Abstract:
A method and apparatus for cleaning an image carrier that is used for transferring a toner image onto a recording medium which includes a black image step in which a black image, i.e., an image of the entire area which is filled with toner, is formed on the image carrier and is transferred onto a cleaner that engages the image carrier for removing dust therefrom, wherein, prior to the black image step, a black page step is carried out in which a black page is printed by transferring a black image from the image carrier onto the recording medium.

Description:
This non-provisional application claims priority under 35 U.S.C. §119(a) on European Patent Application No. 08156248.0 filed in the European Patent Office on May 15, 2008, which is herein incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a method and apparatus for cleaning an image carrier that is used for transferring a toner image onto a recording medium, wherein a black image, i.e., an image, the entire area of which is filled with toner, is formed on the image carrier and is transferred onto a cleaner that engages the image carrier for removing dust therefrom. 
     The invention is applicable, for example, to electrographic or magnetographic copiers and printers, and, more generally, to all kinds of image forming apparatus in which a toner image is transferred from an image carrier onto a recording medium. 
     It is a general problem with such image forming apparatus that certain types of recording media, especially certain types of paper, such as offset paper, home-cut paper that has been cut into sheets in a special apparatus on the premises of the user, and the like, tend to release a considerable amount of dust inside the apparatus. The dust is deposited on sensitive component parts of the apparatus and disturbs the image forming process. Among other contaminates, dust is deposited on the image carrier, so that the image carrier has to be cleaned from time to time. 
     U.S. Pat. No. 5,227,844 discloses a cleaning process in which a specific cleaning sheet is fed through the apparatus in place of the regular recording medium in order to collect and remove the dust. 
     A cleaning process as defined above has been employed by the applicant. In this process, a cleaner, typically in the form of a drum, e.g., a so-called spiral cleaner, is arranged at the periphery of the drum-type or belt-type image carrier, so as to remove the dust from the surface of the image carrier and to collect the dust on the cleaner. However, with an increasing amount of dust being deposited on the cleaner, the cleaning capacity degrades, so that the dust can no longer be removed completely from the image carrier. For this reason, the cleaner has to be regenerated from time to time. This is achieved by means of a so-called black image process in which a black image is formed on the image carrier and is then transferred onto the cleaner. As a consequence, the dust that has been deposited on the cleaner is buried in a layer of toner, and this restores the capacity of the cleaner to collect more dust. The term “black image” is used for simplicity and is to indicate that the entire surface of the image carrier that is normally used for carrying an image is covered with a continuous layer of toner, but it will be understood that the actual color of the toner (normally black) is not important. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method for improving the quality of printed images. 
     In order to achieve this object, the invention is characterized in that, prior to the black image step, a black page step is carried out in which a black page is printed by transferring a black image from the image carrier onto the recording medium. 
     It has been found that the quality of the printed images may sometimes be degraded, at least for the first few images that are printed after the conventional black image cleaning process has been carried out. This effect has been traced back to the following mechanism. When the black image process is carried out at a time when a curtain amount of dust has remained on the surface of the image carrier, this dust tends to retain the toner on the surface of the image carrier, so that, in the black image process, the toner layer is not entirely transferred onto the cleaner as desired, but certain residues of toner remain on the surface of the image carrier. Then, when the next images are printed onto the recording medium, the toner residues on the image carrier result in the dark or “dirty” background on the sheets of the recording medium. Of course, the amount of dust that remains on the surface of the image carrier and causes this effect will be particularly large when the cleaning capacity of the cleaner has decreased after a long time of use and, accordingly, it is time to perform the next cleaning cycle. 
     According to the present invention, at least one black page step precedes the black image cleaning step. In the black page step, the surface of the image carrier is covered with a continuous layer of toner, as in the black image step, but this toner layer is not transferred onto the cleaner but onto the recording medium, so that a black page is printed which will then have to be discarded. The advantage is that, in this transfer process, not only the toner layer but also the dust that had been deposited on the image carrier is transferred onto the recording medium, so that the dust is removed from the image carrier very efficiently. In this way, the image carrier is prepared for the subsequent black image process. Then, the toner layer is applied onto a practically clean surface of the image carrier and, consequently, is transferred completely onto the cleaner, without leaving on the image carrier any toner residues that would impair the quality of the images subsequently printed. 
     In this way, a quality of the printed images, especially of those images that are printed immediately after a cleaning process, can be improved significantly, even in a situation in which a relatively large amount of dust had accumulated on the image carrier. Consequently, the present invention also permits, for a given quality of the recording media, a reduction in the frequency at which the cleaning processes have to be performed. Conversely, for a given frequency of the cleaning processes, the present invention permits the accommodation of “critical” recording media which tend to release large amounts of dust. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the present invention will now be described in conjunction with the drawings, wherein: 
         FIG. 1  is a schematic view of an image forming apparatus to which the invention is applicable; 
         FIG. 2  illustrates a black page process; and 
         FIG. 3  illustrates a black image process. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows an example of an image forming apparatus having a photoconductive drum  10 , a drum-type image carrier  12  and a transport system  14  arranged for feeding sheets  16  of a recording medium, e.g., paper, through a transfuse nip  18  formed between the image carrier  12  and a heated fuse roller  20 . 
     An image forming system  22  is arranged at the periphery of the photoconductive drum  10  for creating an electrostatic charge image on the surface of the drum. By way of example, the image forming system  22  may comprise a laser exposure system for exposing and discharging the drum in accordance with image information supplied thereto. 
     A developer station  24  is arranged for developing the electrostatic charge image with (black) toner, thereby to form a toner image on the surface of the electrographic drum  10 . At a nip formed between the drum  10  and the image carrier  12 , the toner image is transferred onto the surface of the image carrier  12  by means of adhesion and/or electrostatic forces in a cold process. When the toner image thus formed on the surface of the image carrier  12  reaches the transfuse nip  18 , it is transferred in a hot process onto the sheet  16  and is fused thereon by the heat generated by the fuse roller  20 . 
     As has symbolically been shown in  FIG. 1 , a first toner image  26  is just being transferred onto the sheet  16  while a second toner image  28  is just being formed at the developer station  24  and transferred onto the image carrier  12 . 
     A spiral cleaner  30  is arranged at the periphery of the image carrier  12  at a position downstream of the transfuse nip  18  and has the purpose to remove from the surface of the image carrier any dust that may have been released by the sheets  16 . 
     The transport system  14  comprises several pairs of transport rollers  32  and guide plates  34  defining a transport path for the sheets  16 . Some of the transport rollers  32  are driven under the control of a control unit  36  which also controls the image forming system  22  as well as the operation of the other components of the image forming apparatus. 
     When the recording sheets  16  are successively passed through the apparatus and printed, they will release a certain amount of dust which will be deposited on the image carrier  12  and then collected by the spiral cleaner  30 . As a consequence, after a certain time of operation, depending on the quality of the recording sheets, the amount of dust collected on the surface of the spiral cleaner  30  will have become so large that the cleaning capacity decreases, and it is necessary to restore the cleaning capacity of the spiral cleaner  30 . To this end, a cleaning sequence is performed that will now be described in conjunction with  FIGS. 2 and 3 . This cleaning process comprises a black page process as a first step, illustrated in  FIG. 2 , and a black image process as a second step, illustrated in  FIG. 3 . 
     In the black page step, shown in  FIG. 2 , the controller  36  activates the image forming system  22  to form an image that consists of a completely black image. When this image is developed by the developer station  24 , a uniform toner layer  38  is formed on the surface of the photoconductive drum  10 , and the toner is transferred onto the image carrier  12 , so that a uniform toner layer  40  is also formed on the image carrier  12  in the circumferential area between the nip with the drum  10  and the transfuse nip  18 . In the transfuse nip, the toner is transferred in a hot process onto a sheet  16  that has been supplied by the transport system  14 , so that the sheet will leave the apparatus as a black page. 
     The toner layer  40  on the image carrier  12  accommodates any dust particles that may be present on the surface of the image carrier, and when the toner layer  40  is transferred onto the sheet  16 , the dust particles are transferred onto the sheet together with the toner and are thus removed from the apparatus. 
     If necessary, several black pages may be printed in this way, until essentially all dust has been removed from the surface of the image carrier  12 . 
     In the example shown, the recording medium consists of several cut sheets  16  which are fed through the apparatus one after the other, with slight gaps therebetween, and in a normal printing process, the images formed by the image forming system  22  will only have a length corresponding to that of the sheets  16 . However, if several black pages are printed in the black page process, the image forming system  22  may be activated continuously, so that the toner layers  38  and  40  are not interrupted by any gaps. 
     When the surface of the image carrier  12  has become sufficiently clean, a black image process will be performed, as is illustrated in  FIG. 3 . 
     Just as in the black page process, continuous toner layers  38  and  40  are formed on the surface of the drum  10  and on the image carrier  12 . In this case, however, the transport system  14  is controlled to suspend the supply of recording medium sheets for a certain time, so that the transport path is empty. As a result, the toner layer  40  is not transferred onto any recording medium, but is passed on until it reaches the nip formed between the image carrier  12  and the spiral cleaner  30 . If necessary, the fuse roller  20  may be somewhat retracted from the image carrier in order to prevent toner from being transferred onto the fuse roller. 
     The toner that reaches the spiral cleaner  30  is transferred from the image carrier  12  onto the cleaner and forms a toner layer  42  on the surface of the cleaner. 
     As a consequence of the preceding black page step, the surface of the image carrier  12 , when receiving the toner layer  40 , was free of any dust particles which would have had a tendency to retain the toner on the image carrier. Thus, the toner is readily released from the image carrier  12  and transferred onto the cleaner  30  without leaving any toner residues on the image carrier  12 . 
     The black image process may be continued or repeated until the toner layer  42  on the cleaner  30  has reached a sufficient thickness. Then, when new sheets are printed in the same way as in  FIG. 1 , the surface of the image carrier  12  will not only be free of dust but also free of any toner residues, so that an excellent print quality can be achieved. As is known in the art, the toner layer  42  will remain on the cleaner  30  and will bury the dust that has been accumulated thereon, thereby restoring the cleaning capacity of the cleaner, so that new sheets may be printed in excellent quality, until the next cleaning process becomes necessary. 
     The cleaning process that has been described above is applicable also in an apparatus in which images are not printed on separate sheets but on a recording medium in the form of a continues web. Then, of course, care must be taken that no toner is transferred onto the fuse roller  20 , the clean image step shown in  FIG. 3 . 
     If the image forming apparatus is capable of printing on sheets or webs that may have different widths, it will be understood that both, the black image process and the black page process should be performed for the maximum width of the recording medium, so that dust will be removed from the image carrier  12  on the entire (axial) length thereof and the toner layer  42  will be applied on the entire length of the cleaner  30 . In the black page step, sheets or a web with a maximum width should be fed through the transfuse nip  18 . When printing on a continuous web, a known web change mechanism may be used for automatically selecting the web with the largest width. 
     In the example that has been described here, the image forming system comprises the image forming system  22  and the developer station  24 . In a modified embodiment, the direct induction process (DIP), for example, might be employed for forming the toner image directly on the surface of the drum  10  which will then be a DIP drum. 
     Moreover, the invention is not limited to a two-step image transfer process, in which the toner image is first transferred from the drum  10  to the image carrier  12  (serving as an intermediate carrier) and then onto the recording medium. In a modified embodiment, the toner image could be formed directly on the surface of the image carrier  12 . 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.