1. Field of the Invention
The present invention relates to a developing device using a dual-component developer and an image forming apparatus including the same.
2. Description of the Background Art
In recent years, for electrophotographic image forming apparatuses supporting full-color and high-quality images, a dual-component developer (hereinafter simply referred to as the “developer”) which exhibits an excellent charge performance stability as to a toner is in widespread use.
The developer is made up of the toner and a carrier, which are agitated in a developing device and frictionally rubbed with each other to produce an appropriately electrified toner.
In the developing device, the electrified toner is supplied onto a surface of a developing roller. The toner is moved by an electrostatic attraction from the developing roller to an electrostatic latent image formed on a surface of a photoconductor drum.
Thus, a toner image based on the electrostatic latent image is formed on the photoconductor drum.
Further, there has been an increasing demand for the image forming apparatuses that operate faster and that are miniaturized, which is associated with the necessity to electrify the developer quickly and sufficiently and to convey the developer quickly.
To this end, Prior Art 1 proposes a circulative developing device including first and second developer conveying passages divided by a partitioning plate provided in a developer vessel, first and second communicating paths that allow the first developer conveying passage and the second developer conveying passage to communicate with each other at opposite ends, and first and second auger screws that are arranged in the first and second developer conveying passages to convey a developer in directions opposite to each other (for example, see Japanese Unexamined Patent Publication No. 2001-255723).
In the developing device described above, the developer in the first developer conveying passage is conveyed by the first auger screw to an interior wall of the developer vessel located at the most downstream side of the first developer conveying passage, and is pushed toward the first communicating path due to a pressure from the developer conveyed from the upstream side, thereby transferring to the second developer conveying passage.
On the other hand, the developer in the second developer conveying passage is conveyed by the second auger screw to the interior wall of the developer vessel at the downstream side of the second developer conveying passage, and is pushed toward the second communicating path due to a pressure of the developer conveyed from the upstream side, thereby transferring to the first developer conveying passage.
In this manner, the developer is circulated between the first developer conveying passage and the second developer conveying passage.
Prior Art 2 proposes a circulative developing device same as that in the Prior Art 1 except that, on helical blades of first and second auger screws, helical directions of helical blades at an end of the downstream side in a conveying direction of a developer are opposite (for example, see Japanese Unexamined Patent Publication No. 2009-109741).
In the developing device described above, the developer in the first developer conveying passage is conveyed to the helical blade, having the reverse helical direction, of the first auger screw, and then, pushed toward the first communicating path due to a pressure caused between the conveyed developer and a developer conveyed from the upstream side of the first developer conveying passage, thereby transferring to the second developer conveying passage.
On the other hand, the developer in the second developer conveying passage is conveyed to the helical blade, having the reverse helical direction, of the second auger screw, and then, pushed toward the second communicating path due to a pressure caused between the conveyed developer and a developer conveyed from the upstream side of the second developer conveying passage, thereby transferring to the first developer conveying passage.
In this manner, the developer is circulated between the first developer conveying passage and the second developer conveying passage.
However, in the developing devices described in Prior Art 1 and Prior Art 2, the developer receives a violent pressure in an advancing direction, and receives a shear force in such a pressurized state, at the position, facing the communicating path, at the end of the downstream side of the auger screw.
Due to a generation of heat and the shear force caused by a stress, a flow improver, which is an external additive of a toner, is unfavorably be embedded into a particle of a resin constituting the toner, which might cause an extreme deterioration in a flow property of the developer, and hence, a phenomenon in which the conveyance of the developer becomes difficult might be produced.
Consequently, it becomes difficult to supply the developer in a sufficient amount to a photoconductor drum through a developing roller, which might cause a problem that a density of an image printed on a recording medium is reduced.