1. Field of the Invention
The present invention relates to a method of forming images on both sides of a single sheet or recording medium, an apparatus for practicing the method, and an image forming system using the apparatus.
2. Description of the Background Art
Various image forming apparatuses including electrophotographic copiers, facsimile apparatuses, printers and multiplex machines having at least two of their functions are extensively used today. For example, an image forming apparatus of the type capable of producing a duplex print carrying images on both sides thereof is known in the art. This type of apparatus transfers toner images of different colors formed on image carriers to one side of a sheet one above the other, fixes the resulting composite image, and then switches back, the sheet to thereby turn it. Subsequently, the apparatus again feeds the same sheet to the image carriers so as to transfer the next toner images of different colors from the image carriers to the other side of the sheet and then fixes the resulting composite image.
A problem with the image forming apparatus of the type described is that it has to switch back the sheet carrying the image on one side thereof and again feed it to the image carriers, resulting in a long image forming time and therefore low productivity. Another problem is that the sheet carrying the image only on one side thereof is apt to curl due to fixation, making conveyance unreliable.
Japanese Patent Laid-Open Publication No. 8-160703, for example, discloses an image forming apparatus capable of producing a duplex, color print with a single photoconductive element and two intermediate image transfer belts. This image forming apparatus includes a plurality of developing units each storing toner of particular color arranged around the photoconductive element. Toner images of different colors are sequentially formed on the photoconductive element while being sequentially transferred to a first intermediate image transfer belt one above the other, completing a full-color image. Subsequently, the full-color image is transferred from the first intermediate image transfer belt to a second intermediate image transfer belt. Thereafter, another full-color image is formed on the first intermediate image transfer belt in exactly the same manner as the previous full-color image. The two-full-color images are respectively transferred from the second and first belts to both sides of a sheet.
Japanese Patent Laid-Open Publication No. 9-258518 teaches a color image forming apparatus with a duplex print capability and also including a single photoconductive element and two intermediate image transfer belts. A plurality of developing units each storing toner of particular color are arranged around the photoconductive element. Exposure and development are repeated color by color to thereby form a full-color image on the photoconductive element. The full-color image is transferred from the photoconductive element to a first image transfer body and therefrom to a toner image acceptor or second intermediate image transfer body. Subsequently, another full-color image is transferred from the photoconductive element to the first intermediate image transfer body. The two full-color images are respectively transferred to opposite sides of a sheet from the toner image acceptor and first intermediate image transfer body.
However, the apparatus of Laid-Open Publication No. 8-160730 has the following problem left unsolved. In the full-color print mode, a toner image of one color is formed on the photoconductive element and transferred to the first intermediate image transfer body for a single rotation of the element, so that a single full-color image to be transferred to one side of a sheet is completed by four rotations of the element in total. Such a procedure is time-consuming and lower in productivity.
On the other hand, the apparatus of Laid-Open Publication No. 9-258518 uses four exposing units and four chargers for forming a full-color image on the photoconductive element and then transferring it to the intermediate image transfer body, reducing the image forming time. However, the exposing units are arranged within the photoconductive element, which is transparent for light. The photoconductive element therefore must be provided with a particular characteristic and a particular configuration and cannot be made compact. Further, a wasteful space exists around the intermediate image transfer body, increasing the overall size of the apparatus. In addition, charging and development repeated in the vicinity of a position where toner exists make it difficult to insure a stable image forming process.
Although the apparatuses of Laid-Open Publications stated above have some problems left unsolved, they solve to some extent the problem that the switchback of a sheet and the curl of a sheet ascribable to fixation obstruct reliable sheet conveyance. Neither one of them, however, gives consideration to the accurate register of images transferred to opposite sides of a sheet.
Moreover, assume that an intermediate image transfer belt or image carrier, which is passed over rollers, is brought to a stop at the same position every time image formation ends. Then, the belt is apt to curl complementarily to the circumference of each roller. The curl appears in the form of a projection on the belt and obstructs the close contact of an image carried on the belt and a sheet in the event of transfer, thereby rendering the image blurred or otherwise defective on the sheet. This is particularly true with a duplex print type image forming apparatus using non-contact type image transferring means.
It is an object of the present invention to provide a method and an apparatus capable of forming color images on both sides of a sheet in a short period of time, and an image forming system using the same.
It is another object of the present invention to provide a method and an apparatus capable of forming images on both sides of a sheet in accurate register, and an image forming system using the same.
It is a further object of the present invention to provide a method and an apparatus capable of insuring close contact of an intermediate image transfer belt and a sheet by obviating a curl to thereby obviate defective images, and an image forming system using the same. In accordance with the present invention, an image forming method begins with a step of electrostatically transferring toner images of different colors formed on a plurality of image carriers to a first intermediate image transfer body one above the other to thereby form a first composite image. The first composite image is electrostatically transferred from the first intermediate image transfer body to a second intermediate image transfer body and therefrom to one side of a sheet. A second composite image formed on the first intermediate image transfer body in exactly the same manner as the first image is electrostatically transferred from the first intermediate image transfer body to the other side of the same sheet. The first and second composite images are fixed on the recording medium at the same time.
Also, in accordance with the present invention, an image forming apparatus includes a plurality of image carriers on each of which a toner image of particular color is formed. Toner images of different colors formed on the image carriers are transferred to a first intermediate image transfer body one above the other to thereby form a first composite image. The first composite image is transferred from the first intermediate image transfer body to a second intermediate image transfer body. A first image transferring device electrostatically transfers the toner images from each image carrier to the first intermediate image transfer body. A second image transferring device electrostatically transferring the first composite image from the first intermediate image transfer body to the second intermediate image transfer body. A third image transferring device transfers the first composite image from the second image transfer body to one side of a sheet. A fourth image transferring device transfers to the other side of the recording medium a second composite image formed on the first intermediate image transfer body in exactly the same manner as the first composite. A fixing unit fixes the first and second composite images of the sheet at the same time.