1. Field of the Invention
The present invention relates to a developing device including a regulating member for regulating the amount of binary developer to be fed to a developing area, and also relates to an image forming apparatus including the developing device.
2. Description of Related Art
Conventionally, such a developing device as mentioned above is widely used in electrophotographic image forming apparatuses. As shown in FIG. 26, to form a toner image by developing an electrostatic latent image formed on an electrostatic image support 80 such as a photoreceptor drum, for example, a conventional developing device 8 includes a housing 81, a stirring screw 82, a magnet assembly 83, a developing roller 84, and a regulating member 85.
The housing 81 is a casing of the developing device 8, and is fixed to, for example, the frame (not shown) of an image forming apparatus. Moreover, the housing 81 stores a binary developer (simply referred to below as a “developer”) D including toner and magnetic carrier. The stirring screw 82 stirs the developer D in the housing 81, and feeds the developer D to a supply space created in the housing 81.
The magnet assembly 83 is disposed near the electrostatic image support 80, and fixed to the housing 81, for example. Moreover, the magnet assembly 83 is in the shape of, for example, a column or a cylinder, and has a plurality of magnetic poles in its circumferential direction. In the example shown in the figure, a catch pole S2, a regulating pole N2, a feeding pole S1, a developing pole N1, and a separating pole S3 are provided as the magnetic poles. Note that each magnetic pole will be described in detail later.
The developing roller 84 is a typical example of a developer support. The developing roller 84 is in the form of a sleeve, and is rotatable along the outer circumferential surface of the magnet assembly 83. Note that in the example shown in the figure, the developing roller rotates clockwise as indicated by arrow CW. Moreover, the developing roller 84 is disposed near the electrostatic image support 80. Hereinafter, an area where the developing roller 84 and the electrostatic image support 80 face each other at a close distance will be referred to as a “developing area Da”.
The regulating member 85 is disposed in a position at a predetermined distance from the developing area Da counterclockwise along the outer circumferential surface of the developing roller 84, so as to face the developing roller 84 with a predetermined gap (clearance) from the outer circumferential surface.
Next, the magnetic poles of the magnet assembly 83 will be described in detail. The catch pole S2 is disposed so as to be opposed to the supply space of the developer D in the housing 81. Hereinafter, the position of the catch pole S2 will be considered as the most upstream of the path to feed the developer D. The regulating pole N2 is disposed immediately downstream of the catch pole S2, in a position opposed to the regulating member 85. The feeding pole S1 is disposed immediately downstream of the regulating pole N2 between the regulating pole N2 and the developing pole N1. The developing pole N1 is disposed immediately downstream of the feeding pole S1, in a position opposed to the developing area Da. The separating pole S3 is disposed between the developing pole N1 and the catch pole S2, and creates a repelling magnetic field therebetween, thereby creating a low magnetic area LM with a magnetic flux density of, for example, 5 mT or less.
In the developing device 8 thus configured, the developer D is fed in the following manner. First, the stirring screw 82 rotates in the housing 81, thereby frictionally charging the carrier and the toner in the developer D, so that the carrier and the toner electrostatically adhere to each other. Thereafter, the developer D is attracted (supplied) from the supply space in the housing 81 onto the outer circumferential surface of the developing roller 84 by magnetic force of the catch pole S2. Rotation of the developing roller 84 causes the developer D supported on the outer circumferential surface to eventually reach the regulating member 85, so that only the developer D that has passed through the clearance is fed downstream. In this manner, the regulating member 85 regulates the amount of developer D to be fed. Subsequently, the developer D reaches the developing area Da, and is used for developing an electrostatic latent image formed on the electrostatic image support 80, thereby forming a toner image.
Furthermore, the developer D not used in the developing area Da remains attracted onto the developing roller 84, and is fed further downstream. Thereafter, in the low magnetic area LM, the developer D falls from the developing roller 84 into the housing 81.
Note that to feed the developer D through rotation of the developing roller 84, a certain frictional force is required between the developer D and the developing roller 84. The frictional force is expressed by the product of a normal force and a frictional coefficient at the contact interface of the developer D and the developing roller 84. Here, the normal force is mainly a component of a magnetic force based on a magnetic field from the magnet assembly 83, and the component is oriented in the radial direction of the developing roller 84. For example, the distribution of magnetic flux densities for obtaining such a normal force ranges from about tens to hundreds of mT on the outer circumferential surface of the developing roller 84 having the magnet assembly 83 provided therein.
Incidentally, the amount of the developer D that is supplied to the developing roller 84 fluctuates mainly in accordance with a change in the volume of developer D in the housing 81 and/or rotation of the stirring screw 82. However, in the developing device 8, the amount of developer D to be supplied from the housing 81 can be slightly increased and can be regulated under a certain level or more of pressure by the regulating member 85. As a result, a uniform layer of developer D can be formed on the outer circumferential surface of the developing roller 84 regardless of fluctuations in the amount of the developer D that is supplied.
On the other hand, high pressure applied by the regulating member 85 results in stress on the developer D. For example, magnetic force causes the developer D regulated by the regulating member 85 to accumulate immediately before the regulating member 85, as indicated by broken line BL in FIG. 26. Friction and suchlike cause stress on the accumulated developer D. Such a developer D deteriorates over long-term use, and therefore it is necessary for the pressure by the regulating member 85 to be kept appropriately low.
To inhibit accumulation of the developer D, the space immediately before the regulating member 85 is conceivably narrowed as in a developing device 8′ of FIG. 27. As a result, the amount of the developer D that receives stress immediately before the regulating member 85 decreases, but the developer D supplied from the housing 81 to the developing roller 84 is forced in a narrow space until it passes the regulating member 85, so that particles included in the developer D receive high stress. Moreover, stress on the developer D applied by the regulating member 85 might lead to an increase in torque of the developing roller 84 and deterioration of the developer D.
In view of the above problems, Japanese Patent Laid-Open Publication No. 2008-15197 (FIG. 1) describes a developing device including a slip control member in addition to a regulating member. The slip control member is positioned upstream of the regulating member at a predetermined distance along the outer circumferential surface of a developing roller. As a result, pressure is released before the regulating member, thereby inhibiting stress applied by the regulating member.
However, in the configuration of Japanese Patent Laid-Open Publication No. 2008-15197, the developer is accumulated at the upstream end of the slip control member, so that stress is still applied to the developer.