JP2008233702A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To automatically set charging biases to the optimum values, in an image forming apparatus that employs a non-contact type charging system using charging rollers and makes frequencies of the charging biases different from one another for each image formation mode according to the linear velocities of image formation processing. <P>SOLUTION: A magenta image forming part 1M, a cyan image forming part 1C, a yellow image forming part 1Y, and a black image forming part 1Bk include photoreceptor drums 2a, 2b, 2c, and 2d respectively with which charging rollers 3a, 3b, 3c, and 3d are brought into contact, respectively. A charging bias in which a DC voltage and AC voltage are superimposed is applied to each of the charging rollers 3a, 3b, 3c, and 3d. The frequencies of the charging biases are made different according to the linear velocities for each image forming mode. During a period when images are not formed, the charging bias is varied to search a voltage value at which a DC current saturates, and then the charging bias is set on the basis of the found voltage value. The charging bias is set for each of the frequencies. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an electrophotographic image forming apparatus.

  In an electrophotographic image forming apparatus, there are roughly two methods for charging a photosensitive drum as an image carrier. One is a non-contact charging method using corona discharge, and the second is a contact charging method using a charging roller or a charging brush.

In the case of the non-contact charging method using corona discharge, the corona product adheres to the surface of the photoreceptor when charging is repeated. Corona products tend to bind to water molecules in the air in a high humidity environment. When water molecules bind to the corona product, the surface resistance of the photoreceptor decreases. As a result, an “image flow phenomenon” occurs. In order to avoid this, recently, a contact charging method using a charging roller is generally adopted. Examples of the image forming apparatus provided with the charging roller can be seen in Patent Documents 1 and 2.
JP 2001-312123 A JP-A-10-232534

  A charging bias in which an AC voltage is superimposed on a DC voltage is applied to a contact charging type charging roller. When the AC voltage is too low, the uniformity of charging is deteriorated and black spots or streak fogging may occur. On the other hand, if the AC voltage is too high, especially in the case of an organic photoreceptor, the film on the surface deteriorates and is easily scraped. Therefore, the AC bias must be set appropriately.

  On the other hand, in an electrophotographic image forming apparatus, the linear speed of an image forming process may be changed for each image forming mode. For example, when there are a full color mode and a monochrome mode, it is often performed to increase the number of output sheets per unit time by increasing the line speed of an image forming process in the monochrome mode. As the line speed increases, the minimum required frequency of AC bias increases. However, even if the frequency is increased unnecessarily, the charging roller response cannot follow and a phenomenon occurs in which charging becomes difficult. Therefore, the AC bias frequency must be set for each linear velocity.

  When changing the AC bias frequency according to the linear speed of the image forming mode, it is uneconomical to prepare a transformer for each frequency. Such a problem arises. That is, since the output voltage of the high-voltage transformer changes depending on the frequency, if the frequency is changed, the output voltage deviates from the set range, and the image quality deteriorates. In particular, when the photosensitive member is amorphous silicon, the allowable voltage fluctuation range is very narrow, which is likely to cause a problem.

  The present invention has been made in view of the above points, and in an image forming apparatus that employs a non-contact charging method using a charging roller and varies the frequency of the charging bias according to the image forming process linear speed for each image forming mode. An object is to automatically set the charging bias to an optimum value. It is another object of the present invention to prevent the setting of the charging bias from causing inconvenience to the user.

  (1) To achieve the above object, the present invention is an electrophotographic image forming apparatus in which a charging bias in which an AC voltage is superimposed on a DC voltage is applied to a charging roller in contact with a photosensitive drum. In the non-image forming period, the charging bias is varied to search for a voltage value at which the direct current is saturated, the charging bias is set based on the found voltage value, and a plurality of images having different imaging process linear speeds A forming mode is provided, and the frequency of the charging bias is varied according to the linear velocity for each image forming mode, and the voltage value is set for each non-image forming period for each frequency.

  According to this configuration, even if the image forming process is switched to change the image forming process linear velocity, and as a result, the charging bias frequency is changed, the charging bias is set for each frequency in advance. The voltage does not fall outside the set range and the image quality does not deteriorate. The charging bias is set during the non-image forming period, and such an operation does not occur during the image forming period, so that the printing operation is delayed and the user does not feel inconvenience.

  (2) According to the present invention, in the image forming apparatus having the above configuration, the AC voltage output of the charging bias is performed by a single transformer.

  According to this configuration, the cost is not increased because a single transformer is used.

  (3) In the image forming apparatus configured as described above, the non-image forming period is a machine warm-up period or a post-aging period after the end of a print job.

  According to this configuration, the charging bias can be automatically set in a flow in which the image forming apparatus is normally used without providing a special period for setting the charging bias.

  According to the present invention, the charging bias can always be kept at an appropriate value even when a plurality of image forming modes having different imaging speeds are set. Further, since the charging bias is set not in the image forming period but in the non-image forming period, the usability does not deteriorate.

  An embodiment of the present invention is shown in FIGS. 1-3. 1 is a schematic configuration diagram of the image forming apparatus, FIG. 2 is a partial block configuration diagram, and FIG. 3 is a flowchart up to the start of a printing operation.

  The image forming apparatus shown in FIG. 1 is a tandem type full-color printer, and includes four image forming units inside the apparatus main body A. That is, the magenta image forming unit 1M, the cyan image forming unit 1C, the yellow image forming unit 1Y, and the black image forming unit 1Bk. These image forming units are arranged in tandem at predetermined intervals in the horizontal direction.

  The magenta image forming unit 1M is provided with a photosensitive drum 2a, a charging roller 3a, a developing unit 4a containing magenta toner, a transfer roller 5a, and a drum cleaning roller 6a, and an optical unit 7a for exposure. ing. The cyan image forming unit 1C is provided with a photosensitive drum 2b, a charging roller 3b, a developing unit 4b containing cyan toner, a transfer roller 5b, and a drum cleaning roller 6b, and an optical unit 7b for exposure. ing. The yellow image forming unit 1Y is provided with a photosensitive drum 2c, a charging roller 3c, a developing unit 4c containing yellow toner, a transfer roller 5c, and a drum cleaning roller 6c, and an optical unit 7c for exposure. ing. The black image forming unit 1Bk is provided with a photosensitive drum 2d, a charging roller 3d, a developing unit 4d containing black toner, a transfer roller 5d, and a drum cleaning roller 6d, and an optical unit 7d for exposure. ing.

  Each of the photoconductor drums 2a, 2b, 2c, and 2d is formed by forming a photoconductive layer with a negatively charged OPC photoconductor on an aluminum base, and an arrow direction (counterclockwise) by a driving device (not shown). Are synchronously rotated at a predetermined imaging process linear speed.

  A charging bias in which a DC voltage and an AC voltage are superimposed is applied from a charging bias power source (not shown) to the charging rollers 3a, 3b, 3c, and 3d that are in contact with the surfaces of the photosensitive drums 2a, 2b, 2c, and 2d. Due to the applied charging bias, the charging rollers 3a, 3b, 3c, and 3d uniformly charge the surfaces of the photosensitive drums 2a, 2b, 2c, and 2d to a predetermined negative potential.

  The charged surfaces of the photosensitive drums 2a, 2b, 2c, and 2d are exposed by the optical units 7a, 7b, 7c, and 7d. Laser light modulated in accordance with the image signal from the digital image processing unit is output from the laser output unit and sent from the polygon mirror that rotates at high speed to each of the optical units 7a, 7b, 7c, and 7d. Irradiates the surfaces 2a, 2b, 2c and 2d. The rotating photosensitive drums 2a, 2b, 2c and 2d are scanned and exposed by laser light to form electrostatic latent images of respective colors. The digital image processing unit, laser output unit, and polygon mirror are not shown.

  The developing units 4a, 4b, 4c, and 4d attach each color toner to the electrostatic latent images on the photosensitive drums 2a, 2b, 2c, and 2d, and develop them as toner images.

  The transfer rollers 5a, 5b, 5c, and 5d face the photosensitive drums 2a, 2b, 2c, and 2d through the primary transfer nip portions Ta, Tb, Tc, and Td.

  The intermediate transfer belt 8 is sandwiched between the primary transfer nip portions Ta, Tb, Tc, and Td. The intermediate transfer belt 8 is an endless belt and is wound around a driving roller 9, a secondary transfer counter roller 10, and a tension roller 11. The intermediate transfer belt 8 is formed of a dielectric resin film such as polycarbonate, polyethylene terephthalate, or polyvinylidene fluoride.

  After toner transfer to the intermediate transfer belt 8 at the primary transfer nip portions Ta, Tb, Tc, Td, the toner remaining on the surface of the photosensitive drums 2a, 2b, 2c, 2d is drum cleaning rollers 6a, 6b, 6c. , 6c.

  The secondary transfer counter roller 10 faces the secondary transfer roller 12 via the secondary transfer nip portion Te. The intermediate transfer belt 8 and a transfer material (paper, film) are sandwiched between the secondary transfer nip portions Te.

  In the vicinity of the tension roller 11, a belt cleaning device 13 for collecting toner remaining on the surface of the intermediate transfer belt 8 after toner transfer to the transfer material is installed. The belt cleaning device 13 includes a belt cleaning roller 13 a that contacts the intermediate transfer belt 8.

  A fixing device 14 having a fixing roller 14a and a pressure roller 14b is installed downstream of the secondary transfer nip Te in the transfer material conveyance direction.

  When an image formation start signal is issued, the photosensitive drums 2a, 2b, 2c, and 2d rotating at a predetermined image forming process linear speed are charged by the charging rollers 3a, 3b, 3c, and 3d, and the optical units 7a, 7b, An electrostatic latent image is formed by exposure with 7c and 7d. The electrostatic latent images are visualized with toner by developing units 4a, 4b, 4c, and 4d to which a developing bias voltage having the same polarity as the charging polarity of the photosensitive drums 2a, 2b, 2c, and 2d is applied.

  The toner images on the photosensitive drums 2a, 2b, 2c, and 2d are transferred to the primary transfer nip portions Ta, Tb, Tc, and Td, and transfer rollers 5a, 5b, 5c, and 5d to which a primary transfer bias voltage having a polarity opposite to that of the toner is applied. Thus, primary transfer is sequentially performed on the rotating intermediate transfer belt 8.

  The intermediate transfer belt 8 on which the magenta, cyan, yellow, and black toner images are superimposed and transferred in this order to form a full-color toner image transfers the full-color toner image to the transfer material at the secondary transfer nip portion Te. . The transfer material then passes through the fixing device 14, and the toner is fixed to the transfer material by heat and pressure.

  In the case of a monochrome image, only the black toner image formed on the intermediate transfer belt 8 by the black image forming portion 1Bk is transferred to the transfer material at the secondary transfer nip portion Te. Similarly, a monochrome image other than black can be obtained.

  Of the charging bias applied to the charging rollers 3a, 3b, 3c and 3d, an alternating voltage is applied by the high voltage transformer 20 shown in FIG. There is only one high-voltage transformer 20. The high-voltage transformer 20 receives an instruction from the AC waveform generator 21 and generates an AC voltage having a predetermined voltage and a predetermined frequency. An instruction is given from the central processing unit 22 to the AC waveform generation unit 21. The central processing unit 22 belongs to a higher-level control unit, for example, a control unit for the entire image forming apparatus.

  The image forming apparatus has two image forming modes, a full color mode and a monochrome mode. The monochrome mode has a higher imaging process line speed than the full color mode, and the number of output sheets per unit time is larger. The charging bias AC voltage is also higher in the monochrome mode than in the full color mode. The voltage value of the AC voltage is set according to such a frequency difference.

  The AC voltage value is set by searching for a voltage value at which the DC voltage is saturated by changing the charging bias during the non-image forming period. The process is shown in the flowchart of FIG.

  In step # 101 of the flowchart of FIG. 3, when the image forming apparatus is powered on and warmed up, the image forming apparatus is driven at the linear speed in the low speed mode (full color mode). In step # 102, an AC voltage is applied at the charging frequency f1 for the low speed mode.

  In step # 103, the voltage value Vpp of the alternating voltage is gradually increased from zero. Then, the current value Idc of the DC voltage applied while being superimposed on the AC voltage is measured.

  As Vpp rises from zero potential, Idc rises proportionally at first. However, when Vpp reaches a certain voltage, Idc no longer rises. That is, it is saturated.

  The voltage value Vpp when Idc saturates is a necessary and sufficient voltage value, and there is no point in raising it further. Therefore, in step # 104, if Vpp where Idc is saturated can be searched, the determined Vpp is determined as Vpp in the low speed mode.

  In the subsequent step # 105, driving is performed at the linear speed in the high speed mode (monochrome mode). In step # 106, an AC voltage is applied at the charging frequency f2 for the high speed mode.

  In step # 107, the voltage value Vpp of the alternating voltage is gradually increased from zero. Then, the current value Idc of the DC voltage applied while being superimposed on the AC voltage is measured.

  If Vpp at which Idc is saturated can be searched in step # 108, the found Vpp is determined as Vpp in the high-speed mode.

  After setting the charging bias AC voltage for each of the low speed mode (full color mode) and the high speed mode (monochrome mode) in this way, the printing operation (image forming operation) is started in step # 109.

  Since the AC voltage of the charging bias is set for each image forming mode, that is, for each frequency, even if the image forming mode is switched, the voltage does not fall outside the set range and the image quality does not deteriorate. The charging bias is set during the non-image forming period, and such an operation does not occur during the image forming period, so that the printing operation is delayed and the user does not feel inconvenience.

  FIG. 4 shows a modified embodiment of the flowchart up to the start of the printing operation.

  Steps # 111 to # 114 in the flowchart of FIG. 4 are the same as steps # 101 to # 104 in the flowchart of FIG. However, after that, the flowchart of FIG. 4 proceeds to Step # 115 without passing through the step of “driving at the linear speed of the high speed mode (monochrome mode)”. Steps # 115 to # 118 are the same as steps # 106 to # 109 in the flowchart of FIG.

  The flowcharts of FIGS. 3 and 4 are merely examples and do not limit the present invention. The present invention can be implemented in various other flows. For example, a post-aging period after the end of the print job may be selected as the non-image forming period, and the AC voltage value Vpp at which the DC current Idc is saturated may be searched using this period.

  FIG. 5 shows a table of test results for examining the influence of the voltage output value on the image. In the embodiment of the present invention, the Vpp output value was set to 1.0 kV in both the full color mode and the monochrome mode. In either case, there was no problem with the image. As a comparative example, the Vpp output value in the full color mode is 1.0 kV, which is the same as the embodiment, but when the test was performed with the Vpp output value in the monochrome mode being 0.6 kV, which was lower than that in the embodiment, the monochrome mode image was Had lateral muscles and could not withstand use.

  Although the embodiments of the present invention have been described above, various modifications can be made without departing from the spirit of the invention.

  The present invention is widely applicable to an electrophotographic image forming apparatus in which a photosensitive drum is charged by a charging roller.

Schematic configuration diagram of an image forming apparatus Partial block diagram Flow chart to start printing operation Flowchart until start of printing operation according to modified embodiment Table of results of examining the effect of voltage output values on images

Explanation of symbols

2a, 2b, 2c, 2d Photosensitive drums 3a, 3b, 3c, 3d Charging roller 20 High voltage transformer

Claims (3)

An electrophotographic image forming apparatus that applies a charging bias in which an AC voltage is superimposed on a DC voltage to a charging roller that is in contact with a photosensitive drum.
In the non-image forming period, the charging bias is varied to search for a voltage value at which the DC current is saturated, and the charging bias is set based on the found voltage value, and a plurality of image forming processes having different image forming process linear speeds are performed. An image forming apparatus comprising: a mode, wherein the charging bias frequency is varied according to a linear velocity for each image forming mode, and the voltage value is set for each frequency in the non-image forming period. The image forming apparatus according to claim 1, wherein an AC voltage output of the charging bias is performed by a single transformer. The image forming apparatus according to claim 1, wherein the non-image forming period is a machine warm-up period or a post-aging period after the end of a print job.

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