JP3450761B2 - Image forming device - Google Patents

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art In an electrophotographic type and electrostatic recording type image forming apparatus, a developing sleeve (developer carrying body) carrying a developer of a developing device is made to correspond to an image carrying body carrying an electrostatic latent image. Development is generally performed, and 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 this developing bias, as shown in FIG. 11, a component in which an AC component is superimposed on a DC component has been used. The rectangular wave of the AC component shown in FIG.
A frequency of about 2 kHz (1/2 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, as shown in FIG.
In the developing bias as shown in Fig. 3, the highlight part of the image "grips", and when the particle size of the toner of the developer is made small, sufficient image density cannot be obtained. There was a flaw.

In order to improve this drawback, for example, FIG.
It has been proposed by the present applicant that a developing bias in which an AC component is intermittently superposed on a DC component is used as shown in FIG.

The developing bias as shown in FIG. 12 (referred to as blank pulse bias or BP bias in this specification) is composed of a pulse portion and a rest portion (blank portion), as shown in FIG. The developing property is better than the developing bias (referred to as a rectangular bias in this specification), but in order to eliminate the roughness of the highlight part, the frequency of the pulse part is 4 kHz (1/2 cycle = 125 seconds) or more. It was necessary to raise. As a condition for obtaining the density, Vpp of the pulse portion is set to about 2 as in the case of the conventional rectangular bias.
It was necessary to set it to kV or higher.

However, Vpp of the pulse portion exceeds 2 kV,
Further, since the frequency is set to 4 kHz or more, when a conductive foreign substance is mixed in the developing portion, abnormal discharge occurs in that portion, a ring-shaped spot is formed on the image, and the image quality is remarkably deteriorated. There was a problem.

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

An object of the present invention is to perform good development with an appropriate developing bias to prevent the highlight portion from rubbing, to obtain a sufficient image density, and to prevent image defects due to abnormal discharge. Is to provide.

[0010]

The above object can be achieved by an image forming apparatus according to the present invention. In summary, the present invention is
The image carrier and the developer containing toner and carrier are carried.
A developer carrier that is conveyed to a portion facing the image carrier, and
DC when developing the electrostatic latent image formed on the image bearing member
The developing bias for superposing the AC voltage on the developing voltage is carried by the developer.
Bias applying means for applying to the holder, image formation having
In the apparatus, the developing bias causes the toner to carry the image.
The toner and the flying part that applies the voltage to fly to the carrier are
Consists of a return section that applies a voltage to pull back the developer carrier
The substantially rectangular wave shaped vibration part and the AC voltage are cut off.
And a rest portion for flying the toner to the image carrier,
The electric potential of the developer carrying body is stopped during the period from the flight part to the rest part.
In order to prevent exceeding the stop potential,
Image type characterized by a gradual transition to the rest
It is a device .

According to the present invention, preferably the following formula is satisfied :
You ^{| Vpp-2Vcont | · Tb 2} /4 <d 2 / | Q | However, Vpp: peak-to-peak voltage of the developing bias [V], Tb: peak current for pulling back the toner to the developer carrying member
Time for applying pressure [second], Vcont: image contrast potential [V], Q: average amount of triboelectric charge [C / kg] of toner, d: distance between image carrier and developer carrier [m]

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will now be described 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 1 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 2. After that, the photosensitive drum 1 is exposed to a laser beam L corresponding to an image, and an electrostatic latent image is formed on the surface. The electrostatic latent image is developed under the application of a developing bias by the developing device 4 containing a two-component developer containing toner and carrier inside, and visualized as a toner image. In this embodiment, the toner is negatively charged.

The obtained toner image is transferred by the transfer charger 5 onto the paper which is carried by the paper carrying device and is carried to the photosensitive drum 1. After that, the paper on which the toner image has been transferred is sent to a fixing device (not shown) to fix the toner image and output as an image.

In the transfer step, transfer residual toner remaining on the photosensitive drum 1 is cleaned by a cleaning device 6 having a cleaning blade pressed against the photosensitive drum 1. After that, the photosensitive drum 1 is destaticized by the pre-exposure device 11 and is used again for image formation.

The present invention applies a BP bias as a developing bias to the developing sleeve (developer carrying member) 4a of the developing device 4 from a developing power source (not shown) which is a bias applying means at the time of development, and devises the BP bias. There is a feature in doing it.

FIG. 2 shows a Canon CLC1000 copying machine.
A conventional BP bias waveform used in et al.
However, when the BP bias waveform actually created is confirmed with an oscilloscope or the like, as shown in FIG. 3, when the vibration portion changes to the rest portion, the developing sleeve potential exceeds the blank potential by about 50 μs (microsecond). I was shooting.

Therefore, in the present embodiment, as shown in FIG. 4, when the vibrating portion changes to the rest portion, the developing sleeve potential does not overshoot the blank potential and reaches the rest portion gently. BP bias was created.

Specifically, the BP bias of this embodiment is
The vibrating portion is formed by four pulses, and immediately after the rest portion, 900 V is applied as an electric field for pulling back the toner that is the developer carrying member, and -900 V is applied to the blank potential (rest portion potential). 900V against blank potential
Then, -900 V is applied in the direction of causing the toner to fly to the photosensitive drum 1, which is the image carrier, and then the resting section is reached.

The BP bias described above has a characteristic that the slow connection from the vibrating section to the rest section, which is a feature of the present invention, takes time T2 =
It takes 50 μs. The time T1 of each pulse is 6
2.5 μs, 250 μs with 4 pulses. Further, the rest unit is T1 × 12, that is, 750 μs in this embodiment.
By setting to 1, a bias of 1 kHz was obtained in one cycle.

Here, using the biases of FIGS. 3 and 4,
FIG. 5 shows a V-D (development contrast vs. image density) curve when an image is output under the same conditions. As shown in FIG. 5, according to the BP bias (bias of FIG. 4) of this embodiment, it is confirmed that there is no decrease in developability due to overshooting unlike the conventional BP bias (bias of FIG. 3). It was

Considering the phenomenon of deterioration in developability due to the overshoot of the potential, the vibrating portion in the BP bias causes the toner adhered on the developing sleeve due to the reflection force to fly and further accelerates. work. The rest part has a function of adhering the toner accelerated by the vibrating part to the development contrast potential (= blank potential-latent image potential) formed on the photosensitive drum. Therefore, it is expected that the developing efficiency will be lowered.

This was confirmed by another experiment. First, considering the end of the developing process, the end state of the developing process means that the potential on the toner layer (charging potential) of the toner image on the photosensitive drum is the same potential as the potential on the developing sleeve (blank portion potential). Considered synonymous. Therefore, a confirmation experiment was carried out using the experimental apparatus shown in FIG. 6, which is a modified image forming apparatus.

In the experiment, the surface of the photosensitive drum 1 rotating in the direction of arrow R in FIG. 6 is uniformly charged by the charger 2, and the electrostatic latent image on the photosensitive drum 1 is exposed to the 17 gradation potential by the laser beam L. And the latent image is developed with a two-component developer by the developing device 4, and the potential on the toner layer (charging potential) of the 17-tone toner image formed on the photosensitive drum 1 is determined by the potential sensor 3. It was carried out by measuring and comparing the developability thereof. The results are shown in Fig. 7.

FIG. 7 shows the latent image potential for 10 gradations out of 17 gradations and the charging potential by the conventional bias (FIG. 3) and the bias of the present invention (FIG. 4). As is clear from FIG. 7, the charge potential has not reached the blank portion potential with the conventional bias of FIG. Also, since it is in agreement with the result of the V-D characteristic described above, it can be seen that the overshoot portion reduces the developability.

Therefore, the relationship between the bias waveform and the charging efficiency (= charging potential / blank portion potential) when the shape of the image-contacting portion in the vibrating portion and the rest portion was changed was examined. In particular,
Table 1 shows the results using the time from the vibrating part to the rest part (T2) as a parameter. In FIG. 9, the negative value of T2 means the overshoot time.

[0028]

[Table 1] As is clear from Table 1, by increasing the time T2, the developing property (charging potential) of the solid part is improved, but in this experiment, the effect saturates at 50 μs or more, and conversely, the background fogging occurred on the white part. I got it. This is considered to be an adverse effect in that the developing contrast potential becomes large during the time T2, and thus the developability is improved by the solid amount, but conversely, the fog removing potential of the white background portion becomes small. Therefore, in this study, T2 of about 50 μs was preferable.

In the conventional BP bias, in order to achieve the effect of the resting portion, the ratio of the vibrating portion time and the resting portion time must be 1: 4 or more. When the developing sleeve potential reaches the rest portion without overshooting the blank portion potential when changing to, the same effect can be obtained even if the ratio of the vibration portion time and the rest portion time is 1: 1/2. I was able to get it.

From the above, by using a waveform such that the developing sleeve potential does not overshoot the blank potential when it changes from the vibrating portion to the rest portion and gently reaches the rest portion, it is conventionally possible to use peak-to-peak. Voltage V
The image density that could not be obtained without increasing pp to 2 kV is now sufficiently obtained even if Vpp is reduced to 1.8 kV.

In this embodiment, the Vpp of the BP bias can be suppressed to 1.8 kV in this way, and as a result, even if a conductive foreign substance is mixed in the developing portion, it is a problem that has been a problem in the past. No image defects due to discharge occur,
A high-density image can be stably obtained.

Embodiment 2 In Embodiment 1, the BP bias shown in FIG. 4 was used as the developing bias, but the bias effective in the present invention is not limited to this.

The BP bias, developing device and developer suitable for the present invention will be described.

In this embodiment, a two-component developer comprising a non-magnetic toner and a magnetic carrier (magnetic particles) is used as the developer. The mixing ratio is the weight ratio, and the non-magnetic toner is about 5
It was set to be%. The non-magnetic toner has a volume average particle diameter of about 8 μm. The magnetic carrier consists of resin-coated ferrite particles (maximum magnetization 60 emu / g), the weight average particle size is 50 μm, and the resistance value is 10 μm.
Indicates a value of ⁸ Ωcm or more. The magnetic permeability of the magnetic carrier is about 5.0. The amount of triboelectricity of toner is about -2.0 x 1
It is 0 ^-2 C / kg.

The developing device 4 is provided with an opening at a portion of the developing container accommodating the two-component developer in the vicinity of the photosensitive drum 1, and a developing sleeve as a developer carrying member projects to the outside from the opening. . The developing sleeve is rotatably installed in the developing container, and the distance from the photosensitive drum 1 is 500μ.
It is arranged so as to be m. The outer diameter of the developing sleeve is 25 mm, and the peripheral speed thereof is 280 mm / sec.

In this embodiment, as the developing bias, Vpp: peak-to-peak voltage [V] of an alternating voltage applied to the developer carrying member , Tb: peak voltage for returning the toner to the developer carrying member.
Time for applying pressure [sec], Vcont: Image contrast potential [V] (D of developing bias)
Potential difference from the latent image potential when the maximum image density is output from the C voltage), Q: average triboelectric charge amount [C / kg] of toner, d: distance between image carrier and developer carrier [m when a, as a developing bias, | Vpp-2Vcont | · Tb 2/4 <d 2 / | Q | alternating voltage as satisfying an alternating electric field of are intermittently formed is applied.

In the present invention, the triboelectric charge amount of the toner is
It measured using the measuring apparatus shown in FIG.

First, the toner whose triboelectric charge amount is to be measured is combined with a carrier to form a two-component developer, and the amount of 50 to 50 is obtained.
Put in a polyethylene bottle of 100 ml capacity, about 10
Shake by hand for ~ 40 seconds, then add about 0.5 ~ of this developer.
1.5g, 500 mesh bottom conductive screen 14
Put the metal measuring instrument 142 into the container 142
A metal lid 144 is put on the cover. Measuring instrument 14 in this state
2 Weigh the whole and set this as W1 (kg).

Next, the measuring device 142 is installed in the suction device 141 (at least the part of the suction device 141 that is in contact with the measuring device 142 is an insulator), suction is performed from the suction port 147, and the air flow control valve 146 is adjusted. , The pressure of vacuum gauge 145 is 250 m
mAq. In this state, suction is performed sufficiently, preferably for 2 minutes, to remove the toner by suction. Measuring instrument 1 at this time
The potential indicated by the electrometer 149 connected to 42 is read, and this is defined as V1 (V). The weight of the entire measurement container 142 after suction is measured and designated as W2 (kg). Measuring instrument 14
If the capacity of the capacitor 148 connected in parallel with the electrometer 149 is C1 (μF), the toner triboelectric charge amount can be calculated by the following formula: toner triboelectric charge amount (μC / kg) = C1 × V1 × 10
It is calculated as ^-3 / (W1-W2).

In this embodiment, a highlight halftone image and a solid image having an image density of about 0.2 are output,
An experiment was conducted to evaluate the image based on the smoothness of the highlight halftone image and the density of the solid image.

As a result, A = | Vpp-2Vcont | .Tb
^{2/4, B = d 2} / | Q | as, these relationships A <B
Only in the case of, the solid high density was maintained and the highlight reproducibility was good.

This will be described. FIG. 9 is a view showing a force applied to one toner particle on the developing sleeve, and FIG. 10 is an enlarged view thereof. In the figure, q is the charge amount of toner, m is mass, a is acceleration, ΔV is the potential difference between the photosensitive drum and the developing sleeve, and d is the gap between the photosensitive drum and the developing sleeve.

The distance X that the toner can move from the photosensitive drum to the developing sleeve is obtained as follows. The peeling voltage that is pulled back to the developing sleeve is applied to the toner for Tb seconds as the maximum time during the application of the oscillating electric field. The distance X that the toner can move during this period is obtained by the following equation (1): X = | Q | · | Vb−Vcont | · Tb ² / 2d (1)

Here, by setting 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, the toner on the photosensitive drum is set. Repeated vibration biased to. The condition at this time is that X is smaller than the gap d between the photosensitive drum and the developing sleeve.

[0045] Expressed in formula, X = | Vb-Vcont | · Tb 2/2 <d 2 / | Q | ··· (2) and developed under these conditions, the vibrating portion is stripped Even if the voltage is the maximum electric field, the toner cannot be reciprocated sufficiently between S and D, and when it changes to the rest portion, the toner that has the maximum acceleration to adhere to the photosensitive drum has a potential on the developing sleeve that is blank. By designing the developing bias so that it reaches so as not to overshoot the potential, the toner progresses faithfully to the development contrast at the maximum speed in the direction in which the toner flies onto the photosensitive drum created by the vibrating section. Therefore, even when the peak-to-peak voltage Vpp is low, the DC component works to attract the toner in an amount corresponding to the latent image potential to the photosensitive drum,
For this reason, dot loss does not occur, and the toner is repeatedly vibrated intermittently on the photosensitive drum, so that the toner is concentrated on the latent image portion and each dot is faithfully reproduced. It is possible to output a uniform image even with a shallow latent image in which only a non-uniform image can be obtained with the rectangular bias.

Although the embodiments of the present invention have been described above, the present invention is not limited to these embodiments, and various modifications can be made within the technical idea of the present invention.

[0047]

As described above, the present invention provides an image bearing
Image bearing by carrying the body and developer containing toner and carrier
On the image carrier and the developer carrier that conveys it to the part facing the body.
AC voltage to DC voltage when developing the formed electrostatic latent image
Applied to the developer carrier with the development bias
In an image forming apparatus having an ass applying unit,
The bias applies a voltage that causes the toner to fly to the image carrier.
The flying part and the voltage that pulls the toner back to the developer carrier.
Substantially rectangular wave-like vibrating part composed of adding back part
And a pause to cut off the AC voltage and fly the toner to the image carrier
Part, and carries the developer from the flying part to the rest part
To prevent the body potential from exceeding the rest potential
Therefore, since the transition is made from the flying part to the rest part gently , even under conditions of high frequency and low peak-to-peak voltage, image uniformity is high, an image with sufficient density can be obtained, and there are no image defects due to abnormal discharge. It has become possible to stably obtain high-quality images.

[Brief description of drawings]

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

FIG. 2 is a waveform diagram showing a conventional BP bias.

FIG. 3 is a waveform diagram showing an actual waveform of a BP bias shown in FIG.

4 is a waveform diagram showing a BP bias used in the example of FIG.

5 is a diagram showing a V-D curve when an image is output under the same conditions using the biases of FIGS. 3 and 4. FIG.

FIG. 6 is a schematic view showing an experimental apparatus used in the present invention to confirm a decrease in developability at an overshoot portion of a bias by measuring a charge potential of a toner layer.

7 is a diagram showing a latent image potential for 10 gradations and a charging potential by the bias of FIGS. 3 and 4. FIG.

FIG. 8 is a perspective view showing a measuring device used to measure the triboelectric charge amount of toner in a two-component developer according to the present invention.

FIG. 9 is a diagram showing a force applied to one toner particle on a developing sleeve.

FIG. 10 is an enlarged view of FIG.

FIG. 11 is a waveform diagram showing a rectangular bias.

FIG. 12 is a waveform diagram showing an example of a conventional BP bias.

[Explanation of symbols]

1 photosensitive drum 2 Primary charger 4 developing device 4a Development sleeve 5 Transfer charger

─────────────────────────────────────────────────── ─── Continuation of front page (56) References JP-A-6-274016 (JP, A) JP-A-9-127774 (JP, A) JP-A-8-6366 (JP, A) (58) Field (Int.Cl. ⁷ , DB name) G03G 13/06-13/095 G03G 15/06-15/095

Claims (2)

(57) [Claims] 1. An image carrier, including a toner and a carrier.
Development in which a developer is carried and conveyed to a portion facing the image carrier
The agent carrier and the electrostatic latent image formed on the image carrier are displayed.
Development bias in which AC voltage is superimposed on DC voltage when an image is formed
Bias applying means for applying to the developer carrier.
In the image forming apparatus having the above, the developing bias causes the toner to fly to the image carrier.
To the developer-carrying member for applying a voltage
Substantially composed of a return section for applying a pullback voltage
The rectangular wave-like vibrating part and the AC voltage are cut off and the toner is carried on the image.
A rest portion for flying to the holding member, and the potential of the developer carrying member is changed from the flying portion to the rest portion.
In order to prevent the rest section potential from being exceeded,
Image characterized by a gradual transition from the
Forming equipment. 2. The following formula is satisfied: 1.
Image forming device. ^{| Vpp-2Vcont | · Tb 2} /4 <d 2 / | Q | However, Vpp: peak-to-peak voltage of the developing bias [V], Tb: peak current for pulling back the toner to the developer carrying member
Time for applying pressure [second], Vcont: image contrast potential [V], Q: average amount of triboelectric charge [C / kg] of toner, d: distance between image carrier and developer carrier [m]