JP2010256759A - Carrier for developing electrostatic latent image and image forming method using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a carrier for developing an electrostatic latent image, which maintains a stable charging ability and resistance with less deterioration even in the case of large quantities of prints and high-quality printing and is capable of particularly suppressing development memory to form an image of high quality, a manufacturing method thereof, and an image forming method using the same. <P>SOLUTION: In a carrier for developing an electrostatic latent image, surfaces of carrier core members are coated with a resin, and a shape factor (SF-1) of the carrier core members is 130 to 150, and a shape factor (SF-1) of the carrier is 100 to 120, and an exposure rate of carrier core members on surfaces of carrier particles is <4%. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an electrostatic latent image developing carrier and an image forming method using the same.

  In an image forming method using an electrostatic latent image developing method such as an electrophotographic method, generally, an electrostatic latent image is formed by irradiating a photoconductive layer with a light image corresponding to an original, and then the electrostatic latent image is formed. A colored fine powder called toner having a polarity opposite to this is adhered on the image to develop the electrostatic latent image, and if necessary, the toner image is transferred to a transfer material such as paper. A copy is obtained by fixing with pressure or solvent vapor.

  In the process of developing the electrostatic latent image, the toner charged to a polarity opposite to that of the latent image is attracted by electrostatic attraction and adhered onto the electrostatic latent image (in the case of reverse development, the latent image is Toner having a charge of the same polarity as that of the toner). In general, there are two methods for developing such an electrostatic latent image using toner. A two-component developer comprising a toner and a carrier for developing an electrostatic latent image (hereinafter sometimes simply referred to as a carrier) is used. There is a method using a so-called one-component developer that uses a toner alone.

  The present invention relates to a carrier that constitutes a two-component developer. The toner and the carrier are mixed, and a charge is imparted to the toner by the difference in the charging sequence due to the friction between the toner and the carrier. Although the charge imparting property to the toner is more stable than that using a one-component developer without a carrier, it does not mean that there is no problem at all.

  The carrier constituting the two-component developer is usually an insulating carrier having a resin coating layer, and a carrier in which the surface of a carrier core material made of a ferromagnetic material such as iron, nickel, and ferrite is uniformly coated with an insulating resin. It is representative. By coating the resin, there is an effect of preventing the toner fine powder or the like from adhering to the carrier surface and deteriorating the chargeability. In general, although the resistance of the carrier core material used for the resin-coated carrier is low, the resistance of the material used for the resin coating layer (sometimes referred to simply as a coating layer) is high. The resistance can be adjusted. Thereby, it is possible to stabilize the charge imparting property to the toner.

  In recent years, in electrophotographic image forming methods, the range of use has expanded from the act of simple copying to the field of light printing, particularly with the evolution of networks that can easily transmit digital data. In the field of light printing, unlike image copying in a conventional office, high image quality is required to demand added value for printed matter.

  In this field, speeding up, mass printing, and high printing are progressing, and further improvement in developer durability and image quality are required.

  With the increase in speed and mass printing / printing, the solid image area and non-image area are adjacent to each other in the longitudinal direction of the photoconductor. However, there is a problem of so-called “development memory” image defects such as non-uniformity due to the influence of the previous image by the developing roller and a decrease in image density.

  On the other hand, in order to solve this, the developing machine design aimed at improving the toner replacement property so that the toner on the developing roller after development is completely removed from the developing roller and new toner can be carried on the developing roller ( Patent Document 1), and also in a one-component development system, an image forming apparatus that can remove all residual toner after development on a developing roller and is supplied with new toner that is frictionally charged by a layer regulating member (Patent Document 1) 2) has been reported.

  In any case, the problem is solved by improving the toner replacement property on the developing roller by improving the developing device. However, there are problems such as an increase in cost, complexity in mechanical design, a decrease in margin, and an increase in size.

JP 2009-14922 A Japanese Patent Laid-Open No. 9-190070

  The inventors solved the problem by revising the developer design from the viewpoint that the “development memory” is due to the contribution of the toner directly adhered on the developing roller to the development. There are no examples of such studies yet.

  That is, the present invention provides a carrier for developing an electrostatic latent image that can maintain a stable charging ability and resistance with little deterioration even under large-scale printing and high-printing, and can suppress development memory and form a high-quality image. The present invention provides a method and an image forming method using the method.

  The object of the present invention is achieved by adopting the following configuration.

(1)
In the carrier for developing an electrostatic latent image in which the carrier core surface is coated with a resin,
The carrier core has a shape factor (SF-1) of 130 to 150,
The carrier has a shape factor (SF-1) of 100 to 120,
A carrier for developing an electrostatic latent image, wherein a carrier core material exposure rate on a carrier surface is less than 4%.

(2)
The carrier for developing an electrostatic latent image according to (1), wherein a mass ratio of the resin coating the surface of the carrier core material to the carrier core material is 1.0 to 5.0 mass%.

(3)
The core average particle size (R core) is 20 to 50 μm, and the carrier average particle size (R carrier) is
0.5 μm ≦ R core-R carrier ≦ 3 μm
(1) The electrostatic latent image developing carrier according to (1).

(4)
In the method for producing a carrier for developing an electrostatic latent image in which the carrier core surface is coated with a resin,
(1) A method for producing a carrier for developing an electrostatic latent image, which satisfies any one of (3).

(5)
Using the electrostatic latent image developing carrier according to any one of (1) to (3) and a developer containing a toner,
An image forming method comprising developing an electrostatic latent image.

  According to the present invention, a carrier for developing an electrostatic latent image capable of forming a high-quality image that can maintain a stable charging ability and resistance with small deterioration even under a large amount of printing and high printing, and can suppress a development memory, and a manufacturing method thereof, And an image forming method using the same.

1 is a schematic cross-sectional view illustrating an example of a color image forming apparatus used in an image forming method of the present invention.

  The “development memory” is an image in which a solid image portion and a non-image portion are adjacent to each other in the longitudinal direction of the photoconductor, on the first rotation of the developing roller, and a halftone of a large area continues after the second rotation. This is an image defect in which the image density becomes inconsistent due to the influence of the previous image by the developing roller and the image density is lowered.

  The present inventors consider that the occurrence of “development memory” is caused by the fact that weakly charged toner that does not mix with the carrier and adheres directly to the developing roller contributes to development. Since the amount of toner consumed increases as the speed of copying machines increases and printing increases, the amount of toner supplied to the developing device also increases. Therefore, the replenished toner adheres directly to the developing roller without being sufficiently mixed with the carrier.

  When developing a solid image on the first round of the developing roller, both the toner of the magnetic brush and the toner directly adhered on the developing roller are developed on the toner carrier to form an image. The same development as in the first round is also performed in the development of an image in which a halftone of a large area continues after the second round of the developing roller, but the place where the toner that has already adhered directly is developed in the first round of the developing roller. Then, since development is performed only from the toner of the magnetic brush, after the second round of the developing roller, there is a difference in image density between the place where the solid image is developed on the first round of the developing roller and the place where it is not ( (The solid image on the first round of the developing roller appears with a reduced density in the halftone image of a wide area after the second round).

The following is considered effective for suppressing the development memory.
1) Improve toner developability of magnetic brush (suppress development of toner adhering to developing roller)
2) Improvement of mixing property of carrier and toner and rising performance of charging, suppression of toner adhesion on developing roller As a result of the study by the present inventors,
In the carrier for developing an electrostatic latent image in which the carrier core surface is coated with a resin,
The carrier core has a shape factor (SF-1) of 130 to 150,
The carrier has a shape factor (SF-1) of 100 to 120,
It has been found that the carrier core material exposure rate on the surface of the carrier is less than 4%, “development memory” is suppressed and deterioration is small even for long-term use.

  By coating a resin on a deformed core material that easily forms irregularities on the surface having a carrier core material shape factor (SF-1) of 130 to 150, the resin coating layer thickness of the convex region of the core material Is relatively thin, and the resin coating layer other than the protrusions is thick. Since the resistance value is low in the thin resin part, the charge rising property and developability when it is used as a developer can be improved, while on the other hand, the charge retention can be improved in the thick resin part. Therefore, the above 1) and 2) can be achieved, and the development memory can be suppressed.

  If the carrier core material shape factor (SF-1) is smaller than 130, the shape is rounded, so that when used as a carrier, it becomes difficult to provide two functions, and the development memory deteriorates.

  If it is larger than 150, the shape factor (SF-1) of the carrier after resin coating becomes larger than 120, the mixing property of the carrier and the toner is lowered, and development memory is generated.

  When the shape factor (SF-1) of the carrier is 120 or less, the carrier has a relatively round shape and thus has high fluidity and good mixing with the toner. Therefore, the amount of toner adhering to the developing roller can be suppressed, and the carrier can withstand long-term use. When the shape factor (SF-1) of the carrier is larger than 120, the toner mixing property is deteriorated and a problem occurs in durability.

  When the carrier core material exposure rate on the carrier surface is less than 4%, problems such as carrier adhesion are avoided, but when it is 4% or more, the resistance of the core material is lower than that of the coating resin, so that carrier adhesion occurs.

  Claim 1 is satisfied when the mass ratio of the resin covering the surface of the carrier core material to the carrier core material is 1.0 to 5.0 mass%.

The core average particle size (R core) is 20 to 50 μm, and the carrier average particle size (R carrier) is
0.5 μm ≦ R carrier-R core ≦ 3 μm
By using a carrier having a small particle diameter, the contact probability with the toner can be increased by increasing the total surface area of the carrier, and the contact charging ability as a developer can be improved. Further, since the stress due to the mixing of the toner and the carrier is reduced, the deterioration of the developer is also reduced.

  When it is 20 μm or less, carrier adhesion occurs, and when it is 50 μm or more, the image is deteriorated.

  By setting 0.5 μm ≦ R carrier−R core ≦ 3 μm, the resin coating of the convex region of the core material is maintained while maintaining the charge retention capability in the portion where the resin coating layer other than the convex portion of the core material is thick. Since the layer thickness can be further reduced, the charge rising property and developability are improved, and the development memory can be further suppressed.

[Measurement method of carrier core material exposure rate]
Using a Shimadzu X-ray optical analyzer (ESCA-1000 Shimadzu Corporation), the X-ray intensity is 10, 30 mA, analysis depth; in the normal mode, the composition on the surface of the carrier core agent is assumed to be uniform and is the main element 4 The element is selected, and the FE ratio (% in the carrier core material) from the element peak area intensity of the four elements of the quantitative elements Si, Ti, O, and C (silicon = Si2P, carbon = C1s, oxygen = O1s, iron = Fe2P3 / 2) ) (F1) is calculated.

  On the other hand, the carrier having the resin coating layer is similarly subjected to quantitative measurement of four elements to calculate the FE ratio (%) (F2) of the resin coated carrier, and the carrier core material exposure amount is calculated by the following formula.

Carrier core material exposure (%) = (F2 / F1) × 100
[Carrier core material and carrier shape factor (SF-1)]
The shape factor (SF-1) of carrier particles and carrier core particles in the present invention is a numerical value calculated by the following formula.

SF-1 = (maximum length of particle) ² / (projected area of particle) × (π / 4) × 100 (Formula 1)
(Measurement)
The carrier (carrier core material) was photographed with a scanning electron microscope at random at a magnification of 150, and 100 or more particles were photographed, and the photograph image captured by the scanner was converted into an image processing analyzer LUZEX AP (Nireco Corporation). ) To measure. The number average particle diameter is calculated as the average value of the horizontal ferret diameters, and the shape factor is a value calculated by the average value of the shape factor SF-1 calculated by Equation 1.

[Measurement of average particle diameter of carrier core material and carrier]
Typically, it can be measured by a laser diffraction particle size distribution measuring apparatus “HELOS” (manufactured by SYMPATEC) equipped with a wet disperser.

[Material of carrier core]
The core material according to the present invention is preferably made of ferrite, and more preferably represented by the general formula (MO) x (Fe ₂ O ₃ ) y (where y is 30 to 95 mol%). Here, M is preferably one or more selected from Fe, Mn, Mg, Sr, Ca, Ti, Cu, Zn, Ni, Li, Al, Si, Zr, and Bi.

Here, when M is Fe, it means iron ferrite, that is, magnetite. Compared to magnetite, ferrite is a higher-order oxide, and its characteristics are not easily changed by stress. In addition, it is easy to reduce the specific gravity. When Fe ₂ O ₃ is less than 30 mol%, it is difficult to obtain a desired magnetization, and carrier adhesion tends to occur. In particular, ferrite using a specific metal oxide as a raw material has little composition variation among particles, and easily obtains desired characteristics. Further, when the above-described elements are used, the reason is not clear as compared with other elements, but it is easy to inject and coat the resin.

  In consideration of the trend of reducing environmental burdens including recent waste regulations, it is preferable that Cu, Zn and Ni heavy metals are not substantially included.

  For the above reasons, M is preferably one or more selected from Mn, Mg, Sr, Ca, Ti, Li, Al, Si, Zr, Bi, and Mn, Mg, Sr, Ca, Li, Zr, One type or two or more types selected from Bi are particularly preferable.

[Method for producing carrier core material]
When the core material of the present invention is produced, an appropriate amount of raw materials are weighed and then pulverized and mixed with a ball mill or a vibration mill for 0.5 hours or more, preferably 1 to 20 hours. The pulverized material thus obtained is pelletized using a pressure molding machine or the like and then calcined at a temperature of 700 to 1200 ° C.

  You may grind | pulverize without using a pressure molding machine, add water to make a slurry, and granulate using a spray dryer. After calcination, the mixture is further pulverized with a ball mill or a vibration mill, and then water and, if necessary, a dispersant and a binder are added. After adjusting the viscosity, granulation is performed, and the oxygen concentration is controlled. Hold for ˜24 hours and perform main firing (in order to set the carrier core material shape factor (SF-1) to 130 to 150, the temperature is set higher than before). When pulverizing after calcination, water may be added and pulverized by a wet ball mill, a wet vibration mill or the like.

  The pulverizer such as the above-mentioned ball mill and vibration mill is not particularly limited, but in order to disperse the raw materials effectively and uniformly, it is preferable to use fine beads having a particle diameter of 1 mm or less for the medium to be used. Further, the degree of grinding can be controlled by adjusting the diameter, composition and grinding time of the beads used.

  The fired product thus obtained is pulverized and classified. As a classification method, the particle size is adjusted to a desired particle size using an existing air classification, mesh filtration method, sedimentation method, or the like.

  Then, if necessary, the surface can be heated at a low temperature to perform an oxide film treatment to adjust the electric resistance. The oxide film treatment can be performed by heat treatment at, for example, 300 to 700 ° C. using a general rotary electric furnace, batch electric furnace or the like. The thickness of the oxide film formed by this treatment is preferably 0.1 nm to 5 μm. If the thickness is less than 0.1 nm, the effect of the oxide film layer is small, and if it exceeds 5 μm, the magnetization is lowered or the resistance becomes too high, so that it is difficult to obtain desired characteristics. Moreover, you may reduce | restore before an oxide film process as needed.

The core material preferably has a residual magnetization of 15 emu / g (A · m ² / kg) or less.

[Coating resin material]
The carrier according to the present invention is characterized in that its surface is coated with a resin. As the resin to be coated, various resins described later can be used.

  In the present invention, the resin-coated carrier preferably contains a nitrogen-containing compound in the resin-coated layer.

  The resin used for coating the carrier according to the present invention can be appropriately selected depending on the toner to be combined, the environment in which it is used, and the like. The resin is not particularly limited. For example, fluororesin, acrylic resin, epoxy resin, polyamide resin, polyamideimide resin, polyester resin, unsaturated polyester resin, urea resin, melamine resin, alkyd resin, phenol resin, fluoroacrylic resin, acrylic -Styrene resin, silicone resin, and the like. And modified silicone resins modified with resins such as acrylic resin, polyester resin, epoxy resin, polyamide resin, polyamideimide resin, alkyd resin, urethane resin, and fluorine resin. In view of the detachment of the resin due to mechanical stress during use, a thermosetting resin is preferably used. Specific examples of thermosetting resins include epoxy resins, phenol resins, silicone resins, unsaturated polyester resins, urea resins, melamine resins, alkyd resins, and resins containing them.

[Method for producing carrier]
Specific methods for producing the coating layer include a wet coating method and a dry coating method. Each method is described in detail below.

As a wet coating method,
(1) Fluidized bed type spray coating method A method in which a coating solution in which a coating resin is dissolved in a solvent is spray-coated on the surface of magnetic particles using a fluidized bed and then dried to produce a coating layer (2) Immersion Formula Coating Method A method in which magnetic particles are immersed in a coating solution in which a coating resin is dissolved in a solvent and then coated, and then dried to prepare a coating layer. (3) Polymerization Method A reactive compound is dissolved in a solvent. Examples of the method include a method of immersing magnetic particles in a coating solution and applying the coating, and then applying heat to perform a polymerization reaction to prepare a coating layer.

As a dry coating method,
A method in which resin particles are deposited on the surface of particles to be coated, and then a mechanical impact force is applied to melt or soften the resin particles deposited on the surface of the particles to be coated to fix the coated layer. It is.

  Using a high-speed stirring mixer that can impart mechanical impact force to the carrier core material, resin, low-resistance fine particles, etc., without heating or under heating, the impact force is repeatedly applied to the mixture by high-speed stirring. A carrier fixed by dissolving or softening on the surface of the particles is produced. When heating, 60-130 degreeC is preferable. This is because when the heating temperature is excessive, aggregation of carrier particles tends to occur.

  The coating layer forming method in the present invention is most preferably carried out by this method.

The carrier according to the present invention preferably has a magnetization of 30 to 80 emu / g (A · m ² / kg) from the viewpoint of prevention of carrier adhesion and high image quality. More preferably, it is 35-75 emu / g (A * m < ² > / kg), Most preferably, it is 30-70 emu / g (A * m < ² > / kg).

[Manufacture of toner for electrophotography]
As the toner used in the present invention, those commonly used can be used without particular limitation. For example, a styrene-acrylic resin or a polyester resin can be used as a binder resin, and a conventionally used colorant can also be used as a colorant. Further, a release agent or charge control is used as necessary. Agent is added.

  The production method may be a so-called pulverization method or a polymerization method, and in many cases, external additives such as silica fine particles are added after toner particle formation.

(Image forming method)
FIG. 1 is a schematic cross-sectional view showing an example of a color image forming apparatus according to an image forming method using the toner of the present invention.

  First, an outline of an image forming apparatus for color electrophotography equipped with a detection sensor and a secondary transfer device will be described.

  The image forming apparatus GS is called a tandem type color image forming apparatus, and arranges image forming units that form yellow, magenta, cyan, and black color toner images along the moving direction of the intermediate transfer member. The color toner image formed on the image carrier of the image forming unit is transferred onto the intermediate transfer member and superimposed, and then transferred onto the image support at once.

  In the figure, a document image placed on an image reading device SC arranged at a position occupying the upper portion of the image forming device GS is scanned and exposed by an optical system, read by a line image sensor CCD, and read by the line image sensor CCD. The analog signal subjected to photoelectric conversion is subjected to analog processing, A / D conversion, shading correction, image compression processing, and the like in an image processing unit, and then an image data signal is sent to the exposure optical system 3 serving as an image writing unit.

  The intermediate transfer member includes a drum type and an endless belt type, both of which have similar functions. However, in the following description, the intermediate transfer member refers to the endless belt-like intermediate transfer 6. I will decide.

  In the figure, four sets of process units 100 are provided on the peripheral edge of the intermediate transfer body 6 for image formation for each color of yellow (Y), magenta (M), cyan (C), and black (K). It has been. The process unit 100 is arranged in a vertical line along the intermediate transfer body 6 as a color toner image forming unit along the intermediate transfer body 6 with respect to the rotation direction of the vertical intermediate transfer body 6 indicated by the arrows in the drawing. , K in this order.

  Each of the four sets of process units 100 has a common structure, and each includes a photosensitive drum 1, a charger 2 as a charging unit, an exposure optical system 3 as an image writing unit, a developing device 4, and an image. It comprises a photoconductor cleaning device 190 as a carrier cleaning means.

  In the photosensitive drum 1, a photosensitive layer having a layer thickness (film thickness) of about 20 to 40 μm is formed on the outer periphery of a cylindrical substrate formed of a metallic member such as aluminum having an outer diameter of about 40 to 100 mm. Is. The photosensitive drum 1 is rotated in the direction of the arrow in a state where the substrate is grounded by power from a driving source (not shown), for example, at a linear velocity of about 80 to 280 mm / s, preferably 220 mm / s.

  Around the photosensitive drum 1, an image forming unit including a charger 2 as a charging unit, an exposure optical system 3 as an image writing unit, and a developing device 4 is set as a photosensitive drum indicated by an arrow in the figure. 1 with respect to the rotation direction.

  The charger 2 as a charging unit is attached in close proximity to the photosensitive drum 1 in a direction parallel to the rotation axis of the photosensitive drum 1. The charger 2 includes a discharge wire as a corona discharge electrode that applies a predetermined potential to the photosensitive layer of the photosensitive drum 1, and performs a charging action (negative charging in the present embodiment) by corona discharge having the same polarity as the toner. A uniform potential is applied to the photosensitive drum 1.

  An exposure optical system 3 serving as an image writing means rotationally scans laser light emitted from a semiconductor laser (LD) light source (not shown) in a main scanning direction by a rotating polygon mirror (no symbol), and an fθ lens (no symbol). ), A reflection mirror (without a symbol), etc., the photosensitive drum 1 is exposed to an electrical signal corresponding to an image signal (image writing), and an electrostatic latent image corresponding to a document image is formed on the photosensitive layer of the photosensitive drum 1. Form an image.

  The developing device 4 as a developing unit is configured to supply a two-component developer of each color of yellow (Y), magenta (M), cyan (C), and black (K) charged to the same polarity as the charging polarity of the photosensitive drum 1. A developing roller 4a, which is a developer carrier made of, for example, a cylindrical nonmagnetic stainless steel or aluminum material having a thickness of 0.5 to 1 mm and an outer diameter of 15 to 25 mm, is housed. The developing roller 4a is kept in contact with the photosensitive drum 1 with a predetermined gap (for example, 100 to 1000 μm) by an abutting roller (not shown), and rotates in the same direction as the rotational direction of the photosensitive drum 1. At the time of development, a photoconductor drum is applied by applying to the developing roller 4a a DC voltage having the same polarity as the toner (negative polarity in the present embodiment) or a developing bias voltage that superimposes an AC voltage on the DC voltage. The reversal development is performed on the upper exposed portion.

The intermediate transfer member 6 has a volume resistivity of about 1.0 × 10 ^{7 to} 1.0 × 10 ⁹ Ω · cm and a surface resistivity of about 1.0 × 10 ^{10 to} 1.0 × 10 ¹² Ω / □. A semiconductive endless (seamless) resin belt is used. As a resin belt, a semiconductive material having a thickness of 0.05 to 0.5 mm in which a conductive material is dispersed in engineering plastics such as modified polyimide, thermosetting polyimide, ethylene tetrafluoroethylene copolymer, polyvinylidene fluoride, and nylon alloy. A resin film can be used. In addition to this, a semiconductive rubber belt having a thickness of 0.5 to 2.0 mm in which a conductive material is dispersed in silicone rubber, urethane rubber or the like can also be used as the intermediate transfer member 6. The intermediate transfer member 6 is wound around a plurality of roller members including a tension roller 6a and a backup roller 6B facing the secondary transfer member, and is supported so as to be rotatable in the vertical direction.

  The primary transfer roller 7 as a first transfer unit for each color is made of a roller-like conductive member using foamed rubber such as silicone or urethane, for example. The transfer area is formed between the photosensitive drum 1 and the back surface of the intermediate transfer body 6 by pressing the back surface thereof. A DC constant current having a polarity opposite to that of the toner (positive polarity in the present embodiment) is applied to the primary transfer roller 7 by constant current control, and a toner image on the photosensitive drum 1 is formed by a transfer electric field formed in the transfer area. Transferred onto the intermediate transfer body 6.

  The toner image transferred onto the intermediate transfer body 6 is transferred to the image support P. A detection sensor 8 for measuring the density of the patch image toner is installed on the periphery of the intermediate transfer member 6.

  In order to clean the residual toner on the intermediate transfer member 6, a cleaning device 190A is provided.

  Further, a secondary transfer device 70 is provided to clean the patch image toner on the secondary transfer member 7A.

  Next, an image forming process (image forming process) will be described.

  When the image recording is started, the photosensitive drum drive motor (not shown) is rotated to rotate the Y photosensitive drum 1 in the direction indicated by the arrow in the figure, and a potential is applied to the Y photosensitive drum 1 by the Y charger 2. . After the Y photosensitive drum 1 is applied with a potential, the Y exposure optical system 3 performs exposure (image writing) with an electrical signal corresponding to the first color signal, that is, the Y image data. An electrostatic latent image corresponding to a yellow (Y) image is formed on the body drum 1. The latent image is reversely developed by the Y developing device 4 to form a toner image made of yellow (Y) toner on the Y photosensitive drum 1. The Y toner image formed on the Y photosensitive drum 1 is transferred onto the intermediate transfer member 6 by a primary transfer roller 7 as a primary transfer unit.

  Next, a potential is applied to the M photosensitive drum 1 by the M charger 2. After the M photoconductor drum 1 is applied with a potential, the M exposure optical system 3 performs exposure (image writing) with an electric signal corresponding to the first color signal, that is, the M image data. An electrostatic latent image corresponding to a magenta (M) image is formed on the body drum 1. The latent image is reversely developed by the M developing device 4 to form a toner image made of magenta (M) toner on the M photosensitive drum 1. The M toner image formed on the M photoconductor drum 1 is transferred onto the intermediate transfer body 6 by being superimposed on the Y toner image by a primary transfer roller 7 as a primary transfer unit.

  By a similar process, a toner image made of cyan (C) toner formed on the C photoconductive drum 1 and a toner image made of black (K) toner formed on the K photoconductive drum 1 are obtained. The toner images are sequentially superimposed on the intermediate transfer member 6, and a superimposed color toner image composed of Y, M, C, and K toners is formed on the peripheral surface of the intermediate transfer member 6.

  The toner remaining on the peripheral surface of each photoreceptor drum 1 after the transfer is cleaned by the photoreceptor cleaning device 190.

  On the other hand, the image support P as recording paper accommodated in the paper feed cassettes 20A, 20B, and 20C is fed by the feed roller 21 and the paper feed roller 22A provided in the paper feed cassettes 20A, 20B, and 20C, respectively. Secondary transfer is performed as a secondary transfer unit that is transported on the transport path 22 by transport rollers 22B, 22C, and 22D, and is applied with a voltage having a polarity opposite to that of the toner (positive polarity in the present embodiment) through the registration roller 23. The superimposed color toner image (color image) formed on the intermediate transfer body 6 is transferred onto the image support P in a lump in the transfer area of the secondary transfer member 7A.

  The image support P on which the color image has been transferred is heated and pressed at the nip portion formed by the heating roller 17a and the pressure belt 17b of the fixing device 17 and fixed, and is sandwiched by the paper discharge roller 24 and out of the machine. Placed on the paper discharge tray 25.

  After the color image is transferred onto the image support P by the secondary transfer member 7A as the secondary transfer means, the residual toner on the intermediate transfer body 6 that has separated the curvature of the image support P becomes the intermediate transfer body cleaning device 190A. Is removed.

  Further, the patch image toner on the secondary transfer member 7 </ b> A is cleaned by the cleaning blade 71 of the secondary transfer device 70.

  Hereinafter, the present invention will be described more specifically with reference to the following examples, but the present invention is not limited thereto. In the text, “part” means “part by mass”.

[Production of carrier]
(Preparation of carrier core material)
In the following manner, Mn—Mg ferrite particles were produced, and carrier core materials and core materials 1 to 5 having SF-1 of 120 to 160 were obtained.

An appropriate amount of each raw material is blended so as to be 21.0 mol% in terms of MnO, 3.3 mol% in terms of MgO, 0.7 mol% in terms of SrO, and 75.0 mol% in terms of Fe ₂ O ₃ , , Mixed for 10 hours in a wet ball mill, dried, held at 950 ° C. for 4 hours, granulated and dried for 24 hours in a wet ball mill, and placed in a firing furnace with a built-in stirring device. After adding 50% of the volume and maintaining the peripheral speed at 10 m / s and 1200 ° C. for 4 hours, the mixture was crushed and the particle size was adjusted to a particle size of 35 μm to obtain the core material 1.

  The core materials 2 to 5 were similarly produced by changing the firing temperature as shown in the table.

(Core resin coating)
100 parts by weight of “Core 1” produced above and 3 parts by weight of cyclohexyl methacrylate / methyl methacrylate (copolymerization ratio 5/5) copolymer resin were charged into a high-speed mixer equipped with stirring blades, and 120 After stirring and mixing at a wind speed of 10 m / s at 30 ° C. and forming a resin coating layer on the surface of the core particles by the action of mechanical impact force, cooling was performed by lowering the wind speed to 2 m / s to prepare a carrier 1. .

  The coating resin amount for other carriers is as shown in the table.

  Carriers 3, 5, 8 are mixed for 15 minutes, Carriers 2, 7, 9, 12-15 are mixed for 30 minutes, Carrier 10 is mixed for 60 minutes, Carriers 4, 6, 11 are mixed for 90 minutes, Carrier 16 is 150 Each was prepared by mixing for minutes.

(Preparation of developer)
100 parts by mass of each carrier prepared above and 7 parts by mass of black toner were mixed with a stirrer to prepare.

  The toner used was a toner prepared by a polymerization method having a median diameter (D50) of 6.5 μm on a volume basis. Further, 0.5 part of silica particles having a number average particle diameter of 10 nm was added as an external additive.

[Performance evaluation]
A developer and a toner cartridge are prepared using the carrier and toner having the composition shown in the table, and a black (K) toner is prepared using a copying machine (bizhub pro C 6500: manufactured by Konica Minolta Business Technologies, Inc.) remodeled for experiments. A single color image with a printing rate of 20% was copied. In addition, the image quality after 200,000 sheets was initially evaluated.

(Development memory)
An image in which a solid image portion and a non-image portion are adjacent to each other in the longitudinal direction of the photosensitive member, and then an intermediate halftone with a large area is acquired after the initial and after output of 200,000 sheets. The image density between the place where the image was developed and the place where it was not was measured, and the difference was determined. Density measurement was performed using a reflection densitometer “RD-918”.

  An image density difference of 0.02 or less was accepted.

◎: Less than 0.003 ○: 0.003 to less than 0.006 Δ: 0.006 to 0.020 or less ×: Value greater than 0.020 (fogging)
The white density of the image support was measured at 20 locations of A4 size, the average value was defined as the white paper density, and then the absolute image density at 20 locations was similarly measured for the white background portion of the 200,000 evaluation formed images. A value obtained by averaging and subtracting the white paper density from this average density was evaluated as the fog density. If the fog density was 0.006 or less, the fogging was accepted.

  Density measurement was performed using a reflection densitometer “RD-918” (manufactured by Macbeth).

◎: Less than 0.003 ○: 0.003 to less than 0.006 Δ: 0.006 to 0.010 or less ×: Value greater than 0.010 (carrier adhesion)
After the output of 200,000 sheets, the imageless chart was developed, the number of carriers adhering to the surface of the photoconductor was counted by 5 visual observations by magnifying glass, and the average number of adhering carriers per 100 cm ² was regarded as carrier adhesion. Less than 50 were accepted.

  The evaluations were as follows: A: 20 or less, B: 21 or more and less than 50, X: 50 or more.

  As is apparent from Table 3, Examples 1 to 12 in the present invention have practical characteristics, but Comparative Examples 1 to 4 outside the present invention have problems in at least any of the characteristics. .

DESCRIPTION OF SYMBOLS 1 Photosensitive drum 2 Charger 3 Exposure optical system as image writing means 4 Developing apparatus 6 Intermediate transfer body 100 Process unit of yellow (Y), magenta (M), cyan (C), and black (K) 190 Image holding Photoconductor cleaning device as body cleaning means

Claims (5)

In the carrier for developing an electrostatic latent image in which the carrier core surface is coated with a resin,
The carrier core has a shape factor (SF-1) of 130 to 150,
The carrier has a shape factor (SF-1) of 100 to 120,
A carrier for developing an electrostatic latent image, wherein a carrier core material exposure rate on a carrier surface is less than 4%. 2. The electrostatic latent image developing carrier according to claim 1, wherein the mass ratio of the resin covering the surface of the carrier core to the carrier core is 1.0 to 5.0% by mass. The core average particle size (R core) is 20 to 50 μm, and the carrier average particle size (R carrier) is
0.5 μm ≦ R core-R carrier ≦ 3 μm
The electrostatic latent image developing carrier according to claim 1, wherein In the method for producing a carrier for developing an electrostatic latent image in which the carrier core surface is coated with a resin,
A method for producing a carrier for developing an electrostatic latent image, which satisfies any one of claims 1 to 3. Using the electrostatic latent image developing carrier according to any one of claims 1 to 3 and a developer containing toner,
An image forming method comprising developing an electrostatic latent image.

Images