JPH07295320A - Image forming device - Google Patents

Abstract

PURPOSE:To provide an image forming device constituted so that a high-quality image can be stably formed over a long term. CONSTITUTION:By setting the spot diameter of a laser beam in a main scanning direction to be 42mum when recording density is set to be 400 (dpi) and using toner whose volume average grain size is <=8m and whose Macbeth concentration becomes >=1.2 after fixing with respect to the coating quantity of 0.5mg/cm<2>, electrostatic charge is executed by a post-electrostatic charger 101 so that the average electrostatic charge quantity before the toner is transferred becomes >=30mC/Kg. Thus, the high definition image with excellent gradation even at the thin part of image density and with small splashing of the toner is formed extending over the long term.

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,
In particular, the present invention relates to an image forming apparatus that scans an electrophotographic photosensitive member with a light flux controlled in accordance with a recorded image signal to form a latent image and develops the latent image.

[0002]

2. Description of the Related Art Among many types of image forming apparatuses, a laser beam printer that employs an electrophotographic system is well known as a high speed and low noise printer. In this printer, so-called binary recording is performed in which a laser beam is emitted and disappears in response to an image signal to scan the photoconductor. It is well known that the dither method, the density pattern method, and the like are used to express halftones in the binary recording type laser beam printer. However, as is well known, a printer adopting the dither method or the density pattern method cannot obtain high resolution. Therefore, in recent years, a method of forming a halftone image while obtaining a high resolution without lowering the recording density has been proposed.

According to this method, halftone pixels are formed by modulating the width of a pulse signal for driving a laser according to an image signal, that is, the luminous flux emission time length per pixel of the laser is controlled according to the density of the image, Therefore, the photoconductor irradiation time length per pixel of the laser beam scanning the photoconductor is controlled according to the image density. That is, for the low density image portion, the pulse width is shortened to shorten the irradiation time, and for the high density image portion, the pulse width is increased to lengthen the irradiation time. According to such a pulse width modulation method (PWM method), it is possible to form an image with high resolution and high gradation,
Therefore, this system is indispensable especially for a color image forming apparatus which requires high resolution and high gradation.

According to this PWM method, it is possible to perform the area gradation of dots formed by the beam points for each pixel, and it is possible to express the halftone at the same time without lowering the pixel density to be recorded (recording density).

By the way, in order to stably reproduce the gradation by changing the area of the dot of each pixel by using this PWM method, the dot of the beam spot diameter on the photoconductor in the main scanning direction in the main scanning direction of the recording pixel It is desirable that the ratio to the size (spot diameter ratio) be 0.7 or less.

The intensity distribution of the laser beam spot is a Gaussian distribution. Therefore, the beam spot diameter in this specification is 1 / maximum of the maximum intensity on the intensity distribution.
It means the diameter of the contour line in which points having the strength of e ² times are continuous, that is, 1 / e ² diameter.

When the recording density is set to 400 dpi (unit pixel size 63.5 μm), the laser beam spot diameter is preferably 42 μm or less. As described above, in the example in which the laser spot diameter in the main scanning direction is 0.7 times the unit pixel size and the contrast of the exposure distribution within one pixel is at least 80% or more, the latent image formed on the photosensitive drum is also A pattern with high potential contrast is formed according to this exposure distribution. Therefore, even if the image is developed by a developing system having a certain threshold characteristic, the peak of the exposure distribution becomes high immediately from the region where the driving pulse is short, and since it exceeds the developing threshold value, the dots are stably developed. . As a result, the drive pulse turns ON /
It is possible to stably reproduce as a change in dot diameter from a region with a small OFF ratio, and it is possible to reproduce stable area gradation.

The diameter of the laser beam spot on the photoconductor in the main scanning direction is set to 0.7 times or less the unit pixel size in the main scanning direction. It is difficult to make the lower limit of this diameter smaller than 20 μm. However, in order to make the exposure distribution in the sub-scanning direction uniform, the laser spot diameter in the sub-scanning direction should be 1.1 times or more and 1.6 times or less the unit pixel size in the sub-scanning direction as in the conventional case. preferable. Therefore, it is preferable that the shape of the laser beam spot on the photoconductor has an elliptical shape with the short axis substantially in the main scanning direction and the long axis substantially in the sub scanning direction.

Further, it is preferable that the unit pixel has the same size in the main scanning direction and the size in the sub scanning direction in order to equalize the recording density in the main scanning direction and the recording density in the sub scanning direction.

In order to set the size of the laser beam spot on the photosensitive member as described above, a semiconductor laser having an appropriate divergence angle for emitted light and a lens having an appropriate power may be used. Are matters that can be understood by those skilled in the art.

However, even if good gradation reproducibility is obtained by development, there is a problem in that the roughness in the highlight portion due to toner scattering during transfer and fixing does not disappear.

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 contained in the developer and / or the weight average particle size of the toner.

Specifically, when the weight average particle diameter of the toner is M and the particle diameter of the toner is γ, (1/2) M <γ <
A toner containing 90% by weight or more of toner particles in the range of (3/2) M and 99% by weight or more of toner particles in the range of 0 <γ <2M is used.

Further, a toner having a weight average particle diameter of 12 μm or less, preferably 9 μm or less, more preferably 8 μm or less and 4 μm or more is used.

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

Further, when the number of particles having a large particle size in the weight distribution increases, no matter how small the average particle size is, there are many particles having a large particle size that cause scattering in transfer, so the image density is low. It is difficult to reduce the roughness on the part. On the other hand, when the number of toner particles having a small particle size in the weight distribution increases, the amount of toner that adheres to carrier particles and does not separate is relatively increased, and the carrier particles cannot efficiently give an electric charge to the toner. To increase. Further, the toner having a small particle diameter is apt to be fused and fused around the carrier, and fogging and scattering due to carrier deterioration are also increased.

From the above points, it is preferable to use the one having a sharp particle size as shown in FIG. 6 as the volume distribution.

[0018]

However, in the above-mentioned conventional example, although the charge distribution is initially uniform, the number of developers to which sufficient charge is not applied increases as the number of durable sheets increases. In such a state, a uniform image is formed on the drum immediately after being developed, but since the developed image contains toner with low tribo, the image deterioration during transfer becomes remarkable. Therefore, in the image forming method using the PWM, a part to be transferred and a part to be hardly transferred are generated particularly in a part where the image density is low, resulting in an image with a lot of roughness. The non-uniformity of the part where the image density is low makes the image particularly bad. Such an image defect occurs when an area gradation is reproduced up to an extremely highlighted portion by using PWM. In the case of an analog image, it is difficult to obtain high image quality, and nonuniformity of transfer is evenly dispersed, so that the image quality does not significantly decrease unlike a PWM image.

Further, for the purpose of preventing such image deterioration, a method has been proposed in which an electric charge is applied to an image on a drum after development to reduce deterioration at the time of transfer, but the electric charge of the toner becomes too large and the toner charge becomes high. There was a problem of transfer failure in the density area.

As described above, according to the conventional method, it is very difficult to stably form a high quality image for a long period of time.

Therefore, an object of the present invention is to provide an image forming apparatus capable of stably forming a high quality image for a long period of time.

[0022]

The above object can be achieved by an image forming apparatus according to the present invention. In summary, the present invention is
An electrophotographic photosensitive member on which an electrostatic latent image is formed, a light source that emits a light beam, a control unit that controls a light beam emission time length per pixel of the light source according to a recorded image signal, and a light source from the light source. A developing means for scanning the light beam in the main scanning direction on the electrophotographic photosensitive member, and developing an electrostatic latent image formed by the scanning of the light beam with a two-component developer containing toner and carrier particles, and a transfer section. A charging means for charging the toner before being electrostatically transferred to the transfer material, the toner has a weight average diameter of 8 μm or less, and the weight per unit area on the transfer material is 0.5 mg. The image forming apparatus is characterized in that the image density after fixing develops to 1.2 or more and the average amount of charge of the toner by the charging unit is 30 mC / Kg or more when / cm ² .

Preferably, the spot diameter of the light beam on the electrophotographic photosensitive member is 0.7 times or less the unit pixel size in the main scanning direction.

[0024]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An image forming apparatus according to the present invention will be described below in more detail with reference to the drawings.

Embodiment 1 FIG. 2 is a schematic diagram showing an electrophotographic color printer which is an embodiment of the image forming apparatus according to the present invention. This printer is provided with an electrophotographic photosensitive drum 1 as an image bearing member that rotates in the direction of the arrow, and around the photosensitive drum 1,
Rotational developing device 4 provided with charger 2, developing devices 4c, 4m, 4y and 4Bk, transfer discharging device 5b, cleaning means 6
Also, an image forming unit including a laser beam scanner LS and the like arranged above the photosensitive drum 1 in the drawing is arranged.

Each developing unit supplies a two-component developer containing toner particles and carrier particles to the photosensitive drum 1. The developer of the developing unit 4c contains cyan toner, the developer of the developing unit 4m contains magenta toner, the developer of the developing unit 4y contains yellow toner, and the developer of the developing unit 4Bk contains black toner.

The document to be copied is read by a document reading device (not shown). This reading device has a photoelectric conversion element such as a CCD for converting a document image into an electric signal, and outputs image signals corresponding to cyan image information, magenta image information, yellow image information, and monochrome image information of the document, respectively. . The semiconductor laser incorporated in the printer is controlled according to these image signals and emits a laser beam E. The output signal from the electronic computer can be printed out.

The sequence of the entire color print will be briefly described by taking the case of the full color mode as an example. First, the photosensitive drum 1 is uniformly charged by the charger 2. Next, scanning exposure is performed by the laser light E modulated by the cyan image signal, and an electrostatic latent image is formed on the photosensitive drum 1. This latent image is inverted by the cyan developing device 4C that is fixed at the developing position in advance. It is developed.

On the other hand, the transfer material such as paper taken out from the cassette 7 and advanced through the paper feed guide and the paper feed roller is electrostatically wound around the transfer drum 5 by the action of the attraction roller 5g and the attraction charger 5c. To be The transfer drum 5 is rotating in the direction of the arrow in the figure in synchronization with the photosensitive drum 1, and the cyan image developed by the cyan developing device 4c is uniformly charged by the post charger 101, and then transferred at the transfer portion. It is transferred to the transfer material by the charger 5b. The transfer drum 5 continues to rotate as it is to prepare for the transfer of the image of the next color (magenta in FIG. 2).

On the other hand, the photosensitive drum 1 is cleaned by the cleaning means 6, is charged again by the charger 2, and is exposed as described above by the laser beam E modulated by the next magenta image signal to obtain the electrostatic latent image. Is formed. During this time, the developing device 4 rotates and the magenta developing device 4m is placed in a predetermined developing device to perform reversal development of the latent image corresponding to magenta to form a magenta visible image.

Subsequently, the above steps are carried out for the yellow image signal and the black image signal respectively, and when the transfer of the four color visible images (toner images) is completed, the transfer material is discharged by the charger 5h. Is separated from the transfer drum 5 by a separating claw not shown in the figure, and is sent to a fixing device (hot pressure roller fixing device) 9. The fixing device 9 fixes the four-color visible images that are overlaid on the transfer material. In this way, a series of full-color print sequences is completed, and a required full-color print image is formed.

The exposure means is, as shown in FIG. 3, a semiconductor laser section 102 and a polygon mirror 10 rotating at a high speed.
5, f-θ lens group 106, and the semiconductor laser 102 oscillates a laser beam E modulated corresponding to time-series digital pixel signals calculated and output by an image reading device, an electronic computer, or the like. , The surface of the photosensitive drum 1 is exposed. Since the developing devices perform the reversal development in which the toner charged to the same polarity as the charging polarity of the charging device 2 is attached to the light potential portion of the latent image, the laser beam E is applied to the drum 1.
Exposing the areas to which the toner of the above is to be attached.

More specifically, referring to FIG. 3, the semiconductor laser element 102, which is a light source section, is connected to a laser driver 500, which is a light emission signal generator for sending a light emission signal (driving signal) for generating a laser beam, It blinks according to the light emission signal of the laser driver. The laser light flux E emitted from the laser element 102 is made into substantially parallel light by the collimator lens system 103. The collimator lens system 103 is a rack to which the lens system 103 is fixed,
A focus adjusting means 104 including a pinion meshing with the rack and a motor for driving the pinion can move in the direction of arrow A, which is the optical axis direction of the laser light.

Polygon mirror, that is, rotary polygon mirror 105
Rotates in the arrow B direction at a constant speed to scan the parallel light emitted from the collimator lens system 103 in the arrow direction. The f-θ lens group 106 (106a, 106b, 106c) provided in front of the rotary polygon mirror 105 forms a laser beam deflected by the rotary polygon mirror 105 into a spot on the surface to be scanned, that is, the photosensitive drum 1. At the same time, the scanning speed is made uniform on the surface to be scanned.

The direction in which the beam E moves on the drum 1 by the polygon mirror 105, that is, the direction of arrow C is called the main scanning direction. The main scanning direction is a direction intersecting with the moving direction of the drum 1 in the exposure section, preferably a direction substantially at right angles.

On the other hand, the moving direction of the drum 1 in the exposure section is called the sub-scanning direction. Photosensitive drum 1 by main scanning and sub scanning
The surface is raster scanned by the laser beam.

Before the modulation is started corresponding to the image signal to be recorded, the laser beam E is guided through a reflecting mirror 107 to a CCD (solid-state image pickup device) 108 as a detecting means.
The CCD 108 is configured by arranging a large number of photoelectric conversion elements in the direction of arrow C at positions that are substantially equivalent to the surface of the photosensitive drum 1. The CCD 108 is the laser driver 5
00 and the focus adjusting means 104 are connected to a control unit 100.

Further, the signal processor 111 is connected to the laser driver 500 and the controller 100.

In the above structure, when a desired image is formed, first, the image output signal P is input from the signal processing unit 111 to the control unit 100, and the laser driver 50 is used.
The image signal S is input to 0, and the laser element 102 blinks in response to the image signal S. Such a laser beam E
The exposure distribution for the main scanning is formed on the surface of the photosensitive drum 1 by the scanning of 1. The photosensitive drum 1 is rotated by a predetermined amount for each main scanning, and the latent image having the exposure distribution according to the image signal S is formed on the drum 1. Form an image.

The image output signal P is output from the signal processing unit 111 prior to the image signal S, and the output is finished after the output of the image signal S is finished. Further, the control unit 100 stops the operation of the focus adjusting unit 104 while the image output signal P is input from the signal processing unit 111.

Next, the PWM circuit will be described with reference to FIG. The PWM circuit includes a TTL latch circuit 401 for latching an 8-bit image signal, and a TTL logic level for high-speed EC.
Level converter 402 for converting to L logic level, ECLD
/ A converter 403, ECL comparator 404 for generating a PWM signal, level converter 405 for converting the ECL logic level into TTL logic, and clock oscillator 4 for generating a clock signal 2f having a frequency twice that of the image clock signal f.
06, a triangular wave generator 407 that generates a substantially ideal triangular wave signal in synchronization with the clock signal 2f, and the clock signal 2f
It has a 1/2 frequency divider 408 that divides the frequency by 1/2. Moreover, in order to operate the circuit at high speed, ECL logic circuits are arranged everywhere.

The operation of the PWM circuit having such a configuration will be described with reference to FIG. 5 showing signal waveforms. The signal (a) shows the clock signal 2f, and the signal (b) shows the pixel clock signal f having a double period thereof, which is associated with the pixel number as shown in the drawing. Even within the triangular wave generator 407, in order to keep the duty ratio of the triangular wave signal at 50%, the clock signal 2f
Is divided by 1/2 and then the triangular wave signal (c) is generated. Furthermore, this triangular wave signal (c) is at ECL level (
0 to -1 V) to be a triangular wave signal (d).

On the other hand, the pixel signals are OOH (white) to FFH.
It changes in 256 gradation levels up to (black). The symbol H is hexadecimal. And the image signal (e) is the D / A of those
The converted ECL voltage is shown. In FIG. 5, for example, the first
The pixel is the highest density black pixel level FFH, the second pixel is a halftone level 80H, the third pixel is a lower halftone level 40H than the second pixel, and the fourth pixel is a lower halftone density than the third pixel. Each voltage of the adjustment level 20H is shown. The comparator 404 has a pulse width corresponding to the pixel density to be formed by comparing the triangular wave signal (d) and the image signal (e) (in FIG. 5, as an example, the widths of T, t ₂ , t ₃ and t ₄ ). Generate a PWM signal. Where T, t
₂ , t ₃ and t ₄ are set to T> t ₂ > t ₃ > t ₄ .

The PWM signal is converted into a TTL level of 0V or 5V to become a PWM signal (f) (laser drive pulse signal having a width including 0), which is input to the laser driver circuit 500. Thus, the semiconductor laser 102 emits light for each unit pixel for a time corresponding to each pulse width of the signal f to scan and expose the photosensitive drum 1. Since the printer performs reversal development, the higher the density of the pixel, the longer the laser emission time.

In the circuit of FIG. 4, the latch circuit 401
A lookup table (not shown) is provided in the front stage of the. This look-up table is for performing γ correction (gradation correction) of image data, and is a memory that stores the data of the result of γ correction, and accesses the memory by using an image signal of 8 bits per pixel as address data. , And output the image signal of the desired γ-corrected data. Normally, one specific γ correction table is used in one pixel, but a plurality of types of γ correction tables can be switched and used in one pixel as needed. That is, for example, three types of tables are sequentially and repeatedly used for each line scanning by the beam, and the gradation correction can be performed by changing the γ correction in the sub-scanning direction for each line.

Further, the look-up table has a so-called standing γ curve when the toner density is low so as not to be affected by the density peculiar to the toner of each color, for example, four colors of yellow, magenta, cyan and black. A table is set, and when the density is high, a γ table having the opposite characteristic is set and is provided for each forming color. In the preceding stage of such a lookup table, the turbidity of the color of each color toner is corrected. For this purpose, a non-linear color masking circuit, for example a secondary color masking circuit, can be provided.

In the color printer of FIG. 2, the PWM circuit shown in FIG. 4 sequentially inputs the image signals of cyan, magenta, yellow, and black for each page (one page for each original and copy), and for each color. Laser modulation is sequentially performed for each, and one color copy is obtained by four rotations of the drum 1. In the device described later, the PWM circuit is provided for each color.

Furthermore, in this embodiment, a non-linear color masking circuit, for example, a secondary color masking circuit is provided in the preceding stage of the look-up table provided for each color in order to correct the turbidity of the color of each color toner. As will be described in detail later, it is possible to form a higher-definition, higher-quality color image with stable gradation and color reproducibility in combination with a small-diameter toner and a stable small-diameter laser spot. .

Next, in the image forming apparatus shown in FIG.
Set the spot diameter of the laser beam to 7 in the sub-scanning direction.
The volume average particle diameter of the toner when the latent image is formed on the photosensitive drum using a spot of 0 μm and an elliptic diameter of 42 μm in the main scanning direction, and then development, transfer, and heat roller fixing are performed, and the fixing FIG. 7 shows a graph showing the relationship with the diameter of the minimum reproduced dot of the subsequent image.

Here, as a developing condition, an AC bias and a DC bias are superposed for each particle diameter of the toner, or a DC bias is applied.
Although only the bias was used, the type of magnetic particles (carrier), the gap between the sleeve and drum, and the gap between the sleeve and blade were changed, but the diameter of the smallest reproduced dot was hardly affected. This is explained as follows.

That is, in the method of writing for development by controlling the emission time of the laser beam, the density gradation of development can be obtained by decreasing 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. Toner with a small particle size is a thin layer on the paper after transfer, and the attraction force with the paper is also large. Therefore, it is considered that scattering does not easily occur even if the toner image is exposed to the transfer electric field a plurality of times.

The reproducibility of a full-color image in a portion having a low image density remarkably changes the impression of the image. When trying to obtain a full-color image with high gradation and high quality, the impression of the image remarkably differs depending on whether or not dots of about 50 μm are faithfully reproduced.

Therefore, when the recording density is 400 dpi, the laser beam spot diameter in the main scanning direction is 42 μm.
By using a toner having a volume average particle size of 9 μm or less, and more preferably 8 μm or less, dots of about 50 μm are faithfully reproduced as shown in the above description and data, and scattering in transfer is also achieved. Extremely reduced, full-color images with low image density, which could not be obtained by conventional methods, have sufficient gradation, and high-definition images with less blurring and blurring can be obtained.

Due to the above-mentioned effects, particularly 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 less likely to be disturbed even when exposed to the transfer electric field a plurality of times. . In particular, this tendency has a good influence on the roughness and reproducibility in the portion where the image density is low.

For example, when the toner used in this embodiment has a volume average particle size of 6 μm, the toner has a volume distribution of more than 3 μm and less than 9 μm in which 90% or more toner particles are contained. It is preferable that the toner particles are contained in an amount of 99% by volume or more in the range of more than 0 and less than 12 μm.

The volume distribution and volume average particle diameter of the toner are measured, for example, by the following measuring methods. A Coulter Counter-TA-II type (manufactured by Coulter) is used as a measuring device, an interface (manufactured by Nikkaki) and a CX-i personal computer (manufactured by Canon) for outputting a number average distribution and a volume average distribution are connected, and an electric field is connected. A 1% Nacl aqueous solution is prepared from the solution using primary sodium chloride.

As a measuring method, the above-mentioned electric field aqueous solution 100 to 1 is used.
0.1 to 5 ml of a surfactant (preferably alkylbenzenesulfonic acid) as a dispersant is added to 50 ml,
Further, 0.5 to 50 mg of the measurement sample is added.

The electrolytic solution in which the sample was suspended was treated with an ultrasonic disperser for about 1 to 3 minutes, and the Coulter counter-TA was used.
According to the type II, a particle size distribution of particles of 2 to 40 μm is measured by using a 100 μm aperture as an aperture to obtain a volume distribution. The volume average particle diameter of the sample can be obtained from the obtained volume distribution.

In order to produce a toner having a sharp particle size distribution as described above, for example, a predetermined toner material is melt-kneaded, the kneaded product is cooled and then pulverized, and the pulverized powder is precisely classified to obtain a predetermined particle size. A method for preparing a toner having a distribution and / or a volume average particle diameter can be improved.

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

The toner includes colored resin particles (containing a binder resin, a colorant, and optionally other additives) themselves, and colored resin particles to which an external additive is externally added.

Examples of the binder resin used in the toner include a styrene polymer or polyester resin such as a styrene-acrylic acid ester resin or a styrene-methacrylic acid ester resin. In particular, considering the color mixture when fixing color toner,

[0063]

[Chemical 1] (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 (for example, fumaric acid, maleic acid, maleic anhydride, phthalic acid, terephthalic acid). The polycondensed polyester resin is more preferable because it has sharp melting characteristics.

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 Ωc.
m or more, more preferably 10 ⁹ Ωcm to 10 ¹² Ωcm, ferrite particles (maximum magnetization 60 emu / g)
Resin-coated resin can be preferably used.

To measure the resistance value of magnetic particles, for example, ferrite particles or resin-coated ferrite particles, a sandwich type cell having a measuring electrode area of 4 cm ² and an electrode gap of 0.4 cm was used. It is a value obtained by measuring the resistance value of the magnetic particles from the current flowing in the circuit by applying the applied voltage E (V / cm) between both electrodes under pressure.

When an image is output by the PWM method using the spot diameter and the toner diameter described above, high image quality can be obtained without using a post charger in the initial stage. However, if an image is output for a long period of time, the proportion of toner in the developer in which sufficient electric charge is not applied increases. Since the toner to which the sufficient electric charge is not applied is developed, it is possible to obtain the dot reproduction on the drum after the development, even after a long-term use, up to the extreme highlight portion as in the initial stage. However, at the time of transfer, since it is not transferred by the transfer electric field, dots to be transferred and dots to be partially or almost not transferred are generated, and as a result, the image is very rough. This is particularly noticeable because the image forming method that faithfully reproduces dots up to about 30 μm using PWM is used. In the conventional analog method, since the toner is dispersed in the entire highlight portion, the transfer deterioration is less noticeable than in the present embodiment. Therefore, in this embodiment, as described above, the post-charger 101 shown in FIG. 1 is used to increase the charge of the toner to an average value of 30 mc / Kg or more. As a result, it was found that the image was transferred stably at all times and the image did not become rough.

Here, a method of measuring the charge of the toner will be described. To measure the charge of the toner, the toner is sucked into the Faraday cage of the capacity C (F) by suction from the drum after development and after the post-charging, and the change in potential of the Faraday cage ΔE.
Read (V) using an electrometer. By measuring the weight ΔM (Kg) of the attracted toner, the relation of average charge amount = CΔE / ΔM (C / Kg) can be obtained.

The graph of FIG. 8 shows the relationship between the current of the post charger and the toner tribo on the drum. The current of the post charger is the value shown in the graph of FIG. 8 in this embodiment, but this is not important because it depends on the configuration of the charger.
What is important here is the average charge amount of the toner. In this embodiment, if the charge amount of the toner is set in the range of 30 mC / Kg to 50 mC / Kg indicated by the slanted lines, deterioration due to transfer in the highlight portion even after long-term use It turns out that the

The graph of FIG. 9 shows the relationship between the average charge amount of post-charging of the developer after copying 10,000 sheets and the roughness level of highlight.

As shown in the graph, when the average amount of charge after passing post-charging is 30 mC / Kg or less and the distribution of the tribo becomes broad after copying 10,000 sheets, the tribo is sufficiently charged even if post-charging is performed. Toner is not applied, and transfer defects that are particularly conspicuous in the highlight portion are partially present, which is apt to cause roughness, which significantly deteriorates the image quality. On the other hand, when the post-charging charge amount is large and the average charge amount is 50 mC / Kg or more, toner having an extremely high tribo is present, and the same transfer omission as when the tribo is low occurs.

Therefore, FIG. 9 is a graph in which the level of the roughness of an image having a density of 0.2 is subjectively evaluated in five levels, and the relationship with the average charge amount is plotted as an allowable level of 3 or more, as described above by Tribo. 30mC / Kg to 50mC / Kg
It was found that the deterioration due to the transfer in the highlight portion hardly occurs in the range of.

However, when the toner with high tribo is developed in this way, the amount of the toner becomes too large in the high density portion, which may cause transfer failure. Especially for the fourth full-color color, the conditions became severe.

As a result of numerous studies, it has been found that the transfer failure in the high density portion does not occur by reducing the toner amount for obtaining a constant density. When the tribo is set to 30 mC / Kg or more as in this embodiment, the pigment content of the toner is increased so that the Macbeth density is 1.2 or more with respect to the toner amount on the transfer paper of 0.5 mg / cm ^2. By doing so, high-quality images can always be obtained stably from the highlight to the high-density area. The relationship above is that the amount of toner before fixing is measured in advance, adjusted to 0.5 mg / cm ² per unit area, fixing is performed under these conditions, and the density on the fixed paper is measured with a Macbeth reflection densitometer. It was obtained by doing.

The graph of FIG. 10 shows the relationship between the Macbeth density and the level of solid-state voids with respect to the toner amount of 0.5 mg / cm ² . In this case as well, the level was subjectively determined by a five-level evaluation, and a level of 3 or more was set as an acceptable level. As can be seen from the graph, a good image can be obtained when the pigment content is increased so that the density of 1.2 or more can be obtained.

As described above, a toner having a volume average particle diameter of 8 μm or less and having a Macbeth density after fixing of 1.2 or more with respect to a coating amount of 0.5 mg / cm ² was formed by a post charger. Average amount of charge before transfer of toner is 30m
By charging to C / Kg or more, it is possible to obtain a high-definition image with excellent gradation and less scattering in a long time for a long time.

Second Embodiment Next, the second embodiment of the image forming apparatus according to the present invention will be described.
This will be described with reference to FIG.

The image forming apparatus according to this embodiment is a full-color laser beam printer, but unlike the above embodiment, a dedicated image carrier for each color, that is, the electrophotographic photosensitive drum 21Y (for yellow image formation in this embodiment) is used. ), 21M (for forming a magenta image), 21C (for forming a cyan image), 2
1Bk (for black image formation), and dedicated laser beam scanners 23Y and 23 around them
M, 23C, 23Bk, developing units 24Y, 24M, 24
C, 24Bk, post charger 121Y, 121M, 12
1C, 121Bk, transfer discharger 25bY, 25bM,
25bC, 25bBk, cleaning devices 26Y, 26
M, 26C, and 26Bk are arranged.

The transfer material is conveyed through the paper feed guide 50, the paper feed roller 51, and the paper feed guide 52 in this order, and receives the corona discharge from the attraction charger 81 and is reliably attracted to the transport belt 25a. After that, the image formed on each photosensitive drum is superposed on the transfer material by the transfer discharging devices 25bY, 25bM, 25bC, and 25bBk, and the transfer belt is neutralized by the discharging belt 82 and fixed by the fixing device 27 to be a full-color image. An image is obtained.

Even when such a transfer method is used, the laser beam spot diameter in the main scanning direction is 42 μm or less when the recording density is 400 dpi, and the toner has a volume average particle diameter of 8 μm or less and 0.5 mg / cm. The average charge amount before transfer of the toner is 30 mC / K using the toner whose Macbeth density after fixing is 1.2 or more for the coating amount of ²
By providing a means of g or more, it is possible to obtain a high-definition image with excellent gradation and less scattering in a long time for a long period of time.

[0080]

As is apparent from the above description, the image forming apparatus according to the present invention corresponds to the electrophotographic photosensitive member on which the electrostatic latent image is formed, the light source emitting the light flux, and the recorded image signal. Control means for controlling the luminous flux emission time length per pixel of the light source, and an electrostatic latent image formed by scanning the luminous flux from the light source on the electrophotographic photosensitive member in the main scanning direction. Has a developing means for developing the toner with a two-component developer containing toner and carrier particles, and a charging means for charging the toner before being electrostatically transferred to a transfer material at a transfer portion. Has an average diameter of 8 μm or less,
The weight per unit area on the transfer material is 0.5 mg / cm ²
In this case, the image density after fixing develops to 1.2 or more, and the average charging amount of the toner by the charging unit is 30 mC / Kg or more, so that a high-quality image can be stably obtained for a long period of time. It has become possible.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing a characteristic part of a first embodiment of an image forming apparatus according to the present invention.

FIG. 2 is an overall configuration diagram showing a first embodiment of an image forming apparatus according to the present invention.

FIG. 3 is a configuration diagram showing an exposure unit of the image forming apparatus of FIG.

FIG. 4 is a PWM circuit used in the image forming apparatus of FIG.

5 is a waveform diagram of the PWM circuit of FIG.

FIG. 6 is a particle size distribution chart of the toner used in this example.

FIG. 7 is a graph showing the relationship between the volume average particle diameter of toner and the minimum reproduced dot diameter.

FIG. 8 is a graph showing the relationship between post charging current and tribo.

FIG. 9 is a graph showing the relationship between the average charge amount of toner and the roughness level of the highlight portion after copying 10,000 sheets.

FIG. 10 is a graph showing the relationship between the Macbeth concentration and the deblurring level of the solid portion with respect to the toner amount of 0.5 mg / cm ² .

FIG. 11 is a configuration diagram showing a second embodiment of the image forming apparatus according to the present invention.

[Explanation of symbols]

 1 Electrophotographic Photosensitive Member 4 Developing Device 5 Transfer Device 9 Fixing Device 101 Post Charging Device E Laser Luminous Flux LS Laser Scanner

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication location G03G 15/16 (72) Inventor Makoto Kambayashi 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. In the company

Claims (2)

[Claims] 1. An electrophotographic photosensitive member on which an electrostatic latent image is formed, a light source for emitting a light beam, and a control means for controlling a light beam emission time length per pixel of the light source in response to a recorded image signal. Developing means for scanning a light flux from the light source on the electrophotographic photosensitive member in a main scanning direction, and developing an electrostatic latent image formed by the scanning of the light flux with a two-component developer containing toner and carrier particles. Charging means for charging the toner before being electrostatically transferred to the transfer material at the transfer portion, and the toner has a weight average diameter of 8 μm or less, An image forming apparatus characterized in that, when the weight is 0.5 mg / cm ² , the image density after fixing develops to 1.2 or more, and the average charging amount of the toner by the charging means is 30 mC / Kg or more. 2. The image forming apparatus according to claim 1, wherein the spot diameter of the light beam on the electrophotographic photosensitive member is 0.7 times or less the unit pixel size in the main scanning direction.