JP3200562B2 - Image forming device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a copier, a printer, and a recorded image display for developing a visible image by developing an electrostatic latent image formed on an image carrier by an electrophotographic method, an electrostatic recording method, or the like. And an image forming apparatus such as a facsimile.

[0002]

2. Description of the Related Art Conventionally, a dry developer as a developer is carried on the surface of a developer carrier, and the developer is conveyed and supplied to the vicinity of the surface of the image carrier on which an electrostatic latent image is carried. A method of developing an electrostatic latent image while applying an alternating (alternating) electric field between an image carrier and a developer carrier to visualize the electrostatic latent image is well known. Note that, in general, a developing sleeve is often used for the developer carrying member, and therefore, it is referred to as a “developing sleeve” in the following description. In addition, since an image carrier generally uses a photosensitive drum in many cases, In the following description, it will be referred to as "photosensitive drum".

[0003] As a developing method as described above, conventionally, for example, a two-component developer composed of a two-component composition having carrier particles and toner particles is applied to the surface of a developing sleeve in which a magnet is disposed inside. A magnetic brush is formed, and the magnetic brush is rubbed or brought close to the photosensitive drum opposed to the photosensitive drum while maintaining a minute developing interval, and further alternately continuously between the developing sleeve and the photosensitive drum (between SD). A so-called magnetic brush developing method is known in which the application of an electric field causes the toner particles to be repeatedly transferred and reversed from the developing sleeve side to the photosensitive drum side to develop an electrostatic latent image on the photosensitive drum. (See, for example, JP-A-55-3
2060, JP-A-59-165082). Also, for the purpose of simple color development and multiple development,
A non-contact type alternating electric field developing method using a two-component developer is also known (for example, JP-A-56-14268, JP-A-58-68051, JP-A-56-14).
4452, JP-A-59-181362 and JP-A-60-176069). The toner image formed on the photosensitive drum by these developing methods is
After being transferred onto a transfer material such as paper as required, it is fixed by heat, pressure, solvent vapor, or the like, and a copied image is obtained.

[0004] In recent years, for the purpose of further improving transferability and image quality, toner having a substantially spherical shape produced by a polymerization method has been developed. Such a polymerized toner has good releasability from the photosensitive drum due to its shape factor, and as a result, high transfer efficiency can be obtained, and in particular, an effect that a high-density large-area image has high quality can be obtained. . However, on the other hand, the phenomenon that the toner once adhered on the photosensitive drum is easily peeled off on the developing surface, particularly in an area where the amount of adhered toner is small such as a highlight portion and a halftone portion,
As a result, the image gradation may be deteriorated. This phenomenon is due to analog image exposure and digital image gradation reproduction method.
A method of controlling the light emission time of the exposure unit in proportion to the magnitude of an image signal value for each pixel (hereinafter referred to as a PWM method)
Notably occurs when The reason for this is that the toner peeling electric field component from the photosensitive drum during the alternating electric field is larger than the high contrast area in the low contrast area where the developing electric field is low. This was remarkable in a developing method using an agent and a magnetic brush in contact with a photoreceptor.

However, this problem can be solved by a so-called counter developing method in which the rotating direction of the developing sleeve is opposite to the rotating direction of the photosensitive drum at the portion facing the photosensitive drum. That is, according to the counter developing method, the relative speed difference between the photosensitive drum and the magnetic brush on the developing sleeve is larger than in the conventional forward developing method, so that the toner supply amount per unit time increases, Since the efficiency is improved, the image density of the highlight portion and the halftone portion, which are low contrast regions, is secured, and the gradation of the obtained image is improved. It is also a known fact that counter-development can also prevent the problem of the so-called Hakiyose phenomenon, in which, when a low-density portion continues to a high-density portion in forward development, the trailing edge of the high-density portion further increases. It is considered that this crater phenomenon occurs because the high charge amount toner does not enter the developing region of the high density image portion but stays near the photosensitive drum.

As described above, by adopting the counter development system using a two-component developer while maintaining high transferability using a polymerized toner, the gradation in a low density portion can be improved. However, the following problems occur depending on the setting conditions of the magnetic brush on the developing sleeve. For example, when the weight per unit area of the magnetic brush on the developing sleeve is increased to secure the image density, or the toner density of the magnetic brush on the developing sleeve (hereinafter referred to as T / D Ratio, etc.), the rubbing force of the magnetic brush on the photosensitive drum becomes excessive, and the toner layer on the photosensitive drum after development (particularly, highlight portions and halftone portions) is scraped off. As a result, image degradation called so-called unevenness occurs. This phenomenon is also more remarkable when using the polymerized toner than when using the conventional pulverized toner,
Further, when the counter development method was adopted, the image became more noticeable.

[0007]

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a low-density portion of a two-component developer having a polymerized toner when a contact counter magnetic brush development system to which an AC bias is applied is employed. It is an object of the present invention to provide an image forming apparatus which can achieve an improvement in gradation and can obtain an image in which the unevenness of highlights and halftones is particularly well prevented.

[0008]

The above object is achieved by the present invention described below. That is, the present invention is provided for forming an electrostatic latent image corresponding to an image signal by an image exposing means on an image carrier having an initial charge, and developing the electrostatic latent image into a visible image. A rotatable developer carrier for supporting and transporting a two-component developer having a non-magnetic toner and a magnetic carrier generated by a polymerization method, facing the image carrier, and the developer In an image forming apparatus including a developing device having a developing unit in which a plurality of magnetic field generators fixedly provided inside a carrier are arranged, the developing device includes an image carrier and a developer carrier. The two carriers move in opposite directions to each other at the opposing portion, and at the opposing portion,
The two-component electrostatic latent image while applying to the developer carrying member a DC bias obtained by superimposing an AC bias as a developing bias
A magnetic brush developing device for developing a system developer, during development
Is applied to the two-component developer by the image carrier
When a voltage is applied that gives a force in the direction from
Application step, and conversely, the developer carrier on the two-component developer
Voltage for applying a force from
AC bias which alternately repeats the above steps a plurality of times, and
From the developer carrier of the AC bias to the image carrier
The electrostatic latent image on the image carrier is
DC voltage between the image part voltage and the non-image part voltage
Is applied to the developer carrier for a certain period of time.
We, the magnetic carrier, there magnetization in the magnetic field of one kOe within the range of 30~200emu / cm ^3, the resistivity is in the range of ^{1 × 10 10 ~1 × 10 14} Ω · cm, and number average particle diameter Ri range near the 10 to 60 [mu] m, the non-magnetic toner, a shape factor SF-1 of 10
0-140 and SF-2 are within the range of 100-120.
That is generated in the polymerization process are spherical or substantially spherical toner der Rukoto an image forming apparatus according to claim.

According to the present invention having the above structure, even in a system employing a contact counter magnetic brush developing system using an AC bias applying a two-component developer having a polymerized toner, the gradation of a low density portion can be improved. Is achieved, and an image in which the unevenness in the highlight portion and the halftone portion is favorably prevented is obtained.

[0010]

Next, the present invention will be described in more detail with reference to preferred embodiments. In the image forming apparatus of the present invention, a two-component developer having a magnetic carrier and a non-magnetic toner is used. First, the magnetic carrier used in the present invention will be described in detail. As explained in the conventional example,
The phenomenon of crawl unevenness that occurs at the time of image formation occurs when the rubbing force on the photosensitive drum with which the developer magnetic brush contacts is large. The present inventors have conducted intensive studies on the effects of various parameters in the developing area on the rubbing force, and as a result, have confirmed that the following parameters have a strong degree of influence on the rubbing force. (1) Relative speed between photosensitive drum and developing sleeve (2) Distance between photosensitive drum and developing sleeve (SD gap) (3) Weight per unit area of developer on developing sleeve (M / S) (4) Position of developing magnetic pole, magnetic flux density, magnetic force (5) T / D ratio of two-component developer (6) Magnetization amount of carrier

The effect of the magnetic brush on the rubbing force on the photosensitive drum is particularly large according to the above six items.
Therefore, if these values are adjusted in a direction in which the muramura phenomenon occurring in the image is eliminated, the muramura phenomenon itself can be improved. However, the above (1) to (5)
With respect to (3), if it is optimized in the direction in which the creaking phenomenon is eliminated, an adverse effect of image deterioration occurs as described below. Regarding (1), the reduction of the relative speed can improve the muramura phenomenon, but the gradation of the image deteriorates. Regarding (2), the spreading gap can be improved by widening the SD gap, but the above-mentioned cracking phenomenon deteriorates. Regarding (3), by reducing the M / S value, the crawl phenomenon can be improved, but the crater phenomenon and the gradation are deteriorated. Regarding (4), the eccentricity phenomenon can be improved by shifting the position of the developing magnetic pole from the position facing the photosensitive drum or reducing the magnetic flux density, but the eccentricity phenomenon and the carrier adhesion phenomenon are deteriorated. Regarding (5), by increasing the T / D ratio, the creaking phenomenon can be improved, but the problems of toner scattering and fogging are exacerbated.

Regarding (6) the amount of magnetization of the carrier, it has been confirmed that lowering the amount of magnetization has a dramatic effect on the hakimema phenomenon without causing image deterioration.
Further, there was a concern that the carrier adhesion was deteriorated. However, it was found that the deterioration of the carrier adhesion can be prevented by optimizing the lower limit of the magnetization amount. That is, according to the experiments of the present inventors, the SD gap is 300 to 1,
000 μm, M / S 20-50 mg / cm ² , T / D
When the ratio is in the range of 5 to 12%, the amount of magnetization of the carrier is 20%.
0 emu / cm ³ or less, preferably 140 emu / cm ³
If it is below, the rubbing force of the magnetic brush on the photosensitive drum is about ４ or less of the case of Cu—Zn ferrite (about 280 emu / cm ³ ) which is generally used. As a result, it was found that even in a system using a two-component developer having a polymerized toner having a good releasability, it was possible to effectively prevent the unevenness in the highlight area and the halftone area.

Within the range of the amount of magnetization of the magnetic carrier, which can prevent the occurrence of the uneven peeling phenomenon due to the high releasability of the polymerized toner, it is preferably ^{30 to} 200 emu / cm ^{3 in} consideration of the carrier adhesion to the photosensitive drum. Is 80
It was confirmed that it was preferable to set the range to 140 emu / cm ³ . That is, the amount of magnetization of the magnetic carrier is 30 e
When the value is less than mu / cm ³ , not only does carrier adhesion increase, but also it becomes impossible to magnetically apply and transport the developer on the developing sleeve. In particular, when a polymerized spherical toner is used, the transportability is deteriorated due to the shape factor. On the other hand, if it exceeds 200 emu / cm ³ , the rubbing force of the magnetic brush on the photosensitive drum is increased, and in a system using the polymerized toner, the creaking phenomenon is likely to occur.

Further, as a result of the study by the present inventors, it has been found that, by optimizing the amount of magnetization of the magnetic carrier and optimizing the range of the particle diameter and the specific resistance of the magnetic carrier to be used, the photosensitive drum is improved. It has been found that it is possible to solve the phenomenon of uneven spots while reliably preventing carrier adhesion and image deterioration. That is, by setting the number average particle size of the magnetic carrier in the range of 10 to 60 μm, it is possible to prevent the carrier having the fine particle size from adhering to the photosensitive drum, and to reduce the unevenness caused by the large particle size carrier. As a result, the visibility of the unevenness appearing on the image is reduced, and a high-quality image can be obtained. or,
Preferably, the specific resistance of the magnetic carrier is 10 ¹⁰ to 10 ¹⁴ Ω
By setting the diameter within the range of cm, it is possible to prevent carrier adhesion due to charge injection even for carriers having a low magnetization amount, and prevent image deterioration due to charge-up of carriers.

Further, although the use of the magnetic carrier having a low magnetization amount as described above has been described somewhat in the conventional example,
Achieving high image quality is achieved in addition to the prevention of the creaking phenomenon. The reason is that the magnetic interaction between the adjacent magnetic brushes is reduced due to the low magnetization amount, so that the formed magnetic brush ears are dense and short, so that a high resolution image is provided. Can be.

In the magnetic carrier having a low magnetization amount used in the present invention, a magnetic substance produced by a polymerization method comprising at least a binder resin, a magnetic metal oxide and a non-magnetic metal oxide is dispersed. Resin magnetic carriers are preferred. However, the present invention is not limited to this, and any material having the desired magnetization amount, particle size, and specific resistance as described above may be used. For example, the magnetization amount of a conventionally used ferrite carrier or the like is controlled. You may have done it.

Hereinafter, as an example of the magnetic carrier used in the present invention, a method for producing at least a binder resin and a magnetic metal oxide and a non-magnetic metal oxide will be described. The metal oxide dispersed in the binder resin used in the present invention will be described. As the magnetic metal oxide, ferrite which is an iron-based oxide represented by a general formula of MO.Fe ₂ O ₃ or MFe ₂ O ₄ can be preferably used. Here, M is a divalent or monovalent metal ion, Mn, Fe, Ni, Co, Cu, M
g, Zn, Cd, Li and the like correspond to each other, and M can be used alone or as a plurality of metals. For example, magnetite, Mn-Zn ferrite, Ni-Zn ferrite, Mn-Mg ferrite, Li ferrite, Cu
—Zn-based ferrite and the like. Among them,
It is more preferable to use magnetite (Fe ₃ O ₄ ) which is inexpensive and does not contain various metals other than iron.

Other metal oxides usable in the present invention include Mg, Al, Si, Ca, Sc, T
i, V, Cr, Mn, Fe, Co, Ni, Cu, Zn,
Sr, Y, Zr, Nb, Mo, Cd, Sn, Ba, Pb
And the like, a magnetic metal oxide using a single or a plurality,
Non-magnetic metal oxides are exemplified. Non-magnetic metal oxides that can be used in the present invention include, for example, A
l ₂ O ₃ , SiO ₂ , CaO, TiO ₂ , V ₂ O ₅ , Cr
O ₅ , MnO ₂ , Fe ₂ O ₃ , CoO, NiO, CuO, Z
nO, SrO, Y ₂ O ₃ , ZrO _{2 and the} like.

Examples of the binder resin for dispersing and binding the metal oxide include all resins obtained by polymerizing a vinyl monomer. Examples of the vinyl monomer referred to herein include styrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, p-phenylstyrene, p-ethylstyrene, 2,
4-dimethylstyrene, pn-butylstyrene, p-
tert-butylstyrene, pn-hexylstyrene, pn-octylstyrene, pn-nonylstyrene, pn-decylstyrene, pn-dodecylstyrene, p-methoxystyrene, p-chlorostyrene, 3,
Styrene derivatives such as 4-dichlorostyrene, m-nitrostyrene, o-nitrostyrene and p-nitrostyrene, and ethylene and unsaturated monoolefins such as ethylene, propylene, butylene and isobutylene; butadiene,
Unsaturated diolefins such as isoprene, vinyl halides such as vinyl chloride, vinylidene chloride, vinyl bromide and vinyl fluoride; vinyl esters such as vinyl acetate, vinyl propionate and vinyl benzoate; methacrylic acid and methacrylic acid Α-methylene fats such as methyl, ethyl methacrylate, propyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, n-octyl methacrylate, dodecyl methacrylate, 2-ethylhexyl methacrylate, stearyl methacrylate, and phenyl methacrylate Aromatic monocarboxylic esters; acrylic acid and methyl acrylate, ethyl acrylate, n-butyl acrylate, isobutyl acrylate, propyl acrylate, n-octyl acrylate, dodecyl acrylate, 2-ethylhexyl acrylate , Stearyl acrylate, 2-chloroethyl, acrylic acid esters of phenyl acrylate; Mullen acid, maleic acid half ester;
Vinyl ethers such as vinyl methyl ether, vinyl ethyl ether and vinyl isobutyl ether; vinyl ketones such as vinyl methyl ketone, vinyl hexyl ketone and methyl isopropenyl ketone; N-vinyl pyrrole;
N-vinyl compounds such as vinyl carbazole, N-vinyl indole and N-vinyl pyrrolidone; vinyl naphthalenes; acrylic acid or methacrylic acid derivatives such as acrylonitrile, methacrylonitrile and acrylamide;
Acrolein and the like can be mentioned, and those obtained by polymerizing one or more of these are used.

In addition to the resins obtained by polymerizing the above-mentioned vinyl monomers, non-vinyl condensation resins such as polyester resins, epoxy resins, phenol resins, urea resins, polyurethane resins, polyimide resins, cellulose resins, and polyether resins. Resins or mixtures of these with the vinyl resins can be used.

As a method for producing the magnetic carrier used in the present invention from the above materials, a vinyl-based or non-vinyl-based thermoplastic resin, a metal oxide, and other additives such as a curing agent are sufficiently mixed by a mixer. After mixing, heating roll,
Melting using a kneader such as a kneader or extruder,
After kneading, cooling, and then pulverizing and classifying, a magnetic carrier in which the magnetic substance is dispersed may be used. In the present invention, a magnetic carrier produced by a polymerization method described below is preferably used.

The monomers used to obtain a magnetic carrier in which a magnetic material is dispersed by directly mixing and polymerizing a monomer and a metal oxide include, in addition to the above-mentioned vinyl monomers, starting materials for epoxy resins. Bisphenols and epichlorohydrin, phenol resins and aldehydes of phenolic resins, urea and aldehydes of urea resins, melamine and aldehydes, and the like are used. For example, as a method for producing a magnetic carrier using a curable phenol resin, a phenol and an aldehyde are mixed in an aqueous medium in the presence of a basic catalyst, using the above-described metal oxide and, if necessary, a dispersion stabilizer. The suspension is then subjected to suspension polymerization to obtain a magnetic carrier in which a magnetic substance is dispersed.

Generally, in the case of the resin carrier as described above, when the content of the metal oxide is 70% or more,
In particular, because the mechanical strength tends to be brittle,
In order to increase the strength of the carrier, it is preferable to use a crosslinked binder resin. Examples of the method of crosslinking include a method in which a crosslinking component is added during melt-kneading and crosslinking is performed during kneading, a curable resin is directly selected during polymerization, or a monomer containing a crosslinking component is used.

Further, as the magnetic carrier used in the present invention, a carrier obtained by dispersing a magnetic substance obtained by the above-mentioned manufacturing method or the like in a binder resin may be used as it is, but the magnetic carrier is used as a carrier core. Alternatively, an insulating resin may be used as a coating agent and coated on the carrier core surface to be used as a coated magnetic carrier. The coating amount of the coating agent used at that time is 0.5 to 15% by weight.
Is preferably within the range.

The method for measuring the magnetization amount, particle size and specific resistance of the magnetic carrier used in the present invention will be described below. (Carrier Magnetization Amount) The magnetization amount was determined by packing the magnetic properties of the carrier in a cylindrical container in an external magnetic field of 1 kOe using an oscillating magnetic field type magnetic property automatic recording device manufactured by Riken Denshi Co., Ltd. The carrier was placed, and the intensity of magnetization at that time was measured. Thereafter, the amount of magnetization (emu) is multiplied by the true specific gravity of the carrier.
/ Cm ³ ) was calculated.

(Carrier Particle Size) The particle size was determined by randomly extracting 300 carrier particles from a scanning electron microscope FE-SEM (S
-800) and analyzed by an image processing analyzer Luzex3 manufactured by Nireco Co., Ltd., and the Feret diameter in the horizontal direction was calculated as the carrier particle diameter.

(Carrier Specific Resistance) The specific resistance was measured using a measuring device shown in FIG.
The electrodes 30 and 31 are connected to the cell E so as to be in contact with the carrier 33.
Was applied, a voltage 32 was applied between these electrodes, and the current flowing at that time was measured to determine the specific resistance. The specific resistance measurement conditions used in the present invention are as follows: the contact area S between the charged carrier and the electrode is about 2.3 cm ² , the thickness d is about 2 mm,
180 g load on the upper electrode 31, measured electric field strength 5 × 10 ⁴
V / m.

Next, the toner particles used in the image forming apparatus of the present invention will be described. As the non-magnetic toner used in the present invention, a polymerized toner produced by a polymerization method capable of realizing high transfer efficiency as described in the conventional example is used. As a method for producing a toner by a polymerization method, a polymerizable monomer serving as a constituent material of a binder resin, a coloring agent such as a dye or a pigment, a charge control agent, a release agent such as a wax, or the like is used. Dissolve or disperse into a polymerizable composition, disperse in an aqueous continuous phase containing a dispersion stabilizer using a dispersing device to form a dispersion of fine particles, and polymerize this dispersion in the presence of a polymerization initiator. There is a method of obtaining toner particles having a desired particle size and composition by suspension polymerization which solidifies by solidification.

Further, using a liquid medium in which the polymerizable monomer constituting the binder resin is dissolved but the binder resin obtained by the polymerizable monomer is not dissolved, the particles are produced in a solution in which the polymer dispersion stabilizer is dissolved. A so-called toner production method to which dispersion polymerization is applied, and an emulsion polymerization method represented by a soap-free polymerization method in which a toner is produced by directly polymerizing in the presence of a water-soluble polar polymerization initiator, and the like.

The polymerized toner used in the present invention has a shape factor SF-1 of from 100 to 140 and SF-100.
An approximately spherical toner having a number 2 in the range of 100 to 120 is preferable for maintaining high transfer efficiency. In the present invention, 100 shape toner particles are randomly sampled using a scanning electron microscope FE-SEM (S-800) manufactured by Hitachi, Ltd., and the shape factors SF-1 and SF-2 are randomly sampled. The image information is introduced into an image analyzer (Luzex3) manufactured by Nireco Co., Ltd. via an interface, analyzed, and the value calculated by the following equation is used to calculate the shape factor SF-
1 and SF-2.

[0031]

(Equation 1) (AREA: toner projected area, MXLNG: absolute maximum length, PERI: circumference) The above SF-1 indicates the degree of sphericity of the toner, and when the value is larger, the toner gradually changes from spherical to irregular, and SF- 2 indicates the degree of unevenness on the surface of the toner particles. If the value is larger, the unevenness on the surface of the toner becomes remarkable.

Next, the image exposure means used in the image forming apparatus of the present invention will be described. As described above, by using a magnetic carrier having a low magnetization amount, the uneven printing phenomenon in a counter development system using a polymerized toner is improved.
Further, according to the study of the present inventors, the problem of occurrence of the creaking phenomenon is a problem that the digital image gradation reproducing method is used as an image exposure means on the photosensitive drum to reduce the magnitude of the image signal value for each arbitrary pixel. A method of controlling the light emission time of the exposure means in proportion (hereinafter referred to as a PWM method);
It has been found that analog image exposure is more likely to occur as in the case of image gradation deterioration. The reason for this is that, as shown in the center of FIG. 3, in the highlight and halftone areas of the multilevel latent image in the PWM system, the potential contrast of the electrostatic latent image is small, and the adhesion per dot is small. Since the amount of toner is small, the peeling-off unevenness from which the toner has been peeled off when the magnetic brush rubs against the photosensitive drum becomes easy to be seen at the time of development. This is because the electric field component of the toner peeling off the photosensitive drum due to the alternating electric field becomes larger than the high contrast area such as the solid portion, so that the toner is easily peeled off electrostatically.

Therefore, in the present invention, as an image exposure means for reducing the above-described phenomenon, a multi-valued digital image signal corresponding to a binary digital image signal obtained by binarization processing is used. It is preferable to adopt a method of irradiating the image carrier with light. For example, a binary exposure corresponding to a binary digital image signal is performed using an LED array to form a binary latent image. By adopting such image exposure means,
As shown in FIG. 3, since the potential contrast of each pixel becomes the same level in the highlight area and the halftone area and the solid area, the toner amount per pixel in the highlight area and the halftone area is reduced. The number becomes larger than in the case of a multi-valued latent image in the PWM method, and the unevenness in which the toner is peeled off is not conspicuous even by counter development using a two-component developer. Further, since the toner peeling electric field component formed by the developing bias AC and the latent image potential is smaller than that of the PWM system, the toner is less likely to be electrostatically peeled off.

The image exposure means converts a multivalued digital image signal into a binary digital image signal.
By adding a binarizing device, the above-described effects can be obtained even when the image signal is multi-valued. In particular, the error diffusion image signal 2
If the binarization is performed by the binarizing device, an image can be formed by the binary image signal without deteriorating the gradation of the multi-valued image signal, so that a high-quality and high-gradation image can be obtained. Become.

Next, the developing bias used in the image forming apparatus of the present invention will be described. In the image forming apparatus of the present invention, a DC bias on which an AC bias is superimposed is applied as a developing bias to a portion where the image carrier and the developer carrier are opposed. Although the development bias waveform at this time is a so-called rectangular wave, the use of a magnetic carrier having a low magnetization amount improves the hakimema phenomenon as described above, but as a development bias exhibiting a further effect, It is preferable to apply those described below. In other words, the application of the developing bias is a step of applying a voltage for applying a force to the two-component developer in a direction from the image carrier (photosensitive drum) to the developer carrier (developing sleeve) for a certain time. And a step of applying a voltage for applying a force from the developing sleeve to the photosensitive drum to the component-based developer for a certain period of time alternately, and applying a force from the developing sleeve to the photosensitive drum with the AC bias. After applying the voltage,
By applying a DC voltage between the voltage of the image portion and the voltage of the non-image portion of the electrostatic latent image on the photosensitive drum to the developing sleeve for a certain period of time, by repeating a combination cycle of these AC bias and DC bias. (Hereinafter, this developing bias is also referred to as a blank pulse bias).

By applying a developing bias as described above, after applying a DC bias (hereinafter referred to as a blank bias) for causing toner to fly only to the image area, an AC bias for vibrating the toner near the photosensitive drum is applied. In the image area, the T /
As a result, the D ratio is increased. As a result, the toner can be sufficiently and uniformly supplied to the halftone region, and a smooth image with no noticeable unevenness can be obtained. on the other hand,
In the non-image portion, the developer does not vibrate in the vicinity of the photosensitive drum and is slowly pulled back to the developing sleeve side, so that fog does not increase.

Further, in addition to the above-described blank pulse bias, another binary latent image formed by irradiating the image carrier with light corresponding to the binary digital image signal described above is used. Advantages will be described with reference to FIG. That is, in a multi-valued latent image such as PWM, the potential difference between the electrostatic latent image voltage of the highlight portion and the halftone portion and the return bias increases as shown in the center of FIG. Therefore, in order to prevent the toner from vibrating only near the photoconductor drum and causing unevenness, the bias waveform is complicated, such as shortening the time for applying the return bias or increasing the time for applying the feed bias. There is a need to.
On the other hand, when a binary image is formed by the binary image exposure means, the potential difference between the feed bias and the return bias is reduced, so that the time for applying the return bias can be lengthened. Here, the high-voltage power supply used in the development is cheaper if the generated bias waveform is simpler and the frequency is lower, so that the cost of the development power supply can be reduced by using a binary latent image.

Next, the image forming apparatus of the present invention will be described in more detail with reference to the drawings. FIG. 2 is a schematic configuration diagram of an example of an electrophotographic full-color image forming apparatus that can suitably use the image forming apparatus of the present invention. The image forming apparatus shown in FIG. 2 includes a photosensitive drum 3 rotating in the direction of an arrow, and a charger 4 and developing units 1M, 1C, and 1Y around the photosensitive drum 3.
Rotary developing device 1 provided with a transfer device 1Bk and a transfer discharger 1
0, a cleaning unit 12, a photosensitive drum 3, and an image forming unit including a laser beam scanner LS and the like arranged above the drawing. Each developing unit is
A two-component developer of each color containing toner particles and carrier particles is supplied to the photosensitive drum 3. The developer of the developing device 1M contains magenta toner, the developer of the developing device 1C contains cyan toner, the developer of the developing device 1Y contains yellow toner, and the developer of the developing device 1Bk contains black toner.

In the image forming apparatus shown in FIG.
An original reading device having a photoelectric conversion element such as a CD outputs magenta image information, cyan image information, yellow image information, and black-and-white image information of the original to corresponding image signals. A semiconductor laser incorporated in the laser beam scanner is controlled according to these image signals, and emits a laser beam L. According to the image forming apparatus shown in FIG. 2, an output signal from an electronic computer can be printed out.

In the sequence of the entire full-color image forming apparatus, first, the photosensitive drum 3 is uniformly charged by the charger 4. Next, scanning exposure is performed by the laser light L modulated by the magenta image signal, and an electrostatic latent image corresponding to the magenta image signal is formed on the photosensitive drum 3.
This electrostatic latent image is reversely developed by the magenta developing device 1M fixed at the developing position. On the other hand, a transfer material such as paper taken out of the paper feed cassette C and advancing through the paper feed guide 5a, the paper feed roller 6, and the paper feed guide 5b is held by the gripper 7 of the transfer drum 9 and is brought into contact with the transfer drum 9. Is electrostatically wound around the transfer drum 9 by the use roller 8 and its opposite pole. The transfer drum 9 rotates in the direction indicated by the arrow in synchronization with the photosensitive drum 3, and the magenta image developed by the magenta developing device 1M is transferred onto the transfer material by the transfer charger 10 in the transfer section. The transfer drum 9 continues to rotate, and prepares for transfer of an image of the next color (cyan in the apparatus of FIG. 2).

The photosensitive drum 3 is neutralized by the neutralizer 11, the remaining toner is cleaned by the cleaning unit 12, charged again by the charger 4, and is charged by the laser beam L modulated by the next cyan image signal. Receive the same exposure as when magenta image exposure,
An electrostatic latent image is formed on the photosensitive drum 3. During this time, the developing device 1 is rotated and the cyan developing device 1C is fixed at a predetermined developing position, and performs reversal development of the electrostatic latent image corresponding to cyan to form a cyan image. Further, the above steps are performed on the yellow image signal and the black and white image signal, respectively.
The transfer of the color image (toner image) is completed, and the full-color transfer image is transferred to the transfer material. When the transfer is completed, the transfer-receiving material is neutralized by the neutralizers 13 and 14 to release the gripper 7, is separated from the transfer drum 9 by the separation claw 15, and is fixed by the transport belt 16 to a fixing device (for example, a heat roller fixing device or the like). ) 17 is sent. The fixing device 17 fixes the four color toner images overlapping on the transfer material. Thus, a series of full-color image forming sequences is completed,
The required full-color image is formed on the transfer material.

FIG. 1 is an enlarged schematic view of a portion shown as a developing device 1 in FIG. The developing device used in the present invention will be described in more detail with reference to FIG. Since the developing units 1M, 1C, 1Y and 1Bk have the same configuration, only the developing unit 1M (magenta developing unit) will be described here. The developing device 1M includes a developing container 18, and the inside thereof is separated by a partition wall 19 into a developing chamber R1 and a stirring chamber R2.
A toner storage chamber R3 is provided above the stirring chamber R2, and contains toner 20 for replenishment. From the replenishing port 21 below the toner storage chamber R3, an amount of toner corresponding to the toner consumed in the development is dropped and replenished into the stirring chamber R2. On the other hand, the two-component developer 22 in which the toner particles and the magnetic carrier are mixed as described above is contained in the developing chamber R1 and the stirring chamber R2. The two-component developer used in the present invention is a non-magnetic polymerized toner and a magnetic carrier as described above having a T / D ratio of 5 to 12.
%, It is preferable to use a mixture of the toner and the carrier.

A developer conveying screw 2 is provided in the developing chamber R1.
3 is accommodated therein, and transports the developer along the longitudinal direction of the developing sleeve 25 by rotational driving. Also, stirring chamber R
The direction in which the developer is conveyed by the screw 24 housed in 2 is opposite to that by the screw 23. The partition wall 19 is provided with openings on the near side and the back side, and the developer conveyed by the screw 23 has one of the openings.
To the screw 24, and the screw 2
The developer conveyed at 4 is delivered to the screw 23 from the other of the openings.

Further, an opening is provided in a portion of the developer container 18 close to the photosensitive drum 3. The opening is provided with a non-magnetic developing sleeve 25 made of a non-magnetic material such as aluminum or non-magnetic stainless steel, the surface of which is provided with appropriate irregularities. The developing sleeve 25 rotates at a peripheral speed Vb in the direction of arrow b (the direction opposite to the rotating direction of the photosensitive drum), and furthermore, the surface thereof is controlled by a layer thickness regulating blade 28 at the lower end of the opening of the developing container. After the developer whose thickness is regulated is carried, the developer is transported to the developing area 26. The magnetic brush made of the developer carried by the developing sleeve 25 comes into contact with the photosensitive drum 3 rotating at the peripheral speed Va in the direction of the arrow a in the developing area 26 so that the static brush formed on the photosensitive drum 3 is formed. The latent image is developed in the development area 26.

At this time, the peripheral speed ratio of the peripheral speed Vb of the developing sleeve 25 to the peripheral speed Va of the photosensitive drum (hereinafter, simply referred to as the photosensitive drum peripheral speed ratio) is 130 to 200%.
And more preferably 150 to 180%. That is, if the peripheral speed ratio of the photosensitive drum is less than 130%, a sufficient image density cannot be obtained, and if it exceeds 200%, the developer is scattered.

In the developing sleeve 25, a roller-shaped magnet 29 is fixedly arranged. The magnet 29 has a development magnetic pole S1 facing the development area 26. Development magnetic pole S
1 forms a magnetic brush made of a developer on the developing sleeve 25 by a developing magnetic field formed in the developing area 26;
This magnetic brush contacts the photosensitive drum 3 to develop the electrostatic latent image. At this time, together with the toner adhering to the magnetic brush, the toner adhering to the sleeve surface is transferred to the image area of the electrostatic latent image and used for development. In the example shown in FIG. 1, the magnets are N1, N2, and N1 in addition to the developing magnetic pole S1.
3 and S2 poles. With such a configuration, the N3 pole and the S2
The developer applied on the developing sleeve 25 at the pole passes through the layer thickness regulating blade 28, is regulated to an appropriate developer layer thickness, reaches the developing magnetic pole S1, and the developer that has spiked in the magnetic field is removed. The electrostatic latent image on the photosensitive drum 3 is developed.

Thereafter, due to the repulsive magnetic field between the N2 and N3 poles, the developer remaining on the developing sleeve 25 is removed from the stirring chamber R
Fall into 1 The developer that has fallen into the stirring chamber R1 is stirred and transported by the screws 23 and 24. In addition, the magnetic pole configuration of the magnet in the developing sleeve is not limited to the above example,
For example, a configuration in which one of the repulsion magnetic poles is arranged near the opposing layer thickness regulating blade 28 may be used.

[0048]

Next, Reference Examples, Examples and Comparative Examples will be given.
The present invention will be described more specifically. As the image forming apparatus used in the reference examples, examples and comparative examples, an electrophotographic full-color image forming apparatus (see FIG. 2) incorporating the above-described developing device (see FIG. 1) was used. . Reference Example 1 The magnetic carrier, non-magnetic polymerized toner used in Reference Example 1,
A latent image forming method, a developing bias, and the like will be described. First, a mixture of a polymerized toner produced by a suspension polymerization method and a resin magnetic carrier produced by a polymerization method was used as a two-component developer. The T / D ratio of the obtained developer was 8%. As a magnetic carrier, the amount of magnetization in a magnetic field of 1 kOe is 100 emu / cm ³ ,
And the number average particle size is 40 μm, and the specific resistance is 1
A thing of 0 ¹³ Ω · cm was used. As the non-magnetic polymerized toner, the shape factor SF-1 is 115 and SF-2 is 11
0, a substantially spherical toner having a smooth surface, a weight average particle size of 8 μm, and a specific gravity of 1.05 g / cm ³
The toner having an average charge per unit mass of 25 μc / g was used.

[0049] In this reference example, using a two-component developer, such as described above, was developed an electrostatic latent image on the photosensitive drum by the developing apparatus shown in FIG. Developing sleeve 2 at that time
5 has a weight per unit area of 40 mg / cm.
^{And 2} . The gap between the photosensitive drum 3 and the developing sleeve 25 was set to 500 μm. The peripheral speed of the developing sleeve 25 was 100 mm / sec, the peripheral speed of the photosensitive drum 3 was 150 mm / sec, and the ratio of the peripheral speed of the developing sleeve to the photosensitive drum was 150%. Further, the electrostatic latent image on the photosensitive drum 3 is a multi-valued PWM system and has a resolution of 400 dp.
i. The developing bias is obtained by subtracting the potential of the solid image portion from-
230 V and the potential of the non-image portion was -720 V. Furthermore,
The shape of the developing bias, a rectangular wave, V _pp = 2 kV, frequency 2 kHz, and the V _DC = -570V.

Under the above conditions, counter development was performed using a two-component developer having a polymerized toner, and a γ curve showing the relationship between development contrast and image density was obtained. The image density showed a good correlation, and the gradation was good. In addition, in the obtained image, no uneven spots occurred in the halftone region.

Comparative Example 1 In this comparative example, an image was formed in the same manner as in Reference Example 1 except that a magnetic carrier made of Cu-Zn ferrite was used and the T / D ratio of the two-component developer was 6%. Was. The magnetic amount of the magnetic carrier used in this comparative example in a magnetic field of 1 kOe was 250 emu / cm ³ , the number average particle size was 40 μm, and the specific resistance was 10 ⁸ Ω · cm. FIG.
FIG. 7 shows the γ curve obtained in this comparative example. The correlation between the development contrast and the image density was lower in gradation than in Reference Example 1. Further, in the obtained image, ghosting unevenness occurred in the halftone portion region.

Reference Example 2 In this reference example, the same weight average particle diameter as in Reference Example 1 was 8 μm.
m, a specific gravity of 1.05 g / cm ³ , an average charge per unit mass of 25 μc / g, and a substantially spherical polymerization produced by suspension polymerization having a shape factor SF-1 of 115 and SF-2 of 110. Using a toner and a magnetic carrier, a magnetization amount in a magnetic field of 1 kOe is 200 emu / cm ³ ,
Controlled specific resistance to 10 ¹² Ω · cm, number average particle size is 4
A two-component developer prepared using a 0 μm ferrite carrier was used. Further, as a method of forming an electrostatic latent image on the photosensitive drum under development conditions, a binary ED (LED exposure) obtained by binarization processing and a resolution of 400 dpi were used in the same manner as in Reference Example 1. To form an image. That is,
The weight per unit area of the developer on the developing sleeve 25 is 50 mg / cm ² , the gap between the photosensitive drum and the developing sleeve is 500 μm, the developing bias is -230 V for the image portion potential and the non-image portion potential. -720V.
Note that the potential of one dot was the same in both the highlight portion and the solid portion. The shape of the developing bias is a rectangular wave, and V _pp = 2k
V, the frequency was 2 kHz, and V _DC = −570 V.

As a result, as in the case of Reference Example 1, the obtained image was free from unevenness in the halftone region and had good gradation. The configuration in this reference example is
In order to increase the amount of magnetization of the magnetic carrier as compared with Reference Example 1 and to reduce toner scattering from the developing sleeve, T /
Although the D ratio has been lowered, by making it a binary latent image,
As a result, a good image free of unevenness similar to that of Reference Example 1 could be obtained.

Example In this example, first, a mixture of a polymerized toner produced by a suspension polymerization method and a resin magnetic carrier produced by a polymerization method was used as a two-component developer. The T / D ratio of the obtained developer was 8%. As a magnetic carrier, the magnetization amount in a magnetic field of 1 kOe is 150 em
u / cm ³ , a number average particle size of 30 μm, and a specific resistance of 10 ¹³ Ω · cm.
Further, as a non-magnetic polymerized toner, the shape factor SF-1 is 1
15. A substantially spherical toner having a smooth surface with SF-2 of 110, a weight average particle diameter of 6 μm, and a specific gravity of 1.05 g / cm ³ , and an average charge per unit mass of 30 μc / g of toner was used.

Further, in the present embodiment, in forming an image, a binary latent image similar to that in Reference Example 2 was used as an electrostatic latent image on a photosensitive drum, but a blank pulse bias was used as a developing bias. That is, the latent image has a binary ED (LED exposure) and a resolution of 400 dpi, and the potential of the image portion is -2.
The potential of the non-image portion was set to −720 V at 30 V (the potential of one dot was the same for both the highlight portion and the solid portion). The developing bias is a blank pulse bias, one pulse of 6 kHz, a blank portion of 6 kHz, and V _pp = 2
kV and V _DC = −570 V.

As a result, the obtained image was free from unevenness in the halftone region and had very good image characteristics such as gradation. The configuration in this embodiment is
By adopting the binary latent image and the blank pulse bias, not only the unevenness but also other image quality can be improved. Therefore, it is possible to provide an image equal to or more than that of Reference Examples 1 and 2. It has become possible.

[0057]

As described above, according to the present invention,
In a system employing a contact counter magnetic brush development system using an AC bias application using a two-component developer containing a polymerized toner, uniform gradation is achieved without gradation and improvement in low density areas. And a halftone image can be provided.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view of a developing device in an image forming apparatus according to an embodiment of the present invention.

FIG. 2 is a schematic configuration diagram of an example of an electrophotographic color image forming apparatus to which the image forming apparatus of the present invention is applied.

FIG. 3 is a model diagram of a latent image potential and a developing electric field formed by the image forming apparatus of the present invention.

FIG. 4 shows a magnetization amount of 100 emu / cm ³ used in Reference Example 1.
And the amount of magnetization of 250 emu /
⁷ is a γ curve of a carrier of cm ³ .

FIG. 5 is a schematic view of an apparatus for measuring the specific resistance of a magnetic carrier used in the present invention.

[Explanation of symbols]

 1: Developing device 1M: Magenta developing device 1C: Cyan developing device 1Y: Yellow developing device 1Bk: Black developing device 3: Photoconductor drum 4: Charger 11, 13, 14: Static eliminator 5a, 5b: Feed guide 6: Paper feed roller 7: Gripper 8: Contact roller 9: Transfer drum 10: Transfer charger 12: Cleaning means 15: Separation claw 16: Conveyor belt 17: Fixer 18: Developer container 19: Partition wall 20: Toner 21: Toner supply port 22: Developer 23, 24: Developer transport screw 25: Development sleeve 26: Development area 28: Layer thickness regulating blade 29: Magnet 30: Lower electrode 31: Upper electrode 32: Voltage 33: Carrier R1: Development chamber R2: stirring chamber R3: toner storage chamber N1, N2, N3, S1, S2: magnetic pole C: paper feed cassette L: laser beam LS: laser Beam scanner E: cell

──────────────────────────────────────────────────の Continuation of the front page (51) Int.Cl. ⁷ Identification code FI G03G 15/08 507 G03G 9/08 384 9/10 331 (72) Inventor Yoshinobu Baba 3-30-2 Shimomaruko, Ota-ku, Tokyo Inside Canon Inc. (72) Inventor Takeshi Ikeda 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (56) References JP-A-7-295389 (JP, A) JP-A-7-77841 (JP, A) JP-A-8-160671 (JP, A) JP-A-6-51563 (JP, A) JP-A-5-83279 (JP, A) JP-A-7-209952 (JP, A) JP-A-8-95336 (JP, A) JP-A-7-140791 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G03G 15/06-15/095 G03G 9/10 G03G 9/107

Claims (4)

(57) [Claims] An image exposure device is provided to form an electrostatic latent image on an initially charged image carrier by an image exposure means and develop the electrostatic latent image into a visible image. A rotatable developer carrier facing the image carrier, for carrying and transporting a two-component developer having a non-magnetic toner and a magnetic carrier generated by a polymerization method; and In an image forming apparatus provided with a developing device having a developing unit in which a plurality of magnetic field generating devices fixedly provided inside the body are arranged, the developing device includes an image carrier, a developer carrier, both bearing member opposing portion moves in opposite directions, in said opposing portion, Do is applied to the developer carrying member a DC bias obtained by superimposing an AC bias as a developing bias
This is a magnetic brush developing device that develops an electrostatic latent image with the above-described two-component developer , and the application of a developing bias during development applies the image to the two-component developer.
Voltage that gives a force in the direction from the carrier to the developer carrier
Step of applying for a certain time, and conversely, two-component developer
A voltage for applying a force from the carrier to the image carrier for a certain time
AC bias that repeats the application step alternately multiple times
And the image carrier from the developer carrier of the AC bias
After applying a voltage that gives a force toward
Between the voltage of the image part and the voltage of the non-image part of the electrostatic latent image
Applying a DC voltage to the developer carrier for a certain period of time
Performed by, the magnetic carrier, there magnetization in the magnetic field of one kOe within the range of 30~200emu / cm ^3, the resistivity is in the range of ^{1 × 10 10 ~1 × 10 14} Ω · cm There, and number average particle diameter Ri <br/> Ah in the range of 10 to 60 [mu] m, the non-magnetic toner, a shape factor SF-1 of 100-140及
And SF-2 are produced by a polymerization method in the range of 100 to 120.
Made a spherical or an image forming apparatus that substantially spherical toner der wherein Rukoto. 2. The image forming apparatus according to claim 1, wherein the magnetic carrier is a resin magnetic carrier having a carrier core formed by a polymerization method comprising a binder resin, a magnetic metal oxide and a non-magnetic metal oxide. Wherein the image exposure means, according to claim 1 or 2 is irradiated with light corresponding to the digital image signals of two values obtained by binarizing the multivalued digital image signals to the image bearing member Image forming apparatus. 4. An electrostatic latent image is inverted by a two-component developer.
The toner according to any one of claims 1 to 3, which is developed by a developing method.
The image forming apparatus as described in the above.