1. Field of the Invention
The present invention relates to a magnetic powder transporting device, and more particularly to such device capable of preventing the magnetic powder scattering.
2. Description of the Prior Art
As an example of the magnetic powder transporting device there is already known a sleeve-type developing unit for powdered toner for use in the electrophotographic copier, for example disclosed in the U.S. Pat. No. 4,091,765. The abovementioned device is composed of a magnet roller provided on the peripheral surface thereof with magnets and a non-magnetic sleeve covering said peripheral surface, wherein said magnet roller and sleeve being maintained in mutual rotation to transport the powdered toner to a developing area where an electrostatic latent image on a photosensitive member is rendered visible. In said device the toner transportation is achieved either indirectly by iron powder carrier in case of two-component toner consisting of iron powder and toner particles, or directly by the toner particles in case of one-component toner consisting solely of toner particles having iron cores. Said toner transportation is conducted to replenish new toner to the developing area in which the toner is consumed.
The above-mentioned sleeve-type developing unit is advantageous in the significantly smaller weight and dimension in comparison with the cascade developing unit for example disclosed in the U.S. Pat. No. 2,573,881, but is associated with a drawback of causing toner scattering from the lateral ends of the developing area, thus severely soiling the interior of the copier. In the above-mentioned cascade developing unit, the toner particles transported to the developing area are subjected merely to gravity and not influenced by any external force towards the lateral ends of the developing area. In contrast, in the sleeve-type developing unit, the toner particles supported on the sleeve in spicate forms are spread, upon contact with the surface of the photosensitive member, over said surface, thus reaching the lateral ends of the developing area. Said toner spreading, functioning as a partial force toward the lateral ends of the developing area, naturally affects the toner particles present in said lateral end regions where the magnetic field ends, thereby releasing said toner particles from the magnetic field and causing the toner scattering.
The toner scattering is also caused by a fact that, although the magnetic field around the periphery of the sleeve is formed between the adjacent magnetic poles and is generally parallel to the plane perpendicular to the axis of the sleeve whereby the toner particles are scarcely subjected to axial force, the magnetic field at the lateral ends thereof is directed along the axis of the sleeve, thus causing the displacement of toner particles in said direction and facilitating the toner scattering.
In the conventional device as shown in FIGS. 1 and 2 respectively giving a partial view of the sleeve-type developing unit and a partial perspective view of the magnet roller, the toner particles T captured and transported on the surface of a sleeve 2 by a magnet roller 1 are shaped in almost vertical spicate forms over the approximately entire surface of said sleeve 2 until they come into contact with the surface of the photosensitive member 3, but said spicate forms are spread in lateral directions upon such contact. The magnetic field formed on the magnet roller 1 is mostly parallel to the plane perpendicular to the axis of the magnet roller as shown by the arrow A in FIGS. 1 and 2, but at the end portions of the roller there are formed magnetic fields in the axial direction as shown by the line B. (Although the magnetic field at the end portions is in fact complicated, there are only shown the lines in the axial direction for simplicity.) For this reason the toner particles T spread in the above-explained manner are subjected, at the end portions of the magnet roller 1, to the axial force along the magnetic field B, thus easily leaving the end faces of the roller and causing the toner scattering.