Image forming apparatus

Abstract

To suppress a decrease in productivity during double-sided printing.SOLUTION: The engine control unit 301 is configured to be able to execute the first operation (S705) when the predetermined condition is not satisfied, and to be able to execute the second operation (S704) when the predetermined condition is satisfied. The predetermined condition includes at least one of an environmental temperature being equal to or higher than a predetermined temperature, a remaining lifetime of the developing device being less than a predetermined value, designation of the second operation by the user, an image concentration of the immediately preceding print being equal to or greater than a predetermined value, an image of the immediately preceding print including a text other than text, and a maximum concentration of a partial region of the immediately preceding print being equal to or greater than a predetermined value (S721 to S723). The toner image formed in the second operation is transferred to the re-conveyed sheet (S704, S707, S708).SELECTED DRAWING: Figure 8

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus, for example, an image forming apparatus using an electrophotographic method or an electrostatic recording method, and to an image forming apparatus having a switchback mechanism that reverses the conveying direction of a sheet for double-sided printing. [Background technology]

[0002] In recent years, a wide variety of sheet conveying methods for feeding sheets to an image forming unit in an electrophotographic image forming apparatus have been proposed and realized. One of these methods is a so-called automatic double-sided conveying function that enables images to be formed on both sides of an automatically inverted sheet without the user having to re-set the sheet.

[0003] In addition, some image forming devices are equipped with an intermediate transfer body and have an automatic double-sided conveying function, and sheets that have been printed on one side wait in a double-sided unit, and after printing on one side of a fed sheet, printing is performed on the back side of the waiting sheet.

[0004] In such an image forming apparatus, when the paper feed unit runs out of sheets during a double-sided printing job, there may be toner images formed on the intermediate transfer body that do not have a corresponding sheet and toner images that correspond to sheets waiting in the double-sided unit. Here, of the toner images formed on the intermediate transfer body, toner images that do not have a corresponding sheet are called sheet-less toner images.

[0005] When a sheetless toner image passes through the secondary transfer unit, it adheres to the secondary transfer roller, and the toner adhering to the secondary transfer roller may cause stains on the back side of the sheet fed from the duplex unit. Hereinafter, this stain will be called back side stains.

[0006] To prevent such back staining, Patent Document 1 discloses the following technology. That is, when the sheet-less toner image passes through the secondary transfer section, a voltage of the opposite polarity to the voltage applied when transferring the image to the sheet is applied. This keeps the toner adhering to the secondary transfer roller within an acceptable range, and keeps the back staining of the sheet within an acceptable range. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-215369 Summary of the Invention [Problem to be solved by the invention]

[0008] However, when the density of the sheetless toner image is high, the toner adhering to the transfer roller may exceed the allowable range. In such a case, it is preferable to clean the secondary transfer roller to prevent the back of the sheet from being soiled before the secondary transfer of the toner image to the back of the sheet waiting in the duplex unit.

[0009] Here, if the interval between the sheet-less toner image and the succeeding toner image is narrow, the above-mentioned cleaning of the secondary transfer roller may not be performed sufficiently. On the other hand, if printing operations are always performed with a wide image interval in preparation for the toner out situation, there is a problem that printing productivity will decrease.

[0010] The present invention has been made under these circumstances, and has as its object to suppress a decrease in productivity during double-sided printing. [Means for solving the problem]

[0011] In order to solve the above-mentioned problems, the present invention has the following configuration.

[0012] (1) An image forming apparatus comprising: an image carrier; an intermediate transfer body onto which a toner image formed on the image carrier is transferred; a transfer member that transfers the toner image transferred on the intermediate transfer body onto a sheet and forms a transfer nip between the intermediate transfer body and the transfer member; a storage section that stores sheets; a detection means that detects the presence or absence of sheets in the storage section; a double-sided conveying path through which a sheet having an image formed on its front side and conveyed toward the transfer nip passes; and a control means that can perform backside printing and immediate-preceding printing, wherein in the backside printing, a toner image to be transferred to a re-conveyed sheet that is conveyed from the double-sided conveying path to the transfer nip is formed on the intermediate transfer body; and in the immediate-preceding printing, a toner image to be transferred to a target sheet different from the re-conveyed sheet that is conveyed to the transfer nip immediately before the re-conveyed sheet is conveyed to the transfer nip, and the control means an image forming apparatus capable of performing a first operation of performing the backside printing at a first interval from a print, and a second operation of performing the backside printing at a second interval from the immediately preceding print that is wider than the first interval, wherein the control means is configured to be able to perform the first operation when a predetermined condition is not met, and to be able to perform the second operation when the predetermined condition is met, wherein the predetermined condition includes at least one of the following: the ambient temperature is equal to or higher than a predetermined temperature; the remaining life of a developing device is less than a predetermined value; a user has specified the second operation; the image density of the immediately preceding print is equal to or higher than a predetermined value; the image of the immediately preceding print includes something other than text; and the maximum density of a portion of the immediately preceding print is equal to or higher than a predetermined value; and wherein if the detection means detects that there is no sheet corresponding to the target sheet, the toner image formed in the second operation is transferred to the re-conveyed sheet.

[0013] (2) A printing system including an image carrier, an intermediate transfer body onto which a toner image formed on the image carrier is transferred, a transfer member that transfers the toner image transferred onto the intermediate transfer body onto a sheet and forms a transfer nip between the intermediate transfer body and the transfer member, a storage section that stores sheets, a detection means that detects the presence or absence of sheets in the storage section, a double-sided conveying path through which a sheet having an image formed on its front side and conveyed toward the transfer nip passes, and a control means that can perform backside printing and immediate-preceding printing, wherein in the backside printing, a toner image to be transferred onto a re-conveyed sheet conveyed from the double-sided conveying path to the transfer nip is formed on the intermediate transfer body, and in the immediate-preceding printing, a toner image to be transferred onto a target sheet different from the re-conveyed sheet that is conveyed to the transfer nip immediately before the re-conveyed sheet is conveyed to the transfer nip. an image forming apparatus in which a toner image to be printed on the intermediate transfer body is formed on the intermediate transfer body, wherein the control means is capable of performing a first operation of performing the back surface printing at a first interval from the immediately preceding print, and a second operation of performing the back surface printing at a second interval from the immediately preceding print that is wider than the first interval, and the control means is configured to be capable of performing the first operation when a predetermined condition is not satisfied, and to be capable of performing the second operation when the predetermined condition is satisfied, and wherein when the detection means detects that there is no sheet corresponding to the target sheet, the amount of toner adhering to the transfer member as the toner image formed in the immediately preceding print passes through the transfer nip is greater when the predetermined condition is satisfied than when the predetermined condition is not satisfied.

[0014] (3) A printing system including an image carrier, an intermediate transfer body onto which a toner image formed on the image carrier is transferred, a transfer member that transfers the toner image transferred onto the intermediate transfer body onto a sheet and forms a transfer nip between the intermediate transfer body and the transfer member, a storage section that stores sheets, a detection means that detects the presence or absence of sheets in the storage section, a double-sided conveying path through which a sheet having an image formed on its front side and conveyed toward the transfer nip passes, and a control means that can perform backside printing and immediate-preceding printing, wherein in the backside printing, a toner image to be transferred to a re-conveyed sheet conveyed from the double-sided conveying path to the transfer nip is formed on the intermediate transfer body, and in the immediate-preceding printing, the toner image is transferred immediately before the re-conveyed sheet is conveyed to the transfer nip. An image forming apparatus in which a toner image to be transferred to a target sheet other than the re-conveyed sheet, which is transported to a nip, is formed on the intermediate transfer body, wherein the control means is capable of performing a first operation of performing the back surface printing at a first interval from the immediately preceding print, and a second operation of performing the back surface printing at a second interval wider than the first interval from the immediately preceding print, the control means being configured to be capable of performing the first operation when the second operation is not specified by a user, and to be capable of performing the second operation when the user specifies it, and wherein when the detection means detects that there is no sheet corresponding to the target sheet, the toner image formed in the second operation is transferred to the re-conveyed sheet.

[0015] (4) A printing method for a sheet conveyed from a sheet conveyance path to the transfer nip, the printing method comprising: an image carrier; an intermediate transfer body onto which a toner image formed on the image carrier is transferred; a transfer member that transfers the toner image transferred on the intermediate transfer body to a sheet and forms a transfer nip between the intermediate transfer body and the transfer member; a storage section that stores sheets; a detection means that detects the presence or absence of sheets in the storage section; a double-sided conveyance path through which a sheet having an image formed on its front side and conveyed toward the transfer nip passes; and a control means that can perform backside printing and immediate print. In the backside printing, a toner image to be transferred to a re-conveyed sheet conveyed from the double-sided conveyance path to the transfer nip is formed on the intermediate transfer body; and in the immediate print, a target sheet different from the re-conveyed sheet is conveyed to the transfer nip immediately before the re-conveyed sheet is conveyed to the transfer nip. an image forming apparatus in which a toner image to be transferred to a sheet is formed on the intermediate transfer body, wherein the control means is capable of performing a first operation of performing the back surface print at a first interval from the immediately preceding print, and a second operation of performing the back surface print at a second interval from the immediately preceding print that is wider than the first interval; the control means is configured to be able to perform the first operation when the second operation is not specified by a user, and to be able to perform the second operation when the user specifies it; and wherein, when the detection means detects that there is no sheet corresponding to the target sheet, the amount of toner adhering to the transfer member as the toner image formed in the immediately preceding print passes through the transfer nip is greater when the user specifies it than when the user does not specify it.

[0016] (5) An image forming apparatus comprising: an image carrier; an intermediate transfer body onto which a toner image formed on the image carrier is transferred; a transfer member that transfers the toner image transferred on the intermediate transfer body to a sheet and forms a transfer nip between itself and the intermediate transfer body; a storage section that stores sheets; a detection means that detects the presence or absence of a sheet in the storage section; a double-sided conveying path through which a sheet having an image formed on its front side and conveyed toward the transfer nip passes; and a control means that can perform backside printing and immediate-preceding printing, wherein in the backside printing, a toner image to be transferred to a re-conveyed sheet conveyed from the double-sided conveying path to the transfer nip is formed on the intermediate transfer body; and in the immediate-preceding printing, a toner image to be transferred to a target sheet different from the re-conveyed sheet that is conveyed to the transfer nip immediately before the re-conveyed sheet is conveyed to the transfer nip, the toner image being formed on the intermediate transfer body; a second operation of transferring a toner image formed in printing to the re-conveying sheet, and a third operation of performing the cleaning operation after the toner image formed in the back surface printing has passed through the transfer nip and transferring the toner image formed by performing the back surface printing again to the re-conveying sheet, wherein the control means is configured to be able to perform the first operation when a predetermined condition is not satisfied, and to be able to perform the second operation or the third operation depending on the interval between the previous printing and the back surface printing when the predetermined condition is satisfied, wherein the predetermined condition includes at least one of the following: an ambient temperature is equal to or higher than a predetermined temperature; a remaining life of a developing device is less than a predetermined value; a user has designated the second operation; an image density of the previous printing is equal to or higher than a predetermined value; an image of the previous printing includes something other than text; and a maximum density of a partial area of ​​the previous printing is equal to or higher than a predetermined value, and when the detection means detects that there is no sheet corresponding to the target sheet, the toner image formed in the previous printing passes through the transfer nip before the first operation, the second operation, or the third operation.An image forming apparatus characterized by: [Effects of the Invention]

[0017] According to the present invention, it is possible to suppress a decrease in productivity during double-sided printing. [Brief explanation of the drawings]

[0018] [Figure 1] Schematic diagram of the laser printer engine of Examples 1 to 5 [Figure 2] Hardware configuration diagram for Examples 1 to 5 [Figure 3] Functional block diagram of Examples 1 to 3 [Figure 4] Flowchart explaining the prior art for comparison with the first embodiment [Figure 5] FIG. 1 is a communication sequence diagram illustrating a conventional technique for comparison with the first embodiment. [Figure 6] A sequence diagram illustrating a conventional technique for comparison with the first embodiment. [Figure 7] A sequence diagram illustrating a conventional technique for comparison with the first embodiment. [Figure 8] 1 is a flowchart showing a discharge necessity determination process according to the first embodiment; [Figure 9] 1 is a sequence diagram when the discharge necessity determination process of the first embodiment is performed. [Figure 10] 10 is a flowchart showing a discharge necessity determination process according to a second embodiment. [Figure 11] FIG. 10 is a sequence diagram showing a process for determining whether or not discharge is necessary according to the second embodiment; [Figure 12] 10 is a flowchart showing a discharge necessity determination process according to a third embodiment. [Figure 13] FIG. 10 is a sequence diagram showing a process for determining whether or not ejection is necessary according to the third embodiment; [Figure 14] Functional block diagram of Examples 4 and 5 [Figure 15(a)] 10 is a flowchart showing a discharge necessity determination process according to a fourth embodiment. [Figure 15(b)] 10 is a flowchart showing a discharge necessity determination process according to a fourth embodiment. [Figure 16] FIG. 10 is a sequence diagram showing a process for determining whether or not ejection is necessary according to the fourth embodiment; [Figure 17] Flowchart showing discharge necessity determination processing in the fifth embodiment DETAILED DESCRIPTION OF THE INVENTION

[0019] The following describes an image forming apparatus capable of performing double-sided printing on a sheet as a recording medium. Although the sheet may be referred to simply as paper below, paper is merely one example of a sheet, and images can also be formed on sheets other than paper. [Example]

[0020] A first embodiment to which the present invention can be applied will be described below. The overall configuration of a laser printer engine as an image forming apparatus will be outlined with reference to FIG.

[0021] (Outline of image forming device) The laser printer engine 200 (hereinafter simply referred to as printer 200) forms an electrostatic latent image using image light formed based on an image signal sent from a controller unit (not shown), develops the electrostatic latent image, and superimposes and transfers a visible image to form a color visible image. The color visible image is transferred to sheet 2, and the unfixed color visible image on sheet 2 is fixed. The image forming unit is composed of photosensitive drums 5Y, 5M, 5C, and 5K, chargers 7Y, 7M, 7C, and 7K, developers 8Y, 8M, 8C, and 8K, and an intermediate transfer belt 12, each arranged in parallel for each development color. Note that Y represents yellow, M represents magenta, C represents cyan, and K represents black. Hereinafter, the suffixes YMCK will be omitted from the reference numerals except when describing components of specific colors.

[0022] The photosensitive drum 5 as an image carrier, the charger 7 as a charging means, and the developer 8 as a developing means are mounted in a process cartridge 22 that is detachable from the printer 200 body. The developer 8 and a toner container (not shown) may be configured as a cartridge (toner cartridge, developer cartridge) that is detachable from the printer 200 body. The photosensitive drum 5 is configured by applying an organic photoconductive layer to the outer periphery of an aluminum cylinder, and is rotated by the driving force of a drive motor (not shown). The drive motor rotates the photosensitive drum 5 clockwise in response to image formation operations. Exposure light onto the photosensitive drum 5 is sent from a scanner unit 10, and an electrostatic latent image is formed by selectively exposing the surface of the photosensitive drum 5. Each station is equipped with four chargers 7Y, 7M, 7C, and 7K for charging the yellow (Y), magenta (M), cyan (C), and black (K) photosensitive drums 5Y, 5M, 5C, and 5K. Chargers 7Y, 7M, 7C, and 7K are equipped with sleeves 7YS, 7MS, 7CS, and 7KS. To visualize the electrostatic latent images, each station is equipped with four developers 8Y, 8M, 8C, and 8K that develop yellow (Y), magenta (M), cyan (C), and black (K). Developing devices 8Y, 8M, 8C, and 8K are equipped with sleeves (developing rollers) 8YS, 8MS, 8CS, and 8KS. Each of developing devices 8Y, 8M, 8C, and 8K is detachably attached to the printer 200 main body.

[0023] An intermediate transfer belt 12 serving as an intermediate transfer member is in contact with the photosensitive drums 5Y, 5M, 5C, and 5K and rotates counterclockwise during color image formation. The intermediate transfer belt 12 rotates in conjunction with the rotation of the photosensitive drums 5Y, 5M, 5C, and 5K, and receives the transfer of a visible image by a primary transfer voltage applied to the primary transfer rollers 4Y, 4M, 4C, and 4K (hereinafter referred to as primary transfer). The intermediate transfer belt 12 transfers a superimposed color visible image onto the sheet 2 by sandwiching and conveying the sheet 2 together with a secondary transfer opposing roller 18 at the position of a secondary transfer roller 9 serving as a transfer member (hereinafter referred to as secondary transfer). The area where the secondary transfer roller 9 and the intermediate transfer belt 12 contact each other is called the secondary transfer portion (transfer nip). In other words, the secondary transfer roller and the intermediate transfer belt are in contact to form the secondary transfer portion (transfer nip).

[0024] The primary transfer rollers 4Y, 4M, 4C, and 4K are pressed against the photosensitive drums 5Y, 5M, 5C, and 5K with the intermediate transfer belt 12 sandwiched therebetween. A primary transfer power supply (not shown) that applies a primary transfer voltage is connected to the primary transfer rollers 4Y, 4M, 4C, and 4K. The primary transfer power supply applies a primary transfer voltage of positive polarity, which is opposite to the normal charging polarity of the toner, to the primary transfer rollers 4Y, 4M, 4C, and 4K. This causes the toner images on the photosensitive drums 5Y, 5M, 5C, and 5K to be primarily transferred onto the intermediate transfer belt 12 (onto the intermediate transfer body). The secondary transfer rollers 9 have the same configuration and physical properties as the primary transfer rollers 4Y, 4M, 4C, and 4K. The secondary transfer rollers 9 are pressed against the intermediate transfer belt 12 via the sheet 2. A secondary transfer power supply 41 that applies a secondary transfer voltage is connected to the secondary transfer rollers 9.

[0025] The secondary transfer power supply 41 applies a voltage to the secondary transfer roller 9 so as to generate a potential difference between the secondary transfer roller 9 and the intermediate transfer belt 12. An engine control unit 301, which will be described later, is configured to control the secondary transfer power supply 41 to execute a first applying operation and a second applying operation. Here, in the first applying operation, the toner charged to the normal polarity receives an electrostatic force in a direction from the intermediate transfer belt 12 toward the secondary transfer roller 9. On the other hand, in the second applying operation, the toner charged to the normal polarity receives an electrostatic force in a direction from the secondary transfer roller 9 toward the intermediate transfer belt 12. In other words, the secondary transfer power supply 41 can apply voltages of both polarities to the secondary transfer roller 9: a negative polarity, which is the normal charging polarity of the toner, and a positive polarity, which is the polarity opposite to the normal charging polarity of the toner.

[0026] The fixing unit 13, which serves as a fixing means, fixes the transferred, unfixed color visible image while conveying the sheet 2. The fixing unit 13 includes a fixing roller 14 that heats the sheet 2 and a pressure roller 15 that presses the sheet 2 against the fixing roller 14. The fixing roller 14 and the pressure roller 15 are hollow, and a heater is built into the fixing roller 14. That is, the sheet 2 carrying the color visible image is conveyed by the fixing roller 14 and the pressure roller 15, and the toner is fixed to the surface by applying heat and pressure. After the visible image is fixed, the sheet 2 is conveyed to the discharge conveyance path 26 and discharged to the discharge section by discharge conveyance rollers 31, thereby completing the image forming operation. The full-load sensor 39 detects whether the multiple sheets 2 (sheet stack) discharged and stacked on the discharge tray 27 are fully loaded. The conveyance path (conveyance path) for the sheet 2 includes a registration sensor 19, a fixing and discharge sensor 20, and a duplex conveyance sensor 28, which can detect the arrival and passing of the leading and trailing ends of the conveyed sheet 2. Furthermore, a sheet sensor 11 is provided at the paper feed port of the cassette 1 serving as a storage section, as a detection means for detecting the presence or absence of a sheet 2 in the cassette 1. A transport roller 40 and a registration roller (hereinafter referred to as a registration roller) 3 are rollers for transporting the sheet 2. The printer 200 also includes an environment sensor 50 (see FIG. 2, etc.) capable of detecting the temperature and humidity of the environment in which the printer 200 is installed.

[0027] Next, we will explain the double-sided conveyance control when printing on the front side, which serves as the first side, of the sheet 2, and the back side, which serves as the second side opposite to the first side. After the sheet 2 has been printed on the front side and passed through the fixing unit 13, a reversing clutch (not shown) switches the double-sided flapper 32, and the sheet 2 is conveyed to the double-sided reversing path 29. In FIG. 1, the double-sided flapper 32 shown by the solid line indicates the state in which the sheet 2 is conveyed to the double-sided reversing path 29, and the double-sided flapper 32 shown by the dashed line indicates the state in which the sheet 2 is conveyed to the discharge conveying path 26. The double-sided flapper 32 shown by the dashed line also has the function of preventing the sheet 2 conveyed to the double-sided reversing path 29 from traveling backward toward the fixing unit 13.

[0028] When the trailing edge of the sheet 2 reaches the duplex reversing path 29, a reversing clutch (not shown) switches the rotation direction of the reversing roller 30 and the duplex flapper 32, transporting the sheet 2 to the duplex conveying path 33 (duplex conveying path). The sheet 2, whose conveying direction has been reversed, is transported along the duplex conveying path 33 by the duplex conveying roller 37 and the duplex refeed roller 35. In this way, the sheet 2 enters the print conveying path 25 again in an inverted state, where a toner image is transferred and fixed to the backside, and the sheet is discharged to the discharge section. Any contamination on the backside of the sheet 2 fed from the duplex conveying path 33 becomes contamination on the front side (first side) of the sheet 2 after image formation. The sheet 2 transported from the duplex conveying path 33 to the secondary transfer section is also referred to as a retransferred sheet. The printing operation in which a toner image to be transferred to the retransferred sheet transported from the duplex conveying path 33 to the secondary transfer section is formed on the intermediate transfer belt 12 is referred to as backside printing. The sheet transported to the secondary transfer section immediately before the retransferred sheet is transported to the secondary transfer section is also referred to as a target sheet. The printing operation in which a toner image to be transferred to a target sheet is formed on the intermediate transfer belt 12 is called immediately before printing.

[0029] (Hardware configuration) The hardware configuration of the first embodiment will be described with reference to FIG. 2. The printer 200 is composed of an operation / display unit 205, a video controller 204, and an engine control unit 301 as a control means. The video controller 204 receives the status of the printer 200 from the engine control unit 301 and transmits it to the operation / display unit 205. The operation / display unit 205 switches the information displayed based on the received status of the printer 200. The operation / display unit 205 includes an operation panel and operation buttons (not shown). The video controller 204 also receives image information and a print command from the host computer 1000. The video controller 204 analyzes the received image information, converts it into bit data, and sends a print reservation command, a print start command, and a video signal for each page to the engine control unit 301 via a video interface processing unit 330.

[0030] The engine control unit 301 is a control IC configured with a CPU (Central Processing Unit) 207, a ROM (Read Only Memory) 208, a RAM (Random Access Memory) 209, and an IO port 211. 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. The above-mentioned components can access the IO port 211 via a bidirectionally accessible system bus 210. An actuator (not shown) is connected to the IO port 211, and controls each actuator that realizes the sheet 2 conveying operation (hereinafter referred to as the sheet conveying operation), the image forming operation, the initialization operation, etc. Also, a sheet sensor 11 in the cassette 1 is connected to the IO port 211, and detects the presence or absence of a sheet 2 in the cassette 1 via the IO port 211. Furthermore, an environmental sensor 50 is connected to the IO port 211, and detects the ambient temperature and humidity of the environment in which the printer 200 is installed.

[0031] (function block) Next, functional blocks of the first embodiment will be described with reference to FIG. 3. Only the parts related to the first embodiment will be extracted and described here. The engine control unit 301 of the first embodiment is a function of the main control unit that performs the sheet conveying operation and image forming operation of the printer 200. The image formation control unit 307 controls a series of image formation processes from image formation on the photosensitive drum 5 (on the image carrier) to transfer and fixation on the sheet 2, in accordance with the image interval determined by the image formation interval control unit 320. Similarly, the sheet conveying control unit 340 controls the sheet conveying operation from feeding the sheet 2 from the cassette 1 to discharging it outside the machine (hereinafter referred to as sheet conveying control).

[0032] The image formation interval control unit 320 determines the interval between image formations according to the contents of the print reservation command received from the video controller 204 and the operating status of the printer 200. The video interface processing unit 330 processes communication between the engine control unit 301 and the video controller 204. The detection unit 306 inputs signals from the sheet sensor 11 and the environment sensor 50 to the engine control unit 301. The discharge process necessity determination unit 350 (hereinafter simply referred to as the necessity determination unit 350) is a unit that determines in advance whether or not to perform discharge process at the secondary transfer unit when there is no sheet in cassette 1 during double-sided printing, and is also a unit that predicts back stains. The discharge process will be described later. The absence of a sheet in cassette 1 can be called "no paper," "no sheet," etc.

[0033] (Conventional operation flowchart) FIG. 4 is a flowchart of the printing operation performed by the engine control unit 301. In step (hereinafter, S) 101, the engine control unit 301 determines whether or not a print reservation command and a print start command have been received from the video controller 204 via the video interface processing unit 330. If the engine control unit 301 determines in S101 that these commands have not been received, it returns the process to S101. If the engine control unit 301 determines that these commands have been received, it proceeds to S102. In S102, the engine control unit 301 executes pre-processing for the printing operation (hereinafter, referred to as the "pre-rotation sequence"). In S103, the engine control unit 301 outputs a / TOP signal from the image formation control unit 307 to start the printing operation in accordance with the print reservation command for the first page. The engine control unit 301 also determines the image formation interval using the image formation interval control unit 320. In S104, the engine control unit 301 determines whether or not it is time to start the printing operation for the next page in accordance with the image formation interval determined in S103. The engine control unit 301 has a timer (not shown) and manages timing by resetting and starting the timer in S103. If the engine control unit 301 determines in S104 that it is not time to start the next printing operation, it returns the process to S104, and if it determines that it is time to start the next printing operation, it proceeds to S105.

[0034] In S105, the engine control unit 301 determines whether or not a print reservation command and a print start command for the next print operation have been received from the video controller 204 via the video interface processing unit 330. If the engine control unit 301 determines in S105 that they have been received, it returns the process to S103 and starts the print operation for the next page. If the engine control unit 301 determines in S105 that the next print reservation command and print start command have not been received, it proceeds to S106. In S106, the engine control unit 301 executes post-processing for the print operation (hereinafter referred to as the "post-rotation sequence") and ends the print operation.

[0035] (Communication sequence) FIG. 5 shows the communication sequence between the engine control unit 301 and the video controller 204, which alternates between feeding paper from cassette 1 and feeding paper from the duplex conveying path 33, and begins duplex printing on four sheets. In C311, the video controller 204 sends to the engine control unit 301 a print reservation command to feed paper from cassette 1 and eject it to the duplex unit (illustrated as "Cassette ⇒ Duplex") as the first page reservation. This page is reserved for the front side of the first sheet. In C312, the video controller 204 sends to the engine control unit 301 a print reservation command to feed paper from cassette 1 and eject it to the duplex unit as the second page reservation. This page is reserved for the front side of the second sheet. In C313, the video controller 204 sends to the engine control unit 301 a print reservation command to feed paper from the duplex unit and eject it to the ejection unit outside the machine (illustrated as "Duplex ⇒ FD") as the third page reservation. This page is reserved for the back side of the first sheet. These operations are repeated for the remaining reservation IDs in the same manner (C314, C315, C316, C317, C318). As a result, print reservation commands are sent from the video controller 204 to the engine control unit 301 in the following order: front side of 1st sheet ⇒ front side of 2nd sheet ⇒ back side of 1st sheet ⇒ front side of 3rd sheet ⇒ back side of 2nd sheet ⇒ front side of 4th sheet ⇒ back side of 3rd sheet ⇒ back side of 4th sheet.

[0036] Next, in C319, the video controller 204 instructs the engine control unit 301 to start printing for the page reserved by the print reservation command (start printing the first page), and the printer 200 starts the print operation. In C320, after receiving the print start command, the engine control unit 301 starts the image formation sequence, outputs a / TOP signal to the video controller 204, and performs image formation. Meanwhile, the video controller 204 outputs a video signal (not shown) in synchronization with the / TOP signal. After finishing outputting the video signal for the first page, the video controller 204 outputs a print start command for the next page reserved for printing to the engine control unit 301 in C321 (start printing the second page). In C322, after receiving the print start command, the engine control unit 301 starts the image formation sequence, outputs a / TOP signal to the video controller 204, and performs image formation. Thereafter, similar processing is repeated for the remaining pages reserved for printing (reserved pages 3 to 8). As indicated by the reservation commands C311 to C318, having one sheet waiting in the duplex conveying path 33 and feeding the sheet alternately from the cassette 1 and the duplex conveying path 33 will be referred to below as "two-sheet alternating double-sided printing." Note that alternating double-sided printing is not limited to two-sheet alternating double-sided printing, and may be other alternating double-sided printing such as three-sheet alternating double-sided printing.

[0037] FIG. 6 shows the image formation sequence when forming images according to the communication sequence for two-sheet alternating double-sided printing described in FIG. 5. FIG. 6 also shows (i) printer 200 operation (engine operation), (ii) / TOP signal, and (iii) video data. It also shows (iv) a paper feed signal (cassette paper feed signal) when feeding paper from cassette 1, (v) a paper feed signal (double-sided paper feed signal) when feeding paper from double-sided conveyance path 33, (vi) passing the secondary transfer roller position, and (vii) secondary transfer voltage. In the following description of double-sided printing, the first printed side of a sheet 2 fed from cassette 1 and ejected to the double-sided unit is referred to as the front side, and the side printed when fed from the double-sided unit and ejected outside printer 200 is referred to as the back side.

[0038] (Paper included) As described above, when the engine control unit 301 receives a print start command (print ID = 1) for the print reservation command for the front side of the first sheet (first page reservation), it starts the pre-rotation sequence. After completing the pre-rotation sequence, the engine control unit 301 outputs a / TOP signal (100-1-S) to start printing the first sheet. The engine control unit 301 forms a toner image on the intermediate transfer belt 12 in accordance with video data 1-S sent from the video controller 204. A predetermined time after the / TOP signal, the engine control unit 301 outputs a paper feed signal to start sheet transport from cassette 1 (101-1-S) in synchronization with the timing when the toner image formed on the intermediate transfer belt 12 reaches the secondary transfer roller position. When the registration sensor 19 detects the leading edge of sheet 2 fed from cassette 1, the engine control unit 301 accelerates or decelerates the sheet transport so that the leading edge of the toner image formed on the intermediate transfer belt 12 and the leading edge of sheet 2 coincide at the secondary transfer roller position. A predetermined time after the paper feed instruction is given, the toner image formed on the intermediate transfer belt 12 and the fed sheet 2 pass through the secondary transfer section (102-1-S), and the toner image 1-S is transferred from the intermediate transfer belt 12 onto the sheet 2. At this time, a first voltage for printing (hereinafter also referred to as a print voltage) is applied to the secondary transfer roller 9 by the secondary transfer power supply 41 in synchronization with the sheet 2 passing through the secondary transfer section. This causes the toner image on the intermediate transfer belt 12 to be secondarily transferred onto the sheet 2. The toner image transferred onto the sheet 2 is heated and fixed by the fixing section 13, and image formation on the front side of the first sheet is completed. The sheet 2 on which the image formation on the front side of the first sheet has been performed is transported to the duplex unit (duplex conveying path 33).

[0039] Similarly, upon receiving a print start command (print ID=2) for the print reservation command for the front side of the second sheet (second page reservation), the engine control unit 301 outputs a / TOP signal (100-2-S) to start printing the front side of the second sheet. The engine control unit 301 forms a toner image in accordance with the video data 2-S. After a predetermined time has elapsed since the / TOP signal was received, the engine control unit 301 feeds paper from cassette 1 and starts sheet transport (101-2-S). After a predetermined time has elapsed since issuing the paper feed command, the toner image formed on the intermediate transfer belt 12 and the fed sheet 2 pass through the secondary transfer unit, where the toner image 2-S is transferred (102-2-S). Then, the sheet 2 is heated and fixed by the fixing unit 13, and then transported to the duplex unit.

[0040] Next, upon receiving a print start command (print ID=1) for the print reservation command for the back side of the first sheet (reserving the third page), the engine control unit 301 outputs a / TOP signal (100-1-D) to start printing the back side of the first sheet. A predetermined time after the / TOP signal, the engine control unit 301 outputs a paper feed signal to start sheet transport from the duplex unit (101-1-D). A predetermined time after issuing the paper feed command, the toner image formed on the intermediate transfer belt 12 and the fed sheet 2 pass through the secondary transfer unit, where the toner image 1-D is transferred onto the sheet 2 (102-1-D), and is then heated and fixed by the fixing unit 13. The sheet 2 (first sheet) on which image formation has been completed on both the front and back sides is discharged outside the printer 200.

[0041] The engine control unit 301 repeats the above operation for four sheets, and when image formation on all sheets is completed, executes a post-rotation sequence, and after the final sheet 2 (the fourth sheet) passes through the secondary transfer unit, stops the voltage output that has been applied to the secondary transfer roller 9. This completes the series of printing operations.

[0042] (No paper) FIG. 7 shows an image formation sequence that follows the two-sheet alternating double-sided printing communication sequence described in FIG. 5, and during execution of the image formation sequence described in FIG. 6, cassette 1 runs out of sheets for the fourth sheet, making it impossible to continue printing. (i) to (vii) are the same as in FIG. 6. (viii) shows the detection result of sheet sensor 11. The symbols in FIG. 7 are the same as in FIG. 6. The fourth sheet that has run out of paper corresponds to the target sheet described above.

[0043] As shown in Figure 7, the engine control unit 301 detects the absence of the fourth sheet in cassette 1 at timing 110-P using sheet sensor 11, which detects the presence or absence of sheet 2 in cassette 1. At timing 110-P, the / TOP signal (100-4-S) for the front side of the fourth sheet has already been output, completing image formation on the front side of the fourth sheet, and the / TOP signal (100-3-D) for the back side of the next, third sheet has been output, starting image formation on the back side of the third sheet. Here, toner image 4-S is the toner image formed in the immediately preceding print, and toner image 3-D is the toner image formed in the back side print. In other words, the presence or absence of a sheet in cassette 1 is detected by sheet sensor 11 after the immediately preceding print and back side print have started.

[0044] (Through voltage) Here, because the paper-out state has occurred, the fourth sheet 2 is not transported to the secondary transfer unit. Therefore, the toner image on the front side of the fourth sheet (4-S indicated by the dashed line in FIG. 7(vi)) formed on the intermediate transfer belt 12 must be passed through the secondary transfer unit, and the toner image 3-D on the back side of the third sheet formed thereafter must be transferred to the sheet 2 transported from the duplex unit. Note that "passing through" refers to a process in which the toner image is passed through the secondary transfer unit while remaining carried on the intermediate transfer belt 12, without performing secondary transfer (hereinafter referred to as "passing through process"). The engine control unit 301 performs the pass-through process in response to the sheet sensor 11 detecting the paper-out state.

[0045] If toner adheres to the secondary transfer roller 9 when the toner image on the front side of the fourth sheet is passed through, the toner adhering to the secondary transfer roller 9 will adhere to the sheet 2 during the secondary transfer process for the back side of the subsequent third sheet, resulting in staining of the back side of the sheet 2. Normally, when a toner image that is not to be transferred to the sheet 2 is passed through in this manner, the polarity of the voltage applied to the secondary transfer roller 9 by the secondary transfer power supply 41 is changed. Specifically, the engine control unit 301 applies to the secondary transfer roller 9 a second voltage (hereinafter also referred to as a pass-through voltage) whose polarity is opposite to that of the first voltage (print voltage) used when the secondary transfer power supply 41 transfers the toner image to the sheet 2. In other words, the second application operation described above is performed so that the toner image 4-S formed in the immediately preceding print does not adhere to the secondary transfer roller 9 by attracting the toner toward the intermediate transfer belt 12 by electrostatic force.

[0046] However, there is a small but certain percentage of toner particles that do not have normal electrostatic properties (hereinafter referred to as inverted toner). Such toner particles are attracted to the secondary transfer roller 9 and adhere to the surface of the secondary transfer roller 9 even when a through voltage is applied to the secondary transfer roller 9. Inverted toner can also be said to be toner that adheres to the secondary transfer roller 9 when the toner image formed in the immediately preceding print passes through the secondary transfer section. Such inverted toner can cause backside contamination of the subsequent sheet 2, but by optimizing the through voltage, the amount is kept to a small amount, and is usually at a level where the user does not notice the backside contamination (hereinafter referred to as an acceptable level). In this way, even if cassette 1 runs out of paper during double-sided printing, the sheet 2 that has already been printed on one side can be printed on the other side normally and ejected from the machine without being wasted.

[0047] (Discharge processing) The proportion of reversed toner in the cartridge may increase depending on factors such as the environment in which the printer 200 is installed and the degree of cartridge wear. In other words, when certain conditions are met, the proportion of reversed toner may be greater than when the conditions are not met. In such cases, even when a through voltage is applied by the secondary transfer power supply 41, more reversed toner may adhere to the secondary transfer roller 9 than expected. When the amount of reversed toner is greater than expected or when a user requests a further reduction in the level of backside contamination, a discharge process may be performed as a cleaning operation of the secondary transfer roller 9 to reliably prevent backside contamination of subsequent sheets. The following process, which is generally performed after a through voltage is applied to the secondary transfer roller 9, is known as the discharge process. That is, this process (hereinafter referred to as the discharge process) involves rotating the secondary transfer roller 9 by at least the circumference of the secondary transfer roller 9 while applying a third voltage (hereinafter referred to as the discharge voltage) of the same polarity as the first voltage (print voltage) used when transferring a normal toner image to the sheet 2. In the discharge process, the first application operation described above is executed, whereby the reverse toner adhering to the surface of the secondary transfer roller 9 is moved to the intermediate transfer belt 12.

[0048] However, if a paper out occurs when the / TOP signal (100-3-D) for the back side of the subsequent third sheet has already been output and image formation has begun, as shown in Figure 7, such as when printing is performed at the fastest throughput, the following problem occurs: That is, the gap T_gap in the transport direction between the trailing edge of the toner image on the front side of the fourth sheet and the leading edge of the toner image on the back side of the third sheet may not be sufficient to perform the ejection process. Note that the gap T_gap (first gap) is the distance L_gap in the transport direction between the trailing edge of the preceding toner image and the leading edge of the succeeding toner image, expressed in time according to the transport speed.

[0049] In the present invention, if there is a possibility that back staining may not be suppressed to an acceptable level even if a through voltage is applied when a paper-out occurs during double-sided printing, the start timing of image formation is adjusted. In the present invention, by adjusting the start timing of image formation, it is possible to ensure an interval for performing the discharge process. Below, a method for preventing back staining by ensuring an interval for performing the discharge process of the secondary transfer roller 9 will be described.

[0050] (Discharge necessity determination process in Example 1) Fig. 8(a) is a flowchart of the printing operation of the engine control unit 301 of the present invention. Note that the processes of S701 and S706 to S710 in Fig. 8 are the same as the processes of S101 to S106 in the flowchart described in Fig. 4, and therefore a description thereof will be omitted. Note that the engine control unit 301 is assumed to monitor the detection result of the sheet sensor 11 during the loop from S707 to S709 in Fig. 8(a).

[0051] When a print reservation command and a print start command are received in S701, the engine control unit 301 determines in S702 whether ejection is necessary, as described below. The ejection necessity determination is a process for determining whether ejection processing of the secondary transfer roller 9 is necessary if paper runs out during double-sided printing that is about to be performed. In S703, the engine control unit 301 determines whether ejection processing is necessary based on the necessity determination in S702. If the engine control unit 301 determines in S703 that ejection processing is necessary when paper runs out, the process proceeds to S704.

[0052] In S704, when determining the interval between subsequent image formations during printing, the engine control unit 301 adjusts the image interval so that an interval sufficient to perform the discharge process of the secondary transfer roller 9 is ensured even if a paper out occurs. That is, when calculating the interval between subsequent image formations, the engine control unit 301 sets the time required for the discharge process of the secondary transfer roller 9 (hereinafter referred to as the discharge time) T_cln as the time T_delta to be added to the normal image formation interval (described later) (hereinafter referred to as the added time). Specifically, the engine control unit 301 sets T_delta=T_cln. Here, in the first embodiment, the discharge time T_cln is set as the time required for the secondary transfer roller 9 to rotate by at least the circumferential length while the discharge voltage is applied, but the present invention is not limited to this example.

[0053] If the engine control unit 301 determines in S703 that ejection processing is not necessary when paper is out, the process proceeds to S705. In S705, the engine control unit 301 sets the added time T_delta to the normal image formation interval to 0 (T_delta=0) because a normal image formation interval will suffice during subsequent printing. Here, the normal image formation interval is the image formation interval for achieving printing productivity predetermined as a product specification of the printer 200. For example, for a product that prints 60 sheets per minute, the normal image formation interval is 1 second.

[0054] The timing at which the next printing operation can be started in S708 is the timing at which a predetermined time T_next has elapsed since the / TOP signal was output in S707. The image formation interval control unit 320 determines the predetermined time T_next using the normal image formation interval T_intvl and the added time T_delta set in S704 and S705, as in the following formula (1) or formula (2). Note that although the following formulas express each quantity in terms of time, they may also be expressed in terms of distance based on time and the conveying speed. If the next page is the front: T_next=T_intvl formula (1) If the next page is the back: T_next=T_intvl+T_delta Equation (2)

[0055] As described above, the discharge process in the first embodiment is intended to ensure an interval between the toner image on the front side and the toner image on the back side so that the discharge process of the secondary transfer roller 9 can be performed. Therefore, if the next page is the front side, the discharge process is not necessary. Therefore, if the next printing operation is printing the front side, the / TOP signal should always be output at the normal image formation interval T_intvl as shown in equation (1). On the other hand, if the next printing operation is printing the back side, the image formation interval is determined by adding the additional time T_delta to the normal image formation interval T_intvl as shown in equation (2).

[0056] For the back side, if it is predicted that the back side contamination will be below the allowable level, the additional time T_delta is set to 0 (S705), resulting in the same predetermined time T_next (=T_intvl) as for the front side. On the other hand, if it is predicted that the back side contamination will exceed the allowable level, the additional time T_delta is set to the ejection time T_cln (S704), resulting in the predetermined time T_next (=T_intvl+T_cln) including the ejection time T_cln. As a result, if it is predicted that the back side contamination will exceed the allowable level (if the predetermined condition is satisfied), the engine control unit 301 can perform the ejection process after the through process when the sheet sensor 11 detects paper out during the loop from S707 to S709. Furthermore, if it is predicted that the back side contamination will be below the allowable level, there is no need to reserve time for the ejection process, allowing double-sided printing without reducing productivity.

[0057] Note that there are cases where product specifications predetermine that productivity during double-sided printing will be lower than that during single-sided printing due to constraints on the product's transport path configuration, etc. For such products, in the above-mentioned formulas (1) and (2), a larger image formation interval T_intvl' (>T_intvl) can be used instead of the normal image formation interval T_intvl for single-sided printing.

[0058] 8(b) is a flowchart of the ejection necessity determination described in S702 of FIG. 8(a), which is executed by the necessity determination unit 350. In S721 and S722, the necessity determination unit 350 applies a through voltage and estimates the amount of toner adhering to the surface of the secondary transfer roller 9 after the toner image of the front page passes through the secondary transfer unit. In S721, the necessity determination unit 350 determines the installation environment of the printer 200. Specifically, the necessity determination unit 350 determines whether the installation environment is high temperature or not based on the detection result of the environmental sensor 50.

[0059] It is generally known that when the installation environment is a high-temperature environment, the proportion of reverse-colored toner in the toner stored in the toner container increases, and the amount of reverse-colored toner transferred to the surface of the secondary transfer roller 9 during application of the through voltage increases. For this reason, the determination in S721 is made based on the installation environment temperature detected by the environmental sensor 50. If the necessity determination unit 350 determines in S721 that the installation environment of the printer 200 is a high-temperature environment based on the detection result of the environmental sensor 50, the process proceeds to S725. This is the case when it is estimated that a large amount of toner will remain on the surface of the secondary transfer roller 9 after the through voltage is applied. On the other hand, if the necessity determination unit 350 determines in S721 that the installation environment of the printer 200 is not a high-temperature environment based on the detection result of the environmental sensor 50, the process proceeds to S722.

[0060] In S725, the necessity determination unit 350 stores, for example, in RAM 209, the fact that a discharge process is necessary when paper runs out, and then ends the process. The temperature at which a high-temperature environment is determined, in other words, the threshold temperature used in the determination in S721, is determined experimentally in advance based on the correlation between the installation ambient temperature and the proportion of reversed toner in the cartridge. In the first embodiment, an ambient temperature of 28°C or higher is determined to be a high-temperature environment. That is, the threshold temperature is set to 28°C, and the necessity determination unit 350 determines that a temperature detected by the environmental sensor 50 is a "high temperature" if it is equal to or higher than the threshold temperature (28°C or higher). On the other hand, the necessity determination unit 350 determines that a temperature is not high if it is lower than the threshold temperature (lower than 28°C).

[0061] In S722, the necessity determination unit 350 determines whether the developer 8 is near the end of its life based on the remaining life information of the developer 8 attached to the printer 200. If the necessity determination unit 350 determines in S722 that the developer 8 is near the end of its life or has exceeded its life, the process proceeds to S725. This is the case when it is estimated that a large amount of toner has adhered to the surface of the secondary transfer roller 9 after the through voltage has been applied. On the other hand, if the necessity determination unit 350 determines in S722 that the developer 8 is not near the end of its life, the process proceeds to S723.

[0062] It is known that when the developing device 8 is nearing the end of its life or has exceeded its lifespan, the proportion of reverse toner in the toner stored in the toner container increases, and the amount of reverse toner transferred to the surface of the secondary transfer roller 9 during application of the through voltage increases. Therefore, if the developing device 8 is nearing the end of its lifespan, the necessity determination unit 350 stores, for example, in the RAM 209, in step S725, information indicating that a paper-out discharge process is required. The remaining lifespan of the developing device 8 is an index calculated by the engine control unit 301 based on the cumulative usage of consumable parts in the toner cartridge. The remaining lifespan of the developing device 8 is 100% when new and 0% when it reaches the replacement period. The determination of the end of the lifespan of the developing device 8 is experimentally determined in advance based on the correlation between the remaining lifespan information of the developing device 8 and the proportion of reverse toner in the toner cartridge. In the first embodiment, the developing device 8 is determined to be nearing the end of its lifespan when its remaining lifespan is less than 3%. That is, the threshold for determining the remaining life is set to 3%, and the necessity determination unit 350 determines that the developing device 8 is at the "end of life" when the remaining life is less than the threshold (less than 3%). On the other hand, the necessity determination unit 350 determines that the developing device 8 is not at the end of life when the remaining life is equal to or greater than the threshold (3% or greater).

[0063] Here, if the determination in S721 is No and the determination in S722 is also No, it is assumed that the amount of toner adhering to the surface of the secondary transfer roller 9 after the through voltage is applied is not large. However, some users may desire further reduction in backside contamination. For this reason, the determination in S723 is provided. In S723, the necessity determination unit 350 determines whether there is a user specification (backside contamination prevention user specification) that requests further reduction in the level of backside contamination. If the necessity determination unit 350 determines in S723 that there is a backside contamination prevention user specification, the process proceeds to S725, where the fact that a discharge process is required when paper runs out is stored in, for example, the RAM 209. The backside contamination prevention user specification is notified to the engine control unit 301 via the video interface processing unit 330 from a print setting driver in the host computer 1000 or the like. In S724, the necessity determination unit 350 estimates that the amount of toner adhering to the surface of the secondary transfer roller 9 after the through voltage is applied is not large, and there is no user specification to prevent backside contamination, so it stores in RAM 209, for example, that the ejection process is not necessary when there is no paper, and ends the process.

[0064] (Sequence using determination of necessity of discharge processing in Example 1) Fig. 9 shows the image formation sequence when, as in Fig. 7, sheet 2 in cassette 1 runs out after the fourth sheet and printing cannot continue, and applies the processing for the case where it is determined that ejection processing is necessary as a result of the ejection necessity determination explained in Fig. 8. That is, the predetermined time T_next when the next page is the front side is given by formula (1), and the predetermined time T_next when the next page is the back side is given by formula (2) where the added time T_delta is the ejection time T_cln. Note that Figs. 9(i) to (viii) are the same as Fig. 7.

[0065] As shown in FIG. 9, a wider interval T_gap' (second interval) than usual is ensured between the time when the trailing edge of toner image 4-S on the front side of the fourth sheet (which has run out of paper) passes the secondary transfer unit (102-4-S) and the time when the leading edge of toner image 3-D on the back side of the third sheet arrives (102-3-D). This interval is wider than the normal interval T_gap (first interval) shown in FIG. 7(vi) by the amount of the ejection time T_cln of the secondary transfer roller 9. Therefore, after applying the through voltage to the secondary transfer roller 9, the secondary transfer roller 9 can be rotated by more than its circumference while the ejection voltage is applied, thereby performing the ejection process. Therefore, the angle through which the secondary transfer roller 9 rotates while the portion corresponding to the second interval passes the secondary transfer unit is equal to or greater than 360 degrees. Furthermore, the angle through which the secondary transfer roller 9 rotates while the portion corresponding to the first interval passes the secondary transfer unit is equal to or less than 360 degrees.

[0066] The operation of performing backside printing at a first interval T_gap from the immediately preceding printing is referred to as the first operation, and the operation of performing backside printing at a second interval T_gap' from the immediately preceding printing, which is wider than the first interval, is referred to as the second operation. The engine control unit 301 is configured to be able to perform the first operation when a predetermined condition is not met, and to be able to perform the second operation when the predetermined condition is met. Furthermore, when the second operation is performed, the engine control unit 301 is also able to perform a discharge process (cleaning operation) on the secondary transfer roller 9 before the toner image formed in the backside printing reaches the secondary transfer unit. After the discharging process, the toner image 3-D formed in the second operation is transferred onto the rear surface of the third sheet 2, which is the re-conveyed sheet.

[0067] As described above, according to the first embodiment, when a paper out occurs during double-sided printing and there is a possibility that back staining will not be suppressed to an acceptable level even if a through voltage is applied, control is performed as follows. That is, the start timing of image formation is adjusted to ensure an interval at which the discharge process of the secondary transfer roller 9 can be performed. This makes it possible to prevent back staining. Furthermore, because the start timing of image formation is adjusted only in a situation where there is a possibility that back staining will not be suppressed to an acceptable level, there is no unnecessary decrease in productivity during normal times.

[0068] In the configuration of the first embodiment, a sheet sensor 11 is provided in the cassette 1 to detect the presence or absence of a sheet 2, and detects the presence or absence of a sheet 2 in the cassette 1. If the device has a configuration that allows the remaining amount of sheet 2 in the cassette 1 to be determined in stages, separately from the presence or absence of paper in the cassette 1, the above-described determination of whether or not to eject the sheet 2 can be performed, for example, when the remaining amount of sheet 2 is low. For example, the determination of whether or not to eject the sheet 2 may be performed when the remaining amount of sheet 2 is less than 10% of the paper stack capacity. Specifically, in the case of a cassette 1 with a paper stack capacity of 500 sheets, the determination of whether or not to eject the sheet 2 may be performed when the remaining amount of sheet 2 is less than 50 sheets. As a result, when the remaining amount of sheet 2 is 10% or more (50 or more sheets), the predetermined time T_next in equation (1) is set, even if the next page is the back side, thereby reducing the decrease in productivity. In other words, it is possible to further limit the cases in which productivity decreases. The configuration that determines whether or not to eject the sheet 2 when the remaining amount of sheet 2 is low can be applied to each of the embodiments described below.

[0069] In the first embodiment, as described with reference to FIG. 7 , at timing 110-P when the sheet sensor 11 detects the end of the fourth sheet, the / TOP signal (100-4-S) for the front side of the fourth sheet has already been output, completing image formation on the front side of the fourth sheet. The / TOP signal (100-3-D) for the back side of the third sheet has also been output, starting image formation on the front side of the third sheet. This case was described in the first embodiment. However, depending on the layout and dimensions of the components of the image forming apparatus or the paper size, the / TOP signal (100-4-S) for the front side of the fourth sheet may not have been output at timing 110-P when the end of the fourth sheet was detected. In such a case, it is possible to adjust the output timing of the / TOP signal (100-4-S) for the front side of the fourth sheet and subsequent sheets when the end of the fourth sheet occurs, using the determination result by the necessity determination unit 350 described in the first embodiment. By adjusting in this manner, it is possible to operate the secondary transfer roller 9 so that a sufficient interval is secured to perform the discharge process after the through voltage is applied.

[0070] As described above, according to the first embodiment, it is possible to suppress a decrease in productivity during double-sided printing. [Example]

[0071] In the first embodiment, at the start of printing, a determination was made as to whether or not ejection was necessary based on the installation environment of the printer 200 and the remaining life of the developing unit 8. If it was determined that ejection processing was necessary as a result of the determination as to whether or not ejection was necessary, this result was applied to the entire print job, and the interval between / TOP signals from the front to the back was always wide during execution of the job (see T_gap' in FIG. 9(i)).

[0072] In the second embodiment, the necessity of ejection is determined for each page. Specifically, the necessity of ejection is determined using image information of the page in addition to the installation environment of the printer 200 and the remaining life of the developing unit 8, as described in the first embodiment. As a result, when paper runs out during double-sided printing, the start timing of image formation is adjusted only in a situation where there is a possibility that back staining cannot be suppressed to an acceptable level even if a through voltage is applied.

[0073] (Discharge necessity determination process in Example 2) Fig. 10A is a flowchart of the printing operation of the engine control unit 301 of the embodiment 2. The processes of S901, S902, S903, and S908 to S910 in Fig. 10A are the same as the processes of S701, S706, S707, and S708 to S710 in Fig. 8A, and therefore the description thereof will be omitted.

[0074] In FIG. 8(a) of the first embodiment, after the print reservation command and print start command are received in S701 (Yes in S701), and before the pre-rotation sequence (S706), the ejection necessity determination and the additional time T_delta setting are performed. In the second embodiment, after the / TOP signal is output in the process of S903 and the print operation is started in accordance with the print reservation command for the first sheet, the engine control unit 301 determines whether ejection is necessary in S904. The ejection necessity determination in the second embodiment differs from the first embodiment in that the ejection necessity determination is performed for each page using image information for the page for which the / TOP signal is output. Details of the ejection necessity determination in the second embodiment will be described later.

[0075] In S905, the engine control unit 301 determines whether or not the discharge process is necessary. If the engine control unit 301 determines in S905 that the discharge process is necessary, the process proceeds to S906. When determining the interval for image formation of the next page, the image formation interval control unit 320 adjusts the interval so that a sufficient interval is secured for the discharge process of the secondary transfer roller 9 to be performed even if a paper out occurs. In S906, the engine control unit 301 sets the discharge time T_cln of the secondary transfer roller 9 as the additional time T_delta to be added to the normal image formation interval T_intvl when calculating the interval to the next page (T_delta=T_cln).

[0076] If the engine control unit 301 determines in S905 that the ejection process is not necessary, the process proceeds to S907. In S907, the engine control unit 301 sets the addition time T_delta to be added to the normal image formation interval T_intvl to 0 (T_delta=0) because the interval to the next page can be the normal image formation interval T_intvl. In S908, the engine control unit 301 waits for the timing when it is possible to start the printing operation for the next page. The process of S907 is the same as the process using equation (1) described in the first embodiment.

[0077] Fig. 10(b) is a flowchart of the ejection necessity determination performed by the necessity determination unit 350 after the / TOP signal for the page is output, as described in S904 of Fig. 10(a). Note that the processes of S921 to S923, S925, and S926 in Fig. 10(b) are the same as the processes of S721 to S725 in Fig. 8(b), and therefore their explanation will be omitted.

[0078] Before the determination process of S921, in S920, the necessity determination unit 350 determines whether the page following the current page is a back page. The discharge process is performed to ensure a gap between the toner image on the front side and the toner image on the back side so that the discharge process of the secondary transfer roller 9 can be performed. Therefore, if the necessity determination unit 350 determines in S920 that the next page is a front side, the process proceeds to S926, and stores, for example, in RAM 209, that the discharge process is not required.

[0079] After determining in S923 that no user designation has been made, in S924 the necessity determination unit 350 makes a determination using the image information of the page for which / TOP was issued. Here, S924 is executed when the next page is the back side, so the page in question refers to the preceding page, the front side. In S924, the necessity determination unit 350 determines whether the average density of the page in question is equal to or greater than a predetermined value. If the necessity determination unit 350 determines in S924 that the average density of the page in question is equal to or greater than the predetermined value, the process proceeds to S725; if it determines that the average density is less than the predetermined value, the process proceeds to S926.

[0080] When a page with a high average density, such as a solid image, runs out of paper, even if a through voltage is applied, a larger amount of reverse toner adheres to the secondary transfer roller 9 than usual. Therefore, if the average density is high, the process proceeds to S925, and the necessity determination unit 350 stores, for example, in the RAM 209, the information indicating that a discharge process is required. Note that the image information referred to here is the image information received by the video controller 204 from the host computer 1000 that is analyzed and notified to the engine control unit 301 by the video controller 204 for each page. The average density of a page is calculated as the ratio of the actual toner image to the image rendering resolution (e.g., 600 dpi). The predetermined value of the average density is determined experimentally in advance based on the correlation between the average density of a page and the amount of reverse toner adhering to the secondary transfer roller 9 after the through voltage is applied to that page. For example, in Example 2, the average density is determined to be 80% or higher. That is, the predetermined value in the determination process of S924 is set to 80%, and the necessity determination unit 350 determines that the discharge process is "necessary" when the average density is equal to or greater than the predetermined value (80% or greater). On the other hand, the necessity determination unit 350 determines that the discharge process is "unnecessary" when the average density is less than the predetermined value (less than 80%).

[0081] (Sequence using determination of necessity of discharge processing in Example 2) Figure 11 shows the image formation sequence when, as in Figure 6, sheet 2 in cassette 1 runs out on the fourth sheet and printing cannot continue, and processing is performed according to the flow explained in Figure 10. Note that Figures 11(i) to (viii) are the same as Figure 7. In "(iii) Video data" and "(vi) Passing the secondary transfer roller position" in Figure 11, it is shown that only the front side (4-S) of the fourth sheet has a high average density (dark), while the average density of the other pages is low (light).

[0082] The average density up to the back side of the second sheet (2-D) is low. Therefore, according to S924 of the flowchart of the second embodiment described in FIG. 10, it is determined that ejection processing is not required when there is no paper between the front side of the second sheet (2-S) and the back side of the first sheet (1-D), and between the front side of the third sheet (3-S) and the back side of the second sheet (2-D). Therefore, these pages are printed at the normal image formation interval. Even if any of these pages run out of paper, ejection processing is not required when there is no paper, so it is predicted that back smearing will not occur just by applying the through voltage, as in the sequence diagram of FIG. 6.

[0083] On the other hand, because the average density of the front side of the fourth sheet (4-S) is high, it is determined that ejection processing is necessary between the front side of the fourth sheet (4-S) and the back side of the third sheet (3-D). Therefore, a wider gap T_gap' than usual is secured between these pages, and if the fourth sheet runs out of paper, it is possible to apply a through voltage and then perform the ejection processing.

[0084] 10B, in the determination of whether ejection is necessary, the average density of the page is used as image information in the determination process of S924. However, other information such as the following can also be used as image information. That is, [Maximum toner density calculated for each area on the page] The necessity determination unit 350 divides the page into multiple regions and calculates the toner density per unit area for each region. If the toner density per unit area is extremely high in any region, the necessity determination unit 350 infers that a so-called solid image patch has been formed, and determines that discharging processing is necessary. · [Whether the image on the page is text-only or not] The necessity determining unit 350 determines that the ejection process is unnecessary if the image in the page is only text, and determines that the ejection process is necessary if the image contains other content besides text. The necessity determining section 350 may use the image information described above to determine whether or not the ejection process is necessary when paper runs out.

[0085] Thus, the predetermined conditions may include the following conditions in addition to the environmental temperature being equal to or higher than a predetermined temperature, the remaining life of the developing device 8 being less than a predetermined value, the user specifying the second operation, and the image density of the immediately preceding print being equal to or higher than a predetermined value. That is, the predetermined conditions may include the image of the immediately preceding print including non-text and the maximum density of a partial area of ​​the immediately preceding print being equal to or higher than a predetermined value. The predetermined conditions may also include at least one of these conditions. For example, the second operation may be executable when the user specifies the second operation, and the first operation may be executable when the user does not specify the second operation. In other words, the second operation may be executable only when the user specifies the second operation, and the first operation may always be executed when the user does not specify the second operation.

[0086] As described above, according to the second embodiment, when paper runs out during double-sided printing, it is determined for each page using image information whether or not there is a possibility that backside contamination will not be suppressed to an acceptable level even if through voltage is applied. This ensures that the start timing of image formation can be adjusted only between necessary pages to ensure an interval at which the discharge process of the secondary transfer roller 9 can be performed, and further makes it possible to prevent backside contamination while suppressing a decrease in productivity.

[0087] As described above, according to the second embodiment, it is possible to suppress a decrease in productivity during double-sided printing. [Example]

[0088] In the third embodiment, a further modification of the ejection necessity determination using image information described in the second embodiment is shown. In the second embodiment, whether the ejection process is necessary when a paper-out condition occurs is determined according to the ejection necessity determination flowchart in Fig. 10(b). If it is determined that the ejection process is necessary, the ejection time T_cln of the secondary transfer roller 9 is set as the value of the addition time T_delta to be added to the normal image formation interval T_intvl.

[0089] Here, even if the ejection necessity judgment determines that the ejection process is necessary, if the margin on the trailing edge of the page (hereinafter also referred to as the current page) is larger than a predetermined value, the image interval with the subsequent back-side image will actually be large. Therefore, an interval T_gap' or more necessary for the ejection process is ensured. As a result, when paper runs out, the ejection process can be performed after applying the through voltage, eliminating the need for the addition time T_delta that is added to the normal image formation interval T_intvl. In other words, if the margin between the trailing edge of the toner image formed in the previous print and the target sheet is longer than a predetermined length, the engine control unit 301 can execute the first operation even if the predetermined condition is met.

[0090] (Discharge necessity determination process in the third embodiment) Fig. 12 is a flowchart of the printing operation of the engine control unit 301 of the third embodiment. Fig. 12 differs from Fig. 10(a) in that a determination process of S930 is added after the process of S905, and other processes are assigned the same step numbers as Fig. 10(a) and descriptions thereof will be omitted.

[0091] If it is determined in S905 that the ejection process is necessary, the engine control unit 301 determines in S930 whether the margin on the trailing edge side of the page (hereinafter referred to as the trailing edge margin) is equal to or greater than a predetermined value. That is, the engine control unit 301 makes this determination using the amount of margin on the trailing edge side of the page (hereinafter referred to as the trailing edge side of the page) as image information for the page (own page) for which the / TOP signal was issued. As in the second embodiment, the information on the amount of margin on the trailing edge side of the page is obtained by analyzing image information received from the host computer 1000 by the video controller 204, and is notified by the video controller 204 to the engine control unit 301 for each page.

[0092] If the margin amount T_margin on the trailing edge of the page is larger than a predetermined value (e.g., the circumference of the secondary transfer roller 9), the image interval with the subsequent back-side image is substantially larger, as described above. Therefore, it is possible to perform the ejection process after applying the through voltage when paper is out, without widening the image formation interval T_intvl. Therefore, if the engine control unit 301 determines in S930 that the trailing edge margin of the page is equal to or greater than a predetermined value, the process proceeds to S907. That is, even if the ejection necessity determination in S904 determines that the ejection process is necessary, if the trailing edge margin of the page is large, the process proceeds to S907, and the engine control unit 301 sets the value of the added time T_delta to 0 in S907. If the trailing edge margin of the page is less than the predetermined value in S930, the engine control unit 301 proceeds to S906 to ensure time for the ejection process.

[0093] In this way, the predetermined value is set to, for example, the circumferential length of the secondary transfer roller 9, and if the margin amount T_margin is equal to or greater than the predetermined value (equal to or greater than the circumferential length of the secondary transfer roller 9), the engine control unit 301 sets the added time T_delta to 0 (S907). On the other hand, if the margin amount T_margin is less than the predetermined value (less than the circumferential length of the secondary transfer roller 9), the engine control unit 301 sets the ejection time T_cln to the added time T_delta (S906). The subsequent processing is the same as in the second embodiment. Note that although the margin amount T_margin is expressed as a time according to the conveying speed, it may also be expressed as a distance (L_margin).

[0094] (Sequence using determination of necessity of discharge processing in the third embodiment) FIG. 13 shows the image formation sequence when, as in FIG. 11, sheet 2 in cassette 1 runs out on the fourth sheet and printing cannot continue, and is performed according to the flow described in FIG. 12. Note that (i) to (viii) in FIG. 13 are the same as in FIG. 7. The lines "(iii) Video Data" and "(vi) Passing the Secondary Transfer Roller Position" in FIG. 13 show that only the front side (4-S) of the fourth sheet has a high average density, while the average density of the other pages is low. Also, it shows that the rear end of the front side (4-S) of the fourth sheet has a margin area with a margin amount T_margin that is longer than the circumference of the secondary transfer roller 9.

[0095] According to the flowchart in FIG. 12, the flow in FIG. 10(b) determines that the toner discharge process is necessary between toner image 4-S on the front side of the fourth sheet and toner image 3-D on the back side of the third sheet when there is no paper. However, because the engine control unit 301 determines in S930 that the image interval, including the trailing margin, is large, it proceeds to the process in S907 and performs printing at the normal image formation interval T_intvl. Even if the sheet sensor 11 indicates that there is no paper on the fourth sheet, a wide interval T_gap' has already been secured at the secondary transfer roller position, making it possible to perform the discharge process after applying the through voltage. Note that the interval T_gap' in the third embodiment includes the margin amount T_margin, as shown in FIG. 13(vi).

[0096] As described above, according to the third embodiment, by using the margin information at the rear end of the page as image information, the start timing of image formation is kept normal if there is enough space to perform the discharge process of the secondary transfer roller 9. Therefore, it is possible to prevent backside contamination while further suppressing a decrease in productivity.

[0097] As described above, according to the third embodiment, it is possible to suppress a decrease in productivity during double-sided printing. [Example]

[0098] In Examples 1 to 3, when determining the image formation interval during printing operation, a determination was made as to whether or not ejection processing was necessary to avoid backside contamination when paper ran out, and if it was determined that ejection processing to avoid backside contamination was necessary, an image formation interval was set at which the ejection processing could be performed. In other words, if it was determined that backside contamination could not be suppressed below an acceptable level, the image formation interval was extended regardless of whether paper ran out. In other words, the image interval was controlled to be set in advance so that ejection processing could be performed when paper actually ran out, and an interval was set at which the ejection processing could be performed even if paper actually ran out.

[0099] In the fourth embodiment, when a paper out state actually occurs, a determination is made as to whether or not to perform a discharge process to prevent backside contamination, and the processing operation after the paper out state is determined based on the result of the determination. Specifically, when a paper out state actually occurs during double-sided printing, the necessity and timing of the discharge process of toner adhering to the secondary transfer roller 9 are determined based on the paper gap between the pages where the paper out occurred and the result of the determination as to whether or not to perform a discharge process to prevent backside contamination. The paper gap refers to the distance between the trailing edge of the preceding sheet and the leading edge of the succeeding sheet, or the time corresponding to that distance. In the fourth embodiment, the processing operation after the paper out state is determined when the paper out state occurs, so that it is possible to prevent backside contamination while further suppressing a decrease in productivity.

[0100] (function block) Next, the functional blocks of the fourth embodiment will be described with reference to Fig. 14. Only the parts related to the fourth embodiment will be extracted and described here. When the detection unit 306 detects that there is no paper, the image formation control unit 307 of the fourth embodiment makes the following decision. That is, it decides the processing operation after there is no paper, depending on the interval between pages where there is no paper (hereinafter referred to as "paper-out page") determined by the image formation interval control unit 320 and whether or not there is a need for a discharge process to prevent back staining determined by the necessity determination unit 350.

[0101] 14 differs from FIG. 3 in that the necessity determination unit 350 notifies the image formation control unit 307 of information Ifa, and the image formation control unit 307 notifies the video interface processing unit 330 of information Ifb. The information Ifa is information regarding whether or not ejection processing of the secondary transfer roller 9 is necessary when a paper-out condition occurs (information determined in the flowchart of FIG. 10B). The information Ifb is information including a request for reservation-related processing when the sheet sensor 11 detects a paper-out condition, such as a request to cancel the print reservation for sheets after the paper-out condition page, or a request to the video controller 204 to reprint the final back page. Post-processing after a paper-out condition will be described later. The final back page is the back page after the page where the paper-out condition was detected and on which a toner image is formed. For example, if the fourth sheet 2 runs out of paper as in the previous embodiments, the final back page would be the back page of the third sheet 2.

[0102] (Processing after paper runs out) The image forming control unit 307 of the fourth embodiment determines the processing operation after paper out depending on whether or not ejection processing is required to prevent backside contamination when paper out occurs and the paper interval between pages where paper out has occurred. In the fourth embodiment, three processing operations after paper out are selected depending on the situation.

[0103] (Processing action after the first paper out: 1st action) The first is a processing operation selected when the ejection process to prevent backside contamination is not required when paper runs out. Specifically, the engine control unit 301 first applies a through voltage to the secondary transfer roller 9 when the toner image corresponding to the page where paper has run out passes the secondary transfer roller 9. Then, when the toner image corresponding to the final backside page passes the secondary transfer roller 9, it applies a print voltage to the secondary transfer roller 9, and discharges the sheet 2 onto which the toner image corresponding to the final backside page has been transferred, outside the machine. That is, in the first processing, the ejection process is not performed, and the toner image corresponding to the final backside page is transferred to the sheet, completing double-sided printing of this sheet. That is, in this operation, the toner image formed in the backside printing is transferred onto the re-conveyed sheet.

[0104] (Processing action after the second paper out: 2nd action) The second is a processing operation that is selected when a discharge process is required to prevent backside contamination when a paper-out occurs, and the time required for the discharge process by the secondary transfer roller 9 can be secured between sheets. Specifically, the engine control unit 301 first applies a through voltage to the secondary transfer roller 9 when the toner image for the page where the paper-out occurred passes the secondary transfer roller 9. Next, it applies a discharge voltage until the time required for the discharge process to discharge the toner adhering to the secondary transfer roller 9 has elapsed. Finally, it applies a print voltage to the secondary transfer roller 9 when the toner image corresponding to the last back side page passes the secondary transfer roller 9, and ejects the sheet 2 onto which the toner image corresponding to the last back side page has been transferred outside the machine. That is, in this operation, a cleaning operation of the secondary transfer roller 9 is performed before the toner image formed in the back surface printing reaches the secondary transfer section, and the toner image formed in the back surface printing is transferred to the re-transported sheet.

[0105] (Processing action after the third paper out: Third action) Finally, the third is a processing operation selected when a discharge process is required to prevent backside contamination when a paper-out occurs, but the time required for the discharge process by the secondary transfer roller cannot be secured between sheets. Specifically, the interval required for the discharge process cannot be secured between the paper-out page and the last backside page. For this reason, the engine control unit 301 first applies a through voltage to the secondary transfer roller 9 when the toner image corresponding to the paper-out page and the toner image corresponding to the last backside page pass the secondary transfer roller 9.

[0106] Next, the engine control unit 301 applies the ejection voltage until the time required to eject the toner adhering to the secondary transfer roller 9 has elapsed. Finally, the engine control unit 301 performs a reprint operation for the final back side page via the video interface processing unit 330, and ejects the sheet 2 onto which the image for the final back side page has been transferred to the outside of the machine. That is, in the third process, the formation of the toner image for the final back side page is restarted; in other words, the process is restarted from the output of the / TOP signal. This operation is called a reprint operation. In this operation, a cleaning operation is performed after the toner image formed in the back surface printing passes through the secondary transfer section, and the toner image formed by performing the back surface printing again is transferred to the re-transported sheet.

[0107] As described above, the engine control unit 301 of the fourth embodiment is configured to be able to execute the first operation when a predetermined condition is not met, and to execute the second or third operation depending on the interval between the immediately preceding print and the backside print when the predetermined condition is met. Specifically, the second operation is executed when the interval between the immediately preceding print and the backside print is long enough to execute a cleaning operation, and the third operation is executed when the interval is not long enough to execute a cleaning operation. The predetermined conditions are the same as those in the above-described embodiments. Furthermore, when a paper out occurs, a through process in which the toner image formed in the immediately preceding print passes through the secondary transfer unit is executed before any of the first, second, and third operations.

[0108] (Determination of paper out and post-processing determination of paper out in the fourth embodiment) 15A is a flowchart of the printing operation of the engine control unit 301 in the fourth embodiment. Note that the processes of S1401 to S1403 and S1405 to S1407 are the same as the processes of S901 to S903 and S908 to S910 in FIG. 10, and therefore description thereof will be omitted. The engine control unit 301 starts a printing operation such as outputting a / TOP signal in S1403, and while waiting for the timing when the next printing operation can be started in S1405, determines whether paper is out and what processing operation to perform after paper is out in S1404. Note that if the engine control unit 301 determines in S1405 that it is not the timing to start the next printing operation, it returns the process to S1404.

[0109] 15B is a flowchart of the paper out determination and post-paper out processing determination described in S1404 of FIG. 15A, which is performed by the image formation control unit 307. In S1420, the image formation control unit 307 determines whether or not paper has run out based on the detection result of the sheet sensor 11. If the image formation control unit 307 determines in S1402 that paper has not run out, it ends the process.

[0110] If the image forming control unit 307 determines in S1420 that paper has run out, it proceeds to S1421. In S1421, the image forming control unit 307 cancels the reservation for sheets after the page where paper ran out. For example, if paper has run out on the fourth sheet, the reservation for sheets from the fifth sheet onwards is cancelled. The image forming control unit 307 requests cancellation of the print reservation for sheets after the page where paper has run out via the video interface processing unit 330. This causes the video controller 204 to cancel the reservations for all pages except the final back side page to be ejected outside the machine.

[0111] In S1423, the image formation control unit 307 applies a through voltage to the secondary transfer roller 9 using the secondary transfer power supply 41 to pass through the toner image corresponding to the page where paper has run out. In S1424, the image formation control unit 307 determines whether the toner image corresponding to the page where paper has run out has passed the secondary transfer roller 9. If the image formation control unit 307 determines in S1424 that the toner image corresponding to the page where paper has run out has not passed through the secondary transfer roller 9, it returns the process to S1424. If the image formation control unit 307 determines that the toner image corresponding to the page where paper has run out has passed through the secondary transfer roller 9, it proceeds to S1422. In S1422, the image formation control unit 307 determines whether or not toner ejection is necessary. The determination of whether or not toner ejection is necessary when paper has run out in the fourth embodiment is the same process as the flow described in FIG. 10(b) in the second embodiment. The necessity determination unit 350 determines whether or not to eject the toner based on whether the next page is the back side, whether the installation environment is high, whether the remaining cartridge life is nearing the end, whether the user has specified backside soiling prevention, and whether the average density of the page in question is equal to or greater than a predetermined value.

[0112] In S1425, the image forming control unit 307 determines whether or not paper-out discharge processing is necessary based on the determination result of S1422. If the image forming control unit 307 determines in S1425 that discharge processing is "unnecessary," the process proceeds to S1428. In S1428, the image forming control unit 307 determines whether or not the toner image corresponding to the final back side page has reached the secondary transfer roller 9. If the image forming control unit 307 determines in S1428 that the toner image corresponding to the final back side page has not reached the secondary transfer roller 9, the process returns to S1428. If the image forming control unit 307 determines that the toner image corresponding to the final back side page has reached the secondary transfer roller 9, the process proceeds to S1429. In S1429, the image forming control unit 307 applies a print voltage to the secondary transfer roller 9 by the secondary transfer power supply 41 to perform a printing operation for the final back side page. In S1430, the image forming control unit 307 determines whether or not the final back side page has passed the secondary transfer roller 9. If the image forming control unit 307 determines in S1430 that the last back side page has not passed the secondary transfer roller 9, it returns the process to S1430, and if it determines that the last back side page has passed, it ends the process. This series of processes corresponds to the first process (first operation) of the three processing operations after the paper-out condition described above.

[0113] If the image formation control unit 307 determines in S1425 that the discharge process when paper is out is "necessary" based on the determination result of S1422, the process proceeds to S1426. In S1426, the image formation control unit 307 determines whether the time required for the discharge process can be secured between pages where paper has run out. If the image formation control unit 307 determines in S1426 that the required time can be secured, the process proceeds to S1427. In S1427, the image formation control unit 307 applies a discharge voltage to the secondary transfer roller 9 by the secondary transfer power supply 41 until the time required for the discharge process to prevent back staining has elapsed, and the process proceeds to S1428. The time required for the discharge process in the fourth embodiment is the time required to rotate the secondary transfer roller 9 by at least the circumference while the discharge voltage is applied, but is not limited to this example. This series of processes corresponds to the second process (second operation) of the three processing operations after paper has run out described above.

[0114] If the image forming control unit 307 determines in S1426 that the time required for the ejection process cannot be secured between the pages where paper has run out, the process proceeds to S1431. In S1431, the image forming control unit 307 applies a through voltage to the secondary transfer roller 9 from the secondary transfer power supply 41 in order to pass the toner image corresponding to the final back side page through. In S1432, the image forming control unit 307 determines whether the toner image corresponding to the final back side page has passed the secondary transfer roller 9. If the image forming control unit 307 determines in S1432 that the toner image corresponding to the final back side page has not passed the secondary transfer roller 9, the process returns to S1432, and if it determines that the toner image has passed through, the process proceeds to S1433.

[0115] In S1433, the image formation control unit 307 applies a discharge voltage to the secondary transfer roller 9 using the secondary transfer power supply 41. In S1434, the image formation control unit 307 determines whether the time required for the discharge process has elapsed. If the image formation control unit 307 determines in S1434 that the required time has not elapsed, it returns the process to S1434, and if it determines that the required time has elapsed, it proceeds to S1435. In S1435, the image formation control unit 307 requests the video controller 204 via the video interface processing unit 330 to perform a reprint operation for the final back side page, performs a print operation for the final back side page, and ends the process. This series of processes corresponds to the third process (third operation) of the three processing operations after the paper-out condition described above.

[0116] (Sequence using paper out determination and paper out post-processing determination in the fourth embodiment) Fig. 16(a) shows a processing operation sequence after paper-out according to the flow when ejection processing to prevent back staining is not required when paper-out occurs as described in Fig. 15. That is, this is the first case described above.

[0117] The symbols in Figure 16(a) are the same as those in Figure 7. The timing at which the sheet sensor 11 detects that the fourth sheet is out of paper in cassette 1 is set to timing 110-P. As shown in Figure 16(a)(ii), at timing 110-P, the / TOP signal (100-4-S) for the front side of the fourth sheet has already been output, and image formation on the front side of the fourth sheet has been completed. In addition, the / TOP signal (100-3-D) for the back side of the next, third sheet has been output, and image formation on the back side of the third sheet has begun.

[0118] In order to properly print on the back side of the third sheet 2, which has already been printed on one side, without wasting it, the following process is performed. That is, as shown in Figure 16(a)(vii), a through voltage is applied to the secondary transfer roller 9 until the toner image on the front side of the fourth sheet passes through the secondary transfer unit (102-4-S). Then, as shown in Figure 16(a)(vii), a print voltage is applied to the secondary transfer roller 9 until the toner image on the back side of the third sheet passes through the secondary transfer unit (102-3-D), and the sheet 2 onto which the toner image corresponding to the back side of the third sheet has been transferred is discharged outside the machine.

[0119] 16(b) shows the processing operation sequence after paper-out according to the flow when the discharge process is required to prevent backside contamination when paper-out occurs as described in FIG. 15, and the time required for the discharge process of the secondary transfer roller 9 can be secured between sheets. That is, this is the second case described above.

[0120] The symbols in Figure 16(b) are the same as those in Figure 7, and the differences from Figure 16(a) will be explained below. In Figure 16(b), a through voltage is applied to the secondary transfer roller 9 until the toner image on the front side of the fourth sheet passes through the secondary transfer unit (102-4-S) (Figure 16(b)(vii)). After that, a discharge voltage is applied to the secondary transfer roller 9 to discharge the toner adhering to the secondary transfer roller 9. In Figure 16(b), it is assumed that the time required for the discharge by the secondary transfer roller 9 is secured by the time T_gap corresponding to the sheet interval L_gap. After that, a print voltage is applied to the secondary transfer roller 9 until the toner image on the back side of the third sheet passes through the secondary transfer unit (102-3-D), and the sheet 2 onto which the toner image for the back side of the third sheet has been transferred is discharged outside the apparatus.

[0121] 16(c) shows a processing operation sequence after paper-out according to the flow when the discharge process is required to prevent backside contamination when paper-out occurs as described in FIG. 15, and the time required for the discharge process of the secondary transfer roller 9 cannot be secured between sheets. That is, this is the third case described above.

[0122] The symbols in Figure 16(c) are the same as those in Figure 7, and the differences from Figures 16(a) and (b) will be explained below. In Figure 16(c), the time T_gap between the front surface of the fourth sheet and the back surface of the third sheet does not provide the ejection time necessary to avoid back surface contamination. Therefore, a through voltage is applied to the secondary transfer roller 9 until both the toner image on the front surface of the fourth sheet and the toner image on the back surface of the third sheet pass through the secondary transfer unit (102-4-S, 102-3-D) (Figure 16(c)(vii)). After that, an ejection voltage is applied to the secondary transfer roller 9 to eject the toner adhering to the secondary transfer roller 9. After that, a reprint operation for the back surface of the third sheet is requested to the video controller 204 via the video interface processing unit 330, and the printing operation for the back surface of the third sheet is executed (second time 100-3-D).

[0123] As described above, according to the fourth embodiment, when paper-out occurs during double-sided printing and there is a possibility that back-side smearing may not be suppressed to an acceptable level even if through voltage is applied, the timing of the ejection process after the paper-out occurs is changed according to the inter-page gap between pages where paper has run out. This allows the ejection process to be performed only when paper-out occurs, making it possible to further prevent back-side smearing while suppressing a decrease in productivity. Note that in the fourth embodiment, the modifications described in the second embodiment, such as the maximum density and whether or not the image is text, can also be applied to S1422 in FIG. 15(b).

[0124] As described above, according to the fourth embodiment, it is possible to suppress a decrease in productivity during double-sided printing. [Example]

[0125] In the fifth embodiment, a further modification of the determination of the processing operation after paper out described in the fourth embodiment is shown. In the flowchart of the processing operation after paper out in FIG. 15(b), the processing operation after paper out is described in the case where a discharge process is required to prevent backside contamination when paper out occurs, and the paper gap required for the discharge process by the secondary transfer roller 9 cannot be secured. Specifically, since the paper gap between the page where paper out occurred and the subsequent final back side page is small (narrow), and the time required for the discharge process by the secondary transfer roller 9 cannot be secured, a through voltage is applied to the page where paper out occurred and the subsequent final back side page. Then, a discharge voltage is applied for a time longer than that required for the discharge process by the secondary transfer roller 9, and a reprint operation is performed for the subsequent final back side page.

[0126] In the fifth embodiment, even if the paper gap necessary for the ejection process to prevent backside smearing cannot be secured, if the total of the trailing edge margin and the paper gap in the paper-out page is a paper gap that can secure the time necessary for the ejection process, the ejection process to prevent backside smearing between images is executed. This makes it possible to execute the ejection process between the paper-out page and the subsequent final backside page without reprinting the subsequent final backside page, making it possible to prevent backside smearing while suppressing a decrease in productivity.

[0127] (Determination of paper-out state and determination of paper-out post-processing state in Example 5) Fig. 17 is a flowchart of the processing operation after paper is out in the fifth embodiment, and is performed by the image formation control unit 307. Fig. 17 differs from Fig. 15(b) in that the processing of S1440 is added after the processing of S1426, and the other processing steps are given the same step numbers as Fig. 15(b), and the explanations thereof will be omitted.

[0128] If it is determined in S1426 that the time required for the ejection process between sheets cannot be secured, then in S1440 the image formation control unit 307 makes a determination using the amount of margin on the trailing edge of the paper-out page. That is, in S1440 the image formation control unit 307 determines whether the trailing edge margin of the paper-out page is equal to or greater than a predetermined value. As in the third embodiment, the information on the amount of margin is obtained by analyzing the image information received by the video controller 204 from the host computer 1000, and is notified by the video controller 204 to the engine control unit 301 for each page.

[0129] If the image forming control unit 307 determines in S1440 that the margin amount T_margin on the trailing edge of the paper-out page is equal to or greater than a predetermined value, it proceeds to S1427. In this case, the image interval between the paper-out page and the subsequent back-side image is substantially increased, making it possible to perform the ejection process between the paper-out page and the image on the subsequent back-side page. The predetermined value in the fifth embodiment is the circumferential length of the secondary transfer roller 9. Therefore, even if it is determined in S1426 that the time required for the ejection process cannot be secured between the pages where paper has run out, if the margin amount T_margin on the trailing edge of the page is large, it proceeds to S1427 and the ejection voltage is set.

[0130] If the image formation control unit 307 determines in S1440 that the trailing edge margin is less than a predetermined value, it proceeds to S1431. The subsequent processing is the same as in the fourth embodiment. In this way, the predetermined value is, for example, the circumferential length of the secondary transfer roller 9, and the image formation control unit 307 proceeds to processing of S1427 if the margin amount T_margin is equal to or greater than the predetermined value (equal to or greater than the circumferential length of the secondary transfer roller 9). On the other hand, if the margin amount T_margin is less than the predetermined value (less than the circumferential length of the secondary transfer roller 9), the image formation control unit 307 proceeds to processing of S1431.

[0131] As described above, according to the fifth embodiment, by using the margin information of the rear end of the page as image information, the discharge process is performed between images when the total of the rear end margin information and the paper gap ensures a gap that allows the discharge process of the secondary transfer roller 9 to be performed. This makes it possible to further suppress a decrease in productivity while preventing back stains.

[0132] As described above, according to the fifth embodiment, it is possible to suppress a decrease in productivity during double-sided printing.

[0133] <Other embodiments> The present invention can also be realized by supplying a program that realizes one or more functions of the above-described embodiments to a system or device via a network or a storage medium, and having one or more processors in the computer of the system or device read and execute the program. It can also be realized by a circuit (e.g., ASIC) that realizes one or more functions.

[0134] The disclosure of this embodiment includes the following configuration. (Configuration 1) an image carrier; an intermediate transfer member onto which the toner image formed on the image carrier is transferred; a transfer member that transfers the toner image transferred onto the intermediate transfer member onto a sheet, the transfer member forming a transfer nip between itself and the intermediate transfer member; a storage section for storing a sheet; a detection means for detecting the presence or absence of a sheet in the storage section; a double-sided conveying path through which a sheet having an image formed on its front surface passes and is conveyed toward the transfer nip; a control means capable of executing backside printing and immediate-preceding printing; In the back side printing, a toner image to be transferred to a re-conveyed sheet transported from the double-sided transport path to the transfer nip is formed on the intermediate transfer body, and in the immediately preceding printing, a toner image to be transferred to a target sheet different from the re-conveyed sheet, which is transported to the transfer nip immediately before the re-conveyed sheet is transported to the transfer nip, is formed on the intermediate transfer body, the control means is capable of executing a first operation of executing the rear surface printing at a first interval from the immediately preceding print, and a second operation of executing the rear surface printing at a second interval from the immediately preceding print that is wider than the first interval, the control means is configured to be able to execute the first action when a predetermined condition is not satisfied, and to be able to execute the second action when the predetermined condition is satisfied; the predetermined condition includes at least one of the following: an environmental temperature is equal to or higher than a predetermined temperature; the remaining life of a developing device is less than a predetermined value; a user has designated the second operation; the image density of the immediately preceding print is equal to or higher than a predetermined value; the image of the immediately preceding print includes something other than text; and the maximum density of a partial area of ​​the immediately preceding print is equal to or higher than a predetermined value. When the detection unit detects that there is no sheet corresponding to the target sheet, the toner image formed in the second operation is transferred to the re-transported sheet. An image forming apparatus characterized by: (Configuration 2) an image carrier; an intermediate transfer member onto which the toner image formed on the image carrier is transferred; a transfer member that transfers the toner image transferred onto the intermediate transfer member onto a sheet, the transfer member forming a transfer nip between itself and the intermediate transfer member; a storage section for storing a sheet; a detection means for detecting the presence or absence of a sheet in the storage section; a double-sided conveying path through which a sheet having an image formed on its front surface passes and is conveyed toward the transfer nip; a control means capable of executing backside printing and immediate-preceding printing; In the back side printing, a toner image to be transferred to a re-conveyed sheet transported from the double-sided transport path to the transfer nip is formed on the intermediate transfer body, and in the immediately preceding printing, a toner image to be transferred to a target sheet different from the re-conveyed sheet, which is transported to the transfer nip immediately before the re-conveyed sheet is transported to the transfer nip, is formed on the intermediate transfer body, the control means is capable of executing a first operation of executing the rear surface printing at a first interval from the immediately preceding print, and a second operation of executing the rear surface printing at a second interval from the immediately preceding print that is wider than the first interval, the control means is configured to be able to execute the first action when a predetermined condition is not satisfied, and to be able to execute the second action when the predetermined condition is satisfied; when the detecting means detects that there is no sheet corresponding to the target sheet, the amount of toner adhering to the transfer member when the toner image formed in the immediately preceding print passes through the transfer nip is greater when the predetermined condition is satisfied than when the predetermined condition is not satisfied; An image forming apparatus characterized by: (Configuration 3) When the second operation is performed, the control means can perform a cleaning operation of the transfer member before the toner image formed in the back printing reaches the transfer nip. 3. The image forming apparatus according to claim 1, wherein: (Configuration 4) an application unit that applies a voltage to the transfer member so that a potential difference is generated between the transfer member and the intermediate transfer body; the control means is configured to control the applying means to perform a first applying operation and a second applying operation; In the first application operation, the toner charged to the normal polarity is subjected to an electrostatic force in a direction from the intermediate transfer body to the transfer member, and in the second application operation, the toner charged to the normal polarity is subjected to an electrostatic force in a direction from the transfer member to the intermediate transfer body, When the detection means detects that there is no sheet corresponding to the target sheet, the second application operation is executed when the toner image formed in the immediately preceding print passes through the transfer nip. 4. The image forming apparatus according to configuration 3. (Configuration 5) the first application operation is performed in the cleaning operation; 5. The image forming apparatus according to configuration 4. (Configuration 6) When the margin between the rear end of the toner image formed in the immediately preceding print and the target sheet is longer than a predetermined length, even if the predetermined condition is satisfied, if the detection means detects that there is no sheet corresponding to the target sheet, the control means is capable of executing the first operation. 6. The image forming apparatus according to any one of configurations 1 to 5, wherein: (Configuration 7) an image carrier; an intermediate transfer member onto which the toner image formed on the image carrier is transferred; a transfer member that transfers the toner image transferred onto the intermediate transfer member onto a sheet, the transfer member forming a transfer nip between itself and the intermediate transfer member; a storage section for storing a sheet; a detection means for detecting the presence or absence of a sheet in the storage section; a double-sided conveying path through which a sheet having an image formed on its front surface passes and is conveyed toward the transfer nip; a control means capable of executing backside printing and immediate-preceding printing; In the back side printing, a toner image to be transferred to a re-conveyed sheet transported from the double-sided transport path to the transfer nip is formed on the intermediate transfer body, and in the immediately preceding printing, a toner image to be transferred to a target sheet different from the re-conveyed sheet, which is transported to the transfer nip immediately before the re-conveyed sheet is transported to the transfer nip, is formed on the intermediate transfer body, the control means is capable of executing a first operation of executing the rear surface printing at a first interval from the immediately preceding print, and a second operation of executing the rear surface printing at a second interval from the immediately preceding print that is wider than the first interval, the control means is configured to be able to execute the first action when the second action is not designated by a user, and to be able to execute the second action when the second action is designated by a user; When the detection unit detects that there is no sheet corresponding to the target sheet, the toner image formed in the second operation is transferred to the re-transported sheet. An image forming apparatus characterized by: (Configuration 8) When the second operation is performed, the control means can perform a cleaning operation of the transfer member before the toner image formed in the back printing reaches the transfer nip. 8. The image forming apparatus according to configuration 7. (Configuration 9) an application unit that applies a voltage to the transfer member so that a potential difference is generated between the transfer member and the intermediate transfer body; the control means is configured to control the application means to execute a first application operation and a second application operation, wherein in the first application operation, the toner charged to the normal polarity is subjected to an electrostatic force in a direction from the intermediate transfer body to the transfer member, and in the second application operation, the toner charged to the normal polarity is subjected to an electrostatic force in a direction from the transfer member to the intermediate transfer body, When the detection means detects that there is no sheet corresponding to the target sheet, the second application operation is executed when the toner image formed in the immediately preceding print passes through the transfer nip. 9. The image forming apparatus according to configuration 8, (Configuration 10) the first application operation is performed in the cleaning operation; 10. The image forming apparatus according to configuration 9, (Configuration 11) When the margin between the rear end of the toner image formed in the immediately preceding print and the target sheet is longer than a predetermined length, and when the detection means detects that there is no sheet corresponding to the target sheet even if the designation is made, the control means is capable of executing the first operation. 11. The image forming apparatus according to any one of configurations 7 to 10. (Configuration 12) an image carrier; an intermediate transfer member onto which the toner image formed on the image carrier is transferred; a transfer member that transfers the toner image transferred onto the intermediate transfer member onto a sheet, the transfer member forming a transfer nip between itself and the intermediate transfer member; a storage section for storing a sheet; a detection means for detecting the presence or absence of a sheet in the storage section; a double-sided conveying path through which a sheet having an image formed on its front surface passes and is conveyed toward the transfer nip; a control means capable of executing backside printing and immediate-preceding printing; In the back side printing, a toner image to be transferred to a re-conveyed sheet transported from the double-sided transport path to the transfer nip is formed on the intermediate transfer body, and in the immediately preceding printing, a toner image to be transferred to a target sheet different from the re-conveyed sheet, which is transported to the transfer nip immediately before the re-conveyed sheet is transported to the transfer nip, is formed on the intermediate transfer body, the control means is capable of executing a first operation of executing the rear surface printing at a first interval from the immediately preceding print, and a second operation of executing the rear surface printing at a second interval from the immediately preceding print that is wider than the first interval, the control means is configured to be able to execute the first action when the second action is not designated by a user, and to be able to execute the second action when the second action is designated by a user; when the detecting means detects that there is no sheet corresponding to the target sheet, the amount of toner adhering to the transfer member when the toner image formed in the immediately preceding print passes through the transfer nip is greater when the designation is made than when the designation is not made; An image forming apparatus characterized by: (Configuration 13) the transfer member rotates an angle of 360 degrees or more while the portion corresponding to the second interval passes through the transfer nip; 13. The image forming apparatus according to configuration 12. (Configuration 14) the transfer member rotates an angle of less than 360 degrees while the portion corresponding to the first interval passes through the transfer nip; 14. The image forming apparatus according to claim 13, (Configuration 15) an image carrier; an intermediate transfer member onto which the toner image formed on the image carrier is transferred; a transfer member that transfers the toner image transferred onto the intermediate transfer member onto a sheet, the transfer member forming a transfer nip between itself and the intermediate transfer member; a storage section for storing a sheet; a detection means for detecting the presence or absence of a sheet in the storage section; a double-sided conveying path through which a sheet having an image formed on its front surface passes and is conveyed toward the transfer nip; a control means capable of executing backside printing and immediate-preceding printing; In the back side printing, a toner image to be transferred to a re-conveyed sheet transported from the double-sided transport path to the transfer nip is formed on the intermediate transfer body, and in the immediately preceding printing, a toner image to be transferred to a target sheet different from the re-conveyed sheet, which is transported to the transfer nip immediately before the re-conveyed sheet is transported to the transfer nip, is formed on the intermediate transfer body, the control means is capable of executing a first operation of transferring the toner image formed in the back surface printing onto the re-conveying sheet, a second operation of performing a cleaning operation of the transfer member before the toner image formed in the back surface printing reaches the transfer nip and transferring the toner image formed in the back surface printing onto the re-conveying sheet, and a third operation of performing the cleaning operation after the toner image formed in the back surface printing has passed through the transfer nip and transferring the toner image formed by performing the back surface printing again onto the re-conveying sheet, the control means is configured to be able to execute the first operation when a predetermined condition is not satisfied, and to be able to execute the second operation or the third operation depending on the interval between the immediately preceding print and the back surface print when the predetermined condition is satisfied, the predetermined condition includes at least one of the following: an environmental temperature is equal to or higher than a predetermined temperature; the remaining life of a developing device is less than a predetermined value; a user has designated the second operation; the image density of the immediately preceding print is equal to or higher than a predetermined value; the image of the immediately preceding print includes something other than text; and the maximum density of a partial area of ​​the immediately preceding print is equal to or higher than a predetermined value. when the detection means detects that there is no sheet corresponding to the target sheet, the toner image formed in the immediately preceding print passes through the transfer nip before the first operation, the second operation, or the third operation; An image forming apparatus characterized by: (Configuration 16) the control means executes the second operation when the interval between the immediately preceding print and the rear surface print is long enough to execute the cleaning operation, and executes the third operation when the interval is not long enough to execute the cleaning operation. 16. The image forming apparatus according to configuration 15. (Configuration 17) an application unit that applies a voltage to the transfer member so that a potential difference is generated between the transfer member and the intermediate transfer body; the control means is configured to control the applying means to perform a first applying operation and a second applying operation; In the first application operation, the toner charged to the normal polarity is subjected to an electrostatic force in a direction from the intermediate transfer body to the transfer member, and in the second application operation, the toner charged to the normal polarity is subjected to an electrostatic force in a direction from the transfer member to the intermediate transfer body, When the detection means detects that there is no sheet corresponding to the target sheet, the second application operation is executed when the toner image formed in the immediately preceding print passes through the transfer nip. 16. The image forming apparatus according to configuration 15. (Configuration 18) In the third operation, the second application operation is performed when the toner image formed in the back printing passes through the transfer nip before the cleaning operation. 18. The image forming apparatus according to configuration 17. (Configuration 19) the first application operation is performed in the cleaning operation; 19. The image forming apparatus according to Configuration 17 or 18. (Configuration 20) When the detection means detects that there is no sheet corresponding to the target sheet, if the margin between the rear end of the toner image formed in the immediately preceding print and the target sheet is longer than a predetermined length, the control means can perform the second operation even if the cleaning operation cannot be performed between the immediately preceding print and the back side print. 20. The image forming apparatus according to any one of Configurations 15 to 19, [Explanation of symbols]

[0135] 1 cassette 5 Photosensitive drum 9 Secondary transfer roller 11 Sheet Sensor 12 Intermediate transfer belt 33 Double-sided transport path 301 Engine control unit

Claims

1. an image carrier; an intermediate transfer member onto which the toner image formed on the image carrier is transferred; a transfer member that transfers the toner image transferred onto the intermediate transfer member onto a sheet, the transfer member forming a transfer nip between itself and the intermediate transfer member; a storage section for storing a sheet; a detection means for detecting the presence or absence of a sheet in the storage section; a double-sided conveying path through which a sheet having an image formed on its front surface passes and is conveyed toward the transfer nip; a control means capable of executing backside printing and immediate-preceding printing; In the back side printing, a toner image to be transferred to a re-conveyed sheet transported from the double-sided transport path to the transfer nip is formed on the intermediate transfer body, and in the immediately preceding printing, a toner image to be transferred to a target sheet different from the re-conveyed sheet, which is transported to the transfer nip immediately before the re-conveyed sheet is transported to the transfer nip, is formed on the intermediate transfer body, the control means is capable of executing a first operation of executing the rear surface printing at a first interval from the immediately preceding print, and a second operation of executing the rear surface printing at a second interval from the immediately preceding print that is wider than the first interval, the control means is configured to be able to execute the first operation when a predetermined condition is not satisfied, and to be able to execute the second operation when the predetermined condition is satisfied, the predetermined condition includes at least one of the following: an environmental temperature is equal to or higher than a predetermined temperature; the remaining life of a developing device is less than a predetermined value; a user has designated the second operation; the image density of the immediately preceding print is equal to or higher than a predetermined value; the image of the immediately preceding print includes other than text; and the maximum density of a partial area of ​​the immediately preceding print is equal to or higher than a predetermined value. When the detection means detects that there is no sheet corresponding to the target sheet, the toner image formed in the second operation is transferred to the re-transported sheet. An image forming apparatus characterized by:

2. an image carrier; an intermediate transfer member onto which the toner image formed on the image carrier is transferred; a transfer member that transfers the toner image transferred onto the intermediate transfer member onto a sheet, the transfer member forming a transfer nip between itself and the intermediate transfer member; a storage section for storing a sheet; a detection means for detecting the presence or absence of a sheet in the storage section; a double-sided conveying path through which a sheet having an image formed on its front surface passes and is conveyed toward the transfer nip; a control means capable of executing backside printing and immediate-preceding printing; In the back side printing, a toner image to be transferred to a re-conveyed sheet transported from the double-sided transport path to the transfer nip is formed on the intermediate transfer body, and in the immediately preceding printing, a toner image to be transferred to a target sheet different from the re-conveyed sheet, which is transported to the transfer nip immediately before the re-conveyed sheet is transported to the transfer nip, is formed on the intermediate transfer body, the control means is capable of executing a first operation of executing the rear surface printing at a first interval from the immediately preceding print, and a second operation of executing the rear surface printing at a second interval from the immediately preceding print that is wider than the first interval, the control means is configured to be able to execute the first operation when a predetermined condition is not satisfied, and to be able to execute the second operation when the predetermined condition is satisfied, when the detecting means detects that there is no sheet corresponding to the target sheet, the amount of toner adhering to the transfer member when the toner image formed in the immediately preceding print passes through the transfer nip is greater when the predetermined condition is satisfied than when the predetermined condition is not satisfied; An image forming apparatus characterized by:

3. When the second operation is performed, the control unit can perform a cleaning operation of the transfer member before the toner image formed in the back printing reaches the transfer nip.

3. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

4. an application unit that applies a voltage to the transfer member so that a potential difference is generated between the transfer member and the intermediate transfer body; the control means is configured to control the applying means to perform a first applying operation and a second applying operation; In the first application operation, the toner charged to the normal polarity is subjected to an electrostatic force in a direction from the intermediate transfer body to the transfer member, and in the second application operation, the toner charged to the normal polarity is subjected to an electrostatic force in a direction from the transfer member to the intermediate transfer body, When the detecting means detects that there is no sheet corresponding to the target sheet, the second application operation is executed when the toner image formed in the immediately preceding print passes through the transfer nip.

4. The image forming apparatus according to claim 3, wherein the image forming apparatus is a recording medium.

5. the first application operation is performed in the cleaning operation; 5. The image forming apparatus according to claim 4.

6. When the margin between the rear end of the toner image formed in the immediately preceding print and the target sheet is longer than a predetermined length, and when the detection means detects that there is no sheet corresponding to the target sheet even if the predetermined condition is satisfied, the control means is capable of executing the first operation.

3. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

7. an image carrier; an intermediate transfer member onto which the toner image formed on the image carrier is transferred; a transfer member that transfers the toner image transferred onto the intermediate transfer member onto a sheet, the transfer member forming a transfer nip between itself and the intermediate transfer member; a storage section for storing a sheet; a detection means for detecting the presence or absence of a sheet in the storage section; a double-sided conveying path through which a sheet having an image formed on its front surface passes and is conveyed toward the transfer nip; a control means capable of executing backside printing and immediate-preceding printing; In the back side printing, a toner image to be transferred to a re-conveyed sheet transported from the double-sided transport path to the transfer nip is formed on the intermediate transfer body, and in the immediately preceding printing, a toner image to be transferred to a target sheet different from the re-conveyed sheet, which is transported to the transfer nip immediately before the re-conveyed sheet is transported to the transfer nip, is formed on the intermediate transfer body, the control means is capable of executing a first operation of executing the rear surface printing at a first interval from the immediately preceding print, and a second operation of executing the rear surface printing at a second interval from the immediately preceding print that is wider than the first interval, the control means is configured to be able to execute the first action when the second action is not designated by a user, and to be able to execute the second action when the second action is designated by a user; When the detection means detects that there is no sheet corresponding to the target sheet, the toner image formed in the second operation is transferred to the re-transported sheet. An image forming apparatus characterized by:

8. When the second operation is performed, the control unit can perform a cleaning operation of the transfer member before the toner image formed in the back printing reaches the transfer nip.

8. The image forming apparatus according to claim 7,

9. an application unit that applies a voltage to the transfer member so that a potential difference is generated between the transfer member and the intermediate transfer body; the control means is configured to control the application means to execute a first application operation and a second application operation, wherein in the first application operation, the toner charged to the normal polarity is subjected to an electrostatic force in a direction from the intermediate transfer body to the transfer member, and in the second application operation, the toner charged to the normal polarity is subjected to an electrostatic force in a direction from the transfer member to the intermediate transfer body, When the detecting means detects that there is no sheet corresponding to the target sheet, the second application operation is executed when the toner image formed in the immediately preceding print passes through the transfer nip.

9. The image forming apparatus according to claim 8,

10. the first application operation is performed in the cleaning operation; 10. The image forming apparatus according to claim 9,

11. When the margin between the rear end of the toner image formed in the immediately preceding print and the target sheet is longer than a predetermined length, and when the detection means detects that there is no sheet corresponding to the target sheet even if the designation is made, the control means is capable of executing the first operation.

8. The image forming apparatus according to claim 7,

12. an image carrier; an intermediate transfer member onto which the toner image formed on the image carrier is transferred; a transfer member that transfers the toner image transferred onto the intermediate transfer member onto a sheet, the transfer member forming a transfer nip between itself and the intermediate transfer member; a storage section for storing a sheet; a detection means for detecting the presence or absence of a sheet in the storage section; a double-sided conveying path through which a sheet having an image formed on its front surface passes and is conveyed toward the transfer nip; a control means capable of executing backside printing and immediate-preceding printing; In the back side printing, a toner image to be transferred to a re-conveyed sheet transported from the double-sided transport path to the transfer nip is formed on the intermediate transfer body, and in the immediately preceding printing, a toner image to be transferred to a target sheet different from the re-conveyed sheet, which is transported to the transfer nip immediately before the re-conveyed sheet is transported to the transfer nip, is formed on the intermediate transfer body, the control means is capable of executing a first operation of executing the rear surface printing at a first interval from the immediately preceding print, and a second operation of executing the rear surface printing at a second interval from the immediately preceding print that is wider than the first interval, the control means is configured to be able to execute the first action when the second action is not designated by a user, and to be able to execute the second action when the second action is designated by a user; when the detecting means detects that there is no sheet corresponding to the target sheet, the amount of toner adhering to the transfer member when the toner image formed in the immediately preceding print passes through the transfer nip is greater when the designation is made than when the designation is not made; An image forming apparatus characterized by:

13. the transfer member rotates an angle of 360 degrees or more while the portion corresponding to the second interval passes through the transfer nip; 13. The image forming apparatus according to claim 12.

14. the transfer member rotates an angle of less than 360 degrees while the portion corresponding to the first interval passes through the transfer nip; 14. The image forming apparatus according to claim 13.

15. an image carrier; an intermediate transfer member onto which the toner image formed on the image carrier is transferred; a transfer member that transfers the toner image transferred onto the intermediate transfer member onto a sheet, the transfer member forming a transfer nip between itself and the intermediate transfer member; a storage section for storing a sheet; a detection means for detecting the presence or absence of a sheet in the storage section; a double-sided conveying path through which a sheet having an image formed on its front surface passes and is conveyed toward the transfer nip; a control means capable of executing backside printing and immediate-preceding printing; In the back side printing, a toner image to be transferred to a re-conveyed sheet transported from the double-sided transport path to the transfer nip is formed on the intermediate transfer body, and in the immediately preceding printing, a toner image to be transferred to a target sheet different from the re-conveyed sheet, which is transported to the transfer nip immediately before the re-conveyed sheet is transported to the transfer nip, is formed on the intermediate transfer body, the control means is capable of executing a first operation of transferring the toner image formed in the back surface printing onto the re-conveying sheet, a second operation of performing a cleaning operation of the transfer member before the toner image formed in the back surface printing reaches the transfer nip and transferring the toner image formed in the back surface printing onto the re-conveying sheet, and a third operation of performing the cleaning operation after the toner image formed in the back surface printing has passed through the transfer nip and transferring the toner image formed by performing the back surface printing again onto the re-conveying sheet, the control means is configured to be able to execute the first operation when a predetermined condition is not satisfied, and to be able to execute the second operation or the third operation depending on the interval between the immediately preceding print and the rear surface print when the predetermined condition is satisfied, the predetermined condition includes at least one of the following: an environmental temperature is equal to or higher than a predetermined temperature; the remaining life of a developing device is less than a predetermined value; a user has designated the second operation; the image density of the immediately preceding print is equal to or higher than a predetermined value; the image of the immediately preceding print includes other than text; and the maximum density of a partial area of ​​the immediately preceding print is equal to or higher than a predetermined value. when the detection means detects that there is no sheet corresponding to the target sheet, the toner image formed in the immediately preceding print passes through the transfer nip before the first operation, the second operation, or the third operation; An image forming apparatus characterized by:

16. the control means executes the second operation when the interval between the immediately preceding print and the rear surface print is long enough to execute the cleaning operation, and executes the third operation when the interval is not long enough to execute the cleaning operation.

16. The image forming apparatus according to claim 15.

17. an application unit that applies a voltage to the transfer member so that a potential difference is generated between the transfer member and the intermediate transfer body; the control means is configured to control the applying means to perform a first applying operation and a second applying operation; In the first application operation, the toner charged to the normal polarity is subjected to an electrostatic force in a direction from the intermediate transfer body to the transfer member, and in the second application operation, the toner charged to the normal polarity is subjected to an electrostatic force in a direction from the transfer member to the intermediate transfer body, When the detecting means detects that there is no sheet corresponding to the target sheet, the second application operation is executed when the toner image formed in the immediately preceding print passes through the transfer nip.

16. The image forming apparatus according to claim 15.

18. In the third operation, the second application operation is performed when the toner image formed in the back printing passes through the transfer nip before the cleaning operation.

18. The image forming apparatus according to claim 17.

19. the first application operation is performed in the cleaning operation; 19. The image forming apparatus according to claim 17 or 18.

20. When the detection means detects that there is no sheet corresponding to the target sheet, if the margin between the rear end of the toner image formed in the immediately preceding print and the target sheet is longer than a predetermined length, the control means can execute the second operation even if the cleaning operation cannot be executed between the immediately preceding print and the back side print.

16. The image forming apparatus according to claim 15.

Images