Image forming system

Abstract: In an image forming system employing a photosensitive drum (11) on which an electrostatic latent image is formed, and a toner image bearing belt (14) on which a toner image corresponding to the electrostatic latent image is formed by means of a developing device (17), a discharging device (33), such as an AC corona charger, is provided to face the toner image bearing belt superimposed on the photosensitive drum, at a location downstream of the developing area. The surface charges on the toner image bearing member (14) is therefore removed before or when the toner image bearing member separates from the photosensitive drum (11). As a result, the increase of the potential difference across the air layer due to the decrease of the capacitance of the air layer between the toner image bearing member and the photosensitive drum is prevented, and electrical discharge through the air layer, and attendant scattering of the toner image are thereby avoided.

Description: This invention concerns an image forming system. Conventionally, images in recording devices such as printers and copiers were generally formed by Carlson's electrophotography. Electrophotography according to the Carlson's system has been widely employed for forming images in photocopying and printing. Electrophotography uses a drum having a photosensitive surface which passes a charging section where the photosensitive surface is uniformly charged, an exposure section where the photosensitive surface is exposed to light image or pattern of light which correspond to the original (source document) or the print data to form an electrostatic latent image consisting of charged areas and discharged areas, a developing section where toner is attracted to the charged areas of the latent image so that a toner image is created, a transfer section where the toner image is transferred to a recording paper which is brought into contact with the photosensitive drum, and a cleaning section which removes any residual toner on the photosensitive surface. The toner image that has been transferred to the recording paper is fixed by application of heat, for example, and the recording paper is then discharged from the apparatus. A problem associated with the Carlson system is that the photosensitive drum needs to have a sufficient diameter to allow various devices to be disposed on the periphery of the drum, increasing the cost of the drum and the overall bulk of the apparatus. Another problem is that there is a considerable amount of toner remaining on the photosensitive drum after the transfer of the toner image. This residual toner must be removed by a cleaning device, and must be collected and then disposed. When the collected toner is disposed, the toner may be scattered in and out of the apparatus, and the operator's clothing or body may be soiled. A system is disclosed in US-A-3937572, which comprises an image forming system comprising: an electrostatic image bearing member ; a toner image transfer member to receive a toner image upon being passed over the electrostatic image bearing member and thereafter separated therefrom for transferring a toner image to a copy sheet; developing means for developing a latent electrostatic image on the electrostatic image bearing member to form a corresponding toner image on the toner image transfer member; and an optical exposure device to irradiate said electrostatic image bearing member to form said latent image thereon. In the system described above, when the toner image on toner image bearing belt was transferred to and fixed on the recording paper, some disturbances were found to occur in the image reproduced on the recording paper. Upon examination, it has been found that the toner on the toner image bearing belt scattered when the belt separated from the photosensitive drum. An object of the invention is to reduce the scattering of toner which takes place when the belt with the toner image formed thereon separates from the photosensitive drum. FR-A-2625574 discloses an arrangement in a magnetographic image forming system for ameliorating toner scattering. According to the present invention, the image forming system is characterised by a corona charger for applying an electrostatic field to the toner image on the toner image transfer member such as to tend to reduce disturbance of the toner image by the separation of the toner image transfer member from the electrostatic image bearing member; and an optical discharging means disposed between said corona discharger and said developing means to irradiate said electrostatic image bearing member. An embodiment of the present invention will now be described by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a schematic diagram showing the image forming system of an embodiment of the invention; Fig. 2 shows a novel system without the optical discharging located according to the present invention; Fig. 3 is an enlarged view of the part of the toner image bearing belt separating from the photosensitive drum in the image forming system of Figure 5; Figs. 4A to 4F show various process steps of the operation of the image forming system of the present invention; and Fig. 5 shows another embodiment of the present invention. Referring to Figs. 1 to 4F, the illustrated electrophotography apparatus comprises a photosensitive member in the form of a photosensitive drum 11 rotatably mounted on a frame, (not shown). The photosensitive drum is rotated at a constant speed by a motor, (not shown). The photosensitive drum 11 has a photosensitive layer 11b on an electrically conducting support 11a. Suitable materials for the photosensitive layer are selenium photosensitive material, an organic photosensitive material, a zinc oxide photosensitive material and an amorphous silicon photosensitive material. In this embodiment, a composite layer structure of an organic photosensitive material of the positive type comprising a charge transport layer and a charge generating layer formed in that order are employed. The image forming apparatus also comprises a heating roller 12 and a pressure roller 13 which are pressed against each other by a means (not shown). The heating roller 12 may be a hollow metal member enclosing a halogen lamp, or one having a heat-emitting body on a metal surface. An endless toner image bearing belt 14 is passed around the photosensitive drum 11 and the pressure roller 13. More particularly, the toner image bearing belt 14 is in contact, on a first or inner surface thereof, with the peripheral surface of the photosensitive drum 11 over a portion of the photosensitive drum arc, and as the photosensitive drum 11 rotates, the toner image bearing belt 14 moves together with the photosensitive drum 11. Where the toner image bearing belt 14 passes around the pressure roller 13, it passes between the heating roller 13 and the pressure roller 13. The location where the heating roller 12 confronts the pressure roller 13 forms a transfer and fixing section 28. As the photosensitive drum 11 rotates, its surface sequentially passes various processing sections or devices, namely an electrifying or charging device 15, an exposure device 16, a developing device 17 and a discharging device 33. The exposure device 16, the developing device 17 and the discharging device 33 are provided to face the toner image bearing belt 14 superimposed on the photosensitive drum 11. Between the location where the charging device 15 confronts the photosensitive drum 11 and the location where the exposing device 16 confronts the toner image bearing belt 14, the toner image bearing belt 14 is brought into contact with the photosensitive drum 11. Between the location where the developing device 17 ccnfronts the toner image bearing belt 14 and the location where the charging device 15 confronts the photosensitive drum 11, the toner image bearing belt 14 is separated from the photosensitive drum 11. The discharging device 33 is disposed at or downstream of (with respect to the direction of the movement of the toner image bearing belt 14) the location where the developing device 17 confronts the toner image bearing belt 14. In the embodiment illustrated, it is disposed at the location where the toner image bearing belt 14 separates from the photosensitive drum 17. The charging device 15 is a corona discharge device for providing an electrostatic charge uniformly over the photosensitive surface of the photosensitive drum 11. The charging device 15 may alternatively be formed of a brush discharge device, or any other type of charging means. The exposure device 16 exposes the photosensitive drum 11 through the toner image bearing belt to a light image or radiation pattern to form an electrostatic latent image on the photosensitive surface on the photosensitive drum 11. The areas or dots of the photosensitive surface which have been irradiated by light is discharged, while the areas or dots of the photosensitive drum which have not been irradiated are kept charged. This does not mean that each area can assume either of the two distinct states: charged and discharged. That is, there can be intermediate states and each area is discharged to the degree which is dependent on the density of the corresponding area of the image. However, the following description will be made assuming that the latent image consists of charged areas and discharged areas, for simplicity of explanation and illustration. The toner image bearing belt 14 must be transparent to the wavelengths of the light used for the exposure. The exposure device 16 may be a combination of an LED array and a Selfoc lens (commercial name), or a combination of a laser and an optical imaging system. In these case, the light image is produced by electrical signals representing the image. Such electrical signals are supplied from a controller 30 which also performs the overall control of the apparatus. The exposure device 16 may alternatively be a combination of an illuminating device illuminating an original document and an optical imaging system for directing the light reflected at the surface of the original to the photosensitive surface of the photosensitive drum 11. The developing device 17 is installed facing the outer surface of the toner image bearing belt 14 which is moving in contact with the photosensitive drum 11, as described above. The developing device 17 is provided with a toner support 17a, to which toner 18 is attached. The toner 18 is transported so as to develop the electrostatic latent image to form a corresponding toner image on the outer surface of the toner image bearing belt 14. The developing device may be a two-component magnetic brush developer, a one-component magnetic brush developer, or a one-component non-magnetic developer. The discharging device 33 removes the surface charges from the toner image bearing belt 14. In this embodiment, an AC corona charger is employed. A discharge lamp 26 is disposed to irradiate the photosensitive drum 11 through the toner image bearing belt 14, between the location where the developing device 17 is disposed and the location where the discharging device 33 is disposed. The toner image bearing belt 14 is transparent to the wavelengths of light from the discharge lamp 26, as well as to the wavelengths of light from the exposure device 16. It is advantageous that the discharge lamp 26 emits light whose wavelength component substantially overlap substantially with the wavelength component of the exposure device 16, so that the toner image bearing belt 14 which is transparent to the light from the exposure device 15 is also transparent to the light from the discharge lamp 26. The transparency of the toner image bearing belt 14 to the light from the discharge lamp 26 may however be partial. This is because the intensity of the discharge lamp 26 may be increased to be sufficient for the desired function of the discharge. This is in contrast to the light intensity of the exposure device 15 which should not be excessive and should be optimized to produce electrostatic latent image consisting of charged and discharged areas. The disposition of the discharge lamp 26 upstream of the corona charger 33 is considered advantageous for the reason which will be discussed below. A discharging device is provided to remove the surface charges from the toner image bearing belt before or at the time of the separation thereof from the photosensitive drum. It has however been found that with the prior art system, background noise of the image on the recording paper 20 increases progressively with increased use of the apparatus. This is because the charging potential above toner image bearing belt 14 superimposed on the photosensitive drum 11 gradually decreases. The variation of charging potential is due to the fact that charges are transferred to the toner image bearing belt 14 when it separates from the photosensitive drum 11. Fig. 2 is an enlarged view of the part the toner image bearing belt 14 separating from the photosensitive drum 11 in an image forming system, wherein the optical discharge means is conventionally positioned. When the charging potential on the charged areas on the toner image bearing belt 14 superimposed on the photosensitive drum 11 was measured, it was found to decrease gradually as image forming operation of the system was repeated. When the operation of the apparatus was stopped, the charges on the photosensitive drum 11 was removed by irradiation with light, and the potential on the toner image bearing belt 14 superimposed on the photosensitive drum 11 was measured, it was found to have a polarity opposite to the polarity of the potential on the photosensitive drum 11. As the charging polarity of the photosensitive drum 11 in the above image forming system of Figure 1 was positive, a negative potential was detected on the toner image bearing belt 14. This is due to the fact that, as shown in Figure 2, the toner image bearing belt 14 becomes charged in the area which is separating from the photosensitive drum 11. If image forming is continued in this condition, even if the photosensitive drum 11 is uniformly charged, the toner image bearing belt 14 becomes charged, and the potential on the toner image bearing belt 14 superimposed on the photosensitive drum 11 effectively decreased. If the toner image bearing belt 14 is developed in this condition, increased background noise occurs in the uncharged areas. To investigate the reason why the toner image bearing belt 14 was charged, a comparison was first made with the AC corona charger 33 ON and OFF. When the charger 15 was switched ON and the charging potential on the toner image bearing belt 14 superimposed on the photosensitive drum 11 was measured, it was found that the charging potential was stable over a long period of time when the AC corona charger 33 is OFF, but decreases when the AC corona charger 33 was ON. It is thus evident that the AC corona charger 33 is responsible for the decline of charging potential. A further comparison was made with the AC corona charger 33 ON, and the discharge lamp 26 ON or OFF. It was found that the charging potential decreased sharply when discharge lamp 26 was OFF, but that there was less decrease when it was ON. this is due to the fact that when the discharge lamp 26 is ON, the light from the lamp 26 irradiates the whole of the separating area, thereby removing some of the charges on the surface of the photosensitive drum 11 so that it has less charge. it may thus be conjectured that the decrease of charging potential is related to the fact that charges are present on the photosensitive drum 11 before it reaches the separation area. In the system of Figure 2, therefore, charges on the photosensitive drum 11 move onto the toner image bearing belt 14 due to the AC corona charger 33 installed at the position where the photosensitive drum 11 leaves the toner image bearing belt 14, and as a result of the charging of the toner image bearing belt 14, the charging potential on the toner image bearing belt 14 superimposed on the photosensitive drum 11 declines. The AC corona charger 33 was installed to prevent scattering of the toner image formed on the toner image bearing belt 14 in the separating area. To further prevent charging of the toner image bearing belt 14 when the AC corona charger 33 is ON, therefore, it is evident that a discharge lamp or some other optical discharge means for irradiating the photosensitive drum through the toner image bearing belt should be installed in the separation area or upstream of it to remove charges from the photosensitive drum 11 when or before it reaches this separation area. The improvement of the present invention was conceived to overcome the above problems. It aims to provide a means of preventing charging after the toner image bearing belt, superimposed on the photosensitive drum carrying the electrostatic latent image, has been subjected to development, and when or before the toner image bearing belt separates from the photosensitive drum. It thus aims to provide an image forming system which gives stable recordings over a long period of time. In the image forming system of the present invention the discharging device comprises an AC corona charger and an optical discharge means, such as a discharge lamp, is arranged in apposition to the toner image bearing belt between the AC corona charger and the developing device, as described above. Before the AC corona charger removes surface charges from the toner image bearing belt, the optical discharge means causes the charges on the photosensitive drum to disappear, and charging of the toner image bearing belt due to the AC corona charger which otherwise occurs when the toner image bearing belt separates from the photosensitive drum is thereby prevented. Referring to Figure 3, the discharge lamp 26 used as an optical discharge device is disposed between the developing device 17 and the AC corona charger 33 to irradiate the photosensitive drum 11 through the toner image bearing belt 14. With the arrangement described above, the charging potential was found to remain constant for a long period even though AC corona charger 33 is ON. This is due to the fact that charges on the photosensitive drum 11 are removed by the discharge lamp 26 before they reach the separation area, as shown in the figure, and are therefore not transferred to the toner image bearing belt 14 by the AC corona charger 33 which is installed in the separation area. The operation of the image forming system of the present invention for the various processes will be described with reference to Fig. 4A to Fig. 4F. Fig. 4A shows the charging process, Fig. 4B shows the exposure process, Fig. 4C shows the developing process, Fig. 4D shows the optical discharge process, Fig. 4E shows the separation process, and Fig. 4F shows the transfer and fixing process. When the photosensitive drum 11 is rotated, the toner image bearing belt 14 moves. The charging device 15 uniformly charges the photosensitive drum 11 (Fig. 4A). In the exposure process, an electrostatic latent image corresponding to image signals is formed on the photosensitive drum 11 (Fig. 4B). In the developing process, a toner image corresponding to the electrostatic latent image is formed on the toner image bearing belt 14 (Fig. 4C). When the toner image bearing belt 14 on which a toner image has been formed passes the discharge lamp 26, light from the discharging lamp 26 passes through the toner image bearing belt 14 and reaches the photosensitive drum 11 and removes the charges from the surface of the photosensitive drum 11 (Fig.4D). The light from the discharge lamp 26 is prevented from entering the developing area where the development is carried out. This is achieved by shielding, for example by means of the housing of the developing device 17. When the toner image bearing belt 14 separates from the photosensitive drum 11. no charges remain on the photosensitive drum 11 (Fig. 4E). As no charges remain on the photosensitive drum 11, there is no transfer of charge from the photosensitive drum 11 to the toner image bearing belt 14, when the toner image bearing belt 14 is irradiated with positive and negative charges by the AC corona charger 33 in the separation area. Further, when the toner image bearing belt 14 separates from the photosensitive drum 11, its surface charges are removed by the AC corona charger 33. The toner image bearing belt 14 which has separated from the photosensitive drum 11 is then moved on to the transfer and fixing section 28, where the transfer and fixing is performed (Fig. 4F). Referring to Figure 5, reference numerals identical to those in Figure 1 denote identical or similar devices or elements so their description is omitted. This embodiment differs from that of Figure 1, in that a two-ended toner image bearing belt 102 (having ends in the feed direction) is used in place of the endless toner image bearing belt 14 in Figure 1. The exposure device 16 of this embodiment is disposed to face the photosensitive drum 11 directly, i.e. without inter position of the toner image bearing belt 102. The toner image bearing belt 102 is supplied from a supply roller 103, is passed around a free roller 114, around a photosensitive drum 11, and around the heating roller 12, in the direction shown by the arrow L, and is then wound on a take-up roller 104. A controller similar to the controller 30 in Figure 1 is provided but it is omitted from illustration. The operation of the electrophotography apparatus having the above construction will now be described. The photosensitive drum 11 and the heating roller 12 are rotated at a constant peripheral speed in the direction shown by the arrows J and K by a drive means (not shown). Moreover, a tension mechanism (not shown) applies a suitable tension via the free roller 114 to the toner image bearing belt 102. As a result, due to friction between the toner image bearing belt 102 and the photosensitive drum 11 and the contact pressure between the heating roller 12 and the pressure roller 13, the toner image bearing belt 102 moves in the direction L at the same speed as the peripheral speed of the photosensitive drum 11 and the heating roller 12. The operations at the charging section, the exposure section, the developing section, and the transfer and fixing sections are identical to those described with reference to Figures 4A to 4F, but the toner image bearing belt 102 is heated directly, i.e. not through the recording paper 20. After developing, the toner image bearing belt 102 carrying the toner image is irradiated by the discharge lamp 26 and separated from the photosensitive drum 11. The discharging device 33 removes the surface charges from the toner image bearing belt 102. This replaces the rise in the potential difference due to a decrease in the capacitance of the air layer between the toner image bearing belt 102 and the photosensitive drum 11 where they are separating, and prevents discharge through the air layer. Disturbance of the toner image on the toner image bearing belt 102 at the time of separation is thus prevented. After the transfer and fixing, the recording paper 20 is then ejected outside the image forming system, with the toner image facing down, and stacked by a stacker (not shown). After the transfer of the toner image by the recording paper 20, a small amount of toner may remain on the toner image bearing belt 102, and the toner image bearing belt 102 is recovered by the take-up roller 104 with its residual toner still adhering to it. The residual toner recovered by the take-up roller 104, however, has been melted in the transfer and fixing process, and fixed to the toner image bearing belt. There is, therefore, no scattering of toner inside or outside the devices, or over the operator. The embodiments described above are not to be considered exhaustive. For example, the image may also be transferred from the toner image bearing belt to the recording paper electrostatically. Further, instead of transferring the toner image formed on the toner image bearing belt to a recording paper, the image may be made to be seen from outside of the apparatus to serve as a display. Instead of the photosensitive drum, any other member for carrying an electrostatic latent image may be used. Instead of the toner image bearing belt, any other form of medium for carrying a toner image may be used. When the optical discharge device is disposed upstream of the AC corona charger to irradiate the photosensitive drum through the toner image bearing member, the photosensitive drum is discharged before the toner image bearing member passes the AC corona charger. As a result, there will be no transfer of charges from the photosensitive drum to the toner image bearing member at the time when they pass the AC corona charger, so there will be no accumulation of charges on the toner image bearing member as the image forming operation is repeated. The increased background noise in the resultant images are therefore avoided.

Claims: An image forming system comprising: an electrostatic image bearing member (11); a toner image transfer member (14) to receive a toner image upon being passed over the electrostatic image bearing member (11) and thereafter separated therefrom for transferring a toner image to a copy sheet (20); developing means (17) for developing a latent electrostatic image on the electrostatic image bearing member (11) to form a corresponding toner image on the toner image transfer member (14); and an optical exposure device (16) to irradiate said electrostatic image bearing member (11) to form said latent image thereon, characterised by a corona charger (33) for applying an electrostatic field to the toner image on the toner image transfer member (14) such as to tend to reduce disturbance of the toner image by the separation of the toner image transfer member (14) from the electrostatic image bearing member (11); and an optical discharging means (26) disposed between said corona charger (33) and said developing means (17) to irradiate said electrostatic image bearing member (11). An image forming system according to claim 1, wherein said corona charger (33) is an AC corona charger (33). An image forming system according to claim 1 or 2, wherein said corona charger (33) is disposed at the location where the toner image transfer member (14) separates from the electrostatic latent image bearing member (11). An image forming system according to claim 1, 2 or 3, wherein the toner image transfer member (14) is at least partially transparent to light from said optical exposure device (16) and said optical exposure device (16) is arranged to irradiate said electrostatic image bearing member (11) through the toner image transfer member (14). An image forming system according to any preceding claim, wherein the toner image transfer member (14) is at least partially transparent to light from said optical discharging means (26) and said optical discharging means (26) is arranged to irradiate said electrostatic image bearing member (11) through the toner image transfer member (14). An image forming system according to any preceding claim, further comprising a display section permitting said toner image on said toner image bearing member to be seen from outside of the system. An image forming system according to any preceding claim, wherein said electrostatic latent image bearing member is a photosensitive drum and said toner image bearing member is a belt.

Inventors: ITOH SHINICHI C O OKI ELECTRIC, SATO HIROAKI C O OKI ELECTRIC, TANIGUCHI MASAYUKI C O OKI ELE, ITOH, SHINICHI, C/O OKI ELECTRIC IND. CO., LTD., SATO, HIROAKI, C/O OKI ELECTRIC IND. CO., LTD., TANIGUCHI, MASAYUKI, C/O OKI ELECTRIC IND. CO., LT
Applicants: OKI ELECTRIC IND CO LTD, OKI ELECTRIC INDUSTRY CO., LTD.
Application year: 1990
Main classification: G03G15
IPC codes: G03G15