JP2005300751A - Image forming apparatus - Google Patents

Abstract

In an image forming apparatus using a cleanerless system that can reduce the wear of a photoreceptor even when the image forming apparatus is used for a long period of time, ozone (O3) is used.
) And nitrogen oxides (NOx), etc., to prevent the occurrence of image negatives.
An image carrier, a charging unit for charging the image carrier at a contact portion, an exposure unit as a unit for writing an electrostatic latent image on the image carrier charged by the charging unit, A developing unit that supplies toner to the electrostatic latent image formed on the image bearing member for visualization; and a transfer unit that transfers the visualized toner image to a transfer material, and is transferred by the transfer unit. In the image forming apparatus using a cleanerless system in which the toner remaining on the surface of the image carrier is collected by the developing device, the toner developed by the developing unit has a polarity opposite to the charged polarity of the toner. These fine particles are externally added.
[Selection] Figure 1

Description

  The present invention relates to an image forming apparatus using an electrophotographic system, and more particularly, to an image forming apparatus such as a copying machine, a printer, a facsimile.

  Conventionally, image forming apparatuses such as copying machines, printers, and facsimiles using an electrophotographic method generally have an electrophotographic photosensitive member (photosensitive member) that is a rotating drum type image carrier, and the photosensitive member has a predetermined polarity and potential. A charging device for uniformly charging (charging process), an exposure device (exposure process) as a writing means for forming an electrostatic latent image on the charged photosensitive member, and an electrostatic latent image formed on the photosensitive member A developing device (development process) that visualizes an image as a toner image with toner as a developer, a transfer device (transfer process) that transfers the toner image from the surface of the photoreceptor to a recording material such as paper, and a photosensitivity after the transfer process. A cleaning device (cleaning step) that removes a little residual toner (residual developer, transfer residual toner) on the body and cleans the surface of the photoreceptor, a fixing device (fixing step) that fixes the toner image on the recording material, and the like. Preparation Cage, repeated electrophotographic process (charging step, exposing step, developing step, transferring step, cleaning step) is applied is used for image formation in the photosensitive member.

  The toner remaining on the photoconductor after the transfer process is removed from the surface of the photoconductor by the cleaning device, and collected in the cleaning device to become waste toner. However, it is desirable that such waste toner does not come out from the viewpoints of environmental protection and effective use of resources.

  In addition, the cleaning device generally employs a configuration in which a cleaning blade made of urethane rubber is pressed against the photoconductor in a counter direction with a constant pressure. The pressure applied at this time needs to be set to a level at which the toner does not slip through the contact portion between the cleaning blade and the photoconductor, and varies depending on, for example, the weight average particle diameter and fluidity of the toner.

  On the other hand, the cleaning blade not only cleans the transfer residual toner remaining on the surface of the photoconductor, but also has an effect of abrasion and scraping off the surface of the photoconductor by the frictional force acting on the contact portion between the tip of the cleaning blade and the photoconductor. For example, when the photoconductor is an organic photoconductor (OPC), the charge transfer layer (CTL), which is the outermost layer, is worn away and scraped, so that the capacity of the charge transfer layer (CTL) gradually changes, and finally In this case, the surface of the photoreceptor cannot be uniformly charged by the charging means, resulting in an image defect. Therefore, in an image forming apparatus having a cleaning device, the wear of the photoconductor greatly affects the running cost, and it is desirable to reduce the wear by the cleaning blade sufficiently to reduce the running cost.

  Therefore, the cleaning device is eliminated, and the transfer residual toner on the photosensitive member after the transfer process is removed and collected from the photosensitive member by “development simultaneous cleaning” in the developing device and reused. Has been proposed (Patent Document 1).

  In the simultaneous development cleaning, the transfer residual toner on the photoconductor after the transfer process is collected by the developing device during the subsequent development process. That is, the photosensitive member to which the transfer residual toner is attached is continuously charged and exposed to form an electrostatic latent image, and a fog removal bias (DC voltage applied to the developing device and the surface of the photosensitive member is developed during the developing process of the electrostatic latent image. The residual transfer toner remaining on the non-development portion (non-image portion) of the transfer residual toner remaining on the surface of the photoreceptor is removed and collected by the developing device by the fog removal potential difference Vback which is a potential difference between the potentials. It is a method to do.

  According to this method, the transfer residual toner is collected by the developing device and reused for developing the electrostatic latent image in the subsequent steps, so that waste toner can be eliminated. Further, by eliminating the cleaning device, it is possible to significantly reduce the wear of the photoreceptor. Furthermore, it is advantageous in reducing the size of the image forming apparatus by being excellent in usability such as reducing troublesome work during maintenance and being cleanerless.

When a corona charger is used as the charging device in the cleanerless type image forming apparatus employing the simultaneous development cleaning as described above, charging of ozone (O ₃ ), nitrogen oxide (NOx) or the like by corona discharge is performed. Although a large amount of product is generated, the charged product adhering to the photoreceptor cannot be removed because it does not have a cleaning device. For this reason, a film of a charged product is formed on the surface of the photoconductor, and the surface resistance of the photoconductor is remarkably lowered due to adsorption of moisture in the atmosphere, so that the electrostatic latent image written on the photoconductor by the exposure device is disturbed. A phenomenon called image negative occurs.

  Therefore, in a cleanerless type image forming apparatus employing simultaneous development cleaning, for example, a conductive roller is brought into contact with the surface of the photoconductor to reduce discharge as much as possible in order to suppress the generation of charged products. Many have been proposed that use a contact charging method for charging the battery (Patent Document 2).

Japanese Patent Laid-Open No. 07-140791 JP 2001-183905 A

However, even when such a contact charging device is used, the essential charging mechanism uses the discharge phenomenon from the charging member to the photosensitive member, and thus the voltage required for charging as described above. Requires a value equal to or higher than the surface potential of the photoconductor, and a small amount of charged products such as ozone (O ₃ ) and nitrogen oxide (NOx) are generated.

Also, in image forming apparatuses that require high image quality, such as full-color images and graphic images, it is necessary to reduce charging unevenness. For the purpose of charging uniformity, an oscillating voltage in which an AC voltage is superimposed on a DC voltage is applied to a contact charging device. The charging method used (AC charging method) is widely used. In this case, however, further charging products such as ozone (O ₃ ) and nitrogen oxide (NOx) are generated. In some cases, the above-described image negative may occur.

Accordingly, in the present invention, ozone (O ₃ ) or nitrogen oxide (NOx) is used in an image forming apparatus using a cleanerless system that can reduce the wear of the photosensitive member even when the image forming apparatus is used for a long period of time. The purpose of the present invention is to prevent image negatives by suppressing the generation of charged products such as

  In order to achieve the above object, the present invention provides an image carrier, a charging means for charging the image carrier at a contact portion to form an electrostatic image on the image carrier, and charging by the charging means. Exposure means as means for writing an electrostatic latent image on the processed image carrier, developing means for supplying toner to the electrostatic latent image formed on the image carrier for visualization, and a visualized toner image An image forming apparatus using a cleanerless system that collects toner remaining on the surface of the image carrier without being transferred by the transfer means by the developing device. In the toner developed by the developing means, fine particles having a polarity opposite to the charged polarity of the toner are externally added.

  Further, the primary number average particle diameter D1 of the fine particles is 0.5 μm ≦ D1 ≦ 15.0 μm.

  Further, the weight ratio W of the fine particles to the toner is 0.1 wt% ≦ W ≦ 3.0 wt%.

According to the present invention, fine particles having a polarity opposite to the charging polarity of the toner and having a primary number average particle diameter D1 of 0.5 μm ≦ D1 ≦ 15.0 μm, a weight ratio W to the toner is 0.1 wt%. By using toner externally added in the range of ≦ W ≦ 3.0 wt%, fine particles selectively adhere to the surface of the contact charging device to create a minute gap at the contact portion between the contact charging device and the photosensitive member, and contact charging Since the device serves as charging auxiliary fine particles that help to charge the surface of the photoconductor, it is possible to suppress discharge that does not contribute to charging of the surface of the photoconductor. As a result, in an image forming apparatus using a cleanerless system that can suppress the generation of charged products such as ozone (O ₃ ) and nitrogen oxide (NOx), and can significantly reduce the wear of the photoreceptor. In addition, it is possible to prevent image defects such as image negatives.

  The image forming apparatus according to the present invention will be described below in more detail with reference to the drawings.

<Embodiment 1>
FIG. 3 is a cross-sectional view showing a schematic configuration of an embodiment of the image forming apparatus according to the present invention. The image forming apparatus 100 according to the present embodiment is an electrophotographic laser beam printer (printer) that employs a contact charging method, a two-component contact development method, and a cleanerless method.
(Entire printer configuration)
First, the overall configuration of the printer 100 according to the present embodiment will be described with reference to FIG.
(A) Image carrier:
The printer 100 includes a rotating drum type electrophotographic photosensitive member (hereinafter referred to as “photosensitive drum”) as an image carrier. In the present embodiment, the photosensitive drum 1 is an organic photoconductor (OPC) having a negative charging characteristic and has an outer diameter of 30 mm and an arrow having a process speed (peripheral speed) of 130 mm / sec centered on the central support shaft. It is driven to rotate counterclockwise as shown.

As shown in FIG. 4, the photosensitive drum 1 has an undercoat layer 1b for suppressing the interference of light and improving the adhesion of the upper layer on the surface of an aluminum cylinder (conductive drum base) 1a, and a photocharge generation layer 1c. And three layers of the charge transport layer 1d (thickness of about 20 μm) are applied in order from the bottom.
(B) Charging means:
The printer 100 includes a contact charging device (contact charger) 2 as charging means for uniformly charging the surface of the photosensitive drum 1. In the present embodiment, the contact charging device 2 is a charging roller (roller charger), and is charged by utilizing a discharge phenomenon that occurs in a minute gap with the photosensitive drum 1.

  The charging roller 2 is rotatably supported at both ends of a core metal (support member) 2a by a bearing member (not shown), and is biased toward the photosensitive drum 1 by a pressing spring 2e. 1 is brought into pressure contact with a predetermined pressing force. As a result, the charging roller 2 rotates following the rotation of the photosensitive drum 1. A pressure contact portion between the photosensitive drum 1 and the charging roller 2 is a charging portion (charging nip portion, contact portion) a.

  A charging bias voltage of a predetermined condition is applied to the cored bar 2a of the charging roller 2 from the power source S1. As a result, the surface of the rotating photosensitive drum 1 is contact-charged to a predetermined polarity / potential. In the present embodiment, the charging bias voltage for the charging roller 2 is an oscillating voltage obtained by superimposing a DC voltage (Vdc) and an AC voltage (Vac). More specifically, it is an oscillating voltage in which a DC voltage of −500 V, a frequency of 1.3 kHz, a peak-to-peak voltage Vpp of 1.5 kV, and a sinusoidal AC voltage are superimposed. By this charging bias voltage, the surface of the photosensitive drum 1 is uniformly contact-charged to −500 V (dark potential Vd) which is the same as the DC voltage applied to the charging roller 2.

  The charging roller 2 has a longitudinal length of 320 mm and a diameter of 14 mm. Further, as shown in the layer configuration model diagram of FIG. 4, the lower layer 2b, the intermediate layer 2c, and the surface layer 2d are sequentially laminated from the bottom around the outer periphery of the cored bar 2a. The lower layer 2b is a foamed sponge layer for reducing charging noise, and the surface layer 2d is a protective layer provided to prevent leakage even when there are defects such as pinholes on the photosensitive drum 1. is there. More specifically, the specification of the charging roller 2 of the present embodiment is as follows.

Core 2a: Stainless steel round bar with a diameter of 6 mm Lower layer 2b: Foamed EPDM with carbon dispersion, specific gravity 0.5 g / cm ³ ,
Volume resistance value 10 ² to 10 ⁹ Ωcm, layer thickness about 3.5 mm
Intermediate layer 2c: carbon-dispersed NBR rubber, volume resistivity of 10 ² to 10 ⁵ Ωcm,
Layer thickness of about 500μm
Surface layer 2d: tin oxide and carbon dispersed in a fluororesin resin
Volume resistance value 10 ⁷ to 10 ¹⁰ Ωcm,
Surface roughness (JIS standard 10-point average surface roughness Rz) 1.5 μm,
Layer thickness of about 5 μm.
(C) Information writing means:
The printer 100 includes an exposure device 3 serving as an exposure unit as an information writing unit that forms an electrostatic latent image on the surface of the photosensitive drum 1 that has been charged. In the present embodiment, the exposure apparatus 3 is a laser beam scanner using a semiconductor laser. The laser beam scanner 3 outputs a laser beam L modulated corresponding to an image signal sent from a host processing device such as an image reading device (not shown) to the printer side, and is uniformly charged. The surface of the rotating photosensitive drum 1 is subjected to laser scanning exposure (image exposure) at an exposure position (exposure portion) b. By this laser scanning exposure, the potential irradiated with the laser beam L on the surface of the photosensitive drum 1 is lowered, and electrostatic latent images corresponding to image information are sequentially formed on the surface of the rotating photosensitive drum 1. To go.
(D) Developing means:
The printer 100 includes a developing device (developer) 4 as a developing unit that supplies toner according to the electrostatic latent image on the photosensitive drum 1 and reversely develops the electrostatic latent image as a toner image (developer image). In the present embodiment, the developing device 4 is a developing device that employs a two-component contact developing system that performs development while bringing a magnetic brush made of a two-component developer composed of toner and carrier into contact with a photosensitive drum.

  The developing device 4 includes a developing container 4a and a nonmagnetic developing sleeve 4b as a developer carrying member. The developing sleeve 4b is rotatably arranged in the developing container 4a with a part of the outer peripheral surface thereof exposed to the outside of the developing device 4. In the developing sleeve 4b, a magnet roller 4c is inserted in a non-rotating manner. A developer coating blade 4d is provided opposite to the developing sleeve 4b. The developing container 4a contains a two-component developer 4e, and a developer stirring member 4f is disposed on the bottom side in the developing container 4a. Further, replenishment toner is accommodated in the toner hopper 4g.

  The two-component developer (developer) 4e in the developing container 4a is mainly a mixture of a non-magnetic toner and a magnetic carrier and is stirred by the developer stirring member 4f. The toner is triboelectrically charged to a negative polarity by rubbing against the magnetic carrier.

  Here, the resistance value of the magnetic carrier in the present embodiment is 1013 Ωcm, and the volume average particle diameter is about 40 μm. The volume average particle size of the magnetic particles is measured by dividing the range of 0.5 to 350 μm on a volume basis by 32 logarithm using a laser diffraction particle size distribution analyzer HEROS (manufactured by JEOL Ltd.). The number of particles is measured, and the median diameter of 50% volume is taken as the volume average particle diameter from the measurement results.

The resistance value of the nonmagnetic toner is about 10 ¹⁴ Ωcm, the volume average particle diameter D ₄ is about 7.0 μm, and the surface area shape sphericity Φs is 0.95.

For the volume average particle diameter D ₄ of the toner, Coulter Counter TA-II type (manufactured by Coulter Inc.) is used. As a measuring method, 0.1 to 5 ml of a surfactant (preferably alkylbenzene sulfonate) is added as a dispersing agent to 100 to 150 ml of an electrolytic solution composed of 1% NaCl aqueous solution prepared using primary sodium chloride, and further measured. Add 2-20 mg of sample. The electrolytic solution in which the sample is suspended is subjected to a dispersion process for about 1 to 3 minutes with an ultrasonic disperser, and the volume distribution is measured by measuring the volume of toner of 2 μm or more using the Coulter counter TA-II with a 100 μm aperture. And the median diameter of 50% volume is taken as the volume average particle diameter from the measurement results. Surface area shape sphericity Φs of the toner, the ratio of the surface area assuming a toner for BET specific surface area of toner (m ² / g) in sphericity (m ² / g).

  The developing sleeve 4b is disposed in close proximity to the photosensitive drum 1 while maintaining the closest distance (S-Dgap) to the photosensitive drum 1 at 350 μm. A facing portion between the photosensitive drum 1 and the developing sleeve 4b is a developing portion c.

  The developing sleeve 4b is rotationally driven in the direction opposite to the traveling direction of the photosensitive drum 1 in the developing unit c. Due to the magnetic force of the magnet roller 4c in the developing sleeve 4b, a part of the two-component developer 4e in the developing container 4a is adsorbed and held on the outer peripheral surface of the developing sleeve 4b as a magnetic brush layer. This magnetic brush layer is rotated and conveyed as the developing sleeve 4b rotates, and is layered into a predetermined thin layer by the developer coating blade 4d. The magnetic brush layer comes into contact with the surface of the photosensitive drum 1 in the developing section c and is in contact with the photosensitive drum 1. Rub the surface moderately.

  A predetermined developing bias is applied from the power source S2 to the developing sleeve 4b. In the present embodiment, the developing bias voltage for the developing sleeve 4b is an oscillating voltage obtained by superimposing a DC voltage (Vdc) and an AC voltage (Vac). More specifically, it is an oscillating voltage in which a DC voltage of −350 V, a frequency of 8.0 kHz, a peak-to-peak voltage Vpp of 1.8 kV, and a rectangular wave AC voltage are superimposed.

  Thus, the toner in the developer 4e coated as a thin layer on the surface of the rotating developing sleeve 4b and conveyed to the developing section c corresponds to the electrostatic latent image on the surface of the photosensitive drum 1 by the electric field due to the developing bias. The electrostatic latent image is developed as a toner image. In the case of the present embodiment, toner adheres to the exposed bright portion of the surface of the photosensitive drum 1 and the electrostatic latent image is reversely developed.

  The developer thin layer on the developing sleeve 4b that has passed through the developing section c is returned to the developer reservoir in the developing container 4a with the subsequent rotation of the developing sleeve 4b.

  In order to maintain the toner concentration of the two-component developer 4e in the developing container 4a within a substantially constant range, the toner concentration of the two-component developer 4e in the developing container 4a is detected by, for example, an optical toner concentration sensor. Then, the toner hopper 4g is driven and controlled according to the detected information, and the toner in the toner hopper 4g is replenished to the two-component developer 4e in the developing container 4a. The toner supplied to the two-component developer 4e is stirred by the stirring member 4f.

Details of the fine particles externally added to the toner will be described later.
(E) Transfer means / fixing means:
The printer 100 includes a transfer device 5 as a transfer unit. In the present embodiment, the transfer device 5 is a transfer roller. The transfer roller 5 is pressed against the photosensitive drum 1 with a predetermined pressing force, and the pressure nip portion is a transfer portion d. The recording material P is fed to the transfer portion d from a paper feed mechanism portion (not shown) at a predetermined control timing.

  The recording material P fed to the transfer part d is nipped and conveyed between the rotating photosensitive drum 1 and the transfer roller 5. In the meantime, a positive transfer bias having a polarity opposite to the negative polarity that is the normal charging polarity of the toner, that is, +2 kV in the present embodiment, is applied to the transfer roller 5. As a result, the toner image on the surface side of the photosensitive drum 1 is electrostatically transferred sequentially onto the surface of the recording material P that is nipped and conveyed by the transfer portion d.

The recording material P that has received the transfer of the toner image through the transfer portion d is sequentially separated from the surface of the photosensitive drum 1 and conveyed to the fixing device 6. In the present embodiment, the fixing device 6 is a heat roller fixing device, and the recording material P is subjected to a fixing process of the toner image by the fixing device 6 and is output as an image formed product (print, copy).
(F) Cleanerless system (charging auxiliary means):
The printer 100 according to the present embodiment employs a so-called cleanerless system, and is dedicated to removing a residual transfer toner (residual toner) remaining on the surface of the photosensitive drum 1 after the toner image is transferred onto the recording material P. The cleaning device is not provided.

  The transfer residual toner on the surface of the photosensitive drum 1 after the transfer is transported to the developing unit c through the charging unit a and the exposing unit b with the subsequent rotation of the photosensitive drum 1 and is removed by the developing device 4 by simultaneous development cleaning.・ Recovered (cleanerless system).

  In the present embodiment, the developing sleeve 4b of the developing device 4 is rotated in the direction opposite to the traveling direction of the surface of the photosensitive drum 1 in the developing unit c as described above. Such rotation of the developing sleeve 4b is advantageous for collecting the transfer residual toner on the photosensitive drum 1.

  Since the transfer residual toner on the photosensitive drum 1 passes through the exposure part b, the exposure process is performed from the transfer residual toner. Usually, since the amount of the transfer residual toner is small, there is no great influence by performing the exposure process on the transfer residual toner.

  However, as described above, the transfer residual toner includes those having a normal polarity, a reverse polarity (reversal toner), and a low charge amount, and the reverse toner and the charge amount are small. If the toner adheres to the charging roller 2 when passing through the charging portion a, the charging roller 2 may be contaminated with toner more than allowable, resulting in a charging failure.

  Further, in order to effectively remove and collect the transfer residual toner on the photosensitive drum 1 simultaneously with the developing operation by the developing device 4, the charge amount of the transfer residual toner becomes an important factor. That is, the transfer residual toner on the photosensitive drum 1 carried to the developing unit c is a normal polarity of the charge polarity, and the charge amount of the toner that can develop the electrostatic latent image on the photosensitive drum 1 by the developing device. A charge amount is preferred. If the charge polarity of the transfer residual toner is reversed or the charge amount is not appropriate, the toner cannot be removed / collected from the photosensitive drum 1 to the developing device 4, which may cause a defective image.

  Therefore, as a charging auxiliary means, a residual toner equalizing means (uniform residual developer image) for making the transfer residual toner on the photosensitive drum 1 uniform at a position downstream of the transfer portion d in the rotation direction of the photosensitive drum 1. Charging means 8), and the charging polarity of the residual transfer toner at a position downstream of the residual toner uniformizing means 8 in the rotational direction of the photosensitive drum 1 and upstream of the charging portion a in the rotational direction of the photosensitive drum 1. Is provided with toner charge amount control means (developer charge amount control means) 7 for aligning the negative polarity with the normal polarity.

  In general, the untransferred toner remaining on the photosensitive drum 1 without being transferred to the recording material P at the transfer portion d is mixed with reversal toner and toner with an inappropriate charge amount. Therefore, the residual toner is neutralized once by the residual toner equalizing means 8, and then the residual toner is charged to the normal polarity again by the toner charge amount control means 7. Accordingly, it is possible to effectively prevent the transfer residual toner from adhering to the charging roller 2 and to completely remove and collect the transfer residual toner in the developing device 4. Therefore, generation of a ghost image of the transfer residual toner image pattern is strictly prevented.

  In the present embodiment, the residual toner equalizing means 8 and the toner charge amount control means 7 are brush-like members having appropriate conductivity, and are arranged with the brush portion in contact with the surface of the photosensitive drum 1. Yes. A contact portion f between the residual toner uniformizing means 8 and the surface of the photosensitive drum 1 is formed, and a contact portion e between the toner charge amount control means 7 and the surface of the photosensitive drum 1 is formed.

A positive DC voltage is applied to the residual toner uniformizing means 8 from the power source S5, and a negative DC voltage is applied to the toner charge amount control means 7 from the power source S4. The DC voltage applied to each of the residual toner uniformizing means 8 is + 250V, and the toner charge amount control means 7 is -750V DC voltage. The transfer residual toner is sequentially controlled so as to have an appropriate charge amount that can be collected by the developing device 4.
(G) Charge assisting fine particles (toner externally added fine particles):
Toner developed by the developing device 4 is usually externally added with fine particles having a role of imparting fluidity of the toner and a role of controlling the charge amount of the toner. Also in the toner used in the present embodiment, titania (TiO ₂ ) fine particles are 0.7 wt% with respect to the toner weight ratio to impart toner fluidity, and silica (SiO ₂ ) is used to control the toner charge amount. Fine particles are externally added in an amount of 0.5 wt% with respect to the toner weight ratio.

  Further, in the toner used in the present embodiment, in addition to the above two types of fine particles, fine particles that adhere to the surface of the charging roller and play a role of charging assistance (hereinafter referred to as “charging auxiliary fine particles”) are externally added. Yes. The charging auxiliary fine particles are characterized by having a positive charging polarity opposite to the charging polarity of the toner, so that most of them remain on the surface of the photosensitive drum 1 without being transferred to the recording material P by the transfer means 5. . The charging auxiliary fine particles remaining on the surface of the photosensitive drum 1 reach the contact portion a between the charging roller 2 and the photosensitive drum 1, and on the surface of the charging roller 1 due to the relationship between the surface potential of the charging roller 2 and the surface potential of the photosensitive drum 1. It adheres electrostatically (see FIG. 5). Here, in order for the charging auxiliary fine particles to sufficiently adhere to the charging roller 2, when the charge amount of the charging auxiliary fine particles is measured by the blow-off method, the charge amount is desirably about +50 μC / g or more.

  At this time, the transfer residual toner slightly remaining on the surface of the photosensitive drum 1 after passing through the transfer means 5 also passes through the contact portion a between the charging roller 2 and the photosensitive drum 1 together with the fine particles. Since the charge amount is controlled by the means 8 and the toner charge amount control means 7, it passes without adhering to the surface of the charging roller 2 and then collected by the developing device 4. On the other hand, since the charge auxiliary fine particles have a sufficiently large charge amount, they are hardly affected by the residual toner uniformizing means 8 and the toner charge amount control means 7. Further, although a negative DC voltage is applied to the toner charge amount control means 7, the contact area with the photosensitive drum 1 is very small because it is a member made of a brush, and the toner charge amount control means 7 has a charge assist. There is almost no adhesion of fine particles.

  Next, the effect of the charging auxiliary fine particles will be described.

  In the contact charging method of the charging roller 2 or the like, the surface of the photosensitive drum 1 is charged by a necessary and sufficient charge moving from the charging roller 2 to the photosensitive drum 1 at the contact portion a between the charging roller 2 and the photosensitive drum 1. Ideal. However, the photosensitive drum 1 rotates at a constant speed (130 mm / sec in the present embodiment), and the movement of the electric charge is performed so that the surface of the photosensitive drum 1 is charged to the target potential in a short time. Can not.

As a solution to this problem, it is conceivable to reduce the resistance of the charging roller 2 or to increase the area of the contact portion a between the charging roller 2 and the photosensitive drum 1. In the former case, however, when a protrusion exists on the surface of the photosensitive drum 1. Pin fall leak may occur and to some extent (about 105
It is not sufficient because it can only be lowered to Ωcm). In the latter case, there is a demerit such that the size of the apparatus must be increased. Further, in practice, fine irregularities exist on the shapes of the charging roller 2 and the surface of the photosensitive drum 1, and partial resistance unevenness exists, so that the surface of the photosensitive drum 1 cannot be uniformly charged.

  Therefore, even in the case of the contact charging system such as the charging roller 2 for the above reasons, the surface of the photosensitive drum 1 is charged in a necessary and sufficient and uniform manner, and the upstream and downstream of the contact portion a between the charging roller 2 and the photosensitive drum 1. There is no choice but to rely on the discharge generated in the small gap formed. Specifically, an oscillating voltage obtained by superimposing an alternating voltage (Vac) on a direct current voltage (Vdc) is applied to the charging roller 2 (AC charging method), so that the contact portion a between the charging roller 2 and the photosensitive drum 1 is upstream and downstream. A method using a so-called AC discharge that is generated is generally used.

  FIG. 6 is an enlarged view of a contact portion a between the charging roller 2 and the photosensitive drum 1.

  AC discharge occurs when the peak-to-peak voltage (Vpp) of the AC voltage (Vac) applied to the charging roller 2 exceeds a threshold value determined by the resistance of the charging roller 2 or the like.

  FIG. 8A shows the alternating current (Iac) flowing from the charging roller 2 to the photosensitive drum 1 when the alternating voltage (Vpp) applied to the charging roller 2 is changed (the charging roller 2 has Vpp-Iac characteristics).

  In the Vpp-Iac characteristic shown in FIG. 8, the Vpp-Iac curve is bent upward with Vpp (1) as a boundary. Here, the threshold at which Vpp (1) starts discharge (discharge start point, this implementation) Is about 1.2 kVpp), and at Vpp (1) or higher Vpp (for example, Vpp (2)), the origin and the point of Vpp (1) are determined from Iac (Iac (1)) on the Vpp-Iac curve. A value (Iac (1) -Iac (3)) obtained by subtracting Iac (Iac (3)) when it is assumed that the connecting straight line is extended is the discharge current due to AC discharge.

Here, the AC discharge generated upstream and downstream of the contact portion a between the charging roller 2 and the photosensitive drum 1 does not all affect the charging of the photosensitive drum 1, and conversely ozone (O ₃ ) or nitrogen oxidation. It is known that this is a cause of generating charged products such as substances (NOX). That is, with respect to AC discharge generated at a portion where the gap between the charging roller 2 and the photosensitive drum 1 is about 30 μm or less, photons emitted from the surface of the charging roller 2 are oxygen (O ₂ ), nitrogen (N ₂ ), etc. in the atmosphere. If it moves to the surface of the photosensitive drum 1 without colliding with it (AC discharge that contributes to charging), but the gap is larger than that, oxygen (O ₂ ), nitrogen (N ₂ ), etc. in the atmosphere Since it collides with and reacts, it causes generation of charged products such as ozone (O ₃ ) and nitrogen oxides (NOx) (discharge that does not contribute to charging).

  In FIG. 6, the portion shown in (region (1)) is AC discharge that does not contribute to charging, and the portion shown in (region (2)) is AC discharge that contributes to charging. The same applies to the Vpp-Iac characteristic shown in FIG.

  From the above, it is ideal to increase the AC discharge in (region (2)) and decrease the AC discharge in (region (3)). However, as shown in FIG. ) AC discharge is also directly proportional to the AC voltage (Vpp) applied to the charging roller 2 of the AC discharge in (region (2)), so the magnitude of the AC voltage (Vpp), waveform, frequency, etc. It is impossible to realize it even if the conditions are changed.

  Therefore, in the present embodiment, the size of the gap at the contact portion a between the charging roller 2 and the photosensitive drum 1 is obtained by externally adding the above-described charging auxiliary fine particles to the toner and adhering the toner to the charging roller 2. It is possible to create a small gap of about 30 μm or less and increase the amount of AC discharge contributing to charging.

Here, since it is necessary to create a sufficiently small gap when the charging auxiliary fine particles adhere to the surface of the charging roller 2, the size of the charging auxiliary fine particles is the primary number average particle diameter D1.
Is preferably 0.5 μm ≦ D ₁ ≦ 15.0 μm. Primary number average particle diameter D1 of charging auxiliary fine particles
Is smaller than 0.5 μm, a small gap cannot be formed when each of the charge assisting fine particles is present alone, and even after secondary agglomeration, after adhering to the surface of the charging roller 2. Since it is crushed by the pressure applied between the charging roller 2 and the photosensitive drum 1 to form a film, a minute gap cannot be created. Also, the primary number average particle diameter D1
Is larger than 15.0 μm, the charge amount of each charging auxiliary fine particle becomes very large, so that development from the developing device 4 to the photosensitive drum 1 cannot be performed, and accumulated in the developing device 4. End up.

  Further, if the surface roughness of the charging roller 2 is large at this time, the charging auxiliary fine particles are embedded in the charging roller 2, and a minute gap cannot be formed. Therefore, the 10-point average roughness (Rz) of the charging roller 2 is required to be 3.0 μm or less.

  FIG. 7 shows an enlarged view of the vicinity of the contact portion a between the charging roller 2 and the photosensitive drum 1 when the auxiliary charging fine particles adhere to the surface of the charging roller 2. Here, as compared with FIG. 6, since the charge auxiliary fine particles exist also in the contact portion a, a gap is formed, and AC discharge is generated. Therefore, when the AC voltage (Vpp) applied to the charging roller is the same, the AC discharge contributing to the charging generated in (region (2)) increases.

  FIG. 8B shows Vpp-Iac characteristics when the charge assisting fine particles adhere to the charging roller 2. The amount of AC discharge that contributes to charging shown in (region (2)) when the same AC voltage (Vpp) is applied as compared with the case where the auxiliary charging fine particles in FIG. It can be seen that increases.

  In order to uniformly charge the surface of the photosensitive drum 1 when the charging auxiliary fine particles are not present on the surface of the charging roller 2, as shown in FIG. 8A, the amount of AC discharge contributing to the charging of (region (2)). Is Vac (2) (about 1 .mu.A when the image forming apparatus of the present embodiment is used) having a size of Iac (2) -Iac (3) (about 25 .mu.A when the image forming apparatus of the present embodiment is used). It is necessary to apply an AC voltage of 7 kV to the charging roller 2, and the amount of AC discharge that does not contribute to the charging of (region (1)) at this time is Iac (1) -Iac (2) (image of the present embodiment) When the forming apparatus is used, it is generated at a size of about 35 μA).

  On the other hand, when the charge assisting fine particles are attached to the surface of the charging roller 2, discharge occurs in a minute gap formed by the charge assisting fine particles, and as shown in FIG. )) AC voltage (Vpp) applied to the charging roller 2 at (Iac (2) -Iac (3) = Iac (2) -Iac (3)) in order to make the magnitude of the AC discharge contributing to charging equal. Vpp (2) smaller than Vpp (2) (about 1.5 kV when the image forming apparatus of the present embodiment is used) is sufficient, and contributes to the charging of (region (1)) at this time. The AC discharge amount (Iac (1) -Iac (2)) which is not performed is significantly smaller than the former (Iac (1) -Iac (2)) (about 10 μA when the image forming apparatus of this embodiment is used). ) That is, by using the system of the present embodiment, it is possible to significantly reduce AC discharge that does not contribute to charging while maintaining AC discharge that contributes to charging.

  As a method for obtaining the same effect as in the present embodiment, it is conceivable to increase the surface roughness of the charging roller 2, but in this case, the surface of the charging roller 2 may be worn with long-term use, Since the surface roughness of the charging roller 2 is reduced by embedding the toner or fine particles added externally to the toner, it is impossible to stably maintain the effect for a long period of time.

In the image forming apparatus having the cleaning device, it is possible to prevent deterioration of the photosensitive drum 1 by using the same method as in the present embodiment. The size will be limited. Specifically, the primary number average particle diameter D1
When the particle size is 2.0 μm or more, the surface of the photosensitive drum 1 is almost cleaned by the cleaning device (see FIG. 2). Therefore, the primary number average particle diameter D1 of the charging auxiliary fine particles
Is limited to 2.0 μm or less, and even a part of it is collected by the cleaning device, so that the amount of the charging auxiliary fine particles adhering to the charging roller 2 is remarkably reduced and a sufficient effect cannot be obtained. In addition, a method of separately supplying a charging auxiliary fine particle to the surface of the photosensitive drum 1 on the downstream side of the cleaning device may be considered, but disadvantages such as an increase in size and complexity of the device may be considered.

From the above, in the present embodiment, as the auxiliary charging fine particles externally added to the toner, organic fine particles of methyl methacrylate resin (PMMA) having a primary number average particle diameter D ₁ of 4.0 μm were used. Here, the weight ratio of the charge assisting fine particles to the toner varies depending on the kind and size of the charge assisting fine particles, but 0.1 wt% ≦ W ≦ 3.0 wt in consideration of the influence on the charge amount and fluidity of the toner. % Is preferable. In the case of the methyl methacrylate resin (PMMA) used in the present embodiment, 0.5 wt% was optimal from the above.

In addition to the methyl methacrylate resin (PMMA) used in the present embodiment, the charge auxiliary fine particles include inorganic substances such as barium titanate (BaTiO ₃ ), strontium titanate (SrTiO ₃ ), and zinc oxide (ZnO). Fine particles and organic fine particles such as polyamide (PA), polyimide (PI), polyamideimide (PAI), and polyethylene (PE) can be used.

As described above, according to the present embodiment, fine particles having a polarity opposite to the charging polarity of the toner and having a primary number average particle diameter D ₁ of 0.5 μm ≦ D ₁ ≦ 15.0 μm are added to the toner in a weight ratio. By using the externally added toner with W in the range of 0.1 wt% ≦ W ≦ 3.0 wt%, fine particles selectively adhere to the surface of the contact charging device, and minute amounts are formed at the contact portion between the contact charging device and the photoreceptor. A gap is created, and the contact charging device serves as charging auxiliary fine particles that help to charge the surface of the photoconductor, so that discharge that does not contribute to charging of the surface of the photoconductor can be suppressed. Also, from this result, it is possible to suppress the formation of charged products such as ozone (O ₃ ) and nitrogen oxide (NOx), and an image using a cleanerless system that can significantly reduce the wear of the photoreceptor. Also in the forming apparatus, it is possible to prevent image defects such as image negatives.

<Embodiment 2>
Next, a second embodiment of the present invention will be described.

  The basic configuration of the image forming apparatus (printer) according to the present embodiment is the same as that of the first embodiment. Accordingly, elements having the same functions and configurations as those of the printer 100 of the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted.

In the first exemplary embodiment, charge assisting fine particles having a polarity opposite to the charging polarity of the toner and having a primary number average particle diameter D1 of 0.5 μm ≦ D ₁ ≦ 15.0 μm and a weight ratio W to the toner of 0 are used. Using toner externally added in the range of 1 wt% ≦ W ≦ 3.0 wt%, the charging auxiliary fine particles are adhered to the surface of the charging roller 2 to create a minute gap at the contact portion between the charging roller 2 and the photosensitive drum 1. By using a small gap, AC discharge that contributes to charging can be achieved even under a low AC voltage, and AC charging that does not contribute to charging is suppressed, thereby charging ozone (O ₃ ), nitrogen oxides (NOx), etc. It became possible to suppress the product.

  Here, since the charging polarity of the auxiliary charging fine particles is opposite to that of the toner, the developing auxiliary device 4 develops the photosensitive drum 1 from the developing device 4 with a developing fog removal bias (Vback). Therefore, when an image with a relatively low image ratio is output, a large number of charging auxiliary fine particles are developed from the developing device 4 onto the photosensitive drum 1, whereas when the image ratio is high, the charging auxiliary fine particles are 4 is not developed so much on the photosensitive drum 1, and as a result, the amount of auxiliary charging fine particles adhering to the charging roller 2 becomes extremely small, and the effect may be reduced.

  Further, when the image ratio is biased in one image, a portion where a large number of charging auxiliary fine particles are attached and a portion where the charging roller 4 is hardly attached are formed in the main scanning direction of the charging roller 4.

  In the present embodiment, in order to solve the above-described problems, a timing for selectively developing the charging auxiliary fine particles from the developing device 4 to the photosensitive drum 1 is provided at any timing other than image formation.

  Specifically, after the image formation is completed, the high-pressure conditions of the respective image forming means disposed around the photosensitive drum 1 are maintained in the image forming state, and so-called white solid image formation is performed for about one rotation of the photosensitive drum 1. Then, the auxiliary charging fine particles are selectively developed from the developing device 4 onto the photosensitive drum 1, and the position on the photosensitive drum 1 where the development is completed (the last position where the auxiliary charging fine particles are developed on the photosensitive member 1) is charged. After passing through the roller 2, the stopping operation of each image forming means is started (see FIG. 9).

  By providing the timing for supplying the charging auxiliary fine particles to the charging roller 2 as described above, the charging auxiliary fine particles can be stably supplied to the charging roller 2 regardless of whether the image ratio is high or low and the image ratio is uneven. The effect similar to that of Form 1 can be stably supplied. Of course, the timing of supplying the charging auxiliary fine particles to the charging roller 2 may be during the start-up of the image forming apparatus, between image formation (between sheets), interruption every fixed number of sheets, or the like.

  As described above, according to the present embodiment, it becomes possible to adhere the charging auxiliary fine particles to the charging roller 2 stably and uniformly regardless of the output image ratio, the image bias, or the like. The effect can be realized stably.

According to the present invention, fine particles having a polarity opposite to the charging polarity of the toner and having a primary number average particle diameter D ₁ of 0.5 μm ≦ D ₁ ≦ 15.0 μm have a weight ratio W to the toner of 0.5. By using toner externally added in the range of 1 wt% ≦ W ≦ 3.0 wt%, fine particles selectively adhere to the surface of the contact charging device to create a minute gap at the contact portion between the contact charging device and the photoreceptor, Since the contact charging device serves as charging auxiliary fine particles that help to charge the surface of the photoconductor, it is possible to suppress discharge that does not contribute to charging of the surface of the photoconductor. As a result, in an image forming apparatus using a cleanerless system that can suppress the generation of charged products such as ozone (O ₃ ) and nitrogen oxide (NOx), and can significantly reduce the wear of the photoreceptor. In addition, it is possible to prevent image defects such as image negatives.

It is a model diagram showing a schematic configuration of an image forming apparatus having a conventional cleaning device. FIG. 10 is a model diagram showing the behavior of charging auxiliary fine particles when charging auxiliary fine particles are used in an image forming apparatus having a conventional cleaning device. 1 is a model diagram showing a schematic configuration of an image forming apparatus according to the present invention. FIG. 3 is a model diagram illustrating a layer configuration of a photosensitive drum and a charging roller provided in the image forming apparatus of FIG. 2. FIG. 6 is a model diagram showing the behavior of charging auxiliary fine particles when charging auxiliary fine particles are used in the image forming apparatus according to the present invention. FIG. 3 is an enlarged view of a contact portion between a charging roller and a photosensitive drum when charging auxiliary fine particles are not used. FIG. 3 is an enlarged view of a contact portion between a charging roller and a photosensitive drum when charging auxiliary fine particles are used. (A) is a relational diagram showing the relationship between the charging AC voltage and the charging AC current in a state where the charging auxiliary fine particles are not attached to the charging roller, and (b) is a state in which the charging auxiliary fine particles are attached to the charging roller. It is a relationship figure which shows the relationship between a charging alternating voltage and a charging alternating current. 6 is a timing chart showing the operation timing of image forming means around the photosensitive drum, which performs a mode in which charging auxiliary fine particles are attached to the charging roller.

Explanation of symbols

1 Photosensitive drum (image carrier)
2 Charging roller (charging means)
3 Laser beam scanner (exposure means)
4 Developing device (Developing means)
5 Transfer roller (transfer means)
6 Fixing Device 7 Toner Charge Amount Control Unit 8 Residual Toner Uniform Unit 21 Cleaning Unit

Claims (9)

An image carrier, a charging unit for charging the image carrier at a contact portion to form an electrostatic image on the image carrier, and an electrostatic latent image on the image carrier charged by the charging unit. An exposure means as an image writing means, a developing means for supplying toner to the electrostatic latent image formed on the image carrier for visualization, and a transfer means for transferring the visualized toner image to a transfer material. In addition, in an image forming apparatus using a cleanerless system in which the toner remaining on the surface of the image carrier without being transferred by the transfer unit is collected by the developing device,
An image forming apparatus, wherein fine particles having a polarity opposite to the charging polarity of the toner are externally added to the toner developed by the developing means. The image forming apparatus according to claim 1, wherein the primary number average particle diameter D _{1 of the} fine particles satisfies 0.5 μm ≦ D ₁ ≦ 15.0 μm. The weight ratio W of the fine particles to the toner is 0.
3. The image forming apparatus according to claim 1, wherein 1 wt% ≦ W ≦ 3.0 wt%.     The image forming apparatus according to claim 1, wherein the toner has a negative polarity. The image forming apparatus according to claim 1, wherein the fine particles are inorganic fine particles such as barium titanate (BaTiO ₃ ), strontium titanate (SrTiO ₃ ), and zinc oxide (ZnO).     The fine particles are organic fine particles such as polyamide (PA), polyimide (PI), polyamideimide (PAI), methyl methacrylate resin (PMMA), polyethylene (PE) and the like. The image forming apparatus described in 1.     The image forming apparatus according to claim 1, wherein a ten-point average roughness (Rz) of the surface of the charging unit is Rz ≦ 3 μm.     8. The operation according to claim 1, wherein the fine particles are selectively developed from the developing means to the image carrier and attached to the charging means at an arbitrary timing other than image formation. An image forming apparatus according to claim 1. The surface area shape sphericity Φs of the toner is 0.91 ≦ Φs ≦ 1.0, and the volume average particle diameter D _{4 of the} toner is 4.0 μm ≦ D ₄ ≦ 9.0 μm. The image forming apparatus according to claim 1.

Images