1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus, and in particular, to an engine unit capable of transferring a toner image to a recording sheet without corona discharge by adjusting a quantity of light radiated from pre-transfer lamps.
2. Description of the Related Art
An electrophotographic image forming apparatus includes an image forming unit for forming a toner image and a transfer unit for transferring the toner image to a recording sheet fed from a sheet feeding cassette in a main body of the apparatus. Specifically, the image forming unit includes an organic photoconductive (OPC) drum on which a toner image is formed, and the toner image formed on the OPC drum is transferred to the recording sheet by the transfer unit. The recording sheet with the transferred toner image passes through a fixing unit of the main body for fixing the toner image to the sheet, and then is discharged from the main body by a sheet discharging unit.
An earlier engine unit for an electrophotographic image forming apparatus has a developing unit mounted on a specified place in a main body of the image forming apparatus, and the developing unit has an OPC drum, a part of which protrudes out at the bottom of the developing unit. Further, in a sheet path, a transfer roller is mounted with an upper end thereof being protruded, so that the transfer roller comes in contact with the OPC drum, maintaining a constant pressure between the transfer roller and the OPC drum.
Further, a charge roller for uniformly charging a surface of the OPC drum with a high voltage and a developing roller for supplying a toner onto the OPC drum are rotatably fixed such that they come into contact with the OPC drum. A supply roller for supplying the toner provided from a toner container of the developing unit to the developing roller by contact charging is rotatably fixed such that it contacts with the developing roller. An exposure unit for forming an electrostatic latent image on the OPC drum is disposed at a specified place between the charge roller and the developing roller.
At the entrance to the transfer roller, a printed circuit board (PCB) including a plurality of, e.g., 16 pre-transfer lamps (PTLs) is mounted on a body frame under the sheet path. The PTLs are protected by a transparent plastic cover. Being placed in the sheet path, the cover guides a recording sheet being conveyed to the OPC drum side. Before the transfer roller transfers the toner image on the OPC drum to the recording sheet, the PTLs expose the OPC drum to decrease a surface potential of the OPC drum, thereby increasing a transfer efficiency of the transfer roller. Further, a corona discharge unit prevents a spreading phenomenon of the toner attached to the OPC drum, due to the decrease in the surface potential of the OPC drum by the PTLs.
In operation, the charge roller forms a uniform electric charge on the OPC drum. When the OPC drum is exposed to a light beam radiated from the exposure unit, an electrostatic latent image is formed on the OPC drum. Here, the OPC drum has a potential of -800V at a non-image area where the electrostatic latent image is not formed and a potential of -50V at an image area where the electrostatic latent image is formed. The developing roller, being provided with a developing voltage of -300V to -400V, rotates while keeping in contact with the OPC drum, to attach the toner supplied from the supply roller to the electrostatic latent image on the OPC drum. A surface potential of the developing roller provided with the developing voltage of -300V to -400V cancels the potential of -800V of the non-image area on the OPC drum with the passage of time, so that the potential on the non-image area of the OPC drum is in the range between -650V and -700V at that position of the drum's rotation. In this condition, when the recording sheet reaches the transfer roller, a transfer voltage of 1000V is provided to the transfer roller to transfer the toner attached to the OPC drum to the recording sheet. However, the toner may remain untransferred at the border between the image area and the non-image area on the OPC drum. To solve this problem, the PTLs radiate the light beam to the non-image area (having the potential of -650V to -700V) of the OPC drum. Then, the potential of the non-image area becomes similar to the potential of the image area, resulting in a spreading phenomenon of the toner. To prevent the spreading phenomenon of the toner, the corona discharge unit applies a potential of 4000V to the OPC drum. Here, for the corona discharge unit, the engine unit should have a power supply for generating a high voltage for the corona discharge, which increases the size and the cost of the apparatus.
Examples of engine units for image-forming apparatus of the conventional art are seen in the following U.S. Patents. U.S. Pat. No. 4,506,971, to Buell et al., entitled Transfer System, describes an electrophotographic system where the charged photoreceptor is exposed to light to substantially discharge the background around the image. This device employs a corona discharge unit, which as noted, increases the size and cost.
U.S. Pat. No. 4,538,901, to Soumiya, entitled Electrophotographic Copier With a Phantom Image Suppression Function, describes an electrophotographic device with a pre-transfer exposure lamp and a post-transfer exposure lamp, as well as a corona transfer unit and a corona separating device. In this device, the ratio of light from the two lamps is adjusted to be within a certain range.
U.S. Pat. No. 4,978,998, to Aizawa, entitled Image Transferring Device For Image Forming Equipment, describes an electrophotographic device with a pre-transfer lamp and a transfer charger, in which the pre-transfer lamp may be turned off during the trailing edge of the document, to avoid is loss of image in that portion of the printed document. This device also uses a transfer charger.
U.S. Pat. No. 5,038,177, to Parker et al., entitled Selective Pre-Transfer Corona Transfer With Light Treatment For Tri-Level Xerography, describes an electrophotographic device in which a pre-transfer charge corona is used in conjunction with a pre-transfer lamp.
Based on my observation of the art, then, I have found that what is needed is an image forming apparatus which does not employ a corona discharge unit, but which will prevent the spreading of toner due to the decreased surface potential of the OPC drum due to the pre-transfer lamps.