Image forming apparatus and image forming method for controlling a secondary transfer bias according to recording sheet type

An image forming apparatus comprising:

• an image bearer to bear a toner image, the image bearer having a micro-rubber hardness of less than 100;
• a nip forming member contacting the image bearer to form a transfer nip;
• a transfer power source to output a transfer bias including a superimposed voltage, in which an alternating current (AC) voltage is superimposed on a direct current (DC) voltage to transfer the toner image borne on the image bearer onto a recording sheet disposed between the image bearer and the nip forming member;
• and
• a controller to switch a transfer mode between a first mode to transfer the toner image onto a first type sheet having a surface smoothness higher than a surface smoothness of a second type sheet and a second mode to transfer the toner image onto the second type sheet,
• the transfer bias having a transfer peak value to electrostatically move toner from the image bearer to the recording sheet and an opposite-peak value to electrostatically move less toner from the image bearer to the recording sheet than the transfer peak value,
• an opposite-peak duty on a side of the opposite-peak value in the second mode being less than 50%,
• the opposite-peak duty on the side of the opposite-peak value in the first mode being greater than 50%, and
• the opposite-peak duty being tr/T×100%,
• where
• T is one cycle of the transfer bias, and
• tr is a time period during which the transfer bias is on the side of the opposite-peak value relative to an offset voltage of the transfer bias in T.