Abstract: A drop-on-demand ink jet printing apparatus in which the print head has an ink cavity which is filled with ink, and which has an orifice designed so that ink does not flow out under static conditions. A fluid inlet chamber is provided to receive ink from the ink supply and this chamber is separated from the ink cavity by a narrow gap. An electromechanical transducer is mounted adjacent the ink cavity and the inlet chamber. The transducer is selectively energized in response to the print data signals so that, when energized by an electrical signal, the transducer reduces the volume in the ink cavity to eject one ink drop from the orifice and substantially close off the narrow gap to substantially close the flow path from the ink cavity to the inlet chamber during the formation of the drop of ink.

Abstract: The object of this invention is to purge any entrapped air from the ink cavity (17) and nozzle orifice (15) of the print head (10) of a drop-on-demand ink jet printer. Purging is accomplished automatically as a part of the start up operation and also if, during operation, the velocity V.sub.s of the ink droplets (26), as sensed by a sensor (41), drops below a preselected reference value V.sub.r.The print head comprises a tubular piezoelectric transducer (11). The transducer is energized with a series of pulses for a preselected short time period and at a repetition rate substantially equal to a resonant frequency of the ink cavity. This effectively purges any entrapped air from the ink system. Except during purging, the transducer operates asynchronously in drop-on-demand mode in response to discrete binary print signals.