1. Field of the Invention
Exemplary aspects of the present invention generally relate to an image forming apparatus such as a copier, facsimile machine, and printer, and more particularly to an image forming apparatus in which color toner images formed on an image carrier or an intermediate transfer body are collectively transferred onto a recording medium.
2. Description of the Background
Related-art image forming apparatuses, such as copiers, printers, facsimile machines, and multifunction devices having two or more of copying, printing, and facsimile functions, typically form a toner image on a recording medium (e.g., a sheet of paper, etc.) according to image data using an electrophotographic method. In such a method, for example, a charger charges a surface of an image carrier (e.g., a photoconductor); an irradiating device emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; a developing device develops the electrostatic latent image with a developer (e.g., toner) to form a toner image on the photoconductor; a transfer device transfers the toner image formed on the photoconductor onto a sheet; and a fixing device applies heat and pressure to the sheet bearing the toner image to fix the toner image onto the sheet. The sheet bearing the fixed toner image is then discharged from the image forming apparatus.
One example of widely-used image forming apparatuses includes a tandem-type image forming apparatus having multiple image carriers. Colored toner images formed respectively on surfaces of the multiple image carriers are sequentially transferred onto an intermediate transfer body of the transfer device in a superimposed manner to form a full-color toner image on the intermediate transfer body. The full-color toner image thus formed is then transferred onto a recording medium from the intermediate transfer body.
Transfer efficiency of the full-color toner image from the intermediate transfer body onto the recording medium is not 100% in such a tandem-type image forming apparatus, and consequently residual toner adheres to the surfaces of the image carriers and the intermediate transfer body, adversely affecting image density and leading to unnecessary consumption of toner. In particular, a toner image of a specific color formed on the intermediate transfer body as the bottommost layer of the full-color toner image, which consists as described above of superimposed multiple layers of color toner images, tends to remain on the intermediate transfer body. Specifically, adherence of the toner image to the intermediate transfer body prevents the toner image from being transferred onto the recording medium when the full-color toner image is transferred onto the recording medium from the intermediate transfer body. Consequently, a desired color is not achieved in a resultant image formed on the recording medium, causing a deterioration in color reproducibility. Thus, improvement of transfer efficiency of the full-color toner image is demanded.
In order to improve transfer efficiency of the full-color toner image, for example, Published Unexamined Japanese Patent Application Nos. H01-134485, H05-281863, 2002-236392, 2003-162125, and 2006-251717 respectively disclose techniques in which a clear toner image is formed underneath a colored toner image so that the clear toner image is the bottommost layer of a full-color toner image formed on either an image carrier or an intermediate transfer body. Accordingly, the clear toner image formed as the bottommost layer of the full-color toner image remains on the image carrier or the intermediate transfer body when the full-color toner image is transferred onto a recording medium, thereby achieving transfer efficiency approaching 100% of the colored toner image.
In another approach, a clear toner image is formed for the purpose of imparting glossiness to a resultant color image. Specifically, the clear toner image is superimposed on a colored toner image formed on a recording medium to impart glossiness to the resultant color image, thereby achieving color reproducibility like that obtained by photoprinting and so forth. For example, Published Unexamined Japanese Patent Application No. H10-055089 (hereinafter referred to as JP-H10-055089-A) discloses a technique in which a clear toner image is formed underneath a colored toner image so that the clear toner image is the bottommost layer of a full-color toner image formed on an intermediate transfer body. The clear toner image is then transferred onto a recording medium together with the colored toner image so that the clear toner image is formed on the colored toner image on the recording medium.
In a case in which the clear toner image is formed in order to improve transfer efficiency of the full-color toner image as described above, although a part of the clear toner image remains on the intermediate transfer body, the other part of the clear toner image is inadvertently transferred onto the recording medium together with the colored toner image. As a result, the clear toner image is superimposed on the colored toner image on the recording medium. As mentioned above, in this case, the clear toner image is formed not for the purpose of imparting glossiness to the resultant color image as disclosed in JP-H10-055089-A but for improving transfer efficiency of the full-color toner image. Consequently, the clear toner image superimposed on the colored toner image may alter the glossiness or color of the resultant color image from those desired by a user, thereby degrading image quality.
Such deterioration in image quality can be prevented if clear toner that can impart the same degree of glossiness as that of colored toner after being fixed onto the recording medium, or clear toner including fewer impurities, is used for forming the clear toner image to prevent changes in a color of the resultant color image. However, the range of materials available for use in such clear toner is limited, and moreover incurs a cost increase due to removal of impurities therefrom.
Further, in a case in which thick toner layers are transferred onto the recording medium, it is known that toner of a smaller electrostatic force included in an upper portion of the toner layers tends to scatter when subjected to an electric field applied by the transfer device to the toner layers, more often causing irregular images due to toner scattering within the image forming apparatus. In addition, when the thick toner layers are transferred onto the recording medium, heat is not sufficiently applied to all the toner layers in the fixing device, and consequently, the toner image is not appropriately fixed onto the recording medium. A total amount of toner or a thickness of the toner layers to be transferred onto the recording medium is increased particularly in a case in which a full-color toner image formed by superimposing colored toner images of yellow, magenta, cyan, and black one atop the other on the intermediate transfer body is transferred onto the recording medium. Consequently, toner scattering and deterioration in fixing performance tend to occur due to the thick toner layers. Moreover, because formation of the clear toner image as the bottommost layer of the full-color toner image on the intermediate transfer body increases the total amount of toner, toner scattering and deterioration in fixing performance more easily occur.
To solve the above-described problems, a technique for reducing the amount of colored toner is widely known. Specifically, a threshold value to limit the total amount of toner transferred onto the recording medium is set so as to reduce the amount of colored toner when the total amount of all toner, clear and colored, exceeds the threshold value. However, a desired color may not be achieved in a resultant color image if the amount of colored toner is simply reduced. Therefore, a method called under color removal (UCR) is used. In the UCR method, a portion of a toner image where toner of yellow, magenta, and cyan are superimposed one atop the other is converted into a toner image of black, and then the amounts of toner of yellow, magenta, and cyan are reduced. However, an increase in such a portion to be converted into the toner image of black in the UCR method adversely affects color reproducibility. Further, because the total amount of toner is further increased in a case in which the clear toner image is formed as the bottommost layer of the full-color toner image as described above, the threshold value is easily exceeded, thereby more often triggering restriction of the total amount of toner. Consequently, although it might be expected that formation of the clear toner image as the bottommost layer of the full-color toner image improves transfer efficiency of the colored toner image and provides higher image quality with superior color reproducibility, in fact the resultant color image has inferior color reproduction.
It is to be noted that the above-described problems may also occur in image forming apparatuses in which a clear toner image is formed underneath a colored toner image as the bottommost layer of a full-color toner image on an image carrier that transfers the full-color toner image onto a recording medium.