Biasing method and apparatus for electrostatically transferring an image

Abstract

A method and apparatus for electrostatically transferring a toner image from a toner image bearing member (TIBM) to a receiver sheet is disclosed. A receiver sheet is moved within a nip defined between a transfer roller having a resistive outer layer and a toner image bearing member supporting a toner image. To provide for transfer, there is applied an electrical bias to the resistive layer from a conductive member located beneath the resistive layer and in electrical contact with the resistive layer. There are also applied to an outer surface of the resistive layer a first contact voltage to establish an post-nip electrical voltage distribution to a post-nip region of the transfer roller that is immediately downstream of an in-nip region. The post-nip electrical voltage distribution is accomplished by having the first contact voltage be applied at first points on the outer layer of the transfer roller, the first points being within or proximate the post-nip region. There is also applied to the resistive outer layer a second contact voltage to establish an electrical voltage distribution to a pre-nip region of the transfer roller immediately up-stream of the in-nip region. This pre-nip electrical voltage distribution is accomplished by having the contact voltage be applied at second points on the outer layer of the transfer roller, the second points being within or proximate the pre-nip region. The resistivity of the outer layer, the voltage provided to the conductive member, the voltage bias to the TIBM, and the first and second contact voltages establish an in-nip electrical field or voltage suitable for transfer of the toner image to a receiver sheet and for suppression or reduction of pre-nip and post-nip ionization.