Method for determining proper bias voltage for transferring toner image from photoconductive drum to intermediate transfer belt and image forming apparatus incorporating the same

An image forming apparatus comprising:

• electrifying means for electrifying an image carrier;
• exposure means for forming an electrostatic latent image by exposing the electrified image carrier;
• developing means for developing the electrostatic latent image on the image carrier with toner, the developing means having a plurality of developing devices each having toner of a mutually different color;
• transfer means that, in a transfer portion, electrostatically transfers toner images of different colors that are sequentially formed on the image carrier, to a transfer medium, with one color toner image superimposed on another, the transfer means comprising a transfer member for causing the transfer medium to make contact with the image carrier, and voltage applying means for applying a voltage to the transfer member;
• current detecting means for detecting the value of a current flowing when the voltage application by the voltage applying means is performed;
• and
• control means for determining, at a time except during a transfer operation, a value of a transfer voltage to be applied to the transfer member during the transfer operation, based on the detection results obtained when a current detecting operations by the current detecting means were performed,
• wherein, based on a first detection result obtained when the current detecting operation was performed, the control means determines a first transfer voltage value to be obtained when the toner image of a first color is transferred to the transfer medium, when a non-image portion at the time during the formation of an image of a first color in the image carrier passes through the transfer portion before the toner image of the first color is transferred to the transfer medium;
• and
• wherein, based on the first detection result, a potential value of a non-image portion at the time during the formation of an image of each of a second and later colors in the image carrier, and a potential value of the non-image portion at the time during the formation of an image of the first color in the image carrier, the control means determines each transfer voltage value to be obtained when the toner image of a respective one of the second and later colors is transferred to the transfer medium.

A method for determining a proper transfer bias for each one of a plurality of colors to produce electrostatic or electrographic color images, the method comprising:

• providing an image carrier to carry color toner images;
• determining a plurality of nonimage potentials for a nonimage region of the image carrier, wherein each nonimage potential corresponds to a respective one of the plurality of colors;
• for said first color, determining a proper transfer bias voltage by,
• applying a first nonimage potential corresponding to the first color to the photoconductive drum, and
• applying three predetermined voltages to the primary transfer member,
• obtaining three currents corresponding to said three predetermined voltages during rotation of said primary transfer member, and
• using the three currents and predetermined voltages to determine the proper transfer bias voltage for the first color;
• and
• for all subsequent colors, determining a proper transfer bias voltage for each subsequent color by employing the first nonimage potential and the nonimage potential for the subsequent color,
• wherein said proper transfer bias voltages for the first and subsequent colors are determined within no more than three rotations of the primary transfer member.