Image-forming device

Abstract

To suppress occurrence of a ghost image because of simultaneous rotation of an external additive on a photosensitive drum when an image density ratio of an image formed by image formation operation is high.SOLUTION: The ratio of a peripheral speed of a developer carrier relative to a peripheral speed of an image carrier when a control section executes toner band control for forming a toner band in the image carrier when an image density ratio of an image formed by image formation operation is a second ratio that is higher than a first ratio is higher than the ratio of a peripheral speed of the developer carrier relative to a peripheral speed of the image carrier when the control section executes toner band control for forming a toner band in the image carrier when an image density ratio of an image formed by image formation operation is the first ratio.SELECTED DRAWING: Figure 6

Description

Technical Field

[0001] The present invention relates to an image forming apparatus including a developing device that develops an electrostatic latent image formed on an image carrier with a developer containing toner and a carrier.

Background Art

[0002] In an electrophotographic image forming apparatus, when a toner image is formed in a developing portion on a photosensitive drum, a part of fine particles (external additives) added to the toner is also developed together. At this time, the amount of fine particles developed varies as the amount of toner developed varies according to the image ratio of the image portion.

[0003] Although the fine particles developed on the photosensitive drum reach the cleaning portion via the primary transfer portion together with the toner, due to their small particle size, the adhesion force to the photosensitive drum is large, making it difficult to be primarily transferred and also difficult to be removed by the cleaning member.

[0004] Therefore, the fine particles developed on the photosensitive drum remain on the photosensitive drum without being cleaned. When they reach the developing portion again via the charging portion, in a region where there are many fine particles remaining on the photosensitive drum, the toner is more likely to be further developed by the electric field formed by the fine particles. In particular, when forming a uniform image such as a halftone image, a density difference may occur due to the difference in the amount of fine particles remaining on the photosensitive drum, and it may be visually recognized as a ghost image. This phenomenon tends to be prominent when the conditions for easy development of toner or the charge amount of the external additive are large, or when images of the same or similar patterns are repeatedly formed.

[0005] Patent Document 1 describes a technique of discharging external additives during non-image formation with the developing bias changed from that during image formation according to high image ratio printing.

Prior Art Documents

Patent Documents

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2019-66547 [Summary of the Invention] [Problems to be Solved by the Invention]

[0007] In the technology described in Patent Document 1, when too much external additive is discharged onto the photosensitive drum without forming an electrostatic latent image on the photosensitive drum, the difference in the amount of external additive between the image forming portion and the non-image forming portion cannot be filled. As a result, the external additive may be carried around on the photosensitive drum as it is, and the external additive carried around on the photosensitive drum may electrostatically attract the toner in the developing unit and may be manifested as a foggy image.

[0008] The present invention has been made in view of the above problems. An object of the present invention is to provide an image forming apparatus capable of suppressing the occurrence of a ghost image due to the carry-around of an external additive on a photosensitive drum when the image density ratio of an image formed by an image forming operation is high. [Means for Solving the Problems]

[0009] To achieve the above object, an image forming apparatus according to an aspect of the present invention has the following configuration. That is, an image forming apparatus capable of executing an image forming operation for forming an image on a recording material, including an image carrier on which an electrostatic latent image is formed, a developer carrier that carries a developer including toner and carrier for developing the electrostatic latent image formed on the image carrier, a developing container that houses the developer, a developing device having the above components, a transfer member that abuts against the image carrier to form a transfer portion therebetween, and transfers the toner image formed on the image carrier to the recording material at the transfer portion when a transfer bias is applied, and a control unit that executes toner band control for forming a toner band on the image carrier. When the image density ratio of the image formed by the image forming operation is the first ratio, the ratio of the peripheral speed of the developer carrier to the peripheral speed of the image carrier when the control unit executes the toner band control is higher than when the image density ratio is a second ratio higher than the first ratio.

Advantages of the Invention

[0010] According to the present invention, it is possible to suppress the occurrence of ghost images due to the rotation of external additives on the photosensitive drum when the image density ratio of the image formed by the image forming operation is high.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Mode for Carrying Out the Invention

[0012] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. It should be noted that the following embodiments do not limit the present invention according to the claims, and not all combinations of the features described in the first embodiment are essential for the solution means of the present invention. The present invention can be implemented in various applications such as printers, various printing machines, copiers, FAX, multifunction machines, etc.

[0013] [First Embodiment] (Configuration of Image Forming Apparatus) First, the configuration of the image forming apparatus 100 according to the first embodiment will be described with reference to the schematic diagram of FIG. 1.

[0014] As shown in FIG. 1, the image forming apparatus 100 capable of executing an image forming operation for forming an image on a recording material S adopts a tandem method. Although drum cartridges for forming toner images of four colors, yellow, magenta, cyan, and black, are provided side by side, in the following description, the symbols A, B, C, and D are omitted, and only the numbers are shown, which are common to the four-color drum cartridges in FIG. 1.

[0015] The image formation in this image forming apparatus 100 is performed as follows.

[0016] FIG. 2 is a schematic diagram showing the configuration of the photosensitive drum 1A (image carrier) in the four-color drum cartridge in the image forming apparatus 100 according to the first embodiment and the mechanism around it. The mechanisms and configurations shown below are common to the photosensitive drums 1A to 1D.

[0017] The charging roller 2A is a rubber roller that contacts the surface of the photosensitive drum 1A and rotates drivenly, and uniformly charges the surface of the photosensitive drum 1A. A charging bias power supply 82 is connected to the charging roller 2A. The charging bias power supply 82 applies a DC voltage as a charging bias to the charging roller 2A, and charges the photosensitive drum 1A via the charging roller 2A.

[0018] The exposure device 3A is a laser scanner, and emits laser light according to the color-separated image information output from the control unit 11. The developing device 4A develops the electrostatic latent image formed on the photosensitive drum 1A with toner when a developing bias is applied. A developing bias power supply 81 for applying a developing bias is connected to the developing device 4A. Details of the developing device 4A will be described later with reference to FIGS. 3 and 4.

[0019] The toner image developed on the photosensitive drum 1A is primarily transferred to the intermediate transfer belt 62 (intermediate transfer member) by the primary transfer bias applied from the primary transfer roller 61A. The primary transfer roller 61A is disposed to face the photosensitive drum 1A and abuts on the intermediate transfer belt 62. A primary transfer bias power supply 83 for applying a primary transfer bias is connected to the primary transfer roller 61A.

[0020] The intermediate transfer belt 62 abuts on the photosensitive drum 1A to form a primary transfer portion between the photosensitive drum 1A and itself, and when a primary transfer bias is applied, the toner image formed on the photosensitive drum 1A is primarily transferred at the primary transfer portion. By applying a positive primary transfer bias to the intermediate transfer belt 62 by the primary transfer roller 61A, the toner images having respective negative polarities on the photosensitive drum 1A are sequentially and multiply transferred to the intermediate transfer belt 62.

[0021] The secondary transfer unit 65 includes a secondary transfer inner roller 63 and a secondary transfer outer roller 64 (secondary transfer means). A secondary transfer bias power source 84 for applying a secondary transfer bias is connected to the secondary transfer outer roller 64. By applying a positive-polarity secondary transfer bias to the secondary transfer outer roller 64, the full-color toner image formed on the intermediate transfer belt 62 is transferred to the sheet S (recording material). The secondary transfer outer roller 64 abuts on the intermediate transfer belt 62 to form the secondary transfer unit 65 between it and the intermediate transfer belt 62. When the secondary transfer bias is applied, the toner image primarily transferred to the intermediate transfer belt 62 is secondarily transferred to the sheet S by the secondary transfer unit 65.

[0022] After this sheet S is heated, pressurized, and fixed by the fixing device 7, it is discharged outside the image forming apparatus 100. The residual toner remaining on the photosensitive drum 1A after secondary transfer is removed by the cleaner 8A (drum cleaning member).

[0023] The cleaner 8A adopts a blade cleaning method, and the residual toner remaining on the photosensitive drum 1A is cleaned by the cleaning blade 14. The cleaning blade 14 is abutted in the counter direction with respect to the rotation direction of the photosensitive drum 1A. In order to appropriately maintain the lubrication state of the contact portion between the cleaning blade 14 and the photosensitive drum 1A, a toner band is formed on the photosensitive drum 1A in a timely manner. The toner band formed on the photosensitive drum 1A is controlled by the toner band control unit 13 of the control unit 11. Details of the toner band control unit 13 will be described later with reference to FIG. 5.

[0024] (Configuration of the developing device) Subsequently, the configuration of the developing device 4 according to the first embodiment will be described with reference to the cross-sectional view of FIG. 3. Also, the circulation path of the developer in the developing device 4 according to the first embodiment will be described with reference to the perspective view of FIG. 4.

[0025] The developing device 4 has a non-magnetic developing sleeve 41 (developer carrier) that rotates around the outer periphery of a magnet roller 42 (magnetic field generating means) which is fixedly arranged. Further, the developing device 4 has a regulating blade 43 (regulating member) that regulates the height of the magnetic brush formed on the developing sleeve 41, and a developing container 44 that houses a developer containing toner and carrier. The developing sleeve 41 is installed with a predetermined gap from the photosensitive drum 1.

[0026] Furthermore, the developing container 44 is partitioned into a developing chamber 44a and a stirring chamber 44b by a partition wall 44c extending in the vertical direction. A first screw 45a is arranged in the developing chamber 44a, and a second screw 45b is arranged in the stirring chamber 44b. The first screw 45a stirs and conveys the developer in the developing chamber 44a. The second screw 45b stirs and conveys the toner replenished from the toner replenishing layer and the developer in the stirring chamber 44b to equalize the toner concentration.

[0027] The toner remaining amount in the developing container 44 is detected by a toner remaining amount detection sensor 47. The developer conveyed by the first screw 45a and the second screw 45b adheres to the surface of the developing sleeve 41 together with the carrier by the magnet roller 42 fixedly arranged non-rotationally inside the developing sleeve 41. The developer constrained by the magnetic force of the magnet roller 42 on the surface of the developing sleeve 41 has its height regulated by the regulating blade 43, and a predetermined amount of developer is conveyed to the surface of the developing sleeve 41. Generally, the rotation speed of the developing sleeve 41 is operated with a peripheral speed difference relative to the photosensitive drum 1. In this embodiment, the rotation speed of the developing sleeve 41 is set with a 150% peripheral speed difference relative to the peripheral speed of the photosensitive drum 1.

[0028] The arrows shown in Fig. 4 indicate the circulation direction of the developer. The first screw 45a and the second screw 45b convey the developer in opposite directions along the rotation axis direction of the developing sleeve 41. On the partition wall 44c between the developing chamber 44a and the stirring chamber 44b, two communication parts 46 (46a, 46b) are formed at the left and right ends in Fig. 4 to communicate the developing chamber 44a and the stirring chamber 44b with each other. The first communication part 46a formed at the left end in Fig. 4 is an area for delivering the developer from the developing chamber 44a to the stirring chamber 44b. The second communication part 46b formed at the right end in Fig. 4 is an area for delivering the developer from the stirring chamber 44b to the developing chamber 44a. In this way, the developer is circulated through the developing container 44 by the first screw 45a and the second screw 45b via the first communication part 46a and the second communication part 46b. That is, due to the conveying force of the first screw 45a and the second screw 45b, the developer in the developing chamber 44a where the toner is consumed and the toner concentration decreases in the developing process moves into the stirring chamber 44b through the first communication part 46a.

[0029] In this embodiment, the developer accommodated in the developing container 44 is a two-component developer in which a negatively charged non-magnetic toner and a magnetic carrier are mixed. The non-magnetic toner encapsulates a colorant, a wax component, etc. in a resin such as polyester or styrene, and is made into a powder by grinding or polymerization. The magnetic carrier is obtained by applying a resin coat to the surface layer of a core made of resin particles mixed with ferrite particles or magnetic powder. The developer used in this embodiment is a dry two-component development containing a toner and a carrier to which negatively charged fine particles (hereinafter also referred to as external additives) having the same polarity as the charging polarity of the toner are added. In the developing process of developing this developer with respect to the electrostatic latent image formed on the photosensitive drum 1, among the developer carried on the developing sleeve 41, the toner is mainly developed on the image part (the bright part potential part of the electrostatic latent image).

[0030] At this time, fine particles with a negative polarity that is the same as the charging polarity of the toner added to the toner are also developed simultaneously. Also, some of the fine particles added to the toner are agitated by the first screw 45a and the second screw 45b in the developing container 44, resulting in a reduced adhesion force to the toner, and there are also fine particles that separate from the toner.

[0031] Since these fine particles have the same negative polarity as the toner and are likely to be developed on the image area, the amount of fine particles developed on the image area of the photosensitive drum 1 is larger than that on the non-image area. The toner and fine particles developed on the photosensitive drum 1 are primarily transferred onto the intermediate transfer belt 62 in the transfer process. However, a part of the toner and fine particles with a small particle size and a large non-electrostatic adhesion force remain on the photosensitive drum 1 without being transferred.

[0032] Subsequently, the residual transferred toner and fine particles that have reached the cleaning process are cleaned by the cleaning blade 14 for the residual transferred toner. On the other hand, due to the small particle size of the fine particles and their large adhesion force to the photosensitive drum 1, they cannot be completely cleaned and remain on the photosensitive drum 1 as they are.

[0033] The fine particles that have reached the charging process form an electric field in the direction of drawing the toner between the fine particles attached to the photosensitive drum 1 due to the negative charging polarity of the particles themselves and the negative charge received by the charging voltage applied by the charging roller 2. The electric field formed in the direction of drawing the toner between the fine particles becomes stronger as the amount of fine particles attached to the photosensitive drum 1 increases.

[0034] Here, with reference to FIG. 5, the difference in the amount of fine particles attached to the photosensitive drum 1 will be described. In the printing area, since a large number of fine particles are supplied onto the photosensitive drum 1 together with the toner, the amount of fine particles remaining on the photosensitive drum 1 increases. In contrast, in the non-printing area, since no toner image is formed, no toner is supplied, and the amount of fine particles attached is small.

[0035] When the difference in the amount of fine particle adhesion between the printed area and the non-printed area on the photosensitive drum 1 becomes large, a difference also occurs in the force that draws in the toner at the fine particle adhesion area. Therefore, at the time of the next image formation, the electric field formed by the fine particles at the printed area is strong, and the toner is more likely to be drawn in.

[0036] Especially when the same or similar image patterns are continuously formed, since these processes are continuously continued, the accumulation amount of fine particles increases in the printed area on the photosensitive drum 1, resulting in an increase in the toner development amount. As a result, when forming a uniform image such as a halftone image, a density difference occurs between the printed area and the non-printed area, and it is visually recognized as a ghost on the image.

[0037] As described above, the ease of ghost generation is due to the ease of toner development caused by the difference in the accumulation amount of fine particles remaining on the photosensitive drum 1. When an image with a high image ratio is formed, the developer containing fine particles is frequently supplied into the developing device 4. As a result, the concentration of fine particles in the developer in the developing device 4 excessively increases, a large amount of fine particles are developed together with the toner, and the risk of generating the above-described ghost image increases.

[0038] As described above, due to the increase in the amount of fine particles in the developer and the excessive increase in developability due to the decrease in the charge amount in response to the formation of an image with a high image ratio, the difference in the amount of fine particle adhesion between the printed area and the non-printed area on the photosensitive drum 1 becomes large, which is the main cause of the generation of ghost images. Therefore, by reducing the difference in the amount of fine particle adhesion on the photosensitive drum 1A by toner charge control in response to the formation of an image with a high image ratio and making the difference in the amount of fine particle adhesion on the photosensitive drum 1A small, ghost images can be reduced.

[0039] (Toner Charge Control) A procedure for determining the toner band control conditions (peripheral speed of the developing sleeve 41 during toner band control) in this embodiment will be described using the flowchart of FIG. 6. The control in FIG. 6 is executed by the control unit 11 reading a control program stored in the storage unit of the image forming apparatus 100 and controlling various devices. Further, the control in FIG. 6 starts after the image forming apparatus 100 receives an instruction to start an image forming operation for forming an image on the sheet S (recording material).

[0040] When the image forming operation for forming an image on the sheet S is started (S101), the control unit 11 calculates an image ratio (also referred to as an image density ratio) from the video count value of the latest image based on the image information (S102). That is, the image ratio is calculated by the video count value obtained by integrating the output levels for each pixel of the image formed by the image forming operation.

[0041] Subsequently, the control unit 11 calculates the average image ratio output from the image forming apparatus 100 using a method described later based on the image ratio calculated in S102 (S103).

[0042] The method for calculating the average image ratio in S103 is obtained by the following formula (Equation 1).

[0043]

Equation

[0044] As shown in the above formula, after integrating the average image ratio up to the previous sheet up to the number of sheets up to the previous sheet, the image ratio of the latest image is added, and the value averaged by the latest integrated number is defined as the average image ratio. By using the above calculation method, the image ratios to be stored in the storage unit of the image forming apparatus 100 are only Ave_Duty(n - 1) and New_Duty, so that the storage capacity of the storage unit of the image forming apparatus 100 can be suppressed.

[0045] Subsequently, the control unit 11 determines whether or not the average image ratio calculated in S103 exceeds the ghost image generation threshold value (S104). In the present embodiment, the threshold value for determining the generation of a ghost image is set to 10% of the average image ratio, but this is set as an example and is not limited thereto.

[0046] When the average image ratio exceeds the ghost image generation threshold value (S104: Yes), the control unit 11 refers to a table (development sleeve peripheral speed determination table) described in Table 1 below (S105) and determines the peripheral speed of the development sleeve 41 during toner band control (S106). On the other hand, when the average image ratio does not exceed the ghost image generation threshold value (S104: No), the process proceeds to S108.

[0047] Table 1 shows the relationship between the average image ratio and the development sleeve peripheral speed ratio. In Table 1, the development sleeve peripheral speed ratio is the ratio of the peripheral speed of the development sleeve 41 to the peripheral speed of the photosensitive drum 1.

[0048]

Table 1

[0049] Subsequently, the control unit 11 executes toner band control at the peripheral speed of the development sleeve 41 determined in S106 (S107). The toner band width during toner band control execution is desirably the entire width of the image forming area, and the length is one turn or more of the photosensitive drum 1, but is not limited thereto. For example, a modified example in which a toner band is formed only at a location where the adhesion amount of fine particles on the photosensitive drum 1 is assumed to be small may be used.

[0050] Subsequently, the control unit 11 determines whether it is continuous paper feeding (S108). If it is continuous paper feeding (S108), the process returns to S101. On the other hand, if it is not continuous paper feeding, the series of processes according to FIG. 6 is terminated.

[0051] Next, means for reducing the difference in the amount of fine particles attached to the photoreceptor drum 1 corresponding to the concentration of fine particles that varies according to the formation of an image with a high image ratio in this embodiment and the increase in developability will be described below.

[0052] First, the developing bias applied to the developing device 4 according to this embodiment will be described with reference to FIG. 7. FIG. 7 is a diagram showing the waveform of the developing bias output by the developing bias power supply 81 of the developing device 4 according to this embodiment.

[0053] The developing bias power supply 81 as a developing bias applying means applies a developing bias in which a DC component is superimposed on an AC component to the developing sleeve 41. In this embodiment, the AC component of the developing bias is a rectangular wave of 11 kHz. As shown in FIG. 7, this developing bias is provided with a blank portion in which the AC component is intermittently thinned out to become only the DC component. In this specification, the pulse of the rectangular wave existing in the portion corresponding to the blank portion when the AC component is not thinned out, in other words, the pulse that becomes blank (the pulse that no longer exists) due to the thinning out of the AC component is referred to as a "blank pulse". Also, the portion that becomes the DC component by intermittently thinning out the AC component is referred to as a "blank portion".

[0054] Therefore, the developing bias output by the developing bias power supply 81 has a waveform with one cycle consisting of an AC bias portion in which a DC component is superimposed on an AC component and a blank portion following this AC bias portion and consisting of only the DC component.

[0055] In this embodiment, as shown in FIG. 7, a single blank pulse waveform (hereinafter referred to as SBP) in which a blank portion is provided after an alternating current bias portion of a rectangular wave of one cycle (two pulses) is used as a developing bias. In this specification, the number of pulses of the rectangular wave is counted with a half cycle of the rectangular wave as one pulse. Also, the time of the blank portion in one cycle of the developing bias is defined as blank time t1. Further, the total time during which the electric field (pulse) on the developing side of the alternating current bias portion is generated in one cycle of the developing bias is defined as developing time t2.

[0056] The sum of the voltages on the developing side (developer application side) and the developer recovery side (developer return side) in the alternating current bias portion is defined as Vpp, which is set to 1.4 kV in this embodiment.

[0057] And the ratio of the electric field on the developing side to the electric field on the developer recovery side (hereinafter referred to as duty ratio) is 50%. Here, the electric field on the developing side is the electric field by which toner flies from the developing sleeve 41 side to the photosensitive drum 1 side by the alternating current bias portion during one cycle of the developing bias. Also, the electric field on the recovery side is the electric field by which toner is drawn back from the photosensitive drum 1 side to the developing sleeve 41 side by the alternating current bias portion during one cycle of the developing bias.

[0058] As described above, it is common to operate the developing sleeve 41 with a peripheral speed difference with respect to the photosensitive drum 1. In this embodiment, the rotational speed of the developing sleeve 41 is set with a peripheral speed difference of 150% with respect to the peripheral speed of the photosensitive drum 1.

[0059] By providing such a peripheral speed difference to the developing sleeve 41 with respect to the photosensitive drum 1, the developability of the developer with respect to the electrostatic latent image formed on the photosensitive drum 1 can be improved. That is, when the peripheral speed of the developing sleeve 41 is increased, the amount of developer supplied to the electrostatic latent image formed on the photosensitive drum 1 increases, and the amount of fine particles supplied onto the photosensitive drum 1 also increases.

[0060] Therefore, when printing images with a high image density ratio (high image ratio printing) continues and it is detected that the replenishing toner is increasing, during toner band control, the peripheral speed difference of the developing sleeve 41 is increased with respect to normal image forming conditions. As a result, the supply amount of fine particles onto the photosensitive drum 1 is increased, and the unevenness in the adhesion amount of the fine particles carried around on the photosensitive drum 1 is made uniform, thereby suppressing the generation of ghost images caused by the fine particles carried around on the photosensitive drum 1.

[0061] On the other hand, although there is concern that the deterioration of ghost images due to the rotation of fine particles may occur by supplying a large amount of fine particles onto the photosensitive drum 1, the fine particles are supplied to the electrostatic latent image formed on the entire surface of the photosensitive drum 1 by toner band control. For this reason, since the distribution of the adhesion amount of the fine particles carried around on the photosensitive drum 1 becomes uniform, ghost images do not occur.

[0062] (Experimental Example) The effects in this embodiment were confirmed by the following experiment, taking the case where the peripheral speed of the developing sleeve 41 during toner band control was not changed with respect to the average image ratio as a conventional example.

[0063] In advance, at each specific image station, images with image ratios of 5%, 10%, 20%, and 30% were each output 200 sheets in A4 size. Then, after passing 5 sheets of A4-sized images in which vertical bands with an image ratio of 100% having a main scanning width of 30 mm and a sub-scanning width of 210 mm were arranged so as not to overlap at any image forming station, 30HT single-color halftone images were each output in A4 size.

[0064] Thereafter, the reflection density of the vertical band portion and the solid white portions at both ends thereof was measured for each image forming station, and it was calculated as the reflection density difference (= the severity of ghosting).

[0065] The results of comparison with the conventional example are shown in FIG. 8 with the above-described density step on the vertical axis and the image ratio output in advance on the horizontal axis. FIG. 8 is a diagram showing the effects in this embodiment.

[0066] Furthermore, the measurement results of the amount of fine particles adhering to the photosensitive drum 1 that causes the concentration gradient described above are shown in FIG. 9. FIG. 9 is a diagram showing the effect of the difference in the amount of external additive adhering to the photosensitive drum 1 in this embodiment. As shown in FIG. 9, it can be seen that the difference in the amount of fine particles adhering to the photosensitive drum 1 in this embodiment is smaller than that in the conventional example, indicating an improvement.

[0067] Here, the relationship between the difference in the amount of fine particles adhering to the photosensitive drum 1 and the occurrence of the external additive-carrying ghost image will be described using Table 2. Table 2 shows the amount of external additive adhering to the photosensitive drum 1 and the ghost image determination. Specifically, in Table 2, the external additive-carrying ghost image "〇" indicates that it is not visually recognized as a ghost on the image. On the other hand, in Table 2, the external additive-carrying ghost image "×" indicates that it is visually recognized as a ghost on the image.

[0068]

Table 2

[0069] As shown in Table 2, it can be seen that when the difference in the amount of fine particles adhering to the photosensitive drum 1 in this embodiment (the difference in the amount of fine particles adhering to the drum = 0.1 or the difference in the amount of fine particles adhering to the drum = 0.2), the generation of ghost images can be suppressed.

[0070] Also, as can be seen from FIGS. 8 and 9, in this embodiment, it can be seen that the concentration gradient and the difference in the amount of fine particles on the photosensitive drum 1 are smaller than those in the conventional example at each image ratio, and the ghost has been improved.

[0071] In this embodiment, as an example, a method of increasing the peripheral speed ratio of the developing sleeve 41 with respect to the photosensitive drum 1 by increasing the peripheral speed of the developing sleeve 41 during toner band control according to the average image ratio is shown. Specifically, the peripheral speed ratio of the developing sleeve 41 with respect to the photosensitive drum 1 when the average image ratio is a second image ratio higher than the first image ratio is increased compared to the peripheral speed ratio of the developing sleeve 41 with respect to the photosensitive drum 1 when the average image ratio is the first image ratio.

[0072] Not limited to this, as a method of increasing the peripheral speed ratio of the developing sleeve 41 with respect to the photosensitive drum 1, the peripheral speed of the photosensitive drum 1 during toner band control may be decreased according to the average image ratio. Further, as a method of increasing the peripheral speed ratio of the developing sleeve 41 with respect to the photosensitive drum 1, increasing the peripheral speed of the developing sleeve 41 during toner band control according to the average image ratio and decreasing the peripheral speed of the photosensitive drum 1 during toner band control according to the average image ratio may be used in combination.

[0073] When the image density ratio of the image formed by the image forming operation is the first ratio, the ratio of the peripheral speed of the developer carrier to the peripheral speed of the image carrier when the control unit executes toner band control to form a toner band on the image carrier is set as the first peripheral speed ratio. When the image density ratio is a second ratio higher than the first ratio, the ratio of the peripheral speed of the developer carrier to the peripheral speed of the image carrier when the control unit executes toner band control is set as the second peripheral speed ratio.

[0074] In the invention according to the first embodiment described above, since the second peripheral speed ratio is higher than the first peripheral speed ratio, it is possible to suppress the occurrence of ghost images due to the rotation of the external additive on the photosensitive drum when the image density ratio of the image formed by the image forming operation is high.

Explanation of Reference Numerals

[0075] 1 Photosensitive drum 4 Developing device 11 Control unit 13 Toner band control unit 44 Developing container 41 Developing sleeve 62 Intermediate transfer belt 100 Image forming apparatus

Claims

1. An image forming apparatus capable of performing an image forming operation for forming an image on a recording material, an image carrier on which an electrostatic latent image is formed, a developer carrier that carries a developer including toner and a carrier for developing the electrostatic latent image formed on the image carrier, and a developing container that houses the developer, a transfer member that contacts the image carrier to form a transfer portion therebetween, and transfers the toner image formed on the image carrier to the recording material at the transfer portion when a transfer bias is applied, a control unit that executes toner band control for forming a toner band on the image carrier, comprising: When the image density ratio of the image formed by the image forming operation is the first ratio, the ratio of the peripheral speed of the developer carrier to the peripheral speed of the image carrier when the control unit executes the toner band control is higher than when the image density ratio is a second ratio higher than the first ratio. An image forming apparatus characterized by the above.

2. When the image density ratio is the first ratio, the peripheral speed of the developer carrier when the control unit executes the toner band control is faster than when the image density ratio is the second ratio. The image forming apparatus according to claim 1, characterized by the above.

3. When the image density ratio is the first ratio, the peripheral speed of the image carrier when the control unit executes the toner band control is slower than when the image density ratio is the second ratio. The image forming apparatus according to claim 1, characterized by the above.

4. When the control unit executes the toner band control, the length of the toner band formed on the image carrier is equal to or greater than the circumference of the image carrier. The image forming apparatus according to claim 1, characterized in that.

5. The image density ratio is calculated by a video count value obtained by integrating the output levels for each pixel of the image formed by the image forming operation. The image forming apparatus according to claim 1, characterized in that.

Images