Brother MFC620CN Service Manual page 106

Air bubble growth
Liquid ink contains trace amounts of air. The amount varies with the pressure on the ink because
increasing the pressure raises the ink's ability to absorb air from its surroundings and lowering the
pressure forces the ink to give up some of this air.
The ink pressure fluctuations generated by the piezoelectric ceramic actuators repeatedly expand
and contract any air bubbles, causing them to gradually grow larger as the ink pressure changes
repeatedly expand and contract them.
The design therefore compensates by allowing the purging/flushing of air bubbles during printing
to keep the amount of air dissolved in the ink to a minimum.
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3-18 Confidential