Image forming apparatus

Abstract

To provide an image forming apparatus that is configured to execute an adjustment mode of outputting an adjustment chart and adjusting a transfer voltage, and can adjust the transfer voltage to a transfer voltage at which prevention of the occurrence of "break-through" and prevention of the occurrence of "a reduction in image density" are both achieved.SOLUTION: A control section outputs an adjustment chart (S4). The control section reads the adjustment chart with an image reading section (S7) to acquire brightness data of patches on the adjustment chart. The control section determines an adjustment value Na at which the brightness average value of the patches becomes minimum (S10), and checks if "break-through" occurs in the patches at the adjustment value Na (S11). When break-through occurs (Yes in S11), the control section corrects the adjustment value (S12) and reduces the adjustment value by an adjustment amount ΔV compared with a case where the "break-through" does not occur. With this, a transfer voltage set during image formation determined according to "recording material shared voltage Vp+adjustment amount ΔV" is adjusted to achieve both prevention of the occurrence of "break-through" and prevention of the occurrence of "a reduction in image density."SELECTED DRAWING: Figure 8

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus using an electrophotographic method, such as a printer, a copier, a facsimile, or a multifunction machine. [Background technology]

[0002] In an image forming apparatus using an electrophotographic method, such as a printer, a copier, a facsimile, or a multifunction machine, a toner image formed on an image carrier, such as a photoconductor or an intermediate transfer body, is transferred to a recording material. The transfer of the toner image from the image carrier to the recording material is performed by applying a transfer voltage to a transfer member, such as a transfer roller, which contacts the image carrier to form a transfer portion (transfer nip portion). The transfer voltage is determined based on a transfer portion voltage corresponding to the electric resistance of the transfer portion detected during a pre-rotation process before image formation, and a recording material portion voltage corresponding to the type of recording material. In this way, an appropriate transfer voltage can be set according to environmental fluctuations, the usage history of the transfer member, the type of recording material, and the like.

[0003] However, since the types and conditions of recording materials used in image formation are various, the transfer voltage may be excessive or insufficient with a preset default recording material distribution voltage. In this regard, an image forming apparatus has been proposed that includes an adjustment mode for adjusting the set voltage of the secondary transfer voltage according to the recording material actually used in image formation, in the case of an intermediate transfer method using an intermediate transfer belt as an image carrier (Patent Document 1). In the adjustment mode, the secondary transfer voltage is switched to form a plurality of test toner images called patches on one recording material. The recording material on which the plurality of patches are formed is called an adjustment chart. Then, the recording material distribution voltage is changed based on the image average density obtained from the density of each patch formed on the adjustment chart, and the secondary transfer voltage set during image formation is adjusted.

[0004] In an image forming apparatus, when a recording material is discharged near a secondary transfer portion during secondary transfer, the charge polarity of the toner in the corresponding portion is reversed, and the toner is not transferred to the recording material, resulting in a defective image (called punch-through) in which the toner is not transferred to the recording material and the image is left blank in dots. The "punch-through" is more likely to occur as the secondary transfer voltage increases, and is particularly likely to be evident in black halftone images. In the case of a conventional apparatus that adjusts the set voltage of the secondary transfer voltage based on the average image density of the above-mentioned patch, there was a risk that the set voltage of the secondary transfer voltage would be adjusted to a level (absolute value) that would cause "punch-through". In view of this, an apparatus has been proposed that adjusts the secondary transfer voltage set during image formation within a range that can suppress the occurrence of "punch-through" by setting an upper limit value for the recording material distribution voltage (Patent Document 2). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2013-37185 A [Patent Document 1] JP 2020-144289 A Summary of the Invention [Problem to be solved by the invention]

[0006] However, in the past, by setting an upper limit on the recording material distribution voltage, the "punch-through" caused by the transfer voltage being too high did not occur, but the transfer voltage could be adjusted so low that it caused a decrease in image density. [Means for solving the problem]

[0007] According to an embodiment of the present invention, an image forming apparatus includes an image carrier that carries a toner image, an image forming unit that forms a toner image on the image carrier, a transfer member that transfers the toner image from the image carrier to a recording material, a power source that applies a transfer voltage to the transfer member in order to transfer the toner image from the image carrier to the recording material, an acquisition unit that acquires image density information relating to the density of the toner image transferred onto the recording material, and a control unit that is capable of executing an adjustment mode in which a plurality of different test voltages are applied by the power source to output a test chart in which a plurality of test toner images are transferred to the recording material, and a transfer voltage that is set for transferring a toner image from the image carrier to the recording material based on the density of the test toner image transferred to the test chart. the toner image includes a first test toner image transferred to the recording material by applying a first test voltage by the power source, and a second test toner image transferred to the recording material by applying a second test voltage different from the first test voltage by the power source, and the control unit, during the adjustment mode, acquires information regarding a change in the image density variation with respect to a change in the test voltage based on first information regarding a variation in image density acquired in different areas of the first test toner image and second information regarding a variation in image density acquired in different areas of the second test toner image, and adjusts a transfer voltage set for transferring the toner image to the recording material based on the information regarding the change in the image density variation with respect to the change in the test voltage. Effect of the Invention

[0008] According to the present invention, in a configuration in which an adjustment mode is executed in which the transfer voltage set during image formation is adjusted based on a test chart onto which multiple test toner images are transferred, it is possible to adjust the transfer voltage to one that achieves both the prevention of "punch-through" and the prevention of "reduction in image density". [Brief description of the drawings]

[0009] [Figure 1] 1 is a schematic diagram showing an image forming apparatus according to an embodiment of the present invention. [Diagram 2] FIG. 2 is a block diagram showing a schematic configuration of a control system of the image forming apparatus. [Diagram 3] 6 is a flowchart showing secondary transfer voltage control. [Figure 4] 6 is a graph showing voltage-current characteristics obtained in secondary transfer voltage control. [Diagram 5] 11 is a table showing a recording material shared voltage table. [Figure 6] FIG. [Figure 7] 13A and 13B are diagrams showing small chart data, (a) the first page and (b) the second page. [Figure 8] 5 is a flowchart showing an adjustment mode according to the first embodiment. [Figure 9] FIG. 4 is a diagram showing a setting screen for a secondary transfer voltage. [Figure 10] 11 is a graph showing the adjustment value of the secondary transfer voltage and the average luminance value of the patch. [Figure 11] 5A and 5B are diagrams showing a method of acquiring luminance data for calculating a luminance variance value. [Figure 12] 11 is a graph for explaining the relationship between “break-through” and luminance variance value. [Figure 13] 11 is a graph for explaining a method for determining whether or not "punch-through" has occurred. [Figure 14] 11 is a graph for explaining the relationship between the recording material shared voltage and the likelihood of "punch-through" occurring. [Figure 15] FIG. 11 is a table showing upper limit values ​​of recording material voltages. [Figure 16] 13 is a graph for explaining a method of changing an adjustment value when "punch-through" occurs. [Figure 17] 10 is a flowchart showing an adjustment mode according to a second embodiment. [Figure 18] FIG. 11 is a diagram showing a setting screen for a secondary transfer voltage in the second embodiment. [Figure 19] FIG. 4 is a diagram showing a priority image setting screen. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] [First embodiment] <Image forming device> The image forming apparatus of this embodiment will be described below. FIG. 1 is a schematic diagram showing the image forming apparatus of this embodiment. The image forming apparatus 2 of this embodiment is a tandem-type full-color printer that employs an intermediate transfer method and is capable of forming a full-color image using an electrophotographic method. However, the image forming apparatus 2 is not limited to a tandem-type image forming apparatus, and may be an image forming apparatus of another type. Furthermore, the image forming apparatus 2 is not limited to an image forming apparatus capable of forming a full-color image, and may be an image forming apparatus capable of forming only monochrome (black and white or monochromatic) images. Furthermore, the image forming apparatus 2 may be an image forming apparatus for various purposes, such as a printer, various printing machines, a copying machine, a FAX, or a multifunction machine.

[0011] As shown in FIG. 1, the image forming apparatus 2 has a feed section 4, an image forming section 5, a control section 30, and an operation section 70. Although there is only one feed section 4 in FIG. 1, there may be a plurality of feed sections 4. A temperature sensor 71 capable of detecting the temperature inside the apparatus and a humidity sensor 72 capable of detecting the humidity inside the apparatus are provided inside (see FIG. 2). Furthermore, the image forming apparatus 2 can form a four-color full-color image on the recording material S in response to image information (image signals) from an image reading section 80 as a reading means for reading an image on a sheet or an external device 200 (see FIG. 2). Examples of the external device 200 include a host device such as a personal computer, a digital camera, and a smartphone. The recording material S is a material on which a toner image is formed, and specific examples thereof include plain paper, a synthetic resin sheet that is a substitute for plain paper, cardboard, and a sheet for an overhead projector.

[0012] The image forming section 5 can form an image on the recording material S fed from the feeding section 4 and moving in the conveying path J based on image information. The image forming section 5 has image forming units 50y, 50m, 50c, 50k, toner bottles 41y, 41m, 41c, 41k, exposure devices 42y, 42m, 42c, 42k, intermediate transfer unit 44, secondary transfer device 45, and fixing section 46. The image forming units 50y, 50m, 50c, 50k form images of yellow (y), magenta (m), cyan (c), and black (k), respectively. Elements having the same or corresponding functions or configurations provided corresponding to these four image forming units 50y, 50m, 50c, 50k may be generally described by omitting the suffixes y, m, c, and k of the reference numerals indicating that the elements are for any of the colors. The image forming apparatus 2 can also form a monochrome image, such as a black monochrome image, or a multi-color image using image forming units 50 for a desired monochrome color or for some of the four colors.

[0013] The image forming unit 50 has the following components. First, it has a photosensitive drum 51 which is a drum-type (cylindrical) photosensitive member (electrophotographic photosensitive member). It also has a charging roller 52 which is a roller-type charging member. It also has a developing device 20. It also has a pre-exposure device 54. It also has a cleaning blade 55. The image forming unit 50 forms a toner image on an intermediate transfer belt 44b which will be described later. The image forming unit 50 is integrated into a unit as a process cartridge and is detachably attached to the apparatus main body 10.

[0014] The photosensitive drum 51 is movable (rotatable) while carrying an electrostatic image (electrostatic latent image) or a toner image. In this embodiment, the photosensitive drum 51 is a negatively charged organic photoconductor (OPC) having an outer diameter of 30 mm. The photosensitive drum 51 has an aluminum cylinder as a base and a surface layer formed on the surface thereof. In this embodiment, the surface layer has three layers, an undercoat layer, a photocharge generation layer, and a charge transport layer, which are applied and laminated on the base in the following order. When an image forming operation is started, the photosensitive drum 51 is rotated by a motor (not shown) in the direction of the arrow in the figure (counterclockwise) at a predetermined process speed (circumferential speed).

[0015] The surface of the rotating photosensitive drum 51 is uniformly charged by the charging roller 52. In this embodiment, the charging roller 52 is a rubber roller that comes into contact with the surface of the photosensitive drum 51 and rotates in accordance with the rotation of the photosensitive drum 51. A charging bias power supply 73 (see FIG. 2) is connected to the charging roller 52. The charging bias power supply 73 applies a charging bias (charging voltage) to the charging roller 52 during the charging process.

[0016] The charged surface of the photosensitive drum 51 is scanned and exposed by the exposure device 42 based on image information, and an electrostatic image is formed on the photosensitive drum 51. In this embodiment, the exposure device 42 is a laser scanner. The exposure device 42 emits laser light according to image information of separated colors output from the control unit 30, and scans and exposes the surface (outer circumferential surface) of the photosensitive drum 51.

[0017] The electrostatic image formed on the photosensitive drum 51 is developed (visualized) by supplying toner of the developer by the developing device 20, and a toner image is formed on the photosensitive drum 51. The developing device 20 contains a two-component developer (also simply called "developer") including non-magnetic toner particles (toner) and magnetic carrier particles (carrier). The developing device 20 is supplied with toner from a toner bottle 41. The developing device 20 has a developing sleeve 24. The developing sleeve 24 is made of a non-magnetic material such as aluminum or non-magnetic stainless steel (aluminum in this embodiment). Inside the developing sleeve 24, a roller-shaped magnet roller is fixed and arranged so as not to rotate relative to the main body (developing container) of the developing device 20. The developing sleeve 24 carries the developer and transports it to a developing area facing the photosensitive drum 51. A developing bias power supply 74 (see FIG. 2) is connected to the developing sleeve 24. The developing bias power supply 74 applies a developing bias (developing voltage) to the developing sleeve 24 during the developing process. In this embodiment, the normal charging polarity of the toner during development is negative.

[0018] The intermediate transfer unit 44 is disposed so as to face the four photosensitive drums 51y, 51m, 51c, and 51k. The intermediate transfer unit 44 has an intermediate transfer belt 44b, which is an endless belt, as an image carrier. The intermediate transfer belt 44b is wound around a plurality of rollers, including a drive roller 44a, a driven roller 44d, primary transfer rollers 47y, 47m, 47c, and 47k, and a secondary transfer inner roller 45a. The intermediate transfer belt 44b is movable (rotatable) while carrying a toner image. The drive roller 44a is driven to rotate by a motor (not shown) to rotate (circulate) the intermediate transfer belt 44b. The driven roller 44d is a tension roller that controls the tension of the intermediate transfer belt 44b to a constant value. A force is applied to the driven roller 44d by a spring (not shown) so as to push the intermediate transfer belt 44b toward the outer peripheral surface side, and this force applies a tension of about 2 to 5 kg to the intermediate transfer belt 44b in the process proceeding direction. The secondary transfer inner roller 45a constitutes the secondary transfer device 45, as described later. The intermediate transfer belt 44b receives a driving force transmitted by the driving roller 44a and is driven to rotate in the direction of the arrow in the figure (clockwise) at a predetermined peripheral speed corresponding to the peripheral speed of the photosensitive drum 51. The intermediate transfer unit 44 also has a belt cleaning device 60 as an intermediate transfer body cleaning means.

[0019] The primary transfer rollers 47y, 47m, 47c, and 47k are disposed to face the photosensitive drums 51y, 51m, 51c, and 51k, respectively. The primary transfer roller 47 sandwiches the intermediate transfer belt 44b between itself and the photosensitive drum 51. As a result, the intermediate transfer belt 44b comes into contact with the photosensitive drum 51 to form a primary transfer portion (primary transfer nip portion) 48 between itself and the photosensitive drum 51.

[0020] The toner image formed on the photosensitive drum 51 is primarily transferred onto the intermediate transfer belt 44b by the action of the primary transfer roller 47 in the primary transfer section 48. That is, in this embodiment, a positive primary transfer voltage is applied to the primary transfer roller 47, so that the negative toner image on the photosensitive drum 51 is primarily transferred onto the intermediate transfer belt 44b. For example, when a full-color image is formed, the toner images of yellow, magenta, cyan, and black formed on the photosensitive drums 51y, 51m, 51c, and 51k are multiplexed and transferred onto the intermediate transfer belt 44b in order to be superimposed on each other. A primary transfer power source 75 (see FIG. 2) is connected to the primary transfer roller 47. The primary transfer power source 75 applies a DC voltage of a polarity opposite to the normal charging polarity of the toner (positive polarity in this embodiment) to the primary transfer roller 47 as a primary transfer bias (primary transfer voltage) during the primary transfer process. A voltage detection sensor 75a that detects an output voltage and a current detection sensor 75b that detects an output current are connected to the primary transfer power supply 75 (see FIG. 2). In this embodiment, primary transfer power supplies 75y, 75m, 75c, and 75k are provided for the primary transfer rollers 47y, 47m, 47c, and 47k, respectively, and the primary transfer voltages applied to the primary transfer rollers 47y, 47m, 47c, and 47k can be individually controlled.

[0021] The primary transfer roller 47 has an elastic layer of ion conductive foam rubber (NBR rubber) and a core metal. The outer diameter of the primary transfer roller 47 is, for example, 15 to 20 mm. The primary transfer roller 47 has an electrical resistance of 1×10 5 ~1×10 8 A roller of Ω (N / N (23° C., 50% RH) measurement, 2 kV applied) can be suitably used.

[0022] The intermediate transfer belt 44b is an endless belt having a two-layer structure of a base layer and a surface layer from the inner peripheral surface side. As a material for forming the base layer, a resin such as polyimide or polycarbonate, or various rubbers containing an appropriate amount of carbon black as an antistatic agent can be suitably used. The thickness of the base layer is, for example, 0.05 to 0.15 [mm]. As a material for forming the surface layer, a resin such as fluororesin can be suitably used. The surface layer reduces the adhesion of the toner to the surface of the intermediate transfer belt 44b, making it easier to transfer the toner to the recording material S at the secondary transfer section 45n. The thickness of the surface layer is, for example, 0.0002 to 0.020 [mm]. In this embodiment, the surface layer uses, as a base material, one type of resin material such as polyurethane, polyester, epoxy resin, or two or more types of elastic materials such as elastic rubber, elastomer, butyl rubber, etc. Then, a surface layer is formed by dispersing one or more types of powder or particles, such as fluororesin, or other materials with different particle sizes, as a material that reduces surface energy and enhances lubricity, on the base material. In this embodiment, the intermediate transfer belt 44b has a volume resistivity of 5×10 8 ~1×10 14 [Ω·cm] (23° C., 50% RH), and the static friction coefficient is 0.15 to 0.6 (23° C., 50% RH, HEIDON type 94i). In this embodiment, a two-layer structure is used, but a single layer structure of a material equivalent to the above-mentioned base layer may also be used.

[0023] On the outer peripheral surface side of the intermediate transfer belt 44b, a secondary transfer outer roller 45b that constitutes the secondary transfer device 45 together with the secondary transfer inner roller 45a is disposed. The secondary transfer outer roller 45b abuts against the intermediate transfer belt 44b to form a secondary transfer portion (secondary transfer nip portion) 45n between the intermediate transfer belt 44b. The secondary transfer outer roller 45b abuts against the secondary transfer inner roller 45a via the intermediate transfer belt 44b. The toner image formed on the intermediate transfer belt 44b is secondarily transferred onto the recording material S by the action of the secondary transfer device 45 at the secondary transfer portion 45n. In this embodiment, a positive secondary transfer voltage is applied to the secondary transfer outer roller 45b, so that the negative toner image on the intermediate transfer belt 44b is secondarily transferred onto the recording material S that is sandwiched and conveyed between the intermediate transfer belt 44b and the secondary transfer outer roller 45b. The recording material S is fed from the feeding section 4 in parallel with the above-mentioned toner image forming operation, and is transported to the secondary transfer section 45n by the registration rollers 11 provided on the transport path J in synchronization with the toner image on the intermediate transfer belt 44b.

[0024] Thus, the secondary transfer device 45 includes the inner secondary transfer roller 45a and the outer secondary transfer roller 45b as a transfer member. The inner secondary transfer roller 45a is disposed facing the outer secondary transfer roller 45b via the intermediate transfer belt 44b. A secondary transfer power source 76 (see FIG. 2) is connected to the outer secondary transfer roller 45b. The secondary transfer power source 76 applies a DC voltage of a polarity opposite to the normal charging polarity of the toner (positive polarity in this embodiment) as a secondary transfer bias (secondary transfer voltage) to the outer secondary transfer roller 45b during the secondary transfer process. A voltage detection sensor 76a for detecting the output voltage and a current detection sensor 76b for detecting the output current are connected to the secondary transfer power source 76 (see FIG. 2). The core metal of the inner secondary transfer roller 45a is connected to a ground potential. Then, when the recording material S is supplied to the secondary transfer section 45n, a constant voltage controlled secondary transfer voltage of a polarity opposite to the normal charging polarity of the toner is applied to the outer secondary transfer roller 45b. In this embodiment, for example, a secondary transfer voltage of 1 to 6.5 kV is applied, and a current of about 15 to 100 μA is passed, so that the toner image on the intermediate transfer belt 44b is secondarily transferred onto the recording material S. In this embodiment, the inner secondary transfer roller 45a is connected to a ground potential, and a voltage is applied to the outer secondary transfer roller 45b from the secondary transfer power source 76. Alternatively, a voltage may be applied to the inner secondary transfer roller 45a from the secondary transfer power source 76, and the outer secondary transfer roller 45b may be connected to a ground potential. In this case, a DC voltage of the same polarity as the normal charging polarity of the toner is applied to the inner secondary transfer roller 45a.

[0025] In this embodiment, the secondary transfer outer roller 45b has an elastic layer of ion conductive foamed rubber (NBR rubber) and a core metal. The outer diameter of the secondary transfer outer roller 45b is, for example, 20 to 25 mm. The secondary transfer outer roller 45b has an electrical resistance of 1×10 5 ~1×10 8 A roller of Ω (N / N (23° C., 50% RH) measurement, 2 kV applied) can be suitably used.

[0026] The recording material S onto which the toner image has been transferred is transported to the fixing section 46. The fixing section 46 has a fixing roller 46a and a pressure roller 46b. The fixing roller 46a has a built-in heater. The recording material S carrying the unfixed toner image is heated and pressurized by being sandwiched between the fixing roller 46a and the pressure roller 46b and transported. This causes the toner image to be fixed (melted and adhered) onto the recording material S. The temperature (fixing temperature) of the fixing roller 46a is detected by a fixing temperature sensor 77 (see FIG. 2).

[0027] When the image formation on the recording material S is one-sided, the recording material S on which the toner image is fixed is conveyed through the discharge path 6, discharged as is from the discharge port, and stacked on the discharge tray 8 provided outside the device main body 10. On the other hand, when the image formation on the recording material S is two-sided, the recording material S on which the toner image is fixed is conveyed to the inversion conveyance path 7. In the inversion conveyance path 7, the recording material S on which the toner image is fixed on one side is turned over and supplied again to the secondary transfer section 45n. The recording material S supplied again to the secondary transfer section 45n via the inversion conveyance path 7 has the toner image transferred and fixed on the second side, and then conveyed through the discharge path 6 to be stacked on the discharge tray 8 provided outside the device main body 10. In this way, the image forming apparatus 2 of this embodiment is capable of performing automatic double-sided printing in which images are formed on both sides of one recording material S.

[0028] After the primary transfer, the surface of the photosensitive drum 51 is neutralized by a pre-exposure device 54. In addition, toner (primary transfer residual toner) remaining on the photosensitive drum 51 without being transferred to the intermediate transfer belt 44b during the primary transfer process is removed from the surface of the photosensitive drum 51 by a cleaning blade 55 and collected in a collection container (not shown). The cleaning blade 55 is a plate-shaped member that is abutted against the photosensitive drum 51 with a predetermined pressing force. The cleaning blade 55 is abutted against the surface of the photosensitive drum 51 in a counter direction in which the tip of its free end side faces the upstream side of the rotation direction of the photosensitive drum 51. In addition, toner (secondary transfer residual toner) remaining on the intermediate transfer belt 44b without being transferred to the recording material S during the secondary transfer process, and adhesions such as paper powder are removed from the surface of the intermediate transfer belt 44b by a belt cleaning device 60 and collected.

[0029] An automatic document feeder 81 and an image reading unit 80 are disposed on the upper portion of the device main body 10. The automatic document feeder 81 automatically feeds a sheet such as a document or a recording material S on which an image is formed (for example, an adjustment chart described later) to the image reading unit 80. The image reading unit 80 as an acquisition unit reads an image on the sheet fed by the automatic document feeder 81. The image reading unit 80 is configured to illuminate a sheet placed on a platen glass 82 with a light source (not shown) and read the image on the sheet at a predetermined dot density with an image reading element (not shown). That is, the image reading unit 80 optically reads the image on the sheet and converts it into an electrical signal.

[0030] <Control Unit> As shown in Fig. 1, the image forming apparatus 2 of this embodiment includes a control unit 30, which controls the operation of each unit. The control unit 30 will be described using Fig. 2 while referring to Fig. 1. The control unit 30 is configured by a computer, and includes, for example, a CPU (Central Processing Unit) 31, a ROM (Read Only Memory) 32, a RAM (Random Access Memory) 33, and an input / output circuit (I / F) 34. The CPU 31 is a microprocessor that is responsible for the overall control of the image forming apparatus 2, and is the main system controller.

[0031] The CPU 31 is connected to the feeding section 4, the image forming section 5, the operation section 70, etc. via an input / output circuit (I / F) 34, and exchanges signals with each of these sections while controlling the operation of each of these sections. The ROM 32 stores an image formation control sequence (program) for forming an image on the recording material S. The control section 30 is connected to a charging bias power supply 73, a developing bias power supply 74, a primary transfer power supply 75, and a secondary transfer power supply 76, each of which is controlled by a signal from the control section 30. The control section 30 is also connected to a temperature sensor 71, a humidity sensor 72, a voltage detection sensor 75a and a current detection sensor 75b of the primary transfer power supply 75, a voltage detection sensor 76a and a current detection sensor 76b of the secondary transfer power supply 76, and a fixing temperature sensor 77. Signals detected by each sensor are input to the control section 30.

[0032] The operation unit 70 as an input unit has operation buttons (not shown) and a display unit 70a consisting of a liquid crystal panel or the like. In the case of this embodiment, the display unit 70a is configured as a touch panel and also functions as an input unit. An operator such as a user or a service person can operate the operation unit 70 to execute a job (a series of operations for forming and outputting an image on one or more recording materials S in response to one start instruction) and input various information. The control unit 30 receives a signal from the operation unit 70 and operates various devices of the image forming apparatus 2. The image forming apparatus 2 can also execute a job based on an image signal (image data, control command) from an external device 200 such as a personal computer.

[0033] The control unit 30 has an image formation pre-preparation processor 31a, an ATVC control processor 31b, an image formation processor 31c, and an adjustment processor 31d. The control unit 30 also has a primary transfer voltage storage / calculation unit 31e and a secondary transfer voltage storage / calculation unit 31f. These processing units and storage / calculation units may be provided as part of the CPU 31 or the RAM 33. For example, the control unit 30 (more specifically, the image formation processor 31c) can execute a job as described above. The control unit 30 (more specifically, the ATVC control processor 31b) can execute ATVC control (setting mode) of the primary transfer unit and the secondary transfer unit. The ATVC control will be described later. The control unit 30 (more specifically, the adjustment processor 31d) can execute an adjustment mode that adjusts the setting voltage of the secondary transfer voltage. The adjustment mode will be described later.

[0034] Here, the image forming device 2 executes a job (image output operation, print job) which is a series of operations for forming and outputting an image on a single or multiple recording materials S, which is started by one start instruction. The job generally includes an image forming process, a pre-rotation process, a paper-interval process in the case of forming images on multiple recording materials S, and a post-rotation process. The image forming process is a period in which electrostatic image formation, toner image formation, primary transfer of the toner image, and secondary transfer of the toner image are performed for the image to be actually formed on the recording material S and output, and the image formation time (image formation period) refers to this period. More specifically, the timing of the image formation time differs depending on the position where each of the electrostatic image formation, toner image formation, primary transfer of the toner image, and secondary transfer is performed. The pre-rotation process is a period in which a preparatory operation is performed before the image forming process from when a start instruction is input until the image actually starts to be formed. The paper-interval process is a period corresponding to the period between recording materials S when image formation is performed continuously on multiple recording materials S (continuous image formation). The post-rotation process is a period in which an arrangement operation (preparatory operation) is performed after the image forming process. Non-image formation (non-image formation period) refers to a period other than image formation, and includes the above-mentioned pre-rotation process, paper interval process, post-rotation process, and also the pre-multiple-rotation process, which is a preparatory operation when the image forming device 2 is turned on or when returning from a sleep state.

[0035] <Control of secondary transfer voltage> Next, the control of the secondary transfer voltage will be described. Fig. 3 is a flowchart showing an outline of the procedure for controlling the secondary transfer voltage in this embodiment. Generally, the control of the secondary transfer voltage includes constant voltage control and constant current control, and this embodiment uses constant voltage control.

[0036] First, when the control unit 30 (pre-image formation preparation process unit 31a) acquires job information from the operation unit 70 or the external device 200, it starts the operation of the job (S101). This job information includes image information specified by the operator and information on the recording material S. This information on the recording material S includes information on the size of the recording material S and information on the type (paper type category) of the recording material S, such as "thin paper, plain paper, thick paper, etc." The type of recording material S includes any information that can distinguish the recording material S, such as attributes based on general characteristics such as plain paper, thick paper, thin paper, glossy paper, coated paper, brand, product number, basis weight, thickness, etc. The control unit 30 writes this job information to the RAM 33 (S102).

[0037] Next, the control unit 30 (pre-image formation preparation process unit 31a) acquires environmental information detected by the temperature sensor 71 and humidity sensor 72 (S103). In addition, the ROM 32 stores information indicating the correlation between the environmental information and the target current Itarget for transferring the toner image on the intermediate transfer belt 44b onto the recording material S. The control unit 30 (secondary transfer voltage storage unit / calculation unit 31f) obtains the target current Itarget corresponding to the environment from the information indicating the relationship between the environmental information and the target current Itarget based on the environmental information read in S103. Then, the control unit 30 writes this target current Itarget into the RAM 33 (or the secondary transfer voltage storage unit / calculation unit 31f) (S104). The reason why the target current Itarget is changed according to the environmental information is because the charge amount of the toner changes depending on the environment. The target current Itarget in this embodiment is a secondary transfer current value that can transfer the maximum amount of toner (in this embodiment, the entire surface of the secondary color is solid) for each environment by using the image forming device 2 in advance.

[0038] Next, the control unit 30 (ATVC control processor 31b) acquires information on the electrical resistance of the secondary transfer unit 45n by ATVC control (Active Transfer Voltage Control) before the toner image on the intermediate transfer belt 44b and the recording material S to which the toner image is transferred reach the secondary transfer unit 45n (S105). That is, in a state in which the secondary transfer outer roller 45b and the intermediate transfer belt 44b are in contact with each other, a predetermined voltage of multiple levels is supplied from the secondary transfer power source 76 to the secondary transfer outer roller 45b. Then, the current value when the predetermined voltage is supplied is detected by the current detection sensor 76b, and the relationship between the voltage and the current (voltage-current characteristics) as shown in FIG. 4 is acquired. The control unit 30 writes information on the relationship between the voltage and the current to the RAM 33 or the like. This relationship between the voltage and the current changes depending on the electrical resistance of the secondary transfer unit 45n. In the configuration of this embodiment, the relationship between the voltage and the current is not one in which the current changes linearly (proportional) to the voltage, but one in which the current changes so as to be expressed by a polynomial of the voltage of degree two or higher. Therefore, in this embodiment, the specified voltage or current supplied when obtaining information regarding the electrical resistance of the secondary transfer section 45n is set to multiple levels of three or more points so that the relationship between the voltage and current can be expressed by a polynomial.

[0039] Next, the control unit 30 (secondary transfer voltage storage unit / calculation unit 31f) determines the voltage value to be applied from the secondary transfer power source 76 to the secondary transfer outer roller 45b (S106). That is, the control unit 30 determines the voltage value (Vb) required to pass the target current Itarget in the secondary transfer portion 45n in a state where the recording material S is not present, based on the target current Itarget written in the RAM 33 in S104 and the relationship between the voltage and the current determined in S105. This voltage value (Vb) corresponds to the secondary transfer partial voltage (transfer voltage for the electrical resistance of the secondary transfer portion 45n). Also, the ROM 32 stores information for determining the recording material distribution voltage Vp (transfer voltage for the electrical resistance of the recording material S) as shown in FIG. 5. This information is set as table data of the environmental moisture amount and the recording material distribution voltage Vp for each classification (corresponding to the paper type category) of the basis weight of the recording material S. The table data for determining the recording material distribution voltage Vp as shown in FIG. 5 is determined in advance by experiments or the like. In addition, the control unit 30 (pre-image formation preparation process unit 31a) can determine the amount of moisture in the external environment (which may include the inside of the device body) based on environmental information (temperature and humidity) detected by the temperature sensor 71 and the humidity sensor 72.

[0040] The control unit 30 (pre-image formation preparation process unit 31a) obtains the recording material shared voltage Vp from the table data based on the job information obtained in S101 and the environmental information obtained in S103. When an adjustment value is set by an adjustment mode for adjusting the set voltage of the secondary transfer voltage, which will be described later, the control unit 30 obtains an adjustment amount ΔV corresponding to the adjustment value. When the adjustment amount ΔV is set by the adjustment mode, as will be described later, it is stored in the RAM 33 (or the secondary transfer voltage storage unit / calculation unit 31f). The control unit 30 obtains Vb+Vp+ΔV by adding the above Vb, Vp, and ΔV as the secondary transfer voltage Vtr to be applied from the secondary transfer power source 76 to the secondary transfer outer roller 45b when the recording material S passes through the secondary transfer unit N. Then, the control unit 30 writes this Vtr (=Vb+Vp+ΔV) in the RAM 33 (or the secondary transfer voltage storage unit / calculation unit 31f).

[0041] Here, the recording material shared voltage Vp may change depending on the surface properties of the recording material S in addition to information related to the resistance of the recording material S (such as basis weight). Therefore, the table data may be set so that the recording material shared voltage Vp also changes depending on information related to the surface properties of the recording material S. In this embodiment, information related to the resistance of the recording material S (and further information related to the surface properties of the recording material S) is included in the job information acquired in S101. However, a measuring means for detecting the thickness and surface properties of the recording material S may be provided in the image forming apparatus 2, and the recording material shared voltage Vp may be obtained based on information obtained by this measuring means.

[0042] Next, the control unit 30 (image forming process unit 31c) executes image formation, sends the recording material S to the secondary transfer unit 45n, and applies the secondary transfer voltage Vtr determined as described above to perform secondary transfer (S107). Thereafter, the control unit 30 (image forming process unit 31c) repeats S107 until all images of the job have been transferred to the recording material S and output (S108).

[0043] Incidentally, with regard to the primary transfer section 48 as well, the same ATVC control as described above is performed from the start of a job until the toner image is conveyed to the primary transfer section 48, but a description thereof will be omitted.

[0044] <Adjustment mode overview> Next, a simple adjustment mode (herein simply referred to as "adjustment mode") for adjusting the set voltage of the secondary transfer voltage will be described. Depending on the type and condition of the recording material S used for image formation, the moisture content and electrical resistance value of the recording material S may be significantly different from those of a standard recording material S. In this case, optimal transfer may not be achieved with the set voltage of the secondary transfer voltage using the default recording material distribution voltage Vp that is preset as described above.

[0045] First, the secondary transfer voltage must be a voltage necessary for transferring the toner on the intermediate transfer belt 44b to the recording material S. In addition, the secondary transfer voltage must be suppressed to a voltage that does not cause abnormal discharge. However, depending on the type and condition of the recording material S actually used for image formation, the electrical resistance may be higher than the value assumed as the standard value. In this case, the set voltage of the secondary transfer voltage using the preset default recording material distribution voltage Vp may be insufficient for the voltage necessary for transferring the toner on the intermediate transfer belt 44b to the recording material S. Therefore, in this case, it is desirable to increase the set voltage of the secondary transfer voltage by increasing the recording material distribution voltage Vp, for example.

[0046] On the other hand, depending on the type and condition of the recording material S actually used for image formation, the moisture content of the recording material S may be reduced, and the electrical resistance may be lower than the value assumed as the standard value, making it easier for discharge to occur. In this case, the set voltage of the secondary transfer voltage using the preset default recording material distribution voltage Vp may cause image defects due to abnormal discharge. Therefore, in this case, it is desirable to lower the set voltage of the secondary transfer voltage by, for example, lowering the recording material distribution voltage Vp.

[0047] Therefore, it is desirable for an operator such as a user or a service person to adjust (change) the set voltage of the secondary transfer voltage at the time of job execution (image formation) to an optimal value by adjusting (changing) the recording material assigned voltage Vp according to the recording material S actually used for image formation. In other words, it is sufficient to be able to select the optimal "recording material assigned voltage Vp+ΔV (adjustment amount)" according to the recording material S actually used for image formation. This adjustment can be performed by the following method. For example, an operator outputs an image to be output while switching the secondary transfer voltage for each sheet of recording material S, checks whether there is an image defect generated in the output image, and determines the optimal set voltage of the secondary transfer voltage (more specifically, the recording material assigned voltage Vp+adjustment amount ΔV). However, in this method, the output of the image and the adjustment of the set voltage of the secondary transfer voltage are repeated, so that more recording materials S are wasted and it takes time.

[0048] In the present embodiment, the image forming apparatus 2 can execute an adjustment mode for adjusting the secondary transfer voltage set during image formation. In this adjustment mode, an adjustment chart (test chart) is output on the recording material S actually used for image formation, in which a plurality of patches (test toner images) of representative colors are formed for each voltage (each test voltage) while switching the secondary transfer voltage. Then, the image reading unit 80 reads the patch on the output adjustment chart (on the recording material) to obtain the result (information on the density of the image), and the secondary transfer voltage (more specifically, the recording material distribution voltage Vp + adjustment amount ΔV) set during image formation is adjusted based on this. In the present embodiment, information on the adjustment amount ΔV for setting the secondary transfer voltage that optimizes the density of the solid image can be presented based on the brightness data (density information) of the solid patch (patch of the solid image) on the adjustment chart. This reduces the need for the operator to visually check for the presence or absence of image defects, thereby reducing the burden on the operator, and allows the secondary transfer voltage to be adjusted to a more appropriate setting voltage.

[0049] However, when the set voltage of the secondary transfer voltage is adjusted based on the result of reading the patch as described above, the secondary transfer voltage (absolute value) may be too high, causing "punch-through". This is because it is difficult to determine whether or not "punch-through" occurs using the average image density of the patch obtained from the luminance data of the patch. However, it is known that the recording material distribution voltage at which "punch-through" is likely to occur is correlated with the thickness of the recording material S. Furthermore, the inventors of the present application have found through experiments that it is possible to determine whether or not "punch-through" occurs based on the density variation (more specifically, the luminance dispersion value described later) of the divided regions obtained by dividing the patch (test toner image).

[0050] Therefore, in this embodiment, as will be described later in detail, the occurrence of "punch-through" is determined based on the density of the patch formed on the adjustment chart. Then, the "adjustment amount ΔV" required to adjust the set voltage of the secondary transfer voltage (recording material shared voltage Vp+adjustment amount ΔV) can be made different between the cases where "punch-through" has occurred and the cases where it has not occurred.

[0051] <Adjustment chart> In this embodiment, in the adjustment mode, the output adjustment chart is read by the image reading unit 80 to obtain the luminance data of the patch, and a recommended adjustment amount for the set voltage of the secondary transfer voltage is presented. Also, in this embodiment, in the adjustment mode, the operator can visually check the output adjustment chart and change the adjustment amount presented as described above.

[0052] Considering the visual confirmation by the operator, the larger the patch size of the adjustment chart output in the adjustment mode, the easier it is to confirm image defects. However, if the patch is large, the number of patches that can be formed on one sheet of recording material S is reduced. The shape of the patch can be a square or the like. The color of the patch can be determined depending on the image defect to be confirmed and the ease of confirmation. For example, when the secondary transfer voltage is increased from a low value, the lower limit value of the secondary transfer voltage can be determined from the voltage value at which a patch of a secondary color such as red, green, or blue can be properly transferred. In addition, when the operator visually confirms, the upper limit value of the secondary transfer voltage can be determined from the voltage value at which an image defect occurs in a halftone patch due to a high secondary transfer voltage when the secondary transfer voltage is further increased.

[0053] The adjustment chart used in the adjustment mode in this embodiment will be described. In the adjustment mode in this embodiment, two types of image data (100A, 100B) shown in FIG. 6, FIG. 7(a), and FIG. 7(b) are used to output the adjustment chart 100. FIG. 6 shows image data (hereinafter also referred to as "large chart data") 100A of the adjustment chart to be output to a recording material S having a length in the process proceeding direction of 420 to 487 mm. FIG. 7(a) and FIG. 7(b) show image data of the first and second sheets (hereinafter also referred to as "small chart data") of the adjustment chart to be output to a recording material S having a length in the process proceeding direction of 210 to 419 mm. In this embodiment, only two types of image data shown in FIG. 6, FIG. 7(a), and FIG. 7(b) are set as the image data of the adjustment chart. Then, in the adjustment mode, an adjustment chart corresponding to image data cut out from one of the two types of image data shown in FIG. 6, FIG. 7(a), and FIG. 7(b) according to the size of the recording material S used is output to the recording material S. At this time, in this embodiment, image data having a size obtained by subtracting the margins at the edges of the recording material S from the image data shown in FIGS. 6, 7(a) and 7(b) is cut off.

[0054] In this embodiment, the maximum size (maximum paper passing size) of the recording material S on which the image forming apparatus 2 can form an image is 13 inches by 19.2 inches (vertical feed). In this specification, the direction in which the recording material S is transported in the secondary transfer portion 45n is referred to as the "process progress direction," and the direction approximately perpendicular to the process progress direction is referred to as the "longitudinal direction."

[0055] The large chart data 100A shown in FIG. 6 will be further described. The large chart data 100A corresponds to the maximum paper size of the image forming apparatus 2 of this embodiment, and the image size is approximately 13 inches (longitudinal direction) short side (≈330 mm) × 19.2 inches (≈487 mm) long side (process proceeding direction). When the size of the recording material S is 13 inches × 19.2 inches (vertical feed) or less and A3 size (vertical feed) or more, an adjustment chart corresponding to image data cut from this large chart data 100A according to the size of the recording material S is output. At this time, in this embodiment, image data is cut from the large chart data 100A according to the size of the recording material S based on the center of the leading edge in the process proceeding direction. For example, when the adjustment chart 110 is output for A3 size (vertical feed) (short side 297 mm × long side 420 mm) recording material S, image data with a size of 292 mm short side × 415 mm long side is cut from the large chart data 100A. An image corresponding to this cut-out image data is then output onto an A3-sized recording material S, with a leading edge center reference in the process progression direction, and with a margin of 2.5 mm on each edge.

[0056] The large chart data 100A has one solid blue patch 101, one solid black patch 102, and two halftone (gray (black halftone) in this embodiment) patches 103 arranged in the longitudinal direction. Eleven sets of longitudinal patch sets (101 to 103) are arranged in the process proceeding direction. The solid blue patch 101 and the solid black patch 102 are each a square (one side is approximately parallel to the longitudinal direction) of 25.7 mm×25.7 mm. The halftone patches 103 at both ends each have a width of 25.7 mm in the process proceeding direction, and extend in the longitudinal direction to the very end of the large chart data 100A. The interval between the patch sets (101 to 103) in the process proceeding direction is 9.5 mm. The secondary transfer voltage is switched at the timing when the portion on the adjustment chart corresponding to this interval passes through the secondary transfer unit N. The eleven patch sets (101 to 103) in the large chart data 100A in the process progression direction are arranged within a range of 387 mm in length in the process progression direction so that they fit within a length of 415 mm in the process progression direction when the size of the recording material S is A3 size. In this embodiment, the large chart data 100A also has identification information 104 associated with each of the eleven patch sets (101 to 103) in the process progression direction, for identifying the setting of the secondary transfer voltage applied to each patch set. This identification information 104 corresponds to an adjustment value, which will be described later. In this embodiment, eleven pieces of identification information 104 (-5 to 0 to +5 in this embodiment) corresponding to eleven levels of secondary transfer voltage settings are arranged.

[0057] The size of the patch is required to be a size that allows the operator to easily judge the presence or absence of an image defect, taking into consideration visual confirmation by the operator. The transferability of the blue solid patch 101 and the black solid patch 102 is difficult to judge when the patch size is small, so the patch size is preferably 10 mm square or more, and more preferably 25 mm square or more. In the halftone patch 103, image defects caused by abnormal discharge that occurs when the secondary transfer voltage is increased often become image defects such as white dots. This image defect tends to be easier to judge even for small images compared to the transferability of solid images. However, since it is easier to see if the image is not too small, in this embodiment, the width of the halftone patch 103 in the process progression direction is set to be the same as the width of the blue solid patch 101 and the black solid patch 102 in the process progression direction. In addition, the interval between the patch sets (101 to 103) in the process progression direction may be set so that the secondary transfer voltage can be switched.

[0058] It is preferable that no patch is formed in the vicinity of the leading edge and trailing edge of the recording material S in the process proceeding direction (for example, within a range of about 20 to 30 mm inward from the edge). This is because there may be an image defect that occurs only at the leading edge or trailing edge of the recording material S, and it may be difficult to determine whether the image defect is caused by the secondary transfer voltage. A solid image is an image with a maximum density level. A halftone image is an image with a toner amount of 10% to 80% when the toner amount of a solid image is 100%. A solid image is an image with a maximum density level. In this embodiment, a halftone image is an image with a toner amount of 10% to 80% when the toner amount of a solid image is 100%.

[0059] When the above-described large chart data 100A is used, the longitudinal lengths of the halftone patches 103 at both longitudinal ends become smaller as the size of the recording material S becomes smaller than 13 inches (however, A3 size or larger). Also, when the above-described large chart data 100A is used, the margin at the trailing end in the process progression direction becomes smaller as the size of the recording material S becomes smaller than 13 inches (however, A3 size or larger).

[0060] The small chart data 100B shown in FIG. 7(a) and FIG. 7(b) will be further described. The small chart data 100B corresponds to a size smaller than A3 size, and the image size is approximately 13 inches (≈330 mm) long side (longitudinal direction) × 210 mm short side (process proceeding direction). When the size of the recording material S is A5 (short side 148 mm × long side 210 mm) (vertical feed) or more and smaller than A3 size (vertical feed), an adjustment chart corresponding to image data cut from this small chart data 100B according to the size of the recording material S is output. In this embodiment, image data is cut from the small chart data 100B according to the size of the recording material S based on the center of the leading edge in the process proceeding direction. When the small chart data 100B is used, two adjustment charts are output to increase the number of patches.

[0061] The small chart data 100B is configured to have the same patches as the large chart data 100A. In the small chart data 100B, five longitudinal patch sets (101-103) are arranged in the process progression direction. The five patch sets (101-103) in the process progression direction of the small chart data 100B are arranged in a range of 167 mm in length in the process progression direction. In this embodiment, the small chart data 100B is provided with identification information 104 associated with each of the five patch sets (101-103) in the process progression direction, for identifying the setting of the secondary transfer voltage applied to each patch set. Then, five pieces of identification information 104 (-4 to 0 in this embodiment) corresponding to the lower five levels of secondary transfer voltage settings are arranged on the first sheet based on the small chart data 100B shown in FIG. 7(a). Also, on the second sheet, five pieces of identification information 104 (+1 to +5 in this embodiment) corresponding to the five higher levels of secondary transfer voltage settings are arranged based on the small chart data 100B shown in FIG. 7(b).

[0062] When the above-mentioned small chart data 100B is used, the longitudinal length of the halftone patches 103 at both ends in the longitudinal direction becomes smaller as the size of the recording material S becomes smaller (but smaller than A3 size and A5 size or larger). Also, when the above-mentioned small chart data 100B is used, the margin at the rear end in the process progress direction becomes smaller as the size of the recording material S becomes smaller (but smaller than A3 size and A5 size or larger).

[0063] In this embodiment, in addition to standard sizes, recording material S of any size (A5 size or larger, 13 inches x 19.2 inches or smaller) can be used by the operator specifying the size through, for example, the operation unit 70 or the external device 200.

[0064] <Adjustment mode> Next, the adjustment mode of the first embodiment will be described. Fig. 8 is a flowchart showing the adjustment mode in this embodiment. Fig. 9 is a diagram showing a setting screen for the adjustment mode displayed on the operation unit 70. The control unit 30 executes the adjustment mode when the operator issues an instruction to execute the adjustment mode via the operation unit 70 of the image forming apparatus 2.

[0065] In the adjustment mode, the control unit 30 (adjustment processing unit 31d) displays a setting screen (not shown) for the type and size of the recording material S on the operation unit 70, and prompts the operator to input the type and size of the recording material S on which an image is to be formed (S1). Then, the control unit 30 acquires information on the type and size of the recording material S specified on the operation unit 70 by the operator.

[0066] Next, the control unit 30 allows the operator to set (S2) the center voltage value of the secondary transfer voltage to be applied when outputting the adjustment chart, and whether the adjustment chart is to be output on one side or both sides of the recording material S. Note that in this embodiment, the adjustment chart can be output on both sides of the recording material S even in the adjustment mode so that the secondary transfer voltage during secondary transfer to the front side (first side) and back side (second side) in double-sided printing can be adjusted respectively.

[0067] In order to perform the above settings, the control unit 30 causes the operation unit 70 to display an adjustment mode setting screen 90 as shown in Fig. 9. This setting screen 90 has a voltage setting section 91 for setting a center voltage value of the secondary transfer voltage for the front and back sides of the recording material S. The setting screen 90 also has an output side selection section 92 for selecting whether the adjustment chart is to be output on one side or both sides of the recording material S. The setting screen 90 further has an output instruction section (test page output button) 93 for instructing output of the adjustment chart, a confirmation section 94 (OK button 94a or apply button 94b) for confirming the settings, a cancel button 95 for canceling the change of the settings, and the like.

[0068] When the adjustment value "0" is selected in the voltage setting section 91, a preset set voltage (more specifically, recording material distribution voltage Vp) for the currently selected recording material S is selected. When the adjustment value "0" is selected, 11 patch sets of -5 to 0 to +5 are output when large chart data is used, and 10 patch sets of -4 to 0 to +5 are output when small chart data is used, with the secondary transfer voltage switched. Note that, in this example, it is assumed that the large chart data is used and an adjustment chart having 11 patch sets is output. In this embodiment, the difference in the secondary transfer voltage for each level is set to 150V. The control section 30 acquires information on the settings, such as the center voltage value, set via the setting screen 90 in the operation section 70.

[0069] Next, when the operator selects the output instruction section 93 of the setting screen 90, the control section 30 acquires information regarding the electrical resistance of the secondary transfer section N when no recording material S is present at the secondary transfer section N (S3). In this embodiment, the control section 30 acquires a quadratic or higher polynomial (a quadratic expression in this embodiment) of the relationship between the voltage and current according to the low electrical resistance of the secondary transfer section N by an operation similar to that of the ATVC control described above. The control section 30 writes the information regarding the relationship between the voltage and current to the RAM 33 or the like.

[0070] Next, the control unit 30 outputs an adjustment chart (S4). At this time, the control unit 30 cuts out the chart data as described above based on the information on the size of the recording material S acquired in S1, and outputs an adjustment chart on which 11 patch sets are transferred while changing the secondary transfer voltage by 150V. For example, assume that the recording material shared voltage Vp in the current environment is 900V, and the secondary transfer partial voltage Vb obtained as a result of ATVC control is 1000V. In this case, an adjustment chart on which 11 patch sets are transferred is output while changing the secondary transfer voltage by 150V from 1150V to 2650V. At this time, the control unit 30 detects the current value flowing when the voltage of each voltage level is applied by the current detection sensor 76b, and acquires information on the electrical resistance of the secondary transfer unit N and the recording material S when the recording material S is in the secondary transfer unit N (S5). In this embodiment, the control unit 30 obtains a quadratic or higher degree polynomial (a quadratic expression in this embodiment) of the relationship between the voltage and current according to the electrical resistance of the secondary transfer unit N and the recording material S from the detection results of the current for the 11 levels of voltage. The control unit 30 writes information on this relationship between the voltage and current to the RAM 33 or the like. Note that, while the current when the recording material S is in the secondary transfer unit N is typically detected as the current during transfer of the patch, it may also be detected for each voltage level in the portions of the recording material S without toner before and after the patch.

[0071] Then, the control unit 30 acquires the recording material shared voltage Vp(n) at each voltage level from the relationship (quadratic equation) between the voltage and current when the recording material S is present at the secondary transfer unit N acquired in S5 and the relationship (quadratic equation) between the voltage and current when the recording material S is not present at the secondary transfer unit N acquired in S3 (S6). Here, "n" indicates each voltage level, and is "1 to 11" corresponding to 11 levels (11 patch sets). Also, the voltage value of each voltage level is Vtr(n). Also, the voltage value calculated by applying each current value detected when applying the voltage at each voltage level to the relationship (quadratic equation) between the voltage and current when the recording material S is not present at the secondary transfer unit N acquired in S3 is assumed to be Vb(n). At this time, the recording material shared voltage Vp(n) at each voltage level is expressed as "Vp(n)=Vtr(n)-Vb(n)".

[0072] Next, the output adjustment chart is supplied to the image reading unit 80 by, for example, an operator using the automatic document feeder 81, and the adjustment chart is read by the image reading unit 80 (S7). At this time, the image reading unit 80 is controlled by the control unit 30, and in this embodiment, obtains RGB luminance data (8 bits) of each patch of solid blue and solid black on the adjustment chart. Note that when the adjustment chart is output, the control unit 30 can display on the operation unit 70 a message urging the user to supply the adjustment chart to the image reading unit 80.

[0073] Next, the control unit 30 uses the brightness data (density data) acquired in S7 to calculate the average brightness of each patch of the blue solid and black solid according to the following formula 1 (S8). By the process of S8, the average brightness of the blue solid patch corresponding to each voltage level is calculated, as shown in FIG. 10 as an example. The horizontal axis of FIG. 10 indicates the adjustment value (-5 to 0 to +5) indicating each voltage level, and the vertical axis indicates the average brightness of the blue solid patch. In this embodiment, the brightness data of B brightness was used for the blue solid patch, and the brightness data of G brightness was used for the black solid patch. The brightness average value (average brightness) shown in formula 1 is a parameter that reflects the density. The lower the brightness average value Bave(n), the higher the density of the toner image transferred to the recording material S.

number

[0074] Then, the control unit 30 calculates the luminance variance value of the solid black patch 102 according to the following equation 2 (S9). Here, a method of acquiring luminance data used to calculate the luminance variance value of the solid black patch 102 will be described with reference to FIG. 11. As shown in FIG. 11, a reading area P is set in the image area of ​​each solid black patch 102. In this embodiment, the size of the reading area P is set to 10 mm×10 mm at the center of the patch. In order to calculate the luminance variance value, the reading area P is divided into M areas from P(1) to P(M), and the luminance data of the corresponding divided areas of the adjustment chart read in S7 is stored in the RAM 33 as B(1) to B(M). The size of the divided areas P(1) to P(M) may be the minimum unit of resolution that can be read by the image reading unit 80, and may be, for example, about 300 dpi to 1200 dpi.

[0075] The luminance variance is calculated as shown in Equation 2. "B(m)" is the luminance data of the "mth" (m=1 to M) divided area read out in the black solid patch, and "M" is the total number of divided areas read out. "Bave(n)" is the average luminance value of the black solid patch, and "D(n)" is the luminance variance of the black solid patch. The luminance variance shown in Equation 2 reflects the transferability when the recording material S has unevenness. The larger the luminance variance value D(n) (variance), the greater the difference in transferability of the toner image from the intermediate transfer belt 44b to the recording material S depending on the area, in other words, the greater the density variation.

number

[0076] Next, the control unit 30 obtains an adjustment value Na that minimizes the average luminance value of the blue solid patch obtained in S8 (S10). In the example shown in FIG. 10, the luminance decreases as the adjustment value increases from "-5" to "+2". In this adjustment value range, the electric field required to transfer the blue solid patch is insufficient, and the transferability of the blue solid patch improves as the adjustment value is increased. The adjustment value is further increased to "+3 to +4" to obtain the lowest luminance. If the secondary transfer voltage is larger than necessary, the risk of image damage caused by discharge phenomena such as punch-through increases, so in this embodiment, the smaller "+3" is selected as the adjustment value Na. Obtaining the adjustment value Na in the above manner corresponds to selecting the first test voltage when a blue solid patch with a relatively large average density is formed among multiple blue solid patches.

[0077] Next, the control unit 30 checks whether or not "punch-through" occurs at the adjustment value Na obtained in S10 (S11). In other words, it checks whether or not "punch-through" occurs in the blue solid patch formed at the first test voltage selected in S10.

[0078] A method for determining whether or not "punch-through has occurred" will be described with reference to Figs. 12 and 13. Fig. 12 shows the relationship between the occurrence of punch-through when image formation is performed using the image forming apparatus 2 while changing the secondary transfer voltage, and the luminance dispersion value (calculated by the method of S9). Recording materials 1 to 6 represent different types (paper types) of recording materials used on the market, such as "thin paper, regular paper, thick paper." The punch-through rank on the horizontal axis is a value that visually expresses the occurrence of punch-through on a 5-point scale. Here, the case where no punch-through has occurred is "Rank 5," and the case where the most punch-through has occurred is "Rank 1," with the smaller the rank number, the more punch-through has occurred.

[0079] As can be seen from FIG. 12, the luminance variance value (absolute value) differs depending on the type of recording material (paper type). The more punch-through occurs, the more variation occurs in density and the higher the luminance variance value. In this embodiment, the above characteristics are used to find the slope of the luminance variance value with respect to the adjustment value, and if the slope of the found luminance variance value is greater than the slope of a predetermined line L, it is determined that "punch-through has occurred." The slope of line L is set to "1" based on the data in FIG. 12.

[0080] The details of the "punch-through occurrence" determination will be described with reference to FIG. 13. The slope of the black luminance variance value is calculated in the vicinity of the adjustment value at which the blue luminance average value is the minimum. Specifically, in this embodiment, the "adjustment value (Na) at which the blue luminance average value calculated in S10 is the minimum value," the "next smaller adjustment value (Na-1)," and the "next larger adjustment value (Na+1)" are set. The control unit 30 calculates the slope of the approximation line l of the black luminance variance value in the above adjustment value range by the least squares method, and if the slope of the approximation line l is larger than the slope of the line L, it is determined that punch-through has occurred. In other words, if the increase in density variation of the black solid patch due to the increase in the double transfer voltage near the first test voltage is equal to or greater than a threshold value, it is determined that punch-through has occurred.

[0081] 13, the slope of line l is "2.25", which is greater than the slope (1) of line L, and is therefore determined to be "occurrence of punch-through". Note that in this embodiment, since the determination is made based on the slope of the luminance distribution in the adjustment value, strictly speaking, "occurrence of punch-through" is determined to be when "the punch-through rank worsens by -1 for every adjustment value of +1".

[0082] When it is determined that punch-through has occurred (Yes in S11), the control unit 30 corrects the adjustment value (S12). Fig. 14 shows the relationship between the recording material share voltage of the secondary transfer voltage and the occurrence of "punch-through" when an image on a recording material S is confirmed in a NL environment (temperature 23°C, humidity 5%) using the image forming apparatus 2 of this embodiment.

[0083] As shown in FIG. 14, it has been found through experiments that the thicker the recording material S, the greater the recording material voltage (absolute value) at which "punch-through" occurs. According to the study by the inventors, the recording material voltage at which "punch-through" is likely to occur is well matched with the discharge start voltage obtained from the Paschen curve when the thickness of the recording material S is considered as air (gap). In other words, the relationship shown in FIG. 14 coincides with the cause of "punch-through", that is, when the recording material S is subjected to discharge during secondary transfer, the charge polarity of the toner in the corresponding portion is reversed, and the toner is no longer transferred to the recording material. Therefore, in this embodiment, the above relationship is utilized to set an upper limit value for the recording material voltage according to information on the thickness of the recording material S. This makes it possible to select an adjustment value for the set voltage of the secondary transfer voltage within a range that can suppress the occurrence of "punch-through".

[0084] Specifically, in this embodiment, the control unit 30 extracts, from the recording material shared voltages Vp(n) acquired in S6, those that do not exceed the upper limit set according to information on the thickness of the recording material S. In this embodiment, the relationship between information on the thickness of the recording material S on the market (here, basis weight) and the upper limit of the recording material shared voltage Vp(n) is checked in advance for each type (paper type category) of the recording material S, such as "thin paper, plain paper, thick paper 1, thick paper 2, etc." Then, this relationship between the type of recording material S and the recording material shared voltage Vp(n) is stored in the ROM 32 as table data as shown in FIG. 15. The control unit 30 refers to the table data in FIG. 15 to acquire the upper limit of the recording material shared voltage Vp(n) corresponding to the type of recording material S acquired in S1.

[0085] FIG. 16 shows an image of the adjustment value correction in this embodiment when it is determined that the punch-through has occurred. As shown in FIG. 16, the adjustment value Na calculated with the minimum blue luminance value is "+3", but the range of adjustment values ​​in which the recording material distribution voltage Vp(n) is equal to or less than the upper limit is "+2" or less. In this case, the adjustment value "+2" that is the smallest average blue luminance value equal to or less than "+2" is adopted (set as adjustment value Na'). In other words, when the increase in density variation of the black solid patch due to the increase in the secondary transfer voltage near the first test voltage is equal to or greater than the threshold value, the second test voltage smaller than the first test voltage is set as the transfer voltage to be set during image formation. This second test voltage is the largest test voltage among the multiple test voltages within a range lower than a predetermined voltage (the upper limit of Vb+Vp(n)) that is determined in advance according to information on the thickness of the recording material S used to output the adjustment chart.

[0086] On the other hand, if it is determined that no punch-through has occurred (S11 in FIG. 8), the adjustment value Na calculated in S10 in FIG. 8 is used as is. In other words, if the increase in density variation of the black solid patch caused by the increase in the transfer voltage near the first test voltage is smaller than the threshold value, the first test voltage is set as the transfer voltage to be set during image formation.

[0087] Next, the control unit 30 causes the operation unit 70 to display the "adjustment value Na" or "adjustment value Na'" found thus far on a setting screen 90 (voltage setting section 91) as shown in FIG. 9 (S13). The operator determines whether the displayed adjustment value is acceptable or not based on the display contents of the setting screen 90 and the output adjustment chart, and may change the adjustment value as necessary. If the operator does not want to change the displayed adjustment value, he or she simply selects the confirmation section 94 (OK button 94a or apply button 94b) of the setting screen 90. On the other hand, if the operator wants to change the displayed adjustment value, he or she inputs it into the voltage setting section 91 of the setting screen 90 and selects the confirmation section 94 (OK button 94a or apply button 94b).

[0088] The control unit 30 determines whether or not an instruction to change the adjustment value has been received (S14), and if an instruction to change the adjustment value has been received (Yes in S14), the control unit 30 stores the input adjustment value in the RAM 33 (or the secondary transfer voltage storage unit / calculation unit 31f) (S15). If the adjustment value is not changed and the confirmation unit 94 is selected (No in S14), the control unit 30 stores the adjustment value displayed on the setting screen 90 in the RAM 33 (or the secondary transfer voltage storage unit / calculation unit 31f) (S16). This ends the adjustment mode.

[0089] When executing a job thereafter, until the next time the adjustment mode is executed, the control unit 30 calculates the adjustment amount ΔV according to the adjustment value stored in the adjustment mode as "ΔV = adjustment value × 150V" and uses this to calculate the secondary transfer voltage Vtr during normal image formation.

[0090] In the case where the adjustment amount used for calculating the secondary transfer voltage Vtr is determined based only on the average brightness value as in the past, the average patch brightness value may be minimum above the upper limit value of the recording material distribution voltage, and there is a risk of determining an adjustment amount that may cause "punch-through." In contrast, according to the present embodiment, it is possible to avoid an adjustment amount that may cause "punch-through" and determine an appropriate adjustment amount.

[0091] The information on the upper limit of the recording material voltage Vp(n) used in S12 described above is not limited to being set as table data (see FIG. 15) as in this embodiment. For example, a relational expression (not shown) showing the relationship between information on the thickness of the recording material S and the recording material voltage Vp(n) at which "punch-through" is likely to occur can be obtained in advance and stored in ROM 32. In this case, the information on the thickness of the recording material S can be obtained, and the upper limit of the recording material voltage Vp(n) can be obtained from the relational expression.

[0092] Furthermore, information regarding the thickness of the recording material S is not limited to being classified by the type of recording material S. For example, in the above-mentioned S1 (see FIG. 8), an operator can input values ​​related to the thickness, such as the thickness and basis weight, of the recording material S. Furthermore, in a step corresponding to the above-mentioned S1, values ​​related to the thickness, such as the thickness and basis weight, of the recording material S may be obtained using a measuring means (not shown) that measures values ​​related to the thickness, such as the thickness and basis weight, of the recording material S. As such a measuring means, for example, a known thickness sensor using ultrasonic waves may be provided upstream of the secondary transfer portion N in the process proceeding direction of the recording material S.

[0093] In addition, in this embodiment, blue and black patches are used as patches for obtaining the luminance average value and luminance variance value, but this is not limited to this. For example, instead of blue, secondary colors such as red and green can be used, or solid yellow, magenta, cyan, and black can be used. However, using a dark color such as solid black is more advantageous in determining the presence or absence of "punch-through."

[0094] In the present embodiment, the adjustment mode is executed by an operator via the operation unit 70 of the image forming apparatus 2, but the adjustment mode may be executed via an external device 200 such as a personal computer. In this case, the same settings as those described above can be made via a setting screen displayed on the display unit of the external device 200 by a driver program of the image forming apparatus 2 installed in the external device 200.

[0095] Furthermore, in this embodiment, information on the electrical resistance of the secondary transfer portion N when there is no recording material S at the secondary transfer portion N after the adjustment mode is started is obtained. This makes it possible to obtain information on the electrical resistance of the secondary transfer portion N that is more suited to the situation when determining the adjustment amount for the secondary transfer voltage setting. However, if permissible from the standpoint of accuracy, etc., the result of ATVC control at the start of the job immediately before (closest to) executing the adjustment mode may be used as information on the electrical resistance of the secondary transfer portion N.

[0096] Furthermore, in this embodiment, the adjustment mode is control using the display of the adjustment value corresponding to the adjustment amount ΔV, but control may be performed using the display of the adjustment amount ΔV more directly.

[0097] In addition, in this embodiment, when obtaining the relationship between voltage and current, the current value that flows when a predetermined voltage is supplied is detected, but it is also possible to detect the voltage value that occurs when a predetermined current value is supplied.

[0098] In addition, the rank itself used for the punch-through judgment is based on the visual evaluation by the inventors, and may be a different standard. In addition, the method of punch-through judgment using the brightness variance value is not limited to this embodiment, and it is also possible to judge by, for example, the difference in brightness variance between the front and rear adjustment values ​​instead of the inclination. Furthermore, the adjustment values ​​used for judgment are Na-1, Na, and Na+1 in this embodiment, but may be other than these three points.

[0099] As described above, in this embodiment, the occurrence of "punch-through" is determined based on the density of the patch formed on the adjustment chart, and the adjustment amount ΔV can be made different between the case where "punch-through" occurs in the patch and the case where "punch-through" does not occur. When "punch-through" occurs, the adjustment amount ΔV is suppressed to be lower than when "punch-through" does not occur. As a result, the secondary transfer voltage determined by "recording material shared voltage Vp + adjustment amount ΔV" is adjusted according to the occurrence of "punch-through". Therefore, in a configuration having an adjustment mode that adjusts the secondary transfer voltage set during image formation based on the adjustment chart on which the patch is formed, it is possible to adjust the secondary transfer voltage to one that achieves both the suppression of the occurrence of "punch-through" and the suppression of the occurrence of "image density reduction".

[0100] [Second embodiment] Next, an image forming apparatus according to a second embodiment will be described. In the image forming apparatus according to the first embodiment, when punch-through occurs in the adjustment chart, the adjustment value is lowered so that the secondary transfer voltage can be adjusted to a voltage with a low risk of punch-through. However, lowering the adjustment value makes it easier for the secondary transfer voltage to be set low, which may result in a decrease in the density of the secondary color.

[0101] In consideration of this point, in the second embodiment, when executing the adjustment mode, the operator can select whether to prioritize ensuring the density of the secondary color at the expense of the risk of punch-through, or to prioritize preventing the occurrence of punch-through at the expense of the risk of a decrease in the density of the secondary color.

[0102] The adjustment mode of the second embodiment will be described with reference to Figs. 17 to 19 while also referring to Figs. 1 and 2. Fig. 17 is a flowchart showing the adjustment mode in the second embodiment. Fig. 18 is a diagram showing an example of a setting screen for the adjustment mode. In the flowchart shown in Fig. 17, the same processes as those in the adjustment mode of the first embodiment described above (see Fig. 8) are assigned the same reference numerals, and the following mainly describes processes different from those in the adjustment mode of the first embodiment.

[0103] As shown in Fig. 17, first, the recording material S is designated, and the center voltage and double-sided settings are performed in the same manner as in the first embodiment (S1, S2). In this embodiment, a detailed setting 96 is added to the setting screen 90a in Fig. 18, and when the detailed setting 96 is selected, the control unit 30 (adjustment processing unit 31d) causes the operation unit 70 to display a detailed setting screen 90b shown in Fig. 19.

[0104] In the detailed setting screen 90b in FIG. 19, it is possible to input whether to prioritize the output of a "high density image" or a "half tone image" in the priority image selection 97. A "high density image" is an image that uses a lot of toner, such as a so-called solid image or a secondary color (red, blue, green), and requires a large transfer voltage. A "half tone image" is an image that mainly uses light colors and uses a relatively small amount of toner per unit area of ​​the recording material S. Since punch-through is more noticeable in half tones, if the priority image selection 97 is "half tone", correction will be performed when punch-through occurs.

[0105] After selecting the priority image, the operator selects the confirmation section 98 of the detailed setting screen 90b. The control unit 30 stores the settings input by the operator in the RAM 33 or the like (S100). When the detailed setting is completed, the control unit 30 returns to the setting screen 90a in FIG. 18 and performs the same processes (S3 to S10) as in the first embodiment up to the calculation of the minimum luminance adjustment value (S10).

[0106] In the second embodiment, the control unit 30 determines whether the prioritized image is a "high density image" or a "halftone" (S101) before determining whether punch-through has occurred (S11). If it is a "halftone" (Yes in S101), the control unit 30 determines whether punch-through has occurred as in the first embodiment (S11). On the other hand, if it is a "high density image" (No in S101), the control unit 30 does not determine whether punch-through has occurred and jumps to S13. This determination flow makes it possible to set an appropriate secondary transfer voltage according to the image prioritized by the operator. After these processes, the same flow as in the first embodiment is performed to end the adjustment mode.

[0107] This is preferable because it allows the operator to easily adjust the transfer voltage to the optimum level for the desired usage conditions by selecting whether to prioritize suppressing the decrease in secondary color density or suppressing the occurrence of punch-through.

[0108] In this embodiment, the selection of the priority image (S100) is performed on the setting screen 90a, but this is not limiting. For example, the priority image may be selected based on image information of an image that the operator actually wants to output (e.g., image information from the image reading unit 80 or the external device 200 (see FIG. 2)). If the image information contains many half-tone images, processing at the time of punch-through occurrence (see S12) may be performed.

[0109] In the above-described embodiment, the image forming apparatus 2 is described as an intermediate transfer type in which the toner image is primarily transferred from the photosensitive drums 51y-51k of each color to the intermediate transfer belt 44b, and then the toner image is secondarily transferred from the intermediate transfer belt 44b to the recording material S. However, the present invention is not limited to this. The above-described embodiment is also applicable to a direct transfer type image forming apparatus in which the toner image is directly transferred to the recording material S from the photosensitive drums 51y-51k of each color that rotate while carrying the toner image. [Explanation of symbols]

[0110] 2...image forming apparatus, 30...control section, 44b...image carrier (intermediate transfer belt), 45b...transfer member (secondary transfer outer roller), 51y to 51k...photosensitive drum, 70...input section (operation section), 76...power supply (secondary transfer power supply), 80...acquisition section (image reading section), N...transfer section (secondary transfer section)

Claims

[Claim 1] an image carrier that carries a toner image; an image forming unit that forms a toner image on the image carrier; a transfer member for transferring a toner image from the image carrier to a recording material; a power source that applies a transfer voltage to the transfer member in order to transfer a toner image from the image carrier to the recording material; an acquisition unit that acquires image density information relating to the density of a toner image transferred onto the recording material; a control unit capable of executing an adjustment mode in which a plurality of different test voltages are applied by the power source to output a test chart in which a plurality of test toner images are transferred onto the recording material, and a transfer voltage that is set for transferring a toner image from the image carrier to the recording material based on a density of the test toner image transferred onto the test chart is adjusted; the test toner image includes a first test toner image transferred to the recording material by applying a first test voltage from the power source, and a second test toner image transferred to the recording material by applying a second test voltage different from the first test voltage from the power source, The control unit, in the adjustment mode, obtaining information regarding a change in the image density variation with respect to a change in the test voltage based on first information regarding the image density variation obtained in different areas of the first test toner image and second information regarding the image density variation obtained in different areas of the second test toner image; adjusting a transfer voltage set for transferring a toner image onto the recording material based on information regarding a change in the variation in the image density relative to a change in the test voltage; 1. An image forming apparatus comprising:

Images