Image forming apparatus

Abstract

To accurately perform calibration related to image formation without reducing productivity.SOLUTION: An image forming apparatus comprises image forming units, an intermediate transfer belt, and stretch rollers. The intermediate transfer belt has at least one area of a pair of areas sandwiching a print area in a width direction as a test area. The image forming unit transfers a test pattern image to the test area. The image forming apparatus further comprises image density sensors that output values according to the quantity of reflected light from the test area, and a sensor opposing member that has opposing parts facing the image density sensors with the intermediate transfer belt therebetween. The end edge position in the width direction of the opposing part is on the inside in the width direction of the end edge positions in the width direction of the stretch rollers or is the same in the width direction as the end edge position in the width direction of the stretch rollers.SELECTED DRAWING: Figure 5

Description

[Technical field]

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

[0002] The image forming apparatus includes an intermediate transfer belt. The intermediate transfer belt carries an image formed using toner and rotates to transfer the image onto a sheet. Such an image forming apparatus is disclosed in, for example, Patent Document 1. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2014-59336 A Summary of the Invention [Problem to be solved by the invention]

[0004] Conventionally, in order to calibrate image density, etc., a test pattern image is transferred to the end of the intermediate transfer belt in the width direction perpendicular to the rotation direction. In addition, a reflective optical sensor is used as an image density sensor to detect the density of the test pattern image on the intermediate transfer belt. With this method, calibration can be performed during the execution of a print job, so productivity is not impaired.

[0005] However, if the end of the intermediate transfer belt in the width direction is bent, the positional relationship between the intermediate transfer belt and the image density sensor is shifted, which may cause an inconvenience in that the detection by the image density sensor becomes inaccurate, i.e., calibration may not be performed accurately.

[0006] The present invention has been made to solve the above-mentioned problems, and has an object to provide an image forming apparatus that is capable of accurately performing calibration related to image formation without reducing productivity. [Means for solving the problem]

[0007] In order to achieve the above object, an image forming apparatus according to one aspect of the present invention includes an image forming unit that forms an image using toner, an endless intermediate transfer belt that rotates while carrying an image transferred from the image forming unit on its outer peripheral surface, and a tension roller that is rotatable about an axis extending in a width direction perpendicular to the rotation direction of the intermediate transfer belt, is arranged inside the width edge of the intermediate transfer belt, and contacts the inner peripheral surface of the intermediate transfer belt to tension the intermediate transfer belt. The intermediate transfer belt has a print area inside the width edge of the tension roller, and has at least one of a pair of areas that sandwich the print area in the width direction as a test area. The image forming unit transfers a print image to be printed on a sheet to the print area, and transfers a test pattern image for calibration related to image formation by the image forming unit to the test area. The image density sensor is disposed at a distance from the outer peripheral surface of the intermediate transfer belt, irradiates the test area with light, and outputs a value corresponding to the amount of light reflected from the test area, and a sensor facing member is in contact with the inner peripheral surface of the intermediate transfer belt and has a facing portion facing the image density sensor across the intermediate transfer belt. The edge position in the width direction of the facing portion is located inside in the width direction of the edge position in the width direction of the tension roller, or is the same in the width direction as the edge position in the width direction of the tension roller. Effect of the Invention

[0008] In the present invention, calibration related to image formation can be performed accurately without reducing productivity. [Brief description of the drawings]

[0009] [Figure 1] 1 is a schematic diagram of an image forming apparatus according to an embodiment. [Diagram 2] FIG. 2 is a schematic diagram of an image forming unit according to an embodiment. [Diagram 3] FIG. 2 is a diagram showing a schematic diagram of a tension roller and its surroundings according to an embodiment. [Figure 4]1 is a block diagram of an image forming apparatus according to an embodiment. [Diagram 5] FIG. 2 is a diagram illustrating a sensor facing member and its periphery according to an embodiment. [Figure 6] FIG. 2 is a diagram illustrating a sensor facing member according to an embodiment. [Figure 7] FIG. 2 illustrates the locations of print and test areas according to an embodiment. [Figure 8] 13A and 13B are diagrams for explaining inconveniences that arise when the edge of the sensor facing member is located outside the edge of the tension roller. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, an embodiment of the present invention will be described using a tandem color laser printer as an example. Note that the present invention is not limited to printers, but can also be applied to multifunction machines that have a copy function, etc.

[0011] <Configuration of Image Forming Apparatus> The configuration of an image forming apparatus 100 according to this embodiment is shown in Fig. 1. The image forming apparatus 100 is placed on a flat floor surface FL. The up-down direction of the image forming apparatus 100 is perpendicular to the floor surface FL.

[0012] The image forming apparatus 100 includes a main transport path MP. The image forming apparatus 100 also includes a sheet cassette CA. The sheet cassette CA is detachable from the main body of the image forming apparatus 100. The sheet cassette CA accommodates sheets S to be used in a print job. The main transport path MP runs from a supply position P0 for the sheet S from the sheet cassette CA, through a transfer position P1 and a fixing position P2, to an output tray ET.

[0013] In a print job, a sheet S in a sheet cassette CA is supplied from a supply position P0 to a main transport path MP. The sheet S is transported along the main transport path MP. An image is formed using toner. The image is then printed on the sheet S during transport. In other words, a transfer process of the image onto the sheet S during transport is performed at a transfer position P1. A fixing process of the image onto the sheet S is performed at a fixing position P2.

[0014] The image forming apparatus 100 includes an image forming unit 1. There are four image forming units 1. The four image forming units 1 correspond to the colors cyan, magenta, yellow, and black, respectively. The four image forming units 1 form images using toner of the corresponding color. The following description focuses on one image forming unit 1 and explains its configuration, but the configurations of the four image forming units 1 are the same as each other. Therefore, the description of the configurations of the other image forming units 1 will be omitted, as the following description is incorporated herein.

[0015] As shown in FIG. 2, the image forming unit 1 includes a photoconductor drum 11, a charging device 12, an exposure device 13, a developing device 14, and a cleaning device 15. When the image forming unit 1 forms an image, the photoconductor drum 11 rotates. The charging device 12 charges the outer peripheral surface of the photoconductor drum 11. The exposure device 13 exposes the outer peripheral surface of the photoconductor drum 11 to light, and forms an electrostatic latent image on the outer peripheral surface of the photoconductor drum 11. The developing device 14 supplies toner to the outer peripheral surface of the photoconductor drum 11, and develops the electrostatic latent image into a toner image. The toner image on the outer peripheral surface of the photoconductor drum 11 is primarily transferred to an intermediate transfer belt 2, which will be described later. The cleaning device 15 removes toner that has not been transferred to the intermediate transfer belt 2 and remains on the outer peripheral surface of the photoconductor drum 11.

[0016] 1, the image forming apparatus 100 includes an intermediate transfer belt 2. The intermediate transfer belt 2 is an endless belt. The intermediate transfer belt 2 is supported so as to be rotatable. The intermediate transfer belt 2 is one component of an intermediate transfer unit.

[0017] In the following description, the rotation direction of the intermediate transfer belt 2 is referred to as the belt rotation direction, and the belt rotation direction is denoted by the symbol Dr. The width direction of the intermediate transfer belt 2 is referred to as the belt width direction, and the belt width direction is denoted by the symbol Dw. The belt width direction Dw is a direction perpendicular to the belt rotation direction Dr and perpendicular to the up-down direction (i.e., the horizontal direction). The belt width direction Dw corresponds to the main scanning direction, and the belt rotation direction Dr corresponds to the sub-scanning direction. In Figures 1 and 2, the direction perpendicular to the paper surface is the belt width direction Dw.

[0018] The intermediate transfer belt 2 includes, for example, a base layer and a rubber layer (i.e., an elastic layer) on the base layer. The base layer may be made of polyimide resin or PVDF (polyvinylidene fluoride) mixed with a conductive material such as an ion conductive material and conductive carbon to impart conductivity. The rubber layer may be made of hydrin rubber, chloroprene rubber, polyurethane rubber, or the like. A coating layer may be provided to protect the rubber layer. The coating layer may be made of acrylic, silicone, fluororesin, or the like.

[0019] The image forming apparatus 100 includes a tension roller 3 as one component of the intermediate transfer unit. The tension roller 3 is supported rotatably about an axis extending in the belt width direction Dw. Specifically, the tension roller 3 is attached to a rotating shaft 300 (see FIG. 3) that is rotatable about an axis extending in the belt width direction Dw. The tension roller 3 rotates together with the rotating shaft 300.

[0020] The tension roller 3 is disposed in an area on the inner periphery of the intermediate transfer belt 2. The inner periphery of the intermediate transfer belt 2 is an area surrounded by the inner periphery of the intermediate transfer belt 2, and is an area inside the annular body of the intermediate transfer belt 2. The tension roller 3 contacts the inner periphery of the intermediate transfer belt 2. The tension roller 3 suspends the intermediate transfer belt 2 so that it can rotate. In order to suspend the intermediate transfer belt 2 so that it can rotate, a plurality of tension rollers 3 are provided in the inner periphery of the intermediate transfer belt 2. The number of tension rollers 3 provided is appropriately changed depending on the size of the intermediate transfer belt 2, etc.

[0021] As shown in Fig. 3, the tension roller 3 is disposed inside the edge 2a of the intermediate transfer belt 2 in the belt width direction Dw. Fig. 3 corresponds to a cross section of the tension roller 3 and its periphery cut by a plane parallel to the belt width direction Dw. Fig. 3 is a schematic illustration of the tension roller 3 and its periphery, and does not directly show the actual dimensions and shape.

[0022] Of the tension roller 3, an edge 3a on one side in the belt width direction Dw (i.e., the axial direction of the tension roller 3) is located more inward in the belt width direction Dw than an edge 2a on one side in the belt width direction Dw of the intermediate transfer belt 2, and an edge 3a on the other side in the belt width direction Dw is located more inward in the belt width direction Dw than an edge 2a on the other side in the belt width direction Dw of the intermediate transfer belt 2.

[0023] In this configuration, the tension roller 3 contacts the inner peripheral surface of the intermediate transfer belt 2 on the inside of both end edges 2a in the belt width direction Dw of the intermediate transfer belt 2. One end of the intermediate transfer belt 2 in the belt width direction Dw protrudes outward in the belt width direction Dw from one end edge 3a of the tension roller 3 in the belt width direction Dw. The other end of the intermediate transfer belt 2 in the belt width direction Dw protrudes outward in the belt width direction Dw from the other end edge 3a of the tension roller 3 in the belt width direction Dw.

[0024] One of the multiple tension rollers 3 is connected to a belt motor BM (see FIG. 4). In the following description, the tension roller 3 connected to the belt motor BM is referred to as a drive roller, and the drive roller is denoted by the reference symbol 30. The drive roller 30 rotates by a driving force transmitted from the belt motor BM. The intermediate transfer belt 2 rotates in response to the rotation of the drive roller 30. The other tension rollers 3 rotate in response to the intermediate transfer belt 2.

[0025] The image forming apparatus 100 includes a pair of unit frames Fr as components of the intermediate transfer unit. The pair of unit frames Fr are disposed to face each other in the belt width direction Dw with the intermediate transfer belt 2 therebetween. One end and the other end of a rotation shaft 300 to which the tension roller 3 is attached are rotatably supported by the pair of unit frames Fr, respectively.

[0026] As shown in FIG. 1, the image forming apparatus 100 includes primary transfer rollers 201 as a component of the intermediate transfer unit. There are four primary transfer rollers 201. One primary transfer roller 201 is assigned to each of the colors cyan, magenta, yellow, and black. Each primary transfer roller 201 is disposed in an inner peripheral region of the intermediate transfer belt 2 and supported so as to be rotatable around an axis extending in the belt width direction Dw. Each primary transfer roller 201 is disposed opposite a photoconductor drum 11 carrying an image of the corresponding color, with the intermediate transfer belt 2 sandwiched therebetween. Each primary transfer roller 201 sandwiches the intermediate transfer belt 2 between itself and the photoconductor drum 11 carrying an image of the corresponding color.

[0027] The image forming apparatus 100 also includes a secondary transfer roller 202. The secondary transfer roller 202 is supported rotatably about an axis extending in the belt width direction Dw. The secondary transfer roller 202 is in pressure contact with the outer circumferential surface of the intermediate transfer belt 2 at a transfer position P1. The secondary transfer roller 202 sandwiches the intermediate transfer belt 2 between itself and the drive roller 30, forming a transfer nip between itself and the intermediate transfer belt 2. This forms the transfer nip at the transfer position P1. The main transport path MP passes through the transfer nip.

[0028] In a print job, a sheet S is transported toward a transfer position P1 (i.e., a transfer nip). The sheet S passes through the transfer nip during transport. That is, the intermediate transfer belt 2 contacts the sheet S during transport downstream of the contact position with each photoconductor drum 11 in the belt rotation direction Dr.

[0029] Each image forming unit 1 forms an image using toner of a corresponding color. Each primary transfer roller 201 primarily transfers the image onto the outer circumferential surface of the intermediate transfer belt 2.

[0030] The intermediate transfer belt 2 rotates while carrying on its outer circumferential surface the images that are primarily transferred from the photoreceptor drums 11. While the sheet S is passing through the transfer nip, the sheet S comes into contact with the outer circumferential surface of the intermediate transfer belt 2. A transfer voltage is applied to the secondary transfer roller 202 from a transfer voltage power source (not shown). The secondary transfer roller 202 forms a transfer electric field between itself and the intermediate transfer belt 2, thereby performing a second transfer of the image onto the sheet S that is passing through the transfer nip.

[0031] The image forming apparatus 100 includes a cleaning unit 203. The cleaning unit 203 faces the outer circumferential surface of the intermediate transfer belt 2 downstream of the transfer position P1 in the belt rotation direction Dr. The cleaning unit 203 cleans the outer circumferential surface of the intermediate transfer belt 2.

[0032] The image forming apparatus 100 includes a fixing unit FX. The fixing unit FX includes a heating roller and a pressure roller. The fixing unit FX is disposed at a fixing position P2. The heating roller has a built-in heater. The pressure roller is in pressure contact with the heating roller. The heating roller and the pressure roller are in pressure contact with each other to form a fixing nip at the fixing position P2.

[0033] In a print job, the sheet S passes through the fixing position P2. That is, the sheet S is sandwiched in the fixing nip. The fixing unit FX heats the sheet S as it passes through the fixing position P2. Pressure is applied to the sheet S at the fixing position P2. The fixing unit FX applies heat and pressure to the sheet S to fix the toner image to the sheet S. The sheet S after the fixing process is discharged to an output tray ET.

[0034] The image forming apparatus 100 includes a transport unit, the reference numerals of which are omitted. The transport unit includes a transport roller pair. The transport roller pair includes a pair of rollers. The pair of rollers has a transport nip between the rollers. The transport roller pair rotates to transport the sheet S that has entered the transport nip. The transport unit transports the sheet S along a main transport path MP. The transport unit also transports the sheet S along a double-sided printing transport path DP, which will be described later.

[0035] The image forming apparatus 100 is capable of executing a double-sided print job in which an image is printed on both sides of a sheet S, in addition to a single-sided print job in which an image is printed on only one side of the sheet S. In order to execute a double-sided print job, the image forming apparatus 100 is provided with a double-sided print transport path DP.

[0036] The double-sided printing transport path DP branches off from the main transport path MP at a branching position P3 downstream of the fixing position P2 in the sheet transport direction of the main transport path MP, and merges with the main transport path MP at a merging position P4 upstream of the transfer position P1 in the sheet transport direction of the main transport path MP.

[0037] When the job to be executed is a single-sided print job, the sheet S passes through the transfer nip only once, and a single transfer process is performed on the sheet S while it is passing through the transfer nip. After the first transfer process, the sheet S is discharged directly onto the discharge tray ET.

[0038] When the job to be executed is a double-sided printing job, the sheet S passes through the transfer nip twice in order to perform the transfer process once on each of the front and back sides of the sheet S. Specifically, when the sheet S passes through the transfer nip for the first time, the transfer process is performed on one side of the sheet S. After the first transfer process, the sheet S is switched back after the rear end of the sheet S passes through the branch position P3 and before the sheet S is completely discharged onto the discharge tray ET. As a result, the rear end of the sheet S is drawn into the double-sided printing transport path DP.

[0039] Thereafter, the sheet S is transported along the double-sided printing transport path DP. Then, the sheet S on the double-sided printing transport path DP is returned to the main transport path MP from the junction position P4. The sheet S returned to the main transport path MP is transported along the main transport path MP and passes through the transfer nip again. At this time, the orientation of the front and back surfaces of the sheet S is reversed to the orientation when it passed through the transfer nip the previous time. As a result, when the sheet S passes through the transfer nip for the second time, a transfer process is performed on the other side of the sheet S that is opposite to the one side.

[0040] As shown in Fig. 4, the image forming apparatus 100 includes a control unit 10. The control unit 10 includes processing circuits such as a CPU and an ASIC. The control unit 10 also includes storage devices such as a ROM and a RAM. The control unit 10 controls a print job executed by the image forming apparatus 100. The control unit 10 controls a belt motor BM to rotate the intermediate transfer belt 2 appropriately.

[0041] The image forming apparatus 100 includes a communication unit 101. The communication unit 101 includes a communication circuit, a communication memory, a communication connector, and the like. The communication unit 101 is communicably connected to an external device via a network such as a LAN. An example of the external device is a user terminal. A personal computer (PC), a smartphone, a tablet computer, and the like can serve as the user terminal.

[0042] The control unit 10 communicates with an external device using the communication unit 101. For example, print data of a print job is transmitted from an external device (user terminal) to the image forming apparatus 100. The print data includes image data to be printed in the print job. The control unit 10 controls the print job based on the print data.

[0043] The image forming apparatus 100 includes an operation panel 102. The operation panel 102 includes a touch screen. The operation panel 102 receives settings, instructions, and the like from a user. The operation panel 102 is connected to the control unit 10. The control unit 10 detects the settings, instructions, and the like received by the operation panel 102 from the user.

[0044] The image forming apparatus 100 also includes an image density sensor 4. The image density sensor 4 is used to detect the density and transfer position of an image transferred to the outer circumferential surface of the intermediate transfer belt 2. The image density sensor 4 is connected to a control unit 10. The control unit 10 detects an output value of the image density sensor 4.

[0045] The image density sensor 4 is disposed at a distance from the outer peripheral surface of the intermediate transfer belt 2. The image density sensor 4 is a reflective optical sensor, and has a light emitting section and a light receiving section. The image density sensor 4 irradiates light toward the outer peripheral surface of the intermediate transfer belt 2, and outputs a value according to the amount of light reflected from the outer peripheral surface of the intermediate transfer belt 2 (specifically, a test area 22 described later). The image density sensor 4 changes its output value depending on whether an image is present or not at the detection position. The image density sensor 4 also changes its output value depending on the density of the image present at the detection position. The detection position of the image density sensor 4 is a position on the intermediate transfer belt 2 that faces the image density sensor 4, and is a position where the light from the image density sensor 4 is irradiated.

[0046] Here, as shown in FIG. 5, the image forming apparatus 100 includes a sensor facing member 5. The sensor facing member 5 is disposed on the inner circumferential side of the intermediate transfer belt 2. The sensor facing member 5 contacts the inner circumferential surface of the intermediate transfer belt 2. The sensor facing member 5 suppresses deformation of the detection position of the image density sensor 4 from the inner circumferential region of the intermediate transfer belt 2. In FIG. 5, the tension roller 3 is indicated by a two-dot chain line. Also, in FIG. 5, the light emitted by the image density sensor 4 (including reflected light) is indicated by a dotted arrow. The same applies to FIG. 8, which will be referred to in the following description.

[0047] The sensor-facing member 5 has a configuration as shown in Fig. 6. That is, the sensor-facing member 5 has a base member 6 and a sheet member 7.

[0048] The sensor facing member 5 is a part of a sheet metal member 60. The sheet metal member 60 is installed from one of a pair of unit frames Fr to the other. The sheet metal member 60 is disposed on the inner circumferential side of the intermediate transfer belt 2. The sheet metal member 60 has a portion molded to protrude in a substantially rectangular shape toward the inner circumferential surface of the intermediate transfer belt 2 as a base member 6. That is, a part of the sheet metal member 60 functions as the base member 6. The longitudinal direction of the base member 6 is the belt width direction Dw.

[0049] The sheet member 7 is disposed on the surface of the base member 6 that faces the inner circumferential surface of the intermediate transfer belt 2. That is, the sheet member 7 is disposed between the inner circumferential surface of the intermediate transfer belt 2 and the base member 6. As a result, the sheet member 7 comes into contact with the inner circumferential surface of the intermediate transfer belt 2. The sheet metal member 60 including the base member 6 does not come into contact with the inner circumferential surface of the intermediate transfer belt 2. The sheet member 7 may be adhered to the base member 6.

[0050] 5, the sensor facing member 5 is disposed so that a portion of it faces the image density sensor 4 across the intermediate transfer belt 2. That is, the sensor facing member 5 has a facing portion 50 that faces the image density sensor 4 across the intermediate transfer belt 2.

[0051] 5 and 6 correspond to a cross section of the sensor facing member 5 and its periphery cut by a plane parallel to the belt width direction Dw. Fig. 5 and Fig. 6 are schematic illustrations of the sensor facing member 5 and its periphery, and do not directly show the actual dimensions and shapes, etc. The same applies to Fig. 8 referred to in the following description.

[0052] By providing the sensor facing member 5, the detection position of the image density sensor 4 on the intermediate transfer belt 2 is supported by the facing portion 50. In other words, it is possible to prevent deformation such as bending from occurring on the intermediate transfer belt 2 at the detection position of the image density sensor 4.

[0053] The image density sensor 4 is used in the calibration described below. To perform this calibration accurately, the detection by the image density sensor 4 needs to be accurate. For this reason, it is preferable to suppress deformation of the detection position of the image density sensor 4 on the intermediate transfer belt 2.

[0054] <Calibration Overview> The control unit 10 performs calibration to keep the quality of the output image constant. As part of the calibration, the control unit 10 performs a process of correcting the density and color shift of the output image. To this end, the control unit 10 detects the density of the image transferred to the outer circumferential surface of the intermediate transfer belt 2 based on the output value of the image density sensor 4. The control unit 10 also detects the transfer position (in other words, positional misalignment) of the image transferred to the outer circumferential surface of the intermediate transfer belt 2 based on the output value of the image density sensor 4.

[0055] When performing calibration, the control unit 10 causes each image forming unit 1 to form a test pattern image TP used for the calibration. The test pattern image TP is not printed on the sheet S. For example, the test pattern image TP includes an image used for density correction and an image used for color shift correction. In the drawings, the test pattern images TP are indicated by black areas.

[0056] When performing density correction, the control unit 10 causes each image forming unit 1 to form a test pattern image TP used for density correction. For example, each image forming unit 1 forms a plurality of patches having different densities as a test pattern image TP used for density correction, and transfers them to the intermediate transfer belt 2. The control unit 10 detects the densities of the plurality of patches of each color based on the output value of the image density sensor 4. If the detected density is lower than the target density, the control unit 10 performs a correction to make the printed image darker, and if the detected density is higher than the target density, the control unit 10 performs a correction to make the printed image lighter. In the density correction, a development bias, a transfer bias, and the like may be corrected.

[0057] When performing color misregistration correction, the control unit 10 causes each image forming unit 1 to form a test pattern image TP used for color misregistration correction. For example, each image forming unit 1 forms lines inclined at 45° to the main scanning direction as the test pattern image TP used for color misregistration correction in the main scanning direction, and transfers it to the intermediate transfer belt 2. Also, each image forming unit 1 forms lines parallel to the main scanning direction as the test pattern image TP used for color misregistration correction in the sub-scanning direction, and transfers it to the intermediate transfer belt 2. The control unit 10 detects the line intervals of each color based on the output value of the image density sensor 4. The control unit 10 corrects the exposure start position so that the line intervals of each color become the target intervals.

[0058] <Test pattern image formation area> As shown in Fig. 5 and Fig. 7, the intermediate transfer belt 2 has a print area 21 and a test area 22. In other words, the outer peripheral surface of the intermediate transfer belt 2 is divided into the print area 21 and the test area 22. In Fig. 5 and Fig. 7, in order to clarify the print area 21 and the test area 22, the boundary between the print area 21 and the test area 22 is indicated by a dashed line. Note that Fig. 7 is a plan view of the intermediate transfer belt 2 as viewed in its thickness direction.

[0059] The intermediate transfer belt 2 has a printing area 21 on the inside in the belt width direction Dw of the end edge 3a of the tension roller 3 in the belt width direction Dw. The intermediate transfer belt 2 has at least one of a pair of areas sandwiching the printing area 21 in the belt width direction Dw as a test area 22. For example, the intermediate transfer belt 2 has both of a pair of areas sandwiching the printing area 21 in the belt width direction Dw as the test areas 22. In this configuration, the areas on both sides of the printing area 21 in the belt width direction Dw are the test areas 22.

[0060] Each image forming unit 1 transfers a print image to be printed on the sheet S to the printing area 21. Each image forming unit 1 transfers a test pattern image TP to the test area 22. This allows a print job to be executed in parallel with the execution of calibration, so productivity does not decrease. Note that when only calibration is executed, the test pattern image TP may be transferred to the printing area 21 in addition to the test area 22.

[0061] <Edge position of sensor facing member> The image density sensor 4 irradiates light toward the outer circumferential surface of the intermediate transfer belt 2. The light reflected by the outer circumferential surface of the intermediate transfer belt 2 (including the image on the outer circumferential surface of the intermediate transfer belt 2) is incident on the image density sensor 4. As a result, the density and transfer position of the image on the outer circumferential surface of the intermediate transfer belt 2 are detected.

[0062] Here, in a configuration in which the intermediate transfer belt 2 is stretched by the tension roller 3, the portion of the intermediate transfer belt 2 that contacts the tension roller 3 is pressed from the inner periphery side toward the outer periphery side of the intermediate transfer belt 2. As a result, the portion of the intermediate transfer belt 2 that contacts the tension roller 3 is relatively protruded from the inner periphery side toward the outer periphery side of the intermediate transfer belt 2 from the end of the intermediate transfer belt 2 in the belt width direction Dw (see FIG. 3 and FIG. 5). In other words, the intermediate transfer belt 2 is bent in the belt width direction Dw, starting from a boundary portion between a portion that contacts the tension roller 3 and a portion that does not contact the tension roller 3 (i.e., a portion that overlaps with the edge 3a of the tension roller 3 in the belt width direction Dw). In other words, the portion of the intermediate transfer belt 2 that is outside the edge 3a of the tension roller 3 in the belt width direction Dw is bent toward the inner periphery side of the intermediate transfer belt 2. In the following description, the boundary portion between the portion of the intermediate transfer belt 2 that contacts the tension roller 3 and the portion that does not contact the tension roller 3 is denoted by reference numeral 20, and this portion is referred to as the belt bending portion 20.

[0063] If the belt bending portion 20 rides up on the sensor facing member 5, the positional relationship between the image density sensor 4 and the test area 22 will be shifted, as shown in Fig. 8. Specifically, the distance from the image density sensor 4 to the test area 22 will be shifted. Also, the test area 22 will be significantly tilted with respect to the light emission surface of the image density sensor 4 (i.e., the angle will be shifted). If the distance and angle between the image density sensor 4 and the test area 22 are shifted, it will be difficult to accurately detect the density and transfer position of the test pattern image TP.

[0064] 5, the sensor facing member 5 is disposed at a position where the belt bending portion 20 does not ride over. Specifically, the edge position in the belt width direction Dw of the facing portion 50 of the sensor facing member 5 is more inward in the belt width direction Dw than the edge position in the belt width direction Dw of the tension roller 3. In the sensor facing member 5, an edge 5a on one side in the belt width direction Dw of the facing portion 50 is positioned more inward than an edge 3a on one side in the belt width direction Dw of the tension roller 3, and an edge 5a on the other side in the belt width direction Dw of the facing portion 50 is positioned more inward than an edge 3a on the other side in the belt width direction Dw of the tension roller 3.

[0065] 8, if the edge 5a of the sensor facing member 5 is located outside the edge 3a of the tension roller 3 in the belt width direction Dw, the belt bending portion 20 rides up on the sensor facing member 5, and the distance and angle between the intermediate transfer belt 2 and the image density sensor 4 are shifted. On the other hand, if the edge 5a of the sensor facing member 5 is located inside the edge 3a of the tension roller 3 in the belt width direction Dw, as shown in FIG. 5, the belt bending portion 20 does not ride up on the sensor facing member 5. For this reason, deformation of the detection position of the image density sensor 4 (i.e., the test area 22) on the intermediate transfer belt 2 is suppressed.

[0066] As a result, in this embodiment, the regions on both sides of the print region 21 of the intermediate transfer belt 2 in the belt width direction Dw are set as the test regions 22, but even in this case, it is possible to suppress deviations in the distance and angle between the intermediate transfer belt 2 and the image density sensor 4, and detection by the image density sensor 4 becomes accurate. As a result, calibration related to image formation can be performed accurately without reducing productivity.

[0067] In this embodiment, the edge position in the belt width direction Dw of the sensor facing member 5 is more inward in the belt width direction Dw than the edge position in the belt width direction Dw of the tension roller 3, and is more outward in the belt width direction Dw than the edge position in the belt width direction Dw of the print area 21. This makes it easy to make the image density sensor 4, which has the test area 22 as its detection position, and the sensor facing member 5 face each other with the intermediate transfer belt 2 in between.

[0068] In this embodiment, each image forming unit 1 transfers the test pattern image TP to an area of ​​the test area 22 that overlaps with the facing portion 50 of the sensor facing member 5. This makes it possible to accurately detect the density and transfer position of the test pattern image TP.

[0069] As a modified example (not shown), the edge position in the belt width direction Dw of the sensor facing member 5 may be the same in the belt width direction Dw as the edge position in the belt width direction Dw of the tension roller 3. Even in the configuration of the modified example, deviations in the distance and angle between the intermediate transfer belt 2 and the image density sensor 4 can be suppressed, and detection by the image density sensor 4 becomes accurate.

[0070] In this embodiment, the sheet member 7 has a smaller coefficient of friction with the inner circumferential surface of the intermediate transfer belt 2 than the base member 6. This can reduce resistance to the rotation of the intermediate transfer belt 2. For example, the sheet member 7 is a nonwoven fabric.

[0071] In this embodiment, the sheet member 7 is conductive. That is, the sheet member 7 is a conductive nonwoven fabric. This allows static electricity of the intermediate transfer belt 2 to be released.

[0072] The embodiments disclosed herein are illustrative in all respects and should not be considered as limiting. The scope of the present invention is indicated by the claims, not by the description of the above embodiments, and further includes all modifications within the meaning and scope of the claims. [Explanation of symbols]

[0073] 1 Image forming section 2 Intermediate transfer belt 2a edge 3 Tension roller 3a edge 4 Image density sensor 5 Sensor facing member 5a edge 6 Base material 7 Sheet material 21 Print area 22 Test Area 50 Opposite part 100 Image forming device S Seat TP test pattern image

Claims

1. an image forming unit that forms an image using toner; an endless intermediate transfer belt that rotates while carrying the image transferred from the image forming unit on its outer circumferential surface; a tension roller that is rotatable about an axis extending in a width direction perpendicular to a rotation direction of the intermediate transfer belt, that is disposed on the inner side of an edge of the intermediate transfer belt in the width direction, and that contacts an inner peripheral surface of the intermediate transfer belt to tension the intermediate transfer belt; the intermediate transfer belt has a print area located inward in the width direction from an edge of the tension roller in the width direction, and at least one of a pair of areas sandwiching the print area in the width direction is a test area; the image forming unit transfers a print image to be printed on a sheet to the print area, and transfers a test pattern image for calibration related to image formation by the image forming unit to the test area; an image density sensor that is disposed at a distance from an outer peripheral surface of the intermediate transfer belt, that irradiates the test area with light, and that outputs a value corresponding to the amount of light reflected from the test area; a sensor facing member that is in contact with an inner circumferential surface of the intermediate transfer belt and has a facing portion that faces the image density sensor across the intermediate transfer belt, An image forming apparatus, wherein the widthwise edge position of the opposing portion is located inside the widthwise edge position of the tension roller, or is the same in the width direction as the widthwise edge position of the tension roller.

2. 2. The image forming apparatus according to claim 1, wherein the widthwise edge position of the opposing portion is located inside the widthwise edge position of the tension roller and outside the widthwise edge position of the printing area.

3. The image forming apparatus according to claim 2 , wherein the image forming section transfers the test pattern image onto an area of ​​the test area that overlaps with the facing portion.

4. The image forming apparatus according to claim 1 , wherein a portion of the intermediate transfer belt that is located outside an edge of the tension roller in the width direction is bent toward an inner peripheral side of the intermediate transfer belt.

5. The sensor facing member is A base member; a sheet member disposed on a surface of the base member facing an inner circumferential surface of the intermediate transfer belt and thereby contacting the inner circumferential surface of the intermediate transfer belt; The image forming apparatus according to claim 1 , wherein the sheet member has a smaller coefficient of friction with the inner circumferential surface of the intermediate transfer belt than the base member.

6. The image forming apparatus according to claim 5 , wherein the sheet member is conductive.

Images