Charge electrode

Abstract

There is provided a charge electrode for charging ink droplets for continuous ink jet printing. The charge electrode has a generally cylindrical main body defining a generally cylindrical passage for the ink droplets, said passage extending along a travel axis that represents, in use, the position of an ink jet in ingress to the electrode and a direction of travel of the ink droplets once these have detached from the ink jet. The charge electrode comprises two distinct axially disposed regions: a first region for charging the droplets as required; and, a second region for screening the charged droplets from any electric fields which could undesirably affect the trajectory of the ink jet and/or the droplets. The second region may fully surround at least a portion of the travel axis. The invention therefore provides improved screening for the ink jet and/or the droplets. This may in turn enable better control of the charge applied to the charged droplets, and/or of their travel trajectory.

Claims

1. A charge electrode for continuous ink jet printers comprising: a passage, defined by the charge electrode, for forming and shielding charged ink droplets, said passage extending along a travel axis that represents, in use, a position of an ink jet that enters the charge electrode and from which ink droplets detach inside the charge electrode, and a direction of travel of the ink droplets before they exit the charge electrode, wherein the charge electrode comprises directly abutting first and second axially sequentially disposed regions, wherein the first region is configured to induce a charge on ink droplets selected for printing by capacitive coupling, and the second region is configured to shield the charged ink droplets by surrounding at least a segment of said travel axis, wherein the charge electrode comprises an axial extension measuring 5 mm or longer.

2. The charge electrode of claim 1 , wherein the charge electrode comprises a cylindrical body defining a body cylinder axis parallel with said travel axis.

3. The charge electrode of claim 2, wherein said cylindrical body comprises an outer stepped profile defining a cylinder length of increased diameter with respect to a rest of the cylindrical body, wherein said first region comprises said cylinder length and wherein said second region comprises at least a portion of the rest of the cylindrical body.

4. The charge electrode of claim 2, wherein the second region comprises an outlet aperture for the ink droplets; wherein the outlet aperture is located opposite an inlet aperture; wherein a size of the inlet aperture is less than that of the outlet aperture.

5. The charge electrode of claim 2, wherein the charae electrode further comprises a viewing aperture for monitoring formation of the ink droplets; wherein said viewing aperture is provided on a side of said cylindrical body.

6. The charge electrode of claim 5, wherein said viewing aperture comprises elongated shape that extends in the direction of travel and/or cylinder axes.

7. The charge electrode of claim 5, wherein the first and the second axially sequentially disposed regions are located one on either side of said viewing aperture.

8. The charge electrode of claim 5, wherein the first and second axially sequentially disposed regions define opposite ends of the charge electrode.

9. The charge electrode of claim 5, wherein the first and second axially sequentially disposed regions adjoin each other.

10. The charge electrode of claim 1, wherein said passage is cylindrical and defines a passage cylinder axis parallel with said travel axis.

11. The charge electrode of claim 10, wherein the charge electrode is a coaxial charge electrode and a cylinder axis of a cylindrical body of the charge electrode and/or the passage cylinder axis overlap with said travel axis, or one with the another.

12. The charge electrode of claim 1, wherein said first region comprises an inlet aperture for the ink jet.

13. The charge electrode of claim 1, wherein said passage comprises a step defining a stepped profile for the passage; wherein said step separates and/or identifies said first and second axially sequentially disposed regions; wherein the passage is narrower in correspondence of the first region than the second region.

14. The charge electrode of claim 1, wherein the charge electrode comprises a mounting and/or orientation feature for mounting and/or registering in place, in use, the charge electrode on a printhead or a printhead deck.

15. The charge electrode of claim 14, wherein the mounting and/or orientation feature is in the form of a flat provided on the charge electrode; wherein said flat is provided on said second region.

16. The charge electrode of claim 1, wherein a length of the charge electrode is greater than 6 mm; or, greater than 7 mm; or, greater than 8 mm; or, greater than 9 mm; or, greater than 10 mm; wherein said lengths relate to said second region.

17. The charge electrode of claim 1, wherein the a length of said segment surrounded by said second region is 2 mm or longer.

18. The charge electrode of claim 1, wherein the charge electrode comprises an axially elongated shape.

19. The charge electrode of claim 1, wherein the first region is structured and/or configured to be different compared to the second region.

20. The charge electrode of claim 1, wherein the first region is better structured and/or configured to induce said charge on the charged ink droplets than the second region.

21. The charge electrode of claim 20, wherein the second region is better structured and/or configured to shield said charged ink droplets than the first region.

22. The charge electrode of claim 1, wherein the first region is configured to induce said charge on said ink droplets selected for printing for the ink droplets selected for printing to be deflected by one or more deflection plates for printing onto a moving substrate at predetermined positions.

23. Apparatus for a printhead comprising a charge electrode according to claim 1.

24. A printhead comprising: a nozzle for generating a continuous ink jet and ink droplets detached therefrom; a charge electrode as claimed in claim 1 or apparatus according to claim 23; one or more deflection plates for deflecting the charged ink droplets charged by said charge electrode for printing on a moving substrate associated with the printhead; and a gutter for recirculating any uncharged ink droplets.

25. The printhead of claim 24, wherein the nozzle is configured to emit the continuous ink jet at a velocity V, the printhead is arranged to form the ink droplets at a frequency F, and the ink droplets are broken off from the continuous ink jet at a droplet pitch DP=V/F; and wherein a length of the charge electrode is greater than 15 droplet pitches.

26. The printhead of claim 25, wherein the length of the charge electrode is less than 70 droplet pitches.

27. An apparatus comprising the printhead as claimed in claim 24.

28. The apparatus of claim 27, wherein said apparatus is a continuous ink jet printer.

29. A method of printing, the method comprising: generating a continuous ink jet; forming a plurality of ink droplets from said continuous ink jet; using a charge electrode as claimed in claim 1, selectively charging the ink droplets for printing on a moving substrate; deflecting any charged ink droplets to print them on the moving substrate at predetermined positions.

Description

(1) The invention will now be described purely by way of example in connection with the appended drawings in which:

(2) FIG. 1 is a side view of a portion of a charge electrode of the prior art;

(3) FIG. 2 is a top view of the deck of a continuous ink jet printhead of the prior art, showing the charge electrode of FIG. 1;

(4) FIG. 3 is a first perspective view of a charge electrode according to an embodiment of the present invention;

(5) FIG. 4 is a second perspective view of the charge electrode of FIG. 3 viewed from an opposite end;

(6) FIG. 5 is a top view of the deck of a continuous ink jet printhead according to an embodiment of the present invention, incorporating the charge electrode of FIGS. 3 and 4;

(7) FIG. 6 (which is made up of FIGS. 6a, 6b and 6c) shows alternative embodiments of charge electrodes in accordance with the present invention; and

(8) FIG. 7 is a cross-sectional representation of the charge electrode of FIGS. 3 and 4, with most dimensions stated (in millimetres—mm).

(9) Where possible, corresponding features have been labelled below with the same reference numerals. This applies to the prior art charge electrodes and to the described embodiments.

(10) Where possible, indices ′, ″, ′″ and ″″ have been used below to identify corresponding features across different embodiments of the invention.

(11) Continuous ink jet printers supply pressurised ink to a printhead drop generator where a continuous stream of ink emanating from a nozzle 1 (shown in FIG. 2) is broken up into individual regular drops by, for example, an oscillating piezoelectric element housed in the drop generator.

(12) The drops or droplets are directed past a charge electrode 2 where they are selectively and separately given a predetermined charge before passing through a transverse electric field provided across a pair of deflection plates 3. A side view of a charge electrode according to the prior art is shown in FIG. 1, and FIG. 2 shows the arrangement of a printhead 10 also according to the prior art, incorporating the charge electrode of FIG. 1.

(13) Each charged drop is deflected by the field by an amount that is dependent on its charge magnitude before impinging on a substrate S whereas the uncharged drops proceed without deflection and are collected at a gutter 4 from where they are recirculated to the ink supply for reuse.

(14) The charged drops bypass the gutter 4 and hit the substrate S at a position determined by the charge on the drop and the position of the substrate S relative to the printhead.

(15) Typically, the substrate S is moved relative to the printhead 10 in one direction and the drops are deflected in a direction generally perpendicular thereto, although the deflection plates may be oriented at an inclination to the perpendicular to compensate for the speed of the substrate (the movement of the substrate relative to the print head between drops arriving means that a line of drops would otherwise not quite extend perpendicularly to the direction of movement of the substrate). It will be understood that in FIG. 2 (and corresponding FIG. 5) the substrate S travels in a direction perpendicular to the sheet of paper.

(16) In continuous ink jet printing, a character is printed from a matrix comprising a regular array of potential drop positions. Each matrix comprises a plurality of columns (strokes), each being defined by a line comprising a plurality of potential drop positions (e.g. seven) determined by the charge applied to the drops by the charge electrode 2.

(17) Thus each usable drop is charged according to its intended position in the stroke. If a particular drop is not to be used then the drop is not charged and it is captured at the gutter 4 for recirculation. This cycle repeats for all strokes in a matrix and then starts again for the next character matrix.

(18) Ink is delivered under pressure to the printhead 10 by an ink supply system (only part of which is shown in FIG. 2) that is generally housed within a sealed compartment of a cabinet that includes a separate compartment for control circuitry and a user interface panel. The system includes a main pump that draws the ink from a reservoir or tank via a filter and delivers it under pressure to the printhead 10. As ink is consumed the reservoir is refilled as necessary from a replaceable ink cartridge that is releasably connected to the reservoir by a supply conduit. The ink is fed from the reservoir via a flexible delivery conduit 11 to the printhead 10. The unused ink drops captured by the gutter 4 are recirculated to the reservoir via a return conduit (not shown) by a pump (also not shown). The flow of ink in each of these conduits is generally controlled by solenoid valves and/or other like components (also not shown).

(19) As it can be seen from FIGS. 1 and 2, conventional charge electrodes 2 mounted in this kind of ink jet printers are in the form of a slotted cylindrical barrel 7. The barrel 7 typically has a diameter d of a few millimetres, and 6 millimetres in the case of the electrode of FIG. 1. This is consistent with the dimensions provided on FIG. 2. The barrel 7 is mounted on a printhead deck 9 with the axis a of the barrel 7 perpendicular to the front deck surface visible in FIG. 2.

(20) The length of the charge electrode 2 in the direction of travel of the ink drops corresponds therefore to 6 millimetres. This is an upper limit in the prior art, with most conventional charge electrodes measuring 5 millimetres or under. This limit is dictated by the requirement of providing a compact printhead 10 while allowing a sufficient gap between the charge electrode 2 and the deflection plates 3.

(21) While the charge electrode 2 is supplied with a maximum voltage of around 250V to charge the drops, the deflection plates are supplied with much higher voltages known as Extra High Tension (EHT), for example between 6,000-8,000V, to deflect the charged drops.

(22) The distance between the deflection plates 3 and the charge electrode 2 in the direction of travel of the ink drops is such that, as it will be apparent, the deflection plates 3 and the charge electrode 2 do not become electrically bridged.

(23) As it can be seen in FIG. 1, a slot 6 having a width w of less than 1 millimetre is cut halfway through an upper end 8 of the cylindrical barrel 7, on a plane that contains the axis a of the cylinder 7. The slot 6 extends axially downwards for a depth de which is about the size of the diameter d of the barrel 7 (ie about 6 millimetres).

(24) The charge electrode 2 is disposed on the printhead deck 9 as shown in FIG. 2, ie such that the ink drops transit through the head 8 of the barrel 7 across the slot 6 in a direction substantially perpendicular to the axis a of the barrel 7.

(25) As described above, the arrangement is such that the ink drops are broken from a continuous ink jet ejected by the nozzle 1 within the charge electrode. Therefore, the drops can at most transit within the charge electrode for a distance equal to the diameter of the charge electrode 2, ie 6 millimetres in the present case. Such a distance will normally, however, be less than 6 millimetres since the drops will form at a location some way through the electrode It is important that the charge electrode 2 be driven by appropriate circuitry (not shown) such that the required electric field is precisely generated by the charge electrode 2 when a drop is about to break off at this internal location.

(26) The present invention arises from the appreciation by the inventors that the current designs of charge electrode 2 may not adequately protect the continuous ink jet entering the charge electrode 2 and/or the charge ink drops formed therein, or that in any event it is possible to improve the charge electrode designs of the prior art.

(27) In particular, the present invention arises from the appreciation that it may be possible to improve the shielding or screening effect of the charge electrode 2 with respect to any polluting or stray electric fields such as that originated by the deflection plates 3 at the moment of formation of any charged drops and their initial travel stages.

(28) Another source of undesired electric field contamination may be any electronic circuitry mounted on the rear side of the deck 9. Otherwise, the printhead 10 may be close to foreign electronic equipment and this equipment may constitute a source of undesired electric fields.

(29) In relation to any selectively charged ink droplets flying through the charge electrode 2 just after they have been formed within the charge electrode, the electric field present in the electrode between the walls of the slot 6 and the unbroken ink filament at the time of inducing the required charge on each charged droplet may also, at least in principle, undesirably deflect the intended path of the charged ink drops. This effect too may additionally and/or alternatively be reduced or eliminated in embodiments of the present invention, as it will be further explained below.

(30) In addition, the elongated arrangement of charge electrodes according to embodiments of the present invention may make it simpler to induce the required charge on the droplets due to the capacitive coupling between the walls of the passage and the ink filament. For example, the prior art may require a distance of about 300 microns between the charge-inducing wall of the electrode and the ink filament, depending on the size of the ink jet which is determined, as it will be clear to the skilled person, by the size of the printing orifice. In embodiments of charge electrodes according to the invention said distance may be 375 microns or greater. This will be described further below in connection with FIG. 7 which shows an inlet aperture of 750 microns, ie twice the 375 microns. This in turn may reduce the electrode alignment requirements, or its susceptibility to electrostatic steerage, as it will further be described below.

(31) Charge electrodes 2′, 2″, 2′″, 2″″ according to different embodiments of the invention are shown in FIGS. 3-7. These charge electrodes 2′, 2″, 2′″, 2″″ differ from those typical of the prior art in that they are examples of elongated coaxial charge electrodes capable of tunneling the ink filament on entry to the electrodes and the ink drops on exit therefrom, thereby screening the ink filament and/or the drops from any unwanted electric fields before the drops are formed, the moment they charge or while they are in flight through the electrodes. It will be understood that the term ‘coaxial’ as used in the present context refers to the ink jet and the ink drops being positioned at least partially on an axis of the charge electrode. This arrangement is different to that shown in FIGS. 1 and 2 since in FIGS. 1 and 2 the axis a of the charge electrode 2 does not coincide with the direction of travel of the ink jet and drops, this being instead generally perpendicular thereto and crossing the axis a at one intersection (not shown).

(32) By providing elongated coaxial, or in some embodiments generally tubular, charge electrodes 2′, 2″, 2′″, 2″″, it is now possible to dispose the charge electrodes such that the ink filament and the ink drops when formed will now travel parallel to their axes a′, a″, a′″, a″″ rather than in a direction perpendicular thereto, as was the case with the prior art.

(33) Accordingly, additional useful screening of the ink jet and/or the ink drops can now be obtained by extending the length of the charge electrodes 2′, 2″, 2′″, 2″″ in the direction that, in use, would be the direction of travel of the ink drops. Simultaneously, the coaxial capacitive coupling afforded by the coaxial configuration of these charge electrodes allows the charging walls of the slots/passages 6′, 6″, 6′″, 6″″ to be advantageously generally located at an increased distance from the ink filament. A small distance between such walls and the ink filament is usually required to induce an appropriate amount of charge on the ink filament. However, this may cause electrostatic steerage of the filament. An increased distance between the filament and said walls for a given amount of capacitive coupling may thus be beneficial, since any unwanted electrostatic steerage of the ink jet in the charge electrodes 2′, 2″, 2′″, 2″″ may then simultaneously be reduced or eliminated.

(34) It should be noted that it would be of no or very limited benefit to extend the length of the charge electrode 2 of the prior art. This would have only have allowed the charge electrode 2 to provide added surface to capture the break-offs of the ink drops, but would have required an overall longer printhead 10 without providing or improving any shielding or screening of the charged droplets. This is clearly undesirable, especially when, as discussed above, phasing can be used to ensure that the ink drops are charged as and when required. The improved screening provided by embodiments of charge electrodes according to the present invention may instead turn this potential disadvantage of the prior art into a desired feature.

(35) By providing an elongated coaxial design of charge electrodes 2′, 2″, 2′″, 2″″ or, in some embodiments, generally tubular design of charge electrodes 2′, 2″, 2′″, 2″″, the axial length of the electrodes 2′, 2″, 2′″, 2″ can usefully be increased to improve the shielding or screening effect of the tunnel crossed by the ink jet first and the traveling ink drops after these have detached from the ink jet inside the charge electrodes.

(36) Simultaneously, the distance between the charging walls of the electrode and the ink jet may usefully be increased as discussed above.

(37) A longer, generally coaxial or tubular electrode 2′, 2″, 2′″, 2″″ will provide additional screening compared to a shorter electrode of the same type. Accordingly, the inventors have realised that it is now acceptable to provide an overall longer printhead 10′ to accommodate any additional length (and the benefits associated with it) of the charge electrodes 2′, 2″, 2′″, 2″″. Such increments in the length of the charge electrode may for example be 1 millimetre, or a subunit of that, for example one tenth or one hundredth of a millimetre. This is in contrast with the prior art shown in FIGS. 1 and 2, for which an additional length of charge electrode in the direction of travel of the drops would have merely represented a way of capturing additional lengths of drop break-offs as discussed above. This advantage may be preserved by embodiments of the present charge electrodes, but it would be ancillary with respect to the provision of improved screening. For this reason, it will be apparent that the charge electrodes 2′, 2″, 2′″, 2″″ may also enable satisfactory phasing to be performed using less performant, and thus potentially less expensive, apparatus.

(38) As shown in FIG. 5, which depicts a printhead 10′ that mounts the charge electrode 2′ shown in FIGS. 3 and 4, the distance between the nozzle 3 and the gutter 4 is 52 millimetres, while the distance between the nozzle 3 and the gutter 4 in the prior art printhead 10 shown in FIG. 2 is shorter, at 47.3 millimetres. The provision of a longer printhead is justified by the improved screening at the charge electrode.

(39) With reference to FIGS. 3, 4 and 7, the first illustrated embodiment of charge electrode 2′ defines a passage 6′ for inducing a charge on the ink filament in ingress to the charge electrode 2′ and thus for charging the ink droplets. The passage 6′ extends along an axis a′ that represents, in use, a travel axis of the ink droplets, and screens the ink jet and the charged droplets. As best shown in FIG. 7, the charge electrode 2′ can be thought as it being divided into two distinct regions R1 and R2 at different axial locations along the electrode 2′.

(40) The first axial region R1 is responsible for receiving and inducing a charge on the ink filament. This may only happen until the filament is broken into the individual ink drops. However, for simplicity and clarity of representation, the length of the region R1 in FIG. 7 is depicted as extending until the distal end of a viewing aperture 25′ provided on the side of the electrode 2′. This will generally correspond to the maximum break-off distance, assuming that the overall system is designed for the break-off events to take place at locations within said viewing aperture. The first axial region R1 includes a cylindrical portion of the passage 6′ of diameter 0.75 mm (this is twice the required capacitive distance between the passage's wall and the ink filament of 375 microns) which extends for the whole length of the region R1 which is 6.55 mm, as shown in FIG. 7. In this representation, therefore, the first axial region R1 extends up to and includes the electrode region in which the drop break-offs may be expected. It would alternatively be necessary to locate the drop break-off location and draw accordingly the distal boundary of region R1.

(41) The second axial region R2 is responsible for screening the flight of any charged droplets while they transit through the electrode 2′. The minimum distance traveled by the charged droplets across the electrode 2′ is the axial length of the region R2 of 3.95 mm depicted in FIG. 7 (this has been calculated as 10.50 mm corresponding to the overall length of the charge electrode minus 6.55 mm which is the length of the first axial region R1). The drops will however typically form at some location visible from the viewing aperture 25′. The charge electrode 2′ therefore typically provides screening to the charged droplets for a distance in excess of the 3.95 mm shown in FIG. 7, for example for 5 mm, and the second axial region R2 would accordingly extend for this length. The second axial region R2 of the charge electrode 2′ thus surrounds a segment of said travel axis a′ so that, in use, the charged ink drops are tunneled and screened by the charge electrode 2′ as they travel through it. The second axial region may thus be defined as an electrode region dedicated to exclusive transit of formed drops (ie the ink filament cannot reach said second region R2 by definition).

(42) In the embodiment shown in FIGS. 3, 4 and 7, end sections 21′, 22′ of the charge electrode 2′ fully surround corresponding portions of the electrode axis a′ at 360° with respect to plans orthogonal to the electrode axis a′.

(43) In the alternative embodiments 2″, 2′″, 2″″ shown in FIG. 6 at no location along the respective axes a″, a′″ a″″ do the charge electrodes 2″, 2′″ 2″″ fully surround any portion of the axes around a 360° angle with respect to plans orthogonal to these axes.

(44) The charge electrode 2″ of FIG. 6a comprises through-wall cut-away slits c1 and c2 extending part-way through the length L″ of the electrode 2″, from either edge of the electrode inwardly and parallel to the axis a″ so as to overlap circumferentially for a length l″.

(45) The charge electrode 2′″ depicted in FIG. 6b has a through-wall cut-away spiral c3 extending axially and circumferentially from one edge to the other of the charge electrode 2″.

(46) The charge electrode 2′″ depicted in FIG. 6c has two through-wall cut-away slits c4 and c5 extending part-way through the length L″″ of the electrode 2″″ from either edge of the electrode inwardly and parallel to the axis a″ so as to meet at keyhole aperture 25″″. A viewing aperture in the form of a keyhole aperture 25′ is also present in the embodiment shown in FIGS. 3, 4 and 7 and will be described in further detail below.

(47) With continued reference to FIGS. 3-7, the charge electrodes 2′, 2″, 2′″, 2″″ consist of respective generally cylindrical bodies 7′, 7″, 7′″, 7″″ with respective straight passages 6′, 6″, 6′″, 6″″ for the ink filament and the ink drops.

(48) The electrodes 2′, 2″″ shown in FIGS. 3, 4 and 6c are in addition provided with respective viewing apertures 25′, 25″″ for confirming the break-offs of the individual ink droplets and viewing the first instants of their flight through the electrodes 2′, 2″″. In these embodiments, the viewing apertures 25′, 25″″ are in the form of substantially rectangular keyhole apertures, with rounded edges in the case of the embodiment of FIGS. 3 and 4. These apertures that allow visual inspection of the drop formation by using stroboscopic light, as is known in the art. In the embodiments of FIGS. 3 and 4, it is the end section 22′ that fully surrounds a portion of the axis a′ and comprises the viewing aperture 25′.

(49) FIG. 5 shows the charge electrode 2′ of FIGS. 3, 4 and 7 and its keyhole aperture 25′ in the context of the assembled printhead 10′. The keyhole aperture 25′ has an elongated shape in the direction of the travel of the drops and defines two ends 23′, 24′ of the charge electrode 2′ on either side of the keyhole aperture 25′. The first end 23′ defines an inlet aperture 26′ to the charge electrode 2′ for the ink filament, while the second end 24′ defines an outlet aperture 27′ for the ink drops. In FIG. 5, the inlet aperture 26′ is located on the right hand side of the keyhole aperture 25′, adjacent the nozzle 1.

(50) In the case of the charge electrode 2′ of FIGS. 3, 4 and 7, the generally cylindrical body 7′ has an outer stepped profile 28′. The outer stepped profile 28′ defines a cylinder length in the form of a cylinder head 29′ of increased diameter with respect to the rest of the cylindrical body 30′.

(51) In this described embodiment, the inlet aperture 26′ to the ink jet is provided on the cylinder head 29′. The inlet aperture 26′ is smaller than the outlet aperture 27′ and this may aid with the installation and positioning of the charge electrode 2′ on the printhead deck 9′ of the printhead 10′ shown in FIG. 5. Unless the charge electrode has been accurately positioned, the ink jet will not be able to access and cross the passage 6′ in the form of ink drops detached from the ink jet inside the electrode.

(52) The passage 6′ is also generally cylindrical and coaxial, in this embodiment, with the generally cylindrical body 7′ of the charge electrode 2′. The passage 6′ has an internally stepped profile. In this embodiment, said step (located at a distance of 6.55 mm from the inlet as shown in FIG. 7) divides and allows a viewer immediately to identify the first and second axial regions R1 and R2 shown in FIG. 7. The passage 6′ has a first cross-section cs1 in connection with the cylinder head 29′ and along the portion of the end section 22′ comprising the viewing aperture 25′. The passage has a second and distinct cross section cs2 in connection with the second region R2.

(53) In this described embodiment, the outlet aperture 27′ is circular has a diameter (1.4 mm as shown in FIG. 7) which is approximately one third the outer diameter of the generally cylindrical body 7′ at the distal end 24′ (4 mm, as shown in FIG. 7). However, other geometries are possible and may vary greatly in detail.

(54) The charge electrode 2′ has a mounting and orientation feature 35′ for mounting the charge electrode 2′ on the printhead 10′ shown in FIG. 5 such that the viewing aperture 25′ is registered in the required position. In this described embodiment, the mounting and orientation feature is in the form of a flat 35′ provided on the generally cylindrical body 7′ of the charge electrode 2′. The flat 35′ is provided on the outlet end 24′ of the charge electrode 2′ and extends part-way along the length L′ of the generally cylindrical body 7′, parallel to the axis a′. It will be understood, however, that other mounting and/or orientation features are also possible.

(55) The length L′ of the charge electrode 2′ shown in FIGS. 3, 4 and 7 is 10.5 millimetres, and this can be appreciated with reference to the dimensions provided on FIGS. 5 and 7. Other lengths are however also possible.

(56) The charge electrodes described herein are made of suitable conductive materials such as steel, as is known in the art, and will be provided with suitable connections to a conditioning amplifier and/or other control circuits. These however are not described herein.

(57) The nozzle 1 described above is configured to emit the continuous ink jet from a 60 micron orifice at a velocity V equal to, in this case, about 20 m/s. Other velocities are however also possible, typically in the range between about 18 and 25 m/s. Alternative orifice widths are also possible, for example 40, 50 or 70 microns. It will be understood that these smaller orifices would require generally proportionally smaller electrodes, both in width and length, and that the larger orifice would require a proportionally larger electrode.

(58) The printhead 10′ described above is arranged to form the ink droplets at a frequency F equal to, in this case, 77 kHz. Other frequencies are however also possible, typically in the range between about 64 kHz to 128 kHz.

(59) In the described embodiment of printhead 10′, therefore, the ink droplets are accordingly generated at a droplet pitch DP=V/F equal to about 260 microns. The length L′ of the electrode 2′ can thus be expressed in terms of droplet pitch. The length L′ of the charge electrode 2′ shown in FIGS. 3 and 4 thus measures just above 40.3 droplet pitches. This accommodates wide variations of the location of drop formation along the ink jet. The possibility that the drops may form outside the charge electrode is therefore reduced. The (minimum) length of the second axial region R2 shown in FIG. 7 in terms of droplet pitches is just above 15 droplet pitches. This corresponds to about 4 drops per millimetre.

(60) While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described herein. The description is not intended to limit the invention, whereas the invention is instead defined according to the scope of the appended claims.