JP2668440B2 - Image forming device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus capable of forming substantially stable density in a highlight portion of a digital full color printer output using a laser beam. About.

2. Description of the Related Art As a high-speed and low-noise printer, there is a laser beam printer employing an electrophotographic system. A typical application is binary recording of images such as characters and figures. This point, the letters,
Since the recording of graphics does not require halftones, the printer structure can be simplified. There is a printer that can form a halftone image even with such a binary recording method. As such a printer, one using a method such as a dither method or a density pattern method is well known.

However, it is difficult to obtain high resolution with a printer that employs the dither method or the density pattern method. Therefore, in recent years, a printer has been developed which employs a binary recording method and performs halftone formation by pulse width modulation (PWM) of a laser beam with an image signal. According to this PWM method, a high-resolution and high-gradation image can be formed. High resolution and high gradation are necessary for color image formation.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention However, various problems arise in a PWM laser beam printer. It has to do with pulse width modulation of the laser beam.

FIG. 1 shows a typical electrophotographic color printer. This printer includes an electrophotographic photosensitive drum 3 as a photosensitive member that rotates in the direction of an arrow, and a rotary developing device 1 including a charger 4 and developing devices 1M, 1C, 1Y, and 1BK around the photosensitive drum 3. And an image forming means including a laser beam scanner disposed above the transfer discharger 10, the cleaning means 12, and the photosensitive drum 3 in the drawing.

The sequence of the entire color printer will be briefly described by taking a full-color mode as an example. First, the photosensitive drum 3 is uniformly charged by the charger 4. Next, an image of a document (not shown) is exposed by a laser beam E modulated by a magenta image signal, an electrostatic latent image is formed on the photosensitive drum 3, and then, the document is previously set at a developing position. Development is performed by the magenta developing device 1M.

On the other hand, the transfer material that has advanced through the paper feed guide 5a, the paper feed roller 6, and the paper feed guide 5b is held by the gripper 7 in synchronization with a predetermined timing, and is electrostatically charged by the contact roller 8 and its opposite pole. It is physically wound around the transfer drum 9. The transfer drum 9 rotates in the direction of the arrow in synchronization with the photosensitive drum 3, and the developed image developed by the magenta developing device 1M is transferred to a transfer material by a transfer charger 10 in a transfer section. The transfer drum 9 continues to rotate as it is to prepare for the transfer of the next color (cyan in FIG. 1).

On the other hand, the photosensitive drum 3 is neutralized by the charger 11, cleaned by the cleaning means 12, charged again by the charger 4, and is exposed by the next cyan image signal. During this time, the developing device 1 rotates, and the cyan developing device 1C is fixed at a predetermined developing position to perform a predetermined cyan development.

Subsequently, the above steps are performed for yellow and black, respectively, and when the transfer for four colors is completed,
The four-color image on the transfer material is neutralized by the chargers 13 and 14,
When the gripper 7 is released, it is separated from the transfer drum 9 by the separation claw 15 and is sent to the fixing device (heat pressure roller fixing device) 17 by the conveyor belt 16 to complete a series of full-color printing sequence, and to obtain the required full-color printing. An image is formed.

As shown in FIG. 2, the laser beam scanner forming the exposure means comprises a semiconductor laser section 102, a polygon mirror 105 rotating at a high speed, and an f-θ lens 106. The semiconductor laser section 102 is composed of: Upon receiving a time-series digital pixel signal that is calculated and output by an electronic computer or the like of the image reading device, a PWM-modulated laser beam is oscillated corresponding to the signal to expose the surface of the photosensitive drum 3.

More specifically, referring to FIG. 2, a solid-state laser element 102 as a laser light source which is a light source unit is connected to a laser driver 500 which is a light emission signal generator for sending a light emission signal for generating laser light. , According to the emission signal of the laser driver. The laser light flux emitted from the solid-state laser element 102 is made into substantially parallel light by the collimator lens system 103. The collimator lens system 1
03 can be moved by a predetermined amount in the direction of arrow A, which is the optical axis direction of the laser beam, by the focus adjustment means 104 described later.

The polygon mirror, that is, the rotary polygon mirror 105 rotates at a constant speed in the direction of arrow B to reflect the parallel light emitted from the collimator lens system 103 and scan it in the direction of arrow C, which is a predetermined direction. F- provided in front of the rotating polygon mirror 105
The θ lens group 106 (106a, 106b, 106c) forms an image of the laser light beam deflected by the polygon mirror 105 on a surface to be scanned, that is, a predetermined position on the photosensitive drum 3, and the scanning speed on the surface to be scanned. Use constant speed.

The laser beam L is guided to a CCD (solid-state imaging device) 108 as a detecting unit via a reflecting mirror 107, and is scanned on the photosensitive drum 3 as a surface to be scanned. The CCD 108 is configured by arranging a large number of photodetectors in the direction of arrow C at positions substantially optically equivalent to the surface of the photosensitive drum 3 and the light source unit.
In addition, the CCD 108 has a laser driver 500 and a focus adjusting unit 104.
Is connected to the control unit 100 for controlling the

Further, an image processing unit 111 is connected to the laser driver 500 and the control unit 100.

In the above configuration, when a desired image is formed, first, the image output signal P is input from the image processing unit 111 to the control unit 100, and the image signal S is input to the laser driver 500. Decrease 102.

The laser light emitted from the solid-state laser element 102 is converted into substantially parallel light by a collimator lens system 103, and is further converted into an arrow C by a rotating polygon mirror 105 rotating in the direction of arrow B.
The scanning is performed in the same direction, and an image is formed in a spot shape on the photosensitive drum 3 by the f-θ lens group 106. An image-scanning exposure distribution is formed on the surface of the photosensitive drum 3 by the scanning of the laser beam L as described above, and the photosensitive drum 3 is rotated by a predetermined amount for each scanning to generate an image signal S on the drum 3. Is formed and recorded as a visible image on a transfer material by a well-known electrophotographic process.

The image output signal P is output from the image processing unit 111 prior to the image signal S, and the output ends after the output of the image signal S ends. Further, the control unit 100 stops operating while the image output signal P is input from the image processing unit 111. Therefore, the pixel size and the contrast can be kept constant during the image forming operation.

Next, the operation of the focal position adjusting means 104 for the laser beam L will be described.

First, the operation signal is sent from the control unit 100 to the laser driver 500.
And from the laser driver 500, FIG. 15 (a)
A rectangular wave that turns on and off at regular intervals as shown in (1) is generated for a predetermined period, and the solid-state laser element 102 blinks in response to this signal. The laser light from the solid-state laser element 102 is
Scanning is performed as described above, and the light is reflected by the reflecting mirror 107 and disposed at a position optically equivalent to the photosensitive drum 3.
The image is projected and scanned on the CCD 108.

The control unit 100 resets the accumulated charge of each image of the CCD 108 before the laser beam L scans the CCD 108, and reads out the electric charge as an electric signal after the electric charge is accumulated in each pixel of the CCD 108 by spot scanning of one line. .

When the laser light is flickered from the solid-state laser element 102 and scanned once, the CCD 108 is located at a position optically equivalent to the photosensitive drum 3. Therefore, the exposure distribution on the surface of the CCD 108 is, as shown in FIG. 3 shows the distribution shape of the intensity depending on the spot diameter of. Therefore, the output of each pixel of the CCD 108 has a distribution as shown in FIG.
To send to. The control unit 100 calculates and measures the contrast V according to the following formula: V = (θmax−θmin) / (θmax + θmin), where θmax is the maximum value of the output of the CCD 108 and θmin is the minimum value.

In this case, the contrast V becomes larger as the spot diameter in the scanning direction becomes smaller. Therefore, when the preset value V ₀ is compared with the V calculated by the equation (1), the V is not equal to the predetermined value V _0. Sends a drive signal from the control unit 100 to the focus adjustment unit 104 to move the collimator lens system 103 in the direction of arrow A by a predetermined amount. And the collimator lens system
The contrast V is measured at each position where 103 is moved, and if the collimator lens system 103 is fixed at a position where this value is equal to V ₀ , the defocus of the optical system is corrected and the scanning spot diameter of the laser light flux L is corrected. Can be minimized.

FIG. 3 is a circuit diagram of the PWM circuit, and FIG. 4 is a timing chart showing the operation of the PWM circuit.

In FIG. 3, a PWM circuit is a TTL latch circuit 401 for latching an 8-bit image signal, and a TTL logic level is changed to a high-speed ECL.
Level converter 402 for converting to logic level, ECLD / A converter 403, ECL comparator 404 for generating PWM signal, ECL
Level converter 40 that converts logic levels to TTL logic levels
5. A clock signal 2f having twice the frequency of the pixel clock signal f
Generating a clock oscillator 406, a triangular wave generator 407 that generates a substantially ideal triangular wave signal in synchronization with the clock signal 2f,
And a 1/2 divider 408 for dividing the clock signal 2f by 1/2. In addition, ECL logic circuits are placed everywhere to operate the circuit at high speed.

 The operation of this configuration will be described with reference to FIG.

The signal indicates the clock signal 2f and the signal indicates the pixel clock signal f having a double period thereof, which is associated with the pixel number as shown in the figure. Even within the triangular wave generator 407, in order to maintain the duty ratio of the triangular wave signal at 50%, the triangular wave signal is generated after the clock signal 2f is once divided by two. Further, this triangular wave signal has an ECL level (0 to -1).
V) and becomes a triangular wave signal.

On the other hand, the pixel signal changes from OOH (white) to FFH (black) at 256 gradation levels. The symbol H is in hexadecimal. And the image signal D / A them for some image signal values
The converted ECL voltage level is shown. For example, the first pixel is FFH at the black pixel level, the second pixel is 80H at the halftone level,
The third pixel shows the voltage of the intermediate tone level of 40H, and the fourth pixel shows the voltage of the intermediate tone level of 20H. The comparator 404 compares the triangular wave signal and the image signal to generate a PWM signal having a pulse width T, t ₂ , t ₃ , t ₄ according to the pixel density to be formed. Then, this PWM signal is converted to a TTL level of 0V or 5V, becomes a PWM signal, and is input to the laser drive circuit 500.

In the circuit shown in FIG. 3, a look-up table (not shown) is provided at a stage preceding the latch circuit 401. This look-up table is for performing γ correction of image data, and is a memory in which the data of the result of γ correction is stored, and the memory is accessed by using an image signal of 8 bits per pixel as address data to obtain a desired γ An image signal of the corrected data is output. Usually a specific one in one screen
Although one gamma correction table is used, a plurality of types of gamma correction tables can be switched and used in one screen as needed. In other words, for example, three types of tables are sequentially and repeatedly used for each line scan by the beam, and the gamma correction in the sub-scanning direction is changed for each line to perform gradation correction.

When the density of the toner is low, the look-up table is set to a so-called standing γ table so that the lookup table is not affected by the specific density of the toner of each color, for example, yellow, magenta, cyan, and black. When the density is high, a γ table having the opposite characteristic is set and provided for each forming color. However, in the preceding stage of such a look-up table, non-linearity is used to correct the turbidity of the color of each color toner. A color masking circuit, for example, a secondary color masking circuit can be provided.

The above-described PWM method is characterized in that halftones of dots are performed for each pixel, and halftones can be simultaneously expressed without lowering the density of pixels to be recorded.

However, even in this PWM system, as shown in FIG. 5, it was found that the exposure distribution on the surface to be scanned 106 is affected by the spot diameter of the laser and changes as shown in the figure.

FIG. 6 shows a recording pixel density of 400 dpi (pixel size of 63.5 μm).
m), when the laser spot diameter is 70 μm (main scanning Gaussian distribution spot 1 / e ² diameter), 1/4 pixel equivalent per pixel, 1/2 pixel equivalent time, when laser drive time is pulse width modulated The exposure distribution on the surface to be scanned is shown.

Normally, the laser spot diameter is 1.1 for the pixel size (63.5 μm at 400 dpi) so that the exposure distribution is most uniform when the influence of adjacent pixels is taken into consideration when the entire surface of each pixel is exposed.
Optimum size is from 1.6 times to 1.6 times (about 70 μm at 400 dpi)
~100μm1 / e ² diameter).

However, when the above-mentioned spot size conventionally used is used, as shown in FIG. 6, the exposure distribution on the surface to be scanned has a small amplitude due to the ON / OFF of the laser and a low contrast, and the exposure distribution on the surface to be scanned is low. The tendency is that the average exposure amount changes as a whole.

When the halftone is expressed by the above-described method, the tendency that the surface potential on the photosensitive drum is entirely changed with respect to the driving pulse width of the laser in accordance with the above exposure distribution is obtained. The image output is strongly affected by the VD characteristic of the developing system used, and there has been a problem that the output image density does not change linearly with respect to the drive pulse width (PWM signal) of the laser.

Therefore, in order to solve this, as described above, the VD characteristic of the developing system is taken into consideration, and the image signal itself is adjusted to this characteristic so that the output image density changes linearly.
A correction reference table for correcting the PWM signal was created in the image processing unit to solve the problem.

However, the larger the correction amount by this correction table, the larger the correction error due to the loss of image information, the accompanying gradation skip, or the change in V-D characteristics due to environmental changes. And so on.

Accordingly, it is an object of the present invention to provide an image forming apparatus which solves the above-mentioned problems.

It is another object of the present invention to provide a high-quality image forming apparatus which is excellent in gradation in an image portion having a low image density and has no roughness.

It is another object of the present invention to provide a color image forming apparatus in which toner scatter is small even when each color toner is multiply transferred onto a transfer material such as transfer paper.

Still another object of the present invention is to provide a color image forming apparatus that forms a high-definition full-color toner image or a multi-color toner image.

Means for Solving the Problems The above objects are achieved by an image forming apparatus according to the present invention. In summary, the present invention provides a photoconductor, a laser light source that emits laser light, a pulse width modulation circuit that controls the light emission time per pixel of the laser light source according to an image signal, and a laser that is emitted from the laser light source. It has a scanning optical system that deflects light and forms an image on a photoconductor, and a developing unit that develops a latent image formed on the photoconductor with toner. By controlling the light emission time per pixel, gradation is expressed. In the image forming apparatus, the maximum spot diameter per pixel of the laser light emitted from the laser light source is smaller than 0.7 times the pixel size in the scanning direction of the scanning optical system, and the volume average particle diameter of the toner is M, if the particle size of the toner is r, 90% by volume or more of the toner in the developing means is (1/2) M <r <(3
/ 2) An image forming apparatus is provided which is included in the range of M and 99% by volume or more is included in the range of 0 <r <2M.

In the present invention having the above-described configuration, the size of the spot irradiated on the surface to be scanned is made sufficiently small with respect to the density to be recorded, and the particle size distribution of the toner to be used is made sharp to obtain a volume average particle size. By setting the diameter to less than 12 μm, even when the pulse width modulation of the laser drive is performed, even a portion where the image density is low can be faithfully reproduced, and stable area gradation in each pixel is possible.

Hereinafter, an image forming apparatus according to the present invention will be described with reference to the drawings.

FIG. 1 shows an electrophotographic printer capable of implementing the present invention. Since the configuration and operation of the printer have been described above,
Detailed description is omitted.

Also in this embodiment, the laser beam scanner shown and described in FIG. 2 and the PWM circuit described in connection with FIGS. 3 and 4 are preferably used.

That is, as described above, the laser beam scanner is composed of the semiconductor laser unit 102, the polygon mirror 104, the F-θ lens 100, etc., and the semiconductor laser unit 102 is arithmetically output by the image reading device or the electronic calculator. When a time-series digital pixel signal is input, a PWM-modulated laser beam is oscillated according to the input signal to expose the drum surface. In this embodiment, the PWM circuit shown in FIG. 3 uses the image signals of yellow, magenta, cyan, and black for the pages (one page for each of the original and the copy).
Each color is sequentially input, laser modulation is sequentially performed for each color, and one copy is made by four rotations of the drum. In the apparatus shown in FIG. 13 described later, a PWM circuit is provided for each color. The focus correction circuit described with reference to FIG. 2 performs focus control for each color.

Furthermore, in the present embodiment, a non-linear color masking circuit, for example, a secondary color masking circuit, is provided at the preceding stage of the look-up table provided for each color to be formed, in order to correct the color of each color toner. As will be described later in detail, a high-definition, high-quality color image with stable gradation and color reproducibility is formed in combination with a small-diameter toner and a stable small-diameter laser spot constructed according to the present invention. can do.

Further, the laser beam printer according to this embodiment will be described. As described above, the exposure distribution for one scanning of an image is formed on the surface of the photosensitive drum 3 by the scanning of the laser beam L, and the photosensitive drum is further scanned for each scanning. By rotating the drum 3 by a predetermined amount, a latent image having an exposure distribution according to the image signal is formed on the drum 9 and recorded as a visible image on a recording sheet by a well-known electrophotographic process.

By the way, when a pulse width modulated laser beam is imaged on a small spot on the photosensitive drum 3 by an optical system as shown in FIG. The upper beam spot diameter is about 1.1 to 1.6 of the recording pixel size as conventionally used.
If it is doubled, as shown in FIG. 5, even if the laser beam is turned ON / OFF with a 50% pulse width, the exposure distribution on the photosensitive drum surface becomes as shown in FIG. 5 (B). ,
The contrast at the maximum and minimum values of the exposure amount is only about 30%, and gradation reproduction due to a change in the dot area of each pixel obtained by the subsequent development process cannot be performed stably.

In order to stabilize the dot area gradation expression by pulse width modulation, various experiments by the present inventors have revealed that, for example, when the laser beam is turned ON / OFF with a pulse width of 50%, exposure on the drum surface is performed. About 80 contrast in distribution
% Was found to be possible.

In view of this, in the present embodiment, the spot diameter on the drum surface is set to be the spot diameter (Gaussian distribution spot 1 / e ² ) <pixel size × 0.7 with respect to the recording pixel size.

Figure 6 shows the recording density of 400 dpi (pixel size 63.5 μm)
When the laser beam spot diameter is 70 μm (A), which is 1.1 times the pixel size, 50 μm (B), which is 0.8 times, respectively,
It shows the exposure distribution on the drum surface when it is 0.7 times 42 μm (C).

From Fig. 6, the contrast of exposure distribution when ON / OFF with 50% pulse width is about 30% (A) and about 60%, respectively.
(B), about 80% (C), which was made possible by making the laser beam spot diameter (1 / e ² diameter) 0.7 times or less the pixel size.

FIGS. 7A and 7B show changes in the dot shape obtained by the subsequent development process when the drive pulse width of the laser is changed from 10% to 100% at each spot diameter. ). At this time, the spot diameter in the sub-scanning direction was set to 70 μm, which is 1.1 times the conventional pixel size in order to make the exposure distribution in the sub-scanning direction uniform.

In general, a well-known developing system using a powder having a particle size of about 10 to 12 μm, as shown in FIG. It has the characteristic of having.

Therefore, in the method of low contrast of the exposure distribution when the laser is turned on / off in one pixel as shown in FIG. 6 (A), the surface potential of the photosensitive drum also follows the exposure distribution of FIG. 6 (A). Since the surface potential changes as a whole with respect to the driving pulse width of the laser, the surface potential of the photosensitive drum exceeds a certain threshold value according to the VD characteristic of the developing system in FIG. From the beginning, the image is rapidly developed. As a result, as shown in FIG. 7A, the developed dot diameter tends to rapidly develop a large dot shape from a certain number of gradations. The graph (A) in FIG. 9 shows the change in the drive pulse width in one pixel at this time and the image density after development obtained at that time. Development system V-D
It can be seen that the characteristics are strongly affected.

On the other hand, according to the present invention, the laser spot diameter is set to 0.7 times the pixel size and the laser is turned on / off within one pixel.
In an example where the contrast of the exposure distribution at the time is increased to at least 80% or more, the latent image formed on the photosensitive drum also has an ON / OFF pattern having a high potential contrast according to the exposure distribution. Therefore, even if developed with a developing system having a certain threshold characteristic, the peak of the exposure distribution immediately rises from the short region of the drive pulse, and since it exceeds the development threshold value, it is stable as dots. It is developed (FIG. 7 (B)). As a result, a change in dot diameter can be stably reproduced from a region where the ON / OFF ratio of the drive pulse is small, and stable area gradation can be reproduced in development.

The graph (B) in FIG. 9 shows the relationship between the laser drive pulse and the image density when using the spot size according to the present invention. As is clear from the figure, the graph is not significantly affected by the developing system. It can be seen that stable area gradation is possible with respect to the pulse width.

However, even when the density reproducibility by development is obtained, the roughness in the highlight portion due to scattering after transfer and fixing may not be eliminated by the developer.

As a result of numerous experiments, it has been found that the above problems can be solved by adjusting the particle size distribution of the toner and / or the volume average particle size of the toner contained in the developer.

Specifically, according to the image forming apparatus of the present invention, when the volume average particle diameter of the toner is M and the particle diameter of the toner particle is r, (1/2) M <r <(3/2 ) M range (ie, M ±
A toner containing 90% by volume or more of toner particles (in the range of (1/2) M) and 99% by volume or more in the range of 0 <r <2M (that is, the range of M ± M) is used. It

Further, according to the present invention, the volume average particle size is less than 12 μm,
Preferably, a toner having a size of 9 μm or less, more preferably 8 μm or less is used.

In the present invention, the volume distribution and the volume average particle diameter of the toner are measured, for example, by the following measurement methods.

As a measuring device, a Coulter counter TA-II type (manufactured by Coulter) is used, and 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) are connected, and an electrolytic solution is connected. To prepare a 1% NaCl aqueous solution using primary sodium chloride.

As a measurement method, 0.1 to 5 ml of a surfactant (preferably an 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 the sample is suspended is treated with an ultrasonic disperser for about 1 to 3 minutes. Using the Coulter Counter TA-II, a 2 to 4
The volume distribution is determined by measuring the particle size distribution of the 0 μm particles.

From the obtained volume distributions, the volume average particle size of the sample is obtained.

If the toner has a volume distribution exceeding the above range, the effect cannot be sufficiently exhibited even if the particle size is changed.

Further, when the number of particles in the range of large particle size increases, toner particles of large particle size, which cause scattering in transfer, exist even if the average particle size is made small. It is difficult to reduce. on the other hand,
When the number of toner particles having a small particle diameter increases, the amount of toner that adheres to magnetic particles and does not separate increases, and the magnetic particles cannot efficiently give triboelectric charge to the toner, resulting in increased scattering and fogging from the developing device. Further, the toner having a small particle diameter is likely to cause fusion, and is fused around magnetic particles (carrier).
Fog and scattering due to carrier deterioration also increase.

From the above points, it is necessary to use a volume distribution having a sharp particle size as shown in FIG.

In the image forming apparatus shown in FIG. 1, by using an elliptical spot having a laser beam spot diameter of 70 μm in the sub-scanning direction and a beam diameter of 42 μm in the scanning direction, writing similar to that in the following embodiment is performed on the photosensitive drum. FIG. 11 is a graph showing the relationship between the volume average particle diameter of the toner and the diameter of the dot for the minimum reproduction of the image after fixing when development, transfer, and fixing with a hot-press roller are performed.

Here, as the developing conditions, AC bias and DC bias are superposed for each particle size of toner, only DC bias is applied, and types of magnetic particles (carrier), sleeve / drum, sleeve / blade are changed. However, it hardly affected the diameter of the minimum reproduction dot. This is explained as follows.

That is, in the method of writing the latent image by controlling the emission time of the laser beam, the density gradation of development can be obtained by reducing the spot diameter. However, after a process of transferring and fixing a plurality of times to obtain a full-color image, toner having a large particle diameter scatters, resulting in a broad dot diameter, but toner having a small particle diameter does not scatter. No image distortion. That is, the toner having a small particle size is a thin layer on the paper after the transfer, and the adsorbing force with the paper is also large. Therefore, even if the toner image is exposed to the transfer electric field a plurality of times, it is considered that scattering does not easily occur.

The reproducibility of a full-color image in a portion where the image density is low particularly changes the impression of the image significantly. 50 μm when trying to obtain a high-quality image with gradation in a full-color image
The impression of the image is significantly different depending on whether the dots around m are faithfully reproduced.

Therefore, when the recording density is 400 dpi, the laser beam spot diameter in the scanning direction is 42 μm or less, preferably the volume average particle diameter is 9 μm or less, and more preferably 8 μm or less. In this way, the dots around 50μm are faithfully reproduced, the scattering at the transfer is also extremely reduced, and the gradation of the full-color, low-density parts, which cannot be obtained with the conventional device, is sufficient. Fewer high-definition images can be obtained.

Due to the above-mentioned effects, especially when the toner having the volume average particle diameter of 8 μm or less is used, the dots of 50 μm or less are faithfully reproduced, and the image is not disturbed even when exposed to the transfer electric field a plurality of times. In particular, this tendency has a favorable effect on roughness and reproducibility in a portion where the image density is low.

For example, the toner used in the present invention has a volume average particle diameter of 6
In the volume distribution of the toner, the toner contains 90% by volume or more of toner particles in a range of more than 3 μm and less than 9 μm, and 99% by volume or more of particles in a range of more than 0 and less than 12 μm. It is important to contain

In order to generate a toner having a sharp particle size distribution according to the present invention, for example, a predetermined toner material is melt-kneaded,
A method of preparing a toner having a predetermined particle size distribution and / or volume average particle size by cooling and pulverizing the kneaded product and precisely classifying the pulverized powder can be used.

As a method for precisely classifying the pulverized powder, it is classified by a classifying means such as a fixed wall type air classifier, and further, the obtained classified powder is a multi-division classifying device utilizing the Coanda effect, for example,
A method of preparing a toner having a predetermined particle size distribution and / or volume average particle size by precisely removing fine powder and coarse powder simultaneously by a multi-segment classification means such as an elbow jet classifier manufactured by Nippon Mining Co., Ltd. .

In the present invention, the toner is a colored resin particle itself (containing a binder resin, a colorant, and other additives as necessary), and a colored resin to which an external additive such as hydrophobic colloidal silica fine powder is externally added. Contains particles.

Examples of the binder resin used for the toner include a styrene-based polymer or a polyester resin such as a styrene-acrylate resin or a styrene-methacrylate resin. In particular, when considering the color mixing at the time of fixing the color toner, (Wherein R is an ethylene or propylene group, x and y are positive integers of 1 or more, and the average value of x + y is 2 to 10.) or a bisphenol derivative represented by the formula or a substituted product thereof. And a carboxylic acid component such as a divalent or higher carboxylic acid or an acid anhydride thereof, or a lower alkyl ester thereof (eg, fumaric acid, maleic acid, maleic anhydride, phthalic acid, terephthalic acid, etc.). Copolycondensed polyester resins are more preferred because they have sharp melting properties.

Colorants suitable for the purpose of the present invention include the following pigments or dyes. CI with poor lightfastness in the present invention
Colorants such as Disperse Y164, CISolvent Y77 and CISolvent Y93 are not recommended.

As the dye, for example, CI Direct Red 1, CI
Direct Red 4, CI Acid Red 1, CI Basic Red 1, CI Modern Red 30, CI Direct Blue 1, CI Direct Blue 2, CI Acid Blue 9, CI Acid Blue 15, CI Basic Blue 3, CI Basic Blue 5, There is CI Modant Blue 7.

As pigments, Naphthol Yellow S, Hansa Yellow G, Permanent Yellow NCG, Permanent Orange GTR, Parazolone Orange, Benzidine Orange G,
Vermant Red 4R, Watching Red Calcium Salt, Brilliant Carmine 3B, Fast Violet B, Methyl Violet Lake, Phthalocyanine Blue, Fast Sky Blue, Indanthrene Blue BC
There is.

In particular, pigments such as disazo yellow, insoluble azo,
Copper phthalocyanine is preferred, and a basic dye or an oil-soluble dye is preferred as the dye.

Particularly preferably, CI Pigment Yellow 17, CI Pigment Yellow 15, CI Pigment Yellow 13, CI Pigment Yellow 14, CI Pigment Yellow 12, CI
Pigment Red 5, CI Pigment Red 3, CI Pigment Red 2, CI Pigment Red 6, CI Pigment Red 7, CI Pigment Blue 15, CI Pigment Blue 16, or a Ba salt having a phthalocyanine skeleton substituted with two or three carboxybenzamide methyl groups. Is a copper phthalocyanine pigment.

Dyes include CI Solvent Red 49, CI Solvent Red 52, CI Solvent Red 109, CI Basic Red 12, CI Basic Red 1, and CI Basic Red 3b.

As for the colorant content, for the yellow toner that is sensitive to the transparency of the OHP film, the binder resin
12 parts by weight or less with respect to 100 parts by weight, preferably 0.5 part
~ 7 parts by weight is desirable. When the amount is less than 12 parts by weight, reproducibility of green, red and skin color, which are mixed colors of yellow, is poor.

For magenta toner and cyan toner, binder resin
15 parts by weight or less for 100 parts by weight, more preferably 0.1 part by weight
It is preferably up to 9 parts by weight.

In the case of a black toner using two or more colorants in combination, the addition of a total colorant amount of 20 parts by weight or more tends to cause spent to the carrier, and furthermore, a large amount of the colorant is exposed on the toner surface. The fusion of the toner to the photosensitive drum increases, and the fixing property becomes unstable. Therefore, in the black toner, the amount of the colorant is 3 to 1 with respect to 100 parts by weight of the binder resin.
5 parts by weight are preferred.

Preferred combinations of colorants for forming a black toner include a combination of a disazo yellow pigment, a monoazo red pigment and a copper phthalocyanine blue pigment. The compounding ratio of each pigment is preferably such that the ratio of yellow pigment, red pigment and blue pigment is 1: 1.5 to 2.5: 0.5 to 1.5.

When the toner used in the present invention is negatively chargeable, it is also preferable to add a charge control agent to stabilize the negatively chargeable characteristics. At that time, a colorless or light-colored negative charge controlling agent which does not affect the color tone of the toner is preferable. As the negative charge control agent, for example, a metal complex of an alkyl-substituted salicylic acid,
For example, organic-attributable complexes such as chromium or zinc complexes of di-tert-butylsalicylic acid can be mentioned. When the negative charge control agent is blended in the toner, the binder resin 100
It is preferable to add 0.1 to 10 parts by weight, preferably 0.5 to 8 parts by weight based on parts by weight.

In the present invention, when the developer used is a two-component developer, the carrier is preferably magnetic particles. The magnetic particles have a particle size of 30 to 100 μm, preferably 40 to 80 μm, and an electric resistance value of 10 ⁷ Ωcm or more, preferably 10 ⁸ Ω or more, and more preferably 10 ⁹ to 10 ¹² Ωcm. Particles (maximum magnetization 60 emu / g) coated with a resin can be preferably used.

The measurement of the resistance value of magnetic particles, for example, ferrite particles or resin-coated ferrite particles, is performed by measuring the electrode area.
Using a sandwich type cell with a 4 cm ² gap between the electrodes and a 0.4 cm gap between electrodes, the applied voltage E (V / cm) between both electrodes was applied to one of the electrodes under a pressure of 1 kg, and the magnetic current was passed from the circuit. It is a value obtained by measuring the resistance value of particles.

 Hereinafter, the present invention will be further described with reference to examples.

FIG. 12 is an enlarged sectional view showing the vicinity of one developing device of the rotary developing device 1 used in the laser beam printer shown in FIG. 1, and the developing device is disposed at a developing position facing the photosensitive drum 3. ing.

The developing device is a developing sleeve that is close to the photosensitive drum.
It has 22. The developing sleeve 22 is made of a non-magnetic material such as aluminum or SUS316. The developing sleeve 22 has a horizontally long opening formed in the longitudinal direction of the container on the lower left wall of the developing container 36, with a substantially right half circumferential surface protruding into the container 36, and a left substantially half circumferential surface exposed outside the container to be rotatably bearing. It is provided horizontally and is driven to rotate in the direction of arrow b.

In the developing sleeve 22, a fixed permanent magnet (magnet) 23 as a fixed magnetic field generating means positioned in the illustrated position and orientation is disposed. The magnet 23 has four magnetic poles: an N-pole 23a, an S-pole 23b, an N-pole 23c, and an S-pole 23d. The magnet 23 may be an electromagnet instead of a permanent magnet.

On the upper edge side of the developer supply device opening where the developing sleeve 22 is disposed, the base is fixed to the side wall of the container, and the tip end side is projected to the inside of the container 36 from the upper edge position of the opening, and along the length of the upper edge of the opening,
A non-magnetic blade 24 as a developer regulating member is provided. The blade 24 is formed, for example, by bending SUS316 so that the cross section thereof has a shape of “C”. Further, a magnetic particle limiting member 26 is provided in which the upper surface is brought into contact with the lower surface side of the non-magnetic blade 24 and the front end surface is the developer guide surface 261. The portion constituted by the non-magnetic blade 24 and the magnetic particle limiting member 26 and the like is a regulating portion.

The developer contains magnetic particles 27 and non-magnetic toner 37. A seal member 40 is provided to seal toner accumulated in a lower portion of the developing container 36. The seal member 40 has elasticity, is bent in the rotation direction of the sleeve 22, and elastically presses the surface side of the sleeve 22. This sealing member 40
Has an end on the downstream side in the sleeve rotation direction in the contact area with the sleeve so as to allow the developer to enter the inside of the container.

In the developing container, floating toner generated in the developing process is applied to the photosensitive drum side by applying a voltage having the same polarity as that of the toner to prevent the toner from scattering. A toner replenishing roller 60 that operates according to the output obtained by the above-described operation is disposed. As the sensor, for example, a volume detection method of a developer, a piezoelectric element, an inductance change detection element, an antenna method using an alternating bias, and a method of detecting an optical density can be used. The non-magnetic toner 37 is supplied by stopping the rotation of the roller 60. The fresh developer supplied with the toner 37 is mixed and conveyed by the screw 61 while being conveyed.
Be stirred. Accordingly, a tribo is applied to the replenished toner during this conveyance. The threshold plate 63 is cut out at both ends in the longitudinal direction of the developing device. At this portion, the fresh developer conveyed by the screw 61 is delivered to the screw 62.

The S pole 23d is a transport pole. The collected developer after the development is collected in the container, and the developer in the container is further transported to the regulating section. In the vicinity of the S pole 23d, the fresh developer conveyed by the screw 62 provided near the sleeve and the recovered developer after development are exchanged.

The carrying screw 64 is for equalizing the amount of the developer in the axial direction of the developing sleeve. The developer carried on the sleeve as the sleeve rotates rotates the screw 64.
Part of the developer layer that was transported in the axial direction of the sleeve by the "convex" in the axial direction on the sleeve is the opposite direction to the transport direction of the developer on the sleeve through the M space in FIG. It is reversed and pushed back. Screw 64 is screw 62
The developer is conveyed in the opposite direction.

The configuration of such a developing device is also effective when magnetic particles and non-magnetic or weakly magnetic toner are mixed in the developer container.

Distance between the end of the non-magnetic blade 24 and the surface of the developing sleeve 22
d ₂ is 50 to 900 μm, preferably 150 to 800 μm. If this distance is smaller than 50 μm, the magnetic particles are clogged between them and the developer layer is likely to be uneven, and the developer necessary for good development cannot be applied, and only a developed image with thin density and unevenness can be obtained. It tends not to be obtained. Above distance
d ₂ is 40 to prevent blade clogging due to unnecessary particles such as toner aggregates and dust mixed in the developer.
0 μm or more is preferred. If the thickness is larger than 900 μm, the amount of the developer applied on the developing sleeve 22 increases, so that a predetermined developer thickness cannot be regulated, the magnetic particles adhere to the photosensitive drum increases, the developer circulates, and the developer limiting member 26 , And the toner is less liable to be fogged.

Angle θ1 in the figure is −5 ° to 35 °, preferably 0 ° to 25 °
゜. When θ1 <−5 °, the magnetic force acting on the developer,
The developer thin layer formed by the mirror power and / or the cohesive force tends to be sparse and uneven. When the angle exceeds θ> 35 °, the amount of the developer applied to the non-magnetic blade increases, and the predetermined amount of the developer is increased. It becomes difficult to obtain.

Even when the sleeve 22 is driven to rotate in the direction of arrow b, the movement of the magnetic particle layer becomes slower as it moves away from the sleeve surface due to the balance between the magnetic force, the restraining force based on gravity, and the conveying force in the moving direction of the sleeve 22. . Of course, some fall under the influence of gravity.

Accordingly, by appropriately selecting the arrangement positions of the magnetic poles 23a and 23d and the fluidity and magnetic characteristics of the magnetic particles 27, the magnetic particle layer is conveyed toward the magnetic pole 23a closer to the sleeve to form a moving layer. Due to the movement of the magnetic particles, the magnetic particles and the toner are transported to the developing area with the rotation of the sleeve 22 and are subjected to the development.

FIG. 13 shows another embodiment of the image forming apparatus capable of carrying out the present invention.

In this embodiment, the image forming apparatus is a full-color laser beam printer, but unlike the previous embodiment, a dedicated image carrier for each color, that is, the photosensitive drum 3Y in this embodiment.
(Yellow), 3M (magenta), 3C (cyan), 3BK
(Black), around which dedicated laser beam scanners 80Y, 80M, 80C, 80BK, developing unit 1
Y, 1M, 1C, 1BK, transfer discharger 10Y, 10M, 10C, 10BK,
Cleaning devices 12Y, 12M, 12C, and 12BK are arranged.

The transfer material passes through the paper feed guide 5a and is sequentially conveyed to the paper feed roller 6 and the paper feed guide 5b, receives corona discharge from the attraction charger 81, and is reliably attracted to the transport belt 9a.

Thereafter, the images formed on the respective photosensitive drums are transferred by the transfer dischargers 10Y, 10M, 10C, and 10BK, the charge is removed from the conveyor belt 9a by the charge remover 82, and the images are fixed by the fixing device 17 to obtain a full-color image. To be

Even when such a transfer method is used, a recording density of 40
0dpi and the laser beam spot shape in the scanning direction is 42μ
By using a toner having a volume average particle diameter of less than 12 μm, preferably 9 μm or less, more preferably 8 μm or less under the above-mentioned developing conditions by using the developing device shown in FIG. However, a high-definition full-color image with excellent gradation and less scattering was obtained.

In this embodiment, an AC bias using ferrite particles is used.
Although the positive DC bias development method is used, DC bias development using ordinary iron powder may be used.

The same effect was obtained with a toner having an average particle diameter of 8 μm or less even when the developing device having the configuration shown in FIG. 14 was used and the developing sleeve was rotated with the photosensitive drum and the counter as the developing device.

Particle size distribution and particle size satisfying the conditions of the present invention, for example,
Using a toner having a volume average particle diameter of 6 μm, the distance d ₂ between the end of the non-magnetic blade 24 and the surface of the developing sleeve 22 is set to 600 μm.
The distance between the surface of the developing sleeve 22 and the photosensitive drum 3 is 450 μm.
m.

In the above embodiment, the photosensitive drum 3 is a laminated type organic photoconductive (OPC) drum, and the charged latent image potential is -600V. The development was performed using a bias power source obtained by superimposing a DC voltage of -300 V on a rectangular wave alternating voltage having a frequency of 1700 Hz and a peak-to-peak value of 1500 V.

The latent image is written by color separation of the developed image, and the laser beam spot diameter is 42 μm in the main scanning direction 1 / e ² and the sub-scanning direction is 1 with the semiconductor laser as the light source in the order of magenta, cyan, yellow, and black on the photosensitive drum. Set / e ² to 70 μm,
The light emission time is controlled by the aforementioned PWM control, and 200 lines / inc
Write the image of 256 gradations with h, repeat development and transfer in order, finally fix and obtain a full-color image, faithfully reproduce the low image density part, high definition and high image quality without roughness Image was obtained.

On the other hand, the laser beam spot diameter is set in the main scanning direction.
When the image was formed under the same conditions with a toner having a volume average particle diameter of 12 μm, the reproducibility at a portion where the image density was low was poor, and the entire image was rough. I could only get a conspicuous image.

As described above, the original image is color-separated, a latent image is formed on the photosensitive drum for each color using a laser light beam as a light source, and each color toner image visualized by development using toner is transferred to a transfer member. And then transfer
In a multi-color electrophotographic apparatus for obtaining a multi-color image, the spot size in the scanning direction of the beam spot is equal to the spot size (1 / e ² ) <pixel size ×
0.7, and a toner having a particle size distribution of 8 μm or less with a sharp particle size distribution is used as a developer, and a method of pulse width modulation of a light beam with an image signal as a halftone forming method is used. (1) Sufficient gradation can be obtained in a portion having a low image density, and (2) the roughness of the entire image including a portion having a low image density is greatly reduced, and an effect of obtaining a high-quality full-color image can be obtained. is there.

 Hereinafter, specific examples of the present invention will be described.

Example 1 Polyester resin obtained by condensing a propoxylated bisphenol moiety and fumaric acid (weight average molecular weight 15,000, number average molecular weight 3300) 100 weight rhodamine pigment 3 weight parts Negative charge control agent (of dialkyl-substituted salicylic acid) 4 parts by weight The above materials are melt-kneaded, the melt-kneaded material is cooled, the cooled material is pulverized, classified by a fixed wall type air classifier, and further classified by a multi-division classifier utilizing Coanda effect. Thus, a negatively chargeable magenta toner having a volume average particle size of 6 μm was prepared. The resulting magenta toner had a sharp particle size distribution of 95% by volume in the range of more than 3 μm and less than 9 μm, and substantially 100% by volume in the range of more than 0 μm and less than 12 μm.

100 parts by weight of the magenta toner and 0.4 parts by weight of the negative triboelectrically-chargeable hydrophobic colloidal silica were mixed to prepare a magenta toner externally added to silica. Next, a weight average particle diameter of 50 μm coated with a styrene-acrylate copolymer
A two-component developer for forming a magenta toner image was prepared by mixing 94 parts by weight of ferrite magnetic particles (having an electric resistance value of 10 ¹⁰ Ω · cm) and 6 parts by weight of the magenta toner having silica added thereto.

Similarly, a two-component developer for forming a cyan toner image and a two-component developer for forming a yellow toner image
A component developer and a two-component developer for forming a black toner image were prepared.

Each two-component developer was placed in a 100 ml polyethylene container and stirred vigorously by hand about 30 times, and then the triboelectric charge of the toner was measured. The toner of each color was about -30 μc / g.
The value of was shown.

The two-component developer was introduced into the developing device of the color image forming apparatus shown in FIG. In this embodiment, in the developing device, the distance d ₂ between the end of the non-magnetic blade 24 and the surface of the developing sleeve 22 is set to 600 μm, and the distance between the developing sleeve 22 and the photosensitive drum 3 is changed.
The distance d ₁ between was 450μm.

The photosensitive drum 3 is a stacked organic photoconductive (OPC) drum, and has a charged latent image potential of -600V.

Frequency 1700Hz, peak-to-peak value as bias power supply
Development was carried out using a rectangular voltage alternating voltage of 1500 V with a DC voltage of -300 V superimposed thereon.

To write a latent image, the original image is color-separated, and the spot diameter of the laser beam is 42 μm in the main scanning direction 1 / e ² and 1 in the sub-scanning direction, using the semiconductor laser as the light source in the order of magenta, cyan, yellow, and black on the photosensitive drum. Set / e ² to 70 μm,
The light emission time is controlled by controlling the PWM
An image of 256 gradations was written at / inch, reversal development and electrostatic transfer were repeated in order, and finally, the image was fixed by a hot-press roller fixing device to obtain a full-color image. The portion with low image density (highlight portion) was faithfully reproduced, and a high-definition, high-quality image without roughness was obtained.

When the full-color image was observed, dots of about 50 μm were faithfully reproduced corresponding to the latent image.

Examples 2 to 4 In the same manner as in Example 1, the volume average particle size was 5 μm and 6.8 μm.
m and 8 μm of the toners shown in Table 2 were prepared.
When a full-color image was formed in the same manner as described above, good results were obtained.

Comparative Example Table 3 having a volume average particle size of 12 μm was prepared in the same manner as in Example 1.
Toners of respective colors shown in are obtained.

A two-component developer was prepared in the same manner as in Example 1, and a color image was formed in the same manner as in Example 1. As compared with Example 1, the reproducibility in a portion having a low image density was There was, however, only a noticeable rough image.

The two-component developer was placed in a polyethylene container having a volume of 100 ml, and the toner was vigorously stirred about 30 times, and the triboelectric charge amount of the toner was measured.
The value was 18 μc / g, which was lower than that of Example 1.

When the obtained full-color image was observed, the smallest dot faithfully reproducing the latent image was about 90 μm. The dots below that were very splattered.

Example 5 Toners of respective colors shown in Table 4 having a volume average particle diameter of 9 μm were prepared in the same manner as in Example 1, and a two-component type developer was prepared in the same manner as in Example 1.

When a color image was formed in the same manner as in Example 1,
Although the image was slightly inferior to that of Example 1, a portion with a low image density (highlight portion) was faithfully reproduced, and a high-definition and high-quality image with less shading was obtained.

When observing the full-color image, dots 60 μm before were faithfully reproduced corresponding to the latent image, and dots around 50 μm were also faithfully reproduced corresponding to the latent image.

EFFECTS OF THE INVENTION The image forming apparatus according to the present invention configured as described above can increase the contrast between the bright portion and the dark portion of the latent image corresponding to the low-density pixel, and provides good gradation in the image portion with low image density. It is excellent and can form a high-quality image without greasiness, and has less toner scattering in the transfer process and further in the fixing process, good gradation expression, high-definition and stable color reproducibility. The feature is that a beautiful color image can be obtained.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an embodiment of an image forming apparatus according to the present invention. FIG. 2 is a perspective view of a scanning optical device used in the apparatus of FIG. FIGS. 3 and 4 are timing charts showing the PWM circuit used in the device of FIG. 1 and its operation. FIG. 5 is an exposure distribution diagram on a photosensitive medium according to a conventional example. FIG. 6 is a comparison diagram of an exposure distribution on a photosensitive medium according to the present invention and a conventional example. FIG. 7 is a diagram showing a comparison between a gradation expression based on a dot area change according to the present invention and a conventional example. FIG. 8 is a graph showing a typical example of the VD characteristic of a known developing system. FIG. 9 is a graph showing the relationship between the pulse width modulation method according to the present invention and the image density. FIG. 10 is a graph showing a particle size distribution chart of the toner used in the present invention. FIG. 11 is a graph showing the relationship between the average particle diameter of the toner and the minimum reproduced dot diameter. FIG. 12 is a schematic sectional view of one embodiment of the developing device. FIG. 13 is a configuration diagram of another embodiment of the image forming apparatus according to the present invention. FIG. 14 is a schematic sectional view of another embodiment of the developing device. FIGS. 15 (a) and (b) are the ON / OFF state of the laser light source and the output distribution of each pixel of the CCD. FIG. 16 is a distribution diagram of a laser spot diameter. 3: Photosensitive medium (photosensitive drum) 4: Charger 1M, 1C, 1Y, 1BK: Developer 101: Laser drive circuit 102: Laser light source 104: Polygon mirror

Front page continuation (72) Inventor Masahiro Ito 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (72) Inventor Kenichi Takeda 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (72) Inventor Kazuhisa Koshimo 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (56) References JP-A-63-284975 (JP, A) JP-A-62-293888 (JP, A) JP-A-63-85581 (JP, A) JP-A-62-206965 (JP, A) JP-A-63-58584 (JP, A)

Claims (1)

(57) [Claims] 1. A photosensitive member, a laser light source that emits laser light, a pulse width modulation circuit that controls the light emission time per pixel of the laser light source according to an image signal, and a laser light that is emitted from the laser light source. An image that has a scanning optical system that deflects and forms an image on a photoconductor, and a developing unit that develops a latent image formed on the photoconductor with toner, and that can express gradation by controlling the light emission time per pixel In the forming apparatus, in the scanning direction of the scanning optical system, the maximum spot diameter per pixel of the laser light emitted from the laser light source is smaller than 0.7 times the pixel size, and the volume average particle diameter of the toner is M, Assuming that the particle diameter of the toner is r, 90% by volume or more of the toner in the developing means is (1/2) M <r <(3 /
2) An image forming apparatus characterized in that it is included in the range of M, and 99% by volume or more is included in the range of 0 <r <2M.