JP2005091651A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus which has high reliability without degrading the quality of an output image even when a large-sized transfer material such as long-length paper is continuously printed, and is small in size and low in cost. <P>SOLUTION: A developing part forming a toner image by developing a latent image formed on an image carrier while having developer agitated by an agitating member and a carrying part carrying the material to be transferred to which a toner image is to be transferred are provided. Then, motion time for an agitating mode, carrying out agitation of the developer by the agitating member without forming the toner image is adjusted according to the size of the material to be transferred (S5 to S12). <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an image forming apparatus using an electrophotographic system, such as a copying machine, a printer, a facsimile machine, or a complex machine thereof.

  In recent years, in image forming apparatuses such as full-color printers and color copiers, in addition to increasing the quality of output images, downsizing and cost reduction of the apparatuses have been promoted. As an image forming apparatus that achieves such an object, an apparatus that stores a two-component developer and is provided with a developing unit using a biaxial agitation method is disclosed (for example, see Patent Document 1).

Here, the developing unit of the biaxial agitation system mainly includes a developing roller facing the photosensitive drum, and two agitating screws that agitate and convey the two-component developer in the longitudinal direction of the developing roller. It has been done.
Specifically, one stirring screw stirs and conveys the developer from the front side in the longitudinal direction to the back side, and the other stirring screw stirs and conveys the developer from the back side to the front side. The toner constituting the developer is charged to a desired state by the circulation in the longitudinal direction by the two stirring screws. In the developing unit, as the toner in the developer (mainly composed of toner and carrier) is consumed, fresh toner is appropriately supplied from the toner supply unit to the developing unit.

  As described above, since the developing unit of the biaxial agitation method has a simple configuration with a small number of components, the image forming apparatus can be reduced in size and cost.

Japanese Patent Laid-Open No. 2002-148876 (FIG. 2-3)

  The conventional image forming apparatus described above has a problem that the image quality of an output image is lowered when a plurality of transfer materials such as long paper are continuously conveyed and continuously printed.

  Here, the long paper is a transfer paper whose length in the transport direction (sub-scanning direction) is larger than the DLT size (about 17 inches), such as a poster image (pop image) affixed to the store. Say. Such a long sheet is normally handled as an irregular transfer material in the apparatus, and is rarely fed from the sheet feed cassette unit, and is often fed from the manual feed unit. In the case of a poster image as a long paper, in particular, there are many solid images with a large solid area of green, blue, and red, and a high-quality output image without image density unevenness is required.

When these long sheets are continuously printed, uneven density in the longitudinal direction (main scanning direction) is likely to occur. This is due to the following reason.
In the case of continuously printing the transfer material, the transfer material is continuously fed and conveyed with a predetermined conveyance interval (between sheets). Meanwhile, the photosensitive drum is driven to rotate without stopping its rotation. Also on the photosensitive drum, toner images are formed to be transferred to each transfer material at intervals in accordance with the transfer material continuously conveyed.

  Here, the interval between the toner images formed on the photosensitive drum is substantially equal to the interval between the sheets of the transfer material. Then, the stirring mode in the developing unit is performed at a time corresponding to the interval. That is, the stirring mode is to stir the developer by the stirring member without performing the developing process (formation of a toner image on the photosensitive drum) in the developing unit. As a result, the state of the developer in the developing portion that has lost the balance (mainly the uniformity in the longitudinal direction with respect to the toner charge amount and the toner concentration) in one development step is recovered.

Note that fresh toner is appropriately supplied into the developing section even during the stirring mode. Further, in the normal developing process, the developer is always stirred by the stirring member.
Further, the conveyance interval of the transfer material in continuous printing is determined in advance for each size of the transfer material in order to relate to the productivity of the apparatus (the number of prints per unit time). On the other hand, in the case of an irregular transfer material such as a long sheet, when the manual feeding is performed, the conveyance interval is the maximum fixed size (for example, A3 size, DLT size, etc.). It is often set to be equal to the conveyance interval.

  When the material to be continuously printed is long paper, the developer balance after one (one) development process is completed as compared with the material to be transferred of A3 size or DLT size. The collapse becomes larger. This is because in the case of long paper, the time required for one development process increases and the amount of toner consumption also increases. As a result, image density unevenness in the main scanning direction is often observed in the output image of long paper.

  Such a problem is a common problem even if the developing unit agitation method is other than the biaxial agitation method. In other words, in the developing section using a two-component developer, it is necessary to stir the developer by any method. In any agitation method, the longer the development process of one time, the greater the disruption of the developer balance immediately after that. Then, image density unevenness occurs in an image formed in a state where the developer balance is lost.

  The present invention has been made to solve the above-described problems. Even when a large transfer material such as long paper is continuously printed, the image quality of the output image is not deteriorated and is reliable. An object of the present invention is to provide a compact and low-cost image forming apparatus.

  According to a first aspect of the present invention, there is provided an image forming apparatus that develops a latent image formed on an image carrier and forms a toner image while stirring the developer with a stirring member; A transfer unit that transfers a transfer material to which a toner image is transferred, and the operation time of the stirring mode in which the developer is stirred by the stirring member without forming the toner image, and the size of the transfer material It adjusts according to.

  According to a second aspect of the present invention, there is provided the image forming apparatus according to the first aspect of the invention, wherein the adjustment of the operation time in the stirring mode is carried out when the plurality of transfer materials are continuously carried. Adjustment.

  An image forming apparatus according to a third aspect of the present invention is the image forming apparatus according to the first or second aspect, wherein when the size of the transfer material in the transport direction is larger than a predetermined value, The operation time of the stirring mode is lengthened.

  An image forming apparatus according to a fourth aspect of the present invention is the image forming apparatus according to any one of the first to third aspects, wherein the operation time in the stirring mode is set to a size of an image area in the toner image. It adjusts accordingly.

  According to a fifth aspect of the present invention, in the image forming apparatus according to the fourth aspect of the present invention, when the image area is larger than a predetermined value, the operation time of the stirring mode is lengthened. is there.

  According to a sixth aspect of the present invention, in the image forming apparatus according to the fourth or fifth aspect, the size of the image area is based on a latent image formed on the image carrier. It is what you want.

  In the present invention, even when a transfer material having a large size such as long paper is continuously printed, the developer is agitated in accordance with the size in the developing unit. Accordingly, it is possible to provide a highly reliable image forming apparatus in which the image quality of the output image does not deteriorate without complicating the structure of the developing unit.

Embodiment.
Hereinafter, the best mode for carrying out the present invention will be described in detail with reference to the drawings. In addition, in each figure, the same code | symbol is attached | subjected to the part which is the same or it corresponds, The duplication description is simplified or abbreviate | omitted suitably.

FIG. 1 is an overall configuration diagram illustrating an image forming apparatus according to an embodiment.
In FIG. 1, 1 is a main body of a color printer as an image forming apparatus, 2 is an optical unit (writing unit) that emits laser light based on image information, and 20Y, 20M, 20C, and 20BK are colors (yellow, magenta, Cyan, black), 21 is a photosensitive drum as an image carrier housed in each of the process cartridges 20Y, 20M, 20C, and 20BK, 22 is a charging unit that charges the photosensitive drum 21, 23 Is a developing unit that develops an electrostatic latent image formed on the photosensitive drum 21, 24 is a transfer roller that contacts the inner peripheral surface of the transfer belt 30, and 25 is a cleaning that collects untransferred toner on the photosensitive drum 21. Reference numeral 30 denotes a transfer belt that conveys a transfer material P onto which a toner image formed on the photosensitive drum 21 is transferred. 32Y, 32M, 32 , 32BK is a toner supply unit that supplies toner of each color to the developing unit 23 of each process cartridge 20Y, 20M, 20C, and 20BK, 61 is a paper supply unit that stores a transfer material P such as transfer paper, and 66 is a transfer target. A fixing unit for fixing an unfixed image on the material P, 71 is a manual sheet feeding unit on which an irregularly shaped material to be transferred P is placed, and 72 is a material for the material to be transferred P placed on the manual sheet feeding unit 71. A size detection sensor 73 detects the size, 73 is a conveyance roller as a conveyance unit that conveys the transfer material P placed on the manual sheet feeding unit 71, and 100 is a control unit of the apparatus main body 1.

  Here, in each process cartridge 20Y, 20M, 20C, and 20BK, a photosensitive drum 21, a charging unit 22, a developing unit 23, a cleaning unit 25, and the like are integrally held. Then, image formation of each color (yellow, magenta, cyan, black) is performed on the photosensitive drum 21 in each of the process cartridges 20Y, 20M, 20C, and 20BK.

Hereinafter, an operation during normal color image formation in the image forming apparatus will be described.
Each of the four photosensitive drums 21 rotates in the clockwise direction in FIG. First, the surface of the photosensitive drum 21 is uniformly charged at a position facing the charging unit 22 (a charging process). Thereafter, the surface of the photosensitive drum 21 on which the charged potential is formed reaches the irradiation position of each laser beam.

  On the other hand, in the optical unit 2, laser light corresponding to the image signal is emitted from the LD light source corresponding to each color. The laser light is incident on the polygon mirror 3 and reflected, and then passes through the lenses 4 and 5. The laser light after passing through the lenses 4 and 5 passes through different optical paths for each color component of yellow, magenta, cyan, and black (this is an exposure process).

The yellow component laser light is reflected by the mirrors 6 to 8 and then irradiated to the surface of the photosensitive drum 21 of the first process cartridge 20Y from the right side of the drawing. At this time, the yellow component laser light is scanned in the rotation axis direction (main scanning direction) of the photosensitive drum 21 by the polygon mirror 3 that rotates at high speed. Thus, an electrostatic latent image of a yellow component is formed on the photosensitive drum 21 charged by the charging unit 22.
Similarly, the magenta component laser light is reflected by the mirrors 9 to 11 and then irradiated to the surface of the photosensitive drum 21 of the second process cartridge 20M from the right side of the paper, thereby forming an electrostatic latent image of the magenta component. The The cyan component laser light is reflected by the mirrors 12 to 14 and then irradiated to the surface of the photosensitive drum 12 of the third process cartridge 20C from the right side of the paper, thereby forming an electrostatic latent image of the cyan component. The black component laser light is reflected by the mirror 15 and then is irradiated onto the surface of the photosensitive drum 21 of the fourth process cartridge 20BK from the right side of the drawing to form a black component electrostatic latent image.

Thereafter, the surface of the photosensitive drum 21 on which the electrostatic latent images of the respective colors are formed further rotates and reaches a position facing the developing unit 23. Then, the toner of each color is supplied from the developing unit 23 onto the photosensitive drum 21, and the latent image on the photosensitive drum 21 is developed (this is a developing step).
Thereafter, the surface of the photosensitive drum 21 after the development process reaches a position facing the transfer belt 30. Here, the transfer roller 24 is installed at each facing position so as to contact the inner peripheral surface of the transfer belt 30. Then, the toner images of the respective colors formed on the photosensitive drum 21 are sequentially transferred onto the transfer material P conveyed by the transfer belt 30 at the position of the transfer roller 24 (transfer process).

  The transfer belt 30 is stretched and supported by a driving roller and three driven rollers. Then, the transfer belt 30 travels in the direction of the arrow in the drawing by the driving roller.

Then, the surface of the photosensitive drum 21 after the transfer process reaches a position facing the cleaning unit 25. The untransferred toner remaining on the photosensitive drum 21 is collected by the cleaning unit 25 (this is a cleaning process).
Thereafter, the surface of the photosensitive drum 21 passes through a static elimination unit (not shown), and a series of image forming processes is completed.

On the other hand, from the paper supply unit 61 as a conveyance unit, the transfer material P fed by the paper supply roller 62 is guided to the position of the registration roller 64 after passing through the conveyance guide 63. The transfer material P guided to the registration roller 64 is transported toward the contact portion between the transfer belt 30 and the suction roller 27 while the transport timing is controlled.
Thereafter, the transfer material P sequentially passes through the positions facing the four photosensitive drums 21 while being conveyed to the transfer belt 30 running in the direction of the arrow in the drawing. Thus, the toner images of the respective colors are transferred onto the transfer material P, and a color image is formed.

Thereafter, the transfer material P on which the color image is formed is separated from the transfer belt 30 and guided to the fixing unit 66. In the fixing unit 66, the color image is fixed on the transfer material P at the nip portion between the heating roller 67 and the pressure roller 68.
Then, the transfer material P after the fixing process is discharged out of the apparatus main body 1 as an output image by a paper discharge roller 69.
Thus, a series of image forming processes is completed.

Note that the transfer material P fed from the paper feed unit 6 in the form of a paper feed cassette unit is mainly a standard size (for example, A4 size, A3 size, DLT size, etc.) and is predetermined by the apparatus. Size.) On the other hand, when feeding an irregularly sized transfer material P such as long paper, the paper is mainly fed from the manual paper feed unit 71.
In this case, the transfer material P fed by the conveyance roller 73 is guided from the manual sheet feeding unit 71 to the position of the registration roller 64 after passing through the conveyance guide 74 and the size detection sensor 72. Thereafter, the transfer material P guided to the registration roller 64 operates in the same manner as the transfer material P conveyed from the paper feeding unit 61. Here, the size of the transfer material P conveyed from the manual sheet feeding unit 71 in the conveyance direction is detected by the time passing through the size detection sensor 72.

When performing continuous printing (image formation on a plurality of transfer materials P based on a single job command), the above-described series of image formation processes are continuously performed.
Specifically, the transfer material P is continuously fed from the paper feeding unit 61 or the manual paper feeding unit 71 with a predetermined conveyance interval. In the meantime, the photosensitive drum 21 rotates without stopping, and the above-described image forming process is continuously performed.

Next, a process cartridge as an image forming unit in the image forming apparatus main body 1 will be described in detail with reference to FIGS. 2A and 2B. FIG. 2A is a cross-sectional view showing the process cartridge, and FIG. 2B is a cross-sectional view taken along the line AA showing the developing unit 23.
The four process cartridges installed in the apparatus main body 1 have substantially the same structure except that the color of the toner T to be stored is different. Therefore, the four process cartridges are illustrated with the alphabets (Y, M, C, BK) being omitted. To do.

  As shown in FIG. 2A, the process cartridge 20 mainly includes a photosensitive drum 21 as an image carrier, a charging unit 22, a developing unit 23, and a cleaning unit 25, which are integrally stored in a case 26. Has been. The developing unit 23 includes a developing roller 23a, a first stirring screw 23b, a second stirring screw 23c, a doctor blade 23d, a toner concentration sensor 29, and the like, and a developer composed of carrier C and toner T is accommodated therein. Has been. The cleaning unit 25 includes a cleaning blade 25a that contacts the photosensitive drum 21, a cleaning roller 25b, and the like.

The image forming process described above will be described in more detail.
The developing roller 23a rotates in the direction of the arrow in FIG. 2A. As shown in FIG. 2B, the toner T in the developing unit 23 is supplied from the toner supply unit 32 by the first stirring screw 23b and the second stirring screw 23c arranged so as to interpose the partition wall 23e therebetween. The toner T is circulated in the longitudinal direction while being mixed with the carrier C together with the toner T (circulation in the direction of the arrow in FIG. 2B). The frictionally charged toner T is supplied onto the developing roller 23a together with the carrier C.
The toner T in the toner bottle 33 is appropriately supplied from the supply port 26a into the developing unit 23 as the toner T in the developing unit 23 is consumed. Consumption of the toner T in the developing unit 23 is detected by a toner concentration sensor 28 as an optical sensor facing the photosensitive drum 21 and a toner concentration sensor 29 as a magnetic permeability sensor provided in the developing unit 23. The

Thereafter, the toner T carried on the developing roller 23a passes through the position of the doctor blade 23d and then reaches the position facing the photosensitive drum 21 (developing area). At the opposite position, the toner T adheres to the electrostatic latent image formed on the surface of the photosensitive drum 21. Specifically, the toner T adheres to the surface of the photosensitive drum 21 by an electric field formed by a potential difference between the latent image potential in the region irradiated with the laser light L and the developing bias applied to the developing roller 23a.
Most of the toner T adhering to the photosensitive drum 21 is transferred onto the transfer material P. The toner T remaining on the photosensitive drum 21 is collected in the cleaning unit 25 by the cleaning blade 25a and the cleaning roller 25b.

  In the above-described stirring mode, an electrostatic latent image is not formed on the photosensitive drum 21, and toner is not actively attached from the developing roller 23a to the photosensitive drum 21. Therefore, in the developing unit 23, only the developer T and C are agitated and conveyed by the first agitating screw 23b and the second agitating screw 23c as the agitating members. Here, in the agitation mode, by adjusting the electric field strength in the development region by changing the development bias, it is possible to improve the margin for preventing background contamination on the photosensitive drum 21.

Here, in the present embodiment, other conditions in the image forming process are as follows.
The outer diameter (sleeve diameter) of the developing roller 23a is 18 mm, the gap between the developing roller 23a and the photosensitive drum 21 is 0.4 to 0.6 mm, and the gap between the developing roller 23a and the doctor blade 23d is 0. It is set to be 4 to 0.6 mm. The peripheral speed of the photosensitive drum 21 is 60 to 250 mm / second, and the peripheral speed ratio (linear speed ratio) of the developing roller 23a is set to 1.1 to 1.8.
Further, as the carrier C, a ferrite carrier having an average particle diameter of 50 to 70 μm is used. As the toner T, a toner having an average particle diameter of 6 to 7 μm using a polyol resin or a polyester resin as a base resin is used. Here, the amount of the additive is set to 0.5 to 2.0% by weight. The developer in the developing unit 23 is controlled so that the carrier coverage is 30 to 80% and the toner charge amount (Q / M) is 10 to 30 μC / g.

Next, with reference to FIG. 3 and FIG. 4, the control in the case of continuously printing long paper in the above-described image forming apparatus will be described in detail.
Referring to FIG. 3, first, when a plurality of transfer materials P are placed on the manual sheet feeder 71 and a job command for continuous printing is issued (step S1), the size is detected by the size detection sensor 72 at the start of printing. Reading is performed (steps S2 to S3). Specifically, the length of the transfer material P in the transport direction (sub-scanning direction) is detected. This detection result is transmitted to the control unit 100.

  Thereafter, the toner supply time calculated based on the image area corresponding to the image information transmitted to the optical unit 2 (the area occupied by the toner dots in the image transferred to the transfer material P) is the control unit 100. (Step S4). Here, the toner supply time is a reference time for supplying toner from the toner supply unit 32 to the developing unit 23.

Thereafter, it is determined whether the length of the transfer material P in the transport direction (sub-scanning direction) is larger than the DLT size (17 inches) (step S5).
As a result, if it is determined that the length of the transfer material P in the transport direction is equal to or less than the DLT size, the transfer of the transfer material P is assumed that the continuously-transferred transfer material P is not long paper. The speed (seconds / sheet) is calculated (step S6). Here, the conveyance speed is obtained by referring to the PPM table held in the nonvolatile memory of the control unit 100. For example, if the transfer material P is A3 size (standard), the A3 size transport speed in the PPM table is used as the transport speed in the manual sheet feeder 71 as it is.

Next, based on the determined transport speed, the time for the continuous print transport interval (inter-paper) and the stirring mode operation time are determined (steps S7 to S8). Here, the time between the paper and the stirring mode operation time are both equal (inter-paper time = stirring mode operation time = h1).
Thereafter, the present flow is terminated and the next print process is performed (step S9).

  If it is determined in step S5 that the length of the transfer material P in the transport direction is larger than the DLT size, it is assumed that the transfer material P to be continuously printed is a long sheet of paper and the image is further printed. It is determined whether the image area related to the formation is larger than a predetermined value α (step S10). Here, the image area is calculated based on image information related to the electrostatic latent image formed on the photosensitive drum 21. The predetermined value α can be set to the same value as the image area of the A3 all-solid image, for example.

As a result, if it is determined that the image area is larger than the predetermined value α, it is assumed that the balance of the developer after the development process of one time (one sheet) is finished increases, and the interval between continuous prints is increased. And the stirring mode operation time are determined (steps S11 to S12). Here, the time related to the paper and the stirring mode operation time are both equal and are set to be larger than those in steps S7 and S8 (time between paper = stirring mode operation time = h2> h1. .)
Thereafter, the present flow is terminated and the next print process is performed (step S9).

  On the other hand, if it is determined in step S10 that the image area is equal to or smaller than the predetermined value α, it is assumed that the balance of the developer after the completion of one development process does not become large, and step S7. The following flow is performed. That is, adjustment to increase the paper interval time and the stirring mode operation time is not performed.

With reference to FIG. 4, an embodiment of conveyance driving and development driving controlled based on the control flow of FIG. 3 will be described.
Referring to FIG. 4A, when the size of the transfer material P is A3 size, the conveyance drive motor (not shown) installed in the paper feeding unit 61 or the manual paper feeding unit 71 takes time related to the paper. The transfer material P is continuously conveyed and driven by repeatedly turning on and off so that becomes h1. In synchronism with this conveyance drive, a development drive drive motor (not shown) that drives the developing unit 23 and a main drive motor (not shown) that drives the photosensitive drum 21 and the like are started to drive. As a result, a stirring mode operation time h1 (indicated by a double-headed arrow) equivalent to the paper interval time h1 is secured.

  Referring to FIG. 4C, when the size of the material to be transferred P is long paper, the conveyance drive motor repeatedly turns on and off so that the time between the papers becomes h2, and the material to be transferred P Are continuously transported. The development drive motor and the main drive motor are driven to synchronize with this transport drive. As a result, a stirring mode operation time h2 (indicated by a double-headed arrow) equivalent to the inter-paper time h2 is secured.

Here, FIG. 4B is a timing chart in the case where the size of the transfer material P is long paper based on the conventional control flow. Comparing FIG. 4 (B) and FIG. 4 (C), it can be seen that a sufficient agitation time is ensured for each development step when continuous printing is performed on long paper according to this embodiment. .
As a result, the dispersion in the longitudinal direction of the toner charge amount (Q / M) and the toner concentration (TC) after the completion of one development process is repaired. As a result, good image quality without image density unevenness can be obtained for all continuously printed images.

  As described above, in the present embodiment, even when a large transfer material such as long paper is continuously printed, the developing unit 23 sufficiently agitates the developer according to the size. I'm doing it. Thereby, it is possible to provide a highly reliable image forming apparatus in which the image quality of the output image does not deteriorate without complicating the structure of the developing unit 23.

  It should be noted that the present invention is not limited to the above-described embodiment, and it is obvious that the embodiment can be modified as appropriate within the scope of the technical idea of the present invention, other than suggested in the embodiment. In addition, the number, position, shape, and the like of the constituent members are not limited to the above-described embodiment, and can be set to a suitable number, position, shape, and the like in practicing the present invention.

1 is a configuration diagram illustrating an image forming apparatus according to an embodiment of the present invention. FIG. 2 is a side cross-sectional view showing an image forming unit in the image forming apparatus of FIG. 1. FIG. 2 is a top cross-sectional view illustrating a developing unit in the image forming apparatus of FIG. 1. 3 is a flowchart illustrating control in the image forming apparatus in FIG. 1. 2 is a timing chart showing control in the image forming apparatus of FIG. 1.

Explanation of symbols

1 image forming apparatus main body (apparatus main body), 2 optical section,
20, 20Y, 20M, 20C, 20BK Process cartridge,
21 photosensitive drum (image carrier), 22 charging unit,
23 developing section, 23a developing roller,
23b 1st stirring screw (stirring member),
23c 2nd stirring screw (stirring member), 23e Partition wall,
24 transfer roller, 25 cleaning section, 30 transfer belt (conveyance section),
32, 32Y, 32M, 32C, 32BK toner supply unit,
33 toner bottle, 61 paper feeding unit (conveying unit), 66 fixing unit,
71 Manual paper feed unit (conveyance unit), 72 Size detection sensor,
73 Conveying roller (conveying unit).

Claims (6)

A developing unit that develops a latent image formed on the image carrier while stirring the developer with a stirring member to form a toner image;
A transport unit that transports a transfer material onto which the toner image is transferred,
An image forming apparatus, wherein an operation time of a stirring mode in which the developer is stirred by the stirring member without forming the toner image is adjusted according to the size of the transfer material. The image forming apparatus according to claim 1, wherein the adjustment of the operation time in the stirring mode is an adjustment of a conveyance interval when a plurality of the transfer materials are continuously conveyed. 3. The image forming apparatus according to claim 1, wherein when the size of the transfer material in the transport direction is larger than a predetermined value, the operation time of the stirring mode is lengthened. The image forming apparatus according to claim 1, wherein the operation time in the stirring mode is adjusted according to a size of an image area in the toner image. The image forming apparatus according to claim 4, wherein when the image area is larger than a predetermined value, the operation time in the stirring mode is lengthened. 6. The image forming apparatus according to claim 4, wherein the size of the image area is obtained based on a latent image formed on the image carrier.

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