JP2007148165A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enable to successfully form high quality images for a long time by a simple configuration. <P>SOLUTION: A charging control means 30 controls charging conditions for a charging means 2 used for a photoreceptor drum 1 on the basis of information measured by a transfer current measuring means 10. Charging for preventing drum ghost is set only when necessary. This prevents the current value of a charging bias from unnecessarily increasing, and thereby prolongs the lifetime of a photoreceptor drum 1. Also, by efficiently removing drum ghost on the photoreceptor drum 1, satisfactory images is always provided. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an image forming apparatus using an electrophotographic method or an electrostatic recording method such as a copying machine or a printer.

  In general, in an image forming apparatus using an electrophotographic method or the like, the surface of a photoconductor is uniformly charged by a charging device. However, in recent years, it is less ozone than a conventional non-contact type corona charger. -Because of the advantages such as low power, a contact-type charging device that contacts a charging member such as a charging roller to which a voltage (charging bias) is applied to the photosensitive member to charge the surface of the photosensitive member to a predetermined polarity / potential. Practical use is in progress. Such a contact-type charging device is configured to output a charging bias in which an AC component and a DC component are superimposed to a photoconductor via a contact charging member such as a conductive roller, and between the contact charging member and the photoconductor. The surface of the photosensitive member is charged with a discharge product caused by a corona discharge phenomenon generated in a minute gap.

  The surface of the photoreceptor uniformly charged by such a charging device is exposed by an exposure device such as a laser to form an electrostatic latent image, and the electrostatic latent image is developed by a developing device, whereby a toner image is formed. Further, after the toner image is transferred onto a transfer material such as paper by a transfer device, it is fixed as a permanently fixed image on the transfer material by the heating and pressurizing action of the fixing device.

  In addition, a process of cleaning the surface of the photoconductor is performed before the next image forming operation is started. A history of a previously formed image (hereinafter referred to as drum ghost) is formed on the surface of the photoconductor after the cleaning process is completed. ) Remains. Therefore, a so-called “run-in” action of the AC component of the AC / DC superimposed bias is applied during charging of the charging device, so that the remaining potential of the photoconductor that has made one round of processing is removed and the target potential is obtained. By performing uniform charging, the drum ghost as described above is eliminated.

  On the other hand, when transferring the developed image formed on the photosensitive member to the transfer material, a constant voltage is applied from the transfer device to any image pattern. If the pattern is different, it will fluctuate. In other words, since the developer has an insulating property, if the amount of the developer is large, the load becomes difficult to flow, and if the amount of the developer is small, a large amount of current is applied to the photoreceptor via the transfer material such as paper. Flowing. Therefore, the potential of the photoconductor partially varies depending on the current flowing through the photoconductor, and drum ghost occurs when the potential difference of the photoconductor cannot be erased by the charging device. For example, in the case of an image pattern in which the image ratio is different in the conveyance direction of the transfer material, the amount of current flowing into the photosensitive member is likely to change and a potential difference is likely to occur. In this case, drum ghost appears remarkably.

  With respect to such a drum ghost, the so-called “run-in” action described above increases as the AC component Vpp value and the Vf value (frequency value) of AC / DC superimposed charging in the charging bias are increased. Further, the smaller the Vpp value and Vf value of the charging bias, the smaller the “run-in” effect on the drum ghost. Therefore, in order to solve the drum ghost problem, it is conceivable to set the Vpp value and the Vf value of the charging bias sufficiently large in advance. However, if they are set excessively, the photoconductor is scraped by discharge. In particular, this is a hindrance in achieving a longer life of the photoreceptor.

  In the apparatus disclosed in Patent Document 1 below, the ghost potential on the surface of the photoconductor is measured, and the pre-exposure amount is changed in accordance with the measured ghost potential, thereby removing the photoconductor potential. ing. However, for example, when an organic photoreceptor is used when the toner is negatively charged and the transfer is positively output, the positive charge cannot be eliminated by pre-exposure, and drum ghosts are similarly generated. .

JP 2000-75677 A

  SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an image forming apparatus that can easily obtain a high-quality image by eliminating the drum ghost while improving the life of the photosensitive member.

  In order to achieve the above object, in the image forming apparatus according to the present invention, the electrophotographic photosensitive member is charged by applying a charging bias to the electrophotographic photosensitive member and a contact charging member in contact with the electrophotographic photosensitive member. In the image forming apparatus comprising: a charging device; and a transfer device that moves a developer image formed on the charging surface of the electrophotographic photosensitive member to a transfer material, a transfer that detects a current value flowing through the transfer device An image forming unit configured to form an image on the transfer material; and a charge control unit configured to switch a charging bias in the charging device in accordance with a transfer current detected by the transfer current detection unit. The charging bias is changed based on the mode and the detection result of the transfer current detecting means.

  According to the developing device having such a configuration, the charging control unit controls the charging condition of the charging unit with respect to the electrophotographic photosensitive member based on the image forming mode at that time and the information measured by the transfer current measuring unit, and the drum Since it is executed only when charging for ghost countermeasures is required, the current value of the charging bias is not unnecessarily excessive, and the life of the photoconductor is extended and the electrophotographic photoconductor The upper drum ghost is efficiently removed so that a good image is always obtained.

  As described above, the present invention controls the charging condition of the charging unit with respect to the electrophotographic photosensitive member by the charging control unit based on the image formation mode at that time and the information measured by the transfer current measuring unit, and the current value of the charging bias. Is used to extend the life of the electrophotographic photosensitive member by avoiding unnecessary excesses, and to efficiently remove drum ghosts on the electrophotographic photosensitive member to always obtain a good image. Therefore, with a simple configuration, a high-quality image can be obtained satisfactorily over a long period of time, and the reliability of the image forming apparatus can be greatly improved at low cost.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
As shown in FIG. 1, a photosensitive drum 1 as an electrophotographic photosensitive member is provided with an organic photosensitive photoconductive layer, and has a predetermined peripheral speed (in the clockwise direction) in the direction of arrow R1 (clockwise direction). It is configured to be rotated at a process speed. In the periphery of the photosensitive drum 1, a contact charging roller 2, an exposure device 3, a developing device 4, a transfer roller (transfer charging device) 5, and an electrostatic separation charging device constituting a charging device in order along the rotation direction R 1. A (separation charger) 6 and a cleaning device 7 are disposed, and a fixing device 8 is disposed on the downstream side of the electrostatic separation charging device 7 in the conveyance direction of the transfer material P.

  Among these, the contact charging roller 2 is connected to a charging bias power source 2 a in which AC / DC is superimposed, and the charging bias applied from the charging bias power source 2 a to the contact charging roller 2 is the outer peripheral surface of the photosensitive drum 1. The surface of the photosensitive drum 1 is uniformly charged with a discharge product based on a corona discharge phenomenon generated in a minute gap between the contact charging roller 2 and the photosensitive drum 1. In this manner, the charging bias applied to the contact charging roller 2 is appropriately switched by a charge control device 30 described later.

In this embodiment, the charging bias value output via the contact charging roller 2 is set as shown in Table 1 below.

  The above-described exposure apparatus 3 irradiates the surface of the photosensitive drum 1 charged by the contact charging roller 2 with the image exposure L corresponding to the image information, and includes a laser driver 20, a laser diode 21, A polygon mirror 22 and a reflection mirror 23 are provided. That is, a laser beam modulated (intensity) corresponding to the time-series electric digital image signal of the image information input to the laser driver 20 is output from the laser diode 21 and passes through the polygon mirror 22 and the reflection mirror 23. Then, image exposure L is performed on the photoconductive layer on the outer peripheral surface of the photosensitive drum 1, and an electrostatic latent image corresponding to the input image information is formed.

  Further, the developing device 4 includes a developing sleeve 4a as a developer carrying member, and the toner (one component magnetic developer) stored in the developing container of the developing device 4 is on the outer peripheral surface of the developing sleeve 4a. The electrostatic latent image formed on the photosensitive drum 1 is coated with a thin layer, and toner is attached to the electrostatic latent image in the developing region so as to be visualized as a toner image.

  Furthermore, the transfer material P such as recording paper is stored in a stack in the paper feed cassette 40, and the paper feed roller 41 is driven on the basis of the paper feed start signal to The material P to be transferred is fed one by one, passes through a transport roller 42, a paper path 44, and a registration roller 43, and is a transfer that is a contact nip portion between the photosensitive drum 1 and the transfer roller 5 as a contact transfer member. It is introduced into the site T at a predetermined timing. That is, the registration roller 43 causes the transfer material P to be synchronized with the timing at which the leading edge of the toner image on the photosensitive drum 1 reaches the transfer site T and the timing at which the leading edge of the transfer material P reaches the transfer site T. The conveyance timing is controlled.

  The material to be transferred P introduced into the transfer site T is nipped and conveyed around the transfer site T. At this time, a predetermined transfer bias is applied to the transfer roller 5 from the transfer bias power source 11. A transfer bias having a polarity opposite to that of the toner is applied to the transfer roller 5, whereby the toner image on the outer peripheral surface of the photosensitive drum 1 is electrostatically transferred to the surface of the transfer material P at the transfer site T. In addition, a transfer current meter 10 serving as a transfer current detecting means detects a transfer current flowing through the photosensitive drum 1 via the transfer roller 5 and the transfer material P.

The transfer roller 5 in this embodiment is composed of an elastic layer 5a of an ion conductive sponge rubber (NBR rubber) and a cored bar 5b, and has a resistance value measured in an N / N environment (23 ° C., 50% RH). A roller of 9.0 × 10 ⁹ is used with 2 kv applied. Furthermore, as a voltage control means for the transfer bias applied to the transfer roller 5, so-called ATVC control (Active Transfer Voltage Control) as disclosed in, for example, Japanese Patent Laid-Open No. 2-123385 is adopted. The ATVC control is a means for optimizing the voltage applied to the transfer roller 5 at the time of transfer. In order to prevent the occurrence of transfer failure or paper trace due to the resistance fluctuation of the transfer roller 5, the ATVC control is performed during the previous multi-rotation process. A desired constant current voltage is applied from the transfer roller 5 to the photosensitive drum 1, and the resistance value of the transfer roller 5 is measured by holding the voltage value at that time, and the transfer roller 5 is always appropriate according to the measurement result. It is provided with a configuration for controlling to apply a proper transfer voltage.

  The transfer material P that has received the transfer of the toner image at the transfer portion T in this manner is separated and conveyed from the photosensitive drum 1 and conveyed and introduced into the fixing device 8 through the paper path 45 to heat and press the toner image. Take the process.

  On the other hand, the transfer residual toner remaining on the outer peripheral surface of the photosensitive drum 1 after the toner image transfer is removed by the cleaning device 7 described above. Further, the drum ghost (residual charge) on the outer peripheral surface of the photosensitive drum 1 is removed by the AC component applied from the contact charging roller 2 as the above-described charging device, and is uniformly charged to the target potential.

  Here, as described in the section of the prior art, the drum ghost (residual charge) remaining on the photosensitive drum 1 in each image forming process is AC / DC in the charging bias applied from the contact charging roller 2. The higher the AC component Vpp value and Vf value of superimposed charging, that is, the greater the effect of uniform charging by so-called “running” by the charging bias, the better the level is suppressed, and AC / DC superimposed charging AC at the charging bias is suppressed. The smaller the component Vpp value and the Vf value, that is, the smaller the effect of uniform charging by the “run-in” due to the charging bias, the lower the level of the component. This is considered to be due to the difference in the charge eliminating action of the contact charging roller 2.

  Here, FIG. 2A shows an experimental result representing the relationship between the transfer current and the drum ghost, and FIG. 2B shows an experimental result representing the relationship between the charging setting and the ghost. The original pattern shown in FIG. 3A was used to determine the quality of the image when examining FIG. That is, an example of an image model formed by using the original pattern according to FIG. 3A is shown in FIG. 3B. As shown in FIG. The density difference that does not occur becomes a drum ghost, and appears as a density difference in the subsequent halftone image area.

  First, FIG. 2A is an experimental result of determining the transfer current value dependency of the drum ghost, and the drum ghost deteriorates as the transfer current increases. On the other hand, as shown in FIG. 2B, the drum ghost was reduced by increasing the charging frequency of the charging bias applied from the contact charging roller 2. It is considered that the so-called “running” action of the potential is increased by increasing the frequency of the charging bias.

  Therefore, in order to solve the problem of the drum ghost, it is conceivable to set the charging frequency of the charging bias high in advance. However, as described in the section of the prior art, if the charging bias frequency is increased, It has been found that it affects the life of the photosensitive drum, and it is difficult to achieve both a longer life of the photosensitive drum and a drum ghost. In general image forming apparatuses, the charging bias frequency is generally set to be low in consideration of running cost.

  Under such circumstances, in this embodiment, the transfer current value detected by the transfer ammeter 10 connected to the transfer roller 5 is output to the charge control device (CPU) 30. In the charging control device 30, the transfer current value detected by the transfer ammeter 10 is compared with a preset reference current value. If the transfer current value is larger than the reference current value, the ghost countermeasure charging setting is set. A configuration is adopted in which the frequency of the charging bias output to the contact charging roller 2 is controlled so as to increase.

  An example of charging bias control using such a charging control device 30 will be described with reference to a flowchart shown in FIG.

  First, the user selects whether the print mode to be started is “character mode” or “graphic mode” in which halftones are frequently used (step S1). When the “graphic mode” is not selected, image formation is performed with the normal charge setting similar to the conventional one (steps S2 and S3). On the other hand, when the “graphic mode” is selected (Yes in step S1), image formation is once started with normal charge setting similar to the conventional one (step S4), and the transfer paper P is transferred to the transfer portion. After entering T, measurement of the transfer current is started (step S5).

  As a result of the measurement of the transfer current, if the detected transfer current value is less than 6 μA (No in step S6), it is confirmed whether the image formation is completed (step S7), and if not completed (step S7). No) The measurement of the transfer current value is continued (step S5). On the other hand, when the detected transfer current value is 6 μA or more (Yes in step S6), the process proceeds to drum ghost countermeasure charging setting (step S8), and the frequency of the charging bias applied to the contact charging roller 2 is reached. Is increased to 1400 Hz.

  As described above, in this embodiment, the charging control unit 30 controls the charging condition of the contact charging roller 2 with respect to the photosensitive drum 1 based on the image forming mode and the information measured by the transfer ammeter 10 as the transfer current measuring unit. It is executed only when the charging setting for drum ghost countermeasure is necessary. Accordingly, the current value of the charging bias is not unnecessarily excessive, and the life of the photosensitive drum 1 is extended, and the drum ghost on the photosensitive drum 1 is efficiently removed, so that a good image is always obtained. can get.

  As mentioned above, although the embodiment of the invention made by the present inventor has been specifically described, the present invention is not limited to the above embodiment, and it goes without saying that various modifications can be made without departing from the scope of the invention. Nor.

  For example, in the above-described embodiment, the control is performed so as to change the frequency of the charging bias, but it is also possible to control so as to change the current value of the charging bias or both of them.

  The above-described embodiment is based on the premise that a one-component magnetic developer is used. However, the present invention can also be applied to the case where a one-component nonmagnetic developer or the like is used. It has been confirmed that formation can take place.

  Further, in the above-described embodiment, the image forming mode is selected by the user, but control is performed so that the control device of the apparatus main body unit determines the image forming mode based on the input image data or the read image data. It is also possible to make it.

  The image forming apparatus according to the present invention described above can be widely applied to a wide variety of image forming apparatuses including a copying machine and a printer.

1 is a schematic configuration diagram schematically illustrating the structure of an image forming apparatus to which the present invention is applied. It is the table | surface showing the generation | occurrence | production state of a drum ghost, Comprising: (a) shows the experimental result of the relationship between a transfer electric current and a drum ghost image, (b) shows the experimental result of the relationship between a charging frequency and a drum ghost image. Is. It is a model figure of a drum ghost image sample. It is a flowchart of pre-exposure amount control based on charge control in an embodiment of the present invention.

Explanation of symbols

1 Photosensitive drum (electrophotographic photosensitive member)
2 Contact charging roller (charging device)
2a Charging bias power supply 3 Exposure device (exposure means)
4 Developing device 5 Transfer roller (transfer device)
10 Transfer ammeter (Transfer current measuring means)
30 Charge control device

Claims (3)

An electrophotographic photosensitive member, a charging device that charges the electrophotographic photosensitive member by applying a charging bias to a contact charging member in contact with the electrophotographic photosensitive member, and a charging processing surface of the electrophotographic photosensitive member. A transfer device that moves the developed developer image to a transfer material,
A transfer current detecting means for detecting a current value flowing through the transfer device; and a charge control means for switching a charging bias in the charging device according to the transfer current detected by the transfer current detecting means,
The image forming apparatus, wherein the charging control unit is configured to change the charging bias based on an image forming mode for forming an image on the transfer material and a detection result of the transfer current detecting unit. .   The image forming apparatus according to claim 1, wherein the charging control unit is configured to change a frequency of the charging bias. The image forming apparatus according to claim 1, wherein the charging control unit is configured to change a current value of the charging bias.

Images