Abstract:
In an image formation apparatus for sequentially forming toner images on an image carrier and transferring the toner images onto a transfer material on a transfer material support member which is moved in an endless manner, when the transfer material is brought into contact with the image carrier other than in a transfer process, the surface of the image carrier is maintained at the same polarity as that of toner transferred to the transfer material. Thus, toner image transferred to the transfer material can be prevented from being offset onto the image carrier.

Description:
This application is a continuation of application Ser. No. 082,580, filed Aug. 7, 1987, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image formation apparatus which has a plurality of developing units, and develops a latent image formed on an image carrier to obtain a toner image and sequentially transfers and overlaps toner images on a transfer material to obtain a desired image. 
     2. Related Background Art 
     In a conventional image formation apparatus, a transfer material is supported on a drum-like transfer material supporting member, and toner images formed on an image carrier can be multiple-transferred a plurality of times to a single transfer material. 
     For example, in a color image formation apparatus, yellow(Y), magenta(M), cyan(C), and black(BK) developing units are arranged, and the transfer material support member is rotated four times so as to transfer toner images of four colors, e.g., yellow, magenta, cyan, and black images since a toner image of one color on an image carrier is transferred per rotation of the transfer material support member. 
     However, after an image of one color is transferred per rotation of the support member, an image formation process for the next color is not yet completed upon the next rotation depending on the size of the transfer material. Therefore, the support member is rotated once more, and a transfer operation for the next color is performed upon the second next rotation. More specifically, the transfer drum is subjected to a rotation for the transfer process and an extra rotation. Therefore, in order to complete the transfer processes for the four colors, at least three extra rotations are necessary. 
     During the extra rotation without the transfer process, since the toner image which has been already transferred onto the transfer material is again in contact with the surface of the image carrier, toner is offset from the toner image on the transfer material onto the surface of the image carrier due to this contact, thus blurring the transferred image. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to solve the above problem. 
     It is another object of the present invention to prevent offset of toner on a transfer material to an image carrier during a non-transfer operation between transfer operations. 
     According to the present invention, when the transfer material is brought into contact with an image carrier in a state other than the transfer process, the surface of the image carrier is kept at a current of the same polarity as that of the toner transferred to the transfer material. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a timing chart when no extra rotation is necessary; 
     FIG. 2 is a timing chart when an extra rotation is necessary; 
     FIG. 3 is a sectional view of a color multiple recording apparatus to which the present invention is applied; 
     FIG. 4 is a graph showing a surface potential of a photosensitive drum of the present invention; and 
     FIGS. 5 to 7 are flow charts for explaining the operation of the apparatus according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention will now be described in detail with reference to the accompanying drawings. FIG. 3 shows a multi-color image formation apparatus (to be referred to as a printer hereinafter) to which the present invention is applied, and employs an electrophotography method. 
     In FIG. 3, a photosensitive drum 1 constituting an image carrier is rotated in a direction indicated by arrow 2. A charger 12, a developing unit group 13, a transfer drum 20, a cleaning device 27, and the like are arranged around the photosensitive drum 1. Laser optical systems 10 and 11 form an image on the drum 1 in accordance with color-separated image information signals. In addition, a polygonal mirror 10 and a laser reflection mirror 11 are arranged. The developing unit group 13 consists of rotary developing units 131, 132, 133 and 134 respectively using Y, M, C, and BK developing agents. 
     When an image of a specific color is formed on the drum 1, the developing unit 131, 132, 133, or 134 of the corresponding color approaches the drum 1 to form an image. When an image of a different color is formed, the developing unit group 13 is rotated in the direction indicated by arrow 135, so that the developing unit of the desired color comes closer to the drum 1. 
     A cassette 15 stores transfer materials 14 (normally, paper sheets are used). A transfer material 14 (to be referred to as a transfer sheet 14 hereinafter) in the cassette 15 is fed from the cassette 15 by a paper feed roller 16, passes by a pair of first register rollers 17 and a pair of second register rollers 18, and is conveyed to a paper guide 19. The transfer sheet is synchronized by the first and second register rollers 17 and 18 upon passage, and its leading end is then gripped by a gripper 201 on the transfer drum 20. The gripped transfer sheet passes between an attraction charger 23 and a press roller 22, and is attracted on the entire surface of an attraction sheet 202 on the transfer drum 20. An image on the drum 1 is transferred to the attracted transfer sheet by a transfer charger 24. The transfer process is repeated a plurality of times corresponding to the number of colors. After the transfer process is completed, the gripper is opened, and the transfer sheet is then guided toward a convey section 29 by a separation pawl 28. The transfer sheet guided to the convey section 29 passes between a pair of fixing rollers 30 and 31 to fix the transferred image, and is discharged onto a tray 32. After the transfer process, a residual image on the drum 1 is cleaned by the cleaning device 27, so as to prepare for the next image formation. The gripper on the transfer drum is opened/closed by cams 25 and 21. 
     FIG. 4 is provided for explaining a surface potential on the drum in this embodiment. 
     The drum surface is uniformly changed to VD (-650 V) by the charger 12 shown in FIG. 2, and the surface potential of a portion a irradiated with a laser beam is decreased to VL (-150 V). Upon development, toner which is charged to a negative (-) potential is applied to the portion a on the drum surface while applying a developing bias VB (-500 V). A halftone portion in which toner is electrostatically attached to the drum is reproduced by controlling the contrast potential VC=VL-VB(300 V) by laser beam modulation so as to control the amount of attracted toner. 
     Since a portion on which no laser beam is radiated maintains the drum surface potential VD, no toner is attached and is left white. 
     FIGS. 1 and 2 show timing charts when the present invention is applied to the printer having the above arrangement and processes. 
     FIG. 1 shows a case wherein no extra rotation is necessary, i.e., wherein the transfer material is short in a transfer rotational direction. A description will be made under the assumption that the transfer sheet in this case is an A4-sized sheet. FIG. 2 shows a case wherein an extra rotation is necessary, i.e., wherein the transfer sheet is long. A description will be made under the assumption that the transfer sheet in this case is an A3-sized sheet. 
     First, the case of FIG. 1 will be described. In order to form a Y image of the first color, the charger 12 is energized for a period corresponding to the sheet size to uniformly charge the drum surface. The laser beam is radiated in accordance with image information, and a latent image of the Y (yellow) image is formed on the photosensitive drum 2. The Y developing unit is moved in advance to the developing position, and starts a developing operation to visualize the Y image. The developing operation is started earlier than the leading end of the image at the developing position S by a time t2 in consideration of the time t2 until a developing sleeve reaches a predetermined rotating speed. At a transfer position T, the transfer charger 24 is energized in synchronism with the moving timing of the transfer sheet held on the transfer drum, and the Y image can be transferred by the single rotation of the transfer drum. 
     After the Y image is developed, the rotary developing unit group is rotated to prepare for next development of an M (magenta) image, so that the M developing unit is moved to the developing position S. Upon the second rotation of the transfer drum, the M image is formed on the photosensitive drum and is transferred in the same manner as described above. 
     In this manner, when the transfer sheet is an A4-sized sheet (JIS standard: 210 mm×297 mm), since a latent image developing/transfer process can be completed during a single rotation of the transfer drum, transfer images of four colors ca be obtained by four rotations of the transfer drum. 
     When the transfer sheet is an A3-sized sheet (having an area twice that of the A4-sized sheet JIS standard: 297 mm=420 mm), since the transfer size is prolonged as shown in FIG. 2, output times of the charger and the laser beam for forming a latent image, a developing time, and a transfer time are respectively prolonged accordingly. For this reason, the time required for moving the next developing unit to the image transfer position and the developing time cannot fall within a time required for a single rotation of the transfer drum. Therefore, the extra rotation of the transfer drum is performed (the second rotation of the transfer drum in FIG. 2), and the movement of the developing unit is performed during the extra rotation. 
     Therefore, the latent image forming developing/transfer process is performed during the first rotation of the transfer drum for the Y image, during the third rotation for the M image, during the fifth rotation for the C (cyan) image, and during the seventh rotation for the BK (black) image in the timing chart shown in FIG. 2. During the second, fourth, and sixth rotations of the transfer drum, the M, C, and BK developing units are respectively moved to the developing position. 
     During the even-numbered rotation of the transfer drum, the potential of the drum surface is controlled by the primary charger so that the toner image which has already been transferred onto the transfer sheet is not offset onto the surface of the photosensitive drum. In this embodiment, as shown in FIG. 4, since negative polarity (-) toner is used, the drum surface is charged to a negative potential, i.e., -650 V. During the extra rotation, in order to increase an attraction force or bias between the transfer sheet and the toner on the transfer sheet, the transfer charger is energized. 
     In this embodiment, the extra rotation is performed during the time required for exchanging the rotary developing units in the case of an A3-sized sheet. In a copying machine, this also applies to the case wherein an image scan optical system is returned to the home position. 
     According to the present invention as described above, the surface potential of the photosensitive drum is controlled and the transfer charger is energized during the extra rotation of the transfer drum, so as to prevent the toner transferred on the transfer sheet from being offset onto the photosensitive drum. 
     In the above embodiment, the charger 12 is operated as shown in FIG. 2. In this case, as indicated by an alternate long and two short dashed line in FIG. 2, after the charger 12 is started, it can be kept ON without being turned off. Another charging means may be arranged in addition to the charger 12. 
     FIGS. 5 to 7 are flow charts for realizing the operation shown in FIGS. 1 and 2. FIG. 5 shows steps for determining whether or not the size of the selected transfer sheet requires an extra rotation of the transfer drum. 
     FIG. 6 shows a flow for driving the charger 12 during the even-numbered rotation of the transfer drum so that the toner transferred onto the transfer sheet is not offset onto the photosensitive drum during the extra rotation. In the flow chart shown in FIG. 6, since the toners of four colors including black toner are transferred onto the transfer sheet, the transfer process is completed after the transfer drum is rotated seven times. 
     The flow chart of FIG. 7 shows a case wherein no extra rotation is necessary. Each time the transfer drum is rotated, toners of respective colors, i.e., yellow, magenta, cyan, and black, are transferred onto the transfer sheet. After the transfer drum is continuously rotated four times, the transfer process is completed. Since no extra rotation is performed in the case of FIG. 7, the toner transferred onto the transfer sheet does not face the photosensitive drum other than in the transfer process.