Image forming method and apparatus

A color image forming apparatus comprising a rotating image holding member, and a charging unit, an imagewise exposing unit, developing units and a transferring unit that are disposed around the image holding member in this order;

• the developing units including Y, M, C and K developing units that form Y, M, C and K toner images, respectively, by developing electrostatic images on the image holding member with Y, M, C and K toners in association with rotation of the developing unit and the image holding member;
• wherein each toner and the image holding member are selected to provide plots of attachment force F [C] between the toner and the image holding member versus square of charges q2 [C2] giving a linear approximation of F=K×q2+F0 . . . (1) (wherein K denotes a proportionality factor and F0 denotes an intercept), so that plotted values of F fall within a range of ±10% of F given by the linear approximation in a range of attached toner amount on the image holding member of 150 to 600 μg/cm2.

A color image forming apparatus comprising four image forming units including a Y image forming unit, an M image forming unit, a C image forming unit and a K image forming unit, and a transferring unit;

• each image forming unit including an image holding member, a charging unit, an imagewise exposing unit and a developing unit for forming a toner image of a corresponding color on the image holding member;
• the transferring units transferring and superposing Y, M, C and K toner images, which are formed on the respective image holding members, in a non-fixed state onto a transfer medium;
• and in each of the image forming units, the toner and the image holding member are selected to provide plots of attachment force F [C] between the toner and the image holding member versus square of charges q2 [C2] giving a linear approximation of F=K×q2+F0 . . . (1) (wherein K denotes a proportionality factor and F0 denotes an intercept), so that plotted values of F fall within a range of ±10% of F given by the linear approximation in a range of attached toner amount on the image holding member of 150 to 600 μg/cm2.