JP2015230380A - Image forming apparatus and image forming method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of improving image quality.SOLUTION: A laser printer 100 includes a photoreceptor drum 9, an electrifying device 1 which electrifies the surface of the photoreceptor drum 9, an optical scanner 2 which exposes the electrified surface on the basis of image data including a plurality of image areas each having at least one exposure pixel, to form a latent image, a developing device 3 which adheres toner to the latent image formed on the surface, to develop the latent image, a transfer device 5 which includes a transfer roller 5a to which a transfer current for generating an electrostatic force between a toner image developed on the surface and a recording paper P is supplied and transfers the toner image to the recording paper P, and a printer controller 12 which sets the exposure amounts of a plurality of exposure areas, so that the exposure amounts of the plurality of exposure areas are different from each other on the basis of the image data and controls the transfer current on the basis of the average value of the set exposure amounts of the plurality of exposure areas.

Description

  The present invention relates to an image forming apparatus and an image forming method, and more particularly, an image forming apparatus and an image forming apparatus that develops a surface of a charged image carrier after exposure and develops, and transfers an image manifested on the surface onto a transfer material. It relates to a forming method.

  2. Description of the Related Art Conventionally, there is known an image forming apparatus that exposes the surface of a charged image carrier based on image data, attaches toner to the surface, and transfers a toner image that has become apparent on the surface to a transfer material ( For example, see Patent Document 1).

  However, the image forming apparatus disclosed in Patent Document 1 has room for improvement in image quality.

  The present invention relates to an image carrier, a charging unit that charges the surface of the image carrier, and an exposure unit that forms an electrostatic latent image by exposing the charged surface based on image data including a plurality of exposure pixels. A developing means for developing toner by attaching toner to the electrostatic latent image formed on the surface, and a transfer current for generating an electrostatic force between the toner attached to the electrostatic latent image and the transfer material And the transfer means for transferring the toner image to the transfer material, and the exposure of the plurality of exposure pixels so that the exposure amounts of at least two exposure pixels of the plurality of exposure pixels are different from each other based on the image data. An image forming apparatus comprising: a control unit configured to control the transfer current based on an average value of exposure amounts of the plurality of exposure pixels.

  According to this, image quality can be improved.

1 is a diagram illustrating a schematic configuration of a laser printer according to an embodiment. It is a figure for demonstrating the optical scanning device in FIG. It is a figure for demonstrating a printer control apparatus and a scanning control apparatus. FIG. 4A is a diagram for explaining the image processing unit, and FIG. 4B is a diagram for explaining the image processing unit. It is a figure for demonstrating a transfer roller. It is a graph which shows the target surface potential of the image area | region of the photoreceptor drum surface corresponding to each image part of image data. 4 is a flowchart for explaining control by a printer control device. It is a figure for demonstrating the method of setting a transfer electric current. It is a figure for demonstrating the method to set the transfer current in a comparative example. It is a figure for demonstrating the example (modification 1) which correct | amends an exposure amount setting value based on the measured value of a target surface potential and a surface potential. It is a figure for demonstrating the example (modification 2) which sets the environmental zone used as the parameter | index of whether to control the transfer current. It is a figure for demonstrating the example (modification 3) which controls a transfer current based on the width | variety of a transfer material.

  Hereinafter, an embodiment of the present invention will be described with reference to FIGS. FIG. 1 shows a schematic configuration of a laser printer 100 according to an embodiment.

  The laser printer 100 includes an optical scanning device 2, a photosensitive drum 9, a charging device 1, a developing device 3, a transfer device 5, a cleaning device 6, a static elimination device 7, a registration roller 8, a paper feed tray, and a paper feed roller. A paper device (not shown), a fixing device (not shown), a paper discharge device (not shown) including a paper discharge roller and a paper discharge tray, a scanner 10 (see FIG. 3) as a document reading device, a printer control device 12, and the like I have.

  The photosensitive drum 9 is a cylindrical member, and a photosensitive layer is formed on the surface thereof. That is, the surface of the photosensitive drum 9 is a surface to be scanned. The photosensitive drum 9 is rotated in the direction of the arrow in FIG.

  The charging device 1, the developing device 3, the transfer device 5, the cleaning device 6, and the charge removal device 7 are each disposed near the surface of the photosensitive drum 9. The charging device 1, the developing device 3, the transfer device 5, the cleaning device 6, and the charge removal device 7 are arranged in this order along the rotation direction of the photosensitive drum 9.

  The charging device 1 uniformly charges the surface of the photosensitive drum 9.

  The optical scanning device 2 scans the surface of the photosensitive drum 9 charged by the charging device 1 with a laser beam modulated on the basis of image data (image information) from a host device such as a personal computer, for example. An electrostatic latent image corresponding to the image data is formed on the surface 9. The electrostatic latent image formed here moves in the direction of the developing device 3 as the photosensitive drum 9 rotates. The configuration of the optical scanning device 2 will be described later.

  The developing device 3 causes a toner to adhere to the electrostatic latent image formed on the surface of the photosensitive drum 9 to make it visible. Here, the electrostatic latent image to which the toner is attached (hereinafter also referred to as “toner image” for convenience) moves in the direction of the transfer device 5 as the photosensitive drum 9 rotates.

  A recording paper P as a transfer material is stored in the paper feed tray. A paper feed roller is disposed in the vicinity of the paper feed tray. The paper feed roller takes out the recording paper P one by one from the paper feed tray and conveys it to the registration roller 8. The registration roller 8 temporarily holds the recording paper P taken out by the paper feed roller, and the recording paper P is moved between the photosensitive drum 9 and the transfer roller 5 a of the transfer device 5 in accordance with the rotation of the photosensitive drum 9. Send to the gap.

  In order to electrically attract the toner on the surface of the photosensitive drum 9 to the recording paper P, the transfer roller 5a is given a potential having a polarity opposite to that of the toner. Therefore, an electrostatic force is generated between the toner adhering to the electrostatic latent image and the recording paper P, and the toner image is transferred to the recording paper P. The recording sheet P transferred here is sent to a fixing device. The transfer device 5 including the transfer roller 5a will be described in detail later.

  In the fixing device, heat and pressure are applied to the recording paper P, whereby the toner is fixed on the recording paper P. The recording paper P fixed here is sent to a paper discharge tray via a paper discharge roller, and is sequentially stacked on the paper discharge tray.

  The cleaning device 6 removes toner (residual toner) remaining on the surface of the photosensitive drum 9.

  The neutralization device 7 neutralizes the surface of the photosensitive drum 9. The surface of the photosensitive drum 9 whose surface has been neutralized returns to the position facing the charging device 1 again.

  Next, the configuration of the optical scanning device 2 will be described. As shown in FIG. 2 as an example, the optical scanning device 2 includes a light source 19, a polygon mirror 18, a scanning lens 17, a PD 16 (photo detector) as a light receiving element, a scanning control device 15, and the like.

  In the following, for convenience, the direction corresponding to the main scanning direction is abbreviated as “main scanning corresponding direction”, and the direction corresponding to the sub scanning direction is abbreviated as “sub scanning corresponding direction”.

  The light source 19 includes at least one light emitting unit, and emits laser light toward the deflection reflection surface of the polygon mirror 18.

  As the light source 19, a single LD (Laser Diode), an LDA (Laser Diode Array) including a plurality of LDs arranged in one dimension or two dimensions, or a single VCSEL (Vertical Cavity Surface Emitting) may be used. Laser) or VCSELA (surface emitting laser array) including a plurality of VCSELs (surface emitting lasers) arranged one-dimensionally or two-dimensionally may be used.

  As an example, the polygon mirror 18 has a hexahedral mirror with an inscribed circle radius of 18 mm, and each mirror serves as a deflection reflection surface. The polygon mirror 18 deflects the laser light from the light source 19 while rotating at a constant speed around an axis parallel to the sub-scanning corresponding direction.

  An optical system (also referred to as a pre-deflector optical system) that forms an image of laser light emitted from the light source 19 between the light source 19 and the polygon mirror 18 in the vicinity of the deflection reflection surface of the polygon mirror 18 in the sub-scanning corresponding direction. May be provided. Examples of the optical element constituting the pre-deflector optical system include a coupling lens, an aperture member, a cylindrical lens, and a reflection mirror.

  The scanning lens 17 is disposed on the optical path of the laser light deflected by the polygon mirror 18. Then, the laser light passing through the scanning lens 17 is irradiated (condensed) on the surface of the photosensitive drum 9 to form a light spot. This light spot moves in the longitudinal direction of the photosensitive drum 9 as the polygon mirror 18 rotates. That is, the photosensitive drum 9 is scanned. The moving direction of the light spot at this time is the “main scanning direction”. The rotation direction of the photosensitive drum 9 is the “sub-scanning direction”.

  The optical system disposed on the optical path between the polygon mirror 18 and the photosensitive drum 9 is also called a scanning optical system. In the present embodiment, the scanning optical system is composed of a scanning lens 17. The scanning optical system may have a plurality of scanning lenses. Further, at least one folding mirror may be arranged on at least one of the optical path between the scanning lens 17 and the photosensitive drum 9.

  The PD 16 is arranged on the optical path of the laser beam that is deflected by the polygon mirror 18 and passes through the scanning lens 17. The PD 16 may be disposed on the downstream side in the scanning direction with respect to the photosensitive drum 9 or may be disposed on the upstream side in the scanning direction.

  Therefore, the laser light from the light source 19 is deflected by the rotating polygon mirror 18 and is irradiated onto the photosensitive drum 9 which is a scanned medium via the scanning lens 17. The irradiated laser beam becomes a light spot on the photosensitive drum 9, and an electrostatic latent image is formed on the photosensitive drum 9.

  Further, the laser beam deflected by the polygon mirror 18 is incident on the PD 16 after one line scanning is completed or before one line scanning is started. When the PD 16 receives the laser beam, the PD 16 converts the received light amount into an electrical signal, and outputs the electrical signal to the scanning control device 15 as a synchronization signal.

  Here, the printer control device 12 will be described.

  As shown in FIG. 3, the printer control device 12 controls the above-described components of the laser printer 100 as a whole, and controls bidirectional communication with a host device (for example, a personal computer) via a network or the like. (Not shown), an image processing unit 12a, an exposure amount setting unit 12b, and a transfer current control unit 12c (see FIG. 3).

  As shown in FIG. 4A, the image processing unit 12a includes an image processing unit (IPU: Image Processing Unit), a controller unit, a memory unit, and the like.

  As shown in FIG. 4B, the image processing unit includes a density conversion unit, a filter unit, a color correction unit, a selector unit, a gradation correction unit, a gradation processing unit, and a unit control unit that comprehensively controls each unit. (Not shown).

  The density conversion unit converts RGB image data from the scanner 10 or the personal computer into density data using a lookup table, and outputs the density data to the filter unit.

  The filter unit performs image correction processing such as smoothing processing and edge enhancement processing on the density data input from the density conversion unit, and outputs the result to the color correction unit.

  The color correction unit performs color correction (masking) processing on the image-corrected density data input from the filter unit, and outputs the result to the selector unit.

  Under the control of the unit control unit, the selector unit selects any one of C, M, Y, and K for the color-corrected density data input from the color correction unit, and outputs it to the gradation correction unit To do.

  The gradation correction unit sets a γ curve for obtaining linear characteristics for the C, M, Y, and K density data input from the selector unit.

  The gradation processing unit performs gradation processing such as a tethering process on the density data set with the γ curve input from the gradation correction unit.

  The image processing unit outputs image data before image processing or image data after image processing (density data) to the controller unit as necessary.

  The controller unit performs processing such as rotation, repeat, aggregation, compression and decompression on the image data from the image processing unit, and then outputs the processed data to the image processing unit.

  In the memory unit, various data such as the lookup table are stored in advance.

  In the image processing unit 12a, image data that has undergone a series of processes as described above, tag data that identifies object information, and the like are output to the exposure amount setting unit 12b.

  The exposure amount setting unit 12b sets the exposure amount for each exposure pixel of the image data, outputs the image data after setting the exposure amount, tag data, and the like to the scanning control device 15, and sets the exposure amount for each exposure pixel. The value is output to the transfer current control unit 12c (see FIG. 3). The exposure amount setting unit 12b and the transfer current control unit 12c will be described in detail later.

  The optical scanning device 2 including the scanning control device 15 scans the surface of the photosensitive drum 9 based on the image data, the tag data, and the like after the exposure amount setting from the exposure amount setting unit 12b. An electrostatic latent image is formed on the surface.

  As will be described in detail below, the scanning control device 15 inputs image data, tag data, and the like from the exposure amount setting unit 12b as necessary, generates drive information for the light source 19, and uses the drive information. The light source 19 is driven.

  As shown in FIG. 3, the scanning control device 15 includes a reference clock generation circuit 402, a pixel clock generation circuit 405, a light source modulation data generation circuit 407, a light source selection circuit 414, a write timing signal generation circuit 415, and a light source drive circuit 400. It has. The arrows indicate the flow of representative signals and information, and do not represent the entire connection relationship of each block.

  The reference clock generation circuit 402 generates a high frequency clock signal that serves as a reference for the entire scanning control device 15.

  The pixel clock generation circuit 405 mainly includes a PLL circuit, and generates a pixel clock signal based on the synchronization signal s1 and the high frequency clock signal from the reference clock generation circuit 402. The pixel clock signal has the same frequency as the high-frequency clock signal and the phase matches the synchronization signal s1. Therefore, by synchronizing the image data with the pixel clock signal, it is possible to align the writing position for each scan. The pixel clock signal generated here is supplied to the light source drive circuit 400 as one of the drive information and also supplied to the light source modulation data generation circuit 407 as a clock signal of the write data s16 as one of the drive information. Used.

  The light source modulation data generation circuit 407 converts the image data into a PM + PWM signal so that an optimum latent image is formed based on the image data and tag data from the exposure setting unit 12b.

  The light source selection circuit 414 is a circuit used when the light source 19 includes a plurality of light emitting units. When the image surface of the scanning light reaches the scanning end, the light source selecting circuit 414 includes a plurality of light emitting units used to detect the start of the next scanning. The light emitting unit is selected from (for example, 32) light emitting units, and a signal designating the selected light emitting unit is output. The output signal s14 of the light source selection circuit 414 is supplied to the light source drive circuit 400 as one of the drive information. When a single light emitting unit is used as the light source 19, the light source selection circuit 414 is not necessarily provided.

  The write timing signal generation circuit 415 obtains the write start timing based on the synchronization signal s1, and outputs the output signal s15, which is the timing signal, to the light source drive circuit 400 as one of the drive information.

  The light source driving circuit 400 generates a driving current (for example, a pulse current) of the light emitting unit of the light source 19 based on the driving information and supplies the driving current to the light emitting unit.

  Incidentally, examples of the transfer device include a corona transfer device using corona discharge, and a contact transfer type transfer device including a transfer belt and a transfer roller.

  Since the corona transfer device is a transfer device with small fluctuations in electrical resistance due to the environment, the transfer electric field is stabilized by adopting a constant voltage application method. As described above, the corona transfer device has an advantage that it is hardly influenced by the environment and the image ratio when the constant voltage application method is adopted.

  However, since it is necessary to apply a high voltage of several kV to 10 kV in the corona transfer device, a large amount of ozone is generated along with the corona discharge, affecting the human body and reacting with air to generate nitrogen oxides. There is a problem of generating and adversely affecting image formation. That is, when the corona transfer device is used, the airflow design and the ozone removal design are serious problems.

  On the other hand, a transfer device of a contact type transfer system including a transfer roller and a transfer belt allows a transfer material to pass through a nip formed by pressing a conductive rubber roller or a conductive rubber belt to which a voltage is applied to a photosensitive drum, At that time, transfer is performed by generating an electric field in a very fine gap between the photosensitive drum and the conductive rubber roller or the conductive rubber belt. In the case of such a contact transfer method, even if a constant voltage application method is adopted, transfer can be performed with a transfer voltage of about 2 kV or less, and there is no problem of ozone generation.

  However, as a characteristic of conductive rubber, its volume resistivity varies greatly depending on the environment. That is, the volume resistivity of the conductive rubber is 1 to 2 orders of magnitude higher in a low temperature and low humidity environment (for example, 10 ° C., 15%) than in a normal temperature and normal humidity environment (for example, 23 ° C., 65%). Below (for example, 30 ° C. and 90%), it is 1 to 2 orders of magnitude lower than that in the normal temperature and humidity environment.

  For this reason, an appropriate transfer voltage varies depending on the environment, and a higher voltage is required in a low temperature and low humidity environment. In addition, resistance variation with the passage of use time due to repeated conduction also occurs. If measures are taken to suppress resistance fluctuations over time and the environment, the constant voltage application method does not depend on the environment or the image ratio. However, in this case, there is a risk of increasing the cost and size.

  On the other hand, the constant current application method is a method of applying a constant charge to the transfer material by applying a constant current to the conductive rubber, regardless of the volume resistivity of the conductive rubber.

  In this constant current application method, the high potential portion on the photosensitive drum is a non-image region, and when a device that adheres toner to the low potential portion (image region) by reversal development is used, the photosensitive drum, the conductive rubber, A large amount of current flows in the high potential portion (non-image region) where the potential difference is large, and the current flowing in the low potential portion (image region) decreases. Therefore, image data having a low image ratio (high ratio of the high potential portion) requires more current to obtain an optimum image than image data having a high image ratio (high ratio of the low potential portion). To do.

  In consideration of the above points, the present embodiment employs a contact-type transfer method and a constant current application method in order to perform optimum transfer for any image.

  Specifically, in the present embodiment, the transfer device 5 including the transfer roller 5a that is brought into pressure contact with the surface of the photoconductor drum 9 and rotates with the photoconductor drum 9 is employed, as described in detail below. The constant transfer current applied to the transfer roller 5a is controlled to an appropriate value.

  As shown in FIG. 1, the transfer device 5 includes, in addition to the transfer roller 5a, a guide portion 5b that conveys the recording paper P after image transfer to the fixing device, and a retracting mechanism for facilitating the removal of jammed paper. The transfer roller 5a and the guide portion 5b are retracted in the direction of arrow A with the support shaft 11 of the retracting mechanism as a fulcrum.

  As shown in FIG. 5, the transfer roller 5a has a configuration in which a conductive rubber 14 is wound around the entire circumference of a metal core 13, and as shown in FIG. 1, the transfer roller 5a. A transfer current is applied to the metal core 13 by a power source 5c. The transfer roller 5a is pressed against the photosensitive drum 9 by a pressure spring 5d.

  In the present embodiment, as described above, the constant current application method is employed in order to avoid the influence due to the fluctuation of the volume resistivity of the conductive rubber of the transfer roller 5a. On the other hand, the transfer residual toner left on the photosensitive drum 9 after the transfer is scraped off by the cleaning device 6 and accommodated in the cleaning device 6. Further, after the cleaning, the photosensitive drum 9 is neutralized by the neutralizing device 7 to remove residual charges.

  Here, in the laser printer 100, when image formation is performed, first, the surface of the photosensitive drum 9 is uniformly charged to, for example, negative polarity by the charging device 1. In this state, an electrostatic latent image corresponding to image data (image information) including exposed pixels and non-exposed pixels is formed on the surface of the photosensitive drum 9 by the optical scanning device 2. In this case, on the surface of the photosensitive drum 9, the region exposed with the laser light has a lower potential, and the region not exposed with the laser light remains at a high potential.

  The electrostatic latent image formed on the surface of the photosensitive drum 9 by the optical scanning device 2 is developed with the toner from the developing device 3 to be visualized. Here, the developing device 3 is a developing device that performs so-called reversal development, and attaches toner to the low potential portion of the surface of the photosensitive drum 9 and does not attach toner to the high potential portion.

  The toner image (image) formed on the surface of the photosensitive drum 9 is transferred to a transfer material by the transfer device 5, and the transfer material onto which the toner image has been transferred is separated from the photosensitive drum 9 and is fixed to a fixing device (not shown). ).

  Here, it is desirable to set the exposure amount of each exposure pixel based on the image data in order to improve the reproducibility of the image, particularly in image formation that requires high resolution. In this case, the potentials of the portions corresponding to a plurality of exposure pixels having different exposure amount setting values on the surface of the photosensitive drum exposed after charging are different from each other. As a result, a difference in potential difference occurs between the surface of the photosensitive drum and the surface of the transfer roller, and as a result, a variation occurs in the electrostatic force generated between the toner image manifested on the surface of the photosensitive drum and the transfer material. .

  The exposure amount of each exposure pixel means a time integral value of the exposure intensity for the exposure pixel (a product of the exposure intensity and the exposure time when the exposure intensity is constant), and is also referred to as an integrated light amount of the exposure pixel. The exposure intensity can be controlled by adjusting the current value of the drive current to the light source (the amplitude of the pulse current). Note that the exposure intensity can also be obtained by conversion from the PM + PWM output signal of the scanning control device 15. The exposure time can be controlled by adjusting the lighting time of the light source (pulse width of the pulse current).

  In general, in an electrophotographic image forming apparatus, when an image developed on a photosensitive drum is transferred to a transfer material, a transfer current that optimizes the transfer rate of the image is generated on the photosensitive drum. It differs depending on the ratio (image ratio) of the high potential portion (non-image portion) and the low potential portion (image portion) in the longitudinal direction. Therefore, it is conceivable to control the transfer current based on the image area ratio ((area of image portion / (area of image portion + area of non-image portion))).

  However, when setting the exposure amount of each exposure pixel based on the image data, even if the transfer current is controlled based only on the image area ratio, the potential difference between the surface of the photosensitive drum and the surface of the transfer roller Variations cannot be reduced. In this case, variation in the transfer rate of toner from the photosensitive drum to the transfer material increases, and the toner image formed on the surface of the photosensitive drum based on the image data after setting the exposure amount is used as the transfer material. The image cannot be transferred with high accuracy, and the reproducibility of the image is lowered.

  Therefore, in the present embodiment, first, in order to form an image with high resolution and good reproducibility, the exposure amount of each exposure pixel is set based on the image data after image processing. This setting is performed by the exposure amount setting unit 12b described above. For example, the exposure amount setting unit 12b sets the exposure amount of the exposure pixel at the edge portion including at least one exposure pixel in the image data to be larger than the exposure amount of the other exposure pixels.

  In this case, as an example, as shown in FIG. 6, a plurality (three in FIG. 6) of low potential portions (image portions) having different potentials are generated on the surface of the photosensitive drum 9.

  Therefore, in the present embodiment, an average value of the exposure amounts of all the exposure pixels of the image data is calculated, and the transfer current is controlled based on the calculated average value. The transfer current is set by the transfer current control unit 12c described above.

  Hereinafter, an image forming method using the laser printer 100 will be described with reference to FIG. The flowchart of FIG. 7 is based on a series of processing algorithms executed by the printer control device 12.

  In the first step S1, for example, it is determined whether or not a recording start is instructed by the user. This instruction is performed, for example, by inputting an original print command on the operation unit of the laser printer 100 or by clicking a print command displayed on the display of a personal computer connected to the laser printer 100. If the determination in step S1 is affirmed, the process proceeds to step S2. On the other hand, if the determination in step S1 is negative, the same determination is made again.

  In step S2, image data from the scanner 10 or a personal computer is acquired.

  In the next step S3, the image processing unit 12a performs image processing on the acquired image data.

  In the next step S4, the exposure amount of each exposure pixel is set based on the image data after image processing. The case shown in FIG. 6 will be described as an example. The product of the exposure intensity LP1 for the image portion 1 of the image data and the exposure time t1 is set as the exposure amount EP1 for the image portion 1, and the exposure intensity for the image portion 2 of the image data. The product of LP2 and the exposure time t2 is set as the exposure amount EP2 for the image portion 2, the product of the exposure intensity LP3 for the image portion 3 of the image data and the exposure time t3 is set as the exposure amount EP3 for the image portion 3, and non- The exposure amount for the image portion (white portion) is set to zero. Each image portion includes at least one exposure pixel.

  In the next step S5, the exposure amount setting value (the exposure amount setting value by the exposure amount setting unit 12b) of each exposure pixel is acquired. In the case shown in FIG. 6, the exposure amount setting value for the image portion 1 is EP1, the exposure amount setting value for the image portion 2 is EP2, the exposure amount setting value for the image portion 3 is EP3, and for the non-image portion (white portion). The exposure amount setting value is zero.

  In the next step S6, an average value of exposure amount setting values of all exposure pixels is calculated. The average value of the exposure amount setting value is exposure amount of all exposure pixels / total exposure area (total area of all exposure pixels). Taking the case shown in FIG. 6 as an example, the average value EPave of the exposure dose is (EP1 + EP2 + EP3) / (Q1 + Q2 + Q3). Note that Q1 is the area of the image part 1, Q2 is the area of the image part 2, and Q3 is the area of the image part 3.

  In the next step S7, the transfer current is set based on the average value of the exposure setting values. The case shown in FIG. 6 will be described as an example. The exposure potential of the image portion 1 corresponding to the exposure amount EP1 (surface potential after exposure: target surface potential) is VL1, and the exposure potential of the image portion 2 corresponding to the exposure amount EP2. VL2 and when the exposure potential of the image portion 3 corresponding to the exposure amount EP3 is VL3, a current such that the surface potential of the transfer roller 5a becomes an average value VLave = (VL1 + VL2 + VL3) / 3 of VL1, VL2, and VL3. Then, the current is set to the transfer current It (see FIG. 8). In FIG. 6, Vc is a white portion (non-image portion) potential. Here, the exposure potential of each image portion depends on the exposure amount of the image portion, and the average value VLave of the exposure potential may be considered to have a linear relationship with the average value EPave of the exposure amount. In this case, VLave can be said to be a value based on EPave.

  In the next step S8, a paper feeding process by the paper feeding device, a charging process by the charging device 1, an exposure process by the optical scanning device 2, a development process by the developing device 3, a transfer process by the transfer device 5, a cleaning process by the cleaning device 6, A recording control process including a discharging process by the discharging device 7, a fixing process by the fixing device, a discharging process by the discharging device, and the like is performed.

  At this time, in the exposure step, a portion of the surface of the charged photosensitive drum 9 corresponding to the exposure pixel is exposed with an exposure amount set for each exposure pixel of the image data, and an electrostatic latent image is formed. In the developing step, toner adheres to the electrostatic latent image formed on the photosensitive drum 9, and the toner image becomes obvious. Furthermore, when the recording paper P passes through the transfer nip formed by the photosensitive drum 9 and the transfer roller 5a in the above transfer process, a transfer current It is applied to the transfer roller 5a, and The toner image is transferred onto the recording paper P. In this case, in FIG. 6, the potential difference between each image portion and the surface potential (VLave) of the transfer roller 5a can be set to a predetermined value (in FIG. 8, | VLave−VL1 | = | VLave−VL3 |) or less. The toner image in the image area can be transferred at a transfer rate that approximates the optimal transfer rate according to the exposure amount setting value (exposure potential). That is, the deviation amount of the potential difference between each image portion and the surface of the transfer roller 5a from a suitable value corresponding to the optimum transfer rate can be reduced, and the toner image can be properly transferred.

  In the transfer process, a control signal S corresponding to the transfer current It is output from the transfer current control unit 12c to the power source 5c of the transfer device 5, and the transfer current It is applied to the transfer roller 5a from the power source 5c.

  When step S8 is executed, the flow ends.

  On the other hand, in the comparative example shown in FIG. 9, the exposure amount of each exposure pixel is set based on the image data, and the transfer current is set based only on the image area ratio.

  More specifically, in the comparative example, transfer is performed so that the surface potential of the transfer roller 5a becomes a potential VL1 ′ that is closest to the exposure potential VL1 corresponding to EP1 that is the maximum exposure amount among the exposure amounts of the respective image portions. A current It ′ is set. That is, the transfer current It ′ is set based on the exposure amount EP1 for the image portion 1. In this case, | VL2-VL1 '| and | VL3-VL1' | become very large with respect to | VL1-VL1 '|. That is, the variation in the voltage difference between each image portion and the surface potential of the transfer roller 5a increases, and in particular, the transferability of the toner images in the image portion 2 and the image portion 3 deteriorates. Even if the transfer current is set based on the exposure amount of the image portion 2 or 3 instead of the exposure amount of the image portion 1, it is the same as when the transfer current is set based on the exposure amount of the image portion 1. In addition, the transferability of image portions other than the image portion is still deteriorated.

  The laser printer 100 of the present embodiment described above includes a photosensitive drum 9, a charging device 1 for charging the surface of the photosensitive drum 9, and a plurality of images each having at least one exposure pixel on the charged surface. An optical scanning device 2 that forms a latent image by exposure based on image data including a region, a developing device 3 that develops the toner by attaching the toner to the latent image formed on the surface, and a toner that is exposed on the surface A transfer device 5 including a transfer roller 5a to which a transfer current for generating an electrostatic force is supplied between the image and the recording paper P, and transferring the toner image onto the recording paper P; A printer controller 12 that sets the exposure amounts of the plurality of exposure regions so that the exposure amounts of the exposure regions are different from each other, and controls the transfer current based on an average value of the exposure amounts of the set plurality of exposure regions; It is provided.

  In the image forming method using the laser printer 100 of the present embodiment, the surface of the photosensitive drum 9 is charged, and the charged surface is exposed based on image data including a plurality of exposure pixels to form a latent image. Then, toner is attached to the latent image formed on the surface, and a transfer current for generating an electrostatic force between the toner attached to the latent image and the transfer material is supplied to the transfer roller 5a to the transfer material. An image forming method for transferring a toner image, the step of setting exposure amounts of a plurality of exposure pixels so that exposure amounts of at least two exposure pixels among the plurality of exposure pixels are different from each other based on image data; And a step of controlling the transfer current based on the average value of the exposure amount of the exposed pixels.

  In the laser printer 100 and the image forming method of this embodiment, the surface potential of the transfer roller 5a is set to the average value of the potentials of a plurality of image portions (portions corresponding to a plurality of image regions of image data) on the surface of the photosensitive drum 9. The transfer current can be set so as to approximate the difference between the surface potential of the transfer roller 5a and the potential difference between the plurality of image portions on the surface of the photosensitive drum 9, and the toner images in the plurality of image portions can be reduced. The transfer property to the transfer material can be improved.

  As a result, the image quality can be improved.

  Incidentally, the surface potential of the photosensitive drum after exposure varies depending on the potential characteristic (sensitivity) of the photosensitive member.

  In the first modification, therefore, a surface potential sensor (not shown) is provided at a position on the surface of the photosensitive drum 9 that faces the region between the scanning position (exposure position) by the optical scanning device 2 and the developing position by the developing device 3. It is installed. Then, the potential of the surface of the photosensitive drum 9 on which the electrostatic latent image is formed by the optical scanning device 2 is measured by the surface potential sensor, and the exposure amount (EP) -surface potential (VL) characteristic as shown in FIG. (Hereinafter also referred to as EP / VL characteristics). Therefore, the set value is corrected by weighting the set value of the exposure amount on the basis of the EP / VL characteristics to set the transfer current based on the corrected exposure amount. The “surface potential deviation amount” means the deviation amount of the surface potential (measured value) measured by the surface potential sensor from the target surface potential.

  For example, when the surface of the photosensitive drum 9 is exposed at a set value EPa = α × EP (EP is a reference exposure amount, for example, a default exposure amount) corresponding to the target surface potential Vt, When the measured value is Va (≠ Vt), the set value EPa is weighted with | Vt−Va | and corrected to EPa ′ = β × EP (β ≠ α). That is, the exposure amount is corrected from EPa to EPa ′ so that the measured value by the surface potential sensor becomes the target surface potential Vt. Β is a value that depends on α and | Vt−Va |.

  The exposure amount correction described above is preferably performed at a preparatory operation before the operation of the laser printer 100, when the power is turned on, every predetermined time (periodically) or every predetermined number of printed sheets, and a more appropriate transfer current is set. .

  For example, the exposure amount set for each exposure pixel of the predetermined image data is corrected every predetermined time t or every predetermined number of printed sheets n, and the transfer current is set based on the average value of the corrected exposure amount. It is preferable. In this case, it goes without saying that the smaller the t and n, the finer the control and the higher quality image can be obtained.

  It is preferable to correct the exposure amount setting value based on the exposure amount-surface potential characteristics in the modified example 1 described above between step S4 and step S5 in the flowchart of FIG. In this case, the reproducibility of the image can be further improved.

  By the way, the sensitivity (potential characteristics) of the photosensitive drum varies depending on the surrounding environment (temperature and humidity). Therefore, in the second modification, even when the surrounding environment changes greatly, as in the first modification, the exposure amount-surface potential characteristic (EP / VL characteristic) is changed from the exposure amount to the set value of the exposure amount by the variation in the surface potential due to the environmental change. Weighting is performed to control the transfer current more appropriately.

  Specifically, a temperature / humidity sensor is provided in the laser printer 100, and the temperature and humidity measured by the temperature / humidity sensor are transferred to the control unit of the printer control device 12, and an exposure amount-surface potential characteristic measurement operation is performed. Instead of the temperature / humidity sensor, a temperature sensor and a humidity sensor may be provided.

  In Modification 2, three types of environmental zones L / L, N / N, and H / H as shown in FIG. 11 are set, and the combination of temperature and humidity measured values by the temperature and humidity sensor is one. When the process shifts to another environmental zone, the exposure amount-surface potential measurement operation described in the first modification and the exposure amount set value based on the exposure amount-surface potential characteristics are corrected. In particular, when the deviation from the environment in which the normal measurement operation is performed becomes large without setting the environment zone, the exposure amount setting value is based on the exposure amount-surface potential measurement operation and the exposure amount-surface potential characteristics. It is good also as correcting.

  The setting of the environment zone is not limited to three types, and may be two types or four or more types.

  By the way, when the constant current application method is adopted, there arises a problem that the transferability changes depending on the width of the transfer material. As shown in FIG. 12, when a transfer material (for example, recording paper P) whose width is narrower than the width of the transfer roller 5a is used, only the region (transfer material existence region) where the transfer material exists on the photosensitive drum 9 is used. In addition, a part of the transfer current also flows in a region where there is no transfer material on the photosensitive drum 9 (a region where there is no transfer material). This is because the transfer material absence region, which has a relatively low resistance value, allows current to flow more easily than the transfer material presence region because the transfer material has a high resistance value.

  Therefore, in the third modification, the transfer (Ip + 2 × Ie) obtained by adding the current 2 × Ie flowing in the transfer material non-existing region to the optimum transfer current Ip when it is assumed that the current flows only in the transfer material existing region is transferred. A current Iq (hereinafter also referred to as a correction value Iq) is set, and the transfer current Iq is applied to the transfer roller 5a. As a result, the surface potential of the transfer roller 5a can be set to a potential corresponding to the optimum transfer current Ip, and image reproducibility can be improved.

  Specifically, for each of a plurality of transfer materials having different sizes (widths), the relationship between the width of the transfer material absence region and the current flowing in the transfer material absence region is acquired in advance, and the size of the transfer material used The width of the transfer material absence area corresponding to is detected, and the correction value Iq is obtained. For example, the current Ie is set by weighting the current Ic flowing through the white portion (non-image portion) of the potential Vc in FIG. 6 so as to be γ × Ic.

  Note that the width of the transfer material absence region can be known from the size of the transfer material used in advance by the control unit of the printer control device 12.

  Further, since the transfer material has a resistance value that varies depending on the thickness and material, the transfer material information may be reflected when obtaining Ip and Iq.

  Further, for example, when a manual feed tray is provided in a laser printer, even when an irregular shaped transfer material is used, the position of the transfer material can be detected by electrically detecting the position of the manual feed guide (which defines the width of the transfer material) of the manual feed tray. It is possible to get the width.

  It is preferable to perform the correction of the transfer current in the third modification described above between step S7 and step S8 in the flowchart of FIG.

  The transfer current may be switched at the timing between pages so as not to affect the transfer of the toner image (image), or may be performed at the timing at which the non-image area in the page is transferred. That is, the transfer current is preferably switched at a timing other than the timing at which the toner-attached portion on the surface of the photosensitive drum 9 is transferred to the transfer material. The switching of the transfer current includes switching from the default transfer current to the initially set transfer current, or switching from the set transfer current to the reset transfer current.

  Further, in the above-described embodiment and each modified example, the description has been given by taking the photosensitive drum that is a drum-shaped photosensitive member as an example, but the invention is not limited thereto, and for example, a belt-shaped photosensitive member or the like may be used. The transfer device 5 including the transfer roller 5a has been described as an example. However, the transfer device 5 is not limited to this, and a transfer device including another transfer member such as a transfer belt or a transfer brush may be used.

  In the above-described embodiment and each modification, an optical scanning device is used as an exposure device that exposes the photosensitive drum. However, the present invention is not limited to this. For example, at least in a direction parallel to the longitudinal direction of the photosensitive drum. An optical print head including a plurality of light emitting units arranged separately may be used. That is, the photosensitive drum 9 may be exposed by rotating the photosensitive drum 9 with respect to the light from the optical print head. Also in this case, the image quality can be improved by controlling the transfer current in the same manner as in the above-described embodiment and modifications.

  In the above-described embodiment and each modification, a semiconductor laser such as an LD or a VCSEL is used as the light-emitting portion of the light source 19, but instead of this, other lasers, LEDs (light-emitting diodes), organic EL elements Etc. may be used.

  Moreover, in the said embodiment and each modification, although the exposure amount setting part 12b is provided separately from the image process part 12a, you may provide as a part of image process part.

  In the embodiment and the modification, the laser printer 100 is employed as the image forming apparatus of the present invention. However, the present invention is not limited to this. For example, the image forming apparatus of the present invention may be a color printer including a plurality of photosensitive drums.

  The image forming apparatus of the present invention may be an image forming apparatus using a silver salt film as an image carrier. In this case, a latent image is formed on the silver salt film by optical scanning, and this latent image can be visualized by a process equivalent to a developing process in a normal silver salt photographic process. Then, it can be transferred to photographic paper by a process equivalent to a printing process in a normal silver salt photographic process. Such an image forming apparatus can be implemented as an optical plate making apparatus or an optical drawing apparatus that draws a CT scan image or the like.

  The present invention can also be applied to an image forming apparatus such as a digital copying machine in addition to the above-described laser printer and color printer.

  Below, the thought process that led to the present invention will be briefly described.

  In recent years, there has been an increasing demand for higher image quality and higher stability in electrophotographic processes. As a method for realizing high image quality, a small electrostatic latent image is formed by a method of reducing the size of an exposure beam or a method of changing the intensity of light output of a pixel based on image information, thereby increasing resolution.

  However, in the conventional image forming apparatus, even when an electrostatic latent image with increased resolution is formed, deterioration factors such as a transfer process are dominant and an effect as expected is not obtained.

  There are various transfer methods in the electrophotographic process. Basically, a potential opposite to that of the toner on the image carrier is applied to the transfer member, and the toner image is electrostatically attracted. Is transferred to a transfer material. The magnitude of this electrostatic attraction is determined by the difference between the surface potential of the image carrier and the surface potential of the transfer member.

  However, when the image data includes a plurality of exposure pixels having different exposure conditions (for example, exposure amount), the potential of the portion corresponding to each exposure pixel on the surface of the image carrier is a potential corresponding to the exposure amount of the exposure pixel. For example, the image area ratio alone disclosed in Patent Document 1 cannot cope with the surface potential distribution of the image carrier.

  In this case, even if the transfer current is fixed to one value or the transfer current is controlled according to the image area ratio, the electrostatic force attracts toner to the transfer material from the portion corresponding to each exposed pixel on the surface of the image carrier. The magnitude of the attractive force will deviate greatly from the preferred value. As described above, when the magnitude of the electrostatic attractive force for attracting the toner to the transfer material is greatly deviated from the preferable value, an appropriate transfer condition cannot be obtained and the image quality is deteriorated.

  The present invention has been made in view of such circumstances, and the object of the present invention is to perform a good transfer under appropriate transfer conditions even in various images, thereby stably increasing the high An object of the present invention is to provide an image forming apparatus capable of obtaining a quality image.

  DESCRIPTION OF SYMBOLS 1 ... Charging device (charging means), 2 ... Optical scanning device (exposure means), 3 ... Developing device (developing means), 5 ... Transfer device (transfer means), 12 ... Printer control device (control means), 100 ... Laser Printer (image forming apparatus).

JP-A-8-83006

Claims (10)

An image carrier;
Charging means for charging the surface of the image carrier;
Exposure means for exposing the charged surface based on image data including a plurality of exposure pixels to form an electrostatic latent image;
Developing means for developing by attaching toner to the electrostatic latent image formed on the surface;
A transfer means for supplying a transfer current for generating an electrostatic force between the toner attached to the electrostatic latent image and the transfer material, and transferring the toner image to the transfer material;
Based on the image data, the exposure amount of the plurality of exposure pixels is set such that the exposure amount of at least two exposure pixels among the plurality of exposure pixels is different from each other, and an average value of the exposure amounts of the plurality of exposure pixels is set. An image forming apparatus comprising: a control unit capable of controlling the transfer current based on the control unit. Further comprising a potential detecting means for detecting a potential of the surface portion exposed at an exposure amount set for each of the plurality of exposure pixels;
The control unit can correct a set value of the exposure amount of the exposure pixel based on the detected potential, and can control the transfer current based on an average value of the exposure amount after correction of the plurality of exposure pixels. The image forming apparatus according to claim 1.   The image forming apparatus according to claim 2, wherein the control unit periodically controls the transfer current. Environmental information detection means for detecting environmental information around the image carrier,
The image forming apparatus according to claim 2, wherein the control unit determines whether to control the transfer current based on the detected environment information.   The control means controls the transfer current in consideration that a part of the transfer current flows in both the region facing the transfer material and the region not facing the image carrier. The image forming apparatus according to claim 1.   6. The control unit according to claim 1, wherein the control unit switches the transfer current at a timing other than a timing at which an image portion of the toner image is transferred to the transfer material. Image forming apparatus.   The control means switches the transfer current, the timing at which a non-image portion of the toner image is transferred to the transfer material, or the transfer to the next transfer material after the transfer to one transfer material is completed. The image forming apparatus according to claim 6, wherein the image forming apparatus is performed at a predetermined timing until the start. The surface of the image carrier is charged, the charged surface is exposed based on image data including a plurality of exposure pixels to form an electrostatic latent image, and toner is attached to the electrostatic latent image formed on the surface. An image forming method for transferring a toner image to the transfer material by supplying a transfer current for generating an electrostatic force between the toner adhering to the surface and the transfer material to a transfer means,
Setting the exposure amounts of the plurality of exposure pixels so that the exposure amounts of at least two exposure pixels among the plurality of exposure pixels are different from each other based on the image data;
And a step of controlling the transfer current based on an average value of exposure amounts of the plurality of exposure pixels, and an image forming method. Detecting a potential of a portion of the surface corresponding to each of the plurality of exposure pixels exposed at a set exposure amount, and correcting a set value of the exposure amount of the exposure pixel based on the detected potential Further including
The image forming method according to claim 8, wherein in the step of controlling the transfer current, the transfer current is controlled based on an average value of exposure amounts after correction of the plurality of exposure pixels. The surface of the image carrier is charged, the charged surface is exposed based on image data including a plurality of exposure pixels to form an electrostatic latent image, and toner is attached to the electrostatic latent image formed on the surface. An image forming apparatus for supplying a transfer current for generating an electrostatic force between the toner adhering to the electrostatic latent image and the transfer material to a transfer unit to transfer the toner image to the transfer material;
Based on the image data, the exposure amount of the plurality of exposure pixels is set such that the exposure amount of at least two exposure pixels among the plurality of exposure pixels is different from each other, and an average value of the exposure amounts of the plurality of exposure pixels is set. An image forming apparatus comprising control means capable of controlling the transfer current based on the image forming apparatus.

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