Electrophotographic Printing Apparatus With Repellent Bands on the Photoconductive Member and the Developing Roller to Prevent Buildup of Toner Therebetween

The present invention relates to an electrophotographic printing apparatus equipped with a liquid development unit for developing a latent electrostatic image, formed on a surface of a photoconductive member, with liquid developer.

A conventional electrophotographic printing apparatus is disclosed in U.S. Pat. No. 4,268,597, which issued to Klavan et al. on May 19, 1981. The printing apparatus includes a liquid development unit which has a container for storing liquid developer and a developer roller rotatably provided and partially submerged in the liquid developer. The developer roller is located opposite a photoconductive member such that there is a small gap between opposing surfaces of the developer roller and photoconductive member.

In a developing operation, as the developer roller rotates, a film of the liquid developer is formed on the surface of the developer roller at a position close to the photoconductive member. The liquid developer film is partially attracted and then adheres to an electrostatic image which has been formed on the surface of the photoconductive member by electrostatic force.

In such a development unit, it is important to make the gap between the developer roller and photoconductive member small and constant in order to develop a fine electrostatic image. To this end, it is effective to provide a pair of spacer members between the developer roller and photoconductive member; the Klavan et al. patent discloses, in FIGS. 29 and 30, spacer annuli provided on both ends of the photoconductive member.

The conventional printing apparatus has, however, a disadvantage in that the liquid developer stagnates between the developer roller and the photoconductive member initially at the portions where the spacer members are provided due to the surface tension of the liquid developer. As a result, the gap tends to be stopped up with the liquid developer, causing the surface of the photoconductive member to be improperly developed. More specifically, the liquid developer adheres to the area of the photoconductive member's surface where the electrostatic image is not formed.

Therefore, an object of the present invention is to provide an electrophotographic printing apparatus capable of developing a latent electrostatic image with liquid developer, with high accuracy.

Another object of the present invention is to provide an electrophotographic printing apparatus capable of preventing the liquid developer from stopping up the spacing between a photoconductive member and a developer roller, even if spacer members are provided therebetween.

An electrophotographic printing apparatus according to the present invention comprises a photoconductive member including a surface having an image forming area and a non-image forming area means for forming an electrostatic image on the image forming area of the photoconductive member; a developer roller for developing the electrostatic image with a liquid developer, the developer roller including a surface facing the surface of the photoconductive member, the developer roller surface having a developing area facing said image forming area and a non-developing area facing the non-image forming area; a spacer member for determining a gap between the surface of the photoconductive member and the surface of the developer roller, the spacer member being in contact with both the non-image forming area of the photoconductive member and the non-developing area of the developer roller; and a repellent member provided on at least one of the surface of the photoconductive member, at a position between the image forming area and the portion in contact with the spacer member in the non-image forming area, and the surface of the developer roller, at a position between the developing area and the portion in contact with the spacer member in the non-developing area.

FIG. 1 shows schematically an embodiment of the present invention.

FIG. 2 is a perspective view showing a development unit used in the embodiment of the invention shown in FIG. 1.

FIG. 3 is a perspective view showing a photoconductive drum used in the embodiment of the invention shown in FIG. 1.

FIG. 4 is a cross-sectional view taken along a line IV--IV of FIG. 1 showing an operation of the embodiment of the invention.

Referring to FIG. 1, an electrophotographic printing apparatus includes a photoconductive drum 1, as a photoconductive member which, as shown in FIG. 1, rotates in a counterclockwise direction around its rotational axis. In the order of the electrophotographic printing process, a charging unit 20, an exposure unit 21, a development unit 2, a transfer unit 22, a cleaning unit 24, and an erasing unit 25 are provided around the photoconductive drum 1, as is well known in the art.

The development unit 2 includes a container 3 for storing liquid developer (toner) 5 and a developer roller 4 located parallel to the photoconductive drum 1 and partially submerged in the liquid developer 5. The developer roller 4 rotates in a clockwise direction, as show in FIG. 1, around its axis. A doctor blade 6 is provided for scratching off excess liquid developer which adhered to a surface of the developer roller 4. With the rotation of the developer roller 4, a film of the liquid developer is formed on the surface of the developer roller 4 at a position opposite to the photoconductive drum 1. In this embodiment, a hollow sleeve is used as the developer roller 4.

In operation, the charging unit 20 uniformly charges a photoconductive surface of the drum 1. Next, the exposure unit 21 exposes a light beam L to the photoconductive surface of the drum 1 to form an electrostatic image which corresponds an image to be printed. Then the development unit 2 develops the electrostatic image with the liquid developer 5 which is attracted toward the photoconductive drum 1 by the electrostatic force of the electrostatic image, so that droplets of the liquid developer 5 adhere to the electrostatic image on the photoconductive drum surface. Thus, the electrostatic image becomes a visible image.

The transfer unit 22 transfers the visible image formed on the drum surface to a recording sheet 23, which is driven to a position between the photoconductive drum 1 and transfer unit 22. After that, the visible image transferred to the sheet 23 is fixed to the sheet 23 by applying heat and pressure, as is well known in the art. Then, the cleaning unit 24 cleans the untransferred liquid developer on the surface of the drum 1. Finally, the erasing unit 25 discharges the surface of the drum 1 so as to erase the electrostatic image formed thereon.

Referring to FIG. 2, the development unit 2 has a pair of spacer films 7A and 7B, as spacer members, at both axially ends of the developer roller 4 for determining a spacing between the photoconductive drum 1 and the developer roller 4. Both ends of each of the spacer films 7A and 7B are fixed to an upper surface of the container 3 so that an intermediate portion of each of the spacer films 7A and 7B is in contact with the surface of the developer roller 4. The developer roller 4 has a cylindrical developing area 4D at its axially central portion, the developing area 4D facing a cylindrical image forming area 1D (FIG. 3) (used to form the electrostatic image) of the photoconductive drum 1. The area other than the developing area 4D is referred to as a non-developing area 4N.

A pair of repellent bands 4A and 4B are provided on a circumferential surface of the developer roller 4 at portions between the developing area 4D and each of the axially located end portions contacting the spacer films 7A and 7B. The repellent bands 4A and 4B are formed by coating the surface of the developer roller 4 with repellent material, such as fluorine resin.

Referring to FIG. 3, the photoconductive drum 1 also has a pair of repellent bands 1A and 1B provided on its circumferential surface at positions facing to the repellent bands 4A and 4B of the developer roller 4 when the development unit 2 and photoconductive drum 1 are installed as shown in FIG. 1. The repellent bands 1A and 1B are formed by coating the surface of the photoconductive drum 1 with repellent material, such as fluorine resin, similarly to the repellent bands 4A and 4B. As is apparent from FIG. 3, the image forming area 1D of the drum 1 is located between the repellent bands 1A and 1B. The area other than the image forming area 1D is referred to as a non-image forming area 1N.

Referring to FIG. 4, the photoconductive drum 1 and development unit 2 are assembled such that the surfaces of the drum 1 and developer roller 4 have a small gap G produced by means of the spacer films 7A and 7B. To stabilize the gap G, spring means (not shown) is provided to urge the drum 1 and roller 4 toward each other.

As the developer roller 4 rotates, the liquid developer film 5D is formed around the developer roller 4 so as to develop the electrostatic image formed on the image forming area 1D of the photoconductive drum 1 in the manner described in U.S. Pat. No. 4,268,597. In this developing operation, liquid developer 5A and 5B tend to stagnate between the surfaces of the drum 1 and roller 4 at the positions where the spacer films 7A and 7B are provided due to the surface tension of the developer 5A and 5B. The repellent bands 1A, 1B, 4A and 4B prevent the liquid developer 5A and 5B from growing and thereby stop up the gap spacing G between the surfaces of the image forming area 1D and developing area 4D. Therefore, the clearance between the surface of the image forming area 1A and the liquid developer film 5D is kept constant so that the liquid developer 5 adheres only to the portions where the electrostatic image is formed in the image forming area 1D.

In this embodiment, the repellent bands 1A, 1B, 4A and 4B are provided on both the developer roller 4 and photoconductive drum 1. However, similar advantage can be obtained even if repellent bands are provided on only the developer roller 4 or the photoconductive drum 1.

As described above, the present invention makes it possible to prevent the liquid developer from stopping up the gap between the photoconductive member and developer roller in the image forming area. As a result, high quality developing can be obtained.