JP2012132948A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device capable of preventing sandy image failure which may occur in an image forming device having charging means of a contact charging system.SOLUTION: An image forming device comprises a photoreceptor 2, first charging means 11, second charging means 12 at the downstream side in the movement direction of the photoreceptor from the first charging means, a first power source S1 for applying voltage to the first charging means, a second power source S2 for applying voltage to the second charging means, exposure means 3 and developing means 4. The first power source S1 applies an oscillating voltage in which a DC voltage and an AC voltage are superimposed to the first charging means 11. The second power source S2 applies a DC voltage to the second charging means 12 in which the potential difference between the DC voltage applied by the first power source S1 to the first charging means 11 and the DC voltage applied by the second power source S2 to the second power charging means 12 is less than the discharge starting voltage Vth.

Description

  BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as an electrophotographic copying machine or a laser beam printer, and more specifically, an image having a contact charging type charging unit that contacts and charges an electrophotographic photosensitive member. The present invention relates to a forming apparatus.

  Conventionally, in an electrophotographic image forming apparatus, as a charging means for charging an electrophotographic photosensitive member (photosensitive member), a non-contact charging type charging device using a corona discharge phenomenon such as a scorotron charger is generally used. It has been used a lot.

  In contrast to such a contact charging method, recently, a contact charging method in which a conductive roller-type charging member is brought into contact with a photosensitive member and a charging voltage (charging bias) is applied to the charging member is used.

  For example, a conductive rubber roller (charging roller) is brought into contact with a photosensitive drum, which is a drum-type photosensitive member, and is rotated along with the rotation of the photosensitive drum. The photosensitive drum is uniformly charged by supplying a voltage to the mandrel serving as the shaft of the charging roller. At this time, the outer peripheral surface of the photosensitive drum is charged by discharge at a minute gap generated between the charging roller and the photosensitive drum. The contact charging method using such a charging roller has advantages that the discharge interval is narrower than that of the non-contact charging method, so that ozone is hardly generated and the applied voltage is smaller than that of the non-contact charging method. .

  Specifically, there is a DC charging method in which a DC voltage obtained by adding a required charging potential Vd (target charging potential) of the photosensitive drum to the discharge start voltage Vth is applied as the charging voltage. For example, the discharge start voltage Vth is about 550 V in an environment of a temperature of 23 ° C. and a humidity of 50% when the charging roller is brought into pressure contact with the OPC photoreceptor.

  In addition, there is the following AC charging method for the purpose of improving the fluctuation of the potential due to the increase of the usage amount of the environment and apparatus (Patent Document 1). In the AC charging method, the charging voltage has a DC voltage (DC component) corresponding to a required charging potential Vd (target charging potential) of the photosensitive drum and a peak-to-peak voltage Vpp that is at least twice the discharge start voltage Vth. An oscillating voltage in which an AC voltage (AC component) is superimposed is applied. The waveform of the alternating current component of the oscillating voltage is a sine wave, a rectangular wave, a triangular wave, or the like. Alternatively, this oscillating voltage may be a rectangular wave voltage formed by periodically turning on and off the DC power supply.

  In general, the DC charging method is advantageous in that it consumes less power and is less expensive than the AC charging method, and the device is small and space-saving. However, the DC charging method has a narrower discharge area than the AC charging method and does not have the effect of leveling the AC discharge current. It is easy to generate. Further, in the DC charging method, it is difficult to control to a desired charging potential Vd as compared with the AC charging method, and the stability of the surface potential of the photosensitive drum may be deteriorated. This is because the applied voltage required to obtain the desired charging potential Vd changes due to fluctuations in the electrical resistance value during manufacture of the environment, the material of the charging roller, etc., and an increase in the amount of use of the apparatus. As a countermeasure, there is a means for setting a charging voltage based on the measured potential by installing a potential sensor that always measures the surface potential of the photosensitive drum in the vicinity of the photosensitive drum. However, this means requires a space for installing the potential sensor, and there is a problem that the cost increases.

  On the other hand, the AC charging method can perform uniform charging (static elimination) processing and is effective for improving the image quality. In addition, under the condition that the AC voltage (AC component) of the charging voltage is discharged, the surface potential of the photosensitive drum after charging converges to the same potential as the DC voltage (DC component) of the charging voltage. An electric potential sensor becomes unnecessary. However, the AC charging method increases the amount of discharge current compared to the DC charging method, promotes the deterioration of the photosensitive drum, and easily causes image defects called “image flow” due to discharge products. As a countermeasure, it is conceivable to minimize the AC discharge by minimizing the AC voltage of the charging voltage (that is, minimizing the peak-to-peak voltage of the AC voltage).

JP-A 63-149669

  However, if the peak-to-peak voltage of the alternating voltage of the charging voltage is minimized as described above, the AC discharge current flowing between the charging roller and the photosensitive drum is reduced, but the discharge becomes unstable and locally excessive. Sandy image defects are likely to occur due to discharge (abnormal discharge).

  The reason why the above abnormal discharge occurs is considered as follows. In other words, a voltage is applied to the charging roller to such an extent that discharge begins to occur in the contact portion (nip) between the charging roller and the photosensitive drum and in the minute gap in the vicinity thereof. In this case, due to factors such as the distribution of the electrical properties of the charging roller or the geometrical shape of the surface of the charging roller or the photosensitive drum, discharge within the region where the discharge between the charging roller and the photosensitive drum may occur. Discharge may occur only in a part. This partial discharge may cause a current to flow intensively in the discharge space where the impedance is lowered, and may cause partial discharge.

  Such abnormal discharge causes the photosensitive drum to have an abnormal potential locally, and no toner adheres to that part, or conversely, a large amount of toner adheres. Image defect.

  Accordingly, an object of the present invention is to provide an image forming apparatus capable of suppressing sandy image defects that may occur in an image forming apparatus having a contact charging type charging means.

  The above object is achieved by the image forming apparatus according to the present invention. In summary, the present invention relates to a photosensitive member, a first charging unit that contacts and charges the surface of the moving photosensitive member, and a moving direction of the photosensitive member relative to the first charging unit. On the downstream side, a second charging unit that contacts and charges the surface of the moving photoconductor, a first power source that applies a voltage to the first charging unit, and the second charging unit A second power source that applies a voltage to the photosensitive member, an exposure unit that exposes the charged photosensitive member to form an electrostatic latent image, and a developing unit that develops the electrostatic latent image formed on the photosensitive member. The first power supply applies an oscillating voltage obtained by superimposing a DC voltage and an AC voltage to the first charging means, and the second power supply is configured such that the first power supply is the first power supply. A DC voltage at which the potential difference from the DC voltage applied to the charging means is less than the discharge start voltage is applied to the second charging. An image forming apparatus and applying to the stage.

  According to the present invention, it is possible to suppress sandy image defects that may occur in an image forming apparatus having a contact charging type charging unit.

1 is a schematic cross-sectional view illustrating a schematic configuration of an image forming apparatus according to an embodiment of the present invention. 1 is a schematic cross-sectional view illustrating a schematic configuration of a charging roller provided in an image forming apparatus according to an embodiment of the present invention. FIG. 6 is an operation sequence diagram of the image forming apparatus according to the embodiment of the present invention. (A) Block of charging voltage application system for each of first charging roller, (b) second charging roller, and (c) third charging roller provided in an image forming apparatus according to an embodiment of the present invention. It is a circuit diagram. FIG. 4 is a graph schematically showing the surface potential of the photosensitive drum after passing through the first charging unit in the image forming apparatus according to the embodiment of the present invention. FIG. 6 is an image diagram of a halftone image formed with the surface potential of the photosensitive drum shown in FIG. 5. (A) The relationship between the surface potential of the photosensitive drum and the charging voltage applied to the second charging roller, (b) is an explanatory view schematically showing the surface potential of the photosensitive drum after passing through the second charging unit. is there. It is an image figure of the halftone image formed in the state of the surface potential of the photosensitive drum shown in the right figure of FIG.7 (b). (A) The relationship between the surface potential of the photosensitive drum and the charging voltage applied to the third charging roller, (b) is an explanatory view schematically showing the surface potential of the photosensitive drum after passing through the third charging unit. is there. FIG. 10 is an image diagram of a halftone image formed in the state of the surface potential of the photosensitive drum shown in the right diagram of FIG. FIG. 6 is a block circuit diagram of a charging voltage application system for a first charging roller according to another embodiment of the present invention. It is a graph which shows the relationship between the peak voltage Vpp of the alternating voltage applied to the 1st charging roller for explaining the method of detecting a discharge start voltage, and the DC current value which flows into the 1st charging roller. FIG. 4 is an explanatory diagram schematically illustrating a relationship between a distance between a gap between a charging roller and a photosensitive drum and a potential. It is a graph which shows the relationship of the AC electric current which flows into the charging roller with respect to time on the conditions which abnormal current generate | occur | produces. (A) The relationship between the surface potential of the photosensitive drum and the potential of the DC component of the developing bias, (b) A schematic diagram of the output halftone image. It is a graph which shows the relationship between the frequency of the alternating voltage of charging voltage, and a sandy ground disappearance electric current value.

  The image forming apparatus according to the present invention will be described below in more detail with reference to the drawings.

Example 1
1. FIG. 1 shows a schematic configuration of an image forming apparatus according to an embodiment of the present invention. The image forming apparatus 1 of this embodiment is an electrophotographic laser beam printer that employs a contact charging method.

  The image forming apparatus 1 includes a drum-type electrophotographic photosensitive member (photosensitive member) as an image carrier, that is, a photosensitive drum 2. In this embodiment, the photosensitive drum 2 is an organic photoconductor (OPC) photosensitive drum having a negative charging property. In this embodiment, the outer diameter of the photosensitive drum 2 is 30 mm, and the photosensitive drum 2 is driven to rotate in the direction of arrow R1 (counterclockwise) in the drawing at a process speed (circumferential speed) of 130 mm / sec around the center support shaft. In this embodiment, the photosensitive drum 2 has a configuration in which a photocharge generation layer and a charge transport layer (thickness of about 20 μm) are sequentially applied from the bottom to the surface of an aluminum cylinder (conductive drum base). .

  The image forming apparatus 1 includes a first charging roller 11, a second charging roller 12, and a third charging roller, which are roller-type charging members, as a contact charging type charging unit that charges the surface of the photosensitive drum 2. 13 The first, second, and third charging rollers 11, 12, and 13 are in contact with the photosensitive drum 2, respectively. The first, second, and third charging rollers 11, 12, and 13 are arranged in this order from the upstream side to the downstream side in the moving direction of the surface of the photosensitive drum 2. The first, second, and third charging rollers 11, 12, and 13 charge the surface of the photosensitive drum 2 by utilizing a discharge phenomenon that occurs in a minute gap between the first and second charging rollers 11, 12, and 13. The contact portions (contact regions) between the first, second, and third charging rollers 11, 12, and 13 and the photosensitive drum 2 are first, second, and third charging nips, respectively. The charging rollers 11, 12, and 13 are respectively connected to the surfaces of the charging rollers 11, 12, and 13 and the surface of the photosensitive drum 2 on the upstream side and downstream side of the charging nip in the moving direction of the surface of the photosensitive drum 2. A minute gap (charging gap) is formed between them. Each of the charging rollers 11, 12, and 13 performs a charging process on the surface of the photosensitive drum 2 by electric discharge generated at least one of the upstream micro-gap and the downstream micro-gap. Here, for convenience, each of the charging nips of the first, second, and third charging rollers 11, 12, and 13 and an area including a minute gap that generates discharge on the upstream side and the downstream side thereof are respectively defined on the photosensitive drum 2. The first, second, and third charging portions (charging positions) a1, a2, and a3 on which charging processing is performed are assumed. The first, second, and third charging rollers 11, 12, and 13 are disposed in contact with the photosensitive drum 2 so that the respective rotation axes are substantially parallel to the rotation axis of the photosensitive drum 2.

  With reference to FIG. 2, the configuration of the first, second, and third charging rollers 11, 12, and 13 will be described using the first charging roller 11 as an example. In this embodiment, the first, second, and third charging rollers 11, 12, and 13 have substantially the same dimensions and materials, respectively.

  The first charging roller 11 is a conductive elastic roller. The first charging roller 11 includes a cored bar 11a, a lower layer (conductive elastic layer) 11b formed concentrically on the outer periphery of the cored bar 11a, and an intermediate layer (high resistance) formed on the outer peripheral surface thereof. Coating layer) 11c and a surface layer (protective coating layer) 11d formed on the outer peripheral surface thereof. The intermediate layer 11c is for preventing charging failure and the like. The surface layer 11d is for preventing the first charging roller 11 from being fixed to the photosensitive drum 2.

  The first charging roller 11 has both ends in the longitudinal direction of a cored bar (supporting member) 11a rotatably supported by bearing members 11e, and the surface of the photosensitive drum 2 by a pressing spring 11f as urging means. Is pressed at a predetermined pressure. As a result, the first charging roller 11 is pressed against the surface of the photosensitive drum 2, and in this embodiment, the first charging roller 11 rotates in the direction of arrow R2 (clockwise) in the drawing following the rotation of the photosensitive drum 2. A pressure contact portion (contact region) between the photosensitive drum 2 and the first charging roller 11 is a first charging nip.

More specifically, in this embodiment, the first charging roller 11 has a length in the longitudinal direction of 320 mm and a diameter of 14 mm. Further, as described above, the first charging roller 11 has a three-layer configuration in which the lower layer 11b, the intermediate layer 11c, and the surface layer 11d are sequentially laminated from the bottom around the outer periphery of the cored bar 11a. The metal core 11a is formed of a stainless steel round bar having a diameter of 6 mm. The lower layer 11b is formed of foamed EPDM in which carbon is dispersed, has a specific gravity of 0.5 g / cm ³ , a volume resistance value of 10 ² to 10 ⁹ Ωcm, and a layer thickness of about 3.5 mm. The intermediate layer 11c is made of NBR rubber in which carbon is dispersed, has a volume resistance value of 10 ² to 10 ⁵ Ωcm, and a layer thickness of about 500 μm. The surface layer 11d has a structure in which tin oxide and carbon are dispersed in a fluorine compound alcohol-soluble nylon resin, the volume resistance value is 10 ⁷ to 10 ¹⁰ Ωcm, and the surface roughness (JIS standard 10-point average surface roughness Rz) is 1. 0.5 μm and the layer thickness is about 5 μm. As the material of the lower layer 11b, a material in which conductive carbon is dispersed in SBR or the like can be used.

  As described above, in the present embodiment, the first, second, and third charging rollers 11, 12, and 13 have substantially the same dimensions and materials. Each of the second and third charging rollers 12 and 13 has, roughly, core bars 12a and 13a, lower layers 12b and 13b, intermediate layers 12c and 13c, and surface layers 12d and 13d, and is supported by bearings 12e and 13e. At the same time, the pressure springs 12f and 13f are pressed against the surface of the photosensitive drum 2. However, the configuration of each charging member is not limited to that of the present embodiment.

  As shown in FIG. 1, the first, second, and third charging rollers 11, 12, and the core bars 11a, 12a, and 13a have a first power source S1 and a second power source as charging voltage application units, respectively. A charging voltage (charging bias) under a predetermined condition is applied from the power source S2 and the third power source S3. In the present embodiment, the first, second, and third charging rollers 11, 12, and 13 are connected to the first, second, and third charging rollers 11f, 12f, and 13f, and the core bars 11a, 12a, and 13a, respectively. A charging voltage is applied from the third power source S1, S2, S3.

  In the present embodiment, the first power supply S <b> 1 applies an oscillating voltage obtained by superimposing a DC voltage and an AC voltage to the first charging roller 11. On the other hand, in the present embodiment, the second power source S2 and the third power source S3 apply only a DC voltage to the second charging roller 12 and the third charging roller 13, respectively. The surface of the rotating photosensitive drum 2 is substantially uniformly charged to a predetermined potential having a predetermined polarity by a charging voltage applied mainly to the first charging roller 11. In this embodiment, the surface of the photosensitive drum 2 is subjected to a charging process substantially uniformly at −600V. The setting of the charging voltage applied to the first, second, and third charging rollers 11, 12, and 13 will be described in detail later.

  The image forming apparatus 1 includes an exposure device 3 as an exposure unit (information writing unit) that forms an electrostatic latent image (electrostatic image) on the surface of the charged photosensitive drum 2. In this embodiment, the exposure apparatus 3 is a laser beam scanner using a semiconductor laser. The exposure device 3 outputs a laser beam L modulated in accordance with an image signal sent from the host processing device such as an image reading device (not shown) connected to the image forming device 1 to the image forming device 1. . Then, the exposure device 3 performs scanning exposure (image exposure) on the surface of the rotating photosensitive drum 2 subjected to charging processing with the laser light L at an exposure portion (exposure position) b. As a result, the potential of the portion irradiated with the laser light L on the surface of the photosensitive drum 2 is lowered, and electrostatic latent images corresponding to image information are sequentially formed on the surface of the rotating photosensitive drum 2.

  The image forming apparatus 1 includes a developing device 4 as a developing unit. The developing device 4 supplies toner onto the photosensitive drum 2 in accordance with the electrostatic latent image on the photosensitive drum 2, and develops the electrostatic latent image as a toner image (developer image). In this embodiment, the developing device 4 is charged with the charged polarity (negative polarity in this embodiment) of the photosensitive drum 2 on the exposed portion (bright portion) on the photosensitive drum 2 whose potential has been lowered by exposure after being charged. The electrostatic latent image is developed by a reversal development method in which charged toner is attached to the surface. Further, in this embodiment, the developing device 4 is an electrostatic latent image in which a magnetic brush made of a two-component developer including toner (nonmagnetic toner particles) and a carrier (magnetic carrier particles) is brought into contact with the photosensitive drum 2. A two-component contact development system that performs the above development is employed.

  The developing device 4 includes a developing container 4a and a developing sleeve 4b as a developer carrying member that carries a two-component developer and conveys it to a developing portion (developing position) c that is a portion facing the photosensitive drum 2. . The developing sleeve 4b is rotatably disposed in the developing container 4a with a part of the outer peripheral surface thereof exposed to the outside of the developing device 4. The developing sleeve 4b is made of a nonmagnetic material. In the developing sleeve 4b, a magnet roller 4c fixed in a non-rotating manner with respect to the developing container 4a is inserted. In addition, a developing blade 4d is provided as a developer regulating means for applying the two-component developer while regulating the amount thereof on the developing sleeve 4b so as to face the developing sleeve 4b. The developing container 4a contains a two-component developer 4e, and a developer stirring member 4f is disposed on the bottom side in the developing container 4a. Further, replenishing toner is accommodated in a toner hopper (not shown) as a replenishing developer container.

The two-component developer 4e in the developing container 4a is mainly a mixture of toner (nonmagnetic toner particles) and carrier (magnetic carrier particles), and is stirred by the developer stirring member 4f. In this embodiment, the carrier has a volume resistivity of about 10 ¹³ Ωcm and a particle size (volume average particle size) of about 40 μm. Using a laser diffraction particle size distribution measuring device HEROS (manufactured by JEOL Ltd.), the range of 0.5 to 350 μm was measured by dividing into 32 logarithms, and the carrier particle size (volume average particle size) with a volume of 50% ). In this embodiment, the toner is normally triboelectrically charged to the negative polarity by rubbing with the carrier. That is, in this embodiment, the normal charging polarity of the toner is negative.

  The developing sleeve 4b is disposed facing the photosensitive drum 2 with the closest distance (S-Dgap) to the photosensitive drum 2 being maintained at 350 μm. A facing portion between the photosensitive drum 2 and the developing sleeve 4b is a developing portion (developing position) c. In this embodiment, the developing sleeve 4b is rotationally driven so that the moving direction of the surface of the developing sleeve 4b and the moving direction of the surface of the photosensitive drum 2 in the developing unit c are opposite to each other. Due to the magnetism of the magnet roller 4c in the developing sleeve 4b, a part of the two-component developer 4e in the developing container 4a is attracted to the outer peripheral surface of the developing sleeve 4b and held as a magnetic brush layer. The magnetic brush layer is conveyed as the developing sleeve 4b rotates, and is layered into a predetermined thin layer by the developing blade 4d. The magnetic brush layer comes into contact with the surface of the photosensitive drum 2 at the developing unit c and rubs the surface of the photosensitive drum 2 appropriately.

  A developing voltage (developing bias) under a predetermined condition is applied to the developing sleeve 4b from a developing power source S4 as a developing voltage applying unit. In the present embodiment, the developing voltage is an oscillating voltage obtained by superimposing a DC voltage (Vdc) and an AC voltage (Vac). More specifically, it is an oscillating voltage in which a DC voltage of −450 V, a frequency of 9.0 kHz, a peak-to-peak voltage of 1.8 kV, and a rectangular wave AC voltage are superimposed.

The toner in the developer 4e coated as a thin layer on the rotating developing sleeve 4b and transported to the developing section c is generated by an electric field formed between the photosensitive drum 2 and the developing sleeve 4b by the developing voltage. 2 selectively adheres corresponding to the electrostatic latent image. Thereby, the electrostatic latent image on the photosensitive drum 2 is developed as a toner image. The toner charge amount of the toner image formed on the photosensitive drum 2 is about −25 μC / g under the environment of a temperature of 23 ° C. and an absolute water content of 10.6 g / m ³ . The thin layer of the two-component developer on the developing sleeve 4b that has passed through the developing section c is returned to the developer reservoir in the developing container 4a with the subsequent rotation of the developing sleeve 4b.

  In this embodiment, the toner concentration of the two-component developer 4e in the developing container 4a is maintained within a substantially constant range as follows. That is, the toner concentration of the two-component developer 4e in the developing container 4a is detected by an optical toner concentration sensor or the like, and the driving of the toner hopper is controlled according to the detected information, so that the toner in the toner hopper is developed. The two-component developer 4e in the container 4a is supplied. The toner supplied to the two-component developer 4e is stirred by the stirring member 4f.

  The image forming apparatus 1 includes a transfer roller 5 that is a roller-type transfer member as transfer means. The transfer roller 5 is pressed against the photosensitive drum 2 with a predetermined pressing force. A pressure contact portion (nip) between the transfer roller 5 and the photosensitive drum 2 is a transfer portion (transfer position) d. The recording material P is fed to the transfer portion d from the feeding mechanism portion (not shown) of the recording material P at a predetermined control timing.

  The recording material P fed to the transfer part d is nipped between the photosensitive drum 2 and the transfer roller 5 and conveyed. Meanwhile, a positive transfer voltage (transfer bias) having a polarity opposite to the negative polarity that is the normal charging polarity of the toner is applied to the transfer roller 5 from a transfer power source S5 as a transfer voltage application unit. In this embodiment, a DC voltage of +600 V is applied to the transfer roller 5 from the transfer power source S5 as the transfer voltage. As a result, the toner image on the photosensitive drum 2 is sequentially electrostatically transferred onto the surface of the recording material P that is nipped and conveyed between the photosensitive drum 2 and the transfer roller 5.

  The recording material P to which the toner image has been transferred is sequentially separated from the surface of the photosensitive drum 2 and conveyed to a fixing device 6 as a fixing unit. In this embodiment, the fixing device 6 is a heat roller fixing device. The recording material P is output from the image forming apparatus 1 as an image formed product (print, copy) after undergoing a toner image fixing process by the fixing device 6.

  Further, the toner (transfer residual toner) remaining on the surface of the photosensitive drum 2 after the transfer of the toner image onto the transfer paper P is removed and collected from the surface of the photosensitive drum 2 by the cleaning device 7 in the cleaning unit e. Instead of using the cleaning device 7 as means for removing the transfer residual toner, a means for optimizing the charge of the transfer residual toner is provided, and the transfer residual toner whose charge has been optimized thereby is developed in the developing device 4. You may employ | adopt the cleanerless system collect | recovered simultaneously.

  Next, the operation sequence of the image forming apparatus 1 will be further described with reference to FIG.

(1) Initial rotation operation (front multiple rotation process)
This is a starting operation period (starting operation period, warming period) when the image forming apparatus 1 is started. When the power switch is turned on, the photosensitive drum 2 is rotationally driven, and a preparatory operation for a predetermined process device, such as raising the fixing device 6 to a predetermined temperature, is executed.

(2) Printing preparation rotation operation (pre-rotation process)
This is a preparatory rotation operation period before image formation from when the print signal is turned on until an image formation (printing) process operation is actually performed. This operation is executed following the initial rotation operation when a print signal is input during the initial rotation operation. When there is no print signal input, the drive of the main motor is temporarily stopped after the end of the initial rotation operation, the rotation drive of the photosensitive drum 2 is stopped, and the image forming apparatus 1 is in standby (standby) until the print signal is input. Kept in a state. When a print signal is input, a print preparation rotation operation is executed. In this embodiment, a program for calculating and determining the peak-to-peak voltage value of the AC voltage applied to the first charging roller 11 in the charging process of the printing process is executed during the printing preparation rotation operation period. The charging voltage value calculation and determination control will be described in detail later.

(3) Printing process (image forming process, image forming process)
When the predetermined print preparation rotating operation is completed, the toner image on the photosensitive drum 2 is subsequently formed, the toner image formed on the photosensitive drum 2 is transferred to the recording material P, and the toner by the fixing device 6 is used. An image fixing process or the like is performed, and an image formed product is output from the image forming apparatus 1. In the case of the continuous printing (continuous printing) mode, the printing process is repeatedly executed for the set predetermined number of prints (n).

(4) Inter-sheet process In the continuous printing mode, after the rear end portion of one recording material P passes through the transfer portion d, the transfer is performed until the leading end portion of the next recording material P reaches the transfer portion d. This is a non-sheet passing state period of the recording material P in the part d.

(5) Post-rotation operation The drive of the main motor continues for a while after the printing process for one recording material P (the last recording material P in the continuous printing mode) is completed in the case of printing only one sheet. In this period, the photosensitive drum 2 is driven to rotate and a predetermined preparation operation is executed.

(6) Standby When the predetermined post-rotation operation is completed, the drive of the main motor is stopped, the rotation of the photosensitive drum 2 is stopped, and the image forming apparatus 1 is kept in the standby state until the next print start signal is input. Be drunk. In the case of printing only one sheet, after the printing on one recording material P (the last recording material P in the continuous printing mode) is completed, the image forming apparatus 1 goes into a standby state through a post-rotation operation. When a print start signal is input in the standby state, the image forming apparatus 1 proceeds to the pre-rotation process.

  The time of the printing process of (3) is the time of image formation, the initial rotation operation of (1), the pre-rotation operation of (2), the inter-sheet process of (4), and the post-rotation operation of (5). Is during non-image formation.

2. Charging Voltage Application System FIGS. 4A, 4B, and 4C are block circuit diagrams of the charging voltage application system for the first, second, and third charging rollers 11, 12, and 13, respectively.

  As shown in FIG. 4A, the first power source S1, which is a charging voltage applying means for applying a charging voltage to the first charging roller 11, includes a DC power source (DC power source) 11g and an AC power source (AC power source) 11h. And have. A predetermined oscillating voltage obtained by superimposing a DC voltage and an AC voltage is applied from the first power supply S1 to the first charging roller 11 via the cored bar 11a. As a result, the peripheral surface of the rotating photosensitive drum 2 is charged almost uniformly to a predetermined potential.

  As shown in FIG. 4B, the second power source S2, which is a charging voltage applying means for applying a charging voltage to the second charging roller 12, has a DC power source 12g. That is, it does not have an AC power source. A DC voltage is applied from the second power source S2 to the second charging roller 12 via the cored bar 12a. As a result, the potential of the portion charged to the first polarity (negative polarity in this embodiment) side on the peripheral surface of the photosensitive drum 2 is made closer to the desired charging potential Vd.

  As shown in FIG. 4C, the third power source S3, which is a charging voltage applying means for applying a charging voltage to the third charging roller 13, has a DC power source 13g. That is, it does not have an AC power source. A DC voltage is applied from the third power source S3 to the third charging roller 13 via the core bar 13a. As a result, the potential of the portion charged to the second polarity (positive polarity in this embodiment) side on the peripheral surface of the photosensitive drum 2 is made closer to the desired charging potential Vd.

  The second power source S2 and the third power source S3 have the same configuration although the output DC voltage value is different.

  As shown in FIGS. 4A, 4B, and 4C, the operations of the first, second, and third power sources S1, S2, and S3 are controlled by a common control circuit 14 serving as control means. The That is, the control circuit 14 has a function of controlling the DC voltage value applied to the first charging roller 11 from the DC power supply 11g and the peak-to-peak voltage value of the AC voltage applied to the first charging roller 11 from the AC power supply 11h. Have. The control circuit 14 has a function of controlling a DC voltage value applied to the second charging roller 12 from the DC power supply 12g. Further, the control circuit 14 has a function of controlling a DC voltage value applied to the third charging roller 13 from the DC power supply 13g.

  In addition, the control circuit 14 includes a memory unit (not shown) as storage means in which an environment table for obtaining the discharge start voltage Vth in advance for the environment is stored. The control circuit 14 is connected to an environmental sensor (temperature / humidity sensor) (not shown) as environmental detection means provided in the image forming apparatus 1. The control circuit 14 has a function of executing a program for calculating and determining the peak-to-peak voltage value of the AC voltage applied to the first charging roller 11 in the charging process of the printing process. In the present embodiment, the control circuit 14 determines that the AC voltage is based on the information on the environment table stored in the memory unit and the temperature / humidity information on the ambient environment of the image forming apparatus 1 input from the environment sensor. The peak-to-peak voltage value is calculated and determined.

  Here, the discharge start voltage Vth will be described with reference to FIG. FIG. 13 is a diagram schematically illustrating the relationship between the distance between the gap between the charging roller and the photosensitive drum and the potential.

When the gap distance is Z, the photosensitive drum film thickness is d, and the relative permittivity of the photosensitive drum is εr, and a DC voltage Vp is applied to the charging roller, the voltage Vg in the gap is expressed by the following formula ( 1).
Vg = Z / (Z + d / εr) × Vp (1)

In this system, if the voltage in the gap when the discharge is started is Vg0, the direct current voltage applied to the charging roller is the discharge start voltage Vth. Therefore, the above equation (1) is expressed by the following equation (2): To be rewritten.
Vg0 = Z / (Z + d / εr) × Vth (2)

On the other hand, the voltage Vg in the air gap is expressed by a function of the air pressure p of the air gap and the distance Z of the air gap according to Paschen's law.
Vg = f (p, Z) (3)

When the discharge start voltage Vth is applied, since Vg0 = Vg in the above formula (2) and the above formula (3), the following formula (4) is derived.
Vth = (Z + d / εr) / Z × f (p, Z) (4)

  Thus, the discharge start voltage Vth is a value determined from the gap distance Z, the photosensitive drum film thickness d, the photosensitive drum relative dielectric constant εr, the gap pressure p, and the like.

3. Control of Charging Voltage for First Charging Roller Next, a method for controlling the charging voltage applied to the first charging roller 11 during the printing process will be described.

  In the present embodiment, the charging voltage applied to the first charging roller 11 is a vibration voltage obtained by superimposing a DC voltage of −600 V and an AC voltage having a peak-to-peak voltage Vpp of 1200 V and a frequency f of 1.5 kHz. .

  Here, it is assumed that the control is performed in an environment of a temperature of 23 ° C. and a humidity of 50%.

In the present embodiment, as described above, the environment table in which the discharge start voltage Vth is obtained in advance for the environment is stored in the memory unit of the control circuit 14. As described above, the temperature and humidity information of the atmosphere environment of the image forming apparatus 1 is input to the control circuit 14 from the environment sensor. Then, the control circuit 14 determines the peak-to-peak voltage Vpp of the AC voltage applied to the first charging roller 11 by the following formula using the value of Vth read from the environment table corresponding to the current environment.
Vpp = 2 × Vth + 100 [V]

  In the above environment (temperature 23 ° C., humidity 50%), since Vth = 550V, the peak-to-peak voltage Vpp of the AC voltage applied to the first charging roller 11 is 1200V.

  The peak-to-peak voltage Vpp of the AC voltage applied to the first charging roller 11 determined in this way is a set value when the discharge current is minimized, and the AC discharge current amount is about 20 μA.

  If the photosensitive drum 2 is charged under these conditions, the discharge becomes unstable as described above, and sandy image defects may occur. That is, in this case, the discharge may occur only in a part of the region where the discharge between the first charging roller 11 and the photosensitive drum 2 can occur. This partial discharge may cause a current to flow intensively in the discharge space where the impedance is lowered, and may cause partial discharge. Due to the abnormal discharge generated in this way, the photosensitive drum 2 is locally at an abnormal potential, resulting in a sandy image defect.

  FIG. 5 schematically shows the surface potential of the photosensitive drum 2 after passing through the first charging portion a1 when the photosensitive drum 2 is charged with the above setting. In the figure, the surface potential of the photosensitive drum 2 is the vertical axis, and the photosensitive drum at a certain position in the rotational axis direction of the photosensitive drum 2 (assuming that there is an abnormal potential on the negative polarity side and the positive polarity side described later). The position in the circumferential direction of 2 is on the horizontal axis. The schematic diagrams of the surface potential of the photosensitive drum shown in FIGS. 7, 9 and 15 are also shown in the same manner.

  As shown in FIG. 5, the surface potential of the photosensitive drum 2 is converged to −600 V (normal potential) that is a desired charging potential Vd as a whole. However, since the amount of AC discharge current is small, it receives abnormal discharge and locally becomes abnormal potential. The abnormal potential at this time has a potential difference of about 1000 V from the normal potential, the abnormal potential on the negative polarity side (normal charging polarity side of the toner) is about −1600 V, and the positive polarity side (normal charging polarity of the toner) The abnormal potential on the reverse polarity side is about + 400V.

  If image formation is performed in this state, a white spot-like part and a high-density spot-like part exist on the image, resulting in a sandy image defect as shown in FIG.

4). Sandy Image Defect Next, the mechanism when a sandy image defect occurs will be described in more detail. Here, in accordance with this embodiment, the photosensitive drum is uniformly charged to a negative polarity with a charging roller, and the negatively charged toner is adhered to the portion that has been neutralized by the exposure device by the developing device. A model image forming apparatus is used as a model.

  FIG. 14 shows the relationship of the AC current flowing through the charging roller with respect to time when the photosensitive drum is charged using the charging roller by the AC charging method. Here, a sine wave is used as the waveform of the AC voltage applied to the charging roller.

  Here, FIG. 14 shows the current that flows through the charging roller when an AC voltage that minimizes AC discharge is applied to the charging roller. The peak-to-peak voltage Vpp of the AC voltage applied to the charging roller is slightly higher than twice the discharge start voltage Vth (preferably 50 V to 300 V, more preferably 100 V to 200 V, and still more preferably 100 V). .

  In this case, as shown in FIG. 14, even when a sinusoidal AC voltage is applied to the charging roller, there is a short peak due to discharge immediately after the peak of the waveform of the AC current flowing through the charging roller. The waveform of is not a sine wave.

  Further, in this case, abnormal discharge as indicated by A and B may occur in the vicinity of the discharge peak in the AC current waveform. As shown in FIG. 14, the abnormal discharge on the negative polarity side (normal charging polarity side of the toner) such as A and the abnormal discharge on the positive polarity side (side opposite to the normal charging polarity of the toner) such as B Both discharge and discharge may occur.

  FIG. 15A illustrates the relationship between the surface potential of the photosensitive drum and the potential of the developing sleeve (the potential of the DC component of the developing voltage) when the photosensitive drum is charged under the above conditions and a halftone image is output. The relationship between the potential difference between them and the toner image formed is schematically shown. FIG. 15B schematically shows an image output as a result. For example, the halftone image has 125 levels in 256 gradations.

  A portion on the photosensitive drum that has undergone abnormal discharge on the negative polarity side, such as A, has an abnormally large absolute potential on the negative polarity side. For this reason, even if a halftone image or a solid image is formed, the toner does not move to the photosensitive drum side in terms of potential in the developing unit, and the image in that portion is whitened. On the other hand, a portion on the photosensitive drum that has received an abnormal discharge on the positive polarity side, such as B, has an abnormally large potential on the positive polarity side. For this reason, even when a halftone image or a solid white image is formed, a large amount of toner moves to the photosensitive drum side in terms of potential in the developing unit, and the image in that portion becomes a solid high-density dot. . This becomes a sandy image defect when an image is output.

  As described above, in the contact charging type image forming apparatus using the AC charging type, if the amount of AC discharge current is minimized in order to suppress the deterioration of the photoreceptor and the image flow, a sandy image defect may occur. is there.

  FIG. 16 also shows the relationship between the frequency of the AC voltage applied to the charging roller and the amount of AC discharge current (hereinafter also referred to as “sand-disappearance current value”) necessary to prevent the occurrence of sand-like image defects. Show. The sand-disappearance current value with respect to the frequency rises in a quadratic function. This indicates that sandy image defects become more prominent as the frequency of the AC voltage increases.

  When the output speed of the image forming apparatus is increased, the frequency of the AC voltage applied to the charging roller is increased so as not to cause image moiré with the screen processing during latent image formation. In this case, a sandy image defect is likely to occur due to the relationship between the frequency and the sandy land disappearance current value. Therefore, in order to achieve high speed and high image quality of the image forming apparatus, it is strongly desired not to generate sandy image defects even if the AC discharge current is minimized. That is, one of the objects of this embodiment is to suppress sandy image defects that may occur in an image forming apparatus having a contact charging type charging unit. One of the more detailed purposes of the present embodiment is that, in an image forming apparatus having a contact charging type charging means, high speed and high image quality are achieved, and even when the AC discharge current is minimized, a sandy image defect It is to be able to suppress the occurrence of.

5. In this embodiment, in order to eliminate the abnormal potential causing the sand-like image defect, the second charging roller 12 and the third charging roller 13 are respectively described below. Apply such a voltage.

  First, FIG. 7A shows the relationship between the surface potential of the photosensitive drum 2 after passing through the first charging unit a1 and the DC voltage applied to the second charging roller 12. FIG. On the other hand, FIG. 7B shows the relationship between the change in the surface potential of the photosensitive drum 2 when passing through the second charging portion a2 and the surface potential of the photosensitive drum 2 after passing through the second charging portion a2. Indicates.

  In the present embodiment, the second charging roller 12 is disposed for the purpose of eliminating the abnormal potential on the negative polarity side. Then, a DC voltage of −100 V is applied to the second charging roller 12 as a charging voltage.

  That is, the DC voltage applied to the second charging roller 12 is a potential difference between the DC voltage applied to the first charging roller 11 (desired charging potential Vd) and the DC voltage applied to the second charging roller 12. ΔV2 is set to be Vth−50 [V]. Here, since the charging potential Vd is −600 V and the discharge start voltage Vth is 550 V, the DC voltage applied to the second charging roller 12 is positive with respect to −600 V (the polarity opposite to the polarity of the abnormal potential). Side) was set to −100 V having a potential difference of 500 V (= 550 V−50 V).

  As described above, the surface potential of the photosensitive drum 2 after passing through the first charging portion a1 converges to the desired charging potential Vd of −600 V (normal potential) as a whole, and this normal potential. And a potential difference ΔV2 between the DC voltage applied to the second charging roller 12 is 500V. Here, since the discharge start voltage Vth is 550 V, no DC discharge occurs in the normal potential portion on the photosensitive drum 2 from the second charging roller 12. This portion passes through the second charging portion a2 while the surface potential remains at −600V.

  On the other hand, the portion having an abnormal potential on the negative polarity side on the photosensitive drum 2 is a potential of about -1600V, and the potential difference ΔV2m between this potential and the DC voltage applied to the second charging roller 12 is about 1500V. is there. Here, since the discharge start voltage Vth is 550 V, a DC discharge is generated in a portion having an abnormal potential from the second charging roller 12 to the negative polarity side on the photosensitive drum 2. Then, after this portion passes through the second charging portion a2, the surface potential is determined by the potential difference ΔV2m ′ between the abnormal potential on the negative polarity side and the DC voltage applied to the second charging roller 12 being the discharge start voltage Vth. It converges to be equivalent to, and becomes -650V here.

  Further, the portion having an abnormal potential on the positive polarity side on the photosensitive drum 2 is a potential of about + 400V, and the potential difference ΔV2p between this potential and the DC voltage applied to the second charging roller 12 is about 500V. . Here, since the discharge start voltage Vth is 550 V, no DC discharge is generated in the portion having an abnormal potential from the second charging roller 12 to the positive polarity side on the photosensitive drum 2. This portion passes through the second charging portion a2 while the surface potential is about + 400V.

  Accordingly, the surface potential of the photosensitive drum 2 after passing through the second charging portion a2 is as shown in the right diagram of FIG. 7B. When image formation is performed in this state, since the abnormal potential on the negative polarity side on the photosensitive drum 2 is reduced, white spots on the image are reduced, but the positive polarity side on the photosensitive drum 2 is reduced. There is a high-density spot-like portion due to the abnormal potential, resulting in a sandy image defect as shown in FIG.

  Next, FIG. 9A shows the relationship between the surface potential of the photosensitive drum 2 after passing through the second charging unit a2 and the DC voltage applied to the third charging roller 13. On the other hand, FIG. 9B shows the relationship between the change in the surface potential of the photosensitive drum 2 when passing through the third charging portion a3 and the surface potential of the photosensitive drum 2 after passing through the third charging portion a3. Indicates.

  In the present embodiment, the third charging roller 13 is disposed for the purpose of eliminating the abnormal potential on the positive polarity side. Then, a DC voltage of −1100 V is applied to the third charging roller 13 as a charging voltage.

  That is, the DC voltage applied to the third charging roller 13 includes a DC voltage applied to the first charging roller 11 (that is, a desired charging potential Vd), a DC voltage applied to the third charging roller 13, and The potential difference ΔV3 is set to Vth−50 [V]. Here, since the charging potential Vd is −600 V and the discharge start voltage Vth is 550 V, the DC voltage applied to the third charging roller 13 is negative with respect to −600 V (the polarity opposite to the polarity of the abnormal potential). -1100V having a potential difference of 500V (= 550V-50V).

  The surface potential of the photosensitive drum 2 after passing through the first charging unit a1 and the second charging unit a2 has converged to a desired charging potential Vd of −600 V (normal potential) as a whole. The potential difference ΔV3 between the normal potential and the DC voltage applied to the third charging roller is 500V. Here, since the discharge start voltage Vth is 550 V, DC discharge does not occur in the portion of the normal potential on the photosensitive drum 2 from the third charging roller 13. This portion passes through the third charging portion a3 with the surface potential kept at −600V.

  Further, as described above, the surface potential of the portion that has become an abnormal potential on the negative polarity side after passing through the first charging portion a1 on the photosensitive drum 2 is the surface potential when passing through the second charging portion a2. It has converged to -650V. Therefore, the potential difference ΔV3m between this potential and the DC voltage applied to the third charging roller 13 is 450V. Here, since the discharge start voltage Vth is 550 V, DC discharge does not occur in the portion that has an abnormal potential from the third charging roller 13 to the negative polarity side on the photosensitive drum 2. Then, this portion passes through the third charging portion a3 while the surface potential remains at −650V.

  On the other hand, the portion having an abnormal potential on the positive polarity side after passing through the first charging portion a1 on the photosensitive drum 2 has a potential of about +400 V even after passing through the second charging portion a2, as described above. It is. Therefore, the potential difference ΔV3p between this potential and the DC voltage applied to the third charging roller 13 is about 1500V. Here, since the discharge start voltage Vth is 550 V, DC discharge is generated in a portion having an abnormal potential from the third charging roller 13 to the positive polarity side on the photosensitive drum 2. Then, after this portion passes through the third charging portion a3, the surface potential is determined by the potential difference ΔV3p ′ between the abnormal potential on the positive polarity side and the DC voltage applied to the third charging roller 13 being the discharge start voltage Vth. It converges to be equivalent to, and becomes -550V here.

  Accordingly, the surface potential of the photosensitive drum 2 after passing through the third charging portion a3 is as shown on the right side of FIG. 9B. When image formation is performed in this state, both the negative side abnormal potential and the positive side abnormal potential on the photosensitive drum 2 are alleviated, so that white spotted portions and high density spotted portions on the image are reduced. Both parts are reduced, and a good image as shown in FIG. 10 is obtained.

  Here, in this embodiment, the potential difference ΔV2 between the DC voltage applied to the first charging roller 11 and the DC voltage applied to the second charging roller 12 is set to Vth−50 [V]. Similarly, in this embodiment, the potential difference ΔV3 between the DC voltage applied to the first charging roller 11 and the DC voltage applied to the third charging roller 13 is Vth−50 [V]. However, the potential differences ΔV2 and ΔV3 for reducing the sand-like image defect due to the abnormal potential on the photosensitive drum 2 may be selected as long as they are less than the discharge start voltage Vth.

  In order to reduce the potential difference between the normal potential and the abnormal potential on the photosensitive drum 2 after passing through the first, second, and third charging units a1, a2, and a3, it is better that the potential differences ΔV2 and ΔV3 are large. .

  However, if the potential difference ΔV2 or ΔV3 is too close to the discharge start voltage Vth, there is a concern that the potential difference ΔV2 or ΔV3 exceeds the discharge start voltage Vth due to a change in impedance accompanying a rapid environmental change or the like. When the potential difference ΔV2 or ΔV3 exceeds the discharge start voltage Vth, DC discharge from the second charging roller 12 or the third charging roller 13 to the normal potential on the photosensitive drum 2 occurs. For this reason, density changes due to potential fluctuations of the photosensitive drum 2, image density unevenness due to potential unevenness, and the like may occur.

  Table 1 shows the results of evaluating the level of white spots on the halftone image and the DC discharge level to the normal potential when the potential difference ΔV2 is varied.

  When the potential difference ΔV2 is Vth−250 [V], the abnormal potential on the negative polarity side cannot be sufficiently reduced, so that white spots on the halftone image are conspicuous, resulting in an image defect. On the other hand, when the halftone image is output with the potential difference ΔV2 set in a range from Vth−200 [V] to Vth [V], the abnormal potential on the negative polarity side can be sufficiently reduced. Vitiligo spots were at acceptable levels or were not seen.

  On the other hand, when the potential difference ΔV2 is Vth [V], the density of the halftone image may fluctuate high due to DC discharge from the second charging roller 12 to the normal potential. On the other hand, when the halftone image is output with the potential difference ΔV2 set in a range from Vth−250 [V] to Vth−50 [V], the fluctuation in the density of the halftone image is at an acceptable level. Or did not happen.

  Table 2 shows the results of evaluating the level of high density spots on the halftone image and the level of DC discharge to normal potential when the potential difference ΔV3 is varied.

  When the potential difference ΔV3 is Vth−250 [V], the abnormal potential on the positive polarity side cannot be sufficiently reduced, and high density spots on the halftone image are conspicuous, resulting in an image defect. In contrast, when the halftone image is output with the potential difference ΔV3 set in a range from Vth−200 [V] to Vth [V], the abnormal potential on the positive polarity side can be sufficiently reduced. High density spots were at acceptable levels or were not seen.

  On the other hand, when the potential difference ΔV3 is Vth [V], the density of the halftone image may fluctuate low due to DC discharge from the third charging roller 13 to the normal potential. On the other hand, when the halftone image is output with the potential difference ΔV3 set in a range from Vth−250 [V] to Vth−50 [V], the fluctuation in the density of the halftone image is at an acceptable level. Or did not happen.

From the above results, the potential differences ΔV2 and ΔV3 are preferably set as follows.
Vth−200 [V] ≦ ΔV2 ≦ Vth−50 [V]
Vth−200 [V] ≦ ΔV3 ≦ Vth−50 [V]

  Within this range, sandy image defects are reduced, while image density caused by variation in density due to potential fluctuation of the photosensitive drum 2 or potential unevenness due to DC discharge by the second and third charging rollers 12 and 13. Unevenness or the like does not occur, and a sufficiently good image can be formed.

  In this embodiment, the second and third charging rollers 12 and 13 are provided to control the abnormal potentials on both the negative polarity side and the positive polarity side on the photosensitive drum 2 to be close to the desired charging potential Vd. . Thereby, as described above, it is possible to effectively suppress sandy image defects due to the abnormal potential on the bipolar side. However, as described above, even if the abnormal potential on one polarity side is made close to the desired charging potential, an effect of suppressing some sandy image defects can be obtained. Accordingly, if desired, only one more charging roller is provided on the downstream side in the movement direction of the surface of the photosensitive drum 2 with respect to the first charging roller 11, and the abnormal potential on the negative polarity side or the positive polarity side is set as described above. Only one of them may be controlled to approach the desired charging potential Vd.

  That is, the image forming apparatus 1 includes at least a first charging unit 11 that contacts and charges the surface of the moving photosensitive member, and a downstream side in the moving direction of the photosensitive member with respect to the first charging unit. And a second charging means 12 for contacting and charging the surface of the moving photosensitive member. In addition, the image forming apparatus 1 includes at least a first power source S1 that applies a voltage to the first charging unit 11 and a second power source S2 that applies a voltage to the second charging unit 12. Then, the first power supply S1 applies an oscillating voltage obtained by superimposing a DC voltage and an AC voltage to the first charging unit 11. The second power source S2 applies a DC voltage to the second charging unit 12 such that the potential difference ΔV2 from the DC voltage applied to the first charging unit 11 by the first power source S1 is less than the discharge start voltage. Preferably, the image forming apparatus 1 further includes a third charging unit 13 that contacts and charges the surface of the moving photosensitive member downstream of the second charging unit 12 in the moving direction of the photosensitive member. And a third power source S3 for applying a voltage to the third charging means 13. The third power source S3 applies the following DC voltage to the third charging unit 13. That is, the potential difference between the first power source S1 and the DC voltage applied to the first charging unit 11 is less than the discharge start voltage, and the potential source applies the first power source S1 to the first charging unit 11. With the DC voltage as a reference, the potential difference ΔV2 is a DC voltage that has a potential difference of opposite polarity.

  As described above, according to this embodiment, an appropriate charging voltage calculated from the discharge start voltage is applied to a plurality of charging rollers (in this embodiment, the first, second, and third charging rollers 12, 13, 14). Thus, according to this embodiment, it is possible to suppress sandy image defects that may occur in an image forming apparatus having a contact charging type charging unit. For example, even in the case of increasing the image quality by increasing the frequency of the AC voltage of the charging voltage to minimize the amount of AC discharge current in order to cope with higher speed in an AC charging type image forming apparatus, the generation of a sandy image is generated. Can be effectively suppressed. Therefore, according to the present embodiment, in the image forming apparatus having the contact charging type charging means, high speed and high image quality are achieved, and even when the AC discharge current value is minimized, the occurrence of sandy image defects is generated. Can be suppressed.

Example 2
Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of the first embodiment. Accordingly, elements having the same functions and configurations as those of the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted.

  In Example 1, the peak-to-peak voltage Vpp of the AC voltage applied to the first charging roller 11 was determined based on the value of the discharge start voltage Vth obtained in advance in each environment. On the other hand, in this embodiment, the image forming apparatus 1 has a mechanism for measuring the DC current value flowing through the first charging roller 11, and determines the value of Vth based on the measurement result of the current value. .

  FIG. 11 is a block circuit diagram of a charging voltage application system for the first charging roller 11 in this embodiment.

  A first power supply S1 that is a charging voltage application unit that applies a charging voltage to the first charging roller 11 includes a DC power supply (DC power supply) 11g and an AC power supply (AC power supply) 11h. A predetermined oscillating voltage obtained by superimposing a DC voltage and an AC voltage is applied from the first power supply S1 to the first charging roller 11 via the cored bar 11a. As a result, the peripheral surface of the rotating photosensitive drum 2 is charged almost uniformly to a predetermined potential. In this embodiment, the image forming apparatus 1 is a DC current value measuring circuit (hereinafter referred to as a unit “measurement”) as a DC current measuring unit that measures a DC current value flowing through the first charging roller 11 via the photosensitive drum 2. Circuit ”) 15. Information on the DC current value measured by the measurement circuit 15 is input from the measurement circuit 15 to the control circuit 14.

  The control circuit 14 has a function of controlling the DC voltage value applied to the first charging roller 11 from the DC power supply 11g and the peak-to-peak voltage value of the AC voltage applied to the first charging roller 11 from the AC power supply 11h. In this embodiment, the control circuit 14 calculates the peak-to-peak voltage value of the AC voltage applied to the first charging roller 11 in the charging process of the printing process from the DC current value information input from the measurement circuit 15. And a function for executing a program to be determined.

  Next, control for determining the peak-to-peak voltage value of the AC voltage applied to the first charging roller 11 during the printing process will be further described. This control is performed as follows in the print preparation rotation operation (pre-rotation step).

  The peak-to-peak voltage of the AC voltage applied from the AC power source 11h to the first charging roller 11 in a state where a DC voltage equivalent to that during the printing process (here, -600 V) is applied from the DC power source 11g to the first charging roller 11 Apply while increasing Vpp. At this time, the image forming apparatus 1 is operated without creating an image. At this time, a DC voltage is applied to the transfer roller 5, and after passing through the transfer portion d, the surface potential of the photosensitive drum 2 becomes about −350V.

  FIG. 12 is a plot of the DC current value flowing through the first charging roller 11 against the peak-to-peak voltage Vpp of the AC voltage applied to the first charging roller 11.

  As shown in FIG. 12, when the peak-to-peak voltage Vpp of the AC voltage applied to the first charging roller 11 is low (Vpp ≦ 400 V in FIG. 12), the photosensitive drum before and after passing through the first charging unit a1. The surface potential of 2 does not change. Therefore, the DC current value flowing through the first charging roller 11 is zero.

  When the peak-to-peak voltage Vpp of the AC voltage applied to the first charging roller 11 is increased (in FIG. 12, 400 V ≦ Vpp ≦ 1100 V), the surface potential of the photosensitive drum 2 after passing through the first charging portion a1 changes. Begin to. As the peak-to-peak voltage Vpp of the AC voltage applied to the first charging roller 11 increases, the amount of change in the surface potential of the photosensitive drum 2 also increases, and the value of the DC current flowing through the first charging roller 11 also increases.

  When the peak-to-peak voltage Vpp of the AC voltage applied to the first charging roller 11 is further increased (Vpp ≧ 1100 V in FIG. 12), the surface potential of the photosensitive drum 2 after passing through the first charging portion a1 is the first. This is equivalent to the DC voltage applied to the charging roller 11 (here, -600 V). Even if the peak-to-peak voltage Vpp of the AC voltage applied to the first charging roller 11 is further increased, the amount of change in the surface potential of the photosensitive drum 2 does not change. Therefore, the value of the DC current flowing through the first charging roller 11 with respect to the peak-to-peak voltage Vpp of the AC voltage applied to the first charging roller 11 is constant.

  Here, paying attention to the applied voltage when the DC current value does not change, the peak-to-peak voltage Vpp of the AC voltage applied to the first charging roller 11 at this time starts to discharge on both the negative polarity side and the positive polarity side. It vibrates with the touch width of the voltage Vth. That is, at this time, it is understood that Vpp is twice Vth. Therefore, the discharge start voltage Vth in this environment can be detected by utilizing this characteristic.

In actual operation, the control circuit 14 applies a DC voltage of −600 V to the first charging roller 11 and applies an AC voltage while increasing the peak-to-peak voltage Vpp in increments of 50 V. At this time, the control circuit 14 detects the DC current value measured and input by the measurement circuit 15. Here, as shown in FIG. 12, it is assumed that the DC current value becomes 35 μA when Vpp = 1100 V, and the DC current value does not increase thereafter. The control circuit 14 calculates Vth by dividing Vpp (1100 V in this environment) when the DC current value ceases to change by 2. That is, in this environment, Vth = 550V. The control circuit 14 stores the calculated Vth in the memory unit. Then, the control circuit 14 uses the value of Vth read from the memory unit to determine the peak-to-peak voltage Vpp of the AC voltage applied to the first charging roller 11 according to the following equation, as in the first embodiment.
Vpp = 2 × Vth + 100 [V]

  In this environment, since Vth = 550V, the peak-to-peak voltage Vpp of the AC voltage applied to the first charging roller 11 is 1200V.

  The peak-to-peak voltage Vpp of the AC voltage applied to the first charging roller 11 determined in this way is a set value when the discharge current is minimized, and the AC discharge current amount is about 20 μA.

  In the present embodiment, the charging voltages applied to the second and third charging rollers 12 and 13 for reducing sandy image defects are set in the same manner as in the first embodiment.

  As described above, according to the present embodiment, the same effects as those of the first embodiment can be obtained, and charging that is more suitable for the current state of the image forming apparatus is performed based on the discharge start voltage detected in the image forming apparatus. The voltage can be controlled.

  As mentioned above, although this invention was demonstrated according to the specific Example, this invention is not limited to the above-mentioned Example. For example, the image forming apparatus does not directly transfer the toner image from the photosensitive member to the recording material, but transfers the toner image from the photosensitive member to the intermediate transfer member that temporarily holds and conveys the toner image from the intermediate transfer member to the recording material. It may be configured to transfer.

2 Photosensitive drum 11 First charging roller 12 Second charging roller 13 Third charging roller S1 Power source of first charging roller S2 Power source of second charging roller S3 Power source of third charging roller

Claims (6)

A photoreceptor,
A first charging means for contacting the surface of the moving photoreceptor and charging it;
A second charging means for contacting and charging the surface of the moving photoreceptor on the downstream side in the moving direction of the photoreceptor relative to the first charging means;
A first power supply for applying a voltage to the first charging means;
A second power source for applying a voltage to the second charging means;
Exposure means for exposing the charged photosensitive member to form an electrostatic latent image;
Developing means for developing the electrostatic latent image formed on the photoreceptor;
Have
The first power source applies an oscillating voltage obtained by superimposing a DC voltage and an AC voltage to the first charging unit, and the second power source is applied by the first power source to the first charging unit. An image forming apparatus, wherein a DC voltage having a potential difference with a DC voltage that is less than a discharge start voltage is applied to the second charging unit. When the potential difference between the DC voltage applied by the second power source to the second charging unit and the DC voltage applied by the first power source to the first charging unit is ΔV2, and the discharge start voltage is Vth. ,
Vth−200 [V] ≦ ΔV2 ≦ Vth−50 [V]
The image forming apparatus according to claim 1, wherein the relationship is satisfied.   Further, a third charging means for contacting and charging the surface of the moving photoconductor downstream of the second charging means in the moving direction of the photoconductor, and a third charging means. A third power source for applying a voltage, wherein the third power source has a potential difference from a DC voltage applied to the first charging means by the first power source that is less than a discharge start voltage, and The potential difference is determined based on a DC voltage applied to the first charging unit by the first power source and a DC voltage applied to the second charging unit by the second power source and the first power source 3. The image forming apparatus according to claim 1, wherein a DC voltage having a potential difference opposite to a potential difference from a DC voltage applied to the first charging unit is applied to the third charging unit. When the potential difference between the DC voltage applied by the third power source to the third charging unit and the DC voltage applied by the first power source to the first charging unit is ΔV3 and the discharge start voltage is Vth. ,
Vth−200 [V] ≦ ΔV3 ≦ Vth−50 [V]
The image forming apparatus according to claim 3, wherein the relationship is satisfied.   Information on the discharge start voltage read from the storage means corresponding to the atmosphere environment of the image forming apparatus as the discharge start voltage; The image forming apparatus according to claim 1, wherein an image forming apparatus is used.   Current value measuring means for measuring a current value flowing through the first charging means when a voltage is output from the first power source to the first charging means, and the current value measurement as the discharge start voltage; The image forming apparatus according to claim 1, wherein information on the discharge start voltage calculated from a measurement result of the unit is used.