Image forming apparatus featuring a controller for switching a first transfer bias to a second smaller transfer bias while the print material is passing through a transfer nip portion

An image forming apparatus comprising:

• an image carrier that carries a toner image;
• a transfer member forming a transfer nip portion together with said image carrier, for transferring the toner image carried on said image carrier to a print material passing through the transfer nip portion;
• a sensor for detecting a position of a print material;
• a power source for applying a transfer bias to said transfer member;
• a controller for controlling the transfer bias outputted from said power source;
• and
• a detector for detecting a size of the print material,
• wherein said controller receives information relating to a size of the print material from the detector,
• wherein when transferring the toner image to a first print material, and when a position located a first length away from a trailing end of the first print material toward a leading end of the first print material in a conveying direction is passing through the transfer nip portion, said controller outputs a signal for decreasing an output voltage of said power source,
• when transferring the toner image to a second print material with the length in the conveying direction being shorter than the first print material, and when a position located a second length away from a trailing end of the second print material toward a leading end of the second print material in the conveying direction is passing through the transfer nip portion, said controller outputs a signal for decreasing the output voltage of said power source, and
• wherein the second length is longer than said first length.