JP2008191268A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent paper from being soiled with toner resulting from a decrease in surface potential outside a sheet of paper the end shape of which is not uniform. <P>SOLUTION: In the image forming apparatus, an end information capturing means captures paper end shape information, based on which the paper section information capturing means subsequently divides the charging area of the surface of a photoreceptor into a paper presence area and a paper non-presence area. A charging means then increases the surface potential of a photoreceptor to a second potential higher than a first potential assigned for development. An exposure laser output means exposes the surface of a photoreceptor to laser beams of laser powers differing between the paper presence area 301 and paper non-presence area 302 (latent image non-formation area and latent image formation area). <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an image forming apparatus such as an electrophotographic apparatus or an electrostatic recording apparatus.

  Conventionally, in electrophotography, the surface of a photosensitive member (drum) is uniformly charged to a predetermined polarity, and then image exposure is performed by light irradiation based on predetermined document information to form an electrostatic charge image (electrostatic latent image). Form. Then, the electrostatic image is developed to form a toner image, the toner image is transferred to a recording material such as paper, and the toner image is fixed on the recording material by heating and pressurizing with a fixing roller to form an image ( Print).

  Conventionally, a corona charger has been generally used as a charging means for charging the photosensitive member to a predetermined polarity and potential. In this method, a corona charger is disposed in a non-contact manner on the photoconductor, and the surface of the photoconductor is exposed to the corona discharged from the corona charger, so that the photoconductor is charged to a predetermined polarity and potential. In particular, in recent years, compared to this non-contact type corona charging system, it has advantages such as low ozone generation, so that a charging member (contact charging member) to which a charging bias is applied is brought into contact with the photosensitive member to be charged. Many charging methods based on the contact charging method for charging the surface of the photoreceptor to a predetermined polarity and potential have been proposed and put into practical use.

  In such a contact charging type charging device, there are various charging members to be brought into contact with a photosensitive member as a member to be charged, such as a roller type (charging roller), a fur brush type, a magnetic brush type or a blade type (charging blade). There is an improvement proposal. Among them, from the viewpoint of uniform charging, a roller charging method using a conductive roller is widely used because it is preferable in terms of charging stability. The charging roller as the conductive roller is manufactured using a conductive or medium resistance rubber material or foam. There is also a charging roller obtained by laminating these to obtain desired characteristics.

  Such a charging roller has elasticity in order to obtain a certain contact state with the photosensitive member. When this charging roller is brought into pressure contact with the photosensitive member, a certain voltage or more is applied to the charging roller. Is applied to the photosensitive member, the surface potential of the photosensitive member begins to rise, and thereafter, the surface potential of the photosensitive member increases linearly in accordance with the applied voltage. The threshold voltage at which the surface potential starts to rise is defined as a charging start voltage Vth (this charging start voltage Vth is obtained based on Paschen's law).

In order to obtain the photoreceptor surface potential V0 necessary for the electrophotographic image forming process, it is necessary to apply a DC voltage of V0 + Vth to the charging roller. This charging method is called a DC charging method. In order to obtain charging uniformity, an AC charging method is also used in which an AC voltage having a peak-to-peak voltage that is twice or more Vth is superimposed on the DC voltage of V0 + Vth. This utilizes a so-called smoothing effect due to alternating current, whereby the potential of the photosensitive member converges to V0, which is the median value of the peak of the alternating voltage, and suppresses the influence of fluctuations in the resistance value of the rubber roller due to the environment or the like. be able to. By such a method, as described above, the surface of the photoreceptor is uniformly charged to a predetermined polarity, image exposure is performed to form an electrostatic charge image, and this charge image is developed to form a toner image. Following this, in the process of transferring the toner image to the recording material, an elastic roller such as a rubber material or a foam that rotates in contact with the photosensitive member is used in the same manner as described above for the advantage of low ozone generation and paper transportability. A method is generally used in which a toner image is transferred to a recording material by applying a predetermined voltage to the roller. Finally, image formation is performed by fixing the toner image on the recording material by heating and pressing with a fixing roller in the fixing step.
Japanese Patent Laid-Open No. 9-22165

  In the image forming apparatus, as shown in FIG. 9, the charging area 902 (axial direction) of the photoreceptor 901 by the charging roller is normally set larger than the transfer area 904 (axial direction) by the transfer roller 903. However, the surface potential of the photoconductor 901 is different between the portion where the paper 905 (recording material) intervenes and the portion where the paper 905 (recording material) intervenes due to the fact that the polarity of charging is opposite to the polarity of transfer or the influence of transfer on the photoconductor 901. Will make a difference. More specifically, there is a distinction between a portion 906 that receives the transfer bias indirectly through the sheet 905 and a portion 907 that receives the transfer bias directly. However, the thickness of the sheet 905 shown in FIG. 9 is actually thin, and the portion 908 of the transfer roller 903 where the sheet 905 is not interposed comes into contact with the portion 907.

  Therefore, there is a problem that the surface potential of the photoreceptor relative to the latter does not rise to a predetermined potential after passing through the charging step as compared with the former. In general, the width of the used paper varies, and the length of the transfer roller in the axial direction is not less than the maximum width of the used paper. Therefore, it is impossible to prevent such a situation from occurring. Therefore, due to the decrease in the surface potential, the toner is developed on the surface of the photoreceptor outside the paper, or the transfer roller is contaminated with the toner. For example, the toner consumption increases more than necessary, or the paper is conveyed on the paper. In some cases, the toner is developed and transferred near the end in the direction orthogonal to the direction (hereinafter referred to as the end of the paper), and the paper may become dirty.

  In particular, when the paper is not a fixed shape, that is, when the shape of the edge of the paper is not a straight line such as rectangular paper, or when the paper (for example, rectangular paper) is misaligned, the leading edge or the trailing edge of the paper is The position of the paper edge differs during one rotation of the photoconductor, such as when it is not parallel to the photoconductor (axial direction of the photoconductor drum) (the position of the paper edge relative to the photoconductor changes as the photoconductor rotates). ), The paper is smeared.

  A method of neutralizing the charge of the so-called transfer memory by disposing a preliminary (auxiliary) charging device having a polarity opposite to that of transfer on the upstream side of the contact charging unit with respect to the rotation direction of the photosensitive member (for example, Patent Document 1). In addition to this, for example, a method of arranging a photostatic device for initializing preliminary charging at the time of neutralization has been devised. However, the provision of the preliminary charging device and the photostatic device as described above complicates the device configuration, leading to an increase in the size of the device or an increase in cost.

  The present invention has been made in view of the above circumstances, and has a simple configuration without providing a preliminary charging device or a light neutralization device, and is outside the sheet on the surface of the photoreceptor, in particular, as described above. In the case of a paper whose position is not constant, unnecessary toner development, toner contamination of the transfer roller due to a decrease in surface potential outside the paper, or the occurrence of paper contamination due to these, can be prevented. An object of the present invention is to provide an image forming apparatus capable of obtaining a printed image.

  In an image forming apparatus according to the present invention, a toner image developed on a photoconductor is transferred to a sheet that is passed between the photoconductor and a transfer member. Charging means for charging to the potential of the laser, exposure laser output means configured to output a laser beam for exposing the photosensitive member having a plurality of types of laser power, and controlling the laser power in the exposure laser output means Laser power control means, edge information acquisition means for acquiring edge shape information indicating the edge shape in the sheet passing width direction of the paper, and edge shape information acquired by the edge information acquisition means , Paper classification for obtaining paper classification information for classifying a paper existing area where paper exists in a charged area on the surface of the photoconductor and a non-paper existing area where paper does not exist And a latent image classification information for classifying a latent image forming area for forming the developing electrostatic latent image and a non-latent image forming area not forming the electrostatic latent image in the sheet presence area. A latent image classification information acquisition unit, and the charging unit performs potential increase charging to increase the surface potential of the photoconductor to a second potential higher than the first potential at the time of development, and the laser power control unit includes: Based on the paper classification information acquired by the paper classification information acquisition means and the latent image classification information acquired by the latent image classification information acquisition means, the exposure laser output means is supplied with the non-paper existence area, non-latent information. A laser beam having a different laser power is output for each of the image forming area and the latent image forming area to expose the potential rising charging portion on the surface of the photosensitive member, and Lowering the potential of the latent image forming region to a predetermined electrostatic latent image forming potential lower than the first potential, and lowering the potential of the non-latent image forming region in the potential increasing charging unit to the first potential; The potential of the non-paper existing area in the potential increasing charging unit is lowered to a predetermined potential equal to or higher than the first potential.

  According to the above configuration, the edge shape information in the sheet passing width direction is acquired by the edge information acquisition unit, and based on the edge shape information by the sheet classification information acquisition unit, in the charged region on the surface of the photoreceptor. Paper classification information for dividing the paper presence area and the non-paper presence area is acquired. In addition, the latent image classification information acquisition unit acquires latent image classification information that classifies the latent image forming area and the non-latent image forming area in the sheet presence area. Then, the charging means performs a potential increase charging so as to raise the surface potential of the photosensitive member to a second potential higher than the first potential at the time of development, and the above-mentioned paper classification is controlled by the control of the exposure laser output means by the laser power control means. Based on the information and the latent image classification information, a laser beam having a different laser power for each of the non-paper existence area, the non-latent image formation area, and the latent image formation area is output from the exposure laser output means, and the photosensitive member surface The potential rising charging portion is exposed to light, and the potential of the latent image forming area in the potential rising charging portion is lowered to a predetermined electrostatic latent image forming potential lower than the first potential, and non-latent in the potential rising charging portion. The potential of the image forming area is lowered to the first potential, and the potential of the non-paper existing area in the potential rising charging unit is lowered to a predetermined potential that is equal to or higher than the first potential.

  Accordingly, the boundary between the non-paper existence area and the paper existence area can be set to a shape corresponding to the edge shape of the paper, that is, the non-paper existence area and the paper existence reflecting the edge shape of the paper. Since it is possible to perform exposure with different laser powers on the areas (non-latent image forming areas), it is possible to perform a simple configuration without providing a preliminary charging device or a light neutralizing device, and the edge of the sheet with respect to the photoconductor It is possible to prevent unnecessary toner development and toner contamination of the transfer roller due to a decrease in surface potential outside the paper in the case of a paper whose position is not constant, thereby preventing the occurrence of paper smearing due to these, resulting in high image quality. A printed image can be obtained.

  In the above configuration, if the drum rotation distance of the photoconductor from the exposure position of the photoconductor to the transfer position by the transfer unit is a first distance, the edge information acquisition unit is configured to transfer the transfer information in the paper conveyance path. It is preferable that it is disposed upstream of the position at a position spaced apart by a second distance longer than the first distance.

  According to this, when the drum rotation distance of the photosensitive member from the exposure position to the transfer position is the first distance, the end is located at a position separated from the transfer position in the paper conveyance path by a second distance longer than the first distance on the upstream side. Since the copy information acquisition means is provided, the time required to output the exposure laser reflecting the acquired edge shape information of the paper, the exposure pattern of the photoconductor and the edge shape position of the conveyed paper If the second distance is the same as the first distance, it is possible to increase the transmission speed or processing speed of the end shape information. However, it is difficult to match the timing, but by setting the second distance to be longer than the first distance, the end shape information is obtained before the exposure laser output starts, and the end shape is obtained. The paper classification information (and latent image classification information) reflecting the information is input (transmitted) to the laser power control means, and based on this information, a time for preparing (processing) to output with different laser power in each region is obtained. It is possible to easily realize a configuration that enables this.

  In the above configuration, it is preferable that the end information acquisition unit is disposed at a position spaced apart by the second distance upstream from the transfer position and at a position downstream of the registration roller. Item 3).

  According to this, since the end portion information acquisition means is disposed at a position separated by the second distance upstream from the transfer position and at a position downstream of the registration roller, the end information acquisition means is provided with the end information acquisition means. Before acquiring the part shape information, the leading edge of the sheet is aligned by the registration rollers and the conveyance timing of the sheet toward the photoconductor is adjusted, and the edge shape information is acquired for the sheet conveyed by adjusting the timing. Since the operation is performed, the paper conveyance timing and the photosensitive drum rotation timing can be accurately matched (the positional deviation between the edge of the conveyed paper and the exposure pattern on the surface of the photosensitive member does not occur). Furthermore, it is possible to prevent unnecessary toner development, toner contamination of the transfer roller, or paper contamination due to these.

  In the above configuration, it is preferable that the edge information acquisition unit acquires the edge shape information in a non-contact manner with respect to the sheet.

  According to this, since the edge shape information is acquired by the edge information acquisition unit in a non-contact manner with respect to the sheet, when the edge shape information is acquired by the edge information acquisition unit, the edge information acquisition unit It is possible to prevent the orientation of the paper and the conveyance timing from deviating due to contact with the paper (this paper has an arbitrary shape such as an uneven shape at the end).

  In the above configuration, the edge information acquisition means is a row-shaped optical sensor in which predetermined light-emitting elements and light-receiving elements are arranged to face each other in the sheet passing width direction with the sheet conveyance path interposed therebetween. (Claim 5).

  According to this, since the edge information acquisition means is a row-shaped optical sensor in which the predetermined light emitting element and the light receiving element are arranged to face each other in the sheet passing width direction with the sheet conveyance path interposed therebetween, A configuration for acquiring the part shape information in a non-contact manner can be easily realized by using a simple configuration.

  In the above configuration, it is preferable that the laser power control unit changes a laser power of a laser beam with respect to the non-paper existing area according to the shape of the end portion.

  According to this, since the laser power of the laser beam for the non-paper existing area is changed by the laser power control means according to the end shape, that is, when the end shape is uneven, for example. As the width of the non-paper existing area changes (the width in the drum rotation direction), it becomes possible to perform exposure to the non-paper existing area and the paper existing area (non-latent image forming area) with higher accuracy. As a result, unnecessary toner development, toner contamination of the transfer roller, or paper contamination due to these can be prevented.

  Further, in the above configuration, the transfer bias applying means for applying a transfer bias to the transfer member, and a transfer bias control means for controlling a transfer bias applying operation by the transfer bias applying means, further comprising the transfer bias control means Is a current value control for controlling the value of a transfer current applied to the transfer member as a control of a transfer bias application operation by the transfer bias application unit based on the end shape information acquired by the end information acquisition unit. It is preferable to perform (Claim 7).

  According to this, a transfer bias applying means for applying a transfer bias to the transfer member and a transfer bias controlling means for controlling a transfer bias applying operation by the transfer bias applying means are further provided. Based on the edge shape information acquired by the part information acquisition means, current value control for controlling the value of the transfer current applied to the transfer member is performed as control of the transfer bias application operation by the transfer bias application means. The transfer current value can be changed in accordance with the edge shape, that is, the transfer bias application level to the transfer member can be adjusted in accordance with the paper edge shape. It is possible to prevent toner contamination of the developing and transfer rollers, or the occurrence of paper contamination due to these.

  According to the present invention, it is possible to perform exposure with different laser powers on the non-paper existence area and the paper existence area (non-latent image formation area) reflecting the edge shape of the paper. Unnecessary toner development or transfer roller due to a decrease in surface potential outside the paper in the case of a paper having a simple configuration without a charging device or a light neutralizing device and the paper edge position relative to the photoconductor not constant Therefore, it is possible to prevent the occurrence of toner contamination, or the occurrence of paper stains due to these, and as a result, a high-quality printed image can be obtained.

  FIG. 1 is a cross-sectional view schematically showing an internal configuration of an image forming apparatus according to the present invention. The image forming apparatus according to the present invention is a multifunction machine, a printer, a facsimile machine, or the like that develops an electrostatic latent image using toner by an electrophotographic method. In the present embodiment, the printer 1 will be described as an example of the image forming apparatus. In the printer 1, an image forming unit 2 is provided in the printer main body 10. The image forming unit 2 forms (prints and prints) an image on a sheet, and is a photosensitive as an image carrier made of, for example, OPC (Organic Photoconductor) configured to be rotatable in the direction of an arrow shown in FIG. The photosensitive drum 3 includes a charging unit 4, an exposure unit 5, a developing unit 6, a cleaning unit 7, and the like disposed around the photosensitive drum 3.

  The charging unit 4 charges the surface of the photosensitive drum 3 to a predetermined potential by the charging roller 41. The exposure unit 5 is a so-called laser scanner unit, which collimates a laser beam output from a laser diode based on image data transmitted from an image data storage unit 40 and the like which will be described later, and scans it with a polygon mirror. After passing through the lens group, the surface of the photosensitive drum 3 is irradiated to form an electrostatic latent image on the surface of the photosensitive drum 3. Note that the development system of the present embodiment is a so-called “reversal development” system, and the drum surface is exposed according to this reversal development system to form an electrostatic latent image.

  The developing unit 6 includes a toner cartridge 61 that is a so-called container that stores toner as a developer, and a developing unit 62 that includes a developing roller 621 and the like. Thus, by attaching the toner supplied from the toner cartridge 61, the electrostatic latent image becomes visible as a toner image. The cleaning unit 7 cleans the toner remaining on the surface of the photosensitive drum 3 by using, for example, a cleaning blade 701 (see FIG. 3) after toner transfer in the transfer unit 9 described later.

  In addition, the printer 1 includes a paper feed unit 8 that feeds paper toward the image forming unit 2 (photosensitive drum 3), a transfer unit 9 that transfers the toner image on the photoconductive drum 3 to a paper, and the image transferred to the paper. A fixing unit 11 for fixing the toner image and a paper edge detection unit 12 are provided. The paper feed unit 8 includes a paper feed cassette 81 that stores sheets of various sizes, a pickup roller 82 for taking out the stored sheets, a transport path 83 that is a path for transporting the sheets, and a sheet in the transport path 83. A conveyance roller 84 for conveying, a registration roller 841 for waiting the conveyed sheet in front of the photosensitive drum 3 (image forming unit 2), and the like are provided, and sheets fed out from the sheet feeding cassette 81 one by one. The toner is conveyed toward a nip portion between a transfer roller 91 and a photosensitive drum 3 which will be described later. The paper feeding unit 8 conveys the sheet on which the toner image has been transferred to the fixing unit 11 via the conveyance path 85, and further, the sheet fixed by the fixing unit 11 is conveyed to the printer main body by the conveyance roller 86 and the discharge roller 87. 10 is conveyed to a paper discharge tray 13 provided at the top.

  The transfer unit 9 includes a transfer roller 91, and is manifested on the photosensitive drum 3 in a state where the transfer roller 91 is pressed against the photosensitive drum 3 via a conveyed paper (paper P described later). The toner image is transferred onto a sheet. A fixing unit 11 is provided at an appropriate position downstream of the transfer unit 9 in the conveyance path 85. The fixing unit 11 fixes the toner image transferred to the paper. In the present invention, there are main characteristic points regarding the configuration (exposure method) relating to the exposure of the surface of the photosensitive drum 3 by the exposure unit 5, which will be described in detail later.

  The paper edge detection unit 12 acquires data for detecting the shape of the paper edge of the paper being conveyed, and is provided at an appropriate position on the conveyance path 83. However, the “paper edge” indicates the paper edge on both sides in the direction (width direction; paper passing width direction) orthogonal to the paper conveyance direction. Specifically, the paper edge detection unit 12 includes a plurality of line sensors (linear sensors; optical sensors) that cross the entire conveyance path 83 in the width direction, that is, a line sensor in which a plurality of light emitting elements are arranged. This is a line sensor pair composed of a line sensor in which a plurality of light receiving elements are arranged, and these are arranged opposite to each other (in parallel) across a conveyance path 83 (paper to be conveyed). Note that these line sensors can detect the edge of the sheet in a non-contact manner with respect to the sheet, and have at least a length equal to or greater than the maximum width of the sheet that can be conveyed in the conveyance path 83. ing.

  The sheet edge detection unit 12 is not limited to the above configuration, and may be composed of three or more line sensors, or may be composed of one line sensor. In the case of this single line sensor, for example, it may be a CCD line sensor in which a plurality of image sensors, for example, a CCD (Charge Coupled Device) is arranged. Further, as described above, it is possible to detect the edge of the paper by making contact (contact) with the paper instead of non-contact.

  Further, a configuration may be adopted in which a partial line sensor is provided only at the sheet edge position (a range position where a preset sheet edge is assumed to pass). The line sensor may not be a straight line sensor, but may be a line sensor having a curved shape or a combination of a curved line and a straight line. Of course, the line sensor is not necessarily used, and any line sensor can be used as long as it can detect the shape of the paper edge.

  Such a paper edge detection unit 12 acquires paper edge information along with the movement of the conveyed paper, and transmits the acquired paper edge information to an edge shape recognition unit 721 described later. .

  FIG. 2 is a block diagram illustrating an example of an electrical configuration of the printer 1. As shown in FIG. 1, the printer 1 includes a network I / F (interface) unit 30, an image data storage unit 40, an operation panel unit 50, a recording unit 60, a control unit 70, and a sensor unit 80. The network I / F unit 30 controls transmission / reception of various data to / from an information processing apparatus (external device) such as a PC connected via a network such as a LAN. The image data storage unit 40 temporarily stores image data transmitted from a PC or the like via the network I / F unit 30. However, the image data stored in the image data storage unit 40 is classified (sorted) into a development area 3022 that forms an electrostatic latent image for development and a background area 3021 that does not form an electrostatic latent image, as will be described later. Latent image classification information to be included. The operation panel unit 50 is provided on the front portion of the printer 1 or the like, and functions as an input key for inputting various operation instructions (commands) from the user, or displays predetermined information. The recording unit 60 includes the image forming unit 2, the paper feeding unit 8, the transfer unit 9, and the fixing unit 11, and records image information on a sheet based on image data stored in the image data storage unit 40 ( Printing). The sensor unit 80 is a variety of sensors for detecting the state of each part of the printer 1, and includes the line sensor of the paper edge detection unit 12.

  Incidentally, the image forming unit 2 includes a charging bias application unit 21, a laser power output unit 22, and a development bias application unit 23. The charging bias application unit 21 (Am 21 described later) is provided in the charging unit 4 and applies a predetermined charging bias (bias voltage) to the charging roller 41.

  A laser power output unit 22 (LP22 to be described later) is provided in the exposure unit 5 and has a laser beam having a predetermined laser power on the surface of the photosensitive drum 3 (for example, a laser beam output by a laser diode (not shown)). To be exposed. The laser power output unit 22 is configured to be able to output a laser beam by switching a plurality (plural types) of laser power. Here, the laser power output unit 22 changes (modulates) the output level of a certain laser beam to various levels to obtain a laser beam having the plurality of laser powers. In this case, exposure is performed by one laser beam emitted from one laser power output unit. However, the present invention is not limited to this, and, for example, a configuration in which a plurality of laser power output units capable of outputting laser beams having different constant laser powers is used to obtain the laser beams having the plurality of laser powers. In this case, exposure is performed by a plurality of laser beams emitted from a plurality of laser power output units.

  The developing bias applying unit 23 (Ad23 described later) is provided in the developing unit 6 and applies a predetermined developing bias (bias voltage) to the developing roller 621, specifically, to a developing sleeve (not shown). It is. Further, the transfer unit 9 includes a transfer bias applying unit 92 (At 92 described later). The transfer bias application unit 92 applies a predetermined transfer bias (bias voltage) to the transfer roller 91. However, this transfer bias has a polarity opposite to that of the charging bias.

  The control unit 70 reads out and executes a ROM (Read Only Memory) that stores various control programs, a RAM (Random Access Memory) that temporarily stores data or functions as a work area, and the control program. It consists of a microcomputer or the like, and transmits / receives various control signals to / from each of the above-mentioned function units to control the operation of the entire printer 1.

  The control unit 70 includes a charging output control unit 71, a laser power control unit 72, a development output control unit 73, and a transfer output control unit 74. The charging output control unit (Sm) 71 controls the output of the charging bias by the charging bias application unit (Am) 21. This Sm71 charges the surface of the photoreceptor by charge control by constant voltage control. In this case, the output electric field by the charging control is a DC electric field.

  The laser power control unit (Sp) 72 controls the laser beam output (laser power) by the laser power output unit (LP) 22. By the output control by Sp72, laser beams having a plurality of types of laser powers Lp1 to Lpn (at least Lp1 to Lp3 described later) are emitted from LP22. The development output controller 73 controls the development bias output from the development bias application unit (Ad) 23. The transfer output controller (St) 74 controls the output of the transfer bias by the transfer bias applying unit (At) 92. This St 74 performs current value control for controlling the value of the transfer current (transfer current) applied to the transfer roller 91 as control of the transfer bias application operation by the transfer bias application unit 92.

  The laser power control unit 72 includes an end shape recognition unit 721 and an output information storage unit 722. Details of the end shape recognition unit 721 and the output information storage unit 722 will be described later.

  FIG. 3 is a diagram illustrating a configuration around the photosensitive drum 3 in the printer 1. As described above, the charging unit 4, the exposure unit 5, the developing unit 6, the cleaning unit 7, and the transfer unit 9 are provided around the photosensitive drum 3, and the charging roller 41, LP22, developing roller 621, transfer roller 91, and cleaning blade 701 are arranged close to each other.

  Here, the photosensitive drum 3 uses a positive (+) charged single layer type OPC (organic photoreceptor), the drum diameter φ is about 30 mm, the rotational peripheral speed v is about 118 (mm / sec), and the image forming potential. That is, the surface potential (referred to as surface potential V0) during toner development (image formation) is about 550 (V). The charging roller 41 is formed, for example, with an epichlorohydrin rubber layer on the surface of a predetermined metal shaft, and is pressed against the photosensitive drum 3 with a predetermined load (pressing force). The effective charging width of the charging roller 41 in the roller axial direction (photoconductor axial direction) is, for example, about 225 mm.

  As a developing method using the developing roller 621 (developing unit 6), so-called magnetic one-component jumping development is adopted. The developer here is a one-component developer, and is composed of a resin, to which an external additive is applied, and whose charge polarity and charge amount (of toner particles) are controlled. During development, toner is carried on the developing roller 621 (developing sleeve) by the magnetic force of a developing magnet roller (not shown) provided inside the developing roller 621, and the toner on the developing roller 621 is transferred to the surface of the photosensitive drum 3. The latent image is developed (visualized) by adhering to the electrostatic latent image. However, the developing roller 621 is disposed with a certain gap from the photosensitive drum 3 (is held in non-contact with the photosensitive drum 3). The transfer roller 91 is made of, for example, a metallic shaft and a conductive foam, and is pressed against the photosensitive member with a predetermined load. The effective transfer width in the roller axial direction of the transfer roller 91 is, for example, about 218 mm.

  In such a configuration, a charging bias is applied to the charging roller 41 by Sm71 and Am21, and the surface of the photosensitive drum 3 is charged to a predetermined charging potential as the charging roller 41 rotates (rotating contact). In particular, in the charging unit 4, the surface of the photosensitive drum 3 is charged so as to be higher than the image forming potential (the surface potential V 0) of the photosensitive drum 3. However, charging by the charging unit 4 is referred to as primary charging, and the charging potential due to the primary charging is referred to as primary charging potential Vf. The bias application to the charging roller 41 here is performed by controlling only the DC voltage.

  Subsequently, when the photosensitive drum 3 rotates and the portion charged to the primary charging potential Vf reaches the position of the exposure point 31 by LP22, the surface of the photosensitive drum 3 is exposed by Sp72 and LP22, and the electrostatic latent image is discharged. An image is formed. Subsequently, a developing bias is applied to the developing roller 621 by Ad23, and the electrostatic latent image is visualized by toner. The potential on the developing roller 621 is a potential obtained by superimposing alternating current on direct current. Then, a transfer bias is applied to the transfer roller 91 by St73 and At92, and the toner image on the photosensitive drum 3 is transferred to the sheet P conveyed to the nip between the photosensitive drum 3 and the transfer roller 91. After this transfer, the transfer residual toner is scraped off and removed from the surface of the photosensitive drum 3 by the cleaning blade 701.

  In the above series of image forming operations, the surface potential of the photosensitive drum 3 is changed even after the electrical history (transfer history; transfer memory) due to transfer passes through the charging unit 4 due to the presence or absence of paper at the time of transfer or the difference in transfer output. It will be maintained. That is, for example, when a continuous image forming operation is performed on two or more sheets, the difference in the charged state of the surface of the photosensitive drum 3 between the inside and outside of the sheet, which is the transfer history of the first sheet, remains after the primary charging process. Maintained. However, depending on the diameter of the photosensitive drum, a problem may occur due to maintenance of the transfer history from the middle of the first sheet of paper (timing after one rotation of the photosensitive drum after the transfer of the leading edge of the sheet). Specifically, the sheet-corresponding portion on the surface of the photosensitive drum 3, that is, the portion inside the sheet width (hereinafter referred to as the sheet inside) indirectly contacts the transfer roller 91 via the sheet, while the portion outside the sheet width (hereinafter referred to as the sheet width). Since the sheet does not exist, the portion of the sheet outside is in direct contact with the transfer roller 91, and each portion receives a transfer bias to have a different surface potential level (on the drum surface on the outside of the sheet where no sheet exists). The transfer history enters the part).

  After this transfer operation, the photosensitive drum 3 is further rotated to perform the next image forming operation, and the portion in the different charged state reaches the charging unit 4. Then, with respect to these different charged states, the charging roller 41 is charged so that the potential rises to a primary charging potential Vf higher than the surface potential V0. At this time, the charging state is raised to the primary charging potential Vf while the different charging state due to the influence of the previous transfer, that is, the transfer history remains. That is, for example, as shown in FIG. 4, in detail, the electric potential of the portion corresponding to the outer side of the sheet (outer region 201) in the charging region by the charging roller 41, that is, the primary charging region on the surface of the photosensitive drum 3 corresponds to the inner side of the sheet. The surface potential is raised to the primary charging potential Vf while the potential difference 203 between the outer region 201 and the inner region 202 remains, which is lower than the potential of the portion (inner region 202). Generally, in the charging roller 41 and the transfer roller 91, since the charging bias and the transfer bias cannot be controlled in the axial direction (because the bias is applied uniformly), the potential difference 203 remains.

  In order to solve this problem, in the present invention, the surface potential of the photosensitive drum 3 in the axial direction is controlled by the exposure operation by the exposure unit 5. That is, with the LP 22 capable of outputting a plurality of laser powers, the inner region 202 and the outer region 201 on the surface of the photosensitive drum 3 are exposed with different laser powers in order to eliminate the problem caused by the potential difference 203. However, in the inner area 202, a predetermined image is formed, that is, toner development is performed. Therefore, a portion of the inner area 202 where the development is performed (development area) and a portion other than the development area (this is referred to as a background area). ) And different laser power. That is, the outer region, the background region, and the development region are exposed with different laser powers.

  By the way, there is a case where it is desired to form an image on a sheet (expressed as an arbitrary sheet) having an arbitrary shape, for example, a curved shape (a wave shape as shown in FIG. 5), instead of a regular sheet such as a general A4 or B5 size. is there. However, the optional paper is not limited to this, and the shape of the paper edge (the entire paper) is an arbitrary shape such as a circle, an ellipse, a triangle, a rectangle or a square, other rectangles or polygons, or other uneven shapes. It may be.

  In other words, when the sheet is conveyed (moved) with one rotation of the photosensitive drum 3, the length of the sheet end in the conveying direction (the length of the edge) is from the circumference of the photosensitive drum 3. Also, a long paper (arbitrary paper) may be used. Considering this, the photosensitive member is used when the paper (standard-size paper) is displaced in its direction by conveyance or the like and the leading edge or the trailing edge of the paper is not parallel to the photosensitive drum 3 (in the drum axis direction). For example, the present invention can be applied to the case where the position of the sheet end differs in one rotation of the drum 3, that is, the position of the sheet end (edge) with respect to the surface of the photoreceptor drum 3 changes as the photoreceptor drum 3 rotates. is there. These cases are collectively expressed as a sheet having a non-constant sheet edge shape.

  In the present invention, as described above, the outer region, the background region, and the development region are different from each other as described above with respect to a sheet having a non-constant sheet shape (such as an arbitrary sheet or a regular sheet having a misaligned orientation). It is configured to expose with laser power.

  Specifically, for example, the charged region on the surface of the photosensitive drum 3 is irradiated with a laser beam having a laser power as shown in FIG. That is, in the outer region 301 corresponding to the outer region 201 (for example, a region from each edge of the paper edge to the edge of the exposure range in the width (width X) indicated by the symbol X) (described later), the output level of the laser power Lp1 (described later) The laser power is lower than the laser power Lp2), and the laser is applied to the background region 3021 corresponding to the background region and the development region 3022 corresponding to the development region in the inner region 302 corresponding to the inner region 202, respectively. Exposure is performed at output levels of power Lp2 and laser power Lp3. However, the laser power Lp3 is a higher output level than the laser power Lp2.

  Further, in FIG. 5, for the convenience of explanation, the development area is indicated by one large “A” character, and the laser power for this image is one kind of laser power Lp3. For example, it is a plurality of characters and pictures smaller than A, and the laser power becomes a level different from, for example, the laser powers Lp4 to Lpn according to the arbitrary image (this embodiment will be described with one type of laser power Lp3). ).

  Further, the laser power Lp1 of the outer region 201 is “laser power for controlling the outer region potential”, the laser power Lp2 of the background region 3021 is “laser power for controlling the background region potential”, and the laser power Lp3 of the developing region 3022 is “developing region potential”. The laser power for control or the laser power for image formation ”is appropriately expressed, and the laser power for controlling the outer region potential and the laser power for controlling the background region potential are collectively expressed as“ laser power for controlling the surface potential ”as appropriate.

  Thus, by exposing the surface of the photosensitive drum 3 with different laser power for each region, a surface potential (V0) in which the potential difference 203 between the outer region 201 and the inner region 202 is eliminated is obtained as shown in FIG. be able to. That is, the laser power Lp1 is a laser power for controlling the outer region potential necessary for lowering the surface potential by the amount indicated by reference numeral 221 so as to obtain the surface potential V0, and the laser power Lp2 is designated so as to obtain the surface potential V0. The laser power for controlling the background region potential necessary for lowering the surface potential by the amount indicated by 222, and the laser power Lp3 by the amount indicated by reference numeral 223 for developing the toner (forming an electrostatic latent image). This is the laser power for controlling the development area potential necessary for lowering the surface potential. By exposing with each of these laser powers, the transfer history of the photosensitive member can be canceled simultaneously with the development (image formation). .

  However, the shape of the outer area 301 (area shape) is not constant as in the case of a standard sheet, that is, in the case where the width of the sheet is constant, but as shown in the shaded area in FIG. ) Will vary depending on the shape. Therefore, the LP 22 has the outer region potential control laser power Lp1 and the background region potential control laser power Lp2 so that the outer region 301 and the region shape of the background region 3021 corresponding to the region shape of the outer region 301 can be obtained. Each is exposed at.

  For this purpose, the edge shape of the conveyed paper is detected before exposure is performed, and the detected edge shape is fed back to the exposure, that is, the photosensitive drum so as to obtain the detected edge shape. It is necessary to irradiate the laser beam on the three surfaces (laser irradiation). In order to detect the paper edge shape before exposure, paper image data is acquired using the paper edge detection unit 12 described above, and exposure control is performed by the main control unit 70 based on the acquired data.

  Returning to FIG. 2, the edge shape recognition unit 721 recognizes (discriminates) the shape of the sheet edge based on the sheet edge information obtained by the sheet edge detection unit 12 (sensor unit 80). is there. Here, the end shape recognizing unit 721 is configured such that the light emitted from the light emitting side line sensor is blocked by the paper in the light emitting side and light receiving side line sensors arranged opposite to each other with the conveyance path 83 interposed therebetween. Based on the element data of the part not received by the line sensor and the element data of the part received without being blocked by the paper, the boundary position (boundary data) between the paper edge (edge) and the outside of the paper is detected. Thus, the shape of the edge of the sheet is recognized.

  The detection of the boundary position is not limited to this method. For example, when the CCD line sensor is used, a bright part image (a white image whose luminance is higher than a predetermined value) and a dark part image (a black image) in the paper image. It is also possible to use a method of detecting pixel data at the boundary between

  The output information storage unit 722 stores laser power output information (which may be expressed as a laser output pattern) corresponding to each region (outer region 301, background region 3021, and development region 3022) output from the LP22. is there.

  The laser power control unit 72 uses the output information of the laser power (here, Lp1 to Lp3) corresponding to each of the areas stored in the output information storage unit 72 to recognize the edge of the paper recognized by the end shape recognition unit 721. A plurality of types of laser beams having different laser powers according to the part shape are output to the LP 22. As a result, the surface potential V0 as described with reference to FIG. 4 is formed on the drum surface as the photosensitive drum 3 rotates.

  By the way, the timing from when the paper edge (for example, the paper edge on the leading edge side of the paper) is detected until the paper is conveyed to the transfer position (nip portion; contact point between the photosensitive drum 3 and the transfer roller 91). It is necessary to match the timing (referred to as the paper rotation timing) with the timing (referred to as the drum rotation timing) for rotating the photosensitive drum 3 from the laser exposure (for example, at the exposure start end) to the transfer position based on the recognized edge shape information. There is. In other words, it is possible to match the exposure pattern of the photosensitive drum 3 and the edge shape position of the conveyed paper with respect to the time until the exposure laser output is performed by reflecting the acquired edge shape information of the paper. It is necessary to meet the exposure timing.

  For this reason as well, in the present embodiment, the sheet end detection unit 12 (line sensor) is disposed at a position upstream by a predetermined distance from the position (transfer position) of the photosensitive drum 3. This predetermined distance requires at least a processing time until the detected end shape is fed back to the exposure, so that the rotational speed (circumferential speed) of the photosensitive drum 3 and the paper transport speed through the transport path 83 are the same. Is longer than the circumferential length (drum rotation distance; arc distance) of the drum surface from the exposure position (exposure point 31) to the transfer position of the photosensitive drum 3 in the rotation direction of the photosensitive drum 3. The distance. That is, if the predetermined distance is the same as the drum rotation distance, the edge shape information acquired by the sheet edge detection unit 12 is transmitted to the main control unit 70 to perform information processing (transmission speed or Even if the processing speed is increased, it is difficult to meet the above exposure timing (when the drum peripheral speed and the paper transport speed are the same), but the predetermined distance is longer than the drum rotation distance. Thus, before starting the output of the exposure laser, the edge shape information is acquired, and the sheet classification information (and latent image classification information) reflecting the edge shape information is sent to the main control unit 70 (laser power control unit 72). It is possible to easily realize a configuration capable of obtaining time for transmission (preparation (information processing)) to transmit and output with different laser power in each region based on this information.

  Further, it is preferable that the sheet edge detection unit 12 is disposed upstream from the transfer position by a distance longer than the rotation distance, and is disposed downstream of the registration roller 841. As a result, before the paper edge detection unit 12 detects the paper edge, the registration roller 841 aligns the front edge (start edge) of the paper and adjusts the conveyance timing of the paper toward the photosensitive drum 3. Since the edge of the sheet is detected with respect to the conveyed sheet, the sheet conveyance timing and the drum rotation timing can be accurately matched (the actual position of the conveyed sheet, that is, the sheet edge). Thus, it is possible to further prevent unnecessary toner development, toner contamination of the transfer roller, or paper contamination due to these.

  However, the method of adjusting the timing is not limited to the method using the registration roller, and includes, for example, a detection unit configured electrically or mechanically to detect the conveyance position of the paper (for example, the front end of the paper). Based on this, the sheet conveyance timing and the drum rotation timing may be matched. In this regard, the timing may be adjusted in consideration of the transport time from the time when the leading edge of the sheet is detected using the line sensor (sheet end detection unit 12) to the transfer position.

By the way, in the primary charging by the charging unit 4 described above before exposure, the charging unit 4 sets the primary charging potential Vf to a potential satisfying the condition shown in the following expression (A) with respect to the image forming potential V0. It is preferable to set the primary charging potential Vf so that the potential difference between the primary charging potential Vf and the image forming potential V0 is at least greater than 300 V (in this case, the potential is increased to a potential higher than the potential V0).
| Primary charging potential Vf (V) −Image forming potential V0 (V) |> 300 (V) (A)
However, the symbol || indicates an absolute value.

  This is because the larger the potential difference between the image forming potential V0 and the primary charging potential Vf, that is, when the surface potential of the photosensitive member is raised to the primary charging potential Vf and then lowered to the image forming potential V0 as in this embodiment. The larger the drop of this is, the easier it is to remove the potential difference between the outer region and the inner region by the exposure (as compared to the case where the drop is slightly small that raises the potential slightly and lowers it to the image forming potential V0). )

  Thus, after the transfer history is canceled in the exposure unit 5 and the surface potential state where the electrostatic latent image is formed is reached, the surface potential portion reaches the developing roller 621 as the photosensitive drum 3 rotates further. Then, toner development is performed on the electrostatic latent image, and the photosensitive drum 3 is further rotated and transferred by the transfer roller 91. After this transfer, the toner is cleaned, and the primary charging for the next image forming operation, that is, the potential is raised to the primary charging potential Vf.

  In FIG. 5, the potential (V0) on the inner side (background region) of the paper is set to 500 (V), for example, and the potential on the outer side (outer region) of the paper is set to the same potential of 500 (V). Laser power Lp2: about 0.065 (mW) as the laser power for controlling the surface potential for each, and a laser power Lp1: about 0.058 (mW) slightly lower than this 0.065 (mW) To do.

  In this regard, FIG. 6 is a graph showing the relationship between the surface potential of the photosensitive drum during primary charging (primary surface potential; corresponding to the primary charging potential Vf) and exposure energy. According to FIG. 6, for example, when the drum surface is raised to a primary charging potential Vf of 1000 V by primary charging, when the potential of 1000 V is reduced to a target potential (V0) of 500 V, for example, the exposure energy is approximately 0. Exposure with a laser power of 0.08 (mW) is necessary. For example, when the potential is raised to a primary charging potential Vf of 800V, the exposure energy is reduced when the potential of 800V is lowered to the potential (V0) of 500V. Has been shown to require exposure at a laser power of approximately 0.06 (mW). That is, when the primary charging potential Vf on the inner side and the outer side of the paper are different from each other with a potential difference of, for example, 800 V, 760 V, etc., in order to reduce this to the surface potential V0, 0.065 (mW), 0.058 It can be seen that different laser powers are required (mW).

  FIG. 7 is a flowchart regarding the exposure operation on the surface of the photosensitive drum according to the present embodiment. In a series of image forming operations of charging, exposing, developing and cleaning the photosensitive drum 3, first, the surface potential of the photosensitive drum 3 is raised to the primary charging potential Vf by primary charging (step S1). On the other hand, paper edge information is acquired by the paper edge detection unit 12, and the shape of the paper edge is recognized by the edge shape recognition unit 721 based on the paper edge information (step S2). Then, by the control by the laser power control unit 72, the output information of the laser power (here, Lp1 to Lp3) corresponding to each of the regions stored in the output information storage unit 72 is used, and the end shape recognition unit 721 uses the output information. A plurality of types of laser beams having laser powers Lp1 to Lp3 corresponding to the recognized sheet edge shape are output from the LP22. The surface of the photosensitive drum 3 is exposed by a laser beam corresponding to the edge shape of the paper and different for each region, and the surface potential of the development region is set to a potential at which an electrostatic latent image is formed. The surface potential with the region is equalized to the potential V0 (step S3). Next, in this surface potential state, the developing unit 6 develops the toner by attaching the toner to the electrostatic latent image (step S4). Then, transfer onto the paper P is performed by the transfer unit 9 (step S5), and residual toner on the drum surface is removed by the cleaning unit 7 (step S6). For example, when printing on the next sheet in a certain print job (NO in step S7), the operations in steps S1 to S6 are repeated, and when the print job is completed (YES in step S7), the flow ends. It becomes.

  As described above, according to the image forming apparatus (printer 1) of the present invention, the edge shape information indicating the edge shape in the sheet passing width direction is acquired by the sheet edge detection unit 12 (edge information acquisition unit). Based on the edge shape information, the edge shape recognition unit 721 (paper classification information acquisition unit) determines the inner area 302 (paper existence area) and the outer area 301 (non-paper) in the charging area on the surface of the photosensitive drum 3. Sheet classification information for classifying the existing area) is acquired. In addition, the image data storage unit 40 (latent image classification information acquisition unit) has latent image classification information for classifying the development area 3022 (latent image formation area) and the background area 3021 (non-latent image formation area) in the sheet existence area. To be acquired. The charging unit 4 (charging means) performs potential increase charging so as to increase the surface potential of the photosensitive drum 3 to a second potential (Vf) higher than the first potential (V0) at the time of development. By controlling the laser power output unit 22 (exposure laser output means) by the unit 72 (laser power control means), the non-paper existence area, the non-latent image formation area, and the latent image are determined based on the paper classification information and the latent image classification information. A laser beam having a different laser power for each of the image forming areas is output from the exposure laser output means, and the potential rising charging portion on the surface of the photosensitive member is exposed. The potential is lowered to a predetermined electrostatic latent image forming potential lower than one potential, and the potential of the non-latent image forming region in the potential increasing charging portion is reduced to the first potential. Do is, (including the case variant described below (A)) the potential of the non-paper-existing region in said potential increase charging unit to a predetermined potential above the first potential is reduced.

  Accordingly, the boundary between the non-paper existence area and the paper existence area can be set to a shape corresponding to the edge shape of the paper, that is, the non-paper existence area and the paper existence reflecting the edge shape of the paper. Since it is possible to perform exposure with different laser powers for areas (non-latent image forming areas), a simple configuration without a separate precharging device or photostatic device, In the case of paper whose end position is not constant (arbitrary paper, fixed-size paper, etc.), unnecessary toner development or toner contamination of the transfer roller 91 due to a decrease in surface potential outside the paper, or due to these Occurrence of paper stains can be prevented, and as a result, a high-quality printed image can be obtained.

  Further, when the photosensitive drum rotation distance from the exposure point 31 (exposure position) to the transfer position (nip portion) is the first distance, the first distance upstream from the transfer position in the transport path 83 (paper transport path). Since the edge information acquisition means is disposed at a position separated by a longer second distance, the time required to perform exposure laser output reflecting the acquired edge shape information of the paper is determined by the exposure pattern of the photoconductor. And the edge shape position of the conveyed paper can be easily matched with the exposure timing, that is, if the second distance is the same as the first distance, the edge portion Although it is difficult to adjust the timing even if the transmission speed or processing speed of the shape information is increased, the end shape is set before the exposure laser output starts by setting the second distance to be longer than the first distance. Information is acquired, and paper classification information (and latent image classification information) reflecting the edge shape information is input (transmitted) to the laser power control means, and output at different laser power in each region based on this information. Thus, it is possible to easily realize a configuration capable of obtaining time for preparation (processing).

  In addition, since the end information acquisition means is disposed at a position separated by a second distance upstream from the transfer position and at a position downstream of the registration roller 841, the end information obtained by the end information acquisition means Prior to the acquisition of information, the registration roller 841 aligns the leading edge of the sheet and adjusts the conveyance timing of the sheet toward the photoconductor, and the edge shape information acquisition operation is performed for the sheet conveyed with this timing adjusted. Therefore, the sheet conveyance timing and the photosensitive drum rotation timing can be accurately matched (the position of the edge of the conveyed sheet and the exposure pattern on the surface of the photosensitive member are not misaligned). Furthermore, it is possible to prevent unnecessary toner development, toner contamination of the transfer roller 91, or paper contamination due to these.

  Further, since the edge shape information is acquired by the edge information acquisition means in a non-contact manner with respect to the paper, when the edge information acquisition means acquires the edge shape information, the edge information acquisition means and the paper (this It is possible to prevent the orientation of the paper and the conveyance timing from deviating due to contact with the paper (because the paper has an arbitrary shape such as an uneven shape).

  Further, the edge information acquisition means is a row-shaped optical sensor (line sensor) in which the predetermined light emitting element and the light receiving element are arranged to face each other in the sheet passing width direction with the sheet conveyance path interposed therebetween. A configuration for acquiring end shape information in a non-contact manner can be easily realized by using a simple configuration using the line sensor. Various configurations can be added or changed without departing from the spirit of the present invention. For example, the following modifications can be taken.

  (A) In the above embodiment, as shown in FIG. 4, the surface potential of the outer region 201 is lowered to the same surface potential V0 as that of the inner region 202 (background region) by exposure (with laser power Lp1). The surface potential of the outer region 201 is not limited to the surface potential as long as the toner is not developed on the photosensitive member during development (no problem occurs in the printing operation), and is more than the surface potential V0 (development start potential) during normal development. It is only necessary that the surface potential is large and the surface potential is too high and the surface potential is smaller than the surface potential at which so-called upper fog occurs, which is an event in which the toner adheres to the photosensitive member in a state where the developing state is not normal. As a result, the effect of transfer due to potential unevenness at the time of primary charging and the presence or absence of paper (transfer history, surface potential difference between the inside and outside of the paper) is affected by the laser power for controlling the surface potential. ) Is exposed and is set to the surface potential V0, resulting in uniformization.

  (B) In the above embodiment, as shown in FIG. 5, the laser power output of the outer region 301 is constant Lp1, but not limited to this, it is a laser power output that can obtain a drum surface potential equal to or higher than the surface potential V0. Further, for example, the laser power output may be changed (different) according to the size of the width X of each position in the rotation direction of the photoconductor in the outer region 301. That is, the laser power output to the outer region 301 may be changed according to the paper edge shape. In this case, for example, exposure control may be performed so that the laser power output level is reduced at a position where the width X is small and the laser power output level is increased at a position where the width X is large.

  As a result, the outer region 301 and the inner region 302 (background region) are more accurately obtained in accordance with the width X of the outer region 301 when the end shape is uneven, for example, depending on the end shape of the paper. 3021), and further, it is possible to further prevent unnecessary toner development, toner contamination of the transfer roller 91, or paper contamination due to these. Furthermore, the transfer bias applied to the transfer roller 91 may be changed according to the size of the width X (according to the shape of the paper edge). In this case, specifically, for example, at a position where the width X is small (that is, a position where the width X of the outer region 301 on the exposure surface of the photosensitive drum 3 is in contact with the transfer roller 91 by rotation of the photosensitive drum 3). The transfer current applied to the transfer roller 91 is reduced, and at a position where the width X is large (when the portion where the width X is large is rotated to a position where it is in contact with the transfer roller 91 by rotation of the photosensitive drum 3), The transfer output controller 74 controls the transfer bias application operation of At 92 (current value control) so as to increase the transfer current applied to the transfer roller 91. As a result, the transfer current (transfer output) can be changed according to the paper edge shape, that is, the transfer bias application level to the transfer member can be adjusted according to the paper edge shape. Further, it is possible to prevent unnecessary toner development, toner contamination of the transfer roller 91, or paper contamination due to these.

Incidentally, as the width of the sheet passing through the photosensitive drum 3 (considered as a standard sheet) is larger and the electric resistance value of the transfer roller 91 is smaller, the transfer current flowing rate to the outer side of the sheet increases (the transfer current is larger than the sheet). To try to flow to the outside of the paper with a lower resistance). The surface potential is determined by the relationship between the transfer current flowing in and the area of the flowed-in portion. For example, when an A4 size paper having a paper passing width of 210 mm is passed, the area of the transfer roller 91 that directly touches the photosensitive member is reduced, and current is concentrated there to increase the charge density. The shift amount to the transfer polarity side (corresponding to the potential difference 203 shown in FIG. 4) increases. For example, when a transfer roller having an electric resistance value of 10 ⁷ and 10 ⁸ (Ω) (when 1 kVdc is applied) is used and the transfer current is set at two levels of −12 (μA) and −9 (μA), the paper width is set as a parameter. When the primary charging potential inside the paper (within Vf) is set to 800 (V), for example, the difference ΔVf (V) between this voltage and the primary charging potential outside the paper (outside Vf) (= inside Vf−outside Vf) The state of) is shown in FIG. As shown in this figure, the smaller the electric resistance value, the larger ΔVf, and the larger the sheet width, the larger ΔVf. The larger the transfer current, the larger ΔVf, and the maximum value of ΔVf is about 40 ( V), and (outside Vf) at this time is about 760 (V).

  (C) In the above-described embodiment, the case where a sheet (arbitrary sheet) with a non-constant sheet edge shape or a regular sheet with a misalignment passes through the photosensitive drum 3 has been described. The same applies to the case where the paper passes, that is, the case where the edge shape of the paper is linear and the width X is constant in the photosensitive member rotation direction.

  (D) The printer 1 is not limited to a configuration that performs monochrome printing as illustrated in FIG. 1, and may be a configuration that performs color printing (color printer).

1 is a cross-sectional view schematically showing an internal configuration of an image forming apparatus (printer) according to the present invention. FIG. 2 is a block diagram illustrating an example of an electrical configuration of the printer illustrated in FIG. 1. It is a figure which shows the structure around the photoconductive drum of the said printer. It is a schematic diagram explaining the state of the surface potential by charging and exposure of the surface of the photosensitive drum. It is a graph which shows three-dimensionally an example of multiple types of laser power (Lp1-Lp3) of the laser beam output from the laser power output part which concerns on this embodiment. It is a graph showing the relationship between the surface potential (Vf) of the photosensitive drum during primary charging and the exposure energy. 5 is a flowchart relating to an exposure operation on the surface of the photosensitive drum according to the present embodiment. FIG. 5 is a graph showing a relationship between a difference ΔVf (V) between a primary charging potential inside a sheet and a primary charging potential outside the sheet, and a sheet width, an electrical resistance value, and a transfer current that are passed through. It is a schematic diagram for explaining the influence of transfer on a conventional photoreceptor (problem in which a difference occurs in the surface potential of the photoreceptor due to transfer).

Explanation of symbols

1 Printer (image forming device)
2 Image forming section 21 Charging bias applying section 22 Laser power output section (exposure laser output means)
23 Development bias application unit 3 Photosensitive drum (photosensitive member)
4 Charging part (charging means)
41 Charging roller 5 Exposure unit 6 Development unit 62 Development unit 621 Development roller 7 Cleaning unit 841 Registration roller 9 Transfer unit 91 Transfer roller (transfer member)
92 Transfer bias applying unit (transfer bias applying means)
12 Paper edge detection unit (edge information acquisition means)
40 Image data storage unit (latent image classification information acquisition means)
70 Main control unit 71 Charging output control unit 72 Laser power control unit (laser power control means)
721 Edge shape recognition unit (paper classification information acquisition means)
722 Output information storage unit 73 Development output control unit 74 Transfer output control unit (transfer bias control means)
80 Sensor unit 201, 301 Outside area (non-paper existing area)
202, 302 Inner area (paper existence area)
3021 Background area (non-latent image forming area)
3022 Development area (latent image formation area)
203 Potential difference X width

Claims (7)

In an image forming apparatus for transferring a toner image developed on a photoconductor to a sheet passed between the photoconductor and a transfer member,
Charging means for charging the surface of the photoreceptor to a predetermined potential;
Exposure laser output means configured to be capable of outputting a laser beam for exposing the photoreceptor having a plurality of types of laser power; and
Laser power control means for controlling the laser power in the exposure laser output means;
Edge information acquisition means for acquiring edge shape information indicating an edge shape in the sheet passing width direction of the paper;
Based on the edge shape information acquired by the edge information acquisition means, sheet classification information for dividing a sheet existence area where a sheet exists in a charging area on the surface of the photoconductor and a non-sheet existence area where no sheet exists. Paper classification information acquisition means to acquire;
Acquisition of latent image classification information for acquiring latent image classification information for classifying a latent image forming area for forming the developing electrostatic latent image and a non-latent image forming area for which the electrostatic latent image is not formed in the sheet existence area. Means and
The charging means performs potential increasing charging to increase the surface potential of the photoconductor to a second potential higher than the first potential at the time of development;
The laser power control means includes
Based on the paper classification information acquired by the paper classification information acquisition means and the latent image classification information acquired by the latent image classification information acquisition means, the exposure laser output means is supplied with the non-paper existence area, non-latent image. A laser beam having a different laser power is output for each of the formation region and the latent image formation region to expose the potential rising charging portion on the surface of the photoreceptor, and the potential of the latent image forming region in the potential rising charging portion is set. And lowering the potential of the non-latent image forming area in the potential increasing charging portion to the first potential, and reducing the potential in the potential increasing charging portion to the predetermined electrostatic latent image forming potential lower than the first potential. An image forming apparatus, wherein the potential of the non-paper presence area is lowered to a predetermined potential equal to or higher than the first potential. When the drum rotation distance of the photoconductor from the exposure position of the photoconductor to the transfer position by the transfer unit is a first distance,
The end information acquisition means includes
2. The image forming apparatus according to claim 1, wherein the image forming apparatus is disposed at a position spaced apart from the transfer position by a second distance longer than the first distance in the sheet conveyance path. The end information acquisition means includes
3. The image forming apparatus according to claim 2, wherein the image forming apparatus is disposed at a position spaced apart from the transfer position by the second distance and downstream of the registration roller. The end information acquisition means includes
The image forming apparatus according to claim 1, wherein the edge shape information is acquired in a non-contact manner with respect to a sheet. The end information acquisition means includes
The image forming apparatus according to claim 4, wherein the predetermined light emitting element and the light receiving element are row-like optical sensors in which the predetermined light emitting element and the light receiving element are arranged opposite to each other in the sheet passing width direction with the sheet conveyance path interposed therebetween. The laser power control means includes
The image forming apparatus according to claim 1, wherein a laser power of a laser beam with respect to the non-paper existing area is changed according to the end shape. Transfer bias applying means for applying a transfer bias to the transfer member;
A transfer bias control means for controlling a transfer bias application operation by the transfer bias application means,
The transfer bias control means includes
Based on the end shape information acquired by the end information acquisition means,
The image formation according to claim 1, wherein current value control for controlling a value of a transfer current applied to the transfer member is performed as control of a transfer bias application operation by the transfer bias application unit. apparatus.

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