This invention relates to a drop formation control device in an ink jet printer, in which reliability is improved by eliminating the production of mist so that smudges of the print head section are prevented.
An ink jet printer is known in the art which, for instance, comprises: a drop generator for vibrating an ink stream jetted through the nozzle from the ink chamber, to form ink drops; charging electrodes for charging ink drops according to printing signals; deflecting electrodes for deflecting ink drops according to the amounts of charge applied thereto, so as to fixedly stick them onto a recording sheet; and a gutter for receiving ink drops which move straight through the deflecting electrodes because they have not been charged, to return them into the ink tank.
When, in the printer described above, ink is vibrated at a frequency of 100 to 150 KHz while being continuously supplied, under a predetermined pressure, to the ink chamber by the pump, ink drops are formed at a rate of 100 to 150.times.10.sup.3 drops/second corresponding to the drop-forming frequency. The ink drops are charged according to printing signals while passing through the charging electrodes. Therefore, the ink drops are deflected according to the amounts of charge by the deflecting electrodes so as to fixedly stick on the recording sheet and thereby carry out a printing operation.
FIG. 1 shows the control which is performed by the printer at the end of a printing operation. When the printing operation is stopped at the time instant t.sub.1, the ink pressure, e.g. 3 Kg/cm.sup.2, is reduced to a value, e.g. 1.5 Kg/cm.sup.2, which prevents the entrance of bubbles through the nozzle normally caused by a negative pressure which occurs when the three-way valve means is switched, whereby the ink flow-out speed v is reduced to v.sub.2 from v.sub.1 (the ink flow-out speed v being represented by k.sqroot.p (v=k.sqroot.p) where p is the ink pressure and k is a proportional constant). When the ink flow-out speed v is reduced to v.sub.2, the three-way valve means is switched so that the supply of ink to the nozzle is closed, as a result of which the nozzle is disconnected from the pressure of the supply pump and ink will not drop from the nozzle of the drop generator. When the flow of ink is stopped at the time instant t.sub.3, the drop-forming frequency f is reduced to 0 Hz (with the piezo-electric element of the drop generator being deenergized), formation of ink drops is stopped, and the relevant operations are also stopped.
However, the conventional ink jet printer suffers from the following problem: Even during the period of time (about one second) from t.sub.1 to t.sub.2 after the ink flow-out speed has been reduced to v.sub.2, the formation of ink drops is carried out at the same drop-forming frequency f as that in the steady-state operation in which the ink flow-out speed is v.sub.1. Accordingly, the ink flow-out speed and the drop-forming frequency, which are the ink drop forming conditions, are not in agreement with each other, and therefore the formed drops are not uniform, thus producing a mist. As a result, the print head section is smudged. This causes problems in the printer, thus lowering its reliability.