US Patent 7917068 Developing device and image forming apparatus

A developing device is disposed facing an image bearing body that bears a latent image. The developing device includes a developer bearing body that bears a developer for developing the latent image. The developer includes mother particles and external additives. A liberation amount T (weight parts) of the external additives liberated from the mother particles with respect to 100 weight parts of the mother particles, a surface roughness Rz (m) of the developer bearing body, and a circumferential speed Vd (mm/s) of the developer bearing body satisfy the following relationships:1.326×10−1≦T≦2.142×10−1 (weight parts),7.1×10−6≦Rz≦15.0×10−6 (m),161.5≦Vd≦189.2 (mm/s), and4.98×10−6≦(T×Rz/Vd)≦1.99×10−5 (s).