Image forming apparatus

Abstract

To suppress back staining when performing double-sided printing continuation processing while suppressing a decrease in productivity of printing.SOLUTION: The image forming apparatus 100 is capable of executing a continuation process in which the toner image is secondary-transferred onto a second surface of the sheet 2 in the double-side feeding portion in the case where the sheet 2 in the feeding portion 1 runs out during execution of the double-side print job or in the like case. In the case where the continuous process is executed, while the non-transferred toner image on the intermediary transfer member 12 passes through the secondary transfer portion N2, the controller 200 effects control so that a predetermined bias such that the potential of the outer roller 9 is on the side of the normal charge polarity of the toner relative to the inner roller 18 is applied to the secondary transfer portion N2.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus such as a printer, a copying machine, a facsimile machine, or a multifunction machine that uses an electrophotographic method or an electrostatic recording method, or a combination machine that has a plurality of functions among these. [Background technology]

[0002] Conventionally, image forming apparatuses, such as electrophotographic printers, include intermediate transfer-type image forming apparatuses equipped with an intermediate transfer body. In intermediate transfer-type image forming apparatuses, a toner image formed on an image carrier is primarily transferred onto the intermediate transfer body, followed by secondary transfer onto a sheet, forming an image on the sheet. An intermediate transfer belt stretched over multiple tension rollers is widely used as the intermediate transfer body. Primary transfer is performed, for example, by applying a primary transfer voltage to a primary transfer member disposed opposite the image carrier via the intermediate transfer belt. As described in Patent Document 1, a configuration is also known in which a voltage maintenance element is connected to a contact member, such as a tension roller, that contacts the inner circumferential surface of the intermediate transfer belt, thereby maintaining a primary transfer potential above a predetermined value. Secondary transfer is performed, for example, by applying a secondary transfer voltage to a secondary transfer member disposed opposite one of the tension rollers via the intermediate transfer belt. As the secondary transfer member, a secondary transfer roller that contacts one of the tension rollers via the intermediate transfer belt to form a secondary transfer section is often used.

[0003] Image forming devices are sometimes equipped with an automatic duplex conveying function that allows images to be automatically formed on both sides of a sheet without the user having to reset the front and back of the sheet. In such image forming devices, if the paper feed section runs out of sheets during a duplex print job, there is a method for completing printing on the back side of a sheet that has already been printed on its front side and is waiting in the duplex unit. This prevents sheets with printed front sides from going to waste. This operation is also referred to here as "duplex printing continuation processing" (or simply "continuation processing").

[0004] However, when performing continuous double-sided printing as described above, the toner image on the intermediate transfer belt that has not been transferred to the sheet passes through the secondary transfer unit before the sheet waiting in the double-sided unit is fed to the secondary transfer unit. At this time, the toner of the toner image on the intermediate transfer belt adheres to the secondary transfer roller, which may cause contamination of the back side of the sheet fed from the double-sided unit (hereinafter simply referred to as "back contamination").

[0005] Patent document 2 discloses a method for suppressing adhesion of toner to the secondary transfer roller by applying a voltage of the opposite polarity to that applied when the toner image is transferred to the sheet to the secondary transfer roller when the calibration toner image on the intermediate transfer belt passes through the secondary transfer section. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-231942 [Patent Document 2] Japanese Patent Application Laid-Open No. 2006-215369 Summary of the Invention [Problem to be solved by the invention]

[0007] In the configuration for performing continuous double-sided printing as described above, it is important to suppress the above-mentioned backside contamination, but it is also important not to reduce printing productivity.

[0008] For example, if there is sufficient time before secondary transfer to the back side of a sheet waiting in the duplex unit, it is conceivable to clean the secondary transfer roller during that time. However, if the interval between sheets is increased in advance to prepare for the possibility that the paper feed unit will run out of sheets during duplex printing, cleaning the secondary transfer roller as described above can suppress back contamination, but printing productivity will decrease.

[0009] SUMMARY OF THE INVENTION It is therefore an object of the present invention to suppress backside contamination when continuous double-sided printing is performed while suppressing a decrease in printing productivity. [Means for solving the problem]

[0010] The above object is achieved by the image forming apparatus according to the present invention. In summary, the present invention provides an image forming unit that forms a toner image on an image carrier using toner whose normal charging polarity is a predetermined polarity, a circumferentially movable intermediate transfer body onto which the toner image is primarily transferred from the image carrier in a primary transfer unit, an inner roller provided on the inner circumferential surface of the intermediate transfer body, an outer roller that contacts the inner roller via the intermediate transfer body and forms a secondary transfer unit that secondarily transfers the toner image from the intermediate transfer body to a sheet, an application unit that applies a bias to the secondary transfer unit, a feed unit on which a sheet is set, a transport unit that transports the sheet from the feed unit to the secondary transfer unit, a double-sided transport unit that inverts the sheet that has passed through the secondary transfer unit and transports it to the secondary transfer unit, and a double-sided print that secondarily transfers the toner image from the intermediate transfer body to a first side and a second side of the sheet, wherein after a first toner image formed in a first image forming area on the intermediate transfer body is secondarily transferred to a first side of a first sheet transported by the transport unit to the secondary transfer unit, An image forming apparatus having a control unit capable of controlling the image forming unit, the application unit, the transport unit, and the double-sided transport unit to perform double-sided printing in which a second toner image formed in a second image forming area on an intermediate transfer body is secondarily transferred to a second side of a second sheet transported by the double-sided transport unit to the secondary transfer unit, wherein the control unit is capable of controlling the image forming unit, the application unit, the transport unit, and the double-sided transport unit to perform a continuation process in which the second toner image is secondarily transferred to the second side of the second sheet if the first sheet is not transported to the secondary transfer unit when performing the double-sided printing, and the control unit, when performing the continuation process, controls the application unit to apply to the secondary transfer unit a predetermined bias such that the outer roller has a potential of the predetermined polarity relative to the inner roller while the first image forming area is passing through the secondary transfer unit, and such that the absolute value of the potential difference between the inner roller and the outer roller is less than a discharge threshold. [Effects of the Invention]

[0011] According to the present invention, it is possible to suppress the backside contamination when performing continuous double-sided printing while suppressing a decrease in printing productivity. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a schematic cross-sectional view showing a general configuration of an image forming apparatus. [Figure 2] FIG. 2 is a schematic cross-sectional view showing a schematic configuration of an image forming unit. [Figure 3] FIG. 2 is a block diagram showing an outline of a control configuration of the image forming apparatus. [Figure 4] FIG. 10 is a schematic diagram for explaining a communication sequence during double-sided printing. [Figure 5] FIG. 10 is a timing chart for explaining an image formation sequence during double-sided printing. [Figure 6] FIG. 10 is a timing chart for explaining an image forming sequence when continuous double-sided printing is performed. [Figure 7] FIG. 10 is a graph showing the relationship between the voltage value of the through bias and the amount of back staining. [Figure 8] FIG. 10 is a graph showing the relationship between the voltage value of the inter-paper bias and the amount of back staining. [Figure 9] FIG. 10 is a schematic diagram illustrating an example of a setting screen for double-sided printing. [Figure 10] FIG. 10 is a flowchart illustrating a mode selection process for double-sided printing. [Figure 11] FIG. 10 is a schematic diagram for explaining a bias application configuration of another example of an image forming apparatus. [Figure 12] FIG. 10 is a graph illustrating the setting of a through bias in an image forming apparatus according to another example. DETAILED DESCRIPTION OF THE INVENTION

[0013] The image forming apparatus according to the present invention will be described in more detail below with reference to the drawings.

[0014] [Example 1] 1. Overview of image forming equipment FIG. 1 is a schematic cross-sectional view showing the overall configuration of an image forming apparatus 100 according to this embodiment. The image forming apparatus 100 according to this embodiment is a tandem laser printer employing an intermediate transfer method capable of forming full-color images using electrophotography. The image forming apparatus (laser printer engine) 100 forms an image on a sheet 2, which is a sheet-like recording material (recording medium, transfer material), based on an image signal transmitted from an external device 300 (FIG. 3). The external device 300 may be a host computer (such as a personal computer), a digital camera, an image reader, or the like, but in this embodiment, it is assumed to be a host computer. Since paper is typically used as the sheet 2, the sheet 2 is sometimes referred to as paper. However, the sheet 2 is not limited to paper and may be made of materials other than paper, such as plastic sheets, or materials containing materials other than paper.

[0015] Image forming apparatus 100 has four image forming stations (stations, image forming units) SY, SM, SC, and SK that form images of yellow (Y), magenta (M), cyan (C), and black (K), respectively. The four image forming stations SY, SM, SC, and SK are arranged side by side along the direction of movement of the primary transfer surface of intermediate transfer belt 12, which will be described later. Elements provided for each color and having the same or corresponding functions or configurations may be described collectively by omitting the Y, M, C, or K suffixes to the reference numerals indicating that the element is for one of the colors. Figure 2 is a schematic cross-sectional view showing the general configuration of image forming station S. In this embodiment, the image forming unit S is configured to include photosensitive drums 5 (5Y, 5M, 5C, 5K), charging rollers 7 (7Y, 7M, 7C, 7K), exposure devices 10 (10Y, 10M, 10C, 10K), developing devices 8 (8Y, 8M, 8C, 8K), drum cleaning devices 14 (14Y, 14M, 14C, 14K), etc., which will be described later.

[0016] The photosensitive drum 5, a rotatable drum-type (cylindrical) photosensitive member (electrophotographic photosensitive member) serving as an image carrier, is driven to rotate by a driving force transmitted from a drive motor of a drive unit 121 (FIG. 3) serving as a driving means. The photosensitive drum 5 is driven to rotate in the direction of arrow R1 (clockwise) in the figure at a predetermined peripheral speed (process speed). The surface of the rotating photosensitive drum 5 is uniformly charged to a predetermined potential of a predetermined polarity (negative in this embodiment) by a charging roller 7, a roller-type charging member serving as a charging means. During charging, a predetermined charging voltage (charging bias) of the same polarity (negative in this embodiment) as the charging polarity of the photosensitive drum 5 is applied to the charging roller 7 by a charging power source 131 (FIG. 3) serving as a charging voltage application means (charging voltage application unit). The charged surface of the photosensitive drum 5 is scanned and exposed by an exposure device 10 serving as an exposure means, and an electrostatic latent image (electrostatic image) is formed on the photosensitive drum 5.

[0017] The electrostatic latent image formed on the photosensitive drum 5 is developed (visualized) by a developing device 8 as a developing means, which supplies toner as a developer, thereby forming a toner image (toner image, developer image) on the photosensitive drum 5. The developing device 8 has a developing roller 81 as a developer carrier (developing member) that carries and transports the toner stored therein to supply the toner to the photosensitive drum 5. During development, a predetermined developing voltage (developing bias) of the same polarity (negative in this embodiment) as the charge polarity of the photosensitive drum 5 is applied to the developing roller 81 by a developing power supply 132 ( FIG. 3 ) as a developing voltage application means (developing voltage application unit). In this embodiment, toner charged to the same polarity (negative in this embodiment) as the charge polarity of the photosensitive drum 5 adheres to the exposed portion of the photosensitive drum 5, which has been uniformly charged and then exposed to light, reducing the absolute value of the potential (reverse development method). In this embodiment, the normal charge polarity of the toner, which is the main charge polarity of the toner during development, is negative.

[0018] An intermediate transfer belt 12, which is an endless belt and serves as an intermediate transfer body, is disposed facing the four photosensitive drums 5Y, 5M, 5C, and 5K. The intermediate transfer belt 12 is stretched around a plurality of tension rollers, including a tension roller 16, a pre-secondary transfer roller 17, and a drive roller 18, and is tensioned under a predetermined tension. The drive roller 18 is driven to rotate by a drive force transmitted from a drive motor of a drive unit 121 (FIG. 3), and the intermediate transfer belt 12 receives a drive force, thereby rotating (moving in a circular motion) in the direction of arrow R2 in the drawing (counterclockwise) at approximately the same peripheral speed as the peripheral speed of the photosensitive drums 5. The drive roller 18, tension roller 16, and pre-secondary transfer roller 17 form a primary transfer surface of the intermediate transfer belt 12 onto which the toner image is transferred from the photosensitive drums 5. Primary transfer rollers 4Y, 4M, 4C, and 4K, which are roller-type primary transfer members serving as primary transfer means, are arranged on the inner circumferential surface of the intermediate transfer belt 12, corresponding to the respective photosensitive drums 5Y, 5M, 5C, and 5K. The primary transfer rollers 4 press the intermediate transfer belt 12 toward the photosensitive drums 5, forming primary transfer portions (primary transfer nip portions, primary transfer positions) N1 (N1Y, N1M, N1C, and N1K) that are contact portions between the photosensitive drums 5 and the intermediate transfer belt 12. The tension rollers other than the drive roller 18 and each primary transfer roller 5 are rotated in accordance with the rotation of the intermediate transfer belt 12.

[0019] The toner images formed on the photosensitive drums 5 are transferred (primary transfer) onto the rotating intermediate transfer belt 12 by the action of the primary transfer rollers 4 at the primary transfer section N1. A primary transfer power supply 133 (FIG. 3) serving as a primary transfer voltage application means (primary transfer voltage application section) is connected to the primary transfer rollers 4. During primary transfer, the primary transfer power supply 133 applies a predetermined primary transfer voltage (primary transfer bias) to the primary transfer rollers 4. The primary transfer voltage (primary transfer bias) is a DC voltage of opposite polarity (positive polarity in this embodiment) to the normal charging polarity of the toner. For example, when forming a full-color image, the yellow, magenta, cyan, and black toner images formed on the photosensitive drums 5 are sequentially transferred onto the same image forming area (area where a toner image can be formed) on the intermediate transfer belt 12 so as to be superimposed on each other. Furthermore, toner remaining on the photosensitive drums 5 after primary transfer (primary transfer residual toner) is removed and collected from the photosensitive drums 5 by a drum cleaning device 14 serving as a photosensitive body cleaning means. In this embodiment, the drum cleaning device 14 scrapes and collects residual toner from the surface of the rotating photosensitive drum 5 using a cleaning blade 14a as a cleaning member arranged to contact the surface of the photosensitive drum 5.

[0020] A secondary transfer roller (secondary transfer outer roller) 9, a roller-type secondary transfer member serving as a secondary transfer means, is disposed on the outer peripheral surface of the intermediate transfer belt 12, facing the drive roller 18, which also functions as a secondary transfer opposing roller (secondary transfer inner roller). The secondary transfer roller 9 is pressed against the drive roller 18 and contacts the drive roller 18 via the intermediate transfer belt 12, forming a secondary transfer portion (secondary transfer nip portion, secondary transfer position) N2, which is the contact portion between the intermediate transfer belt 12 and the secondary transfer roller 9. In this embodiment, the secondary transfer roller 9 is driven to rotate in accordance with the rotation of the intermediate transfer belt 12, but may also be configured to be driven to rotate. At the secondary transfer portion N2, the toner image formed on the intermediate transfer belt 12 is transferred (secondary transfer) onto the sheet 2, which is being conveyed between the intermediate transfer belt 12 and the secondary transfer roller 9, by the action of the secondary transfer roller 9. A secondary transfer power supply 134 (FIG. 3) serving as a secondary transfer voltage application means (secondary transfer voltage application portion) is connected to the secondary transfer roller 9. During the secondary transfer, a predetermined secondary transfer voltage (secondary transfer bias), which is a DC voltage of the opposite polarity (positive polarity in this embodiment) to the normal charging polarity of the toner, is applied to the secondary transfer roller 8 by a secondary transfer power supply 134. In this embodiment, the drive roller 18 is electrically grounded (connected to ground potential). Note that in this embodiment, the tension 16 and the pre-secondary transfer roller 17 are also electrically grounded. Furthermore, deposits such as toner remaining on the intermediate transfer belt 12 after the secondary transfer (secondary transfer residual toner) are removed and collected from the intermediate transfer belt 12 by a belt cleaning device 15, which serves as an intermediate transfer body cleaning means. The belt cleaning device 15 is disposed facing the intermediate transfer belt 12, downstream of the secondary transfer portion N2 and upstream of the primary transfer portion N1 (the most upstream primary transfer portion N1Y) in the rotation direction of the intermediate transfer belt 12. In this embodiment, the belt cleaning device 15 uses a cleaning blade 15a as a cleaning member arranged to contact the surface of the intermediate transfer belt 12 to scrape and collect deposits from the surface of the rotating intermediate transfer belt 12.

[0021] The sheets 2 are stored in a paper feed unit (paper feed cassette, feeding unit) 1. The sheets 2 are fed one by one from the paper feed unit 1 by a paper feed roller 40 or the like as a paper feed member, and are conveyed to a registration roller 3 as a conveying member. The sheet 2 is then conveyed by the registration roller 3 along a paper feed conveying path 25 so as to synchronize with the toner image on the intermediate transfer belt 12, and is supplied to the secondary transfer unit N2. In this embodiment, the paper feed roller 40 constitutes a conveying unit that conveys the sheet 2 from the paper feed unit 1 toward the secondary transfer unit N2. The paper feed unit is not limited to a paper feed cassette, and may be, for example, a manual feed tray provided in the image forming apparatus 100 instead of or in addition to the paper feed cassette.

[0022] The sheet 2 onto which the toner image has been transferred is conveyed via a pre-fixing conveyance path 23 to a fixing device 13 serving as a fixing unit. In this embodiment, the fixing device 13 includes a fixing roller 13a that heats the sheet 2 and a pressure roller 13b that presses the sheet 2 against the fixing roller 13a. The fixing roller 13a and the pressure roller 13b are each hollow, and a heater is built into the fixing roller 13a. The fixing device 13 heats and presses the sheet 2 bearing the unfixed toner image, thereby fixing (melting, color mixing, and bonding) the toner image onto the sheet 2. After the toner image has been fixed, the sheet 2 is conveyed via a paper discharge conveyance path 26 and is discharged (output) by a paper discharge conveyance roller 31 or other conveyance member to a paper discharge unit (paper discharge tray) 27 located outside the device body 110 of the image forming apparatus 100. Double-sided printing will be described later.

[0023] In this embodiment, the photosensitive drum 5 is an OPC (organic photoconductor) drum configured by applying an organic photoconductive layer to the outer periphery of an aluminum cylinder.

[0024] In this embodiment, the exposure device 10 is configured as a laser scanner device. The exposure device 10 selectively exposes the surface of the uniformly charged photosensitive drum 5 in accordance with image information (image signals), thereby forming an electrostatic latent image on the photosensitive drum 5.

[0025] In this embodiment, the primary transfer roller 4 is made of a nickel-plated steel rod having an outer diameter of 6 mm and a volume resistivity of 1×10 6 The primary transfer roller 4 is an elastic roller covered with a foam sponge body whose main components are NBR and epichlorohydrin rubber, adjusted to a resistance of Ω·cm and a thickness of 4 mm. The primary transfer roller 4 is pressed against the photosensitive drum 5 with the intermediate transfer belt 12 sandwiched between them.

[0026] In this embodiment, the secondary transfer roller 9 is made of a nickel-plated steel rod having an outer diameter of 8 mm and a volume resistivity of 1×10 8 The secondary transfer roller 9 is an elastic roller covered with a foam sponge mainly composed of NBR and epichlorohydrin rubber, adjusted to a resistance of Ω·cm and a thickness of 5 mm. In other words, in this embodiment, the outer diameter of the secondary transfer roller 9 is 18 mm. In this embodiment, the secondary transfer roller 9 contacts the outer peripheral surface of the intermediate transfer belt 12 with a pressure of 50 N, forming a secondary transfer portion N2. In this embodiment, the secondary transfer roller 9 is rotated in accordance with the rotation of the intermediate transfer belt 12. In this embodiment, when the toner image on the intermediate transfer belt 12 is secondarily transferred to the sheet 2, a positive secondary transfer voltage of approximately 2500 V is applied to the secondary transfer roller 9 by the secondary transfer power supply 134.

[0027] In this embodiment, the intermediate transfer belt 12 is an endless belt having a circumference of 700 mm and a thickness of 90 μm, and is formed from polyimide resin mixed with carbon as a conductive agent. In this embodiment, the intermediate transfer belt 12 exhibits electrical conductivity and is characterized by small resistance fluctuations due to ambient temperature and humidity. While polyimide resin is used as the material for the intermediate transfer belt 12 in this embodiment, the material is not limited to this and may be other thermoplastic resins. Examples of suitable materials include polyester, polycarbonate, polyarylate, acrylonitrile-butadiene-styrene copolymer (ABS), polyphenylene sulfide (PPS), polyvinylidene fluoride (PVdF), and mixtures thereof. Furthermore, the conductive agent is not limited to carbon and may be, for example, conductive metal oxide fine particles. In this embodiment, the volume resistivity of the intermediate transfer belt 12 is 1×10 9 The volume resistivity is Ω·cm. The volume resistivity was measured using a ring probe type UR (model MCP-HTP12) in a Hiresta-UP (MCP-HT450) manufactured by Nitto Seiko Analytech Co., Ltd. The measurement conditions were set to an indoor temperature of 23°C, indoor humidity of 55%, applied voltage of 100V, and measurement time of 10 seconds. In this example, the volume resistivity of the intermediate transfer belt 12 is 1×10 7 ~10 10 A range of Ω·cm is preferable. Note that "~" in the numerical range means that the numerical values ​​before and after it are included.

[0028] In addition, a registration sensor 19 capable of detecting the leading and trailing edges of the conveyed sheet 2, a fixing discharge sensor 20, and a duplex conveyance sensor 28 are arranged in the conveyance path of the sheet 2 in the image forming apparatus 100. In addition, a sheet presence / absence sensor 11 is provided at the paper feed opening of the paper feed unit 1 as a sheet presence / absence detection means for detecting the presence or absence of the sheet 2 in the paper feed unit 1. In addition, a full-load detection sensor 39 is provided at the paper discharge opening of the apparatus main body 110 for detecting whether the paper discharge unit 27 is fully loaded with sheets 2. In addition, an environment sensor 50 (FIG. 3) capable of detecting the temperature and humidity of the usage environment (installation environment) of the image forming apparatus 100 is provided within the apparatus main body 110.

[0029] In this embodiment, in each image forming unit S, the photosensitive drum 5, and the charging roller 7, developing device 8, and drum cleaning device 14 acting as process means thereon, are integrally configured as a process cartridge 22 that is detachable from the apparatus main body 110. The process cartridge 22 can be replaced by the user, for example, when the toner in the developing device 8 runs out. In this embodiment, the apparatus main body 110 of the image forming apparatus 100 corresponds to the portion of the image forming apparatus 100 excluding the process cartridges 22 (22Y, 22M, 22C, 22K).

[0030] Furthermore, in this embodiment, the image forming apparatus 100 does not have a mechanism for separating the secondary transfer roller 9 from the intermediate transfer belt 12 inside the apparatus main body 110 .

[0031] 2. Double-sided unit Next, we will explain the configuration and operation of a duplex unit (duplex conveying mechanism) 70 as a duplex conveying means (duplex conveying section) for printing on both sides of the sheet 2 in this embodiment. The image forming apparatus 100 in this embodiment is configured to be able to perform duplex printing (automatic duplex printing) using the duplex unit 70.

[0032] After the toner image is transferred onto the front side of the sheet 2 and the sheet passes through the fixing device 13, the position of the duplex flapper 32 is switched by a reversing clutch (not shown), and the sheet is conveyed to the duplex reversing path 29. When the rear end of the sheet 2 reaches the duplex reversing path 29, the rotation direction of the reversing roller 30 is switched by the reversing clutch, and the position of the duplex flapper 32 is switched, and the sheet 2 is conveyed to the duplex conveying path 33. The sheet 2 whose conveying direction has been reversed is conveyed along the duplex conveying path 33 by the duplex conveying roller 37 and the duplex refeed roller 35. The reversing clutch, the duplex flapper 32, the duplex reversing path 29, the reversing roller 30, the duplex conveying path 33, the duplex conveying roller 37, the duplex refeed roller 35, and the like constitute a duplex unit 70.

[0033] In this way, the sheet 2 is conveyed upside down to the registration rollers 3, and is then conveyed again along the paper feed conveying path 25 by the registration rollers 3 and supplied to the secondary transfer unit N2. Then, the toner image is transferred and fixed onto the back side of the sheet 2, and the sheet 2 is discharged to the paper discharge unit 27.

[0034] In this embodiment, during double-sided printing, the distance in the conveying direction of the sheets 2 between the preceding sheet 2 conveyed to the secondary transfer portion N2 and the next sheet 2 (herein also referred to as "paper gap") is 20 mm. In other words, in this embodiment, the paper gap during double-sided printing is shorter than 56.5 mm, which is the length of one revolution (circumferential length) of the secondary transfer roller 9. Printing productivity can be increased by making the paper gap as short as possible.

[0035] 3. Control Configuration Next, a description will be given of the control configuration of the image forming apparatus 100 in this embodiment. Fig. 3 is a block diagram showing an outline of the control configuration of the image forming apparatus 100 in this embodiment.

[0036] The image forming apparatus 100 includes an operation display unit 205, a video controller (image processing unit) 204, and an engine control unit (control unit) 200.

[0037] The video controller 204 transmits information indicating the status of the image forming apparatus 100 received from the engine control unit 200 to an operation display unit (operation panel) 205. The operation display unit 205 switches the display on the operation display unit 205 based on the received information indicating the status of the image forming apparatus 100. The operation display unit 205 has a display unit for displaying information to a user (operator) under the control of the engine control unit 200, and an input unit such as operation buttons for inputting information to the engine control unit 200 based on operations by the user (operator). The operation display unit 205 may be configured with a touch panel that functions as both a display unit and an input unit. The video controller 204 also receives image information and a print command from the host computer 300. The video controller 204 analyzes the received image information, converts it into bitmap data, and sends a print reservation command, a print start command, and a video signal to the engine control unit 200 for each page via a video interface unit (not shown).

[0038] The engine control unit 200 is configured with a control IC having a CPU 207 as an arithmetic processing unit, ROM 208 and RAM 209 as storage units, and an I / O port 211 as an input / output unit. The CPU 207 loads programs and various data from the ROM 208 and executes the programs by using the RAM 209 as a work area, thereby providing overall control of the image forming apparatus 100. The CPU 207, ROM 208, and RAM 209 are accessible to the I / O port 211 via a bidirectionally accessible system bus 210. Various actuators of the image forming apparatus 100 are connected to each I / O port 211. For example, the drive unit 121, charging power supply 131, developing power supply 132, primary transfer power supply 133, secondary transfer power supply 134, exposure device 10, and a reversing clutch (not shown) of the duplex unit 70 are connected to the I / O port 211. The engine control unit 200 controls various actuators via an I / O port 211 to perform operations such as conveying the sheet 2, forming an image, and initializing. The I / O port 211 is also connected to, for example, a sheet presence / absence sensor 11 in the paper feed unit 1. The engine control unit 200 acquires, via the I / O port, a signal indicating the detection result of the sheet presence / absence sensor 11 as to whether the sheet 2 is present in the paper feed unit 1. The I / O port 211 is also connected to, for example, an environmental sensor 50. The engine control unit 200 acquires, via the I / O port 211, a signal indicating the detection result of the environmental sensor 50 as to the temperature and humidity of the environment in which the image forming apparatus 100 is used. The environment may be at least one of the temperature and humidity inside or outside the image forming apparatus 100. In this embodiment, the environmental sensor 100 detects the temperature and humidity inside the image forming apparatus 100. In addition, although not shown in the figure, the I / O port 211 is connected to a registration sensor 19, a fixing paper discharge sensor 20, a double-sided conveyance sensor 28, a full-load detection sensor 39, etc., and the engine control unit 200 can acquire signals indicating the detection results of each of these sensors.

[0039] Although not shown, in this embodiment, the charging power supply 131, the developing power supply 132, the primary transfer power supply 133, and the exposure device 10 are provided independently for each image forming unit S. However, at least one of these may be shared by multiple image forming units S. The drive unit 121 is also provided with a drive motor as a drive source for driving drive targets such as the photosensitive drum 5, the drive roller 18, various rollers for transporting the sheet 2, and the fixing device 13. The drive motor may be provided independently for each drive target, or a drive motor may be shared by multiple drive targets.

[0040] In this embodiment, the secondary transfer power supply 134 is configured to output a bias using constant voltage control, which adjusts the output voltage to a target voltage. In this embodiment, the secondary transfer power supply 134 is configured to be able to output a positive bias (positive bias) and a negative bias (negative bias). That is, in this embodiment, the secondary transfer power supply 134 has a positive bias output unit and a negative bias output unit.

[0041] For convenience, FIG. 3 also shows a parts counter 60, which will be described in the second embodiment.

[0042] Here, the image forming apparatus 100 executes a job (print job, image formation sequence) that is a series of operations for forming and outputting an image on one or more sheets 2, initiated by a single start instruction. A job generally includes an image formation process, a pre-rotation process, a sheet-to-sheet interval process (when forming images on multiple sheets 2), and a post-rotation process. The image formation process is a period during which electrostatic images, toner images, and toner image transfer are performed for the images that will actually be formed and output on the sheets 2. The image formation process refers to this period. More specifically, the timing of the image formation process differs depending on the positions where the electrostatic image formation, toner image formation, and toner image transfer processes are performed. The pre-rotation process is a period during which preparatory operations are performed before the image formation process, from when a start instruction is input until the actual start of image formation. The sheet-to-sheet interval process is a period corresponding to the interval between sheets 2 when image formation is performed continuously on multiple sheets 2 (continuous image formation). The post-rotation process is a period during which a sorting operation (preparatory operation) is performed after the image formation process. Non-image formation 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-multi-rotation process, which is a preparatory operation when the image forming device 100 is turned on or when it returns from a sleep state.

[0043] 4. Double-sided printing 4 is a schematic diagram for explaining the communication sequence between the engine control unit 200 and the video controller 204 when paper is fed alternately from the paper feed unit 1 and from the duplex unit 70 (duplex conveying path 33) to perform double-sided printing on four sheets. In this embodiment, the engine control unit 200 functions as a main control unit that controls the conveying operation of the sheet 2 and the image forming operation in the image forming apparatus 100.

[0044] First, the video controller 204 transmits to the engine control unit 200 a reservation command (print reservation command) for feeding paper from the paper feed unit 1 and discharging it to the duplex unit 70 as a first page reservation (C311). Next, the video controller 204 transmits to the engine control unit 200 a reservation command for feeding paper from the paper feed unit 1 and discharging it to the duplex unit 70 as a second page reservation (C312). Next, the video controller 204 transmits to the engine control unit 200 a reservation command for feeding paper from the duplex unit 70 and discharging it to the paper discharge unit 27 outside the image forming apparatus 100 as a third page reservation (C313). This page is reserved for the back side of the first page. The transmission of the above reservation commands is repeated in the same manner for the remaining reservation IDs (pages 4 to 8) (C314, C315, C316, C317, C318).

[0045] Next, the video controller 204 outputs to the engine control unit 200 a start command (print start command) for the first page reserved by the reservation command (C319). After receiving the start command, the engine control unit 200 starts the image formation sequence (rotation of the photosensitive drum 5, charging process), and outputs a / TOP signal to the video controller 204 at a predetermined timing when image formation is possible (C320). Then, the video controller 204 outputs a video signal for the first page in synchronization with the / TOP signal (not shown), and under the control of the engine control unit 200, the image forming apparatus 100 starts image formation (exposure process, development process) for the first page in accordance with the video signal. After the video controller 204 finishes outputting the video signal for the first page, it outputs a start command for the next reserved page (C321). Thereafter, the start command and / TOP signal are output repeatedly in the same manner for the remaining reserved pages (pages 3 to 8), and image formation for the remaining pages is performed.

[0046] As indicated by the reservation commands C311 to C318, having one sheet 2 waiting in the duplex unit 70 and alternately feeding paper from the paper feed section 1 and the duplex unit 70 is also referred to here as "alternating two-sheet duplex printing."

[0047] 5 is a timing chart showing an image formation sequence when an image is formed according to the communication sequence for two-sheet alternating double-sided printing described with reference to FIG. 4. Here, in double-sided printing, the side of the sheet 2 onto which a toner image is transferred when the sheet is fed from the paper feed unit 1 and discharged to the double-sided unit 70 is referred to as the "front side." The side of the sheet 2 onto which a toner image is transferred when the sheet is fed from the double-sided unit 70 and discharged outside the image forming apparatus 100 is referred to as the "back side." That is, in double-sided printing, the side onto which a toner image is first transferred of a sheet 2 fed from the paper feed unit 1 and supplied to the secondary transfer unit N2 is the front side (first side). The side of the sheet 2 onto which a toner image is transferred when the sheet 2 is fed from the double-sided unit 70 and supplied to the secondary transfer unit N2 is the back side (second side). Furthermore, with regard to the image forming area, the toner image, or the sheet 2, the "leading edge" and "trailing edge" refer to the leading edge and trailing edge in the direction of movement (conveyance direction). For simplicity, the following description will be given assuming that a toner image is formed over the entire area of ​​each image forming area on the intermediate transfer belt 12. Therefore, for example, the period during which a toner image on the intermediate transfer belt 12 passes through the secondary transfer portion N2 corresponds to the period during which the image forming area on the intermediate transfer belt 12 where the toner image can be formed passes through the secondary transfer portion N2.

[0048] When the engine control unit 200 receives a start command (print ID=1) in response to the reservation command for the front side of the first sheet, it controls the engine control unit 200 to start a pre-rotation sequence (pre-rotation process). After the pre-rotation sequence ends, the engine control unit 200 outputs a / TOP signal (100-1-S) to start an image formation operation (image formation process) for the front side of the first sheet. The engine control unit 200 controls the engine control unit 200 to form a toner image on the intermediate transfer belt 12 in accordance with a video signal (1-S) sent from the video controller 204. After a predetermined time has elapsed since the / TOP signal, the engine control unit 200 outputs a cassette feed signal to start conveying the sheet 2 from the paper feed unit 1 (101-1-S). At this time, the engine control unit 200 controls the sheet 2 to be supplied to the secondary transfer unit 2 in time with the toner image formed on the intermediate transfer belt 12 reaching the secondary transfer unit N2. In this embodiment, when the registration sensor 19 detects the leading edge of the sheet 2 fed from the paper feed unit 1, the engine control unit 200 controls the registration rollers 3 to appropriately accelerate or decelerate the conveyance of the sheet 2. As a result, the engine control unit 200 controls the leading edge of the toner image formed on the intermediate transfer belt 12 to coincide with the leading edge of the sheet 2 at the secondary transfer unit N2. A predetermined time after the engine control unit 200 outputs a cassette feed signal (paper feed instruction), the toner image formed on the intermediate transfer belt 12 and the sheet 2 fed from the paper feed unit 1 pass through the secondary transfer unit N2 (102-1-S). This transfers the toner image (1-S) from the intermediate transfer belt 12 to the sheet 2. At this time, while the toner image on the intermediate transfer belt 12 is passing through the secondary transfer unit N2, the engine control unit 200 controls the secondary transfer roller 9 to apply a first secondary transfer voltage (herein also referred to as a "print bias") for transferring the toner image to the sheet 2. In this embodiment, the application of the print bias is started before the sheet 2, which is transported to the secondary transfer portion N2 for the secondary transfer of the toner image of the first page (front side of the first sheet), reaches the secondary transfer portion N2. Then, the application of the print bias is continued until the sheet 2, which is transported to the secondary transfer portion N2 for the secondary transfer of the toner image of the last page (back side of the fourth sheet), passes through the secondary transfer portion N2.The toner image on the front side of the first sheet transferred onto the sheet 2 is heated and fixed onto the sheet 2 by the fixing device 13. This completes the image formation on the front side of the first sheet. The sheet 2 on which the image formation on the front side of the first sheet has been completed is transported to the duplex unit 70.

[0049] Similarly, upon receiving a start command (print ID=2) for the reservation command for the front side of the second sheet, the engine control unit 200 outputs a / TOP signal (100-2-S) to start the image formation operation for the front side of the second sheet. The engine control unit 200 controls the formation of a toner image on the intermediate transfer belt 12 in accordance with a video signal (2-S) sent from the video controller 204. After a predetermined time has elapsed since the / TOP signal was sent, the engine control unit 200 outputs a cassette feed signal to start conveying the sheet 2 from the paper feed unit 1 (101-2-S). After a predetermined time has elapsed since the engine control unit 200 output the cassette feed signal (paper feed instruction), the toner image formed on the intermediate transfer belt 12 and the sheet 2 fed from the paper feed unit 1 pass through the secondary transfer unit N2 (102-2-S). This causes the toner image (2-S) to be transferred from the intermediate transfer belt 12 to the sheet 2. The toner image on the front side of the second sheet transferred onto the sheet 2 is heated and fixed onto the sheet 2 by the fixing device 13. This completes the image formation on the front side of the second sheet. The sheet 2 on which the image formation on the front side of the second sheet has been completed is transported to the duplex unit 70.

[0050] Next, upon receiving a start command (print ID=1) for the reservation command for the back side of the first sheet, the engine control unit 200 outputs a / TOP signal (100-1-D) to start image formation operation for the back side of the first sheet. The engine control unit 200 controls the formation of a toner image on the intermediate transfer belt 12 in accordance with a video signal (1-D) sent from the video controller 204. Furthermore, after a predetermined time has elapsed since the / TOP signal, the engine control unit 200 outputs a double-sided sheet feed signal to start conveying the sheet 2 from the double-sided unit 70 (101-1-D). After a predetermined time has elapsed since the engine control unit 200 output the double-sided sheet feed signal (sheet feed instruction), the toner image formed on the intermediate transfer belt 12 and the sheet 2 fed from the double-sided unit 70 pass through the secondary transfer unit N2 (102-1-D). This causes the toner image (1-D) to be transferred from the intermediate transfer belt 12 to the sheet 2. The toner image on the back side of the first sheet that has been transferred onto the sheet 2 is heated and fixed onto the sheet 2 by the fixing device 13. This completes image formation on the front and back sides of the first sheet. The first sheet 2, on which image formation has been completed on both its front and back sides, is discharged outside the image forming apparatus 100.

[0051] The above-described operation is repeated for four sheets. The engine control unit 200 controls to execute a post-rotation sequence (post-rotation process) when all image formation (toner image formation) is completed. The engine control unit 200 also controls to stop the output of the print bias applied to the secondary transfer roller 9 after the sheet 2 onto which the final toner image (toner image on the back side of the fourth sheet) has been transferred passes through the secondary transfer unit N2. In this way, double-sided printing of four sheets is completed.

[0052] As described above, in this embodiment, the paper gap during double-sided printing is 20 mm, which is shorter than 56.5 mm, which is the length (circumferential length) of one revolution of the secondary transfer roller 9. Note that this paper gap during double-sided printing is represented by the paper gap between the sheet 2 fed from the paper feed unit 1 and the next sheet 2 fed from the duplex unit 70. As shown in FIG. 5, during double-sided printing, the paper gap between the first sheet 2 (front side) and the second sheet 2 (front side) fed consecutively from the paper feed unit 1, and the paper gap between the last sheet 2 (back side) fed consecutively from the duplex unit 70 and the sheet 2 (back side) just before it, may be longer than other paper gaps due to factors such as switching the conveyance direction of the sheets 2 in the duplex unit 70. A configuration in which the distance between sheets during double-sided printing is shorter than the length of one revolution of the secondary transfer roller 9 means, in other words, that during double-sided printing, the first period from when the rear end of the sheet 2 fed from the paper feed unit 1 reaches the secondary transfer unit N2 until the front end of the next sheet 2 fed from the double-sided unit 70 reaches the secondary transfer unit N2 is shorter than the second period required for the secondary transfer roller 9 to make one revolution.

[0053] 5. What happens when the paper feed section runs out of sheets Fig. 6 is a timing chart showing the image formation sequence when the sheet 2 in the paper feed unit 1 runs out after the fourth sheet during execution of the image formation sequence according to the communication sequence for two-sheet alternating double-sided printing described using Fig. 4. In Fig. 6, the same events and timing as those in Fig. 5 are assigned the same reference numerals as in Fig. 5, and detailed explanations will be omitted.

[0054] 6, at timing 110-P, the engine control unit 200 obtains a detection signal from the sheet presence / absence sensor 11 indicating that the fourth sheet 2 is not present in the paper feed unit 1. At this timing 110-P, the / TOP signal (100-4-S) for the front side of the fourth sheet has already been output, and image formation (formation of a toner image) on the front side of the fourth sheet has already been completed. Also, at this timing 110-P, the / TOP signal (100-3-D) for the back side of the third sheet, which follows the front side of the fourth sheet, has already been output, and image formation (formation of a toner image) on the front side of the third sheet has already started.

[0055] In this embodiment, when the sheet 2 in the paper feed unit 1 runs out during double-sided printing, the image forming apparatus 100 performs a "double-sided printing continuation process" in which printing on the front side of the sheet 2 waiting in the double-sided unit 70 is completed and printing on the back side of the sheet 2 is completed. After the double-sided printing continuation process is performed, the user typically replenishes the paper feed unit 1 with sheets 2, and printing resumes, printing the front side of the fourth sheet and the back side of the fourth sheet, and the job is completed.

[0056] Here, because there are no more sheets 2 in the paper feed unit 1, the fourth sheet 2 is not transported to the secondary transfer unit N2 (the nip between the intermediate transfer belt 12 and the secondary transfer roller 9). Therefore, the toner image (4-S) on the front side of the fourth sheet formed on the intermediate transfer belt 12 must pass through the secondary transfer unit N2. Then, the toner image (3-D) on the back side of the third sheet formed on the intermediate transfer belt 12 must be transferred to the sheet 2 transported from the duplex unit 70.

[0057] In this way, a toner image that has been primarily transferred onto intermediate transfer belt 12 and that passes through secondary transfer section N2 without being secondarily transferred to sheet 2 is also referred to here as a "non-transferred toner image." If toner adheres to secondary transfer roller 9 when non-transferred toner image (4-S) on the front side of the fourth sheet passes through secondary transfer section N2, that toner may adhere to sheet 2 when secondary transfer of toner image (3-D) on the back side of the subsequent third sheet is performed, potentially causing "backside contamination."

[0058] In a configuration in which continuous double-sided printing is performed, it is important to suppress the above-mentioned backside contamination, but it is also important not to reduce printing productivity.

[0059] For example, if there is sufficient time between the time when the non-transferred toner image on the intermediate transfer belt 12 passes through the secondary transfer portion N2 and the time when the secondary transfer to the back side of the sheet 2 waiting in the duplex unit 70 is performed, it is possible to clean the secondary transfer roller 9 during that time. For example, it is possible to transfer the toner adhering to the secondary transfer roller 9 to the intermediate transfer belt 12 by alternately applying positive and negative voltages to the secondary transfer roller 9 for a certain period of time, and then collect the toner from the intermediate transfer belt 12 using the belt cleaning device 15. However, in the intermediate transfer type image forming apparatus 100, as described above, when it is detected that the sheet 2 in the paper feed unit 1 has run out during a double-sided print job, the toner image to be transferred to the front side of that sheet 2 and at least part of the toner image to be transferred to the back side of the sheet 2 waiting in the duplex unit 70 may already have been formed. Therefore, if the interval between sheets is increased in advance in preparation for the case where the sheet 2 in the paper feed unit 1 runs out during double-sided printing, cleaning the secondary transfer roller 9 as described above can prevent backside contamination, but printing productivity will decrease.

[0060] Therefore, it is required to suppress the backside contamination when continuous double-sided printing is performed while suppressing the decrease in printing productivity.

[0061] 6. Through bias Therefore, in this embodiment, the engine control unit 200 controls the secondary transfer roller 9 to apply a second secondary transfer voltage (also referred to herein as a "through bias") when the non-transferred toner image on the intermediate transfer belt 12 passes through the secondary transfer portion N2. Typically, the through bias is applied to the secondary transfer roller 9 throughout the period when the entire non-transferred toner image passes through the secondary transfer portion N2. However, applying the through bias to the secondary transfer roller 9 while at least a portion of the non-transferred toner image passes through the secondary transfer portion N2 provides a corresponding effect. In this embodiment, the secondary transfer voltage is changed from the print bias to the through bias before the non-transferred toner image on the intermediate transfer belt 12 reaches the secondary transfer portion N2. Then, after the non-transferred toner image passes through the secondary transfer portion N2 and before the sheet 2, onto which the next back-side toner image is secondarily transferred and fed from the duplex unit 70, reaches the secondary transfer portion N2, the secondary transfer voltage is changed from the through bias to the print bias.

[0062] The through bias is a voltage of opposite polarity to the print bias. In other words, it prevents toner from adhering to the secondary transfer roller 9 by electrostatic force. Then, secondary transfer is performed on the back side of the third sheet. This allows the back side of the third sheet 2 to be printed and then ejected from the image forming apparatus 100 without wasting the third sheet 2, whose front side has already been printed. The setting of the through bias is described in more detail below.

[0063] 7. Experiment on the optimum value of through bias An experiment was conducted to evaluate the effectiveness of through bias in suppressing backstaining. Three sheets 2 were loaded into the paper feeder 1, and four double-sided prints (two sheets alternately printed on each other) were performed. The amount of backstaining on the third sheet 2 after the fourth sheet 2 was removed was evaluated. A magenta solid image (maximum density level image) was formed on the front side of the fourth sheet 2. When the fourth sheet 2 was removed, the magenta non-transferred toner image on the intermediate transfer belt 12 passed through the secondary transfer section N2. The value of the through bias applied to the secondary transfer roller 9 while the magenta non-transferred toner image on the intermediate transfer belt 12 was passing through the secondary transfer section N2 was varied, and the amount of backstaining caused by the magenta toner on the third sheet 2 was measured. High-whiteness paper GFC081 (sold by Canon Marketing Japan) was used as the sheet 2. The amount of backstaining was measured using a whiteness photometer TC-6DS / A30 (manufactured by Tokyo Denshoku). The experiment was carried out in an environment with a temperature of 23° C. and a relative humidity of 55%. The process cartridge 22 used in the experiment had a remaining life of 50% or more. The results are shown in FIG.

[0064] When the through bias was -500V to -100V, the amount of backside contamination was less than 1%, which was barely noticeable. However, when the through bias was -750V, the amount of backside contamination was 3%, which was visible. Furthermore, when the through bias was -1000V, the amount of backside contamination was 7%, which was clearly visible. On the other hand, when the through bias was -50V, the amount of backside contamination was 1.2%, and although slight backside contamination occurred, it was barely noticeable and at an acceptable level. However, when the through bias was +100V, the amount of backside contamination was 10%, which was clearly visible.

[0065] From the above experimental results, it can be seen that if the amount of backside contamination is 1% or less, it is at a level that is hardly noticeable to the user. It can also be seen that if the through bias is -500V to -100V, the amount of backside contamination can be suppressed to 1% or less.

[0066] Next, the charge amount of the toner adhering to the secondary transfer roller 9 when back contamination occurred was measured. During the above-mentioned experiment, the operation of the image forming apparatus 100 was stopped urgently when the magenta non-transferred toner image on the intermediate transfer belt 12 was passing through the secondary transfer portion N2, and the charge amount of the magenta toner adhering to the secondary transfer roller 9 was measured. The measurement was performed using an E-Spart Analyzer (manufactured by Hosokawa Micron Corporation). For comparison, the charge amount of the magenta toner on the intermediate transfer belt 12 before passing through the secondary transfer portion N2 and the charge amount of the magenta toner adhering to the secondary transfer roller 9 were also measured. The results are shown in Table 1.

[0067] [Table 1]

[0068] The charge amount of the magenta toner on the intermediate transfer belt 12 before reaching the secondary transfer portion N2 was -35 μC / g. When the through bias was -1000 V, the charge amount of the toner adhering to the secondary transfer roller 9 was +40 μC / g. On the other hand, when the through bias was +100 V, the charge amount of the toner adhering to the secondary transfer roller 9 was -30 μC / g.

[0069] This is thought to be because, when the through bias is -1000V, discharge occurs between the toner on the intermediate transfer belt 12 and the secondary transfer roller 9, causing the toner, whose charge polarity is reversed and becomes positive, to adhere to the secondary transfer roller 9. On the other hand, when the through bias is +100V, it is thought that the negative toner on the intermediate transfer belt 12 is electrostatically attracted and adheres to the secondary transfer roller 9.

[0070] From the above results, it was found that the through bias should be set as follows to prevent toner of the non-transferred toner image on the intermediate transfer belt 12 from adhering to the secondary transfer roller 9. In other words, it was found that a through bias of the same polarity as the normal charging polarity of the toner (opposite polarity to the print bias) and with an absolute value less than the discharge threshold should be applied to the secondary transfer roller 9. Furthermore, the through bias applied to the secondary transfer roller 9 is preferably -500V to -100V. If the through bias applied to the secondary transfer roller 9 is greater than -100V and less than 0V, the potential difference at the secondary transfer portion N2 may be insufficient, and the level of backside contamination prevention effect will be slightly reduced. Generally, the voltage at which discharge starts (discharge start voltage, discharge threshold) Vth is calculated based on Paschen's law using the following equation: Vth = f(p × d) (p is gas pressure (Torr), d is distance between electrodes (m))

[0071] The through bias applied to the secondary transfer roller 9 should have the same polarity as the normal charging polarity of the toner (opposite polarity to the print bias) and an absolute value less than the discharge start voltage. In the configuration of this embodiment, if the voltage applied to the secondary transfer roller 9 is between -500V and -100V, the absolute value of the potential difference between the drive roller 18 and the secondary transfer roller 9, which constitute the secondary transfer unit N2, is less than the discharge threshold. This prevents negatively charged toner on the intermediate transfer belt 12 from adhering to the secondary transfer roller 9, allowing the toner to pass through the secondary transfer unit N2. This makes it possible to achieve a level of backside contamination that is barely noticeable to the user. Note that the voltage whose absolute value is less than the discharge threshold is not limited to the above range and varies depending on the atmospheric pressure of the usage environment and the distance between the intermediate transfer belt 12 and the secondary transfer roller 9 based on Paschen's law.

[0072] Next, a similar experiment was conducted on the front side of the fourth sheet 2, but with a secondary-color solid image of yellow and magenta instead of magenta, and the amount of backside contamination on the third sheet 2 was measured. The results were comparable to those for a primary-color solid image of magenta (Figure 7), and it was confirmed that the amount of backside contamination was 1% or less when the through bias was -500V to -100V. When the through bias was -1000V, the toner adhered to the secondary transfer roller 9 was mostly magenta toner, with almost no yellow toner observed. This is thought to be due to the following reason: The upper layer of the secondary-color solid image on the intermediate transfer belt 12 is covered with magenta toner. Therefore, discharge occurs between the magenta toner on the intermediate transfer belt 12 and the secondary transfer roller 9, and only the magenta toner, whose charge polarity is essentially reversed to a positive polarity, adheres to the secondary transfer roller 9. On the other hand, when the through bias was set to +500 V, it was confirmed that both magenta toner and yellow toner adhered to the secondary transfer roller 9. This is thought to be because both the negatively polarized magenta toner and yellow toner on the intermediate transfer belt 12 were attracted by the positively polarized through bias and adhered to the secondary transfer roller 9.

[0073] From the above results, it can be seen that in the configuration of this embodiment, if the through bias is −500 V to −100 V, discharge between the toner on the intermediate transfer belt 12 and the secondary transfer roller 9 can be suppressed even for solid images of secondary colors, and the occurrence of backside staining can be suppressed. Furthermore, from the above results, it can be seen that the same effect can be obtained for other images such as solid images of tertiary colors, character / thin line images, and halftone images.

[0074] Furthermore, by setting the through bias to -500V to -100V, it is possible to suppress adhesion of toner to the secondary transfer roller 9, thereby reducing the need for cleaning the secondary transfer roller 9 between sheets of paper.

[0075] For example, when the through bias is -1000V, positive toner adheres to the secondary transfer roller 5. Therefore, in this case, it is necessary to apply a positive secondary transfer voltage to the secondary transfer roller 9 for a period of at least one revolution of the secondary transfer roller 9, or to apply positive and negative secondary transfer voltages alternately. In other words, in this case, the distance between sheets needs to be at least the length of one revolution of the secondary transfer roller 9 (56.5 mm). If the paper gap is increased in this way in preparation for the possibility of running out of sheets 2 in the paper feed unit 1 during double-sided printing, printing productivity will decrease.

[0076] In contrast, in this embodiment, when the sheet 2 in the paper feed unit 1 runs out during double-sided printing, a voltage having the same polarity as the normal charging polarity of the toner (opposite polarity to the print bias) and an absolute value less than the discharge threshold is applied to the secondary transfer roller 9 as a through bias while the non-transferred toner image on the intermediate transfer belt 12 passes through the secondary transfer unit N2. This prevents the toner of the non-transferred toner image from adhering to the secondary transfer roller 9. Therefore, in a configuration in which the sheet interval is shorter than one rotation of the secondary transfer roller 9 to increase printing productivity, backside contamination can be prevented even if the toner image is secondarily transferred to the sheet 2 fed from the duplex unit 70 immediately after the non-transferred toner image on the intermediate transfer belt 12 passes through the secondary transfer unit N2. This makes it possible to both prevent a decrease in printing productivity and prevent backside contamination when the sheet 2 in the paper feed unit 1 runs out during double-sided printing.

[0077] The value of the through bias is not limited to the value in this embodiment, and may be changed depending on the usage environment and the amount of charge on the toner. It may also be changed depending on the electrical resistance of the intermediate transfer belt 12 and the electrical resistance of the secondary transfer roller 9. A similar effect can be obtained by applying to the secondary transfer roller 9, as the through bias, a voltage with the same polarity as the normal charging polarity of the toner (opposite polarity to the print bias) and an absolute value less than the discharge threshold.

[0078] In this embodiment, the case where a double-sided printing continuation process is performed when the sheet 2 in the paper feed unit 1 runs out during execution of a double-sided print job has been described. The operation for suppressing backside contamination during the double-sided printing continuation process in this embodiment can also be applied to the case where a jam occurs during execution of a double-sided print job and a double-sided printing continuation process is performed. In this case, for example, when a jam is detected by the registration sensor 19 or the like, the engine control unit 200 temporarily stops the operation of the image forming apparatus 100. Then, typically, the jammed sheet 2 is removed by the user (operator), and the engine control unit 200 resumes the operation of the image forming apparatus 100. In this operation after the resumption, the engine control unit 200 can control the double-sided printing continuation process to be performed in the same manner as described above. This can achieve the same effect as when the sheet 2 in the paper feed unit 1 runs out during execution of a double-sided print job.

[0079] In this embodiment, of the drive roller (inner secondary transfer roller) 18 and the secondary transfer roller (outer secondary transfer roller) 9 that constitute the secondary transfer unit N2, a secondary transfer voltage is applied to the secondary transfer roller 9 from the secondary transfer power supply 134, and the drive roller 18 is electrically grounded. Alternatively, of the inner secondary transfer roller and the outer secondary transfer roller that constitute the secondary transfer unit N2, a secondary transfer voltage can be applied from the secondary transfer power supply 134 to the inner secondary transfer roller, and the outer secondary transfer roller can be electrically grounded. In this case, a voltage of the same polarity as the normal charging polarity of the toner can be applied as a print bias to the inner secondary transfer roller. In this case, a voltage of the opposite polarity to the normal charging polarity of the toner (opposite polarity to the print bias) and whose absolute value is less than the discharge threshold can be applied as a through bias to the inner secondary transfer roller. In other words, in the continuous double-sided printing process, a through bias, which is a predetermined bias that causes the secondary transfer outer roller to have a potential on the normal charging polarity side of the toner relative to the secondary transfer inner roller and makes the absolute value of the potential difference between the secondary transfer inner roller and the secondary transfer outer roller less than the discharge threshold, is applied to the secondary transfer section N2 by the secondary transfer power supply 134.

[0080] Furthermore, in this embodiment, the case where the paper gap during double-sided printing is shorter than the length of one revolution of the secondary transfer roller 9 has been described as an example. However, even if the paper gap during double-sided printing is the same as or longer than the length of one revolution of the secondary transfer roller 9, the use of a through bias similar to that of this embodiment can suppress the adhesion of toner of the non-transferred toner image to the secondary transfer roller 9. This eliminates the need to eject toner from the outer secondary transfer roller 9 after the non-transferred toner image has passed. Therefore, even in a configuration where the paper gap during double-sided printing is the same as or longer than the length of one revolution of the secondary transfer roller 9, control using a through bias can be applied in double-sided printing continuation processing, as in this embodiment.

[0081] As described above, in this embodiment, the image forming apparatus 100 includes an image forming unit S that forms a toner image on an image carrier (photosensitive drum) 5 using toner whose normal charging polarity is a predetermined polarity, a circumferentially movable intermediate transfer body (intermediate transfer belt) 12 onto which the toner image is primarily transferred from the image carrier 5 at a primary transfer portion N1, an inner roller (drive roller) 18 provided on the inner circumferential surface side of the intermediate transfer body 12, an outer roller (secondary transfer roller) 9 that contacts the inner roller 18 via the intermediate transfer body 12 and forms a secondary transfer portion N2 that secondarily transfers the toner image from the intermediate transfer body 12 to a sheet 2, an application unit (secondary transfer power source) 134 that applies a bias to the secondary transfer portion N2, a feeding unit (paper feeding unit) 1 on which the sheet 2 is set, and a transport unit (paper feeding roller) that transports the sheet from the feeding unit 1 to the secondary transfer portion N2. a duplex conveying section (duplex unit) 70 that inverts the sheet 2 that has passed through the secondary transfer section N2 and conveys it to the secondary transfer section N2; and a control section (engine control section) 200 that can control the image forming section S, the application section 134, the conveying section 40, and the duplex conveying section 70 to perform duplex printing in which a toner image is secondarily transferred from the intermediate transfer body 12 to the first and second sides of the sheet 2, in which a first toner image formed in a first image forming area on the intermediate transfer body is secondarily transferred to the first side of the first sheet 2 conveyed to the secondary transfer section N2 by the conveying section 40, and then a second toner image formed in a second image forming area on the intermediate transfer body is secondarily transferred to the second side of the second sheet 2 conveyed to the secondary transfer section N2 by the duplex conveying section 70. In this embodiment, when double-sided printing is performed, if the first sheet 2 is not transported to the secondary transfer portion N2, the control portion 200 can control the control unit 200 to perform a continuation process of secondarily transferring the second toner image onto the second side of the second sheet. When performing the continuation process, the control portion 200 controls the application unit 134 to apply to the secondary transfer portion N2 a predetermined bias (through bias) such that the outer roller 9 has a potential of the predetermined polarity relative to the inner roller 18 and the absolute value of the potential difference between the inner roller 18 and the outer roller 9 is less than a discharge threshold while the first image forming area passes through the secondary transfer portion N2.In this embodiment, when performing double-sided printing, the first period from when the trailing edge of the first sheet 2 in the transport direction reaches the secondary transfer portion N2 until when the leading edge of the second sheet 2 in the transport direction reaches the secondary transfer portion N2 is shorter than the second period required for the outer roller 9 to make one revolution. The predetermined bias is preferably a bias such that the absolute value of the potential difference between the inner roller 18 and the outer roller 9 is 100 V or more and 500 V or less. In this embodiment, the control unit 200 controls the printer to execute the continuation process if the first sheet is not transported to the secondary transfer portion N2 because the sheet 2 set in the feed unit 1 has run out. However, the control unit 200 can also execute the continuation process if the first sheet is not transported to the secondary transfer portion N2 because a sheet 2 jam occurs in the transport path of the sheet 2 from the feed unit 1 to the secondary transfer portion N2. In this embodiment, the voltage application unit 134 applies a bias to the secondary transfer portion N2 via the outer roller 9. In this embodiment, the inner roller 18 is grounded.

[0082] As described above, according to this embodiment, it is possible to suppress the backside contamination when performing continuous double-sided printing while suppressing a decrease in printing productivity.

[0083] [Example 2] Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of the image forming apparatus of embodiment 1. Therefore, in the image forming apparatus of this embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of embodiment 1 are assigned the same reference numerals as those of embodiment 1, and detailed descriptions thereof will be omitted.

[0084] In this embodiment, the paper gap during double-sided printing is longer than the length of one revolution of the secondary transfer roller 9. Specifically, in this embodiment, the outer diameter of the secondary transfer roller 9 is 18 mm, the length of one revolution (circumferential length) of the secondary transfer roller 9 is 56.5 mm, and the paper gap during double-sided printing is 80 mm.

[0085] Typically, the toner of the toner image primarily transferred onto the intermediate transfer belt 12 is negatively charged. Therefore, as described in the first embodiment, applying a negative voltage whose absolute value is less than the discharge threshold to the secondary transfer roller 9 as a through bias can prevent the toner from adhering to the secondary transfer roller 9. However, for example, when the process cartridge 22 is used until the end of its life, a small but certain proportion of toner will exist whose charging characteristics deviate from the normal charging characteristics due to toner deterioration. Such toner is typically charged with a polarity opposite to the normal charging polarity, and is also referred to herein as "reverse toner."

[0086] For example, consider the case where the fourth sheet 2 is removed during double-sided printing of four sheets, as described in Example 1. The positive polarity reverse toner particles described above are electrostatically attracted to the secondary transfer roller 9 and may adhere to the surface of the secondary transfer roller 9, even when a through bias of -300 V is applied to the secondary transfer roller 9. When a positive polarity print bias is applied to the secondary transfer roller 9 during secondary transfer of the toner image on the back side of the third sheet, the positive polarity reverse toner particles that had adhered to the secondary transfer roller 9 are subjected to an electrical repulsive force, and adhere to the back side of the sheet 2, causing back contamination.

[0087] Here, an experiment was conducted to evaluate the amount of backside contamination similar to that described in Example 1. A process cartridge 22 with 10% remaining life was used in the experiment. The charge distribution of the magenta toner immediately after primary transfer onto the intermediate transfer belt 12 was measured in advance using an E-Spart Analyzer, and it was confirmed that there was a small amount of positively charged reverse toner.

[0088] In this experiment, the through bias was set to -300V. After the fourth sheet 2 was removed, the voltage (also referred to here as the "paper interval bias") applied to the secondary transfer roller 9 was changed during the period from when the non-transferred toner image (a magenta solid toner image) on the front side of the fourth sheet passed through the secondary transfer section N2 until the toner image on the back side of the third sheet reached the secondary transfer section N2 (the period corresponding to the paper interval), and the amount of back staining on the third sheet 2 was evaluated. The toner image, evaluation environment, and measuring equipment used for evaluation were the same as those used in the experiment described in Example 1. The results are shown in Figure 8.

[0089] First, when the paper bias was 0V, the amount of backside smear was 3.5%. When the paper bias was increased to +700V, the amount of backside smear was reduced to 1.5%, and when the paper bias was increased to +1000V or more, the amount of backside smear was reduced to 1% or less, a level that was barely noticeable. On the other hand, when the paper bias was made negative, there was almost no improvement in backside smear, and it remained at about 3%.

[0090] The above experimental results show that the inverted toner adhering from the intermediate transfer belt 12 to the secondary transfer roller 9 when the through bias is applied can be electrostatically repelled and moved from the secondary transfer roller 9 to the intermediate transfer belt 12 by applying a voltage of +1000 V or more to the secondary transfer roller 9 as an inter-paper bias. The effect of the inter-paper bias applied to the secondary transfer roller 9 tends to saturate when it is increased to a certain extent, and in the configuration of this embodiment, an inter-paper bias of +2000 V or less is sufficient. In this embodiment, the absolute value of the inter-paper bias is smaller than the absolute value of the print bias.

[0091] On the other hand, when a negative inter-sheet bias is applied to the secondary transfer roller 9, the following occurs: The reverse toner adhering to the secondary transfer roller 9 continues to be electrostatically attracted to the secondary transfer roller 9. Therefore, when a positive print bias is applied to the secondary transfer roller 9 during the secondary transfer of the toner image on the back side of the third sheet, the reverse toner moves from the secondary transfer roller 9 to the sheet 2, causing backside contamination.

[0092] Therefore, when the gap between sheets during double-sided printing is longer than the length of one revolution of the secondary transfer roller 9, a voltage (inter-sheet bias) of opposite polarity (same polarity as the print bias) to the normal charging polarity of the toner is applied to the secondary transfer roller 9 between the sheets. This allows the reverse toner adhering to the secondary transfer roller 9 to move from the secondary transfer roller 9 to the intermediate transfer belt 12, thereby preventing back staining. The reverse toner that has moved from the secondary transfer roller 9 to the intermediate transfer belt 12 can be collected by the belt cleaning device 15.

[0093] The reversed toner may occur when the process cartridge 22 is nearing the end of its life (when the toner has deteriorated) or when the process cartridge 22 is used in a high-temperature, high-humidity environment.

[0094] Therefore, in this embodiment, the image forming apparatus 100 is configured to be able to perform double-sided printing in a first mode and a second mode.

[0095] In the first mode, from the viewpoint of print productivity, the paper interval is set shorter than the length of one revolution of the secondary transfer roller 9. Then, if the sheet 2 in the paper feed unit 1 runs out during double-sided printing, a through bias is applied to the secondary transfer roller 9 while the non-transferred toner image on the intermediate transfer belt 12 is passing through the secondary transfer unit N2, as described in the first embodiment. This suppresses backside contamination.

[0096] On the other hand, in the second mode, the paper interval is set to be longer than the length of one revolution of the secondary transfer roller 9. If the sheet 2 in the paper feed unit 1 runs out during double-sided printing, as described in the first embodiment, a through bias is applied to the secondary transfer roller 9 while the non-transferred toner image on the intermediate transfer belt 12 passes through the secondary transfer unit N2. Furthermore, a paper-interval bias is applied to the secondary transfer roller 9 during the period corresponding to the paper interval between the sheet 2 onto which the non-transferred toner image was to be transferred and the sheet 2 fed from the duplex unit 70 after the non-transferred toner image on the intermediate transfer belt 12 passes through the secondary transfer unit N2. That is, the paper-interval bias is applied to the secondary transfer roller 9 during the period from the time the non-transferred toner image passes through the secondary transfer unit N2 until the next sheet 2 fed from the duplex unit 70 onto which the back-side toner image is to be secondarily transferred reaches the secondary transfer unit N2. The paper-interval bias is preferably applied for a period of at least one revolution of the secondary transfer roller 9 to clean the entire circumference of the secondary transfer roller 9. However, during at least a portion of the period corresponding to the inter-sheet interval, the inter-sheet bias can be applied to the secondary transfer roller 9 for the applicable period, and a corresponding effect can be obtained. The inter-sheet bias is a voltage of opposite polarity (same polarity as the print bias) to the normal charging polarity of the toner. In other words, a separate predetermined bias is applied to the secondary transfer unit N2 by the secondary transfer power supply 134 as the inter-sheet bias, so that the secondary transfer outer roller has a potential opposite polarity to the normal charging polarity of the toner relative to the secondary transfer inner roller. This suppresses backside contamination. Note that, as explained in the first embodiment, the same applies when double-sided printing is continued due to a jam occurring during the execution of double-sided printing.

[0097] Here, the engine control unit 200 can automatically switch between the first mode and the second mode based on information regarding the usage status of the process cartridge 22 and the temperature and humidity environment in which the image forming apparatus 100 is installed. Alternatively, the first mode and the second mode can be selected by a user (operator). As described above, reversed toner is unlikely to occur during normal use, but it may occur when the process cartridge 22 is nearing the end of its life or when used in a high-temperature, high-humidity environment. Therefore, under normal use environments, selecting the first mode can sufficiently suppress backside contamination and increase print productivity. On the other hand, under use environments in which the occurrence of reversed toner is predicted as described above, selecting the second mode can suppress backside contamination due to reversed toner, although print productivity will be lower than in the first mode.

[0098] FIG. 9 is a schematic diagram showing an example of a setting screen 401 displayed on the operation display unit 205 when the user is allowed to select a double-sided print mode.

[0099] As shown in FIG. 9, the setting screen 401 has a first selection section 411 for selecting the first mode (productivity priority mode) described above, and a second selection section 412 for selecting the second mode (backside contamination reduction priority mode) described above. The user can select either mode in consideration of the usage status of the image forming apparatus 100 (such as the usage history and environment of the process cartridge 22). The setting screen 401 shown in FIG. 9 may be displayed when called up by a predetermined operation by the user on the operation display unit 205. Also, for example, the first mode may be selected by default. The engine control unit 200 controls to execute a double-sided print job in the double-sided print mode selected on the setting screen 401.

[0100] FIG. 10 is a flowchart showing an example of a control procedure for a double-sided print job, including mode switching control when the engine control unit 200 selects the double-sided print mode.

[0101] First, when information about a double-sided print job (such as a start command or image information) is input from the operation / display unit 205 or an external device, the engine control unit 200 acquires information about the usage history of the process cartridge 22 from the parts counter 60 (FIG. 3) (S101). Examples of information about the usage history (accumulated usage amount) of the process cartridge 22 include information about the remaining amount of toner (or the amount of toner used) in the developing device 8, the number of rotations and rotation time of the rotating members of the developing device 8, and the number of rotations and rotation time of the photosensitive drum 5. Any index can be used as long as it can estimate the deterioration of the developing device 8 (more specifically, the toner) due to use. Any combination of these pieces of information can be used, and they may be sequentially stored in the parts counter 60, which includes a memory unit, as the remaining life of the process cartridge 22, for example. Any method, such as a known method, can be used to acquire (detect) information about the usage history of the process cartridge 22 (developing device 8). The engine control unit 200 also acquires the temperature and humidity detection results (environmental information) from the environmental sensor 50 (S102). The engine control unit 200 can calculate the absolute moisture amount based on the temperature and humidity detected by the environment sensor 50.

[0102] The engine control unit 200 determines whether the remaining life of the process cartridge 22 is greater than a predetermined first threshold value based on the acquired information about the usage history of the process cartridge 22 (S103). The first threshold value can be set in advance depending on the tendency for reversed toner to occur due to toner deterioration, etc. If the engine control unit 200 determines in S103 that the remaining life is greater than the first threshold value ("Yes"), it then determines whether the absolute moisture content is less than a predetermined second threshold value based on the absolute moisture content calculated from the acquired temperature and humidity (S104). The second threshold value can be set in advance depending on the tendency for reversed toner to occur in a high-temperature, high-humidity environment, etc. If the engine control unit 200 determines in S104 that the absolute moisture content is less than the second threshold value ("Yes"), it proceeds to the process of S105.

[0103] If the answers to S103 and S104 are "Yes," the engine control unit 200 determines to execute the double-sided print job in the first mode (S105). On the other hand, if the answers to S103 and S104 are "No" (the remaining life is equal to or less than the first threshold and the absolute moisture content is equal to or greater than the second threshold), the engine control unit 200 determines to execute the double-sided print job in the second mode (S106). Then, the engine control unit 200 starts image formation for the double-sided print job in the mode set in S105 or S106 (S107), and ends the operation of the job when the output of all images specified in the job has been completed (S108).

[0104] The mode for a double-sided print job may be selected based on either the usage history information of the process cartridge 22 (developing device 8) or the environment.

[0105] Also, in S103, for example, if the remaining life values ​​of all the process cartridges 22 are greater than the first threshold value, it can be determined as "Yes." Also, in S103, for example, if the remaining life value of at least one process cartridge 22 is equal to or less than the first threshold value, it can be determined as "No." However, this is not limited to this, and the number of process cartridges 22 (developing devices 8) with a predetermined remaining life or less can be set appropriately when switching between the first mode and the second mode based on factors such as the likelihood of backside contamination.

[0106] Furthermore, in this embodiment, the remaining life (for example, 100% when new and 0% when end of life) is used as information regarding the usage history of the process cartridge 22 (developing device 8), but the amount of usage (for example, 0% when new and 100% when end of life) may also be used. In this case, control may be performed so that double-sided printing is performed in the first mode when the amount of usage of the process cartridge 22 is less than a predetermined threshold, and in the second mode when the amount of usage is equal to or greater than the predetermined threshold. It is sufficient if the mode can be switched depending on whether the information (value) regarding the usage history of the process cartridge 22 (developing device 8) crosses a threshold.

[0107] Furthermore, in this embodiment, it is determined whether the absolute moisture content is smaller than a predetermined threshold, but it may also be determined whether it is greater than the predetermined threshold. In this case, if it is determined that the moisture content is greater than the predetermined threshold ("Yes"), double-sided printing is performed in the second mode, and if it is determined that the moisture content is equal to or less than the predetermined threshold ("No"), double-sided printing is performed in the first mode. It is sufficient if the mode can be switched depending on whether the environmental information (value) crosses the threshold.

[0108] Thus, in this embodiment, the control unit 200 can control the continuous processing to perform double-sided printing in a first mode in which the first period from when the trailing end of the first sheet 2 in the conveying direction reaches the secondary transfer unit N2 to when the leading end of the second sheet 2 in the conveying direction reaches the secondary transfer unit N2 is shorter than the second period required for the outer roller 9 to make one rotation, and in a second mode in which the first period is longer than the second period. In this embodiment, when performing double-sided printing in the second mode and performing continuous processing, the control unit 200 controls the application unit 134 to apply a predetermined bias (through bias) to the secondary transfer unit N2 while a first image forming area, where an untransferred toner image on the intermediate transfer body may exist, passes through the secondary transfer unit N2. The control unit 200 also controls the application unit 134 to apply another predetermined bias (inter-sheet bias) to the secondary transfer unit N2, which causes the outer roller 9 to have a potential opposite to a predetermined polarity (normal toner charging polarity) relative to the inner roller 18, during the period from when the first image forming area passes through the secondary transfer unit N2 until the second sheet 2 reaches the secondary transfer unit N2. The other predetermined bias is preferably a bias that results in an absolute value of a potential difference between the inner roller 18 and the outer roller 9 of at least 1000 V. The control unit 200 can also control the application of the other predetermined bias for at least the period required for the outer roller 9 to make one rotation.

[0109] Furthermore, image forming apparatus 100 may have an input unit (operation display unit) 205 that inputs instructions to control unit 200 based on an operation by an operator, and control unit 200 can control double-sided printing to be selectively performed in the first mode or the second mode in accordance with the instructions input by input unit 205. Furthermore, image forming apparatus 100 may have an acquisition unit (parts counter) 60 that acquires information regarding the usage history of developing device 8 that supplies toner to image carrier 5, and control unit 200 can control double-sided printing to be selectively performed in the first mode or the second mode based on the acquisition result by acquisition unit 60. In this case, control unit 200 can control double-sided printing to be performed in the first mode when the accumulated usage amount of developing device 8 indicated by the acquisition result is a first usage amount, and to be performed in the second mode when the accumulated usage amount indicated by the acquisition result is a second usage amount that is greater than the first usage amount. Furthermore, the image forming apparatus 100 may have an acquisition unit (environment sensor) 50 that acquires information about the environment, and the control unit 200 can control the apparatus to selectively perform double-sided printing in the first mode or the second mode based on the results acquired by the acquisition unit 50. In this case, the control unit 200 can control the apparatus to perform double-sided printing in the first mode when the humidity indicated by the acquired results is a first humidity, and to perform double-sided printing in the second mode when the humidity indicated by the acquired results is a second humidity higher than the first humidity.

[0110] As described above, according to this embodiment, it is possible to suppress backside contamination when performing continuous double-sided printing while suppressing a decrease in printing productivity, depending on the usage status of the image forming apparatus 100, etc.

[0111] [Example 3] Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of the image forming apparatus of embodiment 1. Therefore, in the image forming apparatus of this embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of embodiment 1 are assigned the same reference numerals as those of embodiment 1, and detailed descriptions thereof will be omitted.

[0112] In this embodiment, the configuration regarding application of the primary transfer voltage and the secondary transfer voltage is different from that in the first embodiment.

[0113] 11 is a schematic diagram for explaining the configuration for applying the primary transfer voltage and the secondary transfer voltage in this embodiment. In this embodiment, the intermediate transfer belt 12 has a multi-layer structure of at least two layers. In this embodiment, the surface of the intermediate transfer belt 12 that comes into contact with the primary transfer roller 4 (back layer) has a lower electrical resistance than the surface of the intermediate transfer belt 12 that comes into contact with the photosensitive drum 5 (surface layer). In this embodiment, the intermediate transfer belt 12 has a surface resistivity of 1.0×10 10 Ω / □, the surface resistivity of the back layer is 1.0×10 5 The one with Ω / □ was used.

[0114] In this embodiment, the drive roller 18 for driving the intermediate transfer belt 12 to rotate is disposed as an opposing roller to the secondary transfer roller 9, and is connected to a primary transfer power supply 133 for applying a voltage to the primary transfer rollers 4Y, 4M, 4C, and 4K. The other tension rollers 16 and 17 that support the intermediate transfer belt 12 are also connected to the primary transfer power supply 133, just like the drive roller 18. In the first embodiment, the drive roller 18 and the other tension rollers 16 and 17 were all directly grounded. In contrast, in this embodiment, a primary transfer voltage is applied to the drive roller 18 and the other tension rollers 16 and 17. In this configuration, when a toner image is primarily transferred onto the intermediate transfer belt 12, the primary transfer voltage is applied from the primary transfer power supply 133 to the primary transfer rollers 4Y, 4M, 4C, and 4K, the drive roller 18, and the other tension rollers 16 and 17. This makes it possible to maintain the potential of the intermediate transfer belt 12 at a predetermined potential or higher, suppressing fluctuations in the primary transfer potential at each primary transfer portion N1 and achieving stable primary transfer performance.

[0115] In this embodiment, the primary transfer voltage is applied to the drive roller 18, which serves as the secondary transfer opposing roller, so consideration must be given to the optimum through bias for suppressing backside contamination when performing continuous double-sided printing. Figure 12 shows an example of through bias control in this embodiment.

[0116] For example, when the image formation sequence shown in FIG. 6 is performed at an ambient temperature of 23°C and humidity of 55%, a primary transfer voltage of 250 V is applied to each of the primary transfer rollers 4, the drive roller 18, and the tension rollers 16 and 17. Then, when the fourth sheet 2 is removed, a through bias of -250 V is applied to the secondary transfer roller 9. When the non-transferred toner image passes through the secondary transfer portion N2, the potential difference between the drive roller 18 and the secondary transfer roller 9 is 500 V. In other words, a voltage of the same polarity as the toner and less than the discharge start voltage is applied to the secondary transfer roller 9. This makes it possible to suppress adhesion of toner to the secondary transfer roller 9.

[0117] As shown in FIG. 12, the primary transfer voltage may be changed depending on the usage of the process cartridge 22. This is because toner degradation due to repeated use progresses, and a higher primary transfer voltage is used to obtain good transferability. For example, if a similar image formation operation is performed after the cumulative usage of the process cartridge 22 reaches 15,000 images, a primary transfer voltage of 300 V is applied. In this case, by applying a through bias of −200 V, the potential difference between the drive roller 18 and the secondary transfer roller 9 can be maintained at 500 V, as in the above case. This prevents toner from adhering to the secondary transfer roller 9. By applying a through bias as shown in FIG. 12, the amount of backside contamination can be maintained at 1% or less throughout the life of the process cartridge 22.

[0118] In this way, by adjusting the potential difference between the secondary transfer roller 9 and the drive roller 18 so that it is less than the discharge start voltage based on Paschen's law, and by applying a voltage of the same polarity as the normal charging polarity of the toner to the secondary transfer roller 9, it is possible to suppress the occurrence of backside contamination when performing continuous double-sided printing.

[0119] As for the inter-paper bias, the potential difference between the inter-paper bias and the primary transfer voltage may be set as described in the second embodiment in the same manner as the through bias described above.

[0120] Furthermore, in this embodiment, a configuration has been described in which a primary transfer voltage is applied to the drive roller (secondary transfer inner roller) 18, but for example, the drive roller (secondary transfer inner roller) 18 may be configured to be electrically grounded via a voltage maintaining element such as a resistor or varistor. In this case, too, the same effects can be obtained by setting the potential difference between the drive roller (secondary transfer inner roller) 18 and the secondary transfer roller (secondary transfer outer roller) 9 in the same manner as described above with respect to the through bias and inter-paper bias.

[0121] As described above, in this embodiment, the image forming apparatus 100 has a voltage maintaining means (primary transfer power supply 133, resistor, varistor, etc.) that maintains the potential of the inner roller 18 at a potential of a polarity opposite to a predetermined polarity. In this embodiment, the voltage maintaining means is configured to have a power supply (primary transfer power supply) 133 that applies a bias to the primary transfer member (primary transfer roller) 4 that transfers the toner image from the image carrier 5 to the intermediate transfer body 12.

[0122] As described above, by applying the present invention to the image forming apparatus 100 having the configuration related to the application of the primary transfer voltage and the secondary transfer voltage as in this embodiment, it is possible to obtain the same effects as in the first and second embodiments.

[0123] [others] Although the present invention has been described above with reference to specific embodiments, the present invention is not limited to the above-described embodiments.

[0124] The through bias setting described in the above embodiment is also effective in preventing any toner not transferred to the sheet on the intermediate transfer body from adhering to the outer secondary transfer roller. In the above embodiment, the toner of the non-transferred toner image on the intermediate transfer body during continuous double-sided printing is prevented from adhering to the outer secondary transfer roller. Similarly, any toner, such as the toner of the toner image during calibration for image quality adjustment or the toner of the toner image remaining on the intermediate transfer body due to a jam during continuous single-sided printing, can be prevented from adhering to the outer secondary transfer roller.

[0125] In addition, in the second embodiment, when performing double-sided printing in the second mode and performing double-sided printing continuation processing, an inter-sheet bias is applied to the secondary transfer roller in a period corresponding to the inter-sheet gap after the non-transferred toner image has passed the secondary transfer unit. In contrast, for example, in the second mode, an inter-sheet bias can be applied to the secondary transfer roller in at least some (or all) of the inter-sheet gaps other than when performing double-sided printing continuation processing. This allows reverse toner, which may adhere to the secondary transfer roller during printing, to be moved to the intermediate transfer belt between the sheets, thereby cleaning the secondary transfer roller.

[0126] In addition, the operations and displays described in the above embodiments as being performed by the operation and display unit provided in the image forming apparatus may be performed by an operation unit or display unit of an external device connected to the image forming apparatus so as to be able to communicate with the image forming apparatus. In this case, an input / output unit provided in the image forming apparatus functions as an input unit that inputs signals for various settings to the control unit and an output unit that outputs signals for the control unit to perform various displays. [Explanation of symbols]

[0127] 1 Paper feed section (feed section) 9 Secondary transfer roller (outer roller) 11 Seat presence sensor 12 Intermediate transfer belt (intermediate transfer body) 18 Drive roller (inner roller) 70 Double-sided unit 100 Image forming device 200 Engine control unit (control unit)

Claims

1. an image forming unit that forms a toner image on an image carrier using toner whose normal charging polarity is a predetermined polarity; an intermediate transfer member that is capable of rotating and onto which a toner image is primarily transferred from the image carrier at a primary transfer portion; an inner roller provided on the inner peripheral surface side of the intermediate transfer body; an outer roller that contacts the inner roller via the intermediate transfer body and forms a secondary transfer portion that secondarily transfers a toner image from the intermediate transfer body to a sheet; an application unit that applies a bias to the secondary transfer unit; a feeding section in which sheets are set; a conveying unit that conveys a sheet from the feeding unit toward the secondary transfer unit; a double-sided conveying unit that inverts the sheet that has passed through the secondary transfer unit and conveys the sheet to the secondary transfer unit; a control unit capable of controlling the image forming unit, the application unit, the transport unit, and the double-sided transport unit to perform double-sided printing in which a toner image is secondarily transferred from the intermediate transfer body to a first surface and a second surface of a sheet, wherein a first toner image formed in a first image forming area on the intermediate transfer body is secondarily transferred to a first surface of a first sheet transported by the transport unit to the secondary transfer unit, and then a second toner image formed in a second image forming area on the intermediate transfer body is secondarily transferred to a second surface of a second sheet transported by the double-sided transport unit to the secondary transfer unit; An image forming apparatus having the control unit is capable of performing control so as to execute a continuation process of secondarily transferring the second toner image onto the second surface of the second sheet when the first sheet is not transported to the secondary transfer unit during the double-sided printing; The image forming apparatus is characterized in that, when performing the continuous processing, the control unit controls the application unit to apply a predetermined bias to the secondary transfer unit such that, while the first image forming area passes through the secondary transfer unit, the outer roller has a potential of the predetermined polarity relative to the inner roller, and the absolute value of the potential difference between the inner roller and the outer roller is less than a discharge threshold.

2. The image forming apparatus according to claim 1, characterized in that, when performing double-sided printing, a first period from when the trailing end of the first sheet in the transport direction reaches the secondary transfer unit to when the leading end of the second sheet in the transport direction reaches the secondary transfer unit is shorter than a second period required for the outer roller to make one rotation.

3. the control unit is capable of controlling the double-sided printing to be performed in a first mode in which the first period is shorter than the second period and in a second mode in which the first period is longer than the second period; The image forming apparatus according to claim 2, characterized in that, when performing the double-sided printing in the second mode and the continuous processing, the control unit controls the application unit to apply the predetermined bias to the secondary transfer unit while the first image forming area is passing through the secondary transfer unit, and controls the application unit to apply another predetermined bias to the secondary transfer unit such that the outer roller has a potential of an opposite polarity to the predetermined polarity relative to the inner roller between the time when the first image forming area passes through the secondary transfer unit and the time when the second sheet reaches the secondary transfer unit.

4. 4. The image forming apparatus according to claim 1, wherein the predetermined bias is a bias such that an absolute value of a potential difference between the inner roller and the outer roller is 100V or more and 500V or less.

5. 4. The image forming apparatus according to claim 3, wherein the other predetermined bias is a bias such that the absolute value of the potential difference between the inner roller and the outer roller is at least 1000V.

6. 4. The image forming apparatus according to claim 3, wherein the control section controls the application of the other predetermined bias for at least a period required for the outer roller to make one revolution.

7. The image forming apparatus according to any one of claims 1 to 3, characterized in that the control unit controls the continuous processing to be executed when the first sheet is not transported to the secondary transfer unit because there are no more sheets set in the feeding unit.

8. The image forming apparatus according to any one of claims 1 to 3, characterized in that the control unit controls the continuous processing to be executed when the first sheet is not transported to the secondary transfer unit due to a sheet jam occurring in the sheet transport path from the feeding unit to the secondary transfer unit.

9. an input unit for inputting instructions to the control unit based on an operation by an operator; 4. The image forming apparatus according to claim 3, wherein the control unit controls the double-sided printing to be selectively performed in the first mode or the second mode in response to an instruction inputted through the input unit.

10. an acquisition unit that acquires information about a usage history of a developing device that supplies toner to the image carrier, the developing device being included in the image forming unit; 4. The image forming apparatus according to claim 3, wherein the control unit controls the double-sided printing to be selectively performed in the first mode or the second mode based on the result of the acquisition by the acquisition unit.

11. The image forming apparatus according to claim 10, characterized in that the control unit controls the double-sided printing to be performed in the first mode when the cumulative usage amount of the developing device indicated by the acquired result is a first usage amount, and to be performed in the second mode when the cumulative usage amount indicated by the acquired result is a second usage amount greater than the first usage amount.

12. an acquisition unit that acquires information about the environment; 4. The image forming apparatus according to claim 3, wherein the control unit controls the double-sided printing to be selectively performed in the first mode or the second mode based on the result of the acquisition by the acquisition unit.

13. The image forming apparatus according to claim 12, characterized in that the control unit controls the double-sided printing to be performed in the first mode when the humidity indicated by the acquired result is a first humidity, and to be performed in the second mode when the humidity indicated by the acquired result is a second humidity higher than the first humidity.

14. 4. The image forming apparatus according to claim 1, wherein the applying unit applies a bias to the secondary transfer unit via the outer roller.

15. 15. The image forming apparatus according to claim 14, wherein the inner roller is grounded.

16. 15. The image forming apparatus according to claim 14, further comprising a voltage maintaining means for maintaining the potential of the inner roller at a potential of a polarity opposite to the predetermined polarity.

17. 17. The image forming apparatus according to claim 16, wherein the voltage maintaining means includes a power source for applying a bias to a primary transfer member that transfers a toner image from the image carrier to the intermediate transfer member.

Images