| abstract |
A constant electric field is applied to a drop on demand print head using coincident force address of selected ink drops. This field can be generated by applying one electric potential to the print head, and a different electric potential to a platen which lies on the opposite side of the recording medium. This field does not need to be modulated, or turned on for each drop to be ejected. As a result, a simple high voltage power supply can be used to generate the electric field. No high voltage switching equipment is required. Also, the spacing between nozzles can be small, as the field applied to a nozzle does not need to be separated from fields applied to adjacent nozzles. The electric field is set to be insufficient to cause ink drops to be drawn from the print head when the ink in the nozzles in the quiescent position. The drop selection method causes the ink meniscus of selected drops to protrude from the front surface of the print head. Charge accumulates at the meniscus of the protruding drop, because the drop radius is small, and because the drop meniscus is the closest point to the opposite electrode. This charge concentrates the force produced by the electric potential field onto the selected drop. This force, in combination with the ink pressure, overcomes the surface tension of the ink, and causes the selected drop to separate from the body of ink. The selected drop then accelerates towards the platen, striking the recording medium. By this means, a drop of can be printed on a print medium even when the drop selection method does not impart sufficient kinetic energy to the selected drop to cause the selected drop to overcome surface tension forces and separate from the body of ink. |