JP3976936B2 - Image forming apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an image forming apparatus such as a copying machine, a printer, and a facsimile that performs color image formation using an electrophotographic system.
[0002]
[Prior art]
As an image forming apparatus for forming a color image, for example, a toner image of each color formed on a photosensitive drum as an image carrier is sequentially superimposed on a transfer material such as paper held on a transfer drum (transfer film). An image forming apparatus for forming a color image by transferring the image has been put into practical use.
[0003]
In such an image forming apparatus, an electrostatic latent image formed on a photosensitive drum is developed with a first color toner (for example, cyan) based on an input image signal to form a toner image, and the toner image Is transferred to a transfer material such as paper held on a transfer drum (transfer film). This transfer process is similarly performed for the other three colors, that is, magenta, yellow, and black toners, and four color toner images are sequentially superimposed and transferred onto a transfer material, thereby obtaining a color image.
[0004]
[Problems to be solved by the invention]
By the way, when the color image forming apparatus as described above is, for example, a color copying machine, it does not occur so much in a text original, but in a thin halftone original such as a photographic original, the color image formed by copying is fine. Density unevenness is likely to occur. In general, this density unevenness is said to be “gastling”.
[0005]
The density unevenness is visually noticeable particularly in cyan and magenta among cyan, magenta and yellow colors.
[0006]
Such a phenomenon is a characteristic phenomenon in an electrophotographic image forming apparatus. As shown in FIG. 8, the copy density (copy reflection density) of the actual copy line A with respect to the ideal copy line is shown. Indicates a feature that the density is lower than the original density (original reflection density) in the low density area, the copy density increases rapidly from the middle density of the original density, and reaches the maximum density quickly.
[0007]
In the case of a one-component developer, the adhesion between the toner and the developing sleeve is strong, and in the case of a two-component developer, the adhesion force between the toner and the carrier (mainly, the reflection force of the toner onto the developer carrying member) is strong. Because the toner charge amount distribution is non-uniform, when they are peeled off by the developing bias and are going to fly to the photosensitive drum, the toner in one place is easy to fly, and the toner in other places is hard to fly. As a result, the above-described density unevenness occurs.
[0008]
Further, according to the study of the present inventor, it has been found that what makes the phenomenon of density unevenness more conspicuous is that the “density” of one toner is large. That is, in the halftone area, the amount of toner developed is overwhelmingly smaller than that of the solid part.
[0009]
For example, let 10 be the amount of dye required to produce a certain concentration. The amount of pigment possessed by one dark toner is 7. The amount of the dye of one light toner is set to 3, which is about half. Then, even if there is no problem with the above-described toner charge amount distribution, the density can only be 14 or 7 with the dark toner, and thus the degree of density unevenness inevitably increases due to the density difference with respect to the target value 10. The light toner becomes 12 or 9, so the density difference can be reduced and density unevenness can be reduced.
[0010]
That is, if the “density” of one toner is small, even if the toner adhesion amount suddenly increases as described above, the density unevenness (glossiness) in the halftone area should be inconspicuous. However, if the “density” of one toner is small, there arises a problem that the maximum reflection density (see FIG. 8) decreases.
[0011]
Therefore, an object of the present invention is to provide an image forming apparatus capable of forming a high-quality image while suppressing the occurrence of density unevenness (gray) even in the case of a thin halftone image or the like.
[0012]
[Means for Solving the Problems]
  In order to achieve the above object, the present invention relates to an electrophotographic image forming apparatus that forms a color image by superimposing a plurality of color toner images, and includes a first cyan toner, a first magenta toner, a yellow toner, and a black toner. A first developing unit that develops the toner image of each color on the image carrier with each color toner, a second cyan toner having a maximum reflection density lower than that of the first cyan toner, and a maximum reflection density of the first toner. And a second developing means for developing the second cyan toner and a toner image by the second magenta toner on the image carrier with a second magenta toner having a magenta color smaller than one magenta toner. For the 1 cyan toner and the 2nd cyan toner, the same electrostatic latent image formed on the image carrier is first developed with the 2nd cyan toner. Thereafter, development is performed with the first cyan toner, and for the first magenta toner and the second magenta toner, the same electrostatic latent image formed on the image carrier is formed with the second magenta toner. Develop first and then develop with the first magenta tonerThe difference between the halftone potential of the electrostatic latent image formed on the image bearing member and the DC component of the developing bias at the time of development is calculated as the difference at the time of development with the second cyan toner and the second magenta toner. Is larger than that when developing with the first cyan toner and the first magenta toner.It is characterized by that.
[0013]
  The second cyan toner and the second magenta tonerThe mostThe large reflection density is the first cyan toner or the first magenta toner.The mostIt is characterized by being less than half of the large reflection density.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[0017]
<Embodiment 1>
FIG. 1 is a schematic configuration diagram illustrating an image forming apparatus according to Embodiment 1 of the present invention (in this embodiment, a full-color copying machine including a transfer drum).
[0018]
In FIG. 1, A is a printer unit, and B is an image reading device (image scanner) mounted on the printer unit A.
[0019]
In the image reading section B, reference numeral 20 denotes a fixed platen glass. The platen glass 20 is placed on the upper surface of the platen glass 20 with the surface on which the original G is to be copied facing down, and a document plate (not shown) is placed thereon. Set. An image reading unit 21 is provided with a document irradiation lamp 21a, a short focus lens array 21b, a CCD sensor 21c, and the like.
[0020]
When a copy button (not shown) is pressed, the image reading unit 21 is driven forward from the home position on the left side of the original table glass 20 to the right side along the lower surface of the glass on the lower side of the original table glass 20. When a predetermined reciprocating end point is reached, the actuator is driven backward and returned to the initial home position.
[0021]
In the forward drive process of the image reading unit 21, the downward image surface of the set original G on the platen glass 20 is sequentially illuminated and scanned from the left side to the right side by the original irradiation lamp 21a, and the original of the illumination scanning light is scanned. The surface reflected light is imaged and incident on the CCD sensor 21c by the short focus lens array 21b.
[0022]
The CCD sensor 21c includes a light receiving unit, a transfer unit, and an output unit (not shown). In the light receiving unit, an optical signal is changed to a charge signal and is sequentially transferred to the output unit in synchronization with a clock pulse. In the output section, the charge signal is converted into a voltage signal, amplified and reduced in impedance, and output. The analog signal thus obtained is converted into a digital signal by well-known image processing and output to the printer unit A. That is, the image information of the original G is photoelectrically read as a time-series electric digital pixel signal (image signal) by the image reading unit B.
[0023]
On the other hand, in the printer unit A, a primary charger 2, a developing device 4, a transfer drum 5, a cleaning device 6, a pre-exposure lamp 7, and the like are installed around the photosensitive drum 1 as an image carrier. A fixing device 9 is installed on the downstream side of the transfer drum 5 in the transfer material conveyance direction, and a laser beam scanning exposure type exposure device (laser scanning device) 3 is installed above the printer device A.
[0024]
In the present embodiment, the photosensitive drum 1 is a negatively charged organic photoreceptor, and has a photoelectric layer on a conductive drum substrate (not shown) such as aluminum, and is in the direction of the arrow (counterclockwise) at a predetermined process speed. In this embodiment, the primary charger 2 receives a uniform negative charging process.
[0025]
The exposure device 3 performs laser scanning exposure L on the surface of the photosensitive drum 1 based on the image signal input from the image reading unit 21 to form an electrostatic latent image.
[0026]
FIG. 2 is a schematic block diagram showing the exposure apparatus 3. When the exposure device 3 performs laser scanning exposure L on the surface of the photosensitive drum 1, first, the solid-state laser element 25 is lightened or darkened at a predetermined timing by the light emission signal generator 24 based on the image signal input from the image reading unit 21 ( ON / OFF). The laser light, which is an optical signal emitted from the solid-state laser element 25, is converted into a substantially parallel light beam by the collimator lens system 26, and the photosensitive drum 1 is further moved by the rotary polygon mirror 22 that rotates at high speed in the direction of arrow c. By scanning in the direction of the arrow d (longitudinal direction), a laser spot is imaged on the surface of the photosensitive drum 1 by the fθ lens group 23 and the reflection mirror 27 (see FIG. 1). By such laser scanning, an exposure distribution corresponding to the scanning is formed on the surface of the photosensitive drum 1, and further, by scrolling a predetermined amount perpendicular to the surface of the photosensitive drum 1 for each scanning, an image is formed on the surface of the photosensitive drum 1. An exposure distribution according to the signal is obtained.
[0027]
  That is, the surface of the photosensitive drum 1 is scanned on the surface of the photosensitive drum 1 by scanning the light of the solid laser element 25 that emits light ON / OFF corresponding to the image signal by the rotating polygon mirror 22 that rotates at high speed. An electrostatic latent image of each color corresponding to the scanning exposure pattern is sequentially formed.In the present embodiment, there are four colors of cyan, magenta, yellow, and black.
[0028]
  As shown in FIG. 3, the developing device 4 includes a cyan developing device 4Ca containing cyan toner,Light cyan color with maximum reflection density smaller than the cyan tonerCyan developer 4Cb containing cyan a toner, magenta developer 4Ma containing magenta toner,Light magenta with a maximum reflection density smaller than the magenta toner.A magenta developing device 4Mb containing magenta toner, a yellow developing device 4Y containing yellow toner, and a black developing device 4K containing black toner are supported on a rotatable rotating member 4A and formed on a photosensitive drum 1. The electrostatic latent image is developed and visualized (visualized).In this embodiment, the cyan electrostatic latent image is developed in the order of the cyan developing device 4Cb and the cyan developing device 4Ca, and the magenta electrostatic latent image is developed in the order of the magenta developing device 4Mb and the magenta developing device 4Ma. . The yellow electrostatic latent image is developed by the yellow developing device 4Y, and the black electrostatic latent image is developed by the black developing device 4K.
[0029]
As the cyan developing unit 4Ca, cyan developing unit 4Cb, magenta developing unit 4Ma, magenta developing unit 4Mb, yellow developing unit 4Y, and black developing unit 4K, in this embodiment, a two-component developing unit as shown in FIG. 4 is used. It was.
[0030]
The two-component developing device includes a developing sleeve 30 that is rotationally driven in the direction of arrow e, and a magnet roller 31 is fixedly disposed in the developing sleeve 30. The developing container 32 is provided with a regulating blade 33 for forming a thin layer of the developer T on the surface of the developing sleeve 30.
[0031]
Further, the interior of the developer container 32 is divided into a developing chamber (first chamber) Rl and a stirring chamber (second chamber) R2 by a partition wall 36, and a toner hopper 34 is disposed above the stirring chamber R2. Yes. Conveying screws 37 and 38 are installed in the developing chamber Rl and the stirring chamber R2, respectively. The toner hopper 34 is provided with a replenishing port 35, and when toner is replenished, the toner t drops and is replenished into the stirring chamber R2 through the replenishing port 35.
[0032]
On the other hand, the developer T in which the toner particles and the magnetic carrier particles are mixed is accommodated in the developing chamber Rl and the stirring chamber R2.
[0033]
As the toner t, a known product in which a colorant or a charge control agent is added to a binder resin can be used, and the volume average particle diameter of the toner t is 6 μm.
[0034]
The volume average particle diameter of the toner t can be measured by, for example, the following measurement method.
[0035]
As a measuring device, for example, using a Coulter counter TA-II type (manufactured by Coulter), an interface (manufactured by Nikka) and a CX-i personal computer (manufactured by Canon) for outputting number average distribution and volume average distribution are connected, As the electrolytic solution, 1% NaCl aqueous solution is prepared using primary sodium chloride.
[0036]
As a measurement method, 0.1 to 5 ml of a surfactant (preferably alkylbenzene sulfonate) is added as a dispersant to 100 to 150 ml of the electrolytic aqueous solution, and 0.5 to 50 mg of a measurement sample is further added. The electrolytic solution in which this sample is suspended is subjected to a dispersion treatment for about 1 to 3 minutes with an ultrasonic disperser. As an aperture with a measuring device (Coulter Counter TA-II type), particles of 2 to 40 μm are used with a 100 μm aperture. Measure the particle size distribution and determine the volume distribution. The volume average particle diameter of the sample is obtained from the obtained volume distribution.
[0037]
On the other hand, as the magnetic carrier, a magnetic particle whose surface is coated with a very thin resin is preferably used, and the average particle size is preferably 5 to 70 μm. The average particle size of the carrier is indicated by the maximum horizontal chord length, and the measurement method is to randomly select 300 or more carriers by microscopy, measure the diameter, and take the arithmetic average to obtain the carrier particle size of this embodiment. It was.
[0038]
Further, the developer T in the developing chamber Rl is conveyed in the longitudinal direction of the developing sleeve 30 by the rotational driving of the conveying screw 37. The developer T in the stirring chamber R <b> 2 is transported in the longitudinal direction of the developing sleeve 30 by the rotational driving of the transport screw 38. The developer conveying direction by the conveying screw 38 is opposite to that by the conveying screw 37.
[0039]
The partition wall 36 is provided with openings (not shown) on the front side and the back side that are perpendicular to the paper surface, and the developer T transported by the transport screw 37 is fed from one of the openings to the transport screw. The developer T that has been delivered to the delivery screw 38 and carried by the delivery screw 38 is delivered to the delivery screw 36 from the other one of the openings. The toner is charged to a polarity for developing the latent image by friction with the magnetic particles.
[0040]
A developing sleeve 30 made of a non-magnetic material such as aluminum or non-magnetic stainless steel is provided in an opening provided in a portion of the developer container 32 close to the photosensitive drum 1 and is in the direction of arrow e (counterclockwise). The developer T, which is rotated and mixed with toner and carrier, is carried and conveyed to the developing unit C. The magnetic brush of developer T carried on the developing sleeve 30 contacts the photosensitive drum 1 rotating in the direction of arrow a (clockwise) at the developing portion C, and the electrostatic latent image is developed at the developing portion C.
[0041]
A vibration bias voltage obtained by superimposing a DC voltage on an AC voltage is applied to the developing sleeve 30 by a power source (not shown). The dark portion potential (non-exposed portion potential) and the bright portion potential (exposed portion potential) of the electrostatic latent image are located between the maximum value and the minimum value of the vibration bias potential. As a result, an alternating electric field whose direction changes alternately is formed in the developing portion C. In this alternating electric field, the toner and the carrier vibrate vigorously, and the toner adheres to the surface of the photosensitive drum 1 corresponding to the latent image by shaking off the electrostatic constraint on the developing sleeve 30 and the carrier.
[0042]
The difference between the maximum value and the minimum value (the peak-to-peak voltage) of the vibration bias voltage is preferably 1 to 5 KV, and the frequency is preferably 1 to 10 KHz. Further, a rectangular wave, a sine wave, a triangular wave, or the like can be used as the vibration bias voltage waveform.
[0043]
And, the DC voltage component is a value between the dark part potential and the light part potential of the electrostatic latent image, but the absolute value is closer to the dark part potential than the minimum light part potential. This is preferable in preventing fog toner from adhering to the dark potential region. The minimum gap between the developing sleeve 30 and the photosensitive drum 1 (the minimum gap position is in the developing portion C) is preferably 0.2 to 1 mm.
[0044]
The amount of the developer T that is regulated by the regulating blade 33 and conveyed to the developing unit C is the developing sleeve of the magnetic brush of the developer T formed by the magnetic field in the developing unit C by the developing magnetic pole S1 of the magnet roller 31. It is preferable that the height on the surface 30 be an amount that is 1.2 to 3 times the minimum gap value between the developing sleeve 30 and the photosensitive drum 1 with the photosensitive drum 1 removed.
[0045]
The developing magnetic pole S1 of the magnet roller 31 is disposed at a position facing the developing portion C, and a magnetic brush of developer T is formed by a developing magnetic field formed by the developing magnetic pole S1 on the developing portion C, and this magnetic brush is photosensitive. The dot distribution electrostatic latent image is developed in contact with the drum 1. At this time, the toner adhering to the ears (brushes) of the magnetic carrier and the toner adhering to the sleeve surface instead of the ears are transferred to the exposed portion of the electrostatic latent image and developed.
[0046]
The peak value of the strength of the developing magnetic field generated by the developing magnetic pole S1 on the surface of the developing sleeve 30 (the magnetic flux density in the direction perpendicular to the surface of the developing sleeve 30) is preferably 500 to 2000 Gauss. Further, the magnet roller 31 has Nl, N2, N3, and S2 poles in addition to the developing magnetic pole Sl.
[0047]
Here, a developing process for visualizing the electrostatic latent image on the surface of the photosensitive drum 1 by the two-component magnetic brush method using the developing device 4 and a circulation system of the developer T will be described.
[0048]
The developer T pumped up at the N2 pole by the rotation of the developing sleeve 30 is transported from the S2 pole to the Nl pole, and the layer thickness is regulated by the regulating blade 33 in the middle thereof to form a developer thin layer. Then, the developer T spiked in the magnetic field of the developing magnetic pole S1 develops the electrostatic latent image on the photosensitive drum 1. Thereafter, the developer T on the developing sleeve 30 falls into the developing chamber Rl due to the repulsive magnetic field between the N3 pole and the N2 pole. The developer T that has fallen into the developing chamber Rl is stirred and conveyed by the conveying screw 37.
[0049]
The transfer drum 5 has a transfer sheet 5c made of, for example, a polyethylene terephthalate resin film stretched on the surface thereof, and is placed in contact with and separated from the photosensitive drum 1. The transfer drum 5 is rotationally driven in the arrow direction (clockwise). In the transfer drum 5, a transfer charger 5a, a separation charger 5b, and the like are installed.
[0050]
Next, an image forming operation of the above-described image forming apparatus will be described.
[0051]
The photosensitive drum 1 is driven to rotate in the direction of arrow a (counterclockwise) at a predetermined peripheral speed (process speed) around the center support shaft, and in the present embodiment, the photosensitive drum 1 is negatively charged by the primary charger 2 in this embodiment. The uniform charging process is applied.
[0052]
  Then, a laser beam modulated in response to an image signal output from the image reading unit B to the printer unit A side and output from the exposure device (laser scanning device) 3 to the uniformly charged surface of the photosensitive drum 1. As a result of the scanning exposure L, electrostatic latent images of respective colors corresponding to the image information of the document G photoelectrically read by the image reading unit B are sequentially formed on the photosensitive drum 1.The cyan electrostatic latent image initially formed on the photosensitive drum 1 is first reversely developed by the developing device 4 using the above-described two-component magnetic brush method by the cyan developing devices 4Cb and 4Ca, and the first color cyan toner. Visualized as an image. When the cyan toner image of the first color is transferred to the transfer material P on the transfer drum 5 (transfer sheet), the magenta electrostatic latent image formed on the photosensitive drum 1 is The reversal development is performed by the magenta developing devices 4Mb and 4Ma to form a visible image as the second color magenta toner image.
[0053]
On the other hand, in synchronization with the formation of the toner image on the photosensitive drum 1, the transfer material P such as paper stored in the paper feed cassette 10 is fed one by one by the paper feed roller 11 or 12, and the registration roller 13 is fed to the transfer drum 5 at a predetermined timing, and the transfer material P is electrostatically attracted to the transfer drum 5 by the suction roller 14. The transfer material P electrostatically adsorbed on the transfer drum 5 moves to a position facing the photosensitive drum 1 by the rotation of the transfer drum 5 in the direction of the arrow (clockwise), and is transferred to the back side of the transfer material P by the transfer charger 5a. A charge having a polarity opposite to that of the toner is applied, and a toner image (cyan toner image) on the photosensitive drum 1 is transferred to the surface side.
[0054]
After this transfer, the transfer residual toner remaining on the photosensitive drum 1 is removed by the cleaning device 6 and used for forming a toner image of the next color.
[0055]
  When the magenta toner image of the second color is transferred to the transfer material P on the transfer drum 5 (transfer sheet), the yellow developing device 4Y containing the respective yellow and black toners and the black developing are stored in the same manner. The yellow and black electrostatic latent images on the photosensitive drum 1 are developed by the device 4K, and the yellow toner image and the black toner image sequentially formed on the photosensitive drum 1 are transferred to the transfer drum 5 (transfer sheet) by the transfer charger 5a. A full-color image is formed by sequentially transferring the image onto the transfer material P.
[0056]
During the image formation, the same electrostatic latent image was developed with cyan and cyan a toners, and the same electrostatic latent image was developed with magenta and magenta a toners.
[0057]
Then, the transfer material P is separated from the transfer drum 5 (transfer sheet) by the separation charger 5 b, and the separated transfer material P is conveyed to the fixing device 9 through the conveyance belt 8. The transfer material P conveyed to the fixing device 9 is heated and pressed between the fixing roller 9a and the pressure roller 9b to fix the full color image on the surface, and is then discharged onto the tray 16 by the paper discharge roller 15. The
[0058]
Further, the transfer residual toner is removed from the surface of the photosensitive drum 1 by the cleaning device 6, and the surface of the photosensitive drum 1 is further neutralized by the pre-exposure lamp 7 to prepare for the next image formation.
[0059]
  As described above, in the present embodiment, development is performed with toners of six colors of cyan, cyan a, magenta, magenta a, yellow, and black. As shown in FIG. 5, the cyan developing device 4Ca hasMaximum reflection density is about 1.6Cyan toner is used, and the cyan developer 4CbMaximum reflection density is 0.4, 0.6, 0.8, 1.0, 1.2, 1.4The cyan a toner is used and the effect of the halftone density unevenness (glossiness), which is the above-described problem, is evaluated. The result shown in FIG. 6 is obtained. 5 and 6, toner a is cyan toner, and toner b is cyan a toner.
[0060]
As is apparent from this evaluation result, it has been found that when the maximum reflection density of cyan a toner is smaller than 0.8, which is half of the maximum reflection density of cyan toner, the halftone density unevenness (glossiness) is eliminated. . In the present embodiment, the maximum effect is obtained when the maximum reflection density of cyan a toner is 0.6.
[0061]
The relationship between the magenta toner and the magenta toner is the same. When the maximum reflection density of the magenta toner is smaller than 0.8, which is half of the maximum reflection density of the magenta toner, the density unevenness of the halftone image ( (Gastsuki) is gone.
[0062]
As described above, in this embodiment, each color toner image is developed on the photosensitive drum 1 with cyan, magenta, yellow, and black toners, and the maximum reflection density is lower than cyan toner. A toner image of each color of cyan a toner and magenta a toner is developed on the photosensitive drum 1 with a toner and magenta a toner having a maximum reflection density smaller than magenta toner. The same electrostatic latent image formed on the photosensitive drum 1 is first developed with cyan a toner and then developed with cyan toner, and magenta toner and magenta a toner are formed on the photosensitive drum 1. The same electrostatic latent image is developed first with magenta toner and then developed with magenta toner. Even if the Futon image without density unevenness (roughness), it is possible to obtain a high-quality image.
[0063]
<Embodiment 2>
This embodiment will also be described using the image forming apparatus of the first embodiment shown in FIGS.
[0064]
In the first embodiment described above, development was performed in the order of cyan a toner → cyan toner and magenta a toner → magenta toner. However, cyan toner is developed by reversing the order of cyan a toner and cyan toner, and magenta a toner and magenta toner. When the development was performed in the order of cyan a toner and magenta toner to magenta toner, the effect as obtained in the first embodiment was not obtained.
[0065]
According to the study by the present inventor, since the difference between the halftone portion potential and the DC component of the developing bias is small, the potential difference is filled with the charge of the first developing toner, so that the maximum reflection density After the halftone portion was developed with a large cyan toner and magenta toner, the cyan a toner and magenta a toner were hardly developed to the halftone portion, and it was found that the effects of the present invention could not be obtained.
[0066]
In this regard, as in the first embodiment, when development is performed in the order of cyan a toner → cyan toner, and magenta a toner → magenta toner, the density is reduced after development of cyan a toner and magenta a toner having a small maximum reflection density. The halftone portion where unevenness (gassiness) is likely to occur is almost filled with cyan a toner and magenta a toner, while in a solid black portion where density is desired, the solid black portion potential and the DC component of the developing bias Since the difference is large, it is not filled in the first development, and a sufficient amount of cyan toner and magenta toner having the largest maximum reflection density adheres in the second time.
[0067]
In order to enhance the above process, in the present embodiment, when developing cyan a toner and magenta a toner in the first development, the difference between the halftone portion potential and the DC component of the development bias is increased. When developing the cyan toner in the first development, the difference between the DC component of the halftone portion and the development bias, which is different from the DC component of the development bias at the first development, is reduced. Other configurations and operations are the same as those in the first embodiment.
[0068]
FIG. 7 shows the evaluation results of the development bias direct current component of cyan a toner and magenta a toner and halftone density unevenness (gray) in this embodiment. In FIG. 7, toner b is cyan a toner and magenta a toner.
[0069]
That is, in the present embodiment, the charged potential (Vd) of the photosensitive drum 1 is set to −650 V and the solid black potential (Vl) is set to −100 V based on the result of FIG. Development was performed in the order of cyan toner, magenta toner, and magenta toner, and the DC component of the developing bias of cyan toner and magenta toner was set to −500 V, and the halftone portion potential was −350 V as a result of measurement.
[0070]
  In this way, the development amount of cyan a toner is increased (the difference between the halftone portion potential and the DC component of the development bias isEnlarge), The effect of eliminating the uneven density of the halftone (guzziness) was great.
[0071]
In this embodiment, when the bias direct current component during development of cyan a toner and magenta a toner is large and the development amount of cyan a toner and magenta a toner having a small maximum reflection density is too large, cyan a toner, The solid black portion potential starts to be filled only with the magenta a toner, and the development amount of the cyan toner and the magenta toner having the largest maximum reflection density to the evening black portion is reduced.
[0072]
【The invention's effect】
  As explained above, according to the present invention,Depending on the toner of each color of the first cyan toner, the first magenta toner, the yellow toner, and the black toner,A toner image of each color is developed on the image carrier, and further, a second cyan toner having a maximum reflection density lower than the first cyan toner and a second magenta color having a maximum reflection density lower than the first magenta toner. The toner image of each color of the second cyan toner and the second magenta toner is developed on the image carrier by the toner, and the first cyan toner and the second cyan toner are the same static image formed on the image carrier. The electrostatic latent image is first developed with the second cyan toner and then developed with the first cyan toner. For the first magenta toner and the second magenta toner, the same electrostatic image formed on the image carrier is used. The latent image is first developed with the second magenta toner and then developed with the first magenta toner, thereby eliminating density unevenness even in the case of a halftone image, and thereby producing a high-quality image. It is possible to obtain.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram showing an image forming apparatus according to first and second embodiments of the present invention.
FIG. 2 is a schematic block diagram showing an exposure apparatus (laser beam scanner) of the image forming apparatus according to Embodiments 1 and 2 of the present invention.
FIG. 3 is a schematic configuration diagram illustrating a developing device of an image forming apparatus according to Embodiments 1 and 2 of the present invention.
FIG. 4 is a schematic cross-sectional view showing a configuration of a two-component developing device.
FIG. 5 is a diagram showing the relationship between document reflection density and copy reflection density in the first embodiment.
FIG. 6 shows an evaluation result in the first embodiment.
7 is a diagram showing an evaluation result in Embodiment 2. FIG.
FIG. 8 is a diagram showing a relationship between document reflection density and copy reflection density in a conventional example.
[Explanation of symbols]
1 Photosensitive drum (image carrier)
2 Primary charger
3 Exposure equipment
4 Development device
4Ca cyan developing device (first developing means)
4Cb cyan a developing device (second developing means)
4Ma magenta developing device (first developing means)
4Mb magenta a developing device (second developing means)
4Y yellow developing device (first developing means)
4K black developer (first developer)
5 Transfer drum
5a Transfer charger
5b Separate charger
9 Fixing device
30 Development drum

Claims (2)

In an electrophotographic image forming apparatus that forms a color image by superimposing a plurality of color toner images,
First developing means for developing a toner image of each color on the image carrier with toner of each color of first cyan toner, first magenta toner, yellow toner, and black toner;
The second cyan toner and the second cyan toner have a maximum reflection density smaller than the first cyan toner and a second magenta toner whose maximum reflection density is smaller than the first magenta toner. Second developing means for developing a toner image of magenta toner on the image carrier,
For the first cyan toner and the second cyan toner, the same electrostatic latent image formed on the image carrier is first developed with the second cyan toner, and then with the first cyan toner. Develop,
As for the first magenta toner and the second magenta toner, the same electrostatic latent image formed on the image carrier is first developed with the second magenta toner, and then the first magenta toner is used. Develop with
The difference between the half-tone portion potential of the electrostatic latent image formed on the image carrier and the DC component of the developing bias at the time of development is determined when developing with the second cyan toner and the second magenta toner. An image forming apparatus, wherein the image forming apparatus is larger than that when developing with the first cyan toner and the first magenta toner . The second cyan toner, the maximum reflection density of the second magenta toner, the first cyan toner is less than half of the maximum reflection density of the first magenta toner, the image forming apparatus according to claim 1.

Images