Developing apparatus for electrostatic charge images

Abstract

An improved apparatus is provided for developing an electrostatic charge image on a support being moved through a magnetic brush-forming field between a permanent magnet and a rotating cylinder that transports a powder mixture of toner particles and magnetizable carrier particles into said field from a supply of the mixture held in a tray. A wall of the tray bordering the path of the mixture on the cylinder is formed with an edge over which an excess of powder escapes instead of accumulating objectionably in a region between the support and the cylinder. As toner becomes deficient in the tray it is replenished by fresh powder mixture properly proportioned for use, which mixture is supplied into a shorter part of a trough that also receives the powder returned down the cylinder from the magnetic brush region. Screw conveyor means in this trough move its contents to an off-center opening therein from which mixed toner and carrier particles fall and are guided toward the center of the tray so as to be mixed with the material in the tray by especially formed mixing elements rotating therein. Further intensive mixing is assured by subjecting the mixture passing from the tray to the field of a permanent magnet mounted outside the tray in closely spaced relation to a lower portion of the surface of the cylinder. A central mixing element carries a brush for sweeping a window in the tray bottom through which the toner concentration is optically scanned. The rotary mixing elements have shafts mounted in powder shedding roller bearing bushings fitted into the side walls of the tray.