Image forming apparatus

Abstract

An image forming apparatus has an image former to form a toner image on an image bearer. A primary transferer transfers the toner image from the image bearer to the outer surface of an intermediate transferer as primary transfer by applying a primary transfer bias. A secondary transferer transfers the toner image from the outer surface of the intermediate transferer to a sheet as secondary transfer by applying a secondary transfer bias. A cleaner cleans the toner residual on the surface of the intermediate transferer after the secondary transfer by applying a cleaning bias having a polarity opposite to electric charges of the toner. A controller causes the intermediate transferer to rotate continuously and causes the primary transferer to apply the primary transfer bias until a toner image forming area on the outer surface of the intermediate transferer passes the primary transferer after the secondary transfer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic front view in vertical section of an image forming apparatus according to a first embodiment of the invention.

FIG. 2 is a partial enlarged vertical section showing the periphery of an intermediate transfer belt shown in FIG. 1.

FIG. 3 is a partial enlarged vertical section showing the periphery of an image forming unit shown in FIG. 2.

FIG. 4 is a flow chart showing an image forming operation by the intermediate transfer belt, the image forming units and their peripheral devices.

FIG. 5 is a schematic left side view in vertical section of an image forming apparatus according to a second embodiment of the invention.

FIG. 6 is a partial enlarged vertical section showing the periphery of an image forming unit shown in FIG. 5.

Claims

1. An image forming apparatus, comprising: an image forming unit including an image bearing member and adapted to form a toner image on the image bearing member using toners of a plurality of different colors,a rotatable intermediate transfer member,a primary transfer device for transferring the toner image from the image bearing member to the outer surface of the intermediate transfer member as primary transfer by applying a primary transfer bias,a secondary transfer device for transferring the toner image from the outer surface of the intermediate transfer member to a sheet as secondary transfer by applying a secondary transfer bias,a cleaning device for cleaning the toner residual on the outer surface of the intermediate transfer member after the secondary transfer by applying a cleaning bias having a polarity opposite to electric charges of the toner, anda controller for causing the intermediate transfer member to continuously rotate and causing the primary transfer device to apply the primary transfer bias from the start of the application of the primary transfer bias until a toner image forming area on the outer surface of the intermediate transfer member passes the primary transfer device after the secondary transfer.

2. An image forming apparatus according to claim 1, wherein the image forming apparatus is a tandem-type image forming apparatus in which the intermediate transfer member is an endless belt and a plurality of primary transfer devices and a plurality of image forming units are arranged in a row along a turning direction of the belt, and the controller causes any one of the plurality of primary transfer devices to apply the primary transfer bias.

3. An image forming apparatus according to claim 2, wherein the controller causes the plurality of primary transfer devices to alternately apply the primary transfer biases of minus and plus polarities or plus and minus polarities from an upstream side with respect to the turning direction of the intermediate transfer member.

4. An image forming apparatus according to claim 1, wherein the image forming apparatus is a one-drum type image forming apparatus in which the intermediate transfer member is an endless belt and the image forming unit successively develops the toner images of the plurality of colors on the single image bearing member.