Image forming apparatus and process cartridge

An image forming apparatus comprising:

• a photosensitive drum, having a CTL layer, on which a desired electrostatic latent image is formed;
• and a developing unit which accommodates a two-component developer comprising a toner and a carrier, provided with a developing sleeve which holds said two-component developer in a position opposing said photosensitive drum;
• wherein the external diameter of said photosensitive drum is 20 through 70 mm, and the film thickness of said CTL layer is 20 through 40 μm;
• the external diameter of said developing sleeve is 10 through 30 mm;
• a DC developing bias only is applied to said developing sleeve;
• the drawn amount of the two-component developer which is drawn onto said developing sleeve and arrives at said opposing position is 40 through 70 mg/cm2;
• the magnetic flux density in the normal direction of a main pole formed at said opposing position, of a plurality of magnetic poles formed on said developing sleeve, is 80 through 140 mT, and the magnetic flux density in the normal direction of a magnetic pole P2 formed adjacent to said main pole on the downstream side is 60 through 140 mT;
• the gap between said photosensitive drum and said developing sleeve at said opposing position is 0.2 through 0.5 mm;
• the linear speed ratio of said developing sleeve with respect to said photosensitive drum at said opposing position is 1.2 through 2.5;
• the toner concentration of the two-component developer accommodated in said developing unit is controlled so as to be 4 through 14 wt %;
• the weight-average particle size of said toner is 3.5 through 7.5 μm;
• and
• said carrier has a weight-average particle size of 20 through 60 μm, a static resistance of 1010 through 1016 Ω·cm, and a saturation magnetization of 40 through 90 emu/g.

A process cartridge installed detachably in the main body of an image forming apparatus which comprises:

• an image carrier which holds an electrostatic latent image on the surface thereof;
• a developer carrier, having internally fixed magnetic field generating means which rotates while holding a two-component developer comprising a magnetic carrier and a toner on the surface thereof to oppose said image carrier;
• and developing electric field generating means which generates a developing electric field between said image carrier and said developer carrier;
• the electrostatic latent image on said image carrier being converted into a toner image by the action of said developing electric field, using said toner of said two-component developer held on said developer carrier,
• wherein the photosensitive drum and the developing unit are integrated;
• the volume-average particle size of said toner is 5.5 through 8.0 (μm);
• the volume-average particle size of said magnetic carrier is 20 through 40 (μm);
• the gap between said image carrier and said developer carrier is 0.3 through 0.6 (mm), and the tolerance of the gap is within ±0.125 (mm);
• and
• b 0.2 through 0.7 (wt %) of hydrophobic silica having a particle size of 100 (nm) or above, 1.0 through 2.0 (wt %) of hydrophobic silica having a particle size of 20 (nm) or below, and 0.7 through 1.0 (wt %) of titanium oxide are added to said toner.