Continuous ink jet printer print head

Abstract: A print head for a continuous ink jet printer has a nozzle 5 through which ink is emitted and broken up into droplets under the action of a piezoelectric oscillator. A charge electrode 3 applies charge to selected droplets and a deflection electrode 4 deflects the path 20 of the charged droplets to cause printing on a substrate. A gutter 6 collects droplets which are not required for printing. A charge electrode 3, together with one of the deflection electrodes 4 is movable in a direction transverse to the path of the droplets selectively under the action of pressurised ink fed from an ink supply 14. Similarly, the gutter 6 is movable in the same way.

Description: The present invention relates to so-called "continuous ink jet printers" of the type in which a stream of ink is emitted under pressure from a nozzle and, by the action of a piezoelectric oscillator, is broken up into droplets which can be selectively charged and then deflected in an electric field onto a substrate. Such printers are well known in the art. Although such printers have been available for many years, problems arise still during the start-up of such a printer. Frequently, the stream of ink issuing from the nozzle is unstable at start-up and this can cause ink to impinge on components of the print head undesirably. In particular, ink impinging on the electrode used to charge the droplets can cause unstable conditions to persist and charging to be inaccurate with the result that droplets are not correctly placed on the substrate. Also during start-up, there is a need to ensure that guard droplets and non-printable droplets pass correctly into the gutter which is provided for their collection. Additionally, when the printer is of the type in which uncharged droplets are "printed" and charged droplets are either guard drops or non-printable drops, when the printer is first switched on and the stream of ink starts to issue from the nozzle, it is desirable to avoid wastage of ink or the unnecessary application of ink to part of a substrate which will then not be to be used (see US-A-4305079). According to the present invention, there is provided a print head for a continuous ink jet printer, the print head having ink supply means for supplying ink under pressure to a nozzle through which ink is emitted in use and broken up into droplets by the action of a piezoelectric oscillator, a charge electrode for applying electrostatic charge to selected droplets in use, a deflection electrode for deflecting the path of charged droplets, and a gutter for collecting droplets not required for printing, wherein the charge electrode and/or the gutter are movable in a direction transverse to the path of the droplets, the movement being controlled by the action of pressurised ink selectively supplied from the ink supply means to a hydraulic actuator or actuators coupled to the charge electrode and/or the gutter. Thus, during start-up, the charge electrode may be withdrawn from its normal operating position laterally to avoid being spattered by ink droplets. The gutter may also be able to be positioned so that all droplets issuing from the nozzle during the start-up phase, whether charged or not, pass into the gutter and do not pass to the underlying substrate. Advantageously, the charge electrode is mounted for movement with a deflection electrode. The invention also includes a method of operating a print head according to the present invention wherein the charge electrode is moved laterally out of its normal operating position so that it is withdrawn from proximity to the stream of droplets. A further method using a print head according to the invention involves moving the gutter laterally to a position in which all droplets enter the gutter during start-up. One example of a print-head according to the present invention will now be described with reference to the accompanying drawing which illustrates the print head in side view. The print head has an electronics sub-system 1 by means of which are controlled the piezoelectric oscillator in a droplet generator 2, together with the application of appropriate voltages to charge electrodes 3 and deflection electrodes 4,4' and by means of which appropriate signals are given to valves (not shown) in the printer cabinet (not shown) controlling the flow of ink to the droplet generator 2. The droplet generator 2 has a nozzle plate 5 with a plurality of closely spaced nozzles arranged in a row (normal to the plane of the drawing) and from which issue, in use, streams of ink 20 (the plane of which is thus normal to the plane of the figure) which, under the action of the piezoelectric oscillator, break up into individual droplets for printing purposes. The droplets pass individual charge electrodes 3 (seen end-on in the drawing), also arranged in a row in the same direction, where they are selectively charged and then passed between the pair of deflection electrodes 4, 4' which establish, in use, an electric field by means of which charged droplets are deflected from their straight-line path into a gutter 6. In the start-up position of the gutter 6 (not shown in the drawing) even uncharged droplets (which in the present case are used for printing) pass into the gutter. In use ink is supplied from a supply means 14 to an ink chamber (not shown) within the drop generator 2 above the nozzle plate 5, via a feed line 7 and is also supplied to first 8 and second 9 hydraulic actuators, via lines 8'' and 9'', through the action of the appropriate valves (not shown) mounted in the printer cabinet (not shown). The first actuator 8, which is supported on a bracket 10, has a piston 8' which is arranged to bear against one end of a lever arm 11 at the other end of which is mounted the gutter 6. The second actuator 9 is also mounted on the bracket 10 and has a piston 9' which engages, via a pair of links 12, 12', a flexible support bracket 13 for the charge electrodes 3 and the deflection electrode 4. By selectively operating the valves which control the supply of pressurised ink to the first and second actuators 8, 9 respectively, the gutter 6 can be withdrawn from the "catch-all" position into the position shown in the drawing which is an operating position in which only charged droplets are deflected into the gutter, non-charged droplets being allowed to pass onto the substrate for printing, and the charge electrodes 3 and deflection electrode 4 can be moved rightwards from the position shown in the drawing, to a position in which the charge electrodes 3 are closely adjacent the streams of droplets 20 and the deflection electrode 4 is in the appropriate position relative to the other deflection electrode 4'. This position is defined by an adjustable stop screw 15 which bears against an abutment 16 on the side of the nozzle plate 5. The start/stop sequence described below uses four solenoid valves; jet, bleed, charge electrode actuator and gutter actuator, none of which are shown in the drawing. The jet solenoid valve (aka the feed solenoid) is a two-way solenoid valve which is mounted in the print head and controls the flow of ink to the drop generator 2 through the feed line 7. The bleed solenoid is a similar type of valve to the jet/feed solenoid valve and is also mounted in the print head and controls flow through a bleed line (not shown). When open, it allows a flow of ink through the bleed line from the drop generator 2 primarily to remove ingressed air during start up. During shut down it is also opened to cause a very quick jet shut off by de-pressurising the drop generator. This is helped by connecting the bleed to vacuum source (not shown) which is used to draw ink from the gutter 6. The charge electrode actuator valve is a three-port solenoid valve mounted in the ink cabinet. When activated ink is supplied to the actuator 9 so that the charge electrode 3 moves into the print position. When de-activated, the charge electrode 3 returns to its 'safe', jet start position (as shown in the drawing). The gutter actuator valve is similar to the charge electrode actuator valve and is mounted in the cabinet. When activated, it causes ink to flow to the gutter actuator 8 which moves the gutter 6 into the print position (as shown). When deactivated the gutter 6 is in the "catch all" position needed for jet start up and shut down, rightwards of the position shown in the drawing. The start up sequence is as follows: With both the gutter and charge electrode actuator solenoid valves off (the gutter in the catch all position, the charge electrode in the jet start position) the feed pressure and gutter pumps start. Following a jet start request, the jet solenoid valve opens. The jets start (which causes the pressure to drop). However, the actuators 8,9 require a certain pressure to operate so if the pressure drops below this value the sequence must wait until the pressure reaches this value. After ten seconds, the bleed valve opens for ten seconds which causes another drop in the pressure. Again, the pressure control system can ignore this drop, so long as it is above the minimum pressure. Once the bleed valve closes the pressure control system can establish the pressure required for the current operating parameters. Once the correct pressure is established the charge electrodes 3 are moved rightwards into the operating position by activating the charge electrode solenoid valve. At this point, modulation, phasing, jet velocity measurement and charging can start. Once this has been completed the jets should be being deflected into the back of the gutter 6. At this stage it is safe to move the gutter to the print position shown, by activating the gutter actuator 8. At this stage printing can start. This sequence is summarised in Appendix A. The jet stop sequence begins with the gutter actuator 8 closing so that the gutter 6 returns to the catch all position. It is then safe to stop charging, phasing and modulation and move the charge electrodes 3 to the 'safe' position by de-activating the charge electrode actuator 9. Like the jet start sequence, the jet stop sequence begins with setting the pressure. Once this has been established the bleed solenoid valve opens. After ten seconds, the jet solenoid valve closes shortly followed by the bleed solenoid valve. As with the start sequence, the pressure control system need not try to maintain the generating pressure and pressure control faults should be ignored. After the jets have been turned off, the pumps should continue to run, to clear the gutter, before being turned off. This sequence is summarised in Appendix B.

Claims: A print head for a continuous ink jet printer having an ink supply means (14) for supplying ink under pressure to the print head, the print head comprising a nozzle (5) through which ink is emitted in use and broken up into droplets by the action of a piezoelectric oscillator, a charge electrode (3) for applying electrostatic charge to selected droplets in use, a deflection electrode (4) for deflecting the path (20) of charged droplets, and a gutter (6) for collecting droplets not required for printing, the charge electrode (3) and/or the gutter (6) being movable in a direction transverse to the path (20) of the droplets, characterised in that the movement of the charge electrode (3) and/or the gutter (6) is controlled by the action of pressurised ink selectively supplied from the ink supply means (14) to a hydraulic actuator or actuators (8,9) coupled to the charge electrode (3) and/or the gutter (6). A print head according to claim 1, wherein the charge electrode (3) is mounted for movement with a deflection electrode (4). A print head according to claim 1 or claim 2, wherein the charge electrode (3) and/or the gutter (6) is/are biased to a non-operating position. A print head according to any of claims 1 to 3, wherein the gutter (6) is positionable so that all droplets issuing from the nozzle, whether charged or not, pass into the gutter (6). A method of operating a print head according to claim 1, wherein the charge electrode (3) is moved laterally out of its normal operating position so that it is withdrawn from proximity to the stream of droplets (20). A method according to claim 5, wherein, during start-up, the charge electrode (3) is withdrawn from its normal operating position laterally to avoid being spattered by ink droplets. A method of operating a print head according to claim 1, wherein, during start-up, the gutter (6) is moved laterally to a position in which all droplets enter the gutter (6), whether charged or not.

Inventors: MARSDEN RICHARD, ZABA JERZY MARCIN, MARSDEN, RICHARD, ZABA, JERZY MARCIN
Applicants: DOMINO PRINTING SCIENCES PLC
Application year: 1996
Main classification: B41J2
IPC codes: B41J2