WO2025257576A1

WO2025257576A1 - Printer and associated method

Info

Publication number: WO2025257576A1
Application number: PCT/GB2025/051313
Authority: WO
Inventors: David Andrew Horsnell; Michael Jeffrey Stamp; Nigel Edward Sherman
Original assignee: Videojet Technologies Ltd; Videojet Technologies Inc
Current assignee: Videojet Technologies Ltd; Videojet Technologies Inc
Priority date: 2024-06-14
Filing date: 2025-06-13
Publication date: 2025-12-18
Anticipated expiration: 2026-12-14
Also published as: GB202408545D0

Abstract

A continuous inkjet printer (1) comprises: a print head (3) which comprises: a cleaning chamber (26) selectively sealable by a sealing mechanism (34); a nozzle (22) for generating and ejecting a stream of ink droplets (36) for printing; at least one electrode (27, 28, 30) for guiding the stream of ink droplets; and a gutter (32) for receiving droplets of ink which are not used for printing; wherein the continuous inkjet printer is configured to enter a safe condition in response to an occurrence of an abnormality.

Description

Printer and method Technical Field _{The present disclosure relates to a continuous inkjet printer and a method of operating} _{the same. More particularly, but not exclusively, the present disclosure relates to a} _{continuous inkjet printer which performs a failsafe operation so as to enter a safe,} _{printable, condition in response to an occurrence of an abnormality.} Background In inkjet printing systems, the print is made up of individual droplets of ink generated at a nozzle and propelled towards a substrate. There are two principal systems: droplet on demand, where ink droplets for printing are generated as and when required; and continuous inkjet (CIJ) printing, in which droplets are continuously produced and only selected ones are directed towards the substrate, the others being recirculated to an ink system. CIJ printers supply pressurised ink to a print head droplet generator where a continuous stream of ink emanating from a nozzle is broken up into individual regular droplets by, for example, an oscillating piezoelectric element. The droplets are directed past a charge electrode, where they are selectively and separately given a predetermined charge, before passing through a transverse electric field provided across a pair of deflection plates, the pair comprising a high voltage (or extra high _{tension (EHT)) plate and a zero or negative voltage plate (the ‘ground’ plate). Each} charged droplet is deflected by the field by an amount that is dependent on its charge magnitude before impinging on the substrate, whereas the uncharged droplets proceed without deflection and are collected at a gutter from where they are recirculated to the ink system. The charged droplets bypass the gutter and hit the substrate at a position determined by the charge on the droplet and the position of the substrate relative to the print head. Typically the substrate is moved relative to the print head in one direction and the droplets are deflected in a direction generally perpendicular thereto, although the deflection plates may be oriented at an inclination to the perpendicular to compensate for the speed of the substrate (the movement of the substrate relative to the print head between droplets arriving means that a line of droplets would otherwise 69420664-1 not quite extend perpendicularly to the of movement of the substrate). The various components of the print head are typically contained within a cover tube or print head casing. In CIJ printing, a character is printed from a matrix comprising a regular array of potential droplet positions. Each matrix comprises a plurality of columns (strokes), each being defined by a line comprising a plurality of potential droplet positions (e.g. seven) determined by the charge applied to the droplets. Thus, each usable droplet is charged according to its intended position in the stroke. If a particular droplet is not to be used then the droplet is not charged and it is captured at the gutter for recirculation. This cycle repeats for all strokes in a matrix and then starts again for the next character matrix. Ink is delivered under pressure to the print head by an ink system that is generally housed within a sealed compartment of a cabinet that includes a separate compartment for control circuitry and a user interface panel. The ink system includes a main pump that draws the ink from a reservoir or tank (often referred to as a mixing tank) via a filter and delivers it under pressure to the print head. As ink is consumed, the reservoir is refilled as necessary from a replaceable ink cartridge that is releasably connected to the reservoir by a supply conduit. The ink is fed from the reservoir via a flexible delivery conduit to the print head. The unused ink droplets captured by the gutter are recirculated to the reservoir via a return conduit by a pump. The flow of ink in each of the conduits is generally controlled by solenoid valves and/or other like components. As the ink circulates through the system, there is a tendency for it to thicken because of solvent evaporation, particularly in relation to the recirculated ink that has been exposed to air in its passage between the nozzle and the gutter. In order to compensate for this, “make-up” solvent is added to the ink as required from a replaceable solvent cartridge to maintain the ink viscosity within desired limits. The ink and solvent cartridges are filled with a predetermined quantity of fluid and generally releasably connected to the reservoir, or mixing tank, of the ink supply system so that the reservoir can be intermittently topped-up by drawing ink and/or solvent from the cartridges as required. 69420664-1 _{One of the problems faced by operators existing CIJ printers is that of undesirable} build-up of deposits within, and around, the print head. Deposits include ink ‘fur’, created by non-volatile ink components which remain after the fluid component of the ink (and solvent mixture) has evaporated. Such deposits risk the accuracy of printing, the operation of the print head, and, in extreme circumstances, may result in the blocking of an ink ejection aperture of the print head (e.g. rendering the print head non- operational for at least a period of time). Deposits can exist internally within the print head, but also externally. For example, an (external) end face of the print head (e.g. in which the ink aperture is located) may also be liable to the build-up of deposits due to ink ‘splash-back’ during printing. CIJ printers generally operate in high throughput environments for which the printers, and inks, need to be able to keep up with high production line speeds, fast drying time requirements and virtually non-stop production. _{It is generally desirable to provide a CIJ printer which is robust in the high throughput} _{environments so as to maximise production output.} _{It is an object of the present disclosure, among others, to provide such a CIJ printer.} Summary to a first aspect of the present disclosure, there is provided a continuous inkjet printer which comprises: a print head which comprises: a cleaning chamber selectively sealable by a sealing mechanism; a nozzle for generating and ejecting a stream of ink droplets for printing; at least one electrode for guiding the stream of ink droplets; and a gutter for receiving droplets of ink which are not used for printing; wherein the continuous inkjet printer is configured to enter a safe condition in response to an occurrence of an abnormality. The safe condition refers to a condition in which the continuous inkjet printer is not damaged and can quickly start up to resume printing after the removal of the abnormality, without requiring complicated recovery procedures or service/maintenance from a service technician. The continuous inkjet printer automatically enters the safe condition, by for example performing a failsafe operation. 69420664-1 The failsafe operation may be by the continuous inkjet printer in response to the occurrence of the abnormality without any human intervention. By entering a safe condition in response to an occurrence of an abnormality, the continuous inkjet printer is able to protect itself from being damaged or negatively impacted otherwise by the abnormality, and can resume printing quickly after the removable of the abnormality, thereby reducing the downtime and thus improving the production output of the printer. The at least one electrode may comprise a charge electrode for imparting a charge to at least some of the stream of ink droplets and a deflection electrode for deflecting at least some of the stream of ink droplets. The deflection electrode and at least a part of the gutter may be disposed in the cleaning chamber. The deflection electrode may be a high-voltage deflection electrode. The charge electrode and the nozzle may be in fluid communication with the cleaning chamber. The sealing mechanism may seal an ink aperture of the print head through which the stream of ink droplets pass for printing onto a substrate in use. The print head may be a self-cleaning print head. The continuous inkjet printer may be operable to receive power from an external power supply, and the abnormality may comprise an interruption in power from the external power supply. The interruption in power from the external power supply may comprise a failure of the external power supply and/or a disconnection between the continuous inkjet printer and the external power supply. The continuous inkjet printer may be configured such that the sealing mechanism seals the cleaning chamber in response to the interruption in power from the external power supply. In other words, the safe condition of the continuous inkjet printer may comprise a condition in which the cleaning chamber is being sealed by the sealing mechanism. The operation of sealing the cleaning chamber by the sealing mechanism may be 69420664-1 considered as a failsafe operation of continuous inkjet printer. Using the sealing mechanism to seal the cleaning chamber reduces the drying rate of any residual ink within the print head. Depending upon a temporal length of the interruption, the print head may be able to resume printing without requiring a time-consuming deep cleaning. The sealing mechanism may have a first configuration in which the cleaning chamber is sealed, and a second configuration in which the cleaning chamber is not sealed. The sealing mechanism may be biased to the first configuration. The sealing mechanism may be mechanically biased to the first configuration. The sealing mechanism may be configured to transition from the first configuration to the second configuration by receiving electrical power to overcome the bias. The continuous inkjet printer may comprise a power storage device for storing electric power. The power storage device may comprise a capacitor and/or a battery. The sealing mechanism may be configured to seal the cleaning chamber by using the electrical power stored by the power storage device, in response to the interruption in power from the external power supply. The continuous inkjet printer may comprise a cleaning mechanism for cleaning the print head using a cleaning fluid. The cleaning mechanism may be configured to perform a cleaning operation to the print head by using the electrical power stored by the power storage device, in response to the interruption in power from the external power supply. In other words, the safe condition of the continuous inkjet printer may comprise a condition in which the print head has been cleaned by the cleaning operation. The cleaning operation may be considered as a failsafe operation of the continuous inkjet printer. By using the electrical power stored by the power storage device to clean the 69420664-1 print head, the residual ink within the head can be largely removed, thereby allowing the continuous inkjet printer to quickly resume printing once the interruption stops. The cleaning mechanism may be configured to at least partially fill the cleaning chamber with the cleaning fluid, during the cleaning operation. The cleaning mechanism may be further configured to drain the used cleaning fluid from the cleaning chamber after a soaking period, during the cleaning operation. The cleaning mechanism may be further configured to dry the cleaning chamber after the used cleaning fluid is drained, during the cleaning operation. The continuous inkjet printer may comprise a mixer tank configured to supply ink to the print head, and the abnormality may comprise an ink level within the mixer tank dropping below a threshold. The continuous inkjet printer may comprise a gutter pump configured to draw the droplets of ink which are not used for printing through the gutter to the mixer tank. The continuous inkjet printer may be configured to continuously circulate ink from the mixer tank, through a part of an ink supply line between the mixer tank and the print head, and through a venturi pump, and back to the mixer tank during a printing operation. The continuous circulation may cause the venturi pump to draw ink from an ink cartridge or solvent from a solvent cartridge to the mixer tank. The threshold may be a threshold ink level for maintaining the continuous circulation through the venturi pump. In other words, when the ink level within the mixer tank is above the threshold, the continuous circulation through the venturi pump can be maintained, thereby allowing _{the venturi pump to draw ink/solvent from respective cartridges to the mixer tank. Once} the ink level within the mixer tank falls below the threshold, the continuous circulation through the venturi pump would stop immediately or shortly. 69420664-1 The continuous inkjet printer may be to receive the ink cartridge and the solvent cartridge in use. The continuous ink jet printer may be configured to stop the supply of ink from the mixer tank to the print head so as to stop the print head from printing, in response to the ink level within the mixer tank dropping below the threshold. _{In other words, the safe condition of the continuous inkjet printer may comprise a} condition in which the supply of ink from the mixer tank to the print head is stopped. The operation of stopping the supply of ink from the mixer tank to the print head so as to stop the print head from printing may be considered as a failsafe operation of the continuous inkjet printer. The threshold may be a first threshold. The continuous ink jet printer may be further configured to generate a warning signal, in response to the ink level within the mixer tank dropping below a second threshold, wherein the second threshold indicates a higher ink level than the first threshold. According to a second aspect of the present disclosure, there is provided a method of operating a continuous inkjet printer, comprising: printing on a substrate using a print head of the continuous inkjet printer, wherein the print head comprises: a cleaning chamber selectively sealable by a sealing mechanism; a nozzle for generating and ejecting a stream of ink droplets for printing; at least one electrode for guiding the stream of ink droplets; and a gutter for receiving droplets of ink which are not used for printing; and the continuous inkjet printer automatically entering a safe condition in response to an occurrence of an abnormality. The method may further comprise: receiving power from an external power supply. The _{abnormality may comprise an interruption in power from the external power supply to} the continuous inkjet printer. The continuous inkjet printer automatically entering the safe condition may comprise: automatically sealing the cleaning chamber, by the sealing mechanism, in response to the interruption in power from the external power supply. 69420664-1 The continuous inkjet printer may a power storage device storing electric power, and a cleaning mechanism for cleaning the print head using a cleaning fluid. The continuous inkjet printer automatically entering the safe condition may comprise: automatically perform a cleaning operation, by the cleaning mechanism, to the print head by using the electrical power stored by the power storage device, in response to the interruption in power from the external power supply. The continuous inkjet printer may comprise a mixer tank configured to supply ink to the _{print head. The abnormality may comprise an ink level within the mixer tank dropping} below a threshold. _{The method may further comprise: continuously circulating ink from the mixer tank,} through a part of an ink supply line between the mixer tank and the print head, and through a venturi pump, and back to the mixer tank during the printing. The continuous _{circulation may cause the venturi pump to draw ink from an ink cartridge or solvent} _{from a solvent cartridge to the mixer tank. The threshold may be a threshold ink level} for maintaining the continuous circulation through the venturi pump. The continuous inkjet printer automatically entering the safe condition may comprise: automatically stopping the supply of ink from the mixer tank to the print head so as to stop the print head from printing, in response to the ink level within the mixer tank dropping below the threshold. The expression “automatically” used in the present disclosure means that the corresponding action (e.g., the continuous inkjet printer entering a safe condition, sealing the cleaning chamber, performing a cleaning operation, or stopping the supply of ink from the mixer tank to the print head) takes place without human intervention. According to a third aspect of the present disclosure, there is provided a continuous inkjet printer comprising: a print head which comprises: a nozzle for generating and ejecting a stream of ink droplets for printing; at least one electrode for guiding the stream of ink droplets; and a gutter for receiving droplets of ink which are not used for printing; 69420664-1 a mixer tank configured to supply to the print head, wherein the continuous inkjet printer is configured to continuously circulate ink from the mixer tank, through a part of an ink supply line between the mixer tank and the print head, and through a venturi pump, and back to the mixer tank during a printing operation, and the continuous circulation causes the venturi pump to draw ink from an ink cartridge or solvent from a solvent cartridge to the mixer tank; and a controller, wherein the controller is configured to stop the supply of ink from the mixer tank to the print head so as to stop the print head from printing, in response to an ink level within the mixer tank dropping below a threshold for maintaining the continuous circulation through the venturi pump. According to a fourth aspect of the present disclosure, there is provided a method of operating a continuous inkjet printer, comprising: printing on a substrate using a print head of the continuous inkjet printer, wherein the print head comprises: a nozzle for generating and ejecting a stream of ink droplets for printing; at least one electrode for guiding the stream of ink droplets; and a gutter for receiving droplets of ink which are not used for printing; supplying ink from a mixer tank of the continuous inkjet printer to the print head through an ink supply line between the mixer tank and the print head during the printing; continuously circulating ink from the mixer tank, through a part of the ink supply line, and through a venturi pump, and back to the mixer tank during the printing, wherein the continuous circulation causes the venturi pump to draw ink from an ink cartridge or solvent from a solvent cartridge to the mixer tank; detecting an ink level within the mixer tank; and in response to detecting that the ink level within the mixer tank has dropped below a threshold for maintaining the continuous circulation through the venturi pump, stopping, by a controller of the continuous inkjet printer, the supply of ink from the mixer tank to the print head so as to stop the print head from printing. Optional and/or preferred features as set out herein may be used either individually or in combination with each other where appropriate and particularly in the combinations as set out in the accompanying claims. The optional and/or preferred features for each aspect of the disclosure set out herein are also applicable to any other aspects of the disclosure, where appropriate. 69420664-1 Brief Description of the Drawings _{In may be more fully understood, a number of embodiments of} the disclosure will now be described, by way of example, with reference to the accompanying drawings, in which: _{Figure 1 is a schematic illustration of a CIJ printer according to an aspect of the} disclosure; _{Figure 2 is a schematic illustration of a CIJ printer according to another aspect of the} disclosure; _{Figure 3 is a schematic illustration of a print head used in the CIJ printer of Figure 1;} _{Figure 4 is a schematic illustration of a print head used in the CIJ printer of Figure 2;} Figure 5 illustrates processing steps of a method for cleaning the print head of Figure 3; Figure 6 illustrates processing steps of a method for cleaning the print head of Figure 4; Figure 7 is a simplified schematic diagram of a fluid system for the printer of Figure 1, incorporating the print head of Figure 3; Figure 8 schematically illustrates a working principle of a sealing mechanism used in the print head of Figure 3; _{Figure 9 schematically illustrates processing steps of a method of operating a CIJ} _{printer, according to a further aspect of the present disclosure.} _{In the figures, like parts are denoted by like reference numerals. It will be appreciated} that the drawings are for illustration purposes only and are not drawn to scale. 69420664-1 Detailed Description of the Preferred _{Figure illustrates a CIJ printer 1 according to an aspect of the} disclosure. The printer 1 comprises a printer body 2 (which may be referred to as a cabinet) connected to a print head 3 by an umbilical cable 4. The printer body 2 houses _{an ink system 5 and a printer controller 6 (hereinafter, “controller”). The printer body 2} also has an interface 7 (e.g. a display, keypad, and/or touch screen) for use by an operator. Although the controller 6 is disposed in the printer body 2 in the illustrated embodiment, in other embodiments the controller 6 may be disposed in the print head 3. In either case, the controller 6 is provided in electrical communication with the print head 3, and preferably also the printer body 2. The print head 3 is arranged to print on a substrate provided adjacent to the print head 3. The printer 1 typically comprises two cartridge connections for engagement with respective fluid cartridges. In particular, the printer 1 comprises an ink cartridge connection for engagement with an ink cartridge 8 and a (separate) solvent cartridge connection for engagement with a solvent cartridge 10. The cartridge connections typically each comprise a fluid port arranged to connect to a fluid pathway within the printer 1 to allow fluid to flow between the cartridges 8, 10 and other parts of the inkjet printer 1, such as the ink system 5 and the print head 3 (via the umbilical 4). The _{solvent cartridge 10 comprises unused solvent.} In operation, ink from the ink cartridge 8 and solvent from the solvent cartridge 10 can be mixed within the ink system 5 to generate printing ink of a desired viscosity that is suitable for use in printing. This ink is supplied under pressure from the ink system 5 to the print head 3 and unused ink is returned from the print head 3 to the ink system 5 (via the umbilical 4). When unused ink is returned to the ink system 5 from the print head 3, air may be drawn in with ink from a gutter of the print head 3. The air may then _{become saturated with solvent in the gutter line. The umbilical cable 4 comprises} flexible tubes bundled together with electrical wires (not shown). The print head 3 can be cleaned. In the illustrated example, the print head 3 is a self- cleaning print head. Without operator intervention, the print head 3 can be sealed, and a cleaning fluid (e.g. fresh and/or used solvent, or a fresh and/or used blend of 69420664-1 solvents) be flushed through at least the print head 3, in order to clean the print head 3. In other embodiments, the print head may not be a self-cleaning print head, but be _{cleaned using a wash station. Turning to Figure 2, a CIJ printer 1a according to such} an embodiment is schematically illustrated. The printer 1a shares many features in common with the printer 1 shown and described in connection with Figure 1, and corresponding components are denoted using the same numerals. The printer 1a comprises a wash station 12. The wash station 12 is for cleaning print head 3a, and specifically for cleaning one or more components of the print head 3a. As shown in Figure 2, the print head 3a is placed into the wash station 12 when it is desired to clean the print head 3a (e.g. in a cleaning configuration). The print head 3a may be described as being docked in the wash station 12 in this configuration. With the print head 3a inserted into the wash station 12, the print head 3a can be cleaned in a number of different ways. In one embodiment, and as shown in Figure 2, the wash station includes a cleaning tank for receiving the print head 3a, and one or more fluid outlets (e.g., nozzles) 14, 16, 18, disposed around the cleaning tank, can eject a cleaning fluid into and/or onto the print head 3a in order to clean the print head 3a and/or constituent components thereof. In other embodiments, with the print head 3a placed into a wash station, the print head 3a can be cleaned by virtue of ejecting cleaning fluid through a nozzle of the print head 3a itself. The used cleaning fluid can then be captured by the wash station (or, in some embodiments, by the print head). It will be appreciated that either embodiment may further comprise various features and functionalities, for example the ability to drain used cleaning fluid and/or dry the print head after cleaning. _{By way of a further example, it will be appreciated that the wash station need not be} coupled to the printer body itself. Instead, the wash station could be an entirely separate component to the printer body. As envisaged in the embodiment of Figure 2, it may be an operator that manually places the print head 3a into the wash station 12. Alternatively, in further variants, the print head may automatically (e.g. by way of a robot arm) be placed into a wash station. 69420664-1 Turning to Figure 3, a schematic cross-section side view through part of the self- cleaning print head 3, as shown in Figure 1, is provided. Figure 3 shows the print head 3 comprising a nozzle body 20, charge electrode 27, chamber 26, low voltage electrode 28, deflection electrode 30, gutter 32 and sealing mechanism 34. An ink jet 36 (e.g. a stream of ink droplets), not deflected by deflection electrode 30, is also schematically indicated in Figure 3. Briefly stepping through each of the aforementioned components in turn: the nozzle body 20, as mentioned, defines the nozzle 22. The nozzle 22 is for generating and _{ejecting a stream of ink droplets for printing. The nozzle body 20 may also be referred} _{to as a droplet generator, and the nozzle 22 may otherwise be described as an} _{aperture of the droplet generator. The ink jet 36 is shown being expelled by the nozzle} 22, although these ink droplets would not be used for printing because they pass straight into the gutter 32. _{Downstream of the nozzle and nozzle body 20, 22, the charge electrode 27 is provided.} As the stream of ink droplets 36 is directed past the charge electrode 27, they are selectively and separately given a pre-determined level of charge by the charge electrode 27. Downstream of the charge electrode 27, low voltage and deflection electrodes 28, 30 are provided. The low voltage electrode 28 may otherwise be described as a grounded, or negative potential, electrode. The deflection electrode 30 may be described as a high voltage electrode. The electrodes 28, 30 may collectively be described as a pair of (deflection) electrodes. An electric field is selectively generated between the low voltage and deflection electrodes 28, 30 for guiding the stream of ink droplets 36 away from the gutter 32 and towards an ink aperture for printing onto a _{substrate in use. The ink aperture is labelled 38. The ink aperture 38 may be described} as being selectively opened and closed by the sealing mechanism 34. Downstream of the electrodes 28, 30, the gutter 32 is provided. The gutter 32 comprises a gutter aperture through which droplets of ink not used for printing (e.g. of ink jet 36) are received and subsequently recirculated back into the ink system. 69420664-1 Also shown in Figure 3 is a sealing 34. The purpose of the sealing mechanism 34 is to selectively seal the chamber 26 within the print head 3. The chamber 26 is thus a selectively sealable volume which can be at least partly filled with cleaning fluid, and subsequently drained, in order to clean one or more components disposed within the chamber 26. As indicated in Figure 3, components disposed within the chamber 26 include the gutter 32 and electrodes 28, 30. Although the chamber 26 is schematically shown as not including the charge electrode 27, fluid within the _{chamber 26 can also enter the charge electrode 27. The charge electrode 27 can thus} also be cleaned by filling the chamber 26 with cleaning fluid. In some interpretations a _{cavity defined by the charge electrode 27 may be considered to form part of the} chamber 26. The chamber 26 may be described as a cleaning chamber, or a cleaning volume. The sealing mechanism 34 may otherwise be described as a closure. The sealing mechanism 34 may take one of a number of different forms. The sealing mechanism 34 may comprise: a rotatable sealing mechanism (e.g. a rotatable body that rotates about an axis to selectively open/seal chamber 26), a slidable sealing mechanism which selectively opens/seals the chamber 26, or slidable housing pieces which selectively change the volume of the chamber. Turning to Figure 4, a schematic cross-section side view through part of the print head 3a, shown in Figure 2, is provided. As will be appreciated by comparing Figure 4 with Figure 3, many of the features are shared in common and will therefore not be described in detail. However, of note, for the print head 3a the sealing mechanism 34 of Figure 3 is omitted. As such, the chamber 26 in Figure 4 is not sealable by a sealing mechanism which forms part of the print head 3a itself. Save for this difference, all features are the same as that shown in Figure 3. Also of note, the chamber 26 shown in the Figure 4 embodiment may not be a strictly defined chamber (e.g. a volume). Instead, the chamber 26 may indicate a zone onto which cleaning fluid is sprayed, during a cleaning cycle, to clean constituent components of the print head 3a. In one example, the chamber 26 may represent a zone of the print head 3a which is exposed when an outer cover (e.g. sheath) of the print head 3a is removed. 69420664-1 Although not illustrated in Figures 3 or one or more ports, preferably a plurality of ports, may be disposed in fluid communication with the chamber 26. These ports may provide one or more of: a supply of cleaning fluid for cleaning the chamber, a supply of air for pressure balancing (e.g. during printing, to replenish fluid drawn out of the chamber by the gutter), drainage of used cleaning fluid from the chamber, and a supply of air for drying the chamber. One or more of the gutter 32 and the nozzle 22 may also provide the aforementioned port functionalities, although in preferred embodiments the gutter 32 and nozzle 22 are not used to provide the chamber with such filling, draining or drying functionalities (the chamber 26 instead comprising one or more ‘dedicated’ multifunction fill/drain/dry ports). Turning to Figure 5, steps of a method 50 are shown illustrating a cleaning cycle for the self-cleaning print head 3. The method 50 comprises a fill stage 52, soak stage 54, drain stage 56 and a dry stage 58. During the fill stage 52, the chamber 26 is at least partly filled with cleaning fluid. By virtue of the chamber 26 being partly filled with cleaning fluid, components within the chamber 26 which are reached by the liquid level of the cleaning fluid are contacted by the cleaning fluid. In the soaking stage 54, the cleaning fluid is held in the chamber. As suggested by the name, the soaking stage 54 provides a dwell period, (e.g. a period of time) in which the cleaning fluid acts to remove deposits/build-up from the components and the chamber 26 itself. The soaking stage 54 may comprise percolating air or otherwise disturbing the cleaning fluid in the chamber 26 to improve the clean. In the draining stage 56, the used cleaning fluid from within the chamber 26 is drained. The chamber 26 is thus emptied of used cleaning fluid (and any deposit/build-up entrained within the used cleaning fluid). In the drying stage 58, the chamber 26, and any components provided in fluid communication with the chamber 26, are dried. Although there are a number of different ways in which the drying can take place, one such example is that a supply of air be provided through one or more ports in communication with the chamber 26. 69420664-1 It will be appreciated that the method 50 be modified in various different ways, and may be repeated in multiple cycles to provide a deeper clean. For example, the method 50 may run multiple times in succession for a deeper clean of the print head 3a. It will also be appreciated that a sealing step, in which the sealing mechanism sealing the chamber, precedes the fill stage 52. Turning to Figure 6, steps of a method 80 are shown illustrating a cleaning cycle for the print head 3a (e.g. a print head used with a washing station). The method 80 comprises a docking stage 82, spray stage 84, soak stage 86 and a drying stage 88. In the docking stage 82, the print head 3a is docked in the wash station 12. Print head 3a may otherwise be described as being inserted into the wash station 12. The motion of inserting, or docking, the print head 3a into the wash station 12 may also create a seal around the print head 3a. Said seal may contain cleaning fluid, used in the cleaning cycle, within the washing station 12 (e.g. reducing the risk that cleaning fluid escape between the print head 3 and the wash station 12). In the spray stage 84, cleaning fluid is sprayed (e.g. ejected) onto and/or into the print head 3a, specifically a chamber thereof and any constituent components which are exposed. The cleaning fluid acts to remove deposits/build-up from the components and the chamber. In the soaking stage 86, the cleaning fluid is left ‘applied’ to the print head for a dwell period (e.g. a period of time). During this time the cleaning fluid acts to remove deposit build-up from the print head 3a. In the drying stage 88, the print head 3a, the chamber and any constituent components are dried. The drying can take a variety of different forms. Like the method 50, the method 80 may be modified in various different ways and may be repeated multiple times to provide a deeper clean. For example, the drying stage 88 may be omitted entirely. It will be appreciated that the docking stage 82 need not be repeated, where multiple cycles are run in succession, for a deeper clean of the print head 3a. That is to say, the print head 3a may remain docked, in-situ, such that 69420664-1 the method 80 only be partially the spray and soak stages 84, 86 may be repeated). _{Figure 10 shows a schematic diagram of a fluid system for the CIJ printer 1 of Figure 1,} incorporating the print head 3. As described above, the ink system 5 of the CIJ printer _{1 is contained within the main printer body 2. With reference to Figure 10, the ink} system 5 comprises at least the components that form part of a main ink block 11. The _{ink system 5 may further comprise a cartridge module 15 and a cleaning module 13,} although the cleaning module 13 may be a generally separate module to the rest of the _{ink system 5.} Beginning with the main ink block 11, the main ink block 11 comprises a mixer tank 17 (which may also be referred to as an ink feed, or ink supply, tank) configured to supply ink along a main supply line 19. The main supply line 19 is configured to carry ink from the ink mixer tank 17, along the umbilical 4, to the print head 3. Within the print head 3 _{the ink is provided to the nozzle body 20 under pressure (under the influence of an ink} pump 21) and forms an ink jet. The ink jet begins as a constant stream of ink and, under the influence of surface tension and vibrations applied in the nozzle body 20 _{(e.g. by a piezoelectric oscillator), gradually separates into a series of ink droplets 36.} _{The ink is drawn from the mixer tank 17 by the ink pump 21. Ink also passes through a} filter 23, downstream of the ink pump 21, disposed along the main supply line 19. The _{first filter 23 is for removing any particles (e.g. sediment) contained within the mixer} tank 17. A Venturi line 24 is connected to the main supply line 19 downstream of the first filter 23. Provided along the Venturi line 24 is a Venturi pump 24a (e.g. a restriction). In operation, fluid (e.g. an ink mixture) is continuously circulated from the mixer tank 17, through the main supply line 19, through the Venturi line 24, and so through the Venturi _{pump 24a, before being returned to the mixer tank 17. This continuous circulation,} combined with the Venturi pump 24a, creates suction to draw fluids into the mixer tank _{17 via a refill line 25, which extends between the cartridge module 15 and the Venturi} _{pump 24a. Fluids are drawn into the mixer tank 17 through the Venturi pump 24a and} _{a downstream portion 24b of the Venturi line 24. The Venturi pump 24a is a pump} which creates vacuum based on the Venturi effect, and may also be referred to as a vacuum pump. 69420664-1 _{The cartridge module 15 comprises an ink cartridge 8 and a solvent cartridge 10, which} can be connected to the refill line 25 allowing ink or solvent (depending upon the on/off _{configurations of the valves 60, 61, 62, 63) to be drawn by the Venturi line 24, into the} mixer tank 17. By using a Venturi in this way (i.e. as a jet pump), the main system ink pump 21 can generate both positive pressures (e.g. to supply ink to the print head 3) and negative vacuum pressure (e.g. to draw ink or solvent from the cartridges 8, 10 into the mixer tank 17 via the refill line 25). _{The cartridge module 15 further includes a solvent pump 67 and non-return valves 68,} 69. The solvent pump 67 can pump solvent from the solvent cartridge 10 into (i) the _{print head 3 via a flush line 70 for cleaning the nozzle body 20, (ii) a solvent tank 49, or} (iii) the cleaning module 13 via the flush line 70 and a cleaning module inlet line 72. The cleaning module 13 is in fluid communication with the cleaning chamber 26 via two _{conduits 204, 214. The two conduits 204, 214 are used to supply the cleaning} _{chamber 26 with cleaning fluid and to drain used cleaning fluid from the chamber 26,} _{respectively. The used cleaning fluid can be drawn through a draw line 87 by a gutter} _{pump 46. The gutter pump 46 can also draw unused ink droplets received by the} _{gutter 32 and solvent used for flushing the nozzle body 20.} _{Downstream of the gutter pump 46, a tank valve 48 is provided. The tank valve 48} selectively places the gutter pump 46 in fluid communication with either the mixer tank _{17 or the solvent tank 49. The solvent tank 49 may be described as a ‘used’ solvent} reservoir insofar as it contains solvent which is not fresh. During printing operations, the tank valve 48 places the gutter pump 46 in fluid communication with the mixer tank 17. The ink mixture (e.g. the stream of ink droplets 36) received by the gutter 40 is thus returned to the mixer tank 17 and can be recirculated/reused at a later time. During non-printing operations (e.g. such as priming, cleaning operations etc.), the tank valve 48 may place the gutter pump 46 in fluid communication with the solvent tank 49. This is to avoid cleaning fluid, such as ‘used’ solvent, undesirably contaminating (e.g. altering the viscosity of) the ink mixture in the mixer tank 17. It would be understood that the CIJ printer 1a of Figure 2 may have a fluid system similar to that shown in Figure 7, but without the cleaning module 13. 69420664-1 _{For the print head 3 of Figure 3, the cleaning module 13, the conduits 204, 214 and the} _{ports in the chamber 26 which are in fluid connection with the conduits 204, 214 may} _{be collectively referred to as a “cleaning mechanism” for cleaning the print head 3. For} the print head 3a of Figure 4, the wash station 12 may be considered as a cleaning mechanism for cleaning the print head 3a. _{In use, a CIJ printer is typically installed at a fixed location to print marks (e.g., codes} _{and/or dates) on the packaging of products placed on a moving industrial production} _{line. Generally speaking, the CIJ printers are plug-in machines, and mains power} supply is used to power the CIJ printers. From time to time, various types of abnormalities may happen to interrupt the normal printing operations of a CIJ printer. _{The abnormalities include, for example, power outage, power plug being accidently} _{pulled off socket, ink running low in particular fluid tank(s) of the printer etc. The} _{abnormalities sometimes can make a more serious impact on the CIJ printer than} simply stopping the printing. In some circumstances, even after the removal of the abnormalities, complicated and time-consuming recovery procedures and/or service/maintenance from a professional service technician may be required in order _{for the CIJ printer to resume operation. This would significantly increase the downtime} _{of the CIJ printer and reduce the throughput of the production line.} _{To solve this problem, the CIJ printers 1, 1a according to the present disclosure are} _{configured to perform failsafe operations in response to the occurrence of} _{abnormalities without any human intervention. The failsafe operations cause the CIJ} _{printers 1, 1a to enter a safe condition. In the safe condition, the printers 1, 1a are} protected from being damaged and can quickly and reliability start up after the removal of the abnormality without requiring lengthy recovery procedures or service/maintenance. The safe condition may also be referred to as a printable _{condition or a quick start-up condition. In this way, the downtime of the CIJ printers 1,} _{1a can be reduced, thereby improving the production output of the printers. Several} examples of the abnormalities and the failsafe operations are described below in more detail. _{The first example is applicable to the CIJ printer 1, which receives power from an} _{external power supply (e.g., the mains power supply). In this example, the abnormality} 69420664-1 _{includes an interruption in power from power supply. The interruption may} _{be caused by a failure of the external power supply (e.g., power outage) and/or a} _{disconnection between the CIJ printer 1 and the external power supply (e.g., the power} _{plug of the printer being accidently pulled off a power socket). If the printer 1 was} printing immediately before the power interruption, the nozzle 22 and the nozzle body 20 (i.e., droplet generator) as well as other components of the print head 3 would be wet with ink. Over time the solvent within the wet ink would evaporate, leaving crusty _{deposits which may obstruct the nozzle and can be difficult to remove. The crusty} deposits can result in the CIJ printer 1 not working, even after power is re-supplied from the external power supply. To reduce the downtime of the CIJ printer 1, the CIJ printer 1 is designed such that the sealing mechanism 34 automatically seals the cleaning chamber 26 without human intervention in response to the interruption in power from the external power supply. More specially, the sealing mechanism 34 automatically seals the ink aperture 38, _{thereby sealing the cleaning chamber 26. As a result, the wet ink remaining within the} print head at the time of power interruption would be isolated from the ambient and _{tends to dry up at a much slower rate. If a temporal length of the power interruption is} _{not particularly long, the print head 3 may not need a time-consuming deep cleaning} _{before restarting printing. Therefore, sealing the cleaning chamber using the sealing} mechanism is a failsafe operation of the CIJ printer 1. Figure 8 illustrates a working principle of the sealing mechanism 34. The sealing mechanism has a first configuration in which the cleaning chamber 26 (or the ink aperture 38) is sealed, and a second configuration in which the cleaning chamber 26 (or the ink aperture 38) is not sealed. The sealing mechanism 34 is structured such _{that it is biased (e.g., mechanically biased) towards the first configuration, and that} _{electrical power is required for the sealing mechanism 34 to transition from the first} configuration to the second configuration and to remain at the second configuration. In other words, the electrical power is needed to overcome the mechanical bias. The _{sealing mechanism 34 can be implemented suitably in various different ways based} upon the working principle described above. For example, the sealing mechanism 34 _{may have a spring-loaded movable (e.g., rotatable, slidable) body. A gear, when} _{driven by electrical power, may be used to cause a movement of the plate along one} direction so as to seal the ink aperture 38. A spring may be used to pull the plate in an 69420664-1 opposite direction so as to keep the ink 38 open. Other arrangement of the sealing mechanism 34 is possible. The CIJ printer 1 may include an internal power supply, such as a power storage _{device which stores electric power. The power storage device may be a battery and/or} a capacitor, although the capacitor typically has a smaller power storage capacity than the battery. The power storage device may be charged when the CIJ printer 1 is connected to the external power supply. In the event that the CIJ printer 1 includes a power storage device, the sealing mechanism 34 may be made differently such that electrical power also causes the _{sealing mechanism 34 to transition from the second configuration to the first} _{configuration. For example, the sealing mechanism 34 may use a motor and a worm} _{gear drive arrangement to drive a rotatable body. Thus, in response to the interruption} in power from the external power supply, the power storage device may supply power _{to the sealing mechanism 34, causing it to transition to the first configuration, thereby} _{sealing the cleaning chamber 26 (or the ink aperture 38). It would be understood that} in the event that the CIJ printer 1 includes a power storage device, the controller 6 may actively stop one or more of the fluid pumps within the ink system 5 (as shown in _{Figure 7), and to actively stop the nozzle body 20 from generating the stream of ink} _{droplets 36, before controlling the sealing mechanism 34 to seal the cleaning chamber} 26 (or the ink aperture 38). A second example of the failsafe operation is also applicable to the CIJ printer 1 which includes a power storage device as described above. In this example, the abnormality _{again includes an interruption in power from the external power supply. In response to} the power interruption, the CIJ printer 1 performs a cleaning operation to the print head 3 as a failsafe operation, by using the electric power stored by the power storage device. By performing the cleaning operation, the amount of ink deposits within the _{print head 3 can be greatly reduced, thereby enabling a faster and more reliable start-} _{up of the printer once power is re-supplied from the external power supply. It would be} _{understood that the cleaning operation may be performed after the sealing mechanism} _{34 seals the cleaning chamber 26 (or the ink aperture 38) as described above.} 69420664-1 Depending upon the capacity of the storage device, the cleaning operation may _{vary. For example, if the power storage device has a limited storage capacity, the} cleaning operation may simply include filling the cleaning chamber 26 with the cleaning _{fluid as per step 52 of method 50 (Figure 5). The cleaning fluid is allowed to stay within} _{the cleaning chamber 26 until the power interruption stops. This prevents the formation} of crusty ink deposits with the print head 3. If the capacity of the power storage device _{allows, the cleaning operation may further include draining the used cleaning fluid from} the cleaning chamber 26, e.g., after a soaking period, as per step 56 of method 50 (Figure 5). If the capacity of the power storage device is sufficient, the cleaning operation may also include drying the cleaning chamber as per step 58 of method 50 (Figure 5), and, in addition, the cleaning cycle as shown in Figure 5 may be repeated _{several times to provide a deeper clean of the print head 3.} A third example of the failsafe operation is applicable to both the CIJ printers 1, 1a. In this example, the abnormality includes an ink level within the mixer tank 17 dropping _{below a critical threshold level for maintaining a continuous circulation of ink through} _{the Venturi pump 24a. When this happens, the ink cartridge 8 is typically empty or} almost empty. The abnormality may be detected by a level sensor within the mixer tank 17. _{As described above, the continuous circulation of ink mixture, which is from the mixer} _{tank 17, through the main supply line 19, through the Venturi line 24, and so through} _{the Venturi pump 24a before being returned to the mixer tank 17, creates a negative} _{vacuum pressure at the Venturi pump 24a to draw ink and solvent from the cartridges} _{8, 10 into the mixer tank 17.} _{If the ink level of the mixer tank 17 was allowed to drop below the critical threshold} level for maintaining the continuous circulation, the continuous circulation through the _{Venturi pump 24a would stop (immediately or very shortly) and the Venturi pump 24a} _{would lose the negative vacuum pressure. As a result, the Venturi pump 24a would not} _{be able to draw ink from the ink cartridge 8 to the mixer tank 17, even after an operator} _{replaces the empty ink cartridge 8 with a full ink cartridge. To re-fill the mixer tank 17} _{and to resume printing, the operator may need to manually prime the Venturi pump 24a} by inserting the full ink cartridge into the place of the solvent cartridge 10, and operating the solvent pump 67 to draw ink from the full ink cartridge to the mixer tank 69420664-1 _{17 via the valves 62, 61, the refill line the downstream portion 24b of the Venturi} _{line 24. Once the mixer tank 17 is replenished and the continuous circulation of ink} _{mixture through the Venturi pump 24a can be established, the operator may then re-} _{insert the ink cartridge into its correct place. The manual priming process of the Venturi} is relatively complicated and is prone to error. If the operator is not experienced, _{professional service from a service technician may be needed, and this would} _{significantly increase the downtime of the CIJ printer.} _{To reduce the downtime of the CIJ printer, the CIJ printer 1 or 1a is designed such that} when the ink level within the mixer tank 17 drops below the critical threshold level, the _{controller 6 performs a failsafe operation by stopping the supply of ink from the mixer} _{tank 17 to the print head 3 so as to shut down the printing operation of the print head 3.} This may be done by switching off the ink pump 21. In the meantime, the controller 6 _{may generate a signal (e.g., a beep sound or a message on the interface 7), prompting} _{the operator to replace the empty ink cartridge 8. Once the ink cartridge 8 is replaced,} _{the controller 6 may switch the ink pump 21 back on, which would immediately resume} _{the continuous circulation of ink mixture from the mixer tank 17. In this way, the} _{negative vacuum pressure at the Venturi pump 24a has been maintained and the} _{Venturi pump 24a can draw ink from the replaced ink cartridge 8 into the mixer tank 17} straightaway without requiring any priming. Therefore, this failsafe operation can significantly reduce the downtime of the CIJ printer. In practice, the controller 6 may generate an advanced warning to the operator when the ink level within the mixer tank 17 drops below another threshold level which is higher than the critical threshold level. The advanced warning may often be ignored by the operator because of the desire to continue the printing operation. In that case, the failsafe operation of forcefully shutting down the printing operation when the ink level within the mixer tank 17 drops below the critical threshold level is useful to reserve the printable condition of the CIJ printer. While the above provides several examples of the failsafe operations, it would be appreciated that other types of failsafe operations are possible so as to allow the CIJ _{printer to enter a safe, printable, condition in response to an occurrence of an} abnormality. 69420664-1 _{Figure 9 schematically illustrates steps of a method of operating a CIJ} printer (e.g., the CIJ printer 1). At step S1, a print head (e.g., the print head 3) of the CIJ printer is printing on a substrate. The print head comprises: a cleaning chamber (e.g., the chamber 26) _{selectively sealable by a sealing mechanism (e.g., the sealing mechanism 34); a} _{nozzle (e.g., the nozzle 22) for generating and ejecting a stream of ink droplets (e.g.,} _{the stream 36) for printing; at least one electrode (e.g., the electrodes 27, 28, 30) for} _{guiding the stream of ink droplets; and a gutter (e.g., the gutter 32) for receiving} droplets of ink which are not used for printing. _{At step S2, the CIJ printer automatically enters a safe condition in response to an} occurrence of an abnormality. Step S2 may include one or more of the following sub-steps S2-1 to S2-3. _{The CIJ printer may receive power from an external power supply. The abnormality} _{may comprise an interruption in power from the external power supply to the} continuous inkjet printer. _{At step S2-1, the sealing mechanism automatically seals the cleaning chamber in} response to the interruption in power from the external power supply. _{The CIJ printer may comprise a power storage device storing electric power and a} _{cleaning mechanism (e.g., the cleaning module 13) for cleaning the print head using a} _{cleaning fluid. The abnormality may comprise an interruption in power from the} external power supply to the continuous inkjet printer. _{At step S2-2, the cleaning mechanism automatically performs a cleaning operation to} the print head by using the electrical power stored by the power storage device, in response to the interruption in power from the external power supply. _{The CIJ printer may comprise a mixer tank (e.g., the mixer tank 17) configured to} _{supply ink to the print head through an ink supply line between the mixer tank and the} _{print head during the printing operation of step S1. During the printing operation, ink} 69420664-1 _{from the mixer tank may be continuously through a part of the ink supply line,} and through a venturi pump, and back to the mixer tank. The continuous circulation _{may cause the venturi pump to draw ink from an ink cartridge or solvent from a solvent} _{cartridge to the mixer tank. The abnormality may comprise an ink level within the mixer} _{tank dropping below a threshold for maintaining the continuous circulation through the} venturi pump. _{At step S2-3, the CIJ printer automatically stops the supply of ink from the mixer tank to} the print head so as to stop the print head from printing, in response to the ink level within the mixer tank dropping below the threshold. It would be appreciated that the steps may be performed in a temporal order that is different from the order of description. The terms “having”, “containing”, “including”, “comprising” and the like are open and the terms indicate the presence of stated structures, elements or features but not preclude the presence of additional elements or features. The articles “a”, “an” and “the” are intended to include the plural as well as the singular, unless the context clearly indicates otherwise. Although the disclosure has been described in terms of preferred embodiments as set _{forth above, it should be understood that these embodiments are illustrative only and} that the claims are not limited to those embodiments. Those skilled in the art will be able to make modifications and alternatives in view of the disclosure which are contemplated as falling within the scope of the appended claims. Each feature disclosed or illustrated in the present specification may be incorporated in the disclosure, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein. 69420664-1

Claims

CLAIMS: 1. A continuous inkjet printer, comprising: a print head which comprises: a cleaning chamber selectively sealable by a sealing mechanism; a nozzle for generating and ejecting a stream of ink droplets for printing; at least one electrode for guiding the stream of ink droplets; and a_{gutter for receiving droplets of ink which are not used for printing;} _{wherein the continuous inkjet printer is configured to enter a safe condition in} _{response to an occurrence of an abnormality.} _{2. The continuous inkjet printer of claim 1, wherein the continuous inkjet printer is} _{operable to receive power from an external power supply, and the abnormality} comprises an interruption in power from the external power supply. _{3. The continuous inkjet printer of claim 2, wherein the interruption in power from} the external power supply comprises a failure of the external power supply and/or a disconnection between the continuous inkjet printer and the external power supply. _{4. The continuous inkjet printer of claim 2 or 3, wherein the continuous inkjet} printer is configured such that the sealing mechanism seals the cleaning chamber in response to the interruption in power from the external power supply. _{5. The continuous inkjet printer of claim 4, wherein the sealing mechanism has a} first configuration in which the cleaning chamber is sealed, and a second configuration _{in which the cleaning chamber is not sealed, and wherein the sealing mechanism is} biased to the first configuration. _{6. The continuous inkjet printer of claim 5, wherein the sealing mechanism is} configured to transition from the first configuration to the second configuration by _{receiving electrical power to overcome the bias.} _{7. The continuous inkjet printer of any preceding claim, wherein the continuous} _{inkjet printer comprises a power storage device for storing electric power.} 69420664-1

_{8. The continuous inkjet printer 7, wherein the power storage device} comprises a capacitor and/or a battery. _{9. The continuous inkjet printer of claim 7 or 8, wherein the sealing mechanism is} _{configured to seal the cleaning chamber by using the electrical power stored by the} _{power storage device, in response to the interruption in power from the external power} supply. _{10. The continuous inkjet printer of any preceding claim, wherein the continuous} inkjet printer comprises a cleaning mechanism for cleaning the print head using a cleaning fluid. _{11. The continuous inkjet printer of claim 10 as dependent from claims 4 and 7,} _{wherein the cleaning mechanism is configured to perform a cleaning operation to the} print head by using the electrical power stored by the power storage device, in response to the interruption in power from the external power supply. _{12. The continuous inkjet printer of claim 11, wherein the cleaning mechanism is} _{configured to at least partially fill the cleaning chamber with the cleaning fluid, during} the cleaning operation. _{13. The continuous inkjet printer of claim 12, wherein the cleaning mechanism is} further configured to drain the used cleaning fluid from the cleaning chamber after a soaking period, during the cleaning operation. _{14. The continuous inkjet printer of claim 13, wherein the cleaning mechanism is} _{further configured to dry the cleaning chamber after the used cleaning fluid is drained,} during the cleaning operation. _{15. The continuous inkjet printer of any preceding claim, wherein the continuous} _{inkjet printer comprises a mixer tank configured to supply ink to the print head, and the} _{abnormality comprises an ink level within the mixer tank dropping below a threshold.} _{16. The continuous inkjet printer of claim 15, wherein:} 69420664-1 the continuous inkjet printer is to continuously circulate ink from the mixer tank, through a part of an ink supply line between the mixer tank and the print _{head, and through a venturi pump, and back to the mixer tank during a printing} _{operation, and the continuous circulation causes the venturi pump to draw ink from an} _{ink cartridge or solvent from a solvent cartridge to the mixer tank; and} _{the threshold is a threshold ink level for maintaining the continuous circulation} through the venturi pump. _{17. The continuous inkjet printer of claim 16, wherein the continuous ink jet printer} is configured to stop the supply of ink from the mixer tank to the print head so as to stop the print head from printing, in response to the ink level within the mixer tank _{dropping below the threshold.} _{18. The continuous inkjet printer of claim 17, wherein the threshold is a first} _{threshold, and the continuous ink jet printer is further configured to generate a warning} signal, in response to the ink level within the mixer tank dropping below a second threshold, wherein the second threshold indicates a higher ink level than the first threshold. _{19. A method of operating a continuous inkjet printer, comprising:} printing on a substrate using a print head of the continuous inkjet printer, wherein the print head comprises: a cleaning chamber selectively sealable by a sealing mechanism; a nozzle for generating and ejecting a stream of ink droplets for printing; at least one electrode for guiding the stream of ink droplets; and a gutter for receiving droplets of ink which are not used for printing; and t_{he continuous inkjet printer automatically entering a safe condition in response} to an occurrence of an abnormality. _{20. The method of claim 19, further comprising:} receiving power from an external power supply, wherein the abnormality comprises an interruption in power from the external power supply to the continuous inkjet printer. _{21. The method of claim 20, wherein the continuous inkjet printer automatically} entering the safe condition comprises: 69420664-1 _{automatically sealing the chamber, by the sealing mechanism, in} response to the interruption in power from the external power supply. _{22. The method of claim 21, wherein the continuous inkjet printer comprises a} _{power storage device storing electric power, and a cleaning mechanism for cleaning} _{the print head using a cleaning fluid, and wherein the continuous inkjet printer} automatically entering the safe condition comprises: a_{utomatically perform a cleaning operation, by the cleaning mechanism, to the} print head by using the electrical power stored by the power storage device, in response to the interruption in power from the external power supply. _{23. The method of any one of claims 19 to 22, wherein the continuous inkjet printer} comprises a mixer tank configured to supply ink to the print head, and the abnormality comprises an ink level within the mixer tank dropping below a threshold. _{24. The method of claim 23, further comprising:} continuously circulating ink from the mixer tank, through a part of an ink supply line between the mixer tank and the print head, and through a venturi pump, and back to the mixer tank during the printing, wherein the continuous circulation causes the venturi pump to draw ink from an ink cartridge or solvent from a solvent cartridge to the _{mixer tank; and wherein the threshold is a threshold ink level for maintaining the} continuous circulation through the venturi pump. _{25. The method of claim 24, wherein the continuous inkjet printer automatically} entering the safe condition comprises: automatically stopping the supply of ink from the mixer tank to the print head so as to stop the print head from printing, in response to the ink level within the mixer tank dropping below the threshold. 69420664-1