Ink jet printhead

Abstract: An ink jet printhead comprises a housing (1) having a chamber (2) to which ink is supplied in use and a row of orifices through which droplets are ejected in use under the action of an actuator (5,6,9) bounding a portion of the chamber. The actuator (5,6,9) comprises a (6) body having a plurality of sockets (8), a membrane (5) or membranes disposed so as to close one side of each of the sockets (8), and a piezoelectric or electrostrictive transducer (9) disposed within each socket (8) and mounted on the membrane (5) closing the one side of the socket (8).

Description: The present invention relates to ink jet printheads and, more particularly, to ink jet printheads for use in continuous ink jet printers having multiple streams of ink issuing through corresponding orifices. In order to create streams of droplets from the ink issuing through multiple nozzles or orifices it is known to use a method known as "velocity modulation" to vibrate the chamber from which the jets emerge, and it is also known to pressure modulate the fluid itself by means of plural piezoelectrically actuated drive rods or a single piezoelectrically actuated armature. Attempts have been made to utilise a unimorph structure in order to create pressure waves within the chamber, but investigations have shown that, at the required frequency of around 64kHz, the use of the single unimorph causes uneven break-up length along the array of jets which in turn results in poor definition at best. Analysis of the modal vibration of the unimorph indicated that the unimorph was vibrating in complex modes which appeared to have displacement anti-nodes all along the length of the unimorph. The present invention has, as an object, the desire to overcome such non-uniformity in the array of jets. EP-A-0084891 discloses a multi-jet single head continuous ink jet printer with multiple actuators mounted on a base and operated in phase. According to the present invention, there is provided an ink jet printhead for a continuous ink jet printer, comprising a housing having a chamber to which ink is supplied in use, the chamber having a row of orifices through which streams of ink are continuously ejected in use and broken up into droplets under the action of an actuator bounding a portion of the chamber, characterised in that the actuator comprises a body having a plurality of sockets, a membrane or membranes disposed so as to close one side of each of the sockets, and a piezoelectric or electrostrictive transducer disposed within each socket and mounted on the membrane closing the one side of the socket. Preferably, a single membrane closes all the sockets and is bonded to a face of the actuator body. The membrane is stainless steel in a preferred embodiment. Each transducer is preferably adhesively bonded to the membrane at the centre of the side of the respective socket. The necessary electrical connections can be provided by wire bonding conductors to the transducers. These measures ensure repeatability and uniformity, enabling plural printheads to be arranged in end-to-end relationship to form an elongate assembly of orifices. One example of a printhead constructed in accordance with the present invention will now be described with reference to the accompanying drawings in which: Figure 1 illustrates, diagrammatically, an ink jet printhead droplet generator with the parts exploded; Figure 2 is a longitudinal sectional view of the actuator for the printhead; and, Figure 3 is a plan view of the actuator. Figure 1 shows the actuator in an exploded isometric view, the actuator comprising a stainless steel body 1 which provides a chamber 2 for containing ink to be printed through multiple droplet streams 3 emitted through plural orifices in a stainless steel nozzle plate 4 which is attached to one face of the body 1. On the other side of the body 1 to the nozzle plate 4 there is disposed a rectangular stainless steel foil or membrane 5 which is clamped to the face of the body 1 by a rigid clamping plate 6. In Figure 1 the clamping is shown as being provided by bolts 7. Prior to being clamped onto the body 1 the foil or membrane 5 is bonded to the (as shown) underside of the clamping plate 6. The clamping plate 6 has, as shown, four circular bores 8 which extend through it and the foil or membrane 5 has on it a correspondingly arranged row of disk-like unimorph piezoelectric crystals 9. The diameter of the unimorph crystals 9 is less than the bores 8 so that there are provided, in effect, a plurality of clamped foil disks along the length of the chamber 2. In this arrangement the crystals 9 can be energised simultaneously to flex their respective disks at the same time thus creating a uniform disturbance in the ink within the chamber. Such a multiple-unimorph structure has the advantage that a circular foil disk has a much higher fundamental mode of vibration than the rectangular foil of the prior design. For example, a 30mm by 6mm rectangle of 180 micron thick steel has a fundamental vibration frequency of 27.4kHz whereas a 6mm diameter disk of the same material has a fundamental vibration frequency of 49.3kHz. It is advantageous that the nearest higher mode of vibration is 1.6 times the fundamental frequency because the disk will be likely to vibrate purely in the fundamental mode when excited at 50kHz. A further advantage of the construction is that it may be made modular so that a long array of nozzles may be accommodated simply by increasing the length of the chamber and the foil/membrane and the number of piezoelectric crystals or transducers. Crosstalk is also minimised by the bonding of the foil to the clamping plate 6.

Claims: An ink jet printhead for a continuous ink jet printer, comprising a housing (1) having a chamber (2) to which ink is supplied in use, the chamber (2) having a row of orifices through which streams of ink are continuously ejected in use and broken up into droplets under the action of an actuator (5,6,9) bounding a portion of the chamber, characterised in that the actuator comprises a body (6) having a plurality of sockets (8), a membrane or membranes (5) disposed so as to close one side of each of the sockets, and a piezoelectric or electrostrictive transducer (9) disposed within each socket and mounted on the membrane closing the one side of the socket. A printhead according to claim 1, wherein a single membrane (5) closes all the sockets (8) and is bonded to a face of the actuator body (6). A printhead according to claim 1 or claim 2, wherein the membrane (5) is stainless steel. A printhead according to any of claims 1 to 3, wherein each transducer (9) is adhesively bonded to the membrane (5) at the centre of the side of the respective socket (8). A printhead according to any of claims 1 to 3, wherein electrical connections are provided by conductor wires bonded to the transducers (9). A continuous ink jet printer having a printhead according to any of claims 1 to 5.

Inventors: CHAMBERLAIN COLIN EDWARD, TOMLIN MATTHEW BRIAN, ZABA JERZY MARCIN, CHAMBERLAIN, COLIN EDWARD, TOMLIN, MATTHEW BRIAN, ZABA, JERZY MARCIN
Applicants: DOMINO PRINTING SCIENCES PLC
Application year: 1995
Main classification: B41J2
IPC codes: B41J2