JP2012022057A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the image quality of a half tone image from being degraded due to discharge products generated in electrification by an electrifying device.SOLUTION: It is determined whether or not the image quality of a toner image formed on the surface of a photoreceptor drum is degraded due to the discharge products generated in electrification by the electrifying device, based on a stop time where an image forming operation is not executed and the ratio of a time where the image forming operation is executed just before the stop time to the stop time. When it is determined that the image quality is degraded, it is determined whether or not the half tone image is in a previously set predetermined form in image data. When it is determined that the half tone image in the predetermined form is present, the surface position of the photoreceptor drum facing the electrifying device is specified and the start timing of exposure by an exposure device is delayed so that an electrostatic latent image corresponding to the half tone image is not formed in the specified surface position.

Description

  The present invention relates to an image forming apparatus that forms a toner image by electrophotography.

  In an image forming apparatus such as a copying machine or a printer that forms a toner image by an electrophotographic method, an electrostatic latent image is usually obtained by uniformly charging a photosensitive drum with a charger and exposing the charged photosensitive drum surface. An image is formed. The electrostatic latent image on the photoconductive drum is developed by the developing device into a toner image. The toner image is fixed after being transferred to the recording sheet.

The charger normally charges the surface of the rotating photosensitive drum uniformly by corona discharge. It is known that when the surface of the photosensitive drum is charged by corona discharge, discharge products such as ozone and nitrogen oxide are generated around the charger. Since the discharge product around the charger flows out of the charger due to the rotation of the photosensitive drum, there is usually no problem.
However, after the image forming apparatus continuously performs the toner image forming operation for a relatively long time and then enters the state in which the toner image forming operation is not performed for a long time, the charger is operated for a long time. A large amount of discharge products drifting to the surroundings may adhere to the surface of the photoreceptor near the charger during the subsequent stoppage of operation, and as a result, trap sites may be formed on the surface of the photoreceptor drum.

  When trap sites are formed on the surface of the photosensitive drum, charge transport is hindered, so that sensitivity on the surface portion is reduced, and light is not attenuated even when an electrostatic latent image is exposed. The toner does not adhere to the portion of the photosensitive drum surface where the light is not attenuated, and the toner image formed on the recording sheet has white spots. Such white spots are not particularly noticeable in a character image, a solid black image, and the like, and thus are less likely to be recognized as image noise. However, a halftone image such as a photographic image (halftone image) may be recognized as image noise.

  For this purpose, for example, Patent Document 1 discloses a configuration in which moisture is removed from the air in the charging space and dried, and then discharged to the outside of the apparatus by a fan. With such a configuration, discharge products generated around the charger can be reliably discharged out of the apparatus together with air. Therefore, even if the halftone image forming operation is executed after the image forming operation is not executed for a long time, white spots can be prevented from occurring.

JP 2009-63865 A

However, in the configuration disclosed in Patent Document 1, after moisture is removed from the air in the charging space and dried, the air must be sucked by the fan, which requires a complicated configuration. There is a possibility that the size is increased and the economic efficiency is impaired.
In addition, a method of preliminarily rotating the photosensitive drum and removing discharge products adhering to the surface of the photosensitive drum by the cleaning device before performing the image forming operation without providing an ozone suction mechanism such as a fan. Has also been proposed. In this case, the photosensitive drum has to be rotated for a relatively long time before the image forming operation is executed, which causes a problem that the start of the image forming operation is delayed.

  The present invention has been made in view of such a problem, and an object of the present invention is to generate image noise such as white in a halftone image when there is a possibility that discharge products may adhere to the surface of the photoreceptor. It is still another object of the present invention to provide an image forming apparatus that can prevent the image from being formed, and that can form a halftone image quickly, and that the size of the apparatus is large and the economy is not impaired.

  In order to achieve the above object, an image forming apparatus according to the present invention exposes the surface of a rotating photoconductor uniformly with a charger while exposing it to an electrostatic latent image corresponding to the image data. An image forming apparatus that forms a toner image by developing and developing the electrostatic latent image with a developing device, and when the image forming execution instruction is received when the photoconductor is in a rotation stopped state, A surface position specifying means for specifying the surface position of the photoconductor facing the charger, and a predetermined halftone image set in advance in image data to be subjected to the image formation execution instruction and to form a toner image A halftone image determining means for determining whether the halftone image exists in the form of the surface, and the surface specified by the surface position specifying means when the halftone image determining means determines that the halftone image exists As an electrostatic latent image corresponding to the halftone image in location is not formed, and having a an exposure delay control means for performing control to delay the start timing of exposure by the exposing unit.

  In the present invention, when an instruction for image formation based on image data of a halftone image is received when the photoconductor is in a stopped state, a discharge product is formed at the surface position of the photoconductor facing the charger in the rotation stopped state. Even if is attached, an electrostatic latent image corresponding to a halftone image is not formed in that portion. Therefore, when there is an instruction to form a halftone image, an operation for removing the discharge product is not required, and a high-quality halftone toner image can be quickly formed.

  Preferably, the discharge is performed based on a relationship between a stop time of the image forming operation until receiving the image forming execution instruction and a ratio of an execution time of the image forming operation preceding the stop of the image forming operation to the stop time. The exposure delay control unit further includes an image quality deterioration determining unit that determines whether the image quality of the toner image is deteriorated by the product, and the exposure delay control is performed when the image quality deterioration determining unit determines that the image quality of the toner image is deteriorated. Control by means is executed.

Preferably, the image quality deterioration determination unit includes a control execution time by the exposure delay control unit, an image formation operation stop time until receiving the image formation execution instruction, and an image preceding the image formation operation stop. It is set based on the relationship with the ratio of the execution time of the forming operation to the stop time.
Preferably, the image forming apparatus further includes preliminary rotation control means for performing rotation control for rotating the photoconductor for a predetermined time in order to remove discharge products attached to the surface of the photoconductor.

Preferably, the exposure delay is determined when the halftone image determination unit determines that there are a plurality of halftone images in the image data, and when the halftone image is equal to or greater than the circumference of one rotation of the photoconductor. Control by the preliminary rotation control means is executed instead of execution by the control means.
Preferably, the exposure delay control means stops execution of control for delaying an exposure start timing by the exposure means based on an ambient environmental temperature and environmental humidity.

  Preferably, the halftone image determination means includes a pixel having a gradation of 20 to 150 in a gradation of 0 to 255, or a pixel having a density of 0.1 to 0.8 over a length of 10 mm or more along the main scanning direction. A region that is continuous and continuous in a length of 2 mm or more along the sub-scanning direction is determined as a halftone image.

1 is a schematic front view showing a schematic configuration of a monochrome copying machine according to an embodiment of an image forming apparatus of the present invention. FIG. 2 is a schematic diagram illustrating a configuration of an encoder provided in an image process unit of the monochrome copying machine. 2 is a block diagram of a control system provided in an image process unit of the monochrome copying machine. FIG. It is an example of the 1st table memorize | stored in the control part of the control system. It is an example of the 2nd table memorize | stored in the control part of the control system. 7 is a flowchart illustrating a specific example of image quality deterioration prevention control executed at the start of printing in the control unit of the control system. It is a flowchart which shows the subroutine of exposure timing control performed in the control part of the control system. (A) is a schematic diagram showing a state where the surface of the photosensitive drum is developed in order to explain a toner image formed on the surface of the photosensitive drum by normal control in which exposure timing control is not executed. FIG. 6B is a schematic diagram illustrating a state where the surface of the photosensitive drum is developed in order to explain a toner image formed on the surface of the photosensitive drum when the exposure timing control is performed.

Hereinafter, embodiments of an image forming apparatus according to the present invention will be described.
<Configuration of image forming apparatus>
FIG. 1 is a schematic front view showing a schematic configuration of a monochrome copying machine according to an embodiment of an image forming apparatus of the present invention. The monochrome copying machine is provided with an image processing unit 10 that forms a toner image and transfers and fixes the toner image on a recording sheet, a paper feeding unit 20 provided below the image processing unit 10, and an image processing unit 10. And an image reading unit 30.

The monochrome copying machine forms and fixes a toner image on a recording sheet supplied from the paper feeding unit 20 based on the image data of the original read by the image reading unit 30 in the image processing unit 10.
In the image reading unit 30, when reading an image of a document placed on the manual placement platen glass 33, the scanner unit 31 moves along the original on the manual placement platen glass 33 in the direction indicated by the arrow A 1. Is done. In this case, the slider 34 moves in the same direction as the scanner unit 31 at half the moving speed. As a result, the light irradiated from the lamp 31 a mounted on the scanner unit 31 and reflected on the document surface is reflected by the first mirror 31 b mounted on the scanner unit 31 and is mounted on the slider 34. The light receiving surface of the CCD line sensor 37 is irradiated by the second mirror 34a and the third mirror 34b.

The image processing unit 10 includes a photosensitive drum 11 disposed on the right side from the front side to the back side. The photosensitive drum 11 can be rotated in the arrow Z direction by a main motor 48.
On the left side of the photosensitive drum 11 from the front side to the back side, a charger 12 that uniformly charges the surface of the photosensitive drum 11 is disposed so as to face the photosensitive drum 11. When a high voltage is applied, the charger 12 generates corona discharge with the photosensitive drum 11 to charge the photosensitive drum 11.

An exposure device 13 that irradiates the surface of the charged photosensitive drum 11 with laser light L to form an electrostatic latent image is provided on the side downstream of the charger 12 in the rotation direction of the photosensitive drum 11. ing.
A developing device 14 and a transfer roller 15 for developing the electrostatic latent image formed on the surface of the photosensitive drum 11 are located downstream of the position where the laser beam L is exposed by the exposure device 13 in the rotation direction of the photosensitive drum 11. Are arranged in order.

The exposure device 13 exposes the surface of the charged photosensitive drum 11 with a laser beam L modulated in accordance with image data. As a result, an electrostatic latent image is formed on the surface of the photosensitive drum 11.
The developing device 14 includes a developing roller that transports toner inside, and the toner is transported on the peripheral surface of the developing roller by the rotation of the developing roller to which the developing bias voltage is applied, and faces the photosensitive drum 11. At the position, it adheres to the electrostatic latent image formed on the photosensitive drum 11. As a result, a toner image is formed on the surface of the photosensitive drum 11.

The transfer roller 15 is in contact with the photosensitive drum 11, and a transfer nip is formed between the transfer roller 15 and the photosensitive drum 11. A transfer bias voltage is applied to the transfer roller 15.
A recording sheet is conveyed to the transfer roller 15 from a paper feeding unit 20 provided below the image processing unit. The paper feeding unit 20 is provided with a pair of registration rollers 25 that convey the recording sheet to the transfer nip at a predetermined timing synchronized with the rotation of the photosensitive drum 11.

The paper feed unit 20 is provided with a paper feed cassette 21 in which recording sheets are accommodated, and one recording sheet in the paper feed cassette 21 is conveyed to a registration roller 25 by a paper feed roller 22.
When the recording sheet conveyed to the registration roller 25 is conveyed to the transfer nip by the registration roller 25, the toner image formed on the photosensitive drum 11 is subjected to an electric field action by the transfer roller 15 to which a transfer bias voltage is applied. Thus, the image is transferred onto the recording sheet passing through the transfer nip.

The surface of the photosensitive drum 11 to which the toner image has been transferred is cleaned by the cleaning unit 18. As a result, the toner remaining on the surface of the photosensitive drum 11 can be removed from the surface of the photosensitive drum 11.
The recording sheet that has passed through the transfer nip is conveyed along the conveyance guide 46 to the fixing device 17 disposed above the transfer roller 15. A suction fan 47 is provided on the right side of the conveyance guide 46 to suck the recording sheet so that the recording sheet is conveyed along the conveyance guide 46.

  The fixing device 17 includes, for example, a fixing roller 17a provided with a heater therein, and a pressure roller 17b pressed against the fixing roller 17a, and fixing is performed between the fixing roller 17a and the pressure roller 17b. A nip is formed. The recording sheet on which the toner image is formed is fixed by being heated and pressurized while passing through the fixing nip. The recording sheet on which the toner image is fixed is discharged onto a paper discharge tray 41 provided between the image processing unit 10 and the image reading unit 30 by a pair of paper discharge rollers 19.

The image processing unit 10 is provided with a temperature sensor 43 and a humidity sensor 44 for detecting the ambient temperature and humidity around the copying machine, respectively.
An encoder 42 for detecting the rotational position of the surface of the photosensitive drum 11 is provided on the rotating shaft of the photosensitive drum 11 rotated by the main motor 48.
FIG. 2 is a schematic diagram showing the configuration of the encoder 42. The encoder 42 has a rotating disc 42a that rotates integrally with the rotating shaft 11b of the photosensitive drum 11, and the rotating disc 42a is attached coaxially with the rotating shaft 11b. The rotating disk 42a is provided with a predetermined number of through-holes 42b over the entire circumference at regular intervals in the circumferential direction.

The encoder 42 also includes a photosensor 42c in which a light emitting portion and a light receiving portion are provided facing each other across a region through which the through hole 42b of the rotating disc 42a passes. The photo sensor 42 c is provided at a position corresponding to the surface position of the photosensitive drum 11 facing the charger 12.
The photosensor 42c outputs a number of pulses corresponding to the amount of rotation of the photosensitive drum 11. The number of pulses output from the photosensor 42c is counted by a counter (not shown), and the counter is reset when the rotation of the photosensitive drum 11 is stopped. ing. Accordingly, when the photosensitive drum 11 in the rotation stopped state starts to rotate, the counter counts up. However, the instantaneous value of the counter when the photosensitive drum 11 rotates is the charge in the photosensitive drum 11 in the rotation stopped state. This indicates how far the surface position facing the charger 12 (hereinafter referred to as the control target position) is away from the charger 12 in the rotation direction of the photosensitive drum 11.

The position at which the surface of the photosensitive drum 11 is exposed by the exposure device 13 is separated from the charger 12 by a predetermined distance, and the distance is known. Therefore, it can also be determined that the control target position on the surface of the photosensitive drum 11 has passed the exposure position by the exposure device 13 based on the count value by the counter.
<Control system configuration>
FIG. 3 is a block diagram of a control system provided in the image processing unit 10. The image processing unit 10 is provided with a control unit 45 that controls an image forming operation (printing operation).

The control unit 45 is given an output of the image processing unit 38 provided in the image reading device 30. Based on the output of the CCD line sensor 37, the image processing unit 38 processes the document image into image data necessary for forming a toner image on the photosensitive drum 11 and outputs the image data to the control unit 45.
The control unit 45 rotates and stops the photosensitive drum 11 when executing the printing operation, the main motor 48, the charger 12, the exposure device 13, the developing device 14, and the paper feeding unit 20 (paper feeding roller 22, registration roller). 25) and the like. The control unit 45 executes image quality deterioration prevention control for preventing image quality deterioration when there is a possibility that image quality deterioration occurs due to a discharge product generated by charging by the charger 12 at the start of the printing operation. It has become.

  The control unit 45 executes normal printing operation control (normal control) when the image quality deterioration prevention control is not executed. The normal control is the same as the control during a general printing operation, and the charging device 12 and the exposure device 13 are respectively set to predetermined timings in a state where the photosensitive drum 11 is rotated by the main motor 48 and the developing device 14 is driven. And a recording sheet is conveyed to the transfer nip by a registration roller 25 at a predetermined timing.

Further, when the printing operation for a plurality of recording sheets is continuously performed, after the printing operation for one recording sheet is finished, the exposure device 13 is driven and the recording sheet is conveyed by the registration rollers 25 thereafter. Are started at predetermined timings.
The control unit 45 is provided with a timer and a storage unit that stores various tables, data, and the like. The timer performs a normal timing operation by starting and ending timing, and measures each of the time when the printing operation is performed and the stop time of the photosensitive drum 11 where the printing operation is not performed. Note that the print execution time measured by the timer and the stop time of the photosensitive drum 11 are stored in the storage unit.

  When a toner image corresponding to the same image data is formed on a plurality of recording sheets, the control unit 45 controls image quality deterioration prevention and normal print operation (normal control) at the start of the print operation. And either one of them is selectively executed. Whether to perform the image quality deterioration prevention control or the normal control is determined based on the time during which the print operation is not executed immediately before the print operation instruction. Details will be described later.

The image quality deterioration prevention control executed at the start of the printing operation includes exposure timing control executed when the ratio of halftone images in one image data instructed to print is equal to or less than a predetermined value. There are two controls: preliminary rotation control that is executed when the ratio of the halftone image in the image data is larger than a predetermined value.
The exposure timing control is a control for delaying the exposure start timing so that a halftone image is not formed at a control target position on the surface of the photoconductive drum 11 in a rotation stop state of the photoconductive drum 11 where no image forming operation is performed. . The preliminary rotation control is control in which the cleaning unit 18 removes the discharge products attached to the surface of the photosensitive drum 11 by rotating the photosensitive drum 11 for a predetermined time before starting the image forming operation.

The storage unit of the control unit 45 stores in advance a first table and a second table that are used when determining whether to perform normal control or image quality deterioration prevention control. FIG. 4 shows an example of the first table.
The first table shows a stop time in which the image forming operation is not executed immediately before the image forming operation is instructed, a time in which the image forming operation is executed immediately before the stop time (operating time), and a stop time of the photosensitive drum 11. This indicates whether the control unit 45 executes normal control or image quality deterioration prevention control based on the ratio (operation ratio). However, in the first table, the operation ratio is shown as a ratio of operation time when the stop time is converted per 120 minutes. In addition, the case where the image quality deterioration prevention control is executed without executing the normal control is shown as the execution time T1 of the image quality deterioration prevention control. Accordingly, when the execution time T1 is shown in the first table, it means that the image quality deterioration prevention control is executed over the time T1.

In the first table, when the printing operation stop time is 10 minutes (min) or less, normal control is executed if the operation ratio is less than 50%, and image quality deterioration prevention control is performed if the operation ratio is 50% or more. Is shown to run for 1 minute.
Similarly, when the stop time of the image forming operation is 10 to 30 minutes (more than 10 minutes and less than 30 minutes), if the operation ratio is 20% or less, normal control is executed, and the operation ratio is 20 to 50%. If it is (greater than 20% and less than 50%) and 50% or more, it is shown that the image quality deterioration prevention control is executed for 1 minute and 2 minutes, respectively, without executing the normal control.

Further, when the stop time of the image forming operation is 30 to 120 minutes (more than 30 minutes and less than 120 minutes), the normal control is executed if the operation ratio is 20% or less, and the operation ratio is 20 to 50%. If it is 50% or more, it is indicated that the image quality deterioration prevention control is executed for 2 minutes and 5 minutes, respectively, without executing the normal control.
Further, when the stop time of the image forming operation is 120 minutes or more, the normal control is executed if the operation ratio is 20% or less, and the normal control is performed if the operation ratio is 20 to 50% or 50% or more. It is shown that the image quality deterioration prevention control is executed for 5 minutes and 10 minutes, respectively, without executing.

  If the rotation of the photosensitive drum 11 is stopped for the stop time of the image forming operation in any condition in which the execution of the image quality deterioration prevention control is indicated in the first table, the photosensitive member facing the charger 12 is stopped. The discharge product generated when the surface of the photosensitive drum 11 is charged by the charger 12 during the previous printing operation may adhere to the surface of the drum 11 during the stop time of the photosensitive drum 11. .

When the surface of the photosensitive drum 11 to which the discharge product adheres becomes a trap site and the printing operation is started after that, even if the trap site is exposed by the exposure device 13, the charge transport is hindered to cause light attenuation. Therefore, there is a possibility that the halftone image quality of the printed image is degraded. For this reason, image quality deterioration prevention control is executed.
FIG. 5 shows an example of the second table stored in the storage unit of the control unit 45. This second table shows which of the normal control and the image quality deterioration prevention control is executed based on the relationship between the temperature around the copying machine and the humidity detected by the temperature sensor 43 and the humidity sensor 42, respectively. Yes.

  In the second table, it is shown that when the humidity detected by the humidity sensor 44 is 20% or less, the image quality deterioration prevention control is executed regardless of the temperature detected by the temperature sensor 43. When the humidity is 20 to 40% (greater than 20% and less than 40%), normal control is executed if the temperature is 25 ° C. or higher, and image quality deterioration prevention control is executed if the temperature is lower than 25 ° C. It has been shown.

Furthermore, when the humidity is 40 to 60% (greater than 40% and less than 60%), normal control is executed if the temperature is higher than 15 ° C., and image quality deterioration prevention control is executed if the temperature is 15 ° C. or lower. It has been shown. When the humidity is 60% or more, it is shown that normal control is executed regardless of the temperature.
When the environmental humidity becomes high, the influence of the photosensitive drum 11 by the discharge product is reduced. Therefore, when the environmental humidity is 60% or more, normal control is executed. When the environmental humidity is lower than 60%, it is determined whether to execute the normal control or the image quality deterioration prevention control based on the relationship with the environmental temperature.

<Control by control unit>
FIG. 6 is a flowchart showing image quality deterioration prevention control executed by the control unit 45 at the start of a printing operation when toner images corresponding to the same image data are formed on a plurality of recording sheets. When the execution of the printing operation is instructed, the image reading unit 30 is driven and the original image data is acquired by the CCD line sensor 37 as in the case of the normal printing operation. Then, data processing of the document image acquired in the image processing unit 38 is executed.

  When the execution of the printing operation is instructed, the control unit 45 stores the stop time of the photosensitive drum 11 immediately before the instruction and the time when the printing operation is executed immediately before the stop time of the photosensitive drum 11. To obtain the operating ratio (step S11). In this case, the operation ratio is converted into a ratio to the stop time of the photosensitive drum 11 per 120 minutes.

  When calculating the operation ratio, the control unit 45 acquires the first table from the storage unit (step S12), and based on the calculated operation ratio and the acquired stop time of the photosensitive drum 11, the first table. From this, it is determined whether to perform normal control or image quality deterioration prevention control (step S13). If execution of normal control is indicated in the first table (“YES” in step S13), normal print operation control is executed without executing image quality deterioration prevention control (step S28).

On the other hand, when the execution time T1 of the image quality deterioration prevention control is shown in the first table (“NO” in step S13), the execution time T1 of the image quality deterioration prevention control shown in the first table. Is acquired (step S15).
Next, the control unit 45 acquires the temperature and humidity around the copying machine from the outputs of the temperature sensor 43 and the humidity sensor 44 (step S16). Further, the second table is acquired from the storage unit (step S17), and based on the acquired humidity and temperature, it is determined whether to execute normal control or image quality deterioration prevention control from the second table (step S17). S18). If execution of normal control is indicated in the second table (“YES” in step S18), control of normal printing operation is executed (step S28). This is because depending on the environmental humidity and the environmental temperature, the influence of the photosensitive drum 11 by the discharge product is small, and it is not particularly necessary to execute image quality deterioration prevention control.

  On the other hand, when the image quality deterioration prevention control is indicated in the second table (“NO” in step S18), the image data of the document output from the image processing unit 38 is acquired (step S19). The image processing unit 38 uses, for example, 256 gradations (0 to 255) for each pixel of the document based on image data obtained by processing the output of the line sensor 37 provided in the image reading device 30 by gamma correction or the like. Output as image data.

  The controller 45 determines from the acquired image data whether a halftone image exists in the document image (step S20). In this case, for example, a process of extracting pixels in the gradation value range of 20 to 150 as halftone pixels from the pixel row of each line along the main scanning direction is performed on one original image, and the extracted intermediate A region in which tone pixels are continuous over a length of 10 mm or more along the main scanning direction and is continuous over 2 mm or more along the sub-scanning direction is determined as a halftone image.

  The presence of the halftone image is not limited to the configuration that is determined based on the gradation of the pixel as described above, and may be configured to be determined based on the density obtained from the gradation of each pixel, for example. . In this case, for example, pixels having a density (ID) in the range of 0.1 to 0.8 continue for 10 mm or more along the main scanning direction and continuously for 2 mm or more along the sub scanning direction. If it is, it is determined that a halftone image exists.

If it is not determined in step S20 that a halftone image exists (“NO” in step S20), control of a normal printing operation is executed (step S28).
If it is determined in step S20 that a halftone image exists (“YES” in step S20), the position of the halftone image in the sub-scanning direction and the length in the sub-scanning direction are obtained and stored (step S21). .

Next, it is determined whether there are a plurality of halftone images, or whether the length of the halftone images continuous in the sub-scanning direction is equal to or greater than the circumference of one revolution of the photosensitive drum 11 (step S22).
If there is one halftone image in the image data and the length of the halftone image continuous in the sub-scanning direction is not greater than or equal to the circumference of the photosensitive drum 11 (step) The exposure timing control is executed (“NO” in S22) (step S23). In this exposure timing control, the exposure timing of the exposure device 13 is controlled so that the halftone image is not exposed at the control target position on the photosensitive drum 11.

The exposure timing control is executed over the execution time T1 of the image quality deterioration prevention control acquired in step S15 (step S24). When the execution time T1 has elapsed ("YES" in step S24), thereafter, normal print operation control is executed (step S28).
When the printing operation is executed by the exposure timing control, the toner image formed on the surface of the photosensitive drum 11 is transferred to the recording sheet as in the case of the normal printing operation. The toner remaining on the surface is removed by the cleaning unit 18. At this time, if discharge products adhere to the surface of the photosensitive drum 11, they are removed by the cleaning unit 18.

  Therefore, when the printing operation based on the exposure timing control is executed over the execution time T1, if the discharge product adheres to the surface of the photosensitive drum 11, the discharge product is removed by the cleaning unit 18. Can do. For this reason, when exposure timing control over the execution time T1 is executed, the printing operation by normal control is executed thereafter.

  When there are a plurality of halftone images in the image data, or the length of one halftone image existing in the image data in the sub-scanning direction is greater than or equal to the circumference of one circumference of the photosensitive drum 11 If so ("YES" in step S22), preliminary rotation control for rotating the photosensitive drum 11 to remove the discharge products adhering to the surface of the photosensitive drum 11 before executing the printing operation is performed. Execute (Step S25).

  When there are a plurality of halftone images in the image data, it is not easy to prevent each halftone image from being formed at the control target position by exposure timing control. In addition, when the length of one halftone image continuous in the sub-scanning direction is equal to or longer than the circumference of one circumference of the photosensitive drum 11, the exposure timing control is executed even if the exposure timing control is executed. The halftone image cannot be prevented from being formed.

  For this reason, in order to remove the discharge products adhering to the surface of the photosensitive drum 11 by the cleaning unit 18, the photosensitive drum 11 is executed over the execution time T1 of the image quality deterioration prevention control acquired in step S15 ( Step S26). As a result, the discharge product adhering to the surface of the photosensitive drum 11 is removed by the cleaning unit 18. When the execution time T1 has elapsed ("YES" in step S26), thereafter, normal print operation control is executed (step S28).

  FIG. 7 is a flowchart showing a subroutine of exposure timing control. In the exposure timing control, first, a counter that counts the number of pulses output from the encoder 42 is reset in order to identify the control target position of the photosensitive drum 11 that has stopped rotating (step S31 in FIG. 7 and thereafter). The same). Therefore, when the photosensitive drum 11 is rotated, the control target position can be specified by calculating the distance of the control target position from the facing position of the charging device 12 based on the count value of the counter.

  Next, when the printing operation in the normal control is started, the specified control target position of the photosensitive drum 11 and the position in the sub-scanning direction of the halftone image temporarily stored in step S21 overlap each other. Is determined (step S32). That is, when exposure of the exposure device 13 is started in accordance with the start timing of the printing operation in the normal control, it is determined whether the halftone image is exposed at the control target position of the photosensitive drum 11.

When the control target position of the photoconductive drum 11 and the position of the halftone image overlap each other (“YES” in step S32), the exposure of the halftone image overlaps the control target position of the photoconductive drum 11. In order to avoid this, a time for delaying the exposure start timing by the exposure apparatus 13 (exposure delay time) is calculated (step S33).
Thereafter, the control of the printing operation is started. In this case, the timing at which the exposure device 13 exposes the surface of the photosensitive drum 11 is equal to the calculated exposure delay time than the timing at which the printing operation in the normal control is started. Delayed (step S34). In addition, the start of conveyance of the recording sheet by the registration roller 25 is delayed in accordance with the exposure delay time.

  As a result, the electrostatic latent image formed on the surface of the photoconductive drum 11 is not likely to form a halftone image at the control target position of the photoconductive drum 11. Accordingly, there is no possibility that image noise such as white spots will occur in the halftone image of the toner image developed by the developing device 14 thereafter. In this case, since the conveyance timing of the recording sheet by the registration roller 25 is also delayed, the toner image is transferred onto the recording sheet without deviation.

  The printing operation based on such exposure timing control is executed over the execution time T1. Therefore, when the electrostatic latent images of the same image data are continuously formed on the surface of the photosensitive drum 11, the subsequent electrostatic latent image is formed after the formation of the preceding electrostatic latent image is completed. The exposure start timing until the start is delayed by the time obtained by adding the calculated exposure delay time to the inter-image timing time required as the recording sheet conveyance timing.

As a result, each of the electrostatic latent images continuously formed on the surface of the photoconductive drum 11 has no fear that a halftone image is formed at a control target position of the photoconductive drum 11, and therefore, a plurality of recording sheets. There is no possibility that image noise such as white spots will occur in the toner images formed on the respective toner images.
FIG. 8A is a schematic diagram showing a state in which the surface of the photoconductive drum 11 is developed in order to explain a toner image formed on the surface of the photoconductive drum 11 by the normal control in which the exposure timing control is not executed. is there. FIG. 8B is a schematic diagram showing the surface of the photosensitive drum 11 in a developed state in order to explain the toner image formed on the surface of the photosensitive drum 11 when the exposure timing control is executed. FIG.

  In FIG. 8A, a trap site is formed by the attachment of the discharge product at the control target position 11a on the surface of the photosensitive drum 11, and the toner image formed on the surface of the photosensitive drum 11 is as follows. A halftone image HT that extends over the entire length in the main scanning direction exists in the character image area. In this case, when the printing operation by the normal control is started with respect to the rotating photosensitive drum 11, the exposure device 13 exposes the surface of the charged photosensitive drum 11 at a predetermined timing set in advance. To do.

  Usually, when a toner image for one recording sheet is formed on the surface of the photosensitive drum 11, the photosensitive drum 11 is rotated a plurality of times. For this reason, if the control target position 11a of the photoconductive drum 11 is included in the halftone image HT of the toner image formed on the surface of the photoconductive drum 11, no light attenuation occurs at the control target position 11a. Since the toner does not adhere, a white image noise pattern WP is generated in the halftone image HT.

Similarly, when a toner image for each of a plurality of recording sheets is continuously formed, if the control target position 11a of the photosensitive drum 11 is included in the halftone image HT of the toner image, the halftone image HT There is a possibility that a white image noise pattern WP may occur.
On the other hand, by performing exposure timing control, as shown in FIG. 8B, the control target position 11 a of the photosensitive drum 11 becomes halftone of the toner image formed on the surface of the photosensitive drum 11. There is no fear of being included in the image HT, and it is possible to prevent the occurrence of a white image noise pattern WP in the halftone image HT.

  Thereafter, in the case where an electrostatic latent image is continuously formed on the surface of the photosensitive drum 11, the same exposure timing control is executed. In this case, a time for delaying the recording sheet conveyance timing ( The calculated exposure delay time is added to the (inter-image timing time). Accordingly, there is no possibility that the control target position 11a of the photosensitive drum 11 is included in the halftone image HT of the toner image, and there is no possibility that the white image noise pattern WP is generated in the halftone image HT.

  As described above, in the situation where the discharge product is attached to the photoconductor drum 11, the discharge product is removed from the photoconductor drum 11 when forming a toner image including a halftone image. The image forming operation can be immediately executed without executing the operation. In addition, there is no risk of white spots occurring in the formed toner image, and a high-quality toner image can be obtained.

[Modification]
When there are a plurality of halftone images arranged in the main scanning direction, exposure timing delay control can be executed if any of the halftone images satisfies the above-described conditions.
In the above embodiment, the case where the present invention is applied to a monochrome copying machine has been described. However, the present invention is not limited to such a configuration. Applicable to forming apparatus. Further, the present invention can be applied not only to an image forming apparatus that forms a monochrome image but also to an image forming apparatus that forms a color image.

  In the present invention, an electrostatic latent image corresponding to image data is formed by exposing the surface of a photoreceptor charged by a charger with an exposure device, and the electrostatic latent image is developed with toner by a developer. In an image forming apparatus for forming a toner image, it is useful as a technique for suppressing deterioration in the image quality of a halftone image when a discharge product generated during charging by a charger adheres to a rotation-stopped photoconductor. .

DESCRIPTION OF SYMBOLS 10 Image process part 11 Photoconductor drum 12 Charging device 13 Exposure apparatus 14 Developing device 20 Paper feed part 25 Registration roller 30 Image reading device 37 CCD line sensor 38 Image processing part 42 Encoder 43 Temperature sensor 44 Humidity sensor 45 Control part

Claims (7)

While the surface of the rotating photoconductor is uniformly charged by a charger, it is exposed by an exposure device to form an electrostatic latent image according to image data, and the electrostatic latent image is developed with toner by a developing device. An image forming apparatus for forming a toner image by:
Surface position specifying means for specifying the surface position of the photoconductor facing the charger when receiving an image formation execution instruction when the photoconductor is in a rotation stop state;
Halftone image determination means for determining whether a halftone image exists in a predetermined form set in advance in image data to be subjected to the image formation execution instruction and to form a toner image;
When the halftone image determining means determines that the halftone image exists, an electrostatic latent image corresponding to the halftone image is not formed at the surface position specified by the surface position specifying means. Exposure delay control means for executing control for delaying the start timing of exposure by the exposure means;
An image forming apparatus comprising: Based on the relationship between the stop time of the image forming operation until receiving the image forming execution instruction and the ratio of the execution time of the image forming operation preceding the stop of the image forming operation to the stop time, the discharge product An image quality deterioration determining unit that determines whether the image quality of the toner image has deteriorated;
The image forming apparatus according to claim 1, wherein when the image quality deterioration determination unit determines that the image quality of the toner image is deteriorated, the exposure delay control unit performs control.   The image quality deterioration determining means includes a control execution time by the exposure delay control means, a stop time of the image forming operation until receiving the image forming execution instruction, and an image forming operation preceding the stop of the image forming operation. The image forming apparatus according to claim 2, wherein the image forming apparatus is set based on a relationship between a ratio of the execution time to the stop time.   4. The preliminary rotation control means for executing rotation control for rotating the photoconductor for a predetermined time in order to remove discharge products adhering to the surface of the photoconductor. The image forming apparatus according to one item.   The determination by the halftone image determination means is performed by the exposure delay control means when there are a plurality of halftone images in the image data and when the halftone image is equal to or greater than the circumference of one rotation of the photoconductor. The image forming apparatus according to claim 4, wherein control by the preliminary rotation control unit is executed instead of execution.   The said exposure delay control means stops execution of the control which delays the start timing of exposure by the said exposure means based on surrounding environmental temperature and environmental humidity. The image forming apparatus described in 1. Image forming apparatus.   The halftone image determination means includes pixels of 20 to 150 gradations in gradations of 0 to 255, or pixels having a density of 0.1 to 0.8 that are continuous over a length of 10 mm or more along the main scanning direction. The image forming apparatus according to claim 1, wherein a region continuous in a length of 2 mm or more along the sub-scanning direction is determined as a halftone image.

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