1. Field of the Invention
This invention relates to an electrophotographic printing apparatus capable of printing images by electrophotographic processing and a method of starting up this apparatus.
2. Description of the Related Art
Electrophotographic printing apparatuses print an image by processing a single photosensitive object primarily through charging, exposure, development, and transfer, which constitute an important part of electrophotographic processing.
Electrophotographic printing apparatuses generally use a photosensitive drum as a photosensitive object. Along the periphery of this photosensitive drum, a charging device, an exposure device, a development device, and a transfer device are arranged sequentially to complete the printing apparatus.
In the electrophotographic printing apparatus with this arrangement, the photosensitive drum undergoes a particular process, passing under each device, with the result that it is subjected to a series of electrophotographic processes during a drum revolution. Specifically, the charging device charges the surface of the photosensitive drum (the photosensitive surface) to, for example, -500 V. Then, the exposure device exposes the charged photosensitive surface according to an image to be printed. As a result of this, an electrostatic latent image is formed on the photosensitive surface of the drum. The development device develops this electrostatic image as follows, for example. A cylindrical development sleeve holds charged toner of the same polarity as that of the charged potential of the photosensitive drum, and permits the toner to come into contact with the photosensitive surface of the drum as it rotates. Here, by applying a development bias to the development sleeve, the bias being of the same polarity as that of the charged potential of the photosensitive drum and lower in voltage (for example, -200 V), the interaction between the electrostatic image and the development bias allows the toner to selectively adhere to the photosensitive drum. That is, toner does not adhere to the charged portions on the drum because the drum is at a higher potential, whereas it adheres to the exposure-discharged portions because the drum is at a lower potential. As a result, a toner image is formed on the photosensitive surface of the drum.
Then, printing paper is placed on the photosensitive surface of the drum. The transfer device supplies to the printing paper charges of the opposite polarity to that of the toner (for example, +500 V). As a consequence, the toner attaching to the photosensitive surface of the drum adheres to the printing paper, causing the toner image to be transferred to the paper. After this, the cleaning device removes the residual toner from the surface of the photosensitive drum.
When the electrophotographic printing apparatus thus constructed is not in operation, the potential of the surface of the photosensitive drum is indefinite. With the photosensitive drum at an indefinite potential, when the drum passes under the development device at the start of printing, toner adheres to the portions of lower voltages. To avoid this, the charging device is caused to start charging at the same time that the photosensitive drum starts rotating in order to minimize indefinite potential portions on the photosensitive surface of the drum.
However, when the drum is not in motion, the area between the charging position under the charging device and the development position under the development device cannot be charged, permitting toner to adhere to the area. The toner thus adhering to the photosensitive drum is collected by the cleaning device as waste toner, resulting in a waste of toner. In the transfer device that has a portion making contact with the photosensitive drum, such as a roller or brush, the toner first adhering to the drum adheres to the transfer device. The toner attaching to the transfer device then adheres to the back of the printing paper as the paper passes through between the photosensitive drum and the transfer device, smearing the back of the paper. Further, changes in the electric characteristics of the roller and brush lead to decreased transfer efficiency, causing poor transfer.