WO2025257575A1

WO2025257575A1 - Printer and associated method

Info

Publication number: WO2025257575A1
Application number: PCT/GB2025/051312
Authority: WO
Inventors: David Andrew Horsnell; Bartosz Marcin OPRZADEK; Tim HUTCHINSON; 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: GB202408530D0

Abstract

There is provided a continuous inkjet printer (1, 1a), comprising: a print head (3, 3a) comprising: a nozzle (22) for generating and ejecting a stream of ink droplets (36) for printing; at least one electrode (24, 28, 30) for guiding the stream of ink droplets (36); a gutter (32) for receiving droplets of ink which are not used for printing; and an optical sensor (40) including a light transmitter (42) and a light detector (44), wherein the light transmitter (42) is configured to emit light within the print head, and the light detector (44) is configured to generate sensor data (46, 48) indicative of an amount of light detected by the light detector; and a controller (6) configured to determine an overall cleanliness level of the interior of the print head based upon the sensor data (48).

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 enables an inspection of a characteristic of used cleaning _{fluid so as to determine a cleanliness level of its print head, and a continuous inkjet} printer which determines an overall cleanliness level of its print head by using an optical _{sensor arranged within the print head.} 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 69715475-1 movement of the substrate relative to print head between droplets arriving means that a line of droplets would otherwise not quite extend perpendicularly to the direction 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. 69715475-1 CIJ printers generally operate in high 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. _{A problem faced by operators of 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. The print head of a CIJ printer can be cleaned in various ways. However, it is generally _{difficult to assess a cleanliness (or dirtiness) level of the interior of the print head. It is} desirable to provide a CIJ printer which allows an operator to know whether and/or when the print head should be cleaned, so as to reduce the down time of the CIJ printer. Further or alternatively, it is desirable to provide a CIJ printer which allows an operator _{to know whether a particular cleaning operation is sufficient for removing the build-up of} deposits from the print head. It is an object of the present disclosure, among others, to provide such an improved CIJ printer. Summary to a first 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; and} 69715475-1 a cleaning mechanism for the print head by supplying a cleaning fluid to the print head and removing used cleaning fluid from the print head after a soaking period; wherein the continuous inkjet printer is configured to enable an inspection of a characteristic of the used cleaning fluid during the soaking period, so as to determine a cleanliness level of the print head. Advantageously, the continuous inkjet printer allows a cleanliness level of its print head to be determined, by inspecting a characteristic of the cleaning fluid which has been used _{for cleaning the print head (“used cleaning fluid”), and the inspection of the used cleaning} fluid takes place during the soaking period (in which a part or the whole of the print head is soaked in the cleaning fluid) before the used cleaning fluid is drained or removed otherwise from the print head. Inspecting the characteristic of the used cleaning fluid during the soaking period allows the cleanliness level of the print head to be determined more accurately, as compared to inspection carried out on used cleaning fluid which has been removed from the print head, due to possible remnant particles or dirt existing within removal conduit(s). The characteristic of the cleaning fluid may comprise an electrical characteristic and/or an optical characteristic of the used cleaning fluid. The electrical characteristic may be indicative of an electrical conductivity of the used cleaning fluid. A high electrical conductivity of the used cleaning fluid generally indicates a large amount of ink present within the used cleaning fluid, which in turn indicates a low cleanliness level of the print head. _{The optical characteristic may be indicative of a transmittance of the used cleaning fluid.} The transmittance is the fraction of incident light is transmitted through the used cleaning fluid. If fresh cleaning fluid is clear, then it would be understood that a print head with a low cleanliness level would introduce more dirt and/or ink to the cleaning fluid, thereby causing the used cleaning fluid to have low transmittance (e.g., opaque). Further or alternatively, the optical characteristic may comprise a visual appearance such as a colour of the used cleaning fluid. 69715475-1 _{The print head may further comprise a cleaning chamber selectively sealable by a} _{sealing mechanism, and the cleaning mechanism may be configured to clean the print} head by at least partially filling the cleaning chamber with the cleaning fluid and draining the used cleaning fluid from the print head after the soaking period. The soaking period refers to a time period in which the cleaning fluid remains within the cleaning chamber. In other words, the continuous inkjet printer may be configured to enable an inspection of a characteristic of the used cleaning fluid contained within the cleaning chamber, so as to determine a cleanliness level of the print head. 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 continuous inkjet printer may be configured to measure an electrical characteristic} of the used cleaning fluid by using at least two components selected from the nozzle, the at least one electrode and the gutter. Advantageously, the measurement of the electrical characteristic of the used cleaning fluid does not require any further sensor to be placed within the print head, thereby allowing the print head to have a relatively small volume and to be made with low costs, and further improving the reliability of the continuous inkjet printer. It would be understood that the used cleaning fluid provides the only electrically conductive path between the at least two components which are otherwise insulated from one another. The at least two components are used as electrodes to measure the electrical characteristic of the used cleaning fluid. _{The continuous inkjet printer may comprise an ink build-up sensor for detecting an} accumulation of ink on a print head surface, and the ink build-up sensor may comprise a light transmitter and a light detector which are configured such that the accumulation 69715475-1 of ink reduces an amount of transmitted that is detected by the light detector. The _{continuous inkjet printer may be configured to measure the characteristic of the used} cleaning fluid by using the light transmitter and the light detector. _{The ink build-up sensor may be configured to detect the accumulation of ink on an} internal surface of the print head that is downstream of the at least on electrode for guiding the stream of ink droplets and external to an interior of the gutter. The continuous inkjet printer may be configured to measure an electrical characteristic of the used cleaning fluid by using the light transmitter and the light detector as electrodes. Alternatively, the continuous inkjet printer may be configured to measure an optical characteristic (e.g., transmittance) of the used cleaning fluid by using the light transmitter and the light detector. _{The continuous inkjet printer may be configured to measure the characteristic of the used} cleaning fluid during the soaking period and to determine a cleanliness level of the print head based upon the measured electrical characteristic of the used cleaning fluid. _{The cleaning mechanism may be configured to repeatedly clean the print head under a} _{plurality of cleaning cycles, and the continuous inkjet printer may be configured to} measure the characteristic of the used cleaning fluid during each of the plurality of cleaning cycles and to determine a cleanliness level of the print head based upon a difference of the measured characteristic of the used cleaning fluid between adjacent cleaning cycles. The continuous inkjet printer may further comprise: a controller which is configured to determine the cleanliness level of the print head based upon the inspection of the characteristic of used cleaning fluid, and to stop the cleaning mechanism from cleaning the print head based upon the determined cleanliness level of the print head. _{The print head may comprise a housing which at least partially encloses the cleaning} _{chamber, and at least a part of the housing may be configured to enable an inspection} 69715475-1 of an optical characteristic of the used fluid contained within the cleaning chamber. _{The at least a part of the housing may be transparent or translucent.} _{According to a second aspect of the present disclosure, there is provided a method of} operating a continuous ink jet printer. The method comprises: supplying a cleaning fluid to a print head of the continuous ink jet printer; inspecting a characteristic of the cleaning liquid during a soaking period in which at least a part of the print head is soaked in the cleaning fluid; determining a cleanliness level of the print head based upon the inspected _{characteristic of the cleaning liquid; and removing the cleaning fluid from the print head} after the soaking period. The print head may be a continuous inkjet print head. The print head may comprise a cleaning chamber selectively sealable by a sealing mechanism. Supplying the cleaning fluid to the print head may comprise at least partially filling the cleaning chamber with the cleaning fluid. Inspecting a characteristic of the cleaning liquid may comprise measuring the characteristic of the cleaning fluid. The method may further comprise: terminating a cleaning process of the print head based upon the determined cleanliness level of the print head. _{According to a third aspect of the present disclosure, there is provided a continuous inkjet} printer comprising: a print head comprising: a nozzle for generating and ejecting a stream of ink _{droplets for printing; at least one electrode for guiding the stream of ink droplets; a gutter} _{for receiving droplets of ink which are not used for printing; and an optical sensor} including a light transmitter and a light detector, wherein the light transmitter is configured to emit light within the print head, and the light detector is configured to generate sensor data indicative of an amount of light detected by the light detector; and a controller configured to determine an overall cleanliness level of the interior of the print head based upon the sensor data. 69715475-1 The overall cleanliness level of the interior of the print head represents a general cleanliness level of the print head and can be used as an indicator to prompt a cleaning operation of the entire print head. The overall cleanliness level is in contrast to a local cleanliness level of the print head at a particular region of the print head. By determining an overall cleanliness level of the interior of the print head based upon the sensor data, the continuous inkjet printer uses the optical sensor in a new way without _{requiring further sensing elements, thereby allowing the print head to have a relatively} small volume and to be made with low costs. It would be understood that within this aspect, the overall cleanliness level of the interior of the print head is determined when the print head is not within a cleaning cycle (i.e., when the interior of the print head is not immersed in a cleaning fluid). The light transmitter and the light detector may be configured such that the overall cleanliness level of the interior of the print head affects an amount of the emitted light from the light transmitter that is detected by the light detector. _{The controller may be configured to determine the overall cleanliness level of the interior} of the print head based upon a gradual decrease in the amount of the emitted light detected by the light detector as indicated by the sensor data. To be associated with the overall cleanliness level of the interior of the print head, the amount of the emitted light detected by the light detector is supposed to gradually decrease (i.e., without abrupt changes) with continued operation of the print head. This is because during operation of the print head, thin films of ink deposits and dirt gradually _{build up on the internal surfaces of the print head (and also on the light transmitter and/or} the light detector), thereby obscuring the optical sensor and causing the amount of light detected by the light detector to gradually decrease. In some cases, deposits may build up in localised areas of the print head and growth of the deposits may suddenly block the light path between the light transmitter and the light detector (depending upon the particular location of the optical sensor). The blockage may cause an abrupt change (e.g., a step change) in the amount of emitted light detected by the light detector, and is not associated with the overall cleanliness level of the interior of the print head. In other 69715475-1 words, any part of the sensor data indicates an abrupt change in the amount of light detected by the light detector (e.g., faulty data) cannot be used to determine the overall cleanliness level of the print head. _{The controller may be configured to identify faulty data within the sensor data based} upon a variation of the sensor data, and to disregard the faulty data in determining the overall cleanliness level of the interior of the print head. The faulty data may indicate a step or abrupt change of the amount of light detected by the light detector. The controller may be configured to identify the faulty data based upon a temporal variation of the sensor data generated by the optical sensor. _{The optical sensor may comprise a plurality of the light detectors, and the controller may} _{be configured to identify the faulty data based upon a spatial variation of the sensor data} generated by the plurality of the light detectors at the same time. The faulty data may comprise the sensor data generated by one or more of the light detectors which indicates a lesser amount of detected light than the sensor data generated by remaining ones of the plurality of sensors. The optical sensor may be an image sensor comprising a two-dimensional array of the light detectors. _{The controller may be also configured to detect a localised accumulation of ink in the} interior of the print head based upon the sensor data. A localised accumulation of ink indicates a local cleanliness level of the print head at a particular region of the print head, and is different from an overall cleanliness level of the interior of the print head. The localised accumulation of ink may be associated with the faulty data. The controller may be configured to infer the localised accumulation of ink based upon the faulty data. 69715475-1 The localised accumulation of ink may an internal surface of the print head that is downstream of the at least one electrode and external to an interior of the gutter. The localised accumulation of ink may be on an internal surface of the print head that is adjacent to an inlet of the gutter. The gutter may be positioned between the light transmitter and the light detector. In this way, the localised accumulation of ink on an external surface of the gutter would be effective to reduce the amount of the emitted light that is detected by the light detector. The optical sensor may comprise a transmissive optical sensor. Alternatively, the optical sensor may comprise a reflective optical sensor. The light transmitter may be configured to emit light to an internal surface of the print _{head, and the light detector may be configured to detect an amount of the emitted light} that is reflected by the internal surface of the print head. During operation of the print head, it is expected that thin films of ink deposits and dirt gradually build up on the internal surface of the print head (e.g., in a blanket manner) and cause general degradation/dulling of the internal surface of the print head. The build- up of deposits and dirt gradually enhance the scattering of the emitted light (e.g., diffuse reflection), thereby causing the amount of light detected by the light detector to gradually decrease. The light detector may be configured to detect an amount of specular reflection of the emitted light. The print head may comprise a chamber, and at least some of the at least one electrode, the nozzle and at least a part of the gutter may be arranged within the chamber. The light transmitter may be configured to emit light to an internal surface of the chamber. The light transmitter may be configured to emit light to multiple regions of the internal surface of the print head. The optical sensor may comprise a plurality of the light detectors each configured to detect an amount of the emitted light that is reflected by a respective one of the multiple regions of the internal surface. 69715475-1 The controller may be configured to a warning signal for prompting a user to initiate a cleaning operation of the print head, if the determined overall cleanliness level of the interior of the print head has dropped below a threshold. The print head may be a self-cleaning print head, and the controller may be configured to trigger a cleaning operation of the print head if the determined overall cleanliness level of the interior of the print head has dropped below a threshold. _{The controller may be configured to predict when the print head will require a cleaning} operation based upon the sensor data. The controller may be configured to predict when the print head will require a cleaning operation based upon a temporal variation of the sensor data. _{The controller may be configured to assess a cleaning efficacy of a cleaning operation} of the print head, based upon the sensor data generated prior to and after the cleaning operation. _{At least a part of an interior surface of the print head may be of a high contrast colour} over a colour of the ink droplets. At least a part of an interior surface of the print head may be of a white colour. _{The print head may further comprise a cleaning chamber selectively sealable by a} _{sealing mechanism. The controller may be configured to determine a configuration of the} sealing mechanism based upon the sensor data. The controller may be configured to determine the configuration of the sealing mechanism based upon the sensor data, when the light transmitter is switched off. In this way, the sensor data indicates an amount of ambient light detected by the light detector. _{According to a fourth aspect of the present disclosure, there is provided a method of} operating a continuous inkjet printer, the continuous inkjet printer comprising a print head and a controller, and the print head comprising a nozzle for generating and ejecting a 69715475-1 stream of ink droplets for printing, at one electrode for guiding the stream of ink droplets, a gutter for receiving droplets of ink which are not used for printing, and an optical sensor including a light transmitter and a light detector. The method comprises: _{emitting light, by the light transmitter, within the print head; generating, by the light} detector, sensor data indicative of an amount of light detected by the light detector; and determining, by the controller, an overall cleanliness level of the interior of the print head based upon the sensor data. The method may further comprise: predicting, by the controller, when the print head will require a cleaning operation based upon the sensor data. The method may further comprise: assessing, by the controller, a cleaning efficacy of a cleaning operation of the print head, based upon the sensor data generated prior to and after the cleaning operation. The method may further comprise: identifying, by the controller, faulty data within the sensor data based upon a variation of the sensor data, wherein the faulty data is disregarded in determining the overall cleanliness level of the interior of the print head. _{According to a fifth aspect of the present disclosure, there is provided a print head} _{comprising: 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 at least a part of an interior surface} of the print head is of a high contrast colour over a colour of the ink droplets. The at least a part of an interior surface of the print head may show the high-contrast colour over the colour of the ink droplets under light in one or more of the visible, infrared and ultraviolet ranges. _{According to a sixth aspect of the present disclosure, there is provided a print head} _{comprising: a nozzle for generating and ejecting a stream of ink droplets for printing; at} least one electrode for guiding the stream of ink droplets; a gutter for receiving droplets _{of ink which are not used for printing; and a housing which encloses the nozzle, the at} least one electrode and the gutter; wherein at least a part of the housing is configured to 69715475-1 enable an inspection of an optical of a cleaning fluid when the cleaning fluid is held within the print head for cleaning the print head. According to a seventh aspect of the present disclosure, there is provided a print head _{comprising: 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 draining conduit operable to establish a fluid} communication path with the cleaning chamber, wherein the fluid communication path is for draining used cleaning liquid from the cleaning chamber, and wherein at least a part of the draining conduit is configured to enable an inspection of an optical characteristic of the used cleaning fluid when the used cleaning fluid is drained from the cleaning chamber. The print head may comprise a gutter for receiving droplets of ink which are not used for printing. The draining conduit may be the same as or different from the gutter. The print head may be a continuous inkjet print head. 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. For example, optional or preferred features of each of the _{first and second aspects of the disclosure may be applicable to one another, and optional} or preferred features of each of the third and fourth aspects of the disclosure may be applicable to one another. Brief Description of the Drawings In order that the disclosure 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 embodiment of the} disclosure; 69715475-1 _{Figure 2 is a schematic illustration of a CIJ printer according to another embodiment 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 schematically illustrates changes of the conductivities of used cleaning fluid in} relation to repeating cleaning cycles; Figure 8 schematically illustrates processing steps of a method of operating a CIJ printer, _{according to an aspect of the present disclosure;} _{Figure 9 is a schematic illustration of an optical sensor usable in the print head of Figure} 3 or Figure 4; _{Figure 10 is a schematic illustration of sensor data generated by the optical sensor of} Figure 9; _{Figure 11 is a schematic illustration of another optical sensor usable in the print head of} Figure 3 or Figure 4; Figure 12 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. Detailed Description of the Preferred Embodiments 69715475-1 Figure 1 schematically illustrates a CIJ 1 according to an embodiment 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 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. In operation, ink is delivered under pressure from the ink system 5 to the print head 3 and recycled back via flexible tubes which are bundled together with other fluid tubes and electrical wires (not shown) into the umbilical cable 4. In order to maintain correct consistency of the ink, the ink system 5 may be operable to mix ink removed from the _{cartridge 8 with solvent removed from the cartridge 10 and to mix them together to obtain} an ink having the correct viscosity and/or density for a particular printing application. 69715475-1 The print head 3 can be cleaned. In the 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 solvents) be flushed through at least part of 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. 69715475-1 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 24, 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 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 24 is provided. As the stream of ink droplets 36 is directed past the charge electrode 24, they are selectively and separately given a pre-determined level of charge by the charge electrode 24. Downstream of the charge electrode 24, 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 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. Also shown in Figure 3 is a sealing mechanism 34. The purpose of the sealing mechanism 34 is to selectively seal the chamber 26 within the print head 3. The chamber 69715475-1 26 is thus a selectively sealable volume 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 24, fluid within the chamber 26 can also enter the charge electrode 24. The charge electrode 24 can thus also be cleaned by filling the chamber 26 with cleaning fluid. In some interpretations a cavity defined by the charge electrode 24 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, an inflatable balloon seal that 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. Although not illustrated in Figures 3 or 4, one or more ports, preferably a plurality of ports, may be disposed in fluid communication with the chamber 26. These ports may provide 69715475-1 one or more of: a supply of cleaning 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). For the print head 3 of Figure 3, the ports in fluid communication with the chamber 26 for _{suppling cleaning fluid to the chamber 26 and for draining used cleaning fluid from the} chamber 26 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. 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). 69715475-1 In the drying stage 58, the 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. It will be appreciated that the method 50 may 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. 69715475-1 Like the method 50, the method 80 may 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 the _{method 80 only be partially repeated (e.g. the spray and soak stages 84, 86 may be} repeated). To determine a cleanliness level of the print head 3 or 3a, each of the CIJ printer 1 or 1a enables an inspection of a characteristic of the cleaning fluid which has been used for _{cleaning the print head 3 or 3a. The inspection may take place during the soaking stage} 54 or 86. The characteristic of the used cleaning fluid may be an electrical characteristic (e.g., electrical resistivity, electrical conductivity etc.) of the used cleaning fluid. Fresh cleaning _{fluid (e.g., solvent) typically has a low electrical conductivity or may even be non-} conducting. In comparison, the used cleaning fluid has a higher electrical conductivity due to the presence of ink, and its conductivity increases along with the increased _{amount of ink therein. During a cleaning cycle (Figure 5 or 6), the amount of ink present} _{within the used cleaning fluid is associated with the cleanliness level of the print head (3} _{or 3a). Therefore, the electrical characteristic of the used cleaning fluid is an indicator of} the cleanliness level of the print head. The characteristic of the used cleaning fluid may also be an optical characteristic (e.g., transmittance, colour etc.) of the used cleaning fluid. The “transmittance” of the used _{cleaning fluid refers to the fraction of incident light that is transmitted through the used} _{cleaning fluid. Fresh cleaning fluid (e.g., solvent) is typically a clear fluid, thereby allowing} _{most (if not all) of the incident light to pass through. The presence of ink and dirt within} _{the cleaning fluid makes the used cleaning fluid opaque and reduces the transmittance} of the used cleaning fluid. An increased amount of ink and/or dirt causes the used cleaning fluid to have a lower transmittance. Similarly, the amount of ink within the cleaning fluid also affects the colour of the used cleaning fluid. For example, if the ink used by the print head is of a black colour and the fresh cleaning fluid is transparent, _{then the colour of the used cleaning fluid may be different shades of a grey colour or} nearly black depending upon the amount of ink therein. As described above, during a 69715475-1 cleaning cycle (Figure 5 or 6), the of ink within the used cleaning fluid is associated with the cleanliness level of the print head (3 or 3a). Therefore, the optical _{characteristic of the used cleaning fluid is also an indicator of the cleanliness level of the} print head. _{For the print head 3, the inspection may be carried out on the used cleaning fluid that} _{remains within the sealed chamber 26. The CIJ printer 1 may use existing structures of} the print head 3 as electrodes to measure the electrical characteristic of the used _{cleaning fluid held by the sealed chamber 26. The electrodes may be suitably selected} _{insofar as the used cleaning fluid provides the only electrically conductive path between} _{the electrodes which are otherwise insulated from one another. For example, the CIJ} printer 1 may measure the electrical conductivity between the low voltage electrode 28 and the deflection electrode 30. In another example, the CIJ printer 1 may measure the electrical conductivity between the nozzle 22, and one of the charge electrode 24, the low voltage electrode 28, the deflection electrode 30 and the gutter 32. Alternatively, the CIJ printer 1 may measure the electrical conductivity between the charge electrode 24, and one of the low voltage electrode 28, the deflection electrode 30 and the gutter 32, or measure the electrical conductivity between the gutter 32 and one of the low voltage electrode 28 and the deflection electrode 30. The controller 6 then determines the cleanliness of the print head 3 based upon the measured electrical conductivity of the used cleaning fluid. The determination may be based upon a lookup table accessible to the controller 6. The lookup table may have data including electrical conductivity of the used cleaning fluid and an associated _{cleanliness level of the print head 3. Alternatively, the controller 6 may dynamically} determine the cleanliness level without using any lookup table. As shown in Figure 7, with repeating cleaning cycles (X axis), the conductivity (Y axis) of the used cleaning _{fluid measured during each cleaning cycle reduces (assuming that fresh cleaning fluid is} used in each cycle). When the print head 3 gets cleaner, further cleaning cycles would _{have reduced effects and thus the conductivity curve would become flatter. Therefore,} _{the controller 6 can determine a cleanliness level of the print head 3 based upon a} _{difference of the measured conductivity of the used cleaning fluid between adjacent} cleaning cycles, and considers the print head 3 clean if the difference drops below a threshold. Once the print head 3 is deemed clean, the controller 6 may terminate the cleaning process of the print head 3. 69715475-1 Further or alternatively, the print head 3 may have an optical sensor installed therein and the optical sensor may be used to measure the optical characteristic of the used cleaning _{fluid held by the sealed chamber 26. In an example, the optical sensor may include a} light transmitter and a light detector separated by the used cleaning fluid. A ratio of the amount of light detected by the light detector over a total amount of light emitted by the light transmitter is the transmittance of the used cleaning fluid. The controller 6 then determines the cleanliness of the print head 3 based upon the measured transmittance, in a way similar to that described above in relation to electrical conductivity. _{For the pint head 3a, the inspection of the characteristic of the used cleaning fluid may} be carried out when the print head 3a is soaked in the cleaning fluid held by the wash _{station 12 during the soaking stage 86. Electrodes or an optical sensor may be attached} to an internal wall of the wash station 12 to measure the electrical/optical characteristic of the used cleaning fluid. The cleanliness of the print head 3a may be determined (either by the controller 6 of the CIJ printer 1a or by a processor (if any) of the wash station 12) in a similar way to that described above for the print head 3. _{To assist with cleanliness determination of the print head 3, the print head 3 may be} _{configured to allow an operator to manually inspect a characteristic of the used cleaning} _{fluid. In an example, the print head 3 has a housing which at least partially defines the} _{cleaning chamber 26, and a part or the whole of the housing is made of a transparent or} _{translucent material. The transparent/translucent material of the housing provides an} _{inspection chamber or a bulb which allows the operator to manually inspect the optical} _{characteristic (e.g., transmittance, colour etc.) of the used cleaning fluid held by the} cleaning chamber 26. The operator may inspect the optical characteristic of the used cleaning fluid using naked eyes, or using additional equipment which emits light into the _{transparent/translucent material of the housing and detects light passing through the} _{used cleaning fluid. In another example, the CIJ printer 1 has a draining conduit which} _{can establish a fluid communication path with the cleaning chamber 26 for draining the} _{used cleaning fluid from the cleaning chamber 26. At least a part of the draining conduit} _{may be made of a transparent or translucent material, thereby allowing an operator to} manually inspect the optical characteristic (e.g., transmittance, colour etc.) of the used _{cleaning fluid when the used cleaning fluid is drained from the cleaning chamber through} 69715475-1 the draining conduit. The draining be part of the print head 3, or may be part of the umbilical cable 4 connecting the print head 3 and the printer body 2. Figure 8 schematically illustrates processing steps of a method of operating a CIJ printer (e.g., the CIJ printer 1 or 1a). At step S1, a cleaning fluid is supplied to a print head (e.g., the print head 3 or 3a) of the CIJ printer. This may take place during the fill stage 52 (Figure 5) or the spray stage 84 (Figure 6). _{At step S2, a characteristic (e.g., an electrical/optical characteristic) of the cleaning liquid} is inspected during a soaking period (e.g., the soaking stage 54 or 86) in which at least a part of the print head is soaked in the cleaning fluid. Inspecting the characteristic of the cleaning liquid may comprise measuring the characteristic of the cleaning fluid. _{At step S3, a cleanliness level of the print head is determined based upon the inspected} characteristic of the cleaning liquid. The determination may be carried out by a controller (e.g., the controller 6) of the CIJ printer. At step S4, the cleaning fluid is removed from the print head after the soaking period. For the print head 3, the cleaning fluid is drained from the cleaning chamber 26. For the print head 3a, the cleaning fluid is drained from a cleaning tank of the wash station 12. It would be appreciated that step S2 takes places before step S4 in which the used cleaning fluid is removed from the print head. It would be appreciated that the steps may be performed in a temporal order that is different from the order of description. For example, steps S3 and S4 may be performed simultaneously or that step S4 may be performed prior to step S3. The method may further comprise an optional step of terminating a cleaning process of the print head based upon the determined cleanliness level of the print head. In the embodiments described above, the cleanliness level of a print head is determined by inspecting a characteristic of used cleaning fluid. Further or alternatively, a cleanliness 69715475-1 level of the print head may also be using an optical sensor installed within _{the print head. This may take place when the print head is not soaked in cleaning fluid.} Figure 9 schematically illustrates an exemplary optical sensor 40 for determining the cleanliness level of the print head 3 or 3a. The optical sensor 40 includes a light _{transmitter 42 and a light detector 44. The light transmitter 42 emits light toward the light} _{detector 44 within the print head, and the light detector 44 generates sensor data} _{indicative of an amount of light detected by the light detector 44. The light may be in the} visible, infrared, or ultraviolet range, or combinations thereof. In this example, the light detector 44 is an image sensor comprising a two-dimensional array of light detecting _{elements. Other arrangements are possible. As used herein, a light path 43 is a volume} _{between the light transmitter 42 and the light detector 44 that conforms to their} perimeters as though nothing were present that could block light. Anything disposed in _{the light path 43 would partially block of the light path 43.} In this example, the optical sensor 40 can be used to detect a localised accumulation of ink 41 which partially blocks the light path 43, and can also be used to determine an overall cleanliness level of the interior of the print head. The overall cleanliness level of the interior of the print head represents a general cleanliness level of the print head and can be used as an indicator to prompt a cleaning operation of the entire print head. The overall cleanliness level is in contrast to a local cleanliness level of the print head. _{The localised accumulation of ink 41 detectable by the optical sensor 40 is on an internal} surface of the print head that is downstream of the low-voltage electrode 28 and the _{deflection electrode 30 and external to an interior of the gutter 32. More specifically, the} _{localised accumulation of ink 41 is on an internal surface of the print head that is adjacent} _{to an inlet of the gutter 32. With reference to Figure 9, the gutter is positioned between} _{the light transmitter 42 and the light detector 44. During operation of the CIJ printer 1 or} 1a, an ink droplet may not travel along its flight path as intended, and any anomalously under-deflected ink droplet may be steered toward an upper surface (which faces the deflected droplet flight paths) of the gutter 32 close to the gutter inlet, thereby contributing _{to the accumulation of ink 41 in this location.} _{Figure 10 schematically illustrates sensor data generated by the light detector 44. In this} _{example, the sensor data takes the form of an image, and comprises a shadow area 46} 69715475-1 corresponding to a shape of the accumulation of ink 41, and a non-shadow area 48. _{The shadow area 46 appears because the localised accumulation of ink 41 partially} blocks the light path 43, and reduces the amount of emitted light detectable by _{corresponding pixels of light detector 44. With the localised accumulation of ink 41} _{growing over time, a boundary of the shadow area 46 would move outwards over time.} _{Therefore, a size of the shadow area 46 corresponds to a size of the localised} accumulation of ink 41. The non-shadow area 48 is brighter than the shadow area 46. However, the brightness _{of the non-shadow area 48 tends to gradually decrease with continued operation of the} _{print head (if not cleaned), due to thin films of deposits (ink and/or dirt) gradually building} up on the internal surfaces of the print head and also on the surfaces of the light transmitter 42 and the light detector 44. The gradual build-up of thin films of deposits _{obscure the optical sensor 40 and cause the brightness of the non-shadow area 48 of} the light detector 44 to gradually decrease. Therefore, a brightness level of the non- shadow area 48 indicates an overall cleanliness level of the interior of the print head. _{The controller 6 receives the sensor data generated by the light detector 44, and can} _{distinguish between the shadow area 46 and the non-shadow area 48 due to the different} _{brightness levels therebetween. The controller 6 then detects the localised accumulation} _{of ink 41 based upon a size of the shadow area 46, and detects the overall cleanliness} _{level of the interior of the print head based upon a brightness level of the non-shadow} _{area 48. For example, the controller 6 can be programmed to access a lookup table that} _{has data including light intensity received in the non-shadow area 48 and an associated} overall cleanliness level of the print head. In determining the overall cleanliness level of the print head, the shadow area 46 of the sensor data may be treated as faulty data, which shall be disregarded in determining the overall cleanliness level of the print head. In this example, the faulty data may be identified based upon a spatial variation of the sensor data generated by the plurality of the light detecting elements of the light detector 44 at the same time. As the localised accumulation of ink 41 grows over time, it is expected that a part of the non-shadow area would become part of the shadow area later. In other words, the growth of the localised 69715475-1 accumulation of ink 41 would cause an change (e.g., a step change) in the amount of emitted light detected by particular ones of the light detecting elements at a particular _{time. The sensor data generated by some of the light detecting elements with the abrupt} _{decrease of brightness may be treated as the faulty data which shall be disregarded in} determining the overall cleanliness level of the print head. In the event that the light detector 44 includes a single light detecting element (as compared to a two-dimension array of the light detecting elements as described above), the sensor data does not have any shadow area or non-shadow area. The faulty data may be identified based upon a temporal variation of the sensor data (i.e., an abrupt _{decease of brightness level) generated by the light detector which indicates a sudden} blockage of the light path 43. The controller 6 may disregard the faulty data and use the _{remaining data (which indicate a gradual decrease in the amount of emitted light} detected by the light detector) to determine a rate of deterioration of the overall cleanliness level of the print head. The rate of deterioration of the overall cleanliness level can be used in combination with an overall cleanliness level determined at a _{previous time point to deduce overall cleanliness level in the future.} The optical sensor 40 may also be used within the print head 3 to measure the electrical conductivity of used cleaning fluid when the used cleaning fluid remains within the sealed _{chamber 26. In particular, the light transmitter 42 and the light detector 44 may be used} as two electrodes. _{It would be appreciated that the location of the optical sensor 40 may be adjusted such} that the light path 43 is unlikely to be blocked by any localised accumulation of ink. The _{optical sensor 40 described above is a transmissive optical sensor. It would be} appreciated that a reflective optical sensor can also be used. As shown in Figure 11, the _{reflective optical sensor includes a light transmitter 42 which emits light 62 to an internal} _{surface 60 of the print head 3 or 3a and a light detector 44 which detects the light 64} reflected by the internal surface 60. During operation of the print head, thin films of _{deposits gradually build up on all internal surfaces of the print head (e.g., in a blanket} manner) and cause general degradation/dulling of the internal surface 60. The build-up of deposits gradually enhance the scattering 66 of the emitted light (e.g., diffuse reflection), thereby causing the amount of light 64 detected by the light detector to _{gradually decrease. In other words, for a reflective optical sensor, the light detector 44} 69715475-1 _{may be used to detect an amount of reflection 64 of the emitted light 62. The} internal surface 60 of the print head which reflects the emitted light 62 may be an internal surface of the chamber 26, a surface of the deflection electrode 30 which faces the low voltage electrode 28, or a surface of the low-voltage electrode etc. The reflective optical _{sensor may include at least one light transmitter emitting light to multiple regions of the} internal surface of the print head, and a plurality of the light detectors each configured to detect an amount of the emitted light that is reflected by a respective one of the multiple _{regions of the internal surface. In this way, a spatial variation of the sensor data} generated by the light detectors at the same time can be used to detect faulty data which shall be disregarded in determining the overall cleanliness level of the print head. _{If the overall cleanliness level of the print head as determined by the controller 6 has} dropped below a threshold, the controller 6 may generate a warning signal in the interface 7 prompting a user to initiate a cleaning operation of the print head, and/or may trigger a cleaning operation of the print head for the self-cleaning print head 3. The _{cleaning operation may not be triggered immediately when the print head 3 is used in} printing. Instead, the controller 6 may wait until the current printing operation stops and then trigger the cleaning operation of the print head. _{The controller 6 may be able to predict when the print head will require a cleaning} _{operation based upon the sensor data or the determined overall cleanliness level of the} print head. For example, the controller 6 may determine the overall cleanliness level of _{the print head at some point in time, and may then use known parameters and/or} determine a rate of deterioration of the overall cleanliness level to predict when a cleaning operation might be necessary. The known parameters may be stored in a lookup table which includes data for a make and model of the print head 3 or 3a, type of _{ink used, and/or ambient conditions, associated with a print head run time. The lookup} table may be stored locally or remotely and is accessible to the controller 6. The controller 6 may generate a signal indicative of a recommended time for carrying out the cleaning operation. In this manner the controller 6 provides advance notice, which the operator may use to plan appropriate cleaning operation of the print head. This, in turn, may save time and/or expense. For example, knowing that a cleaning operation of the print head _{will be due soon, the controller 6 or the operator may initiate the needed cleaning} operation during an already-scheduled shutdown. This helps avoid additional downtime of the CIJ printer. 69715475-1 Further or alternatively, the controller 6 may assess a cleaning efficacy of a cleaning _{operation of the print head 3 or 3a, based upon the sensor data generated prior to and} after the cleaning operation. To assist with cleanliness determination of the print head 3 or 3a, at least a part of an interior surface of the print head may be of a high-contrast colour over a colour of the ink _{droplets. In the event that black ink is used, the interior surface of the print head may be} _{of a white colour. Generally speaking, a high-contrast colour refers to a colour which} allows the ink droplets deposited on the interior surface of the print head to be easily _{discerned. The high contrast may appear under light in one or more of the visible, infrared} and ultraviolet ranges. In addition, the optical sensor 40 may be used to determine a configuration of the sealing _{mechanism 34 in the print head 3 (Figure 3). As described above, the purpose of the} sealing mechanism 34 is to selectively seal the chamber 26. The chamber 26 would be darker when the sealing mechanism 34 seals the chamber 26 as compared to a scenario where the chamber 26 is not sealed by the sealing mechanism 34. This is because, by sealing the chamber 26, the sealing mechanism 34 also preventing ambient light from entering the chamber 26. In this use of the optical sensor 40, the light transmitter 42 may be switched off without emitting any light. As such, the controller 6 is able to determine a configuration of the sealing mechanism 34 based upon an amount of light detected by the light detector 44. For example, if the sensor data generated by the light detector 44 indicates that almost no light has been detected by the light detector 44, then the _{controller 6 would deduce that the chamber 26 is currently being sealed by the sealing} mechanism 34; if the sensor data generated by the light detector 44 indicates that some light has been detected by the light detector 44, then the controller 6 would infer that the sealing mechanism 34 is in a configuration which does not seal the chamber 26. _{Figure 12 schematically illustrates processing steps of a method of operating a CIJ} printer (e.g., the CIJ printer 1 or 1a). The CIJ printer comprising a print head (e.g., the print head 3 or 3a) and a controller (e.g., the controller 6), and the print head comprising 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 24, 28, 30) for guiding the stream of ink droplets, a gutter (e.g., the gutter 32) for receiving droplets of 69715475-1 ink which are not used for printing, and sensor (e.g., the sensor 40) including a light transmitter (e.g., 42) and a light detector (e.g., 44). At step M1, the light transmitter emits light within the print head. At step M2, the light detector generates sensor data indicative of an amount of light detected by the light detector. _{At step M3, the controller 6 determines an overall cleanliness level of the interior of the} print head based upon the sensor data. The overall cleanliness level may be determined based upon a gradual decrease in the amount of the emitted light detected by the light detector as indicated by the sensor data. The method may comprise the following optional steps: At step M4, the controller 6 may predict when the print head will require a cleaning operation based upon the sensor data. At step M5, the controller 6 may assess a cleaning efficacy of a cleaning operation of the print head, based upon the sensor data generated prior to and after the cleaning operation. 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. 69715475-1

Claims

CLAIMS: _{1. A continuous inkjet printer, comprising:} a print head comprising: a_{nozzle for generating and ejecting a stream of ink droplets for printing;} _{at least one electrode for guiding the stream of ink droplets;} a gutter for receiving droplets of ink which are not used for printing; and a_{n optical sensor including a light transmitter and a light detector, wherein} the light transmitter is configured to emit light within the print head, and the light detector is configured to generate sensor data indicative of an amount of light d_{etected by the light detector; and} a controller configured to determine an overall cleanliness level of the interior of the print head based upon the sensor data. _{2. The continuous inkjet printer of claim 1, wherein the controller is configured to} _{determine the overall cleanliness level of the interior of the print head based upon a} gradual decrease in the amount of the emitted light detected by the light detector as indicated by the sensor data. _{3. The continuous inkjet printer of claim 1 or 2, wherein the controller is configured} to identify faulty data within the sensor data based upon a variation of the sensor data, _{and to disregard the faulty data in determining the overall cleanliness level of the interior} of the print head. _{4. The continuous inkjet printer of claim 3, wherein the controller is configured to} _{identify the faulty data based upon a temporal variation of the sensor data generated by} the optical sensor. _{5. The continuous inkjet printer of claim 3 or 4, wherein the optical sensor comprises} a plurality of the light detectors, and the controller is configured to identify the faulty data _{based upon a spatial variation of the sensor data generated by the plurality of the light} _{detectors at the same time.} 69715475-1

_{6. The continuous inkjet printer of claim, wherein the controller is also} _{configured to detect a localised accumulation of ink in the interior of the print head based} upon the sensor data. _{7. The continuous inkjet printer of any preceding claim, wherein the light transmitter} is configured to emit light to an internal surface of the print head, and the light detector is configured to detect an amount of the emitted light that is reflected by the internal surface of the print head. _{8. The continuous inkjet printer of any preceding claim, wherein the controller is} configured to predict when the print head will require a cleaning operation based upon the sensor data. _{9. The continuous inkjet printer of any preceding claim, wherein the controller is} configured to assess a cleaning efficacy of a cleaning operation of the print head, based upon the sensor data generated prior to and after the cleaning operation. _{10. The continuous inkjet printer of any preceding claim, wherein at least a part of an} _{interior surface of the print head is of a high contrast colour over a colour of the ink} droplets. _{11. The continuous inkjet printer of any preceding claim, wherein the print head} further comprises a cleaning chamber selectively sealable by a sealing mechanism, wherein the controller is configured to determine a configuration of the sealing mechanism based upon the sensor data. _{12. A method of operating a continuous inkjet printer, the continuous inkjet printer} _{comprising a print head and a controller, and the print head comprising a nozzle for} generating and ejecting a stream of ink droplets for printing, at least one electrode for guiding the stream of ink droplets, a gutter for receiving droplets of ink which are not used for printing, and an optical sensor including a light transmitter and a light detector, wherein the method comprises: emitting light, by the light transmitter, within the print head; generating, by the light detector, sensor data indicative of an amount of light _{detected by the light detector; and} 69715475-1 determining, by the controller, cleanliness level of the interior of the print head based upon the sensor data. _{13. 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; and} a cleaning mechanism for cleaning the print head by supplying a cleaning fluid to the print head and removing used cleaning fluid from the print head after a soaking period; wherein the continuous inkjet printer is configured to enable an inspection of a characteristic of the used cleaning fluid during the soaking period, so as to determine a cleanliness level of the print head. _{14. The continuous inkjet printer of claim 13, wherein the characteristic of the} _{cleaning fluid comprises an electrical characteristic and/or an optical characteristic of the} used cleaning fluid. _{15. The continuous inkjet printer of claim 14, wherein the electrical characteristic is} indicative of an electrical conductivity of the used cleaning fluid. _{16. The continuous inkjet printer of claim 14 or 15, wherein the optical characteristic} is indicative of a transmittance of the used cleaning fluid. _{17. The continuous inkjet printer of any one of claims 13 to 16, wherein the print head} further comprises a cleaning chamber selectively sealable by a sealing mechanism, and _{the cleaning mechanism is configured to clean the print head by at least partially filling} _{the cleaning chamber with the cleaning fluid and draining the used cleaning fluid from} the print head after the soaking period. _{18. The continuous inkjet printer of any one of claims 13 to 17, wherein the} continuous inkjet printer is configured to measure an electrical characteristic of the used cleaning fluid by using at least two components selected from the nozzle, the at least _{one electrode and the gutter.} 69715475-1 _{19. The continuous inkjet printer of any one of claims 13 to 18,} _{wherein the continuous inkjet printer comprises an ink build-up sensor for} detecting an accumulation of ink on a print head surface, and the ink build-up sensor comprises a light transmitter and a light detector which are configured such that the accumulation of ink reduces an amount of transmitted light that is detected by the light detector, and w_{herein the continuous inkjet printer is configured to measure the characteristic} of the used cleaning fluid by using the light transmitter and the light detector. _{20. The continuous inkjet printer of any one of claims 13 to 19, wherein the} continuous inkjet printer is configured to measure the characteristic of the used cleaning fluid during the soaking period and to determine a cleanliness level of the print head based upon the measured electrical characteristic of the used cleaning fluid. _{21. The continuous inkjet printer of any one of claims 13 to 20, wherein the cleaning} mechanism is configured to repeatedly clean the print head under a plurality of cleaning cycles, and the continuous inkjet printer is configured to measure the characteristic of the used cleaning fluid during each of the plurality of cleaning cycles and to determine a cleanliness level of the print head based upon a difference of the measured characteristic of the used cleaning fluid between adjacent cleaning cycles. _{22. The continuous inkjet printer of any one of claims 13 to 21, further comprising: a} controller which is configured to determine the cleanliness level of the print head based upon the inspection of the characteristic of used cleaning fluid, and to stop the cleaning _{mechanism from cleaning the print head based upon the determined cleanliness level of} the print head. _{23. The continuous inkjet printer of any one of claims 13 to 22 as dependent from} claim 17, wherein the print head comprises a housing which at least partially encloses the cleaning chamber, and at least a part of the housing is configured to enable an inspection of an optical characteristic of the used cleaning fluid contained within the cleaning chamber. 69715475-1

_{24. The continuous inkjet printer of 23, wherein the at least a part of the housing} is transparent or translucent. _{25. A method of operating a continuous ink jet printer, comprising:} supplying a cleaning fluid to a print head of the continuous ink jet printer; inspecting a characteristic of the cleaning liquid during a soaking period in which at least a part of the print head is soaked in the cleaning fluid; determining a cleanliness level of the print head based upon the inspected characteristic of the cleaning liquid; and removing the cleaning fluid from the print head after the soaking period. 69715475-1