Image forming apparatus

Abstract: A copier includes a transfer charger (56) for electrostatically transferring a toner image which is formed on a surface (50a) of a photoelectric drum (50) to a sheet and a separation charger (57) for separating the sheet with a toner image transferred thereto from the drum surface (50a). A transfer guide (75) is provided on a conveying path upstream of the transfer charger (56) to guide the sheet toward a transfer point (62) provided relative to the photosensitive drum. A separation supporter (76) for supporting the sheet is provided above the separation charger (57). A transfer output from the passing of a leading edge portion of the sheet through the transfer point (62) to the supporting of it by the separation support (76) is switched to a High level, a transfer output from the supporting of the leading edge portion of the sheet until just before the trailing edge of the sheet is moved away from the transfer guide (75) is set to a Middle level, and a transfer output from the moving of the trailing edge portion of the sheet away from the transfer guide (75) until it is passed through the transfer point (62) is set to a Low level. By doing so it is possible to uniformly print a sheet from a leading edge to a trailing edge and form a better-quality image.

Description: The present invention relates to an image forming apparatus for forming a developing agent image on an image carrier and, by transferring the developing agent image on a transfer material, outputting the image onto the material and, in particular, to an image forming apparatus capable of effecting a transfer output switching in accordance with the conveying position of a transfer material. A printer apparatus such as an electrophotographic copying apparatus and an electrostatic recording apparatus as for example disclosed in U.S. Pat. No. 5,225,879 has a transfer charger which is a DC corona discharger and a separation charger which is an AC corona discharger. The transfer charger and the separation charger are arranged to be separated from a photoconductive drum which constitutes the image-bearing member. The transfer charger discharges a back surface of a paper sheet via the DC corona to electrostatically transfer a toner image from the photoconductive drum to the paper sheet. Then the separation charger discharges the paper sheet via the AC corona to electrostatically separate the paper sheet with the toner image from the photoconductive drum. In the conventional printer apparatus of this type, when the sheet was passed through a transfer area between the drum surface and the respective charger, the outputs of the transfer charger and separation charger were controlled at all times to given levels. For the reason, in a state in which the sheet was supported by both a transfer guide provided upstream of the transfer charger and a separation supporter provided relative to the separation charger and intimately contacted with the drum surface, it was possible to obtain a better transfer process. In the case where a sheet left in a high humid environment for a longer period of time is passed through the transfer area, if the leading edge portion of the sheet is passed over from the transfer guide to the separation supporter, then the leading edge portion of the sheet is separated away from the drum surface due to a decrease in stiffness of the sheet, so that there occurs a partial loss in intimate contact between the drum surface and the sheet and hence an imperfect image transfer. Further, if such partial loss in close contact between the sheet and the drum sheet occurs due to a partial separation of the trailing edge of the sheet from the transfer guide, then the trailing edge portion of the sheet moves nearer the transfer charger and separation charger. As a.result, the toner image on the sheet oscillates under a strong influence from the corona charges of the transfer charger and separation charger and the transfer capability of the toner image is adversely affected, so that an imperfect image, such as a partial image loss, occurs. US 5 541 718 discloses an apparatus according to the preamble of claim 1. An apparatus without guide immediately downstream of the transfer means is known from JP-A-60-134 264. It is accordingly the object of the present invention to provide an image forming apparatus for electrostatically transferring a developing agent image which is formed on an image carrier onto a transfer material and, by doing so, outputting the image onto the material, which can uniformly transfer a developing agent image from a leading edge through to a trailing edge of the transfer material and create a better-quality image. This object is achieved by an image forming apparatus according to claim 1. This summary of the invention does not necessarily describe all necessary features so that the invention may also be a sub-combination of these described features. The invention can be more fully under stood from the following detailed description when taken in conjunction with the accompanying drawings, in which: FIG. 1 is a cross-sectional view showing main portion of an image forming apparatus according to an embodiment of the present invention; FIG. 2 is a diagrammatic view showing a transfer charger, separation charger and its surrounding structure incorporated into the copier of FIG. 2; FIG. 3 is a view for explaining a change in abutting angle of a sheet against a drum surface when a diameter of a photosensitive drum is varied; FIG. 4 is a view for explaining the behavior of a trailing edge of the sheet when the trailing edge portion of the sheet is passed between the transfer charger and the photosensitive drum; FIG. 5 is a graph showing a variation in transfer level against transfer outputs corresponding to a leading edge portion, middle portion and trailing edge portion of a sheet; FIG. 6 is a block diagram showing a control system for effecting the output control of a transfer charger and separation charger; and FIG. 7 is a timing chart for effecting an output control of a transfer charger. An embodiment of the present invention will be explained in more detail below with reference to the accompanying drawing. FIG. 1 is a diagrammatic view showing a major section of an image forming apparatus of the present invention. Around a photosensitive drum 50 serving as an image carrier, a charger 52 is arranged to uniformly charge the photosensitive drum surface 50a. The drum surface 50a charged by the charger 52 is exposed, by a light exposure device not shown, to a beam corresponding to an image signal to form an electrostatically latent image thereon. A developing unit 54a for developing the electrostatically latent image is provided on a downstream side of the charger 52 along a moving direction of the photosensitive drum 50. A transfer unit 56 and separation unit 57 are arranged downstream of the developing unit 54a. A cleaning unit 58 is provided downstream of the transfer unit 56 and separation unit 57 and, after a developing agent image has been transferred by the transfer unit 56 from the drum surface 50a to a medium such as a sheet P, eliminates the developing agent image remaining on the drum surface 50a. The sheet P is sent by a pair of register rollers 65 onto the photosensitive drum in a predetermined timing, followed by the transferring of the developing agent image. The image-transferred sheet P, being separated from the drum surface 50a, is sent by a transfer unit 67 onto a fixing unit not shown. The above-mentioned transfer charger 56, separation charger 57 and associated surrounding members will be explained below with reference to FIG. 2. The transfer charger 56 has a corona discharge wire 56a and its surrounding shield casing 56b. The separation charger 57 has a corona discharge wire 57a and its surrounding shield casing 57b. The shield casing 56b of the transfer charger 56 and shield casing 57b of the separation charger 57 are assembled into an integral structure with a partition section shared therebetween. A transfer guide 75 is provided on the conveying path at a position upstream of the transfer charger 56, that is, on the entry side of the transfer charger 56 so as to guide the sheet P. A separation supporter 76 for guiding the sheet P is provided above the shield casing 57b of the separation charger 57 so as to close an upper opening of the separation charger 57. Thus, the sheet P is passed from the transfer guide 75 to the separation supporter 76 and conveyed in a state to be intimately contacted with the drum surface 50a. It is to be noted that the transfer guide 75 and separation supporter 76 constitute, together with the above-mentioned register roller pair 65 and conveying unit 67, a conveying means for conveying the sheet P past the image transfer point 62. By such an arrangement with the sheet P supported by a forward end 75a of the transfer guide 75 and upper surface of the separation supporter 76, the sheet P is intimately contacted with the drum surface 50a as indicated by a solid line in FIG. 2. It is, therefore, possible to obtain a better image under a better transfer condition. When the leading edge of the sheet P is occupied between the forward end 75a of the transfer guide 75 and an angled area 76a of the separation supporter 76 situated on the most upstream side, that is, the leading edge portion of the sheet P is placed in a not-supported state, a phenomenon "sagging" occurs at the leading edge portion of the sheet P as indicated by a dashed line in FIG. 2. This phenomenon is prominent, in particular, in a sheet P left under a high moist environment for a longer period of time. At that place, a gap is created relative to the drum surface 50a, so that it impairs an intimate contact of the leading edge of the sheet P with the drum surface 50a. This causes imperfect image transfer. The transfer guide 75 for guiding the sheet P toward the image transfer point 62 is positioned in a state upwardly tilted toward the photosensitive drum 50 so that the leading edge of the guided sheet P may be advanced at a give angle toward the drum surface 50a. If, therefore, the diameter of the photosensitive drum 50 is smaller as indicated by the dash line in FIG. 3, the sheet P is advanced at an abrupt angle. If, therefore, the diameter of the photosensitive drum 50 is made smaller, then the leading edge of the sheet P is liable to be bent down upon abutting against the drum surface 50a, so that the leading edge portion of the sheet P sags. When the sheet P is passed through the image transfer point 62 and, as shown in a solid line, the trailing edge portion of the sheet P is moved apart from a forward end 75a of the transfer guide 75, then the trailing edge portion of the sheet P sags under its own weight. At this time, a gap G is created between the trailing edge portion of the sheet P and the drum surface 50a. In this way, if intimate contact fails between the drum surface 50a and the trailing edge portion of the sheet P, an image imperfection occurs at the trailing edge portion of the sheet P corresponding to a width B between the corona discharge wire 56a of the transfer charger 56 and the forward end 75a of the transfer guide 75. That is, when the leading or trailing edge of the sheet P approaches away from the drum surface 50a toward the transfer charger 56 and separation charge 57 side, the toner image on the sheet P is discharged, while oscillating due to a stronger influence by the corona discharge at the transfer charger 56 and separation charger 57, so that the transferring of the toner image is adversely affected and hence there occurs an image imperfection. According to the present embodiment, therefore, in order to effect better transfer across the whole length of the sheet P, the outputs of the transfer charger 56 and separation charger 57 are switched in accordance with the conveying position of the sheet P. In this connection, examination is made on a variation in transfer level when the output of the transfer varies at the leading edge portion, middle portion and trailing edge portion of the sheet as viewed along the sheet conveying direction. FIG. 5 shows a result of tests conducted. From the graph of FIG. 5 it is found that, when the transfer output is set to about 6 to 9 µA (8 µA in particular), an adequate transfer level is reached at the middle of the sheet, that is, at the portion of the sheet P intimately contacted with the drum surface 50a. For the leading edge section of the sheet P, on the other hand, the sheet P is not intimately contacted wit the drum surface 50a due to the sagging of the leading edge portion of the sheet P and, in particular, image imperfection is liable to occur under a high humid environment. In order to achieve an adequate transfer level, it is required that the leading edge portion of the sheet P be set to a high transfer output level (about 10 to 14 µA, in particular, 12 µA). For the trailing edge of the sheet, the sheet is not intimately contacted with the drum surface 50a and, under a low humid environment, transfer spots or marks are liable to occur during the transfer process. In order to achieve an adequate transfer level, it is necessary to set the trailing edge of the sheet to a lower transfer output (about 3 to 6 µA, in particular, 4 µA) than the middle portion of the sheet. Stated in another way, it is possible to achieve an adequate transfer level over a full length, and along the conveying direction, of the sheet P by setting the leading edge portion of the sheet to a relatively high transfer output of about 10 to 14 µA, the middle portion of the sheet to a transfer output of about 6 to 9 µA and the trailing edge portion of the sheet to a relatively low transfer output of about 3 to 6 µA. That is, it is possible to form an image of better quality over a full length of the sheet P by switching the transfer output in accordance with the conveying position of the sheet P. Further, according to the present invention, at the time of the switching control of the transfer output with respect to the above-mentioned trailing edge portion of the sheet, the "separation" output by the separation charger 57 is also switched simultaneously. That is, for the trailing edge portion of the sheet, a portion of a charge created on the sheet P at the image transfer point 62 is discharged by the separation charger 57, in the case where the resistance of the sheet P is lower under a high humid condition, in particular, and imperfect image transfer is liable to occur. It is, therefore, necessary to, while effecting the switching control of the transfer output with respect to the sheet's trailing edge portion, switch the "separation" output to the low level. For the leading and trailing edge portions of the sheet P as distinct from the middle portion of the sheet P, different proper values are necessary to achieve an adequate transfer level and, according to the present invention, it is possible to detect the size of a sheet P being conveyed, monitor the conveying position of the sheet P passing through the transfer/separation path on the basis of the result of the size detection and effect the switching control of the transfer output and "separation" output in accordance with the conveying position of the sheet P. FIG. 6 shows a block diagram showing a control system for switching the transfer output of the transfer charger 56 and "separation" output of the separation charger 57 in accordance with the conveying position of the sheet P. A controller 80 includes a pre-register roller switch 66, a register roller clutch 83 for switching the rotation and stopping of the register roller pair 65, a cassette sensor 84 for detecting the size of the sheet P to be fed by detecting the sheet cassette 55, a transfer transformer 85 comprised of a high voltage transformer for applying a DC voltage to the transfer charger 56, and a control transformer 86 comprised of a high voltage transformer for applying a DC voltage and AC voltage to the separation charger 57. Further, a transfer output switching device 81 connected to the transfer transformer 85, as well as a separation output switching device 82 connected to the separation transfer 86, is connected to the controller 80. Thus, the transfer output control by the transfer charger 56 is switched to a High level (12 µA) immediately before the leading edge of the sheet P reaching the image transfer point 62. At a time point when the leading edge of the sheet P reaches the angled area 76a of the separation supporter 76, the transfer output is switched from the High level (12 µA) to a Middle level (8 µA). Subsequently, immediately before the trailing edge of the sheet P is passed through the forward end 75a of the transfer guide 75, the transfer output is switched from the Middle level (8 µA) to a Low level (4 µA) and at a time when the trailing edge of the sheet P is passed through the image transfer point 62, the transfer output is turned OFF. On the other hand, the "separation" output control by the separation charger 57 is switched simultaneously with the transfer output control by the trailing edge of the sheet P. That is, the "separation" output, like the transfer output by the trailing edge of the sheet P, is switched, by the trailing edge of the sheet P, from a High level (45K/-200V) to a Low level (3KV/0V), except for the case where, between the sheet P and an adjacent sheet, the output is not rendered OFF. The switching control of the transfer output and "separation" output as set out above are repeated at each passage of the sheet P. In this connection it is to be noted that the surface potential on the photosensitive drum 50 applied by the charging device 52 is set to -750V and a development bias on the developing roller 54 of the developing device 54 is set to -500V. With reference to FIGS. 1 and 7 an explanation will be given in more detail below about the switching timing of the transfer output on the present embodiment. FIG. 7 shows a timing chart at a time of switching the transfer output of the transfer charger 56. FIG. 1 shows a positional relation of respective constituent elements for deciding the switching timings of the transfer outputs. The "separation" output control by the separation charger 57 is done, by the trailing edge of the sheet, in the same way as the transfer output, and, here, any detailed explanation on the switching control of the "separation" output is omitted below. In FIGS. 1 and 7, A represents a time taken for the sheet to be conveyed from a nip position of the register roller pair 65 to the forward end 75a of the transfer guide 75; B, a time taken for the sheet to be conveyed from the forward end 75a of the transfer guide 75 to the image transfer point 62; C, a time taken for the sheet to be conveyed from the image transfer point 62 to the angled area 76a of the separation support 76; and D, a time taken for the transfer output to be given by the transfer charger 56. When a print key of the operation panel, not shown, is depressed and the sheet P is fed from a sheet cassette, the controller 80 obtains size data of the sheet P on the basis of an output from the cassette sensor 84 and obtains a sheet passing timing via the pre-register roller switch 66. The controller 80 calculates the trailing edge of the sheet P on the basis of the sheet size data and sheet passing timing. In other words, the controller calculates a transfer output time D. And the sheet conveying times A and B are calculated and the register roller pair 65 serving as a reference of the transfer output switching timing is turned ON. Then at a time point when a time A+B taken from the register roller pair 65 to the image transfer point 62 passes, the transfer output of the transfer charger 56 goes "High" and is turned ON. At this time, it is predicted that the leading edge of the sheet P will sag under its own weight. Then at a passage of the time C, that is, when the leading edge of the sheet P is passed over to the separation supporter 76, the transfer output of the transfer charger 56 is switched to a Middle level. At a passage of a time (D-(B+C)), that is, when the trailing edge of the sheet P passes through the forward end 75a of the transfer guide 75, the transfer output of the transfer charger 56 is switched from a Middle to a Low level. At this time, the separation charger 57 is switched to a Low level. And the time B after the transfer output of the transfer charger 56 is switched to the Low, that is, immediately after the trailing edge of the sheet P is passed through the image transfer point 62, the transfer output of the transfer charger 56 is rendered OFF. When the pre-register roller switch 66 detects the leading edge of a subsequent sheet P, then the above-mentioned operation is repeated. If, for the manual sheet feeding, the print key on the operation panel, not shown, is depressed, the register roller pair 65 is turned ON, the transfer output to the transfer charger 56 goes High (at A+B) and is tuned ON. At a time point when the pre-register roller switch 66 serving as a timing reference is turned OFF, the transfer output time D is calculated with respect to the sheet P and the same control as set out above is done. Subsequently, the above-mentioned operation is performed each time the register roller pair 65 is turned ON. According to the present invention, as set out above, the transfer output and separation output are switched in accordance with the conveying position of the sheet P, that is, of the leading edge portion, middle portion and trailing edge portion. By doing so it was possible to achieve a uniform transfer capability over the sheet P and create a better-quality image over the full length of the sheet P. The present invention is not restricted to the above-mentioned embodiment and various changes or modifications of the present invention can be made without departing from the scope of the present invention as defined by the appending claims.

Claims: An image forming apparatus for forming an image on a transfer material, comprising developing agent image forming means for forming a developing agent image on an image carrier (50), conveying means (64) for conveying a transfer material through a transfer area along the image carrier, transfer means (56) for transferring the developing agent image on the image carrier to the transfer material by supplying a transfer charge to the transfer material conveyed through the transfer area, a first guide (75) for guiding the transfer material thereon on an upstream side of the transfer area along a conveying direction of the transfer material, and a second guide (76) for guiding the transfer material thereon on a downstream side of the transfer area along a conveying direction of the transfer material, characterized by control means (80) for controlling a transfer charge amount supplied by the transfer means such that a transfer charge amount supplied by the transfer means to the transfer material being guided by the first guide and conveyed along the conveying direction is a first charge amount until a leading edge of the transfer material reaches the second guide, and the transfer charge amount supplied by the transfer means to the transfer material from the first charge amount is switched to a second charge amount, which is less than the first charge amount, when the leading edge of the transfer material reaches the second guide, and the transfer charge amount supplied by the transfer means to the transfer material is switched from the second charge amount to a third charge amount, which is less than the second charge amount, when a trailing edge of the transfer material is separated from the first guide. An image forming apparatus according to claim 1, further comprising separation means (57) for separating the transfer material from the image carrier after the developing agent image is transferred by supplying a separation charge to the transfer material, and that the separation means switches the separation charge at the same time the transfer charge amount is switched to the third charge amount.

Inventors: OONO MICHITERU, OONO, MICHITERU, Oono, Michiteru, KK Toshiba,Int.Prop.Div.
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1998
Main classification: G03G15
IPC codes: G03G15