Color image forming apparatus having bias controller for cleaning transfer roller

An apparatus for forming a multi-color image composed of at least two component colors on a sheet material, comprising:

• an electrically chargeable photoconductor on which a toner image is formed, the photoconductor having a transfer section at which the toner image is transferred from the photoconductor to the sheet material, wherein the photoconductor is rotatable and has a radius of curvature larger than 40 mm at the transfer section;
• image forming means comprising means for charging the photoconductor, means for imagewise exposing the charged photoconductor so as to form a latent image, means for developing the latent image so as to form a color toner image, the image forming means forming at least two component color toner images one after another on the photoconductor so that a latter color toner image is superimposed on a charged former color toner image on the charged photoconductor, wherein the toner images on the photoconductor have an electric polarity;
• means for transferring the toner images onto the sheet material, the transferring means comprising
• a transferring roller whose surface is made of an elastic foamed material in which an average cell diameter is 1 .mu.m to 300 .mu.m, the transferring roller coming in contact with the sheet material and pressing the sheet material onto the photoconductor at the transfer section during a transferring operation so that the toner images are transferred at a time as the multi-color image onto the sheet material, and
• an electric bias member for applying an electric bias onto the transferring roller, wherein
• during the transferring operation, the electric bias member applies onto the transferring roller an opposite electric bias of a constant current having the opposite electric polarity to that of the toner image, and
• during a cleaning operation in which the transferring roller is brought in contact with the photoconductor at the transfer section so as to shift toner adhered on the transferring roller to the photoconductor, the electric bias member applies onto the transferring roller alternately an identical electric bias of a constant voltage having the identical electric polarity with that of the toner image and an opposite electric bias of a constant current having the opposite electric polarity to that of the toner image, six times or more.