JP2023042795A - Charging device with variable AC voltage frequency - Google Patents

Abstract

To reduce the wear of a photoreceptor compared to a case in which the frequency of an alternating voltage applied to a charging device is constantly high (large).SOLUTION: An image forming apparatus comprises: a photoreceptor having a surface to form a toner image; a charging device to charge the surface of the photoreceptor based on an alternating voltage; a conveyance path to convey the toner image; a sensor to measure a toner density on the conveyance path; and a controller to change a frequency of the alternating voltage applied to the charging device based on the toner density measured.SELECTED DRAWING: Figure 2

Description

2. Description of the Related Art Some image forming apparatuses are equipped with a charging device that is arranged in a non-contact manner with respect to a photoreceptor. In such an image forming apparatus, a voltage obtained by superimposing an AC voltage component on a DC voltage component is applied to the charging device to charge the surface of the photoreceptor.

FIG. 1 is a schematic diagram of an example image forming apparatus. FIG. 2 is a schematic diagram of an example photoreceptor charging system. FIG. 3 is a graph showing an example of the relationship between the AC voltage frequency and the wear rate of the photoreceptor when the surface of the photoreceptor is charged by a non-contact charging device. FIG. 4 is a diagram showing example print data. FIG. 5 is a schematic diagram showing an example of the relationship between the toner image formed on the surface of the photoreceptor and the frequency of the AC voltage applied to the charging device. FIG. 6 is a schematic diagram showing an example of the relationship between the flow of print jobs and the frequency of the AC voltage applied to the charging device. FIG. 7 is a graph showing an example of the relationship between the frequency of the AC voltage applied to the charging device and the background density when the charging device charges the surface of the photosensitive member. FIG. 8 shows the separation distance between the charging device and the photoreceptor and the AC voltage applied to the charging device when the background density becomes higher than the threshold density T when the surface of the photoreceptor is charged by the charging device. 4 is a graph showing an example of the relationship with frequency; FIG. 9 is a flowchart illustrating an example of frequency switching processing by the control unit. FIG. 10 is a flowchart illustrating an example of second frequency setting processing by the control unit. FIG. 11 is a table showing the survey results of the life extension survey. FIG. 12 is a table showing the survey results of the life extension survey. FIG. 13 is a schematic diagram of another example photoreceptor charging system.

An example image forming apparatus includes a photoreceptor having a surface on which a toner image is formed, a charging device that charges the surface of the photoreceptor based on an alternating voltage, a transport path that transports the toner image, and a toner image on the transport path. and a controller for changing the frequency of the AC voltage applied to the charging device based on the measured toner density. In this image forming apparatus, the controller changes the frequency of the AC voltage applied to the charging device based on the toner concentration on the conveying path measured by the sensor. Moreover, the frequency of the AC voltage applied to the charging device can be lowered (reduced). As a result, the abrasion of the photoreceptor can be suppressed as compared with the case where the frequency of the AC voltage applied to the charging device is always high (large).

Another example of a photoreceptor charging system of an image forming apparatus includes a non-contact charging device that is spaced apart from a photoreceptor and charges the surface of the photoreceptor based on an alternating voltage; and a control unit that changes the frequency of the AC voltage applied to the charging device based on the electrostatic latent image. In this photoreceptor charging system, the controller changes the frequency of the AC voltage applied to the charging device based on the electrostatic latent image formed on the surface of the photoreceptor. The frequency of the AC voltage applied to the charging device can be lowered (smaller) when the electrostatic latent image to be applied does not include an image such as a photograph. As a result, the abrasion of the photoreceptor can be suppressed as compared with the case where the frequency of the AC voltage applied to the charging device is always high (large).

An example image forming apparatus will be described below with reference to the drawings. In the description based on the drawings, the same elements or similar elements having the same function are denoted by the same reference numerals, and overlapping descriptions are omitted.

FIG. 1 is a schematic diagram of an example image forming apparatus 1 . As shown in FIG. 1, an example image forming apparatus 1 is an apparatus that forms a color image using four color toners of magenta, yellow, cyan, and black. The image forming apparatus 1 includes a conveying device 10 , a plurality of photoreceptors 20 , a plurality of developing devices 30 , a transfer device 40 , a fixing device 50 , a discharging device 60 and a control section 70 .

The conveying device 10 conveys a sheet of paper M as a recording medium on which an image is formed. The multiple photoreceptors 20 have photoreceptors 20M, 20Y, 20C, and 20K. Each of the photoconductors 20M, 20Y, 20C, and 20K forms an electrostatic latent image for forming toner images of magenta, yellow, cyan, and black, respectively. Since the photoreceptors 20M, 20Y, 20C, and 20K have basically the same configuration, they will be collectively described as the photoreceptor 20 unless otherwise described separately. The photoreceptor 20 has a surface 21 on which an electrostatic latent image is formed (surrounding surface).

A developing device 30 , a charging device 22 , an exposure device 23 , and a cleaning unit 24 are provided on the periphery of the photoreceptor 20 . The charging device 22 is a charging roller provided in a non-contact manner with respect to the photoreceptor 20, and is a charging means for charging the surface of the photoreceptor 20 to a predetermined potential. The exposure device 23 exposes the surface 21 of the photoreceptor 20 charged by the charging device 22 according to an image to be formed on the paper M. FIG. As a result, an electrostatic latent image corresponding to the image to be formed on the paper M is formed on the surface 21 of the photoreceptor 20 . The cleaning unit 24 collects toner remaining on the photoreceptor 20 .

The multiple developing devices 30 include developing devices 30M, 30Y, 30C, and 30K. Developing devices 30M, 30Y, 30C, and 30K are arranged on respective circumferences of photoreceptors 20M, 20Y, 20C, and 20K, and static electricity is formed on the surfaces of photoreceptors 20M, 20Y, 20C, and 20K, respectively. The electrostatic latent image is developed to form a toner image. Since the developing devices 30M, 30Y, 30C, and 30K have basically the same configuration, they will be collectively described as the developing device 30 unless otherwise described separately.

The developing device 30 has a developing roller 31 that supplies toner to the surface 21 of the photoreceptor 20 . The electrostatic latent image formed on the surface 21 of the photoreceptor 20 is developed by the developing roller 31 supplying toner to the surface 21 of the photoreceptor 20 . As a result, a toner image corresponding to the electrostatic latent image, that is, a toner image corresponding to the image to be formed on the paper M is formed on the surface 21 of the photoreceptor 20 .

The transfer device 40 conveys the toner image developed by the developing device 30 and transfers it onto the sheet M. As shown in FIG. The transfer device 40 includes an intermediate transfer belt 41 for primarily transferring the toner image formed on the surface 21 of the photoreceptor 20 and secondarily transferring the toner image onto the sheet M onto which the toner image has been primarily transferred. A toner image is formed on the intermediate transfer belt 41 by primary transfer of the toner image from the surface 21 of the photoreceptor 20 to the intermediate transfer belt 41 . Therefore, each of the photoreceptor 20 and the intermediate transfer belt 41 constitutes a transport path for transporting the toner image.

The fixing device 50 heats and presses the paper M onto which the toner image has been transferred from the intermediate transfer belt 41, thereby fixing the toner image on the paper M to the paper M. As shown in FIG. The discharging device 60 discharges the paper M on which the toner image is fixed by the fixing device 50 to the outside of the apparatus.

The control unit 70 is an electronic control unit having a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), and the like. The control unit 70 loads a program stored in the ROM into the RAM and executes it by the CPU to perform various controls. The control section 70 may be composed of a plurality of electronic control units, or may be composed of a single electronic control unit. The control unit 70 performs various controls in the image forming apparatus 1 .

FIG. 2 is a schematic diagram of an example photoreceptor charging system 80 . As shown in FIGS. 1 and 2, example photoreceptor charging system 80 is a system for charging surface 21 of photoreceptor 20 . The photoreceptor charging system 80 includes a charging device 22 , a power supply 81 , a toner density sensor 82 and a controller 70 .

The charging device 22 is charging means for charging the surface 21 of the photoreceptor 20 to a predetermined potential. The charging device 22 is a non-contact charging device. The charging device 22 is separated from the photoreceptor 20 so as to form a gap G with the photoreceptor 20 . Photoreceptor 20 and charging device 22 extend in parallel. Therefore, the distance between the surface 21 of the photoreceptor 20 and the outer peripheral surface of the charging device 22 (that is, the length of the gap G in the facing direction between the photoreceptor 20 and the charging device 22) is It is set approximately constant. In order to ensure uniform charging of the photoreceptor 20, the length of the gap G in the facing direction between the photoreceptor 20 and the charging device 22 may be 10 [μm] or more and 100 [μm] or less. Since the charging device 22 is provided in a non-contact manner with respect to the photoreceptor 20 in this way, abrasion of the photoreceptor 20 due to friction is prevented.

A power supply 81 applies a voltage to the charging device 22 for charging the photoreceptor 20 . The voltage applied from the power supply 81 to the charging device 22 is a voltage in which a direct voltage (DC voltage) and an alternating voltage (AC voltage) are superimposed. When a voltage is applied to charging device 22 , discharge occurs between charging device 22 and photoreceptor 20 . This discharge causes an alternating current to flow from the charging device 22 to the photoreceptor 20 , and the portion of the surface 21 of the photoreceptor 20 facing the charging device 22 is charged. As the photoreceptor 20 rotates, the surface 21 of the photoreceptor 20 is charged all around.

The power supply 81 includes a frequency converter (not shown) and has the function of converting the frequency of the AC voltage applied to the charging device 22 . The frequency of the AC voltage is controlled by the controller 70 . Also, the DC component of the voltage applied from the power supply 81 to the charging device 22 has a negative bias voltage of -900 [V] to -300 [V], for example. On the other hand, the AC component of the voltage applied from the power supply 81 to the charging device 22 has, for example, a peak-to-peak voltage of 1500 [V] to 3000 [V]. When such a voltage is applied to the charging device 22, discharge occurs between the charging device 22 and the photoreceptor 20, and the portion of the surface 21 of the photoreceptor 20 facing the charging device 22 is charged.

The surface 21 of the photoreceptor 20 is charged by the charging device 22 while rotating the photoreceptor 20 in the rotation direction D. As shown in FIG. The surface 21 of the photoreceptor 20 charged by the charging device 22 is exposed to light output from the exposure device 23 . The exposure device 23 includes, for example, a laser beam scanning device, and scans a part of the surface 21 with a laser beam according to the image to be formed on the paper M. As shown in FIG. As a result, the potential of the portion of the surface 21 of the photoreceptor 20 exposed by the exposure device 42 changes, and an electrostatic latent image corresponding to the print data is formed on the surface 21 of the photoreceptor 20 . After that, toner is supplied from the developing roller 31 of the developing device 30 to the surface 21 of the photoreceptor 20 on which the electrostatic latent image is formed by the exposing device 23 . As a result, a toner image corresponding to the print data is formed on the surface 21 of the photoreceptor 20 .

The control unit 70 controls application of voltage to the charging device 22 by the power supply 81 . Also, the control unit 70 controls the frequency of the AC voltage that the power supply 81 applies to the charging device 22 . That is, the control unit 70 changes the frequency of the AC voltage that the power supply 81 applies to the charging device 22 .

FIG. 3 is a graph showing an example of the relationship between the AC voltage frequency and the wear rate of the photoreceptor 20 when the surface 21 of the photoreceptor 20 is charged by the non-contact charging device 22 . As shown in FIG. 3, the higher the frequency of the AC voltage applied to the charging device, the higher the wear rate of the photoconductor. The lower the frequency of the AC voltage applied to the charging device, the higher the wear rate of the photoconductor. getting low. This is because when the frequency of the AC voltage applied to the charging device increases, the AC current flowing from the charging device to the photoreceptor increases, thereby accelerating wear of the photoreceptor. It is believed that when the frequency is lowered, the amount of alternating current flowing from the charging device to the photoreceptor is reduced, thereby suppressing wear of the photoreceptor.

Therefore, the control unit 70 performs frequency switching processing in order to suppress wear of the photoreceptor 20 . The frequency switching process is a process of changing the frequency of the AC voltage applied to the charging device 22 between the first frequency FH and the second frequency FL. The second frequency FL is a frequency lower than the first frequency FH. In the frequency switching process, the control unit 70 controls the surface 21 of the photoreceptor 20 that forms an electrostatic latent image of a picture and a toner image in the rotation direction D of the photoreceptor 20 (conveying direction of the paper M: printing direction). , the frequency of the AC voltage applied to the charging device 22 is set to the first frequency FH. On the other hand, when the controller 70 charges the portion of the surface 21 of the photoreceptor 20 that does not form a toner image (electrostatic latent image) in the rotation direction D of the photoreceptor 20 (conveying direction of the paper M), The frequency of the AC voltage applied to the charging device 22 is set to the second frequency FL. Portions of surface 21 of photoreceptor 20 that do not form a toner image of an image include, for example, portions where text toner images are formed and portions where no toner images are formed.

A portion forming a toner image (electrostatic latent image) of the image on the surface 21 of the photoreceptor 20 in the rotation direction D of the photoreceptor 20 (conveying direction of the paper M: printing direction) is developed in, for example, a print memory. It can be specified by analyzing the print data.

For example, the print data developed in the print memory is divided into a plurality of divided areas in the printing direction, and the distribution of image density of pixels in each divided area is obtained. Each divided area is an area extending in the scanning direction perpendicular to the printing direction. Among the plurality of divided areas, an area where the image density distribution is represented by binary values of black and white is determined as an area having no image print data. On the other hand, among the plurality of divided areas, areas having various density distributions of three or more image density distributions are determined to be areas having image print data. Then, the surface 21 of the photoreceptor 20 corresponding to the area having the print data of the image is determined to be the part for forming the toner image (electrostatic latent image) of the image, and the photosensitive member corresponding to the area having no print data of the image is exposed. The surface 21 of the body 20 is determined to be the portion on which the toner image (electrostatic latent image) of the image is not formed. In this case, by increasing the number of divisions in the printing direction of the print data developed in the print memory or by narrowing the division width in the printing direction of the print data developed in the print memory, the surface 21 of the photoreceptor 20 It is possible to determine with high accuracy the portion of the image where the toner image (electrostatic latent image) is to be formed.

Further, for example, halftone dot or halftone data is extracted from the print data developed in the print memory. Then, a portion having halftone dot or halftone data is determined as an area having image print data, and a portion having no halftone dot or halftone data is determined as an area having no image print data. Then, the surface 21 of the photoreceptor 20 corresponding to the area having the print data of the image is determined to be the part for forming the toner image (electrostatic latent image) of the image, and the photosensitive member corresponding to the area having no print data of the image is exposed. The surface 21 of the body 20 is determined to be the portion on which the toner image (electrostatic latent image) of the image is not formed.

FIG. 4 is a diagram showing example print data 100 . The print data 100 shown in FIG. 4 is print data for one page. In the print data 100, from the beginning to the end of the page, there are a first non-picture containing portion 102 that does not contain the image 101, a picture containing portion 103 that contains the image 101, and a second non-picture containing portion 103 that does not contain the image 101. Portions 104 are arranged in this order. The first non-picture-containing portion 102 may include header margins, text, and the like. The image-containing portion 103 may include text or the like in addition to the image 101 . The second non-illustrated portion 104 may include footer margins, text, and the like.

FIG. 5 is a schematic diagram showing an example of the relationship between the toner image formed on the surface 21 of the photoreceptor 20 and the frequency of the AC voltage applied to the charging device 22. As shown in FIG. In FIG. 5, the surface 21 of the photoreceptor 20 is shown as being developed in a plane. 5 shows a case of printing the print data 100 shown in FIG. As shown in FIGS. 4 and 5, when printing the print data 100 shown in FIG. 4, an electrostatic latent image L corresponding to the print data 100 is formed on the surface 21 of the photoreceptor 20 . The electrostatic latent image L formed on the surface 21 of the photoreceptor 20 is formed along the rotation direction D of the photoreceptor 20 in a first non-image-containing printing area L1 corresponding to the first non-image-containing portion 102 of the print data 100. , an image-including print area L2 corresponding to the image-including portion 103 of the print data 100, and a second non-image-including print area L3 corresponding to the second non-image-including portion 104 of the print data 100. FIG. On the surface 21 of the photoreceptor 20, the upstream side of the electrostatic latent image L in the rotation direction D of the photoreceptor 20 is a pre-region L0 where the electrostatic latent image L is not formed. The downstream side of the electrostatic latent image L is a post area L4 where no electrostatic latent image is formed. Note that the pre-area L0 and the post-area L4 are areas charged by the charging device 22 .

Then, when charging the image-including printing area L2 of the surface 21 of the photoreceptor 20, the control unit 70 sets the frequency of the AC voltage to be applied to the charging device 22 to the first frequency FH. When charging the area other than L2, the frequency of the AC voltage applied to the charging device 22 is set to the second frequency FL. Areas other than the image-including printing area L2 are a pre-area L0, a first non-image-including printing area L1, a second non-image-including printing area L3, and a post area L4.

FIG. 6 is a schematic diagram showing an example of the relationship between the flow of print jobs and the frequency of the AC voltage applied to the charging device. FIG. 6 shows an example of the flow of a print job for printing two pages of print data on a sheet of paper M. As shown in FIG. In this print job, pre-processing J1, first-page printing J2, inter-page processing J3, second-page printing J4, and post-processing J5 are performed in this order. The pre-processing J1 is a preparatory process for executing a print job, and is performed during a period from when the control unit 70 receives print data to when the print job is started. The pre-processing J1 is also called a pre-rotation before starting a job. The inter-page process J3 is a process for waiting for the next page of paper M to be conveyed between pages. Post-processing J5 is a termination process for stopping the image forming apparatus 1, and is performed during a period from when the print job is finished until the image forming apparatus 1 is stopped. The post-processing J5 is also called a post-rotation after the end of the job. In pre-processing J1, inter-page processing J3, and post-processing J5, for example, without forming an electrostatic latent image and a toner image on the surface of the photoreceptor 20, the photoreceptor 20 is idly rotated, and the charging device 22 charges the photoreceptor. 20 surface is charged.

Two pages of print data to be printed by the print job shown in FIG. 6 are composed of a non-image-containing portion 111 that does not contain an image and an image-containing portion 112 that contains an image. The non-pictured portion 111 may include header margins, text, and the like. The picture-containing portion 112 may include text or the like in addition to the picture. Then, when charging the area corresponding to the image-containing portion 112 of the first page print J2 and the image-containing portion 112 of the second page print J4 on the surface 21 of the photoreceptor 20, the control unit 70 controls the charging device 22 is set to the first frequency FH. On the other hand, the control unit 70 charges the area of the surface 21 of the photoreceptor 20 that does not correspond to the image-containing portion 112 of the first page print J2 and the image-containing portion 112 of the second page print J4. 22 is set to the second frequency FL. Areas that do not correspond to the image-containing portion 112 of the first page printing J2 and the image-containing portion 112 of the second page printing J4 are the pre-processing J1, the non-image-containing portion 111 of the first page printing J2, and the inter-page processing J3. , the non-image-containing portion 111 of the printing J4 of the second page, and the area corresponding to the post-processing J5.

By the way, referring to FIG. 3, the lower the frequency of the AC voltage applied to the charging device, the more the abrasion of the photoreceptor 20 is suppressed. However, background contamination can occur if the frequency of the AC voltage applied to the charging device is too low. Background contamination is smearing of the non-printing portion (background) of the paper M with unwanted toner. Background contamination is, for example, the phenomenon in which the entire sheet M, or a partial area of the sheet M, becomes gray with unwanted toner.

Therefore, the relationship between the frequency of the AC voltage applied to the charging device 22 and the background density when the surface 21 of the photosensitive member 20 is charged by the charging device 22 was investigated. The background density is one of the evaluation indexes of background contamination, and is the toner density of the area of the intermediate transfer belt 41 where no toner image is formed. In this investigation, non-exposure development was performed while the frequency of the AC voltage applied to the charging device 22 was changed stepwise, and the toner concentration sensor 82 (see FIGS. 1 and 2) measured the background concentration on the downstream side of the photoreceptor 20. , the toner density of the intermediate transfer belt 41 was measured. The toner concentration sensor 82 is a sensor arranged near the intermediate transfer belt 41 on the downstream side of the photoreceptor 20 to measure the toner concentration of the intermediate transfer belt 41 on the downstream side of the photoreceptor 20 . In non-exposure development, the surface 21 of the photoreceptor 20 is charged by the charging device 22 , but the surface 21 of the photoreceptor 20 is not exposed by the exposure device 23 . This is the process of supplying toner to the . In non-exposure development, no electrostatic latent image is formed on the surface 21 of the photoreceptor 20 , so no toner image is formed on the surface 21 of the photoreceptor 20 . Therefore, the toner density sensor 82 measures the toner density of the area of the intermediate transfer belt 41 where no toner image is formed.

Further, the distance between the charging device 22 and the photoreceptor 20 increases as the photoreceptor 20 wears. Therefore, the above background density was measured for three cases in which the distance between the charging device 22 and the photosensitive member 20 was 21 [μm], 50 [μm], and 98 [μm]. An example in which the distance between the charging device 22 and the photoreceptor 20 is 21 [μm] is an example modeling a state in which the photoreceptor 20 is not worn. This is an example of modeling a state in which the body 20 is worn, and the example in which the separation distance is 98 [μm] is an example of modeling a state in which the photoreceptor 20 is considerably worn. The measurement results are shown in FIGS. 7 and 8. FIG. FIG. 7 is a graph showing an example of the relationship between the frequency of the AC voltage applied to the charging device 22 and the background density when the charging device 22 charges the surface 21 of the photoreceptor 20 . 8 shows the distance between the charging device 22 and the photoreceptor 20 and the background density of the charging device 22 when the surface 21 of the photoreceptor 20 is charged by the charging device 22 and the background density becomes higher than the threshold density T. 2 is a graph showing an example of the relationship between the frequency of the AC voltage applied to . The threshold concentration T is a threshold appropriately set for determining whether background contamination is determined. Note that FIG. 7 shows only two examples in which the separation distance between the charging device and the photoreceptor is 21 [μm] and 98 [μm] in order to make the graph easier to see.

As shown in FIG. 7, when the frequency of the AC voltage applied to the charging device 22 is lowered, the background density increases. higher than T. Also, as shown in FIGS. 7 and 8, as the separation distance between the charging device 22 and the photoreceptor 20 increases, the background density becomes higher than the threshold density T. Voltage frequency increases.

Therefore, the controller 70 changes the frequency of the AC voltage applied to the charging device 22 based on the background density measured by the toner density sensor 82 . In this example, the controller 70 changes the second frequency FL based on the background density measured by the toner density sensor 82 .

The control unit 70 performs a second frequency setting process. The second frequency setting process is a process of changing the second frequency FL based on the background density measured by the toner density sensor 82 . In the second frequency setting process, the control unit 70 sets the second frequency FL within a frequency range in which the toner density of the area of the intermediate transfer belt 41 on which no toner image is formed is equal to or lower than the threshold density T. In this case, the control unit 70 may set the second frequency FL to the minimum frequency (lowest frequency) in the frequency range.

The control unit 70 acquires the background density measured by the toner density sensor 82 when the non-exposure development is performed as the toner density of the area of the intermediate transfer belt 41 where no toner image is formed. Then, the control unit 70 sets the second frequency FL within a frequency range in which the background density measured by the toner density sensor 82 is the threshold density T or less. In this case, the control unit 70 may set the second frequency FL to the minimum frequency in the frequency range in which the background density measured by the toner density sensor 82 is equal to or lower than the threshold density T. For example, the control unit 70 may perform non-exposure development while changing the frequency of the AC voltage applied to the charging device 22 in stages, and measure the background density with the toner density sensor 82 . Then, the control unit 70 sets the second frequency FL to any frequency within a frequency range in which the background density measured by the toner density sensor 82 is equal to or lower than the threshold density T, among the plurality of frequencies that are changed stepwise. can be set to In this case, the control unit 70 sets the second frequency FL to the minimum frequency in the frequency range in which the background density measured by the toner density sensor 82 is equal to or lower than the threshold density T, among the plurality of frequencies that are changed stepwise. can be set to

Further, even after the second frequency FL is once set, when the abrasion of the photoreceptor 20 progresses and the separation distance between the charging device 22 and the photoreceptor 20 increases, the toner density measured by the toner density sensor 82 reaches the threshold density. may exceed T. Therefore, the control unit 70 performs the second frequency setting process at the initial setting of the image forming apparatus 1, periodically, or at any timing. Then, the control unit 70 prevents the background density measured by the toner density sensor 82 from exceeding the threshold density T when the background density measured by the toner density sensor 82 exceeds the threshold density T. Increase the frequency FL.

FIG. 9 is a flowchart showing an example of frequency switching processing by the control unit 70. As shown in FIG. The frequency switching process shown in FIG. 9 is, for example, repeatedly performed after the control unit 70 receives print data until printing of the print data is completed. As shown in FIG. 9, in the frequency switching process, when the control unit 70 receives the print data, the surface of the photoreceptor 20 forming a toner image (electrostatic latent image) of the image in the rotation direction D of the photoreceptor 20 It is determined whether or not the portion 21 is to be charged (step S1). Whether or not to charge the portion of the surface 21 of the photoreceptor 20 that forms the toner image (electrostatic latent image) of the image can be determined, for example, by analyzing the print data developed in the print memory. When it is determined that the portion of the surface 21 of the photoreceptor 20 that forms the toner image (electrostatic latent image) of the image is to be charged (step S1: YES), the control unit 70 sets the frequency of the AC voltage applied to the charging device 22 to A voltage is applied to the charging device 22 by setting the first frequency FH (step S2). On the other hand, when it is determined that the portion of the surface 21 of the photoreceptor 20 that does not form the toner image (electrostatic latent image) of the image is to be charged (step S1: NO), the control unit 70 changes the AC voltage applied to the charging device 22. A frequency is set to a second frequency FL that is lower than the first frequency FH (step S3), and a voltage is applied to the charging device 22. FIG.

FIG. 10 is a flowchart showing an example of the second frequency setting process by the control section 70. As shown in FIG. The second frequency setting process shown in FIG. 10 is performed at initial setting of the image forming apparatus 1, periodically, or at arbitrary timing. As shown in FIG. 10, in the second frequency setting process, the controller 70 performs non-exposure development while changing the frequency of the AC voltage applied to the charging device 22 stepwise, and the toner density sensor 82 detects the background density. Measure (step S11). Next, the control unit 70 specifies a frequency range in which the background density measured by the toner density sensor 82 is equal to or less than the threshold density T, among the plurality of frequencies that are changed stepwise (step S12). Then, the control unit 70 sets the second frequency FL to the lowest frequency in the identified frequency range (step S13).

Thus, in the image forming apparatus 1 of this example, the controller 70 controls the frequency of the AC voltage applied to the charging device 22 based on the toner density on the intermediate transfer belt 41 measured by the toner density sensor 82 . change at least the second frequency FL of Therefore, when the toner density on the intermediate transfer belt 41 is low, at least the second frequency FL among the frequencies of the AC voltage applied to the charging device can be lowered. As a result, wear of the photoreceptor 20 can be suppressed as compared with the case where the frequency of the AC voltage applied to the charging device 22 is always high.

Further, the image forming apparatus 1 of this example includes a non-contact charging device 22 that is spaced apart from the photoreceptor 20 and charges the surface 21 of the photoreceptor 20 based on an AC voltage. The frequency of the AC voltage applied to the charging device 22 is changed based on the electrostatic latent image formed on the surface 21 of the photoreceptor 20 . Therefore, when the electrostatic latent image formed on the surface 21 of the photoreceptor 20 does not include an image such as a photograph, the frequency of the AC voltage applied to the charging device 22 can be lowered. As a result, wear of the photoreceptor 20 can be suppressed as compared with the case where the frequency of the AC voltage applied to the charging device 22 is always high.

Further, in the image forming apparatus 1 of this example, the control section 70 changes the second frequency FL based on the background density measured by the toner density sensor 82 by the second frequency setting process. That is, the control unit 70 sets the second frequency FL within a frequency range in which the background density is equal to or lower than the threshold density T by the second frequency setting process. Therefore, it is possible to appropriately suppress abrasion of the photoreceptor 20 while suppressing occurrence of background contamination due to abrasion of the photoreceptor 20 or the like.

Here, when the frequency switching process for changing the frequency of the AC voltage applied to the charging device 22 between the first frequency FH and the second frequency FL and the second frequency setting process for changing the second frequency FL are performed. , extension of the life of the photoreceptor 20 was investigated. In this investigation, using three types of print data with a ratio of image to text: image: text = 0: 100, image: text = 10: 90, image: text = 20: 80, The life of the photoreceptor 20 was measured. FIG. 11 is a table showing the conditions of Examples 1-3. As shown in FIG. 11, in Example 1, neither the frequency switching process nor the second frequency setting process is performed, and the frequency of the AC voltage applied to the charging device 22 is kept constant at the first frequency FH. Example 2 is a case where the frequency switching process is performed but the second frequency setting process is not performed. Example 3 is a case where both the frequency switching process and the second frequency setting process are performed. Then, using the life of the photoreceptor 20 in Example 1 as a reference life, the life extension rate of the life of the photoreceptor 20 in Examples 2 and 3 with respect to the reference life was calculated. Calculation results are shown in FIG.

As shown in FIGS. 11 and 12, in Examples 2 and 3 in which the frequency switching process was performed, the life of the photoreceptor 20 was significantly extended compared to Example 1 in which the frequency switching process was not performed. Also, in Examples 2 and 3, the life of the photoreceptor 20 was significantly increased as the ratio of text to stroke increased. Furthermore, in Example 3 in which the frequency switching process was performed, the life of the photoreceptor 20 was significantly extended compared to Example 3 in which the frequency switching process was not performed.

It is to be understood that not necessarily all aspects, advantages and features described herein will be achieved or included in any one particular example and embodiment. Indeed, while various examples have been described and shown herein, it should be apparent that other examples may be modified in their arrangement and details. I claim all modifications and variations coming within the spirit and scope of the claimed subject matter.

For example, in the above example, the controller 70 changes the second frequency FL based on the background density measured by the toner density sensor 82 . However, the controller 70 may change not only the second frequency FL but also the first frequency FH based on the background density measured by the toner density sensor 82 .

For example, in the above example, the background density measured by the toner density sensor 82 when non-exposure development is performed is described as the toner density of the area of the intermediate transfer belt 41 where no toner image is formed. However, even in the case of non-exposure development, if an area in which no toner image is formed is known, the toner density measured in that area by the toner density sensor 82 can be used to determine whether the toner image is not formed on the intermediate transfer belt 41 . It may be the toner density of the area.

For example, in the above example, the charging device 22 is described as being of a non-contact type that is separated from the photoreceptor 20 . However, like the charging device 22A of the photoreceptor charging system 80A of another example shown in FIG.

For example, in the above example, the toner concentration sensor 82 has been described as measuring the toner concentration on the intermediate transfer belt 41 . However, as described above, not only the intermediate transfer belt 41 but also the photoreceptor 20 constitute the transport path for transporting the toner image. Therefore, the toner density sensor 82 may measure the toner density on the photoreceptor 20 instead of the toner density on the intermediate transfer belt 41 as the toner density on the transport path for transporting the toner image. Both the toner density on 41 and the toner density on photoreceptor 20 may be measured.

For example, in the above example, the image forming apparatus has been described as an apparatus that forms a color image using four color toners of magenta, yellow, cyan, and black. However, the image forming apparatus may be an apparatus that forms a monochromatic image using a monochromatic toner. Here, an image forming apparatus that forms a monochrome image, for example, does not have an intermediate transfer belt, and directly transfers a toner image from a photoreceptor onto a sheet. However, as described above, the photoreceptor also constitutes the transport path for transporting the toner image. For this reason, in an apparatus that does not have an intermediate transfer belt, such as an image forming apparatus that forms a monochrome image, the toner density sensor measures the toner density on the photoreceptor as the toner density on the transport path for transporting the toner image. may

Claims (15)

a photoreceptor having a surface for forming a toner image;
a charging device that charges the surface of the photoreceptor based on an alternating voltage;
a transport path for transporting the toner image;
a sensor for measuring the toner density on the conveying path;
a control unit that changes the frequency of the AC voltage applied to the charging device based on the measured toner density;
Image forming device. the conveying path has an intermediate transfer belt onto which the toner image is transferred from the photoreceptor;
the sensor measures the toner concentration on the intermediate transfer belt;
The image forming apparatus according to claim 1. The sensor measures the toner concentration in a region of the transport path where no toner image is formed.
The image forming apparatus according to claim 1. an exposure device for forming an electrostatic latent image on the surface of the charged photoreceptor for forming the toner image on the photoreceptor;
The control unit changes the frequency of the AC voltage from a first frequency to a second frequency lower than the first frequency, based on the electrostatic latent image formed by the exposure device.
The image forming apparatus according to claim 1. wherein the control unit changes the second frequency based on the measured toner concentration;
The image forming apparatus according to claim 4. the control unit increases the second frequency when the measured toner concentration exceeds a threshold concentration;
The image forming apparatus according to claim 5. The control unit sets the second frequency within a frequency range in which the toner density in a region of the transport path where no toner image is formed is equal to or less than a threshold density.
The image forming apparatus according to claim 5. The control unit sets the second frequency to the minimum frequency in the frequency range.
The image forming apparatus according to claim 7. the frequency of the AC voltage is set to the first frequency when there is an image in the toner image to be formed;
the frequency of the alternating voltage is set to the second frequency when the portion of the toner image to be formed is empty or no toner image is formed;
The image forming apparatus according to claim 4. The charging device is spaced apart from the photoreceptor to form a gap therebetween, and the second frequency is dependent on the size of the gap between the charging device and the photoreceptor. is set by the control unit at
The image forming apparatus according to claim 5. The control unit increases the second frequency as the gap between the charging device and the photoreceptor increases.
The image forming apparatus according to claim 10. a non-contact charging device that is spaced apart from the photoreceptor and charges the surface of the photoreceptor based on an alternating voltage;
a control unit that changes the frequency of the AC voltage applied to the charging device based on the electrostatic latent image formed on the surface of the photoreceptor;
A photoreceptor charging system for an image forming apparatus. The control unit acquires the toner density of the conveying path when no toner image is formed on the conveying path for conveying the toner image,
wherein the control unit changes the frequency of the AC voltage applied to the charging device based on the acquired toner concentration;
13. The photoreceptor charging system of claim 12. the transport path has an intermediate transfer belt that transports the toner image to be printed;
the control unit acquires the toner concentration on the intermediate transfer belt;
14. The photoreceptor charging system of claim 13. The control unit changes the frequency of the AC voltage from a first frequency to a second frequency lower than the first frequency based on the electrostatic latent image formed by the exposure device,
the control unit increases the second frequency when the obtained toner concentration is equal to or higher than a threshold concentration;
14. The photoreceptor charging system of claim 13.

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