JP2015090418A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus configured to prevent a dot-like white spot from being conspicuous in an image on the whole.SOLUTION: An image forming apparatus (color printer 1) which transfers a toner image formed on a photoreceptor drum 61 to a sheet S, to form an image on the sheet S has a first mode (normal-printing mode) for forming an image with first resolution, and a second mode (low-resolution mode) for forming an image with second resolution lower than the first resolution. The image forming apparatus forms an image in the second mode when a white-spot generation condition of generating a dot-like white spot in an image is satisfied.

Description

  The present invention relates to an image forming apparatus that forms an image on a recording sheet by transferring a developer image formed on a photoreceptor to the recording sheet.

  In general, a photosensitive drum, a charging device for charging the photosensitive drum, an exposure device for exposing the charged photosensitive drum, and supplying toner to the exposed photosensitive drum to form a toner image on the photosensitive drum. There is known an image forming apparatus that includes a developing roller or the like and forms an image on a recording sheet by transferring a toner image formed on a photosensitive drum onto a recording sheet such as paper (see, for example, Patent Document 1). .

Japanese Patent No. 4699810

  By the way, in the image forming apparatus, for example, when a deposit such as paper dust or an external additive of toner adheres to the surface of the photosensitive drum, the toner that should originally be placed on the portion where the deposit has adhered is not loaded. Thus, when the toner image on the photosensitive drum is transferred to the recording sheet, dotted white spots may appear in the image on the recording sheet. Such dot-like white spots are particularly noticeable in an image including a solid-colored portion.

  The present invention has been made in view of the above background, and an object of the present invention is to provide an image forming apparatus capable of making dot-like white spots inconspicuous as a whole image.

In order to achieve the above object, an image forming apparatus of the present invention is an image forming apparatus that forms an image on a recording sheet by transferring a developer image formed on a photoreceptor to the recording sheet. A first mode for forming an image with a resolution and a second mode for forming an image with a second resolution lower than the first resolution can be executed.
The image forming apparatus forms an image in the second mode when a white spot generation condition that can cause a spot white spot in the image is satisfied.

  According to such a configuration, the image is formed in the second mode with a low resolution when the white spot generation condition that can cause dot white spots in the image is satisfied, so that the image is formed particularly when the print duty is high. As a whole, the dotted white spots can be made inconspicuous.

  Here, in the present specification, the white spot is a portion on the recording sheet where the toner is supposed to be placed (a portion where a part of the image is to be formed), but the toner is not placed at all. A portion where the background color of the recording sheet appears, or a portion whose color is lighter than the original color due to the amount of toner that is supposed to be placed on the recording sheet not being normally loaded even if the toner is placed. For this reason, the white portions are not always white.

  In the above-described image forming apparatus, the whiteout occurrence condition may include a condition that the humidity is equal to or lower than the first threshold value.

  In the above-described image forming apparatus, the white spot occurrence condition may include a condition that a parameter relating to the usage time of the photosensitive member is equal to or greater than a second threshold value. The parameter relating to the usage time of the photosensitive member is, for example, the total number of rotations of the photosensitive member that is rotatably provided.

  The image forming apparatus includes a charging device for charging the photosensitive member, and a developer carrying member for supplying a developer to the photosensitive member to form a developer image on the photosensitive member. The white spot generation condition may include a condition that an absolute value of a difference between a charging bias applied to the charging device and a developing bias applied to the developer carrier is equal to or more than a third threshold value.

  The image forming apparatus described above may be configured to form an image in the second mode when a white spot occurrence condition is satisfied on condition that a signal permitting execution of control for improving white spot is received. it can.

  According to the present invention, dotted white spots can be made inconspicuous as a whole image.

1 is a diagram illustrating a schematic configuration of a color printer as an example of an image forming apparatus. It is explanatory drawing of an example of the mechanism in which white spots generate | occur | produce in the image on paper, and is a figure (a)-(e) which shows the case of the high humidity environment where white spots do not occur easily. It is explanatory drawing of an example of the mechanism in which white spots generate | occur | produce in the image on paper, and is a figure (a)-(e) which shows the case of the low humidity environment where white spots are easy to generate | occur | produce. FIG. 4 is an enlarged view of a part of an image formed on a sheet shown as an example, and is a diagram showing a case of a normal print mode and a diagram (b) showing a case of a low resolution mode. It is a block diagram of a color printer. It is an explanatory diagram that white spots are likely to occur when the difference between the charging bias and the developing bias is large. It is a flowchart which shows the control in the control apparatus in 1st Embodiment. It is a flowchart which shows the control in the control apparatus in 2nd Embodiment. It is a flowchart which shows the control in the control apparatus in 3rd Embodiment.

[First Embodiment]
Next, the first embodiment will be described in detail with reference to the drawings as appropriate. In the following description, the direction is based on a user who uses the color printer 1 as an example of the image forming apparatus. Specifically, the left side of FIG. 1 as viewed from the user is “front”, the right side of FIG. 1 as viewed from the user is “rear”, and the front side of FIG. 1 is “right”. ”, The left side of the page is“ left ”. Further, the vertical direction in FIG.

  As shown in FIG. 1, the color printer 1 forms an image on a sheet S by transferring a toner image (developer image) formed on a photosensitive drum 61 (described later) onto the sheet S as an example of a recording sheet. The main body housing 2 mainly includes a paper feeding unit 3, an image forming unit 4, and a control device 10.

  The paper feed unit 3 is configured to supply the paper S to the image forming unit 4, and is provided in the lower part in the main body housing 2. The paper feed unit 3 mainly includes a paper feed tray 31 that stores the paper S, a paper pressing plate 32, and a paper feed mechanism 33. The paper feed unit 3 brings the paper S in the paper feed tray 31 upward by the paper pressing plate 32, separates the paper S one by one by the paper feed mechanism 33, and supplies the separated image to the image forming unit 4.

  The image forming unit 4 is configured to form an image on the paper S supplied from the paper supply unit 3, and mainly includes an exposure unit 5, a process unit 6, a transfer unit 7, and a fixing unit 8. ing.

  The exposure unit 5 is provided in the upper part of the main body housing 2 and includes a plurality of laser light sources (not shown) corresponding to each color of yellow, magenta, cyan, and black, a polygon mirror, a plurality of lenses, a plurality of reflecting mirrors, and the like. ing. The exposure unit 5 exposes the surface of the charged photosensitive drum 61 by scanning the surface of each photosensitive drum 61 at a high speed with a plurality of light beams (see chain lines) emitted based on the image data.

  The process unit 6 is disposed between the paper feed tray 31 and the exposure unit 5, and corresponds to the holder 60, a photoconductor drum 61 as an example of four photoconductors arranged in the front-rear direction, and each photoconductor drum 61. The charger 62 and the developing cartridge 63 are mainly provided one by one.

  The holder 60 is a member that holds the photosensitive drum 61 and the like, and is configured to be detachable from the main body housing 2 through an opening that is formed when the front cover 21 provided in the main body housing 2 is opened. Thereby, the photosensitive drum 61 can be replaced by replacing the holder 60. The holder 60 supports the developing cartridges 63 in a detachable manner. Thereby, the developing cartridge 63 can be individually replaced in a state where the holder 60 is pulled out from the inside of the main body housing 2.

The photosensitive drum 61 is a member in which a photosensitive layer 61B is formed on the outer peripheral surface of a cylindrical drum body 61A having conductivity and the drum body 61A is grounded (see FIG. 2 and the like), and an arrow in FIG. It is provided to be rotatable in the direction indicated by.
The charger 62 is an example of a charging device, and includes a charging wire, a grid electrode, and the like that are not shown. The charger 62 generates corona discharge by applying a positive charging bias to uniformly positively charge the surface of the photosensitive drum 61.

  Each of the developing cartridges 63 includes a developing roller 64 as an example of a developer carrier, a supply roller 65, a layer thickness regulating blade 66, and a toner storage unit 67 that stores positively chargeable toner as an example of a developer. And mainly. The developing roller 64 is a roller configured to be able to carry toner, and supplies toner to the exposed photosensitive drum 61 by applying a positive developing bias to form a toner image on the photosensitive drum 61.

  The transfer unit 7 is provided between the paper feed tray 31 and the process unit 6, and mainly includes a driving roller 71, a driven roller 72, an endless conveyance belt 73, and four transfer rollers 74. . The conveyor belt 73 is stretched between the driving roller 71 and the driven roller 72, and the respective photosensitive drums 61 are arranged to face each other. The transfer roller 74 is disposed to face the photoconductive drum 61 with the conveying belt 73 interposed therebetween, and the toner on the photoconductive drum 61 is transferred to the sheet S conveyed between the photoconductive drum 61 by applying a negative transfer bias. Transfer the image.

  The fixing unit 8 is provided behind the process unit 6 and the transfer unit 7 and mainly includes a heating roller 81 and a pressure roller 82. The fixing unit 8 heat-fixes the toner image on the sheet S conveyed between the heating roller 81 and the pressure roller 82.

  The image forming unit 4 forms an electrostatic latent image on the photosensitive drum 61 by uniformly and positively charging the surface of the rotating photosensitive drum 61 with the charger 62 and then exposing with the exposure unit 5. To do. Further, the toner in the toner container 67 is supplied to the developing roller 64 via the supply roller 65 and is carried on the developing roller 64 as a thin layer having a constant thickness between the developing roller 64 and the layer thickness regulating blade 66. Let In this process, the toner is positively frictionally charged between the developing roller 64 and the supply roller 65 or between the developing roller 64 and the layer thickness regulating blade 66. The image forming unit 4 supplies the toner on the developing roller 64 to the electrostatic latent image formed on the photosensitive drum 61 to visualize the electrostatic latent image, and the toner image on the photosensitive drum 61. Form.

  Thereafter, the image forming unit 4 carries the paper S supplied from the paper feeding unit 3 onto the conveying belt 73 between the photosensitive drum 61 and the transfer roller 74, so that the image forming unit 4 is carried on the photosensitive drum 61. The transferred toner image is transferred onto the paper S. Then, the sheet S on which the toner image is transferred is conveyed between the heating roller 81 and the pressure roller 82 to thermally fix the toner image. The sheet S on which the toner image is thermally fixed, that is, the sheet S on which the image is formed is discharged onto the sheet discharge tray 22 by the transport roller 23 and the discharge roller 24.

  The control device 10 is a device for controlling the operation of the color printer 1 by controlling the paper feed unit 3, the image forming unit 4, and the like, and is provided at an appropriate position in the main body housing 2. The control device 10 includes a central processing unit (CPU), a random access memory (RAM), a read only memory (ROM), an input / output interface, and the like (not shown), and includes preset programs and sensor detection results. Control is executed by performing arithmetic processing based on the above.

  Here, an example of a mechanism that causes white spots in the image on the paper S in the color printer 1 will be described. 2 and 3 to be referred to are schematic diagrams for explaining an example of a mechanism that causes white spots. For example, the thickness of the drum body 61A and the photosensitive layer 61B, the light flux B, and the toner T The diameter does not reflect the actual size.

  As shown in FIG. 2A and FIG. 3A, the adhering matter D such as paper dust generated from the paper S or an external additive peeled from the toner on the surface of the photosensitive drum 61 (photosensitive layer 61B). When the surface of the photosensitive drum 61 is charged by the charger 62, the charged surface of the photosensitive drum 61 is positively charged substantially uniformly including the portion where the deposit D is attached.

  Next, as shown in FIGS. 2B and 3B, the surface of the photosensitive drum 61 is exposed with the light beam B from the exposure unit 5. At this time, as shown in FIG. 2 (b), in the case of a high humidity environment, it is considered that the resistance value of the deposit D is lowered due to moisture absorption or the like. When the periphery is exposed, the potential of the portion to which the deposit D is adhered is such that positive charges are scattered from the surface of the deposit D so that the potential of the portion to which the deposit D adheres follows the surrounding potential. descend. As a result, as shown in FIG. 2C, after exposure, the potential of the portion to which the deposit D has adhered becomes substantially the same as that of the surrounding exposed portion.

  When toner is supplied from the developing roller 64 to the photosensitive drum 61 in this state, as shown in FIG. 2D, the toner T is supplied to the entire exposed portion including the portion to which the deposit D has adhered as shown in FIG. Is done. Thereafter, when the toner image on the photosensitive drum 61 is transferred to the paper S, white spots do not occur in the image formed on the transferred paper S as shown in FIG. , White spots are unlikely to occur).

  On the other hand, as shown in FIG. 3B, in the case of a low humidity environment, the resistance value of the deposit D is considered to be high, so even if the periphery including the portion to which the deposit D is attached is exposed, As shown in FIG. 3C, after the exposure, the electric potential of the portion where the adhering material D adheres is higher than the surrounding exposed portion, as shown in FIG.

  Even if toner is supplied from the developing roller 64 to the photosensitive drum 61 in this state, the potential of the portion where the deposit D has adhered is high, so that the deposit D even after development as shown in FIG. The toner T is hardly supplied to the portion where the toner adheres. In this state, when the toner image on the photosensitive drum 61 is transferred to the paper S, the image on the photosensitive drum 61 is attached to the image on the paper S after the transfer, as shown in FIG. Dot-like white spots SP corresponding to the portion to which the kimono D is attached will occur.

  Thus, in the color printer 1, it is considered that white spots SP are likely to occur in the image on the paper S particularly in a low humidity environment with the deposit D attached to the surface of the photosensitive drum 61. The size of the deposit D that adheres to the photosensitive drum 61 is approximately 200 μm or less.

Hereinafter, the characteristic part of the invention will be described in detail.
The color printer 1 is controlled by the control device 10 so that the normal printing mode as an example of the first mode for forming an image on the paper S at the first resolution and the second lower than the first resolution. The low-resolution mode as an example of the second mode for forming an image on the paper S at the resolution can be executed.

  As shown in FIGS. 4A and 4B, the image formed on the sheet S includes the minimum elements E1 and E2 indicated by hatching with toner and minimum elements E1 and E2 without toner. It is comprised by the combination. As shown in FIG. 4A, the normal print mode is a mode in which an image is formed on the paper S with the minimum element E1 being 2 × 2 dots, and the low resolution mode is as shown in FIG. 4B. In this mode, the minimum element E2 is 4 × 4 dots and an image is formed on the paper S. As an example, the resolution (first resolution) in the normal print mode can be 600 dpi, and the resolution (second resolution) in the low resolution mode can be 300 dpi. Here, the dots DT shown as one square mass in FIGS. 4A and 4B are the minimum units constituting the minimum elements E1 and E2, and are exposed by the light flux emitted from the exposure unit 5. The smallest possible size, in other words, a size approximately equal to the diameter of the light beam imaged on the surface of the photosensitive drum 61.

  In the present embodiment, the color printer 1 is configured to change the developing bias applied to the developing roller 64 in accordance with the deterioration of the toner in the developing cartridge 63, specifically, the deterioration of the charging performance of the toner. ing. When the charging performance of the toner is reduced, the positive charge of the toner after frictional charging is reduced, so that the toner is easily placed on the photosensitive drum 61. As a result, a larger amount of toner is placed on the photosensitive drum 61 than before the charging performance is deteriorated, so that a phenomenon occurs in which the density of the image transferred onto the paper S becomes higher. Therefore, since the color printer 1 makes the image density uniform regardless of the toner charging performance, the toner charging performance is lowered, specifically, the elapsed time since the preset development cartridge 63 is replaced with a new one. Accordingly, the developing bias is lowered.

As shown in FIG. 5, the color printer 1 includes a humidity sensor 9.
The humidity sensor 9 is a sensor for detecting the humidity in the main body housing 2 and is provided at an appropriate position in the color printer 1 (main body housing 2). The humidity H in the main body housing 2 detected by the humidity sensor 9 is output to the control device 10.

  The control device 10 is configured to be able to execute white spot improvement control for forming an image in the low resolution mode when a white spot occurrence condition that may cause dot-like white spots (image defects) in the image is satisfied. . More specifically, the white spot improvement control mainly includes a condition determination step (first step) and an image formation step (second step). The control device 10 mainly includes a condition determining unit 11 that executes a condition determining step, an image forming unit 12 that executes an image forming step, and a storage unit 19.

  The condition determining step executed by the condition determining means 11 is a step of determining whether or not a white spot occurrence condition is satisfied. Specifically, the whiteout occurrence condition is that the humidity H in the main body housing 2 is equal to or lower than the first threshold value Hth, and that the total number of rotations NR of the photosensitive drum 61 is equal to or higher than the second threshold value NRth. The condition determination step includes a condition and a condition that the absolute value of the difference VD between the charging bias Vg applied to the charger 62 and the developing bias Vb applied to the developing roller 64 is equal to or greater than a third threshold value VDth. The humidity H is equal to or lower than the first threshold value Hth, the total number of rotations NR is equal to or higher than the second threshold value NRth, and the absolute value of the difference VD between the charging bias Vg and the developing bias Vb is the third threshold value. This is a step for determining that the white spot occurrence condition is satisfied when the voltage is equal to or higher than VDth.

The humidity H in the main body housing 2 is acquired from the humidity sensor 9. For example, the first threshold Hth can be set to 30% RH or the like.
The reason why the whiteout occurrence condition includes the condition that the humidity H is equal to or lower than the first threshold value Hth (H ≦ Hth) is because, as described above, whiteout is likely to occur in a low humidity environment. It is.

  The total number of rotations NR of the photosensitive drum 61 is an example of a parameter related to the usage time of the photosensitive drum 61. More specifically, the total number of rotations of the photosensitive drum 61 after the photosensitive drum 61 is replaced with a new one. Means. A specific method for acquiring the total number of rotations NR is not particularly limited. For example, the total number of rotations NR may be acquired from a sensor or the like that counts the total number of rotations NR. The total number of rotations NR may be calculated and acquired from information on the number of rotations per unit time set in advance.

  In the condition determining step of the present embodiment, the case where the total number of rotations NR is equal to or greater than the second threshold NRth means that when forming a color image using the four-color photosensitive drum 61, the four-color photosensitive drum. 61, when at least one total number of rotations NR is equal to or greater than the second threshold NRth. When a monochrome image is formed using only the black photosensitive drum 61, the black photosensitive drum 61 This is a case where the total number of rotations NR is equal to or greater than the second threshold NRth. The second threshold value NRth can be set to 10,000 rotations as an example. The second threshold value NRth may be the same value among the four color photosensitive drums 61, or may be a different value.

  The reason why the whiteout occurrence condition includes the condition that the total number of rotations NR is equal to or greater than the second threshold NRth (NR ≧ NRth) is that the total number of rotations NR increases, in other words, the usage time of the photosensitive drum 61. This is because if the length of the photosensitive drum 61 becomes longer, deposits such as paper dust adhere to the surface of the photosensitive drum 61 and white spots are likely to occur.

  The difference VD between the charging bias Vg and the developing bias Vb is such that the developing bias is lowered according to the elapsed time after the color printer 1 replaces the developing cartridge 63 with a new one. Increases accordingly.

  In the condition determining step of the present embodiment, the case where the difference VD is equal to or larger than the third threshold value VDth means that when four color photosensitive drums 61 are used to form a color image, four sets of chargers 62 and developing units are used. This is a case where the difference VD between at least one set of the charging bias Vg and the developing bias Vb of the roller 64 is equal to or greater than the third threshold value VDth, and a monochrome image is formed using only the black photosensitive drum 61. In some cases, the difference VD between the charging bias Vg corresponding to black and the developing bias Vb is equal to or greater than the third threshold value VDth. The third threshold value VDth can be set to 400 V, for example. Note that the third threshold value VDth may be the same value or a different value between the four sets of the charger 62 and the developing roller 64.

  Here, the reason why the whiteout occurrence condition includes the condition that the bias difference VD is equal to or greater than the third threshold value VDth (VD ≧ VDth) will be described with reference to FIG. In FIG. 6, Vg is a charging bias (surface potential of the photosensitive drum 61 after charging), VL is a surface potential of the photosensitive drum 61 after exposure, and Vb (Vb1, Vb2) is a developing bias. Further, A is a portion on the surface of the photosensitive drum 61 where deposits adhere, and it is exposed together with the surrounding portion, but the potential after exposure is higher than the potential (VL) of the surrounding portion.

  In the configuration in which the developing bias Vb is lowered in accordance with the elapsed time since the developing cartridge 63 is replaced with a new one, for example, as shown in FIG. 6, when the developing bias is lowered from Vb1 to Vb2, deposits adhere. The potential of the portion A may be higher than the developing bias Vb2. In this case, since it becomes difficult for the toner to be supplied to the portion A to which the adhered matter has adhered, it is considered that white spots are likely to occur. Therefore, in the present embodiment, the condition that the bias difference VD is equal to or greater than the third threshold value VDth is included in the whiteout occurrence condition.

  The developing bias Vb may be in two stages (see Vb1 and Vb2) as shown in FIG. 6, or may be in three stages or more. Further, the developing bias Vb may be linearly lowered according to the elapsed time after the developing cartridge 63 is replaced with a new one. In addition, the condition determination step may be executed for each input print job or may be executed for each image data per page of the paper S included in the print job.

  In the image forming step executed by the image forming unit 12, when it is determined that the white spot occurrence condition is satisfied in the condition determining step, the image forming unit 4 is mainly controlled, not in the normal printing mode (for example, 600 dpi), This is a step of forming an image in a low resolution mode (for example, 300 dpi).

Next, a control flow in the control device 10 will be described with reference to a flowchart.
As illustrated in FIG. 7, when a print job including image data is input to the color printer 1, the control device 10 causes the humidity H in the main body housing 2 to be equal to or lower than a first threshold value Hth (for example, 30%). It is determined whether or not (S101). When the humidity H is equal to or lower than the first threshold Hth (S101, YES), the control device 10 determines whether or not the total number of rotations NR of the photosensitive drum 61 is equal to or higher than a second threshold NRth (for example, 10,000 rotations). Determine (S103).

  When the total number of rotations NR is equal to or greater than the second threshold value NRth (S103, YES), the control device 10 sets the development bias Vb applied to the developing roller 64 and the charging bias Vg applied to the corresponding charger 62 to each other. It is determined whether or not the difference VD (= | Vg−Vb |) is greater than or equal to a third threshold value VDth (for example, 400 V) (S105).

  When the bias difference VD is equal to or greater than the third threshold value VDth (S105, YES), the control device 10 controls the image forming unit 4 and the like to control the low resolution mode (for example, 300 dpi (see FIG. 4B)). ) To form an image (S107). In the present embodiment, the order in which steps S101, S103, and S105 are executed is arbitrary.

  On the other hand, if the humidity H exceeds the first threshold Hth in S101 (NO), if the total number of rotations NR is less than the second threshold NRth in S103 (NO), or if the bias difference VD is third in S105. If it is less than the threshold value VDth (NO), the control device 10 controls the image forming unit 4 and the like to form an image in the normal print mode (for example, 600 dpi (see FIG. 4A)) (S109).

According to the present embodiment described above, when dot-like white spots (image defects) occur in the image on the paper S, the dot-like white spots can be made inconspicuous as a whole image.
This principle will be described in detail. As shown in FIG. 4A, in the normal printing mode, the minimum element E1 constituting the image is made up of 2 × 2 dots, and whether or not toner is placed is set for each minimum element E1. Also, as shown in FIG. 4B, in the low resolution mode, the minimum element E2 constituting the image is composed of 4 × 4 dots, and whether or not toner is placed is set for each minimum element E2.

  In the color printer 1, the color density of a certain part in the image formed on the paper S is set to all the minimum elements in a dither matrix composed of minimum elements (for example, a matrix consisting of 4 × 4 minimum elements). It is expressed in a range from a print duty of 100% in which toner is placed to a print duty of 0% in which toner is not placed on all the minimum elements. As the color becomes lighter, the number of minimum elements (minimum elements of the background color of the paper S (hereinafter referred to as white)) on which toner is not placed increases.

  When the printing duty is low, for example, less than 50%, the area of the white portion in the entire image increases due to the increase in the number of white minimum elements, so even if dot-like white spots of the same color occur, the resolution Regardless, the white spots are not noticeable. On the other hand, when the print duty is high, the area of the white portion occupying the entire image is reduced by reducing the number of white minimum elements, so that the area of dotted white spots is relatively large and becomes conspicuous.

  When the printing duty is high (about 81% as an example in FIG. 4), in the normal printing mode shown in FIG. 4A, the area of the white portion (minimum element E1) is shown as a substantially circular point in FIG. Since the area of the spot-like white spot SP is smaller, the spot-like white spot SP becomes relatively conspicuous. On the other hand, in the low resolution mode shown in FIG. 4B, the area of the white portion (minimum element E2) is approximately equal to the area of the dotted white spot SP (in some cases, the minimum element E2 and the white spot). Since the SPs tend to overlap), the dotted white spots SP (white dots) are relatively inconspicuous.

  In the present embodiment, since the image is formed in the low resolution mode when the white spot occurrence condition in which the spot white spot SP is likely to occur is satisfied, as described above, the white portion (minimum element) when the print duty is particularly high. The area of E2) is about the same as the area of the dotted white spots SP, and the dotted white spots SP can be made inconspicuous as a whole image.

[Second Embodiment]
Next, a second embodiment will be described. In the embodiment described below including this embodiment, differences from the first embodiment will be described in detail, and the same components and control processes as those in the first embodiment will be denoted by the same reference numerals. The description will be omitted as appropriate.

  In the color printer 1, text, graphics, and images are set as the types of images that can be formed on the paper S. These image types are set, for example, when the user performs an operation of selecting text, graphics, or an image from a drop-down list on a printer driver setting screen installed on a personal computer PC (see FIG. 5) or the like. . The set image type information (signal) is output to the color printer 1 (control device 10). Here, the image is a high-definition image such as a color photograph, the graphic is an image including a solid color or figure such as a presentation material, and the text is a character such as a sentence. It is an image consisting of

  The control device 10 is provided on the condition that a signal indicating that the image type is a graphic or an image among text, graphics and images, that is, a signal indicating that the graphic or image is set as the image type is received. For the first time, the white spot improvement control (condition determination step and image formation step) is executed. In other words, the control device 10 is configured not to execute the white spot improvement control when the signal indicating that the text is set as the image type is received.

Next, a control flow in the control device 10 in the present embodiment will be briefly described.
As shown in FIG. 8, when a print job including image data is input to the color printer 1, the control device 10 determines whether the set image type is graphic or image (S201). .

  When the set image type is a graphic or an image (S201, YES), the control device 10 determines that the humidity H is equal to or lower than the first threshold value Hth (S101), as in the first embodiment. , YES), the total number of rotations NR is equal to or greater than the second threshold NRth (S103, YES), and the bias difference VD is equal to or greater than the third threshold VDth (S105, YES). An image is formed in the mode (S107).

  On the other hand, if the image type set in S201 is text (NO), if the humidity H exceeds the first threshold Hth in S101 (NO), the total number of rotations NR is less than the second threshold NRth in S103. (NO) or when the bias difference VD is less than the third threshold value VDth in S105 (NO), the control device 10 forms an image in the normal printing mode (S109).

  According to the present embodiment described above, as in the case of the first embodiment, it is possible to make dot-like white spots less conspicuous in the entire image. In the present embodiment, as a type of image, whiteout is more conspicuous than in the case of text in which the background color of the paper S is increased as a whole image, and whiteout improvement control is performed when graphics and images are set ( The condition determining step and the image forming step) are executed, so that there is an advantage that the resolution of the image is not unnecessarily lowered when forming an image composed of characters.

[Third Embodiment]
Next, a third embodiment will be described.
The control device 10 is configured to execute white spot improvement control (condition determining step and image forming step) on condition that a signal permitting execution of control for improving white spot is received.

  Specifically, in the present embodiment, for example, a control in which a user performs an operation such as checking a check box on a setting screen of a printer driver installed on a personal computer PC (see FIG. 5) or the like to improve white spots. By permitting the execution of (white spot improvement control), a signal to that effect is transmitted to the color printer 1 (control device 10). Then, the control device 10 executes the white spot improvement control for the first time on condition that the signal permitting the execution of the white spot improvement control is received.

Next, a control flow in the control device 10 in the present embodiment will be briefly described.
As shown in FIG. 9, when a print job including image data is input to the color printer 1, the control device 10 determines whether or not it has received a signal (permission signal) that permits execution of white spot improvement control. (S301).

  When the permission signal is received (S301, YES), the control device 10 determines that the humidity H is equal to or lower than the first threshold value Hth (S101, YES), as in the first embodiment, and When the total number of rotations NR is equal to or greater than the second threshold NRth (S103, YES) and the bias difference VD is equal to or greater than the third threshold VDth (S105, YES), an image is formed in the low resolution mode. (S107).

  On the other hand, when the permission signal is not received in S301 (NO), when the humidity H exceeds the first threshold Hth in S101 (NO), or when the total number of rotations NR is less than the second threshold NRth in S103 ( NO) or when the bias difference VD is less than the third threshold value VDth in S105 (NO), the control device 10 forms an image in the normal print mode (S109).

  According to the present embodiment described above, as in the case of the first embodiment, it is possible to make dot-like white spots less conspicuous in the entire image. Further, in this embodiment, when the permission signal is received, the white spot improvement control (condition determination step and image forming step) is executed. Therefore, for example, when the white spot becomes a problem, the low resolution mode is executed for the first time. Until then, there is an advantage that the resolution of the image is not unnecessarily lowered.

  Although the embodiment has been described above, the present invention is not limited to the embodiment. About a concrete structure, it can change suitably in the range which does not deviate from the meaning of invention as follows.

  In the embodiment, when the white spot occurrence condition is satisfied, specifically, the humidity is equal to or lower than the first threshold, the total number of rotations of the photosensitive drum is equal to or higher than the second threshold, and the bias In the case where the difference is equal to or larger than the third threshold value, the image is formed in the low resolution mode, but the present invention is not limited to this. For example, the image forming apparatus may be configured to form an image in the low resolution mode when only that the humidity is equal to or lower than the first threshold is satisfied. Further, the image forming apparatus may be configured to form an image in the low resolution mode when only that the total number of rotations of the photosensitive drum is equal to or greater than the second threshold value is satisfied. Further, the image forming apparatus may be configured to form an image in the low resolution mode when only that the difference in bias is equal to or greater than the third threshold value is satisfied. Further, the image may be formed in the low resolution mode when the humidity is equal to or lower than the first threshold and the total number of rotations of the photosensitive drum is equal to or higher than the second threshold. Alternatively, the image may be formed in the low resolution mode when the humidity is equal to or lower than the first threshold and the difference in bias is equal to or higher than the third threshold. Further, the image forming apparatus may be configured to form an image in the low resolution mode when the total number of rotations of the photosensitive drum is equal to or greater than the second threshold value and the difference in bias is equal to or greater than the third threshold value.

  In the embodiment, the total number of rotations NR of the photosensitive drum 61 from when the photosensitive drum 61 is replaced with a new one is exemplified as the parameter related to the usage time of the photosensitive member. However, the present invention is not limited to this. For example, the parameters related to the usage time of the photoconductor include the driving time of the photoconductor drum after the photoconductor drum is replaced with a new one, and the number of sheets on which an image is formed after the photoconductor drum is replaced with a new one (total print Number). The parameter relating to the usage time of the photosensitive member may be the total number of rotations of the developing roller, the supply roller, and the like since the photosensitive drum is replaced with a new one.

  In the above-described embodiment (second embodiment), the configuration is described in which, when the user selects a graphic or an image as the image type, a signal indicating that the image type is a graphic or an image is output to the image forming apparatus. However, the present invention is not limited to this. For example, when a device other than an image forming apparatus such as a personal computer analyzes image data and determines that the type of an image formed by the image data is a graphic or an image, the image type is a graphic or an image. The signal may be output to the image forming apparatus. In such a configuration, even if the type of image is formally text, the font is a specific font, the size is a predetermined size or larger, and the characters are solidly painted. In some cases, white spots are conspicuous, and therefore, it may be exceptionally determined to be a graphic or an image. Further, when the image forming apparatus itself analyzes input image data and determines that the type of the image is a graphic or an image, the image forming apparatus itself may be configured to execute white spot improvement control. Alternatively, the image data may be analyzed to extract a portion where white spots are conspicuous from the image, and the image may be formed with only the portion having a low resolution.

  In the embodiment, the white spot improvement control is executed on the condition that the signal indicating that the image type is graphic or image is received in the second embodiment, and the white spot improvement control is executed in the third embodiment. However, the present invention is not limited to this. However, the present invention is not limited to this. For example, it is configured to execute the white spot improvement control on the condition that a signal permitting the execution of the white spot improvement control is received and a signal indicating that the image type is graphic or image is received. Also good.

  In the above-described embodiment, the humidity sensor 9 is provided so as to detect the humidity in the main body housing 2. However, the present invention is not limited to this. For example, the humidity sensor is a humidity outside the main body housing, that is, a color printer. You may provide so that the humidity of the installed environment may be detected.

  In the above-described embodiment, the photosensitive drum 61 is illustrated as the photosensitive member. However, the photosensitive drum 61 is not limited thereto, and the photosensitive member may be a photosensitive belt or the like. In the embodiment, the photoconductor is configured to be replaceable. However, the present invention is not limited to this, and a configuration in which the photoconductor cannot be replaced may be used. In this case, the parameter relating to the usage time of the photosensitive member may be, for example, the total number of rotations of the photosensitive member after the start of use of the color printer.

  In the above-described embodiment, the scorotron-type charger 62 including the grid electrode has been exemplified. However, the present invention is not limited to this. For example, a corotron-type charger may be used. Moreover, in the said embodiment, although the charger 62 provided with a charging wire was illustrated as a charging device, it is not limited to this, For example, a charging device is provided with the needle-shaped electrode arranged instead of the charging wire. A charger or the like may also be used. The charging device may be a charging roller.

  In the above embodiment, the exposure unit 5 (exposure apparatus) configured to expose the photosensitive drum 61 by scanning the light beam at high speed is exemplified, but the present invention is not limited to this. For example, the exposure apparatus may be an exposure apparatus that is provided one by one so as to face each photoconductive drum and has a blinking portion such as a plurality of light emitting diodes (LEDs) in a portion facing the photoconductive drum. Good.

  In the embodiment, as shown in FIG. 4, the minimum element E1 constituting the image in the normal print mode is 2 × 2 dots, and the minimum element E2 constituting the image in the low resolution mode is 4 × 4. Although an example using dots is shown, the present invention is not limited to this. For example, the minimum element constituting the image in the normal printing mode (first mode) is 1 dot, and the minimum element constituting the image in the low resolution mode (second mode) is 2 × 2 dots. Good.

  In the embodiment, the color printer 1 that includes a plurality of photoreceptors, charging devices, developer carriers, and the like as the image forming apparatus and can form a color image is exemplified. However, the present invention is not limited to this. For example, the image forming apparatus may be a printer that includes one photoconductor, one charging device, one developer carrier, and the like and that can form only a monochrome image. The image forming apparatus is not limited to a printer, and may be, for example, a copier or a multi-function machine including a document reading apparatus such as a flat bed scanner.

  In the above embodiment, an example in which the present invention is applied to an image forming apparatus using a positively charged toner has been described, but the present invention is not limited to this. That is, the present invention can also be applied to an image forming apparatus that uses a negatively charged toner (developer). In this case, the polarity of the bias is opposite to that of the above embodiment, and the magnitude of the bias is the magnitude of its absolute value.

DESCRIPTION OF SYMBOLS 1 Color printer 10 Control apparatus 11 Condition judgment means 12 Image formation means 61 Photosensitive drum 62 Charger 64 Developing roller S Paper SP White spot T Toner

Claims (6)

An image forming apparatus for forming an image on a recording sheet by transferring a developer image formed on a photosensitive member to the recording sheet, the first mode for forming an image at a first resolution, and the first mode An image forming apparatus capable of executing a second mode for forming an image at a second resolution lower than the resolution,
An image forming apparatus, wherein an image is formed in the second mode when a white spot generation condition that can cause a spot white spot in an image is satisfied.   The image forming apparatus according to claim 1, wherein the whiteout occurrence condition includes a condition that the humidity is equal to or lower than a first threshold value.   The image forming apparatus according to claim 1, wherein the white spot occurrence condition includes a condition that a parameter relating to a usage time of the photosensitive member is equal to or greater than a second threshold value.   The image forming apparatus according to claim 3, wherein the parameter relating to the usage time of the photosensitive member is a total number of rotations of the photosensitive member that is rotatably provided. A charging device for charging the photosensitive member; and a developer carrying member for supplying a developer to the photosensitive member to form a developer image on the photosensitive member;
The white spot generation condition includes a condition that an absolute value of a difference between a charging bias applied to the charging device and a developing bias applied to the developer carrier is a third threshold value or more. The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus.   2. The image forming apparatus according to claim 1, wherein an image is formed in the second mode when the white spot generation condition is satisfied on condition that a signal permitting execution of control for improving white spot is received. 6. The image forming apparatus according to any one of items 5.

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