JPH09185231A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form a high-quality image by an electrophotographic image forming device. SOLUTION: This device is provided with a photoreceptor drum 20, a laser scanner unit 30 forming an electrostatic latent image on the drum 20, a developing roller 56 carrying toner 53, forming a nip part between the drum 20 by being moved in an opposite direction to the moving direction of the drum 20 while being brought into contact with the drum 20 and forming a toner image by developing the electrostatic latent image at the nip part by using the toner 53 and a transfer roller 60 transferring the formed toner image on a paper P. Then, the surface part of the drum 20, the surface part of the roller 56 and the toner 53 are respectively constituted of such a material that the electrostatic charge quantity Q2 of the toner 53 on the drum 20 after the developing action becomes larger than the electrostatic charge quantity Q1 of the toner 53 on the roller 56 before the developing action by being mutually rubbed at the nip part when the developing action is executed by the roller 56.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the technical field of electrophotographic image forming apparatuses such as laser beam printers, facsimiles, and copiers. This belongs to the technical field of image forming apparatuses that perform development while moving.

[0002]

2. Description of the Related Art In an electrophotographic image forming apparatus of this type, when a transfer means such as a transfer roller is arranged above a photosensitive drum, the apparatus can be opened from the upper side as compared with the case where the transfer means is arranged below. It has the advantage of being able to access the paper path, making it easier to deal with paper jams, and also having the structure of exposing the paper feed cassette upwards, which has the advantage of making it easier to feed paper. A structure in which the path is substantially linear is also possible, and there is an advantage that printing can be easily performed even on special paper such as thick paper and envelope. Since various advantages are obtained in this way, conventionally, in this type of image forming apparatus, a structure in which the transfer unit is arranged above the photosensitive drum has been desired.

On the other hand, as a developing method in this type of image forming apparatus, a so-called impression developing method is widely adopted in which a developing roller formed of a conductive elastic roller for conveying toner is pressed against a photosensitive drum to perform development. There is. When the structure in which the transfer means is arranged above the photosensitive drum is adopted and the impression developing method is used, the paper conveying direction (drum rotation direction) and the toner are properly conveyed to the photosensitive drum. Depending on the relationship with the direction of, the contact part (nip part) between the developing roller and the photosensitive drum
In, the two are configured to move in opposite directions.

[0004]

However, in the image forming apparatus constructed such that the moving direction of the developing roller in such a nip portion is opposite to the photosensitive drum, the toner on the developing roller is Since it is used for developing the electrostatic latent image on the photosensitive drum in the area immediately before reaching the nip portion, the developing efficiency is set to 1 in order to improve the image quality.
When it is set to 00%, there is no toner between the developing roller and the photosensitive drum in the nip portion, and both surfaces contact each other.

On the other hand, an external additive which enhances the fluidity of the toner is generally added to the toner, and the toner has the function of improving the slippage between the developing roller and the photosensitive drum in the nip portion. In a configuration in which toner is not supplied to the nip portion between the two, such as the image forming apparatus having the configuration, the rubber forming the surface portion of the developing roller and the surface of the photosensitive drum come into direct contact with each other, and the rotational torque of both is not There is a problem in that the number of rules is increased, and finally printing failure occurs on the output image.

The present invention has been made in view of the above problems, and provides an image forming apparatus capable of forming a high quality image by using a developing roller which moves in the opposite direction at the nip portion of the photosensitive member. Is an issue.

[0007]

In order to solve the above-mentioned problems, an image forming apparatus according to a first aspect of the present invention includes a photoconductor, a latent image forming means for forming an electrostatic latent image on the photoconductor, and a developer. And a contact portion with the photoconductor to move in a direction opposite to the moving direction of the photoconductor to form a nip portion with the photoconductor, and the electrostatic latent image formed at the nip portion is formed. The developing device is provided with a developer carrying unit that develops with the developer to form a visible image, and a transfer unit that transfers the formed visible image onto a recording medium. The surface portion of the carrying means and the developer are rubbed against each other at the nip portion during the development by the developer carrying means, so that the charge amount of the developer on the photoreceptor after the development is The charge amount of the developer on the developer carrying means before Characterized in that it is respectively composed of listening made material.

According to the image forming apparatus of the first aspect,
First, the latent image forming means forms an electrostatic latent image on the photoconductor. Next, by a developer carrying means for carrying a developer,
The electrostatic latent image formed on the photoconductor at the nip portion is developed with a developer to form a visible image on the photoconductor.
Then, the visible image formed on the photoconductor is transferred to a recording medium or the like by the transfer unit. Image formation is performed as described above. Here, particularly during development, when the surface portion of the photoconductor, the surface portion of the developer carrying means and the developer are rubbed against each other at the nip portion, the charge amount of the developer on the photoconductor after development is It becomes larger than the charge amount of the developer on the previous developer holding means, and accordingly, the developing bias is slightly offset by the charge of the developer having a large charge amount on the photoconductor, and Development is stopped halfway depending on the difference between the charge amount of the developer and the charge amount of the developer on the photoconductor. That is, the developer with low charge on the developer carrying means is less likely to be developed, and the developing efficiency is slightly lower than 100%. As a result, the developer that has not moved to the photoconductor remains on the developer carrying means, so that the developer carrying means is prevented from directly contacting the photoconductor at the nip portion. That is, for example, if the developer is charged to the saturated charge amount on the developer carrying means, the charge amount of the developer on the developer carrying means before the development and the charge of the developer on the photosensitive body after the development are charged. The developing bias is not canceled by the charge of the developer on the photosensitive member, and the low-charged developer on the developer carrying means is also developed, so that the developing efficiency is almost 100%. Therefore, at the time of solid black printing, the surface portion of the developer carrying means directly contacts the photoconductor at the nip portion where the developer does not exist. On the other hand, according to the configuration of the present invention, since the developer carrying unit and the photoconductor do not come into direct contact with each other as described above, abrupt increase in rotation torque of the photoconductor and irregular fluctuation due to this contact are reduced. To be done.

An image forming apparatus according to a second aspect is the image forming apparatus according to the first aspect, in which the layer thickness of the developer is regulated on the developer carrying means and the developer is rubbed against the first layer. It is further characterized by further comprising a layer thickness regulating blade for charging to a charge amount, and the developer on the photoconductor is charged to a second charge amount larger than the first charge amount.

[0010] According to the image forming apparatus of the second aspect,
The layer thickness regulating blade regulates the layer thickness of the developer on the developer carrying means, and at this time, the developer on the developer carrying means is rubbed by the layer thickness regulating blade and charged to the first charge amount. On the other hand, the developer moved from the developer carrying means onto the photosensitive member is rubbed at the nip portion, so that the first
The second charge amount larger than the charge amount is charged.

An image forming apparatus according to a third aspect is the image forming apparatus according to the first aspect, wherein the second charge amount is
It is characterized in that it is equal to the saturated charge amount of the developer. According to the image forming apparatus of the third aspect, the developer moved from the developer carrying unit onto the photoconductor is charged until the charge amount of the developer is saturated by rubbing the developer in the nip portion.

An image forming apparatus according to a fourth aspect is the image forming apparatus according to any one of the first to third aspects, wherein the surface portion of the photoconductor is made of a positively chargeable organic photoconductor. The surface portion of the developer carrying means contains urethane, and the developer is a positively charged toner containing a predetermined amount of a charge control agent for positive charging.

According to the image forming apparatus of claim 4,
By using a positively charged toner and a positively chargeable photoconductor, they are relatively easily rubbed against each other at the nip portion, so that the amount of charge of the developer on the photoconductor after development can be carried by the developer before development. It can be larger than the charge amount of the developer on the means.

An image forming apparatus according to a fifth aspect is the image forming apparatus according to any one of the first to fourth aspects, in which the surface portion of the photosensitive member, the surface portion of the developer carrying means, and the developing unit are provided. Each of the agents is made of a material in which the amount of charge of the developer on the photoreceptor after the development is larger than the amount of charge of the developer on the developer holding means before the development by 30% or more. Is characterized by.

According to the image forming apparatus of claim 5,
During development, when the surface of the photoconductor, the surface of the developer carrying means and the developer are rubbed against each other at the nip portion, the charge amount of the developer on the photoconductor after development is The charge amount of the developer on the developer carrying means is 30% or more, and accordingly, the developing bias is canceled by the charge of the developer having a large charge on the photoconductor, and the charge on the developer carrying means is charged. A developer having a low viscosity is less likely to be developed, and the development efficiency is, for example, about 70% to 90%. As a result,
Since the developer that has not been transferred to the photoconductor remains on the developer carrying means, there is some toner between the developer carrying means and the photoconductor in the nip portion, and the developer carrying means and the photoconductor are in direct contact with each other. Is prevented, and printing defects due to fluctuations in the rotation torque of the photoconductor are reduced.

An image forming apparatus according to a sixth aspect of the present invention is the image forming apparatus according to any one of the first to fifth aspects, wherein the transfer means is arranged above the photoconductor and The image forming means is arranged below the photoconductor.

According to the image forming apparatus of the sixth aspect,
Since the transfer means is arranged above the photoconductor and the latent image forming means is arranged below the photoconductor, the apparatus is opened from the upper side for recording as compared with the case where the transfer means is arranged below. A structure for accessing the path of the medium becomes possible, and a structure for exposing the paper feed cassette and the like upward becomes possible.

An image forming apparatus according to a seventh aspect of the present invention is the image forming apparatus according to the sixth aspect, further comprising a conveying unit that conveys the recording medium along a path extending in a substantially linear shape inside the image forming apparatus. Further, it is characterized in that the transfer means transfers the formed visible image to the conveyed recording medium.

According to the image forming apparatus of claim 7,
Since the path of the recording medium has a substantially linear shape, it is possible to easily construct a structure capable of printing on special paper such as thick paper and envelope.

[0020]

Embodiments of the present invention will be described below with reference to the drawings. In FIG. 1, a laser beam printer 1 according to an embodiment of the present invention includes a main body case 2, a feeder unit 10 for feeding a sheet P as an example of a recording medium for image formation, and an image forming unit. A photosensitive drum 20 as an example of a photosensitive member on which processes such as exposure, development, transfer, and recovery are sequentially performed; and a fixing unit 70 for fixing the transfer image transferred from the photosensitive drum 20 to the paper P to the paper P. And a paper discharge tray 77 for discharging the paper P on which the image is fixed along the transport path PP.

The laser beam printer 1 is also provided with a driving means (not shown) for rotating the photosensitive drum 20, and on the photosensitive drum 20 rotated by the driving means in order along the periphery of the photosensitive drum 20. A laser scanner unit 30 as an example of a latent image forming unit for forming an electrostatic latent image on the surface of the photosensitive drum 20, and a toner image as an example of a developer for developing the electrostatic latent image formed on the photosensitive drum 20. A developing unit 50 having a developing roller 56 as an example of a developer carrying unit, and a transfer roller 60 as an example of a transfer unit for transferring a toner image as an example of a visible image developed on the photosensitive drum 20 to a sheet P. And the transfer roller 60
Cleaning roller 42 that temporarily adsorbs the residual toner remaining on the photosensitive drum 20 after the transfer by using the photosensitive drum 20 and returns the residual toner to the developing unit 50 at a predetermined timing using the photosensitive drum 20, and the photosensitive drum 20 after the transfer. A static elimination lamp 41 for removing the remaining residual potential and a charger 40 as an example of a charging unit for charging the photosensitive drum 20 after static elimination so as to form an electrostatic latent image are configured. .

Next, each of the constituent elements of the laser beam printer 1 will be described in detail with reference to FIGS. 1 to 4. In FIG. 1, the feeder unit 10 includes a sheet pressing plate 11 having a width dimension substantially the same as that of the sheet P, which is disposed in the upper feeder section case 3 located at the rear end of the main body case 2. The paper pressing plate 11 is swingably supported at its rear end. A compression spring 12 is provided at the front end of the paper pressing plate 11, and the paper pressing plate 11 is elastically biased upward by the compression spring 12.
The paper pressing plate 11 includes a paper feed roller 13 extending in the left-right direction.
However, it is rotatably supported. The paper feed roller 13 is configured to be rotationally driven at a paper feed timing by a drive system (not shown). In the feeder unit 10, a paper feed cassette 14 capable of accommodating a plurality of sheets P of standard cut paper is detachably mounted in an inclined manner in the feeder case 3, and by rotation of the paper feed roller 13, Among the paper P stored in the paper feed cassette 14, the upper paper P is fed one by one. Further, the feeder unit 10 is provided with a separating member 15 below the paper feeding roller 13 in order to prevent double feeding of the paper P, and the separating member 15 is elastically attached to the paper feeding roller 13 by a compression spring 16. Being energized. A pair of registration rollers 17 and 18 that align the leading ends of the fed paper P are rotatably pivoted downstream of the paper feed roller 13 in the conveying direction (from the rear to the front in FIG. 1). There is.

In FIG. 1 and FIG. 2, the photosensitive drum 20
Is composed of a positively chargeable material, for example, an organic photoreceptor having a positively chargeable polycarbonate as a main component. More specifically, as shown in FIG. 2, the photosensitive drum 20 is, for example, a cylindrical cylindrical sleeve 21 made of aluminum.
The main body of which has a predetermined thickness (for example, about 20 μm) in which a photoconductive resin is dispersed in polycarbonate on the outer peripheral portion thereof.
The photoconductive layer 22 is formed into a hollow drum, and is rotatably supported by the main body case 2 with the cylindrical sleeve 21 grounded. That is, the photosensitive drum 2
The toner 53 charged in the positive polarity is developed by the reversal development method with respect to the electrostatic latent image formed in the positive polarity (positive charge). The photosensitive drum 20 is
The drive means is configured to rotate and drive clockwise in a side view.

In FIG. 1, the laser scanner unit 3
Reference numeral 0 denotes a laser generator 31 disposed below the photosensitive drum 20, which generates a laser beam L for forming an electrostatic latent image on the photosensitive drum 20, and a polygon mirror (a pentahedron) driven to rotate. Mirror 32, a pair of lenses 33 and 34, and a pair of reflection mirrors 35 and 36. The laser scanner unit 30 is attached to the photosensitive drum 20.
Since the laser beam printer 1 is disposed below the laser beam printer 1, the laser beam printer 1 can be compact in size and can be emitted from the laser scanner unit 30 without interfering with sheet transportation. An electrostatic latent image can be formed on the photosensitive drum 20 with the laser light L.

The charger 40 is composed of, for example, a scorotron type charger for positive charging which causes corona discharge from a charging wire made of tungsten or the like. In the present embodiment, a cleanerless system is adopted, but the charger 40 is arranged so as to face the photosensitive drum 20 in a non-contact manner so that the residual toner on the photosensitive drum 20 does not adhere to the charger 40. ing.

The charge eliminating lamp 41 is constituted by including a light source such as an LED (laser light emitting diode), an EL (Electro Luminescence), a fluorescent lamp, etc., and removes the charges remaining on the photosensitive drum 20 after the transfer (eliminating the charge). ), The residual charge affects the next electrostatic latent image,
It finally functions to prevent the image from appearing on the sheet P.

By changing the bias voltage, the cleaning roller 42 once absorbs the residual toner 53 remaining on the photosensitive drum 20 after the transfer by the transfer roller 60, and the next exposure and development performed on the photosensitive drum 20. The residual toner 53 that has been absorbed is discharged to the photosensitive drum 20 at a timing that does not hinder transfer or the like, so that the residual toner 53 is returned from the photosensitive drum 20 to the developing unit 50. The cleaning roller 42 is
For example, it is composed of a conductive foamed elastic body made of silicon rubber or urethane rubber to which a bias voltage can be applied. Incidentally, the residual toner 53 by the developing roller 56
The cleaning roller 42 is provided so that the toner can be efficiently collected. However, instead of or in addition to the cleaning roller 42, a cleaning brush for smoothing the residual toner 53 on the surface of the photosensitive drum 20 may be provided. good.

1 and 2, the developing unit 50
Includes a double-cylindrical toner box 51 detachably mounted in the developing unit case 4. The toner box 51 accommodates an agitator 52 that is driven to rotate and a positively charging toner 53 that has electrical insulation. In front of the toner box 51, a toner storage chamber 54 for storing the toner 53 supplied by the rotation of the agitator 52 through the toner supply port 51a formed in the toner box 51.
Are formed. In the toner storage chamber 54, a supply roller 55 is horizontally arranged in the longitudinal direction and is rotatably supported. Further, a developing roller 56 is horizontally arranged in the longitudinal direction and is rotatably supported so as to partition the front side of the toner storage chamber 54 and contact the supply roller 55 and the photosensitive drum 20, respectively.

The supply roller 55 is made of a conductive elastic foam material such as silicon rubber or urethane rubber, and the resistance value at the contact portion with the developing roller 56 is approximately 5 × 10 ⁴ to 1 × 10 5. It is set to ⁹ Ω.

Further, the developing roller 56 constitutes a nip portion N by contacting the photosensitive drum 20 as shown in FIG. 2, and is a conductive rigid roller made of silicone rubber, urethane rubber or the like. In particular, in the present embodiment, since the photosensitive drum 20 made of an organic photoconductor whose main components are positively chargeable toner and positively chargeable polycarbonate, urethane rubber is used as the material of the developing roller 56. This urethane rubber is rubbed against each other by the toner 53 and the photosensitive drum 20 at the nip portion N, so that the toner 53 on the photosensitive drum 20 after development is rubbed against each other.
The urethane rubber component is blended so that the amount of charge of the toner is larger than that of the toner 53 on the developing roller 56 before development by 30% or more, that is, the former is 1.3 times or more the latter. Has been done.

The resistance value from the central electrode of the developing roller 56, to which the developing bias voltage is applied, to the nip portion N on the outer periphery is set to about 5 × 10 ⁴ to 1 × 10 ⁷ Ω. The supply roller 55 and the developing roller 56 are configured to be rotationally driven clockwise by a drive mechanism.

The developing roller 56 is configured to perform development on the photosensitive drum 20 using the positively charged toner 53 and to collect the residual toner 53 returned to the photosensitive drum 20 by the cleaning roller 42.

More specifically, for example, for the photosensitive drum 20 in which the residual toner 53 remains about 10% of the toner amount before the transfer after the transfer, the residual toner 53 is first removed by the laser scanner unit 30. The exposure should be performed so that the laser light reaches the bottom sufficiently. Next, in the developing unit 50, the residual toner 53 is attached to the unexposed portion of the photosensitive drum 20 by utilizing the potential difference between the exposed portion and the unexposed portion existing regardless of the presence of the residual toner 53 on the photosensitive drum 20. If so, the residual toner 53 is moved (collected) to the developing roller 56, and at the same time,
If the residual toner 53 has already adhered to the exposed portion of the photosensitive drum 20, it should be adhered (developed) as it is,
If the residual toner 53 does not adhere to the exposed portion of the photosensitive drum 20, the positively charged toner 53 is moved (developed) from the developing roller 56. That is, the developing unit 50 is configured so that the developing cycle and the collecting cycle are performed almost simultaneously.

As shown in FIG. 2, the developing unit case 4 of the developing unit 50 is provided with a toner storage chamber 54, and the toner storage chamber 54 is formed by providing a large upper space S above the supply roller 55. Has been done. For this reason, the toner 53 of the toner box 51 is not supplied to the toner supply port 51a.
Even when a large amount of toner is supplied to the toner storage chamber 54 via the toner, the toner 53 is not packed and solidified.
The toner 53 is always in powder form, its fluidity is maintained, and the toner supply by the supply roller 55 can be stabilized.

In FIG. 1 and FIG. 2, a thin plate-like elastic layer thickness regulating blade 57 made of stainless steel or phosphor bronze is attached downward to the developing unit case 4.

The bent portion 57a formed at the lower end of the layer thickness regulating blade 57 is in contact with the developing roller 56 in a pressed state, and is supplied from the supply roller 55 to be supplied to the developing roller 56.
The layer thickness of the toner 53 adhered to the surface of the toner layer in a layered manner is a predetermined thickness of about one layer (about 7 to 12 μm) by the layer thickness regulating blade 57.
m). That is, the toner amount on the developing roller 56 is configured to be 0.5 mg / cm ² or less. The layer thickness regulating blade 57 preferably has a curvature radius of about 0.4, for example, so that the toner amount on the developing roller 56 is set to about 0.45 mg / cm ^2.
It is in mm. The developing bias and the developing roller 5
The minimum required amount of toner on the developing roller 56 is determined by the number of rotations of 6 and the like. For example, when the amount of toner on the developing roller 56 is about 0.3 mg / cm ² , the developing bias within a practical range is obtained. Also, it is possible to perform development with a sufficient toner density by using the rotation speed of the developing roller 56. As described above, since the layer thickness of the toner layer is regulated by the layer thickness regulating blade 57 so that a layer having a toner amount of 0.5 mg / cm ² or less is formed on the developing roller 56, The layer thickness of the toner 53 is less than 2 layers based on the toner particles, that is, 1 layer or 1 layer or more. As a result, most of the toner 53 is firmly rubbed by the developing roller 56 and the layer thickness regulating blade 57, and is positively charged. The toner 53 after the layer thickness regulation on the developing roller 56 is 1
The toner 53 that has not been developed is not deposited on the upper portion of the nip portion N between the photosensitive drum 20 and the developing roller 56 because it has a single layer or a little more than one layer, and thus the developing roller 5 does not have the toner 53.
6 is attached and collected.

As shown in FIG. 3, particularly in this embodiment, when the toner 53 is rubbed on the developing roller 56 by the layer thickness regulating blade 57, the toner 53 on the developing roller 56 is saturated with the toner 53. The layer thickness regulating blade 57 and the developing roller 56 are configured to be charged to a first charge amount Q1 that is lower than the amount. On the other hand, the developing roller 5
The toner 53 that has moved from 6 to the photosensitive drum 20 is rubbed at the nip portion N, so that the second charge amount Q2 (Q2> Q) equal to the saturated charge amount larger than the charge amount Q1.
1), and preferably, the developing roller 56, the photosensitive drum 20, and the toner 53 are charged to a charge amount Q2 (Q2 ≧ 1.3 × Q1) which is 30% or more larger than the charge amount Q1.
The material has been selected. Such charge amounts Q1, Q2
And the developing roller 5 satisfying the conditions defined by Q3.
6, the specific configuration and material combination of the photosensitive drum 20, the toner 53, and the layer thickness regulating blade 57 are set empirically or experimentally so that the charge amounts Q1, Q2, and Q3 of the toner 53 are set on the developing roller 56 and the photosensitive drum 20. It is determined by the above measurement or by a computer simulation or the like.

When Q2> Q1 as described above, the developing bias is slightly offset by the charge of the toner 53 having a large charge amount on the photosensitive drum 20, and the development is stopped in accordance with the difference between the charge amount Q1 and the charge amount Q2. Will be stopped at. That is, the toner 53 having a low charge on the developing roller 56 is hard to be developed, and the developing efficiency is slightly lower than 100%, for example, about 80% to 90%. As a result, as shown in FIG. 3, since the toner 53 which is not moved from the developing roller 56 to the photosensitive drum 20 exists in the nip portion N, the surface portion of the developing roller 56 in the nip portion N directly contacts the photosensitive drum 20.
Can be prevented. That is, for example, when the toner 53 is charged to the saturated charge amount on the developing roller 56, the charge amount Q1 of the toner 53 before the development on the developing roller 56 and the charge amount Q2 after the development on the photoconductor are different from each other. Since they are equal (that is, Q2 = Q1), the photosensitive drum 20
The developing bias is not canceled by the electric charge of the upper toner 53, and the low-charged toner 53 on the developing roller 56 is also developed, and the developing efficiency becomes almost 100%. As a result, the surface portion of the developing roller 56 directly contacts the photosensitive drum 20 in the nip portion N where the toner 53 does not exist.

Further, in the present embodiment, when the charging amount Q2, which is larger than the charging amount Q1 by 30% or more, is used, the charging amount of the residual toner 53 returned to the developing roller 56 is It becomes larger than the charge amount Q1 of the toner 53 on the roller 56 before development. In this case, when the residual toner 53 is moved (collected) to the developing roller 56, the electric repulsive force received from the electric charge of the toner 53 before development on the developing roller 56 becomes small, and the developing roller 5
6 makes it easier to apply the recovery bias to the residual toner 53. Therefore, in this case, the ability of the developing roller 56 to collect the residual toner 53 can be improved.

Further, as shown in FIG. 3, the developing roller 56
The layer thickness of the toner 53 is regulated so that the amount of the toner 53 is 0.5 mg / cm ² or less.
Therefore, the layer thickness of the toner 53 on the developing roller 56 is less than two layers based on the toner particles, that is, one layer or a little more than one layer. As a result, since the surface of the developing roller 56 and the toner 53 are in contact with each other, the toner 53 is not peeled off from the developing roller 56 by the physical force caused by the contact between the photosensitive drum 20 and the developing roller 56, and the toner 53 is removed. It does not stay on top of the nip.

On the other hand, in FIG. 4, the layer thickness of the toner 53 is larger than that of the layer thickness regulating blade 57 so that the amount of the toner 53 carried on the developing roller 56 exceeds 0.5 mg / cm ^2. A state in which the layer thickness regulating blade 157 having the bent portion 157 a regulates the layer thickness is shown. In FIG. 4, the layer thickness of the toner 53 on the developing roller 56 is two layers or more based on the toner particles. As a result, according to the comparative example shown in FIG. 4, the toner reservoir 53a is generated in the upper portion of the nip portion N.

As described above, referring to FIG. 3 and FIG. 4, the structure of the present embodiment prevents contact between the developing roller 56 and the photosensitive drum 20 at the nip portion N and at the same time prevents toner accumulation. It turns out that it is extremely excellent.
Note that, instead of the layer thickness regulating blade 57, as another example of the layer thickness regulating means, an elastic body may be pressed against the developing roller 56 by using a spring, or by a blade not having the bent portion 57a as described above. The layer thickness may be regulated.

The transfer roller 60 is provided so as to be in contact with the upper side of the photosensitive drum 20, is rotatably supported, and is composed of a conductive foamed elastic body made of silicon rubber or urethane rubber. The resistance value at the contact portion of the transfer roller 60 with the photosensitive drum 20 is about 1 × 10 ⁶ to 1 ×.
It is set to 10 ¹⁰ Ω. That is, the transfer roller 60 is
Since it is in contact with the surface of the photosensitive drum 20, its resistance value is increased so that the photoconductive layer 22 formed on the photosensitive drum 20 is not destroyed by the voltage applied to the transfer roller 60. The toner image on the photosensitive drum 20 is configured to be reliably transferred to the paper P.

The fixing unit 70 is provided on the downstream side of the photosensitive drum 20 in the conveying direction, and is composed of a heating roller 71 and a pressing roller 72 having a well-known halogen lamp built therein, and the toner image transferred onto the lower surface of the paper P. Is pressed while being heated and fixed on the paper P.

A pair of transport rollers 75 and a paper discharge tray 77 for transporting the paper are respectively provided on the downstream side of the fixing unit 70 in the transport direction. According to the present embodiment, as shown in FIG. 1, the paper feed roller 13, the photosensitive drum 20, the fixing unit 70, and the paper discharge tray 77 constitute an example of a conveyance unit, and the conveyance extends in a substantially linear shape. The sheet P fed from the sheet cassette 14 is conveyed along the path PP.

In FIG. 1, the toner 53 in the present embodiment is of a positively chargeable type, and is, for example, a non-magnetic one-component toner made of a pulverized toner or a polymerized toner made of styrene-acryl or the like having a nearly spherical shape. The particle size is about 6 to 12 μm. The toner 53 is configured to include a base toner and silica as an example of an external additive (fluidity imparting agent) added to the base toner. The base toner is, for example, resin, wax, carbon black or CCA.
(Charge control agent). Silica, which is an example of an external additive, is a toner surface modifier and has an effect of improving fluidity. By adding amino-modified silica as the positively chargeable silica to the positively chargeable elementary toner, a good positively chargeable toner can be obtained. However, even if negatively chargeable silica is added to the positively chargeable base toner, the positively chargeable toner 53 can be obtained. In the present embodiment, in particular, a considerable amount of silica is used as an external additive in the toner 53 in order to obtain the high fluidity toner 53 corresponding to high image quality (for example, 600 dpi). When the high-fluidity toner 53 is used in this way, generally, toner accumulation is more likely to occur in the upper part of the nip portion N where the photosensitive drum 20 and the developing roller 56 rub against each other, but the layer thickness of the toner 53 is a layer thickness regulating blade. Since one layer or a little more than one layer is regulated by 57, it is possible to prevent toner accumulation in the upper part of the nip portion N. In addition to the function of imparting fluidity, the external additive such as silica can have functions of preventing toner blocking, improving cleaning properties, preventing scratches on the photosensitive drum, improving image density, and improving image quality.
Incidentally, as the external additive other than silica, colloidal silica,
Fine powders of titanium oxide, aluminum oxide (alumina), etc. may be mentioned. Further, examples of CCA for the positively chargeable toner 53 used in the present embodiment include nigrosine and triphenylmethane.

Increasing the amount of such a positively chargeable charge control agent used increases the speed at which the toner 53 reaches saturation. Therefore, the layer thickness regulating blade 57 is adjusted by adjusting the amount of the charge control agent contained in the base toner of the toner 53.
It is possible to set the charge amount (Q1) of the toner 53 before development, which has been rubbed against, to a predetermined value smaller than the saturated charge amount.
According to the present embodiment, urethane rubber is used as the material of the developing roller 56. However, when silicon rubber is used as the material of the developing roller 56 instead of this, the silicone rubber has a charging amount. Is saturated with urethane rubber faster than urethane rubber. Therefore, by containing a smaller amount of charge control agent than in the case of urethane rubber, the toner 53 before development rubbed by the layer thickness regulating blade 57 is charged. The amount (Q1) can be set to a predetermined value smaller than the saturated charge amount. If a negatively chargeable toner is used,
As the CCA for negative charging, a metal complex of a monoazo dye, a salicylic acid derivative, a tetraphenylboron derivative or the like may be used.

Next, the operation of the laser beam printer 1 configured as described above will be described with reference to FIGS. 1 and 5 to 8. In the present embodiment, it is assumed that the image is formed by the developing process of the reversal developing method.

In FIG. 1, the photosensitive drum 20 is rotationally driven clockwise by a driving means, and the supply roller 55 and the developing roller 56 are rotationally driven clockwise, respectively. As a result, the respective particles of the toner 53 are rubbed by the supply roller 55 and the developing roller 56 as shown in FIG.
The toner 53 charged to the positive polarity (charge amount Q1) is rubbed by the developing roller 56 and the photosensitive drum 20 in the nip portion up to the saturated charge amount (charge amount Q2) due to the pressing friction against the toner. While being charged, it is attached to the electrostatic latent image formed on the photosensitive drum 20 by the laser light L and is developed by the reversal development method.

In the present embodiment, the layer thickness regulating blade 57
As a result, a toner layer having a toner amount of 0.5 mg / cm ² or less is formed on the developing roller 56. Therefore, since the layer thickness of the toner 53 on the developing roller 56 is one layer or a little more than one layer, most of the toner is rubbed firmly against the developing roller 56 and the layer thickness regulating blade 57, and the saturated charge amount of the toner 53 is increased. Will be charged to a smaller predetermined charge amount (Q1).

Here, for example, as the transmission density (image density) necessary for the image formed on the paper P, for example, "2.
In order to obtain “0”, as shown in FIG. 6, the amount of toner 53 attached to the paper P, that is, the amount of developing toner is about 0.78 m.
g / cm ² is required. To this end, as described above, the amount of toner on the developing roller 56 is set to about 0.45 mg / cm ² by the layer thickness regulating plate 57, while the developing efficiency is set to about 70 to 90%, and the developing roller 56 is set. The peripheral speed of is equal to or more than twice the peripheral speed of the photosensitive drum 20. Since the developing efficiency is set to about 70 to 90%, the toner 53 is never lost in the nip portion. That is, if the developing efficiency is set to 100% and the fluid toner 53 is completely eliminated in the nip portion, the developing roller 56 and the photosensitive drum 20 will be removed.
Directly contacting each other, the rotational torques of the two moving in opposite directions in the nip portion fluctuate irregularly, resulting in defective printing. Incidentally, the toner amount of about 0.45 mg / cm ² on the developing roller 56, in order to obtain approximately 0.78 mg / cm ² as a developing toner amount required on the photosensitive drum 20, the developing efficiency is about 90
%, The peripheral speed of the developing roller 56 is set to the photosensitive drum 20.
The peripheral speed of the developing roller 56 is adjusted to about 2.5 times that of the photosensitive drum 20 when the developing efficiency is 70%, and the peripheral speed is adjusted according to the developing efficiency. .

The charge amount of the toner 53 is about 25 μC / g in a low temperature and low humidity environment at a temperature of about 10 ° C. and a humidity of about 20%, and from a high temperature and high humidity environment of about 32 ° C. and a humidity of about 80%. It varies up to about 20 μC / g. Therefore, when the developing roller 56 is driven in the same rotation direction as the photosensitive drum 20, as shown in FIG. 7, whichever of the low temperature and low humidity environment indicated by the one-dot chain line and the high temperature and high humidity environment indicated by the two-dot chain line is shown. Even in an environment, the effective developing bias voltage of the developing roller 56 is set to "about 200 V" so that a predetermined developing toner amount (about 0.78 mg / cm ² ) can be obtained with a developing efficiency of about 90%. It At the same time, since the electrostatic latent image voltage which is the voltage of the electrostatic latent image formed on the photosensitive drum 20 is "about 100 V", the developing bias voltage of the developing power source E applied to the developing roller 56 is set to "about 300 V". To be done.

When the image forming process is started under the voltage setting as described above, first, the charge eliminating lamp 41 is operated by the photosensitive drum 20.
After the upper residual charge is wiped out, the surface of the photosensitive drum 20 is uniformly charged by the charger 40 for positive charging to, for example, about +800 V as shown in FIG. Then, in this state, the laser light L emitted from the laser generator 31
Is a lens 33 while being main-scanned by the polygon mirror 32.
And 34 and reflection mirrors 35 and 36 to irradiate the photosensitive drum 20 to form an electrostatic latent image on the photosensitive drum 20. At this time, the voltage of the portion corresponding to the electrostatic latent image on the photosensitive drum 20 is changed by the irradiation of the laser light L.
For example, as shown in FIG. 8, the voltage drops to about + 100V. To the surface of the developing roller 56, as the developing bias voltage of, for example, about +300 V is applied as shown in FIG. 8, the toner 53 of positive polarity is attached with a layer thickness of just over one layer. 53 is not attracted to a charging voltage (about +800 V) higher than its own voltage, but is attracted to an electrostatic latent image voltage (about +100 V) lower than it, so that the toner 53 on the developing roller 56 is exposed to light. Only the electrostatic latent image formed on the drum 20 is attached and developed. That is, at the nip portion between the photosensitive drum 20 and the developing roller 56, the developing roller 56 is moved in the direction opposite to the photosensitive drum 20, and the impression development is performed. At this time, in particular, since the toner layer of one layer or a little stronger than one layer is supplied to the developing roller 56 by the layer thickness regulating blade 57 to the nip portion, almost no toner accumulation occurs at the upper portion of the nip portion, and at the same time, the developing process is performed. Efficiency is about 70-90%
Therefore, there is a slight amount of toner 53 between the developing roller 56 and the photosensitive drum 20, and they do not come into direct contact with each other.

The toner image of the electrostatic latent image developed with the toner 53 is transferred onto the paper P by the transfer roller 60,
The fixing processing is performed by the fixing unit 70 and the sheet is discharged to the sheet discharge tray 77. At this time, the transport path PP for transporting the paper P fed from the paper feed cassette 14 is formed in a substantially linear shape, and the paper P is image-formed while being transported by the substantially linear transport path PP. Even if a thick paper such as a postcard or an envelope or an OHP film is used as the paper P, a clean and reliable image can be formed on the paper P.

On the other hand, in FIG. 1, the residual toner 53 which is not transferred to the sheet P when passing through the transfer roller 60 and remains on the photosensitive drum 20 is changed by changing the bias voltage of the cleaning roller 42, so that the cleaning roller 42 is changed. Is once absorbed by. Then, by changing the bias voltage of the cleaning roller 42 at a timing that does not interfere with the next exposure, development, transfer, etc. performed on the photosensitive drum 20, the residual toner once absorbed is removed from the cleaning roller 42 by the photosensitive drum. It is discharged to 20.
When the residual toner 53 returns to the developing unit 50,
Residual toner 5 on the photosensitive drum 20 by the developing roller 56
3 is recovered. In parallel with this collecting operation, the undeveloped toner 53 on the developing roller 56 is charged to a charge amount Q2 when it is rubbed at the nip portion and adheres to the exposed portion of the photosensitive roller 20, so that the development is properly performed. Done.

As a result of the above, according to the laser beam printer 1 of the present embodiment, although toner having high fluidity is used, almost all toner is accumulated in the nip portion between the developing roller 56 and the photosensitive drum 20. Since the developing roller 56 and the photosensitive drum 20 do not come into direct contact with each other even when solid black printing is performed, it is possible to prevent a white background fog caused by toner accumulation and a printing failure caused by fluctuation of the rotation torque of the photosensitive drum 20, A high quality image can be formed.

According to the above embodiment, the effects of reducing the occurrence of toner accumulation and reducing the developing efficiency to less than 100% while approaching 100% are recognized. A remarkable effect is recognized.
Further, although only the formation of a monochrome image has been described, the present invention effectively functions also when forming a color image. Further, although the photoconductor is composed of the photoconductor drum, even if the photoconductor is a belt-shaped photoconductor, the same effect can be expected as long as it moves in the direction opposite to the moving direction of the developing roller in the nip portion. Furthermore, in the present embodiment, the laser beam printer has been described. However, in the case of an electrophotographic image forming apparatus using toner, a copying machine, a facsimile device, and the like are also provided as in the case of the present embodiment. The invention works effectively.

[0058]

According to the image forming apparatus of the first aspect, the developing efficiency can be made slightly lower than 100% while preventing the developer accumulation, so that the nip is caused by the developer remaining on the developer carrying means. In this part, it is possible to prevent the developer carrying means from directly contacting the photoconductor. As a result, it is possible to obtain the effect of forming a high-quality image with reduced printing defects due to fluctuations in the rotation torque of the photoconductor and reduced white background fogging due to developer accumulation.

According to the image forming apparatus of the second aspect,
By rubbing with a layer thickness regulating blade, the developer on the developer carrying means can be charged to a first charge amount, and by rubbing at the nip portion, the developer on the photosensitive member can be charged to a first charge amount larger than the first charge amount. It can be charged to two charge amounts, and as a result, printing defects due to fluctuations in the rotation torque of the photoconductor can be reduced.

According to the image forming apparatus of the third aspect,
By rubbing at the nip portion, the developer on the photoconductor can be charged to a saturated charge amount, and print defects due to fluctuations in the rotation torque of the photoconductor can be reduced without lowering the developing performance.

According to the image forming apparatus of the fourth aspect,
By using the positively charged toner and the positively chargeable photoconductor, it is possible to relatively easily reduce the printing failure due to the fluctuation of the rotation torque of the photoconductor.

According to the image forming apparatus of the fifth aspect,
Since the developing efficiency can be made lower than 100%, for example, about 70% to 90% while preventing the developer accumulation, the developer remaining on the developer carrying means causes the developer carrying means to directly contact the photosensitive member at the nip portion. Can be prevented. As a result, it is possible to obtain the effect of forming a high-quality image with reduced printing defects due to fluctuations in the rotation torque of the photoconductor and reduced white background fogging due to developer accumulation.

According to the image forming apparatus of the sixth aspect,
By opening the device from the upper side to access the path of the recording medium, it becomes easy to handle jams of the recording medium, and by exposing the paper feed cassette, etc., to feed paper. It will be easier.

According to the image forming apparatus of claim 7,
Since the path of the recording medium is substantially linear, it is possible to easily print on special paper such as thick paper and envelope. As a result, according to the present invention, it is possible to realize an electrophotographic image forming apparatus which is easy to use and is capable of forming a high quality image by using the developer carrying means which moves in the opposite direction at the nip portion with respect to the photoconductor.

[Brief description of the drawings]

FIG. 1 is a vertical sectional side view of a laser beam printer according to an embodiment of the present invention.

FIG. 2 is an enlarged side view of a developing unit and a photosensitive drum portion of the laser beam printer of FIG.

FIG. 3 is an enlarged side view of a developing roller, a layer thickness regulating blade, and a photosensitive drum portion of the laser beam printer of FIG.

FIG. 4 is an enlarged side view of a developing roller, a layer thickness regulating blade, and a photosensitive drum portion in a comparative example.

5 is an enlarged side view of a developing unit of the laser beam printer of FIG.

6 is a characteristic diagram showing a relationship between a developing toner amount and a transmission density in the laser beam printer of FIG.

7 is a characteristic diagram showing the relationship between the effective bias voltage and the amount of developing toner in the laser beam printer of FIG.

FIG. 8 is an explanatory diagram showing the potential of each part of the development processing by the reversal development method performed in the laser beam printer of FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Laser beam printer 10 ... Feeder unit 13 ... Paper feed roller 14 ... Paper feed cassette 20 ... Photosensitive drum 30 ... Laser scanner unit 42 ... Cleaning roller 50 ... Development unit 53 ... Toner 53a ... Toner reservoir 55 ... Supply roller 56 ... Development Roller 57 ... Layer thickness regulating blade 60 ... Transfer roller 70 ... Fixing unit 77 ... Paper ejection tray

Claims (7)

[Claims] 1. A photoconductor, a latent image forming means for forming an electrostatic latent image on the photoconductor, and a developer carrying a developer and moving in a direction opposite to the moving direction of the photoconductor while being in contact with the photoconductor. And a developer carrying means for forming a nip portion with the photoconductor and developing the electrostatic latent image formed in the nip portion with the developer to form a visible image. And a transfer means for transferring a visible image to a recording medium, wherein the surface portion of the photoconductor, the surface portion of the developer carrying means and the developer are the nip at the time of development by the developer carrying means. And a charge amount of the developer on the photoconductor after the development is larger than that of the developer on the developer carrying means before the development. An image forming apparatus characterized by being provided. 2. A layer thickness regulating blade for regulating the layer thickness of the developer on the developer carrying means and rubbing the developer to charge it to a first charge amount. The image forming apparatus according to claim 1, wherein the developer is charged to a second charge amount larger than the first charge amount. 3. The image forming apparatus according to claim 2, wherein the second charge amount is equal to a saturated charge amount of the developer. 4. The surface portion of the photosensitive member is made of a positively chargeable organic photosensitive member, the surface portion of the developer carrying means contains urethane, and the developer is a predetermined amount of charge control for positive charging. The image forming apparatus according to claim 1, wherein the image forming apparatus comprises a positively charged toner containing an agent. 5. The surface portion of the photoconductor, the surface portion of the developer carrying means, and the developer have a charge amount of the developer on the photoconductor after the development that is the developer carrying before the development. The image forming apparatus according to any one of claims 1 to 4, wherein each of the image forming apparatuses is made of a material having a charge amount of 30% or more larger than that of the developer on the means. 6. The transfer unit is arranged above the photoconductor, and the latent image forming unit is arranged below the photoconductor. The image forming apparatus according to item 1. 7. The image forming apparatus further comprises a conveying unit that conveys the recording medium along a path that extends in a substantially linear shape, and the transfer unit conveys the formed visible image. The image forming apparatus according to claim 6, wherein the image is transferred onto the recording medium.