Patent Publication Number: US-2023161284-A1

Title: Image forming apparatus capable of forming image on sheet with uneven surface, transfer current adjustment method

Description:
INCORPORATION BY REFERENCE 
     This application is based upon and claims the benefit of priority from the corresponding Japanese Patent Application No. 2021-189947 filed on Nov. 24, 2021, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     The present disclosure relates to an image forming apparatus and a transfer current adjustment method. 
     In an image forming apparatus of an electrophotographic method, an image may be formed on a sheet with an uneven surface such as a sheet of embossed paper. Toner is hardly adhered to a lower part of the uneven surface of the sheet. As a result, in the image forming apparatus, when an image is formed on a sheet with an uneven surface, an image forming condition, such as a transfer current that is supplied to a transfer portion that transfers a toner image to the sheet, is adjusted for the purpose of restricting the degradation of the formed image. 
     In addition, there is known, as a related technology, an image forming apparatus that adjusts the image forming condition based on the detection result of the sheet surface shape. In this image forming apparatus, the image forming condition is adjusted based on a variation width of a voltage that is applied to a transfer portion connected to a constant current power supply when a sheet passes through a position where a toner image is transferred to the sheet by the transfer portion. 
     SUMMARY 
     An image forming apparatus according to an aspect of the present disclosure includes a transfer portion, a transfer processing portion, a change processing portion, a first acquisition processing portion, a detection processing portion, a second acquisition processing portion, and an adjustment processing portion. The transfer portion transfers a toner image formed on an image-carrying member to a sheet. The transfer processing portion, by using the transfer portion, sequentially transfers, to the sheet, a plurality of specific toner images which each include: a first toner layer of a first color formed on the image-carrying member; and a second toner layer of a second color formed on the first toner layer, wherein the first color is any one of colors C, M, and Y, and the second color is any one of the colors C, M, and Y and is different from the first color. The change processing portion changes a current value of a transfer current that is supplied to the transfer portion, each time a specific toner image is transferred by the transfer processing portion. The first acquisition processing portion acquires a captured image of the sheet. The detection processing portion detects a specific image that corresponds to each specific toner image included in the captured image of the sheet acquired by the first acquisition processing portion. The second acquisition processing portion acquires, for each of specific images detected by the detection processing portion, a skewness of a histogram of gradation values of a color mixture of the second color and a third color in pixels included in each of the specific images detected by the detection processing portion, the third color being a color different from the first color and the second color among the colors C, M, and Y. The adjustment processing portion adjusts the transfer current based on: the skewness acquired by the second acquisition processing portion for each of the specific images; and a current value of the transfer current corresponding to the skewness. 
     A transfer current adjustment method according to another aspect of the present disclosure is executed in an image forming apparatus including a transfer portion that transfers a toner image formed on an image-carrying member to a sheet, and includes a transfer step, a change step, a first acquisition step, a detection step, a second acquisition step, and an adjustment step. In the transfer step, a plurality of specific toner images are sequentially transferred, by using the transfer portion, to the sheet, wherein each of the plurality of specific toner images includes: a first toner layer of a first color formed on the image-carrying member; and a second toner layer of a second color formed on the first toner layer, the first color being any one of colors C, M, and Y, the second color being any one of the colors C, M, and Y and different from the first color. In the change step, a current value of a transfer current that is supplied to the transfer portion is changed each time a specific toner image is transferred in the transfer step. In the first acquisition step, a captured image of the sheet is acquired. In the detection step, a specific image that corresponds to each specific toner image included in the captured image of the sheet acquired in the first acquisition step, is detected. In the second acquisition step, a skewness of a histogram of gradation values of a color mixture of the second color and a third color in pixels included in each of the specific images detected in the detection step, is acquired for each of specific images detected in the detection step, the third color being a color different from the first color and the second color among the colors C, M, and Y. In the adjustment step, the transfer current is adjusted based on: the skewness acquired in the second acquisition step for each of the specific images; and a current value of the transfer current corresponding to the skewness. 
     This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description with reference where appropriate to the accompanying drawings. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. Furthermore, the claimed subject matter is not limited to implementations that solve any or all disadvantages noted in any part of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a cross-section diagram showing a configuration of an image forming apparatus according to an embodiment of the present disclosure. 
         FIG.  2    is a block diagram showing a system configuration of the image forming apparatus according to the embodiment of the present disclosure. 
         FIG.  3    is a cross-section diagram showing a configuration of an image forming portion of the image forming apparatus according to the embodiment of the present disclosure. 
         FIG.  4    is a diagram showing an example of a histogram acquired by the image forming apparatus according to the embodiment of the present disclosure. 
         FIG.  5    is a diagram showing an example of the histogram acquired by the image forming apparatus according to the embodiment of the present disclosure. 
         FIG.  6    is a diagram showing relationship between an unevenness depth level and a secondary transfer current acquired by the image forming apparatus according to the embodiment of the present disclosure. 
         FIG.  7    is a flowchart showing an example of a transfer current adjustment process executed by the image forming apparatus according to the embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The following describes an embodiment of the present disclosure with reference to the accompanying drawings. It should be noted that the following embodiment is an example of a specific embodiment of the present disclosure and should not limit the technical scope of the present disclosure. 
     [Configuration of Image Forming Apparatus  100 ] 
     First, a description is given of a configuration of an image forming apparatus  100  according to an embodiment of the present disclosure with reference to  FIG.  1    and  FIG.  2   . 
     It is noted that, for the sake of explanation, a vertical direction in a state where the image forming apparatus  100  is usably installed (the state shown in  FIG.  1   ), is defined as an up-down direction D 1 . In addition, a front-rear direction D 2  is defined on the supposition that the left side of the image forming apparatus  100  in  FIG.  1    is a front side (front). Furthermore, a left-right direction D 3  is defined based on the image forming apparatus  100  in the installation state viewed from the front side. 
     The image forming apparatus  100  is a multifunction peripheral having a plurality of functions such as a scan function for reading image data from a document sheet, a print function for forming an image based on image data, a facsimile function, and a copy function. It is noted that the present disclosure is applicable to an image forming apparatus such as a printer, a facsimile apparatus, and a copier. 
     As shown in  FIG.  1    and  FIG.  2   , the image forming apparatus  100  includes an ADF (Auto Document Feeder)  1 , an image reading portion  2 , an image forming portion  3 , a sheet feed portion  4 , an operation/display portion  5 , a storage portion  6 , and a control portion  7 . 
     The ADF  1  conveys a document sheet that is a reading target of the scan function. The ADF  1  includes a document sheet setting portion, a plurality of conveyance rollers, a document sheet pressing member, and a sheet discharge portion. 
     The image reading portion  2  realizes the scan function. The image reading portion  2  includes a document sheet table, a light source, a plurality of mirrors, an optical lens, and a CCD (Charge Coupled Device). 
     The image forming portion  3  realizes the print function. Specifically, the image forming portion  3  forms, by an electrophotographic method, a color or monochrome image on a sheet supplied from the sheet feed portion  4 . 
     The sheet feed portion  4  supplies a sheet to the image forming portion  3 . The sheet feed portion  4  includes a sheet feed cassette, a manual feed tray, and a plurality of conveyance rollers. 
     The operation/display portion  5  is a user interface of the image forming apparatus  100 . The operation/display portion  5  includes a display portion and an operation portion. The display portion is, for example, a liquid crystal display and displays various types of information in response to control instructions from the control portion  7 . The operation portion is composed of, for example, operation keys or a touch panel through which various types of information are input to the control portion  7  in response to user operations. 
     The storage portion  6  is a nonvolatile storage device. For example, the storage portion  6  is a storage device such as: a nonvolatile memory such as a flash memory or an EEPROM; an SSD (Solid State Drive); or an HDD (Hard Disk Drive). 
     The control portion  7  comprehensively controls the image forming apparatus  100 . As shown in  FIG.  2   , the control portion  7  includes a CPU  11 , a ROM  12 , and a RAM  13 . The CPU  11  is a processor that executes various calculation processes. The ROM  12  is a nonvolatile storage device in which various information such as control programs for causing the CPU  11  to execute various processes are preliminarily stored. The RAM  13  is a volatile or nonvolatile storage device that is used as a temporary storage memory (working area) for the various processes executed by the CPU  11 . The CPU  11  comprehensively controls the image forming apparatus  100  by executing the various control programs preliminarily stored in the ROM  12 . 
     It is noted that the control portion  7  may be a control portion provided independently of a main control portion that comprehensively controls the image forming apparatus  100 . In addition, the control portion  7  may be formed as an electronic circuit such as an integrated circuit (ASIC). 
     [Configuration of Image Forming Portion  3 ] 
     Next, a configuration of the image forming portion  3  is described with reference to  FIG.  1    to  FIG.  3   . Here,  FIG.  3    is a cross-section diagram showing a configuration of a plurality of image forming units  20 , an intermediate transfer belt  26 , and a secondary transfer roller  27 . 
     As shown in  FIG.  1   , the image forming portion  3  includes four image forming units  20 , a laser scanning unit  25 , the intermediate transfer belt  26 , the secondary transfer roller  27 , a fixing device  28 , and a sheet discharge tray  29 . In addition, as shown in  FIG.  2   , the image forming portion  3  includes a power supply  41  and an image capturing portion  42 . 
     Of the four image forming units  20 , an image forming unit  21  (see  FIG.  3   ) forms a Y (yellow) toner image. Of the four image forming units  20 , an image forming unit  22  (see  FIG.  3   ) forms a C (cyan) toner image. Of the four image forming units  20 , an image forming unit  23  (see  FIG.  3   ) forms an M (magenta) toner image. Of the four image forming units  20 , an image forming unit  24  (see  FIG.  3   ) forms a K (black) toner image. That is, the image forming portion  3  forms an image on a sheet using toners of colors C, M, Y, and K. As shown in  FIG.  1    and  FIG.  3   , the four image forming units  20  are arranged in order of yellow, cyan, magenta, and black from the front side of the image forming apparatus  100  along the front-rear direction D 2 . 
     As shown in  FIG.  3   , each of the image forming units  20  includes a photoconductor drum  31 , a charging roller  32 , a developing device  33 , a primary transfer roller  34 , and a drum cleaning portion  35 . In addition, each of the image forming units  20  includes a toner container  36  shown in  FIG.  1   . 
     On a surface of the photoconductor drum  31 , an electrostatic latent image is formed. For example, the photoconductor drum  31  has a photosensitive layer formed from amorphous silicon. Upon receiving a rotational driving force supplied from a motor (not shown), the photoconductor drum  31  rotates in a rotation direction D 4  shown in  FIG.  3   . This allows the photoconductor drum  31  to convey the electrostatic latent image formed on its surface. 
     Upon receiving a supply of a predetermined charging voltage, the charging roller  32  electrically charges the surface of the photoconductor drum  31 . For example, the charging roller  32  charges the surface of the photoconductor drum  31  to a positive polarity. The surface of the photoconductor drum  31  charged by the charging roller  32  is irradiated with light that is emitted from the laser scanning unit  25  based on image data. This forms an electrostatic latent image on the surface of the photoconductor drum  31 . 
     The developing device  33  develops the electrostatic latent image formed on the surface of the photoconductor drum  31 . The developing device  33  includes a pair of stirring members, a magnet roller, and a developing roller. The pair of stirring members stir developer stored inside the developing device  33 , wherein the developer includes toner and carrier. As the developer is stirred, the toner included in the developer makes friction with the carrier included in the developer, and the toner is charged to the positive polarity. The magnet roller draws up the developer stirred by the pair of stirring members and supplies the toner included in the developer to the developing roller. The developing roller conveys the toner supplied from the magnet roller to a position facing the photoconductor drum  31 . In addition, upon receiving an application of a predetermined developing bias voltage, the developing roller supplies the toner conveyed to the position facing the photoconductor drum  31 , to the photoconductor drum  31 . This allows the electrostatic latent image formed on the surface of the photoconductor drum  31  to be visualized (developed). It is noted that the toner is supplied from the toner container  36  to the developing device  33 . 
     The primary transfer roller  34 , upon receiving a supply of a predetermined primary transfer current, transfers a toner image formed on the surface of the photoconductor drum  31  to an outer peripheral surface of the intermediate transfer belt  26 . As shown in  FIG.  3   , the primary transfer roller  34  is disposed to face the photoconductor drum  31  across the intermediate transfer belt  26 . 
     The drum cleaning portion  35  removes the toner that has remained on the surface of the photoconductor drum  31  after the transfer of the toner image by the primary transfer roller  34 . 
     The laser scanning unit  25  emits light based on the image data, to the surfaces of the photoconductor drums  31  of the image forming units  20 . 
     The intermediate transfer belt  26  is a belt member of an endless shape to which toner images formed on the surfaces of the photoconductor drums  31  of the image forming units  20  are transferred. The intermediate transfer belt  26  is stretched by a drive roller  26 A (see  FIG.  3   ) and a stretch roller  26 B (see  FIG.  3   ) with a predetermined tension. The intermediate transfer belt  26  rotates in a rotation direction D 5  shown in  FIG.  3    when the drive roller  26 A rotates upon receiving a rotational driving force supplied from a motor (not shown). This allows the intermediate transfer belt  26  to convey the toner image formed on the outer peripheral surface thereof to a secondary transfer position P 1  (see  FIG.  3   ) where the toner image is transferred to a sheet by the secondary transfer roller  27 . It is noted that the outer peripheral surface of the intermediate transfer belt  26  from which the toner image has been transferred by the secondary transfer roller  27  is cleaned by a belt cleaning portion  26 C shown in  FIG.  3   . 
     The secondary transfer roller  27  transfers the toner image that has been transferred to the outer peripheral surface of the intermediate transfer belt  26 , to a sheet supplied from the sheet feed portion  4 . As shown in  FIG.  3   , the secondary transfer roller  27  is disposed to face the drive roller  26 A across the intermediate transfer belt  26 . The secondary transfer roller  27  is biased by a biasing member (not shown) towards the drive roller  26 A so that the secondary transfer roller  27  comes in contact with the intermediate transfer belt  26  with a predetermined nip pressure. The secondary transfer roller  27 , at the secondary transfer position P 1  (see  FIG.  3   ) where it comes in contact with the intermediate transfer belt  26 , transfers the toner image formed on the intermediate transfer belt  26  to the sheet. The secondary transfer roller  27  is an example of a transfer portion of the present disclosure. In addition, the intermediate transfer belt  26  is an example of an image-carrying member of the present disclosure. 
     The fixing device  28  fixes the toner image transferred to the sheet by the secondary transfer roller  27 , to the sheet. As shown in  FIG.  1   , the fixing device  28  includes a fixing roller  28 A and a pressure roller  28 B. The fixing roller  28 A is heated to a predetermined fixing temperature by a heater (not shown). The fixing roller  28 A is rotated at a predetermined speed. The pressure roller  28 B is biased by a biasing member (not shown) towards the fixing roller  28 A so that the pressure roller  28 B comes in contact with the fixing roller  28 A with a predetermined nip pressure. Between the fixing roller  28 A and the pressure roller  28 B is formed a fixing nip portion P 2  (see  FIG.  1   ) that heats and pressurizes a sheet. The toner image transferred to the sheet is heated and pressurized so as to be fixed to the sheet when the sheet passes through the fixing nip portion P 2 . The fixing device  28  is an example of a fixing portion of the present disclosure. 
     The sheet to which the toner image has been fixed by the fixing device  28  is discharged to the sheet discharge tray  29 . 
     The power supply  41  is a constant current power supply that supplies a secondary transfer current having a predetermined current value to the secondary transfer roller  27 . Specifically, the power supply  41  supplies the secondary transfer current having a current value set by the control portion  7 . For example, the secondary transfer current is a current of a negative polarity. The secondary transfer current is an example of a transfer current of the present disclosure. 
     The image capturing portion  42  captures an image of a sheet that has been conveyed via the secondary transfer position P 1  (see  FIG.  1   ) where the toner image is transferred to a sheet by the secondary transfer roller  27 . In other words, the image capturing portion  42  reads an image of a sheet that has been conveyed via the secondary transfer position P 1 . Specifically, the image capturing portion  42  captures the image of the sheet at a downstream of the secondary transfer position P 1  in a sheet conveyance path R 1  (see the two-dot chain line with an arrow shown in  FIG.  1   ) that extends from the sheet feed cassette to the sheet discharge tray  29  via the secondary transfer position P 1  (see  FIG.  1   ) and the fixing nip portion P 2  (see  FIG.  1   ). For example, as shown in  FIG.  1   , the image capturing portion  42  is disposed at a downstream of the fixing nip portion P 2  in the conveyance path R 1 . The secondary transfer position P 1  is an example of a transfer position of the present disclosure. It is noted that the image capturing portion  42  may be disposed at an upstream of the fixing nip portion P 2  in the conveyance path R 1 . 
     For example, the image capturing portion  42  is a CIS (Contact Image Sensor) that includes a light emitting portion and a light receiving portion. The light emitting portion emits light toward a surface of a sheet that is conveyed along the conveyance path R 1 . The light receiving portion receives the light that has been emitted from the light emitting portion and reflected on the surface of the sheet, and outputs an electric signal that corresponds to an amount of received light. 
     The electric signal output from the light receiving portion of the image capturing portion  42  is converted into a digital signal (image data) by an analog front-end circuit (not shown). For example, the analog front-end circuit converts the electric signal output from the light receiving portion of the image capturing portion  42  into image data that represents colors of the pixels by R, G, and B of  256  gradations. The image data output from the analog front-end circuit is input to the control portion  7 . 
     Meanwhile, in the image forming apparatus  100 , an image may be formed on a sheet with an uneven surface such as a sheet of embossed paper. The toner is hardly adhered to a lower part of the uneven surface of the sheet. As a result, in the image forming apparatus  100 , when an image is formed on a sheet with an uneven surface, an image forming condition such as the secondary transfer current is adjusted for the purpose of restricting the degradation of the formed image. 
     In addition, there is known, as a related technology, an image forming apparatus that adjusts the image forming condition based on the detection result of the sheet surface shape. In this image forming apparatus, the image forming condition is adjusted based on a variation width of a voltage that is applied to a transfer portion connected to a constant current power supply, when a sheet passes through a position where a toner image is transferred to the sheet by the transfer portion. 
     Here, when the secondary transfer current is adjusted based on the detection result of the sheet surface shape for the purpose of restricting the degradation of the formed image, the secondary transfer current may become excessive. When the secondary transfer current is excessive, an abnormal image including what is called white spots may be generated in the toner image transferred to the sheet. 
     On the other hand, as described in the following, the image forming apparatus  100  according to the embodiment of the present disclosure can restrict the degradation of the formed image, as well as restrict the generation of the abnormal image, when an image is formed on a sheet with an uneven surface. 
     [Configuration of Control Portion  7 ] 
     Next, a configuration of the control portion  7  is described with reference to  FIG.  2   . 
     As shown in  FIG.  2   , the control portion  7  includes a transfer processing portion  51 , a change processing portion  52 , a first acquisition processing portion  53 , a detection processing portion  54 , a second acquisition processing portion  55 , and an adjustment processing portion  56 . 
     Specifically, a transfer current adjustment program for causing the CPU  11  to function as the above-described portions is preliminarily stored in the ROM  12  of the control portion  7 . The CPU  11  functions as the above-described portions by executing the transfer current adjustment program stored in the ROM  12 . 
     It is noted that the transfer current adjustment program may be recorded on a non-transitory computer-readable recording medium such as a CD, a DVD, or a flash memory, and may be read from the recording medium and installed in a storage device such as the storage portion  6 . In addition, a part or all of the transfer processing portion  51 , the change processing portion  52 , the first acquisition processing portion  53 , the detection processing portion  54 , the second acquisition processing portion  55 , and the adjustment processing portion  56  may be composed of an electronic circuit such as an integrated circuit (ASIC). 
     The transfer processing portion  51  uses the secondary transfer roller  27  to sequentially transfer, to a sheet, a plurality of specific toner images which each include: a first toner layer of a first color formed on the intermediate transfer belt  26 ; and a second toner layer of a second color formed on the first toner layer, wherein the first color is any one of colors C, M, and Y, and the second color is any one of the colors C, M, and Y and is different from the first color. 
     For example, the first toner layer is a toner image of C (cyan). In addition, the second toner layer is a toner image of M (magenta). In this case, the first color is C (cyan), and the second color is M (magenta). In addition, the specific toner image is a toner image of B (blue) that is a color mixture of the first color and the second color. 
     For example, in the image forming apparatus  100 , first image data that is used by the image forming unit  22  to form a plurality of first toner layers is preliminarily stored in the storage portion  6 . The first image data includes a plurality of first images that correspond to the plurality of first toner layers formed on the intermediate transfer belt  26  by the image forming unit  22 . For example, each of the first images is a rectangular image of a predetermined size. In addition, each of the first images is a single-color image of C (cyan) having a predetermined specific density. 
     In addition, in the image forming apparatus  100 , second image data that is used by the image forming unit  23  to form a plurality of second toner layers is preliminarily stored in the storage portion  6 . The second image data includes a plurality of second images that correspond to the plurality of second toner layers formed by the image forming unit  23  on the first toner layers, respectively. For example, each of the second images has the same shape as each of the first images. In addition, each of the second images is a single-color image of M (magenta) having the specific density. 
     The transfer processing portion  51  sequentially transfers the plurality of specific toner images to a sheet by using the image forming unit  22 , the image forming unit  23 , the laser scanning unit  25 , the intermediate transfer belt  26 , the secondary transfer roller  27 , the sheet feed portion  4 , the first image data, and the second image data. Specifically, the transfer processing portion  51  forms the plurality of first toner layers in alignment along a rotation direction D 4  (see  FIG.  3   ) on the photoconductor drum  31  of the image forming unit  22 , and sequentially transfers the plurality of first toner layers onto the intermediate transfer belt  26 . In addition, the transfer processing portion  51  forms the plurality of second toner layers in alignment along the rotation direction D 4  (see  FIG.  3   ) on the photoconductor drum  31  of the image forming unit  23 , and sequentially transfers the plurality of second toner layers onto the plurality of first toner layers formed on the intermediate transfer belt  26 , respectively. This forms a plurality of specific toner images in alignment along a rotation direction D 5  (see  FIG.  3   ) on the intermediate transfer belt  26 . Subsequently, the transfer processing portion  51  sequentially transfers the plurality of specific toner images from the intermediate transfer belt  26  to a sheet conveyed by the sheet feed portion  4 . This forms, on the sheet, a plurality of toner images in each of which the layers of the specific toner image are arranged upside down. 
     For example, when a predetermined conveyance timing arrives, the transfer processing portion  51  causes a sheet stored in the sheet feed cassette to be conveyed along the conveyance path R 1 . Subsequently, the transfer processing portion  51  sequentially transfers the plurality of specific toner images to the sheet that is conveyed upon the arrival of the conveyance timing. 
     For example, the conveyance timing is a timing when an instruction to execute a print process for forming an image on a sheet has been input. It is noted that the conveyance timing may be a timing when a predetermined user operation has been performed on the operation/display portion  5 . 
     The change processing portion  52  changes the current value of the secondary transfer current that is supplied to the secondary transfer roller  27 , each time a specific toner image is transferred by the transfer processing portion  51 . 
     For example, the change processing portion  52  increases, in units of a predetermined reference amount, the current value of the secondary transfer current that is supplied from the power supply  41 , each time a specific toner image is transferred by the transfer processing portion  51 . For example, the reference amount is 10 μA (microampere). 
     It is noted that the change processing portion  52  may decrease, in units of the reference amount, the current value of the secondary transfer current that is supplied from the power supply  41 , each time a specific toner image is transferred by the transfer processing portion  51 . In addition, each time a specific toner image is transferred by the transfer processing portion  51 , the change processing portion  52  may change the current value of the secondary transfer current that is supplied from the power supply  41 , to a current value predetermined for each specific toner image in an order of transfer. 
     The first acquisition processing portion  53  acquires a captured image of a sheet. 
     Specifically, the first acquisition processing portion  53  acquires, by using the image capturing portion  42 , the captured image of the sheet to which the plurality of specific toner images have been transferred sequentially by the transfer processing portion  51 . 
     It is noted that the first acquisition processing portion  53  may acquire the captured image of the sheet by using the image reading portion  2 . For example, when a predetermined user operation is received after a sheet to which the plurality of specific toner images have been sequentially transferred by the transfer processing portion  51  is discharged to the sheet discharge tray  29 , the first acquisition processing portion  53  may use the image reading portion  2  to capture an image of a sheet that is placed on the document sheet table or a sheet that is conveyed by the ADF  1 . 
     The detection processing portion  54  detects a specific image that corresponds to each specific toner image included in the captured image of the sheet acquired by the first acquisition processing portion  53 . 
     For example, the detection processing portion  54  detects, as the specific image, a colored area (an area of a color that is different from a base color of the sheet) having the same shape as the specific toner image included in the captured image of the sheet. 
     The second acquisition processing portion  55  acquires, for each of specific images detected by the detection processing portion  54 , a skewness of a histogram of gradation values of a color mixture of the second color and a third color in the pixels included in each of the specific images detected by the detection processing portion  54 , the third color being a color different from the first color and the second color among colors C, M, and Y. 
     For example, when the first color is C (cyan) and the second color is M (magenta), the third color is Y (yellow). In this case, the color mixture of the second color and the third color is R (red). 
     For example, the second acquisition processing portion  55  acquires a histogram of gradation values of R (red) that indicates an appearance frequency for each gradation value of R in the specific image, based on gradation values of R of the pixels included in the specific image detected by the detection processing portion  54 . Specifically, the second acquisition processing portion  55  acquires the histogram of gradation values of R by totaling, for each gradation value of R, the number of appearances of a pixel having a gradation value of R in the specific image. Subsequently, the second acquisition processing portion  55  calculates the skewness of the histogram based on the acquired histogram. 
     Here,  FIG.  4    and  FIG.  5    show examples of the histogram of gradation values of R acquired by the second acquisition processing portion  55 . 
       FIG.  4    shows an example of the histogram of gradation values of R acquired by the second acquisition processing portion  55  when the specific image indicates the specific toner image transferred to a first sheet having an uneven surface. The first sheet is a sheet whose surface includes a flat part and a plurality of recesses. 
     In addition,  FIG.  5    shows an example of the histogram of gradation values of R acquired by the second acquisition processing portion  55  when the specific image indicates the specific toner image transferred to a second sheet having an even surface. 
     As shown in  FIG.  5   , when the specific image indicates the specific toner image transferred to the second sheet, the skewness of the histogram of gradation values of R is substantially  0  (zero). 
     On the other hand, as shown in  FIG.  4   , when the specific image indicates the specific toner image transferred to the first sheet, the skewness of the histogram of gradation values of R is a value of the positive side. The value becomes higher as the difference in height between the flat part and the recesses of the first sheet becomes larger. In addition, the value becomes higher as the current value of the secondary transfer current becomes lower. This is because the transfer of the first layer to the recesses tends to be insufficient since the distance to the recesses from the first layer is larger than that from the second layer, and the second toner layer transferred to the sheet tends to be exposed correspondingly. 
     The adjustment processing portion  56  adjusts the secondary transfer current based on: the skewness acquired by the second acquisition processing portion  55  for each specific image; and the current value of the secondary transfer current corresponding to the skewness. 
     For example, each time the second acquisition processing portion  55  acquires a skewness, the adjustment processing portion  56  determines the unevenness depth level of sheet corresponding to the skewness. For example, in the image forming apparatus  100 , the unevenness depth level is determined as one of six levels from level 1 (most shallow) to level 6 (most deep) depending on the height of the skewness. It is noted that when the difference in sheet surface height is large, the skewness acquired by the second acquisition processing portion  55  may be a value of the negative side. In this case, the adjustment processing portion  56  may determine the unevenness depth level as the maximum (level 6). 
     In addition, the adjustment processing portion  56  sets, as a new current value of the secondary transfer current, the lowest current value among current values of the secondary transfer current for the unevenness depth level “1” corresponding to the skewness acquired by the second acquisition processing portion  55 . 
     Here,  FIG.  6    shows an example of relationship between the unevenness depth level corresponding to the skewness acquired by the second acquisition processing portion  55  and the current value of the secondary transfer current.  FIG.  6    shows an example of relationship between the unevenness depth level and the current value of the secondary transfer current in a case where six specific toner images are transferred to the first sheet, and the current value of the secondary transfer current is increased in units of  10 pA (microampere) from a reference current value each time a specific toner image is transferred. In the example shown in  FIG.  6   , a current value obtained by adding 30 μA (microampere) to the reference current value is set as a new current value of the secondary transfer current. 
     It is noted that the adjustment processing portion  56  may set, as a new current value of the secondary transfer current, the lowest current value among current values of the secondary transfer current for a case where the skewness acquired by the second acquisition processing portion  55  is equal to or lower than a predetermined reference value. 
     [Transfer Current Adjustment Process] 
     In the following, with reference to  FIG.  7   , a description is given of an example of the procedure of a transfer current adjustment process executed by the control portion  7  in the image forming apparatus  100 , as well as a transfer current adjustment method of the present disclosure. Here, steps S 11 , S 12 , . . . represent numbers assigned to the processing procedures (steps) executed by the control portion  7 . It is noted that the control portion  7  executes the transfer current adjustment process when the conveyance timing has arrived. 
     &lt;Step S 11 &gt; 
     First, in step S 11 , the control portion  7  causes a sheet stored in the sheet feed cassette to be conveyed along the conveyance path R 1 . 
     &lt;Step S 12 &gt; 
     In step S 12 , the control portion  7  sequentially transfers a plurality of specific toner images to the sheet that is conveyed by the process of step S 11 . Here, the process of step S 12  is an example of a transfer step of the present disclosure, and is executed by the transfer processing portion  51  of the control portion  7 . 
     Specifically, the control portion  7  forms the plurality of first toner layers in alignment along the rotation direction D 4  (see  FIG.  3   ) on the photoconductor drum  31  of the image forming unit  22 , and sequentially transfers the plurality of first toner layers onto the intermediate transfer belt  26 . In addition, the control portion  7  forms the plurality of second toner layers in alignment along the rotation direction D 4  (see  FIG.  3   ) on the photoconductor drum  31  of the image forming unit  23 , and sequentially transfers the plurality of second toner layers onto the plurality of first toner layers formed on the intermediate transfer belt  26 , respectively. This forms the plurality of specific toner images in alignment along the rotation direction D 5  (see  FIG.  3   ) on the intermediate transfer belt  26 . Subsequently, the control portion  7  sequentially transfers the plurality of specific toner images from the intermediate transfer belt  26  to a sheet conveyed by the sheet feed portion  4 . 
     &lt;Step S 13 &gt; 
     In step S 13 , the control portion  7  changes the current value of the secondary transfer current that is supplied to the secondary transfer roller  27 , each time a specific toner image is transferred by the process of step S 12 . Here, the process of step S 13  is an example of a change step of the present disclosure, and is executed by the change processing portion  52  of the control portion  7 . 
     Specifically, the control portion  7  increases, in units of the reference amount, the current value of the secondary transfer current that is supplied from the power supply  41 , each time a specific toner image is transferred by the process of step S 12 . 
     &lt;Step S 14 &gt; 
     In step S 14 , the control portion  7  acquires, by using the image capturing portion  42 , the captured image of the sheet to which the plurality of specific toner images have been transferred. Here, the process of step S 14  is an example of a first acquisition step of the present disclosure, and is executed by the first acquisition processing portion  53  of the control portion  7 . 
     &lt;Step S 15 &gt; 
     In step S 15 , the control portion  7  detects the specific image from the captured image of the sheet acquired in step S 14 . Here, the process of step S 15  is an example of a detection step of the present disclosure, and is executed by the detection processing portion  54  of the control portion  7 . 
     Specifically, the control portion  7  detects, as the specific image, a colored area having the same shape as the specific toner image included in the captured image of the sheet. 
     &lt;Step S 16 &gt; 
     In step S 16 , the control portion  7  acquires, for each of specific images detected in step S 15 , a histogram of gradation values of a color mixture of the second color and the third color in the pixels included in the specific image. 
     Specifically, the control portion  7  acquires a histogram of gradation values of R for each specific image, based on gradation values of R of the pixels included in the specific image detected in step S 15 . 
     &lt;Step S 17 &gt; 
     In step S 17 , the control portion  7  acquires, for each of the histograms acquired in step S 16 , the skewness of each histogram. Here, the processes of steps S 16  and S 17  are an example of a second acquisition step of the present disclosure, and are executed by the second acquisition processing portion  55  of the control portion  7 . 
     &lt;Step S 18 &gt; 
     In step S 18 , the control portion  7  adjusts the secondary transfer current based on: the skewness for each specific image acquired by the process of step S 17 ; and the current value of the secondary transfer current corresponding to the skewness. Here, the processes of step S 18  is an example of an adjustment step of the present disclosure, and is executed by the adjustment processing portion  56  of the control portion  7 . 
     For example, each time a skewness is acquired by the process of step S 17 , the control portion  7  determines the unevenness depth level corresponding to the skewness. 
     In addition, the control portion  7  sets, as a new current value of the secondary transfer current, the lowest current value among current values of the secondary transfer current for the unevenness depth level “1”. 
     As described above, in the image forming apparatus  100 , a plurality of specific toner images are sequentially transferred to a sheet, wherein each of the plurality of specific toner images includes: the first toner layer of the first color formed on the intermediate transfer belt  26 ; and the second toner layer of the second color formed on the first toner layer. In addition, each time a specific toner image is transferred, the current value of the secondary transfer current is changed. In addition, a skewness of a histogram of gradation values of a color mixture of the second color and the third color is acquired for each of the specific images included in the captured image of the sheet to which the plurality of specific toner images have been transferred. Furthermore, the secondary transfer current is adjusted based on: the skewness acquired for each specific image; and the current value of the secondary transfer current corresponding to the skewness. This makes it possible to adjust the current value of the secondary transfer current to a minimum current value capable of restricting degradation of the formed image. With this configuration, when an image is formed on a sheet with an uneven surface, it is possible to restrict degradation of the formed image, as well as restrict generation of an abnormal image. 
     It is to be understood that the embodiments herein are illustrative and not restrictive, since the scope of the disclosure is defined by the appended claims rather than by the description preceding them, and all changes that fall within metes and bounds of the claims, or equivalence of such metes and bounds thereof are therefore intended to be embraced by the claims.