JP2000089547A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stabilize a quantity of the electrostatic charges of developer in a developer storage part and to reduce texture soiling which occurs with time in use. SOLUTION: This device has a conductive roller 42 which is between a doctor blade 44 and a photoreceptor drum 1 and opposite a developing roller 41 with a gap G between them. The gap G is altered by moving the conductive roller 42 in a direction to be brought into contact with/separated from the developing roller 41.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a copying machine, a facsimile, and a printer having a developing device using a two-component magnetic brush developing method.

[0002]

2. Description of the Related Art A two-component magnetic brush developing method is a method in which a developer comprising a toner and a carrier is held and conveyed on a rotatable developer carrier having magnetic poles, and is brought into contact with an image carrier to form a latent image. This is a development method that is becoming more and more important.

A developing device using this type of system has a charge amount Q / M per unit mass of toner in the device.
[μc / g] has a certain distribution due to variations in the surface characteristics and mass of the toner and carrier. For this reason, the charge amount of the toner includes those that are too high or too low. When the charge amount of the toner is high, the image density cannot be obtained due to the large adhesive force with the carrier, and the charge amount of the toner is low. When the polarity is low or opposite, the toner adheres to the non-image area of the photoreceptor, causing a background stain.

In order to solve such an image quality problem, a method of applying an AC bias to a developing bias has been adopted. It is known that when an AC bias is applied as the developing bias, the developing ability is improved, and it is also known that image quality such as dot uniformity is improved by lowering the resistance of the carrier of the developer. Here, the developing capacity is defined herein as the ratio of the amount of toner moving to the image carrier after development to the amount of toner conveyed on the developer carrier.

The improvement of the developing ability of the AC developing bias is considered as follows. In the case of the DC bias, the toner having a high charge amount has a larger image force with the carrier of the magnetic brush, and does not contribute to the development. Here, by applying the AC bias, the electrostatic force acting on the toner instantaneously is repeatedly generated, so that the toner is easily separated from the carrier. Therefore, the toner having a high charge amount also contributes to the development, and as a result, the development amount increases.

Taking advantage of this property, the toner charge amount Q / M
Various methods have been proposed to enable uniform development even if the particles have a certain distribution. For example, in Japanese Unexamined Patent Application Publication No. 7-311497, the waveform parameters of the AC developing bias are set in consideration of the distribution of the charge amount of the toner. In this way, selective development of the charge amount of the toner due to the AC bias is avoided in the highlight portion, and uniform development without unevenness is performed.

However, since the developing bias is an alternating current, a force for moving the toner alternately in the direction of the photoconductor and the direction of the developing sleeve acts as a developing electric field, so that weakly charged and reversely charged toner which is easily separated from the carrier is developed. The probability increases. Generally, these toners are poorly charged and reversely charged poorly charged toners, and cause abnormal images such as background stains.

Here, in Japanese Patent Application Laid-Open No. Hei 9-15971, an electric field of the same polarity as that of the photosensitive drum is applied to a conductive roller disposed opposite to a developing roller, and defective charged toner is collected by the conductive roller. Thereafter, a method has been proposed in which the toner is returned to the developer accommodating portion in the developing device, thereby preventing the occurrence of background contamination due to the flying of the defectively charged toner onto the surface of the photoreceptor.

[0009]

However, according to the method disclosed in Japanese Patent Application Laid-Open No. Hei 9-15971, the collected defectively charged toner is returned to the developer accommodating portion (called a developer pool) in the developing device again, and the normal toner is collected. The toner is stirred together with the charged toner. If this operation is repeatedly performed, the ratio of the poorly charged toner in the developer in the developer accumulation portion increases. FIG. 11 shows the phenomenon in a graph.
FIG. 11 shows the distribution of the amount of charge of the toner in the developing device at the beginning of use. FIG. 12 shows the distribution of the amount of charge of the toner in the developing device after running 10000 sheets. In this graph, the horizontal axis represents the charge amount Q / M, and the vertical axis represents the number of toners. From these graphs, FIG.
In FIG. 11, which shows the initial state, the charge amount Q /
It can be seen that a peak occurs near 0 of M.

As described above, when the above-described conventional technique is employed, the proportion of the poorly charged toner in the developer gradually increases, and the balance of the charge amount of the entire developer deteriorates.
As a result, as shown in the graph of FIG. 13, there has been a problem that the occurrence rate of background stains over time increases and image quality deteriorates.

The present invention provides an image forming apparatus which solves the above-mentioned conventional problems, stabilizes the charge amount of the developer in the developer accommodating section, and reduces background stains generated over time. It is intended to be.

[0012]

SUMMARY OF THE INVENTION To achieve the above object, the present invention provides a developer carrier disposed close to an image carrier, and a DC bias and an AC bias applied to the developer carrier. A developing bias applying unit configured to apply a superposed developing bias, and a developer regulating member configured to adjust a thickness of the developer applied on the developer carrier. In the image forming apparatus for developing the electrostatic latent image, between the developer regulating member and the image carrier, and having an electrode member disposed to face the developer carrier with a gap, The electrode member is moved in a direction in which the electrode member comes into contact with or separates from the developer carrier, and the gap is variable.

Further, in order to achieve the above object, the present invention provides a developer carrying member disposed close to an image carrying member;
Developing bias applying means for applying a developing bias in which a DC bias is superimposed on an AC bias to the developer carrier; and a developer regulating member for adjusting the thickness of the developer applied on the developer carrier. An image forming apparatus that develops an electrostatic latent image formed on the image carrier by a developing device provided with the developer regulating member and the image carrier; Between
And an electrode member that is disposed so as to face the developer carrier with a gap therebetween, the electrode member is moved in a direction of coming into contact with and separating from the developer carrier, and the gap is set in accordance with an image area ratio. It is characterized by being variable.

It is to be noted that the present invention is directed to a position where the electrode member protrudes into a magnetic brush formed on the developer carrying member;
When moved between the position separated from the magnetic brush,
It is effective. Further, the present invention is effective when the distance between the electrode member and the developer carrier is changed and at the same time the DC voltage value applied to the electrode member is changed.

[0015]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is an explanatory diagram illustrating an embodiment of an image forming apparatus according to the present invention.

In FIG. 1, the image forming apparatus includes a photosensitive drum 1 as an image carrier, and a charging device 2 for charging the surface of the photosensitive drum 1 with a charging roller or the like around the photosensitive drum 1; An exposure device 3 for forming a latent image on a uniformly charged surface of the photosensitive drum 1 by a laser beam or the like; a developing device 4 for forming a toner image by attaching charged toner to the latent image on the photosensitive drum 1; A transfer device 5 for transferring the toner image formed on the photosensitive drum 1 to the recording paper 6 by a transfer roller or the like, a cleaning device 7 for removing toner remaining on the photosensitive drum 1 after the transfer, and a transfer device 5 on the photosensitive drum 1 The static eliminators 8 for removing the residual potential are arranged in the rotation direction of the photosensitive drum 1 in order.

The recording paper 6 is conveyed from a paper supply unit (not shown) having a paper supply tray or the like to the transfer device 5 by a conveyance roller or the like. After the toner image formed on the photoreceptor is transferred onto the recording paper 6 by the transfer device 5, the toner image is sent to a fixing device (not shown), and the toner is fixed on the recording paper to obtain an image.

FIG. 2 is a system block diagram of an image forming apparatus for forming an image based on image data from the outside such as a printer, and shows a flow of image information. at first,
When image information from the computer 10 or the like is input via the interface 11, the image data converted into the bitmap image is transferred to the LD driving unit 12, and the laser light is turned on and off in pixel units. The transferred pixel unit data is also sent to the control unit 13 at the same time, and is converted into an image area ratio of the output image. That is, the image area ratio of the output image can be calculated from the image formation size and the number of pixels. The control unit 13 that performs such processing is an image area ratio detecting unit, and the image forming apparatus includes an image area ratio detecting unit for an output image. It should be noted that the image area ratio detecting means configured as described above can be easily provided in a digital copier or a multifunction peripheral having a printer function or a facsimile function.

FIG. 3 is an explanatory view showing an embodiment of the developing device 4. The developing device 4 includes a developing roller 41 as a developer carrier and a stirring member 43 which are arranged to face the photosensitive drum 1. A doctor blade 44 as a developer regulating member, a two-component developer 45 composed of a toner and a carrier, and a developer accommodating section 46 for accommodating the developer. The developing roller 41 includes a magnet roller (not shown) fixed inside, and a non-magnetic sleeve (not shown) rotating on the peripheral surface thereof.

In the developing device 4, the developer in the developer accommodating portion 46 is agitated by the agitating member 43 composed of an agitating screw or the like, and the toner and the carrier are repeatedly contacted to be charged by friction. The developer 45 is pumped on the sleeve of the developing roller 41 and is transported in the rotation direction of the sleeve. When the developer on the sleeve passes through the doctor blade 44, the excess developer is scraped off, and a magnetic brush having a substantially predetermined thickness is formed on the developing roller 41. Further, a developing bias in which a DC bias is superimposed on an AC bias is applied to the developing roller 41, and the developing bias applying device 47 is provided outside the developing device 4.

The developing device 4 is provided with a conductive roller 42 as an electrode member facing the developing roller 41 in a region from the doctor blade 44 to the photosensitive drum 1 in the rotation direction of the developing roller 41. . A DC bias having the same polarity as that of the non-image portion of the photoconductor is applied to the conductive roller 42, and the bias applying device 48 is provided outside the developing device 4 similarly to the developing bias applying device 47.

As the developing roller 41, a developing bias in which a DC bias is superimposed on an AC bias is used.
The set values are asymmetric rectangular wave, peak-to-peak voltage: Vpp = 2 [kV], frequency: f = 5 [kHz]
z], duty ratio: 20%, integrated average voltage: Va =
−600 [V].

Here, each parameter of the developing bias will be described.
This will be described with reference to FIG. In FIG.
The horizontal axis is time, and the vertical axis is the force of the toner toward the photosensitive drum 1.
Is the voltage value with the direction in which the voltage is applied upward. Smell this figure
And the toner receives the maximum force toward the photosensitive drum 1
When the voltage value is V₁, Toner goes to developing sleeve 4a
The voltage value when receiving the maximum force is V₀And V₁Has
Time to continue ₁, V₀T is the time₂And
Then each parameter is

[Formula 1] Becomes Here, the integral average value V _a represents the voltage value of the time average of the AC bias. Further, the conductive roller 42 has the same surface potential as the non-image portion of the photoconductor,
-800 [V] is applied.

In the image forming apparatus of the present embodiment, the conductive roller 42 is opposed to the developing roller 41 with a gap G therebetween. The gap G is set to be equal to or slightly larger than the height of the spikes of the magnetic brush formed in the region facing the conductive roller 42 on the developing roller 41. That is, the surface of the conductive roller 42 is set at a position where the surface thereof is slightly out of contact with the magnetic brush on the developing roller 41.

The conductive roller 42 set as described above
The gap G between the developing roller 41 and the developing roller 41 is configured to be variable according to the present invention.
2 is supported so as to be movable in a direction of coming and going with respect to the developing roller 41. The conductive roller 42 is located at the position of the gap set as described above (hereinafter, referred to as position A) and at the position of the smaller gap (hereinafter, referred to as B).
It is called position. ). However,
The conductive roller 42 does not directly contact the developing roller 41. In addition, the conductive roller 42 at the A position
Assuming that the gap between the developing roller 41 and G1 is G1, the gap G1
Usually, the width is such that the conductive roller 42 does not come into contact with the magnetic brush, but if the height of the spikes of the magnetic brush becomes higher than expected, slight contact does not matter at all. Further, when the gap between the conductive roller 42 and the developing roller 41 at the position B is G2, the gap G1 has a width that ensures contact with the magnetic brush even if the height of the spike is the lowest. As a specific example, when the height of the magnetic brush on the developing roller 41 is about 0.6 [mm] on average, the gap G1 between the conductive roller 42 and the developing roller 41 is 0.8 mm. [mm] and the gap G
2 is 0.4 [mm]. The values of the widths of G1 and G2 are not limited to those described above, and appropriate values are set according to the height of the magnetic brush, which depends largely on the magnetic pole, the developer, the developer regulating member, and the shape of the developing roller. Must be set.

Next, an example of a mechanism for changing the gap G will be described with reference to FIG. In FIG. 5, the conductive roller 4
An eccentric cam 50 (one not shown) is rotatably provided on the second shaft 42a, and the eccentric cam 50 is pressed against the peripheral surface of the developing roller 41 by a spring 52 as elastic means. The eccentric cam 50 is configured to be rotated and stopped by a driving device (not shown) without being rotated by the rotation of the conductive roller 42. When the conductive roller 42 is at the position A, the eccentric cam 50 has the longest length L1 from the center of rotation to the contact point with the developing roller 41, as shown in FIG. On the other hand, when the conductive roller 42 is at the position B, the eccentric cam 50 has the shortest length L from the center of rotation to the contact point with the developing roller 41 as shown in FIG.
It is 2. When the length is L1, the gap G between the conductive roller 42 and the developing roller 41 is G1, and when the length is L2, the gap G is G2. With such a configuration, the gap G between the conductive roller 42 and the developing roller 41 can be switched between G1 and G2 by rotating the eccentric cam 50 so as to make a half rotation at a certain timing, and the control thereof is also simple. The movement of the conductive roller 42 is not limited to the eccentric cam, but may be performed by a solenoid or the like.

When the gap G between the conductive roller 42 and the developing roller 41 is set to G1, the developing device 4 having such a configuration can collect defectively charged toner. An alternating electric field is generated between the conductive roller 42 and the developing roller 41, and the toner in the magnetic brush receives the force in the direction of the conductive roller 42 and the force in the direction of the developing roller 41 alternately to vibrate. He has worked and is easily separated from his career. In contrast to the integrated average value of the AC developing bias Va = −600 [V], the conductive roller 42 is applied with −800 [V] which is the same as the surface potential in the non-image portion of the photosensitive drum 1 by the bias applying device 48. The normally charged toner is
However, since the toner having a small charge amount Q / M (weakly charged toner) and the uncharged toner have poor responsiveness to an electric field,
It is trapped on the surface of the conductive roller 42. further,
Since the reversely charged toner receives a force in the direction of the conductive roller 42, it also adheres to the surface thereof. Thus, only the poorly charged toner such as the weakly charged toner, the uncharged toner and the oppositely charged toner can adhere to the conductive roller 42 and be recovered by the action of the AC electric field.

Next, when the gap G between the conductive roller 42 and the developing roller 41 is switched to G2, the recovered defective charged toner can be returned to the magnetic brush. In this case, since the surface of the conductive roller 42 and the magnetic brush are completely in contact, the rotation of the developing roller 41 and the conductive roller 42 causes the spikes of the magnetic brush to move through the conductive roller 4.
2 By rubbing the surface, the poorly charged toner held on the surface is scraped by the ears of the magnetic brush,
It will be supplied in a magnetic brush.

Thus, the moving conductive roller 42
The developing device having the function of collecting the defectively charged toner in the magnetic brush formed on the developing roller 41 and supplying the defective charged toner to the magnetic brush.

Therefore, the recovery / supply operation of the defectively charged toner is set to be switched according to the area ratio of the output image. That is, when the image area ratio such as a character image is low, by collecting the defectively charged toner in the magnetic brush immediately before development, it is possible to prevent the toner from adhering to the non-image portion (stain on the background). At this time, the collected defectively charged toner is held while being adhered to the surface of the conductive roller 42. On the other hand, when the image area ratio is high, such as a photographic image or a figure, defective charging toner adhering to the surface of the conductive roller 42 is supplied to the magnetic brush.
The higher the image area ratio, the higher the probability that the supplied defective charged toner becomes a toner constituting an image, and the defective charged toner can be consumed similarly to the normal toner.

FIG. 7 is a flowchart showing an example of control in the image forming apparatus according to the present invention. In FIG. 7, when image formation is started, the control unit 13 detects the image area ratio X prior to image output (step 1). The detected image area ratio X is compared with a reference area ratio Z, for example, 15% by the control unit 13 (step 2). When the area ratio is X ≦ Z in a character image or the like, defective charging toner is collected. The gap G between the conductive roller 42 and the developing roller 41 is set to G1 (step 3).
On the other hand, when X> Z in a photographic image or the like, the conductive roller 42 is used to return the collected defectively charged toner to the magnetic brush.
G is switched from G1 to G2 (step 4). Thereafter, by outputting an image (Step 5), the operation of collecting and supplying the defectively charged toner can be controlled according to the image area ratio X.

The image forming apparatus configured as described above has the following configuration.
As a result of measuring the charge amount distribution of the toner in the developing device after running 0K sheets, there is no peak where the charge amount Q / M is near 0 as shown in the graph of FIG. Was found to be. Therefore, when the background stain occurrence rate when the image area ratio X is X ≦ Z in a character image or the like is examined, a graph shown in FIG. 8 is obtained.
It can be seen that the occurrence of background soiling over time is suppressed as compared with the conventional case of FIG.

FIG. 9 is an explanatory diagram of a developing device 4 showing another embodiment of the image forming apparatus according to the present invention. Only a bias applying device 48 is different from the developing device 4 shown in FIG. Other configurations are the same, and the same members are denoted by the same reference numerals. The developing device 4 of the present embodiment includes a conductive roller 4
2, a bias applying device 48 for applying a DC bias having the same polarity as the non-image portion of the photoreceptor is configured to change the magnitude of the DC bias voltage Vg.

FIG. 10 is a flow chart showing a control example of the present invention using the developing device shown in FIG. This example is similar to the control flow of FIG. 7, and the differences will be described here. In this control, when the image area ratio X is compared with the reference area ratio Z in the determination in step 2 and X ≦ Z, the gap G between the conductive roller 42 and the developing roller 41 is set to G1, and at the same time, FIG. The magnitude of the DC bias voltage Vg applied to the conductive roller 42 is also switched to V1 using the developing device shown (step 3). That is, in order to increase the collection efficiency of the defectively charged toner, the absolute value of Vg may be made smaller.
Is switched to V1 of -600 [V]. As a result, the amount of the weakly charged toner to be collected can be increased.

In addition, if the collection efficiency of the above-mentioned defectively charged toner is increased, the amount of the defectively charged toner adhering to the surface of the conductive roller 42 increases. Therefore, it is necessary to increase the supply efficiency in order to eliminate this. Therefore, when X> Z, the gap G between the conductive roller 42 and the developing roller 41 is set to G2, and at the same time, the magnitude of the DC bias voltage Vg applied to the conductive roller 42 is set to V2 (where | V1 | <| V2 | ) (Step 4). By this switching,
Due to the action of the magnetic brush scraping and the action of the electric field, more badly charged toner can be supplied to the magnetic brush and consumed as part of the image. Note that V in this example
2 is set to -1000 [V]. Note that the magnitude of the DC bias voltage Vg, -600 [V] of V1 and -1000 [V] of V2, are examples, and are not limited thereto.

Even when such control is performed, the occurrence rate of the background stain when the image area ratio X is X ≦ Z in a character image or the like is shown in FIG. 8, and the occurrence of the background stain can be suppressed over time. In the above embodiment, the gap G between the conductive roller 42 and the developing roller 41 is switched between two stages, G1 and G2.
May be finely classified, and the rate may be switched to a plurality or steplessly. The switching can also be performed by the eccentric cam 50.

[0037]

According to the first aspect of the present invention, the distance between the electrode member disposed opposite to the developer carrying member and the developer carrying member is variable, and the electrode member is used to collect and supply the defectively charged toner. By providing the function, it is possible to stabilize the charge amount of the developer in the developer accommodating portion, and it is possible to prevent an increase in background stain over time.

According to the second aspect of the present invention, the distance between the electrode member and the developer carrier is switched depending on the image area ratio, so that the amount of charge of the developer in the developer accommodating portion is stabilized, and the background over time is measured. An increase in dirt can be prevented.

According to the third aspect of the present invention, the electrode member is slightly separated from the magnetic brush formed on the developer carrier, so that the defectively charged toner can be reliably collected, and the electrode member is caused to protrude into the magnetic brush. As a result, the collected defective toner can be reliably returned to the magnetic brush.

According to the fourth aspect of the present invention, the magnitude of the gap between the electrode member and the developer carrier is switched, and at the same time, the DC voltage value applied to the electrode member is switched, so that the magnetic force on the developer carrier is changed. The efficiency of collecting and supplying the defectively charged toner to the brush can be improved.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram illustrating an embodiment of an image forming apparatus according to the present invention.

FIG. 2 is a system block diagram of the image forming apparatus.

FIG. 3 is a schematic configuration diagram illustrating an example of a developing device.

FIG. 4 is a waveform chart showing parameters of a developing bias waveform according to the embodiment.

FIG. 5 is an explanatory diagram showing an example in which an electrode member is made variable.

6A is an explanatory diagram showing a state where the electrode member is farthest from the developing roller, and FIG. 5B is an explanatory diagram showing a state where the electrode member is closest to the developing roller.

FIG. 7 is a flowchart illustrating an example of control according to the present invention.

FIG. 8 is a graph showing a background stain generation rate due to image formation with a small image area ratio in the present invention.

FIG. 9 is a schematic configuration diagram illustrating another embodiment of a developing device.

FIG. 10 is a flowchart showing another embodiment of the control in the present invention.

FIG. 11 is a graph showing an example of toner charge distribution of an initial developer.

FIG. 12 is a graph showing a toner charge distribution when the developer of FIG. 11 is used over time in a conventional image forming apparatus.

FIG. 13 is a graph showing a background stain generation rate due to image formation with a small image area ratio by a conventional apparatus.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Photoconductor drum 4 Developing device 13 Control part 41 Developing roller 42 Conductive roller (electrode member) 44 Doctor blade G Gap

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Osamu Sato 1-3-6 Nakamagome, Ota-ku, Tokyo Inside Ricoh Company (72) Inventor Yoshiko Ishii 1-3-6 Nakamagome, Ota-ku, Tokyo Stock F-term in Ricoh Company (reference) 2H073 BA04 BA13 CA03 2H077 AB02 AD02 AD13 AD36 AE01 DA57 DB08 EA03 GA03

Claims (4)

[Claims] A developing device configured to apply a developing bias obtained by superimposing a DC bias on an AC bias to the developer supporting member; An image forming apparatus that develops an electrostatic latent image formed on the image carrier by a developing device including a developer regulating member that adjusts a thickness of the developer applied on the carrier; An electrode member disposed between the regulating member and the image carrier and opposed to the developer carrier with a gap therebetween, in a direction in which the electrode member comes into contact with and separates from the developer carrier; Wherein the gap is changed. 2. A developer carrier disposed close to an image carrier, a developing bias applying means for applying a developing bias in which a DC bias is superimposed on an AC bias to the developer carrier, and the developer An image forming apparatus that develops an electrostatic latent image formed on the image carrier by a developing device having a developer regulating member that adjusts a thickness of a developer applied on the carrier; An electrode member that is provided between the developer regulating member and the image carrier, and that is opposed to the developer carrier with a gap between the developer regulating member and the image carrier; An image forming apparatus, wherein a member is moved in a direction of coming into contact with or separating from the developer carrier, and the gap is varied according to an image area ratio. 3. The image forming apparatus according to claim 1, wherein a position between the position at which the electrode member enters a magnetic brush formed on the developer carrier and a position at which the electrode member is separated from the magnetic brush. An image forming apparatus characterized by being moved by (1). 4. The image forming apparatus according to claim 1, wherein a distance between the electrode member and the developer carrier is changed, and simultaneously, a DC voltage value applied to the electrode member is changed. Characteristic image forming apparatus.