Image forming device

Abstract

To prevent fogging toner during image formation, while preventing carrier adhesion in a period from when rotation drive of a developing roller is started until when the rotation speed of the developing roller reaches a speed during image formation.SOLUTION: In an image forming apparatus, an absolute value of a difference (Vback) between a surface potential of a developer carrier to which a development bias is applied by development bias application means and a surface potential of an image carrier to which an electrification bias is applied by electrification bias application means in a period from when driving means starts rotation drive of the developer carrier until when the rotation speed of the developer carrier reaches a speed during image formation, is smaller than the absolute value of the difference (Vback) between the surface potential of the developer carrier to which the development bias is applied by the development bias application means and the surface potential of the image carrier to which the electrification bias is applied by the electrification bias application means during image formation.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus equipped with a developing device that develops an electrostatic latent image formed on an image carrier. [Background technology]

[0002] The image forming apparatus includes a photosensitive drum (image carrier), a charging device that charges the image carrier, and a developing device having a developing roller (developer carrier) that carries a developer containing toner and carrier to develop the electrostatic latent image formed on the image carrier.

[0003] Patent Document 1 describes an image forming apparatus that changes Vback between sheets during a continuous image forming job according to the number of sheets to be imaged. Note that Vback refers to the potential difference between the surface potential (non-image portion potential) Vd of the photoconductor drum charged by the charging device and the DC component Vdc of the developing bias. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2012-128320 A Summary of the Invention [Problem to be solved by the invention]

[0005] According to the inventors' research, it was found that the phenomenon of carrier adhesion (carrier adhesion) occurring in the non-image areas of the photosensitive drum is more likely to occur during the non-image formation period immediately after the start of the pre-rotation operation and the start of the rotation drive of the developing roller, compared to during image formation. This is related to the fact that during image formation, the rotation speed of the developing roller is stable because the start-up of the rotation drive of the developing roller is completed, whereas during the non-image formation period of the pre-rotation operation, the rotation speed of the developing roller is not stable because the rotation drive of the developing roller is still starting up.

[0006] In general, when the absolute value of Vback is reduced, carrier adhesion tends to be suppressed more than when the absolute value of Vback is increased. Therefore, in order to suppress carrier adhesion during the non-image formation period of the pre-rotation operation, it is conceivable to uniformly reduce the absolute value of Vback after the start of the pre-rotation operation. On the other hand, when the absolute value of Vback is reduced, the force attracting the toner to the developing roller becomes weaker than when the absolute value of Vback is increased, so that toner (fog toner) is more likely to adhere to the non-image portion of the photosensitive drum.

[0007] The present invention has been made in view of the above problems, and an object of the present invention is to provide an image forming apparatus capable of suppressing carrier adhesion during the period from when the rotational drive of the developing roller starts until the rotational speed of the developing roller reaches the speed during image formation, while suppressing fog toner during image formation. [Means for solving the problem]

[0008] In order to achieve the above object, an image forming apparatus according to one aspect of the present invention has the following configuration: That is, the image forming apparatus includes a developing device having an image carrier, a developing container that contains a developer containing toner and a carrier, and a rotatable developer carrier that carries the developer to develop an electrostatic latent image formed on the image carrier, a driving means that drives the developer carrier to rotate, a developing bias application means that applies a developing bias to the developer carrier, and a charging bias application means that applies a charging bias to the image carrier, and the developer carrier that is driven to rotate by the driving means is rotated by a predetermined amount after the developer carrier starts to be driven to rotate by the driving means. The absolute value of the difference between the surface potential of the developer carrier to which the developing bias is applied by the developing bias application means and the surface potential of the image carrier to which the charging bias is applied by the charging bias application means at a position facing the developer carrier during a period until the speed of the developer carrier reaches the rotation speed of the developer carrier during image formation is the absolute value of the difference between the surface potential of the developer carrier to which the developing bias is applied by the developing bias application means and the surface potential of the image carrier to which the charging bias is applied by the charging bias application means at a position facing the developer carrier during image formation. the charging bias application means applies the charging bias to the image carrier so that the surface potential of the image carrier at the opposing position facing the developer carrier becomes a first potential during the period from when the driving means starts to rotate the developer carrier until the rotation speed of the developer carrier driven by the driving means reaches the rotation speed of the developer carrier during the image formation, and after the rotation speed of the developer carrier driven by the driving means reaches the rotation speed of the developer carrier during the image formation, the charging bias application means applies the charging bias to the image carrier so that the surface potential of the image carrier at the opposing position facing the developer carrier becomes a second potential different from the first potential. Effect of the Invention

[0009] According to the present invention, it is possible to suppress carrier adhesion during the period from when the developing roller starts to be rotated until the rotational speed of the developing roller reaches the speed during image formation, while suppressing fog toner during image formation. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram for explaining a configuration of an image forming apparatus according to a first embodiment. [Diagram 2] 1 is a cross-sectional view illustrating a configuration of a developing device according to a first embodiment. [Diagram 3] FIG. 4 is a diagram showing a sequence of an image forming operation (image creating operation) according to the first embodiment. [Figure 4] FIG. 10 is a diagram showing a sequence of an image forming operation (image creating operation) according to the second embodiment. [Diagram 5] FIG. 13 is a diagram showing a sequence of an image forming operation (image creating operation) according to the third embodiment. [Figure 6] FIG. 13 is a diagram showing a sequence of an image forming operation (image creating operation) according to the fourth embodiment. [Figure 7] FIG. 11 is a diagram showing a sequence of an image forming operation (image creation operation) in a conventional example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, the embodiments of the present invention will be described in detail with reference to the accompanying drawings. Note that the following embodiments do not limit the scope of the present invention, and not all of the combinations of features described in the embodiments are necessarily essential to the solution of the present invention. The present invention can be implemented in various applications, such as printers, various printing machines, copiers, FAX machines, and multifunction machines.

[0012] [First embodiment] (Configuration of Image Forming Apparatus) FIG. 1 is a diagram showing the configuration of an in-line type (four-drum type) color image forming apparatus 100. As shown in FIG.

[0013] Image forming apparatus 100 has four image forming sections (image forming units): an image forming section that forms a yellow image, an image forming section that forms a magenta image, an image forming section that forms a cyan image, and an image forming section that forms a black image. These four image forming sections are arranged in a row at regular intervals. Hereinafter, when the subscripts A, B, C, and D are added to a reference symbol, they indicate the respective yellow, magenta, cyan, and black members (or devices), and when there is only a reference symbol without a subscript, they indicate a member (or device) that is not related to color.

[0014] Each image forming unit is provided with a photoconductor drum 1A, 1B, 1C, or 1D, which is an image carrier. The photoconductor drums 1A, 1B, 1C, and 1D are negatively charged organic photoconductors having a photosensitive layer on a drum base made of aluminum or the like, and are rotated at a predetermined process speed by a drive device.

[0015] Around each of the photoconductor drums 1A, 1B, 1C, and 1D, there are arranged charging rollers 2A, 2B, 2C, and 2D, developing devices 4A, 4B, 4C, and 4D, and primary transfer rollers 61A, 61B, 61C, and 61D, respectively.

[0016] Here, an image forming unit using a charging roller 2 made of conductive rubber as a charging device will be described in detail. The charging roller 2 is rotatably supported by bearing members at both ends of the core metal, maintaining a constant axial distance. The charging roller 2 is also biased toward the photosensitive drum 1 by a pressure spring, and is pressed against the surface of the photosensitive drum 1 with a predetermined pressure. The charging roller 2 rotates following the rotation of the photosensitive drum 1, and a charging bias of predetermined conditions is applied to the core metal of the charging roller 2. As a result, the surface of the photosensitive drum 1, which rotates in contact with the charging roller 2, is contact-charged to a predetermined polarity and potential.

[0017] Furthermore, above each of the photoconductor drums 1A, 1B, 1C, and 1D, there is disposed exposure means (exposure devices) 3A, 3B, 3C, and 3D, respectively, for forming an electrostatic latent image on the surface of the charged photoconductor drum 1. The exposure devices 3 are electrically connected to an image processing unit 9, and form an electrostatic latent image in response to an output image signal transmitted from the image processing unit 9.

[0018] The developing means (developing devices) 4A, 4B, 4C, and 4D contain yellow toner, cyan toner, magenta toner, and black toner, respectively. The developing rollers 44A, 44B, 44C, and 44D are rotated by the respective driving means (developing motors) and develop the electrostatic latent images formed on the surfaces of the photoconductor drums 1A, 1B, 1C, and 1D to form toner images.

[0019] At positions facing the image forming units, an intermediate transfer body, rotatable endless intermediate transfer belt 62, is disposed. The intermediate transfer belt 62 is tensioned by a drive roller 65, a secondary transfer opposing roller 63, and a tension roller 66. The intermediate transfer belt 62 is rotated (moved) in the direction of the arrow (counterclockwise direction) by the drive of the drive roller 65 connected to a motor. The toner images that reach positions facing the primary transfer rollers 61A, 61B, 61C, and 61D of the intermediate transfer belt 62 are transferred from the photosensitive drum 2 to the intermediate transfer belt 62 by a primary transfer voltage.

[0020] The secondary transfer opposing roller 63 contacts the secondary transfer roller 64 via the intermediate transfer belt 62 to form a secondary transfer portion. A belt cleaning device 67 is disposed on the outer side of the intermediate transfer belt 62 to remove and collect residual toner remaining on the surface of the intermediate transfer belt 62.

[0021] In addition, downstream of the secondary transfer section where the secondary transfer opposing roller 63 and the secondary transfer roller 64 abut in the rotation direction of the intermediate transfer belt 62, a fixing device 7 is provided to perform heat and pressure processing to fix the toner to the transfer material.

[0022] Cleaning members (drum cleaning devices) 8A, 8B, 8C, and 8D for cleaning residual toner on the surfaces of the photoconductor drums 1A, 1B, 1C, and 1D are arranged around the photoconductor drums 1A, 1B, 1C, and 1D, respectively.

[0023] The configuration of the developing device 4 is shown in a cross-sectional view in FIG.

[0024] The developing device 4 has a developing container 45 that contains a developer containing toner and a carrier. The developing container 45 is partitioned into an agitating chamber 46a and a developing chamber 46b by a partition wall 43, and the developer is circulated between the agitating chamber 46a and the developing chamber 46b. A screw 45a that agitates and transports the developer in the agitating chamber 46a is disposed in the agitating chamber 46a. A screw 45b that agitates and transports the developer in the developing chamber 46b is disposed in the developing chamber 46b.

[0025] The developing device 4 also includes a developer regulating member 42. The developer regulating member 42 regulates the amount of developer carried by the developing roller 44.

[0026] The developer contained in the developing device 4 (developing container) is a two-component developer in which negatively charged non-magnetic toner and magnetic carrier are mixed. The non-magnetic toner is a resin such as polyester or styrene that contains colorants and wax components, and is pulverized or polymerized to form a powder. The magnetic carrier is a core made of resin particles kneaded with ferrite particles or magnetic powder, with a resin coating on the surface.

[0027] We will now explain the process of developing toner onto the photoconductor drum 1 in the development area. After the photoconductor drum 1 is uniformly charged to a charging potential Vd [V] by the charging roller 2, the image area is exposed by the exposure device 3 to an exposure potential Vl [V].

[0028] A high-voltage power supply (charging bias application means) applies a DC voltage (charging DC bias) or a voltage obtained by superimposing an AC voltage on a DC voltage (charging DC+AC bias) to the charging roller 2. The charging DC bias or charging DC+AC bias is applied to the photoconductor drum 1 via the charging roller 2 that is in contact with the photoconductor drum 1.

[0029] Further, a DC voltage (developing DC bias) or a voltage obtained by superimposing an AC voltage on a DC voltage (developing DC+AC bias) is applied to the developing roller 44 by a high-voltage power supply (developing bias application means).

[0030] When the DC component voltage of the developing roller 44 is Vdc [V], the absolute value of the difference between the exposure potential V1 and this potential is called Vcont, |Vdc-V1|, which creates an electric field that carries the toner to the image portion.

[0031] Moreover, the absolute value |Vdc-Vd| of the difference between the DC voltage Vdc and the charging potential Vd is called Vback, which creates an electric field that pulls the toner back in the direction from the photosensitive drum 1 toward the developing roller 44. This is provided to suppress the phenomenon in which the toner (fog toner) adheres to the non-image portion of the photosensitive drum 1 (the so-called fogging phenomenon).

[0032] When the absolute value of Vback is increased, the force attracting the toner to the developing roller becomes stronger compared to when the absolute value of Vback is decreased, so that toner (fog toner) tends to be less likely to adhere to non-image areas of the photosensitive drum 1. On the other hand, when the absolute value of Vback is decreased, the phenomenon in which carrier adheres to non-image areas of the photosensitive drum (carrier adhesion) tends to be suppressed compared to when the absolute value of Vback is increased.

[0033] Note that the charging potential Vd of the photosensitive drum 1 has a dark decay. Therefore, the charging potential Vd immediately after the charging bias is applied to the photosensitive drum 1 and the charging potential Vd at the time when the charged position of the photosensitive drum 1 reaches the position facing the developing roller 44 due to the rotation of the photosensitive drum 1 are different values.

[0034] The first embodiment is concerned with a phenomenon (fogging phenomenon) occurring at the position where the photosensitive drum 1 faces the developing roller 44. For this reason, the charging potential Vd does not refer to the value of the charging potential Vd immediately after a charging bias is applied to the photosensitive drum 1, but refers to the value of the charging potential Vd at the time when the charged position of the photosensitive drum 1 reaches the position facing the developing roller 44 due to the rotation of the photosensitive drum 1. This also applies to other potentials such as the latent image potential V1 of the photosensitive drum 1.

[0035] (Conventional example) Before describing the sequence of the image forming operation (image creation operation) according to the first embodiment, the sequence of the image creation operation in a conventional example will be described with reference to Fig. 7. Fig. 7 shows the rotational drive of the developing roller 44 at the start of the image creation operation, the developing DC bias applied to the developing roller 44, the value of the charging potential Vd at the position where the photosensitive drum 1 faces the developing roller 44, and a timing chart in the conventional example.

[0036] In the operation of the conventional example shown in FIG. 7, the pre-rotation operation starts, and the rotational drive of the photosensitive drum 1 starts. At the same time, the application of a charging bias to the charging roller 2 starts, and the photosensitive drum 1 is uniformly charged (Vd=−900 V) at the position facing the developing roller 44.

[0037] When the photosensitive drum 1 rotates and the charged position of the photosensitive drum 1 reaches the position facing the developing roller 44, application of a developing DC bias (Vdc=-740V) to the developing roller 44 begins. Then, after the charging potential Vd of the photosensitive drum 1 and the developing DC bias Vdc become stable, the developing roller 44 begins to be rotated. In the conventional operation, the fog removal potential difference Vback (=160V) at this time is controlled to be the same value as Vback during image formation.

[0038] In the conventional operation, the carrier adhesion amount and the fog toner amount were measured when the absolute value of Vback was changed to 60 V, 110 V, 160 V, and 210 V. The results of measuring the carrier adhesion amount and the fog toner amount when the absolute value of Vback was changed during the period from when the rotational drive of the developing roller 44 started until the rotational speed of the developing roller 44 reached the same speed as during image formation are shown in Table 1. Moreover, in the conventional operation, the results of measuring the carrier adhesion amount and the fog toner amount when the absolute value of Vback during image formation was changed are shown in Table 2.

[0039] Regarding the carrier adhesion items in Tables 1 and 2, "◯" indicates that the degree of carrier adhesion is at a level that is acceptable for image quality, and "×" indicates that the degree of carrier adhesion is at a level that is unacceptable for image quality. Regarding the fog toner items in Tables 1 and 2, "◯" indicates that the degree of fog toner is at a level that is acceptable for image quality, and "×" indicates that the degree of fog toner is at a level that is unacceptable for image quality. Note that an unacceptable level for image quality means that image defects have occurred.

[0040] [Table 1]

[0041] [Table 2]

[0042] In this experiment, the image forming apparatus 100 was used to stop the operation during the image formation of a solid white image (i.e., an image with an image ratio of 0%), and the carrier and fog toner adhering to the photosensitive drum 1 were collected with tape. The amount of carrier adhesion was measured by counting the number of carrier particles in an area of ​​3 cm x 3 cm, and the number of carrier particles was counted by 1 cm. 2 The number of carrier particles adhering per unit area was calculated. The amount of fog toner was measured by measuring the density using an X-Rite 500 color reflection densitometer manufactured by X-Rite Corporation.

[0043] During the period from when the developing roller 44 starts to be rotated until the rotation speed of the developing roller 44 reaches the same speed as during image formation, the degree of carrier adhesion reaches a level that is unacceptable for image quality when Vback is 160 V or higher. In contrast, during image formation, the degree of carrier adhesion reaches a level that is unacceptable for image quality when Vback is 210 V or higher.

[0044] On the other hand, as for the fog toner, in the period from when the developing roller 44 starts to be rotated until the rotation speed of the developing roller 44 reaches the same speed as during image formation, if Vback is 60 V or less, the level of the fog toner becomes a level that is unacceptable in terms of image quality. In contrast, during image formation, if Vback is 110 V or less, the level of the fog toner becomes a level that is unacceptable in terms of image quality.

[0045] From the above results, it can be seen that the optimal Vback at which image defects due to carrier adhesion and fog toner do not occur differs between the period from when the rotational drive of the developing roller 44 starts until the rotational speed of the developing roller 44 reaches the same speed as during image formation and during image formation.

[0046] Next, the sequence of the image forming operation (image creation operation) in the first embodiment will be described with reference to Fig. 3. Fig. 3 shows the rotational drive of the developing roller 44 at the start of the image creation operation, the developing DC bias applied to the developing roller 44, the value of the charging potential Vd at the position where the photosensitive drum 1 faces the developing roller 44, and a timing chart in the first embodiment.

[0047] At the same time that the pre-rotation operation starts and the rotational drive of the photosensitive drum 1 starts, the application of a charging bias to the photosensitive drum 1 starts, and the photosensitive drum 1 is uniformly charged to a first charging potential (Vd1=-850V). Then, when the charged position of the photosensitive drum 1 reaches a position facing the developing roller 44 due to the rotation of the photosensitive drum 1, the application of a developing DC bias (Vdc=-740V) to the developing roller 44 starts. After a certain time has elapsed since the application of the developing DC bias, the charging potential Vd of the photosensitive drum 1 and the developing DC bias become stable, and the rotational drive of the developing roller 44 starts. At this time, Vback (start-up Vback) is 110V.

[0048] Next, after the rotation speed of the developing roller 44 reaches the same speed as that during image formation and before the start of the image formation operation, the charging DC bias is changed, and the charging potential Vd of the photosensitive drum 1 is uniformly charged to the second charging potential (Vd2=-900V), which is the same value as that during image formation. Vback at this time becomes 160V.

[0049] 3, during the period from when the developing roller 44 starts to be rotated until it reaches the speed during image formation, carrier adhesion can be suppressed by setting Vback (start-up Vback) to 110 V. Also, during image formation, fog toner and carrier adhesion during image formation can be suppressed by setting Vback (imaging Vback) to 160 V.

[0050] In the first embodiment described above, the rise Vback is set to be smaller than the image formation Vback. According to the first embodiment, the carrier adhesion during the period from when the rotational drive of the developing roller 44 starts until the rotational speed of the developing roller 44 reaches the speed during image formation can be suppressed, while the fog toner during image formation can be suppressed.

[0051] 3, when changing from the rising Vback to the image forming Vback, an example is described in which the charging DC bias is changed while the developing DC bias (Vdc=-740V) to the developing roller 44 is kept constant, but the present invention is not limited to this. If the relationship is satisfied in which the rising Vback is smaller than the image forming Vback, a modified example in which the developing DC bias is changed in addition to changing the charging DC bias may be used.

[0052] [Second embodiment] Next, a second embodiment will be described. The basic configuration of the second embodiment is the same as that of the first embodiment. Therefore, elements having substantially the same or corresponding functions and configurations as those of the first embodiment are given the same reference numerals, and detailed descriptions are omitted, and only the components unique to the second embodiment will be described in detail.

[0053] The sequence of the image forming operation (image creation operation) in the second embodiment will be described with reference to Fig. 4. Fig. 4 shows the rotational drive of the developing roller 44 at the start of the image creation operation, the developing DC bias applied to the developing roller 44, the value of the charging potential Vd at the position where the photosensitive drum 1 faces the developing roller 44, and a timing chart in the second embodiment.

[0054] In the first embodiment described above, the charging potential Vd of the photosensitive drum 1 is changed after the rotation speed of the developing roller 44 reaches the same speed as that during image formation and before the start of the image formation operation. However, if the charging potential Vd of the photosensitive drum 1 is changed immediately after the rotation drive of the developing roller 44 reaches the same speed as that during image formation, there is a possibility that the charging potential of the photosensitive drum 1 will be changed in a state in which the rotation speed of the developing roller 44 is not sufficiently stable. In such a state, since the Vback is not controlled to the optimum Vback for suppressing carrier adhesion and fog toner with respect to the rotation speed of the developing roller 44, there is a risk of carrier adhesion and fog toner occurring.

[0055] On the other hand, after the developing roller 44 makes one rotation (about 100 msec in the second embodiment), the rotation speed of the developing roller 44 is sufficiently stable. For this reason, it is preferable to change the charging potential of the photosensitive drum 1 after 100 msec has elapsed since the rotation speed of the developing roller 44 reaches the same speed as during image formation, and before the image formation operation starts.

[0056] On the other hand, from the viewpoint of productivity, it is preferable to perform the control before the start of the image forming operation in a short time. Therefore, after the rotation speed of the developing roller 44 reaches the same speed as that during image formation, Vback is changed to the same value as that during image formation at least until the photosensitive drum 1 rotates once (216 msec in the second embodiment). This prevents the start point of the image forming operation from being delayed more than necessary due to the change in Vback (change in the charging DC bias) after the rotation speed of the developing roller 44 reaches the same speed as that during image formation, and thus suppresses a decrease in productivity.

[0057] 4, in the second embodiment, the time t from when the rotation speed of the developing roller 44 reaches the same speed as during image formation until the charging potential Vd of the photosensitive drum 1 is changed is set as follows: That is, the time t is controlled to be 100 msec or more, calculated from the time when the rotation speed of the developing roller 44 reaches the same speed as during image formation, until the time required for the photosensitive drum 1 to make one rotation.

[0058] As a result, in the second embodiment, it is possible to suppress the carrier adhesion and the fog toner before the start of the image forming operation while suppressing the decrease in productivity. Also, in the second embodiment, like the first embodiment, it is possible to suppress the carrier adhesion during the period from the start of the rotation drive of the developing roller 44 until the rotation speed of the developing roller 44 reaches the speed during the image formation, while suppressing the fog toner during the image formation.

[0059] [Third embodiment] Next, a third embodiment will be described. The basic configuration of the third embodiment is the same as that of the first embodiment. Therefore, elements having substantially the same or corresponding functions and configurations as those of the first embodiment are given the same reference numerals, and detailed descriptions are omitted, and only the components unique to the third embodiment will be described in detail.

[0060] The sequence of the image forming operation (image creation operation) in the third embodiment will be described with reference to Fig. 5. Fig. 5 shows the rotational drive of the developing roller 44 at the start of the image creation operation, the developing DC bias applied to the developing roller 44, the value of the charging potential Vd at the position where the photosensitive drum 1 faces the developing roller 44, and a timing chart in the third embodiment.

[0061] In the first embodiment described above, the charging potential Vd of the photosensitive drum 1 is changed from the first charging potential Vd1 to the second charging potential Vd2 so that the charging potential Vd becomes the same as Vback during image formation after a certain time has elapsed after the rotation speed of the developing roller 44 reaches the same speed as during image formation. Specifically, the first charging potential Vd1 is −850V, and the second charging potential Vd2 is −900V.

[0062] However, as in the first embodiment, if a step occurs in the charging potential on the photosensitive drum 1 by changing the charging potential Vd of the photosensitive drum 1 in a short period of time, there is a risk that carrier adhesion or fog toner will occur at the position where the charging potential on the photosensitive drum 1 is changed.

[0063] Therefore, in the third embodiment, as shown in Fig. 5, Vback during the period from when the rotation speed of the developing roller 44 reaches the same speed as that during image formation to when the image forming operation starts is controlled to a value different from the rising Vback and the image creation Vback. Specifically, the absolute value of Vback during the period from when the rotation speed of the developing roller 44 reaches the same speed as that during image formation to when the image forming operation starts is set to be larger than the absolute value of the rising Vback and smaller than the absolute value of the image creation Vback. Note that the rising Vback refers to the Vback during the period from when the rotation speed of the developing roller 44 starts to when it reaches the speed during image formation. Also, the image creation Vback refers to the Vback during image formation (after the start point of the image forming operation).

[0064] As a result, in the third embodiment, the step of the charging potential Vd on the photosensitive drum 1 can be reduced, and carrier adhesion and fog toner can be suppressed. Also, in the third embodiment, similar to the first embodiment, carrier adhesion can be suppressed during the period from when the rotational drive of the developing roller 44 starts until the rotational speed of the developing roller 44 reaches the speed during image formation, while fog toner can be suppressed during image formation.

[0065] [Fourth embodiment] Next, a fourth embodiment will be described. The basic configuration of the fourth embodiment is the same as that of the first embodiment. Therefore, elements having substantially the same or corresponding functions and configurations as those of the first embodiment are given the same reference numerals, and detailed descriptions are omitted, and only the components unique to the fourth embodiment will be described in detail.

[0066] The sequence of the image forming operation (image creation operation) in the fourth embodiment will be described with reference to Fig. 4. Fig. 6 shows the rotational drive of the developing roller 44 at the start of the image creation operation, the developing DC bias applied to the developing roller 44, the value of the charging potential Vd at the position where the photosensitive drum 1 faces the developing roller 44, and a timing chart in the fourth embodiment.

[0067] In the first embodiment described above, Vback is controlled to be constant during the period from when the rotational speed of the developing roller 44 starts to when the developing roller 44 reaches the same speed as during image formation.

[0068] However, the optimal value Vback for carrier adhesion and fog toner varies depending on the rotation speed of the developing roller 44. In particular, immediately after the development roller 44 starts to rotate, the development roller 44 is rotated at a slower speed than during image formation, so Vback is greater than the optimal value and there is a risk of carrier adhesion occurring. Also, immediately before the development roller 44 reaches the same speed as during image formation, Vback is smaller than the optimal value and there is a risk of fog toner occurring.

[0069] Therefore, in the fourth embodiment, as shown in FIG. 6, carrier adhesion and fog toner are suppressed by controlling Vback according to the rotation speed of the developing roller 44 which is driven to rotate by a driving means (developing motor).

[0070] In the fourth embodiment, the same control as in the first embodiment is performed for the operation before starting the rotational drive of the developing roller 44. Therefore, the description of the operation before starting the rotational drive of the developing roller 44 is omitted, and the operation after starting the rotational drive of the developing roller 44 will be described in detail below.

[0071] After a certain time has elapsed since the development DC bias was applied to the development roller 44, the first charging potential (Vd1=-850V) and the development DC bias (Vdc=-740V) of the photosensitive drum 1 become stable, and the rotational drive of the development roller 44 begins. At this time, Vback (start-up Vback) is 110V.

[0072] The rotation speed of the developing roller 44 increases from the time when the developing roller 44 starts to be rotated until the time when the rotation speed of the developing roller 44 reaches the same speed as during image formation after the developing roller 44 starts to be rotated. Therefore, as the rotation speed of the developing roller 44 increases, the charging DC bias is changed in stages so that the charging potential of the photosensitive drum 1 becomes the second charging potential (Vd2=-900V). When the stepwise change of the charging DC bias is completed, Vback becomes 160V.

[0073] This makes it possible to control Vback to an optimum value according to the rotation speed of the developing roller 44, thereby suppressing carrier adhesion and fog toner during the period from when the developing roller 44 starts to be rotated until it reaches the same speed as during image formation.

[0074] As a result, in the fourth embodiment, the step of the charging potential Vd on the photosensitive drum 1 can be reduced, and carrier adhesion and fog toner can be suppressed. Also, in the fourth embodiment, similar to the first embodiment, carrier adhesion can be suppressed during the period from when the rotational drive of the developing roller 44 starts until the rotational speed of the developing roller 44 reaches the speed during image formation, while fog toner can be suppressed during image formation.

[0075] [Other embodiments] The present invention is not limited to the above-described embodiments, and various modifications (including organic combinations of the respective embodiments) are possible based on the spirit of the present invention, and these are not excluded from the scope of the present invention.

[0076] In the above embodiment, when Vback is changed at each timing, the charging DC bias is changed while the developing DC bias Vdc to the developing roller 44 is kept constant, but this is not limited to the above example. As long as Vback is changed so as to satisfy the magnitude relationship of Vback at each timing in the above embodiment, a modified example in which the developing DC bias is also changed in addition to changing the charging DC bias may be used.

[0077] In the above embodiment, the image forming apparatus 100 is configured to use the intermediate transfer belt 62 as shown in Fig. 1, but the present invention is not limited to this. It is also possible to apply the present invention to an image forming apparatus configured to transfer the image by directly contacting the recording medium with the photosensitive drum 1 in sequence. [Explanation of symbols]

[0078] 1 Photoconductor drum 2. Charging device 4. Developing device 44 Developing roller 45 Developer container 100 Image forming device

Claims

1. An image carrier; a developing device including a developer container that contains a developer containing a toner and a carrier, and a rotatable developer carrier that carries the developer to develop an electrostatic latent image formed on the image carrier; A driving means for rotating the developer carrying member; a developing bias applying means for applying a developing bias to the developer carrying member; a charging bias applying means for applying a charging bias to the image carrier; Equipped with an absolute value of a difference between a surface potential of the developer carrier to which the developing bias is applied by the developing bias application means, and a surface potential of the image carrier to which the charging bias is applied by the charging bias application means, at a position facing the developer carrier, during a period from when the driving means starts to rotate the developer carrier until the rotation speed of the developer carrier rotated by the driving means reaches the rotation speed of the developer carrier during image formation, is smaller than an absolute value of a difference between a surface potential of the developer carrier to which the developing bias is applied by the developing bias application means, and a surface potential of the image carrier to which the charging bias is applied by the charging bias application means, at a position facing the developer carrier, during image formation, after the driving means starts to rotate the developer carrier until the rotation speed of the developer carrier rotated by the driving means reaches the rotation speed of the developer carrier during image formation, The charging bias application means applies the charging bias to the image carrier so that a surface potential of the image carrier at a facing position facing the developer carrier becomes a first potential during the period from when the driving means starts to rotate the developer carrier until the rotation speed of the developer carrier rotated by the driving means reaches the rotation speed of the developer carrier during the image formation, and applies the charging bias to the image carrier so that a surface potential of the image carrier at a facing position facing the developer carrier becomes a second potential different from the first potential after the rotation speed of the developer carrier rotated by the driving means reaches the rotation speed of the developer carrier during the image formation.

1. An image forming apparatus comprising:

2. The charging bias application means applies the charging bias to the image carrier so that a surface potential of the image carrier at a position facing the developer carrier becomes the second potential after the developer carrier rotates once after the rotation speed of the developer carrier rotated by the drive means reaches the rotation speed of the developer carrier during the image formation.

2. The image forming apparatus according to claim 1,

3. the charging bias applying means applies the charging bias to the image carrier so that a surface potential of the image carrier at a position facing the developer carrier becomes the second potential after 100 msec has elapsed since the rotation speed of the developer carrier rotated by the driving means reaches the rotation speed of the developer carrier during the image formation.

2. The image forming apparatus according to claim 1,

4. The charging bias application means applies the charging bias to the image carrier so that a surface potential of the image carrier at a position facing the developer carrier becomes the second potential during a period from when the rotation speed of the developer carrier driven to rotate by the drive means reaches the rotation speed of the developer carrier during the image formation until when the image carrier makes one rotation.

4. The image forming apparatus according to claim 1, wherein the first and second electrodes are arranged in a first direction.

5. The charging bias applying means applies the charging bias to the image carrier so that a surface potential of the image carrier at a position facing the developer carrier becomes the second potential during a period from when the rotation speed of the developer carrier driven to rotate by the driving means reaches the rotation speed of the developer carrier during the image formation until 216 msec has elapsed.

4. The image forming apparatus according to claim 1, wherein the first and second electrodes are arranged in a first direction.

6. A surface potential of the developer carrier to which the developing bias is applied by the developing bias application means during the period from when the driving means starts to rotate the developer carrier until the rotation speed of the developer carrier rotated by the driving means reaches the rotation speed of the developer carrier during the image formation, and a surface potential of the developer carrier to which the developing bias is applied by the developing bias application means during the image formation after the rotation speed of the developer carrier rotated by the driving means starts to rotate the developer carrier and reaches the rotation speed of the developer carrier during the image formation are the same.

6. The image forming apparatus according to claim 1, wherein the first and second electrodes are arranged in a first direction.

Images