JP3308690B2 - 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 an image forming apparatus for developing an electrostatic latent image formed by, for example, an electrophotographic method or an electrostatic recording method.

[0002]

2. Description of the Related Art In recent years, digitalization of copying machines and printers has been advanced along with full colorization and systemization. For example, a laser beam is scanned, and the laser beam is turned on and off.
2. Description of the Related Art Devices such as a laser beam printer that forms a latent image on a photosensitive drum by F to record a desired image have been widely known. A typical use is for binary recording of characters, figures, and the like. Records of that point, characters, figures, etc.,
Since no halftone is required, the printer structure can be simplified. By the way, there is a printer which can form a halftone even with such a binary recording system. As such a printer, a printer employing a dither method, a density pattern method, or the like is well known. However, as is well known, high resolution cannot be obtained with a printer employing a dither method or a density pattern method. Therefore, in recent years, a method of forming a halftone pixel in each pixel without lowering the high recording density has been proposed. In this method, halftone is formed by modulating a laser beam with a pulse width (PWM) using an image signal. According to this method, a high-resolution and high-gradation image can be formed.

[0003]

However, when an image is output by a digitized copying machine as in the prior art, roughness occurs in a halftone area having a reflection density of 0.3 or less. . This roughness did not occur so much in a text document or the like, but occurred in a low density area such as a photographic image.

[0004] Then, when the cause of the occurrence of roughness was examined, the following was found. When a latent image of a highlight portion is formed by a normal dot latent image, when viewed microscopically, the latent image on the photoconductor is not a broad latent image like an analog latent image but a local latent image. When it tries to further reproduce than the low concentration, the maximum contrast V ₀ as the latent image accent Figure 6 from the effects of the film thickness of the photosensitive member becomes gradually smaller. For example, when trying to reproduce an image having a reflection density of about 0.2, V ₀ of the latent image at that time becomes 150
It becomes about 200V. Also, in the case of reverse development, the surface potential of the non-image portion to take the head, since it is 100~200V set higher than the DC component of the developing bias, the developing bias when V ₀ is 150 to 200 DC The potential difference Vcont with the component is about 0 to 50V. The reason why Vcont is 0 to 50 V is a very unstable contrast whether the toner is applied to the photoconductor side or the sleeve side. For this reason, when the latent image is developed with the two-component developer, the contact state of the magnetic brush is high in development efficiency, and roughness is likely to occur due to dropout of dots corresponding to unevenness of the ears of the clean magnetic brush. .

The present invention has been made in view of the above-mentioned drawbacks of the prior art, and has as its object to reduce the density of the magnetic brush and to reduce the minimum recording area (S).
Density data (O) that is lighter than the same data (Ms) corresponding to
To Ms) are not used, so that density unevenness is less likely to occur in the highlight portion, and an image forming apparatus capable of obtaining a favorable halftone image in the entire density region is provided.

[0006]

Means for Solving the Problems The above-mentioned problems are solved,
In order to achieve the object, an image forming apparatus according to the present invention includes:
Supports developer containing non-magnetic toner and magnetic carrier
Based on the input density data.
Developer support means for transporting the latent image to a developing unit for developing the latent image,
The magnetic brush on the developer supporting means is
Magnetic field generating means for generating a magnetic field with a developing main pole to be standing
And a peak of the developing main pole.
Of the magnetization of the magnetic carrier when the magnetic field strength of
Is less than 100 emu / cm ³ ,
The recording area is S, and the volume average particle diameter of the non-magnetic toner is r.
And S> 10r ² is satisfied .

[0007]

According to this structure, the dark portion in the highlight portion is
Degree unevenness is less likely to occur, and good
A halftone image can be obtained .

[0008]

Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. First Embodiment First, an outline of the present embodiment will be described. When a dot latent image is developed, it is sharper than an analog latent image, but becomes a local latent image, so that it is more likely to occur than a non-uniform (dot missing) image corresponding to the ears of a magnetic brush. Therefore, when developing a dot latent image,
It is desired that the state of the ears of the magnetic brush be denser.

Therefore, in this embodiment, when a magnetic field having a peak magnetic field strength of the magnetic pole of the magnetic field generating means closest to the image carrier and the developer supporting means is applied as a magnetic carrier, a magnetization of 100 emu or less per cubic centimeter is applied. The magnetic brush density becomes dense by having S> 10r ² with the minimum recording area (S) of each pixel with respect to the volume average particle diameter (r) of the toner, and Since density data (O to Ms) lighter than the same data (Ms) corresponding to the minimum recording area (S) is not used, density unevenness is less likely to occur in the highlight portion, and a satisfactory halftone is obtained in all density regions. A tone image can be obtained.

FIG. 1 is a side sectional view showing the structure of an image forming apparatus according to a first embodiment of the present invention. First, the platen 10
The original G is set with the surface to be copied facing downward. Next, copying is started by pressing the copy button. The original irradiation lamp, the short focus lens array, and the CCD sensor form an integrated unit 9 and operate while irradiating the original, so that the original back reflection light of the illumination scanning light is imaged by the short focus lens array. Incident on the CCD sensor. The CCD sensor includes a light receiving unit, a transfer unit, and an output unit. The light signal is converted into an electric signal in the CCD light receiving section, and is sequentially transferred to the output section in synchronization with the clock pulse in the transfer section. The output section converts the electrification signal into a voltage signal, amplifies the signal, reduces the impedance, and outputs the signal. The analog signal obtained in this manner is subjected to well-known image processing, converted into a digital signal, and sent to a printer unit. The printer section receives the image signal and forms an electrostatic latent image in the following manner. The photosensitive drum 1 is driven to rotate around a central support shaft at a predetermined peripheral speed. During the rotation process, the photosensitive drum 1 is subjected to a uniform charging process of positive or negative polarity by a charger 3, and an image signal is applied to the uniformly charged surface. Corresponding to O
By scanning the light of the solid-state laser element 103 which emits N, OFF light with the rotating polygon mirror 104 rotating at high speed, an electrostatic latent image corresponding to the original image is sequentially formed on the surface of the photosensitive drum 1, and FIG. FIG. 3 is a diagram showing a schematic configuration of a laser scanning unit 100 that scans a laser beam in the device described above. When a laser beam is scanned by the laser scanning unit 100, first, a solid-state laser element 102 is generated by a light emission signal generator 101 based on an input image signal.
Blink at a predetermined timing. The laser light emitted from the solid-state laser element 102 is converted into a substantially parallel light beam by a collimator lens system 103, and further scanned in a direction indicated by an arrow c by a rotating polygon mirror 104 rotating in a direction indicated by an arrow b. 105a, 105b, 1
05c, an exposure distribution for one scan of an image is formed on the non-scanning surface 106 such as a photosensitive drum, and by scrolling the non-scanning surface 106 by a predetermined amount perpendicular to the scanning direction for each scan, An exposure distribution according to the image signal is obtained on the non-scanning surface 106.

In this embodiment, since the multi-value recording by the area gradation of one pixel is performed by using the laser PWM method (pulse width modulation), the PWM method will be briefly described. FIG. 4 is a circuit block diagram showing an example of the pulse width gradation circuit, and FIG. 5 is a timing chart showing the operation of the pulse width modulation circuit. In FIG. 4, reference numeral 401 denotes a TTL (transisitor transi) for latching an 8-bit digital image signal.
stor logic) latch circuit, 402 is a level converter for converting a TTL logic level to a high-speed ECL (emitter coupled logic) logic level, and 403 is an A / D converter for converting the ECL logic level to an analog signal. 404 is an ECL comparator for generating a PWM signal, and 405 is an ECL comparator.
A level converter for converting the CL logic level to a TTL logic level; 406, a clock oscillator for transmitting a clock signal 2f; 407, a triangular wave generator for generating a substantially ideal triangular wave signal in synchronization with the clock signal 2f; This is a half-period in which the image clock signal f is generated by dividing the frequency of 2f by １ ／. Thereby, the clock signal 2f
Has a period twice as long as the image clock signal f. In order to operate the circuit at high speed, EC
An L logic circuit is provided.

The circuit operation having such a configuration will be described with reference to the timing chart of FIG. The signal a indicates the clock signal 2f and the signal b indicates the image clock signal f, which is related to the image signal as shown in the figure. Also, inside the triangular wave 407, the clock signal 2f is once reduced to １ ／ in order to keep the duty ratio of the triangular wave signal at 50%.
The frequency is divided to generate a triangular wave signal c. Further, this triangular wave signal c is converted into an ECL level (0 to -1 V) to become a triangular wave signal d.

On the other hand, the image signal is 00h (white) to FFh
It changes at 256 gradation levels up to (black). The symbol '
h 'indicates hexadecimal notation. The image signal e indicates the ECL voltage level obtained by D / A converting some image signal values. For example, the first pixel is FFh at the black pixel level, and the second pixel is 80% at the halftone level.
h, the third pixel indicates a voltage of 40h at the halftone level, and the fourth pixel indicates a voltage of 20h at the halftone level. The comparator 404 compares the triangular wave signal d with the image signal e to obtain pulse widths T, t2, and t according to the pixel density to be formed.
3. Generate a PWM signal such as 3, t4. And this P
The WM signal is converted to a TTL level of 0 V or 5 V, becomes a PWM signal f, and is input to the laser driver circuit 500.

By changing the exposure time per pixel in accordance with the PWM signal value thus obtained, it is possible to obtain 256 gradations per pixel. In FIG.
11 is a developing sleeve, 12 is a magnet roller fixedly arranged in the developing sleeve, 13 and 14 are agitating screws, 15 is a regulating blade arranged to form a thin layer of developer on the surface of the developing sleeve, and 16 is a developing container It is.
Here, the developing step of visualizing the electrostatic latent image by a two-component magnetic brush method using the developing device and the circulation system for the developer will be described below. First, the developer pumped at the N2 pole with the rotation of the developing sleeve 11 is changed from the S2 pole to the N1 pole.
In the process of transporting with the poles, a thin layer is formed on the developing sleeve 11 by being regulated by a regulating blade 15 arranged substantially perpendicular to the developing sleeve 11. Here, when the developer formed in a thin layer is conveyed to the developing main pole S1, poles are formed by magnetic force. The electrostatic latent image is developed with the spike-shaped developer, and then N
The developer on the developing sleeve 11 is returned into the developing container 16 by the repulsive magnetic field of three poles and N2 pole.

A DC bias and an AC bias are applied to the developing sleeve 11 from a power supply (not shown). In this embodiment, Vpp = 2000 V and f = 2000H as AC components.
z is applied. In general, in the two-component developing method, when an AC bias is applied, the development efficiency is increased and the image quality is high. Therefore, in this embodiment, the improvement of the image quality by applying the AC bias is also realized.

The developer is composed of 8 μm non-magnetic toner and 50 μm
And a magnetic carrier having a magnetization strength of 100 emu or less per cubic centimeter when a magnetic field strength of a developing main pole of 1000 G is applied. Thus, the toner image formed on the photosensitive drum 1 is electrostatically transferred onto a transfer material by the transfer charger 7. Thereafter, the transfer material is electrostatically separated by the separation charger 8 and transported to the fixing device 6, where it is thermally fixed to output an image.

On the other hand, the surface of the photosensitive drum 1 after the transfer of the toner image is subjected to removal of adhered contaminants such as untransferred toner by the cleaner 5 and is repeatedly used for image formation. By the way, in the developing device having the configuration used in the present invention, when the above-described peak magnetic field strength of the developing main pole S1 is applied (at 1000 G in the figure), the magnetization strength σd per volume is varied. Examination using a magnetic carrier revealed that as σd increased, the ear density decreased as shown in FIG. A DC magnetization BH characteristic automatic recording device BHH-50 manufactured by Riken Denshi Co., Ltd. is used for measuring the magnetic characteristics of the magnetic carrier.

[0018]

[Table 1]

As shown in Table 1, the density of spikelets is 5 / mm ²
With the above, a good image without roughness can be obtained, and 7 lines / mm
If it is ² or more, a better image can be obtained. The rough feeling greatly depends on the visual characteristics of the human eyes. If the ear density exceeds the above-mentioned ear density, a noise component generated on the image becomes almost invisible.

This depends on the visual limit of the human eye. FIG. 9 shows the spatial frequency υ (line / mm) and the number L (density difference) of recognizable levels. The density difference L is represented by the following equation (1).

[0021]

(Equation 1)

In general, the fluctuation range of the density in the highlight portion of the image density (reflection density) of 0.2 to 0.3 where the roughness is conspicuous is about 0.02, and from FIG. 9, the spatial frequency is about 2.3. (Line / mm), it is difficult for the human eye to recognize the above-described density fluctuation, and the spatial frequency is about 2.7 (line / mm).
It can be seen that if it is higher than this, it can hardly be recognized.

That is, the density of the magnetic brush is 5.3 (lines / m).
In the case of m ² ) (2.3 × 2.3 = 5.3) or more, the frequency component of the roughness becomes higher for the above-mentioned reason, so that it becomes difficult for human eyes to recognize. From the above results, it can be seen from FIG. 7 that the magnetization intensity σd per volume is 1
It can be seen that a good image without roughness can be obtained when the value is not more than 00 emu.

Further analysis of the noise component of the image present in the highlight portion reveals that there is a factor that causes roughness even if the magnetic brush of the developing portion is made as dense as possible. This has no correlation with the density of the magnetic brush,
It has been found that when the area of each pixel developed by the toner on the photoreceptor becomes smaller than a certain value, the formation of the pixel suddenly becomes unstable, which is caused.

FIG. 8 shows the measurement of the fluctuation of the pixel area obtained by developing with the toner of each particle size when the dot system of the pixel is changed by the laser PWM method. It is known that, in the method of expressing gradation by modulating the area of a pixel for the PWM method, the fluctuation amount of the area of each pixel has a correlation with the noise component of the pixel. In FIG. 8, the value (σ / A) obtained by dividing the standard deviation (σ) of the variation of the area of each pixel when the constant density data is output as the fluctuation amount by the average (A) of the pixel area in the density data. ).

As is apparent from FIG. 8, it is confirmed that the smaller the pixel area is, the larger the fluctuation amount is, and the more unstable the pixel formation is. Also, it can be seen that smaller pixels can be formed more stably as the toner particle size becomes smaller. Next, when examining the correlation between the noise amount shown in FIG. 8 and the feeling of roughness felt by human eyes, the fluctuation amount of the pixel area is about 0.
If it exceeds 35, the image becomes rough.

Generally, in a highlight portion where the image density (reflection density) in which the roughness is conspicuous is 0.2 to 0.3, when the fluctuation amount of the pixel area becomes 0.35 or more, the image density (reflection density) becomes about 0. 1 will occur. Whether this density variation of about 0.1 is perceived by the human eye is related to the visual limit of the human eye shown in FIG.

Normally, when an image is formed by the laser PWM method, high-density recording of about 150 dpi or more is performed so that the recording density of pixels does not give a grainy feeling to human eyes. This recording density is about 6 (line / mm) or more when converted to the spatial frequency (υ) shown in FIG. In such a region where the spatial frequency is high, the recognizable density fluctuation is about 0.1, and as shown in FIG. 8, when the fluctuation amount of the pixel area becomes 0.35 or more, the image may feel rough. Matches.

Therefore, in order to obtain a high-quality image by eliminating the feeling of roughness on the image, the image may be formed within a range where the fluctuation amount of the pixel area does not exceed 0.35 as shown in FIG. It is shown that. The minimum pixel area (S) at which the fluctuation amount does not exceed 0.35 substantially depends on the volume average particle diameter (r) of the toner to be used, and is expressed by the following equation (2).

S = 10r ² (2)

[0031]

[Table 2]

Table 2 shows that the volume average particle diameter (r) of the toner and the value of the above equation at that time agree well with the results of FIG. From the above results, in the present invention, the minimum minimum recording area (S) of each pixel is larger than the volume average particle diameter (r) of the toner to be used, as shown in the following equation (3): S> 10r ² . By setting (3), since an area having a pixel area that is uneasy for image formation is not used, an extremely high quality image without roughness can be obtained even in a highlight portion.

In the present embodiment, the volume distribution and the volume average particle diameter of the toner used are, for example, those measured by the following measuring methods. Using a Coulter Counter TA-2 type (manufactured by Coulter) as a measuring device, an interface (manufactured by Nikkaki) for outputting a number average distribution and a volume average distribution and a CX-i personal computer (manufactured by Canon) were connected, and an electrolytic solution was used. Prepare a 1% NaCl aqueous solution using primary sodium chloride.

The measuring method is as follows.
In 150 ml, 0.1 to 5 ml of a surfactant (preferably alkylbenzenesulfonate) is added as a dispersant,
Further, 0.5 to 50 mg of a measurement sample is added. The electrolyte solution in which the sample was suspended was subjected to a dispersion treatment for about 1 to 3 minutes by an ultrasonic disperser, and the above-mentioned Coulter Counter TA-2 was used for 2 to 40 minutes using a 100 μm aperture as an aperture.
The volume distribution obtained by measuring the particle size distribution of the μm particles is determined.

From the obtained volume distribution, the volume average particle size of the sample is obtained. As described above, according to the first embodiment, a dense magnetic brush can be formed on the phenomenon sleeve, and the density data (O to Ms) is lighter than the density data (Ms) corresponding to the minimum recording area (S). ) Is not used, so that the density unevenness in the highlight portion hardly occurs, and a good halftone image can be obtained in the entire density region. <Second Embodiment> In the first embodiment described above, the minimum recording area of each pixel used for image formation is set within the range of the above-described expression, so that image noise in a highlight portion is significantly reduced. Although it has been described that the density can be reduced, in the present embodiment, a uniform image can be reproduced even in a lighter density region by introducing a technique of pseudo gradation expression in addition to the area gradation expression by the laser PWM method. It was made.

The minimum recording area (S) of a pixel indicates the area of the isolated pixel (for example, an isolated dot on a circle) when the pixel is formed separately from an adjacent pixel, and When a screen on a line is connected to adjacent pixels (for example, a vertical screen), the area of the screen on the line is divided by the number of pixels required to form the screen.

For example, the image density data M (0 to 25)
If the image density data Ms corresponding to the above-described minimum recording area S is determined for 5), density data (0 to Ms-1) lighter than the density data Ms cannot be recorded as it is. Therefore, the light density data (0 to M
s-1) is binarized with a specific threshold value (eg, 1/2 Ms) to obtain light density data (0 to Ms-1).
Is binarized to (0 or Ms), and the light density data (0 to
Ms-1).

Further, by applying a well-known error diffusion method in which the binarization error generated at this time is distributed to peripheral pixels to preserve the density, more stable and accurate gradation expression can be achieved. Now, the present invention is not limited to the above two embodiments, and forms a multi-valued latent image on the photoconductor by modulating the laser emission time of each pixel based on the input density data. The present invention can be applied to any image forming apparatus that develops an electrostatic latent image on a multi-valued latent image on a photoreceptor using a two-component developer composed of a toner containing a magnetic carrier and a colorant.

For example, in a color image forming apparatus having a plurality of developing devices (yellow, magenta, cyan, black, etc.), a gradation image is mainly required to have high image quality, and furthermore, a Since noise components at the time of development of each color are accumulated, how to reduce the noise components generated at the time of development is a major issue. Therefore, the effect of applying the present invention to this type of image forming apparatus is significant.

[0040]

As described above, according to the present invention,
Make the magnetic brush ears dense in the developing section, and S> 10
Since r ² is configured to hold, density unevenness hardly occurs in the highlight portion, so that a good halftone image is obtained in the whole density region.

[Brief description of the drawings]

FIG. 1 is a process diagram of an image forming apparatus showing an embodiment of the present invention.

FIG. 2 is a diagram of a developing device used in an embodiment of the present invention.

FIG. 3 is a schematic view of a laser scanning unit used in an embodiment of the present invention.

FIG. 4 is a block diagram of a PWM circuit used in an embodiment of the present invention.

FIG. 5 is a timing chart showing the operation of FIG. 4;

FIG. 6 is a surface potential diagram schematically illustrating a latent image in a high density portion and a highlight portion.

FIG. 7 shows the relationship between the value of magnetization and the density of the brush ears of a magnetic brush when the developing magnetic field is 1000 Gauss.

FIG. 8 shows a relationship between a pixel area and a noise amount.

FIG. 9 is a diagram illustrating visual characteristics of human eyes.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 photosensitive drum 2 pre-exposure lamp 3 primary charger 4 developing device 5 cleaner 6 fixing device 7 transfer charger 8 separation charger 9 document scan unit 10 document table 11 developing sleeve 12 magnet roller 13 14 stirring screw 15 regulating blade 16 developing Container 17 Partition wall 18 Supply toner 19 Developer 20 Supply port 101 Light emission signal generator 102 Solid-state laser element 103 Collimator lens system 104 Rotating polygon mirror 105 fθ lens group 106 Scanned surface

──────────────────────────────────────────────────続 き Continuation of front page (72) Inventor Hiroyuki Suzuki 3-30-2 Shimomaruko, Ota-ku, Tokyo Inside Canon Inc. (56) References JP-A-2-132459 (JP, A) JP-A-5 -303284 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G03G 15/09

Claims (3)

(57) [Claims] 1. Development including non-magnetic toner and magnetic carrier
Supports the agent and on the photoreceptor based on the input density data
To transport the multi-level latent image formed on the developing unit to the developing unit
And a magnetic brush on the developer supporting means.
A magnetic field is generated by providing a main developing pole that stands up in the developing section.
And a magnetic field generating means, when the magnetic field strength of the peak of the main developing pole is applied.
The intensity of magnetization of the non-conductive carrier is 100 emu / cm ³ or less.
Yes, the minimum recording area of each pixel is S, the volume average particle size of non-magnetic toner
When the diameter is r, S> 10r ² is satisfied.
Developing device. 2. A developing device according to claim 1, wherein said developing device comprises a plurality of developing devices.
Using different colors of toner stored in the developing device
An image forming apparatus for forming a color image . 3. A density data corresponding to the minimum recording area S.
Data is Ms, and lighter density data is 0 to Ms.
When, 0 and an image forming apparatus according to claim 2, wherein a binarizing means for processing binarization either one of Ms.