Image forming apparatus

Abstract

To form a frameless image on various kinds of recording media.SOLUTION: When a sheet as a medium is thinner than a predetermined thickness and double-sided printing is executed in borderless printing in which an image is formed without a margin on the sheet, the printer controller 130 of the image forming apparatus does not cause one image forming unit 101 arranged on the most upstream side in the conveyance direction of the sheet among the plurality of image forming units 101 to form a toner image in a predetermined range from at least the leading end of the second surface after printing is performed on the first surface of the sheet.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to an image forming apparatus. [Background technology]

[0002] When performing borderless printing in an image forming apparatus, problems can arise if the rigidity of the recording medium is low. For this reason, conventionally, in image forming apparatuses capable of borderless printing, when borderless printing is difficult due to the type of recording medium, printing is performed with a border (see Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-282325 Summary of the Invention [Problem to be solved by the invention]

[0004] However, with conventional technology, borderless printing cannot be performed depending on the type of recording medium, which reduces user convenience.

[0005] Therefore, one or more aspects of the present disclosure aim to enable borderless image formation on various types of recording media. [Means for solving the problem]

[0006] An image forming apparatus according to one aspect of the present disclosure includes a belt for transporting a medium, a plurality of image forming units arranged in order in a transport direction of the medium and forming a plurality of developer images using developers of a plurality of colors, a plurality of transfer units for transferring the plurality of developer images in order to the transported medium, a fixing unit for fixing the plurality of developer images to the medium, a duplex unit for transporting the medium, on which the plurality of developer images have been fixed and which has been inverted, to the belt, and a duplex unit for transporting the medium to the belt, the plurality of image forming units for forming a plurality of first developer images, the plurality of transfer units for transferring the plurality of first developer images to a first side of the medium, the fixing unit for fixing the plurality of first developer images to the first side, the reversing unit for reversing the medium, and the reversing unit for reversing the medium. and a control unit that performs double-sided image formation by transporting the medium onto the belt, causing the multiple image forming units to form multiple second developer images, causing the multiple transfer units to transfer the multiple second developer images to a second side of the medium, which is the side opposite the first side, and causing the fixing unit to fix the multiple second developer images to the second side, wherein when the medium is thinner than a predetermined thickness and double-sided image formation is performed using borderless image formation in which an image is formed on the medium without leaving any margins, the control unit does not cause one image forming unit among the multiple image forming units that is located most upstream in the direction of transport of the medium to form a developer image, at least within a predetermined range from the tip of the second side. [Effects of the Invention]

[0007] According to one or more aspects of the present disclosure, borderless images can be formed on a variety of recording media types. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a cross-sectional view schematically illustrating a configuration of an image forming apparatus according to an embodiment. [Figure 2] FIG. 2 is a block diagram showing the configuration of a control system of the image forming apparatus. [Figure 3]10A and 10B are block diagrams showing an example of a hardware configuration. [Figure 4] 4 is a flowchart showing a printing operation of the image forming apparatus. [Figure 5] FIG. 10 is a schematic diagram showing a printing defect that occurs on the second side of double-sided printing. [Figure 6] FIG. 2 is a cross-sectional view of the photosensitive member and transfer roller of the black (K) image forming unit. [Figure 7] 10 is a table showing the results of an experiment in which an image forming apparatus that generates printing defects was used to perform double-sided printing of borderless, all-black printing on paper of different thicknesses. [Figure 8] 1 is a schematic diagram illustrating an example of double-sided printing according to the present embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0009] Embodiment FIG. 1 is a cross-sectional view schematically showing the configuration of an image forming apparatus 100 according to an embodiment. Image forming apparatus 100 is, for example, a color printer that forms an image on a medium, such as paper, using an electrophotographic method. Inside the image forming apparatus 100, four image forming units 101K, 101Y, 101M, and 101C, which are four independent printing mechanisms corresponding to black (K), yellow (Y), magenta (M), and cyan (C), are arranged in a line in the paper conveying direction D. In other words, the image forming units 101K, 101Y, 101M, and 101C are The toner image forming unit 100 forms a plurality of developer images using toners of a plurality of colors.

[0010] Note that image forming units 101K, 101Y, 101M, and 101C are configured similarly except for the color of toner they use. Therefore, hereinafter, when there is no need to particularly distinguish between image forming units 101K, 101Y, 101M, and 101C, each of image forming units 101K, 101Y, 101M, and 101C will be referred to as image forming unit 101.

[0011] Image forming unit 101K is a printing mechanism (image forming mechanism) that forms a black toner image, image forming unit 101Y is a printing mechanism that forms a yellow toner image, image forming unit 101M is a printing mechanism that forms a magenta toner image, and image forming unit 101C is a printing mechanism that forms a cyan toner image. The number of image forming units 101 is not limited to four.

[0012] In image forming apparatus 100, as shown in FIG. 1, image forming unit 101K is disposed at the most upstream position in the medium transport direction D. The order of arrangement of the image forming units 101Y, 101M, and 101C arranged downstream of the image forming unit 101K is not limited to the example shown in FIG.

[0013] As described above, the image forming units 101K, 101Y, 101M, and 101C have the same structure except for the color of the toner contained therein, so the structure will be described here using the image forming unit 101K.

[0014] The image forming unit 101K includes a photosensitive member 102 as an image carrier, a charging roller 103 as a charging section, a developing roller 104 as a developing section, a supply roller 105 as a supply section, a developing blade 106 as a regulating section, a cleaning blade 107 as a cleaning section, and a static eliminator 108 as a static eliminator.

[0015] An LED (Light Emitting Diode) head 110K serving as an exposure unit is disposed above the photosensitive member 102 of the image forming unit 101K. The LED head 110K emits light in accordance with input image data, thereby forming an electrostatic latent image on the surface of the photosensitive member 102, which has been uniformly charged by the charging roller 103. The exposure unit is not limited to the LED head 110K, but may be a laser scanning optical system equipped with a semiconductor laser or the like. It should be noted that LED heads 110Y, 110M, and 110C are also disposed at positions corresponding to the image forming units 101Y, 101M, and 101C, respectively. Here, when there is no need to particularly distinguish between the LED heads 110K, 110Y, 110M, and 110C, each of the LED heads 110K, 110Y, 110M, and 110C will be referred to as an LED head 110.

[0016] The image forming unit 101K also includes a toner cartridge 109. The toner cartridge 109 has a toner storage section 109a for storing toner therein. The toner stored in the toner cartridge 109 is supplied to the toner storage section 109a, and the toner is frictionally charged by the supply roller 105 and the developing roller 104.

[0017] The toner on the developing roller 104 is thinned by the developing blade 106 and carried to the contact area between the photoreceptor 102 and the developing roller 104 .

[0018] At the contact point between the photosensitive member 102 and the developing roller 104, toner moves from the developing roller 104 to the electrostatic latent image formed on the photosensitive member 102, and a toner image corresponding to the electrostatic latent image is formed on the surface of the photosensitive member 102.

[0019] A transfer roller 112K is disposed below the photosensitive member 102 with the transfer belt 111 sandwiched therebetween. Transfer rollers 112Y, 112M, and 112C are also disposed at positions corresponding to the image forming units 101Y, 101M, and 101C, respectively. Transfer rollers 112K, 112Y, 112M, and 112C transfer a plurality of toner images onto a single sheet of paper in turn. Here, when there is no need to particularly distinguish between the transfer rollers 112K, 112Y, 112M, and 112C, each of the transfer rollers 112K, 112Y, 112M, and 112C will be referred to as the transfer roller 112.

[0020] The transfer roller 112K is pressed against the photosensitive member 102 with the transfer belt 111 sandwiched therebetween, and a transfer nip portion is formed between the transfer roller 112K and the photosensitive member 102.

[0021] The transfer belt 111 is stretched between a drive roller 113 and a driven roller 114. When the drive roller 113 rotates, a sheet of paper adsorbed on the transfer belt 111 is transported. In other words, the transfer belt 111 is a belt that transports a sheet of paper.

[0022] The toner image formed on the photoreceptor 102 is transferred onto a sheet of paper at the transfer nip. Any toner that is not transferred onto the paper is scraped off by a cleaning blade 107 and discarded. After the unnecessary toner is scraped off by the cleaning blade 107, the photoconductor 102 is neutralized by the neutralization device 108.

[0023] The same operations as above are performed in the image forming units 101Y, 101M, and 101C and the transfer rollers 112Y, 112M, and 112C, and a toner image corresponding to the input image data is transferred onto a sheet of paper that has been conveyed.

[0024] Then, the sheet of paper is separated from the transfer belt 111 and transported to a fixing unit 115 serving as a fixing section. The fixing unit 115 heats and melts the toner image on the sheet of paper, thereby fixing the toner image on the sheet of paper.

[0025] Here, the configuration relating to paper transport will be described. The sheets are stored in a tray 120 and are picked up one by one by a pickup roller 121.

[0026] The fed sheet of paper is sent to the transfer belt 111 by a pair of conveying rollers 122A and a pair of conveying rollers 122B.

[0027] When the image forming unit 101, transfer roller 112, and fixing unit 115 have completed the transfer and fixing of the toner image onto the first side of a sheet of paper, the sheet of paper is transported to a double-sided printing unit 124 as a double-sided unit by a separator (not shown) and a pair of discharge rollers 123A.

[0028] The duplex unit 124 reverses the face of a sheet of paper onto which multiple toner images have been fused. Then, the double-sided printing unit 124 conveys the sheet of paper, on which the multiple toner images have been fixed and which has been turned over, to the transfer belt 111. For example, a sheet of paper transported into the double-sided printing unit 124 is reversed (switched back) by the duplex roller pair 125A, so that the positions of the first side and the second side behind the first side are reversed, and the sheet is transported to the transport roller pair 122A by multiple duplex roller pairs 125B and 125C.

[0029] The sheet of paper is sent to transfer belt 111 by conveying roller pair 122A and conveying roller pair 122B, and a toner image is transferred and fixed onto the second surface thereof by image forming unit 101, transfer roller 112 and fixing unit 115.

[0030] Although an example in which reversal (switchback) is performed by the duplex roller pair 125A has been described above, the first embodiment is not limited to such an example. For example, a sheet of paper on which a plurality of toner images have been fixed may be reversed by discharge rollers 123C and conveyed to double-sided printing unit 124 via discharge rollers 123B and conveyance rollers 123A.

[0031] When the transfer and fixing of the toner image onto the second side is completed, the sheet is discharged to the stacker 126 by a separator (not shown) and discharge roller pairs 123B and 123C.

[0032] The numbers of pickup roller 121, conveying roller pairs 122A and 122B, discharging roller pairs 123A, 123B and 123C, and duplex roller pairs 125A, 125B and 125C are merely examples and are not limited to those shown in FIG.

[0033] FIG. 2 is a block diagram showing the configuration of a control system of the image forming apparatus 100. As shown in FIG. The printer control unit 130 is a control unit that controls the overall functions of the image forming apparatus 100 .

[0034] For example, the printer control unit 130 executes double-sided printing, which is double-sided image formation. Specifically, printer control unit 130 causes transfer belt 111 to transport a sheet of paper, causes multiple image forming units 101 to form multiple first toner images, causes multiple transfer rollers 112 to transfer the multiple first toner images onto a first side of the sheet of paper, causes fixing unit 115 to fix the multiple first toner images onto the first side, and causes duplex printing unit 124 to invert the sheet of paper. Then, printer control unit 130 causes transfer belt 111 to transport the inverted sheet of paper, causes multiple image forming units 101 to form multiple second toner images, causes multiple transfer rollers 112 to transfer the multiple second toner images onto a second side, which is the side opposite to the first side of the sheet of paper, and causes fixing unit 115 to fix the multiple second toner images onto the second side.

[0035] Here, in this embodiment, when double-sided printing is performed using borderless image formation (borderless printing), which forms an image on a sheet of paper thinner than a predetermined thickness and without leaving any margins on that sheet of paper, the printer control unit 130 does not cause the black image forming unit 101K, which is one of the multiple image forming units 101 and is located most upstream in the transport direction of the sheet of paper, to form a toner image, at least within a predetermined range from the tip of the second side.

[0036] Specifically, within the predetermined range, the printer control unit 130 replaces the black toner image by subtractive color mixing by overlapping multiple toner images formed by multiple image forming units 101Y, 101M, and 101C, excluding image forming unit 101K. In addition, the printer control unit 130 may replace the black toner image by subtractive color mixing by overlapping multiple toner images formed by multiple image forming units 101Y, 101M, and 101C, excluding image forming unit 101K, on ​​the entire second surface. Furthermore, when a sheet of paper is thinner than a predetermined thickness and double-sided printing is performed using borderless printing, the printer control unit 130 may replace the black toner image by subtractive color mixing by overlapping multiple toner images formed by multiple image forming units 101Y, 101M, and 101C, excluding image forming unit 101K, on ​​not only the second side but also the entire first side of the sheet of paper.

[0037] High voltage control unit 132 controls the high voltage, which is a relatively high voltage used in image forming apparatus 100 .

[0038] The receiving unit 127 receives print data, which is image formation data, from a host device UD connected to a network such as a LAN (Local Area Network), and setting data including the paper thickness of the recording medium, whether borderless printing is enabled, whether double-sided printing is enabled, paper type, paper size, etc., which are set in the printer driver of the host device UD. The print data and setting data received by the receiving unit 127 are sent to an image information creating unit 131 included in the printer control unit 130.

[0039] The image information creation unit 131 generates image data required for printing, which is image formation, from the print data and setting data. The image information creation unit 131 can switch between four-color printing and three-color printing depending on the paper thickness, whether borderless printing is required, and whether double-sided printing is required.

[0040] Here, the difference between the image data created by the image information creating unit 131 when performing four-color printing and the image data created when performing three-color printing will be described.

[0041] When performing four-color printing, image information creation unit 131 creates four sets of image data for black (K), yellow (Y), magenta (M), and cyan (C). Four image forming units 101K, 101Y, 101M, and 101C form toner images based on the image data for their corresponding colors. These toner images are then transferred onto a sheet of paper in succession, forming the desired color image.

[0042] On the other hand, when three-color printing is performed, image information creation unit 131 forms toner images corresponding to the black toner image formed by black (K) image forming unit 101K using three colors of toner, yellow (Y), magenta (M), and cyan (C), based on the image data for black (K), and forms a black (hereinafter sometimes referred to as process black) image by overlapping these toner images. Therefore, during three-color printing, the three image data for yellow (Y), magenta (M), and cyan (C) are the image data for each color plus process black data for forming a black image using process black.

[0043] In this case, image data other than the image data corresponding to the process black toner image formed with the three colors of toner, yellow (Y), magenta (M), and cyan (C), may be formed appropriately according to the print data. In other words, image data for the single colors of yellow, magenta, and cyan may not be created depending on the print data.

[0044] Returning to the explanation of FIG. 2, the belt motor 140 controls the driving of the driving roller 113 in accordance with instructions from the printer control unit 130 . The developing motor 141 controls the driving of the photosensitive member 102, the developing roller 104, and the supply roller 105 in the image forming unit 101 according to instructions from the printer control unit 130. The transport motor 142 controls the driving of the transport roller pair 122A and 122B in accordance with instructions from the printer control unit .

[0045] The pickup motor 143 controls the driving of the pickup roller 121 in accordance with instructions from the printer control unit 130 . The discharge motor 144 controls the driving of the discharge roller pairs 123A, 123B, and 123C in accordance with instructions from the printer control unit 130. The duplex motor 145 controls the driving of the duplex roller pairs 125A, 125B, and 125C in accordance with instructions from the printer control unit 130.

[0046] The charging voltage generating unit 146 generates and stops the charging voltage to the charging roller 103 in accordance with instructions from the high voltage control unit 132 . The developing voltage generating unit 147 generates and stops the development voltage to the developing roller 104 in accordance with instructions from the high voltage control unit 132 . A supply voltage generating unit 148 generates and stops the development voltage to the supply roller 105 in accordance with instructions from the high voltage control unit 132 . The transfer voltage generating unit 149 generates and stops the transfer voltage to the transfer roller 112 in accordance with instructions from the high voltage control unit 132 .

[0047] As shown in FIG. 3A, part or all of the printer control unit 130 and high-voltage control unit 132 described above can be configured with a memory 10 and a processor 11 such as a CPU (Central Processing Unit) that executes a program stored in memory 10. Such a program may be provided over a network or may be provided recorded on a recording medium. That is, such a program may be provided as a computer program product, for example.

[0048] In addition, part or all of the printer control unit 130 and the high voltage control unit 132 can also be configured with a processing circuit 12 such as a single circuit, a composite circuit, a processor operated by a program, a parallel processor operated by a program, an ASIC (Application Specific Integrated Circuit), or an FPGA (Field Programmable Gate Array), as shown in FIG. 3(B). As described above, printer control 130 and high voltage control 132 can be implemented by processing circuitry.

[0049] FIG. 4 is a flowchart showing the printing operation of the image forming apparatus 100. The flowchart shown in FIG. 4 starts when the receiving unit 127 receives print data and setting data from the higher-level device UD.

[0050] The image information creating unit 131 acquires the setting data from the receiving unit 127 (S10). As described above, the setting data indicates the paper thickness, whether borderless printing is required, whether double-sided printing is required, the paper type, the paper size, and the like.

[0051] Next, the image information creation unit 131 references the setting data to determine whether the printing to be performed this time is borderless printing (S11). If the printing to be performed this time is borderless printing (Yes in S11), the process proceeds to step S12, and if the printing to be performed this time is not borderless printing (No in S11), the process proceeds to step S15.

[0052] In step S12, the image information creation unit 131 determines whether the printing to be performed this time is double-sided printing by referring to the setting data. If the printing to be performed this time is double-sided printing (Yes in S12), the process proceeds to step S13, and if the printing to be performed this time is not double-sided printing (No in S12), the process proceeds to step S15.

[0053] In step S13, the image information creation unit 131 determines whether the printing to be performed this time is to use thin paper by referring to the setting data. If the printing to be performed this time is to use thin paper (Yes in S13), the process proceeds to step S14, and if the printing to be performed this time is not to use thin paper (No in S13), the process proceeds to step S15.

[0054] In step S14, the image information creating unit 131 executes three-color printing without using black (K) toner. On the other hand, in step S15, the image information creating unit 131 executes four-color printing using black (K) toner.

[0055] In the present embodiment, the setting data includes information on the paper thickness, but the present embodiment is not limited to this example. For example, the user may set the paper thickness using an operation panel (not shown) of the image forming apparatus 100.

[0056] Next, the reason why four-color printing and three-color printing can be performed in this embodiment will be explained. Here, double-sided printing is described as a process in which a toner image is first transferred and fixed on the first side of a sheet of paper, and then a toner image is transferred and fixed on the second side, which is the reverse side of the paper. Figure 5 is a schematic diagram showing a printing defect that occurs on the second side of double-sided printing when borderless printing in all black is performed in double-sided printing in another image forming device that has a similar configuration to image forming device 100 but does not perform three-color printing. When performing borderless black printing on both sides, we confirmed that within 3 mm from the leading edge of the second side of the double-sided print, areas P1, P2, and P3, as shown in Figure 5, occur where the black toner is not printed on the paper PA.

[0057] Next, the cause of the printing defect shown in FIG. 5 will be explained. FIG. 6 is a cross-sectional view of the vicinity of the photosensitive member 160 and transfer roller 161 of the black (K) image forming unit in the image forming apparatus used in FIG. Here, FIG. 6 shows the state immediately before a black toner image Im is transferred onto the second side of the double-sided printing paper PA. In double-sided printing, after the toner image transferred to the first side of the double-sided printing is fixed, the paper PA is bent when the paper is reversed (switched back) in a unit such as the double-sided printing unit 124 shown in Fig. 1. For example, in the image forming apparatus 100 shown in Fig. 1, the paper is bent upward when the duplex roller pair 125A reverses the paper.

[0058] As a result, as shown in Figure 6, the leading edge ED of the paper PA reaches the transfer nip portion in an upwardly curved state. As a result, the leading edge ED of the paper PA and the leading edge of the toner image Im come into contact before the toner image Im is transferred, and the leading edge of the toner image Im is scraped off by the leading edge ED of the paper PA. This causes the printing defect shown in Figure 5.

[0059] FIG. 7 is a table showing the results of an experiment in which an image forming apparatus in which the printing defects shown in FIG. 5 occur was used to perform double-sided printing of borderless, all-black printing on paper of different thicknesses.

[0060] In this experiment, 80gsm Excellent White (manufactured by Oki Electric), 120gsm ColorCopy 120 (manufactured by Mondi), 160gsm ColorCopy 160 (manufactured by Mondi), 200gsm ColorCopy 200 (manufactured by Mondi) and 300gsm ColorCopy 300 (manufactured by Mondi) were used.

[0061] If an image defect occurs at the leading edge of the paper as shown in FIG. 5, it is judged as "X", and if no such image defect occurs, it is judged as "O".

[0062] The experimental results shown in Figure 7 confirmed that when the paper thickness is thin (here, less than 160 gsm), image defects such as those shown in Figure 5 occur on the second side of double-sided printing.

[0063] As described above, the causes of the image defects shown in FIG. 5 are borderless printing, double-sided printing, and paper that is thinner than a predetermined thickness. Therefore, in this embodiment, when the printing to be performed satisfies all of these factors, in other words, when the printing satisfies predetermined conditions, three-color printing is performed using process black without using black (K), thereby preventing image defects such as those shown in Figure 5.

[0064] In the image forming units 101Y, 101M, and 101C for yellow (Y), magenta (M), and cyan (C), which are arranged downstream of the black (K) image forming unit, when a sheet of paper passes through the black (K) transfer nip, the sheet is electrostatically attracted to the transfer belt 111, and the state in which the leading edge of the sheet of paper is bent upward is corrected. As a result, the leading edge of the sheet of paper does not come into contact with the leading edge of the toner image, and the image defect shown in Figure 5 does not occur.

[0065] As shown in the experimental results in Fig. 7, paper with a thickness of less than 160 gsm produces image defects such as those shown in Fig. 5, so in this embodiment, paper with a thickness of less than 160 gsm is determined to be thin paper. This threshold value varies depending on the configuration of image forming apparatus 100, such as image forming unit 101, and is not limited to this value.

[0066] Furthermore, the paper thickness is not limited to being set numerically. For example, an option corresponding to the paper thickness may be selectable on the operation panel of the image forming apparatus 100 or in the printer driver of the host device UD, such as "thin paper" for paper less than 160 gsm and "heavy paper" for paper 160 gsm or more. In this case, the option does not need to be set with a threshold value. For example, if the setting option "thin paper" is less than 170 gsm, this includes the threshold value of 160 gsm, so three-color printing can be performed even when this "thin paper" option is selected. In other words, three-color printing can be performed when an option including paper with a thickness that causes image defects is selected.

[0067] In the above embodiment, three-color printing is performed without using black (K) toner when printing meets predetermined conditions, but the range in which images are formed using the three colors yellow (Y), magenta (M), and cyan (C) may be limited. As shown in FIG. 5, image defects occur within 3 mm from the leading edge of the second side of the paper in double-sided printing. Therefore, as shown in Figure 8, in the leading edge portion PA#1 of the second side of the double-sided print, within 3 mm from the leading edge, images are formed in three colors: yellow (Y), magenta (M), and cyan (C), and in the remaining portions, i.e., the first side of the double-sided print and the portion PA#2 3 mm behind the leading edge of the second side of the double-sided print, four-color printing may be performed: black (K), yellow (Y), magenta (M), and cyan (C).

[0068] For example, there may be differences in color in the printed result when an image is formed in three colors in double-sided printing and when an image is formed in four colors in single-sided printing. However, by forming an image in three colors only within 3 mm of the leading edge of the second side of double-sided printing as described above, it is possible to reduce the occurrence of differences in color due to printing conditions.

[0069] Furthermore, for the purpose of preventing differences in color, images on both the first and second sides of double-sided printing may be formed in three colors if the printing satisfies predetermined conditions.

[0070] As described above, according to this embodiment, when borderless printing, double-sided printing, and the thickness of the paper is thinner than a predetermined thickness, printing defects can be prevented by forming images in three colors, yellow, magenta, and cyan, without using black, at least in a predetermined range including the leading edge of the second side of the double-sided printing. [Explanation of symbols]

[0071] 100 Image forming device, 101 Image forming unit, 111 Transfer belt, 112 Transfer roller, 115 Fixing unit, 124 Double-sided printing unit, 130 Printer control unit

Claims

1. a belt for transporting the medium; a plurality of image forming units arranged in order in a transport direction of the medium, the image forming units forming a plurality of developer images using developers of a plurality of colors; a plurality of transfer units that sequentially transfer the plurality of developer images onto the transported medium; a fixing unit that fixes the plurality of developer images onto the medium; a duplex unit that conveys the medium, with the developer images fixed thereon and inverted, to the belt; a control unit that causes the belt to transport the medium, causes the multiple image forming units to form multiple first developer images, causes the multiple transfer units to transfer the multiple first developer images to a first side of the medium, causes the fixing unit to fix the multiple first developer images to the first side, causes the reversing unit to reversal the medium, causes the reversed medium to be transported to the belt, causes the multiple image forming units to form multiple second developer images, causes the multiple transfer units to transfer the multiple second developer images to a second side that is opposite to the first side of the medium, and causes the fixing unit to fix the multiple second developer images to the second side, thereby executing double-sided image formation; When the medium is thinner than a predetermined thickness and borderless image formation is performed in which an image is formed without a margin on the medium, the control unit does not cause one image forming unit, among the plurality of image forming units, that is arranged most upstream in the direction of transport of the medium to form a developer image at least within a predetermined range from the leading edge of the second side. An image forming apparatus comprising:

2. the one image forming unit forms a black developer image using a black developer, The control unit causes the black developer image to be substituted by subtractive color mixing within the predetermined range by overlapping the plurality of developer images formed by the plurality of image forming units excluding the one image forming unit.

2. The image forming apparatus according to claim 1, wherein:

3. the one image forming unit forms a black developer image using a black developer, The control unit replaces the black developer image by subtractive color mixing by superimposing the plurality of developer images formed by the plurality of image forming units excluding the one image forming unit on the entire second surface.

2. The image forming apparatus according to claim 1, wherein:

4. When the medium is thinner than a predetermined thickness and borderless image formation is performed in which an image is formed on the medium without providing a margin, the control unit replaces the black developer image on the entire first side by subtractive color mixing by overlapping a plurality of developer images formed by the plurality of image forming units excluding the one image forming unit.

3. The image forming apparatus according to claim 2, wherein:

Images