In the supply of ink into the nozzle head in an ink jet recorder, it is necessary that pressure of the ink be maintained constant and air bubbles be not mixed in the ink. As a matter of fact, however, it is said that the concentration of air (oxygen) dissolved in the ink in the state that the ink is only exposed to the air is increased to 5 to 6 ppm from 1 ppm for a period of 30 minutes. And, the entrained air accumulates at such as a bent area in the piping to become air bubbles which in the nozzle head cause an undischarge of ink or a fluctuation in ink pressure which in turn brings about a poor or defective printing.
Accordingly, the ink furnishing unit of this type in the prior art includes a deaerator that is provided for a recovery tank which reserves the ink recovered from the purge section and abounding with air bubble (see, for example, JP P2003-127435 A) or a deaerator that is disposed midway in the ink supply circuit (see, for example, JP P2004-17517 A).
In the technique shown in JP P2003-127435 A and designed to permit recycling the ink recovered at the purge section, it is difficult to deaerate the entire recovery tank uniformly so that it has not been possible to remove air bubbles completely from the ink in the recovery tank.
While in this regard the technique shown in JP P2004-17517 A permits the entire volume of ink furnished from the supply tank into the main tank to be evenly deaerated only by passing through its piping, this techniques does not allow such ink to be directly furnished to the nozzle head. Since the ink is once stored in the air-open main tank for stabilization of its supply pressure and then is furnished to the nozzle head, the amount of air entrained in the ink while in the main tank increases with the lapse of time, causing the problem that the ink discharge at the nozzle head tends to be defective.
With these taken into account, it is an object of the present invention to provide an ink furnishing unit in an ink jet recorder that can efficiently deaerate the ink furnished to the nozzle head including the ink recovered from the purge section so that the defective printing due to the mixing of air bubbles in the ink may be prevented.