US Patent 7190926 Developer bearing body and image forming apparatus

In an electrophotographic image forming apparatus, a contact-type developing roller contacts a photosensitive drum, a developing blade, a supply roller or the like. If the surface of the developing roller is excessively hard, there is a possibility that a pressure applied between the developing roller and the photosensitive drum or the like may increase, and may cause a filming, i.e., a phenomena in which a toner is degraded and welded to the surface of the developing roller. The filming can be prevented when a JIS-A hardness HA (degree) of a resilient layer of the developing roller and a dynamic hardness HS (mN/μm2) of a surface layer of the developing roller satisfy the following relationship: HA×HS<15.