JP2001343821A - Developing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent fogging and blotting caused by the break, the thickening or the scattering of a thin line from occurring. SOLUTION: In this developing device, a latent image on a latent image carrier 1 is developed by applying AC superposed bias obtained by superposing AC on DC bias to a developer carrier 2 and forming a developer thin layer on the developer carrier 2 by a layer forming member 4. Since the maximum value Vmax of the AC superposed bias 5 and 6 being applied to the developer carrier 2 is set to be lower than the electrification potential V0 of the latent image carrier 1, developer is prevented from adhering to a non-image part and the occurrence of the fogging is restrained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a latent image formed by applying an alternating current superimposed bias obtained by superimposing an alternating current to a direct current bias to a developer carrier and forming a thin layer of developer on the developer carrier by a layer forming member. The present invention relates to a developing device that develops a latent image on a carrier.

[0002]

2. Description of the Related Art FIG. 9 is a view for explaining the magnitude of an AC superimposed bias applied to a developer carrier of a developing device. Conventionally, in a developing device, a DC bias is applied to the developer carrier in order to form a thin developer layer on the developer carrier using a layer forming member and move and adhere the developer to an image portion of the latent image carrier. Although the voltage is applied, an alternating current is superimposed to vibrate the developer to facilitate the movement from above the developer carrier to the image area (see, for example, Japanese Patent Publication No. 58-32375). This developing device is a so-called non-contact jumping development, in which a gap is provided between the developer carrier and the latent image carrier, and the developer is ejected from the developer carrier to the image portion of the latent image carrier. amplitude (V _max -V _min) is set to a size that exceeds the width of the non-image portion potential V ₀ which an image portion potential V _oN of the image bearing member as shown in FIG. This is because the bias threshold value sufficient to cause the developer to adhere to the image portion of the latent image carrier becomes higher than the potential of the non-image portion, and conversely, the threshold value is sufficient to remove the developer adhered to the non-image portion. This is because the bias threshold becomes lower than the potential of the image section.

[0003]

However, in the conventional developing device, since the developer adheres to the non-image portion of the latent image carrier and the peeling cannot be sufficiently realized, the fog in which the developer adheres to the non-image portion is not achieved. Occurs, or thin lines are interrupted in halftone images,
There is a problem that bleeding due to fattening and scattering occurs and image quality is deteriorated. In particular, in the formation of a color image, when such fogging and thin lines are interrupted, thickened, and bleeding due to scattering occur, each color material is superimposed and output, so that a satisfactory color cannot be obtained in halftone. is there. In order to reduce these problems, it is necessary to control the gap between the developer carrier and the latent image carrier with high accuracy.

[0004]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and it is an object of the present invention to prevent the occurrence of fogging and thin lines due to interruption, fattening, and scattering.

For this purpose, the present invention relates to a latent image carrier by applying an AC superimposed bias obtained by superimposing an alternating current to a direct current bias to a developer carrier and forming a thin layer of developer on the developer carrier by a layer forming member. Wherein the maximum value of the AC superposition bias applied to the developer carrier is set lower than the charging potential of the latent image carrier.

Further, the developer carrier is in contact with the latent image carrier, and the DC bias is set lower than an intermediate potential between the charging potential of the latent image carrier and the exposure potential. The minimum value of the AC superposition bias is set lower than the exposure potential of the latent image carrier.

[0007]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram for explaining an embodiment of a developing device according to the present invention, FIG. 2 is a diagram for explaining an AC superimposed bias of the developing device according to the present invention, and FIG. FIG. 4 is a diagram for explaining the relationship between the bias and the surface potential of the latent image carrier. FIG. 4 shows the development γ, the voltage V _PP between the maximum value and the minimum value of the AC superposition bias, and the threshold value Vth.
FIG. 4 is a diagram for explaining the relationship of FIG. In the drawing, 1 is a latent image carrier, 2 is a developer carrier, 3 is a supply member, 4 is a layer forming member, 5 is a DC bias power source, and 6 is an AC bias power source.

[0008] In FIG. 1, the latent image bearing member 1, for example, contact with the charging roller non-image portion surface potential by direct current (not shown) bias V _a is applied (non-image portion potential to be rotated, charging potential) V ₀ is set, in which an electrostatic latent image is formed by the writing is performed by the exposure data image portion potential (exposure potential) V _ON. The developer carrier 2 is for affixing a developer to an electrostatic latent image written on the surface of the latent image carrier 1 by exposure data in contact with the latent image carrier 1 and developing the electrostatic latent image. The supply member 3 is arranged so as to rotate in contact with the developer carrier 2, and supplies the developer to the developer carrier 2. The layer forming member 4 has a developer thin layer on the developer carrier 2. It is an elastic regulating member that forms a layer. The DC bias power supply 5 and the AC bias power supply 6
And applies an AC superimposed bias supply member 3, a DC bias and V _dc, this maximum value of the AC superimposed bias alternating current is superimposed, the minimum value V _max, is represented by V _min, the present invention the maximum value V _max of the AC superimposed bias as shown in FIG. 2 by adjusting the DC bias V _dc or ac bias maximum and minimum values among the voltage V _PP of (= V _max -V _min) is the latent image This is set so as to be lower than the charging potential V ₀ of the carrier 1.

For example, if the DC bias V _dc is set to a value exactly intermediate between the charging potential V ₀ of the latent image carrier 1 and the exposure potential V _ON ,
As shown in FIG. 2A, the voltage V _pp (= V _max −V _min ) between the maximum value and the minimum value of the AC bias is equal to the width of the charging potential V ₀ of the latent image carrier 1 and the exposure potential V _ON ( = V ₀ −V _ON ). When the DC bias V _dc is further reduced, the result becomes as shown in FIG. In this case, the voltage V _pp between the maximum value and the minimum value of the AC bias is the latent image carrier 1
Can be increased within a range that does not exceed the charging potential V ₀ .

As an example of each of the above-mentioned biases, when a DC bias V _a = -1200 V applied to the charging roller is set, the charging potential V ₀ of the latent image carrier 1 is −600.
V, the maximum value V _max of the developing bias (AC superimposed bias)
= −500 V or more, the exposure potential V _ON of the latent image carrier 1 −30 V, and the minimum value of the developing bias V _min = −10.
It is set to 0V or less. Then, the maximum value-minimum value voltage V _PP = 400 V, the frequency f = 2 kHz, and the DC bias V
It is controlled to _dc = -300V. As an AC superimposed bias source applied to the developer carrier 2, for example, a square wave having a duty of 50% can be realized with a simple power supply circuit at relatively low cost, but other than this, a trapezoidal wave, a triangular wave, a sine wave, etc. It can be used, and the duty is arbitrarily variable.

As described above, the maximum value V of the AC superposition bias is
_{When max} is set to be lower than the non-image portion charging potential V ₀ of the latent image carrier 1, when the minimum value V _min of the AC superimposed bias exceeds the exposure potential V _ON as shown in FIG.
In the image area, the developer adheres to the latent image carrier 1 according to the direction of the electric field, but the direction of the electric field from the latent image carrier 1 to the developer carrier 2 does not change. The phenomenon that the developer peels off is not observed. Further, the developer does not adhere to the non-image portion.

On the other hand, as shown in FIG.
_ONThe minimum value V of the AC superposition bias _minIs low
Indicates the latent image in the image area according to the direction of the electric field of the developer.
Although it adheres to the carrier 1, the exposure potential V_ONAnd AC superimposed bi
Minimum value V of ass_minIn the area where
Is separated from the latent image carrier 1 and returned to the developer carrier 2
In, the developer excessively adhered to the image area is appropriately peeled,
It is possible to reduce density unevenness and image collapse. Non-painting
In the image area, as in the case shown in FIG.
No adhesion occurs.

Maximum value V of AC superimposed bias_maxWithin limits
At the voltage V between the maximum value and the minimum value _PPAnd DC bias
V_dcCan be set arbitrarily.
Either situation occurs, but the developer in the non-image area
Good image formation without fogging
It works.

In the image section, FIG.
In the case shown in (b), there is a difference in the inclination of the development γ, and FIG.
In the setting shown in (b), the peeling of the developer occurs, so that the γ value becomes small. That is, although the development γ fluctuates, it is possible to obtain a good image quality with high gradation and little image unevenness by design. In any case, in the present invention, good image quality without occurrence of background fog can be obtained by eliminating the adhesion of the developer to the non-image portion, and the effect is large in that respect.

The latent image bearing member OPC charge potential V ₀ which is a surface potential meter (TREK, Inc.: Model1344 etc.) can be measured by, varies with the DC bias V _a applied to the charging roller DC bias V _a and the surface thereof FIG. 3 is a graph showing the relationship between the potentials V ₀ and V _ON . As described above, the surface potentials V ₀ and V _ON can be determined by setting the DC bias V _a , and the DC bias V _dc and the voltage V _PP between the maximum value and the minimum value can be determined.
And the DC bias V _a can be set arbitrarily. For example, after detecting the density of the developer on the intermediate transfer medium, the DC bias V _dc or the maximum value / minimum value at which a desired density is obtained can be obtained. set the voltage V _PP, it is possible to set the DC bias V _a relational expression of advance data input V ₀ over V _a depending on their values.

[0016] In FIG. 4, the horizontal axis represents the potential V, the vertical axis represents the deposition density of the developer, development with the DC bias V _dc .gamma.1
On the other hand, the development γ1 ′ due to the AC superimposed bias of the voltage V _PP between the maximum value and the minimum value becomes more sluggish, and the gradation is improved. In addition, when the development γ _min when the minimum value V _min of the AC superposition bias is a DC bias and the development γ _max when the maximum value V _max is a DC bias, when the AC superposition bias of this amplitude is applied, As shown in the drawing, the characteristics fall asleep between them, so that as the amplitude increases, the characteristics fall asleep more. That is, the development γ1 ′ due to the AC superposition bias is the voltage V between the maximum value and the minimum value of the AC superposition bias.
Depending on the size of _PP , the threshold value Vth1 also shifts to Vth1 'accordingly. Here, the threshold values Vth1 and Vth1 'are bias values at which the developer starts to adhere. Therefore, this threshold V
When the bias is set lower than th1 and Vth1 ', the developer does not adhere. When the bias exceeds the thresholds Vth1 and Vth1', the developer adheres, and the degree of the adherence increases as the bias exceeds the threshold. Become. As is apparent from this, in order to eliminate fog and increase the peeling effect, it is necessary to set the bias at least lower than this threshold. In addition, this threshold value is shifted not only by changing the DC bias _Vdc but also by changing the voltage V _PP between the maximum value and the minimum value of the AC superimposed bias.

FIG. 5 is a view showing another embodiment of the developing device according to the present invention, and 7 is a DC bias power supply. In the embodiment shown in FIG. 5, an AC superimposed bias voltage in which the voltages of a DC bias power supply 5 and an AC bias power supply 6 are superimposed is applied to the developer carrier 2, and the DC bias power supply 7 supplies a DC bias to the supply member 3. Voltage is applied. The bias voltage applied to the supply member 3 forms an electric field for supplying the developer to the developer carrier 2.

FIG. 6 is a view showing an example of a schematic overall structure of a developing device according to the present invention, in which 11 is a developing chamber, 12 is a sub-hopper, 13 is a table, 14 is a developing device, 15 is an agitator mechanism, 16 Denotes a toner supply port, and 17 denotes a toner cartridge. In the case of a full-color developing device, Y, M, C,
Although there is a developing device for Bk, FIG. 6 shows only one developing device.

In FIG. 6, the latent image carrier 1 is made of an elastic roller having a photosensitive layer formed on the surface, and the surface is made of another member.
For example, a backup roller that supports the elastic roller from the inside at a position where it contacts the charging device is provided.
The developing device 2 is provided, for example, opposite to the latent image carrier 1, the developing device housing 14 is fixed to the base 13, and the toner supply port 1 from the toner cartridge 17 is provided in the sub hopper 12.
An agitator mechanism 15 for stirring and transporting the developer supplied through the developing chamber 6 to the developing chamber 11, a supply member 3 for supplying the developer transported from the agitator mechanism 15, and a developer supplied to the surface in elastic contact with the supply member 3 And a layer forming member 4 for regulating the thickness of the thin developer layer on the surface of the developer carrier 1.

The developer carrier 2 and the supply member 3 are elastically contacted, rotate in the opposite direction with a difference in peripheral speed, and rub the developer to a predetermined thickness (for example, several hundred μm) on the surface of the developer carrier. The thickness of the developer layer is formed. At this time, the developer is charged to a predetermined polarity by the friction between the developer carrier 2 and the supply member 3, and is further charged by the layer forming member 4.
The thickness is restricted to about 0 μm, and at this time, the layer is charged to the same polarity even by friction with the blade. The developer carrier 2 and the latent image carrier 1 rotate in the forward direction, and contact develop the electrostatic latent image on the latent image carrier while slipping due to a difference in peripheral speed.

For this reason, an AC superimposed bias is applied to the developer carrier 2 to form an image by adhering the developer to the latent image carrier 1 as described above. In order to supply the developer to the developer carrier 2, a bias for forming an electric field is applied. For example, for the supply member 3, the developer carrying member 2 is provided with I _S = −2 μA in the yellow and magenta developing units, and in the cyan developing unit with I _S = −3.
A constant current voltage power supply is connected so that a constant current of I _S = −5 μA flows in the μA black developing device. Further, a voltage is applied to the developer carrier 2 and the supply member 3 only when the latent image on the latent image carrier 1 is developed, and the voltage is not applied during other periods.

Next, the members constituting the developing device will be described in detail with reference to specific examples. First, the developer carrying member is manufactured by forming unevenness on the surface of an aluminum shaft by shot blasting and then performing a nickel electroless plating process. In the formation of unevenness, a shot blast of $ 4
Shot ball pressure of 2k
g / cm ² , a nozzle distance of 30 cm, and a nozzle reciprocated so that shot blasting is performed over the entire shaft of an aluminum shaft rotated at 20 rpm.
Irregularities are formed by performing shot blasting for 0 second. What can be used for shot blasting is not limited to ceramic beads, and glass beads or iron beads such as stainless steel can also be used. When the surface roughness after such shot blasting was measured, the surface roughness Rz
= 7.5 μm and Pc = 230. When the surface was cut and observed at a magnification of 500 to 1000 with an electron microscope (SEM), the surface unevenness was a crater-like and uniform uneven surface was formed.

The shot-blasted aluminum shaft was subjected to nickel electroless plating to form a plating layer, thereby completing a developer carrier. Plating was performed with a plating thickness of 3 to 5 μm, and after washing, degreasing, and pretreatment, nickel electroless plating was performed, followed by cleaning and drying. When the surface roughness of the developer carrier after such plating treatment was measured, Rz = 7 μm
m, Pc = 220. Observation of the surface at this time with an electron microscope at a magnification of 500 to 1000 revealed that the plating was formed cleanly on the underlying shot blast surface, and the dimple shape observed after the shot blast remained with good reproducibility.

In addition to a developer carrier having a nickel-plated surface, a resist layer provided on the surface by alumite treatment or plasma treatment after blasting can also be used. Further, depending on the application, it is also possible to use a developer coat having a resin coat in which a conductive agent is dispersed on the surface or an insulative resin coat treatment. Since the coat layer is worn, the durable life is slightly shortened.

The layer forming member 4 made of an elastic regulating member is formed by attaching a rubber tip to a rigid metal plate. A 1.5 mm thick stainless steel plate is used as the rigid metal plate, and urethane rubber is used as the rubber chip. The urethane rubber has conductivity of a volume resistivity of 10 ⁵ Ωcm by dispersion of carbon black. When the urethane rubber has a large volume resistivity, the same potential is not obtained even when the urethane rubber is brought into contact with the developer carrier, the effect of shielding the developer by the electric field is not obtained, and the transport surface of the developer carrier does not have line-shaped unevenness. Or, only lines with extremely low contrast are formed. Ideally line-shaped uneven conveying surface can be formed as a result of the evaluation by changing the volume resistivity of the urethane rubber, 10 ⁹ .omega.c
m or less. The hardness Hs of urethane rubber is JIS
At A, Hs is preferably 55 to 80 degrees. When the rubber hardness is high, the rubber elasticity does not function, so that the rubber carrier does not follow the developer carrier, and it is difficult to form a line-shaped uneven conveyance surface on the developer carrier. When the contact is made, the rubber vibrates, so that the linear concavo-convex transfer surface, which should be formed corresponding to the frequency of the AC superimposed bias, is disturbed by the rubber vibration.

For example, urethane rubber having a rubber hardness of 70 degrees is attached to the tip of the rigid metal plate by injection molding, and after injection molding, the portion in contact with the developer carrier is ground to form an R shape. The step of the layer forming member is produced during the inspection step, and a desired step, position and size are produced according to the shape and the grinding amount of the grinding wheel. In addition, a step can be formed at a desired position and size by a mold used for injection molding. 1.5 mm from the contact position as a layer forming member
A step was created at a position of 0.1 mm and a size of 0.1 mm. Also,
The surface roughness of the layer forming member is created by changing the roughness of the grindstone used in the grinding process.
0.3 μm, and the surface roughness on the downstream side is Ra = 0.08 μm. The thus formed layer forming member is brought into edge contact with the developer carrier. The layer forming member was provided with a long hole for positioning so that the contact with the edge was always at a constant angle and parallel to the developer carrier by a positioning pin. The edge contact makes it possible to form a thin layer with a small contact load, and the area of a wedge portion (triangular portion between the layer forming member and the developer carrying member) into which the developer enters is reduced, so that the development is performed. Agent clogging was less likely to occur, and a uniform line-shaped uneven transfer surface in the longitudinal direction could be formed.

As the supply member, a urethane foam roller is disposed in pressure contact with the developer carrier, and is rotated at a constant peripheral speed ratio in the against direction with respect to the developer carrier. The volume resistivity of urethane foam is 10 ⁵ to 10 ⁸ Ω
cm is better. If the resistance value is too high, the charge does not follow, so that there is no effect of the supply bias. If the resistance value is too small, it leaks between the developer carrying member, which is not preferable. In this embodiment, urethane foam of 10 ⁷ Ωcm was used. The contact nip between the urethane foam and the developer carrier is 2 to 2.
4.5 mm is good. When the contact nip is small, the supply power of the developer decreases, and when the contact nip is large, the driving torque of the developer carrier increases, and image quality is deteriorated due to banding or the like. In this embodiment, the thickness is 3.5 mm. The peripheral speed ratio with respect to the developer carrying member is preferably 0.3 to 1 in the case of against rotation. If the peripheral speed ratio is small, the supply property is insufficient,
If it is large, the driving torque increases and the image quality deteriorates. In this embodiment, it is set to 0.53. The cell diameter of the urethane foam is preferably 10 to 50 times the volume average particle diameter of the developer. If the volume average particle diameter of the developer is small, the cells are clogged with the developer and the supply property is insufficient. If the diameter is large, scuffing due to the cell diameter occurs on the image and image quality is deteriorated. In this embodiment, urethane foam having a cell diameter of 120 μm and about 17 times the volume average particle diameter of the developer of 7 μm was used.

After assembling the developing device, a developer mainly composed of a polyester resin was sealed in the developing device. The base particles of the developer are produced by kneading a polyester resin, a pigment, a charge control agent, and a wax at a high temperature, and then pulverizing and classifying the particles. A particle size analyzer Coulter Counter (manufactured by Coulter: TA- In the measurement according to II), the volume average particle size is 7 μm in volume measurement. In the present embodiment, a developer obtained by externally adding 3% by weight of silica fine particles to the base particles was used.

Next, a description will be given of the line-shaped uneven transfer surface on the developer carrier formed by the developing device according to the present invention. FIG. 7 is a view for explaining the line-shaped uneven transfer surface on the developer carrying member, where 30 is a developer layer and 31 is the surface of the developer carrying member.

The elastic rubber provided at the tip of the layer forming member 4 is a semiconductive rubber having a volume resistivity of 10 ⁹ Ωcm or less, preferably 10 ^{5 to} 10 ⁷ Ωcm. The layer forming member 4 is in contact with the metallic developer carrier 2,
When an AC bias is applied to the developer carrier 2, the developer carrier 2 and the layer forming member 4 fluctuate at the same potential. When this potential fluctuation is 0 V, the developer is conveyed through the nip portion under the force of entering the contact portion (nip portion) between the developer carrier 2 and the layer forming member 4 and is transported at −400 V
In the case of (1), the developer is not conveyed because the developer cannot pass through the nip portion by receiving a force opposite to the force entering the nip portion. Such an electric potential change causes an ON / OFF shutter action on the developer, which occurs at a cycle of an AC bias, so that a concavo-convex developer layer is formed in a line on the transport surface of the developer carrier.

For example, V _pp = 400 V, V _dc = −200 V
When set to 0 to -400V, _Vdc = 0V
Is set in the range of + 200V to -200V. The frequency f of the AC superimposed bias is
It can be set corresponding to the secondary pitch frequency f _g2 of the developer carrier driving gear. The pitch frequency of the developer carrier driving gear is defined as a pitch n (m) of the developer carrier driving gear.
m) and the vibration period T calculated from the peripheral velocity m (mm / s)
Is calculated from the reciprocal of

T = n / m, f _g1 = 1 / T, where n: gear pitch (mm), m: image forming speed (mm / s) The developer carrier driving gear secondary pitch frequency f _g2 is the gear pitch frequency This value is twice the value of f _g1 , and is a value mainly indicating the effect of banding that occurs when the gear shaft is eccentric.
It is better that the frequency of the AC superposition bias is higher than the secondary pitch frequency as well as the primary pitch frequency.
As one embodiment, the drive gear secondary pitch frequency f _g2 is 2
5.4 Hz, and the frequency f of the AC superimposed bias is 2 kHz.

The line-shaped uneven transfer surface on the developer carrier is
The width is determined by the frequency and the peripheral speed of the AC superimposed bias applied to the developer carrier. As an example, assuming that the peripheral speed of the developer carrier is 360 mm / s and the AC superimposed bias applied to the developer carrier is 2 kHz, the line formed on the developer carrier according to the voltage fluctuation is shown in FIG. 7
0.18mm interval, line width 0.09mm as shown in
Formed. This determination can be made by visual observation of the transport surface formed on the developer carrier, but more objectively, the line after tape transfer is measured with a microdensitometer (Abe Design Co., Ltd.). , MTF measured several times
If the average of the values is 10 or more, it can be determined as a line shape. M
The TF value is calculated by calculating the average value Ion of the line density maximum value and the average value Ioff of the line density minimum value when five lines are measured, and is simply calculated by the following equation.

MTF value = ｛(Ion−Ioff) / (Ion +
Ioff)｝ × 100 While applying a bias using the developing device of this embodiment,
When the developer carrier was driven to form a thin layer of developer on the developer carrier, the transport surface became linear, and a 12 mm wide tape was used with care so as not to disturb the linear developer on the transport surface. And the MTF value was measured with a microdensitometer.

The irregular pitch of the line-shaped uneven conveyance surface can be controlled by the frequency design of the AC bias. An effect of stabilizing the transport amount can be obtained such that the transport amount of the developer is always fixed by correcting the unevenness of the transport. As described above, since the transport amount can be made uniform, an effect that so-called banding is reduced is achieved.

There is another cause for the occurrence of banding, which is caused by the difference in the amount of development caused by unevenness in the transport of the developer carrier at the contact portion between the latent image carrier and the developer carrier. Thus, the grayscale difference appears on the image. Such banding occurs even if the peripheral speed fluctuates at the time of entering the developing nip portion even when the thickness of the transport amount is substantially constant. On the transport surface where no AC bias is applied to the developer carrier, the developer filling ratio (the ratio of the space to the developer in the development nip portion) is as high as 80% or more, and the developer in the developer carrier feeding direction is high. Freedom of movement (space)
When there is a difference in the amount of development at the development nip portion because there is almost no color difference, it becomes the density on the image as it is.

On the other hand, in the developing device of the present invention, since the unevenness on the line is positively formed on the conveying surface by the AC superimposed bias, the developer filling rate is at least 50%.
In particular, since the degree of freedom of movement of the developer in the direction of transport of the developer carrier is high, the developer can freely move before and after the transport direction according to the developing bias, and as a result, the developer adheres to the surface of the latent image carrier. , The electrostatic latent image is faithfully reproduced and no banding occurs. Further, when an elastic photoconductor is used as the latent image carrier, the latent image carrier is elastically deformed in the nip portion, and as a result, the space for moving the developer is further increased, so that the occurrence of banding can be suppressed.

As described above, the influence of gear feeding unevenness can be reduced by forming the line-shaped uneven transfer surface positively, and the developer can be moved freely by reducing the developer filling rate in the developing nip portion. The synergistic effect of being able to increase the degree makes it possible to substantially eliminate the occurrence of banding, and to obtain a good image without noise in an image formed by superimposing many colors like a color printer.

Further, since the transport surface is formed in a line-shaped unevenness, the developer attached to the non-image area is scraped off by the unevenness, and fog and scattering are reduced. In addition, the period of the unevenness on the transport surface is formed on the developer carrier according to the frequency of the AC superimposed bias, and when there is the same transport amount, the thickness of the developer in the convex portion is compared with a general thin layer transport surface. About twice as much. When such an uneven developer layer comes into contact with the latent image carrier, the developer is easily moved in accordance with the bias electric field because the developer filling rate is low in the concave portion, and the developer attached to the non-image portion is It is easy to peel off on the body side. Further, in the convex portion, the developer attached to the non-image portion and the convex portion come into contact at least once, and at this time, the attached developer is scraped off by the van der Waals force and moved to the developer carrier side. Become. Fogging and scattering can be almost eliminated from such an action.

Further, according to the present invention, there is an effect that the clogging between the layer forming member and the developer carrying member is reduced by the crushing effect of the developer aggregate. The developer in the developing device is present in the form of aggregates having a certain size due to standing or the like. Due to the agitator agitation, such aggregates are mechanically crushed to a certain size and supplied to the developer carrier. However, when the aggregates enter the nip portion between the layer forming member and the developer carrier, they pass through. And clogging may occur. At the location where this clogging occurs, the transport amount is reduced, resulting in transport unevenness such as vertical band unevenness and vertical streak, resulting in shading unevenness on the image. Further, the clogged developer accumulates at that location, so that the developer is repeatedly contacted with the developer carrier, causing filming of the developer carrier. However, in the present invention, an AC superimposed bias is applied to the developer carrying member, and in the first half of the nip portion between the layer forming member and the developer carrying member, the developer is vibrated by potential fluctuations to break up the aggregates, and the primary The developer enters between the developer carrying layer forming members in a state close to the particles. For this reason, it easily passes through the nip portion between the layer forming member and the developer carrying member,
Alternatively, since the toner is regulated and flows behind the developing device, a good image can be obtained without clogging of the developer.

As described above, the layer forming member and the developer carrier fluctuate at the same potential due to the AC superimposed bias. When the potential is high or low with respect to the developer, the developer slips through, and the low or high potential is applied. In this case, since the developer is cut off, a line-shaped uneven transfer surface is formed. Therefore, when the layer forming member is insulative (10 ¹⁰ Ωcm or more), the potential with respect to the developer carrier is not increased due to charge-up or the like. This is not preferable because the line-shaped conveyance surface cannot be stably formed.

Next, an image forming apparatus equipped with the developing device according to the present invention will be described. FIG. 8 is a diagram showing an example of the configuration of an image forming apparatus equipped with a developing device according to the present invention. This image forming apparatus is configured to use four color toners (yellow Y, cyan C, magenta M, and black K). Is a device capable of forming a full-color image by using a developing device based on a liquid developer.

In FIG. 8, reference numeral 100 denotes an image carrier cartridge in which an image carrier unit is incorporated. In this example, the photosensitive member (latent image carrier) 140 is configured as a photosensitive member cartridge, and the photosensitive member (latent image carrier) 140 is rotationally driven in a direction indicated by an arrow in FIG. Photoconductor 140
Has a thin cylindrical conductive substrate and a photosensitive layer formed on the surface thereof. Around the photoconductor 140,
Along the rotation direction, the charging roller 16
0, a developing device 10 (Y, C, M, K) as a developing means,
The intermediate transfer device 30 and the cleaning unit 170 are provided.

The charging roller 160 is in contact with the outer peripheral surface of the photosensitive member 140 to uniformly charge the outer peripheral surface. On the outer peripheral surface of the uniformly charged photoconductor 140, a selective exposure L1 according to desired image information is performed by the exposure unit 40, and an electrostatic latent image is formed on the photoconductor 140 by the exposure L1. . The electrostatic latent image is developed by applying a developer by the developing device 10.

As a developing device, a developing device 10 for yellow is used.
Developing device 10C for Y and cyan, developing device 10 for magenta
A developing device 10K for M and black is provided. These developing units 10Y, 10C, 10M, and 10K are:
Each of them is configured to be swingable, and only the developing roller (developer carrier) 11 of one developing device is selectively
40 can be contacted. Therefore, these developing units 10 apply any one of the toners of yellow Y, cyan C, magenta M, and black K to the photoconductor 14.
0 to develop the electrostatic latent image on the photoconductor 140. The developing roller 11 is formed of a hard roller, for example, a metal roller having a roughened surface. The developed toner image is transferred onto the intermediate transfer belt 36 of the intermediate transfer device 30. After the transfer, the cleaning unit 170
The cleaner includes a cleaner blade for scraping off the toner T remaining on and adhering to the outer peripheral surface of the photoconductor 140, and a receiving portion for receiving the toner scraped off by the cleaner blade.

The intermediate transfer device 30 includes a driving roller 31 and
Four driven rollers 32, 33, 34, 35 and an endless intermediate transfer belt 36 stretched around these rollers.
And The drive roller 31 has a gear (not shown) fixed to an end thereof and the drive gear 19 of the photoconductor 140.
0, the photosensitive drum 140 is driven to rotate at substantially the same peripheral speed as the photosensitive member 140.
6 is driven to circulate at substantially the same peripheral speed as the photoconductor 140 in the direction of the arrow shown in the figure.

The driven roller 35 is disposed at a position where the intermediate transfer belt 36 is pressed against the photosensitive member 140 by its own tension with the driving roller 31.
A primary transfer portion T1 is formed at a pressure contact portion between the intermediate transfer belt 36 and the intermediate transfer belt 36. The driven roller 35 is disposed near the primary transfer portion T1 on the upstream side in the circulation direction of the intermediate transfer belt 36.

The driven roller 31 has an intermediate transfer belt 36
A primary transfer voltage is applied to the conductive layer of the intermediate transfer belt 36 via the electrode roller (not shown). The driven roller 32 is a tension roller, and urges the intermediate transfer belt 36 in the tension direction by an urging means (not shown). The driven roller 33 is a backup roller that forms the secondary transfer portion T2. A secondary transfer roller 38 is opposed to the backup roller 33 via an intermediate transfer belt 36. A secondary transfer voltage is applied to the secondary transfer roller 38, and the secondary transfer roller 38 can be moved toward and away from the intermediate transfer belt 36 by a contact / separation mechanism (not shown). The driven roller 34 is a backup roller for the belt cleaner 39. The belt cleaner 39 includes a cleaner blade 39a that comes into contact with the intermediate transfer belt 36 and scrapes off the toner remaining on and attached to the outer peripheral surface thereof, and a receiving portion 39b that receives the toner scraped off by the cleaner blade 39a. ing. The belt cleaner 39 can be moved toward and away from the intermediate transfer belt 36 by a contact / separation mechanism (not shown).

The intermediate transfer belt 36 is composed of a multilayer belt having a conductive layer and a resistance layer formed on the conductive layer and pressed against the photosensitive member 140. The conductive layer is formed on an insulating substrate made of a synthetic resin, and a primary transfer voltage is applied to the conductive layer via the above-described electrode roller. The conductive layer is exposed in the form of a strip by removing the resistive layer in the form of a strip at the side edge of the belt, and the electrode roller comes into contact with the exposed portion.

In the course of circulating the intermediate transfer belt 36, the toner image on the photosensitive member 140 is transferred onto the intermediate transfer belt 36 in the primary transfer portion T1, and the toner image transferred onto the intermediate transfer belt 36 is Is transferred to a sheet (recording material) S, such as a sheet, supplied between the secondary transfer roller 38 and the secondary transfer unit T2. The sheet S is fed from the sheet feeding device 50, and is supplied to the secondary transfer portion T2 at a predetermined timing by the gate roller pair G. 51 is a paper feed cassette, and 52 is a pickup roller.

The toner image is fixed by passing the sheet SH onto which the toner image has been transferred in the secondary transfer portion T2 and the fixing device 60, and is formed on the case 80 of the apparatus main body through the paper discharge path 70. The sheet is discharged onto the sheet receiving section 81. In this image forming apparatus, as a paper discharge path 70,
It has two paper discharge paths 71 and 72 independent of each other, and the sheet that has passed through the fixing device 60 is discharged through one of the paper discharge paths 71 or 72. The paper discharge paths 71 and 72 also constitute a switchback path.
When an image is formed on both sides of a sheet, the discharge path 71
Alternatively, the sheet once entering the sheet 72 is fed again to the secondary transfer portion T2 through the return path 73.

The outline of the operation of the entire image forming apparatus as described above is as follows. (1) When a print command signal (image formation signal) from a host computer (not shown) (personal computer or the like) (not shown) is input to the control unit 90 of the image forming apparatus, the photosensitive member 14
0, each roller 11 of the developing device 10 and the intermediate transfer belt 36 are driven to rotate. (2) The outer peripheral surface of the photoconductor 140 is uniformly charged by the charging roller 160. (3) The outer peripheral surface of the uniformly charged photoconductor 140 is subjected to selective exposure L1 according to the image information of the first color (for example, yellow) by the exposure unit 40 to form an electrostatic latent image for yellow. Is done. (4) Only the developing roller of the developing unit 10Y for the first color, for example, yellow, contacts the photoconductor 140, whereby the electrostatic latent image is developed, and the yellow toner image of the first color is changed to the photoconductor. 140 is formed. (5) A primary transfer voltage having a polarity opposite to the charging polarity of the toner is applied to the intermediate transfer belt 36, and the toner image formed on the photoconductor 140 is transferred onto the intermediate transfer belt 36 in the primary transfer portion T1. Is done. At this time, the secondary transfer roller 38 and the belt cleaner 39
6 away. (6) After the toner remaining on the photoconductor 140 is removed by the cleaning unit 170, the photoconductor 140 is neutralized by the neutralization light L2 from the neutralization unit 41. (7) The above operations (2) to (6) are repeated as necessary. That is, according to the content of the print command signal,
The second color, the third color, and the fourth color are repeated, and a toner image corresponding to the content of the print command signal is formed on the intermediate transfer belt 36.
It is formed by being superposed on the top. (8) The sheet S is fed from the sheet feeding device 50 at a predetermined timing, and immediately before or after the leading end of the sheet S reaches the secondary transfer portion T2 (in short, at a desired position on the sheet S, the intermediate transfer is performed). When the toner image on the belt 36 is transferred, the secondary transfer roller 38
6, the secondary transfer voltage is applied, and the toner image on the intermediate transfer belt 36 (basically, a full color image in which four color toner images are superimposed) is transferred onto the sheet S. Further, the belt cleaner 39 comes into contact with the intermediate transfer belt 36, and the toner remaining on the intermediate transfer belt 36 after the secondary transfer is removed. (9) The toner image is fixed on the sheet S by passing the sheet S through the fixing device 60, and thereafter, the sheet S is directed to a predetermined position (in the case of non-double-sided printing, the sheet S is directed to the sheet receiving portion 81 to perform double-sided printing In the case of (1), the sheet is conveyed to the return path 73 via the switchback path 71 or 72).

When a solid image and a 40% halftone image were formed on the entire surface by the above-described image forming apparatus equipped with the developing device according to the present invention, a uniform image without density unevenness was obtained. The unevenness in the vertical direction was determined based on the standard that the density difference was within 0.2 by a visual observation and a densitometer measurement (manufactured by X-rite: 404). The halftone image was 0.05 or less, which was good. Banding unevenness occurring in the lateral direction of the image was not visually confirmed, and was very good. The fog evaluation was evaluated on the basis of a developer consumption of 2 g or less when printing 1000 continuous white solid sheets, and was a sufficient level of 0.5 g / 1000 sheets. Furthermore, continuous printing of 100,000 sheets was performed as an evaluation of durability printing, but no filming occurred on the developer carrying member, and a good image was obtained after 100,000 sheets as in the initial case.

[0054] The evaluation as described above V _a = -1200V,
V ₀ = −600 V is constant and is always | V ₀ | ≧ | V _max
Table 1 shows the results obtained by changing the waveforms of f, V _pp and V _dc while maintaining the relationship |.

[0055]

[Table 1]

In each case, the image characteristics were good and there was no problem in practical use. In addition, each comparative example is as follows. Using the same developing unit and (Comparative Example 1) Example, V _pp = 400V development bias applied to the developer carrying member, V _dc =
The same evaluation was performed under the condition of −500 V and | V ₀ | <| V _max |. As a result, fog occurred on the non-image portion, and the image was not able to withstand actual use from the beginning. (Comparative Example 2) Using the same developing device as in the embodiment, V _pp = 800 V and V _dc = developing bias applied to the developer carrier.
The same evaluation was performed under the conditions of −300 V and | V ₀ | <| V _max |. As a result, fog occurred on the non-image portion, and the image was not able to withstand actual use from the beginning. (Comparative Example 3) Using the same developing device as in the embodiment, the developing bias applied to the developer carrier was set to a DC bias, and V _dc
The same evaluation was carried out at −300 V. As a result, fog occurred on the non-image portion, and the image was not able to withstand actual use from the beginning.

Note that the present invention is not limited to the above-described embodiment, and various modifications are possible. For example, in the above embodiment, the maximum value V
_max is set on the basis of the charging potential V ₀ , which is the non-image portion surface potential of the latent image carrier, and the minimum value V
_min is the exposure potential V which is the surface potential of the image portion of the latent image carrier.
_{Although ON} is set as a reference, it may be set based on a point at which development γ rises with respect to the surface potential of the latent image carrier. That is, the threshold value V at which the developer actually starts to adhere
th does not always coincide with the surface potentials V ₀ and V _ON as described with reference to FIG. For example, the device usage conditions, determine the respective thresholds Vth in response to environmental, as a condition to eliminate the fog at non-image portion, the maximum value V _max of the AC superimposed bias to the charge potential V ₀ latent image bearing member point development γ rises, is as to that is lower than the threshold Vth _0.

[0058]

As is apparent from the above description, according to the present invention, an AC superimposed bias in which an AC is superimposed on a DC bias is applied to the developer carrier, and the developer is applied on the developer carrier by the layer forming member. In a developing device that forms a thin layer and develops the latent image on the latent image carrier, the maximum value of the AC superimposed bias applied to the developer carrier is set lower than the charging potential of the latent image carrier. Can be prevented from adhering to the developer, and generation of fog can be suppressed.

The developer carrying member is in contact with the latent image carrying member, and the DC bias is set lower than the intermediate potential between the charging potential of the latent image carrying member and the exposure potential. Since the value is set lower than the exposure potential of the latent image carrier, appropriate development γ can be set, and a uniform image without density unevenness can be formed.

[Brief description of the drawings]

FIG. 1 is a view for explaining an embodiment of a developing device according to the present invention.

FIG. 2 is a diagram illustrating an AC superimposed bias of the developing device according to the present invention.

FIG. 3 is a diagram for explaining a relationship between a DC bias applied to a charging roller and a surface potential of a latent image carrier.

FIG. 4 is a diagram for explaining a relationship among a development γ, a voltage V _PP between a maximum value and a minimum value of an AC superimposed bias, and a threshold value Vth.

FIG. 5 is a view showing another embodiment of the developing device according to the present invention.

FIG. 6 is a view showing an example of a schematic overall structure of a developing device according to the present invention.

FIG. 7 is a view for explaining a linear concavo-convex transfer surface on a developer carrying member.

FIG. 8 is a view showing one configuration example of an image forming apparatus equipped with a developing device according to the present invention.

FIG. 9 is a diagram for explaining the magnitude of an AC superimposed bias applied to a developer carrier of the developing device.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Latent image carrier, 2 ... Developer carrier, 3 ... Supply member, 4
... Layer forming member, 5 ... DC bias power supply, 6 ... AC bias power supply

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Kinro Koga 3-5-5 Yamato, Suwa City, Nagano Prefecture Inside Seiko Epson Corporation (72) Fumio Takashiro 3-5-5 Yamato Suwa City, Nagano Prefecture Seiko -Epson F term (reference) 2H073 AA01 BA04 BA06 BA13 BA23 BA41 BA45 CA02 CA22 2H077 AB04 AD06 AD13 AD36 EA14 EA15 FA01 FA13 FA14 FA22 FA25 GA13

Claims (4)

[Claims] An AC superimposed bias in which an AC is superimposed on a DC bias is applied to a developer carrier, and a layer forming member forms a thin developer layer on the developer carrier to form a latent image on the latent image carrier. In the developing device for developing, a maximum value of an AC superimposed bias applied to the developer carrier is set lower than a charging potential of the latent image carrier. 2. The developing device according to claim 1, wherein said developer carrier comes into contact with said latent image carrier. 3. The developing device according to claim 1, wherein the DC bias is set lower than an intermediate potential between a charging potential of the latent image carrier and an exposure potential. 4. The developing device according to claim 1, wherein a minimum value of said AC superposition bias is set lower than an exposure potential of said latent image carrier.