Image forming apparatus having at least two-color image print function and method for controlling the same

A method for forming a single color image from a multi-color image forming apparatus having an image carrier, at least first developer including a first color developing agent and second developer including a second color developing agent, comprising the steps of:

• charging the image carrier with a predetermined surface potential;
• forming a single color image on the charged image carrier by the first color developing agent of the first developer;
• recharging the image carrier, the single color image having a prescribed surface potential level;
• and
• applying a bias voltage to the second developer for reducing the difference in potential level between the single color image and the second developer and avoiding contamination of the second color developing agent of the second developer by the first color developing agent from the single color image.

An image forming apparatus for forming both single image and multi images on an image carrier in response to image signals, the image forming apparatus comprising:

• first image forming means responsive to a first image signal and including a first developer for forming the first image of the image carrier, the first image having a prescribed potential level;
• second image forming means responsive to a second image signal and including a second developer for forming the second image on the image carrier;
• and
• control means for applying a bias voltage to the second developer for reducing the difference in potential level between the first image and the second developer in order to avoid the degradation of the image density of the first image when only the first image signal is received by the apparatus.