Printing apparatus

IS 2 312 151 T3

DESCRIPTION

Printing apparatus.

The present invention relates to an ink jet recording apparatus, and in particular to an apparatus for cleaning, maintaining and supplying ink to a print head of an ink jet printer.

There are certain types of printers that make use of so-called piezoelectric ink jet printing technology. A piezoelectric print head (also sometimes referred to as a PZT print head) of such a printer is characterized in that it has a plurality of miniature jet chambers or "jets" arranged in close proximity in an array. Each jet is arranged to project ink from a respective orifice of a formation of holes defined by the print head, and the jets are selectively energized by a controller to project (or not project depending on the case) "pixels" of ink. The ink is projected onto a substrate to be printed, with the relative displacement between the print head and the substrate causing the projected ink to deposit onto the substrate from a hole along an elongated path. The print head is arranged within the printer such that the jet array extends at a predetermined angle (for example 90 °) relative to the direction of relative movement between the print head and the substrate. The controller selects a first combination of jets through which the ink is projected, and then a second combination of jets, etc., causing the relative movement between the substrate and the print head that a two-dimensional image is printed on the substrate.

Piezoelectric 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 films and labels.

Today's industrial printers use a range of different inks in combination with a variety of different models of piezoelectric print heads. Some use liquid ink, others use solid ink that is heated to change to the liquid phase inside the printer, being the liquid ink, at the moment it is expelled by the jets.

Print head arrays used in industrial applications typically have a length ranging from 10mm to 70mm. In high resolution print heads, up to a total of 512 jets are arranged in a 70mm linear array on a print head. It is clear therefore that the jets must be very small. Each jet ejects a very small droplet (on the order of a picoliter) of ink each time it is energized by the controller.

Although such printers can produce images with sharp resolution, they are subject to image degradation due to one or more of the jets becoming blocked by dirt or other contaminant, or otherwise failing as a result of a jet not being in use. filled with ink to be adjacent to the hole at the time the ink is to be projected (commonly referred to as the "debuffed" state), so that ink is no longer properly ejected. This jet failure (often referred to as "jet withdrawal") results in a degraded image that has one or more non-printed lines running along the printed image in the direction of relative movement between the print head and the substrate. A contaminant that does not completely block a jet can also cause problems by causing an ink droplet to deflect as it is ejected from the jet, such deflection causing a distorted image.

In certain industrial applications, such as printing on cardboard boxes or trays, an element that strongly contributes to contamination is dust, whether it is airborne or that is deposited on the surface of the cardboard. The piezoelectric board process requires that the jet array face of the print head be in very close proximity to the target substrate. Also, the formation of static electricity in the production process can occur in the area around the printer, which can cause dust to be positively attracted to the printing side. Thus, especially in industrial applications, contamination of dispensers is a major problem.

Given the aforementioned problems, it is common practice, especially in industrial applications, to follow a procedure commonly referred to as "purging" in order to maintain or restore proper function of the print head jets. The purge procedure consists of forcing the ink through all the print head jets, with the intention of cleaning and removing any contaminants from the jets and ensuring that the jets are correctly primed, this is full of ink to be placed adjacent to the holes. After execution of the purge procedure, the jets must be in a stable operating state (usually referred to as the "prime" state) so that high quality printing can be started or restarted.

In known industrial printers, the purge procedure is typically user initiated. The purging action is typically performed by placing an ink supply on the printhead under sufficient positive pressure to force a certain amount of ink through all of the printhead jets. This can be accomplished by the user by manually applying pressure to an ink reservoir that communicates

ES 2 312 151 T3 with the ink from the print head, or by applying a pressure using a pump on the reservoir. Typically positive pressure is applied for a period of a few seconds.

In industrial applications, such as printing on boxes or print trays, the print head is often mounted adjacent to a print line so that the boxes pass in close proximity to the print head. In these applications, the print head is very often mounted such that the jet array is vertical or at a certain predetermined angle relative to the horizontal. When the ink is purged through the jets, it collects in an orifice plate of the print head in which the orifice of the jets are formed, and begins to descend through the orifice plate by action of gravity. Ink flow is relatively slow, with the ink taking a few seconds to flow down to the print head. Of course, the longer the print head is in the vertical direction, the longer it takes for the ink flow to descend.

Due to the relatively large volume of ink (compared to the volume of ink normally projected by a single jet) that is purged from the orifices, the jets cannot properly eject ink to create an image until substantially all of the ink has flowed down away. from the dispensers or has been moved in any other way away from the dispensers. In general, certain measures are taken to remove the purged ink. For example, the purged ink is soaked in a cloth and then discarded. This can easily cause ink to drip onto the floor or onto surrounding factory equipment, creating unwanted soil that is difficult to remove and clean. Discarded purged ink is a significant percentage of total ink consumption. Also, cleaning the purged ink as indicated typically takes several seconds. The total time taken to complete a single ink purge procedure is therefore substantial.

Accordingly, although the purge procedure is generally successful in recovering the jets, it can cause a variety of problems. First, the purging procedure generally requires manual intervention and takes at least several seconds. It should be appreciated that the print head cannot print images properly while it is being purged. Consequently, it is often necessary to stop an associated process (for example a production line) until the purge procedure has been completed. Second, 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 alternatives have been proposed to improve ink purging procedures. For example, a known purge procedure described in British Patent GB 2339170 provides an opening in a plate defining the holes through which ink is projected, the opening being located, in use, vertically below the jets. Either in response to the detection of jet withdrawal requiring corrective treatment, or at intervals for maintenance reasons before the jets stop, an ink supply tank is pressurized with a pump. or manually to make the ink flow out of the holes in the print head. After purging, the ink flows under the orifice plate by gravity through the opening. The ink is sucked through the opening and channeled into a reservoir for filtering and subsequent reuse. This reduces the waste of ink, but since the ink flows solely by gravity it should be recognized that it takes at least several seconds for the ink to flow away through the jets to the opening. Even on production lines operating at modest speed, the time it takes for the purged ink to be removed from the orifice plate is too long for the purge to be performed between two successive product printing cycles. It will therefore be recognized that this known purging procedure is too time consuming to avoid the necessity of having to stop the production line while 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 that is located adjacent to ink jets that they will only be purged during the purge procedure. During the purge, a sufficiently high pressure is applied to the ink so that it flows into the collection system rather than dripping down the jets. Thus, the purged ink is withdrawn from the position adjacent to the jets without requiring manual intervention, but only at the expense of providing complex mechanical assembly to achieve the necessary relative displacement between the jets and the ink collection system. The system described has the ability to control the shape of a pressure pulse that is used to purge ink from the jets, the pulse having a "smooth" profile (a slow rising leading edge and a fast rising trailing edge) or a “hard” profile (one quick-lift front edge and one quick-lift rear edge). In the examples described, the slowly rising leading edge portion even lasts for 100 milliseconds or 200 milliseconds. It is stated that it is desirable to abruptly terminate the air supply causing the ink to flow out of the jets (the rapidly falling trailing edge of the pulses). This is supposed to cause the ink stream from the jets to stop abruptly. It is stated that the arrangement described eliminates ink dripping from the jets.

European patent specification EP 1016530 describes another arrangement for cleaning and preventing clogging of the orifice plate of an ink jet printer. The described device incorporates a cleaning assembly that can be moved relative to the orifice plate to a position in which a closed chamber is formed on the orifice plate. A fluid is then pumped through the closed chamber to clean up any contaminants by removing it from the orifice plate or any individual orifice in the plates.

IS 2 312 151 T3

The highly complex arrangement of EP 1016530 is presented as an improvement on an earlier proposal described in US Patent 4970535. That patent discloses an arrangement in which an ink jet orifice plate cleaner is moved to connect. with the orifice plate to provide a closed air passageway through which air is directed to provide the required cleaning effect. Also here, the setup described is complex, requiring relative shifting between a "ready to print" setting and a "clean" setting. Also, it is stated in EP 1016530 that air cleaning in the manner suggested in US 4970535 does not provide acceptable results.

European Patent Specification EP 604029 describes another cleaning arrangement in which a stream of air is directed through an orifice plate of ink jets. The jets are located behind an opening in the plate, the ink projecting as it passes through the opening before being deposited on a substrate to be printed. Air flows between the aperture plate and the orifice plate and thus passes through the apertures. The air flow is maintained during printing at a sufficiently slow air flow speed so as not to carry out a significant ink projection. The purpose of this air flow is to prevent the formation of contaminants on the orifice plate, not to clean the ink from the orifice plate. Cleaning the print head requires a separate operation and involves the release of a latch mechanism to enable the orifice plate to swing from a "ready to print" position to a position in which it can be easily cleaned. Placing the orifice plate behind another plate means that no orifice plate is open to the space through which the substrate to be printed moves relative to the orifice plate. As a result, the distance through which the ink must be projected is relatively large.

US Patent 5184147 describes an ink jet print head maintenance system incorporating various components that can be moved relative to each other including a slow moving elongated mechanical wiper and an "air knife". The air knife generates a narrow air stream that sweeps through the orifice plate. Thus, the air knife requires relative displacement between the narrow air stream generating structure and the orifice plate. Also, the air knife is exposed only as one of a plurality of complementary cleaning mechanisms within a highly complex overall assembly.

In the absence of a simple fast-running jet purge system, in many applications purging generally requires stopping an associated process, for example a production line. As a result, users prefer to start the purge procedure as little as possible. This can translate into a compromise between production chain efficiency, on the one hand, and image quality, on the other. In practice it is common for operators to wait for the print quality to deteriorate significantly before starting the purge procedure. It should also be appreciated that the longer dust and debris are allowed to form on the print head, the more likely it is that the purging and cleaning of the jets will be necessary to fully restore print quality.

It is an object of the present invention to provide an improved ink jet recording apparatus that responds to one or more of the problems outlined above.

This objective is solved by the features of claim 1.

This apparatus makes it possible to rapidly move the purged ink from the print head by using a mount that is permanently fixed in position and which is not interposed between the print head and a substrate onto which the ink is to be projected from the print head. . Thus a compact and mechanically simple printing arrangement is provided which can be quickly cleaned, allowing cleaning to be carried out after the completion of the printing operation and before the start of the subsequent printing operation even if both Print operations are separated by a relatively short period of time, for example one second or less.

Certain preferred embodiments are defined in the appended claims.

Such an arrangement makes it possible to refill a series of ink tanks each of which feeds a set of different jets using only one arrangement to supply ink to all the tanks.

Preferably, the air curtain is directed to flow in a direction perpendicular to the elongated hole formation. An ink receiving opening may extend along the side of the hole array, with the air curtain being directed through the hole array toward the aperture. The aperture may be defined between a clearance between a baffle located on the side of the aperture away from the hole formation and an edge of a surface of the print head within which the holes are formed.

Preferably, an edge of the baffle defining a first side of the clearance is set back with respect to the surface of the print head within which the holes are shaped, such that said edge of the baffle is farthest from the space within which The substrates to be printed are presented as said edges of the surface of the print head that defines a second side of the free space. The backing of the deflector edge can range from 0.1mm to 3mm, for example 1mm.

IS 2 312 151 T3

The baffle preferably defines a baffle surface extending from the baffle edge defining the first side of the clearance, the baffle surface being inclined over the air curtain to deflect the air curtain into said space within which they are presented. the substrates to be printed. The baffle surface may be inclined at an angle between 10 ° and 35 °, for example 20 °, from the direction in which the air curtain flows through the print head, the angle of inclination between the print head being measured. baffle surface and a line extending from said edge within the flow direction. The baffle surface preferably extends from said baffle edge to a downstream edge located on the side of the baffle edge remote from the hole formation, a further surface of the apparatus extending from the baffle downstream edge in a direction away from the baffle. said space within which the substrates to be printed are presented. The additional surface is preferably inclined relative to the surface of the deflector at an inclined angle of between 70 and 155 °, for example 110 °.

Preferably, an ink receiving channel is defined behind the baffle, with the ink receiving channel communicating with the interior of the free space and extending to a lower edge of the baffle. An ink collector may be located below a lower end of the ink receiving channel. The lower end of the channel may be positioned to supply ink to a formation in which ink accumulates 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 manifold and can be adjusted in position with respect to the formation so that ink from the formation can flow over the screw and through the screw into the manifold. Preferably, the manifold comprises an overflow tube, means for detecting ink flowing through the overflow tube, and means for signaling a fault if an ink overflow is detected. The detection means preferably comprises an emitter and at least one detector of which is positioned so as to be at least partially covered by the overflow ink, and means for signaling a fault if an output of the detector indicates an overflow of ink. The emitter and the detector may be arranged to project from a support surface which is located below the print head and which is open to said space within which the substrates to be printed are presented, the emitter and the detector being connected to a detection circuit sensitive both to the presence of the overflow ink and to the presence of an object located in front of the print head that reflects the emissions from the emitter to the detector. The emitter and detector can be angled relative to each other and angled upward. The ink receiving opening may have a width between 0.5mm and 2mm, for example 1mm.

The air curtain can be generated from an elongated slot extending along said one side of the hole array by pumping air through the holes. The slot may be defined between a body adjacent the print head and an edge of a plate attached to the body, an air inlet communicating with a defined space between the body and the plate. The groove may have a width of between 0.1mm and 0.3mm, for example 0.2mm. Air can be supplied to the slot at a pressure between 100,000 and 60,000 Pa above atmospheric pressure, for example 300,000 Pa.

Preferably, the pulse of pressure applied to the ink has a duration from the beginning of the leading edge to the beginning of the trailing edge of less than 1 second, for example less than 300 ms or less than 100 ms, or within the range of 10 at 50 ms.

Pressure can be applied through a valve operable between a first state in which a source of compressed air is connected to an ink supply arrangement, and a second state in which the ink supply arrangement is connected. to an exhaust duct that communicates with the atmosphere by means of an air flow throttle. Preferably the air flow throttle is manually adjustable to enable control of the slope of the trailing edge of the pressure pulse. The pressure setting can be applied after a plurality of printing operations have been carried out by projecting ink from the holes, for example after each printing operation.

The volume of ink supplied to the reservoir can be controlled in response to an ink demand initiated by the detection of an ink level in any one of the sections of the reservoir below the predetermined level, the ink supply control means being operative to supply a predetermined volume of ink in response to a demand for ink, to suspend the supply of ink for a predetermined period, and to supply more ink if an ink demand is indicated after the end of the predetermined period. The tank sections can communicate with a single department to which compressed air is supplied via a single air inlet to pressurize the ink from the tank sections, a separation screen being located over the air inlet within the compartment to distribute the incoming air evenly over all sections of the tank. The reservoir sections are preferably defined within a common partitioned body to divide the interior of the body into the reservoir sections, each septum defining an edge over which ink can spill over a reservoir section located on one side of the septum up to a section of the septum located on the other side of the septum. Preferably, each section of the tank has housed within it a float that supports a magnet, causing the displacement of the float by means of the change of the ink level, the displacement of the magnet with respect to the sensor of the magnetic field supported within the wall of the section. from the reservoir, 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 can be a Hall effect device. The float and magnet may be supported by a pivot-mounted arm located on the wall of the tank section or the float and the magnet can be supported by a pivot-mounted arm located on a tank lid.

IS 2 312 151 T3

In the following, certain embodiments of the present invention will be offered, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a schematic front view of an ink jet printer cleaning apparatus in accordance with the present invention.

Figure 2 is a schematic top view of the apparatus shown in Figure 1;

Figure 3 is a schematic representation of the components associated with the print head cleaning apparatus illustrated in Figures 1 and 2;

Figures 4 and 4a illustrate the shape of a pressure pulse applied to ink within a reservoir incorporated in the arrangement illustrated in Figure 3;

Figure 5 is a schematic front perspective view of the lower portions of the apparatus illustrated in Figures 1 and 2;

Figure 6 is a top view of the components shown in Figure 5;

Figures 7, 8, 9 and 10 are schematic views of a waste ink collector and detector assembly incorporated in the apparatus schematically illustrated in Figures 1 and 2;

Figure 11 is a schematic illustration of an alternative ink reservoir shown in Figure 3;

Figures 12 and 13 are schematic views of two alternative ink level sensing arrangements that may be incorporated into the reservoir arrangement shown in Figure 11;

Figures 14, 15, 16 and 17 illustrate in greater detail an embodiment of the invention presenting the general characteristics of the arrangement illustrated in Figures 1 and 2;

Figure 14 is a horizontal section through the print head assembly illustrated in front view in Figure 15 and a perspective view of Figures 16 and 17 and showing a partial section on line 17-17 of Figure 14; and Figures 18, 19, 20 and 21 illustrate a practical embodiment of a tank arrangement of the general type illustrated in Figure 11, with Figures 19 and 20 being seen on lines 19-19 and 20-20 of the Figure 18 and Figure 21 being a section on lines 21-21 of Figure 20.

With reference to Figures 1 to 3, there is schematically illustrated a printing apparatus according to the invention which comprises a printing head 1 having a front surface 2 within which a linear array of holes is formed 3, a supply device 4 for generating an air stream (represented by arrows 5) sweeping through surface 2, and a deflector plate 6 located downstream of surface 2. In use, surface 2 faces a path along which substrates to be printed are transported (not shown). The ink is projected from the holes 3 onto said substrates. The print head 1 can be of any conventional type. The front surface 2 of said print head which defines the holes 6 is generically designated as "hole plate", and that term will be used hereinafter to refer to the surface 2 in which the holes are constituted. 3. In the described embodiment, the orifice plate 2 is flat and extends vertically, although it should be appreciated that in other embodiments of the invention the plate hole 2 need not be flat and may be mounted to be inclined relative to the vertical, at any angle between 0 ° and 90 °.

The air supply device 4 is supplied with compressed air via a conduit 7, the air supply being controlled by a solenoid-operated valve 8. When the valve 8 is open, the air curtain 5 is set to sweep any ink or contaminants existing on the orifice plate 2 into an orifice 9 defined between a downstream edge 10 of the orifice plate and an upstream edge 11 of baffle plate 6. The ink swept from the orifice plate 2 is directed by the air flow into the free space 9 and then flows downwards by gravity to drip from a dropping point 12 located on the lower edge of the baffle plate 6. Said ink is collected in a collector 13.

As shown in Figure 2, the air supply device 4 has a rearwardly extending portion (with respect to the orifice plate 2) that is coupled to the air supply conductor 7 and an open terminal portion arranged to direct the air curtain 5 within a plane indicated by the line A parallel to the orifice plate 2. The open end of the air supply device 4 defines an elongated outlet with a substantially rectangular cross section. The outlet can be for example 0.2 mm wide. Such an arrangement efficiently channels a well-defined air curtain through the orifice plate 2 towards the downstream edge 10 and into the free space 9.

IS 2 312 151 T3

The upstream edge 11 of the baffle plate 6 is set back from the surface 2 of the orifice plate, the edge 11 lying on a plane indicated by line B in Figure 2, the plane indicated by line B being parallel to the plane. indicated by line A. The separation between plane B and the plane including surface 2 of the orifice plate is less than 1 mm. The width of the clearance 9 (which is the gap between the edge 11 of the baffle plate 6 and a line through the edge 10 of the orifice plate 2 extending perpendicular to the line B) ranges from 0.5 mm and 1.5 mm, for example it is 1 mm.

The baffle plate 6 extends in a parallel direction in a plane indicated by the line C, the plane C being inclined with respect to the plane of the orifice plate 2 at an angle α which in the illustrated embodiment is 20 ° but in general it will oscillate between 10 ° and 35 °. The baffle plate 6 has a width (a dimension in the direction away from the print head parallel to the line C) of about 5 mm. The downstream edge of plate 6 is truncated to define the angle β shown in Figure 2. The angle (which is equal to the included angle defined between the surfaces extending from the downstream edge of plate 6) will typically be order of 110 ° but in general it will oscillate between 70 ° and 155 °.

The downstream edge of the plate 6 is truncated in order to improve the air flow over the orifice plate 2. When printing for example a cardboard box that defines a flat surface close to the mounting of the print head, the presence of the box increases the presence to the air flow. If the downstream edge of plate 6 were not truncated so that an extended surface was defined extending parallel to plane A (corresponding to the sum of angles α and β equal to 180 °), a small elongated free space would be defined between the print head assembly and the case downstream of the plate 6. Such clearance would produce a resistance to the air flow that could break the flow of the air curtain through the orifice plate 2.

Figure 3 illustrates the interconnection of the print head 1, the air supply device 4 and the waste ink collector 13 with associated components. Ink is supplied to the print head 1 from a reservoir 14. Reservoir 14 comprises a first inlet 15 connected via one-way control valves 16 and 17 (to ensure unidirectional flow of ink) with a conduit 18 leading from a main ink supply storage container (not shown) to a conduit 19 that is coupled by an ink recycling mechanism to the manifold 13. The reservoir 14 also has a second inlet 20 connected by means of a valve 21 to a compressed air supply conduit 22 and to an air exhaust conduit 23 ending in a limiter valve 24 that can be manually adjusted to modify the speed. to which air can be expelled through duct 23. The valve is controllable to adopt either an "open" state in which conduits 20 and 22 are in communication, or a "closed" state in which conduits 20 and 23 are in communication.

The conduit 19 is coupled to the ink collector 13 by means of a conduit 25 that communicates with a pump 26, a conduit 27 into which the pump 26 discharges the ink from the collector 13, and a filter unit 28 that ensures that only the ink that is clean enough to be reused is discharged into tank 14.

The air pressure supplied to the supply device 24 will be of the order of 300,000 Pa. The valve 8 will either be closed or fully open to provide the supply pressure to the interior of the air supply device 4. The compressed air supplied to valve 8 will be suitably regulated to maintain the desired pressure and the supplied air will be suitably clean and filtered. The same air supply is used to provide compressed air to conduit 22 connected to valve 21, but the pressure applied to the surface of the ink in reservoir 14 can be limited as described with reference to Figures 4 and 4. 5. In the sense that in the described embodiment the pressure is limited to 100.00 Pa above atmospheric pressure, but will be limited to a range of between 20,000 to 80,000 Pa or 40,000 to 60,000 Pa, for example.

Valves 8 and 21 and pump 26 are controlled by a controller 29. When a substrate is to be printed, valve 8 is closed and valve 21 is closed. Thus there is no air flowing through the orifice plate 2 and consequently there is no risk that the projected ink jets will be deflected from their intended path. At a time when it is not printed, a purge process is executed in which initially valve 8 is opened to establish air flow through orifice plate 2, and then valve 21 is opened to apply a pressure. Positive pressure to the ink inside the reservoir 14 causing the ink to be discharged out through the holes 3 on the surface of the orifice plate 2. That ink is then displaced by the air flowing through the orifice plate and collects in the free space 9 located on the downstream side on the orifice plate. The air flow through the orifice plate 2 is then cut off by closing the valve 8. The printer is then ready for the next printing cycle. It may take time for the ink to travel into the collector 13 but since it is trapped in the free space 9 on the downstream side of the orifice plate 2 this does not impede the normal operation of the printer. Pump 26 is used to periodically transfer ink from manifold 13 to reservoir 14. For example, pump 26 may be activated for a predetermined period of time after each purge procedure.

The time that elapses between the start of the purge procedure and the discharge of substantially all the purged ink over the free space 9 is the sum of the direction of the period within which the ink is purged from the existing holes in the orifice plate 2 and the period of time taken for the purged ink to be swept into the free space 9. Minimizing the period during which a positive pressure is applied to the ink in the reservoir is therefore desirable and, as illustrated in Figures 4 and

IS 2 312 151 T3

4a in the embodiment described in the invention a short duration pulse of positive pressure is applied to the ink in reservoir 14. Figure 4 represents an idealized implementation that could be achieved using extremely fast activation components, while Figure 4A represents performance achieved with readily available standard components.

The upper half of Figure 4 shows a diagram of voltage versus time representing a control voltage applied to valve 21, a voltage 0 corresponding to valve 21 closing and 24 volts corresponding to valve 21 fully opening A control tension pulse of duration T having a steep rising leading edge and a steeply dropping trailing edge is applied to valve 21. The valve 21 is however arranged in the "closed" position to connect the exhaust conduit 23 to the inlet 20 of the reservoir. As a result, when the valve 21 is closed, the air can exit the reservoir 14 at a speed determined by the regulation of the limiter valve 24.

The lower half of Figure 4 shows the variation of the positive pressure applied to reservoir 14 over time. It will be appreciated that the pressure rises rapidly as soon as the valve 21 opens but falls relatively slowly when the valve 21 closes, the rate of fall being determined by the regulation of the limiter valve 24. Thus, the pressure pulse applied to the ink within reservoir 14 has a relatively rapidly rising leading edge and a relatively slow falling or trailing edge. It is desirable for the leading edge to rise relatively quickly inasmuch as it serves to minimize the duration of the purge cycle. A slow-falling trailing edge is desirable since, if the trailing edge is steep, the abrupt withdrawal resulting from the force that causes the ink to be flushed out of the orifices may cause individual jets to be depressed or may cause air is sucked into the print head which could disturb the print head operation. An assortment will be debunked if it is not completely filled with ink with the ink forming a meniscus at the spray hole. In general, the period T will be more than 1 second and shorter periods of time can be used successfully. Preferably T will be less than 100 ms and satisfactory results have been obtained with pressure pulses generated using a voltage pulse of duration T between 10 ms and 30 ms.

The result depicted in Figure 4 is idealized in the sense that the pressure begins to respond substantially instantaneously to the control pulse. In practice, such a result cannot be achieved by using readily available components at appropriate costs. Figure 4A illustrates the performance achieved in a practical embodiment of the invention in which valve 21 was obtained from MAC Valve Europe, part number 34AA BA GD FA-1BA with a specific activity time of 3.4 ms ( time taken to respond to a valve open control input) and a 15 ms off time (time taken to respond to a valve close control input). Limiter 24 was obtained from SMC UK part number AS1001F-04 and had a simple manually adjustable screw arrangement. The upper part of Figure 4A represents a current drawn by the valve, and the lower part represents the pressure at the inlet 20.

It should be noted that there is a delay of several milliseconds after valve 21 begins to draw current (time ti) before valve 21 begins to open, but then the valve opens rapidly and the pressure within the inlet 20 of the reservoir rises rapidly. The pressure rise rate is cut off when the pressure rises to the supply pressure in line 22. Given this cut in the speed of the pressure rise, and the short duration of the pressure pulse, the maximum pressure applied to the ink tank can be substantially below the supply pressure, for example only 50,000 Pa with a pressure supply of 100.00 Pa. Similarly, there is a delay after the current over valve 21 begins to drop (time t₂) before valve 21 begins to close. Once valve 21 begins to close, there is an initial rapid drop in pressure within inlet 20, but then there is a relatively slow drop in pressure within inlet 20 as air flows out through the inlet 20. limiter 24. The initial rapid drop in pressure reduces the period during which the ink is being purged, while the subsequent slow drop in pressure avoids nozzle debulking problems.

In the case illustrated in Figure 4A, the control output on valve 21 has a duration (t_₂-t_₁) of 20 ms. The resulting pressure pulse lasts for approximately 30 ms until the initial rapid pressure drop, with the pressure dropping further over a period of several tens of milliseconds. A pressure pulse duration of 30 ms has produced good results, but acceptable results have been obtained with the particular print head used using pressure pulse durations of between 10 and 50 ms and longer pressure pulses will be appropriate with different pressure heads and associated equipment.

Although a pressure pulse lasting 30 ms is preferred, even with a short duration pulse the ink may continue to be purged from the holes in the print head for a substantial period depending on the hydrodynamic characteristics of the overall mount. For example, the ink may continue to purge for more than 100 ms after the end of the pressure pulse. The air curtain that cleans the orifice plate must be held for a sufficient time to ensure that all purged ink has been displaced out of 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 will be reduced. In a very dirty environment, a decision could be made to maintain the air curtain except during printing, although there will be a trade-off between cleaning efficiency and the cost of the compressed air supplying the air curtain.

IS 2 312 151 T3

Figures 5 and 6 show in greater detail a possible arrangement of the collector 13 that collects the ink that drips from the dripping point 12 located at the bottom of the deflector plate 6. Figure 5 is a schematic perspective view from the front of manifold 13, while Figure 6 is a top view showing that manifold 13 has a first front wall portion 30 extending parallel to surface 2 of the orifice plate and a second front wall portion 31 extending parallel to the outer face of the baffle plate

6. The manifold 13 extends beyond the edges of the print head and the baffle plate so that the horizontal spacing between the upper edge of the front wall portion 30 and the orifice plate 2 ranges from 0.5 to 2. , 5mm and preferably is about 1mm. The same spacing is maintained between the upper edge of the second portion 31 of the front wall and the plane of the front surface of the baffle plate 6. The upper edge of the front wall portion 31 is contoured to follow the lower edge of the dropping point 12 as shown in Figure 12. Thus, the upper edge of the first and second front wall portions 30 and 31 is in close proximity to the dropping point 12 and the lower edge of the orifice plate 2. This facilitates the rapid transfer of ink by capillary action into the collector 13.

The collector 13 is shaped to cause the collected ink to flow backward away from the front wall portions 30 and 31 to empty into a cup-shaped sump from where it is sucked and withdrawn by the pump 26 (Figure 3). Ink can be recycled by pumping through filter unit 28 which feeds it directly into the reservoir that is closely coupled to the print head as shown in Figure 3, or, as an alternative, to a main supply container. which may be located at a relatively remote location and from which ink is discharged to reservoir 14 to keep the ink level in reservoir 14 within acceptable limits. The manifold 13 can be modified to support the components that make it possible to detect a problem resulting from the overflow of the manifold 13 and the detection of products moving in close proximity to the orifice plate 2. As shown in Figures 7 to 10 , an emitter 32 and a detector 33 are mounted so that they protrude from the first portion 30 of the front wall of the manifold 13, which is the portion immediately below the orifice plate 2. The emitter and detector are mounted so that they protrude a small distance, for example 2 mm, from wall 30. Emitter 32 and detector 33 are mounted at an angle γ (typically approximately 70 °) to the plane. of the front portion 30 of the manifold 13 so that they are slightly angled relative to each other. The emitter and receiver are also mounted at an angle δ of approximately 5 ° to the horizontal. It is preferable to mount the detector and emitter from above in this way to reduce the risk of unwanted signals being detected as a result, for example, of reflection from a conveyor or similar element on which the substrates to be printed (eg boxes) are transported beyond the orifice plate.

The emitter 32 and the detector 33 are connected to the controller 29 of Figure 3. The signals received by the detector 33 include a component that represents an amount of light emitted by the emitter 32 and transmitted directly to the detector 33. Said direct communication between a sender and a detector is generically referred to as "cross talk." The signal received by the detector may also include an additional component that represents the light that has been emitted by the emitter 32 and reflected back to the detector from an object in front of the emitter / detector pair. This second component of the detected signal can be processed by the controller 29 to provide a signal that represents the presence of an object in front of the print head, which under normal circumstances will be an object that defines a surface on which information will be displayed. be printed. By appropriate regulation of the emitter / detector circuitry, the arrangement can be mounted to limit the range of separation distances from the sensors at which the "product" can be detected. In some printing processes it is advantageous to ignore products that are beyond a certain maximum acceptable separation distance from the print head.

Thus the emitter / detector pair as shown in Figures 7 to 10 can be used to show the presence of a box or similar object on which a motif is to be printed. However, however, the emitter / detector pair can also be used to detect the overflow of the collected ink. To accomplish this, an overflow outlet 34 is defined in the front wall portion 30 immediately above the detector 33. Under normal circumstances, the ink levels in the manifold 13 will be such that ink cannot flow through the overflow outlet 34. Such circumstances are depicted in Figure 9. If, however, as a result of a failure the collected ink is not discharged by the manifold 13 through the conduit 25, the ink level will rise such that the ink will flow through. from output 34 onto detector 33. The overflow ink runs down, by the force of gravity, on the front face 30 of the collector 13 and impinges on the upper part of the detector 33. The ink then flows around the circumference of the detector, thus blocking part of the side wall of the detector. This produces a change in the "cross talk" component of the signal output by the detector and by proper processing of this signal the controller 29 can detect ink overflow. As a result the controller 29 may state or otherwise communicate a warning of the fault condition.

In the embodiment of the invention illustrated in Figure 3, the reservoir 14 from which ink is discharged onto the print head 1 is shown as a single reservoir. Such an arrangement is acceptable if the vertical extent of the hole formation 3 of the orifice plate 2 is limited to, for example, 10 mm. If a greater vertical extent of the hole formation is required, it is desirable to divide the orifice plate into vertically spaced sections with each section being supplied from a separate tank section, the tank section being positioned at different heights so that the Relative virtual positions of each tank section / orifice plate section pair are substantially the same. Thus the orifice plate receives ink from multiple ink supply conduits, each conduit sucking a section of the reservoir over a section

ES 2 312 151 T3 of respective orifice plate. This prevents the hydrostatic pressure from having too great a pressure difference between the holes at the top of the formation and the holes at the bottom of the formation. Such hydrostatic pressures can determine that either the uppermost holes are not properly primed or that ink is unintentionally discharged from the holes adjacent to the bottom of the formation. Figure 11 illustrates a reservoir 14 divided into four vertically spaced sections with each section feeding a respective group of orifices.

Referring to Figure 11, the schematically illustrated reservoir 14 comprises an uppermost section 35 of the reservoir, a lowermost section 36 of the reservoir, an upper intermediate section 37 of the reservoir, and a lower intermediate section 38 of the reservoir. Ink can be supplied to the uppermost section 35 from the first inlet 15 of the reservoir (see Figure 3). The uppermost section 35 of the reservoir has an overflow so that if that section becomes overfilled the ink will overflow into the upper intermediate section 37. Similarly, section 37 overflows into section 38 and section 38 overflows and penetrates into section 36. Under normal circumstances, the section below 36 will never overflow. Each of the sections is connected to a respective outlet 39 which in turn is connected to a respective group of associated print head jets. The vertical arrangement of each tank section with respect to the respective group of orifices is substantially the same so that the same pressure differentials will apply in the event that each of the four groups of orifices constitutes the single orifice array of the tank. print head. Typically, the interior of a compartment within which each of the tank sections 36 to 38 is housed will be maintained at normal atmospheric pressure. During a purge procedure, however, that pressure will increase by approximately 100.00 Pa as a result of compressed air being pumped into the reservoir through inlet 20. A screen plate 40 is disposed over the inlet 20 to evenly distribute the incoming compressed air throughout all sections of the reservoir.

Each tank section is provided with a sensor arrangement schematically represented in Figure 11 by circles 41. Each sensor provides an output to controller 29 (Figure 3) representative of the ink level within the respective tank section. If any one of the sensors indicates that the ink level within the respective tank section has dropped below a certain lower limit, the ink is pumped into the tank to be initially discharged into the upper section 41 of the tank. . If it is that section that has been indicated as empty, ink is supplied until the level sensor for that section indicates that the level has risen to a predetermined upper limit. In such circumstances the ink does not overflow from the uppermost section 35 of the reservoir. If, however, a level sensor associated with one of the other three tank sections indicates that the respective section needs to be refilled, ink continues to be supplied from the uppermost section 35 but cascades down the series of tank sections. deposit until it reaches the deposit that needs to be refilled. As soon as that reservoir has been refilled to a predetermined level, the supply of ink to the uppermost section stops.

The volumes of ink discharged from and supplied to the various sections of the reservoir are relatively small. As a result, if for example the lowermost section 36 required to be refilled and ink was continuously pumped into the uppermost section until the lowermost section 36 was full, it could result that as much ink was discharged onto the lowermost section. raised at the time the lower section 36 was full, so that the lower section could overflow once all the ink already discharged has overflowed down to the uppermost section. To prevent this from happening, ink can be supplied to the uppermost section 35 in a controlled manner. For example, whenever a signal is produced for an ink demand by one of the sensors 41, a controlled volume of ink could be supplied, the volume being limited to ensure that no overflow occurs. If after a predetermined delay there is still a demand for ink, the same volume could be supplied again, repeating the cycle until the moment when the signal indicating a demand for ink has disappeared. For example the ink could be pumped into the uppermost section of the reservoir for a fixed period and then the supply of ink could be stopped for a second fixed period. This procedure avoids the risk of overflow.

Figure 12 is a schematic illustration of an ink level sensor that could be used to detect the level of ink in each of the tank sections 35 through 38. A Hall effect magnetic sensor 42 is mounted on the outer surface of the wall 43 of the ink tank. Mounted within the tank is a float 44 with the float 44 supported on a lower arm 45 which is pivotally supported on an upper arm 46 fixed to the interior wall of the tank. Lower pivot arm 45 supports magnet 47. The ink level when the reservoir is substantially empty is indicated by line 48. If the ink level rises the float moves upward with the ink, causing the magnet 47 to oscillate away from the wall 43 and thus move away from the Hall effect detector 42. The Hall effect detector 42 may be connected to a sensing circuit that signals that the reservoir is substantially empty as soon as the magnet 47 moves in close proximity to the wall 43. Thus the output of Hall effect sensor 42 can be used to control the supply of ink to the reservoir.

With reference to Figure 13, an alternative ink level sensor illustrated in Figure 12 will be described. In the arrangement of Figure 13, a float 49 is mounted on an arm 50 which is mounted to pivot about an axis. of pivot 51, the pivot axis being supported on a member 52 that forms part of the reservoir cap. A Hall effect sensor 53 is mounted on cap member 52. Arm 50 supports a bipolar magnet 54 arranged such that rotation of arm 50 about pivot 51 substantially alters the magnetic field to which Hall effect detector 53 is exposed. Thus, an output of the detector can be used.

Hall effect 53 for controlling the supply of ink to the reservoir in which float 49 is located, the orientation of float 49 of Figure 13 corresponding to an empty state of the reservoir.

With reference to Figures 14, 15, 16 and 17, they show certain structural details of an embodiment of the invention that operates as described with reference to Figures 1 to 3. A one-piece machined and molded body 55 defines a baffle plate 56 corresponding to baffle plate 6 of Figures 1 to 3 and an air inlet 57 which in use is connected to an air supply conduit corresponding to the air duct. air supply 7 of Figures 1 to 3. A print head body 58 is mounted on the body 55, the print head body defining an orifice plate 59 corresponding to the orifice plate 2 of Figures 1 to 3. A linear array of orifices extends downwardly through the center of orifice plate 59 at the position indicated by numeral 60. A plate 61 is fixed by screws 62 to the body 55, the plate 61 with the body defining a channel 63 that communicates with the air inlet 57 and from which an air curtain is directed through the orifice plate 59 when the air inlet 57 is connected to a compressed air supply. It should be noted that, as in the embodiment of Figure 2, the surface facing the substrates to be printed is truncated on the downstream side of the baffle plate. While in Figure 2, the truncation is located from a sharp edge located on the downstream side of plate 6, in Figure 14, the truncation occurs from a surface that extends parallel to the plate of orifices 59 from the downstream edge of the baffle plate 56.

In use, when ink is purged from orifices 60 and a curtain of air is directed through orifice plate 59, that air pushes the purged ink to the downstream edge 64 of orifice plate 59 and the ink flows. through a clearance defined between the downstream edge 64 and an upstream edge 65 of the deflection plate 56 to enter a collection channel 66 defined behind the deflector plate 56. The ink then flows down channel 66 for collection and recirculation. A substantial volume of ink can be retained within channel 66 so that even if a relatively large volume is purged onto orifice plate 59, all of that volume can be diverted to and retained within channel 66 while awaiting downward flow. of the ink retained within the ink collector at the foot of the baffle plate 56. In the illustrated embodiment, the clearance between edges 64 and 65 is 1mm, and the channel 66 into which the clearance opens has a rectangular cross-section with a length of 4mm and a width of 1mm. .

The ink flowing under the channel 66 flows over a projection 67 disposed on a cavity formed in the base of the mount that forms an ink collection container. A socket head screw is located inside the container and can be manually adjusted so that it just touches the projection 67 over which the ink flows, thereby facilitating ink flow within the manifold and minimizing the risk of a large ink droplet forming at the base of channel 66 and thereby minimizing the risk of channel 66 filling with ink from so that some of the ink could emerge in the forward direction from channel 66.

Referring now to Figures 18 to 21, there is illustrated an embodiment of a four-section tank functionally equivalent to that described with reference to Figure 11. The assembly comprises a machined mold 68 divided by three partitions 69, 70 and 71 into a higher section 72, a lower section 73, an upper intermediate section 74 and a lower intermediate section 75. A slot 76 is formed in each of the partitions 69, 70 and 71, each slot defining a lower edge 77 over which ink can overflow from a reservoir to the immediately adjacent lower reservoir. A respective ink flow outlet passageway 78 communicates with the base of each of the tank sections. Each tank section receives a level sensing assembly that includes a float 79. The level sensing assemblies may be of the type described with reference to Figure 12, with each float controlling the position of a magnet whose position is in turn sensed by a Hall effect detector (not shown) mounted in a recess of a surface. outside of housing 68.

The mode of operation of the arrangement illustrated in Figures 18 to 21 is as described with reference to Figure 11, that is, the ink is supplied to the uppermost section 72 whenever any one of the floats 79 falls to a level. indicative that the reservoir within which that float is located is substantially empty.

The housing 68 shown in Figures 18 to 21 is in use closed by a top plate (not shown) having a central air inlet opening, a screen plate being positioned immediately below the closure plate to distribute evenly. Incoming air evenly over the fourth tank sections. This avoids the possibility of a sudden burst of air displacing the ink from a section of the reservoir below the air inlet.