IMAGE FORMING APPARATUS

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic constitutional view of an embodiment of an electrophotographic image forming apparatus according to the present invention.

FIG. 2 is a schematic sectional view showing details of an image forming portion of an image forming apparatus.

FIG. 3 is a schematic constitutional view of another embodiment of an electrophotographic image forming apparatus according to the present invention.

FIG. 4 is a graph showing a distribution of an amount of charge of fog toner on a photosensitive drum.

FIG. 5 is a schematic view for illustrating a collecting constitution of positive toner in the present invention.

FIGS. 6(
a) and 6(b) are schematic views showing positive toner collecting models.

FIG. 7 is a time chart showing a sequence of a positive toner collecting mode in Embodiment 1 of the present invention.

FIG. 8 is a graph showing a relationship between a rotational speed of a developer carrying member and an amount of toner subjected to development.

FIGS. 9 and 10 are time charts showing sequences of positive toner collecting modes in Embodiment 2 and Embodiment 3, respectively, of the present invention.

FIG. 11 is a schematic constitutional view showing an embodiment of a conventional electrophotographic image forming apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinbelow, the image forming apparatus according to the present invention will be described more specifically with reference to the drawings.

Embodiment 1
[General Constitution of Image Forming Apparatus]

First, a general constitution and operation of an image forming apparatus of this embodiment will be described. In this embodiment, the image forming apparatus employs an intermediary transfer method and a simultaneous development and cleaning method and a general constitution of the image forming apparatus is shown in FIG. 1.

Referring to FIG. 1, an image forming apparatus 200 is a train-of-four tandem-type image forming apparatus and includes a plurality of image forming portions (or stations) P. These image forming portions P are constituted by first to fourth image forming stations P (PY, PM, PC and PK) for forming images of yellow, magenta, cyan, and black, respectively. In this embodiment, during passage of an intermediary transfer member 51 constituting a transferring device 5 through the respective image forming stations P while being moved in a direction of an indicated arrow, respective color images are superposed on the intermediary transfer member 51 at the respective image forming stations P. These multiple toner images superposed on the intermediary transfer member 51 are transferred onto a recording material S such as a transfer sheet as a recording medium. The intermediary transfer member and the recording medium are herein inclusively referred to as a transfer medium.

In this embodiment, constitutions of the respective image forming stations P are substantially identical to each other except that development colors are different from each other. Accordingly, in the case where a particular distinction among the stations P is not required, suffixes Y, M, C and K added for identifying the image forming stations P are omitted and the image forming stations P are collectively described. Further, the respective image forming stations P (PY, PM, PC and PK) have the same constitution as those described above with reference to FIG. 2.

More specifically, referring to FIGS. 1 and 2, as an image bearing member in this embodiment, the image forming station P includes a cylindrical image bearing member 1 comprising an organic photoconductor (OPC) having a negatively chargeable surface layer (hereinafter, referred to as a “photosensitive drum”). The photosensitive drum 1 is rotationally driven in an indicated arrow direction. A charging device 2 as a charging means is provided in contact with the photosensitive member 1. In this embodiment, the charging device 2 includes a charging roller 2 as a contact charging device and electrically charges the photosensitive drum 1 by applying a charging bias voltage to the charging roller 2. An exposure device 3 as an exposure means is disposed downstream from the charging device 2 with respect to the rotational direction of the photosensitive member 1 and exposes the surface of the photosensitive drum 1 to light to form an electrostatic image. In this embodiment, the exposure device 3 is a laser scanning exposure optical system including a laser light source, a polygon mirror, etc.

A developing device 4 is provided further downstream from an exposure position of the photosensitive drum 1 so as to be adjacent to the photosensitive drum 1. In this embodiment, the developing device 4 employs, as a developer, a two-component developer principally containing resinous toner particles (toner) and magnetic carrier particles (carrier). The developer is carried on a roller-type developer carrying member (developing sleeve) 41 containing therein a magnet and regulated by a developing blade 42 as a developer layer thickness regulation member to be conveyed to the photosensitive drum 1. More specifically, a magnetic brush with the developer is formed on the developing sleeve 41 and brought into contact with the surface of the photosensitive drum 1. Simultaneously, a developing bias voltage is applied from a power source S4 to the developing sleeve 41 to create an electric field between the developing sleeve 41 and the photosensitive drum 1, whereby toner in the magnetic brush is transferred onto the photosensitive drum 1 depending on the electrostatic image to form a development image, i.e., a transfer image.

In this embodiment, reverse development is effected by using negatively chargeable toner. The reverse development is developing method in which a latent image is formed by performing image signal-dependent light exposure with respect to the photosensitive member surface negatively charged uniformly to lower a potential and is then developed with the negatively chargeable toner. A toner hopper (toner supply means) 43 supplies the toner in a necessary amount to the developing device 4 via a transfer buffer 44 on an as needed basis.

An intermediary transfer belt 51 as the intermediary transfer member charging a transferring device 5 is stretched around stretching rollers 54 (54a, 54b, 54c) and a backup roller 55 and moved (rotationally driven) in an indicated arrow direction in contact with the photosensitive drum 1. The intermediary transfer belt 51 is interposed between the photosensitive drum 1 and a primary transfer roller 52 as a primary transfer means, so that a primary transfer nip (primary transfer portion) N1 (N1Y, N1M, N1C, N1K) is created between the photosensitive drum 1 and the intermediary transfer belt 51. A secondary transfer roller 53 as a secondary transfer means is disposed opposite to the backup roller 55 via the intermediary transfer belt 51 and forms a secondary transfer nip (secondary transfer portion) N2 between it and the intermediary transfer belt 51.

The image forming apparatus 200 includes, as a supplying means of the recording material S, a cassette 9 containing the recording material S and registration rollers 92 for supplying the recording material S fed and conveyed from the cassette 9 by a feeding and conveying roller 91 and the like to the secondary transfer nip N2. In the conveying direction of the recording material S, a fixing device 6 is disposed downstream from the secondary transfer nip N2. Further, an intermediary transfer member cleaner 56 is provided downstream from the secondary transfer nip N2 with respect to a movement direction of the intermediary transfer belt 51.

In this embodiment, as shown in FIG. 2, at a position downstream from the primary transfer portion N1 with respect to the rotational direction of the photosensitive drum 1, a brush-like residual developer uniformizing means (upstream auxiliary charging means) 81 for uniformizing a charge amount of the transfer residual toner on the photosensitive drum 1 is provided as an auxiliary charging device 8. Further, at a position downstream from the upper auxiliary charging means 81 and upstream from the charging roller 2 with respect to the rotational direction of the photosensitive drum 1, a residual developer charge amount control means (downstream auxiliary charging means) 82 for assisting re-charging of the transfer residual toner to be collected by the developing device 4 is provided.

The operation of the above described image forming apparatus 200 will be described.

The photosensitive drum 1 is rotationally driven in the arrow direction and after being negatively charged uniformly by the charging roller 2 at its surface, is exposed to light by the exposure device 3 to form an electrostatic latent image corresponding to a color-separated image obtained by color-separation of an input original at the surface of the photosensitive drum 1. A toner image formed at the surface of the photosensitive drum 1 is primary-transferred onto the intermediary transfer belt 51 rotating at the same speed as that of the photosensitive drum 1 by the action of the primary transfer roller 52 in the primary transfer nip N1.

The above described operation is performed at each of the image forming stations P (PY, PM, PC, PK), so that toner images formed on the respective photosensitive drums 1 are successively transferred onto the intermediary transfer belt 51 in a superimposing manner. In the case of a full-color mode, the toner images are primary-transferred onto the intermediary transfer belt 51 in the order of those of Y (yellow), M (magenta), C (cyan), and K (black). Also in the cases of a single color mode or two or three color mode, necessary color toner or toners are successively transferred onto the intermediary transfer belt 51 in the superimposing manner in the same process as that described above. The resultant toner image formed in the superimposing manner are secondary-transferred by the action of the secondary transfer roller 53 onto the recording material S supplied to the secondary transfer nip N2 by the registration roller and the like. The recording material S onto which the toner images are secondary-transferred is heated and pressed by the fixing device 6 to fix thereon the toner images and is discharged to a sheet discharge tray 92.

On the other hand, the toner image on the photosensitive drum 1 after the primary transfer is electrically charged uniformly to the opposite polarity by the upstream auxiliary brush (upstream auxiliary charging means) 81 as the auxiliary charging device 8. Thereafter, the toner image is electrically charged to the normal polarity by the downstream auxiliary brush (downstream auxiliary charging means) 82 as the auxiliary charging device 8 in order not to deposit the transfer residual toner on the charging roller 2. Then, the toner image is collected in the developing device 4 by a voltage Vback during the developing operation by the developing device 4. The voltage Vback is a potential difference between a potential at a non-image portion on the photosensitive drum 1 and a DC voltage (Vdc) applied to the developing sleeve. From the intermediary transfer belt 51 after the secondary transfer onto the recording material is completed, the secondary transfer residual toner deposited on the intermediary transfer belt surface is removed and collected by the intermediary transfer member cleaner 56 having the cleaning blade.

[Process Operation]

(a) Drum and Charging

In this embodiment, the photosensitive drum 1 is a drum of a negatively chargeable organic photoconductor (OPC), which has an outer diameter of about 30 mm and is rotated at a process speed of 130 mm/sec in a counterclockwise direction as shown in FIGS. 2 and 3.

The charging roller 2 is rotatably held at both end portions of a core metal and is pressed against the surface of the photosensitive drum 1 at a predetermined pressing force and rotated by the rotation of the photosensitive drum 1.

In this embodiment, by applying an oscillating voltage in the form of an AC voltage consisting of a sinusoidal wave having a frequency of 1300 Hz and a peak-to-peak voltage of Vpp of 1300 V biased with a DC voltage of −550 V from a power source S1 to the charging roller 2, a peripheral surface of the photosensitive drum 1 is electrically charged uniformly in a contact charging manner. The resultant potential is a potential at the non-image portion.

(b) Auxiliary Charging

In this embodiment, as the auxiliary charging device 8, the upstream and downstream auxiliary charging means, i.e., the brush-like residual developer uniformizing means (upstream auxiliary brush) 81 and the brush-like residual developer charge amount control means (downstream auxiliary brush) 82 are provided.

To the upstream auxiliary brush 81 as the upstream auxiliary charging means, from a power source S2, an oscillating voltage in the form of an AC voltage biased with a DC voltage to positively charge the transfer residual toner mixedly including weakly negative polarity toner and positive toner.

Further, to the downstream auxiliary brush 82 as the downstream auxiliary charging means, from a power source S3, a DC voltage is applied to negatively charge the transfer residual toner so that the positively charged transfer residual toner can be collected and held and the transfer residual toner can be collected by the developing device 4 in combination with the charging roller 2. Further, both the upstream auxiliary brush 81 and the downstream auxiliary brush 82 are reciprocated at a predetermined period in a main scanning direction in order to obviate localization of toner deposition amount in the main scanning direction varying depending on an image to be formed.

In this embodiment, an oscillating voltage in the form of an AC voltage having a frequency of 1300 Hz and a peak-to-peak voltage Vpp of 200 V biased with a DC voltage of 250 V is applied from the power source S2 to the upstream auxiliary brush 81 and a DC voltage of −750 V is applied from the power source S3 to the downstream auxiliary brush 82.

In this embodiment, the exposure device 3 for forming the electrostatic latent image at the surface of the electrically charged photosensitive drum 1 is a laser beam scanner using a semiconductor laser. This laser beam scanner outputs laser light modulated in correspondence with an image signal sent from a host processing device such as an image reading device or the like to a printer side, thus subjecting the uniformly charged surface of the photosensitive drum 1 to laser scanning exposure. The exposed portion is an image portion. A potential at the image portion is set to about −150 V in this embodiment.

(d) Development

In this embodiment, a two-component developer in the form of a mixture of negatively chargeable polymerized toner having a particle size of 6 μm and a magnetic powder dispersed carrier having a particle size of 40 μm and an electric resistance of about 10¹³Ω·cm is used. Further, in this embodiment, the developer is supplied to the developing sleeve 41 as the developer carrying member while being conveyed and stirred by a developer conveying and stirring member in the developing device 4. The developer supplied to the developing sleeve 41 is coated no the developing sleeve 41 in a thin layer by a developer amount regulating means (developing blade) 42. The developing sleeve 41 supplies the toner to the photosensitive drum 1 in an opposing area by a counter development method in which a rotation direction of the developing sleeve 41 is opposite (counter) to that of the photosensitive drum 1.

Further, in this embodiment, a developing bias voltage applied from a power source S4 to the developing sleeve 41 is an oscillating voltage in the form of an AC voltage biased with a DC voltage. More specifically, the oscillating voltage is in the form of an AC voltage having a frequency of 1300 Hz and a peak-to-peak voltage Vpp of 1600 V biased with a DC voltage of −400 V.

(e) Transfer

In this embodiment, the transferring device 5 is of the type wherein the toner is transferred from the photosensitive drum 1 to the recording material S through the intermediary transfer belt (ITB) 51 as the intermediary transfer member.

The intermediary transfer belt 51 is pressed against the photosensitive drum 1 at a predetermined pressing force by the primary transfer roller 52 at the primary transfer portion N1 at which the toner is transferred from the photosensitive drum 1 onto the intermediary transfer belt 51. Further, the recording material S is pressed against the intermediary transfer belt 51 at a predetermined pressing force by the secondary transfer roller 53 at the secondary transfer portion N2 at which the toner is transferred from the intermediary transfer belt 51 onto the recording material S. In this embodiment, a primary transfer voltage of about 400 V is applied from a power source S5 and a primary transfer nip load is about 300 g. Further, a secondary transfer voltage is about 1000 V and a secondary transfer nip load is about 500 g.

(f) Fixing

In this embodiment, as the fixing device 6, a roller fixing device is used at a process speed of 130 mm/sec and an average fixing temperature of 190° C.

[Collecting Constitution of Reversely Charged Toner]

In this embodiment, a reversely charged toner collecting mode is executed during non-image formation (e.g., post-rotation) after a series of image forming operations.

First, without using the present invention, an image having an image duty (print ratio) of 5% was formed on an A4-size recording sheet. Image formation was performed by 250 sets of image forming jobs each including continuous printing on 200 sheets, i.e., 50,000 sheets in total. The print ratio is a proportion of an actually image formed area to a maximum image forming area. As a result, when a fog density on the photosensitive drum 1 was measured by a reflection density meter, the fog density was about 2.5%, thus remarkably lowering an image quality. In this case, a distribution of charge amount of the fog toner is as shown in FIG. 4, wherein a large amount of the reversely charged toner as a positive toner component T is present. Further, it was confirmed that on the photosensitive drum 1, a deposited matter of fused toner other than the fog toner was present, i.e., filming occurred.

In this embodiment, the collecting mode of the reversely charged toner (i.e., the positive toner) to be executed in the constitution of the present invention was executed during the post-rotation (i.e., during the non-image formation) after the continuous image formation on 200 sheets (after one job). Further, the printing on 50,000 sheets was performed while the fog toner as the positive toner component carried by the photosensitive drum 1 was collected.

The constitution in accordance with the present invention in the positive toner collecting mode will be described with reference to FIGS. 5 to 7. The positive toner collecting mode is executed by control by means of a controller 300 as a control means.

According to the present invention, during the post-rotation after the job is completed, a rotational speed of the developing device 4, i.e., the developing sleeve 41 is decreased to 50% or less of that during ordinary image formation and the photosensitive drum 1 and the intermediary transfer belt 51 are rotated at rotational speeds equal to those during the ordinary image formation. In this embodiment, the rotational speed of the developing sleeve 41 is 10% of that during the ordinary image formation. In this case, a voltage is not applied to the charging roller 2, a surface potential of the photosensitive drum 1 is 0 V, and a voltage consisting of a DC voltage of −200 V and an AC voltage of 1600 V is applied to the developing sleeve 41 as a positive toner collecting bias. In the positive toner collecting mode, operation states of the respective constitutional factors are shown in a time chart of FIG. 7. Further, as shown in FIG. 6(a), the positive toner on the photosensitive drum 1 is collected in the developing device 4 by the potential difference between the surface of the photosensitive drum 1 and the developing sleeve and the oscillating voltage and is stirred together with the developer in the developing device 4 to be subjected to a subsequent developing process while possessing the normal charge polarity. On the other hand, the toner on the developing sleeve 41 is transferred onto the photosensitive drum 1. However, the peripheral speed of the developing sleeve 41 is decreased, so that consumption of the positive-polarity toner can be suppressed.

Further, to the primary transfer portion N1, the voltage of 400 V kept as it is after the image formation is applied thereby to transfer the toner consumed at the developing nip portion onto the intermediary transfer belt 51 as it is and is then collected by the intermediary transfer member cleaner 56.

When the fog density on the photosensitive drum 1 after the image formation using the collecting mode in the present invention is performed in the same manner as that described above, i.e., on continuous 200 sheets for each job and 50,000 sheets in total was measured, the fog density was about 1%, so that it was possible to obtain a good quality image. Further, a level of the filming generated by the positive toner carried on the photosensitive drum 1 and strongly adhered to the photosensitive drum 1 by the pressing force from the charging roller 2 was also improved.

FIG. 8 shows a relationship between a rotational speed of the developing sleeve 41 and an amount of toner electrically charged to the normal charge polarity. With respect to the developer at an initial stage, when the rotational speed of the developing sleeve 41 is decreased to about 30% or less of that during the ordinary image formation, an amount of toner subjected to development on the photosensitive drum 1 is significantly decreased. With respect to the developer after being subjected to the image formation on 50,000 sheets, when the rotational speed is decreased to about 50% or less of that during the ordinary image formation, the amount of toner subjected to development on the photosensitive drum 1 tends to be significantly decreased. Therefore, in order to reduce consumption of the toner electrically charged to the normal charge polarity in the positive toner collecting mode, a deceleration ratio of the driving speed of the developing device 4 may preferably be 50% or less and a toner consumption reducing effect is largest when the drive of the developing device 4 is completely stopped. Further, a collecting performance for the positive toner little depends on the driving speed of the developing device 4.

According to this embodiment, a lifetime (about 40,000 sheets) of the photosensitive member (OPC) can be prolonged by 10,000 sheets or more to significantly reduce running cost.

Embodiment 2

With reference to FIG. 9, another embodiment of the present invention will be described. A fundamental structure (constitution) of an image forming apparatus of this embodiment is identical to that of the image forming apparatus 200 in Embodiment 1, so that members substantially having the same constitution and function as those for the image forming apparatus 200 in Embodiment 1 are represented by identical reference numerals or symbols and detailed description thereof will be omitted with the help of the description in Embodiment 1.

In this embodiment, a constitution in which in the case where positively charged toner (positive toner) is deposited on a charging roller 2 having a negative polarity, the positive toner deposited on the charging roller 2 is discharged on a photosensitive drum 1 and is collected simultaneously with positive toner carried on the photosensitive drum 1 is shown.

Similarly as in Embodiment 1, as a constitution during post-rotation, a developing sleeve 41 was stopped immediately after completion of a job and the photosensitive drum 1 and an intermediary transfer belt 51 were rotated at rotational speeds equal to those during image formation.

In this case, in this embodiment, the surface of the photosensitive drum 1 was negatively charged by applying a DC voltage of −250 V to a downstream auxiliary brush 82 as an auxiliary charging device 8. At this time, a potential at the surface of the photosensitive drum 1 was about −200 V. Further, to the charging roller 2, as a bias for discharging the positive toner onto the photosensitive drum 1, a DC voltage of 0 V and an AC voltage having a frequency of 1300 Hz and a peak-to-peak voltage of 1300 V were applied. Further, to the developing sleeve 41, a bias for collecting the positive toner carried on the photosensitive drum 1 and the above described positive toner discharged from the charging roller 2 is applied. This bias includes a DC voltage of −400 V and an AC voltage of 1600 V. To a primary transfer portion N1, a voltage of 400 V kept as it is after the image formation is applied.

In this embodiment, a reversely charged toner collecting mode for performing an operation by the constitution of this embodiment is executed during post-rotation after image formation is continuously performed on 200 sheets (after completion of one job). In this collecting mode, image formation on 50,000 sheets was performed while fog toner consisting of the positive toner carried on the photosensitive drum 1 and the positive toner deposited on and contaminating the charging roller 2 were collected.

By the above described constitution, the filming level improved in Embodiment 1 was improved to the extent that it was retained at a level equal to the initial level. Further, a degree of the charging roller contamination, so that a fluctuation in dark-part potential Vd by the image formation on 50,000 sheets and a fluctuation in toner collecting potential Vback by the fluctuation in Vd were alleviated.

For this reason, when a fog density on the photosensitive drum at the time when the image formation on 50,000 sheets was performed, the fog density was 1% or less, so that a higher quality image was obtained in this embodiment.

Further, when the image formation was continued, the above described image level was retained until 80,000 sheets, so that it was possible to significantly improve lifetimes of image forming stations.

Embodiment 3

With reference to FIG. 10, another embodiment of the present invention will be described. A fundamental structure (constitution) of an image forming apparatus of this embodiment is identical to that of the image forming apparatus 200 in Embodiment 1, so that members substantially having the same constitution and function as those for the image forming apparatus 200 in Embodiment 1 are represented by identical reference numerals or symbols and detailed description thereof will be omitted with the help of the description in Embodiment 1.

In this embodiment, a constitution in which positive toner held by a downstream auxiliary brush 82 as an auxiliary charging device 8 and positive toner deposited on a charging roller 2 are collected simultaneously with positive toner carried on a photosensitive drum 1 is shown.

Similarly as in Embodiments 1 and 2, as a constitution of the positive toner collection, a developing sleeve 41 is stopped during post-rotation after completion of a job and the photosensitive drum 1 and an intermediary transfer belt 51 are rotated at rotational speeds equal to those during image formation.

In this embodiment, the toner collecting mode includes a first reversely charged toner collecting mode 1 and a second reversely charged toner collecting mode 2. First, in the first reversely charged toner collecting mode 1, a bias is not applied to the charging roller 2 and on the other hand, pulse voltages are applied to an upstream auxiliary brush 81 and the downstream auxiliary brush 82 as the auxiliary charging device 8. As a result, negative toner and positive toner deposited on the auxiliary brushes 81 and 82, respectively, are discharged onto the photosensitive drum 1.

In this case, the negative toner is collected by an intermediary transfer member cleaner 56 after being transferred onto an intermediary transfer belt 51 since the negative toner is strongly charged negatively when it is discharged from the auxiliary charging device 8 and generates positive toner when it is collected in the developing device 4. On the other hand, the positive toner is collected by the developing device 4.

In a subsequent second reversely charged toner collecting mode 2, a negative potential is applied to the surface of the photosensitive drum 1 by the downstream auxiliary brush 82 to cause the positive toner deposited on the charging roller 2 to be discharged onto the photosensitive drum 1 similarly as in Embodiment 2, so that the positive toner is collected by the developing device 4.

More specifically, ordinarily, when an image forming operation is performed, a DC voltage of 200 V is applied to the upstream auxiliary brush 81 as the auxiliary charging device 8, so that the negative toner as the transfer residual toner is deposited on the upstream auxiliary brush 81. Further, a DC voltage of −750 V is applied to the downstream auxiliary brush 82, so that the positive toner as the transfer residual toner and the positive toner changed in polarity from the negative toner by the upstream auxiliary brush 81 are held by the downstream auxiliary brush 82. Further, a DC voltage of −550 V is applied to the charging roller 2, so that the positive toner is deposited on the charging roller 2.

In the first reversely charged toner collecting mode 1, by alternately applying potentials of 200 V and 0 V ten times with a period of 200 msec to the upstream auxiliary brush 81, the negative toner is subjected to electrical oscillation to be eliminated from the brush, thus being discharged on the surface of the photosensitive drum 1. Further, to the downstream auxiliary brush 82, potentials of 0 V and −200 V are alternately applied ten times with a period of 200 msec, so that the positive toner is discharged on the surface of the photosensitive drum 1. By abruptly applying the potentials changed between 0 V to −200 V, a large current transiently flows from the brush into the photosensitive drum 1. For this reason, the surface of the photosensitive drum 1 has a potential of −200 V or more, so that a potential difference is generated.

The positive toner of the downstream auxiliary brush 82 is deposited and discharged on the surface of the photosensitive drum 1 electrostatically by the potential difference. In this case, the positive toner is discharged with a period shifted by half period so that the discharge position does not overlap with the discharge position of the toner discharged from the upstream auxiliary brush 81.

In this case, a voltage is not applied to the charging roller 2, so that the toner discharged from the auxiliary charging device 8 is passed through the charging roller 2 without stopping. To the developing sleeve 41, as a collecting voltage for the positive toner, a DC voltage of −400 V and an AC voltage of 1600 V are applied. The negative toner discharged from the upstream auxiliary brush 81 is transferred onto the intermediary transfer belt 51 at a transfer position and collected by the intermediary transfer member cleaner 56.

Then, in the second reversely charged toner collecting mode 2, to the downstream auxiliary brush 82, a voltage of −250 V is applied, so that the surface of the photosensitive drum 1 is negatively charged. Further, to the charging roller 2, a DC voltage of 0 V and an AC voltage having a frequency of 1300 Hz and a peak-to-peak voltage of 1300 V are applied, so that the positive toner on the charging roller 2 is discharged on the photosensitive drum 1. The positive toner discharged on the surface of the photosensitive drum 1 is collected in the developing device 4 and stirred together with the developer in the developing device 4, so that the resultant developer is subjected again to the developing process.

By the above described constitution, the filming level at the time of performing image formation on 50,000 sheets was improved to the extent that it was retained at a level equal to the initial level. Further, a degree of the charging roller contamination, so that a fluctuation in dark-part potential Vd by the image formation on 50,000 sheets and a fluctuation in toner collecting potential Vback by the fluctuation in Vd were alleviated.

Embodiment 4

Another embodiment of the present invention will be described. A fundamental structure (constitution) of an image forming apparatus of this embodiment is identical to that of the image forming apparatus 200 in Embodiment 1, so that members substantially having the same constitution and function as those for the image forming apparatus 200 in Embodiment 1 are represented by identical reference numerals or symbols and detailed description thereof will be omitted with the help of the description in Embodiment 1.

In the embodiments 1 to 3 described above, at regular time intervals, the sequence of the positive toner collecting mode is performed during the post-rotation between jobs. However, contamination of the photosensitive drum 1, the charging roller 2, and the upstream and downstream auxiliary brushes 81 and 82 is caused by generation of the positive toner in the developing device 4 by collecting the toner deteriorated by the continuous image formation and the strongly negatively charged toner through the simultaneous development and cleaning. For this reason, a degree of initial contamination is slight but the above described sequence is frequently performed, so that downtime can be rather increased.

In this embodiment, storing means (400Y, 400M, 400C, and 400K), i.e., IC tags (memories) are provided to the image forming portions, i.e., the image forming stations P, and the number of sheets subjected to image formation at an associated image forming station P is written and counted in an associated storing means as IC tag information. The thus counted sheet number stored in the IC tag is read by a controller. Based on the counted sheet number, a table showing a relationship between an image forming sheet number and an execution frequency stored in a read-only memory (ROM) provided as a storing device to the image forming apparatus is referred to. Then, a judgment as to whether the sequence of the collecting mode including the constitutions of the present invention described in Embodiment 1 to Embodiment 3 is executed or not is made.

More specifically, the sequence is performed after completion of a series of jobs based on the image formation sheet number stored in the above described table.

In this embodiment, the positive toner collection of the positive toner is performed by the constitution described in Embodiment 3. Specifically, in this embodiment, the positive toner collection described above is performed in accordance with the following setting.

That is, the positive toner constitution was performed by executing a collecting mode every 20,000 sheets from a first sheet to a 10,000-th sheet, every 1,000 sheets from a 10,001-th sheet to a 20,000-th sheet, and every 500 sheets from a 20,001-th sheet to a 40,000-th sheet with respect to 40,000 sheets which was a lifetime of a conventional photosensitive drum 1.

In this case, with respect to levels of fog and contamination, it was possible to keep good levels until about 80,000 sheets similarly as in Embodiment 3. When the positive toner collecting mode is executed every 200 sheets, the positive toner collecting mode is executed 250 times, when the image formation on 50,000 sheets is performed. However, in this embodiment, the positive toner collecting mode was executed until 50,000 sheets, so that it was possible to significantly educe downtime compared with the constitutions of Embodiment 1 to Embodiment 3. Particularly, it was possible to significantly reduce the frequency of the positive toner collection at an initial stage.

Embodiment 5

In this embodiments 1 to 3 described above, at regular time intervals, the sequence of the positive toner collecting mode is performed during the post-rotation between jobs. However, contamination of the photosensitive drum 1, the charging roller 2, and the upstream and downstream auxiliary brushes 81 and 82 is caused by collecting, in the developing device 4, the toner deteriorated by the continuous image formation and the strongly negatively charged toner through the simultaneous development and cleaning. That is, the contamination is caused by generation of the positive toner in the developing device 4 by collecting the above described toners. For this reason, a degree of initial contamination of the photosensitive drum 1, the charging roller 2, and the upstream and downstream auxiliary brushes 81 and 82 is slight but the above described sequence is frequently performed, so that downtime can be rather increased.

Further, the contamination of the upstream and downstream auxiliary brushes 81 and 82 as the auxiliary charging device 8 or the charging roller 2 with the positive toner carried on the photosensitive drum 1 is frequently caused in the case of forming a high duty image. This is because the amount of the transfer residual toner in the case of forming the high duty image is higher than that in the case of forming a low duty image. For this reason, in the case of forming the low duty image of a print ratio of 5% as in Embodiment 4, it is possible to reduce the downtime by the above described constitution. However, in the case of continuously forming the high duty image when the frequency of the positive toner collecting mode at the initial stage is low, the contamination of the photosensitive drum 1, the charging roller 2, and the upstream and downstream auxiliary brushes 81 and 82 is caused before execution of a subsequent positive toner collecting mode. As a result, an image quality can be lowered. In the present invention, the positive toner collecting mode is executed at the time when the print ratio reaches a predetermined value (a reference value determined in advance). In this embodiment, the predetermined value is 10%.

In this embodiment, image formation on 50,000 sheets similarly as in Embodiment 4 was performed in such a manner that a 5%-duty A4-size image was formed on 200 sheets and thereafter a 20%-duty A3-size image was formed on 10 sheets. First, the image formation was performed by the method of Embodiment 4 and when the fog density on the photosensitive drum 1 was measured at the time when the image formation on 50,000 sheets was completed, the fog density was 1.5%, so that a resultant image quality was lowered compared with the cases where the 5%-duty image was formed by the methods of Embodiment 1 to Embodiment 4.

In addition to the method of Embodiment 4, the positive toner collecting mode was also executed after the job for the high duty image was completed. In this case, when the image formation on 50,000 sheets was performed in a similar manner and then the fog density on the photosensitive drum 1 was measured, the fog density was 1% or less to provide a good image. Further, the contamination of the photosensitive drum 1, the charging roller 2, and the auxiliary charging device 8 was well alleviated.

According to this Embodiment, it was possible to improve the lifetime of the image forming portion P while the contamination of the photosensitive drum 1, the charging roller 2, and the auxiliary charging device 8 was alleviated and the downtime was reduced.

Embodiment 6

In this embodiments 1 to 3 described above, the sequence of the positive toner collecting mode is performed during the post-rotation between jobs. However, contamination of the photosensitive drum 1, the charging roller 2, and the upstream and downstream auxiliary brushes 81 and 82 is caused by collecting, in the developing device 4, the toner deteriorated by the continuous image formation and the strongly negatively charged toner through the simultaneous development and cleaning. That is, the contamination is caused by generation of the positive toner in the developing device 4 by collecting the above described toners. For this reason, a degree of initial contamination of the photosensitive drum 1, the charging roller 2, and the upstream and downstream auxiliary brushes 81 and 82 is slight but the above described sequence is frequently performed, so that downtime can be rather increased.

In this embodiment, as shown in FIG. 2, as shown in FIG. 2, ammeters 81a and 82a as detecting means for detecting amounts of currents passing through the upstream and downstream auxiliary brushes 81 and 82, respectively, as the auxiliary charging device 8 are provided and detected values are monitored by a controller 300 as a control means. Values of the currents passing through the upstream and downstream auxiliary brushes 81 and 82 are ordinarily about 20 μA±10 μA, and these brushes hold the toner and external additive to be contaminated with the toner and external additive, thus increasing electric resistances to lower the amounts of the currents. In this embodiment, a threshold value is taken as 10 μA and a judgment as to whether the contamination of the brushes is caused or not is made by the above described ammeters 81a and 82a and the controller 300. As a result, a judgment as the whether the positive toner collecting mode should be executed or not is made.

When the controller 300 judges that the brushes are contaminated, if a reserved sheet number for an executing job is 100 sheets or less, the positive toner collecting mode is executed after completion of the job. If the reserved sheet number is not less than 100 sheets, the positive toner collecting mode is executed immediately after the image formation on 100 sheets is performed.

In this embodiment, image formation was performed in such a manner that a job for forming a 5%-duty A4-size image on 200 sheets and a job for forming a 20%-duty A3-size image on 10 sheets were performed in an arbitrary order with an arbitrary number of executions. First, when the fog density on the photosensitive drum 1 at the time when the image formation on 50,000 sheets was performed is measured, the fog density was about 1.0% or less, thus resulting in a good level. Further, filming or the like was not caused on the photosensitive drum 1 and there was no contamination of the charging roller 2 and the auxiliary charging device 8. When the sheet number exceeded 85,000 sheets, the fog density first exceeded 1.0% and the filming started to occur on the photosensitive drum 1 to some degree. In this embodiment, the positive toner collecting mode executed during the image formation was one time per 500 sheets to 1500 sheets.

According to this embodiment, it was possible to effectively prevent the deposition of the positive toner causing the contamination of the photosensitive drum 1, the charging roller 2, and the auxiliary charging device 8 and it was possible to prevent fog on the photosensitive drum 1, so that it was possible to prolong the lifetime of the image forming station P. Further, by executing the positive toner collecting mode on the basis of the result of detection of the current amount of the auxiliary charging device 8, it was possible to compatibly realize the reduction in downtime and the long lifetime of the image forming station P.

In the above described embodiment, the present invention is described so that it is applicable to the image forming apparatus of the intermediary transfer type but the image forming apparatus of the present invention is not limited thereto.

More specifically, for example, the present invention is also similarly applicable to an image forming apparatus of the type wherein the toner image on the photosensitive drum 1 is directly transferred onto the recording medium, i.e., the transfer material S as schematically shown in FIG. 3. The image forming apparatus shown in FIG. 3 includes an image forming portion having the same constitution as that of the image forming portion of the image forming apparatus described with reference to FIGS. 1 and 2. The image forming apparatus shown in FIG. 3 is also capable of achieving the same action and effect as those of the image forming apparatuses of the above described embodiments.

In the present invention, the developer may also be a developer, other than the above described developer, such as a mono-component developer. A developing means using the mono-component developer is well known in the art.

While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the purpose of the improvements or the scope of the following claims.

This application claims priority from Japanese Patent Application No. 255070/2006 filed Sep. 20, 2006, which is hereby incorporated by reference.

IMAGE FORMING APPARATUS

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