Liquid jet printing apparatus using a raster of drops to effect printing

An ink jet array printer adapted to print by depositing small drops of ink in accordance with printing information on a surface to be printed during continuous movement relatively to the apparatus of the surface, comprising one or several rows of ink jet printing guns, each gun having means for supplying printing ink under pressure to an orifice, means for forming regularly spaced drops in the ink stream issuing from the orifice, charge electrode means for charging the drops, means for applying to the charge electrode means, under the control of the printing information, a periodic printing voltage waveform whose period is sufficient to span the formation of a "raster" of consecutively formed drops, drop deflection means for providing transverse to the direction of relative movement of the apparatus and the printing surface a substantially constant electrostatic field through which the drops pass towards the printing surface thereby to deflect electrically charged drops transversely to said direction of relative movement to an extent dependent upon the charge levels on the drops, and drop intercepting means for collecting drops other than those drops charged for printing on the printing surface, the drops charged for printing in the printing guns during each period of the voltage waveform being deposited in respective line sections formed by contiguous drops which sections together present a printed line transversely of the direction of relative movement, the printed lines being formed in contiguity successively at the frequency of the printing voltage waveform applied to the charge electrode means, characterized in that there are provided means for generating the print voltage waveform applied to the charge electrode means of each printing gun which are adapted to provide in said waveform at least two successive sets of voltage levels for application to successively formed drops which arrange the raster drops in a group in time order of drop formation for each set of voltage levels so that corresponding drops in each of the groups formed in the raster, if charged for printing, have similar differences of voltage level and have similarly spaced print locations in the line section of drops printed by the printing gun and the line section is formed along its length at successive locations by corresponding drops from successive groups, and further characterised in that the means for generating the print voltage waveform include first correction voltage evaluating means and second correction voltage evaluating means for correcting the voltage level of the print voltage waveform for application to each drop formed which is charged for printing, of which the first correction voltage evaluating means is adpated to evaluate a correction voltage dependent upon the print status of the drop whose print voltage is to be corrected and the print status of the preceding drop, and, the second correction voltage evaluating means is adapted to evaluate a correction voltage which corrects for the effect of mutual electrostatic and aerodynamic forces of a number of raster drops in accordance with the print status thereof, said raster drops being in the immediate vicinity of the drops whose charging voltage level is to be corrected and being a significant influence on the flight path of that drop, and means are provided for employing the same set of voltages evaluated by the second correction voltage evaluating means for a particular drop for each of the corresponding drops in the groups.