JPH1152705A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device capable of reproducing a thin line and an isolated point satisfactorily even when a positively electrified photoreceptor drum and positively electrified toner are used. SOLUTION: The positively electrified photoreceptor drum and developer provided with styrene acrylic toner and at least charge control agent such as nigrosine or triphenylmethane are used, a developing roller is brought into press-contact with the photoreceptor drum by the pressing force of about 700 gf on one side, so that impression developing is performed. As a result, the electrified amount of the toner 53 on the developing roller obtained after the toner passes through a nipping part between the photoreceptor drum and the developing roller is larger than that of the toner on the developing roller obtained before the toner passes through the nipping part, the potential of a white solid part and that of a developing bias are adequately kept as shown in a figure A while the potential of the while solid part is not excessively reduced as shown in the figure B. Consequently, not only the thin line and the isolated point are reproduced well but fogging is prevented from occurring.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention belongs to the technical field of an electrophotographic image forming apparatus such as a laser beam printer, a facsimile, and a copying machine. The image forming apparatus belongs to the technical field of developing an image by using the toner carried and conveyed in the nip portion.

[0002]

2. Description of the Related Art Conventionally, as a developing method in an electrophotographic image forming apparatus of this type, a conductive elastic roller is used as a developing roller for carrying toner, and the developing roller is pressed against a photosensitive drum to develop the image. Is widely used.

In this developing method, the developing roller contacts the surface of the photosensitive drum, on which an electrostatic latent image is formed by being uniformly charged by a corona charger and further exposed by exposure means, and further developing. By generating a predetermined developing electric field between the roller and the photosensitive drum, the toner is moved to the electrostatic latent image side to perform development.

[0004] In this developing method, both positively charged toner and negatively charged toner can be used, but in the past, negatively charged toner was mainly used due to stability of charging characteristics.

Therefore, in a printer that performs image exposure using a laser beam or the like, since the reversal development method is used, the charging polarity of the photosensitive drum is negative in accordance with the negatively charged toner, and the corona is charged. The polarity of the discharge voltage of the charger is also negative.

However, when a corona charger is used, the corona charger ionizes oxygen molecules in the atmosphere to generate ozone (O ₃ ). It is known that the amount of generated ozone is about one order of magnitude greater than that of electric discharge, which is not preferable for environmental health.

In order to solve this problem, a printer using a positively-chargeable dispersion type organic photoreceptor and a positively-chargeable toner has been proposed. According to this printer, it is possible to use a charger for discharging positive polarity as the corona charger, so that the generation of ozone can be reduced.

[0008]

However, in the above-mentioned conventional printer, if a positively charged dispersion type organic photoreceptor is used, the surface potential cannot be completely reduced.
In the case of reversal development, there is a problem that thin lines and isolated points are difficult to reproduce. That is, unless a very high-precision and expensive laser optical system is used, the spot diameter of the laser light applied to the photoconductor becomes larger than a theoretical value due to an error of the laser optical system, and the power of the laser light is increased. Were dispersed, and the surface potential of the photoreceptor could not be sufficiently reduced.

In particular, there is a problem that reproducibility is particularly deteriorated in impression development using a non-magnetic one-component toner which is hardly affected by an edge effect.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and is intended to reproduce fine lines and isolated points well even when a positively charged photosensitive drum and a positively charged toner are used. An object of the present invention is to provide an image forming apparatus capable of performing the above.

[0011]

According to a first aspect of the present invention, there is provided an image forming apparatus comprising: an organic photosensitive member having a positively charged surface portion; and an electrostatic latent image formed on the moving photosensitive member. And a developer having a positively-charged toner and a positively-charged charge control agent, while carrying and transporting the developer, and pressed in the direction of the photoconductor to form the photoconductor. A developer carrying means for forming a nip portion between the photosensitive member and the photosensitive member in contact therewith, and developing the electrostatic latent image formed at the nip portion with the developer to form a visible image. The pressing force of the developer holding means against the photoreceptor is such that a measured value at the time of rest is set to about 700 gf or less at both ends of the developer holding means.

According to the image forming apparatus of the first aspect,
First, an electrostatic latent image is formed on the photoconductor by the latent image forming means on the moving photoconductor. At this time, since the organic photoreceptor is charged to a positive polarity, the generation of ozone is less than in the case where the organic photoreceptor is charged to a negative polarity, which is preferable in environmental hygiene. Next, the formed electrostatic latent image is developed with a developer by developer carrying means,
A visible image is formed on the photoreceptor. During this development, when the surface of the photoreceptor, the surface of the developer carrying means and the developer are rubbed against each other in the nip, the developer on the developer carrying means becomes positively charged toner and positively charged toner. Charge control agent is charged to a positive polarity and a predetermined charge amount, and the charge amount of the developer on the developer carrying means after development is equal to the charge amount of the developer on the developer carrying means before development. It becomes larger than the charge amount. Therefore, when the developer and the photosensitive layer of the photosensitive member come into contact with each other, the charge of the photosensitive layer moves to the developer, and the potential difference between the photosensitive member and the developer carrying means increases. As a result, good development can be performed even on fine lines and isolated points, and good reproducibility can be obtained. In addition, the pressing force of the developer carrying means against the photoreceptor is about 70
Since it is set to 0 gf or less, the rise in the charge amount of the developer on the developer carrying means after the development is appropriately suppressed, and the potential difference between the surface potential of the unexposed portion of the photoconductor and the developing bias is appropriately adjusted. Keep at the value. As a result, fogging of the unexposed portion by the positive polarity toner is reliably prevented.

According to a second aspect of the present invention, in the image forming apparatus of the first aspect, the developer carrying means has an elastic layer made of urethane or silicon on a rotating shaft. It is characterized by being.

According to the image forming apparatus of the second aspect,
Since the developer carrying means has a configuration in which an elastic layer made of urethane or silicon is formed on a rotating shaft,
When the pressing force of the developer carrying means against the photoreceptor is set to about 700 gf or less at both ends of the developer carrying means as a measured value at rest, development by the elastic force of the elastic layer is performed. The agent is appropriately pressurized in the nip to maintain an appropriate charge amount after development. As a result, fogging of the unexposed portion by the positive polarity toner is reliably prevented.

According to a third aspect of the present invention, in the image forming apparatus according to the first or second aspect,
The developer includes a styrene acrylic toner as a positively charged toner, and at least one of nigrosine and triphenylmethane as a charge control agent for positively charging.

According to the image forming apparatus of the third aspect,
By providing the developer with a styrene acrylic toner as a positively charged toner and nigrosine or triphenylmethane as a charge control agent for positive charging, the charging characteristics of the toner can be improved by developing on a developer carrying unit after development. The charge amount of the developer becomes larger than the charge amount of the developer on the developer carrying unit before development. Therefore, when the toner comes into contact with the photosensitive layer of the photoreceptor, the charge on the photosensitive layer moves to the toner side, the potential of the photoreceptor decreases, and good development can be performed even at a fine line or an isolated point. And good reproducibility is obtained. Further, the pressing force of the developer holding means against the photoreceptor is set at about 700 gf or less at both ends of the developer holding means at the time of rest, so that the developer holding means after the development is The increase in the charge amount of the developer is appropriately suppressed, and the potential difference between the surface potential of the unexposed portion of the photoconductor and the developing bias is maintained at an appropriate value. As a result, fogging of the unexposed portion by the positive polarity toner is reliably prevented.

According to a fourth aspect of the present invention, in the image forming apparatus according to any one of the first to third aspects, the mixing ratio of the toner and the charge control agent in the developer and The pressing force of the developer carrying means against the photoreceptor is such that the amount of charge of the developer on the developer carrying means after the development by being mutually rubbed in a nip portion during the development by the developer carrying means. 5. The apparatus according to claim 4, wherein the charge amount is set to be larger than a charge amount of the developer on the developer holding unit before the development.
According to the image forming apparatus described in the above, by setting the compounding ratio of the toner and the charge control agent in the developer and the pressing force of the developer carrying unit against the photoconductor to predetermined values, The amount of charge of the developer on the developer carrying means after the development due to the mutual rubbing at the nip portion during the development by the means is the amount of charge of the developer on the developer carrying means before the development. Larger than. Therefore, the electric charge of the photosensitive layer is moved to the toner side to lower the potential of the photoreceptor, so that good development can be performed even on fine lines or isolated points, and good reproducibility can be obtained. Further, since the measured value of the pressing force of the developer carrying means against the photoreceptor is set to about 700 gf or less at both ends of the developer carrying means at rest, the developer carrying means after the development is The increase in the charge amount of the developer is appropriately suppressed, and the potential difference between the surface potential of the unexposed portion of the photoconductor and the developing bias is maintained at an appropriate value. as a result,
Fogging of the unexposed portion by the positive polarity toner is reliably prevented.

According to a fifth aspect of the present invention, in the image forming apparatus according to any one of the first to fourth aspects, the transport direction of the developer in the developer carrying means is the same as that of the first aspect. In the nip portion, the direction is opposite to the moving direction of the photoconductor.

According to the image forming apparatus of the fifth aspect,
The developer carrying means conveys the developer in a direction opposite to a moving direction of the photoconductor at a nip portion with the photoconductor. Therefore, when the transfer position of the visible image and the conveyance path of the recording material are set above the image forming apparatus, the developer is supplied to the image forming apparatus by the developer carrying unit in the developer container. It will be conveyed upward from below. As a result, the developer or the like having deteriorated charging characteristics does not accumulate at a position below the developer carrying means, and good development is performed for a long period of time.

[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 forming an image, and an image forming apparatus. A photosensitive drum 20 as an example of a photoconductor in which processes such as exposure, development, transfer, and collection for forming are sequentially performed, and fixing for fixing a transfer image transferred from the photosensitive drum 20 to the sheet P to the sheet P The image forming apparatus includes a unit 70 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 including a motor, a gear train, and the like (not shown) for rotating the photosensitive drum 20. The driving means is further rotated around the photosensitive drum 20 in order. A laser scanner unit 30 as an example of a latent image forming unit for forming an electrostatic latent image on the photosensitive drum 20 to be moved; and an electrostatic latent image formed on the photosensitive drum 20 as an example of a developer. A developing unit 50 having a developing roller 56 as an example of a developer carrying means for developing with a toner, a transfer roller 60 for transferring a toner image developed on the photosensitive drum 20 to a sheet P, and a transfer roller 60 The residual toner remaining on the photosensitive drum 20 after the transfer is transferred to the developing unit 50 at a predetermined timing using the photosensitive drum 20 by a cleanerless method. A cleaning roller 42 for attracting the temporary residual toner Sutame, after the transfer the photosensitive drum 2
Static elimination lamp 41 for removing the residual potential remaining at 0
And a charger 40 as an example of a charging unit for charging the photosensitive drum 20 after static elimination so that an electrostatic latent image can be formed.

Next, each component constituting the laser beam printer 1 will be described in detail with reference to FIGS.

In FIG. 1, the feeder unit 10
A paper pressing plate 11 having substantially the same width as the paper P is provided in the upper feeder case 3 located at the rear end of the main body case 2. The paper pressing plate 11 is pivotally supported at its rear end so as to be swingable. Paper pressing plate 11
A compression spring 12 is provided at the front end of the paper pressing plate 11, and the paper pressing plate 11 is elastically urged upward by the compression spring 12. A paper feed roller 13 extending in the left-right direction is rotatably supported by the paper pressing plate 11. Paper feed roller 13
Is configured to be rotationally driven at a paper feed timing by a drive system (not shown). Feeder unit 10
Further, in the feeder unit case 3, a paper feed cassette 14 capable of storing a plurality of sheets P made of standard cut paper is detachably mounted in an inclined manner, and the paper feed cassette 13 is rotated by rotation of the paper feed roller 13. It is configured to feed one sheet at a time from the upper sheet P among the sheets P accommodated in the sheet P. Further, the feeder unit 10 is provided with a separating member 15 below the sheet feeding roller 13 to prevent double feeding of the sheet P, and the separating member 15 is elastically attached to the sheet feeding roller 13 by a compression spring 16. Being energized. A pair of registration rollers 17 and 18 for aligning the leading end of the fed sheet P are rotatably supported on the downstream side of the sheet feeding roller 13 in the conveyance direction (in FIG. 1, from the rear to the front). I have.

Referring to FIG. 1 and FIG.
Is made of a positively-chargeable material, for example, an organic photoreceptor mainly containing positively-charged polycarbonate. More specifically, as shown in FIG. 2, the photosensitive drum 20 is, for example, a cylindrical aluminum sleeve 21.
With a predetermined thickness (for example, about 20 μm) obtained by dispersing a photoconductive resin in polycarbonate on the outer periphery of the main body.
, 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 to a positive polarity is developed by a reversal development method with respect to a positive polarity (positively charged) electrostatic latent image formed on 0. The photosensitive drum 20
The drive unit is configured to be driven to rotate clockwise in a side view.

In FIG. 1, the laser scanner unit 3
Reference numeral 0 denotes a laser generator, which is disposed below the photosensitive drum 20 and generates a laser beam L for forming an electrostatic latent image on the photosensitive drum 20, and a polygon mirror (pentahedron) driven to rotate. (Mirror) 32, a pair of lenses 33 and 34, and a pair of reflection mirrors 35 and 36. Move the laser scanner unit 30 to the photosensitive drum 20
By arranging the laser beam printer on the lower side, the overall length in the transport direction can be shortened, the laser beam printer 1 can be made compact, and the laser beam can be emitted from the laser scanner unit 30 without interfering with the paper transport. An electrostatic latent image can be formed on the photosensitive drum 20 with the laser beam L thus formed.

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

The static elimination lamp 41 is provided with a light source such as an LED (laser light emitting diode), an EL (Electro Luminescence), a fluorescent lamp, etc., and removes the charge remaining on the photosensitive drum 20 after transfer (static elimination). ), The remaining charge affects the next electrostatic latent image,
It functions so as not to appear in an image finally formed on the paper P.

By changing the bias voltage, the cleaning roller 42 temporarily absorbs the residual toner 53 remaining on the photosensitive drum 20 after the transfer by the transfer roller 60, and performs the next exposure, development, The absorbed residual toner 53 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 made of a conductive foamed elastic body made of silicon rubber or urethane rubber to which a bias voltage can be applied. The cleaning roller 42 is provided so that the collection by the cleaner-less method is efficiently performed. However, instead of or in addition to the cleaning roller 42, a cleaning brush for leveling the residual toner on the surface of the photosensitive drum 20 is provided. May be provided.

In FIG. 1 and FIG.
Includes a double cylindrical toner box 51 detachably mounted in the developing unit case 4. The toner box 51 contains an agitator 52 that is driven to rotate, and a positively charged toner 53 that has electrical insulation. At the front side of the toner box 51, a toner storage chamber 54 for storing the toner 53 supplied by the rotation of the agitator 52 via a toner supply port 51a formed in the toner box 51.
Are formed. In the toner storage chamber 54, a supply roller 55 is disposed horizontally in the longitudinal direction, and is rotatably supported. Further, a developing roller 56 is horizontally disposed in the longitudinal direction 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, and is rotatably supported.

The developing unit 50 includes a helical spring 1.
The pressing force of the developing roller 56 against the photosensitive drum 20 is determined by the urging force of the helical spring 100 and the elastic force of the developing roller 56 itself. Details will be described later.

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

The developing roller 56 is a conductive rigid roller made of silicon rubber, urethane rubber, or the like. In this embodiment, particularly, the developing roller 56 mainly contains positively chargeable toner and positively chargeable polycarbonate. Since the photosensitive drum 20 made of an organic photosensitive member is used, urethane rubber is used as a material of the developing roller 56.

The resistance value from the central electrode of the developing roller 56 to which the developing bias voltage is applied to the contact portion 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 driven to rotate clockwise by a driving 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 onto the photosensitive drum 20 by the cleaning roller 42.

More specifically, for example, the laser scanner unit 30 first applies the residual toner 53 to the photosensitive drum 20 in a state where the residual toner 53 remains about 10% of the amount of the toner before the transfer after the transfer. Exposure is performed so that the laser beam reaches the bottom sufficiently. Next, in the developing unit 50, the residual toner 53 adheres to the unexposed portion of the photosensitive drum 20 by using 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, the residual toner 53 is 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 to perform the development cycle and the collection cycle 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. The toner storage chamber 54 is formed by providing a large upper space S above the supply roller 55. Have been. Therefore, the toner 53 in the toner box 51 is
Even when a large amount is supplied to the toner storage chamber 54 via the
The toner 53 is always in a powder state, and its fluidity is maintained, so that the supply of the toner by the supply roller 55 can be stabilized.

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

A 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 the developing roller 56.
The layer thickness of the toner 53 adhered in a layer on the surface of the toner is adjusted to a predetermined thickness (about 7 to
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 0.3 m so that the amount of toner on the developing roller 56 is set to about 0.4 mg / cm ² , for example.
m. The developing bias and the developing roller 56
The minimum required amount of toner on the developing roller 56 is determined by the rotation speed of the developing roller 56. For example, if the amount of toner on the developing roller 56 is about 0.3 mg / cm ² , the developing bias and Development can be performed with a sufficient toner concentration using the number of rotations of the developing roller 56. As described above, the layer thickness of the toner layer is regulated by the layer thickness regulating blade 57 so as to form a layer having a toner amount of 0.5 mg / cm ² or less on the developing roller 56.
6, the layer thickness of the toner 53 is 2
It is less than one layer, that is, one layer or more than one layer. 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. Since the toner 53 on the developing roller 56 after the layer thickness regulation is one layer or a little more than one layer, the toner 53 not subjected to the development does not accumulate on the contact portion with the photosensitive drum 20 without developing. The toner adheres to the roller 56 and is collected.

As shown in FIG. 3, in the present embodiment, particularly, when the toner 53 on the developing roller 56 is rubbed by the layer thickness regulating blade 57, the toner 53 on the developing roller 56 is substantially rubbed before being rubbed. From the charge amount Q0 close to zero to the first charge amount Q1 (Q
The layer thickness regulating blade 57 and the developing roller 56 are configured to be charged to 1> Q0).

Further, the toner 53 on the developing roller 56
By being rubbed in the nip portion, the charging sequence is performed such that the toner is charged to a second charge amount Q2 (Q2> Q1) larger than the charge amount Q1 of the toner 53 on the developing roller 56 before being rubbed in the nip portion. Is selected.

The transfer roller 60 is provided so as to be in contact with the upper side of the photosensitive drum 20, is rotatably pivotally supported, and is made of a conductive elastic foam made of silicon rubber or urethane rubber. The resistance value of the transfer roller 60 at the contact portion 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
Since it is in contact with the surface of the photosensitive drum 20, by increasing the resistance value, the photoconductive layer 22 formed on the photosensitive drum 20 is not broken by the voltage applied to the transfer roller 60. The toner image on the photosensitive drum 20 is reliably transferred to the sheet P.

The fixing unit 70 is provided downstream of the photosensitive drum 20 in the transport direction and includes a heating roller 71 having a built-in halogen lamp and a pressing roller 72. The fixing unit 70 heats the toner image transferred to the lower surface of the paper P. The sheet is pressed and fixed on the sheet P.

The pair of transport rollers 75 and the discharge tray 77 for transporting the paper are provided downstream 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 for conveying the paper P fed from the paper feed cassette 14. Numeral 77 is arranged in a substantially straight line to form a transport path PP indicated by a dashed line in FIG.

In FIG. 1, the toner 53 in the present embodiment is, for example, a non-magnetic one-component toner made of a pulverized toner or a polymerized toner made of styrene acrylic having a nearly spherical shape, and has a particle diameter of 6 to 12 μm. It is about.

The toner 53 includes a base toner and silica as an example of an external additive (fluidity imparting agent) added to the base toner. The elementary toner is, for example, a resin,
Wax, carbon black and CCA (charge control agent)
In particular, nigrosine or triphenylmethane is used as CCA.

The charging characteristic becomes positive due to the action of CCA. Silica as an example of the external additive is a toner surface modifier, has an effect of increasing the fluidity of the toner 53, and generally has a charging characteristic of being negatively charged. However, as a positively chargeable silica having a charge polarity charged to a positive polarity, silica treated with an aminosilane coupling agent and the like are known. However, positively chargeable silica generally does not have a sufficient fluidity improving effect, and thus, negatively chargeable silica having a small polarity is often used as an external additive. The difference between positively chargeable silica and negatively chargeable silica is
If the charge amount of the toner externally added to the elementary toner increases in the positive direction, it is positively charged silica, and if it increases in the negative direction, it is negatively chargeable silica.

In the present embodiment, since the toner 53 has such a configuration, when the toner 53 is sufficiently rubbed at the nip portion between the developing roller 56 and the photosensitive drum 20, it is included in the elementary toner. The charging characteristic of the CCA becomes dominant, and the toner 53 is charged to the positive polarity which is the charging characteristic of the elementary toner.

In addition to the function of imparting fluidity, the external additives include toner blocking prevention, cleaning property improvement,
It is possible to provide functions such as prevention of damage to the photosensitive drum, improvement of image density, and improvement of image quality. In addition, as an external additive other than silica, fine powder such as colloidal silica, titanium oxide, and aluminum oxide (alumina) may be mentioned.
According to the present embodiment, not only the positively chargeable external additive,
This is advantageous because a negatively chargeable external additive can be used.

In particular, in this embodiment, the toner 53
Styrene-acryl toner and nigrosine or triphenylmethane as CCA, the charge amount of the toner 53 on the developing roller 56 is larger after passing through the nip portion than before passing through the nip portion. Then, such toner 53 is supplied to the photosensitive drum 2 at the nip portion.
When it comes into contact with the surface of the photosensitive drum 20, the charge of the photosensitive layer of the photosensitive drum 20 moves to the toner 53, and the surface potential of the photosensitive drum 20 decreases.

As a result, the potential difference between the electrostatic latent image on the photosensitive drum 20 and the developing roller 56 increases, and the reproducibility of fine lines and isolated points improves.

For example, the charged potential on the surface of the photosensitive drum 20 is 700 V, the developing bias is 300 V, and the exposed portion potential is 1
When the voltage is set to 00 V, the potential at one dot point at a resolution of 600 dpi is substantially the same as the developing bias as shown in FIG.

Therefore, as shown in FIG. 4C, when the charge amount of the toner 53 after passing through the nip is equal to the charge amount of the toner 53 before passing through the nip, the charge of the toner 53 does not move. Since the potential of the electrostatic latent image does not change, one dot point having the same potential as the developing bias is not reproduced well.

Further, as shown in FIG. 4D, the charge amount of the toner 53 after passing through the nip is changed to the toner 5 before passing through the nip.
3, the charge of the toner 53 moves to the electrostatic latent image side of the photosensitive drum 20, and
, The potential of one dot point becomes higher than the developing bias. As a result, one dot point is not reproduced at all.

On the other hand, as shown in FIG. 4B, the charge amount of the toner 53 after passing through the nip is
When the charge amount is larger than the charge amount of No. 3, the charge on the photosensitive layer of the photosensitive drum 20 moves to the toner 53 side, the potential of the electrostatic latent image on the photosensitive drum 20 becomes low, and the potential of one dot point is also lower than the developing bias. It will be low enough. As a result, one dot point is reproduced well.

However, the toner 53 after passing through the nip
When the charge amount of the photosensitive drum 20 becomes too large, the transfer of the charge from the photosensitive drum 20 to the toner 53 increases, and the potential of the electrostatic latent image on the photosensitive drum 20 sometimes becomes too low.

For example, as shown in FIG. 5A, when the potential of the electrostatic latent image on the photosensitive drum 20 is appropriately reduced, the potential difference Δ between the potential of the solid white portion (unexposed portion) and the developing bias is obtained.
Since there is sufficient V, the positive polarity toner does not transfer to the solid white portion and does not cause fogging.

However, as shown in FIG. 5B, when the potential of the electrostatic latent image on the photosensitive drum 20 decreases and the potential difference ΔV decreases, the positive polarity toner is transferred to a solid white portion and fog occurs. A problem arises.

Therefore, in the present embodiment, in order to reduce the amount of frictional charge of the toner 53 in the nip portion, the pressing pressure of the developing roller 56 against the photosensitive drum 20 is reduced as compared with the related art.

As shown in FIG. 1, the pressing pressure in this embodiment depends on the spring constant of the helical spring 100 for urging the developing unit 50, the elastic force of the developing roller 56, and the like. Therefore, the helical spring 100 having a different spring constant is used.
Are prepared, these are replaced, and the occurrence of fogging at that time is compared to determine an optimum pressing pressure.

When an experiment was conducted by actually preparing a plurality of helical springs 100, about 700
When it is set to be equal to or less than gf, it has been found that the pressure on the photosensitive drum 20 including the elastic force of the developing roller 56 itself becomes appropriate, and the occurrence of fog can be prevented.

In the measurement method in this experiment, shafts for pressing pressure measurement are provided at both ends of the developing roller 56, the apparatus is stopped, and a spring balance is attached to these shafts to separate the developing roller 56 from the photosensitive drum 20. Pull,
The value of the spring balance when the developing roller 56 was separated from the photosensitive drum 20 was measured as the pressing pressure.

FIGS. 6 and 7 show a comparison of the degree of fogging when the pressing pressure is set to be greater than about 700 gf and when the pressing pressure is set to about 700 g or less.

In this experiment, black solid printing was performed and transmission densities were compared. In addition, the solid white printing is performed, and the photosensitive drum 20 in a portion after development and before transfer (a portion indicated by W in FIG. 2)
Was transferred to a white sheet of paper using a mending tape, and the same mending tape was simultaneously pasted on the same white sheet of paper without toner, and the reflectance was measured to determine the difference. This difference is a measured value of the fog on the photosensitive drum 20.

FIG. 6A shows that the pressing pressure is about 700 g.
f is a graph measured transmission density in the case of less than f,
FIG. 6B is a graph showing the measured transmission density when the pressing pressure is set to be larger than 700 gf. From these graphs, it can be seen that the transmission density is lower when the pressing pressure is about 700 gf or less.

FIG. 7A shows that the pressing pressure is about 7
FIG. 7B is a graph showing the measured value of the fog measured by the above-described method when the pressure is set to 00 gf or less, and FIG. 7B is a graph showing the measured value of the fog when the pressing pressure is set to be larger than 700 gf. As can be seen from these graphs, the degree of the decrease in the potential V ₀ of the solid white portion on the photosensitive drum 20 after development depends on the pressing pressure being 700 gf.
When the pressing pressure is set to about 700 gf or less, the pressure is reduced from 700 V to 600 V, whereas the fog near the developing bias 300 V remains the lowest when the pressing pressure is set to about 700 gf or less. It is.

Examples will be described below. In the apparatus of the present embodiment, the pressing pressure of the developing roller 56 against the photosensitive drum 20 is 500 gf on one side, and the slew rate is 6 pp.
m, the process speed (peripheral speed of the photosensitive drum 20) is 35
mm / sec, the peripheral speed of the developing roller 56 is set to 70 mm / sec.
c, the peripheral speed of the supply roller 55 is set to 17 to 35 mm / sec, the developing bias is set to 300 V, the charging potential of the photosensitive drum 20 is set to 700 V, styrene acrylic toner is used as the toner, and nigrosine as the CCA is 0.2 W.
t% (or 0.2% by weight of triphenylmethane), the charge amount Q / M per unit mass of toner (μC / g) and the toner mass per unit area M / A ( mg / cm ² ), photosensitive drum 2
The surface potential V ₀ (V) of the white solid portion of ₀ was measured. (Before countermeasures) Before passing through the nip After passing through the nip Q / M: 20 (μC / g) 35-40 (μC / g) M / A: 0.5 (mg / cm ² ) 0.5 (mg / cm ² ) V ₀ : 700 (V) 450 (V) (after countermeasure) Before passing through the nip After passing through the nip Q / M: 20 (μC / g) 25 (μC / g) M / A: 0.5 (mg / cm ² ) 0.5 (mg / cm ² ) V ₀ : 700 (V) 600 (V) As described above, while the charge amount of the toner 53 after passing through the nip is increased, the surface potential of the photosensitive drum 20 is extremely reduced. Was confirmed to be suppressed.

The following effects were also obtained by using such a developer of the charging series.

When the rotation direction of the photosensitive drum 20 and the rotation direction of the developing roller 56 are set to be opposite to each other as in the present embodiment, the circulation of the toner 53 becomes good, and good development can be performed for a long period of time. Can be.

However, in the case of such a rotation direction, when the developing efficiency is close to 100%, the toner 53 is reduced in the nip portion between the developing roller 56 and the photosensitive drum 20, and
When so-called black solid printing is performed, there is a problem that a blank white portion with a wave pattern is generated immediately in a black solid portion.

Therefore, as a result of various investigations, it was found that the use of the developer of the charging series as in the present embodiment does not cause the above-mentioned white spots in the wavy pattern even during solid black printing.

Although the cause is unknown, no white spots in the wavy pattern of the solid black print were observed in the above-described embodiment.

As described above, according to the present embodiment, not only can a fine line or an isolated point be reproduced well, but also white spots in a wave pattern of black solid printing can be prevented. A good image without fogging in the portion can be provided.

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

In FIG. 1, the photosensitive drum 20 is driven to rotate clockwise by a driving means in a side view, and the supply roller 55 and the developing roller 56 are driven to rotate clockwise, respectively. As a result, as shown in FIG. 3, each particle of the toner 53 is rubbed between the supply roller 55 and the developing roller 56 and the developing roller 56 of the layer thickness regulating plate 57.
The toner 53 charged to a positive polarity (charge amount Q1) due to the pressing friction against the toner is rubbed by the developing roller 56 and the photosensitive drum 20 at the nip portion to a charge amount Q2 larger than the charge amount Q1. While being charged, the laser light L adheres to the electrostatic latent image formed on the photosensitive drum 20 and is developed by a reversal developing method.

In this embodiment, the thickness regulating blade 57
Accordingly, a toner layer having a toner amount of 0.5 mg / cm ² or less is formed on the developing roller 56. Accordingly, the layer thickness of the toner 53 on the developing roller 56 becomes one layer or a little more than one layer, so that most of the toner is firmly rubbed by the developing roller 56 and the layer thickness regulating blade 57 and the saturation charge amount of the toner 53 Is charged to a smaller predetermined charge amount (Q1).

When the image forming process is started, first, the residual charge on the photosensitive drum 20 is wiped out by the discharging lamp 41, and then the surface of the photosensitive drum 20 is charged by the positive charger 40, for example, as shown in FIG. As shown in (A), it is uniformly charged to about + 700V. Then, in this state, the laser light L emitted from the laser generator 31 is
Is irradiated onto the photosensitive drum 20 via the lenses 33 and 34 and the reflection mirrors 35 and 36 while being main-scanned by
An electrostatic latent image is formed on the photosensitive drum 20. At this time,
The voltage of the portion corresponding to the electrostatic latent image on the photosensitive drum 20 decreases to about +100 V, for example, as shown in FIG. The positive polarity toner 53 adheres to the surface of the developing roller 56 with a layer thickness of just over one layer in a state where a developing bias voltage of, for example, about +300 V is applied as shown in FIG. 53 is a charging voltage higher than its own voltage (about +70
0V), the toner is attracted to a lower electrostatic latent image voltage (about +100 V), and the toner 53 on the developing roller 56 adheres only to the electrostatic latent image formed on the photosensitive drum 20. And developed.

According to such a principle, as shown in FIG. 4B, if the potential at one dot point is sufficiently reduced, good development is performed.

Further, as shown in FIG. 5A, by setting the pressing pressure between the developing roller 56 and the photosensitive drum 20 to about 700 gf or less on one side as described above, the frictional charge amount of the toner 53 is reduced. By doing so, the movement of the electric charge from the photosensitive drum 20 to the toner 53 is suppressed, and the potential difference between the developing bias and the solid white portion is sufficiently ensured, so that no fogging occurs.

After the toner image of the electrostatic latent image developed with the toner 53 is transferred to the sheet P by the transfer roller 60,
The sheet is subjected to a fixing process by the fixing unit 70 and is discharged to a sheet discharge tray 77.

At this time, a transport path PP for transporting the paper P fed from the paper feed cassette 14 is formed substantially linearly.
Since the image is formed while the paper P is transported along the substantially straight transport path PP, even if thick paper such as a postcard or an envelope, an OHP film, or the like is used as the paper P, the paper P is clean and reliable on the paper P. Image can be formed.

On the other hand, in FIG. 1, the residual toner 53 remaining on the photosensitive drum 20 without being transferred onto the paper P when passing through the transfer roller 60 is changed by changing the bias voltage of the cleaning roller 42 as described above. Is temporarily absorbed by the cleaning roller 42. By changing the bias voltage of the cleaning roller 42 at a timing that does not hinder the next exposure, development, transfer, and the like performed on the photosensitive drum 20, the residual toner once absorbed is removed from the cleaning roller 42 by the photosensitive drum. 20
The photosensitive drum 20 is further discharged by the developing roller 56.
The upper residual toner 53 is collected.

In parallel with the collecting operation, the undeveloped toner 53 on the developing roller 56 is charged to the charge amount Q2 when rubbed at the nip portion and adheres to the exposed portion of the photosensitive drum 20. , And development is appropriately performed.

As a result, according to the laser beam printer 1 of the present embodiment, since the surface potential of the photosensitive drum 20 decreases, the reproducibility of fine lines and isolated points can be improved, and the solid white color can be improved. Fogging in the part can be prevented.

Although only the formation of a monochrome image has been described according to the above embodiment, the present invention functions effectively also in the case of forming a color image. Further, although the developing roller 56 and the photosensitive drum 20 are configured to rotate in the same direction (opposite in the contact surface), these rollers move in opposite directions (move in the same direction in the contact surface). May be configured. Further, although the photoconductor is constituted by a photoconductor drum, the same effect can be expected even if the photoconductor is a belt-shaped photoconductor. Furthermore, in this embodiment, a laser beam printer has been described. However, as long as an electrophotographic image forming apparatus using toner, a copier, a facsimile apparatus, and the like can be used in the same manner as in this embodiment. The invention works effectively.

[0087]

As described above, according to the image forming apparatus of the first aspect, the photosensitive member of the developer carrying means uses the developer having the positively charged toner and the charge control agent for the positively charged toner. Is set to about 700 gf or less at both ends of the developer carrying means as a measured value at rest, so that the charge amount of the developer on the developer carrying means after the development is changed to the developing amount before the development. Greater than the charge amount of the developer on the developer carrying means, and good reproducibility of fine lines and isolated points,
An increase in the charge amount of the developer on the developer carrying means after the development is appropriately suppressed, and the potential difference between the surface potential of the unexposed portion of the photoconductor and the developing bias can be maintained at an appropriate value. As a result, fogging of the unexposed portion by the positive polarity toner can be reliably prevented.

According to the image forming apparatus of the second aspect,
Since the developer carrying means has a configuration in which an elastic layer made of urethane or silicon is formed on a rotating shaft,
When the pressing force of the developer carrying means against the photoreceptor is set to about 700 gf or less at both ends of the developer carrying means as a measured value at rest, the developing force is generated by the elastic force of the elastic layer. The agent is appropriately pressurized in the nip portion, so that the charge amount after development can be maintained at an appropriate level, and fogging of the unexposed portion by the positive polarity toner can be reliably prevented.

According to the image forming apparatus of the third aspect,
Since the developer is provided with a styrene acrylic toner as a positively charged toner and nigrosine or triphenylmethane as a charge control agent for positive charging, the charging characteristics of the toner can be improved by developing on a developer carrying means after development. The charge amount of the developer becomes larger than the charge amount of the developer on the developer carrying means before development, and fine lines and isolated points can be reproduced well. Further, the pressing force of the developer holding means against the photoreceptor is set at about 700 gf or less at both ends of the developer holding means at the time of rest, so that the developer holding means after the development is The increase in the charge amount of the developer above is appropriately suppressed, and the potential difference between the surface potential of the unexposed portion of the photoconductor and the developing bias can be kept at an appropriate value. Can be reliably prevented.

According to the image forming apparatus of the fourth aspect,
The compounding ratio of the toner and the charge control agent in the developer and the pressing force of the developer carrying unit against the photoconductor,
The amount of charge of the developer on the developer carrying means after the development due to the mutual rubbing at the nip portion during the development by the developer carrying means causes the development on the developer carrying means before the development. Since the setting is made to be larger than the charge amount of the agent, fine lines and isolated points can be satisfactorily reproduced. The pressing force of the developer holding means against the photoreceptor is set at about 700 gf or less at both ends of the developer holding means at a standstill. The increase in the charge amount of the developer above is appropriately suppressed, the potential difference between the surface potential of the unexposed portion of the photoreceptor and the developing bias can be kept at an appropriate value, and fogging by the positive toner on the unexposed portion can be maintained. Can be reliably prevented.

According to the image forming apparatus of the fifth aspect,
Since the developer carrying means conveys the developer in a direction opposite to a moving direction of the photoconductor in a nip portion with the photoconductor, the transfer position of the visible image and a conveyance path of a recording material are changed. When the image forming apparatus is located above the image forming apparatus, the developer or the like having deteriorated charging characteristics does not accumulate at the position below the developer carrying means, and good development can be performed for a long time.

[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 a schematic enlarged side view of a developing unit and a photosensitive drum portion showing a charge amount of toner at each position in the laser beam printer of FIG.

FIG. 4 is a diagram showing an example of an electrostatic latent image potential for a number of dot points and one dot point on a photosensitive drum, where (A) is a diagram showing an example of the conventional electrostatic latent image potential before development;
FIG. 4B is a diagram illustrating an example of the electrostatic latent image potential after development according to an embodiment of the present invention, and FIG. 4C is a diagram illustrating the conventional electrostatic latent image after development in which the charge amount of toner does not change before and after passing through a nip. FIG. 4D is a diagram illustrating an example of the image potential, and FIG. 4D is a diagram illustrating an example of the electrostatic latent image potential after the conventional development in which the charge amount after passing through the nip is lower than that before passing through the nip.

5A and 5B are diagrams illustrating an example of a potential difference between a surface potential of a photosensitive drum and a developing bias potential, wherein FIG. 5A illustrates an example of a case where the surface potential is appropriate, and FIG. It is a figure showing an example in the case of having done.

FIG. 6A is a diagram illustrating the measured transmission density when the pressing pressure between the developing roller and the photosensitive drum according to the present invention is set to about 700 gf or less on one side, and FIG. 6B is a diagram illustrating the pressing pressure larger than the pressing pressure; FIG. 7 is a diagram illustrating a measured transmission density in the case of the above.

FIG. 7A is a diagram showing a measurement of fog on the photosensitive drum when the pressing pressure between the developing roller and the photosensitive drum according to the present invention is set to about 700 gf or less on one side, and FIG. 7B is larger than the pressing pressure; FIG. 4 is a diagram illustrating measurement of fog on a photosensitive drum when a pressing pressure is used.

FIG. 8 is an enlarged side view of a developing unit of the laser beam printer of FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Laser beam printer 10 ... Feeder unit 13 ... Feeding roller 14 ... Feeding cassette 20 ... Photosensitive drum 30 ... Laser scanner unit 42 ... Cleaning roller 50 ... Developing unit 53 ... Toner 55 ... Supply roller 56 ... Developing roller 57 ... Layer Thickness regulating blade 60 Transfer roller 70 Fixing unit 77 Discharge tray 100 Winding spring

Claims (5)

[Claims] An organic photosensitive member having a positively charged surface portion, a latent image forming means for forming an electrostatic latent image on the moving photosensitive member, a positively charged toner, and a positively charged charge controlling agent. The provided developer, while carrying and transporting the developer, is pressed in the direction of the photoconductor, contacts the photoconductor, forms a nip portion between the photoconductor, and formed in the nip portion Developing means for developing the electrostatic latent image with the developer to form a visible image, and a pressing force of the developer holding means against the photoreceptor has a measured value at rest. The image forming apparatus according to claim 1, wherein both ends of the developer carrying means are set to about 700 gf or less. 2. An image forming apparatus according to claim 1, wherein said developer carrying means has an elastic layer made of urethane or silicon on a rotating shaft. 3. The developer according to claim 1, wherein the developer comprises a styrene acrylic toner as a positively charged toner, and at least one of nigrosine and triphenylmethane as a charge control agent for positively charged toner. 1
Alternatively, the image forming apparatus according to claim 2. 4. The developing agent according to claim 1, wherein a mixing ratio of the toner and the charge controlling agent in the developer and a pressing force of the developer holding unit against the photoconductor are rubbed against each other at a nip portion during development by the developer holding unit. Thus, the charge amount of the developer on the developer holding unit after the development is set to be larger than the charge amount of the developer on the developer holding unit before the development. The image forming apparatus according to any one of claims 1 to 3, wherein 5. The image forming apparatus according to claim 1, wherein a direction in which the developer is transported by the developer carrying unit is a direction opposite to a direction in which the photoconductor moves in the nip portion. An image forming apparatus according to claim 1.