ES2350699T3 - INK REGENERATION SYSTEM FOR PRINTING INK INJECTION. - Google Patents

Abstract

Ink circulation system (1) for use in a fall in the demand for an inkjet printing apparatus, said system comprising: - an ink supply subtank (20) having a first ink level sensor (26) to control a first ink-free level surface in the supply tank (20), the first level sensor controlling a supply pump (73) for replenishing the supply subtank with fresh ink when the ink is consumed; - an ink return subtank (30) having a second ink level sensor (36) that controls a second ink free level surface in the return subtank (30); - at least one printhead (10) having an ink input hydraulically connected to the supply subtank (20) and an ink output hydraulically connected to the return subtank (30); - creating the height difference between the first and second ink-free levels a hydrostatic pressure difference that provides a driving force for the flow of ink through the print head, in which the system additionally comprises: - a path of refreshment from the return subtank to the supply subtank comprising a circulation pump (76) and an active through-flow degassing unit (60) to control a level of dissolved gas in the ink, whereby the circulation pump ( 76) is controlled by the second level sensor (36).

Description

FIELD OF THE INVENTION

The present invention relates to a drop deposition apparatus and especially to an inkjet printing apparatus. More specifically, the invention relates to ink supply systems for inkjet printers. BACKGROUND OF THE INVENTION

Printers are used to print the production of computers or a similar type of devices that generate information about a recording medium such as paper. Commonly available types of printers include impact printers, laser printers and inkjet printers. The term "inkjet" covers a variety of physical and hardware processes, but basically these printers transfer ink from an ink supply to the recording medium in a pattern of fine ink drops. Inkjet printheads produce drops continuously or on demand. "Continuously" means that a continuous stream of ink drops is created, for example, by pressurizing the ink supply. "On demand" differs from "continuously" in that ink droplets are only ejected from a print head by manipulating a physical process to momentarily overcome the surface tension forces that hold the ink in the print head. The ink is kept in a nozzle forming a meniscus. The ink remains in position unless some other force exceeds the surface tension forces that are inherent in the liquid. The most common practice is to suddenly increase the pressure on the ink, expelling it from the nozzle. A category of inkjet printheads of drops on demand uses the physical phenomenon of electrostriction, a change in the size of the transducer in response to an applied electric field. Electrostriction is stronger in piezoelectric materials and, therefore, these printheads are known as piezoelectric printheads. The very small dimensional change of the piezoelectric material is used in a large area to generate a volume change that is large enough to squeeze a drop of ink from a small chamber. A piezo printhead includes a multitude of small ink chambers, arranged in a cluster, each having an individual nozzle and a percentage of the transformable wall area to create the volume changes required to eject a drop of ink from the nozzle, in accordance with the principles of electrostriction.

The present invention deals with the way in which ink is supplied to the ink chambers, the conditioning of the ink and the impact of the conditioning of the ink on the operation of an inkjet printhead.

Document US-A-2003/0011664 describes a regeneration station for refilling inkjet printer cartridges. AIR CAUGHT IN THE INK CHAMBERS

It is known that the presence of air bubbles in the ink chamber of the piezo printhead often causes malfunctions in the printhead. If air is present in the ink chamber, the desired pressure changes resulting from the piezoelectric deformation of part of the walls of the ink chamber will be absorbed by the air, leaving the ink pressure unaffected. The force of the surface tension of the ink in the nozzle keeps the meniscus and no drops will be ejected from the ink chamber. At the frequencies at which the piezoelectric transducers work in the piezoelectric printhead, that is, in the range of kHz to MHz, not only air bubbles, but also the air dissolved in the ink, can cause such malfunctions. as described above. Concepts for avoiding air bubbles in the ink chamber have been described in the state of the art by creating an air trap upstream of the ink chamber, that is, before the ink enters the ink chamber . Solutions have been proposed in EP-A-0 714 779 and US 4,929,963 in the form of air buffers or gas separators that allow air bubbles to rise and be evacuated from the ink in an intermediate tank before that the ink is supplied to the printhead.

EP-A-1 403 059 and US 6,231,174 describe a principle of removing air bubbles from an ink supply path by circulating ink during a dedicated circulation purge operation. These circulation purge operations are executed before or after printing. In US 5,771,052 a deaerator tube is described as an internal part of an inkjet printhead. The deaerator tube is an air permeable and ink impermeable tubular membrane that allows air to be removed from the ink, through the membrane, through a vacuum source.

US 4,555,709 describes an ink reconstitution system and a method for ink drop printers, in which an essentially closed supply system supplies ink under pressure to a print bar, and a receiver collects the unprinted ink . Unprinted ink is supplied to a temporary support or reconstitution tank that has high and low level detection means, and a pump is operated in response to the liquid level when it reaches the high level detection means to deliver a quantity of liquid collected from the support tank back to the supply system. Reconstitution is carried out by measuring a predetermined amount of reconstitution liquid in the support tank

or replenishment, each time the low level detection means operate, so that the amount of replenishment is always related to the amount of ink returned by the receiver and, therefore, subject to evaporation. CONTRAPRESSION CONTROL IN THE NOZZLE IN QUICK SCAN APPLICATIONS

A second point of attention in ink supply systems is the pressure in the nozzle, which is critical for a print head that is well tuned and works well. Inkjet printheads work best with a slightly negative pressure on the nozzle or back pressure. In practice, this is often achieved by maintaining a height difference between the ink-free surface in a ventilated ink supply tank and the meniscus in the nozzle. That is, the ink-free surface in the vented supply tank is gravimetrically maintained a couple of centimeters below the level of the meniscus in the nozzle. This height difference establishes a hydrostatic pressure difference to control the back pressure in the nozzle. In alternate motion printhead configurations, the ink supply tank is positioned off-axis, that is, it does not scan, because otherwise the lower position of the ink supply tank compared to the printhead would interfere with the transport path of the printing medium. A flexible tube is used to connect the ink supply tank with the offset axis with the print head on the axis, as described in, for example, US 4,929,963. During the acceleration and deceleration of the print head pressure waves are created in the tubes, which can significantly alter the pressure balance in the meniscus, and can cause the nozzle to leak in the event of a decrease in negative pressure , or the meniscus rupture in the case of an increase in the negative and inlet pressure in the ink channel. Many approaches to back pressure control have been proposed in alternative motion printhead applications. Backpressure regulation mechanisms in the form of buffers or pressure dampers mounted together with the print head on the reciprocating slide are described in EP-A-1 120 257 and US 6,485,137. For accelerations and decelerations of the slide above 1G, the response time of these devices is insufficient. In EP-A-1 142 713 a ventilated subtank is used. The subtank serves as a local ink reservoir near the printhead and is filled intermittently from a main tank positioned outside the shaft. The solution provides better control of the back pressure of the nozzle, maintaining a difference in the local hydrostatic pressure between the ink-free surface of the ventilated subtank and the meniscus. DEGRADATION WITH THE TIME OF THE INK PROPERTIES IN PRINT HEADS (ESPECIALLY FOR INACTIVE NOZZLES DURING A LONGER PERIOD OF TIME)

Although the properties of inkjet ink can be well controlled in manufacturing and are maintained at a reasonable level during transport and storage, some properties of the ink can be degraded when the ink is used in an ink system or maintained. in the printhead. For example, inkjet inks containing VOCs (volatile organic compounds) often suffer from evaporation of some of the VOCs in the meniscus of the ink in the nozzle. The viscosity of the ink will change locally in the nozzle, having a negative effect on its injection properties and potentially causing a nozzle failure. The time it takes for an ink to degrade in a way that causes a nozzle failure is often indicated as its latency period. Latency problems are often avoided or recovered by regular maintenance of the nozzles, for example, by purging the nozzle so that "fresh" ink enters the nozzle. Together with these problems, it has been found that if the ink retention time in an ink supply system is too long, for example, during production interruptions or overnight, effects such as the dispersion deposit may occur. , self-curing, etc. In many cases, reliable operation of an inkjet printer after a production interruption or production shutdown is only achieved after an extensive start-up procedure, including purging a considerable amount of degraded ink retained in the all or part of the ink supply system, to ensure that the ink in the ink chambers of the print head is of good quality and that it works reliably in the print head. Often, these amounts of purged ink are not reusable in the printer setting.

For the production of inkjet printing equipment, where high printing speeds and reliability are of the utmost importance, the conditioning of the ink is critical. The solutions proposed in the state of the art only partially solve some of the problems described above. Therefore, it is an object of the present invention to provide an ink system, incorporated in an inkjet printer, which provides the ink in optimal conditions immediately after startup and maintains it in optimal conditions during printing. DESCRIPTION OF THE INVENTION

It is an object of the invention to provide an ink system for inkjet printers that provides optimally conditioned and continuously regenerated ink to the inkjet printheads. In an embodiment of the invention, this object is realized by active degassing of the ink in a continuous ink circulation system.

Specific features of the preferred embodiments of the invention are indicated in the claims.

Other advantages and embodiments of the present invention will become apparent from the following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a schematic view of a first embodiment of an ink system according to the present invention.

Figures 2A, 2B and 2C show the ink-free surface in an acceleration or deceleration subtank and the preferred position of the subtank ink inputs and outputs.

Figure 3 shows a schematic view of an alternative embodiment of a subtank.

Figure 4 shows a schematic view of an embodiment of an ink transport system according to the invention, suitable for connection to an end-head type printhead.

Figure 5 shows the flow of ink through a printhead as a function of the pressure difference between the return subtank and the supply subtank, in a specific embodiment of the invention.

Figure 6 shows the efficiency of the removal of dissolved gas as a function of the ink flow through the active degassing unit, in a specific embodiment of the invention.

Figure 7 shows an alternative embodiment of a degassing unit.

Figure 8 shows an alternative embodiment of replenishing the ink system with fresh ink.

Figure 9 shows an alternative embodiment of an ink circulation system, especially suitable for multiple printhead configurations.

Figure 10 shows an alternative embodiment of an ink circulation system with improved operation of a degassing unit.

Figure 11 shows an embodiment of a degassing unit with a controllable vacuum connection. DETAILED DESCRIPTION OF THE INVENTION

The applicability of the present invention is wide ranging. APPLICABILITY REGARDING PRINTER CONFIGURATION

The invention can be applied in printers with alternate motion printhead configurations known in the market as SOHO, that is, inkjet printers of small offices and domestic, and in the large format market, for example, for applications of points of sale, advertising, etc. In this type of printing apparatus, the inkjet printheads move in a first direction, the direction of rapid scanning, through the recording medium, while the printing ink drips into the recording medium. Between two fast scanning operations, the recording medium moves in a second direction, the slow scanning direction perpendicular to the fast scanning direction, to present an unprinted portion of the recording medium below the scanned area path Quick scan print head print. Multiple printheads can be mounted on a single slide that moves back and forth along the fast scanning direction. Numerous printer configurations and printing methods that include reciprocating printheads have been described and are commercially available.

Unlike the alternate motion settings of the print head, fixed grouping configurations are also known. In the fixed grouping configuration, the printheads are stationary and only the recording medium moves in a feed direction, while printing the printheads. Stationary printheads can print a specific scanning area of the recording medium, for example, for the printing of variable data of name and address labels within a dedicated area on preprinted forms, or stationary heads can be placed in a grouping to print the page across, for example, for digital printing of packaging material or labels in a single digital press pass.

Except for SOHO printing devices, almost all inkjet printing devices use an ink system that supplies ink from a replaceable ink supply tank to the inkjet printheads. The ink is ejected in the form of individual drops from the nozzles of the print head, according to a predefined model. Depending on the application, this model may represent an image in a poster printing application, a conductive structure in an application for printed electronics, glue tracks in a binding application, etc. The present invention can be implemented in any of these inkjet printing devices. APPLICABILITY REGARDING HEAD TECHNOLOGY

Inkjet printing is a generic term for a plurality of different printing technologies in which all eject ink drops from a nozzle of the print head in the direction of a recording medium. The most important inkjet printhead technologies today include continuous inkjet, thermal inkjet of drops on demand and piezoelectric inkjet of drops on demand. In the technology of inkjet of drops on demand we can also distinguish between printheads of the type of end puller, printheads of the side puller type and printheads of the type of through flow, depending on its design. The printheader printheads are characterized by having the nozzles at the end of the ink chambers, while the printhead printheads are characterized by having their nozzles on one side of the ink chambers. The printheads of end puller and side puller require an ink connection to provide the ink through an ink collector to a plurality of individual ink chambers, each having drive means for expelling a drop of ink to Through your mouthpiece. The ink supplied to the printhead is retained in the printhead until it is ejected from a nozzle. Through-flow printheads, on the other hand, are characterized by having a continuous flow of ink through the ink chambers, that is, ink flows through an ink inlet into a supply manifold, through of a plurality of individual ink chambers, which terminate in a manifold from where the ink leaves the ink print head through an ink outlet. Only a small part of the volume of ink that flows continuously through the ink chambers is used to eject drops of ink from the nozzle, for example, less than 10%. Hybrid printhead designs are also known, for example, printhead type printheads where the ink collector has an ink inlet and an ink outlet. Here the ink contained in the end puller ink chambers is retained in the print head until it is used; The ink in the ink collector can be continuously renewed.

The present invention is independent of the technology of the inkjet printhead or the type of printhead. Although the embodiments described in detail in the following sections of the detailed description will mainly deal with piezoelectric printheads of the hybrid type, that is, an end puller with through-flow characteristics, the invention is also applicable to other types of printheads. impression, as will be apparent from the additional description. APPLICABILITY REGARDING INK INJECTION INKS

The "inks" used for inkjet printing processes are no longer limited to color printing materials for image reproduction, but currently also include structuring materials for printing OLED screens, conductive electronic materials for printed RFID tags, adhesive materials, etc. Especially piezoelectric inkjet technology is often used to inject a variety of liquid materials other than traditional printing inks, because the physics behind piezoelectric inkjet, that is, electrostriction, does not place restrictions on the composition Chemistry of the liquid material to be injected. This is not the case with thermal inkjet technology, which requires a local "evaporation" of the ink, or continuous inkjet technology, which requires the "electrostatic charge" of the ink drops.

From a chemical composition point of view, inkjet inks are often classified into families based on the carrier material, for example water, used to transport the functional material, for example, pigments. Examples of families of ink based on the carrier used include water-based inks, solvent inks, oil-based inks, UV or EB curable inks, hot melt inks, and recently introduced eco-solvent and bio inks, both of which are intended as a Environmentally safe use.

From the description of the background of the invention, it is known that the performance and reliability of inkjet printing systems increase with the use of degassed inks, because the unwanted air bubbles that develop in the chambers of ink seriously disturb the process of generating the drops, and can even cause failures of the process of ejection of the ink. Therefore, the use of degassed ink in the printing process is preferred. Although the present invention is described in more detail with reference to a UV curable ink, the invention is not limited to UV curable inks, but can also be used to improve the performance of other types of ink.

From the background of the invention, it is also known that some ink dispersions are easily deposited when retained for too long without stirring. A typical example is a pigmented ink that uses titanium dioxide as a white pigment. These inks require continuous circulation to maintain the adjustment of the ink dispersion for injection purposes. FIRST PERFORMANCE -Description

A schematic diagram of an ink system 1 incorporating the invention is shown in Figure 1. The ink system 1 can be divided into an off-axis ink system 2 and a slide ink system 3. The division into two separate parts can be advantageous in inkjet printers with alternate motion printheads. Here, the slide ink system 3 can be placed together with the printheads in a single reciprocating slide, and the ink system outside the axis 2 can be stationary with respect to the alternative movement operation of the printheads . In fixed-group printhead configurations, such as in single-pass digital presses, both parts can be stationary. The slide ink system 3 includes an inkjet printhead having two nozzle groups 10a and 10b, both groups being interwoven to provide a print resolution that is twice the intrinsic resolution of the individual groups of the nozzles The printhead has an ink inlet 11 for receiving ink from a supply subtank 20, and an ink outlet 12 for returning the ink to a return subtank 30.

The printhead may have conditioning means, generally indicated by reference number 15 in Figure 1, for example, heating elements for operating the ink at an elevated temperature, or a heat sink for cooling of the electronics and other parts that dissipate heat. The conditioning means 15 have their own electrical or fluid connections to a separate conditioning circuit, which is generally indicated by 19 in Figure 1. For example, the printhead 10 can be connected to a fluid circulation system in which a high temperature conditioning liquid circulates to (pre) heat the print head to its operating temperature. The fluid circulation system can pass to other components of the ink system that can benefit from (pre) heating, for example, the supply subtank 20 when the ink can (pre) be heated before being supplied to the printhead 10. The (pre) heating the ink in the supply subtank 20 has the advantage of reducing the solubility of the gas in the ink, an issue that will be discussed later when active degassing is explained. A practical example may be that the fluid circulation system includes extrusion parts, through which a conditioning fluid flows at an elevated temperature and over which the supply subtank 20 can be mounted to create a heat exchange interface between the conditioning fluid and the supply subtank.

The supply subtank 20 includes a closed container 29 for containing ink, an ink inlet 21 for replenishing the ink in the container, an ink outlet 22 for supplying ink to the printhead, a pressure connection 23 for apply a pressure to the closed container and one or more ink level sensors 25, 26, 27 to monitor the ink-free surface in the container 29. These sensors can send an analog signal, for example, that represents a continuous level measurement , or a digital signal, for example, in case of a level switch. In the further description of the invention, both types of sensors, or combinations of types of sensors can be used. With reference to Figure 1, the three ink level sensors can be configured as a minimum level sensor 25 used to start the ink replenishment process in combination with a maximum level sensor 27 used to stop the replenishment process; there can only be one operating level sensor 26 with a range of hardware or software hysteresis to create similar functionality; there may be a combination where a single operating level sensor 26 is used and the level sensors 25 and 27 are used as low flow and overflow alarm indications, or even another combination.

In general, the objective of the sensors is to monitor the ink level in the container 29 and activate the start and stop of the ink replenishment process, as well as the signaling of the alarm conditions, such as overflow or underflow of the level of ink in the subtank. Multiple embodiments of ink supply subtanks have been described in EP-A-1 142 713, all of which can be used with the present invention. The return subtank 30 may be a "copy" of the supply subtank 20, with similar characteristics to perform equivalent functions, for example, the ink inlet 31 receives ink from the print head, the ink outlet 32 drains ink from the Return sub-tank 30, the ink level sensors 35, 36 and 37 monitor the ink-free surface in the return sub-tank 30 and control the drainage process. Preferably, the entrances and exits of the subtanks are located at the bottom of the closed container and on an axis of symmetry of the container that is perpendicular to the direction of rapid scanning. The reason why it becomes clear from the investigation of the ink-free surface in the containers when the containers accelerate or decelerate. During acceleration and deceleration, the ink-free surface tilts or decreases due to the inertia of the mass of ink. This is shown in Figures 2A, 2B and 2C. In a plane of symmetry of the container, perpendicular to the direction of acceleration or deceleration, the height of the ink-free surface 28 is constant, resulting in a constant hydrostatic pressure in that position. Because the hydrostatic pressures of the ink are part of the mechanism for creating a return pressure in the nozzles of the print head, it is advantageous to have at least the ink outlet 22 of the supply subtank 20 and the ink inlet 31 of the return subtank 30 placed in a plane of symmetry of the container, perpendicular to the direction of rapid scanning. For reliable and stable operation, it can also be advantageous to have the ink level sensors of the sub-tanks measuring in these symmetry planes. The ink-free surface 28 during acceleration or deceleration is shown in Figures 2A to 2C. Figure 2A shows a steady state ink free surface 28 when the supply subtank 20 does not accelerate or decelerate. Figure 2B shows the situation of a supply subtank 20 that accelerates and Figure 2C shows the situation of a supply subtank 20 that decelerates. In all three examples, the height of the ink-free surface 28, and therefore also the hydrostatic pressure, in the indicated plane of symmetry is constant. As shown in the figures, the ink outlet 22 is preferably placed in the plane of symmetry. In addition, the volume of air in the closed containers 29 and 39 of the respective sub-tanks also acts as a high frequency damper that reduces external noise. The supply subtank 20 and the return subtank 30 can be provided as separate mechanical parts or can be integrated into a single assembly, that is, the functionality of the two subtanks can be integrated into a single molded plastic part. It is preferred that the subtanks be placed just above the corresponding printhead. This position is advantageous because the tube and other ink connections between the subtanks and the printhead will have a minimum of horizontal ink transport sections that may be responsible for the pressure variations induced during the acceleration and deceleration of the slide of the print head

With continuous reference to Figure 1, the off-axis ink system 2 is now described. The off-axis ink system has a supply side and a return side. On the supply side, the off-axis ink system includes a main ink tank 70, a supply container 40 and a degassing unit 60. On the return side, the off-axis ink system includes a container of return 50. The supply side and the return side are hydraulically connected through a serial connection of a check valve 74, a filter 75 and a pump 76, between the ink outlet 52 of the return container 50 and the ink inlet 41 of the supply container 40. The pump 76 must be suitable for pumping inkjet inks and must withstand the back pressure resulting from the pressure difference between the pressure in the supply container 40 and the pressure in the container return 50. A suitable pump may be a KNF Neuberger NF60 micromembrane liquid pump. The filter 75 is preferably a filter that stops any material stuck in the return ink so that it does not re-enter the supply path. A suitable filter may be a Pall MAC type filter. Preferably, a MACCA0303 is used for UV curable inks and the objective is a filtration ratio of 3 μm. This hydraulic connection allows the re-entry of the return ink back into the supply chain, thus creating an ink circulation system. The supply container 40, with the ink inlet 48, is also hydraulically connected to a main ink tank 70 to supply fresh ink to the circulation system through a serial connection of a check valve 71, a filter 72 for filter solid particles of dimensions above 3 to 5 μm of the ink and a pump 73 which can be a peristaltic pump suitable for pumping inkjet inks and resisting back pressure in the supply container 40. The supply container 40 and the return vessel 50 are designed so that they can be pressurized through a pressure connection 43, respectively

53. They may also contain one or more ink level sensors to monitor the ink-free surface in the containers. The embodiment depicted in Figure 1 uses simple ink level sensors 46, respectively 56, with a hardware or software hysteresis around its switching level, to create activation signals to start and stop filling, respectively the operations of drainage, but other level detection methods that can be used to activate filling or drainage operations can be applied. On the supply side of the ink system outside the axis 2, an active throughflow degassing unit 60 is provided. The degassing unit has an ink inlet 61 that receives ink from the ink outlet 42 of the supply container 40 , and an ink outlet 62 that supplies degassed ink to the ink supply side of the slide ink system 3. The degassing unit also has a vacuum connection 63 to apply the vacuum used to degas the ink. The degassing unit is of a type of through flow that continuously removes dissolved gas from the ink during ink circulation, up to an asymptote value of gas dissolved in the ink. The asymptote value can be a function of the applied vacuum, the flow rate of ink through and, of course, the degasser specifications. An example of a through-flow degassing unit suitable for inkjet inks may be a MiniModule hollow fiber membrane type degassing unit available from Membrana GmbH. Finally, the off-axis ink system is connected to the slide ink system through two valves. A refill valve 24 connects the supply side of the ink system off axis 2 with the ink inlet 21 of the supply subtank 20 of the slide ink system 3. A drain valve 34 connects the return side of the ink system ink off axis 2 with ink output 32 of return subtank 30 of the slide ink system

3. The reset valve 24 and the drain valve 34 may reside on the slide, that is, on the slide ink system 3, or be stationary on the printer, that is, on the ink system outside of axis 2 Preferably, they reside in the slide, very close to the sub-tanks, so that they can stop the dynamic pressure waves, generated in the tubes used to connect the slide ink system 3 with the ink system outside the axis 2, which they spread in the subtanks and also in the printhead.

By way of example, the degassing unit 60 in the embodiment of Figure 1 is shown as part of the off-axis ink system, but the degassing unit can also reside on the sliding ink system, which has the advantage of that the length of the ink path from the degassing unit to the print head is reduced and, therefore, reduces the risk of ink degradation before the ink reaches the nozzle of the print head.

An even more preferred embodiment of an active degassing unit is shown in Figure 7. Figure 7 only shows the active degassing module of the ink system 1 and can be integrated into the ink system shown in Figure 1, as part of the sliding ink system or as part of the off-axis ink system . Figure 7 shows, in addition to the active degassing unit itself and the vacuum pressure connection 63, a vacuum brake valve 64 and a filter 65. The advantages of this alternative embodiment will be explained later when degassing is described. Activate in more detail. FIRST PERFORMANCE -Printing mode

The operation of the embodiment according to figure 1 in the normal printing mode is now described. An ink flow through the printhead 10 is made by establishing a pressure difference between the pressure P2 in the supply subtank 20 and the pressure P3 in the return subtank

30. These pressures are applied through a pressure connection 23 in the supply subtank 20, respectively, a pressure connection 33 in the return subtank 30. To create a positive flow from the supply subtank 20 through the head of printing 10 and in the return subtank 30, the pressure in the supply subtank 20 is controlled at a slightly higher value than the pressure in the return subtank 30. The ink flow rate through the print head is it can control through the pressure difference P3-P2, but it also depends on the hydraulic resistance of the fluid conduits to and from the print head, as well as the flow rate of the ink through these conduits, and the internal hydraulic resistance in the printhead. In practice, a pressure difference of 2.5 mbar can already provide a flow rate greater than 300 ml / h through the printhead 10. Figure 5 shows a graph illustrating the increase in through flow of printhead 10 as the pressure difference P3-P2 increases. Figure 5 is illustrative only, because the exact graphics depend on the viscosity of the ink, the hydraulic resistance, etc.

The back pressure on the nozzles of the printhead is controlled by the same pressure values P2 and P3 used to establish the ink flow through the printhead 10. In a preferred hydrodynamic symmetric construction of the slide ink system 3, it is that is, with a balanced hydraulic resistance before and after the nozzles of the print head, the back pressure in the nozzle is equal to ((P2 + P3): 2) + (pgh), where pgh is the hydrostatic pressure of the column of ink between the ink-free surface in the subtanks and the meniscus in the nozzles. In embodiments where the subtanks and printhead are mounted on a single slide, h values typically range between 20 cm and 50 cm. Any deviation from this preferred symmetrical construction of the ink transport system 3 causes dynamic unbalanced pressure drops and unbalanced hydrostatic pressures in the supply path versus the return path. This imbalance can be calculated previously or can be calibrated in advance so that finally the back pressure in the nozzles is perfectly controllable with the pressure in the supply subtank 20 and in the return subtank 30. It is an advantage that both the flow rate of the ink as the back pressure are controllable with only two pressure values, that is, the pressure in the supply subtank 20 and the pressure in the return subtank 30. In embodiments where the subtanks and the printhead are mounted in a single slide, the pressure values P2 and P3 are chosen so that the hydrostatic pressure of the ink column between the ink-free surface in the sub-tanks and the meniscus in the nozzles is largely compensated, and a small back pressure in the nozzle. In a specific embodiment used to verify the invention, the pressure values in the normal printing mode were -30 mbar for P2 and -33 mbar for P3. These pressure values and a difference in height between the ink-free surface in the subtanks and the nozzles of approximately 30 cm produce a back pressure in the nozzles of approximately -1.5 mbar and an ink flow rate above 300 ml / h.

To maintain a continuous flow of ink through the printhead 10, the supply subtank 20 needs to be continuously refilled and the return subtank 30 has to be drained continuously to maintain the ink levels in the constant subtanks. After all, the back pressure on the nozzles depends to some extent on the hydrostatic pressure of the ink columns on the supply and return side of the print head. And although the hydrostatic pressures can be calibrated in advance and taking into account when the set points for P2 and P3 are determined, they must be kept constant during operation. Fortunately, printheads have an operational backpressure window in which the ejection process can work. An operational backpressure window is expressed as a hydrostatic pressure range and can reach up to ± 10 cm of water column around its working point, for the operation of the printheads in a stationary system with constant parameters of the printing process . But the parameters of the printing process are rarely constant and also vary within a tolerance window around its working point, for example, manufacturing tolerances of the print head or variation of dynamic pressure drops in the ink tubes. These tolerance windows consume part of the operating window available for backpressure of the print head. In practice, variations of the ink-free surface in the sub-tanks are preferably limited to ± 1 cm, preferably ± 0.5 cm, more preferably ± 0.1 cm. This operating window, therefore, provides space for the intermittent on / off of the ink refill in the supply subtank 20 and the drainage of the ink in the return subtank 30. Intermittent replenishment concepts can be performed using valves Fast switching with switching speeds in the range of 1 to 10 Hz and having a small diaphragm. The switching can be activated by a single level operating switch with a small hysteresis that defines the target operating window. Fast switching with low flow rates is close to a concept of continuous replenishment, just as pulse width modulated power drives approach analog power drives, but it is cheaper and easier to control. In the embodiment of Figure 1, the reset valve 24 is opened and closed under the control of one or more of the level 25, 26 or 27 level sensing sensors of the supply subtank 20. Depending on the operating back pressure window. , the ink level detection sensors in the supply subtank 20 may be configured to allow a minimum to maximum height difference of ± 1 cm, more preferably ± 0.5 cm, more preferably ± 0.1 cm.

An alternative embodiment to control the continuous flow of ink through the printhead 10 is to maintain the pressure values P2 and P3, apply it to the supply subtank 20, respectively, to the return subtank 30, equalize them and use the hydrostatic control of the ink-free surface of the respective sub-tanks to create a hydrostatic pressure difference between the ink-free surface in the supply sub-tank 20 with respect to the return sub-tank 30. The hydrostatic pressure difference replaces the active pressure difference P3-P2. The hydrostatic pressure difference can be made through a different position of the ink level sensors in the respective sub-tanks, feasible because the continuous ink flow controls the ink level in the sub-tanks towards the position of the level sensors. Ink in that subtank, or it can be done through a difference in height of the subtanks from each other. This embodiment is advantageous when small pressure differences already create a desired ink flow rate through the print head, in which case the hydrostatic difference is easily implemented without serious mechanical consequences in the implementation of the embodiment, and is advantageous because only A pressure value P2 = P3 is made available to the slide ink system.

The ink in the supply subtank 20 on the slide ink system is replenished from a supply container 40 located outside the shaft and through a through-flow degassing unit. A pressure P4 can be applied to the supply vessel 40 through a pressure connection 43. The pressure P4 in the supply vessel 40 is adjusted higher than the pressure P2 in the supply subtank 20 to force an ink flow of the Supply vessel 40 to the supply subtank 20 when the reset valve 24 is opened. The pressure difference P4-P2 between the supply vessel 40 and the supply subtank 20 is chosen based on the desired flow rate of the permissible disturbance of the ink-free surface in the supply subtank 20 during replenishment, of a known flow resistance in the ink path from the supply container 40 to the supply subtank 20, of the pressure drop in the unit of degassing 60, and of the difference in hydrostatic height between the supply vessel 40 and the supply subtank 20. The pressure P4 can be chosen in a range of 200 mbar to 1000 mbar. A practical example for the value of pressure P4, in combination with P2 equal to -30 mbar, can be +400 mbar. It is preferred that the pressure difference P4-P2 can create an ink flow rate of at least 1000 ml / h between the supply container 40 and the supply subtank 20. This preferred minimum ink flow rate is related to the active degassing unit 60, which needs a minimum through flow to function properly, as will be described later.

On the return side of the ink system 1, the ink that is returned from the printhead 10 enters the return subtank 30, where the ink level increases. The ink level in the return subtank 30 has a hydrostatic contribution in the regulation of the back pressure in the nozzles and, therefore, the ink level in the return subtank 30 must be kept within the limits, in a similar manner at the level of ink in the supply subtank 20 that needs to be kept within the limits. The ink in the return subtank 30 in the slide ink system 3 is drained into the return vessel 50 located off the shaft. A pressure P5 can be applied to the return vessel 50 through a pressure connection

53. The pressure P5 in the return vessel 50 is adjusted to be lower than the pressure P3 in the return subtank 30 to force an ink flow from the return subtank 30 to the return vessel 50 when a valve is opened. drain 34. The drain valve 34 opens and closes under the control of one or more of the detection sensors of level 35, 36 or 37 of the return subtank 30. Depending on the operating back pressure window for the print head 10, the ink level detection sensors in the return subtank 30 can be configured to allow a minimum to maximum height difference of ± 5 cm, more preferably ± 1 cm, more preferably ± 0.5 cm. The pressure difference P5-P3 between the return vessel 50 and the return subtank 30 is chosen based on the desired flow rate, the permissible disturbance of the ink-free surface in the return subtank 30 during drainage, of a known flow resistance in the ink path from the return subtank 30 to the return vessel 50, and the difference in hydrostatic height between the return vessel 50 and the return subtank 30. The pressure P5 can be chosen in a range from -100 mbar to -950 mbar. A practical example for the value of pressure P5, in combination with P3 equal to -40 mbar, can be -300 mbar. It is preferred that the pressure difference P5-P3 can create an ink flow rate of at least 1000 ml / h between the return subtank 30 and the return vessel 50. The ink returned to the return vessel 50 is used to replace the supply container 40, which is now described.

To ensure a constant supply of ink and drainage of the ink from the slide ink system 3, the supply container 40 of the ink system outside the axis 2 needs to have continuously available ink, while the return container 50 of the system Ink off axis 2 needs to have continuously available drainage capacity. This is achieved by filling and draining operations of the supply container 40 and the return container 50, respectively. These operations are less critical with respect to maintaining the precise ink levels in the containers 40 and 50. The supply container 40 can be replenished through the ink inlets 41 and 48 from two sources: a hydraulic connection with the ink container. return 50 through the ink outlet 52 will replenish the supply container 40 with return ink from the print head, and a hydraulic connection with the main ink tank 70 will replenish the supply container 40 with fresh ink to compensate the ink which was ejected from the printhead. One of the possible procedures may be that the replacement of the supply container 40 is actuated by an ink level sensor 46 and begins with ink from the return container 50, by default and if possible. If during this replenishment process the ink level in the return container 50 is insufficient to continue supporting the replenishment process, that is, an underflow condition occurs, the replenishment is interrupted through the return container 50 and the replenishment it is assumed by the main ink tank 70, until the amount of return ink in the container 50 is again sufficient to continue supporting the replenishment process through the return container 50. The cause of an underflow condition in the Return container 50 is the ink consumption by the printhead 10. When the ink is consumed, that is, when printing, the total amount of ink circulating in the ink system 1 gradually decreases and the ink in one of the ink buffer elements of the ink circulation system, that is, one of the sub-tanks or one of the containers, will suffer a condition of its b overflow, that is, below your normal ink operating level. It is preferable to allow this underflow condition to only occur in the return container 50, because the level of ink in the return container 50 is the least critical for the operation of the entire circulation system. The line between having an underflow condition or not in the return container 50 is somewhat arbitrary, but can be chosen, for example, to guarantee the replacement of the ink in the supply container 40 during the time period of a replacement operation. of the main tank, that is, during a period of time in which the supply container 40 cannot be replenished through the main tank 70. An underflow condition in the return container 50 can be detected by an ink level sensor 56.

The process of replenishing fresh ink through the pump 73 can operate under the control of the underflow detection in the return container 50, under the control of a printer controller that controls the amount of ink consumed by the print head for printing, or manually operated in the case of emptying the main tank by an operator to replace it with a new one.

An alternative to the progressive replacement of the supply container 40 from the main ink tank 70, when the ink is consumed and printed by the print head, is a one-time replenishment with the entire contents of a main ink tank. A possible embodiment of this alternative is shown in Figure 8. In Figure 8 the return container 50 is also used as a buffer container. The return container 50, which is at a negative pressure P5, can aspirate an entire ink cartridge 80 when the cartridge 80 is hydraulically coupled with the return container 50, provided that the return container 50 can store the ink volume in the cartridge 80. The advantage of this embodiment is that a quantity of fresh ink is loaded into the circulation system 1 is a one-time action of an operator, after which this operator has time to replace the ink cartridge 80 Empty with a new one. Preferably, there is a valve 84 between the ink cartridge 80 and the return container 50 to control the start and stop of the loading process. In the loading process there is no pump involved, which is also an advantage; The negative pressure P5 in the return vessel 50 establishes a pumping action. Depending on the ink consumption in the ink system, that is, the amount of ink printed by the print head (s), it may be advantageous to replace the cartridge 80 with a drum when the ink consumption is high. The cartridges typically provide an amount of ink that up to about 1 or 2 liters. Drums, on the other hand, can easily provide amounts of ink above 2 liters.

FIRST REALIZATIONS -Non-printing modes

The pressure P2 in the supply subtank 20 can be selected from at least three predetermined values P21, P22 and P23 corresponding to different operating conditions of the printhead 10. These predetermined pressure values for the supply subtank 20 cooperate with a set parallel of predetermined values P31, P32, P33 for the pressure P3 in the return subtank 30. A first operating condition of the printhead corresponds to a normal printing condition described above. For this purpose, a set of valves (see Figure 1) could be operated to link the default values P21 and P31 to their respective sub-tanks.

A second operating condition of the printhead may be a purge operation, where the pressures applied to the nozzles is such that the ink leaves the nozzles without actuating the nozzles. For a purging operation, equal positive pressures are applied to the supply subtank 20 and the return subtank 30. In this case there is no through flow in the printhead 10, and all the ink available in the supply subtank 20 and in return subtank 30 is purged through the print head nozzles. It is clear that a purge condition can also be created by two positive but uneven pressures, in which case a throughflow will be created in the printhead

10. In the embodiment of Figure 1, the predetermined pressures P22 and P32, whether the same or different, can be selected to create the purging conditions. The inkjet printhead can be purged with pressures between 50 mbar and 500 mbar. A practical example for the embodiment in Figure 1 may be to set P22 and P32 equal to 150 mbar.

A third operating condition of the printhead 10 is used to create a perspiration nozzle plate before cleaning the nozzle plate during the maintenance of the printhead. A perspiration nozzle plate can help absorb or remove any dirt on the nozzle plate before cleaning the nozzle plate with a cleaning brush. The pressure required for a nozzle to begin perspiring is typically a little less negative than the operating back pressure, that is, just outside the operating back pressure window in the direction of the positive pressure. The perspiration of a nozzle plate can be observed with pressures between 0 mbar and 50 mbar in the meniscus, therefore, slightly positive, while the back pressure of the nozzle for normal printing is slightly negative. As for the purge operation, the perspiration of the nozzle plate can be observed with the same pressure values P23 and P33 in the supply subtank 20 and the return subtank 30, respectively, in which case there is no flow to through printhead 10, which is not a requirement for this operating mode. A practical example for the embodiment in Figure 1 and with a height difference h between the ink-free surface in the sub-tanks and the nozzle plate of about 30 cm, P32 and P33 can be equal to -26 mbar to create a Pressure

slightly positive on the mouthpiece.

As shown in Figure 1, the different predetermined values for the pressure in the supply and return subtank can be provided from a pressure generation subsystem, which is shown very schematically as several pressure regulator pictograms, and a valve assembly The valves may be part of the subtank assembly they belong to, and as such, they are part of the ink transport system 3. In this case, each subtank assembly is connected to a plurality of pressure tubes from the regulation system of the pressure, which may be located off the axis. On the other hand, the valves can be placed off-axis, in which case each subtank assembly on the slide has only one connection of the pressure tube to the configuration of the valve outside the axis. It is very much up to the printer configuration to determine which pressure distribution configuration is preferable. If several printheads are used, each with its individual supply and return subtank, a single pressure bar could be used for the distribution of each of the preset pressure values to all application points of the plurality of printhead subtanks. Additional optimization is possible if all printheads in the printhead configuration always operate in the same way, that is, print simultaneously, purge simultaneously or clean simultaneously. In this case, the pressure switching can be done off-axis and only one pressure bar is necessary to distribute the selected predetermined pressure to all application points. ACTIVE DEGASIFICATION

It is known from the prior art that the reliability of injection of the printheads can be significantly increased by providing degassed ink to the printhead. In the field of inkjet printing, degassing is also known as air removal or deaeration. It is the process of reducing the amount of gas, for example, oxygen or nitrogen or other gases, dissolved in the ink. The embodiment of the invention depicted in Figure 1 includes an active degassing unit 60 for controlling the amount of gas in the ink. The term "active" refers to the property of being able to control the level of removal of dissolved gas from the ink to an objective value, often referred to as a control setpoint. The process parameters may be available to control the level of dissolved gas withdrawal can be the vacuum pressure used with the degassing unit, the flow rate of the ink through the degassing unit, the type of semipermeable membrane used in the degassing unit, etc. Active control of the level of withdrawal of dissolved gas has the following advantages. On the one hand, the amount of air dissolved in the ink can be controlled to be as low as possible to avoid cavitation of the ink during rapid pressure changes in the ink chambers of the print head, for example, variations in the MHz range for piezoelectric inkjet printheads. On the other hand, the amount of air dissolved in the ink can be controlled so that it is not too low due to the chemical stability of the ink can become a problem. For example, a UV curable ink may begin spontaneous (thermal) curing when the amount of oxygen in the ink is too low. With active degassing, the level of removal of dissolved gas from the ink can be controlled within minimum and maximum levels. It has also been found that the level of dissolved gas in the ink is susceptible to changes during its permanence in the ink system. For example, the ink supply side of an ink system may comprise several components that are not "air tight" and therefore allow gas exchange between the ink and its surroundings. This is, of course, a relatively slow process, but when the ink is hours, days or weeks in an unused ink system, this aeration process becomes relevant.

An ink system according to the invention, therefore, includes an active through-flow degassing unit 60 that controls the ink circulating continuously towards a target dissolved gas level. An example of a through-flow degassing unit suitable for inkjet inks is a MiniModule hollow fiber membrane type degassing unit available from Membrana GmbH. Hollow Celgard® fibers are hydrophobic and provide a surface area for a liquid and a gas phase to come into direct contact with each other without the liquid entering the pores. These hollow fibers do not suffer from clogging, a problem can be the porous membrane type degassing units. Generally, in the through-flow degassing units, the removal of dissolved gas is a function of the through-flow rate of the ink, the type of ink, the applied vacuum pressure P6 and, of course, the construction of the ink itself. degassing unit It has been found that the level of removal of dissolved gas in the ink reaches an asymptotic value after two or three steps of the ink through the degassing unit. An active through-flow degassing unit as part of an ink circulation system allows the ink system to provide degassed ink of the correct quality to the print head almost instantaneously and continuously. The supply of degassed ink is independent of print performance (ink consumption speeds), maintenance or purge operations, printer reset, stops to change media, etc. The printer can print reliably from the first centimeter of the print medium. It has also been found that the dissolved gas removal process works effectively only with a minimum through flow of ink. Measures of removal of dissolved gas as a function of the through flow through the degassing unit are shown in Figure 6. The figure shows that the most efficient operating window of the degassing unit is above a through flow of 1000 ml / h

An alternative to the selection of an asymptote value for the level of gas withdrawal dissolved in the circulation ink is the use of an aeration module combined with a degassing unit. The aeration module can be inserted into the ink circulation circuit at the front of the degassing module and bring the level of gas dissolved in the ink back to an equilibrium or saturation level. This aeration module may comprise, for example, a depressurization valve that reduces the pressure of an available pressurized air connection to a pressure value suitable for the injection of air into a component already pressurized in the ink system. For example, if the aeration module is connected to the ink supply container 40, which is pressurized at a pressure P4, the air must be injected at a pressure above P4. Between the depressurization valve and the supply vessel 40 a control valve is placed to control the air injection process, for example, on / off. In addition to the depressurization valve and the control valve, there may be agitation means to accelerate the process of dissolving gas in the ink. The degassing unit that is downstream of the ventilation module always receives the ink with an equilibrium amount of dissolved gas, and always outputs the ink with a reproducible level of dissolved gas removed, the level being dependent on the manufacturing configuration or of the operating configuration of the degassing unit. The aeration module can be inserted into the ink circulation system 1 in a position after the return subtank 30 and before the degassing unit 60, and is preferably inserted near the return container 50.

In the embodiment of the invention shown in Figure 1, the through-flow degassing unit 60 is a separate module in the ink circulation system 1. There are several advantages related to this configuration. First, there is an advantage to maintenance, providing the degassing unit 60 as a replaceable module in the ink system 1, unlike, for example, a degassing unit integrated in the print head. This advantage is important because a degassing unit 60 may have a shorter life than the printheads in the printer and may require regular maintenance, such as cleaning, backwashing, etc. Secondly, there is an advantage to its suitability for use, that is, the characteristics of the degassing unit can be chosen depending on the type of ink, the flow rates, or other parameters of the printer. All these considerations make an individual degassing unit favorable.

An alternative embodiment for the active degassing module is shown in Figure 7. The alternative embodiment includes a vacuum brake valve 64 and a filter 65. The vacuum brake valve 64 breaks the vacuum applied to the degassing unit 60 in the event that ink circulation stops for any reason, for example, for example. , stopping the machine, or when the ink flow through the degassing unit 60 is below a minimum value. Some types of ink have been found to degrade when retained in an operating degassing unit for too long. For example, UV curable inks begin to cure when they are held too long in the degassing unit under vacuum pressure. Gels begin to form inside the ink that can significantly alter the injection performance of the printhead. Therefore, a second precaution is taken to reduce the risk of gels entering the print head, that is, the additional filter 65 is placed between the degassing unit 60 and the supply subtank 20, physically placed as far as possible. possible near the supply subtank 20. Filter 65 filters the gels out of the ink.

Figure 11 shows an even more preferred embodiment of the degassing unit 60, wherein the vacuum connection 63 of the degassing unit 60 is connected to a control valve 66 which allows controlling the vacuum applied to the degassing unit 60 a a target value of vacuum pressure. The control valve 66 controls the vacuum pressure by switching between a fixed vacuum P6 and the atmospheric pressure Patm. A suitable valve for this type of control may be a Rocker 3/2 way valve available from Bürkert Flduid Control Systems (United Kingdom). The advantage of controlling the vacuum applied to the degassing unit 60 is that the vacuum pressure can be adjusted based on a series of operating parameters of the ink circulation system, for example, the ink flow rate through of the degassing unit 60, the type of ink used, the temperature of the ink, the average number of ink passes through the circulation system, etc. The embodiment of Figure 11 can also be used in an on / off switching mode for the application of the fixed vacuum P6 or the atmospheric pressure Patm to the degassing unit 60. The use of on / off of the 3 / valve 2-way can be controlled by operational events, for example, during a traffic break, during periods of non-printing, etc. ALTERNATIVE REALIZATION OF THE SUBTANK

In the first embodiment, the supply subtank 20 and the return subtank 30 are separate modules with a similar operating mode. An alternative design is shown in Figure 3. Whenever possible, the reference numbers in Figure 1 are reused for features with similar functionality. A subtank of the printhead 90 is provided with a first compartment I and a second compartment II separated by a wall 91 fixed to a bottom of the subtank of the printhead 90 and used as an overflow from the compartment I to the compartment II. The ink continuously overflows from compartment I to compartment II through the overflow wall 91. Thus, the level of ink in compartment I is constant and is not measured, while the level of ink in compartment II is not constant and, therefore, is measured with ink level sensors 35, 36 or 37, which have functions similar to those described together with the return subtank 30 of the first embodiment. The measurement of the ink level in compartment II can control the refill valve 24 and / or the drain valve 34 to maintain the level of ink in the compartment II in the print head subtank 90 within an allowable operating window ( see previous descriptions). The reset valve 24 and the drain valve 34 can be chosen to be reduction valves that reduce the total ink supply pressure and the drain pressure of, for example, 400 mbar and -300 mbar, respectively, so that a continuous and constant flow of the ink through the subtank of the printhead 90 can be established. This is different from the first embodiment described together with Figure 1, wherein the reset valve and the drain valve were switching valves. and where they operate on a high frequency opening / closing base.

The printhead subtank 90 has an ink outlet 22 linked to the ink inlet 11 of the printhead to provide ink from the compartment I to the printhead, and an ink inlet 31 linked to the ink outlet 12 of the print head for return ink from the print head to compartment II of the printhead subtank 90. The height difference between the ink levels in compartment I and compartment II of the printhead subtank 90 creates a difference in hydrostatic pressure DP between the ink outlet 22 and the ink inlet 31, so that an ink flow is spontaneously established from the ink outlet 22 through the printhead 10 and the return to the ink inlet Ink 31. ΔP is functionally comparable with the pressure difference P3-P2 in the first embodiment of the invention.

The pressure connection 93 can be used to superimpose a pressure on the pressure of the printing ink, established through valves 24 and 34, both for non-printing operation or to adjust the printing conditions. For example, a purge operation or forced perspiration of the nozzle plate.

A variant to the overflow wall 91, as shown in Figure 3, can be a wall that extends from the bottom of the subtank to the top of the subtank, and which has only one opening that serves as an overflow opening. . Statically this variant would be equivalent to wall 91 in Figure 3, but dynamically prevents a large amount of ink from spilling from compartment I to compartment II by accelerating and decelerating the subtank in a slide of the printhead, thus distorting the balance of hydrostatic pressure.

The use of additional partitions in compartment II used as breakwaters will further stabilize the ink-free surface in compartment II as subtank 90 moves reciprocally on the slide.

The valve 24 can be replaced by a continuous operation pump, since it mainly serves to maintain a condition of continuous overflow from compartment I to compartment II. Ink level control in compartment II can be performed with valve 34 only. REALIZATIONS FOR SPECIFIC PRINTER CONFIGURATIONS: STATIONARY PRINT HEAD

In a stationary inkjet printhead configuration, dividing an ink system into an off-axis ink system and a sliding ink system can be somewhat artificial, since there are no scanning components. However, it can be advantageous to keep the components that operate very closely with the printhead, such as the supply subtank and the return subtank, physically grouped together with the printhead in a "sliding" subset. One of the obvious advantages is the less static or dynamic pressure drop between the sub-tanks and the printhead. PERFORMANCES FOR SPECIFIC PRINTER CONFIGURATIONS: MULTIPLE PRINT HEADS

Although Figure 1 shows an ink system that includes only one printhead, it is apparent to one skilled in the art that multiple printheads can also be included. Different configurations of ink systems are possible.

The off-axis ink system may be common for all printheads, while the slide ink system of Figure 1 doubles several times according to the number of printheads in the configuration. It may be advantageous to have an individual supply subtank and a return subtank dedicated to each printhead, as this would allow for the individual maintenance of the printhead, the individual control of the back pressure and the flow control, and the individual damping of the forces of inertia of acceleration or deceleration of the ink. The additional ink tube resulting from the use of the individual supply and return subtanks for each printhead can be reduced by mechanically integrating the subtanks and the printhead into a single functional and compact subset.

In stationary printhead applications or in less critical reciprocal motion printhead applications, a series of components in the slide ink system can be grouped together. The advantage is a simpler ink system with fewer components together. As an example, the return subtanks of the multitude of alternate printheads, which build a printhead with a single page width, can be combined into a single return subtank, which serves all printheads in the Page width head assembly. This configuration allows individual control of the back pressure through the pressure in the individual supply subtanks that are still assigned to each of the individual printheads, but the purge would be organized for all printheads in the printhead assembly Wide page printing simultaneously. A number of other combinations are possible, depending on the functional specifications that the person skilled in the art would integrate into the ink system and its operation.

A mechanical simplification of the slide ink system in alternate motion printhead configurations is also possible. The plurality of supply subtanks, one for each printhead, can be combined into a single supply subtank that serve all printheads. The only supply subtank can still be part of the slide ink system and can be mounted on the reciprocating slide back and forth, along with the printheads. This embodiment has the advantage of limiting the number of subtanks in the slide and still prevents pressure waves in the ink tubes between the slide ink system and the off-axis ink system from entering the printheads. Between the single supply subtank and the plurality of printheads, a plurality of valves can be used to individually cut the printhead from the ink supply. In normal printing mode each valve opens to allow ink supply from the single subtank to the printheads. The closing of the valves is advantageous in the non-printing mode. For example, if a printhead of the plurality of printheads in the slide needs to be purged during maintenance, the pressure in the single supply subtank, and thereby the back pressure in the nozzles, rises and the ink leaves the printhead nozzles. If the corresponding valves with the printheads that do not require a purge operation are closed, these printheads are separated from the high ink pressure in the single supply subtank, excluding them from the purging operation and saving ink quantities significant. Generally speaking, when multiple printheads that print with the same ink are used, a single off-axis ink system supplies and distributes the ink to the multitude of ink printheads in the slide ink system. If n printheads are involved, each requires a minimum through-flow of ink from the printhead to function properly, then the off-axis ink system must be designed to supply that minimum amount of ink flow to the system of ink Sliding ink where the ink flow will be distributed. ALTERNATIVE REALIZATION OF "COMMON RAIL"

With reference to Figure 9, an embodiment of an ink circulation system is described, especially suitable for multi-head configurations and embodying a number of design alternatives mentioned above. The alternative embodiment includes an ink supply subtank 20 and an ink return subtank 30 having similar functionality, as described above. The supply subtank 20 and the return subtank 30 are equipped with an ink level sensor 26 and an ink level sensor 36, respectively. A preferred embodiment of the level sensors 26 and 36 may include an ultrasonic level sensor with a switching output or an analog output, as available from Hans Turck GmbH & Co (Germany) or a floating element having a magnet, arranged in the subtank, and associated therewith a set of Hall detectors, arranged outside the subtank along a vertical wall. The number of Hall detectors in the set determines the degree of binaries towards continuous measurement. The level sensors can be used to maintain a height difference between the ink-free surface in the supply subtank 20 and the ink-free surface in the return subtank 30. This height difference creates a hydrostatic pressure difference ΔP which It is the driving force for the flow of ink through the print head, as will now be explained. The supply subtank 20 provides ink to a supply busbar 28, which can be, for example, an extruded profile of an ink-resistant material (for example, stainless steel). The supply busbar 28 has multiple connections to the ink inlets of the multiple printheads 10. The ink outlets of the multiple printheads 10 are connected to a return busbar 38, which in turn is connected to the return subtank 30. The supply busbar 28 and the return busbar 38 replace a significant amount of ink tube between subtanks and printheads and, therefore, offer a significant advantage. In addition, the busbars can be sized to reduce the resistance to the flow of the ink path from the supply subtank 20, through the printheads 10 and back to the return subtank 30 to almost zero. The printheads 10 are connected to the busbar 28 and 38 through actuating valves that can deactivate each individual printhead 10 of the ink system, as shown in Figure 9. The valves have two main advantages: (1) in a non-operational mode of the printer, the printhead can be isolated from the ink system, thereby reducing the risk of ink leaking from the ink system through the nozzles of the printhead, for example, due to to a loss of back pressure in the nozzles, and (2) in a maintenance operation, the print heads that do not require purging, which can leave them isolated from the ink system before applying the increased purge pressure in the ink system , thus reducing the amount of ink spent for purging. The back pressure on the nozzles of the multiple printheads is actively controlled by the pressure P0 applied on the ink-free surfaces of the supply subtank 20 and the return subtank 30. The ink system closes through a path of ink from the return subtank 30 back to the supply subtank 20 through the pump 76, the degassing unit 60 and the filter 65. Preferred embodiments of the pump, the degassing unit and the filter have been described in the sections previous. The pump 76 operates under the control of the level sensor 36 of the return subtank 30, similar to the operation of the drain valve 34 in embodiments described above. The ink circulation system of Figure 9, as described so far, can be placed on the slide of an inkjet printing device. The embodiment is especially suitable for industrial inkjet printing devices, where a robust alternative motion slide to support the ink circulation system is no problem. Outside the shaft, a supply container 40 and a pump 73 are placed to fill the supply subtank 20 with fresh ink, when the ink is consumed in the printheads 10. The pump 73 operates under the control of the level 26 sensor of the subtank. Supply 20. A pump is used instead of a replacement valve as in previous embodiments, because the ink in the supply container 40 is maintained at ambient pressure. The supply container 40 comprises a coupling for a main ink tank, for example, of the drum type, which is automatically emptied when coupled. An embodiment may, for example, provide a blade in the coupling that automatically breaks a seal in the drum when the drum is engaged; The drum is emptied by gravity.

The embodiment as shown in Figure 9 has multiple advantages: reduction of the number of connections and fluid tubes, circulation and degassing of local ink (in slide), ink circulation system with fewer components, ink circulation pumped in place of pressurized ink circulation, which is safer in case of problems, minimal interaction between the ink supply part of the slide and the ink supply part outside the axis, only a supply subtank and a return subtank for multiple printheads in the configuration, etc.

It is clear that the concept of a busbar is not limited to the described ink circulation system, but that the concept can be applied in other configurations where a plurality of inkjet printheads must be connected to a common supply or return of ink. “OPTIGASS” ALTERNATIVE REALIZATION

With reference to Figure 10, an alternative embodiment of an ink circulation system is described, especially suitable for improving the operation of the degassing unit. It has been said previously that for optimal operation of an active degassing unit of the through flow a minimum flow of ink is required through the degassing unit. According to Figure 6, this minimum ink flow is approximately 1000 ml / hr. In the previous embodiments of the ink circulation system, the ink flow through the degassing unit was also the ink flow through the print head. In a number of applications, an optimal ink flow through the degassing unit cannot be an optimal ink flow through the print head. The ink circulation system shown in Figure 10 provides a solution to this problem, since it allows a higher flow through the degassing unit than the flow through the print head. The embodiment in Fig. 10 includes an ink supply subtank 20 and an ink return subtank 30 having a functionality similar to that described above. The supply subtank 20 and the return subtank 30 are equipped with an ink level sensor 26 and an ink level sensor 36, respectively. A preferred embodiment of the level sensors 26 and 36 may include an ultrasonic level sensor with a switching output or an analog output as available from Hans Turck GmbH & Co (Germany) or a floating element having a magnet , arranged in the subtank, and associated therewith, a set of Hall detectors, arranged outside the subtank along a vertical wall. The number of Hall detectors in the set determines the degree of binaries for continuous measurement. The level sensor can be used in Figure 1, that is, the level sensor 28 is used to control the reset valve 24 and the level sensor 38 is used to control the drain valve 34. The level sensors are also used. they can use to maintain a height difference between the ink free surface in the supply subtank 20 and the ink free surface in the return subtank 30. This height difference creates a hydrostatic pressure difference ΔP which is the driving force for ink flow through the print head 10. The pressure difference ΔP controls the speed of ink flow through the print head 10. The back pressure on the nozzles of the print head 10 is actively controlled by the pressure P0 applied to the ink-free surfaces of the supply subtank 20 and the return subtank 30. The ink circulation system further comprises a container return 50 for draining the return subtank 30 when the drain valve 34 is opened, driven by a negative pressure difference P5-P0. The return container 50 can be loaded with a quantity of fresh ink from a cartridge 80, driven by a negative pressure P5 in the return container 50. The amount of fresh ink is to replace the amount of ink printed by the printhead 10 Therefore, the ink level in the return container 50 can be measured with a level sensor 56. A preferred embodiment of the level sensor 56 may be a T / LL level sensor 55 available from Fozmula (Kingdom Attached) or a level sensor of ultrasonic type similar to that used for sub-tanks (see above). The ink path supplied by the component described so far with reference to Figure 10, that is, from the reset valve 24 to the return container 50, is also indicated as the main path. The ink system is closed through a conditioning path from the return container 50 back to the reset valve 24, the conditioning path comprising a circulation pump 76, a degassing unit 60 and a filter 65. Preferred embodiments of the pump, the degassing unit and the filter have been described in the previous sections. Parallel to the conditioning path, a deflection path is provided, comprising a flow restriction 78 that allows the ink to flow from the filter outlet 65 back to the inlet of the circulation pump 76, therefore, by diverting the main path through the printhead 10. Embodiments of a flow restriction 78 may include a restriction valve, a reduction valve, a spring loaded check valve or a simple constriction in the ink tube. The operation of the deviation path is now explained. The circulation pump 76 operates continuously and pumps ink at a given flow rate through the degassing unit 60 and the filter 65 to provide the ink in the branch of the main path and the deflection path. Two situations may occur. In a first situation, the reset valve 24 is closed and all the ink from the conditioning path flows to the deflection path, through the flow restriction 78 and against the flow resistance provided by the flow restriction 78 The operation of the circulation pump 76 increases the ink pressure in the branch of the main path and the deflection path to a value that counteracts the pressure resistance of the flow restriction 78. The circulation pump 76 makes continuously circulate the ink in a closed loop with the filter 65 and the degassing unit 60. This closed loop circulation is therefore indicated as the conditioning circulation. In a second case, the reset valve 24 is opened and the ink from the filter 65 flows to the supply subtank 20, against a back pressure P0 in the supply subtank 20, which is generally lower than the back pressure set by the restriction of flow 78. Circulation pump 76 now supplies ink to the main path. The ink flow rate through the main path is determined by the difference in pressure between the ink-free surfaces in the subtanks 20 and 30, and can also be determined by the absolute pressure P0 in the subtanks 20 and 30. This Ink circulation is known as print circulation. During the printing circulation, the ink flow through the flow restriction 78 may be insignificant or completely cut, depending on the specific embodiment used to perform the flow restriction 78. In operation, the reset valve 24 is operated in intermittently at high frequency, creating a pseudo-continuous controllable ink flow through the main path during printing. That is, in operation, the reset valve 24 is functionally comparable with a controllable flow restriction. The reset valve 24 and the flow restriction 78, therefore, allow two parallel ink flows, that is, a print flow through the main path (including the print head) and a conditioning flow through of the deviation path. From these two ink flows, the printing flow is controllable and the conditioning flow delivers the surplus of what comes from the circulation pump. The main advantage of this alternative embodiment, therefore, is that the ink flow through the degassing unit can be adjusted independently of the ink flow through the print head and, therefore, the degassing unit can operate at optimum speed, regardless of the flow restrictions applicable to the flow rate through the print head.

In an alternative embodiment that serves the same purpose, that is, optimal degassing conditions, the deflection path is placed between the outlet of the degassing unit 60, that is, before the reset valve 24, and the inlet of ink to the return container 50, that is, after the drain valve 34. The ink content of the return container 50 is now also included in the conditioning flow. REALIZATIONS FOR SPECIFIC PRINTER CONFIGURATIONS: MULTIPLE COLORS

In a color inkjet printer, each color is printed with a different printhead or set of printheads. Each color has its own ink system with an off-axis part and a sliding part. Each ink system can support one or a multitude of printheads of the same color. The multitude of printheads that print the same color can be mounted in an alternate motion module through the printing medium and the rows of printing that are wider than the width of a single printhead, or can be placed alternated in a full page width printhead assembly. PERFORMANCES FOR SPECIFIC PRINTER CONFIGURATIONS: EXTREME TYPE TYPE PRINT HEAD

Until now, the present invention has been described with printheads of the flow-through type. The advantages of a continuous ink circulation with continuous active degassing are really important with the use of printheads type of through flow, since the ink in the printhead is continuously regenerated with fresh ink and conditioned ink. Prior art ink systems for printhead type printheads with only one ink inlet often have a supply of an ink chain path from a main ink tank or cartridge to the printhead. These ink systems do not have ink circulation and, therefore, the ink in the printheads, tubes and other components that cannot be continuously regenerated. An embodiment of the present invention for an end puller printhead can be very similar to the embodiment depicted in Figure 1, except that the printhead is not connected in series between the supply subtank and the return subtank, but in parallel with a direct access between the supply tank and the return tank. Figure 4 shows the slide ink system of an embodiment of the invention for printheads of the end puller type. For the inkjet ink supply systems, the invention can have the following advantages. First, the amount of ink retained in the ink system that does not regenerate through circulation is limited to the amount in the print head of the end handle. Consequently, in the case of failure or maintenance of the nozzle, the amount of "waste" ink that must be purged through the print head is also reduced before fresh ink is available in the nozzles. Furthermore, as the degassing properties of the ink can degrade over time while it resides in the supply tubes and intermediate deposits of the ink system, the regeneration and circulation of the ink limits the amount of "initial ink residue" that, for example after a production stop for a weekend, cannot be rejuvenated and, therefore, has to be purged through the print head. A second advantage is that a constant and optimal operating point can be provided for the in-line through-flow degassing unit, resulting in a better controlled dissolved gas removal level from the ink. Many degassing units are not suitable for operation at low flow rates, inherent in the one-way ink supply systems for printhead end heads, due to their high degassing feature.

Having described in detail the preferred embodiments of the invention, it will now be apparent to those skilled in the art that numerous modifications can be made therein without departing from the scope of the invention as defined in the appended claims. BRIEF DESCRIPTION OF THE REFERENCE NUMBERS 1 Ink system 2 Off-axis ink system 3 Sliding ink system 10 Printhead 10a, 10b Nozzle grouping 11 Ink inlet 12 Ink outflow 13, 14 Electrical or fluid connections 15 Conditioning means 19 Conditioning circuit 20 Supply sub-tank 30 Return sub-tank 40 Supply container 50 Return container 21, 31, 41, 51, 61 Ink inlet 22, 32, 42, 52, 62 Ink outlet 23, 33, 43, 53, 63, 93 Pressure connection 24 Refill valve 34 Drain valve 84 Ink charge valve 25, 35 Minimum level sensor 26, 36, 46, 56 Operating level sensor 27, 37 Level sensor maximum 28 Ink-free surface 29, 39, 49, 59 Closed container 48, 58 Fresh ink inlet 70 Main ink tank 71, 74 Check valve 72, 75, 65 Filter 73.76 Pump 78 Flow restriction 60 Unit throughflow degassing active 64 Vacuum break valve 66 Vacuum control valve 80 Ink cartridge 90 Print head subtanch 91 Overflow wall

Claims (9)

1. Ink circulation system (1) for use in a fall in the demand for an inkjet printing apparatus, said system comprising: An ink supply subtank (20) having a first ink level sensor (26) to control a first ink-free level surface in the supply tank (20), the first level sensor controlling a pump supply (73) for the replacement of the supply subtank with fresh ink when the ink is consumed; - an ink return subtank (30) having a second ink level sensor (36) that controls a second ink free level surface in the return subtank (30); - at least one printhead (10) having an ink inlet hydraulically connected to the supply subtank (20) and an ink outlet hydraulically connected to the return subtank (30); -creating the height difference between the first and second ink-free levels a hydrostatic pressure difference that provides a driving force for the flow of ink through the print head, in which the system further comprises: - a refreshment path from the return subtank to the supply subtank comprising a circulation pump (76) and an active through-flow degassing unit (60) to control a level of dissolved gas in the ink, whereby the Circulation pump (76) is controlled by the second level sensor (36).

2.

Ink circulation system according to claim 1, wherein the level of dissolved gas in the ink is controlled between a minimum level and a maximum level.

3.

Ink circulation system according to claim 2, wherein the minimum level or the maximum level are dependent on an ink parameter.

Four.

Ink circulation system according to any one of claims 1 to 3, wherein the through-flow degassing unit

(60) also comprises an ink inlet (61) to receive an ink flow, an ink outlet (62) to supply a degassed ink flow, a vacuum connection (63) to apply a vacuum, and a semipermeable membrane to provide a surface area for the ink and the vacuum to come into direct contact with each other without the ink penetrating the semipermeable membrane.

5.

Ink circulation system according to claim 4, which also comprises a valve (66) for controlling the vacuum applied to the degassing unit (60).

6.

Ink circulation system according to claim 4 or 5, wherein the semipermeable membrane is a hollow fiber type membrane.

7.

Ink circulation system according to any one of claims 1 to 6, wherein the supply subtank (20) is hydraulically connected to the input of a printhead through a supply busbar (28).

8.

Ink circulation system according to any one of claims 1 to 7, wherein the return subtank is hydraulically connected to the output of a printhead through a return busbar (38).

9.

Inkjet printing apparatus comprising an ink circulation system according to any one of claims 1 to 8.