JP2020201313A - Image forming apparatus - Google Patents

Abstract

To achieve both image formation with a uniform density across the entire area in a conveyance direction of a long recording material, and prevention of deterioration of a developing roller and a photoconductor drum.SOLUTION: A control unit sets a speed ratio to a first speed ratio (S3), and starts an image forming operation (S4). When the length in a conveyance direction of the recording material is equal to or more than a predetermined length (YES in S5), and an image ratio is equal to or more than a predetermined value (NO in S6), upon arrival of a sub scanning direction position at a change position (YES in S7), the control unit changes a peripheral speed ratio from the first speed ratio to a second speed ratio larger than the first speed ratio (S8). By increasing the peripheral speed ratio, even if insufficiency of toner supplied to a developing roller occurs during the image forming operation, an image density does not fall below a density at which a user can determine a difference in density by visual inspection until the rear end in the conveyance direction of the recording material is reached. Thus, image formation with a uniform density across the entire area in a conveyance direction of a long recording material, and prevention of deterioration of a developing roller and a photoconductor drum can be achieved.SELECTED DRAWING: Figure 4

Description

The present invention relates to an image forming apparatus using electrophotographic technology such as a printer, a copier, a facsimile or a multifunction device.

Recently, for example, a POP advertisement (Point Of Purchase advertising) for sales promotion has been printed on a recording material by an image forming apparatus using electrophotographic technology. When printing a POP advertisement, a long paper (for example, 900 mm to 1200 mm) having a longer transport direction than a standard paper is used as a recording material, and an image having a high image ratio (for example, a solid image) is used on this long paper. Is formed. However, although it is particularly remarkable when a solid image is formed on a long paper, the latter half in which an image is formed thereafter is compared with the first half of the long paper in which an image is formed first in terms of the transport direction. In some cases, the density of the image gradually decreased. Therefore, an image forming apparatus has been proposed in which a uniform image density can be obtained over the entire transport direction when forming an image on long paper (Patent Document 1). In the apparatus described in Patent Document 1, when an image is formed on long paper, the peripheral speed of the rotational speed (peripheral speed) of the photosensitive drum and the rotational speed (peripheral speed) of the developing roller is higher than when the image is formed on standard paper. After increasing the ratio, the image forming operation is started.

Japanese Unexamined Patent Publication No. 2016-109951

In the apparatus described in Patent Document 1 described above, even when the density of the image does not gradually decrease, the image forming operation is started after increasing the peripheral speed ratio as described above for long paper. It was. However, when the peripheral speed ratio is increased, the deterioration of the developing roller and the photosensitive drum is accelerated, and in particular, the developing roller is deteriorated and the toner transportability by the developing roller tends to be deteriorated. Therefore, in the case of a device that mainly forms an image on long paper, the image density of the developing roller is relatively short from the start of use as compared with the case of a device that mainly forms an image on standard paper. As a result, the development rollers may be replaced more frequently because they cannot be sufficiently secured.

The present invention has been made in view of the above problems, and it is possible to both form an image with a uniform density over the entire transport direction of a long recording material and suppress deterioration of a developing roller and a photosensitive drum. An object of the present invention is to provide an image forming apparatus.

The image forming apparatus according to the present invention includes a rotating image carrier, a charging means for charging the surface of the image carrier, and an exposure means for exposing the charged image carrier to form an electrostatic latent image. , The developer carrier that carries the toner and rotates to develop the electrostatic latent image formed on the image carrier into a toner image by the toner, and the toner image on the image carrier is transferred to the recording material. The transfer means for transfer, the developing container for accommodating the toner, the supply means for rotating and supplying the toner in the developing container to the developing agent carrier, the first driving means for driving the image carrier, and the above. When the second driving means for driving the developer carrier and the toner image to be transferred to the recording material having a transport direction length of a predetermined length or more are formed on the developer carrier, the image ratio of the recording material is predetermined. If it is larger than the value, the speed ratio between the rotation speed of the image carrier and the rotation speed of the developer carrier is set as the first speed ratio from the tip of the recording material in the transport direction to the changed position, and the recording material is used. From the changed position to the rear end in the transport direction, a control means for setting the speed ratio to a second speed ratio larger than the first speed ratio is provided.

According to the present invention, it is possible to form an image at a uniform density over the entire transport direction of a recording material having a transport direction length of a predetermined length or more, and to suppress deterioration of a developer carrier and an image carrier. It can be easily realized to achieve both.

The schematic diagram which shows the structure of the image forming apparatus which concerns on this embodiment. The schematic diagram for demonstrating the developing apparatus. A control block diagram for explaining a control unit. The flowchart which shows the density | concentration adjustment processing of this embodiment. The graph for demonstrating the density adjustment process of this embodiment. It is a graph which shows the conventional example, (a) is the density change of an image according to the sub-scanning direction position, (b) is the toner supply amount and required toner amount by sub-scanning direction position.

[Image forming device]
First, the configuration of the image forming apparatus of the present embodiment will be described with reference to FIG. The image forming apparatus 100 shown in FIG. 1 is an intermediate transfer type full-color printer in which a plurality of image forming units PY, PM, PC, and PK are arranged along the moving direction of the intermediate transfer belt 10. As shown in FIG. 1, four image forming portions PY, PM, PC, and PK are formed inside the image forming apparatus 100 from the upstream side in the moving direction of the intermediate transfer belt 10 in yellow (Y) and magenta (M). , Cyan (C), and black (K) are arranged in this order. Although not shown, the image forming apparatus 100 records according to image data sent from a document reading device connected to the apparatus main body 100A, an external device such as a personal computer communicably connected to the apparatus main body 100A, and the like. A full-color image can be formed on the material S. Examples of the recording material S include various types of sheet materials such as plain paper, thick paper, rough paper, uneven paper, coated paper, glossy paper, paper such as photographic paper, plastic film, and cloth.

The image forming portions PY to PK have photosensitive drums 1Y, 1M, 1C, and 1K, respectively. The photosensitive drums 1Y to 1K as the image bearing members are, for example, electrophotographic photosensitive members having a photosensitive layer formed on the outer peripheral surface of an aluminum cylinder, and are shown by a drum drive motor M2 (see FIG. 2) as a first driving means. It is rotated at a predetermined process speed in the direction of the middle arrow R1. First, when the image forming operation is started, the surfaces of the rotating photosensitive drums 1Y to 1K are uniformly charged by the charging rollers 2Y, 2M, 2C, and 2K, respectively. Next, an electrostatic latent image is formed on the surface of the charged photosensitive drums 1Y to 1K by the exposure devices 3Y, 3M, 3C, and 3K. The charging rollers 2Y to 2K as the charging means charge the surface of the photosensitive drums 1Y to 1K to a charging potential (for example, -600V) having the same polarity as the toner. The exposure devices 3Y to 3K as the exposure means scan the surface of the photosensitive drums 1Y to 1K by scanning a laser beam obtained by ON-OFF modulation of a scanning line image signal obtained by developing images of each color with a rotating mirror (not shown). A possible laser scanner. The exposure apparatus 3Y to 3K forms an electrostatic latent image of the exposure portion potential (for example, -100V) on the surface of the photosensitive drum 1Y to 1K by irradiating the laser beam. In the exposure devices 3Y to 3K, the amount of laser light is variable by the laser power supply V2 (see FIG. 2).

The electrostatic latent image on the photosensitive drums 1Y to 1K (on the image carrier) is developed into a toner image by the developing devices 4Y, 4M, 4C, and 4K in which toners of each color are stored. The developing devices 4Y to 4K will be described later (see FIG. 2).

Primary transfer rollers 5Y, 5M, 5C, and 5K are arranged so as to abut on the inner peripheral surface of the intermediate transfer belt 10 at positions facing the photosensitive drums 1Y to 1K with the intermediate transfer belt 10 interposed therebetween. It forms T1. The toner images of each color formed on the photosensitive drums 1Y to 1K are intermediately transferred by the primary transfer unit T1 by applying a primary transfer voltage, which is a voltage opposite to the charging polarity of the toner, to the primary transfer rollers 5Y to 5K. It is first transferred to the belt 10. The toner remaining on the photosensitive drums 1Y to 1K after the primary transfer is removed by the drum cleaning devices 6Y, 6M, 6C, and 6K.

The intermediate transfer belt 10 rotates while carrying the toner image primarily transferred from the photosensitive drums 1Y to 1K as described above. In the case of the present embodiment, the intermediate transfer belt 10 is stretched by a plurality of tension rollers 12 and a secondary transfer inner roller 13. One of the tension rollers 12 is a tension roller that applies a constant tension to the intermediate transfer belt 10, and the other of the tension rollers 12 is rotationally driven by a motor (not shown) to rotate the intermediate transfer belt 10 in the direction of arrow R2 in the drawing. It is a drive roller that rotates to.

The recording material S is appropriately taken out one by one from a storage cassette (not shown) or the like according to the start of the image forming job, and the taken out recording material S is transferred to the secondary transfer unit T2 according to the formation timing of the toner image. Be transported. The secondary transfer portion T2 is a transfer nip portion formed by the secondary transfer inner roller 13 and the secondary transfer outer roller 14 arranged so as to face each other with the intermediate transfer belt 10 interposed therebetween. The secondary transfer outer roller 14 as a transfer means abuts on the intermediate transfer belt 10 supported by the secondary transfer inner roller 13 to form the secondary transfer portion T2. By applying the secondary transfer voltage to the secondary transfer outer roller 14, the toner image on the intermediate transfer belt 10 is secondarily transferred to the recording material S which is sandwiched and conveyed by the secondary transfer unit T2.

The toner, paper dust, and the like that cannot be completely transferred by the secondary transfer unit T2 and remain on the intermediate transfer belt 10 are removed by the belt cleaning device 15. The belt cleaning device 15 is placed on the tension roller 12 across the intermediate transfer belt 10 at a position downstream of the secondary transfer unit T2 and upstream of the image forming unit PY with respect to the moving direction of the intermediate transfer belt 10. They are placed facing each other. At this position, the belt cleaning device 15 brings a blade, a fur brush, or the like (not shown) into contact with the intermediate transfer belt 10 to clean the surface of the intermediate transfer belt 10.

The recording material S to which the toner image is transferred is conveyed to the fixing device 200, and the recording material S is heated and pressurized by the fixing device 200 to fix the toner image on the recording material S. After that, the recording material S is discharged to a discharge tray (not shown) provided outside the main body of the apparatus.

Next, the developing devices 4Y to 4K will be described with reference to FIG. However, the developing devices 4Y to 4K are configured to be substantially the same except that the toner colors used are different from yellow, magenta, cyan, and black. Therefore, in the following, the yellow developing device 4Y will be described as an example, and the description of the other developing devices 4M, 4C, and 4K will be omitted.

As shown in FIG. 2, the developing apparatus 4Y includes a developing container 41, a developing roller J1, a toner supply roller J2, and a developing blade J3. The developing container 41 contains, for example, a one-component magnetic toner having a negative charge polarity as a developing agent. In the case of the present embodiment, the developer may be a one-component toner and may be magnetic or non-magnetic. The developing container 41 has an opening at the upper portion facing the photosensitive drum 1Y, and the developing roller J1 is rotatably provided so that a part of the developing container 41 is exposed to the opening. The developing roller J1 as the developing agent carrier is rotated in the same direction on the surface facing the photosensitive drum 1Y by the roller driving motor M1 as the second driving means in a state where the toner is supported (arrow R3 direction). The developing roller J1 is an elastic roller having elasticity formed of, for example, an ester-based urethane rubber.

The toner supply roller J2 as a supply means is rotatably provided in the developing container 41, and can rotate to supply the toner contained in the developing container 41 (inside the developing container) to the developing roller J1. The toner supply roller J2 is, for example, a sponge roller, and can be rotated by the roller drive motor M1 in the direction of arrow R4 in the drawing at the same speed as the developing roller J1. In the case of the present embodiment, the faster the rotation speed of the toner supply roller J2, the more the amount of toner supplied to the developing roller J1 (the amount of toner supplied per unit time) can be increased.

The developing blade J3 is formed of a rubber material in a plate shape, and is provided in the developing container 41 so as to come into contact with the developing roller J1 at a constant pressure. The toner supplied by the toner supply roller J2 and carried on the developing roller J1 is regulated to a uniform thickness by the developing blade J3. Then, as the developing roller J1 rotates, the toner carried on the developing roller J1 is rubbed against the developing blade J3, so that the toner is charged (for example, negatively charged).

While the developing roller J1 is in contact with the photosensitive drum 1Y, a developing voltage of, for example, "-400V" is applied by the developing power supply V1 as a voltage applying means. In the present embodiment, the potential difference (development contrast), which is the absolute value of the difference between the exposed portion potential of the photosensitive drum 1Y and the developing voltage, can be changed according to the developing voltage applied by the developing power source V1. For example, when the exposed part potential is "-100V" and the developing voltage is "-400V", the developing contrast is "300V". When a developing voltage is applied to the developing roller J1, the negatively charged toner adheres to the electrostatic latent image (exposure part potential: about -100V) formed on the photosensitive drum 1Y. Is developed into a toner image. The development contrast may be changed by changing the exposure portion potential according to the laser voltage applied by the laser power supply V2, or changing both the development voltage and the laser voltage.

[Control unit]
As shown in FIG. 1, the image forming apparatus 100 has a control unit 300, and the control unit 300 as a control means performs various controls of the image forming apparatus 100 such as an image forming operation. The control unit 300 will be described with reference to FIG. In addition to the illustrations, the control unit 300 is also connected to, for example, a motor for driving the charging rollers 2Y to 2K, the primary transfer rollers 5Y to 5K, the fixing device 200, the intermediate transfer belt 10, and the like. However, since it is not the main purpose of the invention, illustration and description will be omitted.

As shown in FIG. 3, the control unit 300 as a control means includes a CPU (Central Processing Unit) 301 and a memory 302. The memory 302 has a ROM (Read Only Memory) and a RAM (Random Access Memory). Various programs such as an image forming job and various data are stored in the ROM. The CPU 301 may execute various programs stored in the ROM to operate the image forming apparatus 100. In addition, work data and input data are stored in the RAM. The CPU 301 can refer to various data stored in the ROM or RAM based on various programs or the like.

In the case of the present embodiment, the control unit 300 can control the roller drive motor M1, the drum drive motor M2, the developing power supply V1, and the laser power supply V2 described above. That is, the rotation speeds of the developing roller J1 and the toner supply roller J2 are adjusted by the control unit 300 controlling the roller drive motor M1. The rotation speed of the photosensitive drum 1Y is adjusted by the control unit 300 controlling the drum drive motor M2. The development voltage applied to the developing roller J1 is adjusted by the control unit 300 controlling the developing power supply V1, and the exposure unit potential of the photosensitive drum 1Y is adjusted by the control unit 300 controlling the laser power supply V2. That is, the control unit 300 can control the development power supply V1 and the laser power supply V2 to change the development contrast described above.

By the way, in the conventional image forming apparatus 100 having the above-described configuration, when an image (solid image) having an image ratio of 100% is formed with respect to a recording material S such as long paper, the transport direction of the recording material S (solid image) ( The density of the image may gradually decrease from the middle of the sub-scanning direction). This is a sufficient amount for the developing roller J1 to develop the electrostatic latent image of the photosensitive drum 1Y due to insufficient toner supply to the developing roller J1 by the toner supply roller J2 in the developing apparatus 4Y shown in FIG. This is because the toner of the above cannot be supported.

As an example, FIGS. 6 (a) and 6 (b) are used for changes in the conventional image density when an image (solid image) having an image ratio of 100% is formed on a long paper having a length of 1200 mm in the transport direction. I will explain. In FIG. 6A, the vertical axis indicates the image density, and the horizontal axis indicates the position in the sub-scanning direction of one recording material S. In FIG. 6B, the vertical axis represents the amount of toner and the horizontal axis represents the position in the sub-scanning direction. Then, in FIG. 6A, the solid line shows the density change of the image according to the position of the sub-scanning direction in the transport direction (sub-scanning direction), and the dotted line is the image density (for example, the density difference can be visually discriminated by the user). 1.1) is shown. FIG. 6A shows the detection result (reflection density) detected by an optical sensor (not shown) as the image density. In FIG. 6B, the solid line shows the toner supply amount (cumulative value) supplied from the toner supply roller J2 obtained in the experiment to the developing roller J1. The dotted line indicates the amount of toner required to maintain the image density constant over the entire transport direction (sub-scanning direction) of the recording material S, which is obtained from the image data (specifically, the video count value). Here, a case where the development contrast is "300 V" is shown as an example.

In the present embodiment, the long paper refers to a recording material S having a length in the transport direction longer than that of standard paper (for example, A3 size) and having a predetermined length or more (for example, 800 mm or more). .. The image ratio of the recording material S is the area ratio of the image in the image-formable region of the recording material S when converted into an image having the maximum density. When the image ratio of the recording material S is low, the ratio of the blank portion is higher than when the image ratio is high (for example, 80% or more), so that the toner consumption per recording material is small.

Point A shown in FIG. 6A represents the density of the image formed at a position where the length in the transport direction is 420 mm (corresponding to A3 size). Point B represents the density of the image formed at a position where the length in the transport direction is 800 mm. Point C represents the density of the image formed at a position where the length in the transport direction is 1000 mm. As can be understood from FIG. 6A, the density of the solid image formed from the tip (0 mm) of the recording material S to the position of 800 mm is maintained at “1.3”. This is because, as shown in FIG. 6B, the amount of toner supplied by the toner supply roller J2 to the development roller J1 (points A'and B') is the amount of toner required to maintain the image density (points A'and B'). This is because it exceeds the required toner amount (A ″ point, B ″ point).

In the case of the present embodiment, the amount of toner supplied from the tip of the recording material S to an arbitrary position in the transport direction (position in the sub-scanning direction) is "M / S (weight of toner on the developing roller J1 per unit area). ) × Axial length of the developing roller J1 × Position in the sub-scanning direction ”. For example, the amount of toner supplied from the tip of the recording material S to the position in the sub-scanning direction 42 cm is "M / S (1.3 mg / cm ² ) x length of the developing roller J1 in the direction of the rotation axis (29.7 cm) x sub". Scanning direction position (42 cm) = 1.6 g ". This toner supply amount represents the toner supply capacity of the toner supply roller J2 to the developing roller J1.

On the other hand, the required amount of toner required to maintain the image density constant is obtained from the image data. The amount of toner required to form a solid image on one of the A3 size recording materials S was "1.05 g". That is, even in the case of long paper, from the tip of the recording material S to the position of "420 mm" (equivalent to A3 size) in the sub-scanning direction, "toner supply amount (1.6 g)> required toner amount (1.05 g). ) ”, And the toner supply amount exceeds the required toner amount, so that the image density is maintained.

However, in the case of long paper, as shown in FIG. 6A, the image density starts to decrease when the position in the sub-scanning direction exceeds the position of 800 mm (point B). Then, when the position in the sub-scanning direction passes the position of 1000 mm (point C), the density starts to fall below the density (for example, 1.1) at which the user can visually discriminate the density difference. The reason why the image density starts to decrease at point B is that “toner supply amount <required toner amount” (point B ″) as shown in FIG. 6 (b), that is, the density is maintained in the developing roller J1. This is because a state of insufficient toner that does not carry enough toner to form an image occurs. The difference between the "required toner amount" and the "toner supply amount" when "required toner amount> toner supply amount" is hereinafter referred to as "toner shortage amount". In the case of the present embodiment, when the toner shortage amount becomes "0.25 g" or more, a density difference (for example, 0.2 or more) that can be visually discriminated by the user occurs (point C ").

In this way, when an image is formed on the recording material S such as long paper, the image density starts to decrease as the position in the sub-scanning direction approaches the rear end in the transport direction of the recording material S, and the user can visually check the image. The density difference may be lower than the density that can be discriminated, and the density of the image may gradually decrease. Therefore, as already described, in order to obtain a uniform image density over the entire transport direction when forming an image on long paper, conventionally, when forming an image on long paper, it is developed with photosensitive drums 1Y to 1K. The image forming operation is started after increasing the peripheral speed ratio with the roller J1. However, if the peripheral speed ratio is increased, the development rollers J1 and the photosensitive drums 1Y to 1K deteriorate faster, and the development rollers J1 and the photosensitive drums 1Y to 1K may be replaced more frequently. In the present specification, the fact that the image density is uniform means that even if there is a slight difference in density in the printed matter (recording material on which the image is formed), the density difference cannot be visually determined by the user. If there is a degree difference in density, it is considered that the image density is uniform.

[Concentration adjustment processing]
Therefore, in view of the above points, in the present embodiment, when forming an image on the recording material S such as long paper, one recording material S is used without changing the peripheral speed ratio prior to the start of the image forming operation. The peripheral speed ratio can be changed even during the image formation operation. As a result, after the start of the image forming operation, the peripheral speed ratio is changed before the image density of one recording material S becomes so thin that the user cannot visually grasp it, so that the recording material S covers the entire transport direction. It is possible to obtain a uniform image density. Hereinafter, the concentration adjusting process of the first embodiment will be described with reference to FIGS. 1 and 3 with reference to FIGS. 4 and 5. The density adjustment process is executed by the control unit 300 at the start of the image forming job. FIG. 4 is a flowchart showing the density adjustment process of the present embodiment.

As shown in FIG. 4, the control unit 300 determines whether or not the image data for one recording material has been acquired (S1). When the image data for one recording material has not been acquired (NO in S1), the control unit 300 waits for the progress of the process until the image data for one recording material is acquired. When the image data for one recording material is acquired (YES in S1), the control unit 300 calculates the image ratio of the recording material S for each image based on the acquired image data (S2). In the case of the present embodiment, the image ratio of the recording material S is the amount of image data in one image forming operation according to the number of pixels (video count value) of the output image used by the exposure apparatus 3Y to 3K at the time of exposure. Can be obtained depending on whether you used. The video count value is an integrated value when the level (0 to 255 levels) for each pixel of the input image data is integrated for one surface of the output image. In the present embodiment, the video count value of the output image formed for each recording material S is obtained, and the image ratio is calculated using this.

Then, the control unit 300 sets the speed ratio (peripheral speed ratio) between the rotation speed (peripheral speed) of the photosensitive drum 1Y (1M to 1K) and the rotation speed (peripheral speed) of the developing roller J1 as the first speed ratio. (S3), the image forming operation is started (S4). Here, in the image forming apparatus 100, when the photosensitive drum 1Y (same for 1M to 1K) is worn and the photosensitive layer becomes thin, charging failure occurs, and toner is applied to the unexposed portion on the photosensitive drum 1Y. Adhering image defects may occur. Further, when the developing roller J1 is worn and the surface becomes non-uniform, the thickness of the toner supported on the developing roller J1 becomes non-uniform, which causes image defects such as streaks and uneven density. In this embodiment, as an example, in order to prevent the photosensitive drum 1Y and the developing roller J1 from being worn as much as possible, the rotation speed (peripheral speed) of the developing roller J1 is the rotation speed (peripheral speed) of the photosensitive drum 1Y at the start of the image forming operation. It is set to be almost the same as (speed). That is, the above-mentioned peripheral speed ratio (first speed ratio) is set to "100%". In this embodiment, the laser power supply V2 controls the exposure potential to "-100V", the developing power supply V1 controls the developing voltage to "-400V", and the developing contrast is "300V" (the first described later). Potential difference) is set.

The control unit 300 determines whether or not the length of the recording material S for forming an image in the transport direction is equal to or greater than a predetermined length (S5). When the length of the recording material S in the transport direction is smaller than the predetermined length (NO in S5), the control unit 300 jumps to the process of step S10. When the length of the recording material S in the transport direction is equal to or greater than the predetermined length (YES in S5), the control unit 300 determines whether or not the image ratio of the recording material S obtained above is equal to or less than the predetermined value (S6). ). When the image ratio of the recording material S is equal to or less than a predetermined value (YES in S6), the control unit 300 jumps to the process of step S10. As the process of step S10, the control unit 300 determines whether or not the image formation job has been completed (S10). When the image formation job is completed (YES in S10), the control unit 300 ends the density adjustment process. If the image formation job is not completed (NO in S10), the control unit 300 repeats the process of step S10 until the image formation job is completed.

On the other hand, when the image ratio of the recording material S is larger than a predetermined value (NO in S6), the control unit 300 determines whether or not the sub-scanning direction position in the recording material S during the image forming operation has reached the changed position (NO). S7). When the sub-scanning direction position in the recording material S has not reached the changed position (NO in S7), the control unit 300 repeats the process of step S7 until the changed position is reached. On the other hand, when the sub-scanning direction position in the recording material S reaches the changed position (YES in S7), the control unit 300 changes the peripheral speed ratio from the first speed ratio to the second speed ratio larger than the first speed ratio. (S8), the image forming operation is continued. For example, the control unit 300 changes the peripheral speed ratio from "100%" (first speed ratio) to "150%" (second speed ratio). In the present embodiment, for example, the roller drive motor M1 is controlled to increase the rotation speed of the developing roller J1. After that, the control unit 300 repeats the process of step S10 described above until the image formation job is completed.

The above-mentioned change position changes depending on the image ratio of the recording material S. For example, when the image ratio of the recording material S is the first ratio, it is set as the first change position, and when the image ratio of the recording material S is larger than the first ratio, it is the transport direction than the first change position. It is the second change position near the tip. This change position is determined when calculating the video count value.

That is, the control unit 300 integrates the amount of toner required to form an image of the input image data for each image formation line in the sub-scanning direction of the recording material S when calculating the image ratio (see S2). To do. One line capable of forming an image with one pixel (dot) in the transport direction of the recording material S is, for example, about 9922 lines in the case of the A3 size recording material S because the length in the transport direction is 420 mm (“420 mm /”. Length per dot (25.4 mm / 600 dpi) "). Further, the number of dots (number of pixels) per image forming line in this case is about 7016 dots at the maximum. Then, the number of dots for image formation per actual image formation line is accumulated, and the toner usage amount per dot is multiplied by this to obtain the toner usage amount for each image formation line. The amount of toner used for each line of image formation is integrated over a plurality of lines extending from the tip of the recording material S in the transport direction to the sub-scanning direction, and is the "necessary toner" required to maintain the density at the intermediate position of the recording material S. Corresponds to "quantity" (see FIG. 6 (b)).

Then, each time the number of image forming lines increases, the control unit 300 includes a "toner supply amount" (see FIG. 6B) stored in the memory 302 (see FIG. 3) in advance in the sub-scanning direction and the above "necessary". Compare with "toner amount". At that time, the position from the tip in the transport direction to the image forming line where "toner supply amount <required toner amount" is determined as the "changed position".

When the peripheral speed ratio is changed to the second speed ratio (see S8), the control unit 300 sets the potential difference (development contrast) between the exposed unit potential and the developing voltage of the photosensitive drum 1Y (1M to 1K) as the first potential difference. May be changed to a second potential difference smaller than the first potential difference (S9). As an example, the control unit 300 controls the development voltage to "-300V" by the development power supply V1 while maintaining the exposure unit potential at "-100V", and changes the development contrast to "200V" (second potential difference). .. That is, when the peripheral speed ratio is increased, toner is likely to be discharged from the developing roller J1 to the photosensitive drum 1Y (1M to 1K), and fog is likely to occur on the photosensitive drum. In order to prevent this fog, the development contrast is reduced by increasing the peripheral speed ratio (that is, changing to the second speed ratio), and the toner from the developing roller J1 to the photosensitive drum 1Y (1M to 1K) is charged. It is preferable to suppress the exhalation.

As described above, in the present embodiment, when the transport direction length of the recording material S is smaller than the predetermined length (NO in S5) or when the image ratio of the recording material S is equal to or less than the predetermined value (YES in S6), the image The image forming operation is continued without changing the peripheral speed ratio set before the start of the forming operation. On the other hand, when the length of the recording material S in the transport direction is equal to or longer than the predetermined length (YES in S5) and the image ratio of the recording material S is larger than the predetermined value (NO in S6), the image forming operation is performed. After the start, the peripheral speed ratio set before the start is changed at the change position in the middle of the recording material S.

FIG. 5 is a graph for explaining the concentration adjustment process of the present embodiment. The vertical axis of FIG. 5 indicates the amount of toner, and the horizontal axis indicates the position in the sub-scanning direction. In FIG. 5, the solid line shows the toner supply amount (cumulative value) supplied from the toner supply roller J2 to the developing roller J1 in the conventional example to which this embodiment is not applied (see FIG. 6B). The dotted line indicates the amount of toner required to maintain the image density constant over the entire transport direction (sub-scanning direction) of the recording material S, which is obtained from the image data (specifically, the video count value). Then, the one-point chain wire is developed from the toner supply roller J2 when the present embodiment is applied, that is, when the peripheral speed ratio is changed from the first peripheral speed ratio to the second peripheral speed ratio at the changed position (here, 800 mm). The toner supply amount (cumulative value) supplied to the roller J1 is shown.

When the peripheral speed ratio is increased as described above, the amount of toner supplied from the toner supply roller J2 to the developing roller J1 per unit time increases. Therefore, as shown in FIG. 5, when the peripheral speed ratio is increased at the change position of the sub-scanning direction position of 800 mm, the toner supplied from the toner supply roller J2 to the developing roller J1 is supplied at the subsequent sub-scanning direction positions. The amount (cumulative value) increases compared to the conventional example. Then, in the case of the example shown in FIG. 5, when the position in the sub-scanning direction exceeds the position of 800 mm, the image density starts to decrease, but unlike the conventional example, the image reaches the rear end in the transport direction of the recording material S. The concentration does not fall below the concentration at which the user can visually discriminate the concentration difference (for example, 0.95). That is, the reason why the image density decreases is that, as described above, "toner supply amount <required toner amount", and a state of toner shortage occurs in the developing roller J1. However, in the case of this embodiment, unlike the conventional example, the toner shortage amount can be suppressed to "0.25 g" or less even when the recording material S reaches the rear end in the transport direction. That is, the image formation on one recording material S is completed before the toner shortage amount (0.25 g) that causes a density difference that can be visually discriminated by the user occurs.

The second speed ratio is the toner supply amount (so that the image density does not decrease even if the sub-scanning direction position exceeds the position of 800 mm in FIG. 5, that is, “toner supply amount <required toner amount” does not occur. The cumulative value) may be changed to a larger speed ratio that can be increased.

Next, in the present embodiment, the reason why the changed peripheral speed ratio (second speed ratio) is set to "150%" will be described. As shown in FIG. 6B, when the peripheral speed ratio at the start of the image forming operation is "100%", the toner supply amount (B') and the required toner are at the position where the sub-scanning direction position is "800 mm". The amount (B ″) is the same amount (2.0 g). Then, at the position where the sub-scanning direction position is "1200 mm" (rear end in the transport direction of the recording material S), the toner supply amount (C') is "2.5 g", and the required toner amount (C "") is "3". It is "0.0 g". That is, when the position in the sub-scanning direction is between "800 mm" and "1200 mm", the required toner amount is increased by "1.0 g" and the supplied toner amount is increased by "0.5 g". Then, in order to suppress the density difference to a concentration difference (for example, 0.2) or less that can be visually discriminated by the user until the position reaches the position of "1200 mm", the toner shortage amount (required toner amount-toner) is suppressed. (Supply amount) must be "0.25 g" or less. As described above, the required toner amount increases by "1.0 g" between "800 mm" and "1200 mm". Therefore, in order to prevent the toner shortage amount from exceeding "0.25 g" by the time it reaches the position of "1200 mm", the amount of supplied toner is set to "0.75 g (1.0 g-0.25 g)". Need to increase. When the peripheral speed ratio is "100%", the amount of supplied toner increases by "0.5 g" as described above. Therefore, in order to increase this by "0.75 g", the peripheral speed ratio is set to "0.5 g". It is necessary to change to "1.5 times (0.75 ÷ 0.5)".

As described above, in the case of the present embodiment, during the image forming operation of the recording material S whose length in the transport direction is equal to or longer than the predetermined length, the rotation speed of the photosensitive drum depends on the size of the image ratio of the recording material S. The peripheral speed ratio with the rotation speed of the developing roller is changed from the middle of the recording material S. Specifically, when the image ratio of the recording material S is larger than a predetermined value, an image is formed from the tip of the recording material S in the transport direction to the change position with the peripheral speed ratio as the first speed ratio, and the change in the middle of the recording material S is performed. From the position, an image is formed with a second velocity ratio larger than the first velocity ratio. By increasing the peripheral speed ratio, it is possible to increase the amount of toner discharged from the developing roller J1 to the photosensitive drum 1Y (1M to 1K). Therefore, even if the toner supply roller J2 insufficiently supplies the toner to the developing roller J1 during the image forming operation, the image density is visually different by the user until the rear end in the transport direction of the recording material S. It is possible not to fall below the concentration at which can be discriminated. As a result, an image can be formed with a uniform density over the entire transport direction of the long recording material S. Further, since the peripheral speed ratio is changed only when the toner supply to the developing roller J1 by the toner supply roller J2 may be insufficient, deterioration of the photosensitive drums 1Y to 1K and the developing roller J1 can be suppressed.

<Other embodiments>
In each of the above-described embodiments, after the toner images of each color are first transferred from the photosensitive drums 1Y to 1K of each color to the intermediate transfer belt 10, the composite toner images of each color are collectively secondarily transferred to the recording material S. The image forming apparatus of the above has been described, but the present invention is not limited to this. For example, it may be a direct transfer type image forming apparatus that directly transfers from the photosensitive drums 1Y to 1K to the recording material S. Further, an image forming apparatus (for example, a monochrome machine) capable of forming a monochrome toner image may be used.

1Y (1M, 1C, 1K) ... Image carrier (photosensitive drum), 2Y (2M, 2C, 2K) ... Charging means (charging roller), 3Y (3M, 3C, 3K) ... Exposure means (exposure device), 14 ... Transfer means (secondary transfer outer roller), 41 ... Development container, 100 ... Image forming apparatus, 300 ... Control means (control unit), J1 ... Developer carrier (development roller), J2 ... Supply means (toner supply roller) ), M1 ... Second drive means (roller drive motor), M2 ... First drive means (drum drive motor), V1 ... Voltage application means (development power supply)

Claims (6)

With a rotating image carrier,
A charging means for charging the surface of the image carrier and
An exposure means that exposes the charged image carrier to form an electrostatic latent image,
A developer carrier that carries toner and rotates, and develops an electrostatic latent image formed on the image carrier into a toner image with toner.
A transfer means for transferring the toner image on the image carrier to the conveyed recording material, and
A developing container that stores toner and
A supply means that rotates to supply the toner in the developing container to the developing agent carrier, and
The first driving means for driving the image carrier and
A second driving means for driving the developer carrier and
When forming a toner image to be transferred to a recording material having a length in the transport direction of a predetermined length or more on the developer carrier, if the image ratio of the recording material is larger than a predetermined value, the recording material is transported. The speed ratio between the rotation speed of the image carrier and the rotation speed of the developer carrier is set as the first speed ratio from the tip of the direction to the change position, and the speed is from the change position of the recording material to the rear end in the transport direction. A control means for setting the ratio to a second speed ratio larger than the first speed ratio is provided.
An image forming apparatus characterized in that. When the image ratio of the recording material is the first ratio, the control means changes from the first speed to the second speed ratio at the first change position, and the image ratio of the recording material is larger than the first ratio. In the case of the second ratio, the first speed ratio is changed to the second speed ratio at the second change position closer to the tip in the transport direction than the first change position.
The image forming apparatus according to claim 1. The control means increases the rotational speed of the developer carrier when the speed ratio is increased.
The image forming apparatus according to claim 1 or 2. The control means slows down the rotational speed of the image carrier when the speed ratio is increased.
The image forming apparatus according to any one of claims 1 to 3, wherein the image forming apparatus is characterized in that. A voltage applying means for applying a developing voltage to the developer carrier is provided.
When the speed ratio is the first speed ratio, the control means has the exposed portion potential of the image carrier exposed by the exposure means and the development applied to the developer carrier by the voltage applying means. When the absolute value of the difference from the voltage is the first potential difference and the speed ratio is the second speed ratio, the exposure portion potential of the image carrier exposed by the exposure means and the voltage application means. The absolute value of the difference from the developing voltage applied to the developer carrier is defined as the second potential difference smaller than the first potential difference.
The image forming apparatus according to any one of claims 1 to 4, wherein the image forming apparatus is characterized in that. The supply means is rotationally driven together with the developer carrier by the second drive means.
The image forming apparatus according to any one of claims 1 to 5, wherein the image forming apparatus is characterized in that.

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