Field of the Invention
The present invention relates to a development device used in an electrophotographic recording system with use of magnetic toner.
Prior dry-type development devices are disclosed in U.S. Pat. Nos. 4,267,248, 4,297,970, and 4,309,498. Describing these development devices with reference to FIG. 2, an electrostatic latent image carrier 1 (hereinafter referred to as a photosensitive drum) has a development roll 2 provided in a confronting relation thereto. The development roll 2 comprises a nonmagnetic cylindrical sleeve 3 and a magnet roll 4 having a plurality of magnetic poles which axially extend and have polarities alternately different from each other in the circumferential direction. The magnet roll 4 is provided in the sleeve 3, and both are relatively rotatable. A developer tank 5 is provided around the development roll 2, and a development region P formed between the photosensitive drum 1 and the development roll 2 both arranged in a confronting relation keeping a slight space therebetween.
In such a development device, a magnetic toner developer 6 is attracted onto the circumferential surface of the sleeve 3 by magnetic force of the magnet roll 4 in the developer tank 5. The sleeve 3 and the magnet roll 4 are rotated in the direction of the arrows A and B respectively at different speeds of revolutions, whereby toner chains are formed around the development roll 2 and carried from an outlet 7 provided on one end of the development roll 2 located upward of the circumference thereof to the development region P.
In the development region P, the toner chains are frictionally brought into contact with the electrostatic latent image formed on the photosensitive drum 1 in rotation in the direction of the arrow E, whereby part of the developer 6 electrostatically adheres onto the photosensitive drum 1, carried to a transfer region (not shown), and transferred onto a paper sheet. By contrast, remaining developer 6 which does not adhere onto the photosensitive drum 1 is again retrieved from an inlet 8 provided on the other end of downward the development roll 2 downward the circumference thereof. Here, broken lines shown in FIG. 2 schematically show lines of magnetic force formed by the magnet roll 4.
However, in such prior development devices, part of the developer adhered onto the photosensitive drum due to electrostatic force is subjected to larger magnetic force than the attraction force due to the above electrostatic force by a moving magnetic field produced by the rotation of the magnet roll prior to the transfer of the latent image onto the paper sheet. As a result, this part of the developer is moved onto the photosensitive drum or falls off downward of the photosensitive drum and drops on a printing paper sheet carried to the transfer region. The former deteriorates the picture quality formed on the printing surface while the latter brings about background stains on the printing surface. Namely, both exert bad influences upon the picture quality.
For resolving these problems, a method is provided wherein the developer tank is formed with a magnetic substance and thereby the moving magnetic field does not exert a magnetic influence upon the photosensitive drum. However, this causes scattering of the developer. The scattering phenomenon will be described with reference to FIGS. 3(a), 3(b) and 3(c). The figures are schematics illustrating relations between the lines of magnetic force in the vicinity of the development roll and the developer. Therefore, the same numbers shall be applied to the same portions as the prior examples, omitting the description therefor.
First, as shown in FIG. 3(a), the magnetic flux density is concentrated on an edge part 9a of the magnetic developer tank 9 due to the magnetic force from the magnet roll 4, so that the developer on the development roll 2 is captured near the concentrated region. The top end of the captured developer 6 located on the side of the development roll 2 is magnetized to an N magnetic pole in the vicinity of the edge part 9a of the developer tank 9, while the other end of the developer 6 on the developer tank edge part is magnetized to an S magnetic pole, as shown in FIG. 3(a).
Given a state shown in FIG. 3(b) due to rotation of the magnet roll 4 in the direction of the arrow B, the top end of the developer 6 is moved in the direction of the arrow R by movement of the lines of magnetic force due to positional movements of the N and S poles. In addition, when the N and S poles reach positions shown in FIG. 3(c) due to the rotation of the magnet roll 4, a polarity of the edge 9a of the developer tank 9 is changed to the same S magnetic pole as the polarity of the top end of the developer 6 located on the side of the developer tank 9. Therefore, the developer 6 is scattered in the arrow T direction as shown in FIG. 3(c).
Further, in FIGS. 3(a), 3(b), 3(c), even if the N and S poles are assumed to be reversed, the same situation as that shown in FIGS. 3(a), 3(b), and 3(c) is produced and the developer 6 is likewise scattered. Accordingly, the magnetic roll 4 is continuously rotated, whereby part of the developer 6 carried on the development roll 2 is continuously scattered. As a result, a problem is produced in which a surface of a printing sheet (not shown) is stained with the developer 6, and thereby the printing quality is sharply deteriorated.