This invention relates to pulsed droplet deposition apparatus. Typical of this kind of apparatus are pulsed droplet ink jet printers, often also referred to as "drop-on-demand" ink jet printers. Such printers are known, for example, from U.S. Pat. Nos. 3,946,398 (Kyser and Sears), 3,683,212 (Zoltan) and 3,747,120 (Stemme). In these specifications an ink or other liquid channel is connected to an ink ejection nozzle and a reservoir of the liquid employed. A piezo-electric actuator forms part of the channel and is displaceable in response to a voltage pulse and consequently generates a pulse in the liquid in the channel due to change of pressure therein which causes ejection of a liquid droplet from the channel.
The configuration of piezo-electric actuator employed by Kyser and Sears and Stemme is a diaphragm in flexure whilst that of Zoltan takes the form of a tubular cylindrically poled piezo-electric actuator. A flexural actuator operates by doing significant internal work during flexure and is accordingly not efficient. It is also not ideally suitable for mass production because fragile, thin layers of piezo-electric material have to be cut, cemented as a bimorph and mounted in the liquid channel. The cylindrical configuration also generates internal stresses, since it is in the form of a thick cylinder and the total work done per ejected droplet is substantial because the amount of piezo-electric material employed is considerable. The output impedance of a cylindrical actuator also proves not to be well matched to the output impedance presented by the liquid and the nozzle aperture. Both types of actuator, further, do not readily lend themselves to production of high resolution droplet deposition apparatus in which the droplet deposition head is formed with a multi-channel array, that is to say a droplet deposition head with a multiplicity of liquid channels communicating with respective nozzles.
Another form of pulsed droplet deposition apparatus is known from U.S. Pat. No. 4,584,590 (Fishbeck and Wright). This specification discloses an array of pulsed droplet deposition devices operating in shear mode in which a series of electrodes provided on a sheet of piezo-electric material divides the sheet into discrete deformable sections extending between the electrodes. The sheet is poled in a direction normal thereto and deflection of the sections takes place in the direction of poling. Such an array is difficult to make by mass-production techniques. Nor does it enable a particularly high density array of liquid channels to be achieved as is required in apparatus where droplets are to be deposited at high density, as for example, in high quality pulsed droplet, ink jet printers.