JP5265157B2 - Image forming apparatus and exposure amount adjusting method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enhance reproducibility in a low gradation area even when the bright potential VL of a photoreceptor is varied corresponding to the photoreceptor in an electrophotographic image forming apparatus. <P>SOLUTION: Writing exposure from an exposure means to a photoreceptor is set to a predetermined multiple of a basic light quantity Eb as which half value exposure (exposure in which potential of the photoreceptor becomes Vo/2) from the dark potential Vo of the photoreceptor is set, for example, twofold to threefold in the conventional manner. In this invention, the potential of the photoreceptor that is the half value exposure is obtained from (Vo-VL)/2+VL when the bright potential of the photoreceptor is defined as VL. Therefore, the reproducibility in the low gradation area is enhanced, especially, in the photoreceptor having the high bright potential VL of the photoreceptor by considering the bright potential VL of the photoreceptor which is varied due to the photoreceptor. <P>COPYRIGHT: (C)2009,JPO&amp;INPIT

Description

  The present invention relates to an electrophotographic image forming apparatus and an exposure amount adjusting method thereof.

  In the electrophotographic image forming apparatus, dielectric breakdown of the photoreceptor film is a big problem. For this reason, conventionally, the pressure resistance effect has been obtained by increasing the thickness of the photosensitive member. However, increasing the film thickness causes deterioration in image quality, resulting in a trade-off between breakdown voltage and image quality. Therefore, in order to obtain a pressure-resistant effect while maintaining the image quality, a Si—C layer or a Si—N layer having higher electrical insulation than amorphous silicon (a-Si), which is a material of the photoconductor film, is provided on the photoconductor. A photoconductor interposed between a film and a base material layer such as aluminum has been developed in Patent Document 1.

  However, as a result, there is a problem that the photoreceptor light potential VL and the residual potential become higher than the conventional one. Specifically, as shown in FIG. 6, the photosensitive member bright potential VL is written with a sufficient exposure amount when the potential of the photosensitive member charged by the charging means is the photosensitive member dark potential Vo. It is the potential of the photoreceptor film at the time of exposure. FIG. 6 is a graph showing the relationship of the amount of decrease in the photoreceptor surface potential with respect to the exposure amount.

  The photoconductor light potential VL and the residual potential depend on the film quality of the photoconductor film (having parameters such as the amount of photocarrier generation with respect to the surface charge density, carrier moving speed, photoconductive layer thickness, etc.) and the degree of charge injection prevention. It is determined. If the degree of charge injection prevention is high, the function of preventing carriers from being injected into the conductive substrate such as aluminum is enhanced, and the pressure resistance of the photoreceptor film is enhanced. On the other hand, the photoreceptor light potential VL and residual potential are increased. Will have an effect.

The photoreceptor light potential VL is about 0 to 20 V in the case of the a-Si. Due to the breakdown voltage layer, in the example of FIG. 6, the photosensitive member bright potential VL is 80 V for the photosensitive member A, 30 V for the photosensitive member B, and the difference between them is 50 V. Conventionally, the writing exposure amount from the exposure means to the photosensitive member is at least twice the basic light amount Eb, with the half light exposure amount required for the photosensitive member dark potential Vo being halved as the basic light amount Eb. The photosensitive member bright potential VL is obtained by performing exposure with the amount of light. In the example of FIG. 6, Vo = 350V, and the half-exposure amount (= basic light amount Eb) is an exposure amount of 0.32 μJ / cm ² that becomes 175V of Vo / 2.

On the other hand, Patent Document 2 suppresses an increase in the photoreceptor light potential VL due to film abrasion of the photoreceptor film (photoconductive layer) by setting the exposure amount to 3 to 10 times the half exposure amount, There has been proposed an image forming apparatus in which the film thickness dependence of the photoconductor light potential VL is reduced to enable good image formation over a long period of time.
Japanese Patent Publication No. 2-333149 Japanese Patent Laid-Open No. 2002-40766

  However, in order to obtain both the withstand voltage and the high image quality, when using the photoreceptor having the withstand voltage layer as described above, the exposure amount is set based on the half exposure amount of the photoreceptor dark potential Vo as in the past. As a result, the photoconductor having the photoconductor bright potential VL having a high withstand voltage layer is underexposed and the low gradation reproducibility is lost.

  Specifically, as an evaluation method, since the exposure amount greatly contributes to dot reproducibility, dot reproducibility is used as an evaluation index. Then, as shown in FIG. 7, a pattern having a dot occupation area ratio within a fixed area of 6.25% (1/16) is exposed, and this is used as an input area ratio and developed on the photoconductor. The dot occupation area ratio of the obtained toner image is measured, and this is used as the output area ratio, and how much the output area ratio changes with respect to the input area ratio is evaluated. As the conditions for image formation, the material of the photoreceptor film is the a-Si, the diameter of the photoreceptor is 40 mm, the linear velocity is 400 mm / sec, the photoreceptor dark potential Vo is 350 V, and the LSU beam diameter is 60 μm. .

Under these conditions, as shown in FIG. 6, the photosensitive bodies A and B both have a basic exposure potential of 0.32 μJ / cm ^2, which is a half-exposure amount of the photosensitive body dark potential Vo = 350V (Vo / 2 = 175V). When the exposure is performed with the same amount of exposure as the light quantity Eb, the output area ratio is greatly different as shown in FIG. For example, when exposure is performed at 0.96 μJ / cm ² , which is the same three times the exposure amount, the output area ratio is 3.3% for the photoconductor A having a high photoconductor light potential VL, and is smaller than the input. Photosensitive body B having a low body potential VL outputs 7.5% more than the input. In addition, the exposure amount necessary to reproduce the correct 6.25% dot occupation area ratio is four times that of the photoconductor A, but about 2.7 times that of the photoconductor B. Become. Therefore, for example, when the photoconductor A is exposed with an exposure amount suitable for the photoconductor B, the black solid image becomes a gray image. As described above, when the exposure amount is determined only from the half-exposure amount of the photoreceptor dark potential Vo, the dot reproducibility varies.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an image forming apparatus capable of improving the reproducibility of a low gradation region with respect to variations in the photoreceptor bright potential, and an exposure amount adjusting method thereof.

In the image forming apparatus and the exposure amount adjusting method of the present invention, the writing exposure amount from the exposure means to the photosensitive member is reduced by half the exposure amount from the photosensitive member dark potential Vo (the exposure amount at which the photosensitive member potential becomes Vo / 2). In the image forming apparatus and the exposure amount adjusting method in which the basic light amount Eb is set to a predetermined multiple of the basic light amount Eb, the photoconductive layer of the photoconductor is made of amorphous silicon, and the photoconductive layer of the photoconductor is electrically conductive. An Si—C layer or Si—N layer, which is higher in electrical insulation than the amorphous silicon and increases the photoconductor light potential VL, is interposed between the photoconductive substrate layer and the photoconductor potential is (Vo−). (VL) / 2 + VL, the exposure amount is set to the basic light amount Eb.

In the image forming apparatus and the exposure amount adjusting method according to the present invention, the exposure amount written from the exposure unit to the photosensitive member is set to the basic light amount Eb in advance with the half light exposure amount from the photosensitive member dark potential Vo as the basic light amount Eb. In the image forming apparatus and the exposure amount adjusting method set to a predetermined multiple, between the photoconductive layer and the conductive base material layer of the photoconductor, electrical insulation is higher than the photoconductive layer, and the photoconductor That at least one of the insulating layer for increasing the bright potential VL and the charge injection blocking layer is interposed, and the exposure amount when the photosensitive member potential becomes (Vo−VL) / 2 + VL is set to the basic light amount Eb. Features.

  According to the above configuration, in the electrophotographic image forming apparatus, the writing exposure amount from the exposure unit to the photoconductor is conventionally half the exposure amount from the photoconductor dark potential Vo (the photoconductor potential is Vo / 2). Is set to a predetermined multiple of the basic light amount Eb, for example, 2 to 3 times, whereas in the present invention, the photosensitive member potential to be the half exposure amount is set to be a photosensitive amount. When the body light potential is VL, it is obtained from (Vo−VL) / 2 + VL.

  Therefore, in consideration of the photoconductor light potential VL, which varies depending on the photoconductor, the reproducibility of the low gradation region can be improved particularly on the photoconductor having a high photoconductor light potential VL.

Further, when the photoconductive layer of the photoconductor is made of amorphous silicon (a-Si), the surface hardness is high compared to a selenium photoconductor, an organic photoconductor, etc., and the durability and handling properties are excellent. Have When a Si—C layer or Si—N layer having higher electrical insulation than the amorphous silicon (a-Si) is interposed between the photoconductive layer of the photoconductor and a conductive substrate layer such as aluminum. In addition, it is possible to improve the image quality because it prevents the photoexcited carriers generated in the photoconductive layer from flowing into the aluminum base layer while increasing the breakdown voltage against the dielectric breakdown of the photoconductor.

  In the image forming apparatus of the present invention, a standard value Vref is set for the photoconductor light potential VL, and the photoconductor dark potential Vo and the developing bias VB in the developing unit are biased by a difference ΔV of VL−Vref. It is characterized by being.

  According to the above configuration, the standard value Vref is set for the photosensitive member light potential VL, and when the actual photosensitive member light potential VL deviates from the standard value Vref, the difference from the developing bias VB in the developing unit. Becomes smaller and the reproducibility of a solid image becomes worse. Therefore, the developing bias VB is increased by the difference ΔV of VL−Vref. On the other hand, if the potential difference between the developing bias VB and the photoconductor dark potential Vo becomes smaller than a certain value, fogging occurs. Therefore, if the photoconductor dark potential Vo is correspondingly increased, such a problem can be prevented. Occurrence can be prevented.

  Furthermore, the image forming apparatus of the present invention is characterized in that the predetermined multiple is 2.25 to 8 times, preferably 2.8 to 3.4 times, more preferably 3.15 times.

  According to the above configuration, if the exposure amount is twice the basic light amount Eb substantially corresponding to the half-exposure amount, there is a possibility that underexposure will occur when the film thickness of the photosensitive film becomes thin due to aging, etc. While exposure may also be underexposure due to variations in the amount of light of the exposure means and sensitivity of the photoconductor film, the film thickness dependence of the photoconductor light potential VL is suppressed by 2.25 times or more. Such an exposure amount with a low possibility of underexposure can be obtained, and on the other hand, even if the magnification is too large, the photoconductor film will be light-fatigue, so it is necessary to make it 8 times or less, preferably 4 times or less. It becomes. Further, if it is 2.8 to 3.4 times, low gradation dot reproducibility can be obtained, and 3.15 times can obtain a sufficient exposure amount and ensure light fatigue. Most preferably.

  As described above, according to the image forming apparatus and the exposure amount adjusting method of the present invention, in the electrophotographic image forming apparatus, the writing exposure amount from the exposure unit to the photosensitive member is conventionally from the photosensitive member dark potential Vo. The half exposure amount (exposure amount at which the photoreceptor potential is Vo / 2) is set as a basic light amount Eb, and is set to a predetermined multiple of the basic light amount Eb, for example, 2 to 3 times. The photosensitive member potential that is the half-exposure amount is obtained from (Vo−VL) / 2 + VL, where the photosensitive member bright potential is VL.

  Therefore, in consideration of the photoconductor light potential VL, which varies depending on the photoconductor, the reproducibility of the low gradation region can be improved particularly on the photoconductor having a high photoconductor light potential VL.

  FIG. 1 is a longitudinal sectional view showing a mechanical configuration of a copying machine 31 which is an image forming apparatus according to an embodiment of the present invention. The copying machine 31 is generally disposed on the upper side of the main body 32 for image formation, the scanner unit 33 serving as a document reading device, and disposed on the upper side of the scanner unit 33. And ADF 34 provided.

  In the main body 32, one or a plurality of (three in FIG. 1) paper feed trays 41, 42, 43 or one of the recording papers 41a, 42a, 43a, 44a loaded in the manual feed tray 44 is a take-in roller 41b, 42b, 43b, and 44b are taken out one by one, adjusted in timing by registration rollers 45 and 46, and then conveyed to the image forming unit 47. In the image forming unit 47, a charger 47b, a laser writing unit 47c, a developing unit 47d, a cleaning unit 47e, a transfer unit 47f, and the like are arranged around the photosensitive member 47a, and the recording paper 41a, 42a, 43a, 44a is disposed on the recording paper 41a. Image formation is performed by electrophotography. The toner images thus formed on the recording papers 41 a, 42 a, 43 a, 44 a are fixed by the fixing unit 48 and discharged from the discharge rollers 49, 50 onto the paper discharge tray 51.

  Document image data given to the laser writing unit 47 c is read by the scanner unit 33 and / or the ADF 34. The scanner unit 33 is a bed-type document reading device that irradiates illumination light onto a book or a single document placed on the document placement table 52 and reads the reflected light. On the other hand, the ADF 34 is a document feeding type document reading device that sequentially reads the sheet document 54 stacked on the document tray 53 and reads the document image, and the read document is discharged onto the discharge tray 55. The document image data read in this way is subjected to image processing such as density adjustment and contour enhancement in an image processing unit (not shown), and is given to the laser writing unit 47c.

  It should be noted that in this embodiment, the writing exposure amount in the laser writing unit 47c, which is the exposure means, is determined including not only the photosensitive member dark potential Vo but also the photosensitive member light potential VL, and the photosensitive member 47a. That is, the exposure amount when the potential becomes (Vo−VL) / 2 + VL is set to the basic light amount Eb. The actual exposure amount is set to 2.25 to 8 times, preferably 2.8 to 3.4 times, more preferably 3.15 times the basic light amount Eb, as will be described later.

  It should also be noted that the standard value Vref is set for the photoconductor light potential VL, and the photoconductor dark potential Vo and the developing bias VB in the developing device 47d as developing means are set by the difference ΔV of VL−Vref. That is biased.

  Specifically, when the photoreceptor A with VL = 80 V and the photoreceptor B with VL = 30 V shown in FIG. 6 are used, the photoreceptor A has a higher photoconductor light potential VL than the photoreceptor B. Since the reproducibility of the solid image is deteriorated, Vref = 30V, and the development bias VB is increased by the difference ΔV = 50V.

However, when the developing bias VB is increased and the potential difference between the photosensitive member dark potential Vo and the developing bias VB becomes small, fogging occurs. Therefore, for the photosensitive member A, Vo = 400V. Thus, when the relationship between the exposure amount and the amount of decrease in the photoreceptor surface potential is obtained in the same manner as in FIG. 6, the photoreceptor A is as shown in FIG. 2, and the photoreceptor B is as shown in FIG. Then, from the above equation, 240V for photoconductor A, the exposure amount 0.26μJ / cm ² and 0.29μJ / cm ² when a 190V for photoconductor B, and each of the basic amount Eb Is set.

  FIG. 4 shows the change in the output area ratio when the exposure amount is changed in the same manner as in FIG. 8 using these values as the evaluation pattern for the pattern shown in FIG. It is understood that the characteristics of the output area ratios of the photoconductors A and B substantially match with the multiple change of the basic light amount Eb. Therefore, the basic light amount Eb is determined in consideration of not only the photoconductor dark potential Vo but also the photoconductor light potential VL which varies depending on the photoconductor, so that the photoconductor A having a high photoconductor light potential VL is low. The reproducibility of the gradation range can be improved (dot reproducibility does not vary).

From FIG. 4, if the exposure amount is twice the basic light amount Eb substantially corresponding to the half exposure amount, there is a possibility that underexposure will occur when the film thickness of the photosensitive film becomes thin due to aging, etc. Although there is a possibility of underexposure due to variations in the amount of light of the laser writing unit 47c and variations in the sensitivity of the photoreceptor film, the film thickness dependence of the photoreceptor light potential VL can be increased by 2.25 times or more. The amount of exposure with no possibility of underexposure can be suppressed, and on the other hand, even if the magnification is too high, the photoconductor film is light fatigued, so that it is 8 times or less, preferably 4 times or less. It is good to do. Particularly preferably, the dot reproducibility of low gradation can be obtained when the magnification is 2.8 to 3.4 times. Among them, from FIG. 4 to 3.15 times, that is, the brightness to the photoconductors A and B. By setting the exposure amount at the point to 0.82 μJ / cm ² and 0.91 μJ / cm ² , the light fatigue of the photoconductors A and B is surely suppressed, and the dot occupation area ratio is ± 1 with respect to the input area ratio. %, It is possible to obtain good dot reproducibility.

  FIG. 9 shows a cross-sectional structure of the photoconductors A and B. These photoconductors A and B are amorphous silicon photoconductors, and the basic structure of the photoconductive layer is an insulating layer 2 made of SiN: H or the like on a conductive substrate 1 made of aluminum or the like, Si (P): H. A charge injection blocking layer 3 made of, for example, a carrier excitation / transport layer (photoconductive layer) 4 made of Si: H or the like, and a surface protective layer 5 made of SiC: H or the like are sequentially laminated thereon. Thus, between the carrier excitation / transport layer (photoconductive layer) 4 and the conductive substrate 1, the insulating layer 2 and the charge injection blocking layer 3 having higher electrical insulation than amorphous silicon (a-Si) are provided. By laminating, the breakdown voltage of the photoconductors A and B against breakdown is increased, and the outflow of photoexcited carriers generated in the carrier excitation / transport layer (photoconductive layer) 4 to the conductive substrate 1 is hindered. Can be achieved.

  In addition, the amorphous silicon photoconductor has advantages such as higher surface hardness and superior durability and handleability compared to a selenium photoconductor and an organic photoconductor. In detail, since the surface hardness of an amorphous silicon photoconductor is high, it is difficult to cause wear deterioration of the photosensitive layer even when repeated image formation is performed, and scratches or pressure marks are not formed on the surface of the photosensitive layer. This is because they are less likely to occur and are easy to handle when assembled to an image forming apparatus.

  Furthermore, it should be noted that in this copying machine 31, an IC memory 47g is mounted on the photoconductor 47a, and information such as the basic light amount Eb and the charging ability that are different for each photoconductor is stored in the IC memory 47g. Is stored, and the control means that reads it changes the writing exposure amount in the laser writing unit 47c.

  FIG. 5 is a block diagram showing a functional configuration of the control means in the copying machine 31 of the present embodiment. When the photoconductor 47a is replaced or when the power is turned on, the reading unit 21 reads information on the basic light amount Eb from the IC memory 47g. The information on the basic light amount Eb is multiplied by the multiple stored in the multiple storage unit 23, 3.15, in the multiplier 22, and the actual exposure amount data is obtained and stored in the exposure amount setting unit 24. The The laser writing unit 47c is controlled by the exposure amount setting unit 24, and the light emission luminance, that is, current, of the semiconductor laser is controlled to irradiate the photoconductor 47a with light corresponding to the original image with a desired exposure amount.

  The IC memory 47g also stores the photosensitive member bright potential VL. Information on the photosensitive member bright potential VL read by the reading unit 21 in the control means is stored in the subtractor 25 by Vref. The difference ΔV is obtained by subtracting the standard value Vref stored in the storage unit 26 and further added to the bias voltage VB stored in the VB storage unit 28 in the adder 27 to obtain the actual bias voltage. And stored in the development bias setting unit 29. The developing unit 47d develops the electrostatic latent image on the photoreceptor 47a with the bias voltage VB set by the developing bias setting unit 29.

  With this configuration, even when the unit of the photoconductor 47a is replaced due to a life or trouble, the control unit can read out necessary information from the IC memory 47g and perform appropriate control. That is, even when the unit is exchanged between the photosensitive member A (Eb = 0.26) and the photosensitive member B (Eb = 0.29), an appropriate exposure amount is set and the same dot can be reproduced. it can. Note that the IC memory 47g may store only the information on the photoreceptor light potential VL, and the basic light quantity Eb may be calculated by the control means.

1 is a longitudinal sectional view showing a mechanical configuration of a copying machine that is an image forming apparatus according to an embodiment of the present invention. 4 is a graph for explaining how to obtain a basic light amount in one photoconductor according to the present invention. It is a graph for demonstrating how to obtain | require the basic light quantity in the 2 photoconductor by this invention. It is a graph which shows the change of the output area rate of two photoconductors when changing the exposure amount based on the basic light quantity calculated | required by this invention. FIG. 3 is a block diagram showing a functional configuration for controlling an exposure amount and a developing bias in the copying machine according to the embodiment of the present invention. It is a graph which shows the relationship of the decreasing amount of the photoreceptor surface potential with respect to the exposure amount of the photoreceptor from which the photoreceptor bright potential used for evaluation of this invention differs. It is a front view of the pattern used for evaluation of the present invention. It is a graph which shows the change of the output area rate of two photoconductors when changing the exposure amount based on the basic light quantity calculated | required by the prior art. It is a figure for demonstrating the cross-sectional structure of the photoreceptor used by this invention.

DESCRIPTION OF SYMBOLS 21 Reading part 22 Multiplier 23 Multiple number memory | storage part 24 Exposure amount setting part 25 Subtractor 26 Vref memory | storage part 27 Adder 28 VB memory | storage part 29 Development bias setting part 31 Copier 32 Main body part 33 Scanner part 34 ADF
47 Image forming unit 47a Photoconductor 47b Charger 47c Laser writing unit 47d Developer 47e Cleaning unit 47f Transfer unit 47g IC memory

Claims (6)

In the image forming apparatus in which the exposure amount written from the exposure unit to the photosensitive member is set to a predetermined multiple of the basic light amount Eb, with the half light exposure amount from the photosensitive member dark potential Vo as the basic light amount Eb.
The photoconductive layer of the photoconductor is made of amorphous silicon. Between the photoconductive layer of the photoconductor and the conductive base material layer, the photoconductive layer is higher in electrical insulation than the amorphous silicon and raises the photoconductor light potential VL. An Si-C layer or Si-N layer to be interposed,
The photoreceptor potential is
(Vo-VL) / 2 + VL
The image forming apparatus is characterized in that the exposure amount at the time is set to the basic light amount Eb. In the image forming apparatus in which the exposure amount written from the exposure unit to the photosensitive member is set to a predetermined multiple of the basic light amount Eb, with the half light exposure amount from the photosensitive member dark potential Vo as the basic light amount Eb.
At least one of an insulating layer and a charge injection blocking layer between the photoconductive layer of the photoconductor and the conductive base material layer, which is higher in electrical insulation than the photoconductive layer and raises the photoconductor light potential VL. Intervened,
The photoreceptor potential is
(Vo-VL) / 2 + VL
The image forming apparatus is characterized in that the exposure amount at the time is set to the basic light amount Eb. The standard value Vref is set for the photoconductor light potential VL, and the photoconductor dark potential Vo and the developing bias VB in the developing unit are biased by a difference ΔV of VL−Vref. 3. The image forming apparatus according to 1 or 2 . The predetermined multiple is 2. The image forming apparatus according to claim 1, wherein the image forming apparatus has a magnification of 8 to 3.4 times, preferably 3.15 times. In the exposure amount adjustment method of the image forming apparatus, the exposure amount written from the exposure unit to the photosensitive member is set to a predetermined multiple of the basic light amount Eb, with the half light exposure amount from the photosensitive member dark potential Vo as the basic light amount Eb.
The photoconductive layer of the photoconductor is made of amorphous silicon. Between the photoconductive layer of the photoconductor and the conductive base material layer, the photoconductive layer is higher in electrical insulation than the amorphous silicon and raises the photoconductor light potential VL. An Si-C layer or Si-N layer to be interposed,
The photoreceptor potential is
(Vo-VL) / 2 + VL
The exposure amount adjustment method for the image forming apparatus is characterized in that the exposure amount at the time is set to the basic light amount Eb. In the exposure amount adjustment method of the image forming apparatus, the exposure amount written from the exposure unit to the photosensitive member is set to a predetermined multiple of the basic light amount Eb, with the half light exposure amount from the photosensitive member dark potential Vo as the basic light amount Eb.
At least one of an insulating layer and a charge injection blocking layer between the photoconductive layer of the photoconductor and the conductive base material layer, which is higher in electrical insulation than the photoconductive layer and raises the photoconductor light potential VL. Intervened,
The photoreceptor potential is
(Vo-VL) / 2 + VL
The exposure amount adjustment method for the image forming apparatus is characterized in that the exposure amount at the time is set to the basic light amount Eb.