Printing apparatus

An inkjet printer in which a printhead defines an elongate array of orifices from which jets of ink are projected into a space within which substrates to be printed are presented. An air curtain generator fixed in position relative to the printhead extends along one side only of the array of orifices so as to direct a curtain of air across the array a of orifices and across an open portion of the printhead on the side of the array of orifices remote from the air curtain generator. The air curtain cleans the printhead. A pressure pulse may be applied to ink within the printhead sufficient to discharge ink form each orifice when not in use to project ink. The pressure pulse has a relatively rapidly rising leading edge and a relatively slowly falling edge. An ink reservoir is coupled to the printhead for supplying ink to the orifices. The orifices may be arranged in at least two vertically offset groups, the reservoir being divided into vertically offset reservoir sections including a lowermost section and an upper section with each reservoir section being coupled exclusively to a respective group of orifices. The upper reservoir section is arranged to overflow into the immediately adjacent reservoir section which is spaced therebelow. Each reservoir section includes an ink level detector, and ink is supplied to the reservoir section which is vertically uppermost if any level detector detects an ink level below a predetermined level.

This invention relates to inkjet-printing apparatus, and in particular to apparatus for cleaning, maintaining and supplying ink to a printhead of an ink jet printer.

Types of printers exist which make use of so called piezo-electric ink-jet printing technology. A piezo-electric printhead (also sometime referred to as PZT printhead) of such a printer is characterised in that it has a plurality of miniature jetting chambers or “jets” closely arranged in an array. Each jet is arranged to project ink from a respective one of an array of orifices defined by the printhead, and the jets are selectively energised by a controller to project (or not project as the case may be) “pixels” of ink. The ink is projected onto a substrate to be printed, relative movement between the printhead and the substrate resulting in ink projected from one orifice being deposited along an elongate path on the substrate. The printhead is arranged within the printer such that the array of jets extends at a predetermined angle (for example 90°) to the direction of the relative motion between the printhead and the substrate. The controller selects a first combination of jets through which ink is projected, and then a second combination of jets, etc, relative movement between the substrate and the printhead resulting in a two dimensional image being printed on the substrate.

Piezo-electric ink jet printers are used in many office and industrial applications. Industrial printing applications include packaging printers, which print directly onto substrates such as cardboard boxes, trays, flexible film and labels.

Current industrial printers use a range of different inks in conjunction with a variety of different models of piezo-electric printheads. Some utilise liquid ink, others utilise solid ink which is heated so as to phase-change to liquid within the printer, the ink being liquid at the time it is ejected from the jets.

Printhead arrays used in industrial applications typically range in length from 10 mm to 70 mm. In high resolution printheads, as many as 512 jets are arranged in a 70 mm long linear array on a printhead. It will therefore be appreciated that the jets are very small. Each jet ejects a very small (picolitre order of magnitude) droplet of ink each time it is energised by the controller.

Whilst such printers can produce fine resolution images, they are subject to image degradation due to one or more of the jets becoming blocked by dust or other contaminant, or otherwise fail as the result of a jet not being filled with ink to adjacent the orifice at the time the ink is to be projected (commonly referred to as a “de-primed” state), so that they no longer eject ink properly. This jet failure (often referred to as “jet drop out”) results in a degraded image having one or several unprinted lines running throughout the printed image in the direction of relative movement between the printhead and the substrate. A contaminant that does not completely block a jet can also cause problems by causing the deflection of a droplet of ink as it is ejected from the jet, such deflection resulting in a distorted image.

In certain industrial applications such as printing onto cardboard boxes or trays, a significant contributor of contamination is dust either airborne or resting on the surface of the cardboard. The piezo-electric printing process requires the printhead jet array face to come into very close proximity with the target substrate. Furthermore there can be a build up of static electricity in the production process in the area around the printer, which can result in dust being positively attracted to the print face. Thus particularly in industrial applications contamination of jets is a major problem.

Given the problems referred to above, it is common practice, particularly in industrial applications, to follow a procedure often referred to as “purging” in order to maintain or restore the proper function of the jets in the printhead. The purging procedure involves forcing ink through all the jets of the printhead, with the intention of flushing out and removing any contaminants from the jets and ensuring that the jets are correctly printed, that is filled with ink to adjacent the orifices. After performance of the purging procedure, the jets should be in a stable working condition (commonly referred to as a “printed” state) such that high quality printing can be commenced or recommenced.

In known industrial printers, the purging procedure is typically initiated by the user. The purging action is typically brought about by putting an ink supply to the printhead under sufficient positive pressure to force some ink through all the jets of the printhead. This can be achieved by the user applying pressure manually to an ink reservoir communicating with ink in the printhead, or by applying pressure using a pump to the reservoir. Typically, positive pressure is applied for a period of a few seconds.

In industrial applications, such as printing onto cardboard boxes or trays, the printhead is often mounted adjacent a production line, so that the boxes pass in close proximity to the printhead. In these applications the printhead is most often mounted such the jet array is vertical, or at some predetermined angle to the horizontal. When ink is purged through the jets, it collects on an orifice plate of the printhead in which the jet orifices are formed, and begins to flow down the orifice plate under gravity. The flow of ink is relatively slow, the ink taking a number of seconds to flow down to the bottom of the printhead. Of course, the longer the printhead in the vertical direction, the longer the downward flow of ink takes.

Since a relatively large volume of ink (as compared with the volume of ink normally projected by an individual jet) is purged out of the orifices, the jets are not able to jet ink properly to create an image until substantially all the ink has flowed down away from the jets or has otherwise been moved away from the jets. Generally steps are taken to remove purged ink. For example, purged ink is soaked into a cloth which is then disposed off. This can easily result in ink being dripped onto the floor or surrounding factory equipment, creating unwanted mess, which is difficult to remove and clean. The wasted purged ink is a considerable percentage of the total ink consumption. Furthermore, wiping away purged ink in this way typically takes several seconds. The total time taken to complete a single ink purge procedure is thus substantial.

Accordingly, although the purge procedure is generally successful in recovering jets, it can cause a number of problems. Firstly the purge procedure generally requires manual intervention and takes at least several seconds. It will be appreciated that the printhead cannot properly print images whilst it is being purged. Therefore it is therefore often necessary to stop an associated process (for example a production line) until the purge procedure has been completed. Secondly, the amount of ink consumed in the process is relatively large, often being several times greater than the amount of ink used to print an image.

Various proposals for improving ink purge procedures have been proposed. For example, one known purging procedure described in British Patent GB 2339170 provides an aperture in a plate defining the orifices from which ink is projected, the aperture being positioned in use vertically beneath the jets. Either in response to detection of jet dropout requiring remedial treatment, or at intervals for maintenance purposes before jet dropout an ink feed reservoir is pressurized with a pump or manually to cause ink to flow out of the printhead orifices. After purging, ink flows down the orifice plate under gravity towards the aperture. The ink is sucked through the aperture and ducted to a reservoir for filtering and subsequent re-use. Ink wastage is thus reduced, but as the ink flows only under gravity it will be appreciated that it takes at least several seconds for the ink to flow away from the jets to the aperture. Even on production lines operating at modest speed the time taken for purged is to be removed from the orifice plate is too long for purging to be carried out between two successive product print cycles. Thus it will be appreciated that this known purging procedure takes too long to avoid the necessity of stopping the production line whilst purging occurs.

International patent specification WO 89/04255 describes an ink jet priming system in which pressurized air is used to purge ink into an ink collection system which is located adjacent ink jets to be purged only during the purging procedure. During purging, a sufficiently high pressure is applied to the ink such that it streams into the collection system rather than trickling out of the jets. Thus purged ink is removed from adjacent the jets without requiring manual intervention, but only as the result of providing a complex mechanical assembly to achieve the necessary relative movement between the jets and the ink collection system. The described system has the capability of controlling the shape of a pressure pulse that is used to purge ink from the jets, that pulse having a “soft” profile (slowly rising leading edge and rapidly falling trailing edge) or a “hard” profile (rapidly rising leading edge and rapidly falling trailing edge). In the described examples, the slowly rising leading edge part of the pulse has a duration of 100 milliseconds or 200 milliseconds. It is stated that it is desirable to abruptly terminate the supply of air which causes the ink to flow out of the jets (the rapidly falling trailing edge of the pulses). This is said to cause the streaming of ink from the jets to abruptly cease. It is stated that the described arrangement eliminates the trickling of ink from the jets.

European Patent Specification EP 1016530 describes another arrangement for cleaning and preventing obstruction of the orifice plate of an ink jet printer. The device described incorporates a cleaning assembly which is moveable relative to the orifice plate to a position in which a closed chamber is formed over the orifice plate. A fluid is then pumped through the closed chamber so as to clean any contaminants from the orifice plate surface or any individual orifice in the plates.

The highly complex arrangement of EP 1016530 is presented as an improvement of an earlier proposal which is described in U.S. Pat. No. 4,970,535. That U.S. patent discloses an arrangement in which an inkjet orifice plate cleaner is moved into engagement with the orifice plate so as to provide a closed air passageway through which air is directed to provide the required cleaning effect. Once again the described assembly is complex, requiring relative movement between a “ready to print” confirmation and a “cleaning” configuration. Furthermore it is stated in EP 1016530 that air cleaning in the manner suggested in U.S. Pat. No. 4,970,535 does not provide acceptable results.

European patent specification EP 604029 describes another cleaning arrangement in which an air curtain is directed across an ink jet orifice plate. The jets are located behind an aperture in the plate, projected ink passing through that aperture before being deposited upon a substrate to be printed. Air flows between the apertured plate and the orifice plate and thus across the orifices. The airflow is maintained during printing at an airflow rate that is sufficiently slow as to not significantly effect ink projection. The purpose of this airflow is to avoid the build up of contaminants or the orifice plate, not to clean ink off the orifice plate. Cleaning of the printing head requires a separate operation, involving release of a latch mechanism to enable the orifice plate to swing away from a “ready to print” position to a position in which it can be readily cleaned. Positioning the orifice plate behind another plate means that none of the orifice plate is open to the space through which the substrate to be printed travels relative to the orifice plate. As a result the distance through which ink must be projected is relatively large.

U.S. Pat. No. 5,184,147 describes an inkjet printhead maintenance system incorporating various relatively moveable components including a slowly moving elongate mechanical wiper and an “air knife”. The air life generates a narrow stream of air which is swept across the orifice plate. Thus the air life requires relative movement between the structure generating the narrow stream of air and the orifice plate. Furthermore, the air knife is provided as only one of a number of complimentary cleaning mechanisms in an overall assembly of great complexity.

In the absence of a simple fast operating jet pinging systems in many applications purging generally requires the stopping of an associated process, for example a production line. As a result, users prefer to initiate the purge procedure as seldom as possible. This can result in a compromise between production line efficiently on the one hand and image quality on the other. In practice, it is common for operators to wait for printing quality to deteriorate significantly before initiating the purge procedure. It will also be appreciated that the longer dust and debris is allowed to build up on the printhead, the more purging and cleaning of the jets is likely to be necessary to fully restore print quality.

It is an object of the present invention to provide an improved ink jet printing apparatus which addresses one or more of the problems outlined above.

According to a first aspect of the present invention, there is provided an apparatus for cleaning an inkjet printer printhead which defines an elongate array of orifices front which in use jets of ink are projected into a space within which substrates to be printed are presented, comprising an air curtain generating means fixed in position relative to the printhead and extending along one side only of the array of orifices so as to direct a curtain of air across the array of orifices and across a portion of the printhead which is open to the said space and which is on the side of the array of orifices remote from the air curtain generating means.

The apparatus defined in the preceding paragraph makes it possible to rapidly displace purged ink from the printhead using an assembly which is permanently fixed in position and which is not interposed between the printhead and a substrate onto which ink is to be projected from the printhead. A compact and mechanically simple printing arrangement is thus provided which can be rapidly cleaned, enabling cleaning to be performed after completion of one printing operation and before the initiation of a subsequent printing operation even if the two printing operations are spaced by only a relatively short period of time for example 1 second or less.

According to a second aspect of the present invention, there is provided an apparatus for maintaining an inkjet printer printhead in a working state, the printer defining an array of orifices from which in use jets of ink are projected, the apparatus comprising means for applying a pressure pulse to ink within the printhead sufficient to discharge ink from each orifice, the pressure pulse having a relatively rapidly rising leading edge and a relative slowly falling trailing edge.

The use of a pulse shape as defined above ensures that contaminants are readily dislodged from the jets by the flow of ink generated by the pulse and yet there is minimal risk of the jets becoming de-primed when the pulse is terminated.

According to a third aspect of the present invention, there is provided all inkjet printer ink supply apparatus for supplying ink to a printhead defining an array of orifices from which ink may be projected, the apparatus comprising a reservoir coupled to the printhead for supplying ink to the orifices, and means for supplying ink to the reservoir, the orifices being arranged in at least two vertically offset groups, the reservoir being divided into vertically offset reservoir sections including a lowermost section and at least one upper section with each reservoir section being coupled exclusively to a respective group of orifices, and the or each upper reservoir section being arranged to overflow into the immediately adjacent reservoir section which is spaced therebelow, wherein each reservoir section includes means for detecting the level of ink within that section, and the ink supply means is arranged to deliver ink to the reservoir section which is vertically uppermost if any level detecting means detects an ink level in the respective reservoir section below a predetermined level.

An arrangement as defined in the preceding paragraph makes it possible to replenish a series of ink reservoirs each of which feeds a different set of jets using only one arrangement for delivering ink to all the reservoirs.

Preferably, the air curtain is directed to flow in a direction perpendicular to the elongate array of orifices. An ink-receiving opening may extend along the side of the array of orifices, the curtain of air being directed across the array of orifices towards the opening. The opening may be defined by a gap between a deflector on the side of the opening remote from the array of orifices and an edge of a surface of the printhead in which the orifices are formed.

Preferably, an edge of the deflector which defines a first side of the gap is set back relative to the surface of the printhead in which the orifices are formed such that the said deflector edge is further away from the space within which substrates to be printed are presented than the said edge of the surface of the printhead which defines a second side of the gap. The set back of the deflector edge may be from 0.1 mm to 3 mm, for example 1 mm.

The deflector preferably defines a deflector surface extending from the deflector edge which defines the first side of the gap, the deflector surface being inclined to the curtain of air so as to deflect the curtain of air towards the said space within which substrates to be printed are presented. The deflector surface may be inclined at an angle of from 10° to 35°, for example 20°, to the direction in which the air curtain flows across the printhead, the angle of inclination being measured between the deflector surface and a line extending from the said edge in the direction of flow. The deflector surface preferably extends from the said deflector edge to a downstream edge on the side of the deflector edge remote from the array of orifices, a further surface of the apparatus extending from the downstream edge of the deflector in a direction away from the said space within which substrates to be printed are presented. The further surface is preferably inclined to the deflector surface at an inclined angle of from 70 to 155°, e.g. 110°.

Preferably an ink-receiving channel is defined behind the deflector, the ink-receiving channel opening into the gap and extending to a lower edge of the deflector. An ink collector may be located beneath a bottom end of the ink-receiving channel. The bottom end of the channel may be positioned to deliver ink to a formation on which ink will accumulate and which is in contact with or closely spaced from a surface defined by the collector. A manually adjustable screw may be mounted on the collector and adjustable in position relative to the formation such that irk on the formation can flow onto the screw and via the screw into the collector. Preferably, the collector comprises an overflow, means for detecting ink flowing through the overflow, and means for signalling a fault if overflowing ink is detected. The detecting means preferably comprises an emitter and a detector at least one of which is positioned to be at least partially covered by overflowing ink, and means for signalling a fault if an output of the detector indicates an overflow of ink. The emitter and detector may be arranged to project from a support surface which is located below the printhead and which is open to the said space within which substrates to be printed are presented, the emitter and detector being connected to a sensing circuit sensitive both to the presence of overflowing ink and to the presence of an object in front of the printhead which reflects emissions from the emitter to the detector. The emitter and detector may be angled towards each other and angled upwards. The ink-receiving opening may have a width of from 0.5 mm to 2 mm, for example 1 mm.

The air curtain may be generated from an elongate slot extending along the said one side of the array of orifices by pumping air through the slot towards the orifices. The slot may be defined between a body adjacent the printhead and an edge of a plate secured to the body, an air inlet communicating with a space defined between the body and the plate. The slot may have a width of from 0.1 mm to 0.3 mm, for example 0.2 mm. Air may be delivered to the slot at a pressure of from 100,000 to 600,000 Pa (1 to 6 bar) above atmospheric pressure, for example 300,000 Pa (3 bar).

Preferably, the pressure pulse applied to the irk has a duration from initiation of the leading edge to initiation of the trailing edge of less than 1 second, e.g. less than 300 ms, or less than 100 ms, or within the range of 10 to 50 ms.

The pressure may be applied through a valve which is switchable between a first condition in which a source of compressed air is connected to an ink supply arrangement, and a second condition in which the ink supply arrangement is connected to an exhaust conduit which communicates with the atmosphere via an airflow restrictor. Preferably the airflow restrictor is manually adjustable to enable control of the slope of the trailing edge of the pressure pulse. The pressure pulse may be applied after a predetermined number of printing operations have been performed by projecting ink from the orifices, for example after each printing operation.

The volume of ink delivered to the reservoir may be controlled in response to an ink demand initiated by detection of an ink level in any one reservoir section below the predetermined level, the ink supply control means being operative to deliver a predetermined volume of ink in response to an ink demand, to suspend ink delivery for a predetermined period, and to deliver further ink if an ink demand is indicated after the end of the predetermined period. The reservoir sections may communicate with a single compartment to which compressed air is delivered via a single air inlet to pressurise the ink in the reservoir sections, a baffle being positioned over the air inlet within the compartment to distribute incoming air evenly over all the reservoir sections. The reservoir sections are preferably defined within a common body partitioned to divide the interior of the body into the reservoir sections, each partition defining an edge over which ink can overflow from a reservoir section on one side of the partition to a reservoir section on the other side of the partition.

Preferably each reservoir section has housed within it a float which supports a magnet, displacement of the float by a changing ink level causing displacement of the magnet relative to a magnetic field sensor supported in a wall of the reservoir section, the magnetic field sensor providing an output indicating a demand for ink if the magnet assumes a predetermined position relative to the magnetic field sensor. The magnetic field sensor may be a Hall effect device. The float and magnet may be supported on an arm pivotally mounted on the reservoir section wall, or the float and magnet may be supported on an arm pivotally mounted on a lid of the reservoir.

Referring toFIGS. 1 to 3, a printing apparatus according to the invention is schematically illustrated which comprises a printhead1having a front surface2in which a linear array of orifices3is formed, an air supply device4for generating a curtain of air (represented by arrows5) which sweeps across the surface2, and a deflector plate6located downstream of the surface2. In use, the surface2faces a path along which substrates to be printed (not shown) are transported. Ink is projected from the orifices3onto such substrates. The printhead1may be of any conventional type. The front surface2of such printheads which defines the orifices3is generally referred to as an “orifice plate” and that term will be used hereinafter for the surface2in which the orifices3fare formed. In the described embodiment, the orifice plate2is planar and extends vertically, although it will be appreciated that in other embodiments of the invention the orifice plate2need not be planar and may be mounted so as to be inclined to the vertical, at any angle between 0° and 90°.

The air supply device4is supplied with compressed air via a conduit7, the supply of air being controlled by a solenoid-actuated valve. When the valve8is open, the air curtain5is established so as to sweep any ink or contaminants on the orifice plate2towards a gap9defined between a downstream edge10of the orifice plate and an upstream edge11of the deflector plate6. Ink swept from the orifice plate2is directed by the flow of air into the gap9and thereafter flows downwards under gravity to drip from a drip point12on the lower edge of the deflector plate6. Such ink is collected in a collector13.

As shown inFIG. 2, the air supply device4has a rearwardly extending (relative to the orifice plate2) portion which is coupled to the air supply conduit7and an open ended portion arranged so as to direct the curtain of air5in a plane indicated by line A which is parallel to the orifice plate2. The open end of the air supply device4defines an elongate outlet of substantially rectangular cross section. The outlet may be for example 0.2 mm, wide. Such an arrangement efficiently channels a well-defined curtain of air across the orifice plate9towards the downstream edge10and the gap9.

The upstream edge11of the deflector plate6is set back from the orifice plate surface2, the edge11lying on a plane indicated by line B inFIG. 2, the plane indicated by line B being parallel to the plane indicated by line A. The spacing between plane B and the plane including the orifice plate surface2is less than 1 mm. The width of the gap9(that is the spacing between the edge11of the deflector plate6and a line through the edge10of the orifice plate2extending perpendicular to the line B) is between 0.5 mm and 1.5 mm, for example 1 mm.

The deflector plate6extends in a direction parallel to a plane indicated by line C, the plane C being inclined to the plane of the orifice plate2by an angle α which in the illustrated embodiment is approximately 20° but will generally be in the range of 10° to 35°. The deflector plate6has a width (the dimension in the direction away from the printhead parallel to the line C) of approximately 5 mm. The downstream edge of the plate6is cut back to define the angle β shown inFIG. 2. That angle (which is equal to the included angle defined between the surfaces extending from the downstream edge of the plate6) will typically be of the order 110° but will generally be in the range of 70° to 155°.

The downstream edge of the plate6is cut back in order to improve the flow of air over the orifice plate2. When printing for example a cardboard box which defines a flat surface close to the printhead assembly, the presence of the box increases air flow resistance. If the downstream edge of the plate6was not cut back such that an extensive surface was defined extending parallel to the plane A (corresponding to the sum of the angles α and β being equal to 180°), a small elongate gap would be defined between the printhead assembly and the box downstream of the plate6. Such a gap would result in air flow resistance that could disrupt the flow of the air curtain across the orifice plate2.

FIG. 3illustrates the interconnection of the printhead1the air supply device4and the waste ink collector13to associated components. Ink is supplied to the printhead1from a reservoir14. The reservoir14comprises a first inlet15connected via one-way check valves16and17(to ensure unidirectional flow of ink) to a conduit18leading from a main ink supply storage vessel (not shown) and to a conduit19which is coupled by an ink recycling mechanism to the collector13. The reservoir14also has a second inlet20connected via a valve21to a compressed air supply conduit22and an air exhaust conduit23terminating in a restrictor valve24which may be manually adjusted to vary the rate at which air can be exhausted through the conduit23. The valve is controllable to assure either an “open” condition in which conduits20and22are in communication, or a “closed” condition in which conduits20and23are in communication.

The conduit19is coupled to the ink collector13by a conduit25leading to a pump26, a conduit27into which the pump26delivers ink from the collector13, and a filtering unit28which ensures that only ink which is sufficiently clean for re-use is deliver to the reservoir14.

The pressure of air supplied to the air supply device4will typically be of the order of 3 bar (300,000 Pa). The valve8will either be closed or fully open so as to deliver the supply pressure to the interior of the air supply device4. The compressed air delivered to the valve8will be appropriately regulated to maintain the desired pressure and the supplied air will be appropriately cleaned and filtered. The same supply of air is used to deliver compressed air to conduit22connected to valve21but the pressure applied to the surface of the ink in the reservoir14may be limited as described with reference toFIGS. 4 and 5. In that in the described embodiment the pressure is limited to 1 bar above atmospheric pressure, but will be limited to a range of from 0.2 to 0.8 bar or 0.4 to 0.6 bar for example.

The valves8and21and the pump26are controlled by a controller29. When a substrate is to be printed the valve8is closed and the valve21is closed. Thus no air flows across the orifice plate2and there is therefore no risk of projected jets of ink being deflected from their intended path. At a time when not printing, a purge procedure is executed in which initially the valve8is opened so as to establish a flow of air across the orifice plate2, and then the valve21is opened to apply a positive pressure to the ink within the reservoir14, causing ink to be discharged out of the orifices3onto the surface of the orifice plate2. That ink is then displaced by the air flowing across the orifice plate and gathers in the gap9on the downstream side of the orifice plate. The flow of air across the orifice plate2is then cut off by closing the valve8. The printer is then ready for the next printing cycle. It may take sometime for ink to run down into the collector13but as it is retained in the cap9on the downstream side of the orifice plate2this does not impede the normal operation of the printer. The pump26is used to periodically transfer ink from the collector13to the reservoir14. For example, the pump26may be turned on for a predetermined period after each purge procedure.

The time which elapses between initiation of a purge procedure and delivery of substantially all of the purged ink to the gap9is the sum of the duration of the period within which ink is purged from the orifices in the orifice plate2and the period of time taken for the purged ink to be swept into the gap9. Minimisation of the period for which positive pressure is applied to ink in the reservoir is therefore desirable and accordingly as illustrated inFIGS. 4 and 4Ain the described embodiment of the invention a short duration pulse of positive pressure is applied to the ink in the reservoir14.FIG. 4represents an idealised performance which could be achieved using extremely fast-acting components, whereasFIG. 4Arepresents the performance achieved with readily available standard components.

The upper half ofFIG. 4shows a voltage versus time diagram representing a control voltage applied to the valve21, a zero voltage corresponding to closure of the valve2and 24 volts corresponding to full opening of the valve21. A control voltage pulse of duration T having a steeply rising leading edge and a steeply falling trailing edge is applied to the valve21. The valve21is arranged however when “closed” to connect the exhaust conduit23to the reservoir inlet20. As a result when the valve21is closed air can bleed out of the reservoir14at a rate determined by the setting of the restrictor valve24.

The lower half ofFIG. 4shows the variation of the positive pressure applied to the reservoir14with time. It will be seen that the pressure rises rapidly as soon as the valve21is opened but falls relatively slowly when the valve21is closed, the rate of fall being determined by the setting of the restrictor valve24. Thus the pressure pulse applied to ink within the reservoir14has a relatively rapidly rising leading edge and a relatively slowly falling trailing edge. It is desirable for the leading edge to be relatively rapidly rising as this serves to minimise the duration of the purge cycle. It is desirable to have the slowly falling trailing edge as, if the trailing edge is steep, tie resultant sudden removal of the force causing irk to be purged out of the orifices can cause individual jets to be de-primed or can cause air to be sucked into the printhead which could disrupt printhead operation. A jet will be de-primed if it is not full of ink with the ink forming a meniscus at the jet orifice. Generally the period T will not be more than 1 second and much shorter periods of time can be used with success. Preferably T will be less than 100 ms and good results have been achieved with pressure pulses generated using a voltage pulse of duration T between 10 ms and 30 ms.

The performance represented inFIG. 4is idealised in that the pressure starts to respond substantially instantaneously to the control pulse. In practice, such a performance cannot be achieved using readily available and appropriately priced components.FIG. 4Aillustrates the performance achieved in one practical embodiment of the invention in which the valve21was obtained from MAC Valve Europe, part number 34AA BA GD FA-1BA with a specified on time of 3.4 ms (time taken to respond to a valve open control input) and an off time of 15 ms (time taken to respond to a valve close control input). The restrictor24was obtained from SMC UK, part number A51001F-04 and had a simple manually adjustable screw arrangement. The upper part ofFIG. 4Arepresents current drawn by the valve, and the lower part represents the pressure in the inlet20.

It will be noted that there is a delay of several milliseconds after the valve21begins to draw current (time t1) before the valve21starts to open, but thereafter the valve opens quickly and the pressure in the reservoir inlet20rises rapidly. The rate of rise of the pressure tails off as the pressure rises towards the supply pressure in conduit22. Given this tail off in the rate of pressure rise, and the short duration of the pressure pulse, the maximum pressure applied to the ink reservoir may be substantially below the supply pressure, e.g. only 0.5 bar with a supply pressure of 1 bar. Similarly, there is a delay after the current to valve21begins to fall (time t2) before the valve21starts to close. Once the valve21begins to close, there is an initial rapid fall in the pressure within inlet20, but thereafter there is a relatively slow fall off in the pressure within inlet20as air flows out through the restrictor24. The initial rapid fall in pressure reduces the period for which ink is being purged, whereas the subsequent slow fall in pressure avoids problems with jet de-priming.

In the case illustrated inFIG. 4A, the control input to the valve21has a duration (t2-t1) of 20 ms. The resultant pressure pulse has a duration of about 30 ms until the initial rapid fall in pressure, the pressure then falling father over a period of several tens of milliseconds. A pressure pulse duration of 30 ms has produced good results, but acceptable results have been achieved with the particular printhead used using pressure pulse durations in the range 10 to 50 ms and larger pressure pulses will be appropriate with different printheads and associated equipment.

Although a pressure pulse of only 30 ms duration is preferred, even with such a short duration pulse ink may continue to be purged from the printhead orifices for a substantial period dependent upon the hydrodynamic characteristics of the overall assembly. For example, ink may still be purged more than 100 ms after termination of the pressure pulse. The air curtain which cleans the orifice plate should be maintained for a sufficient duration to ensure that all purged ink has been displaced off the orifice plate, for example for a duration of 200 ms or 300 ms. The more efficient the cleaning the better, as the risk of dust sticking to the orifice plate is reduced. In a very dirty environment, a decision might be taken to maintain the air curtain except during printing, although there will be a trade-off between cleaning efficiency and the cost of compressed air supplying the air curtain.

FIGS. 5 and 6show in greater detail one possible arrangement of the collector13which collects ink dripping from the drip point12at the bottom of the deflector plate6.FIG. 5is a schematic perspective view from in front of the collector13, whereasFIG. 6is a view from above showing that the collector13has a first front wall portion30extending parallel to the orifice plate surface2and a second front wall portion31extending parallel to the outer face of the deflector plate6. The collector13extends beyond the edges of the printhead and the deflector plate such that the horizontal spacing between the top edge of the front wall portion30and the orifice plate2is in the range of 0.5 to 2.5 mm and preferably is approximately 1 mm. The same spacing is maintained between the upper edge of the second front wall portion31and the plane of the front surface of the deflector plate6. The upper edge of the front wall portion31is contoured so as to follow the lower edge of the drip point12as shown inFIG. 12. Thus the upper edge of the first and second front wall portions30and31is in close proximity to the drip point12and the lower edge of the orifice plate2. This facilitates the rapid transfer of ink by capillary action into the collector13.

The collector13is shaped so as to cause collected ink to run backwards away from the front wall portions30and31and into a cup-shaped sump from where it is sucked away by the pump26(FIG. 3). Ink may be recycled by pumping it through the filtering unit28directly into the reservoir which is close-coupled to the printhead as shown inFIG. 3, or alternatively to a main supply vessel which may be positioned at a relatively remote location and from which ink is delivered to the reservoir14so as to maintain tie ink level within the reservoir14within acceptable limits.

The collector13may be modified to support components which enable detection of a problem resulting in overflow of the collector13and detection of products moving in close proximity to the orifice plate2. As shown inFIGS. 7 to 10, an emitter32and a detector33are mounted so as to protrude from the first front wall portion30of the collector13, that is the portion immediately below the orifice plate2. The emitter and detector are mounted so as to protrude by a small distance for example 2 mm from the wall30. The emitter32and detector33are mounted at an angle γ (typically approximately 70°) to the plane of the front portion30of the collector13so as to be angled slightly towards one another. The emitter and detector are also mounted as an angle δ of approximately 5° to the horizontal. It is preferred to angle the emitter and detector upwards in this way so as to reduce the risk of unwanted signals being detected as a result of for example reflection from a conveyor or the like on which substrates to be printed (e.g. boxes) are transported past the orifice plate.

The emitter32and detector33are connected to the controller29ofFIG. 3Signals received by the detector33include a component that represents an amount of light emitted by the emitter32and transmitted directly to the detector33. Such direct communication between an emitter and detector is generally referred to as “cross talk”. The signal received by the detector may also include a further component which represents light that has been emitted by the emitter32and reflected back to the detector from an object placed in front of the emitter/detector pair. This second component of the detected signal may be processed by the controller29to provide a signal representing the presence of an object in front of the printhead which in normal circumstances will be an object defining a surface on which information is to be printed. By appropriate setting of the emitter/detector circuitry the arrangement may be set up to limit the range of distances away from the sensors that a “product” can be sensed. In some printing processes it is advantageous to ignore products that are beyond a certain maximum acceptable distance away from the printhead.

Thus the emitter/detector pair as shown inFIGS. 7 to 10can be used to detect the presence of a box or the like on which a pattern is to be printed. In addition however the emitter/detector pair can be used to detect overflow of collected ink. To achieve this, an overflow outlet34is detailed in the front wall portion30immediately above the detector33. In normal circumstances, ink levels within the collector13will be such that ink cannot flow through the overflow outlet34. Such circumstances are represented inFIG. 9. If however as a result of failure collected ink is not discharged from the collector13via the conduit25, the ink level will rise such that ink will flow through the outlet34onto the detector33. Overflowing ink runs down, under gravity, the front face30of the collector13and impinges on the top of the detector33. The ink then flows around the circumference of the detector, thus blocking some of the sidewall of the detector. This results in a change in the “cross talk” component of the signal output by the detector and by appropriate processing of this signal the controller29can detect the overflow of ink. As a result the controller29can enunciate or otherwise communicate a warning of fault condition.

In the embodiment of the invention illustrated inFIG. 3, the reservoir14from which ink is delivered to the printhead1is shown as a single vessel. Such an arrangement is acceptable if the vertical extent of the array of orifices3in the orifice plate2is limited to for example 10 mm. If a greater vertical extent of the array of orifices is required, and arrays having a vertical height of 70 mm are well known, it is desirable to divide the orifice plate into vertically spaced sections with each section being supplied from a separate reservoir section, the reservoir section being positioned at different heights such that the relative virtual positions of each reservoir section/orifice plate section pair are substantially the same. Thus the orifice plate receives ink from multiple ink supply conduits, each conduit sucking one reservoir section to a respective one orifice plate section. This avoids hydrostatic pressure presenting too great a pressure difference as between orifices at the top of the array and orifices at the bottom of the array. Such hydrostatic pressures can result either in the uppermost orifices not being correctly primed or ink being discharged unintentionally from orifices adjacent the bottom of the array.FIG. 11illustrates a reservoir14divided into four vertically spaced sections with each section feeding a respective group of orifices.

Referring toFIG. 11, the schematically illustrated reservoir14comprises an uppermost reservoir section35, a lowermost reservoir section36, an upper intermediate reservoir section37and a lower intermediate reservoir section38. Ink can be delivered to the uppermost section35from the first reservoir inlet15(seeFIG. 3). The uppermost reservoir section35has an overflow such that if that section is overfilled ink will overflow into the upper intermediate section37. Similarly section37overflows into section38and section38overflows into section36. In normal circumstances, the lowermost section36will never overflow. Each of the sections is connected to a respective outlet39which in turn is connected to a respective group of the jets of the associated printhead. The vertical disposition of each reservoir section relative to the respective group of orifices is substantially the same so that the same pressure differentials will apply in the case of each of the four groups of orifices making up the single array of orifices in the printhead. Normally the interior of a compartment in which each of the reservoir sections36to38is housed will be held at normal atmospheric pressure. During a purge procedure however that pressure will be increased by approximately 1 bar as a result of compressed air being pumped into the reservoir via inlet20. A baffle plate40is arranged over the inlet20so as to distribute incoming compressed air evenly across all of the reservoir sections.

Each reservoir section is provided with a sensor arrangement schematically represented inFIG. 11by circles41. Each sensor provides an output to the controller29(FIG. 3) representative of the level of ink within the respective reservoir section. If any one sensor indicates that the ink level within the respective reservoir section has fallen below a predetermined lower limit, ink is pumped into the reservoir so as to be delivered initially into the uppermost reservoir section41. If it is that section which has been indicated as empty, ink is supplied until the level sensor of that section indicates that the level has risen to a predetermined upper limit. In such circumstances ink does not overflow from the upper reservoir section35. If however a level sensor associated with one of the other three reservoir sections indicates that the respective section needs to be refilled, ink is still delivered to the uppermost section35but cascades down the series of reservoir sections until it reaches the reservoir which requires refilling. As soon as that reservoir has been refilled to a predetermined level the delivery of ink to the uppermost section is terminated.

The volumes of ink which are discharged from and delivered to the various reservoir sections are relatively small. As a result, if for example the lowermost section36required refilling and ink was pumped continuously into the uppermost section until the lowermost section36was full, it could be that so much ink would have been delivered to the uppermost section by the time that the lowermost section36was full that the lowermost section could overflow once all the ink already delivered had overflowed down to the lowermost section. To avoid this happening, ink can be delivered to the uppermost section35in a controlled manner. For example, whenever a demand for ink is signaled by one of the sensors41, a controlled volume of ink could be delivered, the volume being limited to ensure that overflow cannot occur. If after a predetermined delay a demand for ink is still indicated the same volume could again be delivered, the cycle being repeated until such time as the signal indicating a demand for ink has disappeared. For example ink could be pumped into the uppermost reservoir section for a set period and then the delivery of ink could be arrested for a second set period. Such a procedure avoids the risk of overflow.

FIG. 12is a schematic illustration of an ink level sensor which could be used to sense the level of ink in each of the reservoir sections35to38. A Hall effect magnetic sensor42is mounted on the outside surface of a wall43of the ink reservoir. Mounted within the reservoir is a float44, the float44being supported on a lower arm45that is pivotally supported on an upper arm46secured to the inner wall of the reservoir. The pivotal lower arm45supports a magnet47. The ink level when the reservoir is substantially empty is indicated by line48. If the ink level rises the float moves up with the ink, causing the magnet47to swing away from the wall43and hence to move away from the Hall effect detector42. The Hall effect detector42can be connected to a sensing circuit which signals that the reservoir is substantially empty as soon as the magnet47moves into close proximity to the wall43. Thus the output of the Hall effect sensor42can be used to control the supply of ink to the reservoir.

Referring toFIG. 13, an alternative ink level sensor to that illustrated inFIG. 12will be described. In the arrangement ofFIG. 13, a float49is mounted on an arm50which is mounted to pivot about a pivot axis51, the pivot axis being supported on a member52which forms part of the lid of the reservoir. A Hall effect sensor53is mounted on the lid member52. The arm50supports a bipolar magnet54arranged such that rotation of the arm50about the pivot51substantially alters the magnetic field to which the Hall effect sensor53is exposed. Thus an output from the Hall effect sensor53can be used to control the delivery of ink to the reservoir in which the float49is positioned, the orientation of the float49inFIG. 13corresponding to a reservoir empty condition.

Referring toFIGS. 14,15,16and17, constructional details of an embodiment of the invention operating as described with reference toFIGS. 1 to 3are shown. A single piece cast and machined body55defines a deflector plate56corresponding to the deflector plate6ofFIGS. 1 to 3and an air inlet57which in use is connected to an air supply conduit corresponding to the air supply conduit7ofFIGS. 1 to 3. A printhead body58is mounted on the body55, the printhead body defining an orifice plate59corresponding to the orifice plate2ofFIGS. 1 to 3. A linear array of orifices extends down the centre of the orifice plate59at the position indicated by numeral60. A plate61is secured by screws62to the body55, the plate61defining with the body a channel63which communicates with the air inlet57and from which a curtain of air is directed across the orifice plate59when the air inlet57is connected to a supply of compressed air. It will be noted that, as in the embodiment ofFIG. 2, the surface facing substrates to be printed upon is cut back on the downstream side of the deflector plate. Whereas inFIG. 2, the cutback is from a sharp edge on the downstream side of the plate6, inFIG. 14the cutback is from a short surface extending parallel to the orifice plate59from the downstream edge of the deflector plate56.

In use, when ink is purged from the orifices60and a curtain of air is directed across the orifice plate59, that air pushes the purged ink to the downstream edge64of the orifice plate59and the ink flows through a gap defined between that downstream edge64and an upstream edge65of the deflection plate56so as to enter a collection channel66defined behind the deflector plate56. Ink then flows down the channel66for collection and re-circulation. A substantial volume of ink can be retained within the channel66so that, even if a relatively large volume of ink is purged onto the orifice plate59, all of that volume can be deflected into and retained within the channel66pending the downward flow of the retained ink into the ink collector at the foot of the deflector plate56. In the illustrated embodiment, the gap between the edges64and65is 1 mm, and the channel66into which that gap opens has a rectangular cross-section with a length of 4 mm and a width of 1 mm.

Ink flowing down the channel66flows onto a projection67arranged over a cavity formed in the base of the assembly which forms an ink collection vessel. A grub screw is positioned within that vessel which can be manually adjusted so as to just touch the projection67onto which ink flows, thereby facilitating the flow of ink into the collector and minimising the risk of a large drop of ink forming at the base of the channel66and thereby minimising the risk of the channel66becoming filled with ink so that some ink could emerge in the forward direction from the channel66.

Referring now toFIGS. 18 to 21, an embodiment of a four section reservoir functionally equivalent to that described with reference toFIG. 11is illustrated. The assembly comprises a machined casting68divided by three partitions69,70and71into an uppermost section72, a lowermost section73, an upper intermediate section74and a lower intermediate section75. A slot76is formed in each of the partitions69,70and71, each slot defining a lower edge77over which ink can overflow from one reservoir to the immediately adjacent lower reservoir. A respective ink outflow passageway78communicates with the base of each of the reservoir sections. Each reservoir section receives a level sensing assembly including a float79. The level sensing assemblies can be of the type described with reference toFIG. 12, each float controlling the position of a magnet the position of which is in turn sensed by a Hall effect sensor (not shown) mounted in a recess on an outer surface of the casing68.

The mode of operation of the arrangement illustrated inFIGS. 18 to 21is as described with reference toFIG. 11, that is ink is supplied to the uppermost section72whenever any one of the floats79falls to a level indicating that the reservoir within which that float is located is substantially empty.

The casing68shown inFIGS. 18 to 21is in use closed by a top plate (not shown) having a central air inlet aperture, a baffle plate being located immediately beneath the closure plate so as to distribute incoming air evenly over all of the four reservoir sections. This avoids the possibility of a sudden inrush of air displacing ink from a reservoir section located beneath the air inlet.