JP2963853B2 - Image forming device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus that develops an electrostatic latent image formed on an electrostatic latent image carrier and visualizes the electrostatic latent image using an electrophotographic method.

[0002]

2. Description of the Related Art In an image forming apparatus using an electrophotographic method, an electrostatic latent image formed on an electrostatic latent image carrier is developed by a developing device and visualized as a toner image. FIG. 8 shows a main part of an example of an image forming apparatus provided with a conventional developing device.

As shown in FIG. 8, this image forming apparatus includes a photosensitive drum 1 coated with an α-Si photosensitive layer as an electrostatic latent image carrier. In this embodiment, the photosensitive drum 1 has a diameter of 80 mm. , Rotate in the direction of the arrow at a speed of 320 mm per second,
The primary charger (not shown) disposed around the
Primary charging is uniformly performed at 00V. Next, the photosensitive drum 1 is subjected to image exposure through an optical system or exposure based on image information by a light emitting element (not shown) such as a laser or an LED.
An electrostatic latent image is formed on the photosensitive drum 1. The electrostatic latent image formed on the photosensitive drum 1 is developed by a developing device provided around the photosensitive drum 1.

In this developing apparatus, a developing sleeve 2 as a developer carrier, a magnet roller 3 and a magnetic blade 4 are provided in a developing container 5 containing a magnetic toner as a developer. The developing sleeve 2 has a diameter of 25 mm, and is rotatably disposed in the opening facing the photosensitive drum 1 in the direction of the arrow. A magnet roller 3 is fixedly disposed in the developing sleeve 2.

The developing sleeve 2 carries the toner and transports the toner to a developing section facing the photosensitive drum 1. During the transport, the layer thickness of the toner is regulated by the magnetic blade 4, and the toner is fixed on the developing sleeve 2. Is applied to form a thin toner layer.

The toner applied to the thin toner layer on the developing sleeve 2 as described above is rubbed by the magnetic blade 4 and the developing sleeve 2 in the process so far, and a charge of -10 μC / g is generated. Granted.

The photosensitive drum 1 and the developing sleeve 2 are arranged in a non-contact manner with a gap of 50 to 500 μm in the developing section, and a gap between the developing sleeve 2 and the photosensitive drum 1 (between SD) in the developing section. , By a bias power supply, frequency: 1 KHz to 8 KHz, amplitude: 400 to 3000 V,
A developing bias in which a DC voltage having an integrated average value of the waveform V _DC : 50 to 300 V and an AC voltage are superimposed is applied, and a developing electric field is generated. As the AC voltage,
A sine wave, a triangular wave, a sawtooth wave, a rectangular wave, and a bias such that the 値 value of the peak-to-peak voltage of the waveform is different from the integrated average _VDC (hereinafter, this bias is referred to as a duty bias ( Duty bias) is conventionally known.

The charged toner on the developing sleeve 2 is transferred from the surface of the developing sleeve 2 to the surface of the photosensitive drum 1 by the force received from the developing electric field in the developing section, and the electrostatic latent on the photosensitive drum 1 is changed. Regular development of the image is performed.

It is known that the density of the above-mentioned bias is improved by changing it to a rectangular wave instead of a sine wave or a triangular wave. This is because, for the same peak-to-peak voltage (hereinafter referred to as V _PP ), the rectangular wave has a larger effective value.

Furthermore, it is known that the uniformity of the halftone image quality is improved by using a duty bias instead of a rectangular wave.

This will be described with reference to FIG. FIG. 9 is a diagram showing the potential on the photoconductor, with the vertical axis representing the potential. V _D is the dark potential on the photoreceptor and is developed into a solid black. V _L
Is a light potential and is not developed. The intermediate V _H is the halftone potential and the developing bias V _DC (200 V in the figure)
Is developed into a half-tone gray according to the difference between.

The optical system for realizing such a halftone potential relationship is not only an analog optical system but also an optical system.
Also includes digital optical systems.

This is because it is difficult to achieve a binary potential relationship even with a digital optical system except in the case where a halftone is formed in a complete area gradation, and the halftone is intensity-modulated. This is because it is realized using a typical difference in exposure amount. This has become more remarkable with the recent improvement in resolution (small spot diameter of laser or the like).

A rectangular wave and a duty wave are compared with respect to a developing bias when developing such a halftone potential. In FIG. 9, a rectangular wave is indicated by a dashed line, and a 35% duty wave is indicated by a one-point dashed line. It should be noted that the 35% duty wave means that the time during which the potential is displaced to the development promoting side of _VDC is 35% of one cycle.
% Square wave.

As is apparent from the figure, the developing electric field and the developing time for developing V _H are equal to the potential V _{H-sqr in} the case of a rectangular wave.
And the time is t _H-sqr , whereas in the case of a Duty wave, V
_H-Duty and t _H-Duty .

Here, V _{H -sqr} <V _{H -Duty} , and t
_By making the duty wave such that _H-sqr > t _H-Duty ,
In particular, it is known that uniform development of halftone can be performed.

The reason for this is that the toner tribo on the developing sleeve is not sufficiently uniform and has a distribution.

In the case of the above-mentioned duty wave, in order to develop the halftone with a higher electric field and in a shorter time than the rectangular wave, the toner to be developed into the halftone has a relatively high triboelectricity. It becomes toner. As a result, the adhesion to the drum is increased, so that the developed toner image is less likely to be disturbed, and uniform halftone development becomes possible.

The developing electric field for developing V _D and the developing time are V _D-sqr and t _D-sqr for the rectangular wave, and the duty is
The bias is set to V _D-Duty and t _D-Duty .

[0020]

However, when the duty bias of the prior art is used, when the toner tribo on the developing sleeve is low, a sufficient image density cannot be obtained, and the uniformity of halftone is insufficient. There was a disadvantage that it was. This is particularly the case when magnetic toner having a leak site such as magnetite is used as the toner, when used at a high temperature, or when the image forming apparatus is left unused and then started to be used. It becomes more prominent in situations where the toner tribo is attenuated and lower.

The reason why the density is not obtained is that the development time t _D-Duty of the duty bias for developing V _D described with reference to FIG. 9 is short, so that it becomes difficult for low tribo toner to reach the drum. It is thought to be.

Here, when the bias is made closer to a rectangular wave in order to lengthen t _D-Duty , the density tends to come out.
As described above, the uniformity of the halftone is further reduced.

The reason why the uniformity of the halftone becomes insufficient when a low tribo toner is developed even when the duty bias is used is as follows.

FIG. 10 is the same as FIG. 9 except that only the potential relationships of V _D , V _H , and V _L and the duty bias are extracted.

[0025] The V _H-Duty becomes electric field to the first V _H, the toner on the developing sleeve is jumped out overcomes the image force of the sleeve, and, t receive an electric field in the drum direction
Reach V _H on the drum during _H-Duty .

When the AC bias is changed and the toner is to be peeled off from the drum, when the electric field between _VH and the drum becomes stronger than a certain level, the toner which has once adhered cancels out the mirror image force with the drum. Try to fly back on the sleeve.

Here, if the toner tribo is high, if V
_{If the} toner jumps out of the drum again when the difference between _H and 200 V or more is obtained, the toner will fly back to the sleeve when a bias higher than V _th1 is applied as the threshold bias in FIG. In the area I indicated by the dotted pattern, the sleeve returns under the force of this area.

On the other hand, if the toner tribo is low, the threshold value is low. If it is assumed that the toner jumps out of the drum when a difference of 125 V is reached, the force of the region II (region indicated by oblique lines) higher than the potential indicated by V _th2 In response, he returns to the sleeve. In the drawing, the area II indicated only by oblique lines is larger than the area I indicated by a dotted pattern in terms of area. Therefore, in the case of a low tribo, the toner which should have developed _VH is easily pulled back from the drum again, and
Even when the y bias is used, the uniformity of the halftone cannot be made sufficient.

Here, if the duty is further applied to improve the uniformity of the halftone and the bias is applied as shown by the broken line in FIG. 10, the area of the hatched region III indicated by the dotted line can be reduced. Although the uniformity is improved, it becomes more difficult to obtain a density for the reasons described above.

When the low tribo toner is used,
Even when the duty bias is used, the uniformity of the density and the halftone is insufficient. At this time, the rectangular wave is more advantageous for the density, but the uniformity of the halftone is further deteriorated. On the other hand, in order to achieve the uniformity of the halftone, it is preferable to use a more remarkable Duty bias, but there is a problem that the density is hardly obtained.

Accordingly, an object of the first invention of the present application is to provide an image forming apparatus that can achieve both good density and halftone uniformity and obtain good image quality even when developing low tribo toner. To provide.

Further, the object of the second invention according to the present application is to
In particular, it is an object of the present invention to provide an image forming apparatus capable of obtaining an image excellent in density and uniformity.

Further, the object of the third invention according to the present application is to secure uniformity by shortening the halftone development time, increase the density by increasing the development time of the dark potential portion,
Another object of the present invention is to provide an image forming apparatus capable of realizing the maintenance of the uniformity of the density by shortening the retraction time.

The object of the fourth invention according to the present application is as follows.
An object of the present invention is to provide an image forming apparatus capable of securing a dark potential portion density and maintaining a halftone portion uniformity with a simple circuit configuration.

Still another object of the fifth invention of the present application is to provide an image forming apparatus capable of securing the density of the dark potential portion and maintaining the uniformity of the halftone portion with a simpler circuit configuration. It is in.

The object of the sixth invention according to the present application is to
It is an object of the present invention to provide an image forming apparatus capable of sufficiently maintaining uniformity in a density region in a halftone image in which even a relatively small amount of unevenness is recognized as unsightly.

It is a further object of the present invention to provide an image forming apparatus capable of obtaining sufficient density and uniformity even in various use environments having different amounts of charge of the developer. is there.

The object of the eighth invention according to the present application is to
An object of the present invention is to provide an image forming apparatus capable of sufficiently obtaining density and uniformity even when a small peak-to-peak voltage is used.

Further, according to the ninth aspect of the present invention, even with a low tribo toner, it is possible to obtain sufficient density and uniformity, and to form an image capable of obtaining a good image without fog. It is intended to provide a device.

[0040]

According to a first aspect of the present invention, there is provided an electrostatic latent image carrier on which an electrostatic latent image is formed, and a developer carrier for carrying a developer. And applying a predetermined developing bias to the developer carrier to cause the developer carried on the developer carrier to adhere to the electrostatic latent image on the electrostatic latent image carrier to form a visible image. In the image forming apparatus, the developing bias applied to the developer carrier is an alternating voltage, and the developer is applied in a direction from the developer carrier to the latent image carrier in one cycle of the alternating voltage. The period required for the transition from the peak of the skip voltage in the moving direction to the peak of the return voltage in the direction of moving the developer from the latent image carrier toward the developer carrier is from the peak of the return voltage. Required for shifting to the peak of the skip voltage That longer than the period, the skip voltage, low density portion and the high density portion period whose values may contribute to the development of both of the image formed is alternately a voltage one period of 50%
The entire period in which the value is less than and which can contribute to the development of the high density portion is achieved by using a developing bias waveform set so as to be 50% or more of one cycle of the alternating voltage.

According to the second aspect of the present invention,
The object is that in the first invention, the skip voltage is:
The period during which a value that can contribute to the development of both the low-density portion and the high-density portion of the formed image is less than 45% of one cycle of the alternating voltage, and the entire value that can contribute to the development of the high-density portion is The period is achieved by using a developing bias waveform set so as to be 55% or more of one cycle of the alternating voltage.

Further, according to the third invention of the present application, the object is that in the first invention or the second invention, the developer is transferred from the developer carrier to the latent image carrier within one cycle of the alternating voltage. The bias displacement per unit time from the peak of the skip voltage in the direction of moving the developer in the direction toward the peak of the return voltage in the direction of moving the developer in the direction from the latent image carrier to the developer carrier is: , Is achieved by being steep in the beginning and gradually becoming gentler.

According to the fourth aspect of the present invention,
The object of the first or second aspect of the present invention is that, in one cycle of the alternating voltage, the peak of the return voltage in the direction in which the developer moves in the direction from the latent image carrier to the developer carrier is determined. The bias displacement per unit time up to the peak of the skip voltage in the direction in which the developer is moved in the direction from the developer carrier to the latent image carrier is achieved by being substantially constant.

Further, according to a fifth aspect of the present invention, the object is any of the first to third aspects, wherein the latent image is transferred from the developer carrying member within one cycle of the alternating voltage. Bias per unit time from the peak of the skip voltage in the direction of moving the developer toward the carrier to the peak of the return voltage in the direction of moving the developer from the latent image carrier to the developer carrier Is achieved by changing approximately exponentially.

According to the sixth aspect of the present invention,
The object of the present invention is that, in any one of the first to fifth inventions, the low-density portion means that the image density to be formed is 0.
8 or less.

Further, according to a seventh aspect of the present invention, the object is to provide the image forming apparatus according to any one of the first to sixth aspects, wherein the developer carried on the developer carrying member to form an image is provided. The average charge amount on the developer carrying member is −15 μC /
g to 15 μC / g.

According to the eighth aspect of the present invention,
The above object is achieved by any one of the first to seventh inventions, wherein the developer is a one-component developer.

Further, according to a ninth aspect of the present invention, the above object is achieved by any one of the first to eighth aspects, wherein the developer is a magnetic developer.

[0049]

According to the first aspect of the present invention, the period t _H-slope in which the skip voltage takes a value that can contribute to the development of both the high density portion and the low density portion is 50% of one cycle of the alternating voltage. Even if the charge amount of the developer is low overall, the low-density portion is developed by the developer having a higher charge amount, and the adhesive force of the developer to the latent image carrier increases. The developed developer image is less likely to be disturbed, and sufficient uniformity of the density of the low density portion is maintained. On the other hand, in the high-density portion, the amount of the low-charged amount of the developer tends to decrease and the image density tends to decrease. However, the developer adheres to the latent image carrier within one cycle of the alternating voltage. The transition period from the peak of the skip voltage to the peak of the return voltage for returning the developer to the developer carrier is longer than the transition period from the peak of the return voltage to the peak of the skip voltage, and the period t _H-slope Since the entire period t _D-slope that takes a value that can contribute to the development of the high-density area is longer than that of the high-density area, the bias voltage reaches the same potential as that of the low-density area and then becomes the same potential as the high-density area. The time required to reach this is longer than before, and in this period, a voltage that is a weak electric field but can contribute to the development of the high density portion is applied. Therefore, unless a reverse bias is applied,
Since the developer continues to fly while maintaining its speed, even a developer with a short distance that can fly in a certain time, such as a developer with a low charge amount, can reach the latent image carrier, and if the density is improved, Conceivable. Further, as described above, the transition period from the peak of the skip voltage to the peak of the return voltage is longer than the transition period from the peak of the return voltage to the peak of the skip voltage, and the period t _D-slope is 50% or more.
The retraction time in the high-density portion is less than 50%, and it is considered that the developer is hard to be pulled back and a sufficient density can be obtained. In addition, since the return voltage affecting the high-density portion also contributes to the retraction of the developer in the low-density portion, the retraction time is short even when the charge amount of the developer is low, and the developer is not returned. As described above, the period t _H-slope is less than 50% as described above, and the uniformity of the concentration in the low concentration portion is sufficiently maintained.

According to the second aspect of the present invention,
In the first aspect, the period t of the skip voltage is used.
_Since the developing bias waveform is set so that _H-slope is less than 45% of one cycle of the alternating voltage and the period t _D-slope is 55% or more of one cycle of the alternating voltage, the bias voltage is lower than the low density part. A sufficient period is taken from when the potential reaches the same potential to reach the same potential as the high-concentration portion, and the concentration in the high-concentration portion is sufficiently increased. The retraction time is also shortened, and the uniformity of the concentration in the low concentration portion is sufficiently maintained.

Further, according to the third invention of the present application, in the first invention or the second invention, the displacement of the bias per unit time from the peak of the skip voltage to the peak of the return voltage is as follows. , Initially steep,
As the temperature gradually decreases, uniformity of low-density areas is maintained by selectively flying high-charged developer during steep periods, and high-density is maintained by maintaining forward bias during gentle periods. Parts, and maintain uniformity of the low concentration parts by shortening the retraction time.

According to the fourth aspect of the present invention,
In the first invention or the second invention, the bias displacement per unit time from the peak of the return voltage to the peak of the skipping voltage is made substantially constant. To secure the concentration and maintain the uniformity of the low concentration part.

Further, according to a fifth aspect of the present invention, in any one of the first to third aspects, the bias per unit time from the peak of the skip voltage to the peak of the return voltage is provided. Is changed approximately exponentially, and with a simpler circuit configuration, a sufficient difference between the period t _H-slope and the period t _D-slope is ensured to increase the concentration of the high concentration portion. And maintain the uniformity of the concentration in the low concentration part.

According to the sixth invention of the present application,
In any one of the first to fifth inventions,
The low-density portion is a density region in which the formed image density is 0.8 or less, and it is recognized that even a slight unevenness is very unsightly, but as described above, uniformity is maintained and a good image is obtained.

Further, according to a seventh aspect of the present invention, in any one of the first to sixth aspects, the developer carrier of the developer carried on the developer carrier to form an image is provided. The above average charge amount is −15 μC / g to 15 μC
/ G, so that even in various use environments, the density of the high-density portion is increased and the uniformity of the density of the low-density portion is maintained.

According to the eighth aspect of the present invention,
The object of the present invention is that in any one of the first to seventh inventions, since the developer is a one-component developer, the developer layer on the developer carrier is formed thin, and the The gap between the agent carriers can be reduced, and the small peak-to-peak voltage secures the concentration in the high concentration portion and maintains the uniformity in the low concentration portion.

Further, according to a ninth aspect of the present invention, the above object is achieved in any one of the first to eighth aspects because the developer is a magnetic developer. Since the developer is also carried on the developer carrier by the magnetic field generating means disposed inside the developer carrier, development with a developer having a relatively low charge amount is performed, and the high-density portion is secured in density and the low-density portion is developed. A good image without fog is obtained while maintaining uniformity.

[0058]

Embodiments of the present invention will be described below with reference to the accompanying drawings.

(First Embodiment) First, a first embodiment of the present invention will be described with reference to FIGS. FIG. 1 is a diagram that best illustrates the features of the present invention, and shows a developing bias waveform that balances image density and uniformity of halftone even when developing a low tribo toner. Hereinafter, it is referred to as slope bias.

The potential relationship between V _D , V _H , and V _L is shown in the same manner as in FIG. 9, and the horizontal axis represents time to explain the change in the AC developing bias. It will be described in chronological order.

In this embodiment, the developing device used in the conventional example is used. The toner tribo on the sleeve usually has an average tribo of -10 .mu.C / g. Therefore, when the developing device was used for a long time and the agent was deteriorated to some extent and the tribo was lowered to about -8 μC / g, the developing device was left unused for 2 days and then started to be used. Yes,
The situation will be described as being about −6 μC / g.

First, the developing bias falls in potential from point a and shifts to point b. Here, in order to develop V _D or V _H , V _D-slope or V in FIG.
_It is necessary to set so that the potential difference of _H-slope is large and the toner on the sleeve is allowed to jump out of the mirror image force.

Next, the developing bias from the point b continues between the minimum value V _min of the section c of the AC component, but the length of the section c is the same as the Duty bias shown in the image in the conventional example, the total period 35%.

The operation up to the point where the developing bias rises from the point d is the same as the duty bias. However, at the point e where the bias rises to _{VH, the} developing bias is controlled so as to slowly rise by changing the rising speed. In the present embodiment, this bias is referred to as a slope bias because of its slowly rising state.

[0065] developing bias since e point, it becomes V _D and the same potential gently at the rising point f, until the time here is set to be one cycle of 50% or more. Here, t _D-slope in FIG. 1 is set to 55%. And a maximum value V _max at the point g, falls similarly to again point a continuously during a period h. Although a transformer for generating such a developing bias will not be described in detail, a clock may be generated and an increase in voltage between clocks may be controlled. The present inventor has found that the use of the above-mentioned bias improves the image density and the uniformity of halftone even when developing a low tribo toner, and the reason will be described below.

[0066] First, the image density is considered to develop the V _D. Compared to FIG. 10, it can be seen that the time t _D-slope for developing V _D is longer than in the case of the duty bias. For this reason, it is considered that a toner, such as a low tribo toner, which receives a weak force from the developing electric field, that is, a toner having a short flight distance in a given time can reach the drum, which leads to an improvement in density. For example, although only a conceptual diagram is shown in FIG. 2, the flying distance of the toner flying under a constant electric field is a quadratic function of time, and the horizontal axis represents time and the vertical axis represents distance, as shown by a solid line in the figure. Even if the electric field is cut off at the point A, the toner keeps its speed and continues to fly unless reverse bias is applied (actually, the speed is reduced due to air resistance). extend.

For example, suppose that the time until the point A is t _D-Duty . t _D-Duty is 35% of one cycle, and t
_{Since D-slope} is 55% of one cycle, t _D-slope is t _D-Duty
About 1.5 times longer. Assuming that a point 1.5 times longer than point A is point B, the flying distance of the toner at this time is about twice as long as the flying distance to point A in the figure.

[0068] Of course, in practice, there is also a collision between the toner particles that air resistance and flight experienced by the toner, also the bias takes bias is on the side of developing the V _D is the weak electric field from point e to point f Because these are not taken into account, the extremely simple calculated value cannot be said to be correct. However The idea, the image density of V _D is estimated to be improved in order to toner that can arrive at V _D in such a reason increases.

According to the study of the present inventor, t
_{When the D-slope} was set to 50% or more of one cycle, the effect of improving the image density was remarkable.

Although the reason for this is not fully understood, in particular, in the development of the low tribo toner according to the present invention, whether or not the toner having low tribo and low responsiveness to the electric field is developed largely depends on the length of the development time. Therefore, if the development time is less than half of one cycle, it is considered that the retraction time is more than half, and the retraction is easy and sufficient density cannot be obtained.

For the above reasons, it is considered that this bias in which t _{D-slope is set} to 50% or more has a sufficient effect of improving the density.

Next, the improvement of the halftone uniformity will be described. Considering first the toner developed in the halftone portion V _H, so to point e is the same as Duty bias of Figure 10, the toner is V _H of approximately the same triboelectric
It is considered that the toner has arrived and adhered. In the present embodiment, these toners are considered to have a relatively high tribo in the tribo distribution such that the average tribo is -6 .mu.C / g, but it is considered that the toner is still less than -10 .mu.C / g. .

The toner having a slightly lower triboelectric charge is V _H
After the development, the developing bias turns to the pull-back side, but the potential difference from V _H is 125 V as described in the conventional example.
It is assumed that the toner adhering to V _H starts to fly backward when.

At this time, if the pull-back strength is a duty bias, the toner is pulled back according to the size of the area II indicated by oblique lines in FIG.
In the case of e-bias, it is pulled back according to the size of the area of the region IV indicated by oblique lines in FIG. Obviously FIG.
The shaded area in FIG. 1 is better than the shaded area II in FIG.
Since the IV is small, it is considered that the use of the slope bias can weaken the retraction of the toner, thereby improving the halftone uniformity.

Originally, the duty bias is a bias capable of developing a toner having a higher tribo than the average tribo with respect to V _H. However, especially in a situation where the average tribo is low, even when the duty bias is used, the duty bias may be reduced. The tribo of toner that can be developed in _H must be low. By using the slope bias even in this situation, the average developing the toner triboelectric slightly higher than triboelectricity with respect to V _H, and sufficient not toner high triboelectric charge to the developing bias with respect to these toners By providing a slope to the toner, the pullback of the toner can be weakened, and the uniformity of halftone can be improved.

In the present invention, the halftone portion means that the formed image density is 0.8 or less.

When the halftone uniformity is a problem,
The higher the concentration, the more the uniformity is a little worse,
On the contrary, in a thin area, it is recognized that some unevenness is very noticeable.

In particular, in order to uniformly reproduce a highlight portion having a density of 0.2 to 0.4, it is considered preferable to design the developing bias with the potential for reproducing such a density being VH. Can be

[0079] While albeit at slightly by the time developing method sometimes V _H is equal to or less than V _DC, basically V _H is above V _DC. With respect to such a design problem, the present invention may be applied after appropriately determining V _H for each image forming apparatus.

As described above, the halftone V _H
While the time for developing t _H-slope is less than 50% of one period of the rise time t _D-slope for developing the V _D is displaced to the side to pull back the toner in the developing bias so that one period of more than 50% By using a slope bias which is sometimes controlled with a gentle inclination, a sufficient image density and uniformity of halftone can be achieved at the same time even when developing a low tribo toner.

Here, if the differences from the conventional developing bias are clarified, sine, triangular, saw and square waves are shown in FIGS. 3 (a), (b), (c) and (d), respectively. time t _H way, both of developing the halftone V _H
And the time t _D during which V _D is developed is 50% or more of one cycle, while the Duty wave has t _H , t _D as shown in FIG.
_D is less than 50%, which is different from the slope bias. (This relationship does not change significantly even when the bias is slightly reduced.) The bias disclosed in Japanese Patent Application No. 4-283816 is similar to the present bias at first glance, but the bias gradually rises. In this case, the change in the bias toward the development promoting side is defined. In this case, the change in the bias toward the development inhibiting side in which the toner is pulled back is defined, which is fundamentally different.

Hereinafter, more detailed experimental conditions will be described. As the developing device, the developing device described with reference to FIG. 8 is used.

An experiment was conducted with an average tribo of about −6 μC / g of the toner. The amount of triboelectric charge of the toner depends on the formulation of the toner, the particle size, the addition of a charge control agent, the surface property of the developing sleeve 2, It depends on the distance between the regulating member 4 and the developing sleeve 2, the strength of toner packing (toner density), and the like. In this embodiment, a styrene acrylic magnetic toner having an average particle diameter of 8 μm was used, and silica was externally added to impart fluidity.

The developing sleeve 2 was used by subjecting a surface of a sleeve base made of non-magnetic stainless steel to blasting with about 400 glass beads. The regulating member 4 was made of magnetic stainless steel and had a thickness of 1.2 mm. The gap between the regulating member 4 and the developing sleeve 2 was 200 μm. At this time, initially, the triboelectric charge amount of the toner was about -10 μC / g, and the applied amount was about 0.8 to 1.2 mg / cm ² .

After the endurance of the developing device, the tribo is -8 μC
/ G, which is about −6 μC / g when used under standing conditions.

The magnet roller 3 is magnetized to four poles, and the developing pole located in the developing area faces the photosensitive drum 1 and has a developing sleeve 2 with a magnetic force of about 800 to 1200 gauss.
The magnetic toner above is raised. In this state, a jumping phenomenon occurs due to the action of the alternating electric field described above. The gap at the closest position between the developing sleeve 2 and the photosensitive drum 1 is 2
The developing sleeve 2 was rotated in the same direction as the photosensitive drum 1 and its peripheral speed was about 1.5 times the peripheral speed of the photosensitive drum 1.

[0087] developing bias this time was V _PP = 1400 V, and frequency 2.2KHz with those described in FIG. (Integrated DC value is not 100V but about 100V ~ 15
It is about 0V. Note that the actual waveform used is about 20 to 40 .mu.sec for the transition from point a to point b and point d to point e in FIG. 1, so that the actual waveform is shown in FIG.

When image formation was performed under these conditions, the image density was about 0. 4 compared to the duty bias shown in the figure.
10 higher 1.25 was obtained. Also, the halftone uniformity was better than the duty bias.

As another experimental example, an example in which a latent image is formed under the same conditions as in FIG. 1 and a non-magnetic toner (one-component non-magnetic developer) is used using a developing device having the configuration shown in FIG. It is described below.

The developing device 5 'shown in FIG. 5 contains a non-magnetic toner therein, agitates the toner with a stirring blade 7 and supplies the toner to the coating roller 6, and the coating roller 6 applies the toner to the developing sleeve 2. 'Is configured to supply. The toner is charged by friction with the developing sleeve 2 ′ and the elastic blade 4 ′ rotating in the direction of the arrow, and the charged toner rubs off the elastic blade 4 ′ while adhering to the developing sleeve 2 ′ by the mirroring force, and the developing area Transported to

The toner remaining in the developing area without being used for development is returned into the developing device 5 ′ by rotation of the developing sleeve 2 ′, and is scraped off by friction with the coating roller 6.
The charge polarity of the toner is negative as in the case of the first embodiment.

The elastic blade 4 'is made of silicone rubber, and the elastic blade 4' is applied to the developing sleeve 2 'with a linear pressure of 15 to
Counter contact was performed at 20 g / cm. As the toner of the developing device 5 ', a styrene acrylic nonmagnetic toner colored with carbon black and having an average particle diameter of 8 [mu] m was used, and silica was externally added to impart fluidity.

At this time, the triboelectric charge amount of the toner in the initial state is about −15 μC / g, and the applied amount is about 0.4 to 0.8 mg / g.
cm ² . Although this toner charge amount is higher than the charge amount of the magnetic toner of the first embodiment of −10 μC / g,
This is not a problem because the magnetic toner is attracted to the developing sleeve even by the magnetic force, but the non-magnetic toner of this embodiment adheres to the developing sleeve only by the mirror force, so the charge amount is increased and the mirror force is increased. If the height is not strong, it is easy to fly on the photosensitive drum, and fogging occurs.

After such a developing device is durable under high humidity,
When used in a standing state, the tribo was about -10 μC / g or slightly lower.

The developing device is provided with a gap between the developing sleeve 2 'and the photosensitive drum 1 of about 180 .mu.m with respect to the photosensitive drum. 2 'rotated. The developing bias shown in FIG. 1 is used.

Compared to the previous experimental example, the triboelectricity is higher, so that the mirroring force on the sleeve is large and it is difficult to separate from the developing sleeve. Therefore, the SD distance is reduced to increase the substantial electric field strength. At the same time, the retraction is slightly increased, so that no fog occurs even with non-magnetic toner. Of course, it is better to adjust the bias frequency and V _DC for fog.

When image formation was performed under such conditions, the image density was reduced by about 0.0 as compared with the duty bias.
5 higher 1.30 was obtained. Also, the uniformity of halftone was improved. However, since the tribo was higher than that of the previous experimental example, the effect was slightly smaller than that of the previous experimental example.

Although the above description has been made using one-component development as a development method, the present invention is also applicable to the case where an alternate bias is applied to well-known two-component development.

At this time, since the coating of the developer on the developing sleeve becomes thick, the SD gap is set to 500 to 100.
It is well known that the thickness is preferably 0 μm, and the peak-to-peak voltage must be increased accordingly.

Even when two-component development was used, the effect in a low tribo condition was the same as in the other experimental examples.

(Second Embodiment) Next, a second embodiment of the present invention will be described with reference to FIG. The same parts as those in the first embodiment are denoted by the same reference numerals, and description thereof is omitted.

[0102] developing method in this embodiment with reference to various developing method used in the first embodiment, the developing bias is used as shown in FIG. 5, the frequency at V _PP is 1400 V 2.3
KHz and _VDC 150V.

The present embodiment differs from the first embodiment in that the rise is substantially constant, so that the circuit configuration of the transformer is simplified.

However, t _D is 50% of one cycle, and t _H is 45%.
%, The difference between the two is small, and the effect on density and halftone uniformity is slightly inferior to the first embodiment. Further, since the time of the development promotion side V _min is as short as 45 μsec, it is difficult to obtain the density. For example, it is necessary to make the gap between the developing device and the drum smaller than that in the first embodiment.

At that time, it was confirmed that the effect of this embodiment is the same as that of the first embodiment.

(Third Embodiment) Next, a third embodiment of the present invention will be described with reference to FIG. The same parts as those in the first embodiment are denoted by the same reference numerals, and description thereof is omitted.

In this embodiment, the developing bias shown in FIG. 6 is used, V _{PP is} 1400 V, and the frequency is 2.2 K.
Hz and _VDC 150V.

Since the rise of the bias is made to rise by the exp function, the circuit configuration of the transformer can be formed by a CR circuit. Therefore, the configuration is easier than in the previous embodiment.

Further, t _D is 53% of one cycle, and t _H is 38%.
And substantially the same effects as in the first embodiment can be expected.

Actually, substantially the same effect can be obtained by using the bias shown in FIG. 7 instead of the bias shown in FIG. 4 in the first embodiment.

Although the normal development has been described with the polarity of the toner being negative, the reversal development may be used.

Further, the present invention can be applied to the development to the toner of the positive polarity, and in this case, the one obtained by inverting the bias waveforms in FIGS. 4, 6 and 7 may be used.

Although the photosensitive drum has been described with reference to the α-Si drum, it is needless to say that the photosensitive drum can be applied to a well-known OPC drum. In this case, the charging potential can be made relatively higher than that of α-Si. since, made larger than the present embodiment the difference between V _D and V _H.

Therefore, there is a merit that the latitude of setting the bias waveform is widened.

[0115]

As described above, the first embodiment according to the present application is described.
According to the invention, when the developing bias is displaced to the voltage on the side where the developer is pulled back, the bias is given a gentle slope, and the developing period for the low-density portion is less than 50% of one cycle while the high-density portion is developed. Is set to be 50% or more of one cycle, it is possible to weaken the return to the developer attached to the low-density portion and to promote the arrival of the low-charge amount developer to the high-density portion. It is possible to achieve both a sufficient image density and uniformity of the low density portion.

According to the second invention of the present application,
In the first aspect, the developing bias waveform is set so that the period for developing the low-density portion is less than 45% of one cycle and the period for developing the high-density portion is 55% or more of one cycle. In addition, the period from when the bias voltage reaches the same potential as the low-concentration portion to when the bias voltage reaches the same potential as the high-concentration portion can be sufficiently long, and the concentration in the high-concentration portion can be sufficiently increased. Also, the pullback time can be shortened,
The uniformity of the concentration in the low concentration part can be sufficiently maintained.

Further, according to the third invention of the present application, in the first invention or the second invention, the displacement of the bias per unit time from the peak of the skip voltage to the peak of the return voltage is an initial value. Are steep, and gradually become gradual. In the steep period, the uniformity of the low-density portion can be maintained by selectively flying the high-charged amount of the developer. In this case, it is possible to secure the density of the high-density portion by maintaining the forward bias, and to maintain the uniformity of the low-density portion by shortening the pullback time.

Further, according to the fourth invention of the present application,
In the first invention or the second invention, the bias displacement per unit time from the peak of the return voltage to the peak of the skipping voltage is made substantially constant. And the uniformity of the low concentration portion can be maintained.

Further, according to the fifth aspect of the present invention, in any one of the first to third aspects, the bias displacement per unit time from the peak of the skip voltage to the peak of the return voltage. Is approximately exponentially changed, so that with a simpler circuit configuration, it is possible to sufficiently secure the difference between the period for developing the low-density portion and the period for developing the high-density portion, and And the uniformity of the concentration in the low concentration part can be maintained.

According to the sixth invention of the present application,
In any one of the first to fifth inventions,
The low-density portion is a density region in which the formed image density is 0.8 or less and is recognized as being very unsightly even with some unevenness. Thereby, a good image can be obtained.

Further, according to the seventh invention of the present application, in any one of the first to sixth inventions, the developer carrier of the developer carried on the developer carrier to form an image is provided. The above average charge amount is −15 μC / g to 15 μC
/ G, so that even in various use environments, it is possible to maintain an increase in the density of the high-density portion and uniformity of the density of the low-density portion.

Further, according to the eighth invention of the present application,
The object of the present invention is that in any one of the first to seventh inventions, since the developer is a one-component developer, the developer layer on the developer carrier is formed thin, and the The gap between the agent carriers can be reduced, and the small peak-to-peak voltage can ensure the concentration of the high concentration portion and maintain the uniformity of the low concentration portion.

Further, according to a ninth aspect of the present invention, the above object is achieved in any one of the first to eighth aspects because the developer is a magnetic developer. Since the developer is also carried on the developer carrier by the magnetic field generating means disposed inside the developer carrier, development with a developer having a relatively low charge amount is performed, and the high-density portion is secured in density and the low-density portion is developed. A good image without fog can be obtained while maintaining uniformity.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a developing bias used in a first embodiment of the present invention.

FIG. 2 is a diagram illustrating the application time of a developing bias and the flying distance of toner.

FIG. 3 is a diagram illustrating a difference between the present invention and a conventional example.

FIG. 4 is a diagram of an actual waveform of a developing bias used in the first embodiment of the present invention.

FIG. 5 is a diagram showing a configuration of a main part of the image forming apparatus used in the first embodiment of the present invention.

FIG. 6 is a diagram of an actual waveform of a developing bias used in a second embodiment of the present invention.

FIG. 7 is a diagram of an actual waveform of a developing bias used in a third embodiment of the present invention.

FIG. 8 is a diagram of a main part of a conventional image forming apparatus.

FIG. 9 is a diagram illustrating a developing bias of a conventional example.

FIG. 10 is a diagram showing another developing bias in the conventional example.

[Explanation of symbols]

_{Reference Signs List} 1 photosensitive drum (electrostatic latent image carrier) 2 developing sleeve (developer carrier) t _D-slope skipping voltage The entire period in which the value can contribute to development in a high density part t _H-slope skipping voltage is high in one cycle of the density portion and the low density portion both periods V _D dark potential whose values may contribute to the development of (high-density portion of the potential) V _H halftone potential (low density portion potential) V _max alternating voltage moving the developer in a direction toward the latent image bearing member from the developer carrying member in one cycle of the peak V _min alternating voltage in the direction of returning the voltage to move the developer toward the developer carrying member from the latent image bearing member The peak of the skip voltage in the direction

Claims (9)

(57) [Claims] 1. An image forming apparatus comprising: an electrostatic latent image carrier on which an electrostatic latent image is formed; and a developer carrier for carrying a developer, wherein a predetermined developing bias is applied to the developer carrier. In an image forming apparatus for forming a visible image by attaching a developer carried on the developer carrier to an electrostatic latent image on the electrostatic latent image carrier, the developing bias applied to the developer carrier is alternately applied. From the latent image carrier to the developer carrier from a peak of a skip voltage in a direction of moving the developer in a direction from the developer carrier to the latent image carrier in one cycle of the alternating voltage. The time required for the transition to the peak of the return voltage in the direction of moving the developer toward the body,
A period longer than a period required for the transition from the peak of the return voltage to the peak of the skipping voltage, in which the skipping voltage takes a value that can contribute to the development of both the low density portion and the high density portion of the image to be formed. Is characterized by using a developing bias waveform set so that it is less than 50% of one cycle of the alternating voltage and the entire period of time which can contribute to the development of the high density portion is 50% or more of one cycle of the alternating voltage. Image forming apparatus. 2. The period in which the skipping voltage takes a value that can contribute to the development of both the low-density portion and the high-density portion of the image to be formed is less than 45% of one cycle of the alternating voltage. The total period during which the value that can contribute to the
2. The image forming apparatus according to claim 1, wherein a developing bias waveform set to be 5% or more is used. 3. A direction from a latent image carrier to a developer carrier from a peak of a skip voltage in a direction of moving the developer from the developer carrier to the latent image carrier in one cycle of the alternating voltage. 3. The image forming apparatus according to claim 1, wherein a displacement per unit time until a return voltage peaks in a direction in which the developer is moved is steep at an initial stage and gradually decreases. 4. A direction from a developer carrier to a latent image carrier from a peak of a return voltage in a direction of moving the developer from the latent image carrier to the developer carrier within one cycle of the alternating voltage. 3. The image forming apparatus according to claim 1, wherein a displacement per unit time until a peak of a skip voltage in a direction in which the developer is moved is substantially constant. 5. A direction from a latent image carrier to a developer carrier from a peak of a skip voltage in a direction of moving the developer from the developer carrier to the latent image carrier in one cycle of the alternating voltage. 4. The image forming apparatus according to claim 1, wherein a displacement per unit time until a peak of the return voltage in a direction in which the developer is moved changes substantially exponentially. 6. The image forming apparatus according to claim 1, wherein the low density portion has an image density of 0.8 or less. 7. An average charge amount of the developer carried on the developer carrier to form an image formed on the developer carrier is −15 μC / g.
The image forming apparatus according to any one of claims 1 to 6, wherein the pressure is 15 to 15 C / g. 8. The image forming apparatus according to claim 1, wherein the developer is a one-component developer. 9. The image forming apparatus according to claim 1, wherein the developer is a magnetic developer.