1. Field of the Invention
The present invention relates to an image forming system, a control apparatus, and an image forming apparatus.
2. Description of the Related Art
As a typical image forming apparatus forming toner images on both faces of a recording sheet such as a recording paper, a copying machine is disclosed in Japanese Patent Application Laid-open No. 2005-115355. In that copying machine, a single imaging unit operates to form a toner image to be transferred onto one face of a recording sheet and to form a toner image to be transferred onto the other face of the recording sheet. The imaging unit includes a photosensitive member functioning as an image carrying member, a charging unit that uniformly charges the photosensitive member, a latent image writing unit that writes a latent image on the uniformly charged photosensitive member by means of optical scanning, and a developing unit that develops the latent image formed on the photosensitive member. In the duplex mode of the copying machine in which a toner image is formed on each face of a recording sheet, a first toner image that has been formed by the imaging unit on the surface of the photosensitive member is transferred firstly onto a first intermediate transfer belt and then is further transferred onto a second intermediate transfer belt. Besides, at a slightly earlier timing than the timing of the second transferring operation, a second toner image is formed by the imaging unit on the surface of the photosensitive member. The second toner image is then transferred onto the first intermediate transfer belt. Subsequently, a recording sheet is fed to an area facing the first intermediate transfer belt and the second intermediate transfer belt. As the recording sheet passes over that area, the first toner image gets transferred from the second intermediate transfer belt onto one face of the recording sheet and the second toner image gets transferred from the first intermediate transfer belt onto the other face of the recording sheet.
In such a configuration, as compared to the second toner image, the first toner image is subjected to the transferring operation for one additional number of times. Hence, regarding the first toner image, the amount of toner loss occurring due to the residual toner left during the transferring operations is larger as compared to the second toner image. Thus, in case the first toner image and the second toner image are formed under the same imaging conditions (e.g., a latent image writing condition, a developing condition, etc.), then the image density in the first toner image falls below the image density in the second toner image. In regard to that issue, in the copying machine disclosed in Japanese Patent Application Laid-open No. 2005-115355, the imaging conditions for the first toner image and the imaging conditions for the second toner image are separately corrected in the following manner. Firstly, a first test toner image is formed on a recording sheet in the same way as the first toner image described above, while a second test toner image is formed on another recording sheet in the same way as the second toner image described above. Then, the user is prompted to perform the operation of separately setting the recording sheet with the first test toner image and the recording sheet with the second test toner image on a scanner to scan those test toner images. Once the image density of each test toner image is obtained by means of the operation, the imaging conditions for the first toner image and the imaging conditions for the second toner image are corrected so as to eliminate the difference in image densities of the two test toner images. As a result, the first toner image and the second toner image can be formed with substantially identical image densities.
Meanwhile, as an image forming apparatus forming toner images on both faces of a recording sheet, there is known a configuration in which a first imaging unit and a first transferring unit corresponding to one face of the recording sheet are disposed separately from a second imaging unit and a second transferring unit corresponding to the other face of the recording sheet. In that configuration, the toner image to be transferred onto one face of the recording sheet and the toner image to be transferred onto the other face of the recording sheet are simultaneously formed. Hence, as compared to the copying machine disclosed in Japanese Patent Application Laid-open No. 2005-115355 in which a single imaging unit forms the two toner images at different timings, it becomes possible to speed up the image forming operation. Besides, since the number of transferring operations required for transferring a toner image from the image carrying member onto the first face of a recording sheet can be kept identical to the number of transferring operations for transferring a toner image onto the second face, it becomes possible to eliminate the difference in image densities that may occur due to the difference in the number of transferring operations.
However, in this type of image forming apparatuses, there are times when a difference occurs in the image densities on the two faces of a recording sheet due to reasons other than the difference in the number of transferring operations.
For example, it is common that the first imaging unit and the second imaging unit have individual differences in the imaging capability. In such a case, even if exactly the same imaging conditions are set in each of the first imaging unit and the second imaging unit, there occurs a difference in the amount of toner deposit per unit area of toner images formed on the surface of the respective image carrying members. That difference causes a difference in the image density on the first face and the image density on the second face of a recording sheet.
Moreover, consider the case when the first transferring unit and the second transferring unit have different toner transfer rates. Then, even if it is assumed that the first imaging unit and the second imaging unit have no individual differences in the imaging capability and that the same amount of toner deposit is achieved on the respective image carrying members, there still occurs a difference in the image density on the first face and the image density on the second face of the recording sheet.
Regarding the issue of a difference occurring in the toner transfer rates of the first transferring unit and the second transferring unit, one reason could be the implementation of a configuration in which fixing of a toner image is performed individually on each face of a recording sheet. In such a configuration, the inventor(s) of the present invention found by experiment that the toner transfer rates of the first transferring unit and the second transferring unit tend to differ by a large margin. More particularly, as common practice, only after a recording sheet has passed through the first transferring unit that transfers a toner image on one face of the recording sheet and has passed through the second transferring unit that transfers a toner image on the other face of the recording sheet, both faces of that recording sheet are simultaneously subjected to the fixing operation. However, depending on the apparatus configuration, while a recording sheet having passed through only one of the two transferring units and having an unfixed toner image formed on only one face thereof is being conveyed for the subsequent operation, there are times when the unfixed toner image is disrupted. To solve that problem, a configuration is sometimes implemented in which fixing of a toner image is performed individually on each face of the recording sheet. However, in such a configuration, the inventor(s) of the present invention found by experiment that, when a recording sheet gets heated by a fixing unit before entering a transferring unit at the downstream side, water evaporation that accompanies the heating leads to the formation of small wrinkles on the surface of the recording sheet thereby reducing the toner transferability during the toner transferring operation performed by the transferring unit at the downstream side. Once the toner transferability is reduced; then, irrespective of whether a substantially identical amount of toner is deposited on the toner images formed on a first image carrying member and a second image carrying member, there occurs a difference in the image density on the first face and the image density on the second face of the recording sheet.
Regarding the issue of a difference occurring in the toner transfer rates of the first transferring unit and the second transferring unit, another reason could be the implementation of a configuration in which the toner concentration is decreased with an increase in the cumulative usage time of the magnetic carrier. More particularly, as the cumulative usage time of the developer carrier contained in a developing unit goes on increasing, the developer carrier goes on losing its toner adsorption capability. Hence, a scumming effect tends to occur in the toner image by which the toner gets deposited on the non-image portion as well. In order to prevent the scumming effect from occurring, there are times when a configuration is implemented in which the toner concentration is decreased with an increase in the cumulative usage time of the magnetic carrier. As the toner concentration decreases, the carrier particles and the toner particles undergo sliding friction in a more active manner thereby leading to an increase in the average toner charge. By extension, the toner adsorption capability of the developer carrier can be enhanced so as to prevent the scumming effect from occurring. However, an increase in the average toner charge results in an enhanced adsorption force between the toner and the image carrying member. That reduces the toner transferability during the toner transferring operation. For that reason, it was found that a large difference occurs in the image density on the first face and the image density on the second face of the recording sheet.
Therein, the inventor(s) of the present invention entertained the idea of implementing following configurations. Consider the case when the image densities on the two faces of a recording sheet differ from each other due to the fact that the number of transferring operations required for transferring a first toner image from the image carrying member onto one face of the recording sheet is different than the number of transferring operations required for transferring a second toner image onto the other face. That problem is eliminated by implementing a configuration in which a first imaging unit and a first transferring unit corresponding to one face of the recording sheet are disposed separately from a second imaging unit and a second transferring unit corresponding to the other face of the recording sheet. Further, consider the case when the image densities on the two faces of a recording sheet differ from each other due to the fact that there exists a large difference in the imaging capability of the first imaging unit and the imaging capability of the second imaging unit, and that there exists a large difference in the toner transfer rate of the first transferring unit and the toner transfer rate of the second transferring unit. That problem is eliminated by implementing a configuration in which, based on the difference in the image density of a toner image transferred onto the recording sheet by the first transferring unit and the image density of a toner image transferred onto the recording sheet by the second transferring unit, the imaging conditions for the first imaging unit and the imaging conditions for the second imaging unit are corrected so as to reduce the difference in image densities. With that, even if there exists a large difference in the imaging capability of the first imaging unit and the imaging capability of the second imaging unit or even if there exists a large difference in the toner transfer rate of the first transferring unit and the toner transfer rate of the second transferring unit, it becomes possible to reduce the difference in image densities on the two faces of the recording sheet. However, since it is imperative to form test toner images on the recording sheet, the user is forced to unintentionally consume the recording sheet.