JP2007114418A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus having charging rollers capable of uniformly charging the photoreceptor surface even in image formation on a large number of sheets, by coping with factors in decreasing efficiency in charging toner, the external additive agent of toner, an abrasive material, the filler of paper, and so on. <P>SOLUTION: The image forming apparatus includes the plurality of charging rollers which charge the photoreceptor surface in contact with the photoreceptor while rotated having a peripheral speed difference between the photoreceptor and them. In the image forming apparatus, the dynamic friction coefficient of the upstream charging roller in the rotating direction of the photoreceptor is set greater than the dynamic friction coefficient of the downstream charging roller. Thus, the surface contaminants of the charging roller is collected by the upstream charging roller. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile, or a composite machine using an electrophotographic system, and more particularly to an image forming apparatus in which a photosensitive member is charged using a charging roller. is there.

  In image forming apparatuses such as copying machines, printers, facsimiles, and composite machines using electrophotography, charging by the charging device on the surface of the photosensitive member, exposure based on image data, and electrostatic latent image formed by the exposure The image is formed by steps such as development by the developing device, transfer of the toner image formed by development onto the paper, and fixing of the toner image on the paper by heating.

  Of these, charging devices using corona discharge, such as corotron and scorotron, have been used as charging devices for charging the surface of the photosensitive member. However, these devices generate ozone that adversely affects the environment. Recently, since this ozone is considered to be an environmental problem, a charging roller system that generates a very small amount of ozone is becoming mainstream.

  However, the charging roller system is a toner that passes through the cleaning member, silica that is an external additive to the toner, alumina, titanium oxide as an abrasive for polishing the surface of the photoreceptor, and a filler for paper. Calcium carbonate, talc, kaolin, etc. adhere to the surface of the charging roller, increasing the surface resistance and lowering the charging efficiency. For example, in an image forming apparatus that forms a large number of 300,000 images, it has a long service life. There is a problem with the charging method.

  In order to deal with such a problem, for example, in Patent Document 1 and Patent Document 2, a cleaning member to which a voltage is applied so as to generate an electric field in the charging direction is provided on the upstream side of the charging roller in the rotation direction of the photosensitive member ( Patent Document 1) and a charging blade in which a DC voltage is applied to an AC voltage are also provided on the upstream side in the rotational direction (Patent Document 2).

  In addition, the charging roller system gives charge to the surface of the photoconductor by discharging at a minute gap at both ends of the nip formed by the charging roller in contact with the photoconductor, so that the charging efficiency is higher than that of the scorotron charging system. There is also a problem that it is low and difficult to charge uniformly.

  Therefore, the charging roller can be secured by increasing the outer diameter of the charging roller to ensure a minute gap range, or by arranging a plurality of charging rollers as shown in Patent Document 3, and further to the photosensitive member. Uniform charging has been performed.

JP-A-5-2322843 JP-A-9-281773 JP 2004-325679 A

  However, in the techniques proposed in Patent Documents 1 and 2, there are toners that pass through the cleaning blade and the charging blade, toner external additives, and abrasives and paper fillers. There is a possibility that the surface resistance increases and the charging efficiency decreases.

  Further, Patent Document 3 describes that a conductive elastic roller is used as the charging roller and the potential leveling roller, but the toner adhering to the surface of the charging roller, the toner external additive, and the abrasive There is no description about dealing with the sheet filling material and the like, and it is difficult to maintain the charging efficiency in the image formation of a large number of 300,000 sheets as described above.

  Therefore, in the present invention, it is possible to deal with the factors that lower the charging efficiency such as toner and toner external additives, and abrasives and paper fillers, so that the surface of the photoreceptor can be uniformly charged even in the formation of a large number of sheets. An object of the present invention is to provide an image forming apparatus including the charged roller.

In order to solve the above problems, an image forming apparatus according to the present invention provides:
A photosensitive member that forms and carries a toner image by an electrophotographic method, and a plurality of charging rollers that contact the photosensitive member while rotating at a peripheral speed ratio with the photosensitive member to charge the surface of the photosensitive member. In the image forming apparatus,
The dynamic friction coefficient of the charging roller provided on the upstream side in the rotation direction of the photosensitive member is set to be larger than the dynamic friction coefficient of the charging roller provided on the downstream side, and surface contaminants on the charging roller are collected by the upstream charging roller. It is characterized by.

  By doing so, the charging roller provided on the upstream side in the rotation direction of the photosensitive member can be used for toner passing through the cleaning member, an external additive to the toner, an abrasive on the surface of the photosensitive member, a paper filler, etc. It collects contaminants on the surface of the charging roller and prevents surface contamination of the charging roller provided on the downstream side in the rotation direction of the photoconductor. Therefore, the surface of the downstream charging roller can be kept clean for a long period of time to maintain a predetermined charging property. An image forming apparatus that can uniformly charge the surface of the photoreceptor can be provided.

  One means for increasing the dynamic friction coefficient of the upstream charging roller is to make the surface roughness of the upstream charging roller larger than the surface roughness of the downstream charging roller.

  The means for increasing the dynamic friction coefficient of the upstream charging roller may be such that the peripheral speed ratio of the upstream charging roller to the photosensitive member is larger than the peripheral speed ratio of the downstream charging roller to the photosensitive member. The peripheral speed of the upstream charging roller is preferably faster than the peripheral speed of the photoconductor.

  Further, the means for increasing the dynamic friction coefficient of the upstream charging roller is such that at least the surface layer constituent material of the upstream charging roller and the downstream charging roller is different, and the dynamic friction coefficient of the constituent material of the upstream charging roller is determined by downstream charging. It may be made of a material larger than the dynamic friction coefficient of the constituent material of the roller. In this case, at least the surface layer of the charging roller is made of foam rubber, and the dynamic friction coefficient of the foam rubber of the upstream charging roller is set to the downstream charging The foaming state may be changed so as to be larger than the dynamic friction coefficient of the foamed rubber of the roller.

  The charging bias applied to the charging roller is configured by superimposing alternating current on direct current, and the alternating voltage value at the charging bias applied to the upstream charging roller is made larger than the alternating voltage value applied to the downstream charging roller. Thus, the electrical recovery force of the charging roller provided on the upstream side in the rotation direction may be increased. In this case, the DC voltage value at the charging bias applied to the upstream charging roller is set as the downstream charging roller. It may be larger than the DC voltage value applied to, and the combined use of both can increase the electrical recovery power.

  As described above, according to the present invention, since the external additive or the like that has passed through the cleaning blade is collected by the charging roller provided on the upstream side in the rotation direction of the photosensitive member, the external additive to the charging roller on the downstream side in the rotation direction. It is possible to provide an image forming apparatus with reduced adhesion and stable charging performance over a long period of time.

  Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the drawings. However, the dimensions, materials, shapes, relative arrangements, and the like of the components described in this embodiment are not intended to limit the scope of the present invention unless otherwise specified, but are merely illustrative examples. Not too much.

  FIG. 1 is a diagram showing an outline of a photosensitive drum 11 and image forming process means arranged around the photosensitive drum 11 in the image forming apparatus of the present invention. For example, around the photosensitive drum 11 using an amorphous silicon photosensitive member, the rotational direction of the photosensitive drum 11 with respect to the two rollers (A) 14 and the roller (A) 14 along the rotational direction indicated by an arrow. A charging roller comprising a roller (B) 13 arranged on the downstream side and charging the surface of the photosensitive drum 11, and further exposing the photosensitive drum 11 on the downstream side based on the image data to form an electrostatic latent image. An exposure device 15 for developing, a developing device 17 for developing an electrostatic latent image formed by exposure, a transfer roller 19 as transfer means for transferring a toner image formed by development onto a sheet, and a toner image A cleaning roller 25 for cleaning toner remaining on the photosensitive drum 11 after being transferred to the paper, a cleaning blade 21, and the photosensitive drum 11 Such as erase means 23 for removing the remaining charges is provided.

  Among these, for example, the amorphous silicon photoconductor 11 has an outer diameter of 30 mm, an amorphous silicon film thickness of 20 μm, a length of 254 mm corresponding to A4, and a peripheral speed of 150 mm / sec. The charging rollers 13 and 14 have a diameter of 12 mm, a length of 220 mm, and a conductive rubber wound around a core metal having an outer diameter of 6 mm. The surface potential of the photosensitive drum 11 is about An alternating current (AC) of 1.4 kHz and a frequency of 1.5 kHz is superimposed on a 400 V direct current (DC) so as to be 250 V.

  In forming an image, the surface of the amorphous silicon photosensitive drum 11 is first charged by the charging rollers (B) 13 and (A) 14 to which a bias in which AC is superimposed on DC is applied as described above, and then printing is performed. A latent image is formed by exposure by the exposure unit 15 using the optical signal converted based on the data. Then, the latent image is developed by the developing unit 17 to be visualized as a toner image, transferred onto a sheet by the transfer roller 19, and then conveyed to a fixing device (not shown) to fix the toner image. At this time, the residual toner remaining on the photosensitive drum 11 without being transferred is removed from the surface of the photosensitive drum 11 by the cleaning roller 25 and the cleaning blade 21, and then transported to a waste toner bottle and remains on the photosensitive drum 11. The charged charges are removed by the erasing means 23.

  As described above, in a system using a plurality of charging rollers (B) 13 and (A) 14, the charging roller (A) 14 disposed on the upstream side in the rotation direction of the photosensitive drum 11 is usually the photosensitive drum 11. It is used as a surface potential leveling roller, and finally supplying a uniform charge to the surface of the photoconductor greatly depends on the performance of the charging roller (B) 13 disposed on the downstream side in the photoconductor rotation direction. .

  Therefore, in the case where the charging roller (A) 14 is a surface potential leveling roller on the photosensitive drum 11, a bias voltage (in this example, as an example) in which an alternating current is superimposed on a direct current applied to the charging roller (B) 13. It is only necessary to supply the charging roller (A) 14 with an applied voltage capable of setting a surface potential relatively close to the surface potential of the photosensitive member determined by Vdc = 400 V, Vpp = 1.4 KV). As long as the surface potential can be raised to a predetermined voltage as described above, there is no need to define the voltage to be applied on the 14 side.

  In the present invention, paying attention to the fact that the charging roller (B) 13 is highly dependent on the uniform charging as described above, the dynamic friction coefficient of the charging roller (A) 14 is made larger than the dynamic friction coefficient of the charging roller (B) 13, As described above, the toner slipping through the cleaning member, silica as an external additive to the toner, alumina, titanium oxide as an abrasive for polishing the surface of the photoreceptor, and carbonic acid as a filler for paper Substances that contaminate the surface of the charging roller such as calcium, talc, and kaolin are collected by the charging roller (A) 14 to prevent surface contamination of the charging roller (B) 13.

  That is, by making the dynamic friction coefficient of the charging roller (A) 14 larger than the dynamic friction coefficient of the charging roller (B) 13, the charging roller (A) 14 rubs the photosensitive drum 11 more strongly, and charging Since the amount of the material that contaminates the surface of the roller is constant, the amount of the surface contamination toward the charging roller (B) 13 decreases accordingly.

  As a specific method for changing the dynamic friction coefficient between the charging roller (A) 14 and the charging roller (B) 13, for example, (1). The material of the charging roller (A) 14 and the charging roller (B) 13 is changed. Ordinarily, the charging roller is wound with, for example, foamed ethylene-propylene rubber (EPDM) or the like made conductive around the core metal as described above. It is turned (2). Changing the foaming state of the foamed rubber, (3). Changing the surface roughness Rz of the charging roller (A) 14 and the charging roller (B) 13, (4). For example, the peripheral speed ratio of the charging roller (A) 14 and the charging roller (B) 13 to the photosensitive drum 11 may be changed.

  In the following, each method will be described in detail. The present inventors charged the photosensitive drum 11 when the surface resistance was increased by the contamination of the charging roller with the contaminants as described above. The effect on gender was examined.

The graph of FIG. 2 shows the result. In the graph of FIG. 2, the horizontal axis represents the surface resistance (unit: logΩ) of the charging roller, and the vertical axis represents the surface potential V ₀ (unit: V) of the photosensitive drum 11. A bias in which an alternating current (AC) of 1.4 kHz and a frequency of 1.5 kHz is superimposed on a 400 V direct current (DC) is applied to the charging roller so that the surface potential V ₀ is about 250 V, and the charging roller has a surface resistance of 4 The surface potential of the photosensitive drum 11 is changed by changing the voltage from about 0.5 (log Ω) to about 7.3 (log Ω).

  As a result, it was found that when the surface resistance value of the charging roller was 7 (log Ω) or more, the discharge efficiency was lowered and the surface potential of the photosensitive drum 11 was lowered. That is, when the surface resistance of the charging roller (B) 13 is 7 (log Ω) or less at the time of printing 300,000 sheets as described above, this indicates that a predetermined charging property can be maintained. Even after printing 300,000 sheets by changing the dynamic friction coefficient of the roller (A) 14 and the charging roller (B) 13 and collecting the substances that contaminate the surface by the charging roller (A) 14, It shows that the surface resistance of the charging roller (B) 13 should be 7 (log Ω) or less.

  Although the method of measuring the dynamic friction coefficient is difficult, it is difficult to directly measure the dynamic friction coefficient between the photosensitive drum 11 and the charging rollers (A) 14 and (B) 13. An index indicating the difference in the dynamic friction coefficient by measuring the driving current of the motor that drives the roller (A) 14 and the charging roller (B) 13 and the difference in the driving current between the charging roller (A) 14 and the charging roller (B) 13. It was decided that.

  Specifically, first, the photosensitive drum 11 is rotated at a peripheral speed of 60 mm / sec, and the charging roller (A) 14 and the charging roller (B) 13 are rotated in the same direction as the photosensitive drum 11 at a 1.5 times peripheral speed. The current value of the DC brushless motor for rotating and rotating each charging roller was measured, and the value was used as an index indicating the difference in the dynamic friction coefficient.

The charging roller (A) 14 and the charging roller (B) 13 drive the charging roller (A) 14 by using the second method of changing the foaming state of the foam rubber described in the method for changing the dynamic friction coefficient. Conduction wound around the core of the charging roller (A) 14 so that the current value of the motor is 1.2 times, 1.6 times and 2 times the current value for driving the charging roller (B) 13. sexual foamed ethylene - compounding of the foamed rubber in the propylene rubber (EPDM), and the like number of foam cells controlled under the conditions at the time of molding, the cell with 142 pieces ^/ cm 2, 220 pieces ^/ cm ^2, 327 ^/ cm 2 Then, 100,000 sheets were printed, and changes in the surface resistance of the charging roller (A) 14 and the charging roller (B) 13 were investigated.

  The graph of FIG. 3 shows the result. In FIG. 3, the horizontal axis represents the dynamic friction coefficient ratio of the charging roller (A) 14 and the charging roller (B) 13 (that is, the driving current ratio of the motor that drives both charging rollers), and the vertical axis represents the surface resistance ( Unit: log Ω). Further, ■ represents the charging roller (A) 14 and ● represents the charging roller (B) 13. The surface resistance of the initial charging roller (B) 13 was 5 (log Ω).

  As is apparent from the graph of FIG. 3, when the dynamic friction coefficient is 1.2 times, the surface resistance of the charging roller (A) 14 is about 6.9 (log Ω) and the surface resistance of the charging roller (B) 13 is about 6 The surface resistance of the charging roller (A) 14 is about 7.4 (logΩ) and the surface resistance of the charging roller (B) 13 is about 5.8 (logΩ). In the double ratio, the surface resistance of the charging roller (A) 14 is about 7.4 (log Ω), and the surface resistance of the charging roller (B) 13 is about 5.4 (log Ω).

  That is, when the dynamic friction coefficient is 1.2 times and 1.6 times, the surface resistance 5 (log Ω) of the initial charging roller (B) 13 is 1.0 (log Ω), 0.8 ( If the printing of 300,000 sheets, which is the object of the present invention, is performed as it is, the surface resistance of the charging roller (B) 13 becomes 7.4 (logΩ) even at 1.6 times, and 7 (logΩ). ). Accordingly, in order to maintain the surface resistance 5 (log Ω) of the charging roller (B) 13 in the initial state or not to exceed at least 7 (log Ω), the charging roller (A) 14 and the charging roller (B ) It is understood that the dynamic friction coefficient ratio of 13 needs to be set to 2 times or more.

  In addition, the charging roller (A) 14 increases the dynamic friction coefficient so that the above-described toner or silica as an external additive of the toner, titanium oxide or alumina as an abrasive, calcium carbonate as a paper filler, talc. The resistance value of the charging roller (B) 13 is increased by being contaminated with kaolin and the like, but the increase of the resistance value of the charging roller (B) 13 is suppressed to a low level. Assuming that the external additive and paper filler slipping through the charging roller are constant, the sum of the external additives recovered by the charging roller (A) 14 and the charging roller (B) 13 is constant, so that the charging roller (A) 14 This is because as the recovery amount increases, the recovery amount (attachment amount) at the charging roller (B) 13 decreases and the resistance rise is suppressed.

  Further, the method of changing the dynamic friction coefficient is not limited to the method of changing the foaming state of the foamed rubber as described above, but the materials of the charging roller (A) 14 and the charging roller (B) 13 may be changed. By changing, the dynamic friction coefficient ratio between the charging roller (A) 14 and the charging roller (B) 13 may be set to be twice or more according to the experimental result. That is, as described above, the charging roller is formed by winding a rubber containing a conductive material around a cored bar. The rubber includes foamed ethylene-propylene rubber (EPDM), foamed ethylene-propylene rubber (EPDM), and rubber. A tube-covered one, solid type ethylene-propylene rubber (EPDM), or the like can be used.

  In these materials, foamed ethylene-propylene rubber (EPDM) has a larger dynamic friction coefficient than foamed ethylene-propylene rubber (EPDM) covered with a rubber tube, and foamed ethylene-propylene rubber (EPDM) covered with a rubber tube. Since the dynamic friction coefficient is larger than that of solid type ethylene-propylene rubber (EPDM), foamed ethylene-propylene rubber (EPDM) is used for the charging roller (A) 14 and foamed ethylene-propylene is used for the charging roller (B) 13. A rubber (EPDM) covered with a rubber tube or a solid type ethylene-propylene rubber (EPDM) is used. Similarly, a foamed ethylene-propylene rubber (EPDM) covered with a rubber tube is used for the charging roller (A) 14, and a solid type ethylene-propylene rubber (EPDM) is used for the charging roller (B) 13. Also good.

  In this way, as described above, the surface resistance of the charging roller (B) 13 can be maintained at a required value without being contaminated even after printing a large number of sheets. Can be provided.

  Next, the third method of changing the dynamic friction coefficient is a method of changing the surface roughness Rz of the charging roller (A) 14 and the charging roller (B) 13. The influence of the roughness on the charging property of the photosensitive drum 11 was examined.

  As an important performance of the charging roller, the uniform charging property of the photosensitive drum 11 can be improved. This uniform charging property can be determined by measuring the fog caused by development. Therefore, first, charging rollers having a surface roughness Rz of 5 μm, 10 μm, 15 μm, and 20 μm are prepared, and a bias is applied to these charging rollers so that the surface potential of the photosensitive drum 11 is changed from 200 V to 340 V. The state of fogging was examined.

The results are shown in the graph of FIG. In the graph of FIG. 4, the horizontal axis represents the surface potential (V ₀ ) of the photosensitive drum 11, the vertical axis represents fog, ◆ represents the charging roller surface roughness Rz of 5 μm, ■ represents the same 10 μm, and ▲ represents the same 15 μm. ● is also 20 μm, and the fog needs to be 0.010 or less in terms of image quality.

  From the graph of FIG. 4, it can be seen that the lower the surface roughness Rz of the charging roller, the better the level of fog and the better the uniform charging property. Further, when the surface potential of the photosensitive drum 11 is set in a range of 240 to 300 V, the allowable surface roughness Rz of the charging roller is 10 μm or less.

  Based on the above results, after adjusting the surface resistance of the charging rollers (A) 14 and (B) 13 to about 5 (log Ω) and further fixing the surface roughness Rz of the charging roller (B) 13 to 10 μm, The results of examining the relationship between the number of printed sheets and the surface resistance of the charging rollers (A) 14 and (B) 13 by changing the surface roughness Rz of the charging roller (A) 14 to 10 μm, 15 μm and 20 μm are shown in FIGS. FIG.

  In these graphs, the horizontal axis is the number of printed sheets (× 1000), and the vertical axis is the surface resistance (unit: logΩ) of the charging rollers (A) 14 and (B) 13, which is the upper limit of the surface resistance of the charging roller. A certain 7 (log Ω) position is indicated by a thick horizontal line. Further, ◆ represents the value of the charging roller (A) 14 and ● represents the value of the charging roller (B) 13.

  First, as shown in the graph of FIG. 5, when the surface roughness Rz of the charging roller (A) 14 is 10 μm, the charging roller (A) 14 has 200,000 prints and the surface resistance is 7 (logΩ), which is the upper limit described above. ) And the charging roller (B) 13 has approximately 250,000 sheets, and the surface resistance similarly reaches the upper limit of 7 (log Ω).

  On the other hand, in the graphs of FIGS. 6 and 7 in which the surface roughness Rz of the charging roller (A) 14 is 15 μm and 20 μm, the charging roller (A) 14 has 200,000 prints, respectively, and the surface resistance exceeds the upper limit described above. 7 (log Ω), the charging roller (B) 13 reaches 7 (log Ω) for 300,000 printed sheets when the surface roughness Rz in FIG. 6 is 15 μm, and the surface roughness Rz in FIG. 7 is 20 μm. Then, even when the number of printed sheets is 300,000, the surface resistance is about 6 (log Ω) and does not reach 7 (log Ω).

  That is, when the surface roughness Rz of the charging roller (A) 14 is the same as the surface roughness Rz of the charging roller (B) 13, the surface resistance of the charging roller (B) 13 is the upper limit up to 300,000 printed sheets. (Log Ω) and the surface potential of the photosensitive drum 11 cannot be charged to a predetermined potential. However, by setting the surface roughness Rz of the charging roller (A) 14 to 15 μm or more, the number of printed sheets is 300,000. However, it can be seen that the surface potential of the photosensitive drum 11 can be charged to a predetermined potential.

  This is because when the surface roughness Rz of the charging roller (A) 14 is set to 15 μm or more, as described above, the charging roller (A) 14 causes the toner or the silica as an external additive of the toner and the oxidation as the abrasive. This indicates that titanium, alumina, paper fillers such as calcium carbonate, talc, and kaolin are collected, so that the charging property of the charging roller (B) 13 can be maintained even when the number of printed sheets is 300,000. That is, when the surface roughness Rz of the charging roller (A) 14 is made larger than that of the charging roller (B) 13, the dynamic friction coefficient of the charging roller (A) 14 is made larger than that of the charging roller (B) 13. As described above, surface contamination of the charging roller (B) 13 can be prevented.

  Although the method of changing the surface roughness Rz is generally used, the charging roller is generally a roller in which a conductive material is wound around a core metal as described above. The rotated rubber is polished to make the outer diameter uniform, and the surface roughness Rz can be controlled by changing the polishing conditions. Usually, in rough polishing, the surface roughness Rz is about 20 μm, but the surface roughness Rz can be increased to about 5 μm by finishing. Therefore, the surface roughness may be controlled to be 15 μm or more as described above by controlling the surface roughness according to the feed speed of the polishing machine and the type of grindstone in the polishing process.

  By doing so, the surface resistance of the charging roller (B) 13 can be maintained at a required value without being contaminated even after printing a large number of sheets, so that image formation having a highly durable charging roller is possible. An apparatus can be provided.

  A fourth method of changing the dynamic friction coefficient is a method of changing the peripheral speed ratio of the charging roller (A) 14 to the photosensitive drum 11, and the rotational speed is increased by increasing the peripheral speed in this way. As a result, the rubbing force of the charging roller is increased, and surface contamination of the charging roller (B) 13 can be prevented.

  Accordingly, the peripheral speed ratio of the charging roller (A) 14 to the photosensitive drum 11 is set to follow (constant speed), 1.5 times, and 2.0 times, and the charging roller (A) after printing 100,000 sheets. 14, (B) The surface resistance value of 13 (namely, the state of surface contamination of the charging roller) was examined.

  The result is shown in FIG. In FIG. 8, the horizontal axis is the peripheral speed ratio of the charging roller (A) 14 to the photosensitive drum 11, the vertical axis is the surface resistance (log Ω) of the charging rollers (A) 14 and (B) 13, and The charging roller (A) 14 and ● are values of the charging roller (B) 13. The initial surface resistance of the charging roller (B) 13 (before starting printing) is 5 (log Ω), and the surface resistance after printing 100,000 sheets is plotted for each peripheral speed ratio.

  As apparent from the graph of FIG. 8, when the peripheral speed ratio of the charging roller (A) 14 to the photosensitive drum 11 is driven (constant speed), the surface resistance of the charging roller (A) 14 is about 6.5 ( log Ω), the surface resistance of the charging roller (B) 13 is almost 6.3 (log Ω), and the surface resistance of the charging roller (A) 14 is about 7.2 (log Ω) and 7 at 1.5 times. (Log Ω) is exceeded, the surface resistance of the charging roller (B) 13 is about 5.8 (log Ω), and the difference is greatly widened. In the case of 2.0 times, the surface resistance of the charging roller (A) 14 is about 7.5 (log Ω), the surface resistance of the charging roller (B) 13 is about 5.5 (log Ω), and the difference Is getting bigger.

  That is, when the peripheral speed ratio is constant and 1.5 times, the surface resistance 5 (log Ω) of the initial charging roller (B) 13 is about 6.3 (log Ω) and about 5.8 by printing 100,000 sheets. (Log Ω), which is about 1.3 (log Ω) and about 0.8 (log Ω) higher than the initial surface resistance of 5 (log Ω). Therefore, when printing 300,000 sheets, which is the object of the present application, is almost tripled, the peripheral speed ratio is 8.9 (log Ω) at a constant speed, 7.4 when the peripheral speed ratio is 1.5 times, (LogΩ) and 7 (logΩ) are exceeded. Accordingly, in order to maintain the surface resistance 5 (log Ω) of the charging roller (B) 13 in the initial state or not to exceed at least 7 (log Ω), the charging roller (A) 14 and the charging roller (B ) It is understood that the dynamic friction coefficient ratio of 13 needs to be set to 2 times or more.

  From this result, in order to maintain the initial resistance value 5 (log Ω) of the charging roller (B) 13 after printing 300,000 sheets, the charging roller (B) 13 shown in FIGS. Considering the rate of increase in surface resistance, it can be seen that the peripheral speed ratio of the charging roller (A) 14 to the photosensitive drum 11 is preferably set to 2.0 times or more.

  In this way, as described above, the surface resistance of the charging roller (B) 13 can be maintained at a required value without being contaminated even after printing a large number of sheets. Can be provided.

  As described above, the dynamic friction coefficient of the charging roller (A) 14 is made larger than the dynamic friction coefficient of the charging roller (B) 13 by various methods, so that toner, toner external additives, paper fillers, etc. Can be recovered more by the charging roller (A) 14, and the charging roller (B) 13 can be prevented from being contaminated and the photosensitive drum 11 can be charged stably over a long period of time.

  As a means for preventing the surface contamination of the charging roller (B) 13, as in the fifth embodiment, the toner that has passed through the cleaning means by the charging roller (A) 14 and the external additive of the toner, and the paper If most of the filler or the like can be collected, surface contamination of the charging roller (B) 13 can be prevented. Therefore, it is conceivable to change the bias applied to each of the charging rollers (B) 13 and (A) 14.

  That is, by setting the charging bias to be applied to the charging roller (A) 14 to be larger than the charging bias to be applied to the charging roller (B) 13, the toner that has passed through the cleaning, the external additive of the toner, and the paper It is possible to increase the electrical recovery power for fillers.

  Specifically, for example, a Vdc voltage of +350 V is applied to the charging roller (A) 14 and a Vdc voltage of +450 V is applied to the charging roller (B) 13. ), The charging roller (B) 14 has a larger positive voltage than the charging roller (B) 14, and the repulsive force is larger than that of the charging roller (A) 13. However, the ability to collect positively charged particles is higher. Therefore, by combining with the method described above, it is possible to further increase the ability to collect contaminants by the charging roller (A) 14.

  In the above description, the case where there are two charging rollers has been described as an example. However, this is not limited to two, and it is obvious that three or more charging rollers may be provided.

  According to the present invention, it is possible to provide an image forming apparatus including a charging roller that can stably and uniformly charge the photosensitive drum 11 over a long period of time and that does not generate substances that adversely affect the environment such as ozone.

FIG. 2 is a diagram showing an outline of a photosensitive drum 11 and image forming process means arranged around the photosensitive drum 11 in the image forming apparatus of the present invention. 6 is a graph showing how the charging property of the photosensitive drum 11 is affected when the surface resistance is increased due to contamination of the charging roller by a contaminant. 6 is a graph showing the resistance values of the respective charging rollers when the ratio of the dynamic friction coefficient is changed. 6 is a graph showing the results of investigating the influence of the surface roughness of the charging roller on the surface potential of the photosensitive drum in the state of fog after development. It is a graph which shows the result of investigating the relationship between the number of printed sheets and the surface resistance of the charging rollers (A) 14 and (B) 13 when the surface roughness Rz of the charging roller (A) 14 is 10 μm. It is a graph which shows the result of investigating the relationship between the number of printed sheets and the surface resistance of the charging rollers (A) 14 and (B) 13 when the surface roughness Rz of the charging roller (A) 14 is 15 μm. It is a graph which shows the result of having investigated the surface resistance Rz of charging roller (A) 14 and (B) 13 by making surface roughness Rz of charging roller (A) 14 into 20 micrometers. 6 is a graph showing the surface resistance values of the charging rollers (A) 14 and (B) 13 after printing 100,000 sheets by changing the peripheral speed ratio of the charging roller (A) 14 to the photosensitive drum 11.

Explanation of symbols

11 Photosensitive drums 13 and 14 Charging roller 15 Exposure unit 17 Development unit 19 Transfer roller 21 Cleaning blade 23 Erase means 25 Cleaning roller

Claims (8)

A photosensitive member that forms and carries a toner image by an electrophotographic method, and a plurality of charging rollers that contact the photosensitive member while rotating at a peripheral speed ratio with the photosensitive member to charge the surface of the photosensitive member. In the image forming apparatus,
The dynamic friction coefficient of the charging roller provided on the upstream side in the rotation direction of the photosensitive member is set to be larger than the dynamic friction coefficient of the charging roller provided on the downstream side, and surface contaminants on the charging roller are collected by the upstream charging roller. An image forming apparatus.   2. The image forming apparatus according to claim 1, wherein a surface roughness of the upstream charging roller is larger than a surface roughness of the downstream charging roller.   2. The image forming apparatus according to claim 1, wherein a peripheral speed ratio of the upstream charging roller to the photosensitive member is larger than a peripheral speed ratio of the downstream charging roller to the photosensitive member.   The image forming apparatus according to claim 3, wherein a peripheral speed of the upstream charging roller is faster than a peripheral speed of the photosensitive member.   The upstream charging roller and the downstream charging roller differ in at least the surface layer constituent material, and the dynamic friction coefficient of the constituent material of the upstream charging roller is made of a material larger than the dynamic friction coefficient of the constituent material of the downstream charging roller. The image forming apparatus according to claim 1, wherein:   At least the surface layer of the charging roller is made of foamed rubber, and the foaming state is changed so that the dynamic friction coefficient of the foamed rubber of the upstream charging roller is larger than the dynamic friction coefficient of the foamed rubber of the downstream charging roller. The image forming apparatus according to claim 5.   The charging bias applied to the charging roller is configured by superimposing alternating current on direct current, and the alternating voltage value at the charging bias applied to the upstream charging roller is made larger than the alternating voltage value applied to the downstream charging roller. The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus.   8. The image forming apparatus according to claim 1, wherein a DC voltage value at a charging bias applied to the upstream charging roller is larger than a DC voltage value applied to the downstream charging roller.

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