Developing device for use in an image forming apparatus

A developing device for developing an electrostatic latent image formed on a rotating photosensitive member by bringing developer into contact with said photosensitive member, comprising:

• a magnet member which is nonrotatably fixed in a sleeve, and said sleeve being rotated around said magnet member and retaining the developer on its outer periphery;
• said magnet member being provided with four sequential magnetic poles, said four poles including an upstream magnetic pole arranged upstream of a magnet portion confronting said photosensitive member, first and second magnetic poles of an identical polarity, said first magnetic pole being disposed adjacent to an upstream of said second magnetic pole in a rotational direction of said sleeve, said first and second magnetic poles provided at the portion confronting said photosensitive member, and a downstream magnetic pole arranged downstream of the confronting portion, said upstream and downstream magnetic poles having a polarity opposite to the polarity of the first and second magnetic poles;
• the developer being successively retained between said first and second magnetic poles and on an outer peripheral surface of said sleeve so as to include an upstream part retained by said first magnetic pole, an intermediate part retained by an intermediate portion between said first and second magnetic poles and a downstream part retained by said second magnetic pole;
• wherein the upstream part and the intermediate part of the developer are brought into contact with said photosensitive member, while the downstream part of the developer is held out of contact with said photosensitive member;
• and
• wherein a spacing between the second magnetic pole and the downstream magnetic pole is such that the downstream part of the developer is magnetically drawn to the downstream magnetic pole, and the four poles are arranged such that the developer directly between the second magnetic pole and the photosensitive member is drawn toward the downstream magnetic pole and does not contact the photosensitive member.

An image forming apparatus including a developing device for developing an electrostatic latent image formed on a rotating photosensitive member by bringing developer into contact with said photosensitive member, said developing device comprising:

• a magnetic member which is nonrotatably fixed in a sleeve, said sleeve being rotated around said magnet member and retaining the developer on its outer periphery;
• said magnet member being provided with four sequential magnetic poles, said four poles including an upstream magnetic pole arranged upstream of a magnet portion confronting said photosensitive member, first and second magnetic poles of an identical polarity, said first magnetic pole being disposed adjacent to and upstream of said second magnetic pole in a rotational direction of said sleeve, said first and second magnetic poles provided at the portion confronting said photosensitive member, and a downstream magnetic pole arranged downstream of the confronting portion, said upstream and downstream magnetic poles having a polarity opposite to the polarity of the first and second magnetic poles;
• the developer being successively retained between said first and second magnetic poles and on an outer peripheral surface of said sleeve so as to include an upstream part retained by said first magnetic pole, an intermediate part retained by an intermediate portion between said first and second magnetic poles and a downstream part retained by said magnetic pole;
• wherein the upstream part and the intermediate part of the developer are brought into contact with said photosensitive member, while the downstream part of the developer is held out of contact with said photosensitive member;
• and
• wherein a spacing between the second magnetic pole and the downstream magnetic pole is such that the downstream part of the developer is magnetically drawn to the downstream magnetic pole, and the four poles are arranged such that the development of the electrostatic latent image is completed by the intermediate portion of the developer which is between the first and second magnetic poles.

An image forming apparatus including a developing device for developing an electrostatic latent image formed on a rotating photosensitive member by bringing developer into contact with said photosensitive member, said developing device comprising:

• a magnetic member which is nonrotatably fixed in a sleeve, said sleeve being rotated around said magnet member and retaining the developer on its outer periphery;
• said magnet member being provided with four sequential magnetic poles, said four poles including an upstream magnetic pole arranged upstream of a magnet portion confronting said photosensitive member, first and second magnetic poles of an identical polarity, said first magnetic pole being disposed adjacent to and upstream of said second magnetic pole in a rotational direction of said sleeve, said first and second magnetic poles provided at the portion confronting said photosensitive member, and a downstream magnetic pole arranged downstream of the confronting portion, said upstream and downstream magnetic poles having a polarity opposite to the polarity of the first and second magnetic poles;
• the developer being successively retained between said first and second magnetic poles and on an outer peripheral surface of said sleeve so as to include an upstream part retained by said first magnetic pole, an intermediate part retained by an intermediate portion between said first and second magnetic poles and a downstream part retained by said magnetic pole;
• wherein the upstream part and the intermediate part of the developer are brought into contact with said photosensitive member, while the downstream part of the developer is held out of contact with said photosensitive member;
• and
• wherein a spacing between the second magnetic pole and the downstream magnetic pole is such that the downstream part of the developer is magnetically drawn to the downstream magnetic pole, and the four poles are arranged such that the developer directly between the second magnetic pole and the photosensitive member is drawn toward the downstream magnetic pole and does not contact the photosensitive member.