JP3576744B2 - Image forming method - Google Patents

Description

【００７７】
【表１】

【００７８】
表１に示すように、本実施例１では、全トナーと全キャリアの表面積の比Ｓを２．５以上として、電界強度２×１０^６ Ｖ／ｍにおける現像剤による電流値を２．３×１０−１０Ａ以下としたので、かぶりのない良好な画像を得ることができた。
【００７９】
比較例１
２成分現像剤のトナー粒径ｒｔ 、キャリア粒径ｒｃ およびトナー濃度ｎを変えた以外は実施例１と同様にして、所定電界強度における現像剤による電流値を測定し、また画像形成を行ないってかぶりの有無を評価した。
【００８０】
表１に示されるように、この比較例１では、全トナーと全キャリアの表面積の比Ｓが２．５未満であり、現像剤による電流値も比較例１−ｃを除き、２．５×１０^−１０ Ａを超えた。画像のかぶりが生じ、良好な品質の画像が得られなかった。
【００８１】
実施例２
本発明では、磁性キャリアとしては、たとえば表面酸化、未酸化の鉄、ニッケル、コバルト、マンガン、クロム、希土類などの金属からなるフェライト、あるいは、それらの酸化物からなるフェライトなどを用いることができ、その製法は問われない。磁性キャリアは周知の方法で樹脂被覆することができる。
【００８２】
本実施例では、ネオジウム、サマジウム、バリウム等を含むフェライト粒子に樹脂被覆した、重量平均粒径が２０〜１００μｍ、好ましくは２０〜７０μｍであり、５×１０^４ Ｖ／ｍの電界強度で１０^１２Ωｃｍ以上の比抵抗を有する磁性キャリアを用いた。
【００８３】
磁性キャリアの比抵抗（体積抵抗率）はセルに磁性キャリアを充填し、この充填したキャリアに接するように１対の電極の一方、他方を配し、これらの電極間に電圧を印加して、そのときに流れる電流を計測することにより測定した。比抵抗の測定条件は、充填したキャリアと電極の接触面積が約２．３ｃｍ^２ 、キャリア充填厚さが約２ｍｍ、上部電極の荷重が１８０ｇ、印加電圧が１００Ｖであった。この場合、磁性キャリアが粉末であるため充填率に変化が生じることがあり、それにともない比抵抗が変化するので、そうならないようにキャリアの充填に慎重を要する。
【００８４】
実施例１と同様、所定電界強度における現像剤による電流値を測定し、また画像形成を行ないってかぶりの有無を評価した。結果を表２に示す。
【００８５】
【表２】

【００８６】
表２に示されるように、実施例２では、５×１０^４ Ｖ／ｍの電界強度において１０^１２Ωｃｍ以上の比抵抗を有する磁性キャリアを用いたので、２×１０^６ Ｖ／ｍの電界強度における現像剤による電流値を２．３×１０^−１０ Ａ以下とすることができ、かぶりのない良好な画像を得ることができた。
【００８７】
比較例２
本比較例は、５×１０^４ Ｖ／ｍの電界強度における比抵抗が１０^１２Ωｃｍ未満の磁性キャリアを使用した以外、実施例２と同様にした。キャリアの比抵抗、所定電界強度における現像剤による電流値、および画像のかぶりの有無の結果を表２に示す。
【００８８】
表２に示されるように、比較例２では、画像のかぶりが生じ、良好な品質の画像が得られなかった。
【００８９】
【発明の効果】
以上説明したように、本発明の画像形成方法によれば、表面抵抗が１０⁹〜１０¹⁴Ωｃｍの低抵抗層を有する像担持体を使用したので、電荷注入式の接触帯電手段により効率良く像担持体を帯電でき、また帯電後の像担持体上に形成した静電潜像を２成分現像剤により現像するに際し、２×１０⁶Ｖ／ｍの電界強度において現像剤を通じて流れる電流値が２．３×１０^-10Ａ以下であり、磁性キャリアの抵抗はトナーの抵抗よりも小さく、磁性キャリアの抵抗が５×１０ ⁴ Ｖ／ｍの電界強度において１０ ¹² Ωｃｍ以上であるような電気的特性を有する現像剤を用いたので、潜像をかぶりを生じることなく現像して、高品質な画像を得ることができる。
【図面の簡単な説明】
【図１】本発明の画像形成方法の実施に使用する画像形成装置の一例を示す概略断面図である。
【図２】図１の画像形成装置に設置されたレーザスキャナ部を示す模式図である。
【図３】図１の画像形成装置に設置された２成分現像器を示す断面図である。
【図４】従来の画像形成方法で使用する画像形成装置の一例を示す概略断面図である。
【符号の説明】
１ 感光ドラム
４ ２成分磁気ブラシ現像器
５ クリーナ
１１ 現像スリーブ
１２ マグネットローラ
１９ ２成分現像剤
３１ 磁気ブラシ帯電器
７１ 転写ベルト[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an image forming method for developing an electrostatic latent image formed on an image carrier with a two-component developer to obtain an image.
[0002]
[Prior art]
Conventionally, an image forming method is known in which an image carrier is charged and exposed to form an electrostatic latent image, and the latent image is developed with a two-component developer to obtain an image. A conventional image forming method will be briefly described using the image forming apparatus (copier) of FIG. 4 as an example.
[0003]
When the original G is placed on the original plate 0 of the copying machine shown in FIG. 4 with the surface to be copied facing downward and the copy button is pressed, copying of the original G, that is, image formation is started. The copying machine has an optical scanning unit 9 in which a document irradiating lamp, a short focus lens array, and a CCD sensor are integrated. When the copy button is pressed, the unit 9 irradiates the document G with the irradiating lamp. While scanning, the reflected light of the irradiation light from the original surface forms an image by the short focus lens array and is incident on the CCD sensor.
[0004]
The CCD sensor is composed of a light receiving section, a transfer section and an output section. The light receiving section converts an incident light signal into a charge signal, and the transfer section sequentially transfers the charge signal to an output section in synchronization with a clock pulse and outputs the signal. The section converts the charge signal into a voltage signal, amplifies the signal, reduces the impedance, and outputs the signal. The image signal (analog signal) thus obtained is converted into a digital signal by well-known image processing, and then sent to a printer unit of a copying machine.
[0005]
In the printer section, first, an electrostatic latent image is formed on the surface of the photosensitive drum 1. The photosensitive drum 1 is driven to rotate around a center support shaft at a predetermined peripheral speed, and is subjected to a uniform charging process by a corona charger 3 so that the surface becomes, for example, -650 V during the rotation process. Next, the solid-state laser element of the laser scanning unit 100 receives the image signal (digital signal), generates a laser beam by ON / OFF light emission, and uses a rotating polygon mirror to apply a laser beam L to the surface of the photosensitive drum 1. Scanning is performed, and an electrostatic latent image corresponding to a document image is sequentially formed on the surface of the photosensitive drum 1.
[0006]
When this electrostatic latent image is developed by the developing device 4, it is visualized as a toner image on the photosensitive drum 1.
[0007]
In general, there are roughly four types of developing methods. One of them is to coat a non-magnetic toner of a one-component developer on a developing sleeve (developer carrying member) with a blade or the like, and then apply the non-magnetic toner to the developing sleeve. A non-magnetic one-component non-contact development method in which the toner is conveyed to a photosensitive drum and developed in a non-contact state with respect to the photosensitive drum. The second method is to coat a magnetic toner of a one-component developer on a developing sleeve by a magnetic force. Similarly, it is a magnetic one-component contact developing method in which the toner is conveyed to a photosensitive drum and developed by contact. The third and fourth uses a two-component developer in which a non-magnetic toner and a magnetic carrier are mixed, coats this on a developing sleeve by magnetic force, and conveys it to the photosensitive drum by the developing sleeve. A two-component contact development method (two-component magnetic brush development method) in which development is performed in a contact state, and a two-component non-contact development method in which development is performed in a non-contact state with respect to a photosensitive drum. The two-component magnetic brush development method is often used from the viewpoint of high image quality and high stability.
[0008]
The developing device 4 is configured as a two-component magnetic brush developing device. The developing device 4 includes a developing container 16 containing a two-component developer, and carries the developer in an opening of the developing container 16 facing the photosensitive drum 1 and conveys the developer to a developing unit facing the photosensitive drum 1. A developing sleeve 11 is provided. By setting the closest region between the developing sleeve 11 and the photosensitive drum 1 to a distance of about 500 μm, development is performed so that the developer conveyed to the developing unit is in contact with the photosensitive drum 1. . A magnet roller (not shown) is fixedly arranged in the developing sleeve 11, and a regulating blade (not shown) is arranged for the developing sleeve 11.
[0009]
As the toner of the developer used in the two-component developing method as described above, for example, a negatively charged toner having an average particle diameter of about 6 μm manufactured by a pulverization method and a titanium oxide having an average particle diameter of about 20 nm are used in a weight ratio. In this case, a toner externally added by about 1% is used. The carrier of the developer 19 has, for example, a saturation magnetization of 205 emu / cm.³  A magnetic ferrite carrier having an average particle size of about 35 μm is used. The developer is used at a mixing ratio of the toner and the carrier of 6:94 by weight.
[0010]
In the developing device 4, the developer in the developing container 16 is pumped up onto the developing sleeve 11 by the magnetic poles of the magnet roller with the rotation of the developing sleeve 11, and is conveyed toward the developing section facing the photosensitive drum 1. In this transporting process, the thickness of the developer is regulated by the regulating blade and is formed on the developing sleeve 11 as a thin layer of the developer. When the developer thin layer is conveyed to the developing main pole located at the developing portion of the magnet roller, the developer thin layer is raised toward the photosensitive drum 1 by the magnetic force of the developing main pole to form a magnetic brush. Contacts the surface of the photosensitive drum 1 to develop the electrostatic latent image on the photosensitive drum 1, and the electrostatic latent image is visualized as a toner image.
[0011]
During this development, a developing bias in which a DC voltage and an AC voltage are superimposed from a bias power supply (not shown), for example, a DC voltage of -500 V, an AC voltage having a peak-to-peak voltage Vpp of 1500 V and a frequency f of 2000 Hz is applied to the developing sleeve 11. A voltage is applied. In general, in the two-component magnetic brush development method, when an AC voltage is applied, the development efficiency increases and the image becomes high-quality, but fogging easily occurs. By providing a potential difference, fog is prevented.
[0012]
The toner image formed on the photosensitive drum 1 by the development is electrostatically transferred by the transfer charger 8 onto the transfer material P conveyed to the photosensitive drum 1. The transfer material P to which the toner image has been transferred is electrostatically separated from the photosensitive drum 1 by the separation charger 7 and then conveyed to the fixing device 6, where the transfer material P is heated and pressed to fix the image, and the print image is fixed. Is output outside the image forming apparatus. After the transfer of the toner image, the cleaner 5 removes contaminants such as untransferred toner adhered to the surface of the photosensitive drum 1 and electrically initializes the surface of the photosensitive drum 1 by the pre-exposure lamp 2. Used for forming.
[0013]
[Problems to be solved by the invention]
By the way, with the recent increase in environmental awareness, a method of directly charging the photosensitive drum using a contact charging member without using corona discharge for charging the photosensitive drum 1 is being used. Above all, the injection charging method is extremely excellent since the discharge amount when charging the photosensitive drum is extremely small.
[0014]
The injection charging means that a charge is directly injected by a contact charging member to a trap potential of the material on the surface of the photosensitive drum, or a charge injection layer in which conductive particles are dispersed is provided on the surface of the photosensitive drum. Means charging and charging the conductive particles with a contact charging member. This injection charging method reduces the surface resistance of the photosensitive drum by 10⁹  -10¹⁴It has been found that when the resistance is as low as about Ωcm, the charging efficiency is improved. It should be noted that even a photosensitive drum such as an amorphous silicon drum aiming at high durability has such a surface resistance.
[0015]
However, the surface resistance of the photosensitive drum is set to 10⁹  -10¹⁴When it is set to about Ωcm, there is a problem that image fogging occurs. According to the study of the present inventors, this fog is caused by the surface resistance of the photosensitive drum being 10%.⁹  -10¹⁴It has been found that this is particularly likely to occur when a two-component contact developing method is carried out with a low resistance of Ωcm.
[0016]
An object of the present invention is to obtain a high-quality image by efficiently charging a photosensitive drum and developing an electrostatic latent image formed on the charged photosensitive drum by a two-component developer without causing fogging. It is an object of the present invention to provide an image forming method which is enabled.
[0017]
[Means for Solving the Problems]
The above object is achieved by an image forming method according to the present invention. In summary, the present invention provides 10⁹-10¹⁴An image carrier having a low resistance layer of Ωcm on its surface is charged and exposed to form an electrostatic latent image, and the latent image contains a toner and a magnetic carrier carried on a developer carrier having a built-in magnet. In an image forming method in which an image is developed by applying a developing bias including a DC voltage to obtain an image in a developing unit in which the image carrier and the developer carrier are opposed to each other by using a two-component developer, 10⁶The current flowing through the developer at an electric field strength of V / m is 2.3 × 10^-TenA or lessWherein the resistance of the magnetic carrier is smaller than the resistance of the toner, and the resistance of the magnetic carrier is 5 × 10 ^Four 10 at an electric field strength of V / m ¹² Ωcm or moreAn image forming method using a developer having such electrical characteristics.
[0018]
According to the present invention, preferably, the average particle diameter of the toner of the developer is rt (μm), and the bulk density is ρt (g / cm).^Three), The average particle size of the magnetic carrier is rc (μm), and the bulk density is ρc (g / cm).^Three), And the toner concentration is n (wt%),
S = {n / (100−n)} × (rc / rt) × (ρc / ρt)
S is 2.5 or moreIs done.
[0019]
The image carrier can be charged by a contact charging member. The contact charging member may be a sleeve containing a magnet holding magnetic particles in contact with the image carrier, and the magnetic particles are rotated by rotation of the sleeve, and a voltage is applied to the sleeve to apply a voltage to the image carrier. The image carrier can be charged by injecting charges into the image carrier. The toner has a shape factor SF-1 of 100 to 130 and an SF-2 of 100 to 115, and a part or all of the toner can be formed by a polymerization method.
[0020]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0021]
According to the image forming method of the present invention, as in the related art, a two-component developer containing a toner and a carrier is carried on the surface of a developing sleeve (developer carrying member) in which a magnet is disposed, and is conveyed to a developing unit. A two-component contact developing method is used in which a magnetic brush of a developer is formed on a developing portion by a developing magnetic pole of a magnet to develop an electrostatic latent image on a photosensitive drum (image carrier).
[0022]
In the present invention, the surface resistance is 10⁹  -10¹⁴A photosensitive drum having a low resistance layer of Ωcm is used, and 2 × 10⁶  The current flowing through the developer at an electric field strength of V / m is 2.3 × 10^-10  It is a great feature to use a developing agent having A or less.
[0023]
That is, the present invention reduces the surface resistance of the photosensitive drum to 10⁹  -10¹⁴It is summarized that the charging efficiency of the photosensitive drum is improved by making the resistance as low as about Ωcm, and a high-quality image without fog is obtained by using a developer having a high resistance.
[0024]
Hereinafter, examples of the present invention will be described.
[0025]
Example 1
FIG. 1 is a schematic configuration diagram showing an example of an image forming apparatus used to carry out the image forming method of the present invention.
[0026]
The image forming apparatus (copier) includes a printer unit having a photosensitive drum 1, a developing unit 4, a laser scanning unit 100, and the like below an exposure unit having a document table 10 and an optical scanning unit 9. When the original G is placed on the table 10 with the surface to be copied facing downward and the copy button is pressed, copying of the original G, that is, image formation is started.
[0027]
The light scanning unit 9 is configured by integrally incorporating a document irradiation lamp, a short focus lens array, and a CCD sensor. When the copy button is pressed, the unit 9 irradiates the document G with the irradiation lamp. After scanning, the reflected light of the irradiation light from the document surface forms an image by the short focus lens array and is incident on the CCD sensor.
[0028]
The CCD sensor is composed of a light receiving section, a transfer section and an output section. The light receiving section converts an incident light signal into a charge signal, and the transfer section sequentially transfers the charge signal to an output section in synchronization with a clock pulse and outputs the signal. The section converts the charge signal into a voltage signal, amplifies the signal, reduces the impedance, and outputs the signal. The image signal (analog signal) thus obtained is converted into a digital signal by well-known image processing, and then sent to a printer unit.
[0029]
In the printer section, first, an electrostatic latent image is formed on the surface of the photosensitive drum 1. The photosensitive drum 1 is driven to rotate around a center support shaft at a predetermined peripheral speed, and is subjected to a uniform charging process by a charger 31 so that the surface thereof becomes, for example, -650 V during the rotation process. Next, the solid-state laser element of the laser scanning unit 100 receives the above image signal (digital signal), generates a laser beam L by ON / OFF light emission, and uses a rotary polygon mirror to irradiate the surface of the photosensitive drum 1 with the laser beam. Scanning is performed, and an electrostatic latent image corresponding to a document image is sequentially formed on the surface of the photosensitive drum 1.
[0030]
FIG. 2 shows a schematic configuration of a laser scanning unit 100 including the above-described solid-state laser element. In the laser scanning unit 100, first, the solid-state laser element 102 is caused to blink at a predetermined timing by the light emission signal generator 101 based on the input image signal. The laser light emitted from the solid-state laser element 102 in this manner is converted into a substantially parallel light beam by the collimator lens system 103, and is further scanned in the direction of the arrow c by the rotating polygon mirror 104 rotating in the direction of the arrow b. The lens group 105a, 105b, 105c forms a spot-like image on the scanned surface 106 of the photosensitive drum 1. By such scanning of the laser beam, an exposure distribution for one scan of the image is formed on the surface to be scanned 106, and the surface to be scanned 106 is scrolled by a predetermined amount in a direction perpendicular to the scanning direction for each scan. As a result, an exposure distribution according to the image signal is obtained on the scanned surface 106. That is, an electrostatic latent image is formed.
[0031]
In the present invention, a commonly used organic photoreceptor or the like can be used as the photosensitive drum 1;⁹  -10¹⁴A material provided with a surface layer of a material having a resistance of Ωcm, an amorphous silicon photoconductor, or the like can be used. According to the photosensitive drum having such a surface layer or the like, it is possible to carry out direct charging by a charge injection method, which is effective in preventing generation of ozone and reducing power consumption.
[0032]
In this embodiment, the photosensitive drum 1 is a negatively chargeable organic photosensitive member, and is provided with five first to fifth photosensitive layers from below on an aluminum drum base having a diameter of 30 mm.
[0033]
The first layer, which is the lowermost layer of the photoreceptor layer, is an undercoat layer provided for absorbing defects and the like of the aluminum substrate, and is formed of a conductive layer having a thickness of 20 μm. The second layer thereon is a positive charge injection preventing layer, which serves to prevent positive charges injected from the aluminum substrate from canceling out negative charges charged on the surface of the photoreceptor layer. The positive charge injection preventing layer is made of an amylan resin and methoxymethylated nylon,⁶  It is formed on a medium resistance layer whose resistance is adjusted to about Ωcm.
[0034]
The third layer is a charge generation layer, which is a layer having a thickness of about 0.3 μm in which a disazo pigment is dispersed in a resin, and generates positive and negative charge pairs upon exposure to light. The fourth layer is a charge transport layer, which is formed on a P-type semiconductor in which hydrazone is dispersed in a polycarbonate resin. Therefore, the negative charges charged on the photosensitive drum surface cannot move through the charge transport layer, and only the positive charges generated in the charge generation layer can be transported to the photosensitive drum surface.
[0035]
The fifth layer, which is the outermost layer, is a charge injection layer.₂  Is formed from a coating layer of a material in which is dispersed. More specifically, SnO having a particle diameter of 0.03 μm, which is doped with antimony, which is a light-transmitting conductive filler, and has a reduced resistance (conducted).₂  The particles are dispersed in an insulating resin at 70% by weight to prepare a coating liquid, which is applied to a thickness of about 3 μm by an appropriate coating method such as a dipping method, a spray method, a roll coating method, and a beam coating method. It was done.
[0036]
In the present embodiment, a contact charger of a charge injection system, particularly a charger of a magnetic brush charging system, is used as the charger 31. The magnetic brush charger 31 has a magnet 31b fixedly and non-rotatably installed inside a rotatable non-magnetic sleeve 31a having an outer diameter of 16 mm. The magnetic brush is raised to form a magnetic brush, and the magnetic brush contacts the surface of the photosensitive drum 1. The magnetic particles, which have been made into a magnetic brush, are conveyed by the rotation of the non-magnetic sleeve 31a, and charge is applied to the photosensitive drum 1 through the magnetic particles by applying a charging voltage to the non-magnetic sleeve 31a. Is charged to a potential corresponding to the charging voltage.
[0037]
The charging of the photosensitive drum 1 by the magnetic brush of the magnetic particles is performed by adjusting the nip width of the contact portion of the magnetic brush with the photosensitive drum 1 to about 6 mm, and changing the charging bias applied to the nonmagnetic sleeve 31a to a DC voltage of -700V. Satisfactory implementation was achieved by using a bias in which alternating voltages of a rectangular wave having a frequency of 1000 Hz and an amplitude of 800 V were superimposed.
[0038]
The rotation direction of the non-magnetic sleeve 31 (therefore, the rotation direction of the magnetic brush) is preferably a counter direction in the direction B with respect to the photosensitive drum 1 rotating in the direction A, as shown in FIG. Become good. Also, the higher the rotation speed of the non-magnetic sleeve 31a, the more the charging of the photosensitive drum 1 tends to be uniform. In this embodiment, the rotation speed of the non-magnetic sleeve 31a is set to 150 mm / sec with respect to the rotation speed of the photosensitive drum 1 of 100 mm / sec.
[0039]
The magnetic particles (magnetic carrier) used in the magnetic brush charger 31 have an average particle diameter of 10 to 100 μm and a saturation magnetization of 20 to 250 emu / cm.³  , Resistance is 10²  -10¹⁰Ωcm is preferable, and considering the case where an insulation defect such as a pinhole is present in the photosensitive drum 1, a more preferable resistance is 10Ω.⁶  -10¹⁰It is about Ωcm. In this embodiment, the average particle size is 25 μm, and the saturation magnetization is 200 emu / cm.³  , Resistance is 5 × 10⁶  Ωcm magnetic particles were used.
[0040]
The resistance value of the above magnetic particles is such that the low area is 228 mm.²  2 g of magnetic particles in a metal cell of 6.6 kgf / cm²  The measurement was performed by applying a voltage of 100 V while pressing with a load of.
[0041]
Magnetite is dispersed as a magnetic material in the resin, and carbon black is dispersed for conductivity and resistance adjustment. The one that has been adjusted, the one that has been subjected to resistance adjustment by coating the surface of a single magnetite with a resin, or the like is used.
[0042]
The electrostatic latent image formed on the photosensitive drum 1 is developed by a two-component developing device 4 installed around the photosensitive drum 1 and is visualized as a toner image.
[0043]
FIG. 3 is a schematic configuration diagram of the developing device 4. The developing device 4 is configured as a two-component magnetic brush developing device. The developing device 4 includes a developing container 16 containing a two-component developer 19 containing a toner and a magnetic carrier. The developer 19 is carried in an opening of the developing container 16 facing the photosensitive drum 1. A developing sleeve 11 that conveys the developing unit 11 to the developing unit facing the photosensitive drum 1 is installed, and the developing sleeve 11 is rotated in a direction in which the facing unit to the photosensitive drum 1 moves in the same direction. By setting the closest region between the developing sleeve 11 and the photosensitive drum 1 to a distance of about 500 μm, it is set so that the developer 19 conveyed to the developing section can be developed while being in contact with the photosensitive drum 1. I have.
[0044]
A magnet roller 12 is fixedly arranged in the developing sleeve 11, and a regulating blade 15 made of a magnetic material is arranged perpendicular to the top of the developing sleeve 11. Further, developer stirring and conveying screws 13 and 14 are provided in the developing container 16.
[0045]
The electrostatic latent image formed on the photosensitive drum 1 is developed by the two-component magnetic brush developing method using the developing device 4 as follows. First, the developing sleeve 11 is rotated. With the rotation, the developer 19 in the developing container 16 is pumped onto the developing sleeve 11 by the magnetic pole N3 of the magnet roller 12, and the pumped developer is transferred from the magnetic pole S2 to N1. Transported to During this transport process, the thickness of the developer is regulated by the regulating blade 15 and is formed on the developing sleeve 11 as a thin developer layer. When the developer thin layer is conveyed to the developing main pole S1 of the magnet roller 12, the developer thin layer is raised toward the photosensitive drum 1 by the magnetic force of the developing main pole S1 (in FIG. (Indicated by a parallel line extending from the developing sleeve 11 to the photosensitive drum 1 at the portion). The ears of the developer (magnetic brush) come into contact with the surface of the photosensitive drum 1 to develop the electrostatic latent image on the photosensitive drum 1, and the electrostatic latent image is visualized as a toner image.
[0046]
At the time of this development, a developing bias in which a DC voltage and an AC voltage are superimposed is applied from a bias power supply 17 to the developing sleeve 11 and the photosensitive drum 1. For example, a DC voltage of -500 V, an AC voltage having a peak-to-peak voltage Vpp of 1500 V and a frequency f of 2000 Hz. In general, in the two-component magnetic brush developing method, when an AC voltage is applied, the developing efficiency is increased and the image is of high quality, but fogging is liable to occur. Therefore, fog is prevented by providing a potential difference between the DC voltage applied to the developing sleeve 11 and the surface potential of the photosensitive drum 1.
[0047]
The toner image formed on the photosensitive drum 1 by the development is transferred onto the transfer material P conveyed from the paper feed cassette 80. On the lower side of the photosensitive drum 1, a transfer belt 71 that is wound around a driving roller 72 and a driven roller 73 and that rotates in the direction of arrow D in FIG. The transfer material P is taken out from the paper feed cassette 80 and fed onto the transfer belt 71 at an appropriate timing in synchronization with the rotation of the photosensitive drum 1, and the photosensitive drum 1 and the transfer belt 71 abut at a predetermined timing. Conveyed to the transferred transfer section. A transfer charging blade 74 is provided inside the transfer portion of the transfer belt 71, and the transfer charging blade 74 presses the transfer belt 71 in the direction of the photosensitive drum 1 and supplies power to the transfer charging blade 74 from a high-voltage power supply (not shown). As a result, the transfer material P is charged from the back side with a polarity opposite to that of the toner, and the toner image formed on the photosensitive drum 1 is electrostatically transferred onto the transfer material P.
[0048]
In this embodiment, a 75 μm-thick polyimide resin sheet is used as the transfer belt 71. As the transfer belt 71, a resin sheet such as a polycarbonate resin, a polyethylene terephthalate resin, a polyvinylidene fluoride resin, a polyethylene naphthalate resin, a polyetheretherketone resin, a polyethersulfone resin, a polyurethane resin, or a fluorine-based or silicone resin A system rubber sheet can be suitably used. The thickness of the transfer belt 71 is not limited to 75 μm, but may be about 25 to 2000 μm, preferably 50 to 150 μm.
[0049]
The transfer charging blade 74 has a resistance of 10⁵  -10⁷  Ω, a thickness of 2 mm and a length of 306 mm were used. At the time of the transfer, the current applied to the transfer charging blade 74 was +15 μA, and the power was supplied by controlling the constant current.
[0050]
As described above, the transfer material P to which the toner image has been transferred is separated from the transfer belt 71, and then conveyed to the fixing device 6, where the transfer material P is heated and pressed to fix the image, thereby obtaining a print image. It is output outside the image forming apparatus. After the transfer of the toner image, the photosensitive drum 1 removes contaminants such as toner remaining on the surface of the photosensitive drum 1 by a cleaner 5 and is repeatedly used for image formation.
[0051]
The image formation is carried out through the above steps. In the present invention, 2 × 10⁶  The current flowing through the developer at an electric field strength of V / m is 2.3 × 10^-10  It is a great feature to use a developer having an electrical characteristic (resistance) such as A or less.
[0052]
The electrical characteristics of the developer 19 are 2 × 10⁶  2.3 × 10 at a given electric field strength of V / m^-10  A current value out of the range of A or less, that is, 2.3 × 10^-10  When a characteristic having a current value exceeding A is applied, a DC voltage (developing sleeve potential Vdc) of a developing bias applied to the developing sleeve 11 of the developing device 4 at the time of development is injected into the photosensitive drum 1 through the developer, and The potential of the drum converges to the developing sleeve potential Vdc, and the fogging voltage Vback on a white background portion on the photosensitive drum 1 becomes small, so that fogging cannot be prevented.
[0053]
In order to measure the current value by the developer at the above-mentioned predetermined electric field strength, the measurement is performed by setting conditions simulating the development state in the developing device in FIG. That is, instead of the photosensitive drum 1, a conductive cylinder made of aluminum having a diameter of 30 mm and a length of 300 mm is arranged, and a developing sleeve 1 having a diameter of 16 mm and a length of 300 mm is loaded on the aluminum cylinder to carry a developer. With the developing sleeve and the aluminum cylinder stationary without rotating, apply a voltage of 2000 V between them and measure the value of the current flowing between them at that time. It is obtained by doing. However, the current component due to the charge transfer accompanying the movement of the toner is not included, and this is subtracted from the measured value.
[0054]
In the present invention, the magnetic carrier used for the two-component developer 19 is preferably a carrier having a small particle diameter of 100 μm or less, and more preferably a carrier having a number average molecular weight of 10 to 60 μm. The average particle size of the magnetic carrier is indicated by the maximum length in the vertical direction. In the present invention, the carrier is photographed with a microscope at a magnification of 50 to 1000 times, and 3,000 or more particles are obtained from the carrier particles in the obtained photographic image. Were randomly selected, their major axes were measured, and the arithmetic average was calculated.
[0055]
As the toner used in the developer together with the magnetic carrier, a conventionally known toner can be used. In the present invention, the shape factor SF-1 is particularly 100 to 130, and the shape factor SF-2 is 100 to 115. Are preferred.
[0056]
The shape factors SF-1 and SF-2 were randomly sampled from 100 toner particle images using FE-SEM (S-800) manufactured by Hitachi, Ltd., and the image information was manufactured by Nicole Corporation via an interface. This is defined as a value obtained from the following equation by introducing it into an image analyzer (Luzex3) and analyzing it.
[0057]
SF-1 = ｛(MXLNG)²  / AREA ×× (π / 4) × 100
SF-2 = ｛(PERI)²  / AREA ×× (π / 4) × 100
However, AREA: toner projection area, MXLNG: absolute maximum length,
PERI: circumference
[0058]
The shape factor SF-1 indicates the degree of spherical shape of the toner. If the shape factor SF-1 is larger than 140, the shape of the toner gradually changes from spherical to irregular. SF-2 indicates the degree of unevenness of the toner, and if it is larger than 120, the unevenness of the toner surface becomes remarkable.
[0059]
The expected effect of the shape of the toner is to minimize the influence of the magnetic brush charger 31 for charging the photosensitive drum 1 on the toner surface, thereby suppressing the generation of reactive low molecular weight components in the toner. is there. Therefore, the shape of the toner is preferably spherical, which can minimize the surface area.
[0060]
The effect of the present invention can be enhanced by using a toner partially or entirely formed by a polymerization method. In a toner having a surface portion formed by a polymerization method, the surface portion is present as a pre-toner (monomer composition) particle in a dispersion medium, and polymerization is generated, so that a considerably smooth surface can be obtained. If the shape factor SF-1 of the toner exceeds 140 or SF-2 exceeds 120, fogging of an image may increase or durability of the toner may slightly deteriorate.
[0061]
In the present invention, a toner having a core / shell structure can be suitably used. The toner having the core / shell structure and the shell portion formed by the polymerization method has an advantage that the production is easy. The effect of the core / shell structure is, needless to say, that the anti-blocking property can be imparted without impairing the excellent fixability of the toner.
[0062]
The volume average particle diameter of the toner is preferably 4 to 15 μm. The volume average particle diameter of the toner can be measured, for example, by the following measurement method.
[0063]
A call counter TA-II type (manufactured by Coulter) is used as a measuring device, and an interface (manufactured by Nikkaki) for outputting a number average distribution and a volume average distribution and a CX-i personal computer (manufactured by Canon) are connected to this. As the electrolytic solution, a 1% NaCl aqueous solution is prepared using sodium chloride (reagent first grade).
[0064]
0.1 to 5 ml of a surfactant (preferably an alkylbenzene sulfonate) is added as a dispersant to 100 to 150 ml of the above electrolyte solution, and 0.5 to 50 mg of a toner of a measurement sample is further added and suspended. After the electrolytic solution in which this sample was suspended was subjected to dispersion treatment for about 1 to 3 minutes using an ultrasonic disperser, the particle size distribution of toner particles of 2 to 40 μm was measured using the above-mentioned call counter TA-II using a 100 μm aperture. Measurement is performed to determine the volume distribution of the toner. The volume average particle diameter of the toner is obtained from the thus obtained volume distribution of the toner.
[0065]
It is preferable that the surface of the toner is coated with an external additive so that the influence of the charger 31 is released to the external additive to some extent.
[0066]
The external additive used in the present invention preferably has a particle size of 1/10 or less of the weight average particle size of the toner particles from the viewpoint of durability when added to the toner. The particle size of the external additive means an average particle size obtained by observing the surface of the toner particles with a microscope. The external additive is used in an amount of 0.01 to 10 parts by weight, preferably 0.05 to 5 parts by weight, based on 100 parts by weight of the toner.
[0067]
Examples of the external additives include the following. Metal oxides such as aluminum oxide, titanium oxide, strontium titanate, cerium oxide, magnesium oxide, chromium oxide, tin oxide, and zinc oxide; nitrides such as silicon nitride; carbides such as silicon carbide; calcium sulfate, barium sulfate, and carbonic acid Metal salts such as calcium; fatty acid metal salts such as zinc stearate and calcium stearate; carbon black; These external additives may be used alone or in combination. Preferably, those subjected to a hydrophobic treatment are used.
[0068]
In the present invention, a two-component developer in which the toner and the carrier are mixed as described above is used, and it is preferable that the surface area ratio of the entire toner and the entire carrier in the developer is 2.5 or more.
[0069]
The average particle size of the toner is rt (μm), and the bulk density is ρt (g / cm).³  ), The average particle size of the carrier is rc (μm), and the bulk density is ρc (g / cm).³  ), And assuming that the toner concentration is n (wt%),
S = {n / (100−n)} × (rc / rt) × (ρc / ρt) ≧ 2.5
It is.
[0070]
S is the ratio of the surface area of all toners to the surface area of all carriers when the toner concentration is n (wt%). Generally, since the resistance of the toner is higher than the resistance of the carrier, the resistance of the developer can be increased as the value of S increases. Therefore, as the S value increases, the current value of the developer at the above-described predetermined electric field intensity can be reduced. If the S value is less than 2.5, it is difficult to reduce the current value, and therefore, the S value is preferably 2.5 or more.
[0071]
Hereinafter, specific examples of the present invention will be described.
[0072]
Example 1
In this embodiment, the bulk density ρt of the toner is set to 1.1 g / cm.³  And the bulk density ρc of the carrier is 5.0 g / cm.³  , The surface area ratio S of all toners and all carriers was controlled to 2.5 or more according to the present invention. The current value of the developer by the developer at a predetermined electric field strength was measured by the method described above, and an image was formed by the image forming apparatus shown in FIG. 1 using the developer for development. evaluated. Table 1 shows the results.
[0073]
In this embodiment, the bulk density ρt of the toner is set to 1.1 g / cm.³  And the bulk density ρc of the carrier is 5.0 g / cm.³  , And the current value due to the developer at a predetermined electric field intensity was measured by the method described above. An image is formed by the image forming apparatus shown in FIG. 1 using the toner and the developer with the carrier. The presence or absence of fogging was evaluated. Table 1 shows the results.
[0074]
The bulk density of the toner and the carrier was measured by a method according to the method described in JIS K5101. Using a densitometer TC-DS (TOKYO DENSHOKU CO., LTD.) As a reflection densitometer, the reflection density of the transfer paper before image formation and the reflection density of the fog portion on the transfer paper after image formation are measured. The concentration was determined as follows.
[0075]
Fog density (%) = reflection density (fogged portion on transfer paper)-reflection density (transfer paper)
[0076]
The fogging criteria were as follows:

[0077]
[Table 1]

[0078]
As shown in Table 1, in the first embodiment, the ratio S of the surface area of all the toner and the entire carrier is set to 2.5 or more, and the electric field strength is 2 × 10⁶  Since the current value of the developer at V / m was 2.3 × 10 −10 A or less, a good image without fog could be obtained.
[0079]
Comparative Example 1
In the same manner as in Example 1 except that the toner particle size rt, the carrier particle size rc, and the toner concentration n of the two-component developer were changed, the current value of the developer at a predetermined electric field intensity was measured, and image formation was performed. The presence or absence of fogging was evaluated.
[0080]
As shown in Table 1, in Comparative Example 1, the ratio S of the surface areas of all toners and all carriers was less than 2.5, and the current value due to the developer was 2.5 ×, except for Comparative Example 1-c. 10^-10  A exceeded. Image fogging occurred, and good quality images could not be obtained.
[0081]
Example 2
In the present invention, as the magnetic carrier, for example, surface oxidation, unoxidized iron, nickel, cobalt, manganese, chromium, ferrite composed of a metal such as a rare earth, or ferrite composed of an oxide thereof can be used. The manufacturing method is not asked. The magnetic carrier can be coated with a resin by a known method.
[0082]
In this embodiment, the ferrite particles containing neodymium, samarium, barium and the like are coated with a resin, the weight average particle diameter is 20 to 100 μm, preferably 20 to 70 μm, and 5 × 10⁴  10 at V / m electric field strength¹²A magnetic carrier having a specific resistance of Ωcm or more was used.
[0083]
The specific resistance (volume resistivity) of the magnetic carrier is determined by filling a cell with a magnetic carrier, disposing one or the other of a pair of electrodes so as to be in contact with the filled carrier, and applying a voltage between these electrodes. It was measured by measuring the current flowing at that time. The measurement condition of the specific resistance is that the contact area between the filled carrier and the electrode is about 2.3 cm.²  The carrier filling thickness was about 2 mm, the load on the upper electrode was 180 g, and the applied voltage was 100 V. In this case, since the magnetic carrier is a powder, the filling rate may change, and the specific resistance changes accordingly. Care must be taken in filling the carrier to prevent such a change.
[0084]
As in Example 1, the current value of the developer at a predetermined electric field strength was measured, and image formation was performed to evaluate the presence or absence of fog. Table 2 shows the results.
[0085]
[Table 2]

[0086]
As shown in Table 2, in Example 2, 5 × 10⁴  10 at an electric field strength of V / m¹²Since a magnetic carrier having a specific resistance of Ωcm or more was used, 2 × 10⁶  The current value due to the developer at an electric field strength of V / m is 2.3 × 10^-10  A or less, and a good image without fog could be obtained.
[0087]
Comparative Example 2
This comparative example is 5 × 10⁴  The specific resistance at an electric field strength of V / m is 10¹²Example 2 was repeated except that a magnetic carrier of less than Ωcm was used. Table 2 shows the results of the specific resistance of the carrier, the current value by the developer at a predetermined electric field strength, and the presence or absence of image fog.
[0088]
As shown in Table 2, in Comparative Example 2, image fogging occurred, and an image of good quality could not be obtained.
[0089]
【The invention's effect】
As described above, according to the image forming method of the present invention, the surface resistance is 10⁹-10¹⁴Since an image carrier having a low-resistance layer of Ωcm was used, the image carrier can be efficiently charged by a charge injection type contact charging means, and a two-component electrostatic latent image formed on the charged image carrier was used. When developing with a developer, 2 × 10⁶The current flowing through the developer at an electric field strength of V / m is 2.3 × 10^-TenA or lessThe resistance of the magnetic carrier is smaller than the resistance of the toner, and the resistance of the magnetic carrier is 5 × 10 ^Four 10 at an electric field strength of V / m ¹² Has electrical characteristics such as Ωcm or moreSince the developer is used, the latent image can be developed without causing fogging, and a high-quality image can be obtained.
[Brief description of the drawings]
FIG. 1 is a schematic sectional view showing an example of an image forming apparatus used for carrying out an image forming method of the present invention.
FIG. 2 is a schematic diagram illustrating a laser scanner unit installed in the image forming apparatus of FIG.
FIG. 3 is a sectional view showing a two-component developing device installed in the image forming apparatus of FIG.
FIG. 4 is a schematic sectional view showing an example of an image forming apparatus used in a conventional image forming method.
[Explanation of symbols]
1 Photosensitive drum
4 Two-component magnetic brush developer
5 Cleaner
11 Developing sleeve
12 Magnet roller
19 Two-component developer
31 Magnetic brush charger
71 Transfer belt

Claims (6)

An image carrier having a low resistance layer of 10 ⁹ to 10 ¹⁴ Ωcm on its surface is charged and exposed to form an electrostatic latent image, and the latent image is formed on a developer carrier having a built-in magnet. An image forming method for developing an image by applying a developing bias including a DC voltage to a developing unit in which an image carrier and a developer carrier are opposed to each other with a two-component developer containing a magnetic carrier, The current flowing through the developer at an electric field strength of 2 × 10 ⁶ V / m is 2.3 × 10 ⁻¹⁰ A or less , and the resistance of the magnetic carrier is smaller than the resistance of the toner. An image forming method using a developer having an electrical property such that the resistance of the developer is 10 ¹² Ωcm or more at an electric field strength of 5 × 10 ⁴ V / m . The average particle size of the toner of the developer is rt (μm), the bulk density is ρt (g / cm ³ ), the average particle size of the magnetic carrier is rc (μm), the bulk density is ρc (g / cm ³ ), Is the concentration of n (wt%),
S = {n / (100−n)} × (rc / rt) × (ρc / ρt)
2. The image forming method according to claim 1, wherein S represented by is 2.5 or more. 3. The image forming method according to claim 1, wherein the charging of the image carrier is performed by a contact charging member. The contact charging member is formed of a sleeve containing a magnet for holding magnetic particles in contact with the image carrier, and the magnetic particles are rotated by rotation of the sleeve, and a voltage is applied to the sleeve to charge the image carrier. 4. The image forming method according to claim 3 , wherein the image carrier is charged by being injected. The image forming method according to any one of claims 1 to 4 , wherein the shape factor SF-1 of the toner is 100 to 130, and SF-2 is 100 to 115. The image forming method according to any one of claims 1-5 which form part or all of the toner in the polymerization.

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