1. Field of the Invention
This invention relates to an image forming apparatus, and more particularly to an apparatus that forms a full-color image on a recording sheet utilizing an electrophotographic technique.
2. Description of the Related Arts
Conventionally, as disclosed in U.S. Pat. No. 4,931,839, an image forming apparatus is proposed that sequentially develops electrostatic images formed on a photoreceptor with yellow toner, magenta toner, cyan toner and black toner to sequentially transfer each of the toner images onto a transfer belt in a transfer station (primary transfer) for forming a composite image. In said image forming apparatus, the composite image on the transfer belt is further transferred to a sheet in a composite image transfer station (secondary transfer).
In said image forming apparatus, a secondary transfer roller, that can be brought into contact with and retracted from the transfer belt, is provided in the composite image transfer station. While a composite image is being formed on the transfer belt, the secondary transfer roller is retracted from the transfer belt. Then, after the composite image is formed on the transfer belt, the secondary transfer roller is brought into contact with the transfer belt through a sheet so that the composite image is transferred onto the sheet.
Furthermore, Japanese Laid-open Patent Application Hei 3-37693 discloses a full-color image forming apparatus provided with a cleaning blade that can be brought into contact with and retracted from the transfer belt. In this apparatus the cleaning blade is retracted from the transfer belt during the primary transfer for forming the composite image, and is brought into contact with the transfer belt when the leading end of the composite image passes the position wherein the secondary transfer is performed, thereby the toner remaining on the transfer belt is removed.
However, when the secondary transfer roller or the cleaning blade is retracted from the transfer belt during the first primary transfer for the next print, problems such as vibrations occurs on the transfer belt, resulting in that uneven primary transfer images for the next print occurs. Moreover, when the cleaning blade is brought into contact with the transfer belt during secondary transfer to the copying paper, a problem of uneven secondary transfer images occurs.
Even further, when the secondary transfer roller and the cleaning blade are brought into contact with or retracted from the transfer belt, if both the secondary transfer roller and cleaner are brought into contact with or retracted from the transfer belt at the same time, vibration occurring on the transfer belt becomes stronger and, as a result, there was a possibility of bad influence spreading to toner images on the transfer belt.
On the other hand, if the secondary transfer roller is brought into contact with the transfer belt for more time than necessary, there is a possibility that toner remaining on the transfer belt is adhered to the secondary transfer roller and the toner adhered to the secondary transfer roller is transferred to the rear side of the copying paper. Moreover, if the time the cleaning blade makes contact with the transfer belt is short, there is a possibility that toner may remained on the transfer belt after the cleaning by the cleaning blade. Therefore, it was very difficult to maintain the contact and retraction operation of the secondary transfer roller and the cleaning blade while eliminating these problems.
Furthermore, another problem also occurs in which an electrical discharge and electromagnetic noise are generated when the secondary transfer roller is brought into contact with or retracted from the transfer belt while being applied with a voltage for the secondary transfer.