Developing unit and image forming apparatus employing the same

An image forming apparatus, comprising:

• a photosensitive medium;
• a charging member which charges the photosensitive medium;
• an exposure unit which exposes the photosensitive medium and forms an electrostatic latent image;
• a developing unit to form a visible image corresponding to the electrostatic latent image on the photosensitive medium, the developing unit comprising:
• a housing to accommodate a developer,
• a magnetic cylinder having a catch pole to pick up the developer and a main pole to hold a carrier of the picked-up developer,
• a sleeve rotatably installed on a circumference of the magnetic cylinder to form a magnetic brush having a predetermined width on the circumference by the developer picked up by the catch pole, and
• a sealing member provided at opposite end parts in an axial direction of the sleeve to prevent a leak of the developer,
• wherein a width Ws of the sleeve, a magnetized width WM of the main pole, a magnetized width Wc of the catch pole, and a width WB of the magnetic brush satisfy a condition of WS>WM>WB>WC;
• a transfer unit which transfers an image formed in the developing unit to a printing medium;
• and
• a fusing unit which fuses the image transferred to the printing medium.

A developing roller usable with a developing unit of an image forming apparatus, the developing roller comprising:

• a magnetic cylinder comprising a plurality of magnetic poles to form a magnetic brush;
• and
• a sleeve rotatably disposed to surround the magnetic cylinder and to define a first area where the magnetic brush is formed and a second transfer area at end parts of the sleeve where the magnetic brush is not formed,
• wherein the plurality of magnetic poles have at least two different magnetic widths such that an amount of developer to form the magnetic brush is uniform along the first area.

A developing unit to develop a latent image on a photosensitive medium, the developing unit comprising:

• a housing to hold a developer;
• a developing roller, comprising:
• a magnetic cylinder having a catch pole to pick up the developer and a main pole to hold a carrier of the picked-up developer, and
• a sleeve rotatably installed on a circumference of the magnetic cylinder to form a magnetic brush having a predetermined width with the picked up developer;
• and
• a sealing member provided at opposite end parts of the sleeve to prevent a leak of the developer through the end parts,
• wherein a magnetized width WM of the main pole is greater than a magnetized width Wc of the catch pole, and a width WB of the magnetic brush corresponds to the magnetized width Wc of the catch poles such that an amount of developer on the magnetic brush is uniform.