Developer in an image forming device having a binding resin and magnetic powder

Claim 1

An image forming device for developing electrostatic images held on an electrostatic image holding member, comprising: a developer chamber containing a developer for developing the electrostatic images, comprising a magnetic toner comprising a binder resin and magnetic powder, said developer containing 17-60 % by number of magnetic toner particles having a particle size of 5 microns or smaller, containing 1-23 % by number of magnetic toner particles having a particle size of 8-12.7 microns, and containing 2.0 % by volume or less of magnetic toner particles having a particle size of 16 microns or larger; wherein the magnetic toner has a volume-average particle size of 4-9 microns, and the magnetic toner particles having a particle size of 5 microns or smaller has a particle size distribution satisfying the following formula: N/V=-0.04N+k, wherein N denotes the percentage by number of magnetic toner particles having a particle size of 5 micron or smaller, V denotes the percentage by volume of magnetic toner particles having a particle size of 5 microns or smaller, k denotes a positive number of 4.5-6.5, and N denotes a positive number of 17-60; toner-carrying means having a surface to hold a toner layer thereon and to carry the toner layer to a developing zone; the toner layer being formed of the magnetic toner particles supplied from the developer chamber, said toner-carrying means being made of a non-magnetic material; magnetic means for generating a stationary magnetic field at the developing zone through the non-magnetic toner-carrying means toward the surface of the electrostatic image-holding member; means for forming the layer of the magnetic toner particles of substantially uniform thickness on the surface of the toner-carrying means; and means for maintaining a space between the toner-carrying means and the electrostatic image-holding member at the developing zone within a predetermined range to form a space gap between the electostatic image-holding member and the surface of the layer of the magnetic toner particles on said toner-carrying means.