EP0568419B1 - Optimisation method for operating an ink-jet printer and printer implementing the method - Google Patents

Description

The present invention relates to a method for optimizing the operation of an inkjet printer, as well as to a printer using such a method.

In a deflected continuous inkjet printer, the technique consists in carrying out a continuous jet of calibrated drops of ink projected by a print head, these drops then being electrostatically charged so that some are deflected by an electric field. The printing device and the support to be printed on being in relative displacement, one can thus obtain a matrix of dots printed on the support. The unused ink drops are collected in a gutter and then recycled in the ink supply circuit of the printer.

However, the recovery of the ink in the gutter is a critical point in the operation of an inkjet printer, in order to avoid any smearing on the support causing a degradation of the print quality. In the many applications of this type of printer, including the industrial marking of bar codes or logos, it is necessary to obtain impeccable print quality.

Currently, there are several solutions for getting ink from unused drops from the gutter to a recovery tank.

An accumulation of ink in the immediate vicinity of the gutter allows the ink to be recovered in the liquid phase, independently of the air which tends to enter the gutter between two drops collected or by even entrainment of said drops. The intermediate ink deposit in the vicinity of the gutter can be done by gravity, but a problem arises when the print head operates in all directions of space. Another drawback of this solution lies in the risk of overflow of the gutter, filled with ink, at the slightest error in the operation of the printer.

A second technique consists in drawing in the ink accumulated in the gutter by the Venturi effect, following a restriction of the pipe immediately downstream of the opening of the gutter. A driving fluid is injected into the gutter in the manner of a hydro-ejector, but the operating point of such a system is closely linked to the ambient operating conditions of the printer, that is to say the ink temperature and pressure.

According to a third technique, the ink recovered in the gutter is sucked up by means of a pump which discharges this ink into a recovery tank. For this, a depression is created in the gutter, slightly greater than that necessary for the entrainment of the ink. A drawback stems from the increase in the quantity of air sucked in relation to the quantity of ink sucked into the two-phase liquid-gas mixture present in the gutter, causing an oversizing of the recovery circuit. In addition, the optimal operating point of this system is often unknown and variable depending on the temperature and pressure conditions. The two-phase flow of an incompressible fluid causes discontinuous and random pressure losses along the recovery pipe: local accumulations of liquid, expansion of gas .... This does not make it possible to predict, thanks to a model, the behavior of the ink and therefore the pressure losses to be compensated between the inlet of the recovery pipe at the outlet of the gutter and the reservoir of recovery proper.

Document FR-A-2 545 042 discloses an ink jet pressurization device which applies to continuous ink jet printers. This device comprises three elements of the ink circulation circuit: a section corresponding to the supply circuit, a section corresponding to the recirculation circuit of the ink recovered in a gutter and a section used to bring the ink by gravity. . It uses an ink supply pump from an ink tank and a recirculation pump for returning the recovered ink to the ink tank.

The present invention aims to solve the drawbacks mentioned in connection with the previous solutions, by proposing a method of extracting a two-phase mixture at the lowest possible suction level, compatible with full recovery of the liquid phase.

For this, the object of the invention is a method of optimizing the operation of an inkjet printer, comprising an ink supply circuit of at least one printhead and a recovery circuit ink not used for printing comprising a gutter connected to a sealed tank by a pipe and air suction means located above the ink in the tank, to ensure a depression there, characterized in that it performs on the one hand the control of the recovery rate ink by measuring the pressure P in the reservoir, by means of a sensor, by detecting any reduction in the pressure P in the reservoir, reflecting an anomaly in the rate of recovery of the ink, and on the other hand, the control of the operation of the suction means either at their minimum suction flow rate compatible with the nominal ink recovery flow rate, or at their maximum suction flow rate during anomaly detection of said flow rate. ink recovery.

The invention also relates to an inkjet printer using such a method.

One of the advantages of the invention comes from the fact that the as the amount of gas carried in the recovery circuit and the energy consumed by the drive members are minimal, the life of the printer is extended. This is due to the permanent adaptation of the air intake level to the pressure losses observed in real time, in the recovery line.

• la figure 1 est un exemple d'un circuit d'alimentation et de récupération en encre d'une imprimante, selon l'invention;
• la figure 2 est un diagramme temporel de fonctionnement d'une pompe utilisée selon la méthode de l'invention.

Other characteristics and advantages of the invention will appear on reading the description which follows, illustrated by the figures below referenced:

• FIG. 1 is an example of a circuit for supplying and recovering ink from a printer, according to the invention;
• Figure 2 is a timing diagram of the operation of a pump used according to the method of the invention.

The elements performing the same functions for the same results have the same references in the different figures.

FIG. 1 is a schematic representation of a circuit for supplying and recovering ink from a deviated continuous jet printer, according to a non-limiting exemplary embodiment, of the type described in French patent 2,545,042, filed by Applicant and modified according to the present invention. In this example, the continuous jet printer, intended to project a jet 21 of ink drops from a print head 2 onto a scrolling support below, comprises a circuit A, which is used for supply of the print head 2 connecting a sealed ink tank 17 to the head 2. The circuit R for recirculating unused ink drops connects a recovery gutter 22 to the ink tank 17. Any device for addition of fresh ink or solvent known in the art can be easily inserted into the printer.

The print head 2 is supplied with ink by circuit A, which comprises a pump 1, actuated by a motor not shown and connected to the reservoir 17 by a pipe 31. This pump is intended to pressurize the ink leaving the reservoir, which is then directed through a line 10 to an accumulator 18, maintaining the ink at a constant pressure during printing by the head 2. This ink can be filtered, by a filter 6 for example placed on the line 11 connecting the accumulator 18 to the head 2, before arriving directly at the head 2. Said printing head 2 projects an ink jet 21 which, by appropriate stimulation, breaks up into drops of ink which are then charged electrostatically to be deflected to the medium to be printed. The drops not used for printing and which are therefore not deflected are recovered in the gutter 22 placed under the ink jet. The ink thus recovered must be conveyed to reservoir 17, via circuit R.

This recovery circuit R comprises a line 220, connecting the outlet of the gutter 22, which is at atmospheric pressure, to the reservoir 17, which is at a lower pressure, the ink in the line 220 being put under vacuum by means suction 23 of the air pocket prevailing in the reservoir 17 above the liquid ink. These means can be constituted by a pump, actuated by a motor not shown and connected to the air pocket of the tank 17 by a pipe 230, sucking the air and directing it through a pipe 303 to the outside of the tank. The exterior of the reservoir 17 being at atmospheric pressure, the means 23 thus create a vacuum in the reservoir, which has a sealed cover 170.

The fluid conveyed in line 220 is a two-phase mixture composed of ink recovered in the gutter 22 and of air entrained by the suction of the ink in this same gutter. So that the pressure drops in the recovery line 220 result from the pressure drop which is a function of the liquid flow and the pressure drop which is a function of the gas flow. These pressure losses result in the difference between the atmospheric pressure prevailing at the level of the gutter 22, upstream of the pipe 220, and the pressure P prevailing in the reservoir 17, downstream of said pipe 220. To impose the flow of the liquid phase of this conveyed fluid, that is to say of the ink, it is necessary to impose a determined value of the pressure P in the reservoir 17, lower than atmospheric pressure, and which we will call by the following depression in the tank 17, in order to balance the pressure drops of the liquid and gas flows simultaneously.

This vacuum in the reservoir 17 being provided by the pump 23, which removes a certain amount of air from the volume of the bag placed above the level of ink in the reservoir 17, can be controlled by acting on the pump.

In the case of an increase in pressure losses in line 220, it is necessary to increase the vacuum in the tank to maintain the flow rate of the fluid circulating in line 220. If it is not possible to increase the value of this vacuum au- beyond a value necessary to balance the pressure drops, one of the fluid flows, liquid or gas, will decrease. No problem will arise as long as the flow rate of the liquid phase is sufficient to properly remove the ink from the gutter 22 or line 220, the flow of each of the phases continues to behave more or less like an incompressible flow. The flow rate of the gas phase will be favored over that of the liquid phase because the minimum viscosity of the gas phase is lower than the minimum viscosity of the other phase. This phenomenon will cause an accumulation of liquid in at least one location of the gutter 22 or the recovery line 220 and therefore a malfunction of the printer.

The invention proposes a method for optimizing the operation of the printer, consisting in measuring the pressure P prevailing in the ink tank 17 by means of a pressure sensor 5, and in detecting any increase in pressure drops occurring resulting in an increase in the pressure P in the reservoir 17, that is to say in a decrease in the vacuum in the line 220.

However, in the case where the suction pump 23 is of the constant vacuum type - a turbine for example - a problem will arise when the pressure drops will increase in the line 220 and the pump will not be able to create a sufficient vacuum in the reservoir 17. The flow of the ink will decrease until the gas has a predominant behavior of compressible fluid, finally stopping the flow of the ink, which will accumulate in the line 220 until the gutter 22, which risks overflowing catastrophically.

This is why the invention uses a volumetric pump, which imposes a constant volumetric flow rate for extracting air from the reservoir 17 but the pressure of which varies. Thus, when the pressure drops in line 220 increase, this causes a increase in the vacuum in the tank 17 which can be detected by the sensor 5.

Following a flow problem in line 220, the increase in vacuum in the reservoir 17, linked to the extraction of air at constant volumetric flow, is not enough to quickly solve the problem. For this, we voluntarily move away from the operating point of the pump 23 by increasing its volumetric flow rate, thereby reducing the pressure P in the reservoir 17 until rebalancing the pressure drops in the line 220 necessary for restoring the flow rate of ink.

The flow restored, the proportion of liquid in the two-phase mixture of the ink will return to its initial value in the line 220 and the pressure drops will decrease, which will result in an increase in the pressure P in the reservoir 17 From there, the volumetric flow rate of the pump 23 can be reduced.

Several methods can be used to vary the volumetric flow rate of a pump. You can vary its displacement, or vary the speed of rotation of its drive motor. In the case of a DC motor, the armature supply voltage is varied. However, a variation of the excitation voltage is not suitable because the pump must rotate at its maximum speed of rotation while it provides maximum vacuum. In the case of a stepping motor, the supply frequency of the motor is varied, but correspondingly the current is increased at the same time as the frequency because the torque demanded is an increasing function of the speed.

According to another method varying the average speed of the pump while maintaining a speed of constant operation of the stepping drive motor, ie at constant frequency and current, the motor is stopped at each revolution. As shown in FIG. 2, the pump or the suction means 23 in general operate periodically, each period being able to decompose into a time T1, during which the means 23 do not cause any suction of air into the reservoir 17, and a time T2 of rotation at constant speed, during which the means cause an aspiration. In the case of a positive displacement pump driven in rotation by a motor, FIG. 2 shows the angle of rotation M of the motor, on the ordinate, as a function of time, on the abscissa. During a cycle C of revolution of the motor, an increase in the duration of time T1 decreases the average flow rate of the pump, without variation of the speed of rotation of the drive motor, while a reduction in T1 increases the average flow rate of the pump. The electronic motor control circuit varies the durations T1 and T2 of the motor cycle and it controls the various parameters of the printer - pressure P read by the sensor 5, temperature T of the ink measured by a sensor 26 - and control other actuators - solenoid valves 19 and 28 for example - during certain non-suction times T1 of duration compatible with the measurements. In addition, using a software program taking into account the pressure P in the reservoir 17 and other parameters, the circuit uses the time T1 of no suction to determine the duration before restarting the engine.

Whatever mode is chosen to vary the average flow rate of the positive displacement pump 23, a decrease in the average flow rate of the pump, respectively an increase, should be interpreted subsequently as a decrease in the average speed of the motor. or an increase in stopping time at constant speed, respectively an increase in the average speed of the motor or a reduction in stopping time.

According to the invention, the printer will automatically adjust to its optimal operating mode of ink recovery, considered as a two-phase mixture, according to the method described below.

When the printer starts or in a reinitialization phase, according to a first phase, the means 23 operate at their maximum suction flow rate, thus ensuring a fluid flow rate for recovering unused drops and recovered in the gutter 22. The sensor 5 reads the pressure P in the reservoir 17 which is stored in memory as representative of the correct functioning of the recovery circuit R. Then in a second step, the suction flow of the means 23 is gradually decreased on a ramp, increasing the duration of the time T1 of non-aspiration with respect to the time T2, while the decreasing pressure P is measured, stored and compared with a sliding average of the last three previously measured pressure values. Thus is defined a first range of values of the pressure P significant of a fluid ink recovery flow in the line 220. This step is carried out by the control circuit of the pump drive motor and continues until that the depression in the tank 17 becomes incompatible with the last sliding average measured.

At this instant, the values of the ink temperature and of the pressure in the accumulator 18 are memorized because they are representative of the viscosity of the ink to be recovered. In addition, the flow of suction of the means 23 is stored in the form of an image, which can be, in the case of a pump, the average speed of rotation of its drive motor, or the duration of time T1, acquired at the last reading cycle of the pressure P in the reservoir 17, by the sensor 5. This image is stored as being representative of the minimum suction flow rate of the pump compatible with good recovery of the ink in the gutter 22.

Then, in a third step, the suction flow of the means 23 immediately returns to its maximum value, reducing the duration of time T1 to a minimum. This step can be performed by the engine control device in the case of a positive displacement pump. The duration of time T1 is reduced to the minimum to increase the suction flow rate of the means 23, as long as the pressure P measured in the reservoir 17, has not yet reached the value memorized at start-up. By sucking up a larger amount of air again in the reservoir 17, the correct ink recovery rate will be restored with minimal pressure drops. The pressure P will increase to approach the atmospheric pressure. A second pressure interval is then defined, separate from the first, representative of a fluid flow in line 220, capable of no longer ensuring correct recovery of the ink, without overflow of the gutter 22.

In a fourth step, the duration of the non-suction time T1 goes up to its last stored value causing a pressure P in the reservoir 17 still included in the first interval, with a margin of about 6% above this value.

After this automatic adjustment of the printer, its mode of normal operation takes place in the following two steps.

In a first step, the duration of time T1 results from the value previously memorized by the printer in its start-up phase and from the evolution of the temperature and viscosity conditions of the ink during printing. In a second step, activated by the detection of a reduction in vacuum in the reservoir 17 incompatible with correct recovery of the ink, the duration of the time T1 is reduced to its minimum value until the pressure P n 'has not adopted its last memorized value ensuring correct recovery.

By experimentation on a prototype printer according to the invention, it has been observed that a decrease - conversely an increase - in temperature or an increase - conversely a decrease - in the pressure set point of the accumulator 18 implies an increase - conversely, a reduction - of the suction flow rate by the pump 23 to maintain correct recovery of the ink. As already described in the patents filed by the Applicant, the pressure set point of the accumulator 18 influences the operation of the pump 1 for sending the ink to the print head. Indeed the pressure in the accumulator increases with the viscosity of the ink, which is a decreasing function of the temperature. the higher the viscosity, the more difficult the ink flow in the recovery line.

This is why the method according to the invention correlates the suction flow rate of the pump 23 to the temperature, to the pressure set point of the accumulator 18 and to the image of this flow rate previously described. Predicting the behavior of the printer thus made allows avoid increases in the maximum speed of the pump due to drift in conditions outside the recovery circuit R.

Claims (10)

1. Method to optimize the operation of a deflected continuous flow ink-jet printer, comprising a circuit for the supply of ink to at least one printing head (2) and a circuit for the collection of the ink not used for the printing, comprising a gutter (22), a conduit (220) connecting the gutter (22) to an imperviously sealed ink tank (17) and suction means (23) to draw in the air pocket extending above the ink to set up a depression (P) in the ink tank (17), characterized in that said method consists in carrying out, firstly, the checking of the rate of collection of the ink by the following steps of:

   - measuring the pressure prevailing in tile ink tank (17) by means of a sensor (5);

   - detecting any increase in the pressure (P) in the ink tank (17), indicating an anomaly in the ink flow rate of the conduit (220);

   - and secondly, the control of the operation of the suction means (23), on the one hand, at their minimum suction rate compatible with an optimum flow rate of the ink in the conduit (220) and, on the other band, at their maximum suction rate when anomalies are detected in the ink flow rate in the conduit (220).
2. Method according to claim 1, characterized in that said suction means (23) work periodically, each period comprising:

   - a first period of time (T1) during which said suction means (23) cause no suction in the ink tank (17);

   - a second period of time (T2) during which said suction means (23) bring about a suction in the ink tank (17) at constant operating conditions, the rate of suction of said suction means (23) being decreased and increased respectively through the increasing and the decreasing, respectively, of the first period of time (T1).
3. Method according to claim 2, characterized in that the control of the operation of said suction means (23) at their optimum suction rate, for efficient collection of the ink in the gutter (22) is done automatically, first of all when the system is started up and during the stages of initialization of the printer, according to:

   - a first step during which said suction means (23) operate at their maximum suction rate, the pressure (P) in the ink tank (17) being measured and memorized;

   - a second step during which the suction rate of said suction means (23) is decreased gradually according to a gradient, by an increase in the duration of the first period of time (T1) in the ink tank (17) and the pressure is measured, until said pressure (P) becomes incompatible with the last measured sliding value, defining a first range of pressure values corresponding to a fluid rate of collection of the ink;

   - a third step during which the suction rate of said suction means (23) is raised immediately to its maximum value, by a reduction of the duration of the first period of time (T1) to its minimum value, until the measured pressure (P) again reaches its value memorized at the starting up of the system, and defines a second range of pressure values, separate from the first one, corresponding to an inefficient collection of the ink;

   - a fourth step during which the duration of the first period (T1) of time is raised to its last memorized value ensuring a value of pressure (P) included in the first defined range, with a margin of 6% above this value.
4. Method according to claim 3, characterized in that it comprises, during the normal operation of the printer, a first step during which the duration of the first time of non-suction (T1) by the suction means (23) in the ink tank (17) is fixed as a function of its value memorized in the starting up phase and of the changes in temperature and viscosity of the ink during the printing and a second step activated by the detection of a decrease in pressure (P) incompatible with efficient collection of the ink, during which the first period of time (T1) is reduced to its minimum value until the pressure (P) resumes its last memorized value providing for efficient collection.
5. Method according to claim 4, characterized in that the value of the first period of time (T1), fixed during the first step, increases if the instructed value of pressure at another point of the circuit, such as the accumulator (18), falls while the temperature (T) of the ink rises.
6. Method according to any one of the claims 2 to 5, characterized in that, during the first time (T1) of non-suction by said suction means (23), having a duration compatible with the measurements, the parameters of operation of the printer constituted by the pressure (P) and the temperature (T) of the ink are checked and the duration of the following first periods of time (T1) is determined.
7. Method according to any one of the claims 1 to 6, characterized in that said suction means (23) are comprised of a constant flow pump (23) that is activated by an electronically controlled motor and that has a mean suction rate capable of varying from a minimum value when the rate of collection of the ink is optimal and of increasing to a value sufficient to ensure a depression in the ink tank (17) restoring the rate of collection of the ink after an anomaly has been detected.
8. Method according to claim 7, characterized in that the suction rate of the pump (23) varies as a function of and in the same direction as the mean speed of its driving motor.
9. Method according to claim 8, characterized in that the driving motor of the pump (23) is of the stepping type, with constant operating conditions, the control device of which causes the mean suction rate of the pump to vary by the stopping of the motor at each cycle of revolution and by variation of the stopping time (T1) in relation to the time of rotation (T2) at constant speed at each cycle, the mean suction rate of the pump being decreased through an increase in the duration of the stopping time (T1), and this suction rate being conversely increased through a decrease in said stopping time (T1).
10. Deflected continuous ink jet printer having its operation optimized by a method according to any one of the claims 1 to 9, the printer comprising:

    - an ink tank (17),

    - a supply circuit (A) having first suction means (1) for the ink contained in the ink tank (17) for supplying ink to at least one printing head (2),

    - a circuit (R) for the recovery of the ink not used for printing and having a gutter (22), a conduit (220) connecting the gutter (22) to the ink tank (17) and second suction means (23) for recovering the unused ink from the gutter (22) to the ink tank (17),

    characterized in that:

    - the ink tank (17) is a tight tank equipped with a sensor (15) making it possible to measure the pressure prevailing in the ink tank (17) and making it possible to detect any decrease in the pressure in the tank (17) indicating an anomaly in the ink flow rate of the conduit (220),

    - second suction means (23) of the recovery circuit (R) exerting their action on the air pocket in said tank (17) above the ink in order to ensure a pressure drop there,

    - control means are provided for bringing about the operation of the second suction means (23) of the recovery circuit (R) on the one hand at their minimum suction flow rate compatible with an optimum ink flow in said conduit (220) and on the other at their maximum suction flow rate during detections of anomalies in the ink flow rate in the conduit (220).

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