1. Field of the Invention
The present invention relates to a developing apparatus for forming an image in an electrophotographic method. More specifically, the present invention relates to a developing apparatus for developing, by using a developer composed of toner and carrier, an electrostatic latent image formed on an image bearing body of a so-called electrophotographic image forming apparatus such as an electrostatic copying machine or a laser beam printer, as well as to an image forming apparatus provided with the developing apparatus.
2. Description of the Related Art
In heretofore conventional image forming apparatuses such as copying machines or printers, development of an electrostatic latent image formed on a photosensitive body acting as an image bearing body is effected with use of toner supplied from a developing apparatus. As typical developing apparatuses designed for use in such image forming apparatuses, there are known a mono-component developer development type of developing apparatus and a dual-component developer development type of developing apparatus. The former employs a developer composed solely of toner, whereas the latter employs a developer composed of toner and carrier.
FIGS. 9 and 10 show an example of a developing apparatus of the type that employs a dual-component developer containing toner and carrier for development.
Broadly speaking, this developing apparatus 100 is composed of: a developer reservoir for storing therein a developer 2 containing toner and carrier; a developing roller 110 arranged face to face with a photoreceptor (photoconductor drum) 1 acting as an image bearing body; and a first agitating/conveying member 150 and a second agitating/conveying member 160 for conveying the developer with agitation, which are arranged side by side on the side opposite from the photoconductor drum 1 and the developing roller 110 arranged in a face-to-face manner.
The developing roller 110 is composed of a rotatably-driven developing sleeve 110a formed of a cylindrically shaped non-magnetic member and a magnet member 110b disposed inside the developing sleeve 110a. The magnet member 110b has a plurality of magnetic poles N, S, . . . . The developing sleeve 110a constituting the developing roller 110 magnetically attracts the developer at a surface of the developing sleeve 110a under a magnetic force of the magnet member 110b. Upon driving the developing sleeve 110a to rotate in an arrow-indicated direction, the developer thus attracted thereto is conveyed toward a development area where the developing roller 110 and the photoconductor drum 1 confront each other. Following the completion of a development process, the developer is directed into the developer reservoir. The developer 2 is caused to stand magnetically in a spicate or ear-like form at the N pole of the magnet member 110b, thereby forming a so-called magnetic brush. The developer rising in the form of magnetic brush is slidingly rubbed against the surface of the photosensitive body 1, so that an electrostatic latent image formed on the photoreceptor 1 is developed by dint of toner.
As the developing sleeve 110a is rotated, the developer is conveyed toward the development area opposed to the photoconductor drum 1. In the middle of the conveyance, the amount of the developer is regulated to a predetermined level by a regulating member 170 disposed in the developing apparatus with its front end facing the surface of the developing sleeve 110a. That is, the regulating member 170 serves to make substantially uniform the amount of the developer to be conveyed toward the development area opposed to the photoconductor drum 1.
Moreover, the developer reservoir has an upright partition wall 120 formed therein for dividing the interior region in which are disposed the first and second agitating/conveying members 150 and 160 into two sections: a first developer conveyance section 130 and a second developer conveyance section 140. That is, the first agitating/conveying member 150 and the second agitating/conveying member 160 are rotatably driven in the first developer conveyance section 130 and the second developer conveyance section 140, respectively.
The first and second agitating/conveying members 150, 160 are each composed of a rotary shaft 150a, 160a and a plurality of elliptic bladed members 150b, 160b spacedly arranged about the rotary shaft 150a, 160a. The bladed members 150b, 160b are each inclined relative to the rotary shaft 150a, 160a. Moreover, as shown in FIG. 10, the partition wall 120 has its lengthwisely opposite ends cut out to create opening portions 120a and 120b for allowing the passage of the developer in the arrow-indicated directions. Correspondingly, the first and second agitating/conveying members 150, 160 are provided with a bladed member 150c, 160c facing the opening portion 120a, 120b at one end thereof, respectively, so that the bladed member 150c, 160c inclines in the opposite direction to the bladed members 150b, 160b in order to allow the developer 2 to travel in the arrow-indicated directions smoothly.
In the developing apparatus 100 thus far described, as the first and second agitating/conveying members 150 and 160 are rotated in the first and second developer conveyance sections 130 and 140, respectively, the developer 2 is conveyed in these sections 130 and 140 while being agitated by the bladed members 150b and 160b of the first and second agitating/conveying members 150 and 160 in a manner such that the toner and carrier contained therein are admixed thoroughly. Note that the developer conveyance direction set for the first developer conveyance section 130 is opposite to that set for the second developer conveyance section 140. At this time, at both ends of the interior region of the developing apparatus 100, the developer 2 is transferred to and fro in circulation between the first developer conveyance section 130 and the second developer conveyance section 140 through the opening portions 120a and 120b formed on both sides of the partition wall 120. During the developer circulation, in the first developer conveyance section 130, the developer 2 is supplied to the developing roller 110 arranged face to face with the first developer conveyance section 130.
Then, as the developing roller 110 is rotated, as has already been explained, the developer 2 attracted thereto is conveyed toward the development area opposed to the photoconductor drum 1, and, in the middle of the conveyance, the amount of the developer 2 is regulated by the regulating member 170. The toner contained in the developer 2 is used to develop an electrostatic latent image formed on the photoconductor drum 1. In the course of repeated development steps, the concentration of toner contained in the developer 2 is gradually reduced through consumption. In order to give heed to the concentration of toner, for example, a toner concentration sensor is disposed in the apparatus. In response to a detection result, fresh toner is added to the developer 2 on an as needed basis.
Disclosed in Japanese Unexamined Patent Publication JP-A 10-319721 (1998) is a developing apparatus having a mechanism for facilitating the mixing of toner and carrier. In this construction, a developer is circulated by two juxtaposed screws that convey the developer in opposite directions. Toner is added to the developer in a circulating state, whereupon the fresh toner and the carrier contained in the developer are blended with each other. Moreover, at least one of the two screws is provided with a circulating portion for effecting circulation of the carrier and a projection for receiving the toner and conveying it toward the circulating portion. The replenished toner is conveyed by the projection in a direction toward the circulating portion wherein the toner is blended with the carrier contained in the developer. At this time, being conveyed by the projection so as to find its way into the lower part of the circulating developer, the toner can be blended with the carrier contained in the developer satisfactorily.
According to the invention disclosed in JP-A 10-319721, toner is added directly to the developer in a circulating state after a development process. Therefore, if the toner has not been agitated properly until added to the developer, inconveniently, the developer with poorly-charged toner will be supplied to the developing sleeve from the agitating/conveying portion. As a result, the poorly-charged toner is attached to a region other than a image-forming region, thus causing a so-called fogging phenomenon, or the toner is scattered out of the developing apparatus, thus causing a smear in the interior of the image forming apparatus.
In addition, in the invention disclosed in JP-A 10-319721, toner is conveyed to the development area on an as-is basis. It is thus likely that the toner is mixed and dispersed in the developer in an insufficient manner, thus causing lack of uniformity in toner concentration. This gives rise to a problem of uneven density in a resultant image.