken from a recovery gutter (11) and sucked through a recovery pipe (27) by means of a vacuum pump (7) ; characterized in that it further comprises a second outlet connected to a wetting line (45) connected to the vacuum pump (7), and a viscometer (V) placed below the ink tank (1) which essentially consists of a container (14) comprising three electrodes (32), (33), (34) which immerse therein at different levels whose function is to detect a high level (y) and a low level (x ) which makes it possible to measure the filling time (tr) between (x) and (y) and therefore the viscosity of the ink, this viscometer cooperating with means for adding solvent when a viscosity set value considered limit has been reached.

The invention also relates to such an ink supply circuit, further equipped with a second auxiliary circuit comprising a combination of solenoid valves capable of unblocking the ink drop ejection orifice and cutting it off. blunt inkjet.

Finally, it relates to such a circuit provided with means for treating solvent vapors before they are discharged into the ambient environment.

The invention will be better understood with the aid of the explanations which follow and of the appended figures among which:

- Figure 1 schematically shows an embodiment of an ink supply circuit of a printhead according to the prior art.

FIG. 2 schematically represents an exemplary embodiment of a device according to the invention, capable of measuring the viscosity of the ink, cooperating with the pressurization circuit of FIG. 1.

For the sake of clarity, the same elements are given the same references in all the figures.

FIG. 2 therefore schematically represents an ink supply circuit for an ink jet printhead 10. Such a circuit essentially comprises a circuit ensuring the pressurization of the ink and a circuit ensuring a depressurization, intended to suck at the level of the recovery gutter 11 of the ink drops 100 not used.

The ink is stored in a sealed tank 1 which, according to the invention, includes an internal partition 22 arranged parallel to the side walls of the tank, therefore perpendicular to the bottom (ab) of the latter.

The level 38 of the ink always, less than the height 37 of the internal partition 22, evolves from this level to the bottom of the reservoir. The low level 21 of the ink is detected by means of a sensor 42. This ink is conveyed by means of a supply line 18 passing through the bottom (ab) of the reservoir 1 to a pump 5 driven by a motor 4 The flow rate of the pump 5 is proportional to the speed of rotation of the motor 4. The ink delivered by the pump 5 passes through a filter 8, a two-way pipe 190, one called the supply pipe carrying the reference 20 which routes it to the print head 10, the other called return line bearing the reference 19 which brings the ink through a calibrated leak 17 to the supply line 18.

Leak 17 creates a pressure drop proportional to the ink flow circulating in the latter.

In normal operation, the flow rate in the supply line 20 is completely negligible, compared to the flow rate of the pump 5 which passes through the leak 17. The pressure of the line 18, downstream of the leak 17, is close to atmospheric pressure, which implies by the flow generated by the leak 17 and by the pump 5, that the pipes 19 and 20 are at a pressure higher than atmospheric pressure and almost proportional to the speed of rotation of the engine 4 .

A pressure sensor 9 is interposed on the pipe 19. This sensor 9 in an exemplary embodiment comprises electrical contacts 44 and 43 delimiting the pressure zone to be respected and a feeler 450. This combination makes it possible to obtain a signal and the implementation of an associated electronic device (not shown) which permanently enslaves the speed of the motor 4 so that the pressure in the pipes 19 and 20 keeps a fixed and determined value whatever the variation of the parameters that govern it, such as internal pump leaks or improper calibration of the leak 17.

This first auxiliary circuit referenced R1 therefore ensures a stable pressure.

According to a characteristic of the invention, a vacuum circuit cooperates with the pressurization circuit described above, the function of which is to recover at the level of the gutter 11 the ink drops emitted by the head 10 and not used for the impression.

For this, a so-called vacuum pump 7 of the same type as the so-called pressurizing pump 5 is used. These two pumps 5 and 7 can be connected to the same motor 4.

The pump 7 is a positive displacement pump which normally generates a flow of liquid. It may for example be a gear pump. The ink drops 100, recovered at the level of the gutter 11, are sucked through a so-called suction pipe 27.

This result can only be obtained if the pump 7 is capable of generating an air flow and this as soon as the system is started, when the suction line 27 is still empty of ink.

This type of pump (geared for example) may have internal leaks. In the case of liquids these are negligible, but they are predominant with gases.

On the other hand, such a pump can function perfectly with a gas if its internal elements remain permanently wetted by a liquid, which then closes the leaks. According to the invention, this result is obtained thanks to the combination of means which is now described. The bottom (a, b) of the tank 1 is equipped with a so-called wetting pipe 45 which conveys through a leak 25 of the ink taken from this tank 1 to the vacuum pump 7, which has the effect to permanently wet its internal components. Under these conditions, it can fully fulfill its suction function: namely, sucking air through the pipe 27. The flow of ink from the pipe 45 is limited by the leak 25 so that it always remains below the volume flow generated by the pump 7. In this way the latter can generate a vacuum in the pipe 27, an extension 26 plugged in A is intended to be connected if necessary to a device for measuring the viscosity of the ink as will be described later. This operation of the pump 7 takes place and therefore this depression is created, even if the pipe 27 and its extension 26 contain air in high proportion.

The pump 7, permanently wetted with ink from the reservoir 1 through the leakage 25 therefore sucks in air and recovered ink from the recovery gutter 11. The ink and the air are discharged to the sealed tank 1 by a so-called discharge pipe 28 and according to a characteristic of the invention this pipe 28 is connected to a rigid pipe 24 and perpendicular to the bottom (a, b) of the tank 1, located inside compartment C delimited by the partition 22. This repressed ink obviously comes from the gutter 11 but also from the wetting line 45. Also, even if no ink flow is recovered at the level of the gutter 11, a minimum flow ink return to the sealed reservoir 1 by the rigid pipe 24 is maintained by the ink from the wetting circuit 45. This results in obtaining a constant level 37, the ink overflowing above the internal partition 22.

The air is also discharged into the sealed tank 1. It is discharged to the outside through a coil 39, a non-return valve 470 and a pipe 46 which plunges into a liquid contained in a tank 3 which traps the solvent contained in the air before being released into the open air through the orifice 36. In fact, the air recovers at the level of the gutter 11 circulates with the ink in the pipes 27 and 28, and becomes saturated with solvent especially if the latter is very volatile. The role of the coil is to condense as much as possible the solvent contained in the air, solvent which then joins by gravity the tank 1. The air cannot be completely rid of all the solvent vapors therefore crosses the bubbling tank 3 which traps these vapors by dissolution.

If the gutter 11 is blocked, the line 27 reaches the maximum depression that the pump 7 can generate. If the printer is stopped at this time, the line 27 will suck ink from the reservoir 1 which can only be replaced by the air coming from the orifice 36. The non-return valve 470 in this case prevents the return of bubbling liquid in the ink, which would imply a complete purging of the circuits.

The R2 connection circuit to the head is now described. The body 10 of this print head supporting the orifice 41 creating the jet 100 can be pressurized at the request of the operator by a solenoid valve 13 connected to the head by a pipe 47. A solenoid valve 12 called purge connects the pipe 27 of the vacuum circuit to a pipe 48 which is at the same pressure as that of the body 10.

In normal operation the solenoid valve 13 is in the open position and the solenoid valve 12 in the closed position.

The latter performs three essential functions. Firstly, after a prolonged stop of the machine, the line 20, especially if it is long, may contain degraded ink. It may be useful in this case to replace it with fresh ink coming from the reservoir 1. The flow of the ink jet, cannot allow this operation to be done quickly while the opening of the solenoid valve 12 allows to create a significant flow in the line 20, and in the body 10 and therefore to refresh the ink quickly.

Secondly, the presence of this solenoid valve 12 allows an easy uncorking of the orifice 41, if it is for example closed by a dust. The solenoid valve 13 is closed for this and the solenoid valve 12 is opened so that the body 10 is then in depression. Under these conditions, it is possible to suck solvent through the orifice 41 and thus expel the dust towards the pipe 48.

Thirdly, the function of the solenoid valve 12 is to ensure, during the procedure for stopping the printer, a frank stop of the ink jet. In fact, when the jet is operating, the pipes 47 and 48 which are under pressure, swell slightly, especially if they are made of plastic and of great lengths. When the solenoid valve 13 is closed to cut the jet, the pressure remanence in 47 and 48 means that the jet cannot be cut immediately, given its low flow rate. This inevitably leads to pollution of the environment of the jet, which is to be avoided. The original architecture of the ink supply circuit in accordance with the invention overcomes these drawbacks. When the jet is to be cut off, the solenoid valve 12 is opened shortly before the solenoid valve 13 closes. When the latter closes, the pressure persistence in the lines 47 and 48 does not may remain as a result of the prior opening of the solenoid valve 12. The latter is then closed a short time after the solenoid valve 13 is closed. The jet has cut off frankly and the environment is not soiled. The opening and closing sequence of the solenoid valves is programmed in a known manner.

FIG. 2 schematically represents an exemplary embodiment of a device capable of measuring the viscosity of the ink, called a viscometer V in the following description cooperating, in accordance with the invention, with the pressurization circuit described above

by means of FIG. 1. It is a third circuit R3.

This viscometer (V) essentially consists of a container 14 connected to compartment C delimited inside the tank 1 positioned by construction at a level higher than that of the container 14. It should be remembered that the level 37 of the liquid is kept constant in this compartment C. This container 14 has three electrodes 32, 33, 34 which plunge into it at different levels and which serve to detect a low level and a high level by electrical conduction. Remember that the ink is conductive. The ink will be above the low level (x) when the electrodes 32 and 33 are in short circuit; however the ink will be at the high level (y) when the electrodes (32) and (34) are short-circuited.

The flow of ink through line 30 makes it possible to fill the container 14, the pressure balances at the surfaces of liquid 37 in the compartment C and 40 in the container 14 is achieved by means of a pipe 35 containing only l air and solvent vapors.

The differences in altitudes between the constant level 37, the end x of the electrode 33 (corresponding to the low level) the end y of the electrode 34 (corresponding to the high level), the volume of the container 14 and the diameter of the leak 29 being known, the filling time (tr) between x and y depends only on the viscosity of the ink.

A solenoid valve 15 cooperates with the viscometer (V) in the manner described below. The solenoid valve 15 connects the bottom of the container 14 with point A, that is to say with the pipe 26 of the vacuum circuit described by means of FIG. 1.

The ink from the container 14 is then sucked through the pipe 31, which makes it possible to empty the container 14. This requires that the emptying rate in the pipe 31 is greater than the filling rate in the inlet pipe 30. When the low level is reached, the solenoid valve 15 is closed, the container 14 is refilled to allow a new measurement.

When the viscosity increases due, in particular, to a loss of solvent in the ink, by evaporation, the filling time (tr) increases. As soon as it reaches a setpoint considered as a limit, the viscosity is then corrected by adding solvent.

For this, a solenoid valve 16 is provided to connect a reserve 2 containing pure solvent with point A, that is to say the line 26 which is in depression. This solvent is then sent by the pump 7 to the tank 1.

It is also possible to permanently slave the operation of the solenoid valves 15 and 16 as a function of the parameters collected at the level of the electrodes 32, 33 and 34 of the viscometer V and therefore to permanently correct the viscosity of the ink by adding solvent when this is necessary.

The sealed container 2 is connected to the pipe 35 which allows the solvent to be replaced by air without bringing it into contact with the ambient air, which has the great advantage of avoiding the risks of environmental pollution, in particular undesirable odors. It should be noted that the air which replaces the liquids used such as the ink in the reservoir 1, and the solvent in the container 2 only comes from the recovery gutter 11. The excess air which must exist, s' chappe through the aperture 36 after bubbling in the reservoir 3. The containers 1 and 2 are therefore in slight overpressure due to the height of liquid in the container 3.

As in the case of FIG. 1, the coupling circuit of the two solenoid valves 12 and 13 is provided and operates as described above.

• - l'obtention d'une pression stable;
• - le contrôle de la viscosité de l'encre;
• - le débouchage de l'orifice d'éjection du jet d'encre;
• - l'arrêt franc du jet.

An inkjet printer equipped with an ink supply circuit according to the invention makes it possible to solve the main problems posed by this type of machine, namely in particular:

• - obtaining a stable pressure;
• - control of the viscosity of the ink;
• - the unblocking of the ink jet ejection orifice;
• - the free stop of the jet.

All these results are obtained by means of a simple device, easy to implement by means of only two pumps and four solenoid valves.

The invention applies to any type of inkjet printer.

Claims (9)

1. Ink feed circuit of an ink jet print head, of the type comprising an ink reservoir (1), the base (ab) of which comprises a first outlet connected to a pipe system (18) to conduct the ink towards a pressurizing pump (5), feeding the body (10) of the print head through a feed pipe system (20), an inlet connected to a back-flow pipe system (28) through which travels the unused ink which is taken up at the level of a recovery trough (11) and sucked through a recovery pipe system (27) by means of a pump (7) which causes a depression, characterized in that it further comprises a second outlet connected to a wetting pipe system (45) connected to the pump (7) which causes a depression, and a viscosimeter (V) placed below the level of the ink reservoir (1), which is essentially constituted by a vessel (14) comprising three electrodes (32), (33), (34) which are immersed therein at different levels, and the function of which is to detect an upper level (y) and a lower level (x), which allows the filling time (tr) to be measured betwen (x) and (y) and hence the viscosity of the ink, this viscosimeter co-operating with means for the addition of solvent when a prescribed value of the viscosity, which is considered as the limit, is reached.

2. Feed circuit according to Claim 1, characterized in that these means for the addition of solvent are constituted by a set of valves (15) and (16), arranged such that when the filling of the vessel (14) through the pipe system (30) is completed, the electrovalve (15) connects the base of the vessel (14) with the point (A) situated on the pipe system for creating a depression (26 - 27). The vessel (14) then empties through an emptying pipe system (31); the electrovalve (15) therefore being closed, the vessel (14) may fill up again.

3. Feed circuit according to one of Claims 1 and 2, characterized in that when the prescribed value is reached, the electrovalve (16) connects a reserve (2) containing pure solvent with the point (A), such that solvent is then conveyed by the pump (7) towards the reservoir (1).

4. Feed circuit according to one of the preceding claims; characterised in that the reserve (2) of solvent, being fluid-tight is connected to a pipe system (35) which is common to the reservoir (1) and to the vessel (14), such that the solvent is replaced by air without being in contact with the ambient air, this air originating from the trough through the pump (7).

5. Feed circuit according to one of the preceding claims, characterized in that the surplus air escapes through an orifice (36) after bubbling in a reservoir (3).

6. Feed circuit according to one of the preceding claims; characterized in that it further comprises means able to ensure the unstopping of the orifice (41) for the ejection of drops of ink, and the clean cut-off of the jet.

7. Feed circuit according to Claim 6, characterized in that these unstopping means are constituted by a set of electrovalves (12) and (13) fulfilling three functions: on the one hand the opening of the electrovalve (12) allows a large flow to be created in the body (10) and thus ensures a purging of ink; on the other hand the closing of the electrovalve (13) and the opening of the electrovalve (12) allows a suction of solvent added by the operator at the level of the ejection orifice (41), which suction ensures an unstopping of this orifice; finally, at the moment of cut-off of the jet, the opening of the electrovalve (12) before the closing of the electrovalve (13) prevents the retention of pressure in the pipe systems (47) and (48) connecting the set of electrovalves (12) and (13) to the body (10), thus ensuring a clean cut-off of the jet.

8. Feed circuit according to one of the preceding claims; characterized in that it further comprises means capable of measuring the pressure existing in the feed pipe system (20).

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