JP7638926B2 - Image forming device - Google Patents

Description

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.

The image forming device 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.

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 area potential) Vd of the photosensitive drum charged by the charging device and the DC component Vdc of the developing bias.

JP 2012-128320 A

The inventors' research has revealed that the phenomenon of carrier adhesion (carrier adhesion) occurring in 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 complete, 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.

In general, when the absolute value of Vback is reduced, carrier adhesion tends to be suppressed compared to 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 toner to the developing roller becomes weaker compared to when the absolute value of Vback is increased, so toner (fog toner) is more likely to adhere to the non-image areas of the photosensitive drum.

The present invention has been made in consideration of the above problems. The object of the present invention is to provide an image forming device that can 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.

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 image forming apparatus includes a developing device that drives the developer carrier to rotate and a developing bias application means that applies a charging bias to the image carrier, and the image forming device includes a developing device that drives the developer carrier to rotate and a developing bias application means that applies a charging bias to the image carrier, and the developing ... device that applies a charging bias to the image carrier. 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 the period until the speed of the developer carrier reaches the rotation speed of the developer carrier during image formation is the rotation speed of the developer carrier that is rotationally driven by the driving means after the driving means starts rotating the developer carrier. The charging bias application means applies the charging bias to the image carrier so that the surface potential of the image carrier at the 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 the surface potential of the image carrier at the 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.

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 rotation speed of the developing roller reaches the speed during image formation, while suppressing fog toner during image formation.

1 is a diagram for explaining a configuration of an image forming apparatus according to a first embodiment; FIG. 2 is a cross-sectional view illustrating a configuration of the developing device according to the first embodiment. 5 is a diagram showing a sequence of an image forming operation (image creating operation) according to the first embodiment; FIG. 10A to 10C are diagrams illustrating a sequence of an image forming operation (image creating operation) according to a second embodiment. 13A to 13C are diagrams illustrating a sequence of an image forming operation (image creating operation) according to a third embodiment. 13A to 13C are diagrams illustrating a sequence of an image forming operation (image creating operation) according to a fourth embodiment. 11A and 11B are diagrams illustrating a sequence of an image forming operation (image creation operation) in a conventional example.

The following describes in detail an embodiment of the present invention with reference to the accompanying drawings. Note that the following embodiment does not limit the scope of the present invention, and not all of the combinations of features described in the present embodiment are necessarily essential to the solution of the present invention. The present invention can be implemented in a variety of applications, including printers, various printing machines, copiers, fax machines, and multifunction machines.

[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.

Image forming device 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 components (or devices), and when there is only a reference symbol without a subscript, they indicate a component (or device) that is not related to color.

Each image forming section is provided with a photoconductor drum 1A, 1B, 1C, or 1D, which is an image carrier. Photoconductor drums 1A, 1B, 1C, or 1D are negatively charged organic photoconductors with a photosensitive layer on a drum base such as aluminum, and are rotated at a predetermined process speed by a drive device.

Charging rollers 2A, 2B, 2C, 2D, developing devices 4A, 4B, 4C, 4D, and primary transfer rollers 61A, 61B, 61C, 61D are arranged around each photoconductor drum 1A, 1B, 1C, 1D, respectively.

Here, we will explain in detail the image forming unit that uses the charging roller 2 made of conductive rubber as a charging device. The charging roller 2 is rotatably held by bearing members at both ends of the core metal, maintaining a constant axial distance. The charging roller 2 is also urged 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.

In addition, above each photoconductor drum 1A, 1B, 1C, 1D, there is disposed exposure means (exposure device) 3A, 3B, 3C, 3D for forming an electrostatic latent image on the surface of the charged photoconductor drum 1. The exposure device 3 is electrically connected to the image processing unit 9, and forms an electrostatic latent image in response to an output image signal sent from the image processing unit 9.

Each developing means (developing device) 4A, 4B, 4C, and 4D contains yellow toner, cyan toner, magenta toner, and black toner, respectively. Each developing roller 44A, 44B, 44C, and 44D is rotated by each driving means (developing motor), and develops the electrostatic latent image formed on the surface of the photosensitive drum 1A, 1B, 1C, and 1D to form a toner image.

A rotatable, endless intermediate transfer belt 62, which acts as an intermediate transfer body, is disposed at a position opposite each image forming unit. 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, which is connected to a motor. The toner image that reaches the position opposite each of the primary transfer rollers 61A, 61B, 61C, and 61D on the intermediate transfer belt 62 is transferred from the photosensitive drum 2 to the intermediate transfer belt 62 by the primary transfer voltage.

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

Furthermore, downstream of the secondary transfer section where the secondary transfer opposing roller 63 and the secondary transfer roller 64 come into contact in the rotation direction of the intermediate transfer belt 62, a fixing device 7 is installed to perform a heat and pressure process to fix the toner to the transfer material.

A cleaning member (drum cleaning device) 8A, 8B, 8C, 8D is arranged around each photoconductor drum 1A, 1B, 1C, 1D to clean residual toner from the surface of the photoconductor drum 1A, 1B, 1C, 1D.

The configuration of the developing device 4 is shown in the cross-sectional view of Figure 2.

The developing device 4 has a developing container 45 that contains a developer containing toner and a carrier. The developing container 45 is divided into an agitation chamber 46a and a developing chamber 46b by a partition 43, and the developer is circulated between the agitation chamber 46a and the developing chamber 46b. A screw 45a is disposed in the agitation chamber 46a to agitate and transport the developer in the agitation chamber 46a. A screw 45b is disposed in the developing chamber 46b to agitate and transport the developer in the developing chamber 46b.

The developing device 4 also has a developer regulating member 42. The developer regulating member 42 regulates the amount of developer carried by the developing roller 44.

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 then pulverized or polymerized into a powder. The magnetic carrier is a core made of resin particles kneaded with ferrite particles or magnetic powder, with a resin coating applied to the surface.

The process of developing toner onto the photosensitive drum 1 in the development area is explained below. After the photosensitive 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].

A DC voltage (charged DC bias) or a voltage obtained by superimposing an AC voltage on a DC voltage (charged DC + AC bias) is applied to the charging roller 2 by a high-voltage power supply (charged bias application means). Then, the charging DC bias or the charging DC + AC bias is applied to the photosensitive drum 1 via the charging roller 2 that is in contact with the photosensitive drum 1.

In addition, a DC voltage (development DC bias) or a voltage in which an AC voltage is superimposed on a DC voltage (development DC + AC bias) is applied to the developing roller 44 by a high-voltage power supply (development bias application means).

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

The absolute value of the difference between the DC voltage Vdc and the charging potential Vd, |Vdc-Vd|, is called Vback, and 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 toner (fog toner) adheres to the non-image areas of the photosensitive drum 1 (the so-called fogging phenomenon).

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 toner (fog toner) tends to adhere less to the non-image areas of the photosensitive drum 1. On the other hand, when the absolute value of Vback is decreased, the phenomenon of carrier adhering to the non-image areas of the photosensitive drum (carrier adhesion) tends to be suppressed compared to when the absolute value of Vback is increased.

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 is different from 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.

In the first embodiment, the focus is on the phenomenon (fogging phenomenon) that occurs 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 to the value of the charging potential Vd at the point where 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.

(Conventional example)
Before describing the sequence of the image forming operation (image forming operation) according to the first embodiment, the sequence of the image forming operation in the conventional example will be described with reference to Fig. 7. Fig. 7 shows the rotation drive of the developing roller 44 at the start of the image forming 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.

In the conventional operation shown in FIG. 7, the pre-rotation operation starts, and the photosensitive drum 1 starts to be rotated. 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=-900V) at the position facing the developing roller 44.

When the photoconductor drum 1 rotates and the charged position of the photoconductor drum 1 reaches a 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 photoconductor drum 1 and the developing DC bias Vdc have stabilized, the developing roller 44 begins to rotate. In the conventional operation, the fog removal potential difference at this time, Vback (= 160V), is controlled to be the same value as Vback during image formation.

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. Table 1 shows 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 development roller 44 started until the rotational speed of the development roller 44 reached the same speed as during image formation. Table 2 shows the results of measuring the carrier adhesion amount and the fog toner amount when the absolute value of Vback was changed during image formation in the conventional operation.

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. Also, 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.

In this experiment, the image forming apparatus 100 was used to stop the operation during 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 photoconductor 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 adhering carrier particles per ^cm2 was calculated. The amount of fog toner was measured by density using an X-Rite 500 color reflection densitometer manufactured by X-Rite Corporation.

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

On the other hand, for fog toner, if Vback is 60V or less during the period from when the developing roller 44 starts to rotate until the rotational speed of the developing roller 44 reaches the same speed as during image formation, the level of fog toner will be at an unacceptable level for image quality. In contrast, during image formation, if Vback is 110V or less, the level of fog toner will be at an unacceptable level for image quality.

The above results show that the optimal Vback at which image defects caused by 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.

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.

At the same time that the pre-rotation operation starts and the rotation 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, as the photosensitive drum 1 rotates, the charged position of the photosensitive drum 1 reaches a position facing the developing roller 44, and the application of a developing DC bias (Vdc = -740V) to the developing roller 44 starts. After a certain time has passed 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 rotation drive of the developing roller 44 starts. The Vback (start-up Vback) at this time is 110V.

Next, after the rotation speed of the developing roller 44 reaches the same speed as during image formation but 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 = -900 V), which is the same value as during image formation. Vback at this time is 160 V.

By executing the control shown in FIG. 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 110V. Also, during image formation, fog toner and carrier adhesion during image formation can be suppressed by setting Vback (imaging Vback) to 160V.

In the first embodiment described above, the start-up Vback is set to be smaller than the image formation Vback. According to this first embodiment, it is possible to suppress 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, while suppressing fog toner during image formation.

In the control shown in FIG. 3, when changing from the rise Vback to the image creation Vback, an example is described in which the charging DC bias is changed while the development DC bias (Vdc = -740 V) to the development roller 44 is kept constant, but this is not limited to the example. As long as the relationship that the rise Vback is smaller than the image creation Vback is satisfied, a modified example in which the development DC bias is changed in addition to changing the charging DC bias may also be used.

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 denoted by the same reference numerals, and detailed descriptions are omitted. Only the components unique to the second embodiment will be described in detail.

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.

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 during image formation, but 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 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, there is a risk of carrier adhesion and fog toner occurring because the rotation speed of the developing roller 44 is not controlled to the optimal Vback for suppressing carrier adhesion and fog toner.

On the other hand, after the developing roller 44 rotates once (about 100 msec in the second embodiment), the rotational 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 rotational speed of the developing roller 44 reached the same speed as during image formation, and before the start of the image formation operation.

On the other hand, from the viewpoint of productivity, it is preferable to carry out control before the start of image formation operation in a short time. Therefore, after the rotation speed of the developing roller 44 reaches the same speed as during image formation, Vback is changed to the same value as 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 formation operation from being delayed more than necessary due to a change in Vback (change in the charging DC bias) after the rotation speed of the developing roller 44 reaches the same speed as during image formation, thereby suppressing a decrease in productivity.

Therefore, in the second embodiment, as shown in FIG. 4, 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 so that it is 100 msec or more 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.

As a result, in the second embodiment, it is possible to suppress carrier adhesion and fog toner before the start of the image formation operation while suppressing a decrease in productivity. Also, in the second embodiment, as in the first embodiment, it is possible to suppress carrier adhesion during the period from when the rotational drive of the development roller 44 starts until the rotational speed of the development roller 44 reaches the speed during image formation, while suppressing fog toner during image formation.

[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.

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.

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 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.

However, as in the first embodiment, if the charging potential Vd of the photoconductor drum 1 is changed in a short time, causing a step in the charging potential on the photoconductor drum 1, there is a risk of carrier adhesion or toner fogging at the position where the charging potential on the photoconductor drum 1 is changed.

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 during image formation until the image formation operation starts is controlled to a value different from the start-up 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 during image formation until the image formation operation starts is set to be greater than the absolute value of the start-up Vback and smaller than the absolute value of the image creation Vback. Note that the start-up 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 formation operation).

As a result, in the third embodiment, the step in 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, as in the first embodiment, carrier adhesion can be suppressed during the period from when the rotational drive of the developing roller 44 begins until the rotational speed of the developing roller 44 reaches the speed during image formation, while fog toner can be suppressed during image formation.

[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. Only the components unique to the fourth embodiment will be described in detail.

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.

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 it reaches the same speed as during image formation.

However, the optimal value of Vback for carrier adhesion and fog toner varies depending on the rotation speed of the developing roller 44. In particular, immediately after the developing roller 44 starts to rotate, the rotation speed of the developing roller 44 is slower than during image formation, so Vback is larger than the optimal value and there is a risk of carrier adhesion occurring. Also, immediately before the developing 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.

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).

In the fourth embodiment, the operations until the start of rotational driving of the developing roller 44 are controlled in the same manner as in the first embodiment. Therefore, the explanation of the operations until the start of rotational driving of the developing roller 44 is omitted, and the operations after the start of rotational driving of the developing roller 44 are explained in detail below.

After a certain time has elapsed since the development DC bias was applied to the development roller 44, the photoconductor drum 1 reaches a stable first charging potential (Vd1 = -850V) and development DC bias (Vdc = -740V), and the development roller 44 starts to rotate. At this time, Vback (start-up Vback) is 110V.

The rotational speed of the developing roller 44 increases from the time when the developing roller 44 starts to be rotated until the time when the rotational 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 rotational 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 in the charging DC bias is completed, Vback becomes 160V.

This allows Vback to be controlled 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.

As a result, in the fourth embodiment, the step in 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, as in the first embodiment, carrier adhesion can be suppressed during the period from when the rotational drive of the developing roller 44 begins until the rotational speed of the developing roller 44 reaches the speed during image formation, while fog toner can be suppressed during image formation.

[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.

In the above embodiment, an example was described in which the charging DC bias is changed while the developing DC bias Vdc to the developing roller 44 is kept constant when Vback is changed at each timing, but this is not limited to the above. If Vback is changed 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.

In the above embodiment, as shown in FIG. 1, the image forming apparatus 100 is configured to use the intermediate transfer belt 62, 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 recording medium by directly contacting the photosensitive drum 1 in sequence.

Reference Signs List 1 Photoconductor drum 2 Charging device 4 Developing device 44 Developing roller 45 Developing container 100 Image forming apparatus

Claims (6)

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,
an image forming apparatus, characterized in that 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 a first potential during the period from when the driving means starts to rotate the developer carrier to when the rotational speed of the developer carrier rotated by the driving means reaches the rotational 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 position facing the developer carrier becomes a second potential different from the first potential after the rotational speed of the developer carrier rotated by the driving means reaches the rotational speed of the developer carrier during the image formation. 2. The image forming apparatus according to claim 1, wherein 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 rotational speed of the developer carrier driven by the drive means reaches the rotational speed of the developer carrier during the image formation. 2. The image forming apparatus according to claim 1, wherein 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 100 msec after the rotational speed of the developer carrier driven in rotation by the drive means reaches the rotational speed of the developer carrier during the image formation. 4. The image forming apparatus according to claim 1, wherein the charging bias application means applies the charging bias to the image carrier so that the surface potential of the image carrier at a position facing the developer carrier becomes the second potential during the period from when the rotational speed of the developer carrier driven by the driving means reaches the rotational speed of the developer carrier during the image formation until when the image carrier rotates once. 4. The image forming apparatus according to claim 1, wherein 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 within 216 msec after the rotational speed of the developer carrier driven in rotation by the drive means reaches the rotational speed of the developer carrier during the image formation. 6. The image forming apparatus according to claim 1, wherein the 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 to when the rotational speed of the developer carrier driven by the driving means reaches the rotational speed of the developer carrier during the image formation is the same as the 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 driving means starts to rotate the developer carrier and the rotational speed of the developer carrier driven by the driving means reaches the rotational speed of the developer carrier during the image formation.

Images