JPH1152609A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device in which thin lines and isolated points can be favorably reproduced even in the case where a photosensitive drum of positive chargeability and toner of positive chargeability are used. SOLUTION: Using a photosensitive drum 20 of positive chargeability and toner 53 of positive chargeability, a development roller 56 is pressed to the photosensitive drum 20 to perform impression development. Developer is composed to be provided with styrene acryl toner, charge control agent such as nigrosine and triphenylmethane at least, and outer additive agent such as silica of a grain diameter of about 30 nm are provided. As a result, charging amount of the toner 53 after passing a nip part of the photosensitive drum 20 and the development roller 56 becomes larger than charging amount of the toner 53 on the development roller 56 before passing the nip part, and a potential at a white surface part is not excessively reduced, but the potential at the white surface part and a development bias potential can be kept proper. Thin lines, and isolated points can thus be favorably reproduced, and generation of fogging can be prevented.

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. Was dispersed and the surface of the photoreceptor could not be sufficiently dropped.

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. Forming a nip portion between the photoconductor and the developer while carrying and transporting the developer, and forming the electrostatic latent image formed at the nip portion on the nip. A developer carrying means for developing a visible image by developing with a developer, the developer comprising a positively charged toner, a positively charged charge control agent, and an external additive having a particle size of about 30 nm or more. And an agent.

According to the image forming apparatus of the first aspect,
First, an electrostatic latent image is formed on a moving photosensitive member by a latent image forming unit. 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 by a developer by a developer carrying means, and a visible image is formed on the photoconductor. 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. Because it is equipped with a charge control agent for the positive, it is charged to a predetermined amount of positive polarity,
The charge amount of the developer on the developer holding unit after the development is larger than the charge amount of the developer on the developer holding unit before the development. 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. Moreover, since the external additive of the developer has a particle size of about 30 nm or more, the increase in the charge amount of the developer on the developer carrying means after the development is appropriately suppressed,
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 second aspect of the present invention, in the image forming apparatus of the first aspect, a styrene acrylic toner is used as a positively charged toner, and at least nigrosine or triphenylmethane is used as a positively charged charge control agent. Or any one of the above.

According to the image forming apparatus of the second aspect,
Using styrene acrylic toner as the positively charged toner,
By using at least one of nigrosine and triphenylmethane as the charge control agent for positive charging, the charge amount of the developer on the developer carrying means after development can be surely maintained. It becomes larger than the charge amount of the upper developer. Therefore, even if it is a fine line or an isolated point, development is performed well, and good reproducibility is obtained. Moreover, the external additive of the developer has a particle size of about 30.
Since it is at least nm, the 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 is kept at an appropriate value. 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 compounding ratio of the toner, the charge control agent, and the external additive in the developer and the particle size of the external additive are such that the particles are rubbed against each other in a nip portion during the development by the developer carrying means, so that The charge amount of the developer on the developer carrying means is set to be larger than the charge amount of the developer on the developer carry means before the development.

According to the image forming apparatus of the third aspect,
By setting the compounding ratio of the toner, the charge control agent, and the external additive in the developer and the particle size of the external additive to predetermined values, they are rubbed against each other in the nip portion during development by the developer carrying means. As a result, the charge amount of the developer on the developer holding unit after the development becomes larger than the charge amount of the developer on the developer holding unit before the development.
Therefore, by contact with the photosensitive layer of the photoconductor, the electric charge of the photoconductor layer is moved to the toner side, the potential of the photoconductor is reduced, and good development is performed even at a fine line or an isolated point. Good reproducibility is obtained. Moreover, since the external additive of the developer has a particle size of about 30 nm or more, the increase in the charge amount of the developer on the developer carrying means after the development is appropriately suppressed,
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 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 fourth 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.

[0019]

Embodiments of the present invention will be described below with reference to the drawings.

Referring to 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 paper 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 comprising a motor, a gear train, etc. (not shown) for rotating the photosensitive drum 20, and is further rotated by the driving means along the periphery of 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 downstream of the sheet feeding roller 13 in the conveying direction (from left to right in FIG. 1). Have been.

In 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.

The cleaning roller 42 temporarily absorbs the residual toner 53 remaining on the photosensitive drum 20 after the transfer by the transfer roller 60 by changing the bias voltage, 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.

Referring to 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 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 the present embodiment, particularly, the developing roller 56 mainly includes 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 of the developing roller 56 from the central electrode to which the developing bias voltage is applied to the outer peripheral contact portion 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 on 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 portion 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, 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 substantially rubbed before the rubbing. 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 is
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, urethane rubber or the like. 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, and heats the toner image transferred to the lower surface of the sheet P. The sheet is pressed and fixed on the sheet P.

A pair of transport rollers 75 and a 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 this embodiment is, for example, a non-magnetic one-component toner composed 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.

Further, 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. I have. 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 has insufficient fluidity improving effect,
Negatively charged silica having a small polarity is often used as an external additive. The difference between positively chargeable silica and negatively chargeable silica is that if the charge amount of the toner when externally added to the base toner increases in the positive direction, the chargeable silica is positively chargeable silica. It is that it is.
In addition, the larger the particle size of the external additive such as silica, the lower the fluidity, but in the present embodiment, for the reason described later, the larger the particle size of the external additive such as silica generally having good fluidity. Large ones are used. Details will be described later.

In the present embodiment, since the toner 53 has such a structure, when the toner 53 is sufficiently rubbed in the nip portion between the developing roller 56 and the photosensitive drum 20, the CCA contained in the raw toner is reduced. The charging characteristic of the toner 53 becomes dominant, and the toner 53 is charged to a 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 charging potential on the surface of the photosensitive drum 20 is set to 700 V, the developing bias is set to 300 V, and the exposure section potential is set to 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
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 transfers to a solid white portion, and fog occurs. A problem arises.

Therefore, in this embodiment, as the external additive,
Silica, alumina, or titanium oxide having a particle diameter of about 30 nm larger than the particle diameter of the toner 53 is used.

By using the external additive having such a large particle diameter, the surface of the photosensitive drum 20 and the toner 53 are hardly brought into direct contact with each other, and an extreme decrease in the surface potential of the photosensitive drum 20 is prevented. Is what you do.

It is considered that as the particle size of the external additive increases, the direct contact between the photosensitive drum 20 and the toner 53 decreases. However, as the particle size increases, the fluidity deteriorates, and as a result, the toner is not sufficiently charged. There is a problem that 53 appears and fogging occurs at the lower part of the solid black image.

In general, particles having a particle size of 10 to 20 nm have good fluidity, and particles having a particle size of 40 nm or more have poor fluidity. In the present embodiment, as a result of conducting an experiment using silica or the like having various particle diameters, when the particle diameter is about 30 nm or more,
It has been found that an extreme decrease in the potential of the photosensitive drum 20 can be suppressed. Furthermore, it has been found that fluidity can be ensured by using an external additive having a particle size of 10 to 20 nm or more in combination with an external additive having a particle size of 30 nm or more.

When the external additive having such a particle size is used,
FIGS. 6 and 7 show a comparison of the degree of fogging with the case without using.

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 is a graph showing the measured permeation density when an external additive having the above-mentioned particle diameter is used. FIG. 6B is a graph showing an external additive having a smaller particle diameter than the external additive. It is the graph which measured the transmission density at the time of using. From these graphs, it can be seen that the use of the external additive lowers the transmission density.

FIG. 7A is a graph showing the measured value of fogging measured by the above-mentioned method when the external additive having the above-mentioned particle size is used, and FIG. 4 is a graph showing measured values of fogging when an external additive having a smaller particle size than the agent is used. From these graphs, the degree of decrease in the potential V ₀ of the solid white portion on the photosensitive drum 20 after development is from 700 V to 450 V when no external additive is used. If used, 70
From 0 V to 600 V, it can be seen that the relaxation was achieved.

Examples will be described below. In the apparatus of the present embodiment, the slew rate is 6 ppm, the process speed (the peripheral speed of the photosensitive drum 20) is 35 mm / sec, the peripheral speed of the developing roller 56 is 70 mm / sec, and the peripheral speed of the supply roller 55 is 17 to 35 mm / sec. And the developing bias is 3
00V, the charging potential of the photosensitive drum 20 was 700 V, and styrene acrylic toner was used as the toner.
The toner unit mass of the toner layer on the developing roller 56 is determined using 0.2 wt% of nigrosine (or 0.2 wt% of triphenylmethane) and 0.5 to 1.5 wt% of silica having a particle size of 30 nm. Charge amount per hit Q
/ M (μC / g), toner mass per unit area M / A
(Mg / cm ² ), the surface potential V ₀ (V) of the solid white portion of the photosensitive drum 20 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 charged series developer.

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, when the rotation direction is set as described above, 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 having a wavy pattern occurs 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 a wavy pattern even in 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 fine lines and isolated points be satisfactorily reproduced, it is also possible to prevent white spots in corrugated patterns of black solid printing, and furthermore, it is possible to achieve white solid printing. 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 FIGS.
This will be described with reference to FIG. 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 in a side view by driving means, and the supply roller 55 and the developing roller 56 are driven to rotate clockwise, respectively. As a result, as shown in FIG. 8, the particles of the toner 53 are 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, after the residual charge on the photosensitive drum 20 is wiped out by the charge removing lamp 41, the surface of the photosensitive drum 20 is charged by the positive charger 40, for example, as shown in FIG. As shown in FIG. Then, in this state, the laser generator 31
The laser light L emitted from the lens 33 and 34 and the reflection mirror 3 while being main-scanned by the polygon mirror 32.
Irradiation is performed on the photosensitive drum 20 through 5 and 36, and 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 falls 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 higher than its own voltage (approximately +700
V), 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.

Also, as shown in FIG. 5A, similarly, if there is a sufficient potential difference between the developing bias and the solid white portion, no fogging will occur.

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, the transport path PP for transporting the paper P fed from the paper feed cassette 14 is formed in a substantially straight line.
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 this collecting operation, the undeveloped toner 53 on the developing roller 56 is charged to the charge amount Q2 when rubbed in 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 is reduced, the reproducibility of fine lines and isolated points can be improved, and the white solid portion can be further improved. Fog can be reliably prevented.

Although only the formation of a monochrome image has been described in the above embodiment, the present invention functions effectively also in the formation of 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.

[0085]

As described above, according to the image forming apparatus of the first aspect, the developer is composed of a positively charged toner, a positively charged charge controlling agent, and an external additive having a particle diameter of about 30 nm or more. Since the composition is provided with the agent, fogging in the white peta portion can be ensured, and fine lines and isolated points can be reproduced well. Further, the white spot phenomenon in black solid printing can also be prevented.

According to the image forming apparatus of the second aspect,
Using styrene acrylic toner as the positively charged toner,
Since at least one of nigrosine and triphenylmethane was used as the charge control agent for positive charging, the charge amount of the developer on the developer carrier after development was surely kept on the developer carrier before development. Can be larger than the charge amount of the developer. As a result, fine lines and isolated points can be favorably developed, and since the external additive of the developer has a particle size of about 30 nm or more, the developer on the developer carrier after the development is charged. An increase in the amount can be suppressed moderately, 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,
The compounding ratio of the toner, the charge control agent, and the external additive in the developer and the particle size of the external additive are rubbed against each other at a nip portion during development by the developer carrying means. Since the charge amount of the developer on the developer carrying means is set to be larger than the charge amount of the developer on the developer carrying means before the development, it is possible to favorably develop fine lines and isolated points. In addition, since the external additive of the developer has a particle diameter of about 30 nm or more, the increase in the charge amount of the developer on the developer carrier after the development is appropriately suppressed, and the positive polarity with respect to the unexposed portion is improved. Fogging due to toner can be reliably prevented.

According to the image forming apparatus of the fourth aspect,
Since the direction of transport of the developer of the developer carrying means is opposite to the direction of movement of the photoreceptor in the nip portion, circulation of the developer is improved, and fine lines and isolated points are improved over a long period of time. And the white spot phenomenon in black solid printing can be prevented.

[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. 6 (A) is a diagram showing a measured transmission density when an external additive having a particle size of about 30 nm according to the present invention is used, and FIG. 6 (B) shows an external additive having a particle size smaller than the external additive. It is the figure which measured the transmission density at the time of using.

FIG. 7A is a view showing a measurement of fog on a photosensitive drum when an external additive having a particle size of about 30 nm according to the present invention is used,
(B) is a view showing a measurement of fog on the photosensitive drum when an external additive having a smaller particle size than the external additive 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: paper discharge tray

Claims (4)

[Claims] An organic photoreceptor having a positively charged surface, a latent image forming means for forming an electrostatic latent image on the moving photoreceptor, a developer carried and conveyed, and contact with the photoreceptor. A developer carrying means for forming a nip between the photosensitive member and the electrostatic latent image formed at the nip with the developer to form a visible image; The image forming apparatus according to claim 1, wherein the developer includes a positively charged toner, a charge control agent for positively charging, and an external additive having a particle size of about 30 nm or more. 2. The developer according to claim 1, wherein 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 charged toner. 1
An image forming apparatus according to claim 1. 3. The mixing ratio of the toner, the charge control agent, and the external additive in the developer and the particle size of the external additive are determined by being mutually rubbed in a nip portion during development by the developer carrying means. The charge amount of the developer on the developer carrying means after the development is set to be larger than the charge amount of the developer on the developer carry means before the development. The image forming apparatus according to claim 1 or 2. 4. The apparatus according to claim 1, wherein a direction in which the developer is transported by the developer carrying means is a direction opposite to a direction in which the photoconductor moves in the nip portion. An image forming apparatus according to claim 1.