JP2002049191A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form an image obtaining both of high resolution and high gradation by using light toner and dark toner. SOLUTION: In this image forming device where image exposure and development with the light toner and image exposure and development with the dark toner are performed based on image data, the image data for the dark toner and the image data for the light toner are generated from image density data, and an image forming cycle for the dark toner is set to be smaller than that for the light toner.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus for forming an image using toner by electrophotography,
In particular, the present invention relates to an image forming apparatus that develops an image by using two types of toners of the same color, a light color toner and a dark color toner, and forms an image.

[0002]

2. Description of the Related Art In an image forming apparatus using an electrophotographic method, an image forming body is uniformly charged by a charging means and then exposed to an image to form a latent image. The latent image portion is developed by the subsequent developing means to form a toner image. In a digital image forming apparatus, image exposure is performed by, for example, dots that have been subjected to pulse width modulation, and reversal development in which toner adheres to the dot portions is performed. The amount of toner adhering to the dot portion is determined by the area and potential state of the dot portion on the image forming body, the developing bias for the dot portion, the state of the charge of the toner, and the like.

A high-density image portion recorded on an image forming body by an image exposure means has a large number of dots per area,
The size of the dots is also large. On the other hand, the low-density image portion has a small number of dots per area and a small size of one dot. Thus, after development, the amount of toner adhered to the high density image portion on the image forming body is large, and the amount of toner adhered to the low density image portion is small. The difference in the density of the recorded image is expressed by the amount of toner adhering to the area of the image formed on the image forming body by development.

[0004]

However, it is difficult to form an image with good gradation with one type of toner. Particularly in a highlight portion (low-density portion), the attachment of a very small amount of toner per area is unstable due to the small size of the dot, so that image unevenness is likely to occur and it is difficult to obtain stable gradation. In the high-density portion, the toner is in an excessively toner-adhered state, and the image density is easily saturated.

The present invention is applied to a monochrome or color image forming apparatus, and includes two kinds of toners of the same color, a light color and a dark color (for example, cyan (C), magenta (M), yellow (Y), It is an object of the present invention to provide an image forming apparatus that performs development using any of black (K) and that forms an image having both high resolution and high gradation.

[0006]

SUMMARY OF THE INVENTION The object of the present invention is to provide a developing device using a dark toner of the same color and a developing device using a light toner on the periphery of an image forming body to perform superposition development using dark and light toners. In the image forming apparatus for transferring the superposed dark and light toner images onto a transfer material, the image forming cycle using the dark toner is made shorter than the image forming cycle using the light toner. This is achieved by an image forming apparatus.

The image forming cycle is a cycle formed by dither processing or pulse width modulation processing.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The image forming apparatus of the present invention has
The image density information is decomposed into light toner image data and dark toner image data, and a light toner image is formed by image exposure based on the light toner image data and development using the light toner. Is an image forming apparatus which forms a dark toner image by image exposure based on image formation and development using a dark toner, and forms an image by superimposing a light toner image. First, the structure of a preferable image forming apparatus will be described.

(1) In the present embodiment, as shown in the sectional structural views of FIGS. 1 and 2, an image is formed by superimposing light and dark toner images on an image forming body by repeating charging, image exposure and development. The image forming apparatus is a color image forming apparatus that forms a color image by superimposing light and dark toner images of Y, M, C, and K colors in a tandem system, and is characterized by a small shape.

The color image forming apparatus shown in FIG. 1 is a tandem type color image forming apparatus using an intermediate transfer member, and a yellow (Y), magenta (M) ), Cyan (C) and black (K) are provided, and each process unit 100 forms Y, M, C, and K toner images using light and dark toners. Then, the toner image is superimposed and transferred on the transfer belt 14a,
The transferred color toner image is collectively transferred onto a recording sheet as a transfer material, fixed, and discharged outside the apparatus.

Since the four process units 100 have a common structure, only one of them will be described. The photoreceptor drum 10 as an image forming body includes a light-transmitting conductive layer and an organic photosensitive layer (OPC) on the outer periphery of a cylindrical base formed of a light-transmitting member such as glass or light-transmitting acrylic resin. Is formed.

The photosensitive drum 10 is rotated in the counterclockwise direction indicated by an arrow with the light-transmitting conductive layer grounded by power from a driving source (not shown) or by the transfer belt 14a.

Numerals 11A and 11B denote scorotron chargers as charging means, which are mounted so as to be opposed to and close to the photosensitive drum 10 in a direction perpendicular to the moving direction of the photosensitive drum 10 and have the same polarity as the toner. A uniform potential is applied to the photosensitive drum 10 by the discharge.

The exposure optical systems 12 (L) and 12 (H)
Each is a linear exposure element (not shown) in which a plurality of LEDs (light emitting diodes) as light emitting elements for image exposure are arranged in an array in parallel with the axis of the photosensitive drum 10 and a selfoc as an equal-magnification imaging element. A lens (not shown) is configured as an exposure unit attached to a holder. Exposure optical systems 12 (L) and 12 (H) are attached to a cylindrical holding member and housed inside the base of the photosensitive drum 10. In addition, as the exposure element, a linear element in which a plurality of light emitting elements such as FL (phosphor emission), EL (electroluminescence), and PL (plasma discharge) are arranged in an array is used. Reference numeral 12 (L) denotes an image exposure unit that performs image exposure using image data for light toner, and 12 (H) denotes an image exposure unit that performs image exposure using image data for dark toner.

An exposure optical system 12 (L), which is an image exposure means for light toner, is provided with a scorotron charger 11A and a developing device 13 having light toner built therein.
Between (L) and the developing unit 13 (L), the photosensitive drum 10 is disposed inside the photosensitive drum 10 in a state provided on the upstream side in the rotation direction of the photosensitive drum 10. The exposure position of the exposure optical system 12 (H), which is an image exposure means for dark toner, on the photosensitive drum 10 is on the downstream side of the developing device 13 (H), and incorporates a scorotron charger 11 B and dark toner. Developing device 13
(H), the photosensitive drum 1 is provided on the upstream side in the rotation direction of the photosensitive drum 10 with respect to the developing device 13 (H).
0.

The developing device 13 (L) is a developing device which contains a one-component light toner or a two-component developer of a light toner and a magnetic carrier and performs reversal development by contact or non-contact. The developing device 13 (H) incorporates a one-component dark toner or a two-component developer of a dark toner and a magnetic carrier, which enables superposition development without destroying the toner image, and performs soft development or more preferably non-development. This is a developing device that performs reversal development by contact development.

In forming an image, image data of each color of Y, M, C, and K is divided into image data for light toner and image data for dark toner, which will be described in detail later. Image exposure is performed by the optical system 12 (L), and development with light toner is performed by the developing device 13 (L). The image data for the dark toner is subjected to image exposure by an exposure optical system 12 (H), and a developing device 13 (H).
As a result, development with dark toner is performed, and a toner image with dark toner is formed on the photosensitive drum 10 on the toner image with light toner. This toner image is transferred onto a transfer belt 14a described later at a transfer position. The transfer residual toner remaining on the drum after the transfer is cleaned by a cleaning device 19 that electrostatically collects the toner.

Process unit 1 for four colors of Y, M, C and K
The transfer belt 14a in which 00 is opposed in parallel has a volume resistivity of 10 ¹⁰ to 10 ¹⁵ Ω · cm and a surface resistivity of 10 ¹⁰ to 10 ¹⁵ Ω.
・ Cm ² endless belt, for example, modified polyimide,
Thermosetting polyimide, ethylene tetrafluoroethylene copolymer, polyvinylidene fluoride, dispersed conductive material in engineering plastics such as nylon alloy, outside the semi-conductive film substrate of 0.1 to 1.0 mm thickness, preferably Thickness 5 as a toner filming prevention layer
This is a seamless belt having a two-layer structure and having a fluorine coating of about 50 μm. In addition, as the base of the transfer belt 14a, a semiconductive rubber belt having a thickness of 0.5 to 2.0 mm in which a conductive material is dispersed in silicon rubber or urethane rubber can be used. Transfer belt 14a
Is stretched in contact with a driving roller 14d, a driven roller 14e, a tension roller 14k, and a backup roller 14j. At the time of image formation, the driving roller 14d is rotated by a driving motor (not shown) to rotate the driving roller 14d. At the transfer position, the transfer belt 14a is pressed against the photosensitive drum 10 by the primary transfer device 14c, and the transfer belt 14a is rotated in the direction indicated by the arrow in the drawing.

A primary transfer device 14c, which is a transfer roller as a transfer means for each color, is provided to face the photosensitive drum 10 for each color with the transfer belt 14a interposed therebetween, and the transfer belt 14a and the photosensitive member for each color are provided. A transfer area (no reference numeral) for each color is formed between the transfer area and the drum 10. Primary transfer unit 14 for each color
By applying a DC voltage having a polarity opposite to that of the toner (positive polarity in this embodiment) to c, and forming a transfer electric field in the transfer area, the toner image on the photosensitive drum 10 for each color is transferred onto the transfer belt 14a. Transfer to

The static eliminator 14m, which is a static eliminator for each color, comprises:
Preferably, the primary transfer unit 1 is constituted by a corona discharger.
The transfer belt 14a charged by 4c is neutralized.

The process unit 1 of black (K) is started by starting a photoconductor drive motor (not shown) by starting image recording.
00 is rotated in the direction indicated by the arrow in the figure, and at the same time, the application of the potential to the K photoconductor drum 10 is started by the charging action of the K scorotron charger 11A.

After a potential is applied to the K photosensitive drum 10, an electrical signal corresponding to the first color signal, that is, the image data for the K light toner, is applied by the K light toner exposure optical system 12 (L). Is started, and an electrostatic latent image for light toner corresponding to the K image of the original image is formed on the surface of the K photosensitive drum 10.

The latent image for light toner is subjected to reversal development in a contact state by a developing device 13 (L) for light toner of K, and a toner image of light toner of K is formed in accordance with the rotation of the photosensitive drum 10 of K. It is formed. Then the scorotron charger 11
B and the exposure optical system 12 for the dark toner of K
(H), an electrostatic latent image for dark toner corresponding to the image of K is formed, and the developing device 13 (H) for dark toner forms the K latent image.
Is formed on the previously formed toner image of the light K toner.

The K toner image formed of the light and dark toners formed on the K photoreceptor drum 10 as the image forming body by the above-described image forming process is the first toner image in the K transfer area (no sign). The image is transferred onto the transfer belt 14a by a K primary transfer device 14c as a transfer unit.

Next, the transfer belt 14a is synchronized with the C toner image, and the cyan (C) process unit 10
0, the light color of C formed on the photosensitive drum 10 of C,
Toner image composed of dark toner is C transfer area (no sign)
, The primary transfer device 14c of C as the first transfer means
As a result, a toner image composed of light and dark toners C is superposed on the toner image composed of light and dark toners K.

By the same process, the image is synchronized with the superposed toner images of K and C, and is formed on the photosensitive drum 10 of M by the process unit 100 of magenta (M) by the third color signal. M corresponding to the image data of
In the M transfer area (no symbol), the M primary transfer unit 14c, which is the first transfer means, transfers the M toner images of the K and C toner images from above the K and C toner images. A toner image composed of light and dark toners is formed by being superimposed, synchronized with the superimposed toner images of K, C, and M.
The toner image formed of the light and dark toners of Y corresponding to the image data of Y based on the fourth color signal is formed on the photosensitive drum 10 of the first color in the Y transfer area (no sign). The Y primary transfer device 14c forms a toner image composed of Y light and dark toners on the K, C and M toner images in a superimposed manner, and the transfer belt 14a
A superimposed color toner image of K, C, M and Y is formed thereon.

The transfer residual toner remaining on the peripheral surface of the photosensitive drum 10 for each color after the transfer is cleaned by a cleaning device 19 which is a cleaning means of the image forming body for each color.

In synchronization with the formation of the color toner image on the transfer belt 14a, the recording paper P is transferred from the paper supply cassette 15 as the transfer material storage means via the timing roller 16 as the transfer material supply means to the second transfer. The superimposed color toner image on the transfer belt 14a is recorded by the secondary transfer device 14g, which is conveyed to a transfer area (no symbol) of the secondary transfer device 14g, which is a unit, and to which a DC voltage having a polarity opposite to that of the toner is applied. It is transferred onto the paper P at once.

The recording paper P on which the color toner image has been transferred is
The charge is removed by a charge removing electrode 16b, which is a separating means formed of a sawtooth electrode plate, conveyed to the fixing device 17, and heat and pressure are applied between the fixing roller 17a and the pressure roller 17b, so that the recording paper P After the toner image is fixed,
It is discharged to a tray outside the device.

The transfer residual toner remaining on the peripheral surface of the transfer belt 14a after the transfer is cleaned by a cleaning device 19a which is a transfer belt cleaning means provided opposite to the driven roller 14e across the transfer belt 14a. .

In the present embodiment, the process is such that the light toner adheres to the photosensitive drum 10 of the process unit 100, and then the dark toner adheres. Good transfer of dark toner is achieved. Even if light toner previously attached due to image exposure is scattered, it is not noticeable. The following effects are obtained: the effect is small even if the color mixture of the light toner is present in the developing device 13 (H) located on the downstream side.

In the present embodiment, Y, M,
Although the development using light and dark toners is performed for the four colors C, K, and C, the development is performed using only the dark toner for Y, M, and C, and the development using only the light and dark toners is performed only for K. Thus, the excellent effects of the present invention can be obtained. In this case, the developing device is an image forming apparatus provided with five sets.

As described above, a small-sized image forming apparatus in which color reproduction is easily performed by widening a reproduction area using light and dark toners is shown in FIG.
Explained by. FIG. 2 is a cross-sectional configuration diagram of an image forming apparatus showing another embodiment having the same effect. Pale
Although the process units 100 for forming Y, M, C, and K toner images using dark toner are arranged in parallel with each other as in FIG. 1, in the present embodiment, an intermediate transfer member is used. Instead, the toner image is superimposed on the transfer material. Only the differences from FIG. 1 will be described. In the image formation, the recording paper P discharged from the paper feed cassette 15 is synchronized with the toner image formation on the process unit 100 by the timing roller 16.
Is transported to the transport belt 14 via the

The transported recording paper P is charged to the same polarity as the toner by the paper charger 14m, and the rotating transport belt 1 is rotated.
The toner image formed of the light and dark toners of Y formed on the process unit 100Y is transferred onto the recording paper P by the primary transfer unit 14c, and the process units 100M and 100C are transferred onto the toner image. , 100K, M, C, and K toner images formed of light and dark toners are transferred in an overlapping manner to form a color toner image.

The recording paper P carrying the color toner image is neutralized by the separation electrode 14n, separated from the transport belt 14 and transported to the fixing device 17, where the color toner image on the recording paper P is fixed and Discharge to an external tray.

In the embodiment shown in FIG.
For M and C, development using only the dark toner may be performed, and only K may be developed using the light and dark toner. Alternatively, Y may be developed using only the dark toner, and K, C and M may be developed using the light toner.
It is also possible to adopt a configuration in which development with dark toner is performed, and the excellent effects of the present invention can be obtained. Further, the present invention can also be applied to a monochrome image forming apparatus of K single color.

(2) In the present embodiment, as shown in the sectional configuration diagrams of FIGS. 3 and 4, charging, image exposure,
An image forming apparatus that forms a light toner image and a dark toner image by development and superimposes a light toner image and a dark toner image. In particular, Y, M, C,
K is a color image forming apparatus that forms a color image by superimposing a color light toner image formed by superimposing the respective colors and a color dark toner image. It is a configuration suitable for

FIG. 3 shows an image forming apparatus in which a light toner image and a dark toner image are superimposed on an intermediate transfer member and then transferred onto recording paper, and FIG. This is an image forming apparatus in which a formed color image composed of a light toner and a color image composed of a dark toner are superimposed on a recording sheet. Members having the same functions as those already described in FIGS. 1 and 2 are denoted by the same reference numerals.

The color image forming apparatus shown in FIG. 3 includes a process unit 100L for forming a color image using light toner on the peripheral portion of the transfer belt 14a and a process unit 100L for forming a color image using dark toner. ,
A process unit 100H for forming a color image using dark toner is arranged in parallel. Since the process units 100L and 100H have a common structure,
The process unit 100L will be described. Y, M, C, and K are arranged from the upstream side in the rotation direction along the photosensitive drum 10.
Scorotron charger 1 as charging means for four colors
1. Four sets of an exposure optical system 12 (L) and a developing unit 13 (L) are provided. The photosensitive drum 10 whose peripheral surface has been cleaned by the cleaning device 19 is uniformly charged by the scorotron charger 11Y, and is further exposed to light toner by the exposure optical system 12 (L) Y based on Y image data for light toner. A latent electrostatic image of Y is formed, and a developing device 13 (L) Y containing a light toner forms a Y toner image with the light toner.

An M toner image of light toner is superposed on the Y toner image of light toner by the scorotron charger 11M, the exposure optical system 12 (L) M, and the developing device 13 (L) M. Further, a scorotron charger 11C, an exposure optical system 12
(L) C, a C toner image of light toner is superposed by the developing unit 13 (L) C, and a scorotron charger 11K, an exposure optical system 12 (L) K, and a developing unit 13 are formed thereon.
(L) The K toner image of light toner is superimposed and formed by K.

Similarly, for the process unit 100H, Y, M, C,
A scorotron charger 11 for K4 color, an exposure optical system 12 (H) for exposing an image based on image data for dark toner, and a developing device 13 (H) containing dark toner, all of Y, M, C using dark toner. , K4 color toner images are superposed.

The color toner image of the light toner formed on the process unit 100L is transferred onto the transfer belt 14a by the primary transfer device 14c, and is transferred to the transfer belt 14a.
The color toner image of the dark toner formed on the process unit 100H is transferred onto the color toner image of the light toner transferred thereon so as to overlap. Light transferred by being superimposed on the transfer belt 14a,
The color toner image by the dark toner is
The sheet is collectively transferred by a secondary transfer unit 14g onto a recording sheet P fed through the printer 6 and is discharged by a discharging electrode 16b. Thereafter, the sheet is fixed by a fixing device 17 and discharged to the outside of the device.

FIG. 4 shows another embodiment, in the same manner as in FIG. 3, a process unit 100H for forming a color toner image using dark toner and a process unit 100L for forming a color toner image using light toner. Are arranged side by side facing the conveyor belt 14. In the image formation, the recording paper P discharged from the paper supply cassette is conveyed to the conveyance belt 14 via the timing roller 16 in synchronization with the formation of the toner image by the dark toner on the process unit 100H. The recording paper P is transported in close contact with the transport belt 14 by the paper charger 14m, and a color toner image formed by the dark toner formed on the process unit 100H is transferred onto the recording paper P by the primary transfer unit 14c. Process unit 100L synchronized with above
The color toner image of the light toner formed thereon is transferred in an overlapping manner. The recording paper P carrying the color toner image of the light and dark toners is neutralized by the separation electrode 14n, separated from the conveyor belt 14, conveyed to the fixing device 17, and discharged out of the device after being fixed.

The image forming apparatus shown in FIGS. 3 and 4 can obtain continuous high-quality images of gradation by using the process units 100L and 100H. The light and dark toners can be easily used according to the image or the purpose of use, for example, by forming an image only by using the image.

(3) In the present invention, Y, M, C, K
The image density data for each color is divided into image density data for light toner and image density data for dark toner. The image density data for light toner is
By (L), the image density data for the dark toner is subjected to image exposure on the photosensitive drum 10 by the exposure optical system 12 (H) for the dark toner.

FIG. 5 is a block diagram of the image processing system according to the present embodiment. An original image is read by an image reading means using a solid-state imaging device such as a CCD (F1). The amplified analog image signal output from the CCD is converted to an 8- to 10-bit digital signal by A / D conversion.
After the conversion, shading correction and subsequent image processing (1) such as color space conversion, logarithmic conversion, black generation, and color correction are performed (F2). The image data may be read from the stored memory and subjected to the same processing.

The image data obtained in the image processing (1) is
By checking the density distribution of the data and the density difference between adjacent dots, it is possible to determine whether the original image is a halftone image such as a photograph or a picture or a character image such as a character or a line drawing. (F3).

The image data obtained in the image processing (1) is
The image data for light toner (F5A) and the image data for dark toner (F5B) are determined based on the determination result as to whether the image is a halftone image or a character image, or based on the specified output form (F3). Is created.

FIG. 6 is an explanatory view showing a state in which image data is decomposed into light and dark toner image data. FIG. 6 (a) is a halftone image, FIG. It shows image data for dark toner, and shows a state in which the composition ratio of image data for light and dark toner is changed depending on whether the image data is a halftone image or a character image. That is, for halftone images, image data that uses more light toner is created, and for character images, image data that uses more dark toner is created.

The decomposed light and dark toner image data are subjected to γ correction for each toner (F6A, F6B),
Subsequently, multi-value processing is performed (F7A, F7B).

In the present invention, the following processing is performed in the multi-value processing section. That is, for images with dark toner,
A process for reducing the recording cycle is performed with emphasis on forming a high-resolution and high-density image. For an image using light toner, a process of increasing the recording cycle is performed with emphasis on the formation of a high-gradation image. By performing such a process, an excellent image obtained with both high resolution and high gradation can be formed.

The processing for increasing or decreasing the image forming cycle is performed by dither processing or pulse width modulation processing.

FIG. 7 shows an example in which the dither cycle is changed using multi-valued dither. For an image using dark toner, fine shading is performed as a 2 × 2 matrix dither, and a pulse is applied. A process for reducing the recording cycle is performed in combination with the multi-level modulation by the width modulation, and the image using the light toner is roughly shaded as a 4 × 4 matrix dither, and the multi-level modulation by the pulse width modulation is performed. Processing to increase the recording cycle is performed in combination.

FIG. 8 shows one pixel / 2 using pulse width modulation.
An example in which pixels are changed is shown. For an image using dark toner, a process of making a fine recording as one pixel PWM to reduce a recording cycle is performed, and for an image using light toner, In other words, processing is performed to increase the recording cycle by increasing the recording unit as two pixels PWM.

At this time, P is used in combination with the error diffusion method.
Laser scanning is performed with an elliptical spot diameter such that the laser spot for performing WM processing and exposing the image on the photoreceptor has a short axis in the scanning direction and a long axis in the sub-scanning direction.

The light-toner image data which has been subjected to the multi-value processing is subjected to image exposure on the photosensitive drum 10 by the exposure optical system 12 (L) (F8A), and the dark-toner image which has been subjected to the multi-value processing is obtained. The data is subjected to image exposure on the photosensitive drum 10 by the exposure optical system 12 (H) (F8B).

(4) FIG. 9 is a graph showing the amount of toner adhered on the photosensitive drum 10 when the light toner and the dark toner of the present embodiment are used. For the high-density portion, only the dark toner is used according to the image data for the dark toner, and the toner adhesion amount is reduced while the reproducibility is maintained.
For the low-density portion, by increasing the size of the recording dots of the low-toner image data and using only the light toner, a sufficient amount of adhered toner is maintained, thereby maintaining stable reproducibility. . The light toner and the dark toner are used in combination with the image data for the low and dark toners in the middle between the high density portion and the low density portion, so that the gradation is maintained and the image unevenness is prevented.

FIG. 10 is an explanatory diagram showing the state of toner adhesion at the low to high density portions. According to the present invention, as shown in FIG. 9, the difference in the amount of adhered toner between the high-density portion and the low-density portion is reduced, and the transfer failure and the fixing disturbance failure are eliminated.

The image forming apparatus using the light toner and the dark toner has been described above. In order to obtain a sufficient effect according to the present invention, for example, the image forming apparatus shown in FIG.
It is desirable that the relationship between the toner adhesion amount and the print image density shown in FIG. The density difference between the light and dark toners is caused by the difference in the amount of the pigment contained in the toner particles.
Further, the amount of toner adhered at the time of saturation depends on the toner particle size. For example, with respect to dark toner, the recording density becomes saturated when the amount of toner attached to white paper is about 1 mg / cm ² .
Saturated concentration value _Dmax (H) in a saturated state = 1.7 to 2.5
It is necessary to be. As shown in FIG. 11, the density value at the point of substantially saturation is defined as D _max (H). To be exact, the concentration gradually increases as shown by the broken line.

On the other hand, the density of the light toner does not reach a saturated state when the amount of toner attached to white paper is 1 mg / cm ² , for example. When the light toner has the same toner adhesion amount as the dark toner reaching the saturation density value, that is, the recording density D of the light toner when the dark toner concentration is saturated.
It is necessary that (L) is between 0.3 and 0.8.
The ratio between the recording density D _S (L) when a light toner having a toner adhesion amount of, for example, 0.5 mg / cm ² before the toner adhesion amount on the white paper reaches the saturation density value,
That is, the gradient ratio D of the recording density between the light toner and the dark toner
It is necessary to be between _{S (L) / D S (} H) = 0.2~0.5.

The recording density needs to have the above-described relationship between the dark toner and the light toner, but the toner particle diameter and the charge amount may be the same for the light and dark toners. preferable. For toner particle size,
Because both light and dark toners ensure image quality and developability,
It is preferable that the volume average particle size is between 3 and 10 μm, and even if there is a difference, it is ± 20% or less. The charge amount is between 5 and 30 .mu.C / g for both the light and dark toners. In particular, when the toner particle size is small, a high charge amount is required. Preferably, there is. By satisfying these conditions,
A similar γ characteristic can be obtained between the light and dark toners, and the design can be made so that the sum of the adhesion amounts of the light toner and the dark toner in the middle to high density areas is almost the same and the change in the recording density is linear. It will be easier.

Note that the gradation characteristics shown in FIG. 11 also change depending on the toner particle size and the contained coloring material. In the case of a small particle size toner, the toner may be saturated even with a smaller amount of adhesion of 0.5 mg / cm ² , but the saturation density and the slope ratio are important in designing a light toner and a dark toner.

[0063]

According to the present invention, the image forming cycle for the dark color toner is set to be light for the dark color toner image data and light toner image data for the same color in the processing of the multi-value processing section during image processing. By setting it shorter than that for the toner, an excellent image can be formed that achieves both high resolution and high gradation.

[Brief description of the drawings]

FIG. 1 is a cross-sectional configuration diagram of an image forming apparatus according to an embodiment.

FIG. 2 is a cross-sectional configuration diagram of the image forming apparatus according to the embodiment.

FIG. 3 is a cross-sectional configuration diagram of the image forming apparatus according to the embodiment.

FIG. 4 is a cross-sectional configuration diagram of the image forming apparatus according to the embodiment.

FIG. 5 is a block diagram of an image processing system.

FIG. 6 is an explanatory diagram showing a state in which image data is decomposed.

FIG. 7 is an explanatory diagram of dither processing.

FIG. 8 is an explanatory diagram of a pulse width modulation process.

FIG. 9 is a graph showing the amount of adhered toner.

FIG. 10 is an explanatory diagram showing the state of toner adhesion.

FIG. 11 is a graph showing a relationship between a toner adhesion amount and a print image density.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Photoconductor drum 11A, 11B Scorotron charger 12 (L), 12 (H) Exposure optical system 13 (L), 13 (H) Developing unit 14 Conveyor belt 14a Transfer belt 14c Primary transfer unit 14g Secondary transfer unit 15 Paper cassette 16 Timing roller 17 Fixing device 100Y (M, C, K) Process unit

Continued on the front page F-term (reference) 2C262 AA04 AA24 AA26 BB03 BB06 BB16 BB27 BB44 GA32 GA36 GA40 GA42 2H027 EA02 EA04 EB03 EC20 ED04 EE02 EF09 2H077 BA10 DB25 EA24 5C074 AA05 BB02 BB26 GG03 DD03

Claims (3)

[Claims] A developing device using a dark toner of the same color and a developing device using a light toner at the periphery of the image forming body, and performing superposition development using the dark and light toners;
An image forming apparatus for transferring a dark toner image and a light toner image onto a transfer material, wherein an image forming cycle using the dark toner is shorter than an image forming cycle using the light toner. 2. The image forming apparatus according to claim 1, wherein the image forming cycle is formed by dither processing. 3. The image forming apparatus according to claim 1, wherein the image forming cycle is formed by a pulse width modulation process.