Bead-carry-out phenomenon in two-component development system of electrophotography

During the operation of a magnetic two-component development system used in electrophotography, some carrier particles adhere to a photoreceptor surface and cause serious image defects. This phenomenon is called "bead-carry- out" or "carrier development." The author has observed the phenomenon using a high-speed CCD camera and has measured the number density of carrier particles that adhere to the photoreceptor surface after the development process under various conditions of the development voltage, toner particle concentration, and size of the carrier particles. The experimental results indicated the following characteristics. (1) Some particles were separated from the top of bead chains immediately after the chains separated from the photoreceptor at the outlet of the development nip. (2) A threshold voltage existed for the occurrence of bead-carry-out. (3) The threshold was low when the diameter of the carrier particles was small and the toner particle concentration was low. (4) The number density of the adhered carrier particles increases with an increase in the applied voltage and a decrease in the toner particle concentration. It has been clarified by a separate experiment that the effective conductivity of the bulk mixture of carrier and toner particles was significantly related to these characteristics. Because the carrier particles are conductive and toner particles are insulative, the latter disturbs the electrical conduction in the chain, and therefore, the electrical charge at the top of the chain induced by the voltage application is decreased at a critical concentration of the toner particles in the chain. This condition causes a reduction in the Coulombforce applied to the top of the chain and improves the beadcarry-out phenomenon. Quantitative characteristics of these features have also been elucidated by a numerical simulation. Some countermeasures against this phenomenon were also proposed based on the experiment and calculation.