JP2001194876A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent a highlight part form becoming rough, to obtain sufficient image density and to obtain an image having no defect caused due to abnormal discharge by applying appropriate developing bias. SOLUTION: BP bias consisting of a quiescent part and an oscillating part is applied as the developing bias. The force of toner in a flying direction to a photoreceptor drum becomes maximum when the oscillating part is applied and electric field in the flying direction is formed. The amount of the toner adhering to an image part depends on toner speed accelerated by the electric field when the BP bias changes from the oscillating part to the quiescent part. The oscillating part is formed by four pulses. Just after the quiescent part, a voltage of +900V is applied to the potential of the quiescent part for a time T1 as the electric field to return the toner to a developing sleeve, thereafter, -900V is applied for a time T2, and further +900V is applied for the time T1, finally -900V is applied for the time T1 as the electric field in the direction where the toner is made to fly to the photoreceptor drum, then the quiescent part arrives. T1 is set greater than T2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrophotographic or electrostatic recording type image forming apparatus such as a copying machine or a printer, and more particularly to an image forming apparatus characterized by its developing unit.

[0002]

2. Description of the Related Art In an electrophotographic or electrostatic recording type image forming apparatus, a developing sleeve (developer carrying member) carrying a developer of a developing device is made to correspond to an image carrying body carrying an electrostatic latent image. In general, a developing bias is applied to the developing sleeve in order to form a developing electric field between the image carrier and the developing sleeve.

As shown in FIG. 15, a developing bias in which an AC component is superimposed on a DC component has been used as the developing bias. A rectangular wave of the AC component shown in FIG.
A frequency of about 2 kHz (１ ／ cycle = 250 μsec) and a peak-to-peak voltage (Vpp) of about 2 kV were used, and a good developed image was obtained.

[0004]

However, FIG.
The disadvantages of the development bias are that the highlight part of the image is "rough" and that if the particle size of the developer toner is reduced, sufficient image density cannot be obtained. There were drawbacks.

To improve this disadvantage, for example, FIG.
It has been proposed by the present applicant to use a developing bias in which an AC component is intermittently superimposed on a DC component as shown in FIG. However, this developing bias has the following problems.

A developing bias (referred to as a blank pulse bias or a BP bias in the present specification) as shown in FIG. 16 is compared with a developing bias (referred to as a rectangular bias in the present specification) as illustrated in FIG. And developability is good, but as a condition to eliminate roughness of the highlight part,
The frequency of the pulse part is 4 kHz (1/2 cycle = 125 seconds)
It was essential to raise it above. Also, as a condition for obtaining the density, it was necessary to set Vpp of the pulse portion to 2 kV or more as in the case of the conventional rectangular wave.

When Vpp of the pulse portion exceeds 2 kV and the frequency becomes 4 kHz or more, when a conductive foreign matter enters the developing portion, abnormal discharge occurs in that portion, and a ring-shaped spot is formed on the image. Formed, and there is a problem that the image quality is remarkably deteriorated.

Therefore, when the BP bias is used,
It is required to suppress Vpp smaller than in the case of the conventional rectangular bias.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an image forming apparatus capable of performing satisfactory development with an appropriate developing bias to prevent roughness of a highlight portion, obtain a sufficient image density, and have no image defects due to abnormal discharge. It is to provide.

[0010]

The above object is achieved by an image forming apparatus according to the present invention. In summary, the present invention provides:
An image carrier that carries an electrostatic latent image, and a developing device that develops the electrostatic latent image on the image carrier, the developing device includes a toner and a carrier that face the image carrier. A developer carrying member carrying a two-component developer having the same, and bias applying means for applying a developing bias to the developer carrying member;
The cycle of the waveform of the developing bias voltage applied by the bias applying unit includes a vibrating section and a pause section, and the vibration bias immediately before changing from the vibration section to the pause section causes the toner to fly to the image carrier. Direction and the application time is T
1, and the vibration bias immediately before that is a direction in which the toner is pulled back to the developer carrier, and the application time is set to T2.
In this case, the image forming apparatus has a relationship of T1> T2.

According to the present invention, preferably, the vibrating portion of the developing bias voltage has a substantially rectangular wave. The oscillating portion of the developing bias voltage has substantially two cycles or more.
The integral value of the oscillating portion of the developing bias voltage is substantially zero with the rest portion being zero. The time difference between the oscillation section and the pause section of the developing bias voltage is 1: 1/2 to 1:15. Further, the following equation: | Vpp−2Vcont | d · Tb ² <d ² / | Q | where Vpp: peak-to-peak voltage [V] of the developing bias, Tb: developer carrying of an alternating voltage applied to the developer carrying body Maximum time [sec] to pull toner back to body, Vcont: Image contrast potential [V], Q: Average amount of triboelectric charge of toner [C / kg], d: Distance between image carrier and developer carrier [m] is satisfied.

[0012]

Embodiments of the present invention will be described below in more detail with reference to the drawings.

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

In FIG. 1, a photosensitive drum 131 as an electrostatic latent image carrier rotates in the direction of arrow R in the figure, and its surface is uniformly charged by a primary charger 132. afterwards,
The photosensitive drum 131 is exposed to laser light L corresponding to an image, and an electrostatic latent image is formed on the surface. The electrostatic latent image is developed by a developing device 133 using a two-component developer composed of toner and carrier under the application of a developing bias, and is visualized as a toner image. In this embodiment, the toner is negatively charged. The obtained toner image is transferred by a transfer charger 134 onto paper conveyed by a paper conveyance device (not shown), and then fixed by a fixing device 135 to be output as an image.

In the transfer step, transfer residual toner remaining on the photosensitive drum 131 is cleaned by a cleaning device 136 having a cleaning blade 161 pressed against the photosensitive drum 131. Thereafter, the photosensitive drum 131 is neutralized by the pre-exposure device 137 and is subjected to the image formation again.

According to the present invention, during development, a BP bias is applied as a developing bias to a developing sleeve (developer carrier) 133a of a developing device 133 from a developing power source 138 as a bias applying means, and the BP bias is devised. There is a feature. FIG. 2 shows the developing bias used in this embodiment. The developing bias voltage is a BP bias (blank pulse bias) composed of a pause portion (blank portion) and a vibrating portion, and the vibrating portion is formed from four pulses. In FIG. 2, if the maximum voltage application time in the direction of pulling the toner back to the developing sleeve is Tb, Tb ≧ T1
It is.

The vibrating portion generates an electric field that returns the toner to the developing sleeve of the developing device 133 immediately after the pause portion.
The voltage +900 V with respect to the potential of the rest part is set to time Tb = T1.
After that, a voltage of -900 V is applied for a time T2, and a voltage of +900 V is applied for a time T2. The application of T1 is performed, and then the pause section is established.

In this embodiment, the pause time is 6 × (T1
+ T2) time. The waveform was selected such that T1 = 1.5 × T2. Since one cycle of T1 + T2 was selected to be 8 kHz, in this embodiment, the vibrating part was 250 μm.
s (microseconds), the pause is 750 μs, and T
1 = 75 μs, T2 = 50 μs, and the bias was a BP bias having a cycle of 1000 μs including the vibrating part and the rest part and a frequency of 1 kHz.

FIG. 3 shows a waveform of a BP bias conventionally proposed. The bias in FIG. 3 is the peak-to-peak voltage V
pp was set to 2 kV (the bias in FIG.
kV). FIG. 4 shows a VD (development contrast vs. image density) curve when an image is output under the same conditions using the biases shown in FIGS.

As shown in FIG. 4, by using the BP bias of FIG. 2 which is the developing bias in the present invention, the density which could not be obtained unless the BP bias Vpp was raised to 2 kV in the prior art, is reduced to 1 Vpp. Even when the voltage was lowered to 0.8 kV, a sufficient voltage was obtained.

In this embodiment, Vpp of the BP bias can be suppressed to 1.8 kV as described above. As a result, even if conductive foreign matter is mixed into the developing portion, the abnormalities which have been a problem in the past are considered. No image defects caused by discharge,
High-density images can be obtained stably.

The following are conceivable reasons why the concentration can be sufficiently obtained. FIG. 5 is a view showing a force applied to one toner particle on the developing sleeve, and FIG. 6 is an enlarged view thereof. In the figure, q is the charge amount of the toner, m is the mass, a is the acceleration, ΔV is the potential difference between the photosensitive drum and the developing sleeve,
d is a gap between the photosensitive drum and the developing sleeve.

Here, consider the force that the toner receives during development. The force in the direction in which the toner flies from the developing sleeve to the photosensitive drum is maximized when an electric field in the direction in which the toner flies to the photosensitive drum is formed while the bias of the vibrating portion is applied. . It is considered that the amount of toner adhering to the image portion depends on the speed of the toner accelerated by the electric field when changing from the vibration portion to the blank portion (pause portion). When the voltage of the vibrating section is applied and the voltage for causing the toner to fly from the developing sleeve toward the photosensitive drum is applied immediately before moving to the pause section, the speed at which the toner stationary on the developing sleeve is accelerated is Let us compare the present invention with the conventional case.

The speed of the accelerated toner can be expressed by V = | q | / m × ΔV / d × TL. Assuming that the contrast required for development is 350 V, ΔV is 1350 V with the conventional bias. In contrast, with the bias of the present invention, ΔV is 1250
V. Other values are the same for the conventional bias and the bias of the present invention. Therefore, assuming that the toner speed at the moment when the blank portion is formed by the conventional bias application is V2 and the toner speed by the bias application of the present invention is V1, V1 = 1.23 × V2, and even if Vpp is low, It is considered that the flying speed of the toner has been increased, and as a result, a sufficient image density can be obtained even when Vpp is reduced.

Embodiment 2 In Embodiment 1, the BP bias shown in FIG. 2 was used as the developing bias. However, the bias effective for the present invention is not limited to the bias shown in FIG.

FIG. 7 shows the BP bias used as the developing bias in this embodiment. In the BP bias according to the present embodiment, the vibrating portion has a plurality of vibrations. In the figure, T1 = 50 μs,
The waveform was selected such that T2 = 25 μs and T1 = 2 × T2. As with the BP bias of FIG. 2, the same effect as in the first embodiment can be obtained even when the vibration is performed a plurality of times by setting the integral value to zero.

According to the BP bias shown in FIG.
Even at 6 kV, a sufficient density could be obtained, and no image failure due to abnormal discharge occurred. Also, by using such a BP bias, an image having less roughness and less image unevenness could be obtained as in the first embodiment.

Embodiment 3 Another embodiment of the image forming apparatus of the present invention will be described with reference to FIG.

The image forming apparatus according to the present embodiment is a color copying machine having a digital color image reader at the upper part and a digital color image printer at the lower part.

In the reader section, a document 30 is placed on a document glass table 31 and a reflected light image from the document 30 obtained by exposing and scanning with an exposure lamp 32 is condensed on a full-color sensor 34 by a lens 33 to form a color image. Obtain a color separation image signal. The color separation image signal is amplified by an amplifier circuit (not shown), processed by a video processing unit (not shown), and sent to a printer unit.

In the printer section, a photosensitive drum 1 serving as an image carrier is a photosensitive member having a surface protective layer, and is installed so as to be rotatable in a direction indicated by an arrow in FIG. Around the photosensitive drum 1, a pre-exposure lamp 11 for initializing the surface of the photosensitive drum 1, and a charger 2 for uniformly charging the surface of the photosensitive drum 1
(A corona charger in this example), a laser exposure optical system 3 for forming an electrostatic latent image on the surface of the photosensitive drum 1 according to image information, and a color for developing the electrostatic latent image formed on the photosensitive drum 1 Developing devices 4 containing different kinds of developers (toners)
(4Y, 4C, 4M, 4K) fixedly arranged developing device, light detecting means 1 for detecting toner amount on photosensitive drum 1
3. A cleaner 6 for removing remaining s and other developer on the photosensitive drum 1 is disposed.

In this embodiment, the laser exposure optical system 3 comprises a polygon mirror 3a, a lens 3b, a mirror 3c, etc., is modulated by a color-separated image signal from the reader unit, and is subjected to image scan exposure by a laser output unit. The converted laser light E is converted into an optical signal, and the converted laser light E is used as a polygon mirror 3a.
And is projected on the surface of the photosensitive drum 1 through the lens 3b and the mirror 3c to form an electrostatic latent image corresponding to a color image signal of each color.

At the time of image formation in the printer section, the photosensitive drum 1 is rotated in the direction of the arrow in FIG.
1, the surface of the photosensitive drum 1 is neutralized and initialized, and then the surface of the photosensitive drum 1 is uniformly charged negatively by the charger 2, and the laser beam E corresponding to the image signal of each color separated by the exposure optical system 3. Are sequentially irradiated on the surface of the photosensitive drum 1 to form an electrostatic latent image in a predetermined color order.

Next, a predetermined developing device is operated in the order of cyan (C), magenta (M), yellow (Y), and black (K) in a predetermined developing order to develop the latent image on the photosensitive drum 1. Then, toner images are sequentially formed on the photosensitive drum 1 by using a negative toner based on a resin. Here, each of the developing devices 4C, 4M, 4Y, and 4K of the developing device includes an eccentric cam 24C,
By the operations of 24M, 24Y, and 24K, a required developing unit is configured to selectively approach the photosensitive drum 1 and perform a developing operation in accordance with the color of the formed latent image.

On the other hand, the recording material cassette 7a, 7b or 7
From c (may be manual feed), pickup roller,
A recording material such as transfer paper fed by a transport system including a paper feed guide, a paper feed roller, and the like is wound around the transfer device 5 in synchronization with a predetermined timing.

In this embodiment, the transfer device 5 includes a transfer drum 5a having a diameter of 180 mm as a recording material carrier, a corona charger 5b for transferring a toner image on the photosensitive drum 1 to the recording material, and a recording material. A transfer having an attraction charger 5c as an attraction and charging means for attracting to the drum 5a, an attraction roller 5g as a counter electrode, an inner corona charger 5d and an outer corona charger 5e, and is rotatably supported by a shaft. A recording material carrying sheet 5f made of a dielectric material serving as a recording material carrying means is integrally stretched in a cylindrical shape in a peripheral opening area of the drum 5a. As the recording material carrying sheet 5f, a dielectric sheet such as a polycarbonate film is used.

The transfer drum 5a is rotated in the direction of the arrow in FIG. 1 in synchronization with the photosensitive drum 1, and a cyan toner image obtained by developing with a cyan developing unit 4C is recorded on a recording material carrying sheet. Transfer charger 5 at the transfer section
b. The transfer drum 5a continues to rotate as it is, and prepares for transfer of an image of the next color (for example, magenta).

The photosensitive drum 1 on which the toner image has been transferred
Is cleaned by a cleaner 6 to remove residual toner and the like, is uniformly charged again by the charger 2, and receives the above-described image exposure by a laser beam modulated by a magenta image signal of the next color. This latent image is developed by a magenta developing device and visualized as a magenta toner image. The magenta toner image is superimposed on a recording material on a recording material carrying sheet 5f from a cyan toner image by a transfer charger 5b in a transfer section. Is transcribed. The transfer drum 5a continues to rotate as it is, and prepares for the transfer of an image of the next color (for example, yellow).

Subsequently, the above-described image forming and transferring processes are performed for yellow and black. When the superposition transfer of the four color toner images onto the recording material is completed, the recording material is discharged by the separating charger 5h. Then, the transfer drum 5a is operated by the action of the push-up roller 8b and the separation claw 8a.
The toner image is fixed to the fixing device 9 (a heat roller fixing device in this example) by the conveying means, and is discharged onto an external tray 10. Thus, a series of full-color printing sequences is completed, and a required full-color print image is obtained.

When images are formed on both sides of the recording material, the conveyance path switching guide 19 is driven immediately after the recording material exits the fixing device 9 to transfer the recording material to the reversing path 21a via the conveyance vertical path 20. Lead once. Thereafter, by the reverse rotation of the reversing roller 21, the recording material is retreated in a direction opposite to the direction in which the recording material is fed, with the rear end of the fed sheet being the leading end, and
Store in 2. Thereafter, the recording material is again conveyed from the intermediate tray 22 to the transfer device 5, and an image is formed on the other surface of the recording material by the above-described image forming process.

In the transfer drum 5a after the recording material is separated, the powder scatters and adheres to the recording material carrying sheet 5f.
In order to prevent oil from adhering to the recording material, cleaning is performed by the fur brush 14 and the backup brush 15 and the oil removing roller 16 and the backup brush 17 that face each other via the recording material carrying sheet 5f. Such cleaning is performed before or after image formation, and at any time when a jam (paper jam) occurs.

In this embodiment, the gap between the recording material carrying sheet 5f and the photosensitive drum 1 is reduced by operating the eccentric cam 25 at a desired timing and operating the cam follower 5i integrated with the transfer drum 5a. It can be set arbitrarily. For example, during standby (standby) or when the power is off, the distance between the transfer drum 5a and the photosensitive drum 1 is increased.

Next, the processing process of the image processing unit will be described with reference to the block diagram of FIG.

In the image processing section 40 shown in FIG. 9, the digital image data 42 is converted into an analog image signal 403 by the D / A converter 402 and input to one terminal of the comparator 411. The timing signal generation circuit 407 generates a pixel clock 404 according to the input reference clock signal 43.
Also, a screen clock 408 to the pattern signal generator 409 is created and output. Pattern signal generator 4
09 is a pattern signal 4 based on the screen clock 408.
10 is output and input to the other terminal of the comparator 411.

The digital pixel signal 42 is a pixel clock 40
4, and the D / A converter 402 outputs an analog image signal 403 in synchronization with the pixel clock 404. The screen clock 408 is a clock signal obtained by multiplying the pixel clock 404 by an integer, and defines the cycle of the pattern signal 410, for example, a triangular wave.

Analog image signal 403 and pattern signal 4
10 is compared by the comparator 411. When the analog image signal 403 is larger, it is set to 0;
Pulse-width modulated binary image data 41 is created and output.

FIG. 10 shows a timing chart of each part in FIG. Accordingly, the image processing process will be further described.

Here, the screen clock 408 is a clock having a cycle twice as long as the pixel clock 404. Digital image signal is hexadecimal 00 (white) to FF
The pulse waveform of the binarized image data 41 modulated by the pattern signal 410 when changing stepwise to (black) is shown. By changing the amplitude of the pattern signal in this manner, the relationship between the input level of the digital image data 42 and the pulse width of the binary image data 41 can be changed.

The binarized image data 41 is input to an image forming unit of a full-color copying machine, controls an exposure width by a laser beam, and projects a laser spot having an exposure width according to the image data on a photosensitive drum. A latent image is formed. These latent images are developed by the developing units 4Y, 4C, 4M, and 4K described above.

Next, the developing step which is a feature of the present invention will be described in detail. As the developer, a two-component developer composed of a non-magnetic toner and a magnetic carrier (magnetic particles) is used.
The mixing ratio was such that the nonmagnetic toner was about 5% by weight. The non-magnetic toner has a volume average particle size of about 8 μm. The magnetic carrier is composed of resin-coated ferrite particles (maximum magnetization of 60 emu / g), has a weight average particle size of 50 μm, and has a resistance value of 10 ⁸ Ωcm or more. The magnetic permeability of the magnetic carrier is about 5.0.

The developing containers of the developing devices 4 (4Y to 4K) include:
An opening is provided in a portion close to the photosensitive drum 1, and a developing sleeve as a developer carrier projects outside from the opening. The developing sleeve is rotatably incorporated in the developing container, and is arranged so that the distance from the photosensitive drum 1 is 500 μm. The outer diameter of the developing sleeve is 25m
m, and its peripheral speed is 280 mm / sec.

In this embodiment, Vpp: peak-to-peak voltage [V] of the alternating voltage of the developing bias applied to the developer carrier, Tb: toner on the developer carrier of the alternating voltage of the developing bias applied to the developer carrier Vcont: Image contrast potential [V] (D of developing bias)
(Potential difference from latent image potential when maximum image density is output from C voltage), Q: average triboelectric charge amount of toner [C / kg], d: distance between image carrier and developer carrier [m] ], An alternating voltage is applied as a developing bias such that an alternating electric field satisfying the condition of | Vpp−2Vcont | d · Tb ² <d ² / | Q | is intermittently formed.

The toner used in this embodiment has a triboelectric charge of about −2.0 × 10 ⁻² C / kg and a toner of about −3.0 × 10 ^−2.
Two types of C / kg were used.

Next, a method for measuring the triboelectric charge amount (two-component developer) of the toner will be described with reference to FIG. FIG. 11 shows an apparatus for measuring the triboelectric charge amount (triboelectric charge) of the toner.

First, the toner whose triboelectric charge amount is to be measured is combined with a carrier to form a two-component developer,
Put into a 100 ml polyethylene bottle,
Shake by hand for ~ 40 seconds and then apply this developer for about 0.5-
1.5 g, 500 mesh bottom conductive screen 14
3 is placed in the metal measuring container 142,
Is covered with a metal lid 144. Measurement container 14 in this state
2 Weigh the whole and make it W1 (kg).

Next, the measuring container 142 is set on the suction device 141 (at least the portion of the suction device 141 that is in contact with the measurement container 142 is an insulator), suction is performed from the suction port 147, and the air volume control valve 146 is adjusted to form a vacuum. 250m total pressure of 145
mAq. In this state, suction is performed sufficiently, preferably for 2 minutes, to remove the toner by suction. Measurement container 1 at this time
The potential indicated by the electrometer 149 connected to 42 is read, and this is set as V1 (V). Further, the entire measurement container 142 after suction is weighed, and this is defined as W2 (kg). Measurement container 14
Assuming that the capacitance of the capacitor 148 connected in parallel with the electrometer 149 to C2 is C1 (μF), the triboelectric charge amount of the toner is expressed by the following formula: triboelectric charge amount of toner (μC / kg) = C1 × V1 × 10
^-3 / (W1-W2).

In this embodiment, a highlight halftone image and a solid image having an image density of about 0.2 are output,
The smoothness of the highlight halftone image and the density of the solid image were evaluated.

The formation of an electrostatic latent image for image output is as follows. First, when the photosensitive drum is uniformly charged to -650 V to output a highlight halftone image, PWM (pulse width modulation) is performed by a semiconductor laser to lower the surface potential to about -450 V and output a solid image. To about -150 V (V
cont = 350V).

Next, a latent image was developed by a reversal development method using the developing device having the above-described structure and the toner having the charge amount. In this embodiment, the DC voltage of the developing bias is-
It is set to 500V and the amplitude of the alternating voltage given intermittently Vpp
Was fixed at 1800 V, and the maximum withdrawal time Tb was changed. At this time, the time for passing the alternating electric field was set to two periods for each period of the alternating electric field, as shown in FIG. The developer is a two-component developer, and the toner includes the above-mentioned two kinds of toners (the triboelectric charge amount is about −2.0 × 10 ⁻² C / kg and about −3.0 × 10 ⁻² C / kg). Tried). Table 1 shows the results.

[0060]

[Table 1] As shown in Table 1, A = | Vpp−2Vcont | · Tb
^{2/4, B = d 2} / | Q | as, these relationships A <B
Only when the condition was satisfied, the solid was maintained at a high density, and the reproducibility of the highlight was good.

As described above, FIGS. 5 and 6 show the force applied to one toner particle on the developing sleeve.
In the figure, q is the charge amount of the toner, m is the mass, a is the acceleration, ΔV
Is a potential difference between the photosensitive drum and the developing sleeve, and d is a gap between the photosensitive drum and the developing sleeve.

The distance X at which the toner can move from the photosensitive drum to the developing sleeve is obtained as follows. A stripping voltage to be drawn back to the developing sleeve is applied to the toner for a maximum time of Tb seconds during application of the oscillating electric field. The distance X during which the toner can move can be obtained by the following equation (1): X = | Q | · | 1/2 · Vpp−Vcont | · Tb ² / 2d (1)

Here, the condition that the toner developed on the photosensitive drum is not returned to the developing sleeve at the moving distance X when the stripping voltage is applied for the maximum time is set. Repeatedly biased vibration. The condition at this time is when X becomes smaller than the gap d between the photosensitive drum and the developing sleeve.

[0064] is represented by the formula, | Q | · | 1/2 · Vpp-Vcont | · Tb 2 / 2d ∴ | Vpp-2Vcont | · Tb 2/4 <d 2 / | Q | ··· (2) When the development is performed under such conditions, the reciprocation between the SD and the SD cannot be sufficiently performed even by the electric field in which the stripping voltage is the maximum time, and the development immediately before the alternating voltage is raised as described above. By increasing the application time of the voltage on the accelerating side, even when Vpp is low, the DC component acts to attract the toner of an amount corresponding to the latent image potential to the photosensitive drum, so that dot missing does not occur. Further, by intermittently repeating the vibration on the photosensitive drum, the toner is concentrated on the latent image portion, and each dot is faithfully reproduced, so that only a non-uniform image can be obtained with the conventional rectangular bias. Even with such a shallow latent image, a uniform image can be output.

In this embodiment, the applied alternating electric field, that is, the BP bias is as shown in FIG. 12, but the present invention is not limited to this. For example, as shown in FIG. Alternatively, as shown in FIG.
Wavelength application and ten wavelength pauses may be used. Various waveforms such as a triangular wave and a sine wave can be applied even if the waveforms of these vibrating parts are not rectangular waves, and the most appropriate waveform can be selected according to the copying speed and the developing conditions. Further, the ratio between the bias application time and the pause time is preferably 1: 1/2 to 1:15,
Good results were obtained in this range.

Although the embodiments of the present invention have been described above, the present invention is not limited to these embodiments, and all modifications within the technical concept of the present invention are possible.

[0067]

As described above, in the image forming apparatus according to the present invention, the developing bias voltage applied to the developer carrier by the bias applying means is changed to a blank pulse having a vibrating portion and a pause portion in one cycle of the waveform. The bias immediately before the change from the vibrating section to the rest section is the direction in which the toner flies to the image carrier, and the application time is T1.
Further, the immediately preceding vibration bias is in the direction of pulling the toner back to the developer carrying member, and the application time is T2.
Since these vibration biases have a relationship of T1> T2, even under conditions of high frequency and low peak-to-peak voltage, images with high uniformity and sufficient density can be obtained, and images due to abnormal discharge can be obtained. Without defects, high-quality images can be stably obtained.

[Brief description of the drawings]

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

FIG. 2 is a waveform chart showing a BP bias used in the embodiment of FIG.

FIG. 3 is a waveform diagram showing a conventional BP bias.

FIG. 4 is a diagram illustrating a VD curve when an image is output under the same conditions using the biases of FIGS. 2 and 3;

FIG. 5 is a diagram illustrating a force applied to one toner particle on a developing sleeve.

FIG. 6 is an enlarged view of FIG. 5;

FIG. 7 is a waveform diagram showing a BP bias used in another embodiment of the present invention.

FIG. 8 is a configuration diagram showing still another embodiment of the image forming apparatus of the present invention.

FIG. 9 is a block diagram illustrating an image processing unit of the image forming apparatus of FIG. 8;

FIG. 10 is a timing chart of the image processing unit of FIG. 8;

FIG. 11 is a perspective view showing a measuring device for measuring a triboelectric charge amount of toner in a two-component developer.

FIG. 12 is a waveform diagram showing a BP bias used in the embodiment of FIG.

FIG. 13 is a waveform chart showing another example of a BP bias that can be used in the present invention.

FIG. 14 is a waveform chart showing still another example of a BP bias that can be used in the present invention.

FIG. 15 is a waveform diagram showing a rectangular bias.

FIG. 16 is a waveform diagram showing a conventional BP bias.

[Explanation of symbols]

 131 photosensitive drum 132 primary charger 133 developing device 133a developing sleeve 134 transfer charger 135 fixing device 138 developing power supply

Continuation of the front page (72) Inventor Fumitake Hirobe 3-30-2 Shimomaruko, Ota-ku, Tokyo F-term (reference) 2H027 DA38 EA05 EB04 ED09 EE02 2H073 BA07 BA13 BA41 BA45 CA03 CA22 9A001 BB06 HH23 KK16 KK29 KK31 KK32 KK42

Claims (6)

[Claims] 1. An image carrier that carries an electrostatic latent image, and a developing device that develops the electrostatic latent image on the image carrier, wherein the developing device faces the image carrier. A developer carrying member that carries a two-component developer having a toner and a carrier; and a bias applying unit that applies a developing bias to the developer carrying unit; and a developing bias voltage applied by the bias applying unit. The cycle of the waveform includes a vibrating section and a pause section, and the vibration bias immediately before changing from the vibrating section to the pause section is a direction in which the toner is caused to fly to the image carrier. The image forming apparatus is characterized in that the vibration bias immediately before that is a direction in which the toner is pulled back to the developer carrier, and the application time is T2, where T1> T2. 2. The image forming apparatus according to claim 1, wherein the oscillating portion of the developing bias voltage has a substantially rectangular wave. 3. The image forming apparatus according to claim 1, wherein the oscillating portion of the developing bias voltage has substantially two cycles or more. 4. The image forming apparatus according to claim 1, wherein an integral value of said oscillating portion of said developing bias voltage is substantially zero with a pause portion being zero. 5. The image forming apparatus according to claim 1, wherein the time difference between the vibration section and the pause section of the developing bias voltage is 1: 1/2 to 1:15. 6. The following equation: | Vpp−2Vcont | d · Tb ² <d ² / | Q |, where Vpp is the peak-to-peak voltage [V] of the developing bias, and Tb is the alternating voltage applied to the developer carrier. Maximum time [sec] for pulling toner back to developer carrier, Vcont: image contrast potential [V], Q: average toner triboelectric charge [C / kg], d: image carrier and developer carrier 2. The image forming apparatus according to claim 1, wherein the distance [m] is satisfied.