JP3900729B2 - Development device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a developing device, and more particularly to a one-component non-contact developing device used in an electrostatic recording apparatus such as an electrophotographic copying machine or a printer.
[0002]
[Prior art]
In recent years, full-colorization is also progressing in the electrophotographic region. The process generally performed for full-color electrophotography based on the image forming process for each color is as follows. First, a toner image is formed on the image carrier with the first color, and the toner image is once transferred to the intermediate transfer member. Next, a second color toner image is formed on the image carrier after the first color toner image is transferred and cleaned, and the second color toner image is transferred onto the intermediate transfer body on which the first color toner image is transferred. Superimpose the color toner images. This process is repeated for the number of colors, and after all color toner images are formed on the intermediate transfer member, all color toners are collectively transferred from the intermediate transfer member to the recording paper.
[0003]
In the primary transfer portion from the image bearing member to the intermediate transfer member and the secondary transfer portion from the intermediate transfer member to the recording paper, it is necessary that the movement directions of the respective members are the same and the speed is substantially constant. For this reason, there are restrictions on the arrangement of the elements in the image forming apparatus.
[0004]
The moving direction of the toner carrier and the image carrier in the developing unit is generally the same direction (hereinafter referred to as “with development”). FIG. 3 is a schematic view of an image forming apparatus using the intermediate transfer member in this case. The photosensitive member 1 serving as an image carrier is charged with a predetermined polarity by the charging charger 2 and then forms an electrostatic latent image on the surface by laser light emitted from the printer head 3 in the process of rotation. The latent image is then developed by one of the developing units of the developing unit 4 that reaches the developing unit and rotates around the central axis. The toner image thus visualized is primarily transferred onto the intermediate transfer member 5. The transfer residual toner on the photosensitive member 1 is cleaned by the cleaner 6 in the process of rotation, and proceeds to another toner image forming process. On the other hand, the developing unit 4 is rotated 90 degrees along with this, and is used for the development of the second color.
[0005]
In this way, exposure, development, and primary transfer of the four types of color-separated images are sequentially performed, and after the four color toner images are superimposed on the intermediate transfer member 5, the recording roller 7 is transferred by the transfer roller 8. Are collectively transferred. The recording paper 7 is transported to the fixing unit by the transport roller and heated and melted by the fixing device 9, so that the toner image is fixed on the recording paper 7.
[0006]
Here, as a developing method in the developing unit, as shown in FIG. 3, a non-contact method in which the toner carrying member 10 and the photosensitive member 1 are opposed to each other with a predetermined gap is known. The toner carrying member 10 and the photosensitive member 1 face each other without being in contact with each other to form a developing region. The surface of the toner carrying member 10 and the surface of the photosensitive member 1 move in the same direction in the developing region, the toner carrying member 10 supplies toner to the developing region one after another, and the photosensitive member 1 also rotates at substantially the same speed. Thus, it is used for continuous image formation.
[0007]
Further, in the non-contact method, a method of generating an alternating electric field in the development region by the action of a high-frequency development bias is used. That is, by connecting a vibration bias power source (not shown) to the toner carrying member, an alternating electric field is generated in the developing region where the toner carrying member 10 and the photosensitive member 1 face each other, and the toner 14 charged by the toner carrying member 10 Transport into electric field. The conveyed toner 14 is alternately accelerated in the developing direction from the toner carrier 10 toward the photoconductor 1 and in the collecting direction from the photoconductor 1 toward the toner carrier 10, and adheres to the latent image portion on the photoconductor 1. It will be. By such a method, fog prevention and gradation reproducibility can be improved. In general, such an oscillating bias is configured by superimposing an AC bias on a DC bias. Furthermore, the DC bias is generally set to a middle level between the latent image portion potential on the photoconductor 1 and the background potential.
[0008]
On the other hand, in such a full-color image forming apparatus, as compared with a monochrome image forming apparatus, a developing device is required for a plurality of colors, so that the apparatus tends to be large. In view of this point, in a developing device, a one-component developing system is often used from the viewpoint of reducing the size of the device. The image forming apparatus using the one-component development method does not require a toner / carrier stirring mechanism, and can contribute to downsizing as compared with the case of using the two-component development method. In the case of the one-component development method, control unique to the two-component development method such as adjustment of the mixing ratio of the toner and the carrier is unnecessary, which contributes to simplification of the apparatus and cost reduction.
[0009]
Here, in the image forming apparatus shown in FIG. 3, the fixing unit 9 and the developing unit 4 are in a very close positional relationship. This is because in the developing device, a thin layer of toner is formed using gravity, and therefore the rotation direction of the toner carrier 10 is restricted with respect to the direction of gravity. That is, as shown in FIG. 3, it is necessary to rotate the toner carrier 10 in a direction in which the toner carrier 10 moves downward at a portion facing the image carrier 1, thereby the rotation direction of the image carrier 1 and the intermediate transfer member. This is because the rotational direction of 5 and the moving direction of the recording paper 7 are restricted. For this reason, the waste heat generated in the fixing device 9 reaches the developing device unit 4 and raises the temperature of the developing device.
[0010]
In a full-color image forming apparatus, normally, a toner having a low softening point and being easily soluble is used. Accordingly, as described above, when the temperature of the developing device rises, there is a problem in that the toner aggregates and solidifies in the developing device due to such properties of the toner, and appears as image noise. In order to avoid such a problem of image noise, it is conceivable to arrange the fixing device 9 above the developing unit 4, but this leads to complication of the apparatus due to enlargement of the apparatus and extension of the sheet passing path. This is not preferable.
[0011]
Therefore, in order to solve this problem, as shown in FIG. 2, there is a developing method (hereinafter referred to as “counter development”) in which the moving direction of the toner carrier and the image carrier in the developing unit is reversed. It is valid. In this way, the fixing device 9 and the developing unit 4 can be arranged apart from each other, and the above-described troubles due to waste heat do not occur.
[0012]
[Problems to be solved by the invention]
However, when the one-component toner is developed by applying the vibration bias, the image density greatly changes in the counter development as shown in FIG. 2 due to a slight change in the development bias condition as compared with the width development. There is a problem. FIG. 4 shows the results of measuring the output image density (hereinafter referred to as “development characteristics”) while changing the DC bias value of the developing bias to be applied. As shown by this result, in the counter development, a slope γ of the vertical axis (image transmission density) with respect to the horizontal axis (DC bias value) is more established than in the width development. This indicates a problem that an appropriate image cannot be obtained due to variations in the output of the bias power source. Further, from this result, it can be inferred that the transmission density in the case of the counter development is easily affected by the latent image potential change and the toner charge amount due to the change in the use environment.
[0013]
Further, in the case of non-contact development, if the photosensitive member or the toner carrier is swung with respect to the shaft core, the development gap changes due to the change in the distance between the two, resulting in image density unevenness. Is also inherent.
[0014]
In view of the above problems, the present invention can obtain stability of image density even in an image forming apparatus that performs development by one-component non-contact counter development, and is also stable against fluctuations in the development gap. An object of the present invention is to provide an image forming apparatus capable of obtaining an image density.
[0015]
[Means for Solving the Problems]
In order to achieve the above object, according to the first aspect of the present invention, an image carrier and a toner carrier carrying a single-component toner in a thin layer are opposed to each other with a predetermined gap, and development is performed at the opposed portion. A developing device that performs development by applying a bias, wherein the surface of the toner carrier moves in the opposite direction to the surface of the image carrier at the facing portion, and the developing bias removes toner from the toner carrier. A vibration period oscillating between a first electric potential for generating a developing electric field that moves in a direction toward the carrier and a second electric potential for generating a recovery electric field that moves toner in a direction from the image carrier to the toner carrier. and a rest period for maintaining the third potential for a certain period between the potential and the second potential, when the transition rest period from the vibration period, the developing bias der to shift to the third potential after a second potential The third potential is a developing device according to claim potential der Rukoto biased than an intermediate potential between the second potential and the first potential to the first potential.
[0016]
According to the present invention, since collection is promoted even during a period corresponding to the pause period of the development bias waveform, the slope of the development characteristic is lowered, and the latent image potential changes due to the use environment and the variation in the output of the bias power source. It is possible to provide a developing device that can obtain an appropriate image while suppressing the influence, and that can suppress an influence of a change in the developing gap in the developing unit and can obtain an image without halftone dot unevenness .
[0017]
Furthermore, the invention according to claims 2 to 3 is characterized in that the third electric potential or a time ratio for generating the developing electric field in a period other than a predetermined period for maintaining the third electric potential can be changed. The developing device according to claim 1.
[0018]
According to the present invention, the image density can be adjusted without causing a leak in the developing unit by changing the potential of the developing bias in the pause period or the time ratio of applying the first potential in the developing unit.
[0021]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an example of an embodiment of a developing device according to the present invention will be described.
[0022]
First, referring to FIG. 2, the surface of the photoreceptor 1 is negatively charged uniformly by the charging charger 2. Thereafter, the print head 3 emits a beam corresponding to the image of the color, thereby forming a latent image on the beam irradiation site due to the light attenuation effect. In this way, the photoreceptor 1 carries the latent image and reaches the development area.
[0023]
FIG. 1 is a cross-sectional view of an essential part of a developing device according to the present invention, in which a portion of a photoreceptor 1 and a developing device 4K is taken out.
[0024]
In the developing device according to the present invention, the surface of the developing roller 10K, which is a toner carrier, and the surface of the photoreceptor 1 are configured to move in opposite directions in the developing region. The developing roller 10 </ b> K is configured by providing a resistance layer 12 having elasticity around a metal core 11. The toner 14 accommodated in the toner hopper 13 is supplied to the vicinity of the developing roller 10K by the rotation of the supply blade 15, and is conveyed to the developing region facing the photoreceptor 1 as the developing roller 10K rotates.
[0025]
Here, the toner 14 is thinned between the developing roller 10K and the regulating member 16 in contact with the developing roller 10K, is given a negative charge, and is carried on the developing roller 10K by attractive force due to mirror image force. Thereafter, toner is developed on the latent image portion on the photoconductor under a developing electric field formed between the photoconductor 1 and the developing roller 10K by the vibration bias power source 20. The undeveloped toner remaining on the developing roller 10K without being developed passes through the contact area with the seal member 17 and then returns to the toner hopper 13.
[0026]
The developing device 4K is a developing device for black toner, and the above configuration is the same for the developing devices 4Y, 4M, and 4C for yellow, magenta, and cyan.
[0027]
The developing bias to be generated from the developing bias power source 20 of the apparatus, experiments were conducted with three kinds of the developing bias shown in FIG. 5 (a) to (c). The toner is negatively charged. Hereinafter, as shown in FIG. 5 (b) or FIG. 5 (c), the waveform having a period of oscillation (pulse unit) and rest period (blank portion) marked "blank waveform" Furthermore, FIG. 5 (b) As shown in FIG. 5 , “GKB” indicates that the rest period enters after the recovery direction (plus) potential, and the rest period after the development direction (minus) potential as shown in FIG. The case where “” is entered is referred to as “KGB”.
[0028]
Such a vibration waveform is generated by superimposing an AC bias on a DC bias. The value of the DC bias at this time is indicated as “Vdc”. “Vpp” indicates the amplitude of the waveform, and “Duty” indicates the development direction (minus) time ratio in the vibration period. Further, in the blank waveform, the potential level during the rest period is expressed as “blank level”. This blank level is 0% for the Vdc level, + 100% for the maximum amplitude level on the collection side (plus), and on the development side (minus). The maximum amplitude level is set to -100%.
[0029]
FIG. 5 (a), the Figure 6 shows the developing characteristic when using the waveforms shown in FIG. 5 (b) and FIG. 5 (c). Here, the distance between the photosensitive member and the developing roller is fixed to 0.15 mm, the developing roller resistance is 1E5Ω, the vibration period frequency is 3 kHz, Vpp is 1.6 kV, and Duty is 33%. The same time was used, and one blank was inserted after one pulse (this is expressed as “1 pulse 1 blank” or “1P1B”).
[0030]
As a result, it can be seen that the DC bias value at which development starts is small in the KGB type blank waveform. In the case of the continuous waveform, the toner accelerated in the direction of the photosensitive member by the development direction pulse is pulled back in the direction of the development roller by the subsequent recovery direction pulse, whereas in the KGB blank waveform, the blank period following the development direction pulse. However, it is considered that development is promoted due to the subsequent movement toward the photoconductor.
[0031]
On the other hand, for the GKB type in which the way of inserting the blank is different, the recovery is promoted contrary to the above, and the result of FIG. 6 also has the same development characteristics as the with development. That is, also in the counter development method, the stability of the image density can be ensured in the same manner as with the developing by making the waveform having a pause period after the bias waveform is oscillated from the developing direction to the collecting direction.
[0032]
Based on this result, detailed parameters were examined for the GKB blank waveform. In this experiment, development was performed for each change in the time ratio of the pulse part to the blank part, the duty, and the blank level on the basis of Vpp = 1.6 kV, Duty = 35%, blank level = 0%, and 1 pulse / blank. Changes in characteristics were investigated.
[0033]
First, FIG. 7 shows the development characteristics in the case of 2 pulse 1 blank and 1 pulse 2 blank compared with the reference (1 pulse 1 blank). From this result, it can be seen that the development characteristics hardly change with this time ratio change.
[0034]
Next, FIG. 8 shows changes in development characteristics when the duty is changed with respect to the reference. From this result, it can be seen that by changing the Duty, the development characteristics can be shifted without changing its shape.
[0035]
FIG. 9 shows development characteristics when the blank level is changed with respect to the reference. From this result, it can be seen that even if the blank level is changed, the development characteristics can be shifted without changing the shape as in the case of changing the duty.
[0036]
From the above results, it can be said that the image density can be adjusted by adjusting one or both of the duty and the blank level.
[0037]
The image density can also be adjusted by changing the DC bias value. However, if the image density is simply changed by the DC offset value, as shown in FIG. The potential difference between the image portion potential and the developing direction pulse portion potential becomes large, and there is a risk of leakage to the latent image portion. Conversely, when the DC bias value is lowered, the potential difference between the background portion potential and the recovery direction pulse potential is increased, and there is a possibility that leakage to the background portion occurs. Therefore, performing image density adjustment with a DC bias value is not a preferred choice. On the other hand, when adjusting the image density at the duty and blank levels, it can be concluded that the density adjustment is possible without affecting the maximum electric field, and that the adjustment parameter is stable against leakage.
[0038]
The above examination was a result when a developing roller having no shaft core was used. However, in the case of a developing roller having a resistance layer such as an elastic layer on the surface, a certain amount of flare remains at the time of formation. Resulting in. The fluctuation of the development gap due to this flare acts as the fluctuation of the development electric field, and as a result, appears on the image as density unevenness of the development roller cycle. Such density unevenness is likely to occur particularly in a low density halftone dot image portion having a weak developing electric field.
[0039]
Therefore, the improvement of such density unevenness was examined using a GKB blank waveform that was found to be effective in terms of image density control. In this experiment, a developing roller having a flare of 50 μm and a resistance value similar to that in the above experiment was used. As a result, among the waveform parameters, the time ratio between the pulse part and the blank part and the duty are not affected by the density unevenness, whereas the blank level is set on the development side more than Vdc. An effect of improving density unevenness is observed. FIG. 10 shows the result.
[0040]
To summarize the above, in order to perform image density adjustment without causing leakage while securing stability against fluctuations in the development gap using the GKB blank waveform, the blank level is more developed than Vdc. It can be said that it is desirable to adjust the density with the duty in advance. However, if the blank level is set extremely on the development side, it is necessary to set the Duty to be extremely small for density adjustment, although it depends on the frequency. Then, a side effect that the bias waveform cannot be fully raised occurs. Therefore, it can be said that it is desirable to set the blank level in the range of 0% to about -30% and the duty in the range of 20% or more.
[0041]
In the above, the reversal development in which the exposed portion on the photoconductor is the latent image portion has been described. However, the present invention can be similarly applied to regular development in which a portion to be the background portion is exposed. Further, the development toner has been described as being negatively charged during development, but the present invention is also applicable to the case where development is performed with positive charge of the development toner. In this case, in all of the above explanations, the positive and negative of the photoreceptor surface potential and the developing bias potential can be considered in reverse.
[0042]
【The invention's effect】
As described in detail above, in the one-component non-contact counter development, the bias waveform applied to the developing roller is changed to a waveform having a pause period after being oscillated from the developing direction to the collecting direction, thereby obtaining an image equivalent to that of with developing. Concentration stability can be ensured.
[0043]
Even if there is a gap change between the photosensitive member and the developing roller, density unevenness can be suppressed by using the bias waveform to bias the rest period potential further toward the developing side compared to the DC bias value. .
[0044]
Further, when the density adjustment is performed in the image forming apparatus of the present invention, the density control can be performed without adversely affecting the leak by adjusting the development direction time ratio and the rest period potential.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view of a main part showing an example of an embodiment of an image forming apparatus according to the present invention.
FIG. 2 is a schematic diagram of a color image forming apparatus that performs development by counter development.
FIG. 3 is a schematic diagram of a full-color image forming apparatus that performs development with width development.
FIG. 4 is a graph showing development characteristics of counter development and width development when a continuous waveform bias is applied.
5 is a graph showing various bias waveforms, FIG. 5 (a) is a continuous waveform, FIG. 5 (b) is a KGB blank waveform, and FIG. 5 (c) is a GKB blank waveform. Respectively.
FIG. 6 is a graph showing changes in development characteristics depending on the type of bias waveform during counter development.
FIG. 7 is a graph showing the dependence of the development characteristics on the time ratio between the pulse part and the blank part in the GKB blank waveform.
FIG. 8 is a graph showing duty dependency of development characteristics in a GKB blank waveform.
FIG. 9 is a graph showing the dependency of development characteristics on the blank level in a GKB blank waveform.
FIG. 10 is a graph showing the relationship between blank level and halftone dot density unevenness in a GKB blank waveform.
[Explanation of symbols]
1: Photoconductor, 2: Charger charger, 3: Printer head, 4: Development unit, 4C: Cyan toner developer, 4M: Magenta toner developer, 4Y: Yellow toner developer, 4K: Black toner developer, 5: Intermediate transfer member, 6: cleaner, 7: recording paper, 8: transfer roller, 9: fixing device, 10: developing roller, 11: cored bar, 12: elastic resistance layer, 13: toner hopper, 14: toner, 15: supply Blade: 16: Restriction member, 17: Seal member, 20: Development bias power source

Claims (3)

An image carrier and a toner carrier on which a one-component toner is thinly carried are opposed to each other through a predetermined gap, and a developing device that performs development by applying a developing bias at the opposed portion. The surface of the toner carrier moves in a direction opposite to the surface of the image carrier, and the developing bias generates a first electric potential that generates a developing electric field that moves toner in a direction from the toner carrier to the image carrier. And a vibration period that oscillates between a second potential that generates a recovery electric field that moves the toner in a direction from the image carrier to the toner carrier, and a third potential that is between the first potential and the second potential is maintained for a certain period. and a rest period, during the transition rest period from the vibration period, the developing bias der to shift to the third potential after a second potential is, the third potential, the first potential and the second potential Middle of Developing apparatus is characterized in potential der Rukoto biased from the first potential side position.   The developing device according to claim 1, wherein the third potential in the pause period can be changed.   The developing device according to claim 1, wherein a time ratio for generating the developing electric field in a period other than the predetermined period for maintaining the third potential is changeable.

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