Improved electrostatic head with toner-repelling electrode

Abstract

Liquid or dry developing substance comprising charged particles is applied to a base electrode in the form of a rotary cylinder. A sheet of recording paper is fed in contact with the cylinder. A printing head comprises a plurality of printing electrode sets formed into an array such as a rectangular dot matrix in which each electrode set constitutes a row of the matrix. Each electrode set comprises a plurality of shaping electrodes which constitute the individual data points of the row. At least one bias electrode is provided for each electrode set. Row select wires are connected to the bias electrodes of the electrode sets respectively. Shaping electrode or column select wires are connected to the shaping electrodes of all of the electrode sets in such a manner that a particular column select wire is connected to the shaping electrodes of all of the electrode sets in a corresponding column. A high voltage of the same polarity as the charged particles applied to the bias electrodes causes the charged particles to be repelled against the surface of the cylinder and prevents printing. The voltage on the bias electrodes is lowered one row at a time to sequentially enable the rows for printing. The lower voltage also repels the charged particles against the surface of the cylinder but can be overcome by a higher voltage of a polarity opposite to the charged particles applied to the shaping electrodes of the selected row corresponding to the data points of the row which are to be printed. The interaction of the electric fields of the bias electrode and the shaping electrodes causes the attractive field of the shaping electrodes to be present only directly under the faces of the shaping electrodes thereby providing high printing resolution.